## 3 Results and Discussions
### Differences Between Firing and Motoring  
Besides firing engine operation, high-speed PIV is also applied when the engine was operated in motoring mode without combustion. The PIV results were compared between firing and motoring to identify changes in the flow-field when there is a combustion event. One representative cycle out of the 19 tested cycles for each firing and motoring condition were chosen; the flow velocity of the two conditions were shown in Figure 9 and the turbulence intensity were shown in Figure 10. For brevity, images were shown in 10\({}^{\circ}\) CA timing increments, from \(-45^{\circ}\) CA aTDC to \(25^{\circ}\) CAaTDC. From Figure 9, it can be seen that the flow-field between motoring and firing were not very different prior to the ignition timing (\(-15^{\circ}\) CA aTDC) and the differences can be attributed to cyclic variations. The vortices centers are located more centrally in the pent roof which makes the flow across the spark plug stronger. After ignition, the flame boundary in the firing cycle is overlaid on top of the images, which can be seen from \(-5^{\circ}\) CA aTDC onward. During this phase, significant differences between and firing and motoring flow-field can be observed, suggested strong effect of the flame on the dynamics of the unburned region. While in the motoring cycle, the strength of the flow-field is significantly diminished, the existing tumble flow continues to exist and even enhanced by the propagating flame. The flame first propagates in the tumble flow direction and the flow-field is maintained in this direction. From \(5^{\circ}\) CA aTDC, the flame continues to grow with the flow: flame propagation can be seen toward the other side of the pent-roof and interestingly, the flow here is significantly enhanced. This could be because it is in the later stages of combustion where the flame has occupied a much larger volume of the combustion chamber and hence the pressure rise, the higher temperature can contribute to a  
Figure 8: Image processing steps to extract the flame front from the high-speed PIV image.  
more complicated flow-field. This is well reflected in the high turbulence intensity in the pent-roof seen in Figure 10. The flow-field maintain a high level of turbulence intensity throughout the flame development period and at the region where the flame enhanced flow is seen, a very high turbulence intensity is also found, especially close to the flame front. It is interesting that the turbulence did not decrease at any timing, even when the flame kernel just appears. This could be due to the immediate pressure and temperature rise.  
Figure 9: Mean in-cylinder flow velocity of a selected motoring cycle (**left**) and a selected firing cycles (**right**).  
Figure 11 shows the velocity the flow at certain locations as the flame approaches and passes through them. In this case, 5 points at the same vertical distance of 2.5 mm to the TDC location of the piston were selected. Each point is 10 mm away from each other. These points were chosen as they allow the long distance away from each other while still provide a decent data size for analysis. Also, as they are close to the piston top in the period of investigation, only flow velocity in horizontal  
Figure 10: Turbulence intensity of a selected motoring cycle (**left**) and a selected firing cycles (**right**).  
direction is considered. The colored dots represent the time when the flame front passes through the points. The flame crosses point D first followed by C, E, B and A which reflects its movement is influenced by the tumble direction. From the velocity plot, it is quite noticeable that as the flame passes by, the velocity of the flow achieves a local maximum. This likely suggests that as the flame approaches the flow is enhanced but quickly lost its energy as the mixture undergoes combustion.