In recent years, selective catalytic reduction (SCR) has become the mainstream after-treatment technology to reduce oxides of nitrogen (NOx) emission of diesel exhaust.^1–6 The system uses a urea water solution (UWS) as the reductant to convert NOx in the exhaust to N₂ and H₂O by using a catalyst.⁷ The UWS plays a crucial role for NO_X conversion,^8,9 which is a corrosive liquid that freezes below −11°C and crystallizes easily.^10–12 Therefore, the SCR system has strict requirements for the urea dosing system (UDS). As one of the most important components of the SCR system, the performance of the UDS has a direct impact on the efficiency of NOx conversion. The aim of the UDS is to make its flow rate close to the required value. Because of diesel engines work in dynamic operating conditions, the NO_X emission and the required value of the UDS are dynamic processes. When the flow rate is less than the required value, the NO_X cannot be completely converted. If the flow rate is greater than the required value, NH₃ cannot be completely oxidized, that will cause secondary pollution.^13,14 So, it is better to make metering accuracy of the UDS up to ±2% under all operating conditions of diesel vehicles.

中文翻译：

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运行工况和吸入管道参数对选择性催化还原空气辅助尿素计量系统流量特性和计量精度的影响

近年来，选择性催化还原（SCR）已成为减少柴油机尾气氮氧化物（NOx）排放的主流后处理技术。^1-6该系统采用尿素水溶液（UWS）作为还原剂，通过催化剂将排气中的NOx转化为N ₂和H _{2 O。}⁷ UWS 对于 NO _X转化起着至关重要的作用，^8,9它是一种腐蚀性液体，在 -11°C 以下冻结并容易结晶。^10-12因此，SCR系统对尿素添加系统（UDS）有严格的要求。作为SCR系统最重要的组成部分之一，UDS的性能直接影响NOx转化效率。UDS的目的是使其流量接近要求值。由于柴油机工作在动态工况下，NOX_排放量与UDS要求值是一个动态过程。当流量小于要求值时，NO _X不能完全转化。如果流量大于要求值，NH ₃不能完全氧化，会造成二次污染。^13,14因此，柴油车所有工况下UDS的计量精度最好达到±2%。