Optimal Control of Flywheel Hybrid Transmissions

Abstract

Selection of variable transmission ratio in a vehicle with a flywheel energy storage element to maximize the kinetic energy transfer for specified vehicle accelerations is formulated as an optimal control problem. Models for single and parallel power-flow path configurations are presented as generic hybrid propulsion system types. For both systems variation of the ratio of the continuously variable transmission results in inherently nonlinear control, with velocity trajectories found by the solution of two-point boundary value problems. For the single power-flow path system a closed loop controller is synthesized which tracks acceleration command. For the parallel path system, which is representative of conventional power split transmissions, the open loop control yields useful information, indicating tradeoffs between system energy recovery efficiency and component design parameters. Maximum energy recovery during regenerative braking is achieved by minimizing power losses to vehicle drag and transmission elements. Planetary gear geometry is shown to have the strongest influence on efficiency, maximum transmission component loading, and CVT ratio range.