## 2 Experimental Setup and Procedures
This condition was set as representative of the overfueling strategies adopted in downsized engines at high loads: knock occurrence is avoided by delaying the combustion phasing towards the expansion stroke with an increase in the temperature at the turbine inlet. For this reason, in order to preserve exhaust engine hardware, the ECU adopts fuel-enrichment strategies which reduce the exhaust gas temperature, with penalties over fuel consumptions. As an alternative knocking suppression technique to overfueling strategies, water was injected in the engine through emulsions with gasoline. Two different WiGEs were tested: WiGE 10 and WiGE 20. The first one consists of an emulsion containing 10% of water and 90% of gasoline by volume, while the second one contains 20% water and 80% gasoline. As is well-known, an emulsion is a fine dispersion of two liquid phases. Details concerning the adopted emulsification technique can be found in Reference [18]. Briefly, two controlled fluxes of water and oil impact in a cross-section through properly dimensioned micro-channels. As a result of the impact, the water is atomized in microdroplets inside the oil matrix. Depending on the physical properties of the oil (viscosity, surface tension, etc. ), the emerging emulsion is stable for a variable time, which is long enough to be used in inline feeding systems (some hours before water separation). In the present case, the emulsion is produced offline, and a small amount of nonionic surfactant (SPAN80 0.2%v) is added to preserve the emulsion stability. First, the engine was run with gasoline in the reference engine operating condition. Then, at the same speed, gasoline was replaced with WiGE 10. The IMEP and the lambda values were kept the same as the reference condition by fine-tuning the injection duration (DOI) and plenum pressure, for a peer-to-peer comparison with reference gasoline case. Then, the spark timing was advanced until reaching the new knocking limited spark advance. Aim of the present work is to investigate the potential of WiGE injection as a knock mitigation strategy alternative to fuel enrichment. After the tests under a rich relative A/F ratio (\(\lambda\) = 0.9), WiGE injection duration was changed in order to reach a stoichiometric A/F mixture (\(\lambda\) = 1.0), while the IMEP was kept always the same of reference level. A spark timing sweep was performed under stoichiometric condition starting from the reference spark timing until reaching the new knock limit. This procedure was repeated for WiGE 20, and the overall test matrix is shown in the following Table 2. Gasoline and WiGE presented the same injection timings: \(-\)165 CAD ATDC. During the experiments, the turbine inlet temperature (TIT) was always kept below 950 \({}^{\circ}\)C, and the maximum in-cylinder pressure was below 85 bar (\(\pm\)5 bar), as that is the maximum allowable peak pressure to preserve the engine from mechanical failure of components. To this aim, the maximum boost level was automatically controlled in the range 1.8-2.0 bar, acting on the waste-gate valve opening. The coolant temperature was set at 85 \(\pm\) 1 \({}^{\circ}\)C, using a water heat exchanger. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Fuel** & \(\lambda\) [\(\pm\)**0.01**] & **ST [cad atdc]** & **DOI [cad]** & **P\({}_{\text{INT}}\) [bar]** \\ \hline
**gasoline** & 0.90 & \(-\)10 & 195 & 1.72 \\ \hline
**WiGE 10** & 0.90 & \(-\)10 & 230 & 1.74 \\ \hline
**WiGE 10** & 0.90 & \(-\)11 & 226 & 1.70 \\ \hline
**WiGE 10** & 0.90 & \(-\)12 & 223 & 1.70 \\ \hline
**WiGE 10** & 0.90 & \(-\)13 & 220 & 1.69 \\ \hline
**WiGE 10** & 1.00 & \(-\)10 & 215 & 1.78 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Test conditions.