heatengines

A heat engine is a device that converts part of the thermal energy supplied by a hot source into mechanical work by exploiting a temperature difference between a hot reservoir and a cold sink. In a typical cyclic process, the engine absorbs heat Qh from the hot reservoir, performs work W, and rejects heat Qc to the cold reservoir, with Qh = W + Qc. The cycle repeats to produce continuous work, and the operation is constrained by the first and second laws of thermodynamics.

The efficiency of a heat engine is η = W / Qh = 1 − Qc / Qh. The maximum possible efficiency for given temperatures is the Carnot efficiency, η_C = 1 − Tc/Th, where Th and Tc are the absolute temperatures of the hot and cold reservoirs. Real engines have irreversibilities from friction, nonideal heat transfer, and fluid flow, so their efficiencies are always below Carnot. Materials, manufacturing, and engine design also affect performance.

Heat engines are categorized by energy source and working cycle. External-combustion engines burn fuel outside the working fluid, as in steam engines and Stirling engines. Internal-combustion engines burn fuel inside the working chamber, as in Otto (gasoline), Diesel, and gas-turbine Brayton cycles. Common power-generation cycles include the Rankine cycle for steam plants and the Brayton cycle for gas turbines.

Applications span electricity production, aviation, automotive propulsion, and industrial power systems. The concept emerged from the work of Sadi Carnot in the early 19th century and has driven continual improvements in efficiency, emissions, and versatility of engines, across a wide range of fuels and operating conditions.