## 3 Results and Discussion
### Effect of Intake Flap Closure on the Surge Margin  
Another tip-out, tagged as Case D, was tested by using the same throttle actuation as in Case C and closing the intake flap from 100 to 6% opening in one engine cycle, with no delay between the two actuations. Figure 7 depicts the evolution of compressor inlet and outlet pressure (a), air mass flow at the filter outlet along with its corrected values (b) and turbocharger speed (c), and the operating path on the compressor speed map (d) for Case C and D, both with no ASV. Regarding Case D, \(p_{1}\) decreased up to a minimum value of 0.65 bar at around 0.8 s (Figure 7a), then it recovered as the throttle closed and AMF was reduced. It must be clarified that the intake flap was not reopened during the tip-out. Despite the \(p_{1}\) decrease leading to a faster reduction of the air mass flow in Case D from 0.2 to 1.2 s, slight differences between both cases were noticed in terms of corrected AMF (Figure 7b). This was reflected in the operating path on the compressor map (Figure 7d), where it is observed that there was even a margin to apply a more aggressive throttle  
Figure 6: Time evolution of compressor outlet pressure (**a**), air mass flow at filter outlet (**b**), turbocharger speed (**c**), and engine load (**d**) for Cases A, B, and C.  
closure in Case D at the start and at the end of the maneuver, given that the operating path of Case C set the surge limit a priori. Once the capability of intake flap closure to accelerate the maneuver was confirmed, three \(p_{1}\) levels were analyzed (considering its minimum value): 0.33, 0.65, and 0.87 bar. In the first case, load reduction was only achieved through the intake flap, while the throttle was kept open during the whole maneuver. For the other two \(p_{1}\) levels, throttle actuation was re-optimized by following a similar procedure to the one described in Figure 2. It should be stated that intake flap closing was demanded in one cycle, from full opening to its corresponding final position (2, 6, and 12%, in ascending order of \(p_{1}\)), and that no delay between the start of throttle and intake flap actuations was applied. Figures 11-13 (Appendix A) show the evolution of the compressor outlet pressure and its time derivative for three cases per each \(p_{1}\) level: the best tip-out for each intake flap closing, labelled as Case E (\(p_{1}\) = 0.33), F (\(p_{1}\) = 0.65) or G (\(p_{1}\) = 0.87), a tip-out with a more aggressive throttle actuation for the two highest \(p_{1}\) levels, in which surge was detected, and the best case with throttle only (Case C). It must be emphasized that no surge was found for the tip-out managed only with the intake flap (Case E), despite its closing in one cycle. For the sake of readability, a summary of the different cases analyzed is provided in Table 2, where throttle and intake flap actuations, \(p_{1}\) level, and the use of ASV in each case are specified. Figure 7: Time evolution of compressor inlet and outlet pressure (**a**), air mass flow at filter outlet along with its corrected values (**b**) and turbocharger speed (**c**), and operating path on the compressor speed map (**d**) for Cases C and D.  
Figures 8 and 9 depict throttle actuation and time evolution of the compressor inlet and outlet pressure, air mass flow at filter outlet, turbocharger speed, and engine load for five tip-outs: Cases A, C, E, F, and G. It should be reminded that the first one was tested with the anti-surge valve, while the remaining ones were performed without it. As observed in Figure 8a, the throttle could be closed two and six cycles faster in Cases F and G, as compared to Case C, due to \(p_{1}\) decrease up to 0.65 and 0.87 bar, respectively. The reduction of air mass flow and engine load is consequently accelerated. In particular, the maneuver duration in Case F was just 0.4 s longer than in the reference tip-out (Case A), considering, again, the time to complete the 90% of the total load reduction (Figure 9d).