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A MAJOR PROJECT REPORT ON REGENERATIVE BRAKING SYSTEM

Submitted in the partial fulfillment of the award of

Bachelor of Technology
(Rajasthan Technical University, Kota)

In MECHANICAL ENGINEERING Session 2009 2013

Submitted To: Mrs. Rajashree T.B. (H. O. D.)

Submitted By:Imran Khan Vikram Khan Shakeel Khan Nandkishore Sharma Vishal Kumar Gupta

DEPARTMENT OF MECHANICAL ENGINEERING SIDDHI VINAYAK COLLEGE OF SCIENCE & HIGHER EDUCATION ALWAR

CERTIFICATE
This is to certify that Imran Khan, Vikram Khan, Shakeel Khan, Nandkishore Sharma, Vishal Kumar Gupta are student of B.Tech.(Mechanical Engineering) 8th semester, Final Year have submitted Their Project report entitled Fabrication of electricity speed braker under my guidance. generation by

APPROVED BY:Mrs. Rajashree T.B. (H.O.D) Anurag Dixit (Principal)

FORWARDING LETTER

A PROJECT ON REGENERATIVE BRAKING SYSTEM

SUBMITTED BY:Imran Khan Vikram Khan Shakeel Khan Nandkishore Sharma Vishal Kumar Gupta This dissertation is in partial fulfillment as prerequisite to the award of the degree BACHELOR of TECHNOLOGY in Mechanical Engineering from Rajasthan Technical University during the Academic session 2009-2013. Mr. Asgar khan Mr. Dinesh Jangid Designation of Project Guide

CANDIDATE DECLARATION

We hereby declare that the work presented in the report preparation submitted in fulfillment of the award of the degree of Bachelor of Technology in Mechanical Engineering in an authentic record of our on work carried out under the able guidance of Mrs. Rajashree T.B. (H.O.D.) department of mechanical engineering.

Imran Khan Vikram Khan Shakeel Khan Nandkishore Sharma Vishal Kumar Gupta

ACKNOWLEDGEMENT

We express our sincere thanks to all faculties, teaching & non teaching staffs of department of Mechanical Engineering for us kind help in completing our project. It was due to us kind cooperation, valuable suggestions and effective discussions. We would like acknowledge Mr. Rajashree T.B. (H.O.D. Mech. Engg.) for his constant encouragement for the completion of this project successfully.

(Name of the students) Imran Khan Vikram Khan Shakeel Khan Nandkishore Sharma Vishal Kumar Gupta

(Signature of the students)

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 barking 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

1. Introduction
1.1 Need for regenerative brakes

2. Basic idea of Regenerative brakes

2.1 The Motor as a generator

3. Basic elements of the system

3.1 Energy Storage Unit (ESU) 3.2 Continuously Variable Transmission (CVT) 3.3 Controller 3.4 Regenerative Brake Controllers

4. Different types of Regenerative braking

4.1. Electric Regenerative Braking 4.2. Hydraulic Regenerative Brakes 4.3. Fly Wheels 4.4. Use in compressed air 4.5. Regenerative Braking Using Nitilon Spring

5. Applications 6. Comparisons
6.1 Advantages of regenerative braking over conventional braking 6.2 Comparison of Dynamic brakes and Regenerative brakes 6.3 Why Regenerative Brakes are assisted with the Frictional Brake??

7. Conclusion

INTRODUCTION

Brake:A brake is a machine element and its principle object is to absorb energy during deceleration. In vehicle brakes are used to absorb kinetic energy whereas in hoists or elevators brakes are also used to absorb potential energy. By connecting the moving member to stationary frame, normally brake converts kinetic energy to heat energy. This causes wastage of energy and also wearing of frictional lining material. Regenerative Braking System: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 barking 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, youll be powering the vehicle entirely from gasoline. The regenerative braking Regenerative Braking System comes into its own when youre 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. Heat energy, also, has higher entropy than, say, electric, meaning that it is less ordered.)

Definition: Braking method in which the mechanical energy from the load is converted into electric energy and regenerated back into the line is known as Regenerative Braking. The Motor operates as generator.

Regenerative Braking For Hybrid Vehicle: 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.

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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.

