Publications

Liquid-phase diesel spray penetration during end-of-injection transient

Kook, Sanghoon; Pickett, Lyle M.; Musculus, Mark P.; Kattke, Kyle; Gehmlich, Ryan K.

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.