FLYWHEEL BASED KINETIC ENERGY RECOVERY SYSTEMS (KERS) INTEGRATED IN VEHICLES THOMAS MATHEWS Department of Mechanical Engineering, Sir MVIT, Hunasamaranahalli, Bangalore, Karnataka, 562 157, India tljc90@gmail.com NISHANTH D Department of Mechanical Engineering, MIT Manipal, Karnataka, 576 104, India nishanth.d1990@gmail.com Abstract: Today, many hybrid electric vehicles have been developed in order to reduce the consumption of fossil fuels; unfortunately these vehicles require electrochemical batteries to store energy, with high costs as well as poor conversion efficiencies. By integrating flywheel hybrid systems, these drawbacks can be overcome and can potentially replace battery powered hybrid vehicles cost effectively. The paper will explain the engineering, mechanics of the flywheel system and it s working in detail. Each component of the flywheel-based kinetic energy recovery system will also be described. The advantages of this technology over the electric hybrids will be elucidated carefully. The latest advancements in the field, the potential future and scope of the flywheel hybrid will be assessed. Keywords flywheel, transmission, kinetic energy, continuously variable transmission, kinetic energy recovery systems 1. Introduction In a world where almost all its fuel is being depleted, conservation of natural resources has become a necessity in today s world, especially in the field of renewable technology. In an automobile, maximum energy is lost during deceleration or braking. This problem has been resolved with the introduction of regenerative braking. It is an approach to recover or restore the energy lost while braking. The Kinetic Energy Recovery System (KERS) is a type of regenerative braking system which has the capability to store and reuse the lost energy. In recent years, hybrid electric vehicles were developed in order to meet the demand of reducing energy consumption, the increasing fuel prices and the damage caused by fossil fuel emissions to our environment. Currently, the market for hybrid vehicles is largely comprised of hybrid electric vehicles. These vehicles are partially or fully powered by electric motors that are supplied electricity from rechargeable batteries. Unfortunately the poor conversion efficiencies cancel out most of the advantages these battery powered hybrid vehicles bring with them. The flywheel-based kinetic energy recovery system is a possible solution which could potentially replace the electric hybrids. In principle, a flywheel is nothing more than a wheel on an axle which stores and regulates energy by spinning continuously. The amount of energy that flywheels are able to store is dependent upon the weight of the flywheel and how fast it is rotating. This kinetic energy recovery system stores energy as a vehicle brakes and recycles it as the vehicle accelerates again. The KERS was first designed for formula one racing cars. In this paper, we will examine the flywheel based kinetic energy recovery system and explain why it is the fuel efficiency technology of the future. 2. The Mechanism of the Flywheel KERS The flywheel energy storage is based on the principle of regenerative braking. Regenerative braking is a mechanism which reduces the vehicle speed, converting the kinetic energy into another useful form of energymechanical energy, electrical energy or the like. Generally in a battery powered hybrid, an electric motor is connected to the wheels. When the vehicle brakes, the motor rotates in the opposite direction, acting as a generator and in the process slows down the car. The electricity produced is then sent to the battery which stores the energy as chemical energy. When desired, the battery supplies the stored energy back to the wheels, giving an additional boost of power. Unfortunately the many energy conversions- from mechanical to electrical energy and from electrical to chemical energy, reduces the overall efficiency to the system to about 30%. The mechanical hybrid overcomes this drawback. ISSN : 0975-5462 Vol. 5 No.09 Sep 2013 1694
The flywheel hybrid primarily consists of a rotating flywheel, a continuously variable transmission system (CVT), a step up gearing (along with a clutch) between the flywheel and the CVT and clutch which connects this system to the primary shaft of the transmission. When the brakes are applied or the vehicle decelerates, the clutch connecting the flywheel system to the driveline/ transmission is engaged, causing energy to be transferred to the flywheel via the CVT. The flywheel stores this energy as rotational energy and can rotate up to a maximum speed of 60000 rpm. When the vehicle stops, or the flywheel reaches its maximum speed, the clutch disengages the flywheel unit from the transmission allowing the flywheel to rotate independently. Whenever this stored energy is required, the clutch is engaged and the flywheel transmits this energy back to the wheels, via the CVT. Generally the flywheel can deliver up to 60 kw of power or about 80 HP. Fig.1 shows Volvo s flywheel KERS system Layout and Fig. 2 shows a schematic layout of general flywheel hybrid. Fig.1 Volvo s flywheel KERS system Layout [3] Fig.2 Schematic Layout of the Hybrid [2] The important parts of the flywheel hybrid will now be discussed in detail. 