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MECHANICAL DIESEL ENGINES TRAINING MANUAL COURSE EXP-SM100 Revision 0

Diesel Engines

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  • MECHANICAL

    DIESEL ENGINES

    TRAINING MANUAL COURSE EXP-SM100

    Revision 0

  • Field Operations TrainingMechanical

    Diesel Engines

    Training manual: EXP-SM100-EN Last revised: 14/08/2009 Page 2 / 127

    MECHANICAL

    DIESEL ENGINES

    CONTENTS 1. OBJECTIVES ..................................................................................................................6 2. INTRODUCTION .............................................................................................................7 3. REMINDERS ...................................................................................................................9

    3.1. TERMINOLOGY......................................................................................................10 3.2. CHARACTERISTICS ..............................................................................................11

    3.2.1. Geometrical characteristics .............................................................................11 3.2.2. Combustion characteristics .............................................................................11 3.2.3. Engine characteristics .....................................................................................12

    3.3. GENERAL ...............................................................................................................13 3.3.1. Comparison of diesel and petrol engines ........................................................13 3.3.2. Advantages of the diesel engine .....................................................................14 3.3.3. Disadvantages of the diesel engine.................................................................14

    4. DIESEL ENGINE CYCLE (4-STROKE) .........................................................................15 4.1. THEORETICAL CYCLE ..........................................................................................15 4.2. REAL CYCLE..........................................................................................................16

    5. CLASSIFICATION OF DIESEL ENGINES.....................................................................18 5.1. DIRECT INJECTION...............................................................................................19 5.2. INDIRECT INJECTION ...........................................................................................20

    5.2.1. Engine with precombustion chamber ..............................................................20 5.2.2. Engine with swirl chamber...............................................................................21 5.2.3. Engine with air chamber ..................................................................................22

    6. COMBUSTION AND SUPERCHARGING .....................................................................23 6.1. FUELS USED..........................................................................................................23

    6.1.1. General............................................................................................................23 6.1.2. Characteristics of diesel oil..............................................................................23 6.1.3. The combustion process .................................................................................24 6.1.4. Compression of the air ....................................................................................24 6.1.5. Combustion analysis .......................................................................................25

    6.1.5.1. Ignition delay ..............................................................................................26 6.1.5.2. Flame propagation .....................................................................................26 6.1.5.3. Main combustion ........................................................................................26 6.1.5.4. Postcombustion or diffused combustion.....................................................27

    6.2. SUPERCHARGING.................................................................................................27 6.2.1. Volumetric supercharger .................................................................................28

    6.2.1.1. Twin-screw supercharger ...........................................................................28 6.2.1.2. Roots superchargers..................................................................................29 6.2.1.3. Rotary-piston superchargers ......................................................................29

    6.2.2. Centrifugal superchargers ...............................................................................30 6.2.2.1. Role of the turbocharger ............................................................................30 6.2.2.2. Composition of a turbocharger ...................................................................31 6.2.2.3. Turbocharger operation..............................................................................32 6.2.2.4. Disadvantages of a turbocharger ...............................................................32

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    6.2.3. Pressure-wave supercharging (COMPREX system) .......................................36 7. FUEL SYSTEM..............................................................................................................37

    7.1. SUCTION CIRCUIT.................................................................................................38 7.2. LOW PRESSURE CIRCUIT....................................................................................39 7.3. HIGH PRESSURE CIRCUIT ...................................................................................39 7.4. FUEL PUMP............................................................................................................40

    7.4.1. Diaphragm pumps ...........................................................................................40 7.4.2. Piston pump ....................................................................................................40

    7.4.2.1. Single-acting fuel pump..............................................................................41 7.4.2.2. Double-acting fuel pump (Bosch) ...............................................................42

    8. INJECTION SYSTEMS..................................................................................................43 8.1. INJECTION PUMP (BOSCH)..................................................................................43

    8.1.1. General............................................................................................................43 8.1.2. Pumping components......................................................................................45 8.1.3. Delivery valve ..................................................................................................47

    8.1.3.1. Snubber valve ............................................................................................47 8.1.3.2. Ball valve....................................................................................................48

    8.2. INJECTORS AND INJECTOR HOLDERS ..............................................................49 8.2.1. Role and operation of the injector....................................................................49 8.2.2. Different types of injectors ...............................................................................52

    8.2.2.1. Single and multi-hole injector .....................................................................52 8.2.2.2. Pintle injector..............................................................................................53 8.2.2.3. Throttle injector ..........................................................................................54 8.2.2.4. Pilot hole injector........................................................................................54

    8.2.3. Injector assembly and disassembly .................................................................55 8.2.4. Inspecting the disassembled injectors .............................................................56

    8.2.4.1. Sticking needle check ................................................................................56 8.2.4.2. Needle lift check.........................................................................................57 8.2.4.3. Calibration check.......................................................................................58 8.2.4.4. Spray pattern check ...................................................................................58 8.2.4.5. Injector seat sealing ...................................................................................59 8.2.4.6. Refitting the injector holder.........................................................................60

    8.3. PUMP-INJECTORS AND ELECTRONIC SYSTEMS..............................................61 8.3.1. Pump-injectors ................................................................................................61

    8.3.1.1. Mechanical pump-injectors.........................................................................61 8.3.1.2. Electronically controlled pump-injectors .....................................................62

    8.3.2. New diesel injection system ............................................................................62 9. DESCRIPTION OF DIESEL ENGINES..........................................................................64

    9.1. CYLINDER BLOCK-CYLINDER HEAD ASSEMBLY...............................................64 9.1.1. Cylinder block (also known as the engine block).............................................64

    9.1.1.1. The different types of cylinder blocks .........................................................65 9.1.1.2. The different types of liners ........................................................................66

    9.1.2. Cylinder head ..................................................................................................67 9.1.3. Rockers ...........................................................................................................68

    9.1.3.1. Pushrods ....................................................................................................69 9.1.3.2. Tappets ......................................................................................................69

    9.1.4. Camshaft .........................................................................................................70 9.1.5. Valves..............................................................................................................70

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    9.1.6. Timing..............................................................................................................73 9.1.7. Protection covers.............................................................................................75

    9.1.7.1. Oil sump.....................................................................................................75 9.1.7.2. Timing cover...............................................................................................76 9.1.7.3. Rocker cover ..............................................................................................76

    9.2. THE MOVING PARTS IN THE ENGINE .................................................................77 9.2.1. Connecting rod (also known as the conrod) ....................................................77

    9.2.1.1. Big end bearings ........................................................................................78 9.2.1.2. Connecting rod material .............................................................................79 9.2.1.3. Connecting rod lubrication..........................................................................79

    9.2.2. Crankshaft .......................................................................................................79 9.2.2.1. Composition of the crankshaft....................................................................80 9.2.2.2. Required conditions ...................................................................................81 9.2.2.3. Manufacture ...............................................................................................81 9.2.2.4. Bearing lubrication .....................................................................................82 9.2.2.5. Balancing of forces and inertias .................................................................82 9.2.2.6. Angular positioning of the crank pins .........................................................83 9.2.2.7. Firing order.................................................................................................83

    9.2.3. Piston ..............................................................................................................84 9.2.3.1. Piston rings ................................................................................................87

    9.3. Lubricating system ..................................................................................................91 9.3.1. Oil system........................................................................................................91

    9.3.1.1. Pressure and oil bath lubrication system....................................................91 9.3.1.2. Pressure and dry sump lubrication.............................................................92

