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Not many people know about the kinetic energy recovery
system, also known as KERS for
short. This technology has been able to save energy that would otherwise be
normally lost during braking in an electric/hybrid car. In a research paper, it
was written that by integrating flywheel hybrid systems, these
drawbacks can be overcome and can potentially replace battery powered hybrid
vehicles cost effectively. The paper will explain the engineering, mechanics of
the flywheel system and it’s working in detail. Many companies are now trying
to incorporate KERS in their automobiles. F1 racing is another area which has
been impacted by KERS technology. In this paper, I’ve collected all information
one could find about this technology online and assembled it into by the end of
this we shall understand the details of how this technology operates and if it’s
worth the investment of time and money of people.















In a world where almost all its fuel is being
depleted, conservation of natural resources has become a necessity in today’s
world, especially in the field of renewable technology. In an automobile,
maximum energy is lost during deceleration or braking. This problem has been
resolved with the 0introduction of regenerative braking. It is an approach to
recover or restore the energy lost while braking. The Kinetic Energy Recovery
System (KERS) is a type of regenerative braking system which has the capability
to store and reuse the lost energy 1

In the beginning of this paper, we will try and break
down the basic principle of the KERS technology. We will look at different
sources to see what each of them have to say about KERS and if they view KERS
as something highly beneficial for the world or not.

Going deeper into this paper, we’ll get into the
working of the KERS and try to keep it as explainable as possible to you. We’ll
look into different sources to see how different manufacturers have implemented
the use of KERS in their respective industries. We will see how KERS is used in
an average automobile producing industry and how it is used in the racing

Towards the end of the paper, after giving you as much
as detail as one possibly can about the KERS technology, we will try to
understand whether this technology should be implemented by more manufacturers
or not.

At the end, we will formulate a conclusion.







There are two main implementations of the
KERS system and they differ in how the energy is stored. The electrical KERS
uses an electromagnet to transfer the kinetic energy to electric potential
energy that is eventually converted to chemical energy that is stored in a
battery. It then redelivers the stored energy to the drive train by powering a
motor. The electric KERS was what many groups in Formula o began off
endeavoring to actualize into their autos. In any case, the battery used to
store the power is exceptionally inclined to battery fires and can cause
electric stuns. In an accident with the BMW Sauber team in F1, an engineer who was
working on the KERS was burned while testing the system after a practice run, many
groups esteemed the electric KERS to be risky. Alongside different factors, such
as being heavier than other implementations, the electric KERS implementation
is not found inside today’s Formula 1 cars.



The mechanical implementation, shown in
the figure, was initially developed by Flybrid Systems. To harvest the energy
upon braking, the system uses the braking energy to turn a flywheel which acts
as the reservoir of this energy. When needed, the redelivery of the energy is
similar to that of the electric KERS implementation, the rotating flywheel is
connected to the wheels of the car and when called upon provides a power boost.
The mechanical implementation of KERS is known to be more efficient than the
electric equivalent due to the fewer conversions of the energy that are taking
place. 2


In an Article, Top Gear wrote:

has just built a KERS-equipped S60 T5 development mule. At the fore, there’s the company’s older 254hp five-cylinder
petrol engine, powering the front wheels, and astern there’s a Flybrid KERS
system powering the back axle. So, how does it work? Kinetic energy that you’d
ordinarily lose to heat while braking is sent to a flywheel, which can capture 150-watt
hours in around eight seconds of gentle braking. That’s the same amount of
energy you’d need to charge 25 new iPhones captured in a third of the time it’d
take a Toyota Prius.

Once it’s been recovered, it
can be stored for about half an hour or used immediately, either as a
supplement to the engine, or in one great big lump. Chose the former and it’ll
cut consumption by up to 25 per cent. Chose the latter and you get 80hp added
instantly.  with KERS switched on, our
0-60mph time dropped from 7.68 seconds to 6.07 seconds.

