Do piston rings rotate on the piston?

[olddog]

Originally Posted by Barry_R

Low RPM and transient throttle behavior in street engines are actually worse for a gapless strategy. The variations in ring loading make them float in the groove unsettling the sealing and bringing oil and combustion residue into the chamber. Not good for emissions.

I don't understand - my lack of knowledge, I'm sure.

Ring loading variations caused by cylinder pressure changing with the throttle position?

or

Gap-less rings have a variation in loading due to their nature/design?

[Barry_R]

Cylinder pressure forces rings down against the groove, and out toward the cylinder walls. Light throttle or backing off the throttle will reduce pressure to the point where inertia and inter-ring pressures take over and the top rings are not pushed down & out.

The downstroke flow of some residual combustion gasses through a gap help to move scraped oil toward the piston's drainbacks and the sump. Lacking any downward impetus, the oil will cause the second ring to eventually hydroplane and the oil can contaminate the combustion chamber.

[DMXF]

Originally Posted by olddog

I don't understand - my lack of knowledge, I'm sure.

Ring loading variations caused by cylinder pressure changing with the throttle position?

or

Gap-less rings have a variation in loading due to their nature/design?

How data is interpreted can be an art all in itself. Some comparisons I've heard about used a gapless style that has opposing notches at the ends of the compression ring gaps with the thin parts overlapping. These rings tend to be thicker/heavier than other gapless designs because of the stress concentration from the discontinuity of the notch. Most input I've heard is that the weight is the main factor in any dynamic anomolies. This makes sense because if it were due to lack of gap alone, a standard ring should have the same problem in circumferential areas where there is no gap. In addition feedback is that engine builders in the top eschelons of racing simulate a gapless design by having the ends virtually butt (they close it up till they see evidence of rubbing after use, then open up a couple of thousandths from there), so if gapless in itself were a problem they would see it there too.

[DMXF]

Originally Posted by Barry_R

The downstroke flow of some residual combustion gasses through a gap help to move scraped oil toward the piston's drainbacks and the sump. Lacking any downward impetus, the oil will cause the second ring to eventually hydroplane and the oil can contaminate the combustion chamber.

Barry - I've heard about that, the early Z06 engine had an oil consumption problem which was rectified by going to a slightly greater preload oil scraper rings. Erring on the side of a stiffer oil ring was recommended to me with a gapless.

[Excaliber]

It's fairly obvious what might happen with a ring gap that is to close.
BUT, what happens when the ring gap is to wide???

[olddog]

Originally Posted by Excaliber

It's fairly obvious what might happen with a ring gap that is to close.
BUT, what happens when the ring gap is to wide???

The wider the gap the more the leakage (or blow by) would be the first obvious problem. May well be more issues, too.

Somewhere fuzzily back in a memory I no longer trust, I seem to recall that the flow through a gap is a function of the gap cubed. Not sure if this was a gas law or a polymer dynamics relationship, so don't hold me to that.

[Excaliber]

May well be more issues, too.

Well don't stop there,,, and those other issues might be????

...hold's Olddogs feet to the fire...

[olddog]

Some where in this thread someone mentioned that what pressure leaks past the first compression ring is trapped by the second compression ring. If I recall correctly they said something to the affect of the pressure trapped between the two rings, pushes the top ring off the land causing it to flutter and loose it's seal. Maybe they didn't exactly say this, but it was how I interpreted what ever was said. I think this is along the lines of what Barry_R is talking about.

What about using one gap-less and one normal compression ring? Has this been tried? I'm not sure if the top should be the gap-less or not, but it would seem the logical place.

[olddog]

Originally Posted by Excaliber

Well don't stop there,,, and those other issues might be????

...hold's Olddogs feet to the fire...

I'm a student here, not the master. Not even an apprentice yet.

My thought would be that if the leaking gases past the gaps tend to act like a pin that prevents rotation then the wider the gap the more powerful the affect. That is a totally theoretical thought based on the discussion in this thread, which I find very interesting.

I'm not certain that ring rotation is necessary. It would seem to me that the rotation would help to round out the cylinders, by randomizing where the tight spots tend to wear things. However it also occurs to me that at some point in time when the thrust starts to wear the cylinders out of round, the rings are going to stop turning and tend to wear out of round to match the bore. Now I'm way out on a limb, because I never spent the time studying worn out engines and trying to understand all these things.

[DMXF]

Originally Posted by olddog

What about using one gap-less and one normal compression ring? Has this been tried? I'm not sure if the top should be the gap-less or not, but it would seem the logical place.