    9.3.2. Oil pump..........................................................................................................92 9.3.2.1. Gear pumps ...............................................................................................92 9.3.2.2. Vane pump.................................................................................................94 9.3.2.3. Rotor pump ................................................................................................94 9.3.2.4. Piston pump ...............................................................................................95

    9.3.3. Oil filters ..........................................................................................................96 9.4. COOLING................................................................................................................97

    9.4.1. Advantage of cooling.......................................................................................97 9.4.2. Advantages of high temperatures....................................................................97 9.4.3. The different type of cooling systems ..............................................................98

    9.4.3.1. Water cooling .............................................................................................99 9.4.4. Water pump...................................................................................................101 9.4.5. Water circulation in the engine ......................................................................101 9.4.6. Thermostat ....................................................................................................102

    9.4.6.1. Thermostat on the cylinder head outlet ....................................................103 9.4.6.2. Thermostat on the engine inlet.................................................................103

    9.4.7. Radiator.........................................................................................................104 9.4.8. Fans ..............................................................................................................106 9.4.9. Expansion tank..............................................................................................106 9.4.10. Coolant ........................................................................................................107

    10. DIESEL ENGINE MAINTENANCE ............................................................................108 10.1. TIMING GEAR ADJUSTMENT............................................................................109

    10.1.1. Valve operating gear ...................................................................................109 10.1.2. Determining the TDC...................................................................................110

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    10.2. SETTING AN INJECTION PUMP........................................................................113 10.3. REPLACEMENT OF PISTON RINGS.................................................................115 10.4. DEGLAZING .......................................................................................................119

    10.4.1. Purpose of deglazing...................................................................................119 10.4.2. Criterion for correct deglazing .....................................................................119

    10.4.2.1. Obtaining an even surface (peaks and troughs).....................................119 10.4.2.2. Obtaining a rough surface......................................................................120

    10.5. EXHAUST FUMES..............................................................................................120 10.5.1. Black exhaust fumes ...................................................................................121 10.5.2. Blue exhaust fumes.....................................................................................121 10.5.3. White exhaust fumes...................................................................................121

    11. FIGURES...................................................................................................................123 12. TABLES.....................................................................................................................127

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    1. OBJECTIVES This course is not intended to describe the diesel engine (see course EXP-EQ060: Internal Combustion Engines). The aim, when you have completed this course, is to enable you to efficiently maintain the diesel engines present on the sites.

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    2. INTRODUCTION The diesel engine is the result of the work carried out by the German engineer Rudolf Diesel between 1893 and 1897. It is an internal combustion engine and its ignition is not controlled but spontaneous due to the self-ignition phenomenon. Therefore this type of engine does not need spark plugs like the controlled-ignition engines (petrol engine, also called internal combustion engine).

    Figure 1: Spark plug Figure 2: Examples of spark plugs

    Figure 3: Cross section of a diesel engine

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    This is made possible by the use of a very high compression ratio allowing a temperature of around 600C to be obtained. Glow plugs are often used (on small engines) for cold starting by increasing the combustion chamber temperature. However, the engines are not systematically fitted with them.

    Figure 4: Glow plug Most diesel engines run on diesel fuel and also on fuel oil. Some large engines can use heavy fuel oils or tar oils. Today, some engines (generally car engines and agricultural engines) run on biofuels.

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    3. REMINDERS An engine is a mechanical component which converts a source of energy into mechanical work. The engine is an external combustion engine when the working fluid is totally separate from the combustion products. This is the case with the steam engine.

    Figure 5: Ship's propulsion diesel engine The engine is an internal combustion engine when the working fluid consists of combustion products. This is the case with diesel engines and internal combustion engines. The difference between a diesel engine and an internal combustion engine is determined by the process used to ignite the fuel: self-ignition or explosion. A diesel engine is a reciprocating internal combustion engine in which the mixture is ignited by compression. Diesel engines are called self-ignition engines as opposed to petrol engines which are called controlled ignition engines. First of all, the air is compressed to a very high pressure and heats up: when the temperature in the combustion chamber is sufficient, the fuel is injected as finely atomised particles which ignite spontaneously in contact with the air.

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    Diesel engines have a higher thermal efficiency than petrol engines and are preferred for high-power applications (over 3,000 hp): this is why they are used, among other things, for ships' propulsion. In this case they are single-acting engines operating with a two-stroke cycle at very slow engine speeds (120 to 180 rpm). They develop powers of up to 45,000 hp and are capable of burning low-quality fuels (heavy oils) in good conditions (low specific consumption). Compared to petrol-engine power plants, the consumption of these large engines is around 30% lower. The 4-stroke diesel engine is by far the most commonly used for normal applications (generator set, pump drives, etc.)

    3.1. TERMINOLOGY Bore: internal of the cylinder. Stroke (C): distance between the TDC (or UDC) and the BDC (or LDC). Dead centre points: extreme position of the piston at the top or bottom of its stroke. TDC (or UDC): top dead centre (or upper dead centre), the piston is at the highest point on its stroke. BDC (or LDC): bottom dead centre (or lower dead centre), the piston is at the lowest point on its stroke. Combustion chamber (CC): space between the TDC and the cylinder head. Dead volume: it is the volume of the CC when the piston is at the TDC. Cubic capacity per cylinder: volume between the total volume (V) and the volume of the combustion chamber. Power: work done by a machine divided by the time taken to carry it out (W / t). For an engine, we calculate the power output in rpm. The unit of power is the Watt or the Kilowatt. It can also be expressed in horsepower (hp) 1 hp = 736 W Torque: set of two forces F with the same intensity which can be parallel or opposed. Engine torque is the work done by the combustion which applies a pressure P on the surface area S of the top of the piston.

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    3.2. CHARACTERISTICS

    3.2.1. Geometrical characteristics

    Figure 6: Geometrical characteristics

    Cylinder capacity: CDV =4

    2 Total cylinder capacity: Vt = V x number of cylinders Dead volume: VB

    Total volume: B

    A

    VV=

    3.2.2. Combustion characteristics mc: mass of the fuel injected per cycle (in kg/cycle) Q1: heat of combustion (in Joules) Ch: hourly consumption (in kg/h) Cs: specific consumption (in kg/kW.h) Pci: calorific value (in J/kg)

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    3.2.3. Engine characteristics N: rotation speed (in rpm) Ce: effective torque (in N.m) Pe: effective power (in W) We: effective work (in J) e: effective efficiency Q2: heat loss at the exhaust

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    3.3. GENERAL

    3.3.1. Comparison of diesel and petrol engines The basic difference between a diesel engine and a petrol engine is the method of igniting the fuel and of the self-ignition characteristics of this fuel. If a finely atomised fuel is introduced into a mass of air sufficiently compressed so that its temperature reaches a determined value, the combustion is triggered by self-ignition. The self-ignition phenomenon itself results from:

    A very high volumetric ratio (of the order of 16/1 to 24/1)

    The high temperature generated by this ratio ( 600C)

    Stroke in the cycle

    Functions performed in a diesel engine

    Components in

    operation

    Functions performed in a petrol engine

    Components in operation

    1

    Induction

    Air sucked into the cylinder Inlet valve

    Petrol/air mixture drawn into the

    cylinder

    Inlet valves

    Carburettor or

    injectors

    2

    Compression

    Air highly compressed (20 to 30 bars) hence the heating to 600C

    approx.

    Mixture compressed (8 to

    12 bars) hence the heating to 300C

    approx.

