Modeling EHL of Rough Piston Skirts Using Different Viscosity-Grade Lubricants in Initial Engine Startup

An engine lubricant plays a significant role in preventing adhesive wear of the rough interacting surfaces of the piston skirts and the cylinder liner. A fully established elastohydrodynamic lubricating (EHL) film, appropriate viscosity oil and the fairly rough interacting surfaces prevent wear during normal engine operation. The absence of an EHL film and inappropriate viscosity lubricant fail to minimize wear in the initial engine startup. This work considers a fairly viscous Newtonian engine lubricant to model the rough piston skirts hydrodynamic and EHL in the initial engine start up. The isotropic surfaces of the skirts and the cylinder liner having different roughness amplitudes are considered in the basic lubrication models. The flow factors are introduced in the 2-D average Reynolds equation, which is solved numerically to generate the hydrodynamic pressures. The inverse solution technique is used to develop the basic EHL model of the rough surfaces. The secondary piston dynamics and the contact geometry of interacting surfaces are incorporated in the basic lubrication models. The profiles of the piston eccentricities, secondary velocities, film thicknesses and pressures are generated as the function of 720 degrees crank rotation cycle. The study is extended to develop the lubrication models for the low and high viscosity grade engine lubricants separately. The simulation results are analyzed and compared with those of the basic lubrication model. The results show that the different lubricant viscosities alter the secondary displacements of the sliding piston and affect the lubrication of the rough interacting surfaces. The comparative analysis leads to optimize the use of appropriate viscosity-grade engine lubricant for a few low speed initial engine startup cycles.