## 1 Introduction  
The current stringent emissions regulations as well as demands for better fuel economy in passenger vehicles require the combustion process in engines to be much more efficient while being cleaner overall. While other mode of powertrain such as electric or hybrid are developing, their drawbacks mean the combustion engines will still need to be improved. Researchers have proposed many approaches in order to achieve cleaner, more efficient combustion engines, including ultra-lean burning, high compression ratio, high exhaust gas recirculation (EGR), high intake charge pressure and others. Due to its strong effect on the overall combustion process and the complex in-cylinder turbulent environment [1; 2; 3], understanding the interaction of in-cylinder flow-field and flame development will be crucial in optimizing the effects of the aforementioned strategies. Due to its importance and their intricate relationships, researchers have been working toward imaging both flow-field and flame imaging simultaneously. Flow-field measurement techniques have been quite well studied and developed, with diagnostics such as (Particle Image Velocimetry) PIV, (Laser Doppler Velocimetry) LDV and their variations providing useful information for engine researchers. LDV is a point-based measurement technique that provides the temporal flow velocity variations at a high sampling rate. Due to its setup, it is possible for a small probe of LDV to be made and applied to a production engine. For instance, special spark plug modified to contain LDV optics has been made and applied by engine researchers to provide flow-field information near the discharge location [4; 5]. Despite its benefits, LDV cannot provide the important spatial information of in-cylinder flow, which is where PIV excels. In recent time, the advances in laser development has made powerful,high frequency laser available for researchers. Hence, PIV can now be applied to not just capture spatial characteristics of the in-cylinder flow but also its temporal evolution and development [6; 7; 8]. Most of the previous works about in-cylinder flow diagnostics of internal combustion engines were performed under motoring condition without combustion by using optical engines. However, the phenomenon in a practical engine is different from this condition. Figure 1 shows the phenomena of flow-field, flame propagation and its interaction in a spark ignition engine. During intake and compression stroke before ignition, the differences in flow-field between motoring and firing condition need to be discuss. For firing case, residual gas exists and its effect on flow-field, such as mean flow, turbulence intensity and scale, as well as cyclic variation should be investigated. After the ignition timing, one of the most interesting topics is the effect of flow-field on spark discharge, flame propagation speed, heat release and combustion duration. The large-scale flow will stretch the spark discharge and shift the propagating flame. The small-scale vortex will affect flame configuration and reaction zone thickness. The correlation between flow-field and flame propagation in the same cycle can provide the clear indication of the optimum velocity and turbulence distribution in-cylinder to realize faster flame propagation and combustion. As a result, detail analysis in these areas can provide valuable information for engine design to improve the thermal efficiency. Another point of interest is the effect of flame propagation on the flow-field in unburned mixture. Does the flame enhance or decay the turbulence of unburned mixture? Enhancement in the understanding of physical phenomena and accuracy of engine combustion simulations can be obtained from such results. Furthermore, heat loss is the one of the more important issues for thermal efficiency improvement and the information regarding on flame attachment to the wall as well as the corresponding velocity can help explain the heat transfer between flame wall (surface of piston top and cylinder head).