Mechanical Engineering
2165
FLYWHEEL BASED KINETIC ENERGY RECOVERY SYSTEM FOR AUTOS
Andrew Kearns (AJK95@pitt.edu), RJ Liu (YUL89@pitt.edu)
Abstract—Because of the energy crisis, fuel efficiency is
becoming more and more important in the auto industry.
Therefore, automotive engineers are exploring new
approaches to achieve the goal of saving energy. In this
paper, we will carefully examine one of the approaches—
flywheel based kinetic energy recovery system, or KERS,
which stores kinetic energy on a flywheel as a car breaks
and releases that energy while accelerating [7]. KERS
improves fuel economy by storing large amounts of energy
in a flywheel, and it is an important technology because it
can help reduce the damage that humans do to the
environment by burning fossil fuels. This paper will discuss
the kinetic energy recovery system and the way it functions
in detail. It will discuss the mechanics of the system as well
as the physics behind it. In addition we will also discuss the
ethics relating to this technology. Through this paper we
will show the readers a different approach to dealing with
the energy crisis. Not only can we find new ways to make
energy, we can efficiently reuse the energy that a system
already has.
for the cars due to better fuel efficiency [4]. In this paper, we
will examine the flywheel based kinetic energy recovery
system and explain why it is the fuel efficiency technology
of the future.
THE PHYSICS BEHIND KERS
Flywheel kinetic energy recovery system is complicated, but
it is based on some very basic physics. The mechanical
kinetic energy recovery system stores kinetic energy, the
energy of motion, in a flywheel. This can be explained by
the law of conservation of energy, which states that the
energy of a system cannot be created or destroyed. Energy is
transferred from one form to another. For example, the
energy of a pendulum changes from potential energy to
kinetic energy as it swings from its raised position to the
lowest point of its motion, then the kinetic energy is
converted back to potential energy as it swings back up to its
maximum height. Eventually the pendulum will stop
swinging because the energy of the system is lost to air
resistance. Friction between air and the pendulum eventually
converts the kinetic and potential energy of the system into
heat energy. This is the concept that KERS uses. Kinetic
energy is stored in the rotational motion of the flywheel and
then the trasmission transfers the energy so it can be used
again to accelerate the vehicle.
The kinetic energy of a spinning object is equal to the
half the product of the moment of inertia and the square of
its angular velocity. In plain english what that means is the
faster an object spins, the more kinetic energy it has. The
moment of inertia is different for all objects. It depends on
the distribution of mass in an object. For a flywheel, the
moment of inertia depends on its radius and mass. In other
words, the larger and heavier the flywheel is, the greater its
moment of inertia is. Because of this, large and dense
flywheels are able to store large amounts of kinetic energy.
Kinetic energy of rotating objects also translates to linear
velocity, the forward motion of the car. The bottom line is if
a flywheel is capable of storing large amounts of kinetic
energy, it will translate to a greater linear velocity for the
automobile.
The second law of thermodynamics along with
conservation of energy is the reason why mechanical KERS
is superior to electrical KERS. The second law of
thermodynamics states that imbalances of heat energy will
eventually balance out and reach equilibrium. Because of
this, no transfer of energy is one hundred percent efficient.
Energy is always lost to heat. Electrical kinetic energy
recovery systems transfer kinetic energy to chemical energy,
then the chemical energy is changed to electrical energy and
stored for later use. With each transfer of energy, some of
Key Words— flywheel, transmission, kinetic energy,
continuously variable transmission, kinetic energy recovery
system, moment of inertia, transmission, torque, rotational
velocity
INTRODUCING THE MECHANICAL SOLUTION
In this day and age, the whole car industry is facing a great
conflict
between
continuous
development
and
enveronmental issues. In recent years especially, there has
been a push for more fuel efficient vehicles. In part, this is
because of increasing gas prices. More importantly, negative
effects on the environment have caused this trend. Engineers
are taking fuel efficiency to entirely new levels by
developing new ways to get the most out of the fuel we use
to power our automobiles. Cars have become lighter, more
aerodynamic, and more efficient. Hybrid technology brings
electricity into the mix as a more environmentally friendly
source of energy. Hybrid technology introduces an
interesting way to make vehicles more efficient and
environmentally friendly. By combining traditional gas
powered engines with electrical motors, they reduce
mmisions and fuel consumption. One fairly new system
engineers have designed is the flywheel based kinetic energy
recovery system, or KERS. It is a complex system, but the
physics behind it are simple to understand. This kinetic
energy recovery system stores energy as a vehicle breaks,
stores it, and recycles it as the vehicle accelerates again [7].
