The compressor runs continuously while the engine is
running but actual compression of air is controlled by a
governor which stops or starts the compression of air by
loading or unloading the compressor in conjunction with
its unloading mechanism. This is done when the air
pressure in the system reaches the desired maximum or
INTAKE AND COMPRESSION (Loaded)
During the down stroke of the piston, a slight vacuum
created above the piston causes the inlet valve to move
off its seat. Atmospheric air is drawn in through the
compressor intake, by the open inlet valve, and on top
of the piston (Fig. 4). As the piston starts its upward
stroke, the air that was drawn in on the down stroke is
Now, air pressure on top of the inlet valve plus the force
of its spring, returns the inlet valve to its seat. The
piston continues the upward stroke and compresses the
air sufficiently to overcome the discharge valve spring
and unseat the discharge valve. The compressed air
then flows by the open discharge valve, into the
discharge line and on to the reservoirs (Fig. 5).
As the piston reaches the top of its stroke and starts
down, the discharge valve spring returns the discharge
valve to its seat. This prevents the compressed air in
the discharge line from returning to the cylinder bore as
the intake and compression cycle is repeated.
When the air pressure in the reservoir reaches the high
pressure setting of the governor, the governor opens,
allowing air. to pass from the reservoir through the
governor and into the cavity beneath the unloader
pistons. This lifts the unloader pistons and plungers.
The plungers move up and hold the inlet valves off their
seats (Fig. 6).
With the inlet valves held off their seats by the unloader
pistons and plungers, air is merely pumped back and
forth between the two cylinders. When air is used from
the reservoir and the pressure drops to low pressure
setting of the governor, the governor closes and in doing
so exhausts the air from beneath the unloader pistons.
The unloader saddle spring forces the saddle, pistons
and plungers down and the inlet valves return to their
seats. Compression is then resumed.