Bendix Commercial Vehicle Systems 720CC COMPRESSOR - Manual (Page 16)

A-2
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.
This Bendix ® 720cc compressor is a “discharge line
unloader” (DLU)-style compressor, meaning that the
compressor pumps continuously, unlike some compressor
designs which use an “unloader” mechanism in the
compressor head to switch from a pumping mode to a
non-pumping mode. Instead, the control of air delivery to
the vehicle’s air system is managed by using a separate
discharge line unloader valve mounted in parallel with
the compressor, a turbo cut-off style of air dryer and a
governor (see Figure below). The discharge line unloader
valve and governor 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 discharge
line unloader valve such that the air from the compressor
fl ows through the exhaust of the discharge line unloader
valve and to atmosphere (i.e. preventing air delivery to the
reservoirs) 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 discharge line
unloader valve back to building air mode 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 fi ttings, 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 A-3) 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 fi tting to avoid low
points where ice may form and block the fl ow. If, instead,
ice blockages occur at the air dryer inlet, insulation may
be added here, or if the inlet fi tting is a typical 90 degree
fi tting, it may be changed to a straight or 45 degree fi tting.
For more information on how to help prevent discharge
line freeze-ups, see Bendix Bulletins TCH-008-021 and
TCH-008-022 (see pages A-19-21). Shorter discharge
line lengths or insulation may be required in cold climates.
The air dryer contains a fi lter 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 fi lter, designed to minimize the amount
of oil present.
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.
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22
S
D
S
C
UNL
Compressor
Governor
Air Dryer
Front Service
Reservoir
Rear Service
Reservoir
DLU Valve