## 2 Materials and Methods  
The PWI injector (Bosch EV14) used for the present analysis was characterized in terms of the mean injected mass, global spray evolution, drop size and velocity. The main characteristics of the tested injector are reported in Table 1. The static flow rate was measured by the dINJ injection analyzer, a Zeuch method-type injection analyzer specifically designed to operate with low-pressure injection systems [8,22]. The injector under testing was fed with distilled water statically pressurized with nitrogen in the range from 5 bar,g to 11 bar,g (up to 15 bar,g only for the flow tests). Pressurized water was accumulated in a 100-cc reservoir, which was used as rail to directly feed the injector to have negligible pressure fluctuations during the injection event, according to the rules of JSAE2715. The rail structure was used as a fixture for an electric heater used to control water temperature. The feedback thermocouple was installed at the injector inlet in the same position as the Keller PAA M5 HB sensor (20 bar f.s., 50 kHz bandwidth, 1% accuracy) used to monitor the rail pressure. The injector current time history was acquired by a Pico TA189 probe (30 A, 100 kHz bandwidth) and averaged over 30 consecutive injection events. A schematic of the experimental setup is reported in Figure 1.  
In each operating condition, the mean injected volume was measured by a precision balance (Radwag PS 1000/C/2, resolution of 1 mg, accuracy \(\pm\)1.5 mg) during three repetitions of a 3000-shot sequence with a 10 Hz injection frequency. In previous studies (e.g., [8,23]), the effect of the test vessel's pressure and temperature on low-pressure water sprays was investigated, concluding that the air temperature's variation from ambient to 50-60 \({}^{\circ}\)C (typical of boosted conditions with an intercooler) has negligible effects on the spray's evolution and size. On the other hand, boosted pressure levels have the similar effect of a corresponding injection pressure reduction, with the pressure differential across the injector being the main driving force affecting the spray's evolution. As a consequence, in this research, the test vessel's pressure and temperature were maintained at 1 bar,a and 25 \({}^{\circ}\)C for all the operating conditions. In Table 2, the operating conditions used for the flow test are reported, evidencing the imposed water temperature at the injector's inlet. The global spray evolution was investigated by a fast-shutter imaging technique, applied according to an ensemble averaging approach. The imaging apparatus was based on a pulsed Nd-Yag laser (Litron Nano L 200-20, 200 mJ/shot, shot duration 6 ns), which was used as a light source. The laser was synchronized with a fast-shutter, high-resolution CMOS camera (Dalsa Genie Nano M4020, resolution 3008 \(\times\) 4112, 12-bit). According to the ensemble averaging approach, only one image per injection event was acquired at a given delay from the injection event's start (the TTL signal enabling the injector driver was used as a trigger). The statistical analysis of the spray's global development at an assigned \begin{table}
\begin{tabular}{c c} \hline \hline Injector & Bosch GS EV14 \\ Holes & 4 \(\times\) 220 \(\upmu\)m, symmetric to injector axis \\ Static Flow Rate & 3.5 mm\({}^{3}\)/ms @P\({}_{\text{inj}}\) 5 bar,g ; T\({}_{\text{w}}\) = 20 \({}^{\circ}\)C \\ Injector Driver & Darlington TIP121, V\({}_{\text{supply}}\) 14 V. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: PWI injector characteristics. delay was carried out by repeating the image acquisition over a series of consecutive injection events (30 in the present work). The repetition of the aforementioned acquisition sequence at different delays from the injection start allowed for characterization of the complete spray development throughout the entire injection event.