SIDI Fuel Spray Structure Investigation Using Flow Visualization and Digital Particle Image Velocimetry

The instantaneous structure of a spark ignition direct injection (SIDI) engine fuel spray and the effect of injection pressure on spray structure have been investigated using flow visualization and digital particle image velocimetry (DPIV). Fuel spray experiments have been performed within a non-motored research cylinder. Instantaneous images of the fuel spray are illuminated using single Nd:YAG laser pulses (3-5 ns pulse width) formed into a sheet and passed through the fuel spray. These images are captured using a digital camera connected to an image acquisition board and computer. Flow visualization experiments for a production DI fuel injector, four injection pressures (2.1, 3.4, 4.8, and 6.2 MPa), and a 2 ms injector pulse width illustrate the evolution and instantaneous flow structure of the dense transient spray as a function of injection pressure. The experiments show that a minimum injection pressure is required to produce dense and uniform spray distributions with small fuel droplets. For DPIV, a sequence of two synchronized images are captured at user specified time separation, and a two-dimensional cross-correlation calculation of intensity is utilized for pattern recognition. The DPIV software provides spatially resolved two-dimensional flow velocity for the fuel spray. Results from these measurements show that the initial clump of fluid in the fuel spray travels with velocities as high as the highest velocities found in the fuel spray. These clumps most likely impinge on the piston top or cylinder wall prior to evaporation and represent potential sources of hydrocarbon emissions.