Abstract
In gasoline engines, including conventional gasoline direct injection (GDI) engines and newly developed gasoline compression ignition (GCI) engines, flash boiling of the spray occurs during throttling or low load operations. Superheated fuel that is injected into the cylinder, where the gas pressure is lower than the fuel's saturation vapor pressure, experiences a fast phase change. Plume interaction and spray collapse can occur as a consequence of flash boiling. The structure of flashing spray has not been well elucidated experimentally because of strong multiple-scattering effects in conventional laser sheet imaging due to illumination of out-of-laser-plane droplets. Here, the structured laser illumination planar imaging (SLIPI) is implemented for the first time to study flash boiling sprays. Both front-view and side-view cross-sections are examined to reveal spray behaviors during collapsing events. A comparison of the reconstructed 3D spray volume by SLIPI and conventional laser sheet imaging clearly shows the advantage of SLIPI in resolving the inner structure of the collapsed spray. The near-nozzle region on the injector axis is found to be hollow, indicating that spray collapsing occurs a bit downstream of the nozzle instead of immediately at the nozzle. This observation could not be obtained by conventional laser sheet imaging nor by diffused back illumination (DBI) techniques. In this work, the central tip observed in the 2D DBI image at R-p = 0.1 case has been proven to be not a 'central jet on injector axis' formed due to radial collapse, but a longer projection on the image caused by stronger adjacent plume circumferential interactions. Graphic abstract