Modulo_16 - PDFCOFFEE.COM (2024)

Module

16 16.1: Fundamentals 16.2: Engine Performance 16.3: Engine Construction 16.4: Engine Fuel Systems 16.5: Starting and Ignition systems 16.6: Induction, Exhaust and Cooling Systems 16.7: Supercharging/Turbocharging 16.8: Lubricants and Fuels 16.9: Lubrication Systems 16.10: Engine Indication Systems 16.11: Powerplant Installation 16.12: Engine Monitoring and Ground Operation 16.13: Engine Storage and Preservation

Piston Engine Licence Category B1 Issue number 2

Licence Category B1 and B3

16.1 Fundamentals

Copyright notice © Copyright. All worldwide rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form by any other means whatsoever: i.e. photocopy, electronic, mechanical recording or otherwise without the prior written permission of Total Training Support Ltd.

Knowledge levels – Category A, B1, B2, B3 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1, B2 and B3 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category B2 basic knowledge levels.

Objectives: • The applicant should be able to understand the theoretical fundamentals of the subject. • The applicant should be able to give a general description of the subject using, as appropriate, typical examples. • The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. • The applicant should be able to read and understand sketches, drawings and schematics describing the subject. • The applicant should be able to apply his knowledge in a practical manner using detailed procedures.

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

Objectives: • The applicant should be familiar with the basic elements of the subject. • The applicant should be able to give a simple description of the whole subject, using common words and examples. • The applicant should be able to use typical terms.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

Objectives: • The applicant should know the theory of the subject and interrelationships with other subjects. • The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. • The applicant should understand and be able to use mathematical formulae related to the subject. • The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. • The applicant should be able to apply his knowledge in a practical manner using the manufacturers’ instructions. • The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Mechanical, thermal and volumetric efficiencies;

Part-66 Ref. 16.1

Knowledge Levels A B1 B3 1

Operating principles – 2-stroke, 4-stroke, Otto and Diesel; Piston displacement and compression ratio; Engine configuration and firing order.

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Table of Contents Introduction ___________________________________ 6 Type of engines ______________________________ 6 History of aero-gasoline engines __________________ 6 Examples of modern aero-gasoline engines ________ 14 History of Diesel aero engines __________________ 16 Examples of modern aero-Diesel engines _________ 20 Piston engine world records ____________________ 22 Development of power _________________________ 24 Operating principles ___________________________ The 4-stroke cycle engine ______________________ The Otto cycle _______________________________ The Diesel cycle _____________________________ Valve timing ________________________________ Valve operation ______________________________ Pre-Ignition _________________________________ The 2-stroke cycle engine ______________________ Diesel engines ______________________________ Gasoline and Diesel engines – three differences ____

28 28 34 38 42 46 46 48 52 64

Engine configurations __________________________ 80 General ____________________________________ 80 Inline engines ________________________________ 80 Opposed or O-type engines _____________________ 80 V-type engines _______________________________ 82 X-type engines _______________________________ 82 H-type engines _______________________________ 82 Radial engines _______________________________ 86 Rotary engines _______________________________ 90 Wankle engines ______________________________ 92 Cylinder numbering ___________________________ 94 Firing order __________________________________ 96 General ____________________________________ 96 Single-row radial engines_______________________ 96 Double-row radial engines ______________________ 96 Inline and V-engines __________________________ 98 Horizontally-opposed engines __________________ 100 Left-hand rotation ____________________________ 102

Piston displacement ___________________________ 66

Valve configurations __________________________ 104

Compression ratio ____________________________ 68

Camshaft configurations ______________________ 110

Engine efficiencies ____________________________ Mechanical efficiency _________________________ Thermal efficiency ____________________________ Volumetric efficiency __________________________ Efficiencies of Diesel engines ___________________

Gasoline and Diesel engines – a comparison ______ 112

70 70 70 76 78

Glossary of piston engine terms ________________ 118

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Introduction Type of engines Several methods can classify aircraft engines. They can be classed by operating cycles, cylinder arrangement, or the method of thrust production. All are heat engines that convert fuel into heat energy that is converted to mechanical energy to produce thrust. Most of the current aircraft engines are of the internal combustion type because the combustion process takes place inside the engine. Aircraft engines come in many different types, reciprocatingpiston, rotary, two- or four-cycle, spark-ignition, Diesel, and airor water-cooled. Reciprocating engines also have subdivisions based on the type of cylinder arrangement. Manufacturers have developed some designs that are used more commonly than others and are, therefore, recognised as ‘conventional’. Reciprocating engines may be classified according to the cylinder arrangement (inline, V-type, radial, and opposed) or according to the method of cooling (liquidcooled or air-cooled).

History of aero-gasoline engines Pistons in cylinders first saw use in steam engines. Scotland’s James Watt crafted the first good ones during the 1770s. A century later, the German inventors Nicolaus Otto and Gottlieb Daimler introduced gasoline as the fuel, burned directly within the cylinders. Such motors powered the earliest automobiles. They were lighter and more mobile than steam engines, more reliable, and easier to start. Some single-piston gasoline engines entered service, but for use with aeroplanes, most such engines had several pistons, each shuttling back and forth within its cylinder. Each piston also had a connecting rod, which pushed on a crank that was part of a crankshaft. This crankshaft drove the propeller. Engines built for aeroplanes had to produce plenty of power while remaining light in weight. The first American aeroplane builders – Wilbur and Orville Wright, Glenn Curtiss – used motors that resembled those of cars. They were heavy and complex because they used water-filled plumbing to stay cool. A French engine of 1908, the Gnome, introduced air cooling as a way to eliminate the plumbing and lighten the weight. It was known as a rotary engine. The Wright and Curtiss motors had been mounted firmly in supports, with the shaft and propeller spinning. Rotary engines reversed that, with the shaft being held tightly – and the engine spinning! The propeller was mounted to the rotating engine, which stayed cool by having its cylinders whirl in the open air.

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Numerous types of Gnome engines were designed and built, one of the most famous being the 165-hp 9-N Monosoupape (one valve). It was used during the first world war, primarily in the Nieuport 28. The engine had one valve per cylinder. Having no intake valves, its fuel mixture entered the cylinders through circular holes or ports cut in the cylinder walls. The propeller was bolted firmly to the engine and it, along with the cylinders, turned as a single unit around a stationary crankshaft rigidly mounted to the fuselage of the aeroplane. The rotary engine used castor oil for lubrication.

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During the first world war, rotaries attained tremendous popularity. They were less complicated and easier to make than the water-cooled type. They powered such outstanding fighter aeroplanes as German’s Fokker DR-1 and Britain’s Sopwith Camel. They used castor oil for lubrication because it did not dissolve in gasoline. However, they tended to spray this oil all over, making a smelly mess. Worse, they were limited in power. The best of them reached 190 to 210 kW (260 to 280 hp). Thus, in 1917 a group of American engine builders returned to water cooling as they sought a 300-kW (400-hp) engine. The engine that resulted, the Liberty, was the most powerful aircraft engine of its day, with the US car industry building more than 20,000 of them. Water-cooled engines built in Europe also outperformed the air-cooled rotaries and lasted longer. With the war continuing until late in 1918, the rotaries lost favour. In this fashion, designers returned to water-cooled motors that again were fixed in position. They stayed cool by having water or antifreeze flow in channels through the engine to carry away the heat. A radiator cooled the heated water. In addition to offering plenty of power, such motors could be enclosed entirely within a streamlined housing, to reduce drag and thus produce higher speeds in flight. Rolls Royce, Britain’s leading engine builder, built only water-cooled motors. Air-cooled rotaries were mostly out of the picture after 1920. Even so, air-cooled engines offered tempting advantages. They dispensed with radiators that leaked, hoses that burst, cooling jackets that corroded, and water pumps that failed.

Thus, the air-cooled radial engine emerged. This type of aircooled engine arranged its cylinders to extend radially outward from its hub, like spokes of a wheel. The US Navy became an early supporter of radials, which offered reliability along with lightweight. This was an essential feature if aeroplanes were to take off successfully from an aircraft carrier’s flight deck. With financial support from the Navy, two American firms, Wright Aeronautical and Pratt & Whitney began building aircooled radials. The Wright Whirlwind, in 1924, delivered 164 kW (220 hp). A year later, the Pratt & Whitney Wasp was tested at 306 kW (410 hp). Aircraft designers wanted to build aeroplanes that could fly at high altitudes. High-flying aeroplanes could swoop down on their enemies and also were harder to shoot down. Bombers and passenger aircraft flying at high altitudes could fly faster because air is thin at high altitudes and there is less drag in the thinner air. These aeroplanes also could fly farther on a tank of fuel. But because the air was thinner at high altitude, aircraft engines produced much less power. They needed air to operate, and they could not produce power unless they had sufficient air. Designers responded by fitting the engine with a supercharger. This was a pump that took in air and compressed it. The extra air, fed into an engine, enabled it to continue to put out full power even at high altitude.

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Sopwith camel

A Caudron seaplane being craned onto La Foudre in 1914

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Early superchargers underwent tests before the end of the first world war, but they were heavy and offered little advantage. The development of superchargers proved to be technically demanding, but by 1930, the best British and American engines installed such units routinely. In the United States, the Armyfunded work on superchargers at another engine-builder, General Electric. After 1935, engines fitted with GE’s superchargers gave full power at heights above 9,000 m (30,000 ft).

Leaded gasoline improved an aircraft engine’s performance by enabling it to use a supercharger more effectively while using less fuel. The results were spectacular. The best engine of the first world war, the Liberty, developed 300 kW (400 hp). During the second world war, The Rolls-Royce Merlin engine was about the same size – and put out 1,640 kW (2,200 hp). Samuel Heron, a long-time leader in the development of aircraft engines and fuels, writes that “it is probably true that about half the gain in power was due to fuel”.

Fuels for aviation also demanded attention. When engine designers tried to build motors with greater power, they ran into the problem of ‘knock’. This had to do with the way fuel burned within them. An aeroplane engine had a carburettor that took in fuel and air, producing a highly flammable mixture of gasoline vapour with air, which went into the cylinders. There, this mix was supposed to burn very rapidly, but in a controlled manner. Unfortunately, the mixture tended to explode, which damaged engines. The motor then was said to knock.

These advances in supercharging and knock-resistant fuels laid the groundwork for the engines of the second world war. In 1939, the German test pilot Fritz Wendel flew a piston-powered fighter to a speed record of 755 KM/H (469 mph). US bombers used superchargers to carry heavy bomb loads at 10,000 m (34,000 ft). They also achieved a long-range, the B-29 bomber had the range of 9,000 km (5,600 miles). Fighters routinely topped 640 KM/H (400 mph). Airliners, led by the Lockheed Constellation, showed that they could fly non-stop from coast to coast of the USA.

Poor-grade fuels avoided knock but produced little power. Soon after the first world war, an American chemist, Thomas Midgely, determined that small quantities of a suitable chemical added to high-grade gasoline might help it burn without knock. He tried several additives and found that the best was tetraethyl lead. The US Army began experiments with leaded aviation fuel as early as 1922; the Navy adopted it for its carrier-based aircraft in 1926. Leaded gasoline became standard as a high-test fuel, used widely in cars as well as in aircraft.

By 1945, the jet engine was drawing both attention and excitement. Jet fighters came quickly to the forefront. However, while early jet engines gave dramatic increases in speed, they showed poor fuel economy. It took time before engine builders learned to build jets that could sip fuel rather than gulp it. Until that happened, the piston engine retained its advantage for use in bombers and airliners, which needed to be able to fly a great distance without refuelling.

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Supermarine Spitfire

Messerschmitt Bf109

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Modern-day horizontally opposed gasoline aero-engine

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Examples of modern aero-gasoline engines

Rotax two-stroke engines Continental IOF-240 engine

Rotax, build engines for a number of specialty markets, including motorcycles, watercraft, and snowmobiles. The single-cylinder two-stroke model 277, an early Rotax aircraft engine, put out 26 hp, weighed just 30 kg (65 lbs) with reduction drive and exhaust, and cost just over $1,000. From 1975 through today, Rotax has produced more than 170,000 aircraft engines, most of them two-stroke lightweight models. From the 277, Rotax progressed to building ever more capable and powerful models, including the popular 447 and 503 models up to the 582, one of the most technologically advanced two-stroke engines, with rotary valves, oil injection, dual carbs and electronic ignition. The 582, which is still in production today, powers many dozens of light-aircraft models. Total Training Support Ltd © Copyright 2020

When introduced in 2002, the engine was rare in that it had neither magnetos nor mixture control. Instead, the ignition and fuel flow are controlled electronically. In addition to providing the optimal fuel/air mixture – the IOF-240 burns about 19 l (5 gal) per hour – the electronic system allows for more accurate engine analysis and troubleshooting. The IOF-240 was first introduced in the Liberty XL2, a two-seat carbon fibre aeroplane developed from the Europa kit aeroplane, which became the first piston-engine aeroplane certified with FADEC.

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Lycoming O-540 engine Continental TSIO-550 engine

As had been done by other manufacturers – sometimes to good effect – Lycoming expanded the horsepower range of its four-cylinder aircraft engines by adding another pair of opposed cylinders, creating a six-cylinder engine with 50% more potential power. The 540 series engines got their start in 1957 as new larger personal and charter aeroplanes pressed the need for more power. Piper made great use of the engine, most notably in its Navajo line-up, PA-32s, Aztecs, Comanches and Mirages, but numerous other manufacturers opted for 540-power as well, including Aero Commander, Pitts and Robinson Helicopter’s R44. As with the four-cylinder version, the 540 is available in a variety of models, with turbocharging a popular option.

The six-cylinder, fuel-injected TSIO-550 is the turbocharged version of the IO-550, which was first introduced by Continental in 1983. Engines in the IO-550 series produce anywhere from 110 to 270 kW (280 to 360 hp). In the 1990s, at Raytheon’s request, Continental tweaked the IO-550 to optimise the performance of the 225 kW (300 hp) engine that powers the Beechcraft Baron 58. The result was a smooth, reliable engine that was hard to beat in its class. With even better performance at higher altitude thanks to its dual turbochargers, the TSIO-550 powers the three topperforming modern-day single-engine certified piston aeroplanes: the Cessna TTx, Cirrus SR22 and Mooney Acclaim.

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History of Diesel aero engines The aircraft Diesel engine or aero Diesel has not been widely used. Diesel engines were used in airships and were tried in aircraft in the late 1920s and 1930s but were never widely adopted. Their main advantages are their excellent specific fuel consumption and the somewhat higher density of their fuel. Still, these have been outweighed by a combination of inherent disadvantages compared to gasoline-fuelled or turboprop engines. The ever-rising cost of AVGAS and doubts about its future availability have spurred a resurgence in aircraft Diesel engine production in recent years. Several manufacturers built Diesel aero engines in the 1920s and 1930s; the best known were the Packard air-cooled radial, and the Junkers Jumo 205, which was moderately successful but proved unsuitable for combat use during the second world war. The Blohm & Voss Bv 138 trimotor maritime patrol flying boat, however, was powered with the more advanced Junkers Jumo 207 powerplant. It was more successful, with its trio of Diesel Jumo 207s conferring upwards of a maximum 2,100 km (1,300 miles) combat radius. Nearly 300 examples of the Bv 138 were built during the second world war.

The first successful flight of a Diesel-powered aircraft was made on September 18, 1928, in a Stinson model SM-IDX Detroiter, registration number X7654. Around 1936, the heavier but less thirsty Diesel engines were only preferred over gasoline engines when flight time was over 6–7 hours. Entering service in the early 1930s, the two-stroke Junkers Jumo 205 opposed-piston engine was much more widely used than previous aero Diesels. It was moderately successful in its use in the Blohm & Voss Ha 139 and even more so in airship use. In Britain Napier & Son license-built the larger Junkers Jumo 204 as the Napier Culverin, but it did not see production use in this form. A Daimler-Benz Diesel engine was also used in Zeppelins, including the ill-fated LZ 129 Hindenburg. This engine proved unsuitable in military applications, and subsequent German aircraft engine development concentrated on gasoline and jet engines.

The first successful Diesel engine explicitly developed for aircraft was the Packard radial Diesel of 1928–1929, which was laid out in the familiar air-cooled radial format similar to Wright and Pratt & Whitney designs and was contemporary with the Beardmore Tornado used in the R101 airship. The use of a Diesel had been specified for its low-fire-risk fuel.

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The Centurion line of Diesel engines found early success on the Diamond DA-42 twin

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The Soviet second world war era four-engine strategic bomber Petlyakov Pe-8 was built with Charomskiy ACh-30 Diesel engines, but later in the production run Diesels were replaced with radial gasoline engines because of efficiency concerns. The Yermolaev Yer-2 long-range medium bomber was also built with Charomskiy Diesel engines. Other manufacturers also experimented with Diesel engines in this period, such as the French Bloch (later Dassault Aviation), whose MB203 bomber prototype used Clerget Diesels of radial design. The Royal Aircraft Establishment developed an experimental compression ignition (Diesel) version of the RollsRoyce Condor in 1932, flying it in a Hawker Horsley for test purposes.

Several factors have emerged to change this equation. Several new manufacturers of general aviation aircraft developing new designs have emerged. In Europe, in particular, AVGAS has become very expensive. In several (particularly remote) locations, AVGAS is harder to obtain than Diesel fuel. Finally, automotive Diesel technologies have significantly improved in recent years, offering higher power-to-weight ratios more suitable for aircraft application. Certified Diesel-powered light aeroplanes are currently available, and several companies are developing new engine and aircraft designs for the purpose – many of these run on readily available jet fuel (kerosene), or conventional automotive Diesel.

Interest in Diesel engines in the post-war period was sporadic. The lower power-to-weight ratio of Diesels, particularly compared to turboprop engines, weighed against the Diesel engine. With fuel available cheaply and most research interest in turboprops and jets for high-speed airliners, Diesel-powered aircraft virtually disappeared. The near-death of the general aviation market in the 1990s saw a massive decline in the development of any new aircraft engine types. Napier & Son in Britain had developed the Napier Culverin, a derivative of the Junkers Jumo 205, before the second world war, and took up aero Diesel engines again in the 1950s. The British Air Ministry supported the development of the 2,200 kW (3,000 hp) Napier Nomad, a combination of piston and turboprop engines, which was exceptionally efficient in terms of brake specific fuel consumption but judged too bulky and complicated and cancelled in 1955.

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Hybrid Air Vehicles HAV 304 Airlander 10, aerostatic aerodynamic lift 4 × Thielert Centurion 325 hp V8 diesel engine

Thielert TAE 125-02-99 4-cylinder Diesel engine

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Examples of modern aero-Diesel engines

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Piston engine world records First The Wright brothers made the first controlled, sustained flight of a powered, heavier than air aircraft with the Wright Flyer on December 17, 1903, four miles south of Kitty Hawk, North Carolina USA. Fastest On 2nd September 2017, Steve Hinton Jr. established a new speed record for a propeller-driven piston-engine aircraft by flying the heavily-modified second world war North American P-51 Mustang, Voodoo, at an average speed of 855.41 km/h (531.53 mph). A Hawker Sea Fury holds the unofficial record for the fastest piston-engine aeroplane in level flight at 880 km/h (547 mph). Highest The Grob Strato 2C set the world altitude record for manned piston-engine aircraft of 18,552 m (60,897') on 4 August 1995. It was powered by two Teledyne Continental TSIO-550 turbocharged piston engine with Pratt & Whitney PW127 gas generator to provide a constant supply of pressurised air to the piston engine at high altitude. This had the advantage of maintaining power at high altitudes. Each engine drove a 6 m (19' 8") diameter five-bladed propeller.

Largest The Lycoming XR-7755 was the largest piston-driven aircraft engine ever produced, with 36 cylinders totalling about 127 L (7,750 in³) of displacement and a power output of 3,700 kW (5,000 hp) for 2,740 kg (6,050 lbs). It was initially intended to be used in the ‘European bomber’ that eventually emerged as the Convair B-36. Only two examples were built before the project was terminated in 1946. Most common Lycoming Engines is a major American manufacturer of aircraft engines. With a factory in Williamsport, Pennsylvania, Lycoming produces a line of horizontally opposed, air-cooled, four-, six- and eight-cylinder engines including the only FAAcertified aerobatic and helicopter piston engines on the market. The company has built more than 325,000 piston aircraft engines and powers more than half the world’s general aviation fleet, both rotary- and fixed-wing. Lycoming is an operating division of Avco Corporation, itself a subsidiary of Textron

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Grob G-850 Strato 2

North American P51 Mustang Voodoo

Wilbur Wright adjusts the engine of his aircraft, France 1908

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Development of power The power of an internal combustion engine comes from burning a mixture of fuel and air in a small, enclosed space. When this mixture bums, it expands considerably, and the push or pressure created is used to move the piston, thereby rotating the crankshaft. This motion is eventually sent to the wheels that move the vehicle. Since similar action occurs in each cylinder of an engine, we will use one cylinder to describe the steps in the development of power. The one-cylinder engine consists of four basic parts, as shown below. First, we must have a cylinder that is closed at one end; this cylinder is similar to a tall metal can that is stationary within the engine block. Inside this cylinder is the piston – a movable plug. It fits snugly into the cylinder but can still slide up and down easily. This piston movement is caused by fuel burning in the cylinder. The up-and-down movement of the piston is called reciprocating motion. This motion must be changed into rotary motion, so the wheels or tracks of a vehicle can rotate. This change is accomplished by a throw on the crankshaft and the connecting rod which connects the piston and crankshaft throw.

When the piston slides downward because of the pressure of the expanding gas in the cylinder, the upper end of the connecting rod moves downward in a straight line. The lower end of the connecting rod moves down and in a circular motion at the same time. This moves the throw and, in turn, the throw rotates the crankshaft; this rotation is the desired result. The crankshaft and connecting rod combination is a mechanism to change straight-line or reciprocating motion, to circular or rotary motion. Each movement of the piston from top to bottom or from bottom to top is called a stroke. The piston makes two-strokes (an upstroke and a downstroke), as the crankshaft makes one complete revolution.

The throw is an offset section of the crankshaft that scribes a circle, as the shaft rotates. The top end of the connecting rod is connected to the piston and must, therefore, go up and down. The lower end of the connecting rod is attached to the crankshaft. The lower end of the connecting rod also moves up and down, but because it is attached to the crankshaft, it must also move in a circle.

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The piston is connected to the rotating crankshaft by a connecting rod. In view (A), the piston is at the beginning or top of the stroke. As the crankshaft rotates, the connecting rod pulls the piston down. When the crankshaft has rotated one-half turn, the piston is at the bottom of the stroke (green ghost). As the crankshaft rotates in view (B), the connecting rod pulls the piston down. The piston continues moving downward until the motion of the crankshaft causes it to begin moving up. The position of the piston at the instant its motion changes from down to up is known as the bottom dead centre (BDC). The position of the piston at the instant, its motion changes from up to down is known as the top dead centre (TDC). The term ‘dead’ indicates where one motion has stopped (the piston has reached the end of the stroke) and its opposite turning motion is ready to start. These positions are called ‘rock’ positions.

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Basic piston engine 1-27

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Operating principles The 4-stroke cycle engine In this type of engine, four-strokes are required to complete the required series of events or the operating cycle of each cylinder. 1 – Induction (first down-stroke) The inlet valve is open, the piston moves down the cylinder (from TDC to BDC), fuel/air mixture is drawn into the cylinder (the charge). 2 – Compression (first upstroke) Both of the valves are closed, the piston moves back up the cylinder (from BDC to TDC), fuel/air mixture is compressed into the combustion chamber. 3 – Power (second down-stroke) Both valves remain closed, and a spark occurs igniting the compressed fuel/air mixture. The rapid expansion of the burning mixture forces the piston back down the cylinder (from TDC to BDC).

Two complete revolutions of the crankshaft (720°) are required for the four-strokes. Thus, each cylinder in an engine of this type fires once in every two revolutions of the crankshaft. In the following discussion of the four-stroke cycle engine operation, note that the timing of the ignition and the valve events vary considerably in different engines. Many factors influence the timing of a specific engine, and the engine manufacturer’s recommendations in this respect must be followed in maintenance and overhaul. The timing of the valve and ignition events is always specified in its degrees of crankshaft travel. It should be remembered that a certain amount of crankshaft travel is required to open a valve fully; therefore, the specified timing represents the start of opening rather than the fully open position of the valve. An example of the four-stroke cycle can be seen below.

4 – Exhaust (second upstroke) The exhaust valve is open, the piston moves back up the cylinder (from BDC to TDC). The burnt gases having now performed their usual work on the power stroke, are expelled into the atmosphere.

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The gasoline internal combustion engine works on the Otto cycle

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Induction (intake) stroke During the intake stroke, the piston is pulled downward in the cylinder by the rotation of the crankshaft. This reduces the pressure in the cylinder and causes air under atmospheric pressure to flow through the carburettor, which meters the correct amount of fuel. The fuel/air mixture passes through the intake pipes and intake valves into the cylinders. The quantity or weight of the fuel/air charge depends upon the degree of throttle opening. The intake valve is opened considerably before the piston reaches TDC on the exhaust stroke, to induce a higher quantity of the fuel/air charge into the cylinder and thus increase the horsepower. The distance the valve may be opened before TDC, however, is limited by several factors, such as the possibility that hot gases remaining in the cylinder from the previous cycle may flashback into the intake pipe and the induction system. In all high-power aircraft engines, both the intake and the exhaust valves are off the valve seats at TDC at the start of the intake stroke. As mentioned above, the intake valve opens before TDC on the exhaust stroke (valve lead), and the closing of the exhaust valve is delayed considerably after the piston has passed TDC and has started the intake stroke (valve lag). This timing is called valve overlap and is designed to aid in cooling the cylinder internally by circulating the cool incoming fuel/air mixture, to increase the amount of the fuel/ air mixture induced into the cylinder, and to aid in scavenging the byproducts of combustion from the cylinder.

The intake valve is timed to close about 50° to 75° past BDC on the compression stroke, depending upon the specific engine, to allow the momentum of the incoming gases to charge the cylinder more completely. Because of the comparatively large volume of the cylinder above the piston when the piston is near BDC, the slight upward travel of the piston during this time does not have a significant effect on the incoming flow of gases. This late timing can be carried too far because the gases may be forced back through the intake valve and defeat the purpose of the late closing. Compression stroke After the intake valve is closed, the continued upward travel of the piston compresses the fuel/air mixture to obtain the desired burning and expansion characteristics. The charge is fired using an electric spark as the piston approaches TDC. The time of ignition varies from 20° to 35° before TDC, depending upon the requirements of the specific engine to ensure complete combustion of the charge by the time the piston is slightly past the TDC position. Many factors affect ignition timing, and the engine manufacturer has expended considerable time in research and testing to determine the best setting. All engines incorporate devices for adjusting the ignition timing, and the ignition system must be timed according to the engine manufacturer’s recommendations.

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Intake and compression stroke

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Power (combustion) stroke As the piston moves through the TDC position at the end of the compression stroke and starts down on the power stroke, it is pushed downward by the rapid expansion of the burning gases within the cylinder head with a force that can be greater than 15 tons (30,000 psi) at the maximum power output of the engine. The temperature of these burning gases may be between 1,650°C and 2,750°C (3,000°F and 4,000°F). As the piston is forced down during the power stroke by the pressure the burning gases exert upon it, the downward movement of the connecting rod is changed to rotary movement by the crankshaft. Then, the rotary movement is transmitted to the propeller shaft to drive the propeller. As the burning gases are expanded, the temperature drops to within safe limits before the exhaust gases flow out through the exhaust port.

Exhaust stroke As the piston travels through BDC at the completion of the power stroke and starts upwards on the exhaust stroke, it begins to push the burnt exhaust gasses of out of the exhaust port. The speed of the exhaust gasses leaving the cylinder creates low pressure in the cylinder. This low pressure speeds the flow of the fresh fuel/air charge into the cylinder as the intake valve begins to open. The intake valve opening is timed to occur at 8° to 55° before TDC on the exhaust stroke on various engines. Aircraft Systems - 03 – Engine https://www.youtube.com/watch?v=gIdXLMVP6VU

The timing of the exhaust valve opening is determined by, among other considerations, the desirability of using as much of the expansive force as possible and of scavenging the cylinder as completely and rapidly as possible. The valve is opened considerably before BDC on the power stroke (on some engines at 50° and 75° before BDC) while there is still some pressure in the cylinder. This timing is used so that the pressure can force the gases out of the exhaust port as soon as possible. This process frees the cylinder of waste heat after the desired expansion has been obtained and avoids overheating the cylinder and piston. Thorough scavenging is very important because any exhaust products remaining in the cylinder dilute the incoming charge at the start of the next cycle.

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Combustion and exhaust stroke

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The Otto cycle The German engineer, Nicolaus Otto, was the first to develop a functioning four-stroke engine, which is why the four-stroke principle is sometimes known as the Otto cycle. Four-stroke engines using spark plugs are known as Otto engines. The Otto cycle consists of: • • • •

adiabatic compression; heat addition at constant volume; adiabatic expansion; and rejection of heat at constant volume.

To understand how a piston engine works, we must study the basic thermodynamics of gases. Gases have various properties that we can observe with our senses, including the gas pressure P, temperature T, mass, and volume V that contains the gas. Scientific observation has determined that these variables are related to one another, and the values of these properties determine the state of the gas. A thermodynamic process, such as heating or compressing the gas, changes the values of the state variables in a manner which is described by the laws of thermodynamics. The work done by a gas and the heat transferred to it depends on the beginning and end states and on the process used to change the state. It is possible to perform a series of processes, in which the state is changed during each process, but the gas eventually returns to its original state. Such a series of processes is called a cycle and forms the basis for understanding engine operation.

Using the engine stage numbering system, we begin at the lower left with stage 1 being the beginning of the intake stroke of the engine. The pressure is near atmospheric pressure, and the gas volume is at a minimum. Between stage 1 and stage 2, the piston is pulled out of the cylinder with the intake valve open. The pressure remains constant, and the gas volume increases as the fuel/air mixture is drawn into the cylinder through the intake valve. Stage 2 begins the compression stroke of the engine with the closing of the intake valve. Between stage 2 and stage 3, the piston moves back into the cylinder, the gas volume decreases, and the pressure increases because work is done on the gas by the piston. Stage 3 is the beginning of the combustion of the fuel/air mixture. The combustion occurs very quickly, and the volume remains constant. Heat is released during combustion, which increases both the temperature and the pressure, according to the equation of state. Stage 4 begins the power stroke of the engine. Between stages 4 and 5, the piston is driven towards the crankshaft. The volume in the cylinder is increased, and the pressure falls as the gas does work on the piston. At stage 5, the exhaust valve is opened, and the residual heat in the gas is exchanged with the surroundings. The volume remains constant, and the pressure adjusts back to atmospheric conditions.

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The gasoline internal combustion engine works on the Otto cycle

The Otto cycle P-V diagram

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Stage 6 begins the exhaust stroke of the engine during which the piston moves back into the cylinder, the volume decreases, and the pressure remains constant. At the end of the exhaust stroke, conditions have returned to stage 1, and the process repeats itself. During the cycle, work is done on the gas by the piston between stages 2 and 3. The gas does work on the piston between stages 4 and 5. The difference between the work done by the gas and the work done on the gas is the area enclosed by the cycle curve and is the work produced by the cycle. The work times the rate of the cycle (cycles per second) is equal to the power produced by the engine. The area enclosed by the cycle on a P-V diagram is proportional to the work produced by the cycle. Here we have shown an ideal Otto cycle in which there is no heat entering (or leaving) the gas during the compression and power strokes (this is the definition of ‘adiabatic’), no friction losses, and instantaneous burning occurring at constant volume. In reality, the ideal cycle does not occur, and there are many losses associated with each process. These losses are generally accounted for by efficiency factors which multiply and modify the ideal result. For a real cycle, the shape of the P-V diagram is similar to the ideal, but the area (work) is always less than the ideal value.

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The Otto cycle P-V diagram

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The Diesel cycle In the Diesel cycle, heat is supplied at constant pressure, whereas in the Otto cycle heat is supplied at constant volume. Similar in construction to the Otto engine, the Diesel is also a closed cycle internal combustion engine. Still, instead of using a spark to ignite the fuel, ignition is achieved by rapid compression of the fuel-air mixture to a higher pressure than in the Otto engine. The higher compression ratio allows greater efficiencies to be achieved by the Diesel. The Diesel cycle uses the following processes: A to B – Compression stroke – Adiabatic compression of air in the cylinder. No fuel added yet. B to C – Ignition – Isobaric heat addition. Fuel introduced into the compressed air at the top of the compression stroke. Fuel mixture ignited while the pressure is essentially constant. C to D – Expansion (power) stroke – Adiabatic expansion of the hot gases in the cylinder. D to A – Exhaust stroke – Ejection of the spent, hot gases. Induction stroke. Intake of the next air charge into the cylinder. The volume of exhaust gasses is the same as the air charge. Diesel Engine, How it works https://youtu.be/DZt5xU44IfQ

In the true Diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 23:1 resulting in 40-bar (4.0 MPa; 580 psi) pressure compared to 8 to 14 bar (0.80 to 1.40 MPa; 120 to 200 psi) in the gasoline engine. This high compression causes the temperature of the air to rise to 550°C (1,022°F). At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber. For this reason, the Diesel engine is often called a compression-ignition engine. This may be into a (typically toroidal) void in the top of the piston or a pre-chamber depending upon the design of the engine. The fuel injector ensures that the fuel is broken down into small droplets and that the fuel is distributed evenly. The heat of the compressed air vaporises fuel from the surface of the droplets. The vapour is then ignited by the heat from the compressed air in the combustion chamber. The droplets continue to vaporise from their surfaces and burn, getting smaller, until all the fuel in the droplets has been burnt. Combustion occurs at a substantially constant pressure during the first part of the power stroke. The start of vaporisation causes a delay before ignition and the characteristic Diesel knocking sound as the vapour reaches ignition temperature and causes an abrupt increase in pressure above the piston (not shown on the P-V indicator diagram). When combustion is complete, the combustion gases expand as the piston descends further; the high pressure in the cylinder drives the piston downward, supplying power to the crankshaft.

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An aero Diesel engine works on the Diesel cycle How Diesel Engines Work - Part - 1 (Four Stroke Combustion Cycle) https://youtu.be/fTAUq6G9apg How Diesel Engines Work - Part - 2 (Stages of Combustion) https://youtu.be/HapIGjHkBHU How Diesel Engines Work - Part - 3 (Valve Timing Diagram) https://youtu.be/DBDGOvsxpq8

The Diesel cycle PV diagram

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The PV diagram is a simplified and idealised representation of the events involved in a Diesel engine cycle, arranged to illustrate the similarity with a Carnot cycle. Starting at A, the piston is at bottom dead centre, and both valves are closed at the start of the compression stroke; the cylinder contains air at atmospheric pressure.

Ideally, the adiabatic expansion should continue, extending the line C–D to the right until the pressure falls to that of the surrounding air. Still, the loss of efficiency caused by this unresisted expansion is justified by the practical difficulties involved in recovering it (the engine would have to be much larger).

Between A and B, the air is compressed adiabatically – that is without heat transfer to or from the environment – by the rising piston. (This is only approximately true since there is some heat exchange with the cylinder walls.)

After the opening of the exhaust valve, the exhaust stroke follows, but this (and the following induction stroke) are not shown on the diagram. If shown, they would be represented by a low-pressure loop at the bottom of the diagram. At A, the exhaust and induction strokes are complete, and the cylinder is again filled with air.

During this compression, the volume is reduced, the pressure and temperature both rise. At or slightly before B (TDC) fuel is injected and burns in the compressed hot air. Chemical energy is released, and this constitutes an injection of thermal energy (heat) into the compressed gas. Combustion and heating occur between B and C. In this interval, the pressure remains constant since the piston descends, and the volume increases; the temperature rises as a consequence of the energy of combustion. At C fuel injection and combustion are complete, and the cylinder contains gas at a higher temperature than at B. Between C and D this hot gas expands, again approximately adiabatically. Work is done on the system to which the engine is connected. During this expansion phase, the volume of the gas rises, and its temperature and pressure both fall.

The piston-cylinder system absorbs energy between A and B. In essence, this is the work needed to compress the air in the cylinder and is provided by mechanical kinetic energy stored in the flywheel of the engine. Work output is done by the piston-cylinder combination between B and D. The difference between these two increments of work is the indicated work output per cycle. It is represented by the area enclosed by the PV loop. The adiabatic expansion is in a higher-pressure range than the compression because the gas in the cylinder is hotter during expansion than during compression. It is for this reason that the loop has a finite area, and the net output of work during a cycle is positive.

At D, the exhaust valve opens, and the pressure falls abruptly to atmospheric (approximately). This is unresisted expansion, and no useful work is done by it. Total Training Support Ltd © Copyright 2020

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In the Diesel engine, air is compressed adiabatically with a compression ratio typically between 15 and 20. This compression raises the temperature to the ignition temperature of the fuel mixture which is formed by injecting fuel once the air is compressed.

PV diagram for the Ideal Diesel cycle. The cycle follows the letters A to D in a clockwise direction. The horizontal axis is volume of the cylinder. In the Diesel cycle the combustion occurs at almost constant pressure.

The ideal air-standard cycle is modelled as a reversible adiabatic compression followed by a constant pressure combustion process, then an adiabatic expansion as a power stroke and an isovolumetric exhaust. A new air charge is taken in at the end of the exhaust, as indicated by the processes on the diagram PV diagram.

On this diagram the work that is generated for each cycle corresponds to the area within the loop.

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Valve timing The theoretical four-stroke cycle that has been described indicates that the valves open or close when the piston is precisely at TDC or BDC. In practice, the theoretical four-stroke cycle is operationally inefficient mainly due to three factors: • • •

The inertia of the coming fuel/air mixture and the outgoing combustion gases; The burning rate of the fuel/air mixture, which, although rapid, is not instantaneous; and The ineffective crank angle formed between the connecting rod and the crankshaft around the TDC and BDC positions, where for a larger rotary movement of the crankshaft there is a relatively small linear movement of the piston.

These deficiencies can be minimised by varying the valve timing and the point of ignition.

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The modified four-stroke cycle In practice, the opening and closing of the inlet and exhaust valves vary about the TDC and BDC positions. The actual timing depends upon engine type. The following is a typical application. It is usual to represent practical valve operations and the point of ignition on a valve and ignition timing diagram, which indicates the angular position of the crankshaft when each valve commences to open and finishes closing. A typical example of such a diagram is illustrated below. Study the diagram and take special note of the terms, lead, lag and overlap. Valve lead: This indicates that the inlet valve has opened before the piston has reached TDC, also that the exhaust valve opens before BDC. Valve lag: This indicates that the inlet valve closes after the piston has passed BDC and that the exhaust valve closes after the piston has passed TDC. Valve overlap: This is the period when both the exhaust and inlet valves are open together with the exhaust valve closing and the inlet valve opening. One factor that affects engine efficiency is the inertia of the incoming fuel/air mixture – it was slow to start moving when the inlet valve opened. Some delay is inevitable as the mixture is stationary in the induction manifold while the valve is closed. When the valve opens, the charge has to accelerate to the speed of the piston, which can be alleviated by opening the inlet valve early.

Inlet valve lead For the induction stroke, the opening of the inlet valve is initiated before TDC to ensure that it is fully open when the piston commences its downward stroke, so reducing the time between the piston moving down the cylinder and the charge flowing in. Inlet valve lag This inlet valve is kept open as long as possible to induce the maximum cylinder charge. The incoming gas continues to enter the cylinder for some time after the piston has passed BDC, due to its momentum. This delays the closing of the inlet valve after BDC, until a point when the pressure in the cylinder is approximately equal to the pressure in the induction manifold. The first problem; that of the inertia of the mixture, has been effectively dealt with. Ignition timing When ignited the fuel/air mixture burns at a rate which depends on the ratio of fuel to air; rich mixtures burn faster than usual or weak mixtures. Although the mixture burns quickly, combustion is not instantaneous. Therefore, the ignition is arranged to occur before TDC at the end of the compression stroke, so that maximum pressure is achieved shortly after TDC of the power stroke. The point at which ignition occurs must be timed to suit the mixture and the engine RPM. The earlier ignition occurs before TDC; the more advanced the ignition said to be. When occurring later, that is near to TDC; the ignition is said to be retarded. The valve and ignition diagram for a specific engine typically shows ignition occurring at the fully advanced position.

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• • •

A – advanced – for weak mixture or high RPM. N – normal R – retarded – for rich mixture or low RPM.

Note: Ignition is usually timed to suit engine ‘cruise’ RPM. That is the range where the engine spends most of its operating lifetime.

Four-stroke valve timing diagram

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Valve operation The valves are spring-loaded closed and opened by the mechanism comprising cams, tappets, pushrods, and rocker arms geared to the crankshaft. The gear wheel on the camshaft has twice the number of teeth as the gear wheel on the crankshaft, so you can appreciate that the mechanism operates at half crankshaft speed so that each valve opens and closes once for every two revolutions of the crankshaft.

Pre-Ignition Pre-ignition is a fault condition, not a design feature. It is the term used when the charge is ignited before the intended point by means other than the spark and is usually caused by a hot spot in an overheated engine. This leads to a loss of power and a heavy load on the piston, connecting rod and crankshaft components, which could contribute to component damage.

Remembering that in the four-stroke cycle there is only one induction stroke in every two crankshaft revolutions, it is no doubt apparent to you that the inlet valve needs opening only once in two revolutions of the crankshaft. This also applies to the exhaust valve. Valve clearance Valve clearance is measured in thousandths of an inch or millimetre between the rocker arm and the valve stem. The reason for this clearance is that when, for example, the engine is on the compression stroke, the inlet and exhaust valves must remain closed. Without the specified clearance, the mechanism would be rigid, and the slightest maladjustment or expansion of the valve stem would cause the valve to remain slightly open. The mixture would leak from the cylinder with obvious results. It is customary to set the valve clearance of the cylinder being timed to the hot or running clearance as specified in approved maintenance manuals.

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Valve operating mechanism

Valve clearance

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The 2-stroke cycle engine In the two-stroke-cycle engine, the same four events (intake, compression, power, and exhaust) take place in only twostrokes of the piston and one complete revolution of the crankshaft. The two-piston strokes are the compression stroke (upward stroke of the piston) and power stroke (the downward stroke of the piston). Sequence of events (1) – Scavenging (intake) – A fresh change of air is forced into the cylinder intake ports by the blower. Exhaust gases escape through the open exhaust valves. (2) – Compression – As the piston moves upward, the intake ports are covered, and the exhaust valves close. The air is compressed in the cylinder; the piston continues to move towards TDC. (3) – Injection/ignition and (4) – Combustion – When the piston nears the top of its stroke, fuel is injected into the cylinder. The fuel ignites due to the heat of compression. (5) – Expansion (power) – The rapid expansion of burning gases forces the piston down. (6) – Exhaust – As the piston nears BDC, the exhaust valves open, starting the release of exhaust As shown earlier, a power stroke is produced every crankshaft revolution within the two-stroke-cycle engine, whereas the fourstroke-cycle engine requires two revolutions for one power stroke.

It might appear then that the two-stroke-cycle engine can produce twice as much power as the four-stroke-cycle engine of the same size, operating at the same speed. However, this power increase is limited to approximately 70 to 80% because some of the power is used to drive a blower that forces the air charge into the cylinder under pressure. Also, the burned gases are not completely cleared from the cylinder, reducing combustion efficiency. Additionally, because of the much shorter period, the intake port is open (compared to the period the intake valve in a four-stroke is open), a relatively smaller amount of air is admitted. Hence, with less air, less power per stroke is produced in a two-stroke-cycle engine. The following shows the difference between a 2- and 4-stroke cycle. Light-sport two-stroke aircraft engines Light-sport/ultralight aircraft engines can be classified several methods, such as by operating cycles, cylinder arrangement, and air or water-cooled. An inline engine generally has two cylinders, is two-cycle, and is available in several horsepower ranges. These engines may be either be liquid-cooled, aircooled, or a combination of both. They have only one crankshaft that drives the reduction gearbox or propeller directly. Most of the other cylinder configurations used are horizontally opposed, ranging from two to six cylinders from several manufacturers. These engines are either gear reduction or direct drive. There are a growing number of manufacturers of small, two-stroke engines on the market such as Rotax and Limbach.

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The two-stroke cycle

Two-stroke

Four-stroke

1. One cycle equals one crankshaft revolution and four piston strokes

1. One cycle equals two crankshaft revolutions and two piston strokes.

2. Requires a blower

2. Blower is optional.

3. Requires intake

3. Requires only intake exhaust ports or and exhaust valves, intake ports and exhaust valves.

Four-cylinder, horizontally-opposed, air-cooled, two-cycle engine

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Characteristics of two-stroke engines 1) For a given speed and a given output, a two-stroke engine require one half piston displacement, i.e. the piston is half as heavy, half as bulky and half as expensive as a fourstroke engine piston. 2) They are simpler in design and less complicated in valve design and operation than four-stroke engines. 3) Since every revolution produces one power stroke, therefore torque produced at the crankshaft is uniform, necessitating a lighter flywheel 4) Due to the absence of moving parts like cam and followers, rocker arm and other valve actuating mechanisms, it has higher mechanical efficiency in comparison to a 4-stroke. 5) They have poor scavenging due to absence of separate exhaust stroke. This results in less oxygen and less burning of fuel in the cylinder and less output due to diluting of the fresh incoming gases by the left-over exhaust. Hence, the thermal efficiency is quite low. These engines are usually air-cooled. 6) The fuel and lubricating oil consumption are comparatively high due to the loss of fresh gases through the exhaust ports. 7) They are lighter in weight and require lesser space due to the absence of valves and valve gears.

The Rotax inline cylinder arrangement has a small frontal area and provides improved streamlining. The two-cylinder, inline two-stroke engine, which is piston ported with air-cooled cylinder heads and cylinders, is available in a fan or free aircooled version. Being a two-stroke cycle engine, the oil and fuel must be mixed in the fuel tank oil some models. Other models use a lubrication system, such as the 503 oil-injection lubrication system. This system does not mix the fuel and oil as the oil is stored in a separate tank. As the engine needs lubrication, the oil is injected directly from this tank. The typical ignition system is a breakerless ignition system with a dual ignition system used on the 503, and a single ignition system used on the 447-engine series. Both systems are of a magneto capacitor-discharge design. The engine is equipped with a carburetion system with one or two piston-type carburettors. One pneumatic-driven fuel pump delivers the fuel to the carburettors. The propeller is driven via a flange-connected gearbox with an incorporated shock absorber. The exhaust system collects the exhaust gases and directs them overboard. These engines come with an integrated alternating current (AC) generator (12 V 170 W) with external rectifier-regulator as an optional extra. Example: Limbach L 275 EF – 24 hp Two-cylinder, horizontal opposed, air-cooled, two-cycle engine, with a fuel-saving electronic engine management system and mixture lubrication. Suitable for pusher and tractor installations.

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Two-stroke engine operation

Limbach L 275 EF – 24 hp

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Diesel engines In recent years, the development in aircraft engines has been more or less concentrating on Diesel engines. We have seen one-off installations to fully developed engine production lines. Several companies are active in this market primarily due to major concern of long-term availability and the relatively high price of Avgas. Diesel engines can use jet fuel (Avtur or Jet-A). This fuel is available worldwide and can also be made of renewable sources (biomass) which will contribute to a cleaner environment. Last but not least; Diesel engines have an excellent specific fuel consumption compared to their Avgas counterparts, and as the fuel is denser too, the range of a Diesel-powered aircraft is improved. Diesel engines use the compression-ignition principle. Fuel is injected into the combustion chamber (either direct or indirect) under high pressure. Due to compression of air by the piston in the cylinder, temperatures are very high (700 – 900°C) and the fuel ignites almost instantly when injected. Therefore, there is no need for a carburettor, a throttle valve (no carburettor ice!) or a separate ignition system. Starting a Diesel in a cold environment can be difficult; a form of preheating should be used. To implement this, Diesel engines use a glow plug in each cylinder to preheat the cold air before and after starting and thus help the combustion the first couple of minutes after a cold start.

High torque at low RPM Diesel fuel burns slower than gasoline, therefore, restricting the maximum RPM of the engine to around 4,500 RPM. On the contrary, Diesel engines deliver remarkably high torque at low RPM. This is ideal for propeller-driven aircraft. One drawback is that due to higher compression and acting forces in the engine, these engines tend to be a bit heavier than a comparable gasoline engine. Two-stroke Diesel engines overcome this problem, because they have a power stroke for every revolution per cylinder, compared to a four-stroke Diesel (every other revolution per cylinder). Aircraft Diesel engines are usually the inline or flat-four type, but BMW and Packard (among others) developed a radial Diesel engine. Reliable design Diesel engines are simpler (compared to gasoline types). They have no ignition system, are more reliable, durable, have more torque, use less fuel and have higher thermal efficiency. They use denser fuel which gives more range (about 9%) for the same volume. Fuel system As already said, due to the fuel injection method used, there is no carburettor or associated throttle valve. Power is controlled by the amount of fuel injected by the high-pressure fuel pump. This is a very reliable but also very delicate piece of hardware. The fuel must be filtered (below 70 micron) and fed through a water/fuel separator sometimes combined with an electric heater so that any water is dissolved in the fuel and cannot cause blocking of filters due to ice formation.

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Delta Hawk V4 Diesel engine

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With the high-pressure fuel system and injector, there is more fuel fed to the engine than it uses, this extra fuel is heated by the engine and return to the tank in use. The advantage is that warm fuel reduces the possibility of ice formation. Example: Junkers Jumo 205D These engines all used a two-stroke cycle with twelve pistons sharing six cylinders, piston crown to piston crown in an opposed-piston configuration. This unusual configuration required two crankshafts, one at the bottom of the cylinder block and the other at the top, geared together. The pistons moved towards each other during the operating cycle. Intake and exhaust manifolds were duplicated on both sides of the block. There were two cam-operated injection pumps per cylinder, each feeding two nozzles, for four nozzles per cylinder in total.

The Jumo solved this problem to a large extent through a creative arrangement of the ports. The intake port was located under the ‘lower’ piston, while the exhaust port was under the ‘upper’. The lower crankshaft ran eleven degrees behind the upper, meaning that the exhaust ports opened and, even more importantly, closed first, allowing proper scavenging. This system made the two-stroke Jumos run as cleanly and almost as efficiently as four-stroke engines using valves, but with considerably less complexity.

As is typical of two-stroke designs, the Jumos used no valves. Instead, it featured fixed intake and exhaust port apertures cut into the cylinder liners which were uncovered when the pistons reached a certain point in their stroke. Usually, such designs have poor volumetric efficiency because both ports open and close at the same time and are generally located across from each other in the cylinder. This leads to poor scavenging of the burnt charge, which is why valve-less two-strokes generally produce smoke and are inefficient.

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Supercharged 868 hp 17:1 compression

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Turbo and intercooler As Diesel engines will be more substantial in construction compared to a gasoline engine, it pays to increase efficiency and power to weight ratio by adding a turbo or supercharger combined with an intercooler. Air compressed by the turbocharger heats up and is cooled to lower its density by an intercooler, and the extra compressed air can burn more fuel for the same cylinder volume. The increased power is the result. Diesel knock The older type of Diesel engines (tractor type) have a characteristic sound; Diesel knock, especially when the engine is cold and at low RPM. This knock is the same as detonation in a gasoline engine due to unstable combustion and high peak pressures. This knocking was one of the reasons that these older Diesel engines were built quite heavy. Conclusion Modern light-weight Diesel engines have dealt with this typical Diesel knock. They use indirect injection, where fuel is injected into a pre-chamber and two-stage injectors, which prolong combustion. Electronic engine control (FADEC), compression ratios that are not over 1:20, and better fuel-air mixing all have led to almost no knock at all. Diesel engines can be two- or four-stroke types; both types are used in aviation. Automotive Diesel engines are almost exclusively four-strokes, but in marine applications the large propulsion engines are two-strokes.

2-stroke Diesel Several engines that are being developed are of the two-stroke design. A two-stroke Diesel cannot be compared to a twostroke gasoline engine which uses a mix of fuel and oil (1:50) to lubricate the engine by pressure in the crankcase (caused by the moving piston). Burning this mixture results in highly polluting exhaust gasses (blue smoke). Two-stroke Diesel engines have more in common with their four-stroke cousins, but the engine strokes occur in one revolution: intake, compression, power, and exhaust. Timing is crucial here. Some engine designs use intake ports and exhaust valves, whereas others have ports only and the pistons acting toward each other in pairs in the same cylinder. Two-stroke Diesel engines must use some type of blower to start as their intake stroke is not capable of inhaling air. At this point, the intake port is open, but the piston is near or at the bottom dead centre, so there is no displacement of anything. The blower, usually a roots-type, is engine driven and blows air under pressure into the cylinders. Most of these engines also have a turbocharger which takes over from the roots blower when the engine is running, usually after reaching a certain RPM. And as RPM range is rather narrow (900 – 2,700 RPM), the turbo can be perfectly matched to the engine requirements. The two-stroke Diesel engine has some advantages not found in the four-stroke types which are of tremendous advantage to its application as an aero engine.

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Weight and power With twice as many power strokes per engine revolution, a twostroke Diesel engine produces more power than a four-stroke Diesel of the same displacement. A two-stroke engine of the same power as a four-stroke does not have the same weight. Response and acceleration Since every cylinder of a two-stroke engine produces a power stroke for every revolution, there is a quick response to load changes, for example with a constant speed propeller. Durability The two-stroke engine spreads the load; each cylinder is producing two lighter power impulses per two revolutions instead of the single substantial impulse of a four-stroke engine. At normal loads and speeds, there is no load reversal on pistons, rods, and bearings; this continuous downward loading reduces impact load effects. Lighter loading permits two-stroke Diesel engines to use more compact structural and load-bearing parts without overstressing. The lighter power impulses are produced by smaller displacement cylinders – which means smaller pistons and shorter connecting rods for comparable engine performance. Shorter stroke lowers piston speed, a significant factor in cylinder life.

Smoothness Two-stroke engines run smoother than four-stroke engines because two-stroke engines have twice as many power impulses at the same RPM. The lighter, more frequent power impulses mean less damping is required from the flywheel, hence smaller, lighter flywheels can be used. This permits more rapid acceleration and unprecedented transient load response. Lower exhaust temperatures More air goes through a two-stroke engine than a four-stroke for the same amount of fuel consumed. This results in lower exhaust temperatures for two-stroke Diesel engines and more extended valve and turbo life. Higher piston loads The piston loads in a two-stroke Diesel are higher than in a fourstroke, mainly because there is always downward pressure on the piston either by the combustion or by pushing out the exhaust gasses. There is no load reversal to build up the oil film on the piston pin. An innovative manufacturer uses a sort of ball bearing in the piston with pressure lubrication; this design also solves the problem with piston cooling and lubrication.

All of these weight and size advantages are achieved without sacrificing engine life.

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Indirect versus direct fuel injection A DI (direct injection) engine has the Diesel fuel injected straight into cylinder almost at the top of the compression stroke. In the old days, this meant that it burned and expanded very quickly, making a noisy, rattily engine. This is why most Diesel cars were IDI (indirect injection); the rough behaviour was fixed by injecting the fuel into a small pre-combustion chamber which is connected to the cylinder by a narrow passage. This slows down the combustion as the gasses have to escape through the narrow passage into the cylinder. This gives a softer bang and a smoother engine, but the gasses have to work harder, which reduces the efficiency a little. However, the newer DI engines use other techniques to smooth the behaviour of the engine, such as two stage injection and electronic control of the injectors. Performance at altitude A two-stroke Diesel can run without a turbocharger just by using a roots blower to scavenge the engine from exhaust gasses. This roots blower is not a supercharger but supplies the engine with enough air to clean the cylinders for the next fuel injection and power stroke. This is seen as a naturally aspirated engine. Adding a turbocharger would give the advantage of more air; thus, more fuel can be injected, and the engine would be able to sustain its rated power to a higher altitude. More air through the engine also means better scavenging and cooler exhaust valves translating to longer engine life and more reliable engine.

4-stroke Diesel This type of engine uses two revolutions to accomplish its task. Intake, compression in the first revolution and power and exhaust stroke in the second revolution, timing is less critical. Power is lower compared to its two-stroke cousin, but that can be overcome by running it at a higher RPM and using a gearbox to reduce the RPM for the propeller. Everything else being equal, compared with a two-stroke Diesel of the same displacement and RPM, the four-stroke type is heavier and has less power for its weight. Several engines that are being developed are of four-stroke design. They use a gearbox; to get more power, they need more RPM, two-strokes are usually direct driven and have more power/torque at lower efficient propeller RPM. They usually have electronic engine management called FADEC (which must be dual for redundancy, a dual electrical system is also needed). This type of Diesel uses intake and exhaust valves to regulate the gas flow. At the intake stroke, the piston moves downward, and the intake valve is open. When the piston starts to move up again (after reaching BDC) the compression stroke has started, and both intake and exhaust valves must be closed. Temperature and pressure rise quickly in the cylinder.

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Operation When the piston is almost at TDC fuel is injected, and after a slight delay, the finely atomised fuel starts to burn. Temperature and pressure rise even more and push the piston down in what is called the power stroke. The heat of combustion also heats the cylinder.

Indirect-injection engines use lower pressure fuel systems (to 300 bar) and have softer combustion than direct-injection engines. These engines also have lower toxic exhaust emissions, particularly concerning HC and NOX. Optimised pre-chamber engines have a 40% lower particulate emission, mainly because of better air-fuel mixing in the pre-chamber.

In the last stroke, the piston moves up, and the exhaust valve is open, thereby forcing the burnt gases out the cylinder.

Increasing power There must be increased fuel flow to increase power, but there is a limit. Optimum fuel/air ratio is about 1:14.7; thus, the engine needs more air first before adding fuel. One way to do that is with a supercharger or turbocharger. But as compressing air raises its temperature and density the air from a supercharger or turbocharger must be cooled with a radiator (intercooler).

Valve timing During the above process, the valves do not open and close when the piston is precisely at the top or bottom of the cylinder. This is not very efficient. Valve opening and closing occur with some overlap so that the energy of the moving gas is used to intake fresh air and remove the burnt gas from the cylinder in a continuous motion so that optimum cylinder breathing is accomplished without too much energy losses.

Power is then raised and can be held constant up until a certain altitude where the turbo cannot deliver any more pressure and reaches its maximum RPM.

Combustion chambers Both two- and four-stroke Diesel can use either a pre-chamber (indirect-injection or IDI) or direct-injection (DI). Pre-chamber engines use a small space where fuel is injected and connected through a small canal to the main combustion area. Directinjection engines have the fuel injected into the cylinder above the piston.

Fuel injection system As the Diesel engine intakes air during its intake stroke, the way to regulate RPM or power is by varying the amount of fuel injected. Be it a two- or four-stroke engine. This system must regulate the fuel quantity at all speeds and loads. The engine requires the correct amount of fuel at the correct time, pressure, sequence and point in the combustion process.

The advantage of a direct-injection engine is a reduced fuel consumption up to 15% compared with indirect engines. The disadvantage of direct-injection systems are higher combustion noise and restricted maximum RPM, and they require higher injection pressures and a more complex fuel system.

FADEC Modern four-stroke Diesel engines have a full authority digital engine controller (FADEC) that regulates the fuel injection system and accounts for the multitude of variables of the engine concerning smoke, combustion pressure, EGT, torque and engine speed limits.

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Example: Thielert Centurion When it hit the market in the late 1990s, the Thielert turboDiesel engine promised a real revolution. An extensive reworking of a high-tech Mercedes automotive engine, the Centurion, seemed to offer it all: a power-to-weight ratio competitive with gas piston engines, the ability to run on jet-A fuel, turbocharging and excellent fuel efficiency. Diamond Aircraft was the original adopter, designing the next-gen light twin TwinStar seemingly around the engines. Sadly, gearbox problems – the engine uses a reduction drive to get the prop speed down to useful RPM ranges – plagued the engine, with unacceptably short overhaul intervals and sky-high maintenance costs. Today, the Thielert engine is owned by Continental Motors, which has worked hard to improve the engine’s value proposition while reintroducing it in new and retrofit applications.

The Diesel engine’s high compression results in better fuel efficiency and the higher operating RPM of the Centurion allows higher power to be developed from a smaller displacement, in comparison to conventional aircraft piston engines. A Centurion engine complete with CSU, reduction gearbox, turbocharger and FADEC engine management system is considerably more substantial than the more conventional Continental and Lycoming engines with which it competes. However, the Centurion’s lower fuel consumption compensates this weight disadvantage. Even though they lack the magnetos and spark plugs of conventional gasoline (gas) piston engines, Centurion engines are considerably more complicated.

All Centurion engines are water-cooled, turbocharged, and employ a single-lever digital engine management system (FADEC). This simplifies engine management for the pilot, as well as improving reliability as it prevents the engine from being operated improperly. The series utilises either jet fuel or Diesel fuel. The high compression ratio of the engine combined with the digitally controlled fuel injection system mirrors similar advances in automotive technology. Centurion series engines are always fitted with constant-speed propellers which allow the engine to be operated at the optimum speed at all times. However, the standard operating speed is too high for any suitable propeller, and so the propeller is driven through a reduction gearbox. The constant-speed propeller and reduction gear result in a propeller tip speed that is 10-15% lower than comparable conventional avgas engines, reducing propeller noise. Total Training Support Ltd © Copyright 2020

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Light aircraft equipped with Thielert Centurion engine

Thielert Centurion engine

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Example: Austro Engine AE300 The Austro Engine E4 (marketed as the AE 300) is a liquidcooled, inline, four-cylinder, four-stroke Diesel engine which produces 170 hp and is capable of producing 100% power at 11,800' density altitude. The engine, developed by Austro Engine in collaboration with MB Technology GmbH (Mercedes Benz), Bosch General Aviation Technology GmbH and HÖR Technologies, features the very latest in high-pressure common-rail fuel injection technology. It offers even better specific fuel consumption than the first-generation turbo-Diesel engines originally used by Diamond. With more than 2,000,000 accumulated flight hours in the turbo-Diesel powered Diamond Aircraft fleet, Diamond is the most experienced aeroplane manufacturer concerning turboDiesel piston engine operation. This experience has benefitted the development of the Austro Engine, with features such as a FADEC controlled actuator driving a conventional constant speed propeller governor, a high mounted turbocharger to permit gravity oil return, a torsional vibration absorber in place of a wearing clutch, robust electrical harnesses and connectors, heavy-duty reduction gearbox, four-point engine mount and countless detail features to ensure reliability, and reduced maintenance costs. The combination of low fuel burn, independence from leaded fuel and low noise emission, makes the AE300 the most environmentally friendly piston engine available. The ability to operate on globally available jet-fuel makes the AE300 particularly suitable for strategic applications and markets where Avgas 100LL is either expensive or has limited availability. Total Training Support Ltd © Copyright 2020

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Austro Diesel engine AE300

Light aircraft equipped with Austro Diesel engine

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Gasoline and Diesel engines – three differences Spark-ignition versus compression-ignition Where a gasoline engine compresses a pre-mixed air/fuel mixture and which is then ignited by a spark plug (hence the name ‘spark-ignition’), the Diesel engine compresses only air. The compression ratio is, however, much higher than that of the gasoline engine – high enough that the rise in temperature is great enough that when the fuel is injected into the cylinder at TDC, it ignites (hence the name ‘compression ignition’). This fundamental difference in operation divides the gasoline engine and the Diesel engine into the two categories of internal combustion engine; the spark-ignition engine and the compression ignition engine, respectively. Throttle versus no-throttle Since the air and fuel is pre-mixed with a gasoline engine, using a carburettor, for example, the power output of the engine is varied by opening and closing a throttle valve. This lets more, or less pre-mixed air/fuel mixture into the cylinder, the result being that the air and fuel is varied the same amount as the throttle is opened or closed. Although the mixture ratio can be varied in the carburettor, this is not done to change the power setting, but rather, for other reasons, such as cooling and altitude compensation.

Note: automotive Diesel engines do have a throttle, but this is to provide engine shut-off function and to provide engine braking when going downhill. Since the Diesel engine always compresses the same amount of air, the difference when more or less fuel is injected, the air/fuel ratio, alone governs the power of the engine. A Dieselpowered aircraft has no throttle lever. Instead, the lever that the pilot adjusts to change the power is known as the power lever. Air/fuel ratio Gasoline and Diesel have similar stoichiometric ratios (15:1). However, a gasoline engine operates in a range of air/fuel ratio slightly above stoichiometric (lean) and slightly below stoichiometric (rich). A Diesel engine operates in a range of air/fuel ratio which is all lean, typically between 18:1 and 70:1. The difference in air/fuel ratio is what determines the power output of the Diesel engine, and hence the term “quality controlled” as opposed to “quantity controlled”.

The gasoline engine is sometimes referred to as “quantity controlled,” and the Diesel engine “quality controlled” due to the methods of manipulating the power of the engines. A Diesel engine has no throttle. Power is varied by injecting more or less fuel into the combustion chamber during the power stroke. Total Training Support Ltd © Copyright 2020

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Gasoline Vs Diesel - 4 Major Differences https://youtu.be/rXVJG9n6BAs Diesel Engines and Common Rail https://youtu.be/lVAdJlZr8_k Gasoline (Gasoline) Engine vs Diesel Engine https://youtu.be/bZUoLo5t7kg

Aero Diesel engines have no spark plug(s) nor associated ignition system Aero Diesel engines have no carburettor nor throttle valve

Power lever of an aero Diesel-powered aircraft

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Piston displacement The size of an engine cylinder is indicated in terms of bore and stroke. Bore is the inside diameter of the cylinder. Stroke is the distance between top dead centre (TDC) and bottom dead centre (BDC).

To express the displacement of the engine in the metric system, convert cubic inches to cubic centimetres by multiplying cubic inches by 16.39. Notice that 16.39 is constant. 307.44 in3 × 16.39 = 5,038.9416 cm3

The bore is always mentioned first. For example, a 3½ × 4 cylinder means that the cylinder bore, or diameter, is 3½" and the length of the stroke is 4". These measurements are used to calculate displacement.

To convert cubic centimetres into litres, divide the cubic centimetres by 1,000.

Piston Displacement is the volume of space that the piston displaces, as it moves from one end of the stroke to the other. Thus, the piston displacement in a 3½" × 4" cylinder would be the area of a 3½" circle multiplied by 4 (the length of the stroke.)

The displacement of the engine is expressed as 5.0 L in the metric system.

5,038.9416 = 5.0389416 ÷ 1,000

The area of a circle is πR2, where R is the radius of the circle. With S being the length of the stroke, the formula for volume (V) is the following: V = = = =

πR2 × S 3.14 × (1 .75)2 × 4 3.14 × 3.06 × 4 38.43 in3

The total displacement of an engine is found by multiplying the volume of one cylinder by the total number of cylinders. 38.43 in3 × 8 cylinders = 307.44 in3. The displacement of the engine is expressed as 307 in3 in America.

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Cylinder bore and stroke

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Compression ratio The compression ratio of an engine is a measurement of how much the air-fuel charge is compressed in the engine cylinder. It is calculated by dividing the volume of one cylinder with the piston at BDC by the volume with the piston TDC. One should note that the volume in the cylinder at TDC is called the clearance volume. For example, suppose that an engine cylinder has a volume of 80 in3 with the piston at BDC and a volume of 10 in3 with the piston at TDC.

The fuel chemists have overcome knocking by creating antiknock fuels. Oxygen must be present if combustion is to occur in the cylinder. Since air is the source of the supply of oxygen used in engines, the problem arises of getting the proper amount of air to support combustion. This factor is known as the air/fuel ratio. A gasoline engine normally operates at intermediate speeds on a 15:1 ratio; that is, 15 lb of air to 1 lb of gasoline.

The compression ratio in this cylinder is 8 to 1, determined by dividing 80 in3 by 10 in3. The air-fuel mixture is compressed from 80 to 10 in3 or to one-eighth of its original volume. Two significant advantages of increasing the compression ratio are that the power and economy of the engine improve without added weight or size. The improvements come about because with a higher compression ratio, the air-fuel mixture is squeezed more. This means a higher initial pressure at the start of the power stroke. As a result, there is more force on the piston for a greater part of the power stroke; therefore, more power is obtained from each power stroke. Increasing the compression ratio, however, brings some problems. Fuel can withstand only a certain amount of squeezing without knocking. Knocking is the sudden burning of the air-fuel mixture that causes a quick increase in pressure and rapping or knocking noise.

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Compression ratio Volumetric efficiency

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Engine efficiencies Mechanical efficiency Mechanical efficiency is the ratio that shows how much of the power developed by the expanding gases in the cylinder is delivered to the output shaft. It is a comparison between the brake horsepower (bhp) and the indicated horsepower (ihp). It can be expressed by the formula: Mechanical efficiency =

bhp ihp

Brake horsepower is the useful power delivered to the propeller shaft. Indicated horsepower is the total hp developed in the Cylinders. The difference between the two is friction horsepower (fhp); the power lost in overcoming friction. The factor that has the most significant effect on mechanical efficiency is the friction within the engine itself. The friction between moving parts in an engine remains practically constant throughout an engine’s speed range. Therefore, the mechanical efficiency of an engine is highest when the engine is running at the RPM at which maximum bhp is developed. Mechanical efficiency of the average aircraft reciprocating engine approaches 90%. This is the relationship between the actual power produced in the engine (indicated horsepower) and the actual power delivered at the crankshaft (brake horsepower). The actual power is always less than the power produced within the engine. This is due to the following: •

friction losses between the many moving parts of the engine; and

•

in a four-stroke-cycle engine, a considerable amount of horsepower is used to drive the valve train.

Thermal efficiency Any study of engines and power involves consideration of heat as the source of power. The heat produced by the burning of gasoline in the cylinders causes a rapid expansion of the gases in the cylinder, and this, in turn, moves the pistons and creates mechanical energy. It has long been known that mechanical work can be converted into heat and that a given amount of heat contains the energy equivalent of a certain amount of mechanical work. Heat and work are theoretically interchangeable and bear a fixed relation to each other. Heat can, therefore, be measured in work units (for example, ft-lb) as well as in heat units. The British thermal unit (BTU) of heat is the quantity of heat required to raise the temperature of 1 lb of water by 1°F. It is equivalent to 778 ft-lb of mechanical work. A pound of gasoline fuel, when burned with enough air to consume it completely, gives up about 201,000 BTU, the equivalent of 15,560,000 ft-lb of mechanical work. These quantities express the heat energy of the fuel in heat and work units, respectively. The ratio of useful work done by an engine to the heat energy of the fuel it uses, expressed in work or heat units, is called the thermal efficiency of the engine. If two similar engines use equal amounts of fuel, the engine that converts the greater part of the energy in the fuel into work (higher thermal efficiency) delivers the greater amount of power.

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Furthermore, the engine that has the higher thermal efficiency has less waste heat to dispose of to the valves, cylinders, pistons, and cooling system of the engine. High thermal efficiency also means low specific fuel consumption and, therefore, less fuel for a flight of a given distance at a given power. Thus, the practical importance of high thermal efficiency is threefold, and it constitutes one of the most desirable features in the performance of an aircraft engine. Of the total heat produced, 25 to 30% is utilised for power output, 15 to 20% is lost in cooling (heat radiated from cylinder head fins), 5 to 10% is lost in overcoming the friction of moving parts, and 40 to 45% is lost through the exhaust. Anything that increases the heat content going into mechanical work on the piston, which reduces the friction and pumping losses, or which reduces the quantity of unburned fuel or the heat loss to the engine parts, increases the thermal efficiency.

The thermal efficiency of an engine may be based on either bhp or indicated horsepower (ihp) and is represented by the following formula. ihp × 33,000 Indicated thermal efficiency = weight of fuel burned/min × heat value × 778 The formula for brake thermal efficiency is the same as shown above, except the value for bhp is inserted instead of the value for ihp.

The portion of the total heat of combustion that is turned into mechanical work depends to a great extent upon the compression ratio. The compression ratio is the ratio of the piston displacement plus combustion chamber space to the combustion chamber space, as mentioned earlier. Other things being equal, the higher the compression ratio is, the larger is the proportion of the heat energy of combustion turned into useful work at the crankshaft. On the other hand, increasing the compression ratio increases the cylinder head temperature. This is a limiting factor because the extremely high temperature created by high compression ratios causes the material in the cylinder to deteriorate rapidly and the fuel to detonate instead of burning at a controlled rate. Total Training Support Ltd © Copyright 2020

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Example An engine delivers 85 bhp for 1 hour and during that time consumes 50 lb of fuel. Assuming the fuel has a heat content of 18,800 BTU per pound, find the thermal efficiency of the engine:

In addition to energy lost through waste heat, there are the following inherent losses in the piston engine. • •

85 ihp × 33,000 2,805,000 = 0.833 × 18,800 BTU × 778 12,184,50

•

Much energy is consumed when the piston must compress the mixture on the compression stroke. Energy from the fuel is consumed to pull the intake mixture into the cylinder. Energy from the fuel is consumed to push the exhaust gases out of the cylinder.

Brake thermal efficiency = 0.23 or 23% Reciprocating engines are only about 34% thermally efficient; that is, they transform only about 34% of the total heat potential of the burning fuel into mechanical energy. The remainder of the heat is lost through the exhaust gases, the cooling system, and the friction within the engine. The thermal distribution of a reciprocating engine is shown below. A large amount of energy from the fuel is lost through heat and not used in an internal combustion engine. This unused heat is of no value to the engine and must be removed from it. Heat is dissipated in the following ways: • • • • •

The cooling system removes heat from the engine to control engine operating temperature. A significant portion of the heat produced by the engine exits through the exhaust system. The engine radiates a portion of the heat to the atmosphere A portion of this waste heat may be channelled to the passenger compartment to heat it. The lubricating oil in the engine removes a portion of the waste heat.

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Engine power losses

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Volumetric efficiency Volumetric efficiency is a ratio expressed in terms of percentages. It is a comparison of the volume of fuel/air charge (corrected for temperature and pressure) inducted into the cylinders to the total piston displacement of the engine. Various factors cause a departure from a 100% volumetric efficiency. The pistons of a naturally aspirated engine displace the same volume each time they travel from top centre to bottom centre of the cylinders. The amount of charge that fills this volume on the intake stroke depends on the existing pressure and temperature of the surrounding atmosphere. Therefore, to find the volumetric efficiency of an engine, standards for atmospheric pressure and temperature had to be established. The US standard atmosphere was established in 1958 and provides the necessary pressure and temperature values to calculate volumetric efficiency. The standard sea level temperature is 59°F or 15°C. At this temperature, the pressure of one atmosphere is 14.69 lb/29.92" Hg. These standard sea-level conditions determine a standard density. If the engine draws in a volume of charge of this density equal to its piston displacement, it is said to be operating at 100% volumetric efficiency. An engine drawing in less volume than this has a volumetric efficiency lower than 100%. An engine equipped with true supercharging (boost above 30" Hg) may have a volumetric efficiency higher than 100%. The equation for volumetric efficiency is: Volumetric efficiency

• • • • • • •

part-throttle operation; long intake pipes of small diameter; sharp bends in the induction system; carburettor air temperature too high; cylinder-head temperature too high; incomplete scavenging; and improper valve timing.

Volumetric efficiency can be increased in the following ways: • • •

• •

Volume of charge (corrected for temperature and pressure) = Piston displacement

Many factors decrease volumetric efficiency, including:

Keep the intake mixture cool by ducting intake air from outside the engine compartment. By keeping the fuel cool, you can keep the intake mixture cooler. The colder the mixture, the higher the volumetric efficiency, because a cool mixture is denser or more tightly packed. Modify the intake passages. Changes to the intake passages, that make it easier for the mixture to flow through, increase the volumetric efficiency. Other changes include reshaping ports to smooth bends, reshaping the back of the valve heads, or polishing the inside of the ports. Altering the time that the valves open or how far they open can increase volumetric efficiency. By supercharging and turbocharging, you can bring the volumetric efficiency figures to over 100%.

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Modification of inlet passages

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Efficiencies of Diesel engines Thermal efficiency The thermal efficiency of an internal combustion engine is the ratio between the work output and the heat input into the system. Diesel engines are considerably more thermally efficient than gasoline engines. The three main reasons for this are: • • •

Very lean operation (as seen at lower loads) results in lower combustion temperatures and therefore lower heat losses in the cycle. Diesel engines do not run richer than stoichiometric at high loads, unlike gasoline engines, making them more efficient. Gasoline engines typically run about 15–20% over-rich at full load.

higher compression ratio; unthrottled operation; and lean combustion.

As well as the high level of compression, allowing combustion to take place without a separate ignition system, a high compression ratio greatly increases the engine’s efficiency. Increasing the compression ratio in a spark-ignition engine where fuel and air are mixed before entry to the cylinder is limited by the need to prevent damaging pre-ignition. Since only air is compressed in a Diesel engine, and fuel is not introduced into the cylinder until shortly before top dead centre (TDC), premature detonation is not a problem and compression ratios are much higher. Unthrottled operation in the Diesel engine virtually eliminates the pumping loop in the P-V diagram found with Otto cycle (gasoline) engines running at part load, since the piston does not have to work against a vacuum during the intake stroke. The pumping loop represents negative work in the cycle, as seen below.

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Volumetric efficiency Volumetric efficiency is a parameter to find the effectiveness of an engine induction process. It is the ratio between the actual volume of charge inducted into the cylinder and the swept volume of the piston. Regarding volumetric efficiency, Diesel engines are ideally more efficient than gasoline engines. Since the intake is not throttled, and additionally it is usually turbocharged or supercharged, and the engine runs at a relatively slow speed and accelerates gradually. The volumetric efficiency is greatly affected at greater engine speeds which is the operating range of gasoline engines. The opening and closing of the valves cause a fluctuated frequency of flow. The mass of air bounces back when it hits the closed valve. If the valve opens for the next stroke when the air mass is moving against the direction of flow, due to the shock wave generated by a closed valve, a more considerable negative pressure is required to suck in the air. This leads to improper filling of the cylinder, which results in lower volumetric efficiency, called the resonance effect.

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Engine configurations General Engines are also classified according to the arrangement of the cylinders Classifications include: • • • • • • •

inline; horizontal opposed; V; X; H; radial; and Wankle.

The cylinders are numbered. The cylinder nearest the front of an inline engine is number 1. The others are numbered 2, 3, 4, and so on, from front to rear. In V-type engines, the numbering sequence varies by manufacturer. You should always consult the manufacturer’s manual for the correct order. The firing order (which is different from the numbering order) of the cylinders of most engines is stamped on the cylinder block or the manufacturer’s nameplate. If you are unable to locate the firing order and no operation or instruction manual is available, turn the engine over by the crankshaft and watch the order in which the intake valves open.

Inline engines An inline engine generally has an even number of cylinders, although some three-cylinder engines have been constructed. This engine may be either liquid-cooled or air-cooled and has only one crankshaft, which is located either above or below the cylinders. If the engine is designed to operate with the cylinders below the crankshaft, it is called an inverted engine. The inline engine has a small frontal area and is better adapted to streamlining. When mounted with the cylinders in an inverted position, it offers the added advantages of shorter landing gear and greater pilot visibility. With an increase in engine size, the air-cooled, inline type offers additional problems to provide proper cooling; therefore, this type of engine is confined to lowand medium-horsepower engines used in very old light aircraft. An example would be a DeHavilland Gypsy Major engine used on a DeHavilland Tiger Moth Opposed or O-type engines The opposed-type engine has two banks of cylinders directly opposite each other with a crankshaft in the centre. The pistons of both cylinder banks are connected to the single crankshaft. Although the engine can be either liquid-cooled or air-cooled, the air-cooled version is used predominantly in aviation. It is generally mounted with the cylinders in a horizontal position. The opposed-type engine has a low weight-to-horsepower ratio, and its narrow silhouette makes it ideal for horizontal installation on the aircraft wings (twin-engine applications). Another advantage is its low vibration characteristics. The opposed engine is the most commonly found engine configuration in the General aviation world.

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Lycoming O-145 (horizontally opposed)

De Havilland Gypsy II engine (inline)

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V-type engines In V-type engines, the cylinders are arranged in two inline banks generally set 60° apart. Most of the engines have 12 cylinders, which are either liquid-cooled or air-cooled. The engines are designated by a V followed by a dash and the number of cylinders. For example, the Rolls Royce Merlin V-12. This type of engine was used mostly during the second world war, and its use is mostly limited to vintage fighter aircraft. V-type engines could also be inverted. X-type engines An X-engine is an engine comprising twinned V-block engines horizontally opposed to each other. Thus, the cylinders are arranged in four banks, driving a common crankshaft. Viewed head-on, this would appear as an X. X-engines were often coupled engines derived from existing powerplants. The X-4520 had four banks of six air-cooled cylinders. The banks were arranged at 90-degree intervals around a common crankshaft housed in an aluminium, barrel-type crankcase. The cylinders had a 146 mm (5.75") bore, 184 mm stroke (7.25"), and 4.9 to 1 compression ratio. Total displacement was 74 L (4,518 in3) and 987 kW (1,323 hp)

An H engine is effectively two flat engines, one atop or beside the other. The ‘two engines’ each have a crankshaft, which is then geared together at one end for power-take-off. The H configuration allows the building of multi-cylinder engines that are shorter than the alternatives, sometimes delivering advantages on aircraft. The power-to-weight ratio is not as good as more straightforward configurations employing one crankshaft. There is excellent mechanical balance, especially desirable and otherwise difficult to achieve in a four-cylinder engine. An example of an H engine would be a Napier Sabre (H-24 36.7 L, 3,500 hp) or a Rolls-Royce Eagle (H-24 46.2 L, 3,200 hp). Lycoming and Pratt & Whitney also built H engines.

H-type engines An H engine (or H-block) is an engine configuration in which the cylinders are aligned so that if viewed from the front, they appear to be in a vertical or horizontal letter H.

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Typical engine cylinder configurations

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Horizontally opposed engine Radial engine

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Daimler Benz 601 (inverted V12)

Allison X-4520 (24-cylinder X-block engine)

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Radial engines The radial engine consists of a row, or rows, of cylinders, arranged radially about a central crankcase. This type of engine has proven to be very rugged and dependable. The number of cylinders which make up a row can be three, five, seven, or nine. Some radial engines have up to 4 rows of seven or nine cylinders arranged radially about the crankcase, one in front of the other. These are called double row radials. One type of radial engine has four rows of cylinders with seven cylinders in each row for a total of 28 cylinders. Radial engines are still used in some older cargo aeroplanes, warbirds, and crop spray aeroplanes. Although many of these engines still exist, their use is limited. The single-row, nine-cylinder radial engine is of relatively simple construction, having a one-piece nose and a two-section main crankcase. The larger twin-row engines are of slightly more complex construction than the single row engines. For example, the crankcase of the Wright R-3350 engine is composed of the crankcase front section, four crankcase main sections (front main, front centre, rear centre, and rear main), rear cam and tappet housing, supercharger front housing, supercharger rear housing, and supercharger rear housing cover. Pratt & Whitney engines of comparable size incorporate the same basic sections, although the construction and the nomenclature differ considerably.

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Bristol Mercury (single row radial)

Pratt & Whitney Double Wasp R-2800 2-row 18-cylinder radial (up to 2,800 hp (2,090 kW))

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The Curtiss H-1640 Chieftain ‘hexagon’ or ‘inline-radial’ engine

Pratt & Whitney R-4360 Wasp Major 4-row 28-cylinder radial (up to 4,300 hp (3,210 kW))

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Ayres Air Tractor

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Rotary engines Unlike most familiar engines that move a crankshaft to drive a propeller, one of the most common examples was the Gnome Rotary which was designed with a stationary crankshaft around which the cylinders, crankcase and propeller spun. Two brothers, Laurent and Louis Seguin came up with the idea, and their Société des Moteurs Gnome engine was unveiled at the Paris Air Show in 1908. Gnome engines were widely used in first world war aeroplanes and were developed in a range from 50 to 160 hp. Generally, a lower number of moving parts is preferable when it comes to engines, and rotary engines did not stand the test of time.

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Gnome rotary engine

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Wankle engines Although not commonly used, some modern light general aviation aircraft such as motor gliders use Wankle engines, and they are commonly used on military uncrewed aerial vehicles (UAVs). The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion. All parts rotate consistently in one direction, as opposed to the conventional reciprocating piston engine, which has pistons violently changing direction. In contrast to the more common reciprocating piston designs, the Wankel engine delivers advantages of simplicity, smoothness, compactness, high RPM, and a high power-to-weight ratio. This is primarily because three power pulses per rotor revolution are produced compared to one per revolution in a two-stroke piston engine and one per two revolutions in a four-stroke piston engine. At the actual output shaft, there is only one power pulse per revolution. Since the output shaft spins three times as fast as the actual rotor, it makes it roughly equivalent to a two-stroke piston engine of the same displacement. The displacement only measures one face of the rotor since only one face is working for each output shaft revolution. The engine is commonly referred to as a rotary engine. However, this name also applies to other completely different designs, primarily aircraft engines with their cylinders arranged circularly around the crankshaft. The four-stage cycle of intake, compression, ignition, and exhaust occur each revolution at each of the three rotor tips moving inside the oval-like epitrochoid-shaped housing, enabling the three power pulses per rotor revolution. The rotor is similar in shape to a Reuleaux triangle with the sides somewhat flatter.

Wankle engines are in current manufactured by Austro and Mistral.

production

and

are

Austro Engine AE50R The engine is a single rotor four-stroke, air and liquid-cooled, 294 cm3 (17.9 in3) gasoline Wankel engine design, with a mechanical gearbox reduction drive employing a helical gear set with a reduction ratio of 3.225:1. Cooling is predominantly liquid, with forced air cooling for the rotor core. A starter and generator are standard equipment. It employs dual capacitor discharge ignition with variable ignition timing and produces 55 hp (41 kW) Mistral G-200 The engine is a two-rotor four-stroke 3 × 654 cm3 (3 × 39.9 in3) per rotor displacement, liquid-cooled, gasoline Wankel engine design, with a mechanical gearbox reduction drive. It employs dual electronic ignition systems and produces 200 hp at 2,250 propeller RPM. Mistral G-300 The engine is a three-rotor, 3 × 3 × 654 cm3 (39.9 in3) displacement, liquid-cooled, gasoline Wankel engine design, with a mechanical gearbox reduction drive. It employs dual electronic ignition systems and produces 300 hp (224 kW) at 2,250 RPM.

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UAVs are commonly powered by the Wankel engine Austro AE50R Wankle engine

Wankel engine operation

Mistral G-300 Wankle engine

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Cylinder numbering Occasionally, it is necessary to refer to the left or right side of the engine or a particular cylinder. Therefore, it is necessary to know the engine directions and how cylinders of an engine are numbered. The propeller shaft end of the engine is always the front end, and the accessory end is the rear end, regardless of how the engine is mounted in an aircraft. When referring to the right side or left side of an engine, always assume the view is from the rear or accessory end. As seen from this position, crankshaft rotation is referred to as either clockwise or counterclockwise. Inline and V-type engine cylinders are usually numbered from the rear. In V-engines, the cylinder banks are known as the right bank and the left bank, as viewed from the accessory end. The cylinder numbering of the opposed engine begins with the right rear as No. 1 and the left rear as No. 2. The one forward of No. 1 is No. 3; the one forward of No. 2 is No. 4, and so on. The numbering of opposed engine cylinders is by no means standard. Some manufacturers number their cylinders from the rear (continental) and others from the front of the engine (Lycoming). Always refer to the appropriate engine manual to determine the numbering system used by that manufacturer. Single-row radial engine cylinders are numbered clockwise when viewed from the rear. Cylinder No. 1 is the top cylinder. In double-row engines, the same system is used. The No. 1 cylinder is the top one in the rear row. No. 2 cylinder is the first one clockwise from No. 1, but No. 2 is in the front row. No. 3 cylinder is the next one clockwise to No. 2 but is in the rear row. Thus, all odd-numbered cylinders are in the rear row, and all even-numbered cylinders are in the front row. Total Training Support Ltd © Copyright 2020

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Cylinder numbering

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Firing order General The firing order of an engine is the sequence in which the power event occurs in the different cylinders. The firing order is designed to provide for balance and to eliminate vibration to the greatest extent possible. The smoothness of a reciprocating engine is determined by the spacing and the timing of the firing impulses. An engine is inherently smooth when all of the firing impulses are separated by an equal number of degrees of crankshaft rotation; the closer together the firing impulses, the smoother the engine. In radial engines, the firing order must follow a particular pattern since the firing impulses must follow the motion of the crank throw during its rotation. In inline engines, the firing orders may vary somewhat, yet most orders are arranged so that the firing of cylinders is evenly distributed along the crankshaft. Sixcylinder inline engines generally have a firing order of 1-5-3-62-4. Cylinder firing order in opposed engines can usually be listed in pairs of cylinders, as each pair fires across the centre main bearing. The firing order of six-cylinder opposed engines is 1-4-5-2-3-6. The firing order of one model four-cylinder opposed engine is 1-4-2-3, but on another model, it is 1-3-2-4. Single-row radial engines On a single-row radial engine, all the odd-numbered cylinders fire in numerical succession; then, the even-numbered cylinders fire in numerical succession. On a five-cylinder radial engine, for example, the firing order is 1-3-5-2-4, and; on a seven-cylinder radial engine, it is 1-3-5-7-2-4-6. The firing order of a nine-cylinder radial engine is1-3-5-7-9-2-4-6-8

Double-row radial engines On a double-row radial engine, the firing order is somewhat complicated. The firing order is arranged with the firing impulse occurring in a cylinder in one row and then in a cylinder in the other row; therefore, two cylinders in the same row never fire in succession. An easy method for computing the firing order of a 14-cylinder double-row radial engine is to start with any number from I to 14 and add 9 or subtract 5 (called the firing order numbers), whichever produces a result from 1 to 14 inclusive. For example, starting with 8, 9 cannot be added since the answer would then be more than 14; therefore, subtract 5 from 8 to get 3, add 9 to 3 to get 12, subtract 5 from 12 to get 7, subtract 5 from 7 to get 2, and so on. The firing order numbers of an 18-cylinder, double-row radial engine are 11 and 7; that is, begin with any number from 1 to 18 and add 11 or subtract 7. For example, beginning with 1, add 11 to get 12; 11 cannot be added to 12 because the total would be more than 18, so subtract 7 to gets 5, add 11 to 5 to get 16, subtract 7 from 16 to get 9, subtract 7 from 9 to get 2, add 11 to 2 to get 13, and continue this process for 18 cylinders.

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Inline and V-engines The crankshaft in a four-cylinder inline engine has two sets of throws, 180° apart. The throws for cylinders 1 and 4 are together. The throws for cylinders 2 and 3 are together and are 180° from those for 1 and 4. The pistons in cylinders 1 and 4 are at the top of their stroke at the same time those in cylinders 2 and 3 are at the bottom of theirs. This movement of the pistons gives the engines a firing order of 1-2-4-3 or 1-3-4-2. V-8 engines are essentially two four-cylinder banks on a single crankcase, with one cylinder in each bank sharing a crankshaft throw. The left bank fires 1-2-4-3, and the right bank fires 4-31-2. The crankshaft used in a six-cylinder inline engine has three sets of throws, 120° apart. The throws for cylinders 1 and 6 are together, and 120° from those for cylinders 2 and 5. 120° from 2 and 5 are the throws for cylinders 3 and 4. With this arrangement, the pistons in cylinders 1 and 6 come to the top of their stroke together; 120° later, pistons 2 and 5; 120° later, pistons 3 and 4. This type of crankshaft gives the engine a firing order of 1-5-3-6-2-4. A V-12 engine has two banks of six cylinders firing the same sequence, but the right bank starts its firing at the opposite end of the engine.

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Horizontally-opposed engines Both Continental Motors (CMG) and Textron-Lycoming make four- and six-cylinder horizontally opposed engines. Their fourcylinder engines use a 180° crankshaft, and their six-cylinder engines use a 60° crankshaft. Textron-Lycoming also makes an eight-cylinder horizontally-opposed engine with a 90° crankshaft. The right-hand bank of cylinders on CMG engines are offset to the rear of the cylinders on the left side, and cylinder number 1 is the right rear cylinder. The firing order for a four-cylinder CMG engine is 1-3-2-4. The firing order for a six-cylinder CMG engine is 1-6-3-2-5-4. The right-hand bank of cylinders on Textron-Lycoming engines are offset forward, and cylinder number 1 is the right front cylinder. The firing order for a Textron-Lycoming engine is 1-32-4, for a six-cylinder engine it is 1-4-5-2-3-6, and for an eightcylinder engine, it is 1-5-8-3-2-6-7-4.

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Firing order of horizontally opposed engines – Textron-Lycoming and Continental engines

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Left-hand rotation All cylinders of Lycoming aircraft engines are numbered from the front or propeller end to the rear with cylinder No.1 being furthest forward. Most of the engines have the odd number cylinders on the right side and even number cylinders on the left side. However, the TIO and TIGO-541 series engines are numbered with the odd number cylinders on the left side and even number cylinders on the right side of the engine. Engines with the letter ‘L’ in the model prefix, such as LTIO540-J2BD, denotes that the engine has a counter-clockwise rotation of the crankshaft when viewed from the rear of the engine. To work out the left-hand firing order from the righthand firing order, transpose the number 1 cylinder to the rear and read backwards

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Cylinder numbering

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Valve configurations The majority of internal combustion engines also are classified according to the position and arrangement of the intake and exhaust valves, whether the valves are located in the cylinder head or cylinder block. The following are types of valve arrangements with which you may come in contact. I-head – The intake and the exhaust valves are both mounted in a cylinder head directly above the cylinder. This arrangement requires a tappet, a pushrod, and a rocker arm above the cylinder to reverse the direction of valve movement. Although this configuration is the most popular for current gasoline and Diesel engines, it is rapidly being superseded by the overhead camshaft. L-head – The intake and the exhaust valves are both located on the same side of the piston and cylinder. The valve operating mechanism is located directly below the valves, and one camshaft actuates both the intake and the exhaust valves.

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I-head and L-head valve arrangements

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F-head – The intake valves are generally located in the head, while the exhaust valves are located in the engine block. The intake valves in the head are actuated from the camshaft through tappets, pushrods, and rocker arms. The exhaust valves are actuated directly by tappets on the camshaft. T-head – The intake and the exhaust valves are located on opposite sides of the cylinder in the engine block; each requires their own camshaft.

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F-head and T-head valve arrangements

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Sleeve valve – is a type of valve mechanism for piston engines, distinct from the usual poppet valve. Sleeve valve engines saw use in several pre-second world war luxury cars and, in the United States, in the Willys-Knight car and light truck. They subsequently fell from use due to advances in poppet-valve technology, including sodium cooling, and the Knight system double sleeve engine’s tendency to burn much lubricating oil or to seize due to lack of it. The Scottish Argyll company used its own, much simpler and more efficient, single sleeve system (Burt-McCollum) in its cars. After extensive development, this system saw substantial use in British military aircraft engines of the 1940s, such as the Napier Sabre, Bristol Hercules and Centaurus.

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Camshaft configurations Single overhead camshaft – The camshaft is located in the cylinder head. The intake and the exhaust valves are both operated from a shared camshaft. The valve train may be arranged to operate directly through the lifters, as shown in view A below, or by rocker arms, as shown in view B. This configuration is becoming popular for passenger car gasoline engines. Double overhead camshaft – When the double overhead camshaft is used, the intake and the exhaust valves each operate from separate camshafts directly through the lifters. It provides excellent engine performance and is used in more expensive automotive applications.

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Single overhead cam valve arrangements

Double overhead cam valve arrangements

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Gasoline and Diesel engines – a comparison At a time when many oil companies are reducing, if not eliminating, the production of AVGAS, and in an environment in which better fuel economy and green engine technology are a necessity, there is a growing demand for alternatives. One solution is the use of Diesel engines. Diesel engines have many advantages over traditional AVGAS engines. They run on heavy fuels such as JET-A or Diesel fuel. Heavier fuels mean weight per gallon is more than that of AVGAS. However, JET-A and Diesel fuel have a higher BTU (British Thermal Unit) than AVGAS. Simply stated, JET-A and Diesel fuel produce more heat energy per gallon than does AVGAS. Also, JET-A and Diesel fuel are more readily available and easier to manufacture or refine. Internationally, Diesel fuel is approximately one third less than the price of AVGAS. Diesel engines also use their fuel more economically. For example, adjusted for horsepower, the 1.7 Thielert Centurion Diesel engine consumes approximately 25% less fuel per hour at 75% cruise compared to a Lycoming O-360 AVGAS engine at 75% cruise.

Using FADEC (Full Authority Digital Engine Control) with Diesel engines provides another advantage. In traditional AVGAS engine operation, the engine relies on spark plugs, a fuel/air metering unit, and a throttle for proper operation. In a Diesel engine, there are no spark plugs, fuel/air metering units, or throttle. Diesel engines rely on a single-engine fuel control lever and an automatic propeller pitch control in the co*ckpit. High compression ratios (18:1) coupled with high fuel pressures and temperatures, ignite the fuel instantaneously as it leaves the fuel injector. Diesel engines provide the ability to produce sea-level rated horsepower at high altitudes with supercharging or turbocharging, as do AVGAS engines. What that means to the pilot is more power at altitude. A typical normally aspirated engine produces maximum power only at sea level. One disadvantage of using aviation Diesel engines is the upfront expense to make the conversion from a traditional engine. For example, currently, converting a typical Cessna 172 from an AVGAS engine to a Diesel engine is over $50,000 firewall-forward. A new AVGAS engine is approximately $30,000 firewall-forward. Aircraft owners must decide if the advantage of the Diesel engine is worth the additional cost.

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Another disadvantage of Diesel engines is their cold-weather starting and operation. Since the aircraft Diesel engine relies on higher cylinder temperatures to ignite the fuel/air mixture, cold weather starting may be difficult. However, many Diesel engines utilise a preheat oil system to combat cold weather operation issues.

Premier Diesel Conversion https://youtu.be/Wyf7F2L02To Cessna’s New Turbo Diesel 172 Skyhawk https://youtu.be/bEaQO_2Httg

The current state of aircraft engine design has been stagnant. Most of the engines now used in general aviation aircraft were designed in the 1940s, 1950s or early 1960s. For example, the O-235 engine was first certified in 1942, and the GSO-480A1A6 was certificated in 1955. Many of the engines have been updated over the years, including refinement of the basic design, the materials used, and switching to fuel injection instead of carburettors. Continental Engines offers FADEC as an option for some of their engines. The newest Continental IO-240, an AVGAS engine, was certified in 2007 under Type Certificate Data Sheet (TCDS) E7S0. The engine may be delivered without the FADEC option, under a different TCDS. The engine still uses a spark plug in each cylinder and has a fuel injector and FADEC options. The Austro engine from Austria was certified in 2009. The engine is a Diesel engine and runs on JET-A fuel. This is the newest engine certified by the European authorities, which is accepted by the FAA. The engine is similar to the Thielert Diesel engine. It is an option used in the Diamond Twin Star aircraft. Some examples on the following pages compare the features of several engines (Lycoming is AVGAS; DeltaHawk, Thielert, Zoche, SMA and Austro are Diesels). Total Training Support Ltd © Copyright 2020

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Lycoming IO-540 SMA SR305-230 Thielert Centurion 1.7

Lycoming IO-360

Delta Hawk DH200V

Zoche Z002A

Austro AE 300

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Comparison between a gasoline engine and Diesel engine can be made in aspects like working pressures, combustion, operating cycle, compression ratios, thermal efficiency, engine speeds, maintenance cost and running costs as listed below. Gasoline Engines 1

Diesel Engines

The combustion of fuel takes place approximately at constant volume. In other words, it works on the Otto cycle.

The combustion of fuel takes place approximately at constant pressure. In other words, it works on the Diesel cycle.

The thermal efficiency is up to about 26%.

The thermal efficiency is up to about 40%.

Overheating trouble is more in gasoline engine due to low thermal efficiency.

Overheating trouble is less in Diesel engine due to high thermal efficiency.

The starting of the gasoline engine is easy due to the low compression ratio.

The starting of the Diesel engine is slightly more difficult due to a higher compression ratio compared to a gasoline engine.

Diesel Engines

A gasoline engine draws a mixture of gasoline and air during the induction stroke.

A Diesel engine draws only air during the induction stroke.

The carburettor is installed in gasoline engines to mix air and gasoline in the required proportion and to supply it to the engine during the induction stroke.

The injector or atomiser is installed in Diesel engines to inject the fuel at the end of the compression stroke.

The pressure at the end of The pressure at the end of the compression is about the compression is about 10 bar. 35 bar. The charge (i.e. gasoline and air mixture) is ignited with the help of spark plug.

The fuel is injected in the form of fine spray. The temperature of the compressed air is about 600° C at a pressure of about 35 bar.

A gasoline engine has a compression ratio of approximately from 6 to 10.

A Diesel engine has a compression ratio of approximately 15 to 25.

Gasoline Engines

10 As the compression ratio is low, the gasoline engines are cheaper and lighter in weight.

As the compression ratio is high, the Diesel engines are costlier and heavier in weight.

11 The running cost of a gasoline engine is high because of the higher cost of gasoline fuel.

The running cost of a Diesel engine is low because of the lower cost of Diesel fuel.

12 The maintenance cost is lower.

The maintenance cost is more.

13 Gasoline engines are high-speed engines.

Diesel engines are relatively low-speed engines.

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Lycoming O-320 engine (gasoline)

SMA SR305-230 engine (Diesel)

This little four-cylinder engine from Pennsylvania manufacturer Lycoming epitomises the light-aeroplane engine, helped by the fact that it powered some of the most popular light aeroplanes ever, including later models of the Cessna 172 and the Piper PA-28 Cherokee. With four cylinders arranged two per side, big air-cooled heads to keep the cylinders happy, a normally carburetted fuel system and dual magneto ignition, all driving a fixed-blade prop, the 150 hp O-320 took a 1930s design and updated it with the latest materials and manufacturing techniques. The bottom line is the O-320 delivers reliability, affordability and familiarity.

The four-cylinder SR305-230 diesel engine was developed by French company SMA, which is now a subsidiary of Safran. The engine was first flown in 1998 on the French airframe Socata TB-20 and FAA certified in 2002. Recently, the SR305-230 was chosen to power Cessna’s latest version of the immensely popular Cessna 182 Skylane – the Turbo Skylane JT-A. The engine is air- and oil-cooled and compatible with jet-A, which is widely available at airports around the world. In addition to eliminating the growing concern of 100LL availability, the SR305 has an electronic control unit that optimises the performance of the engine in all phases of ground and flight operations, reducing the workload for the pilot. SMA claims the operating cost of the SR305 is reduced by about 40 percent compared with avgas engines because of its longer 2,400 TBO, reduced number of parts and lower fuel consumption. 1-117

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Glossary of piston engine terms A

Aeration

The introduction of air into a liquid.

Atomisation

The formation of a liquid into a spray.

Aerofoil

The shape of the blade section formed to give lift.

Attenuator

A means of stopping fuel flow fluctuation.

Accessories

Rate of change of velocity

Automatic

Self-operating.

Auxiliary

Something helpful which is supplementary.

Axial flow engine

An engine in which the gas flow travels along the centre line of the engine from front to rear.

Velocity Distance or Time (Time)2

Adaptor

A connection for joining two components through which a fluid or electricity can be transferred from one section or component to the other.

AFRCU

Air/fuel ratio control unit.

Allowance

The permitted difference in dimensions to allow for various fits.

Aluminised

Coated with aluminium to resist corrosion.

Aneroid capsule

A metal container from which most of the air has been exhausted. It is then sensitive to variations of outside pressure.

Angle of attack

AoA is the angle that the chord line makes with the direction of airflow.

Annular

Circular, ring formation around components.

Approach minimum

Minimum engine speed for landing aircraft.

B Backlash

The working clearance, measured at the pitch circles of any two gears in mesh.

Baffles

Plates fitted to prevent or control the movement of a fluid in the direction which it would otherwise flow.

Balancing

Adjusting the size and position of weights to bring a rotating assembly into static and dynamic balance.

Barometric pressure

The atmosphere. It is reduced with an increase of altitude.

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Bearing

The part of a mechanism in which a rotating shaft revolves and is supported.

Breather

A duct connecting the crankcase to the atmosphere that prevents the build-up of crankcase pressure.

Blacklight

Ultraviolet light whose rays are in the lower end of the visible spectrum. While more or less invisible to the human eye, they excite or make visible such materials as fluorescent dyes.

British thermal unit

The amount of heat required to raise the (BTU) temperature of one pound of water one degree Fahrenheit.

Brittleness

The liability of a particular metal to fracture on receiving a blow or shock.

Bulkhead

A transverse partition which separates one compartment from another; a typical example is a fireproof bulkhead on the engine.

Burr

A rough, sharp ridge or protection at the edges of a part after it has been worked or machined.

Bush

A hollow cylindrical one-piece bearing, usually phosphor bronze or cast iron.

By-pass ratio

Mass airflow flowing through the bypass duct divided by that passing through the core engine.

Blade angle

The angle between the chord line and the aeroplane of rotation.

Bleed

The removal of air or air contaminated liquid from an enclosed fluid circuit.

Bonding

Bore

Linking together all the metal parts of an aircraft to obtain positive electrical continuity. The internal diameter of a cylindrical part.

Brake specific fuel consumption (BSFC)

The number of pounds of fuel burned per hour to produce one brake horsepower.

Brazing

Uniting two metals using molten brass. The higher temperature is required than for soldering, but the joint is stronger.

C Calibrate

To measure or check against a known accurate master tool or instrument.

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Clapper micrometre

A precision measuring device with a single movable jaw, advanced by a screw. One revolution of the crew advances the movable jaw 0.025".

Clapper, Vernier

A micrometre calliper with a special Vernier micrometre.

Cam

An eccentric projection on a revolving shaft designed to change rotary motion into reciprocating motion.

Cam-ground piston An aircraft engine piston ground in such a way that its diameter parallel to the wrist pin boss is less than its diameter perpendicular to the boss. When the piston reaches its operating temperature, the difference in mass has caused the piston to expand to a perfect circular form. Capillary

A tube possessing a hair-like bore.

Casting

Pouring molten metal into a mould in which it is allowed to solidify the resultant solidified shape.

Centrifugal

The throwing out action of a revolving unit or mass.

Check

To make a comparison of a measurement of time, temperature, size, pressure or any other quantity with the correct [missing text] for that measurement.

Choke bore

A method of boring a cylinder of an aircraft engine in which the top, that portion affected by the mass of the cylinder head, has a diameter slightly less than that of the main bore of the barrel. When the cylinder reaches operating temperature, the mass of the head has caused the bore to expand, so it is straight throughout its length.

Choked

Restricted or blocked, possibly using sonic methods.

Chord

The imaginary line joining the centre leading edge of an aerofoil to the centre trailing edge.

Chuck

A particular type of vice used to hold a job or tool during machining operations.

Clearance

The space provided between two working parts to allow for freedom of movement, lubrication and variation in size of position due to heat or distortion.

Clutch

A device by which two shafts or rotating members may be connected or disconnected while at rest or in relative motion.

Coarse pitch

Largest blade angle normally used.

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Co-axial

Shafts or gears which have the same centre line.

Convection current

The movement set up in heated air or fluid due to the part of the substance moving away from the source of heat and carrying heat with it.

Combustible

Liable to burn to spring suddenly into flame.

Combustion chamber (piston engine)

That portion of the cylinder of a reciprocating engine in which the combustion takes place. It is that portion above the piston.

Convergent

A convergent duct is one that has a gradual reduction in the size of the bore, a passage which narrows in the direction of flow.

Combustion chamber (gas turbine engine)

Gas turbine engines – the assembly which contains the flame.

Corrected (nondimensional) RPM

An oil pressure relief valve with a thermostatic valve to decrease the regulated pressure when the oil warms up. High pressure is allowed to force the cold oil through the engine, but the pressure automatically decreases when the oil warms up.

This is an RPM obtained by correcting an observed RPM using graphs or formulae to allow for the effects of temperature on the engine. We normally correct the international standard atmosphere (ISA) conditions.

Compensated relief valve

Corrosion

The slow wearing-away of a surface, especially metals, by chemical action, e.g., oxidisation.

Corrugated

Having a ridged or wrinkled surface.

Compression ratio

The ratio of volume before compression to the volume after compression.

Crocus cloth(paper)

An abrasive cloth with a very fine dark red abrasive on its surface. It is used for polishing of metals.

Compression ring

The top piston ring used to provide a seal for the gases in the cylinder and to transfer heat from the piston into the cylinder walls.

Cylinder baffles

Conductivity

The ability of volume before compression to the volume after compression.

Thin sheet metal covers and deflectors attached to air-cooled cylinders to force air through the cooling fins to remove the maximum amount of heat.

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D Dead throttle

De-icing

De-inhibit

Density

D Movement of the throttle lever with no corresponding response from the engine. 10° of movement at the maximum rev/min position. Movement corresponding response from the engine. 10° of movement at the maximum rev/min position.

Diaphragm

A flexible partition used to transmit force.

Disconnect

Uncouple or detach pipelines, controls, cables, etc.

Distributor

A component to distribute electrical impulses to the relevant cylinder (or combustion chamber) spark plug.

Divergent

To remove the inhibitor from an item before being put into service (see inhibit).

A divergent duct is one that has a gradual increase in the size of the bore, a passage which widens in the direction of flow.

Dolls eyes

The mass per unit volume of material; expressed as grams/cm3 or kilograms/m3

Black and white blinders used to indicate fuel, oil and air pressures, to pilot.

Double acting

Influenced by pressure on either side.

Dowel

A small diameter raised plug which fits into a matching hole, use for locating items.

Application of a fluid (alcohol) or heat to a component to break up and/or prevent the formation of ice.

Density altitude

That altitude in standard air which compares with existing air density.

Depression

A pressure below standard atmospheric pressure.

Drag

That which locks or unlocks a mechanism.

The resistance offered by the air to a blade section moving through it.

Drain

The almost instantaneous release of heat energy from fuel in an aircraft engine caused by the fuel-air mixture reaching its critical pressure and temperature. It is an explosion rather than a smooth burning process.

A small hole or pipeline leading from a component to atmosphere, to allow fluid to vent, or to empty a tank, cavity or sump.

Drum

The hollow shaft on which the compressor vanes are mounted.

Detent Detonation

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Duct

A cast or drilled hole forming a passage to convey air, oil, fuel, etc., through engine parts, e.g., pressure balance duct in the carburettor.

Efficiency (mechanical)

The ratio of the amount of heat energy converted into useful work to the amount of heat energy in the fuel used.

Dynamic damper

A counterweight on a crankshaft of an aircraft engine. It is attached in such a way that it can rock back and forth while the shaft is spinning and absorb dynamic vibrations. It, in essence, changes the resonant frequency of the engine propeller combination.

Efficiency (volumetric)

The ratio of the volume of the charge taken in at a cylinder, reduced to standard conditions, to the actual volume of the cylinder.

Elasticity

The capacity of a material to return to its original dimensions on the removal of distorting forces.

Empirical

Relying on observation or experiment, not on theory.

End float

The axial movement or a gear or shaft mounted in bearings.

Engine, aircraft

An engine that is used to propel an aircraft. It includes the turbochargers and accessories necessary for its functioning but does not include the propeller.

Engine, dry sump

An engine in which most of the lubricating oil is carried in an external tank and is fed to the pressure pump by gravity. After it has lubricated the engine, it is pumped back into the tank by an engine-driven scavenger pump.

E Efficiency (of a machine)

The proportion that the actual power or effort is of the ideal power, work or effort, expressed as a percentage. An engine that is 70% efficient does 70% of the work it could do were there no losses. Mechanical advantage Velocity ratio Or Work out Energy in

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Engine, external combustion

A form of a heat engine in which the chemical energy in the fuel is converted into heat energy outside of the engine.

Engine, four-cycle (four-stroke)

The most common event cycle for aircraft engines. The four-stroke fiveevent cycle consists of an intake stroke, in which the piston moves inward with the intake valve open and a compression stroke in which the piston moves outward with both valves closed. Near the top of the compression stroke, ignition occurs. The power stroke is an inward stroke of the piston with both valves closed, and the exhaust stroke occurs when the piston moves outward with the exhaust valve open. At this point, the cycle begins again.

Engine, gas turbine

A form of a heat engine in which the burning fuel adds energy to the compressed air and accelerates the air through the remainder of the engine. Some of the energy is extracted to turn the air compressor, and the remainder accelerates the air to produce thrust. Some of this energy can be converted into torque to drive a propeller or a system of rotors for a helicopter.

Engine, horizontally opposed

An engine with cylinders lying flat in two rows, one on either side of the crankcase.

Engine, inline

An engine with all of the cylinders in a single line. The crankcase may be located either above or below the cylinders. If it is above, it is called an inverted inline engine.

Engine, internal combustion

A form of a heat engine in which the chemical energy in the fuel is converted into heat energy inside the engine.

Engine, reciprocating

An engine which converts the heat energy from burning fuel into the reciprocating movement of the pistons. This movement is converted into rotary motion by the connecting rods and crankshaft.

Engine, remanufactured

An engine assembled by the engine manufacturer or his authorised agent built up of used parts which are held to the new parts’ dimensional limits. The engine is given zero-time records and usually the same warranty and guarantee as the new engine.

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Engine, rotary radial

A form of aircraft engine popular during the first world war, in which the propeller was attached to the crankcase and the pistons were attached to an offset cam mounted on the airframe. As the engine runs, the cylinders, crankcase and propellers all spin around.

Engine, static radiating radial

An engine with all of the cylinders out from a small central crankcase. A single-throw crankshaft is used for each row of cylinders. All single-row radial engines have an odd number of cylinders, but two or four rows may be used if more power is required.

Engine, turboprop

A turbine engine which drives a propeller through a reduction gearing arrangement. Much of the energy in the exhaust gases is converted into torque rather than using its acceleration to move the aircraft.

Engine, two-cycle

A reciprocating engine in which a power (two-stroke) impulse occurs on each stroke of the piston. As the piston moves outward, the fuel-air mixture is drawn into the crankcase below the piston while above the piston the mixture is compressed. Near the top of the stroke, ignition occurs and, as the piston moves downward, power is exerted on the crankshaft. Near the bottom of the stroke, exhaust action takes place on one side of the cylinder and intake action occurs on the opposite side.

Engine, V

An engine with cylinders arranged in two rows, attached to the crankcase in the form of a ‘V’, with an angle of between 45° and 60° between the banks.

Engine, wet sump

An engine in which all of the oil supply is carried within the engine itself.

Epicyclic gear

Where a gear or train of gears revolves around the circumference of a larger gear.

Examine

To make a visual survey of the condition of an item.

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E Extrusion

F Forcing plastic metal through a die of required shape using hydraulic pressure.

Flexible coupling

Used to connect two shafts in which perfect rigid alignment is impossible.

Flight manual

Approved information which must be carried in an aircraft. This includes the engine operating limits and any other information that is vital to the pilot.

Flock

Pulverised wool or cotton fibres attached to screen wire used as an air filter. The flock covered screen is lightly oiled, and it holds dirt and dust, preventing it from entering the engine.

F Fatigue

The diminishing resistance to fracture caused by fluctuating stresses.

Fillet

A radius formed at an intersection.

Filtered

The process in which the fluids are separated from the solids.

Fit

Correctly attach one item to another.

Fits

There are four types of fits 1. Force fits, requiring hydraulic pressure or heat to mat the parts. 2. Driving fits, requiring pressure, or hammering to mate the parts. 3. Push fits, requiring to be pushed into position to mate the parts; parts are not free to rotate. 4. Running fits, where the parts are free to rotate.

Fluctuation

To waver.

Flux

A substance used to clean the surface of a job, prevent oxidisation and aid the flow of the material in such processes as fusion, soldering, brazing and welding.

Forging

Shaping the metal by hammer blows.

Fuel/air ratio

The proportion of fuel to air in a combustible mixture.

Flame propagation

The spread of the flame from the point of ignition.

Fulcrum

The points about which a lever is supported or rotates.

Flange

A projecting rim, e.g., cylinder flange.

Flashpoint

The temperature at which vapour will ignite if brought into contact with a flame.

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Gag

To prevent movement (especially rotation).

Gasket

A thin sheet of material sandwiched between surfaces to make a gastight joint.

Horsepower, friction

The amount of horsepower required to turn the engine against the friction of the moving parts and to compress the charges in the cylinders.

Horsepower, Indicated

The total horsepower developed in the engine. It is the sum of the brake horsepower delivered to the propeller shaft and the friction horsepower required to drive the engine.

Gear ratio

The ratio between the output and input speeds of a train of gears.

Generate

To produce, e.g., electrical energy.

Governor

A speed controlling unit.

Housing

Ground idling consumption

The amount of fuel used by the engine at the lowest RPM on the ground.

That part of a mechanism which carries a bearing.

Hydrometrical

Combination of hydraulic and mechanical operation.

Hysteresis

A lag, delay or differential in a function or operating point in a system.

H Half-ball valve

In the shape of a half-ball, positioned over the end of a duct and capable of controlling fluid flow.

Horsepower

Practical measurement of power used for aircraft engines. It is the accomplishment of 33,000 ft-pounds of work in one minute.

Idler gear

A gear in a train of gears which reverses the direction of motion but does not change the overall ratio of the train gear.

Horsepower, brake

The actual horsepower delivered to the propeller shaft of an engine.

Impeller

The rotating member of a centrifugal pump or blower imparts kinetic energy.

Inconel

Heat-resisting steel.

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Inhibit

To cover temporarily with a protective film to resist corrosion.

Kinetic

The energy contained in a body due to motion.

Inspect

To examine and where necessary test, equipment; the review by a supervisor of the work of tradesmen.

Knuckle pin

The hardened steel pin that holds an articulating rod in the master rod of a radial engine.

Insulate

To separate using a non-conductor.

Interference fit

A fit between two parts in which the part being put into a hole is larger than the hole itself. To assemble the parts, the hole is expanded by heating, and the part is shrunk by chilling.

ISA

International standard atmosphere. 1013 mbar, 15°C.

Isolating

To separate one system from another.

Isochronous

A constant time or at a constant speed.

L Labyrinth

Seal formed by a series of passages.

Laminated

Consisting of thin plates, one upon the other.

Lap

To polish using fine abrasive and production of a flat surface.

Lift

Caused by pressure differences on blade surfaces.

Lug

An irregular projection.

M J Jointing

A thin paper gasket.

Journal

That part of a rotating shaft that is supported in a bearing.

Mach number

The ratio of the velocity of a body to the local velocity of sound.

Magnetic flux

Lines of magnetic energy given off from a magnet.

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M Magneto

N A self-contained, permanent-magnet AC generator with a set of current interrupter contacts and a step-up transformer. It is used to supply the high voltage required for ignition in an aircraft engine.

Mandrel

An accurately ground shaft for supporting or locating a hollow part during measurement or machining, the mandrel accurately fits the hole.

Master switch

Switch in an electrical circuit capable of isolating the whole circuit.

Mating

Surfaces that are or will be in contact with others.

Mechanical advantage (of a machine)

The ratio of load to effort.

Meter

An instrument used for measuring.

Metering

A test instrument for measuring the serviceability of the insulation of lowtension electrical wiring.

Motoring cycle

Nimonic

A special heat-resisting alloy.

Nitriding

A form of case hardening in which the steel part is heated in an atmosphere of ammonia. The ammonia breaks down, and its nitrogen combines with aluminium in the steel to form a tough, abrasive-resistant aluminiumnitride surface. Cylinder walls and crankshafts journals are nitrided.

Nominal

A figure or value about which is a permitted variation.

O Observed RPM

The rev/minute as read directly from the co*ckpit tachometer.

Oil, ashless dispersant

A popular mineral oil which contains no ash-forming additives but does contain additives which prevent contaminants clustering together. It keeps the contaminants dispersed throughout the oil.

Oil, detergent

Mineral oil to which, ash forming additives has been added to increase its resistance to oxidation. Because of its tendency to lose carbon deposits, it is not used in aircraft engines.

Turning the engine through a starting cycle without a light up.

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Oil, synthetic

Lubricating oil with a synthetic rather than gasoline base. It has less tendency toward oxidation and sludge formation than gasoline oils. It is extensively used in turbine engines and is gaining popularity in reciprocating engines.

Overhaul, top

The overhaul of the cylinders of an aircraft engine. It consists of grinding the valves, replacing the piston rings and doing anything else necessary to restore the cylinders to their proper condition. The crankcase of the engine is not opened.

Oil control ring

The piston rings below the compression rings used to control the amount of oil between the piston and the cylinder wall. It is usually a multipiece ring typically fits into a groove with holes to drain part of the oil back to the inside of the piston.

Overswing

The tendency of the engine to temporarily exceed maximum rev/min on full throttle opening.

Oil scraper (wiper) ring

A piston ring located at the bottom, or skirt end of a piston, used to wipe the oil either toward or away from the oil control ring, depending on the design of the engine.

Air intake pressure.

Compressor outlet pressure (single spool engines).

Combustion chamber outlet pressure (single spool engines).

Orifice

An opening at the end of a tube or pipe.

Jet pipe pressure (single spool engines).

Overhaul, major

The complete disassembly, cleaning, inspection, repair and reassembly of an engine or other components of an aircraft.

Parameter

A variable quantity, which is measurable and affects other variables, e.g., the parameter of temperature varies mass flow.

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P Permamould crankcase

P An engine crankcase which has been pressure-moulded in a permanent mould rather than being sand-cast. It is thinner and denser than a sandcast crankcase.

Gasoline

A substance containing a form of chemical energy used as fuel for most of our aircraft engines. It is a natural hydrocarbon product which was in ancient times, plant or animal life but was buried under billions of tons of earth. It is obtained as a liquid from deep wells.

Pigmented

Coloured with pigment, colouring matter or dye.

Pinion

The smaller of a pair of high ratio gears.

Piston displacement

The total volume swept by the pistons of an engine in one revolution of the crankshaft.

Pitting

Plane of rotation

The aeroplane at right angles of the rotating shaft.

Planetary gears

A reduction gearing arrangement in which the propeller shaft is attached to an adapter holding several small planetary gears. These gears run between a sun gear and a ring gear, either of which may be driven by the crankshaft and the other is fixed into the nose section. Planetary gears are efficient and do not reverse the direction of rotation between the two shafts.

Plastic

1. 2.

Surfaces are said to be pitted when corrosion, excessive heating, or hammering has caused shallow irregular depressions in the surface, for example, the effect of rust on iron and steel. Pin or hinge about which rotation may take place.

The property whereby a material is easily deformed. A synthetic resin, capable of being moulded.

Plenum

Space is considered to be filled with matter (as opposed to vacuum).

Porous chrome plating

An electrolytically deposited coating of chromium on walls of aircraft engine cylinders. The surface contains thousands of tiny cracks which hold oil to provide for cylinder wall lubrication.

Port

An opening for the inlet and/or outlet of gases.

Pour point

The lowest temperature at which a fluid will pour without disturbance.

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Power; P

The item rate of doing work. It is force times distance divided by time.

Pressurise

To raise the pressure in a compartment.

Pre-ignition

Ignition occurring in the cylinder before the time of standard ignition. It is often caused by a local hot spot in the combustion chamber igniting the fuel-air mixture.

Preventive maintenance

Simple or minor preservation operations and replacement of small standard parts not involving complex assembly operations.

Primary

First.

Priming

To fill a system with its own fluid.

Progressively variable

To alter by easy stages.

Pressure altitude

It is the altitude at which the air pressure, ISA day, is equal to the local air pressure during a ground run, irrespective of your actual height above sea level. Or, the altitude shown when the aircraft altimeter is set to 1013 mbar.

Pressure, brake mean effective (BMEP)

A computed value (not measured) of the average pressure that exists in the cylinder of an engine during the power stroke.

Pressure, indicated mean effective (IMEP)

The average measured pressure in the cylinder of an engine during the power stroke.

Pressure, manifold

The absolute pressure measured at the appropriate point in the induction systems of an aircraft engine and usually expressed in inches of mercury.

Pressure ratio

Q Quill drive

A short drive shaft designed to shear at its waisted portion used to prevent continued driving force being transmitted to seized or partly seized components.

R Radial

Issuing as rays from a common centre.

Rated maximum continuous power

The maximum approved brake horsepower developed by an aircraft engine approved for an unrestricted period.

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Rated take-off horsepower

The approved brake horsepower that is allowed to be developed by an aircraft engine for a period not exceeding five minutes.

Reciprocating motion

Movements backwards and forwards in a straight line.

Recognition

This process consists of dismantling the assembly, renewing worn and unserviceable parts, reassembling and testing.

Reconnect Refit

To correctly replace an item that he previously been removed. To refill a container to a given level, pressure or quantity.

Resonant

A condition in which a mechanical system is allowed to vibrate when its natural frequency is the same as the frequency of the applied force.

Rich mixture

One which has an excess of fuel.

Rigid

Fixed, will not move, stiff, unyielding.

Rod, articulating

The rod in a radial engine that connects the piston to the master rod. It rocks back and forth rather than encircle the crankshaft. It is called a link rod.

The only connecting rod in a radial engine whose big end passes around the crankshaft. All of the other rods connect to the master rod and oscillate back and forth rather than encircling the crankshaft.

Root

End of blade nearest hub.

Rotor

The revolving part of a component.

Correctly couple pipelines, controls cable, etc.

Replenish

Rod, master

Scaler quantity

That which is considered to have magnitude only.

Scoring

The term applied to scratch-like marks found on bearings or cylinder walls and pistons caused by lack of lubrication or by the ingress of dirt between the bearing surfaces.

Secondary

Second.

Servo

That portion of the system which assists in the operation of the primary system.

Shim

A thin piece of metal cut to shape, used between two surfaces to adjust their distance accurately apart.

Shroud

A portion of a component which covers or shields.

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Solenoid

A coil of wire with a movable core.

Specific fuel

The weight of fuel required to produce a unit of output an hour; expressed as pounds of fuel/brake horsepower/hour, or, pounds of fuel/pound thrust/hour.

Standard atmosphere

The conditions for a standard atmosphere (ICAO) are Pressure = 14.7 lbf/in2 absolute = 29.92" Hg absolute Temperature = 15°C

Stator

To be stationary, fixed compressor blades.

Stellite

A tough and wear-resistant metal used for valve faces and stem tips. It contains cobalt, tungsten, chromium, and molybdenum.

Specific gravity (relative density)

The weight of a fluid by comparison to the same volume of water.

Spectrometric oil

A system of oil analysis in which a sample is burned in an arc and the resulting light is examined for its wavelengths. This test can determine the amount of the different metals suspended in the oil and can indicate an impending engine failure.

Stress

An extension which will enter and help to locate one engine part to another, to ensure concentricity.

An applied load. Tensile stress is a force that tends to stretch a body; shear stress, a force that tends to cut through its section and a compressive stress a force that tends to collapse it.

Stroke

The distance that the piston moves from one end of its travel to the other.

Stoichiometric

Chemical combination which ultimately uses all the products of the reaction.

Suction

The production of a partial vacuum causing fluid to move or adhere.

Surge

To move up and down or to and fro, in waves.

Swirl

The rotary motion is given to a fluid.

Synchronise

To cause two or more events to happen at the same time.

Spigot

Splines or serrations

Stagger angle

Stamping

A series of longitudinal ridges on the outer surface of a shaft, separated by grooves, these fit into a similarly grooved and ridged counterpart. The angle formed between the chord line of a compressor blade and the horizontal centre line of the engine. Shaping and/or cutting using dies in a press.

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Tachometer

Engine rev/minute gauge may be in % of maximum rev/minute.

Tertiary

Third.

Test

To make sure, by using the necessary test equipment, that a component functions correctly.

Thrust

A propulsive force that tends to move an aircraft forward.

Thrust bearing

A shaft bearing designed to take an axial load.

The time between overhauls (TBO)

A recommendation of the manufacturer or an aircraft engine as to the amount of time that the engine can operate under average conditions before it should be overhauled. Overhauls at this time will result in the most economical operation.

Tip

End of blade furthest from the hub.

Torque

A force tending to rotate or twist a shaft.

Trim

A small adjustment to fuel flow, e.g., top temperature trimming.

Turbine

That part of an engine which is rotated by the medium of gas flow.

Turbocharger

An air compressor used to increase the pressure of the air entering the fuel metering system; the compressor is driven by a high-speed turbine which is spun by the exhaust gases leaving the engine.

None

V Vacuum

A region in which the gas pressure is considerably lower than atmospheric pressure. A perfect vacuum is practically unobtainable.

Valve clearance, cold

The clearance between the valve stem and the rocker arm of an engine using solid valve lifters when the engine is cold.

Valve clearance, hot

The clearance between the valve stem and the rocker arm of an engine using solid valve lifters when the engine is at operating temperature.

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V Valve Clearance, Timing

Valve float

Valve lag

Valve lead

V The clearance to which a poppet valve using solid lifters is adjusted to set the cam-to crankshaft timing. The valves in one cylinder are adjusted to this clearance, the timing is set, and the valves are then re-adjusted to the cold clearance.

Valve overlap

The angular distance of crankshaft rotation when the piston is passing top centre on the exhaust stroke when the intake and exhaust valves are both open.

Vaporisation

The conversion of fluids or solids into a gas.

Varsol

A gasoline product which is similar to naphtha, used as a solvent for washing aircraft engine parts.

Vector quantity

That which has both magnitude and direction.

The number of degrees of crankshaft rotation after top or bottom centre at which the intake or exhaust valve opens or closes. For example, if the intake valve closes 60° after bottom centre on the compression stroke, it has a valve lag of 60°.

Velocity

Distance divided by time or rate of change of distance.

Velocity ratio (of a machine)

Effort’s distance moved; load’s distance moved

Vent

A small escape pipe which carries off excess pressures or vapours.

The number of degrees of rotation before the top or bottom centre at which the intake or exhaust valve opens or closes. For example, if an intake valve opens 15° before the piston reaches the top centre on the exhaust stroke, it is said to have a 15°-valve lead.

Venturi

A reduction in the bore of a duct, with convergent upstream and divergent downstream walls that increases the speed of the fluid flow.

Vibration

Oscillation, rapid motion to and fro, of a liquid or solid whose equilibrium has been disturbed.

Viscosity

The reluctance of a fluid to flow, or to change shape easily.

A condition in which the frequency of the valve opening exactly corresponds to the resonant frequency of the valve spring. Under these conditions, the valve spring will exert no closing force.

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V Viscosity index

The measure of the change in viscosity of oil with a change in temperature.

Volatile

Easily vaporised.

W Weak mixture

One which has an excess of air.

Windmilling

The act of being turned by the air (motion) stream.

Wipe contact

Where contact is made between a fixed and a moving object, e.g. carbon brushes in a magneto.

Work

The product of force and distance.

Wrist pin

A hardened and polished steel pin that attaches the small end of a connecting rod into a piston.

None

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Licence Category B1 and B3

16.2 Engine Performance

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Power calculation and measurement;

Part-66 Ref. 16.2

Knowledge Levels A B1 B3 1

Factors affecting engine power; Mixtures/leaning, pre-ignition

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Table of Contents Engine performance ____________________________ 6 Work _______________________________________ 6 Horsepower __________________________________ 6 Indicated horsepower (iHP) ______________________ 8 Friction mean effective pressure (FMEP) __________ 12 Mechanical efficiency _________________________ 14 Brake horsepower ____________________________ 16 Shaft horsepower ____________________________ 18 Horsepower conversions_______________________ 18 Power calculation and measurement _____________ Definitions __________________________________ Power measuring machines ____________________ Testing with a hydraulic dynamometer ____________ Testing with a test club propeller _________________ Calibration records ___________________________ Test cell requirements _________________________ Engine instruments ___________________________ The engine test ______________________________

20 20 22 22 26 30 32 32 36

Mixtures and leaning __________________________ General ____________________________________ Air/fuel ratio _________________________________ Specific fuel consumption (sfc) __________________ Stoichiometric mixture _________________________ Lean and rich mixtures ________________________ Fuel metering devices _________________________ Definition of peak EGT ________________________ Limitations of power at peak EGT ________________ Best economy mixture_________________________ Best power mixture ___________________________ Leaning Lycoming O-540/IO-540 ________________

38 38 38 38 38 40 42 44 44 45 45 46

Detonation and pre-ignition _____________________ 48 Detonation __________________________________ 48 Pre-ignition__________________________________ 52 Factors affecting engine power __________________ 54 Temperature and density of air __________________ 54 Humidity ____________________________________ 54 Fuel mixture _________________________________ 54 Compression ________________________________ 55 Fuel metering ________________________________ 56 Idle mixture _________________________________ 60 The induction manifold _________________________ 62 Operational effect of valve clearance ______________ 64 Ignition system _______________________________ 69 Propeller governor ____________________________ 70 Overlapping phases of engine operation ___________ 72 Engine power troubleshooting ___________________ 73

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Engine performance All aircraft engines are rated according to their ability to do work and produce power. This section presents an explanation of work and power and how they are calculated. Also discussed are the various efficiencies that govern the power output of a reciprocating engine.

The standard unit of mechanical power is horsepower (hp). Late in the 18th century, James Watt, the inventor of the steam engine, found that an English workhorse could work at the rate of 550 ft.lb per second, or 33,000 ft.lb per minute, for a reasonable length of time. From his observations came the unit of horsepower, which is the standard unit of mechanical power in the imperial system of measurement. To calculate the hp rating of an engine, divide the power developed in ft.lb per minute by 33,000, or the power in ft.lb per second by 550.

Work A physicist defines work as force times distance. Work done by a force acting on a body is equal to the magnitude of the force multiplied by the distance through which the force acts.

One hp =

Work (W) = Force (F) × Distance (d) or

Several standards measure work. The most common unit is called foot-pound (ft.lb). If a one-pound mass is raised one foot, one ft.lb of work has been performed. The greater the mass is and/or the higher the distance is, the greater the work performed.

ft.lb per sec. 550 As stated above, work is the product of force and distance, and power is work per unit of time. Consequently, if a 33,000 lb weight is lifted through a vertical distance of 1 foot in 1 minute, the power expended is 33,000 ft.lb per minute, or precisely 1 hp.

Horsepower The output from a piston engine is known as power, but what is power? Power is the rate of doing work. These factors to be taken into consideration when calculating power are: • • •

Work is performed not only when a force is applied for lifting; force may be applied in any direction. If a 100 lb weight is dragged along the ground, a force is still being applied to perform work, although the direction of the resulting motion is approximately horizontal. The amount of this force would depend upon the roughness of the ground.

the force exerted; the distance the force moves; and the time required to do the work.

ft.lb per min. 33,000

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Indicated horsepower (iHP) The indicated horsepower produced by an engine is the horsepower calculated from the indicated mean effective pressure and the other factors which affect the power output of an engine. Indicated horsepower is the power developed in the combustion chambers without reference to friction losses within the engine. This horsepower is calculated as a function of the actual cylinder pressure recorded during engine operation.

Where: P = indicated mean effective pressure, in psi L = length of the stroke, in feet or fractions of a foot A = area of the piston head or cross-sectional area of the cylinder, in square inches N = number of power strokes per minute K = number of cylinders

To facilitate the indicated horsepower calculations, a mechanical indicating device, such as is attached to the engine cylinder, scribes the actual pressure existing in the cylinder during the complete operating cycle. The kind of graph shown below can represent this press variation. Notice that the cylinder pressure rises on the compression stroke reaches a peak after the top centre and decreases as the piston moves down on the power stroke. Since the cylinder pressure varies during the operating cycle, an average pressure (line AB) is computed. This average pressure, if applied steadily during the time of the power stroke, would do the same amount of work as the varying pressure during the same period. This average pressure is known as indicated mean effective pressure and is included in the indicated horsepower calculation with other engine specifications. If the characteristics and the indicated mean effective pressure of an engine are known, it is possible to calculate the indicated horsepower rating.

In the formula above, the area of the piston multiplied by the indicated mean effective pressure gives the force acting on the piston in pounds. This force multiplied by the length of the stroke in feet gives the work performed in one power stroke, which, multiplied by the number of power strokes per minute, gives the number of ft.lb per minute of work produced by one cylinder. Multiplying this result by the number of cylinders in the engine gives the amount of work performed in ft.lb. Since hp is defined as work done at the rate of 33,000 ft.lb per minute, the total number of ft.lb of work performed is divided by 33,000 to find the indicated horsepower.

The indicated horsepower for a four-stroke engine can be calculated from the following formula, in which the letter symbols in the numerator are arranged to spell the word “PLANK” to assist in memorising the formula: Indicated horsepower = Total Training Support Ltd © Copyright 2020

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Piston engine indicated power diagram

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Example Given: Indicated mean effective pressure (P) Stroke (L) Bore RPM No. of cylinders (K) Indicated hp

165 lb/in2 6 in. or 0.5 ft. 5.5 in 3,000 12 PLANK = 33,000 ft.lb/min. = = = = =

Find indicated hp. A is found by using the equation: A = ¼ πD2 A = ¼ × 3.1416 × 5.5 in. × 5.5 in. = 23.76 in2 N is found by multiplying the RPM by ½ (1 power stroke for every 2 RPM): N = ½ × 3,000 = 1,500 Now, substituting in the formula: 165 lb/in2 × 0.5 ft × 23.76 in2 × 1,500 × 12 33,000 ft.lb/min. Indicated hp = 1069.20

Indicated hp =

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Friction mean effective pressure (FMEP) The lost mechanical work is regarded as ‘friction’ work. It has the generic name friction, but it contains all the engine losses: pumping losses, actual friction (mechanical rubbing) losses and auxiliary devices (accessory) losses. Through the combustion process, we get a theoretical amount of work which can be used for propulsion, called indicated work Wi. Subtracting the friction losses work Wf, we end up with an effective work We, which can be used for propulsion. We = Wi – Wf (1) The “friction” losses work is made up from: • • •

pumping work Wp rubbing friction work Wr auxiliary devices work Wa Wf = Wp + Wr + Wa (2)

We can also use normalised parameters, like mean effective pressure (MEP), to define “friction” losses. The brake mean effective pressure (BMEP) is what we have at the crankshaft. BMEP is the difference between the indicated mean effective pressure (IMEP) and “friction” mean effective pressure (FMEP). BMEP = IMEP – FMEP (3) where the “friction” mean effective pressure (FMEP) is the sum of pumping mean effective pressure (PMEP), mechanical rubbing mean effective pressure (RMEP) and auxiliary mean effective pressure (AMEP). FMEP = PMEP + RMEP + AMEP (4) Total Training Support Ltd © Copyright 2020

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Mechanical efficiency The mechanical efficiency of the engine is the ratio between the brake mean effective pressure and the indicated mean effective pressure: ƞm =

•

BMEP (5) IMEP

• •

Replacing equation (3) in (5) gives: ƞm =

IMEP – FMEP FMEP = 1 – (6) IMEP IMEP

From equation (6), we can see that the lower is the friction mean effective pressure (FMEP), the higher is the mechanical efficiency of the internal combustion engine. So, reducing the friction losses for a given engine reduces fuel consumption and improve the power output. Also, the heat losses of an internal combustion engine need to be dissipated by the cooling system and the lubrication system. Lowering the FMEP reduces the component size in the cooling and lubrication system, which means further efficiency improvement.

