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Automotive mechanics (volume i)(part 2, chapter8) engine fundamentals

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Engines and engine systems

8 Engine fundamentals
9 Engine construction and components
10 Cooling system and service
11 Engine-lubricating systems
12 EFI fuel systems
13 Carburettor fuel systems
14 Gas fuel systems
15 Intake and exhaust systems


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Chapter 8

Engine fundamentals

Actions within a cylinder
Reciprocating to rotary motion
Cycles of engine operation
Four-stroke cycle (petrol engine)
Two-stroke cycle (petrol engine)
Diesel engine operation
Rotary engine
Multicylinder engines
Engine classifications
Engine systems
Technical terms
Review questions

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110 part two engines and engine systems
The engine converts chemical energy into mechanical
energy. The chemical energy is in the fuel that is
supplied to the engine. The energy is released in the
form of heat when the fuel is burnt within the cylinders
of the engine. This is referred to as combustion. The
resulting gases are very hot and so reach a very high
pressure. It is this high pressure that is used to force
the piston down the cylinder to operate the engine.
Most automotive engines have a number of
cylinders. A small car engine might have as few as two,
while a large engine might have as many as twelve.
Similar actions take place in all cylinders.

Actions within a cylinder
The basic parts of an engine are the cylinder, the piston,
the connecting rod and the crankshaft. The piston, of
aluminium alloy, is a sliding fit in the cylinder. It has
piston rings which provide a gas-tight seal against the
cylinder walls.
Chemical energy to mechanical energy
The way in which chemical energy is converted to
mechanical energy is shown by the movement of the
piston in the cylinder in Figure 8.1. The cylinder
contains air mixed with a small amount of fuel.
Action within the cylinder
1. Compression. The piston is pushed up the cylinder
to compress the air–fuel mixture. The mixture
consists of tiny particles of fuel (petrol), each
surrounded by air. This forms a combustible
2. Combustion. The piston has been pushed almost to
the top of the cylinder to compress the air–fuel

figure 8.1

mixture into a small space. This space above the
piston is known as the combustion chamber,
because this is where burning of the mixture takes
place. The air–fuel mixture is ignited by a spark
from the spark plug.
3. Power. The pressure of the burning gases forces the
piston down the cylinder, and this action provides
the power to operate the engine.
These are the actions that occur within the cylinder of
a petrol engine. A mixture of air and fuel enters the
cylinder and the piston moves upwards to compress it;
the compressed mixture is ignited in the combustion
chamber and the piston is forced downwards. The
actions are repeated over and over to enable the engine
to operate, although many more parts are needed to
make a complete engine.

Reciprocating to rotary motion
The up-and-down movement of the piston in its
cylinder is called reciprocating motion and, for this
reason, piston engines are sometimes referred to as
reciprocating engines.
The piston has straight-line motion, but this must
be changed to rotary motion. A connecting rod
and crankshaft are used for this purpose (Figure 8.2).
The connecting rod connects the piston to the
The crankshaft is a ‘cranked’ (bent) shaft, with a
crank for each cylinder. Each crank has a crankpin
which provides a surface for the bearing in the lower
end of the connecting rod.
The connecting rod has a removable cap, which is
bolted to the end of the connecting rod. This is needed
so that the connecting rod can be installed on the

Actions in a cylinder which convert chemical energy to mechanical energy

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chapter eight engine fundamentals

figure 8.2

The connecting rod and crankshaft change
reciprocating motion to rotary motion

The piston is held to the upper end of the
connecting rod by the piston pin. This passes through
an eye in the connecting rod and allows a wrist-like
action to take place as the crankshaft rotates and the
piston moves up and down.
The parts of a piston, connecting rod and crankshaft
are shown assembled in Figure 8.3. In Figure 8.4, the
parts are shown dismantled.

figure 8.3


Arrangement of a piston, connecting rod and

figure 8.4

Part of a crankshaft, and a piston and
connecting-rod assembly

Piston strokes
The actions of a piston in a cylinder are divided into
strokes. A stroke occurs when the piston moves either
from the top to the bottom of the cylinder, or from the
bottom to the top of the cylinder.
The top of the piston stroke is known as top deadcentre (TDC), and the bottom of the stroke as bottom
dead-centre (BDC) (see Figure 8.5). At each of these
positions, the piston virtually stops and changes its
direction of travel.

