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Abstract

Traditional regenerative braking control strategies only consider how to ensure motor to work along with the battery/motor joint high-efficiency working line, but do not consider continuously variable transmission efficiency. However, continuously variable transmission efficiency varies between 70% and 95% which cannot be neglected. Based on the analysis of relationship among the battery, motor, continuously variable transmission, and synthesis efficiency, the battery/motor/continuously variable transmission joint high efficiency is calculated and obtained, and the regenerative braking control strategy is proposed. Comparing the previous control strategy with battery/motor joint high efficiency, the motor average generating efficiency increases by 2.91%, and braking energy recovery rate increases by 4.09% under Extra Urban Driving Cycle, while the motor average generating efficiency increases by 3.84%, and braking energy recovery rate increases by 5.74% under Federal Test Procedure-72 through offline simulation. Furthermore, under the hardware-in-the-loop test, the average generating efficiency increases by 2.72%, and the braking energy recovery rate increases by 5.03% under Extra Urban Driving Cycle, while the average generating efficiency increases by 3.13%, and the braking energy recovery rate increases by 3.94% under Federal Test Procedure-72. Both simulation and hardware-in-the-loop test results show that the proposed regenerative braking control strategy can realize battery, motor, and continuously variable transmission to work with joint high efficiency which can enhance braking energy recovery rate.

Keywords

Hybrid electric vehicle regenerative braking efficiency optimization control strategy hardware-in-the-loop

Introduction

The regenerative braking is one of the important working modes of hybrid electric vehicle (HEV). On the condition of guaranteeing safe braking, the optimization of regenerative braking control strategy and recovering the most braking energy are the research focuses for regenerative braking of HEV.

Abroad researches on regenerative braking control strategy have been started earlier, and many control strategies have been proposed. Several researchers have proposed three kinds of braking force distribution control strategies to appraise energy recovery efficiency of regenerative braking and comprehensively considered the friction braking, regenerative braking, and antilock brake system (ABS) control.¹ However, they have not discussed the motor efficiency and the continuously variable transmission (CVT) efficiency. Some researchers have improved energy recovery efficiency and taken the driver perception as the design goals of the braking force distribution strategy.² But they also have not considered the synthesis efficiency of regenerative braking system. Domestic researches on regenerative braking control strategy are in the infancy. Some researchers have taken the average regenerative braking force as the goal and selected key point coordinates on the braking control strategy line as the control variables and then optimized and designed the regenerative braking control strategy.³ And the researchers have proposed battery/motor joint high-efficiency working method and formulated the CVT ratio control strategy and regenerative braking control strategy,⁴ while they have not considered the influence of CVT efficiency to synthesis efficiency of regenerative braking system.

By continuously adjusting CVT ratio, HEV not only can obtain the optimum energy consumption under different driving cycles but also can force the motor to work within the high-efficiency working region and enhance energy recovery rate. For traditional regenerative braking control strategies of HEV with CVT, the CVT efficiency is set to be constant such as 0.85.⁴ However, the CVT efficiency varies with its working condition between 70% and 95%, which obviously influences system synthesis efficiency. Furthermore, the high-efficiency working regions of battery, motor, and CVT do not overlap. Therefore, in order to obtain the high working efficiency for HEV during braking, it is very important to maintain battery, motor, and CVT work together within high-efficiency working region.

In this article, the mild hybrid electric Changan Antelope vehicle with the Integrated Starter/Generator (ISG) motor (permanent magnet synchronous motor, PMSM) is taken as the research object, as shown in Figure 1, and the configuration parameters of Integrated Starter/Generator–hybrid electric vehicle (ISG-HEV) are shown in Table 1. Through analyzing system synthesis efficiency, we proposed the CVT ratio control strategy based on battery/motor/CVT joint high-efficiency working line during regenerative braking. And, we simulated and tested the proposed control strategy to verify accuracy and priority of control strategy, which can lay the foundation for HEV regenerative braking system research and development.

Figure 1.

The diagram of mild HEV with ISG motor and CVT.

Table 1.

The configuration parameters of mild hybrid electric vehicle with ISG motor and CVT.