Road 80%

Rake 65%

Other 18%

Road 26% other 9%

Brake 2%

HEAVY LOADED TRUCK

CITY BUS

2.1 Graphical representation of energy usage between two vehicles.

In regenerative breaking system both these problems is solved i.e. Storage of energy and efficient brake specific fuel consumption. Some of the advantages of regenerative braking over conventional braking are as follows:

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Energy Conservation: The flywheel absorbs energy when braking via a clutch system slowing the car down and speeding up the wheel. To accelerate, another clutch system connects the flywheel to the drive train, speeding up the car and slowing down the flywheel. Energy is therefore conserved rather than wasted as heat and light which is what normally happens in the contemporary shoe/disc system. Wear Reduction: An electric drive train also allows for regenerative breaking which increases Efficiency and reduces wear on the vehicle brakes. In regenerative raking, when the motor is not receiving power from the battery pack, it resists the turning of the wheels, capturing some of the energy of motion as if it were a generator and returning that energy to the battery pack. In mechanical brakes; lessening wear and extending brake life is not possible. This reduces the use of use the brake. Fuel Consumption: The fuel consumption of the conventional vehicles and regenerative braking system vehicles was evaluated over a course of various fixed urban driving schedules. The results are compared as shown in figure. Representing the significant cost saying to its owner, it has been proved the regenerative braking is very fuel-efficient. Braking is not total loss: Conventional brakes apply friction to convert a vehicles kinetic energy into heat. In energy terms, therefore, braking is a total loss: once heat is generated, it is very difficult to reuse. The regenerative braking system, however, slows a vehicle down in a different way.

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BASIC IDEA OF REGENERATIVE BRAKES

Concept of this regenerative brake is better understood from bicycle fitted with Dynamo. If our bicycle has a dynamo (a small electricity generator) on it for powering the Lights, we'll know it's harder to peddle when the dynamo is engaged than when it'sSwitched off. That's because some of our peddling energy is being "stolen" by the dynamoAnd turned into electrical energy in the lights. If we're going along at speed and we Suddenly stop peddling and turn on the dynamo, it'll bring us to a stop more quickly than We would normally, for the same reason: it's stealing our kinetic energy. Now imagine a bicycle with a dynamo that's 100 times bigger and more powerful. In theory, it could bring our bike to a halt relatively quickly by converting our kinetic energy into electricity, Which we could store in a battery and use again later. And that's the basic idea behindRegenerative brakes! Electric trains, cars and other electric vehicles are powered by electric motors connected to batteries. When we're driving along, energy flows from the batteries to the Motors, turning the wheels and providing us with the kinetic energy we need to move. When we stop and hit the brakes, the whole process goes into reverse: electronic circuits Cut the power to the motors. Now, our kinetic energy and momentum makes the wheels Turn the motors, so the motors work like generators and start producing electricity instead of consuming it. Power flows back from these motor-generators to the batteries, charging Them up. So a good proportion of the energy we lose by braking is returned to the Batteries and can be reused when we start off again. In practice, regenerative brakes take Time to slow things down, so most vehicles that use them also have ordinary (friction) Brakes working alongside (that's also a good idea in case the regenerative brakes fail). That's one reason why regenerative brakes don't save 100 percent of our braking energy.

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Fig. (c): Basic idea of Regenerative brakes

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ELEMENTS OF THE SYSTEM

There are three basic element required which are necessary for the working of regenerative braking system, these are :

1.Energy Storage Unit (ESU):

The ESU performs two primary functions 1.TO recover & store braking energy 2. TO absorb excess engine energy during light load operation The selection criteria for an effective energy storage includes:1. High specific energy storage density 2. High energy transfer rate 3. Small space requirement The energy recaptured by regenerative braking might be stored in one of three devices: an electrochemical battery, a flywheel, in a regenerative fuel cell.

Regen and Batteries:

With this system, the electric motor of a car becomes a generator when the brake pedal is applied. The kinetic energy of the car is used to generate electricity that is then used to recharge the batteries. With this system, traditional friction brakes must also be used to ensure that the car slows down as much as necessary. Thus, not all of the kinetic energy of the car can be harnessed for the batteries because some of it is "lost" to waste heat. Some energy is also lost to resistance as the energy travels from the wheel and axle, through the drivetrain and electric motor, and into the battery. For example, the Toyota Prius can only recapture about 30% of the vehicles kinetic energy. The Honda Insight is another vehicle in addition to the Prius that is on the market and currently uses regenerative braking. In the Insight there are two deceleration modes: When the throttle is engaged, but the brake pedal is not, the vehicle slows down gradually, and the battery receives a partial charge.

