Electric vehicle braking: Why drum brakes instead of disc brakes?

Continental Group's launch of EPB Si is the most suitable braking solution for electric vehicle attributes.

Safety is the lifeline of a car.

One traffic accident after another caused by brake failure is ringing the alarm for safety and no trivial matters with bloody lessons. For the "zero accident vision" of Continental Group, minimizing accident rates to the maximum extent has become an endless driving force for innovation in the field of safety. Recently, Continental Group launched the most suitable braking solution for electric vehicles - EPB Si, which not only excels in driving safety, but also has some unfamiliarity in driving comfort.

So, what are the new requirements for braking systems in electric vehicles? Why is EPB Si the most suitable braking system for electric vehicles? Why is EPB Si a drum brake instead of a more advanced disc brake?

Recently, Miao Zhehua, an expert in the electronic parking brake system of Continental Group, gave a detailed interpretation.

The four major demands for electric vehicle braking

The car braking system has two main functions: "stepping on the brake" and "pulling the handbrake". One is the driving brake that allows the car to decelerate from a dynamic state to a static state, which is achieved by the driver operating the brake pedal; Another is the parking brake that keeps the vehicle stationary, which is automatically completed by the driver's operation of the parking switch (EPB) or the vehicle's parking brake logic.

Miao Zhehua believes that the braking system of electric vehicles needs to constantly change in four aspects: energy recovery, safety performance, environmental protection, and range, in order to provide safer and more comfortable driving and riding guarantees.

It is reported that electric vehicles use electric drive powertrains instead of traditional internal combustion engines as the core powertrain, and have four new characteristics in the braking system: first, braking energy recovery. The driving motor has the ability to provide both acceleration and deceleration torque to the vehicle, and this ability is applied in the braking process of the vehicle, which is the braking energy recovery technology. This is also the biggest difference between the braking systems of electric vehicles and fuel vehicles. Secondly, The P-gear locking mechanism is no longer a necessary design. In the past and present, people used to use P gear with handbrake or electronic parking EPB system to achieve stationary parking; In the future, electric vehicles will greatly simplify transmission design, The P-gear locking mechanism is no longer a necessary option, which requires the braking system to have stronger parking capabilities to achieve parking efficiency. Thirdly, reducing brake dust is a major challenge. Although electric vehicles do not have the problem of internal combustion engine emissions pollution, the proportion of dust emissions from the braking system in the overall vehicle emissions is increasing, and it will become a key regulatory direction in the future, which poses a challenge to the braking system. Fourthly, reduce resistance and power consumption. Increasing the range of electric vehicles is one of the performance improvements that every vehicle manufacturer strives to achieve, which requires the braking system to minimize the drag torque and the required battery to overcome this resistance.

The most suitable electric braking solution

Miao Zhehua said that the electronic parking brake system EPB Si launched by Continental is the most suitable braking solution for the attributes of electric vehicles. Electric vehicles have four special requirements for braking systems: energy recovery, safety performance, environmental protection, and range, EPB Si can meet all requirements.

Firstly, energy recovery is the process of recovering the heat energy consumed by the brakes and storing it in the form of electrical energy in the battery, converting the driving motor into a generator state, thereby increasing the range of electric vehicles. Through a series of vehicle tests, it was found that vehicles using energy recovery technology have a 90% reduction in demand for friction braking. EPB Si can help optimize and improve the performance of braking energy recovery Miao Zhehua added that.

Secondly, whether it is the comprehensive driving brake or the parking brake, EPB Si is a braking product that is very suitable for electric vehicles.

Miao Zhehua said, "At extremely low speeds (below 15km/h) and extremely high speeds (above 120km/h), the braking torque that the motor can output is relatively limited. When the vehicle speed is in the range of 15km/h to 100km/h, energy recovery can provide most of the deceleration required for vehicle deceleration. Therefore, for the friction braking system of electric vehicles, the most stringent requirement is to decelerate from 120km/h to 100km/h. In this case, due to the generation of braking heat, it is easy to cause the phenomenon of brake thermal degradation."

In order to evaluate the braking performance of the entire vehicle under thermal degradation of the braking system, Continental conducted the most rigorous AMS testing, and the results showed that EPB-Si can ensure excellent braking performance even in high temperature conditions.

