Regenerative braking algorithm for a hybrid electric vehicle with CVT ratio control

Transcription

1 1589 Regenerative braking algorithm for a hybrid electric vehicle with CVT ratio control Hoon Yeo1, Sungho Hwang2, and Hyunsoo Kim2* 1Electronic Brake System Engineering Department, Hyundai Mobis, Yongin, Gyunggi-do, Republic of Korea 2School of Mechanical Engineering, Sungkyunkwan University, Suwon, Gyunggi-do, Republic of Korea The manuscript was received on 8 March 2006 and was accepted after revision for publication on 11 August DOI: / JAUTO304 Abstract: A regenerative braking algorithm is proposed for a hybrid electric vehicle with a continuously variable transmission (CVT) to make the maximum use of the regenerative braking energy. In the regenerative braking algorithm, the regenerative torque is determined by considering the motor capacity, battery state of charge (SOC), and vehicle velocity. The regenerative braking force is calculated from the brake control unit by comparing the demanded brake torque and the motor torque available. The wheel pressure is reduced by the amount of the regenerative braking force and is supplied from the hydraulic brake module. In addition, the CVT speed ratio control algorithm is suggested during braking for optimal motor operation. The optimal operation line is proposed to operate the motor in the most efficient region while keeping the motor speed as low as possible by considering engine noise and friction. It is found from the experiments that the regenerative braking algorithm with CVT ratio control offers an improved battery SOC, which provides increased recuperation energy by 8 per cent for the federal urban driving schedule compared with that of the conventional CVT ratio control. Keywords: HEV, regenerative braking, CVT ratio 1 INTRODUCTION (federal urban driving schedule) and 53 per cent for Japan s driving mode. Mechanical, hydro- The demand for more environmentally friendly and mechanical, or electromagnetic devices such as fuel efficient vehicles has been increased in response flywheels and pressure reservoirs linked with hydroto growing concerns about a clean environment and mechanical machines can be used as an energy saving energy. In this context, the hybrid electric storage device to accomplish regenerative braking [1]. vehicle (HEV) has emerged as a viable solution to However, for passenger car applications, generator meet those requirements in the short to mid term. battery or generator ultra capacitor regenerative In the HEV, braking energy that would normally be braking technology is considered to be the most wasted as heat can be recuperated through the appli- promising option [2]. cation of regenerative braking. Regenerative braking Generally, the regenerative braking system works takes place by storing the kinetic or potential energy together with the conventional friction brake for in the energy storage device and the stored energy the following reasons: (a) the regenerative braking can be used to propel the vehicle, which results in torque is not large enough to cover the required improved fuel economy. Regenerative braking is an braking torque, (b) regenerative braking cannot be effective approach to improve vehicle efficiency, used for many reasons, such as the high state of especially for vehicles in heavy stop and go traffic. charge (SOC) or high temperature of the battery to For example, the ratio of braking energy to total increase the battery life. In these cases, the contraction energy reaches 48.3 per cent for the FUDS ventional friction braking system works to supply the required braking force. Therefore, in order to apply * Corresponding author: School of Mechanical Engineering, regenerative braking, a control algorithm on how to Sungkyunkwan University, 300 Chunchun-dong, Suwon, Gyunggido distribute the braking force into the regenerative , Republic of Korea. hskim@me.skku.ac.kr braking force and mechanical friction force is

