Honda / Acura K20a K24a Engine Forum banner

Piston ring clocking question

4K views 1 reply 2 participants last post by  LotusElise 
#1 ·
Hello everyone, I have a basic question about piston ring clocking. I have seen in service manuals and heard countless times that it's critical to clock the piston rings correctly for the engine to run correctly and last. I have heard and read to not position the ring gaps inline with each other because it causes compression loss and oil consumption, don't put the ring gaps above the wrist pins because the blow by will excessively heat the wrist pins and cause premature failure, don't put the ring gaps on the piston's thrust surfaces because the ends could damage the cylinder walls, etc.

All of that makes sense in theory, but no one seems to consider the fact that the rings actually rotate while the engine is running. I have read a few studies on this and found that the rings rotate at unpredictable rates determined by a million factors, potentially up to several rotations per minute. I have seen this for myself as well. I have always clocked piston rings so the gaps are 180 degrees apart and found after tearing those engines down for whatever reason that the rings are never in the same position I left them in. Here's an article about piston ring rotation. Rotation of piston rings.

So my question is why do people even bother putting the rings in special positions when they rotate while the engine is running anyways? Perhaps people don't realize that the rings rotate?
 
See less See more
#2 ·
..So my question is why do people even bother putting the rings in special positions when they rotate while the engine is running anyways? Perhaps people don't realize that the rings rotate?
Both assumptions meet the reality. Once I was involved in a serious combustion issue during the combustion process development. We saw pre-ignition and during the investigation of that phenomena we found out with liner distributed temperature measurement that the ring turning, which is a normal and necessary thing, isn't normally behaving. The stopped at a certain position for a longer while as on other position, because of liner deformation (elliptical shape). At those moments the likelihood of pre-ignition was increased, as it was oil droplet driven and was just in that moment feed more with oil. Ring turning is not only because of pressure and oil sealing important, it has an huge impact on homogeneous wear of liners for long duration sealing capacity. We saw about 4-5 rpm ring rotation velocity at 1500 rpm on a 160 bar peak cylinder pressure load point. Ring turning is also for Ring development guys from Mahle or the MIT specialists we worked with difficult to simulate or to forecast. As you said the complexity of forces working on turning is a challenging one. One take away is, a round bore under hot and unloaded or loaded conditions helps definitely to keep the rings regularly turning. Another is of course the labyrinth sealing approach works best when the gaps don't align, but as the rings are turning in velocity and direction uncontrolled the likelihood is small they meet for a long time aligning in a gap position. You see round bores under load and heated are a good pre-condition to have a sealed and long living engine.

BTW, like Porsche does it in some engines to prevent of idle issues, we fixed for testing several rings to see what is happening. Fixing one is ok for a shorter period or a car engine, for engine running 8 years continuously at WOT likely not.

To get back to the gapping issue. Normally ring turns very well, but the engine may suffer starting issues once they are gapped. Oil flow into the chamber will be increased dramatically (overpressure in the block, under-pressure during intake stroke, that's a sprayer condition. This can lead to idle issues and CAT damage later. Think of if the rings won't turn as fast as normally from that position because the are sort of fixed by the gapping conditions there. They will start turning, but delayed, in that time compression tests or idle issues are more realistic. Gapping them right is like you give your wife a hug when you go to work instead of leave her without a word. The consequences can be from negligible up to a serious problem once you see her next - unpredictable. Therefore it is good engine builder habit to turn the rings in a more cross gap looking position (e.g. 180° with 2 rings, 120° with three, and so on). Like I mentioned, Porsche fixed the oil ring for reason to prevent of idle oil issues, what a horizontal liner is prone for because of the gravity forces during stand still. They found it the better compromise (emissions, customer experience vs. longevity) for several series production engines.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top