Carnotcyclus

The Carnotcyclus, also known as the Carnot cycle, is an idealized thermodynamic cycle proposed by Sadi Carnot in 1824. It describes the most efficient possible heat engine operating between two heat reservoirs, under the assumptions of reversibility and no dissipative effects. The concept serves as a theoretical benchmark for the efficiency of heat engines.

The cycle consists of four reversible steps: (1) isothermal expansion at the high temperature Th, during which heat Qh is absorbed and work is performed; (2) adiabatic expansion, during which the working substance cools from Th to Tc without heat exchange; (3) isothermal compression at the low temperature Tc, during which heat Qc is expelled to the cold reservoir while work is done on the system; (4) adiabatic compression, increasing the temperature from Tc back to Th without heat exchange, completing the cycle.

The engine’s efficiency depends only on the reservoir temperatures: η = 1 − Tc/Th, with Th and Tc measured on an absolute (Kelvin) scale. This efficiency is the theoretical maximum for any heat engine operating between those two temperatures and is achieved only by a reversible cycle.

The Carnot cycle is an idealization; real engines include irreversibilities such as friction, finite temperature differences in heat transfer, and non-quasistatic processes. Nevertheless, it serves as a fundamental standard for evaluating real devices and helps illustrate the second law of thermodynamics. The cycle is commonly analyzed in P–V and T–S diagrams, showing the trade-off between heat transfer, work, and temperature.

Related concepts include the Carnot refrigerator and Carnot heat pump, which describe ideal reversible devices for transferring heat or moving heat against a temperature gradient with maximum theoretical efficiency for given reservoir temperatures.