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.

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Biodiesel is a type of particularly attractive alternative fuel for diesel engines. Many studies have investigated the combustion and emissions as fueling biodiesel on diesel engines and constant volume chambers. However, the understanding of the processes of biodiesel soot formation/oxidation is still limited. Therefore, in this work, high time-resolved quantitative soot measurements were carried out on a constant volume chamber by fueling soybean biodiesel. Three different ambient oxygen concentrations (21%, 16%, 10.5%) were tested at a conventional ambient temperature (1000 K) of diesel engine combustion and a lower ambient temperature (800 K). Results showed that the soot appearance was delayed at lower ambient temperatures and oxygen concentrations. At 800 K, less soot mass was observed with decreasing in oxygen concentration. However, soot mass increased with decreasing oxygen concentration as the ambient temperature reaching to 1000 K. To further illuminate the opposite trend on soot behavior in different temperature flames, a semiempirical biodiesel soot model was proposed and implemented into computational fluid dynamics (KIVA-3V, Release 2) code. Validation results showed that the proposed biodiesel soot model could successfully reproduce the entire process of soot formation and oxidation under various oxygen concentrations and ambient temperatures. With decreasing temperature, the appearance of intermediate species about soot formation/oxidation was delayed and the time-integrated mass of C 2 H 2 , soot precursors, OH radicals, and soot was reduced. The soot formation mechanism dominated soot evolution and caused a lower soot mass as the ambient temperature decreased. The formation of soot precursors presented a stronger temperature dependence than biodiesel pyrolysis. Regardless of whether the initial ambient temperature was 800 K or 1000 K, soot oxidation was significantly suppressed as the ambient oxygen concentration was reduced. However, the temperature did change the evolutionary tendency of soot formation with decreasing ambient oxygen concentrations. At 800 K, the time-integrated mass of acetylene and soot precursors and the regions of high equivalence ratios were reduced as the ambient oxygen concentration decreased; therefore, the soot formation was inhibited effectively at lower oxygen concentrations. At 1000 K, the time-integrated mass of acetylene and soot precursors and the regions of high equivalence ratios increased with the decrease of ambient oxygen concentration; therefore, the soot formation was motivated at lower oxygen concentrations. It can be concluded that soot formation transition was the responsible factor for the nonconsistent soot behavior, because of ambient oxygen dilution in conventional and low-temperature flames.

A phenomenological soot model of real biodiesel was proposed to investigate the effects of initial ambient temperatures on combustion and soot emission characteristics of soybean biodiesel. Validation experiments were conducted in an optically accessible constant volume chamber under four difference initial ambient temperatures: 1000, 900, 800, and 700 K. Good agreement was observed in the comparison of time-related soot measurement and prediction. Results indicated that ignition delay prolonged with the decrease of the initial ambient temperature. The heat release rate demonstrated the transition from mixing controlled combustion at a high ambient temperature to premixed dominate combustion mode at a low ambient temperature. Although the soot formation and oxidation mechanisms were both suppressed, biodiesel showed less soot tendency at a lower ambient temperature. Temporal and spatial distribution pictures indicated that the drop in ambient temperature did not cool the combustion temperature. The reduction of the soot mass concentration with the decrease of the initial temperature was caused by the shrinked total area of a local high equivalence ratio, in which soot usually generated fast. At 700 K initial ambient temperature, soot emissions were almost negligible; therefore, clean combustion might be achieved at super low initial temperature operation conditions.

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