The air compressor is driven by the vehicle engine and is operating continuously while the engine is running. Actual compression of air is controlled by the compressor unloading mechanism and the governor. The governor is generally mounted on the compressor and maintains the brake system air pressure between a preset maximum and minimum pressure level.
Intake And Compression Of Air (Loaded)
Fig. 4 Operational-Loaded (Intake)
During the down stroke of the piston, a slight vacuum is created between the top of the piston and the head, causing the flat circular inlet valve to move off its seat. (Note the flat square discharge valve remains on its seat). atmospheric air is drawn into the intake cavity and flows past the open inlet valve and into the cylinder (see Fig. 4). As the piston begins its upward stroke, the air that was drawn into the cylinder on the down stroke is compressed. Air pressure on the inlet valve plus the force of its spring, returns the inlet valve to its seat. As the piston continues its upward stroke, air is compressed and forces the discharge valve away from its seat. Air flows past the open discharge valve into the discharge line and on to the reservoirs. (See Fig. 4).
Fig. 5 Operational-Loaded (Compression)
As the piston reaches the top of its stroke and starts down, the discharge valve spring and air pressure in the discharge line 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.
Fig. 6 Operational Unloaded
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.