User's Manual

4
CYLINDER BLOCK SIDE OF VALVE BODY ASSEMBLY
SLIDING INLET
VALVES
PIVOT
ARMS
COMPRESSOR LOADED
COMPRESSOR UNLOADED
DISCHARGE
PORTS
INLET
PORTS
CYLINDER HEAD SIDE OF VALVE BODY ASSEMBLY
DISCHARGE PORTS
DISCHARGE VALVE
INLET PORTS
COMPRESSOR & THE AIR BRAKE SYSTEM
GENERAL
The compressor is part of the total air brake system, more
specifi cally, the charging portion of the air brake system.
As a component in the overall system its condition, duty
cycle, proper installation and operation will directly affect
other components in the 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, lubricated by the engine oil supply and has its inlet
connected to the engine induction system.
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. If an
air dryer is not used to remove these contaminants prior
to entering the air system, the majority, but not all, will
condense in the reservoirs. The quantity of contaminants
that reach the air system depends on several factors
including installation, maintenance and contaminant
handling devices in the system. These contaminants must
either be eliminated prior to entering the air system or after
they enter.
DUTY CYCLE
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.
Refer to Table A in the Troubleshooting section for a guide
to various duty cycles and the consideration that must be
given to maintenance of other components.
COMPRESSOR INSTALLATION
While the original compressor installation is usually
completed by the vehicle manufacturer, conditions of
operation and maintenance may require additional
consideration. The following presents base guidelines.
DISCHARGE LINE
The discharge line allows the air, water-vapor and oil-vapor
mixture to cool between the compressor and air dryer or
reservoir. The typical size of a vehicle's discharge line,
(see column 2 of Table A in the Troubleshooting section)
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.
The discharge line must maintain a constant slope down
from the compressor to the air dryer inlet fi tting or reservoir
to avoid low points where ice may form and block the fl ow.
If, instead, ice blockages occur at the air dryer or reservoir
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. Shorter discharge line lengths or
insulation may be required in cold climates.
While not all compressors and charging systems are
equipped with a discharge line safety valve, this component
is recommended. The discharge line safety valve is
installed in the cylinder head or close to the compressor
discharge port and protects against over pressurizing the
compressor in the event of a discharge line freeze-up.
FIGURE 7 - UNLOADER MECHANISM
UNLOADER
PLUNGER
UNLOADER
PORT
GUIDE PIN
SLIDING
INLET VALVE
UNLOADER AIR
PASSAGE
PIVOT
ARM