Power braking system of automobile structure

　　The characteristic of a power brake system is that the driver's body only serves as a control energy source, not a braking energy source.

　　In a power brake system, the energy used for braking is pneumatic energy generated by an air compressor or hydraulic energy generated by an oil pump, and the air compressor or oil pump is driven by the vehicle engine.

　　There are three types of power brake systems: pneumatic brake systems, pneumatic-hydraulic brake systems, and all-hydraulic power brake systems.

　　The power supply and transmission devices of a pneumatic brake system are all pneumatic. Its control device mainly consists of a brake pedal mechanism and pneumatic control components such as a brake valve. Some vehicles also have a hydraulic operating transmission device connected in series between the pedal mechanism and the brake valve.

　　The power supply and control devices of a pneumatic-hydraulic brake system are the same as those of a pneumatic brake system, but its transmission device includes both pneumatic and hydraulic parts.

　　In an all-hydraulic power brake system, except for the brake pedal mechanism, its power supply, control, and transmission devices are all hydraulic.

　　I. Pneumatic Brake System

　　Pneumatic brake systems are suitable for medium and heavy-duty trucks and buses.

　　1. Pneumatic Brake Circuit

　　There are three types of connecting pipelines between the components of a pneumatic brake system: ① Power supply pipelines, connecting pipelines between the components of the power supply device (such as the air compressor and air storage tank) and between the power supply device and the control device (such as the brake valve); ② Actuating pipelines, connecting pipelines between the control device and the brake actuator (such as the brake chamber); ③ Operating pipelines, connecting pipelines between one control device and another. If there is only one pneumatic control device in the braking system, i.e., only one brake valve, there is no operating pipeline.

　　2. Power Supply Device

　　The power supply device of a pneumatic brake system includes: ① An air compressor that generates pneumatic energy and an air storage tank that stores pneumatic energy; ② A pressure regulator and safety valve that limit the air pressure within a safe range; ③ An air intake filter, exhaust filter, pipeline filter, oil-water separator, air dryer, antifreeze device, etc., that improve the state of the energy transmission medium (air); ④ A multi-circuit pressure protection valve, etc., that protects the remaining circuits when one circuit fails, preventing the loss of pneumatic energy in those circuits.

　　1) Air Compressor and Pressure Regulator

　　The air compressor is directly driven by the engine through belt drive. There are single-cylinder and double-cylinder types. The air compressor of the Dongfeng EQ1090E vehicle is a single-cylinder air-cooled type.

　　When the pressure in the air storage tank reaches a certain value, the pressure regulator can cause the air compressor to idle. When the pressure in the air storage tank drops to a certain value, the pressure regulator can control the air compressor to charge the air storage tank.

　　The working principle of the air compressor unloading device and the pressure regulator controlling the working state of the air compressor is that when the pressure in the air storage tank reaches a certain value, the air pressure acting on the underside of the pressure regulator diaphragm assembly is greater than the pressure of the spring above it. The diaphragm assembly moves upward and drives the core tube to move upward. The valve under the core tube closes. The air storage tank pressure acts on the top of the unloading plunger, causing it to move down and open the intake valve. During the reciprocating motion of the air compressor, the intake valve remains open, and the air compressor idles. When the air pressure in the air storage tank drops to a certain value, the diaphragm assembly moves down under the action of the spring, the core tube opens the valve, the air pressure above the unloading plunger decreases, the plunger moves up, and the intake valve opens and closes normally, and the air compressor charges the air storage tank.

　　2) Filter Pressure Regulator

　　When the pressure in the air storage tank exceeds the specified value, the air compressor outlet is directly connected to the atmosphere through the pressure regulator, releasing the compressed air and stopping the charging of the air storage tank. The pressure regulator is also combined with an oil-water separator into one component, which is the filter pressure regulator.

　　3) Antifreeze Device

　　The compressed air output from the oil-water separator or filter pressure regulator may still contain a small amount of residual water. To prevent the residual water accumulated in the pipelines and other pneumatic components from freezing in cold weather, it is best to install an antifreeze device to add antifreeze to the air circuit when necessary to lower the freezing point of the water.

　　Its basic working principle is that when the winter temperature is below 5°C, the ethanol vapor in the antifreeze device will flow into the circuit with the compressed air. After the condensed water in the circuit is dissolved in the ethanol, the freezing point is lowered.

　　4) Multi-Circuit Pressure Protection Valve

　　The basic function of the multi-circuit pressure protection valve is that the compressed air from the air compressor can be supplied to the air storage tanks of each circuit through the multi-circuit pressure protection valve. When a circuit is damaged and leaks, the pressure protection valve can ensure that the remaining intact circuits continue to be charged.