The kinetic energy recovery system was first designed for
formula one racing cars, and it is now a mandatory system
University of Pittsburgh
Swanson School of Engineering
March 1, 2012
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Andrew Kearns
RJ Liu
the energy is lost to heat. Mechanical KERS simply transfers
kinetic energy from one place to another. The simplicity of
this aproach minimizes the loss of energy in the system. This
brings up an interesting point. Flywheel kinetic energy
recovery system transfers energy, so some energy is lost.
This is not an issue though, because KERS stores energy that
ordinary cars would waste anyway.
amounts of heat. The solution the Flybrid engineers came up
with is to enclose the flywheel in a vacuum [6].
KERS AS A WHOLE
The whole concept of kinetic energy recovery system,
KERS, was introduced a while ago. However, formula one
racing brought the concept into automotive industry in 2006.
This offered the auto industry a brand new way to cope with
energy crisis, and with a rapid pace. This KERS technology
is about contributing to our society. The flywheel system is
the most direct mechanical method to store and recycle
energy. The principle for this system is simple, a flywheel
connected by a continuously variable transmission, or CVT,
to the drivetrain. The system stores energy when the
transmission moves to the ratio that speeds up the flywheel.
This energy is released when the transmission moves toward
the ratio that slows down the flywheel. Since this system
mainly composed by two parts, this paper will explain these
two parts, the flywheel and the clutched flywheel
transmission, separately.
FIGURE 2
FLYWHEEL
COMPUTER GENERATED MODEL OF A FLYWHEEL DESIGNED BY
FLYBRID
[6].
For a functional flywheel system, there are three critical
aspects, the flywheel, the seal, and the safty. The flywheel is
the main energy storage unit. According to sae.org, Bosch
Engineering has developed a flywheel system that is capable
of storing amounts of energy up to 750kJ [1]. This amount
of energy is equivalent to that of a one ton object traveling at
90 miles per hour as (1) shows.
That resolves the friction issues but raised the problem of
getting power in and out without air leaking in. This leads us
to our next topic.
THE SEAL
In order to incase the flywheel in a vacuum chamber, the
seal is essential. The seal is the company’s own patentable
design and Hilton is unwilling to reveal the details of their
sealing technology. Therefore, based on the traditional
sealing method, we came up with an idea of how it may
work. Many high speed turbo chargers use fluid bearings
that consist of a layer of oil that is constantly pumped around
the turbine shaft. Our solution is very similar; we would use
a pump to constantly circulate oil to the area where the
surfaces would be making contact. Not only does this
method seal the machine, but it also allows for minimal
friction and cools the shaft as it spins to prevent it from
overheating.
750kJ = 750000J
1 ton = 907.18474 kg
KE = (1/2)mv 2
v = √(2KE/m)
v = √((2*750000J)/907.18474 kg)
v = 40.6628 m/s
40.6628 m/s = 90.9601 mph
FIGURE 1
SAMPLE CALCULATION SHOWING WHAT KIND OF ENERGY
KERS IS CAPABLE OF STORING
To achieve the maximum energy storage the speed of the
flywheel has to reach 64,500rmp. As the result, the flywheel
is made from carbon filament wrapped around a steel hub.
The reason engineers chose this particular material is that the
tensile strength of the carbon will effectively prevents the
flywheel from shattering under the great force of high
rotational speed. However, that in turn creates windage
losses that greatly reduce the power and produce enormous
THE SAFETY
Obviously, the company knows that one major injury or
fatality associated with the system would spell the end of the
technology and the company, so safety had been a top
priority throughout the project. The company made
tremendous tests to ensure that the whole system was
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Andrew Kearns
RJ Liu
reliable. The enigneers confined the damage to the flywheel
and containment rings, while the other parts will remain
undamaged and able to be re-used. Moreover, they also
attached accelerometers to look for tell-tale signs of
impending failure [6].
flywheel kinetic energy recovery system. This would
undoubtedly have a positive impact on the environment.
