STUDYOF KINETIC ENERGY RECOVERY SYSTEM(KERS) BY BASIL AFRIDIAMITY UNIVERSITY2018TABLE OF CONTENTS I.
ABSTRACTII. INTRODUCTIONIII. HOW DOES KERS WORK?IV. COMPONENTSV. ADVANTAGES OF ELECTRIC KERSVI. DISADVANTAGES OF ELECTRIC KERSVII. ADVANTAGES OF MECHANICAL KERSVIII. DISADVANTAGES OF MECHANICAL KERSIX.
CONCLUSIONREFERENCES I. ABSTRACT Not many people know about the kinetic energy recoverysystem, also known as KERS forshort. This technology has been able to save energy that would otherwise benormally lost during braking in an electric/hybrid car. In a research paper, itwas written that by integrating flywheel hybrid systems, thesedrawbacks can be overcome and can potentially replace battery powered hybridvehicles cost effectively. The paper will explain the engineering, mechanics ofthe flywheel system and it’s working in detail. Many companies are now tryingto incorporate KERS in their automobiles. F1 racing is another area which hasbeen impacted by KERS technology. In this paper, I’ve collected all informationone could find about this technology online and assembled it into by the end ofthis we shall understand the details of how this technology operates and if it’sworth the investment of time and money of people.
II. INTRODUCTION In a world where almost all its fuel is beingdepleted, conservation of natural resources has become a necessity in today’sworld, especially in the field of renewable technology. In an automobile,maximum energy is lost during deceleration or braking.
This problem has beenresolved with the 0introduction of regenerative braking. It is an approach torecover or restore the energy lost while braking. The Kinetic Energy RecoverySystem (KERS) is a type of regenerative braking system which has the capabilityto store and reuse the lost energy 1In the beginning of this paper, we will try and breakdown the basic principle of the KERS technology. We will look at differentsources to see what each of them have to say about KERS and if they view KERSas something highly beneficial for the world or not.Going deeper into this paper, we’ll get into theworking of the KERS and try to keep it as explainable as possible to you. We’lllook into different sources to see how different manufacturers have implementedthe use of KERS in their respective industries. We will see how KERS is used inan average automobile producing industry and how it is used in the racingindustry.
Towards the end of the paper, after giving you as muchas detail as one possibly can about the KERS technology, we will try tounderstand whether this technology should be implemented by more manufacturersor not.At the end, we will formulate a conclusion. III.
HOW DOES KERS WORK? There are two main implementations of theKERS system and they differ in how the energy is stored. The electrical KERSuses an electromagnet to transfer the kinetic energy to electric potentialenergy that is eventually converted to chemical energy that is stored in abattery. It then redelivers the stored energy to the drive train by powering amotor. The electric KERS was what many groups in F1 began using in their cars. Thebattery used to store the power is exceptionally prone to battery fires and cancause electric shocks.
In an accident with the BMW Sauber team in F1, anengineer who was working on the KERS was burned while testing the system aftera practice run, many groups esteemed the electric KERS to be risky. Alongsidedifferent factors, such as being heavier than other implementations, theelectric KERS implementation is not found inside today’s Formula 1 cars. The mechanical implementation, shown inthe figure, was initially developed by Flybrid Systems. To harvest the energyupon braking, the system uses the braking energy to turn a flywheel which actsas the reservoir of this energy. When needed, the redelivery of the energy issimilar to that of the electric KERS implementation, the rotating flywheel isconnected to the wheels of the car and when called upon provides a power boost.The mechanical implementation of KERS is known to be more efficient than theelectric equivalent due to the fewer conversions of the energy that are takingplace.
2 In an Article, Top Gear wrote:”Volvo hasjust built a KERS-equipped S60 T5 development mule. At the fore, there’s the company’s older 254hp five-cylinderpetrol engine, powering the front wheels, and astern there’s a Flybrid KERSsystem powering the back axle. So, how does it work? Kinetic energy that you’dordinarily lose to heat while braking is sent to a flywheel, which can capture 150-watthours in around eight seconds of gentle braking. That’s the same amount ofenergy you’d need to charge 25 new iPhones captured in a third of the time it’dtake a Toyota Prius.
Once it’s been recovered, itcan be stored for about half an hour or used immediately, either as asupplement to the engine, or in one great big lump. Chose the former and it’llcut consumption by up to 25 per cent. Chose the latter and you get 80hp addedinstantly. with KERS switched on, our0-60mph time dropped from 7.68 seconds to 6.07 seconds.And allthis thrust comes from a little box of gears and clutches that weighs 60kg,requires virtually no maintenance, and will last for what the company claim isthe realistic life of the car. The batteries in Volvo’s current petrol/electrichybrid weigh 300kg alone, and will have to be replaced after about a decade.
