## 2 Real-Time Evaluation Method of a Diesel Vehicle SCR System
### Real-Time Evaluation Method of the Real-Time SCR Operation State  
The three operation states of the SCR system happen when the diesel vehicle runs on a real road. According to the emission standards, the NOx emissions can be tested by regulatory test procedures. In China, all the vehicles are tested according to the global unified transient cycle of heavy engines (WHTC) and the global unified steady cycle of heavy engines (WHSC) for evaluation and certification, according to the emission standard GB-17691-2018 Pollutant Emission Limits and Testing Methods for Heavy Diesel Vehicles (China's Sixth Stage). These complex test cycles (WHTC and WHSC) take much more time and require the PEMS (Portable Emission Measurement System) test device. However, as the government officers' conduct on-the-spot inspections of the in-use vehicles in the highways and city road check points, it is difficult and impossible to install the PEMS device on those vehicles. Moreover, for heavy traffic in cities, especially Chinese big cities such as Beijing, Shanghai, etc., there are so many vehicles on real roads and the traffic is so heavy that it is impossible and unthinkable to inspect every vehicle by driving at varied low, medium, high, and super high vehicle speeds like the WHTC/WHSC test cycle. Nowadays, a practical test method for inspectors is to first stop the vehicle and then test the emissions while they still in place by inserting a sampling device into the exhaust tail pipe. Therefore, a practical and simple test method should be investigated to evaluate the SCR system operation states for the standing-still vehicle emission test.  
This work proposes a new and simple test method to evaluate SCR operation states based on their ammonia storage characteristics. The designed test procedure is illustrated in Figure 1:  
1. Start the vehicle;
2. After the vehicle runs normally on the road for a period of time (several minutes), it pulls over and stops, ready for an on-site emission inspection. The engine maintains idle for several minutes while the test sampling device is well placed;
3. The driver rapidly presses the pedal fully for at least 3 s and then slowly releases the pedal until the engine is stable;  
Figure 1: Proposed real-road test method based on operation states of the SCR system.  
Maintain high idling speed (1000-1500 rpm) for about 10 min;
* The driver again presses fully on the brake pedal and slowly releases it.  
This proposed test procedure may reveal the three states of the SCR system. As for the road driving test, when the vehicle first starts (cold start), the exhaust temperature is lower than the activating temperature, and the SCR does not inject urea, so it is in state III, the non-operational state. As the vehicle drives on the road, the exhaust temperature increases up to the catalyst activation temperature, and the SCR works. This is the state I normal operation state. As for the stop check, the full pedal pressing may make sure the exhaust gas temperature ranges in the temperature window, and this is still in state I. After a certain time of idling, the engine exhaust temperature decreases to be lower than the activating temperature, and the SCR stops injection of urea. However, the previously injected ammonia storage may work, and this is state II.