# Study of Discharge Characteristics on Ignition Performance via High-Speed Imaging in a CVCC
## 3 Results and Discussion
### Simultaneous Visualization Results  
Figure 10a-c shows the simultaneous visualization results under different experimental conditions. Each figure contains three rows of images representing three types of coils, and each row of images includes two kinds of photographs. The top one shows the CO radiation images taken by the high-speed infrared camera and the lower one shows the direct photographs taken by the normal high-speed camera. The profile of the spark plug was drawn with a white line to show the location of the spark plug. Figure 9: Heat release under different conditions: (**a**) Basic condition; (**b**) High diluted condition; (**c**) High flow condition. Figure 10: Simultaneous visualization results of different ignition coils (upper: CO radiation images/lower: direct photographs. Basic condition with the N\({}_{2}\) dilution of 24% and flow velocity of 8 m/s, high diluted condition with the N\({}_{2}\) dilution of 28% and flow velocity of 8 m/s, high flow condition with the N\({}_{2}\) dilution of 24% and flow velocity of 18 m/s): (**a**) Basic condition; (**b**) High diluted condition; (**c**) High flow condition. According to Figure 10, it can be found that after the electrical breakdown, a discharge channel was formed at the spark plug gap, meanwhile, the initial flame was generated immediately along with the discharge channel. Then, the discharge channel was stretched downstream with the flow, and the initial flame was developed during the ignition process. As shown in Figure 10a, high current coil B and high energy coil C have the higher luminance of the discharge channel during the ignition process. Furthermore, after 1.5 ms, the initial flame propagates farther than the standard coil A. However, all the coils are holding the initial flame around the spark plug at 2.5 ms. It is found that all the coils can form an attached flame kernel after the discharge duration under the basic condition. This means all the coils in this study can produce the self-sustained flame after the discharge process, so that the flame grows and propagates rapidly. Therefore, the combustion can complete with a shorter duration and high stability. Under the high diluted condition, according to Figure 10b, the initial flame of high current coil B is detached from the spark plug at 2.5 ms, while the standard coil A and high energy coil C can hold the flame attached. Comparing standard coil A with high current coil B, the initial flame of coil B propagates farther than coil A, at 1.5 ms, due to the higher current. However, the propagation distances of the initial flame front are almost the same after 2.0 ms, and the rear of the flame moved downstream. Hence, it is more difficult to form the self-sustained flame for coil B under the low flow and high diluted conditions, which results in a lower ignition probability and lower combustion stability. On the other hand, from the CO radiation images, the initial flame of high diluted condition develops slower than the basic condition. Even in the high energy coil C, comparing the images of basic condition and high diluted condition at 2 ms, the initial flame of the high diluted condition is smaller than that of the basic condition. This may mean that increasing the mixture dilution ratio will decrease the combustion speed, and even an increase in the ignition energy cannot reach the combustion speed of the low diluted condition. An explanation for this is that increasing the dilution ratio only influences the decrease in speed of the flame kernel formation and flame propagation, shortening the discharge duration results in the initial flame kernel being detached from the spark plug, which contributed to lowering the combustion stability.