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C.  FUEL CONTROL UNIT WITHOUT THE TIMED ACCELERATION CONTROL; The acceleration limiter valve controls the quantity of fuel for acceleration during the APU start. The acceleration limiter valve has a diaphragm, piston and guide assembly, bypass valve, spring, adjustment screw, nut, and bypass filter. The cover for the acceleration limiter valve has a connection for control air inlet and a fixed vent orifice. The control air is supplied by the compressor. The fixed vent orifice removes the control air to ambient. The fixed vent orifice is found on the lower right side of the diffuser housing. This location is just forward of the combustor housing.
D.  FUEL CONTROL UNIT WITH THE TIMED ACCELERATION CONTROL; The acceleration limiter valve controls the quantity of fuel for acceleration when the APU speed is less than 20%. The acceleration limiter valve has a diaphragm, piston and guide assembly, bypass valve, spring, adjustment screw, nut, and bypass filter. The cover for the acceleration limiter valve has a connection for the control air inlet and a fixed vent orifice. The control air is supplied by the compressor. The fixed vent orifice removes the control air to ambient. The fixed vent orifice is found on the lower right side of the diffuser housing. This location is just forward of the combustor housing. When the APU speed is more than 20%, the timed acceleration head on the fuel control unit supplies a specified quantity of fuel. The timed acceleration head is made to supply a specified quantity of fuel for a specified APU speed during the acceleration.
E.  The governor controls the fuel flow to the fuel atomizer during the APU usual operation (Fig. 2). The governor supplies the fuel to keep a constant APU speed during different load conditions. The governor has a shaft and sleeve assembly, governor cage, flyweights, spring, adjustment screw, and nut. High pressure fuel is on all sides of the flyweights. The flyweights move the sleeve to cover the shaft ports for control of the bypass of the fuel. This will give the correct quantity of fuel to the fuel atomizer.
5.  APU Control Thermostat
A.  The APU control thermostat in the single thermostat control system acts both as an acceleration control thermostat and as a load control thermostat. The single thermostat control system consists of a thermostat, selector solenoid, immersion thermocouple and an overtemperature control switch. The thermostat projects into the turbine exhaust duct and is comprised of an alloy steel housing, spring-loaded ball valve, thermostatic core and connection for a pneumatic line. The thermostat is connected through the normally open selector solenoid to the acceleration limiter valve located in the fuel control unit. It functions in the acceleration control mode until 95% switch actuation. At 95% speed, the selector solenoid is energized closed, transferring the thermostat to the load control mode. At about 650°C (1200°F) the thermostat opens to bleed air from the bleed air valve to reduce the EGT.
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G00434 A36092
529  APU Fuel System Component Location 
Jun 20/92  Figure 1 (Sheet 1)  49-31-0 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

567  APU Fuel System Component Location 
Jun 20/92  Figure 1 (Sheet 2)  49-31-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

6.  Fuel Solenoid Valve
A.  The fuel solenoid valve is a normally closed electromagnetic shutoff valve. The valve is located in the high pressure fuel line between the fuel control unit and the fuel atomizer and it is attached to the fuel control unit housing (Fig. 1). The valve consists of two body assemblies, a coil, plunger and spring. The valve is energized to pass fuel to the fuel atomizer when the sequencing oil pressure switch closes during a starting operation.
7.  Combustor Unit
A.  The atomizer, combustor cap and the combustion chamber liner comprise the combustor unit. The fuel atomizer is a dual-orifice type mounted on the combustion chamber cap (Fig. 1). The fuel atomizer consists of a screen, flow-divider valve, distributor head, and housing. The distributor head functions as a dividing passageway, with a core in the center leading to a small orifice plate, and an annulus leading to a large orifice plate located at the tip. The distributor head and orifice plates located within the housing are covered by an atomizer nut. Also located within the housing is the screen and flow-divider valve. The screen strains all fuel entering the fuel atomizer. The flow-divider valve functions to direct all fuel at lower pressures to the small orifice which provides proper atomization of fuel under these conditions. At higher fuel pressures, the valve actuates to permit combined flow to both orifices.
　
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