In order to meet the ever-more stringent emission standards, significant efforts have been devoted to the research and development of cleaner internal combustion engines. Diesel combustion and the formation of pollutants are directly influenced by the spatial and temporal distribution of the fuel injected. This study investigated the effects of dwell angle of split injection on diesel combustion and emissions in a high-speed common rail direct injection optical diesel engine. The fuel injection system was characterized through the measurement of the fuel injection rate and quantity for the tested strategies on a fuel injection test rig. In particular, the interaction between the two injection events was identified. Effects of the split injection dwell angle and the interactions of the two consecutive injection events on diesel combustion and exhaust emissions were then investigated in the single cylinder optical engine using heat release analysis and optical diagnostic techniques. The fuel injection process was illuminated by a high repetition copper vapour laser and recorded synchronously by a high speed video camera. The combustion temperature and soot distribution during the combustion process were measured by a recently developed high speed two-colour system. The results indicated that this injection mode has the potential to improve fuel economy and engine performance while substantially reducing the combustion noise, provided that the injection timings are appropriately selected.