9:00-10:45 AM Paper 2097
tpk14@pitt.edu
jmr175@pitt.edu
Abstract— Lightweight modern flywheels have the potential It has been a common application for years to use flywheels to replace the expensive, short-lived, and unrecyclable batteries that hybrid automobiles currently have in them. As fossil fuels become scarcer on our planet, few alternative energy sources have yet to be shown both affordable and practical. While the consumption of fossil fuels is still on the rise, it is imperative to make existing fossil fuel burning automobiles as efficient as possible. The use of flywheels can be very effective at increasing fuel efficiency [1]. Our paper will explore, evaluate, and gauge the effectiveness of lightweight flywheels and their implementations in modern automobiles. This paper will focus primarily on the science behind flywheels and the specific applications in hybrid
“street” cars.
Only recently have flywheels shown their potential in automobiles due to innovative design and stronger materials. Without this technology, the flywheels would not be light enough to be productive in vehicles as small as compact cars. These flywheel systems are more durable than current battery powered vehicles because the process is nearly frictionless. This lack of friction also makes them more “eco-friendly.”
Key words-Alternative Energy, Emissions, Flywheels,
Hybrid Automobiles, Regenerative Braking, Efficiency
I
NTRODUCTION to smooth out fluctuations in power coming from internal combustion engines before entering the transmission.
However it is a much newer implementation of flywheels to use them for regenerative braking in hybrid vehicles. One of the first attempts at using flywheels to power a vehicle originated in Sweden in the 1930’s with the Gyrobus. The
Gyrobus was powered entirely from a one and half ton flywheel which was charged at stops roughly 6 km apart.
The need for an efficient way of traveling arose from fuel shortages during the World Wars, but died off with the
Gyrobus during the 1950’s. The demand for alternative fuels is less now than it was in the early twentieth century but it is once again becoming economically viable to look towards the implementation of modern flywheels in automobiles [2].
In more recent years, due to the price of gasoline and environmental concerns, the utilization of hybrid systems in commercial automobiles has become a popular topic.
Flywheels offer many different advantages over other systems and within the past few years have begun to make their way into Formula 1 race cars. The rules in regard to flywheel systems changed for the 2009 season, and in the
2010 and 2011 season Porsche had success racing a GT3 with the integrated flywheel kinetic energy recovery system
When you think of hybrid automobiles, the Toyota Prius,
Honda Civic, or another mainstream hybrid is likely to come to mind. However, in recent years flywheels have been gaining attention in the automotive industry because of improved efficiency from magnetic levitation and the vacuum chamber the carbon fiber disk is held. Currently there are not any companies that have introduced a hybrid flywheel system into a commercially available automobile, but there are a few with plans to do so in the near future [1].
A main reason companies are looking into flywheel technology is the benefits of lower production cost and the reduced carbon footprint this technology has to offer. This paper will primarily focus on the science, specific applications, and environmental advantages of flywheels in hybrid automobiles. named Williams Hybrid Power (WHP) [3]. This system was created by WHP, a company that specializes in flywheel designs for automobiles. The Porsche GT3 came in 3 rd in the
Nurburgring Long Distance Championship in 2010, partially due to the estimated 20% increase in fuel economy from the flywheel system [4]. In 2011 the Porsche came in 1 st in one of the rounds of the same long distance championship [3].
This success in Formula 1 racing shows promise for commercial hybrid vehicles. While these specific flywheels have been optimized for race cars, WHP and other companies are currently working on developing flywheel systems optimized for commercial road use.
M ECHANICS OF F LYWHEEL
F LYWHEELS IN Y EARS P AST
The basic premise of a flywheel has stayed the same since its first utilization; however the efficiency and storage capacity have expanded immensely from improvements in material science, magnetic bearings, and vacuum sealed chambers.
1
University of Pittsburgh
Swanson School of Engineering April 14, 2012

Jeremy Rittenhouse
Tim Kilcullen
The mechanics of modern flywheels is simple to understand when in a stationary environment, but when put into an automobile many new factors need to be taken into account.
