Regenerative Brake
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Regenerative braking is an
energy recovery Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but mos ...
mechanism that slows down a moving vehicle or object by converting its
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
into a form that can be either used immediately or stored until needed. In this mechanism, the electric
traction motor A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric vehicle, electric or hydrogen vehicles, elevators or electric multiple unit. Traction motors are used in electrically powered rail vehicles (ele ...
uses the vehicle's momentum to recover energy that would otherwise be lost to the brake discs as heat. This method contrasts with conventional braking systems. In those systems, the excess kinetic energy is converted to unwanted and wasted heat due to friction in the
brake A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. Background ...
s, or with
rheostatic brake Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electric locomotive. It is termed " rheostatic" if the generated electrical power is dissipated as heat in brake grid ...
s, where the energy is recovered by using electric motors as generators but is immediately dissipated as heat in resistors. In addition to improving the overall efficiency of the vehicle, regeneration can significantly extend the life of the braking system as the mechanical parts will not wear out quickly.


General principle

The most common form of regenerative brake involves an
electric motor An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
functioning as an electric generator. In electric
railways Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a pre ...
, the electricity generated is fed back into the traction power supply. In battery electric and
hybrid electric A hybrid electric vehicle (HEV) is a type of hybrid vehicle that combines a conventional internal combustion engine (ICE) system with an electric propulsion system (hybrid vehicle drivetrain). The presence of the electric powertrain is intended ...
vehicles, the energy is stored chemically in a
battery Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...
, electrically in a bank of
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
s, or mechanically in a rotating
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...
.
Hydraulic hybrid Hydraulic hybrid vehicles (HHVs) use a pressurized fluid power source, along with a conventional internal combustion engine (ICE), to achieve better fuel economy in automobiles, fuel economy and reductions in vehicle emissions control, harmful emis ...
vehicles use hydraulic motors to store energy in the form of
compressed air Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air is an important medium for transfer of energy in industrial processes, and is used for power tools such as air hammers, drills, wrenches, and o ...
. In a hydrogen
fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
powered vehicle, the electrical energy generated by the motor is stored chemically in a battery, similar to battery- and hybrid electric vehicles.


Practical regenerative braking

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 as
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
-based braking. * The regenerative braking effect drops off at lower speeds and cannot bring a vehicle to a complete halt reasonably quickly with current technology. However, some cars like the
Chevrolet Bolt The Chevrolet Bolt or Chevrolet Bolt EV is a battery electric subcompact hatchback produced by General Motors under the Chevrolet marque. It was developed and manufactured in partnership with LG Corporation. A rebadged European variant was ...
can bring the vehicle to a complete stop on even surfaces when the driver knows the vehicle's regenerative braking distance. This is referred to as One Pedal Driving. * Current regenerative brakes do not immobilize a stationary vehicle; physical locking is required, for example to prevent vehicles from rolling down hills. * Many road vehicles with regenerative braking do not have drive motors on all wheels (as in a
two-wheel drive Two-wheel-drive (2WD) denotes Motor vehicle, vehicles with a Powertrain, drivetrain that allows two wheels to be driven, and receive Engine power, power and torque from the engine, simultaneously. Four-wheeled vehicles For four-wheeled vehicles ( ...
car); regenerative braking is normally only applicable to wheels with motors. For safety, the ability to brake all wheels is required. * The regenerative braking effect available is limited, and mechanical braking is still necessary for substantial speed reductions, to bring a vehicle to a stop, or to hold a vehicle at a standstill. Regenerative and friction braking must both be used, creating the need to control them to produce the required total braking. The GM EV-1 was the first commercial car to do this. In 1997 and 1998 engineers Abraham Farag and Loren Majersik were issued two patents for this ''
brake-by-wire In the automotive industry, brake-by-wire technology is the ability to control brakes through electrical means. It can be designed to supplement ordinary service brakes or it can be a standalone brake system. This technology is widely used on al ...
'' technology. 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 the runaway train accident in Wädenswil, Switzerland in 1948, which killed twenty-one people.


