Energy efficiency analysis and comparison of transmission technologies for an electric vehicle

Electric vehicles are seen as the most promising solution to convert sustainable energy into drive energy. However, there are still some major (technological) challenges, e.g., in terms of maximizing range anxiety, minimizing battery costs and charging time. A possible solution, in order to improve the relative limited range (100-200 km), battery life time and ultimate costs, is utilization, e.g., of a transmission technology between the electric machine and driven wheels. The main research question of this paper is, what are the effects of transmission types (fixed, manual, continuously variable) and parameters (final drive ratio, efficiency) on the overall (battery-to-wheel) efficiency and performance of an electric vehicle? A backwards-simulation model from the wheels (load) to the batteries (source) has been developed to simulate an electric vehicle on a representative drive cycle (NEDC, FTP75). The model incorporates rotating (drive train) inertias, different transmission types, regenerative braking, battery efficiency and a shift strategy optimization algorithm based on Dynamic Programming (DP). The results show that with an optimized shifting strategy compared to a prescribed strategy using a Manual Transmission (MT) a maximum energy saving of 6% is possible. The conventional push-belt Continuously Variable Transmission (CVT) is less energy efficient and needs more battery energy for driving compared to a fixed-gear transmission type. Mainly, caused by the typically lower CVT efficiency. Further, it was found that reducing the final drive ratio improves the overall efficiency. The range in which the electric motor operates was shifted to higher angular velocity resulting in higher efficiency operating points. If the transmission efficiency of all the transmission types are equal assumed, then the CVT is typically the most energy efficient transmission type. For example, an energy saving of 7% is calculated compared to a well-selected fixed-gear ratio transmission type.