## 2 Method
### Experimental Setup  
The engine used for this application is a new generation 4-cylinder, 2-L displacement, direct injection, and turbocharged compression ignition automotive application engine. This engine is already homological and commercialized, and hence is already part of pollutant emissions legislation. However, in line with the idea of minimizing greenhouse gas emissions, the engine is being upgraded from several perspectives, as evidenced in this study, from the turbocharger point of view. Some of the main specifications of the engine are included in Table 1. INCA software from ETAS company controls the engine actuators, such as injectors and VGT. Figure 2 shows the schematic layout of the engine test setup. It comprises a water-charge air cooler (WCAC) in the inlet manifold. Although an exhaust gas recirculation (EGR) control is not included in this work, the engine contains a low-pressure exhaust gas recirculation loop. The information related to the sensors employed in this study is provided in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Engine Type** & **4 Stroke Compression Ignition** \\ \hline N\({}^{o}\) of cylinders & 4 \\ Bore to Stroke & 84 \(\times\) 90 (mm) \\ Compression ratio & 15.45 \\ Turbocharged & Yes (VGT) \\ EGR & Yes (low and high pressure) \\ Injection system & Direct injection (common rail) \\ Charge cooling & Water charge air cooling \\  & Diesel Particle Filter (DFF) \\ After-treatment & Diesel Oxidation Catalyst (DOC) \\  & Selective Catalytic Reduction (SCR) \\ Variable Valve Technology & No \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Engine specification. Figure 1: Scheme of the work developed in this study. Several boost pressure operations in full load conditions have been conducted with Turbocharger 1 (TC\(-\)1) at constant engine speeds (1000 and 1500 RPM). In these tests, the VGT position was closed, until at least one thermomechanical limitation was reached. The results of the experiments are shown in Figure 3, dealing with BMEP, compressor outlet temperature, turbine inlet pressure, break specific fuel consumption (BSFC), and compressor inlet temperature as a function of the achieved boost pressure. The limitations taken into account are detailed in Table 3. These limitations had to be considered during the experiments and in the last modeling activities. Surge represents another limitation in addition to the ones in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline
**Variable** & **Equipment** & **Range** & **Accuracy** \\ \hline Speed & Dynamometer & 6000 RPM & 0.03\% fs \\ Torque & Dynamometer & \(\pm\)415 Nm & 0.05\% fs \\ Cylinder Pressure & AVL Z133 & 0 to 250 bar & 0.3\% lin. \\ Amplifier & Kistler 5011B & \(\pm\)10 V & - \\ Air Flow & AVL Flowsonix Air & 20 to 720 kg/h & 2\% \\ Fuel Flow & AVL 733S Fuel Meter & 0–150 kg/h & 0.2\% \\ Temperature & K-type Thermocouple & \(-\)200 to 1250 \({}^{\circ}\)C & 1.5 \({}^{\circ}\)C \\ Mean Pressure & Kistler Piezo-Resistive Transmitters & 0–10 bar & 0.2\% lin. \\ Gases Analysis & Horiba mexa 7100 DEGR & - & 1–4 fs. \\ \hline \end{tabular}
\end{table} 
Table 2: Engine test bench instruments. Figure 2: Schematic layout of the engine test bench. As depicted in Figure 3, higher boost pressure was achieved at 1000 RPM with the closed VGO configuration, due to the extended margin in the surge line. In other words, for the same engine speed, surge occurred at lower boost pressure at the open configuration, limiting the achieved boost and BMEP. Furthermore, in the closed series, turbine inlet pressure was systematically lowered along the complete 1000 RPM series. All these differences are presumably attributed to the VGO impact on the TC performance. First, the efficiency improvement in the compressor at the low-end operative range diminishes the required p3 (with respect to the open configuration). Secondly, the retarding surge line therefore allows for an even slightly higher boost in combination with lower engine backpressure.