## 2 Materials and Methodology
### Energy Management Control Strategy with Operation Mode  
The energy management control strategy proposed in this study aims to meet the low-, medium-, and high-load requirements. At low load, the EM outputs the power to meet the need of the vehicle. All power is provided by the ICE at medium load, and the extra power exceeding the driver's demand will drive the generator to charge the battery. The ICE is maintained in the optimal operation area at high load; the EM provides different power according to the power demand, and the two power sources simultaneously provide power to meet the power demand of the vehicle. As long as the ICE is in operation, the optimum operating point is always maintained at low, medium, or high load. The power source of this HEV can be divided into three types: motor drive, ICE drive, and ICE plus motor drive. According to the power distribution principle combined with the rule-based control method, the overall power demand can be divided into four energy management control strategies: motor output, engine output, dual output, and rechargeable battery when there is no power demand. Rule-based control rules convert the driver's throttle signal into power demand. The energy management controller determines the energy distribution according to the power demand. There are four types of energy distribution, which are described as follows:  
1. Motor output: The control flow is shown in Figure 2. The operating mode switching point is determined according to the maximum power that the electric motor can output and the efficiency curve of the internal combustion engine. When the demand power does not exceed the switching value of the maximum output power of the electric motor, the electric motor provides power to meet the vehicle's power demand. Conversely, when the required power exceeds the switching value of the maximum output power of the electric motor, it switches to the ICE mode. During the switching process, the ICE rises to the operating point established by the system in the shortest time. The operating point varies depending on the SOC. As the power output of the ICE rises, the energy management controller controls the output of the motor power. The driver does not feel the vibration caused by the engagement of the two power sources when the mode is switched, as shown in Figure 2.  
Figure 1: The architecture diagram of the HEV system. 2. Engine output: The control flow is shown in Figure 3. The ICE mode is activated when the driver's power demand exceeds the switching value of the maximum output power of the electric motor and the SOC is also within the rechargeable range. This allows for excess power to be charged back to the battery using the generator. In addition, when the driver's power demand exceeds the optimal operating point of the internal combustion engine, a transition from the generator state to the powering electric motor state occurs. Finally, the pure combustion engine mode can also charge the battery to the SOC reference value when the vehicle is under low load. Figure 2: Flowchart of the motor output control strategy. 3. Dual output: The control flow is shown in Figure 4. When the driver's power demand exceeds the optimal operating point of the ICE and the SOC is greater than the reference value, the electric motor is used to assist the ICE to meet the vehicle's power demand. Figure 3: Flowchart of the engine output control strategy. 4. Regenerative braking: The control flow is shown in Figure 5. When the driver depresses the brake pedal, the energy management controller determines that the system is in the deceleration mode. At this time, the ICE does not run and the electric motor is driven by the inertia of the vehicle to become a generator. If the SOC is already greater than the reference value, the pure braking mode is used without charging. Moreover, the vehicle speed is judged first. If the vehicle speed is greater than the switching value of regenerative braking, the electric motor is turned on to absorb the braking energy and recharge the battery.