And all
this thrust comes from a little box of gears and clutches that weighs 60kg,
requires virtually no maintenance, and will last for what the company claim is
the realistic life of the car. The batteries in Volvo’s current petrol/electric
hybrid weigh 300kg alone, and will have to be replaced after about a decade. 3








The flywheel hybrid
primarily consists of a rotating flywheel, a continuously variable transmission
system (CVT), a step-up gearing (along with a clutch) between the flywheel and
the CVT and clutch which connects this system to the primary shaft of the transmission.
When the brakes are applied or the vehicle decelerates, the clutch connecting
the flywheel system to the driveline/ transmission is engaged, causing energy
to be transferred to the flywheel via the CVT. The flywheel stores this energy
as rotational energy and can rotate up to a maximum speed of 60000 rpm. When
the vehicle stops, or the flywheel reaches its maximum speed, the clutch
disengages the flywheel unit from the transmission allowing the flywheel to
rotate independently. Whenever this stored energy is required, the clutch is
engaged and the flywheel transmits this energy back to the wheels, via the CVT.
Generally, the flywheel can deliver up to 60 kW of power or about 80 HP. Fig.1
shows Volvo’s flywheel KERS system Layout. 4




primary idea behind the flywheel-based KERS system is to mechanically store the
kinetic energy from the rear driveshaft in another source for use at another
time. This other source is the flywheel.

the clutch is engaged and both discs of the CVT are in contact with the
rollers, kinetic energy transfer can occur. This energy of motion is
transferred to whatever disc is moving slower; if the car is slowing to a stop,
the rollers in the CVT transmit the kinetic energy from the faster rotating
disc connected to the rear driveshaft, to the slower disc connected to the
flywheel. The disc connected to the flywheel begins to spin faster, which in
turns speed up the flywheel. This process is also reversible, where the rollers
can transfer energy from the disc connected to the flywheel to the disc
connected to the rear driveshaft. 56

flywheel is the component which harvests kinetic energy, when the vehicle
brakes, by increasing its rotational speed. The ability of flywheels to store
energy is explained by the relation between the flywheel’s inertia, angular
velocity and kinetic energy. The equation for the energy stored in a flywheel
reads as follows:



E is the energy (Joules); I is the inertia of the flywheel (kgm2 ), and ? is
the angular velocity (rad/sec) of the flywheel.

equation for the inertia of a flywheel is:



m is the mass of the flywheel;


are the inner and outer radius of the
flywheel respectively. Combining equation 1 and 2 we get:



equation 3, a flywheel’s energy is proportional to its mass, and proportional
to the square of its rotational speed or angular velocity. In other words, by
doubling the mass, the energy stored is also doubled, and by doubling the
speed, the energy stored is quadrupled. Thus, by increasing the speed of the
flywheel it will be possible to reduce the mass and size of it, to a level
where its weight is insignificant while analyzing fuel efficiency. In order to
make the system more efficient it is necessary enclose the flywheel in a vacuum
chamber, and in order to eliminate the resistance due to air and reduce
friction it is mounted on magnetic bearings.

amount of energy that can safely be stored in the rotor depends on the point at
which the rotor will warp or shatter. The hoop stress on the rotor is given by:




the tensile stress on the rim of the flywheel;

 is the
density, r is the outer radius of the flywheel and

the angular velocity of the rotating flywheel.

flywheel can be fabricated using different materials based on the maximum
rotational speed requirements and other design constraints. High speed
flywheels for speeds above 30000 rpm are usually composed of high strength
carbon fibre. A large mass is not desired for high speed flywheels because
extra mass means more energy will be needed to accelerate the vehicle. On the
other hand, low speed flywheels with speed values below 20000 rpm, are
generally made of steel or other metals for low cost. The weight of the
flywheel is a very important factor in determining the efficiency of the
system. 9













The flywheel vacuum chamber


The vacuum chamber is another very essential part of
the flywheel hybrid system. The major function of the vacuum chamber is to
minimize the air resistance as the flywheel rotates. Without the vacuum
chamber, the friction caused by air resistance is enough to cause significant
energy losses and heat the carbon fiber rim to its glass transition temperature
10. Vacuum chambers for KERS systems are frequently made of metals like
aluminum, stainless steel, or the like because these metals can provide
adequate strength to withstand differential pressure between an evacuated
interior and the surrounding atmosphere, as well as to provide a barrier to the
passage of atmospheric gases through the chamber wall by diffusion or flow
through structural defects. Fig. shows the flywheel hybrid system designed by





Magnetic Bearings


Another important part of the system is the bearings
on which the flywheel is mounted. Magnetic bearings have replaced mechanical
bearings as they greatly reduce losses due to friction. Mechanical bearings
cannot, due to the high friction and short life, be adapted to modern
high-speed flywheels. Further magnetic bearings are able to operate in vacuum
which leads to even better efficiency. The magnetic bearings support the
flywheel by the principle of magnetic levitation. It is a method by which an
object is suspended with no support other than magnetic fields. A permanent or
electro permanent magnetic bearing system is utilized. Electro permanent
magnetic bearings do not have any contact with the shaft, has no moving parts,
experience little wear and require no lubrication. It is important that the
bearings are able to operate inside a vacuum because the flywheel in a
flywheel-based KERS must rotate at high speeds for maximum efficiency. The best
performing bearing is the high-temperature super-conducting (HTS) magnetic
bearing, which can situate the flywheel automatically without need of
electricity or positioning control system. However, HTS magnets require
cryogenic cooling by liquid nitrogen 11. Fig. shows a magnetic bearing
designed by Waukesha bearings.




