Heat engine
A cyclic device that takes heat from a hot reservoir, turns part of it into work, and rejects the rest to a cold reservoir.
Definition
A heat engine is any device that operates in a repeating cycle to convert heat into mechanical work. In each cycle it absorbs heat Q_h from a hot reservoir, performs net work W, and discards the remaining heat Q_c to a cold reservoir. Because the working substance returns to its original state every cycle, its internal energy is unchanged over a full loop, and the first law reduces to the bookkeeping identity W = Q_h − Q_c.
The cold reservoir is not an engineering defect but a structural necessity: the Kelvin–Planck statement of the second law forbids any cyclic engine from converting heat entirely into work. Some heat must always be rejected. Steam engines, internal-combustion engines, gas and steam turbines, and thermoelectric generators are all heat engines, differing only in their working substance and the path it traces.
Sadi Carnot's central discovery was that the maximum efficiency of a heat engine depends only on the temperatures of its two reservoirs, not on the engine's design or working substance, and equals 1 − T_c/T_h for a reversible engine.
History
The concept crystallised with Sadi Carnot's 1824 analysis of the steam engine, was given mathematical form by Clapeyron in 1834, and became the foundation on which Clausius and Kelvin built the second law of thermodynamics in the 1850s.