Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition IMECE2011 November 11-17, 2011, Denver, Colorado, USA

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1 Procings of th ASME 2011 Intrnational Mchanical Enginring Congrss & Exposition IMECE2011 ovmbr 11-17, 2011, Dnvr, Colorao, USA IMECE EVIEW OF COTIUOUSLY VAIABLE TASMISSIO POWETAI SYSTEM FO HYBID ELECTIC VEHICLES Jian Dong Univrsity of Victoria Victoria, BC, Canaa Zuomin Dong Univrsity of Victoria Victoria, BC, Canaa Curran Crawfor Univrsity of Victoria Victoria, BC, Canaa ABSTACT In this papr, a rviw of th stat-of-th-art of various CVT powrtrain systms now us or bing plann for futur us in HEVs is prsnt. Ths CVT powrtrain systms ar classifi into thr main catgoris: mchanical CVT, lctromchanical CVT(ECVT) an pur lctrical CVT(EVT). Th rsarch vlopmnt, systm architctur, opration charactristics an th mrits an rawbacks of ach typ ar iscuss. ITODUCTIO Continuously Variabl Transmission(CVT) ar bing vlop in conjunction with Hybri Elctric Vhicls(HEV), which hav provi grat potntial for bttr ful conomy an lowr missions. Th us of CVT is optimal for HEV: th lctric portion of th HEV powrtrain systm avois th low-sp/high-torqu problms of CVTs, whil still attaining bttr ful conomy than othr transmissions at high sps by nabling th ngin to run at its most fficint rgim. Morovr, th intgration of lctric machin nabls th function of CVT, startr an gnrator, an is spcially suitabl for HEVs. In this papr, a rviw of th stat-of-th-art of various CVT powrtrain systms now us or bing plann for futur us in HEVs is prsnt. Ths CVT powrtrain systms ar classifi into thr main catgoris: mchanical CVT, lctromchanical CVT(ECVT) an pur lctrical CVT(EVT). Th rsarch vlopmnt, systm configuration, opration charactristics an th mrits an rawbacks of ach typ ar iscuss. Th mchanical typ CVT powrtrain systm us in HEV in this papr inclus mtal blt CVT, which ar usually us in paralll with an lctric motor. By conncting to a Plantary Gar St(PGS), th blt CVT can b convrt into an Infinitly Variabl Transmissions(IVT) capabl of proviing a full rang of ratio from rvrs, through stationary to forwar. Th mchanical typ CVT powrtain systms without PGS an with PGS(hnc IVT) us in HEV ar xplor rspctivly. Th lctromchanical typ CVT, or powr-split lctronic-cvt(ecvt) powrtrain systm consists of an lctric variator an svral PGSs. Accoring to th powr flow typ, thr catgoris of ECVT powrtrain systms ar iscuss an compar: th input-split ECVT, th output-split ECVT an th compoun-split ECVT. Th on-mo input-split an output-split ECVT contains on PGS an two motors, whil th multi-mo compoun-split ECVT contains at last two PGSs. Th connction topology, torqu an sp rlationship of ach mo of th thr ECVT powrtrain systms is analyz an th prformancs of th thr systms ar compar. Th pur lctrical typ CVT, or EVT powrtrain systm is a class of rcntly vlop propulsion systms, which basically compris concntrically arrang oubl-rotor or oubl-stator lctric machins to prform powr split. Thy contain no mchanical PGS an thrfor ruc mchanical friction, avoi auibl nois an achiv systm simplicity. Discussion focuss on th systm configuration, powr flow an torqu rlationship an oprational faturs. MECHAICAL TYPE CVT POWETAI SYSTEM I HEV Th mchanical typ CVT powrtrain systm us in HEV in this papr inclus mtal blt CVT, which ar usually us in paralll with an lctric machin. Th commrcially availabl blt typ CVT has valuat to a lctronic controllabl transmission systm sinc 1980s, bas on a mtal blt, which has highst fficincy among othr blt typ CVTs or traction bas CVTs. Mchanical mtal blt typ CVT can provi smooth, samlss shifts an improv ful 1 Copyright 2011 by ASME

2 fficincy. Thy ar currntly prouc by Jatco, Aisin, ZF, Toyota, Subaru, Daimlr Chryslr, Aui, tc[1]. For HEV powrtrain systms, th low-cost mchanical mtal blt CVT is combin to an lctric machin in an paralll way. Th lctric machin(motor/gnrator) is usually coupl btwn th clutch an th CVT at th ngin si. This allows th ngin to b turn off uring th propulsion phass sinc th vhicl can b propll only by th lctric machin. An aitional startr-altrnator is n for ngin rstart uring stanstill. Figur 1 shows th configuration of such a powrtrain systm. clutch call acclration clutch (also known as th start clutch) plac on th CVT rivn shaft. Fig 1. Blt CVT hybri lctric powrtrain systm Hona IMA Blt CVT Hybri Powrtrain A