Engine efficiency - Wikipedia: "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, ie., Brayton cycle engines). (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.
Modern gasoline engines have an average efficiency of about 25 to 30% when used to power an automobile. In other words, of the total heat energy of gasoline, 70 to 75% is rejected (as heat) in the exhaust or consumed by the motor (friction, air turbulence, heat through the cylinder walls or cylinder head, and work used to turn engine equipment and appliances such as water and oil pumps and electrical generator), and only about 25% of energy moves the vehicle. At idle the efficiency is zero since no usable work is being drawn from the engine."
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