Piston Ring Friction Analysis from Oil Film Thickness Measurements by

The oil film behavior of a series of fifteen lubricating oils was tested in a single cylinder spark ignition engine. The oil film thickness between the piston and cylinder liner of this engine was observed using the Laser Induced Fluorescence (LIF) technique. The data was examined for correlations between physical properties of the oil and characteristics of the oil film. The region of the oil film trace near the top piston ring was analyzed in detail. The film thickness under the top ring decreases with increasing oil viscosity and increasing surface tension. The trend with viscosity is contrary to the classical lubrication theory. These behaviors are explained in terms of the effects of viscosity and surface tension on the transport of oil to the upper cylinder liner. It is this transport process that determines the film thickness under the top ring. Therefore, the supply of oil regulated by the oil control ring and the wiping action of the entire ring pack become important parameters in the top ring friction and wear question. A theoretical model of top ring lubrication was applied to this data. The model uses classical lubrication theory to solve the ring wetting problem with a novel boundary condition involving the surface tension of the lubricant. With ring, engine, and lubricant data as input, the model calculates the way the oil wets the ring and the resulting friction. Thus, friction was estimated for the tested lubricants. Because of lack of visible wetting profiles in the data and lack of experimentally measured friction, confirmation of the model using the data was not possible. Finally, the computer model was employed in a parametric study to find the theoretical trends between film thickness/friction and viscosity/surface tension. Thesis Supervisor: Wai K. Cheng Title: Associate Professor of Mechanical Engineering