Heat engine

A heat engine is a system that transfers thermal energy to perform mechanical or electrical work. It operates by moving a working substance, typically a gas or liquid, from a region of higher temperature to one of lower temperature, converting part of that thermal energy into useful work while rejecting the rest as waste heat. The maximum theoretical efficiency any heat engine can reach, known as the Carnot limit, equals the temperature difference between the hot and cold ends divided by the absolute temperature of the hot end; real engines always fall short of this limit because of friction, heat loss and other irreversible effects. Heat engines appear throughout transport and power generation: internal combustion engines drive most road vehicles, steam turbines and gas turbines generate electricity in thermal power stations, and the same thermodynamic cycle run in reverse describes refrigerators and air conditioners. Reported efficiencies for real heat engines range from roughly 3 percent for some ocean thermal systems to about 60 percent for advanced gas turbines.