## 3 Results
### Spray Drop Size and Velocity  
The PDA raw data for the measuring station corresponding to the injector axis projection on a plane at 50 mm from the nozzle (coordinates of X = Y = 0; Z = 50 mm) are reported in Figure 9a,b for the reference operating condition Pinj = 7 bar,g and Tw = 20 \({}^{\circ}\)C. In these plots, all the records relevant to 3000 consecutive injection events are reported (blue dots), along with the average values computed in 0.1 ms time bins (red dots). The ET's start was used as a time reference. As can be seen, the drops' diameters and Z-velocity time histories evidence how the spray approached the considered measuring station about 4 ms after the ET's start. The main part of the spray structure flowed through the considered measuring station between 4 and 8 ms. During this time window, the drops' velocity ranged between approximately 5 and 22 m/s, while the observed drops' diameter range was predominantly between 15 and 150 m. A significant number of drops with diameters up to 350 m was also observed, possibly related to the defective primary break-up in the first part of the injection process, as was observed in the spray images. The presence of even such a restricted number of relatively large drops inherently had a non-marginal effect on the resulting Sauter mean diameter. After 8 ms from the ET's start, the velocity data rapidly decreased to values around 2-3 m/s for the drops pertaining to the spray tail. This part of the spray's structure was composed of very small droplets (diameter values below 40 m) featuring reduced momentum which continued flowing through the observed position for a long time. When the injector's water temperature was raised to 110 \({}^{\circ}\)C, the drops' velocities and sizes time histories for the same measuring station, reported in Figure 9c,d, were obtained. Marginal effects due to the applied rise in temperature were observed in terms of both the drops' velocities and size ranges for the bulk spray evolution during the 4-8 ms time window, while a significant shift in the mean values was observed below 15 m/s and toward 50 m.  
In Figure 10, the results obtained with four different water temperature levels at the injector inlet are reported for the complete measuring travers at Z = 50 mm and crossing the entire spray structure. In these plots, both the mean and Sauter mean diameter values are included, along with the mean velocity and relative drop count. All these quantities were computed with data relevant to the entire 30-ms time-window to have a full portrait of the spray quality in the examined positions. The effect of the water temperature on the drops' sizes, which was already commented on for the X = Y = 0 and Z = 50 mm position, was substantially confirmed for the entire measuring traverse; a significant but not dramatic effect from 20 \({}^{\circ}\)C to 110 \({}^{\circ}\)C was observed, with size reductions ranging between 8 and 15 \(\upmu\)m in terms of the SMD and between 4 and 9 \(\upmu\)m in terms of the MD. Minor consequences of raising the water temperature were observed in terms of the drop count and mean velocity, which were proven to change only for the central positions of the spray structure. As was observed when commenting on the spray images in the same operating conditions in Section 3.2, the water temperature's rise at the injector inlet from 20 \({}^{\circ}\)C to 110 \({}^{\circ}\)C, assumed to be compatible with standard PFI technology (e.g., injector body, rail and pressure sensor), was not adequate to trigger a net flash boiling mechanism for the injection process. Consequently, the injection process resulted in a spray with a basically unaffected shape but appreciably improved atomization quality. Nevertheless, the drops' size improvement was not as significant as was presumably attainable in the case of completely developed flash boiling conditions [25].