The effect of surface finish on piston ring-pack performance in advanced reciprocating engine systems

Abstract

Frictional losses in the piston ring-pack of an engine account for approximately 20% of the total frictional losses within an engine. Methods of surface texture optimization were investigated to reduce piston ring-pack friction and increase engine brake thermal efficiency. Adverse effects of surface texture optimization on engine oil consumption and durability were also considered. Although many non-conventional cylinder liner finishes are now being developed to reduce friction and oil consumption, the effects of surface finish on ring-pack performance is not well understood. To enable the study of cylinder liner surface texture on predicted piston ring-pack performance, the description of surface texture in MIT's current ring-pack models was improved. Modifications were made to the asperity contact and oil flow resistance sub-models to enable a more accurate and more general description of surface texture. The accuracy of the new sub- models was validated with experimental results obtained from a ring reciprocating tester. With the use of these sub-models, ring pack friction reduction strategies were investigated. The effects of surface skewness, roughness, and honing cross-hatch angle were considered in detail. The analytical results suggest that a negatively skewed surface finish, such as that produced by plateau honing, will lead to a ring-pack friction reduction relative to a conventionally honed surface. This predicted friction reduction was due to a decrease in rough surface asperity contact between the rings and liner. Additional friction reduction was predicted by decreasing the honing cross hatch angle.

(cont.) The smaller angle decreased friction by blocking lubricant flow transport between the ring and liner thereby increasing the lubricant's effective viscosity and the effective lubricant film thickness between the ring and liner. Both of these effects enabled more ring load to be supported by hydrodynamic pressure, reducing ring-pack friction. There are potential adverse effects related to these surface finish modifications including an increase in the engine's susceptibility to scuffing, and an increase in oil consumption. Nonetheless, these modifications in surface finish reduce predicted ring-pack friction by approximately 1-10%.