## 3 Discussion of the Results
Figure 9: Normalized cumulative CO emission along the exhaust line in every WLTC phase in cold and warm tests with baseline ATS and pre-turbine pre-DOC configuration. (**a**) Baseline configuration in cold WLTC, (**b**) Baseline configuration in warm WLTC, (**c**) Pre-turbine pre-DOC configuration in cold WLTC, (**d**) Pre-turbine pre-DOC configuration in warm WLTC. Concerning the CO and HC mole fraction increase, this contribution is positive for base-level engine emissions (low mole fraction) but can turn negative during CO emission peaks related to accelerations because of the inhibition [31]. By contrast, the DOC performance was worsened with respect to the baseline ATS configuration, in which it was dealing with all the engine-out CO emission and operated at lower temperature (Figure 7). In fact, the DOC provided lower CO conversion efficiency than the pre-DOC when these two catalysts worked together. The cause of this unexpected result was found in the different pattern of CO mass flow that the two catalysts must face. It is exemplified in Figure 11, where the CO mass flow and conversion efficiency in pre-DOC (plot (a)) and DOC (plot (b)) are shown during the medium speed phase of the cold WLTC. According to Figure 11a, the pre-DOC abated most of the base-level engine-out CO emissions, with instantaneous efficiencies ranging 60-80%. However, it was unable to deal with the CO emission peaks due to the previously mentioned mass transfer and inhibition limitations. According to this response, the CO emissions at the DOC inlet were mostly concentrated in the peaks related to the fast accelerations, as depicted in Figure 11b. Consequently, and despite the higher instantaneous conversion efficiency against base-level emissions (80-95%) than the pre-DOC, the CO conversion efficiency of the DOC decreased with respect to the baseline ATS configuration and was even lower than the one of the pre-DOC. The dependence on the flow pattern was also demonstrated by the fact that the CO conversion efficiency was better in the DOC than in the pre-DOC during the low- and extra-high-speed phases, where the emission peaks are less prominent and frequent. By contrast, the DOC performance was minimum during the medium- and high-speed phases penalized by the high dynamics of these periods. Figure 11: Instantaneous CO conversion efficiency and mass flow in (**a**) pre-DOC and (**b**) DOC (pre-turbine pre-DOC configuration) during the medium speed phase of the cold WLTC. Figure 10: CO conversion efficiency in each WLTC phase in (**a**) cold and (**b**) warm tests: baseline (DOC) and pre-turbine pre-DOC (overall and contributions from pre-DOC and DOC) configurations. The results corresponding to HC conversion efficiency are shown in Figure 12. Unlike CO case, the pre-turbine pre-DOC configuration scarcely improved the overall HC conversion efficiency, especially under warm conditions. This was due to the pre-DOC characteristics and the HC oxidation behavior. On the one hand, the pre-DOC was not coated with zeolites, so that its HC conversion efficiency during the low-speed phase was very poor, especially in cold WLTC. By contrast, the DOC showed very high HC conversion efficiency during the cold start emphasizing the importance of the adsorption mechanism. On the other hand, the small size of the pre-DOC involved additional limitations to mass transfer, which are more relevant in HC than CO because of the higher diffusion volume of species of high molecular weight [32]. Despite the better fluid-dynamic conditions in pre-turbine location to increase the reaction rate and the advantage to face base-level HC emission first than the DOC, the pre-DOC HC conversion efficiency was almost constant along the WLTC, both cold and warm tests. In fact, a slight decrease in HC conversion efficiency was found from medium- to extra-high-speed phases due to the residence time decrease.