Study and Analysis of a Multi-Mode Power Split Hybrid Transmission
Article Study and Analysis of a Multi-Mode Power Split Hybrid Transmission
Xiaojiang Chen 1,* , Jiajia Jiang 2, Lipeng Zheng 2, Haifeng Tang 2 and Xiaofeng Chen 2 1 Great Wall Motor Austria Research & Development GmbH, 2542 Kottingbrunn, Austria 2 HYCET E-Drive System Co., Ltd., Baoding 071000, China; [email protected] (J.J.); [email protected] (L.Z.); [email protected] (H.T.); [email protected] (X.C.) * Correspondence: [email protected]; Tel.: +43-660-7048-729
Received: 19 May 2020; Accepted: 10 June 2020; Published: 12 June 2020
Abstract: A two-motor power-split dedicated hybrid transmission (DHT) with two planetary gears is proposed for the applications of a hybrid electric vehicle (HEV) and plug-in HEV (PHEV). The proposed DHT can provide electronically controlled continuous variable transmission (eCVT) with two different gear ratios. One of two electric motors is employed to act as a speeder for splitting the input power of internal combustion engine (ICE) and the other acts as a torquer to assist ICE for boosting. Assisted by an electric motor, ICE can always be enhanced to operate at its efficient area for the benefits of fuel economy improvement. The maximum ICE torque is viable to be mechanically transmitted to vehicle wheels from standstill with two different gear ratios. This feature can help reduce the traction motor torque and power sizing significantly. The paper presents detailed theoretical analyses of the proposed eCVT. Comprehensive simulation demonstrations for a pickup truck HEV application are given to address that the vehicle fuel consumption can be considerably reduced without compromising acceleration performance.
Keywords: power split; dedicated hybrid transmission (DHT); planetary gear; electronically controlled continuous variable transmission (eCVT); fuel consumption
1. Introduction Global vehicle manufacturers are demanded to produce more fuel-efficient and low-emission vehicles including electric hybrid vehicles (HEV), plug–in HEV (PHEV) and battery electric vehicles (BEV) in order to satisfy more stringent fuel consumption and emission requirements from global governments. The BEV sale has been growing fast in recent years. However, the further wide acceptance for BEV relies heavily on the battery cost and reliability improvement and battery charging infrastructure development. Comparatively, an HEV or PHEV with a smaller traction battery is not or less dependent on charging facilities, and becomes more viable and market-demanding in a short and medium term. Due to the significant cost reduction of electric machines and their drive electronics over the years, two-motor based dedicated hybrid transmission (DHT) technologies are becoming a fast growing hybrid powertrain trend for HEV and PHEV applications such as Toyota power-split system hybrid synergy drive (HSD) [1–6], Honda intelligent multi-mode drive (iMMD) [7–9] and two-mode electric variable transmission (EVT) of the General Motor (GM) Voltect-2 [3,10–12]. For a HEV application, the battery size can be minimized to cut the overall hybrid powertrain cost by a dual-motor DHT. An HEV assisted by a single electric motor with a small battery can hardly make a very promising fuel economy improvement over its conventional version model. For a PHEV application, onboard traction batteries have to be sized large enough to satisfy EV driving functions. The popular P2 parallel hybrid powertrain architecture [6,13,14] is adopted by some European vehicle manufacturers for PHEV
World Electric Vehicle Journal 2020, 11, 46; doi:10.3390/wevj11020046 www.mdpi.com/journal/wevj World Electric Vehicle Journal 2020, 11, 46 2 of 21 applications. However, the P2 parallel hybrid is not fuel efficient and cost-effective enough for wide HEV adoptions because a P2 motor with limited power due to a small HEV traction battery can only achieve a mild hybridization function. A DHT with dual motors can easily provide strong hybrid function without the need of adjusting power and torque rating of two motor drive systems for both HEV and PHEV applications. A single-motor multi-gear hybrid special gearbox is proposed with two planetary gear sets, 2 clutches and 2 brakes to provide the pure electric mode with two gears and the hybrid mode with four gears intended for HEV and PHEV applications [15]. However, its shifting strategy is relatively complex. Its fuel economy improvement will be limited if a small HEV battery is adopted. A DHT with two motors can have many advantages over a single-motor based parallel hybrid powertrain. Firstly, it can offer smooth and seamless torque transmission between the motors and engine. The overall DHT powertrain cost equipped with a downsized engine can be comparable to a conventional non-hybrid powertrain with a complex multi-geared automatic transmission and turbo-charged engine system due to the significant cost reduction of motor systems in recent years. Furthermore, HEV and PHEV applications can share a common DHT platform without the need of adjusting motor rating. As one of two DHT motors can provide traction assistant power and the other motor can run in generation to compensate power delivery from traction batteries, the traction battery power can be minimized with the significant benefit of prolonging battery life. Additionally, the implementation of the DHT powertrain control strategy is comparatively simple. Moreover, DHT dual motors can offer much more flexibilities for engines to operate in their most efficient region for the benefit of significant fuel economy and emission improvements. Toyota has produced its one-mode power-split series-parallel hybrid synergy drive (HSD) since 1997 [2–5]. The DHT powertrain architecture of the Toyota’s latest HSD with a single planetary gear is shown in Figure1a [ 1]. It has a simple mechanical structure and is cost-effective especially for economy-class vehicle applications. The HSD traction torque capability is limited because the mechanical path of engine torque to DHT output only has one fixed gear ratio. Accordingly, a traction motor generator 2 (MG2) with high torque capabilities has to be employed if a high traction application is required. This will unavoidably result in a larger-sized MG2 motor and inverter, and cost increase in the motor system. Furthermore, the MG2 speed will vary linearly with the vehicle speed due to the MG2’s direct mechanical coupling with a fixed high gear ratio. During highway cruising, HSD will operate in a direct engine drive mode while the traction motor MG2 has to run persistently at a high speed without delivering any active torque assistance and this would cause extra system loss to the hybrid powertrain. Thereby, it will introduce fuel-consumption penalty and not be efficient enough during continuous highway driving. Honda launched its first-generation strong hybrid vehicle of a PHEV accord [7–9] in 2014. The simplified architecture of Honda iMMD shown in Figure1b only employs simple axial transmission gear to achieve eCVT function through a series hybrid mode during city driving and instant highway power boosting. A parallel hybrid mode at high speed is employed by closing the clutch to directly transfer engine torque to the powertrain output shaft with a fixed gear. The maximum traction torque and power of Honda iMMD HEV powertrain fully relies on its traction motor capability. The iMMD traction motor has to be sized to provide full traction torque and power requirements. Moreover, its generator motor must be sized to match completely with its engine rating. The iMMD DHT system will have highest motor sizing requirements compared to Toyota HSD [1–6] and GM Voltec-2 eCVT architecture [10–12]. Hence Honda iMMD HEV eCVT powertrain requires high DHT cost especially for a high-performance HEV application but owns significant benefits in compact transmission mechanical structure and simple powertrain control strategy. GM introduced its innovative two-mode power-split hybrid powertrain EVT DHT technology based on two planetary gears, as shown in Figure1c [ 10–12]. GM dual-mode power-split system can provide 2-geared eCVT functions and result in significant reduction in motor torque and power rating. It is much more scalable for HEV applications with different vehicle platforms. The GM EVT World Electric Vehicle Journal 2020, 11, 46 3 of 21
World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 3 of 20 architecture provides two eCVT hybrid functions with two distinctive gear ratios: an input power spiltcan directly at first hightransfer gear through ratio and its a compoundplanetary mechanic power splital path at second to the lower vehicle gear wheel ratio. in Its two engine gear torqueratios. canAssisted directly by a transfer power-split through motor, its planetarythe maximum mechanical engine pathtorque to can the vehiclebe delivered wheel mechanically in two gear ratios.to the Assistedvehicle wheel by a power-splitfrom vehicle motor, standstill. the maximum Due to the engine engine torque torque can availability be delivered with mechanically two different to gear the vehicleratios from wheel standstill, from vehicle the GM standstill. EVT motor Due topower the engine and torque torque rating availability to satisfy with overall two di ffpowertrainerent gear ratiostorque from and standstill,power demands the GM can EVT be motordramatically power degraded. and torque This rating will to result satisfy in overall significant powertrain overall torquemotor andsystem power cost demands reduction can accordingly. be dramatically Furthermore, degraded. its This traction will resultmotorin MG2 significant in GM overallEVT can motor operate system as a costspeeder reduction to split accordingly. engine torque Furthermore, and power itsduring traction the motorcompound MG2 power-split in GM EVT caneCVT. operate When as the a speeder vehicle todrives split at engine high speed, torque direct and power engine during drive thecan compound be applied power-splitby controlling eCVT. the Whentraction the motor vehicle MG2 drives speed at higharound speed, zero direct speed. engine Accordingly, drive can two be applied motors by in controllingthe GM two-mode the traction EVT motor are not MG2 necessarily speed around required zero speed.to operate Accordingly, at a very twohigh motors speed induring the GM highway two-mode driving. EVT areThis not will necessarily simply motor required control to operate algorithm at a veryand flux-wakening high speed during control highway effort driving. for two Thismotors will at simply high speed. motor As control the MG2 algorithm loss induced and flux-wakening by power- controlsplit balancing effort for torque two motors is much at high lower speed. than As that the of MG2 the lossMG2 induced idling byloss power-split at high speed, balancing the overall torque ispowertrain much lower efficiency than that of ofGM the two-mode MG2 idling EVT loss DHT at high shall speed, be more the overallefficient powertrain than Toyota effi HSDciency and of GMHonda two-mode iMMD DHT EVT DHTduring shall continuous be more highway efficient thancruising. Toyota In HSDaddition, and as Honda motor iMMD power DHT and duringtorque continuousrequirements highway are degraded cruising. and In addition, two motors as motor need power not operate and torque to a requirementsvery high rotational are degraded speed, and a twobidirectional motors need Direct-Current(DC)-to-DC not operate to a very high buck-b rotationaloost speed, converter a bidirectional connecting Direct-Current(DC)-to-DC the onboard traction buck-boostbattery to motor converter inverters connecting DC-link the bus onboard used tractionin Toyota battery HSD to and motor Honda inverters iMMD DC-link is not busnecessarily used in Toyotarequired HSD anymore. and Honda This will iMMD certainly is not necessarilyresult in th requirede system anymore.efficiency Thisimprovement will certainly and resultfurther in cost the systemreduction effi ofciency the GM improvement two-mode and EVT further DHT powertrain. cost reduction of the GM two-mode EVT DHT powertrain.
