Impact of pilot injections on ducted fuel injection performance

This experimental and numerical study evaluates how ducted fuel injection (DFI) and pilot injections interact to impact soot formation and the premixed heat release pressure spike in diesel combustion. Experiments showed that pilot injections reduced the premixed heat release spike of a free spray by approximately 70%, while DFI configurations only experienced a decrease of approximately 25%. Similarly, pilot injections reduced the initial lift-off length (LOL) of the main injection of a free-spray by approximately 30%, while DFI’s initial LOL had little to no change when pilot injections were utilized. Regardless of whether a standalone-main or pilot-main strategy was used, DFI was able to reduce the spatially integrated natural luminosity (SINL) of the flame relative to a free spray, indicating a likely reduction in soot formation. Both duct configurations studied produced steady SINL signals which were approximately 30% and 70% of the free-spray’s, respectively. For DFI, pilot injections further reduced the peak SINL compared to a standalone main by approximately 16%. The decrease in peak SINL correlated with increased spray head penetration rates. The numerical study revealed that pilot injections led to leaner mixtures near the tip of the penetrating spray for all configurations. Thus, unaffected LOLs, leaner penetrating spray tips, and a reduced time for soot formation possibly led to less soot in the head of the transient penetrating spray when DFI is used in conjunction with pilot injections.