## 3 Discussion of the Results
(**a**) Instantaneous engine-out opacity in cold WLTC, (**b**) instantaneous engine-out opacity in warm WLTC, (**c**) cumulative engine-out NOx emission in cold WLTC, (**d**) cumulative engine-out NOx emission in warm WLTC. phases. Accordingly, the effect on the turbine inlet temperature, which is represented in top charts of Figure 7 for cold and warm WLTCs as a function of the ATS configuration, was also negligible. As observed, the pre-DOC inlet temperature is higher than the one in the exhaust manifold in baseline ATS configuration because of the higher exhaust backpressure effects. However, the temperature dropped across the pre-DOC because of the heat losses and thermal inertia, which were not offset by the heat released due to the HC and CO oxidation. Nevertheless, the change in DOC inlet temperature is positive and much more relevant, as illustrated in Figure 7c,d. The turbocharger operation with quasi-steady flow led to lower temperature drop in the expansion, favoring a faster warm-up of the close-coupled DOC. With these fluid-dynamic boundaries, Figure 8 shows the cumulative engine-out and tailpipe emissions of CO and HC in cold and warm WLTCs with baseline ATS and pre-turbine pre-DOC configurations. Complementary, Table 4 summarizes the cumulative engine-out CO and HC emissions at the end of each WLTC as a function of the ATS configuration. The higher exhaust manifold pressure deteriorated the combustion process with pre-turbine pre-DOC configuration due to the increase of the internal EGR. Consequently, a slight decrease in air mass flow was found, with an increase in fuel consumption to reach the vehicle speed target. Therefore, an increase of the equivalence ratio took place in pre-turbine pre-DOC configuration. This engine response made the CO and HC emission increase around 50% both in cold and warm WLTCs with respect to the baseline ATS case. As observed in Figure 8, this response avoided decreasing the tailpipe emissions with respect to the baseline configuration, despite the earlier activation of the pre-DOC, which highly contributed to the emission abatement. These results are next discussed in detail to bring out the involved phenomena. Figure 5: Exhaust manifold pressure and VGT position in cold and warm WLTCs with baseline ATS and pre-turbine pre-DOC configurations. (**a**) VGT position in cold WLTC, (**b**) VGT position in warm WLTC, (**c**) exhaust manifold pressure in cold WLTC, (**d**) exhaust manifold pressure in warm WLTC. Figure 6: Intake manifold pressure and turbocharger speed in cold and warm WLTCs with baseline ATS and pre-turbine pre-DOC configurations during the high- and extra-high-speed phases. (**a**) Intake manifold pressure in cold WLTC, (**b**) intake manifold pressure in warm WLTC, (**c**) turbocharger speed in cold WLTC, (**d**) turbocharger speed in warm WLTC. Figure 7: Pre-DOC, turbine and DOC inlet temperature in cold and warm WLTCs with baseline ATS and pre-turbine pre-DOC configurations. (**a**) Exhaust manifold temperature in cold WLTC, (**b**) exhaust manifold temperature in warm WLTC, (**c**) DOC inlet temperature in cold WLTC, (**d**) DOC inlet temperature in warm WLTC. To understand better the new emission pattern, Figure 9 shows the cumulative CO emission in the relevant locations of the exhaust line normalized against the total engine-out emission in every test. Top charts are referred to the baseline ATS configuration, distinguishing between cold and warm WLTC in plots (a) and (b) respectively. The cumulative engine-out CO emission presents almost identical distribution along the speed phases between cold and warm tests. The main discrepancies appeared in the low-speed phase, especially in cold WLTC. Nevertheless, the medium- and high-speed phases involved almost 60% of the total engine-out CO emissions due to their higher duration and concurrence of sharp engine load demands, which avoid complete in-cylinder CO oxidation.