## 3 Discussion of the Results
As seen in previous works, this location produces a change from pulsating to constant pressure turbocharging that can have a positive impact on the turbine efficiency [27]. At the same time, the rarefaction wave affects the gas exchange process increasing the internal EGR. On the other hand, these changes in the gas exchange induce an increase in the soot emission, as shown in Figure 4a,b by the instantaneous engine-out opacity and found in previous studies regarding pre-turbine ATS [18]. Consequently, higher SCRF soot loading was obtained, what explains the difference in DOC+SCRF pressure drop between pre-turbine pre-DOC configuration and the baseline ATS observed in Figure 3. Due to the change in the flow pattern at the VGT inlet and, mainly, the increase of the tailpipe backpressure, the engine control varied the VGT position (closing it) to reach the target boost pressure  
Figure 3: Pressure drop in DOC+SCRF and pre-turbine pre-DOC in (**a**) cold and (**b**) warm tests with baseline ATS and pre-turbine pre-DOC configurations. Figure 2: (**a**) Vehicle speed trace during WLTC and cumulative fuel consumption in every WLTC phase in (**b**) cold and (**c**) warm tests with baseline ATS and pre-turbine pre-DOC configurations. (the HP- and LP-EGR control strategy was unaltered, so no differences in EGR rate were produced). Figure 5 shows in plots (a) and (b) how the VGT was closer (higher VGT position value) until reaching its maximum closure (90%) in pre-turbine pre-DOC than in the baseline configuration for both cold and warm WLTCs. The closer VGT position and the trends in pressure drop resulted in a relevant increase of the exhaust manifold pressure with pre-DOC, which was more evident in sudden accelerations and as the engine load increased (Figure 5c,d). As for fuel consumption, the penalty of pre-turbine pre-DOC configuration on exhaust manifold pressure was higher in the low-speed phase for warm conditions than cold ones. In fact, this pressure is even lower (more open VGT) along some periods of the low-speed phase with pre-turbine pre-DOC configuration in the cold WLTC. This is because of the benefits of constant pressure turbocharging (higher turbine efficiency) govern on the pressure drop damage for this initial stage in the cold WLTC. Figure 4a,b show that the higher opacity in pre-turbine pre-DOC configuration with respect to the baseline ATS case is present throughout the cold and warm cycles. The penalty is similar between them during medium-, high- and extra-high-speed phases. In contrast, the pre-turbine pre-DOC case increased more the opacity in cold operation than in warm driving during the low-speed phase. Figure 4c,d depict the cumulative engine-out NOx emission. According to the increase of the internal EGR because of the exhaust manifold pressure increase, the engine-out NOx emissions decreased by the installation of the pre-turbine pre-DOC. At the end of the cycle, the NOx emission reduction reached 0.43 g (3%) in the cold WLCT and 0.60 g (3.9%) in the warm case. The most harmful effect of pre-turbine ATS on the engine performance is the marked turbocharger lag during sudden accelerations caused by the monolith thermal inertia [18]. As this effect is critical in nominal-sized ATS movement upstream of the turbine, it is progressively avoided as the monolith size is decreased. Despite the potential for ATS downsizing of the pre-turbine location [28], the complete removal of the turbocharger lag requires very small monoliths complemented with post-turbo counterparts for full pollutant abatement or an electric-assisted turbocharger [29]. In this work, the tested pre-DOC produced a very slight turbocharger lag, as observed in intake manifold pressure and turbocharger speed represented in Figure 6 for the more demanding high- and extra-high-speed  
Figure 4: Engine-out opacity and cumulative NOx emission in cold and warm WLTCs with baseline ATS and pre-turbine pre-DOC configurations.