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Mechanical degradation of polymeric additives in lubricants has been a topic of extensive study since complex formulations were introduced to reduce the temperature dependence of viscosity. Many devices, which are able to shear polymers, have been tested for their abilities to simulate the degradation observed in engines and other lubricated systems. Conclusions drawn from these studies are often ambiguous as they depend on the test protocol and method of data analysis. In this work, a simple expression based on probabilistic arguments is used to describe kinematic viscosity data from a variety of degradation simulators. This expression provides a method of comparing extent and rate of degradation for different simulators.

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.

Polymer additives in commercial engine oils are mechanically degraded in flows involving large stresses. The oil stability is usually characterized by an asymptotic value called the fully-sheared viscosity. Published data from field and laboratory engine tests show that the kinematic viscosity of degraded oil does not approach such an asymptote. This viscosity reaches a minimum due to the compensating effects of mechanical degradation and oxidation. Kinematic viscosity data from various tests are correlated here using a logarithmic function. This correlation is used to compare the kinetics of degradation processes in different tests. These comparisons suggests that multi-grade oils degrade at a consistent rate in different engine tests. Simulation devices using a diesel fuel injector do not give results comparable to field test data. A milder technique for simulating mechanical degradation of engine oils is suggested to improve correlation with field data.