Understanding Soot Mediated Oil Thickening Through Designed Experimentation - Part 5: Knowledge Exhancement in the GM 6.5L

Our basic understanding of the chemical and physical nature of soot, its interaction with lubricant components and its role in promoting wear and oil thickening in heavy duty diesel engines continues to grow.

Our current study in the GM 6.5L engine focuses on examining the effects of variations in base stock type (Group I vs. Group II), viscosity index improver or viscosity modifier (VM) chemistry (OCP vs. dispersant OCP), zinc dithiophosphate (ZDP) type and dispersant type (low MW vs. high MW) on roller follower wear, viscosity growth and other measured responses. In this study, more robust fluids were tested producing very low wear results and minimal viscosity increase of the lubricant. Fluids containing dispersant OCP (DOCP) and high MW dispersant produced a lower degree of wear, whereas varying the ZDP type (1° vs. 2°) showed no effect on wear. The use of Group II base stocks was associated with significantly lower viscosity increases. Failure analysis of the pins shows that the wear scars have a highly polished surface indicating microabrasive wear. Surface analysis of the pins reveals the presence of an antiwear film as evidenced by iron phosphate, iron sulfide and zinc-containing species.