Optical Investigation of the Impact of Pilot Ratio Variations on Natural Gas Diesel Dual-Fuel Combustion

Experiments were performed on a small-bore optically accessible engine to investigate diesel pilot ignition (DPI) and reactivity controlled compression ignition (RCCI) dual-fuel combustion strategies with direct injection of natural gas and diesel. Parametric variations of pilot ratio were performed. Natural luminosity and OH chemiluminescence movies of the combustion processes were captured at 28.8 and 14.4 kHz, respectively. These data were used to create ignition maps, which aided in comparing the propagation modes of the two combustion strategies. Lower pilot ratios resulted in lower initial heat release rates, and the initial ignition sites were generally smaller and less luminous; for increased pilot ratios the initial portion of the heat release was larger, and the ignition sites were large and bright. Comparisons between diesel pilot ignition and reactivity controlled compression ignition showed differences in combustion propagation mechanisms. DPI displayed a steady combustion propagation speed with regularly sized ignition sites. These sites grow into wedges that follow the shape of the diesel jets. From there, the combustion spreads to the spaces between the wedges and fills the field of view. RCCI has more dispersed ignition sites that do not necessarily correspond to the location of the diesel jets. These sites tend to grow rapidly, which suggests autoignition is the dominant combustion propagation mechanism.