Section A8
Paper 139
Disclaimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly
available information and may not provide complete analyses of all relevant data. If this paper is used for any purpose other
than these authors’ partial fulfillment of a writing requirement for first year (freshman) engineering students at the University
of Pittsburgh Swanson School of Engineering, the user does so at his or her own risk.
REGENERATIVE BRAKING AND ITS IMPACT ON OUR ENVIRONMENT
Jacob Schecter, [email protected], Lora 3:00 PM, Brett Haneman, [email protected], Lora 3:00 PM
Abstract—This paper is focused on regenerative braking
systems, which can save energy typically lost to braking and
have it recycled to be used again. A regenerative braking
system is a type of braking system that can be used in hybrid
and electric cars, taking the energy lost during the braking
process and using it to recharge the car’s battery. There are
two main types of regenerative braking systems, flywheels and
electromagnetic, which will be the focus of our paper. These
systems can be used on any type of car, but are more effective
on electric and hybrid cars, where the electricity is actually
used to power the vehicle. This type of technology reduces
the usage of gasoline as well as the pollution released into the
atmosphere. Overall a new system can increase a vehicle’s
efficiency, since less energy is lost from the motor. In our
paper, we will be explaining how regenerative brakes can be
used to increase efficiency and reduce overall fuel usage in
hybrid and electric cars. Our goal is to successfully explain
fully how this process works, provide real world examples of
uses of this product, and explain to the reader why we think
that this is going to be the optimal braking system for the
future.
brake pedals create friction by pressing against the brake
rotors. This is not only a waste of energy, but a waste of gas.
Regenerative braking is a process which takes that
energy which would normally be wasted, and converts it back
into electricity which the engine can use later. The
regenerative system not only has the slowing power of
standard brakes, but also recharges the car’s electric
engine. The consequence of this is that the fuel economy of
a hybrid car is increased by the installation of regenerative
brakes. This is an excellent way to prevent wastefulness in
our society by limiting fossil fuel consumption and
eliminating carbon emissions. Transportation is the second
leading cause of carbon emissions behind only power
generation [1]. The recent growth in popularity of electric
cars are has increased the potential usage of regenerative
brakes. Although they do have their downsides with a limited
driving range and a higher initial price tag, electric cars have
a number of positive aspects as well, such as zero tailpipe
emissions, and long term savings on gasoline due to high fuel
economy. Engineers are constantly working on improving
the technology of these cars to make them more efficient and
more affordable. Electric cars can be the future of
transportation and regenerative brakes are going to make that
a reality.
Key Words — Regenerative Brakes, Brakes, Hybrid and
Electric Cars, Braking Efficiency, Mechanical Engineering
Where they came from
BRAKING CAN HELP YOUR CAUSE
The idea of a type of brake that could convert the kinetic
into potential energy and back again existed since the
1800s. Trolleys were among the first vehicles to successfully
use this type of braking on streets around the 1930s [2]. In
automobiles, these brakes were not commercialized until
Toyota installed them in the first Prius model. The sales of
the Toyota Prius have been increasing yearly as gas prices
continue to increase. The Prius posts a rate of 50 miles-pergallon driving in the city [3]. That is a staggering number to
which hydraulic braking systems don’t ever come close to
reaching. Also, 95% of Prius models sold in the last 10 years
are still on the road today which shows these brakes are just
as, if not more, reliable than hydraulic brakes [3].