E flywheel
0.5I
ω
2
Equation 1 [5]
I disk
=MR 2
Equation 2 [5]
In equation 1 I is the rotational inertia of the rotating flywheel and ω is the angular velocity. For each flywheel we will discuss, the rotational inertia will be approximated by the inertia of a disk which is defined in equation 2. In equation 2 M stands for mass of the disk, and R is the radius of the disk. With these 2 equations it is easy to track the exact amount of power that is stored within a flywheel system, and determine the exact angular velocity needed to store a set amount of energy [6].
When this energy is considered however it is relatively small as the angular velocity is squared (equation 1). When the angular velocity of the disk is increased the disk undergoes much higher centripetal force, and this is where the material science breakthroughs have allowed flywheels to spin at speeds surpassing 60,000 revolutions per minute (rpm). With rpm’s these large, older metal flywheels would fly apart, whereas new carbon fiber and epoxy flywheels are able to withstand the higher forces seen in modern flywheels (figure
1). The potential of carbon fiber and epoxy flywheels have been recognized by many manufactures and is being utilized by nearly all of them, quickly becoming an industry standard
[4]. Although the flywheel itself is made of the carbon fiber epoxy composite, most of the encasing and shafts are made of steel or aluminum [7].
E delivered
=E stored
-E minimum
=.5I(
ω
2 max
-
ω
2 min
)
Equation 3 [4]
In its most basic form a flywheel consists of nothing more than a spinning disk that stores and releases kinetic energy as torque. While this is the basic concept, the modern flywheels implemented into automobiles consist of many more advanced complicated parts. Modern flywheels are commonly suspended in space by magnetic bearings allowing the flywheel to neglect almost all friction. To offset one of the only other outlets for power loss the flywheel is then put inside a vacuum chamber. With these advanced technologies in place modern flywheels are able to boast up to 98% efficiency within a lab setting [6]. With the ability to make flywheels with such high efficiency research into flywheels is now largely focused on increasing the energy density of a flywheel. From equation 1 and equation 2 it can be understood to decrease the weight and size of the system while holding the energy constant, the angular velocity must increase. In equation 1 it can be seen that the angular velocity is squared in relation to the total energy of the flywheel. From this information it can be understood an increase in angular velocity has more effect on the energy stored than the mass of the flywheel itself. From equation 3 however it can be understood that all the energy stored in the flywheel is not available for use. On flywheels there is a minimum speed which flywheel must be kept spinning.
Figure 1 [4] displays the different density and tensile strengths of materials commonly used within flywheels. From this information it also charts what the maximum kinetic energy for a 1 kg ring is.
There are two sets of magnetic bearings in a modern flywheel system. The first sets of bearings, called active magnetic bearings, are located inside of the chamber on the internal shaft, and are used to levitate the flywheel. These bearings are placed above and below the flywheel, and each
“pull” on the flywheel to suspend it in space. The other set are used to rotate the flywheel, and are located on the external shaft. In between these two shafts, pieces of steel are used to create a ratio in the rotation velocity of each shaft. The ratio of velocities between the two shafts allows traditional transmissions to manage such high rotational velocities. The use of magnetic bearings such as these allows the frictional force of the flywheel to be kept at a minimum
[8].
In modern flywheels a vacuum chamber holds the flywheel and allows it to rotate with minimum energy lost due to air resistance. Even in early flywheel systems it was common to reduce the air pressure around the flywheel, and with current flywheels it is necessary to reduce the pressure to a near perfect vacuum as the air resistance would rip the
2
University of Pittsburgh
Swanson School of Engineering April 14, 2012

Jeremy Rittenhouse
Tim Kilcullen flywheel apart if it was not in a new perfect vacuum [2][8].
While getting the vacuum near perfect is not a breakthrough technology the ability to keep the vacuum for extended periods is a technology that continues to develop [1].
I NTEGRATING
While all the technology to have a flywheel work efficiently and safely on steady ground has been well established, having the same system work seamlessly in a car raises a whole new set of engineering challenges. Within cars the flywheel must be impervious to bumps, safe in a collision, and durable. With the natural bumps of driving the flywheel must have robust bearings that can protect it from hitting the sides of the encasing, while still maintaining maximum efficiency. To accomplish this, the flywheel’s magnetic bearings hold the flywheel in place exceptionally well. In a study using a Flybrid system developed by Flybrid Systems
LLP, a flywheel spinning at 64,500 rpm, the maximum safe level, was subjected to over 20 g’s simulating a crash, and was found to be still spinning with no damage [4]. This shows the durability of the Flybrid design which allows it to go through mock crashes without failure.
