Imaging of Fuel-Film Evaporation and Combustion in a Direct-Injection Model Experiment

Abstract

Late-evaporating liquid fuel films within the combustion chamber are considered a major source of soot in gasoline direct-injection engines. In this study a direct-injection model experiment was developed to visualize and investigate the evaporation of fuel films and their contribution to soot formation with different diagnostic techniques. A mixture of isooctane (surrogate fuel) and toluene (fluorescent tracer) is injected by a multi-hole injector into a wind tunnel with an optically accessible test section. Air flows continuously at low speed and ambient pressure through the test section. Some of the liquid fuel impinges on the quartz-glass windows and forms fuel films. Combustion is initiated by a pair of electrodes within the fuel/airmixture. The turbulent flame front propagates through the chamber and ignites pool fires near the fuel films, leading to locally sooting combustion. Laser-induced fluorescence (LIF) of the toluene, excited by laser pulses at 266 nm, is used to image the fuel-film thickness and to visualize the fuel vapor, while laser-induced incandescence (LII), excited at 1064 nm, is used to visualize soot. In complementary line-of-sight imaging, the natural flame luminosity, mainly from soot incandescence, is captured with a high-speed camera and schlieren imaging, combining visualization of the fuel vapor and the sooting flame. The LIF images show that the fuel films remain on the wall surface long after the flame front has passed. The evaporation rate of the individual fuel films seems to be unaffected by combustion, indicating that conductive heat transfer from the wall is the limiting factor in evaporation. The visualization of both natural flame luminosity and LII show that soot formation occurs in small regions but always close to the fuel films.

Visualization of soot formation from evaporating fuel films by laser-induced fluorescence and incandescence

“…Film 6 has an initial mass of 0.37 mg and has evaporated completely at 125 ms aSOI. The fuel-film evaporation was measured with tracer laser-induced fluorescence, as described in [33]. The quartz wall is indicated by the gray stripe on the right.…”

Quantitative fluorescence-based imaging of in-cylinder fuel film thickness at kHz frame rates in a production-type direct-injection spark-ignition engine

“…In our experiment, distinct ''fingerprints'' of fuel films on the cylinder wall were not seen, as opposed to some results in the literature. 27,45,46 Additional experiments would be needed to clarify if the fuel-film outlines were just blurred by the limited optical resolution of the high-speed imaging system.…”

“…Usually, fuel impinging on a surface forms films of a clear shape, which is often referred to as the ''fingerprint'' of the injection. [26][27][28]45,46 In the area framed by the white dashed lines (5) in Figure 11, at 16 and 20.56 ms aSOI fuel-LIF signal is not distinguishable against the background signal anymore. However, at 21.67 ms aSOI, when the first piston ring has just passed this area, high LIF signal is detected on the first ring (6), suggesting that in fact there was a thin fuel film on the cylinder wall.…”

“…To create the fuel films of known thicknesses, precision shims can be placed between two plates and the gap in between is then filled with the fuel/tracer mixture. 20,27 Previous LIF studies of liquid fuel films mostly used imaging systems with low repetition rates (on the order of 10 Hz 20,27,28 ). In recent work we developed a kHzrate LIF imaging system to simultaneously visualize liquid fuel and oil films in an optically accessible DISI engine.…”

Large Eddy Simulations and Tracer-LIF Diagnostics of Wall Film Dynamics in an Optically Accessible GDI Research Engine

“…Aside lubrication problems, fuel wall films with fuel-rich regions in their vicinity are considered important sources of soot formation [18,19,20]. A detailled optical investigation in a wallguided optical SI engine [21] revealed two main mechanisms responsible for soot: 1) early during combustion due to the flame passing through regions with rich mixtures, much of this soot is however oxidized later on, and, 2) pool fires in the vicinity of walls lasting until late in the cycle when oxidation deteriorates [22,23]. Concerns with respect to the respiratory health effects of soot [24,25] have led to enforcement of corresponding legislation in many countries worldwide.…”