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Electric Vehicle Design TaiRan Hsu, Ph.D. Department of Mechanical of Mechanical and Aerospace Engineering San Jose, California October 12, 2011 © TaiRan Hsu
“ ,” , nd , , New York, ISBN: 978007154373 6, 1994 “ Spartan Zero Emissions Hybrid Human Powered Vehicle ZEMHHPV,” by Amandeep Manik, Scott MacPherson, Heath Fields and Mark Rafael, San State University student senior design project report, Electrical Engineering Department, May 2008
Content Part 1 The Basics History of Electric of Electric Vehicles Anatomy of EVs of EVs Design for Power Drive Train
Design for Velocity and Nocharge Cruising Range
Part 2 Hybrid EV EVss and Regenerative Braking System Regenerative Braking Systems (RBS) for Hybrid Gaselectric Vehicles The FlywheelDriven RBS
Electric Vehicles – Past and Present
LightWeight, LowSpeed Neighborhood and Light Utility Vehicles
High Speed, Long Cruising Range (Freewaylegal)
(Limited to streets with low posted speed limits)
Nissan Leaf (2010)
Tesla Roadster
Anatomy of EVs t n e ) S m e M g ( B a a e M t s y y r S e t t a B n i a r T e v i r D
Power Source Charger Batteries Main Controller (BMS)
Throttle Controller S eed Controller
Motor Adaptor
Transmission Drive wheels
DC from Charger to Batteries Turn key closes Main controller leads to Batteries to Speed controller Throttle controls Speed controller. More volts for higher rpm and higher vehicle speed (48 v for ZEM, 84 v for ZEEN)
Design for Power The very first item in EV design is to determine how much electric power required to drive the vehicleat a top velocity with expected payloads for a nocharge cruising range. A simple formula from “rigidbody dynamics” will do the job.
the following forces: 1) The friction forces between the wheeltires and the road surface (Ff ) 2 T e aero ynamic resistance Fr 3) The dynamic forces associated accompanied with any accelerations (Fd)
Fr
g
V, a Fd
Ff
W = mg
Electric Power Requirement
The power (P) required to drive the vehicle at velocity V is:
P
F f
F r F d F g V
where P has the unit of horse power (hp); F in (lb); V in (ft/s) 1 hp = 550 ftlb/s In SI system:P has a unit of Watt (W): 1W = 1 J/s; (1 J = 1 Nm), so 1 W = 1 Nm s, an 1 W = 0.7457 p
Forces on Running EVs Fr
, Fd Ff
W = mg Fg • The total weightof the vehicle (curb wt. + payload) is very important in the design • Normally weight distribution is about 45% on front axel • Total wei ht contributes to the friction force F – the rimar force in determinin 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)
Forces on Running EVs – cont’d Fr
, Fd Ff
W = mg Fg The aerodynamic drag force (Fr) to the vehicle is unavoidable when it is running. It can be expressed as:
Fr = (Cd A V2)/391with negligible wind where Fr is in (lb), A is the frontal area in (ft2) and V is the velocity of the vehicle in (mph) The drag coefficient Cd for typical EVs are: Cd = 0.3 to 0.35 for cars; 0.33 to 0.35 for vans; and 0.42 to 0.46 for pickup trucks Coefficient Cd needs to be modified when there is a relative wind velocity of Vw present: Cw = [0.98 (Vw/V)2 + 0.63 (Vw/V)]Crw – 0.4 (Vw/V) whereVw = average wind velocity (mph); V = vehicle velocity (mph); Crw = relative wind coefficient = 1.4 for most sedans Total aerodynamic drag force on vehicle is: F (C A V2)/391 [0.98 (V /V)2
0.63 (V /V)]C
– 0.4 (V /V)
Fr
Forces on Running EVs – ends V, a Fd
Ff
W = mg Fg Dynamic forces (Fd) to the vehicle needs to be accounted for only if the vehicle changes its velocity e.g., in accelerations or decelerations. The magnitude of these forces is: 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/s 2 of m/s2 in SI system
Gravitational, or body” force (Fg) in determining the required power only when the vehicle travels on sloped roads. It’s magnitude is: 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. Notice All forces are related to the WEIGHT of the vehicle. Minimizing weight is a
Typical Drive Train of EVs
Wheel
Motor & Controller
Manual Transmission
Drive Shaft
Differential Gear
Drive Axles Battery Banks & BMS
Wheel
Design of Drive Train Three useful formula: h
= Tor ue
x m h x Revolution/mile / 315120 x
for selectin motor
Torquewheel = Torquemotor x (Overall gear ratio x Overall drive train efficiency () Speedwheel (mph) = (rpmmotor x 60)/(Overall gear ratio x revolution/mile)
where 0.9, Overall gear ratio = rpmmotor/rpmwheel
Determine the torque of wheels:
Torquewheel = Ff R + Fh where R = radius of driving wheeltire, ft; h = distance between center of gravity (CG) of the vehicle and the wheel axel, ft
Design of Electric System and BMS Electric system and its associated battery management system (BMS) are the cardiology system of human bodies. Most EVs contain a system as illustrated below: Battery Charger e.g., 48 DCV, 15A
Battery Banks e.g. 48 DCV, 4000 Wh ea. 48 DCV
120 VAC Power Source
Reverse Contactor 400 A Max
DC/DC Converter
SPDT Switch
Main Contactor 400 A Max
ElectronicThrottle
Motor Controller 48 DCV, 250A (1 hr)
DC Motor e.g., 10 hp (40 hp peak) 48 – 72 DCV series wound
& Control Switching
Connected to Mechanical Drive System
Major Components in Electrical System Component Name
Picture
Principal Function
Battery charger
15 A wall charger from 110 ACV to . .,
Main contactor
It is a heavyduty safety switch
Reverse contactor
To allow electronically controlled
Motor controller
To control motor speed and allows safe reversin
DC motor
The motor that drives the EV. Should deliver the maximum designed power for the EV
Batteries for Electric Vehicles “ ” No car can run without gas tank. The larger the gas tank the farther the car runs.
