# Evaluating the Real-World NOx Emission from a China VI Heavy-Duty Diesel Vehicle
## 3 Results and Discussion
### The Exploration of Evaluation Methods  
The parameters which have a strong correlation to MAW NOx emission including the MAW duration, the MAW average power, the MAW average vehicle speed, etc. (Table 6) may help us clearly distinguish urban section from rural and motorway sections. Therefore, in order to guarantee the objectivity and accuracy of the evaluation, referring to the baseline already described in Section 2.5 in this paper, three different methods (Method 1-3) were applied to extend the regulatory boundary conditions, details were as follows:  
Method 1--in accordance with the baseline except power threshold (PT), and setting the PT as 10% for valid MAWs. Method 2--in accordance with Method 1 except using the 90 th cumulative percentile of the valid MAWs emissions as the result of the PEMS test. Instead, according to the average vehicle speed of the MAWs, redefining the MAWs as \"Motorway MAWs\" (>70 km/h), \"Rural MAWs\" (\(\leq\)70 km/h and \(\geq\)reference vehicle speed) and \"Urban MAWs\" (<reference \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Test 1** & **Test 2** & **Test 3** & **Test 4** \\ \hline  & \multicolumn{4}{c}{MAW NOx emission [g/kWh]} \\ \hline MAW Duration (s) & 0.910 & 0.739 & 0.817 & 0.842 \\ MAW Average Power (kW) & \(-\)0.800 & \(-\)0.590 & \(-\)0.678 & \(-\)0.730 \\ MAW Cumulative NOx Mass (g) & 1.000 & 1.000 & 1.000 & 1.000 \\ MAW DeNOx Efficiency (\%) & \(-\)0.999 & \(-\)0.998 & \(-\)1.000 & \(-\)1.000 \\ MAW Average Vehicle Speed (km/h) & \(-\)0.876 & \(-\)0.744 & \(-\)0.772 & \(-\)0.775 \\ MAW Average SCR Temperature (\({}^{\circ}\)C) & \(-\)0.885 & \(-\)0.641 & \(-\)0.677 & \(-\)0.751 \\ MAW Fuel Consumption (kg) & 0.995 & 0.623 & 0.709 & 0.807 \\ MAW engine coolant(\({}^{\circ}\)C) & -0.871 & -0.620 & -0.632 & -0.672 \\ MAW NOx Emission (g/kWh) & 1.000 & 1.000 & 1.000 & 1.000 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Pearson correlation coefficient of MAW NOx emission. Figure 12: Heatmap of MAW parameters inTest#3 (Pearson correlation coefficient). vehicle speed), where, the reference vehicle speed is equal to the average vehicle speed of the rural section of the PEMS test. More than that, distributing weighting factors to \"Motorway MAWs\" (0.55), \"Rural MAWs\" (0.25), and \"Urban MAWs\" (0.2) according to the required trip share. The final evaluation results shall be calculated by Equation (4):  
\[EF=90\%ileEF_{Urban MAW}*0.2+90\%ileEF_{Rural MAW}*0.25+90\%ileEF_{Motorway MAW}*0.55 \tag{4}\]  
where: 90%_ile_ is 90 th cumulative percentile. Method 3--in accordance with Method 1 except using the 90 th cumulative percentile of the valid MAWs emissions as the result of the PEMS test. Instead, redefining the MAWs according to the average engine coolant temperature of the MAWs as \"Cold MAWs\" (<reference engine coolant temperature) and \"Hot MAWs\" (\(\geq\)reference engine coolant temperature), where, the reference engine coolant temperature is equal to the average engine coolant temperature of the entire trip. At the same time, distributing weighting factor to \"Cold MAWs\" (0.14), \"Hot MAWs\" (0.86) referring to the WHTC rules. The final evaluation results shall be calculated by Equation (5):  
\[EF=90\%ileEF_{Cold MAW}*0.14+90\%ileEF_{Hot MAW}*0.86 \tag{5}\]  
where: 90%_ile_ is 90 th cumulative percentile. Due to the uncertainty of the PEMS test procedures, such as trip share, vehicle speed or cumulative work, etc., it is hardly for us to find a constant \"Reference Value\" to ensure a better universality of Method 2 or Method 3, so, the key point of these two methods were to find a proper \"Reference Value\". Our data analysis revealed that the average value of a certain section of the test or the entire trip may be a good choice. Table 7 shows the evaluation results of the Method 1-3, As it can be seen:  
For Method 1, when the power threshold (PT) was set as 10%, all of the MAWs of the four PEMS tests were valid, and the data produced during the entire trip was taken into account for the evaluation.