(a) (b)
(c)
Figure 1.1. Three typicaltypical electronicallyelectronically controlledcontrolled continuouscontinuous variablevariable transmissiontransmission (eCVT)(eCVT) dedicateddedicated hybrid transmissiontransmission (DHT) (DHT) architectures: architectures: (a) Toyota(a) Toyota hybrid hybrid synergy synergy drive (HSD); drive (b(HSD);) Honda (b intelligent) Honda multi-modeintelligent multi-mode drive (iMMD); drive (c (iMMD);) GM two-mode (c) GM electrictwo-mode variable electric transmission variable transmission (EVT). (EVT).
Ravigneaux planetary gearsets have attracted many interests in DHT applications [16,17]. [16,17]. ConfigurationConfiguration syntheses for for novel novel hybrid hybrid transmissions transmissions consisting consisting of of a aRavigneaux Ravigneaux gearset gearset and and a asingle single planetary planetary gearset gearset are areaddressed addressed based based on graph-theory on graph-theory and lever and analogy lever analogy method method[16]. A dual- [16]. Amotor dual-motor based DHT based containing DHT containing a modified a modifiedRavigneaux Ravigneaux gearset with gearset a common with aringer common gear, ringer a common gear, acarrier common and carriertwo sun and gears two is sun studied gears for is studieda PHEV forapplication a PHEV application[17]. Two additional [17]. Two brake additional clutches brake are clutchesadopted arefor allowing adopted the for hybrid allowing function the hybrid switching function of EV, switching compound of EV, power compound split and power parallel split hybrid and parallelin a fixed hybrid gear [17]. in a fixed Due gearto the [17 lack]. Due of an to input the lack power-split of an input configuration, power-split configuration, this Ravigneaux this Ravigneauxbased DHT basedarchitecture DHT architecture[17] will fully [17 rely] will on fully its EV rely function on its EV provided function by provided two motors by two to ensure motors its to maximum ensure its traction capability. Once its PHEV battery is depleted, the compound power-split hybrid function can only offer a limited gradeability. This paper will present a novel concept of two mode eCVT hybrid powertrain architecture that contains less component than that of GM EVT. This eCVT DHT can provide two EV drive modes, World Electric Vehicle Journal 2020, 11, 46 4 of 21 maximum traction capability. Once its PHEV battery is depleted, the compound power-split hybrid function can only offer a limited gradeability. WorldThis Electric paper Vehicle will Journal present 2020, 11 a, x novel FOR PEER concept REVIEW of two mode eCVT hybrid powertrain architecture 4 thatof 20 contains less component than that of GM EVT. This eCVT DHT can provide two EV drive modes, twotwo powerpower split split modes modes with with two two separate separate gear ratios:gear ra antios: input an powerinput splitpower at firstsplit gear at andfirst agear compound and a powercompound split atpower second split gear, at andsecond two gear, direct and engine two drivedirect or engine parallel drive hybrid or parallel modes with hybrid two modes same gears. with Detailedtwo same theoretical gears. Detailed analyses aretheoretical carriedout. analyses A comprehensive are carried simulation out. A comprehensive demonstration forsimulation a pickup HEVdemonstration truck application for a pickup is finally HEV presented. truck application is finally presented.
2.2. Proposed eCVT Concept
2.1.2.1. Two-Mode PowerPower SplitSplit eCVTeCVT Architecture TheThe proposedproposed two-modetwo-mode power-split eCVT is basedbased onon twotwo planetaryplanetary gearsets,gearsets, as shownshown inin FigureFigure2 2.. TheThe internalinternal combustioncombustion engineengine (ICE)(ICE) engineengine torquetorque inputinput isis directlydirectly connectedconnected toto thethe ring ring geargear R1R1 ofof firstfirst planetaryplanetary geargear PG1.PG1. The firstfirst motormotor generator (MG1) is directly coupledcoupled to thethe PG1’sPG1’s sunsun geargear S1.S1. The second motor generator (MG2) is attached to the sun gear S2 ofof secondsecond planetaryplanetary geargear PG2.PG2. A clutch CL1 and a brake BK1 are employed between the PG1 ring gear R1 and PG2 carrier C2C2 forfor grantinggranting didifferentfferent drivedrive functions.functions. The PG1 carrier C1 and PG2 ring gear R2 are mechanically connectedconnected permanently to the eCVT output gear transmission transmission that that has has a a fixed fixed gear gear ratio ratio of of k f toto thethe vehiclevehicle wheelwheel output.output.
Figure 2.2. Proposed eCVT DHT architecture. 2.2. eCVT Drive Mode Introduction 2.2. eCVT Drive Mode Introduction The proposed eCVT in Figure2 can provide multi-mode drive functions: pure electric-vehicle The proposed eCVT in Figure 2 can provide multi-mode drive functions: pure electric-vehicle driving (EV), eCVT hybrid mode and parallel hybrid mode (PH) including direct engine drive, driving (EV), eCVT hybrid mode and parallel hybrid mode (PH) including direct engine drive, as as summarized in the following Table1. summarized in the following Table 1. Table 1. Drive modes of proposed electronically controlled continuous variable transmission (eCVT). Table 1. Drive modes of proposed electronically controlled continuous variable transmission (eCVT). Mode CL1 BK1 ICE MG1 MG2 Definition Mode CL1 BK1 ICE MG1 MG2 Definition EV1 Open Close Only MG2 provides pure EV drive EV1 Open Close ○ ○ ● • Only MG2 provides pure EV drive EV2 Close Close ## Both MG1 and MG2 provide EV drive ○ •• eCVT1EV2 Close Open Close Close # ● ● Both MG1Input and power MG2 split provide mode at EV 1st geardrive ••• eCVT1PH1 Open Open Close Close ● ● ● Parallel Input hybrid power or split direct mode engine at drive1st gear at 1st gear ••• eCVT2PH1 Open Close Close Open ● ● ● Parallel hybridCompound or direct power engine split atdrive 2nd at gear 1st gear ••• PH2 Close Open Parallel hybrid or direct engine drive at 2nd gear eCVT2 Close Open ● •••● ● Compound power split at 2nd gear PH2CL: Clutch;Close BK: Open Brake; ICE:● Internal● Combustion● Parallel Engine; MG:hybrid Motor or direct Generator; engine EV: drive Electric at Vehicle;2nd gear eCVT: electronically controlled Continuous Variable Transmission; PH: Parallel Hybrid. CL: Clutch; BK: Brake; ICE: Internal Combustion Engine; MG: Motor Generator; EV: Electric Vehicle; 3. TheoreticaleCVT: electronically Analyses controlled on Drive Continuous Modes Variable Transmission; PH: Parallel Hybrid.
