The flow rate of gas is faster than that of liquid. When the air brake is working, it can exhaust directly, and it doesn't need a circuit like oil, just make the cylinder bigger, and the exhaust speed can be several times faster than that of hydraulic brake.
Second, the power contrast.
The air brake is relatively rough, and the air pressure generated is very high, which can reach eight atmospheres and can output super braking force. The air brake pedal mainly controls the opening of the air valve, so you can generate great braking force without pedaling hard. The oil pressure required by the hydraulic brake is provided by the brake oil. The pressure provided by brake oil is relatively small, without the amplification effect of jack, and it is only suitable for small and medium-sized vehicles. The principle of hydraulic braking is how hard you step on your foot, and with the vacuum booster, the braking effect is incomparable to that of pneumatic braking.
In addition to its large weight and volume, large trucks have to carry heavy objects, and the braking effect of the hydraulic brake itself is unstable, which will have great security risks. And because of the design of the oil circuit, there is no danger that the oil circuit will be blocked. If the brake cylinder leaks oil, the maintenance cost will be high, and the heat generated when the brake pads rub will be transferred to the brake oil. The consequence of this is that air enters the oil circuit, and the brake oil will boil, which will eventually lead to brake failure. Don't worry about this problem of air braking. If the brake pipe falls off or leaks, the brake will still work.
The hydraulic brake has a simple structure, occupies less space, and the oil pipe is easy to leak oil. If it must be installed on a large truck, it is necessary to increase the brake master cylinder and redesign a more powerful vacuum booster to provide the required braking force, but this will sacrifice the braking speed. Besides, in the case of a semi-trailer, the front of the car must be connected to the brake pipe of the trailer, but the metal oil pipe of the hydraulic brake cannot bend and move.
Although there are many and complicated parts in air brake, there is enough space on the big truck and the layout is convenient, which can provide super braking force without sacrificing braking speed. The air brake is designed as an air brake, and even if there is a problem with the brake air pipe, the brake will not fail.
The family car is an oil brake, which transmits braking force through brake oil. Both large trucks and buses are air-braked, and the braking mechanism is driven by compressed air. There are several reasons why big trucks don't use oil brakes.
1, air braking force is large.
Having said that, it is estimated that many people will object: how can the power of compressed air be compared with that of hydraulic pressure? Admittedly, the maximum force that hydraulic mechanism can produce is absolutely beyond the reach of compressed air, but the problem is that there is a difference between oil braking system and automobile hydraulic system. The hydraulic mechanism has a hydraulic pump to provide strong pressure, while the brake system has no hydraulic pump. Its power is the power of the vacuum booster, and the maximum pressure that the vacuum booster can provide will not be greater than one atmosphere. The air braking system is basically at 6-8 atmospheres. Therefore, the air brake can provide greater braking force.
2, the demand of hand brake system
Large trucks are relatively heavy, so the handbrake must provide enough braking force to ensure that the car will not move after parking. If you use an ordinary hand brake, the driver can't pull it tightly with all his strength. Therefore, the hand brake of large trucks is particularly different, called air brake. The above picture shows a brake pump with air brake. There is a big spring inside, which keeps the brake pump in a braking state forever. Before driving, compressed air needs to be introduced to open the spring to release the braking force before driving. When the handle brakes, the control mechanism releases compressed air, and the spring drives the brake push rod to brake the wheel. When the handbrake is released, compressed air is introduced into the system to press the spring back to release the handbrake. This hand brake system is the safest. If the compressed air is used up, just pull the handbrake and loosen the compressed air spring to provide a strong braking force to ensure that the vehicle will not move. We often see the sound of deflation after the handbrake of large trucks. This sound is the sound of releasing compressed air.
If large trucks use hydraulic braking system, it will involve a lot of hardware modification, and the whole braking system will be different. Pneumatic braking system is more suitable for large trucks.
The air pressure with speed difference is stored in the air storage tank through the engine air pump, and then transported to each pipeline through the air path control valve. Gas will naturally produce pressure through concentration, which can be output again without any energy. Air pressure is not only fast, but also does not produce any heat.
Hydraulic braking is different. First, there is a hydraulic pump. Hydraulic oil can't generate natural energy through compression, and only when the hydraulic pump works and exerts pressure can it generate force. When the truck needs to brake, the braking system of several wheels will start at the same time. Because hydraulic oil can't store energy, it can only be achieved by increasing the engine horsepower to increase the pressure of the hydraulic pump. This logic obviously does not hold water. When you brake and accelerate without a gear, it just forms a reaction force.
