## 3 Results and Discussion
### Spray Tip and Flame Penetration  
The temporal evolutions of vapor penetration and LOL, as well as the ID for all the fuels at nominal condition have been shown in Figure 8. The vapor penetration and ID have been measured from schlieren images and LOL from OH* chemiluminescence, as was mentioned previously. The standard deviation for each parameter is represented by shaded areas. A first stage from start of injection until ignition can be observed, where vapor penetration is identical for all the fuels. Only at certain time after ignition differences appear. This first stage corresponds to the ignition delay phase, which extends for each fuel until combustion starts. The ignition delay time has been marked in the figure as a dashed vertical line for each fuel. During the period until ignition, there are not effects on the spray behavior, as expected because the penetration is governed by the momentum flux at the nozzle [34] which only depends on the pressure drop across the nozzle and on the orifice area, fuel effects are negligible. In addition, in Figure 8 the effect of the different fuels upon the indicated parameters can be observed. The dashed-dot horizontal lines correspond to the temporal evolution of LOL. After the  
Figure 7: Comparison between lift-off length (LOL) of diesel and the other fuels tested. ignition, the fuel that ignites first penetrates faster than the others do. In general, it is possible to see that while OMEx has similar behavior to the paraffinic fuels, OME1 is quite far from them. On the other hand, from OH* images, the flame tip penetration can be measured also from the high-speed OH* chemiluminescence imaging as the axial distance from tip injector until the flame front. In Figure 9, the temporal evolution of the flame penetration for all fuels at 900 K and 1500 bar and both oxygen concentrations has been represented. The solid lines correspond to the case of 15% of oxygen concentration and the dotted lines to 21% case  
From this figure, flame tip penetration is seen to undergo a similar time evolution as spray tip penetration during most of the injection period. However, for all cases flame tip becomes constant after a given period, which depends on fuel type and oxygen concentration. For oxygenated fuels, this occurs between 1000 to 1500 us, while for all other paraffinic fuels it happens close to the end of  
Figure 8: Spray tip penetration and lift-off length at 900 K 15% O2 and 1500 bar (Spray A condition by ECN). Vertical dashed lines represent ignition delay. Figure 9: Flame penetration at 900 K and 1500 bar. The solid lines correspond to 15% of Oxygen and dotted lines to 21%. injection event (around 4200 \(\upmu\)s). During that period, the stoichiometric reacting surface stabilizes, and hence the maximum distance where OH* chemiluminescence is recorded does not change with time. For constant ambient and injection conditions this distance has been shown to scale inversely with the stoichiometric mixture fraction, i.e., directly with the stoichiometric A/F ratio [36; 37], which depends on oxygen concentration and fuel composition. Table 1 shows stoichiometric air-fuel ratios, from which one can observe that paraffinic fuels have similar values, resulting in the similar long stabilization distance, while the oxygenated fuels approximately a 50% lower (A/F)st, resulting in a much shorter flame length. Furthermore, differences among Diesel/HVO/Dodecane are small, in agreement with the small (A/F)st differences, OME1 shows a consistently roughly 10% shorter penetration compared to OMEx. Figure 10 has been used to depict the differences in flame structure for each fuel at the nominal operating condition in an instant of time where the combustion is quasi steady (3500 \(\upmu\)s). Such images confirm the previously plotted trends among fuels in stabilized flame length, with paraffinic fuels having a similar length, and a shorter one is observed for oxygenated fuels. On the other hand, observed differences in lift-off length also point out at interesting features.