When the brake pedal is depressed, the battery receives a higher charge, which slows the vehicle down faster. The further the brake pedal is depressed, the more the conventional friction brakes are employed.

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In the Insight, the motor/generator produces AC, which is converted into DC, which is then used to charge the Battery Module. The Insight, as well as all other regenerative systems, must have an electric controller that regulates how much charge the battery receives and how much the friction brakes are used.

Regen and Flywheels:

In this system, the translational energy of the vehicle is transferred into rotational energy in the flywheel, which stores the energy until it is needed to accelerate the vehicle. The benefit of using flywheel technology is that more of the forward inertial energy of the car can be captured than in batteries, because the flywheel can be engaged even during relatively short intervals of braking and acceleration. In the case of batteries, they are not able to accept charge at these rapid intervals, and thus more energy is lost to friction. Another advantage of flywheel technology is that the additional power supplied by the flywheel during acceleration substantially supplements the power output of the small engine that hybrid vehicles are equipped with.

Regen and Fuel Cells:

The third system uses what is known as a unitized regenerative fuel cell, which is designed to both convert hydrogen and oxygen into energy and water, or be reversed to take the energy from the wheels, combine it with water, and produce hydrogen and oxygen. The system as a single unit is substantially lighter than a separate electrolyzer and generator, which makes this system (known as a URFC) especially beneficial when weight is a factor. When the URFC is paired up with lightweight hydrogen storage, it's energy density of about 450 watt-hours per kilogram is ten times that of lead-acid batteries and twice as much as any predictions for the energy density of forthcoming chemical batteries. This means that not only will this technology make lighter hybrids available, it will also give hybrids a driving range that is comparable to that of vehicles today that are equipped with conventional engines. Further benefits of the URFC is that it will be more cost effective than other vehicles because it will not need to be replaced, and it will provide the additional power needed by an electric engine when accelerating onto a highway. Although URFC technology is still in the labs and has not yet been tried out In the electrolysis (charging) mode, electrical power from a residential or commercial charging station supplies energy to produce hydrogen by electrolyzing water.

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The URFC-powered car can also recoup hydrogen and oxygen when the driver brakes or descends a hill. This regenerative braking feature increases the vehicle's range by about 10% and could replenish a low-pressure (1.4-megapascal or 200-psi) oxygen tank about the size of a football. In the fuel-cell (discharge) mode, stored hydrogen is combined with air to generate electrical power. The URFC can also be supercharged by operating from an oxygen tank instead of atmospheric oxygen to accommodate peak power demands such as entering a freeway. Supercharging allows the driver to accelerate the vehicle at a rate comparable to that of a vehicle powered by an internal-combustion engine. on the market, it should solve many of the problems with hybrid vehicles that manufacturers are facing today when it becomes available.

2. Continuously Variable Transmission (CVT):

The energy storage unit requires a transmission that can handle torque and speed demands in a steeples manner and smoothly control energy flow to and from the vehicle wheels. For the flywheel the continuously variable transmission and vehicle because flywheel rotational speed increases when vehicle speed decreases and vice versa. Flywheel can work well with either mechanical or hydrostatic continuously variable transmission.

3. Control System: An ON-OFF engine control system is used. That means that the engine is ON until the energy storage unit has been reached the desired charge capacity and then is decoupled and stopped until the energy storage unit charge fall below its minimum requirement.

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DIFFERENT TYPES OF REGENERATIVE BRAKING

Based on the mode of storage of energy some of the system developed can be listed they are:-

Electric Regenerative braking:

In an electric system which is driven only by means of electric motor the system consists of an electric motor which acts both as generator and motor. Initially when the system is cruising the power is supplied by the motor and when the there is a necessity for braking depending upon drivers applied force on the brake pedal the electronic unit controls the charge flowing through the motor and due to the resistance offered motor rotates back to act as a generator and the energy is energy is stored in a battery or bank of twin layer capacitors for later use. In hybrid system motor will be coupled to another power source normally I.C Engines as shown in the fig (1)

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The main components of this system

Engine Motor/Generator Batteries Electronic control system During acceleration, the Motor/generator unit acts as electric motor drawing electrical energy from the batteries to provide extra driving force to move the car as(Shown in fig 2). With this help from the motor, the cars internal combustion engine that is smaller and with lower peak power can achieve high efficiency. During braking electric supply from the battery is cut off by the electronic system. As the car is still moving forward, the Motor/ Generator unit is acts as electric generator converting cars kinetic energy into electrical and store in the batteries (shown in fig 3) for later use.

