APU Fuel Control

  1. Fuel Flow Divider Assy ( FFDA )
  • Incoming fuel from the A/C fuel system is received by the FCU. The FCU diverts some of the fuel into a hydraulic circuit to operate the IGV-actuator and the SCV. Returned fuel is routed back into the FCU.
  • The FCU also pressurizes and meters the fuel for APU operation. The fuel is passed through the flow divider assy to the primary and/or secondary fuel manifold.
  • When the fuel pressure is greater than 125 psid (8.618 bar) at the primary fuel nozzles due to increased fuel flow, the check valve within the flow divider assy opens and the fuel also flows to the secondary fuel manifold. The secondary fuel manifold than distributes the fuel to the secondary inlet port of the fuel nozzles. The check valve operation can be disabled by by the fuel solenoid, also part of the flow divider assy. The fuel solenoid is normally open (de-energized) but during cold start and high altitude operation ( ít khí ) the ECB energizes the solenoid valve to close during the first 25% speed of the APU. The fuel is then forced through the primary manifold only to achieve a better engine acceleration.
  • Final : Fuel vào Fuel Flow Divider Assy sẽ được chia thành 2 đướng primary fuel manifold và secondary fuel manifold . Tuy nhiên , ở lúc khởi động ban đầu và lúc hoạt động trên cao , lượng khí chưa nhiều , FDA sẽ chỉ cho fuel đi qua primary fuel manifold , điều này được thực hiện bởi lệnh của ECB energize solenoid valve to close , đẫn đến đóng check vvalve, không cho fuel đi qua secondary fuel manifold. Ngoài 2 điều kiện cold start và high altitude operation thì check valve cũng sẽ mở để fuel đi qua secondary fuel manifold.

2. Fuel Control Unit

  • The Fuel Control Unit (FCU) consists of the following main components:
    • a low-pressure inlet fuel filter
    • a high-pressure fuel pump,
    • a high-pressure fuel screen ( high pressure fuel filter ),
    • a pump relieve valve,
    • an actuator pressure regulator (for hydraulic fuel operation)
    • a Torque-Motor Metering Valve (TMMV).
    • a Resistive Temperature Device (RTD),
    • a delta pressure regulator ( Differential Pressure regulation valve )
    • a flowmeter pressurizing valve,
    • a fuel-shutoff solenoid valve.
  • The FCU draws fuel from the A/C fuel system through the inlet fuel filter to the high-pressure fuel pump. The pressurized fuel is passed through the high pressure fuel filter, to trap the normal pump wear debris. The pump relieve valve ensures that exceeding fuel pressure is bypassed to the fuel inlet to prevent damage.
  • The fuel pressure for the IGV actuator operation and the SCV actuator operation is regulated by the actuator pressure regulator.
  • The APU fuel metering is achieved by the TMMV. The TMMV current is controlled by the ECB. Increasing current moves a clevis to increase the outlet port area and decreasing current decreases the outlet port area.
  • The accuracy of the TMMV is enhanced by the flowmeter pressurizing valve signal. The ECB uses this signal to adjust the TMMV current.
  • The RTD is installed downstream of the TMMV. The RTD measures the fuel temperature and allows the ECB to correct the flowmeter information in relation to the fuel temperature.The delta pressure regulator maintains a constant pressure drop across the TMMV. It also routes non-metered fuel back to the pump inlet.
  • The fuel flow rate of the TMMV outlet is measured by the flowmeter pressurizing valve, which is linked to a resolver. The resolver sends an electric signal to the ECB in relation to the fuel flow.The direct current driven three way shutoff solenoid valve receives signals from the ECB. In the normally closed position it provides a shutoff function by redirecting the metered flow back to the pump inlet. When activated by the ECB it opens a path for the metered fuel flow coming out of the flowmeter pressurizing valve.
  • The FCU is attached to the lube pump module with a quick release V-clamp. The high pressure pump is driven by an oil lubricated splined drive shaft.
  • Set the MASTER SW 14KD to the ON position. When you push the APU START switch 2KA, the engine starts to turn. At 7% engine speed, the ECB 59KD energizes the fuel shut off solenoid valve (3-way ) to the open position and fuel from the pressurizing valve ( flowmeter) of the FCU will flow to the Flow Divider Assy. At the same time, the ECB 59KD energizes the ignition system. The fuel flows from the Fuel Divider Assy to the primary inlet of the fuel nozzles, combustion occurs and the engine continues to accelerate.
  • At 50% engine speed (above 20,000 ft at 58%), the ECB 59KD de-energizes the starter motor 8 KA. The engine continues to accelerate under its own power to governed speed. The fuel speed and proportional governors in the ECB 59KD transmit a signal to the FCU. The FCU controls the fuel flow to gain a steady acceleration.
  • As the engine accelerates, the ECB 59KD transmits a signal to the TMMV of the FCU. The TMMV of the FCU then controls the metered fuel flow. The fuel acceleration control in the ECB 59KD keeps the necessary fuel flow until the engine gets to governed speed.The engine accelerates, the fuel flow increases to a set level and the check valve of the Flow Divider Assy opens at 120 psi (8.2737 bar). This permits a fuel flow to the secondary fuel manifold, then the primary and secondary inlet of the fuel nozzles are supplied with fuel.
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