where η = W Q H {\displaystyle \eta ={\frac {W}{Q_{\text{H}}}}} is the efficiency of the real engine, and η I {\displaystyle \eta _{\text{I}}} is the efficiency of the Carnot engine working between the same two reservoirs at the temperatures T H {\displaystyle T_{\text{H}}} and T C {\displaystyle T_{\text{C}}} . For the Carnot engine, the entire process is 'reversible', and Equation (7) is an equality.