Diffusion-Flame / Wall Interactions in a Heavy-Duty DI Diesel Engine

Over the past decade, laser diagnostics have improved our understanding of many aspects of diesel combustion. However, interactions between the combusting fuel jet and the piston-bowl wall are not well understood. In heavy-duty diesel engines, with typical fuels, these interactions occur with the combusting vapor-phase region of the jet, which consists of a central region containing soot and other products of rich-premixed combustion, surrounded by a diffusion flame. Since previous work has shown that the OH radical is a good marker of the diffusion flame, planar laser-induced fluorescence (PLIF) imaging of OH was applied to an investigation of the diffusion flame during wall interaction. In addition, simultaneous OH PLIF and planar laser-induced incandescence (PLII) soot imaging was applied to investigate the likelihood for soot deposition on the bowl wall. Using these diagnostics and a specially designed piston bowl-rim window, high-resolution images were acquired of the leading edge of the jet as it impinged on the combustion-bowl wall in an optically accessible diesel engine of the heavy-duty size-class. The images show that as the jet encounters the wall, the leading edge frequently deforms, flattening the diffusion flame along the wall surface. Despite being close to the wall, the flame appears to remain active for a short time before being extinguished along the portion of the jet that is against the wall. The simultaneous OH-PLIF and PLII-soot images show that this allows the soot within the combusting jet to impinge directly on the wall, making it available for wall deposition. Measurements of the soot/wall deposition and its potential as a contributing pathway to engine-out particulate emissions are discussed in a companion paper.