In this study, air entrainment, fuel evaporation and mixing process of diesel sprays injected by micro-orifices for direct-injection diesel engines were investigated at the end of injection transient and after the end of injection. The mixture formation process was analyzed using a laser absorption scattering (LAS) technique, providing the information of quantified liquid and vapor mass concentration, entrained air concentration and equivalence ratio. The data was obtained at the timings of quasi-steady state, sudden velocity decrease, the end of injection and after the end of injection. Two micro-orifices, which have different orifice diameters, were selected as test nozzles to investigate the end-of-injection characteristics at different nozzle geometries.
In case of smaller orifice diameter, the liquid phase regression was observed around the end of injection, while it was not observed at larger orifice diameter due to denser liquid concentration near the nozzle tip. Subsequently, the fuel evaporation of smaller orifice diameter was enhanced around the end of injection compared to that of larger orifice diameter. At around the end of injection, the fuel concentration and equivalence ratio were decreased near the injector tip region, since the perturbation induced entrained motion (entrainment wave) strongly attacks that region. In contrary, the fuel concentration and equivalence ratio were stagnant at the spray tip region due to the weakened spray momentum. Long time after the end of injection (later than 0.5ms after the end of injection), the flow perturbation transferred to the entire spray, subsequently, mean fuel concentration and equivalence ratio were being uniform to the axial and radial directions both. Based on the above results and the previous researches, a conceptual model was suggested to describe the mixing process of diesel spray around the end of injection.