Early Investigation of Ducted Fuel Injection for Reducing Soot in Mixing-Controlled Diesel Flames 2018-01-0238
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 and reducing the equivalence ratio at ignition. In this work, the feasibility of DFI to reduce soot and to enable leaner lifted flame combustion 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 measurements of combusting sprays were used to quantify duct effects on jet penetration, ignition delay, lift-off length, 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 diameter are decreased, soot reduction benefits are even more substantial. ‘Pre-ignition’ 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, lift-off length, 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 lift-off length 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 lift-off length. Consequently, equivalence ratios near the lift-off length 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.