柴油机燃用甲醇柴油混合燃料仿真

【Abstract】wWw.shuoshilunwen.com Energy shortage and environmental pollution he become two major problems in world today. As oxygen alternative fuels, alcohol one can increase fuel equivalence ratio in the combustion process, decrease the emission of CO and NOx compared with that of diesel oil and achieve almost zero oke release. In addition, owing to its higher latent heat of viporation, the intake air temperature can be lowered to improve intake volumetric efficiency. Alcohol fuels can be refined from coal, natural gas, etc. The abundant coal resources in China provides a broad prospect of development and application. However, presently they are mainly used in gasoline engines considering their difference in physical and chemical properties compared with diesel fuel. This thesis focuses on the application of the very blendid fuel—a combination of methanol and diesel, to direct injection turbocharged diesel engine by adjusting the fuel quality and supply parameters.The primary factor in controlling diesel combustion process lies in the control of mixed gas formation in cylinder, while that formation depends on the fuel spray characteristics, injection rate, air movement and combustion chamber cylinder shape, etc. With the software named FIRE, this thesis numerically simulates the combustion process, including compression, combustion and expansion stroke in direct injection diesel engine’s combustion chamber and then studies the impacts of fuel quality, injection starting pressure, fuel supply advance angles, etc. on the formation of mixed gas, combustion process and diesel engine’s major emissions-NOx and soot. For the spray process, the research is conducted primarily by analyzing the movement of oil droplets through air movement vector, analyzing the distribution of fuel through fuel concentration field in cylinder and fuel equivalence ratio chart. For the combustion process, the evaluation is mainly done through the analysis of fuel concentration field, cylinder temperature and fuel equivalence ratio, accompanied by the NOx and Soot concentration field and mass fraction curve to analyze the combustion emission condition.The results showed that the comparison between simulation and experiment verifies the reliability of the combustion model set up with FIRE at 4100QBZL diesel engine fueled with methanol-diesel blends and a comparative analysis on the combustion and emission characteristics of the blends in different proportions of methanol was make. The combustion heat release rate curve and the cumulative heat release curve showed that the fuel added with methanol embodies the following properties:larger proportion of premixed combustion, higher speed at diffusion combustion, shorter combustion duration as well as poor Engine FLEXIBILITY and decreased power. However, its emission performance was better than diesel fuel, featuring distinctive decline of CO and Soot, slight increase of HC while NOx closingto the original machine.Based on this, M15 methanol-diesel fuel was selected. According to its combustion and emission performance under the original engine parameters, first, an optimization of injection starting pressure was made and its impacts on the injection rate was analyzed. The outcome indicated that when the injection starting pressure dropped, oil supply would increase but spraying performance deteriorate. The experiment and simulation results showed that when the injection starting pressure was 22.5 MPa, the cylinder of M15 mixed fuels had a relatively higher maximum combustion pressure and maximum temperature, a better dynamic parameter but worse NOx emissions; then adjustments of the fuel supply advance angle and optimization of M15 blends emission performance were made under the pressure of 22.5 MPa. The results demonstrated that larger fuel supply advance angle would advance combustion timing, increase combustion pressure, improve heat release rate and heat status of the cylinder which would thus lead to lower emissions of soot, HC, and CO but increase of NOx by a large margin.Considering the above analysis, trade-offs were made and 14°CA BTDC was choosed as the best fuel supply advance angle.