Automotive braking technology has experienced a hundred years of evolution from pure mechanical friction braking to modern electronic control braking system, and now regenerative energy recovery technology is changing the flow of automotive energy.
This paper systematically introduces the basic structure and working principle of automobile service braking system, explains the cooperative working mechanism of service braking (mechanical friction braking) and regenerative braking (energy recovery braking), expounds the contribution of braking energy recovery system to the endurance mileage and its core value in new energy vehicles, and looks forward to the future development trend of intelligent braking system.
- When brakes are no longer just “brakes”
You drive a new energy vehicle on a downhill mountain road, the red light ahead is on, you release the accelerator and lightly step on the brake-the vehicle slows down smoothly, and finally stops steadily in front of the stop line. The whole process is silent, but a line of small print on the dashboard of the cabin reveals a secret: “Energy recovery + 32 km”.
The meaning of this line of words is: In this braking process, your vehicle replenishes the battery through the braking energy recovery system, which is equivalent to driving 32 kilometers. In traditional fuel vehicles, this part of energy is lost to the atmosphere in the form of friction heat generated by brake pads and never returns. In the new energy era, every deceleration is “adding bricks and tiles” to the range.
Automobile braking system is one of the core systems to ensure driving safety, and its exquisite design is far beyond the imagination of ordinary people. A complete braking system involves many subsystems, such as hydraulic transmission, vacuum booster, electronic control, energy recovery and so on. The failure of any link may lead to serious accidents. At the same time, with the popularity of new energy vehicles, braking energy recovery technology (Regenerative Braking System) is giving the braking system a new mission-it is not only the guardian of safety, but also the contributor of endurance.
- Service brake system: the cornerstone of automobile safety
2.1 What is the service brake system?
The service brake system, commonly known as the “brake system”, is a device used by the driver to reduce the speed of the vehicle or stop the vehicle by stepping on the brake pedal. Unlike the parking brake (hand brake), the service brake system is on standby at any time during the driving process, which is the core safety system to ensure the safety of driving.
2.2 Working principle of traditional hydraulic brake system
Hydraulic Brake System is widely used in modern automobiles, and its core principle can be summarized as four words: force transmission by oil. The specific working process is as follows:
① The driver steps on the brake pedal: the brake pedal is connected with the brake Master Cylinder, and the pedal stroke generates mechanical thrust.
② Hydraulic amplification: the brake master cylinder converts the mechanical thrust into brake fluid pressure. Because the liquid is almost incompressible, the small thrust exerted by the driver’s foot can be transmitted through the brake fluid and amplified by tens of times.
③ Pressure transmission: The brake fluid is transmitted to the Brake Caliper of each wheel through the Brake Lines.
④ Friction braking: The brake caliper pushes the Brake Pads to tightly clamp the Brake Rotor fixed on the wheel, and the kinetic energy of the vehicle is converted into heat energy through friction, and the vehicle is decelerated.
⑤ Release the pedal: the driver releases the brake pedal, the brake fluid flows back, the brake pad leaves the brake disc under the action of the return spring, and the braking force disappears.
2.3 Vacuum booster: make the brake more labor-saving
To stop a car weighing 1.5 tons at a reasonable distance without a booster device, the driver needs to exert about 300-500 kilograms of force, which is impossible for most people.
The vacuum booster (Vacuum Brake Booster) uses the vacuum created by the engine intake manifold (or electric vacuum pump) to create a pressure differential across the booster diaphragm, which greatly amplifies the force applied by the driver to the brake pedal. Modern vacuum booster can amplify the pedal force of the driver by 5-8 times, so that the ordinary driver only needs to exert tens of kilograms of force to produce a strong braking force.
For example, a vacuum booster is like a Hercules-the driver just pushes it lightly, and the Hercules will help you put on the brake with ten times the strength. Without it, driving on the brakes would be an extreme sport.
- Regenerative braking: inject new mission into the brake
3.1 What is regenerative braking?
Regenerative Braking is a technology that converts vehicle kinetic energy into electric energy and recharges the battery. Unlike traditional braking, which converts kinetic energy into heat energy and dissipates it in vain, regenerative braking reverses the direction of energy flow from “consumption” to “recovery”.
