10 Cylinder, piston, connecting rod and crankshaft

8



The Motor Vehicle



Connecting rod. The connecting rod transmits the piston load to the crank,

causing the latter to turn, thus converting the reciprocating motion of the

piston into a rotary motion of the crankshaft. The lower end, or ‘big end’, of

the connecting rod turns on the crank pin.

Crankshaft. In the great majority of internal combustion engines this is of

the double-web type, the crank pin, webs and shaft being usually formed

from a solid forging. The shaft turns in two or more main bearings (depending

on the number and arrangement of the cylinders) mounted in the main frame

or ‘crankcase’ of the engine.

Flywheel. At one end the crankshaft carries a heavy flywheel, the function

of which is to absorb the variations in impulse transmitted to the shaft by the

gas and inertia loads and to drive the pistons over the dead points and idle

strokes. In motor vehicles the flywheel usually forms one member of the

clutch through which the power is transmitted to the road wheels.

The foregoing are the fundamental and essential parts by which the power

developed by the combustion is caused to give rotation to the crankshaft, the

mechanism described being that of the single-acting engine, because a useful

impulse is transmitted to the crankshaft while the piston moves in one direction

only.

Most steam engines and a few large gas engines work on the doubleacting principle, in which the pressure of the steam or gaseous combustion

acts alternately on each side of the piston. The cylinder is then double-ended

and the piston takes the form of a symmetrical disc. The force acting on the

piston is transmitted through a ‘piston rod’ to an external ‘cross-head’ which

carries the gudgeon pin. The piston rod passes through one end of the cylinder

in a ‘stuffing-box’ which prevents the escape of steam or gas.



1.11



Method of working



It is now necessary to describe the sequence of operations by which the

combustible charge is introduced, ignited and burned and finally discharged

after it has completed its work.

There are two important‘cycles’or operations in practical use, namely, the

‘four-stroke’, or ‘ Otto’ cycle as it is sometimes called (after the name of the

German engineer who first applied it in practice), and the ‘two-stroke’, or

‘Clerk’ cycle, which owed its early development largely to Sir Dugald Clerk.

The cycles take their names from the number of single piston strokes

which are necessary to complete a single sequence of operations, which is

repeated continuously so long as the engine works.

The first named is by far the most widely adopted except for small motor

cycle and motor boat engines, and for large diesels, for though it leads to

greater mechanical complication in the engine, it shows higher thermal

efficiency, and therefore greater economy in fuel. This cycle will therefore

be described first, the two-stroke cycle being left until Chapter 7.



1.12



The four-stroke cycle



Figure 1.3 shows in a diagrammatic manner a four-stroke engine cylinder

provided with two valves of the ‘mushroom’ or ‘poppet’ type. The cylinder

is shown horizontal for convenience.



General principles of heat engines

Inlet



9



L



IV

EV



(a)



(b)

Compression pressure



Atmospheric pressure

Suction pressure



(c)



Exhaust



(d)



Explosion pressure



Exhaust pressure

VC



Fig. 1.3



VS



The four-stroke cycle



The inlet valve (IV) communicates through a throttle valve with the

carburettor or vaporiser, from which a combustible mixture of fuel and air is

drawn. The exhaust valve (EV) communicates with the silencer through

which the burnt gases are discharged to the atmosphere. These valves are

opened and closed at suitable intervals by mechanisms, which will be described

later.

The four strokes of the complete cycle are shown at (a), (b), (c) and (d).

Below the diagrams of the cylinder are shown the corresponding portions

of what is known as the indicator diagram, that is to say, a diagram which

shows the variation of pressure of the gases in the cylinder throughout the

cycle. In practice such diagrams can be automatically recorded when the

engine is running by a piece of apparatus known as an indicator, of which

there are many types.

The four strokes of the cycle are as follows —

(a) Induction stroke – exhaust valve closed: inlet valve open

The momentum imparted to the flywheel during previous cycles or rotation

by hand or by starter motor, causes the connecting rod to draw the piston

outwards, setting up a partial vacuum which sucks in a new charge of

combustible mixture from the carburettor. The pressure will be below



10



The Motor Vehicle



atmospheric pressure by an amount which depends upon the speed of the

engine and the throttle opening.

(b) Compression stroke – both valves closed

The piston returns, still driven by the momentum of the flywheel, and

compresses the charge into the combustion head of the cylinder. The pressure

rises to an amount which depends on the ‘compression ratio’, that is, the

ratio of the full volume of the cylinder when the piston is at the outer end of

its stroke to the volume of the clearance space when the piston is at the inner

(or upper) end. In ordinary petrol engines this ratio is usually between 6 and

9 and the pressure at the end of compression is about 620.5 to

827.4 kN/m2, with full throttle opening.

Compression ratio =



Vs + Vc

Vc



Vs = π D 2 × L

4



(c) Combustion or working stroke – both valves closed

Just before the end of the compression stroke, ignition of the charge is

effected by means of an electric spark, and a rapid rise of temperature and

pressure occurs inside the cylinder. Combustion is completed while the piston

is practically at rest, and is followed by the expansion of the hot gases as the

piston moves outwards. The pressure of the gases drives the piston forward

and turns the crankshaft thus propelling the car against the external resistances

and restoring to the flywheel the momentum lost during the idle strokes. The

pressure falls as the volume increases.

