| Regenerative braking is one of the fuel saving technologies of hybrid electric vehicles(HEV),which convertes the gravity energy or kinetic energy of the vehicle into other forms of energy and stores in the energy storage device without affecting the braking performance of the vehicle,and generates the braking force at the same time.Due to the use of regenerative braking,it can also reduce the wear of braking friction plates so that the cost of the vehicle can be reduced.HEV equipped with continuously variable transmission(CVT)adjust the motor operating point continously to achieve the optimal motor operating efficiency during regenerative braking.An appropriate CVT control strategy help improve the operating efficiency of the regenerative braking system and further increase the energy recovery rate.In this paper,a four-wheel drive plug-in hybrid vehicle is taken as the research platform.The theoretical analysis of the CVT torque loss and key components efficiency during regenerative braking are conducted.To compensate the torque loss of CVT a CVT control strategy is formulated.The specific researches are as follows:(1)The braking dynamic analysis is conducted which then leads to the ideal braking force distribution relationship.The three common braking force distribution strategies are analyzed contrastively.Combining the limitation of ECE braking regulations on the distribution of front and rear axle braking force,a regenerative braking force distribution strategy based on the peak motor power and peak motor torque is proposed.The simulation of braking force distribution strategy is performed which shows that the proposed braking force distribution strategy can effectively prevent the motor from overloading,(2)In order to reflect the effciency of the transmission system more accurately,the structure and power transmission theory of the metal belt type continuously variable transmission are introduced.The torque loss of the transmission of CVT during torque transmission is divided into three parts and discussed in detail.Then the mathematical model of torque loss of CVT is obtained.The influence of different parameters on the torque loss of CVT is discussed.Finally,the transmission efficiency model of CVT is estalished.(3)Considering the influence of temperature and SOC on the charging efficiency of lithium battery,the efficiency characteristics of lithium battery are analyzed,and the SOC calculation model of lithium battery is established based on the equivalent circuit.A numerical table model of motor efficiency is also established.For the torque loss of the CVT on the regenerative braking torque transmission path,in order to make full use of the regenerative braking power of the motor,a torque compensatory distribution strategy for the front motor is proposed.Then,a battery-motor-CVT joint efficiency model is established.The influencing factors of system efficiency are analyzed and its optimal operating efficiency curve is obtained.The original CVT speed ratio control strategy is modified and the CVT clamping force control strategy is proposed.Finally,the hydraulic system model of CVT is established,including the ratio control valve and clamping force control valve model.(4)Based on MATLAB/Simulink platform,a backward simulation model of CVT plug-in four-drive drive hybrid vehicle regenerative braking system is built,and the regenerative braking force distribution strategy and CVT control strategy proposed in this paper are loaded into the vehicle controller.The simulations are performed under three typical braking conditions and UDDS,NYCC,US06 respectively.The results show that the front motor braking power,system operating efficiency and braking energy recovery rate have increased in different degrees which indicate the proposed CVT control strategy is superior to the CVT control strategy that condisers only battery-motor efficient operating. |
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