Regenerative Braking System by ASLAM

VISVESVARAYA TECHNOLOGICAL UNIVERSITY
BELGAUM
A
TECHNICAL SEMINAR REPORT
ON
REGENERATIVE BRAKING SYTEM
Submitted in partial fulfilment of the requirements of the degree of
Bachelor of Engineering
in
Mechanical Engineering
By
MD ASLAM HUSSAIN
Department of Mechanical Engineering
H.K.B.K College of Engineering
S.No: 22/1, Near Manyata Tech park, Bangalore-560045

ABSTRACT
Regenerative Braking System is the way of slowing vehicle by using the motors as brakes.
Instead of the surplus energy of the vehicle being wasted as unwanted heat, the motors act as
generators and return some of it to the overhead wires as electricity.
The vehicle is primarily powered from the electrical energy generated from the generator,
which burns gasoline. This energy is stored in a large battery, and used by an electric motor that
provides motive force to the wheels. The regenerative braking taking place on the vehicle is a
way to obtain more efficiency, instead of converting kinetic energy to thermal energy through
frictional braking, the vehicle can convert a good fraction of its kinetic energy back into charge in
the battery, using the same principle as an alternator.

CONTENTS
Chapter 1 Page No.
Introduction 01
1.1 Conventional Braking Systems in Automobiles 01
1.2 Regenerative Braking Systems in Automobiles 02
Chapter 2
Necessity of the System 04
Chapter 3
Working Principle 05
3.1 Regenerative Working 05
3.2 How Does a Motor/Generator Work in a Hybrid Cars 06
3.3 Propelling the Vehicle with the Motor/Generator 06
3.4 Slowing the Vehicles and Generating Electricity 07
Chapter 4
Mode of Power Generation 08
Chapter 5
Parts of Regenerative Braking System 09
5.1 Alternator 09
5.2 Inverter 09
5.3 Convertor 09
5.4 Storage Battery 10
5.5 DC Motor 11
Chapter 6
Types of Braking 13
6.1 Disc Brakes 13
6.2 Drum Brakes 14
6.3 Hydraulic Brakes 14
6.4 Electromagnetic Brakes 15
6.5 ABS Brakes 16
Chapter 7
Future of Regenerative Braking System 18
7.1 Scope in Metro 18

Chapter 8 Page No.
Advantage, Disadvantage & Application 20
8.1 Advantages 20
8.2 Disadvantages 20
8.3 Applications 20
Conclusion 21
References

LIST OF FIGURES
Fig. No. Particulars Page No.
Fig.1.1 Conventional Braking System 02
Fig.3.1 Regenerative Brakes 05
Fig.3.2 Working Principle of Regenerative 06
Fig.5.1 Lead Acid Battery 10
Fig.6.1 Disc Brakes 13
Fig.6.2 Drum Brakes 14
Fig.6.3 Hydraulic Brakes 15
Fig.6.4 Electromagnetic Braking System 16
Fig.6.5 ABS Braking System 17
Fig.7.1 Regenerative Braking System in Metro 19

Regenerative Braking System
Department of Mechanical Engineering 1
Chapter 1
INTRODUCTION
Regenerative Braking System is the way of slowing vehicle by using the motors as brakes.
Instead of the surplus energy of the vehicle being wasted as unwanted heat, the motors act as
generators and return some of it to the overhead wires as electricity.
The vehicle is primarily powered from the electrical energy generated from the generator,
which burns gasoline. This energy is stored in a large battery, and used by an electric motor
that provides motive force to the wheels. The regenerative braking taking place on the vehicle
is a way to obtain more efficiency, instead of converting kinetic energy to thermal energy
through frictional braking, the vehicle can convert a good fraction of its kinetic energy back
into charge in the battery, using the same principle as an alternator.
Therefore, if you drive long distance without braking, you’ll be powering the vehicle
entirely from gasoline. Regenerative Braking System comes into its own when you’re driving
in the city, and spending a good deal of your time braking. You will still use more fuel in the
city for each mile you drive than on the highway, though. Thermodynamics tells us that all
inefficiency comes from heat generation. For instance, when you brake, the brake pedals heat
up and a quantity of heat, or energy, is lost to the outside world. Friction in the engine produces
heat in the same way.
In most electric and hybrid electric vehicles on the road today, this is accomplished by
operating the traction motor as a generator, providing braking torque to the wheels and
recharging the traction batteries. The energy provided by regenerative braking can then be used
for propulsion or to power vehicle accessories.
1.1 Conventional Braking Systems in Automobiles
In conventional braking, brakes are applied using a foot pedal which when pressed
transfers the hydraulic pressure from master cylinder to brake pads with help of fluid lines,
brake pads in turn presses against the brake disc to stop the vehicle. In this way the kinetic
energy change of vehicle is completely lost in the form of heat between brake pads and disc
as well as some amount between tyre and road. This heat is not recovered and is lost to
atmosphere.so today there is need of more efficient braking system.

