## 4 Results and Discussion  
In this test procedure, the free pedal is quite important when the vehicle stops for on-site emission inspection. To analyze the measured deviation of the test, first the free pedal was repeated five times (test numbers 1#, 2#, 3#, 4#, 5#), respectively, before and after the stable idling. In this test procedure, each free pedal is maintained 3-5 s. Figure 3 shows the tested NOx of these five free pedal processes before and after the stable idling as well as the deviation. Figure 3(a) gives the test NOx of each test process. The results show that NOx emission changes slightly for different free pedal cases. It shows that the curves before stable idling have the same tendency with good following, and those after stable idling almost exactly coincide with each other. Figure 3(b), reveals the uncertainty of the test. Figure 3(b) presents the NOx maximum of each test case, and Figure 3(c) gives the deviation. It reveals that most deviations from the maximum of NOx are less than 15%, and about 90% of the deviations are no more than 10%. This free pedal procedure depends on the drivers' driving habits, which have a great influence on the test emissions. For practical emission inspection, a deviation of 10% is acceptable. Figure 2: Test devices and test procedures. Figure 4 shows the test results of the whole test procedure including three cycles with a total of 7000 s. It illustrates results for both the engine speed and the vehicle speed. There are several three-time flooring pedals marked by a dashed-line rectangle in this test, and the enlarged view of this three-time flooring pedal condition (in the second cycle) is also given. In addition, it shows the SCR upstream and downstream NOx and the SCR downstream exhaust gas temperature. It shows the exhaust gas temperature varies with the change in engine speed and vehicle speed. There are three states during the test process. State III occurs when, at first, the engine is in cold start. During this state III, the exhaust temperature is lower than 150 \({}^{\circ}\)C. The SCR downstream NOx and the upstream NOx are almost the same, which means there is no reduction in catalytic reaction in the SCR carrier, so it is in the non-operational state. After the vehicle drives on the road, the exhaust gas temperature increases to above 250 \({}^{\circ}\)C, and the SCR downstream NOx sharply decreases. This means the SCR system works and is in state II SCR normal operation. Then, the vehicle stops and runs at idle, therefore the exhaust temperature gradually declines from approximately 250-150 \({}^{\circ}\)C. During the temperature-decreasing stage, the SCR's downstream NOx emissions increase accordingly. This is a transitional stage between states I and III, i.e., state II ammonia storage. Figure 3: Test deviation analysis. The NOx conversion efficiency is also presented in this figure. Here, taking the first cycle test results which range from 0 to 2500 s, as an example, the three-state NOx conversion efficiency characteristic is analyzed. During state III (cold start stage, time 0-200 s), the exhaust temperature is low (<100 \({}^{\circ}\)C) and the NOx conversion efficiency is quite low, nearly close to zero, which means the SCR system does not work in this state. As the vehicle moves, it comes to state I. The temperature increases rapidly along with the sharply increasing conversion efficiency, which quickly rises to its maximum (about 99%) in minutes. During state I, the SCR system works at a very high efficiency. Once the vehicle stops and runs idle for about 1000 s, the exhaust temperature tends to decline. As the time changes from 1000 to 1400 s, the temperature drops from 230 to 180 \({}^{\circ}\)C, while the NOx conversion efficiency maintains a high level (above 95%). However, as the temperature continues to drop, the conversion efficiency declines gradually. At the end of this state II (about 2450 s), the temperature decreases to about 140 \({}^{\circ}\)C, and the conversion efficiency is lower than 30%.