Analysis For Physicochemical Property Of Biodiesel-Diesel Blends And Their Improvement For Spray Characteristics

Biodiesel is a kind of renewable green energy, and its application and development are very important for national energy security and environmental protection. Because of different properties between biofuel and petroleum fuel, many special problems should be solved in the process of biodiesel's application.The compositions of biodiesel were qualitatively and quantitatively analyzed by infrared spectrometry and gas chromatography. The results indicate that the main compositions of biodiesel are methyl linoleate and methyl oleate, the biodiesel contains altogether 14 kinds of fatty acid methyl ester in which the content of saturated fatty acid methyl ester is 29.39% and the content of unsaturated fatty acid methyl ester is 67.66%. According to the biodiesel' compositions, the molecular formula of biodiesel, described by the formula C₁₈H₃₄O₂, can be worked out.Different mixing ratio blended fuel with biodiesel and diesel was used as research subject and then the various physical and chemical parameters of the blended fuel were determined by GB testing methods. The results show that: firstly, the low-temperature fluidity of biodiesel is comparable with 0^# diesel. Secondly, some biodiesel's parameters are distinctly higher than diesel's, such as viscosity, density, surface tension, flash point and distillation temperature, so above mentioned parameters of blended fuel rises with increasing biodiesel's blending ratio. When biodiesel's blending ratio is more than 25%, 50% distillation temperature of blended fuel exceeds the legal GB limit of diesel. Thirdly, the sulfur content of biodiesel is significantly lower than that of diesel. Biodiesel's acid value and copper corrosion are higher than those of diesel. When biodiesel's blending ratio is more than 40%, acid value and copper corrosion of blended fuel exceed the the legal GB limits. Fourthly, oxidative stability of biodiesel is poor. If biodiesel is stored for more than three months at room temperature, peroxide number, acid value and viscosity of biodiesel will rise up. The results of accelerated oxidation test indicate that if biodiesel's blending ratio is more than 50%, oxidative stability of blended fuel will be greatly influenced by biodiesel and will distinctly become inferior. So the blended fuel needs to depend on anti-oxidants to improve its oxidative stability. While biodiesel's blending ratio is less than 25%, the induction period of oxidation stability of blended fuel can meet the GB limit under the condition of no adding any anti-oxidants. Fifthly, swelling volume of nitrile rubber will be larger than that of fluoroelastomer when they are dipped into blended fuel of biodiesel and diesel. After nitrile rubber and fluoroelastomer are dipped for some time, the hardness of nitrile rubber distinctly decreases and that of fluoroelastomer almost doesn't change. Therefore fluoroelastomer is a good sealing material of fuel circuit for engine fueled biodiesel. Finally, owing to a lot of harm caused by some parameters exceeding GB limits, for instance, distillation temperature, density, acid value, copper corrosion, oxidative stability, this paper argues that biodiesel's blending ratio shouldn't exceed 25% in the blended fuel of biodiesel and diesel, otherwise the properties of blended fuel will have an adverse impact on reliability and durability of an automobile.The test results of low-temperature fluidity of biodiesel indicate that cold-flow additives of diesel can make biodiesel' solidifying point decrease but can't make cold filter plugging point of biodiesel reduce. While the method of blending biodiesel with low solidifying point of diesel or the method of adding absolute alcohol to biodiesel can improve the low-temperature fluidity of biodiesel. The test results of oxidative stability of biodiesel show that T502A antioxidant is the best one among four antioxidants, such as T502A, TBHQ, T501, T534. T502A antioxidant can effectively inhibit the formation of peroxide in the oxidative process of biodiesel and make the oxidative stability induction period of biodiesel surpass 6.2h.The difference between the properties of biodiesel and those of diesel will have effect on injection characteristics. In this paper, the injection characteristics of blended fuels, such as BD0, BD25, BD50, BD100, were determined by the direct photographic method. The test results show that with the increasing of biodiesel blending, the spray cone angle of blended fuel decreases and that the peak of the size and number distribution curve for spray droplet gradually reduces, moreover, the cumulative volume distribution curve of spray droplet shifts towards the direction of large liquid droplet and Sauter mean diameter of atomizing droplets is also gradually increasing. So this paper may safely draw a conclusion that the more the mixing ratio of biodiesel in the blended fuel, the poorer the quality of atomization for the blended fuel it is.Mathematics calculation model about spray droplet size distribution was set up by the function of spray droplet size distribution, and that using Fortran language calculated droplet size distribution and Sauter mean diameter of spraying BD0, BD25, BD50 and BD100. The results show that the theoretical calculation results are comparable with test results to prove availability of the model.Improvement experiment for atomization quality of blended fuel was carried out which was aimed at promoting atomization of blended fuel. The results indicate that whether the methods of adding ethanol and ether into blended fuel of biodiesel and diesel or the method of increasing injection pressure is able to make the droplet size distribution curve shift toward the direction of small droplet and to make Sauter mean diameter reduce. Atomization quality of blended fuel was improved eventually. As the influence of physicochemical properties and spray characteristics, this paper regarded BD_e-525 as optimal ratio used biodiesel in an automobile and determined some engine characteristics burned BD_e-525, such as combustion, power, economy, emission. The results show that the combustion characteristics of BD_e-525 is comparable with that of diesel. Under the condition of accelerator fully opening, power, torque and specific fuel consumption of engine burned BD_e-525 is essentially comparable with those of engine burned diesel, furthermore, its exhaust emissions, such as HC, CO, NO_X, are no differences from diesel's, but its smoke is markedly decreased.