Ducted fuel injection (DFI) has been shown to be an effective method to significantly reduce soot formation in mixing controlled compression ignition (MCCI) diesel combustion. This reduction has been demonstrated in both combustion vessels and in an optical engine. The mechanisms driving the soot reduction are to date not fully understood. Optimal duct configurations are also not immediately evident. The objective of this study is to show the effects of two geometric variables, namely distance from fuel injector orifice exit to duct inlet (0.1-6 mm) for a 2x14 mm duct, and duct length variation (8-14 mm) at a given stand-off distance of 0.1 mm.
A 138 μm on-axis single-orifice injector operated at 100-250 MPa was used in a heated, continuous flow, constant pressure vessel with optical access. Results show that the shortest stand-off distance of 0.1 mm yields the greatest soot reduction for a 2x14 mm duct, but interpretation of this result is confounded by the fact that the ambient air entrained into the duct gets cooler and denser at shorter stand-off distances. Also, at the stand-off distance of 0.1 mm, the longer the duct, the greater the soot reduction. Additionally, the performance of a 2x8 mm duct placed at a longer stand-off distance is comparable to a longer duct with a short stand-off distance, i.e. the duct exit locations are similar. The results also show that greater soot reduction correlates with faster penetration, longer ignition delay, and shorter combustion duration. Stated differently, downstream of the duct the DFI diesel flame is faster, leaner, and hotter.
