De Carnot-cyclus

The Second Law of Thermodynamics states that no heat engine can have 100% efficiency. To derive the maximum theoretical efficiency, Sadi Carnot developed a hypothetical heat engine.

Since work is lost as heat in all irreversible processes, maximizing the efficiency of a heat engine means avoiding all irreversible processes.

However, heat flow through a finite temperature drop is an irreversible process. Hence, heat exchange between the engine and the hot and cold reservoirs must be isothermal. Moreover, when its temperature changes, it must change adiabatically.

In a Carnot cycle, the working fluid is assumed to be an ideal gas. First, it undergoes isothermal expansion in thermal contact with a heat reservoir at temperature T-h and absorbs heat Q-h.

The fluid then expands adiabatically, and its temperature drops to the temperature of the cold reservoir, T-c.

Next it is placed in contact with the cold reservoir at temperature T-c and compressed isothermally, rejecting heat Q-c.

Finally, the gas is thermally isolated and compressed adiabatically to reach its initial state at temperature T-h, thus completing the cycle.