47 Cylinder head Ò some overall design considerations

Constructional details of the engine



93



for large quantity production is costly, porosity in the finished casting can

present difficulties, aluminium is more easily damaged in service and rather

more prone to gasket blow-by failure, corrosion may present problems –

especially where there are copper components in the cooling system – and

heat-resistant valve seat inserts are essential.

Cast iron is inherently stiffer, and therefore contains noise better, and is

cheaper. On the other hand, the labour costs in making the moulds and cores

are higher, and more labour may be required for removing the sand cores,

and for fettling.

In a paper presented by D.A. Parker and R.H. Slee, at Symposium 86,

held by AE plc, some particularly interesting comments were made on trends

in engine design, Fig. 3.38. These authors pointed out that many of the

overhead camshaft engines which came into vogue in the nineteen-seventies

had single ohc valve gear with vertical valves and bath tub combustion

chambers in cast iron heads mounted on cast iron crankcases. Screw type

tappet adjusters were used and twin valve-springs obviated a risk of the



1980



(b)

1970



(a)



1990



(c)

Fig. 3.38 Trend in engine design 1970–1990, as illustrated by Parker and Slee, of AE plc



94



The Motor Vehicle



valve’s dropping into the engine in the event of breakage of one, and in any

case, the installation was more compact than with a single larger spring.

By the nineteen-eighties the pistons carrying four-ring pistons had given

way to three-ring types, with shorter skirts to reduce both friction and

reciprocating mass. Aluminium heads became popular, partly to reduce weight,

but also because their better thermal conductivity would help in the struggle

to meet threatened legislation for the elimination of lead from gasoline. For

the latter reason, too, sparking plug position was determined by the need to

shorten flame travel, enabling the ignition to be retarded to reduce octane

number sensitivity of the combustion chamber. Improvements in valve and

port geometry, coupled with the introduction of electronic single-point injection,

helped in the achievement of both better fuel economy and satisfying emissions

regulations.

According to the two authors of the paper, in many instances, by the

nineteen-nineties, not only cylinder heads but also blocks would be of

aluminium alloy, while coolant volumes would be restricted to reduce weight

and shorten the warm-up time. Twin overhead camshafts actuating four valves

per head would provide good specific power and economy. The inherent

stiffness of valve trains of this layout would enable single valve springs to be

used, while self-adjusting tappets would reduce both noise and maintenance

requirements. Because of its superior accuracy of metering, multi-point would

replace both single-point fuel injection and carburettors for cars in all but the

bottom end of the price range. Light-weight, two-ring pistons with integral

hydrodynamic bearing devices for taking the thrust and minimising skirt

area would reduce friction.



3.48



An interesting cylinder head design



An outstandingly good cylinder head design is that of the Dolomite Sprint,

Fig. 3.39. Since it exemplifies the best solution to many of the problems, it

will be described here. It is of aluminium and has four cylinders, four valves

per cylinder, and a single overhead camshaft. The exhaust valves are inclined

16° to one side of the axis of the cylinder and the inlets 19° to the other side.

Their seats are in a penthouse-type combustion chamber.

Since the engine, when installed, is tilted 45° towards the exhaust side,

the inlet valve ports then slope steeply downwards. This facilitates cold

starting, in the following manner. As the crankshaft is turned, any fuel remaining

unevaporated in the manifold runs down into the cylinder, where it is

evaporated by the heat generated during the subsequent compression stroke.

Mixing is further assisted, at TDC, by the squish effect between the flat area

surrounding the slightly dished portion of the crown of the piston and the flat

lower face of the casting, each side of the pairs of valves. Because of the

steepness of the slope of the inlet ports, this fuel runs down positively into

each cylinder in turn so that, once the first cylinder fires, the others will pick

up immediately. The penalties for a slope that is either inadequate or too

steep are respectively mixtures that are either too weak or too rich to fire in

sequence.

A single camshaft, with eight cams, serves all the valves. Each cam actuates

first an inlet and then an exhaust valve. However, whereas the inlet valve is

actuated directly, through the medium of an inverted bucket tappet, the exhaust

valve is opened by a rocker, one end of which follows the cam and the other



Constructional details of the engine



95



(a)



Inl



et

Exhaust



(b)

Fig. 3.39 The Triumph Dolomite Sprint



bears on a pallet, or thick shim, seated in a recess in the top face of the valve

spring retainer. A similar pallet is interposed between the exhaust valve

spring retainer and its tappet.

To reduce the velocity of sliding between the rocker and cam, the pad on

the end of the rocker is curved. This, however, tends to increase the velocities

of both opening and closing of the valve and therefore has to be taken into

account in the design of the cam profile.

