## 3 Results and Discussion
### Radial-Lips Piston  
As previously discussed, this study tested a second bowl geometry (RP) containing a stepped lip at the bowl edge and radial lips. KL distribution and the KL/OH\({}^{\star}\) radial maps are presented in Figures 10 and 11, respectively, to show the temporal and spatial evolution for both pistons using 50 Ox at 15% O\({}_{2}\). The first thing that can be observed is a different aRoHR evolution. The RP piston provides a faster ignition and a much more intense heat release than the CP. This is relevant as it compensates for the delay caused by the oxygen reduction that was previously mentioned. Regarding soot production, higher KL values are reported for the conventional piston during the whole combustion event. The first KL values appear close to the bowl periphery at 7.5 CAD, probably due to the flame-to-flame interaction after impacting the bowl wall, as seen in Figure 12. After this, the high KL cloud evolves toward the center of the bowl until 20 CAD (before the post-injection combustion). The movement of the KL cloud and OH\({}^{\star}\) reactions can be seen clearly in radial maps (left images from Figure 11). During the same interval, the low values of OH\({}^{\star}\) in the radial map confirm that the soot is hardly oxidized, resulting in the high KL cloud mentioned previously. The lack of  
Figure 8: KL factor imaging evolution, KL evolution curves, and aRoHR for 50% Ox in fuel at 21% O\({}_{2}\) and 15% O\({}_{2}\). The results correspond to the average of 10 combustion cycles. Figure 9: KL evolution curves using 0 Ox, 30 Ox, and 50 Ox in the fuel at 21% O\({}_{2}\) and 15% O\({}_{2}\). In contrast, the RP piston shows several differences, which lead to lower KL values. The first KL values also appear at the bowl periphery and evolve toward the center of the bowl, as in the conventional piston case. However, the intensity of KL values is significantly lower for this piston, and the OH* radial map (see the right image from Figure 11) confirms a higher oxidation activity from the periphery to a 15 mm radius during the main injection combustion event. This suggests that less soot is formed but is also significantly oxidized before the beginning of the post-injection combustion event, as seen in Figure 10 at 17.5 CAD. This fact is also visible in the KL radial map of Figure 11. Here, two different KL clouds separated by a dark gap around 20 CAD can be observed, indicating that the oxidation process during the main injection combustion event is sufficient to oxidize a big part of the soot formed. The results report an improvement in oxidizing the in-cylinder soot formation using radial lip geometry with low oxygen concentration inside the combustion chamber compared to the conventional piston. From the sequence of images shown in Figure 10, it is possible to see that, in contrast with CP, the RP geometry reduces the interaction between flames at the periphery of the bowl. As shown in Figure 12, the presence of radial lips prevents the flames from spreading tangentially, creating a backward movement instead. This reduces the flame-to-flame interaction, reducing the formation of high equivalence ratio regions and promoting the use of fresh oxygen close to the center of the bowl [24]. In addition, the stepped lip improves the oxidation of part of the fuel at the squish region compared to the CP geometry. To summarize the performance of the RP in combination with the fuel, KL evolution curves for all fuels tested in this study at two oxygen concentrations (21% and 15%) are reported in Figure 13. The results show the same behavior discussed previously for all the cases. The RP reduces soot formation and improves its oxidation during combustion, independently of the oxygen concentration. However, its effect is more prominent at 15% O\({}_{2}\). This indicates that RP is more effective than CP under low oxygen concentrations.