We know the ideal Otto cycle is modeled by the two ideal processes isentropic (compression and expansion) and isochoric (heat addition and heat rejection). But why the compression and expansion process is considered as isentropic ? It should be the Adiabatic process as there is no sufficient time to exchange the heat to the surrounding as compared to the speed of piston. Also, T*dS >= dQ From here, yes we do get dS >= 0. But how can we conclude there is no entropy change.

Is there any reference explaining how is the real cycle is modeled to make it ideal. How is the process modeled ?

  • $\begingroup$ It might be worth you while to check out books by Chapman, Judge, Ricardo, Massey as the cycles have been discussed and analysed thoroughly. $\endgroup$
    – Solar Mike
    Commented Feb 21 at 7:06

1 Answer 1


Isentropic is both adiabatic and reversable.

We make it reversable simply because it's easier to model a reversible process.

My Internal Combustion Engines by Obert calls it a "hypothetical" cycle, specifically calls the decision to name the compression and expansion strokes ideal (isetropic) just an obvious part of the model, and the decision to model combustion and blowdown as constant-volume as "arbitrary." It's just easier to understand the ideal cycle. We model these on pv and Ts diagrams because it's what we're used to as mechanical engineers.

The actual is just a rounded version of the ideal, and certainly doesn't have vertical entropy lines on the Ts diagram.


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