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Chassis electrification: Brake energy recuperation strategy

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  • 1. Brake Energy Recuperation StrategyBrake energy recuperation strategy systems have evolved throughout the past severalyears, but with the energy crisis looming, these brake systems have lent a great deal ofpotential to regenerative braking as a mainstream technology. Regenerative braking canbe an incredibly complex and confusing subject that might better be left to automotiveengineers and experts in the field, yet at the root of regenerative braking is a systematicand logical foundation.To explain a regenerative brake, in its simplest form it is an avenue to recover energyas an automobile slows down by capturing the kinetic energy and then utilizing it at thatmoment or storing it for use at a later date. In comparison, the typical braking systemin most vehicles today actually wastes the kinetic energy. This happens because theunused energy is turned to heat by friction in the linings of the brake and then lost.Regenerative braking is pretty much used every day. The most common form ofregenerative brake involves using an electric motor as a generator. In electric railsystems the electricity that is generated is fed back into the system, versus a hybridvehicle, where the energy is stored in a battery or capacitor for later use. Energy mayalso be stored in hydraulic hybrid lines or in a rotating fly wheel.Vehicles driven by electric motors use the motor as a generator when using regenerativebraking, so it is operated as a generator during braking and its output is supplied to anelectrical load, then it transfers the energy to the load provided the braking effect.Regenerative braking is used on hybrid gas or electric automobiles to recover some ofthe energy lost during the stopping process. The energy is then saved in a storagebattery and used later to drive the motor whenever the car is in engaged into electricmode.The regenerative braking effect drops off at lower speeds, so the friction brake stillneeds to be utilized in order to bring the vehicle to a complete stop. Manually locking ofthe rotor is also required to prevent the vehicle from rolling down a hill. The frictionbrake is a necessary back up in case the regenerative brake fails.Most road vehicles with regenerative braking only have power on certain wheels, soregenerative braking power only applies to these wheels, therefore in order to provideconsistent braking under difficult conditions like rainy or slippery roads, the frictionbased braking is necessary on the other wheels.The amount of electrical energy capable of dissipation is limited by either the capacity ofthe supply system to absorb the energy or on the state of charge of the battery orcapacitors. No regenerative braking effect can occur if another electrical component onthe same supply system is not currently drawing power and if the battery or capacitorsare already charged. For this reason, it is normal to also incorporate dynamic braking toabsorb the excess energy. 1----------------------------------------------------------------------------------------------------------------------------------- IQPC GmbH | Friedrichstr. 94 | D-10117 Berlin, Germany t: +49 (0) 30 2091 3330 | f: +49 (0) 30 2091 3263 | e: eq@iqpc.de | w: www.iqpc.de Visit IQPC for a portfolio of topic-related events, congresses, seminars and conferences: www.iqpc.de
  • 2. Under emergency braking it is desirable that the braking force exerted be the maximumallowed by the friction between the wheels and the surface without slipping, over theentire speed range from the vehicles maximum speed down to zero. The maximumforce available for acceleration is typically much less than this except in the case ofextreme high performance vehicles. Therefore, the power required to be dissipated bythe braking system under emergency braking conditions may be many times themaximum power which is delivered under acceleration. Traction motors sized to handlethe drive power may not be able to cope with the extra load and the battery may not beable to accept charge at a sufficiently high rate. Friction braking is required to absorbthe surplus energy in order to allow an acceptable emergency braking performance.Dynamic brakes, as opposed to regenerative brakes, dissipate the electric energy asheat by passing the current through large banks of variable resistors. Vehicles that usedynamic brakes include diesel trains and street car systems, among others. This heatcan be used to warm the vehicle interior, or dispensed externally by large radiators tohouse the resistors.The main disadvantage of regenerative brakes when comparing them to dynamic brakesis the need to closely match the generated current with the supply characteristics andincreased maintenance cost of the lines. With direct current, this requires that thevoltage is carefully managed. Only with the development of electronics has this beenpossible with alternating current supplies, where the supply frequency must also beequaled.Hybrid vehicles extensively utilize brake energy recuperation strategy systems.Regenerative braking is an integral part of hybrid and electric vehicles. In a microhybrid, regenerative braking adds more than the basic stop and start system to improvemiles per gallon. However, regenerative braking is not limited to hybrids. Other vehiclescan take advantage of the kinetic energy captured from deceleration in brake energyrecuperation strategy systems.Using the propulsion motor in hybrid, electric and plug in hybrid electric vehicles toprovide regenerative braking is a common design practice. Regenerative braking re-captures and stores part of the kinetic energy that would otherwise be lost to heatduring braking. The captured energy is used to recharge the electric batteries reducingthe fuel consumption in the hybrid architecture. One of the objectives of brake energyrecuperation strategy systems is to keep the battery at a state of charge that allows youto use the electric motors more often.A typical hybrid with stop or start and regenerative braking can provide up to 7 per centfuel economy savings over a driving cycle. The contribution of regenerative braking isabout half of the total savings. Regenerative braking is part of an almost fifty per centgain in fuel economy on the Chevrolet Tahoe or GMC Yukon hybrid over their non hybridequivalents. 2----------------------------------------------------------------------------------------------------------------------------------- IQPC GmbH | Friedrichstr. 94 | D-10117 Berlin, Germany t: +49 (0) 30 2091 3330 | f: +49 (0) 30 2091 3263 | e: eq@iqpc.de | w: www.iqpc.de Visit IQPC for a portfolio of topic-related events, congresses, seminars and conferences: www.iqpc.de
