## 3 Result and Analysis  
### SCR Reaction at Steady Working Point  
At a certain working point taken from the world harmonized steady-state cycle (WHSC), the reaction within SCR at exhaust temperatures between 200 \({}^{\circ}\)C and 500 \({}^{\circ}\)C is shown in Figure 6a, where exhaust mass flow and inlet NO\({}_{\rm x}\) remain constant. At lower exhaust temperatures, the NH\({}_{3}\)-NO\({}_{\rm x}\) reaction rate is hindered by the low temperature, as exhaust temperature increases, the NO\({}_{\rm x}\) conversion rate gradually increases from 87.07% at 200 \({}^{\circ}\)C to 97.81% at 260 \({}^{\circ}\)C. After peaking at 260 \({}^{\circ}\)C, the NO\({}_{\rm x}\) conversion rate decreases with a further increase in the exhaust temperature. With the ammonia-nitrogen ratio fixed at 1:1, a higher proportion of ammonia reacting with oxygen will hinder the reaction of ammonia with nitrogen oxides; the ratio of unclarified nitrogen oxides is almost equal to ammonia consumed by the NH\({}_{3}\)-O\({}_{2}\) reaction. It should be pointed out that this is not the case in practical Cu-zeolite SCR reactions, as the ammonia storage generally provides sufficient ammonia. Figure 5: SCR simulation model. Figure 6: Reactions in SCR at different exhaust temperatures. (**a**) Changes of NO\({}_{\rm x}\) Conversion Rate with Exhaust Temperature. (**b**) Changes of Ammonia Chemical Loss with Exhaust Temperature. Within the exhaust temperature range selected in Figure 6, the reaction between ammonia and oxygen also increases with increasing temperature, as shown in Figure 6b. At an exhaust temperature below 250 \({}^{\circ}\)C, the ammonia-oxygen reaction consumes less than 0.1% NH\({}_{3}\) input and the NH\({}_{3}\)-O\({}_{2}\) reaction is neglectable. As the exhaust temperature increases to over 300 \({}^{\circ}\)C, the NH\({}_{3}\)-O\({}_{2}\) reaction rate begins to increase significantly, from 2.37% at 300 \({}^{\circ}\)C to 7.2% at 350 \({}^{\circ}\)C, 14.03% at 400 \({}^{\circ}\)C, and 32.09% at 500 \({}^{\circ}\)C. For road vehicles, the diesel engine is less likely to operate constantly at a high exhaust temperature range. Even so, with SCR catalyst temperature above 250 \({}^{\circ}\)C, the loss still requires consideration. The effect of exhaust mass flow is shown in Figure 7, with other parameters remaining constant. It should be noted that the NOx inlet in PPM is fixed, thus increasing exhaust mass flow, which increases the NOx quantity in SCR. This is the main factor that affects the NOx conversion rate at different mass flow rates. Increasing the mass flow by 40% reduces the NOx conversion rate to 82.1%, while decreasing it by 40% improves the conversion rate to 97.6%. The effect of exhaust flow change on the ammonia-oxygen reaction is also significant and the NH\({}_{3}\)-O\({}_{2}\) reaction rate decreases with the increase in exhaust mass flow and the ammonia chemical loss decreases from 0.75% to 0.46% between \(-\)40% and +40%. The effect of mass flow itself is less significant than exhaust temperature and SCR carrier length. The effect of SCR carrier length is shown in Figure 8, with other parameters also remaining constant. Longer carrier length means a larger area and longer time for catalyzed reactions, which improves the NH\({}_{3}\)-NO\({}_{x}\) reaction rate and NOx conversion rate, even at lower exhaust temperatures. The conversion rate of NOx increases from 92.1% to 98.2% as the SCR carrier length increases from 100 mm to 200 mm. The NH\({}_{3}\)-O\({}_{2}\) reaction rate is also significantly affected by the change in carrier length. At lower exhaust temperatures, as the carrier length increases from 100 mm to 200 mm, the ammonia chemical loss increases from 0.55% to 0.83%. Since the ammonia-nitrogen ratio is fixed at 1:1, the increase in the NH\({}_{3}\)-O\({}_{2}\) reaction rate hinders the NH\({}_{3}\)-NO\({}_{x}\) reaction at longer carrier length.