Engine systems

7 Engine systems

7.1 Introduction

When the term ‘stroke’ is used it means the move- ment of a piston from top dead centre (TDC) to

The main sections in this chapter that relate to an bottom dead centre (BDC) or the other way round. area of the vehicle start with an explanation of

The following table explains the spark ignition the particular system. The sections then conclude

(SI) and compression ignition (CI) four stroke with appropriate diagnostic techniques and symp-

cycles – for revision purposes! Figure 7.2 shows tom charts. Extra tests and methods are explained

the SI cycle.

where necessary.

Stroke

Spark ignition

Compression ignition

The fuel air mixture is 7.2 Engine operation Air is forced into the

Induction

forced into the cylinder cylinder through the through the open inlet open inlet valve valve because as the

7.2.1 Four stroke cycle because as the piston

piston moves down it moves down it makes

Figure 7.1 shows a modern vehicle engine. Engines

makes a lower pressure. a lower pressure. It is It is acceptable to say

like this can seem very complex at first but keep in acceptable to say the

the mixture is drawn air is drawn into the

mind when carrying out diagnostic work that, with

into the cylinder cylinder

very few exceptions, all engines operate on the

Compression As the piston moves As the piston moves

four stroke principle. The complexity is in the sys-

back up the cylinder back up the cylinder

tems around the engine to make it operate to its

the fuel air mixture is the fuel air mixture is

maximum efficiency or best performance. With

compressed to about compressed in some

this in mind then, back to basics! an eighth of its original engines to about a

volume because the sixteenth of its

The engine components are combined to use

inlet and exhaust valves original volume

the power of expanding gas to drive the engine.

are closed. This is a because the inlet and

Figure 7.1 The BMW 2.8 litre six cylinder engine uses variable camshaft control

Engine systems 99

Figure 7.2 Four stroke cycle compression ratio of

exhaust valves are

in their standard family cars. Larger cars do, how-

8:1, which is typical for

closed. This is a

ever, make use of the ‘Vee’ configuration. The

many normal engines

compression ratio of 16:1, which causes a

opposed layout whilst still used is less popular.

large build up of heat

Engine firing order is important. This means the

order in which the power strokes occur. The fig-

Power At a suitable time

At a suitable time

before top dead centre, before top dead centre,

ures above have the cylinders numbered, and

a spark at the plug

very high pressure

listed by each are some common firing orders. It

ignites the compressed

atomised diesel fuel is

is important to check in the workshop manual or

mixture. The mixture

injected into the

data book when working on a particular engine.

now burns very quickly combustion chamber.

and the powerful

The mixture burns very

expansion pushes the

quickly and the

7.2.3 Camshaft drives

piston back down the

powerful expansion

cylinder. Both valves

pushes the piston back

The engine drives the camshaft in one of three

are closed

down the cylinder.

The valves are closed

ways, gear drive, chain drive or by a drive belt.

The last of these is now the most popular as it

Exhaust The final stroke occurs

The final stroke occurs

as the piston moves

as the piston moves back

tends to be simpler and quieter. Note in all cases

back up the cylinder

up the cylinder and

that the cam is driven at half the engine speed.

and pushes the spent

pushes the spent gases

This is done by the ratio of teeth between the

gases out of the now

out of the now open

crank and cam cogs which is 1:2, for example 20 crank teeth and 40 cam teeth.

open exhaust valve

exhaust valve

● Camshaft drive gears 7.2.2 Cylinder layouts Gears are not used

very often on petrol engines but are used on Great improvements can be made to the perform-

larger diesel engines. They ensure a good pos- ance and balance of an engine by using more than

itive drive from the crankshaft gear to the one cylinder. Once this is agreed the actual layout

camshaft.

of the cylinders must be considered. The layout ● Camshaft chain drive Chain drive is still can be one of three possibilities as follows.

used but was even more popular a few years

ago. The problems with it are that a way must

● In-line or straight

be found to tension the chain and also provide straight line. They can be vertical, inclined or

The cylinders are in a

lubrication.

horizontal.

Camshaft drive belts The cylinders are in two rows at a set have become very popular. The main reasons angle. The actual angle varies but is often 60° for this are that they are quieter, do not need or 90°.

Camshaft drive belt