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Regenerative braking is anenergy recovery mechanism that slows down a moving vehicle or object by converting itskinetic energy orpotential energy into a form that can be either used immediately or stored until needed.
Typically, regenerativebrakes work by driving anelectric motor in reverse to recapture energy that would otherwise be lost as heat during braking, effectively turning thetraction motor into anelectric generator. Feeding power backwards through the system like this allows the energy harvested fromdeceleration to resupply anenergy storage solution such as abattery or acapacitor. Once stored, this power can then be later used to aid forwardpropulsion. Because of the electrified vehicle architecture required for such a braking system, automotive regenerative brakes are most commonly found onhybrid andelectric vehicles.
This method contrasts with conventional braking systems, where excess kinetic energy is converted to unwanted and wasted heat due to friction in thebrakes. Similarly, withrheostatic brakes, energy is recovered by using electric motors as generators but is immediately dissipated as heat inresistors.
In addition to improving the overall efficiency of the vehicle, regeneration can significantly extend the life of the braking system. This is because the traditional mechanical parts like discs, calipers, and pads – included for when regenerative braking alone is insufficient to safely stop the vehicle – will not wear out as quickly as they would in a vehicle relying solely on traditional brakes.
The most common form of regenerative brake involves anelectric motor functioning as an electric generator. In electricrailways, the electricity generated is fed back into thetraction power supply. Inbattery electric andhybrid electric vehicles, the energy is stored chemically in abattery, electrically in a bank ofcapacitors, or mechanically in a rotatingflywheel.Hydraulic hybrid vehicles use hydraulic motors to store energy in the form ofcompressed air. In a hydrogenfuel cell powered vehicle, the electrical energy generated by the motor is stored chemically in a battery, similar to battery and hybrid electric vehicles.[2]
Regenerative braking is not by itself sufficient as the sole means of safely bringing a vehicle to a standstill, or slowing it as required, so it must be used in conjunction with another braking system such asfriction-based braking.
Regenerative and friction braking must both be used, creating the need to control them to produce the required total braking. The GMEV-1 was the first commercial car to do this. In 1997 and 1998, engineers Abraham Farag and Loren Majersik were issued two patents for thisbrake-by-wire technology.[3][4]
Early applications commonly suffered from a serious safety hazard: in many early electric vehicles with regenerative braking, the same controller positions were used to apply power and to apply the regenerative brake, with the functions being swapped by a separate manual switch. This led to a number of serious accidents when drivers accidentally accelerated when intending to brake, such as therunaway train accident inWädenswil, Switzerland in 1948, which killed twenty-one people.
In the 2020s, most vehicles equipped with regenerative braking can completely halt reasonably quickly in One Pedal Driving mode. Some car models do not illuminate the braking light when engaging in regenerative braking, leading to safety concerns. Most regulations do not mandate the illumination of a braking light when the vehicle decelerates through regenerative braking.[5] The One Pedal Driving (OPD) mode also lead to concerns oversudden unintended acceleration (SUA), as the driver could confuse the accelerator as the brake in stressful situations when the latter is seldomly used during OPD operation.[6] The GB 21670-2025 vehicle standard later mandated thatbrake lights must turn on during regenerative braking when deceleration exceeds 1.3 m/s2.[7]
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In 1886 the Sprague Electric Railway & Motor Company, founded byFrank J. Sprague, introduced two important inventions: a constant-speed, non-sparking motor with fixed brushes, and regenerative braking.
Early examples of this system in road vehicles were thefront-wheel drive conversions of horse-drawncabs byLouis Antoine Krieger in Paris in the 1890s. The Krieger electriclandaulet had a drive motor in each front wheel with a second set of parallel windings (bifilar coil) for regenerative braking.[8] The Orwell Electric Truck introduced byRansomes, Sims & Jefferies in England during WW1 used regenerative braking switched in by the driver.
