EXP-PR-EQ110 Rechauffeurs Et Fours

MECHANICAL DIESEL ENGINES

TRAINING MANUAL COURSE EXP-SM100 Revision 0

Field Operations Training Mechanical Diesel Engines

MECHANICAL DIESEL ENGINES CONTENTS

1. OBJECTIVE OBJECTIVES S ...................... .................................. ....................... ....................... ....................... ....................... ....................... ....................... .......................6 ...........6 2. INTRODUCTI INTRODUCTION ON ...................... .................................. ....................... ....................... ....................... ....................... ....................... ....................... ..................7 ......7 3. REMINDERS REMINDERS ...................... .................................. ........................ ....................... ....................... ........................ ....................... ....................... .......................9 ...........9 3.1. TERMINOLOGY......................................................................................................10 3.2. CHARACTERIS CHARACTERISTICS TICS ....................... .................................. ...................... ....................... ....................... ...................... ...................... ...............11 ....11 3.2.1. Geometrical characteristics ............ ......................... ......................... ......................... .......................... .......................... ..............11 .11 3.2.2. Combustion Combustion characteri characteristics stics ...................... ................................. ....................... ....................... ...................... .....................11 ..........11 3.2.3. 3.2.3. Engine Engine characteris characteristics tics ....................... ................................... ....................... ....................... ....................... ...................... ................12 .....12 3.3. GENERAL GENERAL ....................... ................................... ....................... ....................... ....................... ...................... ....................... ....................... ...................13 ........13 3.3.1. Comparison of diesel and petrol engines ........... ........................ ......................... ......................... .................... .......13 13 3.3.2. Advantages Advantages of the diesel diesel engi engine ne ....................... .................................. ....................... ....................... .......................14 ............14 3.3.3. Disadvantages of the diesel engine.................................................................14 4. DIESEL ENGINE CYCLE (4-STROKE) ............ ......................... .......................... ......................... ......................... ....................... .......... 15 4.1. THEORETIC THEORETICAL AL CYCLE ....................... ................................... ....................... ....................... ....................... ....................... .....................15 .........15 4.2. REAL CYCLE CYCLE ...................... .................................. ....................... ....................... ....................... ....................... ....................... ....................... ...............16 ...16 5. CLASSIFICATION OF DIESEL ENGINES.....................................................................18 5.1. DIRECT DIRECT INJECTION INJECTION ..................... ................................. ....................... ...................... ...................... ....................... ....................... .................19 ......19 5.2. INDIRECT INDIRECT INJECTION INJECTION ...................... ................................. ....................... ....................... ...................... ....................... ....................... ............20 .20 5.2.1. 5.2.1. Engine Engine with precombus precombustion tion cham chamber ber ...................... ................................. ....................... ....................... .................20 ......20 5.2.2. Engine with swirl chamber...............................................................................21 5.2.3. Engine with air chamber ............ ........................ ......................... ......................... ......................... .......................... .................... .......22 22 6. COMBUSTIO COMBUSTION N AND SUPERCHAR SUPERCHARGING GING ...................... ................................. ....................... ....................... ....................... .............23 .23 6.1. FUELS USED..........................................................................................................23 6.1.1. General............................................................................................................23 6.1.2. Characteristics of diesel oil..............................................................................23 6.1.3. The comb combustion ustion process process ....................... .................................. ....................... ....................... ....................... ....................... ............24 .24 6.1.4. Compressi Compression on of the air ...................... ................................. ...................... ....................... ....................... ...................... .................24 ......24 6.1.5. Combustio Combustion n anal analysis ysis ...................... .................................. ....................... ....................... ....................... ....................... ...................25 .......25 6.1.5.1. 6.1.5.1. Ignition Ignition delay........... delay ...................... ....................... ....................... ....................... ....................... ....................... ....................... ..............26 ...26 6.1.5.2. 6.1.5.2. Flame propagatio propagation n ...................... ................................. ....................... ....................... ....................... ....................... .................26 ......26 6.1.5.3. 6.1.5.3. Main combu combustion stion.......... ...................... ....................... ....................... ....................... ....................... ....................... ....................26 .........26 6.1.5.4. Postcombustion or diffused combustion.....................................................27 6.2. SUPERCHARGING.................................................................................................27 6.2.1. Volumetric Volumetric supe superchar rcharger ger ...................... ................................. ....................... ....................... ....................... ....................... .............28 ..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 Centrifugal supe supercharg rchargers ers ...................... .................................. ....................... ...................... ...................... .......................30 ............30 6.2.2.1. 6.2.2.1. Role of the turbocharg turbocharger er ...................... .................................. ....................... ....................... ....................... ...................30 ........30 6.2.2.2. Composition of a turbocharger ............ ........................ ......................... ......................... ......................... .................. .....31 31 6.2.2.3. Turbocharger operation..............................................................................32 6.2.2.4. Disadvantages of a turbocharger ............ ........................ ......................... .......................... ......................... .............32 .32 Training manual: EXP-SM100-EN Last revised: 14/08/2009

