When fuel at elevated temperatures is injected into an ambient environment at a pressure lower than the saturation pressure of the fuel, the fuel vaporizes in the nozzle and/or immediately upon exiting the nozzle; that is, it undergoes flash boiling. It is characterized by a two-phase flow regime co-located with primary breakup, which significantly affects the spray characteristics. Under flash boiling conditions, the near nozzle spray angle increases, which can lead to shorter penetration because of increased entrainment. In a multi-hole injector this can cause other impacts downstream resulting from the increased plume to plume interactions.
To study the effect of injector temperature and injection pressure with real fuels, an experimental investigation of the spray characteristics of a summer grade gasoline fuel with 10% ethanol (E10) was conducted in an optically accessible constant volume spray vessel. A gasoline direct-injection injector with six holes typical of a side-injection engine was studied. Optical diagnostics included high-speed photography with alternate frame imaging from Mie-Scattering and Shadowgraph techniques. Ambient conditions representing Early Injection (45°C, 1 bar) and Late Injection (180°C, 4bar) conditions representative of gasoline direct injection events were studied at injector temperatures from 75 to 250°C and at injection pressures of 100, 150, 200 and 250 bar.
Results showed that for early injection condition, increased fuel temperature leads to two primary effects due to flash boiling: (i) an appreciable increase in spray angle near the nozzle exit followed by (ii) a decrease downstream of the nozzle due to the interaction of the plumes and collapsing sprays. For the early injection condition, spray penetration was observed to be minimum at 100°C followed by an increase in penetration at higher temperatures due to the collapsing sprays. For the late injection condition, the spray angle at the exit and downstream of the nozzle decreased with temperature. Besides, increased injection pressures lead to increased spray penetration due to higher injection momentum of the sprays outperforming the plume to plume interactions.