The CFD Study Of Achieving Low Emission Of Diesel Engine By Using Multiple Injection Combined With SCR

With environmental protection consciousness gradually strengthened and increasingly strict emission regulations, diesel engine exhaust emission control technology need to be further enhanced and improved. This subject simulates the process of spray, combustion and the reactions in the SCR after treatment device while cylinder inject multiple by using AVL-FIRE. Then analyzed the NOx and Soot’s emission levels under those conditions. This research has great significance on diesel engine combustion and emission control.First of all, this subject made a deep analysis on the change process of the pressure, temperature, equivalent ratio and NO and Soot’s formation area and conditions inside the cylinder during the engine in the process of combustion by using an injection. The study shows that the high temperature and oxygen enrichment are main formation conditions for NO, while Soot’s formation conditions are high temperature and high equivalent ratio. Therefore, shortening the ignition delay period can reduce the temperature and lead to reduce of NO emissions. Strengthening the gas flow can promote mix of fuel and gas, reduce the Soot emissions. Secondly, the subject analyzes the influences of those parameters on the performance of diesel engine by using the two or three times injection. Research shows that the pilot injection has obviously effects on reducing the NO. The post injections can increase the temperature of power stroke and strengthen the Soot oxidation.The subject used the orthogonal experiment method to make the calculation schemas about the twice, triple times injection control strategies. The optimal solution in each control strategy is established and comparison of multi-objective optimization results. The conclusion is that the optimal multiple injection control strategies is the pilot injection-main injection-post injection under the full load condition which leads to the NO emission rate reduced by 11.79% and the Soot reduced by 29.82%.This subject made a further study of the SCR after-treatment technology by analyzing the distribution of NH3 on the inlet cross section and corresponding NOx conversion efficiency when the urea spray at different angle and different distance. The conclusion is that NH3 concentration of catalyst inlet cross-section reaches the maximum, the NO conversion efficiency reaches highest.