# Evaluating the Real-World NOx Emission from a China VI Heavy-Duty Diesel Vehicle
## 3 Results and Discussion
### The Exploration of Evaluation Methods
Even so, there was also a great difference between the real NOx emission of the entire trip and 90 th cumulative percentile of valid MAWs NOx emission (g/kWh), not only that, both positive error and negative error were existed. The error was as high as 120.7% in Test 1, \(-\)23.1% in Test 2, \(-\)20.4% in Test 3, 34.6% in Test 4. So, we may conclude that just reducing the power threshold (PT) may be not useful enough for an objective evaluation. For Method 2, the average vehicle speed of rural section of the test was used to distinguish \"Urban MAWs\" and \"Rural MAWs\", that because, for N3 category vehicles, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Method** & **Parameters** & **Test 1** & **Test 2** & **Test 3** & **Test 4** \\ \hline \multirow{4}{*}{Method 1} & Real NOx emission of the entire trip (g/kWh) & 0.648 & 0.794 & 0.811 & 0.893 \\ \cline{2-6}  & Power Threshold (\%) & 10 & 10 & 10 & 10 \\  & NOx emission by Method 1 (g/kWh) & 1.43 & 0.611 & 0.645 & 1.203 \\  & Error1 (Method 1 to Real NOx emission) & 120.7\% & \(-\)23.1\% & \(-\)20.4\% & 34.6\% \\  & MAW Number & 12,291 & 12,513 & 13,742 & 10,432 \\  & Valid MAW Number & 12,291 & 12,513 & 13,742 & 10,432 \\  & Valid MAW Ratio & 100\% & 100\% & 100\% & 100\% \\ \hline \multirow{4}{*}{Method 2} & Power Threshold (\%) & 10 & 10 & 10 & 10 \\  & NOx emission by Method 2 (g/kWh) & 0.619 & 0.773 & 0.833 & 0.812 \\ \cline{1-1}  & Error2 (Method 2 to Real NOx emission) & \(-\)4.5\% & \(-\)2.7\% & 2.8\% & \(-\)9.1\% \\ \cline{1-1}  & Reference Vehicle Speed (km/h) & 55.4 & 60.9 & 60.9 & 57.7 \\ \hline \multirow{4}{*}{Method 3} & Power Threshold (\%) & 10 & 10 & 10 & 10 \\  & NOx emission by Method 3 (g/kWh) & 0.661 & 0.784 & 0.763 & 0.955 \\ \cline{1-1}  & Error3 (Method 3 to Real NOx emission) & 2.0\% & \(-\)1.2\% & \(-\)5.9\% & 6.9\% \\ \cline{1-1}  & Reference Engine coolant Temperature (\({}^{\circ}\)C) & 78.0 & 80.8 & 80.4 & 80.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The results of the Method 1–3. there may be no MAW during the urban operation only. As shown in Figure 13, the cumulative work of urban section of each PEMS test was less than the work performed over the WHTC cycle (38.72 kWh). So, for the four PEMS tests, no MAW was merely composed by the urban operation. The less the cumulative work of urban section, the more the data produced in rural section and used to compose the last MAW. Figure 14 shows the distribution of the redefined MAWs of the four PEMS tests by Method 2. As it can be seen, the worse polluting windows were categorized as \"Urban MAW\", most MAWs were categorized as \"Motorway MAW\" because of its longest test duration and maximum cumulative work. The error between the real NOx emission of the entire trip and the NOx emission evaluated by Method 2 was \(-4.5\%\) in Test 1, \(-2.7\%\) in Test 2, \(2.8\%\) in Test 3 and \(-9.1\%\) in Test 4. For Method 3, the average engine coolant temperature of the entire trip was used to distinguish \"Hot MAWs\" and \"Cold MAWs\". As shown in Figure 15, the engine coolant temperature generally rose gradually and then stabilized around a certain value after  
Figure 14: Distribution of the redefined MAWs of the four PEMS tests by Method 2; (**a**) for Table 1. (**b**) for Test 2; (**c**) for Test 3; (**d**) for Test 4. Figure 13: Cumulative work of each section of the four PEMS tests. the engine coolant temperature had reached 70 \({}^{\circ}\)C for the first time during a PEMS test. Usually, there was no urea solution injection during the \"rising period\". So, we renamed the \"rising period\" as \"Cold Operation\". Via the data analysis, we found that taking the average engine coolant temperature of the entire trip as the \"Reference Value\" would lead to an exciting result. Figure 16 shows the distribution of the redefined MAWs of the four PEMS tests by Method 3, only a few of MAWs were regarded as \"Cold MAWs\" which had worse NOx emission.