Optimum Driving System Design for Dual-Motor Pure Electric Vehicles

Hongwei Zhang， Weichi Chen， Zhuangzhuang Shang， Zhifeng Xu，Zepeng Gao， Yong Chen and Hongbin Ren,N

（1. Automobile Research Institute, School of Mechanical Engineering, Beijing Institute of Techno?logy, Beijing 100081, China；2. Automotive Engineering Institute, Guangzhou Automobile Group Co. Ltd., Guangzhou 511434, China；3. Beijing Electric Vehicle Co. Ltd., Beijing 100081, China）

Abstract: A pure electric vehicle driven by dual motors is taken as the research object and the driving scheme of the driving motor is improved to increase the transmission efficiency of existing electric vehicles. Based on the architecture of the transmission system, we propose vehicle perform?ance parameters and performance indexes of a pure electric vehicle, a time?sharing driving strategy of dual motors. First, the parameters of the battery, motor, and transmission system are matched.Then, the electric vehicle transmission model is built in Amesim and the control strategy is de?signed in Simulink. With the optimization goal of improving the vehicle’s dynamic performance and driving range, the optimal parameters are determined through analysis. Finally, the characteristics of the motor are tested on the bench. The results show that the energy?saving potential of the time?sharing driven double motor is higher, and the driving mileage of the double motor drive is in?creased by 4%.

Key words: pure electric vehicle；vehicle powertrain；parameter matching；time sharing drive

Nowadays, with the depletion of energy and the deterioration of the environment, saving en?ergy saving and protecting the environment have become the developmental themes of the current era. The automobile industry has also brought about serious environmental pollution at the same time it has promoted social development.Therefore, under the current technological condi?tions, pure electric vehicles are undoubtedly a path for the automotive industry, moving for?ward. However, with the continuous improve?ment of consumers’ performance requirements for electric vehicles and the deepening of research on motor drive system technology, the shortcom?ings of traditional motor drive systems are be?coming more and more obvious, which has be?come a bottleneck in the development and pop?ularization of electric vehicles. Therefore, it is ne?cessary to research and develop a new type of motor drive system with high efficiency and low cost.

The US government has implemented the Green New Deal and regards electric vehicles as an important component of its national strategy.It plans to popularize 1 million plug?in hybrid electric vehicles (PHEVs) by 2015[1?2]. Japan re?gards the development of electric vehicles as the core content of the “l(fā)ow?carbon revolution”, and plans to popularize “next?generation vehicles”,including electric vehicles, and increase their numbers to 13.5 million by 2020. To achieve this goal, Japan plans to develop at least 17 pure electric vehicles and 38 hybrid electric vehicles[3].

In November 2008, the German government proposed to popularize 1 million pure electric vehicles and plug?in hybrid vehicles in the next 10 years, and declared that the implementation of the plan marked the beginning of an era of electric vehicles in Germany. The promulgation and implementation of the national strategy have a very important guiding role for industrial de?velopment, and will certainly further accelerate the development process of the international elec?tric vehicle industry[4?6].

For the past few years, China has increased the support for pure electric vehicles. Relying on pure electric vehicles, the emergent new energy vehicles, to replace traditional fuel vehicles will greatly reduce China’s dependence on oil imports,solve the problem of China’s energy dependence,improve China’s energy security, reduce resource consumption and prevent environmental pollu?tion, and achieve the goal of protecting the ecolo?gical environment and achieving sustainable de?velopment[7?9].

Power batteries are the source of power for electric vehicles, but due to the current specific energy, specific power, cycle life, and cost of bat?teries, they have become the bottleneck problem that limits the driving range of electric vehicles[10?11]. Before this problem is solved, the energy utilization efficiency of electric vehicle power batteries can be improved by reasonable matching of electric vehicle power drive system parameters, reasonable power battery energy management, reasonable selection of electric vehicle mechanical transmission devices and lay?out schemes, and motor control to further ex?tend the driving range of electric vehicles[12?13].Extending the driving range of electric vehicles is an important subject of electric vehicle research.Before the effective breakthrough of power bat?tery technology, the main research focuses on battery pack control, power electronic control,motor technology, and electric vehicle perform?ance[14]. However, the parameters of electric vehicle power transmission components, such as motor power, torque, efficiency characteristics,and transmission ratio, as well as reasonable matching control between them, have a signific?ant impact on the power and driving range of pure electric vehicles[15?16].

