## 3 Results and Discussion  
### Conventional Piston  
As reported in the scientific literature [6], using \(\mathrm{OME_{X}}\) improves the reduction of in-cylinder soot formation during the combustion process. Furthermore, the combustion speed event increases, reducing its duration when using this fuel. The first part of the analysis aims to describe the behavior of the different blends and the benefits of increasing \(\mathrm{OME_{X}}\) content with the conventional piston (CP). The upper part of Figure 7 shows the distribution of KL values at different CADs for three different \(\mathrm{OME_{X}}\) percentages in the fuel: 0 Ox (top), 30 Ox (middle), and 50 Ox (bottom), at 21% O\({}_{2}\) to illustrate the temporal evolution during the combustion process inside the combustion chamber. The bottom part of the figure shows the apparent rate of heat release (aRoHR) profiles and the average KL curves for the selected fuels from the top dead center (TDC) to 28 CAD aTDC. As color rectangles, the aRoHR plots highlight the duration of the main and post injections (retrieved from high-speed images). The main injection starts at \(\sim\)2.5 CAD aTDC, and, as stated previously, its duration depends on the \(\mathrm{OME_{X}}\) percentage in the blend. During the main injection (frame@5 CAD), producing soot allows for identifying a diffusive flame's characteristic structure. A few  
Figure 6: Details on fuel injection strategy. Figure 7: KL factor imaging evolution, average KL, and aRoHR for 0 Ox, 30 Ox, and 50 Ox at 21% O\({}_{2}\). The results correspond to the average of 10 combustion cycles. CADs later, the sprays impinge on the piston bowl wall, and the burning jets expand tangentially, interacting between them and merging, forming a crown. For 50 Ox, the liquid fuel sprays are still visible around the nozzle at 7.5 CAD aTDC because of the longer injection duration compared to 0 Ox and 30 Ox. For all mixtures, at this stage, the soot production is more relevant than the oxidation process (increase in area and intensity) until around 10 CAD aTDC. After this instant, the reduction in intensity and area of the KL distribution indicates that the soot oxidation processes dominate overproduction. The peak of aRoHR corresponding to the main injection shows clearer staged combustion when increasing the \(\text{OME}_{\text{x}}\) percentage in the blend. The first stage, which corresponds to the first peak, is related to the premixed combustion phase. The second stage, which corresponds to the diffusive combustion stage, is more intense for 30 Ox and 50 Ox due to the longer duration of the main injection when increasing \(\text{OME}_{\text{x}}\) content. When the post-injection starts, the KL curve for 30 Ox and 50 Ox suggests that a big part of the soot produced has already been oxidized. Therefore, the expected benefits of post-injection (increasing late oxidation) are not so evident. In contrast, a significant increase of soot is observed. However, with Diesel, the KL curve suggests higher soot levels at this stage. In this scenario, the contribution of pilot injection has more potential to improve oxidation, and it causes a minor increase in the KL curve. However, the soot remaining from the combustion of the main injection is added to the soot produced during the combustion of the post-injection, resulting in soot levels higher than for the other blends. The fact that the post-injection peak of aRoHR is reduced when increasing \(\text{OME}_{\text{x}}\) percentage in the blend indicates that the oxygenated fuel allowed a complete main injection combustion resulting in less fuel partially or not oxidized. With all discussed above, a clear in-cylinder soot reduction and faster combustion event are reported when increasing the quantity of \(\text{OME}_{\text{X}}\) in the fuel, consistent with what can be found in the literature.