A major fear of flywheels is catastrophic failure, or the rotating disk shatters and all the stored energy is released in an instant. In studies done by both IABG and DARPA the
Defense Advanced Research Projects Agency, it looks to be nearly impossible to contain a failure at full load within the confines of a car and still keep the system light and be economically viable. With this information in mind the design becomes to minimize the risk to an acceptable level rather than full containment of a catastrophic failure [4].
Minimizing the risk of catastrophic failure to an acceptable level is not an impossible task. The composites used in modern flywheels can withstand extremely high stress levels
(figure 1), and can be made to have outer pieces that are scored and more susceptible to breaking off acting as an indicator if lifetime stress levels indicate the possibility of failure. If one of these pieces were to break off the precise balance of the flywheel would be thrown off which could easily be detected by electronic sensors, and a vibrating of the entire flywheel system [4].
T
F
RANSFERRING
LYWHEELS INTO
P
H
OWER TO AND FROM
YBRID
F
C ARS
LYWHEEL
There are two main ways of transferring power from the vehicle to the flywheel and back again. One system uses a continuously variable transmission (CVT), while the other uses motors and generators to transfer the power to and back out of the flywheel (figure 2). Each of these systems has advantages and disadvantages in different ways. The CVT system is advantageous in that there is less energy lost, as the power does not go under any phase changes (figure 2 right). This being said, the CVT system is a heavier system, which limits the overall benefit of the design [3]. On the other hand, the Electrical design allows for many more car designs as the placement of the flywheel does not have to be in line with the transmission and can be placed anywhere in the car [9]. The electrical design also has drawbacks of its own though, it needs 2 extra motor/generators to transfer the power between electric and mechanical phases which by second law of thermodynamics means significant energy will be lost [10] (figure 2 left). With either of these systems an advantage flywheels have over other hybrids is that the amount of power held within the system is easily tracked
(equation 1). This is not the case with other common hybrid systems such as batteries where the stored energy is more of a rough estimate [6].
Figure 2 [4] shows two diagrams of the most common ways flywheels are connected to the drivetrain of a vehicle.
A PPLICATIONS OF
A
F LYWHEELS IN
UTOMOBILES
Within the past few years companies such as Volvo, Jaguar and Ford have begun research into flywheels as a potential kinetic energy recovery system. There are many advantages to flywheel energy storage versus other energy storage systems such as batteries and ultra-capacitors because of high energy density and ability to quickly discharge and recharge in urban traffic [6]. While Lithium-ion batteries do have a higher energy density they are unable to charge and discharge at the rates seen by flywheels and ultra-capacitors.
Another disadvantage of flywheel systems is they see a decrease in energy storage throughout their lifetimes (figure
3).
3
University of Pittsburgh
Swanson School of Engineering April 14, 2012

Jeremy Rittenhouse
Tim Kilcullen force can be considered negligible when compared to the mass of a typical automobile [4].
Figure 3 [9] shows a graph comparing the specific energy versus the specific power of different hybrid systems. The size of the bubble shows a rough estimate of the range these systems can have.
With the power which can be quickly drawn from the high speed flywheel it is possible for a relatively small engine to make a car feel as if it were a much more powerful car. The addition of a flywheel system adds power at low rpm’s and on the acceleration of a vehicle. Volvo estimates the addition of their flywheel system would result in a double digit km/L increase, and with their system which is entirely mechanical using a CVT they believe it will make it a cheap, easily mass produced item [1]. In the specific design Volvo is looking at implementing into their first models of cars the flywheel is charged and discharged through the rear wheels while an internal combustion engine provides power to the front wheels. This design prevents interference between the systems and makes for a simpler overall design [1].
When a flywheel spins it inherently creates gyroscopic forces. In older systems such as that of the Gyrobus the gyroscopic forces were huge, and had a couple of major effects of the way the bus handled. One thing that was quickly noticed by passengers was it made for a smoother ride, but the bus drivers had to compensate with “adaptive driving” [2]. This “adaptive driving” meant that the bus drivers had to reduce speed before large turns, and meant that maneuvers were slower almost feeling delayed compared to other vehicles. In current systems however the mass of the flywheel is extremely small in comparison with the mass of the vehicle meaning these forces are very small on the vehicle. In the Flybrid system which the spinning disk has a mass of 6 kg the gyroscopic forces measure just under
120 NM at its maximum speed of 60,000 rpm [11]. This
Figure 4 [12] shows a graph of hybrid systems and their relative working temperatures. The range of temperatures for which each is rated is the range at which each can operate at 80% capacity.
