Light and Heavy Vehicle Technology

507 Pages·1988·18.15 MB·English

Light and Heavy
Vehicle Technology
M.J. Nunney
CGI A, MSAE, MIMI
Heinemann Newnes
Heinemann Newnes
An imprint of Heinemann Professional Publishing Ltd
Halley Court, Jordan Hill, Oxford OX2 8EJ
OXFORD LONDON MELBOURNE AUCKLAND SINGAPORE
IBADAN NAIROBI GABORONE KINGSTON
First published 1988
© M. J. Nunney 1988
British Library Cataloguing in Publication Data
Nunney, M. J. (Malcolm James)
Light and heavy vehicle technology.
1. Motor vehicle engineering
I. Title
629.2
ISBN 0 434 91473 8
Printed in Great Britain by
Butler & Tanner Ltd, Frome
Preface
The primary aim of this book is to cover the essential and interesting manner, rather than as a mere catalogue
technology requirements of the City and Guilds Motor of facts and figures. The treatment of the subject
Vehicle Craft Studies (381) Part 2, for both light and therefore includes a certain amount of historical back-
heavy vehicles, while at the same time catering for ground, so that modern engine and vehicle technology
the parallel needs of BTEC and other motor vehicle can be seen in its proper perspective. In relating basic
studies. theory to practice, the opportunity has been taken to
At first glance a student may get the impression that include numerous examples of vehicle maintenance
a daunting amount of ground has to be covered in procedure and of diagnosing vehicle misbehaviour in
pursuing these studies, but it is the author's belief that service. It is also hoped that the book will not be found
such an impression can soon be dispelled once a student entirely lacking in light-hearted comment, since, no
generates a real enthusiasm for the motor vehicle and matter how serious a technical subject may be, most
comes to appreciate the continuing interest it can engineers would agree that a sense of humour is indis-
provide. pensable to its enjoyable study.
To help foster this approach the author has endeav- M.J. Nunney
oured to present the subject in a technically informal
IX
Acknowledgements
The illustrations have been chosen for their special Ltd, Goodyear Tyre & Rubber Co., Leslie Hart ridge
relevance to the text and, apart from those originating Ltd, Honda (UK) Ltd, Hope Technical Developments
from the author, grateful acknowledgement is due not Ltd, Interlube (Tecalemit UK), Lada Cars, Lancia
only to the publishers for allowing the use of illus- (Fiat), Laystall Engineering Co. Ltd, Lipe-Rollway
trations from certain of their other technical books, but Ltd, Lucas Ltd (CAV) (Electrical) (Girling) (Kienzle),
also to the following firms and organizations who so Mercedes-Benz (UK) Ltd, Merlin International Ltd,
kindly supplied the remaining illustrations and much Midcyl Productions, Mintex Ltd, Nissan (UK) Ltd,
useful background information: Perkins Engines Ltd, Pirelli Ltd, Pressed Steel Fisher,
RHP Ltd, Renault (UK) Ltd, Renold Power Trans-
Alfa-Romeo (GB) Ltd, Alpha Automotive Productions missions Ltd, Rists Ltd, SAAB (GB) Ltd, Schrader
Ltd, Automotive Products Ltd, Avery-Denison Ltd Automotive Products Division, Seddon Atkinson Ve-
(Schenck), Bendix Ltd, BL-MG, British Rubber hicles Ltd, Self-Changing Gears Ltd, SKF (UK) Ltd
Manufacturers' Association Ltd, Brockhouse Trans- and Ball Bearing Journal, Smallman Lubricants Ltd,
missions Ltd, Brown Brothers Ltd, Champion Sparking Smiths Industries Ltd, Start Pilot Ltd, SU Butec, Suzuki
Plug Co., Chillcotts Ltd (Reinz/Mann), TI Churchill, GB (Cars) Ltd, Alfred Teves GMBH, The Timken
Citroen Cars Ltd, The Colt Car Co. Ltd (Mitsubishi), Co., Toyota (GB) Ltd, TRW Valves Ltd, Turner-
Con-Vel Division of Dana Corp., Cummins Engine Co. Spicer, UK Department of Transport, Volkswagen (GB)
Ltd, David Brown Gear Industries Ltd, Davies Magnet, Ltd (Audi), Volvo Concessionaires Ltd, Woodhead
Deutz Engines Ltd, Dunlop Automotive Division, Manufacturing Co., Yamaha-Mitsui Machinery Sales
Eaton Ltd, Ferodo Ltd, Fiat Auto (UK) Ltd, Ford (UK) Ltd, and The Zenith Carburetter Co. Ltd. Illus-
Motor Co. Ltd, L. Gardner & Sons Ltd, General trations 15.15, 15.35, 19.25e and 27.4 are reproduced
Motors Ltd (AC Delco) (Powertrain Lansing), GKN by permission of the manufacturers © Talbot Motor
(Hardy Spicer) (Kent Alloys) (Laycock) (Pistons) Co. Ltd.
