Properties and oxidation of in-cylinder soot associated with exhaust gas recirculation (EGR) in diesel engines

The properties and oxidation of in-cylinder soot from a diesel engine fueled with n-heptane were explored under the simulated exhaust gas recirculation (EGR) condition, via CO2 addition to the intake air of the engine. A self-developed total cylinder sampling system was employed to obtain the soot samples at the EGR ratios of 0 and 25%, and this study only focused on the soot oxidation that occurred in the soot oxidation dominated (SOD) phase. Computational fluid dynamics (CFD) simulations were performed to determine the local combustion characteristics related to soot oxidation. Soot oxidation rates were evaluated based on in-cylinder soot mass traces and CFD results. Soot properties were characterized in terms of nanostructure, carbon chemical state, and surface functional groups. Detailed analysis of the soot mass indicates that EGR addition enhances the mass-based specific rate of soot oxidation. However, CFD results show that EGR addition reduces the flame temperature and the concentrations of O-2 and OH, and consequently decreases soot surface oxidation rates by O-2 and by OH. The characterization of soot properties shows that EGR decreases the degree of structural ordering and results in the formation of more oxygenated and aliphatic C-H groups on the soot surface. This alteration in physico-chemical properties favors soot oxidation during the combustion process, and results in an increase in the mass-based specific rate of soot oxidation in the SOD phase. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This study investigated the properties and oxidation of in-cylinder soot from a diesel engine fueled with n-heptane under simulated exhaust gas recirculation (EGR) conditions. It was found that EGR addition enhances the mass-based specific rate of soot oxidation, but reduces flame temperature and concentrations of O-2 and OH, leading to decreased soot surface oxidation rates. Additionally, the characterization of soot properties showed that EGR decreases structural ordering and results in the formation of more oxygenated and aliphatic C-H groups on the soot surface, favoring soot oxidation during combustion processes.