Fig.1.1: Conventional Braking System
1.2 Regenerative Braking Systems in Automobiles
In Regenerative braking system instead of wasting the kinetic energy of vehicle in the form
of heat it is converted into electrical energy to be stored in batteries and capacitors or as
mechanical energy of a flywheel having large moment of inertia. In this way a large proportion
of energy of vehicle is saved only to be used later for either accelerating the vehicle or for
different electrical purpose.
1.2.1 Motor Based Regenerative Braking
This type of regenerative system is used in electrical or hybrid electrical vehicles as it
makes use of electric motors. The drive shaft of vehicle is connected to a motor, when current
is supplied to motor it starts rotating and in turn rotates the drive shaft of vehicle. When brakes
are to be applied, the driver presses the brake pedal which cuts off the current supply to motor.
Now motor is no longer providing torque to the driving shaft, instead the inertial kinetic energy
and momentum of vehicle drives the motor, electric motor now starts acting as a generator
resisting the inertial rotational motion of vehicle to slow down the vehicle besides producing
electricity. This electricity can be stored in battery or capacitor.
1.2.2 Flywheel Based Regenerative Braking
This regenerative braking system consists of a flywheel having a large moment of inertia,
so that it requires a large torque for rotational acceleration. There is a provision for engagement

and disengagement of flywheel with the drive shaft. When driver needs to slow down or stop
the vehicle, the flywheel is engaged with the drive shaft with the help of gears. As flywheel
gets engaged power now goes divided between driving shaft and flywheel, flywheel having a
large moment of inertia absorbs the power from engine in the form of rotational kinetic energy
and brings the vehicle to halt and this rotational kinetic energy of flywheel can be used further
to accelerating the vehicle. Since it has huge momentum so it takes a great deal of stopping and
changing its speed takes a lot of effort. If an engine supplies power intermittently, the flywheel
compensates for surplus or deficit power. So flywheel helps to smooth out the power wheel
receives. The main drawback of using flywheels in moving vehicles is their heavy weight.

Chapter 2
NECESSITY OF THE SYSTEM
The regenerative braking system delivers a number of significant advantages over a car
that only has friction brakes. In low-speed, stop-and-go traffic where little deceleration is
required, the regenerative braking system can provide the majority of the total braking force.
This vastly improves fuel economy with a vehicle, and further enhances the attractiveness of
vehicles using regenerative braking for city driving. At higher speeds, too, regenerative braking
has been shown to contribute to improved fuel economy by as much as 20%.
Consider a heavy loaded truck having very few stops on the road. It is operated near
maximum engine efficiency. The 80% of the energy produced is utilized to overcome the
rolling and aerodynamic road forces. The energy wasted in applying brake is about 2%. Also
its brake specific fuel consumption is 5%.
Now consider a vehicle, which is operated in the main city where traffic is a major problem
here one has to apply brake frequently. For such vehicles the wastage of energy by application
of brake is about 60% to 65%. And also it is inefficient as its brake specific fuel consumption
is high.
In regenerative braking system both these problems are solved that is storage of energy
and efficient brake specific fuel consumption.

