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