## 3 Discussion of the Results
Tailpipe CO emissions showed similar pattern than engine-out ones but with more marked differences between cold and warm tests during the low-speed phase because of the faster DOC warm-up in warm WTLC. Consequently, the tailpipe CO emission reached its lowest contribution during this first cycle phase in the warm test in contrast to the cold-start case. The remainder phases showed a progressive decrease of the cumulative tailpipe CO emission from medium- to extra-high-speed phase with increasing differences with respect to the engine-out cumulative emission. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Cold WLTC**} & \multicolumn{2}{c}{**Warm WLTC**} \\  & **Baseline** & **Pre-Turbine Pre-DOC** & **Baseline** & **Pre-Turbine Pre-DOC** \\ \hline CO [g] & 34.1 & 50.2 (+47.2\%) & 29.3 & 44.1 (+50.5\%) \\ HC [g] & 3.9 & 6.2 (+58.9\%) & 3.2 & 4.9 (+53.1\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Cumulative engine-out CO and HC emission in cold and warm WLTCs with baseline ATS and pre-turbine pre-DOC configurations. Figure 8: Cumulative CO and HC emission along the exhaust line in cold and warm WLTCs with baseline ATS and pre-turbine pre-DOC configurations. (**a**) Cumulative CO emission in cold WLTC, (**b**) cumulative CO emission in warm WLTC, (**c**) cumulative HC emission in cold WLTC, (**d**) cumulative HC emission in warm WLTC. This last trend means a CO conversion efficiency increase, as depicted in Figure 10. The black series represents the CO conversion efficiency in baseline ATS configuration (DOC). It is interesting to note that the CO conversion efficiency in the low-speed phase was not the lowest one even in cold WLTC. Despite the warm-up, the DOC inlet temperature exceeded 150 \({}^{\circ}\)C most of time during this phase (Figure 7), which enabled the reaching of the CO light-off. Together with the smooth accelerations during this first cycle period, i.e., lack of engine-out CO emission peaks, the result was a CO conversion efficiency of 44% and 62% for cold and warm tests. These values were higher than those reached in the medium- and high-speed phases, despite the lower DOC temperature during the low-speed phase. The reason lies on the CO emission peaks along the medium- and high-speed phases during fast acceleration because of the mass flow increase along with high CO emission. As a result, mass transfer and inhibition condition the catalyst performance [30]. In fact, the gradual DOC temperature increase governs the recovery of the conversion efficiency till reaching its maximum in the extra-high-speed phase. Complementary, Figure 9c,d represent the normalized cumulative CO emission distribution for the pre-turbine pre-DOC configuration. The main trends in CO engine-out emission and abatement for the baseline case were also found with the pre-turbine pre-DOC configuration. The cumulative CO emission in every exhaust region behaved as their counterpart in the baseline ATS configuration, both in cold and warm WLTCs. The most relevant result is that the percentage tailpipe CO emission was lower than the one obtained in baseline ATS configuration in every cycle phase. A detailed analysis of the CO conversion efficiency with pre-turbine pre-DOC shown in Figure 10 reveals that the presence of the pre-DOC enhanced the overall CO abatement throughout the cycle, especially during the low-speed phase in cold WLTC. The pre-DOC contributed with very high conversion efficiency, which was comparable to that of the DOC in baseline ATS configuration during the cold WLTC. The reason of the high pre-turbine pre-DOC reaction rate was two-fold. First, this catalyst operated at higher temperature than the DOC. In addition, the reactants partial pressure in pre-turbine location was also higher than in the DOC because of the higher gas pressure prior to the turbine. Higher CO and HC mole fraction was also found along with slightly lower O\({}_{2}\) amount due to the higher equivalence ratio.