# Study of Discharge Characteristics on Ignition Performance via High-Speed Imaging in a CVCC
## 3 Results and Discussion
### Quantitative Analysis of the Initial Flame Area and Initial Flame Luminance  
In order to get a deeper understanding of the initial flame kernel, the initial flame area and initial flame luminance were quantitatively measured for all experimental cases. The initial flame area was calculated via CO radiation image binarization. As shown in Figure 11, firstly, a threshold value was set, then the raw infrared image was binarized and the flame contour could be determined. Then the projected initial flame area was achieved by measuring the pixel of the binary image. The average value of the flame area and error bar for all the cases are shown in Figures 12a, 13a and 14a under different conditions. The mean luminance of the initial flame was calculated by the integral luminance value divided by the flame area value, and the results are shown in Figures 12b, 13b and 14b. The blue line stands for the standard coil A, the orange line represents the high current coil B, and the gray line represents the high energy coil C. Both flame area and flame luminance help to evaluate the initial flame generation and reaction intensity. Figure 11: Procedure of determining the initial flame area: (**a**) Raw infrared image; (**b**) Binary image. Figure 12: Basic condition: (**a**) Initial flame area; (**b**) Initial flame luminance. According to Figure 12, which shows the basic condition, the high current coil B and high energy coil C generate almost the same flame area as they have the same discharge current until 1.5 ms. The speed of the initial flame growth of high current coil B decreases after 1.5 ms, while the high energy coil C keeps a high speed until 2 ms. Standard coil A has the lowest initial flame growth speed and results in a smaller initial flame size. However, it can keep the growth speed and almost gets the initial flame size at 2 ms compared with coil B. It can be explained that although a higher discharge current results in a larger initial flame, the initial flame growth slowed down after the end of the discharge of coil B (discharge duration: 1.3 ms). Coil A and coil C remain at the speed of initial flame growth as the discharge continues until 2 ms. Initial flame luminance indicates the above results more clearly. As shown in Figure 12b, the mean luminance of the initial flame for high current coil B decreases immediately after the end of discharge duration at 1.3 ms, while coil A and coil C keep increasing until 2 ms. Under the high diluted condition in Figure 13, the initial flame area growth is slower than the basic condition. Furthermore, high current coil B firstly has a faster speed of initial flame growth than that of the standard coil A. After the end of the discharge duration of coil B (1.3 ms), the growth speed decreases. However, the flame area of coil B is reduced to less than coil A until 2 ms under such high diluted conditions. The flame luminance of coil C has the same trend as that of coil B and is less than coil A at 2 ms. Therefore, coil B, with the shorter discharge duration, gets the worst ignition performance under such conditions. As the image results are shown under the high flow condition, the initial flame has out of the view scope after 1 ms, so that we investigate the flame area and flame luminance before 1 ms under such conditions. The results show the initial flame area keeps increasing rapidly until 1 ms for all the coils in Figure 14, although standard coil A with a lower discharge current has the smallest flame area. Furthermore, the mean luminance of the initial flame for standard coil A starts to decrease after 0.5 ms, which can infer that the enhanced flow increased the speed of the initial flame growth, although this also increased heat transfer between the initial flame kernel and unburned gas. The initial flame kernel generated by the low discharge current will disperse or even be eliminated. Thus, the flame  
Figure 14: High flow condition: (**a**) Initial flame area; (**b**) Initial flame luminance.