Page 23 - Automotive Engineering

CH AP TER 1 .1 Piston-engine cycles of operation

The two methods of calculating power can be
expressed as follows:

pLANn
i:p: ¼
60 000
where i.p. ¼ indicated power (kW)
2
p ¼ effective pressure (kN/m )
L ¼ length of stroke (m)
2
A ¼ cross-sectional area of piston (m )
N ¼ crankshaft speed (rev/min)
and n ¼ number of cylinders
2 p TN
b:p: ¼
60 000
where b.p. ¼ brake power (kW)
p ¼ 3.142
T ¼ engine torque (N m)
and N ¼ crankshaft speed (rev/min)
The imperial power is quoted in horsepower (hp) and is
defined in terms of foot pounds per minute. In imperial
Fig. 1.1-10 Torque variation during crankshaft rotation ( p ¼ units one horsepower is equivalent to 33 000 ft lb per
cylinder gas pressure; F ¼ connecting-rod thrust; R ¼ crank- minute or 550 ft lb per second. A metric horsepower is
throw; r ¼ effective crank radius; T ¼ turning-effort or torque).
defined in terms of Newton-metres per second and is
equal to 0.986 imperial horsepower. In Germany the ab-
breviation for horsepower is PS derived from the trans-
lation of the words ’Pferd-Sta ¨rke’ meaning horse strength.
The international unit for power is the watt, W, or
more usually the kilowatt, kW, where 1 kW ¼ 1000 W.
Conversion from watt to horsepower and vice versa is:

1kW ¼ 1.35 hp and 1 hp ¼ 0.746 kW

1.1.6.5 Engine cylinder capacity

Engine sizes are compared on the basis of total cylinder
swept volume, which is known as engine cylinder
capacity. Thus the engine cylinder capacity is equal to the
piston displacement of each cylinder times the number
of cylinders,
Vn
i:e: V E ¼
1000
where V E ¼ engine cylinder capacity (litre)
3
V ¼ piston displacement (cm )
Fig. 1.1-11 Torque and power variation over engine speed range. and n ¼ number of cylinders

Piston displacement is derived from the combination
will exist. Quoted performance figures should therefore of both the cross-sectional area of the piston and its
always state the standard used. The three most important stroke. The relative importance of each of these di-
standards are those of the American Society of Automo- mensions can be demonstrated by considering how they
tive Engineers (SAE), the German Deutsch Industrie affect performance individually.
Normale (DIN), and the Italian Commissione technica di The cross-sectional area of the piston crown influences
Unificazione nell Automobile (CUNA). the force acting on the connecting-rod, since the product

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