figure 8.5

Piston stroke positions

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112 part two engines and engine systems

Cycles of engine operation
A cycle is a series of events that repeats itself over and
over. The operating cycle of an engine consists of a
number of piston strokes. There are two-stroke engines
and four-stroke engines.
In a four-stroke engine, the cycle has four piston
strokes and so requires two revolutions of the
In a two-stroke engine, only two piston strokes are
needed for a cycle and this requires only one
revolution of the crankshaft.
Most automotive engines are four-stroke engines,
although some large diesel engines operate on the twostroke principle. Some small engines, such as those
fitted to motor mowers, are two-strokes, while others
are four-strokes.
Most motor bikes have four-stroke engines, but
there are also two-stroke motor bike engines. Many
marine outboard engines are two-strokes, but there are
also four strokes.
The complete cycle for a petrol engine, whether it
is a two-stroke or a four-stroke, requires an air–petrol
mixture to be taken into the cylinder, combustion to
take place, burning gases to expand and apply force
to the piston, and finally, the remains of the burnt
gases to be exhausted from the cylinder.


For a four-stroke cycle, also known as the Otto cycle
after its inventor, there are valves in the cylinder head
at the top of the cylinder. These are opened and
closed at the correct times by the actions of the
camshaft. The intake valve admits the fuel mixture into
the cylinder and the exhaust valve releases the burnt
gases from the cylinder.
Figure 8.6 shows the sequence of the four strokes
of a petrol engine during one cycle. These are:
1. Intake. The piston is moving down and air–fuel
mixture is being drawn into the cylinder through
the open intake valve.
2. Compression. The piston is moving up and the
air–fuel mixture is being compressed in the
cylinder. Both valves are closed.
3. Power. Combustion of the air–fuel mixture has
taken place and the piston is being forced down the
cylinder by the pressure of the gases.
4. Exhaust. The piston is moving up and forcing the
burnt gases out through the open exhaust valve.




1. Intake

3. Power

Four-stroke cycle (petrol engine)


figure 8.6


2. Compression

4. Exhaust

The four-stroke cycle of a petrol engine

These strokes constitute a four-stroke cycle. When the
piston reaches TDC on the exhaust stroke, the intake
valve will again open to commence the cycle all over
again. This will continue as long as the engine is
Intake stroke
The intake stroke commences at TDC with the intake
valve open and the exhaust valve closed. The piston
moves downwards to draw a charge of air–fuel mixture
into the cylinder through the open intake port. The
mixture of air and vaporised fuel is provided by the
carburettor or the fuel injection system.
■ The air–fuel mixture is not actually drawn into the
cylinder. As the piston moves downwards, it creates
a partial vacuum (or negative pressure) and
atmospheric pressure forces the mixture into the

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chapter eight engine fundamentals

Compression stroke
When the piston reaches BDC and begins to move
upwards on the compression stroke, the intake valve
closes. The exhaust valve is already closed, so that the
air–fuel charge is compressed as the piston moves up
the cylinder. By the time the piston reaches TDC, the
mixture will be compressed to about one-eighth of its
original volume. The pressure in the cylinder will also
have increased.
Power stroke
Both valves remain closed during the power stroke. As
the piston reaches TDC at the end of the compression
stroke, the ignition system produces a spark at the
spark plug. The spark ignites the air–fuel mixture,
which burns very rapidly to produce gases at high
pressure in the cylinder.
The expanding gases force the piston down the
cylinder. The force is transferred through the connecting rod to the crankpin, causing the crankshaft to


of the engine. The valve-operating mechanism is
referred to as the valve train.
The cylinder head shown carries a camshaft, which
has a cam for each valve. As the camshaft rotates, the
cams operate the rocker arms to open and close
the valves at the appropriate times.
Cam action lifts one end of the rocker arm, and the
rocker arm pivots on its shaft. The other end of
the rocker arm moves down against the tip of the valve
stem and opens the valve. As the cam continues to
rotate, the valve end of the rocker arm rises, and the
spring closes the valve.
Valve trains
There are two different types of valve trains which are
commonly used: those for overhead valves and those
for overhead camshafts. These two basic arrangements
are shown in Figure 8.8. In both of these, there is a
sprocket on the camshaft and one on the crankshaft.
The sprockets are connected by a chain, although
many overhead camshafts have a special type of drive

