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 $\eta_{Carnot}=\eta_{Otto}$?

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Otto cycle is not a Carnot cycle, so there is no reason Carnot efficiency should apply to Otto cycle.

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 $T_C, T_H$).


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