Since the release of the Prius model, other hybrid and
electric cars have incorporated these brakes into their new
models like the Tesla Roadster for example. Regenerative
brakes have been utilized in trains, buses, and cars including
Imagine yourself cruising down the highway, windows
down on a warm summer day when all of a sudden, you see
brake lights. You are forced to slam on the brakes stopping
all of your momentum. You think to yourself, where does all
of that energy generated from acceleration go? Each time you
step on the brake pedal in your car, the energy that was present
in your car is wasted as heat energy. A regenerative braking
system (RBS) is designed to harness energy that would
typically be lost and use it for a constructive purpose. The
most energy is wasted in a traffic situation in which you are
constantly stopping and starting again. Here, the regenerative
brakes restore energy most efficiently. The first law of
thermodynamics states that energy can neither be created nor
destroyed. The kinetic energy that is present in your car, is
only present while your car is in motion, and is then released
as heat energy when you step on the brake pedal and those
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03.03.2017
Jacob Schecter
Brett Haneman
Mercedes and Porsche even in Formula One Racing
vehicles. When put into in Formula One racing vehicles, the
average speed of the car increased, reducing the average lap
time by a fifth of a second. It gave the cars more acceleration
while saving energy [4]. As the technology in these brakes
improves and they become less expensive and more efficient,
more companies will begin to incorporate them into more
vehicles.
kinetic energy. That energy must go somewhere, and most of
it is lost to friction by heating both the road and the car’s
braking pads. Regenerative brakes seek to save some of that
lost energy, and use it to replenish the car’s battery.
Usage
This system has the capabilities to repower various
vehicles. A braking system like this can be used in any
motorized vehicle with an electric or semi-electric
motor. However, these brakes are not perfect and do not
provide as much stopping power in comparison to traditional
brakes. The system has a maximum braking point, since the
motor is running in reverse, while still converting the energy
to store it. To make up for this, these cars have a backup
hydraulic brake system. A built-in braking controller decides
whether the regenerative brakes are going to be able to
decelerate the car at the rate that the brake is engaged. The
system will switch the brakes over to the hydraulic brakes if
a greater force is required than the force applied by the
regenerative brakes. This control has been tested and proven
to work efficiently which means the RBS can function as a
friction brake system whenever needed. This decreases the
differences between the two systems even further as the RBS
can have the same qualities as a traditional brake system that
drivers are all used to. However, that system cannot compare
to an RBS, since it cannot replicate what the regenerative
brakes have to offer.
There are some differences between regenerative brakes
and a conventional braking system, with the most prominent
being their purpose. The only purpose of the hydraulic brake
system is to slow the vehicle or bring the vehicle to a complete
stop. These brakes provide the same functions as friction
brakes, and more. Due to the fact that these benefit the
purpose of the brake, there is no reason why they should not
be installed in every available car today.
FIGURE 1 [5]
Diagram of a Hydraulic Brake
The core principle behind regenerative brakes is that an
electric motor can also function as an electric generator under
certain conditions, and these brakes can capitalize on that
fact. Back EMF, which stands for back electromotive force,
is voltage that opposes the direction of the total current
[6]. Regenerative brakes use this back EMF to cause the car’s
motor to act as a generator and recharge the battery. When a
torque is applied to the car’s drive system in opposition to the
direction of motion, i.e. braking, there is an associated back
EMF that can be harnessed. Since the only real difference
between an electric motor and a generator is the direction of
current flow and motion, it is relatively simple to recharge a
battery.
An electric motor functions by running a current through
a loop of wire in a magnetic field [7]. The loop will
experience a force, causing the wire to rotate. This rotational
motion is then used to power the car. Conversely, if a loop of
wire is rotated in a magnetic field it will experience a current,
which is the central concept behind an electric generator.
HOW THEY WORK
Generation of Electricity
A moving object has an associated amount of kinetic
energy, which can be determined by the equation
E=½*m*v . What this equation means it that if a car is
moving twice the speed of an identical car, it will have four
times the kinetic energy of the slower car, and if two cars are
moving at the same speed, but one is twice has heavy, the
heavy car will have twice the kinetic energy. According to
the Law of the Conservation of Energy, energy cannot be
created or destroyed, it only changes form. When a car
brakes, the car’s speed rapidly decreases, so the car loses
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FIGURE 2 [8]
Diagram of a basic electric motor
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Brett Haneman
This is true of both direct current (DC) and alternating
current (AC) generators, the only real difference being in how
the current is harnessed from the loop and
magnets. Likewise, the only difference between AC and DC
motors is how the current is sent through the loop to be
converted in motion [7]. When braking using regenerative
brakes, the wire is moving faster than the magnetic force
would normally cause the wire to move. This induces a
current, which can be stored as electrical energy in the car’s
battery for later use.
