Reversible Isothermal Heat Addition

In the first process, the cylinder head is brought into contact with a source at temperature . The gas inside the cylinder is also at temperature . The gas expands reversibly and isothermally . During this process, the system absorbs energy as heat from the source. The system changes its state from 1 to 2 on the diagram.

(18.1)

where, for an ideal gas,

Reversible Adiabatic Expansion

In the second process, the cylinder head is insulated and the gas is allowed to expand till its temperature is equal to the sink temperature . The system thus reaches state 3. This is a reversible adiabatic process.

(18.2)

Reversible Isothermal Heat Rejection

In the next process, the system is brought into contact with the sink which is at a temperature . The heat leaves the system and the internal energy further decreases

(18.3)

where, only for an ideal gas,

Through a reversible isothermal process the system reaches state 4.