Type and Mounting. The air compressor is bolted with five bolts to the engine flywheel cover at the right rear. It
is driven at camshaft speed by an accessory drive bolted to the balance shaft gear (Figure 6-37). It receives its
lubricating oil and cooling water through hoses from the engine.
The air compressor runs continuously while the engine is running. While the compressor is running, actual
compression of air is controlled by the compressor governor which acts in conjunction with the unloading mechanism in
the compressor cylinder block. The governor starts and stops the compression of air by loading or unloading the
compressor when the air pressure in the system reaches the desired minimum or maximum pressure.
During the downstroke of each piston, a partial vacuum is created above the piston which unseats the inlet
valve and then allows air drawn from the air box in the engine cylinder block or through an intake strainer to enter the
cylinder above the piston. As the piston starts the upward stroke, the air pressure on top of the inlet valves, plus the inlet
valve return spring force, closes the inlet valve. The air above the piston is further compressed until the pressure lifts the
discharge valve and the compressed air is discharged through the discharge line into the reservoir.
As each piston starts its downstroke, the discharge valve above it returns to its seat, preventing the
compressed air from returning to the cylinder, and the same cycle is repeated.
When the air pressure in the reservoir reaches the maximum setting of the governor, compressed air from
the reservoir passes through the governor into the cavity below the unloading pistons in the compressor cylinder block.
The air pressure lifts the unloading pistons, which in turn lifts the inlet valves off their seats.
With the inlet valves held off their seats, the air during each upstroke of the piston is merely passed back
through the air inlet cavity and to the other cylinder where the piston is on the downstroke. When the air pressure in the
reservoir drops to the minimum setting of the governor, the governor releases the air pressure beneath the unloading
pistons. The unloading piston return spring then forces the piston down and the inlet valve springs return the inlet valves
to their seats and compression is resumed.
Maintenance. The accessory drive should be inspected for tooth wear and bolt tightness whenever abnormal
noise or other sign of malfunction is noticed. The same items should be checked whenever the flywheel cover is
removed for any reason. Repair the compressor itself, if necessary, by replacement of the unit.