regenerative brakes are hybrids or fully electric, cars will be
able to travel longer than ever before without a stop at the
pump. As the price of gasoline continue to increase, the
benefit of regenerative brakes will also continue to increase
as it will save a continually increasing amount of money in
the long term, when compared to cars that only run on
gasoline. This will cause another benefit, the ratio of cost to
savings in a long term time scale. Although the market price
for regenerative brakes may be higher than friction brakes, do
not be deceived. The long-term benefit of having a RBS far
outweighs the initial price of friction brakes. When switching
to regenerative brakes, customers can save an average of
$3,000 less on gas alone [9]. With gas prices higher than ever
before, savings are at an all time high.
In addition, there is a reduction in overall wear and tear
on the braking systems of cars in general. With the use of
regenerative brakes increasing, the demand for hydraulic
friction brakes decreases significantly. It is safe to say that
braking is one of the most important parts of any
transportation method. Without braking long distance travel
would be nearly impossible. Every single time you step on
the brake pedal when using friction brakes, the brakes will not
perform the same way on the next use, as they did on the
previous use, even if it is a slight amount. Brake pad wear is
one of the most prominent maintenance problems in friction
brakes, but with an RBS, this problem is greatly
reduced. With purely electric braking, the wear of the brake
system decreases significantly [10]. This will save consumers
money in the form of a reduction in maintenance costs to
replace worn down brake pads.
While there are downsides to regenerative brakes, the
positive benefits far outweigh the shortcomings. For
instance, large amounts of negative torque on the wheels, that
is, torque that opposes the direction of rotation, can cause the
rear tires to stop rotating and slide across the ground instead
[11]. This can be very dangerous, as the driver no longer has
full control over the car. However, normal brakes have this
same issue, just to a lesser degree. An advanced anti-lock
brake system can reduce this drawback by a significant
amount. In addition, the braking force provided by
regenerative brakes is not currently as strong traditional
hydraulic brakes. Because of this, cars with regenerative
brakes tend to have a backup set of hydraulic brakes that a
computer can have take over in case of emergency. Overall,
the potential drawbacks of regenerative brakes are
significantly overshadowed by the potential benefits, both to
an individual and to society at large.
Efficiency
As with all mechanical systems, efficiency is a primary
concern. Nothing would be used if it was an inefficient way
to handle the situation. Regenerative braking brings a whole
new level of efficiency to the table. As previously mentioned,
the regenerative braking system boosted the Toyota Prius’
fuel consumption to 50 miles-per-gallon. This cannot be
matched without regenerative brakes. Regenerative braking
reduces fuel consumption by an average of 20% which would
decrease consumer spending on fuel as well as save time by
decreasing the frequency of stops at a gas station to refuel
their car [6]. People as a whole will not purchase anything if
it is not better than what they currently have, however, both
of the aforementioned facts provide monetary and time
incentives for the public to purchase cars with regenerative
brakes, increasing the likelihood of people buying these cars..
The overall process of braking with this new system can
save up to and replenish 70% of the energy lost during braking
compared to using a traditional braking system [7]. This
number is currently at zero while using friction brakes and has
no promise of ever increasing. In addition, that part of that
saved energy is regained again the next time the driver brakes,
and the time after that, allowing for a more extensive saving
of energy over the course of a drive. While a 100% efficient
system would be ideal, some energy will invariably be lost to
entropy as heat, and because of this, a 100% efficient system
for anything is impossible. However, with the current
development of technology, this efficiency number will
continue to climb. Vehicles are able to travel farther and
longer than ever before in one tank of gas. As the technology
and usage of these brakes improve, the efficiency will only
increase as the focus transfers mainly onto these brakes
improving in cars.
