investigation of energy efficient hydraulic hybrid propulsion system for automobiles

9
Investigation of energy efcient hydraulic hybrid propulsion system for automobiles Wei Wu * , Jibin Hu, Chongbo Jing, Zhonglin Jiang, Shihua Yuan National Key Laboratory of Vehicular Transmission, Beijing Institute of Technology, Beijing 100081, PR China article info Article history: Received 10 March 2014 Received in revised form 12 May 2014 Accepted 10 June 2014 Available online 5 July 2014 Keywords: Hybrid vehicle Hydraulic hybrid Common pressure rail Hydraulic transformer Free piston engine Regenerative braking abstract The hybrid method is effective for energy savings. This paper presents an energy efcient hydraulic hybrid propulsion system for automobiles. The system consists of hydraulic common pressure rail, hy- draulic transformer and hydraulic pump or motor. The parameter design and propulsion characteristics of the hydraulic hybrid propulsion system were investigated. The simulated and tested results were proposed. Based on the denition of the hydraulic transformer normal power and normal ow, an effective parameter design method was given. The increase of the degree of freedom of the system makes the parameter design become more exible and the proposed method for the parameter design is feasible. The system achieves a constant torque output at the low speed stage and a constant power output at the high speed stage. The ideal vehicle dynamic performance is guaranteed. The hydraulic transformer speed is closely related to the vehicle speed, which should be considered in the hydraulic transformer design. During the conversion from the driving mode to the regenerative braking mode, the hydraulic transformer speed and the hydraulic motor torque uctuate inherently, which is unhelpful for the service life and reliability of the hydraulic components. It is aimed to provide an effective method for the system design. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction To improve fuel economy, energy saving in automobiles is becoming more important. Reduction of fuel consumption in internal-combustion engines is strongly needed. To deal with these problems, many effective methods of fuel use have been proposed for the automotive eld. The development of hybrid vehicles is one effect method of characteristics of low gas emission and energy savings [1]. The HHV (hydraulic hybrid vehicle) is an important branch of the hybrid vehicles. The HHV presents the best solution for the regenerative braking, especially for the heavy goods vehicle [2]. It can provide good fuel saving on city typical drive cycles with frequent stops and starts [3]. Further, the hydraulic free-piston engine for automobiles has proposed a method to avoid idling oil consumption by the engine on-off control [4]. The HHV mainly consists of power engine, hydraulic pump or motor, accumulator and CPR (constant pressure rail). To enlarge the advantage in the regenerative braking of the HHV, the component parameter and control strategy have been investigated [5]. Ricardo [6] suggested a compromise solution for kinetic energy recovery on the premise of a xed displacement hydraulic pump or motor. Both the shifting schedule of the torque coupler and the distribution of the braking force were proposed as the most important impact factors for energy recovery [7]. Energy management and parameter design of the HHV are the main factors that affect the fuel economy [8,9]. For a better fuel consumption, many researches have been done in the energy management optimisation [10e12]. Kim and co- researchers [13,14] studied the effect of control strategy on emis- sion for different congurations of the HHV. Some optimisations were based on known drive cycles [15e17]. Optimisation based on the drive cycle prediction has been studied and the prediction is based on previous vehicle operations on the same route [18]. Model predictive control has also been presented for the energy man- agement problem [19]. To get a better vehicle performance and fuel economy, the design parameter optimisation of key components has been investigated. Two types of the design methodology have been presented with different accumulator sizes [20]. Beachley and co-researchers [21] examined how factors of fuel saving are linked to the size of the energy storage unit for the HHV. Woon and co- researchers have proposed the design method for the series HHV, including the engine power, the pump speed limitation and the * Corresponding author. Room 412, Building 9, Beijing Institute of Technology, Beijing 100081, PR China. Tel.: þ86 10 68914786; fax: þ86 10 68944487. E-mail address: [email protected] (W. Wu). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy http://dx.doi.org/10.1016/j.energy.2014.06.042 0360-5442/© 2014 Elsevier Ltd. All rights reserved. Energy 73 (2014) 497e505

