C8 CI Engines

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15

The compression ignition enginee

diesel engine

Topics covered in thischapter

The four-stroke compression ignition engine (CIE)

The Diesel cycle and the dual combustion cycle

The two-stroke CIEDiesel engine construction

Direct injection engine

Indirect injection engine

Turbulence

Induction stroke turbulence

Combustion in a compression ignition engine

Three phases of combustion

Diesel fuel and products of combustion

Flash point

Pour point

Cloud point

Products of combustion

Emissions limits

Emissions control on CIE

Particulate trap and selective catalyst reduction

Exhaust gas recirculation (EGR)

Rudolf Diesel (1858e1913) is generally accepted as the

person who first developed an internal combustion engine

that worked by injecting fuel into compressed air that

was hot enough to ignite the fuel. His patent used these

words: Compressing in a cylinder pure air to such an

extent that the temperature thereby produced is far higher

than the burning or igniting point of the fuel. The orig-inal diesel engines of the type shown in Fig. 15.1(a)

were large and heavy, and they operated at slow speed;

they were used for stationary engines and ships but

were not considered suitable for use in road vehicles.

Around the 1920s, developments in fuel injection

equipment and other technologies led to the develop-

ment of engines of the type shown in Fig. 15.1(b) that

were suitable for use in road vehicles e these engines

work on a cycle of operations that is known as the

dual combustion cycle. For this reason some authorities

suggest that the vehicle engines that work on the prin-

ciple of compression ignition should be called compres-

sion ignition engines (CIEs). In modern literature the

terms are used interchangeably.

The four-stroke compressionignition engine

The four strokes (Fig. 15.2) are:

1. The inlet valve is opened and pure air is drawn into

the cylinder as the piston moves down. This is the

induction stroke.

2. Both of the valves are closed and the piston moves up

the cylinder, compressing the air so that the

temperature rises above the ignition point of the fuel.

The high pressure and temperature is achieved by

a high compression ratio of approximately 20:1. As

the piston approaches top dead centre (TDC), fuel is

injected so that ignition has started by the time that

the piston starts on the next stroke.

3. Both of the valves are closed and the piston is forced

down the cylinder by the expanding gas and fuel

injection continues for a short period. The fuel is shut

off after a few degrees of crank rotation and the high

pressure of the gas forces the piston down the

cylinder on the power stroke.

4. The exhaust valve is opened and the piston rises in

the cylinder, expelling the spent gas through the

exhaust port e at the end of this stroke the engine is

ready to start the next cycle.

The Diesel cycle and the dualcombustion cycle

The difference between the two theoretical cycles can

be seen in pressureevolume diagrams, which are graphs

that are used to show how the gas pressure and the

cylinder volume are related as the piston moves along

the cylinder.

2011 Allan Bonnick and Derek Newbold. Published by Elsevier Ltd. All rights reserved


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The Diesel cycle

Inthe Dieselcycle the fuel is injectedinto the hot air inthe

cylinder and the pressureremains constantuntil fuel injec-

tionceases.Thisis represented bythe linefrom1 to2 inthe

pressureevolume diagram shown in Fig. 15.3(a). At point

2 the gas in the cylinder continues to expand, pushing the

piston along the remainder of the power stroke. Because

the combustion takes place while the pressure remains

constant, the Diesel cycle is also known as the constant-

pressurecycle. This cycle is close to thesequence of eventsthat takes place in very large diesel engines that operate at

slow speeds of a few hundred revolutions per minute.

The dual combustion cycle

In the dual combustion cycle (Fig. 15.3(b)) the fuel

injection starts at point 1; this causes the pressure to

rise rapidly, as shown by the vertical line that ends at

point 2. In this first stage the combustion has taken place

while the volume in the cylinder remains constant.

Between points 2 and 3 further injection takes place,

pushing the piston along the cylinder, while the pressure

remains constant and the gas expands, pushing the

piston down the cylinder. At point 3 combustion ceasesand the gas in the cylinder continues to expand, doing

work on the piston until it reaches the end of the stroke.

