Engineering an Affordable Electric Vehicle

Transcription

Tinker Engineering Electric Vehicle 1
Engineering an Affordable Electric Vehicle:
Conversion and Range Optimization
© 2012 by Joel Tinker
Reprinted 2012
By Aquosus Potentia
www.aquopotent.net
© 2011 by Robyn Bentley, reprinted 2011 by Aquosus Potentia, www.aquopotent.net
Abstract
Due to the poor economic and environmental record of the U.S., interest in electric vehicles is
beginning to increase. The advantages of electric vehicles (EVs) include the following: (1) EVs do not
require gasoline or oil, and as a result, significant operational cost savings can be realized over time, and
(2) EVs produce zero emissions and are more environmentally friendly than gas-powered vehicles.
Current disadvantages of EVs include (1) initial cost for commercially available vehicles (typically $30,000$60,000) is generally higher than for comparable gasoline-powered vehicles, and too expensive for the
average consumer, and (2) range (distance traveled between battery charges) is limited by battery
performance, vehicle weight, aerodynamic drag, and rolling resistance.
The purpose of this project was to address the cost and range disadvantages listed above, by
converting a gasoline-powered vehicle to electric to reduce cost of obtaining an EV, and improving range
performance of the electric vehicle through reduction of aerodynamic drag and rolling resistance. First,
the vehicle was converted to electric by removing the internal combustion engine and related
components, and installing the electric motor and all electric components. Second, range testing was
accomplished to determine the effects of average speed and battery temperature on range. Next, several
aerodynamic and rolling resistance enhancements, such as bed covers, spoilers, wheel well covers, low
rolling resistance tires, and low-viscosity drive train fluids were investigated in coast-down testing.
Finally, various selected aerodynamic and rolling resistance enhancements were used in additional range
testing.
Results showed that the total cost of the electric truck conversion was $10,000, approximately
1/3 the cost of a commercially available EV. Coast-down testing showed that the aerodynamic bed cover
(unique development in this project) and low rolling resistance tires were the most effective
enhancements. Therefore, these enhancements plus the front spoiler were installed for the final range
testing with enhancements. Range testing with combined enhancements showed a major 21 % increase
in range compared to the baseline vehicle. This project successfully addressed two current issues for
electric vehicles by (1) demonstrating a low-cost method of obtaining an EV and (2) significantly
improving range performance through innovative design modifications.
Introduction and Background
History of Electric Vehicle Development
The electric motor was invented long before the internal combustion engine. Electric vehicles
(EVs) were first used in the mid 1800s and were manufactured in volume in the late 1800s and early
1900s. Alessandro Volta invented the first electric battery in 1800, and Joseph Henry invented the first
primitive direct current (DC) motor in 1830. Battery-powered electric technology was applied to the first
land vehicle by Thomas Davenport in 1834. Early electric vehicles were very successful in urban areas,
due to paved roads, available power, short driving distances, and low speed limits (Leitman & Brant,
2009, p. 35). The peak production year for early EVs was 1912 (34,000 electric cars registered in the
U.S.).
Oil was discovered in Western Pennsylvania by Edwin Drake in 1859. This was the spark that
ignited the oil revolution. Oil quickly became vitally important to the United States economy. As a result
of readily available, inexpensive oil and gasoline, internal combustion engine automobiles exploded onto
the scene in the early 1900s, and EV production dwindled to one company (Detroit Electric) in the 1920s.
During the 1960s, environmental concerns related to use of fossil fuels led to renewed interest in electric
vehicles. However, commercial EV development was slow, and significant progress came from
individuals converting their own vehicles (Leitman & Brant, 2009, pp. 44-46).
In October 1973, an event called “the first oil shock” occurred. The Arabs cut their oil production
by five percent each month in order to meet their price objectives. Oil companies had to scramble in
order to buy as much oil as they could, and consumers did the same. This event caused gas prices to
double, and EV interests began to rise once again. In the 1980s, EV development began to slow once
again due to the “oil shock glut” of 1986. Gas prices dropped dramatically, and EVs seemed to simply
disappear (Leitman & Brant, 2009, p. 47).
