Automotive mechanics (volume II)(Part 1, chapter2) cylinder head and valves

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21

Chapter 2

Cylinder head and valves

Cylinder heads

Engine illustrations

Combustion chambers

Technical terms

Engine valves

Review questions

Valve trains for OHV engines
Hydraulic valve lifters for OHV engines
Valve trains for OHC engines
Hydraulic lash adjusters for OHC engines
Camshafts

Camshaft drives and timing
Drives for DOHC
Tensioners and dampers
Variable valve timing
Valve-timing diagram


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part one engine construction and overhaul

Two different arrangements of cylinder heads and
valves are used in the engines of passenger cars and
light-commercial vehicles. Both these arrangements
have the valves in the cylinder head, but they have
different designs of valve trains.
Overhead-valve (OHV) engines have their camshafts
mounted in the cylinder block and use pushrods to transfer movement from the camshaft to the valve mechanism
on top of the cylinder head. Overhead-camshaft (OHC)
engines have the camshaft mounted directly on top of the
cylinder head. There are many variations of these two
basic arrangements which will be considered.

While some information in this chapter applies
particularly to petrol-type engines, much also applies
to diesel engines. However, there are separate chapters
later on diesel engines.

Cylinder heads
In-line engines of passenger cars and light commercial
vehicles have a single cylinder head that covers all the

figure 2.1

cylinders. Larger in-line engines can have two or more
cylinder heads, each enclosing some of the cylinders.
V-type engines and horizontally opposed engines have
a separate cylinder head for each bank of cylinders.
Cylinder-head casting
Figure 2.1 shows the top and bottom views of a
cylinder head for a V-8 overhead-valve engine.
A cylinder head, such as this, is produced as a casting
of aluminium alloy or cast iron.
During the casting process, molten metal is poured
into shaped moulds. Spaces are left within the casting
for the water-jackets and cooling-system passages, and
holes are left for the intake and exhaust ports. The
underside of the casting is shaped to form the combustion chambers.
The casting is finished by a number of machining
operations that produce a cylinder head with the
following: a flat surface on the underside where it fits
on to the top of the cylinder block; machined surfaces

A cylinder head for a V-8 engine: the upper illustration shows the top of the cylinder head, and the lower
illustration shows the underside with its hemispherical combustion chambers


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chapter two cylinder head and valves

on the top for the camshaft and valve mechanism;
holes for the valve guides; threaded holes for the spark
plugs and for securing other parts; mounting surfaces
for the manifolds and a machined surface on top for
the valve cover.
Cylinder head designs
While all cylinder heads perform the same function,
there are a number of different designs. To a large
extent, this will depend on whether it is for an
overhead-valve engine, an overhead-camshaft engine,
a V-type engine, or a horizontally-opposed engine.
The parts of a cylinder head for a V-type overheadvalve engine were identified in Figure 2.1, and the
parts of a cylinder head for a four-cylinder petrol
engine are shown in Figure 2.2. This is the basic
arrangement for an overhead-camshaft cylinder head
assembly, which includes the camshaft, rocker

assembly, and the valves and associated parts.

The cylinder head and valve assembly in Figure 2.3
are quite different. They are for a horizontally-opposed
engine with four cylinders. The cylinder head shown is
for two cylinders on one side of the engine. There is
another cylinder head of the same design for the two
cylinders on the opposite side of the engine.
The camshaft is supported in the cylinder-head
casting. The rocker arms are of cast aluminium alloy
and are fitted with hydraulic lash adjusters (see later
heading). The various parts can be identified on the
illustration.
Cylinder-head cooling
Cylinder heads for liquid-cooled engines are provided
with water-jackets through which the coolant is
circulated. This absorbs the heat from combustion and
transfers it to the radiator. Particular attention is given
to cooling the exhaust port area, which is the hottest
part of the cylinder head.

cylinder head

figure 2.2

23

Basic arrangement of a cylinder-head assembly for an OHC engine

FORD

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part one engine construction and overhaul

figure 2.3

Cylinder head for two cylinders of a horizontally opposed engine

The coolant is a mixture of distilled or deionised
water and chemical corrosion inhibitors. Chemicals are
needed to prevent corrosion that can restrict coolant
passages, enlarge holes, or cause leaks. This applies
particularly to aluminium parts, which are very
susceptible to corrosion.
■ Air-cooled engines have cooling fins cast into the
cylinder head and these, with the assistance of a
fan, dissipate the heat to the atmosphere.

Combustion chambers
The combustion chamber is the space between the top

of the piston and the cylinder head when the piston is
on top dead-centre (TDC). This is where the air–fuel
mixture is compressed and burnt. In petrol engines,
most of the combustion chamber is formed in the
cylinder head, but the head (top) of the piston can also
be shaped.
In diesel engines, the cylinder head is usually flat
and the combustion chamber is formed in the piston

SUBARU

head, although some diesels have a precombustion
chamber in the cylinder head.
■ TDC is when the piston is at the top of its stroke and
BDC is when it is at the bottom of its stroke.

Turbulence
The cylinder head, valve ports and the combustion
chambers of petrol engines are designed so that the
air–fuel mixture will be subjected to swirl, or
turbulence. This movement occurs while the mixture is
being taken into the cylinder (Figure 2.4) and also
when it is being compressed in the combustion
chamber.
Turbulence mixes the air and fuel and prevents fuel
droplets from settling on the surfaces of the combustion chamber and cylinder walls. Turbulence also
helps to prevent local high-pressure and hightemperature areas during combustion, which could
cause detonation.

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chapter two cylinder head and valves

figure 2.4

Combustion chambers are designed to create
turbulence FORD

25

Hemispherical

Combustion chamber designs
For petrol engines, there are three basic designs of
combustion chambers, although there are variations.
The names generally given to these three designs relate
to their shapes, as shown in Figure 2.5. These are:
1. hemispherical
2. bathtub

Bathtub

3. wedge.