Burning significantly less fossil fuels would means less
pollution. This would make the air we breathe healthier and
it could slow the effects of global warming.
The effects on the environment are not the only ethical
concern when it comes to KERS. Safety is also a very
important point. The bottom line is if a technology is not
safe, it simply cannot be used. Despite the high rotational
speed of the flywheel, the kinetic energy recovery system
has been proven to be completely safe [3]. The engineers
who designed the flywheel were able to design the kinetic
energy recovery system in a way that does not put people at
risk. Besides, formula 1 race cars have safely used flywheel
kinetic energy recovery systems, and production cars do not
produce as much energy as race cars [7]. If the flywheel
kinetic energy recovery system has been proven to be safe
for use in race cars, it will not harm people who are just
driving their car down the street to the supermarket.
TRANSMISSION
CFT is regarded as the first true second generation
transmission system. It replaced the continuously variable
transmission, and it reduced size, weight and cost for mass
production in the future. Weighted at less than 18kg, its plan
view section can fit on an 8”×11” piece of paper [6]. As is
shown in (3), CFT is small and versatile enough that it can
be placed in multiple locations, making it very easy to
install.
ELECTRIC VERSES MECHANICAL
Thanks to the cars such as Toyota Prius, Honda Insight, or
Chevrolet Volt, when speaking of kinetic energy recovery
system, people usually think about vehicles powered by an
electric motor. There is no denying that electric hybrid
vehicles have proved to have great efficiency and therefore
less impact on the environment. However, electric-gasoline
is not the ultimate solution for a hybrid car in comparison
with the flywheel based kinetic energy recovery system.
First of all, electric hybrid cars are expensive. It is obvious
that electric cars are much more expensive than normal cars,
because the material used for electrical car has a high cost
due to the rareness of material itself. Besides, lithium ion
polymer batteries only last 3 years before they die, so battery
replacement will be a major cost for a hybrid car owner.
Flywheel kinetic energy recovery systems are made out
of traditional materials. Therefore they maintain a lower cost
and greater reliability. Secondly, the whole concept of
environmental friendly for an electric hybrid car is kind of
misleading. Since an electric-gasoline car is usually made by
material from all over the world, the carbon consumption to
make an electric hybrid is way more than that of a normal
car. Also, used batteries can cause big issues with the
environment. In comparison with electrical hybrid, the
flywheel based kinetic energy recovery system costs less to
make and it is recyclable, for it is made entirely of metal.
Finally, the most critical reason why flywheel KERS is
superior to the electrical motor is that the flywheel is much
more efficient. Flybridsystems.com states that cars with
flywheel kinetic energy recovery systems have “a better
power to weight ratio than existing automotive hybrid
technologies [6].” The mechanical hybrid approach is
superior to the electrical hybrids. This is due to the second
law of thermodynamics, transforming energy from one form
to another inevitably introduces a loss of energy. For battery
FIGURE 3
DIAGRAM SHOWING THE DIFFERENT LOCATIONS CTF CAN BE PLACED [6]
The CFT transmission uses a series of separated gears and
high-speed clutches that perform a controlled slip to transmit
the drive. Basically, there are three gears in the CFT KERS,
and all of them can be connected to the main vehicle
transmission, therefore, providing a larger number of ratios
between the flywheel and the wheels of the vehicle [6].
(Number of total gear ratios = 3 CFT KERS * number of car
gearbox speeds). To achieve the highest efficiency and
accuracy possible, the gear-changing is controlled by
software which selects the most appropriate gear. Then it
partially engages the high-speed clutch associated with that
gear [6]. Furthermore, by applying hydraulic pressure to
close or open the clutches, the whole system will run
smoothly without torque interruption.