In the 1990s, there was substantial electric vehicle technology improvement. EVs had achieved
higher range, higher top speed, and increased dependability. During the 1990s, California passed laws
(Zero Emissions Vehicles (ZEV) mandate) requiring auto makers to produce zero-emissions vehicles.
Later, the law was changed, effectively releasing auto makers from having to build EVs at all. As a result,
auto makers terminated their programs and began to recall and destroy the EVs they had produced.
Roughly 1000 of these vehicles remained in private hands, due to public pressure. This event caused a
former EV driver and activist Chris Paine to create a full-length documentary, entitled “Who Killed the
Electric Car?,” which premiered at the 2006 Sundance Film Festival and was released in theaters by Sony
Pictures Classics (Paine, 2006).
Understandably, the attack on the World Trade Center, the Pentagon, and United Airlines flight
93 on September 11, 2001, created new concerns about U.S. reliance on imported oil. High gas prices
since 2005 (exceeding $4.00/gallon for brief periods) have caused widespread recognition of the need for
mass production of alternative vehicles, such as EVs, and a decrease of imported oil. General Motors
(GM) recently developed an EV known as the Volt, a plug-in hybrid, and Nissan has developed the allelectric Leaf. Tesla Motors introduced its Tesla Roadster on July 20, 2006. The Roadster has outstanding
performance, achieving a range of 200 miles and a top speed of 125 mph, but its cost ranges from
$30,000-$60,000 (Leitman & Brant, 2009, pp. 65-67).
Figure 1 (p. 6) shows a timeline of electric vehicle development, summarizing many of the events
described in this section. The timeline was developed by the author specifically for this research project.
Comparison of Electric and Gasoline Powered Vehicles
Vehicles with internal combustion engines are complicated and require a large, heavy vessel to
withstand high temperature and high pressure combustion processes. Internal combustion engines
require systems for cooling, exhaust, ignition, fueling, lubrication, and starting. It is difficult to keep all of
these systems working correctly, which means more frequent repairs and higher repair costs. Internal
combustion engines create waste heat and pollution. The input fluids and exhaust gases are toxic, and
emissions contribute to climate change. Internal combustion engines are also only 20 percent efficient
(Leitman & Brant, 2009).
In contrast to gasoline-powered vehicles, EVs require no fossil fuel of any kind and are powered
completely by electricity. As a result, they are much less expensive to operate. EVs are also less complex
than gasoline-powered vehicles, consisting mainly of an electric motor, motor controller, and a single
battery or set of batteries, depending on whether the batteries are lithium-ion or lead-acid. One
disadvantage of EVs is that batteries are expensive, and they must be replaced every five to six years.
Electric motors have only one moving part, while internal combustion engines have hundreds. As a
result, electric motors have high reliability and low maintenance costs. In the past, EVs had low top
speed and low range. However, with Tesla and GM leading the way, new technology is being developed
to give EVs higher top speeds and range (130 mph and 300 miles) (Leitman & Brant, 2009).
Additional advantages of EVs are realized because they are zero-emissions vehicles (ZEVs).
Therefore, they do not put off any toxic solids or exhaust gases. Substitution of EVs for gasoline-powered
vehicles will reduce carbon dioxide, hydrocarbon, and nitrogen oxide emissions, and significantly benefit
the environment. Even the power plants that create energy for the EVs are held to a higher standard
than those of the internal combustion engines. The only possible wasted elements of EVs are the
batteries, which are recyclable. An electric vehicle converts about 70 percent of the charging energy into
motor energy, whereas a typical gasoline-powered vehicle converts only about 20 percent of the energy
in gasoline into engine energy. The U.S. consumes 20.8 million barrels of petroleum a day, of which nine
million barrels is gas. There is a limited amount of oil, and as the supply continues to decrease, gas prices
will continue to increase (Leitman & Brant, 2009, p. 55), further increasing interest in electric vehicles.
Renewed Interest in Electric Vehicles and EV Conversions
Slowly, auto makers are producing more EVs and hybrids, but most U.S. citizens still own
gasoline-powered vehicles. It will most likely be many years before EVs become dominant, due to limited
options for purchase and the initial high cost. Despite the high price to purchase an EV, much of the cost
can be offset through lower operating expenses, since the vehicle does not require gasoline. However,
for the cost of commercial EVs to become competitive with gasoline-powered vehicles, mass production
by numerous auto makers is needed. Progress has been made in recent years, but commercial EV
options remain limited.
