## 1 Introduction  
The environmental impact, in terms of global greenhouse effect and pollutant emissions, of the automotive sector is considerable. Hence, severe limitations are in force and designed for, in particular, the adoption of challenging CO2 emission targets for the next few years [1, 2, 3]. Consequently, the automotive industry is quickly adopting innovative technologies, globally aiming for a rapid increase in automotive powertrain efficiency. The leading method followed to improve powertrain efficiency is electrification, with the adoption of a progressively more significant energy storage capacity and electric propulsion system power. The final stage of this evolution is foreseen to be the battery electric vehicle, which ensures a zero CO2 local emission operation. In this scenario, the internal combustion engine will still play a significant role for many years [4] due to the complexities and costs related to the actual implementation of the electrification path. Accordingly, along with electrification, a significant evolution of the gasoline engine to improve its efficiency is mandatory. The widespread adoption of gasoline direct injection (GDI) in spray-guided or pre-chamber configurations, turbocharging coupled with downsizing and downspeeding, higher compression ratios and application of the Miller cycle seem to be the most interesting innovation lines [5, 6]. Unfortunately, many of the aforementioned technologies cause a drastic increase in the knocking tendency due to the increased charge temperature before and during combustion, thus restraining the potential benefits in terms of engine efficiency. As a matter of fact, particularly for high-performance engines, the knock tendency in high load conditions is currently controlled by reducing the spark advance and by enriching the fuel/air mixture, resulting in an efficiency penalty and in a restriction of the catalyst operating area with an increase of CO\({}_{2}\), CO and HC emissions. In this frame, water injection technology can be used as an alternative way to control the charge temperature at the end of the intake process and during the combustion phase, thus reducing the risk of abnormal combustion in downsized and highly boosted GDI engines. Water injection can potentially enable \(\lambda=1\) operation in high-load and high-speed operation, even adopting high compression ratios [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17], thereby gaining significant benefits in terms of engine efficiency while preserving the after-treatment system efficacy in controlling exhaust emissions. Water injection can be implemented as low-pressure injection in the intake runners (PWI), as high-pressure injection in the combustion chamber (DWI) or as direct injection of a water/gasoline emulsion. Along with the potential benefits in terms of air charge temperature control, clearly water injection has different potential drawbacks, such as a possible lubricant dilution due to the liquid's impact on the cylinder liner. Potentially catastrophic lubricant dilution can be caused by an incorrect match among the spray's global shape with the inlet duct geometry (for PWI systems) or the combustion chamber (for DWI systems) or, in general, by a slow water evaporation rate. Furthermore, the water deposition on the duct and cylinder walls limits the air charge cooling effect, increasing the water flow rate required to effectively control the knock tendency, resulting in water-to-fuel rate ratios in the range of 0.2-0.5 in full load conditions. Among the possible different schemes, port water injection (PWI) is currently considered the most attractive as a compromise between efficacy and cost, being significantly higher for direct water and water/gasoline emulsion injection systems, including the eventual water recovery technologies from the exhaust stream [18].