As seen in figure 4 flywheels can operate effectively at the largest range of temperatures compared to batteries and ultra-capacitors. This shows they are not as temperature sensitive as other forms of energy storage. The ability to operate in a large range of temperatures makes flywheel systems more appealing to consumers in harsher climates where typical hybrids underperform due to adverse weather conditions. This ability also allows for the flywheels to be put in more places within the car as an engineer does not have to design specific heat vents as are needed with some battery packs.
F
LYWHEEL
T HE F
S
LYBRID
YSTEMS
S YSTEM
4
With flywheel technology and the utilization of it in automobiles being a relatively new venture, startup companies are at the forefront of this technology, creating many of the new breakthroughs. Flybrid systems as mentioned earlier, is working with select car companies, including Jaguar, to integrate its system into new production cars in the near future. Their system is currently undergoing crash tests, and the preliminary results have not shown any major flaws with their system. As mentioned earlier this system was subjected to over 20 g’s of force, simulating a crash, with no adverse effects on the flywheel [4]. Their current design for commercial production has a robust design with an estimated life of 250,000 km [11].
The current commercial Flybrid system utilizes a mechanical powertrain with a CVT. This system has maximum power output of 97 kW and weighs in at just 17.2 kg. With the addition of the CVT however the systems
University of Pittsburgh
Swanson School of Engineering April 14, 2012

Jeremy Rittenhouse
Tim Kilcullen weighs in a bulkier 64.9 kg [9]. One common concern for a flywheel system is the rate at which the flywheel loses power when it is not being used. With all of the technology in the Flybrid system the loss of power is only 2% per minute. This loss of power becomes virtually insignificant because of the fact that 95% of stops are under 55 seconds, and the amount of power lost would rarely exceed 2% at a stop [11]. This quick turnover rate of power storage to consumption is another advantage flywheel systems have over battery systems. Flywheels are able to absorb power and expend it at much higher rates than their battery counterparts as seen if figure 3. fuel efficiency of the buses it is being tested on by roughly
13% [8]. While this seems like a relatively small increase compared with other systems, it is still in its preliminary stages, and designed to test the possibility for the future rather create great fuel savings now [8]. KinerStor, the second research study is an attempt to place the Kinergy system into commercial vehicles, namely Land Rover for a cost under 1000 euros. The goal would be to have this system increase fuel efficancy approaching 30%, but the results of these tests are not yet publicly available [13].
L IFETIME COSTS
T HE W ILLIAMS H YBRID P OWER
The Williams Hybrid Power company is another startup company specifically designing flywheels in hybrid automobiles. The WHP’s flywheel system employs an electrical design as opposed to Flybrid’s mechanical design.
The WHP flywheel system is able to output about 120 kW for about 6 to 8 seconds. This design also has been shown to increase fuel efficiency by about 25% [3]. WHP’s flywheel system differs slightly from most flywheel designs. Instead of using permanent magnets on the surface of the flywheel,
WHP’s design embeds a magnetic powder into the composite material of the flywheel. After the flywheel is formed, the powder is magnetized via flash magnetism. This design originally, invented for uranium enrichment centrifuges, has the advantage over other systems that in the case of a catastrophic failure there are no metallic structures which can shoot off with bullet like velocities [3].
Currently without the mass production of flywheels for hybrid systems the cost for a flywheel system is roughly the same as that of a battery system. However it is estimated that once flywheels are massed produced for this purpose they will cost roughly half of what a battery system does now [7].
This is not necessarily the entire cost however as the flywheel will not degrade as quickly as a battery system will.
Over time as this degradation occurs the battery will pay itself off slower as not as much energy is stored, but the flywheel system will still store the same energy as when it was built. Another cost associated with the lifetime cost of batteries is the recycling that can be expensive at the end of their lifetime. Flywheels on the other hand are cheaply and easily recycled [7].
