By the efficiency of a petrol engine, one usually understands the thermal efficiency, or the ratio of the heat returned as work on the crank shaft, to the heat energy in the fuel. This efficiency is vitally important in an aero engine intended for long-distance work, since it directly affects the weight of fuel to be carried.

The necessity for a high thermal efficiency on the B.H.P. almost of necessity involves that of a high mechanical efficiency, that is it requires that the losses due to mechanical friction and to pumping losses shall be small.

But an engine may have a high thermal efficiency and yet have a comparatively low brake mean effective pressure. It may, owing to too small a valve area or to poor design of the induction system, only be able to draw a small weight charge into the cylinder, although it may use this very efficiently once it is in the cylinder.