a torque and speed coupling hybrid drivetrai

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 3, MAY 2006 741

A Torque and Speed Coupling Hybrid Drivetrain—Architecture, Control, and Simulation

Yimin Gao and Mehrdad Ehsani, Fellow, IEEE

Abstract—In this paper, a speed and torque coupling hybriddrivetrain is introduced. In this drivetrain, a planetary gear unitand a generator/motor decouple the engine speed from the vehiclewheel speed. Also, another shaft-fixed gear unit and tractionmotor decouple the engine torque from the vehicle wheel torque.Thus, the engine can operate within its optimal speed and torqueregion, and at the same time, can directly deliver its torque to thedriven wheels. This paper discussed the fundamentals architec-ture, design, control, and simulation of the drivetrain

Index Terms—Architecture, control, design, parallel hybrid, se-ries hybrid, simulation, speed coupling, torque coupling.

I. INTRODUCTION

AT PRESENT, hybrid electric vehicles (HEVs) are recog-nized as one of the most promising technologies in signif-

icantly reducing the petroleum fuel consumption, and toxic andgreenhouse gases emissions. Hybrid drivetrains are usually cat-egorized into series and parallel configurations. In series config-uration, the two power sources are coupled together electricallythrough a power electronic device as shown in Fig. 1 [1]–[3]. Byits power coupling method, electrically coupling hybrid driv-etrain may be the better term to represent its hybridizing fea-ture. On the other hand, in parallel hybrid drivetrain, two powersources are coupled together mechanically through a mechan-ical device, as shown in Fig. 2 [1]–[3]. This drivetrain may becalled a mechanically coupling hybrid drivetrain.

The major advantage of the series hybrid drivetrain or elec-trically coupling hybrid drivetrain is that the engine is mechan-ically decoupled from the vehicle wheels, and thus, can operatein a narrow, high efficient speed and torque region. Its majordisadvantage is that the mechanical power of the engine needsto change its form twice (mechanical to electrical and then tomechanical again) in delivering to the driven wheels, and thusmore energy losses may occur. On the other hand, in parallel hy-brid vehicles or mechanically coupling hybrid drivetrain, the en-gine directly delivers its mechanical power to the driven wheelswithout undergoing energy form change. The major disadvan-tage is that the engine cannot always operate in a narrow speedregion, because of its mechanical coupling to the driven wheels.Thus, the average engine efficiency is lower than that in serieshybrid drivetrain.

To overcome the disadvantages of the series and parallel hy-brid drivetrain, a new hybrid drivetrain, called series-parallel hy-

Manuscript received March 9, 2005; revised October 26, 2005. Recom-mended by Associate Editor J. Shen.

The authors are with the Electrical Engineering Department, Texas A&MUniversity, College Station, TX 77843–3128 USA (e-mail: [email protected]).

Digital Object Identifier 10.1109/TPEL.2006.872375

Fig. 1. Series or electrical coupling hybrid drivetrain.

Fig. 2. Parallel or mechanical coupling hybrid drivetrain.

brid drivetrain, has been developed [5], [6], in which a planetarygear unit is used to decouple the engine speed from the vehiclespeed, and a shaft-fixed gear unit is used to decouple the en-gine torque from the driven wheels. In this way, the engine canpotentially operate in a narrow speed and torque region, and atthe same time, the engine can deliver its mechanical power di-rectly to the driven wheels. In this paper, this drivetrain is calleda torque and speed coupling hybrid drivetrain.

Reference [6] has also given an overview of the drivetrainstructure, operation modes, and control scheme. Since then, fewpublications have been found on the fundamental analysis of thedrivetrain features, especially the proper characteristic of elec-tric machines, drivetrain, and component control. This paperfundamentally analyzed the physical features of the torque andspeed coupling, introduced the drivetrain architecture, discussedthe torque-speed characteristics of the generator/motor and trac-tion motor, and developed the drivetrain control logic and finallypresented the simulation results.

0885-8993/$20.00 © 2006 IEEE

742 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 3, MAY 2006

Fig. 3. Gear set as the torque coupling.

