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-
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The efficiency of an engine is defined as ratio of the useful work done to the heat provided.
$${\displaystyle \eta ={\frac {\mathrm {work\ done} }{\mathrm {heat\ absorbed} }}={\frac {Q_{1}-Q_{2}}{Q_{1}}}}$$
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An engine has many moving parts that produce friction. Some of these friction forces remain constant (as long as the applied load is constant); some of these friction losses increase as engine speed increases, such as piston side forces and connecting bearing … See more

Steam engine
See also: Steam engine#Efficiency See also: Timeline of steam power
Piston engine See more

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 … See more

Air is approximately 21% oxygen. If there is not enough oxygen for proper combustion, the fuel will not burn completely and will … See more

Reciprocating engines
Reciprocating engines at idle have low thermal efficiency because the only usable work being drawn off … See more