REGENERATIVE BRAKING SYSTEM

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.

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, you’ll be powering the vehicle entirely from gasoline.  The regenerative braking

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.

 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.

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.

 

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:

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 vehicle’s 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.

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.

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. 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.

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 vehicle’s 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.
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 vehicle’s speed when braking is initiated.  The remaining braking force is provided by the vehicle’s 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 technology’s 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 vehicle’s 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 Toyota’s 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 brake’s 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 vehicle’s deceleration.

6.1 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 world’s most fuel-efficient vehicles.

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 Toyota’s quest to realize the ultimate ecocar. By working in concert with previously developed electric motor technologies, its application helps Toyota’s 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.

      7.1 Regenerative braking system using Nitinol Spring

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.

 REFERENCES

1)    Automotive  Abstracts , ARAI  Pune.

2)    General Motors Website (www.gm.com).

3)    www.sae.org