# Evaluating the Real-World NOx Emission from a China VI Heavy-Duty Diesel Vehicle
## 3 Results and Discussion
### MAW NOx Emission Analysis  
In this section, the NOx emission we talk about refers to tailpipe NOx emission and all the basic data was from the AVL-M.O.V.E-PEMS. The boundary conditions meet the requirements described in Section 2.5 in this paper. Table 5 shows the number of MAWs, number of valid MAWs, valid MAWs ratio, power threshold, 90 th cumulative percentile of valid MAWs NOx emission (g/kWh), and the real NOx emission of the entire trip (g/kWh), and the difference between entire trip NOx emission and 90 th cumulative percentile of valid MAWs NOx emission of the four PEMS test. The percentage of valid windows was less than 50% when power threshold was 20%, 19%, 18%, respectively in Test 2. So, the final power threshold used to evaluate the NOx emission in Test 2 was 17%. There was a great difference between the real NOx emission of the entire trip and 90 th cumulative percentile of valid MAWs NOx emission (g/kWh). The evaluation results of the four PEMS tests were all lower than the real, the error is \(-89.48\%\) in Test 1, \(-30.59\%\) in Test 2, \(-45.91\%\) in Test 3 and \(-62.29\%\) in Test 4. Not only that, the evaluated NOx emissions were all lower than the required limit (0.690 g/kWh), but for Test 2, Test 3 and Test 4, the real NOx emissions were higher than 0.690 g/kWh. Figure 9(a) shows the distribution, in g/kWh, of the NOx emissions of each MAW versus the average power (% of maximum engine power) of each MAW during Test 3. As it can be seen, the data points in the red rectangle are considered in the final analysis according to the boundary conditions. The higher NOx emissions which are excluded from the final analysis because of the imposed boundary conditions which are mainly concentrated in the MAWs whose average power are higher. Figure 9(b) shows the average vehicle speed and average power (% of maximum engine power) of each MAW. The lower the average power of the MAW is, the lower the  
Figure 9: (**a**) MAWs NOx emissions vs. % of maximum engine power of MAWs during Test 3; (**b**) MAWs vehicle speed vs. % of maximum engine power of MAWs during Test 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Test 1** & **Test 2** & **Test 3** & **Test 4** \\ \hline MAW Number & 12,291 & 12,513 & 13,742 & 10,432 \\ Valid MAW Number & 7562 & 9047 & 6895 & 6709 \\ Valid MAW Ratio (\%) & 61.5 & 72.3 & 50.2 & 64.3 \\ Power Threshold (\%) & 20 & 17 & 20 & 20 \\
90 th cumulative percentile of Valid & 0.068 & 0.551 & 0.438 & 0.337 \\ MAWs NOx emission (g/kWh) & 0.648 & 0.794 & 0.811 & 0.893 \\ Error 90 th Valid MAW to Overall (\%) & \(-89.48\) & \(-30.59\) & \(-45.91\) & \(-62.29\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Information of MAW and real NOx emission of the four PEMS tests. average vehicle speed of the MAW is. As we know, low vehicle speed occurs mainly in the urban section. Figure 11 shows the distribution of the urban start position, rural start position, motorway start position, test end position, start position of the first valid MAW and end position of the last valid MAW on the timeline. As it can be seen, the end positions of the last valid MAWs are all located in the rural section of the four PEMS tests, that means the valid MAWs obtained by the rules described in Section 2.5 in this paper (in accordance with China VI, GB17691-2018) just represent the emission characteristics of rural and motorway sections. So, if we still want to evaluate the NOx emission by MAW method, the data during the urban operation must be taken into account for the evaluation and new rules may be needed. Before that, we have to find main influence factors of the MAW NOx emission. Figure 12 shows a heatmap that reveals the Pearson correlation coefficient between the parameters' mean value or cumulative value of the MAWs in Test 3. Figure 11: Section and first (last) valid MAW distribution information; (**a**) for Test 1; (**b**) for Table 2. (**c**) for Test 3; (**d**) for Test 4.