Total Seal’s Lake Speed Junior recently highlighted piston rings within the context of lubrication and bore finish. He pointed out that reducing piston ring radial width and axial height not only potentially reduces friction but also enhances conformability to the bore surface. At the same time, piston stability becomes a more significant factor.
Speed noted that steel inherently allows a ring to be smaller than a cast iron production but that the latter will offer superior oil retention. “The key is to maintain an oil film between the ring and the cylinder wall,” he remarked, “and in this respect, compared to a steel ring, the cast iron ring provides a margin of error.
“A steel ring with a PVD or DLC coating has no porosity, so it won’t retain oil at all. The key is then the cylinder bore finish.” This is in respect of both the coating (if any) applied to the cylinder wall and the honing pattern. Speed noted that Sulzer Metco’s powder-based, air plasma-sprayed SUMEbore cylinder coating is porous, so it is particularly beneficial in terms of maintaining the required oil film.
Studying bore finish at Total Seal
“The other variable is the fuel”, he added. “Oxygenated fuel [containing methanol or ethanol] causes more fuel ‘wash’ as well as corrosion problems. Running a coated steel ring and using oxygenated fuel makes the bore surface finish even more crucial.”
“The significance of bore finish and its relationship to ring sealing and lubrication cannot be underestimated.”