What is an Air Brake System? Read the Working and Advantages.

The air brake system is a type of power brake system used in automobiles. When the driver applies the brake this air brake system reduces the effort and maintains the effectiveness of the braking. In this article, I will discuss the topic “What is an air brake system”. Read the working and the function of the air brake system.


What is an Air Brake System?

The manufacturers of braking systems offer a variety of air brake equipment However, the simplest system consists of an air compressor, a brake valve, a series of brake chambers, an unloader valve, a pressure gauge, and a safety valve. These are all connected by lines of tubing.

The other braking systems may have additional components such as a stop-light switch, a low-pressure indicator, an air supply valve to supply air for tire inflation, a quick release valve to release air quickly from the front brake chambers when the pedal is released, a limiting valve for limiting the maximum pressure in the front brake chambers and a relay valve to help in quick admission and release of air from rear brake chambers.

The driver controls the braking system using a brake pedal. When the brake pedal is pressed, it sends a signal to release air pressure and apply the brakes. Brake chambers are there to convert the compressed air into mechanical force. Brake chambers contain diaphragms that expand when air pressure is applied, pushing rods to actuate the brakes.

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Working of Air Brake System


This shows the layout of an air brake system for a bus or truck. The air compressor, governor, pressure gauge, safety valve, and reservoir constitute the compressing and the control units whereas the rest of them are termed as application units. The compressed air available on the vehicle is also used for the operation of additional assemblies of the vehicle such as horns, windshield wipers, etc.

The compressor sends compressed air to the reservoirs which are connected to the brake valve. The lines of tubing from the brake valve extend to the front and rear brake chambers. When the drive depresses the pedal, it operates the brake valve, thus admitting compressed air to all the brake chambers. The compressed air operates the diaphragm of the brake chambers thereby applying the brakes.

In India, the air brake system is used on commercial vehicles manufactured by M/S Ashok Leyland Ltd. The front and rear wheels of the vehicle are controlled directly by the foot brake through the diaphragm-operated brake chambers fitted on the axles.

The brake valve is considered the heart of the air brake system. Its main purpose is to control the vehicle brakes in the air pressure system. The brake valve is operated employing linkages from a conventional brake pedal.

Movement of the pedal controls the inlet and exhaust valve assembly of the brake valve which in turn regulates the air pressure in the brake chambers on the vehicle. When the pedal is depressed, compressed air passes through the valve and the pressure is developed in the brake chambers to apply the brakes.

The valve automatically adjusts this pressure in proportion to the degree of pedal movement so that the further the pedal is depressed, the greater the braking pressure in the chambers until the point is reached where full reservoir pressure is delivered to apply the brakes.

In addition to providing precise control of braking pressure during application and releases, the valve imparts a relative reaction to the movement of the pedal so that the driver can sense the degree of the brake application.

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How does the Air Brake Work as a Vaccum Brake?


In the air brakes, the force of brake application is because of the difference of pressures on the opposite sides of the diaphragm. One side of the piston or diaphragm is exposed to the higher pressure while the other to the atmospheric pressure.

It is the potential difference that is utilized to create the braking effect. A similar type of effect can be obtained if one side of the piston or diaphragm is exposed to the atmospheric pressure while the other side to a pressure below the atmospheric pressure.

The pressure below the atmosphere is obtained by exhausting air from it. This is the principle of vacuum brakes. The system may be atmospheric-suspended or vacuum-suspended. If both the ends of the power cylinder are with the atmosphere and at the time of brake application one of them is connected to the inlet manifold, the system is said to be atmospheric suspended.

If both the ends of the power cylinder are normally in communication with the inlet manifold and at the time of brake application one of them is connected to the atmosphere, the system is said to be vacuum suspended. These days vacuum suspended types of brakes are predominently used. It may be mentioned that the servo is also used in place of power cylinders.

In the vacuum-suspended servo, the air is first exhausted from both sides of the piston in a large cylinder and during the application of the brake, the air is then admitted from the atmosphere to one side of the piston. The amount of air entering the cylinder is controlled by the driver.

The piston of the cylinder moves under the pressure differential which in turn moves the hydraulic piston, thus forcing the fluid into brake pipes into the wheel cylinders. The vacuum is obtained from the inlet manifold of the engine. A pressure reduction to the tune of about 3.3 N/cm2 is obtained with this method.

If the diameter of the vacuum piston is 15 cm, then the force developed by the simple vacuum piston is of the order of about  1237.5 N  assuming 10.3 N/cm^2 as the atmospheric pressure. In this type of braking system, a vacuum reserve tank is fitted between the servo and the engine.

The tank and the servo are protected with the help of non-return valves to enable them to hold some vacuum in case of failure of other components. The tank provides a reserve of some vacuum which can be utilized for making the number of stops in quick succession.

Two different arrangements can be had namely in the direct servo system and the direct-acting servo system. In the former type, the servo is mounted in the hydraulic line from the master cylinder and operates on a slave cylinder.

The slave cylinder in Turn is connected to the various wheel cylinders. Fig shows the arrangement of such types of servo brakes in a line diagram. In the case of the direct-acting servo, the unit is connected to th brae pedal through mechanical linkages and directly operates the master cylinder.

