A Regenerative Braking System Using ‘KERS’ a device used to recover, otherwise wasted energy. I. Abstract -Kinetic Energy Recovery System or simply ‘KERS’ is a system developed to recover a modest amount of energy during braking of an automobile or a locomotive. This system tries to harvest energy which is otherwise wasted. KERS works on the principle of Regenerative Braking, which uses the braking energy to rotate a flywheel connected to the differential( in an automobile) through a gear mechanism, which when actuated( while braking )uses the energy to spin the flywheel at more than 60,000 rpm.
There are various methods of achieving this, most commonly used systems are Electrical KERS and Mechanical KERS. Electrical KERS uses a generator and a battery setup to store the energy while braking, where as a Mechanical KERS system uses a flywheel to accomplish the same, which are discussed in full length of this paper effectively, either way this stored energy is then utilized by the driver to achieve a ‘Boost’ or utilize this energy hence reduce his original energy demand. KERS are effectively applied in Formula 1, 24 Hours of Le Mans and other prestigious races because of the energy boost it offers (around 60 kW in F1).
Several companies like Volvo, Mercedes Benz have tried to implement KERS technology to achieve more efficiency and decrease fuel consumption, many hybrid cars use this technology. They have been successfully applied in Tram cars and rail locos in Europe. Other applications even include a bicycle called the ‘Copenhagen Wheel’ to reduce rider fatigue. In the race to increase efficiency and reduce emissions, salvaging every bit of wasted energy is major step forward for a better tomorrow. II. Introduction – Kinetic energy recovery systems (KERS) is a system that attempts to recover wasted energy while braking.
Braking is employed to retard a vehicle or a machine by absorbing kinetic energy and reducing motion, this kinetic energy gets converted into other forms of energy mainly heat, which is dissipated. KERS tries to reduce this generation of heat yet maintain the same retardation rate, whilst converting the kinetic energy into a more useful energy. As we all know work is a higher level energy than heat so thereby making it easier to recover and utilize. This method was first postulated by physicist Richard Feynman in 1950s but it took many years of development and testing to achieve a practical application efficiently.
Early systems incorporated heavy flywheels and other equipment which made it very inefficient and defeated the purpose, although newer and more advanced versions reacted to this drawback by using composite materials and other technologies. III. Why do we need KERS? – No actual heat engine can be 100% efficient, when we are using a naturally perishable matter to power our engines every small drop of fuel counts, more so with ever escalating fuel prices it becomes economically feasible to improve the efficiency of our vehicles and machines.
We can make an internal combustion engine efficient only to an extent as it has various methods to lose heat such as from the exhaust gases , through the cylinder walls and other areas which cannot be helped. One way to recover some wasted energy is KERS which works on the principle of Regenerative Braking. Depending on the application , every watt of power saved through this method counts as a watt used by the vehicle, which results in saving the fuel that is to be burned to generate that watt. Hence the system is a little more efficient Fig. 1 IV.
Types of KERS – Theoretically any procedure that is employed to recover wasted kinetic energy while braking can be called a type of KERS, but two common methods are very familiar in their applications, these are the two technologies applied in the most common KERS application. 1. Mechanical KERS 2. Electrical KERS 3. Pneumatic KERS A. Mechanical KERS It employs a flywheel type of storage for energy. It has an axle coupling gear which meshes with a CVT transmission that actuates power transmission from the differential to be absorbed by the KERS flywheel.
This wheel absorbs the kinetic energy from the differential through this mechanism, hence retarding the vehicle. This is an example of a flywheel KERS layout, showing the basic components and the way they are put together. Advantages :` • Specific power of flywheel is much higher than a battery. • The cost of this setup is relatively less . • The flywheel almost entirely gives back the power with a high efficiency. • Simpler and robust construction, giving a long life, almost equal to the life of the vehicle. Disadvantages: • Friction between the bearings and the gears may cause some decrease in efficiency. It is vulnerable to contaminants and corrosion. • The bearings and seals may give up over time. B. Electrical KERS – It is a more technologically advanced method to implement KERS, it has three main components Motor (or) Generator, batteries and an electronic control unit (ECU). The motor or generator is coupled to the live axle using an electric clutch mechanism which is actuated by the ECU. While braking the kinetic energy is absorbed by the generator which converts it into electric impulse, in this part of the cycle, the motor/generator acts like a generator. This energy is stored in the batteries.
