Physics of supersonic flow in piston ring gap 1

[apex944]

I don't have a degree in physics so I am looking for a more pedestrian explanation. The question is what happens to the air in and around the top piston ring gap at "high" RPMs?
Does it go supersonic as compression forces it through the gap? Does it get forced through? If supersonic, what are the affects this has on the air in this region?
The genesis of this question is due to a discussion I recall having with a race engine builder many years ago, but that I can no longer recall the details of. I believe the general premise was "Gapless rings are pointless as air in the gap goes supersonic anyway" ... but as stated, I don't know what happens to the air in a ring gap, at high compression, and high RPM.

Thanks, Bill

 

[gruntguru]

Sonic or choked flow (Mach 1) will occur at any restriction where upstream absolute pressure is greater than double (approx) the downstream pressure. The only significance is that reducing the downstream pressure does not increase the velocity or flow once sonic velocity is reached. I don't see any significance in terms of gapped vs gapless rings. If one leaks more than the other, the leakage rate will increase as cylinder pressure increases. The leakage mass-flow will be roughly proportional to the density of gas in the cylinder.

je suis charlie

 

[GregLocock]

OK, yes, the limit is due to Mach choking,

https://en.wikipedia.org/wiki/Choked_flow

, and in air for example a pressure ratio of about 2 is enough to accelerate air to M1 in the orifice. It hasn't got much to do with rpms.

As to whether that's a real effect, hmmm, jury (in my head) is out. Have a read of this

https://link.springer.com/article/10.1007/s40095-019-0296-x

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[apex944]

ty Greg

 

[dgallup]

I really don't understand the OP's quote "Gapless rings are pointless as air in the gap goes supersonic anyway". The leakage is still proportional to the gap, supersonic or not.

Greg Locock's link is a nice piece of work, they conclude the second ring gap is the most important for controlling blow by. Of course, they were looking at a medium speed diesel, that would got out the window on a lot of high performance engines that only have one compression ring.

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[TugboatEng]

Modern diesel engines typically have only one compression ring. The top ring is a keystone fire ring, middle is compression, and bottom is oil control.

 

[mc5w]

I take it that the Mach 1 limit is in part because the ring gap is rectangular nozzle instead of a convergent/divergent nozzle. I suspect that besides machining simplicity the ends of the rings are machined with as little rounding of the corners as possible to block gas flow from becoming supersonic.

 

[gruntguru]

Choked flow will start at much the same pressure ratio, regardless of orifice shape.

je suis charlie

 

[SwinnyGG]

Not a lot happens... the ring gaps are not an orifice between high pressure and low pressure; they are an orifice between high pressure and slightly higher pressure.

At operating temperatures properly gapped rings have a 'gap' that's only a few thousandths wide. The gap between the ends of the rings is narrower than the boundary layers of the gas in contact with the rings. There will be very little flow as long as all the parts are in good condition.