The hardware protection air pressure can be set to a certain value, such as the capacity of the air storage tank, and the applied pressure will not exceed this set value, which can better protect the hardware. The braking system of truck is completed by a lot of mechanical linkage work. If the pressure exceeds this value, the part will be deformed and twisted.
Although the hydraulic braking system can also set a certain value through the distribution valve, its working mode is completed by applying pressure to move the piston rod. You can't have better control when you brake lightly or need to stop. You can't stop until you reach a certain value. If it reaches a certain value, it may cause damage to the hardware, because even if the hydraulic cylinder can set the pressure value, the pressure generated is usually very large.
The difference of pressure emission The air brake system will emit gas when it reaches a certain pressure, and this emission is zero cost. The hydraulic braking system must also recover hydraulic oil. Compared with the pneumatic braking system, it needs to add a set of recovery pipeline and a hydraulic oil cooling system. The system is more complex, more costly and more inefficient.
There are more and more components of truck air pressure control, such as braking system, exhaust braking system, airbag damping system, clutch system and so on. Gas can be stored in advance, the flow rate is faster, no heat is generated, no recovery system is needed, and the cost is low, which is impossible for hydraulic braking system.
Why do freight cars use air brakes instead of hydraulic oil?
1. The working principle of the hydraulic brake is to use the master brake cylinder to pressurize the hydraulic oil, and the pressure can make the brake shoe work in the brake cylinder to achieve the purpose of braking.
The invention is simple in structure and small in installation space, and only needs a master brake cylinder, a sub brake cylinder, an oil cup and a connecting pipeline, without other auxiliary equipment.
The characteristics of oil braking are: slow response, soft braking and low strength.
Oil brakes are mostly used in small and medium-sized cars, with small volume and simple structure. After the engine is turned off, the pressure of the oil brake will be released quickly, and there will be no pressure after stepping on the brake several times, and the braking duration will be short.
Therefore, oil brakes are mostly used in small and medium-sized cars, with light weight, small volume, simple structure, small load and no need for too much effort.
2. Air braking: The compressed air generated by the air pump works to push the brake shoe, thus achieving the purpose of braking.
The structure of air brake is complicated. Besides master brake cylinder, slave cylinder and brake pipeline, there are also air pump, air reservoir, high-pressure controller, relay valve and other components, which need a lot of installation space. Moreover, in order to ensure safety, the rear brake slave cylinder is now gas-cut.
The characteristics of air braking are: quick response, rough braking and great strength. After the engine is turned off, the gas stored in the high-pressure gas storage tank can make the brake work for a long time until the air pressure is insufficient and the braking lasts for a long time.
Therefore, air braking is mostly used in large trucks and buses, which has the advantages of heavy weight, large space, heavy load, large braking force, long service distance and fast response speed, and can get timely response in case of emergency braking.
These are the reasons why freight cars use air brakes instead of hydraulic oil.
Pneumatic braking for large cars and hydraulic braking for small cars are now almost standard. In fact, there used to be a kind of brake called pneumatic top hydraulic brake. This braking method is to connect a set of hydraulic braking system in series with the conventional pneumatic braking system (almost the same as the braking system of small cars). This design improves the sudden force of pneumatic braking and makes the braking more linear, and the second is to improve the braking response speed. Compared with hydraulic braking, pneumatic braking has more power (at least eight atmospheres), but its response speed is not as good as hydraulic braking, because gas is compressed after all, and liquid is almost equivalent to incompressible substance. Pneumatic oil brakes used to be common on some trucks of Mitsubishi and Isuzu.
However, due to Japan's geography and landform, after all, it is not overly dependent on heavy trucks, so most of the gas-filled oil brakes are used on light trucks; To put it simply, Japan has a small geographical area and is surrounded by the sea. Therefore, Japan's large-scale transportation can be completed directly by sea, and the cargo ship has sufficient carrying capacity; However, our country, North America and continental Europe are different. Because the geographical depth is too deep, it relies heavily on land transportation, so road transportation becomes extremely important. Therefore, Ouka and Meika are airlocks, and our heavy trucks are airlocks, so our heavy trucks are airlocks. Therefore, the popular air brake in the 1990s gradually disappeared, which proves that the air brake is hard to give up for heavy trucks.
Heavy trucks use pneumatic braking instead of hydraulic braking, not to reduce costs; In fact, the braking system of heavy trucks is far more complicated, high-end and expensive than our passenger cars, because the kinetic energy carried by heavy trucks is much greater than that of passenger cars; Our passenger cars are often above 200nm, and the power of heavy trucks is not an order of magnitude with passenger cars. When the power of heavy trucks is at least 2000N·m, the speed of passenger cars with 0 1 is only 15 seconds slow, while a full-load heavy truck with 0 1 takes at least 50 seconds. Therefore, it is difficult to stop a heavy truck loaded with high speed because it contains too much kinetic energy. The requirement for the braking system is not only sufficient braking force, but also sufficient stability!