2 1590 Hoon Yeo, Sungho Hwang, and Hyunsoo Kim required with respect to the battery SOC, motor vehicle velocity. In addition, a CVT speed ratio control speed, etc. In addition, the regenerative braking algorithm is proposed to obtain the maximum control algorithm should take into account the maxiof efficiency of the regenerative braking. Performance mum use of the regenerative braking energy for the regenerative braking algorithm is evaluated by further reduction of emissions and improvement of the experiment and the experimental results are the fuel economy. compared with the simulation results, such as the Few investigations have been reported for vehicle speed, motor torque, motor speed, CVT regenerative braking. The effectiveness of regenerative speed ratio, battery SOC, and regeneration energy. braking was investigated for the electric vehicle (EV) From the experimental and simulation results, the and HEV by Gao et al. [2]. They suggested a parallel effectiveness of the CVT speed ratio control algorithm braking system which has a structure such that on braking energy recuperation is investigated the regenerative and mechanical braking share the during regenerative braking. braking force of the front axle in a fixed relationship. For regenerative braking of the EV, Wyczalk [3] suggested a mathematical formulation of the 2 DEVELOPMENT OF THE REGENERATIVE regenerative braking energy by considering the BRAKING HYDRAULIC MODULE charging and discharging efficiencies and showed that a significant improvement in regenerative In Fig. 1, a parallel HEV investigated in this study braking could be achieved by adopting a continuously is shown with the regenerative braking system. variable transmission (CVT). In Toyota s Prius, an The engine is connected to a 12 kw motor with a electronically controlled regenerative braking system single shaft. The power from the engine and motor is introduced for a parallel HEV [4]. The hydraulic is transmitted to a transmission via a clutch. As a module in the brake system consists of an ingeniously transmission, a metal belt CVT is used to maintain the engine operation on the minimum fuel condesigned master cylinder, with a stroke simulator sumption region independent of the vehicle speed. to absorb the brake oil flow. Panagiotidis et al. [5] In the HEV in Fig. 1, braking of the drive wheel, i.e. suggested a regenerative braking model for a parallel the front wheel, is performed by using both the HEV using ADVISOR. This model computes the regenerative brake and the friction brake; meanwheel pressure, based on a look-up table yielding while, the rear wheel braking is carried out only by the distribution of the braking forces between the the friction brake. front and rear wheels. However, the literature reveals In the friction brake, hydraulic pressure is supplied little detail of the modelling of the regenerative to the wheel cylinder in order to generate the brake braking system or the effect on the HEV powertrain. force in response to the driver s brake pedal operation. A regenerative braking algorithm and a hydraulic The regenerative brake generates the electric energy module have been proposed for a parallel HEV that is to be charged to a battery based on a reverse equipped with a CVT [6]. In this algorithm, the electromotive force generated by inertial rotation of regenerative braking torque is calculated by conthe traction motor. sidering the battery SOC, vehicle velocity, motor Figure 2 shows a schematic diagram of the capacity, and CVT speed ratio. regenerative braking hydraulic module developed in Meanwhile, regenerative braking offers improved this study. When a driver pushes the brake pedal, fuel economy for HEVs; CVTs have been adopted pedal force is transmitted to the master cylinder in many HEVs to obtain better fuel economy. It through the vacuum booster. In the hydraulic is well known that the CVT provides optimal module in Fig. 2, the conventional master cylinder engine operation for minimum fuel consumption and vacuum booster are used. The master cylinder independent of the vehicle velocity. In normal driving, pressure is supplied to the rear wheel cylinder. In the the CVT ratio is controlled to maintain the engine front wheel, the regenerative braking is performed operation on the optimal operation line for the given corresponding to the required braking force which driving mode. During the braking, a similar control is calculated from the brake control unit (BCU). If strategy can be applied. the regenerative braking force is not large enough In this paper, a regenerative braking control to meet the required braking force, the hydraulic algorithm including the CVT speed ratio control is friction brake works simultaneously to compensate proposed for a parallel HEV. Firstly, the regenerative for the braking force deficiency. Therefore, the braking algorithm is developed by considering the front wheel pressure, which is reduced by the ideal brake force distribution, battery SOC, and amount of the regenerative braking force, is supplied

