# Classification of Thermodynamic Cycles

The thermodynamic cycles are classified into the following groups

1. Reversible cycle
2. Irreversible cycle
1. Reversible cycle : A proces, in which some change in the reverse direction, reverses 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 process constituting the cycle are reversible. Thus in a reversible cycle, the initial conditions are restored at the end of the cycle.
A littlle 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 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.
2. Irreversible cycle : A process, in which change in the reverse direction, does not reverse the process, is called irreversible process. In an irreversible process, there is a loss of heat due to friction, radiation or conduction.
In actual practice, most of the processes are irreversible to some degree. The main causes for the irreversibility are mechanical and fluid friction, unrestricted expansion, heat transfer with a finite temperature difference. Moreover, friction converts the mechanical work into heat. This heat cannon supply back the same amount of mechanical work, which was consumed for its production. Thus, if there is some friction involved in the process, it becomes irreversible. A cycle will be irreversible if any of the processes, constituting the cycle, is irrreversible. Thus in an irreversible cycle, the initial conditions are not restored at the end of the cycle.