Fuel System (CFM56)

Fuel System (CFM56)

  • The fuel system supplies fuel to the combustion chamber at the required flow rate, pressure, and temperature.
  • The fuel flows from the tank, via the fuel pump unit and the fuel/oil heat exchanger, to the Hydromechanical Unit (HMU) and to the fuel nozzles.

Fuel Pump Unit

  • The HP compressor shaft drives the HP fuel pump assembly.
  • Fuel flows through the LP pump, then through the fuel/oil heat exchanger and the HP pump (gear pump).
  • The fuel then divides into a filtered flow for the servo fuel heater and the servo valves of the HMU, and an unfiltered flow for the metering valve of the HMU.

Shut Off Valves

Moving the ENG1 (ENG2) MASTER switch to OFF directly commands the closing of the LP and HP fuel shut off valves for that engine’s fuel system.
It also closes the fuel return valve and opens the bypass valve.

Hydromechanical Unit (HMU)

GENERAL

The FADEC controls the HMU, which :

  • controls fuel flow to the engine combustion chamber
  • controls fuel hydraulic signals to actuators
  • protects against overspeeding

FUEL FLOW

The Fuel Metering Valve (FMV) transforms FADEC orders through a torque motor and servo valve into fuel flow to the engine fuel nozzles. The FMV resolver generates a feedback signal proportional to the FMV position. The bypass valve maintains a constant pressure drop across the FMV to ensure that the metered fuel flow is proportional to the FMV position.

The FADEC computes the fuel flow that will maintain the target N1. As the FADEC maintains this N1, it allows N2 to vary while remaining between N2 minimum and N2 maximum. The FADEC also controls the engine parameters to :

  • Limit acceleration and deceleration ;
  • Avoid engine stall or flameout ;
  • Limit maximum N1 and N2 ;
  • Maintain air bleed pressure requirement. The FADEC computes an N2 correction according to the bleed configuration.

IDG Cooling System

Some of the fuel flowing out of the HMU goes to cool the oil systems of the Integrated Drive Generators (IDGs). It then returns to the fuel pump unit or to the tank. The Fuel Return Valve (FRV), controlled by the FADEC, ensures that this flow is adequate.

At low engine thrust, if the oil going into the IDG is too hot, the cooling fuel is sent back to the tank (300 kg/h). If oil temperature continues to rise, the ECU increases the minimum N2. If oil temperature still keeps rising, the FADEC increases the fuel flow to the tank (from 300 to 600 kg/h, depending on fuel return temperature). The fuel return valve is always mixing hot fuel with cold fuel so that the temperature of fuel returning to the tank stays below 100 °C (from 200 to 400 kg/h, depending on fuel return temperature).

Fuel recirculation to the tank is inhibited (FRV closed) in the following cases :

  • at engine shutdown
  • during takeoff and climb if :
    • wing tank level is below about 300 kg (660 lb).
    • there is fuel overflow in the surge tank
    • fuel feed is by gravity only.
  • when fuel temperature in the wing tank in flight is above 52.5 °C
  • Note: On the ground, high fuel temperature in the wing tank or fuel overflow in the surge tank does not inhibit the fuel recirculation to the wing tank (FRV remains open).
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