Another alternative to purchasing a commercially available EV is to convert a gasoline-powered
vehicle to electric power (the focus of this research project). The U.S. is gaining interest in converting
gas-powered vehicles to electric because it is much less expensive than buying a new hybrid or electric
vehicle. It is also not as complicated as it may sound. Bob Batson says “Building an EV…requires 100-200
hours of labor and $5000-$9000 in components” (2002). This is much less than buying a new electric
vehicle, which could cost anywhere from $30,000-$60,000 (according to EV manufacturers). In
approximately 200 hours of labor, a conversion can be completed. The converted electric vehicle can be
very practical for everyday use (Batson, 2002, p. 27). In the next section, the factors affecting range of
an electric vehicle are described. Understanding of these factors is critical for successful EV conversion,
including selection of the electric motor and components and determination of design enhancements to
improve range.
Figure #1 – Timeline for EV Development
Factors Affecting Range of an EV
To convert a gasoline powered vehicle to electric, the first step for determining (1) the best
choices for the electric motor and components and (2) potential range performance enhancements is to
investigate all of the forces affecting the vehicle:
Total Force = (Acceleration + Climbing + Rolling + Drag + Wind) Resistance
(Eq. 1)
Where
A = Frontal Area
Cw = Relative Wind Factor
a = Acceleration
Fd = Drag Force
Cd = Drag Coefficient
W = Vehicle Weight
Ci = Inertia Conversion Factor
φ = Angle of Incline
Cr = Rolling Resistance Factor
V = Vehicle Speed
The forces acting on the vehicle must be converted to torque, and an electric motor must produce this
torque to overcome all forces acting on the EV:
Torque Required = Force x (5280/2π) / Tire Revolutions per Mile
(Eq. 2)
If the forces acting on the EV (such as aerodynamic drag and rolling resistance) can be reduced, the
torque that must be provided by the motor is decreased, and an increase in range can be achieved.
Equation 1 shows that the forces acting on the vehicle are significantly affected by the weight of
the vehicle, aerodynamics, and rolling resistance. This means the range is also significantly affected by
these factors. Vehicle weight can be a large factor in EV range performance because it takes more energy
to power a heavy vehicle. Weight can be minimized by selecting a small, lightweight vehicle for
conversion and investigating lightweight, high-performance batteries (potential future work).
Aerodynamic Drag
Aerodynamics has a significant impact on the range of an electric vehicle. The greater the drag
force, the higher the motor current required to maintain vehicle speed and the lower the range. The
factors that affect aerodynamic drag are the shape of the vehicle, speed and direction of the wind, and
speed of the vehicle (drag force increases with the square of vehicle velocity).
The aerodynamics of a vehicle mainly depend on the vehicle shape. For example, small sports
cars are much more aerodynamic than large trucks. The shape that causes the least amount of drag is
the shape that disrupts the air flow the least. The ideal aerodynamic shape is that of a tear drop (Gemba,
2007). Aerodynamic drag can be reduced through the use of enhancements such as aerodynamic bed
covers, wheel well covers, and front spoilers.
Although aerodynamic drag has a very significant effect on the choice of a vehicle for conversion
to electric, it is not the most important factor. For example, most trucks have much more space for
batteries than cars. Although pickup truck aerodynamic performance is not as good as that of a car, the
ease of conversion is of more importance.
Rolling Resistance
Rolling resistance is primarily affected by the tires that are used. However, there are many other
factors that can affect the rolling resistance that the vehicle experiences, including the following:
•
Tire design
•
Tire pressure
•
Brake drag
•
Drive train fluid viscosity
•
Road surface condition (Leitman & Brant, 2009)
The typical coefficient of rolling resistance on concrete and paved roads is 0.015, but when the surface is
softer, the rolling resistance significantly increases.
Tires do not have the most significant effect on the range of an EV, but they are one of the
easiest factors to improve. Some tires have two layers of material and are designed to handle wet roads.
