AIR

SERVICE

TRAINING

(ENGINEERING)

LIMITED

Subsidiary of Perth College

BASIC PISTON ENGINES

Part 66 - C/017

PERTH COLLEGE

BRAHAN BUILDING

CRIEFF ROAD

PERTH PH2 1NX

TEL: 01738 552311

FAX: 01738 553369

Air Service Training (Engineering) Ltd

AERONAUTICAL ENGINEERING TRAINING NOTES

These training notes have been issued to you on the understanding that they are intended for your guidance, to enable you to assimilate classroom and workshop lessons and for self-study. Although every care has been taken to ensure that the training notes are current at the time of issue, no amendments will be forwarded to you once your training course is completed. It must be emphasised that these training notes do not in any way constitute an authorised document for use in aircraft maintenance.

All Rights Reserved

The copyright in these technical training notes remain the physical and intellectual property of Air Service Training (Engineering) Ltd, (AST). Copying, storing in hard copy or electronic format, transmission to third parties and use for teaching by establishments other than AST is forbidden, except with the written permission of the AST General Manager.

M Haufe

Training Manager

March 2006

CONTENTS

PAGE

CHAPTER 1 : FUNDAMENTALS

SECTION 1 : Mechanical Efficiency01.01.01

SECTION 2 : Operating Cycles01.02.01

SECTION 3 : Piston Displacement & Compression Ratio01.03.01

SECTION 4 : Two Stroke & Four Stroke

Diesel Engine Operating Cycles01.04.01

SECTION 5 : Engine Configuration & Firing Order01.05.01

SECTION 6 : Power Calculation & Measurement01.06.01

CHAPTER 2 : ENGINE PERFORMANCE

SECTION 1 : Factors Affecting Engine Power02.01.01

SECTION 2 : Mixture/Leaning Engine Pre-Ignition &

Detonation02.02.01

CHAPTER 3 : ENGINE CONSTRUCTION

SECTION 1 : Crankcase Crankshafts Cam Shafts and Sumps03.01.01

SECTION 2 : Accessory Gearbox03.02.01

SECTION 3 : Cylinder And Piston Assemblies03.03.01

SECTION 4 : Connecting Rods, Inlet & Exhaust Valves03.04.01

SECTION 5 : Valve Mechanism03.05.01

SECTION 6 : Propeller Reduction Gearboxes03.06.01

CHAPTER 4 : ENGINE FUEL SYSTEMS

SECTION 1 : Float Chamber Carburettors04.01.01

SECTION 2 : Injection Chambers04.02.01

SECTION 3 : Direct Fuel Injection Systems04.03.01

SECTION 4 : Icing and Heating04.04.01

CHAPTER 5 : SUPERCHARGING TURBOCHARGING

SECTION 1 : Principles and Purpose of Charging05.01.01

SECTION 2 : Internally Driven Superchargers05.02.01

SECTION 3 : Externally Driven Superchargers05.03.01

CHAPTER 6 : STARTING & IGNITION SYSTEMS

SECTION 1 : Starting Systems06.01.01

SECTION 2 : Magneto Types Construction and

Principles of Operation06.02.01

SECTION 3 : Ignition Harnesses & Spark Plugs06.03.01

CHAPTER 7 : LUBRICANTS AND FUELS

SECTION 1 : Properties and Specification of Fuels07.01.01

SECTION 2 : Properties and Specification of Oils07.02.01

SECTION 3 : Fuel Additives07.03.01

SECTION 4 : Safety Precautions07.04.01

SECTION 5 : System Operation07.05.01

SECTION 6 : Oil System Components07.06.01

chapter 1 : fundamentals

section 1 : mechanical efficiency

injection & turbo general

The physicist defines work as Work is 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.

Work (W) = Force (F) x Distance (D).Work is measured by several standards, the most common unit is called footpound. If a 1-pound mass is raised 1 foot, 1 ft-lb (foot-pound) of work has been performed. The greater the mass and the greater the distance, the greater the work.

HORSEPOWER

The common unit of mechanical power is the hp (horsepower). 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 hp, which is the standard unit of power in the English 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.

hp = ft-lb per min or

33,000

= 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 ft in 1min, the power expended is 33,000 ft-lb per min, or exactly 1 hp.

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.

If the weight were attached to a spring scale graduated in pounds, then dragged by pulling on the scale handle, the amount of force required could be measured. Assume that the force required is 90 lbs, and the 100 lb weight is dragged 660 ft in 2 min. The amount of work performed in the 2 min will be 59,400 ft-lb, or 27,700 ft-lb per min. Since 1 hp is 33,000 ft-lb per min, the hp expended in this case will be 29,700 divided by 33,000, or 0.9 hp.

MECHANICAL EFFICIENCY

Mechanical efficiency is the ratio that shows how much of the power developed by the expanding gases in the cylinder is actually delivered to the output shaft. It is a comparison between the brake horsepower and the indicated horse power. 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 fhp (friction horsepower), the power lost in overcoming friction.

The factor that has the greatest effect on mechanical efficiency is the friction within the engine itself. The friction between moving parts in an engine remains practically constant throughout an engines speed range. Therefore, the mechanical efficiency of an engine will be 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%.

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-lbs) as well as in heat units. The Btu (British thermal unit) of heat is the quantity of heat required to raise the temperature of 1 lb of water 1(F. It is equivalent to 778ftlbs of mechanical work. A pound of petroleum fuel, when burned with enough air to consume it completely, gives up about 20,000 Btu, the equivalent of 15,560,000 ft-lbs 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, obviously the engine which converts into work the greater part of the energy in the fuel (higher thermal efficiency) will deliver the greater amount of power. Furthermore, the engine which has the higher thermal efficiency will have less waste heat to dispose of to the valves, cylinders, pistons, and cooling system of the engine. A high thermal efficiency also means a low specific fuel consumption and, therefore, less fuel for a flight of a given distance at a given power. Thus, the practical importance of a 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 friction of moving parts; and 40% to 50% is lost through the exhaust. Anything which increases the heat content that goes into mechanical work on the piston, which reduces the friction and pumping losses, or which reduces the quantity of unburned fuel or the heat lost to the engine parts, increases the thermal efficiency.

The portion of the total heat of combustion which is turned into mechanical work depends to a great extent upon the compression ratio. Compression ratio is the ratio of the piston displacement plus combustion chamber space to the combustion chamber space. Other things being equal, the higher the compression ratio, 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, for the extremely high temperature created by high compression ratios causes the material in the cylinder to deteriorate rapidly and the fuel to detonate.

The thermal efficiency of an engine may be based on either bhp or ihp and is represented by the formula:

indicated thermal efficiency = ihp x 33,000weight of fuel burned/min x heat value x 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.

Example

An engine delivers 85 bhp for a period of 1 hr and during that time consumes 50 lbs of fuel. Assuming the fuel has a heat content of 18,800 Btu per lb, find the thermal efficiency of the engine:

85 x 33,000 = 2,805,000 .833 x 18,800 x 778 12,184.569

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 produced by 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.

Thermal Distribution in an Engine

VOLUMETRIC EFFICIENCY

Volumetric efficiency, another engine 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 departure from a 100% volumetric efficiency.

The pistons of an unsupercharged engine displace the same volume each time they sweep the cylinders from top centre to bottom centre. 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 lbs/sq in, and this pressure will support a column of mercury 29.92 in high. These standard sea-level conditions determine a standard density, and if the engine draws in a volume of charge of this density exactly 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 a high-speed internal or external blower may have a volumetric efficiency greater than 100%. The equation for volumetric efficiency is:

Volumetric = Volume of charge (corrected for temperature and pressure)

Efficiency

Piston displacement

Many factors decrease volumetric efficiency some of these are:

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.

