Carnot cycle

Until the mid-nineteenth century, it was believed that it was possible to build an ideal thermal machine, which would be able to transform all the energy supplied into work, obtaining a total yield (100%).

To demonstrate that this would not be possible, French engineer Nicolas Carnot (1796-1832) proposed a theoretical thermal machine that behaved like a full yield machine, establishing a maximum yield cycle, which was later called Carnot cycle.

This cycle would consist of four processes, regardless of substance:

  • A reversible isothermal expansion. The system receives heat from the heat source (L-M)
  • A reversible adiabatic expansion. System does not exchange heat with thermal sources (M-N)
  • Reversible isothermal compression. The system gives off heat to the cooling source (N-O)
  • Reversible adiabatic compression. System does not exchange heat with thermal sources (O-L)

In a Carnot machine, the amount of heat that is supplied by the heating source and the amount given to the cooling source is proportional to its absolute temperatures, as follows:

Thus, the yield of a Carnot machine is:




= absolute temperature of the cooling source

= absolute temperature of heating source

Thus, it is concluded that for 100% efficiency, all heat coming from the heating source must be turned into work, since the absolute temperature of the cooling source must be 0K.

From this it follows that absolute zero is not possible for a physical system.


What is the theoretical maximum yield of a steam engine whose fluid enters at 560 ° C and leaves the cycle at 200 ° C?