Advantages
IMPORTANCE TO SOCIETY
Overall, there are many advantages of regenerative
brakes over a traditional brake system. Therefore, these
brakes should be used as much as possible and here’s
why. One major advantage they provide is the reduction in
the use of gas. With the increased fuel economy of these
brakes due to the combination of replenishing energy lost to
braking and electric energy used as the cars that benefit from
Sustainability
A sustainable future is one of the foremost impacts that
this technology is striving for. Sustainable is a word used to
describe something that is able to be maintained at a certain
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level. In terms of the environment, it describes something that
is not harmful to nature and does not deplete natural
resources. Regenerative braking is the quintessential example
of an environmentally sustainable technology, as it both
reduces the use of fossil fuels and decreases carbon emissions.
Recycling is getting much attention as a method to preserve
the environment and that is just what these brakes do. In
addition, the greenhouse gas emissions are reduced as an
effect of the increased fuel efficiency produced by these
brakes.
By utilizing regenerative brakes, the miles-per-gallon of
hybrid cars is increased, which leads to a reduction in
greenhouse gas emissions from transportation, as people can
travel farther using the same amount of gasoline. There is a
finite amount of oil available on Earth, and since oil is the
current method of power for the large majority of people, a
solution to reduce or eliminate oil usage is greatly needed.
Additionally, regenerative brakes reduce one of the
largest drawbacks to electric vehicles, namely short travel
distance and long charge times. With the addition of
regenerative brakes, electric cars can travel further as the car’s
battery is partially recharged whenever the driver brakes.
Customers were hesitant to buy electric cars when they were
first introduced since they could not travel very far on one
charge. With the use of regenerative brakes, drivers can go
farther on one charge. This means that electric cars do not
need to be charged as often, which encourages the public to
purchase electric cars, since there is less of a downside.
Greenhouse gas emissions are a large issue and reducing
them is important. Emissions from transportation is the
second largest source of greenhouse gas emissions, making
up 26% of all emissions [12]. In the next section, we address
this issue more thoroughly, however, regenerative brakes help
reduce emissions per mile driven by increasing the vehicle’s
efficiency. These brakes are vital to reducing greenhouse
emissions on a large scale, where even a small percent change
can have large effects. Over time, these brakes can
substantially decrease our environmental impact before it
becomes dangerous. This reduction in the use of fossil fuels
can create a brighter future for our planet.
Since emissions due to transportation made up 26% of
greenhouse gas emissions in 2014, a reduction in these
emissions would be extremely beneficial [1]. These numbers
made transportation the second largest source of greenhouse
gas, after only electricity generation [1]. In addition, 84% of
all transport emissions come from road travel, the type of
transportation which would benefit the most with the
implementation of regenerative brakes [10]. Studies show
that regenerative brakes can improve gas mileage by 20%,
and this would decrease the amount of greenhouse gas
emissions per mile driven by a corresponding amount [13].
This could result in the overall reduction of our carbon
footprint.
In addition to the reduction of greenhouse gas emissions,
the global consumption of nonrenewable fossil fuels would
also be reduced. With humans constantly using oil,
consumers will eventually be forced to transfer over to
electric cars and not be able to use gas at all. This is a problem
that has a solution right in front of us but no one is turning
their heads to look at it. There is a safe, efficient way to
decrease the use of oil with electric cars and these brakes are
making it more appetizing to customers to purchase fuel
efficient vehicles, especially as the price of gasoline rises over
time.