Since the internal combustion engine is a complex machinery, with a multitude of components and systems, there are many sources for energy losses: mechanical rubbing losses: piston assembly, connecting rod, crankshaft, balance shaft, valve train system; pumping losses: intake and exhaust; and auxiliary device losses: oil pump, fuel pump, water pump, alternator, AC compressor, etc.

The moving parts of the engine (piston assembly, connection rod, crankshaft and valve train) account for more than half of the FMEP. Between these components, half of the losses come from the piston assembly: Components

Average values of the friction losses

Piston assembly

49%

Crankshaft system

18%

Connecting rod system

16%

Valvetrain system

17%

The friction losses are not constant; they depend on engine speed and temperature. The lower the temperature, the higher the oil viscosity, the higher the friction losses.

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Engine components

Engine friction distribution graph

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Brake horsepower The indicated horsepower calculation discussed in the preceding paragraph is the theoretical power of a frictionless engine. The total horsepower lost in overcoming friction must be subtracted from the indicated horsepower to arrive at the actual horsepower delivered to the propeller. The power delivered to the propeller for useful work is known as brake horsepower (bhp). The difference between indicated and brake horsepower is known as friction horsepower, which is the horsepower required to overcome mechanical losses, such as the pumping action of the pistons, the friction of the pistons, and the friction of all other moving parts. The measurement of an engine’s bhp involves the measurement of a quantity known as torque or twisting moment. Torque is the product of a force and the distance of the force from the axis about which it acts, or Torque = Force × Distance (at right angles to the force) Torque is a measure of load and is correctly expressed in pound.inches (lb.in) or pound.feet (lb.ft). Torque should not be confused with work, which is expressed in inch.pounds (in.lb) or foot-pounds (ft.lb). There are numerous devices for measuring torque, such as a dynamometer or a torque meter. One straightforward type of device that can be used to demonstrate torque calculations is the Prony brake. All of these torque-measuring devices are used for calculating the power output of an engine on a test stand. It consists mainly of a hinged collar, or brake, which can be clamped to a drum splined to the propeller shaft.

The collar and drum form a friction brake, which can be adjusted by a wheel. An arm of a known length is rigidly attached to or is a part of the hinged collar and terminates at a point that rests on a set of scales. As the propeller shaft rotates, it tends to carry the hinged collar of the brake with it and is prevented from doing so only by the arm that rests on the scale. The scale indicates the force necessary to arrest the motion of the arm. If the resulting force registered on the scale is multiplied by the length of the arm, the resulting product is the torque exerted by the rotating shaft. For example, if the scale registers 200 lb and the length of the arm is 3.18 ft, the torque exerted by the shaft is: 200 lb × 3.18 ft = 636 lb.ft Once the torque is known, the work done per revolution of the propeller shaft can be computed without difficulty by the equation: Work per revolution = 2π × torque If work per revolution is multiplied by the RPM, the result is work per minute or power. If the work is expressed in ft.lb per minute, this quantity is divided by 33,000. The result is the brake horsepower of the shaft. Power = Work per revolution × RPM and bhp = Work per revolution × RPM 33,000 2πr × force on the scales (lb) × length of arm (ft) × RPM 33,000

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Prony brake principle Aero engine in a test stand

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Shaft horsepower Shaft horsepower (SHP) is the power delivered to the propeller shaft of an aircraft powered by a piston engine. This may be measured, or estimated, from the indicated horsepower given a standard figure for the losses in the transmission (typical figures are around 10%). Effective (true) horsepower Effective horsepower (EHP), or true horsepower (THP) is the power converted to useful work. In the case of a road vehicle, this is the power turned into forward motion as measured on a chassis dynamometer. Horsepower conversions 1 horsepower 1 horsepower 1 horsepower 1 horsepower 1 kilowatt

= = = = =

550 33,000 42.44 0.7456999 1.34102

ft.lb/second ft.lb/minute BTU/minute kilowatts horsepower

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2-19 Module 16.2 Engine Performance

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Power calculation and measurement As a technician, you must know the various ways that engines and engine performance are measured. An engine may be measured in terms of cylinder diameter, piston stroke, and the number of cylinders. It may be measured, performance-wise, by the torque and horsepower it develops and by efficiency. Definitions Work is the movement of a body against an opposing force. In the mechanical sense of the term, this is done when resistance is overcome by force acting through a measured distance. Work is measured in units of foot-pounds. One foot-pound of work is equivalent to lifting a 1 lb weight a distance of 1 ft. Work is always the force exerted over a distance. When there is no movement of an object, there is no work, regardless of how much force is exerted Energy is the ability to do work. Energy takes many forms, such as heat, light, sound, stored energy (potential), or as an object in motion (kinetic energy). The energy performs work by changing from one form to another.

Power is the rate at which work is done. It takes more power to work rapidly than to work slowly. Engines are rated by the amount of work they can do per minute. An engine that does more work per minute than another is more powerful. The work capacity of an engine is measured in horsepower (hp). Through testing, it was determined that an average horse could lift a 200 lb weight to a height of 165 ft one minute. The equivalent of one horsepower can be reached by multiplying 165 ft by 200 lb (work formula) for a total of 33,000 ft.lb per minute. The formula for horsepower is the following: HP =

ft.lb per min L×W = 33,000 33,000 × t

L = length, in feet, through which W is moved W = force, in pounds, that is exerted through distance L T = time, in minutes, required to move W through L

Take the operation of a car, for example. It does the following: When a car is sitting still and not running, it has potential energy stored in the gasoline. When a car is set in motion, the gasoline is burned, hanging its potential energy into heat energy. The engine then transforms the heat energy into kinetic energy by forcing the car into motion. Brakes accomplish the action of stopping the car. By the action of friction, the brakes transform kinetic energy back to heat energy. When all the kinetic energy is transformed into heat energy, the car stops.

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Lifting 1 lb through 1 ft requires 1 lb/ft of energy

Horsepower calculation principle

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Power measuring machines Aircraft piston engines require testing after overhaul to ensure that the engines produce the rated power within the parameters and limits set by the manufacturers. There are three basic methods to measure the engine power output, (1) by using a dynamometer which indicates the power absorbed by it, (2) by using a torque measuring system in conjunction with a load dissipating device and (3) by using a calibrated test club propellers (TCPs). The procedures and equipment used in determining that an engine is ready for airworthy service and is in excellent mechanical condition, usually requires the use of a test stand, or test cell, although the aircraft can be used. The method of engine testing or run-in that takes place during overhaul before delivery of the engine is critical to the airworthiness of the engine. It must be emphasised that engine run-in is as vital as any other phase of engine overhaul, for it is how the quality of a new or newly overhauled engine is checked. It is the final step in the preparation of an engine for service. Thus, the reliability and potential service life of an engine are in question until it has satisfactorily passed the cell test. The test serves a dual purpose. First, it accomplishes piston ring run-in and bearing burnishing. Second, it provides valuable information that is used to evaluate engine performance and determine engine condition. Piston rings must be seated correctly in the cylinder in which they are installed, to provide proper oil flow to the upper portion of the cylinder barrel walls with a minimum loss of oil. The process is called piston ring runin (break-in) and is accomplished chiefly by controlled operation of the engine in the high-speed range.

Improper piston ring conditioning, or run-in, may result in unsatisfactory engine operation with high oil consumption. A process called bearing burnishing creates a highly polished surface on new bearings and bushings installed during overhaul. The burnishing is usually accomplished during the first periods of the engine run-in at comparatively slow engine speeds. The failure of any part during engine testing or run-in requires that the engine is returned, repaired, and wholly retested. After an engine has completed test requirements, it is then specially treated to prevent corrosion if it is shipped or stored before being installed in an aircraft. During the final run-in period during testing, the engines are operated on the proper grade of fuel prescribed for the particular kind of engine. The oil system is serviced with a mixture of corrosion-preventive compound and engine oil. The temperature of this mixture is maintained at 105°C to 121°C. Near the end of the final run-in, the corrosionpreventive mixture (CPM) is used as the engine lubricant. The engine induction passages and combustion chambers are also treated with CPM by an aspiration method. CPM is drawn or breathed into the engine. Testing with a hydraulic dynamometer Hydraulic dynamometers (also referred to as water brakes) have been designed to be compact, robust and to allow easy maintenance. Some of these models are available with a streamlined, fabricated base. These engine dynos are fitted with two half couplings, which, with the rotor, are oil-injected onto the shaft. The standard machines run in grease lubricated rolling element bearings.

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Dynamometer showing lever and load cell

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The hydraulic dynamometer working compartment consists of individual semi-circular shaped vanes cast into stainless steel rotor and stators. Water flowing in a toroidal vortex pattern around these vanes creates a torque reaction through the dynamometer casing which is resisted and measured by a precision load-cell torque-testing apparatus involves a load cell with a torque arm arrangement mounted to a specific load. As a force is applied to the load, the armature compresses the load cell. From the load cell data and moment arm length, torque information can be calculated. The dynamometer load is controlled by a ‘butterfly’ water outlet valve, operated by a closed-loop electro-hydraulic servo system. The power absorbed by the dynamometer is carried away by the water in the form of heat. The dynamometer is operated manually by opening or closing the inlet valve to the absorber system, thereby restricting the flow of water to the rotor blades and causing less load on the engine. At a given set throttle position, the engine output speed will vary between idle and redline depending on how open the inlet flow valve to the dynamometer is. With the valve in a completely open state, the engine will stall as it has more load on it than the engine can produce. Measurements of engine power require shaft speed as well as torque produced by the absorber system. A hall effect sensor is implemented on the output shaft of the engine to measure shaft speed. For torque, a load cell is mounted on rod ends at a specific distance away from the axis of rotation

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Computerised testbed displays

Lycoming engine mounted in test cradle and coupled to dynamometer

Lever on LHS – engine power lever Lever on RHS – dynamometer water valve control (resistance)

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Testing with a test club propeller Piston engine testing using TCP, also referred to as the fan method of testing, consists of running the engine on a test stand with a calibrated TCP and test instrumentation as specified by the manufacturer. Engine power output under this test condition is indicated by the engine revolutions per minute (RPM), corrected for the atmospheric conditions prevailing at the time of the test. This method of engine power measurement is critically dependant on the use of a calibrated TCP, test instruments and application of correction factors. TCPs recommended by the engine manufacturers must be used. Where the manufacturer has only provided the specifications of acceptable TCPs, an appropriate choice has to be made and substantiated. An alternate TCP can be used per approved data, which takes into consideration the power dispersal characteristics, ability to withstand prolonged operation under test conditions, and ability to meet enginecooling requirements. TCPs are available in two types: those that have a single set pitch that cannot be altered and those that have multiple preset pitch positions that can be selected and locked to suit engine type and model. TCPs with single set pitch are usually made with square-tipped blades of laminated wood construction. The blades are made wide to provide maximum power absorption and airflow with minimum tip diameter. These TCPs are initially made with larger diameters than required and are “cropped” during initial calibration to meet the power and speed requirements of a specific engine.

The power absorption characteristics of the multiple-pitchsetting TCPs are varied during calibration, by resetting the pitch stops incorporated in the hub. These settings must not be altered without further recalibration at the altered settings. A flight propeller may be modified for use as a TCP. The design of the modification should include an adjustment procedure for use during calibration and regular maintenance actions to ensure its integrity. Once a flight prop has been used as a TCP, it should not be used again for flight purposes because of higher stresses induced during engine testing. The calibration of a TCP is best carried out in the engine test cell using a torque measuring system. However, where engines are tested in the open air, without the confines of test cell walls, a TCP could be calibrated in a similar facility. In general, TCPs are calibrated at corrected takeoff power subject to engine manufacturer’s instructions. The engine instrumentation, installation details including cooling shrouds and safety precautions, are to be per manufacturer’s instructions. Data used for TCP calibration must be corrected for the atmospheric conditions, because a calibrated TCP that is adequately maintained can remain in service for several years, under different atmospheric conditions, without the need for recalibration.

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Engine testing with test club propellers

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Before the start of a calibration engine run, it is recommended that the target brake horsepower (BHPt) and the target speed (RPMt) are calculated. The target brake horsepower (BHPt) is calculated by the following formula for un-supercharged engines at full throttle.

For supercharged engines The target speed (RPMt) is calculated by the following method: In the chart, draw a vertical line from the observed air temperature on the horizontal scale to the correction curve, and from that point of intersection, draw a horizontal line. The intersection of the horizontal line and the vertical scale gives the correction factor (K). RPMt =

RPMr K

Where Where

RPMt

BHPt = target BHP to be achieved during test club calibration engine run; BHPr = rated take off BHP of the engine at sea level conditions; Po = atmospheric pressure in hectopascals; to = air intake temperature, °C; R = engine compression ratio.

RPMr K

= target speed to be achieved during test club calibration engine run; = rated takeoff engine speed at sea level conditions; = correction factor read from the chart below.

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RPM correction factor determination

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A calibrated engine is a new or newly overhauled and tested engine that meets all of the manufacturer’s performance data. A calibrated engine may be used for TCP calibration. Calibrated instruments must be used along with cooling shrouds as specified by the manufacturer. The engine run procedures, performance limitations, and safety precautions specified by the manufacturer must be followed at all times. Run the engine to the target speed (RPMt ) and at this speed; the power developed as measured by the torque measuring system should indicate the target power (BHPt). A tolerance of ±20 RPM can be applied for speed and –2% for BHP during the calibration run. Wooden TCPs can be ‘cropped’ per manufacturers’ instructions to achieve the calibration requirements, whereas for the adjustable pitch propellers, the pitch stops can be adjusted to meet the requirements. When a flight propeller is used as a TCP, the adjustment procedure must be in accord with the document which approves its use as a TCP. The calibration run should be repeated at least three times to ensure consistent results, with each run meeting the calibration requirements. Before the start of a calibration engine run, it is recommended that the target brake horsepower (BHPt) and the target speed (RPMt) are calculated. It is preferable to use a test facility with a torque measuring system to calibrate a TCP because the power developed, and RPM can be measured simultaneously during calibration runs. The following procedure applies to such a facility and may be used where the manufacturer’s instructions for TCP calibration are not available.

Calibration records The following data should be recorded for the TCP after it has met the calibration requirements. For the fixed-pitch wooden TCP: •

• • •

•

Record the date of calibration, tip diameter, the rated power, and speed for which it is calibrated. It is recommended to mark this data on the TCP. Record the facility and torque measuring system or engine serial number used for calibration. Record the chord and pitch angle of each blade at approximately 150 mm (6") intervals. Record the physical condition of the prop, nicks, dents, erosion, and other damage using diagrams where appropriate. Carry out a static balance.

For the adjustable pitch metal TCP: •

• • •

•

Record the date of calibration, the power, and speed for which it is calibrated. It is recommended to mark this data on the TCP. Mark the pitch stop settings so that any changes can be easily detected. Record the facility and torque measuring system or engine serial number used for calibration. Record the physical condition of the prop, nicks, dents, erosion, and other damage using diagrams where appropriate. Carry out static balance and crack-check using fluorescent penetrant inspection (FPI).

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Engine testing with test club propellers

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Test cell requirements The test cell requires an area to mount and hold the engine for testing. The cell needs to have the controls, instruments, and any special equipment to evaluate the total performance of the engine. A test club should be used for testing instead of a flight propeller. A test club provides more cooling airflow and the correct amount of load. Alternatively, the engine will be coupled to a dynamometer. The operational tests and test procedures vary with individual engines, but the basic requirements are generally closely related. Engine instruments The test cell control room contains the controls used to operate the engine and the instruments used to measure various temperatures and pressures, fuel flow, and other factors. These devices are necessary for providing an accurate check and an evaluation of the operating engine. The control room is separate from, but adjacent to, space (test cell) that houses the engine being tested. The safe, economical, and reliable testing of modem aircraft engines depends mostly upon the use of instruments. In engine run-in procedures, the same basic engine instruments are used as when the engine is installed in the aircraft, plus some additional connections to these instruments, and some indicating and measuring devices that cannot be practically installed in the aircraft. Instruments used in the testing procedures are inspected and calibrated periodically, as are instruments installed in the aircraft; thus, accurate information concerning engine operation is ensured. Engine instruments can operate using different methods, some mechanically, some electrically, and some by sensing the direct pressure of air or liquid. Some of the basic instruments are:

• • • • • • • • • •

Carburettor air temperature gauge Fuel pressure gauge Fuel flowmeter Manifold pressure gauge Oil temperature gauge Oil pressure gauge Tachometer Exhaust gas temperature gauge Cylinder head temperature gauge Torque meter

Instrument markings, ranges of operation, minimum and maximum limits, and the interpretation of these markings are general to all the instruments. Generally, the instrument marking system consists of three colours; red, yellow, and green. A red line, or mark, indicates a point beyond which a dangerous operating condition exists. A red arc indicates a dangerous operating range due generally to an engine propeller vibration range. This arc can be passed through, but the engine cannot be operated in this area. Of the two, the red mark is used more commonly and is located radially on the cover glass or dial fan. The yellow arc covets a given range of operation and is an indication of caution. Generally, the yellow arc is located on the outer circumference of the instrument cover glass or dial face. The green arc shows a normal and safe range of operation. When the markings appear on the cover glass, a white line is used as an index mark, often called a slippage mark. The white radial mark indicates any movement between the cover glass and the case, a condition that would cause mislocation of the other range and limit markings.

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Digital engine test ‘run screen’ of engine on a dynamometer

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Another device that measures the actual usable horsepower of an engine is the prony brake. It isn’t often used today but is simple to understand. It is useful for learning the concept of horsepower-measuring tools. It consists of a flywheel surrounded by a large braking device. One end of an arm is attached to the braking device, while the other end exerts pressure on a scale. In operation, the engine is attached to and drives the flywheel. The braking device is tightened until the engine is slowed to a predetermined RPM. As the braking device slows the engine, the arm attached to it exerts pressure on a scale. Based on the reading at the scale and engine RPM, a brake horsepower valve is calculated by using the following formula: 2π × length of arm × engine RPM × scale reading 33,000

Torque is a force that, when applied, tends to result in twisting an object, rather than its physical movement. When the torque is being measured, the force that is applied must be multiplied by the distance from the axis of the object. Torque is measured in pound-feet (not to be confused with work which is measured in foot-pounds). When torque is applied to an object, the force and distance from the axis depend on each other. For example, when 100 ft.lb of torque is applied to a nut, it is equivalent to a 100-lb force being applied from a wrench that is 1-foot long. When a 2 ft-long wrench is used, only a 50 lb force is required. Do not confuse torque with work or with power. Both work and power indicate motion, but torque does not. It is merely a turning effort that the engine applies to the wheels through gears and shafts.

It must be noted that 6.28 and 33,000 are constants in the formula, meaning they never change. For example, a given engine exerts a force of 300 lb on a scale through a 2 ft-long arm when the brake device holds the speed of the engine at 3,000 RPM. By using the formula, calculate the brake horsepower as follows: 2π × 2 × 3000 × 300 =

342.55 brake horsepower 33,000

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Prony brake

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The engine test As previously stated, the engine test at the end of an engine overhaul is essential to do the initial break-in, determine correct operation and engine performance. During the engine test, it would be typical to complete the following operations: • • • • • •

a run-in; an oil consumption run; an oil-pressure relief-valve test and adjustment; an idle speed and mixture adjustment; a magneto timing check; and a performance check.

Engine torque is a rating of the turning force at the engine crankshaft. When combustion pressure pushes the piston down, a strong rotating force is applied to the crankshaft. This turning force is sent to the transmission or transaxle, driveline or drivelines, and drive wheels, moving the vehicle. Engine torque specifications are provided in a shop manual for a particular vehicle. One example, 78 lb/ft @ 3,000 RPM, is given for one particular engine; this engine is capable of producing 78 lb/ft of torque when operating at 3,000 revolutions per minute.

Friction is the resistance to motion between two objects in contact with each other. The reason a sledge does not slide on bare earth is because of friction. It slides on snow because snow offers little resistance, while the bare earth offers a great deal of resistance. Friction is both desirable and undesirable in an automobile or any other vehicle. Friction in an engine is undesirable because it decreases the power output; in other words, it dissipates some of the energy the engine produces. This is overcome by using oil, so moving components in the engine slide or roll over each other smoothly. Frictional horsepower (fHP) is the power needed to overcome engine friction. It is a measure of resistance to movement between engine parts. Frictional horsepower is power lost to friction. It reduces the amount of power left to propel a vehicle. Friction, however, is desirable in clutches and brakes, since friction is exactly what is needed for them to perform their function correctly. One other term you often encounter is inertia. Inertia is a characteristic of all material objects. It causes them to resist change in speed or direction of travel. A motionless object tends to remain at rest, and a moving object tends to keep moving at the same speed and in the same direction. An excellent example of inertia is the tendency of your car to keep moving even after you have removed your foot from the accelerator. You apply the brake to overcome the inertia of the car or its tendency to keep moving.

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Engine run-in test limits Fuel pressure psi – at inlets to carb or injector

Fuel – minimum octane rating aviation grade

O-235-C1, -C1B, -C2A, -C2B, -E

2-5

80/87

lbs/hr 0.9

qt/hr 0.50

Normal 75-85

O-235-F, -G, -J O-235-C2C,-H2C O-290-D, -D2

2-5 2-5 2-5

100/130 80/87 80/87

0.9 0.9 1.0

0.50 0.50 0.56

O-230-A, -E

2-5

80/87

1.2

O-320-B, -D IO-320-A, -E IO-320-B, -D LIO-320-B IO-320-C

2-5 18-28 18-28 18-28 18-28

91/96 80/87 91/96 91/96 100/130

1.2 1.2 1.2 1.2 1.2

LIO-320-C AIO-320-A, -B, -C O-340-A O-360-A, -C (except -A1C, -C3B and -C2D)

18-28 18-28 2-5 2-5

100/130 91/96 91/96 91/96

1.2 1.2 1.3 1.4

Maximum oil consumption

Oil pressure operating – psi

Oil inlet temp °F

Idle 25

Oil outlet* temp °F

Max cylinder head temp bayonet location °F

Full throttle engine speed RPM

165-230

190-210

500

2,800

75-85 75-85 75-85

25 25 25

165-230 165-230 165-230

190-210 190-210 190-210

500 500 500

2,800 2,600 2,600

0.67

75-85

165-230

190-210

500

2,700

0.67 0.67 0.67 0.67 0.67

75-85 75-85 75-85 75-85 75-85

25 25 25 25 25

165-230 165-230 165-230 165-230 165-230

190-210 190-210 190-210 190-210 190-210

500 500 500 500 500

2,700 2,700 2,700 2,700 2,700

0.67 0.67 0.72 0.78

75-85 75-85 75-85 75-85

25 25 25 25

165-230 165-230 165-230 165-230

190-210 190-210 190-210 190-210

500 500 500 500

2,700 2,700 2,700 2,700

*– desired during oil consumption run. **– do not exceed 3,150 RPM – for test stand at 24-25 in.Hg manifold pressure. For oil consumption run, operated 3,100 RPM at 24 in.Hg manifold pressure.

Examples of engine run-in test limits

Recommended run-in schedule RPM

Load

Time (minutes)

1200 1500 00 2000 2200 2400

Propeller load Propeller load Propeller load Propeller load Propeller load Propeller load

10 10 10 10 10 10

Check magneto drop off. Do not exceed 125 RPM on either magneto or 35 RPM between magnetos.

Propeller load Propeller load

15 60

Oil consumption run

Normal rated* Normal rated* *See engine run-in test limits table

Remarks

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Mixtures and leaning General Fuel, which is a hydrocarbon, combines with the oxygen and burns when ignited by the spark plug; however, the nitrogen in the air is an inert gas and does not burn, therefore slowing down the rate of combustion to maintain acceptable temperatures.

Specific fuel consumption (sfc) The thermal efficiency of an engine is an important consideration. However, the specific fuel consumption, which is the number of pounds of fuel burned per hour to produce each brake horsepower, is a more precise measure of engine performance.

The fuel used should have excellent calorific value, that is, the amount of heat from a given weight of fuel, as this affects the payload or range of the aircraft. It should be non-corrosive to the fuel lines and components. It should have good volatility, which is the tendency to evaporate to give a smooth start in cold conditions, but should not vaporise too rapidly when hot, as this would cause vapour locks in the fuel lines.

Stoichiometric mixture The air/fuel ratio determines whether a mixture is combustible at all, how much energy is being released, and how many unwanted pollutants are produced in the reaction. Typically, a range of fuel to air ratios exists, outside of which ignition does not occur. These are known as the lower and upper explosive limits. The air/fuel ratio is an important measure for antipollution and performance-tuning reasons. If exactly enough air is provided to burn all of the fuel completely, the ratio is known as the stoichiometric mixture, often abbreviated to ‘stoich’.

Air/fuel ratio At low engine power and RPM, the gas flow through the cylinders is slow. Due to valve overlap, the incoming gas (mixture) is diluted by the burnt gas still in the cylinders, so the mixture has to be enriched to ensure smooth running is maintained as the RPM are decreased. This mixture requirement is dependent upon: • •

engine speed; and power output.

If you look at the diagram below you will see that the rich or normal cruise range is about 13:1 to 14:1, from there down to the idle, and from there up to takeoff, the mixture is enriched to 10:1. The economy cruise line is a result of pilot selection when flight level and cruise power have been established. Total Training Support Ltd © Copyright 2020

Ratios lower than stoichiometric are considered “rich”. Rich mixtures are less efficient, but may produce more power and burn cooler. Ratios higher than stoichiometric are considered “lean.” Lean mixtures are more efficient but may cause higher temperatures, which can lead to the formation of nitrogen oxides. Some engines are designed with features to allow leanburn. The stoichiometric ratio is the exact ratio of air to fuel at which complete combustion takes place. The stoichiometric ratio of combustion varies for various fuels. If the engine has less air than the stoichiometric ratio, it is a rich mixture, because it is rich in gasoline. A stoichiometric ratio is neither too rich nor too lean. It contains just enough oxygen to burn all the fuel.

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Typical mixture requirements

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Lean and rich mixtures The chemically correct mixture, or air/fuel ratio, of gasoline, is 15:1. This stoichiometric ratio gives the highest combustion temperature when all the oxygen and fuel are used up but is also so hot that it leads to detonation.

At full-power takeoff, a ratio of approximately 10:1 is standard, with most of the extra fuel use for cooling, as there is not enough oxygen in the mixture to burn it.

A richer mixture, or weaker mixture, than stoichiometric, lowers the combustion temperature; slightly rich is better as the extra fuel has a cooling action, a weaker mixture results in a power loss as all the oxygen is not used. For maximum power, most engines run at 12.5:1; this extra fuel ensures that all cylinders get a little richer than stoichiometric because the mixture is not always evenly distributed in the induction manifold. Weak mixture (lean) A weak mixture burns more slowly and at lower temperatures than stoichiometric. Although the power is down, an increase in efficiency due to the cooler burn gives a decrease in fuel consumption so ‘specific fuel consumption’ drops. Consider the economy cruise condition shown in the diagram above. If the mixture is permitted to go too lean, the combustion chamber will experience an oxidising flame. In combination with high power settings, this will result in hight CHT, possible detonation, very hot, burnt or sticky valves and maybe damage to the pistons resulting in a power failure. Rich mixture At a power setting above the cruise range, any increase in RPM and cylinder pressures result in higher mixture temperatures and eventually detonation. To overcome the problem the engine is usually operated with a slightly richer mixture to safeguard against engine damage. Total Training Support Ltd © Copyright 2020

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Fuel metering devices The basic requirement of a reciprocating fuel metering system is the same, regardless of the type of system used or the model engine in which the equipment is installed. It must meter fuel proportionately to air to establish the proper fuel/air mixture ratio for the engine at all speeds and altitudes at which the engine may be operated. In the fuel/air mixture curves shown in the diagram below left, note that the basic best-power and best-economy fuel/air mixture requirements for reciprocating engines are approximately the same. The fuel metering system must atomise and distribute the fuel from the carburettor into the mass airflow. This must be accomplished so that the fuel/air charges going to all cylinders holds equal amounts of fuel. Each one of the engine’s cylinders should receive the same quantity of fuel/air mixture and at the same fuel/air ratio. Due to the drop in atmospheric pressure, as altitude is increased, the density of the air also decreases. A normally aspirated engine has a fixed amount or volume of air that it can draw in during the intake stroke, therefore less air is drawn into the engine as altitude increases. Less air tends to make carburettors run richer at altitude than at ground level, because of the decreased density of the airflow through the carburettor throat for a given volume of air. Thus, a mixture control must be provided to lean the mixture and compensate for this natural enrichment. Some aircraft use carburettors in which the mixture control is operated manually. Other aircraft employ carburettors which automatically lean the carburettor mixture at altitude to maintain the proper fuel/air mixture.

The rich mixture requirements for an aircraft engine are established by running a power curve to determine the fuel/ air mixture for obtaining maximum usable power. This curve is plotted at 100 RPM intervals from idle speed to takeoff speed (diagram below top-right). Since the power range must add fuel to the basic fuel/air mixture requirements to keep cylinder-head temperatures in a safe range, the fuel mixture must become gradually richer as powers above cruise are used (the diagram below left). In the power range, the engine runs on a much leaner mixture, as indicated in the curves. However, on the leaner mixture, the cylinder-head temperature would exceed the maximum permissible temperatures, and detonation would occur. The best economy setting is established by running a series of curves through the cruise range, as shown in the graph in the diagram below bottom-right, the low point (auto-lean) in the curve is the fuel/air mixture where the minimum fuel per horsepower is used. In this range, the engine usually operates on slightly leaner mixtures and operates on richer mixtures than the low-point mixture. If a mixture leaner than that specified for the engine is used, the leanest cylinder of the engine is apt to backfire because the slower burning rate of the lean mixture results in a continued burning in the cylinder when the next intake stroke starts.

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Power versus fuel/air mixture curve

Specific fuel consumption curve

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Definition of peak EGT A simple definition of peak EGT is the chemically correct mixture of fuel and air, which gives 100% utilisation of all the fuel and all the air. From peak EGT, either increasing or decreasing the fuel flow causes a decrease in EGT. When richer than peak EGT, cooling occurs because there is excess fuel, and when leaner than peak, cooling occurs because there is excess air. Peak EGT with a float-type carburetted engine is frequently a vague point because of less efficient distribution (than fuel injection) to the individual cylinders by this type of metering device. As a result, float-type carburetted engines tend to operate smoother at +4° to +10° on the rich side of peak EGT. Whereas, fuel-injected engines at 250 hp and higher provide a more precise peak, and therefore the EGT system is likewise a more precise method of fuel management with fuel injection. Operation at peak EGT, particularly on long flights, can be an advantage not only for purposes of increased range but there is less likelihood of spark plug fouling as well. In cold outside air temperature flight conditions, the mixture distribution is poorer for both fuel injected and carburetted engines. However, with the float-type carburettor operating in below freezing ambient temperatures, the fuel/air distribution is worsened, resulting in a temperature difference between individual exhaust stacks.

It is also important to understand that leaning to roughness at the engine manufacturer’s recommended cruise power is not an indication of detonation but indicates typical characteristics of distribution to the individual cylinders. The roughness indicates that the leanest cylinder has become so lean, it is beginning to miss. This is typical of an engine with a float-type carburettor. Damage to an engine from leaning does not occur at the manufacturer’s recommended cruise power but takes place at higher than cruise power. As far as the pilot is concerned, operating on the lean side of peak EGT can only be accomplished with fuel-injected engines of at least 250 hp or higher because the fuel flows in the lower horsepower engines are so small. It isn’t possible with float-type carburettors because of the fuel/air distribution problem. Limitations of power at peak EGT Lycoming allows leaning to peak EGT at 75% power and below on direct-drive normally aspirated engines (75% power for the O-540 is approximately 195 bhp obtained at approximately 2,450 RPM whereas the IO-540 produces 225 bhp at approximately 2,450 RPM). However, the limit at peak EGT on geared, supercharged power plants is imposed at 65% power or below. With Lycoming turbocharged engines, where the EGT gauge is used to interpret turbine inlet temperature (TIT), the maximum allowable TIT specified in the pilot's operating handbook (POH) should not be exceeded when attempting to find a peak temperature by manual leaning. Where a cylinder head temperature (CHT) is also available, the operator should always cross-check the CHT as a routine procedure when leaning. Remember that whenever CHT reaches the maximum before reaching peak EGT, then CHT rather than EGT should dictate the limit of allowable leaning.

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Best economy mixture The best economy mixture as it relates to the EGT system begins at peak. For all practical purposes with Lycoming engines, peak EGT is right at the edge of best economy mixture and is our only practical point of reference in the best economy mixture range. At the manufacturer’s recommended cruise power, peak EGT causes a slight loss of horsepower usually reflected in two or three miles per hour of airspeed. If the pilot attempts to go leaner than peak EGT (with fuel injection only), the power decreases rapidly as fuel flow decreases. Best power mixture Best power mixture, or sometimes termed maximum power range, as depicted on the EGT gauge, is in the range of plus 38°C on the rich side of peak. Best power mixture provides fastest indicated airspeed for a cruise power setting, although it is generally not considered a practical economic mixture for cruise purposes. However, the best power mixture generally provides a safe amount of fuel for a power setting higher than the engine manufacturer’s recommended cruise, except that needed for takeoff power. Maximum leaning (peak EGT) does not damage an engine at the engine manufacturer’s recommended cruise power. Damage is caused by maximum leaning at higher than recommended cruise power where the manuals do not allow it, and when the aircraft does not have a complete set of reliable engine instruments to protect the power plants. Excessive leaning under such high-power conditions can cause detonation and/or pre-ignition and possible engine failure. The significant advantages of an EGT system to the operator are as follows: Total Training Support Ltd © Copyright 2020

1. It saves fuel – an economy aid. 2. It aids proper mixture control – more precise fuel management. 3. It helps increase range. 4. It can detect some types of engine trouble. 5. It aids peak engine performance at cruise. 6. It helps prevent spark plug fouling. Although the use of the EGT has the advantages listed above, from a pilot’s point of view, there are also some possible disadvantages. Poor mixture distribution to the cylinders (particularly in carburetted engines) is the primary reason for these disadvantages. The EGT probe is to be installed in the leanest cylinder. However, this changes with altitude and power setting, therefore making it very difficult, or perhaps impossible, to choose the best cylinder for probe installation. Without an EGT installation, the pilot can easily lean using the leanest cylinder of a carburetted engine by simply leaning to find engine roughness from the first indication of ‘lean misfire’ and then richening the mixture to smooth engine operation. The EGT system must be in perfect working order to give accurate readings. The probes in the exhaust system deteriorate with age and continuous use. This often causes the gauge to read a temperature that is not accurate, and therefore a peak reading that is not reached soon enough. This results in over leaning to the lean side of peak where operation is not recommended. Frequent maintenance to ensure that temperature probes are in good condition reduces the possibility of inaccuracies. However, the pilot cannot determine the accuracy of this rather critical reading during operation.

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Leaning Lycoming O-540/IO-540 Manual leaning may be monitored by exhaust gas temperature indication, fuel flow indication, and by observation of engine speed and/or airspeed. However, whatever instruments are used in leaning the mixture, the following general rules should be observed by the operator of Lycoming aircraft engines. General rules Never exceed the maximum redline cylinder head temperature limit. For maximum service life, cylinder head temperatures should be maintained below 224 °C (435 °F) during high-performance cruise operation and below 205 °C (400 °F) for economy cruise powers. On engines with manual mixture control, maintain mixture control in “FULL RICH” position for rated takeoff, climb and maximum cruise powers (above approximately 75%). However, during takeoff from a high-elevation airport or during a climb, roughness or loss of power may result from overrichness. In such a case, adjust mixture control only enough to obtain smooth operation – not for economy. Observe instruments for temperature rise. Rough operation due to overrich fuel/air mixture is most likely to be encountered at altitudes above 5,000 ft. Always return the mixture to full rich before increasing power settings.

During let-down flight operations it may be necessary to lean carburetted or fuel-injected engines manually to obtain smooth operation. A. Leaning to exhaust gas temperature gauge Normally aspirated engines with fuel injectors or carburettors. a) Maximum power cruise (approximately 75% power) – never lean beyond 150 °F on the rich side of peak EGT unless aircraft operator’s manual shows otherwise. Monitor cylinder head temperatures. b) Best economy cruise (approximately 75% power and below) – operate at peak EGT. B. Leaning to flowmeter Lean to applicable fuel-flow tables or lean-to indicator marked for correct fuel-flow for each power setting. C. Leaning with manual mixture control (without flowmeter or EGT gage) Carburetted engines. a) Slowly move mixture control from “FULL RICH” position toward the lean position. b) Continue leaning until engine roughness is noticed. c) Enrich until engine runs smoothly, and power is regained.

Operate the engine at maximum power mixture for performance cruise powers and at best economy mixture for economy cruise power; unless otherwise specified in the aeroplane owner’s manual. Total Training Support Ltd © Copyright 2020

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Detonation and pre-ignition Detonation and Pre-Ignition are two unique conditions that can severely damage an aircraft engine. Forces and severe heat that result from the occurrence of either usually requires the complete teardown and repair of the engine. To understand the similarities, differences and causes of either require an understanding of exactly fuel burns within the engine to develop power. Anyone who has ever observed how settled fuel vapours ignite has seen how the flame front progresses from the source of ignition smoothly to the outer edges where flammable vapours reach a point of dilution that stops the burn. This is precisely how fuel must burn within a piston engine to develop power without doing damage. Detonation refers to the condition where remote pockets within the fuel/air mixture explode violently due to rising pressure following regular ignition. Pre-Ignition refers to the condition where either a mistimed spark or another source of ignition exists within the combustion chamber, allowing the burn to start well in advance of the normally timed spark. Pre-Ignition and detonation can often overlap each other, usually from detonation damagecausing pre-ignition. Detonation Detonation (also known as knock, detonation, spark knock, pinging or pinking) is uniquely distinguished by the fact that it cannot occur before the spark plug fires. When the spark initiates burning within the cylinder, the flame front is expected to progress through the cylinder evenly, creating heat and even pressure to push the piston down.

As the burn starts cylinder pressure quickly rises. If combustion chamber parts are hotter than usual, this can cause remote pockets within the cylinder to detonate spontaneously. This can also occur when fuel octane is below the requirements of the engine. Causes of detonation are limited to excessive heat and low octane. Excessive heat can come from improper cooling, high compression from excessive combustion chamber deposits, lean mixture, advanced timing and more. When it is limited to one cylinder the very likely culprit is a partially clogged fuel injector. This allows one cylinder to operate much leaner than the others. Intake leaks can also lean the mixture but are usually noticed during low manifold pressure operation where the symptoms of a leak become much more apparent. Detonation caused by low octane fuel is more likely to affect several cylinders since the contributing factor is present in all cylinders. Detonation can difficult if not impossible for a pilot to detect from within the co*ckpit. Detonation can occur for some time before severe damage occurs, or it can very quickly progress to severe failure depending on its severity. Minor detonation can cause damage that in time will likely progress to an increasingly severe condition. It occurs at the far reaches of the combustion chamber and usually causes the most damage at the edges of the pistons as a result.

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Detonation

Damage caused by extreme detonation

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It causes rapidly rising temperatures at the piston edges, which can allow subsequent detonation to damage ring lands. It can also trigger pre-ignition due to the hot spots. Once this occurs, the damaged piston edge is exposed to severe heat and pressure, which can cause a hole to be burned through the corner of the piston. Leaking combustion gasses pushing through broken ring lands also causes torching at the edge of the piston which quickly progresses to failure of the seal between the combustion chamber and the crankcase. The following are typical by-products of detonation: • • • • •

excessive cylinder head temperatures; burning piston heads; burning exhaust valves (inlet valves are cooled by the incoming fuel/air vapour); carbonising of the piston rings; and a general breakdown of lubricating oil – quantities.

The main factors that cause detonation are: • • •

incorrect air/fuel mixture; high cylinder head temperature; and most importantly, the anti-knock value of the fuel.

The graph shows a typical indicator diagram for a piston engine during its compression and power strokes. Note the high pressure-fluctuation when detonation occurs.

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Typical indicator diagram

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Pre-ignition Pre-Ignition is defined as combustion that begins before it is intended to, before the regularly timed spark. Pre-Ignition can occur alone or as a result of detonation. Hot spots from detonation, improper heat range spark plugs and glowing carbon deposits from lean mixtures are common causes of preignition. Carbon deposits do not usually accumulate when the lean mixture is chronic, but regular deposits can be quickly heated to glowing temperature when a fuel injector suddenly becomes partially clogged. Carbon tracks within a magneto that allows a cylinder to get the spark from another cylinder can also be a cause. Most cases of pre-ignition start at or near the beginning of the compression stroke since a combustible mixture becomes more difficult to ignite as pressure rises. This causes severe stress on the engine and can quickly burn a hole in the piston, most often in the middle. Pre-ignition causes a sudden loss of power as the affected cylinder is working against the normal rotation of the engine. Severe heat results from compressing a burning mixture. No power is extracted from the burn, resulting in all heat energy being absorbed by the cylinder parts. Damage from either detonation or pre-ignition is severe. Once the seal between the piston and the crankcase is breached pressurisation of the crankcase can push crankcase oil overboard, causing additional damage from oil starvation. Engine contamination and the severe stress imposed requires the engine to be completely disassembled. All parts must be assessed for contamination issues, and all stressed parts must be appropriately NDT tested for integrity.

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Pre-ignition within a cylinder

Normal combustion within a cylinder

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Factors affecting engine power What we must consider first, before looking at the principles of operation, what are the factors that affect the operation of the reciprocating engine; they fall into the following categories: • • • • •

temperature and density of air; Humidity; fuel mixture; effects of altitude on fuel mixture; and fuel grading and octane rating.

The carburettor which is fitted to a piston engine is designed to allow air into the combustion chambers and at the same time, allow a calibrated amount of fuel to mix with this air, creating a vapour fuel/air mixture for combustion purposes.

Temperature and density of air The engine’s power output depends on the weight of the air/fuel mixture, and that the amount of air entering the engine cylinders is regulated by the inlet of the carburettor and the density of the air. The density of the air depends on the atmospheric pressure and temperature. Since air density decreases with altitude, then the engine power output will also decrease. Humidity Other factors that must be considered are temperature and humidity. The density of the air/fuel mixture will vary with different temperatures and humidity. High humidity affects engine power. The high level of water vapour in the air reduces the amount of air available for combustion and results in an enriched mixture and reduced power

Fuel mixture The process by which fuel and air are mixed to the correct proportions when applied to the internal combustion engine for combustion is known as carburation.

Although the air/fuel vapour will burn when mixed in proportions by weight from 8:1, to 20:1, the best results are achieved when the ratio is about 15:1 (air/fuel by weight). Although 15:1 is the correct ratio, some means must be provided within the engine control system to change the ratio of this mixture during certain flight conditions. Let’s take, for example, an aircraft that is climbing. We know that at sea level, the air is dense and contains ‘x’ amount of molecules. As we climb, the air molecules become less and density decreases. However, the carburettor is designed to draw in the same volume of air at a set throttle setting, regardless of altitude. This also means that the fuel drawn through the carburettor jets remains the same. Therefore, as the aircraft climbs, the volume of air is the same, the fuel flow is the same, but the density is less, causing the mixture ratio to become richer as altitude increases. A rich mixture will eventually lead to a loss of power, a rough running engine, and a gradual decrease in RPM, especially when a fixed pitched propeller is fitted.

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An extreme over-rich condition is indicated by black smoke coming from the exhaust pipes; unburnt carbon particles cause this. A weak mixture can be just as harmful to the engine. If the air/fuel mixture is too lean, the flame rate during combustion may be so low that combustion is still taking place when the inlet valve is again opening. A good indication of this condition is when popping back through the carburettor and inlet manifold is experienced. However, this condition could also be caused by an ignition timing misalignment. Compression To prevent loss of power, all openings lo the cylinder must close and seal entirely on the compression and power strokes. In this respect, there are three items in the proper operation of the cylinder that must be right for maximum efficiency. First, the piston rings must be in good condition to provide maximum scaling during the stroke of the piston. There must be no leakage between the piston and the walls of the combustion chamber. Second, the intake and exhaust valves must close tightly so that there is no loss of compression at these points. Third, and very important, the timing of the valves must be such that the highest efficiency is obtained when the engine is operating at its normal rated RPM. A failure at any of these points results in significantly reduced engine efficiency.

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Fuel metering The induction system is the distribution and fuel metering part of the engine. Any defect in the induction system seriously affects engine operation. For best operation, each cylinder of the engine must be provided with the proper fuel/air mixture, usually metered by the carburettor. On some fuel-injection engines, fuel is metered by the fuel injector flow divider and fuel-injection nozzles. In establishing the carburettor settings for an aircraft engine, the design engineers run a series of curves similar to the one shown. A curve is run for each of several engine speeds. If, for example, the idle speed is 600 RPM, the first curve might be run at this speed. Another curve might be run at 700 RPM, another at 800 RPM, and so on, in 100-RPM increments, up to takeoff RPM, The points of maximum power on the curves are then joined to obtain the best power curve of the engine for all speeds. The relation between fuel/air ratio and power is illustrated in the graph below left. Note that, as the fuel mixture is varied from lean to rich, the power output of the engine increases until it reaches a maximum. Beyond this point, the power output falls off as the mixture is further enriched. This is because the fuel mixture is now too rich to provide perfect combustion. Note that maximum engine power can be obtained by setting the carburettor for one point on the curve. This best-power curve establishes the automatic rich setting of the carburettor.

In establishing the detailed engine requirements regarding carburettor setting, the fact that the cylinder head temperature varies with fuel/air ratio must be considered. This variation is illustrated in the curve shown in the graph below-right. Note that the cylinder head temperature is lower with the auto-lean setting than it is with the auto-rich mixture. This is precisely the opposite common belief, but it is true. Furthermore, knowledge of this fact can be used to advantage by flight crews. If during cruise, it becomes difficult to keep the cylinder head temperature within limits, the fuel/air mixture may be leaned out to get cooler operation. The desired cooling can then be obtained without going to auto-rich with its costly waste of fuel. The curve shows only the variation in cylinder head temperature. For a given RPM, the power output of the engine is less with the best-economy setting (auto-lean) than with the best-power mixture. The decrease in cylinder head temperature with a leaner mixture holds only through the normal cruise range. At higher power settings, cylinder temperatures are higher with the leaner mixtures. The reason for this reversal hinges on the cooling ability of the engine. As higher powers are approached, a point is reached where the airflow around the cylinders will not provide sufficient cooling. At this point, a secondary cooling method must be used. This secondary cooling is done by enriching the fuel/air mixture beyond the best-power point. Although enriching the mixture to this extent results in a power loss, both power and economy must be sacrificed for engine cooling purposes.

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Power versus fuel/air mixture

Variation of head temperature with fuel/air mixture (cruise power)

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To further investigate the influence of cooling requirements on fuel/air mixture, the effects of water injection must be examined. The graph below right shows a fuel/air curve for a water-injection engine. The dotted portion of the curve shows how the fuel/ air mixture is leaned out during water injection. This leaning is possible because water, rather than extra fuel, is used as a cylinder coolant. Water or, more accurately, the anti-detonant (water/alcohol) mixture is a better coolant than extra fuel. Therefore, water injection permits higher manifold pressures and a still further increase in power. In establishing the final curve for engine operation, the engine’s ability to cool itself at various power settings is, of course, taken into account. Sometimes the mixture must be altered for a given installation to compensate for the effect of cowl design, cooling airflow, or other factors on engine cooling. The final fuel/air mixture curves take into account economy, power, engine cooling, idling characteristics, and all other factors which affect combustion. The graph below right shows a typical final curve for injectiontype carburettors. Note that the fuel/air mixture at idle and at takeoff power is the same in auto-rich and auto-lean. Beyond idle, a gradual spread occurs as cruise power is approached. This spread is maximum in the cruise range. The spread decreases toward takeoff power. This spread between the two curves in the cruise range is the basis for the cruise metering check.

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Fuel/air curve for a water-injection engine

Typical fuel/air curve for injection-type carburettor

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The graph below left shows a typical final curve for a float-type carburettor. Note that the fuel/air mixture at idle is the same in rich and in manual lean. The mixture remains the same until the low cruise range is reached. At this point, the curves separate and then remain parallel through the cruise and power ranges.

There are variations in mixture requirements between one engine and another because of the fuel distribution within the engine and the ability of the engine to cool. Remember that a carburettor setting must be rich enough to supply a combustible mixture for the leanest cylinder. If fuel distribution is poor, the overall mixture must be richer than would be required for the same engine if the distribution were proper.

Note the spread between the rich and lean setting in the cruise range of both curves. Because of this spread, there is a decrease in power when the mixture control is moved from auto-rich to auto-lean with the engine operating in the cruise range. This is true because the auto-rich setting in the cruise range is very near the best-power mixture ratio. Therefore, any leaning out gives a mixture that is leaner than best power.

The engine’s ability to cool depends on such factors as cylinder design (including the design of the cooling fins), compression ratio, accessories on the front of the engine which causes individual cylinders to run hot, and the design of the baffling used to deflect airflow around the cylinder. At takeoff power, the mixture must be rich enough to supply sufficient fuel to keep the hottest cylinder cool.

Idle mixture The idle mixture curve, below right, shows how the mixture changes when the idle mixture adjustment is changed. Note that the most significant effect is at idling speeds. However, there is some effect on the mixture at airflows above idling. The airflow at which the idle adjustment effect cancels out varies from minimum cruise to maximum cruise. The exact point depends on the type of carburettor and the carburettor setting. In general, the idle adjustment affects the fuel/air mixture up to medium cruise on most engines having pressure-injection-type carburettors, and up lo low cruise on engines equipped with float-type carburettors. This means that incorrect idle mixture adjustments can easily give faulty cruise performance as well as poor idling.

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Typical fuel/air mixture curve for float-type carburettor

Idle mixture curve

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The induction manifold The induction manifold provides the means of distributing air, or the fuel/air mixture, to the cylinders. Whether the manifold handles a fuel/air mixture or air alone depends on the type of fuel metering system used. On an engine equipped with a carburettor, the induction manifold distributes a fuel/air mixture from the carburettor to the cylinders. On a fuel-injection engine, the fuel is delivered to injection nozzles, one in each cylinder, which provide the proper spray pattern for efficient burning. Thus, the mixing of fuel and air takes place in the cylinders or at the inlet port to the cylinder. On a fuel-injection engine, the induction manifold handles only air.

Any leak in the induction system affects the mixture reaching the cylinders. This is particularly true of a leak at the cylinder end of an intake pipe. At manifold pressures below atmospheric pressure, such a leak will lean out the mixture. This occurs because additional air is drawn in from the atmosphere at the leaky point. The affected cylinder may overheat, fire intermittently, or even cut out altogether.

The induction manifold is an essential item because of the effect it can have on the fuel/air mixture, which finally reaches the cylinder. Fuel is introduced into the airstream by the carburettor in a liquid form. The fuel must be vaporised in the air to become combustible. This vaporisation takes place in the induction manifold, which includes the internal supercharger if one is used. Any fuel that does not vaporise will cling to the walls of the intake pipes. This affects the effective fuel/air ratio of the mixture which finally reaches the cylinder in vapour form. This explains the reason for the apparently rich mixture required to start a cold engine. In a cold engine, some of the fuel in the airstream condenses out and clings to the walls of the manifold. This is in addition to that fuel which never vaporised in the first place. As the engine warms up, less fuel is required because less fuel is condensed out of the airstream and more of the fuel is vaporised, thus giving the cylinder the required fuel/air mixture for regular combustion.

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Induction manifold components

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Operational effect of valve clearance While considering the operational effect of valve clearance, keep in mind that all aircraft reciprocating engines of current design use valve overlap. The diagram below left shows the pressures at the intake and exhaust ports under two different sets of operating conditions. In one case, the engine is operating at a manifold pressure of 35 in.Hg. Barometric pressure (exhaust backpressure) is 29 in.Hg. This gives a pressure differential of 6 in.Hg (3 psi) acting in the direction indicated by the arrow. During the valve overlap period, this pressure differential forces the fuel/air mixture across the combustion chamber toward the open exhaust. This flow of fuel/air mixture forces ahead of it the exhaust gases remaining in the cylinder, resulting in complete scavenging of the combustion chamber. This, in turn, permits complete filling of the cylinder with a fresh charge on the following intake event. This is the situation in which valve overlap gives increased power.

In engines with collector rings, this inflow through the exhaust port at low power settings consists of burned exhaust gases. These gases are pulled back into the cylinder and mix with the incoming fuel/air mixture. However, these exhaust gases are inert; they do not contain oxygen. Therefore, the fuel/air mixture ratio is not affected much. With open exhaust stacks, the situation is entirely different. Here, fresh air containing oxygen is pulled into the cylinders through the exhaust which leans out the mixture. Therefore, the carburettor must be set to deliver an excessively rich idle mixture so that, when this mixture is combined with the fresh air drawn in through the exhaust port, the effective mixture in the cylinder is at the desired ratio.

In a situation where the manifold pressure is below atmospheric pressure, 20 in.Hg, for example, there is a pressure differential of 9 in.Hg (4.5 psi) in the opposite direction. This causes air or exhaust gas to be drawn into the cylinder through the exhaust port during valve overlap.

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Effect of valve overlap

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At first thought, it does not appear possible that the effect of valve overlap on fuel/air mixture is sufficient to cause concern. However, the effect of valve overlap becomes apparent when considering idle fuel/air mixtures. These mixtures must be enriched 20 to 30% when open stacks instead of collector rings are used on the same engine. This is shown graphically below top-left. Note the spread at idle between an open stack and an exhaust collector ring installation for otherwise identical engines. The mixture variation decreases as the engine speed or airflow are increased from idle into the cruise range. Engine, aeroplane, and equipment manufacturers provide a powerplant-installation that gives satisfactory performance. Cams are designed to give best valve operation and correct overlap. But valve operation is correct only if valve clearances are set and remain at the value recommended by the engine manufacturer. If valve clearances are set wrong, the valve overlap period will be longer or shorter than the manufacturer intended. The same is true if clearances get out of adjustment during operation. Where there is too much valve clearance, the valves will not open as wide or remain open as long as they should. This reduces the overlap period. At idling speed, it will affect the fuel/air mixture, since a less-than-normal amount of air or exhaust gases will be drawn back into the cylinder during the shortened overlap period. As a result, the idle mixture will tend to be too rich.

When valve clearance is less than it should be, the valve overlap period is lengthened. This permits a greater-thannormal amount of air or exhaust gases to be drawn back into the cylinder at idling speeds. As a result, the idle mixture will be leaned out at the cylinder. The carburettor is adjusted with the expectation that a certain amount of air or exhaust gases will be drawn back into the cylinder at idling. If more or less air or exhaust gases are drawn into the cylinder during the valve overlap period, the mixture will be too lean or too rich. When valve clearances are wrong, it is unlikely that they are all wrong in the same direction. Instead, there will be too much clearance on some cylinders and too little on others. Naturally, this gives a variation in valve overlap between cylinders. This, in turn, results in a variation in fuel/air ratio at idling and lowerpower settings, since the carburettor delivers the same mixture to all cylinders. The carburettor cannot tailor the mixture to each cylinder to compensate for variation in valve overlap. The effect of variation in valve clearance and valve overlap on the fuel/air mixture between cylinders is illustrated below bottom-right. Note how the cylinders with too little clearance run rich and those with too much clearance run lean. Note also the extreme mixture variation between cylinders. On such an engine, it would be impossible to set the idle adjustment to give correct mixtures on all cylinders, nor can all cylinders of such an engine be expected to produce the same power. Variations in valve clearance of as little as 0.005" have a definite effect on mixture distribution between cylinders.

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Effect of variation in valve overlap on fuel/air mixture between cylinders

Comparison of fuel/air mixture curves for open stack and collector ring installations

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Another aspect of valve clearance is its effect on volumetric efficiency. Considering the intake valve first, suppose valve clearance is greater than that specified. As the cam lobe starts to pass under the cam roller, the cam step or ramp takes up part of this clearance. However, it doesn’t take up all the clearance as it should. Therefore, the cam roller is well up on the lobe proper before the valve starts to open. As a result, the valve opens later than it should. Similarly, the valve closes before the roller has passed from the main lobe to the ramp at its end. With excessive clearance, then, the intake valve opens late and closes early. This produces a throttling effect on the cylinder. The valve is not open long enough to admit a full charge of fuel and air. This will cut down the power output, particularly at high-power settings. Insufficient intake valve clearance has the opposite effect. The clearance is taken up, and the valve starts to open while the cam roller is still on the cam step. The valve doesn’t close until the riser at the end of the lobe has almost entirely passed under the roller. Therefore, the intake valve opens early, closes late, and stays open longer than it should. At low power, early opening of the intake valve can cause backfiring because of the hot exhaust gases backing out into the intake manifold and igniting the mixture there. Excessive exhaust valve clearance causes the exhaust valve to open late and close early. This shortens the exhaust event and causes poor scavenging. The late opening may also lead to the cylinder overheating. The hot exhaust gases are held in the cylinder beyond the time specified for their release.

When exhaust valve clearance is insufficient, the valve opens early and closes late. It remains open longer than it should. The early opening causes a power loss by shortening the power event. The pressure in the cylinder is released before all the useful expansion has worked on the piston. The late closing causes the exhaust valve to remain open during a more substantial portion of the intake stroke than it should. This may result in good mixture being lost through the exhaust port. As mentioned before, there will probably be too little clearance on some cylinders and too much on others whenever valve clearances are incorrect. This means that the effect of incorrect clearances on volumetric efficiency will usually vary from cylinder to cylinder. One cylinder takes in a full charge while another receives only a partial charge. As a result, cylinders will not deliver equal power. One cylinder will backfire or run hot while another performs satisfactorily. On some direct fuel injection engines, variations in valve clearance will affect only the amount of air taken into the cylinders. This is true when the induction manifold handles only air. In this case, there is no appreciable effect on the distribution of fuel to the individual cylinders. This means that, when clearances vary between cylinders, air charges will also vary, but fuel distribution will be uniform. This faulty air distribution, coupled with proper fuel distribution, will cause variations in the mixture ratio. In all cases, variations in valve clearance from the value specified have the effect of changing the valve timing from that obtained with correct clearance. This is certain to give something less than perfect performance.

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Ignition system The next item to be considered regarding engine operation is the ignition system. Although basically simple, it is sometimes not understood clearly. An ignition system consists of four main parts: • • • •

The basic magneto. The distributor. The ignition harness. The spark plug.

The basic magneto is a high-voltage generating device. It must be adjusted to give a maximum voltage at the time the points break and ignition occurs. It must also be synchronised accurately to the firing position of the engine. The magneto generates a series of peak voltages which are released by the opening of the breaker points. A distributor is necessary to distribute these peak voltages from the magneto to the cylinders in the proper order. The ignition harness constitutes the insulated and shielded high-tension lines that carry the high voltages from the distributor to the spark plugs. The magnetos used on aircraft engines are capable of developing voltages as high as 15,000 V. The voltage required to jump the specified gap in a spark plug will usually be about 4,000 to 5,000 V maximum. The spark plugs serve as safety valves to limit the maximum voltage in the entire ignition system. As spark plug gaps open up as a result of erosion, the voltage at the plug terminals increases. A higher voltage is required to jump the larger gap. This higher voltage is transmitted through the secondary circuit. The increased voltage in the circuit becomes a hazard. It is a possible source of a breakdown in the ignition harness and can cause flashover in the distributor. Total Training Support Ltd © Copyright 2020

The distributor directs the firing impulses to the various cylinders. It must be timed properly to both the engine and the magneto. The distributor's finger must align with the correct electrode on the distributor block at the time the magneto points break. Any misalignment may cause the high voltage to jump to a cylinder other than the one intended. This will cause severe backfiring and general malfunctioning of the engine. The manufacturer has selected the best compromise and specified an alignment with the No. 1 electrode for timing. However, even with perfect distributor timing, the finger is behind on some electrodes and ahead on others. For a few electrodes (cylinders), the alignment is as far from perfect as it can safely be. A slight error in timing, added to this already imperfect alignment, may put the finger so far from the electrode that the high voltage will not jump from finger to electrode, or the high voltage may be routed to the wrong cylinder. Therefore, the distributor must be timed perfectly. The finger must be aligned with the No. 1 electrode precisely as prescribed in the maintenance manual for the particular engine and aircraft. Although the ignition harness is simple, it is a critical part of the ignition system. A number of things can cause failure in the ignition harness. Insulation may break down on a wire inside the harness and allow the high voltage to leak through to the shielding (and to ground), instead of going to the spark plug. Open circuits may result from broken wires or poor connections. A bare wire may be in contact with the shielding, or two wires may be shorted together.

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Any severe defect in an individual lead prevents the high voltage impulse from reaching the spark plug to which the lead is connected. As a result, only one spark plug in the cylinder will fire. This causes the cylinder to operate on single ignition. This is certain to result in detonation since dual ignition is required to prevent detonation at takeoff and during other highpower operation. Two bad leads to the same cylinder will cause the cylinder to go completely dead. On engines with separate distributors, a faulty magneto-to-distributor lead can cut out half the ignition system. Among the most common ignition harness defects, and the most difficult to detect, are high-voltage leaks. However, a complete harness check will reveal these and other defects. Although the spark plug is simple both in construction and in operation, it is, nevertheless, the direct or indirect cause of a great many malfunctions encountered in aircraft engines. Proper precaution begins with plug selection. Be sure to select and install the plug specified for the particular engine. One of the reasons a particular plug is specified is its heat range. The heat range of the spark plug determines the temperature at which the nose end of the plug operates. It also affects the ability of the spark plug to ignite mixtures that are borderline from the standpoint of high oil content or excessive richness or leanness. A great many troubles attributed to spark plugs are the direct result of malfunctions somewhere else in the engine. Some of these are excessively rich idle mixtures, improperly adjusted valves, and impeller oil seal leaks.

Propeller governor The final item to be considered regarding engine operation is the effect of the propeller governor on engine operation. In the curve shown in the graph below, note that the manifold pressure change with RPM is gradual until the propeller governor cut-in speed is reached. Beyond this point, the manifold pressure increases, but no change occurs in the engine RPM, as the carburettor throttle is opened wider. An accurate picture of the power output of the engine can be determined only at speeds below the propeller governor cut-in speed. The propeller governor is set to maintain a given engine RPM. Therefore, the relationship between engine speed and manifold pressure as an indication of power output is lost, unless it is known that all cylinders of the engine are functioning correctly. In fact, on a multi-engine aircraft, an engine can fail and still produce every indication that it is developing power. The propeller governor will flatten out the propeller blade angle and windmill the propeller to maintain the same engine RPM Heat of compression within the cylinder will prevent the cylinder head temperature from falling rapidly. The fuel pressure will remain constant, and the fuel flow will not change unless the manifold pressure is changed. On an engine not equipped with a turbocharger, the manifold pressure will remain where it was. On a turbocharged engine, the manifold pressure will not drop below the value which the mechanical supercharger can maintain. This may be well above atmospheric pressure, depending upon the blower ratio of the engine and the specific conditions existing. Thus, the pilot has difficulty in recognising that he has encountered a sudden failure unless the engines are equipped with torquemeters, or he notices the fluctuation in RPM at the time the engine cuts out.

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Propeller governor and pitch change unit

Effect of propeller governor on manifold pressure

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Overlapping phases of engine operation Up to this point, the individual phases of engine operation have been discussed. The relationship between the phases and their combined effect on engine operation will now be considered. Combustion within the cylinder is the result of fuel metering, compression, and ignition. Since valve overlap affects fuel metering, proper combustion in all the cylinders involves correct valve adjustment in addition to the other phases. When all conditions are correct, there is a burnable mixture. When ignited, this mixture will give power impulses of the same intensity from all cylinders. The system which ignites the combustible mixture requires that the following five conditions coincide if the necessary spark impulse is to be delivered to the cylinder at the proper time. • • • • •

The breaker points must be timed accurately to the magneto (E-gap). The magneto must be timed accurately to the engine. The distributor finger must be timed accurately to the engine and the magneto. The ignition harness must be in good condition with no tendency to flashover. The spark plug must be clean, have no tendency to short out, and have the proper electrode gap.

If any one of these requirements is lacking or if any one phase of the ignition system is maladjusted or is not functioning correctly, the entire ignition system can be disrupted to the point that improper engine operation results.

As an example of how one phase of engine operation can be affected by other phases, consider spark plug fouling. Spark plug fouling causes malfunctioning of the ignition system, but the fouling seldom results from a fault in the plug itself. Usually, some other phase of the operation is not functioning correctly, causing the plug to foul out. If excessively rich fuel/air mixtures are being burned because of either rich carburetion or improperly adjusted idle mixture, spark plug fouling will be inevitable. Generally, these causes will result in fouled spark plugs appearing over the entire engine, and not necessarily confined to one or a few cylinders. If the fuel/air mixture is too lean or too rich on any one cylinder because of a loose intake pipe or improperly adjusted valves, improper operation of that cylinder will result. The cylinder will probably backfire. Spark plug fouling will occur continually on that cylinder until the defect is remedied. Impeller oil seal leaks, which can be detected only by removal of intake pipes, will cause spark plug fouling. Here, the fouling is caused by excess oil being delivered to one or more cylinders. Stuck or broken rings will cause oil pumping in the affected cylinders with consequent plug fouling and high oil consumption. Improperly adjusted cylinder valves cause spark plug fouling, hard starting, and general engine malfunctioning. They may also cause valve failure as a result of high-seating velocities or the valve holding open, with subsequent valve burning.

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Whenever the exact cause of engine malfunctioning is not determined and whenever the real disorder is not corrected, the corrective measure taken will provide only temporary relief. For example, the standard “fix” for engine backfiring is to change the carburettor. However, as a result of many tests, it is now known that the usual cause of engine backfiring is an improperly adjusted or defective ignition system or improperly adjusted engine valves. Backfiring is usually caused by one cylinder, not all the cylinders. To remedy backfiring, first, locate which cylinder is causing it, and then find out why that cylinder is backfiring. Engine power troubleshooting The need for troubleshooting is typically dictated by poor operation of the complete powerplant. In many cases, power settings for the type of operation where the difficulty is encountered indicate which part of the powerplant is the underlying cause.

The power output of an engine is the power absorbed by the propeller. Therefore, the propeller load is a measure of power output. The propeller load, in turn, depends on the propeller RPM, blade angle, and air density. For a given angle and air density, the propeller load (power output) is directly proportional to engine speed. The basic power of an engine is related to manifold pressure, fuel flow, and RPM Because the RPM of the engine and the throttle opening directly control manifold pressure, the primary engine power controls are the throttle and the RPM control. An engine equipped with a fixed-pitch propeller has only a throttle control. In this case, the throttle setting controls both manifold pressure and engine RPM.

The cylinders of an engine, along with the supercharger impeller, form an air pump. Furthermore, the power developed in the cylinders varies directly with the rate of air consumption. Therefore, a measure of air consumption or airflow into the engine is a measure of power input. Ignoring for the moment such factors as humidity and exhaust back pressure, the manifold pressure gage and the engine tachometer provides a measure of engine air consumption. Thus, for a given RPM, any change in power input will be reflected by a corresponding change in manifold pressure.

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With proper precautions, manifold pressure can be taken as a measure of power input, and RPM can be taken as a measure of power output. However, the following factors must be considered: • •

•

Atmospheric pressure and the air temperature must be considered since they affect air density. These measures of power input and power output should be used only for comparing the performance of an engine with its past performance or for comparing identical powerplants. With a controllable propeller, the blades must be against their low-pitch stops, since this is the only blade position in which the blade angle is known and does not vary. Once the blades are off their low-pitch stops, the propeller governor takes over and maintains a constant RPM regardless of power input or engine condition. This precaution means that the propeller control must be set to maximum or takeoff RPM, and the checks made at engine speeds below this setting.

If the engine is equipped with a torque meter, the torque meter reading rather than the engine speed should be used as a measure of power output. Having relative measures of power input and power output, the condition of an engine can be determined by comparing input and output. This is done by comparing the manifold pressure required to produce a given RPM with the manifold pressure required to produce the same RPM at a time when the engine (or an identical powerplant) was known to be in top operating condition.

An example will best show the practical application of this method of determining engine condition. With the propeller control set for takeoff RPM (full low-blade angle), an engine may require 32 in.Hg of manifold pressure to turn 2,200 RPM for the ignition check. On previous checks, this engine required only 30 in.Hg of manifold pressure to turn 2,200 RPM at the same station (altitude) and under similar atmospheric conditions. Obviously, something is wrong; a higher power input (manifold pressure) is now required for the same power output (RPM). There is a good chance that one cylinder has cut out. There are several standards against which engine performance can be compared. The performance of a particular engine can be compared with its past performance, provided adequate records are kept. Engine performance can be compared with that of other engines on the same aircraft or aircraft having identical installations. If a fault does exist, it may be assumed that the trouble lies in one of the following systems: • • • • •

Ignition system. Fuel metering system. Induction system. Power section (valves, cylinders, etc.). Instrumentation.

If a logical approach to the problem is taken and the instrument readings adequately utilised, the malfunctioning system can be pinpointed, and the specific problem in the defective system can be singled out.

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The more information available about any particular problem, the better the opportunity for rapid repair. Information that is of value in locating a malfunction includes: • • • • • •

Was any roughness noted? Under what conditions of operation? What is the time on the engine and spark plugs? How long since the last inspection? Was the ignition system operational check and power check healthy? When did the trouble first appear? Was backfiring or after-firing present? Was the full throttle performance normal?

From a different point of view, the powerplant is, in reality, several small engines turning a common crankshaft and being operated by two common phases: (1) fuel metering and (2) ignition. When backfiring, low power output or other powerplant difficulty is encountered, first find out which system (fuel metering or ignition) is involved and then determine whether the entire engine or only one cylinder is at fault.

Ignition system reasons for backfiring might be a cracked distributor block or a high-tension leak between two ignition leads. Either of these conditions could cause the charge in the cylinder to be ignited during the intake stroke. Ignition system troubles involving backfiring will not usually be centred in the basic magneto since a failure of the basic magneto would result in the engine not running, or it would run well at low speeds but cut out at high speeds. On the other hand, the replacement of the magneto would correct a difficulty caused by a cracked distributor where the distributor is a part of the magneto. If the fuel system, ignition system, and induction system are functioning correctly, the engine should produce the correct BHP unless some fault exists in the basic power section.

For example, backfiring is usually caused by: • • •

valves holding open or sticking open in one or more of the cylinders; lean mixture; or intake pipe leakage.

An error in valve adjustment, which causes individual cylinders to receive too small a charge or one too large, even though the mixture to the cylinders has the same fuel/air ratio.

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Licence Category B1 and B3

16.3 Engine Construction

Knowledge levels — Category A, B1, B2, B3 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1, B2 and B3 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category B2 basic knowledge levels.

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Crankcase, crankshaft, camshafts, sumps;

Part-66 Ref. 16.3

Knowledge Levels A B1 B3 1

Accessory gearbox; Cylinder and piston assemblies; Connecting rods, inlet, and exhaust manifolds; Valve mechanisms; Propeller reduction gearboxes

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Table of Contents General constructional arrangements _____________ 5 Internal arrangements __________________________ 6 Engine type classification _______________________ 6 Engine cylinder numbering _____________________ 12 External component arrangements _______________ 16

Pistons ______________________________________ 86 General assembly and types ____________________ 86 Piston rings _________________________________ 88 Valve mechanisms ____________________________ 94 General arrangement __________________________ 94 Radial engine valve mechanism _________________ 98 Valves ____________________________________ 102 Valve temperatures __________________________ 106 Sleeve valves _______________________________ 114 Tappets ___________________________________ 122 Pushrods __________________________________ 126

Crankcase ___________________________________ 20 In-line engines _______________________________ 20 Radial engines ______________________________ 24 Accessory gearbox____________________________ 26 Crankshaft ___________________________________ Nomenclature _______________________________ Journals ___________________________________ Crank pins __________________________________ Crankshaft arrangements ______________________

32 32 34 36 38

Inlet and exhaust manifolds ____________________ 130 Propeller reduction gearboxes __________________ 134 Parallel spur gears ___________________________ 134 Epicyclic reduction gears ______________________ 134 Compound spur epicyclic ______________________ 134 Gear train/epicyclic __________________________ 134

Bearings ____________________________________ 50 Camshafts ___________________________________ 52 Connecting rods ______________________________ Function ___________________________________ The plain connecting rod _______________________ The fork and blade connecting rod _______________ The master and articulated rod __________________

56 56 58 62 64

Cylinders ____________________________________ General arrangement _________________________ Cylinder heads ______________________________ Cylinder head temperatures ____________________ Cylinder barrels ______________________________ HTCC combustion chamber ____________________ Water-cooled engine cylinder block and liners ______

66 66 68 72 74 80 84

Diesel engines – differences and additions _______ 142 Example: Thielert TAE 125-series _______________ 142 Crankcase _________________________________ 148 Crankshaft _________________________________ 150 Camshaft __________________________________ 150 Pistons ____________________________________ 150 Connecting rods _____________________________ 152 V-ribbed belt _______________________________ 152 Cylinder head _______________________________ 154 Gearbox ___________________________________ 156 Clutch and dual-mass flywheel _________________ 156 3-4

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General constructional arrangements Internal arrangements The internal arrangement of the piston engine includes a cylinder or cylinders which are closed at one end and open at the other, to allow a piston to slide up and down inside the cylinder.

In-line engines In-line engines can be divided into further categories; they are: • • • • • •

The piston is connected to the crankshaft by a connecting rod; its purpose is to convert the reciprocating movement of the piston into the rotary movement of the crankshaft. Located in the cylinder head, at the closed end of the pistoncylinder, are the inlet and exhaust valves. These allow the fuel/air mixture to pass into the cylinder and the exhaust gases to pass out. The spark plug is fitted in the cylinder head.

As the name implies, the cylinders are arranged in a row, running from forward to the aft of the engine, they can be inverted or upright, although most types in use today are of the inverted type, as this allows better forward vision for the pilot. Another advantage afforded by the in-line type is that the frontal area is limited, allowing smaller cowlings, thus reducing the drag factor. With the standard in-line engine, the number of cylinders is generally limited to six, this being the maximum number that can be cooled efficiently by the passing airflow.

A typical piston engine internal arrangement is illustrated below. Each of the components shown will be discussed in detail in this module.

There is only one crankshaft, and in the inverted engine, it is located above the cylinders.

Engine type classification The type classification of the engine depends on the manufacture and also the airframe constructor’s requirements.

Although classed as an in-line engine, the upright cylinder or inverted V-engine differs from the standard in-line engine by having the cylinders arranged on the crankcase in two rows, forming a letter V. The main advantage of this arrangement is that the engine is considerably shorter than the standard in-line type; this is because the two sets of connecting rods can be attached to the same position to the crankshaft pin. Therefore, there is a significant saving in weight without a reduction in power output.

Piston engines are usually classified by their cylinder arrangement, and can be divided into two main categories: • •

in-line engines; and radial engines.

upright in-line; inverted in-line; inverted V; upright V; flat opposed (boxer); and H-type.

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Basic parts of an internal combustion engine

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The flat opposed type (boxer twin) is the most popular of all the in-line engines, used in light aircraft powerplants. This type of engine has the cylinder mounted horizontally; the main advantage of this arrangement is that it is very compact and very flat, allowing it to be installed in small nacelles. Most installations have an even number of cylinders helping to reduce vibration levels. The H-type (flat-4) of the flat opposed engine shown below is an adaptation of the standard flat opposed engine; it gives a higher power output due to the addition of another series of cylinders.

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In-line engine arrangements

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Radial engines The radial engine arrangement has a row or rows of cylinders arranged radially around a central crankshaft. The number of cylinders fitted to each row is odd, usually five, seven, or nine. As the cylinders are located evenly in a circular plane, all the pistons are connected to a single crankshaft; this arrangement reduces the weight of the engine by reducing the number of moving parts, leading to large power output. Two typical radial engine arrangements are shown below. Some radial engines may have more than one row of cylinders, the most popular being the two-row radial engine. The two-row design utilises two rows of seven or nine cylinders. The cylinders in the two-row system are staggered, thus affording the best means of air cooling. This ensures that the front cylinders do not mask off the airflow to the rear row of cylinders. The radial engine, therefore, has the lowest weight to power ratio of all the engines and is found on most sizeable pistonengine aircraft. Its primary disadvantage, however, is its size, producing a drag problem. Cooling is another main problem area. As each engine has several cylinders, some method must be used to locate and identify a particular cylinder within a group. This may be required for troubleshooting, defect location, or maintenance. A standard location and direction are therefore used by all engine manufacturers for cylinder location and numbering purposes.

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Radial engine cylinder arrangements (Pratt & Whitney Wasp)

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Engine cylinder numbering The propeller-shaft end of the engine is always known as the front end, and the accessory gearbox the rear end, irrespective of the engine’s location and installation. The engine is generally viewed from the rear when numbering the cylinders; however, some British made engines may be numbered from the front. Because of the various cylinder arrangements, the numbering system for in-line and radial engines differs. Consider the inline engines illustrated below. No. 1 cylinder is at the rear end, with the highest cylinder number at the propeller shaft (top left). The V-type and opposed in-line engines have two rows of cylinders, these are left and right as viewed from the rear. Because of the two rows, a slightly different numbering system is used, and this is illustrated top-centre. In this case, No. 1 is the first on the right, followed by No. 2 on the left, and again ending up with the highest number near the propeller shaft.

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The radial engine numbering system adopts a different method, again viewed from the rear, single row radial engine cylinders are numbered clockwise, starting with No. 1 cylinder at the top. In the double system, the same system is used except that the No. 1 cylinder is at the top in the rear row, and No. 2 cylinder is, therefore, the next one round (clockwise) in the front row. This numbering system carries on right round the whole engine (remember clockwise is as viewed from the rear). Another way to remember the two-row radial engine numbering system is that all odd numbers are found on the rear row and even numbers on the front row.

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Cylinder numbering

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External component arrangements The diagram below shows a breakdown of a typical in-line piston engine. In this case, the crankshaft is a single piece extending along the whole length of the engine. Connecting the pistons to the crankshaft is a series of connecting rods. The crankshaft is supported within the crankcase, which is divided into two parts by a series of bearings. The cylinders are bolted directly onto the crankcase and attached to the cylinders are the inlet and exhaust manifold tubes. The camshaft, driven from the crankshaft by gear or chain drives, ensures that the inlet and exhaust valves in each cylinder operate at the correct time and in the correct order. Finally, the accessory gearbox is attached to the rear of the engine.

Also attached to the engine is some form of starter motor. In most cases, this is an electrical motor, similar to a motor car starter, but on some older types of engines, a cartridge starter system may be used. The primary structural support for the in-line type engine shown is the crankcase. The structural design of the radial engine, with its cylinder arrangement, may have slight differences in design compared to that shown below; however, it still contains most of the structural components fitted to the in-line type engine.

The following components are driven by the accessory gearbox: • •

• • • •

the oil pump for the lubrication system; the RPM transmitter, also known as the engine tachometer which indicates the rotational speed of the engine; the electrical generator to supply electric power to the aircraft’s electrical system; a magneto that supplies power to the sparking plugs in the correct sequence, known as ignition timing; hydraulic pumps (if fitted); and fuel pumps (if mechanical).

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A large radial engine consists of the following components: • • • • • • •

nose section; crankcase; crankshaft; connecting rods; cylinders; supercharger (found on most radials); and an accessory gearbox.

Some engines may have some sort of propeller reduction gearing housed inside the nose section.

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Crankshaft and camshaft drive connection

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Crankcase In-line engines The crankcase is considered to be the primary structural support for the moving parts of the engine. If we use as an example the flatly opposed crankcase illustrated below, we can see that as well as supporting the cylinders, in this case, three per side, it also contains bearing supports for the crankshaft and camshaft. It is also divided into two halves for ease of assembly of internal components. However, specific issues must be considered in the design and construction of the crankcase assembly. Strength is the most critical feature of the crankcase. Although the crankcase is a balance assembly, high inertia and centrifugal forces are created within the crankcase housing; therefore, the materials from which it is manufactured must be able to withstand these forces and prevent bending or distortion. The crankcase may be subjected to additional loads from the propeller reduction gears, or by propeller rotation loads. Generally, for the lighter types of aircraft, the crankcase is usually manufactured from an aluminium alloy; forged steel is used for crankcases that are used for high power engines. The use of aluminium alloy ensures sufficient strength of the casing, while still retaining a reasonably light structure. Most attachment points for components use the stud method; these have threaded inserts retaining the studs in the crankcase. The threaded inserts are generally made of steel, thus ensuring a secure fixture to the casing.

Oil transfer to the various rotating parts within the crankcase is accompanied by oil ports and channels drilled into the casing, thereby reducing the number of oil pipes required, leading to reduced weight. The crankcase also serves as a support for the oil sump. This type of oil tank/sump, also known as a wet sump arrangement, is relatively popular in the in-line engine arrangement and is similar to the method used in cars. All joints are assembled with oil seals, ensuring that there is no external oil leakage, for in the wet sump system, once the oil has been used for lubrication, it falls into the sump, and the lubricated cycle starts again. Crankcase breathing Any air leaking past the piston rings finds its way into the lower part of the crankcase. As this area is not designed to withstand high pressures (oil leaks could occur), some means must be provided to vent this air overboard from the crankcase. To achieve this, a pipe is generally connected into the crankcase, allowing air to escape to the atmosphere, thus equalising the pressure within the crankcase to ambient pressure. Usually, a wire type filter may be fitted in this tube. This system is known as crankcase breathing.

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Typical in-line engine crankcase arrangement

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6-cylinder opposed engine crankcase

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Crankshaft in assembled engine, with cylinders and other components

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Radial engines The in-line engine crankcase is usually a single or a two-part assembly. The radial engine, however, may contain as many as seven structural parts. As you can see from the illustration below, this arrangement contains four major sub-assemblies: • • • •

the nose assembly; the power assembly; the supercharger (this may not be fitted); and the accessory assembly.

Nose section The nose assembly houses the propeller support bearings, as well as the propeller reduction gearing. In most cases, the nose section is manufactured from aluminium alloy, affording strength with lightness. The use of aluminium alloy does tend to damp out any vibrations caused by the propeller reduction gearing. As the nose section may contain oil components that are required for the reduction gearing, adequate oil sealing must be achieved between the power and nose sections.

As with the in-line engine, the cylinders are generally attached to the casing by nuts and studs. These casings also support the main crankshaft for the radial pistons. Supercharger section Most high-power radial engines have some form of a supercharger to compress the air/fuel mixture as it leaves the carburettor. These usually are internally driven by the power section and are attached to the aft of the power section. The supercharger casing is manufactured from aluminium alloy or a magnesium alloy. It contains various exit orifices for fuel/air mixture pipes to be fitted, allowing the compressed mixture to be ducted to the cylinders. The accessory drive casing is located at the rear end of the casing.

Attachment of the nose section to the power section is achieved by either stud or by nuts and bolts. Power sections The power section could consist of up to four subsections, depending upon the number of rows of cylinders. Because of the extremely high loads experienced in this area, the crankcase is typically manufactured from a steel forging or a high strength alloy.

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Six cylinder opposed engine crankcase

Radial engine crankcase and assembly arrangement

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Accessory gearbox The rear section of a piston engine is usually of cast construction, and the material may be either aluminium alloy, which is used most widely, or magnesium, which has been used less frequently. On some engines, it is cast in one piece and provided with means for mounting the accessories, such as magnetos, carburettors, and fuel, oil, and vacuum pumps, and starter, generator, etc., in the various locations required to facilitate accessibility. Other adaptations cast magnesium cover plate on which the accessory mounts are arranged.

Gear trains, containing both spur- and bevel-type gears, are used in the different types of engines for driving engine components and accessories. Spur-type gears are generally used to drive the more substantial loaded accessories or those requiring the least play or backlash in the gear train. Bevel gears permit angular location of short stub shafts leading to the various accessory mounting pads.

Accessory drive shafts are mounted in suitable bronze bushings located in the diffuser and rear sections. These shafts extend into the rear section and are fitted with suitable gears from which power takeoffs or drive arrangements are carried out to the accessory mounting pads. In this manner, the various gear ratios can be arranged to give the proper drive speed to magneto, pump, and other accessories to obtain correct timing or functioning. In some cases, there is a duplication of drives, such as the tachometer drive, to connect instruments located at separate stations. The accessory section provides a mounting place for the carburettor, or master control, fuel injection pumps, enginedriven fuel pump, tachometer generator, synchronising generator for the engine analyser, oil filter, and oil pressure relief valve.

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The accessory assembly casing contains the necessary bearings and bushes required to support the accessory drive shafts of the various components that may be located on the gearbox. It is usually manufactured from an aluminium alloy. Drive pads are mounted for such engine subsystems as: • • • • • •

the fuel pump; the oil pump; the magneto; the RPM tachometer; the electrical generators; and the starter motor.

Other components that may be fitted are filters and magnetic chip detectors. The accessory section contains the necessary drive shafts to operate the above systems and components; it is manufactured from either aluminium or magnesium alloy. An example of a typical accessory section is illustrated below.

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Typical radial engine accessory section

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The accessory drive shaft is driven by a crankshaft gear, and this, in turn, drives various shafts and gears that are mounted on the rear of the engine. Driveshafts connect from these gears to each component. By using this method, various gear ratios can be arranged to give the proper drive speed of each component. As well as providing mounts for the system components listed previously, there are also mounts for the carburettor and inlet manifold. The location of the accessory section is most important for ease of access during maintenance operations.

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Typical gear drive arrangement

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Crankshaft The purpose of the crankshaft is to receive the axial thrust of the pistons and to convert this thrust to rotary movement through the connecting rod assembly. The crankshaft is usually machined from a chrome-nickel molybdenum steel forging, which is then nitrided to provide excellent wear characteristics on the bearing surfaces. Crankshafts are classified according to the number of cranks. A shaft with one crankpin is called a single-throw crankshaft, and one with six cranks, a six-throw crankshaft. Examples of a single throw crankshaft and a four-throw crankshaft are illustrated. Suitable drives at each end of the shaft transmit the torque to the propeller, sometimes through a reduction gear, and to the accessory drives, e.g., magnetos, oil pumps, and fuel pump. The crankshaft is hollow to provide oilways, with the added advantage that it also makes it lighter. Nomenclature Crankshafts may differ between the two engine arrangements, but the terminology that is used is the same for both types. Illustrated below are typical examples of an engine crankshafts used in a 6-cylinder and a 4-cylinder in-line engine. The crankshaft comprises three major parts: • • •

journals; crankpins; and cheeks or arms.

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Crankshaft nomenclature

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Journals The journals are, in fact, the major supporting parts of the crankshaft, rotating within the main bearings. These bearings fit into location lugs within the crankcase, see the diagram below right. Because they support the crankshaft, they may also be known as the main bearing journals. For a short shaft, there may be only two bearings supporting it, one at each end. However, for a longer shaft, a bearing may be located midway, providing extra support and allowing even distribution of the loads carried by the crankshaft. In some cases, the bearing faces of the journals may be hardened, which is treated to prevent wear from the bearing inner race shells. The lubrication of this bearing is achieved by oil jets within the engine lubrication system.

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Typical crankshaft assembled with connecting rods and pistons (V12)

Crankshaft mounting points

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Crank pins The crankpins are those parts of the crankshaft to which the piston connecting rods or arms are attached. The crankpin may also be known as the connecting rod bearing journal. They are usually hollow in manufacture to reduce the weight of the crankshaft, and to allow for the lubrication of the connecting rod bearing shells, (big end shells). The bearing shells shown are generally manufactured from non-ferrous metals (copper-tin-bronze) and contain oil holes and grooves that match with the oil holes in the crankpin, allowing adequate lubrication of the crankpin surfaces. Excessive wear of these bearing shells results in heavy knocking noises from the crankshaft area with a subsequent low oil pressure indication. Crank cheeks or arms The primary purpose of the crank cheeks or arms is to attach and support the crankpin to the crankshaft. Extension of this arm may facilitate the addition of balance weights when required.

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Bearing shells

Crankshaft nomenclature

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Crankshaft arrangements Crankshaft arrangements differ between in-line and radial engine installations. However, there are different arrangements again within these two categories. The three principle arrangements found are: • • •

in-line four-throw or six-throw; a single throw 360° (radial engine); and a double throw 180° (radial engine).

In-line six-throw The six-throw crankshaft may be found in a standard sixcylinder in-line engine, or two six throw crankshafts may be found in a twelve-cylinder V-type in-line engine. In the example shown, you can see that the shaft has seven journals and six crankpins for connecting rod attachment. The propeller is usually attached to the shaft by a splined drive, and an accessory gear may be found on the rear of the shaft. Double throw The double throw crank illustrated below right has three journals and two crankpins. This arrangement is found in a two-row radial engine configuration, one throw for each row of cylinders. Again, the construction may be single-piece or twopiece.

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Typical crankshaft assembled with connecting rods and pistons (V12)

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Single throw The single-throw crankshaft, found in a single-cylinder row of a radial engine, consists of a single pin for connecting rod attachment, and two-journal bearing support. This type of crankshaft is used with a master connecting rod assembly, but this arrangement will be discussed in greater detail in the next module. The single throw arrangement may also have a twopiece crankshaft. This allows the crank to be split on the assembly of a single piece connecting rod. This type is illustrated below-left, the crankpin being splined together and secured by a bolt. Another example is illustrated below-right, where one cheek is attached to the crankpin and secured by a nut and bolt.

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Two-piece crankshaft arrangement

Radial engine single-throw crankshaft with dynamic damper

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An example of the movement of a single throw crankshaft is shown below-bottom. Here we have a seven-cylinder singlerow radial engine, and the illustration right shows a complete 360° turn and the position of the pistons in the seven cylinders.

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Double throw crankshaft

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Crankshaft balancing Dampers or balance weights are fitted to reduce the vibrations that are created by a rotating crankshaft. In most cases, the dampers or weights are added by the manufacturer during assembly. By adding balance weights to the crank cheeks or arms, the vibrations can be reduced to an acceptable level. A typical example of this type is illustrated. Single throw crankshafts used for single row radial engines may have their vibrations damped by one of the following methods: • •

counterweight; and dynamic balancing.

Counterweight method The counterweight method allows for the addition of balance weights. This is known as static balancing.

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Dynamic counterweights on a crankshaft

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Dynamic balancing method Radial engine crankshafts have a single throw for each row of cylinders, and they must have a large, heavy counterweight for static balance. Horizontally opposed engines, on the other hand, are built symmetrically and do not need counterweights for static balance. However, most of the larger ones use counterweights for dynamic balance, to absorb torsional vibrations. Dynamic counterweights are installed on blades that are forged as integral parts of the crankshaft. They are retained by pins with diameters smaller than the holes in the bushings through which they fit. The small pin in the large hole allows the weight to rock back and forth in a pendulum fashion.

However, to explain the action of the damper, it is best to consider the sequence of action shown below. A pendulum suspended from the crankshaft is subjected to a series of impulses from the engine; it swings from side to side in frequency with these impulses. However, if another pendulum is suspended below the first pendulum, it tends to absorb the impulses and swing itself, leaving the upper pendulum stationary. This, therefore, is the basic principle of the dynamic damper, where the pendulum weights are suspended in arms attached to the crankshaft.

The power produced by a reciprocating engine is supplied by the pistons in a series of pushes, or pulses. When the frequency of these pulses is the same as the resonant frequency of the crankshaft, severe torsional vibration can occur. Dynamic counterweights change the resonant frequency of the crankshaft. The dynamic balancing of crankshafts is a little more complicated. The purpose of dynamic balancing is to allow any out of balance forces created by the rotation of the crankshaft and power impulses to be balanced within themselves. This ensures that any vibration is kept to an acceptable level and can be tuned to provide the engine with a harmonically tuned vibration frequency which is sympathetic to the airframe. The distance the pendulum moves depends on the vibration frequency that corresponds to the power impulses from the engine.

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Dynamic counterweights on a crankshaft

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Dynamic counterweight system detuning In the piston engine, the reciprocating inertia forces (which increase with engine speed) are counteracted by the expansion forces acting on the pistons. Therefore, the loads imposed on the engine parts are represented by the difference between the two forces. Thus, at high speeds where the inertia forces are the greatest, the resultant forces are much higher with low manifold pressure that at a high manifold pressure, cylinder pressure being directly proportional to manifold pressure. When one of these two forces, the inertia force or the expansion forces, is suddenly changed, the effect on the resultant forces can cause the counterweight system to become detuned. Detuning the counterweight system of the engine can occur when the engine operates outside of its normal range and by abrupt throttle inputs. When this happens, the dynamic counterweights cannot follow the spectrum of frequencies for which they were designed, and rapid and severe damage to the counterweights, rollers, and bushings may result, culminating in engine failure. Lycoming has estimated that a load of approximately 90 tonnes is exerted on the rollers resulting in flat spots; this causes the weights to stick, resulting in violent vibration. An example of this is an engine that had become detuned following a propeller strike. Following inspection and return to service, the engine was flown for a further 100 hours; highfrequency vibration caused by the detuning resulted in 3 of the 4 engine mounts failing.

1. Rapid throttle operation Rapid opening or closing of the throttle can cause counterweight detuning. This can occur while adjusting the governors or other checks on the engine, which makes it necessary to run the engine at rated takeoff speed. Also, detuning can occur if the power is suddenly shut off, such as during a simulated engine failure. To avoid detuning during a simulated engine failure, use the mixture control to shut down the engine and leave the throttle at the normal open position until the engine has slowed down through the lack of fuel. Then close the throttle to the idle position. The throttle being open allows the cylinder to fill with air, maintaining the normal compression forces which are sufficient to cushion the deceleration of the engine. Another result of rapid throttle movement is a severe strain on supercharger gears and associated gears because of the inertia force of the high-speed impeller. 2. High engine speed and low manifold pressure Any operating procedure involving high RPM engine speed and low manifold pressure (under 50 kPa (15" Hg)) such as might be the case in a powered-off decent, can cause detuning of the dynamic counterweight system. However, just prior touchdown, during the landing sequence, it is permissible to place the propeller governor control in the high-RPM (low-pitch) position, and the throttle may be closed. At this low airspeed, there is no increase in engine RPM.

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3. Excessive speed and power Any supercharged or turbocharged engine, without automatic manifold pressure controllers, has the inherent capability of operating at power settings beyond the capability of the engine; this is particularly true at low altitude. See the operator’s manual for speed and power limitations for specific engine models. Reference the latest edition of Service Bulletin No. 369 for limits on manifold pressure and speed. 4. Propeller feathering Avoid propeller feathering during flight. If practice feathering must be accomplished, be sure that the throttle of the feathered engine is set at approximate zero thrust position before the mixture control is opened and engine operation resumed. See the aircraft operation manual for specific feathering instructions.

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Bearings The crankshaft is supported in the crankcase by plain bearings. The main bearing bosses are line-bored to ensure that the bearing seats are all in perfect alignment, and bearing inserts are fitted into each of the seats. The inserts have a steel backing and use a lead alloy as the bearing material. The bearing inside diameter is then clad with a thin lead or silver coating to provide a very low-friction surface. The inserts are prevented from turning in their seats by tangs on one end that fit into slots in the bearing seat. Alternatively, dowel pins pressed into the bearing seats fit through a hole in the insert. When the case halves are torqued together, the inserts are an interference fit with the bore. This is done for increased heat transfer and bearing retention. Camshafts of horizontally opposed engines usually ride in linebored holes through the webs in the crankcase and do not use any type of bearing insert or bushing. This arrangement is satisfactory because camshafts are nearly equally loaded from both sides when in operation. It is not unusual to find the tooling marks still on the case bearing surfaces after one or two times between overhaul (TBO) periods. The bearing shells are commonly manufactured from steel with white metal bearing (aluminium/tin) surface linings.

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Bearing shells and crankcase

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Camshafts The camshaft is found generally mounted to the crankcase but will only on in-line engines. The radial engine arrangement will be explained later.

The valve gear is designed so that the inlet valves share the same cam lobe, whereas the exhaust valves have dedicated cam lobes.

The purpose of the camshaft is to lift the inlet and exhaust valves in the correct sequence during engine rotation to ensure the correct firing order. The relationship between the rotation of the camshaft and the rotation of the crankshaft is of critical importance. Since the valves control the flow of air/fuel mixture intake and exhaust gases, they must be opened and closed at the appropriate time during the stroke of the piston. For this reason, the camshaft is connected to the crankshaft via a gear mechanism. A typical camshaft arrangement is shown below. The camshaft is a straight shaft that has a series of lobes at different angles along its length. As the shaft rotates, each lobe pushed its valve arm connecting rod, thus opening its respective inlet or exhaust valve. The camshaft rotates at only half the crankshaft speed. Lubrication results from having the centre of the shaft hollow, allowing oil to lubricate each lobe through an oil hole drilled in the lobe surface. The camshaft is manufactured from a chrome-nickel molybdenum steel forging. It is initially roughed out before being copper plated. Once the plating has been accomplished, the bearing journals, cam lobes, and gear teeth are ground. The camshaft is then carburised and the copper plating removed. Finally, bearing journals are polished. Total Training Support Ltd © Copyright 2020

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Camshaft drive by gear train

Camshaft and valve lifters

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Causes of camshaft failures In order of frequency: • • • • • •

Inactive engines – moisture in the engine causing corrosion. High RPM starts. Cold starts without preheating. Valve action – too loose or too tight. Sticking valves. Engine overspeed.

The lobes of the camshaft, and the tappets that they continually operate against, have always been subject to wear. Corrosion is a known cause of tappet and cam lobe wear. The engines of aircraft that are not flown regularly may be extremely vulnerable to corrosion. When the film of oil drains from the interior parts of the engine after it has been run, those parts become prey to the chemical changes that are caused by moisture, acids, and oxygen. Tappets from engines that have not been operated for long periods have been carefully examined. Under a microscope, it is not unusual to find microscopic pits on the face of the tappet. This is the beginning of trouble. Starting with these very tiny pits, tiny particles of rust also affect the cam lobes. Once started, the process is not likely to stop until the wear reaches a point where these parts are doing an unacceptable job. Some people might question the assertion that engines can attract unusually large amounts of moisture: brief operating periods, low engine oil operating temperatures and condensation all contribute. It might be astonishing to take an engine that has flown 15 to 25 hours over four to six months and drain the oil into a clear container. Total Training Support Ltd © Copyright 2020

The amount of water that settles to the bottom is likely to be more than one would expect. Also, remember that combustion causes acids to collect in the oil. When these are not removed by regular oil changes, the acids, as well as the moisture, promote the growth of microscopic pitting, which eventually leads to excessively worn tappets and cam lobes. Another factor in the unacceptable rise of tappet spalling in general aviation engines may be the component that is put into many of those engines at overhaul. There is an increasing tendency to put reground camshafts and tappets into these engines, to reduce the costs of the overhaul. Although camshafts may be reground, there is a stringent limit on the amount of grinding which can be tolerated. Grind too much, and the hardened surface of the cam lobe is gone. After this kind of grinding, the cam may look great, but it will be wearing on the soft metal, which was once protected by a hardened surface. Lycoming does not recommend the use of reground tappets under any circ*mstances, but many engines overhauled in the field today come back to the owner with reground tappets and camshaft. In some cases, at least, these items are nothing more than good looking scrap. Because of the high percentage of refurbished used parts that go into many overhauls, and many aeroplanes sitting for long periods without being flown, there could be more tappet spalling today than in the past. Ask about the parts which are going into your overhaul. It may be less expensive to pay for new parts at the time of overhaul than it is to pay for replacing worn out parts before the engine has reached its expected TBO.

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Camshaft and lifter wear

Camshaft corrosion

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Connecting rods Function The connecting rod carries the forces produced by the pistons into the crankshaft. These rods must be strong and stiff enough to resist bending during the power stroke, and at the same time, they must be light enough to reduce the inertia as the rod and piston stop, then change direction at the top and bottom of the stroke. The connecting rod is connected to the crankpin at the big end or crankpin end. The other connection is at the piston end and is known as the small end or piston pin end. Connecting rods are generally made from alloy steel, but lowpower output engines may have an aluminium alloy connecting rod. A heavy connecting rod would produce high inertia forces during its reciprocating motion, so, to reduce the weight of the connecting rod, the cross-sectional area is usually a letter H or I in shape. The connecting rods are made from alloy steel forgings. They have replaceable bearing inserts in the crankshaft ends and split type bronze bushings in the piston ends. The bearing caps on the crankshaft end of the rods are retained by two bolts through each cap secured by a crimp nut. On most engines, the big end of the connecting rod is split, and both the body of the rod and the cap have cylinder identification numbers stamped on them to prevent them being mismatched during overhaul. Two-piece bearing inserts are installed in the big end and are held in place by tangs on the inserts that fit into slots cut into the cap and body. The tangs prevent the insert from spinning inside the rod. Some connecting rods prevent the bearing insert spinning by a short dowel pin pressed into the body of the rod that fits into a hole in the insert. Total Training Support Ltd © Copyright 2020

The outside diameter of the bearing insert is approximately a 0.076 mm (0.003") interference fit in the cap and rod bore. This fit helps to prevent the bearing from spinning in the rod, and aids with heat transfer for cooling. High-strength nuts and bolts hold the cap onto the rod body. The torque applied to the nuts on these bolts is critical because it must produce tensile stress in the bolt higher than the stress that is applied by the hammering action the rod receives in operation. The small end of the connecting rod has a bronze bushing pressed into its hole, and the bushing is reamed to the correct fit for the wrist pin. The reaming must be precise to keep the axis of the bushing parallel to that of the large-end bearing. During the overhaul, the lack of parallelism is measured as twist and convergence. These limits must be held to approximately 0.025 mm (0.001") off true over 25 mm (1"). If this tolerance is not held, the piston can be pinched in the cylinder, the full surface of the bearings involved does not bear the load, and the rod and piston pin work back and forth excessively. This condition causes excessive wear and possible parts failure. Connecting rods can be divided into three categories: • • •

plain type, associated with in-line engines or opposed type engines; fork and blade type, as used in the V-type engines; and master and articulated rod type fitted to radial engine cylinder arrangements.

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Connecting rod and piston assembly Connecting rods – small end and big end

Connecting rod, crankshaft and piston assembly

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The plain connecting rod This type is associated with in-line or opposed engines. The connecting rod below has a letter ‘I’ cross-sectional shape and consists of a small end bush and a big end bush, which is always split into two sections known as the big end bearing shells. A retaining cap holds the shells around the crank pin. The cap is either fitted to the connecting rod using nuts and bolts, or studs may be fitted to the connecting rod. The small end bush is usually of a bronze type material and is force-fitted into the small end of the connecting rod. Through this bush is fitted the pin that attaches the piston to the connecting rod (the piston or gudgeon pin). The crankshaft and connecting rods are usually balanced during manufacture; therefore, it is essential to retain an even balance, and any rods that are removed from the crankshaft are replaced in the same order when reassembling after a major overhaul. To assist you, however, you may find that the connecting rods are numbered, corresponding to the cylinders to which they are to be fitted.

Weight code

Weight

Light

B S

Service rod (STD)

D E

Heavy

Weights can be mixed with care. For example, an ‘A’ weight conrod could be installed with an ‘S’ weight, but not with a ‘D or E’ as this would result in vibration. Correct usage Standard and light rods ‘A’ and ‘B.’ Standard and heavy rods ‘D’ and ‘E.’ Incorrect usage Standard with a mixture of ‘A’, ‘B’, ‘D’, ‘E.’

The connecting rods are numbered on the rod and rod end to ensure that they remain a matched pair and in the correct running position within the engine. Lycoming produces conrods in five different weights of categories a careful engine build procedure is required to prevent vibration and ensure an even engine wear characteristic.

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Connecting rod assembly

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Connecting rod assembly

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Piston, cylinder and connecting rod assembly

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The fork and blade connecting rod This method is used in engines where the cylinders are arranged in a V-shape. Here the fork is divided at the big end bearing to provide space for the blade rod to the fitted and secured as shown below left. The forked rod has a two-piece bearing shell and associated retaining caps attached by either bolts or studs. The blade rod fits between the two retaining caps of the forked rod and is retained on the shells by a single retaining cap secured by bolts and studs. An adaptation of this method consists of a master rod and an offset articulated rod; the example shown below right is from a Daimler Benz DB601. The unusual feature of this engine was that the crankpin was fitted with three rings of roller bearings to reduce running friction.

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Connecting rods used in V-type engines

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The master and articulated rod This type of connecting rod arrangement, shown below left is usually fitted to the radial engine configuration. The master rod is fitted to a piston in one cylinder, and the other pistons in that row are then connected to the master rod by the articulated rods. These are attached to the master rod flange by a series of buckle pins. These pins usually are force-fitted on to the master rod flange to allow the articulated rods to move during rotation of the crankshaft. Locking plates either side of the master rod flange ensure that the knuckle pins are retained in the flange. In this connecting rod arrangement, only the master rod big end is fitted to the crankpin, thus reducing the length of the crankshaft. Big end shells usually are lubricated via the hollow crankshaft, and the small end and knuckle pins are usually splash lubricated from oil passages within the connecting rods.

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Master and articulated rod arrangement

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Cylinders General arrangement The portion of the engine in which the power is developed is called the cylinder. The cylinder provides a combustion chamber where the burning and expansion of gases take place, and it houses the piston and the connecting rod. Four significant factors need to be considered in the design and construction of the cylinder assembly. It must: 1. be strong enough to withstand the internal pressures developed during engine operation; 2. be constructed of lightweight metal to keep down engine weight; 3. have excellent heat-conducting properties for efficient cooling; and 4. be comparatively easy and inexpensive to manufacture, inspect, and maintain. The cylinder head of an air-cooled engine is generally made of aluminium alloy because it is a good conductor of heat, and its light weight reduces the overall engine weight. Cylinder heads are forged or die-cast for greater strength. The inner shape of a cylinder head is generally semi-spherical. The semi-spherical shape is stronger than conventionalist design and aids in a more rapid and thorough scavenging of the exhaust gases.

The cylinder used in the air-cooled engine is the overhead valve type. Each cylinder is an assembly of two major parts: cylinder head and cylinder barrel. At assembly, the cylinder head is expanded by heating and then screwed down on the cylinder barrel, which has been chilled. When the head cools, and contracts and the barrel warms up and expands; a gastight joint results. The majority of the cylinders used are constructed in this manner using an aluminium head and a steel barrel. Often, the cylinder is narrower at the top than at the bottom. This is to allow for expansion when the engine is at full working temperature. The top of the cylinder generally gets hotter than the lower part, due to the combustion taking place at the top. The cylinder, therefore, is divided into two areas: • •

the cylinder head; and the cylinder barrel.

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Cylinder head and barrel and piston assembly - sectioned

Cylinder head and barrel assembly

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Cylinder heads The purpose of the cylinder head is to provide a place for combustion of the fuel/air mixture and to give the cylinder more heat conductivity for adequate cooling. The fuel/air mixture is ignited by the spark in the combustion chamber and commences burning as the piston travels toward the top dead centre (top of its travel) on the compression stroke. The ignited charge is rapidly expanding at this time, and pressure is increasing so that, as the piston travels through the top dead centre position, it is driven downward on the power stroke. The intake and exhaust valve ports are located in the cylinder head along with the spark plugs and the intake and exhaust valve actuating mechanisms. After the cylinder head is cast, the spark plug bushings, valve guides, rocker arm bushings, and valve seats are installed in the cylinder head. Spark plug openings may be fitted with bronze or steel bushings that are shrunk and screwed into the openings. Stainless steel Heli-Coil spark plug inserts are used in many engines currently manufactured. Bronze or steel valve guides are usually shrunk or screwed into drilled openings in the cylinder head to provide guides for the valve stems. These are generally located at an angle to the centre line of the cylinder. The valve seats are circular rings of hardened metal that protect the relatively soft metal of the cylinder head from the hammering action of the valves (as they open and close) and the exhaust gases.

The cylinder heads of air-cooled engines are subjected to extreme temperatures; it is, therefore, necessary to provide adequate cooling fin area and to use metals that conduct heat rapidly. Cylinder heads of air-cooled engines are usually cast or forged. Aluminium alloy is used in the construction for several reasons. It is well adapted for casting or for the machining of deep, closely spaced fins, and it is more resistant than most metals to the corrosive attack of tetraethyl lead in gasoline. The most significant improvement in air cooling has resulted from reducing the thickness of the fins and increasing their depth. In this way, the fin area has been increased in modern engines. Cooling fins taper from 2.3 mm (0.09") at the base to 1.5 mm (0.06") at the tip end. The arrangement of these fins increases the total cooling area of the cylinder head by as much as 500%, thus affording a substantial increase in cooling efficiency. Because of the difference in temperature in the various sections of the cylinder head, it is necessary to provide more cooling-fin area on some sections than on others. The exhaust valve region is the hottest part of the internal surface; therefore, more fin area is provided around the outside of the cylinder in this section. Cylinder heads can be manufactured in one of three basic shapes: • • •

flat; peaked; and domed.

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3-69 Module 16.3 Engine Construction

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Cylinder head

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Cylinder head, barrel and valves/valve springs assembly

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Cylinder head temperatures The cylinder head temperatures on an air-cooled engine are critical to prevent damage to the head, either caused by shock cooling or excessive temperature. Shock cooling is detrimental to the health of the engine; Lycoming recommends that changes to CHT should be limited to 10°C (50°F) per minute. Operations that tend to induce rapid engine cooling are dropping parachutists or a glider towing. Typical engine problems include: • • • • •

excessively worn rings grooves and broken rings; cracked cylinder heads; warped exhaust valves; bent pushrods; and spark plug fouling.

Use of excessive temperature can result in: • • • • •

annealing of the cylinder head; loose valve guides; loose valve seats; cracked heads; and loose studs.

To avoid engine damage, most general aviation aircraft take the CHT off the hottest cylinder of the four-cylinder, six-cylinder or eight-cylinder power plant, determined by extensive flight tests. Optional installations offer readings from all cylinders. In Lycoming engines, all cylinders are drilled to accommodate a CHT bayonet-type thermocouple. Minimum in-flight CHT should be 65°C (150°F), and maximum in most direct-drive normally-aspirated Lycoming engines is 260°C (500°F). Some of their higher-powered more complex engines have a maximum limit of 245°C (475°F). Although these are minimum and maximum limits, the pilot should operate their engine at more reasonable temperatures to achieve the expected overhaul life of the power plant. Engines benefit during continuous operation by keeping CHT below 205°C (400°F) to achieve the best life and wear of the power plant. In general, it would be normal during all-year operations, in climb and cruise to see head temperatures in the range of 175°C to 225°C (350°F to 435°F).

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Cylinder arrangement

Cylinder head shapes 3-73 Module 16.3 Engine Construction

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Cylinder barrels The cylinder barrel is machined from a high-strength chromemolybdenum or chrome-nickel-molybdenum steel forging. The bottom end of the cylinder barrel has a skirt machined to fit into the crankcase. The heavy flange just above the skirt is drilled with holes through which the mounting studs pass to attach the cylinder to the crankcase. The cylinder skirt allows the reduction of the engine width by placing a portion of the cylinder stroke area inside the crankcase. For inverted cylinders, the skirt also reduces the amount of engine oil that would otherwise drain down into the combustion area after the engine shutdown. Cooling fins are machined on the outside of the barrel for most of its length. The outer surface of the top of the barrel is threaded so it can be screwed into the cylinder head. Many cylinder barrels are ground so that their diameter at the top of the bore is smaller than at the centre or bottom. The reason for this process, called choke-grinding, is that the highest operating temperature in the cylinder is at the top, where the barrel screws into the head, and when the engine is hot, the upper end of the cylinder expands more than the rest. The smaller diameter at the top causes the bore to become straight when the cylinder is at its operating temperature. This provision drastically reduces upper piston and ring wear and is one of the primary changes responsible for extending engine operating time between overhauls (TBO). The inner surface of the cylinder barrel is ground to a specified dimension. Then the surface is honed with a 45° cross-hatched finish that is specified by the engine manufacturer. The degree of surface roughness is significant because it must be smooth enough not to cause excessive ring wear, yet rough enough that it holds oil for lubrication and proper ring-face mating to the cylinder wall during engine break-in. Total Training Support Ltd © Copyright 2020

The angle of the crosshatch pattern is also crucial because this determines the proper rate of ring rotation. If rings do not rotate, severe cylinder wall and ring damage occur. The exact surface roughness is specified in terms of micro inches RMS (root mean square). For example, a surface finish of 0.010-0.015 mm (15-25 μ inch) RMS means that the highest and lowest deviation from the average surface can range between 0.010 mm and 0.015 mm (15 and 25 millionths of an inch). This is typical for a honed surface. The surface roughness is measured with an instrument called a profilometer. Some cylinder walls are hardened, either by chrome plating or by a process called nitriding. Chrome-plated cylinders are ground, so their diameter is slightly oversize, and then the inside of the barrel is electrolytically plated with hard chromium to the required dimension. The traditional hard chrome, or channel chrome, used on cylinder walls since the second world war has the characteristic of forming a spider web pattern of minute cracks, or channels, on its surface. When the plating is completed, the plating current is reversed, and some of the chrome is removed. The deplating current removes chrome from the cracks at a much faster rate than it does from the crack-free areas or plateaus. The deplating is continued until the cracks, or channels, are about 0.0045" deep and 0.004" wide. These channels hold enough oil to ensure adequate lubrication. Chrome-plated cylinders have the advantage of better heat transfer and a surface that is harder and more corrosion- and scuff-resistant than plain steel. They have the disadvantage that the lubricating film on the surface of the chrome is not as strong as the film that forms on plain steel cylinder walls.

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Cylinder and head assembly

Cylinder head and barrel assembly

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Chromed cylinders are identified by bands of orange paint around the cylinder base or on some of the fins. Nitriding is a process of case hardening in which the cylinder barrel is heated in an atmosphere of ammonia gas. The alloying elements in the steel combine with the nitrogen from the ammonia to form extremely hard nitrides on the surface. Nitriding is not a plating, and it causes a dimensional growth of 0.0102 mm (0.0004") or less. It hardens the surface to a depth of about 0.05 mm (0.002"), with the hardness decreasing gradually from the surface inward. After nitriding a cylinder, it is honed to give it the desired degree of surface roughness. Nitrided cylinders are identified by a band of blue paint around the base or by blue paint on some fins.

An engine that has recently been overhauled is new, or has had a replacement cylinder installed, should be ‘broken-in’ using straight mineral oils (unless turbocharged). When a cylinder is new, the inner wall surface is not smooth as might be imagined. The objective of the break-in procedure is to rub off any high spots, both on the cylinder wall and the piston rings so that the rings can create a tight gas seal for regular operation. This requires the piston ring to break through the oil film and allow a certain amount of metal-to-metal contact between the components. Once this matching has occurred, the break-in is considered to be complete, and minimal contact occurs after that.

One drawback to nitriding is the tendency of cylinders to rust or corrode. Nitrided cylinders must be kept covered with a film of engine oil, and if the engine is out of service for any length of time, the cylinder walls should be protected with a coating of viscous preservative oil. In more recent years, CermiChrome, CermiSteel, and CermiNil cylinder walls have been used. All of the ‘Cermi’ processes involve embedding ceramic particles into the cylinder wall. These surfaces have experienced limited success. The current edition of the ceramic (silicon carbide) impregnated barrel surfaces, known as NiC3TM, consists of coating the inside of the cylinder with nickel by electrolysis. Suspended in the nickel coating are tiny silicon carbide particles. This system is said to provide excellent wear resistance and lubrication. The nickel base-metal provides corrosion resistance. The porous ceramic particles act as the lubrication reservoir in place of the crosshatching of regular steel or nitride cylinders. Total Training Support Ltd © Copyright 2020

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Cylinder head and barrel – bolted arrangement

Cylinder liquid cooling

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Colour coding (Lycoming) •

•

• •

Azure blue – band around the base of the cylinder or on the top edges of the cylinder head fins between the pushrod tubes indicates nitrided cylinders. Orange – band around the base of the cylinder or on the top edges of the cylinder head fins between the pushrod tubes indicates chromed cylinders. Yellow – above the spark plug hole indicates long reach spark plugs (long reach 20.6 mm (13∕16") short reach 12.7 mm (½")). Green – at base of the cylinder indicates that the barrel is 0.254 mm (0.010") oversize. Yellow – at the base of the cylinder indicates that the barrel is 0.508 mm (0.020") oversize.

Black oxide on the inside of the cylinder barrels is used to provide corrosion protection while parts are in storage

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Cylinder head and barrel

3-79 Module 16.3 Engine Construction

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HTCC combustion chamber The ultimate objective of the aircraft engine combustion system is the best possible thermal efficiency and fuel economy, both directly influenced by the efficiency of the combustion process and design of the combustion chamber. Developments over the years have shown ‘fast burn’ combustion chamber designs to be capable of achieving higher net thermal efficiencies with lessened cyclic variation and reduced knock tendency. Design features which promote fast burn in a combustion chamber are: • • • • •

compact chamber design with short flame travel distance; minimum squish height (compression zone between the flame deck and piston at TDC) generation of chamber turbulence, swirl, and higher inlet port velocity; spark plug location and concentration of chamber volume around the ignition source; and large surface to volume ratio in the end gas region.

‘Squish’ is defined as the gas motion resulting from the compression of the gaseous mixture between that part of the piston closest to the combustion chamber dome or valves at TDC. Compact chamber designs are characterised by small squish heights and tend to yield a more rapid flame front with faster burn rates, which is a practical approach for reducing fuel consumption and octane requirements. Higher intake port velocities are used to promote turbulence, which consequently improves the combustion rate. Intake swirl has also been shown to enhance turbulence and reduce cyclic variability.

during that time of the combustion event when the piston is at TDC, and the effect of squish is strongest. Swirl and squish complement each other since swirl reaches its peak early in the combustion process, and squish reaches its maximum strength later as the piston approaches TDC. It is therefore vital that a new combustion chamber design carefully addresses chamber geometry, intake port velocity, swirl, squish, and spark plug location if higher combustion efficiencies are to be attained. The combustion system incorporated into the Continental Voyager 200/300 engines is a compact fast burn high turbulence combustion chamber (HTCC) which operates at an 11.4:1 compression ratio. The exhaust valve is deeply recessed within a bathtub-shaped chamber type depression to promote swirl and turbulence of the fuel/air mixture. The plane of the inlet valve is located in that portion of the cylinder head, where the critical squish zone is created with the opposed flat piston dome. Nominal squish height is 1.016 mm (0.040"). As the piston approaches TDC during the compression stroke, reaching minimum volume in the squish zone and maximum compression of the charge mixture, the high-velocity rotational flow within the swirl chamber is intensified, thus contributing to a more rapid and efficient combustion process.

Research had indicated that while turbulence without swirl enhances combustion, turbulence with swirl produces even faster burning and lower cyclic variability. Squish provides a practical approach for increasing combustion rates, especially Total Training Support Ltd © Copyright 2020

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HTCC combustion chamber

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Research into the effect of the swirl chamber aspect ratio indicated that a minimum BSFC is obtained between 3.5 and 4.0. The aspect ratio of the HTCC swirl chamber is 3.7. Twin spark plugs are positioned in the swirl chamber. The intake port size and geometry were designed consistent with the swirl and inlet port velocities required for efficient combustion. With HTCC, the Voyager 200 and 300 engines have achieved up to 20% better fuel economy and 10% higher horsepower as compared to the air-cooled counterparts at 7:1 compression ratio. Brake thermal efficiencies as high as 36% have been attained naturally aspirated. A 39% brake thermal efficiency has been demonstrated on the 4.9 L (300 in3) engine with reduced exhaust backpressure simulating the effect of higher efficiency turbochargers at altitude. Knock characteristics for the naturally aspirated IOL-200 and IOL-300 engines with conventional 100LL aviation gasoline are considered satisfactory. However, the use of turbocharging, which results in higher inlet air temperatures to the engine, may require operational strategies such as spark retard or reduced manifold pressure to maintain adequate knock margins.

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Continental Voyager 200 liquid-cooled engine

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Water-cooled engine cylinder block and liners Each cylinder block is a single aluminium casting comprising the head and the coolant jacket, ‘wet’ cylinder liners of steel are provided, having shoulders that fit against the crankcase and the cylinder block, respectively, at either end.

Local distortion of each top joint face, as when subjected to sudden increase or decrease in temperature, is reduced by saw-cutting the casting laterally between each combustion space thus, allowing a certain amount of flexibility.

A coolant joint around the base of each liner is made using a rubber ring, spring-loaded in an external groove in the liner. The coolant jackets do not contact the crankcase; any leakage from the joints is carried outside of the engine.

The primary coolant pipe delivers to each block at its outer side towards the rear lower end. Coolant circulates through the block, finally leaving via three outlet holes, one at each end and one at the centre at the upper end, inlet side (inside) to connect with the main outlet pipe discharging either forwards or rearwards according to installation requirements.

A joint ring of aluminium alloy is arranged between the upper shoulder of each liner and the cylinder block. The resulting joints are maintained using fourteen long studs which extend from the crankcase through to the tops of the blocks. The whole reaction of these studs is taken by the cylinder liners and ensures healthy joints at either end. Oil leaks from the crankcase are also prevented by another rubber ring, pressed by the liner flange into a chamfer in the spigot-engaging bore. Except for the four end studs, the remaining ten pass through stainless-steel tubes. These form oil return ways and make coolant joints at either end. They use two rubber rings at their upper end and one at their lower end, held in annular recesses in the block casting and allowing for relative expansion and slight flexibility.

Each main outlet pipe is built up in two sections, having an intermediate gland. A restriction hole at the rear pipe joint tends to reduce circulation at this end thus evening out the temperatures throughout the block. An air vent plug is fitted at the top side of each pipe section, front or rear, for use during filling operations. The individual liners were replaceable during an engine overhaul.

The tube ends are serrated and slightly expanded at their upper ends, the lower ends projecting slightly and extending into a recess in the crankcase where a rubber ring forms an oil-tight joint. Total Training Support Ltd © Copyright 2020

3-84 Module 16.3 Engine Construction

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3-85 Module 16.3 Engine Construction

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Pistons General assembly and types The piston moves in the cylinder transmitting the forces from the burning gases to the crankshaft via the connecting rod. As the piston moves to the bottom of the cylinder, the air/fuel mixture is drawn in through the inlet valve. On its upward stroke, this mixture is compressed and ignited, the resultant burning and expansion forcing the piston back down to the bottom of the cylinder, thus rotating the crankshaft. As the piston moves back up, the exhaust valve opens, and the piston pushes the exhaust gases out into the exhaust manifold.

The compression ratio of an engine can easily be changed by changing the pistons. Installing a piston whose head comes closer to the cylinder head decreases the volume of the cylinder when the piston is at the top of its stroke and increases the compression ratio. The heads of many of the pistons are flat, and if the piston comes near enough to the cylinder head that the valves could touch it, recesses may be cut in the piston head to provide the needed clearance. The compression ratio may be increased by using domed pistons or pistons whose heads are in the shape of a truncated (cut off) cone.

Pistons are generally manufactured from forged aluminium alloy and have a series of circumferential grooves cut into the piston skirt to accommodate a series of rings that are designed to prevent the loss of the compressed gases during operation. Bosses within the piston assembly allow the piston to be attached to the connecting rod small end by a piston pin, also known as a gudgeon pin, see the diagram below right. The piston pin can either be fully floating, being able to rotate and slide within the piston bosses and connecting rods, or of the stationary type, where movement is not permitted, and the pin is locked in place. The inside of the hollow piston has a series of fins for cooling purposes. These fins present a sizeable cooling surface for the lubricating oil to impinge upon and carry away some of the heat generated within the cylinder head into the oil system.

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Piston assembly and nomenclature Piston types

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Piston rings The functions of a piston ring are to seal off the combustion pressure, to distribute and control the oil, to transfer heat, and to stabilise the piston. The piston is designed for thermal expansion, with the desired gap between the piston surface and liner wall. The rings and the ring grooves form a labyrinth seal, which isolates the combustion chamber from the crankcase relatively well. The position and design of the ring pack are shown below. The ring face conforms to the liner wall and moves in the groove, sealing off the route down to the crankcase. The sealing ability of the ring depends on several factors, like ring and liner conformability, pre-tension of the ring, and gas force distribution on the ring faces. Some of the combustion chamber heat energy is transferred through the piston to the piston boundaries, i.e., the piston skirt and rings, from which heat transfers to the liner wall. Furthermore, the piston rings prevent excess lubrication oil from moving into the combustion chamber by scraping the oil from the liner wall during the downstroke. The piston rings support the piston and thus reduce the slapping motion of the piston, especially during cold starts where the clearance is higher than in running conditions. The rings are generally open at one location, at the ring gap, hence easily assembled onto the piston.

A piston ring material is chosen to meet the demands set by the running conditions. Furthermore, the material should be resistant to damage, even in emergency conditions. Elasticity and corrosion resistance of the ring material is required. Any ring coating must work well with the ring and liner materials, as well as with the lubricant. As one task of the rings is to conduct heat to the liner wall, good thermal conductivity is required. Grey cast iron is used as the primary material for piston rings. It is beneficial, as a dry lubrication effect of the graphite phase of the material can occur under conditions of oil starvation. Furthermore, the graphite phase can act as an oil reservoir that supplies oil at dry starts or similar conditions of oil starvation. The piston has four holes located around the oil scraper ring to permit oil flow between the cylinder and the crankcase. The scraper ring removes excess oil and permits oil from the crankcase side of the piston to flow through to the cylinder wall, therefore, depositing an even oil film. Piston rings are divided into two basic types: • •

compression rings; and oil rings.

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Piston ring arrangement

Connecting rod and piston assembly

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Compression rings Compression rings prevent the compressed gases from leaking past the piston within the cylinder. The number of rings fitted to the piston depends mainly on the design requirements of the engine and the pressure created within the cylinder head during operation. In normal circ*mstances, there are three compression rings located at the top of the piston skirt. The positioning of these rings, within the piston skirt grooves, to each other is most important. If new rings are fitted, there will be a gap at the joint. Gases can escape through this gap, and if all three gaps were lined up, then there would be a continuous gas leak past the piston.

Oil rings The oil rings can be divided into two types:

The ring joints should be staggered around the circumference of the piston, as shown in the diagram below top-left, to reduce the possibility of gas leakage.

The oil wiper or scraper rings are generally located at the bottom of the piston skirt, and their purpose is to regulate the amount of oil passing between the piston skirt and the wall of the cylinder during the piston strokes.

Sideways movement of the piston rings within their grooves is essential to allow the rings to expand against the cylinder walls, but too much movement or wear allow gases to escape. The diagram below bottom-left illustrates how side clearance can be measured. Such clearances are specified in the appropriate overhaul manual.

When replacing any type of piston ring, you must follow the manufacturer’s instructions. In some cases, fitting the piston rings in reverse can have an opposite effect to that for which they were designed, and could lead to early failure of the engine.

• •

oil control rings; and oil wiper or scraper rings.

The location of the oil control ring usually is directly below the compression rings. Its purpose is to control the oil from thickness on the cylinder wall. Too much oil on the wall may lead to excessive build-up of carbon deposits within the cylinder head, affecting the operation of the valves and leading to reduced efficiency of the engine.

The diagram below right shows a cross-section through a piston wall. The top three rings are compression rings. At the centre is the oil control ring. The bottom ring is an oil scraper ring. On the right is an enlarged view of a keystone compression ring.

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Compression ring assembly

Piston cross-section

Ring/groove clearance check

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Piston ring positions on a piston

Piston ring interface with cylinder surface

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Engine assembly – sectioned

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Valve mechanisms General arrangement For a reciprocating engine to operate correctly, each valve must open at the proper time, stay open for the required length of time, and close at the proper time. Intake valves are opened just before the piston reaches the top dead centre, and exhaust valves remain open after top dead centre. At a particular instant, therefore, both valves are open at the same time (end of the exhaust stroke and beginning of the intake stroke). This valve overlap permits better volumetric efficiency and lowers the cylinder operating temperature. This timing of the valves is controlled by the valve-operating mechanism and is referred to as the valve timing. The valve lift (the distance that the valve is lifted off its seat) and the valve duration (length of time the valve is held open) are both determined by the shape of the cam lobes. The portion of the lobe that gently starts the valve operating mechanism moving is called a ramp or step. The ramp is machined on each side of the cam lobe to permit the rocker arm to be eased into contact with the valve tip, thus reducing the shock load which would otherwise occur. The valve operating mechanism consists of a cam ring or camshaft equipped with lobes that work against a cam roller or a cam follower. The cam follower pushes a push rod and ball socket, actuating a rocker arm, which in turn opens the valve. Springs, which slip over the stem of the valves and are held in place by the valve-spring retaining washer and stem key, close each valve and push the valve mechanism in the opposite direction.

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Typical in-line engine valve gear mechanism

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Valve operating mechanism

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Engine assembly – sectioned

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Radial engine valve mechanism The valve mechanism of a radial engine is operated by one or two cam rings, depending upon the number of rows of cylinders. In a single-row radial engine, one ring with a double cam track is used. One track operates the intake valves; the other operates the exhaust valves. The cam ring is a circular piece of steel with a series of cams or lobes on the outer surface. The surface of these lobes and the space between them (which the cam rollers ride on) is called the cam track. As the cam ring revolves, the lobes cause the cam roller to raise the tappet in the tappet guide, thereby transmitting the force through the pushrod and rocker arm to open the valve. In a single-row radial engine, the cam ring is usually located between the propeller reduction gearing and the front end of the power section. In a twin-row radial engine, a second cam for the operation of the valves in the rear row is installed between the rear end of the power section and the supercharger section. The cam ring is mounted concentrically with the crankshaft and is driven by the crankshaft at a reduced rate of speed through the cam intermediate drive gear assembly. It has two parallel sets of lobes spaced around the outer periphery; one set (cam track) for the intake valves and the other for the exhaust valves.

The cam rings used may have four or five lobes on both the intake and the exhaust tracks. The timing of the valve events is determined by the spacing of these lobes and the speed and direction at which the cam rings are driven in relation to the speed and direction of the crankshaft. The method of driving the cam varies on different makes of engines. The cam ring can be designed with teeth on either the inside or outside periphery. If the reduction gear meshes with the teeth on the outside of the ring, the cam turns in the direction of rotation of the crankshaft. If the ring is driven from the inside, the cam turns in the opposite direction from the crankshaft. A four-lobe cam may be used on either a seven-cylinder or nine-cylinder engine. On the seven-cylinder, it rotates with the crankshaft, and on the nine-cylinder in the opposite direction. On the nine-cylinder engine, the spacing between cylinders is 40°, and the firing order is 1-3-5-7-9-2-4-6-8. This means that there is a space of 80° between firing impulses. The spacing on the four lobes of the cam ring is 90°, which is higher than the spacing between impulses. Therefore, to obtain proper relation of valve operations and firing order, it is necessary to drive the cam opposite the crankshaft rotation. Using the fourlobe cam on the seven-cylinder engine, the spacing between the firing of the cylinders is greater than the spacing of the cam lobes. Therefore, the cam must rotate in the same direction as the crankshaft.

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Valve-operating mechanism (radial engine) The annular ring with contoured circumference is a radial engine’s cam. Several of its pushrod rollers can be seen.

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Valve operating mechanism – radial engine

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3-101 Module 16.3 Engine Construction

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Valves The valves which are located in the cylinder head perform the following functions: • •

some allow the air/fuel mixture into the cylinder and are known as the inlet valve; and the others allow burnt exhaust gases out of the cylinder and are known as exhaust valves.

They are controlled by a valve train or gear. This mechanism is discussed later. Most aircraft engine valves are known as poppet valves and fall mainly into the types shown below top-right, the flat, mushroom, and the tulip type. Due to the difference in operating temperatures, the inlet and exhaust valves may be manufactured from different materials, such as: • •

inlet valves exhaust valves

– chrome-nickel steel; and – cobalt-chromium steel.

The inlet valve is cooled by the incoming fuel/air mixture and is already closed and sealed as the exhaust valve is opening. However, the exhaust valve seat and head are subjected to the high exhaust gas temperatures. Some exhaust valves, because of the heat exposure and subsequent distress caused by high temperatures, have hollow stems filled with metallic sodium that melts at normal working temperatures. The reciprocating action of the valve throws this liquid sodium from the head end along the valve stem, dissipating the heat from the valve head through the cylinder head fins, below right. Total Training Support Ltd © Copyright 2020

Valves that have this metallic sodium in the stem should not be cut into on any account, as this could cause an explosion and personal injury. The valves must operate in regions of extremely high temperatures and must be made of materials that are not affected by these temperatures. A valve stem may have hardened surfaces to reduce the wear between stem head and rocker, and the cylinder head seat land has a bead of hardened steel welded to it and ground to mate with the valve face. This helps it to withstand the continuous hammering between the face and the valve seat. The exhaust valve seat and face are usually ground to an angle of 30° or 45°. Examples of correct and incorrect mating are shown below bottom-left. A valve is closed by two or more concentrically mounted coil springs, coiled in opposite directions, of a high-grade steel wire that is not affected by the temperatures generated in the cylinder head. Using two or more springs with different vibrations frequencies prevents the valve from bouncing on its seat when it closes. It does this by disrupting the natural frequency of oscillation of the single spring. The valve and springs are held in place on the cylinder head by a valve spring retainer. The retainer fits over the stem of the valve and the ends of the springs. The retainer is locked in place by conical-shaped collet halves that fit into grooves located at the upper end of the valve stem. The springs push the retaining cap against the collets, maintaining the valve in its guide.

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Types of valve

Sodium cooled exhaust valve

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Symptoms of valve sticking •

• • •

Morning sickness – where the first start of the day is rough. The pilot may suspect oil fouling on the sparkplug and lean the engine off to increase the cylinder head temperature, the engine may respond by running smoothly, but it is a dissimilar thermal expansion that has released the valve. Roughness during climb or in cruise – the pilot, may suspect water in the fuel. High magneto drop – typically a 300/400 RPM drop. Intermittent rough idle – this can be caused by carburettor icing at 4ºC (39°F) or below, the pilot should select carb heat. If the engine runs smoothly, it was the formation of ice; if it remains, it is likely to be a valve sticking.

Causes of valve sticking • • •

• • •

Dirty oil. High cylinder temperatures (baffles and cowls have a significant effect on CHT). Improper leaning. Too lean – resulting in a CHT, which is too high. Too rich – resulting in too much lead – the Lycoming engines were designed to run on 80L, 100LL contains four times as much lead, which leads to lead fouling. Air filter – silicon created in the engine results from sand ingestion. Incorrect overhaul procedures Incorrect fuel or oil used.

Recommendations to avoid valve sticking Depending on the degree of intensity, sticking between the valve stem and guide can severely restrict the valve’s opening and closing movements. A sticking valve condition is often identified by an intermittent hesitation, or miss, in engine speed. Valve sticking can be promoted by contaminants in the oil and by combustion residues. These form deposits on the stem and guide that interfere with the stem’s movements. If the valve cannot open or close properly, incomplete combustion results. This, in turn, can lead to the formation of more deposits and increased valve sticking. The wrong grade of fuel can also contribute to valve sticking. For example, the extensive use of fuel with lead content that is higher than recommended can intensify the formation of lead deposits. Lycoming warns against using any brand of automotive fuel in its engines. Several procedures can be undertaken to prevent, or at least minimise, the formation of lead, varnish, and carbon deposits, which are the prime reasons for valve sticking. Make sure the engine operates with a clean air filter. If the engine is exposed to extremely dusty conditions, the time intervals between filter maintenance should be reduced accordingly. It is essential, too, that the air filter has a good seal, and the rest of the air-induction system has no leaks for unfiltered air to enter.

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Valve shapes in cross-section

Valve nomenclature

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Another means of minimising contaminant build-up is to keep the lubrication system clean. Textron Lycoming recommends a 50-hour interval oil change and filter replacement for all engines using full-flow filtration system and 25-hour intervals for an oil change and screen cleaning for pressure screen systems. Refer to SB 480. If the aircraft is not flown regularly, the risk of valve sticking is increased due to the build-up of moisture, acids, gums, and lead sludge in the oil. Operating the engine for sustained periods, as when flying, vaporises harmful moisture and eliminates most of the other contaminants responsible for valve sticking. Infrequent periods of ground running that do not allow the engine to reach operating temperature can also contribute to valve sticking. On the other hand, if the engine is ground-run for too long, overheating may become a problem. Another drawback to prolonged ground running is that the engine operates on a richer mixture than when flying. During flight at cruise power, the mixture is usually leaned, and much of its lead content vaporises. Ideally, the engine should be leaned to peak exhaust gas temperature (EGT) at cruise power settings. This produces optimum combustion and lessens contaminant build-up. The pilot’s operating handbook should be consulted for proper leaning procedures. Even in flight, an engine can overheat. For example, if the baffles that direct cooling air over the cylinders are deteriorated or improperly fitted, the engine can develop hot spots. The baffles or ducts controlling airflow to the oil cooler must also be maintained in good condition.

Rapid engine cool down from low-power altitude changes, lowpower landing approach, engine shut-down too soon after landing or ground runs should be avoided. Before engine shutdown, Lycoming recommends that the engine speed should be maintained between 1,000 and 1,200 RPM until the operating temperatures have stabilised. At this time, the engine speed should be increased to approximately 1,800 RPM for 15 to 20 seconds, then reduced to 1,000-1,200 RPM and shut-down immediately using the mixture control. The engine should be operated at engine speeds between 1,000 and 1,200 RPM after starting and during the initial warmup period. Avoid prolonged closed throttle idle engine speed operation (when possible). At engine speeds from 1,000 to 1,200 RPM, the spark plug core temperatures are hot enough to activate the lead scavenging agents contained in the fuel, which retards the formation of the lead salt deposits on the spark plugs and exhaust valve stems. Avoid rapid engine speed changes after start-up and use only the power setting required to taxi. Valve temperatures The exhaust valve is the one that is prone to sticking due to being exposed to high temperatures. The exhaust valve stem is filled with powdered sodium to assist in the transfer of heat from the valve head, up through the sodium stem, and through the valve guide into the cylinder head and its cooling fins.

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Valve springs Valves are closed by helical-coil springs. Two springs, one inside the other, are installed over the stem of each valve. If only one spring were used on each valve, the valve would surge and bounce because of the natural vibration frequency of the spring. Each spring of a pair of springs is made of round spring steel wire of a different diameter, and the two coils differ in pitch. Since the progressively wound springs have different frequencies, the two springs together rapidly damp out all spring surge vibrations during engine operation. A second reason for the use of two (or more) valve springs on each valve is that it reduces the possibility of failure by breakage from heat and metal fatigue. The valve springs are held in place by steel valve spring retainers, which are special washers shaped to fit the valve springs. The lower retainer seats against the cylinder head and the upper retainer is provided with a conical recess into which the valve keeper (keys) fit

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Valve springs

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Valve gear components and cylinder assembly

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Valve train nomenclature – pushrod arrangement

Chain driven camshaft

Overhead camshaft and valve assembly

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Valve, rocker and cylinder head assembly

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Double overhead cam (DOHC) arrangement (common on aero-Diesel engines)

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Sleeve valves A sleeve valve takes the form of one or more machined sleeves. It fits between the piston and the cylinder wall in the cylinder of an internal combustion engine, where it rotates, slides, or both. Ports (holes) in the side of the sleeves come into alignment with the cylinder’s inlet and exhaust ports at the appropriate stages in the engine’s cycle. after extensive development, saw substantial use in British aircraft engines of the 1940s, such as the Napier Sabre, and various Bristol engines including the Hercules and Centaurus Knight sleeve-valve engine The first successful sleeve valve was patented by Charles Yale Knight and used twin alternating sliding sleeves. It was used in some luxury automobiles, notably Willys, Daimler, MercedesBenz, Minerva, Panhard, Peugeot, and Avions Voisin. Mors adopted double sleeve-valve engines made by Minerva. The higher oil consumption was heavily outweighed by the quietness of running and the very high mileages without servicing. Early poppet-valve systems required decarbonisation at very low mileages.

