PISTON RING ALIGNMENT? 7

[paulrecon]

When inspecting engines returned for warranty complaints, we keep comming upon a large number (approx 5%-10%) of engines where the 1st.compression and 2nd. compression rings have rotated while in the engine so that they have aligned or very nearly aligned their end gaps. In other words, when the pistons are removed from the bore, the end gaps of the 1st and 2nd rings are alined or close to it. Quality assurance and manufacturing engineering insists that the rings were installed in the correct positions, and that they have moved around in the bore while the engine was running. Has any one else come across this problem and what steps did you take to correct it? What forces would cause the rings to move so that the end gaps were aligned? If rings rotate within the bore, why are manufacturers specific about end gap placement when installing?

 

[ivymike]

It is my understanding that piston rings rotate frantically in a running engine unless something is preventing their rotation, such as a pin. Extreme ovalisation of the cylinder bore will also result in ring alignment, since the rings will eventually find a "most relaxed" position in the bore. I'd definitely get an FE analyst to dig up any block FE results that may be around, and take a look to see what the orientation (phase) of the second-order bore deformation is. If your engine is a Vee, I'm guessing your ring gaps all point inboard or outboard at teardown. (Did I guess right?)

As far as fundamental forces causing ring rotation, I'll list a few possibilities that come to me, but my question for you is "what stops them?" We've probably all seen things rotate for no apparent reason when subject to vibration (Coriolis?).
* bore honing - try as you might, the scratches on the bore surface will probably not be perfectly symmetric. There will likely be some amount of "bore rifling" left, at least until the bore is polished by carbon, etc. Why wouldn't the rings rotate 10deg CCW on the way down and 10deg CW on the way back up? Because there is significant gas loading once out of every four strokes that will overwhelm tension on the top ring (and to a lesser extent, the second)
* gas swirl - there may be some amount of rotation in crevice gas flow
* gap thrust - if the gap is not perfectly symmetric, blow-by and blow-back gases may generate thrust, causing the ring to rotate
* bore lobes - the ring will be jostled around by irregularities in the hot bore shape
* bore ovality (see above)
* piston tilt - not as obvious, but the piston will move around in the bore as the engine runs, and the friction load between the piston and each ring varies over time - there can be a net rotating effect
* Coriolis effect (maybe a stretch)
* momentum - the car will go around corners, maybe the rings don't feel like it (probably a stretch)

More fuel:
I know a gentleman who has run an engine with a viewing glass in the side of a cylinder liner so that he could watch the rings rotate - he said they all rotated, but at different rates.

Instantaneous oil consumption measurements on a running engine showed a correlation between oil consumption spikes and ring gap alignments - but I don't recall the SAE paper where I read that.

Installation issues:
I haven't the slightest idea why people are so picky about ring gap placement when rings are initially installed. Perhaps it's just a tradition that hasn't found its way to the grave. Anybody else have input on that one?

 

[EngJW]

Perhaps the gaps are staggered during assembly just to make sure they are not lined up during the first start up. If you tell assembly people to stagger the gaps, there is always someone with no common sense who insists on knowing exactly what position and plus or minus how many degrees.

John Woodward

 

[ivymike]

I think I've found a reference to the paper I mentioned above. Try out Nakashima et al. "Influence of Piston Ring Gaps on Lubricating Oil Flow into the Combustion Chamber", SAE Paper 952546

 

[aviat]

John Schwaner also covers it on his site with a link to a NACA report.

http://www.sacskyranch.com/piston_ring_rotation.htm

 

[patprimmer]

I would expect that 10% of engines would have the gaps aligned within 36 deg on dissasembly, for the reasons already stated and by the laws of chance.

I align gaps at about 180 deg, and along the axis of the motor, purely out of habit, as I know they move constantly.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[motorsportsdesign]

Have you checked the rod bores to see if they are parallel?
Also check the pistons to see that the pin bore is perpendicular to the ring grooves.

Both of these issues cause the piston the have unwanted twisting or rocking that can cause rings to rotate.



Jonathan T. Schmidt

http://www.motorsportsdesign.com

 

[SMOKEY44211]

Most 2 stroke engines pin the rings to prevent rotation with no adverse effects that I'm aware of. The old Hudson Hornet 6 pinned the rings also. The argument that the rotation self cleans the ring groove may have some merit. Leak down tests that I've conducted produced no measurable difference as far as installed gap placement. The gapless rings marketed by Total Seal make it a moot issue.-----Phil

 

[jons999]

I replace lots of oil control rings that are stuck due to oil sludge, carbon buildup etc. It the ring was constantly spinning i wouldn't think that the carbon would build up in there. Do you think the rings had stopped spinning due to the bore being out of round as mentioned above?

Is it possible that they recommend staggering the gaps so that the engine has good compression for initial start up?

-Jon

 

[ivymike]

Both of these issues cause the piston the have unwanted twisting or rocking that can cause rings to rotate.

Is there such a thing as desired piston rocking?

 

[enginesrus]

If you have an oval shape and something like say a ring conforms to that shape. It most likely would not want to turn. In the post above about the observations of ring rotation. Did all the rings rotate the same direction?
If so it may be a vibratory deal. Even bore finish could contribute.