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

Cylinder head and valves 21 Cylinder heads 22 Combustion chambers 24 Engine valves 26 Valve trains for OHV engines 30 Hydraulic valve lifters for OHV engines 31 Valve trains for OHC engines 32 Hydraulic lash adjusters for OHC engines 33 Camshafts 34 Camshaft drives and timing 35 Drives for DOHC 39 Tensioners and dampers 40 Variable valve timing 41 Valvetiming diagram 44 Engine illustrations 45 Technical terms 45 Review questions 46

Cylinder head and valves

Chapter 2

Cylinder heads

Combustion chambers

Engine valves

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

Engine illustrations

Technical terms Review questions

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

trans-fer 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

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 combus- tion 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

figure 2.1 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

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

overhead-valve 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.

figure 2.2 Basic arrangement of a cylinder-head assembly for an OHC engine FORD

cylinder head

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

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 com- bustion chamber and cylinder walls Turbulence also helps to prevent local high-pressure and high- temperature areas during combustion, which could cause detonation.

figure 2.3 Cylinder head for two cylinders of a horizontally opposed engine SUBARU

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

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

approxi-mately 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

able to be located at the centre of the chamber Also, with the spark plug located at the centre, the flame- travel 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

figure 2.4 Combustion chambers are designed to create

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

rela-tively 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

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 ially 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.

spec-■ 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 2.6 A section of a diesel cylinder showing a

combustion chamber in the piston head figure 2.7 An engine valve with the parts named

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.

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

air or air–fuel mixture.

Valve seats and guides

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

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.

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-stem seals

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

figure 2.8 Arrangement of an intake valve and an

exhaust valve with their associated parts

figure 2.9 Valve-spring retainers

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.

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

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.

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

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

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.

■ Old sodium filled valves should not be tampered

with and should be disposed of in a safe manner.

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.

figure 2.13 Cylinder head of a horizontally-opposed

engine with a hollow-stem intake valve and a sodium-filled exhaust valve SUBARU

exhaust valve (sodium filled)

exhaust camshaft

intake valve (hollow stem)

intake camshaft

figure 2.14 Valve angles

(a) interference angle between the valve face

and seat (b) identical angle

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°.

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

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.

Valve trains for OHV engines

The various components used to operate the valves are

referred to as the valve train Generally, an

overhead-valve (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:

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 lubri- cated 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

figure 2.15 Rocker-arm offset helps with valve rotation

piston connecting rod

crankpin

crankshaft valve lifter

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

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

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.

figure 2.17 One bank of a V-type engine with a hydraulic

valve lifter and a ball-pivoted rocker arm

figure 2.18 Hydraulic valve lifters for an OHV engine HOLDEN LTD

(a) Engine valve closed (b) Engine valve open

pushrod plunger

ball valve (open)

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.

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

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

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

figure 2.20 Twin overhead-camshaft arrangement with

bucket tappets TOYOTA

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.

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

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

guide spring retainer

exhaust rocker arm

rocker shaft intake rocker arm oilway

hydraulic lash adjusters intake valve

camshaft

exhaust valve

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.

Lash adjuster in bucket tappet

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

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 cam- shaft (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.

figure 2.22 Hydraulic lash adjuster for a rocker arm

lash adjuster

exhaust valve

intake valve exhaust port intake port

intake camshaft

spring retainer valve spring valve stem seal

valve layout