Improper valve timing.

PROPULSIVE EFFICIENCY

A propeller is used with an engine to provide thrust. The engine supplies bhp through a rotating shaft, and the propeller absorbs the bhp and converts it into thrust hp. In this conversion, some power is wasted. Since the efficiency of any machine is the ratio of useful power output to the power input, propulsive efficiency (in this case, propeller efficiency) is the ratio of thrust hp to bhp. On the average, thrust hp constitutes approximately 80% of the bhp. The other 20% is lost in friction and slippage. Controlling the blade angle of the propeller is the best method of obtaining maximum propulsive efficiency for all conditions encountered in flight.

During takeoff, when the aircraft is moving at low speeds and when maximum power and thrust are required, a low propeller blade angle will give maximum thrust. For high-speed flying or diving, the blade angle is increased to obtain maximum thrust and efficiency. The constant-speed propeller is used to give required thrust at maximum efficiency for all flight conditions.

notes:

section 2 : operating cycles

RECIPROCATING ENGINE OPERATING PRINCIPLES

A study of this section will help in understanding the basic operating principles or reciprocating engines. The principles which govern the relationship between the pressure, volume, and temperature of gases are the basic principles of engine operation.

An internal-combustion engine is a device for converting heat energy into mechanical energy. Gasoline is vaporized and mixed with air, forced or drawn into a cylinder, compressed by a piston, and then ignited by an electric spark. The conversion of the resultant heat energy into mechanical energy and then into work is accomplished in the cylinder. Fig illustrates the various engine components necessary to accomplish this conversion and also presents the principal terms used to indicate engine operation.

The operating cycle of an internal combustion reciprocating engine includes the series of events required to induct, compress, ignite, burn, and expand the fuel/air charge in the cylinder, and to scavenge or exhaust the by-products of the combustion process.

When the compressed mixture is ignited, the resultant gases of combustion expand very rapidly and force the piston to move away from the cylinder head. This downward motion of the piston, acting on the crankshaft through the connecting rod, is converted to a circular or rotary motion by the crankshaft.

A valve in the top or head of the cylinder opens to allow the burned gases to escape, and the momentum of the crankshaft and the propeller forces the piston back up in the cylinder where it is ready for the next event in the cycle. Another valve in the cylinder head then opens to let in a fresh charge of the fuel/air mixture.

The valve allowing for the escape of the burning exhaust gases is called the exhaust valve, and the valve which lets in the fresh charge of the fuel/air mixture is called the intake valve. These valves are opened and closed mechanically at the proper times by the valve-operating mechanism.

The bore of a cylinder is its inside diameter. The stroke is the distance the piston moves from one end of the cylinder to the other, specifically, from TDC (Top Dead Centre) to BDC (Bottom Dead Centre), or vice versa.

Components and Terminology of Engine Operation

operating cycles

There are two operating cycles in general use:

The two-stroke cycle

The four-stroke cycle

At the present time, the two-stroke cycle engine is fast disappearing from the aviation scene and will not be discussed. As the name implies, two-stroke cycle engine require only one upstroke and one downstroke of the piston to complete the required series of events in the cylinder. Thus the engine completes the operating cycle in one revolution of the crankshaft.

Most aircraft reciprocating engine operate on the four-stroke cycle, sometimes called the Otto Cycle after its originator, a German physicist. The four-stroke cycle engine has many advantages for use in aircraft. One advantage is that it lends itself readily to high performance through supercharging.

In this type of engine, four strokes are required to complete the required series of events or operating cycle of each cylinder, as shown in Fig. 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.

Four-stroke Cycle

In the following discussion of the four-stroke cycle engine operation, it should be realized that the timing of the ignition and the valve events will vary considerably in different engines. Many factors influence the timing of a specific engine, and it is most important that the engine manufacturers recommendations in this respect be followed in maintenance and overhaul. The timing of the valve and ignition events is always specified in degrees of crankshaft travel.

In the following paragraphs, the timing of each event is specified in terms of degrees of crankshaft travel on the stroke during which the event occurs. 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 full-open position of the valve.

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 top dead centre on the exhaust stroke, in order to induce a greater quantity of the duel/air charge into the cylinder and thus increase the horsepower. The distance the valve may be opened before top dead centre, however, is limited be several factors, such as the possibility that hot gases remaining in the cylinder from the previous cycle may flash back 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 top dead centre at the start of the intake stroke. As mentioned above, the intake valve opens before top dead centre on the exhaust stroke (valve lead), and the closing of the exhaust valve is delayed considerably after the piston has passed top dead centre 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 fuel/air mixture induced into the cylinder, and to aid in scavenging the by-products of combustion.

The intake valve is timed to close about 50 to 75 past bottom dead centre 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 bottom dead centre, the slight upward travel of the piston during this time does not have a great 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 by means of an electric spark as the piston approaches top dead centre. The time of ignition will vary from 20 to 35 before top dead centre, depending upon the requirements of the specific engine, to ensure complete combustion of the charge by the time the piston is slightly past the top dead centre position.

Power Stroke

As the piston moves through the top dead centre 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 maximum power output of the engine. The temperature of these burning gases may be between 3,000 and 4,000F.

As the piston is forced downward during the power stroke by the pressure of the burning gases exerted 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.

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 bottom dead centre 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 waster heat after the desired expansion has been obtained and avoids overheating the cylinder and the piston. Thorough scavenging is very important, because any exhaust products remaining in the cylinder will dilute the incoming fuel/air charge at the start of the next cycle.

Exhaust Stroke

As the piston travels through bottom dead centre at the completion of the power stroke and starts upward on the exhaust stroke, it will begin to push the burned exhaust gases out the exhaust port. The speed of the exhaust gases leaving the cylinder creates a low pressure in the cylinder. This low or reduced pressure speeds the flow of the fresh fuel/air charge into the cylinder as the intake valves is beginning to open. The intake valve opening is timed to occur at 8 to 55 before top dead centre on the exhaust stroke on various engines.

SECTION 3 : PISTON DISPLACEMENT & COMPRESSION RATIO

PISTON DISPLACEMENT

When other factors remain equal, the greater the piston displacement the greater the maximum horsepower an engine will be capable of developing. When a piston moves from bottom dead centre to top dead centre, it displaces a specific volume. The volume displaced by the piston is known as piston displacement and is expressed in cubic inches for most American-made engines and cubic centimetres for others.

The piston displacement of one cylinder may be obtained by multiplying the area of the cross section of the cylinder by the total distance the piston moves in the cylinder in one stroke. For multi-cylinder engines this product is multiplied by the number of cylinders to get the total piston displacement of the engine.

Since the volume (V) of a geometric cylinder equals the area (A) of the base multiplied by the altitude (H), it is expressed mathematically as:

V = A x H

For our purposes, the area of the base is the area of the cross section of the cylinder or of the piston top.

AREA OF A CIRCLE

To find the area of a circle it is necessary to use a number called pi. This number represents the ratio of the circumference to the diameter of any circle. Pi cannot be found exactly because it is a never-ending decimal, but expressed to four decimal places it is 3.1416, which is accurate enough for most computations.

The area of a circle, as in a rectangle or triangle, must be expressed in square units. The distance that is one-half the diameter of a circle is known as the radius. The area of any circle is found by squaring the radius and multiplying by pi ((). The formula is expressed thus:

Where A is the area of a circle; pi is the given constant; and r is the radius of the circle, which is equal to the diameter or

Example:

Compute the piston displacement of the PWA 14 cylinder engine having a cylinder with a 5.5 inch diameter and a 5.5 inch stroke. Formulas required are:

Total V = V x N (number of cylinders)Substitute values into these formulas and complete the calculation.