2.1. The flywheel The flywheel is the component which harvests kinetic energy, when the vehicle brakes, by increasing its rotational speed. The ability of flywheels to store energy is explained by the relation between the flywheel s inertia, angular velocity and kinetic energy. The equation for the energy stored in a flywheel reads as follows: (1) [2] Where E is the energy (Joules); I is the inertia of the flywheel (kgm 2 ), and ω is the angular velocity (rad/sec) of the flywheel. The equation for the inertia of a flywheel is: (2) [2] Where is the mass of the flywheel; and are the inner and outer radius of the flywheel respectively. Combining equation 1 and 2 we get: E (3) From equation 3, a flywheel's energy is proportional to its mass, and proportional to the square of its rotational speed or angular velocity. In other words, by doubling the mass, the energy stored is also doubled, and by doubling the speed, the energy stored is quadrupled. Thus by increasing the speed of the flywheel it will be possible to reduce the mass and size of it, to a level where its weight is insignificant while analysing fuel efficiency. In order to make the system more efficient it is necessary enclose the flywheel in a vacuum chamber, and in order to eliminate the resistance due to air and reduce friction it is mounted on magnetic bearings. The amount of energy that can safely be stored in the rotor depends on the point at which the rotor will warp or shatter. The hoop stress on the rotor is given by: (4) [6] Where is the tensile stress on the rim of the flywheel; is the density, r is the outer radius of the flywheel and is the angular velocity of the rotating flywheel. ISSN : 0975-5462 Vol. 5 No.09 Sep 2013 1695
The flywheel can be fabricated using different materials based on the maximum rotational speed requirements and other design constraints. High speed flywheels for speeds above 30000 rpm are usually composed of high strength carbon fibre. A large mass is not desired for high speed flywheels because extra mass means more energy will be needed to accelerate the vehicle. On the other hand, low speed flywheels with speed values below 20000 rpm, are generally made of steel or other metals for low cost. The weight of the flywheel is a very important factor in determining the efficiency of the system. 2.2. The flywheel vacuum chamber The vacuum chamber is another very essential part of the flywheel hybrid system. The major function of the vacuum chamber is to minimize the air resistance as the flywheel rotates. Without the vacuum chamber, the friction caused by air resistance is enough to cause significant energy losses and heat the carbon fibre rim to its glass transition temperature [10]. Vacuum chambers for KERS systems are frequently made of metals like aluminium, stainless steel, or the like because these metals can provide adequate strength to withstand differential pressure between an evacuated interior and the surrounding atmosphere, as well as to provide a barrier to the passage of atmospheric gases through the chamber wall by diffusion or flow through structural defects. Fig.3 shows the flywheel hybrid system designed by flybrid. Fig.3 Flybrid flywheel system [5] 2.3. Magnetic bearings Another important part of the system is the bearings on which the flywheel is mounted. Magnetic bearings have replaced mechanical bearings as they greatly reduce losses due to friction. Mechanical bearings cannot, due to the high friction and short life, be adapted to modern high-speed flywheels. Further magnetic bearings are able to operate in vacuum which leads to even better efficiency. The magnetic bearings support the flywheel by the principle of magnetic levitation. It is a method by which an object is suspended with no support other than magnetic fields. A permanent or electro permanent magnetic bearing system is utilized. Electro permanent magnetic bearings do not have any contact with the shaft, has no moving parts, experience little wear and require no lubrication. It is important that the bearings are able to operate inside a vacuum because the flywheel in a flywheel-based KERS must rotate at high speeds for maximum efficiency. The best performing bearing is the high-temperature super-conducting (HTS) magnetic bearing, which can situate the flywheel automatically without need of electricity or positioning control system. However, HTS magnets require cryogenic cooling by liquid nitrogen [1]. Fig.4 shows a magnetic bearing designed by Waukesha bearings. Fig.4 Waukesha magnetic bearing [8] ISSN : 0975-5462 Vol. 5 No.09 Sep 2013 1696