    End of compression stroke

    Fuel injected at high

    pressure which spontaneously ignites in contact with the hot

    air

    Injection pump

    Injectors

    Mixture ignited by electric spark from

    the spark plug

    Distributor or magneto

    Spark plugs

    3

    Power (combustion or explosion)

    Combustion and expansion

    Combustion and expansion

    4

    Exhaust

    Evacuation of the burnt gases Exhaust valves

    Evacuation of the burnt gases Exhaust valves

    Table 1: Comparison of diesel and petrol engines

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    3.3.2. Advantages of the diesel engine

    Better efficiency: due to the higher compression ratio, the combustion is more complete giving a lower specific consumption (on average of 200 g/kW/h compared to 330 g/kW/h for a petrol engine).

    Higher engine torque which remains approximately constant at low speeds.

    Cheaper fuel.

    Lower fire risks since diesel has a higher flashpoint than petrol.

    Exhaust gases less toxic because they contain less carbon monoxide.

    3.3.3. Disadvantages of the diesel engine

    Mechanical components must be overdimensioned.

    High operating noise.

    High temperature in the combustion chambers which requires more efficient cooling.

    Cold starting not as good as with a controlled ignition engine.

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    4. DIESEL ENGINE CYCLE (4-STROKE)

    4.1. THEORETICAL CYCLE From the thermodynamic viewpoint the different cycles of the diesel engine are:

    Figure 7: Theoretical cycle of a diesel engine

    1. Induction of the air: isobaric transformation (constant pressure).

    2. Compression of the air raised to a temperature of 600C: adiabatic transformation (without heat exchange with the external environment).

    3. Injection of the diesel which ignites spontaneously (combustion) due to the heat

    given off during the compression: isobaric transformation.

    4. Power (a): Expansion providing engine work: adiabatic transformation.

    5. Power (b): Pressure reduction: isochoric transformation (constant volume).

    6. Exhaust: Evacuation of the burnt gases: isobaric transformation.

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    4.2. REAL CYCLE It is a combination of the two conventional cycles where part of the fuel burns at a constant volume and the other part burns at constant pressure. The mixed cycle is more or less similar to one of the two conventional cycles, depending on the settings which determine the injection. The constant volume cycle gives a better efficiency and the constant pressure cycle enables lighter engines to be produced since the maximum pressure is lower. There is a relatively marked difference between the real cycles and the theoretical cycles because:

    The working fluid is not a perfect gas and the nature of the gas changes during the cycle.

    The flow of the fluids is not instantaneous due to inertia.

    There are leaks through the components forming the combustion chamber.

    There are heat losses due to the heat exchanges with the external environment.

    In practice, we must take into account the opening of the valves which is not instantaneous, the inertia of the masses in movement, the ignition and the propagation which are not instantaneous (ignition delay), the pressure drops in the pipelines, and the heat given off by the combustion which requires cooling. To overcome these problems, we must:

    Advance the opening of the inlet and exhaust valves to compensate for the inertia of the masses to be moved.

    Delay the closing of the inlet and exhaust valves to benefit from the inertia of the masses in movement and increase the amount of fresh air entering the cylinders.

    Trigger the injection before TDC to compensate for the ignition delay.

    Advantages of these reasons:

    Before Bottom Dead Centre (BBC): reduces the resisting force during the upstroke.

    Exhaust Valve Close (EVC): more complete evacuation of the burnt gases.

    Valve Overlap (VO): more efficient cooling of the exhaust valves and their seats, the upper part of the cylinder and the cylinder head. Better fresh air charge in the cylinder and more complete evacuation of the residual gases.

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    The operating cycle of a four-stroke engine is as follows:

    Figure 8: Four stroke cycle

    1st stroke: induction

    2nd stroke: compression

    3rd stroke: injection/power

    4th stroke: exhaust The actions of the components necessary for the correct operation of the cycle are shown on a control diagram. This illustrates the operation of the cycle (valve opening and closing, lead at the moment of injection, etc.) and also the engine adjustments and settings.

    Figure 9: Practical diagram of a four-stroke cycle

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    5. CLASSIFICATION OF DIESEL ENGINES Diesel engines are classified according to their type of injection and combustion chamber. There are two main families of combustion types:

    Direct injection which designates all the processes where the combustion chamber is not split (the injector sprays directly into the cylinder's main chamber).

    Figure 10: Direct injection

    Indirect injection covers the different divided combustion chamber solutions (the injector sprays the fuel into an auxiliary chamber where the combustion begins), the gases then spread to the main combustion chamber through a passage or connecting ducts.

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    Figure 11: Indirect injection

    5.1. DIRECT INJECTION Two combustion techniques are used:

    Injector energy: used in large slow-running engines, the central injector has between 6 and 8 holes and sprays the fuel at the circumference of the piston's large-diameter shallow combustion chamber. The system operates without swirl but requires the injector to be very precisely positioned (near the chamber) and a very large excess of air.

    Figure 12: Injector energy

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    Swirl: this is the most commonly used process on all modern engines. The swirl is created due to the shape of the inlet duct, the combustion chamber in the piston is smaller, and its shape may vary according to the manufacturer. It is permanently being modified according to the antipollution standards in order to continuously improve the uniformity of the fuel/air mixture. The injector used is of the multi-hole (3 to 6) type.

    Figure 13: Swirl chamber

    Its operating principle is as follows: During induction, the air enters the cylinder through the inlet volute. It gives the air a very intense swirling movement creating a cyclone which continues during the compression. At the end of the compression the injector sends fuel into the piston's spherical chamber. The very short jet is aimed at the wall and spreads over it as a thin film. The fine droplets which are atomised around this jet are combined with oxygen and initiate combustion. This combustion starts with a small quantity of fuel which eliminates the knocking (also known as detonation). The remainder of the fuel which is spread as a thin film slowly evaporates allowing the vapours to mix with the swirling air.

    5.2. INDIRECT INJECTION

    5.2.1. Engine with precombustion chamber The pintle injector is placed on the cylinder head and in a noncooled cavity called the "prechamber". It communicates with the top of the cylinder via one or more small passages and represents between 20 and 30% of the compression volume.

    Figure 14: Pintle injector

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    The fuel injected into this prechamber starts to burn since it contains air that has been previously compressed and the pressure increase resulting from this precombustion expels the mixture towards the cylinder where the combustion continues. This staged combustion reduces operating noise since the injection pressures are moderate (100 to 150 bars) and the compression ratio varies from 12/1 to 15/1. The engine is usually started with the assistance of a glow plug because the compression ratio used does not allow the ambient air to reach the correct temperature when the cylinder head is cold.

    Figure 15: Precombustion chamber

    5.2.2. Engine with swirl chamber This system is a variation of the previous one, the swirl chamber represents almost the totality of the combustion chamber volume. This prechamber communicates with the cylinder via a large-section cone-shaped orifice.

    As in the previous case the injector sprays the fuel directly into the chamber. The compression ratio of these engines is between 15/1 and 18/1 and the injection pressure is between 110 and 130 bars.

    Figure 16: "RICARDO" swirl chamber

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    5.2.3. Engine with air chamber The reserve of air communicates with the cylinder through a large orifice but the injector is placed outside this chamber and positioned so that the fuel jet is sprayed directly into the compressed air leaving the chamber. This gives a high operating flexibility since the air and fuel are vigorously mixed together, thus facilitating combustion. These systems were abandoned several years ago.