The overall temperature of these tires is much lower than regular tires with one layer. Lower
temperature means less rolling resistance and an increase in range. Tires are also greatly affected by
their tread depth. Typically, the deeper the tire tread, the greater the rolling resistance. If a tire is worn
down, the treads will be shallower, resulting in less rolling resistance and an increase in range.
Tire pressure also has a significant effect on rolling resistance. When the tire pressure is slightly low, a
greater area of the tire is in contact with the surface of the road, increasing the rolling resistance. Tires
should be pressurized to the maximum value recommended by the manufacturer to minimize rolling
resistance.
When vehicles are newly purchased, the brakes have not been worn down, and there is a drag
caused by the brakes that increases rolling resistance. Used vehicles have less rolling resistance simply
because they do not have the same intensity of brake drag that a new vehicle has. The viscosity of drive
train fluids can also have a significant effect on the rolling resistance that a vehicle experiences. If the
fluid is too viscous, the gears will have resistance or drag when turning, but if the fluid is not viscous
enough, the vehicles gears will wear down prematurely. Synthetic fluids can be used to reduce gear drag
and improve range.
Purpose and Engineering Goals
It was seen in the Introduction and Background section of this paper that advantages of electric
vehicles include the following: (1) EVs do not require gasoline or oil, and as a result, significant
operational cost savings can be realized over time and (2) EVs produce zero emissions and are more
environmentally friendly than gas-powered vehicles. However, current disadvantages of EVs include (1)
initial cost for commercially-available vehicles (typically $30,000-$60,000) is higher than for comparable
gasoline-powered vehicles, and too expensive for the average consumer and (2) range (distance traveled
between battery charges) is limited by battery performance, vehicle weight, aerodynamic drag, and
rolling resistance.
The purpose of the project was to address the cost and range disadvantages listed above, by
converting a gasoline-powered vehicle to electric to reduce cost of obtaining an EV and improving range
performance of the electric vehicle through reduction of aerodynamic drag and rolling resistance. The
engineering goals of the project were to accomplish the vehicle conversion and range optimization
through the following steps:
1. Conduct an engineering trade study to determine the vehicle to be converted.
2. Conduct the conversion design process: (a) select the electric motor and components to achieve the
required vehicle performance (70 mph top speed, and 40-mile range) and (b) determine the layout of
electric components.
3. Remove the internal combustion engine, gasoline tank, and related components from the original
vehicle.
4. Install the electric motor, controller, electrical components, and batteries in the vehicle.
5. Test the electric vehicle in order to determine top speed, maximum range, and maximum incline.
6. Perform test runs of the electric truck for various average speeds in warm and cold weather
conditions to determine the effects of speed and temperature on range.
7. Measure the aerodynamic drag and rolling resistance of the baseline vehicle.
8. Investigate aerodynamic improvements for the vehicle by creating and testing an aerodynamic bed
cover, flat bed cover, rear wheel well covers, and front spoiler.
9. Investigate rolling resistance improvements by using low rolling resistance tires and low-viscosity
drive train fluids.
10. Repeat range testing with various selected aerodynamic and rolling resistance enhancements to
determine improvement in range.
Procedures and Results
The procedures followed to accomplish the project purpose and engineering goals are described
in this section.
Engineering Trade Study for Vehicle Selection
The first step in the project was to select the gasoline-powered vehicle for conversion to electric. This
selection is critical because the success of the conversion project and the performance of the converted
EV depend on it. A systematic engineering trade study was conducted using the criteria listed below:
•
Difficulty of conversion: Since this was a student project, it was required that the conversion be
achievable within a few months’ time. The factors determining the difficulty of the conversion
were (1) space under the hood of the vehicle, (2) access to parts that were to be removed, (3)
ease of removing the gasoline engine, and (4) ease of installing the new electric motor and
components.
•
Battery space available: The vehicle needs to be able to accommodate the number of batteries
required to accomplish the performance goals.
•
Curb weight: The vehicle needs to be as lightweight as possible to maximize range and top
speed.
•
Aerodynamics: The vehicle should have the lowest aerodynamic drag possible, to maximize
range and top speed.