2.4. The continuously variable transmission (CVT) unit The most important interface that connects the flywheel to the transmission system is the CVT. A smooth transfer of energy to the flywheel from the transmission and vice versa is very essential in order to get maximum performance from the flywheel. The speed ratio between the vehicle and the flywheel constantly changes between acceleration and braking. The reason why a stepped drive unit is not preferred in this system is because it has only a fixed number of gear ratios as opposed to the CVT s which have an infinite number of gear ratios between the maximum and the minimum value which allows a seamless transfer of energy without any loss of power. As the vehicle slows or accelerates, the Toroidal CVT must continuously adjust the ratio between the speed of the vehicle and the rotation of the flywheel. From Fig.4.1 and Fig.4.2 we see that the two roller discs gradually change their position from a lower gear to a higher gear, depending on the energy transferred from the wheels. As the vehicle retards, the energy is transferred to CVT via the clutch, causing the roller discs to shift its position from the output shaft towards the input shaft. The surface contact area is the factor which determines which shaft rotates faster. When both the rollers have equal contact on the toroidal surfaces of the input and output shafts, the gear ratio is 1:1. Any change in the CVT ratio can be viewed as the transfer of kinetic energy between the flywheel inertia and vehicle. The illustrations in Fig.5 and Fig.6 show how the positions of the rollers affect the output on either side of the CVT. Fig.5 CVT in a lower gear [9] Fig.6 CVT in a higher gear [9] 2.5. Step-up gearing and clutch A step up gearing system consisting of epicyclic gears is connected between the CVT and the flywheel unit. The reason why this gearing system is used is because the high speed at which the flywheel rotates (60000 RPM) needs to be reduced to a manageable speed outside the vacuum chamber, in order for the energy to be smoothly transferred back to the CVT. The clutch disconnects the CVT from the flywheel when it is not transferring power to reduce free running losses. Fig.7 shows an exploded in view of a KERS system in CATIA v5, where the clutch and the gearing system are shown. SHAFT FROM FLYWHEEL LOW SPEED CLUTCH CVT EPICYCLIC GEARS Fig.7 Exploded view of a Flywheel KERS system model [3] ISSN : 0975-5462 Vol. 5 No.09 Sep 2013 1697
2.6. The clutch The clutch is used to couple the flywheel hybrid system to the transmission. It engages the system while the flywheel is accelerating from rest and disengaging while the flywheel is rotating and the vehicle is at rest. Torque is transferred through clutch between the flywheel and vehicle. Hence, the power transmitted in the flywheel system can be controlled by a clutch that could continuously manipulate the torque. 3. Advantages and Disadvantages of the Flywheel Hybrid System: As with all new technologies, the flywheel KERS system has its share of advantages and disadvantages. The advantages include high efficiency, low fuel consumption, and low cost compared to electric hybrids. Although the system has a few drawbacks, most of it can be outweighed by the benefits. One main advantage of this KERS system is its weight. A concern with the addition of a KERS to a vehicle is that the weight of the system will increase the vehicle s fuel consumption and defeat the purpose of installing it in the first place. However, due to the lightweight design of the flywheel and accompanying components, the additional weight is insignificant when analysing fuel efficiency. Moreover, the system is contained in a compact package, making it easy to incorporate into the rear of a vehicle. Another advantage is the ability of the flywheel to store energy efficiently. This is because there is no transformation of energy from one form to another which greatly reduces energy losses in the system. Tests have proven that flywheel-based KERS can recover and store over 70% of the vehicle s energy [1]. Probably the only losses that occur in the system might be due to friction and air resistance to the flywheels rotation. However, the magnetic bearings and vacuum chamber mentioned previously have been developed to minimize these effects. The system has low maintenance costs as it reduces brake wear. Energy is transferred from the driveline to the KERS when the vehicle decelerates. When this energy is given to the flywheel, the flywheel acts as a brake, slowing down the vehicle as it recovers the energy. Instead of releasing the energy as heat, the energy is recovered. This process reduces brake wear. As with any technology, the flywheel KERS system has its shortcomings as well. The flywheels found in a kinetic energy recovery system can store up to 400 kj of energy, which means that failure while rotating at 60,000 RPM could cause immense amounts of damage. To address this concern, the flywheel housing acts as a containment chamber in case of failure. The flywheel-based KERS is not designed to be a stand-alone source of power for a vehicle, like batteries are in electric cars. It is designed for temporary energy storage that is to be used frequently and in smaller amounts. Its purpose is to reduce fuel consumption by providing additional power during the acceleration of a vehicle. Periods of acceleration, especially from a stop, are when the efficiency of the vehicle is at its lowest. This is seen when comparing the gas mileage of city and highway driving. The miles per gallon of a vehicle traveling in the city are significantly lower than the miles per gallon of a vehicle on the highway. The start-stop pattern of city driving requires constant changes in speed as drivers move from stoplight to stoplight. 