Exhaust stroke

Overhead valves

As the piston again reaches BDC, the exhaust valve
opens but the intake valve remains closed. The piston
moves upwards on the exhaust stroke to force the burnt
gases out of the cylinder through the exhaust port.
When the piston reaches TDC, the exhaust stroke is
completed and so is the cycle.

With overhead-valve engines, the camshaft is located
low in the engine block and is driven by a chain from
the crankshaft. Tappets (or cam followers) and
pushrods are used to transfer the cam action to rocker
arms on top of the cylinder head.
Because the complete four-stroke cycle takes two
revolutions of the crankshaft, the camshaft must rotate
at half the speed of the crankshaft. This is accomplished by having the camshaft sprocket twice the size
of the crankshaft sprocket.

Valve operation
A section through a cylinder head and valves is shown
in Figure 8.7. There is an intake valve, an exhaust
valve and a mechanism to operate them. The valves are
normally held closed by the valve springs, but are
opened for certain periods during the operating cycle

■ Engines with pushrods in their valve train are
sometimes referred to as pushrod engines to
distinguish them from overhead camshaft engines.
Overhead camshafts
With overhead-camshaft engines, the camshaft is on
the top of the cylinder head and is driven by either a
timing chain or a timing belt from the crankshaft.
Some engines operate with tappets directly between
the camshaft and the valves, as shown in the second
diagram. Other engines (like the one shown in
Figure 9.5) have rocker arms to operate the valves.
Valve timing

figure 8.7

Cylinder head and valves for an overheadcamshaft engine MITSUBISHI

In the basic four-stroke engine described, the valves
are opened and closed at TDC and BDC, as required,

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114 part two engines and engine systems

figure 8.8

Basic valve operation

to allow air–fuel mixture into the cylinder and the
exhaust gases out of the cylinder. However, in an
actual engine, this does not occur right at TDC and
BDC. Valves operate before or after these particular
Figure 8.9 shows the four separate strokes which
make up the two engine revolutions of a four-stroke
cycle. These are part of a circle and represent the
degrees of rotation of the engine’s crankshaft.
1. Intake. The intake valve opens Figure 8.9(a) before
TDC and closes after BDC. This increases the
length of the intake stroke and allows more air–fuel
mixture to enter the cylinder.
2. Compression. Both valves are closed Figure 8.9(b).
While the intake valve closes after BDC and
shortens the compression stroke, it does not affect
3. Power. Both valves are still closed Figure 8.9(c)
and ignition occurs ahead of TDC. The exhaust
valve opens before BDC but, by this time, most of
the force of combustion is lost.
4. Exhaust. The exhaust stroke is lengthened
Figure 8.9(d) by opening the exhaust valve before
BDC and closing it after TDC.

figure 8.9

A valve timing diagram for a four-stroke
engine, drawn as four separate strokes –
i.o. (intake valve opens), i.c. (intake valve closes),
e.o. (exhaust valve opens), e.c. (exhaust valve closes)

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Valve timing diagram
A valve timing diagram is shown in Figure 8.10. This
is drawn as a spiral, starting from the intake stroke so
that two loops of the spiral represent the four strokes,
or one complete cycle of engine operation.
The degrees of crankshaft rotation at which the
valves open and close, either before or after TDC and
BDC, are shown on the diagram. A diagram for any
four-stroke engine can be drawn like this, but the
angles could be different.
To read the diagram, start with the intake stroke
and follow through the normal cycle of events – intake,
compression, power and exhaust.


