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Regenerative Braking Algorithm for a Hybrid with CVT Ratio Control

Hoon Yeo․Sungho Hwang․Hyunsoo Kim*

School of Mechanical Engineering, Sungkyunkwan University, 300 Chunchun‐dong, Suwon, 440-746, Korea

Abstract : A regenerative braking algorithm is proposed to make maximum use of regenerative braking energy for improvement of fuel consumption. In the regenerative braking algorithm, the regenerative torque is determined by considering the motor capacity, battery SOC and vehicle velocity. The regenerative braking force is calculated from the control unit by comparing the demanded brake torque and the motor torque available. The pressure that is reduced by the amount of the regenerative braking force is supplied form the module. In addition, CVT speed ratio control algorithm is suggested during the braking. 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 the engine noise and friction. It is found from the simulation that the regenerative braking algorithm with the CVT ratio control provides improved fuel economy as much as 4 percent for federal urban driving schedule.

Key words : HEV(), REGEN(regenerative braking), CVT(continuously variable )

1) Generally, regenerative braking system works 1. Introduction together with conventional friction brake for the following reasons : (1) the regenerative braking torque The demand for more environmentally friendly and is not large enough to cover the required braking fuel efficient vehicle has been increased in response to torque, (2) the regenerative braking can not be used for growing concerns on the clean environment and saving many reasons such as high state of charge (SOC) or energy. In this context, the hybrid electric vehicle high temperature of the battery to increase the battery (HEV) has emerged as a viable solution to meet those life. In these cases, the conventional friction braking requirements in short to midterm. In the HEV, braking system works to supply the required braking force. energy that would normally be wasted as heat can be Therefore, in order to apply the regenerative braking, a recuperated through the application of regenerative control algorithm on how to distribute the braking force braking. Regenerative braking takes place by storing into the regenerative braking force and mechanical the kinetic or potential energy in the energy storage friction force is required with respect to the battery device and the stored energy can be used to propel the SOC, motor speed, etc. In addition, the regenerative vehicle, which results in the improved fuel economy. braking control algorithm should take into account the maximum use of the regenerative braking energy for * To whom correspondence should be addressed. further reduction of emissions and improvement of fuel [email protected]

1 economy. the vehicle speed. In the HEV in Fig. 1, the braking of Meanwhile the regenerative braking offers improved the , i.e, the front wheel is performed by fuel economy for the HEVs, CVTs have been adopted using both a and a friction brake in many HEVs to obtain minimum fuel consumption. It meanwhile the rear wheel braking is carried out only by is well known that CVT provides optimal engine a friction brake. operation for the minimum fuel consumption In the friction brake, hydraulic pressure is supplied to independent of the vehicle velocity. In normal driving, wheel cylinder so as to generate a brake force in CVT ratio is controlled to maintain the engine operation response to a driver’s brake pedal operation. Whereas, on the optimal operation line for the given driving the regenerative brake generates electric energy which mode. During the braking, the same control strategy can is to be charged to a battery based on a reverse be applied. electromotive force generated by inertial rotation of the In this paper, a regenerative braking control traction motor. algorithm including the CVT speed ratio control is

w proposed for a parallel HEV. Performance of the ij| regenerative braking algorithm is evaluated by hŠŠœ”œ“ˆ›–™ vu†v mmG }ˆŠœœ ” šž›Š ‰––š ›Œ™ w ows š simulation and time response of the vehicle speed, tˆš›Œ™GŠ “•‹Œ™ wœ ”— w

w™–—–™›–•ˆ“ motor torque, motor speed, CVT speed ratio, battery w–žŒ ™ ™Œ ‹œŠ•ŽGˆ“Œ —ˆŠ’ z ›™–’ŒGš”œ“ˆ›–™ SOC and fuel economy are investigated. y Œ“ŒGˆ“Œ t–›–™

2. Development of Regenerative Braking ol}GT lj| Hydraulic Module m™–•›GžŒŒ“GŠ “•‹Œ™G yŒˆ™GžŒŒ“GŠ “•‹Œ™ Fig. 2 Schematic diagram of regenerative braking hydraulic module Figure 2 shows a schematic diagram of the CVT regenerative braking hydraulic module developed in REGEN Hydraulic Motor & + - Pedal Inverter Battery this study. When a driver pushes a brake pedal, pedal Module Generator Engine force is transmitted to master cylinder through a MCU vacuum booster. In the hydraulic module in Fig. 2, the

