Carnot Cycle and Carnot Engine

Sections 4.7.3.1 and 4.10 have already established that reversibility is a special, idealised property that real processes only approximate, and that the Second Law caps how efficient any real heat engine can be. This naturally raises the question: given a fixed pair of hot and cold reservoir temperatures, what is the theoretical BEST — the maximum possible — efficiency any heat engine operating between them could ever achieve? The answer, worked out across the four subsections that follow, is the Carnot cycle, a hypothetical (but theoretically achievable in the limit) engine proposed by the French engineer and scientist Sadi Carnot in 1824, which tur …