3 Regenerative braking algorithm for a hybrid electric vehicle 1591 Fig. 1 Schematic diagram of a parallel HEV Fig. 2 Schematic diagram of the regenerative braking hydraulic module by the proportional reducing valve. The required front wheel pressure is calculated at the BCU. The regenerative hydraulic module consists of a powerpack, accumulator, relief valve, and proportional valve. The powerpack motor is operated only when the accumulator pressure drops below the lower limit, which saves the system power. When regenerative braking is applied, the hydraulic oil supply from the master cylinder to the front wheel cylinder is cut off and the reduced hydraulic pressure is supplied from the powerpack instead. This may cause an unfamiliar brake pedal feeling to the driver. Therefore, a stroke simulator is used to consume the oil flow that is blocked at the master cylinder, which provides the driver with a similar brake pedal feeling. The BCU calculates the regenerative braking force using the signals of the battery SOC, CVT ratio, vehicle velocity, and wheel cylinder pressure. Since

4 1592 Hoon Yeo, Sungho Hwang, and Hyunsoo Kim the vacuum booster in Fig. 2 is operated by the engine induction depression, the engine should be running at all times except in the idle stop. 3 REGENERATIVE BRAKING CONTROL In order to provide the appropriate regenerative braking for the given driving conditions, a control algorithm is required to determine the magnitude of the regenerative torque with respect to the battery SOC, motor speed, etc., corresponding to the driver s demand. The regenerative torque applied to the front wheel T R is represented as [6] T R =i N T REGEN g CVT g gen g N W 1 W 2 (1) where i is the CVT speed ratio, N is the final reduction gear ratio, T REGEN is the regenerative torque by the motor, which is determined from the motor characteristic curve for the given speed, g gen is the generation efficiency, g CVT is the CVT efficiency, and g N is the final reduction gear efficiency. W 1 and W 2 are the weight factors, expressed as W 1 =W 1 (SOC) W 2 =W 2 (Velocity) (2) In Fig. 3(a), the weight factor for the battery SOC is shown. In this study, a weight factor of 1 is used in charging the battery for SOC=0 80 per cent to increase the SOC level. For SOC=80 90 per cent, the weight factor decreases linearly. This protects the battery from overcharging that may affect the battery life. In addition, the magnitude of the regenerative torque varies depending on the vehicle velocity [Fig. 3(b)]. Below V, no regenerative torque 1 is generated. From V to V, the regenerative torque 1 2 increases in proportion to the vehicle velocity. In this study, no regenerative torque is applied below V for 1 the following reasons: (a) the regeneration energy is not large at low speed and (b) the comfort could deteriorate. For velocity above V, the maximum 2 motor torque available is applied to obtain as much regeneration energy as possible. The regenerative braking force at the wheel is obtained as Fig. 3 Weight factors for regenerative braking wheel is braked only by the regenerative brake. Otherwise, the hydraulic friction brake works together with the regenerative brake. The friction braking force required at the front wheel is obtained as F bf FRICTION =F bf F REGEN (4) The front wheel cylinder pressure equivalent to F is expressed as bf FRICTION P f = R t F bf FRICTION 4R b Am b (5) where R is the brake effective radius, A is the wheel b cylinder area, and m is the brake friction coefficient. b In Fig. 4, the flowchart for regenerative braking is shown. For the driver s pedal input, the rear wheel cylinder pressure, in others words the rear braking force F, is obtained from the pressure sensor. The br required front braking force F is determined from bf the ideal braking force distribution curve for the given F. In the meantime, the BCU calculates the br regenerative force F for the input signals of REGEN the battery SOC, vehicle velocity, and CVT ratio by F = T R (3) REGEN R considering the weight factors, W and W, and t 1 2 the generation efficiency g. If the demanded gen where R t is the tyre radius. If F REGEN is larger than the required front wheel braking force F bf, the front front braking force is less than F, only the REGEN regenerative brake is applied and the magnitude of