Fig (2) showing energy consumption from battery .

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Fig (3) showing charging of battery when brake applied.

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Hydraulic Regenerative Brakes:

Hydrostatic Regenerative Braking (HRB) system uses electrical/electronic Components as well as hydraulics to improve vehicle fuel economy. An alternative regenerative braking system is being developed by the Ford Motor Company and the Eaton Corporation. It's called Hydraulic Power Assist or HPA. With HPA, when the driver steps on the brake, the vehicle's kinetic energy is used to power a reversible pump, which sends hydraulic fluid from a low pressure accumulator (a kind of storage tank) Inside the vehicle into a high pressure accumulator. The pressure is created by nitrogen Gas in the accumulator, which is compressed as the fluid is pumped into the space the gas Formerly occupied. This slows the vehicle and helps bring it to a stop. The fluid remains Under pressure in the accumulator until the driver pushes the accelerator again, at which Point the pump is reversed and the pressurized fluid is used to accelerate the vehicle, Effectively translating the kinetic energy that the car had before braking into the Mechanical energy that helps get the vehicle back up to speed. It's predicted that a system Like this could store 80 percent of the momentum lost by a vehicle during deceleration And use it to get the vehicle moving again. Bosch Rexroth has a regenerative braking system that does not require a hybrid Vehicle. In fact, it does not involve electrical storage. The Hydrostatic Regenerative Braking (HRB) system is intended for commercial vehicles and mobile equipment. The Company says that initial measurements show that the HRB system reduces the fuel Consumption in these vehicles by up to 25%.

In the HRB system, braking energy is converted to hydraulic pressure and stored in a highpressure hydraulic accumulator. When the vehicle accelerates, the stored hydraulic energy is applied to the transmission reducing the energy that the combustion engine has to provide. An electronic controller and a hydraulic valve manifold control the process. At present, these hydraulic regenerative brakes are noisy and prone to leaks; however, once all of the details are ironed out, such systems will probably be most useful in large trucks.

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Fig. (d) Hydraulic Regenerative Brake (HRB) Fly Wheels:

Regenerative brakes may seem very hi-tech, but the idea of having "energy-saving Reservoirs" in machines is nothing new. Engines have been using energy-storing Devices called flywheels virtually since they were invented. The basic idea is that the rotating part of the engine incorporates a wheel with a very heavy metal rim, and this drives whatever machine or device the engine is connected to. It takes much more time to get a flywheel-engineturning but, once it's up to speed, the flywheel stores a huge amount of rotational energy. A heavy spinning flywheel is a bit like a truck going at speed: it has huge momentum so it takes a great deal of stopping and changing its speed takes a lot of effort. That may sound like a drawback, but it's actually very useful. If an engine supplies power erratically, , absorbing extra power and making up for temporary lulls, so the machine or equipment it's connected to is driven more smoothly. It's easy to see how a flywheel could be used for regenerative braking. In something like a bus or a truck, you could have a heavy flywheel that could be engaged or from the transmission at different times. You could engage the flywheel everytime you want to brake so it soaked up some of your kinetic energy and brought you to a halt. Next time you started off, you'd use the flywheel to return the energy and get you Moving again, before disengaging it during normal driving. The main drawback of using flywheels in moving vehicles is, of course, their extra weight.

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They save you energy by storing power you'd otherwise squander in brakes, but they also cost you energy because you have to carry them around all the time.The transfer of energy in both directions (captured from the driveline during coasting andbraking, and released to the driveline for boost) is managed through a CVT (Continuously Variable Transmission) gear box. Packaged inside a single housing is a shaft mounted flywheel that is connected via a chain or belt/pulley drive to a series of discs and rollers (the CVT). During braking and coasting, the flywheel spools-up (accelerates as it spins) and absorbs a storehouse of otherwise wasted energy (heat from friction brakes). During power delivery, as the vehicle begins to accelerate, the pent-up energy in the flywheel is released and it turns the shaft. The rollers within the CVT can change position across the discs either retard or augment the torque of the spinning flywheel shaft much like a conventional step-up or step-down gear box.This gearing is necessary because unlike aircraft & to a certain extent watercraft, which travel at arelatively constant load and speed, earth-bound vehicles travel at regularly and greatly varying speeds and loads as they negotiate traffic and to pography. It is this variable output velocity that allows for smooth power transmission from the flywheel to the driveline as the vehicle travels over the roadway. Advanced transmissions that incorporate hi-tech flywheels are now being used as regenerative systems in such things as formula-1 cars, where they're typically referred to as Kinetic Energy Recovery Systems (KERS).

