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u
ElectricVehicleDesign
TaiRanHsu,Ph.D.
DepartmentofMechanicalandAerospaceEngineering
SanJose,California
October12,
2011
TaiRanHsu
nd , , , ,
NewYork,ISBN:9780071543736,1994
SpartanZeroEmissionsHybridHumanPoweredVehicleZEMHHPV,byAmandeep Manik,
ScottMacPherson,HeathFieldsandMarkRafael,SanStateUniversitystudentseniordesign
projectreport,ElectricalEngineeringDepartment,May2008
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Content
Part1TheBasics
HistoryofElectricVehicles
Anatomyof
EVs
DesignforPower
DriveTrain
DesignforVelocityandNochargeCruisingRange
Part2Hybrid
EVs
and
Regenerative
Braking
System
RegenerativeBrakingSystems(RBS)forHybridGaselectricVehicles
TheFlywheelDrivenRBS
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ElectricVehicles PastandPresent
LightWeight,Low Speed
NeighborhoodandLightUtilityVehicles(Limitedtostreetswithlowpostedspeedlimits)
HighSpeed,LongCruisingRange
(Freewaylegal)
NissanLeaf(2010) TeslaRoadster
ChevyVolt(2011) TeslaModelS(2012)
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DesignforPower
TheveryfirstiteminEVdesignistodeterminehowmuchelectricpower
requiredto
drivethevehicle atatopvelocitywithexpectedpayloadsfora nochargecruising
range.Asimpleformulafromrigidbodydynamicswilldothejob.
thefollowingforces:1)Thefrictionforcesbetweenthewheeltiresandtheroadsurface(Ff)
2 T eaero ynamicresistance Fr
3)Thedynamicforcesassociatedaccompaniedwithanyaccelerations(Fd)
g
V,aFr
Ff
Fd
W=mg
Fg
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ElectricPowerRequirement
Thepower(P)requiredtodrivethevehicleatvelocityVis:
) VFFFFP gdrf +++=wherePhastheunitofhorsepower(hp);Fin(lb);Vin(ft/s)
1hp=550ftlb/s
InSIsystem: PhasaunitofWatt(W):1W=1J/s;(1J=1Nm),
so1W=1Nm s,an 1 W=0.7457 p
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ForcesonRunningEVs
Fr ,
Fd
f
W=mg
Fg
Thetotalweight ofthevehicle(curbwt.+payload)isveryimportantinthedesign
Normally weight distribution is about 45% on front axel
required power for EVs with 4 wheels:
Ff =N = (W/4)
where = coefficient of friction, or rolling resistance factor between wheel tires
and road surface:
= 0.015 on a hard surface (concrete)
= 0.08 on a medium-hard surface (asphalt)
= 0.3 on a soft surface (sand)
The-values may vary with speed V (mph) with: = 0.012 (1 + V/100)
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ForcesonRunningEVs contd
Fr ,
Fd
f
W=mg
Fg
Theaerodynamicdragforce(Fr)tothevehicleisunavoidablewhenitisrunning.Itcanbeexpressedas:
Fr =(Cd AV2)/391 withnegligiblewind
whereFr isin(lb),Aisthefrontalareain(ft2)andVisthevelocityofthevehiclein(mph)
ThedragcoefficientCd fortypicalEVsare:
Cd =0.3to0.35forcars;0.33to0.35forvans;and0.42to0.46forpickuptrucks
CoefficientCd needstobemodifiedwhenthereisarelativewindvelocityofVw present:Cw =[0.98(Vw/V)
2 +0.63(Vw/V)]Crw 0.4(Vw/V)
where Vw =averagewindvelocity(mph);V=vehiclevelocity(mph);Crw =relativewindcoefficient=1.4
formostsedans
Totalaerodynamicdragforceonvehicleis:
Fr =(Cd A
V
2
)/391+[0.98
(Vw/V)
2
+0.63
(Vw/V)]Crw 0.4
(Vw/V)
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ForcesonRunningEVs endsV,a
Fr
Ff
Fd
Dynamicforces(F )tothevehicleneedstobeaccountedforonlyifthevehiclechangesitsvelocity
W=mg
Fg
e.g.,inaccelerationsordecelerations. Themagnitudeoftheseforcesis:
Fd =
Ma
where M = mass in slug or kg in SI system; and a = acceleration with (+) sign and deceleration with
(-) sign in unit of ft/s2 of m/s2 in SI system
Gravitational,orbodyforce(Fg)indeterminingtherequiredpoweronlywhenthevehicletravelson
slopedroads.Itsmagnitudeis:Fg = W Sin
in which W = the total weight of the vehicle; = is the inclination of the road surface. A +ve sign for
traveling up the slope and a ve sign for down-slope traveling.
