heatengine

A heat engine is a device that converts part of the energy in heat into mechanical work through a cyclic process. It absorbs heat from a high-temperature source, does work as the working substance expands or moves, and rejects some heat to a lower-temperature sink. The operation is governed by thermodynamics: in a cycle, the net work produced equals the heat input minus the heat rejected, and not all absorbed heat can be converted into work.

The second law imposes a fundamental limit on efficiency. The maximum possible efficiency for an engine operating between two reservoirs is the Carnot efficiency, ηC = 1 − Tcold/Thot, where temperatures are measured in kelvin. Real engines are less efficient due to irreversibilities, friction, and heat losses.

Engines are categorized by how combustion interacts with the working fluid. External-combustion engines, such as steam engines and Stirling engines, burn fuel outside the main working chamber. Internal-combustion engines, including most gasoline and diesel engines, burn fuel inside the engine itself. Common thermodynamic cycles associated with heat engines include the idealized Carnot cycle and practical cycles such as Otto (spark-ignition engines), Diesel (compression-ignition engines), Brayton (gas turbines), and Rankine (steam turbines).

Historically, the development of heat engines spurred advances in thermodynamics, with early milestones by James Watt and later theoretical limits established by Sadi Carnot in the 19th century. Today, heat engines power transportation and electricity generation, while other devices such as heat pumps and refrigerators operate on related thermodynamic principles in reverse.