The effects of the split ratio on the mixture formation and combustion process of a diesel spray in a constant-volume chamber were experimentally investigated. A commercial seven-hole injector was used in this experiment. The effects of the mass-dependent split ratio and dwell time were observed when the total fuel injection was 5.0 mg/hole. Three split ratios were considered: 3:7, 5:5 and 7:3, while the dwell time of 120 µs was fixed for every condition. A laser absorption-scattering technique was adopted to examine the formation of mixtures with regarding to the equivalence ratio. A high-speed video camera was used to observe natural flame luminosity, and a two-colour pyrometer system was employed to evaluate the temperature and soot concentrations in the flame. Among the distribution ratios tested in this study, the 7:3 split ratio exhibited the best performance for the lean mixture formation considering the overall equivalence ratio distribution. The air entrainment wave at the end of injection timing of the first injection caused the fuel near the nozzle to lean at a rapid rate. The soot formation process for the 3:7 and 5:5 split ratios was observed because the second injection fuel caught the flame of the previous injections; this deteriorated the combustion region and influenced soot formation. The result also revealed that for the 7:3 split ratio, accelerated the soot deduction rate to the cycle of soot oxidation during the combustion period.

The effects of dwell time on the mixture formation and combustion processes of diesel spray are investigated experimentally. A commercial multihole injector with a 0.123 mm hole diameter is used to inject the fuel. The injection procedure is either a single or split injection with different dwell times, whereas the total amount of injected fuel mass is 5.0 mg per hole. Three dwell times are selected, that is, 0.12, 0.32 and 0.54 ms, with a split ratio of 7:3 based on previous findings. The vapour phase is observed, and the mixture formation pertaining to the equivalence ratio is analysed using the tracer laser absorption scattering (LAS) technique. A high-speed video camera is used to visualise the spray combustion flame luminosity, whereas a two-colour pyrometer system is used to evaluate the soot concentrations and flame temperature. An analysis of the mixture formation based on the spray evaporating condition reveals a more concentrated area of the rich mixture within a 0.32 ms dwell time. In the shortest dwell time of 0.12 ms, the equivalence ratio distribution decreases uniformly from the rich mixture region to the lean mixture region. In the case involving a shorter dwell time, a suitable position for the second injection around the boundaries of the first injection is obtained by smoothly growing the lean mixture and avoiding the large zone of the rich mixture. Therefore, the shortest dwell time is acceptable for mixture formation, considering the overall distribution of the equivalence ratios. Spray combustion analysis results show that the soot formation rate of the single injection and 0.32 ms dwell time case is high and decreases quickly, implying a rapid reduction in the high amount of soot. Consequently, 0.12 ms can be considered the optimal dwell time due to the ignition delay and relatively low soot emission afforded.