NoticeAll
forces
are
related
to
the
WEIGHT
of
the
vehicle.
Minimizing
weight
is
amajorconsiderationindesign
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TypicalDriveTrainofEVs
Wheel
ManualMotor&Differential
GearDriveShaft
TransmissionController
DriveAxles
Wheel
BatteryBanks
&BMS
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DesignofDriveTrain
Threeusefulformula:
h = Tor ue xm hxRevolution/mile / 315120x for selectin motor
Torquewheel =Torquemotor x(OverallgearratioxOveralldrivetrainefficiency()
Speedwheel (mph)=(rpmmotor x60)/(Overallgearratioxrevolution/mile)
where 0.9, Overallgearratio=rpmmotor/rpmwheel
Determine thetorqueofwheels:
Torquewheel =Ff R+Fh
whereR=radiusofdrivingwheeltire,ft;h=distancebetweencenterofgravity(CG)
ofthevehicleandthewheelaxel,ft
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DesignofElectricSystemandBMS
Electricsystemanditsassociatedbatterymanagementsystem(BMS)arethecardiologysystem
ofhumanbodies.MostEVscontainasystemasillustratedbelow:
BatteryCharger
e.g.,48DCV,15A
BatteryBanks
e.g.48DCV,4000Wh ea.
DC DC
Converter
48DCV
SPDTSwitch
120VAC
Power
Source
MainContactor
400AMax
ReverseContactor
400AMax
Electronic Throttle
Motor
Controller
48DCV,250A(1hr)
&ControlSwitching
DCMotore.g.,10hp(40hppeak)
48 72DCVserieswound
ConnectedtoMechanicalDriveSystem
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MajorComponentsinElectricalSystem
ComponentName Picture PrincipalFunction
Batterycharger 15Awallchargerfrom110ACVto
. .,
Main contactor Itisaheavydutysafetyswitch
Reversecontactor Toallowelectronically controlled
Motorcontroller Tocontrolmotorspeedandallowssafe
reversin
DCmotor ThemotorthatdrivestheEV.Should
deliverthemaximumdesigned power
fortheEV
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BatteriesforElectricVehicles
.Nocarcanrunwithoutgastank.Thelargerthegastankthefartherthecarruns.
Batteriesarewherethevehicledrivingenergyisstored. NoEVorHEVcanrunwithoutbatteries.
, .
Characteristics LeadAcid LithiumIon NickeMetalHydride
CommonBatteries
for
Vehicles
Pbacid Liion NiMH
Voltage(v) 12 3.2to3.6percell 1.4to1.6percell
Electrolytec Surfuric
acid Lithium
salts
Alkaline
TheorecticalEnergyDensity 3542 150250 6070
(kW/kg)
TheoreticalAmphr 45 3to12 5to10
.
Regularchargingtime(hr) 4to8 1.5to2 1
Memoryeffect Atlowvoltage No No
Selfdischarge 2to10%/mo 1%/mo >30%/mo
Cost Low Hig Mo erate
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DesignforVelocity
ThevelocityoftheEVrelatestothespeedofthedrivingmotorandthedrivetrain
o eve c e,ass own y e ormu a:Speedvehicle (mph)=(RPMmotor x60)/(overallgearratioxrevolutions/mile)
where Overallgearratio=RPMmotor/RPMwheel
Revolutions/mile=5280/(d)inwhichd=diameterofwheeltireinft
Thevelocity(orspeed)ofthevehicle(V)inthefollowingformulaisalsorelatedtothe
Obviously,theelectricpower(P)intheaboveequationmustbegreaterthanthepower
requiredto
drive
the
motor
(hpmotor)
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DesignforNochargeRange
ecru s ngrangeo an epen son ow astt eve c etrave sonspec c
roadconditionsandthetrafficenroute.