Chapter 3
WORKING PRINCIPLE
3.1 Regenerative Working
Hybrids and all electric vehicles create their own power for battery recharging through a
process known as regenerative braking. We have explained what regenerative braking is and
how the process works in general terms, but many folks are interested in the deeper nuts and
bolts of electricity generation. They understand that in a hybrid or all electric vehicle the word
"regenerative" in terms of regenerative braking, means capturing the vehicle's momentum
(kinetic energy) and turning it into electricity that recharges (regenerates) on board battery as
the vehicle is slowing down and/or stopping. It is this charged battery that in turn powers the
vehicle's electric traction motor. In an all-electric vehicle, this motor is the sole source of
locomotion.
Any permanent magnet motor can operate as either a motor or generator. In all electrics
and hybrids, they are more precisely called a motor/generator. But the technologically curious
want to know more, and it is often asked “How”, and by what mechanism or process, is the
electricity created?, It's a good question, so before we get started explaining how
motor/generator and regenerative braking work in hybrids and electric vehicles, it is important
to have basic knowledge about how electricity is generated and how a motor/generator
functions. Fig.3.1 showing the working principle of regenerative brakes.
Fig.3.1: Regenerative Brakes

Fig.3.2: Working Principle of Regenerative Brakes
3.2 How Does a Motor/Generator Work in Hybrid Cars
No matter the vehicle design, there must be a mechanical connection between the
motor/generator and the drive train. In an all-electric vehicle there could be an individual
motor/generator at each wheel or a central motor/generator connected to the drive train through
a gearbox. In a hybrid, the motor/generator could be an individual component that is driven by
an accessory belt from the engine, it could be a pancake motor/generator that is bolted between
the engine and transmission, or it could be multiple motor/generator mounted inside the
transmission (this is how the two modes work). In any case, the motor/generator has to be able
to propel the vehicle as well as be driven by the vehicle in regen mode.
3.3 Propelling the Vehicle with the Motor/Generator
Most, if not all, hybrids and electrics use an electronic throttle control system. When the
throttle pedal is pushed, a signal is sent to the onboard computer, which further activates a relay
in the controller that will send battery current through an inverter/converter to the
motor/generator causing the vehicle to move. The harder the pedal is pushed, the more current
flows under direction of a variable resistance controller and the faster the vehicle goes. In a
hybrid, depending upon load, battery state of charge and the design of the hybrid drive train, a
heavy throttle will also activate the internal combustion engine for more power. Conversely,
lifting slightly on the throttle will decrease current flow to the motor and the vehicle will slow
down. Lifting further or completely off the throttle will cause the current to switch direction
moving the motor/generator from motor mode to generator mode and begin the regenerative
braking process.

3.4 Slowing the Vehicles and Generating Electricity
This is really what the regen mode is all about. With the electronic throttle closed and the
vehicle still moving, all of its kinetic energy can be captured to both slow the vehicle and
recharge its battery. As the onboard computer signals the battery to stop sending electricity (via
the controller relay) and start receiving it (through a charge controller), the motor/ generator
simultaneously stops receiving electricity for powering the vehicle and starts sending current
back to the battery for charging.
Remember from our discussion on electromagnetism and motor/ generator action: when a
motor/generator is supplied with electricity it makes mechanical power, when it's supplied with
mechanical power, it makes electricity. But how does generating electricity slow the vehicle?
Friction. It's the enemy of motion. The armature of the motor/ generator is slowed by the force
of inducing current in the windings as it passes over the opposing poles in the magnets in the
stator (it's constantly battling the push/pull of the opposing polarities). It is this magnetic
friction that slowly snaps the vehicle's kinetic energy and helps to brake it.
C

Chapter 4
MODE OF POWER GENERATION
Electromagnetism
Motor power and electricity generation begin with the property of electromagnetism the
physical relationship between a magnet and electricity. An electromagnet is a device that acts
like a magnet, but its magnetic force is manifested and controlled by electricity. When wire
made of conducting material (copper, for example) moves through a magnetic field, current is
created in the wire. Conversely, when electricity is passed through a wire that is wound around
an iron core, and this core is in the presence of a magnetic field, it will move and twist (a very
basic motor).