(d) Exhaust stroke – inlet valve closed: exhaust valve open

The piston returns, again driven by the momentum of the flywheel, and

discharges the spent gases through the exhaust valve. The pressure will be

slightly above atmospheric pressure by an amount depending on the resistance

to flow offered by the exhaust valve and silencer.

It will thus be seen that there is only one working stroke for every four

piston strokes, or every two revolutions of the crankshaft, the remaining

three strokes being referred to as idle strokes, though they form an indispensable

part of the cycle. This has led engineers to search for a cycle which would

reduce the proportion of idle strokes, the various forms of the two-stroke

engine being the result. The correspondingly larger number of useful strokes

per unit of time increases the power output relative to size of engine, but

increases thermal loading.



1.13



Heat balance



It is instructive to draw up in tabular form a heat balance, arranging the

figures in a manner similar to those on a financial sheet. On one side, place

the figure representing the total heat input, in the form of the potential

chemical energy content of the fuel supplied, assuming it is all totally burned

in air. Then, on the opposite side, place the figures representing the energy

output, in the form of useful work done by the engine, and all the losses such

as those due to friction, heat passing out through the exhaust system, and

heat dissipated in the coolant and in general radiated from the engine structure.

To draw up such a heat balance, measurements are taken of rate of mass



General principles of heat engines

Cylinder

block



30



Cooling

Water



25

20



Unburnt



10



Gearbox

Rolling

resistance



5

Oil



Kinetic

energy



15



Exhaust

gas



Percentage of energy supplied



11



Unaccounted

for



0

Rejected



Engine Transmission Useful work

warm-up

warm-up



Cruise



Fuel consumed (g)



50

40



Acceleration



Fig. 1.4(a) Energy usage of a 2-litre car during the first phase (or warm-up) of the

EEC 15-cycle



30

Idle

20



Overrun



Unburnt



10

0



50

Cruise



Fuel consumed (g)



40



Acceleration



Cycle 1



30

20



Idle

Overrun



10



Unburnt



0

Cycle 2



Fig. 1.4(b) Fuel usage of a 2-litre car during the first two stages of the EEC 15-cycle



12



The Motor Vehicle



flow and temperature of coolant and exhaust gas, radiated losses, work done

and friction losses, etc. Inevitably, however, this leaves some of the heat

unaccounted for. This unaccounted loss can, of course, be due to some serious

errors of measurement, but it mostly arises mainly because the fuel has been

incompletely burned.

For a diesel engine, at full load, about 45% of the heat energy supplied

goes to useful work on the piston, though some of this is then lost in friction.

The cooling water takes away about 25% and radiation and exhaust

approximately 30%. Under similar conditions in a petrol engine, approximately

32% of the total heat supplied goes to useful work on the piston, the coolant

takes away about 28% and radiation and exhaust about 40%. The principal

reason for the differences is that the compression ratio, and therefore expansion

ratio, of the petrol engine is only about 10 : 1 while that of the diesel engine

is around 16 : 1.

An extremely detailed analysis of the overall losses of energy, including

those in the transmission, tyres, etc., of a saloon car, powered by a 2-litre,

four-cylinder engine, operated on the EEC15 cycle (Chapter 11), can be

found in Paper 30/86 by D. J. Boam, in Proc. Inst. Mech. Engrs, Vol. 200,

No. D1. One of the interesting conclusions in the paper was that, of the fuel

energy supplied during the first phase (or warm-up) of the EEC cycle, 60%

was used to warm up the engine and transmission,12% was rejected in the

form of carbon monoxide and unburned fuel, and only 8.5% went to produce

useful work, Fig. 1.4(a). Much of the waste was attributable to the use,

during warm-up, of the strangler, or choke. In Fig. 1.4(b), reproduced from

the same paper, the fuel usage for the first two EEC cycles is shown.



1.14



Factors governing the mean effective pressure



The mean effective pressure depends primarily on the number of potential

heat units which can be introduced into the cylinder in each charge.When the

volatile liquid fuels are mixed with air in the chemically correct proportions,

the potential heat units per cubic metre of mixture are almost exactly the

same in all cases, being about 962 kcal/m3 = 4.050 MJ/m3 at standard

temperature and pressure.

The ‘volumetric efficiency’ represents the degree of completeness with

which the cylinder is re-charged with fresh combustible mixture and varies

with different engines and also with the speed.

The ‘combustion efficiency’represents the degree of completeness with

which the potential heat units in the charge are produced as actual heat in the

cylinder. Its value depends on a variety of factors, among the more important

of which are the quality of the combustible mixture, nature of fuel, quality

of ignition, degree of turbulence, and temperature of cylinder walls.

Lastly, the ‘thermal efficiency’ governs the percentage of the actual heat

units present in the cylinder which are converted into mechanical work.

