MCi Crusader II 1989 Maintenance Manual page 137

Date
1-1-89
Page ·-'--_
4 _- 7 _ _ _
MC-9 MAINTENANCE MANUAL
AIR COMPRESSOR
OPERATION
The air compressor is to provide and maintain air under
pressure to operate devices in the air brake and auxiliary air
system. It is a two cylinder, single stage, reciprocating com-
pressor with a rated displacement of 15.5 cubic feet (439 Cu
Dm) air per minute at 1250 RPM.
The compressor assembly is made up of three cast iron
subassemblies: the cylinder head. the cylinder block and the
crankcase. The cylinder head houses the discharge valving
and is installed to the cylinder block. The cyiinder block houses
the cylinder bores and inlet valves and is installed to the crank-
case. The crankcase houses the crankshaft and main bearings.
The cylinder head and block coolant is routed to the com-
pressor from the engine cooling system. Lubrication of the
internal parts of the compressor is provided by the oil feed line
from the engine's pressurized oil system.
The compressor runs continuously while the engine is oper-
ating but actual compression is controlled by the 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 minimum pressures.
A. INTAKE AND COMPRESSION (Loaded): During thedown-
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, past the open inlet
valve. and on top of the piston. See figure 4-7. As the piston
starts its upward stroke, the air is compressed. Now, air pres-
sure on top of the iniet 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 past the open discharge valve, into
the discharge line and on to the reservoirs. Refer to figure 4-8.
Intake and compression cycle is repeated.
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 compres-
sion cycle is repeated.
8. NON-COMPRESSION (Unloaded): When the-air pressure
in the reservoir reaches the high pressure setting of the gover-
nor. 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.
C. INLET VALVES HELO OFF THEIR SEATS: Air is merely
pumped back and forth between the two cylinders (figure 4-9).
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. Com-
pression is then resumed.
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Figure 4-7. Intake Stroke Beginning.
Figure 4-8. Compression Stroke Ending.
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Figure 4-9. Unloading.
MAINTENANCE
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Every 6 months, 1,800 operating hours or 50,000 miles
{80,500 km). remove the discharge fittings and inspect the com-
pressor discharge port and discharge line for excessive carbon
deposits. If excessive build-up is noted in either. the discharge
line must be cleaned or replaced and the compressor checked
more thoroughly. Carefully inspect the air induction system. oil
supply system and if necessary repair or replace compressor.
Check for noisy compressor operation which could indicate a
worn drive gear coupling. Check all compressor mounting bolts