3.1.3. Theoretical EV1 Mode Analyses on Drive Modes The brake BK1 is engaged close and clutch CL1 open during the first EV driving mode – EV1, 3.1. EV1 Mode as shown in the configuration of Figure3. The brake BK1 is engaged close and clutch CL1 open during the first EV driving mode – EV1, as shown in the configuration of Figure 3. World Electric Vehicle Journal 2020, 11, 46 5 of 21 World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 5 of 20
World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 5 of 20
Figure 3. eCVTeCVT DHT DHT configuration configuration in in Electric Electric Vehicle (EV1) mode. Figure 3. eCVT DHT configuration in Electric Vehicle (EV1) mode. The speed relationship of three power sources cancan bebe describeddescribed as:as:
The speed relationship of three power sourcesω R 1 ICE can==00 be described as: (1) (1) − _ =( +1) =0=( +1) (2)(1) ωS1_MG1 = (k1 + 1)ωC1R2 = (k1 + 1)k f ωWHL (2) _ _ =( =− +1) =− =( +1) (3)(2) ωS2_MG2 = k2ωC1R2 = k2k f ωWHL (3) =− − =− − where and are defined as the_ gear ratios of the sun gear to ring gear of PG1 and PG2,(3) where k1 and k2 are defined as the gear ratios of the sun gear to ring gear of PG1 and PG2, respectively; respectively; _ represents the engine angular rotation speed at the PG1 ring gear R1 input shaft, where represents and theare enginedefined angular as the rotation gear ratios speed of at the the sun PG1 gear ring to gear ring R1 gear input of shaft, PG1 and PG2,as ωR1_ICE ωS1_MG1 respectively; _ as the MG1 angular represents speed the at engine PG1 sun angular gear S1 rotation input shaft, speed at _ the PG1 as thering MG2 gear angularR1 input speed shaft, the MG1 angular _ speed at PG1 sun gear S1 input shaft, ωS2_MG2 as the MG2 angular speed at the PG2 at the PG2 sun gear S2 input shaft and as the final vehicle wheel angular speed. sun _ gear as S2 the input MG1 shaft angular and ω speedas at thePG1 final sun vehiclegear S1 wheelinput shaft, angular _ speed. as the MG2 angular speed Torque outputs of three powerWHL sources of ICE, MG1 and MG2 are defined by： at theTorque PG2 sun outputs gear S2 of input three powershaft and sources of as ICE, the MG1 final andvehicle MG2 wheel are defined angular by: speed. ： Torque outputs of three power sources of ICE,_ MG1=0 and MG2 are defined by (4) TR1_ICE = 0 (4) _ _ =0 (5)(4)
T S=− 1__MG 1 =0= 0 _ (6)(5) (5) where represents the final wheelT WHLoutput ==− torquek k T from_ the eCVT DHT powertrain. Based(6) (6)on − 2 f S2_MG2 Equations (4)–(6), MG2 provides active drive torque solely to the powertrain with a fixed gear ratio wherewhere T representsrepresents thethe finalfinal wheelwheel outputoutput torquetorque fromfrom thethe eCVTeCVT DHTDHT powertrain.powertrain. Based onon of − WHL; due to the engine intrinsic drag torque, engine is kept at standstill without active drive Equations (4)–(6),(4)–(6), MG2MG2 providesprovides active active drive drive torque torque solely solely to to the the powertrain powertrain with with a fixeda fixed gear gear ratio ratio of torque− delivered to the drivetrain and MG1 will rotate in synchronization with the vehicle speed as ofk 2k f ; due ; due to the to enginethe engine intrinsic intrinsic drag dr torque,ag torque, engine engine is kept is at kept standstill at standstill without without active driveactive torque drive described− in Equation (2) without active torque delivery. deliveredtorque delivered to the drivetrain to the drivetrain and MG1 and will MG1 rotate will in rotate synchronization in synchronization with the vehiclewith the speed vehicle as described speed as The maximum vehicle speed in the EV1 mode depends on the maximum allowable rotational indescribed Equation in (2)Equation without (2) active without torque active delivery. torque delivery. speed of the sun gears of two planetary gears PG1 and PG2. The gear ratio of and shall be The maximum vehicle speed in the EV1EV1 modemode dependsdepends onon thethe maximummaximum allowableallowable rotationalrotational properly selected to ensure the required maximum EV vehicle speed achievable. EV1 will be used as speed ofof thethe sun sun gears gears of of two two planetary planetary gears gears PG1 PG1 and and PG2. PG2. The gearThe ratiogear ofratiok1 and of k 2 shalland be properlyshall be the prime electric drive mode if EV driving load requests are moderate. selectedproperly to selected ensure to the ensure required the maximumrequired maximum EV vehicle EV speed vehicle achievable. speed achievable. EV1 will be EV1 used will as be the used prime as electricthe prime drive electric mode drive if EV mode driving if EV load driving requests load are requests moderate. are moderate. 3.2. EV2 Mode 3.2. EV2 When Mode electric drive requires a high traction torque that cannot be solely satisfied by MG2, both the clutchWhen electricCL1electric and drivedrive brake requires requires BK1 ashall a high high be traction traction closed torque torqto allowue that that cannotMG1 cannot and be be solely MG2 solely satisfied to satisfied provide by MG2,by EV MG2, bothdriving both the simultaneously,clutchthe clutch CL1 andCL1 brake asand shown BK1brake shall in BK1 the be configuration closedshall be to allowclosed of MG1 Figureto allow and 4. MG2 MG1 to provideand MG2 EV to driving provide simultaneously, EV driving assimultaneously, shown in the configurationas shown in the of Figureconfiguration4. of Figure 4.
Figure 4. eCVT DHT configuration in EV2 mode. Figure 4. eCVT DHT configurationconfiguration in EV2 mode. World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 6 of 20 World Electric Vehicle Journal 2020, 11, 46 6 of 21 In the EV2 mode, the speed relationship of ICE, MG1 and MG2 still complies with Equations (1)–(3).In theHowever, EV2 mode, the wheel the speed drive relationship torque shall ofbe ICE, provided MG1 andby both MG2 MG1 still and complies MG2 as with described Equations by (1)–(3).Equations However, (7) and (8): the wheel drive torque shall be provided by both MG1 and MG2 as described by
Equations (7) and (8): _ =0 (7) TR1C2_ICE = 0 (7) = ( +1) _ − _ (8) TWHL = (k + 1)k T k k T (8) MG1 and MG2 work jointly to provide1 tractionf S1_MG power1 − 2 andf S2_ torqueMG2 together to achieve electric driveMG1 function. and MG2 This workwill help jointly to toalleviate provide the traction thermal power stress and when torque the together vehicle tois driven achieve by electric one motor drive function.alone in a This high will load help condition. to alleviate Optimal the thermal torque stress split when between the vehicle two motors is driven will by help one improve motor alone the inEV a highdrive load efficiency condition. and Optimalgradeability. torque In split addition, between the two overall motors maximum will help EV improve power the and EV torque drive eratingfficiency to andtwo gradeability.motors can be In addition,split. This the will overall obviously maximum result EV in powersmaller and motor torque sizing rating requirements to two motors for can cost- be split.down Thisbenefits. will obviously result in smaller motor sizing requirements for cost-down benefits.
3.3. eCVT1 Mode—Input Mode—Input Power Power Split The eCVT1 mode has the same configurationconfiguration asas EV1EV1 asas illustratedillustrated inin FigureFigure5 5..
Figure 5. eCVT DHT configuration configuration in eCVT1 and parallel hybrid (PH)1 mode.
In this this input input power-split power-split eCVT eCVT mode mode [6], [ 6the], theMG1 MG1 speed speed shall shallbe controlled be controlled in a closed in a speed- closed speed-feedbackfeedback loop to loop maintain to maintain optimal optimal ICE engine ICE engine running running points. points. MG1 MG1automatically automatically provides provides a split a splittorque torque to balance to balance with with the theengine engine torque torque inpu input.t. In Inthe the meantime, meantime, MG2 MG2 generates generates extra extra torque occasionally toto assistassist the the powertrain powertrain traction traction or or balance balance the the power power of onboard of onboard traction traction battery. battery. In such In ansuch input an input power power split case, split the case, speed the relationshipspeed relation of threeship of power three sources power issources described is described by the following by the Equationsfollowing Equations (9) and (10): (9) and (10): + = ( +1) =( +1) ωS1_ MG_ 1 + k1 ωR 1__ICE = (k1 + 1)ωC 1R2 = (k 1 + 1)k f ωWHL (9)(9)
_ =− =− (10) ωS2_MG2 = k2ωC1R2 = k2k f ωWHL (10) The engine input power is split continuously− by the− speeder MG1 into two separate power The engine input power is split continuously by the speeder MG1 into two separate power transmission path: mechanical path and electro-mechanical path. For the electro-mechanical path, the transmission path: mechanical path and electro-mechanical path. For the electro-mechanical path, speeder MG1 will provide a split torque to balance the internal force distribution of the first planetary the speeder MG1 will provide a split torque to balance the internal force distribution of the first gear PG1. In a steady state by neglecting some minor disturbance effects, the MG1 split torque can planetary gear PG1. In a steady state by neglecting some minor disturbance effects, the MG1 split ideally be described in Equation (11): torque can ideally be described in Equation (11): 1 = (11) _ 1 _ T = T (11) S1_MG1 k R1_ICE The MG1 split power to the overall ICE power input1 is further derived by: The MG1 split power to the overall ICE power input1 is further derived by: = = (12) _ _ _ _ _ 1 PS1_MG1 = TS1_MG1ωS1_MG1 = TR1_ICEωS1_MG1 (12) The engine input torque of _ has to be alwaysk1 positive, then the MG1 split power _ <0 when _ <0, i.e., MG1 generative power is provided to either charge battery or The engine input torque of TR1_ICE has to be always positive, then the MG1 split power PS1_MG1 < 0 directly to the MG2 system; _ =0 when _ =0, i.e., no active ICE power is split; when ωS1_MG1 < 0, i.e., MG1 generative power is provided to either charge battery or directly to the _ >0 when _ >0, i.e., MG1 consumes the battery power to provide extra motoring MG2 system; P = 0 when = 0, i.e., no active ICE power is split; P 0 when power to be addedS1_MG with1 ICE inputω powerS1_MG1 to the final eCVT power output. S1_MG1 > World Electric Vehicle Journal 2020, 11, 46 7 of 21
ωS1_MG1 > 0, i.e., MG1 consumes the battery power to provide extra motoring power to be added with ICEWorld input Electric power Vehicle Journal to the 2020 final, 11 eCVT, x FOR power PEER REVIEW output. 7 of 20 Furthermore, the eCVT DHT powertrain output torque will contain two components provided by ICE andFurthermore, MG2 separately the eCVT as descried DHT powertrain in Equation output (13): torque will contain two components provided by ICE and MG2 separately as descried in Equation (13): ! k1 + 1 TWHL = k f TC1R2 = +1TR1_ICE k2TS2_MG2 k f (13) = =( k1 _ − − _ ) (13) ICE will provide a mechanical torque directly to the output shaft of the PG1 carrier C1 with a ICE will provide a mechanical torque directly to the output shaft of the PG1 carrier C1 with a k1+1 fixed gear ratio of regardless of the vehicle speed. The engine mechanical power output to the fixed gear ratio of k1 regardless of the vehicle speed. The engine mechanical power output to the wheel through the planetary mechanical path depends on Equation (14): wheel through the planetary mechanical path depends on Equation (14): k1 + 1 k1 + 1 PICE_MO = +1TR1_ICEωC1R2 = +1k f TR1_ICEωWHL (14) _ = k1 _ = k1 _ (14) The power split ratio of ICE mechanical output power P versus ICE overall input power at The power split ratio of ICE mechanical output power ICE _ _MO versus ICE overall input power PG1at PG1 ring ring gear gear R1 R1 is derivedis derived by: by: +1 +1 1 PICE _MO_ k1 + 1 ω C1R2 k 1 + 1 1 βeCVT 1 ======(15)(15) TR 1_ ICE_ ω R1_ ICE_ k1 ω R 1__ ICE k 1 ϕ eCVT 1 _ where = ωR1_ICE is defined as the ICE eCVT1 gear ratio to the PG1 output, which will be where ϕ = is defined as the ICE eCVT1 gear ratio to the PG1 output, which will be eCVT1 ωC1R2 continuously variable variable with with the the vehicle vehicle speed speed and and driving driving load load demands. demands. At zero At zero vehicle vehicle speed, speed, the =0 theeCVT1 eCVT1 ICE ICE gear gear ratio ratio ϕeCVT is1 isinfinite infinite with with ICE ICE power power split split ratio ratio β eCVT1 = 0.. This This implies implies that the ICE input power is completely converted into electr electricityicity by MG1 through the electro-mechanical path k1 +1 ideally.ideally. WhenWhen thethe powerpower splitsplit ratioratio β eCVT ==11,, i.e.,i.e., ϕ eCVT == ,, the the MG1 MG1 speed speed must must be be zero, zero, i.e., 1 1 k 1 = ω S _ 1_MG1=00. .In In this this special special case, case, the the ICE ICE input input power power wi willll be completely transmitted to thethe vehiclevehicle wheelwheel throughthrough thethe planetaryplanetary mechanicalmechanical pathpath ideally.ideally. The principle of eCVT1 power split can be clarifiedclarified byby thethe planetary-gearplanetary-gear leverlever diagramdiagram asas illustratedillustrated inin FigureFigure6 6..
