Engine efficiency

Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines-

Each of these engines has thermal efficiency characteristics that are unique to it.

Mathematical definition

The efficiency of engine is defined as ratio of the useful work done to the heat provided.

where, is the heat absorbed and is the work done.

Please note that the term work done relates to the power delivered at the clutch or at the driveshaft.

This means the friction and other losses are subtracted from the work done by thermodynamic expansion. Thus an engine not delivering any work to the outside environment has zero efficiency.

Compression ratio

The efficiency of internal combustion engines depends on several factors, the most important of which is the expansion ratio. For any heat engine the work which can be extracted from it is proportional to the difference between the starting pressure and the ending pressure during the expansion phase. Hence, increasing the starting pressure is an effective way to increase the work extracted (decreasing the ending pressure, as is done with steam turbines by exhausting into a vacuum, is likewise effective).