The goals of this study were to optimize multiple-stage fuel injections and optically analyze the effects of multiple injections of fuel on the combustion process in a heavy-duty compressed ignition engine. The engine experiments consisted of pilot injection and post-injection experiments. The pilot injection quantities and timings were varied. The post-injection quantities and number of post injections were analyzed. Combustion images was obtained to calculate the flame temperature and soot density. The results revealed that premixed combustion by a single injection led to engine noise and vibration because of the long ignition delay. The optimal pilot injection strategy improved the engine noise, vibration, thermal efficiency, and nitrogen oxide emissions. However, a pilot injection induced diffusion combustion, and an increase in particulate matter emissions was inevitable. The optimized post-injection promoted the oxidization of incomplete combustion materials and provided a lower peak cylinder pressure, resulting in low nitrogen oxides and particulate matter emissions. The combustion images revealed that the pilot injection caused a moderate increase in flame temperature and high soot production. The post injection, however, effectively reduced the soot density during the late combustion period, resulting in a low soot density at the end of the observation period.