(Salisbury Transmissions) (SDF), Glacier Metal Co.
x
1 The reciprocating piston petrol engine
1.1 MODERN REQUIREMENTS efficient aerodynamic shape. Today, however, a more
important engine requirement than a further increase
General background in top speed is an improved accelerating capability
together with better flexibility in the low to middle
The motor vehicle engine is basically a device for
converting the internal energy stored in its fuel into speed range. A further performance requirement of a
mechanical energy. It is classified as an internal com- new engine design is that it must usually allow for
bustion engine by virtue of this energy conversion possible future increases in cylinder size.
taking place within the engine cylinders.
Good fuel economy
Since the term 'energy' implies the capacity to per-
The overall aim of improving the fuel economy of cars
form work, the engine is thus able to propel the vehicle
is to minimize the amounts of crude oil used to provide
along the road and, within limits, overcome unwanted
petrol for their engines, because of constraints imposed
opposition to its motion arising from rolling friction,
by limited petroleum resources and rising costs. Fuel
gradient resistance and air drag. To facilitate this pro-
economy may also be made the subject of legislation, as
cess the engine is combined with a transmission system,
it already is in America, where each manufacturer has
the functioning of which is discussed later.
to comply with corporate fuel economy standards (or
The vast majority of car engines are of the recipro-
CAFE standards, as they are generally termed). For
cating piston type and utilize spark ignition to initiate
these reasons, further engine requirements are those of
the combustion process in the cylinders. They also
minimum weight so as to reduce total car weight;
operate on the four-stroke principle in which the piston
improved combustion efficiency, better to utilize the
travels one complete stroke for each of the successive
fuel; and reduced friction losses between the working
events of induction, compression, combustion and
parts.
exhaust.
The late Laurence Pomeroy, a distinguished motoring
Low pollution
historian, once summarized the early history of the
Since the late 1960s increasingly stringent legislation
motor car as follows: From 1885 to 1895 men struggled
has been applied to limit the levels of atmospheric
to make the car go. From 1896 to 1905 they contrived to
pollutants emitted from car engines, especially in
make it go properly. Between 1907 and 1915 they
America and Japan. In Britain the Provision of the
succeeded in making it go beautifully!' What then are
Motor Vehicles (Construction and Use) Regulations
the requirements for the petrol engine of the modern
are also now such that there is a requirement for every
car as reflected in some seventy years of further devel-
motor vehicle to be so constructed that no avoidable
opment and, not least, in the light of present-day
smoke or visible vapour is emitted therefrom, and
energy conservation and environmental pollution con-
another that makes it an offence to use a vehicle which
siderations? These requirements can now add up to
emits substances likely to cause damage to property or
quite a formidable list. As we pursue our studies into
injury to persons. In general, legislation is concerned
the whys and wherefores of engine construction and
with carbon monoxide which has toxic effects; unburned
operation, it will become evident that although some of
hydrocarbons, which contribute to atmospheric smog;
the requirements are complementary, others are not,
and nitrogen oxides, which cause irritation to the eyes
and therefore (as in most engineering) some compro-
and lungs. To reduce these harmful emissions, not only
mise has generally to be accepted in the final product.
is very careful control of the combustion process re-
quired in modern engine design, but also various soph-
Modern requirements isticated devices may have to be added for after-treat-
ment of the exhaust gases.
