Experimental investigation of mixing phenomena for ducted fuel injection

Ducted fuel injection (DFI) is a novel fuel injection scheme which has been shown to mitigate soot formation in mixing-controlled compression-ignition (MCCI) engines, while not producing more NOx. However, the underlying physical phenomena leading to soot reduction in DFI are not well understood. To better understand these mechanisms, DFI and free sprays were studied in a high-pressure vessel across various conditions and geometries using optical diagnostics. Schlieren imaging, OH* chemiluminescence, and broadband luminosity were used to characterize the reacting flow, while 2D laser absorption and scattering (LAS) was used to quantify the non-reacting mixing fields. The experiments show that the duct leads to enhanced air entrainment upstream of the theoretical impingement point between the duct and spray, i.e. the jet-pumping effect; that DFI mitigates soot formation by creating a leaner mixture at the lift-off length; that injection pressure does not have a significant impact on the mixing field for a given DFI configuration; and that a reduction in residence time for soot formation between the lift-off length and soot oxidation, influenced by injection pressure, is another driver for DFI performance. Overall, this study provides solid evidence of the physical mechanisms by which DFI reduces soot, enabling future optimization of design and operation.