types of aircraft engines

TABLE OF CONTENTS

Aircraft EnginesChapter 01 essential physiCs revision

» Internal Combustion Principles » The Properties of Gases » Humidity » The Gas Laws » The Equation of Continuity » Bernoulli’s Equation » Mechanisms for Heat Transfer » Newton’s Laws of Motion » Thrust » Power » Torque » Efficiency » Summary of Units

Chapter 02 piston engine prinCiple of operation

» Piston Engines » Principle of Operation » Function of the Key Moving Parts » The Theoretical Four-Stroke Cycle » The Practical Four Stroke Cycle

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Chapter 03 piston engine Designs anD Components

» Multi Cylinder Engines » Classification of Engines » Major Components of a Spark Ignition Engine » Valves and Valve Gear

Chapter 04 piston engine lubriCation anD Cooling

» Engine Lubrication Systems » The Wet Sump System » Engine Cooling Systems

Chapter 05 piston engine starting anD ignition

» Starting and Ignition Systems » Ignition » Principle of Magneto Operation » Controlling the Ignition System » The Need for Pressurised magnetos » Auxiliary Spark Augmentation » Spark Plugs

Chapter 06 piston engine fuel systems

» Piston Engine Fuel » Controlling Detonation » Engine Fuel Supply » The Basic Float Carburettor » Induction and Carburettor Icing » Fuel Injection » Engine Priming Systems » The Engine Air Induction System » The EGT Gauge

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Chapter 07 piston engine superCharging

» Introduction » Density Altitude » Piston Engine Power and Performance » Supercharging » Measurement of Supercharging Output » Types of Supercharging Unit » The Turbocharger » Dual Pressure Control Unit System » Intercoolers » Engine Management

Chapter 08 the Diesel engine

» Design and Construction » Diesel Fuel » Advantages and Disadvantages of Diesel Engines » Principle of Operation » Diesel Fuel Injection Systems » Glow Plugs » Diesel Engine Cooling Systems

Chapter 09 piston engine power, effiCienCy anD Control

» Engine Power » Calculating Power » Piston Engine Efficiency » Summary of Engine Controls and Indicators » Engine Handling

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Chapter 10 propeller aeroDynamiCs

» Introduction » How a Propeller Produces Thrust » Propeller Definitions » Resolving the Aerodynamic Forces » The Effect of TAS on a Fixed Pitch Propeller » The Effect of RPM on a Fixed Pitch Propeller » Blade Twist » The Variable Pitch Propeller » Windmilling » Reverse Thrust » Feathering » Propeller Efficiency

Chapter 11 propeller systems

» Introduction » Propeller Systems - Design and Construction » Power Torque and Thrust » Variable Pitch System » The Pitch Control Unit » The Centrifugal Latch » The Constant Speed Unit » Propeller Feathering Systems » Unfeathering Mechanisms » Reverse Thrust and the Beta Range » Cabin Noise and Synchrophasing » Methods for Indicating Power » Propeller Controls and Operation

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Chapter 12 gas turbine prinCiples

» Introduction » Pressure, Temperature and Velocity in the Working Cycle » Factors Affecting Thrust » The Drive for Greater Engine Efficiency

Chapter 13 gas turbine Designs

» Components » Types of Gas Turbine Engine » Single Spool Axial Flow Turbojet » Twin Spool Bypass Turbojet » High Bypass-ratio Turbofan » Turbo-propeller and Turboshaft Engines » The Requirement for a Reduction Gearbox

Chapter 14 the gas turbine air inlet

» Air Inlet » The Subsonic Inlet » The Supersonic Inlet

Chapter 15 the gas turbine Compressor

» The Centrifugal Compressor » The Axial Flow Compressor » Compressor Aerodynamics » Compressor Stall » Avoiding Surge and Stall » Axial Flow Compressor Construction » Engine Bleed Air Systems » The Auxiliary Gearbox

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Chapter 16 the gas turbine Combustion system

» The Combustion System » Combustor Design and Construction » Fuel Spray Nozzles » Ignition System

Chapter 17 the turbine system

» Principle of Operation » Types of Turbine » The Turbine » Turbine Blade Design and Construction » Active Clearance Control » Fatigue

Chapter 18 the gas turbine exhaust system

» The Exhaust System » Noise Suppression » Thrust Reversing Systems » Reverse Thrust Operation » Temperature Measurement » The Danger from Jet Efflux

Chapter 19 gas turbine lubriCation

» The Oil System » Main Bearing Housings » Type of Gas Turbine Oil System

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Chapter 20 the gas turbine fuel system

