Motor
The rise of regenerative braking in motorsport
by Steve Rendle
8min read
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Formula 1 cars have used regenerative braking technology since 2009 and now multiple motorsport disciplines deploy similar systems, from sportscar racing to IndyCar.
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The first racing cars to feature regenerative brakes were essentially prototypes to evaluate the efficacy of hybrids in motorsport, but advancements in battery technology meant that they would soon become the norm on the road and the racetrack.
The experimental AMC Ambition debuted regenerative braking on the road in 1967 but it was in the Toyota Prius where this technology was commercialised. Currently, the majority of hybrid and electric road cars use this system.
Here’s how regenerative braking revolutionised motorsport powertrains.
A diagram showing regenerative braking feeding energy back into the battery of the Aston Martin Aramco Formula 1 Team AMR24
What is regenerative braking?
Regenerative braking essentially involves converting the kinetic energy of a car’s motion into electrical energy during braking. That electrical energy can then be temporarily stored in a battery before it is released and ‘regenerated’ back into kinetic energy to power the car.
This process takes place via what is most commonly termed a Motor Generator Unit (MGU), which acts as part of an Energy Recovery System (ERS), although in the early days, the system was termed a Kinetic Energy Recovery System (KERS).
During harvesting of energy, under braking the MGU acts as a generator, transforming the rotational kinetic energy of the engine crankshaft into electrical energy, which is transmitted to the battery (also known as an Energy Store, or ES).
When a power boost is required, the electrical energy stored in the battery can be supplied to the MGU, which then operates as an electric motor, applying additional torque to the engine’s crankshaft, effectively increasing the power.
<div>The Panoz Q9 GTR-1 Hybrid pioneered regenerative braking in motorsport when it debuted in 1998. Image courtesy of Robert Dalaudiere<br></div>
The first regenerative braking systems in motorsport
One of the first cars to use regenerative braking in a motorsport context, in the form in which we know it today, was the 1998 Panoz Q9 GTR-1 Hybrid, an endurance racing car that won its class in the inaugural 1998 Petit Le Mans race.
This innovative car used a six-litre V8 internal combustion engine, coupled to a 110kW MGU, to feed energy harvested during braking to a battery pack. Although pioneering in terms of technology, the car’s Achilles’ heel was the weight of the battery pack, which added around 200kg to the overall weight of the car, negating the extra power produced by the hybrid component.
Panoz estimates that advances in battery technology have made modern hybrid racing power units around 75% lighter than the one on its Q9 Hybrid car.
In reality, rather than increasing power, the idea behind the use of the MGU on the Panoz was to enable the car to use less fuel, and therefore to make fewer pitstops, increasing its competitiveness.
Various other sports and GT cars continued to use regenerative braking, but the 2012 Toyota TS030 LMP1 car was perhaps the first car so equipped to enjoy major success, winning three World Endurance Championship (WEC) races in 2012, and another two in 2013.
Hybrid power, along with regenerative braking, became the norm for the LMP1 and later Hypercar classes in the WEC series, and was also introduced to Formula 1 from the dawn of the hybrid power unit era at the start of the 2014 season.
The autumn of 2014 also saw the arrival of the Formula E series, with all-electric power and regenerative braking as part of the technical package from the outset. Extreme E - the off-road electric racing series, featuring SUV-style cars with regenerative braking - began in 2021.
The Toyota TS030 LMP1 car was one of the first hybrid cars to find success in competitive motorsport
KERS in Formula 1
The early days of the Kinetic Energy Recovery System (KERS), which was first introduced to F1 in 2009, saw an MGU which was gear-driven from the front of the engine crankshaft, and recovered energy which, with a fully charged battery, could provide up to 80bhp extra power, available to the driver though the push of a button on the steering wheel. The energy use was regulated via the car’s FIA standard ECU, and could be used for a maximum of 6.67 seconds per lap. The energy-usage strategy was down to the driver, and the additional power could be used in one go, or could be used in several smaller ‘chunks’ during each lap.
This early system was used by most teams primarily to provide an extra power boost on the run to the first corner at the start of a race, though it could also be used to aid overtaking.
A red arrow showing the KERS button on the steering wheel of the 2008 BMW Sauber F1.08b Formula 1 car, which was used to test for the 2009 F1 season in which KERS debuted
As regenerative braking in F1 evolved with the introduction of hybrid power units in 2014, the unit which fulfilled the role of the previous KERS unit became known as the MGU-K, or Motor Generator Unit-Kinetic (current hybrid F1 cars also use an MGU-H - Motor Generator Unit-Heat, which recovers energy from the exhaust system via the turbocharger).
