## 4 Results
### Soot Formation  
In order to study the effect of the split injection strategy on soot formation and oxidation, luminosity of the late combustion phase (60-145 CAD ATDC) was evaluated from the cycle resolved images. It is relatively well known in the literature that diffusion-controlled flames represent a source of soot, which generally does not oxidize prior to exhaust valves opening [29; 34; 35]. Some authors [36; 37] suggested that one of the main reasons why DISI engines did not deliver the level of improvement in power, fuel economy and emissions expected by manufacturers was due to inadequate design and performance of the direct-injection systems, as well as the engine calibration strategies used. In general, DI systems generate fuel impingement on the walls and piston that provokes the well-known pool fire regions [38; 39]. Practically, during the normal combustion process, it is possible to achieve gas temperature and mixture strength conditions that lead to fuel film ignition by the normally propagating flame front. This phenomenon can create diffusion-controlled flames that can persist well after the first combustion phase [40]. Just before the opening of the exhaust valves, these flames produce soot that cannot be completely oxidized due to low temperatures in the cylinder [41]. The trends for the investigated conditions are shown in Figure 15 and Figure 16. Two different instances of the late combustion phase are shown, 75 CAD ASOS (60 CAD ATDC) and 160 CAD ASOS (145 CAD ATDC). It is possible to observe that the delay of the injection timing increased the size and intensity of the pool fire regions. In addition, splitting the injection contributes to reducing these zones with respect to the single injection case. The total area of this region was estimated by an in-house post-processing method explained in the Optical Analysis section. The results are depicted in  
1. Injection strategies (b) Injection strategies of 200  
Figure 14: Preferential flame propagation along the x axis. Figure 13: Preferential flame propagation along the y axis. **Figure 17**, which confirmed that retarding the injection timing produced high fuel impingement in the piston surface and top land region. The highest soot signal was detected for single injection at 200 strategy. In this condition, a more heterogeneous distribution of fuel position in the combustion chamber is shown, compared to split conditions. It is interesting to note that for single injection 200 strategy a peak was detected in the last phase of the combustion process. This phenomenon could be induced by the flame generated in the top land region that returns to the combustion chamber. In addition, this effect was seen in Figure 16 at 160 CAD ASOS. Going further into the specifics of soot formation, the discussion of two types of diffusive flames is useful. The differentiation was detailed in a previous study [28] that also looked at the combination of air-fuel ratio temperature and identified the flame originating in the top land region as leaner and cooler with respect to those formed on the piston surface. It is interesting to note that the triple injection cases tend to feature a higher soot area later during the expansion stroke compared to the double injection strategies. As expected, the effect is more important for the SOI 200 cases, as these settings featured more pronounced impingement on the cylinder liner. Therefore,  
Figure 16: Image sequence of late combustion stages at 75 and 160 CAD ASOS, for the 200-injection strategy. Figure 15: Image sequence of late combustion stages at 75 and 160 CAD ASOS, for the 250-injection strategy. both observations seem to confirm the correlation of the observed trends to the spray impingement pattern on the piston and cylinder liner. Figure 18 shows the total soot area at 75 CAD ASOS and 160 CAD ASOS; the baseline case is represented with a dashed line.