In England, "automatic regenerative control" was introduced to tramway operators by John S. Raworth's Traction Patents 1903–1908, offering them economic and operational benefits[9][10][11] as explained in some detail by his sonAlfred Raworth.These included tramway systems at Devonport (1903),Rawtenstall,Birmingham, Crystal Palace-Croydon (1906), and many others. Slowing the speed of the cars or keeping it in control on descending gradients, the motors worked as generators and braked the vehicles. The tram cars also had wheel brakes and track slipper brakes which could stop the tram should the electric braking systems fail. In several cases the tram car motors were shunt wound instead of series wound, and the systems on the Crystal Palace line utilized series-parallel controllers.[clarification needed][12] Following a serious accident at Rawtenstall, an embargo was placed on this form of traction in 1911;[13] the regenerative braking system was reintroduced twenty years later.[11]
Regenerative braking has been in extensive use on railways for many decades. The Baku-Tbilisi-Batumi railway (Transcaucasus Railway or Georgian railway) started utilizing regenerative braking in the early 1930s. This was especially effective on the steep and dangerousSurami Pass.[14] In Scandinavia the Kiruna to Narvik electrified railway, known asMalmbanan on the Swedish side andOfoten Line on the Norwegian, carries iron ore on the steeply-graded route from the mines inKiruna, in the north of Sweden, down to the port ofNarvik in Norway to this day. The rail cars are full of thousands of tons ofiron ore on the way down to Narvik, and these trains generate large amounts of electricity by regenerative braking, with a maximum recuperative braking force of 750 kN. FromRiksgränsen on the national border to the Port of Narvik, the trains[15] use only a fifth of the power they regenerate.[failed verification] The regenerated energy is sufficient to power the empty trains back up to the national border.[16][failed verification] Any excess energy from the railway is pumped into the power grid to supply homes and businesses in the region, and the railway is a net generator of electricity.[citation needed]
Electric cars used regenerative braking since the earliest experiments, but this initially required the driver to flip switches between various operational modes in order to use it. TheBaker Electric Runabout and theOwen Magnetic were early examples, which used many switches and modes controlled by an expensive "black box" or "drum switch" as part of their electrical system.[17][18] These, like the Krieger design, could only practically be used on downhill portions of a trip, and had to be manually engaged.
Improvements in electronics allowed this process to be fully automated, starting with 1967'sAMC Amitron experimental electric car.[19] Designed by Gulton Industries[20] the motor controller automatically began battery charging when the brake pedal was applied. Many modern hybrid and electric vehicles use this technique to extend the range of the battery pack, especially those using an AC drive train (most earlier designs used DC power).
An AC/DC rectifier and a very large capacitor may be used to store the regenerated energy, rather than a battery. The use of a capacitor allows much more rapid peak storage of energy, and at higher voltages.Mazda used this system in some 2018 cars, where it is branded i-ELOOP.
During braking, thetraction motor connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator (MG) and the motor armatures are connected across the load. The MG now excites the motor fields. The rolling locomotive or multiple unit wheels turn the motor armatures, and the motors act as generators, either sending the generated current through onboard resistors (dynamic braking) or back into the supply (regenerative braking). Compared to electro-pneumatic friction brakes, braking with the traction motors can be regulated faster improving the performance ofwheel slide protection.
For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exertstorque in a direction that is opposite from the rolling direction.
Braking effort is proportional to the product of the magnetic strength of the field windings, multiplied by that of the armature windings.
Savings of 17%, and less wear on friction braking components, are claimed forBritish Rail Class 390s.[21]Caltrain claims 23% of the energy used by itsStadler KISS electric trains are recaptured and returned to the grid.[22] TheDelhi Metro reduced the amount ofcarbon dioxide (CO2) released into the atmosphere by around 90,000 tons by regenerating 112,500 megawatt hours of electricity through the use of regenerative braking systems between 2004 and 2007. It was expected that the Delhi Metro would reduce its emissions by over 100,000 tons of CO2 per year once its phase II was complete, through the use of regenerative braking.[23]
Electricity generated by regenerative braking may be fed back into the traction power supply; either offset against other electrical demand on the network at that instant, used forhead end power loads, or stored inlineside storage systems for later use.[24]
A form of what can be described as regenerative braking is used on some parts of theLondon Underground, achieved by having small slopes leading up and down from stations. The train is slowed by the climb, and then leaves down a slope, so kinetic energy is converted togravitational potential energy in the station.[25] This is normally found on the deep tunnel sections of the network and not generally above ground or on thecut and cover sections of the Metropolitan and District Lines.
What are described as dynamic brakes ("rheostatic brakes" in British English) on electric traction systems, unlike regenerative brakes, dissipate electric energy as heat rather than using it, by passing the current through large banks ofresistors. Vehicles that use dynamic brakes includeforklift trucks,diesel-electriclocomotives, andtrams. This heat can be used to warm the vehicle interior, or dissipated externally by largeradiator-like cowls to house the resistor banks.
