Fuel-Optimal Cruising Strategy for Road Vehicles With Step-Gear Mechanical Transmission

This paper studies the principles and mechanism of a fuel-optimal strategy in cruising scenarios, i.e., the pulse and glide (PnG) operation, for road vehicles equipped with a step-gear transmission. In the PnG strategy, the control of the engine and the transmission determines the fuel-saving performance, and it is obtained by solving an optimal control problem (OCP). Due to a discrete gear ratio, strong nonlinear engine fuel characteristics, and different dynamics in the pulse/glide mode, the OCP is a switching nonlinear mixed-integer problem. This challenging problem is converted by a knotting technique and the Legendre pseudospectral method to a nonlinear programming problem, which then solves the optimal engine torque and transmission gear position. The optimization results show the significant fuel saving of the PnG operation as compared with the constant-speed cruising strategy. The underlying fuel-saving mechanism of the PnG strategy is explained graphically. For a real-time implementation, a near-optimal practical rule that enables a driver and/or an automatic control system to fast select gear positions and engine torque profile is proposed with only slightly deteriorated fuel saving.