## 3 Results and Discussions
### Operating Conditions 1200 rpm--A/F18  
Figure 4 displays the images of combined in-cylinder flow from high resolution PIV and spark development from high-speed spark imaging near the spark plug. The blue area between the spark gap indicates the spark discharge path obtained from the spark high-speed imaging. The gray area indicates the burned area obtained from high-speed flame tomography. The earliest burning cycle, cycle I, is on the left-hand side, the average cycle, cycle II, is in the middle, and cycle III, the one with the longest ignition delay, is on the left of the figure. Cycle I also has a shorter combustion duration compared to cycle II and III. First, the in-cylinder flows of all the cycles appear to move clock-wise, from the intake valves to the exhaust valves which is the general tumble direction for this engine. For all cases, there exists a bulk flow perpendicular across the spark plug. It is quite noticeable that the  
Figure 3: In-cylinder pressure traces (**top**) and mass fraction burnt (**bottom**) of all tested cases and selected cases for the two operating conditions (1200 rpm and 1700 rpm). earliest burning case has the fastest flow across the spark plug. Cycle III, however, only has a slightly slower flow across the plug compared to cycle II. Regarding the spark behavior, there is a general spark movement with the in-cylinder flow across the spark plug for all cycles. The spark also displays a stretching behavior observed in other work. For the earliest burning cycle I, with the strongest flow, a largest spark stretch is also observed. In fact, a break in the spark channel can also be seen when the spark is at its maximum stretch. However, for the two other cycles, the difference is not very noticeable. Hence, in order to get more details about the relationship between the flow and spark behavior, not only for cycle II and III but also for cycle I, we measured and compared the flow speed across the plug with the stretch distance. Figure 5 displays this plot. For the flow speed across the plug, velocity at a representative point is used. This chosen point is 1 mm away from the left side of the spark plug (before the flow crosses the plug), in-line with the spark gap. In particular for this study, only the horizontal velocity at the point will be considered. Furthermore, the time for the velocity to be considered is the instant just before the first spark. An illustration of the location of this point with regard to the spark plug is shown on the plot. Also, in the illustration is the measurement method to determine the spark stretch distance, which is the distance between the first spark discharge and the maximum stretch of this first discharge before the spark channel is broken off. The plot confirms the visual observation of the data, with cycle I  
Figure 4: Near spark combined PIV-flame-spark images of three selected cycles (I, II, and III) at 1200 rpm condition. having the longest stretch and flow followed by cycle II and then III. Hence, a trend of faster flow speed, longer spark stretch, and shorter ignition delay can be deduced from this data. It also appears that the length of this stretch is somewhat proportional to the strength of the flow. While there is a trend for the spark stretch, flow speed, and ignition delay, a slightly faster combustion duration in cycle I in comparison to cycle II and III will require additional information of the flame for a better understanding, due to the fact that the combustion duration of cycle II and III are very similar despite the difference in spark stretch and flow speed. Figure 6 displays images of flame development of these three cycles. The flames in these three cycles all develop toward the exhaust-valve side, which is with the direction of the bulk flow and the flow across the spark plug. Hence, it is reasonable to say that the direction of early flame kernel development is dependent on the flow direction across the spark plug. For all cases, the kernel appears while the spark channel is still presented.