Hello. First, look at the figure describing Carnot's cycle. In 1st step (A → B) and 2nd step (B → C), I fully understand that the work done on the pistol (surrounding) by the gas (system) is dW = pdV where dV = Adl since F = pA is the only force on the pistol from the gas (I assumed there are no other forces). So far so good. However, in 3rd step (C → D) I have a confusion about the work on the pistol by the gas. In this step, external force Fe applies on the pistol downward direction to compress the gas, so eventually, Fe applies on the gas from the pistol. And according to Newton's 3rd law (action-reaction law), the gas also applies the force -Fe on the pistol. As a result, the gas applies not only the force associated with the gas pressure (Fp = pA), but also -Fe . So. I think the work done on the piston by the gas must be (-Fe + Fp)dl, instead of only Fpdl = pdV. Am I wrong? If it is, could you please point out exactly what I am wrong with? Thanks for reading this post and I'm happily waiting for your response.