(a) (b)

Fig. 15.1 (a) Diesel engine, 1897 (Mirlees). (b) Diesel engine, 2005 (MAN)

Inlet valve Injector

First stroke Induction.Inlet valve open pure airdrawn into cylinder as pistonmoves down the stroke.

Second stroke Compression.Inlet and exhaust valvesclosed. Piston moves up thestroke. Air is compresseduntil it is very hot. Fuelsprayed into cylinder andignites.

Third stroke Power.Both valves closed. Further fuelinjected. The burning fuel raisesthe pressure in the cylinder andforces the piston down the cylinderon the power stroke.

Fourth stroke Exhaust.Exhaust valve opens. Pistonraises in the cylinder andexpels the spent gas. Exhaustvalve closes at end of strokeand inlet valve opens readyto start the next cycle.

Fig. 15.2 The four-stroke compression ignition cycle

The compression ignition engine e diesel engine 169


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This theoretical cycle is considered to be close to the

cycle of events that takes place in the compression igni-

tion engines used in motor vehicles. Indicator diagrams

for the Diesel cycle and the dual combustion cycle are

shown in Fig. 15.3.

The two-stroke CIEThe two-stroke CIE shown in Fig. 15.4 employs end-to-

end scavenging, where there is a compressor that feeds

compressed air to a ring of ports around the circumfer-

ence of the cylinder and a pair of exhaust valves that

are operated by a camshaft. The air ports are controlled

by the piston. When the piston is at bottom dead centre

the air ports are uncovered and air under pressure enters

the cylinder. At this stage the cylinder is occupied by the

exhausted gas from the previous power stroke e the

entering fresh air is denser and at higher pressure than

the exhaust and this, coupled with the upward motion

of the piston, pushes the exhaust gas out through the

open exhaust valves. As the piston rises further it covers

the air ports; at this stage the exhaust valves close and the

compression process begins. Towards the end of the

compression stage, fuel is injected and combustion takes

place to drive the piston down the cylinder on the power

stage of the cycle. At the end of the compression stage the

air ports are uncovered, the exhaust valves are opened,

and the process is completed in two strokes of the piston.

The compressor is similar to a supercharger and it is nor-mally gear driven from the crankshaft.

Diesel engine construction

Much of the diesel engine mechanism and structural

details are similar to those found in petrol engines. The

principal differences are concerned with the way in

which combustion takes place and the stronger

components that are required to cope with the high pres-

sures that are needed to produce the temperature required

for combustion. Diesel engines can conveniently be

divided into two types:

1. Direct injection engines

2. Indirect injection engines.

Fig. 15.4 Principle of the two-stroke CIE

(a) (b)

TDC

1 2

BDC

Diesel Cycle

Power

Comp

Pressure

bar

1

2 3

TDC BDC

Dual Combustion Cycle

Power

Comp

Pressure

bar

Fig. 15.3 Diesel and dual combustion cycles

170 A Practical Approach to Motor Vehicle Engineering and Maintenance


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Direct injection engines

In the direct injection engine shown in Fig. 15.5 the fuel

is sprayed directly into the cylinder. The circular space

in the piston crown forms part of the combustion

chamber; it is designed to produce turbulence when

air is forced in towards the end of the compression

stroke. This type of turbulence is known as squish

turbulence because it is produced by the squashingof air as the piston forces the air down into the piston

cavity. The injector is normally of the multi-hole type

and operates at a pressure of approximately 180 bar e

the compression ratio of the direct injection engine is

of the order of 16:1, which is somewhat lower than

that used in indirect injection engines.

Indirect injection engines

In the indirect engine (Fig. 15.6) the fuel is sprayed

into a small pre-combustion chamber that is placed in

the cylinder head above the piston. As the piston

approaches TDC, air is forced into the pre-combustionchamber, which is designed to produce the swirling

action necessary for good combustion.

