Abstract Pool fire and fuel-rich combustion have been considered the primary soot/particulate matter sources for gasoline direct injection (GDI) engines, which is a crucial issue for commercial and passenger vehicles. Flash boiling atomization, achieved by heating the fuel before injection, can notably improve spray atomization and reduce the occurrence of pool fire, thus reduce soot emission under extreme conditions. This investigation compared the performance of subcooled spray combustions and flash boiling spray combustions with the use of an optical engine facility. The optical engine was equipped with an optical liner so that side views of the combustion can be captured with a high-speed color camera. The high-speed measurement data from early injection conditions were then analyzed with the HSV color model to investigate the flame characteristics in the premixed, infrared, and diffusion flame regions. Indicated mean effective pressure (IMEP) and particulate number (PN) under different conditions were also analyzed under different conditions. It was found that the combustion performance using flash boiling sprays is superior to that using subcooled sprays, and the difference between the two combustion modes was discussed with the use of the flame model.