Bendix Commercial Vehicle Systems TU-FLO 550 COMPRESSOR 6/08 - Manual (Page 3)

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force the pistons upward and the inlet valves return to their
seats. Compression is then resumed.
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
GOVERNOR
PORT
AIR
INLET
PORT
INLET
VALVE
OPEN
PISTON
MOVING
DOWN
DISCHARGE
VALVE
CLOSED
GOVERNOR
PORT
INLET
VALVE
CLOSED
PISTON
MOVING
UP
FIGURE 4 - OPERATIONAL-LOADED (INTAKE)
FIGURE 5 - OPERATIONAL-LOADED (COMPRESSION)
GOVERNOR
PORT
DISCHARGE
PORT
AIR
INLET
PORT
FIGURE 6 - OPERATIONAL-UNLOADED
DISCHARGE
VALVE
CLOSED
DISCHARGE
VALVE
OPEN
AIR
INLET
PORT
NON-COMPRESSION OF AIR (UNLOADED)
When air pressure in the reservoir reaches the cut-out
setting of the governor, the governor allows air to pass
from the reservoir, through the governor and into the cavity
above the unloader pistons. The unloader pistons move
down holding the inlet valves off their seats (see Figure
6.)
With the inlet valves held off their seats by the unloader
pistons, air is pumped back and forth between the two
cylinders, and the discharge valves remain closed. When
air pressure from the reservoir drops to the cut-in setting
of the governor, the governor closes and exhausts the air
from above the unloader pistons. The unloader springs
INLET VALVE
HELD OPEN
BY UNLOADER
PISTON
AIR
DISCHARGE
PORT
AIR
DISCHARGE
PORT
UNLOADER
PISTON