The sparking plug is very close to the centre of the top of the combustion

chamber, for efficient combustion. As a result, it has not been possible to

make the diameter of the two inlet valves much larger than that of the



96



The Motor Vehicle



exhausts but, to compensate for this, their lifts are greater – 8.712 mm as

compared with 7.798 mm.

To span the distance between the camshaft and the exhaust valves, the

rockers have to be long. Consequently forged En8 steel, instead of the usual

cast iron, rockers are used. Because of the relatively poor bearing characteristics

of steel, however, oil from a radial hole in the rocker bearing is fed down a

groove on top of the rocker arm to the cam follower pad. To obviate any

possibility of oil dripping down and getting into the exhaust valve guide,

where it could form carbon deposits, a heat-resistant flexible seal is fitted

over the upper end of the guide.

As can be seen from Fig. 3.39 the upper half-bearing for the camshaft and

the semi-circular clamp for the rocker shaft are machined in a single long

diecast aluminium bearing cap secured on the plane of the inclined joint face

of the valve gear cover by a bolt at each end. With this arrangement, the

valve actuation gear can be fitted to the cylinder head, forming a self-contained

sub-assembly, with valve clearances set, all ready for mounting on the engine.

The cylinder head holding down studs on the inlet side of the head are

inclined 74° relative to the cylinder gasket joint face, and this brings their

upper ends out through the diecast cap, adjacent to the rocker shaft and at

right angles to the inclined seating face. Consequently, when the head is

tightened down, the stud pulls the clamp tightly down on to the rocker shaft

so that there is absolutely no possibility of fretting fatigue between the cap

and shaft. Set bolts, perpendicular to the cylinder head gasket joint face, hold

down the exhaust side of the head. With this overall arrangement, the head

can be removed in service without disturbing the valve gear.

The inlet valves are of Silchrome, while the exhausts are either Nimonic

80A with Stellite tips on the ends of their stems, to prevent undue wear, or

of En18 with Nimonic 80A heads welded to them. All stems are chromiumplated to reduce the rate of abrasive wear in the guides.

Sparking plugs of the conical seating type are fitted, because they are

screwed into bosses at the lower ends of tubular housings cored vertically in

the head casting, where plug washers would be difficult both to place and to

retrieve. The gap between the upper end of each of these cored housings and

the valve gear cover is spanned by an aluminium tube with elastomeric seals

moulded around both its ends. The lower seal is a tighter fit in the head

casting than is the upper one in the valve gear cover, so that the tube will not

pull away from the head when the cover is removed. Although 14-mm plugs

are fitted, their hexagons are of 10-mm plug size, so that the tubular housings

can be of small diameter. This, in turn restricts as little as possible the water

passages around the plug bosses.

The good thermal conductivity of the aluminium head, together with

generous cooling passages around the valve seats account for the absence of

valve sinkage when unleaded fuel is used. Lead in fuel is thought to act as

a lubricant between the valve and its seat, so with unleaded fuel and elevated

temperatures the rate of wear of less well-cooled seats can be high with the

result that the valve sinks into them.

A four-valve head layout was chosen because this engine was required to

have high performance. So far, no positive proof of why the four-valve

layout is so efficient has been put forward. However, the central positioning

of the plug probably contributes, and the scavenging on a broad front –

through a pair of exhaust valves – may also help.



Constructional details of the engine



3.49



97



Cylinder block and crankcase arrangement



Until about 1925, cylinder heads and blocks were generally integral, and in

highly rated aero-engines, notably the Rolls-Royce Merlin, the arrangement

was still in use throughout and after the Second World War. However, with

road vehicles, the need for frequent servicing and economy of manufacture

led to the adoption of the separate cylinder head casting. But, with the

introduction of turbocharging and the consequent development of high gaspressures and temperatures, the designers of the Leyland 500 diesel engine,

Fig. 3.40, reverted to the combined head and block casting, bolted to a

separate crankcase.

The elimination of the cylinder head gasket disposes not only of the

barrier to the conduction of heat, but also of the thick sections of the adjacent

faces of the two separate castings, which can cause thermal distortion of the

structure. In particular, with this layout, the cylinder bores and valve seats

should be relatively free from distortion. The absence of cylinder head retaining

studs should also help in this respect, as well as give the designer greater

freedom in arranging the valve and porting layout and cooling passages.

Large covers bolted on each side of the block facilitate the cleaning of sand

from the cooling passages after the casting operation. However, despite all

these potential advantages, the design proved extremely difficult to develop

to the point of complete satisfaction in practice.