II. TORQUE COUPLING AND SPEED COUPLING

A. Torque Coupling

In a mechanical coupling hybrid drivetrain, two powersources may be coupled together by either torque or speed. Thetypical torque coupling is a shaft-fixed gear set as shown inFig. 3. The speed relationship between the inputs and output isexpressed by

(1)

and

(2)

where , , , and are the tooth number of the gears,and are the gear ratios. The torque

relationship between the input and output is

(3)

where and are the efficiency from the corresponding inputto the output, is the index, associated with the power flowdirection. 1, when or is positive (traction), and

1, when or is negative (regenerative).Equation (3) indicates that for a given output torque, one input

torque can be adjusted by another. For example, in hybrid driv-etrain, when an internal combusion (IC) engine is connected toinput 1, an electric motor to input 2, and driven wheels to theoutput, the electric motor torque can be used to adjust the enginetorque to meet the wheel torque requirement. Thus, the enginecan be operated with its optimal torque. Most parallel (torquecoupling) hybrid drivetrains employ this principle.

B. Speed Coupling

Typical speed coupling mechanism is the planetary gear unitas shown in Fig. 4. The speed relationship between the inputsand output are

(4)

Fig. 4. Planetary gear unit.

where , , and are the angular velocities of carrier, sungear, and ring gear, respectively. , where

and are the tooth number of the sun gear and ring gear.It can be seen from (4) that the angular velocity of the carrier isthe summation of the angular velocity of sun gear and ring gear.

The torque relationship between the inputs and outputs areexpressed as

(5)

and

(6)

where is the efficiency from the sun gear to the ring gear whenthe carrier is fixed to the frame ( 0), and is the index,

1 when , 0 when and 1when .

Equation (4) indicates that at a given carrier angular velocity,angular velocity of the ring gear or the sun gear can be adjustedby the angular velocity of the sun gear or ring gear. For example,in a speed coupling hybrid drivetrain, when an IC engine is con-nected to the ring gear, an electric motor to the sun gear, and thecarrier to the driven wheels, the engine speed can be adjusted bythe motor speed at any vehicle speed, and therefore the enginecan be operated within its optimal speed range.

III. HYBRID DRIVETRAIN WITH TORQUE COUPLING

AND SPEED COUPLING

A. Configuration

Using the torque and speed coupling characteristic mentionedabove, a hybrid drivetrain is proposed as shown in Fig. 5. Inthis drivetrain, an IC engine and an electric motor/generator areconnected to the ring gear and the sun gear of the planetary gearunit to constitute a speed coupling hybrid configuration. Thecarrier of the planetary gear unit is the output port, which isconnected to the driven wheels through a shaft-fixed gear unit.From (5) and (6), it can be seen that when 1 (actually,

has to be greater than one since is larger than andis close to one), the generator/motor has the smallest torque andthe carrier has the largest and the engine torque is in the between.

GAO AND EHSANI: TORQUE AND SPEED COUPLING HYBRID DRIVETRAIN 743

Fig. 5. Configuration of the proposed torque and speed coupling hybrid drive-train.

In this way, a small generator/motor can be used since its weightand volume is grossly proportional to its torque capability. Thelock 1 and lock 2 are used to lock the ring gear and sun gear tothe vehicle frame.

The torque output of the planetary gear unit from the carrier iscoupled to the electric traction motor torque through shaft-fixedgears , , and to constitute a torque coupling configura-tion. The total torque from the planetary gear unit and the trac-tion motor is transferred to the driven wheels through gear ,

, and the differential.In this speed and torque coupling drivetrain, the generator/

motor is used to adjust the engine speed and the traction motor toadjust the engine torque. In this way, the engine can be operatedin its optimal speed and torque region.

This drivetrain has plentiful operation modes with the dif-ferent combinations of the operating status of the power sourcesas follows.

1) Engine alone traction: The generator/motor and tractionmotor are de-energized and lock 1 is engaged (lock the sungear to the vehicle frame). Then, the engine alone deliversits power to the drive wheels through the ring gear, carrierof the planetary unit, gear , , , , and the differ-ential.

