Carnot (1824) proved no heat engine can exceed efficiency η = 1 − Tc/Th, regardless of working fluid. The cycle has 4 strokes: isothermal expansion (absorbs Q_h), adiabatic expansion, isothermal compression (rejects Q_c), adiabatic compression. The T-S diagram is a rectangle — entropy is exchanged but the total change is zero, proving reversibility. Real engines (Otto, Diesel) are less efficient.