    Figure 17: SAURER system and LANOVA system

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    6. COMBUSTION AND SUPERCHARGING In a diesel engine the complex combustion process is linked with the following main characteristics:

    Fuel used

    Injection pressure, atomisation quality (injector)

    Injection point and injection rate

    Compression pressure in the engine cylinder (linked with the compression ratio)

    Air temperature and quantity of oxygen at the moment of injection

    Type of combustion chamber (direct injection, prechamber, etc.) and mixture uniformity

    Engine rpm and temperature.

    6.1. FUELS USED

    6.1.1. General The fuels which can be injected into the cylinders of diesel engines are generally:

    Diesel oil and light fuel oil for road or agricultural applications

    Heavy fuel oils and tar oils (which are from lignite tar and bituminous coal) only used for large static engines (marine or industry)

    Biofuels (based on various vegetable oils, rapeseed, palm, etc.).

    6.1.2. Characteristics of diesel oil Diesel is one of the products resulting from the distillation or cracking of crude oils. It is a complex mixture of many hydrocarbons. The following information gives the normal characteristics of diesel oil. It also highlights the importance of using this fuel in diesel engines.

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    Density: it varies according to the origin of the crude oil and type of processing it has undergone (on average: 0.850 kg/dm3 at 15C) and it decreases by 0.0007 for each degree of temperature increase. Calorific value: it is slightly less than that of petrol. Its average value is 10,800 Cal/kg. Volatility: in practice, diesel oil distillation begins around 200C and ends at approximately 370C. Viscosity: around 9.5 mm2/s at 20C. Cetane (C16H34) index: ignition quality. The "cetane index" of the fuel to be studied is the percentage of cetane contained in a mixture which produces the same ignition delay as the fuel tested. Commercial fuels for diesel engines have a cetane index between 45 and 55 (for engine diesel oil the index must be at least equal to 48).

    6.1.3. The combustion process In a diesel engine the air/fuel mixture is never uniform because the fuel is only injected towards the end of the compression stroke. The fuel can only ignite when it enters the cylinder since it must first draw the heat necessary to reach its self-ignition temperature from the compressed air in the combustion chamber and from the chamber walls. In theory, 20 to 22 g of air are needed to burn 1 g of diesel fuel. In practice, an average of 25 to 30 g of air is used to burn 1 g of diesel fuel. An excess of air is always necessary because it:

    provides a better air / fuel mixture,

    ignites the droplets not mixed with air at the time of injection.

    6.1.4. Compression of the air The volume of air present in the cylinder after the "induction" stroke (with an initial "swirl movement according to the shape of the admission duct or depending on whether there is a deflector on the inlet valve) is compressed by the piston rising to TDC. This compression creates a rapid temperature rise which must reach 500C min. for the mixture to ignite spontaneously at the moment of injection.

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    6.1.5. Combustion analysis From the start of injection, there are four successive phases:

    Ignition delay

    Flame propagation

    Main combustion

    Postcombustion or diffused combustion

    A: Start of injection A-B: Ignition delay

    B-C: Rapid combustion (uncontrolled phase) C-D: Main combustion (controlled phase)

    D: End of injection D-E > Postcombustion or diffusion phase

    --------: Curve without injection

    Figure 18: Combustion timing diagram

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    6.1.5.1. Ignition delay The ignition delay (point A-B) is the (very short) preparatory phase for combustion, which separates the start of injection from the start of the ignition of the fuel. This delay, which depends on the type of fuel used (cetane index) and on certain physical and chemical phenomena, can be divided into:

    Physical delay: the time during which the fine droplets of diesel fuel heat up in contact with the air until they vaporise (varies according to air temperature, velocity, size of the droplets and fuel viscosity, etc.).

    Chemical delay: during the time preceding the ignition the fuel combines with the oxygen in the air. The delay is between 0.001 and 0.002 seconds. During this phase there is a constant rise in the compression pressure which is proportional to the rotation angle of the crankshaft (10 to 20).

    6.1.5.2. Flame propagation The fuel mixture has formed (point B-C) and the ignition process has begun at a large number of points at an extremely high velocity (presence of a large excess of oxygen and of a mass of atomised fuel during the ignition delay). The combustion speed (1000 to 1200 m/s) defines the pressure rise in the cylinder and the noise resulting from this phase (rapid combustion or uncontrolled phase).

    6.1.5.3. Main combustion The injection continues (point C-D), the fuel progressively continues to burn and the combustion speed falls whereas the pressure and temperature continue to increase. This is the "controlled" combustion phase (it depends on the volume of fuel injected per degree of crankshaft rotation). During this phase the fuel molecules are broken down (cracking) thus producing:

    Gaseous and light products which burn.

    Heavier products (tarry) which are much more difficult to burn.

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    6.1.5.4. Postcombustion or diffused combustion The end of injection (point D-E) takes place at point "D" when the injector closes, but the remaining fuel mixture continues to burn. The conditions are more and more unfavourable:

    Rarefaction of the oxygen.

    Increasing volume of the combustion chamber (pressure and temperature falling sharply, piston descending to BDC).

    Remaining hydrocarbons difficult to burn.

    The duration of this last phase is linked to the two previous phases (the postcombustion will be increased if the atomisation is poor or if the main combustion is bad). The post combustion phase must be as short as possible, all additional time increases the temperature of the exhaust gases and reduces efficiency.

    6.2. SUPERCHARGING The engine power can be increased (for a same rotational speed) by improving the volumetric efficiency of the air in the cylinders. This is done by various processes. Improving the volumetric efficiency during the "induction" stroke:

    Multiplying the number of valves (3 or 4 per cylinder, 2 of which are inlet valves)

    Installing a variable ignition system

    Designing and modifying the air inlet ducts to obtain a feed by "oscillations" or by "resonance".

    Precompressing or "supercharging" the air: it consists of introducing air into the cylinders at a pressure greater than atmospheric pressure. This principle is now used on most diesel engines. Three types of system are principally used and give similar results, in spite of their different designs.

    Mechanically driven volumetric systems.

    Centrifugal systems where a turbine is driven by the exhaust gases and directly coupled to a supercharger (turbocharger).

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    Pressure wave systems: pressure difference between the inlet and the exhaust (also called "pressure exchanger").

    6.2.1. Volumetric supercharger These systems are driven by the engine and are used to obtain a pressure right from the start of the engine acceleration (high torque at low engine speed), but the power consumption is high at high speeds with respect to a limited air flow. The different types of volumetric superchargers are as follows:

    Twin-screw supercharger

    Roots supercharger

    Rotary-piston supercharger

    6.2.1.1. Twin-screw supercharger Two helical screw rotors driven by the engine rotate in opposite directions inside a housing where they force the air to flow by compressing it on the outlet side.

    Figure 19: Twin-screw supercharger

    a, b, c, d: air suction phase e, f, g: delivery phase

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    6.2.1.2. Roots superchargers Their operation is similar to that of the screw system but, instead of screws, counter-rotating lobes are used.

    Figure 20: Roots-type supercharger

    6.2.1.3. Rotary-piston superchargers

    The belt-driven internal rotor rotates eccentrically inside the outer rotor.

    Figure 21: Rotary piston supercharger

    Chambers Position

    1 2 3 a Induction Start of filling Start of expansion b Filled Filling Expansion c Compression End of filling End of expansion d Start of expansion Suction Start of filling

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    6.2.2. Centrifugal superchargers These are more generally called turbochargers. The advantage of this type of supercharger is its small size and therefore its lower weight. However, the main advantage is that it is driven by the kinetic energy of the exhaust gases. Therefore, the engine can be supercharged without using engine power to drive the supercharger.

