Study On The Combustion And Soot Emission Characteris-tics Of Soybean Biodiesel Under Different Ambient Conditions

With the increase of the petroleum oil price and the release of the more stringent emission regulations, the study on sustainable, low emission alternative fuels, such as bio-diesel, draws attention of nations all over the world. However, most of the studies are still in initial stages, especially in China. Therefore, the study on the combustion and emission characteristics of biodiesel and its blends under different ambient conditions will provide important information for future biodiesel applications.In this thesis, all the experiments were conducted in a constant-volume combustion chamber which is able to simulate the in-cylinder high temperature, pressure environment of a big bore, low rpm diesel engine at top dead center. The combustion pressure and heat release rate, liquid penetration, natural flame luminosity, soot emission and lift-off length were measured experimentally. The soot formation processes were measured by a new technique called FILE (Forward Illumination Light Extinction). FILE is more accurate than traditional light extinction method and can provide 2-D time-resolved quantitative soot measurement with only one optical window. The liquid penetration with combustion was obtained via light-scattering and the natural flame luminosity images were captured using a CCD high speed video camera directly. The natural flame luminosity is very im-portant for combustion analysis because it is controlled by in-cylinder soot mass and tem-perature. The lift-off length was measured using OH chemiluminescence imaging. A high response ICCD camera was adopted with a 310 nm filter to capture the images of OH chemiluminescence and then to calculate the lift-off length. The lift-off length can be used to calculate the air entrainment at the lift-off length during the diffusion combustion process. The simulation of biodiesel combustion was carried out by CFD code KIVA-3V. The properties of biodiesel or its blends were obtained by the biodiesel property calcula-tion software called BDProp. The ignition model and emission models in KIVA-3V were modified respectively to simulate biodiesel. The ambient temperature was varied from 700 K to 1200 K, the ambient oxygen concentration was varied from 21% to 15% to simulate different EGR rates, and the ambient density was kept at 15 kg/m3 unchanged. Four fuels were used including B0, B20, B50 and B100.The experimental results show that the ambient temperature, ambient oxygen con-centration and fuel type have significant effects on biodiesel combustion. The KIVA si-mulation results and the experimental results are in good agreement. Biodiesel has a shorter ignition delay compared to diesel in this study. B100 burns better than BO at a lower oxygen concentration. With increasing ambient temperature and biodiesel content, the peak combustion pressure decreases. The liquid penetration decreases as the ambient temperature and oxygen concentration increases, and increases as the biodiesel content increases. The oxygen concentration has more effect on B100 liquid penetration. INFL (Integrated Natural Flame Luminosity) increases with increasing ambient temperature and decreasing biodiesel content. At the ambient temperature of 800 K and 900 K, the INFL of B100 decreases with decreasing oxygen concentration, while at 1000 K to 1200 K, the INFL of B100 increases with decreasing oxygen concentration, which is different from the INFL result of B0. Biodiesel has a higher air entrainment at the lift-off length than diesel. Compared to B0, the reduction of soot mass is not significant by using B20 and B50 at a lower ambient temperature, while at 1000 K, there is a reduction of 50%. At different am-bient temperatures, the soot mass can be reduced by more than 60% while using B100 compared to B0. Oxygen concentration has a more obvious affect on the soot formation of B100 than B0. At different oxygen concentrations, the use of B100 decreases the soot mass by around 50% than B0 combustion, at a low ambient temperature, the reduction of soot mass reaches more than 90%. The soot mass of B100 at a high ambient temperature of 1200 K is comparable to the soot mass of B0 at a low ambient temperature of 800 K.