## Setup and Methods  
For this study, a 4-cylinder 1.6 L diesel engine, whose main characteristics are summarized in Table 1, was employed. The fuel injection system included a high-pressure pump capable of delivering up to 200 MPa, a common-rail and Denso G4.5s solenoid fuel injectors with 8-holes, 155\({}^{\circ}\) included angle and a flow number of 340 cc/s. As sketched in Figure 1, the engine was equipped with a variable swirl actuator and two cooled exhaust gas recirculation (EGR) systems, i.e., high and low pressure. The high-pressure exhaust gas recirculation (HP-EGR) line extracted the gases from the exhaust manifold through the cylinder head. This HP-EGR route was composed of a valve, a cooler with a bypass route to avoid potential fouling issues at low load conditions, and a mixer introducing these exhaust gases on one side of the intake manifold. Instead, the low-pressure EGR (LP-EGR) line took the gases at the outlet of the close-coupled aftertreatment system and integrated them upstream of the compressor. The amount of recirculated gases in the low-pressure system was controlled by a three way valve, which throttled the intake flow before the compressor when the pressure difference in the LP-EGR line was low. Table 2 lists the main instrumentation used in this work. The engine was coupled to an asynchronous dynamometer controlled by the HORIBA SPARC automation system accessed through the STARS user interface. The dynamometer can be used for engine dynamic tests being able to reproduce driving cycles as the WLTC tested in this work. An AVL 733S fuel balance measured the fuel consumption. ETAS INCA v7.1 was used to register the main actuations performed by the electronic control unit (ECU). The emissions were monitored by an AVL 439 opacimeter placed at the turbine outlet and up to three samples of HORIBA MEXA-7100 DEGR to analyze the composition of the exhaust gases in different points of the exhaust line depending on the ATS layout. Figure 1 shows the case of installed pre-turbine pre-DOC. The same systems measured the CO\({}_{2}\) mole fraction at the compressor outlet, for the determination of the low-pressure EGR rate, as well as at the intake manifold, to obtain the  
Figure 1: Scheme of the engine (with pre-turbine pre-DOC) and experimental setup. combination of both EGR systems when used simultaneously. The air path was monitored with an ABB hot-wire anemometer placed upstream of the engine airbox. Pressure and temperature probes were installed at the most significant sections of the intake and exhaust lines. Finally, a Picoturn sensor was placed in the compressor housing to measure the instantaneous rotational speed of the turbocharger. The experimental campaign was driven to the analysis of the impact of adding a small-sized DOC at the exhaust manifold outlet upstream of the variable geometry turbine (VGT) in order to improve the removal of HC and CO emissions. The nominal ATS was composed of a closed-coupled DOC, coated with Pt and a zeolite layer for HC adsorption, followed by a SCRF. In this work, the urea injection was canceled to focus the analysis on the engine-out NOx emission and CO and HC oxidation in the pre-DOC and DOC. The geometry of these catalysts is detailed in Table 3. The pre-DOC substrate was metallic with Pt coating but without HC adsorption capability (no zeolite layer). Metallic substrates are interesting for its use in pre-turbine location because of their thin walls, which provide high geometric surface area and low pressure drop in comparison to ceramic counterparts. In addition, these substrates have good mechanical durability and resistance to thermal shock at the same time ease the adaptation as electric catalysts to compensate the thermal inertia effects (not considered in this work). The pre-DOC was obtained from a full-size post-turbo metallic substrate cutting it with the diameter of the exhaust manifold outlet (turbine inlet) to reduce the added canning volumes. The original length (85 mm) was kept providing a volume of 60 cm\({}^{3}\).