    Figure 22: Turbocharged engine

    6.2.2.1. Role of the turbocharger To operate, an internal combustion engine uses an air/fuel mixture, this ratio remains constant. To increase the engine power we must increase the quantity of mixture injected into the engine.

    Figure 23: Turbocharger location

    The problem is that the engine cylinders are never 100% filled, they are only filled to 80% or even 85% in the best possible cases.

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    The turbocharger's role is to increase the pressure of the air/fuel mixture in the cylinders to fill them better and thus improve efficiency and power. In fact the engine is supercharged by compressing the air.

    6.2.2.2. Composition of a turbocharger

    Figure 24: Exploded view of a turbocharger A turbocharger is a centrifugal device with a rotor (very accurately balanced in the factory) which rotates at a very high speed (80,000 to 150,000 rpm) and consists of:

    Central housing: (also called the support) it contains the bearings, complete balanced rotor assembly, and the lubrication and cooling system.

    Turbine stage: this is where the combustion gases from the engine cylinders are guided towards an axial impeller. These gases expand and then rotate the turbine impeller before being outlet to the exhaust manifold.

    This part of the turbocharger is subject to high temperatures (>650C) which requires the use of special materials (spheroidal graphite cast iron for the housing, nickel steel for the turbine and ceramic is sometimes now used) and efficient cooling by oil circulation and sometimes by water circulation.

    Compressor stage: the air enters the compressor axially. It is accelerated by the impeller, then diverted through 90 to the diffuser which converts the kinetic energy gained into air pressure and sends it to the inlet manifold. Since the temperatures in this stage are much lower than on the turbine side (80C to 150C), the parts are made of aluminium alloy (impeller, housing).

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    Rotating assembly: the turbine and shaft assembly is called the rotor. The rotor and compressor impeller assembly forms the "rotating assembly" and must be perfectly balanced. This assembly rotates on a frictionless oil film, the sliding bearings fitted in the central housing serve as guides.

    6.2.2.3. Turbocharger operation As we saw above, the turbocharger consists of two parts: on one side there is a turbine driven by the exhaust gases from the engine, and on the other a compressor connected by its shaft to the turbine and placed in the air inlet duct, i.e. upstream of the engine. The turbocharger is also called a centrifugal supercharger. This is because the turbine drives the shaft which rotates the compressor which, due to the effect of the centrifugal force, drives the air towards the periphery and creates a depression at the centre, hence the pressure of the air in the duct increases.

    Figure 25: Turbocharger operation

    6.2.2.4. Disadvantages of a turbocharger The increase in the air pressure causes the temperature of this air to increase. It is therefore necessary to fit an air cooler, or exchanger, between the compressor and the cylinders.

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    Air/air and air/water coolers can be used. In the first case the air is cooled by the outside air, and in the second case it is cooled by the engine cooling system. The major disadvantage of compressing air in a confined volume is that it heats up. The hotter the air, the more it expands and therefore the more space it requires. For this reason an exchanger will be placed just downstream of the compressor to cool the air after it has been compressed by the turbocharger. Therefore, for an equal pressure, there will again be slightly more air in the cylinder.

    Figure 26: Location of the air cooler On small engines (cars, small generator sets, etc.) the air is cooled by an air/air exchanger called an intercooler.

    On large engines the air is often cooled by an air/water exchanger where the water is taken from the engine's cooling system. Figure 27: Air/air exchanger principle (intercooler) Also, the increase in the inlet air pressure increases the pressure of the exhaust gases, i.e. the gases which then drive the turbine. So we enter a vicious circle where the pressure continually increases and the engine would finally explode.

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    Figure 28: Cooling by an intercooler Fortunately, a small device, called a "Waste Gate" prevents this phenomenon. It is placed upstream of the turbine and consists of a spring which, when the pressure becomes too high, is pushed back and opens an alternative duct through which part of the gases can escape without passing through the turbine.

    Figure 29: Waste gate

    The turbocharger increases engine efficiency and reduces fuel consumption. This results in a reduction in the pollutant gas emissions.

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    Figure 30: General diagram of a turbocharger Since the turbocharger is lubricated (in most cases) by engine oil (via the oil pump), we must consider using a good quality oil which is correctly filtered and monitored.

    Figure 31: Supercharging system

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    There are many variants, ranging from single systems to one or two turbochargers, sometimes water-cooled, with a temperature exchanger (air/air or air/water), and with or without maximum inlet pressure control (high pressure: 3 bars).

    6.2.3. Pressure-wave supercharging (COMPREX system) The "COMPREX" process uses the pressure wave generated by a brief contact between the exhaust gas and the inlet air (pressure difference) in the rotor's cells. This system gives a much shorter response time at low engine speed than a turbocharger while retaining comparable performance at high speeds.

    Figure 32: Pressure-wave supercharging

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    7. FUEL SYSTEM The fuel system supplies the injection pump with a sufficient quantity of perfectly filtered, emulsion-free and water-free fuel at a determined pressure. It is also used to stabilise the injection pump temperature and to limit the pressure peaks at the end of injection.

    Figure 33: Fuel system with an in-line pump

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    7.1. SUCTION CIRCUIT The circuit extends from the down pipe in the tank, via the prefilter, up to the fuel pump suction connection. It is only on this part of the circuit that we can encounter faults due to air in the system (incorrectly tightened connection, defective seal, hole in pipe, etc.).

    Figure 34: Suction circuit (and general view)

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    7.2. LOW PRESSURE CIRCUIT From the delivery side of the fuel pump, via the main filter, up to the supply gallery in the injection pump. In this part, any sealing problem results in a leak.

    Figure 35: Low pressure circuit

    7.3. HIGH PRESSURE CIRCUIT From the injection pump outlet to the injectors, it consists of the:

    HP piping and its connections

    Injector holders

    Injectors

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    7.4. FUEL PUMP The fuel supply pressure of a conventional in-line pump varies between 1 bar and 2.5 bars according to the relief valve setting, to guarantee that the pumping components are filled to the maximum, and with a flow rate at least equal to 1.5 to 2 times the injection pump flow rate at nominal full load engine speed. This takes place after drawing the fuel from the tank, through the prefilter and forcing the fuel through the filter element(s) (piston pump or gear pump).

    7.4.1. Diaphragm pumps They are mechanically controlled and generally have a prefiltering bowl.

    Figure 36: Diaphragm pump with prefilter

    7.4.2. Piston pump This is the most commonly used system. The pump is directly mounted on the injection pump and is controlled by the pump's camshaft. It is self-governing due to the piston spring setting value.

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    7.4.2.1. Single-acting fuel pump The injection pump camshaft controls the roller tappet.

    Due to the action of the piston rod (1), the main piston (6) forces the liquid contained in chamber A into chamber B, by opening the valve (7). The volume corresponding to that displaced by the piston rod (1) is sent to the delivery side.

    Figure 37: Mid-travel position

    Since the top of the cam is exceeded, the main spring (3) pushes piston (6) and piston rod (1):

    The fuel in chamber B is sent to the main filter.

    The depression created in chamber A opens the valve (5) and draws in fuel from the tank.

    If the pressure of the diesel fuel at B is less than that exerted by the spring (3), the piston remains in contact with the piston rod (1) and the stroke is complete.

    Figure 38: Suction-Delivery

    If, on the contrary, the diesel fuel pressure at B reaches that exerted by the spring, the piston separates from the piston rod and it only performs a partial stroke. This is the self-governing mechanism.

