*3.4. Engine Switch Control Strategy*

In this study, the engine switch was designed to avoid engine operating in high fuel consumption regions and to avoid the overcharging and discharging of the battery pack. Since the battery has less energy loss due to the internal resistance while the SOC is within 0.4 and 0.6, the design strategy uses this interval as SOC limits. In ECMS, the mode of operation should be determined first. Since the mode 2 is applied to the higher speed, if the SOC does not exceed 0.6, the engine will continue to operate to ensure that the battery system has enough power.

For mode 1 operation, when the vehicle speed is less than 20 km/h, the operating efficiency of engine will be in poor condition. If the SOC is not lower than 0.45, the engine will be shut down and vehicle is driven by electric motor to enhance fuel consumption performance. When the vehicle speed is between 20 km/h and 40 km/h, and the SOC is greater than or equal to 0.55, the engine will be shut down. When the vehicle speed is greater than or equal to 40 km/h, and the SOC is greater than or equal to 0.6, the engine will be shut-down. If SOC is greater than 0.6 and engine remains off, the SOC is monitored and checked until SOC is less than 0.55 and the process is reset to the starting block. The control flowchart of the engine switch is shown in Figure 10. This control logic was applied on the energy management strategies of both HEV models in this study.

**Figure 10.** Engine switch flowchart.
