## 4 Conclusions  
Water injection is promising technology for the mitigation of the knocking tendency of high-power density spark ignition engines, simplifying the adoption of high compression ratio levels and reducing the necessity for mixture enriching and spark retarding. The final goal is fully exploiting the efficiency potential of highly downsized, turbocharged spark ignition engines that have been rapidly spreading on the market in recent years. The mixture temperature's control potential is related to the water's high latent heat of vaporization, and hence, despite the specific technology used to implement water injection, either injected directly in the combustion chamber or in the inlet runner, the water spray evaporation rate is crucial. To be more detailed, rapid and complete water spray evaporation enables controlling the mixture's temperature without significant drawbacks, such as excessive water consumption (water/fuel ratios in the range of 0.2-0.5 are quite common for full load operation) or potentially destructive lubricant oil dilution. Port water injection (PWI) is currently considered the best compromise among system cost, complexity and efficacy. In the present paper, a deep analysis of the effects of both the water injection pressure and temperature on the resulting spray evolution and sizing characteristics was carried out separately. The ranges for both the pressure and temperature were chosen in order to be fully compliant with the low-pressure injection technology, with the water temperature and injection pressure below 110 \({}^{\circ}\)C and 11 bar,g, respectively. The obtained results can be summarized as follows:  
* The injected mass per shot was significantly affected by the injected water's temperature. Increasing the water temperature from 20 \({}^{\circ}\)C to 110 \({}^{\circ}\)C caused a decrease of about 7% for the injected mass, depending on the injection pressure level and injection duration. This evidence seems to suggest the possible necessity of redesigning the injector coil in the case of high-temperature operation. Figure 13: Effect of the injection pressure and water temperature on drops’ diameter density probability functions. (a) P\({}_{\text{inj}}\) = 7 bar,g. (b) T\({}_{\text{w}}\) = 55 \({}^{\circ}\)C.  
In terms of the global spray evolution, no dramatic changes were observed for the spray tip penetration or cone angle in the examined injection pressure and temperature ranges which, in the present analysis, did not trigger flash boiling phenomena. Complete flash boiling development, with injection in sub-atmospheric conditions presumably not relevant for water injection operation, would require water temperature levels possibly not compatible with the standard injection components' integrity or durability. * The spray atomization quality was sensibly affected by the water temperature's increase, with a benefit in terms of the Sauter mean diameter between 8 and 15 \(\upmu\)m in the examined conditions. At the same time, the observed injection pressure effect was more significant, with the Sauter mean diameter reduction being approximately 20 \(\upmu\)m and the injection pressure increasing from 5 bar,g to 11 bar. The obtained results seem to confirm that there is still significant potential in the optimization of the water spray characteristics, mainly in terms of the atomization quality. To be more detailed, adequate water temperature control could assist the increase of the injection pressure in obtaining an adequate sizing level to speed up the evaporation process, contributing to water injection technology spreading. Conceptualization, L.P.; methodology, L.P. and G.B. ; software, G.B. ; validation, L.P., G.B. and G.M.F. ; formal analysis, L.P., G.B. and G.M.F. ; investigation, G.B. and G.M.F. ; resources, L.P.; data curation, L.P., G.B. and G.M.F. ; writing--original draft preparation, L.P.; writing--review and editing, L.P., G.B. and G.M.F. ; visualization, L.P., G.B. and G.M.F.