Research On Energy Optimization Strategy Of Hybrid Electric Vehicle Based On Quadratic Performance Index

The hybrid electric vehicle(HEV)could solve the energy and enviromental issue produced by auto industry,so it has been seemed as the most promising vehicle at present.Due to the electric energy storage system is added into the drivetrain,how to satisfy the dynamic performance and reduce the fuel consumption simultaneously through optimizing the output power from different sources has extremely important realistic meaning.The existing energy management strategy could not take the real-time and optimality into consideration simultaneously.Therefore,the quadratic integral performance index is introduced to solve the real-time energy optimization problem of HEV in this paper.The proposed strategy is obtained by theoretical deduction,which is easy to be applied in engineering application and its fuel consumption is close to that of the global optimal control strategy based on Pontryagin’s Minimum Principle(PMP).Firstly,the engine operating point and drive cycle could be decoupled in the ISG parallel HEV with dual-clutch and the Toyota PRIUS power-split HEV.Then,the method about how to make the engine work on the optimal operating line is thoroughly discussed based on the structure characteristic.By using the vehicle’s longitudinal dynamic model and the operational characteristic of battery-motor system,the mathematic model is derived through the linearization,which is necessary to apply the quadratic optimal control theory.Secondly,the quadratic tracker performance index is designed to satisfy the vehicle’s dynamic performance,restrict the fluctuation range of battery state of charge(SOC)and inhibit large fluctuation of engine power simultaneously when the driver’s desired speed command is approximated as a series of constant signals.The energy optimization strategy is obtained by theoretical deduction,which is consisted of state feedback and command feedforward.In order to overcome the influence on velocity tracking performance generated by the non-ideal factor,such as slope variation,the extended quadratic regulator performace index is designed,which takes the integral of velocity derivation and residual battery energy deviation as extended state variables into consideration.Then,the energy optimization strategy including integral feedback effect is also obtained by theoretical deduction.Moreover,when the driver’s pedal signal is interpreted as a torque command,the requested power is determined based on the pedal signal and vehicle’s speed.By approximating the fuel consumption rate as a quadratic polynomial of engine power and introducing it into quadratic performance index directly,the energy management problem of HEV could be treated as an extended quadratic optimal tracker problem.The approximated optimal control law is derived by utilizing the solution properties of the Riccati equation and adjoint equation,and it is easy to be used in engineering application.The control variables of above three energy optimization strategies are only related with the current system states and the given commands,but not depend on the driving cycles.Therefore,all of them can be used to deal with the real-time optimization control problem of HEV.Finally,the forward-facing simulation models of vehicle and critical components are established base on the ADVISOR software.A large number of simulation tests demonstate that the fuel economy of the proposed energy optimization strategy is nearly the same as that of the PMP-based global optimal control strategy.Furthermore,it also exhibit good adaptability with unchanged weight coefficients under different cargo mass,road slope and initial battery SOC.