| Diesel engines are widely used due to their excellent power,high fuel economy,wide power coverage and high reliability.However,due to its high temperature and oxygen-rich working characteristics in the cylinder,it causes more nitrogen oxide emissions.As the negative impact of nitrogen oxides on the climate is becoming more and more recognized by people,countries have increasingly strict control of NO_x emissions from diesel engines.Based on current emission regulations,most of the new engines can meet current emission standards through the use of selective catalytic reduction and exhaust gas recirculation technologies.However,through the follow-up study on the emission regulations of heavy-duty diesel engines,it is found that the restrictions on NO_xemissions will be further strengthened.Therefore,based on the 4.5L high-pressure common rail diesel engine of a certain type of van truck,the structure simulation optimization and experimental study of its SCR system are carried out,which has certain theoretical value and practical significance for further reducing NO_x emissions.Starting from the theoretical research of SCR,this paper studies the development of domestic and foreign emission regulations and emission test cycles,introduces the latest research results of California’s near-zero emission regulations,and finally studies the relevant content of my country’s current National VI emission regulations.The three-dimensional model and numerical simulation model of the target diesel engine SCR system were established using AVL fire software.Based on the actual object of the target diesel engine SCR system,the original mixer,cyclone mixer,original rectifier and new rectifier of the target diesel engine were drawn using Pro/ENGINEER software.The three-dimensional model,under three typical working conditions,firstly carried out a simulation analysis on the three mixer schemes,and explored the system pressure loss of the dual mixer scheme and its influence on the ammonia distribution characteristics and NOx conversion efficiency.A new type of rectifier was designed and the pressure loss of the new type of rectifier was simulated and analyzed,and its influence on the ammonia distribution characteristics at the inlet section of the SCR carrier and the NO_xconversion efficiency of the SCR system was analyzed.According to the simulation analysis results,the optimal mixer and rectifier scheme was selected,the emission performance test scheme was formulated,and the test bench of the target diesel engine was built to verify the influence of the mixer and rectifier scheme on the engine exhaust back pressure,fuel consumption and conversion efficiency.The results show that the optimized SCR system increases the engine exhaust back pressure,which has a small impact on fuel consumption,and the NO_x conversion efficiency has been further improved.Carry out WHSC and WHTC cycle tests on diesel engines equipped with improved SCR system.Through analysis of the emission data of various pollutants,it is found that it meets the national VI emission standards,and the design has achieved good results.Based on the national VI standard heavy-duty vehicle chassis dynamometer test method,a certain type of van truck equipped with two sets of SCR systems was tested and compared.The test was based on the C-WTVC cycle.The results showed that the SCR of the two SCR systems The carrier temperature distribution is similar.The improved SCR system has a slight increase in comprehensive fuel consumption compared to the original SCR system C-WTVC.After two C-WTVC thermal cycle tests,the NO_x emission factor of the improved SCR system is average compared to the original SCR system.The decrease reached 29 mg/k Wh,Compared with the original SCR system,the ratio of NO_x conversion efficiency greater than 90%is increased by about 3%. |
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