It was used by the Scottish company Argyll for its cars and was later adopted by Bristol for its radial aircraft engines. It used a single sleeve which rotated around a timing axle set at 90° to the cylinder axis. Mechanically simpler and more rugged, the Burt-McCollum valve had the additional advantage of reducing oil consumption (compared to other sleeve-valve designs), while retaining the combustion chambers and big, uncluttered, porting area possible in the Knight system. A small number of designs used a ‘cuff’ sleeve in the cylinder head instead of the cylinder proper, providing a more ‘classic’ layout compared to traditional poppet valve engines. This design also had the advantage of not having the piston within the sleeve, although in practice, this appears to have had little practical value. On the downside, this arrangement limited the size of the ports to that of the cylinder head, whereas in-cylinder sleeves could have much larger ports.

The Burt-McCollum sleeve valve The Burt-McCollum sleeve valve was named for the two inventors who applied for similar patents within a few weeks of each other. The Burt system was an open sleeve type, driven from the crankshaft side, while the McCollum design had a sleeve in the head and upper part of the cylinder, and a more complex port arrangement. The design that entered production was more ‘Burt’ than ‘McCollum.’

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Advantages/disadvantages Advantages The design has high volumetric efficiency due to a considerable port opening. Sleeve valve engines demonstrated better mechanical and thermal efficiency over engines with poppet valve design.

The combustion chamber formed with the sleeve at the top of its stroke is ideal for complete, detonation-free combustion of the charge, as it does not have to contend with compromised chamber shape and hot exhaust (poppet) valves.

The size of the ports can be readily controlled, which is essential when an engine operates over a wide RPM range. The speed at which gas can enter and exit the cylinder is defined by the size of the duct leading to the cylinder and varies according to the cube of the RPM. In other words, at higher RPM the engine typically requires larger ports that remain open for a higher proportion of the cycle; this is relatively easy to achieve with sleeve valves but complicated in a poppet valve system.

No springs are involved in the sleeve valve system; therefore the power needed to operate the valve remains largely constant with the engine’s RPM, meaning that the system can be used at very high speeds with no penalty for doing so. A problem with high-speed engines that use poppet valves is that as engine speed increases, the speed at which the valve moves also has to increase. This increases the loads involved due to the inertia of the valve; it must open quickly, be brought to a stop, reverse direction to close and brought to a stop again.

Good exhaust scavenging and a controllable swirl of the inlet air/fuel mixture in single-sleeve designs. When the intake ports open, the air/fuel mixture can be made to enter tangentially to the cylinder. This helps to scavenge when exhaust/inlet timing overlap is used and a wide speed range required. In contrast, poor poppet valve exhaust scavenging can dilute the fresh air/fuel mixture intake to a higher degree, being more speed dependent (relying principally on exhaust/inlet system resonant tuning to separate the two streams).

Large poppet valves that allow proper air-flow have considerable mass and require a strong spring to overcome their inertia when closing. At higher engine speeds, the valve spring may be unable to close the valve before the next opening event, failing to close completely. This effect, called valve float, can result in the valve being struck by the top of the rising piston. Also, camshafts, push-rods, and valve rockers can be eliminated in a sleeve valve design, as the sleeve valves are generally driven by a single gear powered from the crankshaft. In an aircraft engine, this provided desirable reductions in weight and complexity.

Greater freedom of combustion chamber design (few constraints other than the spark plug positioning) means that fuel/air mixture swirl at top dead centre (TDC) can also be more controlled. This gives improved ignition and flame travel which allows at least one extra unit of compression ratio before detonation, compared with the poppet valve engine. Total Training Support Ltd © Copyright 2020

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Longevity, as demonstrated in early automotive applications of the Knight engine. Before the advent of leaded types of gasoline, poppet-valve engines typically required grinding of the valves and valve seats after 32,000 to 48,000 km (20,000 to 30,000 miles) of service. Sleeve valves did not suffer from the wear and recession caused by the repetitive impact of the poppet valve against its seat. Sleeve valves were also subjected to less intense heat build-up than poppet valves, owing to their greater area of contact with other metal surfaces.

Continental in the United States conducted extensive research in single sleeve valve engines, pointing out that they were eventually of lower production cost and easier to produce. However, their aircraft engines soon equalled the performance of single-sleeve-valve engines by introducing improvements such as sodium-cooled poppet valves. Most of these advantages were significantly eroded as fuels improved up to and during World War II, and as sodium-cooled exhaust valves were introduced in high-output aircraft engines.

In the Knight engine, carbon build-up helped to improve the sealing of the sleeves, the engines being said to improve with use, in contrast to poppet valve engines, which lose compression and power as valves, valve stems, and guides wear. Due to the continuous motion of the sleeve (BurtMcCollum type), the high wear points linked to inadequate lubrication in the TDC/BDC of piston travel within the cylinder are suppressed, so rings and cylinders lasted much longer. The cylinder head is not required to host valves, allowing the spark plug to be placed in the best possible location for efficient ignition of the combustion mixture. In very large engines, where the speed of flame propagation limits size and speed, the swirl induced by ports can be an additional advantage. Lower operating temperatures of all power-connected engine parts, cylinder, and pistons. As long as the clearance between sleeve and cylinder is adequately settled, and the lubricating oil film is thin enough, sleeves are ‘transparent to heat.’

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Disadvantages Perfect, even very good, sealing is challenging to achieve. In a poppet valve engine, the piston possesses piston rings (at least three and sometimes as many as eight), which form a seal with the cylinder bore. During the ‘breaking in’ period, any imperfections in one are scraped into the other, resulting in a good fit. This type of ‘breaking in’ is not possible on a sleevevalve engine, however, because the piston and sleeve move in different directions and, in some systems, even rotate in relation to one another. Unlike traditional design, the imperfections in the piston do not always line up with the same point on the sleeve. In the 1940s this was not a significant concern because the poppet valve stems of the time typically leaked appreciably more than they do today so that oil consumption was significant in either case. The 1922–1928 Argyll single sleeve-valve engine, the 12, a four-cylinder 1,491 cc (91 in3) unit, was attributed an oil consumption of one gallon for 1,945 miles, and 1,000 miles per gallon of oil in the 15/30 four-cylinder 2,610 cc (159 in3). Mike Hewland claimed in 1974 that the progress in lubricating oils, materials, and machining had solved the oil thirst problem, his experimental 500 cc (30.5 in3) single-cylinder engines using less oil than their contemporary poppet valve ‘competitors.’ Some proposed an added ring in the base of the sleeve, between sleeve and cylinder wall. Single-sleeve-valve engines had a reputation of being much less smoky than the Daimler with engines of Knight double-sleeve engine counterparts.

The high oil consumption problem associated with the Knight double sleeve valve was fixed with the Burt-McCollum single sleeve valve, as perfected by Bristol. The models that had the complex ‘junk head’ installed a non-return purging valve on it; as liquids cannot be compressed, the presence of oil in the headspace would result in problems. After adding an expander ring that worked in reserve, Mike Hewland found the oil consumption of his single sleeve valve engines was half that of a similar poppet valve engine. “In this engine, all we really have to lubricate is the crankshaft; the rest seems to lubricate itself” (C&D, July 1974). At the top dead centre (TDC), the singlesleeve valve rotates in relation to the piston. This prevents boundary lubrication problems, as piston ring ridge wear at TDC and bottom dead centre (BDC) does not occur. The Bristol Hercules time between overhauls (TBO) life was rated at 3,000 hours, very good for an aircraft engine, but not so for automotive engines. Sleeve wear was located primarily in the upper part, inside the ‘junk head.’ An inherent disadvantage is that the piston in its course partially obscures the ports, thus making it difficult for gases to flow during the crucial overlap between the intake and exhaust valve timing usual in modern engines. Mike Hewland admitted this was a problem at speeds above 10,000 RPM in his engines aimed at racing, but in the middle range, SSV was always better than a poppet valve engine. A severe issue with large single-sleeve aero-engines is that their maximum reliable rotational speed is limited to about 3,000 RPM. However, the M Hewland car engine was raced above 10,000 RPM without issues. Improved fuel octanes, above about 87 RON, have assisted poppet-valve engines’ power output more than to the single-sleeve engines’.

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Sleeve valve closeup from a Bristol Centaurus Mark 175

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The increased difficulty with oil consumption and cylinderassembly lubrication was reported as never having been solved in series-produced engines. Railroad and other large single sleeve-valve engines emit more smoke when starting; as the engine reaches operating temperature and tolerances enter the adequate range, smoke is significantly reduced. For two-stroke engines, a three-way catalyst with air injection in the middle was proposed as the best solution in an SAE Journal article around the year 2000. If stored horizontally, sleeves tend to become oval, producing several types of mechanical problems. Special cabinets were developed to store sleeves vertically to avoid this problem. Equivalent implementations of modern variable valve timing and variable lift are impossible due to the fixed sizes of the ports and essentially fixed rotational speed of the sleeves. It may theoretically be possible to alter the rotational speed through gearing that is not linearly related to the engine speed; however, it seems this would be impractically complex even compared to the complexities of modern valve control systems.

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Bristol Perseus cylinder sleeve

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Tappets The purpose of the tappet is to convert the rotary action of the camshaft lobe into the reciprocating action of the push rod so that the rocker arm opens the valve at the correct time. The tappet is in contact with the pushrod and the camshaft. A spring in the pushrod assembly keeps the tappet against the camshaft and the rocker arm against the valve. Oil is usually ported up the pushrod to the rocker arm for lubrication purposes.

As the camshaft lobe contacts the tappet body, it moves it to the left, closing the plate valve and shutting off the supply of engine oil to the plunger reservoir. This has now created a hydraulic lock, and further movement of the tappet body causes the pushrod to open the valve. As the lobe passes its point of maximum deflection, the spring extends and allows the body to contact the camshaft, again eliminating any clearance in the valve linkage.

The system, however, has one major drawback, that being the need to have valve clearances. A slight clearance between the rocker arm and the valve stem must be maintained to ensure that the valve can close fully. The clearances are adjusted when the engine is cold, but as the engine heats up, thermal expansion takes place. The clearance may reduce to such a degree that the valve may be held open by the rocker arm, causing eventual damage to the valve seat. As clearances may differ between the inlet valves (cold air/fuel going in) and the exhaust valves (hot gases going out), some method must be used to ensure that the valve clearance is always maintained within laid down limits. This is done using hydraulic tappets. When the valve is in the closed position, the spring holds the plunger against the camshaft, thus eliminating any clearance in the valve linkage. Oil is continuously fed from the lubrication system into the plunger reservoir, through the plate valve to the arm and also down the centre of the pushrod to the rocker arm.

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Camshaft

Tappets

Camshaft and tappets

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Solid lifters/tappets Solid lifters or cam followers generally require the valve clearance to be adjusted manually by adjusting a screw and lock nut. Valve clearance is needed to assure that the valve has enough clearance in the valve train to close completely. This adjustment or inspection was a continuous maintenance item until hydraulic lifters were used. Hydraulic valve tappets/lifters Some aircraft engines incorporate hydraulic tappets that automatically keep the valve clearance at zero, eliminating the necessity for any valve clearance adjustment mechanism. A typical hydraulic tappet (zero-lash valve lifter) is shown.

Hydraulic valve lifters are normally adjusted at the time of overhaul. They are assembled dry (no lubrication), clearances checked, and adjustments are usually made by using pushrods of different lengths. A minimum and maximum valve clearance are established. Any measurement between these extremes is acceptable, but approximately halfway between is desirable. Hydraulic valve lifters require less maintenance, are better lubricated, and operate more quietly than the screw adjustment type.

When the engine valve is closed, the face of the tappet body (cam follower) is on the base circle or back of the cam. The light plunger spring lifts the hydraulic plunger so that its outer end contacts the pushrod socket, exerting a light pressure against it, thus eliminating any clearance in the valve linkage. As the plunger moves outward, the ball check valve moves off its seat. Oil from the supply chamber, which is directly connected with the engine lubrication system, flows in and fills the pressure chamber. As the camshaft rotates, the cam pushes the tappet body and the hydraulic lifter cylinder outward. This action forces the ball check valve onto its seat; thus, the body of oil trapped in the pressure chamber acts as a cushion. During the interval when the engine valve is off its seat, a predetermined leakage occurs between plunger and cylinder bore, which compensates for any expansion or contraction in the valve train. Immediately after the engine valve closes, the amount of oil required to fill the pressure chamber flows in from the supply chamber, preparing for another cycle of operation.

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Dry tappet clearance It is a common misconception that if the engine is installed with hydraulic tappets, it requires no clearance checks, as the hydraulic tappet takes up the clearance; this is incorrect. Any time work is done on the valve train of an engine – grinding valves or seats, replacing valves or valve rockers, or any other component of the valve operating mechanism – the dry tappet clearance should be checked and the pushrod length adjusted to ensure that the correct tappet clearance is maintained. If tappet clearance is allowed to vary too far from the prescribed limits, the engine will not operate properly. For example, if clearances are too small, burned valves or compression loss may result; and if clearances are too great, the engine will become noisy. In both cases, the engine becomes rough, and mechanical failure may be the result. The following is a brief description of procedures to check and adjust the dry tappet clearance. After observing all safety precautions, rotate the engine until the piston is on top dead centre (TDC) of the cylinder to be checked.

7. To check dry tappet clearance, depress the hydraulic unit by pressing on the pushrod end of the rocker, and measuring the clearance between the valve stem and heel of the rockers by using a feeler gauge. Pushrods The pushrod, tubular in form, transmits the lifting force from the valve tappet to the rocker arm. A hardened-steel ball is pressed over or into each end of the tube. One ball end fits into the socket of the rocker arm. In some instances, the balls are on the tappet and rocker arm, and the sockets are on the pushrod. The tubular form is employed because of its lightness and strength. It permits the engine lubricating oil under pressure to pass through the hollow rod, and the drilled ball ends to lubricate the ball ends, rocker-arm bearing, and valve-stem guide. The pushrod is enclosed in a tubular housing that extends from the crankcase to the cylinder head, referred to as pushrod tubes.

1. Remove the rocker box cover and also remove rocker shaft covers on angle head cylinders, valve rockers, thrust washer (angle head only), pushrods, and shroud tubes. 2. Then remove the hydraulic unit from the tappet body. 3. Disassemble the hydraulic unit and flush out all oil from the unit. Also, remove all oil from the tappet body. 4. Reassemble the hydraulic unit and install in the tappet body. 5. Next, replace the shroud tubes using new seals. 6. Install pushrods, valve rockers, and thrust washer on angle head cylinders only.

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Adjustment of tappet clearance

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Rocker arms The rocker arms used on horizontally opposed engines are made of forged steel with bronze bushings, which ride on the rocker arm shaft, pressed into them. The end in which the pushrod rides has a socket that fits the hemispherical end of the pushrod. A hole is drilled on exhaust arms and Continental intake arms from this socket to the bushing to allow oil that flows through the hollow pushrod to lubricate the bushing. The rocker arms transmit the lifting force from the cams to the valves. Rocker arm assemblies are supported by a plain, roller, or ball bearing, or a combination of these, which serves as a pivot. Generally, one end of the arm bears against the pushrod and the other bears on the valve stem. One end of the rocker arm is sometimes slotted to accommodate a steel roller. The opposite end is constructed with either a threaded split clamp and a locking bolt or a tapped hole. The arm may have an adjusting screw for adjusting the clearance between the rocker arm and the valve stem tip. The screw can be adjusted to the specified clearance to make sure that the valve closes fully. Some rocker arms are furnished with an adjuster to allow mechanics to adjust the valve clearance when solid tappets are used. Rocker arms with no adjusters are used in conjunction with hydraulic tappets.

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Rocker assembly

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Inlet and exhaust manifolds An inlet manifold (or intake manifold, or induction manifold) is the part of an engine that supplies the fuel/air mixture to the cylinders. In contrast, an exhaust manifold collects the exhaust gases from multiple cylinders into a smaller number of pipes – often down to one pipe. The primary function of the intake manifold is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s). Even distribution is important to optimise the efficiency and performance of the engine. It may also serve as a mount for the carburettor, throttle body, fuel injectors and other components of the engine. Due to the downward movement of the pistons and the restriction caused by the throttle valve, in a reciprocating spark ignition piston engine, a partial vacuum (lower than atmospheric pressure) exists in the intake manifold. This manifold vacuum is proportional to the power being developed by the engine during operation and is often indicated in the co*ckpit for power indication. This vacuum can also be used to draw any piston blow-by gases from the engine’s crankcase. This is known as a positive crankcase ventilation system, in which the gases are burned with the fuel/air mixture.

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Inlet manifold

Exhaust manifold

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Propeller reduction gearboxes The purpose of a reduction gear is to reduce engine speed to a speed suitable for the efficient operation of the propeller. Epicyclic (sometimes known as planetary) reduction gears are always used on radial engines, and spur gear reduction gears are generally used on in-line engines. However, either type may be fitted to horizontally opposed engines.

If the annulus is free, rotation of the sun wheel causes the planet pinions to rotate about their axles within the annulus gear. With the planet pinion carrier fixed and the propeller shaft attached to the annulus gear, rotation of the planet pinions causes the annulus gear and propeller to rotate in the opposite direction to the sun wheel and at a reduced speed.

Parallel spur gears This type of gear train has the advantage of being mechanically simple and, therefore, relatively cheap to manufacture.

Compound spur epicyclic Compound epicyclic reduction gears enable a more significant reduction in speed to be obtained without resorting to larger components. They may be of either the fixed or free annulus type.

Epicyclic reduction gears A gear train, consisting of a sun (driving) gear meshing with and driving, three or more equi-spaced gears known as planet pinions. These pinions are mounted on a carrier and rotate independently on their axles. Surrounding the gear train is an internally toothed annulus gear in mesh with the planet pinions.

Gear train/epicyclic Some turboprops use a gear train or a combination of the gear train and epicyclic.

If the annulus is fixed, rotation of the sun wheel causes the planet pinions to rotate about their axes within the annulus gear; this causes the planet carrier to rotate in the same direction as sun wheel but at a slower speed. With the propeller shaft secured to the planet pinion carrier, a speed reduction is obtained with the engine shaft (input shaft) and propeller shaft (output shaft) in the same axis and rotating in the same direction.

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Planetary gear arrangement, fixed ring gear driven sun gear

Planetary gear arrangement, driven ring gear fixed sun gear

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Bristol Centaurus reduction gearbox

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Parallel spur gears – external and internal

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An epicyclic gear

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Diesel engines – differences and additions Diesel engine construction is generally the same as a gasoline engine construction. Any differences are as a result of the necessity to make the engine more rugged and capable of withstanding the higher compression ratios which are necessary for the operation of Diesel engines. This discussion uses the Thielert TAE 125 series engine as a typical example of the construction of a modern Diesel aero engine. Example: Thielert TAE 125-series The company Thielert Aircraft Engines GmbH, based in Saxony, Germany, declared insolvency on 24 April 2008. The company was then run by an insolvency administrator. In 2013 the company was sold to Continental Motors, Inc. Continental is owned by AVIC International, which is, in turn, wholly owned by the Government of the People's Republic of China. Thielert was renamed Technify Motors GmbH. These products are known by any of their manufacturers’ names; Thielert, Continental, or Technify. Throughout this description, we use Thielert. TAE 125 engines are in-line 4-cylinder diesel engines that were developed based on automotive engines, with modifications to lower the compression ratio. They use high-pressure direct injection (common rail fuel injection system) and are turbocharged.

For use in aviation, Thielert developed a new engine control system, a propeller reduction gearbox including a torsion damper (or clutch), a new engine mount system and other aircraft-specific accessory parts. Initially, Thielert purchased complete engines directly from the production line of the automotive manufacturer. Later, they were able to obtain the parts required separately. The TAE 125-02 engine models use a Thielert-designed aluminium crankcase to avoid weight increase (the automotive engine has a cast-iron crankcase). Some parts from the automotive baseline engine are still used. The engine’s electronic control system and gearbox, including clutch, are similar to those on the TAE 125-01 engine model. On an aeroplane, propulsion is ensured by a variable-pitch propeller driven by a reduction gearbox. The engine and the propeller pitch are fully controlled by a computerised full authority digital engine control (FADEC), which simplifies its use. In the co*ckpit, the pilot inputs a power rating via a single power lever. FADEC, which integrates the measurements of various sensors, manages the quantity of fuel injected and propeller pitch to obtain the power requested. These engines are installed, according to the variants, on Diamond DA40 and DA42 aeroplanes, Cessna 172, Piper PA28 and Robin DR400. Thielert obtained type certificates for two engine types: TAE 125 and Centurion 4.

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Thielert Centurion engine and propeller

Thielert TAE-125 in Diamond DA42 aeroplane

Thielert TAE 125 series engine

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General The diagram below shows a view of the engine without accessories. The engine shown is a 2.0 litre (125.46 in3) turbocharged, DOHC in-line 4-cylinder, four valves per cylinder, common rail Diesel engine with FADEC. It is operated through single-lever power control, is equipped with a reduction gearbox, overload clutch and a variable pitch propeller. The engine provides a power output of 114 kW (155 hp) The engines are based on an automobile engine design (Mercedes A-class), which allows the aviation market to benefit from the substantial development and tooling budgets of the automobile industry. Issues of reliability, longevity and other potential problems which occur during the development of any new design have been resolved long ago. Time and effort can then be spent on issues unique to the aviation application of the engine, such as providing redundant systems where applicable or tuning the engine for operation at higher altitudes and colder temperatures. By using a mass-production engine, the aviation industry benefits from one produced through a state-of-the-art manufacturing process. How Continental Builds Diesel Engines https://youtu.be/hH9EUlNcA4A

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Thielert TAE 125 series engine components

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Because of the low production numbers and the associated tooling cost per manufactured unit, this has not been possible for aviation in the past. Below is a description of key components of the basic aero Diesel engine, in comparison to its Mercedes A-class automotive engine, from which the Thielert engine is derived. • • •

•

• •

The manufacturer has redesigned the intake, exhaust, and turbocharger systems to suit the application. The crankcase of the TAE 125 is replaced with a lowpressure sand-cast aluminium unit. The crankshaft of the TAE 125 is replaced with a forged unit made from SAE 4340 aircraft steel and has five journal bearings. The valve train consists of four valves per cylinder (two intake and two exhaust) actuated by hydraulic rockers, which require no adjustment during the life of the engine. The lifters are actuated by two overhead camshafts which are driven by the crankshaft through a selftensioning, service-free chain. Four valves per cylinder improves the flow into and out of the combustion chamber, again improving efficiency and power. Pistons run in cylinders which are cast in ductile iron liners, plasma-coated cylinder walls, or wet liners (depending on series) for the TAE 125. The liners and coatings are not serviceable. The crankcase is vented to an air/oil separator, from where oil is then scavenged back to the crankcase. The engines produce a peak output of 135-310 hp at 3,900 RPM, so the manufacturer designed the reduction gearbox with a gear ratio of 1.7:1 to achieve a maximum propeller RPM of 2,300. Also, an overload clutch isolates the engine from the propeller.

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Because of the high compression ratios used on Diesel engines, they do not run as smoothly as gasoline engines and stop very abruptly during shut-down. Isolating the engine from the large mass of the propeller reduces wear on the engine and improves comfort. On the TAE 125-series an integral vacuum pump, driven by one of the camshafts, is installed to drive instrumentation. Finally, the manufacturer installs a proprietary electronic engine management system. This is discussed in the following section.

In addition to being fuelled by Diesel or JET-A1, the engine differs from a conventional aircraft piston engine as follows. • •

•

The TAE 125-series are in-line 4-cylinder engines, not horizontally opposed. The engines are liquid-cooled. This has the advantage that the coolant flow can be controlled, and shock cooling during flight conditions of high airspeed and low power is not a problem. They are intercooled. The intake air is cooled after the turbocharger to improve power and efficiency. The high maximum boost pressure between 2,275 and 2,350 mbar and associated temperature rise of the intake air make this necessary.

The main mechanical subsystems are discussed in more detail in the following paragraphs.

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Thielert TAE 125 series engine

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Crankcase The TAE 125-01 4-cylinder in-line crankcase is a single piece, squeeze-cast aluminium part. The material for the crankcase is the high-strength aluminium AISi8Cu3 and is low-pressure sand-cast followed by T7-heat treatment (solution annealing, quenching and artificial ageing). Because the TAE 125-series engines are liquid-cooled, the crankcase incorporates the cylinders as well as the coolant passages. The photos below show an external view of the crankcase. The TAE 125 cylinders are cast-in ductile iron liners and cannot be replaced or serviced. Cylinder spacing is 90 mm; bore is 80 mm. The newer TAE 125-02 has the plasma coated cylinder running surfaces replace the cast-in iron liners. The critical aluminium cylinder surface is coated with iron-molybdenum by plasma spraying. The remaining coat thickness is 120 µm and provides a reliable running surface with less wear and tear compared with the grey cast-iron. Cylinder spacing is also 90 mm, but the bore is enlarged to 83 mm. The crankcase includes provisions for an internal oil pump and a water pump that is mounted internally but driven externally. The external drive of the water pump is also shown below. The crankcase includes a wet oil sump and contains the oil supply of the engine. On the front end of the crankcase, a gearbox flange is integrated, which also provides a clutch housing.

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Steel liners in the cylinders

Aluminium crankcase

Crankcase with bed plate

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Crankshaft The crankshaft of the TAE 125-01 is shown below top-left. It is made of vacuum-remelted forged steel. The crankshaft has five main bearings and fixed counterweights. The crankcase side is fitted with a double layer bearing. The highly stressed cap side is fitted with sputter bearings. Camshaft The camshaft arrangement of the TAE 125-01 is shown below top-centre. It is a double overhead camshaft (DOHC) arrangement. One camshaft operates the inlet valves; the other operates the exhaust valves. The camshaft is driven by a chain drive from the crankshaft. Pistons The pistons are cast aluminium, with cast-in steel ring carriers. The top of the piston is domed and recessed to optimise combustion. The bottom has a cast-in cooling duct design where the engine oil is injected from spray cooling nozzles to limit the maximum material temperature to 360°C at rated power output. Special chrome rings guarantee excellent wear resistance. The piston walls are equipped with Teflon pads to reduce friction and improve resistance to seizing. The photo below bottom-left shows the pistons of the TAE 12502.

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Crankshaft (4-cylinder engine) Double overhead camshaft arrangement

PTFE coating on piston

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Connecting rods The connecting rods are made of forged 70MnVS4 steel. The mating surface of the two halves at the journal bearing is improved by cracking rather than cutting the connecting rod so that the mating surfaces are perfectly aligned at assembly. In the case of overload, the rod tends to become shorter, but it shows no bending tendency. The rod length depending on engine type is 140 mm for the TAE 125-01 and 147.85 mm for the TAE 125-02. The journal bearing diameter is 46 mm and 50 mm. The bushing in the small end has a diameter of 28 mm and 30 mm, respectively. The highly stressed rod side is fitted with a sputter bearing while the cap is fitted with a double layer bearing. The soft surface of the cap multi-layer bearings allows the embedding of engine oil contaminations. The photo below bottom shows a connecting rod with bearing. V-ribbed belt The engine is equipped with a V-ribbed belt at the rear, which drives the coolant pump and the alternator off a crankshaft pulley. The figure below top-right shows the rear (firewall) end of the engine with the V-ribbed belt. The belt is self-tensioned by a spring-loaded pulley. The belt has a time-between-overhaul lifetime.

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V-ribbed belt

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Cylinder head The combustion chamber shape of the engine is defined mainly by the shape of the piston. Flow into and out of the cylinder is further improved by the orientation of the valves and the shape of the intake and exhaust ports. The photos below show the cylinder head with the valves installed. The shape of the intake port is arranged such that the intake air enters the combustion chamber in a swirling pattern to improve combustion efficiency. The location of the fuel injector is central to the four valves and can also be seen below right. The hole immediately next to the injector hole is for the glow plugs, which are used during pre-heating. The remaining openings in the cylinder head are coolant passageways. On TAE 125-02 engines there are two water jackets which reduce the temperatures in the critical areas around the exhaust valve seats and the injector to less than 200°C. A three-layer steel head gasket is used.

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Cylinder Head

Cylinder head sectional view

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Gearbox All Diesel engines are equipped with a gearbox to reduce RPM from the engine (typically 4,000 RPM maximum of the engine to 2,300 RPM at the propeller). The reduction ratio of the gearbox is of the order of 1.7:1. The figures below show the disassembled gearbox of the Thielert TAE 125-01 as an example. The gearbox contains three gears. The gearbox housing itself is machined aluminium or low-pressure sand-cast aluminium (depending on engine type). The centre gear shaft drives the oil pumps mounted at the front of the gearbox used to scavenge engine oil from the turbocharger to the oil sump and to provide gearbox oil pressure to the propeller governor (CSU). Lubrication of the gearbox is achieved primarily through the splash oil from the gears rotating in the oil bath. However, gearbox oil is also drawn from the gearbox through the pump/gearbox oil pump to provide pressure to the constant speed unit (CSU) that provides oil pressure to the propeller. Return oil from the CSU pressure-relief valve provides additional lubrication to the gearbox. Two bearings are also lubricated by pressurised oil. To reduce weight and the number of parts the gearbox housing is equipped with an integrated hydraulic oil gallery. External oil lines between the oil pumpCSU and scavenge pump and oil sump are eliminated. The reason for this installation is a pressure split in the oil gallery to provide high pressure for propeller control and low pressure for lubrication.

Depending on the engine installation design into the fuselage and the maximum power output, a gearbox oil cooler is necessary to keep the gearbox oil temperature within the limitations. All gearboxes with extended service life use a liquidto-liquid heat exchanger, transferring the gearbox heat into the engine’s water-cooling system. The better-regulated temperature of the gearbox reduces the stress to the gearbox and allows for a quick warm-up. Clutch and dual-mass flywheel Aero Diesel engines operate at relatively high torque. The impulse produced from each of the power strokes imparts a high torque impulse into the gearbox and ultimately the propeller shaft. This causes a high level of stress and vibration in the transmission, which is also felt by the aircraft occupants, especially at low engine RPM. There are two conventional methods to decouple (or “dampen”) these vibrations: • •

a clutch; and a dual-mass flywheel.

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Sectioned gearbox

Gearbox

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Clutch It is common on aero Diesel engines, to have a clutch installed between the engine and the gearbox. The purpose of the clutch is to decouple engine vibration from the propeller during regular operation for passenger comfort and to decouple the rotating mass of the propeller from the crankshaft during start-up and shutdown to limit the impact load to the crankshaft.

The diagrams below illustrate the flywheel, starter gear and crankshaft signal ring, which are integrated into the side of the clutch, which is fixed to the crankshaft. An indexing pin accomplishes indexing with reference to the crankshaft. Because of the gearbox and the clutch, propeller position relative to the clutch is not important.

This torque peak limitation also protects the engine from damage in case of a propeller ground strike. In this event, besides a propeller repair, only a gearbox, clutch and clutchshaft inspection plus a precautionary engine shock-mount exchange are necessary.

Later clutch designs The newer clutch design is configured as a single dry-plate overload unit. All components have been redesigned to provide a longer lifetime and a safe and straightforward installation procedure.

Older clutch designs The torsional vibration damping function is accomplished by six sets of progressively stiffer springs installed in the vibration damper module connected to the sintered-metal friction plate (clutch plate) assembly.

The friction plate surface is made out of organic material instead of sintered metal and provides a much larger surface compared to the older clutch design.

The friction calibration to protect the engine from torque peaks (overload) is accomplished by eight sets of Belleville pressure springs and calibration shims installed between the clutch cage and pressure plate. The accurate adjustment of the specific friction torque to around 300 Nm (depending on engine model) is particularly important as a friction torque outside of the limits may damage the engine, clutch, gearbox, and propeller and can result in a total power loss. It is also not allowed to clean the sinter metal friction plate with any solvent.

The torsional vibration damping inside the friction plate hub is accomplished by four double-coil springs. The pressure cage has been eliminated while the functions have been integrated with the flywheel cage and outer pressure plate. The outer pressure plate is firmly fitted by a bayonet to the flywheel cage. The inner pressure plate is floating inside the flywheel cage in an axial direction, while it is locked in a rotational direction. A single spring collar replaces all Belleville spring sets and applies about 5,000 N pressure to the pressure-friction plate set resulting in an overload friction rate of 350-450 Nm.

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Later clutch design assembly

Clutch installation

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Dual-mass flywheel More recent Diesel engine designs have incorporated a dualmass flywheel (DMF), in place of the clutch assembly, installed between the engine and gearbox, as shown in the figure below. The purpose of the dual-mass flywheel is to dampen and isolate engine vibration from the gearbox and propeller during regular operation. It also partly decouples the rotating mass of the propeller from the crankshaft during start-up and shutdown to damp the impact load to the crankshaft and therefore relieves crankshaft, gearbox, and propeller. As the name implies, the dual-mass flywheel splits the flywheel mass into two parts. One flywheel continues to be driven by the engine’s mass moment of inertia, and the other part increases the mass moment of inertia of the gearbox. The separated masses of the primary and secondary flywheel are linked by a spring damping system. Torsional movement of the masses against each other are damped by the arc damper springs with a progressive damping behaviour. The outer springs damp the first stage, and the inner springs damp the second stage. A starter gear and crankshaft signal ring are installed on the engine side of the DMF. Dual-mass Flywheel https://youtu.be/DbvP5EvpUbA

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Dual-mass flywheel installation

Sectional view of dual-mass flywheel

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Sectioned dual-mass flywheel

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Licence Category B1 and B3

16.4 Engine Fuel Systems

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Carburettors

Part-66 Ref.

Knowledge Levels A B1 B3

16.4.1

16.4.2

16.4.3

Types, construction and principles of operation; Icing and heating. Fuel injection systems Types, construction and principles of operation. Electronic engine control Operation of engine control and fuel metering systems including electronic engine control (FADEC); Systems layout and components.

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Table of Contents 16.4.1 Carburettors_______________________________ 6

16.4.2 Fuel injection systems ______________________ 68

Types, construction and principles of operation _____ 6 Purpose _____________________________________ 6 Venturi principles______________________________ 6 Carburettor types ____________________________ 10 Construction ________________________________ 10 Float chamber mechanism system _______________ 12 Pressure balance duct (PBD) ___________________ 14 Main metering system _________________________ 16 Idling system ________________________________ 20 Slow running, idle and cut off ___________________ 24 Mixture control system ________________________ 26 Pressure balance duct (PBD) ___________________ 28 The diffuser _________________________________ 30 The air bleed ________________________________ 32 Automatic mixture control ______________________ 35 Economy system _____________________________ 38 Power and enrichment jet ______________________ 40 Acceleration control___________________________ 44 Example system: Marvel-Schebler MSA MA-4-5 float type carburettor (now Precision Airmotive) _____________ 48

General ______________________________________ 68 Purpose ____________________________________ 68 Principle ____________________________________ 68 The regulator ________________________________ 74 Fuel control unit ______________________________ 78 The throttle body _____________________________ 82

Icing and heating _____________________________ General ____________________________________ Engine factors that affect Ice formation ____________ Atmospheric conditions that affect ice formation _____ Indications of ice formation _____________________ General practices ____________________________ Types of ice _________________________________ Carburettor heat _____________________________ Fuel icing ___________________________________ Fluid de-icing system _________________________ Total Training Support Ltd © Copyright 2020

Example system: Bendix PS pressure carburettor __ 86 Example system: Bendix RSA injection system _____ 90 Example system: Teledyne Continental injection system _____________________________________ 118 Aero-Diesel injection systems __________________ 142 Injection types - general _______________________ 142 Diesel fuel injection types _____________________ 144 Direct injection ______________________________ 144 Common-rail direct injection (CDI) _______________ 146 Example: CDI engine – Thielert TAE 125 _________ 148 Fuel injectors _______________________________ 158 16.4.3 Electronic engine control __________________ 160

56 56 56 56 58 59 60 61 64 66

Development ________________________________ 160 EEC and FADEC _____________________________ 162 Description _________________________________ 162 Advantages ________________________________ 162 Disadvantages ______________________________ 164 Full authority digital engine control (FADEC) ______ 166 General ___________________________________ 166 System layout and components _________________ 168 4-4

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FADEC operation ____________________________ General ___________________________________ FADEC maps ______________________________ Control loops _______________________________ Redundancy _______________________________ Diagnostics ________________________________

170 170 172 174 178 178

Example system – PowerLink FADEC ___________ General ___________________________________ Low voltage harness _________________________ The ECU __________________________________ Electronic control units (ECUs) _________________ Ignition system _____________________________ FADEC fuel injection system ___________________ FADEC sensor set __________________________ Speed sensor assembly (SSA) _________________ Cylinder head temperature (CHT) sensors ________ Exhaust gas temperature (EGT) sensors _________ Manifold air pressure (MAP) sensors ____________ Manifold air temperature (MAT) sensors __________ Fuel pressure sensors________________________ Power supplies _____________________________ Failsafe operating contingencies ________________ Fault detection______________________________

182 182 184 184 188 192 194 197 198 200 200 202 202 202 206 207 208

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16.4.1 Carburettors Types, construction and principles of operation Purpose Used on light aircraft, they are the simplest and cheapest but prone to icing. It is adversely affected by flight manoeuvres.

A float carburettor accomplishes these functions with five systems: primary metering, idling, acceleration, mixture control, and power-enrichment, or economiser. Many of the examples used here are the Marvel-Schebler MA4-5 carburettor used on engines in the 200-horsepower range, and the Bendix NAS-3 carburettor used on engines up to about 100 horsepower. The Marvel-Schebler carburettor has both a main and a boost Venturi, and the smaller Bendix carburettor has only a single main Venturi.

The primary purpose of float carburettors is to: • • • • •

• •

measure the amount of air entering the engine; meter into this air the correct amount of atomised liquid gasoline; convert the liquid gasoline into gasoline vapours and distribute them uniformly to all cylinders; provide a constant fuel-air mixture ratio with changes in air density and volume; provide an overly rich mixture when the engine is operating at peak power to remove some of the excessive heat; provide a temporarily rich mixture when the engine is rapidly accelerated; and provide for effective fuel metering when the engine is idling.

Venturi principles The carburettor must measure the airflow through the induction system and use this measurement to regulate the amount of fuel discharged into the airstream. The air measuring unit is the Venturi, which makes use of a fundamental law of physics: as the velocity of a gas or liquid increases, the pressure decreases. As shown in the diagram below, simple Venturi is a passageway or tube in which there is a narrow portion called the throat. As the velocity of the air increases to get through the narrow portion, its pressure drops. Note that the pressure in the throat is lower than that in any other part of the Venturi. This pressure drop is proportional to the velocity and is, therefore, a measure of the airflow. The basic operating principle of most carburettors depends on the differential pressure between the inlet and the Venturi’s throat.

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Pressures and temperatures in a venturi

Marvel-Schebler MA4-5 carburettor

Configurations of carburettor 1. Side draft 2. Updraft 3. Downdraft

Venturi, throttle valve and jet

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Operation The carburettor is mounted on the engine so that air to the cylinders passes through the barrel, the part which contains the Venturi. The size and shape of the Venturi depend on the requirements of the engine for which the carburettor is designed. A carburettor or a high-powered engine may have one large Venturi or several small ones. The air may flow either up or down the Venturi, depending on the design of the engine and the carburettor. Those in which the air passes downward are known as downdraft carburettors, and those in which the air passes upward are called updraft carburettors. Some carburettors are made to use a side draft or horizontal air entry into the engine induction system. When a piston moves down on the intake stroke, air flows through the induction system as the pressure in the cylinder is lowered. Air rushes through the carburettor and intake manifold to the cylinder to replace the air displaced by the piston. Due to this low-pressure area, the higher-pressure air in the atmosphere flows in to equalise it. As it does, the airflow must pass through the carburettor’s Venturi. The throttle valve is between the Venturi and the engine. A mechanical linkage connects this valve with the throttle lever in the co*ckpit. Airflow to the cylinders is regulated using the throttle which controls the power output of the engine. More air is admitted to the engine, and the carburettor automatically supplies enough additional fuel to maintain the correct fuel/air ratio. This is because as the volume of airflow increases, the velocity in the Venturi increases, lowering the pressure and forcing more fuel into the airstream. The throttle valve obstructs the passage of air very little when it is parallel with the flow, in the wide-open throttle position.

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Throttle valve and main jet Bendix NAS-3 carburettor

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Carburettor types The float-type carburettor, the most common of all carburettor types, has several distinct disadvantages. The effect that abrupt manoeuvres have on the float action and the fact that its fuel must be discharged at low pressure leads to incomplete vaporisation and difficulty in discharging fuel into some types of supercharged systems. The chief disadvantage of the float type carburettor, however, is its icing tendency. Since the float carburettor must discharge fuel at a point of low pressure, the discharge nozzle is located at the Venturi’s throat, and the throttle valve must be on the engine side of the discharge nozzle. This means that the drop in temperature due to fuel vaporisation takes place within the Venturi. As previously discussed, ice readily forms in the Venturi and on the throttle valve. A pressure-type carburettor discharges fuel into the airstream at a pressure well above atmospheric. This results in better a vaporisation and permits the discharge of fuel into the airstream on the engine side of the throttle valve. With the discharge nozzle located at this point, the drop in temperature due to fuel vaporisation takes place after the air has passed the throttle valve and at a point where engine heat tends to offset it. Thus, the danger of fuel vaporisation icing is practically eliminated. The effects of rapid manoeuvres and rough air on the pressuretype carburettors are negligible since its fuel chambers remain filled under all operating conditions. Pressure carburettors have been replaced mostly by fuel injection systems and have limited use on modem aircraft engines.

Construction A float-type carburettor consists of six subsystems that control the quantity of fuel discharged in relation to the flow of air delivered to the engine cylinders. These systems work together to provide the engine with the correct fuel flow during all engine operating ranges. The essential subsystems of a float-type carburettor are illustrated in the diagram below. These systems are: • • • • • •

a float chamber mechanism system; the main metering system; the idling system; a mixture control system; an accelerating system; and the throttle valve.

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Carburettor components

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Float chamber mechanism system A float chamber is provided between the fuel supply and the main metering system of the carburettor. The float chamber, or bowl, serves as a reservoir for fuel in the carburettor. This chamber provides a nearly constant level of fuel to the main discharge nozzle, which is usually about 1∕8 below the holes in the main discharge nozzle. The fuel level must be maintained slightly below the discharge nozzle outlet holes to provide the correct amount of fuel flow and to prevent fuel leakage from the nozzle when the engine is not operating. The level of fuel in the float chamber is kept nearly constant using a float-operated needle valve and a seat. The needle seat is usually made of bronze. The needle valve is constructed of hardened steel, or it may have a synthetic rubber section which fits the seat. With no fuel in the float chamber, the float drops toward the bottom of the chamber and allows the needle valve to open wide. As fuel is admitted from the supply line, the float rises (floats in the fuel) and closes the needle valve when the fuel reaches a predetermined level. When the engine is running, fuel is continually drawn off the discharge nozzle, so the needle valve finds a sensitive position where the chamber replenishes at the same rate as the fuel is used thus keeping the level constant. With the fuel at the correct level (float chamber), the discharge rate is controlled accurately by the air velocity through the carburettor Venturi where a pressure drop at the discharge nozzle causes fuel to flow into the intake airstream. Atmospheric pressure on top of the fuel in the float chamber forces the fuel out the discharge nozzle. A vent or small opening in the top of the float chamber allows air to enter or leave the chamber as the level of fuel rises or falls. Total Training Support Ltd © Copyright 2020

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Float chamber and float

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Pressure balance duct (PBD) In some carburettors, the float chamber is vented into the relatively still air within the engine cowlings. The air intake inlet is subject to changes in altitude, aircraft attitude and temperature, all of which cause pressure variations through the Venturi, which in turn affects the air/fuel ratio due to the difference between float chamber and air intake pressures. A pressure balance duct is incorporated, to overcome the problem of varying fuel flows, which extends the float chamber vent line to the air intake. Any changes which take place in the air intake are immediately felt within the float chamber, thus maintaining the correct fuel flow for the air going through the intake.

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Float chamber and float with pressure balance duct

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Main metering system The main metering system supplies fuel to the engine at all speeds above idling and consists of: • • • • •

a Venturi; the main metering jet; the main discharge nozzle; a passage leading to the idling system; and a throttle valve

Since the throttle valve controls the mass airflow through the carburettor Venturi, it must be considered a significant unit in the main metering system as well as in other carburettor systems.

As the air flows through the Venturi, its velocity increases. This velocity increase creates a low-pressure area in the Venturi’s throat. The fuel discharge nozzle is exposed to this low pressure. Since the float chamber vents to atmospheric pressure, a pressure drop across the discharge nozzle is created. It is this pressure difference, or metering force, that causes fuel to flow from the discharge nozzle. The fuel comes out of the nozzle in a fine spray, and the tiny particles quickly vaporise in the air.

The Venturi is the heart of the main metering system. Air flowing into the engine must pass through the Venturi. The Venturi performs three functions: • • •

it proportions the fuel/air mixture; it decreases the pressure at the discharge nozzle; and it limits the airflow at full throttle.

The fuel discharge nozzle is located in the carburettor barrel so that its open end is in the throat or narrowest part of the Venturi. A main metering orifice, or jet, is placed in the fuel passage between the float chamber and the discharge nozzle to limit the fuel flow when the throttle valve is wide open. When the engine crankshaft is revolved with the carburettor throttle open, the low pressure created in the intake manifold acts on the air passing through the carburettor barrel. Air flows from the air intake through the carburettor barrel into the intake manifold because of the pressure difference with the atmosphere. The volume of airflow depends upon the degree of throttle opening. Total Training Support Ltd © Copyright 2020

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Carburettors metering systems

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The metering force (pressure differential) in most carburettors increases as the throttle opening is increased. The fuel must be raised in the discharge nozzle to a level at which it discharges into the airstream. A pressure differential of 1.7 kPa (½" Hg) is required to accomplish this. When the metering force is considerably reduced at low engine speeds, the fuel delivery from the discharge nozzle decreases if an air bleed (air metering jet) is not incorporated in the carburettor. The decrease in fuel flow in relation to airflow is due to two factors: •

•

the fuel tends to adhere to the walls of the discharge nozzle and break off intermittently in large drops instead of forming a fine spray; and a part of the metering force is required to raise the fuel level from the float chamber level to the discharge nozzle outlet.

The basic principle of the air bleed can be explained by the simple diagram shown below top-left. In each case, the same degree of suction is applied to a vertical tube placed in the container of liquid. As shown in A, the suction applied on the upper end of the tube is sufficient to lift the liquid a distance of about 40 mm (1.6") above the surface. If a small hole is made in the side of the tube above the surface of the liquid, as in B, and suction is applied, bubbles of air enter the tube, and the liquid is drawn up in a continuous series of small slugs or drops. Thus, air ‘bleeds’ into the tube and partially reduces the forces tending to retard the flow of liquid through the tube. However, the large opening at the bottom of the tube effectively prevents any significant amount of suction from being exerted on the air bleed hole or vent. Similarly, an air bleed hole that is too large in proportion to the size of the tube would reduce the suction available to lift the liquid. Total Training Support Ltd © Copyright 2020

If the system is modified by placing a metering orifice in the bottom of the tube and air is taken in below the fuel level using an air bleed tube, a finely divided mixture of air and liquid is formed in the tube, as shown in C. In a carburettor, a small air bleed is bled into the fuel nozzle slightly below the fuel level. The open end of the air bleed is in the space behind the Venturi wall where the air is relatively motionless and at approximately atmospheric pressure. The low pressure at the tip of the nozzle not only draws fuel from the float chamber but also draws air from behind the Venturi. Air bled into the main metering fuel system decreases the fuel density and destroys surface tension. This results in better vaporisation and control of fuel discharge, especially at lower engine speeds. The throttle, or butterfly valve, is located in the carburettor barrel near one end of the Venturi. It provides a means of controlling engine speed or power output by regulating the airflow to the engine. This valve is a disk that can rotate on an axis so that it can be turned to open or close the carburettor air passage. The carburettor in the diagram below bottom-right uses a single Venturi and a discharge nozzle that sprays fuel out at right angles to the airflow. The discharge nozzle is located in the throat of the Venturi at the point the air pressure is the lowest, and the float bowl is vented to the inlet ram air. There is, therefore, a difference in pressure between that in the float bowl and at the discharge nozzle. This pressure difference meters fuel from the float bowl to the discharge nozzle proportional to the volume of air flowing into the engine.

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Air bleed system principle

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Idling system When the throttle is closed, there is not enough air flowing past the main discharge nozzle to produce a pressure low enough to pull fuel from the float bowl through the main metering jet, so a separate metering system must be used for idling. When the engine is idling, air that flows into the cylinders must pass around the edge of the butterfly valve. Airflow around the throttle butterfly valve is restricted, causing it to travel at a high velocity past the edge of the valve – this high velocity results in low pressure. There is a series of idle discharge holes in the throttle body located where pressure is the lowest when the throttle valve is in the idle range. In the largest of these holes is an idle mixture adjustment needle valve, and it is located where the throttle valve pressure is the lowest at low idle. When this valve is screwed in, flow from the largest hole is shut off, and as it is screwed out, an increasing amount of fuel discharges into the air stream. The secondary, tertiary, and at times a quad opening, act as additional air bleeds at low idle and as additional fuel discharge ports when the throttle transitions from low to high idle. As the throttle opens and its edge passes the secondary, and progressively the other openings, they become exposed to the low pressure and transition to discharging fuel. This additional fuel is needed because the primary idle port fuel flow is decreasing as the throttle opens and additional air passes through the induction system. The throttle edge is moving away from the bore wall causing less squeeze on the air rushing by and consequently a decrease in pressure drop.

As all the idle ports are just ceasing fuel flow, due to the increasing pressure rise, the main metering system begins to flow out the discharge nozzle because of the dropping Venturi pressure. A drilled passage, containing an idle emulsion tube, connects the idle discharge holes to an annulus, or ring, just above the main metering jet that is filled with fuel from the float bowl. In this passage, there is a perforated idle metering tube, and its entrance contains the idle metering jet – this tube screws into an enlarged hole in the carburettor body. In the side of the tube is a small hole that serves as the idle air-bleed. Air from the upper annulus formed between the Venturi and the air bore supplies air at approximately ambient pressure, to a cavity on the outside of the idle tube bleed opening. The bleed air enters the lower annulus behind the Venturi, passes through a bleed air-filtering screen, and then enters the upper Venturi annulus. This air supply not only feeds the idle bleed in the carburettor but also acts as the vent for the fuel bowl. When the throttle is closed, low pressure at the edge of the butterfly valve pulls fuel up through the idle metering jet. At the same time, it pulls air from behind the Venturi through the air bleed holes in the idle metering tube. The air and fuel form an emulsion that is pulled up to the idle discharge holes and discharged into the air going into the cylinders.

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Carburettors idle system 4-21 Module 16.4 Engine Fuel Systems

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Setting the idling conditions of an engine calls for two adjustments: the closed position of the throttle valve adjusts the idle speed or RPM, and the amount of fuel-air mixture discharged determines how smoothly the engine idles. With the engine warmed up and all systems operating correctly, hold the throttle in a position that produces the desired idle RPM, and adjust the idle mixture needle valve until the engine RPM peaks. From this mixture position, enrich the mixture to provide a 50 RPM decrease. With the throttle reset at the desired idle speed, open it slowly to cruise. If the transition is smooth and without hesitation, the mixture is correct, but if there is hesitation in the transition, richen or lean the idle mixture slightly to correct the situation. Next, idle at the desired RPM, apply full carburettor heat, and make sure the engine continues to idle without the tendency to stall. When the idle mixture is set correctly, screw the idle RPM adjustment screw in until it contacts the idle stop on the throttle arm. Advance the throttle until the engine runs in its cruise RPM range to clear the spark plugs of any fouling caused by the idling, and then pull the throttle back. If the controls have been properly adjusted, the engine should return to a smooth idle at the speed for which it was adjusted. If it does not idle properly, repeat the process. A slightly over-rich idle mixture aids in the transition from idle to main metering operation, aids with additional cooling at idle, smooths engine idle by making the leanest cylinders rich enough to fire consistently and compensates for cylinder misfire due to poor exhaust scavenging at low RPM.

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Marvel-Schebler MA4-5 carburettor

Typical carburettors idle adjustment

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Slow running, idle and cut off With the butterfly almost closed in the position for idle or slow running, the amount of air passing through the Venturi is insufficient to cause a large enough pressure drop to draw off fuel. However, the gap between the throttle housing and the edge of the butterfly will now create its own mini Venturi. A line is taken from the ‘U’ tube before the main jet, and then routed to an outlet at the edge of the butterfly valve. In this line, there is a calibrated orifice known as the slow running jet. The flow from this jet ceases as the throttle opens, as the mini Venturi no longer exists. A simple plunger-type valve, sprung loaded in the open position, is situated in the slow running fuel supply line. Controlled from a lever in the co*ckpit, it will, when operated, overcome the spring and cut off the fuel supply to the slow running jet, causing the engine to stop, as the mixture, now flowing to the engine, is so weak that it can no longer support combustion. The operation of idle cut off is only effective when the throttle lever is also in the idle or slow running position. The idle cut off acts as a safety device to prevent the engine running on due to pre-ignition, which may occur when the engine had stopped, and the ignition switched off. The release of the idle cut off in the co*ckpit allows the spring to reposition the valve to open. An alternative design is illustrated here in the Marvel Schebler MA4-5 carburettor the manually operated mixture control valve is rotated to a no-flow position and this cuts the fuel to the engine.

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Slow running idling jet

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Mixture control system As altitude increases, the air becomes less dense. At an altitude of 18,000 ft, the air is only half as dense as it is at sea level. This means that a cubic foot of space contains only half as much air at 18,000 ft as at sea level. An engine cylinder full of air at 18,000 ft contains only half as much oxygen as a cylinder full of air at sea level. The low-pressure area created by the Venturi is dependent upon air velocity rather than air density. The action of the Venturi draws the same volume of fuel through the discharge nozzle at a high altitude as it does at a low altitude. Therefore, the fuel mixture becomes richer as altitude increases. This can be overcome either by a manual or an automatic mixture control. On float-type carburettors, two types of purely manual or co*ckpit-controllable devices are in general use for controlling fuel/air mixtures, the needle type and the back-suction type.

The back-suction-type mixture control system is the most widely used shown in the diagram below right. In this system, a certain amount of Venturi low pressure acts upon the fuel in the float chamber so that it opposes the low pressure existing at the main discharge nozzle. An atmospheric line, incorporating an adjustable valve, opens into the float chamber. When the valve is completely closed, pressures on the fuel in the float chamber and at the discharge nozzle are almost equal, and fuel flow reduces to maximum lean. With the valve wide open, pressure on the fuel in the float chamber is highest, and the fuel mixture is richest. Adjusting the valve to positions between these two extremes controls the mixture. The quadrant in the co*ckpit is usually marked “LEAN” near the back end and “RICH” at the forward end. The extreme back position is marked “IDLE CUT-OFF”, used when stopping the engine.

With the needle-type system, manual control is provided by a needle valve in the base of the float chamber, illustrated in the diagram below left. This can be raised or lowered by adjusting a control in the co*ckpit. Moving the control to “RICH” opens the needle valve wide, which permits the fuel to flow unrestricted to the nozzle. Moving the control to “LEAN” partially closes the valve and restricts the flow of fuel to the nozzle.

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Mixture control system

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Pressure balance duct (PBD) At this time, the float chamber is vented into the relatively still air within the engine cowlings. The air intake inlet is subject to changes in altitude, aircraft attitude and temperature, all of which cause pressure variations through the Venturi, which in turn affects the air/fuel ratio due to the difference between float chamber and air intake pressures. A pressure balance duct is incorporated to overcome the problem of varying fuel flows, which extends the float chamber vent line to the air intake. Any changes which take place in the air intake are immediately felt within the float chamber, so maintaining the correct fuel flow for the air going through the intake.

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Pressure balance duct

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The diffuser With increased power through the cruise range, more air and fuel is required. Opening the throttle butterfly valve allows more air to the cylinders, and this increased airflow passing through the Venturi causes a more significant pressure differential, which in turn causes an increase in the fuel flow from the discharge nozzle. Unfortunately, the inertia flow rate characteristics of air and fuel are different; this difference causes too much fuel to be drawn off, so causing the mixture gradually to become over-rich throughout the cruise range. A diffuser is built into the discharge line between jet and nozzle to overcome this problem; the diffuser consists of a tube with several small holes drilled in rows just below the top of the tube. The air tapped from the pressure balance duct is routed to the space around the outside of the tube. As the demand for more fuel increases the fuel level in the discharge tube descends, establishing a lower level in the tube. This new level exposes the first row of holes in the diffuser above the fuel level, which allows some of the air from the pressure balance duct to bleed through the holes to reduce the pressure drop. This, in turn, reduces the tendency of the fuel flow to enrich the cruise air/fuel ratio. Opening the throttle further causes the next row of holes to be exposed to pressure balance duct air, so allowing an increase in fuel flow while maintaining the correct air/fuel ratio. A secondary feature is that the air helps to emulsify the fuel when bleeding through the diffuser, resulting in a more volatile mixture.

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Diffuser operation

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The air bleed An alternative to the diffuser is the air bleed, shown in the diagram below left. The main jet is placed at the bottom of the discharge tube, and an air-bleed duct routes atmospheric or pressure-balanced duct-pressure air to the lower end of the discharge tube, so atomising the fuel. When the Venturi depression is strong, the fuel is diluted similarly to that of a carburettor fitted with a diffuser.

An air bleed serves more functions than just maintaining a constant fuel-air mixture ratio above high idle. The air introduced as bubbles into the fuel upstream of the discharge nozzle outlet decreases the density of the fuel and makes it easier for the low pressure to pull fuel from the nozzle. This decrease allows a more realistic Venturi pressure drop to initiate fuel flow by the time the idle system is phasing out. The fuel-air emulsion provides a large fuel surface area for rapid vaporisation.

Control of mixture ratios There is a need to have control over mixture ratios for two reasons: • •

flying the aircraft with economy in mind, compensating for changes in the aircraft’s altitude.

Restricting the air bleed causes the mixture to become richer. If the discharge nozzle were vented to the atmosphere with a free air bleed, as in the diagram below top-left, the low pressure would pull a large amount of air into the discharge nozzle. As the airflow into the engine increases, the mixture becomes progressively leaner. By installing a bleed restrictor with the correct size orifice, just enough air will be metered into the fuel to keep the fuel-air mixture ratio constant as the amount of air flowing into the engine changes.

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The air bleed enrichment system restricts the main air bleed when the throttle is opened 70% and above.

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Mixture control at higher altitudes If the aircraft climbs with a fixed throttle setting, the pressure of atmospheric air entering the air intake decreases, which in turn reduces the weight of charge going to the engine. Remember that the air/fuel ratio is a function of weight, i.e. fourteen pounds of air to one pound of fuel at cruise. Although the weight of air decreases with increase in altitude, the volume of air is unaltered; it is this value air rushing through the Venturi that causes the pressure differential change, which in turn decides the fuel flow, and in this case, the mixture enriches with an increase in altitude.

Mixture control economy When flying with a fixed throttle setting and a stable altitude, the pilot is given a choice of a ratio of 14:1 rich cruise or 17:1 lean or weak cruise. The lean cruise ratio is selected by the pilot using the mixture lever, which allows more air to the discharge tube, so reducing the pressure differential, hence the fuel flow. This causes a small drop in engine power output, but in return, there is a cooler burning of the mixture and lower specific fuel consumption.

Air is tapped from the pressure balance duct and is allowed to enter the discharge tube above the diffuser via a control valve to overcome this problem. The amount of air passing the control valve has a direct effect on the pressure differential, which causes a reduction in fuel flow to maintain our 14:1 ratio. The overall effect of the weight of charge entering the cylinders at altitude is a reduction of volumetric efficiency. Therefore, power falls off with a fixed throttle climb to altitude. Note that the amount of air passing the control valve is under the direct control of the pilot using the altitude mixture control lever.

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Automatic mixture control To relieve the pilot of the need to adjust the mixture as the aircraft climbs to altitude, some carburettors are fitted with an automatic mixture control which senses the drop in pressure that occurs as altitude increases and adjusts the fuel/air ratio to suit the less dense air. The mixture control valve is situated above the discharge tube and diffuser, the pressure difference between the float chamber and the Venturi is altered by the amount of atmospheric or balance duct pressure directed to the discharge tube and diffuser. It is just like the manually operated mixture control described previously. However, instead of manual control, we have a servo piston operated by engine oil in a servo unit which moves the linkage to operate the mixture control valve. The supply of pressure oil is directed to either the top or bottom of the piston by a piston valve which in turn is controlled by an aneroid capsule sensitive to atmospheric pressure. Operation As the aircraft climbs and the atmospheric pressure drops, we need less fuel to maintain the air/fuel ratio. Therefore, the mixture control valve needs to select more air to the top of the fuel in the discharge tube, thus reducing the pressure drop between the float chamber and the Venturi, so reducing the fuel flow rate. The drop in atmospheric pressure causes the aneroid capsule to expand, pushing down the piston valve; this lines up the oil pressure inlet with the underside of the piston and the top of the piston lines up with the top return to scavenge.

The upward movement of the servo piston rotates the mixture control valve to allow more air in to reduce the pressure differential and hence reduce the fuel flow. As we do not want the servo piston to go fully up until we are at a much higher altitude, it would lean off the mixture too much at the start of a climb, the aneroid capsule is attached to the same linkage as the servo piston. As the piston moves up, so the aneroid capsule assembly complete with piston valve also move up, thus blocking off both the lines to the top and bottom of the servo piston, so preventing further movement of the servo piston. The mixture control valve is held in the position selected to compensate for the lower atmospheric pressure. As the aircraft climbs, the aneroid capsule, which is anchored firmly to the linkage and held firmly by the servo piston, expands as the outside of the capsule is subjected to lower atmospheric pressure. The piston valve is pushed down, allowing the pressure oil to the underside of the servo piston, and oil from the top of the servo piston to escape to scavenge as it did before. The servo piston then moves up, repositioning the mixture control valve to admit more atmospheric air to the mixture discharge tube to reduce the pressure differential, and therefore reduce the fuel flow. As the servo piston moves up, the aneroid assembly attached to the linkage also moves up, taking the piston valve with it, so blocking off both the oil lines and thus preventing further movement of the servo valve and the mixture control. This operation repeats again and again as the aircraft climbs, so it becomes continuous, progressively opening the mixture control valve to allow more air to weaken the mixture as atmospheric pressure progressively decreases.

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When the aircraft descends, the reverse happens. The aneroid capsule contracts causing the piston valve to rise; this allows the pressure oil to the top of the servo piston and the oil from below the servo piston can escape to be scavenged past the bottom of the piston valve. The servo piston moves down, moving the mixture control valve to reduce the air to the discharge tube so increasing the pressure differential, and therefore supplying more fuel to maintain the correct air/fuel ratio. As the servo piston moves down, it also moves the aneroid capsule assembly with the piston valve until the lands of the piston valve block off both lines to the servo piston. This stops further movements of the mixture control valve until there is a change in the atmospheric pressure, when the whole process starts all over again, either up or down.

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Automatic mixture control

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Economy system With the manual mixture control, the pilot could lean the mixture in the cruise range to achieve a higher fuel economy. With an automatic mixture control, the pilot is given a two-position lever where he can select rich gear lean. If he selects lean, he turns a sleeve which is usually situated around the piston valve with the holes line up with the lines to the top and bottom of the servo piston. By turning the sleeve through 90°, two alternative holes positioned higher up the sleeve are now used to line up with the oil supplies to the servo piston.

The linkage between the aneroid, the servo piston and the mixture control valve is critical and is set by the manufacturers to ensure that: • •

the movement of the servo piston is proportional to changes of atmospheric pressure; and the opening of the mixture control valve is proportional to fuel flow requirements.

In this case, the piston valve has to travel further up before the lands block the lines to the servo piston. The piston valve has to move further up before it is stopped, in which case the mixture control valve allows more air in, thus reducing the pressure differential more than usual, so weakening the mixture still further. On selection of lean in the co*ckpit, the natural position of the piston valve is reset. The system now works as before with changes in atmospheric pressure, but the original pressure drop that it is subjected to is lower and therefore weaker. This economy setting of lean only works in the cruise range, because this linkage to the pilot’s lever is arranged so that selection of power above the cruse range automatically puts the rich/lean selector to rich and the sleeve is rotated back to 90° to where it started.

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Economy system

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Power and enrichment jet An increase in power selection above cruise reaches the best power at an air/fuel ratio of about 12.5:1. This rich mixture ensures that all cylinders get at least the minimum requirement of fuel. Any further increase in power selection requires extra amounts of fuel because the main jet, which is designed for the cruise range, cannot supply enough to keep the air/fuel ratio constant when the larger quantity of air is entering the engine. Therefore, an additional jet may be fitted, known as the power jet, as shown in the diagram below left. It is usually operated by a cam controlled by the pilot’s throttle power lever. If the best power ratio was maintained up to take off, then detonation is liable to occur due to increased pressures and temperatures in the cylinders; the power jet is arranged so that some extra fuel is added by the power jet to cool the mixture up to take-off. To ensure that there is no possibility of detonation occurring at take-off, which is the most critical time for an aircraft and its engine, even more fuel is added for cooling, to give a ratio of around 10:1. This extra fuel does not burn, as there is not enough oxygen available. Instead, the fuel acts as a coolant and results in black smoke from the exhaust, indicating a rich mixture. This cooling fuel is supplied by yet another jet, known as the enrichment jet. It is usually cam operated from the pilot’s power lever similarly to that for the power jet shown in the diagram below right.

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Enrichment jet operation

Power jet operation

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Back suction economiser A back-suction mixture control operates on the principle of varying the pressure drop across the fixed main metering jet to control the amount of fuel that flows to the main discharge nozzle. For rich operation, the float bowl is vented with air from the carburettor inlet that flows behind the Venturi and through the open disk-type mixture control valve. At altitude, the pilot moves the mixture control toward the lean position. This restricts the disk valve and subjects the float bowl to slightly low pressure from the edge of the Venturi. The lower pressure in the float bowl decreases the pressure drop across the main metering jet, decreasing the amount of fuel that flows through it, and leans the mixture. Some carburettors with back-suction mixture controls have a cut-off valve that vents the float bowl of low pressure taken from the centre of the Venturi. This pressure is the same as that at the main discharge nozzle, and no fuel flows through the main metering jet when these pressures are the same. Some float-carburetted engines, particularly helicopter engines, incorporate an automatic mixture control (AMC). This is an aneroid-controlled needle valve in series with the main metering jet. As the barometric pressure decreases with altitude, the aneroid expands, placing the needle valve closer to its seat, thus restricting the fuel flow more than the main metering jet.

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Mixture control knob in a Robinson helicopter

Back suction economiser

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Acceleration control The acceleration system compensates for the lag in fuel flow acceleration upon the rapid opening of the throttle. The fuel flow in the main metering system is denser than the air in the induction system and has less of a differential applied across it. Therefore, the fuel acceleration rate lags behind that of the induction air resulting temporarily in a weak mixture. This is true throughout the throttle travel. One of the specific areas where an accelerator system helps is when the throttle is opened from idle to higher power settings. Both the idle system and the main metering system require a specific amount of airflow for their proper operation. When advancing the throttle from its idling position to the cruise position, there is a range in which the throttle valve is open too much for the idling system to function adequately, and not enough for the main metering system to be in full operation. This so-called flat spot causes the engine to hesitate when the throttle is opened, but this hesitation can be eliminated by using an acceleration system. The simplest acceleration system is an enlarged annulus, or groove, around the main discharge nozzle as is seen below. When the engine is idling, all fuel entering the cylinders is metered through the idle system, and the acceleration well fills with fuel. When the throttle is suddenly opened, all fuel in the well is pulled out through the main discharge nozzle, and the engine receives a momentarily rich mixture that causes acceleration. As soon as the RPM builds up, air flows in through the main air bleed and the fuel metering returns to normal. Acceleration wells are used in carburettors generally mounted on engines of less than about 3.3 L (200 in3) displacement. Total Training Support Ltd © Copyright 2020

Larger engines require a positive discharge of fuel to enrich the mixture during the transition from the idling system to the main metering system. This is because the larger Venturi openings required are less responsive in the lower RPMs. The diagram below right shows a typical piston-type acceleration pump. When the engine is idling, the throttle is closed, the pump plunger is at the top of its stroke, and the pump chamber is full of fuel that was pulled in from the float bowl through the pump inlet check valve. When the throttle is opened, the piston is forced down. The pump inlet check valve closes, and the fuel is pumped out through the pump discharge check valve and is sprayed into the air flowing into the engine through the Venturi. The outlet check is needed to prevent induction air from being drawn into the pump during the pump’s intake stroke, and the inlet check is necessary to prevent a significant portion of the fuel from returning to the fuel bowl when the pump is discharging. The pump piston is mounted on a spring-loaded telescoping shaft. The restriction caused by the pump discharge nozzle prevents all fuel from discharging immediately when the throttle is opened. But the spring compresses and produces a sustained discharge as it extends to provide the engine with a rich mixture until its speed builds up enough for the main metering system to function. This arrangement also allows the pilot to open the throttle rapidly without damage to the acceleration linkage. The discharge valve is either weighted or spring-loaded to prevent the syphoning of fuel out of the pump discharge during main metering operation. If this valve leaks, it causes an enriching of the mixture above idle until it is compensated for with the manual mixture control system.

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Acceleration control

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When the throttle is opened quickly, there is an immediate increase of airflow into the engine, but unfortunately, the response of the fuel metering system is less quick. To overcome this, an accelerator pump is fitted and activated by linkage from the throttle control. The accelerator pump consists of a small chamber connected to the float chamber with a non-return valve (NRV), which allows fuel to flow from the float chamber to the accelerator pump. A fuel line is taken from the pump to the air stream immediately before the Venturi. At the top of the accelerator pump-chamber is a piston-type plunger held in the up position by a spring. The accelerator pump plunger is operated by a cam, which is actuated by the throttle linkage. Any quick movement of throttle causes the plunger to move down, and the NRV closes under the fuel pressure, preventing the return of the fuel to the float chamber. The fuel is forced along the fuel line to just before the Venturi and forced thorough the fixed orifice, thus enriching the mixture until the fuel metering system has caught up with the airflow. The rate of enrichment is therefore dependent upon the speed of movement of the pilot’s throttle.

When the throttle is wide open, the butterfly valve is parallel with the tube and the airflow, the obstruction caused by the valve is minimum, and the flow is at its highest. Butterfly-type throttle valves are used to control the airflow through the fuel metering system., and it offers minimum restriction. When the throttle is closed, the valve nearly shuts off the flow of air into the engine, but an adjustable stop screw prevents it from completely blocking the airflow. The amount of air flowing past the nearly closed valve determines the idle speed of the engine. The size of the Venturi limits the maximum amount of air that can flow into the engine under wide-open throttle conditions. The same model of carburettor can be used on engines of various sizes by changing the Venturi, the main metering jet, the main air bleed restrictor, and idle metering tube.

Throttle – airflow regulation The amount of power produced by a reciprocating engine is determined by the quantity of air and fuel entering the cylinders. Air flowing into the engine at conditions other than full throttle is controlled by a circular butterfly-type valve actuated by the throttle control in the co*ckpit. It is a flat, disk-shaped valve used to control the flow of fluid in a round pipe or tube. When the butterfly valve is across the tube; the flow is shut off. Total Training Support Ltd © Copyright 2020

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Automatic mixture control

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Example system: Marvel-Schebler MSA MA-4-5 float type carburettor (now Precision Airmotive) Idle system With the throttle fly slightly open to permit idling, the suction or vacuum above the throttle on the manifold side is very high. Very little air passes through the Venturi at this time, and hence, with very low suction on the main nozzle, it does not discharge fuel. This high suction beyond the throttle, however, causes the idle system to function as the primary idle delivers into the high suction zone above the throttle. Fuel from the fuel bowl passes through the mixture metering sleeve, fuel channel, power jet, and into the main nozzle bore. Here it passes through the idle supply opening in the main nozzle, through the idle fuel orifice in the idle tube, where it mixes with air which is allowed to enter idle tube through the primary idle air vent and secondary idle air vent.

On idle, some air is drawn from the throttle barrel below the throttle fly through the secondary idle delivery opening. It blends with the idling mixture to the engine as the throttle is opened, coming into play progressively and blending with the primary idle delivery to prevent the mixture from beginning too lean as the throttle is opened and before the main nozzle starts to feed. These carburettors are provided with a third and, possibly a fourth idle delivery in addition to the secondary idle delivery, depending on the application to cover the broader idle range.

The resultant rich emulsion of fuel and air passes upward through the emulsion channel. It is finally drawn into the throttle body through the primary idle delivery opening, subject to the regulation of the idle adjusting needle, where a small amount of air passing the throttle fly mixes with it, forming a combustible mixture for idling the engine. The idle adjustment needle controls the quantity of rich emulsion supplied to the throttle barrel and therefore controls the quality of the idle mixture. Turning the needle counter-clockwise away from its seat richens the idle mixture to the engine and turning the needle clockwise towards its seat leans the idle mixture.

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Variations of the MA-4-5

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Metering All fuel delivery on idle, and also as steady propeller speeds up to approximately 1,000 RPM, is from the idle system. At approximately 1,000 RPM, the suction from the increasing amount of air now passing through primary and secondary Venturi causes the main nozzle to start delivering. The idle system delivery diminishes due to lowered suction on the idle delivery openings, as the throttle fly is opened for increasing propeller speeds until at approximately 1,400 RPM the idle delivery is practically nil. Most of the fuel delivery from that point on to the highest speed is from the main nozzle. However, the fuel feed of any full throttle operation is entirely from the main nozzle. The idle supply opening connects the idle system and main nozzle. The amount of fuel delivered from either the idle system or main nozzle is dependent on whether the suction is higher on the idle system or main nozzle, the suction governed by throttle valve position and engine load. The main nozzle feeds at any speed if the throttle is open sufficiently to place the engine under load, which drops the manifold suction. Under such conditions of low manifold suction at the throttle fly, the main nozzle feeds in preference to the idle system because the suction is multiplied on the main nozzle by the restriction of the Venturi.

Accelerator pump The accelerator pump discharges fuel only when the throttle fly is moved towards the open position and provides additional fuel to keep in step with the sudden inrush of air into the manifold when the throttle is opened. Using an accelerator pump lever connected to the throttle shaft, the accelerator pump plunger is moved downward when the throttle is opened. This forces fuel past the carburettor pump discharge check-valve into the accelerator pump discharge tube which delivers accelerating fuel through the primary Venturi into the mixing chamber of the carburettor. Upon closing the throttle, the accelerator pump plunger moves upward, thus refilling the accelerator pump chamber by drawing fuel from the fuel bowl through the pump inlet screen and pump inlet check valve. On any quick opening of the throttle the pump follow-up spring yields and thus prolongs the pump discharge sufficiently to prevent ‘slugging’ the engine with fuel. As a precaution to prevent fuel from being drawn into the mixing chamber when the accelerator pump is inoperative (any constant throttle position), the accelerator pump discharge check valve assembly mounted in the carburettor is provided with an accelerator pump discharge check valve loaded by a spring.

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Power enrichment (economiser) system Aircraft engines are designed to produce a maximum amount of power consistent with their weight. But since they are not designed to dissipate all of the heat the fuel is capable of releasing; provisions must be made to remove some of this heat. This is done by enriching the fuel/air mixture at full throttle. The additional fuel absorbs this heat as it changes into a vapour. Power enrichment systems are often called economiser systems because they allow the engine to operate with a relatively lean and economical mixture for all conditions other than full power. Mechanical air bleed enrichment system When an increased air velocity passed through the main Venturi, an increased pressure drop occurs which enriches the mixture, and to prevent this enrichment, an air bleed of exact size is used between the float bowl and the discharge nozzle. If the size of the air bleed is increased, a lean the mixture is produced, and if the air bleed is decreased, more fuel is pulled from the discharge nozzle, and the mixture becomes richer. The air for the air bleed comes from the float chamber and passes through the air bleed metering valve. The needle for this valve is held off of its seat by a spring and is closed by an operating lever attached to the throttle shaft. When the throttle is wide open, the lever closes the air bleed valve and enriches the fuel/air mixture.

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Mixture control The mixture control consists of mixture control lever which is attached to the mixture metering valve assembly. The mixture metering valve assembly is provided at its lower end with mixture metering valve, which rotates in stationary mixture sleeve. Mixture metering sleeve is provided with a transverse slot through which the fuel enters and fuel metering is accomplished by the relative position between one edge of the longitudinal flat on the mixture metering valve and one edge of the slot in the mixture metering sleeve. When the mixture control lever is in toward the carburettor throttle flange, a full rich mixture is provided for take-off. With the mixture control lever in the ‘FULL RICH’ position, metering is controlled by the power jet, but in anything other than ‘FULL RICH’ position, metering is accomplished by the relative position of the respective edges of the mixture metering sleeve and mixture metering valve as described above. To make the mixture leaner for altitude compensation, move the mixture control lever away from the carburettor throttle flange. With the mixture control lever in the full lean position, (with mixture control lever in a position farthest from the carburettor throttle flange), no fuel is allowed to enter the nozzle and idle system, thus providing what is known as “IDLE CUT-OFF” to prevent accidents when working around a hot engine. This cut off is accomplished by the fact that the angular opening between the metering edge of the mixture metering valve and the metering edge of the mixture metering sleeve in the “FULL RICH” position is narrower than the total angular travel of the mixture metering valve.

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Variations of the MA-4-5 detail

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Variations of the MA-4-5 detail

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Icing and heating General Induction system icing in piston engines is commonly referred to as carburettor icing. Although that is only one form, such icing can occur at any time, even on warm days, particularly humid ones. If corrective action is not taken, the engine may stop, especially at low power settings during descent, approach or during helicopter autorotation. Engine induction system icing has been assessed as a likely contributory factor in several aircraft accidents which can be hard to investigate as unfortunately the evidence rapidly disappears. Many misconceptions exist within the aviation industry about induction system icing, partly due to it commonly being referred to as “carb icing”. Some pilots believe that fuel-injected engines are immune to induction icing, this is not so. Although the pilot flying with a fuel-injected engine does not have the same threat of icing at the Venturi as those with a carburettor, rain, snow, slush and cold temperatures may cause a blockage (impact ice) to airflow in other parts of the induction system. Engine factors that affect Ice formation Carburettor icing is more likely when ‘Mogas’ is used because of its volatility and water content. Reduced power settings make engines more prone to icing. Induction temperatures are lower, and the partly closed butterfly can be restricted more easily by the ice build-up. This is a particular problem if the engine is de-rated, as in many piston-engine helicopters and some aeroplanes.

A rough carburettor venturi surface is likely to increase carburettor icing severity. Water-cooled engine bodies tend to cool less quickly when power is reduced, which reduces the severity of carburettor icing. If coolant is directed around the carburettor body, the Venturi temperature may remain above freezing Atmospheric conditions that affect ice formation Carburettor icing is not restricted to cold weather. It occurs on warm days if humidity is high, especially at low power settings. Flight tests have produced serious icing at descent power when the air temperature was above 25 °C, even with relative humidity as low as 30%. At cruise power, icing occurred at 20 °C when relative humidity was 60% or more. (Cold, clear winter days are less of a hazard than humid summer days because cold air holds less moisture than warm air.) In areas of Europe where high humidity is common, pilots must always be on the alert for carburettor icing and take corrective action before the situation becomes irretrievable. If the engine fails due to carburettor icing, it may not restart (even if it does, the delay could be critical) Carburettor icing can occur in clear air without any visual warning. The icing risk may be higher in cloud, but the pilot is less likely to be surprised. The chart shows the wide range of ambient conditions in which carb icing is most likely. It shows the much higher risk of serious icing with descent power.

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Icing severity prediction chart

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Aviation weather forecasts do not usually include specific warnings of induction system icing. Pilots must, therefore, use knowledge and experience. Dewpoint readings close to the temperature mean the relative humidity is high. However, the humidity reported at an aerodrome may bear little relation to the humidity at flying altitudes. When dewpoint information is not available, assume high humidity, particularly when: • • • • •

•

in cloud and fog; these are water droplets, and the relative humidity should be assumed to be 100%; in clear air where cloud or fog may have just dispersed, or just below the top of a haze layer; just below cloud base or between cloud layers (the highest liquid water content is at cloud tops); in precipitation, especially if persistent; if the surface and low-level visibility is poor, especially in the early morning and late evening, and particularly near a large area of water; and when the ground is wet (even with dew), and the wind is light.

However, the lack of such indications does not mean low humidity.

Indications of ice formation If the aircraft has a fixed-pitch propeller, the most likely indications of carb icing are a slight drop in RPM and performance (airspeed and/or altitude). The pilot may automatically open the throttle slightly to compensate for a smooth and gradual loss of RPM, and not notice the performance loss. As ice increases, rough running, vibration, further loss of performance ensues, and ultimately the engine will stop. Pilots should routinely compare the RPM gauge with the ASI and altimeter. With a constant speed propeller, or in a helicopter, a reduction in RPM would only occur after a significant power loss. The onset of icing is more insidious, but the performance reduction is shown as a drop in manifold pressure. In steady level flight, an exhaust gas temperature gauge, if fitted, may show a decrease in temperature before any significant decrease in engine and aircraft performance. Some induction systems are fitted with a carburettor or intake temperature gauge which is easily monitored by the pilot, but other indicators are also available.

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General practices Some engines have electric heaters which directly increase the temperature of the carburettor body, encouraging ice to clear. A similar effect may be obtained in a liquid-cooled engine by directing the flow of coolant.

In cruise flight, apply carburettor heat at regular intervals to prevent ice forming. Apply it for at the very least 15 seconds (but considerably more in particular aircraft) to prevent the loss of engine power, or to restore it.

On other air-cooled engines, carb icing is usually cleared by the pilot selecting an alternative air source which supplies air which has been heated in an exhaust heat exchanger to melt the ice obstruction. This source by-passes the regular intake filter.

If the hot air has dispersed ice which has caused a loss of power, reselecting cold air should produce a higher RPM or manifold pressure than the reading before the selection of hot air. This shows that ice has been forming but does not prove that all the ice has melted! Carry out further checks until there is no resultant increase. Then monitor the engine instruments and carry out the routine checks more often. If there is no carb icing, there should be no increase in RPM or manifold pressure above the figure noted before selecting hot air.

Fuel-injected engines generally have an alternate air intake within the engine cowling. This alternate air does not usually pass through a heat exchanger but may be warmed by engine heat. Whenever you apply carb hot air, always select full heat; partial hot air should only be used if explicitly recommended in the flight manual or pilot’s operating handbook. Select carburettor body heat whenever carb icing is likely. Hot air should be selected: • • • •

as a routine, check at regular intervals to prevent ice build-up; whenever a drop in RPM or manifold pressure, or rough engine running, is experienced; when carb icing conditions are suspected; and when flying within the high probability ranges indicated in the chart. However, while hot air is selected, it reduces engine power (as does body heating to a much lesser extent). This power loss may be critical in certain flight phases, for example, during a go-around.

If you select hot air when ice is present, the situation may at first appear worse, because the engine runs roughly as the ice melts and passes through it. Do not be tempted to return to cold air. Allow the hot air time to clear the ice. This time may be over 15 seconds which feels like an exceedingly long time! Unless it is necessary, avoid using hot air continuously at high power settings. However, carburettor heat should be applied early enough before descent to warm the intake. It should remain fully applied during that descent, as the engine is more susceptible to carb icing at low power settings.

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Types of ice Three types of ice form in fuel metering devices, as illustrated in the diagram below. They are: • • •

impact ice; throttle ice; and evaporation ice.

Note: Although not linked directly to the induction system fuel icing should also be considered. Impact ice This forms when there are low temperatures and high moisture content in the air. The air on striking the relatively cold air intake causes ice to form similar to that formed on mainplane leading edges. It builds up the intake lip, the filter and at the first bend in the duct which then causes a restriction to the airflow with a resulting loss of power. In snow, sleet, or sub-zero cloud, ice may build up on air intakes, filters, alternate air valves, etc. It may also form in the rain if either the rain or the aircraft is below zero °C. Impact icing can affect fuel injection systems as well as carburettors and is the main hazard for turbocharged engines. It is unlikely to be removed by selecting carburettor hot air. However, selecting hot air, or alternate air in a fuelinjected engine, bypasses the regular intake and should allow the engine to run normally, although possibly at reduced power. Although injection systems are less prone to ice than other metering devices, they can suffer in particular from impact ice forming in the throttle housing which can affect the impact tubes and the Venturi sensing line causing engine failure.

Throttle ice This forms when the throttle valve is at or near the closed position. The Venturi formed between the edge of the throttle valve and the throttle body causes a reduction in air pressure and temperature, thus causing the moisture in the air to form ice on the edge of the throttle body. The gap is now similar, and the Venturi effect greater so yet more ice forms. Evaporation ice Evaporation ice forms around the discharge nozzle of float chamber carburettors, and pressure injection carburettors and is the most common, first to appear, and the most serious. Where the fuel joins the airstream, vaporisation of the fuel occurs. The heat required for vaporisation is taken from the surrounding air and the carburettor components. This causes a drop in temperature of the moisture-laden air, and ice formation takes place. In the float chamber carburettor, icing is highly likely because the discharge nozzle is situated in the Venturi. Here, the temperature is already lowered by the drop in pressure caused by the Venturi. In the pressure carburettor, the discharge nozzle is usually situated away from the residual engine heat source, and evaporation icing can occur. With humidity of 60% or above, evaporation ice can form on a warm day. The most likely temperature range is surprisingly +5°C to +27°C. Because the air in this range has a high moisture content, coupled with the possibility of a temperature drop of 25°C at the throttle valve edge due to the Venturi effect, and a drop of around 27°C due to vaporisation, means that the moisture in the air can soon turn to ice. Impact ice, which is supercooled water, tends to form at freezing point (0°C).

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Carburettor heat Carburettor heat (usually abbreviated to 'carb heat') is a system used in piston-powered light aircraft engines to prevent or clear carburettor icing. It consists of a moveable flap which draws hot air into the engine intake. The air is drawn from the heat stove, a metal plate around the (very hot) exhaust manifold. A fixed-pitch propeller aircraft will show a decrease in engine RPM, and perhaps run rough, when carburettor ice has formed. However, a constant-speed propeller aircraft will show a decrease in manifold pressure as power is reduced. In light aircraft, the carburettor heat is usually manually controlled by the pilot. The diversion of warm air into the intake reduces the available power from the engine for three reasons: thermodynamic efficiency is slightly reduced since it is a function of the difference in temperature between the incoming and exhaust gases; the quantity of air available for combustion inside the cylinders is reduced due to the lower density of the warm air; and the previously-correct ratio of fuel to air is upset by the lower-density air, so some of the fuel does not burn and exits as unburned hydrocarbons. Thus, the application of carb heat is manifested as a reduction in engine power, up to 15%. If ice has built up, there will then be a gradual increase in power as the air passage is freed up by the melting ice. The amount of power regained is an indication of the severity of ice build-up. It must be kept in mind that the ingestion of small amounts of water into the engine following melting in the carburettor may cause an initial period of rough running for as much as one or two minutes before the power increase is noted.

Again, the pilot will note this as evidence that icing conditions are present. However, more than one pilot, when confronted with a rough running engine has mistakenly turned the carburettor heat back off, thereby exacerbating the situation. Applying carb heat as a matter of routine is built into numerous in-flight and pre-landing checks. In long descents, carb heat may be used continuously to prevent icing build-up; with the throttle closed there is a significant pressure (and therefore temperature) drop in the carburettor which can cause rapid ice build-up that could go unnoticed because engine power is not used. Also, the exhaust manifold cools considerably when power is removed, so if carb icing occurs, there may not be heat sufficient to remove it. Thus, most operational checklists call for the routine application of carb heat whenever the throttle is closed in flight. Usually, the air filter is bypassed when carb heat is used. If the air filter becomes clogged (with snow, ice, or dust debris), using carb heat allows the engine to keep running. Because using unfiltered air can cause engine wear, carb heat usage on the ground (where dusty air is most probable) is kept to a minimum. Altitude has an indirect effect on carburettor ice because there are usually significant temperature differences with altitude. Clouds contain moisture, and therefore flying through clouds may necessitate more frequent use of carb heat. The intake air of an aircraft engine equipped with a supercharger is heated through compression, so the air entering the throttle body is already warmed enough that carb heat is unnecessary.

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Fuel icing Water held in suspension in the fuel may precipitate and freeze in the induction piping, especially in the elbows formed by bends. In fuel metering systems, ice prevention is the result of using warm or hot air. In some cases, the air warmed by the engine is fed into the intake system, instead of cold air from ram effect. In other systems, the air from around the engine is ducted through a heater muff fitted around the exhaust pipe thus supplying hot air for ice prevention or ice dispersal. The usual method of introducing warm or hot air to the intake is selected using a flap or shutter valve similar to that used for selecting filter/unfiltered air. The diagram below top-left shows a typical intake system that may be selected COLD – FILTERED – HOT.

The warm air taken from around the engine via the alternateair valve is usually from the rear of the cylinders. Most modern installations draw the hot air to the alternate-air valve via a muff around the exhaust. See the diagram below bottom-left. On some aircraft, hot air from this muff is also used for cabin heating, so its integrity should be checked frequently because of the danger of carbon monoxide from the exhaust gases.

The alternate-air valve which allows hot or warm air into the intake ducting is usually manually operated from the co*ckpit. In some installations, it either has to be fully open or fully closed as partial warm air may cause ice to form if the resultant temperature is in the danger range mentioned earlier. Some other installations allow varying degrees of hot air induction and the alternate-air valve shutter may be selected partially open, this only usually on systems fitted with an air intake temperature gauge. The alternate-air valve is springloaded to the closed or cold air position. If the cold air ducts are blocked with ice, the depression felt from the induction stroke is felt right back through the system. This causes the alternateair valve to open against the spring pressure independent of co*ckpit selection, thus supplying air for combustion. Most pilot’s manuals for light aircraft call for the heat control to be selected hot for a few seconds every 15 minutes of flight. Total Training Support Ltd © Copyright 2020

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Intake configuration for hot, cold ram and filtered air

Exhaust muff air heating

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Fluid de-icing system Although not very common some aircraft are fitted with a fluid de-icing system to supplement the hot air system previously described. The system, which is controlled from the co*ckpit, consists of: • • •

a tank; a pump; and manual or electrical spray nozzles in the induction pipe.

The de-ice fluid, which has an alcohol base, is used to enrich the fluid air mixture. At high power, this is an advantage, but at low power settings, the enrichment could make the mixture too rich, so the system must be used with care.

FSII is an agent that is mixed with the fuel as it is pumped into the aircraft. The mixture of FSII must be between 0.10% and 0.15% by volume for the additive to work correctly, and the FSII must be distributed evenly throughout the fuel. Simply adding FSII after the fuel has been pumped is therefore not sufficient. As aircraft climbs after take-off, the temperature drops and any dissolved water separates from the fuel. FSII dissolves itself in water preferentially over the fuel, where it then serves to depress the freezing point of water to -43°C. Alternatively, isopropyl alcohol in amounts not to exceed 1% by volume can be added only to aviation fuel.

Fuel system icing inhibitor (FSII) is an additive to aviation fuels that prevents the formation of ice in fuel lines. FSII is sometimes referred to by the registered, genericised trademark Prist HIFLASH LO-FLO. Gasoline can contain a small amount of dissolved water that does not appear in droplet form. As an aircraft gains altitude, the temperature drops and the fuel’s capacity to hold water diminishes. Dissolved water can separate and could become a severe problem if it freezes in fuel lines or filters, blocking the flow of fuel and shutting down an engine.

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16.4.2 Fuel injection systems General Purpose Float carburettors have limitations that have been overcome by pressure carburettors: •

•

Float carburettors are susceptible to carburettor icing because fuel is discharged into the Venturi where it evaporates rapidly there or at the throttle plate. Pressure carburettors do not discharge their fuel in the Venturi but rather downstream of the throttle. Float carburettors are sensitive to certain manoeuvres. Fuel can surge to the top of the float chamber and shut off the fuel flow to the engine. Pressure carburettors meter the fuel by measuring the amount of air flowing into the engine and spraying an appropriate amount of fuel under pressure into this air. These carburettors can even function properly, inverted, for limited periods. Float carburettors produce an increasingly rich mixture as the aircraft goes up in altitude. Some pressure carburettors are equipped with automatic mixture controls that hold the mixture ratio constant as altitude changes. Carburettors soequipped become truly mass metering devices. Large, high-powered reciprocating engines used during World War II required a fuel metering system that was superior to the float carburettor, in that it did not ice up and did not shut off fuel flow to the engine during negative-G manoeuvres. Direct fuel injection was used on some engines, but the most successful and popular fuel metering system was the Stromberg pressure injection carburettor.

Principle This carburettor measured the mass of air entering the engine and metered the correct amount of fuel. The fuel was injected under pressure into the centre, or eye, of the internal supercharger impeller. Some of these carburettors were fitted with antidetonation injection systems that sprayed an alcoholwater mixture into the supercharger along with the fuel. The water evaporated and increased the density of the air flowing into the engine. Modern pressure carburettors are adapted from the pressure injection carburettor but are simplified to meet the needs of smaller engines. Pressure carburettors have the same basic systems as float carburettors: main metering, idling, acceleration, mixture control, and power enrichment. Pressure injection carburettors are distinctly different from floattype carburettors as they do not incorporate a vented float chamber or suction pickup from a discharge nozzle located in the Venturi tube. Instead, they provide a pressurised fuel system that is closed from the engine fuel pump to the discharge nozzle. The Venturi serves only to create pressure differentials for controlling the quantity of fuel to the metering jet in proportion to airflow to the engine.

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Fuel injection servos

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The fuel-injection system has many advantages over a convention carburettor system. There is less danger of induction icing since the drop in temperature due to fuel vaporisation takes place in or near the cylinder. Acceleration is also improved because of the positive action of the injection system. Also, fuel injection improves fuel distribution. This reduces the overheating of individual cylinders often caused by variations in mixture due to uneven distribution. The fuel injection system also gives better fuel economy than a system in which the mixture to most cylinders must be richer than required to ensure that the cylinder with the leanest mixture operates properly. Fuel-injection systems vary in their details of construction, arrangement, and operation. The Bendix and Continental systems are the most common and are discussed as examples later in this section. The illustration in the diagram below represents a pressuretype carburettor simplified so that only the basic parts are shown. Note the two small passages, one leading from the carburettor air inlet to the left side of the flexible diaphragm and the other from the Venturi’s throat to the right side of the diaphragm.

When air passes through the carburettor to the engine, the pressure on the right of the diaphragm is lowered because of the drop in pressure at the Venturi’s throat. As a result, the diaphragm moves to the right, opening the fuel valve. Pressure from the engine-driven pump then forces fuel through the open valve to the discharge nozzle, where it sprays into the airstream. The distance the fuel valve opens is determined by the difference between the two pressures acting on the diaphragm. This difference in pressure is proportional to the airflow through the carburettor. Thus, the volume of airflow determines the rate of fuel discharge. The pressure injection carburettor (PIC) is an assembly of the following units: • • • •

a throttle body; an automatic mixture control; a regulator unit; and a fuel control unit (some are equipped with an adapter)

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Pressure injection carburettor (PIC)

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Throttle body The throttle body contains the throttle valves, main Venturi, boost Venturi, and the impact tubes. All air entering the cylinders must flow through the throttle body; therefore, it is the air control and measuring device. The airflow is measured by volume and weight so that the proper amount of fuel can be added to meet the engine demands under all conditions. As air flows through the Venturi, its velocity increases, and its pressure decreases (Bernoulli’s principle). This low pressure is vented to the low-pressure side of the air diaphragm, chamber B in the regulator assembly in the diagram below. The impact tubes sense carburettor inlet air pressure and direct it to the automatic mixture control, which measures the air density. From the automatic mixture control, the air is directed to the high-pressure side of the air diaphragm (chamber A). The pressure differential of the two chambers acting upon the air diaphragm is known as the air metering force, which opens the fuel poppet valve. The throttle body controls the airflow with the throttle valves. The throttle valves may be either rectangular or disk-shaped, depending on the design of the carburettor. The valves are mounted on a shaft, which is connected by linkage to the idle valve and the throttle control in the co*ckpit. A throttle stop limits the travel of the throttle valve and has an adjustment which sets engine idle speed.

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Bendix Stromberg PD12 downdraft throttle body

Pressure injection carburettor throttle body

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The regulator The regulator is a diaphragm-controlled unit divided into five chambers: • • • • •

A B C D E

impact pressure; Venturi pressure; metered fuel pressure; regulated fuel pressure; and pump delivery pressure.

It contains two regulating diaphragms and a poppet valve assembly. Chamber A is regulated air-inlet pressure from the air intake. Chamber B is boosted Venturi pressure. Chamber C contains metered fuel pressure controlled by the discharge nozzle or fuel feed valve. Chamber D contains unmetered fuel pressure controlled by the opening of the poppet valve. Chamber E is the fuel pump pressure controlled by the fuel pump pressure relief valve. A stem connects the poppet valve assembly to the two main control diaphragms. The purpose of the regulator unit is to regulate the fuel pressure to the inlet side of the metering jets in the fuel control unit. This pressure is automatically regulated according to the mass airflow to the engine. The carburettor fuel strainer, located in the inlet to chamber E, is a fine mesh screen through which all the fuel must pass as it enters chamber D. The strainer must be removed and cleaned at scheduled intervals.

Referring to the diagram below, assume that for a given airflow through the throttle body and Venturi, a negative pressure of 1.7 kPa (1∕4 psi) is established in chamber B. This tends to move the diaphragm assembly and the poppet valve in a direction to open the poppet valve permitting more fuel to enter chamber D. The pressure in chamber C is held constant at 35 kPa (5 psi) (70 kPa (10 psi on some installations)) by the discharge nozzle or impeller fuel feed valve. Therefore, the diaphragm assembly and poppet valve move in the open direction until the pressure in chamber D is 36.2 kPa (51∕4 psi). Under these pressures, there is a balanced condition of the diaphragm assembly with a pressure drop of 1.7 kPa (1∕4 psi) across the jets in the fuel control unit (auto-rich or auto-le an). If nozzle pressure (chamber C pressure) rises to 36.2 kPa (51∕4 psi), the diaphragm assembly balance is upset. The diaphragm assembly moves to open the poppet valve to establish the necessary 36.2 kPa (51∕4 psi) pressure in chamber D. Thus, the 1.7 kPa (1∕4 psi) differential between chamber C and chamber D is re-established. The pressure drop across the metering jets remains the same. If the fuel inlet pressure is increased or decreased, the fuel flow into chamber D tends to increase or decrease with the pressure change causing the chamber D pressure to do likewise. This upsets the balanced condition previously established, and the poppet valve and diaphragm assembly respond by moving to increase or decrease the flow to re-establish the pressure at the 1.7 kPa (1∕4 psi) differential.

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Pressure injection carburettor regulator

Bendix-Stromberg carburettor regulator

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The fuel flow changes when the mixture control plates are moved from auto-lean to auto-rich, thereby selecting a different set of jets or cutting one or two in or out of the system. When the mixture position is altered, the diaphragm and poppet valve assembly reposition to maintain the established pressure differential of 1.7 kPa (1∕4 psi) between chambers C and D, maintaining the established differential across the jets. Under low power settings (low airflows), the difference in pressure created by the boost Venturi is not sufficient to accomplish consistent regulation of the fuel. Therefore, an idle spring is incorporated in the regulator. As the poppet valve moves toward the closed position, it contacts the idle spring. The spring holds the poppet valve off its seat far enough to provide more fuel than is needed for idling. The idle valve regulates this potentially over-rich mixture. At idling speed, the idle valve restricts the fuel flow to the proper amount. At higher speeds, it is withdrawn from the fuel passage and has no metering effect.

If the vapour vent valve sticks in a closed position or the vent line from the vapour vent to the fuel tank becomes clogged, the vapour-eliminating action is stopped. This causes the vapour to build up within the carburettor to the extent that it passes through the metering jets with the fuel. With a given size carburettor metering jet, the metering of vapour reduces the quantity of fuel metered. This causes the fuel/air mixture to lean out, usually intermittently

Vapour vent systems are provided in these carburettors to eliminate fuel vapour created by the fuel pump, heat in the engine compartment, and the pressure drop across the poppet valve. The vapour vent is located in the fuel inlet (chamber E) or, on some models of carburettors, in both chambers D and E. The vapour vent system operates in the following way. When air enters the chamber in which the vapour vent is installed, the air rises to the top of the chamber, displacing the fuel and lowering its level. When the fuel level has reached a predetermined position, the float (which floats in the fuel) pulls the vapour vent valve off its seat, permitting the vapour in the chamber to escape through the vapour vent seat, its connecting line, and back to the fuel tank.

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Pressure injection carburettor regulator

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Fuel control unit The fuel control unit is attached to the regulator assembly and contains all metering jets and valves. The idle and power enrichment valves, together with the mixture control plates, select the jet combinations for the various settings (i.e., autorich, auto-lean, and idle cut-off). The purpose of the fuel control unit is to meter and control the fuel flow to the discharge nozzle. The basic unit consists of three jets and four valves arranged in series, parallel, and series-parallel hook-ups. These jets and valves receive fuel under pressure from the regulator unit and then meter the fuel as it flows to the discharge nozzle. The manual mixture control valve controls the fuel flow. By using proper size jets and regulating the pressure differential across the jets, the right amount of fuel is delivered to the discharge nozzle, giving the desired fuel/air ratio in the various power settings. It should be remembered that the regulator unit regulates the inlet pressure to the jets and the discharge nozzle controls the outlet pressure. The jets in the basic fuel control unit are the auto-lean jet, the auto-rich jet, and power enrichment jet. The basic fuel flow is the fuel required to run the engine with a lean mixture and is metered by the auto-lean jet. The auto-rich jet adds enough fuel to the basic flow to give a slightly richer mixture than best power mixture when the manual mixture control is in the auto-rich position.

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Fuel control unit

Bendix-Stromberg carburettor fuel control unit

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The four valves in the basic fuel control unit are: • • • •

The regulator fill valve is a small poppet-type valve located in a fuel passage which supplies chamber C of the regulator unit with metered fuel pressure. In idle cut-off, the flat portion of the cam lines up with the valve stem, and a spring closes the valve. This provides a means of shutting off the fuel flow to chamber C and thus provides for a positive idle cut-off.

an idle needle valve; a power enrichment valve; a regulator fill valve; and a manual mixture control.

The functions of these valves are as follows. The idle needle valve meters the fuel in the idle range only. It is a round, contoured needle valve, or a cylinder valve placed in series with all other metering devices of the basic fuel control unit. The idle needle valve is connected by linkage to the throttle shaft so that it restricts the fuel flowing at low power settings (idle range). The manual mixture control is a rotary disk valve consisting of a round stationary disk with ports leading from the auto-lean jet, the auto-rich jet, and two smaller ventholes. Another rotating part, resembling a cloverleaf, is held against the stationary disk by spring tension and rotated over the ports in that disk by the manual mixture control lever. All ports and vents are closed in the idle cut-off position. In the autolean position, the ports from the auto-lean jet and the two vent holes are open. The port from the auto-rich jet remains closed in this position. In the auto-rich position, all ports are open. The valve plate positions are illustrated in the diagram below. The three positions of the manual mixture control lever make it possible to select a lean mixture a rich mixture or to stop fuel flow entirely. The idle cutoff position is used for starting or stopping the engine. During starting, fuel is supplied by the primer.

The power enrichment valve is another poppet-type valve. It is in parallel with the auto-lean and auto-rich jets, but it is in series with the power enrichment jet. This valve starts to open at the beginning of the power range. It is opened by the unmetered fuel pressure overcoming metered fuel pressure and spring tension. The power enrichment valve continues to open wider during the power range until the combined flow through the valve, and the auto-rich jet exceeds that of the power enrichment jet. At this point, the power enrichment jet takes over the metering and meters fuel throughout the power range. Carburettors equipped for water injection are modified by the addition of a derichment valve and a derichment jet. The derichment valve and derichment jet are in series with each other and parallel with the power enrichment jet. Auto lean Auto lean equates to the economic cruise condition of the float chamber carburettor. Auto rich This selection equates to the rich cruise condition in the float chamber carburettor.

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Manual mixture control valve plate positions

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The carburettor controls fuel flow by varying two basic factors. The fuel control unit, acting as a pressure-reducing valve, determines the metering pressure in response to the metering forces. The regulator unit, in effect, varies the size of the orifice through which the metering pressure forces the fuel. It is a basic law of hydraulics that the amount of fluid that passes through an orifice varies with the size of the orifice and the pressure drop across it. The internal automatic devices and mixture control act together to determine the effective size of the metering passage through which the fuel passes. The internal devices, fixed jets, and variable power enrichment valve are not subject to direct external control. The throttle body The throttle body contains the main Venturi and the boost Venturi, as shown in the diagram below. The drop in pressure in the main Venturi causes an acceleration of air through the boost Venturi, thus giving a more considerable pressure drop at the throat of the boost Venturi. A greater air metering force is obtained, and therefore an amplified pressure drop is ducted to chamber B of the regulator. The air throttle valve controls the flow of air through the throttle body and is under the direct control of the pilot’s throttle power lever.

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Bendix-Stromberg PT-13G1 Pressure (injection), 3-venturi carburettor Throttle body

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An automatic mixture controller is mounted on the throttle body which controls the impact pressure ducted to chamber A of the regulator to compensate for changes in air density. During a climb for example, when the atmospheric (or impact pressure) drops, an aneroid capsule expands, forcing a needle into the duct which is sampling impact pressure for chamber A. So, with a drop in air pressure, the pressure to chamber A is restricted. The air metering force becomes less, which in turn reduces the opening of the fuel valve of the regulator, thus reducing the regulated fuel pressure. During aircraft descent, an increase in air pressure causes the bellows to contract, withdrawing the needle, allowing the fuel impact pressure to chamber A to increase, to sea-level pressure when the valve is fully open. You may have noticed that the airflow is down, whereas all previous diagrams have shown the airflow up. This is a manufacturer’s choice often dictated by the position of the air intake for a particular installation; the two different applications are known merely as ‘up-draught’ or ‘down-draught’ carburettors.

Adjacent to the discharge nozzle is an automatic accelerator pump; this is a simple diaphragm operated pump. The air pressure downstream of the throttle valve varies according to the throttle position, being lowest at idle and increasing as the throttle is opened. This pressure is ducted to the rear of the pump diaphragm where it moves the pump to the bottom of its stroke, assisted by a spring. At small throttle openings, the fuel discharge pressure is sufficient to overcome the spring and air pressure, causing the pump to move to the top of its stroke, thus filling the pump with a charge of fuel as the throttle is opened. This higher air pressure, plus the spring, then forces the pump to the bottom of its stroke, discharging the charge of fuel to the discharge nozzle. This extra fuel is sufficient with the normal metered flow to overcome any temporary weakening of the mixture.

The discharge nozzle, which is mounted downstream of the throttle valve, does not open until sufficient pressure is available (about 34.5 kPa (5 psi)) to overcome the springassisted diaphragm. Once open, it acts as a pressure regulator to maintain approximately the same pressure with varying degrees of opening for the various throttle settings.

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Example system: Bendix PS pressure carburettor General In the PS series carburettor, as in the pressure-injection carburettor, the regulator spring has a fixed tension, which tends to hold the poppet valve open during idling speeds or until the D chamber pressure equals approximately 27.6 kPa (4 psi). The discharge nozzle spring has a variable adjustment tailored to maintain 27.6 kPa (4 psi). This maintains a pressure condition of 27.6 kPa (4 psi) in chamber C of the discharge nozzle assembly and 27.6 kPa (4 psi) in chamber D. This produces a zero drop across the main jets at zero fuel flow. At a given airflow, if the suction created by the Venturi is equivalent to 115 g (1∕4 lb), the pressure decrease is transmitted to chamber B and the vent side of the discharge nozzle. Since the area of the air diaphragm between chambers A and B is twice as high as that between chambers B and D, the 115 g (1∕4 lb) decrease in pressure in chamber B moves the diaphragm assembly to the right to open the poppet valve. Meanwhile, the decreased pressure on the vent side of the discharge nozzle assembly causes a lowering of the total pressure from 1.8 kg to 1.7 kg (4 lb to 33∕4 lb). The higher pressure of the metered fuel 1.9 kg (41∕4 pounds) results in a differential across the metering head of 115 g (1∕4 lb) (for the 115 g (1∕4 lb) pressure differential created by the Venturi).

The same ratio of pressure drop across the jet to Venturi suction applies throughout the range. Any increase or decrease in fuel inlet pressure tends to upset the balance in the various chambers in the manner already described. When this occurs, the main fuel regulator diaphragm assembly repositions to restore the balance. The mixture control, whether operated manually or automatically, compensates for enrichment at altitude by bleeding impact air pressure into chamber B, thereby increasing the pressure (decreasing the suction) in chamber B. Increasing the pressure in chamber B tends to move the diaphragm and poppet valve more toward the closed position, restricting fuel flow to correspond proportionately to the decrease in air density at altitude. The idle valve and economiser jet can be combined in one assembly. The unit is controlled manually by the movement of the valve assembly. At low airflow positions, the tapered section of the valve becomes the predominant jet in the system, controlling the fuel flow for the idle range. As the valve moves to the cruise position, a straight section on the valve establishes a fixed orifice effect which controls the cruise mixture. When the valve is pulled full-open by the throttle valve, the jet is pulled entirely out of the seat, and the seat side becomes the controlling jet. This jet is calibrated for takeoff power mixtures.

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An airflow-controlled power enrichment valve can also be used with this carburettor. It consists of a spring-loaded, diaphragmoperated metering valve. Refer to the diagram for a schematic view of an airflow power enrichment valve. One side of the diaphragm is exposed to unmetered fuel pressure and the other side to Venturi suction plus spring tension. When the pressure differential across the diaphragm establishes a force strong enough to compress the spring, the valve opens and supplies an additional amount of fuel to the metered fuel circuit in addition to the fuel supplied by the main metering jet.

When the mixture control lever is moved to the idle cut-off position, a cam on the linkage actuates a rocker arm which moves the idle cut-off plunger inward against the release lever in chamber A. The lever compresses the regulator diaphragm spring to relieve all tension on the diaphragm between chambers A and B. This permits fuel pressure plus poppet valve spring force to close the poppet valve, stopping the fuel flow. Placing the mixture control lever in idle cut-off also positions the mixture control needle valve off its seat and allows metering suction within the carburettor to bleed off.

Accelerating pump The accelerating pump of the Stromberg PS carburettor is a spring-loaded diaphragm assembly located in the metered fuel channel with the opposite side of the diaphragm vented to the engine side of the throttle valve. With this arrangement, opening the throttle results in a rapid decrease in suction. This decrease in suction permits the spring to extend and move the accelerating pump diaphragm. The diaphragm and spring action displace the fuel in the accelerating pump and force it out the discharge nozzle. Vapour is eliminated from the top of the main fuel chamber D through a bleed hole, then through a vent line back to the main fuel tank in the aircraft. Manual mixture control A manual mixture control provides a means of correcting for enrichment at altitude. It consists of a needle valve and seat that form an adjustable bleed between chamber A and chamber B. The valve can be adjusted to bleed off the Venturi suction to maintain the correct fuel/air ratio as the aircraft gains altitude. Total Training Support Ltd © Copyright 2020

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Airflow power enrichment valve

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Example system: Bendix RSA injection system General The RSA fuel injection systems are designed to meter fuel in direct ratio to the volume of air being consumed by the engine at any given time. This is accomplished by sensing Venturi suction and impact air pressures in the throttle body, opening or closing the throttle valve results in a change in the volume of air being drawn into the engine. This results in a change in the velocity of air passing across the impact tubes and through the Venturi. When air velocity increases, the pressure at the impact tubes remains relatively constant depending upon the inlet duct configuration, air figure location, etc. The pressure at the Venturi’s throat decreases. This decrease creates a differential (impact minus suction) which is used over the entire range of operation of the fuel injection system as a measurement of the volume of air consumption.

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All reciprocating engines operate most efficiently in a very narrow range of air to fuel (or fuel/air) ratios. The RSA injection system uses the measurement of air volume flow to generate a usable force which can be used to regulate the flow of fuel to the engine in proportion to the amount of air being consumed. This is accomplished by channelling the impact and Venturi suction pressures to opposite sides of a diaphragm. This difference between these two pressures then becomes a usable force which is equal to the area of the diaphragm multiplied by the pressure difference.

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Bendix RSA injection system

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Fuel is supplied to the engine from the aircraft fuel system. This system usually includes a boost pump located either in the fuel tank or the fuel line between the tank and the engine. The engine-driven fuel pump receives fuel from the aircraft system (including the boost pump) and supplies that fuel at a relatively constant pressure to the fuel injector servo inlet. The engine manufacturer specifies the fuel pump pressure setting applicable to the specific fuel injector installation. The fuel injectors are calibrated at that inlet pressure setting. The settings are checked to assure that metered fuel flow is not affected by changes in inlet fuel pressure caused by normal boost pump “ON” or “OFF” operation. The RSA injection system will, if correctly assembled and calibrated, meet all performance requirements over an extremely wide range of inlet fuel pressures. Its heart is the servo pressure regulator. The easiest way to explain the operation of this regulator and its relationship to the main metering jet is to describe a power change which requires a fuel flow change. To begin this explanation, we start from a cruise condition where air velocity through the throttle body is generating an impact pressure minus Venturi suction pressure differential at the theoretical value of ‘2’. This air pressure differential ‘2’ is exerting a force to the right. Fuel flow to the engine, passing through the metering jet, generates a fuel pressure differential (unmetered fuel minus metered fuel pressure). This pressure differential applied across a second (fuel) diaphragm is also creating a force with a value of ‘2’. This value of ‘2’ is exerting a force to the left.

The two opposing forces (fuel and air differentials) are equal, and the regulator servo valve (which is connected to both diaphragms by a stem) is held at a fixed position that allows the discharge of just enough metered fuel to maintain pressure balance. If the throttle is opened to increase power, airflow immediately increases. This results in an increase in the pressure differential across the air diaphragm to a theoretical value of ‘3’. The immediate result is a movement of the regulator servo valve to the right. This increased servo valve opening causes a decrease in pressure in the metered fuel chamber and an increase in the fuel pressure differential across the main metering jet. When this increasing fuel differential pressure force reaches a value of ‘3’ (equalling the air diaphragm force), the regulator stops moving, and the servo valve stabilises at a position which maintains the balance of pressure differentials, i.e., air and fuel, each equalling 3. Fuel flow to the engine is increased to support the higher power level requested. The fuel diaphragm force being generated by the pressure drop across the main metering jet is equal to the air diaphragm force being generated by the Venturi. This sequence of operation is correct for all regimes of power operation and all power changes. The regulator servo valve responds to changes in effective air diaphragm differential pressure forces. It adjusts the position of the servo valve to regulate unmetered to metered fuel pressure differential forces accordingly. Fuel flow through the metering jet, and to the engine, is a function of its size and the pressure differential across it. The servo valve does not meter fuel; it only controls pressure differential across the metering jet.

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Bendix RSA injection system

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Flow divider Metered fuel flow is delivered from the fuel injector servo unit to the engine through a system which usually includes a flow divider and a set of discharge nozzles (one nozzle per cylinder). A few engine installations do not use a flow divider. On these engines, the fuel flow is divided by either a single four-way fitting (4-cylinder engines) or tee which divides the fuel flow into two separate paths. Each path incorporates a three-way fitting (6-cylinder engines). The flow divider consists of a valve, a sleeve, a diaphragm and a spring. The valve is spring-loaded to the closed position in the sleeve. This effectively closes the path of fuel flow from the fuel injector servo to the nozzles and at the same time isolates each nozzle from all of the others at engine shut down. The two primary functions of the flow divider are: • •

to assure equal distribution of metered fuel to the nozzles at and just above idle; and to provide isolation of each nozzle from all the others for a clean engine shut down.

The area of the fuel discharge jet in the fuel nozzles is sized to accommodate the maximum fuel flow required at rated horsepower without exceeding the regulated fuel pressure range capability of the servo pressure regulator. The area of the jet in the nozzle is such that metered fuel pressure at the nozzle is negligible at the low fuel flows required at and just above idle. Metered fuel from the injector servo enters the flow divider and is channelled to a chamber beneath the diaphragm. At idle, fuel pressure is only sufficient to move the flow divider valve slightly open, exposing the bottom of a V slot in the exit to each nozzle. This position provides the Total Training Support Ltd © Copyright 2020

accuracy of fuel distribution needed for smooth idle. As the engine is accelerated, metered fuel pressure at the flow divider inlet and in the nozzle lines increases. It gradually moves the flow divider valve open against the spring pressure until the area of the V slot opening to each nozzle is greater than the area of the fuel restrictor in the nozzle. At that point, responsibility for equal distribution of metered fuel flow is assumed by the nozzles. Since metered fuel pressure (nozzle pressure) increases in direct proportion to metered fuel flow, a simple pressure gauge can be used as a flow meter indicator. If the fuel restrictor in one or more nozzles becomes partially plugged by contaminant, the total exit path for metered fuel flow is reduced. The fuel injector servo continues to deliver the same amount of total fuel flow. Therefore, nozzle pressure increases, indicating fuel flow increase on the flow meter gauge. The cylinder(s) having restricted nozzles run lean, and the remaining cylinders are rich – the result: a rough engine accompanied by high fuel flow indication. The problem may be caused partially blocked nozzle(s). When the mixture control is placed in cut off, fuel pressure to the flow divider drops to zero. The spring forces the flow divider valve to the closed position and immediately interrupts the flow of fuel to each nozzle. This breaks the path of capillary flow, which would allow manifold suction to continue to draw fuel in dribbles from one or more nozzle lines as the engine coasts down. Without the flow divider, this ‘dribbling’ of fuel into one or more cylinders could keep the engine running for a minute or more.

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Bendix RSA injection system servo

Bendix RSA injection system

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Fuel flow gauge

Bendix RSA injection system flow divider

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Bendix RSA injection system flow divider and injector nozzles

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Idle valve The idle valve is connected to the throttle linkage. It effectively reduces the area of the main metering jet for accurate metering of fuel in the idle range. It is externally adjustable and allows the mechanic to properly tune the fuel injector to the engine installation for a proper idle mixture. Idle mixture is correct when the engine gains approximately 25 to 50 RPM from its idle speed setting as the mixture control is placed in cut off. Manual control of idle mixture is necessary because, at the very low airflow through the Venturi in the idle range, the air metering force is not sufficient to control fuel flow accurately. On some engines, according to specific installation requirements, an enrichment jet is added in parallel with the main metering jet. On these installations, the sliding (rotating) idle valve begins to uncover the enrichment Jet at a pre-set throttle position. This parallel flow path increases the fuel/air mixture strength to provide for ‘fuel cooling’ of the engine in the high-power range. In simple terms, this is trading increased fuel consumption for added engine life.

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Bendix RSA injection system idle valve adjustment

Bendix RSA injection system idle valve

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Bendix RSA injection system servo

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The illustration below shows the further operation of the manual mixture control, constant head, constant effort springs, and centre body seal. The manual mixture control is a sliding valve, that can be used by the pilot to reduce the size of the metering jet effectively. With the servo pressure regulator functioning to maintain a differential pressure across the metering jet in proportion to the volume of airflow, the flow through the jet may be varied by changing its effective size. This allows the pilot the option to manually lean the mixture for best cruise power or best specific fuel consumption. It also provides the means to shut off fuel flow to the engine at engine shut down. The constant head idle spring augments the force of the air diaphragm in the idle and off idle range where the air pressure differential is not sufficient to move the servo valve open. The idle spring assures that the regulator servo valve is open sufficiently to allow fuel being metered by the idle valve to flow out to the flow divider. As airflow increases above idle, the air diaphragm begins to move to the right in response to increasing air pressure differential. It compresses the constant head idle spring until its retainer and guide contact the diaphragm plate. From this point onward, in terms of airflow, fuel flow, or power, the constant head idle spring assembly is a solid member moving with the air diaphragm and exerts no force of its own. The constant head spring LS furnished in a selection of strengths so the overhaul technician can adequately calibrate the injector for idle fuel flow and the transition to servo regulator-controlled fuel flow.

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In most installations, the transition from idle to servo regulatorcontrolled fuel flow has to be supplemented with a constant effort spring. This spring also assists the air diaphragm to move smoothly from the low airflow idle range to the higher power range of operation. It is also furnished in a selection of strengths to be utilised by the overhaul technician for proper calibration of the unit. The fuel section of the servo pressure regulator is separated from the air section by a centre body seal assembly. In 1979, a product improvement was made to the seal changing the design from a rubber diaphragm to bellows type. This bellows seal, as shown in the illustration top right, is presently used in all current production and newly overhauled-type fuel injectors. Leakage through the centre body seal causes extremely rich operation and poor cut off. The presence of raw fuel out the impact tubes may indicate possible seal leakage. Failure of this seal requires repair in an overhaul shop. It cannot be replaced in the field.

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Bendix RSA injection system servo regulator

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Injector nozzles Several different nozzles are manufactured to suit the specific installation requirement for the engine. Normally aspirated, which requires the simple nozzle assembly with the air bleed screen and shroud pressed in place, or the configuration of the shroud assembly to accept the supercharger air pressure signal to the nozzle. All nozzles are of the air bleed type. This means that the fuel is discharged inside the nozzle body into a chamber which is vented to either atmospheric air pressure or supercharger air pressure (injector top deck pressure).

A net decrease in metered fuel pressure results and shows up on the flow meter as a lower fuel flow indication. The result is rough idle with low fuel flow indication and higher than regular RPM rise when going into cut-off. The engine will also have inferior cut off, tending to continue chugging for several seconds following the movement of the mixture control to cut off.

The nozzle is mounted into the intake valve plenum of the cylinder head. Its exit is always exposed to manifold pressure, which on a normally aspirated engine is always less than atmospheric. This results in air being drawn in through the air bleed and mixed with fuel in the fuel/air chamber to provide for fuel atomisation. This is particularly important to the idle and low power ranges where manifold pressure is weakest and bleed air intake is greatest. A plugged air bleed in this range allows the exit of the fuel restrictor to be exposed to manifold suction, which effectively increases the pressure differential across the restrictor and causes an increase in fuel flow through that nozzle. Since this nozzle is now, in effect, stealing fuel from the other nozzles (injector servo output flow remains the same) this cylinder runs rich, and the other cylinders are correspondingly lean.

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The illustration below shows the function and operation of the automatic mixture control (AMC). It also expands the description of the manual mixture control and idle valves. The mixture control is shown in the full rich position and the idle valve fully open as it would be at cruise power or above. In the cutaway, the two rotating valve assemblies are spring-loaded together back to back with an O-ring seal in between. Fuel flows through the mixture control valve, through the idle valve and out to the regulator servo valve. The inlet strainer is located underneath the fuel inlet fitting and is installed spring end first, so the open end is mated to the inlet fitting. If the screen becomes blocked by contaminant material, inlet pressure forces it away from the fitting, compressing the spring to allow fuel to bypass the screen if necessary. This screen filter is a mandatory 100% replacement item at overhaul. There is no approved method of cleaning this screen for reuse.

An engine pumps air based on volume, not weight. This volume is determined by the engine displacement, i.e., I0-540 cubic inches per complete four-stroke cycle (intake, compression, power, exhaust for all six cylinders). So, an I0-540 at 2,500 RPM would be consuming (pumping):

The AMC adjusts fuel/air ratio to compensate for the decreased air density as the aircraft climbs to altitude. Fuel/air ratio is expressed in pounds per hour of fuel and air, respectively. The fuel injector meters fuel on a pounds per hour basis, referenced to the volume of airflow, which, converted to velocity passing through the Venturi produces the air metering signal previously discussed.

As the aircraft climbs to altitude, the specific weight of air decreases from 0.0765 pounds per cubic foot until, at 15,000 ft, air only weighs 0.0432 pounds per cubic foot. The engine at 2,500 RPM would still be consuming 390 ft3 per minute, resulting in:

540 × (2,500/2) = 675,000 in3 per minute 675,000/1,728 = 390 ft3 per minute 390 × 0.0765 = 30 pounds per minute. 30 × 60 (min) = 1,800 pph airflow. This would be equivalent to cruise power at sea level. A fuel/air ratio of 0.08 would result in: 1,800 × 0.08 = 150 pph fuel flow.

(390 × 0.0432) × 60 = 1,020 pph airflow. This 1,020 pph airflow produces the same air metering signal across the Venturi that 1,800 pph did at sea level. This air metering signal maintains the 150 pph fuel flow which would result in: 150/1020 = 0.147 fuel/air ratio.

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Bendix RSA injection system automatic mixture control

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Without an AMC, it would be necessary for the pilot to continually lean the mixture manually to maintain the desired 0.08 fuel/air ratio. The AMC works independently of, and in parallel with, the manual mixture control by providing a variable orifice between the two air pressure signals (impact and suction) to modify the air metering signal force. The AMC assembly consists of a contoured needle that is moved in and out of an orifice by a bellows assembly. This bellows assembly reacts to changes in air pressure and temperature, increasing in length as pressure altitude increases. At ground level, the needle is positioned in the AMC orifice so that the orifice is closed, or nearly closed, to allow the maximum impact pressure to the impact pressure side of the air diaphragm. When the aircraft increases altitude, the AMC bellows elongates with air pressure decrease, and the needle is moved into its orifice. This increases the orifice opening between impact air and Venturi suction and allows impact air to bleed into the Venturi suction channel. This reduces the air metering force across the air diaphragm. The needle is contoured such that regardless of altitude (or air density) the correct air metering signal is established across the air diaphragm to maintain a relatively constant fuel/air ratio as air density changes with altitude. The above description applies to the externally mounted AMC, which is used on the RSA-5ABI and the RS-l0FBl – and shown below right.

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Bendix RSA injection system automatic mixture control

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Current production fuel injectors use a bullet-type AMC which is mounted in the bore of the throttle body. The outer diameter of this unit is contoured to perform the function of the Venturi. The function performed, and the principle of operation is the same as the externally mounted unit. The primary differences are: • •

The bellows assembly is exposed to Venturi suction rather than impact pressure. As the needle is moved into its orifice, impact air pressure to the servo regulator is restricted, thus causing a reduction of the air metering force across the air diaphragm precisely as described above.

On fuel injectors with the bullet-type Venturi which do not use an AMC, the bellows and needle assembly are not used, and the interconnecting channelling between impact air and Venturi suction is blocked off.

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Bendix RSA injection system automatic mixture control bellow

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Example system: Teledyne Continental injection system General The Continental®, or CMG, fuel injection system and the RSA system are both continuous-flow injection systems that do the same job, but they accomplish it in different ways. The RSA system measures the volume of air flowing into the engine to determine the amount of fuel to mix with it. The CMG system uses the engine RPM to determine the amount of fuel to send to the injector nozzles. It has four basic components: the engine-driven injector pump, the fuel-air control unit, the fuel manifold valve, and the nozzles. The CMG system consists of: • • • •

the fuel Injector pump assembly; the fuel/air control unit; the fuel manifold valve; and the injection discharge nozzles.

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Teledyne Continental Injection System

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The fuel injector pump assembly The fuel pump is perhaps the most critical and complex part of the system. This is a vane-type constant-displacement pump with some unique features that allow it to accomplish these functions. •

• •

Provides fuel flow that increases with engine speed. This increase in output is modified to furnish enough fuel when the pump turns at idling speed, yet not move too much fuel when it turns at takeoff speed. Removes vapours from the fuel and sends them to one of the fuel tanks. Incorporates a bypass valve that allows fuel from the auxiliary pump to flow to the engine for starting and for operation if the engine-driven pump malfunctions. Provides a regulated low pressure when the engine is idling. Provides a regulated high pressure when the engine is operating at high speed.

The pump is an engine-driven, positive displacement vane-type pump which accepts the fuel under fuel tank booster pump pressure and increases the fuel flow with a rise in engine speed. The fuel enters the pump assembly via a swirl chamber which acts to remove any air from the fuel. The diagram below is a simplified illustration showing the swirl chamber position.

A small fuel line is taken from the pump delivery to the top of the swirl chamber where there is a Venturi in the line, one side of which is open to the swirl chamber. As the fuel passes through the Venturi, the air is drawn in from the swirl chamber and is then passed back to the fuel tank. This ensures that the pump gets neat fuel only. A bypass from the pump delivery is routed back to the inlet side of the swirl chamber. In this bypass, there is a fixed ‘orifice’ which now makes the pump delivery pressure proportional to engine speed instead of just flow as was the case before the bypass was added. The size of the orifice determines the unmetered fuel pressure for any given speed; a smaller orifice would give a higher output pressure and vice versa. The system described works well at speeds from cruise to full power, but at low speed and at idle the fuel flow is low, and the fixed orifice does not provide enough restriction to maintain a constant output pressure. Therefore, a spring-loaded relief valve is fitted in the bypass line between the fixed orifice and the swirl chamber. This adjustable relief valve provides the required restriction for low idle speed and therefore the pump output pressure. As the engine speed increases the fuel pressure takes the relief valve entirely off its seat, and the pressure is then determined only by the fixed orifice.

As the fuel tends to follow the helical swirl pattern, the air bubbles rise to the top of the chamber and are then vented to the fuel tank. The fuel from the swirl chamber enters the positive displacement vane-type pump and is then pumped to the fuel/air control unit.

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Injector pump – normally aspirated

Injector pump – turbo-charged

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A simple plate type non-return valve (NRV) is fitted in the bottom of this swirl chamber, the line from the NRV joins the fuel pump delivery line to the fuel/air control unit. The NRV allows fuel under tank booster pump pressure to bypass the fuel pump to provide fuel for starting or may also be used in the event of main fuel pump failure. When the engine has started, the fuel pump delivery pressure closes the NRV. Many CMG fuel injection systems are used on engines equipped with turbochargers. Turbocharged engines present a unique problem to their fuel metering systems. When the throttle is opened, fuel flow to the engine increases immediately. Exhaust gases drive the turbocharger, and its speed increase lags considerably behind that of the engine. Since the engine receives its increased fuel before the airflow increases, in a turbocharged engine, the mixture becomes overly rich until the turbocharger comes up to speed. To prevent this momentary over-enrichment, the fuel pumps installed on turbocharged engines use a bellows, or aneroid, evacuated to approximately 95 kPa (28" Hg), to control the size of the metering orifice. This bellow is mounted in a compartment inside the pump where it senses the upper-deck, or turbocharger discharge, pressure. When the throttle is opened, the fuel pump turns faster and discharges only a slight increase in fuel flow. Before the turbocharger increases in speed, the upper-deck pressure is low, and the aneroid is expanded, which leaves the size of the return orifice unrestricted so that the pump output does not increase significantly. As the turbocharger speed increases, the upperdeck air pressure increases, and the aneroid compresses, decreasing the size of the orifice in the return line and increasing the pump output to match the rising induction airflow. Total Training Support Ltd © Copyright 2020

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Complete fuel pump arrangement

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The fuel/air control unit Fuel flows from the fuel injector pump into the fuel portion of the fuel-air control unit and the manifold valve. Air flows from the inlet air filter through the air portion of the fuel-air control unit into the intake manifold. Float carburettors, pressure carburettors, and the RSA fuel injection systems all have Venturis in the air passage to the cylinders to measure the amount of air flowing into the engine. However, the CMG system does not use a Venturi. The air passage is a smooth tube whose diameter allows it to supply an adequate amount of air into the cylinders during maximum power conditions. The throttle air valve is a circular butterfly valve across this tube that controls the amount of air entering the engine. An adjustable linkage connects the throttle air valve with the throttle fuel valve. An adjustable stop determines the amount the air valve remains open when the throttle is pulled back to the idling position. This determines the idle RPM.

The throttle fuel valve is a metering valve that covers the metering orifice outlet. When the throttle is wide open, the outlet is uncovered, and the main metering orifice does the metering. When the throttle is closed, the valve allows only enough of the outlet to be uncovered to meter the correct amount of fuel for idling. The linkage between the throttle air valve and the fuel valve is adjusted to control the idling fuel-air mixture.

The fuel control unit consists of a strainer in the fuel inlet, a mixture control valve, a metering plug containing the main metering orifice (jet), a throttle valve, a metered fuel outlet connected to the manifold valve, and a return fuel outlet that returns fuel to the inlet side of the pump. The mixture control is a variable selector valve. When the mixture control is in the “FULL RICH” position; the selector valve sends all fuel to the throttle. When it is in the “IDLE CUT OFF” position, it sends all fuel back to the inlet side of the pump. When it is in any position between these two extremes, some fuel goes to the throttle valve and some returns to the pump thus modulating unmetered fuel pressure to the metering jet.

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4-127 Module 16.4 Engine Fuel Systems

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Teledyne Continental Injection System – fuel/air control unit

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Teledyne Continental Injection System – adjustments

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Fuel manifold valve This valve is similar to, and provides the same function as, the flow divider in the Bendix RSA system, and is shown in the diagram below. The valve is attached to a spring-loaded diaphragm, the top being vented to atmosphere. The fuel enters the valve body just below the diaphragm. The fuel pressure under the diaphragm lifts the valve allowing the fuel to enter the valve through the upper side ducts to the central ventricle duct, where the pressure overcomes a spring-loaded ball valve to pass the fuel to the outlet ports in the valve body and thence to the injection nozzles. The spring-loaded valve ensures a positive cut off the fuel to the injector nozzles on engine shut down. A fuel pressure gauge line is tapped off the fuel line entering the manifold fuel valve to give a fuel pressure indication in the co*ckpit.

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Teledyne injection system

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Teledyne Continental Injection System – fuel manifold

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The injection discharge nozzle The nozzles are similar to those previously described for the Bendix system. A nozzle is shown in the diagram below. The fuel enters the nozzle body through a duct and passes to a calibrated orifice. Next, it mixes with the air, which enters through apertures in the side of the nozzle body through a filter screen. A metal shield protects the screen. The fuel/air mixture then passes to the injector outlet where it joins the manifold airflow immediately before the inlet valve.

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Teledyne Continental Injection System – injector nozzle

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Supercharged and turbocharged engines Super or turbocharging causes a few problems for aircraft fuel systems. Acceleration may result in over fuelling due to turbo lag; in this case, the fuel flow increases before the turbo has built up sufficient air pressure to increase the airflow. This is overcome by replacing the fixed orifice in the fuel pump assembly with a variable orifice operated by an aneroid capsule, as shown in the diagram. The evacuated bellows are subjected to turbo discharge pressure which only moves a needle valve in an orifice to close the orifice, thus increasing pressure and therefore the fuel flow when there is sufficient airflow to maintain the correct air/fuel ratio for a given throttle selection. The other problem relates to the use of nozzles. With turbocharged engines the manifold air pressure is often higher than atmospheric pressure and, in this case, the pressurised air would tend to blow the fuel out of the nozzle air bleed entrance. To overcome this, the nozzle is shrouded and pressurised either from turbo discharge pressure or from ram air.

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Typical naturally aspirated fuel system schematic (with fuel control unit)

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Typical naturally aspirated fuel system schematic (fuel pump with integral mixture control)

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Teledyne Continental injection system – components

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Aero-Diesel injection systems Due to the operating nature of the Diesel engine, that being of compressing only air during the compression stroke, the premixing of air and fuel in a carburettor is not an option for Diesel engines. An aero Diesel engine does not use a carburettor of any type, nor does it use a throttle. Some automotive Diesel engines do incorporate a throttle, but this is to make shutdown of the engine more of a smooth process and to create a suction in the induction manifold for more effective engine braking. Neither of these features is necessary on an aero Diesel engine. Injection types - general Injection systems fall into two categories, defined by where the fuel is injected. These are: • •

Direct injection is a newer technology. It differs from port injection in that the fuel injector delivers fuel directly into the cylinder chamber, rather than through the intake valve. This is necessary on Diesel engines since the lower ignition temperature would cause pre-ignition if the air/fuel mixture is compressed during the compression stroke. By injecting the Diesel fuel directly into the cylinder, it can be timed precisely with the optimum position of the piston before top dead centre (BTDC) as the fuel ignites as soon as it is injected. The precision required of the timing of the fuel injection necessitates an electronic control system. It is for this reason that modern aero Diesel engines are FADEC controlled.

port injection; and direct injection.

Port injection is when the fuel injector is located just before the intake valve. As the intake valve opens, the injector sprays in fuel that combines with the incoming air before this mixture rushes into the cylinder. As there is an injector for each cylinder, an equal amount of fuel is delivered to each piston. The piston and cylinder then induct the mixed air and fuel and compress the mixture. This is the operational principle of the gasoline engines as previously discussed.

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Direct injection

Port injection

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Diesel fuel injection types As all Diesel engines are fuel injected. There are two types of fuel injection systems used on aero Diesel engines. They are: • •

the direct injection system the Common-rail direct injection (CDI) system

Although both types are ‘direct injection’ in the definition previously stated, the direct fuel injection system described here is so-called because it has no electronic control system. The fuel is injected directly from the fuel pump. Direct injection This type of system was used on older Diesel engines (up to and including aero-Diesels used in the second world war). The system is inefficient and therefore is now used only on light duty Diesel engines (e.g. small agricultural machinery). It is therefore discussed here only briefly as a preamble to the Common-rail fuel injection system. The system uses an engine-driven ‘in-line’ pump to distribute fuel to the injectors. The injectors are situated in each cylinder head. The injectors are mechanically operated and have springloaded poppet valves, so they pop open and spray fuel in the cylinder when the fuel line pressure exceeds a specific limit, typically 4500 psi. Electronic controls on later model injection pumps regulate injection timing, fuel mixture and idle speed.