3. Overlap. The intake and exhaust valves are both
open for a few degrees around TDC on the exhaust
stroke, and this is referred to as overlap.
Interpreting a valve-timing diagram
In the valve-timing diagram (Figure 8.10), the exhaust
valve starts to open at 47° before BDC (lead) at the
end of the power stroke, and stays open until 21° after
TDC (lag) on the intake stroke. Leaving the exhaust
valve open longer gives the exhaust gases more time to
leave the cylinder.
By the time the piston reaches 47° before BDC on
the power stroke, the combustion pressures have
dropped considerably. Little power is lost by opening
the exhaust valve early and giving the exhaust gases
extra time to exhaust from the cylinder.
In a similar manner, leaving the intake valve open
for 40° degrees past BDC on the intake stroke (lag)
gives additional time for air–fuel mixture to flow into
the cylinder.
Giving the intake valve lead and the exhaust valve
lag to create 33° of overlap helps with scavenging.
That is, the incoming air–fuel mixture helps clear out
the exhaust gases.
■ The valves are opened and closed relatively slowly
and even with 47° of lag, the exhaust valve does not
open very wide until it is close to BDC. Similarly,
both valves are only open a very small amount
during the 33° of valve overlap.
Piston movement and crankshaft rotation

figure 8.10

Valve timing diagram drawn as a spiral

Lead, lag and valve overlap
Lead, lag and valve overlap are three terms that relate
to valve opening and closing. They can be identified
on valve timing diagrams.
1. Lead. This denotes that a valve opens before TDC
or BDC. The intake valve has lead because it opens
before TDC. The exhaust valve also has lead
because it opens before BDC.
2. Lag. This denotes that a valve closes after TDC or
BDC. The intake valve has lag because it closes
after BDC. The exhaust valve also has lag because
it closes after TDC.

Figure 8.11 shows the relationship between piston
travel and crankshaft rotation. Piston travel is a linear
measurement, while crankshaft rotation is measured in
degrees. It can be seen that towards TDC and BDC,
piston travel is relatively short in relation to the
number of degrees of crankshaft rotation. If the crankshaft is rocked backwards and forwards with the piston
on TDC, the piston will hardly move.
Alternative valve-timing diagram
A valve-timing diagram of the type provided in some
service manuals is shown in Figure 8.12. The diagram
shows the opening and closing times of the exhaust
and intake valves and also the number of degrees that
the valves remain open. All this is in relation to
degrees of crankshaft rotation.
The information on the diagram can be used to
check the valve timing of an engine. A circular plate,
with degrees marked (degree plate) is used. This is

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116 part two engines and engine systems
While the overall valve timing of an engine can be
checked in this way, it is not possible to alter the
timing of the individual valves, which is determined by
the design of the camshaft.


Two-stroke cycle (petrol engine)
The common two-stroke petrol engine is of the threeport design as shown in Figure 8.13. The three ports in
the cylinder are:


1. The intake port. This admits the air–fuel mixture
from the carburettor into the crankcase below the


2. The transfer port. This transfers the air–fuel
mixture from the crankcase to the cylinder above
the piston.


3. The exhaust port. This exhausts burnt gases from
the cylinder above the piston.




figure 8.11

Diagram showing the relationship of piston
movement to crankshaft rotation

figure 8.12

Valve timing diagram showing the opening
and closing of the intake and exhaust valves

attached to the crankshaft. The piston is set on TDC,
and the zero mark on the degree plate set against a
pointer which can be attached to the crankcase. The
crankshaft can be rotated and the opening and closing
of the valves checked.

The ports are opened and closed by the piston as it
moves up and down the cylinder. There are no separate
The four parts of the operating cycle of an engine
(intake, compression, power and exhaust) still occur.
Some events take place in the cylinder above the
piston, while others take place in the crankcase below
the piston at the same time. This enables the complete
operating cycle to be completed during two piston
strokes – one upstroke and one downstroke.
The crankcase is sealed and forms part of the fuel
intake system. A petrol and oil mixture is used. As
well as providing the air–fuel mixture for combustion,
the oil in the mixture also lubricates the piston,
cylinder and the bearings.
Events that occur in the cylinder above the piston
are: compression of the air–fuel mixture, combustion,
the power stroke and the exhaust stroke.

