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Carnot cycle In thermodynamics, a four-stage cycle (named after Sadi Carnot) involving alternate adiabatic and istothermal processes. The former take place without gaining or losing heat, the latter without changing temperature. For example: 1) adiabatic compression of a gas increases its temperature. 2) the gas is then expanded isothermally. 3) Adiabatic exapansion of the gas lowers the temperature. 4) The gas is compressed isothermally, lowering the temperature and completing the cycle. In practical terms, it demonstrates the impossibility of total efficiency in heat engines. Some heat energy always remains unused in a ‘cold sink’. In an internal combustion engine, this can be thought of as the engine itself.