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11/01/2021
Regenerative braking: how does it work?

Automotive components

 

Electric and hybrid vehicles are on the rise for a variety of reasons such as government incentives or the need for manufacturers to reduce emissions, not to mention an increased environmental awareness by most motorists and preferential treatment such as the opportunity to access Low Emission Zones

Nicodemo Angì

Electric and hybrid vehicles are on the rise for a variety of reasons such as government incentives or the need for manufacturers to reduce emissions, not to mention an increased environmental awareness by most motorists and preferential treatment such as the opportunity to access Low Emission Zones. We are aware that these cars work on different mechanical systems, but this is a wide-ranging discussion and needs to be addressed with additional information.

 

Transforming energy

One of the most important characteristics of electric motors is its reversibility, i.e. its ability to consume energy delivering torque as well as produce it by forcing an electric motor into rotation. However, the electricity thus obtained is not 'free': the rotation must overcome the resistance generated by the bearings and aerodynamic friction of the rotating motor in order to turn mechanical energy into electrical energy. Energy, however, is not created out of nothing, so in order to obtain "X" kWh of electrical energy output, something more than "X" kWh of mechanical energy must be introduced. This is a rotational resistance, which is identical to what is obtained from the friction of the brakes. The big difference is that in conventional brakes the mechanical energy is entirely converted into heat, whereas the use of electric motors as brakes allows for a significant recovery in the form of electrical energy. Obviously, some energy will still be dissipated as heat, but in general the conversion is very efficient and the electrical energy obtained can be stored in a battery.

 

Brakes? Idle or overworked

Brakes work far less with this system as the braking force is applied by the electric motor to the wheels via the drive shaft. How does this affect the brakes? The fact is that much of the braking force is supplied not by brake pads and callipers as in traditional systems with but by regenerative braking. Conventional braking is supplied only when extreme or emergency braking is needed as a regenerative system is not powerful enough. The engine, in fact, is not able develop its nominal power during the regeneration phase, and if the vehicle is not four-wheel drive, it will only brake one axle, unlike traditional friction braking. This poses new challenges as the brakes, which are unused - and therefore cold - for most of the time, will then be suddenly and forcefully called into action. Also not to be underestimated is the issue related to oxidized discs, which is always a problem due to prolonged disuse. Rust is not only unsightly but also limits braking power, at least initially. Hence, the industry was quick in introducing countermeasures: Continental, for example, has created discs, rims and drums specifically designed for electric and hybrid cars.

 

Think electric

One of these components, a drum brake of mixed aluminium/steel construction, is already in production and equips Volkswagen's ID.3 rear drive axle, and therefore under little stress (the company says they will last as long as the car) as most of the braking is performed by the regenerative system. The ID.3 regenerative brakes feature sensors to measure real time braking torque, which is essential to coordinate the braking energy between the traditional friction brakes and the regenerative system. Furthermore, Continental introduced a concept that is bound to revolutionise the entire system; the “New Wheel Concept”, which consists of two aluminium (Al) parts: an inner Al carrier star with an Al brake disk and an outer Al rim well with the tire. In contrast to conventional wheel brakes, the New Wheel Concept brake engages the Al disk from the inside. This allows it to have a particularly large diameter, which benefits the braking performance. This design makes it possible to use a lightweight aluminium alloy disc whose large diameter reduces pad pressure with the same braking power, and this, together with the thermal conductivity of this metal, keeps the temperature under control even during intense braking. The calliper is mounted symmetrically on the disc and this has the advantage of reducing noise (EVs tend to have “silent” moving parts) and minimising residual friction, which can reduce mileage.

 

Sensors and bionic structures

This concept paved the way for the creation of Bionic brakes, inspired by organic structures and optimised for minimum weight by removing unnecessary material. Their shape may come as a surprise to a tire specialist, but it is all about reducing the weight of electric and hybrid cars. We are certainly not surprised by the presence of many sensors: these are vital in coordinating friction and regenerative braking. Regenerative braking is more random than conventional braking: it doesn’t work at very low speed, and if the battery is already fully charged (which can happen in both electric and hybrid cars), only the friction brakes will work. Formula 1 and Formula E cars are hybrids, which is why electronically controlled rear brakes are allowed. These innovations are not confined to the racing world: Alfa Romeo's Giulia and Stelvio as well as Jeep’s Renegade and Compass use Continental MK C1, a unit that features an electric pump that pressurizes the hydraulic fluid and a series of valves that set the pressure to be applied to the callipers of each wheel. The brake pedal is connected to a piston that gives the driver the feeling of a traditional system and is used to operate the brake pads in emergencies, in the event of failure of the electric pump. In practice, the system detects the intensity of the braking required by the driver and applies the needed pressure accordingly.

 

Braking by-wire Italian style

Brembo introduced its own version of Brake-By-Wire technology which takes a further step towards fully electric/electronic brakes. This system has rear callipers operated exclusively by small electric motors. The front brakes are also innovative with their mixed actuation system: each wheel has an electro-hydraulic actuator, managed by a control unit, which pressurises the fluid that activates the callipers. A simulator gives the pedal the feel of a conventional system while an emergency circuit acts directly on the callipers in the event of a failure of the electric pump. This design facilitates and speeds up assembling the system, given that the pipes are few and very short, and promises great flexibility: the system's response can be set via software and can be adjusted by the driver. A high response speed is claimed, which is very useful for Adas and even more so for autonomous driving, torque vectoring, emergency braking, ABS and so on.

 

Long life brake disks

Brembo has also invested in areas such as materials and solutions for more conventional brakes, the result of which can be seen in the company’s Greentive coating, which combines high performance, reduced environmental impact and elegance. This finish can be applied to many types of disc and given that wear is minimal, this translates in a significant reduction of particulate matter. The brake discs preserve their shiny look even if they are not used for a long time, as is the case with electric and hybrid cars, and the Brembo logo on the surface acts as a wear indicator. Friction is also reduced by a small component called Brembo Enesys - Energy Saving System. This is a special spring that retracts the pads when the brakes are not used: friction is minimised and thus mileage is increased, which is extremely important in electric and hybrid vehicles. Pads and discs last longer and reduce particulate emissions. One final note: reduced use of friction brakes will increase the moisture absorbed by the hydraulic fluid, which will remain cooler. Therefore, replacing the fluid might be beneficial because in electric and hybrid cars this might last less than discs and pads!

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