# Study of Discharge Characteristics on Ignition Performance via High-Speed Imaging in a CVCC
## 1 Introduction  
The burden of traditional resources and environmental concerns demand high efficiency and low emissions in internal combustion engines (ICEs) for on-road vehicle applications. As spark-ignition (SI) engines fueled with gasoline continue to be the primary power system for light-duty vehicles, there is a need to increase the thermal efficiency and decrease the exhaust emissions for the SI engine. In recent years, several advanced combustion technologies like highly boosted and lean or dilute combustion had increased the challenges of ignition control in SI engines [1, 2]. Prechamber igniter, as a new method, is also challenging the traditional SI systems, and [PERSON] et al. make a comparison between the prechamber and the standard SI systems, revealing that the knock-limited combustion phasing has improved in prechamber igniter method [3]. Therefore, the development of a next-generation spark ignition system is greatly needed to meet these challenges. A suitable ignition strategy will play a central role in optimizing the ignition system. Several studies were conducted to investigate the ignition performance in SI engines. [PERSON] et al. had employed a variable control device of discharge current instead of the conventional ignition system, and they suggested that, ideally, the ignition system should control the discharge current according to engine conditions [4]. [PERSON] et al. used a high-energy continuous discharge ignition system to show that a higher discharge current can help resist blowouts and improve EGR tolerance [5]. The effects of ignition on combustion variation in SI engines had been studied. Combustion variation is affected by the local spatial distribution of mixture concentration around the spark plug [6, 7] and the sparkignition position and timing [8]. Furthermore, it is well known that such combustion instability significantly impacts the subsequent flame propagation and pollutant emissions. To provide a deep insight into the discharge characteristics of the spark plug in the cylinder, the optical diagnostics method is considered an effective way to understand the ignition process, and lot of research has been conducted by using either optically accessible engines or the endoscope. [PERSON] et al. used a conventional ignition system to investigate the flow velocity and discharge channel behavior in an optical SI engine. They observed that higher flow velocities increase the discharge channel stretching the length and decrease in the ignition delay [9]. [PERSON] et al. investigated the discharge channel shortening phenomena including the short-cut and restrike under both ambient air and stoichiometric conditions, and results have shown that the new short circuit of the discharge channel is more easily formed inside the original discharge channel circuit under stoichiometric conditions [10]. [PERSON] et al. investigated the ignition phenomena using an endoscope to realize the in-cylinder optical measurement. It demonstrated that the initial combustion period correlated with spark stretch before the 1 st restrike and spark-stretch rate [11; 12]. [PERSON] et al. used the optical engine to study the effects of the spark plug gap on the flame kernel growth and engine performance by the method of PLIF, and as a result, the flame kernel growth area increased with the spark plug gap increases, while the engine performance increased slightly due to the reduction in cyclic fluctuation as the spark plug gap increased [13]. [PERSON] et al. also used the method of high-speed PLIF and PIV to analyze the spatial and temporal evolution of the fuel distribution on flame kernel development. Results have helped a better understanding of the nature of poor burning cycles at each dilution level [14; 15; 16]. In addition, due to the limit of the engine bench test, the initial flame formation during the ignition process still had not been studied very well.