The combustion that starts in the pre-chamber rapidly

heats the air and the burning fuel, and air is forced into

the space at the top of the piston, where it mixes with the

main body of air to complete the combustion process.

The injection pressure in the indirect injection engine

is of the order of 120 bar e compression ratios between

22:1 and 28:1 are normal in these engines.

TurbulenceTurbulence is required to ensure that all droplets of fuel

are surrounded by sufficient air to provide the oxygen

that is required for efficient combustion. There are

basically two methods of creating turbulence:

1. Turbulence created on the induction stroke

2. Turbulence created on the compression stroke.

Induction stroke turbulence

Figure 15.7(a) shows the arrangement of the inductionports on a modern diesel engine. The tangential port is

designed to set up a rotary motion in the air as it passes

through the port into the cylinder on the induction

stroke, as indicated in Fig. 15.7(b).

Circularcombustionchamber inthe pistoncrown

Coolentjacket

Squish

Air swirl

Piston

InjectorCylinder head

Direct injection

Fig. 15.5 A direct injection engine



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Special shape inthe piston crowninduces swirl asburning mixtureleaves the pre-combustion chamber

Cylinderblock

Piston

Port

Indirect injection

Pre-combustionchamber

Injector

Fig. 15.6 An indirect injection system

1. Exhaust port2. Exhaust valve3. Fuel injector4. Inlet swirl port5. Inlet valve6. Tangential inlet port7. Heater plug

3

4

5

6

7

(a) (b)

2

1

Fig. 15.7 A tangential inlet port



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Combustion in a compressionignition engine

The power output of a compression ignition engine is

determined by the amount of fuel that is injected e

for low power output such as engine idling a very small

amount of fuel is injected, while for high power output

a large amount of fuel is required. The amount of fuelinjected is determined by the length of time for which

the fuel is injected and this is controlled by the design

of the fuel injection system, which is covered in a later

section. For the time being I wish to concentrate on the

process of combustion.

Three phases of combustion

Sir Harry Ricardo, the founder of the Ricardo research

laboratories at Shoreham in Sussex, first put forward

the idea that combustion in a compression ignition engine

takes place in three separate phases. The graph in

Fig. 15.8 shows how pressure, temperature, and heatrelease from combustion changes from the point at which

injection of fuel starts. Images (a)e(d) are photographs of

the cylinder contents from the start of injection at about

258 before TDC, through to full combustion, which

continues some way down the power stroke. The three

phases of combustion shown in Fig. 15.8 are:

1. The first phase, from (a) to (b). This is known as the

delay period. In this period the fuel is sprayed into the

dense high-pressure and high-temperature air, and

a small period of time elapses during which the tiny

particles of fuel are being evaporated. The resultantfuel vapour must then be brought into contact with

oxygen so that combustion can start. The length of the

delay period depends on several factors, such as:

The ignition quality of the fuel (cetane rating).

The relative velocity between the fuel and the air

in the cylinder (turbulence).

The fineness of the atomization of the fuel.

The airefuel ratio.

The temperature and pressure of the air in the

cylinder.

The presence of residual exhaust gas from the

previous cycle.

2. The second phase, from (b) to (c). This is the periodwhen combustion spreads rapidly through the

combustion space, leading to a rapid rise in pressure.

The rate at which pressure rises in this phase governs

a. Start of injection

b. Combustion begins

c. Peak rate of heat release

Crank angle (degrees)

-30 -20 -10

a

b

d

c

Maximum gas pressureapprox. 55 bar

Max gas temperatureapprox. 1850K

10TDC 20 30 40 50 60

d. Combustion by turbulentdiffusion

Fig. 15.8 Compression ignition engine combustion (Lucas CAV)



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the extent of combustion knock, which is a feature of

compression ignition engines and is known as diesel

knock.

3. The third phase, from (c) to (d), is the period when

combustion is fully operational and the flame spreads

to encompass all of the fuel. In this phase the

pressure continues to rise at a more gradual rate until

injection ceases a few degrees later. After this the

expansive working effect of the gas drives the pistondown the remainder of the power stroke.