4. Conclusion Cars with a flywheel based energy recovery system, though significantly more expensive than cars without this system, have more power and better fuel efficiency. According to www.thegreencarwebsite.co, the system could reduce fuel consumption by as much as 20% and give a four-cylinder engine acceleration like a six-cylinder unit [7]. This effectively means that cars with the Flywheel KERS system have better fuel efficiency and more power than the cars without the KERS system. According to Derek Crabb, vice president of Volvo's power train engineering division, Flywheel KERS has the potential to reduce fuel consumption by up to 20% [11]. This shows that a lot of research and development work is being done by well-known car companies to implement this system into their production cars. The statement made by Derek also serves to confirm the efficiency of the system. Flybrid Systems has been working with OEM car makers which include some big names like Jaguar and BMW to develop this KERS system for road cars. If oil prices continue to increase, the consumer would not mind paying a little extra for a car with flywheel KERS system because it gives more power and better efficiency. The consumer will be able to save enough money on fuel to make it profitable. The flywheel KERS system promises to be a technology of the future. It makes every car more powerful and at the same time improves fuel efficiency. Better fuel efficiency directly translates to a cleaner, greener environment. It reduces the negative impact on the environment by decreasing harmful CO 2 emissions. It has been found that the amount of CO 2 emitted during the manufacturing of one flywheel KERS is made up for within the first 12,000 km of driving [4]. In addition, as opposed to a hybrid electric vehicle, a flywheel-based mechanical hybrid does not have the harmful chemicals to dispose of that are found in batteries. ISSN : 0975-5462 Vol. 5 No.09 Sep 2013 1698
5. Future Scope The simplicity of energy transfer in this mechanical KERS system makes it superior to the electrical KERS system. Mechanical hybrids are more powerful, more efficient, and cheaper than electrical hybrids. In the future, automobiles will be much more fuel efficient than the cars of today. Flywheel kinetic energy recovery system technology is definitely practical because many car companies are looking into using the system in average everyday cars. Volvo in partnership with Flybrid, officially announced that they intend to develop and produce a vehicle that uses the flywheel based kinetic energy recovery system. With improvement in technology, KERS will definitely become even more efficient and affordable. The main driving force which will launch flywheel-based kinetic energy recovery systems into the automotive industry is the low cost in comparison with fully hybrid vehicles. Any vehicle could be designed and fitted with a flywheel-based kinetic energy recovery system, but the area most affected by this technology would be any vehicle with a start-stop cycle of driving. This technology has already been tested in FLYBUS (a flywheel hybrid system developed for buses). The Flywheel KERS is a technology of great importance and potential. With more advancements and refinements, this system would increase the efficiency of hybrid vehicles. It can reduce fuel consumption and at the same time increase power. Its lower CO 2 emissions reduce air pollution. Probably the biggest advantage of this system is its ability to be retrofitted. The flywheel KERS does not come without flaws, however, developments still need to be made in reducing the forces that act upon the flywheel. With these forces minimized, the system would have much higher efficiency and would be able to store energy longer. It would rival hybrid electric vehicles in efficiency and range. References: [1] Bjorn Bolund et. Al. Flywheel energy and power storage systems (online) <http://www.inference.phy.cam.ac.uk/sustainable/refs/storage/flywheel.pdf> [2] Brockbank, C., & Cross, D. (2008) (online) Mechanical Hybrid system comprising a flywheel and CVT for Motorsport & mainstream Automotive applications. <http://www.torotrak.com/pdfs/tech_papers/2009/sae_wc_2009_09pfl-0922_kers.pdf> [3] Flywheel hybrid systems (KERS) (online) <http://www.racecar-engineering.com/articles/f1/flywheel-hybrid-systems-kers/> [4] Hilton, J., Cross, D. Flybrid systems: Breakthrough technology for greener driving. The Royal Academy of Engineering. [Online]. Available: <http://innovationnow.raeng.org.uk/innovations/default.aspx?item=6> [5] Home - Flybrid Systems, Web. 25 Jan. 2012 (online) <http://www.flybridsystems.com/index.html>. [6] Micheal Mathew, Design of flywheel for improved energy storage, (online) <http://ethesis.nitrkl.ac.in/1125/1/design_of_flywheel_for_improved_energy_storage_using_computer_aided_analysis.pdf/> [7] Lucas, Paul. "Volvo to Develop Kinetic Energy Recovery System." Green Cars: Cars with CO2 Emissions under 150g/km plus News, Information, Articles and Press Releases. 27 May 2011. Web. 29 Feb. 2012. [8] Magnetic Bearing Systems (online) <http://www.waukbearing.com/en/magnetic-bearing-systems/> [9] Toroidal CVT (Nissan Extroid) (online) <http://www.carbibles.com/transmission_bible_pg3.html/> [10] Vacuum chambers for flywheels, John Micheal Pinneo, Jonathan Forrest Garber <http://www.faqs.org/patents/app/201200> [11] Volvo Cars tests of flywheel technology confirm fuel savings of up to 25% <http://www.greencarcongress.com/2013/04/kers- 20130425.html> ISSN : 0975-5462 Vol. 5 No.09 Sep 2013 1699