Components	Configuration	Parameters
Vehicle parameters	Curb weight, m	915 kg
	Wheelbase, L	2.365 m
	Height of gravity center, h_g	0.525 m
	Front axle apart from gravity center, a	0.935 m
	Rear axle apart from gravity center, b	1.43 m
Engine	JL475Q1	63 kW/6000 r/min
Engine	JL475Q1	110 Nm/3500 r/min
Motor	ISG	10 kW/4000 r/min
Motor	ISG	45 Nm/1000 r/min
Transmission	Nissan CVT (NCVT)	0.498–2.502
Final drive	Gear ratio	4.38
Wheel	Radius	0.272 m
Battery	NiMH	Nominal voltage: 144 V
Battery	NiMH	Capacity: 6.5 A h

ISG: Integrated Starter/Generator; CVT: continuously variable transmission.

Battery/motor/CVT joint high-efficiency working line

Joint efficiency

According to the previous bench experiments of NiMH battery, ISG motor, and CVT, we can obtain the NiMH battery, the ISG motor, and the CVT efficiency characteristic map.⁴

During the efficiency of battery experiment test, the constant current (6.5, 20, 32.5, 40, 65, and 80 A) discharging and charging test was carried out for battery under the natural ventilation environment with (20°C ± 5°C) temperature. The initial state of charge (SOC) was set up as 0.8, 0.7, 0.6, 0.5, and 0.4, respectively. Then, the total voltage and total current of battery pack, the voltage of battery module, SOC, the temperature of battery, the discharging and charging maintenance time, and discharging and charging capacity were recorded. Finally, the discharging and charging efficiency can be computed and interpolated–fitted, as shown in Figure 2.

Figure 2.

Charging and discharging efficiency characteristic map for NiMH battery.

During the efficiency of battery experiment test, the constant speed control (500–6000 r/min, each 500 r/min interval) was implemented by electrical dynamometer. Then, the output torque of ISG motor was set, and the voltage, current, the output voltage and current of Integration Power Unit–Integrated Starter/Generator (IPU-ISG) controller, and the output speed and torque of ISG motor, the running efficiency of ISG motor, and the efficiency of IPU controller were collected using the motor power analyzer. Finally, the comprehensive efficiency of ISG motor system was computed and obtained, as shown in Figure 3.

Figure 3.

Efficiency characteristic map for ISG motor.

During CVT efficiency experiment test, the CVT ratio (2.5, 2, 1.5, 1, and 0.5) was fixed successively in order. Then, the torque and speed were changed according to fixed step, and the relationship between CVT efficiency and torque and speed for the five ratios was obtained. Finally, the CVT transmission efficiency can be obtained by the cubic spline interpolating data, as shown in Figure 4.

Figure 4.

Efficiency characteristic map for CVT.

The battery/motor/CVT joint working efficiency is the product of the NiMH battery efficiency, the ISG motor efficiency, and the CVT efficiency, which is expressed as follows

$η = η_{b} \cdot η_{m} \cdot η_{cvt}$ (1)

The NiMH battery efficiency η_b includes the electric efficiency η_e and the coulomb efficiency η_k. The electric efficiency η_e is expressed as the loss electric energy quantity due to battery inherent resistance, while the coulomb efficiency η_k is expressed as the ratio of output capacity and input capacity under the certain discharging condition as follows

$η_{b} = η_{e} \cdot η_{k}$ (2)

$η_{e} = \frac{E}{U} = \frac{E}{(E + I \cdot R)}$ (3)

$η_{k} = \frac{(I_{d i s} \cdot t_{d i s})}{(I_{c h g} \cdot t_{c h g})} \times 100 %$ (4)

where E is the electromotive force, U is the NiMH battery terminal voltage, I is the NiMH battery current, R is the NiMH battery inherent resistance, I_dis is the NiMH battery discharging current, t_dis is the NiMH battery discharging time, I_chg is the NiMH battery charging current, and t_chg is the NiMH battery charging time.

