1996-10-01

Understanding Soot Mediated Oil Thickening Through Designed Experimentation - Part 2: GM 6.5L 961915

In our earlier work [1], an investigation was conducted to study lubricant formulation effects, engine type and mode of engine operation on the composition and nature of diesel soot and its interactions with the crankcase lubricant. Tests were run in two types of heavy duty diesel engines the Mack EM6-285 and the GM 6.2L.
Part 2 studies the impact of oil composition on the surface and bulk chemistry of soot and on the ability of the fluid to handle soot produced in the GM 6.5L engine. The study also determined what portion of lubricant viscosity growth is related to bulk oil oxidation versus soot contamination. A statistically designed experiment was developed to examine the effects of dispersant level dispersant type, antioxidant level, and detergent metal type on average roller follower shaft wear, viscosity growth and other measured responses. The effect of run order on these measurements is also studied.
Key results of this study are as follows. Higher levels of dispersant were associated with lower wear. No oxidation products were detected in the drain oils. Engine run order was associated with increases in many measured parameters, such as end of test (EOT) soot level and oil consumption. Roller follower shaft wear did not show this same trend, thus refuting the theory that wear is directly related to the amount of soot contamination in the lubricant.
A new theory is proposed, that soot agglomeration, rather than the amount of soot, is responsible for roller follower shaft wear. Oil thickening results from the level of soot, in combination with the “state of the soot in the oil. The latter is noticeably affected by the dispersant level present in the oil.

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