pneumatic or air braking system in automobile | Construction and working of pneumatic braking system

Pneumatic brake system

An air operated brake system or pneumatic brake system is employed predominantly in medium and heavy duty trucks, because of the following advantages.

1. The pressure of the precompressed air allows practically any force required for braking to be developed with a very small effort applied by the driver to the brake pedal.

2. The compressed air can also be used to inflate the tyres, drive the windshield wiper, actuate steering gear booster, open and close doors in buses etc.

3. An air operated brake is the most dependable and convenient device for braking full trailers and semitrailers.

Construction of pneumatic braking system


The simplest air brake system consists of an air compressor, a brake valve, series of brake chambers at the wheels, unloader valve, a pressure gauge and a safety valve, and an air reservoir. These are all connected by tubes.

Some air braking systems may have additional components such as stop light switch, a low pressure indicator, an air supply valve to supply air for tyre inflation, a quick release air quickly from the front brake chambers when the brake 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 the rear brake chambers.

Working of pneumatic braking system

The air compressor operated by the engine forces air at a pressure of 9-10 kscm, through the water and oil separator to the air reservoir. The air pressure in the reservoir is indicated by a pressure gauge. The reservoir contains enough compressed air for several braking operations. From the reservoir the air is supplied to the brake valve. As long as brake pedal is not depressed, brake valves stop the passage of air to brake chambers and there is no braking effect.


When the brake pedal is depressed, the brake valves varies its position and compressed air is admitted into the wheel brake chambers. In the chambers the air acts upon flexible diaphragms, move4s them the pushes out the rods connected with the levers of the brake gear cams. The cams turn and separate the shoes thus braking the wheels.

When the brake pedal is released, the supply of compressed air is cut off from the brake chambers and they are connected to the atmosphere. The pressure in the chambers drops, the brake shoes are returned to their initial position and the wheels run free.The brake valve is equipped with a servo mechanism which ensures that the braking force on the shoes is proportional to the force applied to the pedal.Besides the valve imparts a relative reaction to the movement of the pedal so that the driver can sense the degree of brake application.

Significant components of air brake system


An unloader valve is mounted in the air pressure system between the compressor and reservoir to control the pressure of air in the reservoir. The unloader valve relieves the compressor of its pumping load once the unloader cut out pressure is obtained and seals the reservoir when the compressor has built up a pressure depending upon the setting of the adjusting screw. The unloader then delivers the air delivered by the compressor to the atmosphere thus allowing the compressor to run light whilst the reservoir contains an adequate supply of air.

Air filters


Air filters are used in the air pressure system to prevent particles of foreign matter from entering the operating system airlines or equipment. These are mounted on the chassis and have drain plug to allow the condensate to be easily removed.

Air reservoir

The function of the air reservoir is to store the compressed air so that there will always be an ample supply available for immediate use in brake operation. It provides storage of sufficient compressed air to permit several brake applications even after the engine has stopped and just restarted. It also provides a space where the air is heated during compression might cool, and oil and water vapours condense.

An electro pneumatic switch is also incorporated in the circuit. This switch operates in conjunction with the brake valve the stop light, by completing the stop light electrical circuit when brake is applied.

introduction to brake system of an automobile | components of brake system in an automobile | history and overview of a brake system

Introduction of brake system

The brake system is one of the most significant systems of a vehicle. It has the following basic roles.

1. It should slow a moving vehicle.

2. It should bring a vehicle to a stop.

3. It needs to hold a vehicle stationary when stopped.

4. It permits directional control during maximum braking.

In a vehicle if the brake system does not operate properly the driver and passengers could face an accident. Professionals who service the brake system must be highly skilled experts because the work they do save lives.

During the design stage, if all of the various dynamics are not considered most braking systems will result in improper braking. When the brake system is not functioning correctly it will be up to the driver to compensate risk. In many cases, the response of the human is either too slow or too quick to react to a braking state. In both cases, there is chance for loss of vehicle control.

Brake System Overview

The complete brake system contains of the major components shown in Figure. The complete brake system can be divided into

  • The service brakes slow and stop the moving vehicle.
  • The parking brakes hold the vehicle stationary.
  • The antilock brake system (ABS) is a another major subsystem.01 - BRAKE SYSTEM OF AN AUTOMOBILE - COMPONENTS OF BRAKE SYSTEM IN AN AUTOMOBILE

Many cars now also include traction control as part of the brake system functions. Braking action on an automobile initiates with the driver’s foot on the brake pedal. The driver steps the pedal and the pedal transfers that force to the master cylinder pistons. The brake pedal also increases the force of the driver’s foot through lever. The brake pedal is mounted on a lever with a pivot near the top of the lever. The movement of the pedal causes a pushrod to move against a master cylinder. The master cylinder is mounted inside the engine compartment on the rear bulkhead. The master cylinder is a hydraulic pump that is operated by the driver through the brake pedal. Most brake pedal installations are an example of what is called a second class lever. A second class lever has a pivot point (or fulcrum) at one end and force applied to the other end. A second class lever transmits the output force in the same direction as the input force, and multiplies the input force, based on where the output load is placed. The brake pedal has a 10-inch lever, and the load is 2 inches from the fulcrum (8 inches from the pedal). The pedal ratio, or the force multiplying factor, is the length of the lever divided by the distance of the load from the fulcrum.

Types of braking system

01 - types of braking system in automobile - different types of automobile braking system

History of Brake system


Modern automobile brakes evolved from the comparatively crude brakes of cart. The earliest automobile brakes were pads or blocks applied by levers and linkage to surface of a solid tire on a wooden-spoked wheel. A similar principle of leverage that employment in popular foot lever installations inflated the force of the brake pad applied to the solid tire. These brakes worked well with speeds of 10 mph to 20 mph and minute traffic. Higher performance and gas tires meant that early wagon brakes were temporary on cars. By the tip of the primary decade of the 20th century, cars were exploitation either external contracting band brakes or internal expanding drum brakes. A number of internal increasing band brakes were tried on some early cars. External contracting brakes have a band lined with friction material wrapped around a drum situated on the driveline or on the wheels. The band is anchored at one finish or at the center; levers and linkage tighten the band round the drum for braking force. Servo action on associate external band brake tends to form the brake grab at high brake forces and high drum speed. Different issues related to band brakes enclosed dirt and water harm and loss of friction with external bands and therefore the tendency of those brakes to lock if the drum hot and expanded an excessive amount of. Internal band brakes conjointly damaged from band and drum heating and reduced braking force .As drum brakes evolved, internal-expanding shoe-and-drum brakes became the quality. External contracting band brakes were used as parking brakes till the late Nineteen Fifties, however their days as service brakes were over by the late Twenties.