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This will be my last post, I promise. I agree that a vacuum pump is not EXACTLY the same as an engine. The vacuum pumps that we use do not have any one way valves (turbo, diffusion, cryo, roughing, etc). Let's look at the case where the intake is open, exhaust is closed, and the piston is...

To a first approximation engines are air pumps and, with the butterflies closed, are vacuum pumps. Obvioulsy, a closer look complicates the analysis. My point is simply that there are no real pumping losses for an engine or pump which is not pumping anything.

No, it still works. The intake valve opens as the piston is on the downward stroke, the chamber is at a pressure less than atmosphere (it takes work to to pressurize ambient atmosphere), the intake closes, the piston moves on the upward stroke which is aided by higher ambient atmospheric pressure.

Sorry, but this is wrong. We have numerous types of roughing and high vacuum pumps at my job. Some of them allow monitoring of instantaneous power. Power drops with pressure. Maintaining a vacuum takes very little power. There are no pumping losses if you are not pumping anything. Whatever...

I don't agree. In my experience, a vacuum pump has much lower loss when operating at a high vacuum. It isn't pumping anything. To think of another way... Think of the diesel engine compressing the air like compressing a spring. The engine rotates compressing and releasing, compressing...