The purpose of the fuel injection pump is to deliver an exactly meted amount of fuel under high pressure at the right time to the injector. The system is a direct injection type. The injector injects the fuel directly into the combustion chamber or in some cases a pre-chamber which is connected to the cylinder, unlike those fitted in a gasoline engine which inject the fuel into the intake manifold just before the inlet valve. The injection system which delivers fuel to a diesel engine operates at a much higher pressure than a gasoline injection system; it can be as high as 17,400 psi for a direct injection system (and up to 23,500 psi for the Common-rail systems) compared to 35 – 90 psi for most gasoline fuel injection systems. Another function of the fuel injection pump is to regulate the timing of the fuel pulses. The timing of the fuel pulses is adjusted in response to engine RPM. Because the low-pressure fuel pump (which pulls the fuel from the tank to the input of the high-pressure pump), is driven by the engine, at higher engine RPM its fuel pressure output is higher. These fuel pressure changes are used to either advance or retard the fuel injection timing. Inside the pump mechanism, there is also a cold start device which advances the idle timing manually. A mechanical RPM governor also fitted to the fuel injection pump limits the maximum speed of the engine. A magnetically operated valve or solenoid opens and shuts off the fuel channel between the feed pump and the metering pump

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Diesel direct injection system Diesel Engines and Common-rail https://youtu.be/lVAdJlZr8_k Diesel Common-rail Injection Facts 1 https://youtu.be/cIkMtnd3LGQ Diesel Common-rail Injection Facts 2 https://youtu.be/7qlVrjxtoY0

Fuel Injector

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Common-rail direct injection (CDI) The Common-rail injection is state-of-the-art in Diesel injection technology. Simply put, it is a high-pressure reservoir of fuel which feeds the injectors. The maximum pressure in the reservoir is typically 1,600 bar (23,200 psi) but can be controlled through a valve. The extremely high pressure assures better vaporisation of the fuel and a better burn. Because fuel is available at any desired pressure, the injection duration can also be controlled for a given injected volume. Even multiple injector pulses for a single cycle are possible. The system improves the capability to fine-tune the engine, which results in a more efficient, cleaner burn and an engine which produces more power. With this system, the engine fuel supply is not dependent on the engine rotational speed. System components: • • •

•

A high-pressure pump – with a pressure regulator and an inlet metering valve A rail, or fuel feed (the ‘Common-rail’) – which contains a pressurised reserve of fuel Injectors – which inject precise amounts of fuel into the combustion chamber as required. The injectors can be either solenoid-operated valves or operated by piezocrystal. In some circ*mstances, they can be activated up to 25 times a second. A full authority digital engine control (FADEC) – It precisely controls the flow and timing of the injectors as well as the rail pressure, while continuously monitoring the operating conditions of the engine. The electronic control system is an essential element of the operation of the modern common-rail system.

The term common-rail refers to the single fuel feed line from which the individual feeds for each injector is taken, on the common-rail direct injection (CDI) engine. In the CDI engine, the pressure, which remains permanently available in the fuel line, is built up independently of the injection sequence, in a high-pressure pump with a pressure regulator and inlet metering valve. The common-rail, which contains a pressurised reserve of fuel, acts as an accumulator, distributing the fuel to the injectors at a constant pressure of up to 1,600 bar. The fuel injectors inject precise amounts of fuel into the combustion chamber, as required. An electronic driver unit controls each injector’s opening and closing. This procedure is regulated by the electronic engine management, which separately and precisely controls the injection timing, rail pressure, and the amount of fuel injected for each cylinder. The EEC does this by using data obtained from continuous monitoring of the operating conditions of the engine. Sensor data from the camshaft and crankshaft provide the foundation of the EEC to adapt the injection pressure precisely to demand. Fuel ‘pressure generation’ and fuel ‘injection’ are managed independently of each other. This factor is an important advantage of common-rail injection over conventional fuel injection systems.

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Diesel common-rail direct injection (CDI) system

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Example: CDI engine – Thielert TAE 125 The fuel system installation of the TAE 125 series engines is quite similar to that of a conventional gasoline engine with a few modifications. Depending on the installation, the fuel passes from one of the two fuel tanks or both fuel tanks through the fuel selector and an auxiliary electric fuel pump to the fuel filter module.

Depending on the installation, a thermostat-controlled, heated filter module may be used where the warm returning fuel heats the filter before returning to the tank. Also, an electric fuel boost pump in front of the feeder pump for take-off and landing is used in some installations.

The TAE 125 engine is equipped with high pressure, commonrail fuel injection system. Details of the fuel supply depend on the individual installation. Below is a schematic of a typical fuel system. The fuel passes from the tank through a fuel filter module to the feed pump where fuel pressure is increased to 3.5 bar before continuing to a high-pressure pump where fuel pressure is increased up to 1,350 bar at maximum power. The fuel filter differs in that hot return fuel from the engine is used to preheat cold fuel because of the unique characteristics of Diesel. This is done until the fuel temperature reaches 60°C. The fuel then passes through a feed pump where fuel pressure is increased to 3.5 bar to a high-pressure pump where fuel pressure is increased to 1,350 bar. Pressurisation causes fuel temperature to rise to approximately 70°C. From this pump, fuel flows to the high-pressure rail where it feeds the injectors. Unused fuel is returned through the fuel filter and fuel selector to the tank in use. The returning hot fuel ensures a higher temperature of the fuel in the tank. The diagram below shows a schematic of the fuel system.

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Diesel Common-rail fuel system pictorial

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Aero-Diesel engine and fuel system components – Thielert TAE-125

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Fuel feed pump (low-pressure pump) The fuel feed pump (also known as ‘low-pressure pump’) is a gear-type pump and driven by one of the camshafts (electrically driven on some installations). The fuel pressure of the pump is adjusted to 3.5 bar absolute by an adjusting screw. The diagram below shows the low-pressure fuel pump. The driveshaft coupling to the camshaft as well as the fuel supply and exit is visible. The fuel enters through feed inlet A, fills the tooth gaps, and is thus conveyed, in the direction of the green arrows, towards discharge outlet B. Suction is created by the void caused when the teeth uncouple.

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High-pressure (HP) pump The high-pressure fuel pump supplies fuel to the common-rail. Typical rail pressure is between 380 bar at idle speed and 1,350 bar at maximum RPM. The actual fuel pressure regulation by the FADEC is discussed later. The pump relies on lubrication from the fuel to function. To operate this pump on Jet fuel with lower lubrication characteristics compared to Diesel fuel, all surfaces of internal parts are coated with a special PTFE surface. Without this special treatment, the HP pump will be the first component to fail if the engine is operated using jet fuel, due to the lower lubricity of jet fuel compared to Diesel fuel. The diagrams below show an external view of the pump as well as a schematic. This is a three-radial piston-type pump, lubricated by fuel. It is driven by the engine using an eccentric camshaft. A triangular cam (ring cam on the scheme below, also called polygon) comes into contact with the eccentric section via a bush. In normal operation, each flat side of the polygon is in contact with a piston: it is the camshaft’s eccentricity that generates polygon movement and therefore compression of the pistons one by one. The second camshaft drives the high-pressure fuel pump.

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High pressure pump

High pressure pump operation

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Common-rail fuel pressure regulation The common-rail is a high-pressure fuel reservoir that supplies fuel to the injectors. The figure below depicts the common-rail with pressure control valve and pressure sensor as well as high-pressure fuel feed and injector fuel lines. Fuel enters the common-rail directly from the high-pressure pump. The high-pressure accumulator is common to all cylinders. The rail volume is permanently filled with pressurised fuel. The compressibility of the fuel resulting from the high pressure is utilised to achieve the accumulator effect. This causes a practically constant rail pressure even when fuel leaves the rail for injection. The sensor and the pressure control valve are fitted at either end of the common-rail. The actual fuel pressure in the rail (and therefore to the injectors) is measured by the rail pressure sensor. The FADEC interprets the rail pressure, compares it to a target value, and adjusts the rail pressure control valve to reach the correct pressure. Return fuel flows back to the fuel tank.

By varying the fuel pressure, the injection duration can be varied for a given desired fuel injection volume per cycle, allowing for better combustion at lower RPM. The common rail has these two components, which, along with the EEC, regulate the pressure in the rail at a constant level. •

Pressure control valve The pressure control valve sets the correct pressure in the rail. It is a solenoid-operated valve actuated by the FADEC.

•

Rail pressure sensor The rail pressure sensor measures the instantaneous pressure in the rail and generates an output signal for the FADEC.

Fuel is supplied to the injectors through the injector supply ports. Excess fuel from the injectors (not depicted) is also connected to the return line. The fuel pressure supplied to the injectors is varied continuously. A higher fuel pressure ensures better vaporisation and therefore better combustion, as well as shorter injection duration, allowing delivery of the desired fuel volume at the optimal time.

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Diesel Common-rail, and fuel Injectors

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Fuel injectors The fuel injectors are solenoid-actuated or piezo-electric units. The figure below shows a schematic of a solenoid-actuated fuel injector. Fuel enters the fuel injector and passes into the chamber (1) where the needle prevents it from entering the combustion chamber. At the same time, fuel enters a chamber (2) through an orifice (1) to equalise the extremely highpressure acting on the plunger, which is held closed by spring tension.

How a Common-rail Diesel Injector Works and Common Failure Points - Engineered Diesel https://youtu.be/NUvWnOd5lFw

The FADEC supplies an electronic control signal to the injector solenoid via the connector, which activates the coil, and opens the orifice (2). Fuel now escapes, allowing the plunger to open and fuel to enter the combustion chamber. The fuel that was trapped in chamber (1) is now passed into the fuel return line. Because of the nature of the operation of the fuel injector, there is a high volume of return of fuel flow from each injector. For example, at maximum power, when the engine burns approximately 29 l/hr (99 kW, 135 hp engine), the return fuel flow volume is approximately 82 l/hr. The heat generated by the pressurisation at maximum power is approximately 1.2 kW and is returned to the tank. The fuel system must be designed to accommodate these flow rates. The injectors are not line-serviceable and are replaced as a unit.

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Diesel fuel injector

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16.4.3 Electronic engine control Development A full authority digital engine (or electronics) control (FADEC) is a system consisting of a digital computer, called an electronic engine controller (EEC) or engine control unit (ECU), and its related accessories that control all aspects of aircraft engine performance. FADECs have been produced for both piston engines and jet engines. The goal of any engine control system is to allow the engine to perform at maximum efficiency for a given condition. Originally, engine control systems consisted of simple mechanical linkages connected physically to the engine. By moving these levers, the pilot could control fuel flow, power output, and many other engine parameters. Following mechanical means of engine control came the introduction of analogue electronic engine control. Analogue electronic control varies an electrical signal to communicate the desired engine settings. The system was an evident improvement over mechanical control but had its drawbacks, including common electronic noise interference and reliability issues. Full authority analogue control was used in the 1960s and introduced as a component of the Rolls-Royce/Snecma Olympus 593 engine of the supersonic transport aircraft Concorde. However, the more critical inlet control was digital on the production aircraft. Following analogue electronic control, the next step was to digital electronic control systems.

In the 1970s, NASA and Pratt and Whitney experimented with the first experimental FADEC, first flown on an F-111 fitted with a highly modified Pratt & Whitney TF30 left engine. The experiments led to the Pratt & Whitney F100 and the Pratt & Whitney PW2000 being the first military and civil engines, respectively, fitted with FADEC, and later the Pratt & Whitney PW4000 as the first commercial dual FADEC engine. The first FADEC in service was developed for the Harrier II Pegasus engine by Dowty and Smiths Industries Controls. The development of EEC or FADEC for piston engines lagged behind that of gas turbine engines considerably. Introduced in 1988, the Porsche Mooney PFM was launched. The PFM was powered by a six-cylinder, air-cooled engine with automotivestyle electronic ignition, fuel injection, auto leaning, automatic cooling control a single power lever. Only 41 PFMs were sold The late 1990’s saw companies Lycoming and Continental developing electronic control systems and some other companies providing FADECs or related systems of their own. However, sales of such systems were still very low. The major problem has been the additional cost of the FADEC systems, increasing the price of a light aircraft, such as the Cessna 182, by 20,000 USD, in what is a very price-sensitive market. Only in recent years has the cost of a FADEC controlled engine been acceptable compared to the cost-benefit, and such engines are now the norm for new aircraft.

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Porsche PFM 3200 Engine Based on the power plant used in the iconic Porsche 911 sports car, the six-cylinder Porsche PFM 3200 emerged in the 1980s as a smoother, simpler and more reliable alternative to traditional general aviation piston engines. The PFM appeared in a handful of aeroplanes but debuted in only one production aeroplane: the Mooney PFM, of which a few dozen were produced. With the PFM, which relied on a dual electronic ignition system, the need to manipulate mixture and prop revolutions per minute settings in flight disappeared, and in its place came a single power lever that ensured the engine operated at its most efficient setting without any extra work by the pilot. Its pitfalls — heavier weight, lack of increased performance and faulty gearbox — held the engine back from further prominence. Total Training Support Ltd © Copyright 2020

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EEC and FADEC Description The first electronic fuel control systems had a mechanical backup which would take over the fuel control function in the event of its failure. These were generally called electronic engine control (EEC). Further development of the EEC was the full authority digital engine control (FADEC).

FADEC not only provides for efficient engine operation, but it also allows the manufacturer to program engine limitations and receive engine health and maintenance reports. For example, to avoid exceeding a specific engine temperature, the FADEC can be programmed to take the necessary measures without pilot intervention automatically.

Actual FADEC has no form of manual override available, placing full authority over the operating parameters of the engine in the hands of the computer. If a total FADEC failure occurs, the engine fails.

Advantages

If the engine is controlled digitally and electronically but allows for manual override, it is considered an EEC or ECU solely. An EEC, though a component of a FADEC, is not by itself FADEC. When standing alone, the EEC makes all of the decisions until the pilot wishes to intervene. FADEC works by receiving multiple input variables of the current flight condition, including air density, throttle lever position, engine temperatures, engine pressures, and many other parameters. The inputs are received by the EEC and analysed up to 70 times per second. Engine operating parameters are computed from this data and applied as appropriate. FADEC also controls engine starting and cooling (if liquid-cooled). The FADEC’s primary purpose is to provide optimum engine efficiency for a given flight condition.

• • • • • • •

•

Better fuel efficiency Safer as the multiple channel FADEC computer provides redundancy in case of failure Care-free engine handling, with single-lever control (no mixture lever, no propeller lever) Provides semi-automatic engine starting Can provide engine long-term health monitoring and diagnostics Reduces the number of parameters to be monitored by the pilot Due to the high number of parameters monitored, the FADEC makes possible ‘fault-tolerant systems’ (where a system can operate within required reliability and safety limitation with specific fault configurations) Saves weight

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A typical EEC (or ECU)

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Fuel injection (no carburettor) With this system, the carburettor is eliminated. This is a significant advantage as the chance for carburettor icing is then eliminated.

Disadvantages •

No vapour lock Due to the high-pressure fuel system, vapour lock is a nonevent. Even hot starting a fuel-injected engine is no problem at all, the higher fuel pressure removes any air pockets in the fuel lines. Fuel savings With a FADEC the engine receives the right amount of fuel per cylinder, and the spark plugs are ignited at the right time regarding RPM, throttle setting, ambient temperature and pressure. This results in fuel savings up to 15%, and easier starting and smoother running engine.

•

Full authority digital engine controls have no form of manual override available, placing full authority over the operating parameters of the engine in the hands of the computer. ⎯ If a total FADEC failure occurs, the engine fails. ⎯ Upon total FADEC failure, pilots have no manual controls for engine restart, throttle, or other functions. ⎯ Single point of failure risk can be mitigated with redundant FADECs (assuming that the failure is a random hardware failure and not the result of a design or manufacturing error, which may cause identical failures in all identical redundant components). High system development and validation effort

Diagnostics FADEC analyses many engine parameters electronically, many times per second. It can, therefore, store a trend of all parameters in non-volatile memory. This can then be downloaded after a flight, especially after a problem occurred during the flight, and using proprietary software; trend analysis can be performed and viewed graphically. The technician is thus provided with a powerful diagnostic tool. Single lever control The fully electronic system allows the pilot to control all engine parameters through a single lever. The pilot simply selects the power level with the load lever, and the FADEC regulates all parameters, including propeller pitch, accordingly.

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Teledyne Continental FADEC for aircraft https://www.youtube.com/watch?v=uE9ZCgA1XTU Lycoming's IE2 Electronic Engine https://www.youtube.com/watch?v=GTNoBlW7d1c

Inside a typical dual channel EEC (or ECU)

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Full authority digital engine control (FADEC) General The fuel/air mixture in the combustion chamber needs to be ignited at the right moment to ensure efficient combustion and power generation by the engine. This is the job of the ignition system, be that the old fashion magneto or a modern fully electronic microprocessor-controlled FADEC system. For safety reasons, the ignition system does not rely on the aircraft electrical system. It is dual, where each system operates one of the two spark plugs in each cylinder. Magneto ignition suffers from several problems which have long been resolved with vacuum, RPM and mechanical ignition advancing combined with memory-mapped microprocessorcontrolled FADEC systems or sophisticated electronic ignitions. FADEC comes to good use in combination with a fuel injection system, and only then are the advantages available. FADEC is a memory-mapped microprocessor-controlled ignition with sensors measuring MAP, CHT, EGT, RPM, atmospheric and oil pressure. It controls fuel injection and ignition timing to optimise the power produced by the engine thoroughly. These systems add considerable complexity and additional wiring to the original electrical system as they need (among other things) separate backup power supply from a second battery. This secondary battery must be kept fully charged during flight through a particular Schottky diode (low voltage drop type) and be monitored for charging. Total Training Support Ltd © Copyright 2020

If the aircraft primary electrical system suffers total failure, the FADEC battery can supply the EEC for approximately 2 hours. co*ckpit controls are added, extra switches for a fuel pump, primary and secondary FADEC power supply and fuses to protect these systems. There is no mixture control as the FADEC controls the fuel mixture injection and timing. The engine control unit (ECU) uses a 3D memory map to control the injector for the right amount of fuel and considers different ambient circ*mstances as outside air temperature (OAT) and air pressure (density) concerning RPM and throttle setting and regulates this several times per second. Mixture control is automatic as the ECU senses barometric pressure and compensates the amount of fuel injected. The ignition timing of the spark plugs (spark advance) is also regulated depending on RPM, throttle setting for every load on the engine. This variable igniting timing results in quicker engine starts and smoother operation under variable loads. Starting a FADEC engine consists only of pressing the start button, and it runs, no more choking, priming and endless battery-draining starts. The ECU takes all variables into account and retards ignition and regulates fuel per cylinder for smooth starts, just like a modern car.

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Example hardwired dual-input device thrust-lever angle RVDT

Load selector lever and potentiometers A and B

Thielert (Diesel engine) ECU 4-167

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System layout and components Dual-channel The FADEC system is fully redundant built around two independent control channels. Dual Input, dual outputs and automatic switching from one channel to the other eliminate any dormant failure. Channel selection The ECU always selects the ‘healthiest’ channel as the Active channel based on a fault priority list. The fault priority list contains critical faults such as processor, memory or power failures. During engine run status, each channel within the ECU determines whether to be in the active state or standby state every 30 milliseconds based on a comparison of its health and the health of the cross-channel. Either channel can become active if its health is better than the cross-channel’s health; likewise, it becomes standby if its health is not as good as the cross-channel. If the two channels have an equal health status, the channels alternate between active/standby status on each engine shutdown and the standby channel becomes the active channel on the next start. Channel transfer Assuming the opposite channel is of equal or greater health, channel Active/Standby transfer generally occurs after the engine has been and subsequently shutdown. However, there are some differences between system designs. The system used on the Thielert Diesel engine, for example, the Channel A is always the active channel, unless its health falls below that of Channel B.

In that condition, the channels automatically switch. The health’s of each channel is further continually monitored, and if the health of Channel B subsequently falls below that of Channel A, the channels revert to Channel A being the active channel. The pilot also has a co*ckpit switch and can switch the channels manually if desired. Dual inputs All command inputs to the FADEC system are duplicated. The parameters are exchanged between the two control channels via the cross-channel data link to increase the faulttolerant design Hardwired inputs Information exchanged between aircraft computers, and the ECU is transmitted over digital data buses. Also; signals are hardwired directly from the aircraft where a computer is not used. Dual outputs All the ECU outputs are duplicated, but only the channel in control supplies the engine control signals to the various receptors such as torque motors, actuators or solenoids. Fail-safe control If a standby channel is faulty and the channel in control is unable to ensure one engine function, this control is moved to a fail-safe position.

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FADEC operation General The diagram below shows the FADEC overview. The unit takes inputs from sensors, including the load selected by the pilot (throttle position), adjusts for variables such as air temperature, engine temperature and barometric pressure, to control fuel injection (quantity and timing), propeller pitch, and boost. Engine control is accomplished through engine control “maps” (or databases).

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FADEC maps The figure below shows a FADEC ‘map’ or database which controls manifold pressure. During development, a target boost pressure for various operating conditions is established. The map shows the desired manifold pressure based on engine RPM and barometric pressure. The desired manifold pressure is then adjusted for air temperature and coolant temperature and continuously compared to the actual manifold pressure. Other engine parameters, such as fuel injection, volume and timing as well as propeller control, are controlled similarly. All functions are duplicated if one system fails. FADEC monitors engine health continuously and displays it in the co*ckpit. FADEC systems are also used for engineer troubleshooting by plugging into a laptop and downloading information to identify problems.

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A typical FADEC map – several FADEC databases (maps), many 3-dimensional, are stored in non-volatile memory within the FADEC unit. The FADEC adjusts fuel quantity and injection timing in accordance with the maps

FADEC unit internal – two identical circuit boards provide redundant dual-channel functionality; ECU-A and ECU-B. Only one channel is in control at any time. Switch over occurs when the health of the active channel reduces below that of the standby channel. The health is continuously monitored and determined in accordance with a hierarchy of the relative importance of the many sensors and inputs and circuitry.

The FADEC sensor input plugs and control output plugs

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Control loops Examples of the main control loops commonly used are listed below (this is for a diesel engine): • •

• • •

Boost control Fuel injection control, which consists of; ⎯ fuel injection timing; ⎯ fuel injection duration (volume); and ⎯ idle control circuit Fuel-injection rail-pressure control Propeller control Glow plug control (Diesel engines only)

The tables below list the functions of the control loops as well as the primary input values, what the target values are adjusted for; the actual output compared to the desired output to form a closed-loop system, as well as the actuator the system controls. Boost control Function

Three of the five loops are closed-loop systems, meaning they monitor the actual output of the system and correct it to the desired value. The figure below shows a closed-loop system. Based on a given input value, the mapping (FADEC software) determines a target value. An adjustment for environmental conditions is added to the target value and fed to a controller. The resulting output is compared to the initial target value, and an offset is established, which is added to the target value. Boost control, propeller control and rail pressure control are closed-loop systems. At idle, the fuel injection loop is a closedloop system as well since the engine establishes a target RPM and compares it to actual RPM. During regular operation, injection is not a closed-loop, since the pilot selects the load.

Control manifold pressure by regulating the wastegate

Primary Input

Power lever position

Adjusted for

Barometric pressure, RPM

Feedback

Actual manifold pressure

Controls

Turbocharger wastegate valve

Fuel injection control Function To deliver the appropriate amount of fuel at the appropriate time into the combustion chamber Primary Input

Power lever position, RPM

Adjusted for

Manifold pressure, barometric pressure, air temperature, coolant temperature, fuel rail pressure

Feedback

Actual RPM (only during idle). Feedback is not possible during regular operation because of pilot input

Controls

Time and duration of fuel injector cycles

Glow plug control is an open-loop system.

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Closed loop system

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Fuel injection pressure control Function Regulate fuel rail pressure to allow varying injection duration for constant injection volume Primary Input

RPM/fuel volume

Adjusted for

No adjustment

Feedback

Actual fuel rail pressure

Controls

Fuel-rail pressure-regulator valve

These tables are simplified representations of the control systems and are intended mainly to show the interaction of various parameters and illustrate the importance of the numerous sensors and actuators. Sensors, actuators and the wiring loom The inputs and outputs of the FADEC are shown below. Typically, two manifold pressure sensors and the two barometric pressure sensors are installed in the ECU itself. The two load sensors (potentiometers or RVDTs) are installed in the throttle quadrant to measure the position of the load lever.

Propeller control Function Control propeller and engine speeds by varying the angle of attack of the propeller blade Primary Input

Power lever position

Adjusted for

No adjustment

Feedback

Actual RPM

Controls

Propeller oil pressure valve

The sensors and actuators all connect to the FADEC via the harness. The harness is purpose-designed to fit with the correct length to each sensor and actuator and is equipped with quickrelease connections to facilitate service. Because of the importance of proper FADEC operation, special care is also taken for protection against EM interference and lightning strikes. The entire loom is shielded and is grounded at each termination.

Glow plug control (Diesel engine only) Function To control glow plugs before and during the initial start Primary Input

Coolant temperature

Adjusted for

No adjustment

Feedback

No feedback

Controls

Glow plug relay

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Redundancy To ensure redundancy of the system, FADEC consists of two separate, redundant ECUs. Both ECUs are supplied with manifold pressure and barometric pressure sensors and are connected by a board which houses the switching relays and to which the wiring harness is connected. Both ECUs are operational at all times, but only one is active. Both ECUs monitor their condition and calculate their health level continuously. If the health of one ECU is not 100 per cent, the system switches to the other ECU automatically, the “CHECK ENGINE” light is illuminated, and an entry into the event log is made. The event log is simply a file that stores the date, time, duration and nature of the error. The “CHECK ENGINE” light will continue to be illuminated until it is reset at an authorised service centre. If necessary, the pilot can switch from one ECU to the other via a co*ckpit switch. The default health level for the ECUs is 9. Each parameter has an associated operational limit. Operating outside these limitations results in a reduction in the health level. Operating outside of these limits has no direct effect on the operation of the engine. The health level is reduced, and the “CHECK ENGINE” light is illuminated to warn the pilot of an abnormal condition. The system continues to adjust all parameters to the measured values. As in any other engine, operating outside these limits is a reason for concern because they imply questionable health. At a predefined signal level from a sensor, the system assumes that instead of an actual condition, the sensor has failed. The table below shows input values at which the ECU assumes a sensor failure. In this case, the system substitutes a default value for the lost sensor and attempts to continue running the Total Training Support Ltd © Copyright 2020

engine. The resulting health levels vary based on the importance of the lost sensor. Thus, a crank sensor, which is necessary for the system to determine injection timing, is much more severe than a lost oil temperature sensor. Diagnostics The FADEC also has several methods of storing and logging data. These include: •

•

event log – snap-shot data is captured whenever an abnormal event occurs and is stored in non-volatile memory inside the FADEC; and onboard logger – the capability to connect a laptop directly to the system and log data continuously.

The user can see at a glance which parameters have shown unacceptable values when using such computer software tools, and whether the values were high, low or sensor failures. The graph below shows an analysis of data downloaded from the onboard logger. The logger writes values of previously selected channels and erases the oldest data first. This allows following the trends of those selected channels for the most recent hours of flight. The diagnostic and event log functions are viewed at every service. The data from the onboard logger is only of interest in particular circ*mstances, where investigating a specific problem or reconstruction of a failure is of interest. None of these functions is used during daily operation.

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INPUT

→

Replacement value

Speed/RPM

→

0 RPM

Load (throttle setting)

→

100 %

Manifold pressure

→

1800 mbar

Battery voltage

→

12 V

Coolant temperature

→

60 °C

Oil temperature

→

50 °C

Air temperature

→

35 °C

Fuel temperature

→

30 °C

Oil pressure

→

0.0 bar

Barro pressure (altitude)

→

1013 mbar

Rail pressure

→

1250 bar

Gearbox pressure

→

50 °C

Tachogenerator voltage (engine speed)

→

2.5 V

Propeller governor pressure

→

1 bar

Health level if a sensor fails

Data analysis from onboard logger

FADEC sensor fail conditions

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Example system – PowerLink FADEC General PowerLink™ FADEC is a solid-state digital electronic ignition and electronic fuel injection system developed by Teledyne Continental Motors. It has only one moving part that consists of the opening and closing of the fuel injector. PowerLink continuously monitors and controls ignition, timing, and fuel mixture/delivery/injection and spark ignition as an integrated control system. PowerLink monitors engine operating conditions (crankshaft speed, top dead centre position, the induction manifold pressure, and the induction air temperature) and then automatically adjusts the fuel-to-air ratio mixture and ignition timing accordingly for any given power setting to attain optimum engine performance. As a result, engines equipped with PowerLink neither require magnetos nor manual mixture control. This microprocessor-based system controls ignition timing for engine starting and varies timing with respect to engine speed and manifold pressure.

PowerLink includes the following components: • • • • • • • • • • • • • • •

low voltage harness; cabin harness; best power/best economy switch; electronic control units (ECUs); health status annunciator (HSA) (panel installed in the co*ckpit); electronic ignition system (high voltage harness); electronic sequential port fuel injection system; fuel flow transducer; PowerLink engine sensor array; speed sensor assembly (SSA); cylinder head temperature (CHT) sensor; exhaust gas temperature (EGT) sensors; manifold air pressure (MAP) sensor; manifold air temperature (MAT) sensor; and fuel pressure sensors

PowerLink provides control in both specified operating conditions and fault conditions. The system is designed to prevent adverse changes in power or thrust. In the event of loss of primary aircraft-supplied electrical power, the engine controls continue to operate using a secondary power source. As a control device, the system performs self-diagnostics to determine overall system status. It conveys this information to the pilot by various indicators on the health status annunciator (HSA) panel.

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PowerLink 4-cylinder installation

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Low voltage harness The low voltage harness shown below connects all essential components of the FADEC system. This harness acts as a signal transfer bus interconnecting the electronic control units (ECUs) with aircraft power sources, the ignition switch, speed sensor assembly (SSA), health status annunciator (HSA), temperature and pressure sensors. The fuel injector coils and all sensors, except the SSA and fuel pressure and manifold pressure sensors, are hardwired to the low voltage harness. This harness transmits sensor inputs to the ECUs through a 50pin connector. The harness connects to the engine mounted pressure sensors via cannon plug connectors. The 25-pin connectors connect the harness to the speed-sensor signalconditioning unit. The low voltage harness attaches to the cabin harness by firewall-mounted bulkhead fittings or connectors. Information from the ECUs is conveyed to the HSA and the co*ckpitmounted data port through the same cabin harness/bulkhead connector assembly. The bulkhead connectors also supply the aircraft electrical power required to run the system.

The ECU The ECU is at the heart of the system in providing both ignition and fuel injection control to operate the engine with the maximum efficiency realisable. Each ECU contains two microprocessors (which we refer to as a computer) that control two cylinders. Each computer controls its own assigned cylinder and is capable of providing redundant control for the other computer’s cylinder. The computer continuously monitors the engine speed, and timing pulses developed from the camshaft gear as they are detected by the speed sensor assembly (SSA). Knowing the exact engine speed and the timing sequence of the engine, the computers monitor the manifold air pressure and manifold air temperature to calculate air density and determine the mass airflow into the cylinder during the intake stroke. The computers calculate the per cent of engine power based on engine RPM and manifold air pressure. From this information, the computer can then determine the fuel required for the combustion cycle for either best power or best economy mode of operation. The computer will then precisely time the injection event and the duration of the injector ‘on’ time for the correct fuel to air ratio. The computer then sets the spark ignition event and ignition timing again based on the per cent of the power calculation. Exhaust gas temperature is measured after the burn to verify the fuel to air ratio calculations were correct for that combustion event. This process is repeated by each computer for its own assigned cylinder on every combustion/power cycle. The computers can also vary the amount of fuel to control the fuel-to-air ratio for each cylinder to control both cylinder head temperature (CHT) and exhaust gas temperature (EGT).

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PowerLink system components

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PowerLink 4-cylinder installation

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Electronic control units (ECUs) An electronic control unit (ECU) is assigned to a pair of engine cylinders. On six-cylinder engines, there are three ECUs required, one unit for every pair of cylinders. The ECUs control the fuel mixture and spark respective engine cylinders; electronic control unit opposing cylinders 1 and 2; electronic control unit cylinders 3 and 4, and electronic control unit 3 (if controls cylinders 5 and 6.

timing for 1 controls 2 controls equipped)

Each ECU is divided into upper and lower portions. The lower portion contains an electronic circuit board; the upper portion houses the ignition coils. The electronic circuit board contains two, independent microprocessor controllers which serve as control channels. During engine operation, one control channel is assigned to operate a single-engine cylinder. Therefore, one ECU can control two engine cylinders, one control channel per cylinder. The control channels are independent, and there are no shared electronic components between the control channel pair within one ECU. They also operate on independent and separate power supplies. However, if one control channel fails, the other control channel in the pair within the same ECU is capable of operating both its assigned cylinder and the other opposing engine cylinder as backup control for fuel injection and ignition timing. Each control channel on the ECU monitors the current operating conditions and operates its cylinder to attain engine operation within specified parameters. The following sensors transmit inputs to the control channels across the low voltage harness: Total Training Support Ltd © Copyright 2020

• • • • • •

speed sensor which monitors engine speed and crank position fuel pressure sensors manifold pressure sensors manifold air temperature (MAT) sensors cylinder head temperature (CHT) sensors exhaust gas temperature sensors

All critical sensors are dually redundant with one sensor from each type pair connected to control channels in different ECUs. Synthetic software default values are also used in the unlikely event that both sensors of a redundant pair fail. The control channel continuously monitors changes in engine speed, manifold pressure, manifold temperature, fuel pressure based on sensor input relative to operating conditions to determine how much fuel to inject into the intake port of the cylinder. Fuel injection timing is based on engine speed and crankshaft position. The control channel uses this input to precisely trim the fuel-toair ratios independently for its cylinder’s next combustion event. A solenoid-type electronic fuel injector (one per cylinder) injects the required fuel quantity into each cylinder intake port upstream of the intake valve at the appropriate time. The fuel injector solenoid on the fuel injector is driven directly by the associated control channel. The control channel actuates the fuel injector by commanding the solenoidcontrolled fuel injector valve “ON” or “OFF”.

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Electronic control unit (ECU)

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The control channel calculates the duration of actual fuel injection based upon a volumetric map of the engine’s breathing characteristics. The map is the baseline mixture for the cylinder at any normal engine condition. The control channel compensates this mixture in response to variations in the following: • • • • • • • •

intake manifold pressure; intake air temperature; fuel pressure; cylinder head temperature; exhaust gas temperature; system voltage; engine speed (RPM); and throttle setting

For example, if EGT Sensor input to the ECU indicates that a fuel mixture needs to be either leaned or enriched, the control channel operates its assigned cylinder to adjust the fuel mixture for the cylinder. The required fuel quantity is injected into each cylinder intake port at the appropriate time, with respect to crank position, by the cylinder’s solenoid-controlled fuel injector. The injector’s control coil is driven directly by the associated control channel. The CHT and EGT sensor input help the control channels determine combustion efficiency.

The control channel calculates the air density within the intake chamber of its cylinder. PowerLink contains a volumetric map of the engine’s breathing characteristics as it applies to engine speed and air density to allow PowerLink to precisely match fuel delivery on demand. PowerLink also compensates for changes in altitude by monitoring the intake manifold pressure.

Each channel controls its assigned cylinder in a manner that yields optimum performance for the current operating conditions to prevent exceeding normal operating parameters. The fuel mixture may be enriched or leaned, and ignition timing may be retarded to minimise the extent of limit excursion for the given parameter. In this respect, a FADEC-controlled engine is different from a non-FADEC engine in that an individual cylinder can be leaned or enriched by its control channel without affecting the other cylinders.

Based on these calculations and other relevant input, the control channel adjusts the fuel mixture and ignition timing as needed for its assigned cylinder as required. PowerLink monitors combustion efficiency and the exhaust gas temperature (EGT) using EGT Sensors. PowerLink uses this input to precisely trim the fuel-to-air ratios independently for each cylinder.

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PowerLink 6-cylinder installation

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Ignition system The ignition system consists of the high voltage coils atop the ECU, the high voltage harness and spark plugs. Since there are two spark plugs per cylinder on all engines, a six-cylinder engine has 12 leads and 12 spark plugs. One end of each lead on the high voltage harness attaches to a spark plug, and the other end of the lead wire attaches to the spark towers on each electronic control unit. The spark tower pair is connected to opposite ends of one of the ECU’s coil packs. Two coil packs are located in the upper portion of the ECU. Each coil pack generates a high voltage pulse for two spark plug towers. One tower fires a positive polarity pulse and the other of the same coil fires a negative polarity pulse.

For both spark plugs in a given cylinder to fire on the compression stroke, both control channels must fire their coil pack. Each coil pack has a spark plug from each of the two cylinders controlled by that coil pack’s ECU unit. The ignition spark is timed to the engine’s crank position. The timing is variable throughout the engine’s operating range and is dependent upon the engine load conditions. The spark energy is also varied with respect to engine load. Note: Engine ignition timing is established by the electronic control units and cannot be manually adjusted.

Each ECU controls the ignition spark for two engine cylinders. The control channel within each ECU commands one of the two coil packs to control the ignition spark for the engine cylinders. The figure below illustrates this scenario. The high voltage harness carries energy from the ECU spark towers to the spark plugs on the engine. PowerLink employs a waste spark ignition system. In this type of ignition, each cylinder’s spark plugs are fired twice per engine cycle – once on the compression stroke and again on the exhaust stroke. The control channel in an ECU emits a high voltage pulse through the high voltage harness to fire its top spark plug on the compression stroke and the bottom spark plug on the exhaust stroke for the opposite cylinder. Electronic control unit 1 fires the top and bottom spark plugs for cylinders 1 & 2; electronic control unit 2 fires the top & bottom spark plugs for cylinders 3 & 4, and electronic control unit 3 fires the top and bottom spark plugs for cylinders 5 & 6. Total Training Support Ltd © Copyright 2020

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Ignition control schematic diagram

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FADEC fuel injection system The FADEC fuel injection system is comprised of: • • • • • • •

The internal components of the fuel injector consist of a pintle valve and a spring. The solenoid creates an electromagnetic field to lift the pintle valve and open the path for fuel to flow. The solenoid coil fits over the pintle valve body and is held in place with two jam nuts that thread onto the valve body. When electrical current ceases to flow through the solenoid, the spring force closes the pintle valve and shuts off the flow of fuel from the fuel injector. The valve design and injector end form a self-atomising feed for the fuel.

an engine-driven fuel pump; a fuel distribution block; solenoid-controlled fuel injectors; 20-micron & 10-micron filters; an engine-mounted fuel filter; a fuel bypass solenoid; and a fuel flow transducer.

The positive displacement style fuel pump is directly driven at the same speed as the crankshaft. Therefore, fuel pressure varies directly with engine speed. Fuel pressure is continuously monitored by the ECUs using dual redundant fuel pressure sensors mounted on the fuel distribution block. Fuel is metered to the cylinders under control of the respective electronic control unit (ECU). The ECU monitors changes in air density and engine speed to determine how much fuel is injected into the intake port of the cylinder. The fuel distribution block distributes fuel to each of the fuel injector nozzles. PowerLink controls the fuel supplied to each cylinder using solenoid-actuated sequential port fuel injectors. A fuel injector assembly is located in each cylinder head, one fuel injector per cylinder. The fuel injector is threaded on both ends and the outlet screws into the tapped fuel injector boss in the cylinder head. The fuel injector assembly is made up of two parts: the control coil and the injector, as shown below. Total Training Support Ltd © Copyright 2020

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Fuel injector parts

Fuel injector 4-195 Module 16.4 Engine Fuel Systems

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PowerLink continuously measures the fuel pressure at the fuel injector inlets under all engine operating conditions using dual fuel pressure sensors mounted on the fuel distribution block. With this input, the ECUs control the amount of fuel flow to each cylinder by controlling the duration of time that the solenoid control valve on the fuel injector remains open, allowing fuel to flow through the fuel injector into the intake port. The two control channels within a given ECU directly regulate the amount of fuel delivered to each channel’s assigned cylinder via the fuel injector assemblies. The ECU monitors changes in air density and engine speed to determine how much fuel to inject into the intake port of the cylinder.

At the appropriate crank rotation angle, the control channel fires its injector for the required injection duration to deliver the appropriate amount of fuel for the combustion event. When PowerLink detects the need for more fuel to be injected to a given cylinder, the solenoids are held on (open) for a longer duration permitting more fuel to flow into that given cylinder. The amount of time the injector is held in the on state determines how much fuel is delivered to the cylinder. Note that fuel is injected through each fuel injector for a cylinder as needed as determined by the control channel assigned to that cylinder.

The control channel in the ECUs of PowerLink controls fuel flow through the fuel injector by switching electrical current on and off to the control coil of the fuel injector assembly. When the current is on, fuel flows through the injector. The amount of time the injector is held in the on state determines how much fuel is delivered to the cylinder. In the on state, the solenoid coil creates an electromagnetic field that lifts the pintle valve opening the path for fuel to flow. The on time for a given injection event is referred to as the ‘injection duration.’ The control channel receives information from PowerLink sensors and uses this information to determine the appropriate injection duration for the next air intake cycle. In the off state, electrical current ceases to flow through the solenoid, and the spring force closes the pintle valve which shuts off flow from the fuel injector. Each control channel independently varies its cylinder’s injection duration depending on current engine operating conditions. Total Training Support Ltd © Copyright 2020

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FADEC sensor set The FADEC sensor set includes all sensors used by PowerLink to monitor engine performance. The FADEC Sensor Set Table below lists the sensors and corresponding control channels (abbreviated ‘CC’) for six-cylinder engines.

For PowerLink to operate at optimum performance, all sensors must be operational.

Each control channel performs diagnostic checks on itself and the sensors it utilises. If a fault with one of the sensors is detected, an HSA lamp is illuminated. A laptop computer with the TCM FADEC Diagnostics Software Tool installed can be used to communicate with each control channel to determine specific information regarding detected sensor faults to be obtained. Refer to Chapter 4, Troubleshooting for details on using this diagnostic software tool. Total Training Support Ltd © Copyright 2020

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Speed sensor assembly (SSA) The speed sensor assembly (SSA) provides PowerLink with information about the engine’s crank position and engine speed. The SSA consists of two separate parts: a signal conditioner and a (Hall effect) speed sensor array. Looking from the rear of the engine, the signal conditioner is mounted on the right-hand side (1-3-5 side) magneto drive pad, and the speed sensor array is mounted on the interior bottom of the oil sump.

pulse to the electronic control units (ECUs) to determine engine speed and crank/cam position. The signal conditioner filters both the electrical power supplied to and the signals generated by the speed sensor array. The conditioned signals are passed on to the ECUs, where they are used to coordinate ignition and fuel injection timing.

Two sealed electrical circular connectors, installed in the oil sump walls, one on each side, conduct signals from the speed sensor array to the signal conditioner. A pair of cables extending from the signal conditioner mate with the sumpmounted connectors. The speed sensor detects the camshaft position. The speed sensor array consists of six sensors that detect the speed and position of the camshaft gear. The six sensors are paired into three sets, each having a speed target sensor (for reading the outer track of 12 drilled holes on the camshaft gear and a cam target sensor (for reading the inner track of the camshaft gear). The SSA sensor pairs detect the outer track of targets in the camshaft gear as it rotates past the sensor array generating a signal pulse train that is proportional to engine speed. The sensor sets also detect the top-dead-centre target on the inner track of the camshaft gear generating the cam pulse. This pulse is timed with the piston in cylinder 1 reaching topdead-centre (TDC) on the compression stroke. When the SSA detects an open hole on the outer track of drilled holes, the SSA creates and sends a corresponding electrical Total Training Support Ltd © Copyright 2020

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Speed sensor signal conditioner assembly

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Cylinder head temperature (CHT) sensors Each cylinder is equipped with a Cylinder Head Temperature (CHT) Sensor to monitor and help maintain the temperature of the cylinder within specified operating parameters. The CHT sensor is mounted on each engine cylinder via a bayonet-style adapter which threads into the cylinder head. A spring-loaded locking ring on the sensor reinforces this attachment. The CHT Sensor is hardwired to the low voltage harness. The CHT sensor emits a temperature signal to the corresponding control channel (in the assigned ECU) which monitors and controls the engine cylinder. The ECU uses this signal to control the cylinder head temperature. The signal is conveyed to the control channel via the low voltage harness. Each CHT sensor is independent and operates for a respective engine cylinder. The sensing element in the CHT sensor is a thermistor. This type of device changes resistance with temperature in a linear and repeatable manner. Measuring the resistance of the sensor alloys enables an accurate determination to be made of the temperature at the sensor tip.

Exhaust gas temperature (EGT) sensors Each cylinder’s exhaust port is outfitted with an EGT sensor. The EGT sensor emits a temperature signal to the corresponding control channel (in the assigned ECU) which monitors and controls that engine cylinder. The ECU uses this signal to control the fuel-to-air ratio. The signal is conveyed to the control channel via the low voltage harness. The EGT sensors are hardwired to the low voltage harness. Each EGT Sensor is independent and operates for a respective engine cylinder. The sensing element in the EGT sensor is a K-type thermocouple. Two conductors made of dissimilar metals are fused to form the sensing element. The sensing element generates a small voltage in proportion to the temperature to which it is exposed. By measuring the voltage produced by the sensing element, an accurate determination can be made of temperature at the EGT sensor tip. The EGT sensors are attached to the exhaust system using a worm screw clamp.

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EGT and CHT sensors

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Manifold air pressure (MAP) sensors PowerLink utilises two MAP sensors for measuring the engine’s induction air pressure (manifold air pressure). These sensors are self-contained, non-serviceable units that are threadmounted into tapped bosses on the intake plenum on top of the engine. The low voltage harness uses removable, circular connectors to interface with the MAP sensors. Manifold air temperature (MAT) sensors PowerLink utilises two MAT sensors for measuring the intake manifold air temperature. These sensors are mounted in the intake plenum using compression fittings – the fitting body threads into bosses on the intake plenum manifold. The sensors are hardwired to the low voltage harness. The sensing element in the MAT sensor is a thermistor, similar to the CHT sensor. Fuel pressure sensors PowerLink utilises two sensors for measuring the engine’s fuel pressure. The fuel pressure sensors are self-contained, nonserviceable units that are thread mounted into the fuel distribution block on top of the engine aft of the intake plenum. The low voltage harness uses removable, circular connectors to interface with the fuel pressure sensors.

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MAP sensors

Fuel pressure sensors MAT sensors

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PowerLink FADEC system installed in aircraft

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Power supplies PowerLink is electrically powered and is supplied current by the aircraft’s primary electrical buss and a secondary power source (SPS). The SPS may be: • •

•

a dedicated backup battery a second alternator-battery (either installed on the single-engine or the alternator installed on the other engine in a twin-engine installation) a dedicated, self-exciting backup generator

Electrical power supply summary table Configuration

Description

Dual alternators & dual batteries*

PowerLink can operate indefinitely on the primary power source alternator and also can operator indefinitely on the SPS because it too has an alternator

Single alternator & battery plus a dedicated backup battery

PowerLink can operate indefinitely on the power source having the alternator; PowerLink must be capable of operating on the backup battery for at least 1 hour.

The electrical power supply summary table below describes the various electrical system configurations that can be used with PowerLink to comply with the redundant power requirement.

The backup battery is solely dedicated for this purpose and is not used to supply any other loads or for cranking. Note: This configuration requires use with a battery condition monitor. The HSA prevents the main aircraft bus from drawing on the backup battery. The HSA provides an indication when the backup battery is low (EBAT FL). If the backup battery voltage is low or if the ECU(s) is/are operating on the backup battery, the PPWR FL annunciator illuminates on the HSA. Single alternator & single battery plus a dedicated backup generator

The self-excited dedicated generator allows continued operation of PowerLink while the primary power is interrupted for diagnostics.

*An electrical power system having two electrical busses each supplied by a separate and independent alternator and battery complies with the requirement for two separate power sources

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The SPS is used to supply power to PowerLink independently from the aircraft’s primary bus. If the SPS is a battery, it is continuously charged by the aircraft’s primary power bus. The charging current supplied to the SPS battery is monitored by the HSA, and the charging circuit is protected by a breaker labelled “SPSC” which means secondary power source charge. Electrical power to PowerLink is controlled from the co*ckpit by two separate, independent switches used to interrupt the primary power and secondary power. The cabin harness through the bulkhead connectors to the low voltage harness conducts primary/secondary power. Information from the electronic control units (ECUs) is conveyed to the HSA, and the co*ckpit mounted data port through the same cabin harness/bulkhead connector assembly. The two power supply circuits are isolated from each other, and each has a separate set of breakers and power switch. The primary power switch and breaker set to control the primary power supply to the FADEC system; the secondary power switch and the breaker set to control the secondary power supply to the FADEC system.

Power switchover is instantaneous and automatic. There is now switchover relay or mechanical breaker. There is no interrupted power period to the FADEC system when transitioning from one source to another as long as both sources are above the minimum voltage level and have sufficient current capacity to run the FADEC system. The FADEC will draw 5.2 amperes total at 2,700 RPM and will decrease its current draw to approximately 1.9 amperes at idle speed. These current values do not include the operation of the electric fuel pump. Failsafe operating contingencies PowerLink is functionally redundant: •

•

If a control channel incurs a fault, the other control channel within the same ECU is capable of operating its assigned cylinder as well as the cylinder experiencing the fault condition. All critical sensors are redundant with one sensor from each pair connected to channels in different ECUs. Synthetic software default values are also used. This arrangement supports the functional redundancy of the FADEC system.

Two breakers protect the SSA and HSA power supply circuits, one breaker being assigned to each of the two power supplies. The pilot starts, enables, and stops PowerLink using a conventional aircraft-style ignition switch.

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Fault detection The fault detection function is a total self-checking system intended to inform the pilot when the conditions of the various components of the system require diagnostic attention.

•

Processor faults. A processor fault should be remedied by replacing the applicable ECU.

Each of the sensors and other analogue input signals is tested and enunciated for low range values, high range values and noisy signal operation. The low range CHT sensor check (misfiring cylinder check) is not performed below 2,000 RPM for the cylinders to be allowed to warm up properly before takeoff and to prevent false alarms. The FADEC system detects and annunciates the following. •

•

• •

The occurrence of excessive CHT and EGT values. Both conditions may indicate an engine condition requiring maintenance. A fuel pump transistor-driver faulted condition. This fault precludes automatic operation of the electric boost pump and should be investigated. An in-range fuel pressure fault. If the fuel pressure sensor has failed within the upper and lower electrical limits, a fault is enunciated. This fault may also be enunciated due to a reduction in the fuel pressure from the nominal-operating curve of pressure versus the engine curve. The fuel system should be investigated for clogged filters, degraded or maladjusted fuel pump, and leaks. Improper speed signal faults. This fault is indicative of a speed sensor or low voltage harness failure. A misfiring cylinder. A misfiring cylinder fault is enunciated when CHT falls below 800°F and should be investigated.

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Licence Category B1 and B3

16.5 Starting and Ignition Systems

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective

Part-66 Ref.

Starting systems, pre-heat systems;

16.5

Knowledge Levels A B1 B3 1

Magneto types, construction and principles of operation; Ignition harnesses, spark plugs; Low- and high-tension systems

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Table of Contents High tension ignition system _____________________ 6 General _____________________________________ 6 The primary or low tension (LT) circuit _____________ 6 The secondary or high tension (HT) circuit __________ 6 Ignition harness _______________________________ 8 Operation of the combined circuits _______________ 14 Magnetos ____________________________________ Magneto operation ___________________________ Impulse coupling spark retard ___________________ Lag-angle-retarded breaker magnetos ____________ Ignition switches _____________________________ The contact breaker (CB) ______________________ The distributor _______________________________ Magneto venting _____________________________ Magneto speeds _____________________________ The four-pole magnet _________________________ The dual magneto ____________________________ The rotating armature magneto __________________ Magneto mounting and drives ___________________

16 16 26 28 30 32 34 34 36 38 38 38 40

Ignition harnesses ____________________________ 44 Construction ________________________________ 44 Testing ____________________________________ 44 Spark plugs __________________________________ Construction ________________________________ The heat range of the spark plug ________________ Inspection and servicing of sparking plugs _________ Spark plug inspection _________________________ Damage from excessive temperatures ____________ Other spark plug problems _____________________ Ground checking _____________________________ Total Training Support Ltd © Copyright 2020

50 50 52 56 56 64 64 72

Auxiliary ignition systems ______________________ 74 Low-tension ignition systems ___________________ 82 Ignition timing ________________________________ 84 General ____________________________________ 84 Engine speed ________________________________ 86 Manifold pressure ____________________________ 86 Mixture strength ______________________________ 86 Automatic timing control________________________ 86 Magneto timing (Bendix) _______________________ 86 The ‘E’ (efficiency) gap ________________________ 88 Timing marks ________________________________ 92 Timing magneto to engine ______________________ 92 The magneto synchroniser _____________________ 96 Eastern Technology E25 timing indicator___________ 98 Aero-Diesel engine glow plugs _________________ 100 Starting systems _____________________________ 102 Types of starter _____________________________ 102 Manual cranking inertia starter __________________ 102 Electrical inertia starters_______________________ 104 Direct cranking starters _______________________ 106 Solenoid or pre-engaged ______________________ 106 Electric starter circuit _________________________ 106 Basic starter description_______________________ 108 Bendix drive ________________________________ 112 Starter relay ________________________________ 114 Bonding/earthing straps _______________________ 114 Troubleshooting and maintenance _______________ 116 Brush maintenance __________________________ 116

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Pre-heat systems ____________________________ General ___________________________________ Installed preheaters__________________________ Portable preheaters__________________________

120 120 122 122

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High tension ignition system General The majority of ignition systems employ on piston engines are of the high tension (HT) type, so named because they are designed to produce a very high voltage output from a low voltage input. These HT systems are classified into battery (coil) ignition and magneto ignition, but aero engines with few exceptions employ magnetos. The component common to both systems is a transformer, sometimes referred to as an induction coil or armature. This device uses the principle of electromagnetic induction to produce the very high voltage spark needed to jump the gap of a spark plug. Before discussing the application of armatures, we should recall some basic electromagnetic principles. •

•

A current flowing in a conductor generates a magnetic field around that conductor. The strength of the field depending on the strength of the current. Moving a conductor across the lines of force in a magnetic field induces a voltage in that conductor. If the conductor is part of a closed circuit, a current will flow. A change of magnetism acting on a coil of wire will induce a current in that wire. The strength of the current depends on: ⎯ the strength of the magnetic field; ⎯ the rate of change of magnetism; and ⎯ the number of turns of wire in the coil.

The primary or low tension (LT) circuit A typical primary circuit is shown in the diagram below left. One end of the primary winding is earthed, and the other end has three components connected in parallel to it before they too are earthed. These are: •

•

The contact breaker (CB). An engine operated switch which breaks the current flow in the primary circuit when it is at its highest. The capacitor (condenser) aids the rapid collapse of the primary’s magnetic field. It also reduces arcing at and burning of, the CB points when they are opened and the current tries to continue to flow across the gap. The pilot operated switch. They are known as the ignition, magneto or mag switch. When this switch is closed or “OFF”, the primary circuit is completely earthed, and opening of the circuit – the magneto is ‘dead’.

Unlike most other electrical switches, these operate in the opposite sense. When the contacts are closed, the switch is off. The secondary or high tension (HT) circuit The diagram below right shows the HT circuit superimposed on the LT circuit. The HT circuit consists of a secondary winding, a distributor and sparking plugs.

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Primary circuit Magneto – cylinder harness connections

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The conductor (core) is made from several twisted strands of either ‘tinned copper’ or, as is the current practice, ‘stainless steel’. Its diameter is small because it carries a very low current. The large diameter of the cable arises from the thickness of its insulation material plus a sturdy outer material or braided metal sleeving for additional protection against abrasion and other hazards. The individual leads are routed between the distributor and the sparking plug via support grommets, clips, and sometimes additional protective shielding. The ends of the leads are fitted with metal connectors to suit the distributor and plugs, respectively. Also, each end has an identifying sleeve which is an essential aid to maintenance. Ignition harness On many engines, and not necessarily on the larger ones, the ignition leads from one magneto are enclosed for most of their length in a rigid metal conduit. This is contoured to suit the engine layout, radial or in-line as in the diagram below left, and has at one end, or somewhere along its length, a large diameter flexible conduit. It terminates in an end fitting which enables all leads’ ends to be secured in their right sockets on the distributor. Adjacent to each of the plugs associated with the magnet, smaller flexible conduits are led from outlets in the rigid conduit and terminate in metal plug connectors. We now have an ignition harness assembly which is secured to the engine by bolts or studs passing through lugs which are brazed or welded to the rigid conduit.

The advantage of such an assembly, are: • • • • • • •

servicing is quicker; all leads are attached to the engine as one unit; other engine components are most accessible; the leads have greater protection from damage; they are protected from moisture and deterioration; there is a saving in weight; and positive screening.

Screening (shielding) During regular operation of the ignition system, the variations in HT current flow and the arcing in the distributor and spark plugs, produce unwanted magnetic fields. These can result in serious interference with radio and other equipment. The rigid conduit may be of made from ‘brass’, ‘aluminium alloy’ or ‘corrosive resistant steel’. The flexible conduits and the metal protection of single leads are made from close mesh, braided, tinned copper or tinned phosphor bronze. The harness, as well as protecting the HT leads, serves another important purpose. The materials of the harness are all electronically conducting and are grounded to the engine and airframe. By conducting the unwanted magnetic lines of force to earth, the ignition harness cuts down electrical interference with radio and other electrically sensitive systems in the aircraft. When the radio and other systems are protected in this manner the ignition harness is said to be a shield. Without this shielding or screening, radio communication would become virtually impossible.

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Magneto, harness and spark plugs

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The screening components are in good electrical connection. They are earthed between spark plug body and cylinder head, the magneto mounting and the engine case and, in some cases, by bonding strips connected between the rigid conduit and the engine in addition to its primary fasteners. The use of separate HT leads requires every lead to have its own braided metal sleeve over the whole length and, on large engines, the total weight of the screening material is high. Because of the routing of the leads, the braiding is more vulnerable to damage during operation and maintenance. This can result in ‘leaks’ in the screening. Low tension (LT) or switch leads The construction of these items is similar to that of the HT leads. However, because they carry a low voltage of higher current, the cable core is often larger in diameter and the insulation thinner. For screening and protection, they have an outer covering of metal braid. Suitable connectors couple the ends to the ignition switch and the magneto contact breaker.

Visual inspection There always precedes the testing of any component or system. For our purpose, we shall consider an installation harness and the items we would look for during the visual inspection would include: • • • • •

The electrical tests – which follow the visual inspection – would require the plug leads and distributor block to be disconnected. This allows completing the harness check by examining: •

Servicing and testing of ignition leads and harnesses There are particular servicing and testing procedures which are common to all ignition systems. They are carried out as scheduled servicing at intervals recommended by the manufacturer and/or specified by the aviation authority under which the aircraft is flown. In the case of an ignition fault, some of these inspections and tests are employed during the process of rectification. They fall broadly under these headings: • • •

loose cables, sleeves, and connectors at distributor block and sparking plugs; the insecurity of attachment bolts and screws; perishing of insulation – indicated by hardening and cracking; damage to metal braiding, rigid conduit, plug and distributor connectors; and oil soakage, which is indicated by swollen and softened insulation. (This is generally more relevant to separate unscreened leads.)

• •

the plug nut for freedom of rotation deformation and thread damage, and the plug terminal components for a good condition; and the distributor block for cracks and signs of tracking; and the security and effectiveness of the lead connections to the block.

visual inspection; continuity testing; and insulation testing.

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Ignition system installed

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Dual magneto, harness and spark plugs

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How the Airplane Ignition System Works https://www.youtube.com/watch?v=fTTAHbV7rcw Magnetos | 60 Second Flight Training https://www.youtube.com/watch?v=2YuKx2Pv7aU Magneto Tutorial https://www.youtube.com/watch?v=9JnI8oN4h8I ATPL Training / Piston Engines #15 Ignition Systems https://www.youtube.com/watch?v=_dyDU8unkqw Aviation Maintenance Aircraft Ignition Systems Part 2 https://www.youtube.com/watch?v=1u 2-jMvFww8 Possibly no sound on some computers How to install Magnetos to an Aircraft Engine https://www.youtube.com/watch?v=0l96NU8afPM Check the Timing on aircraft Magnetos https://www.youtube.com/watch?v=4m8IFYpo-W8 Magneto to Engine Timing https://www.youtube.com/watch?v=xpWyRcUHtXs How to Service Aircraft Spark Plugs https://www.youtube.com/watch?v=1gJc6R08AIE Fouled Spark Plugs - Maintenance Mondays - MzeroA Flight Training https://www.youtube.com/watch?v=AmAAq1Yf6qQ SPCT100A Spark Plug Cleaner Tester Demo Video https://www.youtube.com/watch?v=pbs7sSWow64

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Operation of the combined circuits The secondary winding has one end connected to the primary coil and, thus, when the CB points are closed, direct to earth. The other end connects to a distributor. This component, usually an integral part of the magneto, directs the HT pulses to each cylinder in turn in the correct firing order. With the CB points closed and a current flowing in the primary circuit, the opening of the points causes the magnetic field to collapse inwards to the centre of the armature. The lines of force move across the secondary winding and induce a voltage in that circuit. Because the CB points are open and have thus broken the connection to the earth, the secondary circuit is completed through the primary/secondary junction, the primary coil; and then to earth.

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Combined circuits

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Magnetos Magneto operation The magneto is an entirely self-contained ignition generating device. Typically, two magnetos are installed on each aircraft engine for redundancy. When the aircraft engine crankshaft rotates, gears located in the engine accessory case turn the magneto rotor shaft containing permanent magnets. With the rotating shaft, a magnetic field is produced that is transformed into high tension current through primary and secondary coil windings. The high-tension current is distributed to the appropriate cylinder through a distributor block assembly and ignition cables.

Every magneto has a rotating component for varying the magnetic flux flowing through the armature. The type of magneto takes its names from the form of its rotating member. •

•

Rotating armature - in which the armature rotates in the field between the poles of a stationary horseshoe magnet. The contact breaker and capacitor are fixed to the armature and also revolve. This type had certain disadvantages. Rotating magnet - in which a permanent magnet rotates between extensions of the armature core known as pole pieces. All of the other components are stationery. This is now the most widely used type of magneto.

A two-lobe cam and two-pole rotating magnet assembly are used to generate magnetic flux and trigger the high-tension spark energy. Four-cylinder magnetos are driven at engine speed and produce four sparks through 720° of crankshaft rotation. Six-cylinder magnetos are driven at one and a half times engine speed and produce six sparks through 720° of engine crankshaft rotation. Slick magnetos are constant timing ignition devices once the engine has started. The magneto is typically timed to fire at an advance timing position for a maximum power of the aircraft engine.

A basic two-pole magnet is used here to describe the operation of the magneto.

A typical Slick magneto used on the majority of light horizontally opposed engines produces over 25,000 V at normal speed. Although simple in outward appearance and construction, the magneto is a complicated electromechanical device. The size and shape of the rotating magnet head assembly, magnet material selection, pole lamination design, ignition coil design and capacitor design are all equally important in determining the efficiency of the device. Electrically, the magneto is a balanced LRC circuit.

The magnet has now turned through 90° (to the ‘neutral position’), the lines of force are short-circuited through the pole pieces, and the armature magnetic field is zero.

Refer to the diagram below right and assume that the magnet is rotating clockwise. The magnet is fully aligned with the pole pieces (sometimes called the ‘full register position’), and there is maximum flux flow from the north pole through the armature core to the south pole, and the magnetic field around the armature is at a maximum.

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Slick impulse magneto Magneto internal components

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As the magnet rotates through the neutral position, its poles begin to align with the pole pieces, and flux flow commences through the core but in the opposite direction – a flux reversal has taken place. The core flux increases as the magnet turns until, at the next full register position, it again is at a maximum. A further 180° of rotation produces two flux reversals for one revolution of the magnet. These flux reversals are significant because it is at this point in the operation that the maximum primary current is going to be achieved. As the magnet revolves, the core flux changes, as does the magnetic field around the primary winding. It is not the magnitude of the flux, but the rate at which it changes that determines the voltage induced in the primary winding. The maximum rate of exchange occurs when the magnet’s neutral position is just passed – the point where flux reversal occurs. This is where the primary current is at its highest and is the instant when the CB points are opened to break the circuit, thus causing the magnetic field to collapse across the secondary winding. The voltage induced in the secondary depends on: • • •

the strength of the field created by the primary current which in turn depends on the strength of the magnet; the speed of rotation; and the number of turn in the primary winding.

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Lag angle-impulse coupled magnetos The impulse coupling is a mechanical device which is installed between the engine drive and the magneto proper on the majority of rotary magneto installations. Its primary function is to intensify the ignition spark at a relatively low rotative speed of the magneto to assist in engine starting. The construction of the coupling also provides the means of automatically retarding the ignition spark during the starting period. This reduces the possibility of damage to the engine or injury to the operator due to engine backfire. The impulse coupling retards the magneto ignition timing until the engine crankshaft is at its proper position for starting. The lag angle, noted on the magneto data plate, is the impulse coupling’s retard angle measured in degrees. After engine start, the impulse coupling disengages and returns the magneto to normal engine timing. The majority of impulse couplings use the pivoted pawl design, where each pawl is securely fastened to a hub plate, its movement being confined to a turning action in an arc about its pivot point.

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Impulse coupling operation The impulse coupling functions as a mechanical reservoir to store energy which is available at a low rate during the engine starting period. Then, when the point is reached in the engine cycle where the ignition of the fuel mixture should occur, all the accumulated energy is instantly released to the magneto resulting in a strong spark. Since the point at which the energy is released can be controlled in the construction of the coupling, it is possible to provide an automatic retard of the ignition spark during the starting period. The impulse coupling consists of a shell and a hub, connected by a strong spring. One half of the coupling (the shell) is fitted to a drive member on the engine drive shaft, while the other half (the hub) is keyed to the magneto rotor shaft. In operation at slow speeds, a pawl on the magneto half of the coupling engages a stop pin mounted on the magneto frame which acts to prevent further movement of the rotor. In contrast, the engine half of the coupling continues to rotate. The relative change in position winds up the connecting spring. When the point is reached where the ignition spark is desired, the pawl is released, and the drive spring permitted to snap the magneto rotor forward at high speed through its firing position. As the speed of the engine picks up, the centrifugal force acting on the pawls withdraws them to a position where they no longer engage the coupling stop pins, the impulse coupling then acting as a solid drive member.

Twin pawl coupling operation A twin-pawl coupling is used to produce two impulse sparks per revolution of the magneto rotor. Since these pawls are placed symmetrically on the coupling hub plate, each 180° of rotation brings a pawl into position to engage the stop pin. With only slight changes, the twin pawl can be arranged to produce four sparks per revolution. In such a case two stops instead of using just one and are located 180° apart. The pawls are springloaded to make them independent of the gravity operation and to speed up their action. A stiffer drive spring is provided to compensate for the shorter wind up period. The pivoted pawls operate automatically as a result of the two natural forces of gravity and centrifugal. At slow speeds the gravity force dominates the pawl action, holding the pawl in such a position as to force it to engage with the stop pin. With increased rotative speed, there is a corresponding increase in the centrifugal force acting on the pawl, this force gradually overcoming the fundamental gravity forces and thus becoming dominant in the pawl action. Centrifugal force acts to prevent the pawl from engaging the stop pin. If the magneto is to be installed in an attitude where the gravity force cannot operate the pawls during the starting period, to cause the necessary engagement action, small wire springs are attached to the pawls to act opposite the centrifugal force. They are of such strength as to replace the gravity force entirely.

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Sequence of operations of impulse coupling for 90° spark magneto

Sequence of operations of impulse coupling for 180° spark magneto

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Impulse coupling drive springs Two types of impulse coupling drive springs are used in current designs. The most widely used is the torsion-type spring, which resembles the mainspring of a clock. When the coupling is assembled, this spring is given initial tension of approximately one or two turns, the impulse action providing additional tension. In the compression-type spring, the coil spring must be compressed to assemble it into the shell, and additional compression takes place during the impulse action. The strength of the impulse coupling drive spring varies the amount of windup permitted by the coupling design and application. A long wind up can be secured with a single pawl coupling where engagement only occurs once per revolution. A very short windup is obtained in the case of an impulse coupling which is subject to four impulse actions per revolution.

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Torsion-type drive spring

Compression-type drive spring

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Impulse coupling spark retard The impulse coupling acts to intensify the starting spark, and at the same time automatically retard this spark to the extent that it occurs at the approximate top dead centre of the compression stroke of the engine piston. In distinguishing the ignition spark positions, the retarded starting spark is usually termed the impulse spark, while the usual running spark is often called the advance spark. To analyse the method by which the normal spark of the magneto is automatically retarded while the coupling functions, the angular relationship between the coupling and the magneto rotor must be carefully observed. In the diagram below left, the action of the impulse is illustrated in terms of the angles involved in its operation. The first step (a) occurs when the coupling pawl engages the stop pin, at which point the magneto rotor is prevented from further rotation while the coupling shell continues to turn through angle A. During this period the mechanical energy supplied to the magneto is stored in the coupling through the windup of the drive spring. The second step (b) of the impulse action occurs just as the windup period of the drive spring is completed and the pawl kick-off projection on the shell functions to release the pawl from the stop pin. The angle D shown on the coupling end of the assembly indicates the mechanical lag from the instant the pawl kick-off strikes the pawl to the instant the drive lugs of the shell reach the horizontal centreline. Note that in both steps (a) and (b) the magneto rotor is held stationary in a position preceding its spark point by an angle C which includes the edge gap distance. Total Training Support Ltd © Copyright 2020

In the third step (c) the drive spring of the coupling functions to snap the hub of the coupling together with the magneto rotor through its windup angle A, the magneto rotor passes through the entire spark angle C, including the edge gap distance. Since the speed of this action is determined by the strength of the drive spring, a powerful ignition spark can be produced during the starting period of the engine. As the engine speeds up after starting, the impulse action of the coupling ceases, since the pawls no longer engage the stop pin. In the next figure, the coupling is shown as it operates at speeds higher than those permitting impulse action. Since the pawls do not engage the stop pin, no drive spring windup occurs, and the coupling acts as a solid drive member, the magneto rotor turns at the same rate as the coupling shell. Consequently, the magneto rotor passes through its spark position, as indicated by the edge gap distance, when the coupling drive lugs are at angle E before the horizontal centreline is reached. This angle E is the amount the ignition spark has advanced in the change from impulse spark to running spark. Conversely, it is the angular degrees the running spark is retarded during the impulse action and is commonly referred to as the lag angle of the coupling. The angle E of an impulse coupling is determined by the location of the keyway in the hub; the lag angle can be increased or decreased within limits by moving the keyway when making the hub.

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Relationship of impulse coupling to magneto during impulse action

Angular relationship at normal running speed

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Lag-angle-retarded breaker magnetos The retard breaker assembly is an electrical device powered by the aircraft battery, used to aid in starting the engine. At low cranking speed, the retard breaker retards the magneto ignition timing until the engine crankshaft is at its proper position for starting. The lag angle, noted on the magneto data plate, is the retard breaker’s retard angle measured in degrees. When the engine starter disengages, the retard breaker assembly is also disengaged, and the magneto returns to normal engine timing.

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Flux change

Magneto operation

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Ignition switches The type of switch used varies with the manufacturer and the numbers of engines on the aircraft, and they are prominently mounted on the instrument panel in a position convenient for the engine operator. Generally, for one engine, they take the form of either two separate toggle switches in one housing or a single rotary switch. The toggle switches may be identified “L” and “R” for the ignition systems they control but, unlike other electrical switches, are “ON” when the toggle is up.

When the switches are in any position but “OFF”, the magnetos are ‘live’. They are also dangerous, especially for some time after engine shut down when the engine is warm, and fuel vapours may be present. This is why a propeller is never moved by hand until a physical check is made of the magneto switch positions.

Besides controlling their associated ignition systems, they are used during system checks when starting and running the engine. Modern aeroplane engines are required to have a dual ignition system - that is, two separate magnetos to supply the electric current to the two spark plugs contained in each cylinder. One magneto system supplies the current to one set of plugs; the second magneto system supplies the current to the other set of plugs. For that reason, the ignition switch has four positions: • • • •

“OFF”; “L”; “R”; and “BOTH”

With the switch in the “L” or “R” position, only one magneto is supplying current, and only one set of spark plugs in each cylinder is firing. With the switch in the “BOTH” position, both magnetos are supplying current, and both spark plugs are firing.

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Ignition switches

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The contact breaker (CB) This assembly, which is the most highly stressed of the ignition system, consists of a pair of contacts mounted on a base plate. One is fixed and earthed (grounded) to the magneto body. The other is moveable, insulated from the magneto. It is connected to the primary winding by the leaf spring which holds the contacts together. The points are tipped with platinum to resist pitting, burning and the mechanical hammering of continuous fast operation. A cam is keyed to the magnet shaft and has two lobes because, with two flux reversals per revolution, it needs two CB points separations. The CB assembly is mounted adjacent to the cam to enable the lobes as they rotate to contact a non-metallic block positioned so that it separates the points against the pressure of the leaf spring. This separation occurs just after the neutral position of the magnet. Further rotation of the cam allows the points to close. This occurs in the approximate full register position of the magnet and from there until the next cam lobe comes round, the block is clear of the cam profile.

The maker specifies the size of this gap and provides for its adjustment using the assembly’s base plate adjustment screws. There are two types of CB, as shown in the diagram below, bottom-left. The pivoted type (a) is widely used on all piston engines and is found on aero engines. The pivotless type (b) is common to magnetos of American origin. In both types, an oiled felt pad is employed to lubricate the surface of the cam. The differences between the types lie in the applications of the leaf spring and the non-metallic block. On the pivoted CB it is fitted in the rocker arm and is often called the ‘fibre heel’. In the pivotless type, it is employed as a cam follower, frequently with its own spring. In a 6-cylinder engine rotating at 3,000 RPM, the CB has to operate 9,000 times a minute or 150 times a second. The resulting stress on the spring and the points’ faces is very high.

The points need to be closed for as long as possible during the cam’s rotation after contact has been made to ensure sufficient time for the primary current to build up. This period is measured in degrees of cam rotation and is often identified as the ‘dwell angle’. Two factors govern this period. One is the profile of the cam, which is the manufacturer’s responsibility. The other is the gap between the points when full separation has taken place – adjusted by the maintenance technician.

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Contact breakers Contact breaker components

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The distributor This assembly comprises two items, the rotor and the block, both of which are made of non-conducting material. Set within the rotor is a metal conducting path from the centre to a projecting metal lug, sometimes called the brush. Moulded around the block and standing proud in a common plane, are as many equally spaced metal segments as there are cylinders in the engine. When assembled, the lug of the rotor brush lies in the same plane as the segments, and when rotated, it passes but does not touch, each of these segments in turn. Each of the segments is connected to a socket on the outside of the block. It is from these that the HT is fed by cables to the spark plugs. With the magneto functioning, the rotor, during one revolution, distributes a spark to every cylinder of the engine. During that time, the crankshaft of a 4-cylinder engine has had to complete two revolutions. Because the distributor drive comes from the crankshaft, the rotor always rotates at half the engine speed. The distributor components of a simple 2-pole magneto are shown in the diagram below. The rotor arm is secured to the centre of a gear wheel which meshes with another gear mounted in the magnet’s shaft. The ratio of these gears is chosen so that whatever the speed of the magnet shaft with respect to the engine, the rotor turns at half engine speed. This is why the rotor gear wheel is often called ‘the half-speed wheel’. In the centre of the rotor arm is a spring-loaded carbon brush. This is in contact with a broad leaf spring on the armature to which the secondary winding is connected. Whenever a voltage is induced in the secondary, the rotor brush is opposite one of the segments in the block with a small 0.2 mm (0.008") gap between them. The secondary current flows via the carbon Total Training Support Ltd © Copyright 2020

brush through the rotor arm, jumps the small gap and reaches earth via the HT lead and spark-plug gap. Magneto venting The magneto and its components cannot be hermetically sealed for internal cleanliness because they are subjected to temperature and pressure changes in flight; hence they become prone to the effects of condensation. If the interior of the magneto becomes wet, there is the possibility that the high voltage current arcing across the rotor/block gap could be misdirected to another segment or the magneto case. This is ‘flashover’ and often leaves a fine carbon track as the spark evaporates the moisture and burns dirt particles lying on the non-conducting surface of the distributor. The carbon track remains as a continuous cause of misfiring and power loss. The typical arcing which takes place across the distributor components results in unavoidable erosion of their surfaces. In the presence of moisture, arcing also produces corrosive gases which attack the metal and leave high resistance deposits. All magnetos are vented and drained to reduce these effects. The venting has to consider the possibility of inflammable vapours being present in the confines of the engine cowling. Protection from fire due to magneto sparking is achieved by covering the vents with fine mesh gauze discs. These allow air through the magneto and at the same time act as flame traps on the Davy lamp principle.

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Simple two-pole magneto

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Magneto speeds Every cylinder of a four-stroke engine requires a spark for every two revolutions of the crankshaft. The higher the number of cylinders, the faster the magneto must revolve. Considering the two-pole magnet magneto (two sparks per revolution), we see how and why there are speed limitations on its use. Consider this example formula: Magneto speed =

# of cylinders Sparks per rev

× engine speed

In practical terms, this means that a two sparks/rev magneto is suitable only for engines of up to 6 cylinders because with a greater number of cylinders the magnetorotational speeds are too high. If we assume a 6-cylinder engine take-off speed of 3,200 RPM, the magneto would be rotating at 4,800. Higher speeds would be both mechanically and electrically too stressful. To cater for larger engines, we need a magneto capable of providing more sparks per revolution. How this is achieved is the subject of the next section.

Our magneto on a four-cylinder engine would need to be driven at: =

4 x engine speed 2×2

= 1 × engine speed for a six-cylinder engine: =

6 × engine speed 2×2

= 1.5 × engine speed If we consider both these engines running at a speed of say 2,400 RPM, their magneto speeds respectively would be 2,400 and 3,600 RPM.

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Dual magnetos and harness

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The four-pole magnet A four-pole magnet is shown in the diagram below top-left. A cylindrical magnet and a pair of two-fingered pole pieces are clamped together on a non-magnetic steel shaft. The polarities of the magnet are transferred to the pole pieces so that the pole fingers alternate north and south every 90°. The terms ‘full register’ and ‘neutral’ still apply to the magnet positions relative to the pole pieces, but they now occur at 45° intervals. We now have four flux reversals (sparks) for one revolution of the magnet assembly and, correspondingly, we require four lobes on the cam. If you now apply the magneto speed formula, you can confirm that for engines larger than six cylinders, the magneto speeds are much more practical. For even larger engines, a pair of fourfingered pole pieces are employed to produce eight sparks per revolution. Note that a four-pole magnet would be of little use on a small engine as it would turn at half the speed of our two-pole magnet and thus reduce the strength of both the primary field and the sparks at the plugs.

The rotating armature magneto This type of magneto is one you could well meet on older aircraft. It has the armature conductors revolving in the static field of a horseshoe magnet as opposed to the stationary conductors and varying field of the rotating magnet type. Although a widely used and trusted component, it has certain disadvantages and limitations. It is illustrated in the diagram below bottom-left. • • •

It produces two sparks/revolution and cannot be altered; hence it is only suitable for small engines. The windings, capacitor and CB all revolve and are therefore affected by centrifugal forces. The HT is picked up from a slip ring on the rotor by a carbon brush and transferred to the distributor via a second brush.

Apart from its basic generating difference, the operation and maintenance of an ignition system employing this type of magneto are the same as for the rotating magnet type.

The dual magneto You may meet the dual magneto type on some installations. It is sometimes called a duplex magneto. It incorporates two magnetos in one housing by having one rotating magnet and cam common to two sets of coils and contact breakers. On large engines, the two distributors are mounted on the engine separate from the magneto, each with its own drive. For small engines, the magneto casing houses the two distributors as well, and their respective outlet sockets are placed one on each side of the casing. Total Training Support Ltd © Copyright 2020

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Four-pole magneto Dual magneto

Ignition system

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Magneto mounting and drives Magnetos are situated at the rear of the engine either flangeor platform-mounted with their position and method of drive being dependent on the engine type. A layout which is common to horizontally opposed engines is shown in the diagram below. The magnetos are coupled to a shared drive shaft which is supported by bearings in the engine casing, with suitable gearing from the crankshaft ensuring correct magneto speeds. Each side of the casing has a machined face (or pad) and the magnetos each have a flange with curved slots 180° apart. The magneto is fitted with the slots engaging two studs in the pad and secured by nuts and some locking device. The magnetos on a radial engine could be found fitted as in diagram (b). They are platform mounted, and the drives are part of the gear train which powers all the accessories fitted to the casing.

Refer to the diagram (a) and note that the magnetos rotate in opposite directions as a result of the shared drive. Because the maximum rate of change of flux occurs just after the magnet’s neutral position and the internal settings of the magneto must accommodate this, the magnetos must be identified left and right. This might not be relevant to the diagram (b) because the gear train could result in both magnetos turning in the same direction. There are specific ways of assisting a magneto to provide a strong spark for starting, and quite often it is applied to only one magneto of the pair. This difference could apply to both (a) and (b) so that even if both magnetos rotated in the same direction, as in (b), they would still have to be identified L and R.

Each of the illustrations shows a coupling fitted to transmit the drive to the magneto shaft. On a flange mounted magneto, the coupling is of a simple splined or serrated type. However, for a platform-mounted magneto, the coupling must transmit drive and be flexible to allow for small misalignments between magneto and drive shafts. It must also be capable of the same fine angular adjustment that the slotted flange magneto allows. To meet the running engine’s ignition requirements, both magnetos on the engine are the same. Practically, however, some differences exist between them.

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Mountings and drives

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Mountings and drives

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Ignition harnesses Construction These consist of an insulated single-core cable capable of carrying over 12,000 volts. Construction varies with the manufacturer and age of the design but is generally as in the diagram below left. Testing Continuity test Solid electrical contact (continuity) is essential throughout any conductor or circuit and its connections, to ensure minimum resistance to current flow. Any resistance due to poor contact (and conductor damage) may be detected with the aid of a battery and lamp connected in series with the conductor. For general maintenance purposes, the battery and lamp (typically 4.5 volts and 0.4 amps) are housed in a protective casing with a press button, terminals, and two leads for connection to the conductor. It’s called a lamp and battery, continuity tester, or it could be part of a magneto synchroniser. The test procedure Join the two lead ends, press the button and check the brilliance of the lamp. This is a functional and battery check. Insulation testing The insulation efficiency of all electrical equipment is checked at regular intervals by measuring the resistance of the insulation to the passage of current. Because this resistance is affected by the voltage applied to the insulator, a sufficiently high voltage would cause the insulator to break down. This is the basis on which the testing is carried out. Total Training Support Ltd © Copyright 2020

Resistance measurement is usually performed with a test voltage that is very much higher than the standard voltage of the circuit being tested. If the resistance value is high on the test, it follows that there can be little risk of failure under normal working conditions. In its simplest form, an insulation resistance tester consists of a source of constant high voltage and a protected sensitive meter to indicate the current flowing when these two are connected with the circuit under test. This current usually is very small – microamps or a few milliamps. Now, because of the constant voltage, the resistance of the circuit has a direct relationship with the current flowing (Ohm’s law), so the manufacturer calibrates the meter scale in megohms. This makes for more accurate determination of circuit condition. The voltage source of insulation-resistance testers is commonly a hand-operated generator. The steady output voltage obtainable depends on the make and type of tester and can be typically 250, 500, or 1,000 volts. This covers most test requirements. Regardless of the make, they are often called a Megger from a long-standing trading name. The diagram below right illustrates such a tester. For an ignition HT circuit, which can have a working load of over 7,000 volts, more specialised equipment is required; 12,000 volts or more depending on the engine manufacturer’s test requirements. This may be obtained either from a handoperated generator or from the electrical mains in conjunction with transformers.

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Ignition cable construction

Typical megger

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An often-used item of equipment is known as the HT ignition tester. It supplies 12,000 volts from a hand generator when it is rotated at 300 RPM. The meter is calibrated in megohms and often is divided into green and red sectors with the colour change at the 2 megohm point on the scale. A safety push-button is included in the circuit to protect the meter movement. Two test leads, approximately two meters long, made from HT cable and identified “EARTH” and “HT”, complete the equipment. The test procedure We are assuming that our continuity test was satisfactory and that the plug leads and distributor block are still disconnected. The HT tester just described is on a firm base close to the harness assembly. •

•

Starting with No. 1 cylinder, connect the tester’s HT lead to the core of the plug and lead to be tested and the earth lead to the metal harness. Rotate the generator handle slowly and check for sparking, which may be seen or heard, or hunting of the meter needle. Any of these usually indicate insulation breakdown but first, check for the closeness of the HT lead connection to the earth. If there are no such indications, continue slow handle rotation and depress the button. If the meter needle swings hard over or hunts violently, a short circuit or intermittent breakdown is the cause. If the needle shows neither of these signs, increase handle speed gradually to maximum, with the button still depressed, and note the reading.

This type of HT tester was designed for a minimum acceptable reading of 2 megohms, which is relatively common in HT lead testing. If the needle has settled in the red sector, the lead is notionally unacceptable. However, before any decisions are made, all the plug leads should be tested in cylinder number order and a note made of the reading for each lead. The test is then repeated for each lead and the two sets of readings compared. It is standard practice to take at least two readings in any test procedure. Ideally, insulation resistance would be at infinity. However, the high-test voltages, the age and usage of the harness, and the pressure of moisture all tend to reduce the resistance of the insulation; hence the apparent low pass of 2 megohms. Because an engine manufacturer could require specific test equipment and/or a different minimum resistance for their HT components, the relevant manual must always be checked before the test is begun. This basic insulation resistance is entirely satisfactory when applied to separate HT leads. When these are under test, any lead which is consistently below limits must be changed per the manufacturer’s instructions, and, after changing, is tested for continuity and insulation. The replacement lead may be supplied as an assembly ready for fitting, or it may have to be made up from a length of cable and end fittings. If one or two leads are below the minimum resistance, they must be renewed. This is a more difficult task because they have to be withdrawn from the conduit after a new lead length has been soldered to the old. Check the manual for the procedure and detailed information on the assembly of end fittings. Of course, the two tests are required on completion.

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Individual cable assembly

HT ignition tester

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If, during harness and lead testing, a large number of readings are below limits, don’t condemn the leads immediately, check that: • •

• •

the procedure was carried out correctly; the harness/leads were not subjected to excessive humidity. If so, remove it/them to a dry environment, or connect up, run the engine, and test again; the tester is serviceable, (try it on known serviceable and unserviceable equipment); and there were no significant running defects reported before the inspection.

If after these checks more than two leads are still below the limits, change the harness. • •

•

Connect one tester lead to the cable core and the other to the braid. Make sure that the magneto switch is “ON”. If it were “OFF”, we would have a dead short across the tester’s output leads because the switch contacts would be closed. Turn the handle slowly at first to check for shorting or insulation breakdown, then increase to the recommended maximum and note the needle position on the scale. The acceptable minimum is 10 megohms.

Because of the potential for faulty switches or leads to cause a complete cut-off in an ignition system, the inspection and testing must be conducted with great care.

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Spark plugs Construction There are many shapes and sizes of aero-engine spark plug, and they are all of similar construction. A typical modern plug is illustrated in the diagram below left. Plug body This is made from high tensile steel, sometimes plated to resist corrosion and with a standard hexagon to accept a standard socket or plug spanner. Body thread This is a close tolerance thread which fits into the cylinder head. The diameter is expressed in millimetres, e.g. 12 mm, 14 mm or 18 mm. Sealing washer This ensures a gas-tight fit for the thread and is often made of copper. Screen This an extension of the body which completes the screening of the HT lead. Connector thread This accepts the sleeve nut which secures the HT lead to the plug.

Central electrode This is designed to allow for thermal expansion. It conducts the HT pulse from the contact inside the sleeve to the plug nose. The lower end is often of a nickel alloy. Sometimes it incorporates a resistor to help reduce electrode erosion and ignition interference spikes. Ceramic insulator This is secured and sealed into the body during manufacture. It supports and insulates the central electrode. It is extended at the outer end to insulate the plug lead from the surrounding metal sleeve. Ceramics are very brittle heat resisting materials. A detail not apparent in the diagram is the ‘reach’ of the plug. This is the distance from the underside of the sealing washer to the plug nose. It ensures that the electrodes are in the best position for igniting the mixture within the combustion chamber. Some older types of plug are classified as ‘detachable’. That is, the screen sleeve is removable from the body for cleaning purposes. The sleeve insulation material could be mica or a ceramic.

Earth electrode One or more project towards the centre electrode from the plug nose. They may be of nickel alloy but more likely of platinum or iridium.

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Spark plug construction

Typical types of electrode construction

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The heat range of the spark plug The heat range of a spark plug is the principal factor governing aircraft performance under various service conditions. The term ‘heat range’ refers to the classification of spark plugs according to their ability to transfer heat from the firing end of the spark plug to the cylinder head. Spark plugs have been classified as ‘hot’, ‘normal’ and ‘cold’. However, these terms may be misleading because the heat range varies through many degrees of temperature from extremely hot to extremely cold. Thus, the words hot, cold or normal do not necessarily tell the whole story. Since the insulator is designed to be the hottest part of the spark plug, its temperature can be related to the pre-ignition and fouling regions. Pre-ignition is likely to occur if surface areas in the combustion chamber exceed critical limits or if the spark-plug core nose-temperature exceeds 900°C (1,630°F). However, fouling or short-circuiting of the plug due to carbon deposits is likely if the insulator tip temperature drops below approximately 430°C (800°F). Thus, spark plugs must operate between fairly well-defined temperature limits. So, plugs must be supplied in various heat ranges to meet the requirement of different engines under a variety of operating conditions.

Fundamentally, an engine which runs hot requires a relatively cold spark plug, whereas an engine which runs cool requires a relatively hot spark plug. If a hot spark plug is installed in an engine which runs hot, the spark plug tip overheats and causes pre-ignition. If a cold spark plug is installed in an engine which runs cool, the tip of the spark plug collects unburned carbon, causing fouling of the plug. The principal factors governing the heat range of aircraft spark plugs are: • • • • • • •

the distance between the copper sleeve around the insulator and the insulator tip; the thermal conductivity of the insulating material; the thermal conductivity of the electrode; the rate of heat transfer between the electrode and the insulator; the shape of the insulator tip; the distance between the insulator tip and the shell; and the type of outside gasket used.

From the engineering standpoint, each plug must be designed to offer the broadest possible operating range. This means that a given type of spark plug should operate as hot as possible at slow speeds and light load and as cool as possible at cruising and takeoff power. Plug performance, therefore, depends on the operating temperature of the insulator nose, with the most desirable temperature range falling between 540 and 680°C (1,000 and 1,250°F). Total Training Support Ltd © Copyright 2020

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Spark plug heat rage

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Spark plug types

Short- and long-reach plugs Note long-reach are identified by yellow paint on Lycoming cylinder heads Spark plug reach

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All these factors vary with the manufacturer of the plug and its application, so only the type of spark plug approved for an engine may be fitted. The engine manual specifies the plug and its identification always appears on the plug body. The total number of plugs on an engine is generally known as a ‘set’, and those associated with one magneto are frequently called a ‘half set’. The servicing of plugs is always carried out strictly following the manufacturer’s instructions. Assuming no unscheduled removals, the set has a life of a specific number of flying hours – a period which usually coincides with a point in the engine’s maintenance cycle. They would then be removed for servicing in an adequately equipped plug bay. A fully serviced and certified set is fitted to the engine, and their functioning checked when the engine is run. Inspection and servicing of sparking plugs What follows are the details of plug maintenance as applicable to a non-detachable plug. There are seven stages, and they must be carried out in the correct sequence. • • • • • • •

Inspection De-greasing Cleaning Second inspection Spark gap setting Testing Storage

Spark plug inspection Insulator tip deposits The firing end of the spark plug should be inspected for the colour of the deposits, cracked insulator tips and gap size. The electrodes should be inspected for signs of foreign object damage and the massive type also for copper run-out. The standard colour of the deposits usually is brownish grey with some slight electrode wear. These plugs may be cleaned, regapped and reinstalled. A new engine seat gasket should be used. Carbon fouling Dry, fluffy black deposits show carbon fouling. This indicates a rich fuel/air mixture, excessive ground idling, mixture too rich at idle or cruise, or faulty carburettor adjustment. The heat range of the plug is also too cold to burn off combustion deposits. Typical fuel-related causes to look for are over-rich fuel mixture, excessive idle or excessive operation at closed-throttle idle. Other causes might be improper idle mixture setting or improper (too cold) spark plug application. Ignition-related causes of carbon fouling include improper magneto timing, a failing lead or failed spark plug.

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Oil fouling Oil fouling deposits appear as wet, black carbon deposits on the firing end. Oil fouling deposits are conductive at all temperatures and cause plug misfiring under all power conditions. It is not uncommon to find this condition in a mild form on lower plugs of some horizontally opposed engine models or in lower cylinders of radial engines. It may be caused by oil draining by the piston rings and collecting in the combustion chamber during extended engine shut-down periods. Such mild conditions can usually be cleared up by cycling the engine with slow increases of power until misfiring stops. Oily deposits on the top plugs may indicate damaged pistons, worn or broken piston rings, worn valve guides, sticking valves or faulty ignition supply. This same condition in a new or newly overhauled engine may simply indicate that piston rings have not yet correctly seated.

Lead fouling Under normal conditions, the lead oxobromide deposits from the tetraethyl-lead (TEL) of high-octane aviation fuels form an even, fluffy coating ranging from light tan to light brown. A darkening of these colours near the core tip indicates adverse temperature conditions. Maldistribution of the TEL causes severe lead fouling, which appears as hard cinder-like globules of lead on the firing end, and in time gradually fill the firing end cavity. Although mild lead deposits are always present to some degree, highly leaded fuels, reduced fuel vaporisation, operating the engine too cold and spark plugs not suited for the particular operation are the usual causes of severe lead fouling. Extremely fouled plugs should be replaced and the cause of the fouling corrected. Severely fouled spark plugs, like those shown here, operate colder, causing misfires, and also misfire at higher power because of the conductive nature of the deposits at elevated temperatures.

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Bridged electrode A deposit of conductive material between centre and ground electrodes that shorts out the spark plug. The gap may be bridged by ice crystals that form while trying to start, by carbon particles, by lead globules, by metallic particles or by ingesting silica through the air intake. When metallic fusion bridges the electrodes, the plugs must be replaced, but other deposits may simply be removed, and the plugs returned to service. The cause of deposits that short out spark plugs requires corrective action.

Worn electrodes Electrical and gas corrosion wear spark plug electrodes. Under normal conditions, this wear occurs slowly and should be expected. Severe electrode erosion and necking of fine wire ground electrodes indicate abnormal engine operation. Fuel metering, magneto timing and proper heat range should be checked. Spark plug cleaning and rotation at scheduled intervals is usually adequate care until spark plug gap approaches recommended maximum. Spark plugs with worn electrodes require more voltage for ignition and should be discarded when electrodes have worn to half their original size.

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Cracked core nose Regular engine operation cycles thermal shock to the core nose, and insulator materials and designs are chosen to avoid core nose cracks from such thermal shock. However, occasional abnormal engine operation may exceed even the built-in safety factors, resulting in infrequent core nose cracking. The typical cracked core nose condition shown may be caused by improper cleaning or gapping procedures and by detonation. Copper run out is caused by very high temperatures associated with detonation or pre-ignition. It occurs when high temperatures perforate or burn away the end of the nickel centre electrode sheath and expose the copper core. Melted copper then runs onto the tip surface and forms globules or a fused mass across the electrode gap. The engine must be inspected and the plugs replaced with new ones.

Engine parts such as the piston, cylinder head and connecting rod may suffer severe damage. When detonation has occurred, the cylinder must be examined with a borescope and may require replacement. Corrective action is imperative. In such cases, the cylinders should be inspected with the aid of a borescope. It may be desirable to replace the cylinder, especially if backfiring was reported by the flight crew. The reason for this precautionary action is that if the engine was operated under some detonation conditions, but not to the extent that it caused a complete piston failure, the piston rings could be broken. A piston failure requiring a complete engine change may show up at a later date.

A hot spot in the cylinder may cause pre-ignition which can always be detected by a sudden rise in cylinder head temperature or by rough engine operation. When plugs are removed after a period of pre-ignition, they will have burned or blistered insulator tips and severely eroded electrodes. Detonation is the sudden and violent combustion of a portion of the unburned fuel ahead of the flame front occurs partway through the burning cycle when the remaining unburned fuel suddenly reaches its critical temperature and ignites spontaneously. There is severe heat and pressure shock within the combustion chamber that causes spark plugs to have broken or cracked insulator tips along with damage to the electrodes and lower insulator seal.

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Damage from excessive temperatures Overheating of the spark plug barrel, sometimes caused by damaged cylinder baffles or missing cooling air blast tubes, may seriously deteriorate the ignition leads. Any overheating of the spark plug barrel by a defective baffle or exhaust gas leakage at the exhaust pipe mounting flange can generate temperatures in the insulator tip sufficient to cause pre-ignition and piston distress. Other spark plug problems The cure for threads that are stripped, crossed or badly nicked is the replacement of the spark plug with a new one. Dirty threads in the engine may cause the spark plug to seize before it is seated. Dirty threads also cause poor contact between the spark plug, spark plug gasket and the engine seat. This results in reduced heat transfer and causes excessive overheating of the spark plug. This condition can be corrected by making sure that threads are clean, and by observing the torque specifications when installing new plugs. It is helpful to use antiseize compound or plain engine oil on spark plug threads starting two full threads from the electrode. Do not use a graphite-based compound. Connector-well flashover is caused by an electrical path along the surface of the insulator, from contact cap to shield. It occurs when the voltage required to arc across the electrode gap exceeds the voltage required to track over the surface of the insulator. This condition is caused by a too-wide electrode gap, oil, moisture, salt track or other conductive deposit on the terminal well surface or lead-in assembly. When flashover occurs, combustion chamber residues quickly coat the insulator tip and electrodes so that the condition may be interpreted as oil or gas fouling. If the ceramic of the plug is not broken, the plug may be cleaned and reused. Thorough cleaning of the Total Training Support Ltd © Copyright 2020

lead in the assembly may solve the problem, or it may be necessary to replace the assembly to correct the defect. De-greasing Using an approved solvent but without total immersion. It is not the same as cleaning. Cleaning Can only be carried out on a grease-free plug, and it is confined to the nose. There are three methods, each requiring specialised equipment: • • •

sandblasting; chemical cleaning; and vibratory cleaning.

The choice of technique depends on the manufacturer’s recommendation. After this procedure, the nose interior is checked, and the sleeve insulator cleaned. Spark plug cleaning During operation of an aircraft engine, lead and carbon deposits form on the ceramic core, the electrodes, and the inside of the spark plug shell. These deposits are most readily removed by an abrasive blasting machine specially designed for cleaning spark plugs Second inspection This is for the defects noted in the initial inspection and specifically for electrode looseness and erosion. Any defects or excessive erosion cause the plug to be rejected.

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Champion spark plug cleaner and tester

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Spark gap setting The gap must be measured and compared with the figure specified in the manual before the plug is subjected to an electrical test. Depending on the electrode configuration, the gap is measured with either a feeler gauge or a wire go/no go gauge. Except for radial electrodes, the gap is carefully adjusted on the earth electrode only until it is within limits. In service, plug gaps always tend to get larger, so the practice is to aim for the bottom limit and thus keep the gap within limits for longer running time. If a radial electrode’s gap is greater than the upper limit, the gap is rejected because the gap cannot be reduced. Testing All plugs must pass an insulation test before being cleared for service. The test, which is different from the employed for HT leads, is carried out on one of the types of spark plug tester which are available. The plug is screwed fully into a small pressure chamber and a voltage – higher than the maximum output of a magneto – is applied to the centre electrodes. If the plug insulation is sound a constant stream of sparks traverses the plug gap. Irregular sparking would increase faulty insulation. The small chamber is then pressurised with air at 550 kN/m2 (80 psi) and HT reapplied. Sparking should again be continuous if the plug is to pass its test. Some manufacturers might specify a different chamber pressure because of the effect of their particular electrode gap. This last sentence and the test procedure itself require some explanation.

There is a relationship between the size of a spark gap, the voltage applied to it and the air pressure surrounding the gap. At constant pressure, the voltage required to jump a spark gap must be increased if the gap is widened. If the pressure is increased around the gap, then, to maintain correct sparking again, the voltage must be increased – because the electrical resistance of air increases with pressure. The available voltage from the tester has a high maximum value. In the first part of the test – at atmospheric pressure – the voltage required to give steady sparking is comparatively low (not all the available energy is being used) and the insulation is not thoroughly tested. When the gap is under pressure, however, the insulation has the maximum voltage applied across it, and any insulation defects become evident. The majority of plugs have a very similar gap size, and the test pressure quoted earlier is satisfactory for the test. Any gaps which differ much from this would require a different chamber pressure to get the same test benefit from the fixed HT output. For example, a larger gap equals lower pressure. In one type of tester, the plug aperture is at the bottom of the pressure chamber. This enables a second test to be carried out – mainly on detachable plugs – the gas leakage test. A container of white spirits is placed under the plug so that the body/screen joint is immersed. A pressure of 700 kN/m2 (100 psi) is applied to the chamber, and any leakage shows as bubbles in the fluid; this would cause rejection of the plug.

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Storage If any of the original set of plugs were rejected during servicing, they must be replaced with satisfactory plugs of the same type. The whole set is then protected from corrosion and may be stored for a short time, in a cupboard which is heated to avoid condensation.

•

Long term storage or transit requires the plugs to be packed individually with an identification label in clear plastic tubes from which the air is evacuated before sealing. They are packed in a sturdy box with identification and certified serviceable labels on the outside. Plug fitting This is a regular maintenance operation which, like plug removal is only carried out on a cool engine. This avoids damage to plugs or cylinder head inserts which could occur if these components were hot. Pre-fitting tasks • Clean inhibitor from the nose, wipe screen insulator, dry the plug and check the gap. • Ensure a serviceable and correct type of sealing washer is fitted. • Apply anti-seize compound (e.g. graphite grease) lightly to body threats, avoiding electrodes and washer face. Avoid contamination before fitting. • Ensure cylinder head thread is clean. When fitting • Screw plug into cylinder head by hand until washer contacts the head face. If this is not possible, confirm the cleanliness of the insert thread.

•

With an appropriate spanner and torque wrench, tighten the plug to the loading specified in the manual. To ensure that no side loads are placed on the plug, support the spanner end of the wrench. When all plugs are fitted, clean the lead ends and their nut threads and ensure that the screen is still clean. With led end pressed into the plug, fit the nut to the sleeve and tighten with fingers only – this soon detects any cross-threading. Ensure the lead elbow does not twist when a properly fitting spanner is used for the final tightening. Tighten with fingers only – this soon detects any crossthreading. Ensure the lead elbow does not twist when a properly fitting spanner is used for the final tightening.

Further considerations: • Fitting a plug to a hot engine results in torque loading altering as the engine cools. • Over-torqueing a plug can cause plug or insert damage. • Under-tightening a plug can result in loose plugs, gas leakage, and engine inefficiency. • A plug dropped onto a hard surface must not be fitted, even if the visual examination shows no defects. Return it for proper inspection – the insulator could be cracked. • Never use open-ended spanners for plug removal or fitting. • In the rare event that the plug has to be changed and no torque wrench is available, a properly fitting socket or box spanner is used. For balanced leverage, a tommy bar is required, of a length dependent on the plug size.

5-68 Module 16.5 Starting and Ignition Systems

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Spark plug torqueing and correct HT lead fitting

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Plug rotation (every 50 hours) Excessive electrode erosion is caused by magneto constantpolarity firing and capacitance after firing. Constant polarity occurs with even-numbered cylinder magnetos. One plug fires with positive polarity, causing excessive ground electrode wear, while the next plug fires negatively, which causes excessive centre electrode wear. The wear takes place on the surface from which the current leaves. Capacitance after firing wear is caused by the stored energy in the ignition lead unloading after normal timed ignition. To equalise this wear, keep the spark plugs in engine sets, placing them in trays identified by cylinder locations. After the plugs have been serviced, rotate the plugs as illustrated below to correct the polarity wear condition. Capacitance wear is corrected by swopping long leads for short leads.

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Spark plug erosion and rotation

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Ground checking A ground run is essential before the majority of engine maintenance may be certified as having been adequately carried out. Details of the ground running procedures and precautions appropriate to an engine are found in the maintenance manual, and these must always be observed. Because engines vary in these requirements, the following describes a standard procedure, which applies to an unsupercharged engine with a fixed-pitch propeller. By adhering to the sequence of operations, the functioning of newly installed plugs is thoroughly checked as well as the associated ignition components. The first check is made after the engine has been started and is warming up at the recommended RPM. •

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Switch one magneto “OFF”. The engine should continue running but with a small drop in RPM. Switch magneto “ON”. Switch the other magneto “OFF”, and the effect should be the same as for the first magneto. Switch magneto “ON”. Switch both magnetos “OFF” briefly, then back “ON”. The engine should cut dead then pick up. Never leaves the switches “OFF” for too long. It causes an accumulation of unburnt mixture which could explode dangerously when the magnetos are eventually switched “ON”.

When the minimum operating temperatures of oil and cylinder head are reached, the procedure is as follows: • •

•

The throttle is steadily opened to the maximum, and when the engine RPM has stabilised, it is noted. One magneto is switched “OFF”, the RPM will decrease, and when stabilised, the drop is noted, and the switch returned to “ON”. When the RPM is again stable, the same procedure is followed for the other magneto.

Operating on one magneto causes a loss in power and RPM. The manufacturer sets a limit on the amount of drop in RPM which is acceptable. If the results of the check are outside these limits, an investigation is called for. Sometimes an excessive drop may be cleared by a further engine running. If this is unsuccessful one or more plugs may be suspect, and because it is challenging to identify a particular faulty plug, the half set would need to be changed and the engine rerun. Further investigation is called for if the magneto drop persists. Perhaps it is an engine and not an ignition fault which is the cause. The condition of removed plugs can often provide clues to the fault, even after a short ground run.

Having checked both the magnetos are operating and can be earthed, and the switches are functioning satisfactorily, next carry out what is known as a magneto check. This ensures that the sparking plugs function under the high pressure and temperature conditions of full-throttle operation. Total Training Support Ltd © Copyright 2020

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Magneto drop test

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Auxiliary ignition systems The magneto is a generator driven by the engine producing a high voltage current that provides the spark at the plugs. The correct operation of the magneto relies upon engine rotational speed. The magneto produces a strong spark at regular engine speeds but during engine starting operations the engines in not turning over fast enough to produce the necessary strong spark for ignition. This problem is overcome by the use of auxiliary ignition systems.

impulse starters for low power engines; and booster coils for high power engines.

Impulse starters Impulse starters are associated with low powered engines that are usually started by hand. The unit is a spring-loaded coupling through which the engine drives a magneto. The coupling is divided into halves, and it is driven by the engine through a strong spiral spring to the magneto half of the coupling. As the engine is turned to start, the magneto turns with the engine until just before its contact breaker-points are about to open. At this point, a pawl falls against a stop on the magneto endplate and prevents further magneto rotation. As the engine continues to turn the spiral spring winds up until, just after TDC, a cam on the engine coupling releases the pawl.

Booster coils Booster coils can be divided into two categories: • •

There are two types of auxiliary ignition system. They are: • •

The spring then unwinds rapidly and flicks the magneto round fast enough to produce a spark, which is so far retarded that there is no danger of a kick-back form the propeller. As the engine speeds increases, centrifugal forces hold the balanced pawls out of engagement and only engage again on engine shut down.

high tension coils; and low tension coils.

High tension (HT) coil A high-tension booster coil consists of an entirely separate induction coil with its primary windings energised from the aircraft battery, or ground power unit when the circuit is made by pressing the booster coil switch, or engine starter switch. The diagram below shows that an armature and an electrically operated switch are also part of the booster coil. The armature, as in the magneto, has a soft iron core on which are wound primary and secondary windings. The electrically operated switch controls the primary circuit. The moveable contact of the switch is secured to a leaf spring which tends to hold the contacts closed. The hook of a flexible steel plate, upon which is mounted a soft iron pad, is caught under the leaf spring.

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Booster coil circuit

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The primary contacts are normally closed so that when the primary circuit is energised from the external power, the iron core becomes magnetised. The magnetised core immediately attracts the iron pad, causing the hook to open the contact points and break the primary circuit. This causes the magnetic field to collapse across the secondary winding. A high-tension electrical impulse is produced as a result and is fed to the trailing brush of the engine magneto distributor rotor. In this state the armature losses its magnetism thus allowing the leaf spring to close the contact points. As the contact points close, the primary circuit is again energised, and the cycle is repeated and continues to be repeated until the external power is switched off. Thus, an endless stream of high-tension impulses is fed to the distributor of the main magneto and on to the spark plugs. A capacitor is fitted across the contact points to reduce arcing at the points. The unit is designed to supply a continual stream of hightension electrical pulses, each capable of producing a spark at the spark plug. These impulses are directed to the cylinders in the correct firing order through the additional trailing brush on the engine magneto distributor rotor.

Low tension (LT) coil The low-tension booster coil is supplied with current from the aircraft system when the booster coil or starter switch is pressed. The system is similar to the high-tension system, but the separate starter brush on the magneto is not required. A typical example of a low-tension booster coil circuit is illustrated in the diagram below. The primary contacts are normally closed so that, when the primary circuit is energised from the external power, the iron core becomes magnetised, attracts the iron pad, and breaks the circuit. The resultant collapse of the magnetic field induces a voltage into the secondary winding that charges the second capacitor. The movement made by the iron pad and leaf spring when breaking the primary circuit also closes the secondary contacts. The energy in the secondary capacitor then discharges into the primary circuit of the magneto. This primary current flow causes a high-tension voltage to be induced into the secondary windings of the magneto, and a spark is produced at the spark plug.

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Dual magneto

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Shower of sparks The shower of sparks, or SOS method for starting, was originally used on the Ford Model T and later on some aircraft engines. Shower of spark systems consist of two magnetos, but the left mag has two sets of points—one in the normal advance position, and the other in a retarded position. A vibrator switch (think of an old doorbell buzzer that creates a rapid firing sequence of sparks to the coil) is usually mounted on the firewall, taking the advance points out of the loop, while activating the retarded points. Rather than one spark from an impulse coupling, there are nine to 12 sparks per sequence, providing much more opportunity for the spark to fire. These systems were on several Beechcraft and earlier Mooney models and are still in service. The problem with these systems is that the technology available back then only allowed the vibrator to increase the voltage per spark a moderate amount. While an engine can run well on less than a 10 kV spark from a magneto, impulse coupling increases this voltage by 4.8 kV, but there’s still only one spark at the retarded position. Shower of spark systems create a much longer spark sequence. Champion SlickSTART™ The SlickSTART™ was developed in 1997 by Unison Industries and is now owned by Champion Aerospace. A variety of factors can cause an engine to be difficult to start. Improper priming (either too much or too little fuel), frosted spark plugs, fouled spark plugs and sometimes merely the nature of the engine can make starting a chore. Now that you understand how the shower of sparks works, think of the SlickSTART™ device as a more modern, computerised approach to the SOS concept. Total Training Support Ltd © Copyright 2020

The FAA-PMA approved SlickSTART™ magneto booster system integrates solid-state electronics with conventional ignition hardware to deliver optimum spark energy for improved engine starting under all operating conditions. It delivers up to 340% more spark energy during start than conventional impulse coupled or retard breaker systems. This added energy enables the magnetos to fire partially fouled spark plugs, ignite less than optimum fuel mixtures, improve hot engine restarts, and improve starting performance during extremely cold weather operations. By creating an extended, powerful voltage for low-speed starting, SlickSTART™ increases the length of the starter sequence while also increasing the electrical energy that gets to the spark plug by up to 340 times that of an impulse-coupled spark. Moreover, there is inherent drag in any engine; the larger and more powerful often being the most difficult to start. This drag presents inertia that needs to be overcome to get things turning. Cold temps, fuel problems and cold oil are all common problems. But if there are fuel and air, then all that is needed is heat, or in this case a spark, to create fire. Getting enough of a highly charged spark to the spark plug is vital. The SlickSTART™ is a single module that attaches to the aircraft firewall, measuring roughly130 mm (5") tall and 90 mm (3.5") wide. The device generates a shower of sparks electronically by using a capacitive discharge to store energy and shoot it to the magneto when the starter is engaged. Champion calls it a firestorm of electricity and indeed it is, as the intensity of the spark is measured in kilojoules.

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SlickSTART™ booster

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Low-tension ignition systems Low-tension is a type of magneto ignition system designed for aircraft piston engines that operate at high altitude. High-tension ignition systems have undergone many refinements and improvements in design. This includes new electronic systems that control more than just providing ignition to the cylinders. High-tension voltage presents certain problems with carrying the high-voltage from the magneto internally and externally to the spark plugs. In the early years, it was difficult to provide insulators that could contain high voltage, especially at high altitudes, when the air pressures were reduced. Another requirement of high-tension systems was that all weather and radio-equipped aircraft have ignition wires enclosed in shielding to prevent radio noise due to highvoltages. Many aircraft were turbo supercharged and operated at increased high altitudes. The low pressure at these altitudes would allow the high-voltage to leak out even more. Lowtension ignition systems were developed to address these problems. Electronically, the low-tension system is different from the hightension system. In the low-tension system, low-voltage is generated in the magneto and flows to the primary winding of a transformer coil located near the spark plug. There, the voltage is increased to high by transformer action and conducted to the spark plug by very short high-tension leads.

The low-tension system virtually eliminates flashover in both the distributor and the harness because the air gaps within the distributor have been eliminated by the use of a brush-type distributor, and high-voltage is present only in short leads between the transformer and spark plug. Although a certain amount of electrical leakage is characteristic of all ignition systems, it is more pronounced on radio-shielded installations because the metal conduit is at ground potential and close to the ignition wires throughout their entire length. In low-tension systems, however, this leakage is reduced considerably because the current throughout most of the system is transmitted at a low-voltage potential. Although the leads between the transformer coils and the spark plugs of a low-tension ignition system are short, they are hightension, high-voltage conductors and are subject to the same failures that occur in high-tension systems. Low-tension ignition systems have limited use in modern aircraft because of the excellent materials and shielding available to construct high-tension ignition leads and the added cost of a coil for each spark plug with the low-tension system. Low-tension ignition systems are not popular today because most aircraft that fly at high altitudes are turbine powered.

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Ignition timing General Ignition timing is used to determine the correct position of the piston for ignition of the fuel/air mixture. Ignition of the fuel/air gases takes place just before top dead centre (TDC). As the flame spreads through the combustion chamber, the intense heat raises the pressure within the cylinder to a peak value which is reached about 10° past TDC. This pressure forces the piston down. This is known as the power stroke of the fourstroke cycle. It is, therefore, most important that the spark plug delivers the spark at the right moment of the cycle. If peak pressure is reached before the point of ignition, very little torque is produced, and heavy loads are placed upon the crankshaft bearings, because of the acute angle of the crank web. If peak pressure is reached after this point, not only has gas pressure been lost, because of the increase in volume above the piston, but the actual working stroke (power) has been reduced. Most engines have timing reference marks incorporated into the engine crankcase to help ensure that ignition takes place at the correct time. These reference marks can be in the form of a notched arrow, or plain line. The rotating crankshaft, or propeller flange, has a corresponding mark, indicating TDC and also a mark indicating the correct position of the piston (normally No. 1 piston) for ignition to occur. When the two reference marks are aligned, this is known as ignition timing and can be expressed as x degrees before TDC. The actual position before TDC is determined by the manufacturer, after many rigorous tests to ensure that the Total Training Support Ltd © Copyright 2020

ignition timing produces the maximum output from the engine. Examples of ignition timing marks are illustrated below. There are various methods of checking ignition timing. Sophisticated equipment is available, in the form of electronic timing indicators, mechanical indicators, etc. A simple method in use is the light timing system which shows the exact instant when the contact breakers open. A lamp and battery may be used to give a visual indication. The primary side of the contact breaker points is disconnected or otherwise insulated, and the two leads from the lamp and batter are then connected to either side of the breaker points so that the contact breaker forms a switch in the lamp and battery circuit. With the contacts closed, the lamp lights and, at the moment of opening, the light goes out. This type of test gives not only a visual indication but an audible indication as well (the contacts click as they open). The magneto and distributor can be finely adjusted to ensure that the contact breakers are just opening at the correct time when the reference marks on the engine and crankshaft align. When considering ignition timing, other factors must be considered, they are: • • •

engine speed; manifold pressure; and mixture strength.

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Lycoming timing marks

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Engine speed The faster an engine runs, the greater is the arc of crankpin travel during the time taken for the charge to burn. Therefore, with an increase in engine speed, the timing needs to be progressively advanced. With a reduction in engine speed, the timing should be retarded.

The automatic timing adjuster operates the ignition timing changes as a function of engine RPM. The driven member is keyed to the magneto driving shaft, and the driving flange is secured to studs in the driving member. Two driving dogs formed on the gear face of the driving flange transmit the drive from the engine.

Manifold pressure The higher the pressure of a gas, the faster it burns. Thus, the cylinder charge of an engine running at high boost burns quicker than a charge at low manifold pressure. To stop this peak pressure position moving as manifold pressure is increased, the ignition timing should be progressively retarded.

The automatic advance of ignition timing is obtained through two weighted arms inserted between the driving and driven members. The arms are pivoted on the driving member and, when the engine speed is increased, they move outwards under centrifugal force. This movement is governed by a roller attached to each arm, each roller following the profile of a cam riveted to the driven member.

Mixture strength A correct mixture burns faster than either a weak or a rich mixture. Any variation from the correct mixture strength requires an advancement of ignition timing. There is, however, a tendency for these factors to cancel out (e.g. high engine speed generally means high manifold pressure and rich mixture), but the cancellation is not exact. On the low powered engine, the gain from making slight adjustments to the ignition timing during engine running is usually too small to be considered. On the larger engines, however, where the gain can be approachable, ignition timing may be varied to suit all these conditions, and this variation can be achieved by an automatic timing adjuster. Automatic timing control An example of an automatic timing adjuster is illustrated in the diagram below.

Thus, outward movement of the arms causes the driven member to rotate relative to the driving member, advancing the timing. Reduction in speed lowers the centrifugal force, and outward movement of the arms is opposed by compression springs. The roller moves down the profile of the cam retarding the ignition. This is a simple but effective method of controlling the ignition timing; it automatically advances and retards the ignition timing. Magneto timing (Bendix) the magneto must be carefully timed to the engine while fitting to ensure a spark occurs at the spark plug at the exact number of degrees before T.D.C. stipulated by the manufacturer. Before fitting, the magneto must be internally timed ensure it gives maximum efficiency so giving the most energetic spark possible.

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Dual magneto

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The ‘E’ (efficiency) gap This is the angular distance between neutral and the points opening. The flux change in a rotating magneto does not occur at the neutral point, as it would if rotated slowly by hand. This is because the poles of the electromagnet, created by the current flow in the primary windings, oppose the motion that causes the initial current flow, (Lenz’s Law). Because the poles are of opposite polarity, they attract and draw the flux lines of the rotating magnet the long route through the coil, creating significant stress. This stress is relieved at some 10° after the neutral point by the opening of the contact breaker. This causes a tremendous flux change from collapse to the full register in the opposite direction. This, in turn, induces a very high voltage into the secondary windings of the coil which discharge across the spark plug electrodes. The contact breaker points are a mechanical earthing switch operated by a cam on the engine-driven magneto shaft.

Setting the ‘E’ gap A scale, pointer and Magneto Synchroniser are required for this operation. The cover must be removed from the contact breaker points and the plastic plug removed from the timing window to expose the driving gear. •

•

Rotate the magneto in the normal direction of rotation until the red chamfered tooth is in the centre of the timing hole. Turn magneto backwards slightly until the neutral (magnetic lock) point is felt, see below. The scale should be attached to the screw holes of the contact breaker cover and the pointer attached to the cam screw. Set the pointer to zero on the scale. Connect the magneto synchroniser to the magneto, positive lead (red) to points terminal and negative lead (black) to the magneto body. Switch on the synchroniser and carefully rotate the magneto in the right direction until synchroniser light comes on.

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•

• • •

•

Read off the number of degrees indicated by the pointer; this should be within limits laid down in the service manual, see the diagram below-top. If the angle is incorrect, loosen the screws holding the points and with the pointer held at the prescribed ‘E’ gap setting, move the points until the light just comes up. Tighten screws and recheck. Turn the magneto until the points are fully open and check the gap with feeler gauges, 0.3 mm to 0.6 mm (0.012" to 0.024") typical. If not within limits, change the adjustment enough to bring it in. Then recheck ‘E’ gap to make sure this is still within its tolerance, see the diagram below-bottom. On retard breaker magnetos, turn the magneto back to the point of main breaker opening. Reposition pointer at zero degrees. Turn magneto in the normal direction until the pointer is over the required number of degrees (as marked in the centre of breaker compartment). Connect timing light across retard breaker and adjust contacts to open at this point. Turn magneto in the normal direction until the cam follower is on the high point of cam and measure gap. If the gap is not 0.3 mm to 0.6 mm (0.012" to 0.024") readjust breaker and verify that contacts open at the retard angle. A tolerance of +2°/-0° is allowed. Replace breaker assembly if both ‘E’ gap and points gap tolerance cannot be obtained.

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Setting the ‘E’ gap

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Timing marks Most aircraft reciprocating engines have timing reference marks built into them. The actual number of degrees may be found on the engine data plate mounted on the engine case. In most cases, the number is in the region of 25° before the top dead centre, and both magnetos have the same timing with few exceptions such as the TCM C-85. On direct drive engines (no propeller reduction gear), the timing mark is on the edge of the propeller-mounting flange on the crank. The TC (top centre) mark on the flange aligns with the top crankcase split line on most Lycoming engines. On Continental engines, the TC mark on the propeller flange aligns with the lower crankcase split line. These marks, when properly positioned, indicate that the number-one piston in the number-one cylinder is at top dead centre. Other marks on the flange indicate degrees before or after top dead centre. Some engines have the timing marks on the alternator pulley that correspond to marks on the accessory housing. Other engines have the timing marks on the crankshaft or some crankshaft drive gear and can be viewed by removing a plug from the crankcase. Timing magneto to engine Set the engine to TDC Any given piston position is referenced to TDC. This piston position is not to be confused with the hazily defined position called top centre (TC). A piston in top centre has little value from a timing standpoint because the crankshaft position may vary from one to five degrees in this piston position. In other words, the piston is at the top of its travel and is in its ‘no-travel’ zone. This occurs between the time the crankshaft and connecting rod stop pushing the piston upward and continues until the crankshaft has swung the lower end of the connecting rod into a position where the piston is now pulled downward. Total Training Support Ltd © Copyright 2020

Top dead centre, on the other hand, is the point at which the piston is positioned the maximum distance from the centre of the crankshaft journal. It is also in the centre of the ‘no travel zone’. This places the piston in a position at which a straight line can be drawn through the centre of the crankshaft journal, the crankpin, and the piston pin. It is the point from which all other piston and crankshaft positions are referenced. Placing the piston in this position by turning the crankshaft can be accomplished in any one of several ways Remove one spark plug form number one cylinder and place a thumb over the spark plug hole. Rotate the engine in the normal direction of rotation until the compression stroke is reached. Continue turning until the spark advance timing marks are aligned as illustrated in the diagram below. Set the magneto Rotate the magneto drive shaft in the normal direction of rotation until the red chamfered tooth is in the centre of the timing hole. Turn the magneto backwards slightly until the neutral point is felt. Ensure that the gear does not rotate from this position. Fit gasket and install the magneto to the engine. Secure with washers and nuts, finger tight. Connect the synchroniser, positive lead to points terminal, and negative lead to a clean unpainted of the engine. Switch on. Rotate the magneto on its mounting flange to a point where the light comes on, then slowly turn it in the opposite direction until the light goes out. Rotate the magneto back slowly in the normal direction of rotation until the light just comes on. Tighten nuts to the specified torque.

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Synchronisation Turn the engine back to about 30° before TDC to avoid picking up impulse coupling. Couple the synchroniser to both magnetos and switch on. Slowly turn the engine in the normal direction of rotation and check that the timing lights come on exactly together and that the timing marks are in alignment. If the timing is incorrect, reposition magneto/s and check again. Refit cover over contact breaker and plastic plug to timing pole.

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The magneto synchroniser The aircraft magneto timing light model E50 is the industry standard. Designed specifically for the internal timing and synchronisation of aircraft magnetos, the E50 safely absorbs the current from the magneto’s impulse coupling. So, there is no danger of engine firing while adjustments are being made. The E50 timing light takes all the guesswork out of the magneto timing process. It is quick and easy to use; attach the three clips to the magnetos and adjust them until the E50’s two blinking lights are synchronised and the buzzer changes in pitch.

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Time-Rite piston indicator

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Eastern Technology E25 timing indicator 1) Remove all top spark plugs. Install a piston stop into the No 1 cylinder top spark-plug hole. 2) Install timing disc indicator on the aircraft propeller spinner or hub using elastic bands. 3) Turn the propeller slowly in the direction of rotation until piston lightly touches the piston stop. 4) Rotate the disc of the timing indicator until TDC mark is under the point of the weighted pendulum pointer. 5) Slowly turn propeller in the opposite direction until the piston again lightly touches the piston stop. Observe reading on the disc under the pointer and rotate the disc to exactly half of the number of degrees towards the TDC mark. 6) Remove the piston stop from the cylinder and find the compression stroke of the No 1 cylinder by placing a finger over the spark plug hole and rotating the propeller until compression is felt, continue rotation until the pointer is under TDC. You have now found TDC on the compression stroke. 7) To check the magneto timing or to time the magnetos to the engine move the propeller in the opposite direction of rotation past the specified magneto timing setting and then back in the direction of rotation until the desired setting before TDC is under the pointer (this removes the factor of gear backlash). 8) The breaker points should just be starting to open at this setting. Breaker points should be checked with a synchroniser

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Eastern Technology E25 timing indicator

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Aero-Diesel engine glow plugs Diesel engines do not require an ignition system because the engine uses high cylinder compression to cause a spontaneous combustion of the air/fuel mixture. No spark is required as it is in a gasoline engine to ignite the fuel charge. This is one of the significant advantages of the Diesel engine compared to a gasoline engine, in terms of weight saving, maintenance complexity and safety.

The glow plugs are on the longest during cold weather starts versus a warm engine start, which has the shortest glow cycle. Once the engine is running, the glow plugs are extinguished and do not come back on until the next engine start.

A glow plug is located in each cylinder to preheat all surfaces of the cylinder and the initial cold air charge at the start. This promotes the combustion required for starting and for the engine to continue to run. Hence, fuel injection is necessary to control the timing and the duration of the fuel pulse. When the engine is cold-soaked, the engine block can act as a heat sink, pulling enough heat from the compressed air in the cylinder to prevent the engine from starting. Cold engine starting is the only time the glow plugs may be required during an engine run cycle.

There are two types of glow plug in use.

The glow plugs are similar to spark plugs, but instead of having an electrode in the tip, they have a heating element. They look like a long pencil. When turned on, they begin to ‘glow’ similar to the way a toaster works. It is on for only long enough to preheat the cylinder before engine start, and for several seconds after start to ensure proper engine start and run. The FADEC system determines the total time that the glow plugs are energised by analysing the coolant temperature and air temperature inputs. This time ranges from zero to 40 seconds for prestart and may remain energised for up to 50 seconds after start. Total Training Support Ltd © Copyright 2020

Bosch Glow Plugs https://youtu.be/9hIbT8-rS7E

• •

Metal-type Ceramic-type

Metal-type glow plugs have their heating coils mounted in a heat resistant alloy tube. Ceramic-type glow plugs, which have been employed in Diesel engines since 1985, have their heater elements contained in a ceramic material, which is silicon nitride. Ceramic-type glow plugs have greater heat resistance and durability. Actuation of the glow plugs is automatic and is displayed by an annunciator light on the instrument panel. Current draw per glow plug is approximately 15 Amp at 12V (or 7.5 Amp at 24V). Some glow plug types receive a pulse width modulated control signal from a Glow Plug Control unit (GPC), as determined by the FADEC.

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Glow plug construction Location of glow-plug in the cylinder head

Glow plug and glow plug control unit (used on the Thielert TAE 125) 5-101

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Starting systems The starter’s purpose is to provide the aircraft engine rotational force allowing ignition and sustained engine running speed. The starter motor must develop a high cranking torque during the period when the starting switch is closed. The starter motor is designed to engage its pinion with the engine starter ring gear at the beginning of the start cycle and then disengage when the ignition switch is released.

Hand Starting inertial starter Stearman https://www.youtube.com/watch?v=Z5ZkHhy4m34 How It Works ... Aircraft Starter https://www.youtube.com/watch?v=tjDAjo8CtSU

Types of starter Piston engine starters can be divided into two types: • •

inertia starters; and direct cranking electrical starters.

Manual cranking inertia starter Although the inertia type starter is relatively old in design, it was very effective. It has been mainly superseded by direct electrical starters. However, there are still a few types of aircraft with inertia systems. The diagram below-top shows a typical example of this type. The electrical motor drives the flywheel via a centrifugal clutch. While the flywheel is being turned by the electric motor, the hand crank connection also turns. When hand cranking, the centrifugal clutch is disengaged from the flywheel and allows cranking to continue without affecting the electrical motor. The cranking handle receptacle is normally found on the engine cowling together with the engaging lever pull rod.

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Inertia starter

Inertia starter hand crank

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Electrical inertia starters This type of starter uses the same principle as the manual cranking inertia starter; however, in this case, an electrical motor has replaced the crank handle, although the manual engagement operation is retained. The flywheel is accelerated up to speed by the electrical motor, and engagement to the crank of the engine is similar to the method shown below-top. Combination inertia starters Another type of inertia starter is a combination of both electrical and manual types, and sometimes known as the combination hand and electrical starter. Manual engagement As illustrated in the diagram below-bottom, you can see that this method of engagement is similar to the manual inertia type, in respect of its being operated by a hand cable. By pulling the cable, the engaging lever moves the clutch. The starter ring gear and the drive pinion are stationary, thus allowing smooth meshing of the gears. Once the gears fully mesh, a further movement of the engaging lever energises a switch on the electric motor causing the motor, through the motor gear, the drive pinion and the starter gear ring, to rotate the engine. Once the engine has fired, the overrunning clutch disengages the pinion drive from the electric motor and as the engaging lever is released the drive pinion is disengaged from the starter gear ring by the action of the return spring. Correct adjustment of the engaging lever and cable is essential. The drive pinion must be engaged with the starter gear ring before the electrical motor rotates. An adjusting screw for switch operation is provided on the engaging lever assembly.

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Direct cranking starters Most current small reciprocating engines employ the direct cranking method of engine starting. This system comprises two components: • •

the electric motor; and the gear section.

Both of the two methods so far mentioned are usually found on light aircraft with relatively small reciprocating engines.

The significant difference from the inertia system is that the direct cranking method eliminates the need for a flywheel assembly; this is because the engine is cranked directly by the electrical motor. Although an electric motor is used on all occasions, the method of starter engagement can vary from one starter to another. The three primary methods of engagement are: • • •

Larger type engines require a slightly different type of starter arrangement, although this is still a direct cranking method, the type used for this purpose is the Bendix drive starter. Electric starter circuit A simple engine starter circuit is shown in the diagram below top-right. As you can see, the main components within this system are: • • • •

manual; solenoid or pre-engaged; and Bendix drive.

We will look at each in turn and see how starter engagement is affected. Solenoid or pre-engaged A pre-engaged starter is shown in the diagram below top-left, the method of operation is similar to that for the manual method, but in place of the hand-operated engaging lever, an electric solenoid operating the engaging lever is fitted. Turning the engage start switch to ‘on’ energises the solenoid, which then pulls the engaging lever to the left. Because this lever is pivoted about its centre, it causes the drive pinion to move to the right to engage the starter gear ring. Total Training Support Ltd © Copyright 2020

Once engaged, the plunger in the solenoid bridges the motor contacts and power is directed to the electric motor, which then turns the starter ring via the drive pinion.

a battery switch; a starter switch; a starter relay; and an electrical starter.

If we look at the source of electrical power, we can see that this originated from either an external ground power plug or an aircraft battery. The source of ground power can either be a series of separate batteries or a ground power unit (GPU); the source of ground power is connected to the DC busbar of the aircraft to prevent using the aircraft battery. However, at this point, power is only supplied to the DC busbar and down to the starter relay.

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Inertia starter hand crank Electric starter circuit

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Basic starter description The starter motor must develop a high cranking torque during the period when the starting switch is closed. The starter motor is designed to engage its pinion with the engine starter ring gear at the beginning of the start cycle and then disengage when the ignition switch is released. This is accomplished through the use of the starter drive pack which contains a control circuit, solenoid coil, torque limiting clutch assembly, and a return spring. The gear-reduction starter motor consists of six major components: • • • • • •

the starter drive pack, the gear reduction assembly, the drive housing, the armature, the magnet housing assembly, and the commutator assembly.

The drive pack consists of the pinion gear, torque limiting clutch, solenoid coil, and return spring. When the starting circuit is energised, the battery current is applied to the starter terminal. Current flows through the brushes to the commutator through the armature windings to ground. Permanent magnets are used to create a strong magnetic field. The magnetic force created begins to turn the armature. Current is then supplied to the drive pack (engage and hold coil).

As the armature turns, the drive pack pinion extends and meshes with the engine starter ring gear via the electromagnetic coil. The ball clutch torque limiter holds the pinion for start loads but allows slippage should an engine ‘kickback’ occur, preventing damage to the ring gear or starter When the engine achieves a start and the starter switch is released, the pinion gear de-meshes from the starter ring gear via a return spring in the drive pack. The drive pack assembly consists of the torque-limiting clutch assembly, solenoid assembly and the pinion gear. The drive assembly mounts within the drive housing. The driveshaft passes through the drive assembly and engages via helical splines. The gear reduction assembly consists of the drive shaft, planetary gears, metal gear track, and retaining cover. It provides a reduction from the high-speed low-torque motor to the low-speed high-torque drive shaft. The drive shaft transmits torque to the drive assembly via helical splines. The drive end housing encloses the drive mechanism and contains the needle bearing in which the drive shaft rotates. (The drive end housing assembly includes the intermediate housing.) The armature consists of a laminated soft-iron core assembled on the armature shaft, a commutator and the windings. These are wound in slots in the core and connected to the commutator. The commutator is made up of several copper segments insulated from each other and the armature shaft. The armature shaft extends into the reduction-gear assembly and is supported on each end by bushings.

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Starter motor

Starter motor components

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The magnet housing assembly includes the frame which physically supports the other components of the motor and the permanent magnets. The permanent magnets supply the magnetic field required for producing rotary motion The commutator assembly contains one of the armature shaft bushings and the brush holder assembly, which contains the brushes. Each brush is a one-piece composite matrix with integral shunts. They are spring-loaded and ride on the armature commutator as it rotates Starter duty cycle requirements The starter cranks the engine for starting while the battery supplies the power. The engine does not always start on the first attempt, resulting in significant heat generated in starter components. This requires a limitation to the number of start cycles which can be made before a cooling-down period must be applied, referred to as the duty cycle. 1) The engine must be grounded to the aircraft. 2) Engage starter for 10 seconds of power (start), 20 seconds cool down (rest) for up to 20 starts then 10 minutes cool down before next start attempt. 3) Do not apply power to starter unless installed on the engine. Do not free run as this may cause internal damage to the starter

Starter Models Skytech E-DRIVE series Weight: 4.20 kg (9.25 lbs) They are fitted with a lightweight, 4-pole permanent magnet drive motor. Gear reduction at 3.6 to 1. They have a maximum duty cycle – 10% continuous 30 seconds on 20 seconds rest (continuous) 10 sec on 20 seconds rest for 20 starts (intermittent). Operating temperature range -44°C to +68°C (-45°f to +155°f) Operating Speed (max motor output under load) 3,100 RPM. Max drive amperage (70 amps under load) 80 – 125 amps @ 150 RPM while starting (12.0 V- 24.0 V). Operating input (minimum & maximum under load) (12.0 V) – 9.0 V to 14.5 V, (24.0 V).18.0 V to 29.0 V.

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Bendix drive With this type of starter, the method of engagement is automatic, utilising a drive known as a Bendix drive. The armature shaft turns when electrical power is applied to the starter motor. Because of its inertia, the pinion is thrown along the helical splined drive shaft until it engages with the starter ring and turns the engine see the diagram below. On start-up, the engine rotates faster than the starter pinion, so the ring gear moves the pinion back along the shaft (along the helical splines) and disengages the drive pinion from the starter gear ring. The anti-drift spring ensures that the drive pinion is kept away from the rotating starter gear ring when the engine is running and the starter de-energised. The drive spring transmits torque from the starter motor to the drive during the starting sequence.

Whenever any of the above defects are experienced, the starting motor should be removed immediately, and the Bendix Drive cleaned and lubricated. Oil should never be used, because the oil in that location collects dust and dirt, becoming gummy and causing the Bendix drive to stick. In most cases, it is necessary to partially disassemble the starting motor to service the Bendix drive. The latter should be removed from the motor shaft to ensure thorough cleaning and lubricating. Clean the area in front of the Bendix drive pinion before removing the drive from the shaft. Do not use carburettor cleaner or any solvents that could damage the rubber block inside the Bendix drive. Use only clean petroleum-based cleaners such as kerosene or Varsol.

Many starters manufactured by Prestolite and Electro Systems are in use with Lycoming engines. The location of the starting motor on many aircraft engines subjects the Bendix drive of the starter to contamination from dust, dirt and moisture because the drive housing is open, and the engine continuously circulates air around the starting motor.

Thoroughly clean the Bendix drive to remove all dirt and contamination from the screw shaft threads and control nut. If the drive is exceptionally dirty, the drive pinion cup can be removed to ensure a thorough cleaning job. Do not attempt to remove the control nut. Finally lubricate with silicone spray (Crown Industrial 8034).

Typical indications of a dirty Bendix drive are:

The starter will not rotate until the starter relay is closed on the receipt of an electrical signal from the starter switch.

• • •

sluggish operation – operator has to make several attempts before the starting motor cranks the engine; noisy operation – a grinding noise when the starter is energised; and failure to engage.

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Bendix drive operation Electric starter motor with Bendix drive

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Starter relay The main electrical cables that lead from the batter to the starter are heavy-duty and can carry a current flow in some cases over 300 amps, depending on the starting torque of the engine. It would be impractical to take these heavy cables into the co*ckpit to the starter switch, as this would add weight to the system and create a significant circuit voltage drop.

Bonding/earthing straps Up to this point, we have discussed the simple circuit in terms of starter rotation. However, we all know that in most cases the airframe is used as a negative return in a typical electrical system. For the starter to operate effectively, it must be bonded to the aircraft structure. This is achieved by the use of an earth or bonding strap. We can see this illustrated below-right.

By using a starter relay, only lightly loaded cables need to be routed into the starter switch. The cables still receive a DC supply but operate a remote relay that closes the contacts on the heavy-duty cables, thus allowing power directly from the DC bus to the starter.

In practice, bonding leads are kept to a minimum; this is generally achieved by connecting the leads from the body of the starter to an adjacent point on the airframe. The bolts that attach the starter to the engine are not considered to be an adequate bond.

Illustrated below left is a typical example of a starter relay. Here we can see a low current control circuit energising a solenoid coil that pulls down a moveable contractor, thus closing the contacts on the high-power circuit allowing power through to the starter. Once the starting cycle has been completed, the low current circuit is switched off, and the now de-energised coil, assisted by the spring, allows the contractors to open, thus preventing current flowing into the starter.

Selection of the correct material for earth straps is essential; the use of an innocent material could lead to localised corrosion due to electrolytic action at the earth joint. The most common type of earth strap in use is made of aluminium alloy, although copper can be used to earth arts made of stainless steel, copper, brass or bronze.

However, experience has shown that relay contactors have been known to jam in the closed position. This has led to minor fires, overheating of starter cables, and burning out of starter motors. To overcome this problem, Aviation Authorities recommend some other means of disabling the starter circuit in the event of a relay being jammed in the closed position. This usually takes the form of: • •

either a manually operated starter isolation switch in series with the starter relay contacts; or providing two starter relays in series.