Optimum performance
With modern advances in engine design it is not par- Minimum noise level
ticularly difficult to obtain sufficient power to give the Noise is generally defined as unwanted sound. Reducing
car a high top speed, especially since the recent trend interior noise makes a car more attractive to the buyer.
towards car bodies of lighter construction and more Reducing exterior noise to socially acceptable limits has
1
2 LIGHT AND HEAVY VEHICLE TECHNOLOGY
been the subject of increasingly stringent legislation in car. Since reducing engine weight is not always consist-
most countries since the late 1960s, and in Britain is ent with maintaining durability, the need for adequate
included in the Provision of the Motor Vehicles (Con- testing of the engine components is confirmed. Also
struction and Use) Regulations relating to noise. Since special manufacturing techniques may have to be
the engine is an obvious source of noise an important adopted to avoid damage to such items as castings with
requirement is that its design and installation should very thin walls.
minimize noise emission, not only that directly radiated
from the engine itself to the exterior, but also that Compact size
arising from vibrations transmitted through its mounting For the modern car, the manufacturer strives to provide
system to the car body interior. the maximum interior space for the minimum poss-
ible exterior dimensions. Thus the trend is inevitably
Easy cold starting towards having the front wheels driven, with the power
An essential driver requirement of any engine, whether unit (engine and transmission) installed transversely
it be of past or present design, is that it should possess between them; the conventional arrangement was to
good cold starting behaviour and then continue to run have a longitudinally mounted power unit from which
without hesitation during the warming-up period. A the drive was taken to the rear wheels. It follows that
present-day additional requirement is that the cold the requirement now is for a more compact engine.
starting process should be accomplished with the least This is because the engine length is controlled by the
emission of polluting exhaust gases and detriment to distance available between the steerable front wheels,
fuel economy. To monitor the required enrichment of less that required by the transaxle (combined gearbox
the air and fuel mixture for cold starting, increasingly and final drive); its width by the distance available
sophisticated controls are now being applied to carbu- between the radiator and the dash structure, less that
rettor automatic choke and fuel injection auxiliary required by the engine auxiliaries; and its height by the
supply systems. need for a low and sloping bonnet line, which contri-
butes to an efficient aerodynamic body shape.
Economic servicing
An important owner requirement of a car is that its Economic manufacture
engine design should acknowledge the need to reduce This is clearly a most important requirement for any
servicing costs. This aim may be approached by mini- new design of engine, since putting it into production
mizing the number of items that need periodic attention demands a massive initial investment on the part of the
by a service engineer. For example, the use of hydraulic car manufacturer. It is, of course, for this reason that
tappets eliminates altogether the need for adjustment the smaller specialist car manufacturer generally uses
of the valve clearances. It is also promoted by allowing an existing engine from a volume producer. For econ-
ready access to those items of the engine involved in omic manufacture a new design of engine should lend
routine preventive maintenance, such as the drive-belt itself as far as possible to existing automatic production
tensioner, spark plugs, and petrol and oil filters. processes and require the minimum of special tooling.
The cost of materials will be reduced in building a
Acceptable durability smaller engine, and the construction should be as simple
In order to reduce fuel consumption while still main- as possible to minimize the number of parts to be
taining good car performance, it is now the trend to assembled and thereby further reduce manufacturing
develop engines of smaller size with relatively higher costs.
power output. Furthermore, the installation of a turbo-
charger permits an increase in power without imposing
1.2 ENGINE NOMENCLATURE
a corresponding increase in the size or weight of the
engine itself. However, the greater heating effect on To understand the information given in an engine speci-
certain engine components may require changes to fication table, such as those included in a manufac-
their material specifications and also the addition of an turer's service manual or published in the motoring
oil cooler. The components of modern engines have press, it is necessary to become familiar with some
therefore tended to become more highly stressed, so commonly used terms (Figure 1.1). The language' of
that engine testing of ever-increasing severity by the the reciprocating piston engine is summarized in the
manufacturers is now required to maintain durability in following sections.
extremes of customer service.
