Air brake (road vehicle)

Air brakes are used in trucks, buses, trailers, and semi-trailers. George Westinghouse first developed air brakes for use in railway service. A safer air brake was patented by him on March 5, 1872. Originally constructed for use on trains, and still in common use as such, Westinghouse made many alterations to improve his invention, leading to various forms of the automatic brake, thus, its use was expanded to include road vehicles.

Compressed Air Brake System

Introduction

A "Compressed Air Brake System" is a different air brake used for trucks, consisting of a standard disc/drum brake arrangement using compressed air in place of hydraulic fluid. Most types of truck air brakes are drum units, though there is an increasing trend towards the use of disc brakes in this application. The compressed air brakes system works by drawing filtered air from the atmosphere, compressing it, and holding it in high-pressure reservoirs at around 120 PSI. When needed for braking, this high pressure air is routed to the operating cylinders on the brakes, which actuate the braking hardware and slow the vehicle.

Design and Function

A compressed air brake system is divided into a supply system and a control system. The supply system compresses, stores and supplies high-pressure air to the control system as well as to additional air operated auxiliary truck systems (gearbox shift control, clutch pedal air assistance servo, etc.).

upply system

The air compressor is driven off of the engine either by crankshaft pulley via a belt or directly off of the engine timing gears. It is lubricated and cooled by the engine lubrication and cooling systems. Compressed air is first routed through a cooling coil and into an air dryer the dryer device which removes moisture and oil impurities and also may include a pressure regulator ,safety valve and a smaller purge reservoir. As an alternative to the air dryer, the supply system can be equipped with an anti freeze device. and oil separator. The compressed air is then stored in a reservoir (also called a primary tank) from which it is then distributed via a four way protection valve into the front and rear brake circuit air reservoir, a parking brake reservoir and an auxiliary air supply distribution point. The system also includes various check, pressure limiting, drain and safety valves.

Control system

The control system is further divided into two service brake circuits: the parking brake circuit and the trailer brake circuit. This dual brake circuit is further split into front and rear wheel circuits which receive compressed air from their individual reservoirs for added safety in case of an air leak. The service brakes are applied by means of a brake pedal air valve which regulates both circuits. The parking brake is the air operated spring brake type where its applied by spring force in the spring brake cylinder and released by compressed air via hand control valve. The trailer brake consists of a direct two line system: the supply line (marked red) and the separate control or service line (marked blue). The supply line receives air from the prime mover park brake air tank via a park brake relay valve and the control line is regulated via the trailer brake relay valve. The operating signals for the relay are provided by the prime mover brake pedal air valve, trailer service brake hand control (subject to a country's relevant heavy vehicle legislation) and the prime mover park brake hand control.

Aerodynamic Occlusion as Vehicle Air Brake

This example of the air brake consists of a physical structure on the exterior of a vehicle that will increase the vehicle's drag coefficient, and therefore slow it down. Air brakes of this sort are ineffective at normal road vehicle speeds, and therefore are reserved for vehicles which need to quickly decelerate from high speeds, such as race and high performance sports cars.

The Bugatti Veyron features a rear spoiler which is able to automatically move from the standard wing angle to nearly 70 degrees under high speed braking. Top Fuel Dragsters use this same concept of aerodynamic occlusion via a parachute.

See also

*Air brake
*Knorr-Bremse
*Ozone cracking
*Polymer degradation
*Railway air brake

References

State of California, Copyright 2003, California Commercial Driver Handbook, Sec. 5

North American Standard Inspection Program, Commercial Vehicle Safety Alliance (12 December 2005). "Tractor Protection Systems". Retrieved 7 September 2006 from: [http://www.cvsa.org/resourcecenter/documents/2005_02_tractor_protection_systems_rev5.pdf#search=%22emergency%20brake%2C%20air%2C%20require%22 Commercial Vehicle Safety Alliance]