There were exhaust and intake valves in the upper and lower regions of the image, respectively, and the spark plug was located in the center of the cylinder. The yellow inner circle represents the optical area observed by the high-speed camera from the bottom view. Even though the bore size was 77 mm, the optical area was limited to about 62% of the bore because the diameter of the quartz window and the flame in the images were clipped on the boundary of the quartz window.