## 1 Introduction
The vaporization process starts during the compression stroke; therefore, the water deposited against the liner evaporates extracting heat in part from the end gas and in part from cylinder walls with a drop in knock-suppression efficacy. Furthermore, a proper combustion chamber design is necessary to locate the nozzle, considering the limited available space left by inlet and exhaust four valves, the spark plug and the gasoline injector. On the other hand, port water injection does not allow us to fully take advantage of vaporization heat extraction from the end gas, and, at the same time, it has a limited influence on the volumetric efficiency, as the intake temperature is too low to induce water evaporation in the manifold. An alternative way of introducing water in a SI engine is under the form of an emulsion with gasoline. This solution shows some advantages compared to the standard separate injections of water and gasoline. Indeed, the water-in-gasoline emulsion (WiGE) technique requires a single injector per cylinder, a lower number of injection control variables and, consequently, a simpler control of fuel injection. Referring to the WiGE very few papers are available in the technical literature discussing its production, employment in SI engines and relative influence on combustion, performance and pollutant emissions. On the other hand, the use of water-in-diesel emulsion (WiDE) in diesel engines has been found to be an economical technique for improved combustion and fuel economy; the presence of water in diesel engines also leads to a drop in NOx formation and in the rate of soot particles [9]. In the case of WiGE, old research studies [10; 11] focused on the enhanced knock mitigation capabilities of emulsion fuel and its influence on performance of spark-ignition engines. As a starting point of Reference [10], the authors stated that when WiGEs are correctly used and the engine is properly adjusted to give optimum performance, water addition lowers gasoline octane number requirement and reduces the thermal stress on the parts of the cylinder-piston group without any loss of maximum engine power or torque and without increasing the specific gasoline consumption. The study reported in Reference [11] also confirmed the improved gasoline octane number with the employment of water addition in the form of emulsion. In Reference [12], an analysis on the effects of supplementing gasoline with water on spark-ignition engine performance and emissions is reported. Experiments on a single-cylinder engine, engine cycle simulations, and vehicle tests were performed. This research showed that the concept of adding water to gasoline presents some negative aspects consisting in the increased hydrocarbon emissions and decreased vehicle drivability. The resulting main benefits of water-gasoline fuels were the higher octane ratings and the decreased nitric oxide emissions. A recent paper reports the outcomes of a basic research on direct injection of gasoline/water emulsion. It shows the influence of the amount of water in a WiGE on the spray evolution in a high-pressure chamber [13]. The main evidence of this study is represented by the opportunity to optimize the direct injection of WiGE to improve fuel consumption and emissions in DISI engines. On the other hand, the scientific literature is very poor in the use of water-gasoline emulsions in modern turbocharged SI engines, and WIGE potential in fuel enrichment suppression has not yet been explored. Moreover, there is still a lot of progress to be made in the selection of the emulsifiers and in surfactant with the aim to guarantee the stability of the emulsion fuels for a regular on-board vehicle storage before its use [14]. Surfactants have an additional cost, and, according to the literature papers, they can lead to hazardous emissions during combustion, based on their chemical nature [15]; thus, the use of a stable emulsion with a very low level of surfactant is desired.