Other cylinder block and crankcase arrangements appear in Figs. 3.41 to

3.45. The black sectioning represents cast iron and the cross-hatching

aluminium alloy. While the figures are simplified and to a great extent

diagrammatic only, each is represented by one or more examples in past or

present practice.

That shown in Fig. 3.41 is the conventional arrangement used in large

engines of the highest quality, where it is not essential to economise in

machining and fitting. Here a monobloc cylinder casting, with overhead

valves in a detachable head, is bolted to a two-part aluminium alloy crankcase

split on or below the crankshaft centre line. The maximum degree of

accessibility is provided for valves, pistons and bearings.

Fig. 3.42 shows the side-valve arrangement of what has been the most



Fig. 3.40



Fig. 3.41



Fig. 3.42



98



The Motor Vehicle



widely used construction but which is now generally superseded by the

overhead valve arrangement shown in Figs 3.43 and 3.44 with various camshaft

arrangements. Modern designs simplify the cylinder block casting and improve

accessibility of valves and rockers. A single monobloc casting in iron or in

aluminium alloy – in which case wet cylinder liners are used – extends from

the head joint face to usually well below the crankshaft centre line, forming

a rigid beam structure of great depth and stiffness. Well-ribbed webs tie the

crankcase walls together and carry the main and camshaft bearings. The

bottom is closed by a light sump which is often ribbed for cooling purposes

and may carry the main oil filters. For examination and overhaul, pistons and

connecting rods must be withdrawn upwards through the cores unless removed

bodily with the crankshaft.

The corresponding overhead-valve construction is illustrated in Fig. 3.43,

which shows a low camshaft for push rod and rocker operation of the valves.

A dry liner is indicated in this case. The valves all lie in the same longitudinal

plane.

End assembly of the crankshaft is provided for in the construction of

Fig. 3.44. When arranged for three bearings, the centre bearing is mounted

in a circular housing of diameter exceeding the diameter of the crank webs.

The old Lea–Francis design was of this construction, which provides a rigid

and accurately aligned support for the crankshaft. It had high twin camshafts

to operate the inclined overhead valves, which were pitched transversely.

Figures 3.45 and 9.10 show two examples of wet liner construction, one

for a medium-powered petrol engine and the other for a blower-charged,

poppet-exhaust two-stroke ci engine. The air gallery and scavenge ports of

the latter will be noticed. Water joints are made with synthetic rubber rings

(as indicated) and gas pressure joints usually by separate gasket rings to each

bore. A light alloy main casting is indicated in these two cases.



3.50



The aluminium crankcase



Aluminium alloy has always been a potentially attractive material for

crankcases. This is because of its light weight, good thermal conductivity,



Fig. 3.43



Fig. 3.44



Fig. 3.45



Constructional details of the engine



99



prospect for good cooling and, if aluminium pistons could be used in aluminium

bores, the absence of differential expansion would enable tighter clearances

to be adopted in the bores. However, until recently, this light metal has failed

to gain wide acceptance because it has been necessary to fit cast iron liners,

primarily owing to the unsatisfactory bearing properties of aluminium pistons

in bores of the same material. This has increased the cost of production with

a material that, in any case, is expensive. Moreover, there have been other

problems, including the risk of electrolytic corrosion and of the head welding

to the block, as well as a high noise level and, when diecasting is used

because of its suitability for large quantity production, a large rejection rate

owing to porosity.

Some European manufacturers, however, with access to relatively cheap

supplies of aluminium, and others throughout the world who produce expensive

cars, as well as Rootes – later Chrysler, now Peugeot-Talbot – have used

sand-cast crankcases and cylinder blocks. The Rootes car, the Hillman Imp,

was a special case, since weight reduction was of prime importance as the

engine and transmission were to be installed at the rear. An open-top deck

layout was chosen because, originally, diecasting was envisaged.

There are four iron liners, which are preheated to 204°C before they are

inserted in the mould, for the aluminium to be cast around them. To key the

liners in the aluminium, their outer peripheries have spiral grooves

0.331 mm deep machined around them at 3.18 mm pitch. When cast in, they

are arranged in two siamesed pairs – otherwise the block would be unduly

long – so water can flow transversely between only the central two cylinders

and across the ends of the block.

Renault, too, have used cylinder blocks with wet cast iron liners for many

years. One of their later developments has been the 1.47-litre R16 model, in

which a high pressure aluminium diecast cylinder block and crankcase is

employed. Although most of the walls are only 4 mm thick, the casting is

well ribbed and flanged for stiffness. The liners are closely spaced in the

open-top casting, and there are five main bearings – the Imp, being a much

smaller unit, 875 cm3, has only three bearings. An interesting feature of the

Renault engine is the employment of an aluminium head too.

