In the next future, improvements of direct injection systems for spark-ignited engines are necessary for the potential reductions in fuel consumptions and exhaust emissions. The admission and spread of the fuel in the combustion chamber is strictly related to the injector design and performances, such as to the fuel and environmental pressure and temperature conditions. In this paper the spray characterization of a GDI injector under normal and flash-boiling injection conditions has been investigated. The paper is mainly focused both on the capability of the injection apparatus/temperatures controller system to realize flash-boiling conditions, and the diagnostic setup to catch the peculiarities of the spray behavior. The work aims reporting the spray characterization under normal and flash-boiling conditions. Flash-boiling is a phenomenon that occurs in certain engine conditions when the in-cylinder pressure is lower than the saturation pressure of the fuel, and leads to internal boiling and vapor bubbles creation near the nozzle exit.
A customized sensing of the injector nose permitted the temperature control of the nozzle up to 360 K while a remote-controlled thermostatic device allowed the fuel heating from ambient to 393 K. An axially-disposed, 0.200 mm in diameter, single-hole injector was used with l/d ratio equal to 1 and static flow@100 bar: 2.45 g/s, using commercial gasoline. A 1.0 ms duration single pulse strategy was adopted at the injection pressure of 10.0 MPa. The spray evolved in a quiescent optically-accessible vessel pressurize at 0.05, and 0.1 MPa at ambient temperature of the gas (N2).
Images of the sprays were acquired by Mie-scattering technique using a C-Mos cycle-resolved high speed camera as detector, an off-line processing of the captured images gave tip penetrations and cone-angles vs. time from the start of injection. At flash-boiling conditions clear increasing of the spray-cone angle were observed with respect to the normal one producing consequences on the distribution of the fuel in the combustion vessel.