## 3 Result and Analysis
### SCR Reaction at Steady Working Point
It is necessary to further improve the NH\({}_{3}\)-NO\({}_{x}\) reaction rate, and the ammonia-nitrogen ratio in steady-state working points should be greater than 1. It should be noted that a longer SCR carrier can store more ammonia, which may cause larger ammonia leakage at transient working points. Figure 7: Reactions in SCR at different exhaust mass flows. (**a**) Changes of NOx Conversion Rate with Exhaust Mass Flow Adjustment. (**b**) Changes of Ammonia Chemical Loss with Exhaust Mass Flow Adjustment. The effect of DOC on the NO\({}_{\rm x}\) conversion rate after thermal management should also be emphasized. Nitrogen oxides generated during combustion are mostly NO, and the DOC at SCR upstream can partially oxidize NO into NO\({}_{2}\). The NO\({}_{\rm x}\) conversion rate at different SCR inlet NO\({}_{\rm x}\) compositions (NO and NO\({}_{2}\) only; other possible nitrogen oxides were neglected) is given in Figure 9, with the exhaust parameter and ammonia input remaining constant. The NO\({}_{\rm x}\) concentration at the SCR inlet is higher than in Figures 6-8 for better comparison. The effect of exhaust temperature at this working point is still generally positive for NO\({}_{\rm x}\) conversion, while the effect of NO oxidization depends on actual SCR temperature. At lower temperatures with lower catalyst activity, a higher NO\({}_{2}\) proportion can increase the proportion of fast reactions. With exhaust temperature at 200 \({}^{\circ}\)C, the NO\({}_{\rm x}\) conversion rate of 40% NO\({}_{2}\) (60% NO) is 77.57%, significantly higher than the 49.01% of 100% NO; at 250 \({}^{\circ}\)C, the difference caused by NO\({}_{2}\) is limited. However, the consumption rate of NO is higher than NO\({}_{2}\); thus, NO may be completely clarified before complete NO\({}_{2}\) conversion, especially at higher temperatures, and NH\({}_{3}\)-NO\({}_{2}\) reaction rate is significantly low and N\({}_{2}\)O is likely to be generated during this process. With 40% NO\({}_{2}\) at the inlet and exhaust temperature at 300 \({}^{\circ}\)C, the NO\({}_{\rm x}\) emission at the SCR outlet increased to 7.15%, while, with 100% NO, the emission is only 3.38%. It should be noted that the NO\({}_{\rm x}\) at the SCR inlet in Figures 6-8 is completely NO. The effect of DOC after thermal management requires further consideration. In summary, the low exhaust temperature at lower load working points will inevitably decrease the SCR catalyst activity. To improve the NH\({}_{3}\)-NO\({}_{\rm x}\) reaction rate, a higher exhaust temperature is required and a proper NO/NO\({}_{2}\) ratio may also improve the conversion [19]. Figure 8: Reactions in SCR at different carrier lengths. (**a**) Changes of NO\({}_{\rm x}\) Conversion Rate with SCR Carrier Length. (**b**) Changes of Ammonia Chemical Loss with SCR Carrier Length. Figure 9: Total NO\({}_{\rm x}\) conversion rate at different inlet conditions. Methods that improve the reaction between ammonia and nitrogen oxides will also improve the ammonia-oxygen reaction. Thermal management is required to further decrease NO\({}_{\mathrm{x}}\) emission, and the increase in ammonia chemical loss should also be taken into consideration [4].