Air Brakes

Various ways have been found to utilize the air we breathe for our work. Pneumatic drills use air to drill and break the concrete. Heavy loads are supported on air filled rubber tires. Air is also used to provide smooth and comfortable rides in the form of suspension. One such application of air is in the field of brakes, where compressed air is used to arrest the motion of the tires.

Who Invented Air Brake?

George Westinghouse patented a safer air brake on March 5, 1872. These air brakes were first used in the railways. Later, in the early 20^th century, vehicle manufacturers started using air brakes in trucks and other heavy load carrying vehicles. In modern days, you will find air brakes in most of the buses, trucks, trailers and semi-trailers.

Air Brakes Components:

Air brake system uses driver’s control to transmit air pressure to the service brakes. The components of the air brake system are as follows:

·         Air Compressor: It pumps the air to the storage tank or reservoir. The compressor is run by the engine crankshaft through gears or belt drive. The compressor is either air cooled or oil cooled.

·         Governor: The governor controls the pumping of air from the compressor by monitoring the pressure in the air tank. If the air tank reaches the cut-in pressure level (5.85 bar), the governor allows the compressor to pump the air. On the other hand, if the air tank reaches the cut-out pressure (9.3 bar), the governor stops the compressor from pumping air.

·         Air Dryer: Air dryers are installed before the reservoir tank to absorb the moisture from the air. Moisture build up in the tank can lead to brake failures.

·         One-Way Check Valve: It allows the air to flow in one direction only. It is located between the air compressor and the 1^st tank. It prevents the air from going out if the compressor develops a leak.

·         Air Storage Tank: It stores the compressed air pumped by the compressor. The tanks store enough air for braking even if the compressor has stopped working. There is a possibility of moisture and oil entering the tank. Water can cause brake failures, especially in winter where water gets converted into ice. Usually they provide two tanks. First the air is supplied to Wet tank, where the moisture and oil is drained. The moisture free air is then supplied to the Dry tank.

·         Drain Valve: To overcome the problem of moisture and oil getting accumulated in the tank, drain valves are provided at the bottom to drain the water and oil. It usually drains automatically.

·         Safety Valve: Safety valves are imperative in order to make sure that pressure doesn’t build up beyond a point of danger. The valve usually opens if the pressure crosses 10 bar. You would have often heard a loud hissing noise coming out of a truck. The release of pressure through the safety valves causes such noise.

·         Brake Pedal (Foot Valve): Brake pedal or foot valve allows the air pressure from the tank to be applied to the brake chambers. When we press on the brake pedal, the foot valve opens in the storage tank. Higher the force applied on the brake pedal, greater will be the air pressure applied to the brake chambers.

·         Drum Brake: Let’s consider a drum brake in use. When the brake pedal is pressed, the compressed air is supplied to the brake chamber. It is a cylindrical container that pushes the push rod out when pressure is applied. The push rod then moves the slack adjuster, thereby twisting the S-cam. The twisting of S-cam forces the brake shoes to move away from each other and press against the inside of the brake drum.

Alcohol Evaporators:

Some vehicles equipped with air brakes have alcohol evaporators that release alcohol in the storage tank to prevent the formation of ice in the tank.

Emergency Brakes and Parking Brakes:

All vehicles equipped with air brakes must mandatorily have emergency brake and parking brake. Usually spring brakes are used for this purpose. When you are driving, the spring brakes are held back by air pressure. If the air pressure is removed, spring brakes will apply the brakes. When using the parking brake, air pressure is removed off the spring brakes so that spring brakes can apply brake.

The spring brakes will also fully come on if the air pressure falls down due to leakage. Spring brakes will come on if the pressure falls in the range of 1.5 bar to 3 bar.  

Anti-lock Braking System (ABS):

All truck tractors manufactured after March 1, 1997 in US with air brakes are equipped with ABS and also other heavy load vehicles such as trucks, buses have been equipped with ABS after March 1, 1998. ABS is a safety device that prevents the wheels from locking up when heavy brakes are applied.

Related Topics:

• Disc Brakes

• Drum Brakes

• Anti-lock Braking System (ABS)

• Traction Control System (TCS)

• Electronic Brake force Distribution (EBD) 

Air Suspension

Air suspension or air spring is a type of vehicle suspension that is powered by an air pump or a compressor. They are commonly used in heavy duty vehicles, but over the last decade it has also made its mark in the smaller commercial vehicles.