As well as the combustion chamber formed in the
cylinder head, the top of the piston can be crowned or
hollowed. In some cases, the pistons have depressions
for the valves.
■ These designs of combustion chambers do not apply
to diesel engines, which have different cylinder
heads and different pistons.
Hemispherical combustion chamber
In this design, the combustion chamber is approximately the shape of a hemisphere. An intake valve is
on one side of the combustion chamber and an exhaust
valve is on the other. This provides a crossflow – the
air–fuel mixture enters the chamber on one side and
combustion gases exhaust on the other. Because of
this, cylinder heads of this design are also referred to
as crossflow cylinder heads.
The position of the valves allows comparatively
large valves and ports to be used. Two intake and two
exhaust valves are used on some engines. These
arrangements assist with engine breathing.
■ This design is also referred to as a pent roof
combustion chamber and is used in many
passenger-car engines.
As well as crossflow, hemispherical combustion
chambers have an advantage because the spark plug is

Wedge

figure 2.5

Basic types of combustion chambers for

petrol engines

able to be located at the centre of the chamber. Also,
with the spark plug located at the centre, the flametravel distance is reduced and this provides rapid and
effective combustion.
Burning of the fuel starts at the spark plug and
travels rapidly outwards in all directions. This is
known as flame propagation. With this design of
combustion chamber, the flame front of burning fuel
has less distance to travel than in some other designs.
Bathtub combustion chamber
This is a somewhat oval-shaped chamber in the
cylinder head, with the valves side by side. The name
has been derived from its shape, which has been
likened to an inverted bathtub. The spark plug is


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part one engine construction and overhaul

located on one side. This arrangement provides a short

flame path from the spark plug. The valves are usually
vertical in the cylinder head and are in line. A relatively simple valve-operating mechanism is able to be
used.
Turbulence in the combustion chamber is assisted
by the shape of the chamber and the fact that it has a
smaller cross-section than the cylinder. This produces
a squish effect when the air–fuel mixture is
compressed between the piston and the flat part of the
cylinder head.
■ Squish is a term used to describe the squeezing
effect on the gases that increases their velocity and
turbulence.
Wedge-shaped combustion chamber
In this design, the combustion chamber is shaped like a
wedge, tapering away from the spark plug, which is
located at the thick end of the wedge. The valves are
inclined from the vertical, but all the valves are in line.
Wedge-shaped combustion chambers tend to have a
smaller surface area than other designs and so have
less area on which droplets of fuel can condense. This
assists in reducing the amount of fuel that remains
unburnt after combustion and so reduces the
hydrocarbon emissions in the engine’s exhaust.
Diesel cylinder heads
With diesel engines, the face of the cylinder head is
usually flat and the combustion chamber is formed in
the top of the piston instead of in the cylinder head
(Figure 2.6). In some designs, the rim of the piston

figure 2.6

A section of a diesel cylinder showing a
combustion chamber in the piston head

provides squish, which forces the air towards the
centre of the piston and into the combustion chamber.
This causes turbulence as the fuel is being injected into
the cylinder.
Combustion chambers of diesel engines are specially designed to promote turbulence, so that the
compressed air and injected fuel are properly mixed.
There are a number of ways in which this is done,
including the use of precombustion chambers.
■ Combustion chambers for diesel engines are
discussed in Chapter 18: Diesel engines: features.

Engine valves
A valve, with its parts identified, is shown in Figure
2.7. It has two main parts: the stem and the head. The
valve is fitted to a port in the head with its face
providing a gas-tight seal against the seat in the port.
This type of valve is known as a poppet valve or a
mushroom valve.
A portion of a cylinder head with an intake and an
exhaust valve is shown in Figure 2.8. This can be used
to identify the various parts, including the valves,
springs, seals, guides and valve-seat inserts.
The intake valves are larger than the exhaust
valves. This is because the intake air that is being
taken into the cylinder through the intake port is at a
low pressure, while the gases that are being forced

from the cylinder through the exhaust port are at
a much higher pressure. The larger intake port
and valve opening are designed to assist intake air
flow.

figure 2.7

An engine valve with the parts named


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chapter two cylinder head and valves

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of cast iron bush. The guides are pressed into holes
bored through the cylinder head into the valve ports.
Guides can be removed and replaced if they become
worn.
Valve springs
The valves are normally held on their seats by one or
two coil springs which are compressed between the top
of the cylinder and a retainer on the valve stem. The

retainer is held on the end of the valve stem by
conical-shaped collets (Figure 2.9). These are also
known as cotters, keepers or keys.

figure 2.8

Arrangement of an intake valve and an
exhaust valve with their associated parts

Number of valves per cylinder
Some engines have two valves for each cylinder – one
intake and one exhaust. Some engines have three
valves for each cylinder – two intake and one exhaust.
Other engines have four valves for each cylinder – two
intake and two exhaust.
Two intake valves provide better breathing. The
two valves allow larger intake passages and a freer
flow into the cylinder, so that the cylinder receives a
better charge. This increases the engine’s volumetric
efficiency.
Similarly, two exhaust valves enable the engine to
be designed with larger exhaust passages. These
provide a freer flow of exhaust gases from the cylinder
and so there is less gas residue.

figure 2.9

Valve-spring retainers

■ The term breathing refers to the engine’s taking in

air or air–fuel mixture.

The pitch of the coils of valve springs is often
closer at the bottom of the spring than at the top.
Springs can also be made of wire with a specially
shaped section. The purpose of these variations is to
keep the valve on its seat when it closes and prevent
valve bounce. A simple spring could have resonance
that would allow the valve to bounce on its seat under
certain operating conditions. For this reason, two
springs, an inner and an outer (as can be seen in
Figure 2.8), are sometimes used.
Springs can also be tapered, with the top coils being
a smaller diameter than the lower coils. This is done to
reduce the mass of the spring that actually moves when
the valve is opened and closed.

Valve seats and guides

Valve-stem seals

The valve ports in the cylinder head have seats on
which the valves rest when they are closed and this
forms a gas-tight seal. The seats are metal rings
(inserts) that are pressed into recesses that are cut in
the head. The inserts are made of a special iron alloy
that is designed to withstand the high temperatures of
the gases that pass through the exhaust ports.
The valves operate in valve guides that are a form

Oil seals are fitted to the valve stems or to the valve
guides. These prevent excessive oil from passing down
between the valve stem and its guide into the
combustion chamber. Oil seals fitted to the tops of the
valve guides can be seen in Figures 2.8 and 2.10.
The action of a valve stem seal is shown in Figure
2.11. The coil spring on the outside holds the sealing
edge against the valve stem, while the angle at the top


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part one engine construction and overhaul

Oil will pass the intake valves if the seals are worn,
or if the valve guides are worn and there is excessive
valve-to-guide clearance. Also, more oil will tend to
pass through the guides when the engine is operating
under light conditions or no-load conditions, such as
when travelling downhill. Under these conditions, the
intake manifold pressure will be much lower than
atmospheric pressure and this will cause more oil to

pass down through the valve guides.
Valve temperatures

figure 2.10

Valve components
1 cotters, 2 spring retainer, 3 valve spring,
4 valve stem seal, 5 spring seat, 6 exhaust valve, 7 intake
valve HOLDEN LTD

figure 2.11

Intake valves pass air or air–fuel mixture and so run at
a cooler temperature than exhaust valves. The exhaust
valves are in the path of the hot gases that pass through
the exhaust ports and so the heads of exhaust valves
become very hot.
Figure 2.12 shows the typical temperature pattern
of an exhaust valve. During operation, the stem
transfers heat to the guide, so the stem is the coolest
part of the valve. The head near the face of the valve
transfers heat to the valve seat, so that is the coolest
part of the head. The valve seat and guide, in turn, are
cooled by the coolant in the water-jackets that
surround the valve ports.

