Modeling Shear Heating in Piston Skirts EHL Considering Different Viscosity Oils in Initial Engine Start Up

Abstract

A fully established elastohydrodynamic lubricating (EHL) film between the piston and the liner surfaces during normal engine operation minimizes piston slap and prevents adhesive wear. Wear cannot be prevented in the initial engine start up due to the absence of EHL film. During normal engine operation, thermal loading due to combustion dominates piston skirts lubrication. However, in a few initial cold engine start-up cycles, shear heating affects the lubricant viscosity and other characteristics considerably. This study models 2D piston skirts EHL by incorporating shear heating effects due to lubricant flow between the skirts and liner surfaces. The hydrodynamic and EHL film profiles are predicted by solving the 2D Reynolds equation and using the inverse solution technique, respectively. The temperature distribution within the oil film is given by using the 2D transient thermal energy equation with heat generated by viscous heating. The numerical analysis is based on an energy equation having adiabatic conduction and convective heat transfer with no source term effects. The study is extended to low and high viscosity grade engine oils to investigate the adverse effects of the rising temperatures on the load carrying capacity of such lubricants. Numerical simulations show that piston eccentricities, film thickness profiles, hydrodynamic and EHL pressures visibly change when using different viscosity grade engine lubricants. This study optimizes the viscosity-grade of an engine lubricant to minimize the adhesive wear of the piston skirts and cylinder liner at the time of initial engine start up.