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

P_{2} u_{2}^{r} = p_{3} u_{3}^{r}

P_{2} u_{2} u_{2}^{r-1} = p_{3} u_{3} u_{3}^{ r-1}

RT1 u_{2}^{r}-1 = RT2 u3 ^{r-1}

T_{1} / T_{2} = ( u_{3} / u_{2}) ^{r-1}

Similarly for process 4 – 1

T_{1} / T_{2} = ( u_{3} / u_{2}) ^{r-1}

Equating equations (7.4) and (7.5) we get

u_{3} / u_{2} = u_{4} / u_{1}

u_{2} / u_{1}= u_{3} / u_{4}

Thus equation (7.3) becomes

where T_{1} = source temperature and T_{2 }= 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, T_{1 }is to be increased because T_{2} 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.