2) Traction motor alone traction and regenerative braking:The engine is turn-off and the engine clutch is disengaged(disconnecting the engine from the ring gear), lock 2 locksthe ring gear to the vehicle frame, lock 1 releases the sungear from the vehicle frame and the generator/motor isde-energized. Then traction motor alone delivers its powerto the driven wheels or accepts braking power through gear

, , , and the differential.3) Generator/motor alone traction: When the engine clutch is

open, lock 1 is released, lock 2 is locked, and the tractionmotor is de-energized, the generator/motor functions as atraction motor to deliver its power to the driven wheels, orfunctions as a generator to accept braking power from thedriven wheels through the sun gear, carrier of the planetarygear unit, gear , , , , and the differential.

4) Speed coupling traction (Engine plus generator/motor):When the engine clutch is engaged (connecting the engineto the ring gear), lock 1 and lock 2 both are released, and thetraction motor is de-energized, the engine, generator/motorand planetary gear unit constitute the speed coupling con-figuration. In this case, when the generator/motor speed isnegative, it functions as a generator (negative power). Oth-erwise, it functions as motor (positive power).

5) Torque coupling traction (engine plus traction motor):When the lock 2 is released, and lock 1 locks the sungear to the vehicle frame and the generator/motor isde-energized, the engine and the traction motor constitutethe torque coupling configuration. In this case, when thetraction motor has positive torque, both the engine andtraction motor deliver their powers to the driven wheels.On the other hand, when the traction motor has negativepower, the traction motor accepts power from the engine,charging the batteries.

6) Torque coupling traction (Generator/motor plus tractionmotor): When the engine clutch is disengaged and lock2 locks the ring gear to the vehicle frame, the generator/motor and traction motor constitutes a torque coupling con-figuration. In this case, both the generator/motor and trac-tion motor can operate for traction or regenerative braking.

7) Speed coupling and torque coupling (engine plus gener-ator/motor and traction motor) operation mode: When theengine clutch is engaged and the lock 1 and lock 2 are re-leased, the engine, generator/motor and the traction motorconstitute the speed coupling and torque coupling config-uration. In this case, the generator/motor is used to adjustthe engine speed and the traction motor is used to adjustthe engine torque.

In real operation, the vehicle control unit (VCU) should selectthe best operating mode, at a given driver’s command, operatingenvironment, battery state-of-charge (SOC), etc. and give con-trol commands to the components. A control strategy preset inthe VCU is needed to fulfill the best mode selection and powersource control.

IV. SPEED-TORQUE CHARACTERISTICS OF THE GENERATOR

MOTOR AND TRACTION MOTOR

A. Generator/Motor

Equation (5) indicates that the torque acting on the sun gear(generator/motor torque, refer to Fig. 5) is proportional to thetorque acting on the ring gear (engine torque). In order to be ableto carry the engine torque within the full engine speed range, thespeed-torque curve of the generator/ motor should have the con-stant maximum torque within the first and second quadrant (pos-itive torque, positive, and negative speed) as shown in Fig. 6.Among the available motor drives, permanent magnet motorsmay be the best selection, since they naturally have almost con-stant torque characteristic [1], [7], [8].

B. Traction Motor

Functioning as peaking torque source in acceleration andhill climbing, the ideal profile of the maximum torques versusspeeds of the traction motor is constant power within its whole


Fig. 6. Relationship between the engine and generator/motor speed-torquecharacteristics.

Fig. 7. Typical speed-torque characteristics of traction motor.

speed range in order to minimize its power rating for givenacceleration performance as shown in Fig. 7 [1], [4], [10].However, the real torque-speed profile of a traction motor hasa constant torque range in low speeds as shown in Fig. 7. Thecorner speed as shown in Fig. 7 is called base speed. A speedratio, , is usually defined as the ratio of the maximum speedto the base speed, that is . It is said that twomotors have the same speed-torque characteristics only whenthey have the same speed ratio.

Studies have shown that at given vehicle acceleration perfor-mance, increasing the speed ratio of the traction motor can ef-fectively reduce the required motor power rating, thus reducingbattery power rating. For passenger cars, 4 5 is theproper selection. Further increasing beyond this can not getsignificant motor power rating reduction, and at same time causedifficulty in motor design and control [1], [7], [9].

