Introduction to the Air Brake Charging System
Powered by the vehicle engine, the air compressor | air dryer. The typical size of a vehicle's discharge line, |
builds the air pressure for the air brake system. The | (see column 2 of Table A on page 18) assumes a |
air compressor is typically cooled by the engine coolant | compressor with a normal (less than 25%) duty cycle, |
system and lubricated by the engine oil supply. | operating in a temperate climate. See Bendix and/or |
The compressor's unloader mechanism andgovernor | other air dryer manufacturer guidelines as needed. |
(along with a synchro valve for the Bendix® DuraFlo™ | When the temperature of the compressed air that enters |
596 air compressor) control the brake system air | the air dryer is within the normal range, the air dryer can |
pressure between a preset maximum and minimum | remove most of the charging system oil. If the |
pressure level by monitoring the pressure in the service | temperature of the compressed air is above the normal |
(or “supply”) reservoir. When the air pressure becomes | range, oil as |
greater than that of the preset | dryer and into the air system. Larger diameter discharge |
controls the unloader mechanism of the compressor | lines and/or longer discharge line lengths can help reduce |
to stop the compressor from building air and also | the temperature. |
causes the air dryer to purge. As the service reservoir | The discharge line must maintain a constant slope |
air pressure drops to the | down from the compressor to the air dryer inlet fitting |
the governor returns the compressor back to building | to avoid low points where ice may form and block the |
air and the air dryer to air drying mode. | flow. If, instead, ice blockages occur at the air dryer |
As the atmospheric air is compressed, all the water | inlet, insulation may be added here, or if the inlet fitting |
vapor originally in the air is carried along into the air | is a typical 90 degree fitting, it may be changed to a |
system, as well as a small amount of the lubricating oil | straight or 45 degree fitting. For more information on |
as vapor. | how to help prevent discharge line |
The duty cycle is the ratio of time the compressor | Bendix Bulletins |
spends building air to the total engine running time. | pages |
Air compressors are designed to build air (run “loaded”) | insulation may be required in cold climates. |
up to 25% of the time. Higher duty cycles cause | The air dryer contains a filter that collects oil droplets, |
conditions that affect air brake charging system | and a desiccant bed that removes almost all of the |
performance which may require additional | remaining water vapor. The compressed air is then |
maintenance. Factors that add to the duty cycle are: | passed to the air brake service (supply) reservoir. The |
air suspension, additional air accessories, use of an | oil droplets and the water collected are automatically |
undersized compressor, frequent stops, excessive | purged when the governor reaches its |
leakage from fittings, connections, lines, chambers or | For vehicles with accessories that are sensitive to small |
valves, etc. | amounts of oil, we recommended installation of a |
The discharge line allows the air, | Bendix® PuraGuard® system filter, designed to minimize |
the amount of oil present. |
Discharge | Optional “Ping” Tank |
Line | Air Dryer |
The Air Brake Charging System supplies the compressed air for the braking system as well as other air accessories for the vehicle. The system usually consists of an air compressor, governor, discharge line, air dryer, and service reservoir.
Optional Bendix® PuraGuard®
System Filter or PuraGuard®
QC™ Oil Coalescing Filter
Compressor
Governor
(Governor plus Synchro valve | Service Reservoir | ||
for the Bendix® DuraFlo™ 596™ | (Supply Reservoir) | ||
Compressor) |
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| Reservoir Drain |
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