*2.1. Introduction of Hybrid Powertrain System*

The AHS-II two-mode hybrid powertrain consists of a planetary gear, a compound planetary gear, four clutches, an internal combustion engine, and two motor/generators, MG1 and MG2, as shown in Figure 1. The architecture can produce effects which are similar to the continuously variable transmission (CVT). Therefore, it is also known as electrical continuously variable transmission (eCVT). The symbols R, S, and C represent the ring gear, the sun gear, and the carrier, respectively, and the subscripts 1 and 2 represent the compound planetary gear set and the simple planetary gear set. When the vehicle travels on different road conditions, the powertrain system can operate between two eCVT modes and four fixed-gear modes.

**Figure 1.** AHS-II two-mode powertrain architecture.

### *2.2. HEV Simulation Model*

The backward calculation dynamic model of the HEV system was implemented using Matlab/Simulink, as shown in Figure 2. The US FTP-75 (EPA Federal Test Procedure) urban and highway drive cycles were applied as the road conditions for simulation, as shown in Figures 3 and 4. According to the known driving speed, the vehicle dynamic model calculated the required vehicle acceleration and driving torque, and through the energy management control module, the operating mode of the system and the output powers of engine and motor/generators, MG1 and MG2, were determined. The speed and torque of the two motor/generators were determined by the gear ratio of the transmission module. According to the speed and torque of the motor/generators, the battery module calculated the state of charge (SOC) of the battery pack. Finally, the fuel consumption was accumulated by ICE module through an engine two-dimensional lookup table.

**Figure 2.** Two-mode powertrain simulation model.

**Figure 3.** FTP-75 urban driving cycle.

**Figure 4.** FTP-75 highway driving cycle.
