Quote:
Originally Posted by aerohead
would the vacuum below an ascending piston be any better than a vacuum above a decending piston?
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I wondered this myself once, and thought/mentally designed a vacuum pump system that would act on the exhaust, as opposed to using a supercharger on the intake.
With this system, Cam overlap would allow the intake charge to be partially pulled in by the exhaust's vacuum pump (like scavenging, but better), as well as piston downstroke.
The pump would have to be able to move enough air to create enough of a vacuum to have a force sufficient to pull the piston up in the bore faster than it would normally be moving, though, and this amount of vacuum would probably serve to pull oil up through the rings, at best.
Another setup would have the pump just moving enough capacity to completely exhaust the spent gasses, while drawing in more intake charge through the cam overlap, but not enough force of vacuum to actually act on the pistons or rings (notably, anyway). I had proposed that this would work to cancel pumping losses on the top of the cylinders, as opposed to drawing oil and such through PCV vacuums, but I never studied it any more than enough to draw up the system on a napkin and commit it to memory, so no real research was done on it. This was only a few years ago, however, so it's still open to investigation in my head, if I ever feel like it.
I do wonder about the vacuum pump's drag on the motor, though... and if canceling pumping losses would compare to the amount of power needed to run the pump efficiently so that it could work as proposed.. if there would be any actual efficiency gain, or if it would just cancel itself out.