| Under the pressure of energy crisis and air pollution, it is urgent to develop a practical new alternative bio-fuel for transportation sector. Nowadays, with the growth of highway transport, the amount of trucks and passenger cars equipped with diesel engine increases rapidly, and their emissions especially soot emission is currently very high, thus finding a suitable diesel alternative bio-fuel for diesel engine is more urgent. Meanwhile, butanol has been widely regarded as next generation energy for diesel engine due to its diesel fuel similar properties and effects on soot and NOx emissions reduction. However, the bio-butanol cannot receive the competitiveness in the fuel market because of high cost on separation and purification during fermentation process. During the ABE fermentation process, bio-butnaol(the final product) is obtained by further separation and purifying from three flammable materials mixture, acetone-butanol-ethanol,(ABE)(the intermediate products). ABE is proposed to be directly used as alternative diesel fuel in the current work in order to reduce the cost and increase the market completeness of bio-butanol related alternative fuel. Fortunately, it is found that ABE exhibits a better solubility with diesel fuel and water than that of butanol, and its component ratios can be controlled in some degree by adjusting the bacterial strain and fermentation environment al conditions.In order to collect the fundamental and useful information for application of ABE/diesel mixture on diesel engine in the future, the comprehensive investigations of spray and combustion characteristics of ABE/diesel and its related fuels based on laser diagnostics technologies were carried out on a pre-burn type constant volume chamber. The experimental system was mainly formed by constant volume chamber, high speed camera, copper vapor laser, signal synchronization device, optical filters, light diffuser, and in-chamber pressure recording system, etc. Then, the Mie scattering spay images, broad-band natural flame images, CH* chemiluminiscence images, soot images, and heat release history during the process of injection and combustion were captured and analyzed. In addition, the ambient condition in chamber for injection can be controlled though adjusting the pressure and ratios of initial intake gases and injection timing on the chamber system. During testing, the ambient temperature was chose at 700 K, 800 K, 900 K, 1000 K, 1100 K, and 1200 K to achieve low temperature combustion mode and conventional combustion mode, while the ambient oxygen concentration was chose at 11%, 16%, and 21% to represent different EGR ratios for practical spray combustion on diesel engine. Since the concept of ABE is from bio-butanol, ABE component ratios are variable, and property difference between ABE components and diesel is large, the current work studied the impacts of difference between ABE and n-butanol, the difference among four butanol isomers, difference of ABE with varied component ratios, difference of blends with varied ABE ratios on fuel spray and combustion process. The main findings are as follows.With the changing of ambient injection condition, all the tested fuels shows some general trends, such as, with the ambient temperature and oxygen concentration decreasing, the combustion phasing retards, heat release rate increases and slightly reduces, combustion duration reduces and slightly prolongs, spray size increases, flame lift-off length increases, and the fame intensity reduces. In addition, all the tested fuels show very similar spray, combustion, and flame characteristics under high ambient temperature, while the difference between tested fuels was exhibited gradually with ambient temperature reducing. The four butanol-isomer/diesel blends exhibit very similar combustion and flame characteristics trends overall. Specifically, under high ambient temperatures, n-butanol/diesel blend shows almost the same heat release rate curve with other butanol-isomer/diesel blends, but when the ambient temperature reduced to 800 K, the n-butanol shows a stronger sensitivity on ambient oxygen concentration than other ones. Neat ABE performs a very similar spray and combustion characteristics with neat n-butanol, but the higher volatility and larger latent heat value make it show an improved spray performance and further retarded combustion phasing under low ambient temperature. In addition, the more fuel-borne oxygen content and higher flame lift-off length of ABE make it show a higher potential on soot emission reduction. The component ratio of ABE is able to influence the combustion characteristics of ABE/diesel blends significantly. By increasing the acetone content in ABE, ABE/diesel could achieve a very similar combustion phasing and heat release curve with diesel, while remained the capability to reduce soot emission and slightly increase thermal efficiency. As well, the ABE ratio itself could influence the spray and combustion of blends. There is a critical ratio, between 20% and 50%, that controls the spray performance, once ABE content crossed the ratio, an improved and similar spray performance will be achieved, if ABE content is below the ratio, a spray performance similar with neat diesel is remained. Consequently, ABE50 achieves a combustion characteristics close to those of diesel because it obtained a better balance between spray performance and latent heat when ABE ratio changing. For a certain fuel, the flame lift-off length is directly related to ignition delay, which is consistent with reported results. However, it is a different story for different fuels. With the increase of ABE content in ABE/diesel blends the flame lift-off length increases continuously, which is not consistent with the order of ignition delay. A new finding based on the tested result is the flame lift-off length does related to ignition delay but more close to the chemical kinetics prepare part, controlled by cetane number and molecular construction. The longer chemical kinetics preparation part in ignition delay corresponds to a higher flame lift-off length. Furthermore, with the increase of flame lift-off length, the entrained air quantity increased and air-fuel mixing strengthened, which reduce equivalence ratio of air-fuel mixture in combustion area, then reduce the flame intensity and increase the potential of soot emission reduction.Overall, the ABE mixture shows similar performance with n-butanol as an alternative diesel fuel but shows a higher potential on soot emission reduction, thus the existed research experience of butanol is useful for the further investigations. More important is that the spray and combustion characteristics of ABE/diesel blends is able be optimized by adjusting the ABE component ratio or blend ratio. |
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