## 3 Results and Discussion
### \(O_{2}\) Production System Design Process Results
#### 3.2.1 Turbocharging System
In this case, the TCH-2 ON turbocharger is bypassed, for which a sweep of the valve lift with respect to the turbine scale factor was performed to establish the lift that produces the lowest fuel consumption. Figure 12 displays this sweep for openings between 20% and 60%, since in this interval the turbocharger works properly. As can be seen, the lifts from 40% to 60% are very close and produce the lowest fuel consumption, independently of the turbine scale factor, so the selected one is 50% to avoid being at the limit of the range. Continuing with the analysis of the serial turbocharging system, as the N\({}_{2}\) mass flow through TCH-2 ON turbine decreases by its size reduction, there is less suction in the N\({}_{2}\) line leaving the MIEC, thus slightly decreasing the mass flow passing through the TCH-1 NA turbine; for this reason, the behavior of the TCH-1 NA turbocharger is opposite to that manifested by the parallel arrangement. As for the TCH-2 ON turbocharger, examining Figure 10b and Table 3, it is evident that the pressure ratio in the turbine increases even though the inlet pressure decreases, which occurs because the outlet pressure also drops, but at a higher rate. This increment in the pressure ratio and temperature of the flow entering TCH2-ON turbine results in an increase in the power output of the TCH-2 ON turbocharger. Figure 11: Influence of turbocharger size ratio on MEC operating conditions and burner fuel consumption in the configurations studied at full load engine operating point. (**a**) Parallel; (**b**) serial; (**c**) two\(-\)stage compression. Regarding the MIEC operating conditions and fuel consumption, the decreased performance of TCH-1 NA turbocharger implies a decrease in the air mass flow drawn from the atmosphere and in the air pressure after compression, leading to a lower O2 partial pressure on the membrane feed side. The improved power of TCH-2 ON generates a higher vacuum in the O2 flow line and, consequently, a decrease in the O2 partial pressure on the permeate side. Despite the decrease in O2 partial pressures on both membrane sides, the ratio between them depends on the rate of decrease of one with respect to the other. As evidenced in Figure 11b, from 72% to 68% of turbine scale factor, the slopes of both curves are almost equal, and from the last point, the slope of the partial pressure on the feed side is greater than on the permeate; for this reason, the O2 partial pressure ratio increases up to this range and then decreases. The lowest fuel consumption is found at 68% of the TCH-2 NA turbine scale factor, in the range where the ratio of O2 partial pressures is the maximum, and within this, where the temperature of the air entering the MIEC is higher, as concluded in the parallel configuration. * _Two-stage air compression_ (Figure 6): The third turbocharging configuration studied is another serial arrangement but, in this case, the two turbochargers work in full serial mode (i.e., no bypass valve) and both are located in the air flow line to obtain the O2 partial pressure gradient in the MIEC by compressing only the fluid going to the feed side. Since this configuration has the operating principle of the previous one, the TCH-1 NA turbo has the same tendency in performance variation (Figure 10c). Nevertheless, the TCH-2 NA turbocharger power first improves due to the increase in the pressure ratio and temperature of N2 entering TCH-2 NA turbine, but later decreases due to the corrected mass flow decrease. It is worth noting that, this time, the pressure ratio increases due to the rise in the inlet pressure of TCH-2 NA turbine caused by the conditions produced in the MIEC; in addition to the significant decrease in the inlet pressure of TCH-1 NA turbine (outlet of TCH-2 NA turbine). Although the turbocharger behavior in this case is very similar to the previous one, the MIEC and fuel consumption are different.