The Carnot cycle has the maximum efficiency for an ideal reversible refrigerator.
A refrigerator operates as a reversed heat engine: it uses work input to remove heat from a cold reservoir and discharge that heat into a warmer reservoir. The Carnot cycle is the limiting reversible cycle, so it sets the greatest possible coefficient of performance for refrigeration between specified temperatures.
Its ideal refrigeration coefficient of performance is COP_R = T_C/(T_H − T_C), using absolute temperatures. This is not technically an efficiency in the ordinary heat-engine sense; refrigerators are judged by how much heat they remove per unit of work supplied. A smaller temperature gap between the refrigerated space and surroundings produces a higher theoretical COP.
The Carnot cycle consists of two isothermal and two adiabatic reversible processes. Rankine, Diesel, and Otto cycles are important models for other applications, especially power generation and internal-combustion engines, but they are not the universal reversible limit for refrigeration. Real refrigerators perform below the Carnot value because of friction, pressure losses, finite temperature differences, and other irreversibilities.