Physicochemical properties of soot in diesel engine lubricating oil: characterizations and impact on frictional characteristics

The study investigates the effects of the physicochemical properties of engine sump soot on the frictional characteristics of its contaminated engine oils. Moreover, the relationship between soot physicochemical properties and its frictional characteristics was also discussed. The results showed that the properties of the two kinds of lubricating oil (A, B) used in diesel engines affect the properties of engine sump soot (Soot-A, Soot-B), i.e. Soot-A showing smaller aggregate size, larger primary particle diameter, more disordered nanostructure, and higher functional content than those of Soot-B. For both soot particles, as the amount of soot particles added to the formula oil went up from 1 wt.% to 2 wt.%, the wear scar diameter (WSD) and friction coefficient of the wear spot increased. For both soot particles, the main wear form of the steel ball wear area was abrasive wear (more than 90%), accompanied by a small amount of plastic deformation and pitting (less than 2%). With the same amount of soot in lubricating oil, soot with a higher degree of graphitization, low oxygen content and low reactivity may cause smaller WSD and lighter wear. Moreover, a larger soot accumulation would result in deeper plow marks and pits in the abrasion areas.