Chapter 5
PARTS OF REGENERATIVE BRAKING SYSTEM
The followings are the parts of regenerative brake system
1. Alternator
2. Inverter
3. Convertor
4. Storage Battery
5. DC Motor
5.1 Alternator
Principle:
A.C. generators or alternators (as they are usually called) operate on the same fundamental
principles of electromagnetic induction as D.C. generators.
Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating
a magnetic field within a stationary coil. The value of the voltage generated depends on
1. The number of turns in the coil.
2. Strength of the field.
3. The speed at which the coil or magnetic field rotate.
5.2 Inverter
An inverter is an electrical device that converts electricity derived from a DC (Direct
Current) source to AC (Alternating Current) that can be used to drive an AC appliance. The
theory of operation is relatively simple. DC power, from a hybrid battery for example, is fed to
the primary winding in a transformer within the inverter housing. Through an electronic switch
(generally a set of semiconductor transistors), the direction of the flow of current is
continuously and regular broken (the electrical charge travels into the primary winding, then
abruptly reverses and flows back out). The in/out flow of electricity produces AC current in
the transformer's secondary winding circuit. Ultimately, this induced alternating current
electricity flows into and produces power in an AC load (for example an electric vehicles
electric traction motor). A rectifier is a similar device to an inverter except that it does the
opposite, converting AC power to DC power.

5.3 Convertor
More properly called a voltage converter, this electrical device changes the voltage (either
AC or DC) of an electrical power source. There are two types of voltage converters, one is step
up (which increases voltage) and other is step down (which decreases voltage). The most
common use of a converter is to a take relatively low voltage source and step it up to high
voltage for heavy duty work in a high power consumption load, but they can also be used in
reverse to reduce voltage for a light load source.
5.4 Storage Battery
5.4.1 Lead Acid Battery
A lead acid battery is an electrical storage device that uses a reversible chemical reaction
to store energy. It uses a combination of lead plates or grids and an electrolyte consisting of a
diluted sulphuric acid to convert electrical energy into potential chemical energy and back
again. The electrolyte of lead acid batteries is hazardous to your health and may produce burns
and other permanent damage if you come into contact with it.
Fig.5.1: Lead Acid Battery

5.5 DC Motor
5.5.1. DC Motors
A DC motor is designed to run on DC electric power. Two examples of pure DC designs
are Michael Faraday's homo polar motor (which is uncommon), and the ball bearing motor,
which is (so far) a novelty. By far the most common DC motor types are the brushed and
brushless types, which use internal and external commutation respectively to create an
oscillating AC current from the DC source so they are not purely DC machines in a strict sense.
5.5.2. Brushed DC electric motor
The classic DC motor design generates an oscillating current in a wound rotor with a split
ring commutator, and either a wound or permanent magnet stator. A rotor consists of one or
more coils of wire wound around a core on a shaft; an electrical power source is connected to
the rotor coil through the shaft, causing current to flow in it, producing electromagnetism. The
commutator causes the current in the coils to be switched as the rotor turns, keeping the
magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field,
so that the rotor never stops (like a compass needle does) but rather keeps rotating indefinitely
(as long as power is applied and is sufficient for the motor to overcome the shaft torque load
and internal losses due to friction, etc.).
Many of the limitations of the classic commutator DC motor are due to the need for brushes
to press against the commutator. This creates friction. At higher speeds, brushes have
increasing difficulty in maintaining contact. Brushes may bounce off the irregularities in the
commutator surface, creating sparks. (Sparks are also created inevitably by the brushes making
and breaking circuits through the rotor coils as the brushes cross the insulating gaps between
commutator sections. Depending on the commutator design, this may include the brushes
shorting together adjacent sections and hence coil ends momentarily while crossing the gaps.
Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its
circuit is opened, increasing the sparking of the brushes.) This sparking limits the maximum
speed of the machine, as too rapid sparking will overheat, erode, or even melt the commutator.
The current density per unit area of the brushes, in combination with their resistivity, limits the
output of the motor. Brushes eventually wear out and require replacement, and the commutator
itself is subject to wear and maintenance (on larger motors) or replacement (on small motors).

The commutator assembly on a large machine is a costly element, requiring precision assembly
of many parts.
Large brushes are desired for a larger brush contact area to maximize motor output, but
small brushes are desired for low mass to maximize the speed at which the motor can run
without the brushes excessively bouncing and sparking (comparable to the problem of "valve
float" in internal combustion engines). (Small brushes are also desirable for lower cost.) Stiffer
brush springs can also be used to make brushes of a given mass work at a higher speed, but at
the cost of greater friction losses (lower efficiency) and accelerated brush and commutator
wear. Therefore, DC motor brush design entails a trade off between output power, speed, and
efficiency/wear.