» The Engine Fuel System » The Low Pressure Fuel Pump » The High Pressure Fuel Pump » Fuel Control Unit (FCU) » FADEC » Fuel Oil Heat Exchanger » Types of Jet Fuel

Chapter 21 gas turbine Controls anD operation

» Engine Starting » Types of Starter Motor » Gas Turbine Operation and Monitoring » Thrust » Electronic Engine Displays » Display Colour Coding Conventions

Chapter 22 the auxiliary power unit (apu)

» The Auxiliary Power Unit (APU)

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CHAPTER 8: THE DIESEL ENGINE

Design and Construction

Diesel Fuel

Advantages and Disadvantages of Diesels

Principle of Operation

Diesel Fuel Injection Systems

Glow Plugs

Diesel Engine Cooling Systems

Issue 1 8.2

The Diesel Engine

Design and Construction

Diesel engines use the heat generated during the compression stroke to ignite the charge. This is why they are sometimes known as compression ignition engines. Because ignition occurs spontaneously there is no need for spark plugs, distributors and magnetos.

The other main difference between diesel and petrol engines is the way in which the mixture is created. Diesel engines always use direct fuel injection whereas petrol engines may use a carburettor, direct or indirect fuel injection system.

To produce a sufficiently high temperature for spontaneous fuel combustion to occur, diesel engines operate at much higher compression ratios than petrol engines; typically from about 14:1 up to about 28:1. This compares with a maximum ratio of about 9:1 for a petrol engine. This higher compression ratio increases the power output from a given quantity of charge and improves the engine’s thermal efficiency.

figure 8.1The Thielert diesel engine

08

Issue 1 8.3

AGK - Engines The Diesel Engine

Diesel Fuel

Diesel engines use a form of kerosene (Jet A1) for fuel. Gasoline can’t be used because it detonates uncontrollably at such high compression ratios. Diesel fuel is heavier than gasoline - with a specific gravity of about 0.8 to 0.9. It is less volatile and has a higher flash point than AVGAS which makes it a safer and more stable fuel. Its only real disadvantage is that it is a poor lubricant. Consequently the fuel pump has to be specially designed to cope with kerosene.

Kerosene fuels are sometimes categorised according to their Cetane number. This indicates the ignition delay, which is the time interval between start of injection and the start of combustion. The higher the Cetane number the shorter the ignition period.

Advantages and Disadvantages of Diesels

The higher operating pressures require stronger construction which increases the weight of the engine. This has traditionally been a problem because weight is usually of paramount importance. However, the development of strong, light alloys has allowed designers to create diesel engines with much less weight penalty.

In consequence, modern relatively light diesel engines offer a number of advantages:

JJ They are inherently more fuel efficient than gasoline engines.

JJ Their heavier build makes them more robust, so they require less scheduled maintenance.

JJ Their more robust build also makes them ideally suited to turbocharging.

JJ Without the need for an ignition system they are simpler and more reliable.

JJ They use kerosene which is not only cheaper than AVGAS but is less prone to the risk of fire or explosion.

Issue 1 8.4


Principle of Operation

The four stroke diesel engine operates in a manner very similar to the four stroke petrol engine.

JJ Induction. During the induction stroke air rather than mixture is drawn into the cylinder.

JJ Compression. The air is compressed. Its temperature and pressure rise significantly. The temperature rise is so great that it would quickly exceed the flash point of the fuel. If mixture had been used it would detonate. So a diesel has no option but to introduce the fuel at the very last moment. This is why all diesel engines use direct fuel injection. At the very top of the compression stroke, at TDC, a precisely metered quantity of fuel is injected into the cylinder. It immediately ignites. Again this differs from the petrol engine where ignition begins before TDC.

JJ Power. Compared to the power generated by a similar sized petrol engine the force exerted on the piston of a diesel engine is very much greater. This is because, at the point of ignition, the charge is very much more compressed and contained in a much smaller volume. Because fuel is introduced right at the last moment there is no risk of detonation. So, unlike petrol engines diesels don’t normally operate close to the correct stoichiometric ratio; they operate much leaner. With a normally lean mixture and no risk of detonation, diesel engines don’t require a mixture control lever. The power output of the diesel engine is controlled by the amount of fuel injected into the cylinder (though some designs may also have a throttle butterfly). This differs from the petrol engine where power is set by adjusting the amount of fuel and air introduced into the cylinder.

