CONCEPT
Carnot Efficiency
The thermodynamic ceiling on any heat engine, established by Sadi Carnot in 1824: the maximum fraction of heat that can be converted to useful work, set permanently by the temperatures between which the engine operates—and the founding instance of a physical limit that no engineering improvement can exceed.
Carnot efficiency is the maximum possible efficiency of any heat engine operating between a hot reservoir at temperature T
hot and a cold reservoir at T
cold: (T
hot − T
cold) / T
hot, where temperatures are absolute.
Sadi Carnot derived this limit in 1824 by showing that an engine exceeding it would, when coupled to the ideal reversible engine, produce perpetual motion—work extracted from nothing. Since perpetual motion is impossible, the limit is permanent. It does not depend on the fuel, the materials, the mechanism, or the ingenuity of the engineer; it is fixed by the temperatures and by nothing else. This was the first time a physical limit had been derived by reasoning from impossibility rather than from the properties of existing devices. The same argument structure, applied to information a century and a half later, yielded the
thermodynamic cost of computation