Top dead centre
Least weight The top dead centre (TDC) is of general application in
Another important design requirement of the modern engineering; it is any position of a hinged linkage in
petrol engine is that it should be made as light as which three successive joints lie in a straight line. In the
possible. This is because a corresponding reduction in case of a motor vehicle engine, top dead centre refers
car weight can make significant improvements not only to the position of the crankshaft when the piston has
in fuel economy and acceleration capability, but also reached its closest point to the cylinder head. This
in general handling and ease of manoeuvring the results in the main, big-end and small-end bearings
THE RECIPROCATING PISTON PETROL ENGINE 3
and the engine has four cylinders, then the engine
3
capacity is 1500 cm or 1.5 litres (1). This can be simply
stated as:
VH = Vhz
3
where VH is the engine capacity (cm ), Vh is the piston
3
displacement (cm ) and z is the number of cylinders.
Mean effective pressure
This term is used because the gas pressure in the
cylinder varies from a maximum at the beginning of the
Figure 1.1 Engine nomenclature (Yamaha): 1 volume of power stroke to a minimum near its end. From this
combustion chamber; 2 top dead centre (TDC); 3 stroke;
value must, of course, be subtracted the mean or aver-
4 bottom dead centre (BDC); 5 bore
age pressures that occur on the non-productive exhaust,
Piston displacement: volume of gases displaced by the piston
induction and compression strokes. Engine mean effec-
as it moves from BDC to TDC
tive pressure can be expressed in kilonewtons per square
2
metre (kN/m ).
lying in a straight line. A motor vehicle service engineer
Indicated and brake power
often needs to establish top dead centre for checking
The most important factor about a motor vehicle engine
the ignition and valve timing of an engine.
is the rate at which it can do work or, in other words,
the power it can develop. It is at this point that we must
Bottom dead centre
distinguish between the rate at which it might be ex-
The bottom dead centre (BDC) is, of course, the
pected to work (as calculated from the mean effective
opposite extreme of crankshaft rotation when the piston
pressure in the cylinder, the piston displacement, the
has reached its furthest point from the cylinder head.
number of effective working strokes in a given time and
the number of cylinders) and the rate at which it
Piston stroke
actually does work (as measured in practice when the
In a general engineering sense, the stroke is the move-
engine is running against a braking device known as a
ment of a reciprocating component from one end of its
dynamometer).
travel to the other. In the motor vehicle engine the
The significance of this is that the brake power de-
piston stroke, therefore, is the distance travelled by the
livered at the crankshaft is always less than the indi-
piston in its movement from BDC to TDC or, of
cated power, owing to internal friction losses in the
course, vice versa, and is expressed in millimetres (mm).
engine. A simple expression for calculating in kilowatts
(kW) the indicated power of an engine is as follows:
Cylinder bore
In engineering practice the term bore may refer to a P = psAEz
hole through a bushing or pipe, or to the cutting of a
Where P is the indicated power (kW), p is the mean
large-diameter cylindrical hole, or to an actual measure- 2
effective pressure (kN/m ), s is the piston stroke (in), A
ment of the inside diameter of a hollow cylinder. It is 2
is the piston area (m ), E is the number of effective
the last named with which we are concerned here,
working strokes per second and z is the number of
where the bore refers to the inside diameter of the
cylinders. (Since in the four-stroke cycle engine there
engine cylinder expressed in millimetres (mm).