Pros of flywheel systems:

Compact weight and size -- The entire system (the CVT, the flywheel and
the housing) is roughly half the weight and packaging of a battery hybrid system.

Twice as efficient -- Battery-electric structures lose kinetic potential during

The conversion of energy from mechanical to electrical to chemical, and then back again. Its a fundamental of the Second Law of Thermodynamics: transforming energy from one form to another introduces losses. Batteryelectrics are approximately 34 percent efficient. Flywheel drives are allmechanical and suffer no conversion losses. Most of the energy loss that does occur comes from normal friction between moving parts. These systems are about 70 percent efficient.

Lower cost -- Smaller size and weight and reduced complexity make these
arrangements about one quarter the cost of a battery-electric system.

4.4. Use in compressed air

Regenerative brakes could be employed in compressed air cars to refill the air tank During braking. By absorbing the kinetic energy (necessary for braking), using the same for compressing the air and reuse these compressed air while powering the car.

4.5. Regenerative Braking Using Nitilon Spring

From fig it is clear that while braking the kinetic energy is stored in form of potential energy in spring. When the system actually demands for the acceleration this potential energy stored is given back to the wheels to power them.

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Fig (e) Regenerative braking using Nitilon Spring

APPLICATIONS
Some of vehicles using regenerative brake:-

Fig (f): Toyota Prius

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Fig.(g): Ford FUSION

Fig.(h):Tesla Roadster Electric Car

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Fig(i). Truck using Hydraulic Regenerative Brake (HRB)

Fig.(j): Vectrix Electric Maxi-Scooter

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Fig: KERS is used in F1 cars from 2009

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COMPARISIONS
Advantages of regenerative braking over conventional braking
Energy Conservation: The flywheel absorbs energy when braking via a clutch system slowing the car down and speeding up the wheel. To accelerate, another clutch system connects the flywheel to the drive train, speeding up the car slowing down the flywheel. Energy is therefore conserved rather than wasted as heat and light which is what normally happens in the contemporary shoe/disc system. Wear Reduction: An electric drive train also allows for regenerative breaking which increases Efficiency and reduces wear on the vehicle brakes. In regenerative braking, when the motor is not receiving power from the battery pack, it resists the turning of the wheels, capturing some of the energy of motion as if it were a generator a returning that energy to the battery pack. In mechanical brakes; lessening wear and extending brake life is not possible. This reduces the use of use the brake. Fuel Consumption: The fuel consumption of the conventional vehicles & regenerative braking system vehicles was evaluated over a course of various fixed urban driving schedules. The results are compared as shown in figure. Representing the significant cost saying to its owner, it has been proved the regenerative braking is very fuel-efficient. The Delhi Metro saved around 90,000 tons of carbon dioxide from being released into the atmosphere by regenerating 112,500 megawatt hours of electricity through the use of regenerative braking systems b/w 2004 and 2007. It is expected that the Delhi Metrowill save over 100,000 tons of CO2 from being emitted per year once its phase is complete through the use of regenerative braking. The energy efficiency of aconventional car is only about 20 percent, with the remaining 80 percent of its energy being converted to heat through friction. The miraculous thing about regenerative braking is that it may be able to capture as much as half of that wasted energy and put it back towork. This could reduce fuel consumption by 10 to 25 percent. Hydraulic regenerative braking systems could provide even more impressive gains, potentially reducing fuel use by 25 to 45 percent. Braking is not total loss: Conventional brakes apply friction to convert a vehicles kinetic energy into heat. In energy terms, therefore, braking is a total loss: once heat is generated, it is very difficult to reuse. The regenerative braking system, however slows a vehicle down in a different way.

