Engine efficiency is the amount of power developed as compared to the energy input which is measured by the heating value of the fuel consumed. The term efficiency is used to designate the relationship between the result obtained and the effort expended to produce the result. The term compression ratio is frequently used in connection with engine performance. From your study of the principles of internal combustion, you will recall that compression ratio is the ratio of the volume of air above the piston, when the piston is at the BDC position, to the volume of air above the piston when the piston is at the TDC position.

EFFICIENCIES

The principal efficiencies which must be considered in the internal combustion process are cycle, thermal, mechanical, and volumetric.

Cycle Efficiency

The efficiency of any cycle is equal to the out-put divided by the input. The efficiency of the diesel cycle is considerably higher than the Otto or constant volume cycle because of higher compression ratio and because combustion starts at a higher temperature. In other words, the heat input in a diesel engine is at a higher average temperature. Theoretically, a gasoline engine using the Otto cycle would be more efficient than the diesel engine if equivalent compression ratios could be used. However, engines operating on the Otto cycle cannot use a compression ratio comparable to that of diesel engines because fuel and air are drawn together into the cylinder and compressed. If comparable compression ratios were used, the fuel would fire or detonate before the piston reached the correct firing position.

Since temperature and amount of heat content which is available for power are proportional to each other, the cycle efficiency is actually computed by measuring the temperature. The specific heat of the mixture in the cylinder is either known or assumed, and when combined with the temperature, the heat can be calculated at any instant.