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MECHANICAL DIESEL ENGINES CONTENTS

1. OBJECTIVE OBJECTIVES S ...................... .................................. ....................... ....................... ....................... ....................... ....................... ....................... .......................6 ...........6 2. INTRODUCTI INTRODUCTION ON ...................... .................................. ....................... ....................... ....................... ....................... ....................... ....................... ..................7 ......7 3. REMINDERS REMINDERS ...................... .................................. ........................ ....................... ....................... ........................ ....................... ....................... .......................9 ...........9 3.1. TERMINOLOGY......................................................................................................10 3.2. CHARACTERIS CHARACTERISTICS TICS ....................... .................................. ...................... ....................... ....................... ...................... ...................... ...............11 ....11 3.2.1. Geometrical characteristics ............ ......................... ......................... ......................... .......................... .......................... ..............11 .11 3.2.2. Combustion Combustion characteri characteristics stics ...................... ................................. ....................... ....................... ...................... .....................11 ..........11 3.2.3. 3.2.3. Engine Engine characteris characteristics tics ....................... ................................... ....................... ....................... ....................... ...................... ................12 .....12 3.3. GENERAL GENERAL ....................... ................................... ....................... ....................... ....................... ...................... ....................... ....................... ...................13 ........13 3.3.1. Comparison of diesel and petrol engines ........... ........................ ......................... ......................... .................... .......13 13 3.3.2. Advantages Advantages of the diesel diesel engi engine ne ....................... .................................. ....................... ....................... .......................14 ............14 3.3.3. Disadvantages of the diesel engine.................................................................14 4. DIESEL ENGINE CYCLE (4-STROKE) ............ ......................... .......................... ......................... ......................... ....................... .......... 15 4.1. THEORETIC THEORETICAL AL CYCLE ....................... ................................... ....................... ....................... ....................... ....................... .....................15 .........15 4.2. REAL CYCLE CYCLE ...................... .................................. ....................... ....................... ....................... ....................... ....................... ....................... ...............16 ...16 5. CLASSIFICATION OF DIESEL ENGINES.....................................................................18 5.1. DIRECT DIRECT INJECTION INJECTION ..................... ................................. ....................... ...................... ...................... ....................... ....................... .................19 ......19 5.2. INDIRECT INDIRECT INJECTION INJECTION ...................... ................................. ....................... ....................... ...................... ....................... ....................... ............20 .20 5.2.1. 5.2.1. Engine Engine with precombus precombustion tion cham chamber ber ...................... ................................. ....................... ....................... .................20 ......20 5.2.2. Engine with swirl chamber...............................................................................21 5.2.3. Engine with air chamber ............ ........................ ......................... ......................... ......................... .......................... .................... .......22 22 6. COMBUSTIO COMBUSTION N AND SUPERCHAR SUPERCHARGING GING ...................... ................................. ....................... ....................... ....................... .............23 .23 6.1. FUELS USED..........................................................................................................23 6.1.1. General............................................................................................................23 6.1.2. Characteristics of diesel oil..............................................................................23 6.1.3. The comb combustion ustion process process ....................... .................................. ....................... ....................... ....................... ....................... ............24 .24 6.1.4. Compressi Compression on of the air ...................... ................................. ...................... ....................... ....................... ...................... .................24 ......24 6.1.5. Combustio Combustion n anal analysis ysis ...................... .................................. ....................... ....................... ....................... ....................... ...................25 .......25 6.1.5.1. 6.1.5.1. Ignition Ignition delay........... delay ...................... ....................... ....................... ....................... ....................... ....................... ....................... ..............26 ...26 6.1.5.2. 6.1.5.2. Flame propagatio propagation n ...................... ................................. ....................... ....................... ....................... ....................... .................26 ......26 6.1.5.3. 6.1.5.3. Main combu combustion stion.......... ...................... ....................... ....................... ....................... ....................... ....................... ....................26 .........26 6.1.5.4. Postcombustion or diffused combustion.....................................................27 6.2. SUPERCHARGING.................................................................................................27 6.2.1. Volumetric Volumetric supe superchar rcharger ger ...................... ................................. ....................... ....................... ....................... ....................... .............28 ..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 Centrifugal supe supercharg rchargers ers ...................... .................................. ....................... ...................... ...................... .......................30 ............30 6.2.2.1. 6.2.2.1. Role of the turbocharg turbocharger er ...................... .................................. ....................... ....................... ....................... ...................30 ........30 6.2.2.2. Composition of a turbocharger ............ ........................ ......................... ......................... ......................... .................. .....31 31 6.2.2.3. Turbocharger operation..............................................................................32 6.2.2.4. Disadvantages of a turbocharger ............ ........................ ......................... .......................... ......................... .............32 .32 Training manual: EXP-SM100-EN Last revised: 14/08/2009

