Predicting Viscosity-Shear Relationships of VI Improved Lubricants

The majority of crankcase lubricants are now formulated to contain polymeric additives to improve the viscosity temperature properties to provide a better lubricating film in the various bearing systems in an internal combustion engine. These VI (viscosity index) improved lubricants are non-Newtonian under the high shear conditions that exist in most automotive bearing systems. The conditions of interest range from starting the engine at temperatures of as low as -40°C to operating the engine at normal operating conditions including bearing temperatures of 150°C or higher. This paper presents a method for predicting the viscosity shear relationship for a series of SAE multigrade engine oils as a function of temperature and shear stress. The method is demonstrated using three types of polymeric VI improvers currently used in SAE multigrade engine oils. The polymer types include olefin copolymers (OCP), polymethacrylates (PMA), and styrene-isoprene copolymers (SI).