High Shear Rate Viscosity of Engine Oils

This paper describes the development and use of a pressure capillary viscometer for operating at the temperatures and high shear rates typically experienced in operating engines. It has been used to make viscosity measurements on engine oils without significant problems at temperatures ranging from 20 to 150°C and shear rates up to 2×10⁶s^-1. Experimental results with Newtonian fluids have been found to agree extremely well with the mathematical predictions of Newtonian behaviour and the Poiseuille-Hagen equation for capillary flow. High shear rate viscosity measurements have been made for several oils currently of interest to the ASTM task force on high temperature rheology. These include four GM reference oils, nine CEC reference oils, and two Imperial Oil Ltd. Diesel reference oils. These viscometric measurements are discussed in relation to the available engine wear data. The extent of permanent shear on temporary shear stability was also studied for several oils. Permanent viscosity loss resulted in a gain in temporary shear stability. This gain in temporary shear stability partially offsets the permanent viscosity loss during high shear rate lubrication.