应用SCR后处理实现车用柴油机国IV排放

 选择性催化还原（SCR）技术是实现柴油机超低NOx排放最有效的后处理方法之一。本研究采用一台电喷单体泵柴油机，通过燃烧优化使PM排放满足国IV排放要求。为降低原机较高的NOx排放，采用开环控制策略开发一套电控单元控制尿素溶液喷射，通过将32.5%的尿素溶液定量喷射到排气管，提供还原剂，与柴油机排气中的NOx发生选择性还原反应，生成N2和H2O，实现降低NOx排放的目的。由于选择催化还原方法的NOx转化效率高，通常采用开环控制即能实现国IV排放。本研究通过标定实验获得发动机的原机NOx排放、排气温度、排气流量、单位尿素溶液流量的NOx降低量等脉谱，并将其写入电控单元。通过这些脉谱和当前的发动机转速、负荷信号及催化器进出口的温度信号，电控单元可计算出当前工况需要的尿素喷射量。采用该方法，在稳态和瞬态循环工况，研究了SCR后处理的NOx转化效率、氨泄漏及尿素溶液消耗量特性。结果表明，稳态循环ESC、瞬态循环ETC中，NOx平均转化效率分别达到65.2%、65.3%，氨排放小于5ppm；可溶性有机成分（SOF），实验表明，PM减少主要是由于SOF在氧化单元中发生氧化反应；对ETC循环中尿素消耗量的分析表明，采用本研究开发的尿素溶液计量单元及配套本实验使用的发动机的柴油车，运行100km大约消耗32.5%尿素溶液2L。

Development of SCR Post-treatment for Vehicle Diesel Engine to Meet the China Stage IV Emission Norms

The selective catalytic reduction is one of the most efficient technologies for NOx reduction. The experimental investigation was conducted on a six-cylinder diesel engine with electronically-controlled cam-actuated unit pumps. The particulate mass of the diesel engine is greatly decreased to meet the China stage IV emission norms by a proper modification of the combustion system. In order to reduce the serious NOx emission, a dosing controlling unit with open loop strategy is developed. The urea solution with concentration of 32.5% is induced into exhaust pipe by using the dosing injection system and the controlling unit. Thus, the NOx reacts with ammonia from pyrogenation of urea and then they are transformed into H2O and N2. Because of high rate of NOx conversion, the open loop strategy for dosing of urea solution is sufficient for meeting the China stage IV emission norms. Firstly, the maps of NOx emission, the exhaust temperature, the exhaust flow and the reduction of NOx per unit urea solution were acquired in the engine calibration tests, and saved into the flash memory of the dosing controlling unit. Then one can calculate the quality of urea solution to be induced into the exhaust pipe by using the maps, the current speed and torque percent of the diesel engine as well as the inlet and outlet of the catalyst. Moreover, the performances of NOx conversion, NH3 slip and urea solution consumption of the selective catalytic reduction post-treatment were investigated in the European Steady Conditions (ESC) and European Transient Conditions (ETC). The results show that, NOx conversion over 65.2% and 65.3% can be achieved in the ESC and ETC tests, respectively, while NH3 slip is below 5ppm. On the basis of SOF analysis, the PM reduction is the result of SOF reduction with SCR catalyst. Also, in the analysis of the urea solution consumption in the ETC test, a heavy duty truck with the diesel engine in the test will consume 2 liter urea solution with concentration of 32.5% per 100 kilometers under the controlling of the dosing controlling unit.