Total V = V x N Total V = 130.6712 x 14 Total V = 1829.3968

Rounded off to the next whole number total piston displacement equals 1830cuin.

Another method of calculating the piston displacement uses the diameter of the piston instead of the radius in the formula for the area of the base.

Substituting

From this point on the calculations are identical to the preceding example.

Compression Ratio

All internal-combustion engines must compress the fuel/air mixture to receive a reasonable amount of work from each power stroke. The fuel/air charge in the cylinder can be compared to a coil spring, in that the more it is compressed the more work it is potentially capable of doing.

The compression ratio of an engine is a comparison of the volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke. This comparison is expressed as a ratio, hence the term compression ratio. Compression ratio is a controlling factor in the maximum horsepower developed by an engine, but it is limited by present-day fuel grades and the high engine speeds and manifold pressures required for takeoff. For example, if there are 140 cu in of space in the cylinder when the piston is at the bottom and there are 20 cu in of space when the piston is at the top of the stroke, the compression ratio would be 140 to 20. If this ratio is expressed in fraction form, it would be 140/20 or 7 to 1, usually represented as 7:1.

Compression Ratio

To grasp more thoroughly the limitation placed on compression ratios, manifold pressure and its effect on compression pressures should be understood. Manifold pressure is the average absolute pressure of the air or fuel/air charge in the intake manifold and is measured in units of inches of mercury (Hg). Manifold pressure is dependent on engine speed (throttle setting) and supercharging. The engine-driven internal supercharger (blower) and the external exhaust-driven supercharger (turbo) are actually centrifugal-type air compressors. The operation of these superchargers increases the weight of the charge entering the cylinder. When either one or both are used with the aircraft engine, the manifold pressure may be considerably higher than the pressure of the outside atmosphere. The advantage of this condition is that a greater amount of charge is forced into a given cylinder volume, and a greater power results.

Compression ratio and manifold pressure determine the pressure in the cylinder in that portion of the operating cycle when both valves are closed. The pressure of the charge before compression is determined by manifold pressure times the compression ratio. For example, if an engine were operating at a manifold pressure of 30 in Hg with a compression ratio of 7:1, the pressure at the instant before ignition would be approximately 210 in Hg. However, at a manifold pressure of 60 in Hg the pressure would be 420 in Hg.

Without going into great detail, it has been shown that the compression event magnifies the effect of varying the manifold pressure, and the magnitude of both affects the pressure of the fuel charge just before the instant of ignition. If the pressure at this time becomes too high, premature ignition or knock will occur and produce overheating.

One of the reasons for using engines with high compression ratios is to obtain long-range fuel economy, that is, to convert more heat energy into useful work than is done in engines of low compression ratio. Since more heat of the charge is converted into useful work, less heat is absorbed by the cylinder walls. This factor promotes cooler engine operation, which in turn increases the thermal efficiency.

Here, again, a compromise is needed between the demand for fuel economy and the demand for maximum horsepower without knocking. Some manufacturers of highcompression engines suppress knock at high manifold pressures by injecting an antiknock fluid into the fuel/air mixture. The fluid acts primarily as a coolant so that more power can be delivered by the engine for short periods, such as at takeoff and during emergencies, when power is critical. This high power should be used for short periods only.

SECTION 4 : two stroke & four stroke diesel engine operating cycles

diesel engine introduction

The diesel engine is a type of internal combustion engine; more specifically, a compression ignition engine, in which the fuel is ignited by the high temperature of a compressed gas, rather than a separate source of energy (such as a spark plug).

It was invented and patented by Rudolf Diesel in 1892. Diesel intended the engine to use a variety of fuels including coal dust. He demonstrated it in the 1900 Worlds Fair using peanut oil.

When a gas is compressed, its temperature rises (as stated in Charles Law); a diesel engine uses this property to ignite the fuel. Air is drawn into the cylinder of a diesel engine and compressed by the rising piston, at a much higher compression ratio than for a spark-ignition engine. At the top of the piston stroke, diesel fuel is injected into the combustion chamber at high pressure, through an atomizing nozzle, mixing with the hot, high-pressure air. The resulting mixture ignites and burns very rapidly. This contained explosion causes the gas in the chamber to expand driving the piston down with considerable force and creating power in a vertical direction. The connecting rod transmits this motion to the crankshaft which is forced to turn, delivering rotary power at the output end of the crankshaft. Scavenging (pushing the exhausted gas-charge out of the cylinder, and drawing in a fresh draught of air) of the engine is done either by ports or valves.

Diesel engines do not operate well when the cylinders are cold. Some engines utilize small electric heaters called glow plugs inside the cylinder to warm the cylinders prior to starting. Others use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. Once the engine is operating the combustion of fuel in the cylinder keeps the engine warm effectively.

In very cold weather, diesel fuel thickens and increases in viscosity and forms wax crystals or a gel in extreme cold. This can make it difficult for the fuel injector to get fuel into the cylinder in an effective manner, making cold weather starts difficult at times, though recent advances in diesel fuel technology have made these difficulties very rare.

There are two classes of diesel engines; two-stroke and four-stroke. Many larger diesels operate on the two-stroke cycle. Smaller engines generally use the four-stroke cycle.

Normally banks of cylinders are used in multiples of 2,4,6 or 8, and although any number of cylinders can be used as long as the load on the crankshaft is counterbalanced to prevent excessive vibration.

A few aircraft have been built that use diesel engines, such as the Junkers-powered Blohm & Voss Ha 139 of the late 1930s. This is quite rare because of the high importance of power-weight ratios in aeronautical applications, and the development of kerosene-powered jet engines and the closely-related turboprop engines.

The newer automotive diesels have power-weight ratios comparable to the ancient spark-ignition designs common in general aviation aircraft, and have far superior fuel efficiency. Their use of electronic ignition fuel injection, and sophisticated engine management systems also makes them far easier to operate than mass-produced spark-ignition aircraft engines, most of whom still use carburetors. Combined with Europes very favourable tax treatment of diesel fuel compared to gasoline, these factors have seen considerable interest in diesel-powered small general aviation planes, and several manufacturers have recently begun selling diesel engines for this purpose.

Mechanical Cycles

All reciprocating piston engines can either be a four-stroke Otto cycle or the twostroke cleric cycle.

The Basic Four-Stroke Cycle Diesel Engine

From the name, it is fairly obvious that there are four strokes in one complete engine cycle. A stroke is the movement of the piston through the full length of the cylinder and, since one such movement causes the crankshaft to rotate half a turn, it follows that there are two crankshaft revolutions in one complete engine cycle. The four strokes, in correct order, are as follows:

The inlet stroke. With the inlet valve open and the exhaust valve closed, the piston moves from TDC to BDC, creating a low-pressure area in the cylinder. Clean, filtered air rushes through the open inlet valve to relieve this low-pressure area, and the cylinder fills with air.

The compression stroke. With both valves closed, the piston moves from BDC to TDC, compressing the air. During this stroke, the air becomes heated to a temperature sufficiently high to ignite the fuel.

The power stroke. At approximately TDC, the fuel is injected, or sprayed, into the hot, compressed air, where is ignites, burns and expands. Both valves remain closed, and the pressure acts on the piston crown, forcing it down the cylinder from the TDC to BDC.

