Effect of Injection Pressure on Ignition, Flame Development and Soot Formation Processes of Biodiesel Fuel Spray

The effect of injection pressure ranging from 100 to 300MPa on the ignition, flame development and soot formation characteristics of biodiesel fuel spray using a common rail injection system for direct injection (D.I.) diesel engine was investigated. Experiments were carried out in a constant volume vessel under conditions similar to the real engine condition using a single hole nozzle. Biodiesel fuels from two sources namely; palm oil (BDFp) and cooked oil (BDFc) with the commercial JIS#2diesel fuel were utilized in this research. The OH chemiluminescence technique was used to determine the ignition and the lift-off length of the combusting flame. The natural luminosity technique was applied to study the flame development and the two color pyrometry was applied for the soot formation processes. Ignition delay decreased as the injection pressure progressed from 100 to 300MPa. This was as a result of the enhanced mixing achieved at higher injection pressures. The BDFp's higher cetane number facilitated shortest ignition delay when compared to the other fuels under all injection pressures. For all the fuels, the lift-off length increased as the injection pressure increased. The percentage of the stoichiometric air entrained upstream of the lift-off length by the BDFp was the lowest. The integrated, average natural luminosities and flame areas of the BDFp and BDFc were smaller at increasing pressures as compared to that of the diesel fuel flames. At the 100MPa injection pressure, the two color pyrometry measurements indicated that there was no significant difference in the integrated and averaged KL factors for all the fuels. At 200 and 300MPa injection pressures, the BDFp and the BDFc presented much lower integrated and averaged KL factors than diesel. The averaged flame temperatures of the BDFp and the BDFc were found to be lower than that of diesel except at the 300MPa injection pressure. The oxygen contents in the BDFp and the BDFc fuels played a significant role on the soot formation process.