Carnot cycle is an ideal cycle, which is not possible in actual practice because heat addition and rejection at constant temperature is a quasi-static process (very slow process)
and this process cannot be achieved in actual practice.
All processes in this cycle are reversible. The efficiency of Carnot cycle is maximum.
No other cycle will have efficiency more than Carnot cycle efficiency.
Carnot cycle consists of:
Process 1-2: Reversible isotherma heat addition at constanl temperature T1.
Process 2-3; Reversible adiabatic expansion from temperature T1 to temperature T2.
Process 3-4: Reversibie isothermal heal rejection at constant temprature T2.
Process 4-i: Reversible adiabatic compression from temperature T2 to temperature T1.
Heat addition = qA = mRT1logV2/V1 and Heat rejecdon = qR = mRT2logV3/V4
Net work done = qA-Qr
efficiency depends upon T1and T2
when T2=0 efficiency=100%
if T1=T2 means no work is done.