If you’re a fan of Formula 1 racing, you’ve probably heard of regenerative braking. In 2009, the FIA introduced a regenerative braking system called the Kinetic Energy Recovery System (KERS) as an attempt to steer F1 towards greener technologies. Since then, the sport has developed the more efficient Motor Generator Unit Kinetic (MGU-K).

Today, though, you don’t need to be whizzing around the track in a high-end racing car to enjoy the benefits of regenerative braking. But what is regenerative braking, exactly?

electric vehicle charging

What is regenerative braking?

Regenerative braking has been used in trains, trolleybuses and even electric bikes. The idea has been around for well over 100 years, and to understand it, we need to go right back to Albert Einstein, who explained:

“Energy cannot be created or destroyed; it can only be changed from one form to another.”

In that case, then, when you brake and your car slows down, where does that kinetic energy—which pushes the car forward—go?

Answer: it mostly dissipates, via friction caused by tyre grip, as heat and a little noise. In other words, it mainly goes to waste.

Regenerative braking is a way of trying to recoup some of that braking energy. Regenerative braking systems capture the energy normally lost during braking and store it for future use. While they’re found in many modern cars, how they work and what their stored energy is used for depends on the car type.

car emergency braking

How does regenerative braking work in fossil fuelled cars?

Regenerative braking in Formula 1 cars

The MGU-K stores kinetic energy generated under braking and converts it into electricity.

As it’s connected to the crankshaft through the timing gears, when the driver accelerates, the MGU-K can then act as a motor, providing an additional 160 horsepower.

F1 cars

Regenerative braking systems in petrol or diesel road cars

Regenerative braking is far less common in petrol or diesel cars than automatic braking, because it can’t use the helpful features of an electric motor (as we’ll see in a moment). In cars with an internal combustion engine, power recouped by regenerative braking is used to power ancillary systems, reducing fuel usage and the engine’s workload.

Some 2017 Mazda 3 models, for instance, are equipped with an i-ELOOP system, which uses a small generator to collect braking energy. This powers the Mazda’s air conditioning, lights, and infotainment system, increasing fuel efficiency by one mile per gallon.

Volkswagen’s BlueMotion models also use a regenerative braking system, which ensures the battery is only charged under deceleration and braking.

How does regenerative braking work in electric vehicles and hybrids?

Electric motors make ideal partners for regenerative braking systems due to the way they work. While running in one direction, they’re converting electrical energy into kinetic energy to turn your car’s wheels—but when running in the opposite direction, they reverse that transition, converting kinetic energy into electrical energy that can be fed into a charging system for the car's batteries.

So, with regenerative braking, every time you lift your foot off the accelerator and start to brake, the motor changes direction and starts putting energy back into the battery rather than drawing from it.

Some drivers dislike the sensation of the car ‘braking itself’, i.e. feeling like the brakes are fully on as soon as they lift their foot from the accelerator. To address this, some models allow you to adjust the degree of braking regeneration. The Kia e-Niro, for instance, has paddles behind the steering wheel that allow the driver to choose between four different levels of braking energy regeneration.

In hybrids with regenerative braking, captured energy is only used to provide power to the electric motor part of the drivetrain via the vehicle's battery.

hybrid car

What is hydraulic regenerative braking?

Eaton Corporation’s hydraulic launch assist system (HLA), discontinued in 2013, used a reversible hydraulic pump/motor. About 70% of the energy produced during braking was captured to pump hydraulic fluid from a low-pressure reservoir to a high-pressure accumulator, where it compressed nitrogen gas and pressurised the system. During acceleration, fluid in the accumulator drove the motor to transmit torque to the driveshaft. 

They worked with Ford and other companies to trial it in commercial vehicles, particularly heavy vehicles that start and stop frequently (e.g., refuse trucks). Similar systems are still being studied and developed.

Braking systems of the future

With more of us moving to electric vehicles and hybrids, regenerative braking will soon be familiar to many of us. It’s estimated to reduce fuel consumption by between 10 to 25 percent—and that hydraulic regenerative braking systems could do even better, reducing fuel use by 25 to 45 percent. That’s another significant step towards a greener drive and a healthier environment.