Zynerji wrote: ↑
Tue Sep 10, 2019 7:22 pm
Mudflap wrote: ↑
Tue Sep 10, 2019 2:05 pm
Zynerji wrote: ↑
Fri Sep 06, 2019 4:55 am
I'm sure it's been discussed, but why no lapped pistons like R/C engines? Some of them hit 40kRPM already. I'd expect with the r&d put into the combustion chamber of the current engines that someone would have tried this by now.
Because they would be piss poor at sealing.
That seems a bit illogical at first, care to educate on why? My logic would expect a closer fitting piston in an ultra-tolerance manufacturing environment would yield the closest-to-theoretical-perfect results compared to an inherently changing diameter and the slosh of classic piston rings.
Change my mind.
The advantage of piston rings is to be able to tune the contact forces. It is essentially a spring.
As you say you want the piston to be as tight as possible. But since the piston is relatively stiff you could end up with huge pressure when you end up on the tight side of the tolerances. But on the other hand you will always have clearance when you are on the correct side of the tolerances.
Additionally and maybe the bigger concern is that neither the piston nor the engine block deforms uniformly and both are deforming differently during the power stroke and combustion. So you would need to have it tight enough for one stroke, but can't be too tight for another.
Therefore you use a "soft" springy device that seals the piston. Ergo the piston rings. Additionally this can be used to tune and balance the friction and sealing of the piston.
RC engine pistons are pretty forgiving. They are usually pretty high and aren't they two stroke engines anyway?