Valve stem seal installed on top of a valve
guide FORD
figure 2.12

of the seal forms a small reservoir of oil to lubricate
the stem and guide.
Some oil is needed for valve stem lubrication, but
too much oil passing through the guides will cause
problems. Oil will be burnt and carbon deposits will
form in the intake valve ports and on the valve heads.
■ Worn valve guides will also cause excessive oil
consumption and smoke from the exhaust.
Reason for valve stem seals
The intake valve is more likely to pass oil through its
guide than the exhaust valve. This is because the intake
port has low pressure that tends to suck the oil in. The
exhaust port has a higher pressure that tends to keep
the oil out.

The temperature in different parts of an
exhaust valve

The valve temperatures show the importance of
correct valve seating. If a valve does not seat properly
there will be a smaller contact area through which heat
transfer can take place and the face will overheat.
Local hot spots will reach very high temperatures and
the edge of the valve will burn.
The exhaust-valve seat is also subjected to
extremely high temperatures, and for this reason,
exhaust-valve seats in many engines are fitted with
heat-resistant alloy inserts.
Sodium-cooled valves
Some engines have sodium-cooled valves. These have

hollow stems that are partly filled with metallic


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chapter two cylinder head and valves

sodium. This melts at about 90°C and so becomes
liquid at engine operating temperatures.
As the valve moves up and down in its guide, the
liquid sodium is thrown around inside the valve stem.
While doing this, the sodium absorbs heat from the
hotter part of the valve near the head and transfers it to
the cooler part of the valve at the stem.
The cylinder head in Figure 2.13 shows valves with
hollow stems. The intake valve stem is hollow to
reduce its mass. The exhaust valve stem is also hollow,
but it contains sodium for cooling. Sodium valves can
often be recognised by their stems which are larger in
diameter than normal valve stems. Sodium is a highly
reactive element that is safe while it is contained
within the hollow valve stem.

29

The valve seats are often ground to the same angle
as the valve face, but some manufacturers use an
interference angle as shown in Figure 2.14. With this,
the valve face and seat are ground to slightly different
angles. These vary with different engines, and the
interference angle may be 0.5°, or as much as 2°.
A typical specification is 45° for the valve face and
44° for the valve seat.

■ Old sodium filled valves should not be tampered
with and should be disposed of in a safe manner.
intake valve
(hollow stem)

intake
camshaft

figure 2.14

Valve angles
(a) interference angle between the valve face
and seat (b) identical angle

An interference angle is provided by an engine
manufacturer to allow for a quick bedding-in of the
valve face to the seat on new engines, and is not
always used when reconditioning valves and seats.
Valve rotation

exhaust valve
(sodium filled)

exhaust
camshaft

figure 2.13

Cylinder head of a horizontally-opposed
engine with a hollow-stem intake valve and a
sodium-filled exhaust valve SUBARU

Valve-face angles
The faces of most valves are ground at an angle of 45°
to the valve stem, although angles of 30° have been
used for some intake valves. In some engines, the
intake valves have 30° face angles and the exhaust
valves have 45°.

Valve rotation refers to the action of a valve turning a
little as it opens and closes, so that it gradually rotates
and does not always seat in the same place.
Valve rotation has a number of advantages: it
produces a slight wiping action, which tends to keep the
face and seat free of carbon; it helps to prevent sticking
in the valve guide and it distributes the heat around the
valve seat. All these help to increase valve service life.
Valves have a natural tendency to rotate and this is
aided in different ways. One way is to have the rocker
arm slightly offset to the valve stem as shown in

Figure 2.15. This causes the valve to turn slightly each
time it is opened.
Another method is to have positive valve rotators
on the exhaust valves. These are sometimes used on
larger engines. The rotator is similar to a small thrust
ballrace, which is fitted either under or on top of the
valve spring. The balls rest on small ramps and each
time the valve is opened, the balls are forced to move
up the ramps. This has a positive action which causes
the valve to rotate slightly as it is opened and closed.


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part one engine construction and overhaul
adjustment

rocker arm
exhaust
valve

pushrod

exhaust
port

valve lifter
piston

figure 2.15

connecting
rod

Rocker-arm offset helps with valve rotation
FORD

camshaft

crankpin

Valve trains for OHV engines
The various components used to operate the valves are
referred to as the valve train. Generally, an overheadvalve (OHV) engine with its camshaft in the crankcase
has a greater number of moving parts than an
overhead-camshaft (OHC) engine.
A basic arrangement of an in-line OHV engine is
shown in Figure 2.16. The valve train consists of:
1. the camshaft

crankshaft

oil pump

figure 2.16

Basic in-line overhead-valve engine

2. valve lifter
3. pushrod
4. rocker arm.
■ Valve lifters are also referred to as cam followers
or tappets.
Camshaft
In OHV engines, the camshaft is located in bearings in
the cylinder block or crankcase. It is driven from the
crankshaft at half the engine speed. It has a cam for
each valve and for carburettor engines, it has an
additional cam to operate the mechanical fuel pump.
As the cams rotate, they move the valve lifters up and
down and this movement is transferred through the
other parts of the valve train to the valves in the
cylinder head.
Where a distributor is used as part of the ignition
system, there can be a gear on the camshaft for the
distributor drive. This gear is also used to drive the oil
pump.
Valve lifters
The cams and valve lifters change the rotary motion of
the camshaft into linear or straight-line motion of the

pushrods. Rotation of the camshaft moves the valve

lifters up and down, and this movement is transferred
by the pushrods to the rocker arms on top of the
cylinder head.
There are two general types of valve lifters: solid
lifters and hydraulic lifters, although most OHV
engines now use hydraulic lifters. (These are discussed
below, see section ‘Hydraulic valve lifters for OHV
engines’.) Solid lifters are actually small hollow cast
iron cylinders. They are mounted in bores in the
crankcase and are free to rotate. The slow rotation that
occurs distributes the wear from the cam over the face
of the lifter.
Pushrods and rocker arms
Some rocker arms are made of cast steel, some are a
steel pressing and others are of aluminium alloy. The
engine in Figure 2.16 has cast rocker arms that are
mounted on a rocker shaft. The rocker arms are lubricated by oil supplied through the hollow shaft.
Figure 2.17, which is one bank of a V-type engine,
has pressed-steel rocker arms that are supported by ball
pivots mounted on studs in the cylinder head. The
ball is lubricated from an oil gallery in the cylinder


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chapter two cylinder head and valves

31

valve tip. However, they can be adjusted for certain
conditions by turning the nut on the ball stud to raise
or lower the rocker arm.