　　The following figure shows a dual-circuit pressure protection valve, which ensures that one circuit can continue to be charged when the other circuit leaks.

　　The following figure shows a four-circuit pressure protection valve, which ensures that the other three circuits can operate normally at a slightly lower pressure when any one circuit is damaged and leaks.

　　3. Control Device

　　1) Brake Valve

　　The brake valve is the main control device in the pneumatic vehicle braking system. It is used to provide a follow-up action and ensure a sufficiently strong pedal feel, i.e., under a certain input pressure, its output pressure is a certain increasing function of the input control signal—pedal travel and pedal force. The change in its output pressure should be gradual within a certain range. The output pressure of the brake valve can be directly input to the brake chamber as a transmission device as the actuating pipeline pressure, but if necessary, it can also be input to another control device (such as a relay valve) as a control signal.

　　The Jiefang CA1091 vehicle uses a tandem double-chamber piston brake valve. The work of the upper and lower chambers is controlled by the brake pedal, and it can ensure that the other circuit can still work when one circuit leaks.

　　2) Hand Control Valve

　　The hand control valve can control the parking brake of the vehicle and the trailer. Because there is no requirement for gradual control of the parking brake, the hand control valve that controls the parking brake is actually just an air switch.

　　When the control lever is in the position shown in Ⅰ, the intake valve is closed, the exhaust valve is open, and the brake chamber is connected to the atmosphere through the core tube. When the control lever is in the position shown in Ⅱ, the intake valve is open, the exhaust valve is closed, and the brake chamber is connected to high-pressure air.

　　3) Quick release valve and relay valve

　　The function of the quick release valve is to ensure that the brake chamber is quickly vented when the brakes are released. The quick release valve is located on the pipeline between the brake valve and the brake chamber, close to the brake chamber. Because it is close to the brake chamber, the circuit through which the brake chamber exhausts is short, and the venting speed is faster. The state shown in the figure below is that the air inlet is closed and the air outlet is open.

　　The function of the relay valve is to allow compressed air to enter the brake chamber directly through the relay valve instead of through the brake valve, so as to shorten the air supply route and reduce the braking lag time. In the state shown in the figure below, the valve is pressed against both the valve seat of the valve body and the core tube, and both the intake valve and the exhaust valve are closed.

　　4) Shuttle valve (two-way valve)

　　The characteristic of the shuttle valve is that both chambers of the dual-chamber brake valve can input control air pressure to the trailer brake valve through the shuttle valve, ensuring that the trailer brake valve can still receive the braking control signal when one of the two braking circuits of the vehicle is damaged.

　　4. Brake chamber

　　The function of the brake chamber is to convert pneumatic energy into mechanical energy output, and the output mechanical energy is transmitted to the brake cam and other actuating devices to generate braking torque. There are three types of brake chambers: diaphragm type, piston type, and composite type.

　　1) Diaphragm type brake chamber

　　The two chambers of the diaphragm type brake chamber are isolated by a diaphragm, and the connecting fork is connected to the brake adjusting arm.

　　2) Piston type brake chamber

　　The piston type brake chamber has a larger push rod stroke, and its piston service life is also longer than that of the diaphragm type, but the overall structure of the chamber is more complex and the cost is higher, often used in heavy-duty trucks.

　　3) Composite brake chamber

　　The characteristic of the composite brake chamber is that it consists of a service brake chamber and a parking brake chamber, which serves both service braking and parking braking.

　　II. Air-over-hydraulic braking system and full hydraulic power braking system

　　1. Air-over-hydraulic braking system

　　The power supply device and control device of the air-over-hydraulic braking system are pneumatic, and the transmission device is a pneumatic-hydraulic combination. Pneumatic energy is converted into hydraulic energy through a series of power chambers and hydraulic master cylinders, and the hydraulic energy is transmitted to each wheel cylinder to generate braking action.

　　Advantages of the air-over-hydraulic braking system: ① The pneumatic system is compact, shortening the pipeline length and lag time. ② Using hydraulic wheel cylinders as brake actuators reduces unsprung mass. ③ When a vehicle using an air-over-hydraulic braking system tows a trailer, the trailer can use pneumatic braking or hydraulic braking. ④ The brakes of each axle can be actuated by hydraulic or pneumatic actuation.

　　2. Full hydraulic power braking system

　　The full hydraulic power braking system is a power braking device that uses hydraulic energy stored in an accumulator or restricts the circulation of liquid flow to generate hydraulic action.