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Page 1: Investigation of energy efficient hydraulic hybrid propulsion system for automobiles

lable at ScienceDirect

Energy 73 (2014) 497e505

Contents lists avai

Energy

journal homepage: www.elsevier .com/locate/energy

Investigation of energy efficient hydraulic hybrid propulsion systemfor automobiles

Wei Wu*, Jibin Hu, Chongbo Jing, Zhonglin Jiang, Shihua YuanNational Key Laboratory of Vehicular Transmission, Beijing Institute of Technology, Beijing 100081, PR China

a r t i c l e i n f o

Article history:Received 10 March 2014Received in revised form12 May 2014Accepted 10 June 2014Available online 5 July 2014

Keywords:Hybrid vehicleHydraulic hybridCommon pressure railHydraulic transformerFree piston engineRegenerative braking

* Corresponding author. Room 412, Building 9, BeBeijing 100081, PR China. Tel.: þ86 10 68914786; fax

E-mail address: [email protected] (W. Wu).

http://dx.doi.org/10.1016/j.energy.2014.06.0420360-5442/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The hybrid method is effective for energy savings. This paper presents an energy efficient hydraulichybrid propulsion system for automobiles. The system consists of hydraulic common pressure rail, hy-draulic transformer and hydraulic pump or motor. The parameter design and propulsion characteristicsof the hydraulic hybrid propulsion system were investigated. The simulated and tested results wereproposed. Based on the definition of the hydraulic transformer normal power and normal flow, aneffective parameter design method was given. The increase of the degree of freedom of the systemmakesthe parameter design become more flexible and the proposed method for the parameter design isfeasible. The system achieves a constant torque output at the low speed stage and a constant poweroutput at the high speed stage. The ideal vehicle dynamic performance is guaranteed. The hydraulictransformer speed is closely related to the vehicle speed, which should be considered in the hydraulictransformer design. During the conversion from the driving mode to the regenerative braking mode, thehydraulic transformer speed and the hydraulic motor torque fluctuate inherently, which is unhelpful forthe service life and reliability of the hydraulic components. It is aimed to provide an effective method forthe system design.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

To improve fuel economy, energy saving in automobiles isbecoming more important. Reduction of fuel consumption ininternal-combustion engines is strongly needed. To deal with theseproblems, many effective methods of fuel use have been proposedfor the automotive field. The development of hybrid vehicles is oneeffect method of characteristics of low gas emission and energysavings [1]. The HHV (hydraulic hybrid vehicle) is an importantbranch of the hybrid vehicles. The HHV presents the best solutionfor the regenerative braking, especially for the heavy goods vehicle[2]. It can provide good fuel saving on city typical drive cycles withfrequent stops and starts [3]. Further, the hydraulic free-pistonengine for automobiles has proposed a method to avoid idling oilconsumption by the engine on-off control [4].

The HHV mainly consists of power engine, hydraulic pump ormotor, accumulator and CPR (constant pressure rail). To enlarge theadvantage in the regenerative braking of the HHV, the component

ijing Institute of Technology,: þ86 10 68944487.

parameter and control strategy have been investigated [5]. Ricardo[6] suggested a compromise solution for kinetic energy recovery onthe premise of a fixed displacement hydraulic pump or motor. Boththe shifting schedule of the torque coupler and the distribution ofthe braking force were proposed as the most important impactfactors for energy recovery [7]. Energy management and parameterdesign of the HHV are the main factors that affect the fuel economy[8,9]. For a better fuel consumption, many researches have beendone in the energymanagement optimisation [10e12]. Kim and co-researchers [13,14] studied the effect of control strategy on emis-sion for different configurations of the HHV. Some optimisationswere based on known drive cycles [15e17]. Optimisation based onthe drive cycle prediction has been studied and the prediction isbased on previous vehicle operations on the same route [18]. Modelpredictive control has also been presented for the energy man-agement problem [19]. To get a better vehicle performance and fueleconomy, the design parameter optimisation of key componentshas been investigated. Two types of the design methodology havebeen presented with different accumulator sizes [20]. Beachley andco-researchers [21] examined how factors of fuel saving are linkedto the size of the energy storage unit for the HHV. Woon and co-researchers have proposed the design method for the series HHV,including the engine power, the pump speed limitation and the

