Carnot cycle consists of two reversible isothermal processes and two reversible adiabatic processes. Consider a stationary system as in piston cylinder machine as shown in below diagram,
The above Carnot cycle consists of
A reversible isothermal process in which heat Qa enters the system from source A reversibly when cylinder cover is in contact with diathermic cover D.
A reversible adiabatic process, in which a diathermic cover D is replaced by the adiabatic cover C and work Wa is done by the system reversibly at the expense of internal energy of the system.
A reversible isothermal process, in which again an adiabatic cover C is replaced by a diathermic cover D and heat Qb is rejected to the sink B from the system reversibly.
A reversible adiabatic process, in which a diathermic cover D is replaced again by the adiabatic cover C and work Wb is done upon the system reversibly and the internal energy of the system increases.
