Effect of diesel soot contaminated oil on engine wear

Contamination of lubricating oil by diesel soot is one of the major causes of increased engine wear, especially with most engine manufacturers opting for Exhaust Gas Recirculation (EGR) technology to curb oxides of nitrogen (NOx) emissions. The diesel soot interacts with engine oil and ultimately leads to wear of engine parts. Factors which can change or modify the characteristics of the soot surface are expected to play an important role in controlling the interactions with soot. Hence, it is important to study the interactions between soot and oil additives in order to develop high performance diesel engine oils for engines equipped with EGR. A statistically designed experiment was developed to study the effects of soot contaminated engine oil on wear. The variables that were considered were the base stock (groups I and II), dispersant level, and zinc dithiophosphate (ZDP) level. The above three variables were formulated at two levels: low (-1) and high (1), which resulted in 2 3 matrix (8 oil blends). In order to study the non-linear effect of soot, it was considered as a variable and was tested at three levels: low (-1), medium (0), and high (1). This resulted in testing of 24 oil samples. A three-body wear machine was used to simulate and estimate the extent of wear quantitatively. The extent of wear was measured as the actual loss of material, in grams. A second set of experiments were performed on a milling machine (ball-on-flat disk setup). The wear scars formed on the steel balls were examined using a scanning electron microscope (SEM) and were analyzed qualitatively to determine the effect of soot contaminated oils on wear. 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 wear. The analysis indicated that wear increased nonlinearly as the amount of soot increased. Cumulative wear was more for samples with soot contamination than without soot contamination. This showed a detrimental effect of soot on the oil blends wear performance. The SAS analysis showed that the base stock and soot content were the most significant variables affecting wear. Dispersant and ZDP levels were also found to be significant. The highest wear resulted from a sample that had 4% soot. (c) 2006 Elsevier B.V. All rights reserved.