Regenerative Braking

REGENERATIVE BRAKING

Abstract

The purpose of this report is to analyse a recent development in Braking Systems – Regenerative Braking. The report discusses the processes and ideas involved behind regenerative braking, as well as detailing its advantages and uses in modern vehicles (e.g. the Honda Insight).

Introduction

As the natural fuel sources of the Earth continue to be depleted, new methods of technology and innovations are being investigated to allow engineers to design more efficient methods of transportation. The result has been cars like the Honda Insight, which uses advanced mechanisms to provide fuel efficiency.

In modern cities, a typical car in city traffic wastes a lot of energy each time it brakes. In effect, the energy of the forward momentum is lost in the form of heat in the car's brakes. More energy is then needed to reaccelerate the vehicle back to its previous speed. However, recent developments of braking systems have led to a much more efficient and resourceful mechanism of braking – regenerative braking. The idea behind regenerative braking is to recapture some of the energy that would otherwise be wasted during braking. This energy is stored in batteries, so that the energy can later be used on reacceleration. This idea is widely used in both pure electric and hybrid electric vehicles.

How it works

The Honda Insight uses regenerative braking to both recharge its batteries and slow down the vehicle. The car’s Integrated Motor Assisted (IMA) motor helps to propel the Insight is also used to generate electricity and recharge the batteries. The more electricity that is drawn from the motor/generator to charge the batteries, the more drag the motor/generator creates, effectively slowing down the car.

The simple mechanics in the Insight is that, when the brake pedal is pressed lightly, the car will switch to the deceleration mode, and begin the regenerative braking process. Then the motor/generator will begin to charge the battery. The gasoline engine’s crankshaft is still moving with the wheels, but the fuel injection and spark are no longer on and therefore no gasoline is being burned.

However, the actual process involved in the regenerative braking is a good deal more complicated. The process of regeneration is entirely automatic and occurs when the demand speed is reduced.

When the driver applies the brakes, the controller supplies a voltage to the motor to drive it, and, the motor is generating an EMF proportional to the speed of the vehicle. If the motor goes faster, its EMF rises and the current (caused by the difference between the controller's output voltage and the motor's EMF) falls.

In the Insight, if the motor rotates fast enough, the current ...

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However, the actual process involved in the regenerative braking is a good deal more complicated. The process of regeneration is entirely automatic and occurs when the demand speed is reduced.

In the Insight, if the motor rotates fast enough, the current falls to zero, as the EMF then equals the controller's output. If the motor rotates further, the current must go negative (feeding back into the controller) as the motor’s EMF is now greater than the controller's output voltage.

If the controller can accept this current being fed back into it, then braking starts to occur. The controller has to do something with this current. Crude designs simply dump it as resistive heating, but the Insight is more efficient, and feeds the current back into the battery.

Therefore, in the process of regenerative braking, it is actually the battery which is doing the braking, not the controller, since the braking energy gets dumped into the battery.

← The path of energy flow from the wheels to the battery in a Honda Insight.

Other uses of Energy

In the Insight, a system is used in which the motor/generator produces AC that is converted into DC. This DC is then used to charge the battery and slow down the vehicle. However, some systems use this energy for other purposes:

Regenerative Braking 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.

Another advantage of flywheel technology is that the additional power supplied by the flywheel during acceleration supplements the power output of the small engine.

A flywheel system in a hybrid car. →

Regenerative Braking and Fuel Cells:

The third system uses a unitised regenerative fuel cell (URFC), which is designed to be reversed to take the energy from the wheels, combine it with water, and produce hydrogen and oxygen – allowing it to act as a type of revolutionary battery.

The system as a single unit is substantially lighter than a separate electrolyzer and generator, which makes this system especially beneficial when weight is a factor. Also, the system’s energy is ten times that of lead-acid batteries and twice as much as any chemical batteries.

This means that not only will this technology make lighter hybrids available, but 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.

Materials in the System

The most crucial component in the Insight’s regenerative braking system is the IMA motor/generator. As previously mentioned, it operates as a motor and generator, allowing excess energy (such as during braking) to be used to recharge the battery.

The IMA motor is a permanent-magnet type, brushless DC motor.

The only moving part in this type of motor is the rotor, which consists of nothing but permanent magnets. The electromagnets are located around the perimeter of the stator. →

There are several key reasons for designing this type of motor with these materials:

Because the electromagnets are located around the perimeter, the electromagnets can be cooled by conduction to the motor casing, thus requiring no airflow inside the motor for cooling. This means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter.
With no windings on the rotor, they are not subjected to centrifugal forces, thus eliminating chance of fracture.
Inefficiencies caused by brush material resistance, and brush drag friction are eliminated.

The IMA system’s permanent magnet motor is only 60 mm wide. This is made possible by using heavy gauge copper wire for the motor windings. Heavy gage copper wire efficiently conducts electricity from the battery pack to the motor for increased fuel economy.

By relying on copper, one of the world's most durable and recyclable materials, Honda has developed technologies that will help to conserve our limited supply of fossil fuels and make our air cleaner.

Furthermore, the structure and materials of almost all parts in the IMA engine have been designed with the aim of creating a very light engine. This weight reduction is made possible by carburised connecting rods. Carburisation strength enhancement technology also contributes greatly to increasing engine operational speed.

The oil pans of the IMA are made of steel plate or aluminium alloy. Conventional magnesium materials have had problems withstanding the high temperatures of engine oil. In contrast to conventional materials, which experience a significant deformation and failure at 120°C or higher, the newly developed magnesium oil pan ensures sufficient ability to resist failure and deformation up to 150°C.

Also, there has been a large use of plastic parts and plastic materials in the intake manifold, cylinder head cover, water pump pulley, and intake system parts. These changes brought the weight of the engine to less than 60 kg.

← At the heart of the Insight's sophisticated IMA system is a compact, 1.0-litre, 3-cylinder gasoline automobile engine. The engine’s advanced low-friction design is made of lightweight materials such as aluminium, magnesium and special plastics.

Future of the System

As one of the cars with the most advanced system of regenerative braking, the Insight is a prime example of Honda's dedication to bringing significant environmental innovation to the forefront of the automotive industry.

The regenerative braking system will continue to be one of the leading mechanisms in Honda’s, and others, never ending quest in providing fuel efficient vehicles for the future. As such, the system should continue to flourish, becoming ever more efficient as further research into the system continues.

The manufacturer and designer of the Insight system, Honda, believe that the regenerative braking system will continue to be used in cars as a system to recapture otherwise wasted energy and store it in a normal car battery.

There is also a move by the manufacturers to introduce Honda cars that are capable of using regenerative energy in flywheels (see p.2) and storing it in the URFC (see p.3).

Conclusions

The Regenerative Braking system is a fairly recent development in Braking Systems, pioneered by companies such as Honda, and featured in vehicles like the Insight. The system has opened up a new world of possibilities and opportunities for fuel efficiency in peak hour city traffic. As such, it will dramatically cut down on fuel consumption in city areas and result in a much more efficient system of transport, where there is little energy lost as heat every time the vehicle brakes.

Suggestions / Recommendations

The regenerative braking system is still a new concept. Because of this, the system is not as efficient as it can be. Further research in to materials and designs of the system will enable a progressively more effective system of energy capture. This system can also be customised to suit particular environments (eg. city, rainy weather, country) to help the vehicle recapture as much energy as possible from its particular situation.

Bibliography