•
Condition/quality of the vehicle: In order to minimize the work and expense for repairs, the
vehicle should be clean, free of rust, have no dents, and have a working transmission, brakes, and
accessories.
•
Cost: The vehicle needs to be as inexpensive as possible while meeting the other criteria, in
order to maximize use of funding for purchase of electric components and conversion costs.
The vehicles suitable for conversion that were available for purchase in the author’s local area were 1986
Mazda 626 Coupe, 1993 Mitsubishi Eclipse, 1986 Mazda 323 DX, 1988 and 1991 Mitsubishi Trucks, 1990
Geo Metro, and 1994 Toyota Truck. The results of the trade study are shown in Table 1. The 1994
Toyota Truck was selected because of ease of conversion, large space for batteries, low weight (2530 lb.),
great condition of body and transmission, and low cost. Despite the poor aerodynamics of the vehicle,
based on the remaining criteria, the 1994 Toyota Truck was the best fit.
Analysis for Selection of Electric Motor and Components
A motor must produce sufficient torque (torque required) to overcome all forces acting on the EV
(Eq. 2 in the Introduction and Background section). Torque required must be graphed as a function of
vehicle speed for this analysis. To determine the torque available for different electric motors, the
following equations were used (Leitman & Brant, 2009):
Torquewheel = Torquemotor x (Overall Gear Ratio x Overall Drivetrain Efficiency)
(Eq. 3)
Speedvehicle= (RPMmotor x 60) / (Overall Gear Ratio x Revolutions per Mile)
(Eq. 4)
Gear Ratio = RPMmotor/ RPMwheel
(Eq. 5)
The motor torque and revolutions per minute (RPM) versus current were obtained from the motor
manufacturers (such as Advanced DC Motors, Inc. Electric Vehicles Applications Manual), and this data
showed calculation of wheel torque available and vehicle speed for various gear ratios and motor current
values. Torque required and torque available were calculated in a spreadsheet and graphed together as
a function of vehicle speed for each individual electric motor being considered (FB1-4001, ImPulse, WarP,
and Kostov) to help determine if the motor could meet performance requirements. Using the torque
required and torque available graphs and considering the major factors defining vehicle performance, the
FB1-4001 electric motor was chosen, as shown in Figure 2 below. This was due to its sufficient top speed,
sufficient expected range (from previous vehicle conversions), ability to handle more than a 25 percent
incline, and its reasonable price. Other electric components were selected that worked well with the
FB1-4001 motor.
Additional information can be understood from the torque required vs. torque available curves
for the FB1-4001 motor shown in Figure 2. Vehicle operating ranges for each gear and are shown above
and to the left of each intersection point with the 0% incline curve. Also, the top speed for each gear is
given by each intersection point with the 0% incline curve. For example, as labeled in Figure 2, the top
speed for the FB1-4001 motor in the 1994 Toyota, in 3rd gear on level ground, is 70 mph.
Figure #2 – Torque Required and Torque Available Curves for FB1-4001 Motor
in 1994 Toyota Small Pickup Truck
Vehicle Conversion
After the conversion vehicle, electric motor, and components were selected, the internal
combustion engine and all unneeded components were removed from the small pickup truck. In order to
install the electric motor, the transmission had to be attached to the motor, using an adapter plate and
coupler. This work involved several steps as follows: first, the end of the transmission shaft was cut so
that it matched the coupler without contacting the motor shaft, and the adapter plate was trimmed so
that it matched the transmission bell housing. Next, the adapter plate and coupler assembly were
fastened to the electric motor. The motor/adapter assembly was then connected to the transmission.
The motor and transmission assembly was lifted using a hoist, the assembly was installed in the truck,
and the driveshaft was connected to the transmission. Twenty-four 6-volt deep-cycle batteries were
installed. Twenty-one were placed in the truck bed and three under the hood (Figure 5 on next page).
The batteries were connected together with welding cable.
Electric components were installed on a board and placed above the electric motor as shown in
Figure 3 below. These components included the motor controller (“brain” of the EV that regulates
current flow), potentiometer (throttle), contactors (switches activated by ignition switch and
accelerator/throttle), shunt (to allow measurement of voltage and current), inertia safety switch, relay
and fuses. More information for those components is given in Leitman and Brant and in Electric Vehicle
Conversion Installation Guidelines.
