## 2 Models and Methods
### Real Driving Emission Modeling  
The real driving CO\({}_{2}\), CO, NOx, and PN emissions of the test vehicle in RDE Test A and their accumulation over the total trip are given in Figures 3-6, respectively. In these figures, frequent changes in engine operating conditions and many start-stop events can be observed. However, CO\({}_{2}\) emissions seem to be less affected by engine operating dynamics. Using the engine speed and torque recorded in the real driving emission test as input, the steady-state CO\({}_{2}\) emissions can be interpolated in the steady-state CO\({}_{2}\) emission data map from the engine bench tests. Comparing the real driving CO\({}_{2}\) emissions and the interpolated steady-state CO\({}_{2}\) emissions under the same engine speed and torque conditions, it can be found that the difference between the two is not significant. The error of the interpolated total trip cumulative CO\({}_{2}\) emissions relative to the measured data is only 5.9%. In the enlarged view of Figure 3, the engine start-stop events can be recognized by the transient changes in engine speed, but no transient jump-up in CO\({}_{2}\) emissions corresponds to engine start-up; the interpolated CO\({}_{2}\) emissions are still close to the measured CO\({}_{2}\) emissions at the moment when the engine starts up. Thus, the error due to using the interpolated steady-state CO\({}_{2}\) emissions to predict the real driving CO\({}_{2}\) emissions of this EREV should not be too large. In addition, the carbon in the fuel is mainly discharged as CO\({}_{2}\) emissions after in-cylinder chemical reactions (other carbon-containing pollutants in the exhaust are emitted in orders of magnitude lower than CO\({}_{2}\) emissions). Thus, fuel consumption is linearly correlated with CO\({}_{2}\) emissions. This study also uses the interpolated fuel consumption from the engine bench steady-state performance data to predict the real driving fuel consumption. Figure 3: Real driving CO\({}_{2}\) emissions associated with the engine’s start–stop events. Figure 4: Real driving CO emissions associated with the engine’s start–stop events. Figure 5: Real-driving PN emissions associated with the engine’s start–stop events. However, the engine start-stop events significantly affect real driving CO, NO\({}_{\rm x}\), and PN emissions. As shown in Figure 4, some non-continuous singular data points can be observed in the real driving CO emission data, and these singularities are indicative of a tens of times surge in transient CO emissions. In the enlarged view of Figure 4, the moments of these abnormally high CO emission peaks correspond to the engine start-up moments, so the engine start-stop events should cause these abnormally high CO emissions. Since the steady-state engine performance test under laboratory conditions cannot reflect the transient emission characteristics under engine start-stop conditions, a considerable difference exists between the interpolated steady-state CO emissions and the CO emissions caused by the engine start-stop events. In Figure 4, the relative error between the total trip cumulative CO emissions obtained by interpolating the engine steady-state performance data tables and those obtained by cumulating the transient CO emissions in the real driving emission test reaches 29.7%. As for PN emissions, the impact of the engine start-stop events is much more sensitive. In Figure 5, the abnormal transient PN emissions due to the engine start-stop events increase hundreds of times. The total trip cumulative PN emissions obtained by interpolating the engine steady-state performance data tables are less than 10% of those measured in the real driving emissions test, with a relative error of 91.8%. Comparatively, NO\({}_{\rm x}\) emissions are less affected by the engine start-stop events. In Figure 6, the abnormal increment of NO\({}_{\rm x}\) emissions due to engine start-up is seldom observed in the urban and rural trips, but is only more demonstrated in the motorway trips.