In engine tests the phrase ‘thermal efficiency’ is taken comprehensively

to include combustion efficiency as well as conversion efficiency, as in

practice it is impossible to separate them.

They are further combined with the mechanical efficiency where this

cannot be separately measured, as ‘brake thermal efficiency’.

It can be shown theoretically that the conversion efficiency is increased

with an increase in compression ratio, and this is borne out in practice, but



General principles of heat engines



13



a limit is reached owing to the liability of the high compression to lead to

detonation of the charge, or pinking as it is popularly called. This tendency

to detonation varies with different fuels, as does also the limiting compression

ratio which, with low grade fuel, generally lies between 6 and 7 1 . With

2

better fuels a higher compression ratio (8 to 9 1 ) is possible, owing to the

2

greater freedom from risk of detonation. (See Chapter 14.)

It thus follows that for the same volumetric efficiency, compression ratio

and thermal efficiency the mean effective pressure will be practically the

same for all liquid fuels. This is borne out in practice.

The thermal efficiency of an internal combustion engine of a given type

does not depend very much on the size of the cylinders. With small cylinders,

the loss of heat through the jacket may be proportionately greater, but the

compression ratio may be higher.

The highest mean effective pressure obtained without supercharging, and

using petrol as fuel, is about 1103.6 kN/m2, but this is exceptional and very

little below the theoretical maximum. A more normal figure to take in good

conditions with full throttle is about 896 kN/m2.



1.15



Work per minute, power and horsepower

= mean effective pressure, N/m2.

= diameter of cylinders, m.

= length of stroke, m.

= revolutions per minute.

= number of effective strokes, or combustions, per revolution per

cylinder, that is, half for a four-stroke engine.

n = number of cylinders.



Let p

D

L

N

f

Then



Force acting on one piston = p



π D2

newtons

4



Work done per effective stroke = p

Work done per revolution = p



π D2 L

newton-metres = joules

4



π D2

L f joules

4



π D2

L f N joules

4

Since the SI unit of power is the watt (W), or one joule per second, the power

per cylinder in SI units is—

Work done per minute = p



pπD 2 L f N

W

4 × 60



and for the whole engine—

pπD 2 L f nN

W

4 × 60

Incidentally, since 1 hp is defined as the equivalent of 550 ft lbf of work

per second (Section 1.6), it can be shown that the formula for horsepower is

=



14



The Motor Vehicle



precisely the same as that for the power output in watts, except that p, D and

L are in units of 1bf/in2, in and ft, and the bottom line of the fraction is

multiplied by 550.

Following the formation of the European common market, manufacturers

tended to standardise on the DIN (Deutsche Industrie Norm 70 020) horsepower, which came to be recognised as an SI unit. In 1995, however, the ISO

(International Standards Organisation) decreed that horsepower must be

determined by the ISO 1585 standard test method. This standard calls for

correction factors differing from those of the DIN as follows: 25°C instead

of 20°C and 99 kPa instead of 1013 bar, respectively, for atmospheric

temperature and pressure, and these make it numerically 3% lower than the

DIN rating. The French CV (chevaux) and the German PS (pferdestarke),

both meaning ‘horse power’, must be replaced by the SI unit, the kilowatt,

1 kW being 1.36 PS.



1.16



Piston speed and the RAC rating



The total distance travelled per minute by the piston is 2LN. Therefore, by

multiplying by two the top and bottom of the fraction in the last equation in

Section 1.15, and substituting S – the mean piston speed – for 2LN, we can

express the power as a function of S and p: all the other terms are constant

for any given engine. Since the maximum piston speed and bmep (see Section

1.14) tend to be limited by the factors mentioned in Section 1.19, it is not

difficult, on the basis of the dimensions of an engine, to predict approximately

what its maximum power output will be.

It was on these lines that the RAC horsepower rating, used for taxation

purposes until just after the Second World War, was developed. When this

rating was first introduced, a piston speed of 508 cm/s and an mep of

620 kN/m2 and a mechanical efficiency of 75% were regarded as normal.

Since 1 hp is defined as 33000 lbf work per minute, by substituting these

figures, and therefore English for SI metric dimensions in the formula for

work done per minute, Section 1.15, and then dividing by 33 000, we get the

output in horsepower. Multiplied by the efficiency factor of 0.75, this reduces

to the simple equation—

2

bhp = D n = RAC hp rating

2.5

For many years, therefore, the rate of taxation on a car depended on the

square of the bore. However, because it restricted design, this method of

rating for taxation was ultimately dropped and replaced by a flat rate. In the

meantime, considerable advances had been made: mean effective pressures

of 965 to 1100 kN/m2 are regularly obtained; improved design and efficient

lubrication have brought the mechanical efficiency up to 85% or more; and

lastly, but most important of all, the reduction in the weight of reciprocating

parts, and the proper proportioning of valves and induction passages, and the

use of materials of high quality, have made possible piston speeds of over

1200 cm/s.



1.17



Indicated and brake power



The power obtained in Section 1.15 from the indicator diagram (that is,