figure 8.13

Arrangement of a basic two-stroke engine

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Events that occur in the crankcase below the piston
are: the intake of the fuel mixture into the crankcase
and transfer of the fuel mixture from the crankcase
through the transfer port to the cylinder above the
Two-stroke operation
The events occur as follows (Figure 8.14):
1. Piston upstroke. The upward movement of the
piston compresses the fuel mixture in the cylinder
above the piston. At the same time, it creates a low
pressure in the crankcase below the piston, ready
for the intake port to be opened. When the intake
port opens, fuel mixture from the carburettor is
allowed into the crankcase.
2. Top of the stroke. A spark from the spark plug
ignites the fuel mixture near the top of the stroke
and combustion occurs. Meanwhile, the piston has
uncovered the intake port on its way up the
cylinder. This allows fuel from the carburettor into
the crankcase.
3. Piston downstroke. The piston is forced down the
cylinder during the power stroke. Near the end of
the stroke, the top of the piston uncovers the
exhaust port to allow the burnt gases in the top of
the cylinder to escape. At the same time, piston
downward movement is compressing the air–fuel
mixture in the crankcase ready for transfer.
4. Bottom of the stroke. Near the bottom of the stroke,
the piston uncovers the transfer port. This allows
the compressed fuel charge in the crankcase to
transfer from the crankcase below the piston to the
cylinder above the piston. The exhaust port is

uncovered before the transfer port, so most of the
exhaust gases will have escaped before the transfer
port is uncovered to admit a fresh charge of fuel
mixture from the crankcase.
The piston of a two-stroke engine is often designed
with a deflector on its head. This directs the intake
mixture upwards and the exhaust gases downwards
towards the exhaust port. This prevents the incoming
mixture from escaping through the exhaust port and
also assists in blowing out the exhaust gases. This is
known as scavenging.
Two-stroke engine with reed valve
Some two-stroke engines, such as outboard marine
engines, are designed with reed valves. The reed valve
is a small flexible metal plate which covers the intake
port. The reed valve normally covers the port, but it
opens and closes the port automatically as pressure in
the crankcase changes (Figure 8.15).
When the piston is moving upwards, a negative
pressure (below atmospheric) is produced in the
crankcase and the air–fuel mixture from the
carburettor, which is at a pressure close to
atmospheric, lifts the reed valve off its port to enter the
crankcase as shown in Figure 8.15(a).
After the piston reaches TDC and commences to
move downwards on the power stroke, pressure
develops in the crankcase and forces the reed valve
closed. Further downward movement of the piston







(b) Downstroke

(a) Upstroke

figure 8.15

figure 8.14

Operation of a two-stroke engine

Two-stroke engine with a reed valve (a) and (b)
1 exhaust port, 2 transfer port, 3 transfer
passage, 4 crankcase, 5 intake from carburettor, 6 cover,
7 reed valve

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118 part two engines and engine systems
compresses the trapped air–fuel mixture in the
When the piston nears the bottom of its stroke, it
uncovers the transfer port. The pressure which has
built up in the crankcase then forces the mixture
through the transfer passage into the cylinder above the
piston as shown in Figure 8.15(b).
The type of cylinder shown operates in a horizontal
position. The crankcase is fitted with a cover, which
forms a chamber between it and the crankcase. The
carburettor is attached to the cover and the air–fuel
mixture passes from the carburettor into this chamber.
The reed valve is located between the chamber and
the crankcase. In engines with more than one cylinder,
the section of the crankcase for each cylinder is sealed
separately and fitted with a reed valve.

Diesel engine operation
Most diesel engines are four-stroke engines, but there
are some large diesel engines that are two-stroke.
These operate in a different way to small two-stroke
petrol engines.
Diesel engines run on distillate which, in most
engines is injected directly into the cylinder. It does
not have a carburettor or an electronic fuel-injection
system like a petrol engine.
In a diesel, air only is taken into the engine during
the intake stroke. During the compression stroke, the
air is compressed to about 600°C. This is hot enough
to ignite the distillate as it is sprayed into the
combustion chamber. It does not need a spark from a
spark plug to fire the charge.
Four-stroke diesel cycle
The four strokes of a diesel engine are shown in
Figure 8.16. These are:
1. Air intake (downstroke). The intake valve is open
and air is being drawn into the cylinder as the
piston moves downwards. At BDC, the cylinder
will be full of air.