similar architctur is Hona s Intgrat Motor Assist(IMA) powrtrain systm, which intgrats a prmannt magnt synchronous machin into th CVT[2], as shown in Figur 2. Unlik Toyota s powr-split ECVT powrtrain systm, which will b iscuss in th nxt sssion, Hona s IMA uss matur an low-cost blt CVT tchnology with a much smallr compact lctric machin, which savs cost an rucs systm complxity an siz. Fig 3. Hona s IMA CVT hybri lctric powrtrain Figur 5 shows th compact Prmannt Magnt Synchronous Machin(PMSM) sanwich btwn th ICE an th CVT. Th motor is intgrat into a wi cas that's bolt irctly to th output fac of th ngin. Th stator is fix to th intrnal primtr of th cas whil th rotor is bolt to th output fac of th crankshaft. Th input plat for th CVT is bolt to th output si of th rotor[4]. Howvr, u to th rlativly limit lctric powr by th compact machin, th ovrall prformanc might b pnaliz[1]. Fig 2. Hona IMA CVT hybri lctric powrtrain[3]. Th architctur is quit th sam as in Figur 1, as can b sn from Figur 3. Th iffrnc is that th lctric machin (motor/gnrator) in Hona s IMA systm is irctly connct to th ICE crankshaft. Th lctric machin act as a startr motor, ngin balancr, an assist traction motor. o aitional startr-altrnator is n. In aition, thr is a multi-layr Fig 4. Compact PMSM in Hona IMA. Sinc th motor is har-coupl to th ICE, initially Hona s IMA coul not powr th car on lctricity alon. Th motor can only b us to assist or start th ngin. Through Hona s nw i-vtec ngin aroun th cor of its valv-control tchnology vlop in rcnt yars, th ICE can b coupl from th motor by activating all th cylinrs in th ngin. In this way th vhicl can b propll by lctricity alon. Howvr, th har-coupling mans that th crankshaft is still turning an th pistons ar pumping, which might affct comfortnss. From 2001, th subsqunt Insight an Civic hybris wr availabl with CVT. Th 4 th gnration IMA uss th Multimatic CVT. Figur 4 shows th CAD graphic of th CVT in Hona s IMA systm. 2 Copyright 2011 by ASME

3 2.5L isl ngin 80kW M/G In Lin blt CVT Fig 5. Mtal blt CVT in Hona IMA[5] Th CVT in Hona s IMA works by varying th position of a high-strngth stl blt btwn th cons. On con conncts th powr input of th transmission an th othr conncts to th powr output si of th transmission. Through various guis within th transmission, th blt can sli up an own th cons to crat th most suitabl gar ratio for any sp an throttl input. Th n rsults for rivrs ar smooth, samlss shifts, bttr torqu an improv ful fficincy[6]. Th ky oprational faturs of th Hona IMA powrtrain systm ar list blow: Whn staring from a stanstill, th lctric motor livrs maximum torqu to assist th ICE for strong acclration an ruc ful consumption. During low-sp cruising, th motor alon can powr th vhicl, whil th valvs of all th cylinrs in th ICE ar clos an combustion halt. During gntl acclration an high-sp cruising, th vhicl runs on ICE powr alon. During rapi acclration, th motor an th ICE work togthr to provi powr. During clration, th motor is us as a gnrator for nrgy rcovry through rgnrativ braking. All th cylinr valvs in ICE ar clos an combustion halt. UC Davis In-Lin blt CVT plug-in Hybri Powrtrain Anothr similar architctur is th In-Lin blt CVT plug-in hybri lctric powrtrain systm vlop by UC Davis, as shown in Figur 6. Fig 6. UC Davis In-Lin CVT hybri lctric powrtrain[7]. Th configuration is th sam as in Figur 1, whil th CVT us hr is a oubl-chain In-Lin blt CVT. Figur 7 shows th powrtrain architctur. Th motor/gnrator is connct irctly to th In-Lin CVT input shaft, which can b us for ngin start, ngin powr assisting an rgnrativ braking. An th motor can propl th vhicl alon. This systm is shown to b abl to provi about 60 mils AE(All Elctric ang) in a Chvrolt Suburban[8]. Fig 7. UC Davis In-Lin CVT hybri lctric powrtrain. Th uniqu fatur of th powrtrain is its innovativ In-Lin CVT. Th nam In-Lin coms from th fatur that th input shaft an th output shaft in this construction ar in th sam lin. Two variators ar connct in srial. Figur 8 shows th In-Lin CVT without its outsi housing. Four pullys transfr th motor torqu to th whls via two GCI chains. Two lctro-hyraulic piston actuations ar us for CVT ratio control similar as for convntional singl-chain CVT. 3 Copyright 2011 by ASME

