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-
Internal combustion (gasoline, diesel and gas turbine-Brayton cycle engines) and
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.