## 3 Results and Discussion
### Conformity Factors  
The conformity factor (CF) of 1.0 represents the Euro 6 legal emission limit for vehicle type approval and first registration, which is 6.0 \(\times\) 1011 \(\#\)/km for PN emissions and 60 mg/km for NOx emissions regarding vehicles with SI engines. As these values shall apply for laboratory conditions, respectively, the WLTP margins for PN and NOx are introduced to consider the measurement uncertainties of the PEMS for (mobile) RDE tests. Indeed, this study does not evaluate valid RDE tests, but a real-world driving scenario. However, it must be noted that most of the RDE trip requirements are fulfilled by the Stuttgart test route (Table 1).  
For PN, the current margin value is 50%, which equals a CF of 1.5 or 9.0 \(\times\) 1011 \(\#\)/km [22]. For NOx, the PEMS measurement uncertainty is recently quantified by a margin of 43% [23], corresponding to a CF of 1.43 or approx. 86 mg/km.  
Figure 12: Seasonal influence of driving mode on the real-world NOx emissions of the studied PHEV considering hybrid driving mode. (**a**) Summer time; (**b**) autumn time.  
Figure 13 presents the CFs for the PN emissions of each performed real-world driving test with the PHEV under investigation, taking also the different driving situations (urban, rural, motorway vehicle speeds) into account.  
For the total real-world driving test route, the tests no. 10 and 11 exceed the CF of 1.5 (both tests show CFs of approx. 1.7), while the other tests show CFs between 0.9 and 1.2, partially even achieving the laboratory limit value. It must be noted that the tests no. 9-11 represent sub-23 nm PN emission measurements, and that the current Euro 6 legislation including the RDE margins only consider PN emissions up to 23 nm, but not below. Additionally, as the investigated PHEV is not equipped with a GPF, the UFP fraction might increase significantly. This is furthermore indicated by the sharp increase in PN emissions for urban vehicle speeds considering these tests (CFs between 1.5 and 2.3), which could indicate a significant UFP share in cold start emissions. In the first 8 tests, the urban PN emissions are within the not-to-exceed limit of 1.5. For rural driving conditions, all tests reveal CFs below 1.5 and the majority is even below the laboratory limit value. For the tests no. 1, 3, 10 and 11 the CFs for motorway driving exceed the limit of 1.5. On the one hand, this again highlights that the hybrid driving mode can increase PN emissions significantly at motorway vehicle speeds (Section 3.2). On the other hand, considering tests no. 1 and 3, it shows that this increase is not immediately related to the UFP fraction.  
Figure 14 gives an overview of the NO\({}_{\text{x}}\) CFs of each performed real-world driving test with the PHEV under investigation for the different vehicle speed categories.  
Figure 13: Conformity factors (CFs) for PN emissions of the performed real-world driving tests with the studied PHEV.  
Even though the current NO\({}_{\rm x}\) CF for RDE tests is set to 1.43, the \(y\)-axis in Figure 14 only ranges to CF 1.0, as in all performed real-world driving tests, the NO\({}_{\rm x}\) CFs are below 0.4. Regarding the total test route, the NO\({}_{\rm x}\) emissions are at least 5-times lower than the laboratory limit value. Section 3.4 shows that the main part of NO\({}_{\rm x}\) emissions is related to the initial emission peak from the first activation of the ICE, which is also indicated by the relatively high urban CFs (Figure 14). In comparison, the CFs for rural and motorway driving are below 0.1, partly on a near-zero level. Even if high relative differences are observed within the NO\({}_{\rm x}\) CFs, compared to the Euro 6 and the not-to-exceed limits, the NO\({}_{\rm x}\) emissions are generally on a low absolute level, considering all tests and driving situations.