Application of pilot injection strategies for enhanced ignition and combustion of a low reactivity fuel in a compression-ignition engine

The impact of pilot injection on ignition and combustion of a low-reactivity fuel is systematically investigated in an optically accessible light-duty compression-ignition engine. The role of key pilot-main injection control parameters, including the pilot injection timing, proportion and pilot-main injection dwell time, is found through line-of-sight integrated high-speed imaging of cool-flame and OH* chemiluminescence signals as well as planar laser-induced fluorescence imaging of formaldehyde (HCHO-PLIF) and hydroxyl (OH-PLIF). This suite of diagnostics enables visualisation of low-temperature reaction and high-temperature reaction during the extended ignition delay period of a selected low reactivity fuel with only 30 cetane number. The results show significant combustion enhancement associated with the pilot-main injection strategy for all injection timings tested. The enhanced ignition is most effective in the case of retarded injection timings with almost misfiring single injection operation recovered for stable and strong combustion due to the pilot mixture merged with the main fuel jet. This enhancement effect is further promoted when the proportion of pilot injection mass to the total fuel mass is increased or the dwell time between the pilot and main injection is decreased as the pilot mixture interaction with the main fuel jet is enhanced. Compared to the single injection, measured HCHO-PLIF signals are higher and OH-PLIF signals develop at a higher rate due to the pilot injection, indicating the pilot mixture addition enhanced both the low-temperature reaction and high-temperature reaction of the tested low-reactivity fuel.