Quote:
Originally Posted by serialk11r
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.
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Yes, the Roots is a bit of a blunt instrument, but it is available on the shelf. A purpose built unit might use two vane pumps in series to keep the seals in their preferred range. With those, you could vary the eccentricity to get a continuous adjustment of the vacuum at a constant speed.
I don't understand your second paragraph at all.