Heat Engines & Carnot Cycle

Discover the upper efficiency limit set by the second law of thermodynamics

The second law of thermodynamics places an absolute upper bound on engine efficiency: no heat engine operating between two reservoirs at T_H and T_C can be more efficient than a Carnot engine operating between the same reservoirs. The Carnot cycle consists of two reversible isothermal processes (heat exchange with reservoirs) and two reversible adiabatic processes (no heat exchange). Because all steps are reversible, no entropy is generated — this is the theoretical ideal. Real engines (Otto, Diesel, Rankine) suffer irreversibilities: friction, turbulence, finite temperature differences during heat transfer, and non-quasi-static compression. These generate entropy, increasing Q_C and reducing net work. The gap between actual and Carnot efficiency is a direct measure of irreversibility. Importantly, the Carnot efficiency depends only on the reservoir temperatures — higher T_H or lower T_C always improves it.