In 1971, General Motors announced the Chevrolet Vega 2300 engine with

a diecast all-aluminium crankcase and cylinder block, Figs 3.46 and 3.47,

reported in Automobile Engineer, August 1970. Developments that have

made this possible include the Accurad method of diecasting, described in

the November 1969 issue of Automobile Engineer. This entails control of the

cooling of the metal so that areas remote from the point of injection solidify

first, and a two-stage injection process, the second stage of which effectively

compensates for shrinkage of the metal. Secondly, a new alloy, termed

A390, has been introduced by Reynolds Metals, and this combines good

fluidity in the molten condition with fine dispersion of silicon after heat

treatment, which gives good bearing properties and ease of machining –

large particles wear the tools and tear from the surface of the alloy. It contains

16 to 18% silicon, 4 to 5% copper and 0.45 to 0.65% magnesium.

Diecasting is employed because of its suitability for large quantity production and its accuracy: the latter minimises subsequent machining and enables

thin walls to be incorporated – as little as 4.826 mm, but 6.35 mm for the

bores – thus economising in material. The finished block for this 2.3-litre



100



The Motor Vehicle



Fig. 3.46 The diecast aluminium block of the Chevrolet Vega has 6.35 mm thick

cylinder walls, and most of the other walls are 4.826 mm thick



Fig. 3.47 The open-deck design, with siamesed cylinders, was adopted for the diecast

aluminium crankcase of the Vega so that the dies could be easily withdrawn



Constructional details of the engine



101



engine weighs only 36 lb, as compared with 87 lb for the cast iron block of

the comparable Chevrolet L-4 engine. However, because it is diecast and

provision must therefore be made for withdrawal of metal cores, the opentop deck form, with siamesed cylinders, has to be used.

Diecast aluminium heads have not yet been used by any manufacture in

quantity production, because of the problem of withdrawal of the cores.

Experiments, though, have been made with two-piece castings joined by

means of adhesives or by electron beam welding. A cast iron head, however,

makes up for any lack of stiffness of an aluminium block with an open-top

deck and helps to contain the noise of combustion and from the overhead

camshaft and valve gear. On the Vega, it is secured by ten long bolts screwed

into bosses at the bases of the cylinders, so that the bores are rigidly held in

compression.

The cylinder bore treatment in the Vega, to form a wear-resistant and oilretaining surface, and to prevent scuffing, comprises exposure of the hard

silicon particles by an electro-chemical etching process. In addition, the

skirts of the pistons are coated with iron by a four-layer electro-deposition

process, to render the bearing surfaces compatible with those of the bores.

The coats, in order of application, are: zinc, copper, iron and tin. Zinc bonds

well to aluminium, the copper prevents removal of the zinc by the iron

plating process and the tin prevents subsequent corrosion of the iron and

helps in running-in. The iron coating is 0.019 mm thick.



3.51



Camshaft drive



Whatever the type of valve used it is necessary in the four-stroke engine to

drive it from a camshaft which runs at half the speed of the crankshaft, as

each valve is required to function only once in two revolutions of the crankshaft.

The necessary gearing for this purpose is placed, with few exceptions, at the

front of the engine, that is, at the end remote from the flywheel and clutch.

The camshaft or camshafts may be driven by gears, chains or toothed belts,

while a few overhead camshafts have had a ‘coupling rod’ drive. Figure 3.48

shows diagrammatically some typical arrangements of two-to-one drive for

both low and high camshafts.

No. 1 shows the simplest possible arrangement of direct gearing for either

one or two camshafts. The wheel on the camshaft has twice as many teeth as

1



2



5



3



Fig. 3.48



4



6



7



102



The Motor Vehicle



the crankshaft wheel, and therefore revolves at half the speed of the latter.

Where, as is often the case, the distance between the two shafts is considerable,

this arrangement requires undesirably large gear wheels, and this had led to

the adoption of the arrangement shown at 2. Here an intermediate idler

wheel is interposed between the crankshaft wheel and the camshaft wheel.

This idler wheel may be of any convenient size, as the number of teeth in it

does not affect the gear ratio. The camshaft now revolves in the same direction

as the crankshaft, whereas in the former arrangement it did not.

A chain drive is shown at 3. The single chain drives the auxiliaries in

addition to the camshaft, the drive thus being a triangular one, but if the

chain is passed round only the crankshaft and camshaft sprockets, the shorter

run thus obtained is less prone to whip.

