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 is compressed.

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 unloading mechanism.

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 the valve.

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