An analysis of the in-cylinder soots generated from the main- and post-injection combustion in diesel engines

In modern diesel engines, the exhaust soot primarily comes from the main-injection combustion and post -injection combustion. Therefore, to reduce the diesel soot emissions, it is essential to better understand the soots generated from the main-injection combustion (main-soot) and from the post-injection combustion (post-soot). This work focused on the properties of the main-soot and post-soot during the combustion pro-cess, including the primary particle size, nanostructure and soot mass. The in-cylinder soot samples were obtained using a self-developed total cylinder sampling system, and the primary particle size and nanostruc-ture were determined using high-resolution transmission electron microscopy. The isolation of the post-soot was achieved by adding dimethyl ether to the intake gas instead of the real main-injection to create a simu-lated main-injection combustion environment for post-soot formation. Combustion analysis and numerical simulation results showed that the simulated combustion environment for post-soot formation generated by the DME combustion was very similar to that generated by the real main-injection combustion. During the combustion process, although the main-soot and post-soot exhibit similar variations in the primary particle size, the maximum primary particle size of the post-soot is smaller than that of the main-soot (23.38 nm for the main-soot and 20.51 nm for the post-soot). The main-soot and post-soot show almost the same trends in the nanostructure, as characterized by the fringe length, separation and tortuosity, throughout the combus-tion process. The introduction of the post-injection accelerates the reduction of the primary particle size of the main-soot and the increase in the structural order of the main-soot. Because a large number of the main -soot particles are oxidized during the post-injection combustion, the post-soot accounts for a considerable proportion in the engine-out soot (i.e., 42%).& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

To reduce diesel soot emissions, it is important to understand the properties of soots generated from main-injection combustion and post-injection combustion. This study focused on the primary particle size, nanostructure, and mass of main-soot and post-soot during the combustion process. In-cylinder soot samples were obtained using a total cylinder sampling system, and high-resolution transmission electron microscopy was used to determine particle size and nanostructure. Post-soot was simulated by adding dimethyl ether to the intake gas. The results showed that the primary particle size of post-soot was smaller than that of main-soot, and both soots exhibited similar trends in nanostructure throughout the combustion process.