Carnot established in 1824 that the efficiency η C of reversible engines operating between a hot bath at absolute temperature T hot and a cold bath at temperature T cold is equal to 1−T cold/T hot. Carnot particularly considered air as a working fluid and small bath-temperature differences. Plugging into Carnot’s expression modern experimental values, exact agreement with modern Thermodynamics is found. However, in a recently published paper [“Sadi Carnot on Carnot’s theorem”, Am. J. Phys. 70 (1), 42–47, 2002], G ̈u ́emez and others consider a “modified cycle” involving two isobars that they mistakenly attribute to Carnot. They calculate an efficiency considerably lower than η C and suggest that Carnot made compensating errors. Our contention is that the Carnot theory is, to the contrary, perfectly accurate.
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