End-of-injection behavior of diesel sprays measured with X-ray radiography

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 quantitatively probe the fuel mass distribution in diesel sprays at the end of injection. 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 traveling entrainment wave at low ambient density. The spray density declines most rapidly near the nozzle, a behavior that matches the expected entrainment wave behavior. The dilution of the spray plume is most prominent in the central dense region of the spray. Three-dimensional reconstructions of the spray density at the end of injection show that the spray plume considerably widens, enhancing the dilution caused by the reduction in fuel flow.