While renewable energy sources are gaining prevalence,
they are currently dwarfed by the world’s use of
nonrenewable energy sources. As of 2011, only 1.59% of the
world’s energy came from renewable sources [10]. This is a
significant increase from 0.57% of the global consumption a
decade earlier in 2001. It is not increasing at a fast enough
rate to keep pace with our current energy consumption should
the nonrenewable resources run out [10]. However, with
regenerative brakes, there is an additional incentive for the
public to use hybrid and electric cars, because of their
increased efficiency, in addition to the numerous other
benefits that hybrid and electric cars have over gasoline
powered cars. With more vehicles that are less reliant on
fossil fuels, as well as cars that effectively can more
efficiently use these non-renewable sources, society will be
able to consume non-renewable energy sources for longer.
Overall, this process is giving more time for sustainable
energy technology to meet the world’s energy needs.
Environmental Impact
Global warming is becoming more and more noticeable
in our society every day. One of the main causes of global
warming are carbon emissions and land clearing [12]. Global
warming is usually only known for its effect of causing the
temperature of the planet to rise, but it is doing more than just
that. As the overall temperature of the atmosphere increases,
the ice caps are melting and the ocean’s sea level is rising. But
maybe more importantly, the occurrence of storms along with
their severity increase dramatically. All of these factors, if
left unchecked, could end our way of life as we know it
today. A problem like this needs to be addressed before
society causes more damage to Earth than humans can
reverse.
PRACTICAL EXAMPLES
Current Development in Formula 1
There are many developments in the field of
regenerative brakes. For instance, in 2009, Formula 1
allowed their cars to have a type of regenerative brake system
called a kinetic energy recovery system (KERS) [4]. This
KERS is a type of regenerative brake that instead of
converting the mechanical energy into electrical energy to be
stored, then converted back into mechanical energy, the
KERS simply stored the energy as mechanical energy. This
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was achieved via a flywheel, which is a heavy disk that is
rotated when the car brakes, then that rotation continues due
to the disk’s high rotational inertia [2]. This stored rotational
energy is then used to accelerate the car back up to full speed
after the braking is finished. This system was able to increase
acceleration and saved 70% energy typically lost to
braking. At the same time, commercially available
regenerative brakes found in the Toyota Prius, for
comparison’s sake, were only able to restore 30% of braking
energy [4].
and it is important that future children are able to live in the
safe and beautiful world that all other generations were able
to grow up in. Parents strive to create a perfect world for our
children, as well as their children, and even their children and
so on. Going to parks and just enjoying the different views of
nature, of sunsets, and of bodies of water is one of my favorite
activities, and if this wasn’t available to future generations,
that would be a travesty.
Regenerative braking is a safe, easy, and efficient way
to preserve energy and decrease pollution without changing
our daily routine. As such, it should be pursued and improved
upon for the benefit of society as a whole. As such, it is also
a topic in which engineers are constantly working to
improve. With such a little and easy change in our lives the
world’s population could make a huge impact on our
environment for years to come while also saving money and
fossil fuels.
Tesla
A more specific and in-depth case would be Tesla, a car
company that specializes in electric cars, that is using
regenerative brakes in their cars because one of the main
limitations in electric cars is the car’s battery life. The battery
life of an electric car corresponds to the maximum distance
the car can drive, similar to the size of a traditional car’s gas
tank. Since it takes longer to recharge a fully electric car than
it takes for refill the gas tank in a hybrid or gasoline powered
car, the increase in battery life brought by regenerative brakes
is crucial to Tesla’s cars. An example of a car with
regenerative brakes installed would be the Tesla Roadster,
which was first sold to the public in 2008 [11].
To balance against the some of the limitations of
regenerative brakes, additional features were added to make
the car safer. As was mentioned earlier, if large amounts of
negative torque are applied to the rear wheels, which can
occur during braking, especially with regenerative brakes in
comparison to traditional brakes, those wheels can begin to
skid [11]. This can be very dangerous, since the driver no
longer has control over their car’s back tires. To fix this
problem, a safety system that controls the regenerative brakes
will disable these brakes and only use the traditional brakes
instead. This system works in conjunction with the far more
commonplace anti-lock brake system (ABS), which helps
prevent cars from skidding. In addition, if the battery is
already fully charged, additional energy added can be
dangerous, so there is another safety system which can disable
the regeneration feature of the brakes to prevent this from
occurring. Tesla and other companies have proven that
regenerative brakes can work effectively in real-world
situations, and not just in laboratory testing.
