Size Distribution and Structure of Wall-Deposited Soot Particles in an Automotive-Size Diesel Engine

Wall-deposition of soot particles occurs during the cylinder liner wall/flame interaction, which can potentially deteriorate engine oil quality and alter the heat loss rate in a diesel engine. These issues motivate a detailed study on structure and size of the wall-deposited soot particles. A morphological difference between the wall-deposited soot and in-flame soot particles is another focus of this study. We performed thermophoretic soot sampling in the cylinder liner wall using an in-liner-type sampler. Obtained soot particles were imaged by a transmission electron microscope and post-processed to acquire the number of particles, projection area on the sampling grid, and size distribution. The same set of data was also obtained for soot particles within the diesel flame using a probe-type sampler. The results show that the amount of soot particles deposited on the wall is about 2.7% (by means of number of particles and projection area on the grid) of those collected inside the flame. Close examination of soot particles structure suggests that the wall-deposited soot particles are largely different to the in-flame soot particles. It is observed that very small particles (radius of gyration < 10 nm) that are popular within the diesel flame disappear in the images of wall-deposited soot. Also, large particles (radius of gyration > 100 nm) with long stretched structure that are easily found within the flame do not present in the wall-deposited soot but smaller particles with cluster-like agglomerated shape. As a result, the mean radius of gyration of the wall-deposited soot and in-flame soot particles show nearly identical value of 38 nm at the tested conditions of this study, despite significant structural differences. It is suggested that the soot particles with highly concentrated core region are likely deposited on the cylinder liner wall during the flame/wall interaction.