High-Speed UV and Visible Laser Shadowgraphy of GDI In-Cylinder Pool Fire 2016-01-2165
For better understanding, model development and its validation of in-cylinder soot formation processes of Gasoline Direct Injection (GDI) engines, visualization of piston surface fuel wetting, vaporization and soot formation processes of in-cylinder pool fire via high-speed UV (266nm) and visible (445nm) laser shadowgraphy was attempted in an optically accessible Rapid Compression and Expansion Machine (RCEM). A direct-injection, spark-ignition and single-shot combustion event was achieved in the RCEM under engine-equivalent, simplified and well-defined conditions operated with engine speed 600 rpm, compression ratio 9.0, equivalence ratio 0.9 and natural aspiration. The tested fuel was composed of 70% iso-octane and 30% toluene by volume and the UV absorption by toluene enabled visualization of the in-cylinder fuel distribution. Significant UV absorption was caused also by in-cylinder soot particles, which was reasonably distinguishable from the fuel by comparing the UV shadowgraphs with visible shadowgraphs and direct photographs of soot luminosity taken under identical conditions. The obtained high-speed shadowgraphs successfully demonstrated the potential of the visualization technique for qualitative visualization of fuel spray, mixture formation, piston surface fuel wetting and vaporization, qualitative measurement of piston surface fuel film thickness and distinction between pool fire soot and bulk combustion soot, which will be beneficial for fundamental understanding of the process and the model development.