The nanostructure of soot-in-oil particles and agglomerates from an automotive diesel engine

Abstract

The characteristics of soot particles and agglomerates of particles extracted from samples of lubricating oil drawn from the sump of a diesel engine have been investigated. The engine was a high pressure common rail, direct injection diesel designed for light duty automotive applications. Soot from the samples was prepared for imaging by sample dilution with heptane, followed by washing in diethyl ether and in some cases, sample centrifugation. The size and shape of agglomerates were defined from measurements of projected length and width allowing for chain contortion. When used, centrifugation is shown to alter the size and shape of agglomerates, increasing the proportion of chain agglomerates and reducing clusters. Without centrifugation, roughly half of the soot in oil exists in long-chain agglomerates with average length of 130nm and 50nm in width. Clusters with modest branching account for the remaining 46%. The average aspect ratio (L/W) was of 2.9. The diameter of spherical primary particles that form the agglomerates ranges between 10 and 35nm grouped in a Gaussian distribution with a mean value of 20.2nm. All primary particles exhibit an inner core and an outer shell. The inner core is composed by several nuclei of around 4nm diameter. Inner core is of around 8–15nm in diameter and outer shell 4–12nm thick.