Up to the present, the “W” portion of multigrade engine oils were classified only with regard to their low temperature startability by viscosity measurements at -18 C in the Cold Cranking Simulator. Because of the low shear rates encountered at the suction side of the oil pumps, the low temperature pumpability of the oils in the engine were not being considered.
The investigations which were promoted by the DGMK were conducted to correlate the low-temperature pumpability of multigrade oils in a full-scale engine with suitable viscosity measurements and with results of tests in laboratory pumping rigs. Comparative measurements of viscosities were obtained with different viscometers.
A critical shear rate of G = 50 s-1 was found for the borderline pumping conditions of the test engine.
Good correlations were obtained between viscosity data of a rotational viscometer and engine pumping data.
With a special laboratory pumping rig, which was developed for this program, very good correlations were obtained with the low temperature pumpability of the engine. The temperature where borderline pumping occurs can be predicted equally well by the mean flow rates at the beginning of pumping, or by the time to, when the first oil reaches the end of a capillary.