is only one power stroke for every two complete rev-
olutions of the crankshaft, the number of effective
Piston displacement
working strokes per second will correspond to one-half
This term refers to the volume of cylinder displaced or
the number of engine revolutions per second.)
swept by a single stroke of the piston, and is also
As mentioned earlier, a dynamometer is used in an
referred to as swept volume. It is expressed in cubic
3 engine testing laboratory to measure the brake power
centimetres (cm ) and may be simply calculated as
(or effective power) of an engine, because it acts as a
follows:
brake to balance the torque or turning effort at the
crankshaft through a range of speeds. A graph of the
4000 engine power curve can then be drawn by plotting
where Vh is the piston displacement or swept volume brake power values against engine speeds (Figure 1.2).
3
(cm ), d is the cylinder bore (mm) and s is the piston Various standardized test procedures may be adopted
stroke (mm). in engine testing, such as those established by the
American Society of Automotive Engineers (SAE), the
Engine capacity German Deutsche Institut fur Normalisation (DIN)
Here we are referring to the total piston displacement and the Italian Commissione tecnica di Unificazione
or the swept volume of all cylinders. For example, if the nelF Automobile (CUNA). In an engine specification
3
swept volume of one cylinder of an engine is 375 cm table only the maximum brake power and corresponding
4 LIGHT AND HEAVY VEHICLE TECHNOLOGY
It would seem to be generally accepted that the first
internal combustion engine to operate successfully
on the four-stroke cycle was constructed in 1876 by
Nicolaus August Otto (1829-91). This self-taught
German engineer was to become one of the most
brilliant researchers of his time and also a partner in the
firm of Deutz near Cologne, which for many years was
the largest manufacturer of internal combustion engines
in the world.
Although the Otto engine ran on gas, which was then
regarded as a convenient and reliable fuel to use, it
Engine r e v / m i n — ►
nevertheless incorporated the essential ideas that led to
Figure 1.2 Engine power and torque curves
the development in 1889 of the first successful liquid-
fuelled motor vehicle engine. This was the twin-cylinder
crankshaft speed are usually quoted. For example, the Daimler engine, patented and built by the German
high-performance engine used in the Mercedes-Benz automotive pioneer Gottlieb Daimler (1834-1900)
500 SL cars is claimed to develop, according to DIN, who, like Otto, had been connected with the Deutz
180 kW at 4750 rev/min. firm. The Daimler engine was subsequently adopted by
several other car manufacturers and, in most respects,
Engine torque it can be regarded as the true forerunner of the modern
Also usually included on an engine performance graph four-stroke petrol engine (Figure 1.4).
is a torque curve, which is obtained by plotting crank- In this type of engine the following sequence of
shaft torque, or turning effort, against engine speed. events is continuously repeated all the time it is running
(Figure 1.2). The engine torque, of course, is derived (Figure 1.5):
from combustion pressure acting upon the cross-
1 The induction stroke, during which the combustible
sectional area of the piston, the resulting force from
charge of air and fuel is taken into the combustion
which applies a turning effort to the crankshaft through
chamber and cylinder, as a result of the partial vac-
the connecting rod and crankthrow arrangements (Fig-
uum or depression created by the retreating piston.
ure 1.3). Engine torque, therefore, may be considered
The compression stroke, which serves to raise both
as the force of rotation acting about the crankshaft axis
the pressure and temperature of the combustible
at any given instant.
Engine torque T may be expressed in newton metres
(N m), and generally reaches a peak value at some
speed below that at which maximum power is devel-
oped; the reason for this is explained at a later stage.
An engine that provides good pulling power is typically
one in which maximum torque is developed at moderate
engine speeds.
1.3 OPERATING PRINCIPLES
The four-stroke petrol engine
As with the various repeating cycles of events in nature,
so does the motor vehicle petrol engine need to operate
on a constantly repeating cycle known as the four-
stroke principle.
