## 2 Material and Methods
### Experimental Setup  
A HSDI diesel Euro 6d-Temp engine for passenger car applications was used to carry out this work. Table 2 summarizes the main characteristics of the engine. The engine was equipped with variable geometry turbine (VGT) as well as HP- and cooled LP-EGR. The aftertreatment system was composed by a close-coupled diesel oxidation catalyst (DOC) and a wall-flow particulate filter coated as selective catalytic reduction system (SCRf) [35]. This study is focused on the DOC, whose main characteristics are listed in Table 3.  
The engine was installed in a test bench that was equipped with an Elin EBG dynamometer controlled by AVL PUMA Open 2. According to the scheme that is shown in Figure 1, the most relevant characteristics of the experimental facility was the coupling of the engine with Horiba MEDAS, an altitude simulator that was developed by CMT-Motores Termicos [33; 34]. MEDAS provides variable working pressure to the engine as a function of the requested driving altitude. This simulator is coupled to the engine intake, exhaust, and sump, and it communicates with the engine control unit [36; 37]. Additionally, the ambient temperature was imposed by means of the MEDAS temperature module (MTM), which was placed upstream of the MEDAS core module. With this setup, the driving conditions can be emulated in conventional test cells, with a variable altitude from sea-level to 5000 m and ambient temperature ranging between \(-15\) and \(45\ ^{\circ}\)C.  
\begin{table}
\begin{tabular}{l c} \hline \hline Engine type & HSDI diesel \\ Emissions Standards & Euro 6d-Temp \\ Number of cylinders & 4 in line \\ Displaced volume [cm\({}^{3}\)] & 1461 \\ Bore [mm] & 76 \\ Stroke [mm] & 80.5 \\ Number of valves & 2 per cylinder \\ Compression ratio & 15.2:1 \\ Rated power @ speed & 81 kW @ 4000 rpm \\ Rated torque @ speed & 260 Nm @ 1750–2500 rpm \\ Fuel Injection & Common-rail direct injection \\ Turbocharger & VGT \\ EGR & HP- and cooled LP-EGR \\ ATS & Closed-coupled DOC + SCRf \\ \hline \hline \end{tabular}
\end{table}
Table 2: Main specifications of the engine.  
\begin{table}
\begin{tabular}{l c} \hline \hline Substrate & Cordierite \\ Washcoat & Alumina oxide and \(\beta\)-zeolite \\ Coating & Pt \\ Monolith length [mm] & 140 \\ Monolith diameter [mm] & 120 \\ Channel cross-section & Square \\ Cell density [cpsi] & 400 \\ Cell size [mm] & 1.04 \\ Wall thickness [mm] & 0.23 \\ Catalytic area [m\({}^{2}\)] & 3.95 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Main characteristics of the oxidation catalyst.  
The engine was fully instrumented to evaluate its performance and the abatement of CO and HC in the DOC. The engine speed and torque were set by the dynamometric brake. The air mass flow was obtained from the flow-meter that was monitored by engine control unit and accessed by ETAS INCA software. The fuel mass flow was measured with an AVL 733S gravimetric balance. The temperature and pressure were also measured along the flow path in the intake and exhaust lines while using K-type thermocouples and piezoresistive sensors, respectively. The exhaust gas composition was measured using Horiba Mexa-One gas analyzers, which sampled the exhaust gas upstream and downstream of the DOC in order to determine the CO and HC conversion efficiencies. A gas sample was also obtained at the intake manifold allowed for measuring the CO\({}_{2}\) concentration and determining the EGR rate. Only LP-EGR was used in this work.