To ensure effective low resistance connections are made, nonconducting surfaces, paint and anodising films should be removed before the connection of the earthing strap. High resistances at earthing points generates poor starting qualities in the starter system, resulting in the overheating of cables and starter motors. It must be noted, however, that surfaces that have been reduced to bare metal must have some form of protective coating applied after the earth joint has been made. This is typically achieved by the use of a blue colour paint that covers the affected area but also acts as an earthing point identifier.

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Starter relay

Bonding/earthing strap 5-115 Module 16.5 Starting and Ignition Systems

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Troubleshooting and maintenance This information must be regarded as general in presentation and does not reflect any particular type of engine. Reference should always be made to the aircraft maintenance manual for the inspection and maintenance of engine starter systems. Failure of the starter system to operate correctly can be attributed to any of the following: • • • • •

electric power source, i.e. battery condition; starter control switch; starter relay; electrical wiring circuit; or starter motor – mechanical or electrical failure or poor earthing strap.

We consider each in turn and see how the system can be tested, and remedial action carried out. The battery must be fully charged to ensure proper rotation of the starter motor when using the starter relay is indicative of a low voltage battery. A slow turning starter also indicates a low charged battery. The apparent remedy for this situation is to replace the battery, if the aircraft has the facility, to connect an external electrical supply. Where the starter control switch, starter solenoid and electrical circuit are concerned, any investigation into the integrity of these components and wiring should be carried out by a qualified engineer. If all the components and the circuits prove to be satisfactory, then the fault is likely in the starter motor itself.

Dirty commutators can be cleaned carefully with a very fine grade of sandpaper or stone, taking great care not to damage the insulation on the commutator. The best action, however, is to send the motor to an overhaul agency for a complete overhaul to be carried out. Brush maintenance Sparking of brushes reduces the effect of brush area in contact with the commutator bars quickly. The degree of such sparking should be determined. Excessive wear warrants a detailed inspection. The following information pertains to brush seating, brush pressure, high-mica condition, and brush wear. Manufacturers usually recommend the following procedures to seat brushes which do not make good contact with commutators. The brush should be lifted sufficiently to permit the insertion of a strip of No. 000, or finer, sandpaper under the brush, rough side out as in the diagram below. Pull sandpaper in the direction of armature rotation, being careful to keep the ends of the sandpaper as close to the slip ring or commutator surface as possible to a sold rounding the edges of the brush. When pulling the sandpaper back to the starting point, the brush should be raised, so it does not ride on the sandpaper. The brush should be sanded only in the direction of rotation.

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Commutator and brushes (removed) Seating brushes with sandpaper

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After the generator has run for a short period, brushes should be inspected to make sure that pieces of sand have not become embedded in the brush and are collecting copper. Under no circ*mstances should emery cloth or similar abrasives be used for seating brushes (or smoothing commutators), since they contain conductive materials which cause arcing between brushes and commutator bars. Excessive pressure causes rapid wear of brushes. Too little pressure, however, allows ‘bouncing’ of the brushes, resulting in burned and pitted surfaces. A carbon-graphite or light metallised brush should exert a pressure of 1½ to 2½ psi on the commutator. The pressure recommended by the manufacturer should be checked with the use of a spring scale graduated in ounces. Brush spring tension is usually adjusted between 32 to 36 ounces; however, the tension may differ slightly for each specific motor. When a spring scale is used, the measurement of the pressure which a brush exerts on the commutator is read directly on the scale. The scale is applied at the point of contact between the spring arm and the top of the brush, with the brush installed in the guide. The scale is drawn up until the arm just lifts off the brush surface. At this instant, the force on the scale should be read. Flexible low-resistance pigtails are provided on most heavy current-carrying brushes, and their connections should be securely made and checked at frequent intervals

The purpose of the pigtail is to conduct the current, rather than subjecting the brush spring to currents which would alter its spring action by overheating. The pigtails also eliminate any possible sparking to the brush guides caused by the movement of the brushes within the holder, thus minimising side wear of the brush. Carbon dust resulting from brush sanding should be thoroughly cleaned from all parts of the generators after a sanding operation. Such carbon dust has been the cause of several serious fires as well as costly damage to the generator. Operation over extended periods often results in the mica insulation between commutator bars protruding almost to the surface of the bars. This condition is called ‘high mica’ and interferes with the contact of the brushes to the commutator. Whenever this condition exists, or if the armature has been turned on a lathe, carefully undercut the mica insulation to a depth equal to the width of the mica, or approximately 0.5 mm (0.020"). Each brush should be a specified length to work properly. If a brush is too short, the contact it makes with the commutator will be faulty, which can also reduce the spring force holding the brush in place. Most manufacturers specify the amount of wear permissible from a new brush length. When a brush has worn to the minimum length permissible, it must be replaced.

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Brush wear indicator Section of a commutator

Brushes and pigtails

Commutator and brushes (removed)

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Pre-heat systems General Preheating is required whenever the engine has been exposed to temperatures at or below -7°C (20°F) (wind chill factor) for two hours or more. In extremely low temperatures, oil congeals, battery capacity is lowered, and the starter can be overworked. Improper cold weather starting can result in abnormal engine wear, reduced performance, shortened time between overhauls, or failure for the engine to operate correctly. Failure to properly preheat a cold-soaked engine may result in oil congealing within the engine, oil hoses, and oil cooler with subsequent loss of oil flow, possible internal damage to the engine, and subsequent engine failure.

Preheating is about far more than just oil temperature. Proper preheating involves heating the entire engine so that all critical engine parts can be brought into the ‘safe’ temperature range. In an ideal world, you would heat the entire aircraft to reduce wear in everything from pulleys to gear components, avionics to gyroscopic instruments. Preheating can have a considerable impact on the longevity of an aircraft engine. At a minimum, try to get the entire engine above 5°C (40°F) to provide the best environment for its longterm health. And, if you have an option to get some heat into the cabin to warm the avionics and gyros before they spool up, even better.

Superficial application of preheat to a cold-soaked engine can cause damage to the engine. An inadequate application of preheating may warm the engine enough to permit starting but not de-congeal oil in the sump, lines, cooler, filter throughout. Congealed oil in these areas requires considerable preheat. The engine may start and appear to run satisfactorily but can be damaged from lack of lubrication due to the congealed oil blocking proper oil flow through the engine. The amount of damage varies and may not become evident for many hours. However, the engine may be severely damaged and may fail shortly following application of high power. Proper procedures require a thorough application of preheat to all parts of the engine.

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Electrical heater elements bonded to the oil sump

Electrical forced hot air pre-heat system

Fuel powered forced hot air pre-heat system 5-121

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Installed preheaters The most common preheaters are built-in, electric preheating systems. Basic preheaters consist of a small electric pad that is bonded to the oil sump. However, more extensive preheating systems are available; these include heating elements for other parts of the engine, including the case and cylinders. A variety of component options to heat the different parts of the engine are used, including heated intake tube bolts and valve cover bolts. Some systems use heated bands that wrap around the base of each cylinder. With either system, a well-insulated cowl cover is strongly recommended to ensure that the entire engine compartment is kept warm.

Leaving the preheater on for extended periods is controversial, mainly due to the potential for increased corrosion. Different situations dictate different recommendations. Above all, regular flying of the aircraft, frequent oil changes, and the use of anticorrosion additives has the most significant impact on reducing the exposure to corrosion. There are a variety of remote-controlled switches available now (that work via the internet) that make it easier than ever to get the aircraft heated before the pilot even venture out to the airport.

The main problem with the sump-only preheating systems is that they do not always address the critical clearance issues. The oil may be warm, but if the cylinders are cold, you can still have expansion issues and excessive wear at start-up. Also, heating just the oil sump for long periods can do more damage than good. The problem lies in condensation.

Portable preheaters If no electricity is available at the tie-down location, or if you want to avoid the cost of an installed system, there are portable, forced hot-air preheating systems available. This is the most common form of rapid engine heating used by private operators and flight schools.

Condensation occurs anytime warm, moist air flows over a surface colder than the dewpoint. In the case of electric oil sump heaters, the warm air above the oil can condense on the cold parts of the engine, such as the cylinders and camshaft. Since water is a key ingredient for corrosion, leaving only an oil sump heater plugged in for extended periods can lead to premature cylinder and camshaft wear.

These systems usually require both electricity and propane to create a powerful flow of hot air into the engine compartment. The air is either blown into the bottom of the cowl at the exhaust opening or through the front of the cowl at the air inlets.

However, if a complete engine heating system is used in conjunction with an insulated cover, corrosion concerns can be largely eliminated.

One manufacturer makes a portable preheater that requires only fuel and accepts a wide range (100LL, Jet-A, Kerosene). With either solution, the key is to provide enough time to get the entire engine up to a reasonable temperature before attempting a start.

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Electrical built-in pre-heat system kit – crankcase heater pads Electrical built-in pre-heat system components – cylinder head heater bands

Electrical built-in pre-heat system kit – heater pads

Portable forced-air pre-heat system

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Electrical built-in pre-heat system components – cylinder head heater bands

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Licence Category B1 and B3

16.6 Induction, Exhaust and Cooling Systems

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Construction and operation of induction systems including alternate air systems;

Part-66 Ref. 16.6

Knowledge Levels A B1 B3 1 2 2

Exhaust systems, engine cooling systems – air and liquid.

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Table of Contents Induction systems _____________________________ 6 Intercooler ___________________________________ 6 Intakes _____________________________________ 6 Filters _____________________________________ 12 Alternate air systems__________________________ 14 The induction manifold ________________________ 16 Updraft and downdraft induction systems __________ 18 Induction priming systems______________________ 24 Induction system examination ___________________ 26 Exhaust systems _____________________________ General ____________________________________ Radial engine exhausts systems _________________ Inline engine exhausts systems _________________ Horizontally opposed engine exhausts systems _____

28 28 30 32 34

Cooling systems ______________________________ General ____________________________________ Air-cooling radial engines ______________________ Townend ring cowl ___________________________ The NACA cowl ______________________________ Gill ring ____________________________________ Air-cooling inline and horizontally opposed engines __ Augmenters _________________________________ Liquid-cooled engines _________________________ Example system: Continental Voyager ____________ Advantages of liquid cooling ____________________ Liquid cooling system layout ____________________ Short-circuit _________________________________ Cabin-heat circuit ____________________________ Liquid-coolant system components _______________

40 40 42 48 50 50 54 62 66 70 72 76 82 82 84

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Induction systems Aircraft engines are air-breathing, and there must be sufficient air-flow into the engine to provide the oxygen needed to mix with the hydrocarbon fuel so it can release its energy. The air must enter the engine clean and cool so that its’ density is high, yet not cold enough for ice to form in the induction system. The induction system consists of the air intake up to the carburettor/injector and the induction manifold, and from there to the cylinders. The purpose of an induction system is to direct the correct air/fuel mixture into the cylinders, via the atmosphere and carburettor, either by positive pressure from the engine-driven supercharger/turbo-supercharger or by the depression caused by the descending piston during the induction stroke. The induction system generally is similar in both types except for the introduction of the actual supercharger. The main problem on a non-supercharged engine is to obtain an equal charge in each cylinder. The design of the manifold becomes critical, as does the design of the inlet valves. The relative cross-sectional area of the inlet valves and the manifold passages affect the mixture flow. So, the whole efficiency of the engine depends on them, as all the cylinders are the same swept volume and require the same charge. Another problem is the efficiency of the gasoline globules in suspension in the mixture stream; these tend to stick to the walls of the passages. This problem is reduced by imparting swirl or turbulence to the mixture, or by heating the passages as some engines do by passing the induction pipe through the hot engine oil sump.

The reverse of these conditions apply with supercharged engines since a positive pressure exists at all times in the induction manifold, and the increased pressure of air imparts sufficient heat to vaporise the mixture. In many instances, the compressed mixture may require cooling, and an intercooler does this. Intercooler The supercharger compresses the air charge, which results in it becoming hot. To increase engine performance by increasing the air density, it is passed through an intercooler which may be part of the engine cooling system or may be independent of it, with an engine-driven circulating pump, a header tank and a coolant radiator of its own. Intakes Having briefly described the problems of design encountered in normally aspirated and supercharged engines, the start point of any induction system is the air intake assembly. The air intake scoop itself provides air for purposes other than an air feed to the carburettor/injector.

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Atmospheric conditions at altitude give lower air temperatures than at ground level, and the effects of ice on the air intake and carburettor are serious, both as regards restriction of the air intake (thus affecting the whole induction system) and the mechanical jamming of the throttle valves. To prevent this, ice guards, and provision for drawing pre-heated air from the engine nacelle, are used in the air intake. The pilot can control the alternative hot or cold conditions at the intake using shutters, either manually or by electro/pneumatic rams, some of which are operated automatically. Also fitted in the air intake is an air filter. The air filter removes all dust and grit from the air before passing through the induction system. In some cases, the air duct on one side of the filter supplies cold air to the engine accessories such as the alternator or the oil cooler, and the other duct goes to the heat exchanger for the cabin air conditioning system. Two main types of air intake are used on piston engines, namely ram air-type and NACA-type.

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The ram intake The ram air-type consists of a scoop projecting into the air-flow, which has the advantage of raising the air density to the carburettor with increasing airspeed of the aircraft. This recovers some of the power loss due to increasing altitude. Unfortunately, this type of air intake is prone to icing and also generates an appreciable amount of drag. The icing problem can be overcome easily enough, but the drag problem tends to waste a large proportion of any power gained by the ram air effect. For these reasons, the ram air intake is not used on fast aircraft, and the NACA type of intake was developed to supersede it.

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Ram air intake used on light aircraft

Ram air cowling intake on light aircraft

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The NACA intake The NACA air intake is mounted flush with the aircraft’s skin and therefore imposes no drag penalty on the aircraft. It consists of an ogee-shaped opening which develops into a divergent duct which slows down the air-flow and at the same time increases the air density. In some instances, an electrical anti-icing heater element is built into the leading edge. Although the NACA intake was developed for high-speed aircraft with their shock-wave problems, it is becoming increasingly popular on medium- and low-speed aircraft solely for its zero-drag property. Prior submerged inlet experiments showed weak pressure recovery due to the slow-moving boundary layer entering the inlet. This design is believed to work because the combination of the gentle ramp angle and the curvature profile of the walls creates counter-rotating vortices which deflect the boundary layer away from the inlet and draws in the fastermoving air while avoiding the form drag and flow separation that can occur with protruding scoop designs

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NACA intake on a Cirrus SR22

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Filters The propeller stirs up a considerable amount of dust and grit on the ground, and the pumping action of the pistons causes the carburettor or fuel injection system to draw in large amounts of this contaminated air. Sand and dust storms can fill the air with sharp-edged particles up to an altitude of 10,000 to 15,000 ft. Effective filters must be used to trap all of it before it can enter the engine, to prevent the abrasive action of dust from causing excessive wear. Dust and grit could also collect in the carburettor and upset the air/fuel mixture by clogging air and fuel passages. Despite the resultant loss of the ram effect, the majority of air intakes now incorporate an air filter at the air intake entrance to remove particles of dust and dirt. These, if allowed into the engine, can restrict pressure and flow sampling orifices so upsetting the fuel/air ratio of the fuel metering devices. Air filters vary in their composition: • • • •

dry paper elements similar to some cars; paper elements impregnated with phenolic resin; polyurethane foam impregnated with glycol; and on older engines, an oiled fibre material glued to a wire mesh frame.

All filters should be cleaned or replaced at prescribed intervals. Whether one is cleanable or must be replaced depends upon the engine and the environment in which it is working, so adhered to the servicing manual. Some filters can be washed in detergent and dried, others washed in fuel and then oiled. Some can only be tapped to remove loose dirt while others can be cleaned by blowing compressed air in a direction against normal flow. Refer to your maintenance manual for the correct cleaning procedure for your filter. Total Training Support Ltd © Copyright 2020

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Air filters

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Alternate air systems Some installations achieve the best of both worlds in that the intake is modified to give two entrances to the intake duct, a straight-through entrance to take advantage of ram effect and above or below it another entrance containing a filter. If the intake or filter becomes obstructed by ice or other causes, the pilot must manually select alternate air (“ALT AIR”) to “ON” (lever down) to bypass the filter and allow induction air to be taken from inside the engine cowling. Some aircraft have an alternate air source which is provided automatically. In most cases, a simple suction operated, spring-loaded alternative air flap is incorporated in the air-filter box structure, and this automatically opens in the event of the main air intake orifice becoming blocked. As the filter becomes restricted the suction pressure in the intake increases until the sprung loaded door opens permitting unfiltered air to enter the engine. In extreme cases, an alternate air intake is built into the top cowling so that a flap valve may select ram or filtered air, the alternate intake having a dust trap filter fitted. With the alternate intake selected, air heavily laden with dirt has to turn through 90° to enter the intake and the dust and dirt particles, being heavier, tend to go straight on past, the filter then removes the remainder of the particles from the air stream.

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The induction manifold The induction manifold takes the air or fuel/air mixture to the actual cylinders; air in the case of an injection system, fuel/air mixture in the case of a carburettor. With carburettors, the fuel/air distribution has always been a problem mainly due to the varying lengths of the induction tubes. This is the reason why the mixture must be richer than the ideal so that each cylinder is assured of at least a rich enough mixture to prevent detonation. The induction manifold must have no leaks as this adversely affects the fuel/air ratio.

It is crucial to identify and rectify induction leaks as they have an adverse effect on the mixture and cylinder head temperature. As cylinder head temperature is only provided from a master cylinder, the pilot does not have any indication that there is a change in the CHT of an individual cylinder.

Normally-aspirated engines have a lower than atmospheric air pressure in the induction tube. Air is sucked inwards resulting in a lean mixture; this potentially results in increased CHT possibly resulting in valves sticking, mag drops and rough operation. Turbocharged engines run rich due to the induction system being above atmospheric pressure, resulting in a loss of pressure in the induction system. However, the fuel metering system (injection) still delivers the same amount of fuel. The likely effect is lead fouling, rough running and pre-ignition. Some induction manifolds are routed through the oil sump, where the heat from the sump oil assists in vaporisation of the mixture and helps to cool the oil. On some continental 6 cylinder horizontally opposed engines the manifold is divided to supply three cylinders from each branch, and a balance pipe is fitted across the front of the engine to ensure equal pressure in each branch. Yet another factor is that on some engines the induction pipes are turned to an engine, i.e. length and diameter. In this case, extra care must be taken if an induction pipe replacement becomes necessary. Total Training Support Ltd © Copyright 2020

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Induction system Lycoming IO-540

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Updraft and downdraft induction systems Updraft system The updraft induction system utilises an intake manifold with an air distribution system mounted below the engine cylinders. It consists of two runners and a balance crossover tube. It carries induction air to the individual cylinder intake ports. The cylinder intake ports are cast into the cylinder head assembly. Air from the manifold is carried into the intake ports, mixed with fuel from the injector nozzles where it enters the cylinder as a combustible mixture when the intake valve opens. Engine components through which intake air flows following the aircraft air inlet filter/alternate air door are the throttle assembly and manifold, through the induction tubes and into the cylinder intake ports. Air flows through these components in the order listed. The intake manifold is an air distribution system mounted below the engine cylinders. It consists of two runners and a balance tube. It carries induction air to the individual cylinder intake ports. The balance crossover tube is designed to reduce pressure imbalances between the left and right-side induction runners. These potential imbalances can occur due to the air waves accelerating and reflecting in the runners as the individual cylinders run through their respective intake sequences.

Downdraft system The balanced induction system shown in the above figure provides optimum air-flow to each of the individual cylinders across a broad operation rpm range. With balanced air-flow and precisely metered fuel injected into the cylinders, a much smoother and efficient running engine can be achieved. This is due primarily to better-matched fuel to air ratios in all of the cylinders. The cylinder intake ports are cast into the cylinder head assembly. Air from the manifold is carried into the intake ports, mixed with fuel from the injector nozzles where it enters the cylinder as a combustible mixture when the intake valve opens. The intake ports of the crossflow cylinder head design are located on top of the cylinder head while the exhaust ports are located below. This cylinder design is used in conjunction with a balanced induction system mounted above the engine. This design permits the top-mounted downdraft induction of the type shown in the figure above. The separate induction risers for each individual cylinder permits not only a balance of air-flow to optimise the breathing in each of the cylinders, but also serves to isolate any shock waves of air that would tend to migrate between cylinders in the earlier runner induction design.

The cylinder intake ports are cast into the cylinder head assembly. Air from the manifold is carried into the intake ports, mixed with fuel from the injector nozzles where it enters the cylinder as a combustible mixture when the intake valve opens. Total Training Support Ltd © Copyright 2020

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Bottom view of engine with induction manifold installed

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Induction priming systems Some engines are furnished with a fuel priming system, to reduce the time for initial engine starting, where fuel is introduced directly into the induction manifold or induction port on the cylinder head. This obviates the need to vaporise the charge through the carburettor. As a sufficient quantity of air is always contained in the induction system for the initial cylinder charge, a spray of fuel from an external source is mixed with this air. A magneto or booster coil is then operated, and the engine is rotated sufficiently by the starter motor to compress a charge in whichever cylinder or cylinders are on the compression stroke. The high-tension current is fed through the regular magneto distributor to the correct cylinder, and the engine starts. Priming is continued until the carburettor can supply the correct mixture. The amount of priming required under varying temperature conditions is given in the appropriate engine publication.

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Operation of engine primer in co*ckpit

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Induction system examination 1) Check the air filter for cleanliness, normal operation and the absence of gaps or leaks in the filtering element. Check the air filter seal for potential bypass circuits from the filter. Correct or replace, as necessary. 2) Verify the integrity of the airbox by examining for alternate air circuits that can bypass the filtering system. Any holes or bypass circuits found behind the filtering element should be repaired as required. 3) Verify the operation of the alternate air door and the integrity of the seal when in the closed position. Verify the door operating mechanism for security when in the closed location. Replace or repair as necessary. 4) If the operator conducts regular oil analyses, use the silicon content of the most recent analysis and the overall silicon trend to assess the possibility of induction system leaks, or pilot operational issues, such as extensive use of carburettor heat or alternate air during ground operation. 5) Identify induction system inspection requirements for the specific aircraft in service and comply with all requirements for inspection and maintenance of the induction system.

Improper or inadequate maintenance of the air induction components of the aircraft engine installation can and often does result in the engine breathing unfiltered air. Unfiltered air contains particulates which are abrasive to the engine; especially to the cylinder walls and ring faces. Induction system maintenance that emphasises properly sealed filters, alternate air doors, and air ducts can prevent much of that damage. Induction system deficiencies can often be detected through oil analysis which identifies the contamination.

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Induction system bolted to cylinder heads

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Exhaust systems General An exhaust system is briefly a means of leading the high temperature, noxious engine exhaust gases safely away from the aircraft so that they pose no hazard to the airframe or occupants of the aircraft. It can also be a means of obtaining a slight gain in propulsion from 40% of fuel energy customarily lost to exhaust. An exhaust system designed to provide this gain is called an ‘ejector-type exhaust system,’ i.e. the mass of exhaust gases is ejected rearwards at a high velocity thus giving a resultant push reaction forward on the aircraft. Increased performance can also be provided on turbocharged engines; here the exhaust gases must be collected to drive the turbine compressor of the turbo. Although the collector system raises the backpressure of the exhaust system, the gain in horsepower from turbo-supercharging more than offsets the loss in horsepower resulting from increased backpressure. As the exhaust gas temperature varies with the fuel/air mixture, it is standard that the exhausts have a simple thermocouple probe inserted into one or more exhausts. This probe provides co*ckpit indication of the exhaust gas temperature (EGT) to the pilot as an aid to leaning.

The factors influencing the design of an exhaust system are as follows. •

•

The metal or material in contact with the hot exhaust gases must be capable of withstanding temperatures up to 800°C (1,475°F), without losing its strength. Also, it should be non-corrodible. Means must also be provided for cooling the system by allowing air to blow over it. The system must be designed with expansion joints to counteract expansion and contraction, thereby alleviating stresses in the metal. The shape and size of an exhaust system must be arranged so that the gas can get away quickly, thus avoiding excessive backpressure. Sharp bends and rapid changes in pipe section must be avoided. Those parts of the exhaust system projecting through the engine cowling must be shaped to offer a minimum of air resistance (drag).

Exhausts can generally be divided into two categories, those used for inline engines and those used for radial engines.

The main difficulty when designing exhaust systems is to make them withstand the heat from exhaust gases and the expansion and the contraction of the material from which they are constructed.

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Exhaust system on a radial engine

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Radial engine exhausts systems This engine usually requires a main collector pipe of ring formation into which branches deliver the exhaust gases from the cylinders. The collector has one or two main outlets which carry the gases back clear of the aircraft structure. The main collector ring and branches are housed inside the cooling lines so as not to present any external projections which would upset the smooth air-flow. A well-known example of this is the Bristol exhaust system which was designed to form the nose portion of the main engine cowling. Other radial engines have their exhaust system behind the engine. For cooling purposes, it is enclosed in a muff or isolated from the rear of the engine by a trough; the air is fed in from the front of the engine. Each of these two systems has advantages over the other. In essence, one provides a saving in complication and space and helps to keep the accessories cool. The other assists cylinder cooling since the air passing to the cylinders is not pre-heated by passing in the vicinity of the exhaust pipe.

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Radial engine exhaust system and cowling

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Inline engine exhausts systems Engines with inline cylinders of one or more banks usually enable a more straightforward design of the exhaust system to be incorporated. The simplest method is to have short stubpipes from each cylinder, but this can be extremely noisy and can blind the pilot when flying at night due to the flames exiting. A more usual design is one in which short pipes from each cylinder are fed into a standard pipe or manifold with a single outlet; the manifold and branch pipes are enclosed in a duct inside the nacelle. For cooling purposes, air is forced through the duct from a scoop cylinder block in the front and is evacuated in a backwards direction at inner the rear. A small amount of propulsive energy is obtained in this natural way owing to the heat energy absorbed by the air, causing it to work as a jet backwards. This propulsive energy may counterbalance to some extent the drag on the aircraft caused by the cooling air being forced through the duct.

Because of the continual change in temperature of the exhaust pipes, coupled with the corrosive nature of the gases, the inspection of the exhaust system must be done at regular intervals. Special inspections of the systems that contain a heating muff arrangement are essential to ensure that flight crews and passengers do not suffer from the effects of carbon monoxide poisoning caused by leaking heater muffs.

With the inline arrangement, the exhaust pipes are typically bolted directly onto the cylinders and join up to form an exhaust manifold. However, when installed on light aircraft, the hot exhaust gas may form part of the aircraft heating system. Ambient air is ducted into a heater type muff arrangement and allowed to circulate the internal exhaust pipe. The action of the hot exhaust pipe warms this ambient air and allows the heated ambient air through to the aircraft. The exhaust gases are then ducted overboard.

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Hawker Hurricane with stub exhausts

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Horizontally opposed engine exhausts systems The typical collector exhaust system components of a horizontally, opposed engine are shown in a side view. The exhaust system in this installation consists of a down-stack from each cylinder, an exhaust collector tube on each side of the engine, and an exhaust ejector (tailpipe) assembly protruding aft and down from each side of the firewall. The down-stacks are connected to the cylinders with hightemperature locknuts and secured to the exhaust collector tube by ring clamps. A cabin heater exhaust shroud is installed around each collector tube; this muffler can also be used on engines installed with a carburettor to reduce the risk of carburettor ice by making provision for carb heat.

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In-line engine exhaust system with heater muff

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In-line engine exhaust system with heater muff

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Exhaust and induction system on an engine with a turbocharger

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Turbo-supercharged system

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Cooling systems General Excessive heat is always undesirable in both reciprocating engines. If means were not available for its control or elimination, significant damage or complete engine failure would occur. Although the vast majority of reciprocating engines are air-cooled, some diesel liquid-cooled engines are being made available for light aircraft. An internal combustion engine is a heat machine that converts chemical energy in the fuel into mechanical energy at the crankshaft. It does not do this without some loss of energy, however, and even the most efficient aircraft engines may waste 60 to 70% of the original energy in the fuel. Unless most of this waste heat is rapidly removed, the cylinders may become hot enough to cause complete engine failure. Excessive heat is undesirable in the engine for three principal reasons. 1) It affects the behaviour of the combustion of the fuel/air charge. 2) It weakens and shortens the life of engine parts. 3) It impairs lubrication. If the temperature inside the engine cylinder is too high, the fuel-air mixture is pre-heated, and combustion occurs before the desired time. Since premature combustion causes detonation, knocking, and other undesirable conditions, there must be a way to eliminate heat before it causes damage.

One gallon of AVGAS has enough heat value to boil 75 gallons of water; thus, it is easy to see that an engine that burns 4 gallons of fuel per minute releases a tremendous amount of heat. About a quarter of the heat released is changed into useful power. The remainder of the heat must be dissipated so that it is not destructive to the engine. In a typical aircraft powerplant, half of the heat goes out with the exhaust, and the engine absorbs the other half. Circulating oil picks up part of this soaked-in heat and transfers it to the airstream through the oil cooler. The engine cooling system takes care of the rest. Cooling is a matter of transferring the excess heat from the cylinders to the air. However, there is more to such a job than just placing the cylinders in the airstream. A cylinder is provided with cooling fins which can increase the original surface area by approximately 500%; such an arrangement increases the heat transfer by radiation. If too much of the cooling fin area is broken off, the cylinder cannot cool properly, and a hotspot develops. Therefore, cylinders are generally replaced if a specified number of square inches of fins are missing. You should recall from Module 16.3 that the construction of the standard air-cooled cylinder was manufactured with an aluminium cylinder head with an extensive area of fins and a steel cylinder bore with a small area of fins. It should be noted that the greatest fin area is provided in the hottest part of the engine. Throughout engine operation, the pilot is closely monitoring the cylinder head temperature display (CHT) to prevent engine damage.

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A thermocouple is generally located on the cylinder head to provide the pilot with CHT indications. The aircraft manufactures carefully design the engine cowls and baffles to ensure adequate, even air-flow over all of the cylinders, however achieving equal cooling to all cylinders is nearly impossible, and some cylinders run hotter than others. For example, most aircraft fitted with a horizontally opposed engine have front cylinders that run cool due to the large amount of RAM air they receive. The rearmost cylinder is usually the hottest as the air warms up as it travels rearwards and decreases in pressure. All piston engines are cooled by transferring excess heat to the surrounding air. In air-cooled engines, this heat transfer is direct from the cylinders to the air. Therefore, it is necessary to provide thin metal fins on the cylinders of an air-cooled engine to have an increased surface for sufficient heat transfer. Most reciprocating aircraft engines are air-cooled although a few high-powered engines use an efficient liquid-cooling system. In liquid-cooled engines, the heat is transferred from the cylinders to the coolant, which is then sent through tubing and cooled within a radiator placed in the airstream, the coolant radiator must be large enough to cool the liquid efficiently. The main problem with liquid cooling is the added weight of coolant, heat exchanger (radiator), and tubing to connect the components. Liquid-cooled engines do allow high power to be obtained from the engine safely.

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Air-cooling radial engines The early engines were velocity cooled and the radial engine cylinder arrangement, where all cylinders are exposed to the air-flow, is particularly suited to this type of cooling. The photograph below shows a typical example of this type of cooling. These engines had little if any baffling and cooling was entirely dependent on the velocity of air flowing over the cylinders. The propeller accomplished cooling on the ground, and in flight, forward motion provided the necessary air-flow. Velocity cooling left something to be desired in that it did not provide uniform air-flow around the entire cylinder assembly. This deficiency is illustrated in the diagram. Notice the turbulence and lack of air-flow contact on the rear side of the cylinder, which is typical of velocity cooling. As the compression ratios were often little more than 5:1 and maximum engine speed rarely above 2,000 RPM this did not pose a significant problem on these early engines. As the engine performance increased, so too did the compression ratios and the number of rows of cylinders.

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Velocity cooling (1922 Ryan PT 20)

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As the performance of single row radial systems increased, they had a series of baffles located between the cylinders. These force the oncoming cooling air into the deflectors which supplied it to the cooler rear of the cylinder, see the diagram below left. This ensured that the maximum benefit is achieved from the cooling air-flow. When radial engines were produced with multiple rows of cylinders cooling became more critical as the cylinders in the second row would tend to run hotter due to them being screened from the ram air-flow by the first row. To overcome this problem the designers utilised baffles and deflectors, these were installed to guide the oncoming air around the cylinders ensuring they were all impinged by sufficient air-flow.

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Even cooling airflow is achieved using baffles.

Baffles and deflectors fitted to a single row radial engine

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As the engine performance and compression ratios went up, but so did the amount of dissipated heat that had to be removed from the engine. The RPM increased, and this too brought increased cooling requirements. The point was reached where aircraft engines could no longer be adequately cooled by the velocity method. Cowlings were placed around the engines, and baffles were installed between the cylinders. Now the cooling air could be directed around the entire area of the cylinder. Thus, pressure cooling was born, and the results were superior as well as uniform engine cooling, as shown below. The engine cowls are usually of aerofoil section which reduces drag and to improve the cooling of the engine. The two types are generally used are known as the Townend ring or a NACA cowl, and they both serve to direct the air-flow over the cylinder heads. As a refinement, and to provide the pilot with some control of the cylinder head temperatures, gill rings are fitted around the rear of the engine; the angle of opening, adjusted from the co*ckpit, regulates the air-flow over the cylinders.

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Early form of pressure cooling incorporating an NACA cowling

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Townend ring cowl The Townend ring is a narrow-chord cowling ring fitted around the cylinders of an aircraft radial engine to reduce drag and to improve cooling. The Townend ring was introduced in 1929. It caused a reduction in the drag of radial engines and was widely used in high-speed designs of 1930-1935 before the long-chord NACA cowling came into general use. Despite suggestions of it exploiting the Meredith effect, low airspeeds, low-temperature differences and small mass flows make that unlikely, particularly when combined with the lack of flow control as the air exits the cowling. Although superior to earlier cowlings, and uncowled engines in terms of drag and cooling, above 217 knots (402 km/h, 250 mph) the NACA cowling was more efficient and soon replaced it in general use. The exhaust system was often incorporated into the design of the Townend ring, thus reducing exhaust system drag as well as the profile drag of the engine. This integral exhaust can be seen on the Fairey Swordfish illustrated below.

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Townend ring is a complete hollow section which is used as an exhaust collector (Fairey Swordfish)

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The NACA cowl The NACA cowling is a type of aerodynamic fairing used to streamline radial engines for use on aeroplanes and developed by the National Advisory Committee for Aeronautics in 1927. It was a significant advance in aerodynamic drag reduction and paid for its development and installation costs many times over due to the gains in fuel efficiency that it enabled. The cowling enhanced speed through drag reduction while delivering improved engine cooling.

Gill ring A gill ring is a small flap located at the rear of the cowling for a reciprocating engine. Opening the cowl flap increases the amount of cooling air flowing through the engine by decreasing the pressure at the point the air leaves the engine compartment.

The cowling constitutes a symmetric, circular aerofoil, in contrast to the planar aerofoil of wings. It directs cool air to flow through the engine where it is routed across the engine’s hottest parts, that is, the cylinders and heads. Furthermore, turbulence after the air passes the free-standing cylinders is significantly reduced. The sum of all these effects reduces drag by as much as 60%. The test conclusions resulted in almost every radial-engine aircraft being equipped with this cowling, starting in 1932. It is long-chord ring cowling whose trailing edge fairs smoothly into the fuselage or engine nacelle. NACA cowlings are used on the vast majority of modern radial engine installations. The cooling air rams into the open front of the cowling and exits through an annular slot at the rear. This slot is often covered with adjustable cowl flaps to control the amount of cooling air allowed to flow through the engine. The aerofoil shape of the cowling produces a forward aerodynamic force that more than compensates for the cooling drag of the engine.

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NACA cowl with gill ring and eductor system (Douglas DC3/AC47 Dakota)

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Gill ring

Gill type cowl flaps

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Gill type cowl flaps

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Air-cooling inline and horizontally opposed engines Most of the early 4-cylinder, horizontally opposed engines were velocity cooled like the radial engines. However, as these engines increased in size and power, they too had to be pressure cooled. As the engine power and aircraft performance improved, the engine cowlings became streamlined, and as a consequence, the space inside the cowl decreased. The proper cooling of engines in aircraft now became a precise science. The amount of heat energy that must be removed by the cooling air is approximately equal to the horsepower that is driving the propeller. This is why the failure of the cooling baffles can lead to rapid and significant deterioration of the cylinder and other engine components. The baffling installed on the engines of today is the result of considerable study. Special wrap around baffles now guide the cooling air entirely around the cylinder heads and barrels. Other baffles channel cooling air into oil radiators and cooling ducts for various accessories. Rubber seals are provided along the cowling edges of the baffling. These seals are critical since they provide the necessary airtight seal between the baffling and the cowling. Therefore, every baffle and its seal must be in its proper position, and good working condition, otherwise adequate cooling be achieved. It is important to understand that to control the air-flow from the propeller, and ram air there has to be a pressure differential inside the cowling, on most applications this pressure differential is around 100-150 mm (4-6") of water pressure.

The diagram below top-left illustrates a typical air-flow pattern around a modern engine installation. When the aircraft is flying, air enters the cowling and is slowed in the plenum formed by the cowling, engine, baffles and seals. The effect creates a static, or higher-pressure area, above the engine. Since gasses move from high pressure to low pressure, the air then flows down through the cylinders and cross the oil cooler to the lowpressure areas below and behind the engine. The air exits the cowling through cowl flaps or other flaring openings, carrying away excess heat. If the baffles are broken or misshapen, the deformity can reduce the volume of air passing. Some, or all of the air, is rammed in on just one side of the cylinders and guided through the cooling fins by baffles. It is then sucked out by the slipstream cylinders, meaning less than expected cooling for the cylinders or the oil cooler. Seals can create similar problems.

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Cowl flap

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Cowl flap operation

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Cowl flap

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If the seals are not in good condition or are not correctly adjusted, air can ‘bleed up’ and reduce the static pressure, slowing the flow of cooling air and increasing engine temperatures. Higher engine temperatures can foreshadow trouble to come. If the cooling air is not adequately contained and directed, hot spots which promote a lead or carbon buildup on the valve guides can occur, potentially leading to valve sticking problems during start-up. A stuck valve most of the time ends up bending a pushrod and causing an oil leak but can also cause a substantial reduction in engine power and costly damage to the crankcase. Other problems with insufficient cooling include overheating the spark plug barrels, which deteriorates ignition leads and boosts temperatures in the insulator tip high enough to cause pre-ignition and piston distress. Adequate air-flow is particularly important during hot weather to provide proper cooling of the oil cooler; oil that runs too hot breaks down and causes more friction inside the engine. While the first step in diagnosing abnormal engine temperatures in normal operations is making sure the temperature gauge is providing accurate readings, the next step is to check all the seals for fit and condition. If the seals aren’t soft and pliable, replace them. One way to observe how well the seals are performing their stop-gap function is to remove the cowling and look at the residues left where the cowling and seals rub together, having one continuous line of smudge means the seal is doing its job. If there are breaks in the line, which might show up as an unmarked area where the air was rushing through the gap, that could mean leaks and lower static pressure above the engine. Inspect the cowl flaps or flaring openings at the rear of the cowling for excessive leakage, indicated by discolouration.

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Cooling loss due to gaps in the baffle seals

Airflow without baffles

Engine seals

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Cooling airflow pattern around a typical modern engine installation

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Tightly cowled engine

Engine baffles and seals

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Augmenters Some aircraft use augmenters to provide air-flow. Each nacelle has two pairs of tubes running from the engine compartment to nacelle. The exhaust collectors feed exhaust gas augmenter tubes. The exhaust gas mixes with air that has passed over the engine and heats it to form a high temperature, low pressure, jet-like exhaust. This low-pressure area in the augmenters draws additional cooling air over entering the outer shells of the augmenters is heated through contact with the augmenter tubes but is not contaminated with exhaust gases. The heated air from the shell goes to the cabin heating, defrosting, and antiicing system. Augmenters use exhaust gas velocity to cause air flow over the engine so that cooling is not entirely dependent on propeller wash. Vanes installed in the augmenters control the volume of air. These vanes are usually left in the trail position to permit maximum flow. They can be closed to increase the heat for cabin or anti-icing use or to prevent the engine cooling too much during descent from altitude. In addition to augmenters, some aircraft have residual heat doors or flaps that are used mainly to let the retained heat escape after engine shutdown. The nacelle flaps can be opened for cooling than that provided by the augmenters. A modified form of the previously described augmenter cooling system is used on some light aircraft. Augmenter systems are not used much on modern aircraft.

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Augmenter operation

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As shown in the diagram below, the engine is pressure cooled by air taken in through two openings in the nose cowling, one on each side of the propeller spinner. A pressure chamber is sealed off on the top side of the engine with baffles appropriately directing the flow of cooling air to all parts of the engine compartment. Warm air is drawn from the lower part of the engine compartment by the pumping action of the exhaust gases through the exhaust ejectors. This type of cooling system eliminates the use of controllable cowl flaps and assures adequate engine cooling at all operating speeds.

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Liquid-cooled engines Although air cooling is the best method with respect to simplicity and cost, it may be, however, that it is not the most efficient method, especially if the cylinders are of the inline arrangement. With this arrangement, although not exclusive to inline engines, liquid cooling may be preferred to air cooling. Most modern Diesel engines are liquid-cooled. Diesel and JET-A fuel has more energy (BTU) per gallon than has AVGAS and generates more heat than a typical gasoline aircraft engine, so additional cooling is necessary. Water is an outstandingly good cooling medium, with extremely high ability to transfer heat. Its only real shortcoming is that it boils at 100°C at sea level and even lower temperatures at higher altitudes. It is possible to push up the boiling temperature by keeping the entire cooling circuit under pressure. Many of the final types of high-power liquid-cooled engines a mixture used of water plus about 30-60% ethylene glycol, the latter being simply an antifreeze additive, as in cars. At a pressure of 40 lb/in2, such a mixture could circulate at 130°C. In a liquid-cooled engine, the temperature is controlled by circulating a cooling fluid around the space between the cylinder jackets and the cylinder walls, and through passages cored in the cylinder head. Water passages are designed to ensure a calibrated feed to all parts of the engine, so reducing the possibility of hot spots forming.

A simple system consists of the following: 1) The header (or expansion) tank. This tank, positioned at the top of the system, stores the reserve of coolant and provides a head of coolant to the pump. 2) The water pump. This is usually a centrifugal type pump but does not produce a pressure; it is fitted to increase the speed of the coolant flow through the system and so to reduce the quantity of coolant required. 3) The coolant passages through the engine. While the main passages are centred around the combustion chambers and valves; it is sometimes found that the hot coolant is directed to jackets fitted to the induction pipe and supercharger casings. This auxiliary flow assists in vaporising the fuel/air mixture. The coolant flow through the engine is arranged to follow the normal course of the thermal current, i.e. from the base of the cylinders to the cylinder head. 4) The radiator. The radiator is positioned in the slipstream of the propeller and dissipates heat from the coolant into the air. Shutters are positioned in front of the radiator; when the coolant temperature falls below an efficient figure, the shutters are closed, either automatically or manually, to deflect the air-flow around the radiator. 5) The thermostat. This is a thermostatically operated bypass valve which affects the coolant flow through the radiator and limits the minimum coolant temperatures while the engine is running.

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Liquid cooling system layout

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Liquid cooling system layout

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After coolant

A simple liquid cooling system as found on the Rotax 912/914 engine.

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Example system: Continental Voyager The Voyager 200 is a 4-cylinder liquid-cooled 3.27 l (200 in3) engine with a 103.12 mm (4.06″) bore capable of producing 82.0 kW (110 BHP) at 2,750 RPM with a high turbulence HTCC combustion chamber at 11.4:1 compression ratio for minimum fuel consumption. This engine has already made aviation history by powering the Voyager aircraft on the historic recordbreaking non-refuelled, nonstop flight around the world in late 1986. A 6-cylinder 4.92 l (300 in3) version utilises the same cylinder assembly and is capable of being rated at 126-142 kW (170-190 BHP) at 2,700-3,000 RPM. Both engines are capable of providing a 0.375 BSFC across a broad operating range with minimal heat loss to coolant and oil. A brake thermal efficiency as high as 36% has been attained at lean cruise with specific heat loss to coolant and oil limited to only 16% of available fuel energy at best power mixture. A 0.345 BSFC has been demonstrated on the 4.92 l (300 in3) engine under conditions simulating advanced turbocharging techniques at high altitude.

In the Voyager design, a new design cast aluminium cylinder head with an integral water jacket is assembled onto the cylinder barrel using the conventional threaded interference fit. The steel barrel is similar to its air-cooled counterpart except the cooling fins are deleted, cooling of the lower barrel is accomplished by a high flowrate piston oil jet. Parts such as rocker covers, rocker arms, valve springs, connecting rods, and piston pins remain common to the aircooled engine. A parallel flow coolant circuit supplies coolant to each cylinder using a tubular manifold; connectors utilised at the inter cylinder joints provide for relative motion and thermal growth. This unique design concept offers an economical, lightweight, and effective means of providing a liquid-cooled aircraft engine for general aviation. The ease of maintenance is not compromised; disassembly of the entire engine is not required to perform maintenance on a single cylinder. The bottom end of the engine is relatively unchanged.

Unlike conventional automotive engines where the cylinders are formed as an integral part of a single block-type crankcase structure, air-cooled horizontally opposed aircraft engines have utilised separate cylinders attached to the crankcase with threaded fasteners. Early in continentals design study, the decision was made to pursue a liquid-cooled concept which would allow utilisation of the bottom end, common to the existing production air-cooled line. It was not considered economically practical to design a completely new engine.

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Continental Voyager 200 liquid-cooled engine

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Advantages of liquid cooling The lightweight approach to liquid cooling the power section in combination with the efficient weight effective brazed aluminium plate/fin-type heat exchangers now available are significant factors in allowing the liquid-cooled engine concept described here to provide an efficient weight competitive option to the conventional air-cooled engine. The high compression ratio, high turbulence combustion chamber (HTCC) supplements the attractiveness of the Voyager 200 and 300, by providing improved thermal efficiency with substantial fuel savings. The success of the high compression-ratio high-turbulence combustion chamber in the Voyager 200 and 300 engines is a classic example of the advantages offered with liquid cooling. By itself, liquid cooling does not necessarily provide improved fuel consumption; however, as demonstrated in this program, it can serve as the tool which allows engine refinements previously not possible with air cooling. Past attempts at incorporating HTCC into an air-cooled general aviation engine were not as successful, mainly due to the higher cylinder head metal temperatures. This earlier work had indicated that substantial improvements in cylinder cooling would be required for acceptable HTCC operation. Liquid cooling became the solution to this problem. A further reduction in SFC is made possible during take-off and climb operation. Most air-cooled engines are rated at a mixture strength richer than best power mixture to provide a fuel cooling effect during the low-speed take-off and climb. It is not uncommon to encounter a 0.700 BSFC on the larger air-cooled, turbocharged engines. The typical turbocharged air-cooled engine would overheat at the best power fuel/air ratio during take-off and climb. Total Training Support Ltd © Copyright 2020

Liquid cooling allows the engine to be rated at best power mixture since the cylinder assembly is designed to cool adequately under these conditions with proper sizing of the liquid to air heat exchanger. The Voyager 200 and 300 are rated at a 0.425 BSFC, representing a 25% reduction in takeoff and climb BSFC. Rating at best power mixture equates to a lower manifold pressure and reduced peak cylinder firing pressure on turbocharged engines which assists in improved durability. Liquid cooling also offers the potential for increased power output and improved detonation margin. The cooler combustion chamber provides improved knock resistance and allows an increase in compression ratio. Higher boost levels are feasible for turbocharged engines. The conventional air-cooled engine is well known for its cooling anomalies. It is not uncommon to encounter a large spread in cylinder head temperature (CHT) among the cylinders. Nonuniform cooling air-flow distribution within the nacelle and the leakage associated with inter-cylinder and perimeter baffles are contributing factors. Also, due to the nature of the air-cooled cylinder design, it is usually difficult to achieve uniform cooling around the circumference of the head and barrel using the typical sheet metal baffles. Furthermore, the cylinder head metal temps may run 10-38°C (50-100°F) hotter than obtainable on a liquid-cooled design.

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Air cooled cylinder

Liquid cooled cylinder

Liquid cool cylinder Air cool cylinder

Liquid cool cylinder

Comparison of liquid-cooled and air-cooled cylinders

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Liquid cooling provides substantial improvements in cylinder cooling with more uniform temperature distributions in both the head and barrel areas. These temperature characteristics yield less distortion of the cylinder walls and better material strengths which result in lower wear rate and longer engine life. Uniformity of the temperatures around the circumference of the cylinder barrel is improved sufficiently to allow a reduced piston-to-cylinder running clearance as compared to the aircooled equivalent. Inherent with the basic concept of liquid cooling, the uniformity of cooling among the various cylinders on a given engine is now more finitely controlled and not subject to the variables which trouble the air-cooled engines. The spread in individual cylinder head temperatures is reduced to approximately 5°C (40°F) or less. With liquid cooling, the engine manufacturer can effectively maintain control of engine cooling over its life span. Significant reductions in cooling drag are also possible with the liquid-cooled engine. Comparative analysis indicates that a 30%-50% reduction in cooling air mass flow is possible for the higher output engines using a well-designed and adequately installed liquid to air heat exchanger. The lower cooling drag yields higher aircraft flight speeds or reduced fuel consumption for fixed flight speed. For example, a 50% reduction in cooling air-flow equates to a 2-3% increase in speed or optionally a 7-10% decrease in brake-specific fuel consumption (BSFC) and kW required, assuming cooling drag at 15-20% of total aircraft drag.

In a typical air-cooled aircraft engine installation, the cooling air leakage around the perimeter and inter-cylinder baffles may approach 40% of the cooling air-flow required for a new tightly baffled engine. The leakage around a properly installed single heat exchanger should be negligible. Fundamentally, a welldesigned heat exchanger is a far more efficient heat transfer device than the conventional air-cooled engine. With the more uniform cylinder cooling, cooler combustion chamber temperatures, absence of cooling anomalies, and better wear characteristics, significant improvements in engine durability are now attainable. It is projected that these improvements result in a more reliable, highly efficient engine with longer TBO and reduced operating/maintenance costs. Also, performance and operational advantages such as reduced SFC, increased power output, better detonation margins, higher tolerance to operational abuse, less severe cooling transients, and reduced cooling drag, contribute to providing an advanced aircraft engine concept capable of meeting the challenges of future general aviation requirements. Continental concluded from results of the Voyager engine technology demonstration program have demonstrated the feasibility and advantages of this unique liquid-cooled aircraft engine concept. The basic design concept, performance, cooling and durability characteristics have been validated by a comprehensive test and development program which has accumulated well over 5,500 hours test time to date. It is concluded that liquid cooling offers definite advantages over air cooling in the areas of durability and performance. Other advantages, such as lower operating costs and operational improvements, are more subjective and can only be substantiated with the service experience.

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Large radiator used by the liquid cooled EPS Diesel engine

Thielert Centurion liquid cooled Diesel engine

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These advantages and benefits are summarised as follows. • • • • • • • • • • • • •

Liquid cooling system layout The Voyager 200 and 300 utilise a common liquid-cooled cylinder assembly with a coolant jacket formed as an integral part of the cast aluminium cylinder head. This coolant jacket encircles the combustion chamber area and the outer end of the barrel. Unique to the design concept, the coolant jacket extends down along a portion of the cylinder length as illustrated below, terminating at a position adjacent to the bottom of the piston skirt when at TDC. This leaves the lower length of the barrel free of the coolant jacket, thereby resulting in a lightweight but effective cooling system.

More uniform cylinder cooling Cooler combustion chamber metal temperatures Absence of cooling air-flow anomalies Better cylinder wear characteristics Improved durability Reduced fuel consumption Increased power output Better detonation margin Significant reduction in cooling drag Longer life and TBO Precise cooling control over engine life Higher tolerance to operational abuse Less severe cooling transients

A coolant passageway, having an inlet and outlet port is formed within the cylinder head. An engine-driven pump supplies coolant under pressure via an external manifold to a flanged inlet port located on the lower side of each head. Upon entering the head, the coolant is first directed through a central passage to the critical valve bridge area with a portion bypassed immediately around the exhaust port. The intake port is left free of direct cooling along with a small section of the exhaust port. Transfer passages direct the flow around the spark plug ports and valve seat areas, then into the barrel cooling jacket to a flanged outlet port located on top vertical. The coolant is then collected in a manifold and directed to a ram air-cooled heat exchanger.

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Continental liquid-cooled cylinder assembly

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As an integral feature of this system, the lower length of the cylinder barrel is not enclosed by the jacket. Instead, it is cooled by the spray of an oil nozzle directed at the underside of the piston dome. This oil jet is the primary cooling mechanism for the piston and the lower barrel section. Piston oil jets are not uncommon on an air-cooled aircraft engine; however, a proportionally higher oil flow rate is used to cool the lower barrel indirectly. The diagram below top shows a schematic representing piston and cylinder heat balance. Heat is transferred by conduction from the cylinder wall via piston rings and the oil film to the cooled piston. Also, a lesser amount of heat is conducted through the cylinder wall to the crankcase, which is subsequently oil-cooled.

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Piston/cylinder heat balance

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Most liquid cooling systems utilise a series coolant flow circuit. Typical of these systems, coolant enters the block and flows first around the base of each cylinder before being directed to the cylinder head area. This approach tends to over cool the cooler bottom end, and under cool the hotter head area with the cylinder heads increasing in temperature along the flow path as the coolant temperature rises. In a cooling system where the flow is first directed through the head area before circulating the cylinder barrel section, a more uniform cylinder assembly temperature profile is possible. Also, a parallel coolant circuit can provide a more uniform cylinder to cylinder temperature distribution since each cylinder receives essentially the same coolant temperature.

The coolant flow sequence is: 1) Coolant tank to the radiator (via thermostat and pressure 2) 3) 4) 5)

relief valve). Radiator to the coolant pump. Pump to the coolant manifold. Coolant manifold distributes to the individual cylinders. Return coolant manifold to tank.

The cooling system on the Voyager 200 and 300 engines is arranged so that the coolant flows in parallel through the cylinders. By using a parallel system rather than series the pressure drop through the engine is minimised, this loss being 1-2 psi. Pump power demand is reduced in comparison to a series flow system. Engine coolant is a 60/40 mixture of ethylene glycol and distilled or de-ionised water. With the 120°C (250°F) maximum allowable coolant inlet temperature, a 206.8 kPa (30 psia) minimum pressure must be maintained at the coolant pump inlet to prevent boiling and cavitation.

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Coolant enters the engine, driven by the coolant pump. Depending on coolant temperature, it passes through the bypass circuit directly back to the engine, or through the radiator where cools before returning to the engine. Short-circuit Water passes from the pump through the engine to cool the engine and then to a thermostat. Depending on coolant temperature, coolant bypasses the radiator and goes directly back to the pump or is allowed to flow through a radiator. The thermostat is wholly closed at (typically) 84°C and completely open at (typically) 94°C. This ensures that the engine heats up quickly without overheating and prevents the engine from shock cooling. Cabin-heat circuit There is an additional cabin heating circuit which allows hot coolant to flow through a heat exchanger to provide hot cabin air. This circuit is always open, and cabin temperature can be controlled via an air flap from inside the co*ckpit. The cabin heating circuit is always open and aids in cooling. The layout of the radiator and heater circuit, as well as the selection of the heat exchanger, are defined via the installation for each aircraft model.

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Operation of the thermostat and short-circuit

Liquid coolant circuit with cabin heating

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Liquid-coolant system components Heat exchanger The heat exchanger (or ‘radiator’) is constructed of a pair of metal expansion chambers, linked by a core with many narrow passageways, giving a high surface area relative to volume. This core is usually made of stacked layers of metal sheet, pressed to form channels and soldered or brazed together. For many years, radiators were made from brass or copper cores soldered to brass headers. Modern radiators have aluminium cores. The radiator is linked to the engine by rubber hoses. On some engines, the expansion chambers are replaced with an external coolant reservoir (or ‘header tank’, or ‘expansion tank’). The expansion tank is installed at the highest point of the system and ensures the proper coolant level. A pressure relief cap is installed on the heat exchanger (or expansion tank if fitted).

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Heat exchanger operations

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Thermostat The cooling system is controlled by a wax element thermostat, as shown below. The thermostat begins to open at 84°C (typically) and is completely open at 94°C (typically). As the thermostat opens, it progressively switches from the short circuit to the external circuit. The heating circuit is always active.

Cabin heat exchanger A small heat exchanger supplies cabin heat. Engine coolant passes through the heater core and heats the incoming air. The coolant flow through the heater core is always open; cabin heat is controlled by directing air-flow into the cabin or out of the cowl. The exact configuration of the heating system depends on the aircraft installation.

Coolant pump The coolant pump is an impeller unit internal to the engine and is driven by the V-ribbed belt at the rear of the engine. The figure below shows the coolant pump installed in the crankcase. The volume flow rate of the coolant pump under normal operating conditions is 90-100 l/min on the 4-cylinder engine. Coolant reservoir/expansion tank The coolant reservoir (expansion tank) allows the coolant to expand when warm or allows the cooling system to draw additional coolant as needed. A pressure relief valve on top of the expansion tank relieves the pressure exceeding 1.8 bar. The higher pressure, when hot, increases the boiling point of the coolant and prevent boiling and cavitation. The coolant reservoir also contains a low coolant level sensor which sends a signal to the warning lamp “Water level” on the instrument panel if the coolant is low. In most installations, a silicate pouch inside of the coolant reservoir is required to help building-up an aluminium-silicate layer for corrosion protection of the aluminium block and cylinder head. Total Training Support Ltd © Copyright 2020

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Coolant pump and temperature sensor (Thielert TAE 125 aero-Diesel)

Coolant reservoir

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Wax element thermostat

Wax element thermostat operation

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Licence Category B1 and B3

16.7 Supercharging and Turbocharging

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Principles and purpose of supercharging and its effects on engine parameters;

Part-66 Ref. 16.7

Knowledge Levels A B1 B3 1 2 2

Construction and operation of supercharging/turbocharging systems; Systems terminology; Control systems; System protection

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Table of Contents Introduction ___________________________________ 6 Normally aspirated engines______________________ 6 Supercharged engines _________________________ 8 Turbocharged engines _________________________ 8 Turbocharger advantages ______________________ 10 Turbocharger disadvantages ___________________ 10 Supercharger advantages ______________________ 10 Supercharger disadvantages ___________________ 10 Superchargers _______________________________ Altitude effects ______________________________ The two-speed supercharger ___________________ The two-stage supercharger ____________________ Types of supercharger ________________________ The centrifugal supercharger ___________________ Boost and manifold air pressure _________________ System layout _______________________________ Power from supercharging _____________________ Manifold pressure control ______________________ Variable datum control ________________________ Boost reversal _______________________________ Supercharger drives __________________________ The centrifugal friction clutch ___________________

Turbochargers ________________________________ 38 General ____________________________________ 38 The wastegate _______________________________ 40 The wastegate actuator ________________________ 40 Intercooler __________________________________ 40 Construction _________________________________ 46 Operation ___________________________________ 52 Operating characteristics _______________________ 52 The absolute pressure controller _________________ 54 The variable pressure controller _________________ 56 The dual unit control system ____________________ 58 The density controller__________________________ 58 The sloped controller __________________________ 60 The differential pressure controller _______________ 60 The triple unit control system ____________________ 64

12 12 16 18 22 24 26 28 28 32 34 34 34 36

Glossary of supercharger and turbocharger terms __ 66

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Introduction Engines can be categorised into two types depending on their induction method: • •

normally aspirated engines; and supercharged/turbocharged engines.

Normally aspirated engines A normally aspirated (or ‘naturally’ aspirated) engine relies upon the carburettor inlet to suck air into the carburettor where the correct mixing of the fuel and create a combustible vapour. However, we know from previous reading, that the power developed by an engine depends upon the weight (density) of the air/fuel mixture in the cylinders at a given time and setting. As each piston descends during the four-stroke cycle, a negative pressure is created within the cylinder, so the weight of air entering the cylinder depends on the air pressure within the inlet manifold. In a normally aspirated aircraft engine, inlet manifold air pressure is governed by the pressure at altitude, and also by the amount that the throttle is opened. So, with a constant throttle opening as an aircraft climbs, there is a reduction in inlet atmospheric pressure, leading to a reduction in inlet atmospheric pressure, leading to a reduction in power. To prevent this loss of power as the aircraft climbs, it is necessary to supply more air to the manifold; this is achieved by supercharging.

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Normally aspirated engine

Turbocharged engine

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Supercharged engines A supercharger is a mechanically driven, forced induction device that is utilised in a piston engine to enable it to produce more power. Its purpose and function are similar to a turbocharger. The principal difference is that it is mechanically driven by a direct connection to the engine. In contrast, a turbocharger is turbine-driven using engine exhaust gas flow.

To increase power for takeoff and initial climb and to maintain power during high altitude operations, the manifold pressure (inlet) must be raised artificially; and the supercharger does this. However, it must be remembered that too much pressure created in the cylinders may lead to detonation, so correct operating procedures must be maintained at all times.

In a normally aspirated piston engine, intake gases are drawn into the cylinder by the reduced pressure created by the downward stroke of the piston. The mass of air drawn into the cylinder, in part, limits the power production of the engine. A supercharger is an engine-driven mechanical device which powers a compressor. This compressor draws in ambient air, compresses it and then feeds it into the engine intake resulting in a greater mass of air and, proportionally, a greater amount of fuel entering the cylinders on the intake stroke. The additional air and fuel result in a significant increase in the power production of the engine compared to a normally aspirated version of the same engine. Although it takes a significant amount of engine power to drive the supercharger, the power increase due to the effect of the supercharger more than compensates for that loss.

Turbocharged engines A turbocharger is a turbine-driven, forced-induction device that is utilised in a piston engine to enable it to produce more power. In a normally aspirated piston engine, intake gases are drawn into the cylinder by the reduced pressure created by the downward stroke of the piston. The mass of air drawn into the cylinder, in part, limits the power production of the engine. A turbocharger recovers waste energy from the engine exhaust stream and uses it to power a turbine which in turn drives a compressor. This compressor draws in ambient air, compresses it and then feeds it into the engine intake resulting in a greater mass of air and, proportionally, a greater amount of fuel entering the cylinders on the intake stroke. This increase in air and fuel results in the turbocharged engine being more powerful and efficient than its naturally aspirated counterpart.

The supercharger is an engine-driven fan that is usually situated between the inlet manifold and the carburettor. The compressor section can be either driven by the exhaust system via a turbine or be directly driven by the engine crankshaft through a series of gears. An example of the exhaust-driven system is illustrated in the diagram below right.

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Supercharged engine schematic

Turbocharged engine schematic

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Turbocharger advantages Turbochargers are more efficient than superchargers because turbos use waste air that’s already exiting through the exhaust pipe. Though they don’t run entirely ‘free of charge’; it does take energy for the engine exhaust to turn the turbine.

Supercharger disadvantages Superchargers are inefficient because they require quite a bit of engine power to turn. That makes superchargers less fuelefficient than turbos. And finally, because they use a system of pulleys and gears to turn, more parts can break.

But in comparison to a supercharger, turbos use less fuel, and they typically have less total weight than a supercharger. Finally, most turbochargers provide a better total increase in horsepower than superchargers, because their speed can be changed by adjusting the wastegate (which is sometimes an automatic function). Turbocharger disadvantages Most turbochargers suffer from lag. Because it takes a second or two for exhaust gas to spin up the turbine, there is a delay from when you throttle up your engine, to the time the turbine achieves its desired speed and output. Next, turbos provide little to no benefit at idle and low power settings. And finally, turbos can suffer from a power surge. This happens when you rapidly reduce power, and air pressure quickly builds in the intake manifold, causing a temporary flow reversal and vibration. Surge isn’t as much of a problem with modern turbos as it used to be, but it’s something that you need to watch out for, especially if you’re flying older turbocharged aircraft. Supercharger advantages Superchargers have no lag, they boost an engine at low RPM, they run at cooler temperatures than turbos, and they’re relatively cheap in comparison to turbos which can be expensive.

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Turbocharger unit

Supercharger unit

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Superchargers Altitude effects Superchargers are a natural addition to aircraft piston engines that are intended for operation at high altitudes. As an aircraft climbs to a higher altitude, air pressure and air density decrease. The output of a piston engine drops due to the reduction in mass of air that can be drawn into the engine. For example, the air density at 9,100 m (30,000 ft) is 1⁄3 of that at sea level; thus only 1⁄3 of the amount of air can be drawn into the cylinder, with enough oxygen to provide efficient combustion for only a third as much fuel. So, at 9,100 m (30,000 ft), only 1⁄3 of the fuel burnt at sea level can be burnt. An advantage of the decreased air density is that the airframe experiences only about 1⁄3 of the aerodynamic drag. Also, there is decreased back pressure on the exhaust gases. On the other hand, more energy is consumed holding an aeroplane up with less air in which to generate lift.

The pilot controls the output of the supercharger with the throttle and indirectly via the propeller governor control. Since the size of the supercharger is chosen to produce a given amount of pressure at high altitude, the supercharger is oversized for low altitude. The pilot must be careful with the throttle and watch the manifold pressure gauge to avoid overboosting at low altitude. As the aircraft climbs and the air density drops, the pilot must continuously open the throttle in small increments to maintain full power. The altitude at which the throttle reaches full open and the engine is still producing full rated power is known as the critical altitude. Above the critical altitude, engine power output starts to drop as the aircraft continues to climb.

A supercharger can be thought of either as artificially increasing the density of the air by compressing it or as forcing more air than usual into the cylinder every time the piston moves down. A supercharger compresses the air back to sea-levelequivalent pressures, or even much higher, to make the engine produce just as much power at cruise altitude as it does at sea level. With the reduced aerodynamic drag at high altitude and the engine still producing rated power, a supercharged aeroplane can fly much faster at altitude than a naturally aspirated one.

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Effect of altitude with a sea-level supercharger

Atmospheric properties as a function of altitude

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Single-stage, single-speed superchargers are found on many high-powered radial engines and use an air intake that faces forward so the induction system can take full advantage of the ram air. Intake air passes through ducts to a carburettor, where fuel is metered in proportion to the airflow. The fuel/air charge is then ducted to the supercharger, or blower impeller, which accelerates the fuel/air mixture outward. Once accelerated, the fuel/air mixture passes through a diffuser, where air velocity is traded for pressure energy. After compression, the resulting high-pressure fuel/air mixture is directed to the cylinders. Some of the large radial engines developed during the second world war have a single-stage, two-speed supercharger. With this type of supercharger, a single impeller may be operated at two speeds. The low impeller speed is often referred to as the low-blower setting, while the high impeller speed is called the high-blower setting. On engines equipped with a two-speed supercharger, a lever or switch in the flight deck activates an oil-operated clutch that switches from one speed to the other. Under normal operations, takeoff is made with the supercharger in the low-blower position. In this mode, the engine performs as a ground-boosted engine, and the power output decreases as the aircraft gains altitude. However, once the aircraft reaches a specified altitude, a power reduction is made, and the supercharger control is switched to the highblower position. The throttle is then reset to the desired manifold pressure. An engine equipped with this type of supercharger is called an altitude engine.

The impeller blades form their own divergent duct with which starts to convert speed into pressure energy. With air entering the impeller at right-angles to the direction of the rotation, buffeting occurs; some inlet blades are curved to overcome this. Some impellers are shrouded to reduce the friction between the air and the rotation blades. The diagram below shows two types of impeller. On many supercharged engines, the fuel is added to the airstream at the eye of the impeller. This has two advantages; the fuel helps to cool the air, and the air assists in the vaporisation of the fuel. The impeller and diffuser assembly is situated after the throttle valve and before the induction manifold. The impeller is driven by the engine crankshaft through the gearing to increase its speed. To give you an idea, if the supercharger has a pressure ratio of 2 to 1 and it is driven at six times engine speed, then it is reasonable to expect sealevel power to be maintained, up to around 1,500 m (5,000 ft).

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Single-sided impeller

Fully-shrouded impeller Impeller types

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The two-speed supercharger If the engine is required to give sea-level power up to 3,000 m (10,000 ft) where the air density is only 75% of that at sea level, the supercharger has to be driven at 12 times engine speed and use about 200 BHP. By way of contrast, only 15 kW (20 BHP) is required to maintain the pressure up to 1,500 m (5,000 ft). Some superchargers can be driven at two different speeds, e.g. 6 times and 12 times engine speed; the 6 times speed up to 1,500 m (5,000 ft) using only 15 kW (20 BHP), and the 12 times speed up to 3,000 m (10,000 ft) using 150 kW (200 BHP) approximately. The changeover may be made using a selector lever operated by the pilot or automatically by an aneroid capsule. The two speeds are sometimes referred to as M and S gear. The M gear is for medium supercharging up to around 1,500 m (5,000 ft) and the S gear for supercharging up to 3,000 m (10,000 ft). This change over is affected by a clutch mechanism in the drive from the crankshaft. Changing from M to S gear at exactly 1,500 m (5,000 ft) is not ideal as the extra BHP loss causes a loss of power at the propeller shaft. The best time to engage S gear is when the manifold air pressure (MAP) drops 13.7 or 20.7 kPa (2 or 3 psi/5 to 6 inHg).

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Comparison of normally aspirated engine with a two-speed supercharged engine Brake horsepower vs. density altitude

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The two-stage supercharger For very high altitude flying some larger engines are fitted with two-stage superchargers which are in effect two superchargers in series. The first stage or first impeller raises the air pressure or fuel/air mixture pressure, and this pressurised gas is then fed into the eye of the second stage and then to the induction manifold.

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The Rolls-Royce Merlin, a supercharged aircraft engine from World War II. The supercharger is at the rear of the engine at left

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Allison V1710 two-stage supercharger

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Two stage supercharger schematic

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Types of supercharger There are three types of superchargers used in aero engines: • • •

the centrifugal-type; the roots type; and the vane type.

The centrifugal supercharger A centrifugal supercharger is a specialised type of supercharger that makes use of centrifugal force to push additional air into an engine. Increased airflow into an engine allows the engine to burn more fuel, which results in increased power output of the engine. Centrifugal superchargers are generally attached to the engine via a belt-drive or gear-drive from the engine’s crankshaft. The Roots supercharger The two double-lobed impellers rotate in opposite directions and increase the flow of air into the cylinder, thus increasing the volumetric efficiency. As the supercharger is engine driven, it increases speed with increases in engine speed. It is reasonably efficient but heavy and suffers from lubrication problems. The vane supercharger The vane-type supercharger increases airflow by the paddle action of the vanes; it is suitable for some engine but not as efficient as the other two types.

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Types of supercharger

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The centrifugal supercharger The centrifugal-type is the one in common use today. The air enters the eye or centre of the impeller. It is thrown out by centrifugal force which increases its speed into a diffuser or volute casing, where the kinetic energy is converted into pressure by the divergent ducts formed by the vanes of the diffuser. The impeller, which is driven through gearing by the crankshaft at several times engine speed, is a centrifugal type. The air enters the centre of the impeller and is flung outwards by centrifugal force. It then enters the diffuser, which is a casing around the impeller periphery. In this casing, there are fixed vanes which are so arranged that they form divergent ducts as shown below. A divergent duct acts opposite to a Venturi; as air passes into the broader section of the duct, there is a decrease in speed and an increase in pressure. The temperature of the air also increases. The pressurised air is now fed into the induction manifold. The impeller is usually one-sided, and the clearances between it and adjacent casings are kept to a minimum to avoid pressure losses from the tip into the crankcase and to prevent lubricating oil being drawn into the eye of the impeller.

Unfortunately, too substantial a rise in temperature has an adverse effect. It results in a decrease in the density of the air, so defeating the object of the supercharger. The probability of detonation taking place in the cylinder is also higher. Detonation occurs when the temperature generated by the compression stroke reaches a self-igniting temperature. Instead of a flame rate of about 18.3 m/second (60 ft/second) it leaps to 300 m/second (1,000 ft/second) and becomes in effect an explosion; causing damage to pistons, cylinder walls, con rods and bearings. So, on some engines, there is a need to cool the mixture before it enters the cylinders. Dependent upon the size of the supercharger and its speed relative to engine speed, the supercharger can achieve compression ratios of between 1.5:1 and 3:1. As an example, assume that we have a centrifugal-type supercharger driven at six times engine speed, giving us a compression ratio of 2:1. That means the supercharger doubles whatever pressure is fed into it so that the pressure in the induction manifold is doubled. This sort of arrangement allows sea-level power to be maintained up to about 1,500 m (5,000 ft). The induction manifold pressure is sampled and fed to a gauge in the co*ckpit to allow the pilot to monitor his/her engine power.

In all cases the extra air is ducted via the induction manifold to the cylinders, the pressure rise being partially due to the resistance to flow provided by the piston in the cylinder. Where there is a pressure rise, there is also a rise in temperature. Initially, the temperature rise is a good thing as it assists in the vaporisation of the fuel.

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Diffuser and impeller assembly

Single-sided impeller

Fully-shrouded impeller Impeller types

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Boost and manifold air pressure There are two alternative gauges, a boost gauge which is calibrated in psi with 0 being atmospheric at sea level and the pointer being able to move above (+) or below (–) and 0 settings. The other gauge is a manifold air pressure (MAP) gauge. The MAP gauge is calibrated in inches of mercury from absolute zero. As normal atmospheric is approximately 30" of mercury (30 inHg), the MAP gauge reads 30 when the boost gauge reads 0. The calibrations on the gauge are 2 inHg. For 1 psi, so a boost of +4 psi gives a MAP reading of 30 + (4 × 2) = 38 inHg. Boost at sea level atmosphere it is 0, if it is above sea level atmosphere it is positive (+), if it is below sea level atmosphere it is negative (–). With the aid of the supercharger, we can increase the volumetric efficiency of an engine, i.e. a greater charge can be forced into the cylinders during each cycle than is possible without the supercharger which means that we can increase the power of an engine at sea level. Also, we can maintain sealevel conditions by suing supercharged power at altitude. Taking the example mentioned earlier whereby the supercharger doubles the air pressure fed into it, it follows that if a sea-level atmospheric pressure of 14.7 psi is fed in, then 14.7 × 2 = 29.4 psi is delivered. This would show on the boost gauge as +14.7 psi boost. Remember that an atmospheric pressure of 14.7 psi is zero boost. Unfortunately, most engines are not built robustly enough to withstand the temperatures and pressures that a +14-psi boost would generate, however, as an example, most engines may well be able to accept +4 psi boost. Total Training Support Ltd © Copyright 2020

This boost, the maximum the engine can stand for any length of time is known as the rated boost (sometimes referred to as climbing boost) and would be limited to about 1 hour’s duration. It follows that a means of controlling boost at sea level is needed; this is achieved by closing the throttle butterfly, thus reducing the pressure going into the supercharger. If we require +4 psi boost, we calculated above that this is equivalent to 18.7 psi (14.7 psi atmospheric +4 psi boost). Our 2:1 supercharger must, therefore, have an inlet pressure of half 18.7 – 9.35 psi to produce 18.7 psi at its outlet. The throttle lever in the co*ckpit must be positioned to give +4 psi boost. Hence, the throttle valve is only partially open, and we know that this position must allow only 9.35 psi into the supercharger. The pilot does not need to know the details. All they have to do is open the throttle lever until +4 psi is indicated on the boost gauge. Assume that the engine power is set at +4 psi boost and the pilot elects to climb to altitude. As the aircraft climbs, so the atmospheric pressure drops, to say 13.7 psi. If the throttle lever is left in the same position, then the boost starts to fall because the throttle valve was set to partially open to give 9.35 psi when the atmospheric pressure was 14.7 psi. The throttle must be opened progressively as altitude increases to maintain 9.35 psi into the supercharger, to give a continual +4 psi boost indicated on the boost gauge. Eventually, height is reached when the throttle valve is fully open; somewhere around 1,500 m (5,000 ft). This is known as rated altitude and is also the full-throttle height for rated boost, in this case, +4 psi boost, i.e. the throttle valve is fully open.

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The aircraft climbs above this height, but the power starts to drop off, in the same way as a normally-aspired engine with no supercharger loses power as it climbs from sea level.

Reads absolute pressure above zero atmospheric pressure which is marked as zero.

Reads pressure relative to standard sea level atmospheric pressure which is marked as zero.

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System layout An intercooler cools the gas from the first stage of supercharging before passing to the second-stage supercharger impeller. The intercooler has a coolant system which is independent of the main engine coolant system. The system is usually filled with a mixture of ethylene glycol and water. It consists of a radiator in the air stream which cools the coolant and a radiator in a casing through which the first stage supercharged air passes. Two-stage supercharging may be achieved by the use of a turbocharger for the first stage then the intercooler to the carburettor and then to the main supercharger impeller described earlier. Power from supercharging Supercharging increases the pressure in the induction manifold; this increases the power of the engine in two ways: •

•

it increases the weight of the charge delivered to the cylinder on each induction stroke – giving more fuel and more oxygen to burn it in; and it increases the actual compression pressure; this means that the MEP (mean effective pressure) is great, thus giving more power.

A means of controlling the supercharger outlet pressure is needed. This control is achieved by reducing the inlet pressure, by closing the throttle butterfly valve, causing a pressure drop across the throttle valve. This pressure drop gives the required pressure into the supercharger to obtain the desired pressure out. The maximum pressure the engine can withstand for any length of time is known as its rated boost. A higher boost setting may be used for takeoff, known as takeoff boost, which may be used for about five minutes in an emergency. Consider an engine with a supercharge with a pressure ratio of 2:1 and a rated boost of +4 psi. The actual pressure in the induction manifold is 18.7 psi; that is atmospheric pressure of 14.7 psi, plus the rated boost of 4 psi. As the supercharger doubles any pressure fed in, to get 18.7 psi out 9.35 psi must be supplied. The pressure at sea level is 14.7 psi which, if delivered to the supercharger, would result in 29.4 psi in the induction manifold, a boost of 14.7 psi, which is far more than the engine can withstand. To reduce the supercharger inlet pressure, the throttle butterfly valve is closed to a smaller opening, thus providing a significant pressure drop across the valve. When the throttle is closed sufficiently to give 9.35 psi supercharger inlet pressure, there is a reading of +4 psi on the boost gauge.

As the aircraft climbs, the weight of a given volume of air decreases. The supercharger makes up for this deficiency by increasing the volume of the lighter air going into the induction manifold. If the supercharger is designed to give maximum power as the aircraft climbs to a certain maximum altitude, below this altitude the supercharger could deliver more pressure than the engine can stand. Total Training Support Ltd © Copyright 2020

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During a climb to altitude, the atmospheric pressure decreases, and if the throttle is left in the same position, the boost starts to fall as air at a lower pressure is entering the supercharger. The throttle valve must be opened at a few degrees until the +4 psi is re-established on the boost gauge, to restore the supercharger inlet pressure to 9.35 psi. The higher the aircraft goes, the further must the throttle valve be progressively opened to maintain the 4-psi boost as the atmospheric pressure decreases. Eventually, the aircraft reaches a height where the throttle valve is fully open, known as its rated altitude. Above this height, the engine power drops off in the same way as a normally aspirated engine power drops off above sea level.

Unexpectedly, there is a rise in power from a supercharged engine up to rated altitude due to three factors: • •

•

there is a drop in temperature of the air as altitude is increased thus giving a greater weight of charge; there is an increase in volumetric efficiency due to better scavenging of the exhaust because there is less resistance from the reduced atmospheric pressure. Thus, a greater charge may be induced; and there is the ram effect of the aircraft’s forward movement giving a greater mass flow of air into the intake.

The power selected does not have to be rated power. The pilot may, for example, select +2 psi boost, which means that the inlet pressure going into supercharger will be 8.35 psi at sea level. The throttle valve still has to be progressively opened as the aircraft climbs, until it is fully open. This point is known as full throttle height for that particular power selection. The fullthrottle height for +2 psi boost is higher than that for rated +4 psi boost as the induction pressure required is only 16.7 psi, compared with 18.7 psi.

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Manifold pressure control Manifold pressure control is fitted to some systems. This control will automatically partially close the throttle butterfly valve if the manifold or boost pressure rises above the rated value. If the servo piston moves down, it causes a deflection in the linkage to the throttle valve; this causes the throttle valve to close slightly irrespective of the throttle lever position in the co*ckpit. Engine oil pressure is routed to the top or bottom of the servo piston through a piston valve attached to an aneroid capsule. The outside of the capsule is subjected to superchargerdelivery low pressure, and the capsule expands. The operating rod is moved down, thus allowing the pressure oil to the underside of the piston. Oil from the top of the servo piston can escape back to the engine as the servo piston moves up to the top of the servo unit. As the throttle lever is advanced (note the operating rod is joined to allow for normal throttle movement) the supercharger delivery pressure (MAP or boost) increases. The capsule progressively contracts, moving the piston valve up until at rated boost both ducts to the servo piston are closed. The piston is now held firm in a hydraulic lock. If the throttle lever is advanced further, then the supercharger delivery pressure rises above rated boost. The capsule contracts further, allowing pressure oil to the top of the piston. The servo piston moves down, causing a deflection in the linkage to partially close the throttle valve. The supercharger delivery pressure now falls, and the capsule expands causing the piston valve to move down until, at rated boost, both ducts to the servo piston and the compressor delivery are closed to maintain rated boost.

With rated power selected, if the aircraft climbs, then the supercharger outlet pressure falls, causing the capsule to expand. The piston valve moves down, allowing pressure oil to the underside of the servo piston, which moves up; thus, the capsule contracts to seal off both ducts to the servo piston. This operation is repeated as the aircraft climbs and becomes a continual process until the throttle valve is fully open. This point is known as full throttle height for rated boost, or rated altitude. Although rated power is the maximum power allowed for any length of time, e.g. about an hour, a higher power may be used for takeoff, and in an emergency. As an example, an engine may have a rated power (boost) of +4 psi and a maximum takeoff or emergency of +8 psi boost; takeoff power is usually limited to 5 minutes. As the manifold pressure control prevents rated boost being exceeded, a means of overriding the control is necessary. This is usually achieved by a graduated bleed-off of supercharger delivery pressure from around the aneroid capsule. At full throttle, the bleed off is opened by the throttle linkage, so the capsule is not subjected to full supercharger delivery pressure, and it does not control the throttle to the rated power. This fixed datum system (rated boost) does not affect power selections below rated power. If the pilot elects to climb or descend at any power setting below rated, then constant adjustments of the throttle lever are necessary to maintain the selected power.

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A further disadvantage is that there is lost motion of the throttle lever above rated boost when the control is in operation; this is greatest at sea level and decreases with altitude. It also means throttle lever-operated enrichment devices in the carburettor are inoperative, and only a carburettor with pressure-controlled enrichment devices may be used. These shortcomings may be overcome by the use of a variable datum control.

Supercharger control system

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Variable datum control The variable datum control controls the supercharger outlet pressure or manifold pressure at any pressure (power) selected by the throttle lever. The diagram below shows a variable datum control unit. In this case, the aneroid capsule and piston valve assembly is repositioned by a cam operated by the throttle linkage so that a new datum is established for each throttle position. Thus, the boost pressure is determined by the throttle lever position, and the variable datum control maintains that boost setting with changes in altitude and forward speed. Consider a situation where the pilot has selected 0 psi boost, and 0 psi boost has been obtained. The piston valve is in the neutral position. The pilot now advances the throttle lever to select +2 psi boost. The cam attached to the throttle linkage rotates, causing the capsule and the piston valve to move down a certain amount, and pressure oil raises the servo piston. As soon as +2 psi boost is obtained the pressure around the capsule is 16.7 psi, which is sufficient to contract the capsule to the neutral position. If the pressure rises above +2 psi boost, the capsule contracts, allowing oil to the top throttle valve. Supercharger inlet and outlet pressures are reduced. This reduction of outlet pressure is felt on the capsule, which returns the piston valve to the neutral position. If the supercharger outlet pressure fails, say due to an increase in altitude, then the capsule expands. The piston valve lowers, and pressure oil goes to the underside of the servo, which opens the throttle valve until the +2-psi boost is restored. The piston valve then returns to the neutral position.

The pilot now advances the throttle to obtain +3 psi boost. The cam moves the aneroid and piston valve assembly down, thus re-establishing the datum at which the controller operates. As the throttle lever is closed the cam relieves the pressure on the aneroid assembly, and the spring returns the assembly to a lower datum. Thus, the variable datum control controls the manifold pressure or boost pressure to whatever is selected by the operation of the throttle lever without any lost movement and without the need to adjust the throttle for changes in altitude up to full throttle height. Boost reversal There is, however, one problem when the engine is in the idle or slow running range. When the cylinder valves are in the valve overlap condition, if the induction pressure is low and the outside pressure is relatively high, then the exhaust gases flow in the wrong direction, i.e. from the exhaust to the induction manifold. There is now a rise of pressure in the induction manifold; this causes the aneroid capsule to close the throttle, and acceleration of the engine is prevented. To overcome this problem, the cam is so shaped that in the idle range the throttle lever has direct control over the throttle valve and the variable datum control is inoperative. Supercharger drives A supercharger is usually driven by a splined drive from the crankshaft through a gear train to increase its speed relative to engine speed. The gear train may incorporate some form of spring drive or a centrifugal friction clutch or a combination of the two.

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Spring drive A spring drive may be a driveshaft designed to twist and absorb the shocks, or a more complex spring drive unit fitted to overcome the effects of power pulses from the engine.

The centrifugal friction clutch Fitted to relieve excessive loading on the driving gears caused by sudden engine acceleration and deceleration, it also reduces the load on the engine during start-up.

The diagram below shows an example of a spring drive unit. The unit is splined on to a drive shaft from the crankshaft. The inner member has several fingers extending from it, and the outer member also has the same number of fingers extending inwards and located in the slots of the inner member. Two steel blocks are contoured to fit between each pair of the drive’s fingers and have extension pieces which locate in the centre of the spring. The outer member has driving gears on its periphery.

It consists of a driven inner member with several wedgeshaped fingers extending from it. Between each of these fingers are similar wedge-shaped pieces which are free to move radially.

A sudden increase of rotational speed by the inner driving member causes the springs to contract, the inner member still drives the outer member, but the springs absorb the initial kick. When the kick load is relieved, the springs re-start themselves. A sudden decrease in rotational speed causes the outer member fingers to contract the springs, thus relieving the torsional shock load. Thus, the spring drive damps out torsional fluctuations of the drive to the supercharger.

The outer member, which has driven gears machined on to its periphery, is not directly driven by the inner member. On startup and low rotational speed, the inner member rotates taking the free wedge pieces with it. The outer member does not move, and therefore the supercharger does not rotate. With an increase of rotational speed by the inner member, centrifugal force causes the free wedges to be flung out until they contact the inner surface of the outer member causing the supercharger to rotate. During sudden deceleration, the inertia in the supercharger going at about 6 times engine speed tries to turn the engine. However, the free wedges move towards the centre, and then the engine slows down, leaving the supercharger un-driven to slow down of its own accord. On acceleration, if the load is too high, the free wedges slip on their friction drive to the outer member and only drive the supercharger as engine speed increases. Thus, the centrifugal clutch relieves excessive loading on the supercharger driving gear train during acceleration and deceleration.

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Spring drive unit

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Turbochargers General Turbochargers are divided into two groups: •

•

Altitude turbochargers – which are designed to maintain sea-level air pressure to a given altitude, the engine requires no strengthening as it is only ever subjected to sea level air pressure (14.7 psi). Ground boosted turbochargers – designed to give an induction pressure higher than sea level air pressure at all attitudes. Engines with ground boosted turbochargers have to be strengthened or be already strong enough to withstand the higher stresses resulting from higher combustion pressures.

One side of the ‘Y’ pipe going direct to atmosphere the other side going through the turbine. A selection flap known as the wastegate is fitted in the line to the atmosphere. The wastegate determines which route the exhaust gases take, i.e. to the atmosphere or the turbine. This wastegate can be selected partially open or closed so that the amount of exhaust gas passing through the turbine can be controlled to all or nothing, or anywhere between. The movement of the wastegate is affected by a linkage from an actuator, which is a piston and spring assembly.

The turbocharger is an externally driven form of the supercharger and is not driven mechanically by the engine. Mounted in the induction system is a small centrifugal type impeller which increases the airflow to the cylinders, thus providing a greater charge and increasing the volumetric efficiency; thus, the engine develops more power. The impeller is mounted on a common shaft with a small turbine which is located in the exhaust system. As the exhaust gases flow through the turbine, the turbine rotates the joint shaft and thus the impeller. As engine speed increases, so the exhaust gas flow increases, which speeds up the turbine. In turn, this drives the impeller causing an increase in the airflow to the cylinder. The speed of the turbine is controlled by controlling the exhaust flow. The exhaust system is divided by a ‘Y’ section pipe.

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The wastegate For various reasons, the turbocharger does not need to run at full power all the time the engine is running. A method of controlling the turbocharger is affected using a wastegate. The exhaust manifold is divided by use of a ‘Y’ shaped pipe. In one leg of the ‘Y’ pipe, the gases go through the turbine and operate the turbocharger; the other leg of the ‘Y’ pipe goes directly to the atmosphere, and in this leg, there is fitted a flap-type valve known as the wastegate.

The pressure to the piston is varied by bleeding off the operating pressure oil back to the engine sump. The amount bled back to the engine is determined using a controller. There are various types of controller employed. In its simplest form, some turbochargers have an absolute pressure controller which is designed to limit the maximum pressure from the turbocharger outlet.

With the wastegate open all exhaust gases pass to the atmosphere without going through the turbine, whereas with the wastegate closed all exhaust gases pass through the turbine. At any intermediate position of the wastegate, a relative proportion of the exhaust gases pass through both the turbine and the wastegate.

Intercooler As you compress air, it heats up. This is one of the most significant disadvantages for any turbocharger. Aircraft engines already operate at hot temperatures, and hot intake air makes them even worse. Many turbochargers use something called an intercooler to overcome the problem.

The wastegate actuator The wastegate is set between shut and open by linkage from the wastegate actuator. The actuator consists of a piston and rod connected to the operating linkage, a spring above the piston loads the linkage to the wastegate open position, that is, no turbo operation.

An intercooler is a mini air-conditioner that is placed between the turbocharger and the engine. As the hot air moves from the turbo to the engine, it passes through the intercooler, and the temperature drops significantly. The cooler air makes the engine run more smoothly.

The wastegate is closed by engine oil under pressure from the engine lubrication system passing through a restrictor to the underside of the actuator piston. This pressure oil forces the actuator piston to move to overcome the spring pressure, thus closing the wastegate and bringing the turbo into operation. The amount the wastegate closes is dependent upon the pressure of oil to the underside of the piston.

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Turbocharger system schematic - layout and operation

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Operation of a waste gate

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Wastegate and wastegate actuator

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Wastegate and wastegate actuator

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Construction A turbocharger consists of a compressor wheel and an exhaust gas turbine wheel, coupled together by a solid shaft. It is used to boost the intake air pressure of an internal combustion engine. The exhaust gas turbine extracts energy from the exhaust gas and uses it to drive the compressor. In most applications, both the compressor and turbine wheel are of the radial flow type. The flow of gases through a typical turbocharger with a radial flow compressor and turbine wheels is shown below. Centre-housing The turbine-compressor common shaft is supported by a bearing system in the centre housing (bearing housing) located between the compressor and turbine. The centre housing is commonly aluminium alloy. Seals help keep oil from passing through to the compressor and turbine. Turbocharger rotors specifically have unique characteristics due to the dynamics of having a heavy turbine and compressor wheel located at the overhang ends of the rotor. The majority of turbocharger rotors are supported within a couple of floatingring oil film bearings. In general, these bearings provide the damping necessary to support the high gyroscopic moments of the impeller wheels.

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Turbocharger bearing assembly

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Turbocharger bearing assembly detail

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Turbocharger internal – sectional view

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Turbocharger internal – sectional view

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Operation Turbocharger compressors are generally centrifugal compressors consisting of three essential components: compressor wheel, diffuser, and housing. With the rotational speed of the wheel, air is drawn in axially, accelerated to high velocity and then expelled in a radial direction. The diffuser slows down the high-velocity air, largely without losses, so that both pressure and temperature rise. The diffuser is formed by the compressor backplate and a part of the volute housing, which in its turn collects the air and slows it down further before it reaches the compressor exit.

Choke line The maximum centrifugal compressor volume flow rate is normally limited by the cross-section at the compressor inlet. When the flow at the wheel inlet reaches sonic velocity, no further flow rate increase is possible. The choke line can be recognised by the steeply descending speed lines at the right on the compressor map.

Operating characteristics The compressor operating behaviour is generally defined by maps showing the relationship between pressure ratio and volume or mass flow rate. The useable section of the map relating to centrifugal compressors is limited by the surge and choke lines and the maximum permissible compressor speed. Surge line The map width is limited on the left by the surge line. This is basically "stalling" of the air flow at the compressor inlet. With too small a volume flow and too high a pressure ratio, the flow can no longer adhere to the suction side of the blades, with the result that the discharge process is interrupted. The air flow through the compressor is reversed until a stable pressure ratio with positive volume flow rate is reached, the pressure builds up again and the cycle repeats. This flow instability continues at a fixed frequency and the resultant noise is known as "surging".

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Turbocharger operating map

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The absolute pressure controller This controller limits turbocharger output to a specified maximum pressure. At low power settings, the controller has no effect, and all the oil pressure available is used to close the wastegate, thus causing maximum exhaust flow through the turbine. With an increase in throttle settings, more exhaust gases pass through the turbine, increasing the impeller speed and consequently the pressure in the induction manifold. When the predetermined induction pressure is reached, the controller bleeds off engine oil pressure from the underside of the wastegate actuator piston, see the diagram below left. In turn, this partially opens the wastegate, reducing the exhaust flow through the turbine. Thus, the impeller speed is reduced to give the predetermined maximum pressure. The airflow that runs from the turbo-compressor outlet to the inlet of the throttle plate is referred to in short terminology as “Upper Deck Pressure.” All airflow after the throttle plate is referred to as manifold pressure. The operation of the absolute pressure controller is as follows. An aneroid capsule is subject to turbocharger outlet pressure, attached to the capsule is a needle-type bled valve.

The bleed valve is in the oil pressure line downstream of the wastegate actuator. When the predetermined maximum turbo pressure is reached, the aneroid capsule contracts lifting the bleed valve off its seat which allows the engine-pressure oil to bleed back to the engine sump, reducing the oil pressure on the underside of the wastegate actuator piston. The spring pressure causes the piston to move down to open the wastegate, so reducing the exhaust gas flow through the turbine and thus reducing the impeller output and consequently the induction pressure. At low altitude, the maximum pressure is reached very quickly. At high-pressure settings, the wastegate is almost fully open, but as the aircraft climbs and ambient pressure decreases, it will progressively close to maintain pressure. At a certain altitude, the wastegate fully closes, giving maximum turbocharger output. This is known as the critical altitude, which is the equivalent of rated altitude for an internally driven supercharger. Above this critical altitude the turbocharger outlet pressure, and thus engine power, decreases even though the turbocharger is going at maximum speed. There is a further loss of power at altitude due to the increased temperature, which results from the increased impeller speed; the temperature rise causes the air to expand and become less dense. Oil temperature and cylinder head temperature also rise as a result of higher combustion temperatures. As the absolute pressure controller only works for a fixed, predetermined maximum pressure, some systems use a variable pressure controller which sets a maximum pressure for each throttle lever power selection.

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Absolute pressure controller installed on a throttle body

Absolute pressure controller

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The variable pressure controller The variable pressure controller (also known as the variable absolute pressure controller) has a variable datum which controls the bleed valve to give a manifold pressure related to the power selected by the throttle lever. This is achieved by having the bleed valve seat position adjusted by a cam, which is controlled by the throttle linkage. The aneroid capsule operated needle-type bleed valve operates similar to that in the absolute pressure controller, to bleed off wastegate actuator oil pressure for a given induction manifold pressure. Instead of being in a fixed position, the valve seat is adjusted by a cam, operated by the throttle linkage. At small power settings, the seat or datum is held away from the bleed valve by a spring allowing a larger bleed of actuator oil. This bleed remains constant for that throttle setting until the predetermined induction manifold pressure is reached. Then, the aneroid capsule contracts, moving the bleed valve to increase the bleed, thus reducing actuator pressure and opening the wastegate to reduce output from the turbocharger. When the throttle selects higher power settings, the cam rotates and forces the valve seat closer to the needle valve, thus reducing the bleed so resetting the datum. The aneroid pressure lifts the bleed valve off its seat at a given pressure, but as the seat is now temporarily fixed in its new position, this occurs at a higher manifold pressure. So, although the bleed valve responds to delivery pressure, the original bleed valve that it alters is determined by the position of the valve seat which in turn is positioned by the cam. The spring ensures the seat moves away from the valve, when the cam is off load, at low power selections.

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Variable pressure controller Variable absolute pressure controller

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The dual unit control system Some grounded posted turbochargers have a dual unit control system fitted. The unit consists of: • •

a density controller; and a differential pressure controller.

The density controller is designed to prevent the turbocharger form exceeding its maximum limiting pressure and only operates at full throttle up to the critical altitude. The differential pressure controller controls the induction manifold pressure by controlling the wastegate at all other throttle positions. A typical dual unit control system is illustrated in the diagram below. The density controller The density controller shown on the right of the diagram maintains a constant density at full throttle and also compensates for temperature and pressure. The capsule is filled with dry nitrogen which is sensitive to both pressure and temperature. At a predetermined density (temperature and pressure) the capsule, the outside of which is subject to turbocharger outlet pressure, contracts, withdrawing the bleed valve from its seat. Oil from the wastegate actuator can now bleed away, allowing the wastegate to open and diverting the exhaust gate flow to the atmosphere until the turbo outlet pressure drops. When the turbo outlet pressure drops, the capsule in the density controller expands, closing off the bleed valve. The capsule is pre-set so that only the density of the turbo outlet at full throttle affects it. There are two springs, one around the capsule holding the bleed valve on its seat until the capsule contracts compressing the spring, allowing the other spring to open the bleed valve. Total Training Support Ltd © Copyright 2020

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Dual unit control system

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The sloped controller The sloped controller (or ‘slope’ controller) is designed to maintain the rated compressor discharge pressure at wideopen throttle and to reduce this pressure at part throttle settings. A diaphragm, coupled with a spring-supported bellows for absolute pressure reference, is exposed to deck pressure and intake manifold pressure through ports located before and after the throttle, respectively. This arrangement continuously monitors deck pressure and the pressure differential between the deck and manifold pressure due to the throttle being partially closed. If either the deck pressure or throttle differential pressure rises, the controller poppet opens and decreases turbocharger discharge (deck) pressure. The sloped controller is more sensitive to the throttle differential pressure than to deck pressure, thereby accomplishing deck pressure reduction as the throttle is closed.

The differential pressure controller The differential pressure controller has a bleed valve attached to a diaphragm which is spring assisted to the valve closed position. The diaphragm is subjected to turbo outlet pressure on the top and inlet manifold pressure on the bottom, so the diaphragm is responsive to the pressure drop across the throttle valve. At full throttle the pressure drop is at its least, the bleed valve is closed, and the density controller will control (remember the throttle valve is downstream of the impeller). As the throttle is closed less turbocharger output is required; the pressure drop across the throttle increases as the throttle is closed. The downstream throttle pressure, the inlet manifold pressure, decreases and the upstream throttle pressure, which is turbo outlet pressure, is relatively higher. These two pressures acting on the diaphragm cause the bleed valve to open, bleeding off wastegate actuator pressure. The wastegate opens to reduce the turbo outlet pressure to suit the new throttle position. As the throttle is closed, the wastegate opens to reduce the turbo pressure to the power selected. As the throttle is opened, the reverse applies. Small changes in temperature or engine speed may cause some systems to hunt while they try to establish the correct bleed setting. However, the differential pressure controller gives immediate compensation which reduces the effects of small power changes. This hunting is sometimes known as bootstrapping. On yet another ground boosted turbochargers, a triple unit control system is used.

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Sloped controller

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Turbocharged engine

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The triple unit control system This consists of three separate controllers: • • •

an absolute pressure controller, which limits the maximum turbo outlet pressure up to critical altitude; a pressure ratio controller for operation above critical altitude; and a rate controller which controls the rate of increase of turbo outlet pressure during acceleration.

The absolute pressure controller The absolute pressure controller controls the maximum turbo outlet pressure up to the critical altitude; above the critical altitude it is necessary to limit turbo outlet pressure below that of the absolute pressure controller. This is because if the turbo is going flat out trying to maintain the maximum pressure allowed by this controller, the rise in temperature of the air generated by the turbo would result in detonation. Limits on the manifold air pressure are imposed on turbocharged engines above the critical altitude, or they may be done automatically by the pressure ratio controller, as shown in the diagram below. The pressure ratio controller The pressure ratio controller controls turbo outlet maximum pressure above the critical altitude. It consists of an aneroid capsule subjected on the outside to turbo outlet pressure. It is connected to a diaphragm in the upper section of the unit, in the lower section is a bleed valve held on its seat by a spring, and the lower section is open to ambient air. Above the critical altitude, the ambient air pressure tends to flex the diaphragm down, and in the aneroid chamber, the pressure is reduced. This allows the capsule to expand, first touching the end of the bleed valve and then progressively opening the bleed valve, reducing the turbo speed and therefore the turbo pressure. Total Training Support Ltd © Copyright 2020

The higher the aircraft goes above critical altitude, the more the bleed valve opens, and the turbo outlet pressure is thus progressively reduced to keep the inlet manifold temperature below that which may cause detonation. Turbo outlet pressure is thus kept at a set ratio to atmospheric about 2.2:1. The rate controller The rate controller controls the rate at which turbo outlet pressure rises during the engine acceleration, thus preventing over boosting when the throttle is opened. The unit has a bleed valve attached to a diaphragm which is spring assisted to the closed position. Both sides of the diaphragm are subjected to turbo outlet pressure. The lower side has a restrictor in the line so that if the turbo pressure outlet rises too fast, the unrestricted pressure on the top of the diaphragm will increase at a faster rate than the pressure below the diaphragm, because of the restrictor. This results in the diaphragm flexing down, opening the bleed valve to bleed pressure oil for the wastegate actuator. The wastegate opens, causing a reduction turbo speed and outlet pressure. Eventually, the reduced pressure plus spring pressure equals the unrestricted pressure, and the bleed valve closes, allowing the turbo outlet pressure to rise. The rate of turbocharger outlet pressure is now controlled regardless of the rate of acceleration of the engine, so the rate controller is in effect a turbo control.

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Triple unit control system

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Glossary of supercharger and turbocharger terms A Aftercharger

B A radiator placed between the turbocompressor outlet and the engine intake manifold used to cool the intake charge, which is heated by the pressurization of the turbo (also called an intercooler or charge air cooler).

Airflow (cfm)

A measurement of how much air/exhaust is able to flow through the turbo. Airflow is measured in cubic feet per minute.

Area/radius (A/R)

The ratio of the cross-sectional area of the exhaust-turbine inlet/compressor outlet divided by the radius from the centre of the turbo wheel to the centre of the crosssection (right). Differences in compressor A/R do not affect performance very much, but a large turbine A/R will allow big power gains at high engine rpm-this will cause turbo lag at low speeds.

Backplate

Located behind the compressor wheel, this supports the compressor housing, attaches to the turbo centresection, and routes air into the compressor housing.

Backpressure

A build-up of pressure in the exhaust that prevents the free flow of new exhaust gases and slows the speed of the turbo wheel. Backpressure that builds after the compressor in the intake can cause the wheel to suddenly stop spinning (surge).

Blow-off valve

A valve between the turbo and the intake manifold that vents air to avoid turbo surge when a pre-set pressure limit (boost) is surpassed.

Boost

The intake pressure created by the spinning of the compressor wheel inside the housing. Measured in pounds per square inch over the normal atmospheric pressure (14.7:1).

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Boost controller

A mechanical or electrical device that alters the boost-pressure signal sent to actuate the wastegate, allowing higher boost pressures than would normally be permitted.

Boost creep

When the boost rises past the set limit. This is often caused by a wastegate that cannot handle the exhaust flow.

Boost spike

A period of uncontrolled boost when the wastegate and/or blow-off valve cannot act fast enough because of sudden changes in the engine load.

Boost threshold

When engine conditions provide enough exhaust pressure to create boost in the intake manifold.

Boreless turbo

A turbo that uses a compressor wheel that does not have a hole drilled through it. This design increases the strength of the compressor wheel in its highest stress area.

Centre-section

The housing between the exhaust and intake sides of the turbo that houses the turbo shaft and contains the bearings, oiling system, and water-cooling system.

Charge air cooler

A radiator placed between the turbo compressor outlet and the engine intake manifold used to cool the intake charge, which is heated by the compression of the turbo (also called an intercooler or aftercooler).

Choke line

The boundary on the righthand side of a compressor map that indicates the rpm where the turbo efficiency quickly drops. If this happens at low rpm, a larger turbo is needed.

Compound turbos

Two or more turbos that feed into each other in series to build high boost pressures.

Compressor wheel

The fan blades that suck in the intake air and compress it against the compressor housing and backplate.

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C Cool down

E Running the engine until the exhaust temperature is low (less than 300 degrees F) so oil in the centre-section does not overheat when the engine is shut down and the oil flow stops.

EGT

Where flow exits the turbine or compressor wheel.

Exhaust manifold

A collector on the engine head(s) that routes exhaust gases to the turbo.

External wastgate

A wastegate that is not built in to the turbo.

None

E EGR

Exducer

Exhaust-gas recirculation routes some exhaust gas back into the intake manifold after it passes through a water-cooled heat exchanger. EGR reduces emissions of nitrogen oxides.

H Heat soak:

Exhaust-gas temperature that should be monitored to prevent the turbo from overheating, which can lead to failure (1,250 degrees F maximum for extended periods).

When heat from the turbine housing is transferred to the compressor side of the turbo.

I Inducer:

Where flow enters the turbine or compressor wheel.

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I Intake manifold

Intercooler

Internal wastegate

L Routes airflow from the intercooler line into the intake ports in the head(s) of the engine.

Air-to-air or air-to-water radiators used between the turbo and the intake manifold to reduce intake temperature, which is heated by the pressurization inside the turbo (also called aftercharger or charge air cooler). A built-in valve that diverts exhaust gases away from the turbine when a certain boost level is reached on the compressor side of the turbo. Can be mechanically or electrically controlled.

Mass flow rate

A representation of how much air (based on density) is being output by the turbo for use by the engine.

None

O Oil restrictor

A device that reduces the amount of oil delivered to a ball-bearing turbo because it requires less oil pressure than a stock journal-bearing unit. Pressure ratio: The absolute compressor outlet divided by the absolute inlet pressure and represented on the left side of a compressor map.

Oil supply/return lines

Routes engine oil to and from the turbo to lubricate the bearings in the centre-section.

J Journal bearings

None

Hollow brass sleeves suspended in oil that allow the turbo shaft to spin freely inside the centre-section.

None

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None S

None

Shaft

The piece that travels through the centre-section and connects the turbine wheel to the compressor wheel.

Shaft play

Condition in a worn turbo where the shaft is allowed to move (other than spin). Signs of play indicate the turbo is ready to fail by allowing the fan blades to contact the housings or other problems. An abnormal whine or scraping sound can be a sign that a turbo is suffering from shaft play.

Spool

Another term for turbo boost. A turbo is spooled up when it is creating boost in the intake manifold.

Surge

When boost pressure builds up to the point that it causes the compressor wheel to stall. This can be prevented with the use of bypass valves. Also known as bark.

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T Trim

The ratio of the inducer area divided by the exducer area.

Turbine wheel

A wheel that is spun by exhaust gases that pass through the fins and into the housing before dumping into the exhaust pipe.

Turbo lag:

The time it takes for a turbo to spool up after the throttle has been increased.

Turbo speed lines

Lines on a compressor map that represent the rotational speed of the compressor wheel.

Turbo timer

An electronic device that keeps an engine running before shutdown to ensure EGT is low enough to prevent oil from cooking in the centre-section.

Twin turbos

None

Variable-geometry turbo (VGT)

A turbo that uses variable vanes or a sliding nozzle to alter the volume inside the exhaust housing to maximize turbo speed at low engine rpm. Also known as variable-turbine geometry (VTG).

Variable nozzle

A sliding nozzle in the exhaust side of the turbo that can reduce the volume around the turbine fan blades to increase turbo rpm at low engine loads.

Variable vanes

Adjustable blades that route exhaust gases directly into the turbine wheel at low engine rpm to increase spool on tap at low speeds.

A system using two turbos mounted in parallel. Twin turbocharged engine

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W Wastegate

Bypass that diverts excess exhaust gases away from the turbine once a pre-set boost level is reached in the compressor side of the turbo. It can be built into the exhaust-turbine housing (internal) or can be separate from the turbo housing (external).

Water-cooled turbo A turbo assembly that incorporates channels in the centre-section to circulate engine coolant to keep the bearing assembly and related parts cool and prevent heat-soak from the exhaust turbine and housing.

None

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Licence Category B1 and B3

16.8 Lubricants and Fuels

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Properties and specifications;

Part-66 Ref. 16.8

Knowledge Levels A B1 B3 1 2 2

Fuel additives; Safety precautions

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Table of Contents Lubricants ______________________________________ 6 Purpose ______________________________________ 6 Types of lubrication ____________________________ Boundary lubrication ___________________________ Hydrodynamic lubrication _______________________ Elastohydrodynamic lubrication __________________

6 8 8 8

Properties and specifications ___________________ General ____________________________________ Viscosity and viscosity index ____________________ Flashpoint and fire point _______________________ Cloud point and pour point _____________________ Specific gravity ______________________________

10 10 12 14 14 14

Additives ____________________________________ Extreme pressure additive _____________________ Detergent additives ___________________________ Use of automotive engine oil ____________________

16 16 16 16

Engine break-in _______________________________ 18 Oil and filter changes __________________________ Protective coatings ___________________________ Moisture formation ___________________________ Acid formation _______________________________ Geographical operation ________________________

20 20 20 20 20

Diesel engine lubricants _______________________ 22

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Fuels _________________________________________ 24 Safety precautions ____________________________ 52 General ____________________________________ 52 Electrical bonding_____________________________ 52 Misfuelling __________________________________ 52 PED use during refuelling ______________________ 54 Refuelling with passengers on board ______________ 54 Defueling ___________________________________ 54

General _____________________________________ 24 Properties and specifications ___________________ Cloud point _________________________________ Cetane number (Diesel) _______________________ Octane rating (gasoline) _______________________ Fuel grade (gasoline) _________________________ Lubricity____________________________________ Flashpoint __________________________________ Release of energy ____________________________ Heat energy content __________________________ Vapour pressure _____________________________ Critical pressure and temperature ________________ Additives ___________________________________

24 24 24 26 26 26 26 28 30 30 30 32

Avgas _______________________________________ General ____________________________________ History _____________________________________ Avgas 100 __________________________________ Avgas 100LL ________________________________ Avgas 82 UL ________________________________ Avgas density _______________________________

34 34 34 36 36 36 36

The future of general aviation fuels ______________ 37 Mogas ______________________________________ General ____________________________________ Adverse effects ______________________________ Testing for alcohol in Mogas ____________________

39 39 39 39

Diesel and Jet fuel ____________________________ General ____________________________________ Diesel fuel or Jet fuel?_________________________ Cold weather operation ________________________ Diesel pollutants _____________________________

40 40 42 46 48

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Lubricants Purpose •

The primary purpose of a lubricant is to reduce friction between moving parts. Because liquid lubricants or oils can be circulated readily, they are used universally in aircraft engines. In theory, fluid lubrication is based on the actual separation of the surfaces so that no metal-to-metal contact occurs. As long as the oil film remains unbroken, metallic friction is replaced by the internal fluid friction of the lubricant. Under ideal conditions, friction and wear are held to a minimum. Oil is generally pumped throughout the engine to all areas that require lubrication. Overcoming the friction of the moving parts of the engine consumes energy and creates unwanted heat. The reduction of friction during engine operation increases the overall potential power output. Engines are subjected to several types of friction.

•

If we want to reduce friction, we need to change or remove the factors which may harm the surfaces in motion. There are several ways to do just that. If there is sliding friction, rolling element like a ball or needle bearing elements are used. The use of sacrificial surfaces can be used to, such as lead/copper journal bearings. Last but not least, the changing of viscosity, different or improved additives or even changing from oil to grease can reduce friction. In the small area where the sliding or rolling surfaces are lubricated, this happens in one of three modes of lubrication:

Types of lubrication The amount of friction between two parts depends on several factors: • •

• •

• • •

the temperature, either ambient or in the engine itself affects friction; the surface finish, the better the surface is machined or polished, the lower the coefficient friction the surfaces have; the load, the higher the load on a surface the more friction there is; the speed of movement, the increase of speed of sliding surface will increase the friction;

the nature of the movement, sliding or rolling motion have different friction characteristics; and the type of lubricant, the type of oil and its characteristics also affect friction (viscosity).

boundary lubrication; hydrodynamic lubrication (HDL); and elastohydrodynamic lubrication, (EHL)

We will discuss each of these.

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Engine oil

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Boundary lubrication Boundary lubrication occurs when an engine is started, at low speed or even in high load conditions. At this time, the two moving (rolling or sliding) surfaces may come into real contact and damage could result. Some specialists say that 70% of all wear in an engine occurs in this regime.

This is when a full film of oil has separated an engine shaft (crankshaft or camshaft) from its support, and no contact exists between the parts. The oil is keeping the shaft and bearing apart by its viscosity. Also, during hydrodynamic lubrication, there is no friction except in the lubricant itself where molecular structures shear during operation.

To make sure that no damage is done during these regimes, it is reasonable to use a lubricant which is formulated with antiwear or even extreme pressure additives. These additives react with the surfaces in contact due to the high pressure and temperature, forming a chemical film on those surfaces. This film is then sacrificed as the surfaces come into contact so that the film wears off and not the metal surface.

Hydrodynamic lubrication requires that the machined surfaces have a high degree of geometric conformity and relatively low pressure. This situation can be found between rotating crank or camshafts and the journal or sleeve bearings. Once the engine is at operating temperature and shafts are at average engine speeds it should be possible to remain in the hydrodynamic regime forever so that friction is at minimum.

By increasing the viscosity of the lubricant, i.e. increasing its thickness, can reduce boundary friction in some situations. However, care must be taken not to increase viscosity too much as the internal friction of the lubricant increases too and can give rise to higher temperatures.

Elastohydrodynamic lubrication This type of lubrication occurs where surfaces have a low degree of conformity combined with high contact pressures as found in gear drives and rolling bearing elements. The moving surfaces catch the lubricants. Under high pressure, the viscosity increases to such a high level that it forms a semisolid film separating the two moving surfaces.

Hydrodynamic lubrication This is when a full film of oil has separated an engine shaft (crankshaft or camshaft) from its support, and no contact exists between the parts. The oil is keeping the shaft and bearing apart by its viscosity. Also, during hydrodynamic lubrication, there is no friction except in the lubricant itself where molecular structures shear during operation.

As long as these conditions do not change the metal surfaces do not come into contact. These surfaces may deform way before the semi-solid oil or grease film breaks, due to this remarkable property of the lubricant.

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Full fluid film (hydrodynamic) lubrication

Boundary lubrication

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Properties and specifications General While there are several important properties that satisfactory reciprocating engine oil must possess, its viscosity is most important in engine operation. The resistance of an oil to flow is known as its viscosity. Oil that flows slowly is viscous or has a high viscosity; if it flows freely, it has a low viscosity. Unfortunately, the viscosity of an oil is affected by temperature. It was not uncommon for earlier grades of oil to become practically solid in cold weather, increasing drag and making circulation almost impossible. Other oils may become so thin at high temperatures that the oil film is broken, causing a low loadcarrying ability, resulting in rapid wear of the moving parts.

The oil used in aircraft reciprocating engines has a relatively high viscosity required by: •

• •

large engine operating clearances due to the relatively large size of the moving parts, the different materials used, and the different rates of expansion of the various materials; high operating temperatures; and high bearing pressures.

The oil selected for aircraft engine lubrication must be light enough to circulate freely at cold temperatures, yet heavy enough to provide the proper oil film at engine operating temperatures. Since lubricants vary in properties and since no one oil is satisfactory for all engines and all operating conditions, it is imperative that only the approved grade or Society of Automotive Engineers (SAE) rating is used. Several factors must be considered in determining the proper grade of oil to use in a particular engine, the most important of which are the operating load, rotational speeds, and operating temperatures. The grade of the lubricating oil to be used is determined by the operating conditions to be met in the various types of engines.

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Choosing the correct oil

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Viscosity and viscosity index The viscosity of any lubrication oil is its most important characteristic. The resistance of an oil to flow is known as its viscosity. Viscosity can be defined as the force required to overcome the resistance of the oil to deformation or sheer. It is commonly known as the thickness or ‘body’ of the oil. • •

A thin oil which runs freely has a low viscosity. A thick oil that is difficult to pour has a high viscosity.

Typically, aviation grades are; SAE 30/65 weight; SAE 40/80 weight; SAE 50/100 weight; SAE 60/120 weight. The manufacturer, when choosing an oil to use as a lubricant, requires oil that adheres to the metals within the engine, to prevent metal to metal contact of the moving parts. Equally as important is that the oil does not produce an oil drag problem, and so reduce the efficiency of the engine while it is moving. Determining the oil viscosity The viscosity of an oil and how it is determined has to be universally adopted. A universal viscosimeter is used to achieve this. This instrument determines the viscosity of an oil by heating the oil to a predetermined temperature in a tube. The time in seconds that the heated oil takes to pass through a calibrated jet or orifice is recorded as a measurement of the oil’s viscosity. Although by this method we could end up with many values, a simplified selection of values is classified under an SAE system, where the oils are graded into seven groups (SAE 10 to 70) according to the viscosity at either 130°F or 210°F. Therefore, a typical example of an SAE rating for lubricating oil systems for a reciprocating engine would read like this. Total Training Support Ltd © Copyright 2020

“On the ………… engine, SAE 50 oil is to be used when the temperature is above 40°F, and SAE 30 oil below”. In other words, if it is required to operate at high temperatures, a higher-viscosity oil is required, and if low temperatures are to be expected, a low viscosity oil should be used. The effect of temperature Lubricating oils react in different ways when heated; some oils exhibit little change in viscosity, whereas others show a considerable change. Good quality oils tend to exhibit a smaller change in viscosity than others when heated. It is particularly important to select the correct grade of oil for use in a lubrication system if a change in temperature is likely to occur, because of the resulting change in viscosity that can take place. For this reason, the operation temperature of the system must be known before the selection can be made. A thick oil, with a high viscosity, protects heavily loaded parts once it is circulated throughout the system. However, when it is cold, such oil does not flow quickly, causing wear to take due to oil starvation. Alternatively, a thin oil with low viscosity flows quickly to all parts of the system and give proper initial lubrication to the parts when the oil is cold. Such an oil, however, does not have the firm strength that is necessary to protect heavily loaded surfaces or bearings when the oil is hot. Manufacturers now produce lubricating oils with a multigrade rating. This grade rating produces a grade of oil that is compatible for operations between a series of temperature ranges. When using such lubricating oils, the engine manufacturer’s instructions must be strictly adhered to when selecting the correct grade of oil.

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Generally, commercial aviation oils are classified by a number, (such as 80, 100, 140, etc.) that approximates the viscosity as measured by a testing instrument called the Saybolt Universal Viscosimeter. In this instrument, a tube holds a specific quantity of the oil to be tested. The oil is brought to an exact temperature by a liquid bath surrounding the tube. The time in seconds required for exactly 60 cubic centimetres of oil to flow through an accurately calibrated orifice is recorded as a measure of the oil’s viscosity. If actual Saybolt values were used to designate the viscosity of the oil, there would probably be several hundred grades of oil.

temperature and, therefore, are subject to classification in the same grade. The SAE letters on an oil container are not an endorsem*nt or recommendation of the oil by the SAE. Although each grade of oil is rated by an SAE number, depending on its specific use, it may be rated with a commercial aviation-grade number or an Army and Navy specification number. The correlation between these grade numbering systems is shown in the table. Commercial aviation number

Commercial SAE number

Army and navy specification number

1065

1080

100

1100

120

1120

To simplify the selection of oils, they are often classified under an SAE system that divides all oils into seven groups (SAE 10 to 70) according to viscosity at either 130°F or 210°F. SAE ratings are purely arbitrary and bear no direct relationship to the Saybolt or other ratings. The letter W occasionally is included in the SAE number giving a designation, such as SAE 20W. This W indicates that the oil is satisfactory for winter use in cold climates, in addition to meeting the viscosity requirements at the testing temperature specifications. This should not be confused with the W used in front of the grade or weight number that indicates the oil is of the ashless dispersant type. Although the SAE scale has eliminated some confusion in the designation of lubricating oils, it must not be assumed that this specification covers all the critical viscosity requirements. An SAE number indicates only the viscosity grade or relative viscosity; it does not indicate the quality or other essential characteristics. It is well known that there are good oils and inferior oils that have the same viscosities at a given Total Training Support Ltd © Copyright 2020

140

70 Grade designations for aviation oils

Viscosity index A viscosity index is a number that indicates the effect of temperature changes on the viscosity of the oil. When the oil has a low viscosity index, it indicates a relatively significant change of viscosity with increased temperature. The oil becomes thin at high temperatures and thick at low temperatures. Oils with a high viscosity index have small changes in viscosity over a wide temperature range.

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The best oil for most purposes is one that maintains a constant viscosity throughout temperature changes. Oil having a high viscosity index resists excessive thickening when the engine is subjected to cold temperatures. This allows for rapid cranking speeds during starting and prompt oil circulation during initial start-up. This oil resists excessive thinning when the engine is at operating temperature and provides full lubrication and bearing load protection. Flashpoint and fire point Flashpoint and fire point are determined by laboratory tests that show the temperature at which a liquid begins to give off ignitable vapours; flash, and the temperature at which there are sufficient vapours to support a flame; fire. These points are established for engine oils to determine that they can withstand the high temperatures encountered in an engine. Cloud point and pour point Cloud point and pour point also help to indicate suitability. The cloud point of an oil is the temperature at which its wax content, generally held in solution, begins to solidify and separate into tiny crystals, causing the oil to appear cloudy or hazy. The pour point of the oil is the lowest temperature at which it flows or can be poured. Specific gravity Specific gravity is a comparison of the weight of the substance to the weight of an equal volume of distilled water at a specified temperature. As an example, water weighs approximately 8 lb to the gallon; oil with a specific gravity of 0.9 would weigh 7.2 lb to the gallon.

In the early years, the performance of aircraft piston engines was such that they could be lubricated satisfactorily using straight mineral oils, blended from specially selected petroleum base stocks. Oil grades 65, 80, 100, and 120 are straight mineral oils blended from selected high-viscosity index-base oils. These oils do not contain any additives except for exceedingly small amounts of pour point depressant, which helps improve fluidity at extremely low temperatures, and an antioxidant. This type of oil is used during the break-in period of a new aviation piston engine or those recently overhauled. Demand for oils with higher degrees of thermal and oxidation stability necessitated fortifying them with the addition of small quantities of nonpetroleum materials. The first additives incorporated in straight mineral piston engine oils were based on the metallic salts of barium and calcium. In most engines, the performance of these oils concerning oxidation and thermal stability was excellent. However, the combustion chambers of the majority of engines could not tolerate the presence of the ash deposits derived from these metal-containing additives. A non-metallic (i.e., non-ash-forming, polymeric) additive was developed that was incorporated in blends of selected mineral oil base stocks, to overcome the disadvantages of harmful combustion chamber deposits. W oils are of the ashless type and are still in use. The ashless dispersant grades contain additives, one of which has a viscosity stabilising effect that removes the tendency of the oil to thin out at high oil temperatures and thicken at low oil temperatures. The additives in these oils extend operating temperature range and improve cold engine starting and lubrication of the engine during the critical warm-up period permitting flight through wider ranges of climatic changes without the necessity of changing oil.

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Semisynthetic multigrade SAE W15 W50 oil for piston engines has been in use for some time. Oils W80, W100, and W120 are ashless dispersant oils specifically developed for aviation piston engines. They combine non-metallic additives with selected high viscosity index base oils to give exceptional stability, dispersancy, and antifoaming performance. Dispersancy is the ability of the oil to hold particles in suspension until they can either be trapped by the filter or drained at the next oil change. The dispersancy additive is not a detergent and does not clean previously formed deposits from the interior of the engine. Some multigrade oil is a blend of synthetic and mineral-based oil. Semisynthetic, plus a highly effective additive package, that is added due to concern that fully synthetic oil may not have the solvency to handle the lead deposits that result from the use of leaded fuel. As multigrade oil, it offers the flexibility to lubricate effectively over a broader range of temperatures than monograde oils. Compared to monograde oil, multigrade oil provides better cold-start protection and a stronger lubricant film (higher viscosity) at typical operating temperatures. The combination of non-metallic, anti-wear additives and selected high viscosity index mineral and synthetic base oils give exceptional stability, dispersancy, and antifoaming performance. Start-up can contribute up to 80 per cent of normal engine wear due to lack of lubrication during the startup cycle. The easier the oil flows to the engine’s components at start-up, the less wear occurs.

The ashless dispersant grades are recommended for aircraft engines subjected to wide variations of ambient temperature, particularly the turbocharged series engines that require oil to activate the various turbo controllers. At temperatures below 20°F, preheating of the engine and oil supply tank is usually required regardless of the type of oil used. Premium, semisynthetic multigrade ashless dispersant oil is a blend of high-quality mineral oil and synthetic hydrocarbons, with an advanced additive package that has been specifically formulated for multigrade applications. The ashless anti-wear additive provides exceptional wear protection for wearing surfaces. Many aircraft manufacturers add approved preservative lubricating oil to protect new engines from rust and corrosion at the time the aircraft leaves the factory. This preservative oil should be removed at the end of the first 25 hours of operation. When adding oil during the period when preservative oil is in the engine, use only aviation-grade straight mineral oil or ashless dispersant oil, as required, of the viscosity desired. If ashless dispersant oil is used in a new engine, or a newly overhauled engine, high oil consumption might be experienced. The additives in some of these ashless dispersant oils may retard the break-in of the piston rings and cylinder walls. The use of mineral oil can avoid this condition until regular oil consumption is obtained, then change to the ashless dispersant oil. Mineral oil should also be used following the replacement of one or more cylinders or until the oil consumption has stabilised. In all cases, refer to the manufacturers’ information when oil type or time in service is being considered.

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Additives Extreme pressure additive Lubricants containing an extreme pressure additive (oil extreme pressure – OEP) are designed to operate under extreme pressure conditions. They appear to work in the same way as a fatty acid, in that they combine chemically with the surface of the bearing metal being lubricated. Extreme pressure additives are used in oils which lubricate heavily loaded gear trains, such as those fitted to helicopter gearboxes or the accessory gearboxes of large aircraft. Ashless Dispersant oils are formulated from base stocks blended with additives designed with a range of objectives which may include enhancing low-temperature fluidity, hightemperature stability, corrosion inhibition and anti-wear protection. The additive system is ashless and of a dispersant nature offering greater engine cleanliness. Ashless means that the product does not contain any metallic components – this is important because it reduces the formation of harmful metallic ash deposits within the engine. Dispersant means it holds small particles in suspension if they do not dissolve, allowing these particles to be carried away from critical areas and filtered out; this helps keep the engine clean. Ashless dispersant pistonengine oils are approved against SAE J1899 specification (superseding MIL-L-22851D).

Detergent additives Detergent additives are added to lubricating oil to keep the internal parts of components and lubrication systems clean. This additive tends to keep the sludge in suspension and reduce the tendency for it to accumulate in piston ring grooves, oil pipes, and other small oil ways. The additive helps to prevent such problems as piston rings sticking in their grooves, and oil starvation due to blocked oil ways. Use of automotive engine oil This is not permitted. Only an aviation piston engine oil should be used to lubricate engines designed for aviation use. Automotive engine oils are usually formulated with detergents and other additives that may contain metals such as zinc. Ash deposits produced from these metal-containing additives tend to form in the combustion chamber where they can cause preignition. This, in turn, can lead to engine failure; hence aviation oils are formulated from metal-free additives.

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Engine oil and anti-corrosion additives

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Engine break-in Most Lycoming engines must be operated on mineral oil during the first 50 hours or until oil consumption has stabilised. LW16702 additive may be used. If an ashless dispersant oil is used in a new engine, or a newly overhauled engine, high oil consumption might be experienced. The additives in some of these ashless dispersant oils may retard the break-in of the piston rings and cylinder walls. The use of mineral oil can avoid this condition until regular oil consumption is obtained, then change to the ashless dispersant oil. Mineral oil must also be used following the replacement of one or more cylinders or until the oil consumption has stabilised. All turbocharged Lycoming engines must be broken in with ashless dispersant oil only. In engines that have been operating on straight mineral oil for several hundred hours, a change to ashless dispersant oil should be made with a degree of caution. The cleaning action of some ashless dispersant oils tends to loosen sludge deposits and cause plugged oil passages. When an engine has been operating on straight mineral oil and is known to be in an excessively dirty condition, the switch to ashless dispersant oil should be deferred until after the engine is overhauled.

When changing from straight mineral oil to ashless dispersant oil, the following precautionary steps should be taken. •

• •

Do not add ashless dispersant oil to straight mineral oil. Drain the straight mineral oil from the engine and fill with ashless dispersant oil. Do not operate the engine longer than five hours before the first oil change. Check all oil filters and screens for evidence of sludge or plugging. Change oil every ten hours if sludge conditions are evident. Repeat ten-hour checks until the clean screen is noted, then change oil at recommended time intervals.

Straight grade oils Straight monograde oils are designed to be used when breaking-in a new or recently overhauled engine. They are formulated from mineral base stocks, typically further enhanced by a low concentration of antioxidant, and pour point depressant for improved low-temperature performance. These oils are also referred to as running-in or break-in oils. They are designed and formulated to provide the correct level of lubricant breakdown and controlled cylinder wear to help lap and seal the piston rings. Straight monograde piston engine oils are approved against SAE J1966 specification (superseded MIL-L6082E). Some engines use these oils beyond break-in, so if in doubt, please refer to your engine manual/manufacturer.

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Oil and filter changes Oil and filter should be changed regularly, but the use of the aircraft (or the lack of) and other factors dictate if the oil must be changed sooner than prescribed by the manufacturer to prevent any corrosion formation in the engine.

Hence the need for multigrade oils in which we have 15W50 and 20W50 which are thinner at lower temperatures facilitating quicker oil pressure but can be thick enough at engine operating temperatures.

The frequency of aircraft/engine use is one of the prime factors in determining how the oil performs and how often it should be changed. Frequently flown aircraft; think one hour a week and regular (50 hours) oil changes makes any oil look good. This behaviour keeps the oil covering all internal parts. There is some debate that oil will ‘runoff’ engine parts after a while, but oil always sticks to metal and keep it covered. This, however, could not be the case of piston oil compression and scraper rings as they are subject to high temperatures and oil does tend to get burned off, although this is only in minute quantities.

Moisture formation Moisture is formed when the engine oil cools, and water condenses. Regular flying with oil temperatures reaching 100°C makes sure that all water is boiled off. Ground running is just not enough; it is too short for all parts to get up to operating temperatures and, in the end, will cause harm. It also increases water formation and corrosive attack.

Engines flown less than 100 hours a year are candidates for corrosion formation Protective coatings Between aircraft use, engine oil should maintain a coating on all internal parts, if not, the surfaces will begin to oxidise within a short period. If left unattended longer, the oxidation will damage the steel parts of the engine. Frequent oil and filter changes are an excellent way to minimise these effects. Thicker oil would help too as it sticks better to the metal. But this has the disadvantage that it takes a couple of seconds for the oil to be up to pressure and reaching all parts moving, especially in winter.

Acid formation Engine combustion by-products are picked up by the oil and form, when mixed with condensation, acids capable of etching into the metals of the engine, resulting in more corrosion. Frequent oil changes help against acid formation even on a four-monthly basis when not frequently flying (time-limited as opposed to hour limited). Geographical operation The location where the aircraft is used or parked, coastal and or high humidity places, contribute to corrosion. As said above, if flying infrequently and you are in said locations, do more oil changes to minimise possible corrosion, and this helps in keeping the engine in good health.

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Diesel engine lubricants The recommended oil for use with most aero Diesel engines is 10W-40, multigrade engine oil specifically designed for use in the new generation of compression ignition (Diesel) aviation piston engines.

Furthermore, not all aero Diesel engine oils are suitable for all aero Diesel engine types. It is therefore essential to use only the type of oil recommended by the aero Diesel engine manufacturer.

The formulation is suitable in piston engines fuelled by Jet A or Jet A-1. Diesel engine oil 10W-40 is a fully synthetic engine oil containing unique additives to provide piston cleanliness, resulting in a clean, efficient and reliable engine. This includes a powerful surface-active additive that bonds to the surface of highly loaded engine parts, protecting the engine from scuffing damage. Aero Diesel oils of this type are optimised to cope with the demands of this type of engine. Its key performance features include the ability to sustain high bearing loads, neutralisation of acid build-up from the sulphur present in the fuel and high dispersancy to allow for the relatively high loading produced when burning Jet fuel. Oils specifically designed for gasoline-burning engines must not be used in Diesel powered aircraft engines. Oils specifically designed for aero Diesel engines must not be used in spark-ignition or gasoline-powered aircraft engines.

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AeroShell Oil Diesel 10W-40 is approved for use in the following engines. • •

AeroShell Oil Diesel Ultra is a fully synthetic, multigrade engine oil designed for use in the new generation of compression ignition (Diesel) aviation piston engines. The formulation is suitable for use in piston engines fuelled by Jet A or Jet A-1 and is designed for use in the latest highly rated turbocharged Diesel engines under all operating conditions. AeroShell Oil Diesel Ultra must not be used in spark ignition, or Avgas powered aircraft engines.

SMA SR135 Thielert 1.7 Centurion

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Fuels General Aircraft commonly use engines which consume Avgas or engines running Mogas. Some engines are capable or modified, to run either fuel; although some with restrictions. Diesel engines use aviation grade Diesel fuel or can operate on Jet-A fuel if the engine is suitably modified to cope with the lower lubricity of Jet-A fuel.

Properties and specifications Cloud point Cloud point refers to the temperature below which wax in Diesel forms a cloudy appearance. The presence of solidified waxes thickens the oil and clogs fuel filters and injectors in engines. The wax also accumulates on cold surfaces (e.g. pipeline or heat exchanger fouling) and forms an emulsion with water. Therefore, cloud point indicates the tendency of the oil to plug filters or small orifices at cold operating temperatures

Cetane number (or CN) is an inverse function of a fuel’s ignition delay and the time between the start of injection and the first identifiable pressure increase during combustion of the fuel. In a particular Diesel engine, higher cetane fuels have shorter ignition delay periods than lower Cetane fuels. In short, the higher the Cetane number, the easier the fuel combusts in a compression setting (such as a Diesel engine). The characteristic Diesel ‘knock’ occurs when fuel that has been injected into the cylinder ignites after a delay causing a late shock wave. Minimising this delay results in less unburned fuel in the cylinder and less intense knock. Therefore higher-cetane fuel usually causes an engine to run more smoothly and quietly.

Cetane number (Diesel) Cetane number, Cetane rating or CN is an indicator of the combustion speed of Diesel fuel and is a measure of Diesel fuel quality. A cetane number is a measure of the delay of ignition of each Diesel fuel. A higher cetane number indicates that the fuel ignites more readily when sprayed into hot compressed air. European (EN 590 standard) road Diesel has a minimum cetane number of 51.

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Cetane number is a measure of how quickly the fuel ignites after being injected into the combustion zone

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Octane rating (gasoline) An octane rating, or octane number, is a standard measure of the performance of an engine or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating (igniting). In broad terms, fuels with a higheroctane rating are used in high-performance gasoline engines that require higher compression ratios. In contrast, fuels with lower octane numbers (but higher cetane numbers) are ideal for Diesel engines, because Diesel engines (also referred to as compression-ignition engines) do not compress the fuel, but rather compress only air and then inject fuel into the air which was heated by compression. Gasoline engines rely on ignition of air and fuel compressed together as a mixture, which is ignited at the end of the compression stroke using spark plugs. Therefore, high compressibility of the fuel matters mainly for gasoline engines. Use of gasoline with lower octane numbers may lead to the problem of engine knocking. Fuel grade (gasoline) All aircraft fuel has an anti-knock rating, which is a measure of their resistance to detonation. Detonation is the situation where the speed of burning of the mixture suddenly leaps from tens of feet per second to thousands of feet per second because the unburnt mixture reaches its critical temperature and pressure and explodes, causing damage to pistons, cylinders and bearings. The use of additives can increase the anti-knock value of a fuel, the most common being tetraethyl lead (TEL), but as this causes environmental health problems, alternative additives are being developed.

Fuel is graded by its anti-knock properties, i.e.: • •

100L which is green; and 100LL, which is blue.

Both the above fuels contain TEL, and their use may give rise to problems for engines designed for fuel with lower lead content, or unleaded fuel. Unleaded fuel can be used on some engines, but if extra power is obtained by increasing the cylinder pressures, then there is a high risk of detonation. Lubricity In a modern Diesel engine, the fuel is part of the engine lubrication process. Diesel fuel naturally contains compounds that provide lubricity, but because of regulations in many countries (such as the US and the EU), sulphur must be removed from the fuel before it can be sold. The hydrotreatment of Diesel fuel to remove sulphur also removes the compounds that provide lubricity. Reformulated Diesel fuel has a lower lubricity and requires lubricity-improving additives to prevent excessive engine wear. Flashpoint The flashpoint is the lowest temperature at which vapours of a volatile material ignite when given an ignition source. Avoid confusing flashpoint with autoignition temperature which is the temperature at which the vapour ignites without an ignition source The autoignition temperature, by definition, is higher than the flashpoint.

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Fuel

Flashpoint

Autoignition temperature

Ethanol (70%)

16.6°C (61.9°F)

363°C (685°F)

Gasoline

−43°C (−45°F)

280°C (536°F)

Diesel No. 2

>52°C (126°F)

256°C (493°F)

Jet fuel (Jet A/A-1)

>38°C (100°F)

210°C (410°F)

Kerosene

>38–72°C (100–162°F) 220°C (428°F)

Vegetable oil (canola) 327°C (621°F) Biodiesel

424°C (795°F)

>130°C (266°F)

At room temperature, as the flashpoint of gasoline is -45°C, the gasoline instantly ignites when in contact with a flame. The Diesel will put the flame out.