.
Batteries are where the vehicle driving energy is stored.No EV or HEV can run without batteries. , .
Common Batteries for Vehicles Characteristics Voltage (v) Electrolytec
Environmental merits Theorectical Energy Density (kW/kg) Theoretical Amp hr
LeadAcid
Pb acid 12 Surfuric acid 35 42 45
Lithium Ion
Li ion 3.2 to 3.6 per cell Lithium salts non a ueous solution 150 250
Nicke Metal Hydride
NiMH 1.4 to 1.6 per cell Alkaline Potassium h droxide 60 70
3 to 12
5 to 10
1.5 to 2 No 1%/mo Hig
1 No >30%/mo Mo erate
. Regular charging time (hr) Memory effect Self discharge Cost
4 to 8 At low voltage 2 to 10%/mo Low
Design for Velocity The velocity of the EV relates to the speed of the driving motor and the drive train o t e ve c e, as s own y t e ormu a:
Speedvehicle (mph) = (RPMmotor x 60)/(overall gear ratio x revolutions/mile) where Overall gear ratio = RPMmotor/RPMwheel Revolutions/mile = 5280/(d) in which d = diameter of wheeltire in ft The velocity (or speed) of the vehicle (V) in the following formula is also related to the
Obviously, the electric power (P) in the above equation must be greater than the power
Design for Nocharge Range T e cruising range o an EV R epen s on ow ast t e ve ic e trave s on speci ic road conditions and the traffic en route. The cruisin ran e of an EV R can be obtained b usin the followin formula:
R = n E Vav/Pmiles where n = total no. of batteries or cells E= (Theoretical Amph) x (voltage output by each battery or cell, v) from characteristics of the selected batteries (Wh) av = verage ve c e ve oc ty m es r P = Required power to drive the vehicle, W
Design Case A neighborhood electric vehicle with a curb weight at 1200 lbs and is designed to carry a payload of 400 lbs. the vehicle is designed to operate under the following conditions: 1) The vehicle is powered by 2 banks of leadacid batteries with 12 v output by each battery. Each bank consists of 4 batteries connected in series.The DC amph output is 45/battery. 2)Travels on straight flat concrete paved roads with an average slopeav = 3o. 3) Maximum speed Vmax = 35 mph with an average speed Vav = 25 mph (or 36.67 ft/s). The latter is used as the designed velocity 4)The vehicle is designed to acceleration from zero to 25 mph in 30 seconds after each stop. 5) The vehicle has a small front surface area of 8 ft2 with an aerodynamic drag coefficient Cd = 0.3. 6) The vehicle wheeltire diameter is 20 inches.
Design for power requirement:
can be obtained by the equation:
where V = Vav = 25 mph = 25x5280/3600 ft/s = 36.67 ft/s
The friction force:Ff =W /4 = 0.015 x (1600)/4 = 6 lbs The aerodynamic drag force: Fr = (CdAV2)/391 = (0.3 x 8 x 252)/391 = 1.4 lbs The dynamic force Fd = Ma = (1600/32.2) x [(36.67 – 0)/30] = 60.7 lbs
The gravitational force Fg = W Sin = 1600 Sin(3o) = 83.74 lbs Total forces acting on the vehicle is F = 6 + 1.4 + 60.7 + 83.74 = 151.84 lbs Hence the required power P = F V = 151.84 x 36.67 ftlbs/s = 5568 ftlbs/s = 5568 550 p = 10.12 p = 10.12/0.7457 kW = 13.58 kW
Selection of DC motor: se
e ormu a: pmotor =
orquewheel x mp x evo u on m e
x
In the above formula: Torquewheel = Ff R + Fh with R = 10/12 = 0.833 ft and h = 2 ft (estimated) withforces: Ff = 6 lbs, Fr = 1.4 lbs, Fd = 60.7 lbs, and Fg = 83.74 lbs, and Revolution/mile = 5280 (ft/mi)/(2 R) (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)x 1009 rev/mi/(315120x0.9) = 26.39 hp
Design for Nocharge Range R = n E Vav/P w eren = no. o atter es = E = (Theoretical Amph) x (voltage output by each battery or cell, v) from characteristics of the selected batteries (Wh) = 45 (Amph) x 12 (v) = 540 Wh = av = mp P =Required power to drive the vehicle = 13.58 kW = 13580 W
Hence the nocharge cruising range is: R = 8 x 540 x 25/13580 = 0.318 mi This nocharge cruising range R for the EV is UNACCEPTABLY LOW!! One need to either use more and more powerful batteries (n), or cut down the power requirement (P) by reducing the weight(W) of the vehicle.
innocharge cruising range: d e p s r e h g i H
EVs with HigherVelocity and Better Nocharge Cruising Range Maximum velocity (V) and Nocharge cruising range (R) are the two most important design . presence in marketplace. Up till very recently, most EVs could only be used for what is termed as “neighborhood” . Low V and R are primarily attributed to thelimited electric power and the energy storage systems using lessthanefficient batteries.
E ectr c Ve c es on Current Mar et Ford Focus Mitsubishi “I” Nissan Leaf Electric ,
*
Tesla Model S ,
Miles per charge
Up to 100 miles
50 to 85 miles
100 miles
160 to 300 miles depend to battery pack
Seats
Five
Four
Five
Seven
ate
a
ro ecte availability
rea y available
ar y
• Low wei ht • Streamline exterior • Simple drive train • DC to get started u v • 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
