High Shear Rate Rheology of Lower Viscosity Engine Oils Over a Temperature Range of 80° to 150°C Using the Tapered Bearing Simulator (TBS) Viscometer 2010-01-2288

In 2005, the growing emphasis on fuel efficiency coupled with the long-recognized negative effects of viscous friction caused by engine hydrodynamic lubrication, led to considerations of the benefits of lower viscosity engine oils by the SAE Engine Oil Viscosity Classification (EOVC) Task Force. More recently these considerations were given further impetus by OEM enquiry regarding modification of the SAE Viscosity Classification System to include oils of lower viscosity specification than that of SAE 20.

For the EOVC Task Force, such considerations of commercially available, significantly lower viscosity engine oils, also produced a need to reassess the precision of high shear rate viscometry of such engine oils as presently practiced at 150°C - as well as interest in temperatures such as 100° and 120°C believed more representative of engine operating conditions.

This paper presents studies of the precision of high shear rate viscometry on lower-viscosity simulated engine oils at temperatures of 80°, 100°, 120°, and 150°C at a shear rate of one million reciprocal seconds (1.0⋅10⁶s^-1) using the Tapered Bearing Simulator (TBS) Viscometer. These viscosity values were then analyzed for conformity to the MacCoull, Walther, Wright viscosity-temperature and the ability to interpolate high-shear rate viscosities. Lastly, the ability to determine the high shear rate viscosities of oils at various chosen temperatures was applied in appraising the fuel efficiency benefits of such oils by determining the Viscous Fuel Efficiency Index. These values are, in turn, compared both to one another and to the Index values of several hundred North American engine oils collected by the Institute of Materials in 2008 and 2009 for their yearly engine oil database.