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Release of energy Aircraft reciprocating engines are a type of heat engine that changes the chemical energy in fuel and air into heat energy and then changes this heat energy into useful work to drive the pistons. Gasoline (for example) is a liquid hydrocarbon compound obtained by fractional distillation of crude oil. Its chemical formula is C8H18, it nominally weighs six pounds per gallon, and contains approximately 20,000 Btu of heat energy per pound. Heat energy is released by a chemical reaction between the hydrogen and carbon in the gasoline and the oxygen in the air. When the proper amounts of gasoline and air are mixed and the temperature of the mixture is raised to its kindling point, the carbon and some of the oxygen combine to form carbon dioxide, and the hydrogen and the rest of the oxygen combine to form water. This reaction takes place so rapidly that a great deal of heat is released, and it is this heat that performs useful work. In the perfect combination of gasoline and air, two molecules of gasoline (C8H18) and 25 molecules of oxygen (O2) combine to form 16 molecules of carbon dioxide (CO2) and 18 molecules of water (H2O), plus a large amount of heat. Approximately fifteen pounds of air is needed to provide enough oxygen for complete combination with one pound of gasoline. This mixture ratio is expressed as an air/fuel ratio of 15:1, or a fuel/air ratio of 0.067 (1:15). This ratio, which provides the exact and correct number of oxygen molecules to unite with all of the hydrocarbon molecules in the gasoline, is called a stoichiometric mixture.

Gasoline burns with a mixture as rich as 8:1 (0.125) to one as lean as 18:1 (0.056) when atomised fuel is injected into engine induction air. This ratio band is known as the ‘limits of flammability’ for reciprocating aircraft engines. When burned in a mixture richer than 15:1, there is not enough oxygen in the mixture for the fuel to release all of its energy, and an excess of carbon appears as black smoke and soot. When the mixture is leaner than 15:1, there are fewer fuel molecules than what is needed for the available oxygen, so a given volume of fuel-air charge releases less energy. Air is a compressible fluid which is a physical mixture of gases. It is made up of approximately 21% oxygen and 78% nitrogen, by weight, with traces of other gases. The percentages of these gases remain relatively constant throughout the atmosphere, but the pressure produced by each gas decreases with altitude. This fact is especially important in the operation of aircraft engines. The power produced by an engine at sea level decreases as the aircraft goes up in altitude because the air density lessens, resulting in fewer pounds of oxygen to combine with the gasoline. Since the purpose of a reciprocating engine is to convert the maximum amount of heat energy in the fuel into useful work, it would appear that the engine should always be operated with an air-fuel mixture ratio of 15:1. This is true in theory, but many variables prevent this from actually happening. These variables are discussed next.

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Gasoline octane rating symbols used on the aircraft

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Heat energy content Aviation gasoline has a nominal heat energy content of 20,000 Btu per pound, and a nominal weight of 6 lb per gallon. Kerosene, which is a significant component of turbine engine fuel, has a nominal heat energy content of 18,500 Btu per pound, and a nominal weight of 6.7 lb per gallon.

Critical pressure and temperature It is important to rate gasoline according to its ability to resist detonation, and its critical pressure and temperature determine this resistance. Antidetonation characteristics of fuel are measured by the octane rating or the performance number of the fuel.

Vapour pressure The vapour pressure of a liquid is the amount of pressure needed above the liquid to prevent the fuel from evaporating. Vapour pressure is determined by the temperature of the fuel and is rated according to its Reid vapour pressure (RVP), which is measured at 100°F. If fuel has an RVP of 15 psi at 100°F, any time the atmospheric pressure is 15 psi (approximately normal sea level pressure) or less, and the temperature of the fuel is 100°F, the fuel will vaporise. If the RVP is 5 psi, the atmospheric pressure must decrease to 5 psi, which is standard for an altitude of approximately 26,000 ft, before the fuel will vaporise.

The octane rating of a fuel is measured by comparing its performance in a specialised laboratory test engine to that of a mixture of two hydrocarbon products, isooctane, and heptane. Iso-octane is a hydrocarbon compound with an exceptionally high critical pressure and temperature that is used as a highend reference for antidetonation rating, and it has an octane rating of 100. Heptane, as the low reference, has an octane rating of zero. If a fuel performs in the same way as a mixture of 80% isooctane and 20% heptane, it is given an octane rating of 80.

Aviation gasoline is required to have an RVP of 5.5 to 7 psi. If the RVP is too high, the fuel vaporises too quickly; when the temperature of the fuel is high, or the atmospheric pressure in the tank is low, vapours are released from the fuel, and are likely to cause a vapour lock in the lines to the engine.

Fuels that perform better than 100% isooctane are compared with isooctane that contains varying amounts of tetraethyl lead as the reference and are rated in performance numbers rather than octane ratings. Performance numbers are higher than 100.

If the vapour pressure of the fuel is too low, the fuel will not readily vaporise when discharged from the carburettor. The engine will be hard to start, the fuel-air distribution to the cylinders will not be uniform, the engine may not respond rapidly to additional power demands, the lubrication film will be weakened in the upper cylinder range, and fuel efficiency will be decreased. Total Training Support Ltd © Copyright 2020

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Refuelling with AVGAS 100LL

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Additives The antidetonation characteristics of aviation gasoline are improved when tetraethyl lead is added. But when fuel containing tetraethyl lead is burned, lead oxides are left in the cylinder, and these collect in the firing end of the spark plugs and provide a conductive bridge for high voltage to leak across. A lead-fouled spark plug will not ignite the fuel-air mixture. Ethylene dibromide is often mixed with the gasoline. When it burns, its residue combines with the majority of the lead oxides and converts them into volatile lead bromides, which leave the cylinder with the exhaust gases rather than fouling the spark plugs. Tricresyl phosphate (TCP) may also be added to aviation gasoline to minimise lead fouling of spark plugs. It converts the lead deposits into a non-conductive lead phosphate, which is easier to eliminate from the cylinder than is lead bromide. Aromatic compounds such as benzene, toluene, and xylene have been added to aviation gasoline to increase its antidetonation characteristics. However, these additives attack rubber products in the fuel system and must only be used in systems approved explicitly for them. These would be systems where the sealing materials are not susceptible to aromaticcompound deterioration.

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Fuel additives are often used to help prevent lead fouling like this

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Avgas General Avgas has long been used as the fuel for piston-powered aircraft, but aero diesel engines use either Jet or diesel fuel.

• •

100/130 115/145

Aircraft piston engines operate using the same basic principles as spark ignition engines of cars, but they have a much higher performance requirement. Aircraft engines are designed to run at 55% power or more (on take-off even 100%), whereas car engines are run at an average of 30% power or less. The design of the aero-engine is different in terms of strength, think of cylinders, pistons, bearings, crankshaft, etc.

The first number of each set is the lean performance number, and the second is the rich performance number.

Avgas is gasoline fuel developed for reciprocating piston engine aircraft. Common additives to Avgas include tetra-ethyl or alkyl-lead, anti-knock additives, metal de-activators, colour dyes, oxidation inhibitors, corrosion inhibitors, icing inhibitors, and static dissipaters. It is very volatile and extremely flammable at normal operating temperatures. Proper and safe handling of this product is, therefore of the highest importance. Their octane rating defines Avgas grades. Two ratings are applied to aviation gasoline’s (the lean- and the rich-mixture rating) resulting in multiple numbers e.g. Avgas 100/130 (the lean mixture is 100, and the rich mixture is 130).

History In the past, there were many different grades of aviation gasoline in general use, e.g. 80/87, 91/96, 100/130, 108/135 and 115/145, specially designed for high powered turbocharged and supercharged radial engines. However, with decreasing demand, these have been narrowed down to one type, Avgas 100/130, also known as Avgas 100.

When ordinary gasoline below 100% is graded, it carries a single octane number, 87 for instance, but above 100% a performance figure is used.

Fuel octane or performance numbers of fuel are indicated by the engine manufacturers and must be adhered to at all times. Incorrect fuel use can lead to loss of power, overheating, detonation and eventually engine failure.

Aeons ago, an additional grade was introduced to allow one fuel to be used in engines originally designed for grades with lower lead contents: this was called Avgas 100LL, the LL standing for ‘low lead’. Much later, Avgas 82UL was added. The lead was added to increase the fuel’s resistance against detonation inside the engine during combustion. Thus, higher compression (more power) engines could be used

However, the anti-knock qualities differ according to the air/fuel mixture. Therefore, the performance figure is expressed in two numbers, one for a lean mixture and one for a rich mixture. An example is shown below: Total Training Support Ltd © Copyright 2020

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Avgas 100 High lead – coloured green The standard high lead (1 g/litre) high octane fuel for aviation piston engines. There are two specifications for Avgas 100. The ASTM D 910 and UK DEF STAN 91-90. These are almost the same but have some differences in antioxidant content, oxidation stability requirements and lead content. Avgas 100LL Low lead – coloured blue A low lead version of Avgas 100. Still containing about 0.5 g lead per litre of fuel, low lead is a relative term. This grade is listed in the same specifications as Avgas 100, ASTM D 910 and UK DEF STAN 91-90. Avgas 82 UL Unleaded – coloured purple A relatively new grade targeted at the low compression ratio engines not needing high octane Avgas 100(LL) and designed to run on unleaded fuel (0.1 gr/litre). The octane rating can be increased beyond the pure proportion of octane to heptane by adding anti-knock agents, which delay the onset of detonation. Until recently, the most important such additive, for both automotive and aviation use, was the tetraethyl lead. Avgas density Avgas weighs around 6 lbs/US gallons (to be more precise, 5.97 lbs/US gallons per 0.719 g/ml) at standard temperature (15°C).

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The future of general aviation fuels Currently, the two principal types of fuel used in Aviation are Avgas 100LL and Jet A-1; Jet A-1 for turbine engines/Diesel engines and Avgas for spark-ignition piston engines. By far, the most commonly used is Avgas, and it is this which we concentrate on here. Avgas contains TEL – the additive which has recently been banned in automotive fuels in the European Union for environmental reasons. Although the total fuel volume used in aviation is less than 0.5% of that used in the automotive sector in Europe, there is considerable pressure from environmental lobbyists to remove or replace TEL in Avgas and produce an unleaded grade. To understand what is involved, we first need to look at what benefits TEL has. Lead compounds from TEL form a protective layer on the valve seat and prevents the soft valve seats from eroding. Without TEL, small areas of a soft metal valve seat fuses to the valve and be ‘plucked’ from the face of the seat. Once attached to the valve, they form an abrasive surface which further damages the valve seat. This combination of actions is known as valve seat recession (VSR) as the seat of the valve is worn away and recesses into the cylinder head. The solutions to this are to either use a VSR additive or fit hardened valve seats which are resistant to this action. VSR additives are now commonly used in lead replacement petrol on petrol station forecourts, however for several reasons they are not yet approved for use in aviation engines. This means that the only current method of preventing valve seat recession for aviation engines using unleaded fuels would be to fit hardened valve seats. Total Training Support Ltd © Copyright 2020

This is common in newly manufactured Avco Lycoming and Teledyne Continental engines, but some older engines would need modification. The other more significant problem with unleaded fuels is that of octane rating. Octane rating is a measure of how resistant a fuel is to detonation; the higher the octane rating, the more the fuel/air mixture can be compressed without detonation happening. To make this clear, octane rating is not a measure of the amount of energy in the fuel but is a measure of its resistance to detonation. The advantage of higher-octane fuels is that a higher compression ratio or supercharging ratio can be used, which then leads to a higher engine cycle efficiency, which in turn means more power output for a given fuel burn. However, to confuse things further, there are four principal ways to measure octane rating, RON, MON, lean mixture and rich mixture ratings. Road fuels tend to be measured on a RON scale, for which unleaded fuels tend to be 95/98 RON but are only 85/87 MON. Avgas is measured on lean mixture (similar to MON) but also has a rich mixture octane rating. The lean mixture rating is 100 octane (15 octanes higher than the comparable 85 MON for unleaded Mogas). However, Avgas also has a rich mixture rating of 130, which allows higher supercharger boost pressures to be used without detonation occurring.

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This is mainly a problem when using high power settings at low altitude, for example, during take-off. As you can see TEL in Avgas makes a significant difference to the octane rating, and without it, octane ratings would be back down to 80/85 lean mixture – the level for road fuels – instead of 100/130. This is not a problem for most conventional modern normally-aspirated engines as their compression ratios are quite modest and detonation would not be a problem with 80/85 lean mixture octane fuel. However, for those aeroplanes with supercharged or turbocharged engines, the use of low octane unleaded fuels would not be suitable. The only way to operate these engines on current unleaded technology fuels would be to significantly reduce the boost pressure of the supercharging and massively de-rate the engines. This de-rating would be so severe that many of the engines would no longer be powerful enough for the aeroplane in which they are installed. Modern aviation unleaded fuels are currently being developed, such as 82UL in the United States. This is an 82-octane leanmixture rating fuel and is approved for use in modern non-turbo Avco Lycoming engines amongst others. However, it is not yet available in Europe, but also not everyone can use it – the aircraft manufacturer must raise an aircraft modification document to approve its use. Some new Cessnas are approved to use 82UL, but most aircraft types currently do not have manufacturer’s approval.

To date, there are no additives available to replace TEL which increase the octane rating – the additives used in automotive lead replacement fuels only tackle the problem of valve seat recession and do not affect the octane rating of the fuel. Therefore, if Avgas 100LL were to disappear, the only other option currently available to owners with turbo or supercharged engines would be for the aircraft manufacturer raise a modification to replace their engine with either a turboprop or diesel engine. This brings us on to the other recent advance in General Aviation engines; the development by several engine manufacturers of Diesel engine technology. These engines potentially offer several significant advantages over Avgas engines. They return up to 30% better fuel economy, use Jet A-1 rather than Avgas, and have the potential to be retrofitted to many light aircraft, replacing their current Avgas engines. The downside is the cost of engine replacement and aircraft modification. While some applications may be able to take advantage of this technology, this is not a solution for everyone. In summary, aviation engines present many unique challenges to the development of Avgas, and as such, there is yet no firm date to replace Avgas 100LL. However, there can be little doubt that eventually leaded Avgas will be withdrawn from use. However, this does not seem likely until suitable fully developed alternatives are available; a situation that is likely to be several years into the future.

The potential quantity of Avgas piston-engine aircraft worldwide that could use this grade is estimated to be around 60%. However, some of these would probably need fuel system modifications before approval. Total Training Support Ltd © Copyright 2020

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Mogas General Generally, Mogas (also known as ‘autogas’) is cheaper than Avgas (Europe) which is one of the reasons pilots of experimental aircraft wanting to use that fuel. Even some manufacturers advise users to mainly use Mogas in their engines, but that Avgas can be used too. The Rotax four-stroke models (912, 914) run happily on Avgas, but due to the higher lead content oil changes must be done every 50 hours and oil must be mineral or semisynthetic. Nowadays, Lycoming is developing engines capable of running Avgas and Mogas, because of reduced availability of Avgas. Running an aircraft engine on Mogas can introduce unwanted and unexpected side effects because of the possible blending with bio-alcohol. Gasoline pumps should be labelled as such if the fuel that the pump dispenses contains bio-alcohol so that the buyer of the fuel is warned. Adverse effects Bio-alcohol attracts, carries and retains water; this has several side effects in an engine: bio-alcohol is absorbed by the water and is difficult to detect. After engine shutdown water can lead to corrosion on vital parts. Water freezes in cold conditions and during carburetion. Water also causes vapour lock and lowers the vaporisation point of the fuel. If the aircraft is then flown at higher altitudes (also favouring vapour lock), it could lead to an engine failure at the most unexpected time. Other issues with bio-alcohol blended fuels are lack of lubricity; alcohol is a solvent and could clean deposits in the fuel system and carry them to the filters clogging them. Total Training Support Ltd © Copyright 2020

Bio-alcohol also burns leaner and may cause an increase in exhaust gas temperatures and possibly exhaust valve problems. Testing for alcohol in Mogas The following steps describe how to test to see if there is any alcohol in your Mogas: •

•

Using a glass or chemical resistant plastic container, mark ten equally spaced volumes. A graduated cylinder is ideal; however, a non-tapered glass jar will suffice. Add one part of water (approximately 100 ml) into the container, fill to the first mark, and then add nine parts (approximately 900 ml) of automotive gasoline, fill to the top mark. Shake thoroughly, let stand for 10 minutes or until automotive gasoline is again bright and clear. Record the apparent level of the line between the automotive gasoline and water.

Interpreting the above test: •

•

If alcohol is present in the automotive gasoline, the water will absorb it, and the amount of water will appear to increase, indicating the automotive gasoline should not be used in the aircraft. However, if the water level remains the same, no alcohol is present in automotive gasoline, and it can be used in the aircraft.

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Diesel and Jet fuel General Diesel and jet fuel are discussed together in this section because they are very similar in properties. Indeed, jet fuel is commonly used in aero-Diesel engines, due to its lower cost and greater availability. This is only permissible if the engine is type certified to operate with jet fuel. Diesel fuel is more efficient than gasoline because it contains 10% more energy per gallon than gasoline. But there are a few kinds of diesel fuel: Just as gasoline is rated by its octane, diesel fuel is rated by its cetane Standard Diesel fuel (sometimes called Diesel oil) comes in two grades: • •

Diesel No. 1 Diesel No. 2

The higher the cetane number, the more volatile the fuel. Most Diesel vehicles use fuel with a rating of 40 to 55. Automobile manufacturers specify Diesel No. 2 for normal driving conditions. Truckers use Diesel No. 2 to carry heavy loads for long distances at sustained speeds because it is less volatile than Diesel No. 1 and provides greater fuel economy.

Diesel No. 1 flows more easily than Diesel No. 2, so it is more efficient at lower temperatures. The two types of oil can be blended, and most automobile service stations offer Diesel fuel blended for local weather conditions. Most manufacturers of aero Diesel engines recommend Jet fuel, which is clear to straw-coloured. Jet fuel and automotivegrade Diesel fuel differ in the amounts of lubrication inherent in the fuels. Jet fuel is based on either an unleaded kerosene (Jet-A) or a naphtha-kerosene blend (Jet B). Also, jet fuel often has antifreeze and antimicrobial agents, static dissipaters, and corrosion inhibitors added to improve performance in aeronautical engines, in which ambient temperature often varies wildly during flight. Because of these differences, automotive-grade Diesel is not recommended but can be used as a substitute fuel if jet fuel is unavailable. Jet-A is more like Diesel No. 1, while Diesel No. 2 is the more commonly available since it is the one most often used by ground vehicles.

Diesel fuel also is measured by its viscosity. Like any oil, Diesel fuel gets thicker and cloudier at lower temperatures. Under extreme conditions, it can become a gel and refuse to flow at all.

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Diesel fuel sold for cars is Diesel No. 2

The colour of Diesel fuel is very similar to that of Jet fuel

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Diesel fuel or Jet fuel? Jet-A and Diesel No. 1 tend to have lower viscosities. Lower lubricity is likely as the viscosity decreases. Jet fuels have additional specifications that are not required of Diesel fuels, such as the requirement of testing for certain components and volatility requirement. Some of the methods for testing also vary from one fuel to the other. However, the differences noted above are the most significant. Jet-A is a relatively high-sulphur fuel, while automotive Diesel is low sulphur, and environmental requirements are getting more stringent about sulphur in Diesel every year. We are now in the ultra-low sulphur Diesel era. Jet-A is ‘dry.’ Diesel contains additives to lubricate and clean the injector system of a Diesel engine. Diesel fuel is very similar to Jet-A or kerosene fuel. Diesel is a light oil with a density of around 850 gr/l, and it releases 40.9 MJ of energy per litre. Jet fuel/kerosene has just 5% less energy per litre than Diesel No. 1. The basic properties compare so much that either can be used in a regular Diesel engine providing the fuel pump has been adapted to cope with the lower lubricity of jet fuel. The main difference is that the lubrication properties of Diesel are much better and as the fuel is sometimes used as a lubricant for the high-pressure fuel pump, running primarily on Jet fuel can ruin this pump if precautions are not taken.

Diesel is denser, heavier, contains more energy than gasoline. It also is used to lubricate the fuel pump. However, Jet fuel does not have the lubrication properties as standard Diesel. Either engine oil is used to lube the pump, or an additive is added to the fuel when filling the tanks. This is not to say using Jet-A in a Diesel engine would not work. It has worked in Diamond aircraft with Diesel engines without reported problems. The issue is one of liability, since all automotive Diesel has a required cetane rating as per the EU or US specifications, and all automotive Diesel engines have a minimum requirement. However, that requirement does not indicate that the engine does not work with a lower rating, only that the manufacturers do not warrant the engine if a fuel with a lower cetane rating is used. Jet-A has no cetane rating standard because it is not necessary for a gas turbine engine; thus, some manufacturers are very concerned about liability with the increase in popularity of Diesel engines. Jet-A has a widely varying cetane index, which is not controlled or guaranteed, so potentially a particular batch of Jet A fuel could have a cetane rating above or below what is specified for a Diesel engine. The engine manufacturer determines the appropriateness of any of the three different fuels to perform in a particular engine. If the manufacturer indicates the use of only one type of fuel, that is the fuel that should be used.

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Comparison of Jet fuels and Diesel fuels

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Manufacturers seek certification of their engines for use on specific grades of fuel. For example, Continental Motors Group offers their CD-155, a jet fuel piston engine for general aviation with a take-off power of 114 kW (155 HP). The CD-155 is a turbocharged 4-cylinder in-line engine which is EASA and FAA certified. The CD-155 is certified for the use of both jet fuel (Jet-A) and Diesel (DIN EN590) and is running with the two fuels in any mixture ratio. However, in parts of the world where Jet-A and EN590 fuels are not available, alternatives are, Jet-A1, JP-5, DEF STAN 9186, JP 8, DEF STAN 91-91, JP-8+100, Chinese Jet Fuel No 3. The value of high quality, differentiated Diesel fuel https://youtu.be/6tTlbAzEHLQ

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AVGAS 100LL (blue), Jet A (clear/straw), Diesel

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Cold weather operation Diesel is prone to gelling and waxing in cold weather if the right measures are not taken. Usually, the formulation of the fuel sold in petrol stations is changed when winter sets in. But when an aircraft flies at high altitude temperatures can drop below freezing quickly, if it were using automotive summer Diesel, it would experience fuel gelling and probably an engine flameout. Special additives are needed to keep Diesel from gelling at low temperatures, or the exclusive use of Jet fuel is recommended in these operations.

Because kerosene-type fuels tend to absorb water more readily than gasoline, the US military often uses anti-icing (or ‘thermal stability’) additives as a precaution in its jet aircraft. However, some of these additives have questionable environmental attributes and are (or soon will be) banned in some countries.

Sometimes heat exchangers are used to make sure that no ice can develop in the fuel system, causing engines surges during critical phases of the flight. And as the high-pressure fuel pump intakes more fuel than the engine uses, the return fuel is warm/hot and thus helps to warm the fuel still in the tanks preventing gelling and waxing from occurring. Diesel fuel has a cloud point of about -10°C while Jet-A has a cloud point of about -40°C, which would seem to be an advantage. However, piston-powered aircraft do not reach speeds that cause the heating of the fuel in the wing due to friction caused by airflow. It is possible that an aircraft powered by a Diesel engine could reach altitudes where the fuel would begin to freeze in flight, particularly in cold climates where the ground temperature in the winter can be close to the jet fuel freezing point. While the fuel may not freeze solid, other physical properties such as viscosity can change. This may have adverse effects on engine components such as fuel filters, fuel pumps and fuel injectors.

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Diesel gelling in a fuel filter at low temperature

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Diesel pollutants In Diesel automobile engines, the Diesel pollutants are a big issue. Despite the addition of catalytic converters to Diesel cars, the public opposition to Diesel cars has put the sales into decline and even prompted prohibitions of Diesel vehicles in some major cities worldwide. Aero-Diesel engines, however, do not create the same kind of stigma. Aero-Diesel engines are not fitted with catalytic converters, and yet they do not emit anywhere near the same quantities of pollutants as do ground vehicles. The reasons for this are discussed here. Diesel engines, like other internal combustion engines, convert chemical energy contained in the fuel into mechanical power. Diesel fuel is a mixture of hydrocarbons which, during an ideal combustion process, would produce only carbon dioxide (CO2) and water vapour (H2O). Indeed, Diesel exhaust gases are primarily composed of CO2, H2O and the unused portion of engine charge air. The concentrations depend on the engine load, with the content of CO2 and H2O increasing and that of O2 decreasing with increasing engine load. None of these principal Diesel emissions (except for CO2 for its greenhouse gas properties) have adverse health or environmental effects. Diesel emissions also include pollutants that can have adverse health and environmental effects. Most of these pollutants originate from various non-ideal processes during combustion, such as incomplete combustion of fuel, reactions between mixture components under high temperature and pressure, combustion of engine lubricating oil and oil additives as well as

combustion of non-hydrocarbon components of Diesel fuel, such as sulphur compounds and fuel additives. Common pollutants include unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) or particulate matter (PM, or ‘soot’). The total concentration of pollutants in Diesel exhaust gases typically amounts to some tenths of one per cent. •

• •

•

CO2: From a CO2 perspective, diesel engines are less polluting than gasoline engines, which is why in many places across the world, people have been moving to Diesel. Governments (many in Europe) have, in some cases taxed Diesel less to make it more attractive. NOx compounds: Nitrogen oxides are linked to asthma and breathing disorders, and diesel engines produce more of these than gasoline engines Sulphur compounds: These are also linked to asthma and breathing disorders. Jet fuel has higher sulphur content than diesel fuel, and much higher than gasoline/Avgas. Particulate matter (soot): Particulates are a big issue from an environmental and health perspective these days. Particulates are linked to asthma and breathing disorders, and there is some evidence that black carbon particulates are heating the planet and causing glacier melt at an accelerated rate as they absorb heat. Diesels produce much more particulates than gasoline engines.

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Diesel engine emissions

Diesel engine emissions depend upon the flame temperature and the fuel/air ratio

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Very high temperatures in the combustion chamber help reduce the emission of soot but produce higher levels of NOx. Lowering the peak temperatures in the combustion chamber reduces the amount of NOx produced but increases the likelihood of soot formation. Better mixing of the air and fuel is the key to lower emissions. The NO produced rapidly oxidises to NO2 (collectively called NOx). NOx combines with hydrocarbons or volatile organic compounds in the presence of sunlight to form low-level ozone. This leads to smog formation. In the past, fuel systems were mechanical and used injection pressures of 200–300 bar, with one fuel injection per power stroke. The resulting plume of fuel in the combustion chamber had a wide temperature range, due to poor mixing with the air. The combustion in the rich region of the flame produced soot, and the lean regions produced NOx.

Secondly, the use of digital fuel control can manipulate the injection of fuel, in terms of duration and multiple injections per power stroke, to optimise the temperature and mixing of fuel to minimise both soot and NOx. The temperature profile across the plumes is far more limited; this reduces emissions and offers better air utilisation within the cylinder. And finally, the fact that aero Diesel engines are operated at almost a fixed power setting, or at least within a very much narrower band of power settings (close to 100% power most of the time) than do automobile Diesel engines, it is much easier for the engine designer to fine-tune the engine and fuel system to ensure that the engine is not operated in or near either the soot formation or NOx formation regions of the graph shown previously.

Mechanical pumps are still used in modern systems to generate the pressures, but the injection timing is now computer-controlled and delivers exact amounts of fuel. This has enabled the development of engines, which operate with up to six injections per power stroke. This combustion technology lowers the combustion temperature by forming a lean pre-mixture and burning it to reduce NOx and smoke. To overcome this, systems today operate at pressures up to 1,500 bar and have up to 8 holes per injector. This requires the injection holes to be smaller. The fuel plumes in engines with multiple injection holes are smaller than those from a single large injector.

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Avgas from a tanker

Avgas from a selfserve pump

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Safety precautions General Aircraft re-fuelling and de-fuelling are accompanied by attendant hazards which must be managed sufficiently for their mitigation to acceptable levels. The issues are much the same whether the fuel source is a tanker/bowser or a fuel hydrant system. Pressure re-fuelling is normal for multi-crew transport aircraft and business aircraft, but gravity re-fuelling of these types may be available as a backup system. The kerosene fuel used by turbine engine aircraft has a higher flash/ignition point than the aviation gasoline used by piston-engine aircraft, but there are still potential hazards. The primary risk is unintended ignition of fuel vapour, which can occur by a single spark. A sufficient quantity of fuel vapour to create a high risk of ignition may result from spillage arising from procedural errors, leaks, aircraft tank venting or failure of pressurised fuel lines or their couplings. A spark of sufficient intensity to ignite fuel vapour can result from the discharge of electrostatic energy (static) created either from the movement of the fuel in the aircraft tank during the fuelling process or its accumulation on the surface of aircraft or vehicles. Fuel movement during re-fuelling or defueling may lead to a static charge building up in the fuel. If the charge is of sufficiently high potential, it can cause sparking within the aircraft or the ‘origin’ storage tank. The charge density in the fuel and the possibility of sparks inside the tanks are not affected by bonding. However, the use of static dissipater additives in fuel can contribute materially to reducing the risk involved.

Accumulation of surface static charge may occur on either an aircraft or its fuelling vehicle under certain conditions. Electrical bonding must be used to eliminate this hazard. Coupling/uncoupling of hoses must not be undertaken unless electrical bonding is in place (see below). Re-fuelling should not take place during active electrical storms/thunderstorms near the airport. Electrical bonding There must be a cable to link to designated points or to clean unpainted metal surfaces on the chosen airframe. Bonding cables should connect the installation delivering the fuel with the aircraft or installation receiving the fuel. All connections should be made before filler caps are removed before the start of fuelling and then not broken until fuelling is complete and the filler caps have been replaced where applicable. On no account should either the fuelling vehicle (including hydrant dispenser) or the aircraft be bonded to a fuel hydrant pit. It should be noted that fuel hoses, including so-called ‘conductive’ hoses, are not suitable substitutes for dedicated clips and bonding wires. Misfuelling Misfuelling is the introduction of an improper fuel into an aircraft’s tanks. The consequences of misfuelling can range from the benign (fuel system drainage) to the expensive (engine replacement) to the disastrous (engine failure shortly after take-off). Given simple precautions, all are easily preventable. Although the frequency of misfuelling has declined dramatically with the widespread adoption of colour-coded wing decals and standardised fuel nozzles and receptacles, the potential for trouble still exists

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Standardised fuel label colour codes

Jet fuel nozzles have a wide spade top that is theoretically incapable of being inserted in an avgas fuel filler equipped with a restrictor ring

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Avgas fuel-nozzle filler-ports are designed to be less than 50 mm (2") diameter, whereas Jet fuel filler nozzles are over 50 mm (2") diameter. Older avgas aircraft should be fitted with a restrictor ring to prevent inadvertent filling with Jet fuel. The introduction of diesel engines for general aviation holds new potential for misfuelling trouble. Aviation diesel engines are designed to run on jet fuel: Unlike most turbines, they cannot be run safely on Avgas. This is a potentially dangerous problem because Avgas dispensing nozzles fit easily into the large-diameter re-fuelling ports used in diesel aircraft. Until new measures are taken to prevent this physically, owners of dieselpowered aircraft should be particularly cautious when refuelling. PED use during re-fuelling There is a risk that a PED (personal electronic device) may create or induce a spark of sufficient intensity to ignite fuel vapour released during fuelling. However, it is exceptionally remote under normal circ*mstances. A particular concern is the proliferation of below-specification mobile telephone batteries that have the potential to fail dangerously. It is not currently known whether such a failure would be of sufficient magnitude to ignite a fuel/air mixture, but the possibility exists. It is recommended that the circ*mstances under which such an event might occur during re-fuelling should be carefully considered and mitigated.

It also appears that PEDs close to or on-board modern aircraft can interfere with fuel gauges and some navigation equipment and may cause false fire warnings in cargo/baggage holds. Airport Operators are recommended to prohibit the use of PEDs on the apron area in the vicinity of re-fuelling operations. Passengers boarding or disembarking an aircraft should be discouraged from using PEDs. Re-fuelling with passengers on board Aircraft Operators should have their procedures for re-fuelling, including emergency evacuation if re-fuelling is permitted with passengers on board as appropriate in the Operations Manual. Crew stations and duties should be clearly defined as should appropriate passenger communications. Defueling This is an unusual and infrequent operation; the personnel involved, including any flight crew, should be careful to refer to the necessary procedural documentation. Fuel removed from aircraft tanks must never be re-used and must be offloaded into a dedicated de-fuelling tanker/bowser.

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Licence Category B1 and B3

16.9 Lubrication Systems

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective System operation/layout and components.

Part-66 Ref. 16.9

Knowledge Levels A B1 B3 1 2 2

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Table of Contents Lubrication system function _____________________ Reduce friction _______________________________ Provide cooling _______________________________ Cushioning and sealing _________________________ Preservation _________________________________ Cleanliness __________________________________

6 6 6 7 7 8

Oil filter ______________________________________ 46 General ____________________________________ 46 Differential pressure (ΔP) indicator _______________ 52 Oil pressure filter _____________________________ 54 Oil dilution system ____________________________ 58 Precautions when oil diluting ____________________ 58 Period of effectiveness_________________________ 60

Film and boundary lubrication __________________ 10 Function as a hydraulic medium _________________ 12

Pressure control ______________________________ 62 Pressure relief valve __________________________ 62 Pressure regulator ____________________________ 64

System layouts and operation ___________________ 14 Wet sump system ____________________________ 14 Dry sump system ____________________________ 20 Oil pump ____________________________________ General ____________________________________ Vane-type pump _____________________________ Gerotor-type pump ___________________________ Gear-type pump _____________________________ Pressure relief valves _________________________

24 24 24 26 28 32

Oil tank _____________________________________ General ____________________________________ Vent_______________________________________ Hot pot ____________________________________ Stack pipe __________________________________ De-aeration _________________________________ Anti-drain valve ______________________________ Inverted oil system ___________________________

34 34 36 36 36 36 36 38

Oil distribution ________________________________ 66 Crankshaft sealing ____________________________ 76 Defects and troubleshooting ____________________ 78 Crankshaft oil seal leaks _______________________ 78 Oil system indications _________________________ 82 Low oil pressure ______________________________ 82 High oil consumption __________________________ 82 Worn piston rings _____________________________ 82 Worn big end bearing shells ____________________ 82 High oil temperature___________________________ 82 Chip detectors ________________________________ 84 Magnetic chip detectors ________________________ 84 Indicating magnetic-chip detectors _______________ 84 Pulsed chip detectors__________________________ 86 Oil consumption monitoring_____________________ 88

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Oil particle analysis ___________________________ General ____________________________________ Chip detectors _______________________________ Oil filters ___________________________________ Debris particle examination _____________________ Spectrometric oil analysis ______________________ Ferrography ________________________________

90 90 92 92 92 94 94

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Lubrication system function The lubricating oil in the engine does considerably more than reduce friction. It performs five major functions, all of which are necessary requirements. Each of these five basic requirements must be met to the degree specified by the engine manufacturer before the oil can be approved for use.

In a sense, these little ball bearings of oil can be regulated in size by the petroleum refineries. Thus, the different viscosities or grades of lubricating oils that are available. The clearances or space between moving parts in your engine dictate what grade or viscosity of oil the engine must have to provide satisfactory operation and long service life.

The lubricating oil must: • • • • •

Today’s high compression engines subject their lubricating oils to severe stresses and in more ways than one. Laboratory tests have shown conclusively that the oils in today’s engines undergo continuous shearing action from many of the moving parts in the engine. In time this shearing action alters the oils’ original viscosity properties. This change in the oils’ ‘thixotropic’ properties could lead to a reduction in the service life of the engine, hence why engine manufactures have strict oil change frequencies published in their maintenance schedules.

reduce friction between moving parts to a negligible amount; provide necessary cooling to the internal areas of the engine that cannot be reached by external means; cushion moving parts against shock and help seal the piston rings to cylinder walls; protect the highly finished internal parts of the engine from rust and corrosion; and keep the interior of the engine clean and free of sludge, dirt, varnish, and other harmful contaminants.

Provide cooling Moving parts generate friction, which in turn produces heat. While the lubricating oil practically eliminates metal to metal contact, it is subject to its own friction. The constant flow of oil to all of these moving parts carries away the heat fast enough to keep the moving parts at a safe temperature. The upper cylinder walls, pistons and exhaust valve stems are exposed to extreme temperatures during normal combustion. Here, again, the excess heat inside the engine is removed by the lubricating oil. In most instances, the oil temperature observed on the oil temperature gauge is the temperature of the oil after it leaves the oil cooling radiator.

The lubricating oil must perform all five of these functions simultaneously and without compromise to each other. Therefore, the oil must have a high degree of compatibility between each of its five functions. Reduce friction During regular operation lubricating oil is distributed to all moving parts in the engine. The method of delivery, quantity and pressure vary according to the loads imposed on the various parts. In all cases however the oil reduces friction by behaving like millions of tiny ball bearings rolling around between the moving parts of the engine.

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Engines not equipped with oil coolers usually have their oil temperature taken immediately after the oil is removed from the sump where it has rime to cool before redistribution. Here the oil temperature is considerably less than what it is during actual contact with the hot parts. Oil temperatures must be closelyregulated, as operating with too low an oil temperature can be harmful to the engine; the oil does not get hot enough to dry out the moisture it collects during a normal shutdown, start-up and ground operations. The ‘hot’ areas of the engine and turbocharger impose high temperatures on the lubricating oil while it is performing its cooling function.

Also, a perfect gas-tight seal between piston rings and cylinder walls is never wholly attained so that the lubricating oil is subjected to some high-temperature ‘blow-by’ of combustion gasses. Not only does this blow-by contribute to the oxidation situation pointed out earlier, but it also contaminates the oil with the various acids and corrosive lead salts generated during combustion. These contaminants remain in the oil and are not removed by the filter. Each hour of engine operation adds more of these contaminants. After shutdown, water vapour condenses inside the engine, and its subsequent mixture with these corrosive combustion products produces harmful acids which are detrimental to the internal components of the engine such as the camshaft.

These high temperatures subject the oil to ‘co*king’ and ‘oxidation.’ co*king tends to dirty up the oil with carbon particles, while oxidation causes the oil to breakdown and thicken. A ‘full flow’ filter helps to remove much of the co*king effects, but nothing can be done about the oxidation. Both effects cause harmful deterioration of the oil.

Preservation The lubricating oil accomplishes this task in several ways. After shutdown, a coating of oil covers all of the interiors of the engine. This coating protects against rust; however, it slowly drains off in time and eventually exposes the interior of the engine to corrosion. To maintain this protection, the engine should be flown (not run-up); Continental Engines recommends at least once a month if operating inland, and every two weeks when flying near the sea (it should be noted that salt can be found 75-150 miles inland). Additives in the lubricating oil provide some protection against corrosion; however, these additives are not sufficient for long periods of engine idleness. Remember that during regular operation the engine continuously adds more contaminants to the lubricating oil. In time, the protective additives are used up, and the oil becomes saturated with undesirable corrosive agents. The only sure method for continued protection is to change the oil.

Cushioning and sealing An excellent example of cushioning is in the valve train. Here each valve is being thrust open and yanked shut every 1∕20 of a second at standard cruise power. These components would not last long if the shock-absorbing did not protect the parts qualifies of the oil film between these parts. A thin film of oil on the cylinder walls not only lubricates but helps provide the necessary gas-tight seal between the piston rings and cylinder walls. In this function, one can easily see the severe shearing and crushing action imposed on the lubricating oil by just the valve train alone.

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Cleanliness Ashless dispersant types of lubricating oils keep the interior of your engine clean if used continuously after the first 100 hours of the engine’s life. They contain additives that cause the dirt to disperse throughout the oil and to prevent the dirt from precipitating out and collecting in the engine when the oil is at rest. This function is vitally important since the many oil passages in an engine could become clogged with dirt over time and cause oil starvation.

The water is vaporised and passes out through the crankcase breather during ‘dry out’ of the oil, but remember, after shutdown, it returns, and during the humid summer months, this condition is worse than any other time of the year. The more humid the climate, the more water condensation accumulates during shutdown and start-up. All aeroplane engines are not equipped with ‘full flow’ filters, but even those having filters do not enjoy all the protection some owners are often led to believe. The filter can remove only the solid contaminants such as dirt and co*ke.

Also, many assemblies such as hydraulic valve lifters must have clean oil for satisfactory operation; otherwise, they fill up with dirt and sludge and cease to operate. Every hour the engine runs, it adds more dirt to the oil. This dirt comes from a variety of places; dust taken from the atmosphere which is always present even at high altitudes, also, from soot during starting and idling. Then there is the co*ke produced by the hot areas.

The liquid contaminants pass right through the filter and continue to remain in the oil. The only way to get rid of them is to drain the oil.

Highly corrosive lead salts and minute metal particles are other sources. Blow-by gasses contribute several different acids such as sulfuric, formic and others. Water vapour forms each time the engine is shut down and started up. Gasoline dilution occurs during starting, especially during cold weather. When all these contaminants get mixed up in the lubricating oil, the form new mixtures such as sludge, varnish, and highly corrosive acids. Acids are usually harmful only when they are wet or contain water. When the lubricating oil reaches its normal operating temperature, it dries out.

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Corrosion on camshaft and lifters

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Film and boundary lubrication The lubrication process maintains a stable film of oil between close-fitting moving surfaces to keep them apart. The film of oil may be thin but providing it has a suitable viscosity it continues to provide this separation. Lubrication is considered to operate in two phases, as shown in the diagram below. • •

Film lubrication. Boundary lubrication.

Film lubrication Consider figure (c) of the diagram, which is the desired condition, and is the phase where a substantial amount of oil is maintained on the bearing surfaces. This is achieved by using an oil pump to ensure that there is an oil flow and to build up a pressure to maintain a good film of oil between the working parts. The viscosity of the oil has to be such that when hot, it does not drain from the bearings faster than the pump can replace it. (a) Shaft stationery and possible metal to metal contact at the arrow. (b) Shaft begins to move. It tends to climb up the bearing, the point of contact has been moved, but now some oil is present, and boundary lubrication conditions apply. (c) Shaft now up to speed and film lubrication exists with an even distribution of oil around bearing. A high bearing load, e.g. during the power stroke, could briefly produce a boundary lubrication condition

Boundary lubrication Consider the situation shown in figure (b), which arises when film lubrication is breaking down or when mating parts starts to move, for example, on engine starting. It can be followed by lubrication failure or seizure. Several factors can bring about this condition; they include: • • • • •

excessively high bearing loads; excessively high oil temperatures; oil starvation; loss of oil pressure; and oil contamination.

It is a condition which is difficult to prevent, and much of the wear of moving parts is caused before the lubricant is circulating through the oil system. Some oils can resist this condition for a longer time than others. It is one of the reasons why the oils that are specified for an engine must always be used.

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Phases of lubrication

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Function as a hydraulic medium The engine’s lubrication system supplies oil to the propeller governor. Here, an integral oil pump boosts the pressure to control and adjust the blade angle on a variable-pitch propeller, to maintain the correct propeller RPM. Oil is used within the hydraulic tappet (zero-lash valve lifter) which eliminates any clearance in the valve linkage and automatically compensates for any expansion or contraction in the valve train. Hydraulic valve lifters require less maintenance, are better lubricated and operate more quietly.

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Propeller governor

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System layouts and operation There are two ways the lubrication systems of reciprocating engines can be classified; the location in which the oil supply is carried, and the method of lubrication within the engine itself: • •

wet-sump; and dry-sump

The main difference is how the oil is carried. The oil supply can be carried inside the engine itself in a wet-sump system, or it can be carried in a separate tank outside the engine in a drysump system. Both types of systems are used in modern aircraft. Wet sump system Oil is carried in the crankcase of a wet-sump engine, similar to almost all automobile engines. The oil is picked up from the sump by the oil pump and forced through the engine. When it has served its lubricating functions, it drains back into the sump and is picked up and recirculated through the engine. The wet sump system is common to most applications of piston engines and is used in a large number of light aero-engines; it works as follows. An engine-driven pump draws oil from a sump at the bottom of the engine and delivers it under pressure through a filter to the oil circuit. This circuit is the route of the oil through the engine to the bearings and the valve operating gear. After lubricating the rest of the engine by splash and spray, as it escapes from the bearings, the oil drains down to the sump ready for recirculation.

A relief valve is fitted on the outlet side of the pump to protect the system from excess pressure. Also, a sensing device is fitted in this line to check the oil pressure is within limits prescribed for the engine. The crankcase sump can be an integral part of the crankcase or, more often a metal pressing or light alloy casting attached directly to the bottom of the crankcase. It acts as the oil reservoir for the system. The size of the sump, and thus the quantity of oil it can hold, is a compromise between having too much, so that engine warm-up is hindered and having too little so that the oil is contaminated quickly. The sump allows entrapped air to escape from the oil, and it is sufficient depth for water and other contaminants to settle at the bottom. If some cooling assistance is required for the oil, the sump exterior is finned, and slipstream air is passed over it. A specified level of oil is required to be maintained in the sump, and this is most usually checked before flight using a dipstick. For maintenance purposes, a drain plug is fitted at the lowest point of the sump. On Lycoming engines, the sump is of a cast aluminium construction, and the induction pipes are an integral part of the casting. The heat of the oil is used to vaporise the fuel in the induction pipe to provide a more uniform mix of the air and fuel vapour to aid in even combustion.

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A wet sump system

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Wet sump schematic Cast oil sump

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Example: Lycoming wet sump system

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Dry sump system The crankcase of a radial engine is too small to carry the oil supply, and some horizontally opposed engines are mounted in shallow nacelles where there is not enough room for a deep oil sump. Some aeroplanes designed for aerobatic flight cannot use wet-sump engines. Oil is carried in an external oil tank that is higher than the oil inlet to the engine, and it flows to the inlet of the oil pressure pump by gravity. The pump forces oil through the engine where it lubricates and cools, and then drains down into a small collection sump where it is picked up by the scavenger pump and returned to the tank. The scavenged oil is hot, and it usually contains some air, so its volume is greater than the cooler oil that is forced through the engine by the pressure pump. The scavenger pump, therefore, must have a considerably larger volume than the pressure pump. An oil cooler with temperature control is mounted in the line between the scavenger pump and the oil tank. If the oil does not need cooling, it passes around the core of the cooler, but if it is too hot, it is forced to flow through the cooler core where it transfers its heat to the air flowing through the cooler. The standard procedure is to vent the oil tank to the engine crankcase, which is in turn, vented to the outside air through the crankcase breather line. This method of venting provides adequate ventilation of the tank and prevents oil loss that could occur if the tank were vented directly.

One popular series of Continental horizontally opposed engines has a semi-dry sump system. The oil drains into a kidney-shaped steel sump attached to the bottom of the engine, which does not require a scavenger pump. The oil is picked up by an oil pickup tube that extends down into the sump. Most engines installed in aeroplanes that are to be flown inverted use dry-sump lubrication systems. The oil pickup tube in the reservoir is flexible and weighted so it can pick up oil even when the aeroplane is inverted. Many reciprocating aircraft engines have pressure dry-sump lubrication systems and are generally found on aerobatic aircraft. The oil supply in this type of system is carried in a tank. A pressure pump circulates oil through the engine. Scavenger pumps then return it to the oil tank as quickly as it accumulates in the engine sumps. The need for a separate supply tank is apparent when considering the complications that would result if large quantities of oil were carried in the engine crankcase. On multiengine aircraft, each engine is supplied with oil from its own complete and independent system. Although the arrangement of the oil systems in different aircraft varies widely and the units of which they are composed differ in construction details, the functions of all such systems are the same. A study of one system clarifies the general operation and maintenance requirements of other systems. The main units in a typical reciprocating engine dry-sump oil system include; an oil supply tank, an engine-driven pressure oil pump, a scavenge pump, an oil cooler with an oil cooler control valve, oil tank vent, necessary tubing, and pressure and temperature indicators.

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A dry sump system principle

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A dry sump system

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Oil pump General The function of the oil pressure pump or lube pump is to supply oil under pressure to the parts of the engine that require lubrication. Many pump assemblies consist of not only the pressure or lube element but scavenge elements as well, all in one housing usually driven from the accessory or high-speed gearbox. By its nature, an oil pump is designed to provide a volume of flow to the engine. How much pressure it creates is a function of how much resistance to flow there is. The more the flow is restricted, the higher the oil pressure tends to be. For example, as an oil filter starts to clog, the resistance to flow increases in front of the filter and the pressure increases. The three most common oil pumps are the vane, gerotor, and gear types. All are classed as positive displacement pumps because they deposit a fixed quantity of oil in the pump outlet per revolution. All three types of pumps are also self-lubricating. These category pumps are also referred to as constant displacement types because they displace a constant volume per revolution.

Pumping action takes place as the rotor drive shaft and eccentric rotor, which act as one rotating piece, drive the sliding vanes around. The space between each vane pair floods with oil as it passes the oil inlet opening and carries this oil to the oil outlet. As the spaces diminish to a zero clearance, the oil is forced to leave the pump. The downstream resistance to flow determines the pump output pressure unless a relief valve is present to regulate pressure. Vane pumps are considered to be more tolerant of debris in the scavenge oil. They are also lighter in weight than the gerotor or gear pumps and offer a slimmer profile. They may not, however, have the mechanical strength of other type pumps.

Vane-type pump The vane pump illustrated could be a single element type or one element of a multiple pump. Multiple pumps of this type generally contain one pressure element and one or more scavenge elements, all of which are mounted on a common shaft. The drive shaft mounts to an accessory gearbox drive pad and all pumping elements rotate together.

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Vane-type oil pump

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Gerotor-type pump The diagram shows one pumping element mounted on a multiple-element pump main shaft. The gerotor pump sometimes referred to as a gear rotor, utilises a principle similar to the vane pump. The gerotor uses a lobe-shaped drive gear within an elliptically shaped idler gear to displace oil from an inlet to an outlet port

•

The space reaches its minimum volume as it is closed to the discharge port and begins to open to the intake port, repeating the cycle. This action takes place in each of the seven inter-lobal spaces between the inner sixlobe gerotor and the outer seven lobe gerotor, giving a virtually continuous oil flow.

Notice that the inner driving gear has six lobes (teeth) and that the outer idling gear has seven openings. This arrangement allows oil to fill the one open pocket and move inlet oil through the pump as it rotates until a zero clearance force the oil from the discharge port. The principle of operation is that the volume of the missing tooth multiplied by the number of lobes in the outer gear determines the volume of oil pumped per revolution of the outer gear. A complete pumping element is shown, one of several which could be mounted on a single shaft within the same pump housing. The diagram depicts the principle of operation of the gerotor pump. The operation is as follows. •

•

From 0° to 180°, the inter lobe space increases from a minimum to a maximum volume. Most of it is open to the intake port allowing it to fill with oil. As the space reaches its maximum volume, it is closed to the intake port, and it is in a position to open to the discharge port. At 270°, the space decreases in volume, forcing its oil out of the discharge port.

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Gerotor-type oil pump

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Gear-type pump The single element gear type pump takes in inlet oil and rotates in a direction which allows oil to move between the gear teeth and the pump case until the oil is deposited in the outlet. The idler gear seals the inlet from the outlet preventing fluid backup and also doubles the capacity per revolution. This pump also incorporates a system relief valve in its housing which returns unwanted oil to the pump inlet. The second figure below shows a dual pump with both a pressure and a scavenge element. This is the most common pump assembly seen on gas turbine engines, and for large engines, it is normal to have up to seven scavenge pumps. Both pressure and scavenge pumps are of the spur gear type shown below.

The capacity of the pump, i.e. its output in litres/hour, depends on gear size, which is fixed, and rotational speed which varies. At idling speed, for example, the output would be much less than at cruise or take-off speeds. The scavenge pump, which rotates at the same speed as the pressure pump, has larger gears and therefore a larger capacity. This means that it takes oil from the sump faster than the pressure pump can fill it, hence the name ‘dry sump’. Quite often, both pumps are in a common housing with the scavenge gears on the same shafts as those of the pressure pump, but with a wall between them to provide two separate chambers. The scavenge pump gears are longer than those of the pressure pump to get the higher capacity while being the same diameter.

Two meshing gears of equal diameter are housed in a closefitting chamber with one of the gears driven by the engine. When the gears are rotated, oil entering the pump through the inlet port becomes trapped between the teeth of the gears and the wall of the chamber and is then carried round to be discharged through the outlet port. The meshing teeth prevent oil from escaping to the inlet side of the pump. As with the wet sump system, the output pressure is indicated on a co*ckpit gauge.

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Gear-type lubrication – combined pressure and scavenge

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Oil pump circuit inside the accessory gearbox

Accessory gearbox, oil pump and filter assembly

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Oil pump and filter circuit

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Pressure relief valves The oil pressure must be maintained within limits established by the engine manufacturer. When the pressure is too low, not enough oil is forced through the engine to lubricate and cool it properly. The oil temperature rises, and the wear becomes excessive. Pressure higher than that recommended by the engine manufacturer should be avoided because excessive pressure can damage the oil cooler, burst the oil lines, cause excessive leakage at the oil seals, cause excessive oil consumption by supplying excessive amounts of oil to the cylinder-wall area, and cause the hydraulic lifters to keep the valves from seating.

Most pressure relief valves are a simple spring-loaded arrangement and are set to ‘relieve’ at a predetermined pressure. They protect the pump, its drive, and the rest of the system form increases in resistance to oil flow. It can be fitted in the pump casing or the crankcase near to the pump.

A pressure relief valve maintains a constant pressure in the lubricating system as the engine speed changes. At all but idling speeds, the pump moves more oil than is needed by the engine, and restriction to the flow is significant enough to raise the pressure to a value higher than that specified by the engine manufacturer. As soon as the pressure at the inlet to the relief valve creates a force greater than the force of the spring, the valve moves off its seat, and enough oil returns to the sump to maintain pressure in the oil system at the value specified by the engine manufacturer. One advantage of using a relief valve to maintain the oil pressure is that the flow of oil through the engine increases as the clearances between moving parts increase through normal wear, and as the viscosity of the oil changes with temperature. The output volume of the pressure pump remains constant for any constant RPM. As the engine parts wear, more oil flows out through the bearings and other moving parts, and less oil is returned to the sump through the relief valve. Total Training Support Ltd © Copyright 2020

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Gear-type lubrication – with pressure relief valve

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Oil tank General Oil tanks are used with a dry-sump lubrication system, while a wet sump system uses the crankcase of the engine to store the oil. Oil tanks are usually constructed of aluminium alloy and must withstand any vibration, inertia, and fluid loads expected in operation. The tank is usually fitted to the airframe behind the fireproof bulkhead in a position which gives a gravity feed to the pressure pump inlet. This is because spur gear pumps are not particularly effective in suction, and therefore require the inlet line to be full of oil when the engine is at rest. In the design of the tank, the position of the filter neck determines the amount of oil it can hold. This is the quantity which is required for full circulation and maximum possible consumption. An air space is always provided above the oil to cater for the following: • • • •

the increased return flow of the accumulated drain oil in the sump that occurs during engine starting; expansion of oil with temperature increase; frothing due to aeration; and variable pitch propeller oil displacement.

Each oil tank used with a reciprocating engine must have expansion space of not less than the greater of 10 per cent of the tank capacity or 2 litres (0.5 gallons). Each filler cap of an oil tank that is used with an engine must provide an oil-tight seal. The oil tank is usually placed close to the engine and high enough above the oil pump inlet to ensure gravity feed. The tank is ordinarily fitted with a weighted oil pickup pipe which remains submerged in the oil in the tank as the aircraft is put through aerobatic manoeuvres. Oil tank capacity varies with the different types of aircraft, but it is usually sufficient to ensure an adequate supply of oil for the total fuel supply. The tank filler neck is positioned to provide sufficient room for oil expansion and for foam to collect. The filler cap or cover is marked with the word OIL. A drain in the filler cap well disposes of any overflow caused by the filling operation.

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Components of an oil tank

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Vent The tank must be vented to atmosphere to allow for the pressure changes which can arise from variations in oil level and changes in altitude. Usually, it is connected to the engine crankcase, thus preventing loss of oil through the vent. The crankcase has its own breather which incorporates some sort of oil trap. Oil tank vent lines are provided to ensure proper tank ventilation in all attitudes of flight. These lines are usually connected to the engine crankcase to prevent the loss of oil through the vents. This indirectly vents the tanks to the atmosphere through the crankcase breather. The addition of a check valve in the vent line can be used to pressurise the tank to provide an excellent positive pressure of oil flow to the oil pump inlet. Oil contents Different methods are employed for indicating the contents of the tank: • • •

a graduated dipstick attached either to the screw-on filler cap or a separate cap; a visual oil level indicator – sight glass; and an electrical indicating system.

Most GA aircraft oil systems are equipped with the dipstick-type quantity gauge, often called a bayonet gauge. Hot pot After a cold engine start, bringing a tank full of oil up to operating temperature could take a long time. By having a separate compartment, the hot-spot in the tank, which is in the direct path of the oil circulation, only a small portion of the oil, about 10%, circulates through the engine. This oil quickly reaches operating temperature, and a rapid warm-up is Total Training Support Ltd © Copyright 2020

achieved. The remainder of the oil is gradually warmed as running continues. Stack pipe By having the suction pipe inlet projecting above the floor of the tank and often surrounding it with a gauze filer, the circulation of sludge or water may be trapped in the tank is prevented. De-aeration Because the scavenge pump has a higher capacity than the circulating pump, air is always present in the return oil, and this produces a froth. If this were to be passed on to the engine, the pump would not maintain a full flow and the bearings could be partially starved of oil. The return oil is discharged on to a surface, the de-aerator, where it spreads and allows the air bubbles to escape reducing the froth. Where no de-aerator is fitted, it is usually because the oil volume and circulation rate are such that air has time to escape from the returning oil before it is recirculated. Anti-drain valve The oil tank can be fitted in such a position that it gives a gravity feed to the engine. To avoid oil draining from the tank and possibly flooding the engine when it is at rest, a non-return valve (check valve) is fitted just after the pressure pump. It is a spring-loaded valve, which is strong enough to resist the head of oil in the tank. The plate moves off its seat as the pressure pump begins to operate and allows an unrestricted flow of oil to the engines.

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A dry sump system Lycoming AIO-320

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Inverted oil system The inverted oil system, shown in below is supplied as a kit form accessory for Lycoming AEIO-320, -360 and -540 engines, which permits regular engine lubrication, with minimal oil loss, during inverted flight. The system functions in all negative-g flight conditions. It is particularly suited to highperformance aircraft used for an unlimited-class aerobatic competition involving long inverted flight and many negative-g manoeuvres. Aircraft equipped with constant-speed propellers require evaluation before modification for aerobatic flight. During periods of zero oil pressure, some propeller types decrease pitch, while other types increase pitch. Momentary interruption of engine oil pressure during aerobatic flight, which usually occurs during certain manoeuvres, may produce decreased pitch and cause engine overspeed if the propeller is of the decreasing pitch type. For safe engine operation during aerobatic flight, therefore, the propeller should be of the increasing pitch type. The inverted oil system kit, which is shown below, consists of the oil valve, oil separator, sump fitting, breather tee, oil sump strainer fitting, and oil return sump fitting. The oil suction screen, oil suction sump fitting and sump plug are installed in the engine hose, oil lines, and standard fittings.

The inverted oil system, normal flight – refer to the diagram below, right. During normal flight, the weighted ball valve at the top of the oil separator is open, allowing blow-by gasses from the engine crankcase to be vented from the breather port, through the breather tee, to the top of the oil separator, and out through the overboard breather line. The top ball-valve of the oil valve is closed, and the bottom ball valve is open, allowing oil to flow from the sump out through the strainer fitting, to the oil valve, back through the sump fitting to the oil pump and out to engine lubrication points. The inverted oil system, inverted flight – when the aircraft is inverted, engine oil falls to the top of the crankcase. The weighted ball-valve in the oil separator closes, preventing overboard loss of oil through the top of the oil separator. Blowby gasses from the engine crankcase are vented from the sump to the bottom of the oil separator and out through the overboard breather line. The top ball-valve of the oil valve is open, and the bottom ball valve is closed, allowing oil to flow out from the breather port, through the breather tee, to the oil valve, through the sump fitting and the sump screen, to the oil pump and out to engine lubrication points. Any oil in lines which fails to return to the sump during the transition between normal and inverted flight drains into the oil separator. This oil then returns to the sump from the bottom of the oil separator during periods of normal flight.

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Inverted oil system

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Oil Cooler General Oil cooling is a method by which the oil is cooled to maintain the oil temperature to the limits laid down by the engine manufacturer. It consists of an assembly of tubes in a casing, having their ends expanded and sealed together, thus leaving spaces between the tube walls for most of their length. This core, or ‘matrix’ as it is called, is arranged so that the hot oil passes between the outer surfaces of the tubes. Heat is transferred from the oil through the tube walls and is conducted away by air forced through the tubes. The cooler may be at the front or rear of the engine with apertures for ducting arranged to ensure cooling airflow through it. It also ensures that the viscosity of the oil is maintained at the required level to produce adequate lubrication.

The amount of ram air cooling can be regulated by one of the following methods: • •

regulating the flow through the cooler to ensure that an even temperature is maintained; or regulating the ram air supply going through the air portion of the cooler.

In practice, however, both methods are usually adopted.

Oil cooling can be achieved in two ways: • •

The location of the oil cooler within the lubrication system can differ. In the diagram below, the wet-sump arrangement is shown with the cooler in the supply line from the oil pump to the engine. With a dry-sump method, the oil cooler is arranged in the return line to the oil tank. However, in both configurations, the oil is providing the same function.

ram air cooling; and fuel cooling.

Ram air cooling is the most effective for piston engines. This is because a continual supply of cooling air is available during flight. Fuel oil cooling, on the other hand, relies upon a continuous supply of fuel through the cooler; such a system is not generally found on piston engine installations.

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A wet sump cooler system

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Air-cooled oil cooler A typical oil cooler assembly consists of a series of hollow tubes that make up the core of the cooler. The tubes are supported within a double-walled shell. The purpose of the double-walled arrangement will be explained shortly. Oil is allowed to flow around the tubes, while ram air passes through the tubes. This allows for cooling of the oil as it passes around the cooling air tubes. Incorporated into the oil cooler assembly are two components that control the flow of oil through the cooler. One component is the temperature regulating valve, and the other is a bypass valve. An explanation of both functions can be demonstrated below. The oil cooler has three modes of operation: • • •

bypass mode; non-cooling flow mode; and cooling flow mode.

Surge condition This system requires an additional bypass valve and is not used on all types. In the surge condition, we have to assume that the viscosity of the oil is so high that extreme pressure has been created in the oil system. A typical case is operation in extremely cold conditions, just after start-up of the engine.

Cold oil Flow (non-cooling flow) Before considering this flow, a word must be said about the temperature regulator. The regulator contains an expansion element within the assembly that reacts to temperature. As the temperature increases, the bellows extend and vice versa. Therefore, by considering the non-cooling flow, the oil is at the correct operating temperature and is again taking the path of least resistance by going past the open regulator, allowing no oil to pass around the cooling tubes. Hot oil flow (cooling flow) As the temperature of the oil increases, the bellows extend under the action of their servicing element. This movement of the bellows closes off the oil flow and forces the oil through the cooling elements of the oil regulating valve, which, in turn, regulates the flow of oil through the cooling part of the oil cooler. In some installations, a flap arrangement is used to control the incoming ram airflow through the cooler. This flap can be controlled by the aircrew when desired.

If the oil cooler becomes blocked, a high back pressure may be built up in the oil passages, leading to the cooler damage. With a high-pressure condition, the bypass valve (not shown) lifts off its seat and allows oil to bypass the cooler by taking the path of least resistance. The oil then passes onto the oil system directly, thus protecting oil cooler from damage. Total Training Support Ltd © Copyright 2020

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Hot oil

Cold oil

Oil cooler system

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Oil cooler system with oil temperature control valve

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Oil filter General A secondary function of the oil is to cleanse the engine’s internal parts to ensure that any contaminations are removed from the oil systems. Typical examples of contaminations are: • • • •

rust particles; dust; carbon deposits; and metallic material from moving internal parts.

The simplest way to remove these contaminants is by changing the oil. However, this can be a costly exercise, so some means of removing the particles is desired; filtration is the best method. If left unattended, contamination of the oil could lead to failure of the lubrication system, by causing blocked oil ways, blocking oil jets and reducing the close tolerance clearances in such components as ball and roller bearings and shells. Contamination of the oil is not only attributed to foreign particles but can also be caused during normal use. Apart from carrying particles in suspension in the oil, two significant chemical changes take place during its travel around the engine. Firstly oxidation, which occurs due to the mixing of the oil with corrosive lead salts produced by combustion. Secondly, the chemical reaction of water vapour condensing inside the engine and then being mixed with the oil.

Oil filter assemblies are designed in many shapes and forms; the system requirements decide the most suitable design. A typical example of a basic oil filter assembly is shown in the diagram below right. Note that the filter is located downstream, on the pressure side of the pump; this is to ensure that the oil is forced through the filter. Filter elements are generally manufactured from fabric or paper and are usually reinforced by a wire mesh type arrangement. The oil flows through the filter element and past the open check valve assembly. During this stage, a slight pressure drop occurs across the filter element, but this is not considered harmful to the system. However, as contaminants build up on the outer wall of the filter element, the restriction of flow through the element is reduced, causing a back-pressure effect at the bypass valve. At a predetermined setting, the bypass valve opens and allows unfiltered fluid through to the lubrication system. This pressure is felt on top of the check valve and closes the valve, preventing fluid going into the interior of the filter. This action prevents an idling circuit being created and reduces the possibility of contaminants being forced back through the bypass valve.

Neither of these effects can be filtered out of the system; they can only be removed by regular oil changes.

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Oil filter assembly

Oil pump and filter circuit with relief valve and bypass valve

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Primary filters are fitted in both suction and pressure lines in an oil system, to remove foreign particles from the fluid, and to protect the seals and working surfaces in the components. Also, individual components often have a small filter fitted to the inlet connection. Primary filters usually comprise a filter head containing inlet and outlet valves, and a sump (or bowl) which houses the filter element. Installation of the sump normally opens the valves, and removal of the sump normally closes them, so that the filter element can be removed without the need for draining the complete system. The filter element can be made of porous paper or wire gauze. Filters made of porous paper may only be used once. After use, they are destroyed. In modem hydraulic systems, filters are made of wire gauze. This gauze can be made of a copper or a corrosion-resistant steel alloy. The openings in the gauze determine the degree to which the hydraulic system is filtered and is indicated in microns (1 micron = 0.001 mm). The filters in the hydraulic system vary from 5 to 15 microns.

At regular intervals, laid down in the servicing schedule, the oil filter element should be removed and inspected. It is wise at this stage to replace this filter element after cleaning out the filter bowl. In inspection should be carried out on any particles found in the bowl, and a fuller investigation carried out on the engine if large deposits of metal particles are found. We have discussed the screen type systems here; it is a common and effective method of filtration. However, in most cases, especially the wet-sump arrangement, the oil gear pump must also be protected from any contaminants that may cause the pump to seize. A metal screen or strainer is fitted over the oil pump inlet in the oil sump to protect the oil pump from this condition. This screen retains any large particles that may cause damage to the pump. The degree of filtration is not as high as the filter element method, as its only purpose is to retain large particles.

Paper filter elements are usually discarded when removed, but elements of wire cloth may usually be cleaned. Cleaning by an ultrasonic process is usually recommended. However, if a new or cleaned element is not available when the element becomes due for a check, the old element may be cleaned in trichloroethylene as a temporary measure.

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Spacers and screens oil filter

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Filtering assembly

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Oil pump and filter circuit

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Differential pressure (ΔP) indicator Some filters are fitted with a device which senses the pressure differential across the filter element, and releases a visual indicator, in the form of a button, when the pressure differential increases because of the filter becoming clogged. False indication of element clogging, because of high fluid viscosity at low temperature, is prevented by a bi-metal spring which inhibits indicator button movement at low temperatures. Other filters are fitted with a relief valve, which allows unfiltered fluid to pass to the system when the element becomes clogged; this type of filter element must be changed at regular intervals. The ΔP indicator (also known as the ‘clogging indicator’) indicates the difference in pressure between filter inlet and filter outlet. It, therefore, gives information about the filter’s condition to the maintenance personnel. The indicator piston pops out and can be reset by hand. The figure below shows a filter unit where a bypass and shutoff valve and a pressure indicator have been included in the filter housing. If the bowl is screwed open to change the filter, the valve closes automatically, which prevents the system from emptying when changing the filter. After installing the bowl and a new filter, the shutoff valve opens again. Filter units in modem hydraulic power systems all have a shutoff valve.

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Differential pressure indicator – internal components

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Oil pressure filter The oil filter element, or ‘spin-on filter’, should usually be replaced every fifty hours of engine operation unless specified otherwise. This is accomplished by removing the lockwire from the filter and removing the filter from the engine. Before discarding the full-flow filter element, an examination should be undertaken. Remove the outer perforated paper cover and, using a sharp knife, cut through the fold of the element at both ends close to the metal caps. Unfold the pleated element and examine the material trapped in the element for evidence of excessive internal engine wear and damage, such as metal chips or bearing particles. In new or newly overhauled engines some small metallic shavings may be found. These are generally of no consequence and should not be confused with particles produced by impact, abrasion, or pressure. Evidence of excessive metal contamination found in the filter element justifies further examination to determine the cause.

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Oil filter assembly on the accessory gearbox

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To examine the spin-on full-flow filter, Champion tool No. CT470 or Airwolf AFC-470 cutter must be used to open the can. Instructions for operating the can opener are as follows: 1) Secure the filter can in a bench vice. When cutting open a female threaded type filter, use the CT-470-2 or AFC470-2 adapter. Male threaded type filters can also be opened with the cutter by removing the CT-470-2 or AFC-470-2 adapter from the rotating bushing. 2) Secure the cutter on the filter mounting plate. Tighten the knurled head screw until the cutter blade meets the metal filter can surface. 3) Rotate the cutter 360º observing that the cutting blade is penetrating the metal can of the filter. Continue tightening the knurled head screw and rotating the cutter until the filter mounting plate is separated from the can. 4) Remove the element from the filter and cut filter material from the end cap. Carefully unfold the element and examine the material trapped in the filter. 5) After the filter element or spin-on filter has been replaced, properly torqued and lock-wired, run the engine and check for oil leaks.

Oil filter contaminant Inspection Contaminant Typical cause Carbon

Excessive oil temperature, poor oil system maintenance

Aluminium

Piston plugs, oil pump and aluminium bearings in turbocharged engines

Steel

Camshaft and followers

Bronze

Tacho drive bushing, conrod bearings and rocker arm bushes

Copper

Camshaft gears (only on some engines)

On older engines which are not fitted with the spin-on full-flow oil filter, the oil pressure filter had no bypass passage. If excessive contamination is experienced, the soldered joint is designed to burst with the increase in oil pressure allowing dirty oil to flow into the engine, thus preventing oil starvation and engine seizure.

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Oil filter content inspection

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Oil dilution system The object of oil dilution is to facilitate the starting of piston engines in cold weather. Fuel is added to the oil to reduce the viscosity; this reduces the torque necessary to turn the engine and ensures, immediately after the start, an adequate supply of lubricant to all moving parts at approximately normal working pressure. The reduced viscosity also minimises the risk of bursting flexible hoses, couplings and oil coolers when starting.

Therefore, the oil level should be reduced to compensate for the addition of the fuel. Maintenance manual information gives the necessary details. If it has been decided that oil dilution has to be implemented, then the following general guidelines should be observed. •

If carried out regularly irrespective of the atmospheric temperature prevailing, oil dilution also minimises the accumulation of sludge deposit within the engine.

•

If we consider the schematic diagram illustrated below left, we can see that a pipe is connected from the pressure delivery side of the fuel pump and connected to an oil dilution solenoid valve.

•

A metered supply of fuel is directed from the outlet side of the solenoid valve and mixed with the oil supply going to the engine. A switch in the co*ckpit operates the solenoid valve. The oil dilution valve illustrated below right is shown in the deenergised condition where the spring holds the ball against the metering orifice. When the solenoid is energised, the spring loading is removed from the ball, and fuel pressure moves the ball off its seat, allowing a supply of fuel to pass through the metering met, to the oil system. Precautions when oil diluting One of the most obvious precautions when introducing fuel into the oil system during oil dilution operations is the level of oil in the oil tank.

•

The engine should be started in the usual manner unless otherwise recommended. Before opening up, the engine must be warmed up for long enough to ensure that some of the fuel is boiled off. If this is not done, there is a danger of frothing, and much of the oil may be blown out through the engine breathers. The partial boiling off period depends on the installation and the time for which the oil has been diluted. The engine should be run at the specified RPM and for the period recommended in the maintenance manual or operations manual.

In no case should any engine be opened up to a high-power setting until the oil temperature, as well as the coolant temperature on liquid-cooled engines, reaches at least the minimum permitted or take off, and until the oil pressure is normal. If the oil pressure does not start to build up immediately the engine is started, or the pressure falls during the boil-off run, the engine should be stopped. Insufficient oil pressure may be due to cold undiluted oil having found its way into the oil pump suction line, due to too low an oil level before dilution, or possibly to a leaking dilution valve.

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Oil dilution system

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After completion of the routine engine checks and any necessary boiling off period, the engine should be stopped if necessary, and the oil tanks topped up to the required level for flight and the engine restarted without delay. Improper use of the oil dilution system will result in the excess fuel flowing into the oil, preventing effective boiling off. Excessive loss through the engine breathers may then result. The engine should be warmed up for the recommended boiling off period. Then, during the routine ground checks, careful observation should be made to ensure that no loss of oil through the engine breathers is apparent. If it is known or suspected that the oil has been over diluted, a further check that no breather loss occurs at take-off power is essential. Engineering personnel conducting ground running operations should be familiar with the location of the breather outlets or drains.

Period of effectiveness Dilution should remain effective for at least two or three days during the cold weather, providing the engine is not run up. If the engine is run up for ground servicing purposes only, further oil dilution, if still required, should not be carried out until the engine has been run for the required period to boil off any remaining diluent. Exact times for the complete elimination of diluent depend, among other factors, on the oil temperatures during the run, and cannot be quoted for all cases. When no time is quoted, 20 minutes at no less than the recommended RPM should ensure that all diluent has been boiled off; an excessive cumulative percentage of dilution may result if this is not done. After all diluent has been boiled off, the appropriate full dilution should be carried out.

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Oil dilution valve – de-energised position

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Pressure control The simplest form of pressure control uses a pressure relief valve, such as that illustrated below. Pressure relief valve The pressure relief valve has a spring-loaded plunger that keeps the poppet valve on its seat during normal operating conditions. The adjustment screw adjusts the spring tension to a predetermined pressure. When held on its seat, the flow of oil is directed past the poppet to the engine. When the oil pressure is higher than the spring pressure, the poppet valve is pushed off its seat by the oil pressure. The oil then flows past the valve back to the oil pump inlet. As the oil system pressure drops below the valve of the spring, the valve closes, and regular supply is returned. This type of relief valve has several disadvantages; for example, it is only designed to relieve a maximum oil pressure and not to act as a pressure regulator. As the valve opens to relieve oil pressure, there is usually an instant drop in system pressure; the pressure then rises again, and the oil is returned to the pump. This causes the poppet valve to slam back on its seat, aided by the spring. If the relief is set too low, then the valve would chatter (continually opening and closing), and damage to the valve could result. The valve tensions are calibrated by an overhaul agency and should never be adjusted during normal operations.

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Pressure relief valve

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Pressure regulator Some pressure-control systems regulate the oil pressure in the system, irrespective of the pump output. The pressure regulator, in this case, serves two purposes within the lubricating system: • •

it offloads the system during a high-pressure condition (cold day – thick oil); and it regulates the system to a maximum pressure once the oil has warmed up.

An example of an oil pressure regulator is illustrated below. In this example, there are three main components: • • •

a series of springs set at a tension equal to the maximum working systems pressure; a pressure relief valve (A); and a spring adjuster.

The regulator offloads high oil pressures caused by thick viscosity oil during start-up in cold conditions. The high pressure is felt at valve (A) and pushes it off its seat, allowing excess oil pressure to again return to the inlet of the pump. System maximum pressure can be controlled by the adjuster screw that alters the tension of the spring acting on valve (A). Information on adjustment procedures can be found in Chapter 79 (oil) of the maintenance manual. Consider the cavity drain line shown below. Any oil leaking past the seal of the piston seeps into the regulator body. In the absence of a cavity drain, the regulator body could fill with oil and cause a hydraulic lock. Periodic checks of the cavity drain line ensure the integrity of the regulator.

Under normal operating conditions, the springs hold valve (A) on its seat. Note the sense line going into the cavity (B); it senses the system pressure. When the system pressure exceeds the value of the springs, sense oil is moved the piston to the right. Because valve (A) is attached to this piston, the oil pressure is allowed to flow past it back to the oil pump inlet. This action causes a drop in sense pressure, and when it becomes less than the spring pressure value, it allows valve (A) to close, stopping the idling circuit, and causing the normal operation to resume. The regulator controls the oil pressure in the system during normal high-temperature operations. Total Training Support Ltd © Copyright 2020

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Oil distribution Oil distribution within the reciprocating engine is achieved in three ways: • • •

pressure supply; splash; and a combination of both splash and pressure.

Of the three methods listed above, the third is the most commonly used. It is essential that certain parts of the engine are supplied with high-pressure oil to ensure that adequate lubrication is achieved, crankshafts and big end bearings being typical examples. Engine cylinder walls receive a splash supply of oil to ensure adequate lubrication of the pistons within the cylinders. Therefore, most engines use a combination of both splash and pressure supply. Other distinct advantages of using pressure supply are: •

•

satisfactory lubrication at all altitudes and altitudes of flight. Splash supply could not provide adequate lubrication at varying attitudes; and the pump output ensures more than adequate supply of oil to bearings and crankshafts. In most cases, the pump output is higher than the lubricating requirement, so a great supply of oil is provided; hence a greater cooling ability is achieved.

The illustration below shows the lubrication system of a typical reciprocating engine. It shows the lubrication flow and component location within a wet-sump lubricating system. Total Training Support Ltd © Copyright 2020

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The lubrication system illustrated below is a combination of pressure and splash supply. The main bearings, connecting rod bearings, camshaft bearings, valve tappets and pushrods are lubricated by positive pressure. The pistons, piston pins, cams, cylinder walls, valve rockers, valve stems and other internal moving parts are lubricated by oil spray.

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A typical lubrication distribution – wet sump type

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Oil under pressure from the oil pump is fed through drilled crankshaft passages which supply oil to the crankshaft main bearings and camshaft bearings. Connecting rod bearings are pressure lubricated through internal passages in the crankshaft. Valve mechanisms are lubricated through the hollow pushrods, which are supplied with oil from the crankcase oil passage.

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Cylinder walls and piston pins are spray-lubricated by oil escaping from the connecting rod bearings. A pressure relief valve is installed to maintain the correct oil pressure at higher engine speeds. The diagram below shows the splash lubricating areas. Once the oil has lubricated its designated parts, it falls to the lower sump region. In the case of the wet sump system, the oil is then picked up by the supply pump, and then the process starts all over again. In the dry-sump arrangement, a scavenge pump in the sump pumps the used oil back to the oil tank. Hoses and pipelines used in lubricating systems are the same as those used in the aircraft hydraulic systems. As all reciprocating engines have internal passageways, some small and some large, cleanliness is essential to prevent blockage by carbon or dirt particles. In most cases, the oil jet orifices and oil internal passages are of a calibrated size to ensure even distribution of the pressurised oil. Any blockage may cause a reduction in the oil supply to parts of the engine and cause premature failure of bearings or crankshafts. The replacement of engine oil and filters at the engine hours recommended by the manufacturer is essential to the long life of the engine.

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Crankshaft sealing Some method must be provided to maintain the lubricating oil within the crankcase areas, irrespective of the type of engine configuration. Manufacturers of light aircraft engines all adopt the same system, the use of the O-ring seal and plate method. Sealing is achieved by using the O-ring seal fitted either in a machined groove in the crankcase body or being retained in place by a retaining plate. Lubrication of the crankcase at this point is achieved by either splash lubrication (wet sump method), or by oil jets at the journal bearing in the dry-sump method. Although lubricating oil is directed to the seal area, engine arrangements have an oil slinger attached to one crankshaft to throw most of the oil back into the sump allowing a calibrated amount to lubricate the seal. This method reduces the possibility of significant oil leaks at the crankshaft seal. A typical example is illustrated below.

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Crankshaft slinger ring

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Defects and troubleshooting Crankshaft oil seal leaks The ‘nose seal’ provides an oil seal at the forward end of the crankshaft. It can leak for the following reasons: 1) 2) 3) 4) 5)

Age hardening of the rubber. Excessive forward and aft crankshaft play. Long periods of engine inactivity. A blocked breather pipe. Incorrect installation.

Should this seal leak immediately after installation, it is possible it was damaged during the installation process. However, a poor fit between the crankcase or crankshaft and seal could also be responsible for the leak. Before installation of a crankshaft oil seal, it is essential to check the recess into which it fits for the proper size. Excessive wear which enlarges the crankcase bore for any reason may cause the crankshaft oil seal to leak. An undersized crankshaft could result in the same poor fit and a leak. This is usually caused by a rusty or pitted surface which has been polished excessively. When a leak at the crankshaft oil seal develops after many hours of regular operation, it is usually the result of other problems. A leaking crankshaft oil seal is frequently caused by a restricted breather or an oil-slinger clearance that is too tight. The leak might also be caused by a propeller defect which places an abnormal side load on the crankshaft oil seal. To avoid the problem of oil leakage at the crankshaft oil seal because of an engine breather restriction, examination of the breather tube to determine its condition is an excellent idea. Total Training Support Ltd © Copyright 2020

If the tube is in good condition, also remember that the engine expels moisture through the tube. Under freezing conditions, there is some possibility that the moisture may freeze at the end of the tube, and ice builds up until the tube is entirely restricted. Should this happen, pressure may build up in the crankcase until something gives – usually the crankshaft oil seal. Since the airframe manufacturers know this is a possibility, and since they design to prevent engine-related problems of this kind, some means of preventing freeze-up of the crankcase breather is usually incorporated. The breather tube may be insulated, it may be designed, so the end is located in a hot area, it may be equipped with an electric heater, or it may incorporate a hole, notch or slot which is often called a ‘whistle slot.’ Because of its simplicity, the whistle slot is often used and is located in a warm area near the engine where it will not freeze. Aircraft operators should know which method of preventing freeze-up is used and then ensure that the configuration is maintained as specified by the airframe manufacturer. Should leakage at the crankshaft oil seal occur as a result of oil-slinger clearance which is too tight, the problem can initially be identified by checking for excessive end clearance. This can be done with a dial indicator. Remove the prop and then push the prop flange to the extreme aft position and zero the indicator. Then, pull the prop flange full forward and read the travel on the indicator. Compare this figure with the limits listed in the Table of Limits for the appropriate engine model.

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Should the end clearance exceed the limits specified, the oil slinger clearance should then be checked. First, remove the old nose seal, and clean the work area. Again, push the crankshaft to the rear of the engine. Insert a 0.002" feeler gauge about 3∕l6" wide between the oil slinger on the crankshaft and the crankcase. Again, pull the crankshaft forward. If the 0.002" feeler gauge is pinched tight, the required 0.002" to 0.007" clearance has been exceeded. Lack of appropriate clearance is the result of excessive wear on the crankcase thrust face which allows oil to be pumped out past the crankcase oil seal. Overhaul time is usually when the crankcase thrust face might receive a necessary repair. Should the crankcase oil seal be leaking excessively, it may simply mean that overhaul time has arrived early. Fortunately, this is something which does not happen very often. Crankcases with worn or damaged thrust face areas can be repaired by reworking the thrust face area to permit installation of new thrust-bearing washers. These bearings are available as repair items. Thrust-bearing washers may be reused if they do not show wear and if their thickness is sufficient to maintain compliance with the crankshaft and crankcase end-clearance specifications in the engine manufacture’s table of limits.

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Oil leak

Oil seals

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Oil system indications Oil system problems can be divided into these specified areas: • • • • • •

no oil pressure; low oil pressure; high oil consumption; broken oil pipes (oil everywhere!); kinked or deformed flexible oil pipes; and blocked filter.

Worn big end bearing shells This causes a reduction in oil pressure as the oil flow escapes into the crankcase.

Low oil pressure Low oil pressure can be associated with many things, but two main areas can be considered: • •

Worn piston rings This is indicated by blue smoke emitting from exhaust pipes during operation. Oil may block one of the crankcase oil breathers, as gasses escape past the piston rings into the crankcase and blow out the oil.

High oil temperature High oil temperatures can be caused primarily by the restriction of oil flow. This is usually attributed to: • • •

oil supply; and mechanical defects in the engine.

The oil which is supplied can be as a result of the conditions described in the previous section dealing with no oil supply. In this case, a reduction in oil supply rather than no oil supply is produced.

blocked or partially blocked internal pipe bores; lack of oil within the lubricating systems; or restricted oil cooler flows.

One cause may be a worn oil pump, another mis-set oil pressure relief valve, relieving too early. High oil consumption The mechanical defects associated with low oil pressure can also be related to the problem of high oil consumption. Typical examples are: • •

worn piston rings; and worn big end bearing shells.

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Oil quantity is measured with a dipstick. It is not usually indicated in the co*ckpit.

Oil system indications

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Chip detectors There are three types of chip detectors in everyday use: • • •

magnetic chip detectors; indicating magnetic chip detectors; and pulsed chip detectors

Magnetic chip detectors Many scavenge systems contain permanent magnet chip detectors which attract and hold ferrous metal particles which would otherwise circulate back to the oil tank and the engine pressure subsystem, possibly causing wear or damage. Chip detectors are a point of frequent inspection to detect early signs of main bearing failure.

Indicating magnetic-chip detectors The diagram below shows an indicating type magnetic chip detector. It has a warning circuit feature. When debris bridges the gap between the magnetic positive electrode in the centre and the ground electrode (shell), a warning light is activated in the co*ckpit. When the light illuminates, the flight crew must take whatever action is warranted, such as in-flight shutdown, continued operation at flight idle, or continued operation at normal cruise, depending on the other engine instruments readings.

As a general rule, the presence of small fuzzy particles or grey metallic paste is considered satisfactory and the result of normal wear. Metallic chips or flakes are an indication of serious internal wear or malfunction Note: The following safety precautions are required when fitting bayonet-type MCDs: • • •

ensure that serviceable seals are fitted; ensure that the bayonet prongs are in place and secure; and ground run for leak check after fitment.

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Magnetic chip detector

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Pulsed chip detectors A newer type is the electric-pulsed chip detector, which can discriminate between small wear metal particles, both ferrous and nonferrous, considered non-failure related, and larger particles, which can be an indication of bearing failure, gearbox failure, or other potentially serious engine malfunction. The pulsed chip detector looks like the indicating chip detector at the gap end, but its electrical circuit contains a pulsing mechanism which is powered by the aircraft 28 V DC bus.

The resulting burn off prevents a co*ckpit warning light from illuminating by opening the circuit before a time delay relay in the circuit activates to complete the current path to ground. If the debris is a large particle, it will remain in place after the burn-off cycle is completed and a warning light will illuminate in the co*ckpit when the time delay relay closes.

The pulsed detector is designed with either one or two operating modes; manual only or manual and automatic. In the manual mode, each time the gap is sufficiently bridged, regardless of the particle size, the warning light illuminates in the co*ckpit. The operator then initiates the pulse; electrical energy discharges across the gap end in an attempt to separate the debris from the hot centre electrode. This procedure is called bum off. If the light goes out and stays out, the operator considers the bridging a result of a non-failure related cause. If the light does not go out, or repeatedly comes on after being cleared, the operator must take appropriate action, such as reducing engine power or shutting down the engine. In the automatic mode, if the gap is bridged by small debris, a pulse of electrical energy discharges across the gap.

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Oil consumption monitoring There is no single standard oil consumption rate for a given type of engine. Each engine finds its level of oil consumption based on several factors: • • • •

the type of oil used; how the engine is broken in; the type of cylinders it has; and the type of flying for which it is used.

• • •

For example, if the oil consumption is mainly past the rings, the rate may go down if you switch from a single grade oil such as W100 to a multigrade oil such as 15W50. If the oil leakage is past the intake valves or leaks, the rate of oil consumption can go up if you switch to a multigrade oil. The absolute rate of oil consumption is not the critical factor; a significant change in the rate of consumption is more important. If your engine is using a litre (quart) of oil every eight hours and then starts to consume a litre (quart) in four hours under the same type of maintenance and operational conditions, an investigation should be conducted to find the cause of the increase. Low oil consumption is not necessarily a good thing; some engines such as large turbocharged engines require some oil past the rings to aid seating. Some engines tend to throw out the first litre (quart) of oil if you fill the sump to the maximum level, you might want to run one litre (quart) low, although care must be taken to ensure there is sufficient oil for long cross country flights. Formal oil consumption record should be generated for the engine installation. If oil consumption is more than one litre (quart) every three hours of operation or if the oil consumption Total Training Support Ltd © Copyright 2020

trend has changed substantially, conduct the differential compression and borescope examinations. If the oil consumption trend is stable and the oil consumption is less than one litre (quart) every three hours. Record the type of oil used. Record the number of litres (quarts) of oil added. Record oil change interval.

At every oil change, strain the oil and examine for debris. Also, cut open the oil filter and examine it for unusual material content; record the examination results of the strained oil, oil filter or screen. The presence of a substantial amount of material requires investigation to determine the source before further engine operation. Oil consumption can be expected to vary with each engine depending on the load, operating temperature, type of oil used and condition of the engine. A differential compression check and borescope inspection should be conducted if oil consumption exceeds one litre (quart) every three hours or if any sudden change in oil consumption is experienced and appropriate action taken. It is important to note that the current technology of general aviation aircraft reciprocating engines requires a certain level of oil consumption to assure proper lubrication of the cylinder walls and rings. Aircraft engines operate under much greater loads and at higher temperatures than automotive engines and require correspondingly greater oil use. In addition to lubrication, the oil serves as a coolant and as a means to transport contaminants, wear particles, acids and moisture

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from the engine at oil changes. Frequent oil changes based on operating hours or calendar time are critical to engine life.

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Oil particle analysis General Oil particle analysis is a tool to monitor wear material and contaminants in the engine. To be effective, a baseline of at least three analyses must be established from a single source to provide trend characteristics. For those engines with an established oil analysis profile, changes in iron, copper and other tracked elements can indicate unusual wear. In such cases, other diagnostic tools such as differential compression checks, borescope inspections, oil filter/screen examination and oil consumption trends can be useful in identifying the problem. Oil analysis can also detect air filtration or induction system leaks indicated by high silicon content. Note that oil analysis does not provide any indication of cracks, leaks or similar situations that could result in engine problems.

The oil analysis procedure is typically as follows. • •

•

If an oil analysis profile has been established, review the results for indications of wear or contamination. Based on the latest oil analysis, record the results of the profile trend. If the trend indicates an abnormal increase in material amounts, reference the recommended actions provided by the oil analysis laboratory. If no prior oil analysis exists, initiate sampling according to the instructions you receive with the oil analysis kit.

To establish a meaningful database for comparison, the oil samples must be taken on a regular schedule using the same sampling technique and laboratory. The engine must have operated long enough to obtain normal operating temperatures and the oil sample taken within 30 minutes after engine shut down. The tube or funnels used to drain the oil from the oil sump must be clean and free of any foreign material or residue. If the oil sample is taken from the oil as it drains from the sump, allow approximately ⅓ of the oil to drain before taking the sample. If the sample is taken via the oil filler or other location using a sampling tube, it is critical that the sample not be taken from the bottom of the sump, but at a location 2" to 3" above the bottom of the sump Under no circ*mstances should an oil sample be taken from the oil filter canister.

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Oil analysis report

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Chip detectors Magnetic chip detectors are magnetic plugs may be fitted in the scavenge oil return lines. Any ferrous particles carried in the oil is attracted to these plugs and caught. The average particle size caught by such plugs is around 200 microns and above. This may be in the form of a light furring, or it may be flakes of bearing surface material or it may be sharp, spiked steel particles or chips. The AMM usually gives guidelines on how to identify the debris. Magnetic chip detectors are removed as a set during periodic servicing. They are given an initial examination before being placed in a particular container together with documentation that identifies the engine and the position they came from. The replacement magnetic chip detectors are generally delivered as a set in a special container, so this is used to return the removed set. They are sent to a laboratory. In the case where an engine has been rejected then the plugs are left in the engine for dispatch back to the engine overhaul facility. Some magnetic chip detectors are electronically monitored, using one of two principles. One principle is to let ferrous material build up between to electrical poles. When sufficient debris has been accumulated, an electrical circuit closes. Another principle uses induction where a coil assembly monitors the ferrous build up on the magnetic pole.

Oil filters Disposable type oil filters are removed periodically and during fault diagnosis. The technician gives these an initial examination before packing and sending them to the laboratory for examination. Reusable filters are washed through filter papers to collect sediment and debris. The oil filters are capable of collecting ferrous and nonferrous particles. In many cases, the laboratory requires a sample bottle containing the oil from the filter housing. Debris particle examination This examination is used for particles from the magnetic chip detectors and filters. The particles are generally large and in the range of 200 to 500 microns. A binocular microscope is used to identify the size and shape of individual particles. An experienced operator can identify whether the particles are from teeth, bearings or from rotating seals by their general shape and appearance. Examples of this would be circular steel flakes that would be traceable to ball bearings while rectangular steel flakes would be traceable to roller bearings. The particles can be preserved in a fixative on a slide and be retained for future investigation. The engine will be rejected if flakes are found.

In both cases, a too high debris level results in some kind of alert, for instance via a post-flight report or MCDU warning.

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Metal particles in oil filter

Oil analysis kit

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Spectrometric oil analysis Branches of aviation have used spectrometric oil analysis program (SOAP) for many years. It is based on the fact that each element will display a specific pattern of light when a sample is used in a spectrometer. It applies to either reciprocating or turbine engines. A film of oil from the sample is placed and burnt between two graphite electrodes that are supplied with a high voltage AC. The spectrometric analysis for metal content is possible because metallic atoms and ions emit characteristic light spectra when vaporised by an electric arc. As the oil film burns, the light that emits passes through calibrated slits separating it into different wavelengths that depend on the trace elements present in the oil.

Ferrography The most powerful diagnostic tool in oil analysis is analytical ferrography; in fact, the only reliable test that can justify the removal of an engine.

The spectrum produced by each metal is unique for that metal. The position or wavelength of a spectral line identifies the particular metal. The intensity of the line can be used to measure the quantity of metal in a sample. Each wavelength is projected in its colour in the light spectrum. The colour identifies the element, for example, chromium, nickel, etc. and the intensity of the colour light is directly related to the amount of the element present. An alloyed material transmits several colours dependent on the elements they contain.

This is another process that is used to examine oil samples. It is used to identify the presence of ferrous particles and to measure the concentration in the sample.

This process is used to detect and measure the microscopic particles of wear metals in an oil sample in parts per million. The particles are in the range of one to ten microns and are too small to be seen or be captured by the magnetic chip detectors or the filters. The records of the results from the periodic samples from an engine can be recorded to give a wear trend chart. Any sudden increase in a particular element or alloy in a sample alerts the operators and assist in identifying the component that is wearing. Total Training Support Ltd © Copyright 2020

It consists of a detailed microscopic examination of a slide to determine: • • • • •

particle size and relative concentration; metallurgy, both ferrous and nonferrous; wear mechanism that indicates the root cause; component source of wear; and identification of contaminants.

The test can identify particles in the 5 to 100-micron range. The oil sample is passed across a glass slide that is under the influence of a varying magnetic field. This causes the particles to separate and form themselves into groups each containing different micron sizes. The slide is then examined by microscope to assess the nature and concentrations of caught material. Once again, the results can be recorded and compared with previous results to discover if there is any significant rate of change that would reveal an adverse trend. More sophisticated equipment employs optical sensors to measure the various particle concentrations.

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Licence Category B1 and B3

16.10 Engine Indicating Systems

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

Objectives: • The applicant should know the theory of the subject and interrelationships with other subjects. • The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. • The applicant should understand and be able to use mathematical formulae related to the subject. • The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. • The applicant should be able to apply his knowledge in a practical manner using the manufacturers' instructions. • The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Engine speed;

Part-66 Ref. 16.10

Knowledge Levels A B1 B3 1 2 2

Cylinder head temperature; Coolant temperature; Oil pressure and temperature; Exhaust gas temperature; Fuel pressure and flow; Manifold pressure.

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Table of Contents Parameters and abbreviations____________________ 6 co*ckpit engine indication layouts_________________ 6 Analogue engine displays _______________________ 6 Electronic engine displays______________________ 10 Hobbs meter _________________________________ 12 Engine speed (RPM) ___________________________ General ____________________________________ Mechanical tachometers _______________________ Electromechanical tachometers _________________ Inductive probe type (Hall sensor) _______________

14 14 16 18 20

Pressure measuring instruments ________________ Types of pressure ____________________________ Capsules and bellows _________________________ Bourdon tube _______________________________ Strain gauges _______________________________ Piezo-resistive sensors ________________________ Variable frequency signals _____________________

22 22 22 24 26 26 28

Temperature measuring instruments _____________ Non-electrical temperature measurements _________ Resistance thermometers (thermistors) ___________ Indicators __________________________________ DC ratiometer _______________________________ Thermocouples ______________________________

30 30 32 34 36 38

Manifold pressure and boost pressure gauges _____ Manifold pressure ____________________________ Boost pressure ______________________________ Operation __________________________________ MAP sensor ________________________________

40 40 41 42 44

Oil pressure, temperature and quantity ____________ 46 Layout _____________________________________ 46 Oil pressure _________________________________ 48 Oil temperature ______________________________ 52 Oil quantity __________________________________ 54 Cylinder head temperature, exhaust gas temperature, coolant and carburettor temperatures _____________ 56 Cylinder head temperature (CHT) ________________ 56 The indicator ________________________________ 60 Exhaust gas temperature (EGT) _________________ 62 Coolant and carburettor temperatures _____________ 64 Fuel pressure and flow _________________________ 66 Fuel pressure ________________________________ 66 Fuel flow ___________________________________ 68 Fuel used ___________________________________ 74 Diesel engine indicating ________________________ 76 co*ckpit control and instrumentation _______________ 76 FADEC sensors ______________________________ 76

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Parameters and abbreviations

co*ckpit engine indication layouts

The following abbreviations are commonly used:

Analogue engine displays Analogue gauges, often called 'steam gauges', are those mechanical, or electromechanical gauges that indicate with a moving needle against a scale. The diagram below shows a typical piston engine gauge layout in the co*ckpit.

EGT: CHT: RPM: MP or MAP: FF: Press: Temp:

exhaust gas temperature cylinder head temperature engine speed revs per minute manifold air pressure fuel flow pressure temperature

Frequently, two or more indications are incorporated into one unit. For example, oil pressure and oil temperature may share the same indicator unit.

The manifold pressure and RPM gauges are considered to be a 'primary' or 'performance' instruments because they usually provide some indication of the power output of the engine, so it is larger and positioned with the other primary aircraft instruments. On some light aircraft, only the RPM gauge is provided. The 'secondary' or 'condition' gauges (EGT, CHT, fuel flow/pressure, oil pressure/temperature, etc.) are arranged together in a cluster, which makes for easy scanning by the pilot. These indications provide the pilot with a measure of the condition of the engine for any given power level.

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Instrument cluster on a piston engined helicopter

Common piston engine analogue indications

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A piston engine analogue indication cluster Piston engine indications on an EFIS display

An example of a composite indication 10-9

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Electronic engine displays Many modern small aircraft are fitted with electronic engine instrument displays. These are often fitted even if the flight instruments are the more conventional 'steam gauge' type. They contain a processor which converts all sensed inputs into digital format, as requires, and generates the symbols and digits for display on the screen. Some older displays use LED arrays or, on more modern systems, LCD screens. The representations may be in a bargraph format or may be a simulation of the conventional moving pointer and scale. Usually, a combination of digital and analogue style displays are presented. If the aircraft has a modern EFIS type display, then the engine instruments can also be displayed on the EFIS multi-function display, on a section of the screen which is dedicated to engine instrumentation.

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Piston engine indications inset in an EFIS display

An electronic piston engine display unit 10-11

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Hobbs meter 'Hobbs meter' is a genericised trademark for devices used in aviation to measure the time that an aircraft is in use. The meters typically display hours and tenths of an hour, but there are several ways in which the meter may be activated: • •

•

It can measure the time that the electrical system is on which maximises the recorded time. It can be activated by oil pressure running into a pressure switch, and therefore runs while the engine is running. Many rental aircraft use this method to remove the incentive to fly with the master electrical switch off. Another switch can activate it, either an airspeed sensing vane under a wing (as in the Cessna Caravan) or a pressure switch attached to the landing gear (as in many twin-engine aeroplanes). In these cases, the meter only measures the time the aircraft is flying. Metrics such as 'time in service' and 'turbine actual runtime' are kept monitoring overhaul cycles and are usually used by commercial operators. It can be activated when the engine alternators are online (as in the Cirrus SR series).

At these times, the clock runs slower. Depending on the type of flight, tach time can be 10–20% less than Hobbs time. Many organisations, such as flying clubs, charge by tach time to differentiate themselves from fixed-base operators because 10–20% less time recorded makes it 10–20% cheaper to fly (if the hourly rate is the same). In the case where flying clubs use tach time, many charge a dry rate, thus requiring the renter to pay for fuel on top of the hourly tach time rate. On most modern piston engine aircraft, the Hobbs meter is incorporated into the RPM gauge.

Hobbs time is usually recorded in the pilot's logbook, and many fixed-base operators that rent aeroplanes charge an hourly rate based on Hobbs time. Tach (tachometer) time is recorded in the engine's logbooks and is used, for example, to determine when the oil should be changed and the time between overhauls. Tach time differs from Hobbs time in that it is linked to engine revolutions per minute (RPM). Tach time records the time at some specific RPM. It is most accurate at cruise RPM, and least accurate while taxiing or stationary with the engine running. Total Training Support Ltd © Copyright 2020

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A Hobbs meter combined with the RPM gauge

A standalone Hobbs meter

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Engine speed (RPM) General The tachometer often referred to as the 'tach', shows the engine crankshaft RPM. The system used for block testing the engine is the same as the system in the aircraft installation. The figure below shows a tachometer with range markings. The tachometer is calibrated in hundreds of revs per minute (RPM x 100). A blue arc on the tachometer indicates the RPM range within which auto-lean operation is permitted. The bottom of this arc indicates the minimum RPM desirable in flight. The top indicates the RPM at which the mixture control must be moved to auto-rich. A green arc indicates the RPM range within which auto-rich operation is required. The top of the green arc indicates maximum continuous power. All operation above this RPM is limited in time (usually 5 or 15 min.). A red line indicates the maximum RPM permissible during takeoff; any RPM beyond this value is an overspeed condition.

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Tachometer gauge, often combines with an engine hours meter, sometimes known as a Hobbs meter

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Mechanical tachometers Older aircraft have a mechanical cable-driven tachometer. Inside the instrument are a set of flyweights to which a pointer is attached, indicating RPM. These are not used much nowadays, but you may encounter them on older aircraft. The necessary components are a flex drive from a drive outlet on the engine and an indicator in the co*ckpit. The diagram below right shows a basic indicator. It consists of a pivoted weight on a spindle which rotates at the engine speed, causing the centrifugal forces to act on the weight; it tries to assume a horizontal position but is opposed by the springs. Later types have the cable attached to a magnet which rotates inside an aluminium cup. In this cup, eddy currents (generated by the rotating magnet) create a magnetic field coinciding with the magnetic field from the magnet. The cup is fixed to a hairspring and pointer so it cannot rotate, but it is dragged along by the magnetic field, hence the name 'drag cup'. Mechanical drive tachometers have one main drawback, and that is the flexible drive. It has a limited wear life, and it is not recommended for use over more than 7.5 m (25 ft) and is limited to a nine-inch radius bend. The flex drive consists of a flexible outer casing supporting the inner drive. The casing is formed by spirally wound interlocking metal strips of either brass or steel. The interlock is packed with asbestos stringing and crimped with a union nut at each end.

At the remote, receiving end of the control run, the cable passes through an adjustable stop and is connected to the component operating lever by the nipple or bolts. The purpose of the stop is to provide a means of adjusting the length of the conduit, thus altering the range of movement of the control cable. Where a single cable operates two components, a junction box is used. The inner drive is made of a central steel core on which layers of steel wire are wound left then right alternatively. A square drive is then soldered or swaged onto each end. The inner drive is retained in the outer case by slip washers located in a recess in the casing union and clip into a groove on the shank of the drive connector. The two vent holes in the union prevent oil from reaching the indicator. If a flex drive longer than 3.5 m (15 ft) is used, the readout on the gauge is erratic and is in frequent need of attention, as well as having a short working life. There are three types of drive coupling used on tacho generators, they are: • • •

flexible; splined; and two-pin.

The most common type in use is the splined type. The operating speed is reduced by gears in the engine drive system to reduce the mechanical load on the generator. If the generator is running at one quarter engine speed, then the indicator is calibrated to read four times engine speed.

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Principle of the tachometer using the magnetic drag cup

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Electromechanical tachometers You are most likely to encounter electromechanical tachometers. They are preferable to the mechanical type as they do not require a flexible drive to give information to the indicator. A conventional type is known as the magnetic-drag tachometer.

The type of generator shown in the diagram below (known as a 'tachogenerator') has a 2-pole permanent magnet rotor with a three-phase stator coil producing an alternating current whose frequency is related to the speed of rotation.

The tachometer consists of two units: • •

a transmitter; and an indicator.

A three-core cable electrically connects them. The generator, which is driven by the engine, has a three-phase alternating current output. Most tacho generators work on similar principles, but variations in design are common. Each generator consists of a stator and a rotor, the stator being a hollow cylinder of laminated iron which has a three-phase star-connected winding. The conductors of the stator are arranged in slots cut around the inner surface of the cylinder. The rotor will either be a 2-pole or a 4-pole type; a 2-pole is illustrated in the diagram below. The 4-pole version has skewed poles so that when the end of one pole leaves a stator, the other end is entering the next stator which produces a better waveform and an even driving torque and prevents the 'cogging effect'. The 2-pole version achieves the same effect by skewing the stator teeth and the individual coils which make up a phase.

The rotor is a permanent magnet which rotates inside the stator inducting alternating electromotive forces in the stator windings. The rotor is mounted on a spindle and runs in a ball or Oilite® bearing. The Oilite bearings retain oil and allow for long periods for use without maintenance.

The mounting flange also carries the electrical termination. The square drive shaft is flexible enough to allow slight misalignment with the drive coupling. At each end of the rotor shaft are ball bearing assemblies which are pre-packed with grease. The rear bearing is pre-loaded to eliminate end shake. The indicator is a synchronous motor type. The output of the generator turns a rotor inside the indicator. When the speed of the rotor field equals the rotating field of the generator, the two fields lock, and they both rotate synchronised with each other. The measuring device consists of a 4-pole magnet mounted on the end of the motor spindle and a copper drag cup with surrounds the magnet. As the magnet rotates, it tends to turn the copper drag cup which in turn moves the pointers over the scale. The hairsprings give the pointer a dead-beat type of movement, thus reducing the fluctuations of the pointer.

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The indicator instrument case is constructed of steel. It is grounded to form a Faraday's cage, to prevent the magnetic flux from within, interfering with other instruments on the instrument panel. Drag cup or eddy current tachometer https://www.youtube.com/watch?v=WGRjodIvFkM Animation | How speedometer works | Eddy current type https://www.youtube.com/watch?v=3_vfyt6-2Ic

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Inductive probe type (Hall sensor) The inductive probe speed sensor type is a speed sensor which is used to measure the speed of a shaft. Its operational principle is known as the 'Hall effect'. This type of speed sensor is often called a 'Hall sensor'. These sensors are used on engines which have a FADEC control system. The ECU/EEC of the FADEC requires a highly accurate speed signal. It consists of an electromagnetic part named the pick-up which is positioned near to a gear that is rotating on a shaft, e.g. inside the gearbox. An inductive sensor, for example, a crankshaft position sensor, is one that uses an induced electrical current to signal the ECU. For example, the crankshaft speed/position sensor uses induction to produce a sine wave that is sent back to the ECU, which then analyses this data and determines the speed and position of the crankshaft at any given instant. An inductive sensor normally has a permanent magnet, surrounded by a coil of wire, wound into many turns around, but not touching the magnet. Then, some sort of toothed wheel (known as a reluctor wheel, or phonic wheel) is mounted on to the shaft whose speed and position we wish to sense. The toothed wheel is made of ferrous material, and as the wheel moves past the sensor, it 'traps' and 'releases' the flux from the permanent magnet in the sensor.

The sensors on FADEC controlled engines are always dual sensors. As the FADECs ECU/EEC has two channels, the sensors supply speed information to both channels in the ECU/EEC. If one ECU/EEC channel or one of the dual-sensor elements fail, there is a possibility to switch to the other ECU/EEC or sensor and still provide speed indication. The FADEC may also require information on the position of the crankshaft at any moment in time. This gives the FADEC the information of the position of the piston relative to its TDC. On direct fuel-injected engines (i.e. all modern Diesel engines) this information is crucial since the timing of the fuel injection before TDC is critical. If the fuel is injected at the wrong time, the performance of the engine is degraded at best and catastrophic at worst. The crankshaft position signal also allows the FADEC to monitor the fuel injection timing many times per second, adjust as necessary and even change the injection timing according to engine load and RPM. The crankshaft position can be provided by the same inductive probe type sensor that provides the engine RPM signal. This is done by ensuring that one tooth of the phonic wheel or gear that is being measures has an extra concentration of ferrite. This makes the signal have an extra-large 'blip' at each rotation of the wheel. The position of the blip is proportional to the phase of the rotation or crankshaft position.

This movement of the magnetic field generates a pulsing signal in the coil of wire, and this signal is fed back to the ECU.

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Crankshaft with reluctor wheel

Inductive probe for RPM indication

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Pressure measuring instruments Pressure measuring instruments use either a diaphragm or bourdon type of sensor. The choice depends on the pressure the instrument is working with; bourdon types are more suited to handle higher pressures. Reading is either direct, via a pressure line running to the co*ckpit, or indirect with electrical wires and the sensor close to the engine, mounted either on the engine or firewall. All pressure must be measured from some known reference. Absolute pressure is measured from zero pressure or a vacuum. Gauge pressure is measured from the existing atmospheric pressure, and the differential pressure is the difference between two pressures.

When used to measure differential pressure, as it is when used as a fuel pressure gauge, one bellows senses the air pressure at the carburettor inlet, and the other bellows sense the fuel pressure at the carburettor fuel inlet. A differential bellows can be used to measure gauge pressure by leaving one of the bellows open to the atmosphere and the other connected to the pressure to be measured. Capsules and bellows

Types of pressure Absolute pressure instruments - This instrument uses a sealed, evacuated, concentrically corrugated metal capsule, known as an aneroid capsule, or diaphragm, as its pressuresensitive mechanism. The concentric corrugations provide a degree of springiness that opposes the pressure of the air. As the air pressure increases, the thickness of the capsule decreases, and as the pressure decreases, the capsule expands. A rocking shaft, sector gear, and pinion multiply the change in dimension of the capsule and drive a pointer across a calibrated dial. Gauge pressure instruments - Gauge pressure is a measure of existing barometric pressure plus the pressure that has been added to the fluid over and above atmospheric pressure. Total Training Support Ltd © Copyright 2020

Differential pressure instruments - Differential pressure is simply the difference between two pressures. A differential bellows, like that in the diagram top left, is a popular instrument mechanism that can be used to measure absolute, differential, or gauge pressure.

Aneroid capsule - An aneroid capsule is a thin, disk-shaped capsule, usually metallic, partially evacuated and sealed, held extended by a spring, which expands and contracts with changes in atmospheric or gas pressure. It is used to measure small gas pressures such as atmospheric pressure. The gas pressure acts on the outside of the capsule, so when the pressure increases, the capsule contracts. The contraction is connected to a scale, via a quadrant gear and pinion, which amplifies the movement. Bellows –The pressure to be measured is taken into the bellows. As the pressure increases, the bellows expands, and its expansion rotates the rocking shaft and the sector gear. Movement of the sector gear rotates the pinion gear and the shaft on which the pointer is mounted.

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Differential bellows with indication mechanism Aneroid capsule (bellows)

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Bourdon tube A Bourdon tube is typically used to measure gauge pressure. This tube is a flattened thin-wall bronze tube formed into a curve. One end of the tube is sealed and attached through a linkage to a sector gear. The other end is connected to the instrument case through a fitting that allows the fluid to be measured to enter. As the pressure inside the tube increases, it tries to change the cross-sectional shape from flat to round. As the cross-section changes, the curved tube tends to straighten out which, in turn, moves the sector gear, which rotates the pinion gear on which the pointer is mounted. Bourdon tube instruments measure relatively high pressures like those in engine lubricating systems and hydraulic systems

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Bourdon tube Bourdon tube

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Strain gauges These passive electric devices are used to detect forces. The resistance of strain-gauges varies with force applied to it. The metallic wire consists of a chrome-nickel alloy. The length and diameter of the conductor changes as a function of the force. The expanding force increases the resistance, and a shortening force decreases it. One or more strain gauges are bonded to a flexible diaphragm. The diaphragm assembly is housed inside a sealed chamber, into which the pressure to be measured is fed. An increase in pressure causes the diaphragm to stretch, and with it, so does the strain gauges. The increase in resistance of the strain gauges is then measured using a Wheatstone bridge circuit, and the output is indicated on a sensitive voltmeter calibrated in psi. Piezo-resistive sensors P- or N- conducting elements are diffused into a pure silicon substrate. This so-called piezo-resistive effect changes the resistance with a much higher sensitivity than what a metallic strain gauge does. Semiconductor-based sensors are in many different forms. The substrate of the pressure sensor shown, in the diagram bottom right, has a dimension of 3.5 × 3.5 mm (1∕8" × 1∕8"). Inside there is a bridge with four elements. The output of the sensor is measured and indicated in much the same way as that for the strain gauge type.

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Piezo-resistive element

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Variable frequency signals A variable frequency signal has a frequency which is controlled by a specific parameter. A device with a variable output frequency makes such a signal. The frequency varies, under control of the parameter, between a high and a low frequency. These limit frequencies are different from device to device and depend on the design of the device. A control voltage, a variable capacitor, and a variable resistor are, for example, parameters that control the frequency. Frequency counters, microprocessor systems and special moving coil meters are all devices that work with variable frequency signals. The diagram below top shows a very sensitive and accurate pressure transducer used inside air data computers. The oscillator coil assembly oscillates the diaphragm. Its resonant frequency increases with the applied pressure against the vacuum reference inside the transducer. The output frequency, proportional to the pressure, is easily changed inside the computer, into a digital signal. The temperature sensing resistor compensates for influences of the ambient temperature.

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Vibrating diaphragm transducer

Pressure to digital conversion

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Temperature measuring instruments Temperature is one of the most critical measurements in aircraft operation. Operational temperatures range from well below freezing for outside air, fuel, oil, air conditioning and pneumatic air, to around 1,000°C for exhaust gas temperatures. Non-electrical temperature measurements Most solids, liquids, and gases change dimensions proportional to their temperature changes. These dimensional changes may be used to move pointers across a dial to indicate changes in temperature.

A thin-wall, hollow metal bulb is connected to the Bourdon tube by a capillary tube, that has a very small inside diameter. The bulb is filled with a volatile liquid such as methyl chloride which has a high vapour pressure, and the entire bulb, capillary, and Bourdon tube are sealed as a unit. The bulb is placed where the temperature is to be measured. As the temperature changes, the pressure of the vapours above the liquid changes. This pressure change is sensed by the Bourdon tube, which moves a pointer across a dial that is calibrated in degrees Fahrenheit or Celsius.

Bimetallic strip – Most small general aviation aircraft have an outside air temperature gauge protruding through the windshield. This simple thermometer is made of strips of two metals having different coefficients of expansion welded together, side by side, and twisted into a helix, or spiral. When this bimetallic strip is heated, one strip expands more than the other, and the spiral tries to straighten out. A pointer is attached to the metal strip to indicate temperature changes. Gas expansion – Temperature is determined by measuring the pressure of the vapours above a highly volatile liquid. The vapour pressure varies directly as the temperature of the liquid. The Bourdon tube consists of a hollow brass or bronze elliptical-shaped tube formed into a semi-circle. One end is open and connected to the fluid to be measured, and the opposite end is sealed. As pressure is applied, the elliptical tube changes shape and tends to straighten the semi-circular curve. The bourdon tube needs to be attached to a mechanical linkage and pointer to create a useful instrument. Total Training Support Ltd © Copyright 2020

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Remote temperature indication with Bourdon tube

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Resistance thermometers (thermistors) The thermistor type is a portmanteau for 'thermal resistor' type of sensor. A thermistor used for temperature measurement can be used for control, indication and protection. Its standard operating temperature range is from -60 °C to +200 °C. The device is usually a platinum or nickel wire sensor wound on a former made of an insulating material such as mica. This assembly is enclosed within a steel tube. The resistance of the wire changes with changes in temperature. Hence, it acts as the variable resistance element. There are two types: •

•

A negative temperature coefficient (NTC) resistor. Its resistance decreases at increasing temperatures. So, it is called: high-temperature conductor. A positive temperature coefficient (PTC) resistor. Its resistance increases with increasing temperature. So, it is called: low-temperature conductor.

The sensor is often referred to as a temperature 'bulb'. This bulb is immersed in the fluid whose temperature is being measured. The resistance of the nickel-chrome wire varies directly with its temperature. Resistance thermometers can often be found with double windings to act as dual-channel devices in a single unit, particularly for FADEC controlled engines.

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Resistance thermometer probes Resistance thermometer probe

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Indicators Any temperature indicator that uses a temperature-sensitive resistor for its sensor requires a power supply and an indicator that can measure the resistance sensitively enough to indicate as a measure of the temperature. Two conventional methods are used to achieve this: • •

The Wheatstone bridge circuit The ratiometer

Wheatstone bridge circuit A Wheatstone bridge circuit consists of three fixed resistors and one variable resistance, whose resistance varies with temperature. When power is applied to a Wheatstone bridge circuit, and all four resistances are equal, no difference in potential exists between the bridge junctions ('A-B' in the diagram below). However, when the variable resistor is exposed to heat, its resistance increases, causing more current to flow through the fixed resistor R3 than the variable resistor R4. The disproportionate current flow produces a voltage differential between the bridge junctions A-B, causing current to flow through the galvanometer indicator. The greater the voltage differential, the higher the current flow through the indicator and the greater the needle deflection. Since indicator current flow is directly proportional to the temperature, an indicator calibrated in degrees provides an accurate means of registering the temperature.

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Wheatstone Bridge

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DC ratiometer A ratiometer circuit measures current ratios and is more reliable than a Wheatstone bridge, especially when the supply voltage varies. Typically, a simple ratiometer circuit consists of two parallel branches powered by the aircraft electrical system. One branch consists of a fixed resistor and coil, and the other branch consists of a variable resistor and coil. The two coils are wound on a rotor that pivots between the poles of a permanent magnet, forming a meter movement in the gauge. The shape of the permanent magnet provides a larger air gap between the magnet and coils at the bottom than the top. Therefore, the flux density, or magnetic field, is progressively stronger from the bottom of the air gap to the top. Current flow through each coil creates an electromagnet that reacts with the polarity of the permanent magnet, creating a torque that repositions the rotor until the magnetic forces are balanced.

Ratiometer temperature measuring systems are especially useful in applications where accuracy is critical or significant variations of supply voltages are encountered. Therefore, a ratiometer circuit-type temperature-sensing system is generally preferred over Wheatstone bridge circuits by aircraft and engine manufacturers. Notes: 1. Variation in input voltage does not affect readout. 2. An open circuit in the sensor causes the instrument to go to full-scale deflection. 3. A short circuit in the sensor causes the instrument to go to a minimum (off-scale) position. 4. A hairspring is not required (as in a moving coil instrument), any hairspring used is only to take the needle indicator off the scale.

If the resistance of the temperature probe and fixed resistor are equal, the current flow through each coil is the same, and the indicator pointer remains in the centre position. However, if the probe temperature increases, its resistance also increases, causing a decrease in current through the temperature-sensing branch. Consequently, the electromagnetic force on the temperature sensing branch decreases, creating an imbalance that allows the rotor to rotate until each coil reaches a null or balance. The pointer attached to the rotor then indicates the oil temperature.

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DC ratiometer

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Thermocouples Higher temperatures, like those found in the exhaust gases of both piston and turbine engines, are measured with thermocouples. A thermocouple is a loop made of two different kinds of wire welded together at one end to form a hot or measuring junction. For example, Chromel and Alumel wires are used. The coil of a current measuring instrument is connected between the wires at the other end to form a cold, or reference junction. The hot junction is held against the cylinder head in the spark plug gasket, and a voltage is produced in the thermocouple whose amount is determined by the difference in temperature between the hot and cold junctions. This voltage difference causes a current to flow that is proportional to the temperature of the cylinder head. The indicator of a thermocouple system is a sensitive millivoltmeter, calibrated to indicate in °C or °F. The thermocouple oil temperature indicating the system is not powered by the aircraft bus. It is a self-contained and selfgenerating circuit. It derives its power from a pair of dissimilar metals; iron and constantan, which when heated at the hot junction, produce a milli-voltage and cause a current flow through a co*ckpit indicator. The amount of EMF generated depends on the amount of heat which is sensed by the thermocouple. The more heat, the higher the current flow, the bigger the EMF and the greater the temperature reading on the indicator.

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Thermocouple principle

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Manifold pressure and boost pressure gauges The manifold pressure gauge is an engine instrument typically used in piston aircraft engines to measure the pressure inside the induction system of an engine. Engines with a controllable propeller of some sort need a manifold pressure gauge to set the correct power setting with the selected propeller RPM. It reduces fuel consumption and by having a slight over-square condition, for example, 2,200 RPM at 25 inHg (inches of mercury) MAP, engine efficiency is also much improved. Usually, an aneroid capsule or diaphragm connected to a pointer with the scale calibrated in pressure, inHg or hPa/bar. The instrument can be made to show either only boost (pressure above atmospheric) or manifold pressure, from 10 to 30 inHg and for turbocharged engines, the scale even goes higher, up 50 inHg and more. Manifold pressure The manifold pressure gauge is an engine instrument typically used in piston aircraft engines to measure the pressure inside the induction system of an engine.

When you can measure how much air pressure is in the induction system, just before the air/fuel mixture enters into the cylinders, you have a good idea of how much power you are developing. In normally aspirated engines (non-turbo-charged), the manifold pressure gauge has a range of anywhere between 10 – 40 in.hg. In a turbocharged engine, the manifold pressure is allowed to go as high as the engine manufacturer allows. When the engine is shut down, the manifold pressure gauge should read very close to the current atmospheric pressure setting. To equate manifold pressure to aircraft performance, we need to look in section 5, or the performance section of the flight manual (see below). In the sample shown below, you can see that at 8,000 ft pressure altitude, -2°C, and 2,450 RPM we would be developing about 19.5 in.hg in the induction system. You can also see how that would then be related to fuel flow and our true airspeed.

This measurement, read in inHG or psi, is one of the best methods to determine just how much power is being developed by the engine. The more air and fuel we can pump or pull into the cylinders, the more power the engine can develop (which makes us fly faster).

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Boost pressure The reason for a boost gauge is to indicate the pressure of the fuel/air mixture in the induction system of a supercharged engine concerning the standard atmospheric pressure which as we know is approximately 14.7 psi. The boost gauge gives a reading relative to standard sea level pressure. The boost gauge dial is calibrated to show both positive and negative pressures. The pointer only registers zero when the induction pressure is 14.7 psi; if the pressure decreases to 12.7 psi, then the boost gauge indicates -2 psi.

Manifold pressure and RPM gauges usually used together

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Operation The principle of the manifold pressure and boost pressure gauge is that of the aneroid capsule. The capsule is subjected to induction pressure and as the capsule expands or contracts it operates a quadrant which in turn drives the pointer. You can also note this sequence of events if you refer to the diagram below. The bezels of the gauges are often coloured red to indicate that they are associated with the fuel system. Fitted to the bezel is an adjustable lubber mark to indicate the maximum permissible boost of the engine to which the gauge is connected. A grub screw locates the lubber mark in the bezel, or the bezel itself may lock it as it is tightened.

If there is an increase in pressure, the capsule contracts and pull the connecting link down. This pulls on the cross arm rotating the layshaft and quadrant in the direction of the arrow. As the quadrant moves, it actuates a pinion on the pointer spindle which in turn rotates the pointer clockwise indicating a pressure increase. A decrease in pressure causes the capsule to expand. There are difficulties in maintaining the cases of boost gauges free from leakage. An instrument was developed so that the airtight case was not necessary to overcome this difficulty. This instrument is illustrated below.

Induction pressure is fed to the mechanism via a union at the rear of the case, as shown below. The union houses a small gauge filter and a choke. The filter prevents foreign matter from getting into the instrument. The choke, a restricting orifice (hole), is to protect the mechanism form pressure surges, i.e. if the engine backfires. Gauge mechanisms vary considerably in design, but their principle of operation remains the same. Two representative types are described in the following paragraphs. The diagram below illustrates a typical gauge mechanism of the type which uses an airtight case. Induction pressure is fed directly into the case from the union. Although it is evacuated of air, i.e. a vacuum, the capsule is strong enough to resist collapsing due to the surrounding pressure.

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The mechanism is formed by two flexible metal bellows, the outer ends of which are fixed to the machine frame and the inner ends connected by a distance piece. The vacuum bellows in the diagram is exhausted of air, but a spring keeps it distended. The pressure bellows is connected to the union at the rear of the instrument, i.e. induction pressure enters the bellows, but not in the case, applying a force to the evacuated bellows via the connecting link. When the engine is at rest and the induction pressure is at atmospheric pressure then the pressure bellows does not exert any pressure on the vacuum bellows because the pressure inside the pressure bellows is the same as that surrounding it, which is atmospheric. The evacuated bellows locate the distance piece, and the internal force from the spring balances the external force of atmosphere. When the engine runs the induction, the pressure causes the bellows to push or pull the link; depending on whether the pressure is increasing or decreasing. The resultant movement rotates the pointer on the distance piece. The quadrant drives a pinion on the pointer spindle and moves the pointer over the scale. The hairspring on the pointer spindle takes up any backlash between gears and link pins.

MAP sensor The manifold absolute pressure sensor (MAP sensor) is one of the sensors used in an engine's electronic control system. Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provides instantaneous manifold pressure information to the engine's electronic control unit (ECU). The data is used to calculate air density and determine the engine's air mass flow rate, which in turn determines the required fuel metering for optimum combustion and influence the advance or retard of ignition timing. A fuel-injected engine may alternatively use a mass airflow sensor (MAF sensor) to detect the intake airflow. A typical naturally aspirated engine configuration employs one or the other, whereas forced induction engines typically use both; a MAF sensor on the intake tract pre-turbo and a MAP sensor on the charge pipe leading to the throttle body. MAP sensor data can be converted to air mass data. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation.

Although the bellows are subjected to atmospheric pressure externally, any change of atmospheric pressure due to altitude does not affect the position of the bellows. This is because both bellows are of the same surface area, i.e. the effect on one bellows is equally opposed by the effect on the other.

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Manifold pressure gauge internal mechanism

Manifold pressure indicator

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Oil pressure, temperature and quantity Layout The diagram below shows typical locations of the sensors for the oil system pressure and temperature. The pressure indicator is a bourdon tube (older type aircraft) or a temperature-sensitive resistor (thermistor) type. Temperature indicators are remote type systems which use a thermistor sensor which is connected electrically to the indicator. The indicator is a Wheatstone bridge or ration meter.

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A composite analogue oil temperature and pressure indicator

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Oil pressure Bourdon tube / autosyn transmitter Older, and larger, aeroplanes use the bourdon type of transmitter.

Oil pressure indication The oil pressure indicator has markings for normal operating range; this is the green range.

The oil pressure indicating system is an autosyn design, powered by 26 V AC or 115 V AC from the aircraft bus. The oil pressure transmitter receives two input pressure signals; one, engine vent subsystem pressure and the other, engine oilpressure subsystem-pressure. The signals apply pressure to a pair of opposing bourdon tubes which are linked mechanically to an electromagnetic coil.

Another range, which is yellow, is one where the oil pressure is either too low or too high, and the crew must take some action according to a checklist. If the pointer enters the red area, the engine must be stopped immediately.

When the magnet rotates within its electrical field, the indicator magnet also rotates because it is in a similar coil connected in parallel with the transmitter coil.

A low oil pressure warning light is also provided beside the normal oil pressure indication. Too low oil pressure may damage the engine as it is not lubricated sufficiently. The indication is taken from the same location as the oil pressure transmitter and with the same reference pressures. The transmitter can be formed as a switch or as a transmitter which has a fixed trigger point. The warning given is red, and with a few exceptions, the engine must be shut down immediately when it illuminates. Low oil pressures (and warnings) are seen in cold weather with decreased oil flow capability.

By utilising two pressure inputs, this system algebraically subtracts vent pressure from the pressure subsystem fluid pressure, giving a differential oil pressure indication in the co*ckpit. Many engines require this to give an accurate co*ckpit indication due to oil flow through to the engine.

For electronic instruments, the colour markings are the same, but the presentation is different.

Although the oil pressure gauge indication, and the oil pressure low warning switch could be provided from a common pressure sensor, often two separate sensors are provided (a transducer sensor and a pressure switch). This provided redundancy in the event of the failure of one or other of the sensors.

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Bourdon tube pressure type transmitter

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Oil pressure sensor and switch

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Direct reading system

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Oil temperature Two types of sensors exist for oil temperature measurements, the thermocouple and the thermistor. Temperature indicators are remote type systems which use a thermistor type sensor which is connected electrically to the indicator. The indicator is a resistance indicator calibrated in degrees Celsius or Fahrenheit. Oil temperature indication The oil temperature indicator typically ranges from 0 to 200 °C. It has a marked green range that informs the crew that the engine oil temperature is within the normal range. It also has a yellow range which is the caution range where the crew must take some action, e.g. check the other engine indications.

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Oil quantity A direct-reading type indicator can be marked with a scale or a yellow or red index. The oil quantity is measured only when the engine is stationary. Piston engines do not have an oil quantity indication in the co*ckpit. Some engines have a combined oil filler and dipstick which has a scale marked in imperial quarts, US gallons and litres. There is no indication in the co*ckpit for oil quantity. A bayonet cap usually locates the dipstick. Many engines have a sight glass instead of a dipstick. The oil quantity indicator is typically located on the oil tank itself and may have a dual scale, one for the engine running and another scale for the engine not running. For some engines, the aircraft maintenance manual and flight manual specifies a minimum time must elapse (typically 30 minutes) after the engine has been shut down, before reading the oil level.

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Typical dipstick quantity markings

Dipstick oil quantity indicators

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Cylinder head temperature, exhaust gas temperature, coolant and carburettor temperatures Cylinder head temperature (CHT) The cylinder head temperature gauge illustrated below is a dual gauge that incorporates two separate temperature scales. A thermocouple probe is fitted in the cylinder head. Each cylinder should have one, but it is usually installed on the hottest cylinder (whichever that may be, this does not so much depend on the position but the mixture entering the cylinders). Made from iron- or copper-constantan (copper and nickel alloy) and able to measure up to 400 °C. A common alternative location for the thermocouple hot junction is under a spark plug. The sensor is the gasket for the spark plug. Usually, only one cylinder has the temperature sensed in this way. This is the cylinder that is known to run hottest. A blue arc on the gauge (when used) indicates the range within which operation is permitted in auto-lean. The bottom of this arc indicates the minimum desired temperature to ensure efficient engine operation during flight. The top of the blue arc indicates the temperature at which the mixture control must be moved to the "AUTO-RICH" position. The green arc describes the range within which operation must be in auto-rich. The top of this arc indicates maximum continuous power; all operation above this temperature is limited in time (usually 5 to 15 minutes). A red line indicates the maximum permissible temperature.

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CHT indicator

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A thermocouple probe is fitted in the cylinder head. Each cylinder should have one, but it is usually installed on the hottest cylinder (whichever that may be, this does not so much depend on the position but the mixture entering the cylinders). Made from iron- or copper-constantan (copper and nickel alloy) and able to measure up to 400°C. The system consists of three parts: • • •

a thermocouple; the extension leads; the indicator.

Thermocouple The thermocouple is in the form of a bimetallic lug with a lead attached. The lug is made of two joined metals; copper (or iron) and constantan,(an alloy of copper and nickel), The lug is screwed into the cylinder head, or fitted to a bayonet type socket adapter. The lead, which is attached to the lug, has two conductors, one copper (or iron), the other, constantan. The copper conductor is connected to the indicator copper lead via a terminal block.

Extension leads To maintain the continuity of the correct material between the hot junction and cold junction of the circuit, a special extension lead is used to connect the thermocouple to the instrument to give continuity. These leads may be called either extension leads or compensating leads. Copper/constantan extension leads are identifiable by the colour of their outer covering. The conductors are marked in the same way as on the thermocouples, with black sleeves for copper and yellow sleeves for constantan. The thermocouple and its lead have a resistance of about 0.25 ohms, compensating or extension leads measure about 1.75 ohms. These leads are supplied in various lengths for installations where a continuous lead can be used. Extension leads must not be cut, nor spliced nor altered in any way, as the change in resistance alters the calibration of the instrument.

The copper conductor is usually identified with a black sleeve, and the constantan has a yellow sleeve (although colour code standards vary from one manufacturer to another). A common alternative location for the thermocouple hot junction is under a spark plug. The sensor is the gasket for the spark plug. Usually, only one cylinder has the temperature sensed in this way. This is the cylinder that is known to run hottest.

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Thermo couple (bayonet type for cylinder head)

CHT sensor fitting in the cylinder head

CHT sensor fitting using the spark plug gasket

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The indicator The indicator is usually a moving coil millivoltmeter which is very sensitive. An EMF is generated when heat is applied to two different materials; there is also a current flow and a voltage. The sensing element at the cylinder head is referred to as the hot junction, the indicator the cold junction. The amount of EMF generated depends on the amount of heat which is sensed by the thermocouple. The more heat, the higher the current flow, the bigger the EMF and the greater the temperature reading on the indicator. There is a zero adjustment on the indicator in the form of a screw which is fitted to the end of a bimetallic spiral. This spiral compensates for the effect in changes of atmospheric temperature, and the screw sets the zero position of the pointer.

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CHT indicator

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Exhaust gas temperature (EGT) These are fitted on the hottest cylinder exhaust pipe of some carburetted engines and used for leaning the engine. If done correctly, each cylinder should have an EGT indicator. FADEC controlled fuel injected engines cannot function without this vital measurement since they regulate the amount of fuel in combination with RPM and MAP to obtain the best mixture for the power requirement without detonation in the cylinders. The diagram below illustrates a typical thermocouple fitted into an exhaust. Each thermocouple protrudes into the exhaust gas stream and is secured in position with a jubilee band. The materials used for exhaust gas thermocouples (EGTs) are copper (or iron) and constantan, and all extension leads must be of the same material or material of similar thermal/EMF characteristics. The leads to the two conductors are silver soldered, and together they provide the hot junction. The leads have an armoured outer covering of metal braid.

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EGT indicator on a twin-engine aircraft EGT and CHT sensors

EGT sensors (thermocouple) fitted using a jubilee band

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Coolant and carburettor temperatures Either a bourdon tube (older type aircraft) or a temperaturesensitive resistor (thermistor) type is used to sense these temperatures. Carburettor air temperature (CAT) is measured at the carburettor entrance; it is regarded by many as an indication of induction system ice formation. Although it serves this purpose, it also provides many other important items of information. The temperature level of the induction air affects not only the charge density but also the vaporisation of the fuel. In addition to the regular use of CAT, it is found useful for checking induction system condition. Backfiring is indicated as a momentary rise on the gauge, provided it is of sufficient severity for the heat to be sensed at the carburettor airmeasuring point. A sustained induction system fire shows a continuous increase of carburettor air temperature.

The carburettor air temperature gauge indicates the temperature of the air before it enters the carburettor. A bulb senses the temperature reading. In the test cell, the bulb is located in the air intake passage to the engine, and in an aircraft, it is located in the ram-air intake duct. The carburettor air temperature gauge is calibrated in the centigrade scale. The figure below shows a typical carburettor air temperature gauge or CAT. This gauge, like many other multi-engine aircraft instruments, is dual; that is, two gauges, each with a separate pointer and scale, are used in the same case. Notice the range markings used. The yellow arc indicates a range from -15 °C. to +5 °C., since the danger of icing occurs between these temperatures. A green range (where used) indicates the normal operating range from +5 °C. to +40 °C. A red line indicates the maximum operating temperature of 40° C.; any operation at a temperature over this value places the engine in danger of detonation.

The CAT should be noted before starting and just after shutdown. The temperature before starting is the best indication of the temperature of the fuel in the carburettor body and tells whether vaporisation is sufficient for the initial firing or whether the mixture must be augmented by priming. If an engine has been shut down for only a short time, the residual heat in the carburettor may make it possible to rely on the vaporising heat in the fuel and powerplant, and priming would then be unnecessary. After shutdown, a high CAT is a warning that the fuel trapped in the carburettor will expand, producing high internal pressure. When the high temperature is present at this time, the fuel line and manifold valves should be open so that the pressure can be relieved by allowing fuel passage back to the tank. Total Training Support Ltd © Copyright 2020

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Carb temperature indicator

Coolant temperature indicator Carb heat sensor (thermistor) fitted to the carburettor

Temperature sensor (thermistor)

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Fuel pressure and flow Fuel pressure Injection systems have a fuel pressure gauge fitted; the pressure is tapped from the line to the flow divider. The diagram below shows a typical fuel pressure gauge for a nonsupercharged engine. It is calibrated in psi with the face marked to give the best power from takeoff to the selected altitude. At the selected altitude, the fuel pressure can be reduced to its upper limit for the cruise power selected. When the engine has stabilised at its best power, the mixture lever can be adjusted to select the minimum line for that cruise setting, thus giving economical cruising. With turbocharged engines, as the fuel pressure remains constant at varying altitudes, the fuel pressure gauge shows only pressure or in some cases flow as the flow is proportional to the pressure. The pilot adjusts the mixture to the recommended figure for that particular flight condition.

The green arc shows the desired range of operation, which is 10 to 25 psi. The red line at the 25-psi graduation indicates the maximum allowable fuel pressure. Fuel pressures vary with the type of carburettor installation and the size of the engine. When float-type carburettors or low-pressure carburetion systems are used, the fuel pressure range is of much lower value; the minimum allowable pressure is 3 psi, and the maximum is 5 psi with the desired range of operation between 3 and 5 psi.

The fuel pressure gauge is calibrated in pounds per square inch of pressure. It is used to measure engine fuel pressure at the carburettor inlet, the fuel-feed valve discharge nozzle, and the main fuel supply line. In some aircraft installations, the fuel pressure is sensed at the carburettor inlet of each engine, and the pressure is indicated on individual gauges on the instrument panel. The dial is calibrated in 1 psi graduations, and every fifth graduation line is extended and numbered. The numbers range from 0 to 30. The red line on the dial at the 5 psi graduation shows the minimum fuel pressure allowed during flight.

