Figure 7-39.Typical air brake system.
from the engine. Compressors, having a displacement
of approximately 7 cubic feet per minute (cfm), have
two cylinders, while those with a displacement of 12
cfm have three cylinders.
The reciprocal air compressor (fig. 7-40) operates
continuously while the engine is running, but the
governor controls the actual compression. The
operation of the compressor is as follows:
The partial vacuum created on the piston
downstroke draws air through the air strainer and
intake ports into the cylinder.
As the piston starts its upstroke. the intake ports
are closed off, and the air trapped in the cylinder
The pressure developed lifts the discharge valve,
and the compressed air is discharged to the
reservoirs. As the piston starts its downstroke.
pressure is relieved, closing the discharge valve.
The purpose of the compressor GOVERNOR is to
maintain the air pressure in the reservoir between the
maximum pressure desired (100 to 110 psi) and the
minimum pressure required automatically for safe
operation (80 to 85 psi) by controlling the compressor
In the type O-1 governor (fig. 7-41) air pressure
from the reservoir enters the governor through the
strainer and is always present below the tower valve
and in the spring tube. As the air pressure increases, the
tube tends to straighten out and decrease pressure on
When the reservoir air pressure reaches the cutout
setting of the governor (100 to 110 psi), the spring load
of the tube on the tower valve has been reduced enough
to permit air pressure to raise the tower valve off its
seat. This movement of the lower valve raises the
upper valve to its seat, which closes the exhaust port.
Air then flows up through the small hole in the lower