Friction and Wear of the Piston Ring – Cylinder Liner System with Artificially Aged Ultra-low Viscosity Engine Oils

Abstract

This study aims to investigate the performance of artificially aged prototype engine oils through friction and wear experiments. Experiments were performed on a piston ring – cylinder liner model system with boundary conditions derived from real-life operating conditions. The experimental design implemented two prototype oils (SAE 0W-12 and 0W-16) in unaltered and artificially aged form. An additional fully formulated off-the-shelf engine oil (SAE 0W-20) was also aged and analyzed as reference. Oil samples were artificially aged in a custom rig, to simulate long-term in-engine use through thermal cycling at 180 °C. Fourier Transformed Infrared Spectroscopy of the lubricant samples highlighted a depletion of zinc dialkyl-ditiophosphate antiwear additives in all cases, which is comparable to a selected in-service oil. Oxidation was also measurable, albeit lower compared to the in-service sample. Averaged friction coefficients showed a ranking of aged 0W-12 < aged 0W-16 < unaltered 0W-12 < unaltered 0W-16. A decrease in surface roughness was experienced with aged oil samples, whereas unaltered 0W-16 oil produced an unexpected transition in the wear phenomenon and resulted in severe wear.