## 3 Results and Discussion
### \(O_{2}\) Production System Design Process Results
#### 3.2.1 Turbocharging System
The reason is that the cooler between the compression stages leads to a lower air temperature after the turbocharging system compared to the other designs studied (it can be seen in Figure 13), so now the main purpose of the burners is to heat the air up to the MIEC requirement instead of increasing the O2 partial pressure ratio, because the two-stage air compression is high enough to reach a very good outlet air pressure. This change in the burner operating objective explains why the EGR line burner is not needed for this configuration at full engine load and the exhaust gas line burner is sufficient to heat the air to the required  
Figure 12: Sweep of the bypass valve lift of the serial turbocharging system. value at the MIEC. The lowest fuel consumption is obtained when the TCH-2 NA turbine scale factor is around 50%, where the temperature of air entering the MIEC is high enough without further reduction in O\({}_{2}\) partial pressure ratio in the MIEC. It is important to highlight that this study relies on two main parameters related to the MIEC operation: the ratio of O\({}_{2}\) partial pressures between the feed and permeate side, and the temperature of air coming in the membrane. These parameters have a proportional and opposite tendency during the sweep of the scale factors, as shown in Figure 11a-c. The fuel consumption, as well as the air temperature reaching the MIEC, follows a somewhat opposite trend to the O\({}_{2}\) partial pressure ratio achieved by the turbocharging system, as long as the air temperature entering the heat exchanger network is not too far from what the system requires, in which case the fuel consumption will depend on the latter. Once each turbocharging configuration has been examined, the arrangements are compared using Figure 13, which shows the main variables that define the system efficiency, in order to subsequently select the most energy-efficient one. As can be seen in the graph, the two-stage air compression configuration achieves the highest ratio of O\({}_{2}\) partial pressures and the highest air mass flow despite the lower temperature of the air entering the MIEC. This turbocharging layout achieves the best possible conditions for the membrane operation, which results in the lowest fuel consumption in the burners. Before continuing with the heat exchanger network evaluation and the membrane area sweep, the selected turbocharging configuration is evaluated for all engine speeds. As can be seen in Figure 14a, the intake air mass flow entering the MIEC is almost constant for the first three operating points, regardless of the O\({}_{2}\) target for each of them. In this range, the trends of the air temperature and O\({}_{2}\) partial pressure ratio are opposite to that shown in Figure 11c, but this time the fuel consumption in the burners is proportional to the latter because the size of the turbochargers is the same, and therefore the decisive parameter is the temperature. At full load, the air mass flow increases because the growth of the air temperature with respect to the preceding point is not significant, and the other parameters must compensate for the MIEC requirements to generate the O\({}_{2}\). Figure 14b-e show the compressor and turbine maps for the optimum size found of turbochargers TCH-1 NA and TCH-2 NA for all engine operating points. In these, confirming the previous explanation, the first three engine speeds are very close (indeed, 72.7 rpm points overlaps 63.5 point in the turbocharger maps) and the full load point presents more pressure ratio and reduced mass flow. Figure 13: Comparison of turbocharging configurations studied at full load engine operating point.