Mode Control Strategy for a Two-Mode Hybrid Electric Vehicle Using Electrically Variable Transmission (EVT) and Fixed-Gear Mode

A mode control strategy for a two-mode hybrid electric vehicle (HEV) with a fixed-gear mode is proposed, considering wheel torque demand while minimizing fuel consumption. First, the dynamic equations of a power-split HEV are derived, and these equations consist of two electrically variable transmission modes and four fixed-gear modes. Using the dynamic equations, the optimal operation mode is selected from the viewpoint of the maximum available output torque with respect to the driver´s demand and vehicle velocity. Next, an optimized mode selection map is constructed using optimization to minimize fuel consumption while satisfying the required wheel torque. We note that fixed-gear mode uses the fixed-gear mode 1 to implement a higher gear ratio if the vehicle speed is low and if the required output torque is large, as in a conventional automatic transmission shift map, and selects the fixed-gear mode 4 to implement a lower gear ratio if the vehicle speed is high and if the required output torque is low. A vehicle simulation was performed to verify the mode control algorithm of the two-mode HEV, and the results were compared with vehicle experimental results. The simulation results showed that the mode control strategy for the two-mode HEV gave satisfactory performance, similar to the experiment results.