V. DRIVETRAIN CONTROL

A. Control System of the Drivetrain

The control system of the drivetrain is shown in Fig. 5.The vehicle controller unit (VCU) receives the traction orbrake torque commands from driver through the accelerator orbrake pedal, and other necessary operating information suchas battery SOC and vehicle speed. Based on the real-timeinformation received and the control logic, the VCU generatesthe control signals to control the IC engine, generator/motor,

Fig. 8. Engine operating points controlled by the engine throttle and generator/motor torque.

traction motor, as well as two locks, through engine throttleactuator, generator/motor controller, traction motor controller,and lock 1 and lock 2 actuators.

B. Engine Operating Point Control

Engine operating point can be controlled by the enginethrottle and generator/motor torque. It is supposed that theengine is presently operating in point with a speed ,torque , and throttle angle 60 , as shown in Fig. 8.With a fixed generator/motor torque , thus the fixed en-gine torque, increasing the engine throttle angle will cause theengine speed increasing, to point with 70 for example.Similarly, reducing the engine throttle angle will cause enginespeed decreasing, to point at 50 for example. Theengine operating point can also be changed by changing thegenerator/motor torque as shown in Fig. 8. With a fixed enginethrottle, reducing the generator/motor torque, thus the enginetorque, will cause the engine speed increasing from point topoint , or increasing the generator/motor torque will causethe engine speed decreasing from point to point . Thus, theengine operating points can be potentially controlled within itsoptimal region by instantaneously controlling both the enginethrottle and generator/motor torque as shown in Fig. 9.

C. Traction Torque Control

As shown in Fig. 5, driver’s command is transferred to thevehicle control unit through the accelerator or brake pedal. Theposition of the accelerator or brake pedal represents the pro-pelling or braking torque that the driver desires. As mentionedabove, the engine torque can be controlled in its optimal region,thus the torque output from the carrier of the planetary gear unitis limited to a narrow region [refer to (6)]. The traction motoris the only torque source to meet the propelling or regenera-tive braking torque. The propelling torque acting on the drivenwheels can be expressed as

(7)


Fig. 9. Fuel consumption (efficiency) map of typical IC engine and its optimaloperating region.

where and are the torque outputs from the carrier of theplanetary gear unit and the traction motor, respectively and

are the efficiencies from the carrier and the traction motorto the driven wheels, respectively. and are the gearratio from the carrier and the traction motor to the driven wheelsrespectively, and are expressed as

(8)

and

(9)

where to are the tooth number of the gear as shown inFig. 5.

At a given accelerator pedal position, thus the driven wheelspropelling torque , and engine torque, the required tractionmotor torque can be obtained from (7).

Fig. 10(a) shows the maximum torque developed on thedriven wheels of an example drivetrain, the engine torque,traction motor torque, and the generator/motor torque with awide open engine throttle and full load traction motor operating(maximum torque output). The engine rpm is controlled suchthat, at low vehicle speeds, the engine has a constant speed(1200 rpm in this example) and the generator/motor has nega-tive speeds, at medium vehicle speeds, the generator/motor islocked to the vehicle frame and engine speed linearly increaseswith vehicle speed, and at high vehicle speeds, the enginespeed is also constant (3500 rpm in this example) and thegenerator/motor has positive speeds as shown in Fig. 10(b).In this way, the engine operating speeds are constrained to itshigh efficiency range. It is noted that the generator/motor isde-energized in the medium vehicle speed range in order to usethe high engine torque and shut down the energy flow throughthe generator/motor, which may cause more energy loss.

Fig. 10. Torques and speeds of the driven wheels, engine, generator/motor andtraction motor with full open engine throttle and full load traction motor: (a)torque and (b) speed.

The maximum torque on the driven wheels, which cor-responds to the full accelerator pedal position, dictates thevehicle performance, such as acceleration and gradeability. Onthe other hand, with a partially depressed accelerator pedal, thatis, the required propelling torque on the driven wheels is lessthan its maximum, the engine or traction motor or both have toreduce their torques to meet the torque demand. How to controlthe engine, generator/motor and the traction motor to meet thepartial load torque requirement depends on other operating pa-rameters, such as vehicle speed, battery state-of-charge (SOC)etc. Thus, a drivetrain control strategy is needed.

