## 4 Results
### Cycle Resolved Visualization Analysis  
In this study, after the global analysis of the combustion process with the thermodynamic tools, the sequence of images acquired with the high-speed camera were post-processed and analysed to have a better understanding of the spatial behaviour of the combustion process. Figure 9 shows a selection of representative images of the combustion process in the first stage for the baseline case and the two cases of double injection with different second injection phasing. The images are presented in terms of crank angle after the start of the spark (ASOS). By analysing these raw data images, it is possible to note a decrease in terms of flame luminosity as fuel delivery events were retarded. In addition, flame growth is lower for the double 50-50 at 200 CAD BTDC with respect to the other two cases. This behaviour could be associated with differences in the air-fuel mixing as was hypothesized for the thermodynamic analysis. For all the cases tested, flame morphology was studied with the post-processing methodology explained in the Optical Analysis section. Figure 10 shows the results of the area growth normalized by the piston cross-section. For the cases of early injection (Figure 10a) the split improves the combustion process. However, for the cases where the injection was retarded, flame growth showed significant delay for single and double injections. Triple injection featured similar behaviour compared with the baseline, while the double 70-30 shows the best results. Therefore, splitting the injection seems to render the flame propagation process less sensitive to fuel delivery phasing compared to single cases. Figure 8: NOx emissions in ppm (a) and g/kWh (b) for different split injection strategies at fixed SA (15 CAD BTDC) and engine speed (2000 rpm); second injection phasing set at 250 and 200 CAD BTDC. Flame speed values were calculated for each case from the area growth graph. The main hypothesis is to consider the flame perfectly spherical until the wall is intercepted. The maximum value for each injection strategy is shown in Figure 11. The trend reveals a decrease of flame propagation speed when delaying the start of injection (by 50 CAD). The largest differences were found for single and double 50-50, due to the increase of injected fuel during the last phase of the intake stroke. For these operative conditions, the peak values of flame speed were found to be between 20 and 25 m/s. Figure 10: Flame area growth versus crank angle degree for injection strategies of: 250 (**a**) and 200 (**b**). Figure 9: Flame image sequence during the flame propagation stage for baseline and double injection 50-50 at two SOI (250 and 200 CAD BTDC). The next step in the combustion analysis was the calculation of complementary parameters as Heywood circularity factor (HCF) and preferential flame propagation. It is important to note that as the engine has combustion variability, all the parameters were studied by considering the average value of 30 consecutive combustion cycles. The circularity parameter shows the flame deformation with respect to a perfect circle, measured with the perimeter ratio as was explained in the Optical Analysis section. The results were presented in terms of maximum values, as shown in Figure 12. A clear tendency of increased distortion could be seen as injection was retarded (i.e., HCF values close to unity suggest more circular flames). In addition, splitting the injection decreases the flame circularity parameter, with the lower values recorded for the triple injection strategy. Regarding the preferential propagation in a certain direction, it was defined as the case for which the flame propagates faster to one side than the other.