###### Abstract  
In China, where in-use heavy-duty diesel vehicles producing NOx and particulate emissions for air pollution are required to undergo emission spot inspection at check stations, it is significant to adopt a simple method to evaluate emissions due to traffic jams, especially in big cities. To realize convenient vehicle emission spot inspection, this paper investigates the effects of exhaust temperature on the Selective Catalytic Reduction (SCR) de-NOx conversion efficiency and presents a method for evaluating the SCR operation state based on ammonia storage characteristics. The paper proposes a real-road driving test procedure and verifies it by measuring NOx from a heavy-duty diesel vehicle in an on-road driving test. The results show that the SCR de-NOx efficiency experiences three operation states. In state I, SCR works and injects urea, resulting in high de-NOx efficiency (>90%); state III occurs during the cold, starting with the lowest de-NOx efficiency (<50%) due to a lack of NH3; and state II is a transition stage caused by the ammonia storage, with a certain conversion efficiency (50-90%) when SCR de-NOx efficiency linearly relates to exhaust temperature. Whether the SCR works normally can be judged based on the SCRs operation states. This method is simple and easy to implement in different SCR operation states, and it is effective and repeatable. s +
Footnote †: journal: Medical Engineering, Volume 1, Issue  
sulfur resistance, with few changes to the original engine and has become one of the most effective ways to reduce NOx emissions from heavy-duty diesel engines [5,6]. The SCR system is mainly composed of the urea supply system, related sensors, the SCR catalyst, the dosing control unit (DCU), and other components. As the operating environment of an SCR system is rather harsh, it needs to bear high temperature, high pressure, chemical poisoning, scale aging, mechanical external force damage, poor road conditions, etc., which will lead to the performance degradation of an SCR system. When it causes a fault, it will also lead to a further increase in vehicle pollutant emissions. Therefore, it is urgent to test the real-time emissions to evaluate the SCR system performance when diesel vehicles are driving on real roads. For the diesel vehicle, the emissions from the test bench and the real-road driving test are not exactly the same. [PERSON] et al. [7] tested the NOx emission from a Euro 6-compliant diesel passenger car on a chassis dynamometer over both the new European driving cycle (NEDC) and the more realistic worldwide harmonized light-duty test cycle (WLTC), and they found that the NO emission factor was lower for the NEDC than the WLTC. [PERSON] et al. [8] tested emissions from seventy-three Euro 6 diesel passenger cars over both NEDC and WLTC, and they reported that most vehicles met the legislative limit of 0.08 g/km of NOx over NEDC, but the average emission factors rose dramatically over WLTC. These studies reveal that real-road NOx emissions differ from the legislative limits. For the vehicle emission evaluation, real-time experiments include both the test bench and the on-road driving emission test, which is required in emission regulations. Generally, there are two test methods to measure the real-time emissions of diesel vehicles, i.e., the Portable Emissions Measurement System (PEMS) and the NOx sensor monitored by an on-board diagnostic system (OBD). Current China VI Emission Regulation, GB17691-2018 Emission Limits and Measurement Methods of Heavy Diesel Vehicles, requires that the Real Driving Emission (RDE) test be used as a supplementary test procedure to measure the real-time emissions of the vehicles. The test cycle of China VI emission regulation adopts the global unified transient cycle of heavy engines (WHTC) and steady cycle of heavy engines (WHSC).