Ducted fuel injection (DFI) is a developing technology for reducing in-cylinder soot formed during mixing-controlled combustion in diesel compression ignition engines. Fuel injection through a small duct has the effect of extending the lift-off length (LOL) and reducing the equivalence ratio at ignition. In this work, the feasibility of DFI to reduce soot and to enable leaner lifted-flame combustion (LLFC) is investigated for a single diesel jet injected from a 138 μm orifice into engine-like (60-120 bar, 800-950 K) quiescent conditions.
High-speed imaging and natural luminosity (NL) measurements of combusting sprays were used to quantify duct effects on jet penetration, ignition delay, LOL, and soot emission in a constant pressure high-temperature-pressure vessel (HTPV). At the highest ambient pressure and temperatures tested, soot luminosity was reduced by as much as 50%. When ambient temperatures and/or duct diameters are decreased, soot reduction benefits are even more substantial. “Preignition” prior to the duct exit and degraded performance were observed for ducts with excessive standoff distance. Computational simulations of free and “ducted” fuel injections have captured many of these and other trends in jet penetration, LOL, and soot luminosity, thereby elucidating key physics of DFI.
Results indicate that injection of fuel through the duct initially limits air entrainment, resulting in a spray at the duct exit that is faster, cooler, and richer than a comparable free spray, all of which lead to LOL extension. Delayed air entrainment and higher jet momentum at the duct exit can lead to elevated levels of turbulent mixing downstream, persisting up to and beyond the LOL. Consequently, equivalence ratios near the LOL are comparatively lower, reducing soot produced in the burning jet. Application of DFI to achieve significantly lower particulate matter (PM) emissions in heavy-duty diesel engines is promising, though many challenges remain.