ISG motor power is expressed as follows

$P_{m} = U \cdot I = \frac{T_{m} \cdot n_{m}}{9549 \cdot η_{m}}$ (5)

where T_m is the ISG motor torque, n_m is the ISG motor speed, and η_m is the ISG motor efficiency.

After the NiMH battery efficiency, ISG motor efficiency, and CVT efficiency have been computed and obtained, the joint working efficiency map of battery/motor/CVT can be obtained under different SOC and different CVT ratios by setting series of ISG motor speed and torque values. It means that each CVT ratio can be corresponded to one battery/motor/CVT joint working efficiency map.

According to Figures 2 –4 and formulas (1)–(4), we can obtain the battery/motor joint efficiency map and battery/motor/CVT joint efficiency map, as shown in Figures 5 and 6. The distinguishing difference between the two joint working efficiency maps is that the efficiency of the high-speed and high-torque region is much lower than the efficiency of the high-speed and low-torque region in the battery/motor joint working efficiency map. But it is not obvious in the battery/motor/CVT joint working efficiency map. We can draw the conclusion that the battery/motor joint high-efficiency region is not equivalent to the battery/motor/CVT joint high-efficiency region. Therefore, the regenerative braking control strategy should be established by considering the battery/motor/CVT joint high efficiency, which can ensure the highest working efficiency for HEV system with CVT. As shown in Figure 6, we can know that the battery/motor/CVT joint working efficiency is different under different CVT ratios. And, the battery/motor/CVT joint efficiency is the maximum when CVT ratio is 1. Moreover, when CVT ratio varies from 0.498 to 1, the joint high efficiency increases along with CVT ratio to the highest efficiency value until i_cvt = 1; then, the joint efficiency decreases when CVT ratio varies from 1 to 2.508.

Figure 5.

The battery/motor joint efficiency map when SOC = 0.3.

Figure 6.

The battery/motor/CVT joint efficiency map when SOC = 0.3.

Furthermore, the values of battery/motor/CVT joint working efficiency surfaces drastically vary between 40% and 80%. If we can control the battery, motor, and CVT to integrally work within the joint high-efficiency area, we can improve the energy recovery efficiency and obtain more regenerative braking energy during braking, which can further reduce fuel consumption, decrease emissions, and conserve energy.

Determining the battery/motor/CVT joint high-efficiency working line

To recover more braking energy, HEV should work within the battery/motor/CVT joint high-efficiency region during braking. First, it is very important to determine the joint high-efficiency working line which can guarantee the highest synthesis efficiency of the NiMH battery, ISG motor, and CVT.

During regenerative braking, the ISG motor torque is determined according to the braking power. In order to conveniently obtain the battery/motor/CVT joint high-efficiency working line, taken i_cvt = 1, for example, Figure 6 can be converted to be Figure 7. Moreover, Figure 7 can be further converted to be Figure 8. Each highest efficiency point is corresponding to each constant power line. All the highest efficiency points are linked to be the joint high-efficiency working line.

Figure 7.

The battery/motor/CVT joint high-efficiency working line when i_cvt = 1.

Figure 8.

The contour map for battery/motor/CVT joint high-efficiency working line when i_cvt = 1.

Under different CVT ratios, different battery/motor/CVT joint high-efficiency working lines can be obtained by the same method, as shown in Figure 9. The battery/motor/CVT joint high-efficiency working lines under the different CVT ratios vary slightly but with the similar overall variation tendency. Moreover, the battery/motor joint high-efficiency working lines also can be determined by the same method, as shown in Figure 9. Obviously, the battery/motor joint high-efficiency working lines do not completely coincide with the battery/motor/CVT joint high-efficiency working lines. So, the battery/motor joint high-efficiency working line cannot maintain HEV to be the highest efficiency working region when braking. Of course, it is also hard to recover more braking energy. In other words, the traditional regenerative braking control strategy based on the battery/motor joint high-efficiency working line cannot realize the optimal energy recovery. Therefore, just only by controlling the ISG motor to operate along with the battery/motor/CVT joint high-efficiency working lines, we can obtain the highest synthesis efficiency and recover more regenerative braking energy during regenerative braking.

