12
Introduction to the Air Brake Charging System
Powered by the vehicle engine, the
air compressor
builds the air pressure for the air brake system. The air
compressor is typically cooled by the engine coolant system
and lubricated by the engine oil supply.
The compressor's unloader mechanism and
governor
(along with a synchro valve for the Bendix
®
DuraFlo
™
596
air compressor) control the brake system air pressure
between a preset maximum and minimum pressure level
by monitoring the pressure in the service (or “supply”)
reservoir. When the air pressure becomes greater than that
of the preset “cut-out”, the governor controls the unloader
mechanism of the compressor to stop the compressor from
building air and also causes the air dryer to purge. As the
service reservoir air pressure drops to the “cut-in” setting
of the governor, the governor returns the compressor back
to building air and the air dryer to air drying mode.
As the atmospheric air is compressed, all the water vapor
originally in the air is carried along into the air system, as
well as a small amount of the lubricating oil as vapor.
The
duty cycle
is the ratio of time the compressor
spends building air to the total engine running time. Air
compressors are designed to build air (run “loaded”) up to
25% of the time. Higher duty cycles cause conditions that
affect air brake charging system performance which may
require additional maintenance. Factors that add to the duty
cycle are: air suspension, additional air accessories, use
of an undersized compressor, frequent stops, excessive
leakage from fittings, connections, lines, chambers or
valves, etc.
The
discharge line
allows the air, water-vapor and
oil-vapor mixture to cool between the compressor and air
dryer. The typical size of a vehicle's discharge line, (see
column 2 of Table A on page 13) assumes a compressor
with a normal (less than 25%) duty cycle, operating in
a temperate climate. See Bendix and/or other air dryer
manufacturer guidelines as needed.
When the
temperature
of the compressed air that enters
the air dryer is within the normal range, the air dryer can
remove most of the charging system oil. If the temperature
of the compressed air is above the normal range, oil as
oil-vapor is able to pass through the air dryer and into the
air system. Larger diameter discharge lines and/or longer
discharge line lengths can help reduce the temperature.
The discharge line must maintain a
constant slope
down
from the compressor to the air dryer inlet fitting to avoid low
points where ice may form and block the flow. If, instead,
ice blockages occur at the air dryer inlet, insulation may
be added here, or if the inlet fitting is a typical 90 degree
fitting, it may be changed to a straight or 45 degree fitting.
For more information on how to help prevent discharge
line freeze-ups, see Bendix Bulletins TCH-08-21 and
TCH-08-22 (see pages 30-32). Shorter discharge line
lengths or insulation may be required in cold climates.
The
air dryer
contains a filter that collects oil droplets, and
a desiccant bed that removes almost all of the remaining
water vapor. The compressed air is then passed to the air
brake service (supply) reservoir. The oil droplets and the
water collected are automatically purged when the governor
reaches its “cut-out” setting.
For vehicles with accessories that are sensitive to small
amounts of oil, we recommended installation of a Bendix
®
PuraGuard
®
system filter, designed to minimize the amount
of oil present.
Air Dryer
Reservoir Drain
Service Reservoir
(Supply Reservoir)
Compressor
Governor
(Governor plus Synchro valve
for the Bendix
®
DuraFlo
™
596
Compressor)
Discharge
Line
Optional “Ping” Tank
Optional Bendix
®
PuraGuard
®
System Filter or PuraGuard
QC
™
Oil Coalescing Filter
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.
