Liquid Penetration of Diesel and Biodiesel Sprays at Late-Cycle Post-Injection Conditions

The liquid and vapor-phase spray penetrations of #2 diesel and neat (100%) soybean-derived biodiesel have been studied at late expansion-cycle conditions in a constant-volume optical chamber. In modern diesel engines, late-cycle staged injections may be used to assist in the operation of exhaust stream aftertreatment devices. These late-cycle injections occur well after top-dead-center (TDC), when post-combustion temperatures are relatively high and densities are low. The behavior of diesel sprays under these conditions has not been well-established in the literature. In the current work, high-speed Mie-scatter and schlieren imaging are employed in an optically accessible chamber to characterize the transient and quasi-steady liquid penetration behavior of diesel sprays under conditions relevant for late-cycle post injections, with very low densities (1.2 - 3 kg/m ³ ) and moderately high temperatures (800 - 1400 K). Using #2 diesel fuel and a nozzle orifice size of 0.108 mm, our results show that the quasi-steady liquid penetration ranges from 40 mm (3 kg/m ³ , 1400 K) to over 100 mm (1.2 kg/m ³ , 800 K), conditions that can cause wall wetting and oil dilution problems in most engines. Wall wetting would increase when using biodiesel fuels, as the liquid penetration length of biodiesel is about 20% higher than that of #2 diesel. These measured liquid penetration lengths can be reasonably estimated using a simple mixing-limited vaporization model, but there is evidence that atomization effects may become important as densities approach these near-atmospheric levels. The maximum liquid penetration can be limited by implementing short injections that end prior to the steady-state jet development period. However, this transient penetration period is short under these very low density conditions, limiting the maximum injection duration to short injections with small injected quantities, which may not deliver enough fuel for practical application. Multiple short injections can deliver larger quantities of fuel without reaching the steady-state liquid length.