General Electric's experimental 1936steam turbine locomotives featured true regeneration. These two locomotives ran the steam water over the resistor packs, as opposed to air cooling used in most dynamic brakes. This energy displaced the oil normally burned to keep the water hot, and thereby recovered energy that could be used to accelerate again.[26]
The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the generated current with the supply characteristics and increased maintenance cost of the lines. With DC supplies, this requires that the voltage be closely controlled. The AC power supply and frequency converter pioneer Miro Zorič and his first AC power electronics have also enabled this to be possible with AC supplies.[citation needed] The supply frequency must also be matched (this mainly applies to locomotives where an AC supply isrectified for DC motors).
In areas where there is a constant need for power unrelated to moving the vehicle, such as electric train heat orair conditioning, this load requirement can be utilized as a sink for the recovered energy via modernAC traction systems. This method has become popular with North American passenger railroads wherehead end power loads are typically in the area of 500 kW year round. Using HEP loads in this way has prompted recent electric locomotive designs such as theALP-46 andACS-64 to eliminate the use of dynamic brake resistor grids and also eliminates any need for any external power infrastructure to accommodate power recovery allowing self-powered vehicles to employ regenerative braking as well.
A small number ofsteep grade railways have used3-phase power supplies andinduction motors. This results in a near constant speed for all trains, as the motors rotate with the supply frequency both when driving and braking.
Kinetic energy recovery systems (KERS) were used for the motor sportFormula One's2009 season, and are under development for road vehicles. KERS was abandoned for the2010 Formula One season, but re-introduced for the2011 season. By2013, all teams were using KERS withMarussia F1 starting use for the 2013 season.[27] One of the main reasons that not all cars used KERS immediately is because it raises the car's center of gravity, and reduces the amount ofballast that is available to balance the car so that it is more predictable when turning.[28] FIA rules also limit the exploitation of the system. The concept of transferring the vehicle's kinetic energy usingflywheel energy storage was postulated by physicistRichard Feynman in the 1950s[29] and is exemplified in such systems as theZytek, Flybrid,[30] Torotrak[31][32] and Xtrac used in F1.Differential based systems also exist such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS).[33]
Xtrac and Flybrid are both licensees of Torotrak's technologies, which employ a small and sophisticated ancillary gearbox incorporating acontinuously variable transmission (CVT). The CPC-KERS is similar as it also forms part of the driveline assembly. However, the whole mechanism including the flywheel sits entirely in the vehicle's hub (looking like a drum brake). In the CPC-KERS, a differential replaces the CVT and transfers torque between theflywheel, drive wheel and road wheel.
The first of these systems to be revealed was the Flybrid. This system weighs 24 kg and has an energy capacity of 400 kJ after allowing for internal losses. A maximum power boost of 60 kW (82 PS; 80 hp) for 6.67 seconds is available. The 240 mm diameter flywheel weighs 5.0 kg and revolves at up to 64,500 rpm. Maximum torque is 18 Nm (13.3 ftlbs). The system occupies a volume of 13 litres.[citation needed]
Formula One have stated that they support responsible solutions to the world's environmental challenges,[34] and theFIA allowed the use of 60 kW (82 PS; 80 hp) KERS in the regulations for the2009 Formula One season.[35] Teams began testing systems in 2008: energy can either be stored as mechanical energy (as in aflywheel) or as electrical energy (as in a battery orsupercapacitor).[36]
Prior to the introduction of KERS,McLaren had already deployed an early regenerative braking system in their 1998MP4/13 race car. This system didn't send power to the wheels directly, and instead used energy from the brakes to run auxiliary pumps on the engine to combat parasitic losses, resulting in an extra 30 to 40 horsepower for a limited period.[37]
Two minor incidents were reported during testing of KERS systems in2008. The first occurred when theRed Bull Racing team tested their KERS battery for the first time in July: it malfunctioned and caused a fire scare that led to the team's factory being evacuated.[38] The second was less than a week later when aBMW Sauber mechanic was given an electric shock when he touchedChristian Klien's KERS-equipped car during a test at theJerez circuit.[39]
