GDI sprays for s.i. engines: evolution and wall impingement at different operative conditions

Direct injection of gasoline into the engine cylinders has become a very important approach for reducing the fuel consumption and the pollutant emissions. The sprays, generated by suitable injectors, play an important role in direct-injection spark-ignition (DISI) engines. The spray atomisation, the fuel distribution in the combustion chamber, the spray structures and their interaction with the gas and the wall, markedly affect the air-fuel mixing, the combustion process and the pollutant emissions. The impingement behavior of the liquid jet onto a solid wall was investigate acquiring useful details about the wall-guided stratified-charged combustion mode in DISI engines equipped with high-pressure swirled injectors. For different injection conditions in an optically accessible test chamber, such injection pressure, gas backpressure and wall temperature, the evolution of the impinging spray was visualized by image processing technique. Pictures of the spray, lighted by a pulsed laser sheet generated on the second harmonic of a Nd-YAG laser (532 nm), were collected on a CCD camera, with the frame grabber synchronized with the injection command and the laser pulse at different instant from the Start of Injection (SOI). In this paper detailed spatial and temporal evolutions of the impinging spray are discussed, showing interesting peculiarities of jets for the different operating conditions. The initial phase of a spray, emerging from a high-pressure swirled injector, is characterized by large droplets at the spray axis. Pre-spray structure appears due to the sac volume in the nozzle tip, being the axial component of the injection velocity greater than the radial one at the start of the injection. At later time, the radial component effects of the velocity of the incoming fuel, appear and the hollow-cone structure becomes stable. The pre-spray droplets penetrate all the way faster than the spray main body and they impact a wall controlled in temperature simulating the combustion chamber. Then, the main spray, hollow-cone structured, collides the wall covering the pre-spray and shifting the droplets outward respect to the axis of the spray. Air motion, induced by the spray collision on the wall, forms large eddies. Fuel droplets are involved in the curls and transported by the air motion. These phenomena were studied for all the investigated injection conditions and wall temperatures showing peculiar behaviors in the hollow cone structures, eddies formation, outward droplet velocities and spray vaporization.