In this paper, on the basis of the original single?motor drive model, we changed the config?uration of the motor drive and the dynamics parameter matching, and the dual?motor control strategy of the electric motor driven by the double motor are studied in detail. The designed dual?motor pure electric vehicle is an improve?ment of the driving scheme based on the original single?motor?driven electric vehicle. The purpose is to improve the vehicle’s dynamic characterist?ics under the premise of ensuring the original vehicle's power index. Therefore, a new dual?mo?tor coupling device is proposed. The transmis?sion efficiency of the whole vehicle is improved by changing the type of vehicle drive electrical mechanism to improve the driving range.

1 Vehicle Model

Fig. 1 Simple model of dual?motor pure electric vehicle

Fig. 1 shows a simple model of a dual?motor pure electric vehicle. The motors A and B drive the front and rear wheels with time?sharing.When the required power is small, the single mo?tor drive can meet the power requirements.When the demand power is large, and the single motor cannot satisfy the power requirements, the use of the dual motor drive meets the power of the car, in order to save energy and improve the cruising range.

2 Selection of Maximum Power of Motor

The driving motor is the only power source for a pure electric vehicle[17]. It is responsible for the conversion of electrical energy into mechanic?al energy of the vehicle. Its working characterist?ics are closely related to the performance of the vehicle. Therefore, the parameter matching of the motor is the key to studying the power system of the pure electric vehicle. The power of the motor contains rated power and peak power. Generally speaking, when the electric vehicle runs stably,the duration is long and the motor mainly works in the rated power area; when the electric vehicle accelerates or climbs, the motor output power re?quired is large, the running time is short, and the motor mainly works in the overload area. There?fore, it should be considered whether the power demand is in the continuous working area or overload working area of the motor when design?ing the motor parameters. In addition, if the rated power of the matching motor is too small,the motor runs under overload conditions for a long time, which will affect the life of the motor;if the rated power is too large, the motor cannot often be kept in the high?efficiency area, which will reduce the rate of energy utilization of the electric vehicle and increase the mass and volume of the motor.

First, determine the rated motor power at the highest speed

Fig. 2 Relation curve between maximum speed and required power

According to the operating characteristics of the motor, the continuous rated output charac?teristics meet the power requirements of the elec?tric vehicle at the maximum speed, and the peak output characteristics can meet the requirements of the acceleration performance and climbing per?formance of the electric vehicle, as shown in Fig.2.

According to the power requirements of the vehicle under different working conditions, the sum of the two motors’ rated powers of the drive system should be greater than or equal to the vehicle’s maximum speed required power, and the motor’s peak power should be greater than or equal to the maximum value of the vehicle’s maximum grade demand power and acceleration process power demand as

Therefore, according to the structure of the power coupling system, the matching principle of motor A and motor B is that the peak power mainly meets the requirements of the climbing performance and acceleration performance of electric vehicles and the sum of the rated power of motor A and motor B needs to meet the max?imum speed of electric vehicles power. According to the above selection principles, the motor matching is in Tab.1. The vehicle parameters are shown in Tab.2.

Tab. 1 Selection of motor parameters

Tab. 2 Vehicle parameters

The dynamic performance indicators of the whole vehicle are shown in Tab.3.

Tab. 3 Index of dynamic performance

3 Parameter Matching of Dual-Motor Coupling Power Transmission System

In general, the increase of the number of gears is conducive to increasing the chance of us?ing the maximum power of the motor and work?ing in the best efficiency area, which can im?prove the power and driving range of the car.However, the working characteristics of the mo?tor are close to the ideal equal?power driving characteristics, so the number of gears in the transmission system should not be too high, oth?erwise it will complicate the system structure, in?crease the weight and volume of the vehicle and affect the performance of the vehicle.

