# Evaluating the Real-World NOx Emission from a China VI Heavy-Duty Diesel Vehicle
## 3 Results and Discussion
### Section NOx Emission Analysis
For the entire trip, the engine out BSNOx emissions of the four PEMS tests were 6.665, 6.622, 7.363 and 6.759 g/kWh, respectively. The tailpipe BSNOx emissions of rural section and motorway section which were lower than the required limit of NOx emission (0.690 g/kWh) were also lower than that of the entire trip, but, the tailpipe BSNOx emission of urban section was significantly higher than that of other sections. For instance, the tailpipe BSNOx emission of urban section was 8.27 times greater than that of the entire trip in Test 4. For the entire trip, the tailpipe \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Section** & **Test 1** & **Test 2** & **Test 3** & **Test 4** \\ \hline Engine out & Urban & 8.162 & 6.647 & 8.105 & 7.114 \\ NOx emission & Rural & 6.578 & 6.640 & 7.212 & 6.838 \\
[g/kWh] & Motorway & 6.466 & 6.610 & 7.330 & 6.691 \\  & Overall & 6.665 & 6.622 & 7.363 & 6.759 \\ \hline Tailpipe & Urban & 5.054 & 3.519 & 5.637 & 7.388 \\ NOx emission & Rural & 0.395 & 0.370 & 0.344 & 0.271 \\
[g/kWh] & Motorway & 0.059 & 0.502 & 0.413 & 0.317 \\  & Overall & 0.648 & 0.794 & 0.811 & 0.893 \\ \hline \end{tabular}
\end{table} 
Table 4: NOx brake-specific emissions of each section. Figure 6: (**a**) Each section’s cumulative mass of tailpipe NOx emission; (**b**) each section’s contribution ratio to the total mass of tailpipe NOx emission. BSNOx emissions of the four PEMS tests were 0.648 g/kWh, 0.794 g/kWh, 0.811 g/kWh, 0.893 g/kWh, respectively. So, we may conclude that the real-world NOx emissions may get worse as the increase of odometer. Figure 7 shows the instantaneous emissions of the engine output of NOx, tailpipe NOx and the instantaneous vehicle speed of the entire trip in Test 3. As it can be seen, the tailpipe NOx concentration is almost close to the engine output of NOx concentration in the urban section since catalytic converter requires a certain temperature to work efficiently. A small urea solution injection was registered in the urban section because the temperature of catalytic converter was not high enough. The data points which conducted urea solution injection were only 6.3% of the data points recorded in the whole urban section in Test 3 (Figure 8). Figure 9a shows the instantaneous engine NOx emission and the vehicle acceleration profile of the entire trip in Test 3. As it can be seen, there was little abrupt acceleration during the rural or motorway section because the vehicle speed was relatively stable during these two sections. Most abrupt positive acceleration occurred during urban section. As shown in Figure 9b, NOx emission peaks were clearly linked to the vehicle acceleration peaks during the urban section, that means abrupt positive vehicle acceleration would lead to worse engine NOx emission. In fact, the urban section of the four PEMS test were mainly conducted on the city road with traffic jam, roundabouts and traffic light, driving under these circumstances may lead to more frequent \"stop-go\" events where abrupt positive vehicle acceleration would happen. So, the lower temperature of SCR may lead to a smaller urea solution injection and more frequent \"stop-go\" events which may lead to higher engine NOx emission together would cause higher tailpipe NOx emission in the urban section. We see clearly from the above discussion that the urban tailpipe NOx emission plays an important role in the real-world tailpipe NOx emission of N3 category heavy-duty diesel vehicle. Figure 7: The instantaneous emissions of engine output of NOx, tailpipe NOx and vehicle speed of the entire trip in Test 3. Figure 8: The average temperature of selective catalytic reduction (SCR) and the amount of urea solution injection in Test 3; (**a**) for entire trip; (**b**) for urban section. Figure 9: The instantaneous engine NOx emission and the vehicle acceleration in Test 3; (**a**) for entire trip; (**b**) for urban section.