FigureFigure 6. 6. LeverLever schematicsschematics forfor eCVT1eCVT1 andand firstfirst parallelparallel hybridhybrid PH1PH1 mode.mode.
Figure6 6 illustrate illustrate thatthat MG2MG2 willwill alwaysalways operateoperate atat aa negativenegative rotationalrotational speedspeed whenwhen thethe vehiclevehicle drives forwards in in the the eCVT1 eCVT1 mode mode because because of of the the speed speed relationship relationship describe describedd in inEquation Equation (10). (10). In Inthe the meantime, meantime, the the ICE ICE speed speed at at the the PG1 PG1 ring ring gear gear input input shaft shaft shall shall be be continuously continuously and optimally adjusted by MG1 through a feedback speed control. Firstly, an expectedexpected ICE speed shall be calculated inin real time based based on on a a lookup lookup map map versus versus the the inputs inputs of of the the drive drive load load and and vehicle vehicle speed, speed, or by or byan anonline online optimization optimization algorithm. algorithm. Furthermore, Furthermore, the theMG1 MG1 speed speed reference reference shall shall be calculated be calculated based based on onEquation Equation (9), (9),where where the current the current vehicle vehicle wheel wheel speed speed and expected and expected ICE speed ICE are speed employed are employed as inputs. as Eventually MG1 is commanded in a closed-loop control to follow its speed reference in order to maintain the required optimal engine speed. As indicated in the operation Line 1–4 of Figure 6, MG1 will run at a negative speed from startup in generation mode to split the ICE power and gradually move towards zero speed with the increase of the vehicle speed. At zero speed, MG1 has zero power split but has to maintain its split torque described in Equation (11) to allow ICE input power directly be transferred to the wheel completely; World Electric Vehicle Journal 2020, 11, 46 8 of 21 inputs. Eventually MG1 is commanded in a closed-loop control to follow its speed reference in order to maintain the required optimal engine speed. As indicated in the operation Line 1–4 of Figure6, MG1 will run at a negative speed from startup in generation mode to split the ICE power and gradually move towards zero speed with the increase of the vehicle speed. At zero speed, MG1 has zero power split but has to maintain its split torque described in Equation (11) to allow ICE input power directly be transferred to the wheel completely; once MG1 has to run into a positive speed, MG1 starts to consume battery energy in motoring mode to deliver extra positive power adding to existing ICE input power at the powertrain output. If the onboard battery power has to be balanced, MG2 must provide generative power to counteract the MG1 motoring power. In such a case, power re-circulation will happen inside of the planetary system and result in the decrease of eCVT transmission efficiency. With the further increase of MG1 speed at position rotation, overall eCVT efficiency will be even less. Thus, the eCVT1 power split shall be avoided to be applied at high vehicle speed. The eCVT1 operational range of vehicle speed depends on both the restriction of the maximum allowable speed of the PG2 sun gear and overall powertrain efficiency consideration. Based on Equation (13), the engine torque to the final wheel output through planetary gear mechanical pass has a fixed gear ratio even though the engine speed to the final output continuously varies: k1 + 1 kG1_eCVT1 = k f (16) k1
Then eCVT1 is defined as the eCVT mode at the first gear of kG1_eCVT1.
3.4. PH1 Mode The first parallel hybrid (PH1) mode is defined to be the special operational case within the eCVT1 mode when MG1 is controlled at zero speed, as indicated in Line–4 of Figure6. Based on Equations (9), (10) and(15), we can derive: ωS1_MG1 = 0 (17)
ωR1_ICE k1 + 1 ϕeCVT1_PH1 = = (18) ωC1R2 k1
PICE_MO k1 + 1 ωC1R2 βeCVT1_PH1 = = = 1 (19) TR1_ICEωR1_ICE k1 ωR1_ICE
At this operational point in the eCVT1 mode, the ICE power split ratio βeCVT1_PH1 = 1 and the MG1 active split power PS1_MG1 = 0. This implies the whole ICE input power is delivered to the vehicle wheel through the planetary mechanical path if eCVT system component losses are ignored. This special operation point is defined as the first Mechanical Point (MP1) of eCVT [11,12,18]. The torque transmission through the planetary-gear mechanical path is generally more efficient than through electro-mechanical path. Hence this special operation node is employed to implement direct engine drive or parallel hybrid mode of ICE and MG2 at the first gear. In the PH1 mode, the torque relationship of three power sources are the same as described in Equation (11) and Equation (13). Even though the split device MG1 is controlled at zero speed with zero active split power ideally in the PH1 mode, MG1 still has to provide a balanced split torque against ICE torque as defined in Equation (11). The MG1 steady-state split torque will unavoidably cause extra losses including motor winding copper loss and motor inverter loss. In order to balance the thermal stress of MG1 motor stator windings and inverter phases, it is preferable to operate MG1 in a low rotational speed during the PH1 mode.
3.5. eCVT2 Mode—Compound Power Split eCVT The eCVT2 mode has a configuration as illustrated in Figure7. The clutch CL1 is engaged close and the brake BK1 is kept open. World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 9 of 20
World Electric Vehicle Journal 2020, 11, 46 9 of 21 World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 9 of 20
Figure 7. eCVT DHT configuration in eCVT2 and PH2 mode. eCVT2 is a compound power-split hybrid mode [6,10–12,18]. Two planetary gearsets of PG1 and PG2 are restructured into a compound planetary system which allows either MG1 or MG2 to act as Figure 7. eCVT DHT configuration in eCVT2 and PH2 mode. the speeder for splittingFigure the 7. eCVTICE input DHT configurationpower and inthe eCVT2 other and as PH2 the mode. torquer. The basic speed relationshipeCVT2 isof a three compound power power-split sources in this hybrid compound mode [6,10–12,18]. system can Twobe further planetary derived gearsets as: of PG1 and PG2 eCVT2are restructured is a compound into a power-splitcompound hybridplanetary mode system [6,10 –which12,18]. allows Two planetary either MG1 gearsets or MG2 of PG1to act and as _ + _ = ( +1) (20) thePG2 speeder are restructured for splitting into a compoundthe ICE input planetary power system and whichthe other allows as eitherthe torquer. MG1 or The MG2 basic to act speed as the + = ( +1) relationshipspeeder for splitting of three thepower ICE sources input power_ in this and compound the other system as the torquer.can be_ further The basic derived speed as: relationship (21) of three power sources in this compound system can be further derived as: = (22) _ + _ = ( +1) (20) where _ represents theω SICE1_MG engine1 + k1ω speedR1C2_ICE at =the(k 1joint+ 1 )shaftωC1R2 of PG1 ring gear R1 and PG2(20) _ + = ( +1) _ (21) carrier C2. The speed relationship and steady-state torque relationship of ICE, MG1, MG2 and eCVT output can be illustrated by a reconstrucωS2_MG2 +tedk 2ω leverC1R2= diagram= ( k2 + 1 as)ω shownR1C2_ICE in Figure 8. (22)(21) As indicated in Figure 8, the speeds of ICE, MG1, MG2 and the output shaft of the compound where _ represents the ICE engineωC 1speedR2 = k atf ω theWHL joint shaft of PG1 ring gear R1 and PG2(22) carrierplanetary C2. systemThe speed respectively relationship position and steady-state along the sametorque lever. relationship The planetary-gearset of ICE, MG1, MG2 output and speedeCVT where ω represents the ICE engine speed at the joint shaft of PG1 ring gear R1 and PG2 carrier output iscanR linear1C 2_beICE illustrated to the vehicle by a wheel reconstruc speedted with lever a fixed diagram gear asratio shown of in. Two Figure motor 8. speeds of _ C2. The speed relationship and steady-state torque relationship of ICE, MG1, MG2 and eCVT output and As _ indicated, respectively in Figure located 8, the atspeeds two sidesof ICE, of MG1, .MG2 The andICE speedthe output _ shaft isof alongthe compound the same can be illustrated by a reconstructed lever diagram as shown in Figure8. planetaryside of _ system. respectively position along the same lever. The planetary-gearset output speed
is linear to the vehicle wheel speed with a fixed gear ratio of . Two motor speeds of _ and _ , respectively located at two sides of . The ICE speed _ is along the same side of _ .
Figure 8. Lever schematics of compound planet planetaryary system in eCVT2 and PH2 mode.
