Quote:
Originally Posted by Bicycle Bob
I think that the easiest way to test this is with an Eaton electric roots supercharger. It has about the right pressure differential range, but might want some high-vac oil. It could be rigged to go from idle vacuum right into boost if you wanted to, either producing or drawing power. However, doing that would make a variable compression ratio even more desirable.
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A Roots blower is very inefficient in principle because it directly mixes atmospheric and high pressure air before compressing. In certain speed ranges the exhaust port geometry can be tuned to basically use the "wave drag" of the rotor to produce adiabatic compression. When you run it in reverse, this feature actually works against you.
Variable compression ratio is far more effective than controlling throttling losses past some point. At idle, bleeding off 80% of the air from the cylinder with an effective static compression ratio of 10:1, the effective compression ratio is only 2:1. If you could have a 25:1 compression ratio at idle (effectively 5:1 in this example), thermal efficiency would go WAY up.