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?