Effects of End-of-Injection Transients on Combustion Recession in Diesel Sprays

End-of-injection transients have recently been shown to be important for combustion and emissions outcomes in diesel engines. The objective of this work is to develop an understanding of the coupling between end-of-injection transients and the propensity for second-stage ignition in mixtures upstream of the lifted diesel flame, or combustion recession. An injection system capable of varying the end-of-injection transient was developed to study single fuel sprays in a newly commissioned optically-accessible spray chamber under a range of ambient conditions. Simultaneous high-speed optical diagnostics, namely schlieren, OH* chemiluminescence, and broadband luminosity, were used to characterize the spatial and temporal development of combustion recession after the end of injection. As ambient conditions become cooler or contain less oxygen (i.e. less reactive), combustion recession transitions from strong sequential ignition, to weak separated pockets of ignition, and finally to the absence of combustion recession altogether. At ambient conditions that do not favor combustion recession, increasing the end-of-injection transient duration promotes combustion recession. At high temperatures and/or oxygen concentrations, and also with longer end-of-injection transients, soot recession was observed, suggesting a trade-off in the emissions impact of combustion recession. Combustion recession under relatively lean conditions can help to reduce unburned hydrocarbons, but if mixtures are too rich, combustion recession can promote increased soot emissions. A previously developed reduced-order model was also used to provide further insight. The model shows that the shape of the end-ofinjection transient is important. In particular, the minimum pressure at time of needle closing has a strong impact on combustion recession.