Impinging jets of fuel on a heated surface: effects of wall temperature and injection conditions

In spark ignition engines, the nozzle design, fuel pressure, injection timing, and interaction with the cylinder/piston walls govern the evolution of the fuel spray inside the cylinder before the start of combustion. The fuel droplets, hitting the surface, may rebound or stick forming a film on the wall, or evaporate under the heat exchange effect. The face wetting results in a strong impact on the mixture formation and emission, in particular, on particulate and unburned hydrocarbons. This paper aims to report the effects of the injection pressure and wall temperature on the macroscopic behavior, atomization, and vaporization of impinging sprays on the metal surface. A mono-component fuel, iso-octane, was adopted in the spraywall studies inside an optically-accessible quiescent vessel by imaging procedures using a Z-shaped schlieren-Mie scattering set-up in combination with a high-speed C-Mos camera. The arrangement was capable to acquire alternatively schlieren and Mie-scattering images in a quasi-simultaneous fashion using the same optical path. This methodology allowed complementing the liquid phases of the impact, obtained by the Mie scattering, with the liquid/vapor ones collected by the schlieren technique for determining both the phases inside a single cycle. A Delphi solenoid-activated eight-hole electro-injector was used, 0.165 mm in diameter, L/d=2 having a static flow of 15 cc/s @10.0 MPa. This injector is part of a set of six items, chosen by the Engine Combustion Network (ECN) for the gasoline characterization (Spray G), at defined injection conditions. The wall and ambient temperature ranged within 296 to 573 K, under atmospheric gas densities at the injection pressure of 20.0 MPa. The contours of the liquid phase and the vapor/atomized zone, indicative of impact evolution, were extracted by a customized algorithm operating on the data set. Repetition cycles at fixed conditions were carried out for a spread analysis on the events. Spatial and temporal evolutions were measured for the liquid and vapor/atomized phases in terms of fuel slipping (width) and rebounding penetration on the wall (thickness).