Autoignition characteristics of diesel sprays under different injection conditions

An experimental study analyzing the complex relationship between external thermo fluid dynamic parameters and the ignition of diesel sprays, as well as flame propagation, is presented. The experiments were performed in a high-pressure, high-temperature system simulating the top dead center conditions in the combustion chamber of a diesel engine. The only simplification with respect to real diesel engines is relative to the aerodynamic pattern. In fact, in this case, the air flux is almost quiescent and does not influence significantly the spray evolution or the combustion behavior. The influences of two very different injection regimes on the autoignition characteristics of diesel sprays were investigated using imaging techniques. In order to perform this analysis, a careful choice of injection conditions, obtained with two different types of injection apparatus, was necessary. The spray evolution at both low and high temperatures (at the same air density), the temporal and spatial distributions of the ignition points, and the flame growth velocity were measured for the two cases. Analysis of the results revealed the specular behavior of the ignition spatial distributions ill the two cases. A first attempt to correlate these different behaviors with the spray-flame interaction is presented, and its implications are discussed.