Study On Application Technology Of SCR System For Heavy-Duty Diesel Engines

Diesel engines are widely used to medium and heavy vehicles because of higher dynamic performance and lower oil consumption. However, the problems about the pollution of diesel vehicles begin to stand out, especially heavy diesel engine, since we have been attaching great importance to the environment protection, it has been shown that although the number of heavy diesel vehicles accounts for about15%of the total vehicles in our country, the quantity of NOx and PM emissions accounts for more than60%of the total. At present, most of engine enterprises for commercial vehicles in our country tend to adopt SCR technology as the main technical route to meet Euro IV and Euro V standards. However, although the native manufacturers have been starting the application in SCR technology on heavy-duty diesel engines, the studies about Urea-SCR technology are not self-dependent currently. Actually the key technologies are still grasped by foreign manufacturers. As a result, we use them through the introduction of foreign technologies. In addition, we have no enough systematic researches with the problems in use process, so there is still a big gap with advanced foreign technologies. A series of problems come to appear in city bus that used SCR technology at Beijing and Shanghai since the national fourth emission standard put into effect, such as SCR system doesn’t work sometimes at low speed and low temperature, urea solutions form sediments inside the exhaust pipe, etc. these result that NOx emissions exceed the standards limit. So it is obvious that studies of stable and efficient SCR technologies and devices have become the main bottleneck for the popularization and application of SCR technology. This thesis focus on the several key questions in application of SCR technology, which include higher efficiency, urea deposition, low temperature applications, and durability of catalyst and so on. The main contents include:The components of urea sediments and its influence factors, the influence factors affecting the stability of SCR system which include urea deposited and catalyst degradation, are analyzed, and the influence of sulfur contents in fuel on durability of catalyst is studied at the same time. Complex physical and chemical reactions would occur in SCR system, many intermediate products such as cyanic acid, biuret and cyanuric acid are generated when the urea droplets resolve into ammonia. It is assured that the primary components in crystallization are urea and cyanuric acid using TG-IR. The causes why there would be sediment near the nozzle hole, downstream of the exhaust pipe, front of catalyst support, etc. are analyzed combination with problem appeared in application. Unreasonable structures result in several problems like How of poor organization, local low temperature, poor atomization of large droplets, lack of evaporation decomposition, which are the main causes for urea sediment. A scheme for improving the structure of nozzle holder is put forward to solve these problems; it can reduce the amount of urea sediment effectively by the testing results on test bench and road, and6%reduction of NOx emission in ETC cycle is achieved at the same time. The influences of exhaust temperature and rate of urea injection on the urea sediment formation are studied through the experiments and the control strategies of urea injection are also optimized. Degradation coefficient and the threshold value, which indicate the changes of the catalyst performance before and after the endurance test, are put forward to evaluate the catalyst performance before and after the durability experiment, and the influences of the sulfur contents in fuel on SCR catalyst performance are also investigated. It indicates that the vanadium catalysts may well adapted to fuel quality, and the degradation coefficient is feasible to evaluate the catalyst durability via the1000h-bench-endurance-test using diesel oil corresponding to national Ⅱ standard and20.000kilometers’road test.A scheme of a tight coupling SCR system layout for heavy duty commercial vehicle is put forward, and structural optimization is carried out by CTD analysis. Urea injector is integrated with the entrance of catalytic converter by the variable diameter and cone structure at the fixed nozzle location. The closed design of mixing tube top may lead the gas flow to run out along the circumferential direction of the perforated tube, and then the flow will arrive to the front of catalyst by crossing front plate of carrier with perforated structure, and longer tube with large diameter is used to enhance gas flow disturbance in order to promote full mixing of urea solutions and exhaust air. The heat loss could be effectively avoided with high temperature gas around since the mixing section for urea droplets is inside SCR catalyst converter. Several new designs avoid urea droplets jetting to the opposite wall, they are the gradient nozzle with bigger diameter in mixing tube, and the distance from the starting point for urea injection to the opposite wall is larger than jet penetrating distance of urea spray. The large scale vortex will play a leading role in the mixing process in mixing section, while the small turbulence would play a leading role at the mixing holes on the wall when gas flowing across the mixing hole for the closed design of mixing pipe prompts the formation of small turbulence at the positions close the holes. This combination reduces the demand of mixing distance, and lead to a more compact structure design. The uniformity of mixture will be improved because the front support plate of catalyst is perforated structure, and the catalyst efficiency will be improved. NOx conversion efficiency of new structures is improved by17%under the ESC cycle test and21.6%under the ETC cycle test compared to the original structure scheme according to data from bench test. And another results coming from50,000kilometers’ road test indicate that this scheme could solve the problems of crystallization and sediment in SCR system.The influence of urea injection delay on diesel engine emissions performance under transient conditions to deal with the hysteresis nature of the catalyst carrier temperature is studied, and we also analysis the influence of different control strategies for urea injection delay and delay time on the NOx conversion efficiency and the amount of NH3escaped are researched. As we all know, the rate of reduction reaction of NOx is directly related to the work conditions, the appropriate temperature, the appropriate time, and the appropriate amount of urea solution, are all basic conditions to insure good performance of the system. But the bed temperature of the catalyst carrier lags because of its heat capacity as the working conditions of diesel engine changing. The amount of ammonia escape will increase if it is ignored when the exhaust temperature changed from high to low rapidly, which result in secondary pollution; on the other hand, it is unfavorable to the reduction efficiency of NOx. In order to solve this problem, the rise and fall function for delay characteristic of first order system is used. The experimental results show that reasonable falling delay strategies are beneficial for full play of catalyst efficiency, for reducing emissions of NOx under engine transient conditions, and for reducing the Urea consumption, at the same time it could control the escape of NH3effectively.In view of the requirements of WHTC cycle test in Beijing’s fifth stage emission standards, systematic test research work is conducted to control vehicles’ Nitrogen oxides emissions levels at low speed and low load condition. The advantages and disadvantages of the existing technical measures improving emissions performance of diesel engine assembled SCR system at low temperature and low load condition are analyzed. The technical solution that use air inlet throttle valve device to reduce engine’s air inflow at low working condition is put forward on this basis, it could promote combustion temperature in cylinder and thus raise exhaust temperature. It is well known that catalyst activity is very low when exhaust temperature is below250℃for SCR system, and the urea cannot be transferred to ammonia normally when it is lower than200℃. Experimental tests show that exhaust temperature of59.3%of working conditions in WHTC cold start test cycles and52.7%of the working conditions in WHTC warm start test cycles are below200℃in, so the urea cannot be injected in these working conditions. Ordinary vanadium catalyst activity is very low in hot start and cold start conditions because the temperature are basically below280℃The influences of intake throttle valve and DOC technolegy solutions on NOx emissions and fuel consumption in WHTC test cycle are experimental studied on the engine test bench. The results show that influence of intake throttle valve opening degree on exhaust temperature is great, so selecting reasonable inlet throttle valve opening degree can significantly improve exhaust temperature at corresponding working conditions and achieve the reasonable temperature range benefit for reducing NOx emissions in WHTC test cycle. The results comparison of two technical solutions show that intake throttle valve technical solution has obvious advantages, which could reduce the weighed values of Nitrogen oxides emission by41.5%in WHTC test cycle, and its influence on fuel consumption is very small, in addition the cost is low.