Experiments were performed on an small-bore optically-accessible engine investigating diesel pilot ignition (DPI) and reactivity controlled compression ignition (RCCI) dual-fuel combustion strategies. Within these strategies, parameter sweeps were performed with high and low variations of pilot ratio, equivalence ratio, and injection pressure. Using high-speed cameras, natural luminosity and OH chemiluminescence movies of the combustion processes were captured. This data was used to create ignition maps, which aided in comparing the propagation modes of the two combustion strategies.
When comparing the data generated from the parameter sweeps, several trends were apparent. Lower pilot ratios usually resulted in lower initial rates of heat release, and the initial ignition sites were generally smaller and less luminous. Higher pilot ratios usually had the opposite effect where the initial portion of the heat release was larger, and ignition sites were large and bright. Lower equivalence ratios generally resulted in less natural luminosity signal, while higher equivalence ratios resulted in higher signal levels. Injection pressure seemed to have a significant impact on in-cylinder mixing. Higher injection pressures resulted in more dispersed ignition sites, and lower injection pressures had more concentrated ignition sites.
Comparisons between diesel pilot ignition and reactivity controlled compression ignition showed differences in combustion propagation mechanisms. DPI displays 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 suggested autoignition as the dominant combustion propagation mechanism.