# 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-

1. Internal combustion (gasoline, diesel and gas turbine-Brayton cycle engines) and
2. External combustion engines (steam piston, steam turbine, and the Stirling cycle engine).

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

Engine efficiency, transmission design, and tire design all contribute to a vehicle's fuel efficiency.

## Mathematical definition

Main articles: Thermal efficiency and Carnot cycle

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

η = w o r k   d o n e h e a t   a b s o r b e d = Q 1 − Q 2 Q 1 {\displaystyle \eta ={\frac {\mathrm {work\ done} }{\mathrm {heat\ absorbed} }}={\frac {Q_{1}-Q_{2}}{Q_{1}}}}

where, Q 1 {\displaystyle Q_{1}} is the heat ab... ...read more