Air suspension is made of tough rubber and plastics inflated at a certain pressure to withstand the shocks. Air springs have various advantages over a shock absorber and coil spring arrangement. Several automobile manufacturers use air suspension today such as Rolls Royce, Mercedes-Benz, Maybach, Land Rover, Jeep, Lexus, Volkswagen, etc.

Air Suspension System Design:

As already mentioned, that air suspension is made of tough rubber and plastic (polyurethane) bags that are inflated to a correct pressure with the help of a compressor.  

The bags come in 3 different shapes:

·         Double-Convoluted Bag: This type of bag is in the shape of an hour glass or a double cheese burger. It provides better lateral flexibility than other types and has more load capacity. It has a short stroke and a progressive spring rate. It is best suited for front suspension.

·         Rolling Sleeve and Tapered Sleeve Bags:  These have a smaller diameter and also the stroke length is longer. The linear spring rate is higher and more suited for rear end applications.

Rolling Sleeve

Taper Sleeve

Compressor Motor:

Most of the air suspension systems are provided with an electric compressor motor. It draws the air from the atmosphere, compresses it and then sends the compressed air to inflate the bags through a series of compressed air lines. Apart from compressing the air, it also maintains the pressure to be supplied to the bags. The pressure has to be varied for different driving conditions. Therefore, the compressor provides a transition in pressure.

In vehicles fitted with advanced air suspension system, a separate air tank is provided to maintain the compressed air pressure and also to provide a smoother change in pressure. The pressure of the air might vary from 5 bar to 12 bar and the tank capacity might vary from 7 liters to 20 liters depending on the load that a vehicle carries.

Desiccant Air dryer:

The air drawn from the atmosphere might have moisture which can damage the system. To remove the moisture, air dryer is installed to absorb the moisture before it is sent to the system.  

Air Lines:

High pressure lines made of rubber and polyurethane composition are used to carry the compressed air to the bags. Advanced systems often use steel lines.

Valves (Two way or Four way):

Valves play a crucial role in controlling the amount of air to be supplied to the bags. A two way valve system uses one valve per axle. During cornering, let’s assume a vehicle taking a left turn; the left air bag will be more compressed than the right air bag. Two way valve systems allow the air to transfer from left to right, thus increasing the tendency of body rolls. Body rolls bring a huge disadvantage to a air suspension system.

To overcome the problem, four way valve systems were introduced. It offered separate valve for each bag. It also requires a separate air line for each bag, hence canceling the possibility of body rolls due to air suspension.   

Electronic Control System (Solenoids):

Solenoids are used to electronically control the amount of air supplied to an air bag. The electronic control module receives inputs such as ride height, and pressure through sensors and switches the compressor ON and OFF as required to vary the pressure.

There is also a completely manual setup to regulate the rife height and pressure. This is done with the help of pneumatic valves.

There are 3 types of Electronic Control Systems:

·         Pressure Based System: The pressure based electronic control system relies solely on the air pressure inputs. Based on the air pressure inputs, the control module has to calculate the ride height of the vehicle. It is more of an assumption than calculating the exact ride height value. The system might not perfectly vary the ride height required based on the pressure inputs alone.

·         Ride Height Based System: With a ride height control system, the issue of assuming the ride height is overcome. Ride height sensors accurately measure the ride height of individual wheel. But without a pressure based system, the vehicle might face cross loading. Cross loading is the difference in ride height between the tires in one diagonal and the tires in the other diagonal. Two diagonal corner tires might be over inflated while the other two diagonal corner tires might be under inflated. To overcome the issue, air suspensions are fitted with a combo of both pressure based system and ride height based system.

·         Combo System: Both pressure based system and ride height system are installed together to overcome both cross loading and ride height calculation issues.

Advantages of Air Suspension:

·         Superior ride quality. Passengers don’t feel like their car is being driven over bumps.

·         It can improve towing capabilities

·         It can maintain the firmness of a vehicle when carrying heavy loads.

Disadvantages:

The only major disadvantage is the cost of buying an air suspension unit. It is mostly used in luxury cars. People who can spend extra money would generally go for this variant. Air suspension is standard in Mercedes S class and Range Rover.