The vacuum source of the vacuum booster of the brake system of traditional internal combustion engine cars comes from the engine intake manifold, and the vacuum degree can generally reach 005~0.07MPa. For pure electric cars or fuel cell cars modified from traditional models, after the engine assembly is removed, the brake system loses the vacuum assist function because there is no vacuum power source, and the braking force generated by manpower alone cannot satisfy the driving system. Therefore, it is necessary to reform the vacuum booster of the brake system, and the core problem of the reform is how to generate a vacuum source with sufficient pressure. In order to produce sufficient vacuum, in addition to an electric vacuum pump with sufficient displacement, in order to save energy and reliability, it is necessary to design a suitable working time for the electric vacuum pump motor. In order to achieve the same vacuum requirements as fuel cars, the electric vacuum pump needs to generate a vacuum of more than 50kPa within 4~5.
As shown in Figure 1, the electric vacuum booster is installed between the brake pedal and the brake master cylinder, and is directly manipulated by the pedal through the push rod. The force generated by the booster and the pedal is superimposed on the push rod of the master cylinder to increase the output pressure of the master cylinder. The vacuum booster is divided into a front chamber (vacuum chamber) and a rear chamber (connected to the atmosphere) by a piston with a rubber diaphragm (diaphragm). Generally, the vacuum degree of the vacuum chamber is 60~8kPa (that is, the vacuum degree that the vacuum pump can provide) ). The amount of boost that the vacuum booster can provide depends on the difference in air pressure between the front chamber and the rear chamber. When the vacuum degree of the front chamber reaches the maximum value, the vacuum booster can provide the maximum brake boost. The vacuum degree and speed produced by the vacuum pump are related to the working state of the vacuum booster, and the capacity of the vacuum pump is related to the performance of the booster, which in turn affects whether the brake system can work normally under various working conditions.
The control process of the electric vacuum assisted braking system is as follows.
①Turn on the car’s 1V power supply, the pressure delay switch is closed, and the vacuum pump works for about 30s and then the switch is disconnected. At this time, the vacuum in the vacuum tank is about 80kPa
② When the vacuum degree in the vacuum tank drops to 55kPa, the pressure delay switch is closed again.
③When the vacuum degree in the vacuum tank drops to about 34kPa, the pressure alarm will send out a signal.
④ If the vacuum pump control switch is turned on and off for a short period of time, it means that a leak has occurred.
According to this control strategy, an intermittent vacuum generation system is designed. The basic working principle of the system: When the driver starts the car, the 12V power supply is turned on, the pressure delay switch and the pressure alarm start the pressure self-check, if the vacuum tank When the vacuum degree is less than 55Pa, the pressure diaphragm will squeeze the contacts, thus turning on the power supply, and the vacuum pump starts to work; when the vacuum degree increases to 55kPa, the pressure delay switch is turned off, and then the vacuum pump continues to work for about 30s through the delay relay After stopping; each time the driver makes a braking action, the pressure delay switch will self-check to determine whether the electric vacuum pump should work; if the vacuum in the vacuum tank is lower than 34kPa, the vacuum booster cannot provide effective vacuum assistance , The pressure alarm will send out a signal to remind the driver to pay attention to the driving speed.
The electric vacuum pump can also be controlled by an electric control unit, as long as the pressure switch is replaced with an absolute pressure sensor, and the electric vacuum pump is controlled by the control unit to control the relay. Some pure electric cars adopt a closed-loop vacuum control system composed of vacuum booster, vacuum sensor, car controller ECU, electric vacuum pump working relay, and vacuum pump motor to ensure the normal operation of the vacuum booster during braking.