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[3] The APU fuel system consist of a fuel valve, control unit, solenoid valve, filter,
heater and control valve. The fuel valve allows fuel supply from the tank to the
APU engine. This valve operation is controlled by the APU switch on P5 panel, it is
opened by placing the APU switch to the ON position and closed by the switch
placed to the OFF position. The valve can be operated manually without power by
the override handle. The control unit regulates the fuel flow to the combustion
chamber, during starting and normal operation. The solenoid valve allows fuel flow
to the combustion chamber after the initial rotation of the APU. The two fuel filters
are used to prevent contamination of the pump and fuel control unit. The fuel heater
is used to prevent ice from blocking the fuel filter.
[4] The fuel from the tank is delivered to the fuel control unit through the fuel valve.
The fuel control unit supplies and regulates the fuel to the combustion chamber. The
regulated fuel flow controls the acceleration of the APU engine during the starting
operation. When a load is applied to the APU, the fuel control unit meters the fuel
flow to maintain a near constant speed and a safe operating temperature of the unit.
[5] The APU ignition and starting system provides the means of rotating the APU
engine to starting speed and for igniting the fuel-air mixture in the combustion
chamber. This system consists of a start switch, starter, start relay, ignition exciter,
igniter plug, and control circuit. The starter provides initial power for rotating the
APU engine compressor, tubine, and engine-driven accessories to a speed high
enough to obtain good airflow for combustion. The ignition exciter provides the
high voltage required to produce the spark at the ignitor plug, the ignition exciter
electrical components are enclosed in a hermetically sealed metal container. The
igniter plug is installed on the forward side of the combustion chamber, it is used to
provide the high energy spark for igniting the fuel/air mixture.
[6] The start system interfaces with the sequencing oil pressure switch, speed switch,
APU control unit and APU switch. During APU start, the starter is energized and
APU rotates. The ignition exciter is energized by the action of the sequencing oil
pressure switch (this switch ensures that combustion cannot be initiated during an
APU start until a minimum lubricating oil pressure has been build up .During APU
start, with increasing oil pressure, this switch closes at 4 psig oil pressure and
provides 28 volt dc power to the fuel solenoid valve and the ignition exciter).The
starter and ignition circuits are de-energized by the action of the speed switch
during APU engine acceleration. During normal engine operation, the exciter
remains de-energized.
[7] The APU exhaust conducts the engine exhaust gas overboard and reduces the
noise level of the high velocity gas flow. The exhaust duct consists of a heat shield
and a muffler. The muffler is inside the heat shield. A felt metal liner is installed on
the inside surface of the muffler for noise reduction. At the forward end, the heat
shield has a drip pan and is supported by the engine shroud. There is a gap between
the muffler and the APU engine exhaust. At the aft end, the exhaust duct is
supported by the aft fairing .
Fig 18-3 Exhaust duct
EXERCISE
A. Find out the explanations of the fllowing words in your “English-chinese
technical dictionary”
1. jet pipe [ P1-86 ]
2. hoist [ P1-76 ]
3. radial [ P1-121 ]
4. override [ P1-110]
5. circuit [ P1-30 ]
6. energize [ P1-5 ]
7. muffler [ P1-103 ]
8. ignition exciter [ P1-79 ]
9. bleed air [ P1-20 ]
10.bleed air [ P1-20 ]
11. centrifugal flow compressor [ P1-28 ]
12. solenoid [ P1-138 ]
13. fuel flow [ P1-66 ]
14. combustion chamber [ P1-31 ]
15. igniter [ P1-78 ]
B. Translate the followeng sentences into chinese.
1. The auxiliary power unit (APU) is used for providing .
1) compressed air for engine starting,
2) compressed air to the air conditioning system,
3) electrical power to the airplane electrical system.
2. The APU engine is located in the aft end of the fuselage,behind the pressure
bulkhead and below the horizontal stabilizer, the APU engine is completely
enclosed in a titanium shroud.
3. The APU is a single shaft gas turbine engine with a two stage centrifugal
compressor, a single combustion chamber and one radial inflow turbine.
4. The fuel control unit supplies and regulates the fuel to the combustion
chamber, the regulated fuel flow controls the acceleration of the APU
engine during the starting operation.
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
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航空工程英语基础（中英文对照）(25)
