MODELING AND SIMULATION OF UNCOATED AND SOLID–LUBRICANT-COATED

Abstract

The coating of low coefficient of friction protects the lubricant-starved dry surface of the piston skirts against wear in a low-load and low-speed initial engine start-up. Despite a relatively large radial clearance, a physical contact between the skirts and the cylinder liner causes an elastic deformation. A hydrodynamic or elastohydrodynamic (EHL) film may also break due to unusual operating conditions. This study develops the numerical piston dynamics model by incorporating the secondary piston displacements of the skirts during the 720-degree crank rotation cycle. The study also models the 2-D hydrodynamic and EHL of the un-coated and the solid lubricant coated piston skirts separately. The numerical models couple the secondary piston dynamics with the hydrodynamic and EHL models. The contact zone in the boundary-value problem is discretized using the finite difference method and solved Navier /Lame equation numerically by successive-over-relaxes (SOR) method. The secondary piston and oil lubrication parameters are plotted as function of 720-degree (04 stokes) crank rotation cycle. The simulation results show that the dry piston skirts establish a physical contact with the liner during the compression stroke that remains in the expansion and exhaust strokes. The elastic displacements of coating prevent the stress accumulation on the substrate and protect it from adhesive wear. The solid lubricant film facilitates towards achieving an effective lubrication of the skirts and a more uniform pressure distribution to prevent adhesive wear in the low-speed initial engine start up