Originally Posted by
DP-buggyboy
Stroke will be the same as a "long stroke" .21 buggy engine.
So, when I got out of the shower last night after coming home from a race, I had a little flash of an idea of how to use the crankcase to supercharge a 4 stroke engine. You could simply valve it with reed-valves. The intake charge would get sucked into the crankcase when the piston goes up, then pushed back out into a the intake manifold as the piston goes back down. This would happen twice as often as the intake stroke, so I'm guessing it could give a significant boost without adding more rotating parts. Of course, other types of valves could be used, but it's just a generic and random idea at this point.
Has anyone ever seen this used in any type of 4 stroke engine?
The closest I've seen would be the Husqvarna 4 stroke MX engines of the early eighties. They didn't have a regular oil pump (for weight and parasitic loss reduction), but had a reed valve in the bottom of the case to control oil flow. When the piston moved down and pressurized the crankcase, oil would be forced through a small reed valve and on its way through passages to where it was needed. This created the pumping action. Worked OK as long as the oil was changed OFTEN, and was high quality synthetic. They were air cooled, didn't carry much oil (by 4 stroke bike standards) and had a rep for cooking the oil (and failing).
Two stroke (GMC type) diesel engines use an external supercharger to pressurize the crankcase. The engine has exhaust valves in the heads, but no intake valves. The cylinders have ports like a two stroke gasoline engine.
I think you wouldn't have any gains by having the engine draw the intake charge into the crankcase and then forcing it through some other valve (reed or otherwise) into the cylinder (only using the botom of the piston as your 'supercharger'). Remember that the pumping action is the same on both sides of the piston. You'll have losses rather than gains. Now using an external supercharger to pressurize the crankcase on the other hand would be a different story, and would allow you to get plenty of lubrication to the lower end.