The HCCI Combustion Process in a Single Cycle – High-Speed Fuel Tracer LIF and Chemiluminescence Imaging

The Homogeneous Charge Compression Ignition (HCCI) combustion progress has been characterized by means of high-speed fuel tracer Planar Laser Induced Fluorescence (PLIF) combined with simultaneous chemiluminescence imaging. Imaging has been conducted using a high-speed laser and detector system. The system can acquire a sequence of eight images within less than one crank angle. The engine was run at 1200 rpm on isooctane or ethanol and a slight amount of acetone was added as a fuel tracer, providing a marker for the unburned areas. The PLIF sequences showed that, during the first stage of combustion, a well distributed decay of fuel concentration occurs. During the later parts of the combustion process the fuel concentration images present much more structure, with distinct edges between islands of unburned fuel and products. The transition between the evenly distributed fuel oxidation in the beginning and the large structures at the end, is most likely the result of a gradual amplification of small temperature inhomogeneities and an expansion of burned gas compressing the unburned.