Structures of Diesel Sprays in Isobaric Combustion Conditions

A model experiment is presented as a "bridge" case between fundamental research and studies on practical combustion systems. The evolution of a diesel spray in high-temperature (900 K), high-prossure (4 MPa), nearly quiescent (cross flow velocity of 1 m/s), oxidative (air) environment is characterized for two injection conditions that are of interest in light-duty diesel engines (1.6 and 6 mg/stroke/hole). Laser light scattering and emission imaging have been used for this purpose along with chemical analysis of material sampled with a fast sampling valve, at 35 mm of distance from the nozzle. The structure of the combustion processes for the two injection conditions show similar features. The oxidation has progressed significantly just after the autoignition. The main autoignition zone, revealed by the pattern of emission in the 500-800 nm spectral range, is located near to the nozzle between 5 and 15 mm. Soot formation extends in the whole field, very early after the autoignition time and it pervades regions around the centerline too. The carbon conversion to soot is no more than 3% also in the richest mixture formed in the injection typical of full load operation of light duty diesel engine and it proceeds simultaneously with the carbon conversion to CO and CO2. The simultaneous detection of oxidation products and unburned fuel and the nonuniformity of the scattering pattern justifies the occurrence of both oxidation and pyrolysis due to the partial mixing of fuel and air on length scales smaller than the sampling volume. The comparison of the diesel combustion structure presented in this paper with those reported in the literature, relative to injections of much higher fuel quantity, suggests that both the experimental conditions define prototypic classes of diesel-like processes, which should be considered as reference models in the analysis of apparatus dependent combustion processes in real diesel engines.