Mechanical Degradation of Multi-Grade Engine Oils 930690

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.