Hydraulic valve lifters for
OHV engines

figure 2.17

One bank of a V-type engine with a hydraulic
valve lifter and a ball-pivoted rocker arm

head through a drilling in the stud. The pushrod is also
hollow and this carries oil to lubricate the end of the
rocker arm.
Valve trains with solid valve lifters have a small
clearance, referred to as valve clearance, valve lash or
tappet clearance. This is provided between the end of
the rocker arm and the tip of the valve stem. If there
was no clearance in the valve train, the valve would
not close properly and it would quickly burn out.
The cast rocker arms (Figure 2.16) have a screw
adjustment at the pushrod end to adjust the clearance.
The pressed-steel rocker arms (Figure 2.17) have
hydraulic valve lifters and do not have clearance at the

body

Hydraulic valve lifters are quiet in operation because
there is zero lash. That is, there is no free movement in
the valve train. There is no need for clearance between
the rocker arm and the valve stem because the
hydraulic action takes care of any changes in the valve
train due to wear or temperature.
Figure 2.18 shows the construction and operation of
a hydraulic lifter for an OHV engine. There are two
main parts: a hollow body, and a plunger in the body.
The top of the plunger has a cup for the pushrod, and
there is a spring under the plunger which holds it
upwards.
The lifter is supplied with pressure oil from the
engine’s lubricating system. A ball valve under the
bottom of the plunger allows oil into the chamber
beneath the plunger and this holds the plunger against
the pushrod.
Operation
The hydraulic lifter operates as follows:
1. With the engine valve closed, there is no load on
the lifter, and the plunger is held upwards by the
plunger spring.
2. Oil from the oil gallery enters the lifter through
holes in the lifter body and plunger.

pushrod
plunger

roller

oil gallery

ball valve
(closed)

ball valve (open)

camshaft
(a) Engine valve closed

figure 2.18

Hydraulic valve lifters for an OHV engine

(b) Engine valve open
HOLDEN LTD


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part one engine construction and overhaul

3. The oil pressure forces the check valve open to
keep the chamber below the plunger full (Figure
2.18(a)). This removes lash from the valve train.
4. As the cam rotates and raises the lifter, the check
valve closes to trap the oil in the chamber below the
plunger.
5. The lifter moves upwards as an assembly, as shown
in Figure 2.18(b), to open the engine valve in the
usual way.
6. As the cam continues to rotate, the lifter moves
downwards to close the engine valve, and the force
on the plunger is relieved. Any oil lost from the
chamber is replaced by oil through the check valve.
7. The lifter is designed to have a slight leak past the
plunger. This acts as a lubricant between the
plunger and body, and also enables air to be bled
from the lifter.

Valve trains for OHC engines
Overhead camshaft engines have the camshaft
mounted on top of the cylinder head. This provides a
short valve train, although a long timing chain or belt
is needed between the crankshaft and the camshaft or
camshafts.
Having the camshaft overhead also eliminates the
reciprocating parts that are used in the valve trains of
overhead-valve engines. This is an advantage with
smaller high-speed engines where reciprocating parts

could produce vibration.
The valves in overhead camshafts are operated by
rocker arms or, more directly, by bucket-type tappets.
There are a number of different arrangements.

figure 2.19

Overhead-camshaft and valve arrangement
1 rocker cover, 2 spring retainer, 3 spring,
4 seal, 5 exhaust valve, 6 rocker arm, 7 rocker shaft,
8 camshaft, 9 cylinder head, 10 intake valve MITSUBISHI

not only allow a gas leak, but would cause the valve to
burn. The rocker arms in the illustration have an
adjusting screw on the end of the rocker arm to adjust
the valve clearance.
Bucket tappets
Figure 2.20 shows the arrangement for bucket tappets
and a double overhead camshaft. The camshafts are
mounted directly above the valves. Only two valves

Rocker arms
Figure 2.19 shows a section through a cylinder head
with a single overhead-camshaft and rocker arms to
operate the valves. This is the basic arrangement for
OHC engines. It has a hemispherical combustion
chamber with the valves set at an angle in the head. It
has one camshaft, but two sets of rocker arms – one set
for the exhaust valves and one set for the intake valves.
Rocker arms are a form of lever, with one end

against a cam of the camshaft and the other end against
the stem of a valve. As the camshaft rotates, cam
movement is transferred by the rocker arm to open and
close the valve.
There has to be some clearance in the valve train to
allow for changes in temperature, otherwise the valve
could be prevented from seating properly. This would

figure 2.20

Twin overhead-camshaft arrangement with
bucket tappets TOYOTA


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chapter two cylinder head and valves

are shown, but the cylinder actually has four valves –
two intake and two exhaust.
The bucket tappets are located between the cams of
the camshaft and the ends of the valve stems. Cam
movement is transferred directly through the tappet to
the valve.

The bucket tappet is cup-shaped so that it fits over
the end of the valve and spring. It operates in a guide
which protects the valve against side thrusts that it
would receive if the cam operated directly against the
valve. There is a small clearance between the cam and
the tappet and this is adjusted by altering the thickness
of shims or spacers that are in the tappet.
■ Bucket tappets can also be identified in Figure 2.24
and in Figure 2.43, which also has two camshafts.

Hydraulic lash adjusters for
OHC engines
Some overhead-camshaft engines are fitted with
hydraulic lash adjusters that are used to remove the
lash from the valve train. These operate in a similar
way to the hydraulic valve lifters for OHV engines
discussed previously. With hydraulic lash adjusters,
there is no clearance at the valve stem and this
eliminates the need for adjustment.