” 3 IV. COMPONENTS The flywheel hybridprimarily consists of a rotating flywheel, a continuously variable transmissionsystem (CVT), a step-up gearing (along with a clutch) between the flywheel andthe 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 connectingthe flywheel system to the driveline/ transmission is engaged, causing energyto be transferred to the flywheel via the CVT. The flywheel stores this energyas rotational energy and can rotate up to a maximum speed of 60000 rpm. Whenthe vehicle stops, or the flywheel reaches its maximum speed, the clutchdisengages the flywheel unit from the transmission allowing the flywheel torotate independently. Whenever this stored energy is required, the clutch isengaged 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.
1shows Volvo’s flywheel KERS system Layout. 4 Fig.1 Theprimary idea behind the flywheel-based KERS system is to mechanically store thekinetic energy from the rear driveshaft in another source for use at anothertime. This other source is the flywheel.
Whenthe clutch is engaged and both discs of the CVT are in contact with therollers, kinetic energy transfer can occur. This energy of motion istransferred to whatever disc is moving slower; if the car is slowing to a stop,the rollers in the CVT transmit the kinetic energy from the faster rotatingdisc connected to the rear driveshaft, to the slower disc connected to theflywheel. The disc connected to the flywheel begins to spin faster, which inturns speed up the flywheel. This process is also reversible, where the rollerscan transfer energy from the disc connected to the flywheel to the discconnected to the rear driveshaft. 56Theflywheel is the component which harvests kinetic energy, when the vehiclebrakes, by increasing its rotational speed. The ability of flywheels to storeenergy is explained by the relation between the flywheel’s inertia, angularvelocity and kinetic energy. The equation for the energy stored in a flywheelreads as follows: (1)7WhereE is the energy (Joules); I is the inertia of the flywheel (kgm2 ), and ? isthe angular velocity (rad/sec) of the flywheel. Theequation for the inertia of a flywheel is: (2)7Wherem is the mass of the flywheel; and are the inner and outer radius of theflywheel respectively.
Combining equation 1 and 2 we get: (3)Fromequation 3, a flywheel’s energy is proportional to its mass, and proportionalto the square of its rotational speed or angular velocity. In other words, bydoubling the mass, the energy stored is also doubled, and by doubling thespeed, the energy stored is quadrupled. Thus, by increasing the speed of theflywheel it will be possible to reduce the mass and size of it, to a levelwhere its weight is insignificant while analyzing fuel efficiency. In order tomake the system more efficient it is necessary enclose the flywheel in a vacuumchamber, and in order to eliminate the resistance due to air and reducefriction it is mounted on magnetic bearings. Theamount of energy that can safely be stored in the rotor depends on the point atwhich the rotor will warp or shatter. The hoop stress on the rotor is given by: (4)8Where isthe tensile stress on the rim of the flywheel; is thedensity, r is the outer radius of the flywheel and isthe angular velocity of the rotating flywheel.
Theflywheel can be fabricated using different materials based on the maximumrotational speed requirements and other design constraints. High speedflywheels for speeds above 30000 rpm are usually composed of high strengthcarbon fibre. A large mass is not desired for high speed flywheels becauseextra mass means more energy will be needed to accelerate the vehicle. On theother hand, low speed flywheels with speed values below 20000 rpm, aregenerally made of steel or other metals for low cost. The weight of theflywheel is a very important factor in determining the efficiency of thesystem. 9 a.
The flywheel vacuum chamber The vacuum chamber is another very essential part ofthe flywheel hybrid system. The major function of the vacuum chamber is tominimize the air resistance as the flywheel rotates. Without the vacuumchamber, the friction caused by air resistance is enough to cause significantenergy losses and heat the carbon fiber rim to its glass transition temperature10. Vacuum chambers for KERS systems are frequently made of metals likealuminum, stainless steel, or the like because these metals can provideadequate strength to withstand differential pressure between an evacuatedinterior and the surrounding atmosphere, as well as to provide a barrier to thepassage of atmospheric gases through the chamber wall by diffusion or flowthrough structural defects. Fig.
shows the flywheel hybrid system designed byflybrid. b. Magnetic Bearings Another important part of the system is the bearingson which the flywheel is mounted. Magnetic bearings have replaced mechanicalbearings as they greatly reduce losses due to friction. Mechanical bearingscannot, due to the high friction and short life, be adapted to modernhigh-speed flywheels.
Further magnetic bearings are able to operate in vacuumwhich leads to even better efficiency. The magnetic bearings support theflywheel by the principle of magnetic levitation. It is a method by which anobject is suspended with no support other than magnetic fields. A permanent orelectro permanent magnetic bearing system is utilized. Electro permanentmagnetic bearings do not have any contact with the shaft, has no moving parts,experience little wear and require no lubrication. It is important that thebearings are able to operate inside a vacuum because the flywheel in aflywheel-based KERS must rotate at high speeds for maximum efficiency. The bestperforming bearing is the high-temperature super-conducting (HTS) magneticbearing, which can situate the flywheel automatically without need ofelectricity or positioning control system.