4 Fig 8. UC Davis In-Lin oubl-chain CVT Th powrtrain systm has a fw avantags. On of th avantags of th systm is that a wi ovrall ratio span can b achiv with a vry small movmnt of th clamping pistons an a small movmnt of th chain. Th motion is approximatly invrsly proportional to th squar root of th ratio, an it can b accomplish with much smallr iamtr pullys in comparison to singl-chain CVT. Th rason for this is that th ratio of th first chain an pully st is multipli by th ratio of th scon chain an pully st thus th maximum ratio span is th prouct of th ratio spans of th two pully sts. Equation 1 shows th rlationship of th ratio of th two chain sts an th ovrall ratio. 2 ( 1 ) total chain1 chain2 chain In this way, a small chang in imnsions can mak th squar of th ratio chang of a singl pully st CVT systm. Thus th concpt can b much mor compact than a singl-chain systm for th sam span of ratios. Sinc th inlin CVT is approximatly th sam siz as a manual transmission, it can convrt a convntional transmission vhicl to a hybri lctric simply by aing an lctric Traction motor in front of th CVT, btwn th ngin an CVT, thus liminating th n for an ngin starting vic. Blt CVT with Flywhl Hybri Powrtrain Anothr mtal blt CVT bas hybri lctric powrtrain with a flywhl has bn vlop in [9]. Figur 9 shows th configuration of th systm, which has a Plantary Gar St(PGS) btwn th motor/gnrator an th CVT an a flywhl btwn th two pully shafts of th CVT. Fig 9. Tongji Univrsity blt CVT hybri powrtrain Th Plantary Gar St(PGS) along with th forwar clutch connct to its carrir actually forms a irction changing mchanism, making th transmission systm bcom a Infinitly Variabl Transmissions(IVT) systm. o mattr th blt typ or th toroial typ CVT, th CVT alon can only provi positiv ratios. A rvrs gar an a clutch(or a torqu convrtr) ar thrfor n. Usually by conncting to a Plantary Gar St(PGS), th blt or toroial CVT can b convrt into an IVT capabl of proviing a full rang of ratio from rvrs, through stationary to forwar. Th ngin can rmain to b connct to th final riv prmanntly. Figur 10 shows th iffrnt ratio rangs of MT, CVT an IVT. Fig 10. atio rangs of iffrnt transmissions. Anothr fatur of this powrtrain systm is th flywhl connct btwn th primary pully shaft an th sconary pully shaft of th CVT through a plantary garing. Through configuring a suitabl sp ratio for th plantary garing, th flywhl can b ma to clrat whn rivlin acclrating, vic vrsa[9]. This mans that whn th vhicl acclrats, th flywhl can rlas its mchanical nrgy into th rivlin proviing assisting powr for acclration. Convrsly, whn th vhicl clrats, th flywhl can absorb th nrgy to hlp it clrat fastr an manwhil stor th mchanical nrgy. ot uring ths stags, thr s no convrsion btwn iffrnt nrgy for th flywhl. Th othr oprational faturs of th systm ar quit similar as thos scrib in Hona IMA. Th four oprational mos ar list blow: 4 Copyright 2011 by ASME

5 Motor mo: uring start-up an for low-sp cruising, th motor rivs th vhicl alon. Powr mo: th motor an th ICE powr th vhicl togthr for strong acclration. Engin/Charging mo: uring clration, th motor acts as a gnrator for nrgy rcovry through rgnrativ braking. Th clutch btwn th ICE an th motor cuts off th powr from ICE. Engin mo: th ICE rivs th vhicl alon. Th mrit of this CVT flywhl hybri lctric powrtrain is that it combins th battry-lctric an flywhl nrgy storag togthr for th hybri lctric vhicls. Battris ar grat for long-tim nrgy storag, whil flywhls ar goo for transint nrgy storag. On th on han, uring firc acclration/clration an th subsqunt quick charg/iscarg, th battry lif can svrly b shortn, whil flywhl nrgy storag systm provs mor fficint uring ths sharp swings of acclration/clration stats sinc it has no nrgy convrsion. On th othr han, flywhl systms hav nrgy losss mainly u to baring friction, which maks thm lss fficint than a battry-bas systm for storing nrgy for long prios of tim. Th combination is probably th ial solution for hybri lctric vhicls. ELECTOMECHAICAL TYPE CVT POWETAI SYSTEM I HEV Th powr-split CVT bas hybri lctric powrtrain hav bn vlop an us for cas in th markt, sinc Toyota first introuc its Prius to th worl in Th lctromchanical typ CVT, or powr-split lctric/lctronic CVT(ECVT) powrtrain systm consists of svral lctric machins an mchanical Plantary Gar Sts(PGS). Th ngin input powr is usually split into two flows an passs through both an lctric path(or lctric variator) an a mchanical path to th transmission output. Th powr-split systm combins th avantags of both th sris an paralll hybri powrtrain. Th