A steam engine is an external combustion engine (ECE: the fuel is combusted away from the engine), as opposed to an internal combustion engine (ICE: the fuel is combusted within the engine). While gasoline-powered ICE cars have an operational thermal efficiency of 15% to 30%, early automotive steam units were capable of only about half this efficiency.[citation needed] A significant benefit of the ECE is that the fuel burner can be configured for very low emissions of carbon monoxide, nitrogen oxides and unburned carbon in the exhaust, thus avoiding pollution.