The continuously variable transmission (CVT) unit


The continuously variable
transmission (CVT) as used by Flybrid, is mounted between two clutches within
the KERS unit. The clutches allow for disengagement of the CVT from the
flywheel and the vehicle when not in use, and therefore minimizes losses.

The only mechanism for
controlling energy into or out of the flywheel is by controlling the ratio of
the CVT. The CVT is responsible for the smooth variation of ratios. The CVT may
sometimes be referred to as a Toroidal Continuously Variable Transmission (TCVT),
due to the shape of the rotating discs. The main components that make up the
CVT are: the rotating discs, rollers, carriages, and the pistons (levers).

Each roller is mounted in
a carriage and attached to a hydraulic piston. The pressure in the pistons can
be increased or decreased to create a range of reaction torque within the CVT.
The movement of the hydraulic pistons alters the angle of the rollers, where
the angle of the rollers in relation to the centerline of the CVT controls the
transmission ratio. This ratio affects the torque transferred through the CVT.12



Step-up gearing and clutch


A step-up gear takes the 60,000 RPM to a manageable
speed outside the vacuum. The maximum step up of an epicyclic gear or a
magnetic gear is 6:1. The gears are placed just outside the vacuum enclosure
and spin all the time the flywheel is spinning. They emit a continuous high-pitched
sound. The clutch disconnects the CVT from the flywheel when it is not
transferring power to reduce free running losses. 13










The clutch


The clutch is used to couple the flywheel hybrid
system to the transmission. It engages the system while the flywheel is
accelerating from rest and disengages while the flywheel is rotating and the
vehicle is at rest. Torque is transferred through clutch between the flywheel
and vehicle. Hence, the power transmitted in the flywheel system can be
controlled by a clutch that could continuously manipulate the torque. 4



The electric systems allow the teams in Formula One to
be more flexible in terms of placing the various components around the car
which helps for better weight distribution which is of vital importance in
The specific energy of Lithium-ion batteries in
comparison is unrivalled as they can store considerably more energy per kg
which helps reduce the size of RESS.


Lithium-ion batteries take 1-2 hours to charge
completely due to low specific power (i.e rate to charge or discharge)
hence in high performance F1 cars more batteries are required which increases
the overall weight of the batteries.
Chemical batteries heat up during charging process and
this takes place a number of times in KERS units which if not kept under
control could cause the batteries to lose energy over the cycle or worse
even explode.
The specific power is low as the energy needs to be
converted at least two times both while charging or discharging causing
energy losses in the process.


The specific power of flywheels in comparison is much
greater than that of batteries.
The energy lost during transfers amongst the system
components is relatively less due to high efficiency.
The flywheel system can deliver almost the entire
amount of energy stored in it, repeatedly without any decline in
The mechanical system does not need to be replaced as
its life cycle is as good as that of the car.



The specific energy capacity of flywheels is lower than
some of the advanced battery models.
Friction produced in the bearings and seals cause the
flywheel to slow down and lose energy. 14









Apart from increasing overtaking the main purpose of
introducing KERS was to challenge the best engineers in the business to develop
innovative ideas that would directly benefit the mainstream motor industry.
Given the resources and pace of developments in F1, the KERS systems produced by the teams would have taken the car
manufacturers much longer to develop. Both the types of KERS can be retrofitted
in cars albeit with minor modifications. Given the current trend of engine
downsizing they can add substantial amount of performance to the car without
affecting the engine and average. The mechanical system is more efficient than
the electrical systems that use inefficient batteries which makes them more
likely to be induced in cars in the near future. 14

By adopting the cheaper and
lighter flywheel system (the ideal solution if it could be made to fit into the
no-refueling era cars), a more powerful boost, and limiting the number of
activations in a race it would cover all the bases it needs to. It would be
affordable for the all the teams, deliver performances as well as being a more interesting race variable. 

The sidepod solution is quite
unique, and has given us a new envelope to try to drive performance to the rear
of the car. We need to keep thinking out-of-the-box. Compared to ten or 20
years ago, it’s really quite staggering what can be delivered given the
restrictions we have now – it’s a tribute to
imaginative thinking. 15

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