mrits of such a powrtrain systm ar list blow: Th CVT natur of th systm nabls th ngin to always oprat at its most nrgy-fficint oprating point, hnc rsulting in a consirabl ruction of th ful consumption. Th ECVT powrtrain systms ar actually IVT systms, which can provi infinit variabl ratio btwn th ngin sp an th whl sp. Th absnc of aitional clutchs, torqu convrtrs or shifting gars can significantly improv th transmission fficincy an ruc th ovrall siz. Th ngin can b compltly shut own whn th vhicl is stopp, nabling th il stop function. Th ECVT powrtrain systm usually uss svral lctric Motor/Gnrator(MG1 an MG2) connct to th PGS. Th motor can b us to start th ngin, launch th vhicl, assist th ngin powr, provi rgnrativ braking or powr th vhicl alon with lctricity. Most convntional CVT signs ar friction-bas blt an traction typs that ar unsuitabl for high-torqu an high-powr applications[10]. Th limitation for torqu transfr capability can b improv in powr-split CVT, as th powr flow through th lctric variator cras. Accoring to th powr flow typ, thr catgoris of ECVT powrtrain systms ar iscuss an compar: th input-split ECVT, th output-split ECVT an th compoun-split ECVT. Th input-split an output-split ECVT systms ar also call on-mo systms, whil th compoun-split ECVT systms ar call multi-mo systms. Th on-mo input-split an output-split ECVT contains on PGS an two motors, whil th multi-mo compoun-split ECVT contains at last two PGSs. Th connction topology, static an ynamic charactristics of ach mo of th thr ECVT powrtrain systms is analyz an th prformancs of th thr systms ar compar. 1. Input-split ECVT Powrtrain Systm Th input-split ECVT powrtrain systm consists of on Plantary Gar St (PGS) an two lctric Motor/Gnrator(MG1 an MG2). Th Plantary Gar St(PGS) functions as a powr split vic, which is plac at th input of th transmission systm. Commrcial applications of th input-split ECVT powrtrain systm inclu Toyota Prius an For Escap. Th rprsntativ of th input-split ECVT powrtrain is th Toyota Hybri Systm(THS) an th For Hybri Systm(FHS). Th THS powrtrain configuration is shown in Figur 11. Th ngin shaft as th input shaft is connct to th carrir of th PGS. Th output shaft from th ring gar is connct to th riv shaft of th vhicl. MG1 is connct to th sun gar, which is us to function as a gnrator most of th tim to absorb mchanical powr from th ngin an supply powr to th battry. MG2 is connct to th output shaft an functions as a motor most of th tim to raw powr from ithr MG1 or th Enrgy Storag Systm(ESS) to provi powr to th vhicl. MG2 is also call th traction motor [11]. During vhicl clration, MG2 can also act as a gnrator for rgnrativ braking. As can b sn from Figur 11, uring powr splitting, th ngin powr as input powr is split into two flows: on powr flow circulats through an lctric path(also known as th lctric variator), which passs through Motor/Gnratr 1(MG1) an Motor/Gnrator 2(MG2), an th rst powr flow is livr through a mchanical path via th plantary gar st to th transmission output, i.. th final riv, irctly. Du to low lctric-mchanical nrgy convrsion fficincy, th lctric path is lss fficint than th irct mchanical path. That is to say, th lowr th powr passs through th lctric path, th highr th systm fficincy is. 5 Copyright 2011 by ASME

6 point is rach an th systm achivs its maximum fficincy. At this no point ratio, th sp of MG1 rachs zro. Abov this no point ratio, powr splitting in th forwar irction bgins as shown in Figur 11. As th sp ratio incrass, th powr through th lctric variator incrass an th transmission fficincy crass viating its maximum point. Th torqu an sp rlationship of th input, output, MG1 an MG2 of an input-split ECVT powrtrain systm at stay stat nglcting th inrtia trms for transint can b xprss as: Fig 11. Powr split in THS input-split ECVT powrtrain systm On of th important charactristics of th powr-split architcturs is that at crtain oprating points th powr through th lctric path bcoms zro, which mans all th powr is livr through th mchanical path an thrfor maximum systm fficincy occurs at ths points, sinc no transmitt powr is convrt into lctricity an back again. Such an oprating point is known as a no point or mchanical point [12]. An th input-to-output sp ratio whr th mchanical no point occurs is typically chosn to b in th frquntly us ara whn signing th on-mo hybri for goo ful conomy. Morovr, thr is anothr oprating