    Figure 39: Self-governing system

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    7.4.2.2. Double-acting fuel pump (Bosch)

    Under the action of the control cam, the downward movement of the pump piston compresses the spring and opens valves A1 and R1 and sends fuel to the pump. Figure 40: Double-acting pump downstroke

    When the cam lift decreases, the pump moves upwards under the action of the spring and opens valves A2 and R2 and the fuel is sent to the injection pump.

    For each piston movement we obtain a simultaneous suction and delivery of the fuel; the fuel pump is double-acting.

    Figure 41: Double-acting pump upstroke

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    8. INJECTION SYSTEMS The injection pump must deliver a precise volume of fuel to each injector at the right moment and for a determined time via a hydraulic system consisting of a delivery valve, connection and high pressure pipe. However, there are a certain number of conditions to be met:

    The metering must correspond very closely to the engine's requirements (load).

    The metering must be strictly equal for each cylinder.

    The injection must be carried out at a precise moment.

    The injection must take place during a very short lapse of time and without internal leaks.

    The machining precision of the pump, particularly the plungers and cylinders, must be very high.

    The instantaneous pressure reaches a very high value.

    The quantity of fuel to be delivered for each plunger stroke is very variable according to the engine type.

    8.1. INJECTION PUMP (BOSCH)

    8.1.1. General This type of pump is controlled by the camshaft and via roller tappets, the plungers or pumping components have a constant upstroke. This stroke depends on the size of the pump. For example: size A = 7 mm; size MW = 10 mm. The plungers are returned to BDC by springs whose setting depends on the maximum speed of the pump which rotates at half engine speed. The fuel metering is provided by the rotational movement of the plungers with the help of sleeves connected to the adjustable gear sectors which are linked to the control rod (also called control rack".

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    Figure 42: BOSCH injection pump - size A

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    Figure 43: Plunger rotation control mechanism

    8.1.2. Pumping components Inlet of fuel: (filling)

    At BDC the plunger uncovers fuel inlet orifices 0 and 01. The fuel enters chamber V and, via the vertical slot, enters chamber X driven by the supply pressure.

    Prestroke:

    This is the plunger travel between BDC and the start of delivery. Start of delivery:

    The plunger has performed its prestroke and covers inlet orifices 0 and 01.

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    This is the start of delivery and the compressed fuel lifts the delivery valve, thus performing the expansion stroke.

    Working stroke:

    This is the stroke between delivery valve opening and the end of delivery (the delivery orifice is uncovered by the lower part of the helical groove).

    End of delivery:

    As soon as the lower edge of the helical groove uncovers orifice 01 there is a sudden pressure drop and the delivery valve descends onto its seat. The fuel in chambers V and X returns to the supply pressure. The plunger then continues its travel to TDC (cam travel).

    Figure 44: Bosch injection pump plunger

    The quantity of fuel delivered depends on the length of time the plunger covers the delivery orifice 01; this is the working stroke. This time is modified by the plunger's rotation. It varies the end-of-delivery instant determined by the helical groove. The following figures (a, b, c) show the full flow, medium flow and idle positions. In picture d the vertical groove is aligned with orifice 01 and no delivery is possible, this is the stop position.

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    To obtain the desired helical groove position the plunger is rotated using a control mechanism.

    Figure 45: Position of the plungers in the cylinder

    8.1.3. Delivery valve Its role is to quickly reduce the pressure in the HP lines after each injection, to get the injector to close completely while maintaining a determined residual pressure.

    8.1.3.1. Snubber valve Start of delivery: The plunger compresses the fuel, the injection pressure P becomes greater than pressure P' (spring load + residual pressure) and lifts the valve. The fuel flows when the expansion plunger uncovers the orifice. End of delivery: The fuel delivery stops when pressure P becomes less than pressure P. The valve then closes in two phases:

    the lower edge of the plunger comes in contact with the ground part of the seat and the interconnection is interrupted.

    the valve continues to descend until it is fully closed and at the same time draws a new volume into the lines.

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    hdhSV ==4

    2

    V = new suction volume S = surface area of expansion plunger h = descent height

    Figure 46: Snubber valve

    8.1.3.2. Ball valve It consists of two superimposed housings, each containing a ball. At the end of injection the ball in the lower housing falls onto its seat first, creating a pressure drop in the upper housing.

    Figure 47: Ball valve The fuel contained in the delivery lines expands and closes the second ball. The delivery valve (snubber-type or ball-type) normally acts as a nonreturn valve (NRV). It also relieves the pressure in the lines at the end of injection and causes a sudden fuel shutoff to eliminate any droplet effect and any ignition retard called "injection tail-off"; Injection tail-off is the unwanted small injection caused by injection restarting due to an insufficient pressure drop in the fuel lines. To correct this injection tail-off phenomenon the lines must be as rigid as possible and of the same length, and also a greater pressure drop must be created in the injection tube due to the delivery valve.

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    8.2. INJECTORS AND INJECTOR HOLDERS

    8.2.1. Role and operation of the injector The injector (or atomising head) is fixed and placed in a support called the injector holder. It is a high precision component which ensures that the fuel delivered by the injection pump is correctly atomised and distributed in the engine combustion chamber (or the prechamber, depending on the case).

    1: Supply duct 2: Injector holder body 3: Delivery duct 4: Intermediate disc 5: Injector connection nut 6: Delivery duct connection nut 7: Rod filter 8: Fuel leaks recovery connector 9: Valve loading washer 10: Injection pressure spring 11: Push rod 12: Injector positioning needle 13: Injector a: Needle b: Nozzle

    Figure 48: Injector description

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    The injector holder:

    Channels the inlet and return of the diesel fuel

    Supports the nozzle valve spring

    Has an adjustment system for this spring The injector consists of two parts:

    Needle

    Nozzle Operation: The needle is at rest and held on its seat by a spring which abuts in the injector holder. The diesel fuel is inlet via the injector holder in a circular throat (g), then sent to the pressure chamber (V) through the duct (t).

    Figure 49: Detail view of the injector and injector holder When the fuel is delivered by the injection pump, there is a very rapid pressure rise in the pressure chamber (V) until the instant when the needle lifts (this is the start of injection), a force is then applied which is greater than the preload of the pressure spring (R). The fuel is finely atomised until the end of the injection pump delivery, the injector needle then falls back onto its seat and is held in place by the force of the pressure spring (R) and covers the injector nozzle orifice(s) (perfect sealing is essential).

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    Figure 50: Injector holder with pintle nozzle injector

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    8.2.2. Different types of injectors

    8.2.2.1. Single and multi-hole injector This type of injector is normally used on direct injection engines since it is basically used to distribute the fuel.

    Figure 51: Injector with holes open and closed

    This type of injector is used on direct injection engines with high pressure settings (200 to 400 bars). Features:

    Penetration: achieved by high pressure.

    Atomisation: achieved by high pressure.

    Distribution: achieved by the number and configuration of the holes. Manufacture:

    The needle and the nozzle are matched and ground together.

    Manufacturing tolerance: 4 to 6 microns.

    The needle cone is greater than that of the seat to provide good sealing. Disadvantage:

    Highly sensitive to fouling. With direct injection the injector must contribute to the fuel-air mixing due to the almost total lack of turbulence.

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    8.2.2.2. Pintle injector Features:

    Used on indirect injection engines.

    Pressure setting: 100 to 150 bars.

