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I have just completed a university course in tribology, but have a question that pertains to real-life applications.

For pistons in combustion engines, the rings seal the combustion chamber from the crankcase and wipes off excess oil. Now, for my tribology course that includes the workings of hydrodynamic lubrication, we are shown piston rings with a wedge shape that allows the creation of a pressure film on the ring-wall interface.

I am reading Audis self-study PDF on engine basics (obviously noting that this is not written for engineers), but still, the rings are shown with various geometries.

Some are rectangular, and a rectangular surface does not generate a pressure profile. How can this then work? My guess is that the hydrodynamic bearing working of this ring is that it works as a stepped bearing, not fully flat. Am I correct in this, or is something else going on?

Secondly, tapered rings are described that they offer more rapid run-in of the rings. Am I correct in that the ring is worn due to lack of full-film lubrication due to perhaps insufficient oil? In this case, what then causes the run-in to stop and not continue, wearing the rings?

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The pistons that i had to replace rings on in a v8 had 3 rings.

Each ring was a different design based on its position AND function.

The bottom or lowest ring was actually 3 pieces as it had two scraper thin rings and a spring ring to force them against the cylinder wall.

The upper two rings were much harder and designed to need less lubrication because they were designed to provide the seal against the pressure due to combustion.

Once the bottom ring starts to fail and leaves too much oil on the surface, a combination of burnt oil and soot starts to rapidly wear the upper rings and then you get a loss of compression.

One way you can start to see this is to buy a couple of old worn out engines and dismantle them.

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  • $\begingroup$ Indeed, I think I'll have to see if I can find the tools and time to pick apart an old one to see. But I don't see how the upper right receives adequate lubrication. For the speed the piston operates at and the lifetime of engines in general, the surfaces must be fully seperated. Or at least, I struggle to see any other way. And as such I struggle to see how this film forms on straight rings $\endgroup$
    – Erik
    Jun 7, 2023 at 13:47
  • $\begingroup$ Think about how "hard" they need to be in contact... $\endgroup$
    – Solar Mike
    Jun 7, 2023 at 13:56

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