Most preferred batteries are of Li-ion type. On the command of the driver, the ECU directs the power to be given back to the differential, this is when the motor uses the stored electric energy from the battery pack to give an addition power spike to the vehicle. A simple schematic diagram is shown below which explains the process. • Capable of storing more energy Disadvantages: • This system is more complex and expensive • A lot of heat is generated by the batteries and other electric equipment which require additional cooling equipment • Specific Power of batteries is low and they require a minimum charge up time C.
Pneumatic KERS – Systems that store energy as compressed or pressurized air are called pneumatic systems. During deceleration the engine can be modified to act as a compressor which compresses air and stores it in a tank at this time the engine is in compressor mode and during acceleration , the engine can be operated as an Air-Engine reusing this air from pressurized tanks to run the engine. This method is much simpler and easier to use than the previous methods. As the method is also more readily feasible and cheapest of all the methods.
Advantages: • • • • • Simpler construction Storing compressed air or gas requires only tanks Convectional engines can easily be modified to suit this system Cheaper to install and maintain Very feasible for heavy duty applications Fig. 2 Advantages: • The construction is safe, without any moving parts for storage of energy • The efficiency if also higher • The system is more accurate and flexible in meeting demands of the driver Disadvantages: • • Storing large quantity of compressed air posses it’s own risk The power boost and supply is not as effective as previous systems
It is also to be noted that other car companies such as Volvo, Mercedes Benz and Toyota have taken keen interest in this technology and are trying to employ these in their road cars, Volkswagen AG has tried to incorporate KERS in their BlueMotion series of diesel cars in Europe and currently these engines are being tested to solve out all the initial niggles. Although new to conventional IC engine cars, KERS is no stranger to hybrid and alternate fuel cars across the world. They go hand-in-hand with the motive of a hybrid car.
KERS in a modified form has been applied to various tram cars and locomotives in Europe namely Skoda Astra tram and some Bombardier Locomotives, they use a technology similar to electric KERS but the difference being they don’t always store this electric energy, but give back to the main supply. A very innovative application of KERS is a bicycle, called the Copenhagen Wheel developed by students of M. I. T,Boston, MA. It utilizes a mechanical KERS system with a flywheel that is fitted and aligned with the sprocket on the other side, on the rear wheel.
A small CVT is employed to make or break this connection, while braking the CVT engages the flywheel to absorb the kinetic energy of the wheel and while not braking this flywheel gives its energy directly to the wheel and eventually disconnected by the CVT. This cycle is designed in such a way so as it take energy only while braking and not spin under load. Also the flywheel was made of cheap composite materials to keep the weight and cost low. The results of this experiment have been astonishing, the cycle was efficient and could reduce rider fatigue by 20% and also making it easier to cycle for a long distance.
A small disadvantage can be seen that a rotating flywheel beside the wheel can add a minute gyroscopic couple while turning, although the effect of which is negligible Fig. 3 V. Applications – KERS has been widely encouraged in motor sports, more importantly FIA which conducts the Formula One races every season. Formula one has earned the flak of every environmental protection agency known for being so ignorant about the usage of fuel and emission of greenhouse gasses ( In a single race an average F1 car burns around 500 liters of fuel. So ,FIA president Max Mosley has taken an initiative to get rid of this image, using ethanol, smaller engines etc have all been the steps taken to do the same, development and application of KERS was a major step taken in this regard. KERS was first developed to achieve more speed that can be used to boost a performance, almost all the F1 teams have employed electrical KERS as they are more feasible for them and are very low weight (25 Kg) KERS gives about 80 hp boost in an average F1 car, Mclaren becomes the first team to win a championship using KERS .