The exhaust stroke. At approximately BDC, the exhaust valve opens and the piston starts to move from BDC to TDC, driving the burnt gas from the cylinder through the open exhaust valve.

At the completion of the exhaust stroke, the exhaust valve closes, the inlet valve opens and the piston moves down the cylinder on the next inlet stroke. Since there are three non-working strokes to one working stroke, some means of keeping the engine turning over must be provided, particularly in single cylinder engines. It is for this reason, and to ensure smooth running, that a heavy flywheel is fitted to the crankshaft.

Scavenging the Four-Stroke Cycle Diesel

It is necessary, since efficient combustion is desired, to completely clear the burnt gas from the cylinder on the exhaust stroke so providing a full cylinder of fresh air by the completion of the inlet stroke. Air that has burned with fuel has had almost all of its oxygen consumed, and cannot be used again. The clearing of the exhaust gas from the cylinder of an internal combustion engine is known as scavenging.

Both the upward movement of the piston on the exhaust stroke and the valve timing contribute to the scavenging of the burnt gas in the four-stroke engine.

It is usual for the exhaust valve to be opened before BDC, thus allowing a puff of high-pressure gas to escape at high velocity through the exhaust system. The upward movement of the piston ensures that the burnt gas continues this high-speed movement.

At the of the exhaust stroke, the gas will continue to move through the exhaust system simply because of its momentum. This continued movement will cause a low-pressure area to develop behind the fast-moving exhaust gas, and this can be used to draw fresh air into the cylinder if the inlet valve opens at the instant the low-pressure area develops.

In practice it is found that good scavenging can be achieved by opening the inlet valve just before TDC, and closing the exhaust gas to draw fresh air into the cylinder before the inlet stroke actually begins.

The period of crankshaft rotation when both valves are open together is known as valve overlap, and occurs only at TDC.

Suitable positioning of the valves, usually one at each side of the combustion chamber, helps to guide the gases on their correct paths, and assists in achieving complete scavenging.

It is worthwhile to note at this stage that superheated engines usually have more valve overlap than naturally aspirated engines.

The Two-Stroke (Clerk) Cycle Diesel Engine

In engines of this type, there are obviously only two strokes, or one crankshaft revolution, to one complete engine cycle. This means that there are twice as many working strokes per minute in a two-stroke diesel as there are in a comparable four-stroke, working at the same engine speed. Theoretically, then, the two-stroke engine should develop twice the power of a four-stroke of similar bore and stroke and the same number of cylinders but, due to scavenging difficulties, the power output is in the vicinity of one-and-a-half times that of a comparable four-stroke.

Two-stroke diesels may operate on either the scavenge blown principle or the crankcase compression principle. However, crankcase compression two-strokes are rarely seen and only scavenge blown two-strokes will be discussed here.

The Basic Scavenge Blown Two-Stroke Diesel Engine

A scavenge blown engine makes use of an engine-driven air pump, or blower, to supply air to the engine cylinder. An inlet port in the cylinder wall is used instead of an inlet valve, in conjunction with either an exhaust port in the engine cylinder or an overhead exhaust valve.

On the power stroke, the exhaust valve or exhaust port opens first, and the high-pressure exhaust gas escapes. The inlet port then opens, allowing air to sweep through the cylinder clearing the exhaust gas and recharging the cylinder with fresh air. It is usual to close the exhaust valve or port first, allowing a slight pressure to build up in the cylinder before compression begins.

It is general practice to have a number of inlet ports machined in the cylinder liner. These are cut at an angle or tangent to the cylinder liner, to give the incoming air a spiralling action, and are known as tangential ports. A large air jacket, called an air box, usually surrounds the outside of the ports and acts as an air reservoir, ensuring a good supply of air to all ports at all times.

Scavenge Blown Two-Stroke

SCAVENGING METHODS USED IN TWO-STROKE DIESELS

Once the fresh air charge has entered the engine cylinder, it may act in one of three ways as it sweeps towards the exhaust port (or valve), driving the burnt gas ahead of it. The path within the engine cylinder through which the air sweeps as it drives out the exhaust gas classifies the scavenging system as one of the following three:

Cross scavenging This is the simplest system, and is used on many small petrol engines. However, due to its relative inefficiency with regard to scavenging, it is becoming increasingly rare in modern diesel engines.

Scavenging is achieved by situating the inlet and exhaust ports on opposite sides of the engine cylinder. The piston, reciprocating in the cylinder, opens or closes these ports. In some cases, the piston crown is specially shaped to deflect the incoming air upwards throughout the cylinder, in an attempt to obtain complete scavenging.

Cross Scavenging

Uniflow scavenging uniflow scavenging is considered the most efficient system of all, since the incoming air enters at one end of the cylinder and spirals throughout the entire length of the cylinder, to pass out through the exhaust port or the exhaust valve (depending on the design of the particular engine).

Mechanically operated exhaust valves in the cylinder head are usually used with this system, although opposed piston engines use one piston to control the exhaust ports and one to control the inlet ports. Again, the inlet ports are specially shaped to direct the incoming air in the required direction, and are known as tangential ports.

Because of its efficiency, the uniflow scavenging system is used on an extremely wide range of engines.

Uniflow Scavenging

Loop scavenging There are a number of loop-scavenging systems, but the scavenge air paths are basically the same in all cases. Loop scavenging is more efficient than cross scavenging, since the incoming air moves from the inlet port to the top of the cylinder, and down to the exhaust port, driving out the burnt gas.

Piston-controlled ports are used, the exhaust port (or ports) being the upper. The inlet port (or ports) may be directly beneath the exhaust, or may be some distance around the cylinder liner. Regardless of their actual position in relation to the exhaust ports, the inlet ports are so shaped that they direct the incoming air upwards, thus ensuring complete scavenging in the form of a loop.

It is not uncommon for all loop scavenging systems to be referred to as the Schnuerle loop. However, the Schuerle loop system is a specific system in which the air is admitted to the cylinder through two inlet ports, one on each side of the single exhaust port.

Loop Scavenging

NOTES:

SECTION 5 : ENGINE CONFIGURATION & FIRING ORDER

ENGINE LAYOUT

Individual designers have adopted different methods of arranging the cylinders on an aircraft engine to achieve a particular power output. The different arrangements are illustrated. Air-cooled in-line, horizontally-opposed, and radial engines, are all widely used on civil aircraft because of their general reliability and economy. Liquid-cooled Vee engines were widely used on military aircraft because of their high power output and low frontal area, but are rarely found in civil aircraft.

In-line Engines

In-line engines usually have four or six cylinders arranged in an upright or inverted row along the crankcase; it is not usual to have more than six cylinders, because of the difficulty of cooling the rear cylinders and the length of the crankshaft which would be required. In a four-cylinder engine, four power strokes occur every two revolutions of the crankshaft, and must be evenly spaced to provide smooth running. The firing order could be 1, 3, 4, 2 or 1,2,4,3. The camshaft, which is a shaft having a cam for each valve in the engine, would be driven from the crankshaft at half engine speed, and would operate the valves by means of push rods, and rockers. Each of the eight cams (two to each cylinder) would be located on the camshaft to open and close an inlet or exhaust valve in relation to the particular firing order and the valve timing prescribed for that engine. If the engine had six cylinders, there would be six power strokes every two revolutions of the crankshaft, and a cylinder would have to fire every 120( of crankshaft movement. This would necessitate a crankshaft with throws (ie. The offset portions of the crankshaft containing the crankpins). Suitably arranged cams would be provided on the camshaft, which would still be driven at half engine speed. The firing order of a six-cylinder engine is generally 1, 4, 2, 6, 3, 5, but a different order could be used, and the crankshaft throws could be arranged differently.

