The behavior of diesel fuel sprays at the end of injection is poorly understood, yet has important implications regarding diesel engine emissions. Recent research has shown that at the end of injection, an entrainment wave is created, causing the fuel spray to rapidly entrain ambient gas. This rapid entrainment creates a dilute mixture of fuel that may be a source of unburned fuel emissions. In this study, x-ray radiography is used to examine the end-of-injection behavior of diesel sprays. X-ray radiography permits quantitative mass distribution measurements in dense sprays, providing data that cannot be obtained with optical techniques. Analysis of the spray velocity at steady-state suggests an entrainment wave speed of several hundred m/s, which is supported by the appearance of a travelling entrainment wave at low ambient density. The spray density declines most rapidly near the nozzle, behavior that matches the expected entrainment wave behavior. In several cases, the spray distribution in a cross-section across the nozzle axis becomes smoother at the end of injection. Three-dimensional reconstructions of the spray density at the end of injection show that the spray plume widens considerably, enhancing the dilution caused by the reduction in spray mass in the flowfield. Measurements of injector needle motion with x-ray phase contrast imaging show that throttling across the needle seat may cause a smearing of the ideally sharp entrainment wave.

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