Evaporation of gasoline-like sprays from an outwards-opening injector studied with LIEF
Paper in proceedings, 2017
The design and operation of the fuel injection system plays an important role to achieve a suitable fuel distribution in the cylinder. The liquid fuel should evaporate and mix well with air, and avoid overly rich or lean zones as well as wall and piston wetting. Spray imaging in a constant-volume, constant-pressure chamber, provides a well-controlled environment for detailed characterization of spray development under different operational conditions. In this study planar laser-induced exciplex fluorescence is used since it enables separate imaging of both vapor and liquid phase of fuel simultaneously. As a surrogate fuel for gasoline n-hexane is used with fluorobenzene and diethylmethylamine added as exciplex-forming tracers. Fluorescence excitation is carried out with a parallel laser sheet from the fourth harmonic light of a Nd: YAG-laser (266 nm). Exciplex fluorescence images from liquid phase and monomer fluorescence images from vapor phase can be acquired by a single UV-sensitive CMOS camera equipped with a stereoscope having filters selectively transmitting monomer fluorescence at 295 nm and exciplex fluorescence at 355 nm. Sprays from an outwards-opening injector with an initial hollow-cone shape were investigated at gas pressures above atmospheric. The expanding conical fuel sheet breaks up and a torus-shaped vortex structure is formed with the vortices continuing to expand after the end of injection, which is the typical development using this type of injector. Fuel vapor is firstly observed at the same locations as the liquid drops, and is then accumulated into the center of the vortices. The fuel evaporation was found to be very slow at room temperature, whereas the vapor fraction was significant already at the end of injection above 40°C, and an even faster evaporation was observed above 55°C.