Basic knowledge of electric car regenerative braking system
Regenerative braking is unique to electric cars. It converts part of the car’s kinetic energy into electrical energy during deceleration (braking or downhill), and the converted electrical energy is stored in storage devices (such as various batteries, super capacitors, and flywheels). Battery), ultimately increasing the mileage of electric cars. If the accumulator has been fully charged, regenerative braking cannot be achieved, and the required braking force can only be provided by a conventional hydraulic braking system. Almost all electric cars are now equipped with regenerative hydraulic braking systems, which can save braking energy, recover part of braking kinetic energy, and provide drivers with regular braking performance. Figure 1 shows the energy conversion diagram of an electric car.
Generally speaking, the regenerative braking system is activated when the electric car slows down, relaxes the accelerator pedal to cruise on the highway, or depresses the brake pedal to decelerate to a stop. During normal deceleration, the regenerative braking torque is usually maintained at the maximum load. When an electric car is cruising at high speed, its drive motor generally runs under a constant power state, and the drive torque is inversely proportional to the speed of the drive motor or car speed. Therefore, the higher the speed of the drive motor under constant power, the greater the ability of regenerative braking. Low. When the brake pedal is stepped on, the drive motor usually runs at a low speed. At low speeds, the kinetic energy of an electric car is not enough to provide energy for the drive motor to generate the maximum braking torque, so the regenerative braking capacity will also increase with the speed of the car. Decrease and decrease.
Figure 2 shows the regenerative braking and hydraulic braking curves of electric cars. The regenerative braking torque of electric cars usually cannot provide sufficient braking deceleration like the braking system in traditional fuel cars. Therefore, in electric cars In, regenerative braking and hydraulic braking systems usually coexist. Only when the regenerative braking has reached the maximum braking capacity and can not meet the braking requirements, hydraulic braking will work.
The coordination between regenerative braking and hydraulic braking is the key to the problem, and the following special requirements should be considered: In order to make the driver have a smooth feeling when braking, the hydraulic braking torque should be based on the regenerative braking torque. The change is controlled, and finally the driver obtains the desired total torque. At the same time, the control of hydraulic brake should not cause the impact of the brake pedal, so it will not give the driver an abnormal feeling.
The ABS extended ESP function can be used to increase the oil pressure of the electric pump. This requires the ABS ESP module to communicate with the car control system, and the regenerative braking software can be written in the ABS module to drive the oil pump, control the friction brake and control the vacuum source of the brake boost. ABS communicates with the car controller to control the intensity of regenerative braking. The hydraulic braking torque is electronically controlled, and the generated hydraulic pressure is transmitted to the brake wheel cylinder. Therefore, the regenerative hydraulic braking system needs a mechanism to prevent brake failure. In order to improve the reliability of the system and meet the safety standards, the system generally adopts dual-pipe braking. When one of the pipelines fails, the other pipeline must be able to provide sufficient braking force.
Of course, because the ABS software is not open, and some electric car production or modification plants have no ABS development capabilities, many electric cars have built the regenerative braking software in the car controller, and the sensor is connected to the car controller, which increases the overall The signal processing burden of the car controller does not save components and makes the system appear chaotic, but it is also helpless.
In order for the car to brake stably, the braking force on the front and rear wheels must be well balanced. In addition, in order to prevent the car from slipping, the maximum braking force applied to the front and rear wheels should be lower than the maximum allowed (mainly determined by the rolling resistance coefficient).
The regenerative hydraulic hybrid braking system used in electric cars can meet the above requirements, and its structural design is shown in Figure 3. After the driver steps on the brake pedal, the electric pump pressurizes the brake fluid to generate the required braking force. The brake control and motor control work together to determine the regenerative braking torque on the electric car and the hydraulic braking force on the front and rear wheels. During regenerative braking, the regenerative braking control recovers the regenerative braking energy and reverse-charges it into the battery. The ABS and its brake proportional control valve on electric cars (which can be replaced by the extended function EBD of ABS) have the same function as those on traditional fuel cars, that is, they produce the maximum braking force. The electric pump can use the electric energy supply pump of the ESP in the extended function of the ABS in the existing automobile as the pressure source.
As mentioned earlier, the total braking torque on an electric car is the sum of regenerative braking torque and hydraulic braking torque, the distribution ratio between them is shown in Figure 4. The purpose is to maintain the maximum regenerative braking torque while driving The driver provides the same braking sensation as a fuel car. When the brake pedal force is small, only the regenerative braking torque is applied to the driving wheels and is proportional to the brake pedal force. The braking force on the non-driving wheels is provided by the hydraulic brake, and the hydraulic braking force is also related to the brake pedal. The force is proportional; when the brake pedal force exceeds a certain value, the maximum regenerative braking torque is all added to the driving wheels, and the hydraulic braking torque is also applied to the driving wheels to obtain the required braking torque. Therefore, the maximum regenerative braking torque can remain unchanged, so that the kinetic energy of the car can be fully recovered.
If the pipeline pressure of the brake system due to braking is higher (or the brake pedal is stepped deeper), it means that the greater the total braking torque required by the driver’s judgment, the braking torque of the non-driving wheels has been increasing , The braking torque sum of the driving wheels is also increasing. But the friction torque increases much, the regenerative braking torque does not increase, or even decreases, which requires the regenerative braking and ABS system to work in coordination.
In the ABS of a car with two independent front wheels and low rear wheel selection, the brake pressure sensor (hydraulic sensor) monitors the brake pressure (hydraulic or air pressure) of the brake system pipeline. The car with ABS uses a speed sensor and a pressure sensor ( It can also be the brake pedal stroke switch) to collect the brake state signal, and compare the deceleration value calculated according to the car speed with the set deceleration value to control the brake pressure.