So why don't heavy trucks use hydraulic brakes? In fact, the possibility of brake fluid boiling still exists. As shown in the above figure, even the dry boiling point of dot 5 is only 260. Of course, such a high boiling point is enough for passenger cars, but the heat generated by heavy truck braking is too large. It is often seen that the wheels are burnt out when heavy trucks are braking, right? Therefore, when a heavy truck is loaded on a long downhill road, if the heat generated by frequent braking is too easy to cause the brake fluid to boil, a large number of bubbles will appear in the brake pipeline, which will make the brake soft or even lose its braking ability, because once a large number of bubbles appear in the brake pipeline, it is equivalent to adding a useless trip, and the gas is compressible, and the braking ability will be seriously reduced!
In addition to the hidden danger of brake fluid boiling, the heat generated during heavy truck braking can easily lead to the direct melting or falling off of the closed cup of the hydraulic wheel cylinder, which directly leads to the leakage of brake fluid and makes the automobile completely lose its braking ability. Therefore, compared with more solid and direct pneumatic braking, hydraulic braking is fragile after all, at least for heavy trucks. Therefore, whether it is a heavy truck or a heavier train, the braking system is pneumatic braking rather than hydraulic braking, because the heat generated by these large mechanical braking and hydraulic braking systems is hard to bear, and the sealing system is mostly rubber products. How to withstand the high heat of rubber products is a big problem, so it is necessary to ensure the continuous stability in harsh working environment, and pneumatic braking is the first choice for heavy trucks!
Therefore, heavy trucks at home and abroad will choose air braking, which is not only a problem of braking force; The braking force of air cap oil can also meet the requirements, and the pure oil brake can also meet the standards if it is equipped with hydraulic pump and braking force; Therefore, air braking and hydraulic braking cannot be distinguished by braking force. The key is stability, and air braking is more ideal in this respect. Big cars are different from small cars, which are easier to drive fast, so the response speed of braking is very critical, so hydraulic braking is the most suitable; Heavy trucks run slowly, but the kinetic energy is too great, and a strong enough braking force is the most important. Even if a fully loaded heavy truck travels at a speed of 20km/h (slow enough), the braking distance will be very long, so the braking response speed is not the first (not too slow), and the braking force is the key!
There are several reasons why air braking is used in large trucks: the cost of air braking is lower than that of oil braking, which is environmentally friendly, the operation of air braking is easier, the intensity of air braking is greater, the arrangement of air braking is easier, and the transmission speed of compressed air is faster and easier to maintain than oil braking. Not only at home, but also abroad, most large trucks are airlocks.
Why do big trucks use air brakes?
High sensitivity, low maintenance cost and good safety.
Sensitivity 1 As we all know, gas flows faster than liquid. The body of the van is long and big. When using hydraulic braking, it is necessary to make the hydraulic pump very large, and the hydraulic oil of the oil pump should fill the piston, which requires a circuit, so that the braking speed can not keep up. When the air brake is running, it can directly exhaust the gas, and it doesn't need a circuit like oil, just make the cylinder bigger, and the exhaust speed can be several times faster than that of hydraulic brake.
2 system layout
The air pump generates high-pressure gas.
The dryer separates the high-temperature and high-pressure gas cooling gas from the water and controls the air pressure of the whole vehicle.
The four-circuit protection valve distributes gas to each cylinder with six air pressures, and four chambers are blocked. If the air pressure is above six, other cabins can still work normally, and there is enough time to go to the maintenance station for maintenance.
Gas cylinders store high-pressure gas.
Gas distribution of semi-trailer with throttle trailer valve
The brake valve provides pressure for the brake cylinder.
The brake air pump provides braking force to the wheel segment.
safe
No matter which brake pipe leaks, breaks or the valve fails, the braking of the whole vehicle will not fail.
Xiao Liu teaches you to repair the truck and updates it irregularly. Thank you for your attention.
Strong and quick response.
Because the van is very big. Oil must be used a lot. Air is much cheaper than oil. What do you think you will use?
Hello, first of all, because of its special function, trucks have a large total mass during transportation and need to brake quickly, and air braking conforms to this feature, so trucks use air braking more.
The speed of the oil brake is slow, but the equipment used is small, so the vehicle oil brake can not only meet the braking requirements, but also leave room for installation.