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6.  Fuel Solenoid Valve (Fig. 1)
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 A.  The fuel solenoid valve is an electromagnetic shutoff valve that is usually in the closed position. The fuel solenoid valve is installed in the high-pressure fuel line between the fuel control unit and the fuel atomizer. The fuel solenoid valve is attached to the housing of the fuel control unit. The fuel solenoid valve has two body assemblies, a coil, plunger, and spring. The fuel solenoid valve is energized open when the sequencing switch for the oil pressure closes during the APU start. This will permit fuel to flow to the fuel atomizer.
 7.  Combustor Unit
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 A.  The combustor unit has the fuel atomizer, combustor cap and combustion chamber liner. The fuel atomizer is a dual-orifice type with a screen, flow-divider valve, distributor head, and housing. The fuel atomizer is installed on the combustor cap. The distributor head is a dividing passageway. The distributor head has a core and an annulus at its center. The core leads to a small orifice plate and the annulus leads to a large orifice plate at the end of the distributor head. The distributor head and orifice plates in the housing are covered by an atomizer nut. The screen and flow-divider valve are also found in the housing. The screen keeps the unwanted material from the fuel atomizer. When the fuel pressure is low, the flow-divider valve moves the fuel to the small orifice plate for full atomization. When the fuel pressure is high, the flow-divider valve moves the fuel to the two orificed plates.
 8.  Fuel Drain (Fig. 1)
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 A.  The fuel drain is found below the combustion chamber. The fuel drain lets the unburned fuel from the combustion chamber and turbine plenum drain from the APU. A warm airflow from the fuel drain shows that the fuel drain is not clogged.
 B.  APUs WITHOUT THE FUEL DRAIN HOUSING; The fuel drain has a drain union on the combustion chamber cap, a drain line, drain tee and plenum union on the turbine plenum.
 C.  APUs WITH THE FUEL DRAIN HOUSING; The fuel drain has a drain reducer on the combustion chamber cap, a drain line and a fuel drain housing. The fuel drain housing has the upper drain housing, lower drain housing, spring, screen, clamp and drain seal.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / GTCP85-129 SERIES /737-300/400/500/ ENGINES/MAINTENANCE MANUAL ////////////////////////
9.  Fuel Heater (Fig. 1)
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 A.  The fuel heater is installed on the inner surface of the upper shroud between the fuel inlet and the fuel control unit. The fuel heater controls the temperature of the fuel that goes to the APU. Also, the fuel heater prevents the blockage of the low-pressure fuel filter by ice. A main check valve is installed in the fuel line upstream of the fuel heater. The main check valve keeps the fuel in the fuel pump. To start the APU, a positive fuel pressure is kept by the main check valve.
 B.  The check valve for the flow control valve is installed in the fuel heating system. The check valve for the flow control valve closes when fuel heating is not necessary and keeps the fuel out of the bleed air duct. An air regulator valve that monitors the fuel temperature makes the fuel heating system operate automatically. When the fuel temperature in the air regulator valve falls to a specified value, the air regulator valve will open. When the air regulator valve opens, hot air from the APU bleed port goes into the fuel heater.
 10. Operation (Fig. 2)
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 A.  All of the components of the APU fuel system are automatically controlled. When the APU master switch is moved to the START position, the APU fuel valve opens. This permits the fuel from the No. 1 fuel tank to flow to the fuel control unit. The fuel control unit is full of fuel at all times. The fuel pump supplies a high-pressure fuel to the fuel solenoid valve when the APU engine turns. When the oil pressure increases to approximately 4.0 psig (27.6 kPa), the sequencing switch for the oil pressure closes. This energizes the fuel solenoid valve to open. When the fuel solenoid valve opens, high-pressure fuel flows to the fuel atomizer. The atomizer sprays the fuel in the combustion chamber. The APU engine speed and exhaust gas temperature are automatically controlled by the fuel control unit to be in the limits.
 B.  The acceleration of the APU engine is controlled by the acceleration limiter valve. The acceleration limiter valve uses the control air pressure to make an estimate of the fuel pressure. The pressure-actuated diaphragm in the acceleration limiter valve operates a bypass valve. This bypass valve lets fuel go back to the inlet fuel port of the fuel control unit to keep the necessary ratio of fuel flow to control air pressure. During the initial stage of acceleration, the bypass valve does not let the fuel go back to the inlet port of the fuel control unit. A spring force on the diaphragm keeps the bypass valve from operation.
　
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