Figure #3 – Electric Components Used in
Figure #4 – Installed Electric Motor
Vehicle Conversion
Figure #5 – Installation of 6-Volt Lead-Acid Batteries in Truck Bed
Highway Range Testing of Baseline Vehicle
Following completion of the vehicle conversion, range testing of the EV was performed in order
to measure the effect of the following factors on the distance traveled between battery charges:
•
Average and maximum speed
•
Temperature of surroundings
The same route (flat open highway) was used for every test, to remove road conditions as a variable.
After the batteries were fully charged, the vehicle was driven until the batteries were exhausted. The
twenty-four 6V deep-cycle batteries and 12V accessory battery were charged immediately after the test
runs in order to maintain battery life. The initial and final voltage, maximum and average current, battery
and air temperatures, and distance traveled were recorded for each test.
Range testing with various average speeds from 28-50 miles per hour (mph) was done in warm
weather (avg. 69° F) to find the effect of average speed on range. The range was also measured at the
same average speeds in cold weather to determine the effect of low temperatures (avg. 38° F)
on range. In all tests, the maximum speed that was reached was also recorded to determine its effect on
range performance. Range results for warm and cold weather conditions are compared in Figure 6,
indicating approximately 20% lower range for cold conditions. The graph shows that as average speed
increases, range decreases.
Figure #6 – Range as Function of Average Speed for Warm and Cold Battery Conditions
Vehicle Modifications
Due to the high rolling resistance and poor aerodynamics of the pickup truck, modifications were
needed to optimize the range performance. The following aerodynamic modifications were constructed
or implemented for the electric truck:
•
Flat bed cover
•
Aerodynamic (sloping) bed cover
•
Front spoiler
•
Wheel well covers
•
Low rolling resistance tires
•
Low-viscosity drive train fluids
The flat bed cover was designed to prevent turbulent air flow into the bed of the truck and
therefore reduce aerodynamic drag. The bed cover was constructed using 2x3 inch wooden boards and
plywood. The aerodynamic bed cover was an original invention of the author, and was designed to slope
down from the roof of the truck to the back of the bed. (See Figures 7 and 8 on next page.) This design
not only prevents air from flowing into the bed, but it also causes the truck to assume a more ideal
aerodynamic shape (more closely resembling a tear drop), minimizing disruption of the flow and reducing
the drag. The aero bed cover was constructed using 2x3 inch and 2x4 inch wooden boards, and clear
plastic material to cover the framework.
The front spoiler was made from aluminum roofing material. The material was formed to the
shape of the bumper of the truck and bolted into position. The spoiler was designed to reduce air flow
under the truck body and into any cavities that could increase turbulence and drag. (See Figure 7 below.)
The wheel well covers were made from foam material that was cut into the shape of the wheel well. The
purpose of the wheel well covers was to prevent turbulent air from flowing into the rear wheel wells of
the truck. Instead, air should flow smoothly down the side of the truck, reducing drag. Finally, rolling
resistance reductions were sought through the installation of low rolling resistance tires and use of lowviscosity drive train fluids.
Figure #7- CAD Model of Truck with
Figure #8 – Electric Truck with All Enhancements
All Enhancements
Coast-Down Testing
Coast-down testing was performed to estimate the aerodynamic drag coefficient and rolling
resistance coefficient for different electric vehicle configurations. The purpose of the tests was to select
aerodynamic drag and rolling resistance reduction enhancements for implementation and range testing
on the vehicle. The following steps were followed:
1. A section of road was chosen to use for all test runs. The selected test route was flat with
minimal inclines and no turns.
2. The vehicle was accelerated to the desired speed (55 miles per hour), then shifted into neutral
and allowed to coast. The speed of the vehicle was measured every 5 seconds for 60 seconds.
3. Coast-down tests were performed three times in each direction on the test route (total of six
runs for each configuration) to insure that the direction of the test run was not a factor, and the
results were averaged together to calculate the mean. Data for vehicle speed versus time was
graphed for subsequent use as described below.
