Effects of positive or negative dwell times of split injection on diesel spray development and mixture formation processes

An investigation on the effect of dwell time of split injection on a diesel spray evolution and mixture formation process was carried out. A commercial 7-hole injector were used in the experiment to eliminate the possible discrepancies on the spray with single-hole research injector. Laser absorption scattering (LAS) technique was implemented for the measurement of the temporal evolution of fuel evaporation and mixture concentration. The diesel surrogate fuel consists of n-tridecane and 2.5% of 1-methylnaphthalene in volume basis was used. The total amount of fuel injected was initially fixed to 5.0 mg/hole. A split ratio of 9: 1 in mass basis was selected according to the results obtained from a previous study. The dwell time was varied from 120 µs to a negative value of −50 µs. The effects of negative dwell time was not ideal for lean mixture formation when compared to zero or positive dwell time conditions. The collision of the spray tail of the first injection and the head of the second injection created locally rich pockets at the vicinity of the nozzle. Finally, injection rate shaping through modulating the pulses consisted of several injections of negative dwell times was investigated. This rate shaping strategy which showed a gradual decrease of the injection rate at the EOI (end-of-injection) timing yielded positive effect on mixture formation. It also exhibited faster lean mixture formation than that of single injection.