# On the Impact of Particulate Matter Distribution on Pressure Drop of Wall-Flow Particulate Filters
## 4 Discussion of the Results
### Filtration Efficiency and Regeneration  
Experimental data confirmed that the use of pre-DPF water injection does not affect the filtration efficiency of the DPF [36], which was between 99.35% and 99.85% (number-based filtration efficiency) during the soot loading process shown in Figure 2. According to the theoretical and visualization results shown in Section 4.1, this behavior can be justified based on the lack of variation of the soot penetration into the porous wall, keeping a saturated portion that acts as a barrier filter of high collection efficiency. It ensures high filtration efficiency in the entrance region before the onset of the particulate layer. In addition, the sparse thin particulate layer along the inlet channels and its concentration towards the end region also contribute to ensure the filtration performance of the DPF. Thus, the theoretical mass-based filtration efficiency is over 99.95% for all considered particulate layer characteristics, as shown in Figure 13 for operating conditions after the first and 13 th water injections. With respect to the regeneration behavior, previous works showed the lack of relevant influence of pre-DPF water injection on passive and active regeneration processes based on pressure drop and outlet gas temperature evolution [12]. In this work, an active regeneration process have been modeled. Figure 14 shows in (a) the experimental and modeled pressure drop variation along the active regeneration, while (b) is devoted to the temperature fashion. In both tests (baseline and applying consecutive water injections), the DPF was previously loaded up to 60 g. After the soot loading, two consecutive NEDCs were performed before active regeneration. Details on the soot loading can be consulted in [12]. At the beginning of the regeneration, lower pressure drop in the case of the soot loading applying pre-DPF water injections can be clearly observed. This is even  
Figure 12: DPF pressure drop as a function of the particulate layer porosity and the soot mass distribution onset in DPF #A imposing a linear soot mass distribution. Figure 13: Filtration efficiency as a function of the particulate layer porosity and the soot mass distribution onset after different injection events. obtained with higher temperature along the monolith (higher outlet gas temperature), whose effect is the pressure drop increase (lower gas density). The interest for these tests is in fact such a difference in outlet temperature at the beginning of the regeneration. The outlet gas temperature is higher in the case of the DPF sample subjected to water injections. This is due to the fact that a longer thermal stabilization period took place in this case before the regeneration started. Being that the inlet gas temperature is equal during the two regenerations and the outlet one higher in the pre-DPF water injection sample, a faster regeneration process is expected for this last case. This result is also deduced from the pressure drop dynamics, which is properly predicted by the model for both tests. The temperature peak at the DPF outlet is almost equal in both tests, obtaining also consistent modeled results. The gas temperature within the monolith, which is shown in Figure 15, is also very similar in both regenerations. The temperature increase is faster in the pre-DPF water injection case mainly due to higher initial temperature. Nevertheless, this situation does not promote hot spots' appearance keeping the maximum temperature in the same order of magnitude as the baseline regeneration. Nevertheless, a more detailed analysis of the modeling results provides interesting insights. Figure 16a shows the evolution of the DPF soot mass during the regeneration process. The soot mass depletion rate is higher in the case of the pre-DPF water injection during the first seconds of the regeneration, but it gets progressively slower. In fact, the amount of soot still accumulated into the DPF in the case of the pre-DPF water injection is clearly higher from 200 s on.