As indicated in Figure8, the speeds of ICE, MG1, MG2 and the output shaft of the compound Once one speed of three-power sources is determined along with the known vehicle speed, the planetaryspeed lever system line is respectively fixed accordingly position alongin Figure the same8. ICE lever. is preferably The planetary-gearset employed as output main speedactiveω powerC1R2 is Figure 8. Lever schematics of compound planetary system in eCVT2 and PH2 mode. linearsource to in the torque vehicle control wheel mode speed withto meet a fixed the geardriver ratio request. of k f . Two The motor optimal speeds running of ω Spoint1_MG1 ofand ICEωS speed2_MG2, respectively located at two sides of ω . The ICE speed ω is along the same side of ω . can beOnce continuously one speed and of three-power variably adjusted Csources1R2 by is active determined closed-loopR along1C2_ ICEspeed with control the known of either vehicle MG1 speed, orS 2_MG2.MG the2 Once one speed of three-power sources is determined along with the known vehicle speed, speedThe resistive lever line torque is fixed of theaccordingly speed adjuster in Figure of ei8.ther ICE MG1 is preferably or MG2 employedwill automatically as main activerespond power and the speed lever line is fixed accordingly in Figure8. ICE is preferably employed as main active power sourcegenerate in a torque balanced control torque mode against to meet the ICE the applied driver request.torque load The within optimal the running compound point lever. of ICE The speed other source in torque control mode to meet the driver request. The optimal running point of ICE speed canmotor be cancontinuously occasionally and provide variably instantaneous adjusted by active assistant closed-loop torque to speed ICE or control regenerative of either braking MG1 or torque MG2. can be continuously and variably adjusted by active closed-loop speed control of either MG1 or MG2. Thefor mechanical resistive torque energy of recovery the speed during adjuster vehicle of ei brakingther MG1 operation. or MG2 will automatically respond and The resistive torque of the speed adjuster of either MG1 or MG2 will automatically respond and generateBased a balanced on the lever torque schematics against the illustrated ICE applied in torqueFigure load8, this within compound the compound power lever.split eCVT The other can generate a balanced torque against the ICE applied torque load within the compound lever. The other motoroperate can in occasionallythe whole vehicle provide speed instantaneous range without assistant violating torque the to thresholds ICE or regenerative of maximum braking speed torque limit for mechanical energy recovery during vehicle braking operation. Based on the lever schematics illustrated in Figure 8, this compound power split eCVT can operate in the whole vehicle speed range without violating the thresholds of maximum speed limit World Electric Vehicle Journal 2020, 11, 46 10 of 21 motor can occasionally provide instantaneous assistant torque to ICE or regenerative braking torque for mechanical energy recovery during vehicle braking operation. Based on the lever schematics illustrated in Figure8, this compound power split eCVT can operate in the whole vehicle speed range without violating the thresholds of maximum speed limit of sun gears of PG1 and PG2. In this compound planetary lever, either MG1 or MG 2 can be employed as the power-split device in a closed speed-loop control to optimally adjust the required engine speed, however the eCVT transmission performance under either MG1 or MG2 as the speed variator will behave differently. In order to derive the torque relationship of three power sources of ICE, MG1 and MG2 of the reconstructed compound planetary system, speed lever relationships in Equations (20) and (21) are further reorganized into two different forms as discussed in the following sections.
3.5.1. Steady-State eCVT2 Torque Distribution with Speeder MG1 Equations (20)–(22) is reorganized into the form of Equations (23)–(25) if MG1 is adopted as the power-split speeder to ICE input:
ωS1_MG1 + k1ωR1C2_ICE = (k1 + 1)ωC1R2 (23)
k1ωS2_MG2 + (k2 + 1)ωS1_MG1 = (k1 + k2 + 1)ωC1R2 (24)
ωC1R2 = k f ωWHL (25) By carefully examining the lever schematics in Figure8 and speed relationship of Equations (23)–(25), and disregarding the planetary-gear transmission loss and some negligible dynamics coupling factor [6], the speed ωC1R2 is used as the imaginary lever fulcrum, then the resistive steady-state torque response of the MG1 speed loop output will have to contain two terms from both ICE and MG2 applied torque, respectively as described by Equation (26):
1 k2 + 1 TS1_MG1 = TR1C2_ICE + TS2_MG2 (26) k1 k1
In order to derive the final torque output of this compound eCVT transmission, the speeder MG1 shall be used as the imaginary lever fulcrum. The eCVT transmission torque output to the wheel can be derived as in Equation (27) containing two terms from both ICE and MG2 applied torque, respectively [18]:
k1 + 1 k1 + k2 + 1 TWHL = k f TC1R2 = k f TR1C2_ICE + k f TS2_MG2 (27) k1 k1
As we can see, if MG1 is used as the power-split device in the eCVT2 mode, the final gear ratio of ICE torque to the wheel in Equation (27) is completely same as the first gear ratio of the eCVT1 mode described in Equation (16). k1 + 1 kG1_eCVT2 = k f = kG1_eCVT1 (28) k1 Furthermore, the MG1 split torque described in Equation (26) has the same sign as both ICE and MG2 torque. Consequently, MG1 can only provide generative split power to ICE input power when MG1 run into negative rotational speed when the vehicle speed is low. In the meantime, the positive assistant torque from MG2 will intensify the MG1 split torque and power. This will unavoidably cause the electric power recirculation and eCVT efficiency drop. Furthermore, higher torque and power rating to MG1 has to be required as well. Thus, the eCVT2 control mode by using MG1 as the power split device is not preferable operation mode for a high load drive request. Instead eCVT1 is more efficient in response to high load condition at low vehicle speed. World Electric Vehicle Journal 2020, 11, 46 11 of 21
When the MG1 speed is maintained at standstill, as indicated in the operation line-2 of Figure8, the MG1 split power is equal to zero. However, MG1 still have to bear the split torque as described in Equation (26) at zero speed. In such a special case, if the loss due to the MG1 balancing torque described in Equation (26) is negligible, ideally the engine and MG2 power will completely transfer to the final wheel. This can be approximately regarded as a parallel hybrid mode of ICE and MG2 torque with fixed gear ratios in Equation (28), or direct ICE drive if no MG2 torque is applied. This running point is + defined as the first Mechanical Point (MP1) of eCVT2 mode with a fixed gain of k k1 1 . The eCVT2 f k1 MP1 is completely same as the eCVT1 MP1. MP1 can be used as the smooth switching point between eCVT1 and eCVT2. The speed lever line across MP1 shown in Figure8 is not unique but infinite and will vary with ICE speed and vehicle speed.
3.5.2. Steady-State eCVT2 Torque Distribution with Speeder MG2 Equations (20)–(22) can also be reconstructed alternatively by Equations (29)–(31) below if MG2 is adopted as the power-split speeder to the ICE input:
ωS2_MG2 + k2ωC1R2 = (k2 + 1)ωR1C2_ICE (29)
k1ωS2_MG2 + (k2 + 1)ωS1_MG1 = (k1 + k2 + 1)ωC1R2 (30)
ωC1R2 = k f ωWHL (31)
Based on the lever principle and the speed ωC1R2 as the imaginary lever fulcrum, assuming the planetary-gear transmission loss and some negligible dynamics coupling factors [6,18] at steady state are negligible, then the resistive torque generated from the MG2 closed-loop speed control will have to contain two terms from both ICE and MG1 applied torque, respectively:
1 k1 TS2_MG2 = TR1C2_ICE + TS1_MG1 (32) −k2 + 1 k2 + 1
The final torque output of this eCVT transmission can be simply derived in Equation (33) if the speeder MG2 is regarded as the imaginary lever fulcrum.
k2 k1 + k2 + 1 TWHL = k f TC1R2 = k f TR1C2_ICE + k f TS1_MG1 (33) k2 + 1 k2 + 1
Equation (33) indicates that the final gear ratio of ICE torque to the wheel has a completely different + gain of k2 k compared to the first gear ratio of k1 1 k when MG2 is used as the power-split device in k2+1 f k1 f the eCVT2 mode. This gear ratio is defined as the second gear ratio for engine torque transmission to the final wheel output. k2 kG2_eCVT2 = k f (34) k2 + 1 Based on Equation (33), the MG1 positive torque regardless of the MG1 rotation speed direction will provide acceleration assistance to the engine torque. In the meantime, the MG1 positive torque will accordingly reduce the overall MG2 split torque as described in Equation (32). This results in less MG1 overall split power and can improve the eCVT transmission efficiency. Once the MG1 applied 1 torque has the relationship of TS1_MG1 = + TR1C2_ICE, the MG2 overall split torque TS2 MG2 = 0, i.e., k2 1 − MG2 split power is zero. This means all input power from ICE and MG1 will fully transfer to the vehicle wheel ideally. Furthermore, the ICE power split ratio in eCVT2 by MG2 can be further defined as:
k2 TR1C2_ICEωC1R2 k2+1 k2 ωC1R2 k2 1 βeCVT2_ICE = = = (35) TR1C2_ICEωR1C2_ICE k2 + 1 ωR1C2_ICE k2 + 1 ϕeCVT2_ICE World Electric Vehicle Journal 2020, 11, 46 12 of 21
ω where ϕ = R1C2_ICE is defined as the ICE eCVT2 speed ratio and will vary continuously with eCVT2_ICE ωC1R2 the vehicle speed and driving load requests.
