Experimental Investigation on the Oxidation Characteristics of Diesel Particulates Relevant to DPF Regeneration

Diesel engines generally employ diesel particulate filter (DPF) systems to meet increasingly stringent emissions regulations. The development of optimum methodologies for DPF regeneration requires detailed information on the oxidation characteristics of diesel particulate matter (PM) that accumulates on the DPF under realistic engine conditions. An experimental investigation on the oxidation behavior of diesel PM collected from a DPF test system connected to the exhaust stream of a 1.9L, 4-cylinder, light-duty diesel engine is reported. A thermogravimetric analyzer (TGA) was used to measure the instantaneous sample mass and the rate of mass loss during its oxidation for a wide range of conditions, which include initial sample mass, amount of volatile components of soluble organic fraction (SOF) in the sample, oxygen concentration, and various heat treatment schemes in both the inert and oxidizing environments. The global kinetic parameters, i.e., the reaction orders of soot and oxygen, activation energy, and pre-exponential factor, were determined for the diesel PM and surrogate soot samples. Significant differences are observed in the oxidation behavior of surrogate soot and diesel PM. The oxidation rate of surrogate soot decreases continuously as the soot is oxidized, while that of diesel soot is nearly constant until about 80% of the sample mass is oxidized, and then decreases as the sample is completely oxidized. In addition, the oxidation behavior of surrogate soot is found to be essentially independent of the heat treatment schemes used, while that of diesel soot is strongly influenced by them. These differences may be attributable to changes in soot morphology during heating/oxidation and the presence of surface functional groups and heavier SOF components in the diesel PM. The effects of SOF and thermal aging on diesel PM oxidation have also been characterized. Results indicate that the PM oxidation is only weakly influenced by the presence of volatile components of SOF, whereas it is noticeably affected by thermal aging.