1992-02-01

A Comprehensive Model of Piston Skirt Lubrication 920483

This paper describes a comprehensive model of piston skirt lubrication, developed for use in conjunction with piston secondary dynamic analysis, to accurately characterize the effects of the skirt-cylinder oil film on piston motions. The model represents both hydrodynamic and boundary lubrication modes and applies an asperity contact pressure when surfaces are in close proximity with each other. In addition to skirt dimensions and surface roughness properties, the circumferential extent of lubrication, an arbitrary skirt profile and bore distortion are specifiable inputs to the model. The model is also extended to represent the oil starvation at the cylinder end of the skirt by allowing the axial extent of lubrication on the skirt surface to vary circumferentially and with time to satisfy continuity of oil.
Using a finite difference solution of the Reynolds equation and an asperity contact submodel, the model calculates oil and contact pressure distributions in the skirt-bore oil film as a function of all input design parameters and positions and motions of the skirt relative to the cylinder. In the context of a piston secondary dynamic analysis the computed oil and asperity pressures are integrated to calculate axial, lateral forces and moments on the skirt, in the thrust plane.
The model was coupled to a piston secondary dynamics analysis and applied, in a number of parametric studies, to conventional and articulated pistons. Results indicate that the skirt friction predictions of the model correspond to known levels and trends. Further design parameters such as nominal clearance, skirt profile, circumferential lubrication extent as well as oil viscosity are shown to have key influences on the action of the oil films and thereby on piston motions and skirt friction.

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