Conversion to electric energy: the motor as a generator

Electric motor An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
s, when used in reverse, function as generators and will then convert mechanical energy into electrical energy. Vehicles propelled by electric motors use them as generators when using regenerative braking, braking by transferring mechanical energy from the wheels to an electrical load. Early examples of this system were the
front-wheel drive Front-wheel drive (FWD) is a form of engine and transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel drive vehicles feature a transverse engine, rather than the conventional longit ...
conversions of horse-drawn
cabs C mathematical operations are a group of functions in the standard library of the C programming language implementing basic mathematical functions. All functions use floating-point numbers in one manner or another. Different C standards provide d ...
by Louis Antoine Krieger in Paris in the 1890s. The Krieger electric
landaulet Landaulet may refer to: * Landaulet (carriage), horse-drawn carriage * Landaulet (car) A landaulet, also known as a landaulette, is a car body style where the rear passengers are covered by a convertible top. Often the driver is separated from t ...
had a drive motor in each front wheel with a second set of parallel windings (
bifilar coil A bifilar coil is an electromagnetic coil that contains two closely spaced, parallel windings. In engineering, the word ''bifilar'' describes wire which is made of two filaments or strands. It is commonly used to denote special types of winding ...
) for regenerative braking. The Orwell Electric Truck introduced by
Ransomes, Sims & Jefferies Ransomes, Sims and Jefferies Limited was a major British agricultural machinery maker also producing a wide range of general engineering products in Ipswich, Suffolk including traction engines, trolleybuses, ploughs, lawn mowers, combine harves ...
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 as explained in some detail by his son
Alfred Raworth The Southern Railway (SR), sometimes shortened to 'Southern', was a British railway company established in the 1923 Grouping. It linked London with the Channel ports, South West England, South coast resorts and Kent. The railway was formed by ...
. These included tramway systems at Devonport (1903),
Rawtenstall Rawtenstall () is a town in the borough of Rossendale, Lancashire, England. The town lies 15 miles/24 km north of Manchester, 22 miles/35 km east of Preston and 45 miles/70 km south east of the county town of Lancaster. The town is at the cen ...
,
Birmingham Birmingham ( ) is a city and metropolitan borough in the metropolitan county of West Midlands in England. It is the second-largest city in the United Kingdom with a population of 1.145 million in the city proper, 2.92 million in the West ...
, 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. Following a serious accident at Rawtenstall, an embargo was placed on this form of traction in 1911; the regenerative braking system was reintroduced twenty years later. Regenerative braking has been in extensive use on railways for many decades. The Baku-Tbilisi-Batumi railway (
Transcaucasus Railway The Transcaucus Railway (russian: Закавка́зская желе́зная доро́га) was the first railway in South Caucasus.Большая Советская Энциклопедия. Гл. ред. Б. А. Введенский, 2-е ...
or Georgian railway) started utilizing regenerative braking in the early 1930s. This was especially effective on the steep and dangerous
Surami Pass The Surami Pass ( ka, სურამის უღელტეხილი) is a mountain pass in the Likhi Range of Georgia with an altitude of . The pass connects the western and eastern part of Georgia. A railroad (in a tunnel) runs throug ...
. In Scandinavia the Kiruna to Narvik electrified railway, known as
Malmbanan The Iron Ore Line ( sv, Malmbanan) is a long railway line between Riksgränsen and Boden in Norrbotten County, Sweden, owned by Trafikverket (the Swedish Transport Administration). The line also contains two branches, from Kiruna to Svappavaar ...
on the Swedish side and
Ofoten Line The Ofoten Line ( no, Ofotbanen) is a railway line in Narvik, Norway. It runs from the Port of Narvik to Riksgränsen on the Norway–Sweden border, where the line continues as the Ore Line via Kiruna and Gällivare to Luleå. The Ofoten Lin ...
on the Norwegian, carries iron ore on the steeply-graded route from the mines in
Kiruna (; se, Giron ; fi, Kiiruna ) is the northernmost Stad (Sweden), city in Sweden, situated in the province of Lapland, Sweden, Lapland. It had 17,002 inhabitants in 2016 and is the seat of Kiruna Municipality (population: 23,167 in 2016) in Norr ...
, in the north of Sweden, down to the port of
Narvik ( se, Áhkanjárga) is the third-largest municipality in Nordland county, Norway, by population. The administrative centre of the municipality is the town of Narvik. Some of the notable villages in the municipality include Ankenesstranda, Ball ...
in Norway to this day. The rail cars are full of thousands of tons of
iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
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. From
Riksgränsen Riksgränsen, ( no, Riksgrensen), ''The National Border'' in Swedish, is a ski-resort in Kiruna Municipality, Lappland, Sweden, 200 km north of the Arctic Circle. The skiing season is from February to June. From end of May the lifts operate ...
on the national border to the Port of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is sufficient to power the empty trains back up to the national border. 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. Electric cars used regenerative braking since the earliest experiments, but this was often a complex affair where the driver had to flip switches between various operational modes in order to use it. The Baker Electric Runabout and the
Owen Magnetic 1920 Owen Magnetic Touring Car ad, from '' House Beautiful'' magazine The Owen Magnetic was a pioneering American brand of hybrid electric luxury automobile manufactured between 1915 and 1922. Car models of the brand were notable for their use of ...
were early examples, which used many switches and modes controlled by an expensive "black box" or "drum switch" as part of their electrical system. 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's
AMC Amitron The AMC Amitron was an experimental electric subcompact car built in 1967 by American Motors Corporation (AMC) and Gulton Industries. It included many advanced features, including regenerative braking and advanced battery designs, to provide ...
experimental electric car. Designed by Gulton Industries 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 , commonly referred to as simply Mazda, is a Japanese multinational automotive manufacturer headquartered in Fuchū, Hiroshima, Japan. In 2015, Mazda produced 1.5 million vehicles for global sales, the majority of which (nearly one m ...
uses this system in some current (2018) road cars, where it is branded i-ELOOP.