E
NVIRONMENTAL IMPACTS
R ICARDO K INERGY
University of Pittsburgh
Swanson School of Engineering
The Ricardo Kinergy flywheel is very similar to the Flybrid design in that it utilizes a mechanical powertrain. The main difference between the two systems is that the Kinergy design uses a hermetic seal while the Flybrid system uses a high speed vacuum shaft seal [11][13]. This hermetic seal is meant to be a permanent seal lasting for the lifetime of the flywheel, while the Flybrid system has a small vacuum pump that will occasionally evacuate air from the chamber.
This hermetic seal relieves the need for a vacuum pump needed in the Flybrid system [13].
The Kinergy flywheel system has been implemented in two research opportunities thus far. The Flybus is one of these opportunities being supported by the United Kingdom
Government. Ricardo say that the study’s goal is to create a bus transportation system using the Kinergy flywheel system as well as a CVT. This system is estimated to increase the
5
Within the past few years fossil fuels and alternative energy sources have become a popular topic in the news. Currently there is a large amount of research going into hybrid automobiles and removing fossil fuel from the picture, however the goal of eliminating fossil fuels from transportation is still decades away. With this in mind, the issue of the current inefficiencies of internal combustion engines must be addressed. Current automobiles are known to only be 14-26% efficient at changing the potential energy of gasoline into energy that is used in transportation [14].
This is where hybrid technology shows potential with its ability to offset the energy lost to braking and use this energy to save fuel on acceleration. For a few years now the standard for hybrid cars has been battery systems. There are companies looking to change that however with ultracapacitor and flywheel systems being looked at as possible alternatives.
B ATTERY SYSTEMS
April 14, 2012

Jeremy Rittenhouse
Tim Kilcullen
Battery systems control the hybrid market, as they are currently the only system that is commercially available.
However there is some controversy over the use of these hybrid systems; there is research claiming throughout their lifetime, battery hybrids actually have more negative effects on the environment than positive ones. According to a study by CNW marketing research, from the creation of a battery to the end of its functioning life more energy is consumed in creating the automobile than is ever saved during its lifetime
[15]. This study also neglects the fact that the most prevalent batteries found in cars, lithium-ion, can be toxic and expensive to dispose of properly.
Up until a few years ago, Nickel-Cadmium batteries were the battery of choice for the majority of hybrid manufacturers. However, these batteries were severely limited by the rate at which energy could be stored and retrieved. While Lithium ion batteries have higher energy transfer rates they still are somewhat limited in this regard compared with flywheels and ultra-capacitor systems [15].
Lithium ion batteries cost thirty percent more than Nickel-
Cadmium, but during the lifetime of the battery this extra money is paid off as they have a higher energy density and are able to deliver stored energy quicker [16]. The cost per kW for a lithium ion system is in the range from 33-66 dollars [9].
Although batteries have the highest specific energy
(figure 3), batteries suffer from decreased efficiency over their lifetime. This is substantial cause to review other systems in hybrid automobiles which can last the lifespan of the vehicle without the degradation. energy than that of an ultra-capacitor, thus they are able to release their power over a longer period of time.
F LYWHEELS
The use of batteries in hybrid automobiles seems shortsighted because they use more energy in their lifetimes than they save; flywheels on the other hand are a very environmentally friendly alternative. Unlike batteries which contain harsh chemicals, flywheels only contain conventional materials, with a thin layer of carbon fiber [11].
Even in the making of flywheels, there are few waste chemicals produced making the creation of flywheels more environmentally friendly [6]. This simplistic design in comparison with batteries means flywheels are cheaper to make and are easily recycled. To recycle a flywheel the metal is simply melted down and the carbon fiber is ground up and used for plastic composites as a filler [7]. This recycling process is much simpler than the recycling of batteries or ultra-capacitors, which take stages and when not recycled properly can cause environmental harm. It ultracapacitors are not recycled properly they will leach chemicals from the plastics used, but if a battery is left out it can leech toxic chemicals into ground water leaving untold environmental impact [7]. On the other hand if flywheels are not recycled properly there will be few adverse environmental effects as the vast majority of the components are steel or aluminum.
