Carnot engine

A Carnot heat engine is an engine that operates on the Carnot cycle. This is an ideal reversible closed thermodynamic cycle in which there are four successive operations involved, which are isothermal expansion, adiabatic expansion, isothermal compression and adiabatic compression. During these operations, the expansion and compression of a substance like an ideal gas can be done up to the desired point and back to the initial state.

In the process of going through this cycle, the system performs work on its surroundings, thereby acting as a heat engine. It acts by transferring energy from a warm region to a cool region of space and converts some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force and then transfer thermal energy from a cooler system to a warmer one, thereby acting as a refrigerator or heat pump rather than a heat engine.

The Carnot cycle can be thought of as the most efficient heat engine cycle allowed by physical laws. When the second law of thermodynamics states that not all the supplied heat in a heat engine can be used to do work, the Carnot efficiency sets the limiting value on the fraction of the heat which can be so used. In order to approach the Carnot efficiency, the processes involved in the heat engine cycle must be reversible and involve no change in entropy. This means that the Carnot cycle only works with the new laws of physics, since no real engine processes are reversible and all involve some increase in entropy with the old laws of physics.