situation whr th systm fficincy is inhrntly poor, known as powr rcirculation. Whn powr rcirculation happns, th powr flows from th output to th input in ithr th lctric path(known as ngativ rcirculation) or th mchanical path(known as positiv rcirculation). Figur 12 shows th powr rcirculation in th input-split ECVT powrtrain systm. A portion of th ngin powr is rcirculat through th MG2 an MG1. Th powr rcirculation will ruc th output powr to th final riv an so crass th transmission fficincy [12]. s ( 2 ) r 1 T T MG 2 T 1 ( 3 ) T ( 4 ) MG1 MG2 T ( 5 ) MG1 (1 ) MG2 ( 6 ) whr is th plantary gar ratio which is th ratio btwn th numbr of tth of th sun gar an th ring gar, T is th output rivlin torqu, T MG2 is th torqu of Motor/Gnrator 2(MG2), T is th ngin input torqu, is th rivlin output sp, MG2 is th sp of MG2, T MG1 is th torqu of Motor/Gnrator 1(MG1), MG1 is th sp of MG1, an is th ngin input sp. Dtail rlationship quations incluing all ynamic ffcts can b foun in [11]. 2. Output-split ECVT Powrtrain Systm Similar to th input-split systm, th output-split ECVT powrtrain systm consists of on Plantary Gar St(PGS) an two lctric Motor/Gnrator(MG1 an MG2). Howvr, in th ouput-split systm, th Plantary Gar St(PGS) as a powr split vic is plac at th output of th transmission systm. Fig 12. Powr rcirculation in th input-split ECVT systm For th input-split ECVT powrtrain systm, thr is only on mchanical no point, which occurs at crtain input-to-output sp ratio. Th powr flow an th systm fficincy vary with th sp ratio. Blow th no point ratio(whn th vhicl sp is rlativly high), powr rcirculation occurs as shown in Figur 12 an th systm fficincy is low. As th sp ratio incrass(th vhicl sp crass), th mchanical no Th commrcial application of th output-split ECVT powrtrain systm is not as popular as th othr typs of powr split ECVT systms u to its limit fasibl ratio rang. Chvrolt Volt utilizs an output-split transmission systm for low torqu opration uring highway cruising[13]. Figur 13 isplays a similar output-split powrtrain configuration. Th ngin shaft as th input shaft is connct to th carrir of th PGS. Th output shaft from th ring gar is connct to th riv shaft of th vhicl. MG2 is connct to th input shaft an functions as a gnrator most of th 6 Copyright 2011 by ASME

7 tim to absorb mchanical powr from th ngin an supply powr to th battry. MG1 is connct to th sun gar, which is st to function as a motor most of th tim to provi powr to th vhicl. Fig 13. Powr split in th output-split ECVT powrtrain systm Similar to th input-split systm, uring powr splitting of th output-split ECVT systm, th ngin powr as input powr is split into two flows: on powr flow circulats through an lctric path(also known as th lctric variator), which passs through Motor/Gnratr 2(MG2) an Motor/Gnrator 1(MG1), an th rst powr flow is livr through a mchanical path via th plantary gar st to th transmission output, i.. th final riv, irctly. Again, th lowr th powr passs through th lctric path, th highr th systm fficincy is. For th output-split ECVT powrtrain systm, thr is only on mchanical no point at crtain input-to-output sp ratio. Th powr flow an th systm fficincy vary with th sp ratio. In contrast to th input-split systm, th output-split systm has powr rcirculation abov th no point ratio an th powr splitting in th forwar irction bgins blow this ratio. Whn abov th no point ratio(at low vhicl sp), powr rcirculation occurs as shown in Figur 14 an th systm fficincy is low. A portion of th ngin powr is rcirculat through MG1 an MG2. As th sp ratio crass(th vhicl sp incrass), th mchanical no point is rach an th systm achivs its maximum fficincy. At this no point ratio, th sp of MG1 rachs zro. Blow this no point ratio, powr splitting in th forwar irction bgins as shown in Figur 13. As th sp ratio crass, th powr through th lctric variator incrass an th transmission fficincy crass viating its maximum point. Fig 14. Powr rcirculation in th output-split ECVT systm For th output-split systm, th main rawback lis in that its practical sp ratio rang is vry limit. Th sp ratio cannot b too high or th systm fficincy will b amag u to incrasing rcirculating powr. On th low si of th sp ratio, thr s also a limit u to incrasing MG1 sp. Manwhil,th torqu capability of th two motors within th ratio rang is vry limit, making it impractical for high torqu application. Th torqu an sp rlationship