Modeling of Shear Heating in Elastohydrodynamic Lubrication of Piston Skirts by Considering Different Viscosity Oils in Initial Engine Start Up

Abstract

The presence of fully established elastohydrodynamic lubricating (EHL) film between piston skirt and cylinder liner during normal engine operation prevents adhesive wear, piston noise and slap. The absence of EHL in the initial engine start up fails to prevent the same. 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 may be assumed to considerably affect the lubricant viscosity and other characteristics. This study undertakes a 2-D EHL modeling of piston lubrication by incorporating shear heating effects due to lubricant flow between skirts and liner surfaces. The EHL film profile is predicted by solving the 2-D Reynolds equation using inverse solution technique and the finite difference method in fully flooded lubrication conditions. The temperature distribution within oil film is given by using the 2-D transient thermal energy equation with heat generated by viscous heating. The numerical analysis is based on energy equation having adiabatic conduction and convective heat transfer with no source term effects. The study is extended to a number of engine lubricants having different viscosities to investigate the extent of adverse effects due to temperature rise on load carrying capacity of 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 suggests that a lubricant of appropriate viscosity can be optimized keeping in view the vulnerability of piston skirts and cylinder liner to adhesive wear at the time of initial engine start up.