Page 2: Investigation of energy efficient hydraulic hybrid propulsion system for automobiles

Nomenclature

A the vehicle frontal area, m2

CD the air drag coefficientDmax the maximum power factorf the rolling resistance coefficientECF the required operation frequency of the HFPE units, HzFd the feature driving force when the pressure ratio is 1.0,

NFf the rolling resistance force, NFi the road grade force, NFj the acceleration force, NFw the aerodynamic drag force, NFT the driving force, NFTmax the maximum driving force, Ng the acceleration of gravity, 9.8 N kg�1

G the vehicle gravity, NH the high-pressure raili the maximum slope ratioi0 the final drive ratioL the low-pressure railm the vehicle mass, kgncyl the upper bound of the HT cylinder speed, r min�1

nHT the HT cylinder speed, r min�1

nHTmaxthe maximum HT cylinder speed, r min�1

nm the hydraulic motor speed, r min�1

nmr the required hydraulic motor speed, r min�1

NHT the number of the HTNm the number of the hydraulic motorpA the HT port A pressure, MPapB the HT port B pressure, MPapmr the hydraulic motor rated pressure, MPapT the HT port T pressure, MPapA-rh the designed upper bound of the high-pressure rail,

MPapA-rl the designed lower bound of the high-pressure rail,

MPaPHT the HT nominal power, kWQA the HT port A flow rate, L min�1

QB the HT port B flow rate, L min�1

Qm the hydraulic motor flow rate, L min�1

QHT the HT nominal flow, L min�1

QT the HT port T flow rate, L min�1

r the wheel radius, mt the time, sTm the hydraulic motor torque, Nmu the vehicle speed, km h�1

umax the maximum vehicle speed, km h�1

ur the required vehicle speed, km h�1

V the displacement of the swash plate HT basal body,mL r�1

Vm the hydraulic motor displacement, mL r�1

Vmn the nominal displacement of the hydraulic motor,mL r�1

VA the HT ports A displacement, mL r�1

VB the HT ports B displacement, mL r�1

VT the HT ports T displacement, mL r�1

Greeksa the road grade, radamax the maximum road grade, radaA the HT ports A wrap angle, radaB the HT ports B wrap angle, radaT the HT ports T wrap angle, radDpm the pressure difference of the hydraulic motor, MPahmt the total efficiency of the hydraulic motorhHTmax

the total efficiency of the HT at the nominal powerq the HT control angle of the swash plate, deg.qmax the HT control angle at the maximum pressure ratio,

deg.lmax the maximum pressure ratioU,U0,U1 the abbreviation of formulationss the mass factor of rotating components

AbbreviationsHyps hydraulic hybrid propulsion systemCPR common pressure railDOF degree of freedomHT hydraulic transformerHFPE hydraulic free piston engine

W. Wu et al. / Energy 73 (2014) 497e505498

rated pressure [22]. Ramakrishnan and co-researchers [23] inves-tigated the effect of system parameters in series hydraulic hybrid bynumerical simulation. Some other simulation models have beendeveloped to the parameter design and control strategy [24e28].Hardware in the loop simulation was developed for emissionreduction and efficiency improvement [13,14,29]. The noise andvibration reduction of the hydraulic system in the HHV has beenstudied [30]. The locations and orientations of the isolation systembased on themagnetorheological technology affect the efficiency ofthe noise and vibration mitigation [31].