    The pintles can take different forms and different dimensions to vary the distribution.

    Advantages:

    This type of injector is less sensitive to fouling (since there is no small-diameter hole).

    The pintle prevents carbon buildup.

    They are used on swirl-chamber engines because the fuel mixture is prepared by the air swirl and facilitated by the specially-designed injection spray pattern.

    Figure 52: Cylindrical pintle injector The nozzle has a drilled relatively large-diameter central hole and the needle has a slightly smaller-diameter pintle.

    With this system we obtain a conical spray pattern and the dispersion angle depends on the shape of the needle pintle. The pintle also prevents any carbon deposits from building up on the injection hole. Figure 53: Conical pintle injector

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    8.2.2.3. Throttle injector In this type of injector the specific shape of the needle pintle and special spring in the injector holder allow a "preinjection" to be obtained. At the moment of opening, the needle uncovers a narrow annular passage which allows very little fuel to enter (choke effect). As the opening movement progresses (pressure increase), the passage widens and the maximum fuel flow rate is only injected near the end of the needle's travel. This is now the most commonly used type on fast, small-capacity indirect injection engines.

    Figure 54: Throttle injector

    8.2.2.4. Pilot hole injector This is a large overlap pintle (R). Its nozzle has an obliquely drilled capillary hole (P) which terminates under the needle seat (S).

    Figure 55: Injector overlap

    It is used in cylinder heads with prechambers.

    At low engine speeds, and particularly when the engine is being cranked by the starter, the injector slowly lifts and by a value which is often less than its maximum lift: the main part of the flow has time to leak off through the pilot hole since the pintle seal is still intact on its overlap.

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    The injector acts like a hole injector. This injector has a line marked on it which must be positioned facing the inlet coupling on the injector holder.

    8.2.3. Injector assembly and disassembly The injector holder secures the injector into the cylinder head of the engine. It provides the connections with the delivery lines and contains a spring, adjustable by a screw or washers, which determines the injector opening pressure. When fitting the injector to the cylinder head, check the condition of the threads: cylinder head, studs, screws, injector holder, etc. Always replace the sealing parts: copper seals, sealing washers, cup washers, etc. by genuine new parts. Disassembly:

    Note the orientation of the parts before removal

    Slacken the injector holder using a socket

    Unscrew the nut and withdraw the parts. Take care not to drop the needle

    Immerse the removed parts in clean diesel fuel to avoid them being deteriorated

    Reassembly:

    Reassemble by carrying out the disassembly operations in reverse order

    Tighten the nut to the manufacturer's torque loading setting

    Figure 56: Assembling the injector

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    8.2.4. Inspecting the disassembled injectors The injector plays a very important role. It is the first link in the injection system and is responsible for sending the fuel metered by the injection pump into the combustion chamber of each of the engine's cylinders. It is in contact with high temperatures, in very precise pressure, atomisation and orientation conditions which are reliable over time. The operating rate can be more than 2,500 injections per minute on small, fast engines. The injector servicing frequency will be given in the engine owner's manual supplied by the manufacturer. To operate correctly, the correct quality of diesel fuel must be used, it must not contain water or additives and the filtration system must be carefully maintained. The injector holder must be fitted to the cylinder head using the specified torque loading and the engine must not overheat, even for very short periods. The injector setting pressures must be checked at regular intervals.

    8.2.4.1. Sticking needle check When the needle is inserted in the injector body it must descend slowly onto the seat under its own weight. To carry out this check, place the injector body at 45.

    Figure 57: Sticking needle check

    Never touch the injector needle with your fingers: problem of oxidation. The use of an abrasive product for bedding in and grinding needle or injector seats is prohibited.

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    In all cases, the overhaul of an injector must be limited to cleaning operations.

    Clean the injector nozzle with a wooden spatula.

    Clean each injector individually to avoid mixing up the needles and the nozzles.

    Figure 58: Cleaning the injector

    8.2.4.2. Needle lift check The injection quality greatly depends on the lift of the injector needle. The value may increase after a series of repairs. If there is insufficient needle lift there is a risk of insufficient atomisation. However, if there is excessive needle lift the injector becomes fouled with carbon deposits since it closes too slowly, thus allowing combustion gases to enter the injector. To carry out the check, we hold the complete injector in a vice equipped with soft jaws. We place a dial test indicator on the end of the needle and, using very fine-nosed pliers, we measure the possible displacement between its seat and the needle lift gauge. When this last check is completed, reassemble the needle and the body in filtered diesel fuel and leave them in it until the injector is refitted to the injector holder.

    Figure 59: Needle lift

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    8.2.4.3. Calibration check Secure the injector on the test rig. Connect the high pressure line to the injector. Close the pressure gauge stop valve and quickly pump the lever a few times (purge). Open the valve then slowly increase the pressure by pressing the lever until the injector opens.

    Figure 60: Test pump Note the pressure indicated on the pressure gauge and, if a correction is needed, adjust the injector holder adjustment screw or, where applicable, change the thickness of the shims. When checking the injectors, ensure that the fuel spray does not hit your hands since the fuel would penetrate the skin due to the high pressure and could cause serious injuries. Test pump operating precautions:

    Close the valve to isolate the pressure gauge before disconnecting an injector holder which has just been checked.

    When the injector holder is disconnected, gradually open the valve so that the needle of the pressure gauge slowly falls until it returns onto its stop.

    Any sudden pressure drop can distort the pressure gauge reading.

    8.2.4.4. Spray pattern check

    Close the pressure gauge valve.

    Pump the lever rapidly (1 to 2 strokes per second).

    Observe the injector spray pattern.

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    The atomisation must be fine, uniform and symmetrical. The injector must make a humming sound.

    Figure 61: Spray pattern check

    Unwanted jets may sometimes appear during the injector check. If this is the case, close the pressure gauge valve and pump relatively rapidly so that the injector operates, then return to the initial situation and check the spray pattern again. If the spray pattern is still incorrect, replace the injector.

    Figure 62: Unwanted jets

    8.2.4.5. Injector seat sealing

    Wipe the end of the injector

    Increase the pressure to a value less than the injector opening pressure (10 bars less).

    Maintain this pressure for around 10 seconds, there must be no drips from the injector

    If the injector is not sealed, a drop will quickly form at its tip (if this is the case, it should be replaced)

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    8.2.4.6. Refitting the injector holder Replace the copper seal (at the injector tip) and the wave sealing washer (some models are equipped with them).

    Figure 63: Detail view of injector tip seals

    Fit the injector in the cylinder head until it is hand tight

    Screw up the injection line couplings by hand on the injection pump, then on the injector holder to centre the end fittings

    Moderately tighten the couplings on the pump then on the injector holder

    Tighten the injector holder in the cylinder head

    Connect the return line

    If a connection leaks, do not tighten it any further. Unscrew the connection and then screw it up again.

    Figure 64: Different injector tip seals

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    8.3. PUMP-INJECTORS AND ELECTRONIC SYSTEMS

    8.3.1. Pump-injectors

    8.3.1.1. Mechanical pump-injectors These injectors have been known and used by some American automotive manufacturers (GM Detroit Diesel, Caterpillar) for a long time. This type of injection is controlled by the engine camshaft and therefore requires specially adapted cylinder heads but has a major argument in its favour: the injection pressure can exceed 1200 bars.

