## 4 Results and Discussion
This result reveals that the low conversion efficiency occurs due to the decreasing exhaust gas temperature. As the exhaust temperature is low, the SCR is designed to stop urea injection, but the conversion efficiency is not zero because the catalytic reduction reaction still happens due to the ammonia storage. More details of the gradual decrease in the conversion efficiency in state II are investigated in this work. Figure 5 presents the results of state II in different test cycles (cycle 1, cycle 2, and cycle 3). It shows that each state II experiences about 1400-1600 s, and the exhaust gas temperature continues decreasing in this state. For these three different test cycles, the SCR upstream NOx maintains a constant of about 180 ppm while the SCR  
Figure 4: Test results. downstream NOx changes. At the beginning of 200 s, the SCR downstream NOx is low, and the conversion efficiency is high, close to 99%. After then, the SCR's downstream NOx increases, and the conversion efficiency drops gradually. At the end of state II for each test cycle, the SCR downstream NOx rises to about 140 ppm and the conversion efficiency declines to about 20%. Figure 5: Test results for SCR state II. It is worth noting that the moment when the SCR downstream NOx suddenly rises keeps pace with the moment when the conversion efficiency suddenly drops, there is no lag between these two moments. Therefore, it can be proved that the reason why the SCR's downstream NOx emissions increase sharply in a short time in state II during the idling phase of the vehicle is highly related to the conversion efficiency of the SCR. Because the exhaust temperature continues to drop in state II, the urea pump stops working due to the low temperature. Figure 5 shows that the SCR conversion efficiency does not drop directly from the high value to zero. This is because of the ammonia storage of the SCR catalyst carrier. The ammonia releases from the catalyst and takes part in the reduction reaction continuously, so that the conversion efficiency gradually declines and is far lower than that in the normal operation state. The results reveal that the conversion efficiency experiences a period of time where it decreases from a high to a low value. Here, high conversion efficiency is defined as >90%, while low conversion efficiency is defined as <50%. The conversion efficiency is closely related to the exhaust temperature. Figure 6 presents the relationship between the conversion efficiency and the exhaust temperature in state II. The results reveal that the NOx conversion efficiency increases with the increase in the exhaust temperature and that it maintains a maximum value (above 98%) at high temperatures (>180 \({}^{\circ}\)C). For each test cycle, the range from the high to the low conversion efficiency covers a certain exhaust gas temperature. Here, the temperature range corresponding to the high and low conversion efficiencies is defined as Tt. Furthermore, it shows that the conversion efficiency is linearly related to the exhaust gas temperature. In addition, the slope of each line, here named K, varies for each cycle. Figure 6: Relationship between conversion efficiency and temperature in state II. During this linearly increasing stage of the conversion efficiency, the diesel engine runs at idle speed with a stable engine speed and exhaust mass flow rate for each test cycle. The average exhaust mass flow rate and average temperature during the linear increasing stage for each test cycle are also presented, and during this linear rising stage, each cycle has a similar average mass flow rate and exhaust temperature. During the idling stage, the exhaust gas temperature only changes a little, and the average exhaust temperature of each cycle varies within the range of 157-163 \({}^{\circ}\)C by no more than 8 \({}^{\circ}\)C. However, the parameters relating to the conversion efficiency, i.e., Tt and K, change. As the average exhaust temperature rises, Tt linearly increases, but the slope K slightly linearly decreases.