For a new HHV with the HT (hydraulic transformer), Acthen andco-researchers proposed the configuration analysis [32,33]. Thecomparison between the conventional vehicle and the new HHVwas also presented [34]. The results indicated that the newHHV hasadvantage in fuel economy. The energy storage unit of the newHHVhas also been investigated and the series accumulator is much su-perior to the parallel accumulator in the hydraulic transformerpulsation damping [35]. The theory of limit cycles has been appliedto the analysis of the newHHV [36]. The existence of limit cycles andstability of equilibriumpoints in the systemwere discussed indetail.

In a conventional HHV, the operation of the hydraulic pump ormotor is based on the flow balance. The speed and flow of the

hydraulic pump are known since the hydraulic pump is mechani-cally connected to a crank engine. Different from the conventionalHHV, one of the key components of the new HHV is the HT. The HTis performed based on the torque balance of the cylinder, which isdetermined by the CPR pressure, the hydraulic motor pressure, thedisplacements of the HT ports and the friction resistance. Further,there is no mechanical connection between the cylinder of the HTand the engine. The HT control is also different from the conven-tional HHV equipped with the hydraulic pump or the hydraulicthrottle-valve. In this paper, a method for the parameter design ofthe new HHV is presented. The dynamic characteristics of the Hyps(HYdraulic HYbrid Propulsion System) are investigated. The simu-lated and tested results are proposed. It is aimed to provide aneffective method for the hydraulic hybrid propulsion systemdesign.

2. Design parameter analysis of the Hyps

2.1. Analytical formulations for the Hyps

The Hyps for automobiles consists of hydraulic CPR, HT andhydraulic pump or motor. The CPR is composed of hydraulic power

Page 3: Investigation of energy efficient hydraulic hybrid propulsion system for automobiles

Fig. 2. Operation principle of the Hyps for the automobile application. q is the HTcontrol angle of the swash plate. nHT is the HT cylinder speed. nm is the hydraulic motorspeed. Tm is the hydraulic motor torque.

W. Wu et al. / Energy 73 (2014) 497e505 499

engine, high-pressure hydraulic accumulator, low-pressure hy-draulic reservoir and common pressure circuit. The hydraulic po-wer engine is a single piston HFPE (hydraulic free-piston engine)group. The HT is a swash plate hydraulic transformer. The high-pressure oil of the CPR is adjusted by the HT to satisfy therequirement of the hydraulic motor. The adjustment is madethrough a constant power method. Fig. 1 shows the architecture ofthe Hyps for the automobile application. It seems that there is nomechanical connection between the HFPE and the hydraulic motor.The hydraulic motor can also be operated as a pump.

The output torque-speed relation of the Hyps is shown in Fig. 2[32]. It seems that the Hyps can drive the vehicle to move back-wards without the reverse gear. In the first quadrant, the HTcontrol angle is positive and the HT cylinder speed is clockwise.The port B of the HT deliveries the pressure oil to the hydraulicmotor for the forward propulsion. In the second quadrant, the HTcontrol angle is negative. However, the HT cylinder still rotates in aclockwise direction. The Hyps operates in a kinetic energy recov-ery mode and the pressure oil flows to the port T. The hydraulicmotor performs the pump function for the forward braking. In thethird quadrant, the HT control angle is negative and the HT cyl-inder speed is anticlockwise. The port T deliveries the pressure oilto the hydraulic motor for reverse propulsion. In the fourthquadrant, the HT control angle is positive and the HT cylinder stillrotates in an anticlockwise direction. The Hyps operates in thesame kinetic energy recovery mode as the mode in the secondquadrant.

Displacement is one of the most important parameters of thepositive displacement hydraulic component. The HT displacementsare expressed as [33]:

VA ¼ V sinaA2

sin q (1)

Fig. 1. Architecture of the Hyps for the automobile application. H represents the high-pressure rail. L represents the low-pressure rail. (a) Passively differential type. (b)Actively differential type.

