FUEL SYSTEMS Flashcards
Where is the fuel control unit of an engine usually mounted?
To the engine fuel pump on the accessory gearbox.
On turbofan engines, usually hydromechanical component
In a hydromechanical fuel control unit what are all control functions done by?
Fluid pressures and mechanical components.
What are the 2 different types of fuel control unit?
Speed governed fuel control unit (older engines)
Constant thrust fuel control unit (modern engines)
What do speed governed fuel control units mainly use for steady state fuel metering?
N2 demand signal and N2 speed feedback signal.
Constant thrust fuel control, additional thrust feedback signals such as N1 speed, ambient pressure and temp signals
What do temperature signals like CIT or fan inlet temperature convert the air temperatures to in a fuel control unit?
Fuel pressure signals.
What do hydromechanical fuel control units use for operation?
Hydraulically, Pneumatically and mechanically
What 2 parts can the fuel control unit be split into?
Fuel metering section
Computing section
What can the computing section of the fuel control system be further divided into?
Governing section
Limiting section
What does the fuel metering system do in a fuel control unit?
Ensures necessary fuel gets to the fuel nozzles and all fuel not needed for combustion returns to the fuel pump.
What does the governing section of the fuel control unit do?
Makes sure that the selected power is controlled.
What is the limiting section for in a fuel control unit?
ensures the engine always operates within safe limits.
What does FADEC stand for?
Full Authority Digital Engine Control.
What is the digital computer in FADEC system normally called?
ECU or EEC.
What is a FADEC made up of?
EEC or ECU
FMU (fuel metering unit)
Where are the electronic control units and FMU located?
Fan stator case of the engine.
What can a small permanent magnet alternator on the accessory gearbox provide for FADEC?
A power supply.
What are engine interface units (EIU) for?
Transmit the data between the FADEC system and the aircraft.
Control power supply from aircraft to ECU.
What can the engine fuel system be separated into?
- fuel distribution subsystem
- fuel control system
What 3 main tasks does the fuel distribution system have?
- safely supply fuel to combustor
- pressurise fuel
- heat the fuel
What does the low pressure fuel pump do?
Increases the pressure that comes from the tank boost pumps
What two functions does the oil cooler have?
Cold fuel cools the engine lubrication system
Puts fuel above freezing so it doesn’t block the fuel filter
Where is the filter located in comparison to the oil cooler?
Downstream.
Ensures that ice cannot block the filter as fuel is warmed above freezing point
What is a high pressure fuel pump?
Increases fuel pressure so it’s able to vaporise in the combustion chamber
What is the purpose of the fuel control unit?
Meters the fuel which is needed for combustion. Also responsible for supply and shut off.
The fuel control unit needs high pressure fuel to be used as servo pressure for internal mechanisms. To avoid ice build up, what can be added?
An additional servo fuel heater.
What does a fuel flow transmitter measure?
Measure the actual flow of fuel and transmits to cockpit
What is the fuel manifold?
Distributes fuel to individual fuel nozzles on the combustion chamber.
What is the advantage of having an oil cooler downstream of the high pressure pump?
Requires less external fuel lines because the low and high pressure pumps are in one housing but the Danger of fuel leaks is greater in a low pressure system
What PSI does the low pressure fuel pump increase the fuel pressure to before supplying fuel to the high pressure stage?
175psi
Why is it necessary for the low pressure fuel pump to pressurise before the high pressure stage?
So the HP does not have to draw via suction and prevents cavitation at the inlet.
At max engine speed, what does the high pressure pump increase the fuel pressure to?
900psi
On the high pressure pump, when will the overpressure relief open?
When the system reaches 1250psi
Some fuel is released back to the inlet port of the high pressure pump
How much fuel will the pump supply in comparison to the amount that is needed?
Always supplies more than needed.
Fuel that isn’t needed returns via the bypass return line
Fuel that is passed down the bypass line can be used to cool what?
The IDG oil cooler
In low engine power conditions, bypass fuel is returned to the tank. How is it ensured that fuel is not too hot when re-entering?
Cold fuel from the LP stage is added to the hot bypass fuel