  • 3. In some generic hybrids with standard regenerative braking, manufacturers may notblend the electric and hydraulic brake operation. If it is small enough, they wont doanything with the brake systems in terms of compensating for decelerator changes. Themanufacturer simply adds the regenerator on top of the standard hydraulic brakesystem. In contrast, the evolving regenerative brake system blends the capabilities ofthe friction brakes and the regenerative braking from a hybrids motor running as agenerator.To avoid higher risk and higher component and system costs, the goal for the next-generation system was to use proven brake components as much as possible. The onlynew development parts were a vacuum pump and a pedal feel simulator, which aresimply a mechanical spring pack and a cut off device on the pedal.Some of the braking comes from the electric motors in the hybrid power train running asa generator. When the electric motor generates power back into the battery, it puts aload on the driveshaft. The additional braking requirements requested by the driver areprovided by the hydraulic portion of the brakes for the brake energy recuperationstrategy system.Although the system can be used on non hybrid vehicles, it does not provide theregenerative portion of brake blending on those vehicles. In these applications, thesystem provides the driver a simulated pedal touch, computing the drivers decelerationintent and then applying the boosted hydraulic pressure to the brakes. Future powertrains such as gas direct injection with added turbo charging and even diesel engineshave significant mechanical losses due to vacuum pumps. As a result, eliminating theuse of vacuum is under serious consideration. A hydraulic boosted system is analternative to a vacuum boosted brake system. These systems optimize the utilization ofbraking energy in electric and hybrid electric vehicles by controlling the balance betweenhydraulic braking and regenerative braking to offer greater vehicle range.Using a specially designed alternator as the electrical machine for generating theenergy, the system implements a unique charging system strategy. Instead ofconstantly running the alternator to maintain a high level of charge in the battery, thesystem only charges to eighty per cent of its capacity using power from the engine. Thisrelieves the power requirements to drive the alternator from the engine. When thedriver applies the brakes, the intelligent alternator control is activated to provideadditional energy to the battery. The alternator is also engaged in overrun conditions.The system maintains a reserve charge adequate for the power requirements while thecar is idling and sufficient to start the engine under all circumstances. From the brakingside, the vehicles conventional brakes contribute whatever additional braking is neededto meet the drivers commands.Formula one racing has also embraced the brake energy recuperation strategy system,referred to as the kinetic energy recovery system. The device recovers the kineticenergy that is present in the waste heat created by the car’s braking process. It stores 3----------------------------------------------------------------------------------------------------------------------------------- IQPC GmbH | Friedrichstr. 94 | D-10117 Berlin, Germany t: +49 (0) 30 2091 3330 | f: +49 (0) 30 2091 3263 | e: eq@iqpc.de | w: www.iqpc.de Visit IQPC for a portfolio of topic-related events, congresses, seminars and conferences: www.iqpc.de
  • 4. that energy and converts it into power that can be called upon to boost acceleration.There are principally two types of system, battery or electrical, and flywheel ormechanical. Electrical systems use a motor-generator incorporated in the car’stransmission which converts mechanical energy into electrical energy and vice versa.Once the energy has been captured, it is stored in a battery and released when it isrequired.Another utilization of brake energy recuperation strategy systems are trains. The energyput into accelerating a train, as an example, and into moving it uphill is stored in thetrain as kinetic and potential energy. In vehicles with electric traction motors, a greatpart of this energy can be reconverted into electric energy by using the motors asgenerators when braking. The electric energy is transmitted backwards along theconversion chain. This is known as regenerative braking and is widely used in railways.However, the use of dynamic braking does not necessarily mean that the recoveredenergy is used to save energy. Diesel electric trains will often have dynamic brakes tosave the braking pads and the recovered energy is just dissipated in brake resistors.Energy recovery is especially powerful on local and regional lines with frequent stops.Nevertheless, even on high speed traffic regenerative braking offers potential for energyefficiency. Although regenerative braking is in wide spread use in many countries, thereis still a great potential for increasing the share of recovered energy. Energy recovery isonly an option whenever another train in the system can use the energy at the sametime. The probability for this depends on train density and possible transmissiondistance. Want to read the full article? Visit our Download Center: http://bit.ly/chassis-electrification-articlesAbout IQPC:IQPC provides tailored conferences, large events, seminars and internal training programmes formanagers around the world. Topics include current information on industry trends, technicaldevelopments and regulatory rules and guidelines. IQPCs conferences are market leading events, highlyregarded for their opportunity to exchange knowledge and ideas for professionals from various industries.IQPC has offices in major cities across six continents including: Berlin, Dubai, London, New York, SaoPaulo, Singapore, Johannesburg, Sydney and Toronto. IQPC leverages a global research base of bestpractices to produce an unrivaled portfolio of problem-solving conferences. Each year IQPC offersapproximately 2,000 worldwide conferences, seminars, and related learning programs. 4----------------------------------------------------------------------------------------------------------------------------------- IQPC GmbH | Friedrichstr. 94 | D-10117 Berlin, Germany t: +49 (0) 30 2091 3330 | f: +49 (0) 30 2091 3263 | e: eq@iqpc.de | w: www.iqpc.de Visit IQPC for a portfolio of topic-related events, congresses, seminars and conferences: www.iqpc.de