VB ¼ �V sinaB

2sin

�aT2

þ q�

(2)

VT ¼ V sinaT

2sin

�aB2

� q�

(3)

where, VA, VB and VT are the displacements of the HT ports A, B, andT, respectively. V is the displacement of the proposed swash plateHT basal body. The proposed swash plate HT basal body is an axialpiston pump or motor. aA, aB and aT are thewrap angles of the portsA, B, and T, respectively.

IdealflowratesQA,QB andQTof the three ports on theHTare [33]:

QA ¼ nHTVA (4)

QB ¼ nHTVB (5)

QT ¼ nHTVT (6)

Without considering the power loss, the energy conservation ofthe HT is:

ðpAVA þ pBVB þ pTVTÞuHT ¼ 0 (7)

where,pA,pB andpT are the pressures of the HT ports A, B, and T,respectively. The ideal pressure ratios of the proposed swash plateHT are [33]:

pBpA

¼ � sin q

sin�aT2 þ q

� (8)

pTpA

¼ � sin q

sin�aB2 � q

� (9)

The hydraulic motor is described by:

Qm ¼ Vmnm (10)

Page 4: Investigation of energy efficient hydraulic hybrid propulsion system for automobiles

W. Wu et al. / Energy 73 (2014) 497e505500

Tm ¼ DpmVm

2p(11)

Fig. 4. The calculated the HT nominal power for the Hyps.

where, Qm is the hydraulic motor flow rate, Vm is the hydraulicmotor displacement, Dpm is the pressure difference of the hydraulicmotor.

Equation (12) is a well-known longitudinal vehicle dynamicmodel [37], which is used to analyse the propulsion performance ofthe Hyps.

FT ¼ Ff þ Fi þ Fw þ Fj (12)

where, FT is the driving force, Ff is the rolling resistance force, Fi isthe road grade force, Fw is the aerodynamic drag force, Fj is theacceleration force. The details of the forces are [37]:

FT ¼ fmg cos aþmg sin aþ CDAu2

21:15þ sm

dudt

(13)

where,m is the vehicle mass, g is the acceleration of gravity, f is therolling resistance coefficient, a is the road grade, CD is the air dragcoefficient, A is the vehicle frontal area, u is the vehicle speed, s isthe mass factor of rotating components, t is the time.

2.2. Gradeability parameters of the Hyps

The gradeability is an important parameter which reflects thevehicle longitudinal dynamic performance. The aerodynamic dragforce can be ignored since the vehicle speed is lower duringclimbing. Combining Eqs. (8), (12) and (13), the force balance dur-ing climbing is given by:

pANmVmi0 sin q

2pr sin�aT2 þ q

� ¼ mgðf cos aþ sin aÞ (14)

where, Nm is the number of the hydraulic motor, i0 is the final driveratio, r is the wheel radius. Then, the climbing capability can beexpressed as:

i ¼ tan amax ¼ Dmax � fffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffif 2 � Dmax

2 þ 1q

fDmax þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffif 2 � Dmax

2 þ 1q (15)

Fig. 3. The calculated climbing capability.

Dmax ¼ FTmax

G(16)

FTmax¼ Fdlmax (17)

Fd ¼ pANmVmi02pr

hmt (18)

Where, i is the maximum slope ratio, amax is the maximum roadgrade,Dmax is the maximum power factor, G is the vehicle gravityforce,qmax is the HT control angle corresponding to the maximumpressure ratio,FT is the maximum driving force,lmax is themaximum pressure ratio. Fd is defined as the feature driving forcewhen the pressure ratio is 1.0. NmVmi0 is defined as the nominaldisplacement Vmn of the hydraulic motor. hmt is the total efficiencyof the hydraulic motor.