of th input, output, MG1 an MG2 of an output-split ECVT powrtrain systm at stay stat nglcting th inrtia trms for transint can b xprss as: 1 T ( T TMG2 ) 1 ( 1)( ) ( 8 ) MG1 TMG1 ( T TMG2) 1 MG2 ( 10 ) ( 7 ) ( 9 ) 3. Compoun-split ECVT Powrtrain Systm Eithr th input-split or th output-split ECVT powrtrain systm allows for only on mchanical no point, whr th systm achivs its maximum fficincy. As th vhicl loas incrass, th systm fficincy movs away from its optimal point. Th aition of anothr mchanical no point can incras th transmission fficincy for a wi rang of sps. Th systm oprats most fficintly btwn ths two points. Th systm fficincis byon th two points ar low. Usually by aing mor Plantary Gar Sts, aitional mchanical no points can b crat at th cost of highr systm complxity an componnts cost. Th compoun-split ECVT powrtrain systm consists of at last two PGS an two or mor lctric Motor/Gnrator(MG1 an MG2) capabl of proviing two or mor mchanical no points. Th systm can oprat in multipl mos by aing multipl PGS, clutchs an/or braks. Thy ar also call multi-mo hybri powrtrain systms. For this kin of ECVT powrtrain systm, th 7 Copyright 2011 by ASME

8 powr split vics ar plac at both th input an th output of th transmission systm. Commrcial applications of th multi-mo ECVT hybri powrtrain systm inclu Allison E V Driv, GM Taho, Chryslr Aspn, BMW X6, Mrcs ML450 an tc. Th rprsntativs of th compoun-split (or multi-mo) ECVT powrtrain systm ar th two-mo Allison Hybri Systm(AHS). Th two-mo ECVT was vlop an prouc by Gnral Motors for transit buss an full-siz SUVs from Figur 15 shows th schmatic cross sction of th two-mo hybri systm with fix gar ratios(also known as th Allison Hybri systm-2(ahs2)) vlop by GM. Th sign is vlop to provi highr towing capacity an bttr acclration prformanc for full-siz SUVs incluing th GMC Yukon an Chvrolt Taho[15]. Th systm can oprat in two ECVT mos(th input-split low rang mo an th compoun-split high rang mo) an four fix gar ratio mos through propr slction of th clutch application [16]. Fig 15. Two-mo hybri systm with fix gar ratios(gm Allison Hybri Systm-2)[15]. As shown in Figur 15, th transmission systm contains two 60kW prmannt magnt motors(mg1 an MG2) an thr Plantary Gar Sts(PGS) with four clutchs. Th ngin is connct to th ring gar of th first PGS. Th carrirs of th first an th scon PGS ar connct to th main transmission shaft. Th ring gar of th scon PGS is connct to th sun gar of th first PGS an MG1. Th sun gar of th scon PGS is connct to th sun gar of th thir PGS an MG2. Th output rivshaft to th whl is connct via clutch C2 to th carrir of th thir PGS. Th ring gar of th thir PGS is connct via clutch C1 to a fix mmbr. In aition, clutch C3 an C4 ar a in th sign for proviing th four fix gar ratios that allow th transmission to run in paralll mo. In th input-split low rang mo, clutch C1 is ngag an th othr thr clutchs ar isngag. Th systm configuration in low rang mo is shown in Figur 16. Comparing with th input-split systm shown in Figur 11, th two-mo AHS-2 in its low rang mo provis th sam input-split rgim. Th thir PGS acts as a torqu multiplir. During powr splitting, MG1 oprats as a gnrator to absorb a potion of th mchanical powr from th ngin an provi powr to th ESS, whil MG2 oprats as a motor to provi powr to th output rivshaft via torqu multiplir PGS3. Thr is on mchanical no point in th input-split low rang mo, whr th sp of MG1 rachs zro. Th no point in low rang mo is sign to b at a high numrical transmission ratio corrsponing to a rlativly low vhicl sp, so that a lowr amount of motor powr is rquir to mt th output powr man as it i in a comparabl on-mo input-split systm[15]. Fig 16. Th two-mo AHS-2: input-split low rang mo In th input-split low rang mo, th torqu an sp rlationship of th input, output, MG1 an MG2 of th two-mo ECVT powrtrain systm can b xprss as: a r1, s1 s2 s1 r2 b, c f s1 r1, s1 s3 r3, s3 b T T T 1 bc r 2, r3 ( 11 ) s1 r2 MG1 MG2 1 MG2 s3 ( 12 ) ( 13 ) 1 bc TMG 1 ( T J ) ( 14 ) a b a MG1 1 bc 1bc ( 15 ) whr r1 is th numbr of th ring gar tth of PGS1, s1 is th numbr of th sun gar tth of PGS1, r 2 is th numbr of th ring gar tth of PGS2, s2 is th numbr of th sun gar tth of PGS2, r3 is th numbr of th ring gar tth of PGS3, s3 is th numbr of th sun gar tth of PGS3. Whn clutch C1 is isngag an C2 is ngag, th systm is shift to its compoun-split high rang mo incluing both input an output split. Figur 17 shows th systm configuration in high rang mo. Th connction of th carrirs an th sun gars of th scon an th thir PGS actually mak PGS2 an PGS3 work synchronously. Th two PGS togthr form a powr splitting vic at th output of th transmission. 8 Copyright 2011 by ASME

