Model-Based DPF SCR System Optimization

Effect of DOC size and PGM activity The presence of a DOC has two main implications on the SCR system efficiency. The DOC thermal inertia affects the SCR inlet temperature while the production of NO2 over the DOC alters the NOi/NO ratio upstream of the SCR. The latter case is of greater interest since the thermal mass of the DOC is relatively low and it only affects the de-NO c performance during the light-off phase. Chatterjee et al. [52] stated that SCR efficiency is mainly inffuenced by the DOC generated NOa/NO ratio, with the DOC noble metal loading and volume being the two parameters that can be modified independently to optimize the system performance. A computational study performed by Koltsakis et al. [53] relates the NO c conversion efficiency with the DOC PGM loading and volume (Fig. 13.19). 

It is apparent that higher PGM loading leads to an increase of the de-NO efficiency due to a more favorable NO2/NOJC ratio, closer to 50 %. An increase of the DOC volume with a constant specific noble metal loading has also a positive effect on the NO conversion. This is due to the higher PGM amount of the DOC which increases the NO oxidation rate. 

Effect of DPF thermal inertia The effect of the thermal inertia of the components upstream of the SCR is expected to be more pronounced in the case where the SCR is located after the DPF. In this case, it is not only the thermal inertia of the DOC which delays the SCR warm-up but also the thermal mass of the DPF. To examine this effect, four different DPF formulations, with alternating substrate material and volume, were considered along the baseline DOC. The simulated DPF s were the baseline (SiC, 2.5 1), a (Cordierite, 2.5 1), a (SiC, 3.75 1), and a (Cordierite, 3.75 1). The effect of the thermal response on the average NO , conversion efficiency is illustrated in Fig. 13.20. For the basehne DPF volume of 2.5 1, the substrate material of the DPF appears to have an impact of up to 5 % in the 

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