33

There are three general locations for hydraulic lash
adjusters in OHC engines:
1. In the valve end of the rocker arm.
2. In the cylinder head at the end of the rocker arm.
3. In bucket-type tappets.
Adjuster in the rocker arm
Figure 2.21 shows hydraulic lash adjusters fitted in
the ends of the rocker arms which operate against the

valve stems. They have a body with a plunger which is
held against the valve stem by a spring. Oil supplied to
the adjuster keeps the plunger in contact with the valve
and eliminates lash from the valve train.
The detail of a hydraulic lash adjuster can be seen
in Figure 2.22. When the lash adjuster is under load,
the plunger is held outwards against the valve tip by
the plunger spring. Oil is trapped in the high-pressure
chamber by the check ball and this eliminates any lash
in the valve train. The lash adjuster acts like a solid
lifter.
There is a reservoir of oil in the adjuster that
is supplied by the engine’s lubricating system. Any
loss of oil from the high-pressure chamber will allow
the plunger to move back into the body. But, when the
load is released, the spring will move the plunger

intake valve
hydraulic lash
adjusters

oilway
intake rocker arm
rocker shaft

exhaust
rocker arm

camshaft

retainer
spring
guide
exhaust valve

figure 2.21

Section through a cylinder-head assembly for a horizontally opposed engine; the hydraulic lash adjusters are
in the ends of the rocker arms SUBARU


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part one engine construction and overhaul

Lash adjuster in bucket tappet

high-pressure chamber
check ball
rocker arm

plunger

oil reservoir

body

figure 2.22

plunger spring

The arrangement for a bucket tappet is shown in
Figure 2.24, where the lash adjuster is installed inside
the bucket tappet. In this design, the hydraulic action
of the plunger tends to spread the tappet. This holds
the bucket body against the cam on the camshaft and
the plunger against the tip of the valve stem. This gives
zero clearance.

Camshafts

oil path

Hydraulic lash adjuster for a rocker arm
SUBARU

outward and allow oil from the reservoir to enter the
high-pressure chamber. In this way, the high-pressure
chamber will always be full and the plunger will
always be in contact with the tip of the valve.
Lash adjuster in cylinder head
The cylinder-head assembly in Figure 2.23 has a

double camshaft and pivot-type hydraulic lash
adjusters mounted in the cylinder head. The rocker
arms are a roller type, with a roller bearing operating
against the cam to reduce friction and wear.
The lash adjusters operate in the same way as those
in a rocker arm, except that they are stationary and
provide a ball pivot for one end of the rocker arm. The
ball is on the end of the lash-adjuster plunger and this
holds the roller up against the cam.

While the basic purpose of the camshaft is to open and
close the valves, the cams perform a far more detailed
function, because the entire action of the valve
depends on the shape of the cam.
The shape of the cam is referred to as the cam
profile, or cam contour. This will determine when the
valve commences to open, its maximum opening, how
long it remains open, and when it closes. All these are
part of the design of a particular engine.
The three basic camshaft arrangements are: single
overhead camshaft (SOHC), double overhead camshaft (DOHC) and overhead-valve (OHV) camshafts.
Single overhead camshaft (OHC)
A single camshaft is located on the top of the cylinder
head and driven by a toothed belt and toothed pulley,
or by a chain and sprocket. The camshaft in
Figure 2.25 is driven by a toothed belt. It has five
journals which support it on top of the cylinder head
and eight cams to operate the valves.

lash adjuster

exhaust
camshaft

intake
camshaft

spring
retainer
valve spring
E

I

E

I

valve stem seal

exhaust port

intake port

valve layout
exhaust
valve

figure 2.23

intake
valve

Cylinder head with double overhead camshafts and pivot-type hydraulic lash adjusters

HYUNDAI


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chapter two cylinder head and valves

35

Double overhead camshafts (DOHC)
Two camshafts (DOHC) are used in many engines.
Each cylinder of a DOHC engine has four valves – two
intake and two exhaust valves. One camshaft is used
for the intake valves and the other for the exhaust
valves. A cylinder head with two camshafts is shown
in Figure 2.26. These have toothed pulleys that are
driven by a toothed belt.
With V-type engines, two camshafts are used for
each bank of cylinders, so the engine has four camshafts. This arrangement is sometimes referred to as

quad-cam.
■ DOHC arrangements can also be seen in a number
of other figures in this chapter.
Camshafts for overhead valves (OHV)

figure 2.24

Bucket tappet with a hydraulic lash adjuster
1 bucket body, 2 ball seat, 3 check ball,
4 plunger spring, 5 check-ball spring, 6 oil passage,
7 chamber A, 8 plunger, 9 sleeve, 10 chamber B FORD

■ Toothed pulleys are also referred to as sprockets,
although this term is usually reserved for gears that
are used with chains.

figure 2.25

Camshaft for a four-cylinder OHC engine

In overhead-valve (OHV) engines, a single camshaft is
mounted in the cylinder block and valve lifters and
pushrods transfer the cam action to push rods on top of
the cylinder head. The arrangement for an in-line
engine was shown previously in Figure 2.16.
With a V-type engine the camshaft is situated in
the cylinder block directly above the crankshaft, as
shown in Figure 2.27. The camshaft is fitted with a
sprocket that is driven by a short timing chain from
a sprocket on the end of the crankshaft. In the

assembly shown, the oil pump is driven directly by the
crankshaft.

Camshaft drives and timing
Camshafts are driven at half the crankshaft speed by
means of timing gears, timing chains or timing belts

HOLDEN LTD


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part one engine construction and overhaul
camshaft pulleys

bearing caps (8)
camshaft
journals

front bearing
caps (2)
intake camshaft

cylinder
head bolts (10)
exhaust
camshaft

cylinder head
gasket

figure 2.26

Cylinder head assembly with double overhead camshafts

that are connected between the camshaft and the
crankshaft.
The camshaft is designed so that the valves open and
close correctly in relation to each other and in the correct
firing order for the engine. However, the camshaft must
also be timed to the crankshaft so that the valves open
and close at the correct times in relation to piston
movement. This is a function of the camshaft drive.
■ The drive must be correctly timed and timing marks
are provided for this purpose.
Timing gears
Where timing gears are used, the camshaft gear is
twice the size of the crankshaft gear (Figure 2.28). The
crankshaft gear has marks on two adjacent teeth, while
the camshaft gear has marks on only one tooth. For

FORD

correct valve timing, the single marked tooth on the
camshaft gear is meshed between the two marked teeth
on the camshaft gear as shown.
This is a simple arrangement that was used for
OHV engines in passenger vehicles, but which now
has a limited application.

Timing chains
Figure 2.29 illustrates the chain arrangement for a
V-type OHV engine. This is a double-roller chain with
a sprocket on the camshaft and another on the
crankshaft. The sprockets carry timing marks. For
correct timing of the arrangement shown, the timing
mark on each sprocket and the centre of each shaft
should be in line when No. 1 piston is on top deadcentre (TDC).


