STUDYOF KINETIC ENERGY RECOVERY SYSTEM(KERS) BY BASIL AFRIDIAMITY UNIVERSITY2017TABLEOF 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. one research paper.
By integratingflywheel hybrid systems, these drawbacks can be overcome and can potentiallyreplace battery powered hybrid vehicles cost effectively. The paper willexplain the engineering, mechanics of the flywheel system and it’s working indetail. Many companies are now trying to incorporate KERS in their automobiles.F1 racing is another area which has been impacted by KERS technology. In thispaper, I’ve collected all information one could find about this technologyonline and assembled it into by the end of this we shall understand the detailsof how this technology operates and if it’s worth the investment of time andmoney of people. 0II. INTRODUCTION I0n 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 teams started off trying to implementinto their cars.
However, the battery used to store the energy is very prone tobattery fires and can cause electric shocks. After an incident with the BMWSauber team, where an engineer working on the KERS was burned while testing thesystem after a practice run, many teams deemed the electric KERS to be unsafe.Along with other factors such as being heavier than other implementations, theelectric KERS implementation is not found inside today’s Formula 1 cars.
Themechanical implementation, shown in the figure, was initially developed byFlybrid Systems. To harvest the energy upon braking, the system uses thebraking energy to turn a flywheel which acts as the reservoir of this energy.When needed, the redelivery of the energy is similar to that of the electricKERS implementation, the rotating flywheel is connected to the wheels of thecar and when called upon provides a power boost. The mechanical implementationof KERS is known to be more efficient than the electric equivalent due to thefewer conversions of the energy that are taking place..
2 In an Article, Top Gear wrote: Volvohas just 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 0Theflywheel hybrid primarily consists of a rotating flywheel, a continuouslyvariable transmission system (CVT), a step-up gearing (along with a clutch)between the flywheel and the CVT and clutch which connects this system to theprimary shaft of the transmission. When the brakes are applied or the vehicledecelerates, the clutch connecting the flywheel system to the driveline/transmission is engaged, causing energy to be transferred to the flywheel viathe CVT. The flywheel stores this energy as rotational energy and can rotate upto a maximum speed of 60000 rpm. When the vehicle stops, or the flywheelreaches its maximum speed, the clutch disengages the flywheel unit from thetransmission allowing the flywheel to rotate independently. Whenever thisstored energy is required, the clutch is engaged and the flywheel transmitsthis energy back to the wheels, via the CVT. Generally the flywheel can deliverup to 60 kW of power or about 80 HP.
Fig.1 shows Volvo’s flywheel KERS systemLayout. 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. When the clutch is engaged and both discsof the CVT are in contact with the rollers, kinetic energy transfer can occur.This energy of motion is transferred to whatever discis moving slower; if the car is slowing to a stop, the rollers in the CVTtransmit the kinetic energy from the faster rotating disc connected to the reardriveshaft, to the slower disc connected to the flywheel. The disc connected tothe flywheel begins to spin faster, which in turns speed up the flywheel. Thisprocess is also reversible, where the rollers can transfer energy from the discconnected to the flywheel to the disc connected 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 is the tensile stress onthe rim of the flywheel; isthe density, 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 speed flywheelsfor 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 meansmore energy will be needed to accelerate the vehicle. On the other hand, lowspeed flywheels with speed values below 20000 rpm, are generally made of steelor other metals for low cost. The weight of the flywheel is a very importantfactor in determining the efficiency of the system. 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 flow throughstructural defects. Fig.
shows the flywheel hybrid system designed by flybrid. 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 Thecontinuously variable transmission (CVT) as used by Flybrid, is mounted betweentwo clutches within the KERS unit. The clutches allow for disengagement of theCVT from the flywheel and the vehicle when not in use, and therefore minimizeslosses.The onlymechanism for controlling energy into or out of the flywheel is by controllingthe ratio of the CVT.
The CVT is responsible for the smooth variation ofratios. The CVT may sometimes be referred to as a Toroidal ContinuouslyVariable Transmission (TCVT), due to the shape of the rotating discs. The maincomponents that make up the CVT are: the rotating discs, rollers, carriages,and the pistons (levers).
Each rolleris mounted in a carriage and attached to a hydraulic piston. The pressure inthe pistons can be increased or decreased to create a range of reaction torquewithin the CVT. The movement of the hydraulic pistons alters the angle of therollers, where the angle of the rollers in relation to the centerline of theCVT controls the transmission ratio. This ratio affects the torque transferredthrough the CVT.
12 d. Step-up gearing and clutch A step-up geartakes the 60,000 RPM to a manageable speed outside the vacuum. The maximum stepup of an epicyclic gear or a magnetic gear is 6:1. The gears are placed justoutside the vacuum enclosure and spin all the time the flywheel is spinning.
Theyemit a continuous high-pitched sound. The clutch disconnects the CVT from theflywheel when it is not transferring power to reduce free running losses. 13 SHAFT FROM FLYWHEEL LOW SPEED CLUTCH CVT EPICYCLIC GEARS e. The clutch The clutch is usedto couple the flywheel hybrid system to the transmission. It engages the systemwhile the flywheel is accelerating from rest and disengaging while the flywheelis rotating and the vehicle is at rest. Torque is transferred through clutchbetween the flywheel and vehicle. Hence, the power transmitted in the flywheelsystem can be controlled by a clutch that could continuously manipulate the torque.4 V.
ADVANTAGES OF ELECTRIC KERS The electric systems allow the teams 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 be affordablefor 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 to imaginativethinking. 15