## 2 Method
### Experimental Setup
In all, Figure 5 shows that the closed configuration is mainly developed for the left side area of the compressor operative map, aiming to retard the surge line, and improving the efficiency at the low-end. This is the reason why this study mainly focuses on the low-end working conditions at both transient and steady-state operations. The second prototype of VGO TC was also evaluated in this study. This TC is labeled TC\(-\)2 and also employs VGT technology. TC\(-\)2 maps come from the supplier. As for the TC\(-\)1, the maps were adiabatized considering testing standards for diesel engines as per SAE J1826. The resulting maps for closed and open configurations are depicted in Figure 6. The black dotted series, which corresponds to the experimental data for the closed configuration, is also included in the open maps, for purposes of comparison. Here, the trends and conclusions observed at TC\(-\)1 were confirmed: the closed position of the mechanism extended the operative map towards lower mass flows and the efficiency in such area. However, the closed position restricted the inlet section, limiting the maximum compressor mass flow. As in TC\(-\)1, TC\(-\)2 shows the efficiency drop in the closed configuration and high mass flows. Regarding the transient tests, a harsh transient was forced by performing a pedal shift from 2 bar BMEP to full load. Figure 7 shows the case of the lower speed transient, where some improvement can be observed for the closed series regarding turbine inlet pressure: substantially lower p3 was required for similar engine output. Also, the compressor outlet  
of time. For the second, BMEP can be directly converted into torque according to (1), where \(n\) corresponds to the engine speed, \(V_{D}\) corresponds to cylinder displacement, and \(i\) to the number of power strokes per engine revolution, which is 0.5 in the case of a 4-stroke engine. The result from Equation (1) was applied at each time step and was then integrated and multiplied by engine speed, as Equation (2) shows, resulting in the developed work through the considered time. The integrated area was considered between the tip-in start until 4 s later, to ease the comparison between closed and open series. \[\mathrm{T}=\frac{\mathrm{BMEP}}{\mathrm{n}/\mathrm{i}}\mathrm{V_{D}} \tag{1}\]  
\[\mathrm{W}=\mathrm{n}\int\limits_{t_{0}}^{t_{0}+4}\mathrm{T}\,\mathrm{dt} \tag{2}\]  
Regarding BMEP evolution, a slight advantage of 0.1 s was identified towards the closed configuration at 1000 RPM, but no benefit was identified at 1500 RPM. Finally, integrating the trapped area below BMEP evolution showed an absolute improvement of 1.17 kJ, representing a 1.5% improvement of the closed VGO configuration over the open one and in terms of work developed through the maneuver. A summary is included in Table 4 for both transient evolutions. The differences identified may be attributed not only to the compressor technology but also to other uncontrolled variables, such as environmental boundaries or VGT control, which is purposely developed for the VGO open configuration while not explicitly adapted to the VGO closed design. All the previously established attributes may impact on the differences to some extent, motivating the complete engine model development.