3.1 Driveling parameter matching principle

Due to the particularity of the motor’s oper?ating characteristics, increasing the speed ratio may not necessarily increase the maximum speed of the vehicle under certain motor parameters.This is because when the car is driving at the maximum speed, the motor usually works in the constant power area. The calculation equation of torque and power is

It can be seen from Eq.(8) that in the con?stant power area, the traction force of the whole vehicle is only related to the driving speed. Fig.3 shows the curves of the traction force and driv?ing resistance with the vehicle speed under differ?ent gears. The traction force curve of each gear has a coincident part in the constant power area.Under the condition that the motor parameters and driving resistance are fixed, even if the speed ratio changes, the maximum speed correspond?ing to the intersection of the traction force curve and the driving resistance curve will not change.The maximum speed can only be increased by adjusting the motor parameters. The selection of the speed ratio of the automobile transmission system mainly depends on the power perform?ance requirements of the vehicle. The maximum speed ratio mainly depends on the requirements of the maximum grade of the vehicle, and the minimum speed ratio depends on the require?ments of the maximum speed of the vehicle.

Fig. 3 Curves of driving force and driving resistance

3.1.1 Selection of maximum speed ratio

The maximum speed ratio of the drive train is determined by the maximum torque of the mo?tor and the maximum grade demand of the vehicle

In addition, for electric vehicles, while the low gear is mainly used in the starting stage,most of the cases run in high gear, so the minim?um speed ratio of the motor should also be con?sidered when the motor should work in the high?efficiency area at high speed to improve the en?ergy utilization of the car.

3.2 Driving range calculation

In the preliminary design, the constant speed method is used to calculate the driving range.The output power of the starting motor is

When equipped with the same battery, the driving range is only related to the transmission efficiency, and it is a positive correlation.

4 Control Strategy of Dual Motor Working Mode

This paper adopts the instantaneous optim?ization pattern recognition strategy. When the pedal gives a required power, by comparing the electric power when Motor 1 works alone with the electric power when Motor 2 works alone, the electric power in the coupling mode of Motor 1 and Motor 2, and the required power in sequence,we adopt the working mode of the smallest power that can meet the required power. Finally, calcu?lating of the required power of the three working modes of the configuration under the current state in real time, the minimum required power is selected as the current working state. The core of the transient optimization pattern recognition strategy is to find the optimal working point of the current required power. The required torque corresponding to the optimal operating point is the smallest. When the two motors work at the same time, the power of the whole vehicle is the coupling power of the two motors, and the torque distribution is also coupled through the ground friction.

Fig. 4 shows the control strategy architec?ture of a dual?motor pure electric vehicle, and Fig. 5 displays the immediate optimization strategy for pattern recognition. By comparing the power of Motor 1, the power of Motor 2, and the power of Motor 1 and Motor 2 in the coup?ling mode, the pedal will obtain the correspond?ing required torque and select the working mode with the smallest power as the current working state. Fig. 5 is the real?time calculation logic dia?gram of the power demand in the three working mode configuration states. The core strategy based on transient optimization pattern recogni?tion is to identify the current operating point of required power, where the optimal operating point corresponds to the minimum required torque. After finding the best working mode, a torque distribution strategy is obtained for each working mode.

Fig. 4 Strategy of dual motor control

Fig. 5 Recognition of dual motor pattern

5 Analysis of Simulation Results

The New European Cycle Condition(NEDC) is composed of 4 Economic Commis?sion of Europe conditions and one Extra Urban Driving Cycle condition. In this simulation, the target speed is set under the NEDC operating conditions. The simulation results are shown in Fig. 6 and Fig. 7. The maximum speed is 33.57 m/s(136.7 km/h). When the speed is 2.47 m/s, the maximum gradient can reach 33.2%, and when the speed is 5.01 m/s, the maximum gradient can reach 31.2%, which can reach the vehicle’s dy?namic performance index. As can be seen from Fig. 8, the motor can realize time?sharing and switch among the three modes.

Fig. 6 Vehicle speed simulation results under NEDC condi?tions

Fig. 7 Simulation results of maximum climbing speed

Fig. 8 Torque of front and rear shafts under NEDC conditions

5.1 Analysis of motor characteristics under cycling conditions

A simulation model is established in Matlab/Simulink software to compare the energy con?sumption of the single?electric mechanism type and the double?electric mechanism type. Fig. 9 and Fig.10 show the distribution of motor operat?ing points under NEDC and urban dynamomet?er driving schedule (UDDS) operating condi?tions. Fig. 9 clearly shows that the load factor of the operating point of the dual?electric mechan?ism motor under NEDC operating conditions is significantly higher than the load rate of the single motor, and the electric vehicle of the double electric mechanism type is significantly higher than the efficiency of the single motor. As shown in Fig. 10, the two electric mechanism types under UDDS working conditions are the same and the distribution points are also same.As shown in Fig. 10, the two electric mechanism types under UDDS working conditions are the same and the distribution points are also same,but the distribution point of the dual motor is better under NEDC conditions. Therefore, the ef?ficiency of the dual?electric mechanism type is higher.