I recall that is the standard gapless configuration, they all use one gapless and one gapped compression ring. Various manufacturers have tried the gapless in different grooves and I think the top groove is the one most people feel is optimal (?).

[Dominik]

In his book Power Secrets Yunick elborated around the gap and power loss, and 20 years ago when I read it, lost me.

It relates to a more than linear (cube?) loss depending on the gap. I shall dig out that book.

I did, and he also wrote about rings spinning in the grooves (or was is Bill Jenkins?):


Page 50, center column
Smokey Yunick's Power secrets

"If the head gaskets, the rings and the valves are all sealed tightly, there should not be very much leakage when the the high-pressure air (from a leak-down tester) is fed into the cylinder. It is not possible to gain a 100% mechanical seal in the cylinder, but if everything is in top notch condition, there should not be more than about 4% leakage.

..., if the average is over 8%, you don't have an engine,...

This doesn't sound like much to worry about, but you have to remember that only 1/3 of the heat energy (pressure) developed by each cylinder is actually going to push the piston down. So, if 8% of the cylinder pressure is leaking away, you are theoretically losing 24% of the recoverable flywheel horsepower."

[Excaliber]

Smokey was, on so many levels, well ahead of his time. Certainly one of my hero's.

[Barry_R]

Sometimes lost in the discussions lies the fact that gapless rings are not a new invention. They have been around in various overlap joints, bevel joints, and multirail designs since at least the 20s if not earlier.

The ring package is a dynamic sealing device moving at a high rate of speed under high pressures and at high temperatures. Not an easy thing to optimize or quantify. What works in one environment (say an air compressor) is not the best in another.

If you look at the wear pattern of rings at the gap you will find that they are not round. More & wider wear is evident at the tail end alongside the gap. They cut them after lapping so that the unit pressure at the gap is higher - this helps to stabilize the "tails".

[olddog]

Barry,

I don't want to put words in your mouth, but I get the impression that you feel that traditional rings do a good enough job. I'm thinking the positives minus the negatives on gap-less isn't an overwhelming reason to go fix something that isn't a big problem to begin with.

For high performance applications - certainly racing applications - engines don't typically run long enough to wear the bores a whole lot. I have pulled some high mileage engines down that the bores were so worn that a 0.040 over bore wouldn't straighten the bores up and tossed the block. It never occurred to me back then to stick a ring in and measure the gap, but I expect the gap would have increased by .010" to .015." I believe that the gap-less ring would be better at the end of a street engines life. Not that that necessarily makes a good case for there use.

Your thought?

[Jac Mac]

I wonder if the rotation might occur mainly in the warm-up phase of operating the motor, when the piston crown has not expanded to its full diameter at operating temp, during this phase with the flame front on many motors tending to have a toroidal/circular pattern due to the spark plug placement & chamber design the gas pressure increase must tend to flow around the piston top land area therefore inducing ring rotation rather than just over the edge of the piston crown.

[Barry_R]

Gap increases with wear by the factor of PI. So if you have .040 wear (that'd be pretty incredible from a standard bore - never seen anyting like that still runnin' ) your ring gap would have increased by +/- .120"ish...

[Dominik]

We did a 59 Cadillac motor once with STD size pistons where we had to skip the next overbore straight to the following... 30 over wouldn't clean the bore.

Which we only realized once first oversize pistons had arrived. Maybe we should have quietly inserted the 30 over pistons and close it up again. Can you imagine that ring gap?

BTW, the rings cannot be 100% round if your bore is not exactly matched. The rings get more oval when you compress them. Unless they are made to be round when they butt.

But then they might as well come exactly gapped to begin with?

Yunick says they rotate at BDC and TDC

[Bill E]

Rings do rotate. But it is cool they work both ways LOL

[Barry_R]

Originally Posted by Dominik

Unless they are made to be round when they butt.

But then they might as well come exactly gapped to begin with?

They are round when they are made and finished. Then they get "split" at the gap. The gaps on presized rings are surprisingly close considering the variables involved - .020-.025 is common to see. That allows for some bore size variation while remaining in a safe working range - remember that a .001 bore size difference is .003 gap variance. They don't know if you're putting them into a mom & pop driver, a taxi cab, a hot rod, a boat, or a MD truck when they go into the box...

[xb-60]

It's simple....if it's a La Nina weather cycle, the rings rotate clockwise and if it's El Nino, they go anti-clockwise. And if the weather's stable, the bores wear oval shaped and the rings don't rotate. I did a PhD on this.
Cheers,
Glen