figure 8.16

Four-stroke diesel cycle – fuel is injected at
the top of the compression stroke

combustion chamber. The hot air in the combustion
chamber not only forms a combustible mixture with
the fuel, but also ignites it. Pressure resulting from
combustion forces the piston down the cylinder on
the power stroke.
4. Exhaust (upstroke). The exhaust valve is opened
towards BDC of the previous stroke. As the piston
moves upwards on the exhaust stroke, it forces the
exhaust gases from the cylinder through the open
exhaust port.
Diesel engines are fitted with a fuel injection pump
which delivers fuel at high pressure to the injectors
situated in the cylinder head. The amount of fuel can
be varied and the speed of the engine controlled in this

2. Compression (upstroke). Both valves are closed and
air is being compressed in the cylinder. By the time
the piston reaches TDC, the air will be compressed
to about one-sixteenth of its original volume, and
will be hot enough to ignite the diesel fuel that will
be injected into it.

■ Diesel engines are often classed as compressionignition engines because the fuel is ignited by the
heat of compression. However, the name diesel is
commonly used.

3. Power (downstroke). Just before the piston reaches
TDC, a small quantity of fuel is sprayed through
the injector at the top of the cylinder, into the

Some two-stroke diesel engines have ports for both
intake and exhaust, but one of the most commonly
used designs has intake ports and exhaust valves. This

Two-stroke diesel

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arrangement is shown in Figure 8.17. The piston in the
diagram is almost at TDC on the compression stroke.
Fuel is being injected into the combustion chamber and
combustion would have commenced.
The engine does not depend on crankcase compression and a transfer port, but uses a blower to force
air into the cylinder through an intake port. The
exhaust gases leave the cylinder through an exhaust

figure 8.18

figure 8.17

Basic two-stroke diesel engine

Rotary engine
A rotary engine has a rotor in a housing instead of a
piston in a cylinder. The rotary motion of the rotor is
transferred by a unique gear system to an eccentric
shaft which performs the same function as the
crankshaft of a piston engine.
Figure 8.18 shows the action of the rotor in its
housing and demonstrates the four strokes which occur
in the working cylinder. These are similar to the four
strokes of a four-stroke piston engine, but they are
performed in a working chamber formed between the
rotor and its housing.
The inside of the housing has a particular shape to
accommodate the action of the rotor. There are two
ports in the side of the housing – an intake port and an
exhaust port. The rotor covers and uncovers the ports
as it rotates so that air–fuel mixture enters through the
intake port and the exhaust gases discharge through
the exhaust port. There are no valves in the engine.
The four strokes shown in sequence in the figure
1. intake
2. compression
3. expansion (or power)
4. exhaust.

The four strokes in the working chamber of a
rotary engine MAZDA

As the rotor turns in the housing, air–fuel mixture is
taken in through the intake port, it is compressed
between the rotor and the housing and then ignited by
the spark plug. Combustion takes place and the
expanding gases create a force to turn the rotor. As the
rotor uncovers the exhaust port, the exhaust gases are
discharged to complete the cycle.
The shape of the rotor provides three working
chambers and the sequence shown in Figure 8.18
occurs three times during each rotation of the rotor –
once in each of the three working chambers.
■ The rotary engine is covered in more detail in
Volume 2.

Multicylinder engines
A single-cylinder engine operating on the four-stroke
cycle provides only one power stroke for every two
crankshaft revolutions. The power is not uniform
throughout the stroke, but is delivered more as an
impulse, with the greatest power being delivered at the
start of the stroke and decreasing towards the end of
the stroke. To provide a more uniform power flow and
smoother operation, most automotive engines have
more than one cylinder. Four, six and eight cylinders
are commonly used.
With multicylinder engines, the power strokes are
evenly spaced. A four-cylinder engine (four stroke) has
two power strokes for each crankshaft revolution, a

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120 part two engines and engine systems
six-cylinder engine has three, and an eight-cylinder
engine has four. In six- and eight-cylinder engines, the
power strokes overlap to provide a smoother power
With two-stroke engines, each cylinder provides a
power impulse for each crankshaft revolution, so a
two-stroke multicylinder engine will deliver twice as
many power impulses as a similar four-stroke engine
running at the same speed.

