Page 22 - Automotive Engineering

Piston-engine cycles of operation CHAPTER 1.1

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is no noisy injection equipment used on the petrol engine, may be either kilonewtons per square metre (kN/m )or
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unlike that necessary on the diesel engine. bars (note: 1 bar ¼ 100 kN/m ).
The products of combustion coming out of the ex-
haust system are more noticeable with diesel engines, 1.1.6.3 Engine torque
particularly if any of the injection equipment compo-
nents are out of tune. It is questionable which are the This is the turning-effort about the crankshaft’s axis of
more harmful: the relatively invisible exhaust gases from rotation and is equal to the product of the force acting
the petrol engine, which include nitrogen dioxide, or the along the connecting-rod and the perpendicular distance
visible smoky diesel exhaust gases. between this force and the centre of rotation of the
Safety Unlike petrol, diesel fuels are not flammable crankshaft. It is expressed in newton metres (N m);
at normal operating temperature, so they are not a han- i.e. T ¼ Fr
dling hazard and fire risks due to accidents are minimised. where T ¼ engine torque (N m)
Cost Due to their heavy construction and injection F ¼ force applied to crank (N)
equipment, diesel engines are more expensive than and r ¼ effective crank-arm radius (m)
petrol engines.

During the 180 crankshaft movement on the power
stroke from TDC to BDC, the effective radius of the
1.1.6 Engine-performance crank-arm will increase from zero at the top of its
terminology stroke to a maximum in the region of mid-stroke and
then decrease to zero again at the end of its downward
To enable intelligent comparisons to be made between movement (Fig. 1.1-10). This implies that the torque
different engines’ ability to pull or operate at various on the power stroke is continually varying. Also, there
speeds, we shall now consider engine design parameters will be no useful torque during the idling strokes. In
and their relationship in influencing performance fact some of the torque on the power stroke will be
capability. cancelled out in overcoming compression resistance and
pumping losses, and the torque quoted by engine
manufacturers is always the average value throughout
1.1.6.1 Piston displacement or swept the engine cycle.
volume The average torque developed will vary over the
engine’s speed range. It reaches a maximum at about
When the piston moves from one end of the cylinder to mid-speed and decreases on either side (Fig. 1.1-11).
the other, it will sweep or displace air equal to the cylinder
volume between TDC and BDC. Thus the full stroke 1.1.6.4 Engine power
movement of the piston is known as either the swept
volume or the piston displacement. Power is the rate of doing work. When applied to engines,
The swept or displaced volume may be calculated as power ratings may be calculated either on the basis of
follows: indicated power (i.p.), that is the power actually de-
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pd L veloped in the cylinder, or on the basis of brake power
V ¼ (b.p.), which is the output power measured at the
4000
crankshaft. The b.p. is always less than the i.p., due to
frictional and pumping losses in the cylinders and the
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where V ¼ piston displacement (cm ) reciprocating mechanism of the engine.
p ¼ 3:142 Since the rate of doing work increases with piston
d ¼ cylinder diameter (mm) speed, the engine’s power will tend to rise with crank-
and L ¼ cylinder stroke (mm)
shaft speed of rotation, and only after about two-thirds of
the engine’s speed range will the rate of power rise drop
1.1.6.2 Mean effective pressure off (Fig. 1.1-11).
The slowing down and even decline in power at the
The cylinder pressure varies considerably while the gas upper speed range is mainly due to the very short time
expands during the power stroke. Peak pressure will available for exhausting and for inducing fresh charge into
occur just after TDC, but this will rapidly drop as the the cylinders at very high speeds, with a resulting re-
piston moves towards BDC. When quoting cylinder duction in the cylinders’ mean effective pressures.
pressure, it is therefore more helpful to refer to the Different countries have adopted their own
average or mean effective pressure throughout the whole standardised test procedures for measuring engine per-
power stroke. The units used for mean effective pressure formance, so slight differences in quoted output figures

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