Super Charging and how to

[nesikachad]

Grab a big syringe. Collapse the plunger. Now plug the opening and pull the plunger back to generate a vacuum. There is resistance. (As one would expect right?) Now allow air back into the cylinder. Plug it off again and attempt to collapse the plunger once more.

I think you'll find that it takes far more effort to compress the air than it does to overcome a vacuum. Lesser of two evils. . .

Have remember also that gasses will piddle past the rings which will only compound the matter. Simply venting to atmosphere won't do once you start turning higher rpm.

 

[Admiral_dk]

A number of engine configurations do have a equal or rather close to equal volume in the combined crankcase at all positions of the crank .... so far so good .... BUT the gas (air + ) MUST be moved around inside the crankcase, between pistons going up and down and this creates PUMPING LOSS = power lost. A loss that increases with rpm and I wouldn't be surprised if it isn't a linear but for instance a square relationship with rpm's.

With a vacuum in the crankcase, there's a lot less gas molecules to be moved = smaller power loss.

No oil foaming, no oils sweating on the outside = another great plus.

 

[Lakc]

BUT the gas (air + ) MUST be moved around inside the crankcase, between pistons going up and down and this creates PUMPING LOSS = power lost. A loss that increases with rpm and I wouldn't be surprised if it isn't a linear but for instance a square relationship with rpm's.

Thats the best explanation I have heard yet. It still seems unnatural that the pumping losses would be less then the windage losses.

 

[mu38&Bg#]

I have to wonder how the greater pressure differential impacts ring sealing?

 

[nesikachad]

That is the part that I've always had a hard time grasping. It's widely understood that having a negative pressure in the crankcase helps with ring seal. I have no doubt it does but I wonder how exactly.

It would seem to me that the exact opposite would be true. if it were possible to have more pressure in the crankcase than whats on the combustion side it would aid with ring seal as it would mitigate gasses from piddling past the ring. A negative pressure in my mind only encourages additional leakage during combustion.

Of course this "pretend" engine would be completely backwards from everything I've been blabbering about in the above half dozen posts.

I dunno. . .but its interesting/fun to chat about.

This discussion has prompted me to entertain the idea of building a roots blower for my Moki 210 glow engine that I use on my H9 Sundowner. USRA had a class for this plane but they dropped it since H9 stopped producing it. (bummer) So, now it seems it'll just be a good test tube for seeing what it takes to grenade the front off an airplane at 170+mph. . . ;D

Good times!

 

[Admiral_dk]

I'm no expert on this but I believe that I can explain it - so here goes :

On the upwards movement of the piston, the ring is in the bottom of the grove and the compressed gas is leaking a bit down the piston, passing the top of the ring and pressures the back of the ring against the cylinder wall => making the ring seal better against the cylinder wall. During combustion, the much higher pressure on the top of the ring still lets a small bit of the gas leak past the top of the ring and still helps to press it out against the cylinder wall. For this to work the combustion pressure should easily be able to press the ring down against the bottom of the ring grove. If you have a to big pressure down below it can't press the ring down to the bottom of the grove and alas it's not tight => blow-by => increased pressure under the pistons => less seal => etc.

I'm frankly not sure what's going on during inlet and exhaust on a four-stroke, ring vise ....

 

[mu38&Bg#]

I know messing around with my single cylinder model airplanes engines blocking the crankcase vent leads to erratic running. Whether it's due to the ring unseating during the intake stroke or other factors I'm not sure, but the difference is evident.

 

[jpeter]

I know nothing about this but I'm gonna comment anyway: When there's negative pressure in the cylinder, as there is during the intake cycle, and positive pressure in the crankcase wouldn't any leakage in the rings cause air to be sucked from the crankcast to the combustion chamber, carring some oil with it? After a while you'd be out of oil. Some negative pressure in the crankcase would reduce this a lot, maybe.
Regarding the pumping loss question: If the cc was full of oil such that the oil had to slosh from cylinder to cylinder there'd be no question about power loss. Well, air is fluid too and pumping it from cylinder to cylinder takes a lot of work too. Maybe what you need is a tuned crankcase like a tuned exhaust. Maybe with proper tuning you could take advantage of the pumping. (it's a joke guys)