3.6. PH2 Mode When the MG2 speed is controlled to zero speed, the overall MG2 split power is equal to zero. This implies that the ICE input power will be completely transmitted to the vehicle wheel through the planetary mechanical path ideally. This special eCVT2 operation mode is named as the PH2 mode as indicated in the operational line-4 of Figure8. This is defined as the eCVT2 second Mechanical Point (MP2) [10–12]. Based on Equation (35), we can derive:
ωS2_MG2 = 0 (36)
ωR1_ICE k2 ϕeCVT2_PH2 = = (37) ωC1R2 k2 + 1
k2 TR1C2_ICEωC1R2 k2+1 k2 ωC1R2 βeCVT2_PH2 = = = 1 (38) TR1C2_ICEωR1C2_ICE k2 + 1 ωR1C2_ICE In the PH2 mode, the torque dependency of three power sources is same as the equations described in Equations (32) and (33). Even though the MG2 split power split is zero, MG2 must provide a balanced split torque against ICE and MG1 torque input as defined in Equation (32). This MG2 steady-state split torque will unavoidably cause some losses due to motor winding copper loss and motor inverter loss. In a practical application, it is favorable for MG2 to rotate at a low rotational speed when the PH2 mode is employed in order to alleviate the thermal stress unbalance of MG2 motor stator winding and inverter phases. When the vehicle is driven at a high speed, such as steady cruising control, if the engine can directly operate into its most efficient area, the PH2 mode shall be used as the primary optimal control mode to achieve direct ICE drive. MG1 only provides assistant torque in parallel occasionally to satisfy the instantaneous variable torque requests and in the meantime ICE is commanded to satisfy stable and slow-varying torque requests.
4. eCVT Component Sizing Principles The torque and power rating of MG1, MG2 and ICE shall be sized to meet the vehicle performance requirements including acceleration, gradeability, maximum sustainable vehicle speed, etc. An ideal powertrain performance requirement is illustrated in Figure9. At low vehicle speed, constant high traction torque is demanded for satisfying acceleration and gradeability requirements. A constant power is needed above a base speed. The torque-speed curve indicated by inner solid line is assumed to be fully provided by the maximum ICE power capability and the extra power and torque illustrated in the outer dotted line is provided by either MG2 or MG1 to consume onboard traction battery power. As illustrated in Figure9, in the eCVT1 mode, through the MG1 torque split, ICE maximum k1+1 torque can constantly be transmitted to the wheel by the first gear ratio of kG = k from zero 1 k1 f vehicle speed theoretically. Similarly, the ICE maximum torque can also be provided to the wheel k1 by the second gear ratio of kG = k from zero vehicle speed to high speed in the eCVT2 mode. 2 k2+1 f Therefore, the maximum eCVT powertrain torque output can be derived from Equation (13) as:
k1 + 1 TWHL_max = k f TICE_max + k2k f TMG2_max (39) k1 where TWHL_max represents the maximum wheel torque demand of eCVT hybrid powertrain output in order to meet the vehicle performance requirements; TICE_max is the maximum available ICE torque World Electric Vehicle Journal 2020, 11, 46 13 of 21
output;WorldT MGElectric2_max Vehicleis Journal the maximum 2020, 11, x FOR MG2 PEER positive REVIEW torque capability. Based on Equation (39), 12 theof 20 MG2 maximum torque rating can be sized by:
_ =0 (36) 1 k1 + 1 TMG2max = TWHL _max _ TICE_max (40) k2k_ f = −= k1k2 (37) +1 As we can see from Equation (40), due to the support of ICE torque through its mechanical path +1 _ with the first-gear ratio, the maximum= torque and power demand= by MG2 can=1 be dramatically(38) reduced. _ +1 MG2 is only required to provide torque and_ power to_ cover the area between_ the lines of the maximum ICE mechanicalIn the PH2 torque mode, output the torque at first dependency gear and maximumof three power required sources wheel is same torque, as the as equations illustrated in described in Equations (32) and (33). Even though the MG2 split power split is zero, MG2 must Figure9. If PWHL_max is defined to be the maximum required powertrain power to the wheel, then the provide a balanced split torque against ICE and MG1 torque input as defined in Equation (32). This MG2 maximum power demand can be approximately derived by Equation (41): MG2 steady-state split torque will unavoidably cause some losses due to motor winding copper loss and motor inverter loss. In a practical application, it is favorable for MG2 to rotate at a low rotational PWHL_max speed when the PH2 mode is PemployedMG2_max =in korder2k f T MGto 2_maxalleviate the thermal stress unbalance of MG2 (41) TWHL_max motor stator winding and inverter phases. BasedWhen on the the torquevehicle splitis driven principle at a high in Equation speed, such (11), as the steady MG1 cruising maximum control, torque if the capability engine can can be rateddirectly by: operate into its most efficient area, the PH2 mode shall be used as the primary optimal control mode to achieve direct ICE drive. MG1 only provid1 es assistant torque in parallel occasionally to satisfy the instantaneous variable torqueTMG requests1_max = andT inICE the_max meantime ICE is commanded to satisfy (42) k1 stable and slow-varying torque requests. Accordingly, in the eCVT2 mode, the maximum powertrain torque output to the wheel can be derived4. eCVT based Component on Equation Sizing (33): Principles The torque and power rating of MG1, MG2 and !ICE shall be sized to meet the vehicle k k + k + 1 1 k + 1 performanceT requirements including= k 2acceleration,+ 1 2gradeability,T maximum= 1sustainablek T vehicle speed, (43) WHL_eCVT2_max f k + k + k ICE_max k f ICE_max etc. An ideal powertrain performance2 1requirement2 1is illustrated1 in Figure 9.1 At low vehicle speed, constant high traction torque is demanded for satisfying acceleration and gradeability requirements. As we can see from Equation (43), the maximum available torque in the eCVT2 mode is theoretically A constant power is needed above a base speed. The torque-speed curve indicated by inner solid line equal to the maximum ICE mechanical torque output in the eCVT1 mode. In theory, this implied that is assumed to be fully provided by the maximum ICE power capability and the extra power and k1+1 eCVT2 can fully cover the powertrain torque control area below the k TICE line as illustrated torque illustrated in the outer dotted line is provided by either MG2 ork1 MG1f to _maxconsume onboard in Figuretraction9. battery power.
FigureFigure 9. 9.eCVT eCVT powertrain powertrain overall overall torque–speedtorque–speed performance performance demand. demand.
Furthermore,As illustrated based in onFigure Figure 9, in9, MG1the eCVT1 shall bemo ablede, through to provide the torqueMG1 torque and power split, ICE coverage maximum between k1+1 k2 the toquetorque output can constantly line of bek transmittedk f TICE max toand the kwheel+1 k f TbyICE themax firstat gear least. ratio This of can be= verified from by: zero 1 − 2 − vehicle speed theoretically. Similarly, the ICE maximum torque can also be provided to the wheel by k1+1 k2 TWHL MG = k TICE k TICE _ 1_eCVT2_max k1 f _max k2+1 f _max k − (44) k1+k2+1 f k1+k2+1 = TICE = k TMG k2+1 k1 _max k2+1 f 1_max World Electric Vehicle Journal 2020, 11, 46 14 of 21
This fully complies with the MG1 eCVT2 torque output defined in Equation (33). In addition, the maximum power demand from MG1 can be estimated approximately by Equation (45):
k1 + k2 + 1 PWHL_max k1 + k2 + 1 PMG1_max k f TMG1_max = PWHL_max (45) ≈ k + 1 k1+1 (k + 1)(k + 1) 2 k TICE 2 1 k1 f _max
This MG1 power rating definition in Equation (45) can satisfy the maximum power split World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 14 of 20 requirement in the eCVT1 mode as well.
5.5.1. Simulation Simulation Demonstration Parameter Setup A simulation demonstration of the proposed eCVT hybrid powertrain is carried out for a pickup 5.1. Simulation Parameter Setup truck HEV application. The relevant parameters of the pickup truck vehicle are listed in Table 2. A simulation demonstration of the proposed eCVT hybrid powertrain is carried out for a pickup truck HEV application.Table 2. TheVehicle relevant parameters parameters of a hybrid of the elec pickuptric vehicle truck (HEV) vehicle pickup are truck. listed in Table2. Vehicle parameters Definition Table 2. Vehicle parameters of a hybrid electric vehicle (HEV) pickup truck. Half-load vehicle weight in simulation 2357 kg VehicleMaximum Parameters speed >160 Definition km/h Half-load vehicleEV max weight speed in simulation >105 2357km/h kg 0–100Maximum km/h acceleration speed <12>160 s km/h MaximumEV max speedgradeability >35%>105 km/h 0–100Maximum km/h accelerationwheel power >120
AA longitudinallongitudinal two-geared eCVTeCVT DHTDHT transmissiontransmission is proposed withwith thethe schematicschematic mechanicalmechanical structurestructure layoutlayout asas shownshown inin FigureFigure 1010.. The output of the eCVT DHT is directly connected to the finalfinal = 4.1 drivedrive axle with a fixedfixed finalfinal gear ratio of k f = 4.1.. TheThe ICEICE inputinput shaftshaft isis coupledcoupled toto thethe eCVTeCVT DHTDHT throughthrough aa torsionaltorsional damper.damper.
Figure 10. Schematic structure layout of the longitudinal eCVT DHT for a pickup truck HEV. The eCVT DHT component parameters are listed in Table3. The HEV pickup truck is equipped The eCVT DHT component parameters are listed in Table 3. The HEV pickup truck is equipped with a 1.5-L turbo-charged engine with a peak torque-power rating of 264 Nm/110 kW. The gear ratios with a 1.5-liter turbo-charged engine with a peak torque-power rating of 264 Nm/110 kW. The gear of two planetary gears are carefully selected to ensure the maximum operational rotation speed of ratios of two planetary gears are carefully selected to ensure the maximum operational rotation speed PG1 and PG2 sun gears be restricted within 10,000 rpm. The two gear ratios of eCVT DHT to the final of PG1 and PG2 sun gears be restricted within 10,000 rpm. The two gear ratios of eCVT DHT to the final wheel are selected with = 6.235 and = 2.961. Two electric machines of MG1 and MG2 are sized based on the theoretical principles addressed in Section 4. The continuous rating of MG1 and MG2 has significant impact on the vehicle continuous gradeability. A high-voltage (HV) traction battery is sized based on an HEV power-type battery cell with a capacity of 5.5 Ah and an instantaneous peak discharge rate of 30 C. The internal resistance of battery cells is modeled based on a lookup table versus the battery state of charge (SOC) data.
