When a driver applies the brakes, the car's kinetic energy is converted into heat. This energy, which could be put to good use, is simply dumped into the air and goes to waste.
A regenerative braking system attempts to minimise this wastage by capturing some of the kinetic energy, typically by converting it to electricity, and reusing it.
This captured energy can then be used to improve the efficiency of a car; it can extend the battery range of an electric vehicle, or be used in several ways to reduce the fuel consumption of a hybrid or conventional car - and it can even be used to boost performance momentarily.
Why use regenerative braking?
If the driver applies the brakes in a conventional car, the vehicle's kinetic energy - which is the result of its motion - is converted into heat by the friction generated in the braking system. When a brake pad is clamped to the disc, the kinetic energy of the car is converted to heat by the friction occurring between the material in the pad and the surface of the disc.
The heat produced is then lost to the atmospheric air passing over the braking components, or absorbed into the components themselves. This energy, however, could be put to good use instead of simply being wasted - granting efficiency benefits.
How does a regenerative braking system work?
Regenerative braking is most commonly used in electric and hybrid vehicles, where the drive or assistance motor can be used as a generator while braking. The motor then converts the kinetic energy of the vehicle into electrical energy, while the torque required to turn the motor slows the car.
The electrical energy generated can then be stored in a battery or a supercapacitor - as is the case in Mazda's 'i-ELOOP' configuration. It can be used immediately, to power ancillaries and the drive motor, or stored and used later when required.
In some setups, the conventional hydraulic braking system may be actuated at the same time; in this 'parallel' configuration, regenerative braking will be applied to the axle or axles driven by the motor in conjunction with friction braking.
Alternatively, a 'series' regenerative system can be used. In this set-up, the regenerative braking can be used on its own to slow the vehicle when only light braking is required. If more stopping power is required, the hydraulic system can then be gradually introduced to stop the vehicle. This setup requires that the vehicle have some form of electro-hydraulic braking systems, in which the driver's inputs aren't directly related to the actuation of the brakes.
Manufacturers have to work hard to ensure that the transition between these modes isn't noticeable, though, with the aim being to smoothly blend regenerative and physical braking modes. The strength of the regenerative braking can also be adjusted in many cars, to suit the driver's preferences.
Regenerative braking can be even be used in so-called 'micro hybrids', which have an integrated starter/generator for their stop-start system.
Are there other types of regenerative braking systems?
Any system that captures energy which would otherwise be lost during deceleration is a regenerative braking system. While the electric and hybrid variants are the most common, there are other setups.
For example, the PSA Group developed a concept using a compressed air regenerative system dubbed 'Hybrid Air' - and similar hydraulic-based systems have also been tested.
Flywheel-based recovery systems can also be used; in these, a flywheel can be accelerated while braking and then the energy stored by it deployed straight back into the drivetrain. Alternatively, the energy in the flywheel can be used to drive a generator. The electricity produced by the generator can then be used in a drive motor, or to top up the vehicle's batteries.
Early 'Kinetic Energy Recovery Systems' used in Formula 1 were based on such flywheel-type designs but quickly evolved into 'Motor Generator Unit - Kinetic' setups, which adopted a more straightforward approach using a motor integrated into the powertrain which would convert kinetic energy into electricity during braking; the same motor can also be used to provide a boost when required.
A brief history of regenerative braking
The concept of regenerative braking, in automotive applications, has been around since some of the earliest electric vehicles and dates back to the late 1800s. It was also employed in some of the earliest petrol-electric hybrids; the Woods Motor Vehicle 'Dual Power' from 1911 reputedly had a regenerative mode, for example, which could be used to top up its batteries when slowing.
It would be some time until a fully-fledged electro-hydraulic braking system, working in conjunction with a regenerative braking set-up, would arrive. The first road-going example was featured in the General Motors EV1 of 1996; a mass-produced system that arrived in the 1997 Toyota Prius. Today, several manufacturers use such configurations.