BCU HEV-ECU conventional master cylinder and vacuum booster are used. The master cylinder pressure is supplied to the rear wheel cylinder. In the front wheel, the regenerative braking is performed corresponding to the required Fig. 1 Schematic diagram of parallel HEV braking force which is calculated from a brake control In Fig. 1, a parallel HEV investigated in this study is unit (BCU). If the regenerative braking force is not shown with regenerative braking system. The engine is large enough to meet the required braking force, the connected to a 12kW motor with a single shaft. The hydraulic friction brake works simultaneously to supply power from the engine and motor is transmitted to a the insufficient braking force. Therefore, the front transmission via a . As a transmission, a metal wheel pressure which is reduced by the amount of the belt CVT is used to maintain the engine operation on regenerative braking force is supplied by a proportional the minimum fuel consumption region independent of reducing valve. The required front wheel pressure is

2 calculated at BCU. The regenerative hydraulic module SOC=80~90%, the weight factor decreases linearly. consists of powerpack, accumulator, relief valve and This protects the battery from overcharging that may proportional valve. The powerpack motor is operated affect the battery life. In addition, magnitude of the only when the accumulator pressure drops below the regenerative torque varies depending on the vehicle lower limit, which saves the system power. velocity. Below V1, no regenerative torque is generated.

When the regenerative braking is applied, the From V1 to V2, the regenerative torque increases in hydraulic oil supply from the master cylinder to the proportion to the vehicle velocity. In this study, no front wheel cylinder is cut off, and the reduced regenerative torque is applied below V1 for the hydraulic pressure is supplied from the powerpack following reasons : (1) the regeneration energy is not instead. This may cause a unfamiliar brake pedal large at low speed and (2) the comfort could be feeling to the driver. Therefore, a stroke simulator is deteriorated. For a velocity above V2, the maximum used to consume the oil flow that is blocked at the motor torque available is applied to obtain as much master cylinder, which provides the driver a similar regeneration energy as possible. The regenerative brake pedal feeling. The BCU calculates the braking force at the wheel is obtained regenerative braking force using the signals of the TR FREGEN battery SOC, CVT ratio, vehicle velocity and wheel Rt (3) cylinder pressure. Since the vacuum booster in Fig. 2 is where Rt is the radius. If FREGEN is larger than the operated by the engine back pressure, the engine should required front wheel braking force Fbf, the front wheel be running at all times except idle stop. is braked only by the regenerative brake. Otherwise, the hydraulic friction brake works together with the 3. Regenerative Braking Control regenerative brake. The hydraulic braking force

In order to provide an appropriate regenerative required at the front wheel is obtained braking for the given driving conditions, a control Ff HYD = Fbf – FREGEN (4) algorithm is required to determine the magnitude of the The front wheel cylinder pressure equivalent to FfHYD regenerative torque with respect to the battery SOC, is expressed as [4] motor speed and etc. corresponding to the driver’s Rt FfHYD Pf demand. The regenerative torque applied to the front 2rAwc Pb (5) wheel TR is represented as[6] where r is the brake effective radius, Awc is the wheel T i ˜ N ˜T ˜K ˜WW R REG gen 1 2 , (1) cylinder area, Pb is the brake friction coefficient. where i is the CVT speed ratio, N is the final In Fig. 3, the regenerative braking algorithm is reduction gear ratio, TREG is the regenerative torque by shown. For the driver’s pedal input, the rear wheel the motor, which is determined from the motor cylinder pressure, in others words, the rear braking force Fbr is obtained from the pressure sensor. The characteristic curve for a given speed. Kgen is the required front braking force Fbf is determined from the generation efficiency, W1, W2 are the weight factors. ideal braking force distribution curve for the given Fbr. Weight factors W1 and W2 are expressed as In the mean time, the BCU calculates the regenerative W1 = W1(SOC) force FREGEN for the input signals of the battery SOC, W2 = W2(Velocity). (2) vehicle velocity and CVT ratio by considering the In this study, a weight factor 1 is used in charging the weight factors and the generation efficiency. If the battery for SOC=0~80% to increase the SOC level. For

3 demanded front braking force is less than FREGEN, only final reduction gear, the regenerative brake is applied and the magnitude of TR i u N uTREG uKCVT ˜Kgen ˜K f ˜W1W2 (6) the regenerative braking force is limited not to exceed K the demanded braking force. In case that the demanded where KCVT is the CVT efficiency and f is the final front braking force Fbf is larger than FREGEN, the reduction gear efficiency. It is seen from the above hydraulic friction brake works together with the equation that in order to make the maximum use of regenerative brake. The front wheel pressure Pf is recuperation energy, the regenerative braking should be applied from the regenerative hydraulic module. performed following the most efficient process. The

KCVT Battery SOC, CVT efficiency depends on the input torque and Pedal input Velocity, CVT speed ratio[1]. In this study, it is assumed that CVT ratio Rear pressure KCVT and final reduction gear efficiency K f are constant.

