OF KINETIC ENERGY RECOVERY SYSTEM
BY BASIL AFRIDI
HOW DOES KERS WORK?
ADVANTAGES OF ELECTRIC KERS
DISADVANTAGES OF ELECTRIC KERS
ADVANTAGES OF MECHANICAL KERS
DISADVANTAGES OF MECHANICAL KERS
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. one research paper. 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.
I0n 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
At the end, we will formulate a conclusion.
HOW DOES KERS WORK?
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 teams started off trying to implement
into their cars. However, the battery used to store the energy is very prone to
battery fires and can cause electric shocks. After an incident with the BMW
Sauber team, where an engineer working on the KERS was burned while testing the
system after a practice run, many teams deemed the electric KERS to be unsafe.
Along with other factors such as being heavier than other implementations, the
electric KERS implementation is not found inside today’s Formula 1 cars.
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.
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
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
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.
When 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
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:
is the tensile stress on
the rim of the flywheel;
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 flybrid.
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
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
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).
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
SHAFT FROM FLYWHEEL
LOW SPEED 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 disengaging 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.
ADVANTAGES OF ELECTRIC KERS
electric systems allow the teams 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 F1.
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.
OF ELECTRIC KERS
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 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.
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
OF MECHANICAL KERS
specific power of flywheels in comparison is much greater than that of
energy lost during transfers amongst the system components is relatively
less due to high efficiency.
flywheel system can deliver almost the entire amount of energy stored in
it, repeatedly without any decline in efficiency.
mechanical system does not need to be replaced as its life cycle is as
good as that of the car.
OF MECHANICAL KERS
specific energy capacity of flywheels is lower than some of the advanced
Friction produced in the bearings and seals cause the flywheel
to slow down and lose energy..
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