Double-Pulse PLIF Imaging of Self-Ignition Centers in an SI Engine

In this study, the occurrence of auto-ignition centers in a two-stroke SI engine was investigated using planar laser-induced fluorescence (PLIF). An experimental SI engine equipped with glass windows to enable full optical access into the combustion chamber was operated under knocking conditions. The pulsed output of two XeCl excimer lasers was formed into planar light sheets (300 μm × 4 cm), which were spatially overlapped and directed into the combustion chamber of the operating engine. Unburned fuel components fluoresce strongly when illuminated with XeCl laser radiation; burned regions display no fluorescence. Self-ignited regions therefore show up as dark sites in the fluorescence images, indicating local consumption of the fuel. The resulting PLIF images were recorded using fast-gated ICCD cameras. By delaying the second laser pulse a specific time (100 ns-600 μs), image pairs were acquired which allowed the temporal development and mutual influence of hot-spots to be studied. The short laser pulse duration (20 ns), the two-dimensional nature of the imaged region and the strong signals obtained from our detection scheme yield very detailed information of the self-ignited regions. Approximately 20 000 image pairs were recorded. As important quantities, spatial distribution and expansion velocities were extracted from the PLIF images and investigated statistically.