Publications

Ducted fuel injection is a strategy that can be used to enhance the fuel/charge-gas mixing within the combustion chamber of a direct-injection compression-ignition engine. The concept involves injecting the fuel through a small tube within the combustion chamber to make the most fuel-rich regions of the micture in the autoignition zone leaner relative to a conventional free-spray configuration (i.e., a fuel spray that is not surrounded by a duct). This study is a follow-on to initial proof-of-concept experiments that also were conducted in a constant-volume combustion vessel. While the initial natural luminosity imaging experiments demonstrated that ducted fuel injection lowers soot incandescence dramatically, this study adds a more quantitative diffuse back-illumination diagnostic to measure soot mass, as well as investigates the effects on performance of varying duct geometry (axial gap, length, diameter, and inlet and outlet shapes), ambient density, and charge-gas dilution level. The result is that ducted fuel injection is further proven to be effective at lowering soot by 35–100% across a wide range of operating conditions and geometries, and guidance is offered on geometric parameters that are most important for improving performance and facilitating packaging for engine applications.

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Designers of direct-injection compression-ignition engines use a variety of strategies to improve the fuel/charge-gas mixture within the combustion chamber for increased efficiency and reduced pollutant emissions. Strategies include the use of high fuel-injection pressures, multiple injections, small injector orifices, flow swirl, long-ignition-delay conditions, and oxygenated fuels. This is the first journal publication on a new mixing-enhancement strategy for emissions reduction: ducted fuel injection. The concept involves injecting fuel along the axis of a small cylindrical duct within the combustion chamber, to enhance the mixture in the autoignition zone relative to a conventional free-spray configuration (i.e., a fuel spray that is not surrounded by a duct). The results described herein, from initial proof-of-concept experiments conducted in a constant-volume combustion vessel, show dramatically lower soot incandescence from ducted fuel injection than from free sprays over a range of charge-gas conditions that are representative of those in modern direct-injection compression-ignition engines.

Leaner lifted-flame combustion (LLFC) is a mixing-controlled combustion strategy for compression-ignition (CI) engines that does not produce soot because the equivalence ratio at the lift-off length is less than or equal to approximately two. In addition to completely preventing soot formation, LLFC can simultaneously control emissions of nitrogen oxides because it is tolerant to the use of exhaust-gas recirculation for lowering in-cylinder temperatures. Experiments were conducted in a heavy-duty CI engine that has been modified to provide optical access to the combustion chamber, to study whether LLFC is facilitated by an oxygenated fuel blend (T50) comprising a 1:1 mixture by volume of tri-propylene glycol mono-methyl ether with an ultra-low-sulfur #2 diesel emissions-certification fuel (CFA). Results from the T50 experiments are compared against baseline results using the CFA fuel without the oxygenate. Experimental measurements include crank-angle-resolved natural luminosity and chemiluminescence imaging. Dilution effects were studied by adding nitrogen and carbon dioxide to the intake charge. Initial experiments with a 2-hole fuel-injector tip achieved LLFC at low loads with the T50 fuel, and elucidated the most important operating parameters necessary to achieve LLFC. The strategy was then extended to more moderate loads by employing a 6-hole injector tip, where lowering the intake-manifold temperature, reducing the coolant temperature, and retarding the start-ofcombustion timing resulted in sustained LLFC at both 21% and 16% intake-oxygen mole fractions at loads greater than 5 bar gross indicated mean effective pressure. In contrast to the results with T50, LLFC was not achieved under any of the test conditions with CFA.

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Unlike conventional diesel engines, which have a negative ignition dwell, many strategies for low-emissions diesel combustion operate with a positive ignition dwell mode, where the ignition delay exceeds the injection duration. Although nitrogen oxides and particulate matter emissions can be reduced by operating with a positive ignition dwell, unburned hydrocarbon and carbon monoxide emissions typically increase. Sources of these emissions can stem from characteristics of the fuel spray after the end of injection, which may differ significantly from the main injection period where most spray models have been developed. To provide fundamental details of spray mixing during the end-of-injection transient, we have studied liquid-phase spray penetration and evaporation using simultaneous high-speed shadowgraph and Mie-scatter imaging for a single-hole, common-rail injector. Experiments were conducted over a wide range of ambient temperature and density in a constant-volume vessel. The experiments show that during the injection-rate ramp-down, the liquid penetration decreases (recedes towards the injector) from the quasi-steady-state distance for most diesel conditions. A transient jet entrainment model, coupled with the assumption of mixing-limited spray vaporization and direct measurement of the vaporized jet spreading angle, shows that this behavior is caused by a slower fuel delivery interacting with an increased rate of ambient entrainment during the injection-rate ramp-down. This increased mixing travels downstream as an "entrainment wave", permitting complete vaporization at distances closer to the injector than the quasi-steady liquid length. The position of the entrainment wave relative to the quasi-steady liquid length determines how far, and how quickly, the liquid recedes towards the injector. The tendency of recession increases with increasing ambient temperature and density because the transit time of the entrainment wave to the liquid length is shorter than the injection-rate ramp-down transient. Alternatively, the liquid-length recession is zero for conditions with low ambient temperature or density because the entrainment wave does not reach the quasi-steady liquid length until after the end of the injection-rate ramp-down. Copyright © 2008 by the Japan Society of Mechanical Engineers.