Hierarchical Control of Dry Clutch for Engine-Start Process in a Parallel Hybrid Electric Vehicle

Engine start is an important process for a parallel hybrid electric vehicle (HEV), during which the electric motor (EM) must simultaneously provide driver's demand torque and engine-start torque via an engine-disconnect clutch. Therefore, the performance of dynamic and comfort, the engine-start time, and the sliding power of clutch are all supposed to be considered in the control of engine start. This paper introduces a parallel HEV equipped with an automatic dry clutch using an electromechanical actuator, and proposes a hierarchical control method to control the engine start. As upper layer of the controller, a linear quadratic (LQ) optimal control is used to obtain the required torque of EM and clutch based on the current states of the vehicle and engine. As lower layer, a double closed-loop control method is proposed to control the clutch transmitting required torque, which contains a fuzzy controller (FC) as an outer loop and a modified predictive functional controller (mPFC) as an inner loop. Meanwhile, considering the difficulties in measurement of load torque of a running vehicle and actual transmitted torque of a dry clutch, two Kalman filters (KFs) are designed to observe them. Simulations in MATLAB/SIMULINK and hardware-in-loop (HIL) tests were carried out, of which the results show that the hierarchical control method has a satisfying performance.