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chapter two cylinder head and valves

37

camshaft

bearings
valve lifters
camshaft
lifter guides

retainer

camshaft
location
camshaft
sprocket

crankshaft
oil pump
crankshaft sprocket
and pump drive
timing chain

figure 2.27

Cylinder block, crankshaft assembly and drive for a V-type OHV engine

figure 2.28

Camshaft and crankshaft gears with timing
marks

HOLDEN LTD

timing

marks

figure 2.29

Overhead camshaft chains

Timing chain and timing marks for a V-type
OHV engine – the marks on the sprockets are
in line with the centrelines of the shafts FORD

Overhead camshafts can be driven by chains in a
number of different ways. In Figure 2.30, a single
chain drives the camshaft sprocket and also an
auxiliary shaft sprocket. The auxiliary shaft drives the
oil pump. The chain has a damper, or guide, on one
side and a hydraulic tensioner on the other.

There are two timing chains in Figure 2.31, which
has two overhead camshafts. One chain drives an
auxiliary shaft for the oil pump and the other drives the
two sprockets for the two overhead camshafts. This


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part one engine construction and overhaul
timing chain

camshaft
sprocket

damper

tensioner

crankshaft
sprocket

auxiliary
shaft sprocket

figure 2.30

Timing chain for an in-line engine with an
overhead camshaft – the sprockets carry
timing marks (not shown) FORD

arrangement avoids what would otherwise have to be a
long chain drive.

figure 2.31

Timing-chain arrangement for double
overhead-camshaft engine
1 slipper, 2 tensioner, 3 camshaft sprocket, 4 damper,
5 intake camshaft, 6 exhaust camshaft, 7 camshaft sprocket,
8 guide, 9 pump drive shaft, 10 oil pump drive, 11 crankshaft,
12 crankshaft sprocket, 13 tensioner, 14 pump shaft
sprocket, 15 camshaft drive sprocket TOYOTA

Timing belts
Toothed timing belts are used with many overhead
camshafts. They are quieter in operation than chains
and require no lubrication. Belts are also able to be
wrapped further around the pulleys or sprockets than
chains. Belt tensioning pulleys and idler pulleys are
relatively simple as they are able to run on the smooth
back of the belt to guide the belt and keep it tight.
The construction of a timing belt is shown in
Figure 2.32. It consists of a core of glass fibre that has
a high tensile strength to resist stretching, a canvasreinforced tooth section to resist wear, and a rubber
backing that has good heat- and wear-resistant
properties.
Timing belt arrangements
A timing belt for a single overhead camshaft is shown
in Figure 2.33. This has timing marks that are

figure 2.32

Construction of toothed timing belt

MAZDA

identified by arrowheads. The crankshaft pulley is
positioned with the key to the top, and the camshaft
pulley is positioned in line with its timing mark as
shown.
Figure 2.34 shows the timing belt for a
horizontally-opposed engine. This is a long belt that
drives a water pump as well as the two camshafts.
There are two idler pulleys on the back of the belt that


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chapter two cylinder head and valves
timing
belt

camshaft
pulley

39

The arrangement of the timing belt and timing
marks for an overhead camshaft V-type engine is

shown in Figure 2.35. This is a long timing belt that
drives the camshaft pulleys on each bank and also the
coolant pump. An idler pulley on the back of the belt
acts as a guide and a hydraulic tensioner keeps the
belt tight.

Drives for DOHC
tension
pulley

auxiliary
drive pulley

crankshaft
pulley

figure 2.33

Toothed-belt drive and timing marks for an
overhead-camshaft engine FORD

guide the belt and keep it wrapped around the pulleys.
An automatic tensioner pulley keeps the belt tight.

figure 2.34

The drive for double overhead camshafts can be
arranged in a number of different ways. For example,
the upper chain in Figure 2.31 drives both camshafts.
In other engines, a chain or belt is used between the

crankshaft and the exhaust camshaft. A drive chain is
then used between a sprocket on the exhaust camshaft
and a sprocket on the intake camshaft (Figure 2.36).
The exhaust camshaft is timed to the crankshaft, but
the intake camshaft has to be timed to the exhaust
camshaft. For this purpose, there are timing marks on
the chain and on the sprockets.
Another method of driving double camshafts is by
gears. The gears between the camshafts are sometimes
referred to as scissor gears (Figure 2.37). These are
provided with timing marks.
To reduce lash between the gears, the intake camshaft has a thin subgear that is held against its drive
gear. This creates side friction between the two gears.
Both gears are in mesh with the exhaust camshaft
gear, but the teeth of the subgear mesh differently

Timing belt, sprockets and pulleys for a horizontally-opposed engine

SUBARU


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part one engine construction and overhaul
Right bank

Left bank

timing marks

timing marks

water pump pulley

camshaft
sprocket

camshaft sprocket
tensioner pulley
idler

tensioner

timing mark
crankshaft sprocket

figure 2.35

Timing belt and timing marks for a V-type OHC engine

camshaft
pulley

MITSUBISHI

drive gears

exhaust camshaft sprocket

chain

sub-gear

intake
camshaft
sprocket

figure 2.36

Double overhead camshafts with a chain
drive between camshafts HYUNDAI

intake
camshaft

figure 2.37

because the profile of the subgear is different to that of
the drive gear.
The result of this arrangement is that lash between
the two drive gears is dampened out.

Tensioners and dampers
Various methods are used to dampen out the lash and
to provide tension on timing belts and chains.
When operating under a constant load, a chain or

exhaust
camshaft

Double overhead camshafts with a gear drive
between the camshafts TOYOTA

belt will have one slack side and one tight side. At
engine idle, the slack will vary due to the power
impulses of the engine and the chain or belt will tend
to vibrate. Also, each time the engine speeds up or
slows down, the tension will alter and the slack will
move from one side to the other. To prevent this and to
reduce noise, some form of damper or tensioner is
used.


