In direct-injection gasoline engines, evaporating fuel wall-films and the resulting inhomogeneities of the air-fuel mixture near those films make the formation of PAH and soot in subsequent combustion likely. Different optical techniques are needed to visualize the links of this process chain, such as the spray, film formation, evaporation, combustion, and soot formation. In our model experiment, a mixture of isooctane (surrogate fuel) and toluene (fluorescent tracer) is injected by a multi-hole injector into an optically accessible flow channel. Air flows continuously through the channel at room pressure. Combustion is initiated by a spark plug within the fuel/air-mixture cloud. Some of the liquid fuel impinges on the quartz-glass wall on the opposite side and forms wall films. The turbulent flame front propagates along the chamber and ignites pool fires above the wall films, leading to locally sooting combustion. Laser-induced fluorescence (LIF) of the toluene using 266 nm excitation images the fuel-film thickness and visualizes the fuel vapor above the liquid films. Laser-induced incandescence (LII) using 1064 nm excitation visualizes soot. As a complementary visualization of soot, the natural flame luminosity, mainly from soot incandescence, is captured with a high-speed camera. Schlieren imaging combines the visualization of the evaporating liquid and the sooting flame. The LIF images show that indeed the fuel wall-films remain on the surface long after the flame front has passed, leading to subsequent soot formation. In a next step, we will simultaneously visualize the formation of soot precursors (PAH) and soot with LIF and LII, respectively.</p