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Comparison of Dynamic brakes and Regenerative brakes:

Dynamic brakes ("rheostatic brakes" in the UK), unlike regenerative brakes,\dissipate the electric energy as heat by passing the current through large banks of variable resistors. Vehicles that use dynamic brakes include forklifts, Diesel-electric locomotives streetcars. This heat can be used to warm the vehicle interior, dissipated externally by large radiatorlike cowls to house the resistor banks.The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the generated current with the supply characteristics and increased maintenance cost of the lines. With DC supplies, this requires the voltage be closely controlled. Only with the development of powerelectronics has this been possible with AC supplies, where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors). A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors. This results in a near constant speed for all trains as the motors rotate with the supply frequency both when motoring and braking.

Why Regenerative Brakes are assisted with the Frictional Brake??

Traditional friction-based braking is used in conjunction with mechanical regenerative braking for the following reasons: The regenerative braking effect drops off at lower speeds; therefore the friction brake is still required in order to bring the vehicle to a complete halt. Physical locking of the rotor is also required to prevent vehicles from rolling down hills. The friction brake is necessary back-up in the event of failure of the regenerativebrake. Most road vehicles with regenerative braking only have power on some wheels (as in twowheel drive car) and regenerative braking power only applies to suchwheels, so in order to provide controlled braking under difficult conditions (such as in wet roads) friction based braking is necessary on the other wheels. The amount of electrical energy capable of dissipation is limited by either the capacity of the supply system to absorb this energy or on the state of charge of the battery ,capacitors. No regenerative braking effect can occur if another electrical component on the same supply system is not currently drawing power and if the battery or capacitors are already charged. For this reason, it is normal to also incorporate dynamic braking to absorb the excess energy. Under emergency braking it is desirable that the braking force exerted be the maximum allowed by the friction between the wheels and the surface without slipping, over the entire speed range from the vehicle's maximum speed down to zero. The maximum force available for acceleration is typically much less than this except in the case of extreme high-performance vehicles. Therefore, the power required to be dissipated by the braking system under emergency braking conditions may be many times the maximum power which is delivered under acceleration. Traction motors sized to handle the drive power may not be able to cope with the extra load and the battery may not be able to accept charge at a sufficiently high rate. Friction braking is required to absorb the surplus energy in order to allow an acceptable emergency braking performance.

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DESCRIPTION & OPERATION

How regenerative braking system works? Regenerative (or Dynamic Braking) occurs when the vehicle is in motion, such as coasting, traveling downhill or braking. And the accelerator pedal is not being depressed. During Regent, the motor becomes a generator and sends energy back to the batteries. It is explained as follows, because the wheels of a decelerating vehicle are still moving forward, they can be made to turn the electric motor, which then feeds energy to the batteries for storage. The system becomes, in effect, a generator, which provides braking force while it converts the vehicles kinetic energy into a reusable form- electrical energy. When the accelerator pedal is released, the absence of pressure triggers a response from the Energy Storage Unit (ESU). Regenerative braking begins, and the batteries are re-charged by the motor, which is turned by the wheels. In this case, the friction brakes are not engaged. If more vigorous deceleration is required, and the brake pedal is depressed, this engages both sets of brakes. However, to maximize energy efficiency, it is advantageous to apply the regenerative brake as such as possible it therefore tends to do more of its total work in the first part of the braking motion. There are two deceleration modes: 1. Foot off throttle but not on brake pedal in this mode, the charge/assist gauge will show partial charge, and the vehicle will slow down gradually. 2. Foot on brake pedal - In this mode, a higher amount of regeneration will be allowed, and the vehicle will slow more rapidly. During light brake pedal application, only the IMA motor//generator is slowing the car. With heavier brake pedal application, the conventional friction brakes also come into play. When decelerating, regeneration will continue u8ntil engine speed falls to about 1000 rpm. At this point, the driver will typically shift into neutral. In this way braking of the vehicle is obtained.

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EXAMPLE
Regenerative braking of Toyota Prius: Toyota realized that one way to achieve longer vehicle range was to conserve and reuse some of the energy that a vehicle normally loses as heat caused by braking friction. This idea led engineers to apply the principles of regenerative braking. In all Toyota vehicles that feature the regenerative braking system, the regenerative brake is only responsible for a part of the deceleration necessary to stop the vehicle. In an EV, this fraction is determined by the vehicles speed when braking is initiated. The remaining braking force is provided by the vehicles friction brakes. To maximize fuel economy, of course, the regenerative braking system is made to do as much of the braking work as possible.

