Carnot

Carnot is most commonly associated with Sadi Carnot, a French engineer and physicist whose work laid the foundations of thermodynamics. The surname also denotes the cycle and related concepts named after him, including the Carnot efficiency. The Carnot family name is linked to Lazare Nicolas Marguerite Carnot, a French revolutionary statesman and mathematician who was Sadi’s father and earned the epithet the Organizer of Victory.

Sadi Carnot (1796–1832) studied the science of heat engines and, in his 1824 treatise Reflections on the Motive Power of Fire, examined how engine performance depends on temperature differences rather than the particular working substance. He argued that the most efficient engines would operate reversibly, and his analysis introduced the idea that theoretical limits exist for converting heat into work.

The Carnot cycle is an idealized reversible cycle used to model the maximum possible efficiency of a heat engine. It comprises four steps: isothermal expansion at a high temperature Th, adiabatic expansion to a low temperature Tc, isothermal compression at Tc, and adiabatic compression back to Th. The theoretical efficiency of a Carnot engine is given by eta = 1 − Tc/Th, and, importantly, this efficiency depends only on the temperatures of the hot and cold reservoirs, not on the working substance. Real engines exhibit lower efficiency due to irreversibilities and practical losses.

Legacy of Carnot’s work is foundational in thermodynamics. His insights led to the formal statements of the second law, influencing later developments by Clausius and Kelvin. The Carnot cycle and the concept of Carnot efficiency remain central as ideal limits in the study of heat engines.