Mechanisms of post-injection soot-reduction revealed by visible and diffuse-background-illumination soot extinction imaging.

Paper #:
  • 2018-01-0232

Published:
  • 2018-04-03
Abstract:
Small closely-coupled post injections of fuel in diesel engines are known to reduce engine-out soot emissions, but the relative roles of various underlying in-cylinder mechanisms have not been established. Furthermore, the efficacy of soot reduction is not universal, and depends in unclear ways on operating conditions and injection schedule, among other factors. Consequently, designing engine hardware and operating strategies to fully realize the potential of post-injections is limited by this lack of understanding. Following previous work [1], several different post-injection schedules are investigated using a single-cylinder 2.34 L heavy-duty optical engine equipped with a Delphi DFI 1.5 light-duty injector. In this configuration, adding a closely-coupled post injection with sufficiently short injection duration can increase the load without increasing soot emissions. With increasing post-injection duration, the plateau in soot emissions eventually turns upward until the post-injection increases engine-out soot above that for a single-injection strategy at the same load. To gain more insight into in-cylinder processes affecting soot with post-injections, a new optical diagnostic techniques is utilized. Diffuse-Background Illumination Extinction Imaging (DBIEI), has previously been used in a high-pressure constant volume vessel [2], but has not yet been reported in the literature for heavy-duty engines. The DBIEI setup developed for this experiment enables quantitative 2-dimensional (2D) line-of-sight optical thickness (KL) measurements from soot extinction with a temporal resolution of 42 kHz . The high temporal resolution and relatively large field of view quantifies the evolution of in-cylinder soot for roughly the downstream half of one diesel jet of the multi-hole injector throughout each cycle. 2D maps of the local difference in line-of-sight soot with and without a post-injection reveals that at these operating conditions, and for sufficiently short post injections, the vast majority of the soot in the post injection is oxidized thus allowing for increased load with retained soot emission levels compared to the single injection conditions. References: 1: O'Connor, J. and Musculus, M., "In-Cylinder Mechanisms of Soot Reduction by Close-Coupled Post-Injections as Revealed by Imaging of Soot Luminosity and Planar Laser-Induced Soot Incandescence in a Heavy-Duty Diesel Engine," SAE Int. J. Engines 7(2):673-693, 2014, doi:10.4271/2014-01-1255 2: Skeen, S et al. “Extinction-based Imaging of Soot Processes over a Range of Diesel Operating Conditions” 8th U. S. National Combustion Meeting
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