Chapter 6
TYPES OF BRAKING
The different types of braking are
1. Disc Brakes
2. Drum Brakes
3. Hydraulic Brakes
4. Electromagnetic Brakes
5. ABS Brakes
6.1 Disc Brakes
The disc brake is a device for slowing or stopping the rotation of a wheel. A brake disc as
shown in Fig.6.1 (or rotor in usually made of cast iron or ceramic composites (including carbon
and silica), is connected to the wheel and or the axle. To stop the wheel, friction material in the
form of brake pads (mounted on a device called a brake caliper) is forced mechanically,
hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction
causes the disc and attached wheel to slow or stop.
Fig.6.1: Disc Brakes

6.2 Drum Brakes
The drum brake has been more widely used than any other brake design. Braking power
is obtained when the brake shoes are pushed against the inner surface of the drum which rotates
together with the axle. Drum brakes are used mainly for the rear wheels of passenger cars and
trucks while disc brakes are used exclusively for front brakes because of their greater
directional stability.
The backing plate is a pressed steel plate, bolted to the rear axle housing. Since the brake
shoes are fitted to the backing plate, all of the braking force acts on the backing plate. Drum
Brakes are now used mainly for the rear wheels of passenger cars and trucks. When the brake
pedal is depressed while the vehicle is moving backward, the brake shoes expand and contact
the drum.
Fig.6.2: Drum Brakes
6.3 Hydraulic Brakes
The hydraulic brake system used in the automobile is a multiple piston system. A multiple
piston system allows forces to be transmitted to two or more pistons in the manner indicated in
Fig.6.2. Note that the pressure set up by the force applied to the input piston (1) is transmitted
undiminished to both output pistons (2 and 3), and that the resultant force on each piston is
proportional to its area. The multiplication of forces from the input piston to each output piston
is the same as that explained earlier.

The hydraulic brake system from the master cylinders to the wheel cylinders on most
automobiles operates in a way similar to the system illustrated in Fig.6.3.
Fig.6.3: Hydraulic Brakes
6.4 Electromagnetic Brakes
Electromechanical braking systems (EMB), also referred to as brake by-wire, replace
conventional hydraulic braking systems with a completely “dry” electrical component system.
This occurs by replacing conventional actuators with electric motor driven units. This move to
electronic control eliminates many of the manufacturing, maintenance, and environmental
concerns associated with hydraulic systems.
Electromechanical braking systems (EMB), also called brake by wire, replace
conventional hydraulic braking systems with completely “dry” electrical component systems
by replacing conventional actuators with electric motor driven units. This move to electronic
control eliminates many of the manufacturing, maintenance, and environmental concerns
associated with hydraulic systems.

Fig.6.4: Electromagnetic Braking System
6.5 ABS Brakes
Antilock braking system that is between the brake master cylinder and the wheels. The
system prevents an unstable condition of the car under extreme braking conditions. It
modulates the pressure of the brake fluid that is applied to both front brake calipers and/or both
rear calipers, preventing the wheels from "LOCKING UP". Normal brake fluid pressure is
restored when there is no longer a possibility of the wheels locking up. Each wheel has a
SENSOR that the system monitors for each wheels rotation. If one of the wheels is turning

slower than the others, the anti lock system releases the pressure to that wheel. The system is
designed to provide positive feedback by way of a kickback on the brake pedal when the system
has been activated. The system works very well in wet or icy conditions, preventing skids and
loss of directional control. The ABS computer controls all functions of the antilock brakes. If
there is a problem with the ABS system, the ABS INDICATOR LIGHT on the dash
illuminates. In the event of a failure of the antilock brake system, it does have a fail safe
function that allows for normal braking.
Fig.6.5: ABS Braking System