Diesel fuel and products ofcombustion

Diesel fuel

Diesel fuel has a calorific value of approximately

45 MJ/kg and a specific gravity of about 0.8 g/cm3.

The ignition quality of diesel fuel is denoted by the

cetane number; a figure of 50 indicates good ignition

properties. Among other properties of diesel fuel thataffect normal operation are flash point, pour point,

and cloud point or cold filter plugging point.

Flash point

The flash point of a fuel is the lowest temperature at

which sufficient vapour is given off to cause temporary

burning when a flame is introduced near the surface. A

figure of 1258F (528C) minimum is quoted in some

specifications.

Pour point

The pour point of a fuel is the temperature at which thefuel begins to thicken and congeal and can no longer be

poured from a container; a pour point of 2188C is

considered suitable for some conditions.

Cloud point

The cloud point, which is sometimes known as the cold

filter plugging point (CFPP), is the temperature at which

the fuel begins to have a cloudy appearance and will no

longer flow freely through a filtering medium. The cloud

point is normally a few 8C higher than the pour point.

Note

These figures for diesel fuel are approximate and are

presented here as a guide only. Readers who require

more detailed information are advised to contact

their fuel supplier.

Products of combustion

Exhaust gases are the products of combustion and under

ideal circumstances they would comprise carbon

dioxide, steam (water), and nitrogen. However, owing

to the large range of operating conditions that engines

experience, exhaust gas contains several other gases

and substances, such as:

COe carbon monoxide due to excess fuel and

incomplete combustion.

NOxe oxides of nitrogen arising from extremely

high combustion temperature.

HC e hydrocarbons arising largely from incomplete

combustion. PMe particulate material. The bulk of PM is soot,

which is incompletely burnt carbon. Other

particulates arise from lubricating oil on cylinder

walls and metallic substances from engine wear.

SO2 e sulphur dioxide. Some diesel fuels contain

small amounts of sulphur, which combines with

oxygen during combustion to form SO2. This in turn

can combine with water to form sulphurous acid.

CO2e carbon dioxide is not treated as a harmful

emission but it is considered to be a major contributor

to the greenhouse effect and efforts are constantly

being made to reduce the amount that is produced. In

the UK the quantity of CO2that a vehicle produces ina standard test appears in the specification, and

vehicle taxation (road tax) is less for small CO2emitters than it is for large ones.

Emissions limits

In the UK the emissions limits are set by the European

Union. The limits are the subject of constant review e

those shown in Table 15.1 are for the standards known

as Euro 4. The figures apply to vehicles as they leave

the manufacturer; once in service the standards set by

the UK Department for Transport apply and it is their

figures that are used in the annual tests that are knownas the MOT.

At the time of writing the test is conducted by passing

the exhaust gas through an approved apparatus such as

the Hartridge smoke meter, which measures the opacity

of the exhaust gas. Figure 15.9 shows how the opacity of

the exaust gas relates to the Hartridge and the Bosch

scales.

Emissions control on the CIE

The airefuel ratio in compression ignition engines

varies from very weak (probably 50:1) to slightly rich(12:1), and combustion temperatures are high. The

three-way catalyst used with petrol engines is not suit-

able for use with compression ignition engines because

Table 15.1 Emissions limits (g/km)

CO HC HC 1 NOx

NOx

PM

Petrol 1.0 0.10 e 0.08 e

Diesel 0.50 e 0.30 0.25 0.025



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it requires the airefuel ratio to be held near to 15:1 and

alternative methods of dealing with harmful emissions

are used. The two systems that are used on heavy vehi-

cles are:

1. Selective catalyst reduction (SCR) and a particulate

matter (PM) filter.

2. Exhaust gas recirculation (EGR).

Particulate trap and selectivecatalyst reduction

The three-way catalyst used on petrol engines requires

an airefuel ratio of about 15:1. Diesel engines operate

on mixture strengths that may be as low as 40:1, which

means that an alternative system is used to reduce NOx.

