## 3 Results and Discussion
### PN Emissions  
Figure 6 presents a comparison of the average, distance-related PN emission factors of the studied PHEV for real-world driving conditions in the hybrid driving mode (Figure 6a) and in the sport driving mode (Figure 6b). The PN emission factors base on the PEMS raw emission data. Figure 5: Seasonal influence on the real-world fuel consumption of the studied PHEV considering hybrid driving mode. (**a**) Summer time; (**b**) autumn time. The average PN emission factors of the total test track are similar for both the hybrid driving mode (6.25 \(\times\) 10\({}^{11}\) #/km) and the sport driving mode (6.44 \(\times\) 10\({}^{11}\) #/km), but the variance of the hybrid driving mode values is higher than those of the sport driving mode test results. Concerning the PN emission factors for urban and rural vehicle speeds, the emission values for sport mode driving are approx. 6% (urban) and 13% (rural) higher than for hybrid mode driving. For both test series, the average emission factors for urban vehicle speeds are higher than those for the rural speed category and the variances are also comparable. The higher PN emissions for urban vehicle speeds relative to rural vehicle speeds--in spite of higher electric driving shares and comparable fuel consumptions (Figure 4)--can be associated with cold start emission peaks, which appear when the ICE is activated the first time (Figure 3). At cold start conditions and during the warming up phase, the fuel consumption can increase. In addition to that, the three-way catalyst (TWC) is not fully warmed up yet during this phase, which leads to high CO and hydrocarbon (HC) emissions. These effects can also favor particle formation. Furthermore, it is important to note that the PHEV under study is not equipped with a gasoline particle filter (GPF). For motorway vehicle speeds, Figure 6 shows an average emission factor of 8.6 \(\times\) 10\({}^{11}\) #/km for the hybrid driving mode and 6.9 \(\times\) 10\({}^{11}\) #/km for the sport driving mode. Thus, PN emissions are significantly higher (+25%) in the hybrid driving mode compared to the sport driving mode, although lower fuel consumptions and higher electric driving shares are observed within the hybrid driving mode tests (Figure 4 and Table 4). In order to identify possible causes for this anomaly, Figure 7 shows a comparison of the cumulative PN emissions over time for a test drive in the hybrid driving mode (Figure 7a) and a test drive in the sport driving mode (Figure 7b). In addition, the differentials of the fuel consumption are presented as an alternative indicator for the activity of the ICE as the engine speed could not have been tracked during the tests in the sport driving mode. During the first half of the test track, the cumulative PN emissions in the hybrid driving mode test are marginally higher than in the sport driving mode test. In the second half of the test track, which mainly consists of rural and motorway vehicle speed parts, the increase in cumulative particle emissions in the hybrid driving mode test becomes much greater than in the case of the sport driving mode test. The fuel consumption differential curve shows that there are generally less spikes in the hybrid driving mode test which means that the ICE is activated less frequently than in the sport driving mode test. However, the amplitude of the fuel consumption differential spikes is significantly higher for the hybrid driving mode (Figure 7a) than for the sport driving mode (Figure 7b). The authors assume that the observed dynamic operation pattern of the ICE in terms of its regular activation and deactivation causes these higher fuel consumption differentials, which in turn leads to the strong increase in PN emissions during the rural and especially the motorway driving  
Figure 6: Influence of driving mode on the real-world particulate number (PN) emissions of the studied PHEV during summer time. (**a**) Hybrid driving mode; (**b**) Sport driving mode. parts.