Figure 9.

The battery/motor/CVT joint high-efficiency working line under different CVT ratios when SOC = 0.3.

Regenerative braking control strategy

Under regenerative braking mode, the ideal motor torque can always be found on the battery/motor/CVT joint high-efficiency working lines for the certain regenerative braking power, SOC, and CVT ratio. Then, the corresponding speed of ISG motor can be computed, and CVT ratio can be finally determined. Moreover, corresponding to the certain regenerative braking power, the torque and speed of motor are unique. They can be target values to control the ISG motor to work along with the battery/motor/CVT joint high-efficiency working line. Thus, we can guarantee the battery, the motor, and CVT work together within the joint high-efficiency working region.

CVT ratio control based on the joint high-efficiency working line

During regenerative braking, the regenerative braking force F_reg can be interpolated and obtained according to braking severity z. And, the regenerative braking power can be computed by braking severity multiplied with the vehicle speed.³ Then, the motor speed can be interpolated on the battery/motor/CVT joint high-efficiency working line. Furthermore, the CVT target ratio value i_cvt can be computed through the vehicle speed v obtained above, the final drive ratio, and brake intensity z, as shown in formula (6). The computed CVT target ratio is shown in Figure 10

$i_{cvt} = \frac{ω_{m} \cdot r}{(v \cdot i_{f})}$ (6)

where i_cvt is the CVT ratio, ω_m is the ISG motor target speed, r is the wheel radius, v is the vehicle speed, and i_f is the final drive ratio.

Figure 10.

CVT target ratio control.

According to the CVT target ratio, the working radius of the primary belt wheel and required working pressure of the secondary belt wheel cylinder can be computed.⁵

When controlling the CVT ratio, the acceptable charging power of NiMH battery must be considered. We can take the minimum value between the regenerative braking power provided by ISG motor and the acceptable charging power of NiMH battery as the actual required regenerative braking power. In addition, the braking severity limit must also be considered during CVT ratio control. When the braking severity z > 0.7, the motor will not be in the regenerative braking mode due to security factors, and the CVT ratio should be adjusted to be the maximum ratio value i_max. In order to easily start, the CVT ratio also should be adjusted to be i_max when the vehicle speed is lower than critical vehicle speed v₀.⁴

Regenerative braking control strategy

According to the working characteristics of HEV braking system, the braking forces computed by braking force distribution strategy are distributed as the following several kinds of situations to avoid battery to overcharge:^5–7

The SOC value is first judged. If SOC > 0.8, the traditional friction braking and the engine braking work together, but not motor regenerative braking. If SOC ≤ 0.8, the ISG motor can provide the regenerative braking force.

The braking severity z is computed according to the actual vehicle speed obtained from the wheel and the braking intention, which is used to judge the braking control as follows:

When the braking severity 0 ≤ z≤ 0.1, only the motor regenerative braking is adopted;

When the braking severity 0.1 < z < 0.7, the motor regenerative braking and the traditional friction braking work together, and the engine braking can take part in when required;

When the braking severity 0.7 ≤z ≤ 1, the traditional friction braking and the engine braking work together, but not motor regenerative braking.

Modeling and simulation analysis

Modeling of regenerative braking system

Adopted with the theory modeling and the numerical modeling methods, the entire forward simulation model (driver intention model, vehicle control model, and controller model) for continuously variable transmission–hybrid electric vehicle (CVT-HEV) has been established. It includes the vehicle models (driver intention model, working mode transition, and vehicle parameters computing), control modules (clutch engaging/disengaging control, CVT ratio control, and regenerative braking control), and the subsystem models (engine, NiMH battery, ISG motor, final drive, and wheel model).⁴

Simulation on regenerative braking system

The proposed regenerative braking control strategy is comparatively simulated under the battery/motor/CVT and battery/motor joint high-efficient working lines, respectively.^4,6

As the results shown in Figure 11, compared to the battery/motor joint high-efficiency working line, all the characteristic indexes are improved using the battery/motor/CVT joint high-efficiency working line under the Extra Urban Driving Cycle (EUDC). For the battery/motor joint high-efficiency working line, the SOC value reduces from the start value 0.7 to the end value 0.6693, the average motor generation efficiency is 66.46%, and the braking energy recovery rate is 47.52%. In view of the battery/motor/CVT joint high-efficiency working line, the SOC value reduces from the start value 0.7 to the end value 0.6746, the average motor generation efficiency is 69.37%, and the braking energy recovery rate is 51.61%. Comparing with the battery/motor joint high-efficiency working line, the SOC increases with 0.792%, the motor average generation efficiency increases by 2.91%, and the braking energy recovery rate increases by 4.09%.

