**4. Conclusions**

In this work, an experimental study was carried out to quantify the influence of WiGEs on combustion, performance and exhaust emissions at a high-load knock-limited operation of a small turbocharged PFI spark-ignition engine. The emulsions are produced through a micro-channels emulsifier, potentially capable to work inline, without the addition of surfactants. Two different water contents in emulsion were tested, 10% and 20% by volume, respectively. As in the present work, the emulsion is produced offline, and a small amount of nonionic surfactant (SPAN80 0.2%v) is added to preserve emulsion stability. Engine tests are performed with full gasoline and emulsions injected in the intake runners and considering a reference engine point at a speed of 3000 rpm and 16 bar IMEP. For the selected operating condition, the standard ECU calibration is applied in gasoline mode, running the engine under rich air/fuel mixture (λ = 0.9) and a spark advance (SA = −10 CAD AFTDC) at knock limit. Starting from the above reference point, a spark timing sweep is realized for each WiGE up to the new knock-limited condition, keeping the IMEP constant. Further, the cooling and dilution effects of water evaporation in WiGE has allowed us to work at stoichiometric condition. Engine overall performance, in-cylinder pressure traces and pollutant emissions are measured in each tested condition.

The analysis of experimental in-cylinder pressure cycles and burn-rate profiles show that the water presence in the combustion chamber produces a cooling and dilution effect of charge, inducing an increasing slowdown of combustion velocity and a lowering of pressure peak with the water content. On the other hand, the cooling and dilution effects of WiGE allow us to mitigate the knock occurrence and, consequently, to advance the spark timing, reaching an optimized combustion phasing.

A decreasing TIT trend with spark timing is observed, and the measured TIT level under stoichiometric mixture at the most advanced spark timing reaches a quite similar value to the one attained in the reference gasoline condition.

Relevant ISFC benefits are realized with WiGE 10 (3.7%) and WiGE 20 (7.1%) in stoichiometric mixture and optimized combustions.

Concerning the exhaust emissions, a comparison with the reference gasoline mode highlights a slight increase in HC emissions with a corresponding reduction in NO when using WiGEs. When switching at stoichiometric A/F ratio, the more complete combustion results in a certain reduction in HC, while major penalties for NO are found. One order of magnitude reduction in CO levels is obtained.

Summarizing, water-in-gasoline emulsions demonstrated to be a technique to improve fuel consumption at medium/high loads of turbocharged spark-ignition engines, enabling the stoichiometric combustions, while preserving the turbine blades from severe thermal stresses.

**Author Contributions:** Conceptualization, methodology and formal analysis, L.M., C.T., L.T., P.M. and J.B.; experimental setup, investigation, data acquisition and data curation, L.M., C.T. and L.T.; writing—original draft preparation, L.M., C.T., L.T., P.M. and J.B.; writing—review and editing, visualization and supervision, L.M., C.T., L.T., P.M. and J.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by the Region Pays de la Loire, Chaire Connect Talent ODE program for outstanding research in the field of energetics.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors gratefully acknowledge the support of the Region Pays de la Loire, Chaire Connect Talent ODE program, for outstanding research in the field of energetics. The authors thank Alfredo Mazzei and Bruno Sgammato for the technical support in the experimental campaign.

**Conflicts of Interest:** The authors declare no conflict of interest.
