Converting work to heat is an irreversible process, and the purpose of a heat engine is to reverse the effect partially. Heat engines aim to increase the efficiency of the reversal, that is, maximize the work retrieved from heat. If the efficiency of a heat engine were 100%, it would imply reversing the process completely without introducing any other effect. Thus, it would violate the second law of thermodynamics.

What could be the theoretical limit to the efficiency of a heat engine? The French engineer Sadi Carnot devised a hypothetical cycle between the same two hot and cold reservoirs to deduce the limit. This theoretical cycle helps to understand the conventional cyclical systems’ performance limits in transforming heat to work. It also helps define an ideal reversible process via Carnot’s Principle.

The Carnot cycle plays a significant role in developing an important statement of the second law of thermodynamics. Since only two reservoirs are involved in its operation, along with the second law of thermodynamics, it can also be used to define an absolute temperature scale that is truly independent of any substance used for temperature measurement.