Impact of multi-injection strategies on morphology, nanostructure and oxidation reactivity of diesel soot particles

Abstract

The objective of the present study was to investigate and optimize the multi-injection parameters that were able to match the diesel particulate filter (DPF) after-treatment system for reduction of particulate matter (PM) generated from diesel engines. Three representative multi-injection strategies, including main plus post-injection (designated as M+Po), one pilot injection plus M+Po (Pi1+M+Po) and two pilot injections plus M+Po (Pi2+Pi1+M+Po), were selected to generate soot particles in a diesel engine. The morphology and nanostructure of soot particles, including primary particle size, fringe length, thermal stability of organic carbon (OC) and graphitization degree, were analyzed by transmission electron microscopy (TEM) and Raman spectroscopy. The OC content and oxidation reactivity were quantified by thermalgravimetric analyzer (TGA). Results revealed that pilot injection led to the formation of soot particle with smaller primary particle size, more ordered nanostructure, and higher graphitization degree, ascribing to the better air-fuel mixing, higher cylinder temperature, as well as longer residence time. On the contrary, post injection (e.g., M+Po) resulted in soot particles with more OC exhibiting larger size and more ambiguous boundary. Overall, the soot particles with more disordered nanostructure and more OC content showed better oxidation reactivity. Interestingly, the soot particle morphology and nanostructure were observed to be varying under TEM electron irradiation, which might be due to the OC heating dissolution.