A Study Of Electric Vehicle Drive System Equipped With CVT

Drive motor output characteristics of electric vehicle (EV) are better for driving thevehicle, but it should meet the requirement of torque when the vehicle starts or climbsand power for high speed, and ensure its high energy conversion efficiency. Therefore,EV is necessary to have a transmission. Continuously Variable Transmission (CVT)speed ratios have the continuously adjustable character, so they can ensure that the drivemotor, even the entire power transmission system, is running in the best condition whichcan improve the energy transfer efficiency, cut down energy consumption of the vehicle,and prolong the travel distance of electric vehicle.This paper presents EV drive system equipped with continuously variable transmission(CVT-EV), investigates its dynamic performance, matches its parameter, and makes adrive control strategy. On the CVT-EV drive system, the main work can be shown asfollows:(1) According to the drive motor characteristics and the principles of the vehiclelongitudinal dynamics, EV’s rated speed which is an important parameters is defined;the relationships which are between EV’s rated speed, drive motor output peak powerand vehicle acceleration process, and climbing process at maximum climbing slope areinvestigated; at the same time, the choice laws of EV’s rated speed and drive motoroutput peak power are determined. Based on the analysis of requirement power duringcruise driving, the reasonable choice range of drive motor rated power is proposed. Inview of the vehicle dynamic performance, the selection methods of ratio range and thenumber of gear are proposed. And, sensitivity analysis of related factors such as ratedspeed, vehicle parameters and transmission system efficiency on vehicle accelerationprocess is studied, which lay the theoretical foundations for selecting parameters of thevehicle;(2) The relationships between the driving range of electric vehicles and powerbattery’s energy and capacity are studied in this paper; total energy, vehicle parameters,transmission efficiency and other related factors on the sensitivity relationship ofvehicle economy are analyzed, which further determines the capacity, voltage, numberof single cell and total mass of power battery; based on the dynamic performanceanalysis of electric vehicles, output peak power of drive motor and rated speed of EVare determined, and value selection principles of rated power and rated speed are presented; the mechanism of energy losses of power battery, and drive motor duringtransfer process and conversion process are analyzed, and then mathematical model ofpower battery and drive motor is established; on the basis of power performance andeconomic performance of CVT-EV, method of determining transmission ratio of mainreducer is proposed, and a transmission mathematical model of CVT electric vehicle isestablished;(3) Performance requirements of drive motor in the driven process of CVT-EV areanalyzed, and calibration scheme of pedal opening degree in vehicle acceleration isfixed; the drive system efficiency model of CVT-EV in cruise driving is established,which proposes CVT ratios control strategy when vehicles drive at a steady conditionbased on the optimal efficiency of driving system; during the acceleration process andenergy consuming process, the methods of the accelerator pedal opening degree and theCVT ratio combinations numbering are adopted, which takes the three parametervariables reducing to two parameter variables to analyze energy consumption in thevehicle acceleration process and to obtain the optimal accelerator pedal opening degreeand CVT speed ratio of the lowest energy consumption within the speed range; CVTratio changing rate and the effect laws of vehicles speed on CVT-EV accelerationperformance are studied, and CVT ratio control strategy when CVT-EV in dynamicprocess (acceleration) is obtained; considering power performance, economicalperformance and ride performance of CVT-EV, CVT ratio control strategy is determined;the simulation model is established which is given simulation analysis lastly;(4) To make a contrast analysis of the pros and cons of CVT-EV, based on the sameparameters of power battery and drive motor, two gear Automatic Manual Transmission(AMT) ratios are optimally designed, the shift strategy of AMT is draw up, electricvehicle equipped AMT model is established; analysis of economic performance anddynamic performance simulation results which are simulated on EV with CVT modeland EV with AMT model is made.