Insights into the oxidation regime of diesel soot during realistic DPF regeneration

Optimal DPF regeneration methodologies need detailed knowledge of soot oxidation regime under realistic engine conditions. This work experimentally explored the soot oxidation regime during DPF regeneration at inlet temperatures of 300°C and 350°C. A high-resolution transmission electron microscope (HRTEM) and a Raman spectroscopy were used to observe the structure evolution and explain the oxidation regime transformation during regeneration. Results showed that at the DPF-inlet temperature of 300°C, partial soot particles possessed localized hollow interiors with thicker boundaries at the later stage of its oxidation process, indicating the occurrence of internal burning. However, at the DPF-inlet temperature of 350°C, soot particles exhibit no voids and became more ordered over the whole soot oxidation process, which agree well with the behavior induced by surface oxidation. At the early regeneration stage under at 300°C DPF inlet temperature, soot oxidation led to the extension of micropores, increasing the accessibility of oxygen penetration to particle core. Reversely, the oxidation at higher temperature of 350°C and the late oxidation stage of 300°C consumed the micropores and shrank the primary particle size, accompanying with the oxidation-induced graphitization. Oxidation-induced disordering was interesting observed at the early stage of 300°C regeneration, with the ever shorter fringe, larger tortuosity and separation distance, and rising peak area ratio of D1 to G band. Evolutions of structure and defect sites revealed the inclination of internal burning at lower DPF regeneration temperatures, and the prevalence of surface oxidation during the overall process of higher temperatures.