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, 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 and main injection timing.
To gain more insight into in-cylinder processes affecting soot with post-injections, a new optical diagnostic technique is utilized. Diffused back-illumination imaging (DBI) of soot extinction has previously been used in a high-pressure constant volume vessel, but has not yet been reported in the literature for heavy-duty engines. The DBI 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 (FoV) quantifies the evolution of in-cylinder soot for roughly the downstream half of one diesel jet of the multi-hole injector throughout each cycle. The DBI imaging reveals that at these operating conditions, when the post injection is sufficiently short, the majority of the soot in the post injection is oxidized, thus allowing for increased load with similar soot emissions compared to a single-injection condition. A transient increase in entrainment that occurs after the end of injection (the “entrainment wave”) is a candidate explanation for the observed completeness of post-injection soot oxidation. Additionally, semi-quantitative comparisons of soot KL and natural luminosity (NL) trends reveal decreasing KL accompanied by increasing NL. This observation is consistent with an increase in post-injection soot temperature after the end of the post injection, which may further aid oxidation.