NOTE:The cylinders of British engines are usually numbered commencing from the propeller end of the engine, but engines of American manufacture are often numbered in the opposite direction

Horizontally-opposed Engines

The cylinders of a horizontally-opposed engine (usually four or six) are arranged in horizontal banks on opposite sides of the crankcase. Most engines have individual connecting rods operating on separate crankpins, thus the cylinders are staggered. A single camshaft is located either above or below the crankshaft, and is driven at half engine speed to operate the valves in both banks of cylinders. On some engines the inlet valve cams are shared by opposing cylinders, so that the camshaft of a sixcylinder engine may have a total of nine cams, six separate exhaust cams and three shared inlet cams. To minimize the length of the engine, a four-cylinder engine may have three main (crankshaft) bearings and a six-cylinder engine may have four. Because six firing strokes occur every two revolutions of the crankshaft of a six-cylinder engine, the throws of the crankshaft must be arranged at 120( to each other. In the four-cylinder engine the firing order would normally be 1,3,4,2, and the firing order of the six-cylinder engine would normally be 1,4,5,2,3, 6, but different firing orders would be possible on engines with different crankshaft and cam arrangements.

Radial Engines

A radial engine has an odd number of cylinders (usually not more than nine) arranged radially around the crankcase. If greater power is required, two banks of cylinders are used, each cylinder in the rear row being located midway between two front row cylinders to ensure adequate cooling. The crankshaft of a radial engine has only one throw for each bank of cylinders, and all the connecting rods are attached to the single crankpin via a master rod. This fact also dictates the firing order of the engine. On a seven-cylinder engine a firing stroke is required every of crankshaft movement, and since the angle between cylinders is 513/7(, the firing order can only be alternate cylinders in the direction of rotation, ie. 1, 3, 5, 7, 2, 4, 6. To balance the heavy mass of the master rod assembly, counterweights are fitted to the crankshaft, and it is also usual to fit vibration dampers to minimize the effects of any residual vibration. On engines with two banks of cylinders, the crankshaft throws are arranged at 180( to each other.

Except for sleeve-valve engines, the valves are operated by a cam drum which is concentric with, and driven by, the crankshaft. The cam drum has two rows of cams, one for the inlet valves and one for the exhaust valves. On seven-cylinder and ninecylinder engines, there are four equally spaced cams in each row, and the drum rotates at 1/8 engine speed; on three-cylinder and fivecylinder engines, two equally spaced cams on each row, with the drum rotating at engine speed, would be suitable.

Taking a seven-cylinder radial engine as an example, when the inlet valve on No 1 cylinder is open, the next inlet valve to open is on No 3 cylinder (since this is the next cylinder in the firing order). The cams are 90( apart and the drum must, therefore, rotate through an angle of 126/7( (the angle between No 1 and No 3 cylinder is 1026/7() in the direction of rotation to open the required valve on No 3 cylinder. Speed of rotation of the cam drum must be 126/7(1026/7 = 1/8 engine speed (operation of the cam drum on a sevencylinder engine is illustrated. On a ninecylinder engine the spacing of the cylinders is 40(, and successive valves open every 90( of crankshaft movement. Since the cams are 90( apart, the cam drum must rotate in the opposite direction of rotation to the crankshaft, but still at 1/8 engine speed

Engine Cylinder Arrangement

Radial Engine Cam Drum Operation

Numbering of Engine Cylinders

notes:

SECTION 6 : POWER CALCULATION & MEASUREMENT

INDICATED HORSEPOWER

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. To facilitate the indicated horsepower calculations, a mechanical indicating device, attached to the engine cylinder, scribes the actual pressure existing in the cylinder during the complete operating cycle. This pressure variation can be represented by the kind of graph shown. Notice that the cylinder pressure rises on the compression stroke, reaches a peak after top centre, then 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.

The indicated horsepower for a four-stroke cycle 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 = PLANK

33,000

Where:

P = Indicated mean effective pressure in psi

L = Length of the stroke in ft or in fractions of a foot

A = Area of the piston head or cross-sectional area of the cylinder, in sq in

N = Number of power strokes per minute; rpm

2

K = Number of cylinders

In the aforementioned formula, the area of the piston times 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, by the engine. Since hp is defined as work done at the rate of 33,000 ft-lb per min, the total number of ft-lb of work performed by the engine is divided by 33,000 to find the indicated horsepower.

Example:

Given:

Indicated mean effective pressure (P)

= 165 lbs/sq in

Stroke (L)

= 6 in or 5 ft

Bore

= 5.5 in

rpm

= 3,000

No of cylinders (K)

= 12

Indicated hp =

PLANK

33,000 ft-lbs/min

Find indicated hp:

A is found by using the equation

A = ( D2

A = x 3.1416 x 5.5 x 5.5

N is found by multiplying the rpm by :

N = x 3,000

= 1,500 rpm

Now, substituting in the formula:

Indicated hp = 165 x .5 x 23.76 x 1,500 x 12

33,000 ft-lbs/min

= 1069.20

Cylinder Pressure During Power Cycle

BRAKE HORSEPOWER

The indicated horsepower calculation discussed in the receding 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 bhp (brake horsepower). 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 and the friction of the pistons and the friction of all moving parts.

In practice, the measurement of an engines bhp involves the measurement of an engines 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 x Distance

(at right angles to the force)

Torque is a measure of load and is properly expressed in pound-inches (lb-in) or pound-feet (lb-ft) and should not be confused with work, which is expressed in inch-pounds (in-lbs) or foot-pounds (ft-lbs).

Typical Prony Brake

There are a number of devices for measuring torque, of which the Prony brake, dynamometer, and torquemeter are examples. Typical of these devices is the Prony brake, which measures the usable power output of an engine on a test stand. It consists essentially 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 which bears 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 bears on the scale. The scale reads 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.

Example

If the scale registers 200 lbs and the length of the arm is

3.18 ft, the torque exerted by the shaft is:

200 lb x 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 = 2( x 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-lbs per min, this quantity is divided by 33,000; the result is the brake horsepower of the shaft. In other words:

Power= Work per revolution x rpm

and bhp= Work per revolution x rpm

33,000

or bhp= 2( x force on the scales (lbs) x

length of arm (ft) x rpm

33,000

Example

Given:

Force on scales= 200 lbs

Length of arm= 3.18 ft

rpm

= 3,000

= 3.1416

Find bhp:

Substituting in equation

bhp= 6,2832 x 200 x 3.18 x 3,000

33,000

= 363.2

= 363.

As long as the friction between the brake collar and propeller shaft drum is great enough to impose an appreciable load on the engine, but is not great enough to stop the engine, it is not necessary to know the amount of friction between the collar and drum to compute the bhp. If there were no load imposed, there would be no torque to measure, and the engine would run away. If the imposed load is so great that the engine stalls, there may be considerable torque to measure, but there will be no rpm. In either case it is impossible to measure the bhp of the engine. However, if a reasonable amount of friction exists between the brake drum and the collar and the load is then increased, the tendency of the propeller shaft to carry the collar and arm about with it becomes greater, thus imposing a greater force upon the scales. As long as the torque increase is proportional to the rpm decrease, the horsepower delivered at the shaft remains unchanged. This can be seen from the equation in which 2( and 33,000 are constants and torque and rpm are variables. If the change in rpm is inversely proportional to the change in torque, their product will remain unchanged. Therefore, bhp remains unchanged. This is important. It shows that horsepower is the function of both torque and rpm, and can be changed by changing either torque or rpm, or both.