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chapter two cylinder head and valves

Chain devices
In general, there are three devices that are used with
chains:
1. Dampers that act on the back of a chain to prevent
it from vibrating.
2. Guides that direct the chain in relation to the
sprockets.
3. Tensioners that provide a load against the chain to
keep it tight.
These devices can be seen in Figures 2.30 and 2.31.
Dampers, guides and tensioners are used for
slightly different purposes, but they all act in a similar
manner against the back of the chain. In shorter chains,
the action is mainly one of tensioning but, with longer
chains, they also act as guides.
A damper consists of a synthetic rubber pad that
may be either fixed or held against the back of the
chain by a spring blade. A guide is usually a fixed pad
against which the back of the chain operates.
A tensioner is either spring-loaded or provided with
a hydraulic plunger to hold the rubber pad against the
back of the chain. Hydraulic tensioners are selfadjusting to compensate for wear of the pad and
stretching of the chain.
Hydraulic tensioners are operated by engine oil
pressure. The rubber pad that bears against the chain is
attached to a plunger that operates in a small cylinder.
With the engine stopped and no engine oil pressure, a
spring holds the plunger outwards with the rubber pad
against the chain. With the engine running, oil pressure

acts against the plunger to force the pad against the
chain.

41

camshaft to be advanced or retarded to suit the
engine’s operating conditions. With normal valve
timing, the camshaft is designed so that it opens and
closes the valves to give best engine performance at
higher speeds. However, this means that there is some
loss of power at lower engine speeds.
With variable valve timing, the camshaft is advanced
or retarded to suit the engine speed and conditions and
this improves both low- and high-speed performance.
The range of advance will depend on the design of the
particular unit, but this can be as much as 40° of
crankshaft rotation.
■ There are a number of different methods of providing variable valve timing, but they are all used
to improve engine performance.
Variable valve timing with SOHC
A variable camshaft timing unit for a single overhead
camshaft is shown in Figure 2.38. This is bolted to the
camshaft sprocket. The camshaft sprocket is driven in
the normal way by the timing chain and this rotates the
camshaft timing unit as well as the camshaft. However,
while the camshaft is rotating, the timing unit is able
to move the camshaft in relation to the sprocket to
advance or retard the valve timing.
As well as the unit shown, there is a housing bolted
to the front of the engine that contains oil passages and

camshaft
sprocket

timing marks

timing chain

variable
timing
unit

Belt devices
Timing belts use idlers and tensioners in the form of
rollers that run on the back of the belt. The idlers guide
the belt so that it wraps around the crankshaft and the
camshaft pulleys. This increases the length of belt that
is in contact with the pulleys.
There are many arrangements of idlers and
tensioners. The belt in Figure 2.33 has a simple springloaded tensioner that is manually adjustable. Figure
2.34 has idlers and a hydraulic tensioner. Figure 2.35
also has an idler pulley and a hydraulic tensioner.

Variable valve timing
Variable valve timing, also called variable camshaft
timing, is fitted to some engines. It enables the

figure 2.38

Variable camshaft timing unit for an SOHC
engine FORD

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part one engine construction and overhaul

an oil-control valve that is operated by a solenoid. The
timing unit is operated hydraulically by engine oil
pressure and the oil-control valve controls the supply
of engine oil to and from the timing unit.
A partly sectioned camshaft timing unit is shown in
Figure 2.39. Its main parts are:
1. Camshaft sprocket. This is driven by the timing
chain and has the timing unit attached.
2. Inner sleeve. This is bolted to the front of the
camshaft. It has helical gear teeth which mesh with
the ring gear.
3. Ring gear. This ring-shaped gear has external teeth
as well as internal teeth. Its internal teeth mesh with
the teeth on the sleeve and its external teeth mesh
with the teeth inside the housing. The ring gear is
attached to the ring piston which is operated by

engine oil pressure.
4. Housing. This encloses the parts. It has internal
teeth that mesh with the external teeth of the ring
gear. It also provides a cylinder in which the ring
piston operates.
5. Ring piston and return spring. The ring piston is
operated by engine oil pressure to advance the
camshaft. The spring is used to return the ring
piston to retard the camshaft.

ring piston

housing

Operation of the timing unit
The timing chain rotates the sprocket on the timing
unit and this rotation is carried through to the camshaft
by the internal gears of the unit. The gears provide a
connection between the sprocket and the camshaft, but
they do not rotate.
In the retarded position, oil pressure is blocked off
by the solenoid valve and the piston is held in the
forward position by the spring.
In the advanced position, the solenoid valve is opened
so that engine oil flows into the advance unit. The
pressure of oil against the ring piston moves it and the
ring gear rearwards. This turns the camshaft in relation to
the sprocket, so that the camshaft timing is advanced.
The actions of advance and retard are:
1. Camshaft advance. When the piston moves the ring

gear, the helical teeth slide on each other and so the
ring gear causes the sleeve to turn. This moves the
camshaft in a forward (advance) direction in
relation to the housing and sprocket. This occurs
within the advance unit while it is rotating.
■ Camshaft advance occurs because of the helical
shape of the gear teeth.
2. Camshaft retard. When the solenoid is operated to
relieve the oil pressure in the unit, the oil drains
away and the spring force against the piston moves
the ring gear forward. Again the gear teeth slide on
each other, but this time to retard the camshaft.
The solenoid valve is controlled by the engine’s
electronic control module (ECM). At higher engine
speeds, for example above 2000 rpm, the ECM opens
the valve to allow engine oil to flow into the advance
unit. At lower speeds, the solenoid valve closes off
the oil supply and allows oil to drain from the unit.

inner sleeve
teeth on
inner sleeve

■ The engine control module (ECM) is a microcomputer that is responsible for monitoring and
controlling a number of engine functions.
Variable valve timing with DOHC

return
spring

ring gear

camshaft
sprocket

figure 2.39

Construction of a variable camshaft timing
unit FORD

With double overhead camshafts, an advance device is
fitted to the intake camshaft but not to the exhaust camshaft. This is because altering the intake valve timing has
a much greater benefit than altering the exhaust timing.
Figure 2.40 shows a schematic arrangement of two
camshafts of an engine and a control system. The
camshafts are driven by a notched timing belt. The
intake-camshaft pulley has a variable timing unit and
the exhaust camshaft has a normal pulley.


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chapter two cylinder head and valves
oil control valve

43

camshaft sensor
TDC detection
plate
intake camshaft

exhaust
camshaft

ECM

variable timing
actuator
engine oil
supply line

timing belt
crankshaft
sensor

figure 2.40

Variable valve-timing system for a DOHC engine

The components of the system shown are:
1. the variable valve-timing unit
2. an oil control valve
3. electronic sensors

4. an engine electronic control module (ECM).

MAZDA

■ Variable valve-timing systems rely on correct
lubricating oil viscosity for their operation.
Servicing at the recommended intervals with the
recommended grade of oil is critical to maximum
performance. The use of oil additives is not
recommended.