3.2 Physical principles of energy recovery
The basic principle of regenerative braking is the law of electromagnetic induction. When the motor is driven to rotate by external force, its rotor coil cuts the magnetic induction line in the magnetic field, and according to Faraday’s law of electromagnetic induction, the coil will produce induced current.
Specific to the new energy vehicle: when the vehicle is running, the driving motor is normally powered on to convert the electric energy of the battery into mechanical energy (kinetic energy). When the driver releases the accelerator or depresses the brake pedal, the drive system stops energizing, and the wheels drive the drive motor to rotate-the roles are reversed: the wheels become the power source of the generator, and the drive motor turns into the Generator in reverse, converting the kinetic energy of the vehicle into electric energy, which is recharged into the battery.
This process will produce a moment opposite to the direction of the vehicle, which will have a deceleration effect on the vehicle-this is the regenerative braking force. It is this force that plays a role in the “drag feeling” that drivers feel.
3.3 Cooperation between regenerative braking and service braking
Regenerative braking is not a substitute for conventional service braking, but rather a cooperative partner. The braking system of modern electric vehicle adopts the braking energy recovery coordination control system (Brake Energy Recovery Coordination System), which intelligently distributes the proportion of two kinds of braking force according to the driver’s braking intention and vehicle state.
| Braking scenario | Regenerative braking participation | Degree of service brake participation |
| Lightly apply the brake (low deceleration) | High-Primary Brake Source | Low-supplementary supplement |
| Medium braking (medium deceleration) | Medium and high-priority use | Medium — supplement the insufficient part |
| Emergency braking (high deceleration) | Low or closed – unable to meet demand quickly | High — main braking source |
| Long downhill driving | High-continuous and stable recovery | Low-maintain control |
It can be seen from the table that regenerative braking mainly plays a leading role in light and medium braking scenarios, while emergency braking still relies mainly on traditional service braking due to the need to quickly establish maximum braking force. This reflects the brake system design principle of “safety first”.
- Energy contribution of regenerative braking: endurance mileage
4.1 The truth about energy recovery efficiency
Many electric car owners are concerned about how much regenerative braking can contribute to the endurance?
The answer is that it varies from person to person and from scene to scene.
According to industry test data, under comprehensive working conditions (NEDC or CLTC), the contribution of braking energy recovery to the endurance mileage is generally between 10% and 30%. Depending on the following factors:
- Driving style: Aggressive driving with frequent acceleration and deceleration can recover more energy; Mild driving has a relatively small recovery.
- Road conditions: For mountain roads, hills and other sections with frequent uphill and downhill, the energy recovery effect is significantly better than that of plain urban road conditions.
- Traffic conditions: The stop-and-go urban congested road conditions are the “main battlefield” of regenerative braking, and the recovery efficiency can reach more than 30%.
- SOC state of the battery: when the SOC of the battery is close to 100%, the recovery space is limited; when the SOC is between 20% and 80%, the recovery effect is the best.
- Temperature conditions: The battery charging acceptance is reduced in the low temperature environment, and the energy recovery efficiency will be reduced.
4.2 Single pedal driving mode: extreme energy recovery experience
When it comes to regenerative braking, we have to mention the popular One-Pedal Driving mode in recent years.
In this mode, the driver simultaneously decelerates and recuperates energy by controlling the degree to which the accelerator pedal is raised-when depressed for acceleration, lifted for deceleration, and released, the regenerative braking force of the vehicle is sufficient to bring the vehicle to a complete stop in most cases.
Single-pedal mode takes regenerative braking to the extreme, and the driver does not need to switch between accelerator and brake frequently, which not only improves the efficiency of energy recovery, but also reduces the wear of brake pads. However, it should be noted that the single pedal mode can not completely replace the traditional brake pedal, in case of emergency, it is still necessary to step on the brake pedal to obtain the maximum braking force.
- Technical depth analysis: key technologies of regenerative braking system
5.1 Motor control strategy
The core of regenerative braking system is Motor Control Strategy. According to different braking demands, the control strategies are mainly divided into three types:
◆ Speed control mode (RPM Control): control the regenerative braking force by adjusting the generator speed to achieve a smooth sense of deceleration.
◆ Torque control mode (Torque Control): Directly control the output torque of the motor (the negative value is the braking force), with faster response, which is the mainstream control mode at present.
◆ Composite control mode: combine the advantages of speed and torque control modes, and automatically switch the optimal strategy in different speed ranges.
