Shear Heating of High-Viscosity Grade Lubricant in Piston Skirts: EHL at Idling Speeds in Initial Engine Start Up

A few initial cold engine start up cycles at low idling speeds do not prevent wear due to the absence of a fully established elastohydrodynamic lubrication (EHL) film between the piston skirts and the cylinder liner. It happens when the thermal loading due to combustion may be ineffective initially, and shear heating becomes significant as a result of the sliding motion of the piston. This study models the 2-D piston skirts EHL at the idling speeds in the initial engine start up by using a high-viscosity grade engine oil and incorporating the shear heating effects. The 2-D heat transfer equation is used with no source term effects to study the temperature changes and their effects on the viscosity of a Newtonian lubricant at the different idling speeds in the initial start up of an internal combustion engine. The 2-D Reynolds equation is solved numerically to generate the hydrodynamic pressures as the function of 720 degrees crank rotation cycle. Under the flooded lubrication conditions the inverse solution technique is employed to generate the hydrodynamic pressures in the EHL regime. The numerical analysis at the two different idling initial engine start up speeds is presented based on the 2-D heat equation having adiabatic conduction and convective heat transfer with no source term effects. Viscous dissipation coupled with the piston motion, the pressure fields generation, the temperature effects on the viscosity of the lubricant and the subsequent oil film thickness profiles in the contact region are examined. The influence of the low-temperature shear heating on the hydrodynamic and EHL film thickness at the time of initial engine start up are investigated. This study suggests that by using a high-viscosity grade oil in the idling speed engine start up the film temperature rises non-uniformly due to shear heating in the hydrodynamic and EHL regimes. The low temperature rise affects the pressure and temperature dependent oil viscosity, and the secondary transverse eccentric displacements of the piston. Resultantly, the piston skirts lubrication is affected despite the initial engine start up at the idling speeds.