International Journal of Engineering and Techniques - Volume 3 Issue 6, Nov - Dec 2017

RESEARCH ARTICLE OPEN ACCESS

Regenerative Braking in Automobiles

Rushikesh Kharade
(Department of Mechanical Engineering, MIT College of Engineering, Pune)

Abstract:
Normally, the braking system for a car iscentered on hydraulic braking technology. However, thistraditional
braking approach causes a lot of wastage of energysince it produces undesirable heat energy during the braking
action.Consequently, the invention of regenerative braking in electricvehicles has overcome these disadvantages
moreover it helpsto save energy and increase efficiency for the vehicles.Wheneverbrakes are applied, a significant
amount of kinetic energyislost in the form of heat energy due tofriction between brake pads and rotor.
RegenerativeBraking System recovers kinetic energy as much aspossible that is wasted during the process of braking.
The energy is stored and then released under acceleration. Thispaper highlights the two different methods of
recoveringenergy, by converting it intoeither electrical or mechanical energy. The kinetic energygets converted into
of the
electrical energy with the help ofelectric motor while flywheel energytheis kineticenergy
harnesses expended during braking
by converting of
it into the
mechanical energy. vehicle[(9),(10)]. According to the energy
Keywords —Regenerative Braking, Energy recovery system, Electric motor, Generator, Batteries, Flywheel, HEV,
EV.

I. INTRODUCTION
The electric vehicle (EV) invention has been a
miracle, also called as a green vehicle as it produces evaluation, the kinetic energy is anexcess
zero emission to the air which means there is no energy when the electric motor of the vehicle is in
release of toxic gasses from the car which causes the braking conditionsince dissipation of energy as
the ozone layer depletion. Nowadays, the number of heat takes place and hencethere is loss in the overall
EV is increasing according to the market demand. energy of the vehicle. In the case of the EVs and
Besides, the Government's regulations to the EVs, this wasted energy can actually be converted
production of electric car are getting more to a useful energy. As a result, regenerative braking
serious.Each step is taken by the world to save the has been implemented in the car braking system to
Planet Earth from the excessive air pollution and recollect this wasted energy from the vehicle.
the recession on the natural resources like crude oils Additionally,driving condition also play a part in
and natural gasses in the earth. Today, there have the total energy recovered.Generally,regenerative
been a lot of technological advances in fields of braking is more effective in city driving because of
control technology and integrative technology. But, a lot of braking action rather than highway.There
the limitation of the driving range has still been an are many advantages of regenerative braking over
obstacle in development of the Electric Vehicles. the traditional braking system such as:
By using regenerative braking in EVs and HEVs, • More control over braking
this hurdle of driving mileage can be tackled which • More efficient and effective in stop-and-go
improves the driving limit by 8-25% [(11), driving conditions
(12)].The traditional braking system in a vehicle is • Prevents wear on mechanical brake systems
been replaced by regenerative braking system as the
• Better fuel economy
former only utilises friction for kinetic energy
• Save energy
dissipation in order to completely stop a vehicle.
Many studies show that, of the total energy required
for the operation of a vehicle, about a third to half

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 International Journal of Engineering and Techniques - Volume 3 Issue 6, Nov - Dec 2017
II. BASIC THEORY OF VEHICLE Ideally for optimal performance of braking,
BRAKING thedesired braking forces i.e. and , should be
Braking theory and design principles of the proportional to the corresponding vertical loads
conventional vehicle by using a frictional ( and ),as shown in Fig. 1.
mechanism are well established. Itemphasizes on Thus, we have:
the distribution of total braking force on front and + ℎ
rear in order to attain short braking distance and = =
prevent the rear wheel from being locked earlier − ℎ
than the front wheel is locked.A brief description of When the braking forces is applied on the
traditional vehicle braking theory is added in this front and rear wheels follow the above equation,
paper as followings for an easy understanding for both the front and rear wheels will be locked
readers. simultaneously as the braking force applied reaches
the maximum friction capability in the contact area
of the tire and road. At this instant, the deceleration
of the vehicle can be expressed further as:

+ ( + )μ μ
= = = = μ

where μ is the adhesive coefficient between tire and

road.It is defined as the ratio of the maximum
braking force theroad can supply to the vertical load.
The equation can be expressedfurther as:
Figure 1: Forces acting on the while braking
+ μℎ
=
Fig 1 shows the forces acting on the vehicle during − μℎ
braking on a flat road surface, where the resistance
due to rolling and aerodynamic drag are ignored. Using these equations, the ideal braking forces
The normal forces and , acting on the front applied on the front and rear wheels on the roads
and rear wheels can be expressed as: with a various adhesivecoefficient (vehicle
deceleration) are shown in Fig. 2, which are
marked for full load and for unloaded. These
= ( + ℎ ) curves are parabolic. In designing of mechanical
brakingsystem, implementation of the parabolic
= ( − ℎ ) braking force distribution will tend to very complex
structure. Hence, inconventional vehicles, a
straight-line braking force distribution is usually
where j is the vehicle deceleration in m/ , and employed as marked in Fig. 2. In this case, only on
are braking forces acting on the front and rear one road condition (point A, u=0.8 in the example
wheels, and M is the mass of vehicle in kg, and of Fig. 2), the front and rear wheels will be locked
other parameters shown in Fig.1 where,O is the instantaneously.When braking on the road with
centre of gravity of the vehicle. Normally, the adhesive coefficient less than thisvalue, the front
deceleration of the vehicle depends on the total wheels will be locked before the rear wheels. This
braking force as: leads to a great directional stability.However, when
+ braking on the road with large adhesivethe
= =
coefficient ( μ >0.8), the rear wheel
willbrakingforces acting on front and rear wheels,

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 International Journal of Engineering and Techniques - Volume 3 Issue 6, Nov - Dec 2017
and M is the mass to be locked before the front III. REGENERATIVE BRAKING USING
wheels lacked, which causes instability of vehicle. ELECTRIC MOTOR
This instability tendency will worsenwhen the Electric vehicles (EVs) and Hybrid Electric
vehicle is unloaded, where the rear wheels haveless Vehicles (HEVs) use an electric motor as asource to
vertical load, and will be locked much easily, propel themselves. As known, when the motor is
asdemonstrated by point B in Fig. 2.Usuallyin madeto run in reverse direction itbehaves as a
actual practice,the brake system ofpassenger cars is generator. Similarconcept or principle is utilized for
designed such that the rear wheels willnever be regenerative braking.When braking action is to be
locked. This results in small braking forceon the done, motor switches itself to generatormode. The
rear wheels as demonstrated by the straight-line generator then captures the kinetic energy of the
labelled β inFig.2. Nevertheless, this design has the wheel viaa drivetrain. Consequently, it transforms
disadvantage that the actualbraking force the kinetic energy intoelectrical energy which is
distribution is far from the ideal;hence,it leads to then stored in a battery for further use.On the other
low road adhesion utilization and low hand, generator resistance produced from
utilizationefficiency ofpressure of the front wheel. theelectricity generated, slows down the vehicle.
Because of this, some brakedesign regulations insist The greater the power ofMotor or Generator in
on the minimum rear braking force,typical one is generator mode, larger will be the amount
the BCE regulation illustrated by the curvelabelled ofkineticenergy recovered. The capacity of battery
ECE-regulation [12]. to store the energy is also a factor that affects the
amount ofenergy recuperated. When the torque
generated by the generator is less as compared to
the required braking torque,it is enhanced by a
friction brake.

Figure 3: Regenerative Braking using motor and Battery as an

Figure 2: Ideal braking and real braking force on the front and energy storage system
rear wheels, and theminimum braking force on rear wheels
with loaded condition (ECE-regulation) IV. REGENERATIVE BRAKING USING
In design and developmentof a hybrid braking FLYWHEEL
system for EVs and HEVs, the principle mentioned The flywheel is a heavy rotating mass which is
above should be followed in order to ensure the used to store the kineticenergy or the mechanical
vehicle has a good braking performance. At the energy of rotating wheel in the form of rotational
same instant, it should be able to recover as much energy.This process of recoveringenergy is more
braking energy as possible. efficient. The losses involved during
energytransformation are avoided because the
energy is beingtransmitted in the form of
mechanical energy throughout the cycle. Incase of

