Experimental Study Of Flash Boiling Characteristics Of A Gasoline Direct Injection Multi-hole Injector

High-pressure multi-hole injectors have become the state-of-the-art choice forgasoline direct-injection (GDI) engine due to their flexibility in fuel targeting byselection of the number and angle of the nozzle holes. Flash boiling spray has beenshown to have significant effect in improving fuel atomization and combustioncharacteristics. Therefore, the characteristics of flash boiling spray of multi-holegasoline direct injector is researched with Mie scattering in constant volume vessel.Specifically, gasoline, n-pentane, n-hexane, n-heptane and pentane-heptane blends aretested at20,50,80and110℃injector body temperatures for ambient pressures of40,70and100kPa in order to simulate early homogeneous injection strategies forpart-load and wide open throttle engine operation. The characteristics of spraycollapse on flash boiling spray are represented by introducing concepts of cone angleof near-field and cone angle of far-field. The conclusions are drawn as following:Spray plumes contract in the axial direction of the multi-hole injector and reachflash boiling state when increasing the temperature of fuel and reducing theenvironmental pressure. The mechanism of flash boiling spray collapse is studied bymeans of comparing the spray characteristics of single-hole, double-hole and six-holeinjector. Quantities of tiny droplets generated from micro-explosion of flash boilingchange their directions under the action of air flow causing overlapping in6-holeinjector spray; moreover, air entrainment in the chamber exacerbate spray overlapping.Thus, numerous droplets gather in the axis of the injector to form a solid columnarspray and this phenomenon is spray collapse.In the flash boiling spray experiments of n-pentane and n-hexane, once thedegree of superheat of fuels exceed40℃the spray become narrow which is resultingfrom faster evaporation of droplets. When the superheat continue to increase to70℃,the number and the size of droplets involved in overlapping decrease which lead tolower spray brightness and loose spray.In the flash boiling spray experiments of n-heptane and n-hexane, the directcontrol factor of the spray’s penetration and shape is the degree of superheat ratherthan fuel type when fuels reach flare flash boiling. In the flash boiling spray experiments of blends of n-heptane and n-pentane, thetrend of cone angle of near-field is not only related to the volume fraction ofn-heptane but also related to the blends’ temperature. When the temperature of blendsis not more than80℃, decreasing the volume fraction of n-heptane weaken the flashboiling phenomenon and lead to smaller cone angle of near-field. However, increasingthe volume fraction of n-heptane causes the cone angle of near-field first increasedand then decreased if the temperature of blends is heated to110℃.Although the n-hexane and the blends containing50%volume fraction n-heptanehave the closer physical parameters, there are some differences in flash boiling spraycharacteristics between the two fuels. The blends containing50%volume fractionn-heptane is more prone to reach flash boiling than n-hexane, but the spray ofn-hexane is more solid after reaching flash boiling state.