In order to meet EPA's emission requirements for 1999 diesel engines, soot levels in the crankcase oil will increase significantly due to retarded timing to lower NOx. This study uses the Cummins M11 engine at soot levels up to 9% in the crankcase oil to demonstrate how oils can be formulated to prevent valve train wear, extend filter life, and maintain oil pumpability.
The study includes the oil formulation development and the evaluation of API CG-4/SJ oils at 4.5% soot and API CH-4/SJ oils at 9% soot. In addition it includes X-Ray Photoelectron Spectroscopy (XPS) for surface film analysis and Surface Optical Profilometry and Scanning Electron Microscopy (SEM) of the valve train valve-bridges and rocker pads to determine the mechanism of failure. The oil's low temperature rheology as it affects oil pumpability is defined by Mini Rotary Viscometer (MRV TP-1), Scanning Brookfield Test (SBT), and Cold Cranking Simulator (CCS).
The most vulnerable part of the engine to soot aggravated wear is the valve-train system, which has components such as the valve-bridges and injector adjusting screws, operating under boundary lubrication conditions. The key to low valve train wear in the diesel engine is the proper selection of zinc dithiophosphates (ZnDTPs), detergent, dispersant, and V.I. improver type and level. The phosphorous, sulfur, and zinc from the ZnDTP must adsorb, react, and remain as an intact film on the wear surfaces, in spite of soot, which can remove this protective film. Soot dispersancy is critical to filter life and oil pumpability, which are very dependent on ashless dispersant, V.I. improver, and base oil type.