The combustion characteristics and soot distributions of neat butanol and neat soybean biodiesel were explored in an optical constant-volume combustion chamber using natural flame luminosity and forward illumination light extinction methods under various ambient temperatures (800 and 1000 K) and oxygen concentrations (21%, 16%, 10.5%). Results demonstrated that butanol had a higher normalized peak pressure compared with the biodiesel. Soybean biodiesel autoignited earlier than butanol at 21% and 16% oxygen, while a reverse trend was found at 10.5% oxygen. The oxygen concentration had little effect on the autoignition timing for butanol when it was between 16% and 10.5% at 800–1000 K. The natural flame luminosity reduced with lowered oxygen concentration and the flame distribution was notably increased at 10.5% oxygen. At 800 K ambient temperature, there was no soot formation detected for butanol, while the net soot release for soybean biodiesel was reduced with the decrease of oxygen concentration. At 1000 K ambient temperature, the net soot release increased for both butanol and soybean biodiesel with the decrease of oxygen concentration. Compared with butanol, soybean biodiesel had a higher value of normalized time integrated soot mass (NTISM) under all conditions. The value of NTISM increased about 16 times from 800 to 1000 K for the soybean biodiesel at 10.5% oxygen, indicating that low oxygen concentration will deteriorate combustion and increase the difficulty of soot emissions control for the soybean biodiesel under a higher ambient temperature.

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To investigate the effects of different alcohol additives in biodiesel fuel on the spray, combustion characteristics, and soot formation and oxidation, a detailed comparative study between the butanolÀbiodiesel blend and the ethanolÀbiodiesel blend was carried out in an optical constant volume combustion chamber. Two different volumetric blend fuels were tested in this study with different ambient temperatures at the start of injection (from 800 to 1200 K). The volumetric ratios were the 20% butanol/80% soybean biodiesel referred to as B20S80 and 20% ethanol/80% soybean biodiesel referred to as E20S80. Results demonstrated that the microexplosion occurred for B20S80 and E20S80 fuels at 800 and 900 K ambient temperature because of the volatility difference between the additives (butanol or ethanol) and the base fuel (biodiesel). The E20S80 fuel presented higher peak pressure and shorter combustion duration compared to the B20S80 fuel. The autoignition was earlier for the B20S80 fuel at 1000 and 1200 K ambient temperature, while the autoignition of the B20S80 and E20S80 fuels was nearly the same at 800 K ambient temperature. The E20S80 fuel had a lower flame luminosity compared to the B20S80 fuel. The soot distribution was increased downstream of the spray jet with a higher ambient temperature for both tested fuels, and E20S80 had a lower value of normalized time-integrated soot mass (NTISM). Therefore, E20S80 has more advantages to reduce the soot emission compared to the B20S80 fuel. Also, increasing the ambient temperature from 800 to 1200 K led to a rapid increase in the value of NTISM for both tested fuels. Therefore, a lower ambient temperature with the piston at top dead center (TDC) should have more advantages to combustion and soot control in a real diesel engine.

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