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Efficiency of Carnot Cycle

Applying adiabatic law to process 2 – 3 and 4 – 1 we get

P2 u2r = p3 u3r

P2 u2 u2r-1 = p3 u3 u3 r-1

RT1 u2r-1 = RT2 u3 r-1

T1 / T2 = ( u3 / u2) r-1

Similarly for process 4 – 1

T1 / T2 = ( u3 / u2) r-1

Equating equations (7.4) and (7.5) we get

u3 / u2 = u4 / u1

u2 / u1= u3 / u4

Thus equation (7.3) becomes


where                                                  T1 = source temperature and T2 = Sink temperature


Thus Carnot efficiency depends on source and sink temperatures only and does not depend on the working fluid of the system. To increase the efficiency of Carnot cycle, T1 is to be increased because T2 cannot be decreased as generally atmosphere is taken to be sink.


Drawbacks of Carnot Cycle

Following are the drawbacks of Carnot Cycle :

  • Frictionless motion of the piston through cylinder is impossible.
  • An interchangable cylinder head makes engine impractical.
  • Isothermal expansion is a slow process and adiabatic expansion should be very fast.

This is impractical in a single stroke of the piston.