5 Regenerative braking algorithm for a hybrid electric vehicle 1593 the driver s braking command, i.e. the demanded deceleration, the front and rear braking forces are obtained as F bf and F br respectively. 4 CVT SPEED RATIO CONTROL In regenerative braking, the kinetic energy is transmitted to the battery via the CVT and the motor. The regenerative torque at the front wheel T R is represented by equation (1). It is seen from equation (1) that T depends on the CVT ratio R and system efficiency. Therefore, in order to make the maximum use of the recuperation energy, regenerative braking should be performed following the most efficient process. The CVT efficiency g CVT depends on the input torque and CVT speed ratio [7], but in this study it is assumed that g and the CVT final reduction gear efficiency g are constant. N Figure 6 shows the motor efficiency map used in this study. It is noted from Fig. 6 that the motor efficiency varies from 60 to 90 per cent. Therefore, if the CVT ratio is controlled to operate the motor on the most efficient region during regenerative braking, maximum use of the braking energy can be achieved, which results in improved fuel economy. In this study, an optimal operation line (OOL) is proposed for the most efficient motor operation during the braking. The OOL can be determined as follows: for the given regenerative braking power, a point exists where the motor efficiency is highest. Point OOL B is obtained by connecting the best efficiency points for the given braking power (Fig. 6). However, since the motor is connected directly with the engine Fig. 4 Flowchart for regenerative braking for the HEV used in this study, this high engine speed may cause a sound noise and unexpected engine braking effect. Therefore, OOL B is modified to OOL A, the regenerative braking force is limited not to which is used in this study during regenerative exceed the demanded braking force. In case the braking. demanded front braking force F is larger than When the driver pushes the brake pedal, the BCU bf F, the hydraulic friction brake works together calculates the wheel power and determines the REGEN with the regenerative brake. The front wheel pressure desired motor speed v from the OOL. The desired m_d P is applied from the regenerative hydraulic module. motor speed v can be determined at the point f m_d In Fig. 5, an ideal braking force curve used in this where the OOL crosses wheel power (point Q in study is shown. The ideal braking force curve is Fig. 6). If the vehicle speed is V during the braking, obtained from the basic relationship between the the desired CVT ratio i which operates the motor d front brake force and the rear brake force with on the OOL (point Q) is obtained as respect to the demanded deceleration for the HEV in Fig. 1. If the braking forces on the front and rear wheel follow the curve in Fig. 5, the maximum braking stability can be achieved since the braking forces reach the adhesive limitation between the road and the tyres. In Fig. 5, if the point P is chosen for i d = v m_d R t VN (6) The actual CVT ratio i is realized though CVT dynamics, as shown in Fig. 7. In the simulation, the

6 1594 Hoon Yeo, Sungho Hwang, and Hyunsoo Kim Fig. 5 Braking forces at the front and rear wheels Fig. 6 Motor efficiency map with an optimal operating line Fig. 7 CVT ratio control

7 Regenerative braking algorithm for a hybrid electric vehicle 1595 Fig. 8 Schematic diagram of the bench tester following CVT dynamics by Ide et al. [8] is used i(t)=b(i)v m (F p F* p ) (7) where b(i) is the coefficient depending on the speed ratio, v m is the motor speed, F p is the primary actuator force, and F* p is the steady state primary actuator force. In addition, dynamic equations of the motor and battery SOC [6] are used for the simulation. 5 HEV BENCH TESTER To validate the performance of the regenerative braking algorithm experimentally, a bench tester was designed and realized for the target hybrid electric Fig. 9 HEV bench tester vehicle. Figures 8 and 9 show the bench tester designed in this study. The bench tester consists of the engine, motor, CVT, clutch, inertia, regenerative The engine torque and motor torque are measured braking module, hydraulic dynamometer, and control by the torque transducer. The drive torque and load system modules. torque are measured by the torque sensors. In In the bench tester, the power is supplied by addition, the primary and secondary pressures are the internal combustion (IC) engine and the motor measured by the pressure sensor. and is transmitted to the CVT rig. In the CVT, the power is transmitted from the primary pulley to the secondary pulley via a metal belt. The output power 6 EXPERIMENTAL RESULTS AND DISCUSSION from the CVT rig drives the flywheel and is balanced at the hydraulic dynamometer. The engine speeds, In Fig. 10, bench test results of the HEV are shown. i.e. the primary pulley speed and the secondary Experiments are performed for the braking perform- pulley speed, are measured by the rotary encoder. ance from V=50 to V=0 km/h during 15 seconds.

