Mechanical Degradation of Multigrade Oils in Laboratory Engine Tests

Eight SAE 10W-30 oils were formulated with different viscosity index (VI) improvers and used in single and multicylinder engines. A significant number of measurements were made during each engine test to accurately determine changes in kinematic viscosity with time. These measured values appear to decay logarithmically over a large portion of each test. This behavior is qualitatively similar to results reported in other engine tests involving multigrade oils. A coefficient in the data correlation is suggested as a measure of engine severity for a particular formulation. This coefficient and the shear stability index are used to monitor severity changes during the test period. These changes are correlated to an increase in ring gap and a decrease in ring mass for the single cylinder engine. A procedure is suggested for extrapolating severity measures to a value representative of the initial engine configuration. While the ratio of these initial values is nearly constant for fired engine tests, poor correlation is obtained between fired and motored tests on the same engine. Bench tests based on an ultrasonic oscillator and the Bosch diesel injector correlate poorly with all engine tests.