Study On Control Strategy For SCR Technology To Reduce NOx Emission From Heavy-Duty Diesel Engines

As the aggravation of environmental pollution caused by vehicles, a series of legislations are put out to reduce emissions from automobile engines and more stringent. The after-treatment technology is one of the major measures to meet the emerging stringent emission legislation limits. Because of its advantage of higher power performance and lower oil consumption, diesel engines are widely applied in medium/heavy trucks and passenger carriages. However, the NOx and PM, which are the main emissions of diesel, can't be effectively solved for lean burn engines through TWC (Three-Way Catalysts) technology as gasoline engines due to the low NO_x conversion in the presence of oxygen. Thus, it is the time for diesel engines to develop appropriate exhaust after-treatment technology. In all the technology routes studied by diesel engine manufacturers to deal with Euro IV and Euro V standards two methods are concluded, which one is reducing NOx emissions with SCR (Selective Catalytic Reduction) technology on the basis of combustion optimization for diesel engines to reduce PM, and the other to debase NOx emissions with EGR (Exhaust Gas Recirculation) and PM with DPF.The Selective Catalytic Reduction (SCR), using urea as reducing agent, because of the advantage of low oil consumption and strong resistibility, is presently considered the most promising technique for the removal of nitrogen oxides from the exhaust of heavy-duty diesel vehicles. It has contributed to control the emissions from vehicles in Europe. Due to our emission standard system is equivalent to the Europe standard system, in view of China's conditions and the convenience of technology explanting, manufacturers of vehicle and diesel engine are inclined to adopting the SCR technology as the major technology to meet the more stringent exhaust emission standards in future.However, the study about Urea-SCR technology is not self-dependent currently in our country although the native manufacturers have been starting the application in SCR technology on heavy-duty diesel engines. Due to the key technology and patents for SCR system which are held by manufacturers abroad, they have been becoming the bottle-neck of the SCR techonogy extending and application in China. With the diesel vehicles are increasing significantly in our country, it is very urgent to reduce the pollutants from vehicles, and it has a rapidly increasing demand of the advanved SCR catalysts. Therefore, it is urgent for the domestic research of the relative SCR technology and is necessary to have a totally understand of them to prepare for the further study and domestically manufacture. So, it is more and more necessary to research the SCR technology, from the catalyst, the catalyst box design, the urea dosing control strategy and the OBD system, to the entire system in the vehicle integrating and matching.In this paper, it's based on the SCR technology's developing, matching, calibration on diesel engine WP12, and the actually application of the SCR technology is deeply studied with the integration methods of experimental evaluation, theoretical analysis, numerical simulation and the experimental verification. The paper includes: the study of SCR catalyst reaction system, the study of urea dosing control strategy and calibration of urea dosing quality, the development and calibration of OBD system, the main problems and solution measures when the SCR system using in cold area. The following are the brief description and introduce of this paper in writing orders.Firstly, the influence factors of DeNOx efficiency of the SCR catalyst reaction system are totally studied, include optimizing screening of substrate and catalyst, designing and optimizing of catalyst box structure and dripping distribution of exhaust pipe where dosing urea. As the result of contrast test to substrate and catalyst shows, through substrate 400/7 has the biggest pressure loss, it's not obviously and can be the substrate of catalyst in the test temporary from the view of cost and mass production; catalyst sample (B) is superior to catalyst sample (A) both in DeNOx efficiency and preventing NH3 disclosure, and after degradation testing its performance does not decrease obviously, therefore, catalyst sample (B) is chosen as the catalyst for the further test. SCR catalyst converters of box-type which are suitable for heavy-duty engines are designed and optimized by CFD. As the calculation result of the design schemes to several catalyst converters shows, there is a optimum length for inlet pipe area with holes and a optimum angle scale for the area with holes at which every substrate have the most even flow distribution; Furthermore, a flow guidance panel at the bottom of inlet pipe can remove the vortex in the base, and is in favor of improving flow effect and reducing pressure loss. CFD technology is used to have contrast calculation and analysis to the position and angle of the urea dosing system, and the diameter of exhaust pipe of different schemes.Secondly, good dosing strategy not only can shorten calibration period, improve work efficiency, but also save urea consumption at the premise of satisfying emission limits. This paper has testing study about the SCR urea dosing system on the engine test bench, based on the developing, matching, calibration of SCR system on diesel engine of WP12, and aims at verify the system can satisfy China IV emission standard at the condition of dosing reasonable quality of urea and avoiding excessive NH3 escape. It has positive function for making scientific dosing control strategy and efficient and reasonable SCR matching and calibration. The contents of study include the following: urea dosing control strategy, optimum calibration of urea dosing quantity, catalyst dynamic temperature correction and so on. In the foundation of in-depth studying and analyzing of urea dosing strategy, according to the summary of calibration experience, and referring to relative technical material, the method of optimum calibration of urea dosing quality under steady testing state is put forward, the calibration calculation model estimating synthetical NOx emission during ESC test cycle is also established, which might help to control the NOx with a clear goal during SCR urea dosing quantity calibrating. Practice proved that this method can obviously improve working efficiency, save calibration time. Study of urea dosing strategy and dynamic temperature correction of catalyst converter, and ESC and ETC test result indicate that opening dynamic correction can effectively reduce urea consumption at the premise of satisfying emission limits. Under transient state, the secondary pollution caused by leakage of NH3 can also be effectively reduced. But at the same time, urea dosing quality should be optimized fatherly under steady state, so as to reduce the ratio of dynamic correction and improve the utilization effectiveness of SCR system as far as possible. In addition, the volume of catalyst substrate should be further reduced in order to reduce the total amount of substrate thermal storage and improve the catalyst converser's response ability to temperature change under the transient state.Once more, due to the OBD system must be installed during stage IV, the study of control strategy, control algorithm and system development and calibration of OBD system is very necessary and time presses. Based on detection theory and the testing technical requirements of the OBD system, the development and the calibration of OBD1+NOx control stage on the WP12 heavy diesel engine which applied SCR technology is introduced in this paper, and certification test has been passed, which can indicate that the developed OBD fault diagnosis and emission monitoring strategies satisfy the online monitor and diagnosis request to engine.At last, because the vast territory of China, the temperature difference is quite large between north and south in winter, and it may achieve -30℃below in north of Yellow River. Urea will freeze when the environment temperature is lower than -11℃, leading to the disable of SCR system urea supplying, resulting in restriction to the use of vehicle which apply the SCR system in these areas. But this problem can be solved effectively by heating the urea storage unit and flowing pipeline. With the purpose of ensuring the normal running of SCR system in north cold area, heating ability of the heating system must be distribute reasonably during the process of heating urea storage unit and pipeline, so as to maintain temperature of urea above the freezing point but not over-heating. Therefore, it is necessary to have a theoretical analysis and study in practical application condition to the heating function of SCR system. First, mathematical model estimating heat dissipating capacity is established by using heat transfer theory and thermodynamics knowledge, and referring to the relative technical material, which provides reference for the selection of urea pipeline and heating power; And then contrast selection under different heating power is done through simulation test in the cooled lab; At last, the whole system in the vehicle is tested in cold area to ensure the normal running request of the system in cold area of China. The study result of urea heating system indicates that urea tank heating by engine cooling water, urea supply module and pipeline heating by electric can solve the freezing problem of SCR system in cold area of China.The study results have reference value to develop commercial diesel engine which can satisfy China IV emission limits, and have realistic social meaning to China for reducing commercial automobile diesel engine emissions and air pollution.