Bendix Commercial Vehicle Systems BX2150 AIR COMP - Manual (Page 3)

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cylinder bore as the intake and compression cycle is
repeated.
NON-COMPRESSION OF AIR (UNLOADED)
When air pressure in the reservoir reaches the cut-out setting
of the governor, the governor allows air to flow from the
reservoir into the unloader piston cavity.
The unloader piston moves, in response to air pressure and
drives the inlet valve away from its seat and holds it against
its stop.
With the inlet valve held away from its seat, air compression
is stopped and air is free to move back and forth past the
inlet valve in response to piston travel. System pressure will
eventually drop to the cut-in pressure setting of the governor
due to air usage. When the cut-in pressure is reached, the
governor responds by exhausting air from the unloader piston
cavity. Spring force moves the unloader piston away from
the inlet valve and compression is resumed as the inlet valve
returns to its seat.
COMPRESSOR & THE AIR BRAKE SYSTEM
GENERAL
The compressor is part of the total air brake system, more
specifically, 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 fittings, 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 fitting or reservoir
to avoid low points where ice may form and block the flow. If,
FIGURE 3 - UNLOADED CYCLE
FIGURE 2 - COMPRESSION/DISCHARGE CYCLE
INLET
INLET
VALVE
PISTON
DISCHARGE
VALVE
UNLOADER
PISTON
VALVE
STOP
T O
RESERVOIR
INLET
INLET
VALVE
PISTON
DISCHARGE
VALVE
UNLOADER
PISTON
VALVE
STOP
T O
RESERVOIR