Electric railway vehicle operation

In 1886 the Sprague Electric Railway & Motor Company, founded by
Frank J. Sprague Frank Julian Sprague (July 25, 1857 in Milford, Connecticut – October 25, 1934) was an American inventor who contributed to the development of the electric motor, electric railways, and electric elevators. His contributions were especially ...
, introduced two important inventions: a constant-speed, non-sparking motor with fixed brushes, and regenerative braking. During braking, the
traction motor A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric vehicle, electric or hydrogen vehicles, elevators or electric multiple unit. Traction motors are used in electrically powered rail vehicles (ele ...
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 Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electric locomotive. It is termed " rheostatic" if the generated electrical power is dissipated as heat in brake grid r ...
) 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 of
wheel slide protection Wheel slide protection and wheel slip protection are railway terms used to describe automatic systems used to detect and prevent wheel-slide during braking or wheel-slip during acceleration. This is analogous to ABS and traction control systems us ...
. For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
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 for
British Rail Class 390 The British Rail Class 390 ''Pendolino'' is a type of electric high-speed passenger train operated by Avanti West Coast in the United Kingdom, leased from Angel Trains. They are electric multiple units using Fiat Ferroviaria's tilting train ...
s. The
Delhi Metro The Delhi Metro is a mass rapid transit (MRT) system serving Delhi and its satellite cities of Ghaziabad, Faridabad, Gurgaon, Noida, Bahadurgarh and Ballabhgarh, in the National Capital Region of India. The network consists of 10 colour-cod ...
reduced the amount of
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
() 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 per year once its phase II was complete, through the use of regenerative braking. 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 for
head end power In rail transport, head-end power (HEP), also known as electric train supply (ETS), is the electrical power distribution system on a passenger train. The power source, usually a locomotive (or a generator car) at the front or 'head' of a train, p ...
loads, or stored in lineside storage systems for later use. A form of what can be described as regenerative braking is used on some parts of the
London Underground The London Underground (also known simply as the Underground or by its nickname the Tube) is a rapid transit system serving Greater London and some parts of the adjacent ceremonial counties of England, counties of Buckinghamshire, Essex and He ...
, 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 to
gravitational potential energy Gravitational energy or gravitational potential energy is the potential energy a massive object has in relation to another massive object due to gravity. It is the potential energy associated with the gravitational field, which is released (conv ...
in the station. This is normally found on the deep tunnel sections of the network and not generally above ground or on the
cut and cover A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube cons ...
sections of the Metropolitan and District Lines.