Figure 11.

Offline simulation results under EUDC.

As the results shown in Figure 12, compared to the battery/motor joint high-efficiency working line, all the characteristic indexes are also improved for the battery/motor/CVT joint high-efficiency working line under the Federal Test Procedure-72 (FTP-72) cycle. Using the battery/motor joint high-efficiency working line, the SOC value reduces from the start value 0.7 to the end value 0.6513, the average motor generation efficiency is 66.37%, and the braking energy recovery rate is 51.59%. By adopting the battery/motor/CVT joint high-efficiency working line, the SOC value reduces from the start value 0.7 to the end value 0.6546, the average motor generation efficiency is 70.21%, and the braking energy recovery rate is 57.33%. Comparing with the battery/motor joint high-efficiency working line, the SOC increases with 0.507%, the motor average generation efficiency increases by 3.84%, and the braking energy recovery rate increases by 5.74%.

Figure 12.

Offline simulation results under FTP-72.

Therefore, it can be drawn the conclusion that the proposed regenerative braking control strategy with battery/motor/CVT joint high-efficiency working line can realize the NiMH battery, ISG motor, and CVT to work with joint high efficiency which further enhances braking energy recovery rate under guaranteeing entire vehicle braking security condition compared to the traditional regenerative braking control strategies.

Hardware-in-the-loop test

In order to further verify the regenerative braking control strategy and joint high-efficiency working line, the hardware-in-the-loop (HIL) test platform is established,⁴ as shown in Figure 13.

Figure 13.

The HIL test bench diagram.

The HIL test platform includes JL475Q1 engine, 10 kW ISG motor, Nissan continuously variable transmission (NCVT) (0.498–2.502), clutch, cone gear driveline box, brake, driveline box, electric eddy current dynamometer, inertia flywheel, NiMH battery, and so on. The hydraulic braking system is controlled by the braking pedal, the pressure of the front wheel braking system is controlled by the duty cycle of the two high-speed switching valves, and the input pressure of the main braking pump is adjusted by the loading sensing pressure proportioning valve. The control system includes the ISG motor controller-IPU, battery management system-battery control management (BCM), CVT controller-transmission control unit (TCU), and the HEV controller-hybrid control unit (HCU) replaced with the dSPACE/AutoBox during the test. The HIL electric/electronics system includes the two speed–torque sensors, two current sensors, pressure sensor of the hydraulic system, three pedal stroke sensors, and so on. The speed–torque signals are read through the speed–torque meter installed in the IPC and transmitted to dSPACE/AutoBox through the serial communication, but the other signals are converted A/D through the I/O interface of the DS1103 card installed in the dSPACE/AutoBox. The dSPACE/Autobox is high-speed local area network (LAN) connected with a laptop through network cable with 100 M bandwidth.

As the HIL test results shown in Figure 14, when adopting with the battery/motor joint high-efficiency working line, the SOC value reduces from the start value 0.7 to the end value 0.5623, the average motor generation efficiency is 65.77%, and the braking energy recovery rate is 37.58% under the EUDC cycle. When using the battery/motor/CVT joint high-efficiency working line, the SOC value reduces from the start value 0.7 to the end value 0.5713, the average motor generation efficiency is 68.24%, and the braking energy recovery rate is 42.61%. Comparing with the battery/motor joint high-efficiency working line, the SOC increases with 1.60%, the motor average generation efficiency increases by 2.72%, and the braking energy recovery rate increases by 5.03%.

Figure 14.

HIL test results under EUDC.

