Time and space characterization of multi-hole GDI sprays for IC engines by images processing and PDA techniques

In this work a multi-hole GDI injector has been characterized for different operative conditions. The multi-hole approach makes possible a versatile distribution of the fuel in the engine combustion chamber, adequate to the setting requirements. Hollow-, full-cone or ellipsoidal footprint structures can be realized orienting the axes of the jets. The spatial and temporal behaviour of the jets, in terms of overall fuel distribution (penetration, cone angle) and local details (droplet size and velocities, break-up and coalescence phenomena), is indicative of the air/fuel mixture preparation for the combustion in the engine. Aim of this paper is to report the experimental work for characterizing the structure of a multi-jet gasoline spray in terms of droplets atomization and dispersion. A multi-hole injector has been used with a hollow-ellipsoid footprint structure of the injected fuel on a plane perpendicular to the spray axis. Commercial gasoline is injected (density 0.76 kg/dm³) with quantities ranging between 10 to 100 mg/str at injection pressures up to 20.0 MPa. High intensity flashes, synchronized with the injection system, have enlightened the emerging fuel from the nozzle developing in an optically accessible vessel in quiescent air at ambient temperature and atmospheric backpressure and the images of the jets have been captured by a CCD camera. The image processing techniques has enabled to extract the main parameters of the jets for characterizing their evolution. Velocity and droplet size measurements have been achieved by a Phase Doppler Anemometry (PDA) system at different locations along and off- axis of one jet. In the off-axis locations, average axial velocities around 100 m/s are reached for droplets near the nozzle exit while this value decreases to about 80 m/s downstream. Along the spray axis, the velocity increases up to 120 m/s. The droplets diameter, ranging in an interval between 17 and 22 micron, shows an inverse trend assuming higher values for droplets travelling at lower velocities, resulting higher at lower velocities and vice versa highlighting a fragmentation process that is dependent to the droplets speed.