Comparison of dynamic and regenerative brakes

What are described as dynamic brakes ("
rheostatic brake Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electric locomotive. It is termed " rheostatic" if the generated electrical power is dissipated as heat in brake grid ...
s" 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 of
resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
s. Vehicles that use dynamic brakes include
forklift truck A forklift (also called lift truck, jitney, hi-lo, fork truck, fork hoist, and forklift truck) is a powered industrial truck used to lift and move materials over short distances. The forklift was developed in the early 20th century by various ...
s, diesel-electric
locomotive A locomotive or engine is a rail transport vehicle that provides the Power (physics), motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, Motor coach (rail), motor ...
s, and
tram A tram (called a streetcar or trolley in North America) is a rail vehicle that travels on tramway tracks on public urban streets; some include segments on segregated right-of-way. The tramlines or networks operated as public transport are ...
s. This heat can be used to warm the vehicle interior, or dissipated externally by large
radiator Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...
-like cowls to house the resistor banks. General Electric's experimental 1936
steam turbine locomotives Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. ...
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. 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. The supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors). In areas where there is a constant need for power unrelated to moving the vehicle, such as electric train heat or
air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
, this load requirement can be utilized as a sink for the recovered energy via modern AC traction systems. This method has become popular with North American passenger railroads where
head end power In rail transport, head-end power (HEP), also known as electric train supply (ETS), is the electrical power distribution system on a passenger train. The power source, usually a locomotive (or a generator car) at the front or 'head' of a train, p ...
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 the ALP-46 and
ACS-64 The Siemens ACS-64, or Amtrak Cities Sprinter, is an electric locomotive designed by Siemens Mobility for use on the Northeast Corridor (NEC) and the Keystone Corridor in the northeastern United States. The design was based on locomotives Sie ...
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 of steep grade railways have used 3-phase power supplies and
induction motor An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction mot ...
s. This results in a near constant speed for all trains, as the motors rotate with the supply frequency both when driving and braking.


Conversion to mechanical energy


Kinetic energy recovery systems

Kinetic energy recovery systems (KERS) were used for the motor sport
Formula One Formula One (also known as Formula 1 or F1) is the highest class of international racing for open-wheel single-seater formula racing cars sanctioned by the Fédération Internationale de l'Automobile (FIA). The World Drivers' Championship, ...
's 2009 season, and are under development for road vehicles. KERS was abandoned for the
2010 Formula One season The 2010 FIA Formula One World Championship was the 64th season of FIA Formula One motor racing. Red Bull Racing won its maiden Constructors' Championship with a 1–2 finish in 2010 Brazilian Grand Prix, Brazil, while Red Bull Racing's Se ...
, but re-introduced for the 2011 season. By
2013 File:2013 Events Collage V2.png, From left, clockwise: Edward Snowden becomes internationally famous for leaking classified NSA wiretapping information; Typhoon Haiyan kills over 6,000 in the Philippines and Southeast Asia; The Dhaka garment fact ...
, all teams were using KERS with
Marussia F1 The Marussia F1 Team (subsequently Manor Marussia F1 Team) was a Formula One racing team and constructor which was based in Banbury, Oxfordshire and then later Dinnington, South Yorkshire in the United Kingdom and competed with a Russian lice ...
starting use for the 2013 season. 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 of
ballast Ballast is material that is used to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship, ...
that is available to balance the car so that it is more predictable when turning. FIA rules also limit the exploitation of the system. The concept of transferring the vehicle's kinetic energy using
flywheel energy storage Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a c ...
was postulated by physicist
Richard Feynman Richard Phillips Feynman (; May 11, 1918 – February 15, 1988) was an American theoretical physicist, known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of the superflu ...
in the 1950s and is exemplified in such systems as the
Zytek Gibson Technology is an automotive and motorsport company based at Repton, Derbyshire, England. It was founded by Bill Gibson as "Zytek Engineering" in 1981. In 1981 Gibson founded the "Zytech Group" with two main divisions: Zytek Automotive, ...
, Flybrid, Torotrak and Xtrac used in F1. Differential based systems also exist such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS). Xtrac and Flybrid are both licensees of Torotrak's technologies, which employ a small and sophisticated ancillary gearbox incorporating a
continuously variable transmission A continuously variable transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. T ...
(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 the
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...
, drive wheel and road wheel.


Use in motor sport


History

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 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.


