# Study of Discharge Characteristics on Ignition Performance via High-Speed Imaging in a CVCC
## 3 Results and Discussion
### Combustion Results
Due to the small internal volume of the combustion chamber, the small pressure changes can be detected by a highly sensitive pressure sensor, and the results of ignition delay are shown in Figure 8. Ignition delay is defined as the time period between the start of discharge to the MFB1 (mass fraction burn of 1 percent). It can be found that coil B with a high discharge current results in the longest ignition delay in the high diluted condition, while when in the high flow condition, coil A with the extended discharge duration results in the longest ignition delay. The difference in initial flame formation is largely affected by ignition. Hence, given that coil A has the same ignition energy as coil B, it shows that coil A with extended discharge duration favors more in improving the ignition performance than coil B with higher discharge current under high diluted conditions. Nevertheless, the opposite of this result is the case under high flow velocity conditions, as the higher discharge current is more important under the high flow condition. According to the R-W (Rasswei-Withrow) model [27], the pressure changes are only caused by the combustion in the constant volume combustion chamber since the volume is constant in it, the heat release rate can consequently be calculated from the pressure data. Figure 9 illustrates the heat release at different conditions of all the experimental cases and about three types of coils. As shown in Figure 9, combustion time is defined as the time period from the start of discharge to the point of peak heat release. The graph of heat release can reflect the combustion speed via the length of combustion time and  
Figure 8: Ignition delay of all the conditions. Figure 7: Ignition probability of all the conditions. embodies the combustion stability by the fluctuation of combustion time of all the cases. From Figure 9a of the basic condition, all the coils have no significant difference from each other in heat release histories, additionally, the combustion speed and combustion stability have a high level of all the cases; From Figure 9b of the high diluted condition, combustion becomes slower and unstable as compared with the basic condition, the timing of the peak heat release point is put off, which means the combustion time is extended. Furthermore, the combustion fluctuation case-to-case is increased, and the high current coil B has the slowest combustion case and even has misfire cases under the high diluted condition. Although standard coil A and high energy coil C have the same ignition probability, coil C combustion cases keep a smaller combustion fluctuation than that of coil A. In Figure 9c of the high flow condition, the combustion time of the fastest case of coil C shows a slightly shorter compared with the fastest case of coil C under the basic condition, although the combustion fluctuation is increased, which reveals that increasing the flow velocity may improve the combustion speed only when the ignition energy is increased correspondingly. As described in the combustion results, several interesting results can be found. On the one hand, increasing the mixture dilution ratio will decrease the combustion speed. Even increasing the ignition energy cannot reach the combustion speed of low diluted condition, when focusing on the same ignition energy coils, prolonging the discharge duration is more important than increasing the discharge current for the ignition performance. On the other hand, increasing the flow velocity may improve the combustion speed only if increasing the ignition energy correspondingly. Additionally, if the ignition energy is constant, increasing the discharge current has more benefits than prolonging the discharge current for improving the ignition performance. Since the experiments were carried out in the combustion chamber, the combustion results were mainly affected by the ignition process.