## 3 Results and Discussions
### Operating Conditions 1700 rpm--A/F18  
Figure 7 displays the PIV and spark images at the near spark region of three selected cycles at 1700 rpm. Similar to Figure 4 of the 1200 rpm condition, these three chosen cycles have the earliest, average, and latest start of combustion; they will be referred to as cycle A, B, and C, respectively. First impression shows that the general strength of the flow is much higher than at 1200 rpm, as expected from the higher piston speed at 1700 rpm. The general flow direction still goes across the spark plug from intake to exhaust, which is the same in both operating conditions. At the beginning of the spark event, there is a flow across the spark plug for all cycles, however, it is quite weak in cycle B. It is also clearer in the 1700 rpm image that after the flow crosses the plug, it diminishes significantly.  
For cycle A, the strong flow has a strong effect on the spark channel. The channel stretches out quite a bit, comparable to the cycle I of 1200 rpm condition. However, unlike cycle I, the spark channel in cycle A actually breaks off and restrikes close to the end of the spark event. It is likely that the stronger flow in this case overcomes the ignition energy required to sustain the spark channel. These breaks in the spark channel will affect the combustion event negatively as it momentarily stops the supply of energy to grow the flame kernels [11]. As shown on the flow speed versus spark stretch in Figure 8, higher flow speed leads to longer spark stretch. Cycle C displays a somewhat longer stretch compared to cycle B, which can be explained by the lower flow speed near the spark plug. However, the proportion of this relationship in this case is not similar to that of the 1200 rpm condition. This is understandable, as the flow that will affect the stretch behavior of the spark cannot be completely  
Figure 6: PIV-flame-development images after initial kernel development of three selected cycles (I, II, and III) at 1200 rpm condition.  
represented two-dimensionally. Hence, when looking at the spark stretch distance, the velocity of the flow should be used only for qualitative information.  
Despite having a longer spark stretch, cycle C has a slower start for the combustion compared to cycle B. However, the difference between the ignition delay of these two cycles is very small.  
Figure 8: Flow speed and spark stretch relationship of selected cycles at 1700 rpm condition.  
Figure 7: Near spark combined PIV-flame-spark images of three selected cycles (A, B, and C) at 1700 rpm condition.  
Upon closer inspection, at the start of the flame kernel formation, the flow near the spark plug where the flame will develop is weaker in cycle C. This might negate the effect of a small spark stretch advantage. Interestingly, all three cycles have very similar combustion duration. As shown in Figure 9, the flames in all cycles develop toward the intake side, following the tumble bulk flow. While they all have different shapes and grow paths, the overall flow condition seems to be somewhat similar. This could be why the combustion duration is the same across the investigated cycles. This further suggests that it is the bulk flow in the combustion chamber that will influence the combustion duration, while the spark stretch and flow near spark plug will influence the start of the combustion.