Effect of diesel soot on lubricant oil viscosity

Soot related lubricant oil thickening is a primary concern for heavy-duty diesel engines. Engines which produce a relatively low level of particulate matter in exhaust emissions show a significant level of soot contamination in the lubricant. This contamination results in lubricant breakdown. The soot contaminates the lubricant and changes the chemical properties resulting in the lubricant ceasing to perform its functions. This causes an increase in viscosity of the engine oil causing pumpability problems. Hence, it is necessary to study the effects of soot and lubricant oil additives and their interactions on engine oil viscosity.Statistically designed experiments were developed to study the effect of soot contamination on engine oil viscosity. The oil samples used for the study differed in the base stock, dispersant level, and Zinc Dithiophosphate (ZDP) level. These three variables were formulated at two levels: Low (−1) and High (1), which resulted in a 23 matrix (8 oil blends). Soot was considered as a variable at three levels: low/0% weight (−1), medium/2% by weight (0), and high/4% by weight (1). This resulted in 24 oil samples, and soot at three levels helped in determining the non-linear effect of soot on oil viscosity.Experiments were conducted at 40 and 90 °C to study the effect of the various factors on viscosity with temperature variation. The results showed that viscosity of the oil samples increased with increase in soot at both 40 and 90 °C. The analysis indicated a nonlinear behavior of viscosity as the amount of soot increased at 40 °C, whereas a linear variation at 90 °C.The results obtained were analyzed using the general linear model (GLM) procedure of the statistical analysis system (SAS) package to determine the significance of variables on viscosity. The statistical analysis system also highlighted the significance of various interactions among the variables on viscosity. The statistical analysis results at 40 and 90 °C showed that the effect of base stock and ZDP levels were negligible at 40 °C, whereas the dispersant level and soot level influenced the viscosity of the oil samples at both temperatures.