4.6.
Performance Curves
The word performance for an engine is generally used for designating the relationship
between power, speed and fuel consumption. In variable speed engines (like automobile
engines), the rated power at a particular speed does not provide enough information. Under
such situations, the performance curves help to obtain necessary information.
Typical performance curves for internal combustion engines (both petrol and diesel engines)
used in automobiles are shown in Figs. 4.2 and 4.3.
Figure 4.2 shows performance curves for high output, V-8, multiple carburettors (3 dual-
throat carburettors) automotive petrol engine with 7 x 10~3 m3 displacement.
Figure 4.3 shows performance curves for typical automotive CI engine having 6-cylinders
(110 mm x 135 mm) and compression ratio 15:1 that uses 50 cetane fuel.
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Fig. 4.3. Typical performance curves of automotive CI engine.
The two figures reveal that in diesel engines, fuel consumption per kWh is less and markedly
so at the usual range of part-load operation. The torque for diesel engine remains fairly uniform
over a wider range of operating speeds than for petrol engine. This results in better top gear
performance, as the engine is more flexible over a wider speed range. Moreover, a high value of
the torque at lower engine speeds in diesel engine enables the vehicle to run much more slowly
on top gear. The CI engine has an appreciable higher thermal efficiency than the petrol engine
because (a) it has a higher compression ratio and (b) it uses higher air/fuel ratios in order to
avoid incomplete combustion and smoky exhaust. On the other hand, the use of high compres-
sion ratios in petrol engines creates the problem of combustion knock. In petrol engine, the brake
power curve takes a peak, but in CI engine it does not peak because the top speed is limited due
to their heavier reciprocating masses. The friction power curve goes up rapidly at higher speeds
in both cases as it includes fluid friction.
It is a common practice to give the automotive diesel engine three ratings: (a) the maximum
rating for short intervals of operation (fe) the rated output for larger period of operation than
the former and (c) the continuous output rating for operation with no time limit.