Powers and Pressures

FRICTION HORSEPOWER

Friction horsepower is the indicated horsepower minus brake horsepower. It is the horsepower used by an engine in overcoming the friction of moving parts, drawing in fuel, expelling exhaust, driving oil and fuel pumps, and the like. On modern aircraft engines, this power loss through friction may be as high as 10% to 15% of the indicated horsepower.

FRICTION AND BRAKE MEAN EFFECTIVE PRESSURES

The IMEP (indicated mean effective pressure), discussed previously, is the average pressure produced in the combustion chamber during the operating cycle and is an expression of the theoretical, frictionless power known as indicated horsepower. In addition to completely disregarding power lost to friction, indicated horsepower gives no indication as to how much actual power is delivered to the propeller shaft for doing useful work. However, it is related to actual pressures which occur in the cylinder and can be used as a measure of these pressures.

To compute the friction loss and net power output, the indicated horsepower of a cylinder may be thought of as two separate powers, each producing a different effect. The first power overcomes internal friction, and the horsepower thus consumed is known as friction horsepower. The second power, known as brake horsepower, produces useful work at the propeller. Logically, therefore, that portion of IMEP that produces brake horsepower is called BMEP (brake mean effective pressure). The remaining pressure used to overcome internal friction is called FMEP (friction mean effective pressure). IMEP is a useful expression of total cylinder power output, but is not a real physical quantity; likewise, FMEP and BMEP are theoretical but useful expressions of friction losses and net power output.

Although BMEP and FMEP have no real existence in the cylinder, they provide a convenient means of representing pressure limits, or rating engine performance throughout its entire operating range. This is true since there is a relationship between IMEP, BMEP, and FMEP.

One of the basic limitations place don engine operation is the pressure developed in the cylinder during combustion. In the discussion of compression ratios and indicated mean effective pressure, it was found that, within limits, the increased pressure resulted in increased power. It was also noted that if the cylinder pressure was not controlled within close limits, it would impose dangerous internal loads that might result in engine failure. It is therefore important to have a means of determining these cylinder pressures as a protective measure and for efficient application of power.

If the bhp is known, the BMEP can be computed by means of the following equation:

BMEP = bhp x 33,000

LANK

Example

Given:

bhp

= 1,000

Stroke

= 6 in

Bore

= 5.5in

rpm

= 3,000

No of cylinders= 12

Find BMEP:

Find length of stroke (in ft):

L= 0.5

Find area of cylinder bore:

A= (D2

= 0.7854 x 5.5 x 5.5

= 23.76 sq in

Find No of power strokes per min:

N= x rpm

= x 3,00

= 1,500

Then substituting in the equation:

BMEP = 1,000 x 33,000

.5 x 23.76 x 1,500 x 12

= 154.32 lbs per sq in

THRUST HORSEPOWER

Thrust horsepower can be considered as the result of the engine and the propeller working together. If a propeller could be designed to be 100% efficient, the thrust-horsepower and the brake-horsepower would be the same. However, the efficiency of the propeller varies with the engine speed, attitude, altitude, temperature, and airspeed, thus the ratio of the thrusthorsepower and the brake-horsepower delivered to the propeller shaft will never be equal. For example, if an engine develops 1,000 bhp, and it is used with a propeller having 85% efficiency, the thrust-horsepower of that engine-propeller combination is 85% of 1,000 or 850 thrust hp. Of the four types of horsepower discussed, it is the thrust horsepower that determines the performance of the engine-propeller combination.

notes:

CHAPTER 2 : ENGINE PERFORMANCE

SECTION 1 : FACTORS AFFECTING ENGINE POWER

ENGINE DESIGN FEATURES

The aims of an engine designer can be either to produce as much power as possible for a given engine size or weight, or to produce as small and light an engine as possible. with a given power output. In either case this means producing an engine with the best possible power/weight ratio, but reliability and cost are also very important factors to be considered. The power produced by an engine results from the burning of a fuel/air mixture in the cylinders, the greater the weight of mixture burnt the greater will be the amount of energy released. The power produced in the cylinders is initially used to overcome internal friction and to drive accessories such as pumps and generators, and the remainder is available to drive the propeller.

ENGINE POWER

There are a number of ways of increasing the power output of an engine, but these may be resolved into three main methods. These are, increasing the volume of the cylinders, increasing combustion pressure, and increasing the engine speed. The strength and weight of the components used in an engine are the main factors limiting the power produced.

INCREASED VOLUME

The obvious way to increase the volume of the cylinders is to increase their actual size. However, increasing the size of the cylinders would also mean increasing the size, and therefore the weight, of the reciprocating and rotating parts of the engine, and a point will be reached when the forces on these parts will approach the limits of strength of the materials used. The rate of acceleration of the pistons, and their speed of movement, will increase as the length of stroke increases and stresses will become very high. Inertia forces on the crankshaft will also increase with cylinder size. These factors place a physical limit on the size of the cylinders, and the method normally adopted to increase volume above this limit is to increase the number of cylinders. This method also has its limitations, however, resulting mainly from increased complexity, which may affect reliability.

INCREASED PRESSURE

There are two ways of increasing pressure in a cylinder. One method is to increase the compression ratio (ie. the ratio of the total volume of the cylinder with the piston at BDC, to its volume with the piston at TDC). This produces a higher pressure in the cylinder at the end of the compression stroke, and the force exerted on the piston during combustion will also be greater. The second method is to increase the weight of charge which will fill the swept volume (ie. the volume of the cylinder between the TDC and BDC positions of the piston) at standard temperature and pressure, is known as volumetric efficiency, and is expressed as a percentage. Volumetric efficiency may be increased by mechanically raising the pressure of the mixture fed to the inlet valve (ie. supercharging), or, to a more limited extent, by careful design of the induction passages, ports and valves, so as to present as little hindrance as possible to the flow of gases. An increase in volumetric efficiency produces higher pressures in the cylinder throughout the complete cycle of operations; a greater weight of fuel/air mixture being burnt in a given time, and more energy being released by combustion. The extent to which compression ratio and manifold pressure (ie. induction pipe pressure) can be raised, is limited by the strength of the materials used in the engine, and factors known as detonation and pre-ignition which is explained later.

INCREASED SPEED

An increase in engine speed will also result in the burning of a greater weight of fuel in a given time, and will therefore result in the production of more power. However, the higher centrifugal forces, and other stresses set up in the engine, necessitate stronger components, with a disproportionate increase in weight. Again the strength of the materials is the limiting factor.

section 2 : MIXTURES/LEANING ENGINE PREIGNITION & DETONATION

MIXTURE REQUIREMENTS

Air and fuel vapour will burn if mixed in the ratios of between approximately 8:1 and 20 : 1 by weight. However, complete combustion will only occur at a ratio of approximately 15 : 1 (ie. all the hydrogen and carbon in the fuel, and all the oxygen in the air will be used up), and this is known as the chemicallycorrect, or stoichiometric, mixture, which produces the highest combustion temperatures. With weaker mixtures (ie. those containing less fuel), and richer mixtures (ie. those containing more fuel), the excess air or fuel will absorb some of the heat of combustion and lower the temperature of the burning gases.