5.2 Braking smoothness challenge
A key difference between regenerative braking and conventional braking is the continuity and stability of the braking force. The regenerative braking force may fluctuate slightly due to factors such as motor speed and battery charging status. If the control is improper, the driver will feel the frustration of “one meal at a time”, which will seriously affect the driving experience.
A good regenerative braking system needs to solve the problem of “brake feel consistency”. Through fine motor control algorithm, speed estimation accuracy improvement and regenerative braking force feedforward control, engineers make the foot feeling of regenerative braking and traditional braking almost indistinguishable.
5.3 Braking energy storage and conversion efficiency
The complete chain of braking energy recovery is: wheel kinetic energy → motor power generation → power conversion → battery charging.
In the whole chain, there are energy losses in each step: the motor-generator conversion efficiency is about 85% ~ 92%, the power conversion (AC/DC) efficiency is about 95% ~ 98%, and the battery charging efficiency is about 90% ~ 95%. Taken together, the overall efficiency of the braking energy recovery system is usually between 70% and 80%, that is, about 70% to 80% of the braking kinetic energy can eventually be converted into electrical energy stored in the battery.
This means that if the vehicle travels at 100 km/H, about 80% of the kinetic energy from braking to stop can be recovered and reused for driving-in contrast, conventional braking converts 100% of the kinetic energy into heat energy and dissipates it.
- Impact of regenerative braking on new energy vehicles
6.1 Direct improvement of range
Taking a new energy vehicle with a battery capacity of 60 kWh and a comprehensive range of 500 km as an example, with the support of the braking energy recovery system, the braking energy recovery alone can contribute an additional range of about 50-150 km to the vehicle. This means that car owners can travel the equivalent of the distance from Beijing to Tianjin per charge than vehicles without energy recovery.
6.2 Prolonging the service life of the braking system
Regenerative braking takes over most of the braking tasks in light and medium braking scenarios, significantly reducing the reliance on conventional brake pads and discs. Research shows that a good braking energy recovery strategy can extend the replacement cycle of brake pads by 2 to 4 times, which not only reduces the maintenance cost of owners, but also reduces the environmental pressure caused by waste brake pads.
6.3 Optimization of driving experience
Modern regenerative braking systems are highly mature and perform well in terms of braking smoothness. Many car owners reflect that after getting used to the single-pedal driving mode, they feel that the driving experience of traditional fuel vehicles is “not enough to follow hands”. Regenerative braking makes driving more leisurely and more in line with the overall temperament of “intellectualization and electrification” of new energy vehicles.
- Future outlook: intelligent evolution of braking system
Regenerative braking is not the end, but a new starting point of technological evolution. The following trends are reshaping the future of automotive braking systems:
- Electronically controlled hydraulic brake (EHB):
The traditional vacuum booster is replaced by the motor to realize the electronic precise control of braking force, and the interface is reserved for advanced assistant driving (ADAS) and automatic driving. At present, the latest models of BYD, Tesla and other companies have begun to adopt.
- Brake-by-Wire:
Brake fluid and mechanical connections are completely eliminated, and brake calipers are controlled by electronic signals. The reaction speed is compressed from 300 ~ 500ms of traditional hydraulic pressure to less than 100ms, and the braking safety is greatly improved.
- Intelligent cooperative braking (Cooperative Regenerative Braking):
With the help of V2X technology, vehicles can predict the road conditions ahead (traffic lights, congestion, bends) in advance, optimize the energy recovery strategy before the driver steps on the brake, and push the braking energy recovery efficiency to a new height.
- Chassis integrated control:
The braking system is deeply integrated with steering, suspension and driving system to realize the global optimization of vehicle dynamics, so that the energy utilization efficiency and safety performance can be improved synchronously.
Looking back on the history of automobile braking technology for more than 100 years, from the initial pure mechanical braking to the advent of the hydraulic era, and then to the braking energy recovery under the current wave of electrification, every technological transition has made the relationship between human beings and automobiles closer and more intelligent.
Regenerative braking technology tells us that there is no end to the use of energy. Every time you step on the brake lightly, it is the gentle retention of kinetic energy. Every downhill slide is a generous gift of the laws of physics. In the era of electrification and intellectualization, automobiles are no longer just machines that consume energy, but also intelligent terminals that can perceive, think and optimize energy flow.