8 1596 Hoon Yeo, Sungho Hwang, and Hyunsoo Kim Fig. 10 Experimental results The actual velocity (a) follows the target velocity. The regenerative braking does not work any more and engine torque (b) varies according to the motor friction braking begins to work. The battery SOC (f) torque (e). The engine speed (c) changes by the increases when the motor is used as the generator. CVT ratio control (d) corresponding to the required The final battery SOC by the CVT REGEN control braking power. The motor (engine) speed begins to is higher than that of the conventional CVT ratio increase when the braking starts and remains around control, which implies that more electric energy r/min during the braking by the CVT is stored by the CVT REGEN control. The front regenerative ratio control (REGEN control). It is seen wheel cylinder pressure (g) does not build up during from Fig. 10(d) that the CVT ratio by the CVT REGEN regenerative braking since the magnitude of the control is downshifted much earlier than the conventional regenerative braking force is large enough to cover CVT ratio control. This CVT ratio control the required front brake force. The front wheel makes the motor run in a relatively high efficiency pressure is generated only after the regenerative region, i.e. on OOL A. braking stops. The rear wheel cylinder pressure (h) The motor torque (e) shows a negative value during is generated from the brake force distribution curve regenerative braking. It is noted from Fig. 10(e) (Fig. 5) to supply the required rear brake force as that the regenerative braking is carried out for soon as the braking begins. the first 10 seconds of braking. When the vehicle In Fig. 11, the motor operation trajectories by the velocity decreases below the limit velocity V (Fig. 3), 1 CVT REGEN control and conventional CVT ratio

9 Regenerative braking algorithm for a hybrid electric vehicle 1597 train are developed. In the modelling, dynamic models of the internal combustion engine, motor, battery, CVT, tyre, and other vehicle parts are obtained by a modular approach using MATLAB Simulink [6]. Figure 12 shows the MATLAB model of the HEV powertrain. The vehicle data used in the simulations are shown in Table 1. In Fig. 13, simulation results for the CVT REGEN control are compared with those of the conventional CVT control for s of the federal urban driving schedule (FUDS). The vehicle velocity (a) follows the driving schedule V closely for both cases. desired The motor torque (b) shows a positive value when the motor is used to assist the vehicle and shows Fig. 11 Motor operation trajectory a negative value when the motor is used as the generator during regenerative braking. It is seen that control are compared during the braking. As shown the motor speed (c) by the CVT REGEN shows higher in Fig. 11, the motor operation for the CVT REGEN values than those of the conventional CVT control control is carried out more frequently around the during regenerative braking since the motor operation OOL compared with those by the conventional CVT is performed on the relatively high efficiency region control, which results in improved regeneration by the CVT REGEN control. The CVT ratio (d) for efficiency. This can be verified from the increased the CVT REGEN is downshifted faster than the con- battery SOC [Fig. 10(f)]. Since the increased battery ventional control during regenerative braking. The SOC means that more electrical energy is stored in battery SOC (e) by the CVT REGEN control shows the battery, it is expected that the fuel economy of an increased value during regenerative braking. In the parallel HEV in Fig. 1 can be improved by the Fig. 13(f), the regeneration energy stored in each CVT REGEN control. battery is compared. As shown in Fig. 13(f), the In order to evaluate the recuperated energy during stored energy by the CVT REGEN control shows a regenerative braking, simulations are performed. For higher value than that of the conventional CVT ratio the simulation, dynamic models of the HEV power- control. Since the stored energy can be re-used to Fig. 12 MATLAB model of the HEV powertrain