Operation of the timing system
Engine oil under pressure is directed to the oil control
valve. This is a shuttle valve that is operated by an
electrical solenoid. The solenoid moves the valve, as
required, to direct oil to the variable valve-timing unit
and alter the camshaft timing. The solenoid of the
control valve is operated by the engine’s electronic
control module (ECM).
The ECM receives signals from a number of
sources, including the sensor on the crankshaft, which
registers engine rpm, and the sensor on the camshaft,
which detects piston TDC. Input from the sensors is
processed by the ECM, which determines how much
advance or retard the intake camshaft needs for
particular operating conditions. Output signals from
the ECM go to the oil control valve.

Hydraulic system
Figure 2.41 shows the arrangement of the hydraulic

system.
The variable valve-timing unit has a rotor within its
housing. The housing carries the timing pulley and the
rotor is mounted to the end of the camshaft. Movement
of the rotor in relation to the housing will advance or
retard the camshaft timing.
The rotor is shaped to fit into the housing and fitted
with seals to form hydraulic chambers (Figure 2.41(a)).
The spool valve and a section through the timing
unit are shown in Figure 2.41(b). In the diagram, the
spool valve has been moved by the solenoid to direct
oil to the advance side of the rotor. This has turned the
rotor in the housing to fully advance the camshaft.


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part one engine construction and overhaul
spool valve

solenoid

engine oil
pump

ECM

oil return
oil control
valve assembly
pulley

advance
chamber

housing

camshaft
rotor
(a)

figure 2.41

Variable valve timing in maximum advance mode
(a) variable timing unit (b) hydraulic arrangement

To retard the camshaft, the spool valve is moved in
the opposite direction. Pressure oil is directed to the
retard side of the rotor and drained away from the
advance side.
The camshaft does not have to be fully advanced or
retarded. It can be held in any intermediate position by

the spool valve directing pressure oil to both sides of
the rotor. The ECM determines the camshaft timing for
the particular operating conditions.
When the engine is stopped, the rotor is held in the
retarded position by a spring-loaded pin. When
the engine is started, engine oil pressure disengages the
pin so that the rotor can be turned in the housing.
Variable valve opening
With variable valve opening, the intake valve is
arranged to open further at higher engine speeds. This
increases the intake air flow and improves the engine’s
volumetric efficiency. There are different ways of
achieving this.
In one system, each cylinder has an extra intake
rocker arm operated by a high-lift cam on the camshaft. At lower engine rpm the cam moves the rocker
arm up and down, but the rocker has no effect on the
intake valves.

rotor

(b)

MAZDA

At higher rpm, a plunger is moved hydraulically to
connect the high-lift rocker arm to the normal intake
rocker arm. Both rocker arms then operate together,
but the intake valve will now be opened further
because of the high-lift cam.
There is another method that is used with a buckettype tappet. The tappet has a spring-loaded plunger

that provides lost motion at low engine rpm. This has
the effect of reducing the intake valve opening because
the full movement of the tappet is not used.
At higher engine rpm, a wedge is hydraulically
inserted between the plunger and the body of the
tappet. This eliminates the lost motion and so
the intake valve is opened further than at lower engine
speeds. Again, the increased valve opening improves
the air flow and engine efficiency.

Valve-timing diagram
Valve-timing diagrams show how the valves are timed
during the four strokes of an engine (intake,
compression, power, and exhaust). The diagrams show
how the valves are timed to open and close in relation
to top dead-centre (TDC) and bottom dead-centre
(BDC) of the piston strokes.
To illustrate variable valve timing and intake


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chapter two cylinder head and valves

camshaft advance, the diagram in Figure 2.42 has been
drawn with two intake strokes. The diagram shows
how advancing and retarding the intake camshaft
affects the valve timing:
1. Intake valve timing, both opening and closing, can
be varied 40°.
2. Exhaust valve timing does not change.
3. Valve overlap will vary from 5° to 45°.
TDC
intake
opens

This lowers combustion temperatures and reduces
the nitrous oxide emissions from the exhaust.
3. At higher speeds and heavier loads, the timing of
the intake valve closing is varied and this helps
with the volumetric efficiency of the engine.
■ For valve-timing diagrams, see Chapter 8: Engine
fundamentals in Volume 1. For an explanation of
volumetric efficiency, refer to Chapter 9: Engine
measurement and performance in this Volume.
Valve-timing terminology

exhaust closes
5∞ after TDC
intake
(advanced)

intake opens
40∞ before TDC

45

intake
(retarded)

exhaust

Lead, lag and overlap are terms associated with valve
timing. The intake valve opens before TDC and the
exhaust valve opens before BDC. This is referred to as
lead.
The intake valve closes after BDC and the exhaust
valve closes after TDC. This is referred to as lag.
The intake and the exhaust valves are both open for
a few degrees around TDC of the exhaust stroke and
this is referred to as overlap.
■ Advancing or retarding the intake camshaft alters
the intake valve’s lead and also its lag. It also
alters the valve overlap.

intake closes
60∞ after BDC

Engine illustrations
exhaust opens
50∞ before BDC

intake closes
20∞ after BDC

figure 2.42

BDC

Timing diagram for variable-valve timing,
showing the advance and retard for the
intake valves

The valve timing is adjusted by the ECM to suit
driving conditions. In general, at idle speed and light
loads, there are slower piston speeds and slower air
intake movement, so opening the intake valve can be
delayed. The opposite conditions apply at higher engine
speeds. Piston speeds are higher and the intake air flow
is faster, so the intake valve opening can be advanced.
There are also advantages by varying the valve
overlap for different conditions:
1. At low speeds and light loads, less overlap allows
more exhaust gas to be expelled from the cylinder
before the intake valve opens – idling is improved.
2. At medium speeds, increasing overlap recirculates
more exhaust gas within the engine (internal EGR).

Figures 2.43 and 2.44 are engine illustrations in which
some of the features that have been covered in this
chapter can be identified.
The engine in Figure 2.43 has double camshafts
with a chain drive similar to that of Figure 2.31. It has
four valves for each cylinder and bucket-type tappets.

The engine in Figure 2.44 is also a double
overhead-camshaft engine, but the camshafts are belt
driven by a single belt. It has four valves for each
cylinder, roller rocker arms and hydraulic lash
adjusters.
■ Engine features can also be identified in Figures
1.18 and 1.19, located at the end of the previous
chapter.

Technical terms
Bank of cylinders, casting, mould, sprocket, rocker,
retainer, lash adjuster, distilled, deionised,
corrosion inhibitor, susceptible, dissipate,
precombustion, swirl, turbulence, detonation,