The fundamental principles of the system are as for the previous KERS, except that the control system strategy is far more advanced than the KERS control system, integrating the operation of the MGU-K with the MGU-H and the internal combustion engine (ICE) engine management system, plus a brake-by-wire system.
When the MGU-K is harvesting energy, because it is being driven by the engine crankshaft, it creates mechanical drag at the crankshaft, which increases the engine-braking effect normally present (due to the low inertia of the engine) when the driver lifts off the throttle.
The control system can be used to maximise energy recovery and to control the engine-braking effect, and in conjunction with the brake-by-wire system provides consistent brake balance during a lap.
From right to left, a line-up of Mercedes AMG hybrid F1 power units from 2014-2018
Regenerative braking and hybrid use in current F1 cars
The current F1 Technical Regulations, in force until the start of the 2026 season, limit the rotational speed of the MGU-K to 50,000rpm, and the unit’s maximum power output must not exceed 120kW, with a maximum regulated torque of 200Nm. From 2026, the regulation changes will outlaw the use of the MGU-H, which will place more emphasis on the efficiency and operation strategy of the MGU-K.Electronic brake control is necessary for hybrid F1 cars, as there are effectively three forms of braking acting on the rear axle: engine braking provided by the internal combustion engine, braking effect provided by the MGU-K when operating in ‘regen’ mode, plus braking provided by the conventional hydraulic brakes.
As a result, the contribution provided by the hydraulic brakes needs to be adjusted when the MGU-K is recovering energy, as less hydraulic braking effort is required at the rear axle in order to maintain consistent front-to-rear brake bias. Of course, increased hydraulic braking effort is required when the MGU-K is not harvesting energy.
Although the driver can control the sources of braking effort using controls on the steering wheel, a form of automatic control is required to avoid the driver having to make constant adjustments to maintain control of the car. Additionally, the operation of the MGU-K itself needs to be controlled to avoid overcharging the battery.
A ‘brake-by-wire’ electronic control system controls the hydraulic braking effort to provide consistent and predictable braking, avoiding the fluctuations which would otherwise occur when the MGU-K is operating, and hence, importantly, maintaining the driver’s confidence in the handling of the car.
An annotation on a Mercedes F1 steering wheel pointing out the yellow dial used to change the strength of engine braking (L) and the darker yellow dial used to change the brake bias from the front to the rear or vice versa (R)
How regenerative braking is used in fully electric racing
While regenerative braking is extremely important in F1, and in top-flight sports-car racing, it is absolutely pivotal in Formula E, where the MGU is effectively the car’s power unit. Formula E cars have evolved significantly since the introduction of the original Gen1 cars in 2014.
The current Gen3 cars, introduced for the 2022/23 season, use far more complex control systems than their Gen1 ancestors, and the energy-management and regenerative braking systems have come on leaps and bounds. The regeneration capacity of the Gen3 cars is double that of the original Gen1 cars.
With recent regulation changes, there is no longer a requirement for a direct hydraulic connection between the brake pedal and the callipers on F1 and Formula E cars, and this is known as a fully decoupled system. For obvious safety reasons, a back-up hydraulic circuit is still required.
Current Formula E cars use regenerative braking on the front and rear axles while F1 cars only have regeneration on the rear axle
With no mechanical connection between the brake pedal and the hydraulic system to provide feedback to the driver (a pedal-operated sensor or actuator provides an electrical signal to operate a high-pressure hydraulic pump), these fully decoupled systems incorporate a device to simulate pedal pressure, providing feedback at the driver’s foot. The challenge with these systems is achieving consistent performance and ‘feel’ for the driver, with the varying combination of engine, regenerative and hydraulic braking.
Perhaps the current ultimate incarnation of regenerative braking in a motorsport application can be seen in contemporary Gen3 Formula E cars, which use a Front Powertrain Kit (FPK) for regenerative braking on the front wheels (not to power the front wheels at this stage – though there are moves afoot to do so for 2025), in addition to the MGU driving the rear wheels.
This enables regenerative braking on both the front and rear axles. Initially, the plan was to completely dispense with rear hydraulic brakes, but following various challenges with early evolutions of the new system, it was decided to provide the cars with an emergency secondary hydraulic braking system at the rear, using conventional discs and callipers.
Regenerative braking in motorsport has evolved rapidly over the past decade, and the technology developed has been fed back into road-car programmes, as hybrid and electric road cars become more prevalent in the quest for improved efficiency and reduced impact on the environment.