Formula One

Formula One Formula One (also known as Formula 1 or F1) is the highest class of international racing for open-wheel single-seater formula racing cars sanctioned by the Fédération Internationale de l'Automobile (FIA). The World Drivers' Championship, ...
have stated that they support responsible solutions to the world's environmental challenges, and the
FIA FIA is the Fédération Internationale de l'Automobile (English: International Automobile Federation), the world's governing body for all forms of motor sport where four or more wheels are used. Fia or FIA may also refer to: People * Fia Backst ...
allowed the use of KERS in the regulations for the 2009 Formula One season. Teams began testing systems in 2008: energy can either be stored as mechanical energy (as in a
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...
) or as electrical energy (as in a battery or supercapacitor). Two minor incidents were reported during testing of KERS systems in . The first occurred when the
Red Bull Racing Red Bull Racing, also simply known as Red Bull or RBR and currently competing as Oracle Red Bull Racing, is a Formula One racing team, racing under an Austrian licence and based in the United Kingdom. It is one of two Formula One teams owned ...
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. The second was less than a week later when a
BMW Sauber BMW has been involved in Formula One in a number of capacities since the inauguration of the World Drivers' Championship in . The company entered occasional races in the 1950s and 1960s (often under Formula Two regulations), before building ...
mechanic was given an electric shock when he touched
Christian Klien Christian Klien (born 7 February 1983) is an Austrian former Formula One racing driver. In total he has scored 14 points in Formula One. Klien currently competes full-time in the GT World Challenge Europe for JP Motorsport. Racing career Bor ...
's KERS-equipped car during a test at the Jerez circuit. With the introduction of KERS in the 2009 season, four teams used it at some point in the season:
Ferrari Ferrari S.p.A. (; ) is an Italian luxury sports car manufacturer based in Maranello, Italy. Founded by Enzo Ferrari (1898–1988) in 1939 from the Alfa Romeo racing division as ''Auto Avio Costruzioni'', the company built its first car in ...
,
Renault Groupe Renault ( , , , also known as the Renault Group in English; legally Renault S.A.) is a French multinational automobile manufacturer established in 1899. The company produces a range of cars and vans, and in the past has manufactured ...
, BMW, and
McLaren McLaren Racing Limited is a British motor racing team based at the McLaren Technology Centre in Woking, Surrey, England. McLaren is best known as a Formula One constructor, the second oldest active team, and the second most successful Formul ...
. During the season, Renault and BMW stopped using the system.
McLaren Mercedes McLaren Racing Limited is a British motor racing team based at the McLaren Technology Centre in Woking, Surrey, England. McLaren is best known as a Formula One constructor, the second oldest active team, and the second most successful Formul ...
became the first team to win a F1 GP using a KERS equipped car when
Lewis Hamilton Sir Lewis Carl Davidson Hamilton (born 7 January 1985) is a British racing driver currently competing in Formula One for Mercedes. In Formula One, Hamilton has won a joint-record seven World Drivers' Championship titles (tied with Mic ...
won the 2009 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 Heikki Johannes Kovalainen (; born 19 October 1981) is a Finnish racing driver competing in the Japan Rally Championship for Rally Team AICELLO. He raced in Formula One between 2007 and 2013 for the Renault, McLaren, Team Lotus, Caterham and ...
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 placed Giancarlo Fisichella claiming "Actually, I was quicker than Kimi. He only took me because of KERS at the beginning". Although KERS was still legal in Formula 1 in the 2010 season, all the teams had agreed not to use it. New rules for the 2011 F1 season which raised the minimum weight limit of the car and driver by 20 kg to 640 kg, along with the FOTA teams agreeing to the use of KERS devices once more, meant that KERS returned for the 2011 season. This is still optional as it was in the 2009 season; in the 2011 season 3 teams elected not to use it. For the 2012 season, only Marussia and HRT raced without KERS, and by 2013, with the withdrawal of HRT, all 11 teams on the grid were running KERS. In the 2014 season, the power output of the MGU-K (The replacement of the KERS and part of the ERS system that also includes a
turbocharger In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...
waste heat recovery A waste heat recovery unit (WHRU) is an energy recovery heat exchanger that transfers heat from process outputs at high temperature to another part of the process for some purpose, usually increased efficiency. The WHRU is a tool involved in cogen ...
system) was increased from 60 kW to 120 kW and it was allowed to recover 2
mega- Mega is a metric prefix, unit prefix in metric systems of units denoting a factor of one million (106 or 1000000 (number), ). It has the unit symbol M. It was confirmed for use in the International System of Units (SI) in 1960. ''Mega'' comes fro ...
joules The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied. ...
per lap. This was to balance the sport's move from 2.4-litre V8 engines to 1.6-litre V6 engines. The fail-safe settings of the
brake-by-wire In the automotive industry, brake-by-wire technology is the ability to control brakes through electrical means. It can be designed to supplement ordinary service brakes or it can be a standalone brake system. This technology is widely used on al ...
system that now supplements KERS came under examination as a contributing factor in the fatal crash of
Jules Bianchi Jules Lucien André Bianchi (; 3 August 1989 – 17 July 2015) was a French motor racing driver who drove for the Marussia F1 Team in the FIA Formula One World Championship. Bianchi had previously raced in Formula Renault 3.5, GP2 and Form ...
at the 2014 Japanese Grand Prix.


