1.9.Thermodynamic cycles

Unit 1 - Fundamentals of Heat and Thermodynamics

1.9.Thermodynamic cycles
A Thermodynamic cycle or a cyclic process consists of a series of thermodynamic operations (processes) which take place in a certain order and the initial conditions are restored at the end of the processes. When the operations or processes are plotted on P.V.diagram they form a closed figure, each operation being represented by its own curve. Since the area under each curve gives the work done to scale, during each operation, it therefore follows that the network done during one cycle will be given by the enclosed area of the diagram as shown in Figure.

Thermodynamic cycles
Types of Thermodynamic cycles

Reversible cycle
A process in which some change in the reverse direction, reverse the process completely, is known as 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 cycle are reversible. Thus in a reversible cycle, the initial conditions are restored at the end of the cycle.A little consideration will show that, when the operations are performed in the reversed order, they 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 “Heat pump” such as “Refrigerator” because it pumps heat from the cold body to the hot body.

Irreversible cycle
Whenever some changes in the reverse direction reverses the process completely, it is known as reversible process. But if the change does not reverse the process it is called an Irreversible process.In an irreversible process there is a loss of heat due to friction, conduction or radiation. Thus in an irreversible process the initial conditions are not restored at the end of the process.
Last modified: Monday, 22 August 2011, 9:43 AM