A Numerical Analysis for Piston Skirts in Mixed Lubrication: Part II—Deformation Considerations

Abstract

This paper presents a mathematical model for piston skirts in mixed lubrication. It takes into account the effects of surface waviness, roughness, piston skirt surface profile, bulk elastic deformation and thermal distortion of both piston skirts and cylinder bore on piston motion, lubrication and friction. The corresponding computer program developed can be used to calculate the entire piston trajectory and the hydrodynamic and contact friction forces as functions of crank angle under engine running conditions. Complete distributions of the oil film thickness and elastic deformation as well as the hydrodynamic and contact pressures can also be given at any crank angle if needed. This paper is the second part of a series of two papers. The first part (Basic Modeling), presented earlier by Zhu et al. (1991), gave the basic formulation and some preliminary results without bulk deformation considerations. In the present part, the three-dimensional finite element method is used to calculate so-called influence coefficient matrices. These matrices are repeatedly used to compute bulk elastic deformations of piston skirts. Results for 12 different cases are presented, and discussions are given focusing on the influences of elastic and thermal deformations on piston motion, lubrication and friction. An attempt to compare the calculated friction with experimental data is made, and agreement appears good for the two available cases. The computer program presented should be a useful tool for piston design and development.

Finite element method for mixed elastohydrodynamic lubrication of journal‐bearing systems

“…Since the 1970s, several typical modellings considering surface roughness effect for mixed lubrication problems have been developed, such as the longitudinal-roughness flow model by Christensen [8], the average Reynolds equation by Patir and Cheng [9], and the asperity contact model by Greenwood and Tripp [10]. Zhu et al [11] studied the mixed lubrication problem of a piston-cylinder contact, focusing on the convex piston. Wang et al [12] analysed the journal-bearing conformal contacts under steady-state conditions, considering the combined effects of bearing deformation, surface roughness and asperity interaction.…”

Transient Tribo-Dynamics of Thermo-Elastic Compliant High-Performance Piston Skirts

“…Zhu et al [42,43] presented a two-part analysis. In the first part, they developed transient hydrodynamics of rigid bodies in contact.…”

“…The second part extended the approach to include gross piston distortions. They overcame the computation burden of the repeated time-dependent calculations for piston skirt deflection through use of an influence coefficient matrix, derived from their own FEA model [43]. Common to both their contributions was the use of low relaxation Newton-Raphson iterative solution for system dynamics.…”

On the design of lubricant free piston compressors

“…Those authors have shown through an analytical model that piston skirt friction can be significantly reduced if the piston center of rotation is located in a favorable position. Zhu et al [88] numerically investigated piston motion, lubrication, and friction in mixed lubrication regime taking into account effects of surface waviness, roughness, surface profile, bulk elastic deformation, and thermal distortion of both piston skirt and cylinder bore. The proposed model was applied to a four-stroke automotive engine showing that good hydrodynamic lubrication can minimize the possibility of piston impact against the cylinder bore and reduce the frictional loss.…”