Although the chemically-correct mixture strength would theoretically produce the highest temperature, and therefore power, in practice mixing and distribution are less than perfect and this results in some regions being richer and others being weaker than the optimum strength, this variation may exist between one cylinder and another. A slight excess of fuel does not have much effect on power since all the oxygen is still consumed and the excess of fuel simply serves to reduce slightly the effective volumetric efficiency, in fact its cooling effect can be to some extent, beneficial. Weak mixtures, however, rapidly reduce power since some of the inspired oxygen is not being utilised, and this power reduction is much greater than that resulting from slight richness. It is, therefore, quite common to run engines (when maximum power rather than best fuel economy is the objective) at somewhat richer than chemically-correct mixtures (eg. about 12.5 : 1) to ensure that no cylinder is left running at severely reduced power from being unduly weak.

Typical Mixture Requirements

Fuel Consumption and Power

A mixture which is weaker than the chemically-correct mixture, besides burning at lower temperatures, also burns at a slower rate (because of the greater proportion of inert gas in the cylinder). Power output thus decreases as the mixture is weakened, but, because of the increase in efficiency resulting from cooler burning, the fall in power is relatively less than the decrease in fuel consumption. Thus the specific fuel consumption (ie. the weight of fuel used per horsepower per hour) decreases as mixture strength is weakened below 15 : 1. For economical cruising at moderate power, air/fuel ratios of 18 : 1 may be used, an advance in ignition timing being necessary to allow for the slower rate of combustion.

At high power settings, the increase in engine speed and cylinder pressure results in an increase in mixture temperature, and this could lead to detonation. Cooling may be provided by using excess fuel, an air/fuel ratio as low as 10 : 1 often being used at maximum power. This excess fuel, other than acting as a coolant, is otherwise wasted, because there is no oxygen available to burn it.

A richer mixture is also required at low engine speeds. The valves are timed to provide efficient operation at high engine speeds, and at low speeds the exhaust gas velocity is much less, with the result that exhaust gases are left in the cylinder during the period of valve overlap. This residual gas results in dilution of the incoming mixture, which must be progressively enriched as speed is decreased, in order to maintain smooth running.

The mixture requirement is, therefore, dependent upon engine speed and power output. Fuel is supplied to the engine as a liquid, but must be burnt as a mixture of fuel vapour and air, a number of engine and carburettor design features are, therefore, aimed at producing thorough atomisation and mixing of the charge.

Initial atomisation of the fuel in a float-chamber carburettor is achieved in a diffuser or discharge nozzle, by mixing the air and the fuel before they pass into a venturi, but in other carburetion systems the fuel is forced through a discharge nozzle under positive pressure, and better atomisation is achieved.

Vaporisation is often assisted by warming the induction passages, by designing the engine so that much of the induction manifold is either submerged in hot oil in the engine sump, or is surrounded by an exhaustheated jacket.

On some engines the fuel/air mixture passes through a distribution impeller, which is attached to the crankshaft and rotates at engine speed. This has the effect of thoroughly mixing the fuel and air, and assisting in vaporisation.

The carburetion system must control the air/fuel ratio in response to throttle setting, at all selected power outputs from slow-running to full throttle, and during acceleration and deceleration; it must function at all altitudes and temperatures in the operating range, must provide for ease of starting and may incorporate a means of shutting off the fuel to stop the engine. The float chamber carburettor is the cheapest and simplest arrangement and is used on many light aircraft it is very prone to carburettor icing, however, and may be affected by flight manoeuvres. The injection carburettor is a more sophisticated device and meters fuel more precisely, thus providing a more accurate air/fuel ratio, it is also less affected by flight manoeuvres, and is less prone to icing. The direct- (or port-) injection system provides the best fuel distribution and is reputed to be the most economical, it is unaffected by flight manoeuvres and is free from icing.

Any of these carburettor types may be fitted with a manual mixture control, by means of which the most economical cruising mixture may be obtained. However, in order to assist the pilot in selecting the best mixture, some aircraft are fitted with fuel flowmeters, exhaust gas temperature gauges or exhaust gas analysers.

NORMAL COMBUSTION

Normal combustion occurs when fuel/air mixture ignites in the cylinder and burns progressively with a normal pressure increase producing maximum pressure immediately after the piston passes top dead centre of the compression stroke.

A flame front starts at the spark plugs and travels across the combustion chamber at a speed of approximately 70 100 feet per second. The velocity of the flame front is influenced by the type of fuel, the ratio of fuel air mixture, the pressure on the fuel air mixture and the temperature of the fuel air mixture.

When the fuel/air mixture is ignited by means other than the normal spark ignition, the result is abnormal combustion. This abnormal combustion is divided into live distinct types DETONATION and PRE-IGNITION.

DETONATION

When the fuel/air mixture is subjected to a combination of excessively high temperature and high pressure within the cylinder, the spontaneous combustion point of the gaseous mixture is reached. When this critical detonation point is reached, normal progressive combustion is replaced by a sudden explosion, or instantaneous combustion. Due to the pistons position in the cylinder at the time the detonation wave starts, extremely high pressures are reached, often in excess of the structural limits of the cylinder and engine parts. Tests have proven that pressures in excess of 4,000 PSI are reached during detonation. Since these pressures are virtually instantaneous, the effect on the piston is equivalent to a sharp blow with a sledge hammer. This shattering force is what is sometimes heard in an automobile ass it is accelerated rapidly. In an automobile engine this is not so serious, because it is heard and can be readily remedied by reducing the engine power. In aircraft engines it is much more serious because it is difficult to detect above other aircraft noises and corrective action may then be too late to be effective. This form of combustion causes a definite loss of power, engine overheating, pre-ignition and, if allowed to continue, physical damage to the engine.

Although detection of detonation may be extremely difficult, the indications are an otherwise unexplained rise in cylinder head temperature, an unexplained loss of power, especially at the higher power settings, and a whitish-orange exhaust flame accompanied by puffs of black smoke.

PRE-IGNITION

Pre-ignition is defined as ignition of the fuel prior to normal ignition, or ignition before the electrical arcing occurs at the spark plugs.

Pre-ignition may be caused by excessively hot exhaust valves, carbon particles or spark plug electrodes heated to an incandescent or glowing state. In most cases these local hot spots are caused by the high temperatures encountered during detonation.

This form of abnormal combustion has the same effect on the engine as an early or advanced timing of the ignition system, and is so harmful in its effects that an engine will continue to operate normally only for a short period of time. This holds especially true if detonation and pre-ignition are in progress simultaneously. During pre-ignition conditions, cylinder pressures are in excess of the normal limits of the cylinder and engine structure.

One significant difference between pre-ignition and detonation lies in the fact that if the conditions for detonation exist in one cylinder, they may exist in all cylinders but, pre-ignition may exist in only one or two cylinders. This can make pre-ignition rather difficult to detect, because of the possibility of preignition occurring in a cylinder which is not the location of the thermocouple which measures cylinder head temperature. Probably the most reliable indication is a loss of power, but this also may be difficult to determine unless the engine has a torquemeter. Another indication of pre-ignition may be the observation of glowing carbon particles being discharged from the exhaust system.

chapter 3 : engine construction

section 1 : CRANKCASE CRANKSHAFTS CAM SHAFTS AND SUMPS

CRANKCASE

This is the name given to that part of the engine that houses the crankshaft and connecting rods. It provides mounting faces for the cylinders or cylinder blacks, reduction gear, wheel case and other units. It may be a single casing or build-up of several sections depending on the type of engine. It will contain the main bearings which are usually plain metal bearings for in-line engines and roller bearings for radial engines. The engine mountings for in-line engines take the form of our feet and a steel ring is usually used for radial engines. Provision is made at the lowest point of the crankcase for collection of engine oil for recirculation.