Fig. 9 Operating point distribution of NEDC

Fig. 10 Operating point distribution of UDDS

5.2 Analysis of SOC characteristics of power battery

In order to ensure the reliability and safety of the battery and improve the overall perform?ance of the vehicle, the pure electric vehicle uses a battery management system (BMS). In order to prevent over?charging and over?discharging,increase battery life, and prevent potential dam?age, the current and voltage signals of the bat?tery usually require an estimate of SOC[18]. In a short period of time, for low?speed, low?accelera?tion conditions, pure electric vehicles with dual motors have slow and steady power battery dis?charge, higher SOC characteristic change curves,and higher vehicle efficiency. As a result, the en?ergy loss is relatively small, and the range of the vehicle can be improved relatively.

Fig. 11 shows the changes in voltage, cur?rent, and SOC during the battery discharge pro?cess. As can be seen from Fig.11, during the dis?charge process, the current changes are relat?ively regular, and the voltage stays the same. At the end of the discharge, the current increases in the opposite direction, and the voltage increases slightly, which is caused by braking energy recov?ery during braking. The SOC drops from 95% to 85%, and the driving range is S = (SOC/?SOC ×SNEDC) = 153.2 km. In the equation, SOC is the effective capacitance coefficient of the battery pack, where 0.8 is taken, and ?SOC is the vari?able of the battery SOC in a NEDC cycle.

Fig. 11 SOC varying curves of power battery

5.3 Motor speed test

In order to further verify the power perform?ance of electric vehicles equipped with dual mo?tors, three actual acceleration times of electric vehicles (0?30 km/h, 0?40 km/h, 0?50 km/h) are tested and verified, and the simulation accelera?tion time of dual motors is compared with the original vehicle test acceleration time. Fig.12 compares the experimental change curve of the acceleration time of the original pure electric vehicle and the dual?motor drive configuration of the electric vehicle modeling simulation in Cruise software. The result shows that the dynamic per?formance of the dual?motor drive through reason?able parameter matching with the acceleration performance is better than the original model of the single motor drive. The comparison results are shown in Tab. 4.

Fig. 12 Comparison between simulation and experiment

Tab. 4 Simulation and test results

As shown in Fig.13, the maximum efficiency of the test results of the dual?motor drive motor reaches 95%.

The high?efficiency area (area greater than 92%) is not only wider than the single motor drive efficiency as shown in Fig.14, but also more efficient. This means that the wider the high?effi?ciency area of the motor is, the more energy?effi?cient it can perform under different operating conditions and the better the driving range can be improved. The efficiency of dual motors in the low?speed range is also better than that of single motors.

The results of the dual motor simulation test and the single motor test are shown in Tab.4.

The results show that the energy?saving po?tential of the time?sharing driven double motor is higher, and the driving mileage of the double mo?tor drive is increased by 4%.

Fig. 13 Efficiency map of dual motor

Fig. 14 Efficiency map of single motor

6 Conclusions

This paper proposes the dynamic perform?ance of electric vehicles as the constraint and the driving range as the design goal. We have stud?ied and analyzed the relationship between the key components of the electric vehicle power transmission system (motor and transmission system) and the performance of the whole vehicle. We obtain a reasonable matching prin?ciple and matching results of key component parameters of the power transmission system and also establish a theoretical numerical model of key components.

① The simulation results show that the ac?celeration performance, maximum speed, driving range, and other performance indicators of pure electric vehicles all meet the design requirements,and the optimized pure electric vehicle can im?prove the power transmission efficiency of the whole vehicle and thus can increase range;

② Pavement tests were carried out for 3 ac?tual acceleration time periods of 0–30 km/h, 0–40 km/h, 0–50 km/h for electric vehicles, which were very similar to the simulation results, fur?ther verifying the rationality of the simulation model and the correct parameter design.