Near-nozzle spray and spray collapse characteristics of spark-ignition direct-injection fuel injectors under sub-cooled and superheated conditions

There is limited literature available regarding the spray collapse process and the characteristics of flash boiling spray near the critical location of the nozzle exit, even though flash boiling has been proven to produce finer fuel spray with much improved vaporizing quality, and liquid fuel atomizes quickly soon after it is discharged from the nozzle. In this study, the near-nozzle fuel spray of various types of spark-ignition direct-injection prototype multi-hole injectors was investigated under a wide range of sub-cooled and superheated conditions using a high-speed backlit imaging technique. The effects of fuel temperature, ambient pressure, nozzle L/D ratio, and nozzle number, as well as near-nozzle spray characteristics and spray collapse phenomenon under superheated conditions, were studied. Experimental results revealed that both increasing fuel temperature and decreasing ambient pressure resulted in faster fuel jet disintegration and wider fuel plume due to flash boiling. In-nozzle fuel evaporation and outside-nozzle fuel boiling were the primary influences governing flash boiling spray atomization under superheated conditions. Spray collapse length increased with decreasing Pa/Ps ratio; it also increased with the elapse of injection time. Flash boiling spray also has the potential to resolve injector deposit issues with significantly improved end-of-injection performance. (C) 2016 Elsevier Ltd. All rights reserved.