Energy-efficient torque-allocation strategy for a 6 × 6 vehicle using electric wheels

Abstract The utilization of multiple independently controlled electric wheels offers possibilities for enhancing not only the vehicle stability control but also energy efficiency of a vehicle. Previous research on energy-efficient torque allocation mainly focuses on four-wheel-independent-drive vehicles but not multi-axle all-wheel-drive vehicles. In this paper, we propose an energy-efficient torque-allocation strategy for a vehicle equipped with six identical electric wheels, using the Karush–Kuhn–Tucker (KKT) conditions to find candidates for globally optimal solutions. We present and compare the analytical forms of these candidates, including typical modes such as single-wheel-drive, two-wheel even-distribution, and three-wheel even-distribution modes. It shows that reallocating torques other than even distribution has energy-saving effects when the power loss characteristics of an electric wheel are not convex. For the studied electric wheel, the optimal mode revealed by KKT conditions is to progressively increase the number of active wheels with the torque demand. This method can be employed regardless of whether the power loss characteristics with respect to the torque demand are convex or nonconvex. The KKT conditions also allow the establishment of an online rule-based strategy to switch between different modes, depending on the vehicle speed and torque demand. Examples based on the electric wheel's power loss characteristics validate the effectiveness of the KKT-based allocation strategy. Compared with an even-distribution strategy, the electric wheels' energy losses and the vehicle's energy consumption can be reduced by 12.5% and 3.7%, respectively, over a complete driving cycle using the KKT-based strategy.