Numerical investigation of a multiple injection strategy on the development of high-pressure diesel sprays

Computational fluid dynamics results are presented providing information on the influence of multiple injection strategy on fuel vaporization characteristics under conditions typical of direct injection, turbocharged, high-speed automotive diesel engines. The fuel is assumed to be injected from a high-pressure common rail injector nozzle. Focus is given on the number of multiple injections and dwell-time on the evaporating spray plume development. Comparison between the different cases is performed in terms of liquid and vapour penetration curves, the spatial distribution of the air-fuel equivalence ratio and the fuel vapour spatial distribution difference between the cases considered. The results confirm that, under the operating conditions investigated, the liquid penetration length, known to freeze at a distance from the nozzle exit, is not significantly affected by the injection strategy, while vapour penetration follows the time-shift of the dwell-time. Longer dwell-times retard the diffusion of the vapour in the carrier gas. Although injection of small fuel quantities prior to the main pulse does not affect the liquid penetration, it contributes up to 5 per cent more stoichiometric fuel vapour present in the area of observed auto-ignition sites. Post injection and splitting of the main injection in two Pulses modify the vapour distribution by creating two spatially separated fuel-rich zones.