In addition to driving brake performance, electric vehicles have higher requirements for parking brake due to their heavy weight. According to regulations, electric vehicles must be equipped with two independent parking mechanisms. When one of the parking mechanisms fails, the other must be able to safely park the vehicle on an 8% slope. Continental Group's performance testing on slopes shows that, The maximum parking torque output of EPB-Si products can reach 1700Nm, which is sufficient to meet the 20% parking requirement of a 3-ton vehicle model and provides parking safety far higher than regulatory requirements. Therefore, for electric vehicles without a transmission locking mechanism, relying solely on the two EPB Si on the left and right wheels can achieve the functions of two sets of parking mechanisms.

Thirdly, thanks to the enclosed system structure of EPB-Si, 80% of the worn brake pad dust will be retained inside the braking system, which greatly helps to reduce PM10 and protect the environment.

Fourthly, facing the challenge of long range of electric vehicles, on the one hand, battery capacity is the most important factor, and on the other hand, there are two related factors, electrical load and vehicle drive, which affect the range. According to evaluations, for every 1Nm reduction in wheel end drag, it can help the vehicle increase its range by 10km to 15km. EPB-Si adopts a drum brake product structure and has a series of spring components that can help the brake pads maintain a disengagement state from the brake drum. After testing using the Worldwide Uniform Light Vehicle Test Procedure (WLTP), it was found that, The drag torque of EPB-Si products is not higher than 0.5Nm in all working conditions, providing assistance for improving the vehicle's range.

Drum brakes conform to the development trend of electric vehicles

As is well known, drum brakes are generally used in trucks, while disc brakes are mostly used in passenger cars. Why does EPB Si choose drum brakes?

"Disc braking and drum braking each have their own advantages. With the trend of electrification in automobiles, changes in braking systems have always occurred. Drum braking has advantages in energy recovery, safety performance, environmental protection, and range, so the EPB-Si product developed based on drum braking is in line with the future development trend of electric vehicles." Miao Zhehua answered.

It is reported that drum brakes have two major advantages in electric vehicles: firstly, drum brakes have higher parking efficiency than disc brakes. Due to the design of drum brakes being a rotating structure, they have an internal self increasing force effect. As the wheels rotate, this pressure is transmitted to the brake pads, creating greater pressure and a larger contact area between them and the brake disc, achieving higher parking efficiency than disc brakes. This is also the reason why many large commercial vehicles use drum brakes. The second is that the structural closure of drum brakes has good rust prevention ability. The heat accumulated inside the drum brake is difficult to release into the atmosphere due to its closed structure. The accumulation of heat inside causes a significant decline in the performance of friction materials/brake pads, leading to a decrease in braking force. This is also the main reason why drum brake has been less and less used in vehicle models in the past 20 years. However, due to the application of energy recovery in electric vehicles, the brake temperature is significantly reduced, and the problem of drum brake heat decay is fundamentally solved. Another characteristic brought about by the closed structure is that it has better rust and corrosion resistance than disc brakes.

Miao Zhehua added, "The problem of rust is precisely a side effect of the decrease in friction braking strength of electric vehicles. Disc braking is a completely open system, and the friction surface between the brake disc and the friction material is completely exposed to the air. In this case, the brake disc will rust within a few hours. Some vehicles will have rust spots on the brake disc surface after parking overnight, and this rust situation will be more obvious on rainy and humid days. In the past, traditional vehicle models used several brake actions to remove rust, which is also the reason why the first few brakes after starting the vehicle in the morning are more prone to brake noise. For electric vehicles, the strength and frequency of friction braking are not enough to remove the rust produced on the brake disc, so it will bring about A series of brake noise and even insufficient braking force issues. The closed structure formed by EPB Si and brake drum can effectively prevent rust on the friction surface of the brake drum and avoid the braking problems caused by it

It can be seen that due to changes in the carrier, the thermal degradation disadvantage of drum brakes is offset or even transformed into an advantage of increasing driving range due to the energy recovery of electric vehicles.

EPB Si is the latest generation electronic parking brake system launched by Continental Group. This product has been in mass production since 2020 and has been applied in the Volkswagen ID3 On the ID4 model. Miao Zhehua said, "We are also actively negotiating with other vehicle customers, and there will be more models applying EPB Si in the future."

In short, technology is a double-edged sword, and technological innovation that leverages strengths and avoids weaknesses is valuable and grounded innovation. After achieving deep localization, Continental Group is focusing on the Chinese market with more competitive products and providing better and faster services to Chinese consumers.