## 3 Results and Discussion
### Radial-Lips Piston
To quantify the reduction in terms of KL achieved with RP for each fuel, the relative reduction of KL values at 15% O\({}_{2}\) has been calculated according to Equation (1):  
\[\textit{Rel. KL}=\frac{Avg.KL_{CP}-Avg.KL_{RP}}{Avg.KL_{CP}} \tag{1}\]  
The Rel. KL evolution for the three fuels considered in this study is shown in Figure 14. Between 0 and 20 CAD aTDC, the KL reduction achieved with the RP with 0 Ox varies from 10% to 45%. In the same interval, a much higher reduction is achieved when increased the OME\({}_{\text{X}}\) content in the blend. For 30 Ox, it varies between 40% and 75%, while for 50 Ox, it reaches from 30% to 80%, depending on the combustion stage. During the late stages of combustion, the KL reduction using the RP is 17%, 70%, and 70% for 0 Ox, 30 Ox, and 50 Ox, respectively. Therefore, the trend shows a clear improvement in soot reduction when combining the radial-lips design with OME\({}_{\text{X}}\) content in the fuel, especially at low ambient oxygen concentrations, which are representative of low NO\({}_{\text{X}}\) operating conditions. When  
Figure 13: KL evolution curves using 0 Ox, 30 Ox, and 50 Ox in the fuel at 21% O\({}_{2}\) (**left**) and 15% O\({}_{2}\) (**right**) for conventional and radial-lips pistons. comparing 21% and 15% O\({}_{2}\), it is possible to see that the RP can keep the same KL values with 30 Ox and 50 Ox. The level reached with both oxygen concentrations is similar when looking at the two peaks. However, when looking at the interval between peaks and the late oxidation stage, it is possible to see that KL decreases much faster with 21% O\({}_{2}\). This behavior suggests that the oxygen in the fuel strongly impacts the soot formation (and the maximum KL reached) with RP. In contrast, the ambient oxygen controls the oxidation (and the velocity with which the KL decreases). However, this is not observable with 0 Ox. To summarize the differences between piston geometries and all fuels tested, the area below the KL evolution curves from Figure 13 is plotted in Figure 15. The aim is to represent the accumulation of KL reached along the combustion process. The curves corresponding to CP at 21% O\({}_{2}\) for all fuels are plotted with solid lines for reference. The 15% O\({}_{2}\) cases show a higher accumulation of KL when using CP compared to the RP one. The use of this piston achieves a reduction of 37%, 70%, and 78% using 0%, 30%, and 50% of OME\({}_{\text{x}}\) in the fuel, respectively. It is true that without OME\({}_{\text{x}}\) in the fuel (0 Ox), the CP (red-solid line) reaches a 48% lower value for 21% O\({}_{2}\) than RP at the lowest oxygen concentration condition using the same fuel (red-dashed line). However, when combined with OME\({}_{\text{x}}\) in the blend, the RP achieves similar values as the cases using the CP at 21% O\({}_{2}\). Even with 50 Ox, the values reported are below the ones corresponding to the reference case. On the one hand, this indicates that using only the piston geometry with low oxygen concentration inside the combustion chamber is insufficient to achieve or improve the soot levels significantly compared to using a CP geometry with no EGR. On the other hand, it confirms a synergy when combining oxygenated fuels (blended with Diesel) with the RP geometry, as the effect of reducing ambient oxygen concentration over soot is almost negligible. Figure 14: KL values reduction percentage using radial-lips piston compared to conventional piston for all fuels at 15% O\({}_{2}\). Figure 8:0 Normalized average KL closed curves area.