ETHICS
For every single project engineers work on, functionality is
key, but ethics are just as important to professional
engineers. Engineers cannot put a solution to a problem into
effect if it is not ethical. The flywheel based kinetic energy
recovery system is ethical. This is a result of its efficiency
and its safety. Because the kinetic energy recovery system
increases fuel efficiency, our society would burn less fossil
fuel if it was used in all vehicles. Think of how much fuel
could be conserved if every automobile on the road used
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Andrew Kearns
RJ Liu
based hybrid drive systems, there are four energy-sapping
transformations:
1. Kinetic energy is transformed into electrical energy
in a motor/generator.
2. Electrical energy is transformed into chemical
energy as the battery charges up.
3. The battery discharges, transforming chemical into
electrical energy.
4. Finally, the electrical energy passed into the
motor/generator acting as a motor and is
transformed once more into kinetic energy.
The four energy transformations greatly reduce the level of
efficiency. For instance, let us consider a motor that makes
75% under the peak load, both on the way in and out. Also
consider a battery that makes 70% both on the way in and
out. The resulting overall efficiency will be less than 30%.
Unlike the electrical approach, the flywheel based approach
will avoid most of energy loss by keeping the energy in the
same form as energy is stored and released. Simplicity is
superior when it comes to transferring energy. So from what
we have discussed above, we can draw a conclusion that the
flywheel based will be the trend of future design, because it
is easy, economical, and efficient.
TECHNOLOGY OF THE FUTURE
Kinetic energy recovery system can make every car more
efficient and powerful. Because of this, it can also reduce the
negative impact on the environment due to burning fossil
fuels. The flywheel kinetic energy recovery system stores
kinetic energy in a flywheel as a car decelerates. Then as the
car accelerates, the kinetic energy stored in the flywheel is
transferred back to the forward motion of the car as a boost
of acceleration. Due to the simplicity of energy transfer in
the mechanical system, it is superior to the electrical kinetic
energy recovery system. Mechanical hybrids are more
powerful, more efficient, and cheaper than electrical hybrids.
In the future, automobiles will be much more fuel efficient
than the cars of today. What started out as a technology for
formula 1 race cars will be a standard component of
automobiles. Flywheel kinetic energy recovery system
technology is definitely practical because multiple car
companies are looking into using the system in the
production cars that consumers drive. Volvo officially
announced that they intend to develop and produce a vehicle
that uses the flywheel based kinetic energy recovery system.
As technology improves, KERS will become even more
efficient and affordable. It would be ignorant to think that
the flywheel kinetic energy recovery system or a similar
technology will not contribute to their superior efficiency.
KERS TECHNOLOGY IN THE REAL WORLD
Although cars with flywheel based kinetic energy recovery
system are more expensive than cars without flywheel
kinetic energy recovery system, the cars that have the system
have more power and have better fuel efficiency. According
to thegreencarwebsite.co, “the system could reduce fuel
consumption by as much as 20 percent and give a fourcylinder engine acceleration like a six-cylinder unit [8].”
This means that cars that have flywheel kinetic energy
recovery system burn less fuel and have more power than
cars that do not have the system. Another source states that,
“According to Derek Crabb, vice president of Volvo's
powertrain engineering division, Flywheel KERS has the
potential to reduce fuel consumption by up to 20 percent
[5].” This shows that well known car companies are looking
into using flywheel kinetic energy recovery systems in their
production cars. It also confirms the efficiency of the
system. “Flybrid Systems have been working with a number
of OEM car makes including Jaguar Cars to develop
flywheel hybrid systems for road cars [6].” Road cars with
flywheel kinetic energy recovery systems are definitely
going to exist within the next few years. Consumers would
not mind paying extra for a car with flywheel kinetic energy
recovery system because it will have a good amount of
power and it will save them money on fuel in the long run.
By paying a higher fixed price while purchasing a vehicle,
consumers will be able to save enough money on fuel to
make it worthwhile. This will be especially true if gas prices
continue to increase.
REFERENCES
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<http://www.sae.org/mags/aei/5139>.
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[8] Lucas, Paul. "Volvo to Develop Kinetic Energy Recovery
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RJ Liu
Energy Storage Systems in Autonomous Projects." Renewable Energy 39.1
(2012): 149-53. Print.
ACKNOWLEDGEMENTS
We would like to thank Scott and Pramod Jacob, our chair
and co-chair for giving us guidance and advice on this paper.
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