In-line engines
These have their cylinders arranged one behind the
other in a straight line. The cylinder block for a fourcylinder engine is shown in Figure 8.20.

Arrangement of the cylinders
The arrangement of the engine’s cylinders is referred
to as the engine configuration. Cylinders can be
arranged in-line, as a ‘V’, or horizontally opposed.
These arrangements are shown in Figure 8.19. The
engines operate in just the same way, whatever the
arrangement, whether the pistons are moving up and
down or from side to side.
The usual method of identifying the cylinders of an
engine is to number them from front to back as shown
in the illustration. The cylinders of V-type engines are
usually numbered consecutively on one bank and then
the other.

figure 8.20

Cylinder block for a four-cylinder in-line

The cylinders are usually vertical, but in some inline engines, the cylinders are tilted at an angle to the
vertical and so these engines are referred to as slant
engines. Tilting the engine has the effect of reducing
its overall height, and this enables it to be used in a
passenger car with a lower bonnet line, or installed in
a truck where the engine is mounted beneath the cab.
In-line engines can also be designed as flat
engines, with the cylinders horizontal, but these are
mainly used for heavy vehicles. These engines can be
mounted under the cabs of trucks, or under the floors
of buses.
V-type engines
The cylinders of V-type engines are arranged in two
banks, with half the cylinders in each bank. The
cylinders in each bank are in-line, and the banks are set
at an angle to each other. In many engines the angle is
90°, but other angles are also used. The cylinder block
for a V-8 engine is shown in Figure 8.21.
Compared with an equivalent in-line engine, a
V-type engine will be wider and shorter and, generally,
will be more rigid and will have a shorter and more
rigid crankshaft.
Horizontally opposed engines

figure 8.19

Engine configurations
(a) four-cylinder in-line (b) six-cylinder in-line
(c) V-6 (d) V-8 (e) four-cylinder horizontally opposed

These engines have their cylinders arranged in two
horizontal banks with the crankshaft mounted between
them. There are two cylinder blocks – a left block and

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chapter eight engine fundamentals


than an equivalent in-line engine. It also has better
natural balance because the movement of a piston in
one direction is balanced by the movement of a piston
in the opposite direction.

Engine classifications
There are many variations in engine design, but piston
engines can be classified, or grouped, according to
their main design features. The various engine
classifications are:

figure 8.21

Cylinder block and a cylinder head for a
V-type engine with eight cylinders

a right block. The cylinder blocks for a four-cylinder
engine are shown in Figure 8.22. These are made of
aluminium alloy with cast-iron cylinder liners.
Because this is a flat arrangement, the term flat
engine is often applied to horizontally opposed
engines. An engine of this design is wider and lower

1. Number of cylinders. Four, six and eight cylinders
are the ones most commonly used, but there are
also automotive engines with one, two, three, five
and twelve cylinders.
2. Arrangement of the cylinders. Engines are designed
with cylinders that are in-line, at an angle to each
other (V-type engine) or horizontal.
3. Engine capacity. The capacity or size of the engine
is the total volume of all the cylinders of the
engine. This provides a common basis for
comparing the size of engines.

right cylinder

left cylinder

figure 8.22

Cylinder blocks for a horizontally opposed engine


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122 part two engines and engine systems
■ This does not mean that all engines of the same
capacity will produce exactly the same power
output, because some engines are designed for
higher performance than others.

1. Fuel system. Includes a fuel tank and a fuel pump to
supply the fuel to the engine.

4. Arrangement of the valve mechanism. The common
arrangements are overhead valves (OHV) and
overhead camshaft (OHC). Both these have
overhead valves, but the different names are
because the camshafts are located in different parts
of the engine. Some engines have two overhead
camshafts and this arrangement is referred to as
Some engines have two valves per cylinder,
some have three and some have four. This feature is
sometimes used to describe the engine, for
example, a four-cylinder engine with four valves
for each cylinder might be referred to as a sixteenvalve engine.