To r q u e is e q u a l to Fx r
Figure 1.4 Cross-section of a four-stroke petrol engine
Figure 1.3 ' Engine torque or turning effort (Fiat)
THE RECIPROCATING PISTON PETROL ENGINE 5
strokes; nor is the spark timed to occur exactly at the
Air and petrol
mixture beginning of the power stroke. At a later stage the
entering
reasons for these departures in valve and ignition timing
C = >
from the basic four-stroke operating cycle will be made
clear.
The two-stroke petrol engine
As its designation implies, the two-stroke petrol engine
(Figure 1.6) completes its working cycle in only two
strokes of the piston, so that a combustible charge is
ignited at each revolution of the crankshaft. Although
in its simplest construction the two-stroke petrol engine
needs no valves, the induction and exhaust process
must be facilitated by a system of scavenging or forcible
clearing of the cylinder gases. This may either take the
form of a separate engine-driven pump, or utilize the
motion of the engine piston itself in a sealed crankcase.
The flow of gases entering and leaving the cylinder is
controlled by the reciprocating movement of the engine
piston, which thus acts as a slide valve in conjunction
with ports cut in the cylinder wall. Until recent years,
the two-stroke petrol engine had been favoured in
Europe for some inexpensive small cars, but it is now
generally regarded as being obsolescent in car practice
Figure 1.5 The four-stroke petrol engine cycle
because of the difficulty in reducing its harmful exhaust
emissions.
charge as it is compressed into the lesser volume of It is perhaps of interest to recall that the two-stroke
the combustion chamber by the advancing piston. and the four-stroke engine both originated in the late
3 The power stroke, immediately preceding which the 1870s, so it might reasonably be assumed that both
combustible charge is ignited by the sparking plug types of engine started out in life with an equal chance
and during which the gases expand and perform of success. The fact that the four-stroke engine became
useful work on the retreating piston. by far the more widely adopted type can probably be
4 The exhaust stroke, during which the products of explained by its having a greater potential for further
combustion are purged from the cylinder and com- development. This is a criterion that can often be
bustion chamber by the advancing piston, and dis- applied to rival ideas in all branches of engineering.
charged into the exhaust system. The first successful application of the two-stroke
cycle of operation to an early gas engine is generally
It thus follows that one complete cycle of operations
attributed to a Scottish mechanical engineer, Sir Dugald
occupies two complete revolutions of the engine crank-
shaft. Since energy is necessarily required to perform
the initial induction and compression strokes of the
engine piston before firing occurs, an electrical starter
motor is used for preliminary cranking of the engine.
Once the engine is running the energy required for
performing subsequent induction, compression and
exhaust strokes is derived from the crankshaft and
flywheel system, by virtue of its kinetic energy of ro-
tation. Kinetic energy is a term used to express the
energy possessed by a body due to its mass and motion.
The principle of an engine flywheel is therefore to act as
a storage reservoir for rotational kinetic energy, so that
it absorbs energy upon being speeded up, and delivers
it when slowed down.
In the four-stroke cycle, the functions of admitting
the combustible charge before its compression, and
releasing the burnt gases after their expansion, are
performed by the engine inlet and exhaust valves. The
opening and closing of the inlet and exhaust valves are
not, in actual practice, timed to coincide exactly with Figure 1.6 Cross-section of a two-stroke petrol engine
the beginning and ending of the induction and exhaust (Honda)
6 LIGHT AND HEAVY VEHICLE TECHNOLOGY
Clerk (1854—1932). It is for this reason, of course, that strokes per cylinder per revolution. In theory at
the two-stroke cycle is sometimes referred to as the least, this might be expected to produce twice the
Clerk cycle. Dugald Clerk, like several other pioneer performance of a four-stroke engine of equivalent
researchers of the internal combustion engine, was size. Unfortunately, this is not realized in practice
later to achieve high academic distinction, culminating because of the difficulties encountered in effectively
in his election as Fellow of the Royal Society in 1908. purging the exhaust gases from the cylinder and then
The Clerk engine was scavenged by a separate pump- filling it completely with a fresh combustible charge.