Autopart makers

Bosch Motorsport Service is developing a KERS for use in motor racing. These electricity storage systems for hybrid and engine functions include a
lithium-ion battery A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
with scalable capacity or a
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...
, a four to eight kilogram
electric motor An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
(with a maximum power level of ), 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.


Car makers

Automakers including
Honda is a Japanese public multinational conglomerate manufacturer of automobiles, motorcycles, and power equipment, headquartered in Minato, Tokyo, Japan. Honda has been the world's largest motorcycle manufacturer since 1959, reaching a product ...
have been testing KERS systems. At the 2008 1,000 km of Silverstone, Peugeot Sport unveiled the Peugeot 908 HY, a
hybrid electric A hybrid electric vehicle (HEV) is a type of hybrid vehicle that combines a conventional internal combustion engine (ICE) system with an electric propulsion system (hybrid vehicle drivetrain). The presence of the electric powertrain is intended ...
variant of the diesel 908, with KERS. Peugeot planned to campaign the car in the 2009 Le Mans Series season, although it was not capable of scoring championship points. Peugeot plans also a compressed air regenerative braking powertrain called Hybrid Air.
McLaren McLaren Racing Limited is a British motor racing team based at the McLaren Technology Centre in Woking, Surrey, England. McLaren is best known as a Formula One constructor, the second oldest active team, and the second most successful Formul ...
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. In November 2008 it was announced that
Freescale Semiconductor Freescale Semiconductor, Inc. was an American semiconductor manufacturer. It was created by the divestiture of the Semiconductor Products Sector of Motorola in 2004. Freescale focused their integrated circuit products on the automotive, embe ...
would collaborate with
McLaren Electronic Systems McLaren Applied Limited is a British technology company that works in conjunction with companies such as GSK, NHS and more. Its electronic division, McLaren Electronics, manufactures parts such as engine temperature and pressure sensors for F ...
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.
Toyota is a Japanese multinational automotive manufacturer headquartered in Toyota City, Aichi, Japan. It was founded by Kiichiro Toyoda and incorporated on . Toyota is one of the largest automobile manufacturers in the world, producing about 10 ...
has used a supercapacitor for regeneration on a
Supra Supra may refer to: * Supra (feast), an important part of Georgian social culture *Toyota Supra, a sports car/grand tourer produced from 1978 to 2002, and then since 2019 *Supra (footwear brand) *Sydney University Postgraduate Representative Assoc ...
HV-R hybrid race car that won the
Tokachi 24 Hours The Tokachi 24 Hours (十勝24時間レース) was an endurance race held at the Tokachi International Speedway Tokachi International Speedway (十勝インターナショナルスピードウェイ) is a motor racing circuit in Takikubo, Sar ...
race in July 2007. BMW has used regenerative braking on their E90 3 Series as well as in current models like F25 5 Series under the EfficientDynamics moniker. Volkswagen have regenerative braking technologies under the
BlueMotion BlueMotion is a trade name for certain car models from the Volkswagen Group with an emphasis on higher fuel efficiency. Volkswagen introduced the name in 2006 on the Polo Mk4 BlueMotion, and in 2007 a version based on the current Passat was ...
brand in such models as the
Volkswagen Golf Mk7 The Volkswagen Golf (Mk7) is a small family car ( C-segment) produced by German automobile manufacturer Volkswagen, as the seventh generation of the Golf and the successor to the Golf Mk6. It was introduced in Berlin on 4 September 2012, before ...
and Mk7 Golf Estate / Wagon models, other VW group brands like
SEAT A seat is a place to sit. The term may encompass additional features, such as back, armrest, head restraint but also headquarters in a wider sense. Types of seat The following are examples of different kinds of seat: * Armchair (furniture), ...
,
Skoda Škoda means ''pity'' in the Czech and Slovak languages. It may also refer to: Czech brands and enterprises * Škoda Auto, automobile and previously bicycle manufacturer in Mladá Boleslav ** Škoda Motorsport, the division of Škoda Auto respons ...
and
Audi Audi AG () is a German automotive manufacturer of luxury vehicles headquartered in Ingolstadt, Bavaria, Germany. As a subsidiary of its parent company, the Volkswagen Group, Audi produces vehicles in nine production facilities worldwide. Th ...
.


