The effect of after injection on in-cylinder soot particulates in a small bore diesel engine

After injection, which introduces a short injection following the main injection, is proven to be effective in reducing diesel engine soot emissions. The present study aims to investigate the in-cylinder soot reduction mechanism triggered by after injection in a small-bore optical diesel engine with a particular emphasis on the morphology of soot particles. Accordingly, in-bowl soot sampling based on the thermophoresis principle was performed for the direct collection of soot from the reacting diesel flame. The subsequent transmission electron microscope imaging enabled the structural analysis of soot particles, which provides fundamental insight regarding the formation and oxidation processes of in-bowl soot particles. The results show that, with the introduction of after injection, a downsizing trend of soot particles is found regardless of the sampling locations, which was explained by the increased in-cylinder bulk gas temperature and thus higher oxidation rate. A significant after-injection-induced decrease in soot aggregate size also suggests an accelerated oxidation of in-bowl soot particles through the fragmentation of large soot aggregates.