Effect of the jet fuel cetane number on combustion in a small-bore compression-ignition engine

Three custom-made jet fuels with cetane numbers 30, 40 and 51 are investigated in an optically accessible small-bore compression-ignition engine. The low-temperature and high-temperature reactions are visualised using planar laser-induced fluorescence imaging of formaldehyde (HCHO-PLIF) and hydroxyl radicals (OH-PLIF) and is complemented with high-speed, line-of-sight integrated signal imaging of cool-flame and OH* chemiluminescence. For soot measurements, planar laser-induced incandescence (soot-PLII) imaging is performed. The cool-flame chemiluminescence and HCHO-PLIF images show that the low-temperature reaction develops quicker and covers a larger area in the combustion chamber for higher cetane number fuels. Also, the OH* chemiluminescence and OH-PLIF images indicate the transition from low-temperature reaction to high-temperature reaction occurs faster in both spatial distribution/concentration and temporal evolution, indicating the predominant effect of higher fuel reactivity despite lower charge premixing. The results obtained for single injection conditions are extended to pilot-main injection conditions in an attempt to further reduce the charge premixing to the level that its impact becomes measurable. Indeed, the significantly reduced charge premixing condition induces lower HCHO-PLIF and OH-PLIF for CN50 than those of CN40 while the soot-PLII is most intense. The CN30 exhibits the lowest soot-PLII than the other two fuels due to the enhanced charge premixing but the OH-PLIF signals are weaker due to the lower fuel reactivity. This study successfully identifies the important trade-off characteristics between fuel reactivity and charge premixing among the tested fuels and given injection conditions, CN40 shows the most optimised performance with the strong high-temperature reaction and low remaining in-cylinder soot.