**4. Conclusions**

The influence of a pre-turbine pre-DOC on the engine performance and emissions has been analyzed experimentally from driving cycle tests in cold and warm conditions. The pre-DOC consisted of a small-sized metallic monolith placed at the exhaust manifold outlet and whose diameter coincided with that of the turbine inlet.

The results indicate that the addition of the pre-turbine pre-DOC had relevant effects on the engine performance and emission abatement. Although the small monolith volume avoided the turbocharger lag, the reduced cross-section generated a high pressure drop upstream of the turbine affecting the gas exchange process. The direct effect was the deterioration of the combustion leading the engine-out CO, HC and soot emissions to increase as well as a small NOx emission decrease because of the increase of internal EGR.

Concerning the engine performance, the increase in soot emission led the SCRF pressure drop to increase causing, in turn, an additional increase of the exhaust manifold pressure with respect to the baseline ATS configuration. As a result, the fuel consumption was penalized in 3.2% and 3.9% in cold and warm WLTCs, respectively.

The increase of the engine-out CO and HC emissions with respect to the baseline ATS were ranging 50% in cold and warm WTLCs. This huge increase was not able to be balance out by the presence of the pre-DOC upstream of the turbine. The CO tailpipe emissions increased by 8% and 21% in cold and warm WLTCs, respectively. In the same conditions, HC did by 24% and 37%. The tailpipe percentage increments were lower than engine-out ones because of the high pre-DOC conversion efficiency. This was promoted by the positive fluid-dynamic conditions upstream of the turbine concerning high temperature and pressure, the last increasing the partial pressure of the reactant species and, hence, the reaction rate. However, these advantages were only evident to abate base-level emissions. The pre-DOC conversion efficiency fell against engine-out emission peaks generated during fast accelerations due to the high mass transfer limitations of small-sized monoliths. The main concern of this behavior is that the close-coupled DOC also reduced its conversion efficiency compared to the baseline ATS configuration. The higher emission peaks with respect to this configuration and the low base-level emissions because of their removal by the pre-DOC compromised the DOC performance. As a result, the overall pollutants conversion efficiency was improved when using the pre-DOC but was not high enough to offset the increase in engine-out emissions.

Considering these results as a part of the literature context, the potential of pre-turbine ATS configuration to increase the pollutants conversion efficiency under driving conditions is confirmed even for very small monolith sizes. This kind of geometry is also positive to reduce the damage on the turbocharger lag, the main concern of pre-turbine ATS layouts. However, the effects on the engine fluid-dynamics and, hence, on the fuel consumption and pollutants formation, penalizes this solution with respect to full-size pre-turbine ATS approaches. These alternatives can improve further the conversion efficiency because of the inherent lower mass transfer limitations at the same time the fuel consumption is highly reduced if the wall-flow filter is placed upstream of the turbine (high reduction in engine backpressure). The high thermal inertia of the monoliths and its managemen<sup>t</sup> based on e-turbocharger technology would remain as the main challenge to combine high pollutants conversion with fuel savings.

**Author Contributions:** Conceptualization, J.R.S. and P.P.; Formal analysis, P.P. and M.J.R.; Funding acquisition, J.R.S.; Investigation, J.R.S., P.P., J.D.l.M. and M.J.R.; Methodology, P.P. and J.D.l.M.; Project administration, J.R.S. and P.P.; Resources, J.R.S.; Supervision, J.R.S. and P.P.; Visualization, P.P., J.D.l.M. and M.J.R.; Writing—original draft, P.P. and J.D.l.M.; Writing—review and editing, J.R.S., P.P., J.D.l.M. and M.J.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research has been partially supported by FEDER and the Government of Spain through project TRA2016-79185-R and by Universitat Politècnica de València under a gran<sup>t</sup> with reference number FPI-2018-S2-10 to the Ph.D. student María José Ruiz.

**Conflicts of Interest:** The authors declare no conflict of interest.