    1. Adjustment screw 2. Rocker

    3. Mounting flange

    4. Pushrod 5. Control rod 6. Delivery element 7. Roller pushrod 8. Injector camshaft

    Figure 65: Caterpillar pump-injector system

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    8.3.1.2. Electronically controlled pump-injectors The search for power linked with depollution needs for new diesel engines, requires higher and higher injection pressures and favours the development of this product but, of course, no longer with mechanically controlled metering but using an electronic system (variable start, end, duration, injection point individually controlled for each cylinder).

    Figure 66: Electronically controlled pump-injector system

    8.3.2. New diesel injection system The conventional injection systems, now assisted by electronics, are well-implanted with the different engine manufacturers.

    Figure 67: Operation of the electromagnetic injector

    However, future development is already in progress and some totally new "high pressure" systems (without injection pump) are now being tested. These operate at over 1000 bars, with individually electronically controlled electromagnetic injectors allowing a variable injection law to be obtained.

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    Figure 68: "Nippondenso" high pressure system

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    9. DESCRIPTION OF DIESEL ENGINES

    9.1. CYLINDER BLOCK-CYLINDER HEAD ASSEMBLY The cylinder block-cylinder head assembly is a rigid assembly which serves as the bearing point for the internal and external moving components, and some external components (starter, water pump, etc.) are attached to it.

    The cylinder block supports the crankshaft. It has to withstand thrusts, torsions and vibrations.

    The cylinder guides the plunger. It has to withstand pressure, heat and friction.

    The cylinder head forms the top part of the combustion chamber. It also has to withstand pressure and high temperature.

    The assembly must have a good thermal conductivity to quickly eliminate the excess heat. The cylinder block-cylinder head assembly also carries the timing components, and contains the oil and coolant ducts.

    9.1.1. Cylinder block (also known as the engine block) The block is normally made of cast iron or cast aluminium. It is the main body of the engine. Its upper part is machined to form the cylinders or liner housings if the engine has liners. Its top part is machined to form the contact face with the cylinder head: the cylinder head sits on the top contact face to cap the cylinders.

    Figure 69: Cylinder block

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    9.1.1.1. The different types of cylinder blocks Directly bored cylinder block: Some blocks are directly bored, the cylinders and block are in a single piece. If the cylinders wear, the block must be rebored to a higher (oversize) dimension and larger (oversized) pistons must then be used. Block with liners: This system makes the blocks easier to manufacture. It is the system generally found on most of today's industrial engines. It allows different materials to be used for the block and for the cylinders (cast iron cylinders, light alloy block). They are easy to repair (they can be replaced by new liners).

    Figure 70: Block for cylinder liners

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    9.1.1.2. The different types of liners Liners are subject to mechanical fatigue due to the:

    pressure and temperature of the gases

    cooling

    friction of the piston

    reaction of the guides. The metal used to manufacture liners is normally cast iron but steel is sometime used. To reduce wear and friction the inner wall or the liner is given superficial surface treatments (nitriding and, above all, chroming). When the liner is in place, it must seal the combustion chamber and the cooling space (for water-cooled engines). Wet liner: The liners form the walls of the cooling ducts. They are easy to replace but special care must be taken with their sealing. The liners are made of centrifuged cast iron, they are also bored, ground and honed.

    Figure 71: Wet liner

    Dry liner: Thin sleeve pressed into a cast iron or light alloy block.

    It is possible to replace them but they are a tight fit. There is no contact with the coolant. Figure 72: Dry liner

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    9.1.2. Cylinder head The cylinder head closes off the top part of the cylinders, thus forming the combustion chamber. Functions:

    Inlet and outlet of the gases.

    Positioning of the ignition timing components.

    Fast heat removal; the combustion chamber is the hottest part of the engine. The cylinder head is made of cast iron or cast aluminium alloy. Since the mechanical stresses are lower than those of the engine block, the car manufactures have almost totally abandoned cast iron in favour of aluminium, due to its lightness and its very good heat conduction (depending on the size of the engine). The cylinder head has a network of water and oil ducts, and the cylinder block-cylinder head sealing is provided by the cylinder head gasket.

    Figure 73: Detail view of a cylinder head

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    Depending on the engine size and power there may be one cylinder head per cylinder, or one cylinder head for several cylinders.

    Figure 74: Cylinder head for several cylinders

    9.1.3. Rockers Rockers are mechanical parts designed to transmit a movement by changing and reversing its direction.

    Figure 75: A rocker It is a rocking lever which is part of the timing control system reciprocating secondary transmission.

    The transmission is ensured by pivoting around a shaft. Figure 76: A rocker shaft assembly

    A valve is opened by the pressure of the rocker on the heel of the valve. Excessive clearance causes chatter at idle, which can eventually deteriorate the valve heel.

    Figure 77: Valve clearance

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    Insufficient clearance may cause a loss of compression which can rapidly deteriorate the valve seat. It is thus vital that this clearance be regularly checked and adjusted.

    9.1.3.1. Pushrods A pushrod is a timing control component which transmits the cam movement to the rocker e.g. in an overhead valve engine.

    Figure 78: Pushrod

    It generally has a spherical end which sits in the bottom of the tappet, and the valve clearance adjustment screw on the rocker pivots in the cup on its other end. Figure 79: Examples of pushrods

    But it can also have rounded ends (depending on the angle between the rod and the rocker)

    Figure 80: Detail view of the pushrod ends

    9.1.3.2. Tappets Tappets are inserted between the cams and the valves.

    Tappets and pushrods may have to be inserted in the valve train assembly, depending on the position of the camshaft with respect to the valves. The tappets form the link between the rotational movement of the cams and the straight-line movement of the valves. Figure 81: A tappet

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    9.1.4. Camshaft The camshaft is a mechanical device which converts a rotary movement into a longitudinal movement and vice versa. It is one of the most important timing components. It must lift the valves very precisely for a well-determined period which corresponds to the engine timing diagram.

    Figure 82: The camshaft

    It must resist the twisting movement resulting from the pressure of the springs and resist the wear due to friction. The camshaft can be made of a special cast iron casting or made of forged or case-hardened steel.

    It is used in 4-stroke internal combustion engines for the synchronised control of the valves. It consists of a cylindrical rod which has the same number of cams on it as there are valves to be controlled independently or in groups, sliding on the end of the valve stem or on a rocker. It is located beside the crankshaft (overhead valve engine or side valve engine) or on top of the cylinder head (overhead camshaft engine).

    Figure 83: Overhead camshaft engine

    9.1.5. Valves At first sight the valve seems to be a very simple component but, in reality, it is very complex. It is subject to enormous thermal and mechanical stresses. The valves are used to open and close a chamber or a passage when required. They are essential to the operation of four-stroke engines. They are operated by the camshaft and have one or more individual return springs.

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    They must have a large head . This dimension is limited by the free space in the combustion chamber, the valve's weight which must be as low as possible, and by its mechanical strength to resist impacts and distortions.

    Figure 84: Valves

    The tapered bearing surface provides perfect sealing when the valve closes and correct centring which prevents distortion of the valve stem. Figure 85: Detail view of a valve The bearing surface angles are approximately 90.

    The inlet valves which are subjected to lower temperatures can have an angle of 120 which provides less protection against distortion but gives a larger gas port cross-sectional area for a same lift height. Valves are made of a special high-strength steel but are sometimes hollow and sodium-filled to evacuate the heat more efficiently from the head to the stem. They consist of three parts:

    The heel, on which the valve timing gear pushes (in the case of an engine with rockers, it is the rocker which performs this function).

    The stem and the head which sit on a conical-section seat machined in the metal of the eng