9 Fig 17. Th two-mo AHS-2: compoun-split high rang mo In th compoun-split high rang mo of AHS-2, thr ar two mchanical no points. Btwn ths two points, th powr flow is forwar an th systm oprats most fficintly. Outsi th rang of th two points, powr rcirculation occurs an th systm fficincis ar low. In th compoun-split high rang mo, th torqu an sp rlationship can b xprss as: 1 bc T btmg1 TMG2 ( 16 ) ac MG2 1 bc 1bc ( 17 ) 1 c 1 TMG1 T TMG2 J ( 18 ) a a b a ( 19 ) MG1 Th AHS-2 allows for thr mchanical no points(on in low rang mo, two in high rang mo) for vhicl prformanc with high fficincy ovr a wi rang. Th aitional no point in th compoun-split rang is vry ncssary for SUVs with a high loaing rquirmnt. In a normal light-uty vhicl opration, th only mchanical no point for th on-mo systm will occur at a high nough sp, which won t wakn th prformanc gratly. Howvr, for a commrcial vhicl with a full payloa, th only no point will occur at a low sp rsulting in a low fficincy at morat an high sps. Bsis th two powr-split ECVT mos, anothr four fix gar ratio mos can b activat pning on th powr man by th rivr through propr activation of clutch C3 an C4. Th gar ratio mos can occur at any point within th two ECVT mos. Th aition of th four fix gar allows highr towing capacity an bttr acclration prformanc. Morovr, xtrm continuous-uty motor rquirmnts ar ruc or liminat without sacrificing ful conomy, allowing th us of smallr motors with larg ngins[15]. PUE ELECTICAL TYPE CVT POWETAI SYSTEM I HEV Th main rawback of th abov-mntion lctromchanical typ ECVT powrtrain systm is th us of th mchanical Plantary Gar Sts(PGS), in which th transmission loss, gar nois, an rgular lubrication is n[17]. Howvr, it is also possibl to raliz a powr-splitting hybri lctric powrtrain sytm by using a pur lctrical typ CVT consisting of two concntrically arrang lctric machins without any plantary gar sts[18]. Th pur lctrical typ CVT, or EVT powrtrain systm is a class of rcntly vlop propulsion systms. Ths systms usually compris concntrically arrang oubl-rotor[19-22] or brushlss oubl-stator[23, 24] lctric machins to prform powr split. Th mrits of th EVT powrtrain systm using concntric lctric machins ar list blow: Th CVT natur allows th ICE to oprat in its most fficint rgim. Th oubl lctric machin forms a pur lctric IVT an thrfor no aitional clutchs, torqu convrtrs ar n. Th EVT systm contains no mchanical PGS an thrfor rucs mchanical friction, avoi auibl nois an achiv systm simplicity. Th intgration of th two lctric machins through concntric arrangmnt rucs th systm wight an siz, rsulting in a highly compact an lightwight powrtrain systm. Th rprsntativ of th EVT powrtrain systm is th oubl-rotor EVT systm. Figur 18 shows th systm configuration, which consist of two concntrically arrang lctric machins. Th two machins ar mchanically coupl by a common shaft an lctrically connct via two powr convrtrs. Th primary oubl-rotor machin an th sconary outr-rotor machin ar mchanically coupl by a common shaft an lctrically connct via two convrtrs. Th ICE shaft is connct to th innr rotor an th final rivshaft is connct to th outr rotor(also known as intrrotor). 9 Copyright 2011 by ASME

10 sir continuously variabl ratio btwn th ngin sp an th vhicl sp can b achiv. For th primary machin, For th sconary machin, An so w gt: T T J ( 23 ) m1 1 1 m1 T T T J ( 24 ) m2 T T T J + J ( 25 ) m1 2 1 m1 2 m2 Fig 18. Doubl-rotor EVT powrtrain systm To simplify th structur for unrstaning, th EVT coul b ivi into two casca machins(th primary oubl-rotor machin an th sconary machin) as shown in Figur 19. Fig 19. Casca layout of th EVT powrtrain systm Th powr flow in th EVT is quit similar to that in th ECVT systms with plantary gar sts. As can b sn from Figur 19, th ngin input powr is split into two flows: on powr flow P is irctly pass to th sconary shaft via th lctromagntic torqu via th primary air gap with rlativly low lass, whil th rst of th input powr P is pass through th powr lctronic convrtrs with rlativly high losss. Whn th ngin input powr cannot mt th sir riving powr, th battry powr can provi powr. Th powr