## 3 Results and Discussion
### Soot Production  
As was indicated in methodology section, the DBI technique was used to determine the soot production. The DBI images were collected after schlieren. However, OH* chemiluminescence was recorded with the two sets. To verify that both sets are consistent and there are no experimental discrepancies among them, the results obtained for ID from OH* images measured with the first and in the second set are compared in Figure 11. The grey shadow in the bisector represents the uncertainty associated to the time interval between consecutive frames. It can be observed that ID differences between both sets are always less than the time elapsed between two consecutives frames (40 \(\upmu\)s). Thus, it is possible to conclude that both sets are consistent and results can be analyzed together. DBI images were processed to calculate the flame soot in terms of the optical thickness (KL). KL maps were constructed to depict soot evolution throughout the combustion event, by calculating the KL for each axial distance and time-step. A good way to simultaneously evaluate the KL evolution in time and space is transforming the KL map at any given instant into a 1D vector where KL values at any given cross section from the nozzle are accumulated into a single value. Thus, each KL map at any instant is converted into a vector of \"accumulated KL\" values along the flame axis. By compiling those vectors at every time step (i.e., at every frame), a plot such as those of Figure 12 can be obtained. In any of those plots, the abscissa axis reads for time, the ordinates axis indicates axial distance from  
Figure 11: Comparison of ignition delay (ID) obtained in the first set and in the second one. the nozzle and the color at any point (X,Y) in the plot indicates the accumulated KL at a cross section at distance Y from the nozzle, of frames taken at time X after SOI. In Figure 12, the accumulated KL has been represented for the operating condition with greater tendency to soot formation, that is, at the highest temperature (1000 K), the lowest injection pressure (500 bar) and the lowest oxygen concentration (15%). Dashed lines have been used to depict ID, vapor penetration and the flame lift off length. In this figure, it is possible to observe that soot production for OME\({}_{1}\) and OME\({}_{\rm x}\) is below the detection threshold. The color scale for these two fuels has been modified in other to enhance this fact. Therefore, a sootless flame seems to be established. This holds for all operating conditions and, consequently, KL values for those fuels cannot be analyzed further. Dodecane, HVO and OME\({}_{\rm x}\) show smaller KL values than diesel. As for the time evolution, in all three cases it can be observed that initially the largest soot amount is located at the spray tip, in the head vortex area, but when reaching a distance around 70 mm, this soot disappears due to the establishment of the quasi-steady flame The highest KL region appears for the three fuels from 40 to 50 mm from the nozzle. For a given fuel composition, the soot production is related to the amount of air entrainment that occurs upstream of the lift-off [29]. Longer LOL suggests less soot formation, but this depends also on fuel composition. This can be observed in Figure 12, where diesel has the longest lift-off length but also the largest soot formation. The stoichiometric air/fuel ratio (ma/mf) for dodecane and OME\({}_{1}\) is quite different. At 15% of oxygen concentration, stoichiometric air-fuel ratio for dodecane is 20.72 versus 10.03 for OME\({}_{1}\) indicating that oxygenated fuels require less air to oxidize. This explains that OME\({}_{\rm x}\), although with LOL closer to diesel, does not produce soot. This statement is supported by the equivalence ratio (\(\Phi\)) at LOL. For dodecane, \(\Phi\) is 4.9 at baseline operating conditions, while for the OME\({}_{1}\) is 1.0. Furthermore, OME fuels do not have carbon-carbon (C-C) bonds, which contribute to the absence of soot.