## 4 Conclusions  
The technical viability of eliminating the compressor anti-surge valve (ASV) in a turbocharged SI engine, equipped with a low-pressure EGR loop, was assessed in this study, in the interest of cost reduction. To this end, a series of tip-out maneuvers from full to zero load at 1500 rpm was tested, without the compressor ASV, to evaluate two alternative strategies to this anti-surge control system: the optimization of throttle closure to achieve a load decrease as fast as possible without compressor surge, and the application of different closings on the intake flap, located upstream of the compressor, to accelerate the maneuver. The time derivative of the filtered crank-angle resolved pressure signal at the compressor outlet was used as the main surge indicator.  
The optimization of throttle actuation with no ASV, while keeping open the intake flap, enabled to ensure the compressor stable operation for the whole tip-out. Nevertheless, the maneuver duration was extended for around 1s, as compared with the time to reach 10% load in the reference case with ASV. When the intake flap was also actuated, the load decrease could be accelerated. The tip-out duration in the case where the compressor inlet pressure was reduced up to 0.65 bar was just 0.4 s longer than in the reference case. In the case that the compressor inlet pressure was further decreased until 0.33 bar, unnoticeable differences in terms of engine load were observed with respect to the reference. Therefore, it is concluded that the role of the compressor ASV can be assigned to the intake flap, required at the compressor inlet for high EGR operation, without compromising the compressor stable operation and engine load reduction.  
Conceptualization, J.G., J.d.l.M., S.G. and T.B.; methodology, J.G., H.C. and J.d.l.M.; software, H.C.; formal analysis, H.C. and D.G.-D.; investigation, J.d.l.M. and D.G.-D.; resources, S.G. and T.B.; writing--original draft preparation, H.C. and D.G.-D.; writing--review and editing, J.G., J.d.l.M., H.C., D.G.-D., S.G. and T.B.; visualization, D.G.-D.; supervision, J.G., J.d.l.M. and T.B.; project administration, J.G. and S.G.; funding acquisition, S.G. and T.B. All authors have read and agreed to the published version of the manuscript.  
Figure 9: Time evolution of compressor outlet pressure (**a**), air mass flow at filter outlet (**b**), turbocharger speed (**c**), and engine load (**d**) for Cases A, C, E, F, and G.  
**Funding:** This research received no external funding.  
**Data Availability Statement:** The data that support the findings of this study are not available due to privacy restrictions.  
**Acknowledgments:** The authors would like to thank [PERSON], [PERSON], and [PERSON] for their invaluable work during the experimental campaign.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.  
**Nomenclature**  
_Acronyms and abbreviations_  
AMF air mass flow  
ASV anti-surge valve  
CAD crank-angle degrees  
cyc. cycle  
ECU engine control unit  
EGR exhaust gas recirculation  
SI spark ignition  
VGT variable geometry turbine  
VVT variable valve timing  
w. with  
wo. without  
_Symbols_  
\(f\) frequency  
\(p\) pressure signal  
\(\widetilde{p}\) filtered pressure signal (\(f_{C}\) = 30 Hz)  
\(\overline{p}\) moving average pressure signal (\(f_{C}\) = 10 Hz)  
\(\widetilde{\widetilde{p}}\) time derivative of \(\widetilde{p}\)  
\(\widetilde{\widetilde{p}}\) time derivative of \(\overline{p}\)  
_Subscripts_  
1 compressor inlet  
2 compressor outlet  
c cut-off  
corr corrected