## 3 Result and Analysis
### SCR Reaction at Transient Cycle  
The steady-state condition analysis in the previous section investigated the effect of various factors. Under transient working conditions, the exhaust temperature and mass flow rate change frequently and the temperature of the SCR carrier will also be affected, thereby affecting the NH\({}_{3}\)-NO\({}_{\mathrm{x}}\) reaction. Therefore, before analyzing thermal management methods to further decrease NO\({}_{\mathrm{x}}\) emission, it is necessary to analyze the SCR NO\({}_{\mathrm{x}}\) conversion capacity at a transient cycle. The WHTC cycle test is relatively close to the actual driving cycle and the engine operates under frequent varying conditions. The NO\({}_{\mathrm{x}}\) emission of a conventional EAT system under the WHTC cycle without thermal management intervention is shown in Figure 10. High NO\({}_{\mathrm{x}}\) emission levels mainly occur in the first third of the entire cycle, while the emission at the latter part of the cycle remains low. The results of NO\({}_{\mathrm{x}}\) emission before 200 s are not taken into consideration in Figure 10, as thermal management is inevitably required to reduce NO\({}_{\mathrm{x}}\) emission under cold starting conditions. In Figure 10a, compared to the 500-1800 s range (Figure 10b-d), the NO\({}_{\mathrm{x}}\) emission is higher in the 200-500 s range (indicated by the blue line in Figure 10a). The peak NO\({}_{\mathrm{x}}\) emission occurs at 361 s, with a value of 465.4 ppm. The average conversion rate of NO\({}_{\mathrm{x}}\) at 200-500 s is only 62.58%, which may be due to the SCR carrier not having been raised to the appropriate temperature. Even with the NO\({}_{\mathrm{x}}\) emissions of the starting condition, the NO\({}_{\mathrm{x}}\) emissions of 200-500 s account for 39.21% of the total emissions. Figure 10: NO\({}_{\mathrm{x}}\) at LO-SCR inlet and outlet during WHTC cycle at (**a**) 200–500 s, (**b**) 500–1200 s, (**c**) 1200–1500 s, and (**d**) 1500–1800 s.  
In Figure 10b, the NOx content in the engine exhaust gas does not change significantly compared to before (indicated by the red broken line in Figure 10b). The average NOx content is 282.6 ppm in the interval of 500-1100 s, while it is 280.2 ppm in the interval of 200-500 s. However, due to the increase in SCR carrier temperature, the NOx emission decreases significantly. In the interval of 500-1100 s, the peak NOx emission decreases to 349.2 ppm, occurring at 801 s, and the average conversion efficiency of NOx increases to 92.48%. In the interval of 1100-1500 s, as shown in Figure 10c, the NOx content in the engine exhaust increases compared to the previous period, and the average NOx content rises to 457.8 ppm. At the same time, due to the high temperature of the SCR carrier, the NOx emission does not change significantly compared to the interval of 500-1100 s, which further increases the average NOx conversion rate to 93.96%. In the interval of 1500-1800 s, as shown in Figure 10d, the average content of NOx in the engine exhaust gas decreases to 361.6 ppm; except for a few operating points where the NOx emission changes significantly, the overall trend is smooth and the average conversion rate of NOx is 90.01%. The average NOx emission level in the first 500 s is 260 ppm, while the average NOx emission from 500-1800 s is 10.5 ppm. Under the WHTC cycle, there are more low-temperature conditions and more nitrogen oxide emissions. For the SCR system, the high-efficiency window of the catalyst is mainly 250-400 \({}^{\circ}\)C, so the catalytic conversion efficiency of the catalyst is affected under the condition of low exhaust temperature, resulting in the NOx not being efficiently clarified. The high NOx emissions in the first 500 s are mainly due to the low SCR carrier temperature, which only improved after higher exhaust temperature at the latter part of the cycle increased catalyst temperature to higher NOx conversion capability.