Real-Time Transfer Case 4WD Control Development to Improve Vehicle Coasting Stability

The real-time transfer case installed on the four-wheel drive vehicle with a longitudinally mounted engine, can distribute not only the engine drive torque, but also the engine drag torque between the front and rear axles, to improve vehicle traction and stability performance. When under vehicle coasting condition, the engine drag torque may cause vehicle rear wheel slippage on low friction adhesion road, which causes a vehicle to be unstable and spin in. Although the current engine Drag Torque Control technology has the ability to reduce rear wheel longitudinal slippage by engine torque control, but the control effect is poor, the wheel slip ratio is large and the slipping duration is long, the vehicle still has the tendency to spin in. This paper proposed a new transfer case coasting control strategy to avoid this problem. Through the pre-load control based on vehicle coasting characteristics and closed-loop feedback control based on rear axle slip state, the rapid dynamic distribution of engine drag torque between the front and rear axle is realized to avoid single axle slippage and improve vehicle coasting stability. Through test vehicle verification, the proposed control strategy could significantly improve vehicle coasting deceleration and stability performance.