VI. DRIVETRAIN CONTROL STRATEGY

The drivetrain control strategy is the algorithm code preset inthe vehicle control unit, which generates the control signals tothe engine, traction motor, generator/motor as well as the twolocks, based on driver’s command and other operating parame-ters, such as vehicle speed, engine speed, and battery SOC. Thecontrol objectives are: 1) always meet driver’s torque commandinput from the accelerator or brake pedal, 2) always maintain thebattery SOC in reasonable level, for example around 70% and


TABLE IDRIVE TRAIN CONTROL LOGIC

never lower than 30%, and 3) operate the engine in its optimalspeed and torque region as much as possible.

Table I lists the control logic of the engine, generator/motorand traction motor at various operating conditions, such as ve-

hicle speed, driver’s torque command, and battery SOC. Thiscontrol strategy tries to maintain the battery SOC at its highlevel, that is, when the commanded torque is small and the bat-teries are fully charged, only the engine, operating with partial


TABLE IIVEHICLE PARAMETERS

Fig. 11. Vehicle speed, engine power, generator/motor power, traction motorpower and battery SOC in FTP 75 urban driving cycle.

throttle, is used to propel the vehicle, rather than shutting downthe engine and using pure electric motor propelling. This controlstrategy may be suitable for urban driving, where the batteriesshould always be ready for supplying its peak power to the driv-etrain for acceleration.

VII. DRIVETRAIN SIMULATION

Base on the control strategy developed, a 1500-kg passengercar is simulated in FTP 75 urban and highway driving cycles.The simulations were performed on the HEV simulation soft-ware developed in the Advanced Vehicle Systems Research Pro-gram at Texas A&M University. The software was developedfor HEV research with Matlab/Simulink. The parameters of thevehicle simulated are listed in Table II.

Fig. 11 shows the simulation results of vehicle speed, en-gine power, generator/motor power, traction motor power, andbattery SOC in FTP urban driving cycle. It can be seen fromthis figure that the generator/motor always works for generating(negative power) because of the low vehicle speeds. Through theregenerative braking and charging from the engine by the gen-erator/motor, the battery SOC can be easily maintained at highlevel, which ensures the batteries always being able to supplysufficient power to the drivetrain for acceleration.

Fig. 12 shows the engine operating points on the engine fuelconsumption map. This figure indicates that the engine, in mostof the time, operates in its high efficiency area. Engine alonetraction with light load and high battery SOC causes some en-

Fig. 12. Engine operating points on its fuel consumption map in FTP 75 urbandriving cycle.

Fig. 13. Vehicle speed, engine power generator/motor power, traction motorpower and battery SOC in FPT 75 highway driving cycle.

gine operating points away from its high efficiency area. Thefuel consumption of the vehicle in FTP urban driving cycle ob-tained from the simulation is 5.88 l per 100 km, or 40.2 mpg.Compared with similar conventional vehicle, such as ToyotaCamry (1445 kg, 10.3 l/100 km or 23 mpg in urban driving), thefuel consumption is reduced significantly 10.3 5.88 10.342.9 .

Figs. 13 and 14 shows the simulation results of the vehicledriving in FTP75 highway cycle. It can be seen that the gen-erator/motor power is always zero, except in a short time pe-riod of the cycle start. This means that the drivetrain, in mostof the time, works with a pure torque coupling (the sun gearthen the generator/motor is locked to vehicle frame). Simula-tion indicates that the fuel consumption of the vehicle in FTP 75


Fig. 14. Engine operating points on its fuel consumption map in FPT 75highway driving cycle.

highway driving cycle is 4.96 l/100 km or 47.7 mpg. Comparedwith similar conventional vehicle, the fuel consumption reduc-tion is significant (Toyota Camry: 7.63 l/100 km or 32 mpg).

VIII. CONCLUSION

Using planetary and shaft-fixed gear units, a speed and torquecoupling hybrid drivetrain is developed. This drivetrain can po-tentially decouple the engine speed and torque from the vehicledriven wheels. Thus, the engine can be operated in its efficientspeed and torque range. Thus, the drivetrain efficiency can beimproved. With a proper control strategy, the battery SOC canbe easily maintained in its desired level. Simulations show thatthe fuel economy in urban and highway driving cycles can begreatly improved.

REFERENCES

[1] M. Ehsani, Y. Gao, S. Gay, and A. Emadi, Modern Electric, HybridElectric and Fuel Cell Vehicles—Fundamentals, Theory, and De-sign. Boca Raton, FL: CRC, Nov. 2004.

