Polymeric Viscosity Modifiers and Their Effect on Lubricant Flow and Degradation in the Piston Assembly of a Fired Gasoline Engine

摘要

The flow of lubricant through the piston assembly of an automotive engine has a tremendous effect on the tribological performance of the engine, the fuel economy, oil consumption and emissions. A great deal of work has been invested in studying the mechanisms by which lubricant flows through the piston assembly. Many lubrication models indicate that mechanisms of lubricant transport upwards through the piston are currently under-defined. Other experimental work has indicated that mist may form through the piston ring gaps, where oil films and high velocity gas flows interact. It is hypothesised that this mechanism may fill a significant gap in existing lubricant transport knowledge. This work considered the effect of oil misting on lubricant transport, with particular reference to the dependence of this mechanism on the polymeric viscosity modifiers. It has been shown in lab-based simulators that the tendency of a lubricant to form mist is greatly reduced by the presence of polymeric viscosity modifiers. This is attributed to their contribution to the viscoelasticity of the lubricant. Increased formation of mist greatly increases the surface area to volume ratio of the lubricant in the piston assembly. Thus, it is thought that the rates of chemical and physical reaction that cause lubricant degradation may increase. The degradation of lubricant and its flow through a fired gasoline engine was studied. Lubricant was sampled from the sump, top ring zone and mist from the crankcase during an extended run. Chemical analyses (FTIR, GC and GPC) were used to characterise the degradation of the lubricant. The effects of various additives, including viscosity modifiers of different molecular structure, were considered in terms of their rheology and tendency to reduce mist.