Effects of Isotropic Pressure on the High Temperature High Shear Viscosity of Motor Oils

The viscosity of 10W/40 motor oils formulated with different viscosity index (VI) improvers has been measured at pressures up to 200 MPa (2000 bar) over a wide temperature and shear rate range. The response of viscosity to pressure was found to depend on the chemical nature of the VI improver at both low and high shear rates. As a consequence, the ranking of the 150°C viscosities of polymer-thickened oils can be different at high pressures to that observed in conventional atmospheric pressure viscometers. These effects of VI improver type on high pressure viscosity appear to be relevant to wear in journal bearings of fired engines; thus in experiments carried out in the ALI Bearing Distress Test, bearing weight loss of multigrade oils containing different VI improvers correlated better with high pressure, high shear viscosities than with high shear viscosities measured at atmospheric pressure.