## 1 Introduction
investigated the impact of intake air dilution with nitrogen, spark plug orientation, ignition system dwell time, and fuel injector targeting on ignition stability in an optical spray-guided spark ignition direct injection engine and identified the potential reasons for misfires [13]. [PERSON] et al. showed that it is possible to decrease heat loss and improve ignitability with the addition of a flow guide plate to promote a gas flow stream to the spark gap [14]. [PERSON] indicated that under conditions where heat losses and electrical energy are relevant for the initial flame kernel growth, the flow velocity was identified as the most dominant single parameter affecting the initial kernel growth [15]. This enhancement mechanism of the stretch the spark plasma creates most likely is due to the increase in ignition volume, as well as less heat loss to the metal electrodes. In a realistic engine environment, the in-cylinder flow surrounding the spark plug has been shown to have a correlation with the early combustion period, with stronger flow more likely to result in faster combustion. Optical studies in optical engine has also shown that the flow across the spark plug causes the plasma arc to stretch [13; 16]. Moreover, when the spark plug orientation is fixed to match the flow or when the spark plug geometry is optimized for flow interaction, an improvement in lean limit is observed [14]. Hence, it is very likely that only certain characteristics of the flow surrounding the spark plug have some positive correlation with the stretch of the spark plasma, while others might have a negligible effect. In fact, having an understanding on such specific correlation is quite important for combustion performance. As the flow velocity and turbulence intensity reach a very high level, detrimental effects on the early kernel stability is prominent. An increase in misfire rate is attributed to high turbulence level restricting early growth of the kernel and blowing off of the spark plasma channel, which cuts off the energy supply to the mixtures [16]. However, despite the research effort, there is still a gap in the knowledge relating to the exact correlation between flow-field and discharge stretch length. This is mainly the result of the high difficulty in imaging the spark channel and flow-field simultaneously in a realistic operating engine environment. Besides the existing difficulties that all engine optical diagnostics face, such as vibration or interferences, in spark ignition (S.I.) engines with a pent-roof, the spark plug is commonly placed in locations that are difficult for the penetration of a laser in diagnostics, such as particle image velocimetry (PIV) or laser induced fluorescence (LIF). Furthermore, to have the best understanding possible of this ignition process, not only do the spark and the flow-field need to be measured but the resulting kernel should also be visualized in the same cycle. This means the diagnostics have to be carried out in a reactive environment. Previously, we have demonstrated our application of high-speed PIV to visualize both the in-cylinder flow-field and flame front in an optical engine [4; 17]. A newly developed high-speed PIV technique that allows for time-resolved measurement of both flame front and flow field simultaneously was applied to observe the flame structure and its relationship to the engine flow and turbulence [4]. Moreover, the diagnostics were performed for both the vertical plane and the horizontal plane of the combustion chamber with a particular focus on the pent-roof area [17]. This diagnostic technique is very meaningful for investigating the interaction between flow field and flame propagation for each cycle and for discussing the cyclic variation. In this study, to attempt to expand the understanding of the ignition process, particularly the interaction between flow-field and spark, we added high-speed imaging of the spark channel together with high resolution, high-speed combustion PIV.