## 1 Introduction
The additional information in relation to the flame-wall interaction process can also be derived when using with advanced wall thermometry technique [9]. There are a number of diagnostics that one can use to observe the flame, ranging from simple natural emission based such as chemiluminescence or natural-flame luminosity to more complicated techniques such as tomography of hydroxyl planar laser-induced fluorescence (OH PLIF). [PERSON] et al. investigated the effect of the timing of the first and second injection for an evenly split dual injection strategy in an optical engine [10; 11]. Performance parameters derived from in-cylinder pressure data are analyzed alongside high-speed natural flame luminescence images in order to obtain relationships between engine output and the physical properties associated with flame propagation. They have been combined with PIV in attempts to observe flame and flow-field simultaneously and allow for interesting behaviors to be observed [12; 13; 14]. For instance, [PERSON] [13] used oil seeds for PIV that would be evaporated by the flame and hence, creating flame tomography effect to observe the flame. While this provide some information on the flame growth, it was not combined with flow-field and PIV results and the flow-field inside the burned region is missing. Other approaches involve the combination of laser diagnostic techniques: PIV and OH PLIF where there would be multiple cameras and lasers required to be implemented [12]. As OH PLIF is one of the best diagnostics for  
Figure 1: Flow-field, flame propagation, and its interaction in a spark ignition engine. flame front, this set up has high accuracy and allows for derivation of more fundamental parameters such as turbulent burning velocity. However, OH PLIF is difficult to get enough signal intensity at high-speed to detect the change during the cycle. In this study, true simultaneous, crank-angle resolved imaging of the flame front propagation, and the measurement of flow-field was achieved by the application of high-speed Particle Image Velocimetry (PIV). PIV measurement under combustion environment is quite challenging for multiple reasons, including the interferences of the natural luminosity from the flame, the interaction between the PIV seeding materials, and the high temperature flame, as well as complications from direct injection operations. All these issues decrease the optical clarity necessary for successful implementation of PIV. This is also why it is difficult to be able to observe both the flame and flow-field simultaneously in a reacting engine environment. Through careful optimization of a high-speed PIV setup and small modification of our engines, we have been able to observe simultaneously the flame and flow-field inside an operating engine environment. From the same captured PIV images, flow-field information as well as flame characteristics can be processed through a series of commercially-available and inhouse-developed software. The setup has been demonstrated to be able to capture the results at various engine operating conditions and provide valuable information on the relationship of flame development and in-cylinder flow [15]. In this report, we will discuss with more details on the setup, accuracy of the results and its potential errors. Focus will also be placed on the differences between the flow-field of non-reacting and reacting engine environment as well as the cyclic reaction on flame movement and combustion quality with respect to the flow-field.