Fig. 3 shows the variation of the climbing capability withdifferent lmax and Fd/G. Base on the vehicle mass and the maximumclimbing capability, the feature driving force can be calculatedthrough Eq. (15)e(18). When the HT works at the maximumpressure ratio, the hydraulic motor working pressure should besmaller than the maximum pressure:

pA � pmr

lmax(19)

where, pmr is the hydraulic motor rated pressure. The high pressureof the CPR is determined by Eq. (19) since the hydraulic motor ratedpressure is fixed.

2.3. Power parameters of the Hyps

The flow conservation between the HT and the hydraulic motoris:

NmVmnm ¼ NHTnHTjVBj (20)

Table 1The maximum speed of the constant displacement axial piston pumps.

Displacement (mL/r) 32 45 56 63 80Maximum speed (r/min) 6900 6200 5500 5500 5000Displacement (mL/r) 90 107 125 160 180Maximum speed (r/min) 5000 4400 4400 4000 4000

Page 5: Investigation of energy efficient hydraulic hybrid propulsion system for automobiles

Fig. 5. The parameter design flow chart for the Hyps.

Table 3Calculated parameters of the Hyps with the proposed method.

Parameter Presented no.1 Ref. [34] Presented no.2

PHT 172 kW e 172 kWi0 1.0 1.0 4.8QHT 366 L min�1 e 366 L min�1

Vm 236 mL r�1 e 50 mL r�1

NHT � V 2 � 61 mL r�1 2 � 60 mL r�1 1 � 157 mL r�1

Nm � Vmn 4 � 59 mL r�1 4 � 65 mL r�1 2 � 25 mL r�1

W. Wu et al. / Energy 73 (2014) 497e505 501

where,NHT is the number of the HT. Combining Eqs. (2) and (20), thehydraulic motor speed is:

nm ¼ nHTNHTVNmVm

sinaB

2sin

�aT2

þ q�

(21)

Then, the vehicle speed can be expressed as:

u ¼ nHT3pr25i0

NHTVNmVm

sinaB

2sin

�aT2

þ q�

(22)

Combining Eqs. (8) and (13), the longitudinal force balance isexpressed as:

pANmVmi0 sin q

2pr sin�aT2 þ q

� ¼ Gðf cos aþ sin aÞ þ CDA21:15

u2 (23)

Equation (23) can get simplified as:

sin�aT2

þ q�¼

ffiffiffi3

p

2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiU2 � Uþ 1

p (24)

where,

U ¼2pr

�f cos aþ sin aþ CDA

21:15u2

G

�NmpAVmi0=G

(25)

Table 2Parameters of the vehicle proposed by Ref. [34].

Parameter Value Unit

m 1554 kgA 2.26 m2

CD 0.28 e

f 0.008 e

r 0.315 mumax 220 km/h

To prevent the HTcylinder from speeding, the vehicle maximumspeed should be:

umax � 9prNHTV

100Nmi0Vm

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiU2 �Uþ 1

p nHTmax (26)

where,umax is the maximum vehicle speed,nHTmax is the HTmaximum cylinder speed. When the vehicle reaches the maximumspeed, the aerodynamic drag force becomes the main resistanceand the smaller rolling resistance force can be neglected. Then, Eq.(25) is simplified as:

U0 ¼ 2pr CDA21:15umax

2

pAVmNmi0(27)

Combining Eqs. (26) and (27),

PHT �200Fdumax

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�CDAumax

2

21:15Fd

�2

� CDAumax2

21:15Fdþ 1

s

9hHTmax(28)

PHT ¼ pAQHT (29)

QHT ¼ NHTnHTmaxV (30)

where,PHT is the HT nominal power for the Hyps, hHTmax is the totalefficiency of the HT at the nominal power, QHT is the HT nominalflow for the Hyps. The Eq. (28) indicates that the HT nominal poweris mainly determined by the feature driving force and themaximum vehicle speed. Fig. 4 shows the variation of the HTnominal power with the feature driving force and the vehiclespeed. The range of the feature driving force is 1.0e4.0 kN. Thecorresponding vehicle mass and the dynamic factor are1200e2400 kg and 0.3e0.5, respectively. The HT nominal power

Fig. 6. Block diagram of the Hyps control method.

