Effect of Injection Pressure on Flame and Soot Characteristics of the Biodiesel Fuel Spray

The authors studied the effect of injection pressure on nonevaporating spray and spray flame characteristics of biodiesel fuel injected by a common rail injection system in a constant volume combustion vessel. Two biodiesels, biodiesel from palm oil (BDFp) and biodiesel from cooked oil (BDFc) were investigated, including JIS#2 Diesel. Mie scattering technique was employed to investigate nonevaporating spray characteristics. High-speed direct photography and two-color pyrometry were applied for spray flame characteristics. Injection pressures of 100, 200, and 300MPa and ambient environment typical of diesel engine were used. Nonevaporating spray result showed that biodiesel fuels give longer spray tip penetrations and narrower spray angles especially for BDFp. Integrated flame luminosity of BDFp and BDFc show lower values compared to that of diesel at injection pressure of 100MPa, and integrated flame luminosity of BDFp and BDFc is even lower than that of diesel at injection pressures of 200 and 300MPa. Flame luminosity area follows a similar trend as that of integrated flame luminosity. Two-color pyrometry measurements show that the integrated KL factor of BDFp and BDFc yields smaller values than that of diesel at injection pressure of 100MPa, and BDFp and BDFc present even lower values of integrated KL factors than diesel at injection pressure of 200MPa. At an injection pressure of 300MPa, the soot formation is extremely low for BDFp and BDFc. This indicates that the effect of soot reduction by using BDFp and BDFc is increased at high injection pressures. Flame luminosity and KL factor show the same trend at each injection pressure. Flame temperatures of BDFp and BDFc are lower than that of diesel at injection pressures of 100 and 200MPa. BDFp and BDFc give higher flame temperature than diesel at injection pressure of 300MPa. A simplified estimate for fuel jet air entrainment was made to interpret trends between different fuels and injection pressures. Analysis indicates that oxygen from fuel molecule plays a significant role on soot formation propensity at injection pressures of 200 and 300MPa, and poor spray atomization and low mixing rate prohibit the effect of fuel molecule oxygen on soot reduction at injection pressure of 100MPa. Combination of nonevaporating spray and spray combustion measurements provides a unique opportunity to understand the difference of spray flame characteristics between biodiesels and diesel fuel.