A heat engine performs the conversion of heat energy to work by exploiting the temperature gradient between a hot "source" and a cold "sink". Heat is transferred to the sink from the source, and in this process some of the heat is converted into work. The theoretical maximum efficiency of any heat engine is defined by the Carnot Cycle. The carnot heat engine (the ideal imaginary heat engine) has an efficency equal to (T1 - T2)/T1 where T1 is the temperature of the hot source and T2 is the temperature of the cold sink.
Examples of everyday heat engines include: the steam engine, the diesel engine, and the gasoline (petrol) engine in an automobile. All of these familiar heat engines are powered by the expansion of heated gases. The general surroundings are the heat sink, providing relatively cool gases which when heated, expand rapidly to drive the mechanical motion of the engine.
Examples of heat engines:
- Vapor power cycles. In these cycles and engines the working fluid are on gas and liquid:
- Gas power cycles. In these cycles and engines the working fluid are always like gas:
- Carnot refrigerator
- Absorption refrigerator
- Heat pump
From the laws of thermodynamics, we conclude that:
- H = C - W
The efficiency of a heat engine is defined by:
- e = W / H = (C / H) - 1
- ecarnot = 1 - Tc / Th
Empirically, no engine has ever been scientifically shown to run at a greater efficiency than a Carnot Cycle heat engine.