Audi Motor sport division has tried to implement KERS technology in its multiple championship winning r8 in 24hours of Le Mann’s race from which Audi could race the car for a longer duration without stopping for refueling as much as their competitors did. Fig. 5 a typical f1 steering wheel with KERS activation button shown Fig. 4 An exploded view of wheel components of said “Copenhagen Wheel” VII. Design and Packaging of KERS – The design or the way all the components of a KERS unit are arranged and packaged is surprisingly compact and even very space efficient for a device that performs such complex functions.
Crediting its size to advanced electronics and smart materials, even newer and lighter mechanical components a typical KERS unit may weight from 40 to 50Kg even for a heavy application. VI. How and when to use it? – With an automated system there always exists a risk of an unwanted happening or an accident, specially with a device that gives back power to the driving wheels, more than what the engine would normally deliver, this can shock a driver and make him loose control of the vehicle. So to counter this fact most of the KERS are activated by the driver, when and how he wishes to use the extra power, in formula one as entioned earlier the energy is stored as electrical charge in a battery which can be activated by pressing a button on the steering wheel. How ever some mechanical KERS systems use an intelligent system` called KERS control unit (KCU) which , after analyzing various parameters such as throttle position, brake position etc, to automatically control the boost. Fig. 6 A KERS unit of a city bus manufactured by VOLVO. VIII. Limitations of KERS – Although KERS is an innovative new technology it is not free from flaws and it requires further development to answer many of its currently faced problems.
As many engineers from Formula one and other motor sports feel that KERS has a long way to go before it can be easily and effectively employed in everyday road cars, mainly due to the cost of development and other parameters, on an average the KERS unit used in F1 cars costs upwards of a hundred thousand dollars and they weigh only 25kg . In case of electrical KERS the battery systems employed will require to have more storage capacity, or else increasing the weight of the car excessively will have an adverse impact on fuel consumption which would defeat the purpose of installing such a system in place.
The specific power to weight ratio of these units must improve drastically, carbon fiber does the job but it is a very expensive material and it is not accessible to everybody. Many formula 1 drivers have complained about the ineffective braking and spongy braking in their cars which have been fitted with KERS, many teams decided not to use KERS from 2011 as their drivers crashed out or even reported damage to the car. d_regulations/technical_regulations/8699/fia. htm l [7. 2012 Fia rules and regulations [8. ] Volkswagen AG, Germany [9. ] Fig. 5 BMW f1 team. [10. ] Fig. 4, MIT Journal – “ The Copenhagen Wheel “ [11. ] Fig. 6, Volvo trucks, Sweden. [12. ] Fig. 3, Autocar magazine, online edition http://autocar. co. uk * All photographs are a property of their respective owners. IX. Conclusion – As mechanical engineers we must try to make our machines as efficient as possible, as the plethora of new ideas and innovations come up, we must find a way to effectively employ them.
KERS is one such idea to take seriously, it utilizes wasted work, which if converted to heat cannot be utilized as effectively. KERS coupled with DRS(1)* is used in many races to achieve extra performance, car companies like Volvo managed to achieve better efficiency by using KERS in their road cars, although not free from its flaws, It can prove to be an innovative technology that can be used to improve our machines, from rough estimates if a power of up to 60kW can be salvaged from a road going car, it can lead to fuel savings of around 35% . 1)*DRS is drag reduction system which employs a spoiler or a wing to reduce air drag and boost aerodynamic performance References [1. ]http://www. flybridsystems. com/Technology. h tml [2. ]Top gear magazine , august 2010 [3. ]Fig. 1 Volvo automobiles,sweden [4. ]Fig. 2 Magneti Marlelli, italy [5. ]ASME online journal january 2012, “Stopping Power” [6. ]http://www. formula1. com/inside_f1/rules_an