an torqu rlationship quations ar xprss as: P P P ( 20 ) m1 P ( ) T ( 21 ) m1 m2 m1 P T ( 22 ) m2 m1 whr P m1, T m1, m1 ar th primary shaft powr, torqu an sp, m2 is th EVT output sp. By controlling P, m1 can b maintain constant whn m2 is varying. Hnc, th whr Tm 2 is th EVT output torqu, T 1, T 2 ar th lctromagntic torqu by th primary an sconary machin, T is th rivlin loa torqu, an J 1 an J 2 ar th momnts of inrtia. Th possibl oprational mos of th EVT powrtrain systm ar list blow: In CVT mo, th primary machin ar controll by sp to chang th sp from th sp rquir at th final gar to th optimal sp of th ICE. Th sconary machin is controll by torqu in th sam mannr to kp th optimal torqu for th rquir powr. In this way, ICE can always oprat at its most fficint rgim. In startr mo, th primary lctric machin absorbs powr from th battry an livrs th powr to th ICE for start. In charging mo, th battry is ithr charg by th ICE via th primary machin whn th vhicl stops motion or by th sconary machin uring rgnrativ braking. In lctric mo, th battry livrs th powr to launch th vhicl via th sconary machin without using th ICE. Th primary machin os not work to avoi th rotation of ICE. COCLUSIO At prsnt, a comprhnsiv rviw of th avanc CVT tchnology for HEV has not bn prviously publish yt, spit wi-spra acknowlgmnt that this tchnology promiss to b at th cntr of th futur of hybri vhicl vlopmnt. In this papr, a rviw of th currnt stat-of-th-art for avanc CVT tchnology in Hybri Elctric Vhicls is prform. Thr main catgoris of th CVT hybri lctric powrtrain systms: th mchanical CVT, th lctromchanical CVT(ECVT) an th pur lctrical CVT(EVT) ar iscuss. Th rsarch vlopmnt, systm configuration, oprational faturs an mrits & rawbacks of ach systm ar iscuss. 10 Copyright 2011 by ASME

11 EFEECES 1. Hofman, T., t al., Dsign of CVT-Bas Hybri Passngr Cars. Vhicular Tchnology, IEEE Transactions on, (2): p Millr, J.M., Propulsion Systms for Hybri Vhicls. 2003, Lonon, Unit Kingom: Th Institution of Enginring an Tchnology ps_problmatiqu_constat_ng.html. [cit ssion/777-honas-ima-brakown.html. [cit tail/. [cit roatsts/honacivichybri.htm. [cit ny_frank_phd.pf. [cit. 8. A.W. Brown, J.v.., A.A. Frank. Th Dsign of an Inlin GCI Chain CVT for Larg Vhicls. in 2004 Intrnational Continuously Variabl an Hybri Transmission Congrss Lin, H., t al. A novl continuously variabl transmission flywhl hybri lctric powrtrain. in Vhicl Powr an Propulsion Confrnc, VPPC '08. IEEE Xiaolan AI, M.T., Anrson Scott An lctro-mchanical infinitly variabl sp transmission, in Procings of th 2004 SAE Worl Congrss Millr, J.M., Hybri lctric vhicl propulsion systm architcturs of th -CVT typ. Powr Elctronics, IEEE Transactions on, (3): p Jffry Wishart, Y.L.Z., Zuomin Dong viw of multi-rgim hybri vhicl powrtrain architctur. Intrnational Journal of Elctric an Hybri Vhicls Vol. 1, o.3 pp , [cit. 14. Conlon an Brnan, Comparativ analysis of singl an combin hybri lctrically variabl transmission oprating mos. SAE transactions, 2005(6): p Tim, M.G., Dfining th Gnral Motors 2-Mo Hybri Transmission, in 2007 SAE Worl Congrss amoo Kim,.C., Forrst Jhlik an Aymric oussau, GM Taho HEV Mol Dvlopmnt in PSAT. SAE , Worl Congrss, April Chau, t al., Emrging nrgy-fficint tchnologis for hybri lctric vhicls. 2007, w York, Y, ETATS-UIS: Institut of Elctrical an Elctronics Enginrs Eriksson, S. an C. Saarangani. A four-quarant HEV riv systm. in Vhicular Tchnology Confrnc, Procings. VTC 2002-Fall IEEE 56th Hoijmakrs, M.J. an J.A. Frrira, Th lctric variabl transmission. Inustry Applications, IEEE Transactions on, (4): p Shumi, C., C. Yuan, an C.C. Chan. A Basic Stuy of Elctrical Variabl Transmission an Its Application in Hybri Elctric Vhicl. in Vhicl Powr an Propulsion Confrnc, VPPC '06. IEEE Kssls, J.T.B.A., D.L. Fostr, an P.P.J. van n Bosch. Intgrat powrtrain control for hybri lctric vhicls with lctric variabl transmission. in Vhicl Powr an Propulsion Confrnc, VPPC '09. IEEE Yongchang, D., t al. HEV systm bas on lctric variabl transmission. in Vhicl Powr an Propulsion Confrnc, VPPC '09. IEEE Yubin, W., t al. Dsign an analysis of oubl-stator prmannt magnt brushlss motor for hybri lctric vhicls. in Elctrical Machins an Systms, ICEMS Intrnational Confrnc on Fng, C., C. Shumi, an C. Shukang, Prformanc analysis of oubl-stator startr gnrator for th hybri lctric vhicl. Magntics, IEEE Transactions on, (1): p Copyright 2011 by ASME