## 3 Results and Discussion
### NOx Emissions  
Figure 11 shows a comparison of the average, distance-related real-world NOx emissions of the studied PHEV for driving in the hybrid mode in summer (Figure 11a) and driving the sport mode in summer (Figure 11b).  
In total, the average NOx emission factor of the studied PHEV is about 8 mg/km for hybrid driving and almost 6 mg/km for the sport driving mode. While the emissions for rural and motorway vehicle speeds are similar for both driving modes and generally are on a low level, the urban NOx emissions differ significantly. For the sport driving mode, the average urban NOx emission factor is approx. 10 mg/km and the variance within the tests is small (Figure 11b). The average urban NOx emissions for hybrid driving are nearly 16 mg/km, which is 52% higher than for the sport driving mode. Furthermore, the  
Figure 11: Influence of driving mode on the real-world nitrogen oxides (NOx) emissions of the studied PHEV during summer time. (**a**) Hybrid driving mode; (**b**) Sport driving mode.  
Figure 10: Correlation between the particle counter of the portable emissions measurement system (PEMS) and the engine exhaust particle size (EEPS) with a similar cut-off diameter of 23 nm.  
variance of urban NOx emissions is considerably greater within the hybrid driving mode tests, which ranges from approx. 10 mg/km till 24 mg/km (Figure 10(a)).  
Figure 12 presents the average, distance-related real-world NOx emissions of the studied PHEV for hybrid mode driving in summer (Figure 11(a)) and in autumn (Figure 11(b)).  
The average NOx emission factor for the total Stuttgart test route is 6.5 mg/km considering the autumn tests, which is about 18% less compared to the summer tests. The variance of the total emissions is very similar between summer and autumn tests. In comparison with the summer test results, NOx emissions during rural and motorway driving are on an even lower, near zero-level in the autumn tests and thus can be neglected. The average urban NOx emission factor of the autumn tests is approx. 14 mg/km, which is 9% lower than for the summer tests. The variance of the test results for urban driving is also similar to the respective bandwidth of the summer test results, ranging from 7 mg/km to 19 mg/km (Figure 11(b)). As the NOx emissions of the summer tests are generally higher than within the autumn tests considering all vehicle speed classes, the authors conclude that the lower ambient temperatures do not influence NOx emissions in this study. As already discussed regarding the PN emissions, the ambient temperature differences between the summer and autumn tests are small and can be neglected. In contrast, the driving mode seems to be the main influencing factor, as especially for urban vehicle speeds the hybrid driving mode raises the NOx emissions. Similar to PN emissions, the urban NOx emissions are mainly determined by the emission peaks resulting from the activation of the ICE during low TWC temperature situations (Figure 3). Besides the generally higher values of these cold start peak NOx emissions, the greater variances of the urban emissions within the hybrid driving mode tests and sport driving mode tests also prove that the uncertainty regarding the height of these peak emissions increase for hybrid driving.