To determine the drag and rolling resistance coefficients from coast-down data, the starting point is the
force equation (Eq. 1) in the Introduction and Background section. For a vehicle coasting on a level road
with no wind,
Force = CrW + CdAV²
(Eq. 6)
where Cr and Cd are rolling resistance and drag coefficients, W is vehicle weight, A is frontal area, V is
vehicle speed, and M and a are vehicle mass and acceleration. Solve for acceleration to get
a = F/M = -½(CdArV2)/M – Crg
(Eq. 7)
The data from the coast-down testing was inserted into a spreadsheet that was used to curve fit
the acceleration vs. time results and estimate drag and rolling resistance coefficients using the equations
above. Coast-down tests were completed for the baseline electric vehicle, then the modified vehicle with
(1) flat bed cover, (2) aerodynamic bed cover, (3) front spoiler and wheel well covers, (4) low-viscosity
drive train fluids, and (5) low rolling resistance tires. The data were evaluated for all test runs to
determine which enhancements were most effective. It was found that the aerodynamic bed cover, low
rolling resistance tires, and front spoiler were most effective in improving coast-down performance of the
vehicle. Figure 9 (on next page) shows the coast-down test results for the most effective enhancements.
Figure #9 – Coast-Down Data for Electric Vehicle with and without Design Enhancements
Range Testing of Redesigned Vehicle
After the coast-down testing was completed, range testing of the electric vehicle was repeated
with the selected enhancements that were most effective in reducing aerodynamic drag and rolling
resistance (aerodynamic bed cover, low rolling resistance tires, and front spoiler). Range testing with the
enhancements listed showed a major 21 percent increase in range. The maximum range recorded with
these enhancements was 80 miles (65 miles without enhancements) at an average speed of 29 mph. At
an average speed of 50 mph, the range recorded was 50 miles (41 miles without enhancements). Figure
10 on next page shows the significant improvement in range performance achieved with the design
enhancements.
Figure #10 – Range Data for Electric Vehicle with and without Design Enhancements
CFD Modeling with Selected Enhancements
Computational Fluid Dynamics (CFD) analysis was completed for vehicle in the basic configuration
and with the aerodynamic bed cover and front spoiler. This analysis was done to better understand the
aerodynamics of the truck with and without enhancements. The small pickup truck and enhancements
were modeled using the COMSOL multiphysics software package. Since research had shown that a threedimensional (3D) analysis was not feasible, a 2D model was developed. The CFD results show that the
turbulent wake (low pressure) region behind the vehicle is much smaller with the aerodynamic bed
cover, as seen in Figures 7 and 8. The aerodynamic bed cover allows the vehicle to approach the ideal
teardrop shape and provide less disruption of the flow field. Vehicle drag is therefore significantly
reduced through use of the aero cover.
Figure #11 – CFD Results for Baseline Electric Vehicle Showing Large Turbulent Wake Region
Figure #12 – CFD Results for Electric Vehicle with Aerodynamic Bed Cover and Front Spoiler showing
smaller turbulent wake region compared to baseline vehicle
Cost and Economic Analysis
In this section, the operating costs for the small pickup EV are compared to the gasoline-powered
version of the same vehicle (Leitman & Brant, 2009, p. 12). The total EV conversion costs, operating costs,
and repair costs were determined in order to find the total time to recover the conversion costs. The
1994 Toyota electric vehicle pickup averages about .44 kWh per mile. Using the cost of electricity in
Huntsville, Alabama ($.074 per kWh), the operating cost per mile for electricity becomes
(0.44 kWh/ mile) X ($ .074 / kWh) = $ .033 per mile
(Eq. 8)
Assuming a range of 50 miles, and a cost of $1.25 to recharge batteries, the battery charging cost per
mile is $1.25 per charge/50 miles = $.025 per mile. Adding 3.3 cents per mile for battery replacement
gives
Total EV operating cost = $.025 + $.033 + $.033 = $.091 per mile
(Eq. 9)
Next, the EV operating costs for the 1994 Toyota truck conversion are compared to the gasolinepowered version of the same pickup truck. Assuming 20 mpg and a rate of $3.42 per gallon for gas
(national average for regular gas on 11/13/2011), the cost of fuel is
.05 gallons per mile X $3.42 per gallon for gas = $.171 per mile
(Eq. 10)
Periodic maintenance and consumables cost must be added for the gas-powered vehicle. Assuming
$41.67 per month for oil changes, fuel additives, and aligning/balancing tires, and assuming mileage of
12,000 miles per year, additional costs are
$500 per year/12000 miles per year = $ .042 per mile
(Eq. 11)
Adding the fuel and maintenance costs gives
Total gasoline vehicle operating cost = $.042 + $.171 = $.213 per mile
(Eq. 12)
Next, the conversion costs for the Toyota pickup truck EV are presented:
•
Chassis (original vehicle plus repairs):
$1200
•
Conversion parts:
$6305
•
Batteries, including replacement costs:
$2100
•
Construction materials:
$400
•
Total: $10,005
Estimated Savings from EV Operations
•
Savings per mile: $.213 - $.091 =
•
Estimated 12,000 miles per year, with savings per year: $ 1,464
•
Estimated saving in seven years:
•
Therefore, it would take seven years to completely recover costs of conversion if the vehicle
$
.122 (eliminating fuel, oil)
$10,248
were driven regularly, such as a daily commuter vehicle for school or work.
