Piston ring rotation and stuck rings

[pasadena_commut]

Dogma is that in piston engines which are operating normally the rings slowly rotate around the pistons. I'm not entirely clear on why, but evidently there is a net force (let's call it the driving force) tangent to the ring edge (in the plane of the ring), so it rotates. Until it gets stuck, then it doesn't rotate. The rings rotating probably plays some role in keeping the ring lands clean. Typically when one piece of metal rubs over another it will have the effect of knocking loose whatever substances are sticking to them, or breaking those substances down through a shearing action. I could not find any discussion of this hypothetical rotational cleaning action though. Perhaps it is negligible?

Anyway, this makes me wonder about how and when exactly rings seize up. One model would be that when the motor turns off, once nothing is moving, oil and combustion related substances cool and bind tightly to the piston and ring, so that at the next startup the driving force isn't sufficient to rotate the ring. Another model would be that the oil and combustion related substances accumulate while the motor is in operation, slowing the ring rotation, which lets more accumulate, until the ring finally gets stuck even though the motor is operating. It could be one, it could be the other, it could be both. Anybody know?

If the second model is important it suggests that designing the motor to increase the driving force would decrease the rate at which rings stick. Since I don't understand what is providing the driving force in the first place it is hard to say how to increase it. Although, since piston bores are usually honed with what are essentially both right and left handed riflings of very shallow depth, employing just one handedness of rifling at a shallow angle very close to parallel to the bore axis might cause the rings to oscillate back and forth a degree or two on each stroke, rather than rotate. Probably it would mess up the seal though, since I'm confident it is done the way it is now for a very good reason.

 

[Donald]

My guess is Ford and GM and others have put together engines noting the ring positions and run them for awhile then disassembled and noted everything upon disassembly.

They certainly want to make sure the rings do not rotate and end up all lined up.

I think they know the answers to your questions.

 

[MolaKule]

Consider why we gap rings! Free rings seal better because they are able to flex.

A NASA study purpordedly found that a ring rotates once for every 1000 rpm if I recall correctly.

A vectoral force rotates the ring by a combination of piston rocking and reciprocating motions.

 

[Kira]

Likely the force-flow* is analogous to the one used to rotate valves.
Was there not an angle ground into the top of the valve stem, thus creating a 'nudge' along the radius of the stem?

* a term I made up

 

[barryh]

2 stroke rings don't rotate because they are pegged to prevent the ends catching in the ports and yet I don't recall having problems with rings seizing.

 

[pasadena_commut]

A vectoral force rotates the ring by a combination of piston rocking and reciprocating motions.

Once the rings are locked in place wouldn't the piston rock less, maybe not at all? Then the force goes down, maybe to zero. If rotation does act to clean the lands then this introduces (even more) hysteresis into the cleaning process. Once stuck the same amount of cleaning effect from oil and gas plus additives, which was sufficient to keep them moving, is insufficient to get them moving again, since the extra cleaning force from rotation is gone.

Determining if the rings are free or stuck in a normal running engine would not be easy. Still, I wonder if it might not be possible to introduce a "ring rotation" sensor in the side of the cylinder, probably at the level of the top piston ring. It would detect the presence of the ring gap as each ring passes over it. Or rather, gap, no gap. Statistical analysis of this information in an ongoing manner would detect that the ring rotation rate is below optimal, on a per ring basis, hopefully at a stage where some oil or fuel treatment could resolve the situation before it completely locks. How such a sensor would work I'm not even going to speculate. Although, it might be completely pointless - if the first model in the original post is correct it may well be that the ring goes from free to stuck all at once, with no decrease in rotation rate to give a warning.

 

[ZeeOSix]

Once the rings are locked in place wouldn't the piston rock less, maybe not at all? Then the force goes down, maybe to zero. If rotation does act to clean the lands then this introduces (even more) hysteresis into the cleaning process. Once stuck the same amount of cleaning effect from oil and gas plus additives, which was sufficient to keep them moving, is insufficient to get them moving again, since the extra cleaning force from rotation is gone.

If all of the rings were stuck (not just the oil control rings), which also means they are not springing out like they should along with not rotating, then it might be possible for the top of the pistons to rock and contact the cylinder. That's the theory on why severely stuck piston rings can lead to cylinder damage and wear which will make the engine always burn some oil even if the rings became free again.

Statistical analysis of this information in an ongoing manner would detect that the ring rotation rate is below optimal, on a per ring basis, hopefully at a stage where some oil or fuel treatment could resolve the situation before it completely locks.

If the engine is using oil not from obvious external leaks, just use something that cleans ring packs, like Valvoline Restore and Protect as it's proven to free totally stuck piston rings and reduce oil burning. It doesn't have to be complicated.

 

[PandaBear]

Stuck rings likely is the reason for rings not rotating, instead of not rotating is the reason they are stuck. As to why they want you to install rings opening from one direction to another between pieces, my guess is they will likely end up all aligned and gas blow through them straight with lower probability. A random walk on 3 different starting point will likely have alignment of all 3 in much lower probability than they all start at the same place.

Maybe the honing direction of whether it start with clockwise vs counter clockwise leads to which angle has the last hone action thus lead to which direction it rotates? I don't know but is there any research on this?

 

[repairman54]

2 stroke rings don't rotate because they are pegged to prevent the ends catching in the ports and yet I don't recall having problems with rings seizing.

Correct on 2 stroke rings being pinned. Back in the '80's I had issues with rings getting stuck in a 440cc twin free air snowmobile engine. OEM or aftermarket pistons. Pop the jugs off and pop the rings loose and back in business. The cure for that engine was synth. BelRay oil, Every dino oil I tried would do it when the hot engine sat overnight in cold temps, low compression in the morning with a no start. And I mean every dino snowmobile oil no matter the brand. That BelRay synth. oil made me a convert to synth. oils and once switching to it I never again had an issue with that engine. Only synth. oils in all my engines since then, 2 or 4 stroke.