Soot Emissions of Various Oxygenated Biofuels in Conventional Diesel Combustion and Low-Temperature Combustion Conditions

To investigate the effects of oxygenated biofuels on soot formation, oxidation, and distribution, a detailed comparative study using the forward illumination light extinction method was conducted in an optical constant volume combustion chamber. Various ambient temperatures (800 and 1000 K) and ambient oxygen concentrations (21, 16, and 10.5%) were investigated to mimic both conventional diesel combustion and low-temperature combustion conditions. Five oxygenated biofuels were used, including neat soybean biodiesel S100, neat butanol B100, and three alcohol biodiesel blends that contained by volume 20% ethanol E20S80, 20% butanol B20S80, and 50% butanol B50S50. It is found that the composition of the biofuel has a larger effect on soot suppression efficiency at 800 K ambient temperature than that at 1000 K. Soot distribution is observed at larger distances from the injector, and less soot is located near the wall region with the additional oxygen content of biofuels at 21% ambient oxygen concentration. With a declining oxygen concentration, the soot concentration reduces at 800 K but increases at 1000 K. Soot formed in the spray jet region decreases at lower oxygen concentrations, and soot appears mainly near the wall region. Further, the soot distribution is more dispersed over a wider region at lower oxygen concentrations. B100 has shorter ignition delays at 10.5% oxygen concentration than B50S50 and S100 fuels, despite the fact that it has a lower cetane number. Therefore, the conventional correlation between ignition delay and cetane number does not hold for neat butanol at low oxygen concentrations. Soot concentrations are dramatically increased for soybean biodiesel from 800 to 1000 K at 10.5% oxygen, while such increases are not found for B50S50 and B100 fuels, indicating that proper choosing of the fuel will be very important to the high efficiency and clean low-temperature combustion.