Motor speed, rpm W XWWW YWWW ZWWW [WWW \WWW ]WWW Regenerative torque, Rear braking force W TRE GE N vvs

i

iNTREGEN Ideal distribution o f braking fo rce TR W1W2 K T[W h __L kŒ”ˆ•‹Œ‹G‰™ˆ’•ŽG›–™˜œŒ Front braking force, Fbf _]L Regenerative force, FREGEN _[L Motor torque, Nm torque, Motor _YL _WL T_W ~ŒŒ“G—–žŒ™ ]WL ^WL ^_L

Fig. 4 Motor efficiency map with optimal operating line in Yes No Fbf > FREGEN regenerative braking

Regenerative braking In Fig. 4, the motor efficiency map used in this study + Regenerative braking Hydraulic braking is shown. It is noted from Fig. 4 that the motor efficiency varies from 60 percent to 88 percent. Hydraulic braking P = 0 Therefore, if the CVT ratio is controlled to operate the FfHYD=Fbf-FRE GEN F motor on the most efficient region during the braking,

R F maximum use of the braking energy can be achieved, t fHYD Fbf = FRE GE N p f 2rAP b which results in the improved fuel economy. In this study, the optimal operation line (OOL) is proposed for Hydraulic module the most efficient motor operation during the braking. The OOL can be determined as follows: for a given F fHYD regenerative braking power, there exists a point where Fig. 3 Flow chart for regenerative braking the motor efficiency is the highest. The OOL is obtained by connecting the high efficiency points. 4 Regenerative Braking Control When the driver pushes the brake pedal, the BCU calculates the wheel power and determines the desired In regenerative braking, kinetic energy is transmitted motor speed ɓm)d from the OOL. Then, the desired to the battery via the CVT and motor. The regenerative CVT ratio id is obtained by the following equation torque at the front wheel TR is represented as follows Zm )d ˜ Rt from Eq. (1) by considering the efficiency of CVT and id V ˜ N (7)

4 For example, when the demanded regenerative OOL for the REGEN control. torque is obtained as –50 Nm at the motor speed 1500rpm (point A in Fig. 4) from the BCU, the motor operating point is moved by the CVT ratio control from A to B where the OOL crosses with the given iso‐ power curve at the wheel.

5 Simulation Results and Discussion

Performance of the regenerative braking algorithm developed in this study is evaluated by simulation. In the simulation, dynamic models of the HEV powertrain are developed. In the modeling, dynamic models of the I.C engine, motor, battery, CVT, tire and other vehicle parts are obtained by modular approach using MATLAB simulink. (d)

In Fig. 5, simulation results are shown. The actual 30 REGEN(rear) (e) vehicle velocity(a) follows the target velocity closely. CVT REGEN(rear) REGEN(front) The motor torque(b) shows a positive value when the A CVT REGEN(front)

motor is used to drive the vehicle and shows a negative Pressure, bar 0 value when it is used as a generator. It is seen from Fig. 5c that the CVT ratio by the CVT REGEN control ͢͡͡ CVT REGEN is downshifted much earlier than the REGEN control. REGEN This CVT ratio response makes the motor run on ͦ͡ relatively high efficiency region, i.e. on the OOL, as ͡ shown in Fig. 5f. The battery SOC(d) decreases when \WWW W͡ ͢͡͡͡ ͣ͡͡͡ ͤ͡͡͡ ͥ͡͡͡ ͦ͡͡͡ the motor is used to propel the vehicle and increases ͦ͞͡ OOL when the motor is used as a generator. The final battery Motor torque, Nm SOC by the CVT REGEN control is higher than that of OP ͢͞͡͡ the REGEN control, which implies that more speed, rpm energy is stored by the CVT REGEN control. It is seen Fig. 5 Simulation results from Fig. 5e that wheel pressure by the CVT REGEN control shows smaller value than those by the REGEN However, it is noted from Fig. 5f that the motor control since the regenerative torque by the CVT speed reaches up to 4300 rpm. Since the motor is REGEN control is smaller. The front wheel pressure (e) connected directly with the engine for the HEV used in is reduced compared with that of the rear wheel this study, this high engine speed may cause a sound pressure by the amount of the regenerative torque. In noise and unexpected engine braking effect. Therefore, Fig. 5f, the motor operation trajectory is shown. It is in order to reduce the motor(engine) speed while seen that the motor operation for the CVT REGEN maintaining the optimal motor operation, a modified control is carried out around the OOL during the OOL is proposed. In Fig. 6, a modified optimal braking while the motor is operated mostly out of the operation line OOL B is shown with the original

