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Modern ‘DOC-cDPF’ systems for diesel exhaust are employing Pt-, Pd- as well as Pt/Pd alloy- based coatings to ensure high conversion efficiency of CO, HC even at low temperatures. Depending on the target application, these coatings should be also optimized towards NO2 generation which is involved in low temperature soot oxidation as well as in SCR-based deNOx. Zeolite materials are also frequently used to control cold-start HC emissions. Considering the wide variety of vehicles, engines and emission targets, there is no single optimum coating technology. The main target is therefore to maximize synergies rather than to optimize single components. At the same time, the system designer has nowadays a wide range of technologies to choose from, including PGM alloyed combinations (Pt/Pd), multiple layers and zones applicable to both DOCs and DPFs.

A case study of a close-coupled catalyst subjected to exhaust gas conditions typical for a modern engine warm-up phase is studied using a time-efficient 2-dimensional modeling approach. The flow distribution at catalyst inlet is affected by the downstream flow resistance, which is in turn a function of catalyst temperature field. Unlike traditional CFD approaches, the presented model focuses on this interesting coupling between the problems of flow distribution and catalyst thermal response. The results are expressed in terms of time-dependent velocity and temperature distribution as well as conversion efficiency. After a basic understanding of the phenomena, a parametric analysis is performed to assess the significance of various design parameters affecting the cold start performance of close-coupled catalysts. It is shown that the detrimental effect of flow uniformity on light-off is associated to the non-uniform ageing.

The pressure for compact and efficient deNO systems has led to increased interest of incorporating SCR coatings in the DPF walls. This technology could be very attractive especially if high amounts of washcoat loadings could be impregnated in the DPF porous walls, which is only possible with high porosity filters. To counterbalance the filtration and backpressure drawbacks from such high porosity applications, the layered wall technology has already been proposed towards minimizing soot penetration in the wall and maximizing filtration efficiency. In order to deal with the understanding of the complex interactions in such advanced systems and assist their design optimization, this paper presents an advanced modeling framework and selected results from simulation studies trying to illustrate the governing phenomena affecting deNO performance and passive DPF regeneration in the above combined systems.