Page 6: Investigation of energy efficient hydraulic hybrid propulsion system for automobiles

Fig. 7. Test rig for the Hyps. (a) Experimental schematic diagram. (b) Experimentalapparatus.

Fig. 8. Driving performances of the Hyps with the presented parameters. (a) Vehicle speedresults of the test rig during the driving mode.

W. Wu et al. / Energy 73 (2014) 497e505502

and the HT nominal flow for the Hyps can be calculated through Eq.(28)e(29).

3. The Hyps parameter design method and validation

When the vehicle reaches the maximum speed, the hydraulicmotor should not be speeding. Table 1 presents the maximumspeed of the hydraulic motor. The required hydraulic motor speednmr is given by:

nmr ¼ umax

0:377ri0 (31)

Further, when the port A pressure is confirmed, the nominaldisplacement of the hydraulic motor becomes a constant and isgiven by:

Vmn ¼ Nmi0Vm (32)

Then, the required hydraulic motor speed is expressed as:

nmr ¼ 16:66Vmnumax

NmVm2pr(33)

When the gradeability and maximum vehicle speed are given,the relation between the hydraulic motor displacement and itsrequired speed is confirmed.

The parameter design flow chart for the Hyps is shown in Fig. 5.To validate the proposed parameter design method, the vehicleparameters presented in the Ref. [31] are used, as shown in Table 2.

vs. time. (b) Driving force vs. time. (c) Hydraulic motor speed vs. time. (d) Measured

Page 7: Investigation of energy efficient hydraulic hybrid propulsion system for automobiles

W. Wu et al. / Energy 73 (2014) 497e505 503

In the calculation, the maximum pressure ratio is 2.0 and the cor-responding total efficiency of the HT is about 94%. The total effi-ciency of the hydraulic motor is 94%. The high pressure of the CPR is20e40 MPa. The maximum slope ratio is 30%. When the Hyps usesthe actively differential method, the final drive ratio selects 1.0. Thefinal drive ratio uses 4.8 when the Hyps uses the passively differ-ential method in the calculation. The calculated parameters for theHyps are presented in Table 3. The parameters presented in theRef. [34] are also given. In order to validate the calculated results inTable 3, a numerical model of the vehicle equipped with the Hypshas been built in the AMESim environment. The block diagram ofthe Hyps control method is presented in Fig. 6. Further, a test rig,which is mainly composed of HT, hydraulic pump ormotor and CPR,has also been built, as shown in Fig. 7.

Fig. 8 presents the results of the accelerated process underdifferent parameters of the Hypes during the driving mode. Itseems that the drive system can achieve the wanted vehicle per-formance with both parameters presented in Table 3. The increaseof the DOF (degree of freedom) of the Hyps makes the parameterdesign become more flexible. The proposed method for the Hypsparameter design is feasible. The Hyps achieves a constant torque

Fig. 9. Drive train performances of the Hyps during the FTP-75 cycle. (a) Simulated vehicleresults of the test rig during the regenerative braking mode.

at the low speed stage and a constant power at the high speedstage. Due to the constant power characteristic of the HT, the idealvehicle dynamic performance can be guaranteed. Based on thetwo different parameters presented in Table 3, the operationspeeds of the hydraulic motor are different. The design parameterof the hydraulic motor should be optimised considering thevehicle speed range, since the operation efficiency of the hydraulicmotor is closely related to its speed. The optimisation is helpful toimprove the efficiency of the driving system for different vehicletypes. The measured results of the test rig during the driving modeare proposed in Fig. 8(d). In the early stage, due to the smallerinertia of the HT, the HT speed increases more rapidly. Then, thehydraulic motor speed changes the same as the HT speed gradu-ally. Both speeds are closely related to the variation of the HTcontrol angle. During the test, the port A pressure wave affects thestability of the speeds.

