Entrainment Waves in Diesel Jets

Recent measurements in transient diesel jets have shown that fuel in the wake of the injection pulse mixes with ambient gases more rapidly than in a steady jet. This rapid mixing after the end of injection (EOI) can create fuel-lean regions near the fuel injector. These lean regions may not burn to completion for conditions where autoignition occurs after EOI, as is typical of low-temperature combustion (LTC) diesel engines. In this study, transient diesel jets are analyzed using a simple one-dimensional jet model. The model predicts that after EOI, a region of increased entrainment, termed the “entrainment wave,” travels downstream at twice the initial jet propagation rate. The entrainment wave increases mixing by up to a factor of three. This entrainment wave is not specific to LTC jets, but rather it is important for both conventional diesel combustion and LTC conditions. It is shown to be responsible for (i) observed over-mixed regions and (ii) rapid stagnation of mixtures near the injector, (iii) spatial shifts in the location of the onset of soot formation, (iv) increased soot oxidation after the end of injection, (v) decreased penetration of short injections, and (vi) detachment, retreat, and splitting of the liquid part of the vaporizing fuel spray. Finally, the model predicts that a faster ramp-down of injection rate at EOI creates a stronger entrainment wave forming leaner mixtures near the injector more rapidly.