With the introduction of KERS in the 2009 season, four teams used it at some point in the season:Ferrari,Renault,BMW, andMcLaren. During the season, Renault and BMW stopped using the system.McLaren Mercedes became the first team to win a F1 GP using a KERS equipped car whenLewis Hamilton won the2009 Hungarian Grand Prix on 26 July 2009. Their second KERS equipped car finished fifth. At the following race, Lewis Hamilton became the first driver to take pole position with a KERS car, his teammate,Heikki Kovalainen qualifying second. This was also the first instance of an all KERS front row. On 30 August 2009,Kimi Räikkönen won the Belgian Grand Prix with his KERS equipped Ferrari. It was the first time that KERS contributed directly to a race victory, with second placedGiancarlo Fisichella claiming "Actually, I was quicker than Kimi. He only took me because of KERS at the beginning".[40]
Although KERS was still legal in Formula 1 in the 2010 season, all the teams had agreed not to use it.[41] New rules for the 2011 F1 season which raised the minimum weight limit of the car and driver by 20 kg to 640 kg,[42] along with the FOTA teams agreeing to the use of KERS devices once more, meant that KERS returned for the 2011 season.[43] This is still optional as it was in the 2009 season; in the 2011 season 3 teams elected not to use it.[27] For the2012 season, only Marussia andHRT raced without KERS, and by 2013, with the withdrawal of HRT, all 11 teams on the grid were running KERS.
In the2014 season, the power output of the MGU-K (The replacement of the KERS and part of the ERS system that also includes aturbochargerwaste heat recovery system) was increased from 60 kW to 120 kW and it was allowed to recover 2mega-joules per lap. This was to balance the sport's move from 2.4-litre V8 engines to 1.6-litre V6 engines.[44] The fail-safe settings of thebrake-by-wire system that now supplements KERS came under examination as a contributing factor in the fatal crash ofJules Bianchi at the2014 Japanese Grand Prix.
Bosch Motorsport Service is developing a KERS for use in motor racing. These electricity storage systems for hybrid and engine functions include alithium-ion battery with scalable capacity or aflywheel, a four to eight kilogramelectric motor (with a maximum power level of 60 kW or 80 hp), as well as the KERS controller for power and battery management. Bosch also offers a range of electric hybrid systems for commercial and light-duty applications.[45]
Automakers includingHonda have been testing KERS systems.[46] At the2008 1,000 km of Silverstone, Peugeot Sport unveiled thePeugeot 908 HY, ahybrid electric variant of the diesel 908, with KERS. Peugeot planned to campaign the car in the2009 Le Mans Series season, although it was not capable of scoring championship points.[47] Peugeot plans also a compressed air regenerative braking powertrain called Hybrid Air.[48][49]
McLaren began testing of their KERS in September 2008 at the Jerez test track in preparation for the 2009 F1 season, although at that time it was not yet known if they would be operating an electrical or mechanical system.[50] In November 2008 it was announced thatFreescale Semiconductor would collaborate withMcLaren Electronic Systems to further develop its KERS for McLaren's Formula One car from 2010 onwards. Both parties believed this collaboration would improve McLaren's KERS system and help the system filter down to road car technology.[51]
Toyota has used asupercapacitor for regeneration on aSupra HV-R hybrid race car that won theTokachi 24 Hours race in July 2007.[52]
BMW has used regenerative braking on their E90 3 Series as well as in current models like F25 5 Series under the EfficientDynamics moniker.[53] Volkswagen have regenerative braking technologies under theBlueMotion brand in such models as theVolkswagen Golf Mk7 and Mk7 Golf Estate / Wagon models, other VW group brands likeSEAT,Skoda andAudi.[54]
KTM racing bossHarald Bartol has revealed that the factory raced with a secret kinetic energy recovery system (KERS) fitted toTommy Koyama's motorcycle during the 2008 season-ending 125ccValencian Grand Prix. This was against the rules, so they were banned from doing it afterwards.[55]
Automobile Club de l'Ouest, the organizer behind the annual24 Hours of Le Mans event and theLe Mans Series, was "studying specific rules forLMP1 that will be equipped with a kinetic energy recovery system" in 2007.[56] Peugeot was the first manufacturer to unveil a fully functioning LMP1 car in the form of the 908 HY at the 2008 Autosport 1000 km race at Silverstone.[57]
On electric bicycles, regenerative braking can be used in principle. However, as of 2024 it is rarely used on bicycles, mainly because it requires a direct-drivehub motor (while many bicycles use a mid-drive motor which drives the chain), and because it cannot be combined with afreewheel mechanism. Also, the amount of energy regenerated is typically too low to be worthwhile.[58]
Regenerative braking is also possible on a non-electric bicycle. TheUnited States Environmental Protection Agency, working with students from theUniversity of Michigan, developed thehydraulic Regenerative Brake Launch Assist (RBLA).[59]
