Brakes in electric vehicles are full of technology
Electric vehicles place special demands on their brake system. The vehicles are heavier because of their batteries, and the torques are higher, the acceleration stronger. You would think that this causes increased wear of the brake discs and brake pads during braking. In one sense, yes it does, but the mechanical brakes are applied significantly less compared to cars with combustion engines because of the system. The key words are regenerative braking. Depending on the operating conditions, the electric vehicle decelerates using the generator function of the electric motor(s) and energy recovery, or it is at least partially supported in its deceleration. Experts say that there is around a fifth less brake intervention in electric cars. The brake pads are therefore not subject to increased wear. On the contrary, since the two friction partners – the brake pads and the brake disc – are not permanently stressed, pads tend to age more quickly and may vitrify. Rust may form on brake discs. This results in material-related vibrations and noise emissions, such as squeaking or grinding.
This is an interesting point, because these types of noises become the focus, especially in electric cars, as the electric engine is quiet. This means that they impair the acoustic comfort when driving. In addition, the braking performance can decrease when you have to step sharply on the brakes. "The reduced use of conventional brakes can lead to problems with electric cars", says Jannis Dörhöfer, New Mobility Officer at the German TÜV Association. "If brake pads are not heated sufficiently and regularly, the material properties can suffer." This results in a lower coefficient of friction and, as a result, a poorer braking effect. "Therefore, regularly checking and servicing the brake system is recommended on electric vehicles."
For these reasons, HELLA (a joint venture between HELLA and TMD Friction) has special brake pads and brake discs for electric vehicles in its range. They are specially designed for these particular conditions and ensure braking performance and comfort despite "rest breaks". Market coverage in the aftermarket currently already amounts to over 90 percent of brake pads for electric and hybrid vehicles and 85 percent of brake discs.
Safe braking with electric vehicles requires complex interactions
Electric vehicles decelerate using the classic braking system and the recuperation function. In many driving situations, the vehicle brakes completely without the mechanical brake when the brake is actuated and in overrun mode. What sounds simple, however, requires a highly complex interaction between the electrical and hydraulic components. Gently decelerating while driving downhill, before a junction or sharp emergency braking in a dangerous situation? The electromechanics have to recognise what the driver wants to do immediately.
Electric cars do not just decelerate using the classic brake system. The deceleration is supported by recuperation (regenerative braking) and, in some cases, it is even replaced completely.
An electromechanical brake booster with pressure accumulator, an electric vacuum pump and the brake pedal value sensor play a central role. Depending on the requirement, i.e. depending on the brake pedal position and brake pedal pressure, the brake control unit calculates the necessary braking force in milliseconds. Above a certain brake deceleration force, the brake booster activates the mechanical brakes (depending on the vehicle and settings). In addition, there is the intelligent control of ABS and ESP and the integration of driver assistance systems, such as the emergency brake assistant. The advantage: The dual brake system (brake blending = mixed brake control) can be controlled extremely accurately and the brake intervention is always optimally dosed. So, it all interacts perfectly!
HELLA is a specialist in vacuum pumps and electronic brake pedals, and in the corresponding sensor technology. The pedal feel can even be regulated dynamically by hydraulic valves or damping units (simulators).
Electric vehicles: Special brake pads, special brake discs
Basically, at first glance there are no differences between brake pads and brake discs for combustion and electric cars. The layout is similar. But the vehicle manufacturers are constantly adapting the specifications for e-cars. For brake pads, the focus is on quiet and low-vibration pads (Noise, Vibration, Harshness - NVH). Sometimes the brake pads even have a "slimmer" design: they have a smaller and thinner friction surface, which is due to the lower frequency of use. Material can also be saved, which benefits the environment.
In product development, HELLA focuses on environmental protection and environmental compatibility in addition to optimal performance. The brake pads for electric and hybrid vehicles, for example, are all copper-free. The brake pads' copper-free formula is not only environmentally friendly, but has also been designed precisely for the relevant vehicle types, brake systems, engine performance, driving loads and braking characteristics. They have a higher compressibility and tend to have higher coefficients of friction. Due to the lower frequency of use, the brake pads must also function optimally when cold. HELLA tests the coefficient of friction, pedal pressure and heat wear in its own research and development centre. The brake pads also exceed the current requirements set out in ECE R90 regulation.
HELLA offers heavy-duty high-carbon brake discs, specially for very powerful electric and hybrid vehicles. The increased carbon content enables rapid heat dissipation even in extreme situations, which also enables optimum braking performance. The brake discs also ensure low fading behaviour, reduced braking noise, fewer disc impacts and increased braking comfort. To prevent corrosion, the brake discs are coated with a water-based paint. And what's more, they can be installed immediately in the workshop: no protective oil needs to be removed.
Brake fluid with high dry and wet boiling point for electric vehicles
Although electromechanical actuators have long been used in electric parking brakes (EPB) and vehicles with brakes that are operated entirely electrically are being tested, the friction brake system in electric vehicles is still operated with hydraulics. With DOT 5.1 EH, HELLA is therefore developing a special glycol-based brake fluid for electric vehicles. The plan is for it to be characterised by high dry and wet boiling points of at least 260°C and 180°C respectively, low conductivity, high corrosion protection and a low viscosity of maximum 750 cSt at -40°C. This means that the brake fluid is to exceed almost all applicable specifications for common brake fluids from DOT 3 to DOT 5.1, meaning it will be optimally equipped for the high demands of the on-board electronics in today's electric vehicles. Acceleration without torque loss ensures that electric cars reach high speeds within a very short time. Reaching a standstill again quickly from these speeds requires a correspondingly high braking performance, which is ensured at all times by the new brake fluid's above-average dry and wet boiling point.
Brake system on the rear axle of a Tesla. A combination of hydraulic brake with an electric parking brake (EPB).
Euro 7 and brake dust emissions in electric cars: New test procedure
With the introduction of the new Euro 7 exhaust gas regulation from 2025, manufacturers of passenger car brake pads must also record particulate emissions and comply with the relevant values. This regulation is to apply to vehicles with internal combustion engines and to electric and hybrid vehicles. The central aspect here is the interaction between the friction partners – the "pad" and "disc". It strongly depends on the material composition and their dynamic interaction over the brake components' life cycle. To reduce this particulate matter, HELLA and TMD Friction rely on environmentally compatible brake pad compounds.
The measurement of emissions itself is not a simple undertaking, especially since the measured values must be reproducible and comparable. To ensure that the measurement results can be transferred to real-world driving conditions, a uniform WLTP (Worldwide Harmonised Light Vehicles Test Procedure) was developed. This cycle was defined using real vehicle data and it maps various brake operating points over a longer period of time. This ensures a reproducible recording of the brake abrasion. Going forward, braking emissions are to be measured uniformly on test benches using this WLTP braking cycle. Furthermore, it is crucial to accurately map the specific vehicle model on the test bench. For example, the combination of brake pad and brake disc, and the vehicle weight and weight distribution, influence the wear behaviour.
Conclusion: Brakes in electric vehicles – a clean thing!
In conclusion, it can be said that electric vehicles themselves produce significantly fewer emissions. This means the complete elimination of exhaust gases from the engine, lower noise development and significantly lower brake dust emissions related to the system, thanks to proportional brake energy recovery (recuperation). Quiet acceleration, quiet braking – a clean thing!