World Electric Vehicle Journal 2020, 11, 46 15 of 21
wheel are selected with kG1 = 6.235 and kG2 = 2.961. Two electric machines of MG1 and MG2 are sized based on the theoretical principles addressed in Section4. The continuous rating of MG1 and MG2 has significant impact on the vehicle continuous gradeability. A high-voltage (HV) traction battery is sized based on an HEV power-type battery cell with a capacity of 5.5 Ah and an instantaneous peak discharge rate of 30 C. The internal resistance of battery cells is modeled based on a lookup table versus the battery state of charge (SOC) data.
Table 3. Component parameters of eCVT HEV powertrain.
HEV Powertrain Component Parameters Values Displacement 1.5-L turbo-charged gasoline engine ICE Peak torque power 264 Nm@3600 rpm/110 kW Peak efficiency >36% Peak value 140 Nm/65 kW/10,000 rpm MG1 Nominal value 80 Nm/35 kW Peak value 220 Nm/70 kW/10,000 rpm MG2 Nominal value 120 Nm/40 kW
PG1 ratio k1 = 1.92 PG2 ratio k2 = 2.60 Average eCVT mechanical efficiency 95% Gear Final gear ratio of rear-drive axle k f = 4.10 1st gear ratio (MP1 gain) kG1 = 6.235 2nd gear ratio (MP2 gain) kG2 = 2.961 HV Cell 5.5 Ah, 96 in series, 30 C instantaneous max-discharge rate battery Pack capacity 350 V/1.92 kWh, <60 kW (10 s)
ICE, MG1 and MG2 are respectively modeled based on their torque-speed-efficiency performance maps as shown in Figure 11. The maximum engine efficiency is around 36.5%. The MG1 and MG2 motor systems including motor and inverter has a peak efficiency of around 94%. The eCVT control strategy is developed to ensure ICE operate ideally along its optimal performance curve as indicated in the ICE efficiency map of Figure 11 for improving fuel economy. Some important eCVT DHT characteristic parameters are calculated based on the component sizing specifications as listed in Table4. The maximum speed of PG1 and PG2’s sun gears is clamped within 10,000 rpm. The peak torque in both EV and eCVT mode will satisfy the wheel torque requirements in Table2.
Table 4. eCVT dedicated hybrid transmission (DHT) parameters for an HEV pickup truck application.
HEV Powertrain Component Parameters Values EV MG1 maximum speed 8870 rpm @ 105 km/h Motor maximum speed ≈ EV MG2 maximum speed 7900 rpm@105 km/h ≈ EV maximum wheel torque 3820 Nm ≈ eCVT1 maximum wheel torque 3790 Nm Maximum wheel torque capability ≈ ICE max wheel output torque at 1st gear 1560 Nm@k ≈ G1 ICE max wheel output torque at 2nd gear 740 Nm@k ≈ G2 ICE maximum speed 3340 rpm@160 km/h PH2 max speed ≈ MG1 maximum speed 7100 rpm@160 km/h ≈ World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 15 of 20
Table 3. Component parameters of eCVT HEV powertrain.
HEV Powertrain Component Parameters Values Displacement 1.5-liter turbo-charged gasoline engine ICE Peak torque power 264 Nm@3600 rpm/110 kW Peak efficiency >36% Peak value 140 Nm/65 kW/10,000 rpm MG1 Nominal value 80 Nm/35 kW Peak value 220 Nm/70 kW/10,000 rpm MG2 Nominal value 120 Nm/40 kW
PG1 ratio =1.92 PG2 ratio =2.60 Average eCVT mechanical 95% Gear efficiency Final gear ratio of rear-drive axle =4.10 1st gear ratio (MP1 gain) = 6.235 2nd gear ratio (MP2 gain) = 2.961 5.5 Ah, 96 in series, 30 C instantaneous max-discharge HV Cell rate battery Pack capacity 350 V/1.92 kWh, <60 kW (10 s)
ICE, MG1 and MG2 are respectively modeled based on their torque-speed-efficiency performance maps as shown in Figure 11. The maximum engine efficiency is around 36.5%. The MG1 and MG2 motor systems including motor and inverter has a peak efficiency of around 94%. The eCVT Worldcontrol Electric strategy Vehicle is Journal developed2020, 11, 46to ensure ICE operate ideally along its optimal performance curve16 of as 21 indicated in the ICE efficiency map of Figure 11 for improving fuel economy.
(a)
(b) (c)
Figure 11. ICE,ICE, motor motor generator generator (MG)1 (MG)1 and and MG MG22 torque-speed-efficiency torque-speed-efficiency map map ( (aa)) ICE ICE efficiency efficiency map; map; (b) MG1 eefficiencyfficiency map;map; ((cc)) MG2MG2 eefficiencyfficiency map.map.
5.2. Simulation Result Analyses A comprehensive Matlab/Simulink model including driver, vehicle dynamics, eCVT DHT, ICE, MG1 and MG2, battery and control strategy is developed to demonstrate the feasibility of the proposed eCVT-based powertrain. Two driving cycles including a practical Chinese city cycle and the well-known Worldwide harmonized Light vehicles Test Cycle (WLTC) are employed in simulation. A control strategy based on predetermined optimal control profile of ICE torque by rule of thumb is employed to investigate the fuel-saving feasibility of the proposed eCVT hybrid powertrain. Further optimization on control strategy based on online efficiency optimization of the power sources of ICE, MG1 and MG2 will be investigated in the future. However, even the current simple control strategy can fully demonstrate that significant fuel-consumption improvement to the hybrid pickup truck can be achieved by the proposed eCVT with two gears.
5.2.1. Simulation Investigation for an Actual Chinese Drive Cycle Figure 12 presents the simulation results for three cycles of a measured city drive cycle in the Baoding city of China. The eCVT DHT control mode shown in Figure 12a is defined as 0 = EV mode, 1 = eCVT1 and 2 = eCVT2 mode, PH1 and PH2 are included, respectively within eCVT1 and eCVT2 as their special cases. The eCVT powertrain system intermittently operates in EV, eCVT1 and eCVT2 mode. During urban driving, the EV mode is automatically applied whenever the battery SOC reaches above the predefined threshold of 40% and battery charge mode is not requested. The eCVT2 hybrid mode is primarily applied to satisfy the light-to-medium driver torque request and charging electric power to the onboard traction battery through MG2 power splitting. The battery shall request a World Electric Vehicle Journal 2020, 11, 46 17 of 21
Worldcharging Electric mode Vehicle when Journal the 2020 battery, 11, x FOR SOC PEER drops REVIEW below 30% in an EV mode. The battery charging 17 mode of 20 request within an eCVT mode shall be deactivated once the battery SOC exceeds 55%. Based on fuelFigure consumption 12d, the HV (FC) battery illustrated SOC is maintainedin Figure 12a within is calculated its predefined based operationalon the integral window value of of 30%–70%. ICE fuel inThe liter eCVT1 being hybrid divided mode by integrated is only applied traveloccasionally distance in 100 whenever km at every the driver sampling load interval. request exceedsAn average the ICEeCVT2 fuel traction consumption capability. of less than 4 L/100 km can be achieved for this light-load Baoding city-driving cycle if a balanced battery SOC is considered.
(a) (b)
(c) (d)
Figure 12. Simulation results for three Baoding city driving cycles: ( aa)) Baoding city city cycle data, DHT mode and ICE fuel consumption; ( b)) ICE ICE operational operational torque torque location location within within ICE ICE map map and and efficiency; efficiency; (c) ICEICE/MG1/MG2/MG1/MG2 operational torque and speed; ( d) DHT mode, high-voltage (HV) battery state of charge (SOC), power and voltage.