Chapter 7
FUTURE OF REGENERATIVE BRAKING SYSTEM
Regenerative braking system will be used further in all the cars, trucks, electric cars and
in electric locomotive also. Besides regaining the electrical energy, it is also increasing the
effort of braking to control the wheel from skid.
7.1 Scope in Metro
Traction accounts for about 60-80% of total energy consumption in a Metro system. The
quantity of energy consumed by trains is influenced by a wide range of factors, design of train
being one of them. Hence, optimization of overall system design in order to control
consumption of electricity becomes essential. The modern design of Metro Rolling Stock
incorporating three phase induction motors and Converter Inverter enables recovery of a major
portion of consumed electricity by way of using regenerative braking. Metro railways
worldwide have reported an average of about 20% saving in traction energy on account of
regeneration. It also helps to reduce heat load inside tunnel and thus reduce Air Conditioning
load. With increased awareness and commitment for environment metros have also taken it as
Green House Gas reduction initiative. Regenerated energy is mostly used by other trains
powering in the network. The quality of regenerated power is important since the injected
harmonics effect signaling, communication system and other loads connected on the grid.
With smaller inter-station distances, Metro operation is essentially of start/stop nature. Due
to frequent acceleration & de-acceleration requirements, energy demand is very high. Traction
accounts for about 60-80% of total energy consumption in a Metro system. The quantity of
energy consumed by trains is influenced by a wide range of factors, which can be grouped as
1. Design of trains &
2. Design of network,
3. Service planning operation.
Hence, optimization of overall system design in order to control consumption of electricity
becomes essential. The scope of this paper is limited to one of design aspect of trains i.e.
regenerative braking. The modern design of Metro Rolling Stock incorporating three phase
induction motors and Converter Inverter enables recovery of a major portion of consumed
electricity by way of using regenerative braking. Metro railways worldwide have reported an
average of about 20% saving on account of regeneration. By using intelligent blending of

regenerative and pneumatic braking, optimization of energy recovery as well as accurate
control of train movement can be achieved. In addition to saving of electricity, regenerative
braking provides additional benefits in form of lesser wear of wheel and brake pads. In Metro
applications, there is likelihood of having a considerable area of underground operation. An
efficiently employed regenerative braking system helps to reduce heat load inside tunnel and
thus reduce Air Conditioning load. Needless to mention that regenerative braking helps in
mitigating Global Warming by way of reducing carbon emissions due to reduced electricity
requirements from grid.
Energy cost is 5 to 15% of operating cost in Metros and traction power consumption
accounts for 60-80% of this energy cost saving achieved through regenerative braking is an
average of about 20% and hence this is acknowledged as most significant step for energy
saving. Besides energy saving, regenerative braking has indirect benefits in terms of reduction
in maintenance, increase in train availability and reduced heat load generation in underground
corridors.
Fig.7.1: Regenerative Braking System in Metro

Chapter 8
ADVANTAGE, DISADVANTAGE & APPLICATION
8.1 Advantages
1. Improved Performance.
2. Improved Fuel Economy.
3. Reduction in Engine wears.
4. Reduction in Brake Wear Reducing cost of replacement brake linings, cost of labour to
install them, and vehicle down time
5. Engine emissions reduced by engine decoupling, reducing total engine revolutions and
total time of engine operation.
6. Operating range is comparable with conventional vehicles a problem not yet overcome
by electric vehicles
8.2 Disadvantages
1. Added weight extra component can increase weight.
2. Friction brakes are still necessary.
3. Safety primary concern with any energy storage unit of high energy density.
4. Added maintenance requirements dependent on the complex of design.
8.3 Applications
1. For recovering Kinetic energy of vehicle lost during braking process.
2. One theoretical application of regenerative braking would be in a manufacturing plant
that moves material from one workstation to another on a conveyer system that stops at
each point.
3. Regenerative braking is used in some elevator and crane hoist motors.

CONCLUSION
The regenerative braking system used in the vehicles satisfies the purpose of saving a
part of the energy lost during braking. It can be operated at high temperature range and are
efficient as compared to conventional braking system. The results from some of the test
conducted show that around 30% of the energy delivered can be recovered by the system.
Regenerative braking system has a wide scope for further development and the energy
savings. The use of more efficient systems could lead to huge savings in the economy of any
country.

REFERENCES
[1] wikipedia.org/wiki/Regenerative brakes
[3] Auto.howstuffworks.com/auto-parts/brakes/brake-types/regenerative-braking.
[4] S.J.Clegg, “A Review of Regenerative Braking System”, Institute of Transport
Studies, University of Leeds, Working paper of 471, 1996.
[5] Chibulka.J, “Kinetic Energy Recovery System by means of Flywheel Energy Storage”,
Advanced Engineering, Vol. 3, No. 1, 2009, pp. 27-38.
[6] Regenerative braking boosts green credentials". Railway Gazette International. 2 July
2007. Retrieved 11 March 2014.
[7] Transport World The Tramway and Railway World XX. Carriers Publishing. July
December 1906. p. 20. Retrieved 11 March 2014.

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