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Components of Air Brake System

Air brake systems play a crucial role in ensuring the controlled and safe deceleration of large vehicles like trucks, buses, and trains. In this blog post, we will delve into the components that make up an air brake system, exploring their functions and the intricate interplay that ensures the effective operation of these essential safety features.

These components are-

1. Compressor

At the heart of the air brake system is the compressor. This component is responsible for pressurizing the air that powers the entire braking system. As the vehicle operates, the compressor continually replenishes the compressed air supply, ensuring a consistent and reliable source for the braking system.

2. Air Reservoirs

Compressed air is stored in reservoirs strategically placed throughout the vehicle. These reservoirs act as a buffer, providing a readily available source of pressurized air for the braking system. This stored air becomes crucial during braking, where a sudden demand for compressed air is required.

3. Air Dryer

Moisture is the enemy of any pneumatic system, and the air brake system is no exception. The air dryer is tasked with removing moisture from the compressed air to prevent corrosion and freezing, which could impair the proper functioning of the braking system.

4. Brake Pedal and Valve System

The brake pedal, a familiar component in any vehicle, is connected to a series of valves in an air brake system. When the driver applies pressure to the brake pedal, these valves regulate the release of compressed air to the brake chambers.

5. Brake Chambers

Brake chambers are pivotal in converting the energy from the compressed air into mechanical force. They consist of a diaphragm that, when exposed to compressed air, pushes against a rod connected to the brake mechanism, initiating the braking process.

6. Slack Adjusters

Ensuring that brakes maintain the correct clearance is the job of Slack adjusters. These components automatically adjust the distance between the brake shoes and the drum to compensate for wear and tear, ensuring optimal braking performance over time.

7. Foundation Brakes

Located at each wheel, the foundation brakes consist of brake shoes, brake drums, and associated components. When the compressed air reaches the brake chambers, it activates the foundation brakes, causing the brake shoes to press against the drum and create the friction necessary for deceleration.

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Application of Air Brake System

This air brake system is very efficient and can be used in various industries. Most of the air brake systems are used in the automobile vehicles. So, the application of air brake systems is-

  1. One of the primary applications of air brake systems is in commercial trucks and trailers
  2. Urban transit buses, long-distance coaches, and school buses often employ air brake systems.
  3. Nowadays the air brake system is used in the Urban Ac transport buses.
  4. Trains utilize air brake systems for both locomotives and cars. The consistency and reliability of air brakes are essential for managing the immense weight and momentum of a train. They allow for precise control during acceleration, deceleration, and emergency stops.
  5. Heavy equipment used in construction sites and mining operations, such as dump trucks, excavators, and bulldozers, often rely on air brake systems.
  6. Some emergency response vehicles, particularly those designed for specialized functions and heavy-duty applications, may incorporate air brake systems.

Advantages of Air Brake System

The advantages of air brake systems are-

  1. One of the primary advantages of air brake systems is their ability to provide powerful and consistent braking performance, especially in heavy-duty applications.
  2. Air brake systems exhibit excellent fade resistance, meaning they maintain their effectiveness even under prolonged or repeated use.
  3. Air brake systems are known for reducing brake wear, contributing to longer brake life, and decreasing maintenance costs.
  4. In the event of air pressure loss, air brake systems are designed with fail-safe features. If there is a loss of air pressure, the system defaults to a fully engaged brake state, providing a safety mechanism to bring the vehicle to a stop and prevent uncontrolled movement.
  5. Many modern air brake systems are compatible with Anti-lock Braking Systems (ABS), adding an extra layer of safety.
  6. Air brake systems are particularly well-suited for heavy vehicles, including trucks, buses, and trains.

Disadvantages of Air Brake System

Disadvantages of Air Brakes Systems are-

  1. Air brake systems are inherently more complex than hydraulic systems. The inclusion of components like compressors, air dryers, and various valves increases the system’s complexity.
  2. The functionality of air brake systems relies entirely on the availability of compressed air.
  3. The air compressor places an additional load on the engine.
  4. In comparison to hydraulic systems, air brake systems may have a slightly longer response time.
  5. Air brake systems can generate noise during their operation, particularly when the compressor is running.
  6. The initial cost of installing an air brake system can be higher than other braking systems, such as hydraulic systems.
  7. Air brake systems may be less effective at very low speeds.

Conclusion on What is an Air Brake System

In conclusion, while air brake systems offer many advantages, it’s important to be aware of the potential disadvantages associated with their use. Operators and maintenance teams should carefully consider these factors and implement proper maintenance practices to ensure the continued reliability and safety of the braking system.

Frequently Asked Questions(FAQ’S)

Can air brake systems be retrofitted on existing vehicles?

While it is possible to retrofit air brake systems on certain vehicles, it requires careful consideration of the vehicle’s design, compatibility, and adherence to safety standards. Consulting with a qualified professional is recommended.

How do air brake systems ensure safety in case of air loss?

Air brake systems incorporate fail-safe features, such as spring brakes, that automatically engage in the event of air loss. This ensures that the vehicle can come to a controlled stop even if there is a failure in the compressed air supply.

Can air brake systems freeze in cold weather?

In extremely cold conditions, moisture in the air can freeze within the system. To address this, air brake systems are equipped with air dryers to remove moisture and prevent freezing. However, extreme temperatures may still pose challenges.


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