# Carnot vs. Otto

Otto cycle is reversible and according to Carnot theorem should have the same efficiency as Carnot $\eta_{Carnot}=1-T_C/T_H$. The efficiency of Otto is $\eta_{Otto}=1-\frac{T_4-T_1}{T_3-T_2}$ (see figure). How is it possible to compare these two efficiencies and say that $\eta_{Carnot}=\eta_{Otto}$?

The statement "all reversible engines have the same efficiency" applies only to engines that use Carnot cycle (and thus operate with two heat reservoirs of constant temperatures $T_C, T_H$).