Many kinds of additives such as viscosity index (VI) improvers, anti-wear additives, rust-corrosion inhibitors, antioxides, etc. are added in the lubricant for modern engines. In particular, VI improvers of high molecular weight polymer content are added to base oils to give multigrade oil characteristics against temperature-viscosity sensitivity. However, the incorporation of polymeric additives produces significant decrease of thickening effect by the polymeric additives even at normal operating speed (106s-1) of engine components as well as boundary film formation by the sticking layer of polymeric additives on the solid surfaces. The shear thinning effect of polymeric additives causes serious wear due to reduced film thickness while it may reduces friction forces and the sticky-absorbed polymeric layer protects the surfaces.
Abnormally thin film thickness (0.01∼0.1μm) can frequently happen in cam and follower contacts under high fluctuating concentrated loading condition. Such condition must be influenced by the effects of shear-thinning and sticky polymer layer due to the existence of VI improvers.
For this physical picture of film formation, a modeling representing both the existence of sticky-absorbed layer on the solid surface and shear thinning effect away from the layer needs to be developed for cam and follower contacts with the VI improver contented lubricant. Furthermore, elastic deformation of solid surface should not be overlooked for those concentrated high loading components.
This paper investigates the reaction film formation of automobile valve train systems focusing on the balance between frictional efficiency and wear protection under the condition of viscosity index improvers added.