Measurement of the Viscosity of Multigrade Oils in a Running Engine

Conventional laboratory viscometric methods are not adequate to characterise the behaviour of multigrade oils in service, in that they take no account of the high shear rates occurring in running engines. A technique has been developed which permits the measurement of apparent viscosity in the crankshaft bearings of engines running under road conditions. The technique is to isolate a single bearing, provide it with its own oil feed and calibrate flow-rates through it under normal running conditions using single grade oils. Flow rates obtained with multigrade oils can then be converted to apparent viscosities, seen by the bearing under operating conditions.

Results show that temporary viscosity loss due to shear is important even at moderate speeds (3000 rpm) in normal production engines. Shear breakdown characteristics of a range of different chemical types of VI Improvers have been compared. The technique has also been modified to permit determination of the minimum viscosity acceptable to the engine by relating this parameter to temperature increases in the bearings.