# Study of Discharge Characteristics on Ignition Performance via High-Speed Imaging in a CVCC
## 2 Experimental Setup
### Simultaneous Visualization Method  
Most researchers use high-speed photography to investigate the discharge channel behaviors and the initial flame kernel during the ignition process. The discharge channel is relatively easy to detect directly by natural imaging since the discharge channel typically emits a visible glow. However, the initial flame kernel is not easy to detect as it is invisible on a visible band [23; 24]. The Schlieren photography method has been widely used to investigate the initial flame kernel in the combustion chamber. However, this method is hard to distinguish between the preheated zone and initial flame, as the Schlieren method visualizes the density gradient in the optical path as a shadow. The preheat zone caused by the discharge channel can be also visualized. Even under no fuel conditions, the preheated zone was still detected by the Schlieren imaging [25], which has affected the accuracy of the  
Figure 1: Constant volume combustion chamber. (**a**) Front view; (**b**) Cross-sectional view.  
initial flame kernel area. Therefore, in this study, a high-speed infrared camera was used to detect the initial flame as most of the combustion products, such as carbon monoxide (CO), carbon dioxide (CO\({}_{2}\)), and water (H\({}_{2}\)O), absorb specific infrared wavelengths and radiate the infrared when the components turn to be hot. As shown in Figure 2, the infrared camera is a sensor-cooled infrared camera equipped with an indium antimonide (InSb) element, and it can store 1/4-inch filters inside. It is possible to shoot at 1000 fps with total pixels (640 \(\times\) 512 px) and at a maximum of about 30,000 fps.  
Simultaneous visualization method was carried out by a high-speed infrared camera (FLIR X6900 sc) and a conventional high-speed camera (Photron SA-X) in this study, as shown in Figure 3. The infrared dichroic mirror is utilized to reflect the light with 3000- nm wavelength and pass the light with 400-700 nm wavelength. According to the NIST Chemistry WebBook [26], as shown in Figure 4, CO has two high absorbances around 4600 nm and 4700 nm in the spectral range compared with CO\({}_{2}\) and H\({}_{2}\)O. Therefore, the bandpass 4560 \(\pm\) 135 nm filter is utilized in the high-speed infrared camera for capturing the infrared radiation images of carbon monoxide with 5000 frames per second (fps). The conventional high-speed camera is set up for taking the natural imaging of the discharge channel with 20,000 fps in the visible band.  
Figure 4: IR absorbance spectrum.  
Figure 3: Schematics of the imaging method.  
Figure 2: Infrared high-speed camera (FLIR X6900 sc).  
In addition, to confirm the preheat zone of infrared images, the test was conducted under both fuel and no fuel conditions. Under no fuel condition, there was no initial flame kernel generated. Still, the preheat zone will change the temperature at the vicinity of the spark plug via the discharge channel formation. Hence, it is necessary to confirm whether the preheat zone affects the filtered signal in the infrared images or not. As shown in Figure 5, the upper part is the no fuel condition, the lower part is the fuel condition. In each part, the top line is the CO radiation images taken by the infrared camera, and the lower line is the direct images taken by the conventional high-speed camera. The brightness of the initial flame kernel is saturated in the infrared image under fuel condition, while under the no fuel condition, the brightness of CO radiation is very weak and can hardly be confirmed between the spark plug gaps, and it can be found that the preheat zone by the discharge channel can be ignored compared with the initial flame kernel. Thus, the CO radiation images can detect the area of the initial flame kernel more accurately than the Schlieren imaging.