On light vehicles there is a tendency to rely on exhaust

gas recirculation to limit NOxand an oxidation catalyst

to control HC; in addition, a particulate filter may be

used to deal with soot and other particulates. This

system is shown in outline in Fig. 15.10.

There is some debate in the heavy vehicle field about

the most suitable system for exhaust gas after treatment,

and some large vehicles are equipped with an alternative

system of the type shown in Fig. 15.11.

The exhaust gas is first passed through the oxidation

catalyst and particulate filter. The high concentration of

oxygen in the fueleair mixture and the relatively high

temperature allow the oxidation catalyst to convert HC

and COinto CO2and H2O. The gas then enters the partic-

ulate filter, where the soot and other materials are filtered

out. Any PM that is deposited in the filter can be removedlater by active regeneration, which is combustion with

oxygen at approximately 6008C; this is achieved by

a temporary increase in the amount of fuel injected.

The regeneration process is performed by the engine

management system at intervals dictated by operating

conditions. After passing through the oxidation catalyst

and particulate filter, a solution of pure water and urea

is injected into the exhaust stream, where it reacts with

the catalyst to reduce the NOx to water vapour and

nitrogen. In Europe the solution of pure water and urea

is called AdBlue and is carried in a small tank that is

about 20% of the capacity of the main fuel tank, and it

is normally placed next to it, as shown in Fig. 15.12.

CAT

Oxidising catalystParticulate filter

Pressure differentialsensor

Oxygen sensor

P.F.

Fig. 15.10 Light diesel exhaust engine emission control

N0x oxides of nitrogen

Urea injectionNitrogenH2O Water vapourPlus normal exhaust gas

Hc hydrocarbons unburnt fuel

PM soot etc.

Plus normal exhaustgas

1. Particular filter2. Oxidation catalyst

3. Selective reduction catalyst

1 2 3

Fig. 15.11 A heavy diesel engine selective catalyst reduction system

Light absorption coefficient [m-1]

Hartridge smoke units [HSU]

Bosch number [BN]

0.1

10

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

20

30

40

50

60

70

80

90

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.2

2.4

2.6

2.8

3.0

3.5

4.0

5.0

6.0

7.0

8.0

7.0

7.5

Fig. 15.9 Smoke meter scales



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Exhaust gas recirculation (EGR)

Oxides of nitrogen (NOx) are formed when combustion

temperatures are high, as they are in compression igni-

tion engines. Exhaust gas contains considerable

amounts of CO2 and H2O and small amounts added to

the incoming air charge reduce the combustion temper-ature and the production of NOx. An electrically oper-

ated EGR valve of the type shown in Fig. 15.3 that

operates under the control of the engine management

computer is placed between the exhaust and air intake

systems. The engine computer is programmed to recir-

culate exhaust gas when operating conditions are suit-

able e in most cases a quantity of exhaust gas

equivalent to about 15% of the air intake is recirculated

when the engine is running between idling speed and

full load.

Self-assessment questions1. Why are diesel engines sometimes referred to as

compression ignition engines?2. How is ignition of the fuel achieved in a diesel

engine?

3. How does the compression ratio of a diesel

compare with that of a petrol engine?

4. How does exhaust gas recirculation help to reduce

NOxemissions?

5. What is meant by the term particulate matter?

6. How does the airefuel ratio of a diesel engine vary

across the engine speed and power range?

7. Give an approximate value of the airefuel ratio for

a diesel engine at idling speed.

8. What effect on cold weather starting will low

compression pressure have on a diesel engine?9. Why doesnt a three-way catalyst work on diesel

exhaust?

10. Describe the procedure for conducting the MOT

exhaust gas test on a light vehicle equipped with

a turbocharged engine.

11. What is the approximate maximum temperature

reached in a diesel engine?

Fig. 15.13 Exhaust gas recirculation

Fig. 15.12 The AdBlue tank


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