Influence of the SCR reactivity on passive soot oxidation for vanadium-coated and uncoated (S)DPF

In modern diesel exhaust aftertreatment systems (EATS), combining a catalytic coating for selective NOx reduction (SCR) with a diesel particulate filter (DPF) enables simultaneous particulate filtration and NOx reduction. In such systems, soot regeneration is primarily influenced by the NO2 supply in the exhaust gas, particularly at low exhaust temperatures (250-400 °C). Several investigations found that in SCR-coated filters with urea-water solution (UWS) dosing upstream, particle oxidation is partially inhibited by NH3. However, at higher temperatures (>400 °C), this effect seems to be reduced. In this study, numerical modelling approaches based on engine test bench results are utilised to examine the impact of SCR reactivity on passive soot oxidation for a highly porous vanadium-coated SDPF. To identify the interaction mechanism between the SCR reaction and soot oxidation, further investigations were conducted using a filter with a washcoat without SCR-active component. The methodology for determining the oxidation reduction of soot with UWS dosing is based on measuring the CO concentration difference across the particulate filter. The experimental data were determined on a state-of-the-art 13.5-litre six-cylinder rail engine under varying operating conditions, including different soot loadings, UWS injection rates and temperatures. Results show significant reductions in passive soot oxidation rates at both low and high temperatures when NH3 is present. The diffusion of NO2 in the filter wall appears to be the determining factor for the reduction, indicating that the NO2 difference after and before the filter is not the only determining factor for the diffusion effect, but rather the concentration gradient in the filter wall. The findings highlight the complex interaction between SCR reactions and passive soot oxidation. Optimising the design of the EATS can avoid critical soot loads for many applications of particulate filters with SCR activity, even without active regeneration.