## 2 Method
### Experimental Setup
All this directly impacted BMEP, which was slightly higher (about 0.6 bar higher). As a result, BSFC values also showed subtle benefits towards the closed configuration; in the best cases, 2 g/kWh improvement was achieved. No significant turbocharger speed differences were observed. The atmospheric conditions all support the previous discussion: despite higher T1 values in the closed series (which imply higher compressor inlet temperature),  
Figure 3: Results of Experiments for TC\(-\)1: BMEP (**A**), Compressor outlet temperature (**B**), Turbine inlet pressure (**C**), BSFC (**D**), Compressor inlet temperature (**E**), and Turbocharger Speed (**F**). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable** & **Value** & **Units** \\ \hline Engine BMEP & 26 & bar \\ Turbine inlet pressure (p3) & 4.5 & bar \\ Boost pressure (p2) & 3.5 & bar \\ TC speed & 250 & kRPM \\ Turbine inlet temperature & 850 & \({}^{\circ}\)C \\ Compressor outlet temperature & 205 & \({}^{\circ}\)C \\ p3-p2 (pressure difference) & 1 & bar \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Engine limitations. a lower compressor outlet temperature takes place, compared to the open series, which is in line with the improved efficiency of the compressor (at closed configuration) in such operative conditions. However, for 1500 RPM, a lower boost (hence, lower BMEP) was achieved in the closed configuration. The reason is that the compressor outlet temperature limit of 200 \({}^{\circ}\)C, shown in Figure 3B, was reached at a lower boost pressure level. This could be attributed to the unequal ambient temperature. The results of the temperature differences or compressor efficiency (or a combination of both) can be seen in Figure 3E. However, the turbine inlet pressure differences were negligible, as well as BSFC figures. Also, TC speed differences did not show any discrepancy. This shows that no change in the turbine-compressor matching emerged because of the VGO operation (at least for the mass flow range analyzed). In any case, the conclusions may be biased by dispersions arising during the experiments and reducing testing productivity. This is the primary motivation for developing an engine model to overcome this problem. TC\(-\)1 operates with the compressor orifics as fully open or fully closed. The switching between the positions is carried out manually, and a dedicated actuator is used for locking the part. A schematic layout of the VGO technology is depicted in Figure 4. Figure 4A shows the VGO architecture in an open configuration, while Figure 4B shows VGO technology in the closed position. As Figure 4 shows, the minimum diameter at the compressor inlet (\(c\)) was restricted in the closed configuration (\(O>c\)). Following the procedure explained in [27], the compressor maps in the respective configuration were obtained. The resulting maps for TC\(-\)1 under open and closed configurations are included in Figure 5, as well as the speed and efficiency maps. Furthermore, Figure 5 includes, in black dotted format, the experimental information constituting the compressor map for the closed configuration. The closed VGO map has also been translated into the open map plots to show the impact of this mechanism on the open configuration (the default one). The information outside of the dotted area in the closed configuration corresponds to extrapolated information. The experimental points regarding the engine test bench have also been included in the compressor maps. The surge limit was reached in the 1000 RPM series (red asterisks for closed and blue circles for open). However, for the closed configuration, the surge limit was slightly less restrictive and allowed for a further boost in the low-end engine speed, as previously described in Figure 3.  
The open configuration of the compressor showed a wider operative area, specially extended at high, corrected mass flows. The open configuration also showed that the maximum efficiency operative area is wider.