Soot in the Lubricating Oil: An Overlooked Concern for the Gasoline Direct Injection Engine?

Formation of soot is a known phenomenon for diesel engines, however, only recently emerged for gasoline engines with the introduction of direct injection systems. Soot-in-oil samples from a three-cylinder turbocharged gasoline direct injection (GDI) engine have been analysed. The samples were collected from the oil sump after periods of use in predominantly urban driving conditions with start-stop mode activated. Thermogravimetric analysis (TGA) was performed to measure the soot content in the drained oils. Soot deposition rates were similar to previously reported rates for diesel engines, i.e. 1 wt% per 15,000 km, thus indicating a similar importance. Morphology was assessed by transmission electron microscopy (TEM). Images showed fractal agglomerates comprising multiple primary particles with characteristic core-shell nanostructure. Furthermore, large amorphous structures were observed. Primary particle sizes ranged from 12 to 55 nm, with a mean diameter of 30 nm and mode at 31 nm. Particle agglomerates were measured by nanoparticle tracking analysis (NTA). The agglomerates were found to range between 42 and 475 nm, with a mean size of 132 nm and mode at 100 nm. The distribution was shifted towards larger sizes with a minor concentration of very large agglomerates observed around 382 nm. While deposition rate and agglomerate morphology were similar to diesel engines, distinctive amorphous carbon and smaller particles were observed. Hence, existing knowledge for diesel applications might not be directly transferrable.