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Fuel pressure indicator Fuel pressure sensor in the fuel distributor of a fuel injection system

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Fuel flow The fuel flow meter measures the amount of fuel delivered to the carburettor. During engine block- test procedures, the fuel flow to the engine is measured by a series of calibrated tubes located in the control room. The tubes are of various sizes to indicate different volumes of fuel flow. Each tube contains a float that can be seen by the operator, and as the fuel flow through the tube varies, the float is either raised or lowered, indicating the amount of fuel flow. From these indications, the operator can determine whether an engine is operating at the correct fuel/air mixture for a given power setting. In an aircraft installation, the fuel flow indicating system consists of a transmitter and an indicator for each engine. The fuel flow transmitter is conveniently mounted in the engine's accessory section and measures the fuel flow between the engine-driven fuel pump and the carburettor. The transmitter is an electrical device that is connected electrically to the indicator located on the aircraft operator's panel. The reading on the indicator is calibrated to record the amount of fuel flow in pounds of fuel per hour. This instrument is even more important than fuel level as any change in fuel consumption is immediately visible. It is usually manufactured in combination with a MAP indication. Small piston-engine aircraft using carburettors seldom have fuel flow meters. The pilot assumes a flow rate based on the engine RPM and manifold pressure and checks it against the amount of fuel used in a given time.

Vane-type sensor A vane-type sensor is usually located downstream behind the fuel filter and pump measuring the amount of fuel going to the engine. Larger piston engines use a fuel flow meter in the fuel system between the fuel pump and the carburettor. Fuel-injected engines might have a fuel return to the tank in use so for a correct reading the fuel returning must also be measured with a flow sensor. A spring-loaded vane is moved by the fuel flowing to the carburettor. The higher the flow, the further the vane moves. The movement is transmitted to the indicator, which is calibrated in gallons per hour. The reading is only an approximation since it assumes that the fuel is at standard temperature and has a standard density. Fuel pressure as fuel flow Piston engines that are equipped with fuel injection systems have a flow meter indicator that is a fuel pressure gauge. This is, for normally aspirated engines, a bourdon tube instrument that measures the pressure drop across the fuel injector nozzles. The higher the flow, the greater the pressure drop is. Turbocharged engines use a differential pressure gauge to measure the flow. They measure the pressure at the distributor, or manifold valve, and compare it with the upper deck air pressure, the air pressure as it enters the fuel metering system. One major problem with this type of flow indicator is the fact that a clogged injector nozzle decreases the fuel flow, but the pressure drop across the nozzle increases, and it will indicate to the pilot as increased fuel flow.

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Fuel flow sensor and indicator system

Fuel flow indicator

Composite manifold pressure and fuel flow indicator

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Turbine-type sensor Another development in fuel flow instruments is the digital-type system that uses a small turbine wheel in the fuel line to the fuel control unit. As fuel flows through this line, it spins the turbine, and a digital circuit reads the number of revolutions in a specified period and converts this into a fuel flow rate. This flow rate may electronically compensate for any peculiarities of the specific system. Relatively new types of fuel flow sensors/transmitters are available in new aircraft and for retrofit to older aircraft. One type of device found in home-built and experimental aircraft uses a turbine that rotates in the fuel flow. The higher the flow rate is, the faster the turbine rotates. A Hall effect transducer is used to convert the speed of the turbine to an electrical signal to be used by an advanced fuel gauge similar to a fuel computer to produce a variety of calculated readouts and warnings. The turbine in this unit is in line with the fuel flow but is fail-safe to allow adequate fuel flow without interruption if the unit malfunctions. Another fuel flow sensor used primarily on light aircraft also detects the spinning velocity of a turbine in the fuel path. It too has a failsafe design should the turbine malfunction. In this unit, notches in the rotor interrupt an infrared light beam between an LED and phototransistor that creates a signal proportional to the amount of fuel flow. This type of sensor may be coupled with an electronic indicator.

Thermal dispersion sensor Increasing use of microprocessors and computers on aircraft has enabled the integration of fuel temperature and other compensating factors to produce highly accurate fuel flow information. Fuel flow sensing with digital output facilitates this with a high degree of reliability. Thermal dispersion technology provides flow sensing with no moving parts and digital output signals. The sensor consists of two resistance temperature detectors (RTDs). One is a reference RTD that measures the temperature of the fuel. The other is the active RTD. It is heated by an adjacent element to a temperature higher than the fuel. As the fuel flows, the active element cools proportionally to the fuel flow. The temperature difference between the two RTDs is highest at no flow. The RTDs are connected to an electronic assembly that supplies power to the heater and uses sensing circuitry and a microprocessor to control a constant temperature difference between the heated and unheated RTDs. The electrical current to the heater is proportional to the mass flow of the fuel. As mentioned, the reference RTD is used as a temperature sensor to provide a temperature output and allow for temperature compensation of the flow measurement.

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A transducer and microprocessor for control functions are located in the base of this turbine fuel flow sensor. The gauge is menu driven with numerous display options

Fuel flow sensing units using thermal dispersion technology have no moving parts and output digital signals

A turbine flow transducer in this fuel flow sensor produces a current pulse signal from an opto-electronic pickup with a preamplifier

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Ultrasonic flow sensor The development of microelectronics has enabled the design of solid-state flow measuring devices, not only for aircraft fluid flow but for any fluid flowing in pipes. They can be portable as well as fixed devices. Known as 'transmit-time measurement' devices, they operate by sending a pulse of ultrasound through the flowing fluid. For accuracy, two pulses are sent and received by transducers, one in the direction of flow, the other, against the direction of flow. The principle is based upon the fact that one pulse is speeded up by the fluid flow; the other is slowed down.

Some sensors may incorporate an electromagnetic shield, to shield the sensitive instrument from the effects of electromagnetic fields from other aircraft systems, and to confine any harmful emissions created by the ultrasonic signal generated within.

The difference between the time of arrival of a pulse propagated against the flow (upstream direction), and one propagated with the flow (downstream), is used to calculate the velocity of the flow. Given that the fuel pipeline is of a fixed cross-sectional area, the calculated velocity of the fluid can be converted into a volume flow rate. In the case of aircraft fuel flow, the propagation of sound is affected by fuel temperature. Therefore, the sensor is usually combined with a temperature sensor so that the application of a simple algorithm can remove the effect of temperature. Secondly, the temperature signal can be used to convert the volume flow rate into a mass flow rate by the application of the density formula, to provide an optional display parameter in PPH or Kg/hr.

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Principle of operation of ultrasonic flow measurement

A combined fuel flow and temperature sensor, and the indications it provides

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Fuel used With accurate fuel flow knowledge, numerous calculations can be performed to aid the pilot's situational awareness and flight planning. Most high-performance aircraft have a fuel totaliser that electronically calculates and displays information such as; total fuel used, total fuel remaining on board the aircraft, total range and flight time remaining at the present airspeed and rate of fuel consumption. On light aircraft, it is common to replace the original analogue fuel indicators with electronic gauges containing similar capabilities and built-in logic. Some of these 'fuel computers', as they are called, integrate global positioning satellite (GPS) location information. Aircraft with fully digital co*ckpits process fuel flow data in computers and display a wide array of fuel flow related information on demand. The fuel used indication shows the mass of fuel, which was burned since the last engine start on the ground. This allows comparing the different performances of the different engines. It also gives redundant information for the actual fuel quantity. You can calculate the actual fuel quantity by subtracting the amount of fuel used from the amount of fuel in the tank at takeoff. Fuel used indication is usually automatically reset to zero when the engine master switch is set to on, and the aircraft is on the ground. There needs to be a fuel flow transmitter on the engine, and a calculation has to be done, to generate the fuel flow and the fuel used indications. The calculation on modern systems is usually done by the FADEC systems computer (EEC/ECU) by merely integrating the time into actual fuel flow: Fuel flow Kg/hour × time = fuel used.

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Fuel system indicators – analogue and digital

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Diesel engine indicating co*ckpit control and instrumentation While the FADEC system itself is more complicated than the conventional mechanical fuel injection or carburetion system with two magnetos, it reduces the workload for the pilot considerably by eliminating several control elements (mixture, primer, prop control) and the need to monitor several parameters (EGT, CHT). Issues such as shock cooling and detonation simply do not exist. A modern aero-Diesel engine generally has more engine sensors than a conventional gasoline engine. However, due to the FADEC taking care of and automating most functions, it has fewer co*ckpit indications. The interface with the pilot consists of the following: • • • • • •

load selector level; EEC test switch force-EEC switch; EEC status light (one for each EEC); glow plug status light; and engine display for RPM, oil temp, coolant temp, etc.

Some of the sensors are duplicated. Many of these sensors do not exist on more conventional fuel systems and gasoline engines. The following photographs show the sensors located around the engine on the Thielert TAE 125. The power lever connects to the FADEC. The pilot sets the power lever position. Duplicated potentiometers in the base of the power lever sense its position. The potentiometers give resistance to the FADEC. Crankshaft position is one of the most important sensed parameters since the FADEC needs to know this information to inject fuel at the right moment relative to the TDC of the piston. An incorrectly timed injection could be catastrophic. For this reason, the crankshaft position sense is duplicated.

The engine displays are usually reduced to simple composite LED indicators called the central engine display (CED) and auxiliary engine display (AED). Only the most essential parameters of RPM, load lever setting and fuel flow are indicated numerically. Other, less critical parameters such as coolant water level, are indicated with a warning LED only.

FADEC sensors With the use of FADEC, many co*ckpit indications are redundant and not used. However, FADEC requires a large number of sensors around the engine to function.

A Hall effect sensor, which is placed adjacent to a spinning steel disk, is used to measure the crank position. The crank position sensor is also used to calculate the RPM of the crankshaft.

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Diesel engine indication and warning panel Diesel engine indicators

Auxiliary engine display (AED) 10-77 Module 16.10 Engine Indicating Systems

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Licence Category B1 and B3

16.11 Powerplant Installation

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Configuration of firewalls, cowlings, acoustic panels, engine mounts, anti-vibration mounts, hoses, pipes, feeders, connectors, wiring looms, control cables and rods, lifting points and drains.

Part-66 Ref. 16.11

Knowledge Levels A B1 B3 1 2 2

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Table of Contents Engine mounts ________________________________ 6 Engine mount frame ___________________________ 6 Anti-vibration mounts (LORD mounts) ____________ 10 Control cables and rods ________________________ Throttle control ______________________________ Propeller controls ____________________________ Mixture control ______________________________ Carburettor heat _____________________________ General requirements _________________________ Engine and propeller controls ___________________ Vernier-assist engine controls ___________________ Push-pull control rods _________________________ Bowden cables ______________________________ Teleflex helix cable systems ____________________ Ball-bearing control cables _____________________

14 14 14 14 14 14 18 20 22 24 28 34

Nacelles and cowlings _________________________ 36 General ____________________________________ 36 Acoustic panels ______________________________ 40 Firewall _____________________________________ 42

Lifting points _________________________________ 44 Engine removal _______________________________ 46 General ____________________________________ 46 Quick engine change assembly (QECA) ___________ 46 Preparation of reciprocating engines for installation __ 46 QECA build-up of radial engines _________________ 48 Preparation of the engine_______________________ 48 Oil drains ___________________________________ 50 Electrical connections _________________________ 50 Engine controls ______________________________ 52 Hoses and pipes _____________________________ 52 Hoisting ____________________________________ 54 Engine installation_____________________________ 56 Hoisting and mounting the engine ________________ 56 Connections and adjustments ___________________ 58 Helicopter engine removal and installations________ 64 General ____________________________________ 64 Engine controls ______________________________ 64 Clutch actuator _______________________________ 64 Sprag clutch _________________________________ 66 Reciprocating helicopter engine and QECA_________ 68

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Engine mounts Engine mount frame Most aircraft equipped with reciprocating engines use an engine mount structure made of welded steel tubing. The mount is constructed in one or more sections that incorporate the engine mount ring, bracing members (V-struts), and fittings for attaching the mount to the wing nacelle or the fuselage. These are the structural assemblies to which the engine is fastened. They are usually constructed from chrome/ molybdenum 4130 or 4140 steel tubing in light aircraft and forged chrome/nickel/molybdenum assemblies in larger aircraft.

In all cases, a tubular steel framework arrangement is used, where welded tubes form the assembly. If you consider the nose-mounted system below, you can see that this arrangement is a continuation of the fuselage structure and in some cases may also form part of the landing gear support structure.

The engine mounts are usually secured to the aircraft by special heat-treated steel bolts. The importance of using only these special bolts can be readily appreciated since they alone support the entire weight of the engine and propeller in flight and withstand all the stresses imposed by them. The upper bolts support the weight of the engine while the aircraft is on the ground, but when the aircraft is airborne, another stress is added. This stress is torsional and affects all bolts, not just the top bolts. A typical engine mount ring shown below discloses fittings and attachment points located at four positions on the engine mount structure. Each fitting houses a dynamic engine mount. The section of an engine mount where the engine is attached is known as the engine mount ring. It is circular so that it can surround the engine, which is near the point of balance for the engine, or, in the case of Dynafocal mounts, has a focal point near the centre of gravity of the engine. Total Training Support Ltd © Copyright 2020

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Engine mount for a wing-mounted engine

Engine mount frame for a fuselage-mounted engine

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Engine attachment shock mount

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Anti-vibration mounts (LORD mounts) Some method is needed to absorb vibration and/or isolate vibration from the engine. This demand has led to the development of the rubber and steel engine suspension units called shock mounts. This combination permits restricted engine movement in all directions. These vibration isolators are commonly known as flexible, or elastic, shock mounts. A popular manufacturer of the vibration isolators mounts is the LORD company (now Parker). Hence the mounts are often generally known as “LORD mounts”. An interesting feature common to most shock mounts is that the rubber and metal parts are arranged so that, under normal conditions, rubber alone supports the engine. Of course, if the engine is subjected to abnormal shocks or loads, the metal snubbers limit excessive movement of the engine. Two shock mount geometry arrangements exist: •

•

Conical – straight mounts parallel to crankshaft, these types of mounts are common on vertical lift (Helicopter) applications; and Dynafocal – mounts are set at a specified angle to the crankshaft with Type-1 set at 30° and Type-2 set at 18° where All engine mounts converge on a central position along the thrust line and centre of gravity of the engine mass.

You can recognise a conical mount because the enginemounting bolts are all parallel to each other along the longitudinal axis of the aeroplane, and the mount bushings are rather cone-shaped, hence the name. An engine that needs a conical mount has mounting holes that are tapered from both sides and are cut square with the back of the engine case. The advantage of the conical mount is that it does not permit as much movement between the engine and the rest of the aeroplane as a Dynafocal type. In a tightly cowled engine, this reduced movement can be a real benefit. A conical mount only allows about half as much movement of the engine as a Dynafocal mount. In both cases, however, the engine is mounted to the framework using flexible mountings, as shown below. Although still providing thrust loads from the engine to the airframe, they do dampen out any vibrations caused by the propellers or any engine running imbalances. It is essential that these mounts are inspected regularly to ensure that they are not lose or suffering from problems created by oil contamination. The support frame is usually attached to the airframe without the use of these vibrations’ insulators.

The vibration isolators focus their centrelines at a point slightly ahead of but in the same plane as the engine’s centre of gravity. Flight loads are applied equally to each isolator.

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Semi-focalised bed mounts

Dynafocal mounts

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Rubber conical engine-mount bushings

Rubber conical engine-mount bushings assembly (Lycoming 0-235 through 0-320 models)

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Engine attachment shock mount

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Control cables and rods A typical engine control pedestal for a twin-engine aircraft is shown below. The control levers are fitted to allow the pilot to have control over the following functions: • • • •

throttle; propeller; mixture; and carburettor heat.

General requirements Any control system, no matter the type or design, must have certain characteristics. These can be summed up as follows. It must be: • • •

Throttle control This controls the power output of the engine. It is usually marked with a letter “T” or “THROTTLE” with “OPEN” and “CLOSE” captions on the throttle quadrant. Forward movement increases power; rearward movement decreases power.

•

accurate – the range of movement is precise and consistent; positive – with minimal backlash or sloppiness; reliable – it performs its specified function every time it is operated; and effective – irrespective of the distance run.

Propeller controls This is used as a means of altering the pitch of a variable pitch propeller. It works in conjunction with the throttle lever to gain optimum power from the engine. Mixture control This is fitted to adjust the fuel/air mixture during regular operation of the engine. It allows the pilot to enrich or weaken the mixture to keep the engine within the mixture control parameters. The quadrant is marked “FULL RICH”, “LEAN” and “IDLE CUT OFF”. Carburettor heat This control allows for the heat from the exhaust system to warm the carburettor and its air intake, to prevent or reduce any build-up of ice within the inlet manifold.

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co*ckpit controls and instrumentation to engine

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Throttle and mixture control levers on a simple (fixed-pitch propeller) single-engine aircraft

Throttle, propeller and mixture control levers on a complex (variable-pitch propeller) single-engine aircraft

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Engine and propeller controls Because the engine and propeller must work together to produce the required thrust for a turboprop installation, there are a few unique relationships. The turboprop fuel control and the propeller governor are connected and operate in coordination with each other. The power lever directs a signal from the co*ckpit to the fuel control for a specific amount of power from the engine. The fuel control and the propeller governor together establish the correct combination of RPM, fuel flow, and propeller blade angle to provide the desired power. Propeller control levers in the co*ckpit must be arranged to allow for smooth operation of all controls at the same time, but not to restrict the movement of individual controls. The propeller controls must be rigged so that an increase in RPM is achieved by moving the controls forward and a decrease in RPM is caused by moving the controls aft. The throttles must be arranged, so that forward thrust is increased by the forward movement of the control and reverse thrust is increased by an aft movement of the throttle. When operating in reverse, the throttles are used to place the propeller blades at a negative angle.

co*ckpit instruments such as tachometers and manifold pressure gauges must be marked with a green arc to indicate the normal operating range, a yellow arc for take-off and precautionary range, a red arc for critical vibration range, and a red radial line for the maximum operating limit. Alpha range The propeller control system is divided into two types of control; one for flight and one for a ground operation. For a flight, the propeller blade angle and fuel flow for any given power setting are governed automatically according to a predetermined schedule. This is known as the alpha range. Beta range Below the flight-idle power-lever position, the coordinated RPM blade angle schedule becomes incapable of handling the engine efficiently. Here the ground-handling range, referred to as the beta range, is encountered. In the beta range of the throttle quadrant, the propeller blade angle is not governed by the propeller governor but is controlled by the power lever position. When the power lever is moved below the start position, the propeller pitch is reversed to provide reverse thrust for rapid deceleration of the aircraft after landing.

co*ckpit powerplant controls must be arranged to prevent confusion as to which engine they control. Recent regulation changes require that control knobs must be distinguished by shape and colour, as shown below.

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Propeller/engine control lever shapes and colours

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Vernier-assist engine controls The Vernier-assist engine control is used on single-engine aircraft. It has two modes of operation. It can be pushed and pulled for large inputs to the control system, but alternatively, it has a knob which can be turned and for small trim inputs. A cam inside the mechanism then translates this motion into small push or pull inputs. An additional feature of some Vernier-assist controls is an automatic friction lock. This prevents inadvertent movement of the control by the pilot, or creep movement due to vibration. The lock mechanism automatically engages and must be released by pushing a button on the end of the control, whenever the pilot wants to use the control in the push/pull action. Vernier-Assist Throttle Control Assembly https://www.youtube.com/watch?v=5RcjEd7lR4Q

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Vernier assist throttle, propeller and mixture control levers on a complex (variable-pitch propeller) single-engine aircraft

Operation of a Vernier-assist control with friction lock

Vernier assist control mechanism

Operation of a Vernier-assist control (no automatic friction lock)

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Push-pull control rods This system uses a series of rods connected to transmit motion in one direction or another, dependent on selection. The rods are in most cases, straight, as any bends on the rods would be distorted by compression and tension loads, for example: • •

compression loads tend to increase a bend; and tension loads tend to decrease a bend.

The use of curved rods, and the effect of compression and tension on them, would not allow the above conditions to be satisfied (changes of length). The control rods, either solid or tubed, are usually supported by either roller guides or plastic or hard rubber brushes. Due to installation difficulties, rods may have to change direction on numerous occasions. Bell-crank levers and walking-beam levers are used to enable these direction changes.

Swivel ball joint attachment When the control rod is not in line with the control arm, or where a limited amount of angular movement is experienced (maximum 15°), a swivel ball joint is fitted to allow this movement to take place. Wear The control rod system does provide an excellent method of remote control; however, one of the main drawbacks is wear within the system. A slight amount of wear at the rod connections can build up through the system and cause a fair amount of backlash or play. In turn, this can lead to incorrect rigging conditions, out of range movements, and possibly misalignment of control settings.

Clevis and pin attachment The clevis and pin connection arrangement is probably the most popular in limited length control systems. In most cases, the fork end attachment has some means of adjustment for control system rigging purposes. Ball-bearing attachment Where a large amount of rotary movement is required in the control system, the ball bearing assembly is attached to the control rod or bell crank/walking beam assembly. These bearings are pre-packed with lubricant and require little servicing, except for routine inspection for wear and to check the condition of the grease seals.

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Engine control rod assembly

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Bowden cables Flexible Bowden cables were invented by Sir Frank Bowden, the founder of the famous English bicycle company, Raleigh. Although several other companies make Bowden cables, the name Bowden has remained as the generic name. A Bowden cable is a type of flexible cable used to transmit mechanical force or energy by the movement of an inner cable relative to a hollow outer cable housing. The housing is generally of composite construction, consisting of an inner lining, a longitudinally incompressible layer such as a helical winding or a sheaf of steel wire, and a protective outer covering.

The linear movement of the inner cable is generally used to transmit a pulling force, although for light applications over shorter distances a push may also be used. Movement is always in the direction of the lie of the cable, with a push or pull rather than a twist, and can be in either direction. Other types of sheathed mechanical cables exist, but what distinguishes the Bowden cable is the direction of the force. Bowden cable https://youtu.be/HhzvytVW1kk

Bowden cables are mostly fabricated from steel, usually coated by pure tin or zinc. They are flexible, preformed and corrosionresistant; of a 3 by 7, 7 by 7, 7 by 19 or 6 by 19 construction. The cable moves relative to a hollow outer housing, generally made from a spiral steel wire with a plastic outer sheath. The linear movement of the inner cable is most often used to transmit a pulling force, although push/pull cables have gained popularity in recent years. Many light aircraft use a push/pull Bowden cable for the engine controls, and here it is typical for the inner element to be a solid wire, rather than a multi-strand cable. Usually, provision is made for adjusting the cable tension using an in-line hollow bolt (often called a barrel adjuster), which lengthens or shortens the cable housing relative to a fixed anchor point. Lengthening the housing (turning the barrel adjuster out) tightens the cable; shortening the housing (turning the barrel adjuster in) loosens the cable.

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Bowden cables used on engine controls

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The advantages of using flexible cables are the ability to bend and Bowden cables do not require so much space as other types of control installation. The disadvantage is the backlash, which is apparent as lost motion under light push-pull input forces. Backlash is caused by the core member of the cable assembly moving from the inside to the outside of the bends with a change in the direction of movement. It is a function of the clearance between the core and liner. That also explains why flexible cables are not used on installations requiring force to be transmitted over a longer distance.

Teleflex Bowden cables are sealed and resist abrasion and contamination. They should, however, be protected against pinching, shearing and crushing, and the effects of excess heat. The operating end should be shielded against direct spray and excessive dust. If the outer cable becomes damaged, it can allow water to enter. If exposed to low temperatures, icing can be induced, and jamming may occur. Water ingress may also induce corrosive effects to the cable.

The amount of range of movement that is lost due to the backlash is sometimes called “dead-band”. To overcome the problem, many aircraft designers incorporate an amount of over travel into the control lever. This is often called control lever cushion. It allows the control lever to be moved slightly further than is necessary for the engine control. However, the extra travel returns to the normal position after the pilot releases the control. The amount of overtravel is specified in the aircraft maintenance manual and must be carefully adjusted. Although cables are flexible motion-transfer devices, keeping the number of bends to a minimum can attain the best performance and service life. Also, for the best efficiency and longest operating life, the cable should be installed so that it encounters the heaviest load in the pull direction of operation. Where bends are required, as generous a radius as is practically possible should be allowed.

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A Bowden cable assembly

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Teleflex helix cable systems The Teleflex cable system is more complicated than the Bowden cable system in that the operating cable, within the conduit, is many spirally wound cables which surround a core tension cable, giving it support. This allows the cable to transmit a push force as easily as a pull force, doing away with the need for any form of return spring.

A and B are wound in different directions to prevent the cable from twisting under load. Wire of more substantial gauge C, inter-spaced by three turns of wire D, forms the helix which acts as a bearing surface when working in the conduit and by which the end of the cable is gripped in the control unit.

The company began in 1943 with one simple product – a multistrand helical cable and a gear that could convert push-pull motions into rotary motions. Its first use was on Spitfire planes during the second world war, as a flexible cable to adjust the pilot’s radio, which was located behind the co*ckpit and out of the pilot’s reach. The flexible cable was used telescopically to adjust the radio, giving it its name. A typical use of a Teleflex system is the throttle lever to engine fuel-control connection. The Teleflex cable system is a snug fit within the conduit. Because there might be the chance of it becoming seized, due to foreign objects, dirt or freezing, the inner cables must be regularly removed, cleaned and lubricated with a lowtemperature grease. It is also essential that the conduits are thoroughly cleaned using a form of pull-through before the inner cable is reinstalled. At longer intervals, it might become necessary to inspect the outer conduit for signs of damage or kinking; which can cause the control to become tight or ‛notchy’. The cable is made of high tensile steel wires, which enable it to transmit both push and pull. The strand core A transmits the pull, while the first wire wrap B transmits the push. Total Training Support Ltd © Copyright 2020

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Cable Box unit

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Box unit The cable should be tucked into the slot in the pinion, ensuring that the cable helix engages with the pinion teeth to give a wrap of at least 40° for single entry units. On double-entry units, the cable should engage with the pinion to give a wrap of 180°, the cable projecting through the lead-out hole throughout the travel of the control. Ensure that the cable end does not foul the blanked end of the conduit when fully extended. All box units should be packed with recommended grease.

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End fittings Installing sliding end fittings (fork-end type). Unscrew the threaded hexagon plug from the body, screw the lock nut right back, and pass the cable through the plug. Screw the lock spring on to the end of the cable so that 3⁄16" of cable projects. Insert the cable end, with its lock spring, into the bore of the body of the end fitting, and screw the hexagon plug tight down, preventing the body from rotating. Check that the free end of the cable is beyond the inspection hole, but not beyond the fork gap (for a fork-end fitting). Tighten the lock nut and turn up the tab washer. Check that the distance from the face of the body to the end of the sliding tube does not exceed 0.45" (0.35" for the old type without a tab washer). This ensures that the lockspring is tightly compressed. Control end units vary in detail. In the ‛box’ (or wheel) unit, at the control end, the large wire of the cable C is engaged between the teeth of the gear wheel and the unit body. The gear lever is turned by a hand lever or handwheel, thus moving the cable into or out of the box. In some boxes (double entry) the cable engages the pinion for 180°, and an outlet is provided for the free end of the cable, which is protected by a short length of the conduit. These boxes permit much greater movement of the cable. Alternatively, the control may consist of a simple push-pull unit. This is sometimes fitted with a spring-loaded stop to retain the control in any position or sometimes fitted with a spring which returns the control to its original position when released from operation. The cable is attached to the fitting by clamping a short coil of wire, which is screwed to the cable end, between two parts of the control knob. Total Training Support Ltd © Copyright 2020

At the operating (or terminal) end, the fitting may be a box type (with gear wheel) or a simple sliding end fitting, to which the cable is attached similarly to that for a push-pull type of control. This final attachment to the component operated could vary, being either fork end or ball and socket – examples of which are shown. In assembling, the body of the end fitting must not be screwed on to the hexagon plug. The plug should be screwed into the fork, not fork into the plug. Failure to apply this rule would result in the lock spring unscrewing. The same method should be used when removing the fork, and care should be taken not to jam the spring and foul up the wire wrap. Swivelling couplings A swivelling coupling is sometimes used to allow the sliding end fitting and angular movement; this is used when the lever on the component describes an arc. The cable and conduit are connected to control units at each end of the run, and in between, to other units and fittings which are used to direct the run. In many locations, the cables are attached to lever-operated wheel units or push-pull handles. At the receiving end of the run, another wheel unit or sliding endfitting is used to actuate the mechanism.

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Cable

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Ball-bearing control cables Ball-bearing control cables are another form of push-pull cable which may be encountered. However, due to their method of construction, they are flexible in only one plane. Several manufacturers make Ball bearing control cables; two types are the Bowdenflex cable and Flexball cable. The tension and compression loads are transmitted by a flexible centre load rail. This is supported by a set of balls either side, which are kept at regular intervals by two flexible ball-cage strips. Each set of balls runs in an outer guide rail, and the whole assembly is retained in a flexible casing. Orientation flats are formed on the outer casing, parallel to the flat face of the centre load rail, to ensure that the correct plane of flex is evident on installation. End fittings are attached to the centre load-rail to allow input and output loads to be applied. The cable requires no lubrication in service and can operate at temperatures of between -40° and +250°C (-40° and 480°F). Minimum bend radius is around 75 mm (3") while stroke range usually is 25 mm to 100 mm (1-4"). Bending in the wrong plane would result in high friction and excessive wear. Applications are like those of the Teleflex cable although they are far less common.

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Flexball cable

Bowdenflex cable

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Nacelles and cowlings General Nacelles (sometimes called “pods”) are streamlined enclosures used primarily to house the engine and its components. They usually present a round or elliptical profile to the wind, thus reducing aerodynamic drag. On most single-engine aircraft, the engine and nacelle are at the forward end of the fuselage. On multiengine aircraft, engine nacelles are built into the wings. A pusher-type aircraft is designed with a nacelle in line with the fuselage aft of the passenger compartment. Regardless of its location, a nacelle contains the engine and accessories, engine mounts, structural members, a firewall, and skin and cowling on the exterior to fare the nacelle to the wind. Some aircraft have nacelles that are designed to house the landing gear when retracted. Retracting the gear to reduce drag is standard procedure on high-performance/high-speed aircraft. The wheel well is the area where the landing gear is attached and stowed when retracted. Wheel wells can be in the wings and/or fuselage when not part of the nacelle.

The exterior of a nacelle is covered with a skin or fitted with a cowling which can be opened to access the engine and components inside. Both are usually made of sheet aluminium or magnesium alloy with stainless steel or titanium alloys being used in high-temperature areas, such as around the exhaust exit. Regardless of the material used, the skin is typically attached to the framework with rivets. Cowling refers to the detachable panels covering those areas into which access must be gained regularly, such as the engine and its accessories. Cowl flaps are moveable parts of the nacelle cowling that open and close to regulate engine temperature. Composite cowlings are also used, usually with a heatsensitive, intumescent internal coating that expands to create a temporary insulating layer in the event of an engine fire. The ‘scrubbing’ effect of the cooling airflow on the outside surface of the cowlings help maintain structural integrity in the event of an engine fire.

The framework of a nacelle usually consists of structural members like those of the fuselage. Lengthwise members, such as longerons and stringers, combine with horizontal/vertical members, such as rings, formers, and bulkheads, to give the nacelle its shape and structural integrity.

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‘Orange peel’ type cowling for large radial reciprocating engine

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On wing-mounted aircraft, nacelles can be considered as being divided into two sections: (1) The wing nacelle, and (2) the engine nacelle. The wing nacelle is that portion of the nacelle which is attached to the wing structure. The engine nacelle is that portion of the nacelle that is constructed separately from the wing. The diagram below left illustrates a typical nacelle with the separation line identified. Outwardly, the wing nacelle seems to be only a streamlining for the engine nacelle, but that is not its only purpose. On many aircraft, the inboard wing nacelle houses the landing gear when it is in the retracted position. Also, the wing nacelles typically contain lines and units of the oil, fuel, and hydraulic systems, as well as linkages and other controls for the operation of the engine.

The EASA regulation Certification Specification specifies the following for the cowlings. •

•

• •

•

The cowling is attached to the nacelle using screws and/or quick-release fasteners. Some large reciprocating engines are enclosed by “orange peel” cowlings which provide excellent access to components inside the nacelle. These cowl panels are attached to the forward firewall by mounts which also serve as hinges for opening the cowl. The lower cowl mounts are secured to the hinge brackets by quick release pins. The side and top panels are held open by rods, and the lower panel is retained in the open position by a spring and a cable. All of the cowling panels are locked in the closed position by over centre steel latches which are secured in the closed position by spring-loaded safety catches.

•

CS-23

Each cowling must be constructed and supported so that it can resist any vibration, inertia and air loads to which it may be subjected in operation. There must be a means for rapid and complete drainage of each part of the cowling in the standard ground and flight attitudes. No drain may discharge where it would cause a fire hazard. Cowling must be at least fire-resistant (resist fire for at least 5 minutes). Each part behind an opening in the engine compartment cowling must be at least fire-resistant for a distance of at least 61 cm (24") aft of the opening. Each part of the cowling subjected to high temperatures due to its nearness to exhaust system ports or exhaust gas impingement must be fireproof. Each nacelle of a twin-engine aeroplane with turbocharged engines must be designed and constructed so that with the landing gear retracted, a fire in the engine compartment cannot burn through a cowling or nacelle and enter a nacelle area other than the engine compartment. Also, for commuter category aeroplanes, the aeroplane must be designed so that no fire originating in any engine compartment can enter, either through openings or by burn-through, any other region where it would create additional hazards.

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Typical engine and wing nacelle.

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Acoustic panels Some aircraft engines, albeit rare, use acoustic liners to damp engine noise. Liners are applied on the internal walls of the engine nacelle and dissipate acoustic energy. An acoustic liner is a sandwich panel made by: • • •

a porous top layer called the face-sheet; a honeycomb structure providing internal partitions; and an impervious layer, called back-sheet or back-skin.

The most common acoustic liner consists of a single layer honeycomb and perforated facing sheet, as shown below. The facing sheet is porous and typically formed with aramid fibre fabrics, or glass fibre fabrics, reinforced-resin matrix-composite materials (the parameters of the porous sheet are corresponding to the frequency and noise of the engine). The facing sheet is often called the impedance layer, which has many characteristics such as high ratio strength and specific stiffness, low thermal conductivity, anti-noise vibration, anticorrosion, anti-ageing. The core layer is a porous separator material, e.g. honeycomb core. The outer sheet is a rigid and solid composite back sheet. Some special acoustic liners contain a multilayer structure which combines porous composite sheet with the thermoplastic material located in the resonant cavity. The structures of resonant cavities are designed based on the noise frequency, noise reduction coefficient of engine requirements, which make it possible to achieve efficient noise reduction while ensuring a reasonable weight increase. During the propagation of noise through the porous facing sheet into the resonant cavity, the acoustic energy of the noise is absorbed by lining material and converted into heat. Total Training Support Ltd © Copyright 2020

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Single-layer acoustic panel

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Firewall The aircraft’s firewall is a flameproof bulkhead that separates the engine compartment from the rest of the aircraft – whether that is the cabin, for a single-engine aircraft, or the nacelles and wings for twin-engine aeroplanes. The regulations require that engines, auxiliary power units, and fuel-burning heaters are physically isolated from the rest of the aeroplane by firewalls or shrouds. The firewall must prevent any hazardous quantity of liquid, gas, or flame from passing through the firewall to other parts of the aeroplane. It must be protected against corrosion, and each opening in the firewall must be sealed with close-fitting, fireproof grommets, bushings, or other fittings. The firewall’s primary role is, in the event of a fire in the engine compartment, to protect the aircraft’s occupants long enough for the pilot to make an emergency landing. Act promptly, however – the regulations require firewall materials and fittings to resist flame penetration for at least 15 minutes. Because of its strength and location, the firewall also can be a god place to mount battery boxes, voltage regulators, and other engine accoutrements. The point at which the engine nacelle is disconnected from the wing nacelle can easily be identified on most aircraft. To locate the point of disconnect, find the last section of removable engine nacelle cowling farthest from the propeller end of the engine. Usually, the removal of these sections of cowling would expose lines, fittings, electrical connections, cables, and mount bolts.

The firewall is usually the foremost bulkhead of the wing nacelle. It differs from most other aircraft bulkheads in that it is constructed of stainless steel or some other fire-resistant material. Since the fuel tanks are usually contained in the wings, the probable consequences of an engine fire are apparent. Thus, the necessity for sealing all unused openings in the firewall cannot be overstressed. An aircraft engine and its accessories which have been in storage must undergo careful depreservation and inspection before they may be installed in an aircraft. This involves more than removing an engine from its container and bolting it to the aircraft. The EASA regulation Certification Specification CS-23 specifies the following materials for construction of the firewall. • • • • • •

Stainless steel sheet, 0.38 mm (0.015") thick. Mild steel sheet (coated with aluminium or otherwise protected against corrosion) 0.45 mm (0.018") thick. Terne plate, 0.45 mm (0.018") thick. Monel metal, 0.45 mm (0.018") thick. Steel or copper-base alloy firewall fittings. Titanium sheet, 0.41 mm (0.016") thick.

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Firewall of a fuselage-nose mounted engine

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Lifting points All engines are provided with one or more lifting points. These are usually steel eyes attached to the crankcase. If a single hoist point is provided, it is located above the centre of gravity of the engine. Check the aircraft maintenance manual to determine what ancillary components (generator, pumps, oil coolers, etc.) must be removed before hoisting, to maintain the correct centre of gravity position. Some manufacturers specify that the engine mounting frame must be removed at the firewall and hoisted with the engine, others may specify that the mounting frame must be disconnected at the engine vibration absorbers and be left on the aircraft when the engine is removed.

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Engine lifting eye

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Engine removal General Procedures for removing or installing an aircraft engine usually vary widely with the type of aircraft and the type of engine. Thus, no single list of instructions can be provided as a guideline for all engines. Because of the many types of engine installations and a large number of design variations within each type or category, representative examples have been selected to illustrate the most typical installation procedures for reciprocating, turboprop, and turbojet engines. The radial and the opposed engines are used to describe and represent general and typical procedures for all reciprocating engine build-up, removal, preservation, storage, and installation. Although these two types have been included to ensure adequate coverage of engines used in both heavy and light aircraft, much of the information and many of the procedures presented in the discussion of radial engines apply to opposed-type engines. Only the significant differences between the two types are included in the discussion of opposed-type engines. While procedures for specific engines and aircraft are included in this chapter, many pertinent or mandatory references are omitted as they are not relevant for a general discussion. For this reason, always reference the applicable manufacturer’s instructions before performing any phase of engine removal or installation.

Quick engine change assembly (QECA) The QECA system is most commonly used with large radial engines, and for this reason, such engines are used to describe QECA build-up and installation procedures. But it should be emphasised that many of these procedures apply to all other methods of engine build-up and installation. Preparation of reciprocating engines for installation After the decision has been made to remove an engine, the preparation of the replacement engine must be considered. The maintenance procedures and methods used vary widely. Commercial operators, whose maintenance operations require the most efficient and expeditious replacement of aircraft engines, usually rely on a system that utilises the quick engine change assembly, or QECA, also sometimes referred to as the engine power package. The QECA is essentially a powerplant and the necessary accessories installed in the engine mount ring. Other operators of aircraft equipped with radial engines and most opposed-type engines use a slower but less-expensive method. Since engine replacement in these repair facilities often occurs at random intervals, only a few replacement engines (sometimes only one) are kept on hand. Such replacement engines may be partially or wholly built up with the necessary accessories and subassemblies, or they may be stored as received from the manufacturer in packing boxes, cases, or cans and are un-crated and built up for installation only when needed

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Types of engine on maintenance stands

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QECA build-up of radial engines The study of QECA build-up that follows is not designed to outline procedures to be followed in a practical application since most maintenance shops develop build-up procedures tailored to their facilities or use those recommended by the manufacturer. The procedures included in this chapter provide a logical sequence in following a QECA and its components through the stages of a typical build-up to gain a better understanding of units and systems interconnection.

If the engine is installed in a wooden shipping case, it is necessary to carefully break the seal of the protective envelope and fold it down around the engine. Remove the dehydrating agent or desiccant bags and the humidity indicator from the outside of the engine. Also, remove and set safely aside any accessories that are not installed on the engine but are mounted on a special stand or otherwise installed inside the protective envelope.

The QECA consists of several units. Among such units that are common to most present-day aircraft QECAs are the air-scoop, cowl flaps, engine ring cowl, cowl support ring, access panels, engine mount, and the engine, together with all of its accessories and controls.

If the engine is a radial type, the mounting ring bolts must be unfastened from the container, and the engine hoisted slightly to allow the mounting ring to be removed from the engine. Engines other than radial types are usually bolted directly to the container.

Preparation of the engine If the engine is stored in a pressurised metal container, the air valve should be opened to bleed off the air pressure. Depending upon the size of the valve, the air pressure should bleed off in somewhat less than 30-minutes. Prepare the container for opening by removing the bolts that hold the two sections together. Then attach a hoist to the hoisting points and lift the top section clear of the container and place it away from the work area.

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Purpose made aircraft engine shipping containers

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Oil drains Place a large metal pan (drip pan) on the floor under the engine to catch any spilt mixture or oil. Next, secure a clean container in which to drain the oil or corrosion-preventive mixture. Place the container beneath the Y drain located between the oil tank and the oil inlet to the engine, open the valve, and allow the oil to drain. Other points at which the oil system is drained include the oil cooler, the oil return line, and the engine sumps. All valves, drains, and lines must remain open until the oil system has been completely drained. After draining the oil, reinstall all drain plugs and close all drain valves. Then wipe all excess oil from around the drain points. Electrical connections Electrical disconnections are usually made at the engine firewall. This does not always apply when the basic engine is being removed. The electrical leads to such accessories as the starter and generators are disconnected at the units themselves. When disconnecting electrical leads, it is a good safety habit to disconnect the magnetos first and immediately ground them at some point on the engine or the assembly being removed. Most firewall disconnections of electrical conduit and cable are simplified by the use of AN or MS connectors. Each connector consists of two parts; a plug assembly and a receptacle assembly. The outlet is threaded to permit a knurled sleeve nut to be screwed to the outlet and then fastened with safety wire, if necessary, to prevent accidental disconnection during aeroplane operation.

In the junction box, the electrical circuit is completed by fastening two leads to a common terminal. The lead which runs from the junction box to engine is disconnected from the terminal, and the conduit is disconnected from the junction box when preparing to remove the engine. After the safety wire is broken, remove all of it from the sleeve nuts which hold the conduit to the junction boxes, as well as from the nuts on the connectors. Wrap moisture-proof tape over the exposed ends of connectors to protect them from dirt and moisture. Also, do not leave long electrical cables or conduits hanging loose, since they may become entangled with some part of the aircraft while the engine is being hoisted. It is good practice to coil all lengths of cable or flexible conduit neatly and tie or tape them to some portion of the assembly being removed.

A typical plug fitting assembly is shown below right. This also shows a typical junction box assembly, which is used as a disconnect on some aircraft engine installations. Total Training Support Ltd © Copyright 2020

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Electrical connections and other fittings at the firewall Oil system drain point

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Engine controls The engine control rods and cables connect such units as the carburettor or fuel control throttle valve and the mixture control valve with their manually actuated control in the co*ckpit. The controls are sometimes disconnected by removing the turnbuckle which joins the cable ends. A typical assembly is shown in the diagram below, top-left. Typical control linkage consisting of a control rod attached to a bellcrank is illustrated. The control rod in the linkage shown has two rod-end assemblies, a clevis and an eye, screwed onto opposite ends. These rod-end assemblies determine the length of the control rod by the distance they are screwed onto it and are locked into position by check-nuts. An anti-friction bearing is usually mounted in the eye end of a rod. This eye is slipped over a bolt in the bellcrank arm and is held in position by a castle nut safetied with a split pin. The clevis rod end is slipped over the end of a bellcrank arm, which also usually contains an anti-friction bearing. A bolt is passed through the clevis and the bellcrank eye, fastened with a castle nut, and safeties with a split pin. Sometimes linkage assemblies do not include the anti-friction bearings and are held in position only by a washer and split pin in the end of a clevis pin which passes through the bellcrank and rod end. After the engine control linkages have been disconnected, the nuts and bolts should be replaced in the rod ends or bellcrank arms to prevent being lost. All control rods should be removed entirely or tied back to prevent them from being bent or broken if they are struck by the replacement engine or QECA as it is being hoisted. Total Training Support Ltd © Copyright 2020

Hoses and pipes The lines between units within the aircraft and the engine are either flexible rubber hose or aluminium-alloy pipes joined by lengths of hose clamped to them. Lines which must withstand high pressure, such as hydraulic lines, are often stainless-steel tubing. The diagram below bottom-right shows the basic types of line disconnects. Most lines leading from a QECA are secured to a threaded fitting at the firewall by a sleeve nut around the tubing. Hoses are sometimes secured in this manner but may also be secured by a threaded fitting on the unit to which they lead, or by a hose clamp. The firewall fittings for some lines have a quick-disconnect fitting that contains a check valve to prevent the system from losing fluid when the line is disconnected. The metal tubing on some installations may also be disconnected at a point where two lengths of it are joined together by a length of rubber hose. Such a disconnection is made by loosening the hose clamps and sliding the length of rubber hose over the length of tubing which remains on the aircraft. There may be some further variations in these types of disconnections, but basically, they follow the same pattern. Some type of container should be used to catch any fuel, oil, or other fluid that may drain from the disconnected lines. After the lines have drained, they should be immediately plugged or covered with moisture-proof tape to prevent foreign matter from entering them as well as to prevent any accumulated fluid from dripping out.

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Engine control cable and turnbuckle assembly

Types of line disconnects

Engine control linkage assembly

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Hoisting If there has been thorough preparation of the engine for removal, the actual removal should be a relatively speedy operation. If a QECA is being removed, the engine mount accompanies the engine. If only the engine is being removed, the mount remains on the aircraft. Before the engine can be freed from its attachment points, a sling must be installed so the engine’s weight can be supported with a hoist when the mounting bolts are removed. Aircraft engines or QECA’s have marked points for attaching a hoisting sling. The location of these attaching points varies according to the size and weight distribution of the engine. The photograph below left shows a sling supporting an engine which has two attaching points. As a matter of safety, the sling should be carefully inspected for condition before installing it on the engine. Before attaching the sling to the hoist, be sure that the hoist has sufficient capacity to lift the engine safely. A manually operated hoist mounted in a portable frame is shown below right. This hoist assembly is specifically manufactured to remove engines and other large assemblies from aircraft. Some frames are fitted with power-operated hoists. These should be used with care since considerable damage can be done if an inexperienced operator allows a power-operated hoist to overrun. The hoist and frame should also be checked for condition before being used to lift the engine. Before the hoist is hooked onto the engine sling, re-check the aircraft tail supports and the wheel chocks. Fasten lines to the engine at points on the sides or rear so that the engine can be Total Training Support Ltd © Copyright 2020

controlled as it is being hoisted. Hook the hoist onto the sling and hoist the engine slightly – just enough to relieve the engine weight from the mount attachments. Remove the nuts from the mount attachments in the order recommended in the manufacturer’s instructions for the aircraft. As the last nuts are being removed, pull back on the lines fastened to the engine (or force it back by other means if lines are not being used), thus steadying the engine. If bolts must be removed from the mount attachments, be sure the engine is under control before doing so. If the bolts are to remain in the mount attachments, the hoist can be gently manoeuvred upward or downward as necessary after all the nuts have been removed. Meanwhile, gently relax the backward force on the engine just enough to allow the engine gradual forward movement when it is free from the mount attachments. At the point where the hoist has removed all engine weight from the mount attachments, the engine should be eased gently forward, away from the aircraft. If the engine binds at any point, manoeuvre it with the hoist until it slips free. The procedure just discussed applies to the removal of most reciprocating and turbine aircraft engines. Any variation in details will be outlined in the manufacturer’s instructions for the aircraft concerned. Before attempting any engine removal, always consult these instructions. When the engine has been removed, it can be carefully lowered onto a stand. The engine should be fastened to the stand and prepared for the removal of accessories.

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Engine showing hoisting sling attachments

Hoist used for engine removal

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Engine installation Hoisting and mounting the engine When the new or overhauled engine is ready to be hoisted for installation, move the engine stand as close as possible to the nacelle in which the replacement is to be installed. Then attach the sling to the engine and hook the hoist to the sling; then take up the slack until the hoist is supporting most of the engine weight. Next, remove the engine attaching bolts from the stand and hoist the engine clear. The engine stand may be moved and the hoist frame positioned in a way that most easily permits the engine to be hoisted into the nacelle. To prevent injury to the crew or damage to the aircraft or engine, be sure that the engine is steadied when moving the hoist frame. Seldom is an engine nacelle so designed that the engine can be fitted and bolted into place as though it were being mounted on a bare wall. The engine must be guided into position and mated with its various connections, such as the mounting bolt holes and the exhaust tailpipe. This must be done despite such obstacles as the nacelle framework, ducts, or firewall connections and without leaving a trail of broken and bent parts, scratched paint, or crushed fingers.

The nuts on the engine mount bolts must be tightened to the torque recommended by the aircraft manufacturer. While the nuts are being tightened, the hoist should support the engine weight sufficiently to allow alignment of the mounting bolts. If the engine is permitted to exert upward or downward pressure on the bolts, it is necessary for the nuts to pull the engine into proper alignment. This would result in nuts being tightened to the proper torque value without actually holding the engine securely to the aircraft. The applicable manufacturer’s instructions outline the sequence for tightening the mounting bolts to ensure the security of fastening. After the nuts are safetied and the engine sling and hoist are removed, bonding strips should be connected across each engine mount to provide an electrical path from the mount to the airframe. Mounting the engine in the nacelle is, of course, only the beginning. All the ducts, electrical leads, controls, tubes, and conduits must be connected before the engine can be operated.

When the engine has been aligned correctly in the nacelle, insert the mounting bolts into their holes and start all of the nuts on them. Always use the type of bolt and nut recommended by the manufacturer; never use an unauthorised substitution of a different type or specification than that prescribed.

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Engine showing hoisting sling attachments

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Connections and adjustments There are no hard and fast rules that direct the order in which units or systems should be connected to the engine. Each maintenance organisation usually supply a worksheet or checklist to be followed during this procedure. This list is based upon experience in engine installation on each particular aircraft. If this is followed carefully, it serves as a guide for an efficient installation. The following instructions, then, are not a sequence of procedures but are a discussion of correct methods for completing an engine installation. The system of ducts for routing air to the engine varies with all types of aircraft. In connecting them, the goal is to fit the ducts tightly at all points of disconnect so that the air they route cannot escape from its intended path. The duct systems of some aircraft must be pressure-checked for leaks. This is done by blocking the system at one end, supplying compressed air at a specified pressure at the other end, and then checking the rate of leakage. The filters in the air induction system must be cleaned to assure an unrestricted flow of clean air to the engine and its units. Because methods for cleaning air filters vary with the materials used in the filtering element, clean them per the technical instructions relating to the aircraft being serviced. The exhaust system should also be carefully connected to prevent the escape of hot gases into the nacelle. When assembling the exhaust system, check all clamps, nuts, and bolts and replace any in doubtful condition. During assembly, the nuts should be gradually and progressively tightened to the correct torque. The clamps should be tapped with a rawhide mallet as they are being tightened to prevent binding at any point. Total Training Support Ltd © Copyright 2020

On some systems, a ball joint connects the stationary portion of the exhaust to the portion that is attached to the engine. This ball joint absorbs the normal engine movement caused by the unbalanced forces of the engine operation. Ball joints must be installed with the specified clearance to prevent binding when expanded by hot exhaust gases. Hoses used within low-pressure systems are generally fastened into place with clamps. Before using a hose clamp, inspect it for security of welding or riveting and smooth operation of the adjusting screw. A clamp that is severely distorted or materially defective should be rejected. (Material defects include extremely brittle or soft areas that may easily break or stretch when the clamp is tightened.) After a hose is installed in a system, it should be supported with rubber-lined supporting clamps at regular intervals. Before installing metal tubing with threaded fittings, make sure the threads are clean and in good condition. Apply sealing compound, of the correct specification for the system, to the threads of the fittings before installing them. While connecting metal tubing, follow the same careful procedure for connecting hose fittings to prevent cross-threading and to assure correct torque. When connecting the leads to the starter, generator, or various other electrical units within the nacelle, make sure that all connections are clean and properly secured. On leads that are fastened to a threaded terminal with a nut, a lock-washer is usually inserted under the nut to prevent the lead from working loose. When required, connector plugs can be safetied with steel wire to hold the knurled nut in the ‘full-tight’ position.

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Cables through the firewall

Electrical connections at the firewall

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Electrical leads within the engine nacelle are usually passed through either a flexible or a rigid conduit. The conduit must be anchored as necessary to provide a secure installation and bonded when required.

Next, adjust the throttle control so that it has a slight cushion action at two positions on the throttle quadrant: One when the carburettor throttle valve is in the full-open position, and the other when it is closed to the idle position.

All engine controls must be accurately adjusted to assure instantaneous response to the control setting. For flexibility, the engine controls are usually a combination of rods and cables. Since these controls are tailored to the model of aircraft in which they are installed, their adjustment must follow precisely the step-by-step procedure outlined in the manufacturer’s instructions for each particular model of aircraft.

Adjust the cushion by turning the cable turn-buckles equally in opposite directions until the throttle control cushion is correct at the full-open position of the throttle valve. Then, when the throttle arm stop is adjusted to the correct idle speed setting, the amount of cushion should be within tolerance at the idle speed position of the throttle valve. The presence of this cushion assures that the travel of the throttle valve is not limited by the stops on the throttle control quadrant, but that they are opening fully and closing to the correct idle speed as determined by the throttle arm stop.

The diagram below illustrates a simplified schematic drawing of a throttle control system for a reciprocating aircraft engine. Using the drawing as a guide, follow a general procedure for adjusting throttle controls. First, loosen the serrated throttle control arm at the carburettor and back off the throttle stop until the throttle valve is in the fully closed position. After locking the cable drum into position with the locking pin, adjust the control rod to a specified length. Then, attach one end of the control rod to the locked cable drum and re-install the throttle control arm on the carburettor in the serrations that allows the other end of the control rod to be attached to it. This correctly connects the control arm to the cable drum. Now, loosen the cable turnbuckles until the throttle control can be locked at the quadrant with the locking pin. Then, with both locking pins in place, adjust the cables to the correct tension as measured with a tensiometer. Remove the locking pins from the cable drum and quadrant.

Adjustment of the engine controls is basically the same on all aircraft, insofar as the linkage is adjusted to a predetermined length for a specific setting of the unit to be controlled. Then, if cables are used in the control system, they are adjusted to a specific tension with the control system locked. Finally, the full travel of the unit to be controlled is assured by establishing the correct cushion in the controls. In general, the same basic procedure is used to connect the linkage of the manual mixture control. This system is marked at the quadrant and the carburettor for the three mixture positions; (1) idle cut-off, (2) auto lean, and (3) auto rich. The positions of the lever on the control quadrant must be synchronised with the positions of the manual mixture control valves on the carburettor.

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Schematic drawing of throttle control system

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Generally, this adjustment is made simultaneously with the cushioning adjustment by placing the mixture control lever and the mixture control valve in the idle cut-off position before adjusting the linkage. After rigging the engine controls, safety the turnbuckles and castle nuts, and make sure the jam nuts on all control rods are tightened.

After the engine has been completely installed and connected, install the propeller on the aircraft. Before doing so, the thrust bearing retaining nut should be checked for correct torque. If required, the propeller shaft must be coated with light engine oil before the propeller is installed; the propeller governor and anti-icing system must be connected according to applicable manufacturer’s instructions.

On multi-engine aircraft, the amount of cushion of all throttle and mixture controls on each quadrant must be equal so that all are aligned at any specific setting chosen. This eliminates the necessity of individually setting each control to synchronise engine operations. After the engine has been installed, it is necessary to adjust the cowl flaps so that the passage of the cooling air over the engine can be regulated accurately. When the cowl flap adjustments have been completed, operate the system and re-check for opening and closing to the specified limits. Also check the cowl flap position indicators, if installed, to assure that they indicate the true position of the flaps. The oil cooler doors are adjusted in a manner similar to that used to adjust the cowl flaps. In some cases, the procedure is reversed insofar as the door is first adjusted to retract to a specified point, and the limit switch on the motor is set to cut out at this point. Then the jackscrew is adjusted to permit the door to open only a specified distance, and the open limit switch is set to stop the motor when this point is reached.

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Helicopter engine removal and installations General Unlike the majority of light fixed-wing aircraft that are classified as direct drive as the propeller is installed directly to the propeller flange and propeller speed, therefore, is engine speed. In a rotorcraft, the engine needs to be able to run independently to the rotors, and the main rotor and tail rotors need to operate at a different rotational speed. This requires the engine to be connected to the transmission via a pilot operated clutch.

the collective lever and throttle; the clutch actuator; the sheave; V-belts; the clutch and sprag clutch assembly; the drive shafts; the main rotor gearbox; the tail rotor gearbox; and the governor.

Other engine controls include a mixture control located forward and to the right of the cyclic centre post and a carburettor heat control located to the right and aft of the cyclic. R22s with O-360 engines are equipped with carb heat assist.

Engine controls A twist-grip throttle control is located on each collective stick. The controls are interconnected and actuate the throttle valve through a mechanical linkage. The engine throttle is also correlated to collective inputs through a mechanical linkage. When the collective is raised, the throttle is opened, and when the collective is lowered, the throttle is closed. The electronic engine governor makes minor throttle adjustments by rotating the twist grip to maintain RPM within Total Training Support Ltd © Copyright 2020

An overtravel spring located in the throttle linkage allows the pilot to roll the throttle off beyond the idle stop before a ground contact (run-on) autorotation landing. This prevents the throttle from opening when the collective is raised. Correct throttle linkage adjustment may be verified during preflight by rolling the twist-grip through the overtravel spring and holding against the hard-idle stop. The carburettor throttle arm should just barely start to move when the collective is raised full up.

The principal components are: • • • • • • • • •

power-on limits. Manual manipulation of the twist grip is not typically required except during start-up, shut down, autorotation practice, and emergencies.

Clutch actuator After the engine is started, it is coupled to the rotor drive system through vee-belts which are tensioned by raising the upper drive sheave. An electric actuator, located between the drive sheaves, raises the upper sheave when the pilot engages the clutch switch. The actuator senses compressive load (belt tension) and switches off when the vee-belts are appropriately tensioned. The clutch caution light illuminates whenever the actuator circuit is energised, either engaging, disengaging, or re-tensioning the belts. The light stays on until the belts are properly tensioned or wholly disengaged.

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Helicopter engine installations and drive components

Helicopter engine drive components

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Sprag clutch The most commonly used freewheeling unit on helicopters is the sprag clutch. This clutch allows movement in only one direction by having an inner and outer race, which are often at the end of the driveshaft. The sprag assembly is made up of several sprags resembling the rollers in a roller bearing. The sprags, unlike the circular bearings, have a figure-eight shape. The vertical height of each of these sprags is slightly greater than the gap between the inner diameter of the outer race and the outer diameter of the inner race. This engaged position places the sprags against both races at a slight angle. Rotation from the engine on the outer race jams the sprags between the outer and inner races and this interference fit drives the inner race, which is attached to the driveshaft. If the driveshaft attempts to drive the engine, the sprags are relived, and the driveshaft rotates without the engine. The same would happen if the engine stopped. Transmission/drive system Transfers energy from the engine to rotors via the lower sheave which is bolted directly to the engine output shaft. The two V-belts transfer power to the upper sheave when they become tight when the pilot operates the clutch actuator. This moves the upper sheave and driveshaft up to tension the belts to turn the rotors. The airframe supplied gearboxes convert the engine RPM to the ideal main/tail rotor RPM and changes the axis of rotation.

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Helicopter sprag clutch

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Reciprocating helicopter engine and QECA The engine is installed facing aft with the propeller shaft approximately 39° above horizontal. The engine is supported by the engine mount, which is bolted to the fuselage structure. The installation of the engine provides for ease of maintenance by allowing easy access to all accessories and components when the engine access doors are opened. The QECA contains the engine, engine mount, engine accessories, engine controls, fuel system, lubrication system, ignition system, cooling system, and hydromechanical clutch and fan assembly. Removal of helicopter QECA Before removing the helicopter QECA, the engine should be preserved if it is possible to do so. Then, shut off the fuel supply to the engine and drain the oil. Make the disconnections necessary to remove the QECA, and then perform the following steps.

Installation, rigging, and adjustment of helicopter QECA The installation of a new or an overhauled engine is in reverse of the removal procedure. The manufacturer’s instructions for the helicopter must be consulted to ascertain the correct interchange of parts from the old engine to the new engine. The applicable maintenance instructions should be followed. Refer to the maintenance instructions manual and associated technical publications for detailed information concerning rigging the throttle, mixture control, cable tensions, and related data.

1) Attach the engine lifting sling to a hoist of at least a twoton capacity. 2) Raise the hoist to apply a slight lift to the QECA. Loosen both engine-mount lower attachment bolt nuts before leaving the upper attachment bolts. 3) Remove the bolts from the sway braces and remove both engine upper attachment bolts. Then, remove both engine-mount lower attachment bolts and remove the QECA from the helicopter. Mount the power package in a suitable work stand and remove the sling.

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Helicopter engine removal

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Licence Category B1

16.12 Engine Monitoring and Ground Operation

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Procedures for starting and ground run-up;

Part-66 Ref. 16.12

Knowledge Levels A B1 B3 1 3 2

Interpretation of engine power output and parameters; Inspection of engine and components: criteria, tolerances, and data specified by the engine manufacturer.

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Table of Contents Procedures for starting and ground run-up _________ 6 Ground running checks _________________________ 6 Ground run precautions ________________________ 6 Prestart checks and procedures __________________ 6 Starting _____________________________________ 7 Hand cracking ________________________________ 8 Power variation procedures ____________________ 10 Engine shut down ____________________________ 10 Example: Lycoming O-540 / IO-540 ground running, warm-up, and ground test procedure _____________ 11

Magnetos and other ignition components __________ 32 Summary ___________________________________ 34 Other reasons for removal ______________________ 36 Overhaul periods ______________________________ 40 Private category aircraft ________________________ 40 Public transport and aerial work category aircraft ____ 40 Engine inspection procedures for an extension to overhaul life _________________________________ 40 Crop spraying aircraft__________________________ 41 Extensions to overhaul periods __________________ 41

Interpretation of engine power output and parameters ____________________________________________ 14 Engine manufacturer’s power charts ______________ 14 Temperature correction ________________________ 16 Humidity correction ___________________________ 18 Effect of compression ratio _____________________ 20 Cruise operations ____________________________ 22 Turbocharged engines ________________________ 24 CHT and EGT _______________________________ 24 Reasons for engine failure ______________________ Crankshaft__________________________________ Crankcase __________________________________ Camshaft and lifters __________________________ Gears _____________________________________ Oil pump ___________________________________ Bearings ___________________________________ Connecting rods _____________________________ Pistons and rings_____________________________ Cylinders ___________________________________ Valves and valve guides _______________________ Rocker arms and pushrods _____________________ Total Training Support Ltd © Copyright 2020

Engine overhaul_______________________________ 42 Top overhaul ________________________________ 42 Major overhaul _______________________________ 42 General overhaul procedures ____________________ 44 Disassembly ________________________________ 44 Inspection terminology _________________________ 45 Inspection procedures _________________________ 47 Cleaning____________________________________ 48 Repair and replacement________________________ 50

26 26 26 28 28 28 30 30 32 32 32 32 12-4

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Cylinder assembly inspection and overhaul _______ Cylinder head _______________________________ Cylinder barrel _______________________________ Engine break-in ______________________________ Cylinder bore inspection _______________________ Valves and valve springs ______________________ Rocker arms and shafts _______________________ Piston and piston pin __________________________ Refacing valve seats __________________________ Valve reconditioning __________________________ Valve lapping and leak testing __________________ Piston repairs _______________________________

52 52 54 58 64 68 68 70 72 76 80 82

Block testing ________________________________ 116 General ___________________________________ 116 Purpose ___________________________________ 116 Requirements ______________________________ 117 Mobile stand testing of reciprocating engines ______ 118 Block test instruments ________________________ 120 Engine operation _____________________________ 122 General ___________________________________ 122 Engine instruments __________________________ 122 Engine starting ______________________________ 123 Engine warm-up _____________________________ 123 Ground check ______________________________ 124 Propeller pitch check _________________________ 125 Power check _______________________________ 125 Ignition system operational check _______________ 127 Cruise mixture check _________________________ 128 Idle speed and idle mixture checks ______________ 128 Two-speed supercharger check _________________ 129 Acceleration and deceleration checks ____________ 129 Engine stopping _____________________________ 130 Basic engine operating principles _______________ 131 Backfiring __________________________________ 137 After-firing _________________________________ 137

Crankshaft ___________________________________ 86 Connecting rods ______________________________ 88 Visual inspection _____________________________ 88 Checking alignment___________________________ 88 In-situ cylinder care and maintenance ____________ 90 General ____________________________________ 90 Hydraulic lock _______________________________ 90 Valve blow-by _______________________________ 92 Cylinder compression tests _____________________ 93 Direct compression tester ______________________ 94 Differential pressure tester _____________________ 94 Cylinder replacement _________________________ 97 Cylinder removal ____________________________ 100 Cylinder installation __________________________ 101 Valve and valve mechanism ___________________ 106 Valve clearance ____________________________ 108 Valve spring replacement _____________________ 111 Cold cylinder check __________________________ 112

Ground and flight testing ______________________ 138 Pre-oiling __________________________________ 138 Fuel system bleeding _________________________ 139 Propeller check _____________________________ 139 Checks and adjustments after engine run-up and operation __________________________________ 140

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Procedures for starting and ground run-up Ground running checks Ground running checks can be very varied, depending upon the type of engine and manufacturer’s instructions. Typical functional checks are as follows.

Start procedures differ from one type of engine to another. Therefore, the information in this module is general and not directed to any specific type. As we are dealing primarily with light aircraft, it is this type that we will consider for engine running.

If a variable pitch propeller is fitted, check the propeller controls for movement through their range. However, it is recommended that feathering of the propeller is not carried out on the ground, as excessive vibration may occur. •

•

Engines are capable of starting in relatively low temperature without the use of engine heating or oil dilution, depending on the grade of oil used.

Check that the oil pressure and temperature indications are within the correct limits for all engine RPM conditions. Carry out idle and full-power checks with reference to the engine RPM gauge and the manufacturer’s recommendations. Check the operation of the carburettor heating systems. Note that the application of this system will cause a slight drop in RPM.

Prestart checks and procedures Typical checks to make before starting are as follows: •

•

The various covers (wing, tail, co*ckpit, wheel, etc.) protecting the aircraft must be removed before attempting to turn the engine.

These are just a few of the checks that may be required to the carried out during ground running operations. For more information and details of these checks, you should consult the manufacturers’ information.

External sources of electrical power should be used when starting engines equipped with electric starters. This eliminates an excessive burden on the aircraft battery.

Ground run precautions Before any engine ground running operations are carried out, reference must always be made to the manufacturer’s operating instructions, or maintenance manual, as in most cases a starting and after starting checklist should always be followed.

Check the wind direction – face the aircraft into wind, but make sure the wind velocity is within limits for ground running. Check that ground chocks are positioned.

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Starting To start the engine proceeds as follows. 1) Turn the auxiliary fuel pump on if the aircraft is equipped with one. 2) Place the mixture control to the position recommended for the engine and carburettor combination being started. As a general rule, the mixture control should be in the “IDLE CUT OFF” position for pressure type carburettors and the “FULL RICH” position for float-type carburettors. Note: Many light aircraft are equipped with a mixture control pull rod which has no detected intermediate positions. When such controls are pushed in flush with the instrument panel, the mixture is set in the “FULL RICH” position. Conversely, when the control rod is pulled all the way out, the carburettor is in the “IDLE CUT OFF” or “FULL LEAN” position. Unmarked intermediate positions between those two extremes can be selected by the operator to achieve any desired mixture setting. 3) Open the throttle to a position that provides 1,000 to 1,200 RPM. 4) Leave the preheat or alternate air (carburettor air) control in the cold position to prevent damage and fire in case of back power; auxiliary heating should be used after the engine warms up. They improve fuel vaporisation, prevent fouling of the spark plugs, ice formation, and eliminate icing in the induction system. 5) Energise the starter; after the propeller has made at least two complete revolutions, and then turn the ignition switch on. On engines equipped with induction vibration, turn the switch to the “BOTH” position. When starting an engine that uses an impulse coupling magneto, turn the Total Training Support Ltd © Copyright 2020

ignition switch to the “LEFT” position. Place the ignition switch to “START” when the magneto incorporates a retard breaker assembly. Do not crank the engine continuously with the starter for more than one minute. Allow three to five-minutes for cooling the starter between successive attempts. Otherwise, the starter may be burnt out due to overheating. Put the primer switch to “ON” intermittently, or prime with one three strokes of primary pump depending on how the aircraft is equipped. When the engine begins to fire, hold the primer while gradually opening the throttle to obtain smooth operation. After the engine is operating smoothly on the primer, move the mixture control to the “FULL RICH” position. Release the primer as soon as a drop in RPM indicates the engine is receiving additional fuel from the carburettor. Piston engine installations vary considerably, and the method of starting recommended by the manufacturer should always be followed. Engine speed should be kept to a minimum until oil pressure has built up and the engine should be warmed up to minimum operating temperature before proceeding with the required tests. High power should only be used for sufficient duration to accomplish the necessary checks, since the engine may not be adequately cooled when the aircraft is stationary. After all the checks have been carried out the engine should be cooled by running at the recommended speed for several minutes, the magneto switches should be checked for operation, and the engine should be stopped.

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Hand cracking Extreme care is essential when starting piston engines by hand swinging; many accidents have occurred in this way. Both pilots and technicians should be given demonstrations and be checked out on this method of starting. The engine must always be treated as live, and no parts of the arms, legs, or body should be moved into the propeller disc at any time. No attempt should ever be made to start an engine without someone in the co*ckpit to operate the throttles or brakes as necessary, or without chocks placed in front of the wheels. •

•

Sucking in: To prime the engine cylinders, when necessary, the ground crew should stand away from the propeller. Face the pilot and call “switches off”, “petrol on”, “throttle closed”, “suck in”. The pilot should repeat these words, carrying out the appropriate actions at the same time. The ground crew should then set the propeller to the beginning of a compression stroke and turn the engine through at least two revolutions. The propeller must be swung smartly down and across the body. Turn away from the propeller and step away in the direction of the movement of the aircraft. Starting: The ground crew should set the propeller at the start of a compression stroke, stand away from the propeller, face the pilot and call “contact”. The pilot should set the throttle for starting, switch “ON” the magnetos and repeat “contact”. The ground crew should then swing the propeller. If the engine does not start, the ground crew should ensure that the magnetos are switched off before resetting the propeller and switched on again before making another attempt to start the engine.

Blowing out: If the engine fails to start through overrichness the ground crew should face the pilot and call “switches off”, “petrol off”, “throttle open”, “blow out”. The pilot should repeat these words, carrying out the appropriate action at the same time. The ground crew should then turn the propeller several revolutions in the reverse directions of rotation to expel the mixture from the engine. This usually entails swinging the propeller up from the 6 o’clock position, using the opposite hand. The throttle should then be closed, the petrol should then be closed, the petrol turned “ON” and the operations for starting the engine repeated.

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Swinging the propeller to start the engine

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Power variation procedures While running the engine, the following procedures should be observed. • •

Engine shut down Once the ground running procedures have been accomplished, then the engine can be shut down. However, like all types of aircraft engine, the engine will be hot, so a condition known as thermal shock must be avoided. To do this, if the engine has been operating at high RPM, the engine must be allowed to idle before shutting down.

To increase power – first, enrich the mixture, increase RPM, then follow with the throttle. To decrease power – first, reduce throttle, reduce RPM, and then adjust the mixture.

Idling allows the engine to cool gradually to a lower temperature before stopping. This condition can best be judged by using either cylinder temperature gauges, if fitted, or by using the oil temperature gauge. Once these instruments indicate a normal running condition, then the engine can be stopped.

Increasing power – enrich mixture first to ensure protecting the engine against damage from a higher power when previously leaned out for a lower power setting. Next, increase RPM, because, in some models, the engine and propeller would have undesirable pressure and stresses with high manifold pressure and lower RPM. Then, follow with the appropriate manifold pressure now that the mixture and RPM have been correctly set to accommodate the increased throttle. Decreasing power – most models of engines require the basic procedure for a decrease of power by retarding throttle, followed by RPM. In the climb configuration, we recommended full throttle throughout the climb for internal fuel cooling with RPM reductions initially to 3,000 RPM and then 2,750 RPM for a prolonged climb. Turbocharged and supercharged engines require careful application of the basic power sequences as outlined in the beginning. It is also possible to create an over-boost condition on these engines by going to takeoff manifold pressure at cruise RPM, such as might take place in an unexpected goaround. The stresses and pressures on prop and engine would create a threat to both. Total Training Support Ltd © Copyright 2020

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Example: Lycoming O-540 / IO-540 ground running, warm-up, and ground test procedure Remember that these engines are air pressure cooled and depend on the forward movement of the aircraft to maintain proper cooling. Therefore, particular care is necessary when operating these engines on the ground. It is recommended that the following precautions are observed to prevent overheating.

d) (Where applicable) Move the propeller control through its complete range to check operation and return to the “FULL LOW PITCH” position. Full feathering check (twin engine) on the ground is not recommended. However, the feathering action can be checked by running the engine between 1,000 and 1,500 RPM, then momentarily pulling the propeller control into the feathering position. Do not allow the RPM to drop more than 500 RPM. e) A proper magneto check is essential. Additional factors, other than the ignition system, affect magneto drop-off; load-power output, propeller pitch and mixture strength. The important thing is that the engine runs smoothly because magneto drop-off is affected by the variables listed above. Make the magneto check per the following procedures. 1) (Controllable pitch propeller) With propeller in minimum pitch angle, set the engine to produce 5065% power as indicated by the manifold pressure gauge unless other specified in the aircraft manufacturer’s manual. Set the mixture control to the full rich position. At these settings, the ignition system and spark plugs must work harder because of the greater pressure within the cylinders. Under these conditions, ignition problems can occur. Magneto checks at low power settings only indicate fuel-air distribution quality.

Note: Any ground check that requires full-throttle operation must be limited to three minutes, or less if indicated cylinder head temperature should exceed the maximum stated in the operator’s manual. Preparation a) Head the aircraft into the wind. b) Leave the mixture in the “FULL RICH” position. c) Operate the engine on the ground only with the propeller in the minimum blade-angle setting. d) Warm-up at approximately 1,000-1,200 RPM. Avoid prolonged idling and do not exceed 2,200 RPM on the ground. e) The engine is warm enough for takeoff when the throttle can be opened without the engine faltering. Ground check a) Warm-up as directed above. b) Check both oil pressure and oil temperature. c) Leave the mixture at “FULL RICH”.

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Note: Aircraft that are equipped with fixed-pitch propellers, or not equipped with a manifold pressure gauge, may check magneto drop-off with the engine operating at approximately 2,100-2,200 RPM. 2) Switch from both magnetos to one and note drop-off; return to both until engine regains speed and switch to the other magneto and note drop-off, then return to both. Drop-off should not exceed 175 RPM and should not exceed 50 RPM between magnetos. A smooth drop-off past the expected specification of 175 RPM is usually a sign of a too-lean or too-rich mixture. 3) If the RPM drop exceeds 175 RPM, slowly lean the mixture until the RPM peaks. Then retard the throttle to the RPM specified in step e) 1) for the magneto check and repeat the check. If the drop off does not exceed 175 RPM, the difference between the magnetos does not exceed 50 RPM, and the engine is running smoothly, then the ignition system is operating correctly. Return the mixture to full rich. f) Do not operate on a single magneto for too long; a few seconds is usually sufficient to check drop-off and minimises plug fouling.

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Engine ground running

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Interpretation of engine power output and parameters Engine manufacturer’s power charts The power charts provided by manufacturers of typecertificated engines are the standard when determining engine power, but these charts have significant limitations that we must understand. The limitations should be listed on the margins of the charts. The limitations, stated or implicit, are:

For this example, let us determine the power produced with mixture set for best power at 2,000 RPM and 23.6" MAP at 2,300 ft pressure altitude at 14°F (-10°C). Start on the right side of the graph, which shows various combinations of full-throttle manifold pressure vs RPM at standard temperature. Find the 2,000 RPM line and interpolate between the 22" and 24" lines to find the power for 23.6" MAP at 2,000 RPM, labelled as point A in the example on the chart.

1) The mixture must be set for best power. 2) The chart power is for standard temperature. If the temperature is above or below standard, the temperature corrections listed on the chart must be applied. 3) The chart power is for dry air. If the air is humid, power is reduced. 4) The engine configuration must match that of the engine model listed on the chart. Any changes in compression ratio, ignition system, or fuel delivery system may affect the power produced. 5) The engine must be in good condition. An engine with low compression, leaky valves, weak ignition system, or other issues, will not make the power claimed by the chart.

Look to the left to find 109 hp. If you go straight down, you see that this combination of RPM and manifold pressure is predicted to occur at full throttle at about 5,900 ft. Now go to the left side of the chart, which shows the power produced at sea level, at standard temperature. Point B, in the example, indicates 2,000 RPM and 23.6" MAP. Look to the right to see that this power setting would produce 97 hp at sea level with standard temperature. Now we know what power would be produced at 2,000 RPM and 23.6" at sea level, and also at 5,900 ft. The next step is to interpolate to find what power would be produced at 2,300 ft.

The graph below shows a typical power chart (Lycoming IO360-M1A series engine). The chart can be found in the engine operator’s manual. The left side of the chart shows the power that is produced at sea level at standard temperature, as a function of RPM and manifold absolute pressure (MAP). The right side of the chart provides power at full throttle as a function of RPM and MAP at various altitudes.

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Note 4 also gives a temperature correction of “approximately 1 % correction for each 10 °F variation from TS.” The actual temperature in our example (14 °F) is 37 °F colder than the standard temperature of 51 °F. The correction is 1 % for every 10 degrees, so we have a correction of 3.7% of 102 hp which is 4 hp. The predicted power is 102 + 4 = 106 hp, in dry air. This 4 hp temperature correction is shown at points E and F.

Take the 97 hp point from the sea level chart on the left and mark it on the right chart. This is point C. It is at the left edge of the portion of the chart that shows sea level on the scale at the bottom. Draw a straight line from point C (97 hp at sea level) to point A (109 hp at 5,900 ft). Find 2,300 ft on the scale on the bottom and go up from there to see where that altitude intersects the line you just drew – 102 hp (point D in the example). This is the predicted power at 2,000 RPM and 23.6" MAP at standard temperature, in dry air, at 2,300 ft pressure altitude. Temperature correction Some of the power charts have a line at the bottom of the right half of the chart showing standard temperature (TS) as a function of altitude. Find 2,300 ft, go up to the line, then over to the scale on the left. You will see that the standard temperature is 51 °F (10.4 °C). Note 4, at the top left of the chart, provides two ways to correct for non-standard temperature. The temperature correction formula in note 4, which assumes temperatures are in degrees Fahrenheit, is: P = PS × √(460 + TS ÷ 460 + T) [temperatures in °F]. If using degrees Celsius, the formula would be: P = PS × √(273 + TS ÷ 273 + T) [temperatures in °C]. Where P = power at the actual temperature PS = power at a standard temperature from power chart T = actual temperature TS = standard temperature Total Training Support Ltd © Copyright 2020

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Humidity correction Water vapour in the air, humidity, displaces the other constituents of air. The lower oxygen content means less fuel can be burned, so less power is produced. The manifold pressure lines on the power charts are for “dry manifold pressure,” i.e., they are valid for completely dry air. In the real world, with some amount of humidity, the manifold pressure must be corrected before entering the power chart.

Where P = power in humid air Pdry = power in dry air, from power chart MAP = actual manifold pressure PH20 = MAP correction from tables below The water vapour pressure and the approximate power correction for humid air may also be determined using various tables compiled for the purpose.

The amount of water vapour in the air can be determined from the dewpoint. The tables below provide the correction that must be applied to the MAP for various dewpoint values. For example, if the dewpoint is 59 °F (15 °C), and the manifold pressure is 29 inHg, the correction is -0.5", thus we would use a MAP of 28.5 inHg when using the power charts. The dewpoint cannot be higher than the air temperature, and air temperature generally decreases as the altitude increases. Thus, there usually is less water vapour present at altitude than at ground level. If the air is cold enough, the amount of water vapour it can hold is so small that the effect on power is negligible. At 18 °F (-8 °C), even fully saturated air has a vapour pressure of only 0.1 inHg, which is likely smaller than the error in our MAP gauges. We can determine ground-level dewpoints from reported airport weather observations then the following approximate correction may be applied: P = Pdry × (((MAP – PH2O) ÷ MAP) – 0.17) ÷ (1 – 0.17)

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Effect of compression ratio Some engine manufacturers provide modified higher compression pistons in their engines to obtain increased power. The compression ratio is one of the significant factors that determine the thermal efficiency of internal combustion engines. The higher the compression ratio, the greater the amount of power produced from the combustion of a given amount of air and fuel. Engine manufacturer power charts are only valid if the compression ratio is as specified for the engine model listed on the chart. If we have changed the compression ratio of our engine, we can make approximate corrections to the power from the power chart, using the theoretical relationship between efficiency and compression ratio. P2 = P1 × (1 – CR2-0.27) ÷ (1 – CR1-0.27) Where P1 = power with the original compression ratio P2 = power with a new compression ratio CR1 = original compression ratio CR2 = new compression ratio For example, if we had a 150 hp O-320, with a 7.0:1 compression ratio, and we installed 8.5:1 compression ratio pistons, the predicted power with the higher compression ratio pistons is: P2 = 150 × (1 – 8.5-0.27) ÷ (1 – 7.0-0.27) = 161

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Cruise operations The following recommendations are provided by Teledyne Continental motors.

rapidly in response to small changes in the fuel mixture. Adjust single point EGT system’s cruise mixture 50° to 75°F (10° to 24°C) lean of peak. Adjust Multi-point EGT systems so the richest (last to peak) EGT is 25° to 50°F (-4° to 10°C) lean of peak. Note on the cruise power settings chart that lean of peak operation reduces power by up to 10% at the same RPM and manifold pressure setting. Do not increase manifold pressure or RPM to regain reduced power or speed. The result is the same as leaning from a higher power setting. You may also notice in the cruise power settings chart that part of the perceived fuel flow reduction operating at best economy comes simply because the power is reduced.

Takeoff and climb Maintain full-rich fuel flows, leaning only for density altitude compensation. Fuel helps cool cylinders during high power operations. CHT should be substantially less than 460°F, typically between 380° to 440 °F (193° to 226 °C) on typical “hot day” conditions. Oil temperatures should be less than 220°F (105°C). Keep cowl flaps open to assist with cooling. High-power cruise High power cruise is generally defined as power settings between 65 and 74% of rated engine power. In this range, TCM recommends the fuel mixture be leaned for best power settings (see chart below). This can be done by leaning to the values in the POH, or if EGT information is available, lean the mixture by finding peak EGT and adjusting richer to get to best power. The leanest cylinder EGT (first to peak) should be at least 50°F (10°C) rich of peak, preferably 75°F (24°C) rich of peak. Since a 75 to 100°F (24 to 38°C) EGT spread is usual, a single point EGT system should be adjusted to at least 125°F (52°C) rich of peak. Multi-point EGT systems should be adjusted so the cylinder with the leanest (first to peak) EGT is 50° to 75°F (10° to 24°C) rich of peak. For turbocharged engines, turbine inlet temperature (TIT) limitations may restrict leaning. In those cases, comply with the AFM/POH instructions.

For all cruise power settings, CHTs should be in the 360° to 400°F (182° to 205°C) range and oil temperature between 180° and 210°F (82° and 100°C). Add fuel or open cowl flaps, as required, to maintain cooler temperatures. Low power cruise Low power cruise is generally defined as power settings below 65%. In this range, duty cycles and temperatures are lower, and operation over a broader range is generally permitted. Regular operation is still recommended to be in the best power range with lean of peak or best economy reserved for trips where extended range is desired. Adjust fuel mixture in the sequence described under the high-power cruise heading. When operating the engine with the fuel mixture lean of peak, advancing manifold pressure or RPM from standard settings to regain lost power is prohibited.

Lean of peak operation in the best economy range as shown on the cruise power settings chart is permitted on many models and should be used primarily for trips where extended range is desired. Lean of peak operation requires the operator to monitor EGT closely, as power and temperature may change Total Training Support Ltd © Copyright 2020

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Turbocharged engines Turbocharged engines usually operate at increased power with higher air temperatures in the cylinders. Aggressive leaning at high power settings reduces detonation margins and should not be practised. Always stay within the safe engine operational margins outlined in the AFM/POH. In addition to effects on cylinder life, operation at high exhaust gas temperatures also affects turbocharger, wastegate and exhaust system life.

Therefore, attempting to limit EGTs in an attempt to be kind to the engine is wrongheaded. • •

One caveat: Turbocharged engines usually have a turbine inlet temperature (TIT) red line that should be observed, particularly when flying at flight level altitudes. The purpose of the TIT limit is to protect the fast-spinning turbine wheel from blade stretch. The TIT limit is usually either 1,650°F (900°C) or 1,750°F (955°C), depending on the model of turbocharger installed.

CHT and EGT From an engine management perspective, it is crucial to understand that CHT and EGT tell us quite different things about what is going on inside the engine. CHT mainly reflects what is going on in the cylinder during the Otto cycle power stroke before the exhaust valve opens, while EGT mainly reflects what is going on during the exhaust stroke after the exhaust valve opens: •

•

Factors affecting CHT CHT increases when power is increased; it also increases when cooling airflow is decreased. But several other factors also affect CHT.

CHT measures heat energy wasted during the power stroke when the cylinder is under maximum stress from high internal pressures and temperatures. EGT measures heat energy wasted during the exhaust stroke when the cylinder is under relatively low stress.

Recall that during the Otto cycle power stroke, peak internal cylinder pressure and temperature optimally occur at 15° to 20° of crankshaft rotation after top dead centre. Anything that causes it to occur earlier (i.e., closer to TDC) increases CHT, and anything that causes it to occur later (further from TDC) decreases CHT.

High CHTs generally indicate that the engine is under excessive stress. That is why it is so important to manage the powerplant in a fashion that limits CHTs to a tolerable value. By contrast, high EGTs do not indicate that the engine is under excessive stress. They simply indicate that much energy from the fuel is being wasted out the exhaust pipe rather than being extracted in the form of mechanical energy sent to the propeller. High EGTs do not represent a threat to engine longevity. The engine is simply not capable of producing EGTs that are high enough to harm anything. Total Training Support Ltd © Copyright 2020

Limiting CHTs is essential to ensure cylinder longevity. Limiting EGTs accomplishes nothing useful.

For example, advancing the ignition timing so that the spark plug fires earlier causes the power peak to occur earlier and increases CHT. Retarding the ignition timing so that the spark plug fires later causes the power peak to occur later and decreases CHT.

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Alternatively, changing the mixture can affect CHT by changing the rate at which the air/fuel charge burns, and therefore causing the power peak to occur earlier or later. The burn rate of the air/fuel charge is fastest a mixture that is slightly richer than stoichiometric, approximately 50°F (10°C) rich of peak EGT (50°F (10°C) ROP). Either richening or leaning the mixture from that point decreases the burn rate, causes the power peak to occur later and consequently reduces CHT.

Note: The absolute value of EGT is not important. It is quite common for different cylinders of the same engine to indicate quite different EGTs, and that is perfectly normal. What is important is the relative value of EGT for a particular cylinder compared to that cylinder’s peak EGT value. In other words, we do not care whether a cylinder’s EGT is 1,390°F (755°C) or 1,460°F (795°C); what we care about is whether the cylinder’s EGT is 80°F (26°C) ROP or 30°F (-1°C) LOP.

Failure of one spark plug or magneto can also affect CHT because the air/fuel mixture takes longer to burn when it is ignited by only one spark plug instead of two. This causes θ pp to occur later and CHT to decrease.

EGT is also affected by ignition performance. Advanced ignition timing that ignites the air/fuel charge earlier and causes the power peak to occur earlier decreases EGT. Retarded ignition timing that ignites the air/fuel charger later and causes the power peak to occur later increases EGT. Failure of one spark plug or magneto causes the air/fuel charge to take longer to burn, so it causes the power peak to occur later and EGT to increase. (You can see this EGT rise every time you do a preflight magneto check.)

Factors affecting EGT EGT is affected by the mixture. Peak EGT occurs at approximately the “stoichiometric” (chemically correct) mixture of 14.7 pounds (kg) of air for each pound (kg) of fuel, at which there is precisely the right amount of oxygen to oxidise all the hydrocarbon chains in the fuel. Leaner mixtures cause EGT to decrease simply because less fuel produces less energy. Richer mixtures also cause EGT to decrease because excess (unoxidised) fuel absorbs heat energy when it vaporises. Consequently, peak EGT can be used to identify a stoichiometric mixture and decreases in EGT from peak can be used to establish mixtures richer or leaner than stoichiometric (ROP and LOP).

Finally, a burned exhaust valve can increase EGT if it allows some of the ultra-hot gas during the peak-temperature phase of the power stroke to leak past the valve and impinge on the EGT sensor probe located a few inches beyond the cylinder’s exhaust port. Even a badly burned valve permits only a tiny amount of gas leakage. The EGT increase caused by a burned exhaust valve is usually quite small (typically a rise of 1,400°F to 1,500°F EGT (760° to 815°C)) and quite easy to miss unless you pay close attention.

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Reasons for engine failure The following discusses the common reasons for aircraft engine failure.

Crankcase Crankcases are also rarely replaced at major overhaul. They are typically repaired as necessary, align-bored to restore critical fits and limits, and often provide reliable service for many TBOs. If the case remains in service long enough, it will eventually crack. The good news is that case cracks propagate slowly enough that a detailed visual inspection once a year is sufficient to detect such cracks before they pose a threat to safety. Engine failures caused by case cracks are extremely rare.

Crankshaft There is no more severe failure mode than crankshaft failure. If it fails, the engine stops. Yet crankshafts are rarely replaced at overhaul. Lycoming did a study that showed their crankshafts often remain in service for more than 14,000 hours (that’s 7+ TBOs) and 50 years. Continental has not published any data on this, but their crankshafts probably have similar longevity. Crankshafts fail in three ways: • • •

infant-mortality due to improper materials or manufacture; following unreported prop strikes; and secondary to oil starvation or bearing failure.

Over the past 15 years, there has been a rash of infantmortality failures of crankshafts. Both Continental and Lycoming have had major recalls of crankshafts that were either forged from inferior steel or were damaged during manufacture. These failures invariably occurred within the first 200 hours after the new crankshaft entered service. If the crankshaft survived its first 200 hours, we can be confident that it was manufactured correctly and should perform reliably for numerous TBOs.

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Crankshaft failure

Crankcase cracks

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Camshaft and lifters The cam/lifter interface endures more pressure and friction than any other moving parts in the engine. The cam lobes and lifter faces must be hard and smooth to function and survive.

Gears The engine has lots of gears: crankshaft and camshaft gears, oil pump gears, accessory drive gears for fuel pump, magnetos, prop governor, and sometimes alternator. These gears are made of case-hardened steel and typically have a very long useful life. They are not usually replaced at overhaul unless apparent damage is found. Engine gears rarely cause catastrophic engine failures.

Even tiny corrosion pits (caused by disuse or acid build-up in the oil) can lead to rapid destruction (spalling) of the surfaces and dictate the need for a premature engine teardown. Cam and lifter spalling is the number one reason that engines fail to make TBO, and it is becoming an epidemic in the owner-flown fleet where aircraft tend to fly irregularly and sit unflown for weeks at a time.

Oil pump Failure of the oil pump is rarely the cause of catastrophic engine failures. If oil pressure is lost, the engine will seize quickly. But the oil pump is simple, consisting of two steel gears inside a close-tolerance aluminium housing, and usually operates trouble-free.

Cam and lifter problems seldom cause catastrophic engine failures. Even with a badly spalled cam lobe (like the one pictured below right), the engine continues to run and make good power. Typically, a problem like this is discovered at a routine oil change when the oil filter is cut open and found to contain a substantial quantity of ferrous metal. Sometimes a cylinder is removed for some other reason, and the worn cam lobe can be inspected visually.

The pump housing can get scored if a chunk of metal passes through the oil pump – although the oil pickup tube has a suction screen to prevent this. Even if the pump housing is damaged, the pump has typically ample output to maintain adequate oil pressure in flight; the problem is mainly noticeable during idle and taxi. If the pump output seems deficient at idle, the oil pump housing can be removed and replaced without tearing down the engine.

If the engine is flown regularly, the cam and lifters can remain in good condition for thousands of hours. At overhaul, the cam and lifters are often replaced with new ones, although a reground cam and reground lifters are sometimes used and can be just as reliable.

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Worn camshaft and lifters

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Bearings Bearing failure is responsible for a significant number of catastrophic engine failures. Under normal circ*mstances, bearings have a long useful life. They are always replaced at major overhaul, but it is not unusual for bearings removed at overhaul to be in pristine condition with little detectable wear.

Spun bearings are usually infant-mortality failures that occur either shortly after an engine is overhauled (due to an assembly error) or shortly after cylinder replacement (due to lack of preload on the through bolts). Failures occasionally occur after a long period of crankcase fretting, but such fretting is usually detectable through oil filter inspection and oil analysis. They can also occur after extreme unpreheated cold starts, but that is quite rare.

Bearings fail prematurely for three reasons: • • •

they become contaminated with metal from some other failure; they become oil-starved when oil pressure is lost; or main bearings become oil-starved because they shift in their crankcase supports to the point where their oil supply holes become misaligned.

Connecting rods Connecting rod failure is responsible for a significant number of catastrophic engine failures. When a rod fails in flight, it often punches a hole in the crankcase (thrown rod) and causes loss of engine oil and subsequent oil starvation. Rod failure has also been known to cause camshaft breakage. The result is invariably a rapid and often total loss of engine power.

Contamination failures can generally be prevented by using a full-flow oil filter and inspecting the filter for metal at every oil change. So long as the filter is changed before its filtering capacity is exceeded, metal particles are caught by the filter and do not get into the engine’s oil galleries and contaminate the bearings. If a significant quantity of metal is found in the filter, the aircraft should be grounded until the source is identified and the problem corrected.

Connecting rods usually have a long useful life and are not generally replaced at overhaul. (Rod bearings, like all bearings, are always replaced at overhaul.) Many rod failures are infantmortality failures caused by improper tightening of the rod cap bolts during engine assembly. Rod failures can also be caused by the failure of the rod bearings, often due to oil starvation. Such failures are usually random failures unrelated to time since overhaul.

Oil-starvation failures are relatively rare. Pilots tend to be welltrained to respond to decreasing oil pressure by reducing power and landing at the first opportunity. Bearings continue to function correctly at partial power even with fairly low oil pressure.

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‘Thrown’ rod

Worn bearing shells

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Pistons and rings Piston and ring failures usually cause only partial power loss, but in rare cases can cause complete power loss. Piston and ring failures are of two types: • •

Valves and valve guides It is quite common for exhaust valves and valve guides to develop problems well short of TBO. Actual valve failures are becoming much less common nowadays because incipient problems can usually be detected using borescope inspections and digital engine monitor surveillance. Even if a valve fails completely, the result is usually only partial power loss and an on-airport emergency landing.

infant-mortality failures due to improper manufacturer or assembly; and heat-distress failures caused by preignition or destructive detonation events. Heat-distress failures can be caused by contaminated fuel (e.g., 100LL laced with Jet A), or by improper engine operation.

Rocker arms and pushrods Rocker arms and pushrods (which operate the valves) typically have a long useful life and are generally not replaced at overhaul. (Rocker bushings, like all bearings, are always replaced at overhaul.) Rocker arm failure is quite rare. Pushrod failures are caused by stuck valves and can almost always be avoided through regular borescope inspections. Even when they happen, such failures usually result in only partial power loss.

They are generally unrelated to hours or years since overhaul. A digital engine monitor can alert the pilot to preignition or destructive detonation events in time for the pilot to take corrective action before heat-distress damage is done. Cylinders Cylinder failures usually cause only partial power loss, but occasionally can cause complete power loss. A cylinder consists of a forged steel barrel mated to an aluminium alloy head casting. Cylinder barrels typically wear slowly, and excessive wear is detected at annual inspection using compression tests and borescope inspections. Cylinder heads can suffer fatigue failures, and occasionally the head can separate from the barrel. As dramatic as it sounds, a head separation causes only a partial loss of power; a six-cylinder engine with a head-to-barrel separation can still make better than 80% power. Cylinder failures can be infant-mortality failures (due to improper manufacture) or age-related failures (especially if the cylinder head remains in service for more than two or three TBOs). Nowadays, most major overhauls include new cylinders, so age-related cylinder failures have become quite rare. Total Training Support Ltd © Copyright 2020

Magnetos and other ignition components Magneto failure is commonplace. Magnetos are full of plastic components that are less than robust; plastic is used because it is non-conductive. Fortunately, aircraft engines are equipped with dual magnetos for redundancy, and the probability of both magnetos failing simultaneously is exceptionally remote. Magneto checks during pre-flight run-up can detect ignition system failures, but in-flight magneto checks are far better at detecting subtle or incipient failures. Digital engine monitors can reliably detect ignition system malfunctions in real-time if the pilot is trained to interpret the data. Magnetos should be disassembled, inspected, and serviced every 500 hours; doing so drastically reduces the likelihood of an in-flight magneto failure. 12-32

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Valve failure Cylinder failure

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Summary The bottom-end components of piston aircraft engines – crankcase, crankshaft, camshaft, bearings, gears, oil pump, etc., – are very robust. They usually exhibit long useful life that are many multiples of published TBOs. Most of these bottomend components (with the notable exception of bearings) are routinely reused at major overhaul and not replaced on a routine basis. When these items do fail prematurely, the failures are mostly infant-mortality failures that occur shortly after the engine is built, rebuilt or overhauled, or they are random failures unrelated to hours or years in service. The vast majority of random failures can be detected long before they get bad enough to cause an in-flight engine failure simply through routine oil-filter inspection and laboratory oil analysis. The top-end components; pistons, cylinders, valves, etc., are considerably less robust. It is not at all unusual for top-end components to fail before TBO. However, most of these failures can be prevented by regular borescope inspections and by use of modern digital engine monitors. Even when they happen, top-end failures usually result in only partial power loss and a successful on-airport landing, and they usually can be resolved without having to remove the engine from the aircraft and sending it to an engine shop. Most top-end failures are infantmortality or random failures that do not correlate with time since overhaul. The bottom line is that the traditional practice of fixed-interval engine overhaul or replacement is unwarranted and counterproductive. A conscientiously applied program of condition monitoring that includes regular oil filter inspection, oil analysis, borescope inspections and digital engine monitor data analysis can yield improved reliability and much-reduced expense and downtime. Total Training Support Ltd © Copyright 2020

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Top end and bottom end components

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Other reasons for removal The following paragraphs outline the most common reasons for removing and replacing a reciprocating engine. Information aid in determining engine conditions that require removal is included; however, in every case, consult applicable manufacturer’s instructions as the final authority in establishing the basis for an engine replacement.

Sudden reduction in speed Sudden reduction in engine speed can occur when one or more of the propeller blades strike an object at a low engine RPM. After impact, the foreign object is cleared, and the engine recovers RPM and continues to run unless stopped to prevent further damage.

Engine life expired Engine life is dependent upon such factors as operational misuse, the quality of manufacture or overhaul, the type of aircraft in which the engine is installed, the kind of operation being carried out, and the degree to which preventive maintenance is accomplished. Thus, it is impossible to establish definite engine removal times. However, based on service experience, it is possible to establish a maximum expected life span of an engine. Regardless of condition, an engine should be removed when it has accumulated the recommended maximum allowable time since the last overhaul, including any allowable time extension.

While taxiing an aircraft, a sudden reduction in speed can occur when the propeller strikes a foreign object, such as a raised section in the runway, a toolbox, or a portion of another aeroplane. Investigation of engines on which this type of accident occurred has shown that generally no internal damage results when the RPM is low, for then the power output is low. The propeller absorbs most of the shock. However, when the accident occurs at high engine RPM, shocks are much more severe. When a sudden reduction in RPM occurs, the following action should be taken. Make a thorough external inspection of the engine mount, crankcase, and nose section to determine whether any parts have been damaged. If damage is found which cannot be corrected by line maintenance, remove the engine.

Sudden stoppage A sudden stoppage is a very rapid and complete stoppage of the engine. It can be caused by engine seizure or by one or more of the propeller blades striking an object in such a way that RPM goes to zero in less than one complete revolution of the propeller. The sudden stoppage may occur under such conditions as the complete and rapid collapse of the landing gear, nosing over of the aircraft, or crash landing. Sudden stoppage can cause internal damage, such as cracked propeller gear teeth, gear train damage in the rear section, crankshaft misalignment, or damaged propeller bearings. When a sudden stoppage occurs, the engine is usually replaced. Total Training Support Ltd © Copyright 2020

Remove the engine oil screens or filters. Inspect them for the presence of metal particles. Remove the engine sump plugs, drain the oil into a clean container, strain it through a clean cloth, and check the cloth and the strained oil for metal particles. Heavy metal particles in the oil indicate a definite engine failure, and the engine must be removed. However, if the metal particles present are similar to fine filings, continue the inspection of the engine to determine its serviceability.

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When the propeller strikes the ground, and stops the engine, it is known as ‘sudden stoppage’

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If there are no heavy metal particles in the engine oil, give the engine a flight test. If the engine operates properly during this flight, look again for any metal in the oil system. If none is found, continue the engine in service. Recheck the oil screens for the presence of metal after 10 hours of operation and again after 20 hours. If no indication of internal failure is found after 20 hours of operation, the engine probably requires no further special inspections.

Start the engine to see if the operation is smooth and the power output adequate. If the engine operates properly during this ground check, shut the engine down and repeat the inspection for metal particles in the oil system.

Remove the propeller and check the crankshaft, or the propeller drive shaft on reduction-gear engines, for misalignment. Clamp a test indicator to the nose section of the engine. Use the dial-type reversible indicator, which has 1∕ 1,000" (0.025 mm) graduations. Remove the front or outside spark plugs from all the cylinders. Then turn the crankshaft and observe if the crankshaft or propeller shaft runs out at either the front or rear propeller cone seat locations. If there is an excessive run-out reading at the crankshaft or propeller driveshaft at the front seat location, the engine should be removed. Consult the applicable manufacturer’s instructions for permissible limits. Even though the run-out of the crankshaft or propeller driveshaft at the front cone seat is less than allowable limits, the rear cone seat location should be checked. If any run-out is found at the rear seat location, which is not in the same plane as the run-out at the front cone seat location, the engine should be removed. If the crankshaft or propeller drive shaft run-out does not exceed these limits, install a serviceable propeller. Make an additional check by tracking the propeller at the tip in the same plane perpendicular to the axis of rotation to assure that blade track tolerance is within the prescribed limits. Total Training Support Ltd © Copyright 2020

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Metal particles in the oil Metal particles on the engine oil screens or the magnetic sump plugs are generally an indication of partial internal failure of the engine. However, due to the construction of aircraft oil systems, metal particles may have collected in the oil system sludge at the time of a previous engine failure. Furthermore, carbon tends to break loose from the interior of the engine in rock-like pieces which have the appearance of metal. It is necessary to consider these possibilities when foreign particles are found on the engine oil screens or sump plugs.

Unstable engine operation Engines are usually removed when there is consistent unstable engine operation. Unstable engine operation generally includes one or more of the following conditions: • • • •

excessive engine vibration; backfiring, either consistent or intermittent; cutting-out while in flight; or low power output.

Before removing an engine for suspected internal failure as indicated by foreign material on the oil screens or oil sump plugs, determine if the foreign particles are metal by placing them on a flat metal object and striking them with a hammer. If the material is carbon, it will disintegrate, whereas metal will either remain intact or change shape, depending on its malleability. If the particles are metal, determine the probable extent of internal damage. For example, if only small particles are found which are similar to filings, drain the oil system, and refill it. Then ground-run the engine and reinspect the oil screens and sump plugs. If no additional particles are found, the aircraft should be test-flown, followed by an inspection of the oil screens and sump plugs. If no further evidence of foreign material is found, continue the engine in service. However, engine performance should be closely observed for any indication of difficulty or internal failure.

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Overhaul periods Private category aircraft Aircraft in the private category (not exceeding 2,730 kg) can continue in service beyond the overhaul period indefinitely as long as an inspection is carried out by a licensed engineer before the 120% period is used.

However, here are some typical examples:

If these inspections are successful, then the engine may continue in service, being reassessed at 100-hour intervals or annually, whichever comes first. Public transport and aerial work category aircraft Aircraft engines in the public transport and aerial work category may continue to remain in service but are prohibited from being employed in public transport or aerial work above the 120% period and must have their Certificate of Airworthiness endorsed to this effect. Inspections are the same above 120% as for the private category.

External Inspections Overheating; security of attachment of components; cracked support brackets and casings; excessive play in shafts and bearings.

•

Internal Inspections Check filters, magnetic chip detectors, spark plug condition (oiling, etc.).

•

Oil Consumption This check must be carried out during the last ten hours of the overhaul period. If the oil consumption rate is within the manufacturer’s recommendations, then an extension is acceptable.

The periods between overhaul are usually calculated in flying hours. However, during everyday use, the engine may be subjected to different environmental conditions and loads (aerobatic – hard use, private flying – easy use). So, the condition of the engine when the overhaul period is due may differ significantly from one engine to another.

You must read the CAA document CAP 747 GR No. 24: ‘Light Aircraft Piston Engine Overhaul Periods’ (download from the CAA website). Engine inspection procedures for an extension to overhaul life Most of these procedures are laid down by the manufacturers in their overhaul manuals.

•

Therefore, at the discretion of the operator/owner, some manufacturers may permit the engine to exceed the recommended overhaul period, if the engine condition shows it to be justified. However, certain rules cover these conditions.

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Engines that have been installed since new or since major overhaul for more than ten years, and are not due overhaul, must be inspected at yearly or 100-hour intervals, whichever comes first. If the condition of the engine is acceptable at these inspection times, then the engine is allowed to complete its recommended period between overhauls.

For aero engines, the major servicing is known as an overhaul, and this can be defined as a major work operation that involves dismantling, bench testing and removal of working parts to enable the engine to continue in service. So, who decides when an aircraft engine needs to be overhauled? Usually, the national aviation authority accepts the engine manufacturer’s recommended overhaul period, which results from following a laid down procedure approved by that authority.

The inspection, however, must be carried out by a fully qualified, appropriately licensed engineer. An engine may continue in service beyond its recommended overhaul period not exceeding 20% of the recommended period between overhaul life, but only when the appropriately licensed engineer has inspected the aircraft engine just before the termination of its standard period, to assess its conditions. If proved satisfactory, the engine can continue in service and be inspected at 100-hour or yearly intervals, whichever comes first, until the 20% extension is used up.

Light aircraft engines are those which: • •

are installed in an aircraft whose maximum weight does not exceed 2,730 kg; or with an output of 400 hp or less.

Crop spraying aircraft Note that for crop spraying aircraft engines, the 100-hour inspection interval is reduced to 50 hours. Extensions to overhaul periods At the end of the 20% extension, any further extensions depend upon the following. Overhaul periods (light aircraft) Aircraft piston engines, like other mechanical components, have to be serviced at set intervals. However, the degree of servicing differs greatly, for instance, from the servicing that may be carried out on a car engine.

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Engine overhaul Both maintenance and overhaul operations are performed on aircraft powerplants at specified intervals. This interval is usually governed by the number of hours the powerplant has been in operation. Tests and experience have shown that operation beyond this period is inefficient and even dangerous because certain parts are worn beyond their safe limits. For an overhauled engine to be as airworthy as a new one, worn parts, as well as damaged parts, must be detected and replaced during overhaul. The only way to detect all unairworthy parts is to perform a thorough and complete inspection while the engine is disassembled. The principal purpose of the overhaul is to inspect the engine parts. Inspection is the most precise and the most critical phase of the overhaul. Inspection cannot be slighted or performed carelessly or incompletely.

It includes removal of the units, such as exhaust collectors, ignition harness, and intake pipes, necessary to remove the cylinders. The actual top overhaul consists of reconditioning the cylinder, piston, and valve-operating mechanism, and replacing the valve guides and piston rings if needed.

Each engine manufacturer provides specific tolerances to which his engine parts must conform and provides general instructions to aid in determining the airworthiness of the part. However, in many cases, the final decision is left up to the mechanic. He must determine if the part is serviceable, repairable, or should be rejected. Knowledge of the operating principles, strength, and stresses applied to a part is essential in making this decision. When the powerplant mechanic signs for the overhaul of an engine, he certifies that he has performed the work using methods, techniques, and practices acceptable to the CAA surveyor.

Major overhaul Major overhaul consists of the complete reconditioning of the powerplant. The actual overhaul period for a specific engine is generally determined by the manufacturer’s recommendations or by the maximum hours of operation between overhaul, as approved by the authority.

Usually, at this time, the accessories require no attention other than that generally required during ordinary maintenance functions. A top overhaul is not recommended by all aircraft engine manufacturers. Many stress that if an engine requires this much dismantling, it should be completely disassembled and receive a major overhaul.

At regular intervals, an engine should be completely dismantled, thoroughly cleaned, and inspected. Each part should be overhauled per the manufacturer’s instructions and tolerances for the engine involved. At this time, all accessories are removed, overhauled, and tested. Here again, instructions of the manufacturer of the accessory concerned should be followed.

Top overhaul Modern aircraft engines are constructed of such durable materials that top overhaul has mostly been eliminated. Top overhaul means an overhaul of those parts ‘on top’ of the crankcase without completely dismantling the engine. Total Training Support Ltd © Copyright 2020

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Major overhaul

Top overhaul

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General overhaul procedures •

Because of the continued changes and the many different types of engines in use, it is not possible to treat the specific overhaul of each in this module. However, there are various overhaul practices and instructions of a non-specific nature which apply to all makes and models of engines. These general instructions are described in this section.

•

Any engine to be overhauled completely should receive a runout check of its crankshaft or propeller shaft as a first step. Any question concerning crankshaft or propeller shaft replacement is resolved at this time since a shaft whose run-out is beyond limits must be replaced.

Always use the proper tool for the job and the one that fits. Use sockets and box end wrenches wherever possible. If special tools are required, use them rather than improvising. Drain the engine oil sumps and remove the oil filter. Drain the oil into a suitable container; strain it through a clean cloth. Check the oil and the cloth for metal particles.

Disassembly Since visual inspection immediately follows disassembly, all individual parts should be laid out in an orderly manner on a workbench as they are removed. To guard against damage and to prevent loss, suitable containers should be available in which to place small parts, nuts, bolts, etc., during the disassembly operation. Other practices to observe during disassembly include: • •

•

Before disassembly, wash the exterior of the engine thoroughly. Dispose of all safety devices as they are removed. Never reuse safety wire or split pins. Always replace with new safety devices. All loose studs and loose or damaged fittings should be carefully tagged to prevent them from being overlooked during the inspection.

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Inspection terminology Several terms are used to describe defects detected in engine parts during an inspection. Some of the more common terms and definitions are listed here.

Crack – A partial separation of material usually caused by vibration, overloading, internal stresses, defective assembly, or fatigue. Depth may be a few thousandths to the full thickness of the piece.

Abrasion – An area of roughened scratches or marks usually caused by foreign matter between moving parts or surfaces.

Cut – Loss of metal, usually to an appreciable depth over a relatively long and narrow area, by mechanical means, as would occur with the use of a saw blade, chisel or sharp-edged stone striking a glancing blow.

Brinelling – One or more indentations on bearing races usually caused by high static loads or application of force during installation or removal. Indentations are rounded or spherical due to the impression left by the contacting balls or rollers of the bearing.

Dent – A small, rounded depression in a surface, usually caused by the part being struck with a rounded object.

Burning – Surface damage due to excessive heat. It is usually caused by improper fit, defective lubrication, or overtemperature operation.

Erosion – Loss of metal from the surface by the mechanical action of foreign objects, such as grit or fine sand. The eroded area will be rough and may be lined in the direction in which the foreign material moved relative to the surface.

Burnishing – Polishing of one surface by sliding contact with a smooth, harder surface. Usually no displacement nor removal of metal.

Flaking – The breaking loose of small pieces of metal or coated surfaces, which is usually caused by defective plating or excessive loading.

Burr – A sharp or roughened projection of metal usually resulting from machine processing.

Fretting – A condition of surface erosion caused by minute movement between two parts usually clamped together with considerable unit pressure.

Chafing – Describes a condition caused by a rubbing action between two parts under light pressure which results in wear. Chipping – The breaking away of pieces of material, which is usually caused by excessive stress concentration or careless handling.

Galling – A severe condition of chafing or fretting in which a transfer of metal from one part to another occurs. It is usually caused by a slight movement of mated parts having limited relative motion and under high loads.

Corrosion – Loss of metal by chemical or electrochemical action. The corrosion products generally are easily removed by mechanical means. Iron rust is an example of corrosion.

Gouging – A furrowing condition in which a displacement of metal has occurred (a torn effect). It is usually caused by a piece of metal or foreign material between close moving parts.

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Grooving – A recess or channel with rounded and smooth edges usually caused by the faulty alignment of parts.

Upsetting – A displacement of material beyond the normal contour or surface (a local bulge or bump). Usually indicates no metal loss.

Inclusion – Presence of foreign or extraneous material wholly within a portion of the metal. Such material is introduced during the manufacture of rod, bar, or tubing by rolling or forging. Nick – A sharp sided gouge or depression with a V-shaped bottom which is generally the result of careless handling of tools and parts. Peening – A series of blunt depressions in a surface. Pick up or scuffing – A build-up or rolling of metal from one area to another, which is usually caused by insufficient lubrication, clearances, or foreign matter. Pitting – Small hollows of irregular shape in the surface, usually caused by corrosion or slight mechanical chipping of surfaces. Scoring – A series of deep scratches caused by foreign particles between moving parts, or careless assembly or disassembly techniques. Scratches – Shallow, thin lines or marks, varying in degree of depth and width, caused by the presence of fine foreign particles during operation or contact with other parts during handling. Stain – A change in colour, locally, causing a noticeably different appearance from the surrounding area.

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Inspection procedures The inspection of engine parts during overhaul is divided into three categories: • • •

solution of acetic acid at room temperature can be applied for a maximum of one minute. The part should then be rinsed with a solution of 1 oz (30 g) of ammonia in 1 gallon (4.5 l) of water.

visual; magnetic; and dimensional.

Examine all gears for evidence of pitting or excessive wear. These conditions are of particular importance when they occur on the teeth; deep pit marks in this area are sufficient cause to reject the gear. Bearing surfaces of all gears should be free from deep scratches. However, minor abrasions usually can be dressed out with fine abrasive cloth.

The first two methods are aimed at determining structural failures in the parts, while the last method deals with the size and shape of each part. Structural failures can be determined by several different methods. Non-austenitic steel parts can readily be examined by the magnetic particle method. Other methods, such as X-ray or etching, can also be used.

All bearing surfaces should be examined for scores, galling, and wear. Considerable scratching and light scoring of aluminium bearing surfaces in the engine does no harm and should not be considered a reason for rejecting the part, provided it falls within the clearances outlined in the table of limits in the engine manufacturer’s overhaul manual. Even though the part comes within the specific clearance limits, it is not satisfactory for reassembly in the engine unless inspection shows the part to be free from other serious defects.

A visual inspection should precede all other inspection procedures. Parts should not be cleaned before a preliminary visual inspection, since indications of failure may often be detected from the residual deposits of metallic particles in some recesses in the engine. Defects in nonmagnetic parts can be found by careful visual inspection along with a suitable etching process. If it is thought that a crack exists in an aluminium part, clean it by brushing or grit-blasting very carefully to avoid scratching the surface. Cover the part with a solution made from 1¼ lbs (575 g) of sodium hydroxide and 1 pint (0.6 l) of water at room temperature. Rinse the part thoroughly with water after about one minute’s contact with the solution. Immediately neutralise the part with a solution of one-part nitric acid and three-parts of water heated to 100°F (40°C). Keep the part in this solution until the black deposit is dissolved. Dry the part with compressed air.

Ball bearings should be inspected visually and by feel for roughness, flat spots on balls, flaking or pitting of races, or scoring on the outside of races. All journals should be checked for galling, scores, misalignment, or out-of-round condition. Shafts, pins, etc., should be checked for straightness. This may be done in most cases by using V-blocks and a dial indicator. Pitted surfaces in highly stressed areas resulting from corrosion can cause the ultimate failure of the part. The following areas should be scrutinised for evidence of such corrosion: • •

•

interior surfaces of piston pins; the fillets at the edges of crankshaft main and crankpin journal surfaces; and thrust bearing races.

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If pitting exists on any of the surfaces mentioned to the extent that it cannot be removed by polishing with crocus cloth or other mild abrasive, the part usually must be rejected.

Regardless of the method and type of solution used, coat or spray all parts with lubricating oil immediately after cleaning to prevent corrosion.

Parts, such as threaded fasteners or plugs, should be inspected to determine the condition of the threads. Badly worn or mutilated threads cannot be tolerated; the parts should be rejected. However, small defects such as slight nicks or burrs may be dressed out with a small file, fine abrasive cloth, or stone. If the part appears to be distorted, badly galled, or mutilated by overtightening, or from the use of improper tools, replace it with a new one.

While the degreasing solution will remove dirt, grease, and soft carbon, deposits of hard carbon will almost invariably remain on many interior surfaces. To remove these deposits, they must be loosened first by immersion in a tank containing a decarbonising solution (usually heated). A great variety of commercial decarbonising agents are available. Decarbonisers, like the degreasing solutions, previously mentioned, fall generally into two categories, water-soluble and hydrocarbons; the same caution concerning the use of watersoluble degreasers applies to water-soluble decarbonisers.

Cleaning After visually inspecting engine recesses for deposits of metal particles, it is essential to clean all engine parts thoroughly to facilitate inspection. Two processes for cleaning engine parts are: • •

Extreme caution should be followed when using a decarbonising solution on magnesium castings. Avoid immersing steel and magnesium parts in the same decarbonising tank, because this practice often results in damage to the magnesium parts from corrosion.

degreasing to remove dirt and sludge (soft carbon); and the removal of hard carbon deposits by decarbonising, brushing or scraping, and grit-blasting.

Decarbonising usually loosens most of the hard carbon deposits remaining after degreasing; the complete removal of all hard carbon, however, generally requires brushing, scraping, or grit- blasting. In all of these operations, be careful to avoid damaging the machined surfaces. In particular, wire brushes and metal scrapers must never be used on any bearing or contact surface.

Degreasing can be done by immersing or spraying the part in a suitable commercial solvent. Extreme care must be used if any water-mixed degreasing solutions containing caustic compounds or soap are used. In addition to being potentially corrosive to aluminium and magnesium, such compounds may become impregnated in the pores of the metal and cause oil foaming when the engine is returned to service. When using water-mixed solutions, therefore, the parts must be rinsed thoroughly and completely in clear boiling water after degreasing.

When grit-blasting parts, follow the manufacturer’s recommendations for the type abrasive material to use. Sand, rice, baked wheat, plastic pellets, glass beads, or crushed walnut shells are examples of abrasive substances that are used for grit-blasting parts.

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Part-cleaned components

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All machined surfaces must be adequately masked and all openings tightly plugged before blasting. The one exception to this is the valve seats, which may be left unprotected when blasting the cylinder head combustion chamber. It is often advantageous to grit-blast the seats since this cuts the glaze which tends to form (particularly on the exhaust valve seat), thus facilitating subsequent valve seat reconditioning. Piston ring grooves may be grit-blasted if necessary; extreme caution must be used, however, to avoid the removal of metal from the bottom and sides of the grooves. When grit-blasting housings, plug all drilled oil passages with rubber plugs or other suitable material to prevent the entrance of foreign matter.

Flanged surfaces that are bent, warped, or nicked can be repaired by lapping to a true surface on a surface plate. Again, the part should be cleaned to be sure that all abrasive has been removed. Defective threads can sometimes be repaired with a suitable die or tap. Small nicks can be removed satisfactorily with Swiss pattern files or small, edged stones. Pipe threads should not be tapped deeper to clean them, because this practice results in an oversized tapped hole. If galling or scratches are removed from a bearing surface of a journal, it should be buffed to a high finish. In general, welding of highly-stressed engine parts is not recommended for unwelded parts. However, welding may be accomplished if it can be reasonably expected that the welded repair will not adversely affect the airworthiness of the engine. A part may be welded when:

The decarbonising solution generally removes most of the enamel on exterior surfaces. All remaining enamel should be removed by grit- blasting, particularly in the crevices between cylinder cooling fins.

•

After cleaning operations, rinse the part in petroleum solvent, dry and remove any loose particles of carbon or other foreign matter by air-blasting, and apply a liberal coating of preservative oil to all surfaces.

• •

Repair and replacement Damage such as burrs, nicks, scratches, scoring, or galling should be removed with a fine oil stone, crocus cloth, or any similar abrasive substance. Following any repairs of this type, the part should be cleaned carefully to be sure that all abrasive has been removed, and then checked with its mating part to assure that the clearances are not excessive.

•

the weld is located externally and can be inspected easily; the part has been cracked or broken as the result of unusual loads not encountered in regular operation; a new replacement part of an obsolete type of engine is unavailable; and The welder’s experience and the equipment used ensures a first-quality weld and the restoration of the original heat treatment in heat-treated parts.

Many minor parts not subjected to high stresses may be safely repaired by welding. Mounting lugs, cowl lugs, cylinder fins, rocker box covers, and many parts fabricated initially by welding are in this category. The welded part should be suitably stress-relieved after welding. However, before welding any engine part, consult the manufacturer’s instructions for the engine concerned to see if it is approved for repair by welding. 12-50

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Parts requiring the use of paint for protection or appearance should be repainted according to the engine manufacturer’s recommendations. One procedure is outlined in the following paragraphs.

Magnesium parts should be cleaned thoroughly with a dichromate treatment before painting. This treatment consists of cleaning all traces of grease and oil from the part by using a neutral, non-corrosive degreasing medium followed by a rinse. Then the part is immersed for at least 45 minutes in a hot dichromate solution (340g (¾ lb) of sodium dichromate to 4.5 l (1 gallon) of water at 80° to 95°C (180° to 200°F)). Next, the part should be washed thoroughly in cold running water, dipped in hot water, and dried in an air blast. Immediately after that, the part should be painted with a prime coat and engine enamel in the same manner as that suggested for aluminium parts.

Aluminium alloy parts should have original exterior painted surfaces rubbed smooth to provide a proper paint base. See that surfaces to be painted are thoroughly cleaned. Care must be taken to avoid painting mating surfaces. Exterior aluminium parts should be primed first with a thin coat of zinc chromate primer. Each coat should be either air-dried for two hours or baked at 177°C (350°F) for half an hour. After the primer is dry, parts should be painted with engine enamel, which should be air-dried until hard or baked for half an hour at 82°C (180°F). Aluminium parts from which the paint has not been removed may be reprinted without the use of a priming coat, provided no bare aluminium is exposed.

Any studs which are bent, broken, damaged, or loose must be replaced. After a stud has been removed, the tapped stud hole should be examined for size and condition of threads. If it is necessary to re-tap the stud hole, it is also necessary to use a suitable oversize stud. Studs that have been broken off flush with the case must be drilled and removed with suitable stud remover. Be careful not to damage any threads. When replacing studs, coat the coarse threads of the stud with antiseize compound.

Parts requiring a black gloss finish should be primed first with zinc chromate primer and then painted with glossy black cylinder enamel. Each coat should be baked for one and a half hours at 177°C (350°F). If baking facilities are not available, cylinder enamel may be airdried; however, an inferior finish results. All paint applied in the above operations should preferably be sprayed; however, if it is necessary to use a brush, use care to avoid an accumulation of paint pockets.

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Cylinder assembly inspection and overhaul Cylinder and piston assemblies are inspected according to the procedures contained in the engine manufacturer’s manuals, charts, and service bulletins.

One of the best methods to double-check your findings is to inspect using the Zyglo process. Any crack in the cylinder head, except those on the fins which can be worked out, is a reason for rejecting the cylinder.

A general procedure for inspecting and reconditioning cylinders (known as top overhaul) is discussed here to provide an understanding of the operations involved.

Inspect the head fins for other damage besides cracks. Dents or bends in the fins should be left alone unless there is a danger of cracking. Where pieces of the fin are missing, the sharp edges should be filed to a smooth contour.

Cylinder head Inspect the cylinder head for internal and external cracks. Carbon deposits must be cleaned from inside of the head, and paint removed from the outside for this inspection.

Fin breakage in a concentrated area will cause dangerous local hot spots. Fin breakage near the spark plug bushings or on the exhaust side of the cylinder is more dangerous than in other areas. When removing or reprofiling a cylinder fin, follow the instructions and the limits in the manufacturer’s manual.

Exterior cracks show up on the head fins where they have been damaged by tools or contact with other parts because of careless handling. Cracks near the edge of the fins are not dangerous if the portion of the fin is removed and contoured correctly. Cracks at the base of the fin are a reason for rejecting the cylinder. Cracks may also occur on the rocker box or in the rocker bosses.

Inspect all the studs on the cylinder head for looseness, straightness, damaged threads, and proper length. Slightly damaged threads may be chased with the proper die. The length of the stud should be correct within ± 1 mm (1∕32") to allow for proper installation of pal nuts or other safety devices.

Interior cracks will almost always radiate from the valve seat bosses or the spark plug bushing boss. They may extend from one boss to the other entirely and are usually caused by improper installation of the seats or bushings.

Be sure the valve guides are clean before the inspection. Very often, carbon covers pits inside the guide. If a guide in this condition is put back in service, carbon again collects in the pits, and valve sticking results. Besides pits, scores, and burned areas inside the valve guide, inspect them for wear or looseness.

Use a bright light to inspect for cracks and investigate any suspicious areas with a magnifying glass or microscope. Cracks in aluminium alloy cylinder heads are generally jagged because of the granular nature of the metal. Do not mistake casting marks or laps for a crack.

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Most manufacturers provide a maximum wear gauge to check the dimension of the guide. This gauge should not enter the guide at all at either end. Do not confuse this gauge with the Go/No-Go gauge used to check new valve guides after reaming. Inspection of valve seat inserts before they are refaced is mostly a matter of determining if there is enough of the seat left to correct any pitting, burning, scoring, or out-of-trueness. Inspect spark plug inserts for the condition of the threads and looseness; run a tap of the proper size through the bushing. Very often, the inside threads of the bushing are burned. If more than one thread is missing, the bushing is rejectable. Tighten a plug into the bushing to check for looseness. Inspect the rocker shaft bosses for scoring, cracks, oversize, or out-of-roundness. Scoring is generally caused by the rocker shaft turning in the bosses, which means either the shaft was too loose in the bosses or a rocker arm was too tight on the shaft. Out-of-roundness is usually caused by a stuck valve. If a valve sticks, the rocker shaft tends to work up and down when the valve offers excessive resistance to opening. Inspect for out-of-roundness and oversize using a telescopic gauge and a micrometre.

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Cylinder barrel Inspect the cylinder barrel for wear, using a dial indicator, a telescopic gauge and micrometre, or an inside micrometre. Dimensional inspection of the barrel consists of the following measurements: • • • • •

The measurement for out-of-roundness is usually taken at the top of the cylinder. However, a reading should also be taken at the skirt of the cylinder to detect dents or bends caused by careless handling.

the maximum taper of cylinder walls; maximum out-of-roundness; bore diameter; step; and fit between piston skirt and cylinder.

A step or ridge is formed in the cylinder by the wearing action of the piston rings. The greatest wear is at the top of the ring travel limit. The ridge which results is very likely to cause damage to the rings or piston. If the step exceeds tolerances, it should be removed by grinding the cylinder oversize, or it should be blended by hand stoning to break the sharp edge.

All measurements involving cylinder barrel diameters must be taken at a minimum of two positions 90° apart in the particular plane being measured. It may be necessary to take more than two measurements to determine the maximum wear. The use of a dial indicator to check a cylinder bore is shown below left.

A step also may be found where the bottom ring reaches its lowest travel. This step is very rarely found to be excessive, but it should be checked. Inspect the cylinder walls for rust, pitting, or scores. Mild damage of this sort can be removed when the rings are lapped. With more extensive damage, the cylinder must be reground or honed. If the damage is too deep to be removed by either method, the cylinder usually must be rejected. Most engine manufacturers have an exchange service on cylinders with damaged barrels.

The taper of the cylinder walls is the difference between the barrel’s diameter at the bottom and the top. The cylinder is usually worn larger at the top than at the bottom. This taper is caused by the natural wear pattern. At the stroke’s top, the piston is subjected to greater heat, pressure and a more erosive environment than at the bottom. Also, there is greater freedom of movement at the top of the stroke. Under these conditions, the piston wears the cylinder wall.

Check the cylinder flange for warpage by placing the cylinder on a suitable jig. Check to see that the flange contacts the jig all the way around. The amount of warp can be checked by using a thickness gauge. A cylinder whose flange is warped beyond the allowable limits should be rejected.

In most cases, the taper ends with a ridge, as seen below right, which must be removed during overhaul. Where cylinders are built with an intentional choke, measurement of taper becomes more complicated. It is necessary to know exactly how the size indicates wear or taper. The taper can be measured in any cylinder by a cylinder dial gauge as long as there is not a sharp step. The dial gauge tends to ride up on the step and causes inaccurate readings at the top of the cylinder. Total Training Support Ltd © Copyright 2020

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Checking cylinder bore with a dial test indicator

Ridge or step formed in an engine cylinder

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A method for checking cylinder flange warpage

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Engine break-in Each new production engine is given a standard production acceptance test. The last part of this test is the oil consumption run and is conducted at full throttle. The purpose of this test is the initial seating of the piston rings to the cylinder walls. The run is conducted at full power because that is where greatest brake mean effective pressure (BMEP) occurs, and a high BMEP is necessary for good piston ring break-in. The test house at the factory determines initial piston ring seating by the amount of oil consumed by the engine during this run. Only a few hours are involved in the acceptance test, and the new engine is by no means wholly broken in. Finishing up the breakin is the responsibility of the pilot who flies the engine during the first 100 hours of its life. The cylinder walls of a new engine are not mirror-smooth as one might imagine. A special hone is used to put a diamondlike pattern of scratches over the entire area of the cylinder wall. The diagrams below show magnified views of these crosshatch patterns. The cross-hatch treatment of the cylinder walls plays an essential role in the proper break-in of piston rings to cylinder walls.

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The illustration below is considerably exaggerated for effect; in reality, the sawtooth effect would not be that pronounced. Notice that a film of lubricating oil holds the piston ring away from the cylinder wall. Proper break-in of piston ring to cylinder wall requires that the ring rupture or break through this oil film and contact the cylinder wall. During such metal-to-metal contact, the little peaks on the ring face and cylinder wall become white-hot and rub off. This condition continues to occur until the ring face and cylinder wall have established a smooth, compatible surface between each other. At this point, break-in is said to be relatively complete, and minimal metal-to-metal contact occurs after. As the break-in progresses, the degree of metal-to-metal contact regresses.

The diagram below right is an exaggerated illustration of oil film rupture during the normal break-in process. Note that the points or ridges of the honed-in scratches have partially worn away. During the actual oil film rupture, only the ridges on the piston rings and cylinder walls contact each other. The little valleys between the ridges retain a film of oil and thereby prevent a total dry condition between the piston ring and cylinder wall. Notice how BMEP, or combustion pressure, forces the ring against the cylinder wall. This is the key to the break-in process. You can see then that low power (low BMEP) won’t provide the same results, and the break-in process requires more time. However, time in this instance has a detrimental effect on the engine because any prolonged, low power break-in procedure usually leads to glazed cylinder walls.

The film of lubricating oil is there to prevent metal-to-metal contact. Ordinarily, this is what we want. However, during the break-in process, there must be some minute metal-to-metal contact as previously explained. Therefore, rupture of the oil film is necessary. Two factors under the pilot’s control can retard this necessary rupture; low power, and improper lubricating oils during the break-in period. Engine lubricating oils can be divided into two basic categories, compounded (detergent and ashless dispersant) and non-compounded. The compounded oils are superior lubricants with a greater film strength than non-compounded oils. Consequently, only non-compounded oils should be used during the break-in period (unless directed by the engine manufacture, Lycoming require all turbocharged engines to be broken in using ashless dispersant oil). Some owners insist on using additives or super lubricants along with the regular engine oil during the break-in period. They believe that such practice aids the engine during its break-in. However, this practice is wrong and causes harm. Total Training Support Ltd © Copyright 2020

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Oil film rupture during ring seating

Piston ring seating

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During each power stroke, the cylinder walls are subjected to extremely high temperatures, often 2,200°C (4,000°F) or higher. This period is very brief but long enough to cause oxidation of minute quantities of some of the lubricating oil on the cylinder walls. Some of this oxidation settles into the valleys of the honed cylinder wall cross-hatch. Eventually, this situation fills the valleys of the cylinder walls creating a smooth, flat surface. This is also a normal situation; however, the ring break-in process practically ceases when these valleys become filled or glazed over. If this glazed-over process occurs before the break-in is complete, “you’ve had it.” Excessive oil consumption resulting from incomplete ring seating will result, and the only sure remedy is rehoning the cylinder walls; this is both expensive and unnecessary.

Interrupt cruise power every 30 minutes or so with a smooth advance to full available manifold pressure and RPM for 30 seconds then return to original cruise settings (nonsupercharged engines only). This procedure helps to hasten a good break-in. These procedures apply primarily to the breakin period and are not necessary after that. Avoid long power-off let downs, especially during the break-in period. Carry enough power during let down to keep cylinder head temperatures at least in the bottom of the green. Keep ground running time to absolute minimums, especially during warm weather. During the break-in period, it is better to delay departure than to sit at the end of the runway for 15 minutes or more running in high ambient temperatures.

Duration of the break-in period is usually defined as the first 50 hours or until oil consumption stabilises.

Be especially generous with mixture controls and cooling air during break-in. All takeoffs should be with a full-rich mixture except those from altitudes over 5,000 ft, and then take care to lean only enough to restore power lost from overly rich mixtures. Make your climbs just a little flatter in hot weather to assure adequate cooling air.

Oil changes are more critical during the break-in period than at any other time in the engine’s life. Do use full rated power and RPM for every takeoff and maintain these settings until at least 400 ft of altitude above the departing runway is attained. At this point, reduce power to 75% and continue the climb to your cruising altitude. Maintain 65 to 75% power for all cruise operation during the break-in period. Avoid high altitude operation with nonsupercharged engines during the break-in period. At altitudes over 8,000 ft, air density does not permit sufficient cruise power development with non-supercharged engines.

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Cylinder bore inspection The following is a general procedure for the inspection of the cylinder bore.

Inspect each cylinder for signatures of heavy wear, belowbottom-right. Heavy bore wear is identified as a complete loss of visible hone pattern over the full ring travel. It generally has associated low cylinder differential compression or high oil consumption. This generally indicates a need for cylinder repair or replacement or, at a minimum, call for more frequent condition inspections.

Inspect each cylinder for signatures of everyday wear, belowtop-right. Cylinder walls which appear to have a minimum, or no hone pattern are acceptable if the cylinder has good differential compression readings and the engine has acceptable oil consumption.

Inspect each cylinder for signatures of scoring. A predominant amount of cylinder bore scratches or grooves that extend in the direction of piston travel typically leads to low differential compression checks and high oil consumption. This may also be identified by burnt or blistered paint on the exterior of the cylinder barrel and indicates a need for cylinder repair or replacement.

Inspect each cylinder for signatures of light rust, below-top-left. Light rust which has not resulted in pitting of the cylinder wall is acceptable. Several small, localised areas less than 1.5 mm (1/16") in diameter are acceptable as long as the total affected areas in any one cylinder do not exceed 25 mm (1") in diameter. The affected areas must be separated by at least 12.5 mm (½"). Rust above the top ring travel is inconsequential and not cause for cylinder removal. Surface discolouration or staining is acceptable and does not result in any damage to the cylinder barrel or the piston rings.

Cylinder borescope inspections are recommended when reported oil consumption is high, or as routine inspections to monitor cylinder condition. Conducting meaningful borescope inspections requires practice and experience to interpret the limited view available properly.

Inspect each cylinder for signatures of heavy rust, belowbottom-left. Cylinder walls which show heavy rust as characterised by pitting of the cylinder wall surface should be removed for repair or replacement if the cylinder has low differential compression or the engine oil consumption is high. Areas of corrosion where the honed surfaces have been altered are of primary concern. These areas usually are very dark in contrast to the surrounding areas. Small localised areas less than 12.5 mm (½") in diameter are acceptable as long as there are no signatures of scoring or material pick up.

When conducting the borescope inspection, the maintenance technician should examine the cylinder for the presence of rust and overall condition of the cylinder bore and valve area.

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Light rust formation, hone not affected

Typical bore at TBO

Heavy rust formation, surface pitting has altered honed pattern

Heavily worn cylinder bore

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To provide improved rust formation protection in new cylinders, TCM cylinders produced have a manganese phosphate coating. These cylinders have an advanced multi-step hone pattern to aid in oil retention. Note that the phosphated cylinder bore has a dark grey to brownish colour that wears away as hours in service are accumulated. Infrequent or irregular use of the aircraft can easily lead to rust formation, which may result in reduced cylinder life if the engine is not correctly preserved.

The following figures show hone patterns in a new cylinder and at TBO for typical TCM cylinders. As can be seen from the photograph at TBO, below-top-right, cylinders which have a very light or no hone pattern in the upper portion of the bore can function normally, have regular oil consumption and have acceptable differential compression checks. For this reason, the borescope inspection should be used in conjunction with differential compression checks and oil consumption trends to assess engine condition.

Caution: The practice of ground running as a substitute for regular use of the aircraft is unacceptable. Ground running does not provide adequate cooling for the cylinders. Also, ground running introduces water and acids into the lubrication system, which can cause substantial damage over time to cylinders and other engine components such as camshafts. Turning the propeller by hand is not recommended as this wipes off the residual oil.

Scratches or grooves that extend in the direction of piston travel can result from contamination and may lead to low differential compression checks and high oil consumption. Heavy bore wear with a complete loss of visible hone pattern over the full ring travel can result from an over-temperature operation or abrasive wear, as shown below-bottom-right. These signatures, in conjunction with low differential compression checks or high oil consumption, generally indicate cylinder repair or replacement or, at minimum, call for more frequent condition inspections.

Borescope inspections of the cylinder wall are performed to assess the condition of the hone pattern and identify abnormal wear patterns which can contribute to low differential compression readings or increased oil consumption.

Over time, the cylinder wall may develop a glazed coating which is generally beneficial to cylinder life as a rust inhibitor. The glaze is a residue of hydrocarbon constituents and lead deposits which serve as both a rust inhibitor and lubricant. Changes and variations in fuel constituents and types of oil used in recent years may impact this beneficial coating. TCM’s revised hone pattern, reduced oil control ring tension and manganese phosphate coating are intended to offset this impact.

