4.4.
Engine Rating
Generally three methods are adopted to define the rated power of an automobile engine.
(a) Maximum load carried by the engine continuously. This load is indicated either
on the basis of the mean effective pressure, kPa or by the piston displacement in mL per kW
per sec. However, both are identical. Automobile engines using petrol, the m.e.p. varies from
640 kPa. Every engine should be capable of withstanding overload of 10 to 20% in case of
emergency.
(6) Maximum power developed by the engine. In this case the engines are rated in
terms of their maximum capacity, i.e. maximum b.p. that can be developed.
(c) Using conventional formula (RA.C. Rating). For taxation purposes, the Royal
Automobile Club made certain assumptions for finding out the b.p. for a four-stroke automobile
engines. The assumptions are :
Piston speed = 1000 ft/min
m.e.p. = 90 psi
Mechanical efficiency = 75%
Using these values the nominal b.p. of the engine is calculated by the formula :
2
b.p. = (d n)/2.5, where, d is diameter of the cylinder, inches; and n is number of cylinders.
NOTE : The above definition of R.A.C rating is taken from British Standard. If SI unit is
introduced, the expression changes to :
Piston speed 305 m /min, m.e.p. 620 kPa, and Mechanical efficiency 75%
Therefore, b.p. = 460 (d2n), where d in meter.
This b.p., which is much less than obtained in case (b), represents the R.A.C. rating of the
engine.
