## 2 Models and Methods
### Real Driving Emission Tests
The atmospheric pressure measurement range is 0-115 kPa and measurement accuracy is \(\pm\)2%; the atmospheric temperature measurement range is \(-\)40-60 \({}^{\circ}\)C and measurement accuracy is \(\pm\)0.5 \({}^{\circ}\)C; the atmospheric humidity measurement range is 0-100% and measurement accuracy is \(\pm\)1.5%. The HORIBA OBS-ONE provides real-time exhaust flow data based on the exhaust pressure, exhaust temperature, and upstream-downstream differential pressure measured by the exhaust flow meter, which is then combined with the pollutant emission concentration to obtain the exhaust mass data for each pollutant. Engine operating data, such as engine speed, torque, and coolant temperature, are read directly by the onboard diagnostics (OBD) assembled inside the engine control unit. Figure 1 shows the installation of the HORIBA OBS-ONE on the test vehicle. The gas analyzer model, PN measurement module, system control unit, power management model, etc., are placed inside the cabin (Figure 1a), and the sampling pipes and temperature sensors are installed on the exhaust tailpipe (Figure 1b). To reduce the excessive influence of various random factors (traffic flow, driving style, road gradient, etc.) on real driving emission tests, the real driving emission test procedure defines the test boundary conditions, including the test environment (ambient temperature and altitude, etc. ), trip dynamics, test routes, road topography, vehicle test mass, auxiliary equipment, lubricants, and fuels [43; 44]. Real driving emission tests should be conducted consecutively in the order of urban, rural, and motorway sections. The driving speed is below 60 km/h on the urban trip, between 60 km/h and 90 km/h on the rural trip, and greater than 90 km/h on the motorway trip. The mileage of the urban, rural, and motorway sections should account for 34%, 33%, and 33% of the total trip, respectively, and the error in the mileage proportion should be controlled within \(\pm\) 10%. Moreover, the minimum driving distance of the test vehicles in the urban, rural, and motorway sections is 16 km, and the minimum cumulative urban mileage for engine operation of EREVs is 12 km. The altitude difference between the start and end points of the test route must not exceed 100 m, and the standardized cumulative positive altitude gain of the test route is not greater than 1200 m/100 km [44]. Figure 2 shows the test vehicle's driving speed and altitude profiles in the two real driving emission tests conducted in the study, which are referred to as RDE Test A (Figure 2a) and RDE Test B (Figure 2a), respectively. The two real driving emission tests comply with the above provisions specified in the regulation and also meet the requirements for the normal altitude condition (altitude more than 700 m) and the normal temperature condition (ambient temperature more than 0 \({}^{\circ}\)C and less than 30 \({}^{\circ}\)C) defined in the real driving emission test procedure. During these two real driving emission tests, no external charging is carried out, and the charge-sustaining under a power-following strategy is used to operate the engine more frequently to obtain more real-time data on engine fuel consumption and pollutant emissions under limited vehicle driving mileage conditions. In what follows, the data collected in these two real driving emission tests are used to construct real driving emission models of the EREV.