Reversible Cycle

A process in which some change in the reverse direction, reverse the process completely , is known as a reversible process. In a reversible process, there should not be any loss of heat due to friction, radiation or conduction etc. 

A cycle will be reversible if all the processes constituting the cycles are reversible. Thus in a reversible cycle, the initial conditions are resorted at the end of the cycle.

A little consideration will show that when the operations are performed in the reversed order, the cycle draws heat from the cold body and rejects it to the hot body. This operation requires an external power to drive the mechanism according to the second law of thermodynamics.

A machine which operates on a reversed cycle is regarded as a "heat pump", such as a "refrigerator", because it pumps heat from the cold body to the hot body.

Following are the conditions for reversibility of a cycle:

1. The pressure and temperature of the working substance must not differ, appreciably from those of the surrounding at any stage in the process.
2. All the processes, taking place in the cycle of operation, must be extremely low.
3. The working parts of the engine must be friction free.
4. There should be no loss of energy during the cycle of operation.