Mechanical Degradation of Viscosity Modifiers in Heavy Duty Diesel Engine Lubricants in Field Service
Modern multi-grade engine lubricants are formulated to “stay in grade” during field service. Viscosity loss during the early stages of lubricant life is commonly believed to be caused by mechanical degradation of the viscosity modifier in the engine . The Kurt Orbahn shear stability bench test (ASTM D 6278, 30 cycles) has been the industry standard predictor of viscosity loss due to polymer shear in heavy duty diesel engine lubricants. However, the Engine Manufacturers' Association (EMA) has expressed some concern that it underestimates the degree of polymer shear found in certain engines in the field, such as the Navistar 6.0L HEUI (Hydraulic Electronic Unit Injector) Power Stroke engine; a more severe bench test would serve to improve correlation with this and other similar engine designs. This paper offers a new approach for critically examining the relationship between the bench test and field performance.