JJ Exhaust. The exhaust stroke operates in exactly the same way as a petrol engine.

Issue 1 8.5


Diesel Fuel Injection Systems

The direct fuel injection system on a diesel engine must cope with the very high pressures experienced in the cylinder. Typically the injectors must operate at anything up to 1600 bar (24 000psi) and still be able to deliver a fine mist of exactly the right quantity of fuel at exactly the right time.

Two types of injection system are used on aero diesel engines:

JJ Direct Injection. In this system high pressure fuel is fed from a high pressure pump directly to each injector.

JJ Common Rail. Here, a high pressure pump supplies fuel to a common rail from where fuel is distributed to each injector.

Most modern diesel designs use the common rail system because it results in more efficient combustion, cleaner emissions and quieter operation. Figure 8.2 shows a typical diesel cylinder design.

figure 8.2The fuel injector injects a precisely metered amount of fuel directly into the cylinder

Issue 1 8.6


The Direct Fuel Injection System

The direct fuel injection system comprises an engine driven high pressure pump and an injector for each cylinder.

The pump must deliver an exactly metered amount of high pressure fuel to the injector at exactly the right point in the cycle. To do this it must be able to:

JJ Sense engine RPM.

JJ Sense air density.

JJ Provide a means for advancing and retarding the timing of the fuel pulse.

JJ Rapidly generate a series of fuel pulses of sufficient pressure and deliver them in the appropriate sequence to each injector.

figure 8.3Direct fuel injection system

Issue 1 8.7


The injectors are mounted in the cylinder head. They incorporate a mechanically operated spring loaded poppet valve which opens momentarily whenever the fuel line pressure exceeds the opposing force of the spring.

figure 8.4A diesel fuel injector

Issue 1 8.8


The Common Rail Injection System

The common rail system comprises four major components:

JJ A high pressure pump with a pressure regulator and metering valve

JJ A common rail acting as an accumulator for high pressure fuel.

JJ Electro-magnetically operated injectors.

JJ An engine control unit (ECU) which monitors engine conditions and then precisely controls the pressure in the common rail and the opening and closing of the injectors.

The high pressure pump supplies a constant output of high pressure fuel to the common rail. Unlike the injection pump in direct injection system it doesn’t have to generate a series of pulses; nor does it need to time the fuel delivery. Its sole purpose is to pressurise the common rail.

The common rail acts as a reservoir which provides a constant supply of high pressure fuel to each injector. Excess fuel from the rail is returned via a cooler to the aircraft fuel system

The injectors are electrically controlled. A solenoid valve in each opens and closes on command from the engine control unit.

The engine control unit monitors the key engine parameters including the speed and position of the cam and crankshafts to calculate and adjust the output of the high pressure pump and the timing of the injectors.

The great advantage of this system is that the two functions: generating high pressure fuel and delivering metered quantities to the cylinders are now separate. This greatly simplifies the pump design.

Issue 1 8.9


figure 8.5Common rail system

Issue 1 8.10


Glow Plugs

Diesel engines can be difficult to start when cold. This is because in cold conditions the temperature rise on the compression stroke may not be enough to raise the air temperature above the flash point of the fuel.

To overcome this problem diesel engines are usually fitted with glow plugs to increase the temperature in the combustion chamber. There are two types:

JJ Sheathed Glow Plug. The sheathed glow plug contains an electrically heated coil encased in a sheath filled with magnesium oxide powder. The powder protects the coil from vibration and the damaging environment of the combustion chamber.

JJ Ceramic Glow Plug. The ceramic glow plug, Figure 8.6, comprises a special heating element with a very high melting point encased in silicon nitrite. Silicon nitrite is a particularly good heat conductor, so the preheating time is very short. The material can also withstand rapid heating for long periods of time. Consequently this type of glow plug is more heat resistant and longer-lasting than the sheathed type.

figure 8.6A diesel glow plug

Issue 1 8.11


Glow plugs are powered from the battery. A cockpit warning light illuminates to indicate their operation.

Power to Weight Ratio

Compared to a gasoline engine of similar size the aircraft diesel engine has a lower power to weight ratio.

Diesel Engine Cooling Systems

Diesel engines require more cooling than their petrol engine equivalents. Although air cooling can be used, most modern design uses a liquid cooling system.

Liquid coolant systems use a mixture of water and antifreeze. Coolant is pumped around passages within the engine and returned to a storage tank via a thermostatically controlled valve. An air cooled heat exchanger located in the intake helps to disperse the heat energy absorbed the coolant.

types of aircraft engines

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