SOURCES
[1] “Sources of Greenhouse Gas Emissions” US
Environmental Protection Agency, 10.06.16 Accessed 2.2.16
https://www.epa.gov/ghgemissions/sources-greenhouse-gasemissions
[2] T. Cunningham. “Regenerative Braking.” Energy
Education.
Accessed
1.11.2017.
http://energyeducation.ca/encyclopedia/Regenerative_brakin
g
[3] “2017 Prius Features” Toyota, Accessed 2.25.17
http://www.toyota.com/prius/prius-features/
[4] M. Abrams. “Stopping Power.” American Society of
Mechanical Engineers. 1-2012. Accessed 1.11.2017.
https://www.asme.org/engineeringtopics/articles/automotive-design/stopping-power
[6] “Brake Rotor and Caliper” Auto Repair Help (Figure 1)
Accessed
3.31.17
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[6] T. Keller. “Energy Efficiency.” Control Engineering.
9.15.2011.
Accessed
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[7] J. Scott. “How Does an Electric Motor Work.” Power
Transmission Engineering. 10-2016. Accessed 2.28.2017.
http://www.powertransmission.com/issues/1016/work.pdf
[8] “Download Electrical Motor Images Free Here”
Wonderful Engineering. 2014. Accessed 3.28.2017.
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[9] C. Gonzalez “What’s the Difference Between Friction and
Regenerative Car Brakes?” Machine Design. 2017. Accessed
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[10] M. Singh. “Regenerative Braking: A Good Source of
Green Energy.” International Journal on Power Engineering
SAVING ENERGY ONE STEP AT A TIME
Over the next century as we, as a society, are running
out of fossil fuels, conservation of these fossil fuels is
essential. Mass producing a system of brakes like this and
putting them into use is going to cause a major decrease in the
usage of fossil fuels and a decrease in pollution of the
atmosphere especially in major cities. This topic is important
to us because of the impact that it will have on our future
generations. They are the main focus of a sustainable world
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Brett Haneman
and
Energy.
1-2013.
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1.11.2017.
http://infomesr.org/attachments/W13-P-0006.pdf
[11] G. Solberg “The Magic of Tesla Roadster Regenerative
Braking”
Tesla
6.29.2007.
Accessed
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https://www.tesla.com/blog/magic-tesla-roadsterregenerative-braking
[12] “Global Warming” LiveScience, Accessed 3.1.17.
http://www.livescience.com/topics/global-warming
[13] P. Patel, H. Chandra, T. Sahoo. “Study on regenerative
braking system, considerations of design, safety and
associated effects.” IEEE Xplore. 7-2015. Accessed
1.24.2017.
http://ieeexplore.ieee.org/xpls/icp.jsp?arnumber=7386930
ADDITIONAL SOURCES
“Introduction to Regenerative Braking” AASA: Know Your
Parts.
2014.
Accessed
1.24.2017.
http://www.knowyourparts.com/technicalarticles/introduction-regenerative-braking/
N. Luo, J. Jiang, A. Yu. “Research on the control strategy of
the regenerative braking system.” IEEE Xplore. 7-2017.
Accessed
1.11.2017.
http://ieeexplore.ieee.org/xpls/icp.jsp?arnumber=7232021
ACKNOWLEDGEMENTS
We would like to thank our roommates for putting up
with our late hours, and our friends for helping us proofread
the paper. In addition, we would like to thank our writing
instructor and conference co-chair for editing our paper and
helping to make it better.
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