A Study of the Filtration and Oxidation Characteristics of a Diesel Oxidation Catalyst and a Catalyzed Particulate Filter 2007-01-1123

An experimental and modeling study was conducted to study the passive regeneration of a catalyzed particulate filter (CPF) by the oxidation of particulate matter (PM) via thermal and Nitrogen dioxide/temperature-assisted means. Emissions data in the exhaust of a John Deere 6.8 liter, turbocharged and after-cooled engine with a low-pressure loop EGR and a diesel oxidation catalyst (DOC) - catalyzed particulate filter (CPF) in the exhaust system was measured and used for this study. A series of experiments was conducted to evaluate the performance of the DOC, CPF and DOC+CPF configurations at various engine speeds and loads. Pressure drop across the devices, mass of PM deposited in the CPF at the end of loading, upstream and downstream gaseous and particulate emissions, and particle size distributions were measured at different times during the experiments to characterize the pressure drop and filtration efficiency of the DOC-CPF system at each engine load case as functions of loading time.

Two computer models, i.e., the MTU 1-D DOC model and the MTU 1-D 2-layer CPF model were developed as part of this research and calibrated using the data obtained from experiments. The 1-D DOC model employs a three-way catalytic reaction scheme for CO, HC and NO oxidation, and is used to predict CO, HC, NO and NO₂ concentrations downstream of the DOC. The 1-D 2-layer CPF model uses a “2-filters in series” approach for filtration, PM deposition and oxidation in the PM cake and substrate wall via thermal (O₂) and NO₂/temperature-assisted mechanisms, and production of NO₂ as the exhaust gas mixture passes through the CPF catalyst washcoat. Results obtained from the 1-D 2-layer CPF model calibrated to experimental data at 2200 rpm are presented. Comparisons of filtration and oxidation behavior of the CPF at sample engine load cases in both configurations are also presented.

PM oxidation efficiency of the DOC-CPF device increased with increasing CPF inlet temperatures due to temperature dependency of NO₂/temperature-assisted and thermal PM oxidation reactions in the CPF, and was higher in the DOC+CPF configuration compared to the CPF configuration due to higher CPF inlet NO₂ concentrations. Overall CPF filtration efficiencies greater than 90% were observed within 90-100 minutes of loading time (starting with a clean filter) at all engine load cases, due to the fact that the PM cake layer on the substrate wall forms a very efficient filter.