Mechanical Degradation of Viscosity Modifiers in Heavy Duty Diesel Engine Lubricants in Field Service

Abstract

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 [1]. 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. Rather than indirectly measuring polymer shear via viscosity determination, a method for directly measuring olefin copolymer (OCP) molecular weight in heavy duty diesel drain oils was developed. The main advantage of this approach is that we can follow the polymer degradation process that occurs near the end of the oil drain period, when the accumulation of soot and/or oxidation by-products masks changes taking place to the polymer that could be affecting viscosity. Over sixty drain oils taken from a large variety of commercial truck engines were analyzed. In addition, a mathematical model was developed to quantify the relative effects of polymer shear and soot accumulation on the kinematic viscosity of used engine oil, assuming no oxidative thickening mechanism. This study shows that (1) the standard ASTM D 6278 test adequately predicts OCP molecular weight break-down in many modern diesel engine lubricants, but (2) certain engine designs (such as the HEUI fuel injection system) and/or duty cycles degrade polymers to a level near that achieved in the 90-cycle Kurt Orbahn bench test.