Thecruisin ran eofanEV R canbeobtainedb usin thefollowin formula:
R =nE Vav/P miles
where n=totalno.ofbatteriesorcells
E =(TheoreticalAmph)x(voltageoutputbyeachbatteryorcell,v)from
characteristicsoftheselectedbatteries(Wh)av = verageve c eve oc ty m es r
P =Requiredpowertodrivethevehicle,W
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DesignCase
A neighborhoodelectricvehiclewithacurbweightat1200lbs andisdesignedtocarrya
payloadof400lbs.thevehicleisdesignedtooperateunderthefollowingconditions:
1) Thevehicleispoweredby2banksofleadacidbatterieswith12voutputbyeachbattery.
Eachbankconsistsof4batteriesconnectedinseries. TheDCamphoutputis45/battery.2) Travelsonstraightflatconcretepavedroadswithanaverageslope av =3
o.
3) MaximumspeedVmax =35mphwithanaveragespeedVav =25mph(or36.67ft/s).
Thelatterisusedasthedesignedvelocity
4) Thevehicleisdesignedtoaccelerationfromzeroto25mphin30secondsaftereachstop.
5) Thevehiclehasasmallfrontsurfaceareaof8ft2 withanaerodynamicdragcoefficientCd =0.3.
6) Thevehiclewheeltirediameteris20inches.
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Designforpowerrequirement:
canbeobtainedbytheequation:
whereV=Vav =25mph=25x5280/3600ft/s=36.67ft/s
Thefrictionforce: Ff = W/4 =0.015x(1600)/4=6lbs
Theaerodynamicdragforce:Fr =(CdAV2)/391=(0.3x8x252)/391=1.4lbs
ThedynamicforceFd =Ma=(1600/32.2)x[(36.67 0)/30]=60.7lbs
ThegravitationalforceFg =WSin =1600Sin(3o)=83.74lbs
TotalforcesactingonthevehicleisF =6+1.4+60.7+83.74=151.84lbs
HencetherequiredpowerP =FV=151.84x36.67ftlbs/s=5568ftlbs/s
=5568 550 p =10.12 p
=10.12/0.7457kW=13.58kW
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SelectionofDCmotor:
se e ormu a: pmotor =
orquewheel xmp
x
evo u on m e
x
Intheaboveformula:
Torquewheel =FfR +Fh withR=10/12=0.833ftandh=2ft(estimated)
with forces:Ff=6lbs,Fr=1.4lbs,Fd =60.7lbs,andFg =83.74lbs,
andRevolution/mile=5280(ft/mi)/(2R) (ft/rev) = 1009 rev/mi,
an = . , a common assump on, we ave e orsepower o e mo or o e:
Hpmotor =[6x10/12+(1.4+60.7+83.74)x2]x25(mph)x1009rev/mi/(315120x0.9)=26.39hp
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DesignforNochargeRange
R =nE Vav/P
w ere n=no.o a er es=
E =
(TheoreticalAmph)x(voltageoutputbyeachbatteryorcell,v)fromcharacteristicsoftheselectedbatteries(Wh)
=45(Amph)x12(v)=540Wh
= av = mp
P= Requiredpowertodrivethevehicle=13.58kW=13580W
Hencethenochargecruisingrangeis:
R=8x540x25/13580=0.318mi
ThisnochargecruisingrangeRfortheEVisUNACCEPTABLYLOW!!
Oneneedtoeitherusemoreandmorepowerfulbatteries(n),orcutdownthepower
requirement(P)byreducingtheweight (W)ofthevehicle.
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in nochargecruisingrange:
d
Higher
spe
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EVswithHigher VelocityandBetterNochargeCruisingRange
Maximumvelocity(V)andNochargecruisingrange(R)arethetwomostimportantdesign
.presenceinmarketplace.
Uptillveryrecently,mostEVscouldonlybeusedforwhatistermedasneighborhood
.
LowVandRareprimarilyattributedtothe limitedelectricpowerandtheenergystorage
systemsusinglessthanefficientbatteries.
FordFocus
Electric
MitsubishiI NissanLeaf TeslaModelS
E ectr cVe c esonCurrentMar et
, ,
Milesper
charge
Upto100miles 50to85miles 100miles 160 to300miles
dependto
batterypackSeats Five Four Five Seven
ro ec e
availability
a e a rea y
available
ar y
*Qualifyfor$7,500federaltaxcreditwithpossible$5,000staterebate.Source:MakeWayforElectricVehiclesSanJoseMercuryNews,April3,2011
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Low wei ht
Streamline exterior
Simple drive train DC to get started
o run a ove mp
Use more high frequency components (> 400 Hz)
DC motor that gets 96 volts
AC motor that gets 400 volts
Matching controller and motor impedance
Use high energy density batteries