Technology Used in Toyota Prius: The next phase of regenerative breaking technologys development came in its application to the Prius, the platform for the Toyota Hybrid System. Whit the Prius, too, the fraction of breaking torque supplied by the regenerative break is proportional to the vehicles speed when the breaks are applied. Because the Prius battery pack is less than a fifth the size of that of a pure EV, however, regenerative capacity is considerably lower. Squeezing the greatest energy savings out of the Prius regenerative breaking system meant devising a new way to control the interplay of the friction and regenerative breaking systems throughout the breaking action. The solution that Toyotas engineers found was to have the prius regenerative brake supply a continuously varying amount of braking torque as the vehicle decelerates. A pure EV, with its greater battery capacity, can achieve substantial energy saving without modifying the regenerative brakes contr4ibution throughout the braking action. This was not the case for the prius. So Toyota engineers developed a new type of control value to regulate the interplay between the two systems. These values allow continuous variation, so the maximum possible energy is extracted from the vehicles deceleration.

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Demand curve (1) most of the gracing is regenerative. For fast & powerful braking Demand Curve (2). Friction brakes do most of the work.

Two Brake, One Natural Banking Experience: The final step was to make sure that the two brake systems worked smoothly together so that the driver would experience a natural braking feel when the brake pedal was depressed.

The hydraulic brakes and the regenerative brake are essentially two separate systems. Integrating them required the construction of two dedicated computer systems that keep the brakes coordinated at all times. They call these the electronic control units, or ECUs. They continuously control the braking forces being generated by the friction and regenerative systems, ensuring that the total force produced matches the force signaled by the driver. This design gives the brakes a very smooth feel as far as the driver is concerned the brakes work like conventional brakes. In the prius, the Toyota Hybrid System accounts for an 80% gain in fuel efficiency compared to vehicles equipped with conventional gasoline engines. The regenerative braking system adds and additional 20% to this, making the prius one of the worlds most fuel-efficient vehicles.

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Component Used in Toyota Prius for Regenerative Braking System: Brake Pedal:
It is used to apply braking force by the driver.

Hydraulic Booster Unit:

It is composed of the master cylinder and the regulator, responds in two steps. First it signals electronically to the brake ECU that braking force has been demanded. Next, the master cylinder exerts hydraulic pressure on the pedal stroke simulator, and the regulator feeds hydraulic fluid to the hydraulic pressure control unit.

Brake ECU:
The brake ECU senses the braking demand and sends a fraction of this demand to the THSECU for regenerative braking. It also calculates the force necessary to fulfill remaining braking demand and instruct the hydraulic pressure control unit to pass on a corresponding amount of hydraulic fluid

Pedal Stroke Simulator:

It absorbs an amount of hydraulic pressure from master cylinder that corresponds to the amount of braking force applied by the regenerative braking system. As hydraulic pressure is fed back to the pedal, the pedal, the pedal stroke simulator feeds back to the master cylinder.

THS (Toyota Hybrid System) ECU:

It induces regenerative braking, and returns a signal that indicates braking force output back to the brake ECU.

Hydraulic Pressure Control Unit:

It passes on a corresponding amount of hydraulic fluid to a four way cylinder.

Result:
Regenerative braking technology is one more positive step forward in Toyotas quest to realize the ultimate ecocar. By working in concert with previously developed electric motor technologies, its application helps Toyotas electric vehicles and hybrid vehicles (including the recently released prius) to achieve extended ranges and to be friendlier to the environment than ever before. At the same time, this new technology remains unobtrusively in the background; drivers benefit from regenerative braking while enjoying the same firm braking feel found in conventionally equipped vehicles.

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Wheel Rotating Device.

Wheel A Rotating Device B Shafts C.D. Gearbox E Clutch G Spring - H

Shafts.

Gearbox

Clutch

Spring

7.1 Regenerative braking system using Nitinol Spring

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CONCLUSION
Theoretical investigations of a regenerative braking system show about 25% saving in fuel consumption. The lower operating and environment costs of a vehicle with regenerative braking system should make it more attractive than a conventional one. The traditional cost of the system could be recovered in the few years only. The exhaust emission of vehicle using the regenerative braking concept would be much less than equivalent conventional vehicles as less fuel are used for consumption. These systems are particularly suitable in developing countries such as India where buses are the preferred means of transportation within the cities.

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REFERENCES
1) Automotive Abstracts , ARAI Pune. 2) General Motors Website (www.gm.com). 3) www.sae.org