FigureFor this 12c city illustrates driving the cycle operational with many behavior stops atof trathreeffic power lights, control the EV sources mode dominatesof ICE in red, most MG1 of inthe blue driving and MG2 time asin indicatedblack. In the in FigureEV mode, 12c. MG2 ICE isis onlyfully startedin charge when of the battery torque charging and power is requested. delivery forIn thesatisfying eCVT modes, the drive ICE demands; provides ICE powertrain keeps at tractionstandstill torque with 0Nm control torque and inoutput; the meantime MG1 rotates delivers due toextra the powerPG1 mechanical to charge restriction the HV battery without through active powertorque splittingand power by output. either MG1During or the MG2. forced Figure battery 12b chargingindicates and that SOC-balancing ICE is controlled control to operate by eCVT2, within MG2 a narrow runs bandas speeder in blue to ofimplement the pre-determined torque and optimal power splittorque-speed function curveto ICE specified and MG1 in inputs. Figure 11 ICEa. Asis always a result, optimized the overall to ICE deliver efficiency efficient is over torque 30%. through Inefficient its mechanicallow ICE torque pass requests to meet are the prohibited driver demand. by the control In the strategy. meantime, The negativeMG1 assists ICE lineto provide in Figure instant 12b is accelerationdue to friction torque drag or torques regenerati duringve braking ICE fuel torque cut-off occasionally.operation if ICE rotates above 0 rpm. The instant ICE fuelThe consumptionbattery SOC (FC)shown illustrated in Figure in 12-d Figure is 12balanca is calculateded within basedits permissible on the integral operation value window of ICE fuel of 30%–70%.in liter being The divided battery by SOC integrated is integrated travel based distance on the in 100battery km atload every current sampling due to interval. the electric An loads average of MG1, MG2 and low-voltage (LV) 12 V with a 500 W constant LV load assumed in simulation. Due to the HV battery internal impedance and SOC variation impact, the battery voltage changes significantly with electric current load. The battery power is clamped within its permissible power range within ±60 kW. World Electric Vehicle Journal 2020, 11, 46 18 of 21
ICE fuel consumption of less than 4 L/100 km can be achieved for this light-load Baoding city-driving cycle if a balanced battery SOC is considered. Figure 12c illustrates the operational behavior of three power control sources of ICE in red, MG1 in blue and MG2 in black. In the EV mode, MG2 is fully in charge of the torque and power delivery for satisfying the drive demands; ICE keeps at standstill with 0 Nm torque output; MG1 rotates due to the PG1 mechanical restriction without active torque and power output. During the forced battery charging and SOC-balancing control by eCVT2, MG2 runs as speeder to implement torque and power split function to ICE and MG1 inputs. ICE is always optimized to deliver efficient torque through its mechanical pass to meet the driver demand. In the meantime, MG1 assists to provide instant acceleration torque or regenerative braking torque occasionally. The battery SOC shown in Figure 12d is balanced within its permissible operation window of 30%–70%. The battery SOC is integrated based on the battery load current due to the electric loads of MG1, MG2 and low-voltage (LV) 12 V with a 500 W constant LV load assumed in simulation. Due to the HV battery internal impedance and SOC variation impact, the battery voltage changes significantly with electric current load. The battery power is clamped within its permissible power range within 60 kW. ± 5.2.2. Simulation Investigation for A Complete WLTC Cycle The simulation demonstration results for three complete WLTC cycles are illustrated in Figure 13. The overall average fuel consumption reaches about 6 L/100 km with respect to a balanced battery SOC. The EV mode is still used frequently during city driving, and the eCVT2 mode is primarily adopted for battery SOC balancing and fuel-efficient driving. The parallel hybrid mode PH2 in eCVT2 is introduced for direct engine drive at MP2 during highway driving while MG2 is maintained at 0rpm, as indicated in the highlighted band of Figure 13c. ICE is in many cases maintained to follow the predefined optimal performance curve but has more PH2 parallel operation to support highway driving in more efficient way. The overall ICE efficiency is kept above 30%. The battery SOC is controlled within 30%–70% and the battery power is limited within its power boundary of 60–60 kW. − 5.2.3. Acceleration Performance Investigation A full acceleration driver request is introduced to demonstrate the acceleration performance of the pickup HEV vehicle, as illustrated in Figure 14. eCVT1 is primarily selected to achieve fastest acceleration from the vehicle standstill up to the vehicle speed of 105 km/h. The maximum traction torque from both ICE and MG2 applied simultaneously at the beginning and MG2 torque gradually decreases due to overall eCVT power restriction. MG1 operates in a closed-loop feedback speed control to deliver torque and power split to ICE input during the eCVT1 mode. Once the vehicle speed exceeds the eCVT1 speed limit of 105 km/h, ECVT2 is employed instead to keep high-speed driving. ICE still maintains its maximum torque output and MG1 provides assistant torque for maintaining full acceleration instead. Alternatively, MG2 is swapped into closed-loop speed control in order to automatically split the torque and power input from both ICE and MG1. A final acceleration performance with about 9.7 s from 0 to 100 km/h and 4.4 s from 0 to 60 km/h is achieved. World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 18 of 20
5.2.2. Simulation Investigation for A Complete WLTC Cycle The simulation demonstration results for three complete WLTC cycles are illustrated in Figure 13. The overall average fuel consumption reaches about 6 L/100 km with respect to a balanced battery SOC. The EV mode is still used frequently during city driving, and the eCVT2 mode is primarily adopted for battery SOC balancing and fuel-efficient driving. The parallel hybrid mode PH2 in eCVT2 is introduced for direct engine drive at MP2 during highway driving while MG2 is maintained at 0rpm, as indicated in the highlighted band of Figure 13c. ICE is in many cases maintained to follow the predefined optimal performance curve but has more PH2 parallel operation to support highway Worlddriving Electric in Vehiclemore Journalefficient2020 ,way.11, 46 The overall ICE efficiency is kept above 30%. The battery SOC19 of 21is controlled within 30%–70% and the battery power is limited within its power boundary of −60–60 kW.
(a) (b)
World Electric Vehicle Journal 2020, 11, x FOR PEER REVIEW 19 of 20 decreases due to overall eCVT power restriction. MG1 operates in a closed-loop feedback speed control to deliver torque and power split to ICE input during the eCVT1 mode. Once the vehicle speed exceeds the eCVT1(c) speed limit of 105 km/h, ECVT2 is employed instead(d) to keep high-speed driving.Figure ICE 13.13. still SimulationSimulation maintains results results its for formaximum three three Worldwide Worldwide torque harmonized output harmonized and Light MG1Light vehicles vehiclesprovides Test CycleTest assistant Cycle (WLTC) (WLTC) torque cycles: for maintaining(cycles:a) WLTC ( afull) cycle WLTC acceleration data, cycle DHT data, modeinstead. DHT and Alternativel ICEmode fuel and consumption; ICEy, MG2fuel consumption;is (b swapped) ICE operational into(b) ICEclosed-loop torque operational location speed torque within control in orderICElocation mapto automaticallywithin and effi ICEciency; map ( splitcand) ICE efficiency;/theMG1 torque/MG2 (c operational) andICE/MG1/MG2 power torque input operational and from speed; bothtorque (d) DHT ICE and mode, andspeed; HVMG1. (d battery) DHT A final accelerationSOC,mode, power HV performance battery and voltage. SOC, with power about and 9.7voltage. s from 0 to 100 km/h and 4.4 s from 0 to 60 km/h is achieved.
5.2.3. Acceleration Performance Investigation A full acceleration driver request is introduced to demonstrate the acceleration performance of the pickup HEV vehicle, as illustrated in Figure 14. eCVT1 is primarily selected to achieve fastest acceleration from the vehicle standstill up to the vehicle speed of 105 km/h. The maximum traction torque from both ICE and MG2 applied simultaneously at the beginning and MG2 torque gradually
(a) (b)
FigureFigure 14. AccelerationAcceleration simulation simulation results results with with a a full-pedal driver request: ( (aa)) vehicle vehicle speed speed and and drivingdriving mode; mode; ( bb)) ICE/MG1/MG2 ICE/MG1/MG2 torque and speed.
6. Conclusion A novel two-mode power-split eCVT hybrid transmission architecture with two planetary gears for HEV/PHEV applications is proposed. The actuation of one clutch and one brake inside eCVT is controlled for implementing multimode hybridization and EV functions. The detailed theoretical analyses illustrate that the proposed eCVT can offer many advantages over several popular hybrid powertrains already available in market. The direct mechanical path of engine torque with two independent gear ratios can provide a maximum ICE torque output available from standstill with two different gear ratios. Two electric machines can be sized with much less torque and power rating due to the availability of maximum engine torque transmitted mechanically by separate two gear ratios from standstill. The instantaneous torque capability of two-geared eCVT is comparable to a conventional multi-geared transmission powertrain system. Comprehensive simulation demonstrations for a pickup truck HEV application are carried out to prove that significant fuel saving and acceleration performance improvement can be achieved.
7. Patents The proposed eCVT DHT architecture concept in this paper is fully based on the Chinese patents of CN201921174082.6 and CN201910672384.4.
Author Contributions: Conceptualization, X.C. (Xiaojiang Chen) and J.J.; methodology, X.C. and L.Z.; software, X.C. (Xiaojiang Chen); validation, X.C. (Xiaojiang Chen), H.T. and J.J.; formal analysis, X.C. (Xiaojiang Chen); investigation, J.J.; resources, L.Z., and X.C.(Xiaofeng Chen); data curation, H.T.; writing—original draft preparation, X.C. (Xiaojiang Chen); writing—review and editing, J.J., L.Z., and X.C. (Xiaofeng Chen); visualization, X.C. (Xiaojiang Chen).; supervision, L.Z.; project administration, J.J., and X.C. (Xiaofeng Chen); funding acquisition, H.T. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
References World Electric Vehicle Journal 2020, 11, 46 20 of 21
6. Conclusions A novel two-mode power-split eCVT hybrid transmission architecture with two planetary gears for HEV/PHEV applications is proposed. The actuation of one clutch and one brake inside eCVT is controlled for implementing multimode hybridization and EV functions. The detailed theoretical analyses illustrate that the proposed eCVT can offer many advantages over several popular hybrid powertrains already available in market. The direct mechanical path of engine torque with two independent gear ratios can provide a maximum ICE torque output available from standstill with two different gear ratios. Two electric machines can be sized with much less torque and power rating due to the availability of maximum engine torque transmitted mechanically by separate two gear ratios from standstill. The instantaneous torque capability of two-geared eCVT is comparable to a conventional multi-geared transmission powertrain system. Comprehensive simulation demonstrations for a pickup truck HEV application are carried out to prove that significant fuel saving and acceleration performance improvement can be achieved.
7. Patents The proposed eCVT DHT architecture concept in this paper is fully based on the Chinese patents of CN201921174082.6 and CN201910672384.4.
Author Contributions: Conceptualization, X.C. (Xiaojiang Chen) and J.J.; methodology, X.C. and L.Z.; software, X.C. (Xiaojiang Chen); validation, X.C. (Xiaojiang Chen), H.T. and J.J.; formal analysis, X.C. (Xiaojiang Chen); investigation, J.J.; resources, L.Z., and X.C.(Xiaofeng Chen); data curation, H.T.; writing—original draft preparation, X.C. (Xiaojiang Chen); writing—review and editing, J.J., L.Z., and X.C. (Xiaofeng Chen); visualization, X.C. (Xiaojiang Chen).; supervision, L.Z.; project administration, J.J., and X.C. (Xiaofeng Chen); funding acquisition, H.T. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.
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