Statistically designed experiments were developed to investigate the nature of soot, to understand its role in oil viscosity growth, and to study the interactions involved with additives that inhibit viscosity growth. The matrix was designed to examine effects of engine type, mode of operation, and the oil formulations. Mack EM6-285 and GM 6.2L engines operating under both high speed and high torque conditions were used in this study. An API CE\SG quality lubricant was used as the baseline. The detergent sulfonate substrate was varied from standard to three-fold levels; the dispersant TBN contribution ranged from 1.1 to over 3.0. The surface and bulk exhaust soot properties were determined. Colloidal suspension stability and rheology were measured to evaluate the design factor effects on the formation of soot and subsequent effects on oil thickening.
The Mack EM6-285 engine produced less soot, less oil viscosity growth, and less oxidation than the GM 6.2L engine. High speed operation created more oil oxidation than did high torque conditions, especially in the 6.2L engine. Soot levels, particle size, and viscosity growth were greater in the 6.2L engine under high speed operation. The dispersant's nitrogen-sourced alkalinity statistically reduced lubricant viscosity growth. The sulfonate substrate content of the oil had no significant effect on viscosity growth. All soot-in-oil suspensions, over the range of conditions examined, were stable and Newtonian.
The additive chemistries studied provided more than adequate oil viscosity control. Viscosity changes were due to a combination of the amount of soot and soot particle size; direct correlation with the surface area of soot dispersed in the oil was found. Some interesting differences in the soot characteristics were observed.