Development of a High Shear Rate Capillary Viscometer for Engine Oils

Abstract

A high shear rate capillary viscometer is described for measuring the viscosity of engine oils at shear rates and temperatures within the range experienced by automotive engine oils. Although the use of multigrade engine oils is well-established, the lack of commercial viscometers to measure engine oil viscosity at appropriately high temperatures and shear rates has led many laboratories to build their own viscometers. The simplicity of the capillary viscometer is enhanced since the device is used as a relative viscometer rather than as an absolute one. Viscosity is determined from calibration with Newtonian oils instead of calculation from capillary dimensions. Flow properties of the multigrade oil being tested are equated to viscosities of a series of single-grade reference oils. Principal elements of the capillary viscometer are a glass capillary, a pump-activated pressurization system, a glass timing column for flow rate measurements, and necessary temperature and pressure instrumentation. Procedures for using the viscometer are detailed, and the device's usefulness is demonstrated for a group of commercial engine oils. The viscometer can also be used for interpreting the results of a laboratory engine friction test.