## 3 Results and Discussion
### Transient Engine Performance Assessment  
The following assessment regarding transient engine running condition is proposed to analyze further the potential benefits of using VGO technology. Here, the tip-in pedal position changes from 2 bar BMEP to full load in a second. The following tip-out is modeled before tip-in: AFR changes from a constant value of 16 to a pulse width modulation (PWM) \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Speed** & **TC Unit** & **Configuration** & **Maximum BMEP (Bar)** & **Maximum Boost (Bar)** & **Limitation** \\ \hline
1000 & TC\(-2\) & Closed & 13.91 & 1.51 & Surge \\
1000 & TC\(-2\) & Open & 13.19 & 1.44 & Surge \\
1500 & TC\(-2\) & Closed & 27.87 & 2.86 & T2 \\
1500 & TC\(-2\) & Open & 28.24 & 2.87 & T2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Extended boost analysis for TC\(-2\) in steady state operation. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Speed** & **TC Unit** & **Configuration** & **Maximum BMEP (Bar)** & **Maximum Boost (Bar)** & **Limitation** \\ \hline
1000 & TC\(-1\) & Closed & 13.99 & 1.53 & Surge \\
1000 & TC\(-1\) & Open & 13.05 & 1.44 & Surge \\
1500 & TC\(-1\) & Closed & 27.32 & 2.87 & T2 \\
1500 & TC\(-1\) & Open & 27.75 & 2.9 & T2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Extended boost analysis for TC\(-1\) in steady state operation. value which varies from 85 to 16, as shown in Figure 11A. Likewise, the VGT rack position is changed from 85% VGT opening to 40%, as shown in Figure 11B. This transient profile is maintained for both VGO configurations and engine speeds. The objective relies on keeping the same VGT evolution to avoid any impact from the experimental VGT signal evolution since, as previously stated, the actual VGT controller was explicitly developed for the open configuration of the VGO. Also, no boundaries dispersion or initial working conditions affect the simulation. The information in Figures 12 and 13 shows the results for tip-in engine running conditions at 1000 and 1500 RPM engine speeds, respectively. Red solid lines provide information on engine performance with VGO architecture in the closed position. In contrast, solid blue lines show results in VGO open condition. The area plot shows the percentage differences between VGO closed and open configurations through the running simulation time (see Equation (3)). \[\%\ \text{Difference}=\frac{\text{VGO}_{\text{closed}}-\text{VGO}_{\text{ open}}}{\text{VGO}_{\text{open}}}*100 \tag{3}\]  
At 1000 RPM, plotted in Figure 12, as soon as the transient starts, an improvement in BMEP becomes evident, reaching a peak value of 4.6%. BMEP differences originating in boost differences, as Figure 12B shows in absolute terms, and Figure 12F in a percentage. As the boost pressure increases, so does the intake air charge. As both simulated series are assessed under the same AFR evolution, more fuel is injected, leading to the enhanced engine BMEP performance in the VGO closed configuration. This is purely attributed to the differences in the compressor maps depending on the setup. In the closed VGO in particular, the compressor operates at a more efficient region in the map, thus leading to a compressor efficiency improvement of 8% in VGO closed configuration over VGO open architecture, as shown in Figure 12C,F. However, Figure 12H shows that the closed configuration leads to some decline in the turbine efficiency (oscillating between 0% and \(-1.6\%\)). Overall, considering the compressor and turbine performance benefits and drawbacks, there is an improved global performance in engine response. Similarly, at 1500 RPM, engine speed improvements in the transient simulation can be observed in Figure 13. Similar conclusions can be drawn: improved BMEP response, motivated by more considerable boost pressure (and more injected fuel), is supported by the compressor efficiency improvement when the VGO is closed. Enhanced compressor efficiency reaches a peak improvement of about 4%.