At 4 is illustrated a combined chain and gear drive for twin high camshafts.

The chain sprocket ratio is 1 : 1, and the 2 : 1 ratio is provided by the

gearing.

An automatic chain tensioner of the Coventry eccentric type is indicated.

The axis of rotation of the jockey chain wheel can swing eccentrically

round the spindle on which it is mounted to take up slack in the chain, the

desired pressure on the back of the chain being adjustable and automatically

maintained by the clock-type spring, the inner end of which is secured to

the mounting spindle and the outer end to the drum which carries the

jockey-wheel bearing.

A later development is the Renold hydraulically-actuated tensioner, Fig.

3.49(a). When assembled, the cylinder casting is secured by two bolts to the

front wall of the crankcase, and inserted into it is a plunger, carrying the

neoprene-faced slipper together with the spring-loaded piston which forces

the slipper lightly into contact with the chain. When the engine is started, oil

from the pressure-lubrication system, ducted into the cylinder of the adjuster,

forces the slipper firmly against the chain. The piston has a spiral ratchettoothed slot in its skirt, in which registers a peg projecting radially inwards

in the bore of the plunger: its function is to limit the backlash, that is,

movement of the slipper away from the chain.

Various means of driving overhead camshafts are illustrated at 5, 6 and 7



Fig. 3.49(a) The ratchet device on the plunger of the Renold hydraulic chain-tensioner

prevents the chain from going too slack when the oil pressure is low



Constructional details of the engine



103



in Fig. 3.48. Diagrams 5 and 6 show vertical shafts driven by bevel and skew

gears respectively. In either case the 2 : 1 ratio may be obtained in one or two

steps as may be most convenient. In the latter case particularly, ratios 3 : 2

and 4 : 3 are advantageous.

It will be noticed that in the case of the bevel gears a tongue and slot

arrangement is provided so that expansion of the cylinder block will not

affect the meshing of the gears. This provision is not strictly necessary with

skew gears, though the slight axial movement of the vertical gear that may

place will result in a slight variation of timing between the hot and cold

conditions.

No. 7 is the layout on the Jaguar in-line engine, which has stood the test

of time. The 2 : 1 ratio is divided between the two stages, the tooth ratio of

the four chain-wheels being 21: 28 × 20:30. This tends to distribute the wear

and maintains uniformity of pitch. Another advantage is that small sprockets

can be used.

For the lower chain, a nylon-faced damping slipper bears on the driving

strand, to prevent thrash due to torsional oscillations. On the earlier models,

a spring plate tensioner bore on the slack strand, but this was subsequently

superseded by the Renold hydraulic tensioner. For manual adjustment of the

tension in the upper chain, the jockey sprocket is mounted on an eccentric

spindle, as described in detail in Automobile Engineer, May 1956.

Renold also produce a commendably simple double-acting tensioner,

comprising a central cast housing with twin parallel bores in which are

horizontally opposed plungers, each with a shoe on its outer end. Two coil

springs in compression push the plungers, lightly outwards until the shoes

contact the inner faces of both the taut and idle runs of the chain. At the same

time, oil splashing into a pocket on top of the housing is drawn continually,

by the motions of the plungers due to fluctuations in the drive, through a

non-return valve to fill their bores. Movement of one plunger inwards in its

bore instantly seats in the non-return valve, causing the other to move in the

opposite direction so that the light contact with both runs is continuously

maintained.

Another type of tensioner is that used in the Jaguar V-12 engine, Section

4.24, supplied by the Mores Chain Division of Borg-Warner Ltd. Of the four

runs of chain between the sprockets – on the crankshaft, two camshafts and

the jackshaft – three are controlled by damper pads and the fourth by the

tensioner. Because these four runs form a strand 1.674 m long, an extremely

accurate and effective tensioning is essential.

The Morse tensioner, Fig. 3.49(b), supports the run of chain over most of

its length – its shoe is approximately 28 cm long – so the force per unit area

on the shoe is small, and undue noise and wear are therefore avoided. Moreover,

the shoe is a nylon moulding and therefore does not require a separate

facing. Nylon, of course, has a low coefficient of friction and, in this application,

fillers are added to improve both the stiffness and wear characteristics. It is

also resistant to oil, fuels and temperatures of 150°C and even higher.

Furthermore, its flexibility increases with temperature, so, when the engine

is warm, the tensioner readily conforms to changes in shape of the chain run

throughout its life. Finally, so that the chain cannot go slack and, possibly,

ride over the sprocket, a one-way device is incorporated in the tie that holds

the shoe in the bowed condition. A tendency for the chain to go slack can