## 4 Conclusions  
The main objective of this study was to assess the VGO technology to attain a better performance at low engine speeds in either steady state or transient working conditions. First, comparison of sole compressor maps is included to illustrate two VGO prototypes and how the closed vs. open configurations differ in efficiency and pressure ratio terms. For this study, both experimental and modeling-based assessments were performed. Experiments were conducted at 1000 and 1500 RPM to develop and validate a 1D gas dynamics model in GT-Power software. An extended boost pressure study in steady state and transient analysis was established, taking advantage of the robust model and understanding the potential benefits of using VGO architecture. These model assessments were conducted to avoid engine tests operating beyond the engine, turbocharger, or test bench's safety limits, and to avoid any bias from changes in the scope of the experiments, such as environmental conditions. The modeling assessment includes VGO prototypes under two main operating conditions: steady states at low-end and sudden tip-ins to analyze differences in transient response. As a result of the study, the following points were deduced:  
* VGO closed configuration extended the operating characteristic map into lower mass flow rates stable operative range (retarding surge). In other words, the closed VGO improves surge margin and compressor efficiency. In the experiments, VGO closed configuration showed an improvement of approximately 0.6 bar BMEP higher at 1000 RPM. The model showed slightly improved behavior (with respect to the experiments), reaching a gain of 0.94-0.72 bar BMEP (depending on the TC unit) for the open configuration. The main difference between the experiments and the model relies on the daily dispersion of the experiments, which is avoided in the modeling stage. * However, no benefits were identified at 1500 RPM, where T2 was the limitation for engine output. The closed VGO configuration efficiency was predicted to be lower than the open one, about 0.35%, in both theTC units analyzed. The differences in the model were about 0.43-0.37 bar less BMEP under the closed configuration, depending on the TC prototype. * Transients at sea level showed improved breathing capabilities of the engine using VGO-closed by improving engine response at 1000 RPM and 1500 RPM. In this analysis, a VGT evolution profile was fixed (and the same) for both VGO configurations to avoid any impact on the results coming from any kind of VGT control. In conclusion, the time to achieve the same BMEP was reduced in the closed configuration, about 0.12 s and 0.3 s (at 1000 and 1500 RPM, respectively), while the engine output evolution also showed a 1.5-3.2 % improvement in produced work, which, in any case, represents the engine's drivability under highly transient demand. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Speed** & **Configuration** & **Maximum BMEP** & **Time to 90\% BMEP Open** & **Work in 4s** \\
**(RPM)** & & **(Bar)** & **(s)** & **(kJ)** \\ \hline
1000 & Open & 12 & 3.36 & 75.68 \\
1000 & Closed & 13 & 3.24 & 76.87 \\
1500 & Open & 24 & 2.28 & 187.10 \\
1500 & Closed & 25 & 1.98 & 193.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Extended boost analysis for TC\(-\)1 in transient operation. **Author Contributions:** Conceptualization, A.G.-V. and A.D.; methodology, A.G.-V. and A.D.; software, A.G.-V. and A.D.; validation, A.G-V. and A.D.; formal analysis, A.G.-V. and A.D.; investigation, A.G.-V. and A.D.; resources, J.R.S., H.C. and S.G.; data curation, A.G.-V. and J.R.S. ; writing--original draft preparation, J.R.S., H.C., A.G-V. and A.D.; writing--review and editing, A.G.-V. and A.D.; visualization, A.G.-V. and A.D.; supervision, J.R.S., H.C. and S.G.; project administration, J.R.S., H.C. and S.G. All authors have read and agreed to the published version of the manuscript.