Motorcycles

KTM KTM AG (Kraftfahrzeug Trunkenpolz Mattighofen, formerly KTM Sportmotorcycle AG) is an Austrian motorcycle, bicycle and sports car manufacturer owned by Pierer Mobility AG and the Indian manufacturer Bajaj Auto. It was formed in 1992 but traces ...
racing boss Harald Bartol has revealed that the factory raced with a secret kinetic energy recovery system (KERS) fitted to Tommy Koyama's motorcycle during the 2008 season-ending 125cc Valencian Grand Prix. This was against the rules, so they were banned from doing it afterwards.


Races

Automobile Club de l'Ouest The Automobile Club de l'Ouest (English: Automobile Club of the West), sometimes abbreviated to ACO, is the largest automotive group in France. It was founded in 1906 by car building and racing enthusiasts, and is most famous for being the orga ...
, the organizer behind the annual
24 Hours of Le Mans The 24 Hours of Le Mans (french: link=no, 24 Heures du Mans) is an endurance-focused Sports car racing, sports car race held annually near the town of Le Mans, France. It is the world's oldest active Endurance racing (motorsport), endurance r ...
event and the
Le Mans Series The European Le Mans Series (abbreviated as ELMS) is a European sports car racing endurance series inspired by the 24 Hours of Le Mans race and organized by the Automobile Club de l'Ouest (ACO). The European Le Mans Series is similar to the fo ...
, was "studying specific rules for
LMP1 A Le Mans Prototype (LMP) is the type of sports prototype race car used in the 24 Hours of Le Mans, FIA World Endurance Championship, IMSA SportsCar Championship, European Le Mans Series and Asian Le Mans Series. Le Mans Prototypes were crea ...
that will be equipped with a kinetic energy recovery system" in 2007. 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.


Use in civilian transport


Bicycles

Regenerative braking is also possible on a non-electric bicycle. The
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
, working with students from the
University of Michigan , mottoeng = "Arts, Knowledge, Truth" , former_names = Catholepistemiad, or University of Michigania (1817–1821) , budget = $10.3 billion (2021) , endowment = $17 billion (2021)As o ...
, developed the hydraulic Regenerative Brake Launch Assist (RBLA). It is available on electric bicycles with direct-drive
hub motor The wheel hub motor (also called wheel motor, wheel hub drive, hub motor or in-wheel motor) is an electric motor that is incorporated into the hub of a wheel and drives it directly. History * First wheel motor concept: Wellington Adams of ...
s.


Cars

Many hybrid electric and fully electric vehicles employ regenerative braking in conjunction with friction braking, first used in the US by the 1967 AMC Amitron electric concept car. Regenerative braking systems are not able to fully emulate conventional brake function for drivers, but there are continuing advancements. 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.


