Page - 266 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 437 (2) Inputsplit type: In thepowersplit transmission(PST)usingthesingleplanetarygear (PG), thepower issplitat the inputsideoroutputsideof thePSTdependingonthe locationofPG[4,17]. Thesplitpower flowsto themechanicalpathandelectricalpath. Since theefficiencyof theelectricalpath,which consistsof thepowerelectronic(PE)systemincludingMG1,MG2,andtheinverter, ismuchlower thanthatof themechanicalpath, thePSTefficiency increasesas thepowerratioof theelectrical pathdecreases.Whenall thepowerflowsthroughthemechanicalpath, thePSTefficiencyshows thehighestvalue,andthis is calledthe ‘mechanicalpoint’ (MP) [18]. Intheinputsplit typePHEV,thePSTefficiencydecreasesrapidlywhenthevehiclespeedbecomes higher than the speedof themechanicalpoint. This is becausepower circulationoccurs along the electricalpath,whichcausesdecreasedPSTefficiency. Incontrast, in theoutputsplit type, thepower circulationoccurswhenthevehiclespeed is lower thanthemechanicalpoint. Therefore, if thevehicle drivesmostly in the city (urbandynamometerdrivingschedule cycle), inotherwords, at a lowto mediumspeed, it isdesirable touse the inputsplit type thathasarelativelyhighefficiency in the low tomediumspeedregionwithout thepowercirculationwhenthemechanicalpoint ispositionedat highspeed[18]. Considering theabove twoconstraints, the inputsplit configurationusingasingleplanetarygear wasselectedasareference topology indevelopmentofanewPHEV. Apower split structure canbe representedas ageneralized single levermodel [19]. Figure 1 showsa levermodelof the inputsplit type,which involvesoneengineandtwoMGsusingthesingle planetarygear. Since the singlePG isused, there are threenodes in the lever. Now,wecanfinda feasiblePSTconfigurationas follows: (a) IfweuseMG2asamaindrivingmotor, itshouldbeconnectedtotheoutputnode. This isbecause MG2needs tobeoperatedtopropel thevehicledirectly in theelectricvehicle (EV)mode,ascan beobservedinmostPHEVs. (b) In the singlePG, thenodeof the speed lever shouldbepositioned in theorderof the sun (S), carrier (C),andringgear (R),orviceversa fromthe leveranalogy. (c) Themechanicalpointneeds tobe locatedatahighvehiclespeedtoachievehighPSTefficiency in the lowtomediumspeedrange. Therefore,wehavetwopossibleconfigurations,asshowninFigure1. InFigure1, i is the lever distance fromtheoutput to theengineandaandbare the leverdistances fromtheoutput toMG1and toMG2, respectively. It is seenfromFigure1 thatb is equal tozerobecauseMG2needs tobe locatedat theoutput fromtheabovecondition(a). Since theelectricalpowerbecomeszeroat themechanicalpoint,wecanfindthespeedratioof themechanicalpointunder theconditionthat thespeedofMG1orMG2iszero.Wedefinethespeed ratio (SR)as, SR= ωe ωout , (1) whereωe is theenginespeedandωout is theoutputspeed(vehicle speed). Fromthespeed lever in Figure1,whenthespeedofMG1orMG2iszero, thespeedratioat themechanicalpoint isobtainedas SR = a− i a = (a/i)−1 (a/i) atmechanicalpoint. (2) InFigure 2, the speed ratio at themechanical point is shownwith respect to a/i for an input split type transmission [20]. Tomeet condition (c) above, the speed ratio at themechanical point shouldbe theoverdriveratio (regionF) forwhich theoutputspeedof thePSTishigher thanthe input (engine)speed. This implies that the leverdistancesaand ineedtobe in thesamedirectionandthat a/i is larger thanone. It is foundthat leverAsatisfies theaforementionedrequirements,while lever 266