Air Brake Manual

Placing the compressor in the unloading stage is
done by directing air pressure to the inlet valves of
the compressor, holding them open, allowing the air
to be pumped back and forth between the two
cylinders, instead of compressing the air. When the
pressure in the system drops, the inlet valves close,
returning the compressor to the “loading” stage.
The governor must place the compressor in the
“loading” stage at no lower than 80 psi. During the
“unloading” stage, the compressor is able to cool.
It is very important the air that enters the system be
kept as clean as possible. The air must first pass
through a filter to remove any dust particles. The air
filter must be cleaned regularly. A dirty filter will
restrict the flow of air into the compressor, reducing
its efficiency. Some vehicles have the inlet port of
the compressor connected to the intake manifold
and receive air that has been filtered by the engine
air cleaner.
A piston type compressor operates on the same
principle as the intake and compression strokes of
an engine.
· Intake stroke: The downward stroke of the piston
creates a vacuum within the cylinder which causes
the inlet valve to open. This causes atmospheric air
to flow past the inlet valve into the cylinder.
Compressor (Unloading stage)
Inlet
valve
Compressor (Intake stroke)
Inlet
valve
Unload
plunger
Unload
plunger
Piston
Piston
Discharge valve
From governor
Intake air filter
Discharge
valve
Intake air filter
Usually compressors are lubricated from the engine
lubrication system, although some compressors are
self-lubricating and require regular checks of the
lubricant level.
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