Thermodynamics


KERS flywheel

The energy of a flywheel can be described by this general energy equation, assuming the flywheel is the system: : E_\text-E_\text= \Delta E_\text where :*E_\text is the energy into the flywheel. :*E_\text is the energy out of the flywheel. :*\Delta E_\text is the change in energy of the flywheel. An assumption is made that during braking there is no change in the potential energy,
enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
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: :\frac = \Delta E_\text where :*m is the mass of the car. :*v is the initial velocity of the car just before braking. The flywheel collects a percentage of the initial kinetic energy of the car, and this percentage can be represented by \eta_\text. 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 its
angular velocity In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an objec ...
. 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: :KE_\text=\frac


Regenerative brakes

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: : \eta_\text=\frac where :*W_\text is the work into the generator. :*W_\text is the work produced by the generator. 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: :P_\text= \frac where :*\Delta t is the amount of time the car brakes. :*m is the mass of the car. :*v is the initial velocity of the car just before braking. The efficiency of the battery can be described as: : \eta_\text=\frac where :*P_\text=P_\text :*P_\text=\frac The work out of the battery represents the amount of energy produced by the regenerative brakes. This can be represented by: : W_\text=\frac


In cars

A diagram by the
United States Department of Energy The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and manages the research and development of nuclear power and nuclear weapons in the United Stat ...
(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. The efficiency of an electric vehicle, taking into account losses due to the electric network, heating, and air conditioning is about 50% according to
Jean-Marc Jancovici Jean-Marc Jancovici (born 1962) is a French engineering consultant, energy and climate expert, professor, conference speaker, writer, and independent columnist. He is co-founder and associate at the Carbone 4 consultancy firm, and the founding p ...
(however for the overall conversion see Embodied energy#Embodied energy in the energy field). Consider the electric motor efficiency \eta_\text=0.5 and the braking proportion in towns p=0.46 and on motorways p=0.1. Let us introduce \eta_\text which is the recuperated proportion of braking energy. Let us assume \eta_\text=0.6. Under these circumstances, E being the energy flux arriving at the electric engine, E_\text the energy flux lost while braking and E_\text the recuperated energy flux, an equilibrium is reached according to the equations E_\text=(E + E_\text) \cdot \eta_\text \cdot p and E_\text=\eta_\text \cdot E_\text thus E_\text=\frac It is as though the old energy flux E was replaced by a new one E \cdot (1-\eta_\text \cdot p \cdot \eta_\text) The expected gain amounts to \eta_\text \cdot p \cdot \eta_\text 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%.


See also

* Kinetic energy recovery system *
Brake (railway) A railway brake is a type of brake used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them immobile when parked. While the basic principle is similar to that on road vehicle usage, operational ...
*
Electromagnetic brake Electromagnetic brakes (also called electro-mechanical brakes or EM brakes) slow or stop motion using electromagnetic force to apply mechanical resistance (friction). They were originally called "electro-mechanical brakes," but over the years the ...
*
Regenerative (design) Regenerative design is a process-oriented whole systems approach to design. The term "regenerative" describes processes that restore, renew or revitalize their own sources of energy and materials. Regenerative design uses whole systems thinking ...
* Regenerative shock absorber *
Hybrid Synergy Drive Hybrid Synergy Drive (HSD), also known as Toyota Hybrid System II, is the brand name of Toyota Motor Corporation for the hybrid car drive train technology used in vehicles with the Toyota and Lexus marques. First introduced on the Toyota Prius, ...
*
Ram air turbine A ram air turbine (RAT) is a small wind turbine that is connected to a hydraulic pump, or electrical generator, installed in an aircraft and used as a power source. The RAT generates power from the airstream by ram pressure due to the speed o ...
*
Dynamic braking Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electric locomotive. It is termed " rheostatic" if the generated electrical power is dissipated as heat in brake grid r ...
*
Electric motor An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
*
Self-winding watch An automatic watch, also known as a self-winding watch or simply an automatic, is a mechanical watch where the natural motion of the wearer provides energy to wind the mainspring, making manual winding unnecessary if worn enough. It is distingui ...
* Brake run *
Electromagnetic clutch Electromagnetic clutches operate electrically but transmit torque mechanically. This is why they used to be referred to as electro-mechanical clutches. Over the years, EM became known as electromagnetic versus electro-mechanical, referring more a ...
*
Eddy current brake An eddy current brake, also known as an induction brake, electric brake or electric retarder, is a device used to slow or stop a moving object by generating eddy currents and thus dissipating its kinetic energy as heat. Unlike friction brakes, wh ...


References

{{Railway brakes Dynamic braking Electric vehicle technologies Energy recovery Railway brakes