[2] C. C. Chan and K. T. Chau, Modern Electric Vehicle Technology.New York: Oxford Univ. Press, 2001.

[3] I. Husani, Electric and Hybrid Vehicles—Design and Fundamentals.Boca Raton, FL: CRC Press, 2003.

[4] M. Ehsani, Y. Gao, and K. Butler, “Application of electrically peakinghybrid (ELPH) propulsion system to a full size passenger car with sim-ulated design verification,” IEEE Trans. Veh. Technol., vol. 48, no. 6,pp. 1779–1787, Nov. 1999.

[5] A. Nedunadi, M. Walls, and D. Dardalis, “A parallel hybrid drivetrain,”presented at the SAE SP 1466, 1999.

[6] K. Yamaguchi, S. Moroto, K. Kobayashi, M. Kawamto, and Y.Miyaishi, “Development of a new hybrid system—duel system,”presented at the SAE SP 1156, 1996.

[7] M. Ehsani, Y. Gao, and Y. S. Gay, “Characterization of electric motordrives for traction applications,” in Proc. IEEE 29th Annu. Conf.IECON’03, Nov. 2–6, 2003, vol. 1, pp. 891–896.

[8] T. M. Johns, “Torque production in permanent magnet synchronousmotor drive with rectangular current excitation,” IEEE Trans. Ind.Appl., vol. 20, no. 4, pp. 803–813, 1984.

[9] M. Ehsani, K. Rahman, and A. Toliyat, “Propulsion system design ofelectric and hybrid vehicles,” IEEE Trans. Ind. Electron., vol. 44, no.1, pp. 19–27, Feb. 1997.

[10] J. Y. Wong, Theory of Ground Vehicles. New York: Wiley, 1978, pp.132–133.

Yimin Gao received the B.S., M.S., and Ph.D de-grees in automotive engineering from Jilin Univer-sity of Technology, Changchun Jilin, China, in 1982,1986, and 1991, respectively.

He has been a Vehicle Design Engineer with theDongfeng Motor Co. LTD., Shiyan, Hubei, China,from 1982 to 1983, an Associate Professor with theAutomotive Engineering College, Jilin University ofTechnology, from 1991 to 1995. At present, he is aResearch Associate in the Advanced Vehicle SystemsResearch Program, Texas A&M University, College

Station. He is the author of over 20 technical papers in specialty of electricand hybrid electric vehicles. He is the author of the book Modern Electric, Hy-brid Electric and Fuel Cell Vehicles—Fundamentals, Theory and Design (BocaRaton, FL: CRC Press, 2004). His research areas are mainly the fundamentals,architecture, control, modeling, and design of electric and hybrid electric driv-etrain and major components.

Dr. Gao received four outstanding research awards in automotive engineeringfrom the Chinese government. He is a member of SAE (International Society ofAutomotive Engineers).

Mehrdad Ehsani (S’70–M’81–SM’83–F’96) hasbeen at Texas A&M University, College Station,since 1981, where he is the Robert M. KennedyProfessor of electrical engineering and Director ofAdvanced Vehicle Systems Research Program. Heis the author of over 300 publications in specialtypower systems, pulsed-power supplies, high-voltageengineering, power electronics and motor drives,and automotive power and propulsion systems. He isthe co-author of several books on power electronics,motor drives, vehicle power and propulsion systems,

and a contributor to an IEEE Guide for Self- Commutated Converters in 1990,as well as many monographs. He is the author of over 20 U.S. and EC patents.

Dr. Ehsani received the Prize Paper Awards in Static Power Converters andmotor drives at the IEEE-Industry Applications Society in 1985, 1987, and 1992Annual Meetings, was named the Halliburton Professor in the College of Engi-neering at A&M in 1992, was named the Dresser Industries Professor in thesame college in 1994, was named the Dow Chemical Faculty Fellow of theCollege of Engineering at Texas A&M University in 2001, received the JamesR. Evans Avant Garde Award from the IEEE Vehicular Technology Society in2001, the IEEE Field Award in Undergraduate Teaching in 2003, and becamethe holder of Robert M. Kennedy Endowed Chair of Engineering at Texas A&MUniversity in 2004.

a torque and speed coupling hybrid drivetrai

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