3. Cooling system. Includes the radiator, hoses, water
pump, water-jackets in the cylinder block and head,
and also the coolant which is circulated through the
system to prevent the engine from overheating.

5. Type of cooling. Engines can be liquid-cooled or
air-cooled. Liquid-cooled engines have waterjackets and a radiator. Air-cooled engines depend
on a flow of cooling air.

7. Intake system. This includes the air cleaner, ducting
and the intake manifold which provide clean air,
and fuel mixture to the engine.

6. Operating cycle. Engines are classed by their
operating cycle, either as four-stroke engines or
two-stroke engines.
7. Type of fuel used. There are three automotive fuels:
petrol, distillate (for diesel engines) and gas. Most
passenger cars have petrol engines, but many fourwheel drives and light commercial vehicles are
fitted with diesel engines. Heavy trucks and
earthmoving equipment have diesel engines.
Gas fuel comes in two forms: liquefied
petroleum gas (LPG) and natural gas for vehicles
(NGV). Car engines and light truck engines can be
converted to run on LPG and are usually arranged
as dual-fuel engines, capable of running on either
gas or petrol.
Gas is also used for special applications. The
exhaust gases are much cleaner than those from a
petrol engine and so the engines of fork-lifts and
other handling equipment used inside buildings are
fitted with fuel systems that use LPG.

Engine systems
In addition to the actual engine components, an engine
requires a number of systems to enable it to operate.
Some of the engine components previously mentioned
form part of these systems. The systems are:

2. Lubrication system. Uses a pump and passages to
supply oil to all the moving parts of the engine.

4. Starting system. Consists of the battery and starter
motor, which rotates the engine for starting by
means of the ring gear on the engine’s flywheel.
5. Charging system. An engine driven alternator is
used to provide electrical power and to charge the
6. Ignition system. In petrol and gas engines, it
provides the spark at the spark plug to ignite the
mixture in the combustion chamber.

8. Exhaust system. This includes the exhaust
manifold, connecting pipes, catalytic converter,
muffler and the pipes which carry the exhaust to the
rear of the vehicle.
The systems outlined above are required for all
engines, except the ignition system which is not
required for diesel engines.
In addition to the various components and systems
which have been mentioned, other auxiliaries are fitted
to an engine, such as power-steering pumps, airconditioning compressors and, in heavier vehicles,
vacuum pumps or air compressors for braking systems.

Technical terms
Energy, combustible mixture, combustion chamber,
reciprocating, rotary, crankpin, stroke, top deadcentre, TDC, bottom dead-centre, BDC, cycle,
air–fuel mixture, Otto cycle, four-stroke, intake,
compression, power, exhaust, connecting rod, valve
train, cam, camshaft, sprocket, pushrod, overhead
camshaft, OHC, overhead valve, OHV, valve
timing, lead, lag, overlap, two-stroke, port, rocker
arm, transfer port, reed valve, diesel, injector,
injection, blower, rotor, configuration, in-line,
horizontally opposed, engine capacity, liquefied
petroleum gas, LPG, natural gas for vehicles, NGV.

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chapter eight engine fundamentals

Review questions



What are valve lag and valve lead?


Draw a spiral valve-timing diagram showing the
approximate positions at which the valves open
and close.


State the purpose of the ports in a two-stroke


What part does the crankcase play in the
operation of a small two-stroke engine?


What is a piston stroke?


What is meant by reciprocating motion?


How is reciprocating motion changed to rotary
motion in an engine?


Name the parts of a basic engine.


What is a cycle?


Name the strokes of a four-stroke engine.


What is a reed valve?


At what stage are both the valves of an engine


Indicate how the cylinders are arranged in
different types of engines.


What is the speed of the camshaft in relation to
the crankshaft?


What is a rotary engine?



Name two different types of valve-operating

State the main differences between a petrol and
a diesel engine.


What are the different types of fuels that are
used in automotive engines?


Name the main engine systems.


What is a valve-timing diagram?


What is valve overlap?

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