The Design And Optimization Of The Urea-SCR Supply System For Diesel Engine

The automotive industry of our country has been developing rapidly in the last few years, which has caused severe environmental problems. According to the statistical data of the last few years, the vehicle emission has contributed greatly to the environmental pollution and this phenomenon is getting worse. The amount of NOx from diesel engine accounted for60%of the overall vehicle emission, and PM90%in2010. In view of the condition of our country, SCR is deemed as the proper solution to the diesel emission problems to satisfy the emission standard of level four as well as level five in our country after a lot of argumentation.For now, all the SCR systems used on the diesel engine to meet the emission standards are provided by foreign companies. The systems are expensive. Besides, it is difficult to popularize the technology and to progress secondary development for the domestic manufacturers because of the suppliers’blockade on techniques. A new kind of Urea solution supply system is developed, considering the domestic level of manufacture and purchase. This new system can guarantee the accurate measure and good atomization of Urea solution. It has four working modes, and they are priming mode, doing mode, purging mode and off mode, respectively. In addition, the thermal management is discussed in order to solve the freezing problem, which can avoid the jam of the pipe and atomizer.The perfect working status of SCR system is achieving high NOx reduction rate along with low NH3escape rate. These two targets can be obtained if the SCR system is working with good droplet evaporation and thermolysis, sound distribution of airspeed, and sound distribution of reductant concentration. Before the droplets get into the carrier, they must have or almost have evaporated and thermolyzed completely. Good distribution of airspeed can guarantee the enough contact and reaction time between the reactant and catalyzer. The appropriate excess ammonia coefficient can be achieved, only if the concentration distribution of the ammonia before the carrier is good. In this thesis, a three-dimensional simulation model is built up to simulate the transient atomization process. The flow field changing, the reductant forming, and the mixing process of reductant with exhaust gas are analyzed. Allowing for the geometric characteristics of the catalyst in the bench test, four influencing factors are discussed in this thesis. The status of the catalyst can be judged through the influence they have over the performance criteria. Through the controlling of parameters, the simulations are carried out many times and a good layout of the supply system components, which can accord with the current research condition, is confirmed. After all these comparison and discussion, the subsequent structure optimization and sample making will have some principles to refer to.