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6.2.3. Pressure-wave supercharging (COMPREX system) ............. .......................... .......................... ............. 36 7. FUEL SYSTEM SYSTEM........... ....................... ....................... ....................... ....................... ...................... ....................... ....................... ....................... ...................37 .......37 7.1. SUCTION CIRCUIT.................................................................................................38 7.2. LOW PRESSURE CIRCUIT....................................................................................39 7.3. HIGH PRESSURE PRESSURE CIRCUIT CIRCUIT ...................... ................................. ...................... ....................... ....................... ...................... ................39 .....39 7.4. FUEL PUMP............................................................................................................40 7.4.1. Diaphragm Diaphragm pumps ....................... .................................. ....................... ....................... ....................... ....................... ......................40 ...........40 7.4.2. Piston Piston pump ...................... ................................. ...................... ....................... ....................... ...................... ...................... ......................40 ...........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 Delivery valve ...................... ................................. ...................... ....................... ....................... ...................... ....................... ....................47 ........47 8.1.3.1. 8.1.3.1. Snub Snubber ber valve ....................... .................................. ....................... ....................... ....................... ....................... ...................... ............47 .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 52 8.2.2.2. Pintle injector..............................................................................................53 8.2.2.3. 8.2.2.3. Throttle Throttle injector injector ...................... ................................. ....................... ....................... ....................... ....................... ......................54 ...........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. 8.2.4.1. Sticking Sticking needle needle check ...................... ................................. ...................... ....................... ....................... ...................... .............56 ..56 8.2.4.2. Needle lift check.........................................................................................57 8.2.4.3. Calibration check....................... check.......... ......................... ......................... .......................... .......................... .......................... ............. 58 8.2.4.4. 8.2.4.4. Spray pattern pattern check ....................... .................................. ...................... ....................... ....................... ...................... ...............58 ....58 8.2.4.5. 8.2.4.5. Injector Injector seat sealing sealing ...................... .................................. ....................... ....................... ....................... ....................... ...............59 ...59 8.2.4.6. Refitting the injector holder.........................................................................60 8.3. PUMP-INJECTORS AND ELECTRONIC SYSTEMS..............................................61 8.3.1. Pump Pump-injec -injectors tors ..................... ................................ ....................... ....................... ...................... ....................... ....................... ..................61 .......61 8.3.1.1. Mechanical pump-injectors.........................................................................61 8.3.1.2. Electronically controlled pump-injectors ........... ........................ .......................... .......................... ................ ...62 62 8.3.2. New diesel diesel injection injection system ...................... ................................. ...................... ....................... ....................... ....................62 .........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 65 9.1.1.2. The different types of liners........................................................................66 9.1.2. Cylinder Cylinder head ...................... ................................. ....................... ....................... ....................... ....................... ....................... ...................67 .......67 9.1.3. Rockers Rockers ....................... .................................. ....................... ....................... ....................... ....................... ....................... ....................... ...............68 ....68 9.1.3.1. 9.1.3.1. Pushrods Pushrods.......... ...................... ....................... ...................... ...................... ....................... ....................... ...................... ......................69 ...........69 9.1.3.2. 9.1.3.2. Tappets Tappets ...................... .................................. ....................... ....................... ....................... ...................... ....................... .......................69 ...........69 9.1.4. Camshaft Camshaft ...................... .................................. ....................... ....................... ....................... ....................... ....................... ....................... ..............70 ..70 9.1.5. Valves..............................................................................................................70 Training manual: EXP-SM100-EN Last revised: 14/08/2009

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

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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

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

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

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This is made possible by the use of a very high compression ratio allowing a temperature of around 600°C 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. Training manual: EXP-SM100-EN Last revised: 14/08/2009

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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 Ø 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:

V =

π × D

4

2

× C

Total cylinder capacity: Vt = V x number of cylinders

Dead volume: VB Total volume:

ε

=

V A V B

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 ( 600°C) ≈

Stroke in the cycle

1 Induction

2 Compression

End of compression stroke

Functions performed in a diesel engine

Air sucked into the cylinder

Components in operation

Functions performed in a petrol engine

Inlet valve

Petrol/air mixture drawn into the cylinder

Inlet valves Carburettor or injectors

Mixture compressed (8 to 12 bars) hence the heating to 300°C approx.

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

Fuel injected at high pressure which spontaneously ignites in contact with the hot air

Components in operation

Injection pump Injectors

Mixture ignited by electric spark from the spark plug

Distributor or magneto Spark plugs

3 Power (combustion or explosion)

4 Exhaust

Combustion and expansion

Evacuation of the burnt gases

Combustion and expansion

Exhaust valves

Evacuation of the burnt gases

Exhaust valves

Table 1: Comparison of diesel and petrol engines Training manual: EXP-SM100-EN Last revised: 14/08/2009

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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 600°C: 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. Training manual: EXP-SM100-EN Last revised: 14/08/2009

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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/dm 3 at 15°C) 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 200°C and ends at approximately 370°C. 2 Viscosity: around 9.5 mm /s at 20°C.

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 500ºC 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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