Many hybrid electric and fully electric vehicles employ regenerative braking in conjunction with friction braking,[60] Regenerative braking systems (RBS) are not able to fully emulate conventional brake function for drivers, but there are continuing advancements.[61] The calibrations used to determine when energy will be regenerated and when friction braking is used to slow down the vehicle affects the way the driver feels the braking action.[62][63]
The RBS is a key mechanism for electric vehicles to obtain braking energy. This technology seriously affects the economy, emissions, safety and other functions of electric vehicles. By improving the RBS, the kinetic energy recovery rate of the vehicle can be significantly increased, and the driving stability of the vehicle can be improved.[64]
Power consumption is reduced by regenerative braking on streetcars (AE) ortrams (CE) inOranjestad, Aruba. Designed and built by TIG/m Modern Street Railways inChatsworth, USA,[65] the vehicles usehybrid/electric technology: they do not take their power from external sources such as overhead wires when running but are self-powered bylithium batteries augmented byhydrogenfuel cells.[66]
The energy of a flywheel can be described by this general energy equation, assuming the flywheel is the system:
where
An assumption is made that during braking there is no change in the potential energy,enthalpy of the flywheel, pressure or volume of the flywheel, so only kinetic energy will be considered. As the car is braking, no energy is dispersed by the flywheel, and the only energy into the flywheel is the initial kinetic energy of the car. The equation can be simplified to:
where
The flywheel collects a percentage of the initial kinetic energy of the car, and this percentage can be represented by. The flywheel stores the energy as rotational kinetic energy. Because the energy is kept as kinetic energy and not transformed into another type of energy this process is efficient. The flywheel can only store so much energy, however, and this is limited by its maximum amount of rotational kinetic energy. This is determined based upon the inertia of the flywheel and itsangular velocity. As the car sits idle, little rotational kinetic energy is lost over time so the initial amount of energy in the flywheel can be assumed to equal the final amount of energy distributed by the flywheel. The amount of kinetic energy distributed by the flywheel is therefore:
Regenerative braking has a similar energy equation to the equation for the mechanical flywheel. Regenerative braking is a two-step process involving the motor/generator and the battery. The initial kinetic energy is transformed into electrical energy by the generator and is then converted into chemical energy by the battery. This process is less efficient than the flywheel. The efficiency of the generator can be represented by:
where
The only work into the generator is the initial kinetic energy of the car and the only work produced by the generator is the electrical energy. Rearranging this equation to solve for the power produced by the generator gives this equation:
where
The efficiency of the battery can be described as:
where
The work out of the battery represents the amount of energy produced by the regenerative brakes. This can be represented by:
A diagram by theUnited States Department of Energy (DoE) shows cars with internal combustion engines as having efficiency of typically 13% in urban driving, 20% in highway conditions. Braking in proportion to the useful mechanic energy amounts to 6/13 i.e. 46% in towns, and 2/20 i.e. 10% on motorways.
The DoE states that electric cars convert over 77% of the electrical energy from the grid to power at the wheels.[67] The efficiency of an electric vehicle, taking into account losses due to the electric network, heating, and air conditioning is about 50% according toJean-Marc Jancovici[68] (however for the overall conversion seeEmbodied energy#Embodied energy in the energy field).
Consider the electric motor efficiency and the braking proportion in towns and on motorways.
Let us introduce which is the recuperated proportion of braking energy. Let us assume.[69]
Under these circumstances, being the energy flux arriving at the electric engine, the energy flux lost while braking and the recuperated energy flux, an equilibrium is reached according to the equations
and
thus
It is as though the old energy flux was replaced by a new one
The expected gain amounts to
The higher the recuperation efficiency, the higher the recuperation.
The higher the efficiency between the electric motor and the wheels, the higher the recuperation.
The higher the braking proportion, the higher the recuperation.
On motorways, this figure would be 3%, and in cities it would amount to 14%.
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