10 1598 Hoon Yeo, Sungho Hwang, and Hyunsoo Kim Table 1 Vehicle data Engine Displacement 1400 cm3 Maximum torque 140 N m Motor 10 kw motor torque at base r/min 50 N m Battery (Ni MH) Total power 10 kw Vehicle CVT gear ratio range Final reduction gear ratio Vehicle mass 1380 kg Front project area m2 Drag coefficient Tyre radius m Fig. 13 Simulation results for the FUDS (from 0 to 400 s)

11 Regenerative braking algorithm for a hybrid electric vehicle 1599 propel the vehicle, the vehicle fuel economy can be improved by the amount of increased stored energy. Figure 14 shows simulation results of the regeneration energy for FUDS. It is seen from Fig. 14 that the regeneration energy is increased by as much as 8 per cent for the CVT REGEN control compared with the conventional CVT control. More regeneration energy can be retrieved by selecting different OOLs. For instance, if OOL B (Fig. 6) is chosen, a larger increase in the battery SOC can be obtained since the OOL B crosses the motor characteristic map in the relatively high efficiency region. However, since the motor is directly connected with the engine, OOL B may cause higher engine braking friction due to the relatively higher engine speed, which causes a reduced regeneration energy. Therefore, an appropriate motor OOL should be selected by considering the regeneration energy and the engine braking effect. torque. The wheel pressure that is reduced by the amount of the regenerative braking force is supplied from the hydraulic brake module. In addition, the CVT speed ratio control algorithm is suggested during braking for optimal motor operation. The optimal operation line is proposed to operate the motor in the most efficient region while keeping the motor speed as low as possible while considering engine noise and friction. It is found from the experiments that the regenerative braking algorithm with the CVT ratio control (CVT REGEN control) offers an improved battery SOC since the CVT REGEN control makes the motor run on the OOL. From the simulation, it is found that the CVT REGEN control provides an increased recuperation energy of 8 per cent for the federal urban driving schedule compared with that of the conventional CVT ratio control. REFERENCES 7 CONCLUSION 1 LaPlante, J., Anderson, C. J., and Auld, J. Development of a hybrid electric vehicle for the US In this paper, a regenerative braking algorithm is Marine Corps. SAE paper , proposed for a hybrid electric vehicle with a coneffectiveness of regenerative braking for EV and HEV. 2 Gao, Y., Chen, L., and Ehsani, M. Investigation of the tinuously variable transmission (CVT) to make the maximum use of the regenerative braking energy. In SAE paper , Wyczalk, F. A. Regenerative braking concepts for the regenerative braking algorithm, the regenerative electric vehicle a primer. SAE paper , torque is determined by considering the motor 4 Toyota, Prius Manual, capacity, battery state of charge (SOC), and vehicle 5 Panagiotidis, M., Delagrammatikas, G., and velocity. The regenerative braking force is calculated Assanis, D. Development and use of a regenerative from the brake control unit by comparing the braking model for a parallel hybrid electric vehicle. demanded brake torque and the available motor SAE paper , Yeo, H. and Kim, H. Hardware-in-the-loop simulation of regenerative braking for hybrid electric vehicles. Proc. Instn Mech. Engrs, Part D: J. Automobile Engineering, 2002, 216(D11), Kim, H. and Kim, T. Low level control of metal belt CVT considering shift dynamics and ratio valve on off characteristics. KSME Int. J., 2000, 14, Ide, T., Uchitama, H., and Kataoka, R. Experimental investigation on shift speed characteristics of a metal V-belt CVT. In Proceedings of the International Conference on Continuously variable power transmission, Yokohama, Japan, APPENDIX Notation Fig. 14 A wheel cylinder area (m2) F force (N) i CVT speed ratio Comparison of regeneration energy for the N final reduction gear ratio FUDS P pressure (N/m2)

12 1600 Hoon Yeo, Sungho Hwang, and Hyunsoo Kim R radius (m) Subscripts T torque (N m) b V vehicle velocity (m/s) d W weight factor f m b coefficient p g efficiency REGEN m friction coefficient t v rotational speed (r/min) brake desired front motor primary regenerative brake tyre