Hybrid Electromechanical Coupling Systems And Dynamic Coordinated Control Of HEV

As powertrain of hybrid electric vehicles(HEVs),the electromechanical coupling system can achieve combination and distribution of multiple power sources and has gradually become one of research frontiers of HEVs.Dynamic coordinated control is one of methods that ensure transfering power smoothly and decrease torque fluctuation.The theory and application researches on the hybrid electromechanical coupling mechanism and dynamic coordinated control are carried out.The main research contents of this paper are as below.The steady-state and dynamic characteristics of six schemes for planetary gears and fixed axis gears mechanism are analyzed in detail by using equivalent lever method and bond graph respectively.The conclusions are that planetary gears mechanism belongs to speed coupling system and fixed axis gears mechanism to torque coupling system,steady-state and dynamic characteristics of the scheme are both optimum in which the engine is connected to sun wheel,the electric motor to ring gear ring and planetary pinion carrier to driving wheel.On the basis of these conclusions,a hybrid electromechanical coupling scheme is proposed which is composed of the second scheme for planetary gears and fixed axis gears mechanism.This scheme can both realize separate control to two speeds and two torques of power sources.Vehicle dynamic model,steady-state and dynamic models of the engine,steady-state operating efficiency model of the electric motor are established by means of combination of theory and test.According to operating modes divide and power flow analysis,bond graph models in every operating mode such as pure electric driving,engine driving,hybrid driving and braking mode are further obtained,which enhance modeling precision and adaptation to transient driving cycles.Because there is obvious difference between driving condition and braking condition and their control objectives differ,the same control method can't satisfy their needs,so the corresponding energy distribution strategy is designed for driving condition and braking condition respectively.For the former,optimization objective function is established which takes into account battery SOC and transient driving cycles,and torque distribution strategy with instantaneous optimization is proposed aiming at equivalent fuel consumption minimum.Torque optimum distribution among multiple power sources has been accomplished.As for the latter,on the basis of distribution principles of braking force,evaluating indicators are selected and braking force distribution strategy with fuzzy control is devised aiming at braking energy recovery maximum and good riding comfort.Braking energy recovery law has been acquired in different braking condition and different braking strength.The switching process from electric motor driving to simultaneous driving of electric motor and engine is divided into three phases including engine starting,speed synchronization and torque redistribution,the according disturbance observer and sliding mode controller of which are designed.In the face of system uncontinuity and strong nonlinearity during mode switching,for mode switching with engine starting,a control algorithm of torque coordination is put forward by means of combination of disturbance compensation and sliding mode control,which increases system robustness.For mode switching without engine starting,a control algorithm of dynamic coordination is raised by means of compensating engine torque lag by electric motor,which improves power transfer smoothness in switching process.As for the problems that bring about large friction loss,high fuel consumption,significant increase in emissions and strong impact on powertrain,an optimization algorithm of engine on/off is brought by use of combination of rolling optimization and feedback compensation with model prediction.Optimization objective function is established which takes into account not only fuel consumption of engine and electric motor,battery SOC,but also engine on/off status and engine transient model.This heightens precision of equivalent fuel consumption and startability of HEVs,cuts down engine on/off frequency and ensures that engine and battery work in high efficiency area.The electromechanical coupling system and control algorithm of dynamic coordination and so on are validated by simulations and experiments.The results demonstrate that the designed control algorithms can effectively enhance fuel economy,dampen torque fluctuations and improve riding comfort,and ensure power transfer smoothness.