I. Electric Vacuum Assisted Braking System
Automotive braking systems typically employ vacuum boosters or pneumatic boosters. The greater the vacuum level generated by the vacuum pump, the better the braking assist performance, and the less effort the driver needs to exert when pressing the pedal. Therefore, the design or selection of the electric vacuum pump for a vacuum-assisted braking system should aim to ensure that the vacuum level meets the braking performance requirements.
Calculation results show that when the minimum vacuum level of the electric vacuum pump is 37.5 kPa, it can provide braking assistance to the braking system that meets the design requirements. Figure 6-1 shows the basic structure of an electric vacuum-assisted braking system. The vacuum booster is installed between the brake pedal and the master cylinder, and is directly operated by the pedal via a push rod. The arrangement of the vacuum braking system on a certain type of electric vehicle is shown in Figure 6-2. The forces generated by the booster and the pedal are superimposed and act on the master cylinder push rod to increase the output pressure of the master cylinder. The vacuum booster consists of a piston with a rubber diaphragm, divided into a front chamber and a rear chamber (which can be connected to the atmosphere when the atmospheric valve is open). Typically, the vacuum level in the atmospheric chamber is 60–80 kPa (i.e., the vacuum level that the vacuum pump can provide). The amount of assistance provided by the vacuum booster depends on the pressure difference between its atmospheric chamber and variable pressure chamber.
When the pressure in the transformer chamber reaches the external atmospheric pressure, the vacuum booster can provide maximum braking assistance. The magnitude and speed of the vacuum generated by the vacuum pump affect the working state of the vacuum booster, and the capacity of the vacuum pump affects the performance of the booster, which in turn affects whether the braking system can work normally under various operating conditions.
Figure 6-1 Basic Components of an Electric Vacuum Assisted Braking System
Figure 6-2 Arrangement of electric vacuum pump and vacuum tank on the vehicle
The control process of the electric vacuum-assisted braking system is as follows.
① Connect the car's 12V power supply. The pressure delay switch will close. The vacuum pump will work for about 30 seconds and then the switch will open. At this time, the vacuum level inside the vacuum tank will be about 80 kPa.
②When the vacuum level inside the vacuum tank drops to 55kPa, the pressure delay switch closes again.
③ When the vacuum level inside the vacuum tank drops to approximately 34 kPa, the pressure alarm will sound. If the vacuum pump control switch opens and closes very noticeably for a short period, it indicates a leak. Based on this control strategy, an intermittent vacuum generation system was designed. The basic working principle of this intermittent vacuum generation system is as follows: when the driver starts the car, the 12V power supply is connected, and the pressure delay switch and pressure alarm begin pressure self-checking. If the vacuum level inside the vacuum tank is less than 55 kPa, the pressure diaphragm will squeeze the contacts, thereby connecting the power supply and starting the vacuum pump.
When the vacuum level increases to 55 kPa, the pressure delay switch disconnects, and then the vacuum pump continues to operate for approximately 30 seconds before stopping via the time-delay relay. Each time the driver applies the brakes, the pressure delay switch performs a self-check to determine whether the electric vacuum pump should operate. If the vacuum level in the vacuum tank is below 34 kPa, the vacuum booster cannot provide effective vacuum assistance, and the pressure alarm will sound to alert the driver to adjust their speed. The electric vacuum pump can also be controlled by an electronic control unit (ECU). Simply replace the pressure switch with an absolute pressure sensor, and the electric vacuum pump will be controlled by the control unit and relays.
Some pure electric vehicles in China use a closed-loop vacuum control system consisting of a vacuum booster vacuum sensor, a vehicle controller ECU, an electric vacuum pump working relay, and a vacuum pump motor to ensure the normal operation of the vacuum booster during braking.
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