Ignitability of hollow cone gasoline/gasoline-ethanol sprays
Övrigt konferensbidrag, 2009
Powering vehicles by a fuel-blend of ethanol and gasoline (E85) is being considered as one of various possible ways to decrease fossil carbon dioxide emissions. There is great potential to both improve power and increase energy conversion efficiency using such blends in combination with direct injection by modern outward-opening piezo-actuated injectors. However, cold starts when using mixtures with high ethanol contents are problematic. This report presents results of ongoing observations of sprays from a piezo-injector under conditions similar to those that would be encountered in-cylinder during cold starts at -30°C (243 K) ambient temperatures.
Sprays of several fuels (gasoline, E75 and neat ethanol) have been monitored in a constant pressure, constant temperature spray chamber by both laser-induced fluorescence, to obtain understanding of the fuel vaporization process, and particle image velocimetry to map flow velocities and vortex formation inside the sprays and fuel clouds formed by their atomisation. In addition, the ignitability of the ethanol fuel sprays has been evaluated using focused laser light to obtain indications of the likelihood that similar sprays could be ignited in a real engine.
As expected, under the test conditions ethanol evaporates more slowly than the lighter components of gasoline. In experiments at various temperatures, with constant air density, the vortex structure inside ethanol fuel clouds varied substantially between cycles, but remained similar. The clouds consistently formed toroid shapes in which two counter-rotating vortices developed, and the first traces of vapour appeared at the centres of these vortices. In addition, the laser ignition tests showed that under possible in-cylinder conditions with a fairly high compression ratio (~12:1) it would be possible to ignite a stratified neat ethanol spray.