Development and application of a lubricant composition model to study effects of oil transport, vaporization, fuel dilution, and soot contamination on lubricant rheology and engine friction

Abstract

Engine oil lubricants play a critical role in controlling mechanical friction in internal combustion engines by reducing metal-on-metal contact. This implies the importance of understanding lubricant optimization at the piston ring-cylinder liner interface. Lubricating oil composition varies along the liner and throughout the engine. Composition changes occur due to degradation, vaporization, mixing during ring passage, fuel dilution, particulate matter contamination, and combustion gases getting onto the liner causing wear and erosion. These chemical and physical properties change oil composition and in-situ oil properties. The objective of this thesis is to discuss the development of an oil composition model to determine rheological properties at critical rubbing surfaces due to oil transport, vaporization, fuel dilution, and soot contamination. This study will specifically focus on the oil on the cylinder liner because the interaction between piston assembly and cylinder wall is where most of the mechanical friction originates. The first physical process discussed is oil mixing due to piston movement. Axial mixing analysis shows that mixing only occurs when the piston ring is above the oil particle location. Flow rates are calculated at each liner position from using piston speed, film thickness, and pressure gradient parameters. From this basic model of oil transport, chemical processes are applied to each species in each different liner location. For the process of vaporization, due to high temperatures near the top dead center of the piston, light volatile hydrocarbons vaporize and leave the system. Light carbon number species disappear at a faster rate due to their high volatility and vaporization rates. This results in retention of heavier hydrocarbon species near the top zone of the cylinder liner model. Vaporization rates for different species in each liner location are obtained by looking at individual vapor pressures, mass transfer coefficients, and other oil properties. The link between composition and viscosity is a blending equation. The Arrhenius blending equation is used to calculate mixture viscosity from the summation of different species composition and component viscosity values. A combination of composition results shows that near the top dead center or top zone, the viscosity is higher than just considering temperature effects on oil viscosity. The impact of this vaporization component shows that the addition of a non-volatile oil species near the top dead center of the cylinder liner has the ability to flatten the species viscosity versus liner location curve. Other rheology applications were studied for effects of fuel dilution, additive concentrations, and also soot contamination. This new oil composition model solves for in-situ compositional changes for different oil species due to different physical and chemical processes along the cylinder liner. This change in composition causes a change in viscosity of the overall mixture which is solved for with blending equations. Then from mixture viscosity values, friction and wear can be calculated to optimize the lubricant for fuel efficiency.