## 4 Results and Discussion  
In this study, all the optimization processes were carried out on the same device with the following specifications: CPU-Intel Core 7 12700H, GPU-NVIDIA GeForce RTX 3070 and RAM-32 GB. The time required to complete an overall optimization process could be kept within 15 s for each working condition. In order to demonstrate the genetic algorithm optimization process, three operating conditions-1200 rpm-75 N-m, 1800 rpm-300 N-m and 2400 rpm-450 N-m--were also selected. Figure 4 shows the Pareto plots of the targets for the three operating conditions. In this study, a combination of SCR + ASC is used for the aftertreatment system. According to the regulatory requirement that NH\({}_{3}\) slip should not be higher than 10 ppm,  
Figure 4: Optimization result of urea injection (Pareto front). \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Maximum number of iterations & 100 \\ population size & 50 \\ Crossover Percentage & 0.8 \\ Mutation Percentage & 0.1 \\ Mutation Rate & 0.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Parameters and their values. the actual NH\({}_{3}\) slip after ASC is considered when finding the optimum point in the Pareto plot. At SCR inlet temperatures above 275 \({}^{\circ}\)C, the NH\({}_{3}\) conversion efficiencies after ASC were verified on the test rig to be above 90%. At SCR inlet temperatures below 275 \({}^{\circ}\)C, the NH\({}_{3}\) conversion efficiency after ASC is also above 80%. The optimum point is selected to maximize the NOx conversion efficiency as much as possible while ensuring that the NH\({}_{3}\) slip is less than 10 ppm. The SCR inlet temperature is 264 \({}^{\circ}\)C at 1200 rpm-75 N\(\cdot\)m. The optimal result is shown in Figure 4a and the coordinate of the optimal point is (87.5, 9.7). The urea injection corresponding to this point is 42.7 mg/s. That is, the urea injection rate to optimize the performance of the SCR (NOx conversion efficiency of 87.5% and NH\({}_{3}\) slip of 9.7 ppm) was 42.7 mg/s under this operating condition. At operating condition 1800 rpm-300 N\(\cdot\)m, the SCR inlet temperature is 315 \({}^{\circ}\)C. As shown in Figure 4b, the optimum point coordinate is (93.6, 9.4). The urea injection corresponding to this point is 394.1 mg/s. That is, the urea injection rate to optimize the performance of the SCR (NOx conversion efficiency of 93.6% and NH\({}_{3}\) slip of 9.4 ppm) was 394.1 mg/s under this operating condition. At operating condition 2400 rpm-450 N\(\cdot\)m, the SCR inlet temperature was 521 \({}^{\circ}\)C. As shown in Figure 4c, the optimum point coordinate is (96.1, 9.5). This point corresponds to a urea injection of 542.8 mg/s. That is, the urea injection rate to optimize the performance of the SCR (NOx conversion efficiency of 96.1% and NH\({}_{3}\) slip of 9.5 ppm) was 542.8 mg/s under this operating condition. In order to observe the urea optimization results for all working conditions, the NSGA-II is used to optimize the urea injection amount for all working conditions, all the optimal urea injection amounts are obtained and the optimal solution NOx conversion efficiency and NH\({}_{3}\) slip are calculated according to the model. A comparison of the results before and after optimization of the SCR system is shown in Figure 5.  
In Figure 5d, it can be seen that after optimization, when the SCR inlet temperature is in the range of 200 \({}^{\circ}\)C to 300 \({}^{\circ}\)C, the NOx conversion efficiency rises more obviously with the increase in urea injection. When the SCR inlet temperature exceeds 450 \({}^{\circ}\)C, the NOx conversion efficiency shows a decreasing trend. This is mainly due to the fact that when the SCR inlet temperature exceeds a certain value (about 450 \({}^{\circ}\)C), an oxidation reaction of NH\({}_{3}\) occurs through Equation (15). The product of this reaction is NO.