Summary and Conclusions
This project has successfully addressed two disadvantages related to the use of all-electric
vehicles, the high cost of commercially available EVs and the limited range. A small pickup truck was
selected through engineering trade studies for conversion to electric, and the electric motor and
components were chosen through mathematical analysis. The pickup truck was successfully converted at
a total cost around $10,000, approximately 1/3 the minimum cost of a commercially available EV.
The converted electric vehicle was extensively tested to determine the effects of average and
maximum speed and battery temperature on the range. It was found that with increased average and
maximum speed, there was a decrease in range. Range testing in both warm and cold weather
conditions showed that when the temperature is lower than optimum for the batteries (78° F), the range
is negatively affected.
In attempts to improve the range performance of the converted EV, aerodynamic and rolling
resistance enhancements were added to the vehicle and evaluated through coast-down testing and
range testing. Aerodynamic enhancements that were tested included flat and aerodynamic bed covers,
spoilers, and wheel well covers, and rolling resistance enhancements included low rolling resistance tires
and low-viscosity drive train fluids. Coast-down test results showed that (1) the aerodynamic bed cover
was the most effective at reducing drag, followed by the flat bed cover, and (2) the low rolling resistance
tires were very effective at reducing rolling resistance. Final range testing was accomplished with
combined aerodynamic and rolling resistance enhancements, showing a major 21.4 percent average
increase in range compared to the baseline vehicle.
In conclusion, the converted electric vehicle exceeded the design goals for top speed and range
(70 mph and 40 miles), achieving 75 mph top speed and 50 miles range (at avg. 50 mph highway speed).
This project has shown that converting a gasoline-powered vehicle to electric is a practical, affordable
method of obtaining an electric vehicle, providing significant cost savings compared to purchase of
commercially available electric vehicles. The project has also demonstrated that EV conversion is a great
alternative to use of fossil fuel-powered vehicles, helping to protect the environment by reducing
emissions, and providing acceptable speed and range performance. The converted EV with aerodynamic
bed cover has successfully been used as a commuter vehicle for combined highway/city driving and
round trips exceeding 50 miles.
References
Advanced D.C. Motors, Inc. ( 1992). Electric vehicles applications manual. East Syracuse, NY:
Advanced D.C. Motors.
Batson, B. (2002, November). Do it right the first time! EVAmerica. Retrieved from
http://www.evamerica.com/02evamericatechpapersfall20061.pdf
Electric Vehicle Conversion Installation Guidelines. (2005). Wolfesboro, NH: Electric Vehicles of
America.
Leitman, S., & Brant, B. (2009). Build your own electric vehicle. New York: McGraw Hill.
Paine, C. (director). (2006). Who killed the electric car? [Motion picture]. United States: Sony
Pictures.
Tesla Motors. (n.d.). Tesla Motors: Premier Electric Vehicles. Retrieved from
http://www.teslamotors.com/

Engineering an Affordable Electric Vehicle

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