Early-Injection and Time-Resolved Evolution of a Spray for GDI Engines

An extensive experimental study on sprays from an injector for gasoline direct injection (GDI) engines has been performed. Spatial and temporal evolution measurements of a large cone-angle jet, emerging from a high pressure swirled injector, have been carried out in an optically accessible vessel. The spray has been lightened, both along the spray axis and in cross sections perpendicular to it, by a 532 nm Nd-YAG pulsed laser sheet, 80 mm thickness and 12 ns duration. The scattered light has been collected at 90° with respect to the sheet direction by a digital CCD camera with a frame grabber synchronized with the injection command and the laser pulse. A digital delay system has provided a fine temporal shift (up to microseconds range) of the images acquisition with respect to the start of the injection (SOI). Finally, a digital image processing system has provided analysing the images collected by the CCD camera. The emerging spray has been acquired with three spatial scales, providing both the global and local spray behaviour, and with a detailed temporal resolution to characterize the early stage of the jet formation. The initial phase of the spray is characterized by a strong axial component of the velocity with respect to the radial one, resulting in a pre-spray or slug phase. It produces a cylindrical shape of the jet with the characteristic mushroom shape. Large droplets with high momentum are produced, travelling downstream in advance to the main spray. At later time the radial velocity component controls the process and it gives up to the classical hollow-cone shape with a strong interaction with the gas in the vessel. The images give evidence in time of the collapse of the hollow-cone structure hence resulting in a full cone spray. This behaviour is confirmed by the cross section measurements through the spray, carried out in the range 10–40 mm from the nozzle tip. These measurements make evidence of the refilling of the cone with the presence of strong vortexes on the boundary of the jet. The effects of the fuel injection pressure, injection duration and air-flow field interaction on the structure and evolution of the spray have been studied in details. The digital image processing system also has allowed to reconstruct the spray profile and to determine a refilling index.