Carnot Cycle

In 1824 Carnot suggested a particular cycle of operation for a CHPP which avoided all irreversibilities. It consisted of four processes, two isothermal and two adiabatic. The process take place between a heat source at temperature T_h and a heat sink at temperature T_c. The system is a mass of gas behind a piston. The cycle on a p-v diagram is shown below:

In state 'A' the gas is temperature T_h and the cylinder is fully insulated.

 

Adiabatic expansion (A to B)

The gas expands adiabatically and very slowly, i.e., quasi-statically (and therefore reversibly). As the gas expands its U decreases (dU=Q-W) and its temperature drops until it reaches T_c.

Isothermal Compression (B to C)

At C the heat reservoir at temperature T_c is removed and the insulation put back. Then slow compression from C to D. At D the temperature reaches T_h. Again process is reversible

Isothermal expansion (D to A)

At D the heat reservoir at temperature T_h is brought into contact with the cylinder and as a result slow isothermal expansion occurs from state D to A, there by completing the cycle.