The cylinder wall hone pattern consists of a carefully applied pattern of surface scratches introduced at the time of manufacture. These scratches aid in ring-seating by allowing the ring and wall surface to wear into conformity to each other and provide a reservoir of oil for lubrication during ring travel. The cylinder walls and rings are designed to wear over the life of the engine, particularly in the high pressure and temperature combustion area. The visible hone pattern in the upper portion of the bore may disappear during regular operation. Such patterns are normal and not a cause for cylinder removal. Total Training Support Ltd © Copyright 2020

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Using a boroscope to view the interior of cylinders

Typical bore at TBO

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Valves and valve springs Remove the valves from the cylinder head and clean them to remove soft carbon. Examine the valve visually for physical damage and damage from burning or corrosion. Do not reuse valves that indicate damage of this nature. Check the valve face run-out. The locations for checking run-out and edge thickness are shown in the diagram below left.

Examine the valve springs for cracks, rust, broken ends, and compression. Cracks can be located by visual inspection or the magnetic particle method. Compression is tested with a valve spring tester. The spring is compressed until its total height is that specified by the manufacturer. The dial on the tester should indicate the pressure required to compress the spring to the specified height. This must be within the pressure limits established by the manufacturer

Measure the edge thickness of valve heads. If, after refacing, the edge thickness is less than the limit specified by the manufacturer, the valve must not be reused. The edge thickness can be measured with sufficient accuracy by a dial indicator and a surface plate.

Rocker arms and shafts Inspect the valve rockers for cracks and worn, pitted, or scored tips. See that all oil passages are free from obstructions.

Using a magnifying glass, examine the valve in the stem area and the tip for evidence of cracks, nicks, or other indications of damage. This type of damage significantly weakens the valve, making it susceptible to failure. If superficial nicks and scratches on the valve indicate that it might be cracked, inspect it using the magnetic particle or dye penetrant method.

Inspect the shafts for correct size with a micrometre. Rocker shafts are frequently found to be scored and burned because of excessive turning in the cylinder head. Also, there may be some pickup on the shaft (bronze from the rocker bushing transferred to the steel shaft). Generally, this is caused by overheating and too little clearance between shaft and bushing.

Critical areas of the valve include the face and tip, both of which should be examined for pitting and excessive wear. Minor pitting on valve faces can sometimes be removed by grinding.

Inspect the rocker arm bushing for the correct size. Check for proper clearance between the shaft and the bushing. Frequently the bushings are scored because of mishandling during disassembly. Check to see that the oil holes line up. At least 50% of the hole in the bushing should align with the hole in the rocker arm.

Inspect the valve for stretch and wear, using a micrometre or a valve radius gauge. Checking valve stretch with a valve radius gauge is illustrated in the diagram below right. If a micrometre is used, the stretch is found as a smaller diameter stem near the valve neck. Measure the diameter of the valve stem and check the fit of the valve in its guide.

On engines that use a bearing, rather than a bushing, inspect the bearing to make sure it has not been turning in the rocker arm boss. Also, inspect the bearing to determine its serviceability.

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Valve, showing locations for checking run-out and section for measuring edge thickness

Checking valve stretch with manufacturer’s gauge

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Piston and piston pin Inspect the piston for cracks. As an aid to this, heat the piston carefully with a blow torch. If there is a crack, the heat expands it and forces out residual oil, no matter how well the piston has been cleaned. Cracks are more likely to be formed at the highly stressed points. Carefully inspect the base of the pin bosses, inside the piston at the junction of the walls and the head, and the base of the ring lands – especially the top and bottom lands.

Inspect the ring grooves for evidence of a step. If a step is present, the groove must be machined to an oversize width. Use a standard piston ring and check side clearance with a feeler gauge to locate wear in the grooves or to determine if the grooves have already been machined oversize. The largest allowable width is usually 0.5 mm (0.02") oversize because any further machining weakens the lands excessively.

When applicable, check for the flatness of the piston head using a straightedge and thickness gauge. If a depression is found, double-check for cracks on the inside of the piston. A depression in the top of the piston usually means that detonation has occurred within the cylinder.

Examine the piston pin for scoring, cracks, excessive wear, and pitting. Check the clearance between the piston pin and the bore of the piston pin bosses using a telescopic gauge and a micrometre. Use the magnetic particle method to inspect the pin for cracks.

Inspect the exterior of the piston for scores and scratches. Scores on the top ring land are not a cause for rejection unless they are excessively deep. Deep scores on the side of the piston are usually a reason for rejection.

Since the pins are often case hardened, cracks show up inside the pin more often than they will on the outside. Check the pin for bends, using V-blocks and a dial indicator on a surface plate, as pictured below. Measure the fit of the plugs in the pin.

Examine the piston for cracked skirts, broken ring lands, and scored piston-pin holes. Measure the outside of the piston using a micrometre. Measurements must be taken in several directions and on the skirt, as well as on the lands section. Check these sizes against the cylinder size. Several engines now use cam-ground pistons to compensate for the greater expansion parallel to the pin during engine operation. The diameter of these pistons measures several thousandths of an inch larger at an angle to the piston pin hole than parallel to the pin hole.

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Checking a piston pin for bends

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Refacing valve seats The valve seat inserts of aircraft engine cylinders are usually in need of refacing at every overhaul. They are refaced to provide a true, clean, and correct size seat for the valve. When valve guides or valve seats are replaced in a cylinder, the seats must be trued-up to the guide.

The pilot must be tight in the guide because any movement can cause a poor grind. The fluid hose is inserted through one of the spark plug inserts. The three grades of stones available for use are classified as rough, finishing, and polishing stones. The rough stone is designed to true and clean the seat. The finishing stone must follow the rough to remove grinding marks and produce a smooth finish. The polishing stone does just as the name implies and is used only where a highly polished seat is desired.

Modern engines use either bronze or steel seats. Steel seats are commonly used as exhaust seats and are made of a hard, heat-resistant, and often austenitic steel alloy. Bronze seats are used for intake or both seats; they are made of aluminium bronze or phosphor bronze alloys. Steel valve seats are refaced by grinding equipment. Bronze seats are refaced preferably by the use of cutters or reamers, but they may be ground when this equipment is not available. The only disadvantage of using a stone on bronze is that the soft metal loads the stone to such an extent that much time is consumed in redressing the stone to keep it clean.

The stones are installed on special stone holders. The face of the stone is trued by a diamond dresser. The stone should be refaced whenever it is grooved or loaded and when the stone is first installed on the stone holder. The diamond dresser also may be used to cut down the diameter of the stone. Dressing of the stone should be kept to a minimum as a matter of conservation; therefore, it is desirable to have sufficient stone holders for all the stones to be used on the job.

The equipment used on steel seats can be either wet or dry valve seat grinding equipment. The wet grinder uses a mixture of soluble oil and water to wash away the chips and to keep the stone and seat cool; this produces a smoother, more accurate job than the dry grinder. The stones may be either silicon carbide or aluminium oxide.

In the actual grinding job, considerable skill is required in handling the grinding gun. The gun must be centred accurately on the stone holder. If the gun is tilted off-centre, the chattering of the stone results and a rough grind is produced. The stone must be rotated at a speed that permits grinding instead of rubbing. This speed is approximately 8,000 to 10,000 RPM. Excessive pressure on the stone can slow it down. It is not a good technique to let the stone grind at slow speed by putting pressure on the stone when starting or stopping the gun. The maximum pressure used on the stone at any time should be no more than that exerted by the weight of the gun.

Before refacing the seat, make sure that the valve guide is in good condition, is clean, and does not need to be replaced. Mount the cylinder firmly in the hold-down fixture. An expanding pilot is inserted in the valve guide from the inside of the cylinder. An expander screw is inserted in the pilot from the top of the guide as shown in in the diagram below left. Total Training Support Ltd © Copyright 2020

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Valve seat grinding equipment

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Another practice which is conducive to good grinding is to ease off on the stone every second or so to let the coolant wash away the chips on the seat; this rhythmic grinding action also helps keep the stone up to its correct speed. Since it is quite a job to replace a seat, remove as little material as possible during the grinding. Inspect the job frequently to prevent unnecessary grinding.

If the seat contacts the upper third of the valve face, grind off the top corner of the valve seat as shown in the diagram middle below. Such grinding is called “narrowing grinding.” This permits the seat to contact the centre third of the valve face without touching the upper portion of the valve face. If the seat contacts the bottom third of the valve face, grind off the inner corner of the valve seat, as shown in the diagram below-bottom.

The rough stone is used until the seat is true to the valve guide and until all pits, scores, or burned areas are removed. After refacing, the seat should be smooth and true.

The seat is narrowed by a stone other than the standard angle. It is common practice to use a 15° angle and 45° angle cutting stone on a 30° angle valve seat, and a 30° angle and 75° angle stone on a 45° angle valve seat.

The finishing stone is used only until the seat has a smooth, polished appearance. Extreme caution should be used when grinding with the finishing stone to prevent chattering.

If the valve seat has been cut or ground too much, it contacts the seat too far into the cylinder head, affecting its clearance. To check the height of a valve, insert the valve into the guide and hold it against the seat. Check the height of the valve stem above the rocker box or some other fixed position.

The size and trueness of the seat can be checked by several methods. Run-out of the seat is checked with a particular dial indicator and should not exceed 0.05 mm (0.002"). The size of the seat may be determined by using Prussian blue. To check the fit of the seat, spread a thin coat of Prussian blue evenly on the seat. Press the valve onto the seat. The blue transferred to the valve indicates the contact surface. The contact surface should be one-third to two-thirds the width of the valve face and in the middle of the face.

Before refacing a valve seat, consult the overhaul manual for the particular model engine. Each manufacturer specifies the desired angle for grinding and narrowing the valve seat.

In some cases, a Go-No Go gauge is used in place of the valve when making the Prussian blue check. If Prussian blue is not used, the same check may be made by lapping the valve lightly to the seat. Examples of test results are shown in the diagram below-top.

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Fitting of the valve seat

Grinding top surface of the seat

Grinding the inner corner of the valve seat

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Valve reconditioning One of the most common jobs during engine overhaul is grinding the valves. The equipment used should preferably be a wet valve grinder.

Notice that the interference angle is ground into the valve, not the seat. It is easier to change the angle of the valve grinder work head than to change the angle of a valve seat grinder stone. Do not use an interference fit unless the manufacturer approves it.

With this type of machine, a mixture of soluble oil and water is used to keep the valve cool and carry away the grinding chips.

Install the valve into the chuck and adjust it so that the valve face is approximately 50 mm (2") from the chuck, see belowbottom. If the valve is chucked any further out, there is a danger of excessive wobble and also a possibility of grinding into the stem.

Like many machine jobs, valve grinding is mostly a matter of setting up the machine. The following points should be checked or accomplished before starting a grind. True the stone using a diamond nib. The machine is turned on, and the diamond is drawn across the stone, cutting just deep enough to true and clean the stone.

There are various types of valve grinding machines. In one type the stone is moved across the valve face; in another, the valve is moved across the stone. Whichever type is used, the following procedures are typical of those performed when refacing a valve.

Determine the face angle of the valve being ground and set the movable head of the machine to correspond to this valve angle. Usually, valves are ground to the standard angles of 30° or 45°. However, in some instances, an interference fit of 0.5° or 1.5° less than the standard angle may be ground on the valve face.

Check the travel of the valve face across the stone. The valve should completely pass the stone on both sides and yet not travel far enough to grind the stem. There are stops on the machine which can be set to control this travel.

The interference fit in the diagram below-top is used to obtain a more positive seal using a narrow contact surface. Theoretically, there is a line contact between the valve and seat. With this line contact, the entire load that the valve exerts against the seat is concentrated in a tiny area, thereby increasing the unit load at any one spot. The interference fit is especially beneficial during the first few hours of operation after an overhaul. The positive seal reduces the possibility of a burned valve or seat that a leaking valve might produce. After the first few hours of running, these angles tend to pound down and become identical.

With the valve set correctly in place, turn on the machine and turn on the grinding fluid so that it splashes on the valve face. Back the grinding wheel off all the way. Place the valve directly in front of the stone. Slowly bring the wheel forward until a light cut is made on the valve. The intensity of the grind is measured by sound more than anything else. Slowly draw the valve back and forth across the stone without increasing the cut. Move the work head table back and forth using the full face of the stone but always keep the valve face on the stone. 12-76

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Valve installed in the grinding machine

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When the sound of the grind diminishes, indicating that some valve material has been removed, move the work head table to the extreme left to stop the rotation of the valve. Inspect the valve to determine if further grinding is necessary. If another cut must be made, bring the valve in front of the stone, then advance the stone out to the valve. Do not increase the cut without having the valve directly in front of the stone.

Grinding of the valve tip may remove or partially remove the bevel on the edge of the valve. To restore this bevel, mount a vee-way approximately 45° to the grinding stone. Hold the valve onto the vee-way with one hand, then twist the valve tip onto the stone, and with a light touch grind around the tip. This bevel prevents scratching the valve guide when the valve is installed.

An important precaution in valve grinding, as in any kind of grinding, is to make light cuts only. Heavy cuts cause chattering, which may make the valve surface so rough that much time is lost in obtaining the desired finish. After grinding, check the valve margin to be sure that the valve edge has not been ground too thin. A thin edge is called a “feather edge” and can lead to preignition. The valve edge would burn away in a short time, and the cylinder would have to be overhauled again. The diagram below-top-left shows a valve with a regular margin and one with a feather edge. The valve tip may be resurfaced on the valve grinder. The tip is ground to remove cupping or wear and also to adjust valve clearances on some engines. The valve is held by a clamp on the side of the stone as in the diagram below-bottom-right. With the machine and grinding fluid turned on, the valve is pushed lightly against the stone and swung back and forth. Do not swing the valve stem off either edge of the stone. Because of the tendency for the valve to overheat during this grinding, be sure plenty of grinding fluid covers the tip.

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A correctly lapped valve Grinding a valve tip

Interference fit of valve and valve seat

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Valve lapping and leak testing After the grinding procedure is finished, it is sometimes necessary that the valve is lapped to the seat. This is done by applying a small amount of lapping compound to the valve face, inserting the valve into the guide, and rotating the valve with a lapping tool until a smooth, grey finish appears at the contact area. The appearance of a correctly lapped valve is shown in the diagram below-bottom. After the lapping process is finished, be sure that all lapping compound is removed from the valve face, seat, and adjacent areas. The final step is to check the mating surface for leaks to see if it is sealing correctly. Install the valve in the cylinder, holding the valve by the stem with the fingers, and pour kerosene or solvent into the valve port. While holding finger pressure on the valve stem, check to see if the kerosene is leaking past the valve into the combustion chamber. If it is not, the valve reseating operation is finished. If kerosene is leaking past the valve, continue the lapping operation until the leakage is stopped. Any valve face surface appearance that varies from that illustrated in the diagram below-top is correct. However, the incorrect indications are of value in diagnosing improper valve and valve seat grinding. Incorrect indications, their cause and remedy, are shown.

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Incorrectly lapped valves

A correctly lapped valve

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Piston repairs Piston repairs are not required as often as cylinder repairs since most of the wear is between the piston ring and cylinder wall, valve stem and guide, and valve face and seat. A lesser amount of wear is encountered between the piston skirt and cylinder, ring and ring groove, or piston pin and bosses.

After machining, check to be sure that the small radius is maintained at the back of each ring groove. If it is removed, cracks may occur due to localisation of stress. Ring groove oversizes are usually 0.127 mm (0.005"), 0.254 mm (0.010”), or 0.5 mm (0.020”) More than that would weaken the ring lands.

The most common repair is the removal of scores. Usually, these may be removed only on the piston skirt if they are very light. Scores above the top ring groove may be machined or sanded out, as long as the diameter of the piston is not reduced below the specified minimum. To remove these scores, set the piston on a lathe. With the piston revolving at a slow speed, smooth out the scores with number 320 wet-and-dry sandpaper. Never use anything rougher than crocus cloth on the piston skirt.

A few manufacturers sell 0.127 mm (0.005") oversize piston pins. When these are available, it is permissible to bore or ream the piston-pin bosses to 0.127 mm (0.005") oversize. However, these bosses must be in perfect alignment. Small nicks on the edge of the piston-pin boss may be sanded down. Deep scores inside the boss or anywhere around the boss are definite reasons for rejection. Cylinder grinding and honing If a cylinder has an excessive taper, out-of-roundness, step, or its maximum size is beyond limits, it can be reground to the next allowable oversize. If the cylinder walls are lightly rusted, scored, or pitted, the damage may be removed by honing or lapping.

On engines where the entire rotating and reciprocating assembly is balanced, the pistons must weigh within one-fourth ounce of each other. When a new piston is installed, it must be within the same weight tolerance as the one removed. It is not enough to have the pistons matched alone; they must be matched to the crankshaft, connecting rods, piston pins, etc. To make weight adjustments on new pistons, the manufacturer provides a heavy section at the base of the skirt; to decrease weight, file metal evenly off the inside of this heavy section. The piston weight can be decreased easily, but welding, metallizing, or plating cannot be done to increase the piston weight.

Regrinding a cylinder is a specialised job that the powerplant mechanic usually is not expected to do. However, the mechanic must be able to recognise when a cylinder needs regrinding, and he must know what constitutes a good or bad job. Generally, standard aircraft cylinder oversizes are 0.254 mm (0.010”), 0.38 mm (0.015”), 0.5 mm (0.020”), or 0.76 mm (0.030”). Unlike car engines which may be rebored to oversizes of 1.9 mm (0.075”) to 2.54 mm (0.100”), aircraft cylinders have relatively thin walls and may have a nitrided surface, which must not be ground away. Manufacturers usually do not allow all of the above oversizes.

If ring grooves are worn or stepped, they must be machined oversize so that they can accommodate an oversize width ring with the proper clearance.

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Some manufacturers do not allow regrinding to an oversize at all. The manufacturer’s overhaul manual or parts catalogue usually lists the oversizes allowed for a particular make and model engine.

The standard used when measuring the finish of a cylinder wall is known as micro inch root-mean-square, or microinch RMS In a finish where the depth of the grinding scratches are onemillionth (0.000001) of an inch deep, it is specified as 1 microinch RMS*. Most aircraft cylinders are ground to a finish of 15 to 20 microinch RMS. Several low-powered engines have cylinders that are ground to a relatively rough 20- to 30-microinch RMS finish. On the other end of the scale, some manufacturers require a super finish of approximately 4- to 6-microinch RMS.

The standard bore size must be known to determine the regrind size. Usually, this can be determined from the manufacturer’s specifications or manuals. The regrind size is figured from the standard bore. For example, a certain cylinder has a standard bore of 98.425 mm (3.875"). To have a cylinder ground to 0.381 mm (0.015") oversize, it is necessary to grind to a bore diameter of 98.806 mm (3.890") 98.425 + 0.381 (3.875 + 0.015). A tolerance of ±0.127 mm (±0.0005") is usually accepted for cylinder grinding. Another factor to consider when determining the size to which a cylinder must be reground is the maximum wear that has occurred. If there are spots in the cylinder wall that are worn larger than the first oversize, then obviously it is necessary to grind to the next oversize to clean up the entire cylinder. An important consideration when ordering a regrind is the type of finish desired in the cylinder. Some engine manufacturers specify a reasonably rough finish on the cylinder walls, which allows the rings to seat even if they are not lapped to the cylinder. Other manufacturers desire a smooth finish to which a lapped ring will seat without much change in ring or cylinder dimensions. The latter type of finish is more expensive to produce.

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Cylinder grinding is accomplished by a rigidly mounted stone that revolves around the cylinder bore, as well as up and down the length of the cylinder barrel, shown in the diagram belowbottom. Either the cylinder, the stone, or both may move to get this relative movement. The size of the grind is determined by the distance the stone is set away from the centre line of the cylinder. Some cylinder-bore grinding machines produce a perfectly straight bore, while others are designed to grind a choked bore. A choked bore grind refers to the manufacturing process in which the cylinder walls are ground to produce a smaller internal diameter at the top than at the bottom. The purpose of this type of grind or taper is to maintain a straight cylinder wall during operation. As a cylinder heats up during operation, the head and top of the cylinder are subjected to more heat than the bottom. This causes greater expansion at the top than at the bottom, thereby maintaining the desired straight wall.

After the cylinders have been reground, check the size and wall finish, and check for evidence of overheating or grinding cracks before installing on an engine.

After grinding a cylinder, it may be necessary to hone the cylinder bore to produce the desired finish. If this is the case, specify the cylinder regrind size to allow for some metal removal during honing. The usual allowance for honing is 0.0254 mm (0.001"). If a final cylinder bore size of 98.806 mm (3.890") is desired, specify the regrind size of 98.781 mm (3.889"), and then hone to 98.806 mm (3.890"). There are several different makes and models of cylinder hones. The burnishing hone is used only to produce the desired finish on the cylinder wall. The more elaborate micromatic hone can also be used to straighten out the cylinder wall, A burnishing hone, shown in the diagram below-bottom, should not be used in an attempt to straighten cylinder walls. Since the stones are only spring-loaded, they follow the contour of the cylinder wall and may aggravate a tapered condition. Total Training Support Ltd © Copyright 2020

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Cylinder bore grinding

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Crankshaft Carefully inspect all surfaces of the shaft for cracks. Check the bearing surfaces for evidence of galling, scoring, or other damage. When a shaft is equipped with oil transfer tubes, check them for tightness. Some manufacturers recommend supplementing a visual inspection with one of the other forms of non-destructive testing, such as magnetic particle or radiography.

The sludge chamber or tubes must be removed for cleaning at overhaul. If these are not removed, accumulated sludge loosened during cleaning may clog the crankshaft oil passages and cause subsequent bearing failures. If the sludge chambers are formed using tubes pressed into the hollow crankpins, make sure they are reinstalled correctly to avoid covering the ends of the oil passages.

Use extreme care in inspecting and checking the crankshaft for straightness. Place the crankshaft in V-blocks supported at the locations specified in the applicable engine overhaul manual. Using a surface plate and a dial indicator, measure the shaft run-out. If the total indicator reading exceeds the dimensions given in the manufacturer’s table of limits, the shaft must not be reused. A bent crankshaft should not be straightened. Any attempt to do so will result in rupture of the nitrided surface of the bearing journals, a condition that will cause eventual failure of the crankshaft. Measure the outside diameter of the crankshaft main and rodbearing journals. Compare the resulting measurements with those in the table of limits. Some crankshafts are manufactured with hollow crankpins that serve as sludge removers. The sludge chambers may be formed using spool-shaped tubes pressed into the hollow crankpins or by plugs pressed into each end of the crankpin.

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Measuring journal wear

Measuring run-out

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Connecting rods The inspection and repair of connecting rods include: • • • •

Checking alignment Check bushings that have been replaced to determine if the bushing and rod bores are square and parallel to each other. The alignment of a connecting rod can be checked in several ways. One method requires a push-fit arbour for each end of the connecting rod, a surface plate, and two parallel blocks of equal height.

visual inspection; checking of alignment; rebushing; and replacement of bearings.

Some manufacturers also specify a magnetic particle inspection of connecting rods.

To measure rod squareness or twist, insert the arbours into the rod bores, see the diagram below. Place the parallel blocks on a surface plate. Place the ends of the arbours on the parallel blocks. Check the clearance at the points where the arbours rest on the blocks, using a thickness gauge. This clearance, divided by the separation of the blocks in inches, gives the twist per inch of length.

Visual inspection A visual inspection should be done with the aid of a magnifying glass or bench microscope. A rod which is bent or twisted should be rejected without further inspection. Inspect all surfaces of the connecting rods for cracks, corrosion, pitting, galling, or other damage. Galling is caused by a slight amount of movement between the surfaces of the bearing insert and the connecting rod during periods of high loading, such as that produced during overspeed or excessive manifold pressure operation. The visual evidence produced by galling appears as if particles from one contacting surface had welded to the other. Evidence of any galling is sufficient reason for rejecting the complete rod assembly. Galling distortion in the metal and is comparable to corrosion in the manner in which it weakens the metallic structure of the connecting rod.

To determine bushing or bearing parallelism (convergence), insert the arbours in the rod bores. Measure the distance between the arbours on each side of the connecting rod at points that are equidistant from the rod centre line. For exact parallelism, the distances checked on both sides should be the same. Consult the manufacturer’s table of limits for the amount of misalignment permitted. The preceding operations are typical of those used for most reciprocating engines and are included to introduce some of the operations involved in engine overhaul. It would be impractical to list all the steps involved in the overhaul of an engine. It should be understood that some other operations and inspections must be performed. For exact information regarding a specific engine model, consult the manufacturer’s overhaul manual. 12-88

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Checking connecting rod parallelism and squareness

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In-situ cylinder care and maintenance General Each cylinder of the engine is, in reality, an engine in itself. In most cases, the cylinder receives its fuel and air from a common source such as the carburettor. Every phase of cylinder operation, such as compression, fuel mixture, and ignition must function properly since even one type of malfunctioning causes engine difficulty. Engine backfiring, for example, may be caused by a lean fuel/air mixture in one of the cylinders. The lean mixture may be caused by such difficulties as an improper valve adjustment, a slicking intake or exhaust valve, or a leaking intake pipe. Most engine difficulties can be traced to one cylinder or a small number of cylinders. Therefore, engine difficulty can only be corrected after malfunctioning cylinders have been located and defective phases of cylinder operation brought up to normal.

lock which goes undetected at the time it occurs. The piston meets extremely high resistance but is not entirely stopped. The engine falters but starts and continues to run as the other cylinders fire. The slightly bent connecting rod resulting from the partial lock also goes unnoticed at the time it is damaged but is sure to fail later. The eventual failure is almost certain to occur at a time when it can be least tolerated since it is during such critical operations as takeoff and go-around that maximum power is demanded of the engine and maximum stresses are imposed on its parts. Before starting any radial engine that has been shut down for more than 30-minutes, check the ignition switches for “OFF” and then pull the propeller through in the direction of rotation a minimum of two complete turns to make sure that there is no hydraulic lock or to detect the hydraulic lock if one is present. Any liquid present in a cylinder is indicated by the abnormal effort required to rotate the propeller. However, never use force when a hydraulic lock is detected. When engines which employ direct drive or combination inertia and direct drive starters are being started, and an external power source is being used, a check for hydraulic lock may be made by intermittently energising the starter and watching for a tendency of the engine to stall. Use of the starter in this way will not exert sufficient force on the crankshaft to bend or break a connecting rod if a lock is present.

Hydraulic lock Whenever a radial engine remains shut down for any length of time beyond a few minutes, oil or fuel may drain into the combustion chambers of the lower cylinders or accumulate in, the lower intake pipes ready to be drawn into the cylinders when the engine starts. As the piston approaches the top centre of the compression stroke (both valves closed), this liquid, being incompressible, stops piston movement. If the crankshaft continues to rotate, something must give. Therefore, starting or attempting to start an engine with a hydraulic lock of this nature may cause the affected cylinder to blow out or, more likely, may result in a bent or broken connecting rod. A complete hydraulic lock – one that stops crankshaft rotation – can result in serious damage to the engine. Still more serious, however, is the slight damage resulting from a partial hydraulic Total Training Support Ltd © Copyright 2020

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Initial step in developing a hydraulic lock

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To eliminate a lock, remove either the front or rear spark plug of the lower cylinders and pull the propeller through in the direction of rotation. The piston will expel any liquid that may be present.

Check the valve clearance on the affected cylinder. If the valve clearance is incorrect, the valve may be sticking in the valve guide. To release the sticking valve, place a fibre drift on the rocker arm immediately over the valve stem and strike the drift several times with a mallet. Sufficient hand pressure should be exerted on the fibre drift to remove any space between the rocker arm and the valve stem before hitting the drift.

If the hydraulic lock occurs as a result of over-priming before initial engine start, eliminate the lock in the same manner, i.e., remove one of the spark plugs from the cylinder and rotate the crankshaft through two turns.

If the valve is not sticking and the valve clearance is incorrect, adjust it, as necessary.

Never attempt to clear the hydraulic lock by pulling the propeller through in the direction opposite to normal rotation. This tends to inject the liquid from the cylinder into the intake pipe with the possibility of complete or partial lock occurring on the subsequent start.

Determine whether blow-by has been eliminated by pulling the engine through by hand or turning it with the starter again. If blow-by is still present, it may be necessary to replace the cylinder.

Valve blow-by Valve blow-by is indicated by a hissing or whistle when pulling the propeller through before starting the engine, when turning the engine with the starter, or when running the engine at slow speeds. It is caused by a valve sticking open or warped to the extent that compression is not built up in the cylinder as the piston moves toward the top dead centre on the compression stroke. Blow-by past the exhaust valve can be heard at the exhaust stack, and blow-by past the intake valve is audible through the carburettor. Correct valve blow-by immediately to prevent valve failure and possible engine failure by taking the following steps. Perform a cylinder compression test to locate the faulty cylinder.

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Cylinder compression tests The cylinder compression test determines if the valves, piston rings, and pistons are adequately sealing the combustion chamber. If pressure leakage is excessive, the cylinder cannot develop its full power. The purpose of testing cylinder compression is to determine whether cylinder replacement is necessary. The detection and replacement of defective cylinders will prevent a complete engine change because of cylinder failure. Cylinder compression tests must be made periodically.

Be sure that the ignition switch is in the “OFF” position so that there can be no accidental firing of the engine. Remove necessary cowling and the most accessible spark plug from each cylinder. When removing the spark plugs, identify them to coincide with the cylinder. Close examination of the plugs aids in diagnosing problems within the cylinder. Review the maintenance records of the engine being tested. Records of previous compression checks help in determining progressive wear conditions and in establishing the necessary maintenance actions.

Although the engine can lose compression for other reasons, low compression, for the most part, can be traced to leaky valves. Conditions which affect engine compression are: • • • • • •

The two basic types of compression testers currently in use for checking cylinder compression in aircraft engines are the direct compression tester and the differential pressure tester. The procedures and precautions to observe when using either of these types of testers are outlined in this section. When performing a compression test, follow the manufacturer’s instructions for the particular tester being used.

incorrect valve clearances; worn, scuffed, or damaged piston; excessive wear of piston rings and cylinder walls; burned or warped valves; carbon particles between the face and the seat of the valve or valves; and early or late valve timing.

Perform a compression test as soon as possible after the engine is shut down so that piston rings, cylinder walls, and other parts are still freshly lubricated. However, it is not necessary to operate the engine before accomplishing compression checks during engine build-up or on individually replaced cylinders. In such cases, before making the test, spray a small quantity of lubricating oil into the cylinder or cylinders and turn the engine over several times to seal the piston and rings in the cylinder barrel.

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Direct compression tester This type of compression test indicates the actual pressures within the cylinder. Although the particular defective component within the cylinder is difficult to determine with this method, the consistency of the readings for all cylinders is an indication of the condition of the engine as a whole. The following are suggested guidelines for performing a direct compression test:

minute valve leakages can be detected, making possible the replacement of cylinders where valve burning is starting. The operation of the compression tester is based on the principle that, for any given airflow through a fixed orifice, a constant pressure drop across the orifice results. As the airflow varies, the pressure changes accordingly and in the same direction. If air is supplied under pressure to the cylinder with both intake and exhaust valves closed, the amount of air that leaks by the valves or piston rings indicate their condition; the perfect cylinder, of course, would have no leakage.

1) Warm up the engine to operating temperatures and perform the test as soon as possible after shutdown. 2) Remove the most accessible spark plug from each cylinder. 3) Rotate the engine with the starter to expel any excess oil or loose carbon in the cylinders. 4) If a complete set of compression testers is available, install one tester in each cylinder. However, if only one tester is being used, check each cylinder in turn. 5) Using the engine starter, rotate the engine at least three complete revolutions and record the compression reading. Use an external power source, if possible, as a low battery will result in a slow engine-turning rate and lower readings. 6) Recheck any cylinder which shows an abnormal reading when compared with the others. Any cylinder having a reading approximately 103 kPa (15 psi) lower than the others should be suspected of being defective.

The differential pressure tester below left requires the application of air pressure to the cylinder being tested with the piston at top-centre compression stroke. Guidelines for performing a differential compression test are: 1) Perform the compression test as soon as possible after engine shutdown to provide uniform lubrication of cylinder walls and rings. 2) Remove the most accessible spark plug from the cylinder or cylinders and install a spark plug adapter in the spark plug insert. 3) Connect the compression tester assembly to a 690- to 1,035-kPa (100- to 150-psi) compressed air supply. With the shutoff valve on the compression tester closed, adjust the regulator of the compression tester to obtain 550 kPa (80 psi) on the regulated pressure gauge. 4) Open the shutoff valve and attach the air hose quickconnect fitting to the spark plug adapter. The shutoff valve, when open, automatically maintains a pressure of 103 to 137 kPa (15 to 20 psi) in the cylinder when both the intake and exhaust valves are closed.

If a compression tester is suspected of being defective, replace it with one known to be accurate, and recheck the compression of the affected cylinders. Differential pressure tester The differential pressure tester checks the compression of aircraft engines by measuring the leakage through the cylinders. The design of this compression tester is such that Total Training Support Ltd © Copyright 2020

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Differential compression tester – schematic

Differential compression tester

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By hand, turn the engine over in the direction of rotation until the piston in the cylinder being tested comes up on the compression stroke against the 103 kPa (15 psi). Continue turning the propeller slowly in the direction of rotation until the piston reaches the top dead centre. The top dead centre can be detected by a decrease in force required to move the propeller. If the engine is rotated past top dead centre, the 103 to 137 kPa (15 to 20 psi) tends to move the propeller in the direction of rotation. If this occurs, back the propeller up at least one blade before turning the propeller again in the direction of rotation. This backing up is necessary to eliminate the effect of backlash in the valve- operating mechanism and to keep the piston rings seated on the lower ring lands.

If the low compression is not corrected, remove the rocker-box cover and check the valve clearance to determine if the difficulty is caused by inadequate valve clearance. If the low compression is not caused by inadequate valve clearance, place a fibre drift on the rocker arm immediately over the valve stem and tap the drift several times with a 0.5- to 1-kg (1- to 2-lb) hammer to dislodge any foreign material that may be lodged between the valve and valve seat. After staking the valve in this manner, rotate the engine with the starter and recheck the compression. Do not make a compression check after staking a valve until the crankshaft has been rotated either with the starter or by hand to reseat the valve in a normal manner. The higher seating velocity obtained when staking the valve indicates valve seating even though valve seats are slightly egged or eccentric.

Close the shutoff valve in the compression tester and recheck the regulated pressure to see that it is 550 kPa (80 psi) with air flowing into the cylinder. If the regulated pressure is more or less than 550 kPa (80 psi), readjust the regulator in the test unit to obtain 550 kPa (80 psi). When closing the shutoff valve, make sure that the propeller path is clear of all objects.

Cylinders having compression below the minimum specified after staking should be further checked to determine whether the leakage is past the exhaust valve, intake valve, or piston. Excessive leakage can be detected: •

There is sufficient air pressure in the combustion chamber to rotate the propeller if the piston is not on top dead centre. With regulated pressure adjusted to 550 kPa (80 psi), if the cylinder pressure reading indicated on the cylinder pressure gauge is below the minimum specified for the engine being tested, move the propeller in the direction of rotation to seat the piston rings in the grooves. Check all the cylinders and record the readings.

• •

The wheeze test is another method of detecting leaking intake and exhaust valves. In this test, as the piston is moved to top dead centre on the compression stroke, the faulty valve may be detected by listening for a wheezing sound in the exhaust outlet or intake duct.

If low compression is obtained on any cylinder, turn the engine through with the starter or restart and run the engine to takeoff power and recheck the cylinder or cylinders having low compression. Total Training Support Ltd © Copyright 2020

at the exhaust valve by listening for air leakage at the exhaust outlet; at the intake valve by escaping air at the air intake; and past the piston rings by escaping air at the engine breather outlets.

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Another method is to admit compressed air into the cylinder through the spark plug hole. The piston should be restrained at the top dead centre of the compression stroke during this operation. A leaking valve or piston rings can be detected by listening at the exhaust outlet, intake duct, or engine breather outlets.

Some of the reasons for cylinder replacement are: • • • • • •

Next to valve blow-by, the most frequent cause of compression leakage is excessive leakage past the piston. This leakage may occur because of lack of oil. To check this possibility, squirt engine oil into the cylinder and around the piston. Then recheck the compression. If this procedure raises compression to or above the minimum required, continue the cylinder in service. If the cylinder pressure readings still do not meet the minimum requirement, replace the cylinder. When it is necessary to replace a cylinder as a result of low compression, record the cylinder number and the compression value of the newly installed cylinder on the compression check-sheet.

When conditions like these are limited to one or a few cylinders, replacing the defective cylinders should return the engine to a serviceable condition. The number of cylinders that can be replaced on air-cooled, inservice engines more economically than changing engines is controversial. Experience has indicated that, in general, onequarter to one-third of the cylinders on an engine can be replaced economically. Consider these factors when deciding: • • • •

Cylinder replacement Reciprocating engine cylinders are designed to operate a specified time before normal wear requires their overhaul. If the engine is operated as recommended and proficient maintenance is performed, the cylinders typically last until the engine is removed for “high-time” reasons. It is known from experience that materials fail and engines are abused through incorrect operation; this has a severe effect on cylinder life. Another reason for premature cylinder change is poor maintenance. Therefore, exert special care to ensure that all the correct maintenance procedures are adhered to when working on the engine.

low compression; high oil consumption in one or more cylinders; excessive valve guide clearance; loose intake pipe flanges; loose or defective spark plug inserts; and external damage, such as cracks.

• •

time on the engine; priority established for returning the aircraft to service; availability of spare cylinders and spare engines; whether QECA (quick engine change assemblies) are being used; the number of persons available to make the change; and when spare serviceable cylinders are available, replace cylinders when the working hour requirement for changing them does not exceed the time required to make a complete engine change.

The cylinder is always replaced as a complete assembly, which includes piston, rings, valves, and valve springs. Obtain the cylinder by ordering the cylinder assembly under the part number specified in the engine parts catalogue. 12-97

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Except under certain conditions, do not attempt to replace individual parts, such as pistons, rings, or valves. This precaution guarantees that clearances and tolerances are correct. Other parts, such as valve springs, rocker arms, and rocker box covers, may be replaced individually.

Correct procedures and care are essential when replacing cylinders. Careless work or the use of incorrect tools can damage the replacement cylinder or its parts. Incorrect procedures in installing rocker-box covers may result in troublesome oil leaks. Improper torqueing of cylinder holddown nuts or cap-screws can easily result in a cylinder malfunction and subsequent engine failure.

Normally, all the cylinders in an engine are similar; that is, all are a standard size or all a certain oversize, and all are steel bore or all are chrome-plated. In some instances, because of shortages at the time of overhaul, it may be necessary that engines have two different sizes of cylinder assemblies.

The discussion of cylinder replacement here is limited to the removal and installation of air-cooled engine cylinders. The discussion is centred on radial and opposed engines since these are the aircraft engines on which cylinder replacements are most often performed.

Replace a cylinder with an identical one, if possible. If an identical cylinder is not available, it is permissible to install either a standard or oversize cylinder and piston assembly, since this will not adversely affect engine operation.

Since these instructions are meant to cover all air-cooled engines, they are necessarily general. The applicable manufacturer’s maintenance manual should be consulted for torque values and special precautions applying to a particular aircraft and engine. However, always practice neatness and cleanliness and always protect openings so that nuts, washers, tools, and miscellaneous items do not enter the engine’s internal sections.

The size of the cylinder is indicated by a colour code around the barrel between the attaching flange and the lower barrel cooling fin, shown below. In some instances, air-cooled engines are equipped with chrome-plated cylinders. Chrome-plated cylinders are usually identified by a painted band around the barrel between the attaching flange and the lower barrel cooling fin. This colour band is usually international orange. When installing a chromeplated cylinder, do not use chrome-plated piston rings. The matched assembly will, of course, include the correct piston rings. However, if a piston ring is broken during cylinder installation, check the cylinder marking to determine what ring, chrome-plated or otherwise, is correct for replacement. Similar precautions must be taken to be sure that the correct size rings are installed.

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Identification of cylinder size

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Cylinder removal Assuming that all obstructing cowling and brackets have been removed, first remove the intake pipe and exhaust pipes; plug or cover openings in the diffuser section. Then remove cylinder deflectors and any attaching brackets which would obstruct cylinder removal. Loosen the spark plugs and remove the spark-plug lead clamps. Do not remove the spark plugs until ready to pull the cylinder off. Remove the rocker box covers. First, remove the nuts and then tap the cover lightly with a rawhide mallet or plastic hammer. Never pry the cover off with a screwdriver or similar tool. Loosen the pushrod packing gland nuts or hose clamps, top and bottom. Pushrods are removed by depressing the rocker arms with a special tool or by removing the rocker arm. Before removing the pushrods, turn the crankshaft until the piston is at top dead centre on the compression stroke. This relieves the pressure on both intake and exhaust rocker arms. It is also wise to back off the adjusting nut as far as possible because this allows maximum clearance for pushrod removal when the rocker arms are depressed. On some model engines, tappets and springs of lower cylinders can fall out. Provision must be made to catch them as the pushrod and housing are removed. After removing the pushrods, examine them for markings or mark them so that they may be replaced in the same location as they were before removal.

The ball ends are usually worn to fit the sockets in which they have been operating. Furthermore, on some engines’ pushrods are not all of the same lengths. A good procedure is to mark the pushrods near the valve tappet ends “No 1 IN,” “No 1 EX,” “No 2 IN,” “No 2 EX”, etc. On fuel injection engines, disconnect the fuel injection line and remove the fuel injection nozzle and any line clamps which interfere with cylinder removal. If the cylinder to be removed is a master rod cylinder, special precautions, in addition to regular cylinder removal precautions, must be observed. Information designating which cylinder has the master rod is included on the engine data plate. Arrangements must be made to hold the master rod in the mid-position of the crankcase cylinder hole (after the cylinder has been removed). Templates or guides are usually provided by the manufacturer for this purpose, or they are manufactured locally. Under no circ*mstances should the master rod be moved from side to side. It must be kept centred until the guide is in place. Do not turn the crankshaft while the master rod cylinder is removed and other cylinders in the row remain on the engine. These precautions are necessary to prevent bottom rings on some of the other pistons from coming out of the cylinders, expanding, and damaging rings and piston skirts. If several cylinders are to be removed, one of which is the master rod cylinder, it should always be removed last and should be the first installed.

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The next step in removing the cylinder is to cut the lockwire or remove the split pin, and pry off the locking device from the cylinder-attaching cap-screws or nuts. Remove all the screws or nuts except two located 180° apart. Use the wrench specified for this purpose in the special tools section of the applicable manual. Finally, while supporting the cylinder, remove the two remaining screws or nuts and gently pull the cylinder away from the crankcase. Two men must work together during this step as well as during the remaining procedure for cylinder replacement. After the cylinder skirt has cleared the crankcase and before the piston protrudes from the skirt, pro-vide some means (usually a shop cloth) for preventing pieces of broken rings from falling into the crankcase. After the piston has been removed, remove the cloths, and carefully check for piston ring pieces. To make sure that no ring pieces have entered the crankcase, collect, and arrange all the pieces to see that they form a complete ring. Place a support on the cylinder mounting pad and secure it with two cap-screws or nuts. Then remove the piston and ring assembly from the connecting rod. When varnish makes it hard to remove the pin, a pin pusher or puller tool must be used. If the special tool is not available and a drift is used to remove the piston pin, the connecting rod should be supported so that it will not have to take the shock of the blows. If this is not done, the rod may be damaged.

Using a wire brush, clean the studs or cap-screws and examine them for cracks, damaged threads, or any other visible defects. If one cap-screw is found loose or broken at the time of cylinder removal, all the cap-screws for the cylinder should be discarded, since the remaining cap-screws may have been significantly weakened. A cylinder hold-down stud failure would place the adjacent studs under greater operating pressure, and they are likely to be stretched beyond their elastic limit. The engine manufacturer’s instruction must be followed for the number of studs that will have to be replaced after a stud failure. When removing a broken stud, take proper precautions to prevent metal chips from entering the engine power section. In all eases, both faces of the washers and the seating faces of stud nuts or cap-screws must be cleaned and any roughness or buns removed. Cylinder installation See that all preservative oil accumulation on the cylinder and piston assembly is washed off with solvent and thoroughly dried with compressed air. Install the piston and ring assembly on the connecting rod. Be sure that the piston faces in the right direction. The piston number stamped on the bottom of the piston head should face toward the front of the engine. Lubricate the piston pin before inserting it. It should fit with a push. If a drift must be used, follow the same precaution that was taken during pin removal.

After the removal of a cylinder and piston, the connecting rod must be supported to prevent damage to the rod and crankcase. This can be done by supporting each connecting rod with the removed cylinder-base oil-seal ring looped around the rod and cylinder base studs. Total Training Support Ltd © Copyright 2020

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Oil the exterior of the piston assembly generously, forcing oil around the piston rings and in the space between the rings and grooves. Stagger the ring gaps around the piston and check to see that rings are in the correct grooves and whether they are positioned correctly because some are used as oil scrapers, others as pumper rings. The number, type and arrangement of the compression and oil-control rings vary with the make and model of engine. If it is necessary to replace the rings on one or more of the pistons, check the side clearance against the manufacturer’s specification, using a thickness gauge. The ring end gap must also be checked. The method for checking side and end clearance is shown in below. If the ring gauge shown is not available, a piston (without rings) may be inserted in the cylinder and the ring inserted in the cylinder bore. Insert the ring in the cylinder skirt below the mounting flange, since this is usually the smallest bore diameter. Pull the piston against the ring to align it properly in the bore. If it is necessary to remove material to obtain the correct side clearance, it can be done either by turning the piston grooves a slight amount on each side or by lapping the ring on a surface plate. If the end gap is too close, the excess metal can be removed by clamping a mill file in a vice, holding the ring in proper alignment, and dressing off the ends. In all cases, the engine manufacturer’s procedures must be followed.

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Measuring piston ring end gap

Measuring piston ring side clearance

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Before installing the cylinder, check the flange to see that the mating surface is smooth and clean. Coat the inside of the cylinder barrel generously with oil. Be sure that the cylinder oilseal ring is in place and that only one seal ring is used. Using a ring compressor, compress the rings to a diameter equal to that of the piston. Start the cylinder assembly down over the piston, making sure that the cylinder and piston plane remains the same. Ease the cylinder over the piston with a straight, even movement, which moves the ring compressor as the cylinder slips on. Do not rock the cylinder while slipping it on the piston, since any rocking is apt to release a piston ring or a part of a ring from the ring compressor before the ring’s entrance into the cylinder bore. A ring released in this manner will expand and prevent the piston from entering the cylinder. Any attempt to force the cylinder onto the piston is apt to cause cracking or chipping of the ring or damage to the ring lands. After the cylinder has slipped on the piston so that all piston rings are in the cylinder bore, remove the ring compressor and the connecting rod guide. Then, slide the cylinder into place on the mounting pad. If cap-screws are used, rotate the cylinder to align the holes. While still supporting the cylinder, install two cap-screws or stud nuts 180° apart. If the cylinder is secured to the crankcase by conical washers and nuts or cap-screws, position the cylinder on the crankcase section by two special locating nuts or cap-screws. These locating nuts or cap-screws do not remain on the engine, but are removed and replaced with regular nuts or cap- screws and conical washers after they have served their purpose and the other nuts or cap-screws have been installed and tightened to the prescribed torque. Total Training Support Ltd © Copyright 2020

Install the remaining nuts or cap-screws with their conical washers and tighten the nuts or cap-screws until they are snug. Make sure that the conical side of each washer is toward the cylinder mounting flange. Before inserting cap-screws, coat them with a good sealer to prevent oil leakage. Generally, studs fit into holes, and the fit is tight enough to prevent leakage. The hold-down nuts or cap-screws must now be torqued to the value specified in the table of torque values in the engine manufacturer’s service or overhaul manual. A definite and specific sequence of tightening all cylinder fastenings must be followed. Always refer to the appropriate engine service manual. A general rule is to tighten the first two nuts or capscrews 180° from each other; then tighten two alternate nuts or cap-screws 90° from the first two. If locating nuts or cap-screws are being used, they should be torqued first. The tightening of the remaining screws or nuts should be alternated 180° as the torqueing continues around the cylinder. Apply the torque with a slow, steady motion until the prescribed value is reached. Hold the tension on the wrench for a sufficient length of time to ensure that the nut or cap-screw will tighten no more at the prescribed torque value. In many cases, an additional turning of the cap-screw or nut as much as one-quarter turn can be done by maintaining the prescribed torque on the nut for a short time. After tightening the regular nuts or cap- screws, remove the two locating nuts or cap-screws, install regular nuts or capscrews, and tighten them to the prescribed torque. After the stud nuts or cap-screws have been torqued to the prescribed value, safety them in the manner recommended in the engine manufacturer’s service manual.

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Reinstall the pushrods, pushrod housings, rocker arms, barrel deflectors, intake pipes, ignition harness lead clamps and brackets, fuel injection line clamps and fuel injection nozzles, exhaust stack, cylinder head deflectors, and spark plugs. Remember that the pushrods must be installed in their original locations and must not be turned end to end. Make sure, too, that the pushrod ball end seats properly in the tappet. If it rests on the edge or shoulder of the tappet during valve clearance adjustment and later drops into place, valve clearance will be off. Furthermore, rotating the crankshaft with the pushrod resting on the edge of the tappet may bend the pushrod. After installing the pushrods and rocker arms, set the valve clearance. Before installing the rocker-box covers, lubricate the rockerarm bearings and valve stems. Check the rocker-box covers for flatness and resurface them if necessary. After installing the gaskets and covers, tighten the rocker-box cover nuts to the specified torque. Safe-tie those nuts, screws, and other fasteners which require safetying. Follow the recommended safetying procedures.

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Valve and valve mechanism Valves open and close the ports in the cylinder head to control the entrance of the combustible mixture and the exit of the exhaust gases. It is essential that they open and close properly and seal tight against the port seats to secure maximum power from the burning fuel/air mixture for the crankshaft, and to prevent valve burning and warping. The motion of the valves is controlled by the valve-operating mechanism. The valve mechanism includes cam plates or shafts, cam followers, pushrods, rocker arms, valve springs, and retainers. All parts of a valve mechanism must be in good condition, and valve clearances must be correct if the valves are to operate correctly. Checking and adjusting the valve clearance is perhaps the most crucial part of valve inspection, and indeed, it is the most challenging part. However, a visual inspection should not be slighted. It should include a check for the following significant items. Metal particles in the rocker box are indications of excessive wear or partial failure of the valve mechanism. Locate and replace the defective parts. Excessive side clearance or galling of the rocker arm side. Replace defective rocker arms. Add shims when permitted, to correct excessive side clearance.

Replace any damaged parts, such as cracked, broken, or chipped rocker arms, valve springs, or spring retainers. If the damaged part is one which cannot be replaced in the field, replace the cylinder. Excessive valve stem clearance. A certain amount of valve stem wobble in the valve guide is normal. Replace the cylinder only in severe cases. Evidence of incorrect lubrication. Excessive dryness indicates insufficient lubrication. However, the lubrication varies between engines and between cylinders in the same model engine. For example, the upper boxes of radial engines typically run drier than the lower rocker boxes. These factors must be considered in determining whether or not ample lubrication is being obtained. Wherever improper lubrication is indicated, determine the cause, and correct it. For example, a dry rocker may be caused by a plugged oil passage in the pushrod. Excessive oil may be caused by plugged drains between the rocker box and the crankcase. If the pushrod drains become clogged, the oil forced to the rocker arm and other parts of the valve mechanism cannot drain back to the crankcase. This may result in oil leakage at the rocker box cover or in oil seepage along valve stems into the cylinder or exhaust system, causing excessive oil consumption on the affected cylinder and smoking in the exhaust.

Insufficient clearance between the rocker arm and the valve spring retainer. Follow the procedure outlined in the engine service manual for checking this clearance and increase it to the minimum specified.

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Excessive sludge in the rocker box. This indicates an excessive rocker box temperature, which, in turn, may be caused by improper positioning of cowling or exhaust heat shields or baffles. After correcting the cause of the difficulty, spray the interior of the rocker box with dry cleaning solvent, blow it dry with compressed air, and then coat the entire valve mechanism and interior of the rocker box with clean engine oil. Variation in valve clearance not explained by everyday wear. If there is excessive valve clearance, check for bent pushrods. Replace any that are defective. Also, check for valve sticking. If the pushrod is straight and the valve opens and closes when the propeller is pulled through by hand, check the tightness of the adjusting screw to determine whether the clearance was set incorrectly or the adjusting screw has loosened. After adjusting the clearance on each valve, tighten the lock screw or nut to the torque specified in the maintenance manual. After completing all clearance adjustments and before installing the rocker box covers, make a final check of all lock screws or nuts for tightness with a torque wrench. Warped rocker box covers are a common cause of oil leakage. Therefore, the box covers should be checked for flatness at each valve inspection. Resurface any warped covers by lapping them on emery cloth laid on a surface plate. Rocker box cover warpage is often caused by improper tightening of the rocker box cover nuts. Eliminate further warpage by torqueing the nuts to the values specified in the manufacturer’s service manual.

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Valve clearance The amount of power that can be produced by a cylinder depends primarily on the amount of heat that can be produced in that cylinder without destructive effects on the cylinder components. Any condition that limits the amount of heat in the cylinder also limits the amount of power which that cylinder can produce. In determining valve timing and establishing the maximum power setting at which the engine is permitted to run, the manufacturer considers the amount of heat at which cylinder components such as spark plugs and valves can operate efficiently. The heat level of the exhaust valve must be below that at which pitting and warping of the valve occur. The head of the exhaust valve is exposed to the heat of combustion at all times during the combustion period. Also, the head of this valve and a portion of the stem are exposed to hot exhaust gases during the exhaust event. Under regular operation, the exhaust valve remains below the critical heat level because of its contact with the valve seat when closed and because of the heat dissipated through the stem. Any condition which prevents the valve from seating correctly for the required proportion of time will cause the valve to exceed the critical heat limits during periods of high power output. In cases of inferior valve seat contact, the exhaust valve can warp during periods of low power output. Typically, the exhaust valve is closed and in contact with its seat about 65% of the time during the four-stroke cycle. If the valve adjustment is correct, and if the valve seats firmly when closed, much of the heat is transferred from the valve, through the seat, into the cylinder head. Total Training Support Ltd © Copyright 2020

For a valve to seat, the valve must be in good condition, with no significant pressure being exerted against the end of the valve by the rocker arm. If the expansion of all parts of the engine, including the valve train were the same, the problem of ensuring valve seating would be quite easy to solve. Practically no free space would be necessary in the valve system. However, since there is a great difference in the amount of expansion of various parts of the engine, there is no way of providing a constant operating clearance in the valve train. The clearance in the valve-actuating system is exceedingly small when the engine is cold but is much greater when the engine is operating at normal temperature. The difference is caused by differences in the expansion characteristics of the various metals and by the differences in temperature of various engine parts. There are many reasons why proper valve clearances are of vital importance to satisfactory and stable engine operation. For example, when the engine is operating, valve clearances establish valve timing. Since all cylinders receive their fuel/air mixture (or air) from a common supply, valve clearance affects both the amount and the richness or leanness of the fuel/air mixture. Therefore, valve clearances must be correct and uniform between each cylinder. On radial engines, valve clearance decreases with a drop in temperature; therefore, insufficient clearance may cause the valve to hold open when extremely cold temperatures are encountered. This may make cold-weather starting of the engine difficult, if not impossible, because of the inability of the cylinders to pull a combustible charge into the combustion chamber.

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Accurate valve adjustment establishes the intended valve seating velocity. If valve clearances are excessive, the valve seating velocity is too high. The result is valve pounding and stem stretching, either of which is conducive to valve failure. Insufficient clearance may make starting difficult and cause valves to stick in the “open” position, causing blow-by and subsequent valve failure as a result of the extreme temperatures to which the valve is subjected. The engine manufacturer specifies the valve inspection period for each engine. In addition to the regular periods, inspect and adjust the valve mechanism any time there is rough engine operation, backfiring, loss of compression, or hard starting. Because of variations in engine designs, various methods are required for setting valves to obtain correct and consistent clearances. In all cases, follow the exact procedure prescribed by the engine manufacturer, since unknown factors may be involved.

Wright engines incorporate pressure-lubricated valves. Oil under pressure passes through the pushrod and into the centre of the valve-clearance adjusting screw. From this point, oil passages radiate in three directions. To permit proper lubrication, one of the three passages in the adjusting screw must be at least partially open to the passage leading to the rocker arm bearing. At the same time, neither of the other two passages must be uncovered by being in the slot in the rocker arm. Determine the location of the oil passages in the adjusting screw by locating the 0 stamped in three places on its top. If there are only two stamped circles, the third oil passage is midway between the two marked ones. After final tailoring of the valve adjustment, if any one of the three oil passages aligns with or is closer than 2.38 mm (3∕32") to the nearest edge of the slot in the rocker arm, turn the adjusting screw in a direction to increase or decrease the clearance until the reference 0 mark is 2.38 mm (3∕32") from the nearest edge of the slot in the rocker arm, or until the maximum or minimum valve clearance is reached.

For example, there is considerable cam float on many radial engines, and the valve-adjusting procedure for these engines is developed to permit accurate and consistent positioning of’ the cam. Since the ratio of valve movement to pushrod movement may be as much as two to one, each 0.0254 mm (0.001") shift of the cam can result in a 0.102 mm (0.002") variation in valve clearances.

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Pratt and Whitney's engines also incorporate pressurelubricated valves. On these engines, there is no slot in the rocker arm, but the valve-clearance adjusting screw can be turned in or backed out so far that the oil passage from the screw into the rocker arm is blocked. The specific instructions for setting clearance on the Pratt and Whitney engines state that a certain number of threads must show above the rocker arm. For example, on one model engine, at least two threads and not more than five must show, and will, providing the pushrod is the correct length. Correct the pushrod length by pulling off one of the ball ends and changing the washer beneath for a thicker or thinner one. If there is no washer and the rod is too long, correct it by grinding away the end of the rod. Check the engine manufacturer’s service or overhaul manual for the maximum or minimum number of threads which may show on the engine in question.

In making the check with the feeler gauge, do not use excessive force to insert the gauge between the valve stem and the adjustment screw or rocker arm roller. The gauge can be inserted by a heavy force, even though the clearance may be several thousandths of an inch less than the thickness of the gauge. This precaution is particularly important on engines where the cam is centred during valve clearance adjustment, since forcing the gauge on these engines may cause the cam to shift, with subsequent false readings.

When adjusting valve clearances, always use the valve clearance gauge or the dial gauge specified in the “tools” section of the engine manufacturer’s service manual. The specified gauge is of the proper thickness and is so shaped that the end being used for checking can be slipped in a straight line between the valve and the rocker arm roller of the rocker arm. When a standard gauge is used without being bent to the proper angle, a false clearance will be established since the gauge will be co*cked between the valve stem and rocker arm or rocker arm roller.

With a dial gauge, the clearance is the amount of travel obtained when the rocker arm is rotated from the valve stem until the other end of the rocker arm contacts the pushrod.

When a dial gauge and brackets for mounting the gauge on the rocker box are specified, be sure to use them. A dial gauge with a bracket may be used for checking valve clearances on any engine, provided the rocker arm arrangement is such that the pickup arm of the gauge is located over the centre line of the valve stem.

Since valve clearance adjustment procedures vary between engines, a single treatment will not be sufficient. Thus, the procedures for various engines or groups of engines are treated separately in the following paragraphs. However, the procedures are described only to provide an understanding of the operations involved. Consult the engine manufacturer’s instructions for the clearance to be set, the torque to be applied to lock-screws and rocker box cover nuts, and other pertinent details.

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The first step in checking and adjusting valve clearances is to set the piston in the No. 1 cylinder at top-centre compression stroke. Turn the propeller by hand until the valve action or cylinder pressure against a thumb held over the spark plug hole indicates that the piston is coming up on the compression stroke. Insert a piece of aluminium tubing into the spark plug hole and turn the propeller in the direction of rotation until the piston reaches its highest position. Proper precaution must be taken to make sure that the piston is on the compression stroke. After positioning the piston and crankshaft, adjust intake and exhaust valve clearances on the No. 1 cylinder to the prescribed values. Then adjust each succeeding cylinder in firing order, properly positioning the crankshaft for each cylinder.

Valve spring replacement A broken valve spring seldom affects engine operation and can, therefore, be detected only during a careful inspection. Because multiple springs are used, one broken spring is hard to detect. But when a broken valve spring is discovered, it can be replaced without removing the cylinder. During valve spring replacement, the critical precaution to remember is not to damage the spark plug hole threads. The complete procedure for valve spring replacement is as follows: • • • •

Recheck the valve clearances and readjust any that are outside the limits. On this second check, align the oil passages in the adjusting screws of engines incorporating pressure-lubricated valves.

• •

Remove one spark plug from the cylinder. Turn the propeller in the direction of rotation until the piston is at the top of the compression stroke. Remove rocker arm. Using a valve spring compressor, compress the spring and remove the valve keepers. During this operation, it may be necessary to insert a piece of brass rod through the spark plug hole to decrease the space between the valve and the top of the piston head to break the spring retaining washer loose from the keepers. The piston, being at the top position on the compression stroke, prevents the valve from dropping down into the cylinder once the spring retaining washers are broken loose from the keepers on the stem. Remove the defective spring and any broken pieces from the rocker box. Install a new spring and correct washers. Then, using the valve spring compressor, compress the spring and, if necessary, move the valve up from the piston using a brass rod inserted through the spark plug hole. Reinstall the keepers and rocker arms. Then check and adjust the valve clearance. Reinstall the rocker box cover and the spark plug.

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Cold cylinder check The cold cylinder check determines the operating characteristics of each cylinder of an air-cooled engine. The tendency for any cylinder or cylinders to be cold or to be only slightly warm indicates lack of combustion or incomplete combustion within the cylinder. This must be corrected if the best operation and power conditions are to be obtained. The cold cylinder check is made with a cold cylinder indicator (magic wand). Engine difficulties which can be analysed by use of the cold cylinder indicator are: • • •

•

rough engine operation; excessive RPM drop during the ignition system check; high manifold pressure for a given engine rpm during the ground check when the propeller is in the full low-pitch position; and faulty mixture ratios caused by improper valve clearance.

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In preparation for the cold cylinder check, head the aircraft into the wind to minimise uneven cooling of the individual cylinders and to ensure even propeller loading during engine operation. Open the cowl flaps. Do not close the cowl flaps under any circ*mstances, as the resulting excessive heat radiation affects the readings obtained and can damage the ignition leads. Start the engine with the ignition switch in the “BOTH” position. After the engine is running, place the ignition switch in the position in which an excessive RPM drop is obtained. When excessive RPM drop is encountered on both right and left switch positions, or when excessive manifold pressure is obtained at a given engine RPM, perform the check twice, once on the left and once on the right switch position. Operate the engine at its roughest speed between 1,200 and 1,600 RPM until a cylinder head temperature reading of 150° to 170°C (302° to 338°F) is obtained, or until temperatures stabilise at a lower reading. If engine roughness is encountered at more than one speed, or if there is an indication that a cylinder ceases operating at idle or higher speeds, run the engine at each of these speeds and perform a cold cylinder check to pick out all the dead or intermittently operating cylinders. When low power output or engine vibration is encountered at speeds above 1,600 RPM when operating with the ignition switch on “BOTH,” run the engine at the speed where the difficulty is encountered until the cylinder head temperatures are up to 150° to 170°C or until the temperatures have stabilised at a lower value. When cylinder head temperatures have reached the values prescribed in the previous paragraph, stop the engine by moving the mixture control to the “IDLE CUT-OFF” or “FULL LEAN” position. When the engine ceases firing, turn off both Total Training Support Ltd © Copyright 2020

ignition and master switches. Record the cylinder head temperature reading registered on the co*ckpit gauge. As soon as the propeller has ceased rotating, move a maintenance stand to the front of the engine. Connect the clip attached to the cold cylinder indicator lead to the engine or propeller to provide a ground for the instrument. Press the tip of the indicator pickup rod against each cylinder and record the relative temperature of each cylinder. Start with number one and proceed in numerical order around the engine, as rapidly as possible. A firm contact must be made at the same relative location on each cylinder to obtain comparative temperature values. Recheck any outstandingly low values. Also, recheck the two cylinders having the highest readings to determine the amount of cylinder cooling during the test. Compare the temperature readings to determine which cylinders are dead or are operating intermittently. Difficulties which may cause a cylinder to be inoperative (dead) on both right and left magneto positions are: • • • • • • •

defective spark plugs; incorrect valve clearances; leaking impeller oil seal; leaking intake pipes; lack of compression; plugged push rod housing drains; or faulty operation of the fuel injection nozzle (on fuelinjection engines).

Before changing spark plugs or making an ignition harness test on cylinders that are not operating or are operating intermittently, check the magneto ground leads to determine that the wiring is connected correctly.

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Repeat the cold cylinder test for the other magneto positions on the ignition switch, if necessary. Cooling the engine between tests is unnecessary. The airflow created by the propeller and the cooling effect of the incoming fuel/air mixture is sufficient to cool any cylinders that are functioning on one test and not functioning on the next. In interpreting the results of a cold cylinder check, remember that the temperatures are relative. A cylinder temperature taken alone means little, but when compared with the temperatures of other cylinders on the same engine, it provides valuable diagnostic information. On this check, the cylinder-head temperature gauge reading at the time the engine was shut down was 160°C on both tests. A review of these temperature readings reveals that, on the right magneto, cylinder number 6 runs cool and cylinders 8 and 9 runs cold. This indicates that cylinder 6 is firing intermittently and cylinders and 9 are dead during engine operation on the front plugs (fired by the right magneto). Cylinders 9 and 10 are dead during operation on the rear plugs (fired by the left magneto). Cylinder 9 is completely dead. An ignition system operational check would not disclose this dead cylinder since the cylinder is inoperative on both right and left switch positions, A dead cylinder can be detected during run-up since an engine with a dead cylinder requires a higher-than-normal manifold pressure to produce any given RPM below the cut-in speed of the propeller governor. A dead cylinder could also be detected by comparing power input and power output with the aid of a torquemeter. Total Training Support Ltd © Copyright 2020

Defects within the ignition system that can cause a cylinder to go completely dead are: • • •

both spark plugs inoperative; both ignition leads grounded, leaking, or open; a combination of inoperative spark plugs and defective ignition leads.

Faulty fuel injection nozzles, incorrect valve clearances, and other defects outside the ignition system can also cause dead cylinders. Temperature readings Cylinder number Right magneto Left magneto 1 180 170 2 170 175 3 100 170 4 145 60 5 70 155 6 60 45 Readings are taken during a cold-cylinder check In interpreting the readings obtained on a cold-cylinder check, the amount the engine cools during the check must be considered. To determine the extent to which this factor should be considered in evaluating the readings, recheck some of the first cylinders tested and compare the final readings with those made at the start of the check. Another factor to be considered is the normal variation in temperature between cylinders and between rows. This variation results from those design features which affect the airflow past the cylinders.

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Block testing General The information in this chapter on block testing of engines is intended to familiarise you with the procedures and equipment used in selecting for service only those engines that are in top mechanical condition. Like a new or recently overhauled automobile engine, the aircraft engine must be in top mechanical condition. This condition must be determined after the engine has been newly assembled or completely overhauled. The method used is the block test, or run-in, which takes place at overhaul before delivery of the engine. It must be emphasised that engine runin is as vital as any other phase of engine overhaul, for it is how the quality of a new or newly overhauled engine is checked. It is the final step in the preparation of an engine for service. In many instances, an engine has appeared to be in perfect mechanical condition before the engine run-in tests. However, the tests have shown that it was actually in poor and unreliable mechanical condition. Thus, the reliability and potential service life of an engine is in question until it has satisfactorily passed the block test.

Purpose The block test serves a dual purpose: first, it accomplishes piston ring run-in and bearing burnishing; and second, it provides valuable information that is used to evaluate engine performance and determine engine condition. Piston rings must be seated correctly in the cylinder in which they are installed to provide proper oil flow to the upper portion of the cylinder barrel walls with a minimum loss of oil. The process is called piston ring run-in and is accomplished chiefly by controlled operation of the engine in the high-speed range. Improper piston ring conditioning or run-in may result in unsatisfactory engine operation. A process called “bearing burnishing” creates a highly polished surface on new bearings and bushings installed during overhaul. The burnishing is usually accomplished during the first periods of the engine run-in at comparatively slow engine speeds.

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Requirements The operational tests and test procedures vary with individual engines, but the basic requirements are discussed in the following paragraphs. The failure of any internal part during engine run-in requires that the engine be returned for a replacement of the necessary units, and then be retested entirely. If any part of the basic engine should fail, a new unit is installed; a minimum operating time is used to check the engine with the new unit installed. After an engine has completed block-test requirements, it is then specially treated to prevent corrosion. During the final runin period at block test, the engines are operated on the proper grade of fuel prescribed for the particular kind of engine. The oil system is serviced with a mixture of corrosion-preventive compound and engine oil. The temperature of this mixture is maintained at 105° to 121°C. Near the end of final run-in CPM (corrosion-preventive mixture) is used as the engine lubricant; the engine induction passages and combustion chambers are also treated with CPM by an aspiration method (CPM is drawn or breathed into the engine).

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Mobile stand testing of reciprocating engines The mobile stand testing of reciprocating engines is much the same as for the block testing of reciprocating engines. They both have the same purpose; i.e., to ensure that the engine is fit to be installed on an aircraft. Once the engine has been operated on the mobile test stand and any faults or troubles corrected, it is presumed that the engine operates correctly on the aircraft. A typical mobile test stand consists of a frame, engine mount, control booth, and trailer welded or bolted together. The engine test stand mount and firewall are located toward the rear of the trailer deck and afford access to the rear of the engine. The engine test stand mount is a steel structure of uprights, braces, and cross members welded and bolted together forming one unit. The rear stand brace has non-skid steel steps welded in place to permit the mechanic to climb easily to the top of the engine accessory section. The front side of the engine mount has steel panels incorporating cannon plugs for the electrical connections to the engine. Also, there are fittings on the steel panels for the quick connection of the fluid lines to the engine. The hydraulic tank is located on the rear side of the engine test stand mount. Finally, the mobile engine test stand has outlet plugs for the communication system. The control booth is located in the middle of the trailer and houses the engine controls and instrument panels. The most important thing about positioning a mobile test stand is to face the propeller directly into the wind. If this is not done, engine testing will not be accurate.

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Mobile engine test stand

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Block test instruments The block-test operator’s control room houses the controls used to operate the engine and the instruments used to measure various temperatures and pressures, fuel flow, and other factors. These devices are necessary for providing an accurate check and an evaluation of the operating engine. The control room is separate from, but adjacent to, space (test cell) that houses the engine being tested. The safe, economical, and reliable testing of modern aircraft engines depends mostly upon the use of instruments. In engine run-in procedures, the same basic engine instruments are used as when the engine is installed in the aircraft, plus some additional connections to these instruments and some indicating and measuring devices that cannot be practically installed in the aircraft. Instruments used in the testing procedures are inspected and calibrated periodically, as are instruments installed in the aircraft; thus, accurate information concerning engine operation is ensured. Instrument markings indicate ranges of operation or minimum and maximum limits, or both. Generally, the instrument marking system consists of four colours (red, yellow, blue, and green) and intermediate blank spaces. A red line or mark indicates a point beyond which a dangerous operating condition exists, and a red arc indicates a dangerous operating range. Of the two, the red mark is used more commonly and is located radially on the cover glass or dial face.

The blue arc, like the yellow, indicates a range of operation. The blue arc might indicate, for example, the manifold pressure gauge range in which the engine can be operated with the carburettor control set at automatic lean. The blue arc is used only with certain engine instruments, such as the tachometer, manifold pressure, and cylinder head temperature. The green arc shows a standard range of operation. When used on certain engine instruments, however, it also means that the engine must be operated with an automatic rich carburettor setting when the pointer is in this range. When the markings appear on the cover glass, a white line is used as an index mark, often called a slippage mark. The white radial mark indicates any movement between the cover glass and the case, a condition that would cause mislocation of the other range and limit markings. The expanded portion is set off from the instrument to make it easier to identify the instrument markings. Oil temperature indicator During engine run-in at block test, engine oil temperature readings are taken at the oil inlet and outlet. From these readings, it can be determined if the engine heat transferred to the oil is low, normal, or excessive. This information is of extreme importance during the breaking-in process of large reciprocating engines. The oil temperature gauge line in the aircraft is connected at the oil inlet to the engine.

The yellow arc covers a given range of operation and is an indication of caution. Generally, the yellow arc is located on the outer circumference of the instrument cover glass or dial face.

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Three range markings are used on the oil temperature gauge. The red mark on the dial shows the minimum oil temperature permissible for operational ground checks or during flight. The green mark shows the desired oil temperature for continuous engine operation. The red mark indicates the maximum permissible oil temperature. Oil pressure indicator The oil pressure on block-test engines is checked at various points. The main oil pressure reading is taken at the pressure side of the oil pump. Other pressure readings are taken from the nose section and blower section; and when internal supercharging is used, a reading is taken from the high- and low-blower clutch.

The temperature recorded at either of these points is merely a reference or control temperature. However, as long as it is kept within the prescribed limits, the temperatures of the cylinder dome, exhaust valve, and piston are within an acceptable range. Since the thermocouple is attached to only one cylinder, it can do no more than giving evidence of general engine temperature. While it can usually be assumed that the remaining cylinder temperatures are lower, conditions such as detonation are not indicated unless they occur in the cylinder that has the thermocouple attached.

Generally, there is only one oil pressure gauge for each aircraft engine, and the connection is made at the pressure side (outlet) of the main oil pump. Cylinder head temperature indicator During the engine block-test procedures, a pyrometer indicates the cylinder head temperatures of various cylinders on the engine being tested. Thermocouples are connected to several cylinders, and by a selector switch, any cylinder head temperature can be indicated on the pyrometer. There are one thermocouple lead and indicator scale for each engine installed in an aircraft. Cylinder head temperatures are indicated by a gauge connected to a thermocouple attached to the cylinder which tests show to be the hottest on an engine in a particular installation. The thermocouple may be placed in a special gasket located under a rear spark plug or in a special well in the top or rear of the cylinder head. Total Training Support Ltd © Copyright 2020

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Engine operation General The operation of the powerplant is controlled from the co*ckpit. Some installations have numerous control handles and levers connected to the engine by rods, cables, bellcranks, pulleys, etc. The control handles, in most cases, are conveniently mounted on quadrants in the co*ckpit. Placards or markings are placed on the quadrant to indicate the functions and positions of the levers. In some installations, friction clutches are installed to hold the controls in place. Manifold pressure, RPM, engine temperature, oil temperature, carburettor air temperature, and the fuel/air ratio can be controlled by manipulating the co*ckpit controls. Coordinating the movement of the controls with the instrument readings protects against exceeding operating limits. Engine operation is usually limited by specified operating ranges of the following: • • • • • • • •

crankshaft speed (RPM); manifold pressure; cylinder head temperature; carburettor air temperature; oil temperature; oil pressure; fuel pressure; and fuel/air mixture setting.

The procedures, pressures, temperatures, and RPMs used throughout this section are solely for illustration and do not have general application. The operating procedures and limits used on individual makes and models of aircraft engines vary considerably from the values shown here. Total Training Support Ltd © Copyright 2020

For exact information regarding a specific engine model, consult the applicable instructions. Engine instruments The term engine instruments usually includes all instruments required to measure and indicate the functioning of the powerplant. The engine instruments are generally installed on the instrument panel so that all of them can easily be observed at one time. Some of the simple, light aircraft may be equipped only with a tachometer, oil pressure gauge, and oil temperature gauges. The heavier, more complex aircraft have all or part of the following engine instruments: • • • • • • • • • • • • • •

oil pressure indicator and warning system; oil temperature indicator; fuel pressure indicator and warning system; carburettor air temperature indicator; cylinder head temperature indicator for air-cooled engines; manifold pressure indicator; tachometer; fuel quantity indicator; fuel flow meter or fuel mixture indicator; oil quantity indicator; augmentation liquid quantity indicator; fire warning indicators; a means to indicate when the propeller is in reverse pitch; and BMEP (brake mean effective pressure) indicator.

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Engine starting Correct starting technique is an integral part of engine operation. Improper procedures are often used because some of the basic principles involved in engine operation are misunderstood. In general, two different starting procedures cover all engines. One procedure is for engines using float-type carburettors, and the other for engines with pressure-injection carburettors. The specific manufacturer’s procedures for a particular engine and aircraft combination should always be followed. Engine warm-up Proper engine warm-up is essential, particularly when the condition of the engine is unknown. Improperly adjusted idle mixture, intermittently firing spark plugs, and improperly adjusted engine valves all have an overlapping effect on engine stability. Therefore, the warm-up should be made at the engine speed where maximum engine stability is obtained. Experience has shown that the optimum warm-up speed is from 1,000 to 1,600 RPM. The actual speed selected should be the speed at which engine operation is the smoothest since the smoothest operation is an indication that all phases of engine operation are the most stable. Most Pratt and Whitney engines incorporate temperaturecompensated oil pressure relief valves. This type of relief valve results in high engine-oil pressures immediately after the engine starts, if oil temperatures are below 40°C. Consequently, start the warm-up of these engines at approximately 1,000 RPM and then move to the higher, more stable engine speed as soon as oil temperature reaches 40°C.

During warm-up, watch the instruments associated with engine operation – this aids in making sure that all phases of engine operation are normal. For example, engine oil pressure should be indicated within 30 seconds after the start. Furthermore, if the oil pressure is not up to or above normal within one minute after the engine starts, the engine should be shut down. Cylinder head or coolant temperatures should be observed continually to see that they do not exceed the maximum allowable limit. A lean mixture should not be used to hasten the warm-up. Actually, at the warm-up RPM, there is little difference in the mixture supplied to the engine, whether the mixture is in a “RICH” or “LEAN” position since metering in this power range is governed by throttle position. Carburettor heat can be used as required under conditions leading to ice formation. For engines equipped with a float-type carburettor, it is desirable to raise the carburettor air temperature during warm-up to prevent ice formation and to ensure smooth operation. The magneto safety check can be performed during warm-up. Its purpose is to ensure that all ignition connections are secure and that the ignition system permits operation at the higher power settings used during later phases of the ground check. The time required for proper warm-up gives ample opportunity to perform this simple check, which may disclose a condition that would make it inadvisable to continue operation until after corrections have been made.

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The magneto safety check is conducted with the propeller in the high RPM (low pitch) position, at approximately 1,000 RPM Move the ignition switch from “BOTH” to “RIGHT” and return to “BOTH”; from “BOTH” to “LEFT” and return to “BOTH”; from “BOTH” to “OFF” momentarily and return to “BOTH.”

The ground check is made after the engine is thoroughly warm. It consists of checking the operation of the powerplant and accessory equipment by ear, by visual inspection, and by the proper interpretation of instrument readings, control movements, and switch reactions.

While switching from “BOTH” to a single magneto position, a slight but noticeable drop in RPM should occur. This indicates that the opposite magneto has been adequately grounded out. Complete cutting out of the engine when switching from “BOTH” to “OFF” indicates that both magnetos are correctly grounded. Failure to obtain any drop while in the single magneto position, or failure of the engine to cut out while switching to “OFF” indicates that one or both ground connections are not secured.

During the ground check, the aircraft should be headed into the wind, if possible, to take advantage of the cooling airflow. A ground check may be performed as follows:

Ground check The ground check is performed to evaluate the functioning of the engine by comparing power input, as measured by manifold pressure, with power output, as measured by RPM or torque pressure.

Control position

Check:

Cowl flaps

Open.

Mixture

Rich.

Propeller

High RPM.

Carburettor heat

Cold.

Carburettor air filter

As required.

Supercharger control

Low, neutral, or off position (where applicable).

The engine may be capable of producing a prescribed power, even rated takeoff, and not be functioning correctly. Only by comparing the manifold pressure required during the check against a known standard will an unsuitable condition be disclosed. The magneto check can also fail to show up shortcomings since the allowable RPM drop-off is only a measure of an improperly functioning ignition system and is not necessarily affected by other factors. Conversely, the magneto check can prove satisfactory with an unsatisfactory condition present elsewhere in the engine.

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Procedure: • Check propeller according to propeller manufacturer’s instruction. • Open throttle to manifold pressure equal to field barometric pressure. • Switch from “BOTH” to “RIGHT” and return to “BOTH.” Switch from “BOTH” to “LEFT” and return to “BOTH.” Observe the RPM drop while operating on the right and left positions. The maximum drop should not exceed that specified by the engine manufacturer. • Check the fuel pressure and oil pressure. They must be within the established tolerance for the subject engine. • Note RPM. • Retard throttle. In addition to the operations outlined above, check the functioning of various items of aircraft equipment, such as generator systems, hydraulic systems, etc. Propeller pitch check The propeller is checked to ensure proper operation of the pitch control and the pitch-change mechanism. The operation of a controllable pitch propeller is checked by the indications of the tachometer and manifold pressure gauge when the propeller governor control is moved from one position to another. Because each type of propeller requires a different procedure, the applicable manufacturer’s instructions should be followed. Power check Specific RPM and manifold pressure relationship should be checked during each ground check. This can be clone at the time the engine is run-up to make the magneto check. The basic idea of this check is to measure the performance of the engine against an established standard. Calibration tests have Total Training Support Ltd © Copyright 2020

determined that the engine is capable of delivering a given power at a given RPM and manifold pressure. The original calibration, or measurement of power, is made using a dynamometer. During the ground check, power is measured with the propeller. With constant conditions of air density, the propeller, at any fixed-pitch position, always requires the same RPM to absorb the same horsepower from the engine. This characteristic is used in determining the condition of the engine. With the governor control set for full low pitch, the propeller operates as a fixed-pitch propeller. Under these conditions, the manifold pressure for any specific engine, with the mixture control in auto-rich, indicates whether all the cylinders are operating correctly. With one or more dead or intermittently firing cylinders, the operating cylinders must provide more power for a given RPM. Consequently, the carburettor throttle must be opened further, resulting in higher manifold pressure. Different engines of the same model using the same propeller installation and in the same geographical location should require the same manifold pressure, within 1 inHg, to obtain RPM when the barometer and temperature are at the same readings. A higher-than-normal manifold pressure usually indicates a dead cylinder or late ignition timing. An excessively low manifold pressure for a particular RPM usually indicates that the ignition timing is early. Early-ignition can cause detonation and loss of power at takeoff power settings. Before starting the engine, observe the manifold pressure gauge, which should read approximately atmospheric (barometric) pressure when the engine is not running. At sea level, this is approximately 30 inHg, and at fields above sea

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level, the atmospheric pressure is less, depending on the height above sea level. When the engine is started and then accelerated, the manifold pressure decreases until about 1,600 or 1,700 RPM is reached, and then it begins to rise. At approximately 2,000 RPM., with the propeller in low-pitch position, the manifold pressure should be the same as the field barometric pressure. If the manifold pressure gauge reading (field barometric pressure) was 30 inHg before starting the engine, the pressure reading should return to 30 inHg at approximately 2,000 RPM. If the manifold pressure gauge reads 26 inHg before starting, it should reread 26 inHg at approximately 2,000 RPM. The exact RPM will vary with various models of engines or because of varying propeller characteristics. In certain installations, the RPM needed to secure field barometric pressure may be as high as 2,200 RPM. However, once the required RPM has been established for an installation, any appreciable variation indicates some malfunctioning. This variation may occur because the low-pitch stop of the propeller has not been correctly set or because the carburettor or ignition system is not functioning properly. The accuracy of the power check may be affected by the following variables:

Atmospheric temperatures. The effects of variations in atmospheric temperature tend to cancel each other. Higher carburettor intake and cylinder temperatures tend to lower the RPM, but the propeller load is lightened because of the less dense air. Engine and induction system temperature. If the cylinder and carburettor temperatures are high because of factors other than atmospheric temperature, a low RPM results since the power is lowered without a compensating lowering of the propeller load. Oil temperature. Cold oil tends to hold down the RPM since the higher viscosity results in increased friction horse-power losses. The addition of a torquemeter can increase the accuracy of the power check by providing another measurement of power output. As long as the check is performed with the blades in a known fixed-pitch position, the torquemeter provides no additional information, but its use can increase accuracy; in the frequent instances where the tachometer scales are graduated coarsely, the tachometer gauge reading may be a more convenient source of the desired information.

Wind. Any appreciable air movement (5 MPH or more) will change the air load on the propeller blade when it is in the fixedpitch position. A headwind increases the RPM obtainable with given manifold pressure. A tailwind decreases the RPM.

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Ignition system operational check In performing the ignition system operational check (magneto check), the power-absorbing characteristics of the propeller in the low fixed-pitch position are utilised. In switching to individual magnetos, cutting out the opposite plugs results in a slower rate of combustion, which gives the same effect as retarding the spark advance.

functioning. The excessive difference in RPM drop-off between the left and right switch positions can indicate a difference in timing between the left and right magnetos.

The drop in engine speed is a measure of the power loss at this slower combustion rate.

Sufficient time should be given the check on each single switch position to permit complete stabilisation of engine speed and manifold pressure. There is a tendency to performing this check too rapidly with resultant wrong indications. Single ignition operation for as long as 1 minute is not excessive.

When the magneto check is performed, a drop in torquemeter pressure indication is an excellent supplement to the variation in RPM and in cases where the tachometer scale is graduated coarsely, the torquemeter variation may give more positive evidence of the power change when switching to the individual magneto condition. A loss in torquemeter pressure not to exceed 10% can be expected when operating on a single magneto. By comparing the RPM drop with a known standard, the following are determined:

Another point that must be emphasised is the danger of a sticking tachometer. The tachometer should be tapped lightly to make sure the indicator needle moves freely. In some cases, tachometer slicking has caused errors in indication to the extent of 100 RPM Under such conditions the ignition system could have had as much as a 200 RPM drop with only a 100 RPM drop indicated on the instrument. In most cases, tapping the instrument eliminates the sticking and results in accurate readings.

• • •

proper timing of each magneto; general engine performance as evidenced by the smooth operation; and an additional check of the proper connection of the ignition leads.

Any unusual roughness on either magneto is an indication of faulty ignition caused by plug fouling or by malfunctioning of the ignition system. The operator should be sensitive to engine roughness during this check. Lack of drop-off in RPM may be an indication of faulty grounding of one side of the ignition system. Complete cutting out when switching to one magneto is definite evidence that its side of the ignition system is not Total Training Support Ltd © Copyright 2020

In recording the results of the ignition system check, record the amount of the total RPM drop, which occurs rapidly and the amount which occurs slowly. This breakdown in RPM drop provides a means of pinpointing certain troubles in the ignition system. It can save much time and unnecessary work by confining maintenance to the specific part of the ignition system, which is responsible for the trouble. Fast RPM drop is usually the result of either faulty spark plugs or faulty ignition harness. This is true because faulty plugs or leads take effect at once. The cylinder goes dead or starts firing intermittently the instant that the switch is moved from “BOTH” to the “RIGHT” or “LEFT” position.

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Slow RPM drop usually is caused by incorrect ignition timing or faulty valve adjustment. With late ignition timing, the charge is fired too late with relation to piston travel for the combustion pressures to build up to the maximum at the proper time. The result is a power loss greater than usual for single ignition because of the lower peak pressures obtained in the cylinder. However, this power loss does not occur as rapidly as that which accompanies a dead spark plug. This explains the slow RPM drop as compared to the instantaneous drop with a dead plug or defective lead. Incorrect valve clearances, through their effect on valve overlap, can cause the mixture to be too rich or too lean. The too-rich or too-lean mixture may affect one plug more than another, because of the plug location, and show up as a slow RPM drop on the ignition check. Cruise mixture check The cruise mixture check is a check of carburettor metering. Checking the carburettor metering characteristics at 200 to 300 RPM intervals, from 800 RPM to the ignition system check speed, gives a complete pattern for the basic carburettor performance. To perform this test, set up a specified engine speed with the propeller in full low pitch. The first check is made at 800 RPM. With the carburettor mixture control in the “AUTO-RICH” position, read the manifold pressure. With the throttle remaining in the same position, move the mixture control to the “AUTOLEAN” position. Read and record the engine speed and manifold pressure readings. Repeat this check at 1,000, 1,200, 1,500, 1,700, and 2,000 RPM or at the RPMs specified by the manufacturer. Guard against a sticking instrument by tapping the tachometer.

Moving the mixture control from the “AUTO-RICH” position to the “AUTO-LEAN” position checks the cruise mixture. In general, the speed should not increase more than 25 RPM or decrease more than 75 RPM during the change from “AUTORICH” to “AUTO-LEAN.” For example, suppose that the RPM change is above 100 for the 800 to 1,500 RPM checks; it is evident that the probable cause is an incorrect idle mixture. When the idle is appropriately adjusted, the duration is correct throughout the range. Idle speed and idle mixture checks Plug fouling difficulty is the inevitable result of failure to provide a proper idle mixture setting. The tendency seems to be to adjust the idle mixture on the extremely rich side and to compensate for this by adjusting the throttle stop to a relatively high RPM for minimum idling. With a properly adjusted idle mixture setting, it is possible to run the engine at idle RPM for long periods. Such a setting results in a minimum of plug fouling and exhaust smoking, and it pays dividends from the savings on the aircraft brakes after landing and while taxiing. If the wind is not too strong, the idle mixture setting can be checked easily during the ground check as follows: • •

Close throttle; and Move the mixture control to the “IDLE CUT-OFF” position and observe the change in RPM Return the mixture control back to the “RICH” position before engine cut-off.

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As the mixture control lever is moved into idle cut-off, and before normal drop-off, one of two things may occur momentarily. •

•

The engine speed may increase. An increase in RPM, but less than that recommended by the manufacturer (usually 20 RPM), indicates proper mixture strength. A greater increase indicates that the mixture is too rich. The engine speed may not increase or may drop immediately. This indicates that the idle mixture is too lean.

The idle mixture should be set to give a mixture slightly richer than best power, resulting in a 10 to 20 RPM rise after the idle cut-off. The idle mixture of engines equipped with electric primers can be checked by flicking the primer switch momentarily and noting any change in manifold pressure and RPM. A decrease in RPM and an increase in manifold pressure will occur when the primer is energised if the idle mixture is too rich. If the idle mixture is adjusted too lean, the RPM will increase, and manifold pressure will decrease. Two-speed supercharger check To check the operation of the two-speed blower mechanism, set the engine speed to a sufficiently high RPM to obtain the minimum oil pressure required for clutch operation. Move the supercharger control to the “HIGH” position. A momentary drop in oil pressure should accompany the shift. Open the throttle to obtain not more than 30 inHg manifold pressure.

When the engine speed has stabilised, observe the manifold pressure, and shift the supercharger control to the “LOW” position without moving the throttle. A sudden decrease in manifold pressure indicates that the two-speed supercharger drive is functioning correctly. If no decrease occurs, the clutch may be inoperative. As soon as the change in manifold pressure has been checked, reduce the engine speed to 1,000 RPM, or less. If the shift of the supercharger did not appear to be satisfactory, operate the engine at 1,000 RPM for two or three minutes to permit heat generated during the shift to dissipate from the clutches, and then repeat the shifting procedure. Blower shifts should be made without hesitation or dwelling between the control positions to avoid dragging or slipping the clutches. Make sure the supercharger control is in the “LOW” position when the ground check is completed. Acceleration and deceleration checks The acceleration check is made with the mixture control in both auto-rich and auto-lean. Move the throttle from idle to take off smoothly and rapidly. The engine RPM should increase without hesitation and with no evidence of engine backfire. This check will, in many cases, show up borderline conditions that will not be revealed by any of the other checks. This is true because the high cylinder pressures developed during this check put added strain on both the ignition system and the fuel metering system.

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This added strain is sufficient to point out certain defects that otherwise go unnoticed. Engines must be capable of rapid acceleration, since in an emergency, such as a go-around during landing, the ability of an engine to accelerate rapidly is sometimes the difference between a successful go-around and a crash landing. The deceleration check is made while retarding the throttle from the acceleration check. Note the engine behaviour. RPM should decrease smoothly and evenly. There should be little or no tendency for the engine to after-fire. Engine stopping With each type of carburettor installation, specific procedures are used in stopping the engine. The general procedure outlined in the following paragraphs reduces the time required for stopping, minimises backfiring tendencies, and, most important, prevents overheating of tightly baffled air-cooled engines during operation on the ground. In stopping any aircraft engine, the controls are set as follows, irrespective of carburettor type or fuel system installation. Cowl flaps are always placed in the “FULL OPEN” position to avoid overheating the engine and are left in that position after the engine is stopped to prevent residual engine heat from deteriorating the ignition system. Oil cooler shutters should be “FULL OPEN” to allow the oil temperature to return to normal. Intercooler shutters are kept in the “FULL OPEN” position.

Turbocharger waste gates are set in the “FULL OPEN” position. Two-speed supercharger control is placed in the “LOW BLOWER” position. A two-position propeller will usually be stopped with the control set in the “HIGH PITCH” (decrease RPM.) position. Open the throttle to approximately 1,200 RPM and shift the propeller control to “HIGH PITCH” position. Allow the engine to operate approximately one minute before stopping, so that the oil dumped into the engine from the propeller may be scavenged and returned to the oil tank. However, to inspect the propeller piston for galling and wear and for other particular purposes, this propeller may be stopped with the propeller control in “LOW PITCH” (increase RPM) position when the engine is stopped. No mention is made of the throttle, mixture control, fuel selector valve, and ignition switches in the initial set of directions because the operation of these controls varies with the type of carburettor used with the engine. Engines equipped with a float-type carburettor without an idle cut-off unit are stopped as follows. •

• •

Adjust the throttle to obtain an idling speed of approximately 600 to 800 RPM, depending on the type of engine. Close the fuel selector valve. Open the throttle slowly until the engine is operating at approximately 800 to 1,000 RPM.

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•

Observe the fuel pressure. When it drops to zero, turn the ignition switch to the “OFF” position and simultaneously move the throttle slowly to the “FULL OPEN” position. This operation removes the accelerating charge from the induction system and avoids the possibility of accidental starting. When the engine has stopped, place the fuel selector valve in the “ON” position and refill the carburettor and fuel lines by using the auxiliary pump.

An engine equipped with a carburettor incorporating an idle cutoff is stopped as follows. • •

•

Idle the engine by setting the throttle for 800 to 1,000 RPM. Move the mixture control to the “IDLE CUT-OFF” position. In a pressure-type carburettor; this causes the cloverleaf valve to stop the discharge of fuel through the discharge nozzle. In a float-type carburettor, it equalises the pressure in the float chamber and at the discharge nozzle. After the propeller has stopped rotating, place the ignition switch in the “OFF” position.

Basic engine operating principles A thorough understanding of the basic principles on which a reciprocating engine operates and the many factors which affect its operation is necessary to diagnose engine malfunctions. Some of these basic principles are reviewed not as a mere repetition of basic theory, but as a concrete, practical discussion of what makes for good or bad engine performance.

The conventional reciprocating aircraft engine operates on the four-stroke-cycle principle. Pressure from burning gases acts upon a piston, causing it to reciprocate back and forth in an enclosed cylinder. This reciprocating motion of the piston is changed into rotary motion by a crankshaft, to which the piston is coupled using a connecting rod. The crankshaft, in turn, is attached or geared to the aircraft propeller. Therefore, the rotary motion of the crankshaft causes the propeller to revolve. Thus, the motion of the propeller is a direct result of the forces acting upon the piston as it moves back and forth in the cylinder. Four strokes of the piston, two up and two down, are required to provide one power impulse to the crankshaft. Each of these strokes is considered an event in the cycle of engine operation. Ignition of the gases (fuel/air mixture) at the end of the second, or compression, stroke makes the fifth event. Thus, the five events which make up a cycle of operation occur in four strokes of the piston. As the piston moves downward on the first stroke (intake), the intake valve is open, and the exhaust valve is closed. As air is drawn through the carburettor gasoline is introduced into the stream of air forming a combustible mixture. On the second stroke, the intake closes, and the combustible mixture is compressed as the piston moves upward. This is the compression stroke. At the correct instant, an electric spark jumps across the terminals of the spark plug and ignites the fuel/air mixture. The ignition of the fuel/air mixture is timed to occur just slightly before the piston reaches the top dead centre.

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As the mixture burns, temperature and pressure rise rapidly. The pressure reaches a maximum just after the piston has passed top centre. The expanding and burning gas forces the piston downward, transmitting energy to the crankshaft. This is the power stroke. Both intake and exhaust valves are closed at the start of the power stroke. Near the end of the power stroke, the exhaust valve opens, and the burned gases start to escape through the exhaust port. On its return stroke, the piston forces out the remaining gases. This stroke, the exhaust stroke, ends the cycle. With the introduction of a new charge through the intake port, the action is repeated, and the cycle of events occurs over again as long as the engine is in operation. Ignition of the fuel charge must occur at a specific time in relation to crankshaft travel. The igniting device is timed to ignite the charge just before the piston reaches top centre on the compression stroke. Igniting the charge at this point permits maximum pressure to build up at a point slightly after the piston passes over top dead centre. For ideal combustion, the point of ignition should vary with engine speed and with the degree of compression, mixture strength, and other factors governing the rate of burning. However, certain factors, such as the limited range of operating RPM and the dangers of operating with incorrect spark settings, prohibit the use of variable spark control in most instances. Therefore, most aircraft ignition system units are timed to ignite the fuel/air charge at one fixed position (advanced). On early models of the four-stroke-cycle engine, the intake valve opened at top centre (beginning of the intake stroke), it closed at bottom centre (end of intake stroke). The exhaust valve opened at bottom centre (end of power stroke) and Total Training Support Ltd © Copyright 2020

closed at the top centre (end of exhaust stroke). More efficient engine operation can be obtained by opening the intake valve several degrees before top centre and closing it several degrees after bottom centre. Opening the exhaust valve before the bottom centre, and closing it after top centre, also improves engine performance. Since the intake valve opens before topcentre exhaust stroke and the exhaust valve closes after topcentre intake stroke, there is a period where both the intake and exhaust valves are open at the same time. This is known as valve lap or valve overlap. In valve timing, the reference to piston or crankshaft position is always made in terms of before or after the top and bottom centre points, e.g., ATDC, BTDC, ABDC, and BBDC. Opening the intake valve before the piston reaches top centre starts the intake event while the piston is still moving up on the exhaust stroke. This aids in increasing the volume of charge admitted into the cylinder. The selected point at which the intake valve opens depends on the RPM at which the engine normally operates. At low RPM, this early timing results in poor efficiency since the incoming charge is not drawn into and the exhaust gases are not expelled out of the cylinder with sufficient speed to develop the necessary momentum. Also, at low RPM the cylinder is not well scavenged, and residual gases mix with the incoming fuel and are trapped during the compression stroke. Some of the incoming mixture is also lost through the open exhaust port. However, the advantages obtained at normal operating RPM more than make up for the poor efficiency at low RPM. Another advantage of this valve timing is the increased fuel vaporisation and beneficial cooling of the piston and cylinder.

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Delaying the closing of the intake valve takes advantage of the inertia of the rapidly moving fuel/air mixture entering the cylinder. This ramming effect increases the charge over that which would be taken in if the intake valve closed at bottom centre (end of intake stroke). The intake valve is open during the latter part of the exhaust stroke, all of the intake stroke, and the first part of the compression stroke. Fuel/air mixture is taken in during all this time. The early opening and late closing of the exhaust valve goes along with the intake valve timing to improve engine efficiency. The exhaust valve opens on the power stroke, several crankshaft degrees before the piston reaches the bottom centre. This early opening aids in obtaining better scavenging of the burned gases. It also results in improved cooling of the cylinders, because of the early escape of the hot gases. Actually, on aircraft engines, the major portion of the exhaust gases, and the unused heat, escape before the piston reaches the bottom centre. The burned gases continue to escape as the piston passes bottom centre, moves upward on the exhaust stroke, and starts the next intake stroke. The late closing of the exhaust valve still further improves scavenging by taking advantage of the inertia of the rapidly moving outgoing gases. The exhaust valve is open during the latter part of the power stroke, all of the exhaust stroke, and the first part of the intake stroke. From this description of valve timing, it can be seen that the intake and exhaust valves are open at the same time on the latter part of the exhaust stroke and the first part of the intake stroke. During this valve overlap period, the last of the burned gases are escaping through the exhaust port while the new charge is entering through the intake port. Total Training Support Ltd © Copyright 2020

Many aircraft engines are supercharged. Supercharging increases the pressure of the air or fuel/ air mixture before it enters the cylinder. In other words, the air or fuel/air mixture is forced into the cylinder rather than being drawn in. Supercharging increases engine efficiency and makes it possible for an engine to maintain its efficiency at high altitudes. This is true because the higher pressure packs more charge into the cylinder during the intake event. This increase in weight of charge results in a corresponding increase in power. Also, the higher pressure of the incoming gases more forcibly ejects the burned gases out through the exhaust port. This results in better scavenging of the cylinder. The flame fronts start at each spark plug and burn in more or less wavelike forms. The velocity of the flame travel is influenced by the type of fuel, the ratio of the fuel/air mixture, and the pressure and temperature of the fuel mixture. With normal combustion, flame travel is about 100 ft/second. The temperature and pressure within the cylinder rise at a normal rate as the fuel/air mixture burns. There is a limit, however, to the amount of compression and the degree of temperature rise that can be tolerated within an engine cylinder and still permit normal combustion. All fuels have critical limits of temperature and compression. Beyond this limit, they will ignite spontaneously and burn with explosive violence. This instantaneous and explosive burning of the fuel/air mixture or, more accurately, of the latter portion of the charge, is called detonation.

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As previously mentioned, during normal combustion, the flame fronts progress from the point of ignition across the cylinder. These flame fronts compress the gases ahead of them. At the same time, the gases are being compressed by the upward movement of the piston. If the total compression on the remaining unburned gases exceeds the critical point, detonation occurs. Detonation then is the spontaneous combustion of the unburned charge ahead of the flame fronts after ignition of the charge. The explosive burning during detonation results in an extremely rapid pressure rise. This rapid pressure rise and the high instantaneous temperature, combined with the high turbulence generated, cause a scrubbing action on the cylinder and the piston. This can burn a hole completely through the piston. The critical point of detonation varies with the ratio of fuel to air in the mixture. Therefore, the detonation characteristic of the mixture can be controlled by varying the fuel/air ratio. At high power output, combustion pressures and temperatures are higher than they are at low or medium power. Therefore, at high power, the fuel/air ratio is made richer than is needed for good combustion at medium or low power output. This is done because, in general, a rich mixture does not detonate as readily as a lean mixture.

The effects of detonation are often not discovered until after teardown of the engine. When the engine is overhauled, however, the presence of severe detonation during its operation is indicated by dished piston heads, collapsed valve heads, broken ring lands, or eroded portions of valves, pistons, or cylinder heads. The essential protection from detonation is provided in the design of the engine carburettor setting, which automatically supplies the rich mixtures required for detonation suppression at high power; the rating limitations, which include the maximum operating temperatures; and selection of the correct grade of fuel. The design factors, cylinder cooling, magneto timing, mixture distribution, supercharging, and carburettor setting are taken care of in the design and development of the engine and its method of installation in the aircraft.

Unless detonation is heavy, there is no co*ckpit evidence of its presence. Light to medium detonation does not cause noticeable roughness, temperature increase, or loss of power. As a result, it can be present during takeoff and high-power climb without being known to the crew.

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Normal combustion within a cylinder

Detonation within a cylinder

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The remaining responsibility for the prevention of detonation rests squarely in the hands of the ground and flight crews. They are responsible for observance of RPM and manifold pressure limits. Proper use of supercharger and fuel mixture, and maintenance of suitable cylinder head and carburettor -air temperatures are entirely in their control. Preignition, as the name implies, means that combustion takes place within the cylinder before the timed spark jumps across the spark plug terminals. This condition can often be traced to excessive carbon or other deposits which cause local hot spots. Detonation often leads to preignition. However, preignition may also be caused by high- power operation on excessively lean mixtures. Preignition is usually indicated in the co*ckpit by engine roughness, backfiring, and by a sudden increase in cylinder head temperature. Any area within the combustion chamber which becomes incandescent will serve as an igniter in advance of normal timed ignition and cause combustion earlier than desired. Preignition may be caused by an area roughened and heated by detonation erosion. A cracked valve or piston, or a broken spark plug insulator may furnish a hot point which serves as a glow plug.

The hot spot can be caused by deposits on the chamber surfaces resulting from the use of leaded fuels. Normal carbon deposits can also cause preignition. Mainly, preignition is a condition similar to the early timing of the spark. The charge in the cylinder is ignited before the required time for normal engine firing. However, do not confuse preignition with the spark which occurs too early in the cycle. Preignition is caused by a hot spot in the combustion chamber, not by incorrect ignition timing. The hot spot may be due to either an overheated cylinder or a defect within the cylinder. The most apparent method of correcting preignition is to reduce the cylinder temperature. The immediate step is to retard the throttle. This reduces the amount of fuel charge and the amount of heat generated. If a supercharger is in use and is in high ratio, it should be returned to a low ratio to lower the charge temperature. Following this, the mixture should be enriched, if possible, to lower combustion temperature. If the engine is at high power when preignition occurs, retarding the throttle for a few seconds may provide enough cooling to chip off some of the lead, or other deposit, within the combustion chamber. These chipped-off particles pass out through the exhaust. They are visible at night as a shower of sparks. If retarding the throttle does not permit a return to uninterrupted normal power operation, deposits may be removed by sudden cooling shock treatment. Such treatments are water injection, alcohol from the de-icing system, full-cold carburettor air, or any other method that provides sudden cooling to the cylinder chamber.

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Backfiring When a fuel/air mixture does not contain enough fuel to consume all the oxygen, it is called a lean mixture. Conversely, a charge that contains more fuel than required is called a rich mixture. An extremely lean mixture either will not burn at all or will burn so slowly that combustion is not complete at the end of the exhaust stroke. The flame lingers in the cylinder and then ignites the contents in the intake manifold or the induction system when the intake valve opens. This causes an explosion known as backfiring, which can damage the carburettor and other parts of the induction system. A point worth stressing is that backfiring rarely involves the whole engine. Therefore, it is seldom the fault of the carburettor. In practically all cases, backfiring is limited to one or two cylinders. Usually, it is the result of faulty valve clearance setting, defective fuel injector nozzles, or other conditions which cause these cylinders to operate leaner than the engine as a whole. There can be no permanent cure until these defects are discovered and corrected. Because these backfiring cylinders will fire intermittently and therefore run cool, they can be detected by the cold cylinder check. The cold cylinder check is discussed later in this chapter.

After-firing After-firing, sometimes called afterburning, often results when the fuel/air mixture is too rich. Overly rich mixtures are slowburning. Therefore, charges of unburned fuel are present in the exhausted gases. Air from outside the exhaust stacks mixes with the unburned fuel which ignites. This causes an explosion in the exhaust system. After-firing is perhaps more common where long exhaust ducting retains greater amounts of unburned charges. As in the case of backfiring, the correction for After-firing is the proper adjustment of the fuel/air mixture. After-firing can also be caused by cylinders which are not firing because of faulty spark plugs, defective fuel injection nozzles, or incorrect valve clearance. The unburned mixture from these dead cylinders passes into the exhaust system, where it ignites and burns. Unfortunately, the resultant torching or afterburning can easily be mistaken for evidence of rich a carburettor. Cylinders which are firing intermittently can cause a similar effect. Again, the malfunction can be remedied only by discovering the real cause and correcting the defect. Either dead or intermittent cylinders can be located by the cold cylinder check.

In some instances, an engine backfires in the idle range but operates satisfactorily at medium and high-power settings. The most likely cause, in this case, is an excessively lean idle mixture. Proper adjustment of the idle fuel/air mixture usually corrects this difficulty.

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Ground and flight testing Pre-oiling Before the new engine is flight-tested, it must undergo a thorough ground check. Before this ground check can be made, several operations are usually performed on the engine. To prevent failure of the engine bearings during the initial start, the engine should be pre-oiled. When an engine has been idle for an extended period, its internal bearing surfaces are likely to become dry at points where the corrosion- preventive mixture has dried out or drained away from the bearings. Hence, it is necessary to force oil throughout the entire engine oil system. If the bearings are dry when the engine is started, the friction at high RPM will destroy the bearings before lubricating oil from the engine-driven oil pump can reach them. There are several methods of pre-oiling an engine. The method selected should provide an expeditious and adequate pre-oiling service. Before using any pre-oiling method, remove one spark plug from each cylinder to allow the engine to be turned over more easily with the starter. Also, connect an external source of electrical power (auxiliary power unit) to the aircraft electrical system to prevent an excessive drain on the aircraft battery. If the engine is equipped with a hydromatic (oil-operated) propeller, remove the plug and fill the propeller dome with oil. Then reinstall the plug.

Then a line must be disconnected or an opening made in the oil system at the nose of the engine to allow the oil to flow out of the engine. Oil flowing out of the engine indicates the completion of the pre-oiling operation since the oil has now passed through the entire system. To force oil from the pre-oiler tank through the engine, apply air pressure to the oil in the tank while the engine is being turned through with the starter. When this action has forced oil through the disconnection at the nose of the engine, stop cranking the engine and disconnect the pre oiler tank. When no external means of pre-oiling an engine are available, the engine oil pump may be used. Fill the engine oil tank to the proper level. Then, with the mixture in the “IDLE CUT-OFF” position, the fuel shutoff valve and ignition switches in the “OFF” position, and the throttles fully open, crank the engine with the starter until the oil pressure gauge mounted on the instrument panel indicates oil pressure. After the engine has been pre-oiled, replace the spark plugs and connect the oil system. Generally, the engine should be operated within four hours after it has been pre-oiled; otherwise, the pre-oiling procedure typically must be repeated.

In using some types of pre-oilers, the oil line from the inlet side of the engine-driven oil pump must be disconnected to permit the pre-oiler tank to be connected at this point.

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Fuel system bleeding To purge the fuel system of airlocks and to aid in flushing any traces of preservative oil from a pressure carburettor, remove the drain plug in the carburettor fuel chamber which is farthest from the fuel inlet to the carburettor. In its place, screw a threaded fitting to which a length of hose, leading to a suitable container, is attached. Then open the throttle and place the mixture control in the “AUTO- LEAN” or “AUTO-RICH” position, so that fuel will be permitted to flow through the system. After making sure the fuel shutoff and main fuel tank valves are open, turn on the fuel boost pump until there are no traces of preservative oil in the fuel being pumped through the system. The passage of air will be indicated by an audible burp emerging from the end of the hose submerged in the container of fuel. This phenomenon is not to be confused with the numerous small air bubbles that may appear as a result of the velocity of the fuel being ejected from the carburettor. Usually, after approximately a gallon of fuel has been bled off, the system can be considered safe for operation. After completing the bleeding operation, return all switches and controls to their “NORMAL” or “OFF” position, and replace and safety the drain plug in the carburettor. Propeller check The propeller installed on the engine must be checked before, during, and after the engine has been ground operated.

A propeller whose pitch-changing mechanism is electrically actuated may be checked before the engine is operated. This is done by connecting an external source of electrical power to the aircraft electrical system, holding the propeller selector switch in the “DECREASE RPM” position, and checking for an increase of the propeller blade angle. Continue the check by holding the switch in the “INCREASE RPM” position and examining the propeller blades for a decrease in angle. The propellers can also be checked for feathering by holding the selector switch in the “FEATHER” position until the blade angle increases to the “FULL-FEATHER” position. Then return the propeller to a normal operating position by holding the switch in the “INCREASE RPM” position. Propellers whose pitch-changing mechanisms are oil actuated must be checked during engine operation after the normal operating oil temperature has been reached. In addition to checking the increase or decrease in RPM, the feathering cycle of the propeller should also be checked. When an engine equipped with an oil-operated propeller is stopped with the propeller in the “FEATHER” position, never unfeather the propeller by starting the engine and actuating the feathering mechanism. Remove the engine sump plugs to drain the oil returned from the feathering mechanism and turn the blades to their normal position using the feathering pump; or a blade wrench, a long-handled device that slips over the blade to permit returning the blades to normal pitch position manually, can be used.

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Checks and adjustments after engine run-up and operation After the engine has been ground-operated, and again after a flight test, operational factors must be adjusted, as necessary, and the entire installation given a thorough visual inspection. These adjustments often include fuel pressure and oil pressure, as well as rechecks of such factors as ignition timing, valve clearances, and idle speed and mixture, if these rechecks are indicated by how the engine performs. After both the initial ground run-up and the test flight, remove the oil sump plugs and screens and inspect for metal particles. Clean the screens before reinstalling them. Check all lines for leakage and security of attachment. Especially, check the oil system hose clamps for security as evidenced by oil leakage at the hose connections. Also, inspect the cylinder hold-down nuts or cap screws for security and safetying. This check should also be performed after the flight immediately succeeding the test flight.

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Licence Category B1

16.13 Engine Storage and Preservation

LEVEL 3 The knowledge level indicators are defined as follows:

•

LEVEL 1

•

A familiarisation with the principal elements of the subject.

LEVEL 2 • •

A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge.

A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner.

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Certification statement These Study Notes comply with the syllabus of EASA Regulation (EU) No. 1321/2014 Annex III (Part-66) Appendix I, and the associated Knowledge Levels as specified below:

Objective Preservation and depreservation for the engine and accessories/systems

Part-66 Ref. 16.13

Knowledge Levels A B1 B3 2 1

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Table of Contents Engine preservation ____________________________ 6 General _____________________________________ 6 General procedures for preservation_______________ 8 Corrosion-preventive materials __________________ 14 Corrosion-preventive compounds ________________ 14 Dehydrating agents ___________________________ 16 Corrosion-preventive treatment __________________ 18 Fuel system inhibiting _________________________ 20 Blanks and seals _____________________________ 20 Scheduled inspection of stored engines ___________ 20 Engine depreservation _________________________ General procedure ___________________________ Additional considerations ______________________ Inspection and de-preservation of accessories ______ Inspection and replacement of powerplant external units and systems ____________________________

22 22 24 26 27

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Engine preservation General Engines in aircraft that are flown only occasionally may not achieve normal service life because of corrosion. It occurs when moisture from the air and products of combustion combine to attack cylinder walls and bearing surfaces during periods when the aircraft is not used. The procedures for combating this condition consist of coating the vulnerable surfaces with rust inhibitive compounds as herein described. Need for preservation must be evaluated by the owner or operator of the aircraft based on environmental conditions and frequency of aircraft activity. The periods given are recommendations based on normal conditions. In regions of high humidity, active corrosion can be found on cylinder walls of new engines inoperative for periods as brief as two days. The cylinder walls of engines that have accumulated 50 hours or more service in a short period acquire a varnish that tends to protect them from corrosive action. Such engines, under favourable atmospheric conditions, can remain inactive for several weeks without evidence of damage by corrosion. Aircraft operated close to oceans, lakes, rivers and in humid regions have a greater need for engine preservation than engines operated in arid regions. If the engine is not going to be used for an extended period, measures must be taken to protect the engine against heat, direct sunlight, corrosion, and formation of residues. In particular, the water bonded by the alcohol in the fuel causes increased corrosion problems during storage. After each flight, activate choke for a moment before stopping the engine. Close all engine openings such as the exhaust pipe, venting tube, and air filter to prevent the entry of contamination and humidity. Total Training Support Ltd © Copyright 2020

For engine storage of one to four weeks, proceed with preservation before engine stop or on the engine at operating temperature. Let the engine run at increased idle speed. Shut the engine down and secure against inadvertent engine start. Remove air filters and inject approximately 3 cm3 (0.18 in3) of preservation oil or equivalent oil into the air intake of each carburettor. Restart the engine and run at increased idle speed for 10–15 seconds. Shut the engine down and secure against inadvertent engine start. Close all engine openings, such as exhaust pipe, venting tube, and air filter, to prevent the entry of contamination and humidity. For engine storage of engine for longer than four weeks and up to one year, proceed with preservation before engine stop and on the engine at operating temperature. Let the engine run at increased idle speed. Remove air filters and inject approximately 6 cm3 (0.37 in3) of preservative oil or equivalent oil into the air intake of each carburettor. Stop the engine. Remove spark plugs and inject approximately 6 cm3 (0.37 in3) preservation oil or equivalent oil into each cylinder and slowly turn crankshaft 2 to 3 turns by hand to lubricate top end parts. Replace and re-torque the spark plugs. Drain gasoline from float chambers, fuel tank, and fuel lines. Drain coolant on liquidcooled engines to prevent any damage by freezing. Lubricate all carburettor linkages using the proper lubricants. Close all openings of the engine, such as exhaust pipe openings, venting tube, and air intake, to prevent the entry of any foreign material and humidity. Protect all external steel parts by spraying with engine oil.

13-6 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Internal engine corrosion

13-7 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

General procedures for preservation Before an engine is placed in temporary or indefinite storage, it should be operated and filled with a corrosion-preventive oil mixture added in the oil system to retard corrosion by coating the engine’s internal parts. Drain the regular lubricating oil from the sump or system and replace with a preservative oil mixture according to the manufacturer’s instructions. Operate the engine until normal operating temperatures are obtained for at least one hour. Always take the appropriate precautions when turning or working around a propeller. After the flight, remove all the spark plug leads and the top spark plugs. To prevent corrosion, spray each cylinder interior with a corrosion-preventive mixture to prevent moisture and oxygen from contacting the deposits left by combustion. Spray the cylinders by inserting the nozzle of the spray gun into each spark plug hole and playing the gun to cover as much area as possible. Before spraying, each cylinder to be treated should be at the bottom centre position and the oil at room temperature. This allows the entire inside of the cylinder to become coated with the corrosion-preventive mixture. After spraying each engine cylinder at bottom centre, respray each cylinder while the crankshaft is stationary with none of the cylinder’s pistons at top dead centre.

The crankshaft must not be moved after this final spraying, or the seal of corrosion-preventive mixture between the pistons and cylinder walls are broken. Air can then enter past the pistons into the engine. Also, the coating of corrosion preventive mixture on the cylinder walls is scraped away, exposing the bare metal to possible corrosion. The engine should have a sign attached similar to the following: “DO NOT TURN CRANKSHAFT—ENGINE PRESERVED PRESERVATION DATE ____________.” When preparing the engine for storage, dehydrator plugs are screwed into the spark plug opening of each cylinder. If the engine is to be stored in a wooden shipping case, the ignition harness leads are attached to the dehydrator plugs with lead supports. Special ventilatory plugs are installed in the spark plug holes of an engine stored horizontally in a storage container. Any engine being prepared for storage must receive thorough treatment around the exhaust ports. Because the residue of exhaust gases is potentially very corrosive, a corrosion-preventive mixture must be sprayed into each exhaust port, including the exhaust valve. After the exhaust ports have been thoroughly coated, a moisture-proof and oilproof gasket backed by a metal or wooden plate should be secured over the exhaust ports using the exhaust stack mounting studs and nuts.

13-8 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Corrosion prevention fluid

13-9 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

These covers form a seal to prevent moisture from entering the interior of the engine through the exhaust ports. Engines stored in metal containers usually have special ventilatory covers. Another point at which the engine must be sealed is the intake manifold. If the carburettor is to remain on the engine during storage, the throttle valve should be wired open and a seal installed over the air inlet. But, if the carburettor is removed and stored separately, the seal is made at the carburettor mounting pad. The seal used in either instance can be an oil-proof and moisture-proof gasket, backed by a wooden or metal plate securely bolted into place. Silica gel should be placed in the intake manifold to absorb moisture. The silica gel bags are usually suspended from the cover plate. This eliminates the possibility of forgetting to remove the silica gel bags when the engine is eventually removed from storage. A ventilatory cover, without silica gel bags attached, can be used when the engine is stored in a metal container. Although the above procedures should prevent corrosion under favourable conditions, it is recommended that the engine is periodically inspected for evidence of corrosion. An engine awaiting overhaul or return to service after overhaul must be given careful attention. It does not receive the daily care and attention necessary to detect and correct early stages of corrosion. For this reason, some definite action must be taken to prevent corrosion from affecting the engine.

13-10 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Engine preservation kit

13-11 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

After the following details have been taken care of, the engine is ready to be packed into its container. If the engine has not been spray-coated with the corrosion-preventive mixture, the propeller shaft and propeller shaft thrust bearing must be coated with the compound. Then, a plastic sleeve, or moistureproof paper, is secured around the shaft, and a threaded protector cap is screwed onto the propeller retaining nut threads. All engine openings into which dehydrator plugs (or ventilatory plugs if the engine is stored in a metal container) have not been fitted must be sealed. At points where the corrosion-preventive mixture can seep from the interior of the engine, such as the oil inlet and outlet, oil-proof and moisture-proof gasket material backed by a metal or wooden plate should be used. At other points, moisture-proof tape can be used if it is carefully installed. Before its installation in a shipping container, the engine should be carefully inspected to determine if the following accessories, which are not a part of the basic engine, have been removed: • • • • • • • •

spark plugs and spark plug thermocouples; remote fuel pump adapters (if applicable); propeller hub attaching bolts (if applicable; starters; generators; vacuum pumps; hydraulic pumps; propeller governor; and engine-driven fuel pumps.

13-12 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Inhibited engine

13-13 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Corrosion-preventive materials An engine in service is in a sense ‘self-purging’ of moisture since the heat of combustion evaporates the moisture in and around the engine, and the lubricating oil circulated through the engine temporarily forms a protective coating on the metal it contacts. If the operation of an engine in service is limited or suspended for a time, the engine is preserved to a varying extent, depending upon how long it is to be inoperative. This discussion is primarily directed to preserving engines that have been removed from an aircraft. However, the preservation materials discussed are used for all types of engine storage. Corrosion-preventive compounds Corrosion-preventive compounds are petroleum-based products which form a wax-like film over the metal to which it is applied. Several types of corrosion-preventive compounds are manufactured according to different specifications to fit the various aviation needs. The type mixed with engine oil to form a corrosion-preventive mixture is a relatively light compound that readily blends with engine oil when the mixture is heated to the proper temperature.

The desired proportions of lubricating oil and either heavy or light corrosion-preventive compound must not be obtained by adding the compound to the oil already in the engine. The mixture must be prepared separately before applying to the engine or placing in an oil tank. A heavy compound is used for the dip treating of metal parts and surfaces. It must be heated to a high temperature to be sufficiently liquid to coat the objects to be preserved effectively. A commercial solvent or kerosene spray is used to remove corrosion-preventive compounds from the engine or parts when they are being prepared for a return to service. Although corrosion-preventive compounds act as an insulator from moisture, in the presence of excessive moisture, they eventually break down, and corrosion begins. Also, the compounds eventually become dried because their oil base gradually evaporates. This allows moisture to contact the engine’s metal, and aids in corroding it. When an engine is stored in a shipping container, a dehydrating agent must be used to remove moisture from the air in and around the engine.

The light mixture is available in three forms: MIL-C-6529 type I, type II, or type III. Type I is a concentrate and must be blended with three parts of MIL-L-22851 or MIL-L-6082, grade 1100 oil to one part of concentrate. Type II is a ready-mixed material with MIL-L-22851 or grade 1100 oil and does not require dilution. Type III is a ready-mixed material with grade 1010 oil, for use in turbine engines only. The light mixture is intended for use when a preserved engine is to remain inactive for less than 30 days. It is also used to spray cylinders and other designated areas.

13-14 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

AeroShell Fluid 2XN is a corrosion preservative concentrate for protecting infrequently used engines. It uses an ashless anti-corrosion additive package and highly refined mineral base oils to protect internal engine surfaces from the Effects of humidity. It is also designed to neutralize the acidic by-products of oil oxidation and combustion. AeroShell Fluid 2XN can be used neat to long-term storage, but it is typically mixed with one-part AeroShell Fluid 2XN to three parts fresh AeroShell Oil 100 to create an inhibited oil. It can also be sprayed undiluted on pistonengine exhaust ports, Rocker arms and accessories to provide additional protection.

13-15 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Dehydrating agents There are several substances, referred to as desiccants, that can absorb moisture from the atmosphere in sufficient quantities to be useful as dehydrators. One of these is silica gel. This gel is an ideal dehydrating agent since it does not dissolve when saturated. As a corrosion preventive, bags of silica gel are placed around and inside various accessible parts of a stored engine. It is also used in clear plastic plugs, called dehydrator plugs, that can be screwed into engine openings, such as the spark plug holes. Cobalt chloride is added to the silica gel used in dehydrator plugs. This additive makes it possible for the plugs to indicate the moisture content, or relative humidity, of the air surrounding the engine. The cobalt-chloride treated silica gel remains a bright blue colour in low relative humidity. As the relative humidity increases, the shade of the blue becomes progressively lighter, becoming lavender at 30% and fading through the various shades of pink, until at 60% relative humidity it is a natural or white colour. Some types of dehydrator plugs can be dried by removing the silica gel and heating the gel to dry it out, returning it to its original blue colour. When the relative humidity is less than 30%, corrosion does not normally take place. Therefore, if the dehydrator plugs are bright blue, the air in the engine has so little moisture that internal corrosion is held to a minimum. This same cobalt-chloride-treated silica gel is used in humidity indicator envelopes. These envelopes can be fastened to the stored engine so that they can be inspected through a small window in the shipping case or metal engine container. All desiccants are sealed in containers to prevent their becoming saturated with moisture before they are used. Care should be taken never to leave the container open or improperly closed. Total Training Support Ltd © Copyright 2020

13-16 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Silica gel bags

13-17 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Corrosion-preventive treatment Before an engine is removed, it should be operated, if possible, with corrosion-preventive mixture added in the oil system to retard corrosion by coating the engine’s internal parts. If it is impossible to operate the engine before removal from the aircraft, it should be handled as much as possible in the same manner as an operable engine. Any engine being prepared for storage must receive thorough treatment around the exhaust ports. Because the residue of exhaust gases is potentially very corrosive, a corrosionpreventive mixture must be sprayed into each exhaust port, including the exhaust valve. After the exhaust ports have been thoroughly coated, a moisture-proof and oil-proof gasket backed by a metal or wooden plate should be secured over the exhaust ports using the exhaust stack mounting studs and nuts. These covers form a seal to prevent moisture from entering the interior of the engine through the exhaust ports. Engines stored in metal containers usually have special ventilatory covers. When preparing the engine for storage, dehydrator plugs are screwed into the spark plug opening of each cylinder. If the engine is to be stored in a wooden shipping case, the ignition harness leads are attached to the dehydrator plugs with lead supports, as shown in the diagram below. Special ventilatory plugs are installed in the spark plug holes of an engine stored horizontally in a metal container. If the engine is stored vertically in a container, these vent plugs are installed in only the upper spark plug holes of each cylinder, and non-ventilatory plugs are installed in the lower cylinders. Dehydrator plugs from which the desiccant has been removed may be used for this latter purpose. Total Training Support Ltd © Copyright 2020

Another point at which the engine must be sealed is the intake manifold. If the carburettor is to remain on the engine during storage, the throttle valve should be wired open and a seal installed over the air inlet. But, if the carburettor is removed and stored separately, the seal is made at the carburettor mounting pad. The seal used in either instance can be an oil-proof and moisture-proof gasket backed by a wooden or metal plate securely bolted into place. Silica gel should be placed in the intake manifold to absorb moisture. The silica gel bags are usually suspended from the cover plate. This eliminates the possibility of forgetting to remove the silica gel bags when the engine is eventually removed from storage. A ventilatory cover without silica gel bags attached can be used when the engine is stored in a metal container. After these details have been taken care of, the engine is ready to be packed into its container. If the engine has not been spray-coated with the corrosion-preventive mixture, the propeller shaft and propeller shaft thrust bearing must be coated with the compound. Then, a plastic sleeve or moistureproof paper is secured around the shaft, and a threaded protector cap is screwed onto the propeller retaining nut threads. All engine openings into which dehydrator plugs (or ventilation plugs if the engine is stored in a metal container) have not been fitted must be sealed. At points where the corrosion-preventive mixture can seep from the interior of the engine, such as the oil inlet and outlet, oil-proof and moisture-proof gasket material backed by a metal or wooden plate should be used. At other points, moisture-proof tape can be used if it is carefully installed.

13-18 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Before its installation in a shipping container, the engine should be carefully inspected to determine if the following accessories, which are not a part of the basic engine, have been removed: • • • • • • • • •

Spark plugs and spark plug thermocouples; remote fuel pump adapters (if applicable); propeller hub attaching bolts (if applicable); starters; generators; vacuum pumps; hydraulic pumps; propeller governors; and engine-driven fuel pumps.

Ignition harness lead support

13-19 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Fuel system inhibiting The fuel used in turbine engines usually contains a small quantity of water which, if left in the system, could cause corrosion. All the fuel should, therefore, be removed and replaced with an approved inhibiting oil. Uninstalled fuel components should be preserved and placed in airtight bags with a small amount of wrapped silica gel. Pressure-injection carburettors should be drained of all fuel, the mixture placed in the full-rich position and grade #1010 (light lubricating mineral oil) introduced into the fuel inlet. When the preservative oil begins to flow from the uncapped vapour vent opening, all drain and vent plugs should be reinstalled and safety wired, and the inlet and outlet fittings capped for storage. Blanks and seals Approved blanks or seals should be used whenever possible. These are generally supplied with a new or reconditioned engine and should be retained for future use. Pipe connections are usually sealed using a screw-type plug or cap such as AGS 3802 to 3807, and plain holes are sealed with plugs such as AGS 2108; these items are usually coloured for visual identification. Large openings such as air intakes are usually fitted with a specially designed blanking plate secured by the regular attachment nuts, and the contact areas should be smeared with grease before fitting, to prevent the entry of moisture. Adhesive tape may be used to secure waxed paper where no other protection is provided but should never be used as a means of blanking off by itself, since it may promote corrosion and clog small holes or threads.

Scheduled inspection of stored engines Most maintenance shops provide a scheduled inspection system for engines in storage. Typically, the humidity indicators on engines stored in shipping cases are inspected every 30 days. When the protective envelope must be opened to inspect the humidity indicator, the inspection period may be extended to once every 90 days if local conditions permit. The humidity indicator of a metal container is inspected every 180 days under normal conditions. If the humidity indicator in a wooden shipping case shows by its colour that more than 30% relative humidity is present in the air around the engine, all desiccants should be replaced. If more than half the dehydrator plugs installed in the spark plug holes, indicate the presence of excessive moisture, the interior of the cylinders should be resprayed. If the humidity indicator in a metal container gives a safe blue indication, but air pressure has dropped below 1 PSI, the container needs only to be brought to the proper pressure using dehydrated air. However, if the humidity indicator shows an unsafe (pink) condition, the engine should be re-preserved.

13-20 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Dehydrator plug blue showing low humidity

Dehydrator plug pink showing high humidity

Dehydrator plug

Silica gel – dehydrated and saturated

13-21 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Engine depreservation General procedure For an engine which was not installed in an aircraft during storage, the installation procedure described in the appropriate maintenance manual should be carried out, followed by a thorough ground run, and check of associated systems. For an engine which was installed in an aircraft during storage the following actions should be taken: 1) 2) 3)

4) 5) 6) 7)

Remove seals and all desiccant bags. Remove cylinder dehydrators and plugs or spark plugs from upper and lower spark plug holes. Remove oil sump drain plug and drain the corrosion preventive mixture. Replace drain plug, torque, and safe-tie. Remove the oil filter. Install new oil filter, torque, and safe-tie. Service the engine with oil per the manufacturer’s instructions.

8) 9) 10)

Warning: To prevent serious bodily injury or death, accomplish the following before moving the propeller: ⎯ Disconnect all spark plug leads. ⎯ Verify that magneto switches are connected to magnetos and that they are in the off position and P-leads are grounded. ⎯ Throttle position “CLOSED”. ⎯ Mixture control “IDLE-CUT-OFF”. ⎯ Set the brakes and block the aircraft’s wheels. Ensure that aircraft tie-downs are installed and verify that the cabin door latch is open. ⎯ Do not stand within the arc of the propeller blades while turning the propeller.

14)

11) 12) 13)

Rotate propeller by hand several revolutions to remove preservative oil. Service and install spark plugs and ignition leads per the manufacturer’s instructions. Service the engine and the aircraft per the manufacturer’s instruction. Thoroughly clean the aircraft and engine. Perform a visual inspection. Correct any discrepancies. Conduct a standard engine start. Perform the operational test per operational inspection of the applicable maintenance manual. Correct any discrepancies. Perform a test flight per airframe manufacturer’s instructions. Correct any discrepancies before returning aircraft to service. Change oil and filter after 25 hours of operation.

13-22 Module 16.13 Engine Storage and Preservation

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Fitting spark plugs

13-23 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Additional considerations After the engine has been secured to an engine stand, all covers must be removed from the points where the engine was sealed or closed with ventilatory covers, such as the engine breathers, exhaust outlets, and accessory mounting-pad cover plates. As each cover is removed, inspect the uncovered part of the engine for signs of corrosion. Also, as the dehydrator plugs are removed from each cylinder, make a careful check of the walls of any cylinder for which the dehydrator plug colour indicates an unsafe condition. Care is emphasized in the inspection of the cylinders, even if it is necessary to remove a cylinder. On radial engines, the inside of the lower cylinders and intake pipes should be carefully checked for the presence of an excessive corrosion-preventive compound that has drained from the engine and settled at these low points. This excessive compound could cause the engine to become damaged from a hydraulic lock (also referred to as liquid-lock) when a starting attempt is made. The check for an excessive amount of corrosion-preventive compound in the cylinders can be made as the dehydrator plugs are removed from each cylinder. Much of the compound will drain from the spark plug holes of the lower cylinders of a radial engine when the dehydrator plugs are removed. Some mixture remains in the cylinder head below the level of the spark plug hole and can be removed with a hand pump.

A more positive method, however, is to remove the lower intake pipes and open the intake valve of the cylinder by rotating the crankshaft. The latter method allows the compound to drain from the cylinder through the open intake valve. If for some reason, an excessive amount of compound is present in an upper cylinder, it can be removed with a hand pump. The oil screens should be removed from the engine and thoroughly washed in kerosene or an approved solvent to remove all accumulations that could restrict the oil circulation and cause engine failure. After the screens are cleaned, immerse them in clean oil and then reinstall them in the engine. When the cover has been removed from the intake manifold, the silica gel desiccant bags must be removed before installing the carburettor. Take care not to tear one of the bags accidentally. Remove the protective covering from the propeller shaft and wash all the corrosion-preventive compound from inside and outside surfaces. Then coat the propeller shaft lightly with engine oil. As a final check, see that the exterior of the engine is clean. Usually, a quantity of compound runs out of the engine when the dehydrator plugs and oil screens are removed. To clean the engine, spray it with kerosene or an approved commercial solvent.

13-24 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Draining corrosion-preventive compound

13-25 Module 16.13 Engine Storage and Preservation

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Inspection and de-preservation of accessories An engine’s performance is no better than that of its accessories. Though the engine has been completely overhauled and is in top condition, any oversight or error in installing the accessories can result in improper engine operation or even irreparable damage to it. Before de-preserving any of the accessories enclosed with the engine, consult the storage data usually stencilled on the outside of the engine container or the records enclosed with the engine to determine how long the engine and accessories were in storage. Certain accessories that generally accompany an engine from overhaul are considered unsafe for use if their time in storage has exceeded a specified period. This time varies according to the limits prescribed by the manufacturer.

Before installing any replacement accessory, check it visually for signs of corrosion and freedom of operation. Always wipe the mounting pad, flange, and coupling clean before mounting the accessory, and install the proper gasket between the mounting pad and the accessory mounting flange. Lubricate the accessory drive shaft when indicated in the manufacturer’s instructions.

Any accessory that has been removed from the old engine and can be installed on the new one must be given a thorough inspection to determine its condition. This inspection includes a check for general condition, cleanliness, absence of corrosion, and absence of wear as evidenced by excessive ‘play’ in the moving parts. Some accessories must be replaced, regardless of their operating time, if the engine is being changed because of internal failure. Such accessories may have been contaminated by metal particles carried into their operating mechanisms by the engine oil that lubricates them.

13-26 Module 16.13 Engine Storage and Preservation

Issue 2 – July 2020

Inspection and replacement of powerplant external units and systems The engine nacelle must be thoroughly cleaned before it is inspected. The design of an engine nacelle varies with different aircraft. It is a framework covered with removable cowling, in which the engine is mounted. This assembly is attached to the aircraft and incorporates an insulating firewall between the engine and the airframe. The interconnecting wiring, tubing, and linkages between the engine and its various systems and controls pass through the firewall.

Before installing an engine, inspect all tubing in the nacelle for dents, nicks, scratches, chafing, or corrosion. Check all tubing carefully for indications of fatigue or excessive flatness caused by improper or accidental bending. Thoroughly inspect all hoses used in various engine systems. Weather checking (cracking of the outside covering of the hose) sometimes penetrates to the hose reinforcement. Replace any length of hose that shows indications of the cover peeling or flaking or has exposed fabric reinforcement. Replace a hose that shows indications of excessive cold flow.

Inspect the complete engine nacelle for the condition of the framework and the sheet-metal cowling and riveted plates that cover the nacelle. The engine mounting frame assembly should be checked for any distortion of the steel tubing, such as bends, dents, flat spots, corrosion, or cracks. Use the dye penetrant inspection method to reveal a crack, porous area, or other defects.

Cold flow is a term used to describe the deep and permanent impressions or cracks caused by hose clamp pressure.

The engine mounting bolts are usually checked for condition by magnetic particle inspection or other approved process. While the bolts are removed, the bolt holes should be checked for elongation caused by the movement of an improperly tightened bolt.

On older aircraft, check the pulleys in the control system for freedom of movement. It is easy to spot a pulley that is not turning freely, for both it and the cable are worn from the cable sliding over the pulley instead of rolling free. The bearings of a pulley may be checked by inspecting the pulley for excessive play or wobble with the tension removed from the cable. The cable must also be inspected for corrosion and broken strands. Locate any broken strands by wiping the cable with a cloth.

Check the outer surface of all exposed electrical wiring for breaks, chafing, or other damage. Also, check the security of crimped or soldered cable ends. Also, carefully inspect connector plugs for overall condition. Any item that is damaged must be repaired or replaced, depending on the extent of the damage.

Always replace a control rod if it is nicked or corroded deeply enough to affect its strength. If the corrosion cannot be removed by rubbing with steel wool, the pitting is too deep for safety.

Check bonding for fraying, loose attachment, and cleanness of terminal ends. The electrical resistance of the complete bond must not exceed the resistance values specified in the applicable manufacturer’s instructions.

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Inspect the exhaust stacks, collector ring, and tailpipe assembly for security, cracks, or excessive corrosion. Depending on the installation, these units, or parts of them, may be mounted on the engine before it is installed in the aircraft. Check all air ducts for dents and for the condition of the fabric or rubber anti-chafing strips at the points where sections of duct are joined. The dents may be pounded out; the anti-chafing strips should be replaced if they are pulled loose from the duct or are worn to the point at which they no longer form a tight seal at the joint. Thoroughly inspect the engine oil system and perform any required special maintenance upon it before installing a replacement engine. If an engine is being changed at the end of its normal time in service, it is usually only necessary to flush the oil system. However, if an engine has been removed for internal failure, usually some units of the oil system must be replaced and others thoroughly cleaned and inspected. If the engine has been removed because of internal failure, the oil tank is generally removed to permit thorough cleaning. Also, the oil cooler and temperature regulator must be removed and sent to a repair facility for overhaul. The vacuum pump pressure line and the oil separator in the vacuum system must also be removed, cleaned, and inspected. Internal failure also requires that the propeller governor and feathering pump mechanism be replaced if these units are operated by engine oil pressure.

13-28 Module 16.13 Engine Storage and Preservation

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References