Fig. 9 presents the simulated results of the Hyps under the FTP-75 driving cycle. The measured results of the test rig during theregenerative braking mode are also given in Fig. 9 (d). It is shownthat the drive systems with both parameters satisfy the vehiclerequirement. The HT speed is closely related to the vehicle speed,

speed. (b) Simulated HT speed. (c) The HT speed fluctuations enlarged. (d) Measured

Page 8: Investigation of energy efficient hydraulic hybrid propulsion system for automobiles

Fig. 10. Measured hydraulic system characteristics of the Hyps. (a) Measured speeds of the HT and the hydraulic motor. (b) Measured pressures of the HT ports. (c) Measured flowrates of the HT ports. (d) The control signal of the HT control angle during tests.

W. Wu et al. / Energy 73 (2014) 497e505504

which is validated by the simulation and test. It should be consid-ered in the HT design since the speed of the HT is limited. Duringthe conversion from the driving mode to the regenerative brakingmode, the HT speed fluctuation appears. And this fluctuation isinherent, as shown in Fig. 9 (c) and (d). It results in a fluctuation ofthe braking torque of the Hyps during the regenerative brakingmode. Due to the much larger inertia connected with the hydraulicmotor, the HT speed fluctuation appears. However, the speed of thehydraulic motor will fluctuate if the HT inertia is larger than thehydraulic motor. It is unhelpful for the service life and reliability ofthe hydraulic components.

Fig. 10 gives the tested results the Hyps from acceleration todeceleration. The regenerative braking was applied during thedeceleration test. Under the step input of the HT control signal,the HT speed responses quickly. As the HT speed become stable,the hydraulic motor speed tends to be stable. During the regen-erative braking around 120 s, the port B becomes a low-pressureand the port T pressure increases higher. The port A connected tothe accumulator absorbs the kinetic energy and the port A pres-sure increases. It seems that the HT efficiency has a greater in-fluence on the energy recovery efficiency. As soon as the HT stops,the energy recovery terminates. During the test, the system effi-ciency is low. The optimisation of the whole system is needed.Further, an accumulator with a higher energy density is especiallypractical. James [38] has proposed a constant pressure accumu-lator which improves the energy density by 16% over conventionalaccumulators. It eliminates pressure variation in the hydraulicsystem and allows all components of the Hyps to be sized for asingle pressure.

4. Conclusions and future work

In this research, the parameter design and the propulsioncharacteristics of the Hyps were investigated. Two typical types ofthe Hyps for automobiles were presented. The parameter designmethod for the Hyps was given. The simulated and tested resultshave been proposed. The results suggest the following.

(1) The increase of the DOF of the Hyps makes the parameterdesign become more flexible. Based on the definitions of theHT normal power and the HT normal flow, the proposedmethod for the Hyps parameter design is feasible. The speedlimitation of the hydraulic components is considered in themethod.

(2) The Hyps achieves a constant torque at the low speed stageand a constant power at the high speed stage. The idealvehicle dynamic performance can be guaranteed by theHypes.

(3) The HT speed is closely related to the vehicle speed. It shouldbe considered in the HT design since the speed of the HT islimited. During the conversion from the driving mode to theregenerative braking mode, the fluctuations of the HT speedand hydraulic motor torque are inherently.

The results proposed can be used for the hydraulic hybrid pro-pulsion system design of automobiles. However, the dynamiccharacteristics of the hydraulic transformer-controlled hydraulicmotor system and the energy management need more effort toinvestigate. Work on these topics is currently underway in the

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W. Wu et al. / Energy 73 (2014) 497e505 505

National Key Laboratory of Vehicular Transmission at the BeijingInstitute of Technology.

Acknowledgement

This work is supported the National Natural Science Foundationof China (Grant No. 51305034), and the Doctoral Fund of Ministry ofEducation of China (Grant No. 20131101120013). The authors thankreferees for improving the quality of the paper.

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