F
UTURE OF
H
YBRID
F
LYWHEELS
U LTRA CAPACITORS
Ultra-capacitors store energy in a very similar way as any capacitor stores energy; the energy is stored as electric potential energy between two plates. One of the major advantages of using an ultra-capacitor in hybrid automobiles is its ability to release power at a much more quickly than a battery is able to. However, ultra-capacitors still release power at a slower rate than flywheels (figure 3). In a study done by Oxford University, it was estimated that the cost per kWh for ultra-capacitors is the lowest among batteries and flywheels coming in at between 13 and 51 dollars per kW, but when ran through a simulator, the ultra-capacitors came in behind for cost in terms of fuel efficiency [9]. This is caused by a couple of factors. One of these factors being flywheels are able to release their stored energy at a higher rate. On the other hand batteries are able to store more
Flywheels in automobiles look poised to take a significant share of the hybrid market in the near future. The technology to create efficient, safe flywheels is currently available, and there are companies that see the potential flywheels have to offer. Flybrid Systems, Williams’s hybrid power, and
Ricardo all are currently working on bringing their products to market through well-established car companies such as
Jaguar, Ford, and Land Rover [1]. As these systems begin to show up in a new generation of cars the price to produce them will drop significantly with mass production [4]. With a price drop from mass production, flywheels will quickly become a competitive option versus the standard battery powered hybrid.
As advances in the material science of flywheels continue it will be likely to see a continued increase in the maximum angular velocity attained by flywheels [4]. From equation 1 it can be understood that if everything else on the flywheel is held constant an increase in the maximum angular velocity
University of Pittsburgh
Swanson School of Engineering
6
April 14, 2012

Jeremy Rittenhouse
Tim Kilcullen will increase the energy stored on the flywheel. As the capacity of stored energy in flywheel systems increases, the benefits will have a greater impact. Increased power will be able to be transferred to the hybrid automobile over a longer period of time. Because of this, greater efficiency will be added to a system that is already extremely efficient compared to other hybrids.
In a world where fossil fuel depletion is a growing problem, many people have and more will continue to realize the potential of flywheel systems. While flywheels are not a direct source of energy that can replace fossil fuels, they allow for a more efficient automobile that can slow the rate at which society uses fossil fuels. This technology certainly will not end the search for alternative energy, however, the use of flywheel systems in hybrid automobiles will certainly benefit the automotive industry as increased efficiency and power will allow for higher quality hybrid automobiles.
[14]"Fuel Economy: Where the Energy Goes." Fuel Economy: Where the
Energy Goes . U.S. Department of Energy, 29 Feb. 2012. Web. 01 Mar.
2012. <http://www.fueleconomy.gov/feg/atv.shtml>.
[15]DiMauro, Frank J. "Hybrid Cars Versus Conventional
Cars." Greendesignetc.net
. Green Design, 25 Nov. 2008. Web. 1 Mar. 2012.
<http://www.greendesignetc.net/GreenProducts_08_(pdf)/DiMauro_Frank-
Hybrid%20Cars(paper).pdf>.
[16]"Honda's Low-Cost Hybrid Strategy: Lithium Ion Batteries." 301
Moved Permanently . Ed. Bradley Berman. HybridCars.com, 18 Mar. 2010.
Web. 01 Mar. 2012. <http://www.hybridcars.com/news/honda-low-costhybrid-strategy-lithium-ion-batteries-27544.html>.
A
CKNOWLEDGMENTS
We would like to thank our writing instructor for the positive critiques he gave us along the way. We would also like to thank out Chair and Co-Chair who gave us valuable insight into the importance of putting together a research paper. Another valuable source of information was Ryan
Soncini whose weekly meetings laid out exactly what needed to be done.
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[7]Cross, D.; Hilton, J.; , "High Speed Flywheel Based Hybrid Systems for
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[8]Duncan, Graham-Rowe. "Hybrid Cars Give Flywheels a Spin." The New
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[9]Doucette, Reed T., and Malcolm D. McCulloch. "A Comparison of
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[10]Ellis, Chris. "Flywheel Hybrids." 301 Moved Permanently . 06 Apr.
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[11]"Home - Flybrid Systems." Home . Web. 01 Mar. 2012.
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[12]Cibulka, J. "Kinetic Energy Recovery System by Means of Flywheel
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[13]"Delivering Value Through Innovation & Technology." Home . 01 Mar.
2012. Web. 01 Mar. 2012. <http://www.ricardo.com/>.
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