15.11 Fuel Systems Flashcards
What is the primary fuel system and what is the secondary fuel system
Primary - Fuel storage
Secondary - On the engine
What are the two engine fuel subsystems
- Fuel distribution subsystem
- Fuel Control subsystem
What are the 3 main purposes of the engine fuel distribution subsystem
- Safely supply the combustion chamber
- Pressurise fuel enough to combust in combustion chamber
- To heat fuel and prevent ice build up in jet nozzles
Where does the engine fuel distribution system start
Directly behind the low pressure fuel shut off valve
What does the main fuel shut off valve supply
The main fuel supply line
Where is the low pressure fuel pump located
On the auxiliary gearbox
What is the purpose of the low pressure fuel pump
Increase the pressure of the fuel coming from the wing tank jet pumps
Where does the fuel go after the low pressure fuel pump
The oil cooler
What is the purpose of the oil cooler
The cold fuel cools the hot oil of the engine lubrication system and the hot engine oil warm the cold fuel about the melting point of water
What is located downstream of the oil cooler
The fuel filter
What component comes after the fuel filter
The high pressure fuel pump
What is the purpose of the high pressure fuel pump
Increases the pressure of the fuel enough for vaporisation to combust
It is fitted with a pressure relief valve
After the high pressure fuel pump where does the fuel go
The fuel control unit
What is the purpose of the fuel control unit
Meters the fuel needed for combustion
Responsible for supply and shut off to fuel to the fuel nozzles and combustion chamber
Needs high pressure fuel to operate internal control mechanisms
What do some fuel control units have to ensure there is going to be no ice build up
An additional servo fuel heater
When the fuel leaves the fuel control unit where does it flow next
The fuel flow transmitter
What is the purpose of the fuel flow transmitter
Measures the actual fuel flow and transmits signals to the cockpit for fuel flow and fuel used Indication
Where does the fuel flow after the fuel flow transmitter
It is routed to the fuel manifold which distributes the fuel to the individual fuel nozzles on the combustion chamber
What does the low pressure fuel pump increase the pressure of the fuel to
Around 175PSI
What is the secondary purpose of the low pressure fuel pump
To prevent cavitation at the inlet of the high pressure pump
At maximum engine speed the high pressure fuel pump increases the fuel pressure to approx what pressure
900 PSI
The overpressure relief valve open when the pressure reaches approx?
1250PSI
Where would the fuel go after the metering unit if it is not needed for combustion
It would return to the fuel pump by the bypass return line
What is the bypass return line connected to (and to perform what task) from the fuel metering unit
On some engines it’s connected to the IDG oil cooler to cool IDG oil
On most turbofan engines what type of unit is the fuel control unit
A hydromechanical component
Where is the fuel control unit usually mounted
To the engine fuel pump on the auxiliary gearbox
In a hydromechanical unit what are all functions controlled by
Fluid pressures and mechanical components
What are the two different types of fuel control unit
- Speed governed fuel control unit, which you usually find on older engines
- Constant thrust fuel controlled units usually use the N2 demand signal and the N2 speed feedback for steady state fuel metering
The fuel control unit has many pipes and push pull cables connected to it. What are they needed for
Some are needed to transmit throttle demand signal, the temperature signal, the pressure signal and the speed feedback signals for fuel metering and others are needed for the engine compressor
Thrust lever is usually a mechanical deflection of a small power lever at the fuel control unit, by what means is this done
Rack and pinion transmission or rods and levers
How is the N2 speed feedback signal transmitted
Via a mechanical drive shaft from the main fuel pump
What two major sections can the fuel control unit be split into
- The fuel metering section
- The computing section
The computing subsystem can be sub split into: - A governing section
- A limiting section
What are the 3 main tasks of the fuel control unit
- Fuel metering
- Power control
- Engine protection
What does the governing computing section of the computing section do
Makes sure the selected power is controlled
What does the limiting section of the computing section do
Monitors the governing section and makes sure that the engine always operates within safe limits
What does FADEC stand for
Full Authority Digital Engine Control
What is the purpose of the FADEC computer
In a FADEC system a digital computer has full authority over the engine control functions. The digital computer is the heart of the FADEC system (Normally called the EEC or ECU)
What is the second main component of the FADEC system
The Fuel Metering Unit
What is the Fuel Metering Unit also known as
The Hydromechanical Unit (HMU)
What does the FMU control
The FMU is not a controller, it only receives orders from the ECU to move the Fuel Metering Valve
What type of systems does not have a hydromechanical unit
A FADEC system
Where is the ECU usually located
On the fan stator case of the engine
What parameters does the ECU need to operate properly
Needs a demand signal from the thrust lever in the cockpit, engine speed and all important air temperatures and air pressures of the engine.
It also needs electrical power for its operation
Where does the power for the ECU come from
Either from aircraft power or the Permanent Magnet Alternator on the engine
What main tasks does the ECU perform apart from fuel metering and engine limit protection
Performs full power management and gives optimum thrust control for all operating conditions
What subsystems does the ECU control
- The compressor stall protection system
- The turbine and compressor clearance control system
- The thrust reverser system
- The engine starting system
- The engine Indication system
What main components make up a FADEC system
- An ECU
- A Fuel Metering Unit
- Many electrical sensors on the engine
- Many control possibilities for other engine sub systems
What are EIUs
Engine Interface Units
Transmit data from the FADEC system to the aircraft. They also control the power supply from the aircraft to the ECU