As the HIL test results shown in Figure 15, when adopting with the battery/motor joint high-efficiency working line, the SOC value reduces from the start value 0.7 to the end value 0.5511, the average motor generation efficiency is 66.02%, and the braking energy recovery rate is 42.93% under the FTP-72 cycle. For the battery/motor/CVT joint high-efficiency working line, the SOC value reduces from the start value 0.7 to the end value 0.5602, the average motor generation efficiency is 69.15%, and the braking energy recovery rate is 46.87%. Comparing with the battery/motor joint high-efficiency working line, the SOC increases with 1.65%, the motor average generation efficiency increases by 3.13%, and the braking energy recovery rate increases by 3.94%.

Figure 15.

HIL test results under FTP-72.

Obviously, the energy recovery rate adopting with the battery/motor/CVT joint high-efficiency working line is higher compared to the battery/motor joint high-efficiency working line. Although, the HIL test results such as SOC and the average motor generation efficiency are not good as the corresponding simulation results. It dues to the efficiency loss, powertrain loss, and the response lag of whole test bench. Additionally, based on the previous research results, when the CVT ratio upshifts from the lower shift to the higher shift, the oil must be injected into the initial actuator. The filling of oil needs about 200 ms time depending on the pressure.^8,9 Moreover, the response characteristics of electric control system, the dynamic characteristic of electromagnetic valves, and the elastic characteristic of belt wheel and belt can also cause about 110 ms response lag.^6,7 Furthermore, according to the research results of my teamwork,¹⁰ we can know that the response characteristic of CVT ratio is related to the speed when the ratio reduces. When the speed is about 1000 r/min, the time response time lag is obvious. But, as the speed increases, the value of response time lag reduces gradually. It means that the speed has only little influence on the response characteristic of CVT ratio when the speed value exceeds 1500 r/min. Just for this HIL test, we find that there are about 350 ms time response lag. Compared with the existed results,^6,8,10 this 350 ms response lag can be acceptable, and the test is effective.

Therefore, we can also safely draw the conclusion that the proposed regenerative braking control strategy with the battery/motor/CVT joint high-efficiency working line is better than the traditional control strategies with battery/motor joint high-efficiency working line.

Conclusion

For regenerative braking system, the influence of NiMH battery, ISG motor, and CVT efficiency to system synthetic efficiency has been analyzed, and the battery/motor/CVT joint high-efficiency model has been established, and then the battery/motor/CVT joint high-efficient working line has been drawn.

Based on the battery/motor/CVT joint high-efficiency working line, the regenerative braking control strategy for HEV with CVT has been proposed. Subsequently, the forward simulation model of regenerative braking system has been established.

Under EUDC and FTP-72 cycle, the regenerative braking control strategies adopted with the battery/motor and battery/motor/CVT joint high-efficiency working line have been simulated and compared, respectively. The simulation results show that the motor average generating efficiency increases by 2.91%, and the braking energy recovery rate increases by 4.09% when adopting with the battery/motor/CVT joint high-efficiency working line compared to the battery/motor joint high-efficiency working line under the EUDC cycle, while the motor average generating efficiency increases by 3.84%, and the braking energy recovery rate increases by 5.74% under the FTP-72 cycle.

The HIL test results show that the average generating efficiency increases by 2.72%, and the braking energy recovery rate increases by 5.03% when using the battery/motor/CVT joint high-efficiency working line compared to battery/motor joint high-efficiency working line under EUDC cycle. And, the motor average generating efficiency increases by 3.13%, and braking energy recovery rate increases by 3.94% under FTP-72 cycle.

Footnotes

Academic Editor: Rahmi Guclu

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This work was supported by National Natural Science Foundation of China (grant no. 51305473),Project Funded by China Postdoctoral Science Foundation (grant no. 2014M552317),Postdoctoral Science Funded Project of Chongqing (grant no. xm2014032),Foundation and Advanced Research Program General Project of Chongqing City,China (grant no. cstc2013jcyjA60007),and Board of Education Science and Technology Research Project of Chongqing City,China (grant no. KJ120421).

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