The crankcase is subjected to many variations of vibrational and other forces. Since the cylinders are fastened to the crankcase, the tremendous expansion forces tend to pull the cylinder off the crankcase. The unbalanced centrifugal and inertia forces of the crankshaft acting through the main bearing subject the crankcase to bending moments which change continuously in direction and magnitude. The crankcase must have sufficient stiffness to withstand these bending moments without objectional deflections. If the engine is equipped with a propeller reduction gear, the front or drive end will be subjected to additional forces.

Crankcase and Sump of Horizontally-Opposed Engine

Typical Opposed Engine Exploded into Component Assemblies

CRANKSHAFT

The purpose of this component is to change the reciprocating motion of the piston into rotary motion. Crankshafts are usually alloy steel forgings with their journals and crankpins hardened to resist wear. The crankpins and journals are usually hollow, to reduce weight, these spaces being interconnected by drillings in the crank webs to provide passages for lubricating oil. A shaft is classified according to the number of throws or cranks, for instance a six throw shaft has six crankpins. The crankwebs are sometimes extended, the extra metal providing a means of balancing the assembly. Suitable drives at each end of the crankshaft transmit the torque to the reduction gear and the accessory drives.

The simplest crankshaft is the single-throw or 360( type. This type is used in a single-row radial engine. It can be constructed in one or two pieces. Two main bearings (one on each end) are provided when this type of crankshaft is used.

The double-throw or 180( crankshaft is used on double-row or 180( crankshaft is used on double-row radial engines. In the radial-type engine, one throw is provided for each row of cylinders.

Typical Crankshafts

Four-Throw Crankshaft

CRANKSHAFT BALANCE

Excessive vibration in an engine not only results in fatigue failure of the metal structures, but also causes the moving parts to wear rapidly. In some instances, excessive vibration is caused by a crankshaft which is not balanced. Crankshafts are balanced for static balance and dynamic balance.

A crankshaft is statically balanced when the weight of the entire assembly of crankpins, crank cheeks, and counterweights is balanced around the axis of rotation. When testing the crankshaft for static balance, it is placed on two knife edges. If the shaft tends to turn toward any one position during the test, it is out of static balance.

A crankshaft is dynamically balanced when all the forces created by crankshaft rotation and power impulses are balanced within themselves so that little or no vibration is produced when the engine is operating. To reduce vibration to a minimum during engine operation, dynamic dampers are incorporated on the crankshaft. A dynamic damper is merely a pendulum which is so fastened to the crankshaft that it is free to move in a small arc. It is incorporated in the counterweight assembly.

A Single-Throw Radial engine Crankshaft

CAMSHAFTS & CAM DRUMS

CAMSHAFT

The valve mechanism of an opposed engine is operated by a camshaft. The camshaft is driven by a gear that mates with another gear attached to the crankshaft. The camshaft ALWAYS rotates at one-half the crankshaft speed. As the camshaft revolves, the lobes cause the tappet assembly to rise in the tappet guide, transmitting the force through the push rod and rocker arm to open the valve.

The profile of the lobe controls in terms of crankshaft degrees the point of valve opening, the rate of valve opening, the period the valve remains open, the rate of valve closing and the point at which the valve closes.

CAM DRUM

The valve mechanism of a radial engine is operated by one or two cam drums 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, 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 (on which the cam rollers ride) is known as 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 push rod 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 twinrow 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. The cam ring 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 will turn in the direction of rotation of the crankshaft. If the ring is driven from the inside, the cam will turn in the opposite direction from the crankshaft. This method is illustrated.

A study of the table given will show that a four-lobe cam may be used on either a seven-cylinder or nine-cylinder engine. On the seven-cylinder it will rotate in the same direction as the crankshaft, and on the nine-cylinder, opposite the crankshaft rotation. 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. Therefore, to obtain proper relation of valve operations and firing order, it is necessary to drive the cam opposite the crankshaft rotation.

5 Cylinders7 Cylinders9 CylindersDirection of Rotation

Number of lobesSpeedNumber of lobesSpeedNumber of lobesSpeed

3

21/6

4

31/8

1/65

41/10

1/8With crankshaft

Opposite crankshaft

Radial Engines, Cam Ring Table

Using the four-lobe cam on the seven-cylinder engine, the spacing between the firing of the cylinders will be greater than the spacing of the cam lobes, Therefore, it will be necessary for the cam to rotate in the same direction as the crankshaft.

A formula that sometimes is used in figuring cam speed is:

Cam ring speed = ( by the number of lobes on either cam track

Cam Drive Mechanism Opposed-Type Aircraft Engine

Cam Profile

One-half is the speed at which the cam would operate if it were equipped with a single lobe for each valve. It is divided by the number of lobes, which will determine how much the speed will have to be reduced.

In a twin-row, 14-cylinder radial engine which has seven cylinders in each row or bank, the valve mechanism may consist of two separate assemblies, one for each row. It could be considered as two seven-cylinder engines in tandem having the firing impulses properly spaced or lapped. For instance, in a twinrow engine, two four-lobe cam rings may be used. The cams are driven by gears attached to the crankshaft through gear teeth on the periphery of each cam.

Radial Engine Cam Drum Operation

Valve-Operating Mechanism (Radial Engine)

SUMP

The sump on a piston engine is where the oil returns having completed the lubrication of the engine. This can be a pressed steel container bolted onto the lower part of the engine (wet sump). In this system the oil is stored in the sump.

In (dry sump) engines the sump consists of an integral part of the crankcase where scavenge pumps return the oil to a separate storage tank.

section 2 : ACCESSORY GEARBOX

ACCESSORY SECTION

The accessory (rear) section usually is of cast construction, and the material may be either aluminium alloy, which is used most widely, or magnesium, which has been used to some extent. 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.

Recent design practice has been toward standardising the mounting arrangement for the various accessories so that they will be interchangeable on different makes of engines. For example, the increased demands for electric current on large aircraft and the requirements of higher starting torque on powerful engines have resulted in an increase in the size of starters and generators. This means that a greater number of mounting bolts must be provided and, in some cases, the entire rear section strengthened.

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, engine-driven fuel pump, tachometer generator, synchronising generator for the engine analyser, oil filter, and oil pressure relief valve.

ACCESSORY GEAR TRAINS

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 heavier 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.

section 3 : CYLINDER AND PISTON ASSEMBLIES

CYLINDERS

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 takes place, and it houses the piston and the connecting rod.

There are four major factors that need to be considered in the design and construction of the cylinder assembly. These are:

It must be strong enough to withstand the internal pressures developed during engine operation.

It must be constructed of a lightweight metal to keep down engine weight.

It must have good heat-conducting properties for efficient cooling.

It must be comparatively easy and inexpensive to manufacture, inspect, and maintain.

The head is either produced singly for each cylinder in air-cooled engines, or is cast in-block (all cylinder heads in one block) for liquid-cooled engines. The cylinder head of an air-cooled engine is generally made of aluminium alloy, because aluminium alloy 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 may be flat, semispherical, or peaked, in the form of a house roof. The semispherical type has proved most satisfactory because it is stronger 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 shown in the illustration. Each cylinder is an assembly of two major parts:

The Cylinder Head

The Cylinder Barrel

At assembly, the cylinder head is expanded by heating and then screwed down on the cylinder barrel which has been chilled, thus, when the head cools and contracts, and the barrel warms up and expands, a gastight joint results. While the majority of the cylinders used are constructed in this manner, some are one-piece aluminium alloy sand castings. The piston bore of a sand cast cylinder is fitted with a steel liner which extends the full length of the cylinder barrel section and projects below the cylinder flange of the casting. This liner is easily removed, and a new one can be installed in the field.

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 top dead centre 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 casting, 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 ge