Impact of oil aging and composition on the morphology and structure of diesel soot

Hypothesis: Soot is a black powder-like substance consisting of carbonaceous amorphous particles, formed by incomplete combustion of hydrocarbons. In the last decades, industrial research on lubricant oils has grown to develop more efficient formulations, mainly to reduce the formation of soot in oil so to increase engine lifetime. The comprehension of the mechanism of soot formation and particles growth/aggregation in times during real applications is of fundamental importance for the design of better performing lubricants. Experiments: In this work, we report on a multi-technique investigation of soot-in-oil samples, drawn from the oil sump of a direct-injected heavy-duty diesel engine at increasing working times in a standard engine test. Scanning Electron Microscopy, Small Angle X-ray Scattering and Dynamic Light Scattering are employed to study the evolution in term of soot size, morphology and fractal dimensions. Findings: Our results evidence that, in a complete lubricant formulation, exhaust oil viscosity evolution correlates well with the increase of soot amount inside the oil. Moreover, the growth of both primary soot particles and aggregates during the engine test is limited to about 24 and 200 nm, respectively, because of the active role of the dispersant present in the investigated oil.