However, HTS magnets requirecryogenic cooling by liquid nitrogen 11. Fig. shows a magnetic bearingdesigned by Waukesha bearings. c. The continuously variable transmission (CVT) unit The continuously variabletransmission (CVT) as used by Flybrid, is mounted between two clutches withinthe KERS unit. The clutches allow for disengagement of the CVT from theflywheel and the vehicle when not in use, and therefore minimizes losses.The only mechanism forcontrolling energy into or out of the flywheel is by controlling the ratio ofthe CVT. The CVT is responsible for the smooth variation of ratios.
The CVT maysometimes be referred to as a Toroidal Continuously Variable Transmission (TCVT),due to the shape of the rotating discs. The main components that make up theCVT are: the rotating discs, rollers, carriages, and the pistons (levers).Each roller is mounted ina carriage and attached to a hydraulic piston. The pressure in the pistons canbe increased or decreased to create a range of reaction torque within the CVT.The movement of the hydraulic pistons alters the angle of the rollers, wherethe angle of the rollers in relation to the centerline of the CVT controls thetransmission ratio.
This ratio affects the torque transferred through the CVT.12 d. Step-up gearing and clutch A step-up gear takes the 60,000 RPM to a manageablespeed outside the vacuum. The maximum step up of an epicyclic gear or amagnetic gear is 6:1. The gears are placed just outside the vacuum enclosureand spin all the time the flywheel is spinning. They emit a continuous high-pitchedsound.
The clutch disconnects the CVT from the flywheel when it is nottransferring power to reduce free running losses. 13 SHAFT FROM FLYWHEEL LOW SPEED CLUTCH CVT EPICYCLIC GEARS e. The clutch The clutch is used to couple the flywheel hybridsystem to the transmission. It engages the system while the flywheel isaccelerating from rest and disengages while the flywheel is rotating and thevehicle is at rest. Torque is transferred through clutch between the flywheeland vehicle. Hence, the power transmitted in the flywheel system can becontrolled by a clutch that could continuously manipulate the torque. 4 V. ADVANTAGES OF ELECTRIC KERS The electric systems allow the teams in Formula One to be more flexible in terms of placing the various components around the car which helps for better weight distribution which is of vital importance in F1.
The specific energy of Lithium-ion batteries in comparison is unrivalled as they can store considerably more energy per kg which helps reduce the size of RESS. VI. DISADVANTAGESOF ELECTRIC KERS Lithium-ion batteries take 1-2 hours to charge completely due to low specific power (i.e rate to charge or discharge) hence in high performance F1 cars more batteries are required which increases the overall weight of the batteries. Chemical batteries heat up during charging process and this takes place a number of times in KERS units which if not kept under control could cause the batteries to lose energy over the cycle or worse even explode.
The specific power is low as the energy needs to be converted at least two times both while charging or discharging causing energy losses in the process.VII. ADVANTAGESOF MECHANICAL KERS The specific power of flywheels in comparison is much greater than that of batteries. The energy lost during transfers amongst the system components is relatively less due to high efficiency. The flywheel system can deliver almost the entire amount of energy stored in it, repeatedly without any decline in efficiency.
The mechanical system does not need to be replaced as its life cycle is as good as that of the car. VIII. DISADVANTAGESOF MECHANICAL KERS The specific energy capacity of flywheels is lower than some of the advanced battery models.
Friction produced in the bearings and seals cause the flywheel to slow down and lose energy. 14 IX. CONCLUSION Apart from increasing overtaking the main purpose ofintroducing KERS was to challenge the best engineers in the business to developinnovative ideas that would directly benefit the mainstream motor industry.
Given the resources and pace of developments in F1, the KERS systems produced by the teams would have taken the carmanufacturers much longer to develop. Both the types of KERS can be retrofittedin cars albeit with minor modifications. Given the current trend of enginedownsizing they can add substantial amount of performance to the car withoutaffecting the engine and average. The mechanical system is more efficient thanthe electrical systems that use inefficient batteries which makes them morelikely to be induced in cars in the near future. 14By adopting the cheaper andlighter flywheel system (the ideal solution if it could be made to fit into theno-refueling era cars), a more powerful boost, and limiting the number ofactivations in a race it would cover all the bases it needs to. It would beaffordable for the all the teams, deliver performances as well as being a more interesting race variable. The sidepod solution is quiteunique, and has given us a new envelope to try to drive performance to the rearof the car. We need to keep thinking out-of-the-box.
Compared to ten or 20years ago, it’s really quite staggering what can be delivered given therestrictions we have now – it’s a tribute toimaginative thinking. 15