The use of flash boiling atomization has demonstrated potential in improving engine efficiency and reducing particulate matter (PM) emissions because of its stronger phase change process and consequentially finer droplets during the atomization. However, it is unclear how flash boiling has contributed to achieving such capacities in engine applications. This study utilized an optical engine facility operated with both subcooled and flash boiling sprays, with their combustion processes and emission characteristics recorded. To understand the properties of sprays and soot/soot precursor inside the cylinder, a high-speed external laser was used to illuminate the spray and soot. Then, the spray and soot distribution were evaluated by Mie scattering of the scatters, which is, more specifically, an elastic laser scattering (ELS) method for soot detections. Furthermore, to illustrate the soot motions inside the cylinder, the optical flow method was used to qualitatively capture the velocity of soot filaments under both conditions. It was found that flash boiling atomization achieves a better combustion performance with a higher indicated mean effective pressure (IMEP) and lower PM emissions, which is attributed to the stronger vaporization and faster oxidization of the soot/soot precursor under flash boiling conditions.