ing cylinder. A few early motor vehicle two-stroke The scavenging efficiency of the basic two-stroke
petrol engines followed the same principle, but it later petrol engine is therefore poor.
became established practice to utilize the underside of 2 In performing twice as many power strokes per rev-
the piston in conjunction with a sealed crankchamber olution, the two-stroke engine can deliver a smoother
to form the scavenge pump. This idea was patented in flow of power, but this may be less true at low engine
1889 by Joseph Day & Son of Bath and represented the speeds when irregular firing or Tour-stroking' can
simplest type of two-cycle engine. result from poor scavenging.
In the two-stroke or Clerk cycle, as applied by Day, 3 An obvious practical advantage of the basic two-
the following sequence of events is continuously re- stroke engine is the mechanical simplicity conferred
peated all the time the engine is running (Figure 1.7): by its valveless construction, which contributes to a
more compact and lighter engine that should be less
1 The induction-compression stroke. A fresh charge of
expensive to make.
air and fuel is taken into the crankchamber as a
4 Reduced maintenance requirements might reason-
result of the depression created below the piston as it
ably be expected with the basic two-stroke engine by
advances towards the cylinder head. At the same
virtue of point 3. There is, however, the well-known
time, final compression of the charge transferred
tendency for carbon formation to have a blocking
earlier in the stroke from the crankchamber to the
effect on the exhaust ports, which impairs engine
cylinder takes place above the advancing piston.
performance by reducing scavenging efficiency.
2 The power-exhaust stroke. The combustible charge
5 The fuel consumption of the basic two-stroke engine
in the cylinder is ignited immediately preceding the
is adversely affected by the poor cylinder scavenging,
power stroke, during which the gases expand and
which allows part of the fresh charge of air and fuel
perform useful work on the retreating piston. At the
to escape through the exhaust port before final com-
same time, the previously induced charge trapped
pression of the charge takes place.
beneath the retreating piston is partially compressed.
6 There is a greater danger of overheating and piston
Towards the end of the stroke the exhaust gases are
seizure with a two-stroke engine, which can set a
evacuated from the cylinder, a process that is facili-
limit on the maximum usable performance. It is more
tated by the scavenging action of the new charge
difficult to cool satisfactorily, because it does not
transferred from the crankcase.
have the benefit of the second revolution in the four-
The uncovering and covering of the cylinder ports of stroke cycle when no heat is being generated.
the piston, or port timing, is determined by consider- 7 Lubrication of the two-stroke petrol engine is com-
ations similar to those affecting the valve timing of the plicated by the need to introduce oil into the fuel
four-stroke engine and will be explained at a later supply to constitute what is generally termed apetroil
stage. mixture. The working parts of the engine are thus
lubricated in aerosol fashion by oil mist in the air
Four-stroke versus two-stroke engines and fuel charge, and this tends to increase harmful
exhaust emissions. It is for this reason that the two-
The following generalizations may be made concerning
stroke petrol engine is now obsolescent for cars.
the relative merits of two-stroke and four-stroke petrol
engines in their basic form:
Scavenging: further details
1 The two-stroke engine performs twice as many power Frequent reference has been made to the inherently
poor scavenging efficiency of the basic two-stroke petrol
engine. The word 'basic' has been used deliberately and
J^-^Spark ing plug
is intended to apply to the Day type of early two-stroke
/ ^ - A about to tire
engine, which had a deflector-head piston to promote
Exhaust a cross-scavenging effect on the burnt charge leaving
the cylinder (Figure 1.8a). This not entirely successful
Induction
cum scheme persisted until the mid 1920s, when Dr E.
compression
Schnuerle of Germany developed an alternative loop-
scavenging system. In this the deflector on the piston
head is omitted and two transfer ports with angled
passages are disposed on either side of, instead of
opposite, the exhaust port (Figure 1.8b). The loop-
Figure 1.7 The two-stroke petrol engine cycle scavenge effect produced is such that before the two