Diesel-Spray Ignition and Premixed-Burn Behavior 2000-01-0940

The temporal and spatial evolution of the ignition and premixed-burn phases of a direct-injection (DI) diesel spray were investigated under quiescent conditions. The diagnostics used included temporally resolved measurements of natural light emission and pressure, and spatially resolved images of natural light emission. Temporally resolved natural light emission measurements were made with a photo-multiplier tube and a photodiode, while the images were acquired with an intensified CCD camera.

The experiments were conducted in an optically accessible, constant-volume combustion vessel over a range of ambient gas temperatures and densities: 800-1100 K and 7.3-45.0 kg/m³. The fuel used was a ternary blend of single-component fuels representative of diesel fuel with a cetane number of 45. The fuel was injected with a common-rail injector at high pressure (140 MPa).

The results provide new information on the evolution of the two-stage ignition/premixed-burn phases of DI diesel sprays. For conditions with longer ignition periods, the low-temperature (cool-flame), first-stage ignition chemistry (indicated by chemiluminescence and slow heat release) occurs mainly downstream of any liquid-phase fuel, over a broad region. As the ignition period shortens, the first stage chemistry moves closer to the liquid-phase region, eventually surrounding the inner, liquid fuel containing region of the spray for the shortest ignition periods examined. When the transition to the second stage of ignition and the premixed-burn occurs, the transition is fast throughout the region of first stage chemistry. Soot (incandescence) is first observed near the peak in the premixed-burn pressure spike. The timing of ignition events based on pressure histories was found to depend on pressure measurement and analysis techniques, which has important implications for comparing model predictions with measured engine pressure data.