The Safety and Dynamic Performance of Blended Brake System on a Two-Speed DCT Based Battery Electric Vehicle 2016-01-0468

Regenerative braking has been widely accepted as a feasible option to extend the mileage of electric vehicles (EVs) by recapturing the vehicle’s kinetic energy instead of dissipating it as heat during braking. The regenerative braking force provided by a generator is applied to the wheels in an entirely different manner compared to the traditional hydraulic-friction brake system. Drag torque and efficiency loss may be generated by transmitting the braking force from the motor, axles, differential and, specifically in this paper, a two-speed dual clutch transmission (DCT) to wheels. Additionally, motors in most battery EVs (BEVs) and hybrid electric vehicle (HEVs) are only connected to front or rear axle. Consequently, conventional hydraulic brake system is still necessary, but dynamic and supplement to motor brake, to meet particular brake requirement and keep vehicle stable and steerable during braking. Therefore, a complicated effect on the safety and performance of braking, mainly relating to tyre slips and locks, vehicle body bounces and braking distance will be applied by the blended brake system.

In this paper, the brake energy recovery potentials of typical driving cycles are presented. Relevant critical limitations are introduced to define the available brake force distribution range for front and rear axles. Then the distribution strategies are compared and analyzed to achieve a satisfied balance between braking performance, driving comfort and energy recovery rate. Next, the required motor brake force is tuned, according to the response time and efficiency loss in transfer process which obtained in testing bench. At last, solutions for some special cases are proposed, for instance, motor brake torque interruption when downshifting occurs on long downhill.

A credible conclusion is gained, through experimental validation of optimized brake force distribution strategy on a two-speed DCT based BEV testing rig, that the selected force distribution strategy help the blended brake system achieve a comfortable and safety braking during all driving conditions.