A Comprehensive Piston Skirt Lubrication Model Using a Mass Conserving EHL Algorithm

Abstract

Elasto-Hydrodynamic Lubrication (EHL) analysis of a fully flooded piston skirt-liner conjunction is a useful methodology for design analysis of pistons. However, under typical engine operating conditions, oil present in the clearance region between the skirt and liner is sufficient to wet only a portion of the' piston skirt (partial skirt lubrication). The reduction in dampind due to partial skirt lubrication is an important consideration to address issues related to piston slap noise, liner cavitation and other noise and vibration aspects. The existing simulation methodology for EHL analysis of a fully flooded piston skirt uses a finite-difference scheme to solve the coupled Reynolds, Greenwood-Tripp and elasticity equations in order to calculate the nodal oil film pressures, contact pressures and elastic deformations respectively. Detection of cavitation zones within the oil film done via implementation of the Half-Sommerfeld boundary condition. The limitations of this boundary condition are that a) it violates the mass-conservation property of the Reynolds Equation and b) it does not rigorously track the boundaries of the cavitation zones in the oil film. As a result, this methodology cannot be applied towards partial skirt lubrication and needs to be enhanced. This is achieved via a finite-volume solution to the Reynolds Equation which takes into account a) mass conservation based on oil supply and b) more comprehensive treatment for the rupture and reattachment boundaries which define the cavitation zone. Additionally, this methodology enables to specify partial skirt lubrication by means of an oil supply to the piston skirt as a boundary condition. Results are presented from parametric studies that show comparisons between usage of the finite-difference and finite-volume schemes.