5 optimal operation line OOL A. The OOL B is obtained ͢͡͡ by connecting the points near the best efficiency region CVT REGEN OOL B for the given braking power while keeping the motor ͦ͡ speed as low as possible. Motor speed, rpm W XWWW YWWW ZWWW [WWW \WWW ]WWW ͡ W ͡ ͢͡͡͡ ͣ͡͡͡ ͤ͡͡͡ ͥ͡͡͡ ͦ͡͡͡

vvs h Nm torque, Motor ͦ͞͡

T[W vvs i __L

_]L ͢͞͡͡ _[L Motor torque, Nm torque, Motor _YL Motor speed, rpm _WL T_W ]WL ^WL ^_L Fig. 7 Motor operation trajectories for FUDS

Fig. 6 Modified OOL Figure 8 shows comparison of the fuel economy and final battery SOC for FUDS. It is found that the fuel In Fig. 7, the motor operation trajectories are economy is improved as much as 4 percents by the compared for FUDS. As shown in Fig. 7, the motor CVT REGEN control for the OOL A and OOL B while operation by the CVT REGEN control is carried out the final battery SOCs show higher values compared more frequently around the optimal operation line for with the REGEN control. the OOL A and OOL B compared with those by the REGEN control, which provides improved fuel CVT REGEN OOL A economy. % CVT REGEN OOL B ͢͡͡ 105 REGEN REGEN

ͦ͡

100 ͡ ͡ ͢͡͡͡ ͣ͡͡͡ ͤ͡͡͡ ͥ͡͡͡ ͦ͡͡͡ ͧ͡͡͡

Motor torque, Nm torque, Motor ͦ͞͡ 95

͢͞͡͡ Fuel Economy Battery SOC ͢͡͡ CVT REGEN OOL A Fig. 8 Comparison of fuel economy and final battery SOC ͦ͡ for FUDS 6 Conclusion ͡ ͡ ͢͡͡͡ ͣ͡͡͡ ͤ͡͡͡ ͥ͡͡͡ ͦ͡͡͡ ͧ͡͡͡ A regenerative braking algorithm is proposed to make maximum use of regenerative braking energy for Motor torque, Nm torque, Motor ͦ͞͡ OOL improvement of fuel consumption. In the regenerative

͢͞͡͡ braking algorithm, the regenerative torque is determined Motor speed, rpm by considering the motor capacity, battery SOC and vehicle velocity. The regenerative braking force is

6 calculated from the brake control unit by comparing the On‐Off Characteristics,” KSME Int. Journal, 2000, demanded brake torque and the motor torque available. Vol. 14, pp. 645~654. The wheel pressure that is reduced by the amount of the 2 Ide, T., Uchitama, H., Kataoka, R., “Experimental regenerative braking force is supplied form the hydraulic Investigation on Shift speed Characteristics of a Metal V‐belt CVT,” Proc. Int. Conf. on brake module. In addition, CVT speed ratio control Continuously Variable Power Transmission, 1996. algorithm is suggested during the braking. The optimal 3 Yeo, H., Kim, H., “Hardware in the Loop Simulation operation line is obtained to operate the motor in the most of Regenerative Braking for a Hybrid Electric efficient region while keeping the motor speed as low as Vehicle,” Proc Instn Mech Engrs, Vol. 216, Part D: J possible by considering the engine noise and friction. It is Automobile Engineering, 2002, pp.855~864. found from the simulation that the regenerative braking 4 Song, H., Lee, H., Kim, H., "CVT Power algorithm including the CVT ratio control provides Transmitting Characteristics and Control Logics for improved fuel economy as much as 4 percent for federal Negative Torque", Transactions of the Korean Society of Automotive Engineers, Vol. 10, Number urban driving schedule. 1, 2002, pp.255~264. References ACKNOWLEDGEMENT-The authors are grateful for 1 Kim, H., Kim, T., ”Low Level Control of Metal Belt the support provided by a grant from the Brain Korea CVT Considering Shift Dynamics and Ratio Valve project in 2004.

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