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 International Journal of Engineering and Techniques - Volume 3 Issue 6, Nov - Dec 2017
recovering the energy through motor/generator establishing suitable gear accelerating mode, ratio
andbattery system, many energy losses occur between flywheel and wheel.The gear ratio varies
as mechanical energy isbeing transformed continuously to speed up thewheels. Thus, the
into electrical energy while charging thebattery and vehicle is powered by a flywheel by shuttingdown
during discharging, the electrical energy the engine.Thus, the energy is retainedby the
getsconverted into mechanical form.The energy flywheelassisted by a regenerative braking system.
generated is expressed as: Flywheels have a highspecific power as compared
1 to the conventional battery andthus prove to be a
= better energy storage system.
2
where,
E=rotational energy of flywheel
I=moment of inertia of flywheel
=angular velocity of flywheel

The energy stored by the flywheel depends

upon itsmass, radius and rotational velocity. Hence,
the maximumenergy stored by flywheel can be
enhanced by increasingmoment of inertia and
angular velocity of the flywheel. In orderto recover
energy or to start energy transfer throughflywheel,
the angular momentum of the flywheel must be Figure 4: Flywheel-Regenerative Braking System
varied. For varyingangular momentum, angular
velocity or moment of inertiamust be varied V. ADVANTAGES OF REGENERATIVE
constantly. Thus a Continuous VariableSystem BRAKING
(CVT) is used to transmit the power. Itconsistsof 1. Fuel economy is improved as it lowers the
two pulleys which are connected by a belt. One fuelconsumption.
Pulleyis acts as a driving pulley whereas the other 2. Reduce the carbon footprint and thus
acts as a drivenpulley. The continuous movement of environmentfriendly.
the sheaves of pulleychanges the effective diameter 3. Improved vehicle performance as it boosts
of belt and hence gear ratio changes theacceleration instantly.
accordingly.Flywheel is connected to one end of the 4. Regenerative braking system reduces the
CVT whereas it’sanother end is linked to driving overallwork done by the engine and
axle i.e. the wheel.The CVT changes the angular therefore reduces thewear of engine parts.
velocity of theflywheel establishing different gear 5. Regenerative braking complements the
ratio between flywheeland wheel. The range of frictionalbraking and reduces the wear and
CVT determines the maximumangular velocity that tear of brakeassembly parts.
can be achieved by a flywheel.During normal mode,
vehicle is powered by the engine byengaging the
clutch between engine and variabletransmission. In VI. FLYWHEELS: A BETTER
regenerative braking system, flywheel isengaged to ALTERNATIVE TO BATTERIES
the transmission while the engine clutch 1. Flywheel has better efficiency ascompared
isdisengaged. The variable transmission to battery as the losses do not occurduring
continuously variesgear ratio so as to speed up the energy transformation.
flywheel. The angularvelocity of wheel gets 2. It has higher specific power and thus can
reduced and that of flywheel increases.Thus, the deliverenergy at high rate.
energy is stored in flywheel in the form of 3. They are less sensitive to the temperature
mechanical energy. In energy recoverymode or variationwhereas conventional batteries
accelerating mode, flywheel clutch is engaged by

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 International Journal of Engineering and Techniques - Volume 3 Issue 6, Nov - Dec 2017
experiences powerfades at high above discussedstorage system proves flywheel as a
temperatures. better energy storagesystem. There is always a
4. The operating life or cyclic age of flywheel scope for improvement in terms of technology.
is more as compared to battery. Thus, the effort should be made to layout
5. They have comparatively less impact on anappropriate design of flywheel which can
theenvironment and are easy to recycle. withstand a large amountof the stress developed in
flywheel. Also, the research should be made to
implement such a material for the flywheel which
VII. THE FUTURE FOR REGENERATIVE has good strength and comparatively less weight
BRAKING sothat they can be feasible for the practical aspects
of vehicle.Some modern flywheels are made of
There are a number of subjective decisions one carbon-compositematerials and are proved to be
has to make concerning the regenerative system more feasible and reliable.By introducing an
profile. Some people like regenerative system to efficient transmission system, it wouldincrease the
work all the way to 0 mph, bringing the car to stop. overall efficiency of regenerative braking.Further
Some like to coast that last 2 to 3 mph. almost research could help in making this technology
morereliable and efficient.
everyone likes the car to regenerate when you take
your foot off of the throttle pedal, but there are IX. REFERENCES
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