20
375 Caterpillar Standard and Aftercooled/Filtered Operator’s Manual and Parts List
SECTION 2
Engine Air Filter1.
Compressor Air Filter2.
Air Inlet Valve3.
Compressor 4.
Engine5.
Figure 2-8: Air Inlet System – Caterpillar
Normal Operation – 80 to 100 psig (5.6 to 6.9
bar) or 80 to 150 psig (5.6 to 10.3 bar) for H
machines or 80 to 200 psig (5.6 to 13.8 bar)
for HH machines
When the warm-up control selector valve handle is moved
to the RUN position, the sump pressure rises above 80
psig (5.6 bar). At this time, the inlet valve remains fully
open for maximum air output. The engine will continue to
run at full speed during this phase of operation.
Modulation – 100 to 110 psig (6.9 to 7.5 bar)
or 150 to 165 psig (10.3 to 11.4 bar) for H
machines or 200 to 220 psig (13.8 to 15.2
bar) for HH machines
If the demand on the compressor is less than its rated
capacity, the service line pressure will rise above 100 psig
(6.9 bar) - low or single pressure rating: 150 psig (10.3
bar) for “H” rating or 200 psig (13.8 bar) for “HH” dual or
high pressure rating. The pressure regulating valve
gradually opens, applying pressure to the inlet valve piston
and engine speed control. This causes the inlet valve to
partially close and reduces the engine speed. As the
pressure increases, the inlet valve piston will further close
the inlet valve and the engine speed will decrease until it
reaches its preset idle speed. When the demand on the
compressor increases, the sump pressure falls below 110
psig (7.6 bar) or 165 psig (11.4 bar) for “H” machines, or
220 psig (15.2 bar) for “HH” ones. The pressure regulating
valve closes, the air inlet valve opens fully, and the engine
speed increases to its preset full load rating.
Between the pressure regulating valve and the inlet valve,
there is a small orice that vents a small amount of air into
the atmosphere when the pressure regulating valve is
open. This allows changes in air output to conform to air
demand. This orice also discharges any accumulated
moisture from the regulator.
Shutdown
The blowdown valve is normally closed. At shutdown the
back pressure in the compressor inlet signals the
blowdown valve to vent the sump pressure into the
atmosphere.
AIR INLET SYSTEM, 2.7
FUNCTIONAL DESCRIPTION
Refer to Figure 2-8. The air inlet system consists of two air
lters, a compressor air inlet valve and interconnecting
piping to the engine and the compressor.
The air lters are three-stage dry element type lters that
are capable of cleaning extremely dirty air. However, when
operating in dirty environments, the lters should be
checked more frequently.
See Air Filter Maintenance on page 33 for Air Filter
Maintenance Procedures.
INSTRUMENT PANEL GROUP, 2.8
FUNCTIONAL DESCRIPTION
Refer to Figure 2-9. The instrument panel group consists
of a molded panel containing an: air pressure gauge,
ignition/start switch, annunciator light, hourmeter, and an
idle warm-up control. A high-low pressure selector valve is
located on the panel for “H” and “HH” compressors.
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