15.11 Fuel Systems Flashcards
What must the fuel system be able to do and how is it achieved?
It must be able to increase or decrease the power to obtain the required thrust for any operating condition. This is achieved by varying the flow of fuel to the combustion chamber
Gas turbine control systems are generally divided into three categories what are they?
Hydro mechanical, hydro mechanical/electronic, full authority digital engine (or electronics) control (FADEC)
What is the hydro mechanical/electronic fuel control?
It is a hybrid of two types of fuel control but can function solely as a hydro mechanical control. In dual mode inputs and outputs are electronic and fuel flow is set by server motors
What does FADEC give the computer?
Complete control
How does hydro mechanical fuel control work?
It has no electronic interface assisting in computing or monitoring the fuel flow
Why aren’t hydro mechanical fuel controls used as often?
There use is limited giving way to electronic base controls
What is the function of the engine fuel system?
Pressurise the fuel, meter fuel control and deliver fuel to the combustion section
The total engine fuel and control system consists of the following components what are they?
The vane fuel pump assembly is a fixed displacement fuel pump that provides high-pressure fuel to the engine fuel control system.
The filter bypass valve in the fuel pump allows fuel to bypass the fuel filter when the pressure drop across the fuel filter is excessive.
What does the electronic engine control system consist of?
Hydromechanical fuel control, EFCU and aircraft power lever angle (PLA) potentiometer
What does the engine generated control signals include?
fan spool speed, gas generator spool speed, inner turbine temperature, fan discharge temperature, and compressor discharge pressure
Where is the PLA potentiometer mounted?
Throttle quadrant
What does the PLA potentiometer do?
Transmits an electrical signal to the ECFU which represents engine thrust demand in relation to throttle position
Where is the EFCU located?
Remotely and airframe mounted
Why was the FADEC system developed?
To further enhance fuel control on most new turbine engines
Give two features on the FADEC system?
No hydromechanical fuel control backup system, uses electronic sensors that provide engine parameter information to the EEC
How does the EEC use information from the FADEC system?
To calculate the amount of fuel flow by adjusting the fuel metering valve
What is the EEC
A computer that controls the operation of the engine
The EEC houses two electronic channels what are they?
A and B
How does the EEC work?
The EEC computer uses data provided by the engine sensors and aircraft systems to control the engine operation. It receives electronic signals from the flight deck to set engine power or thrust. The throttle lever angle resolver supplies the EEC with a signal in proportion to the thrust lever position
What does the crosstalk logic do?
Compares data from channels A and B to figure out the best to control the output
What type of memory does the EEC have?
Volatile and non-volatile
Where is the fuel metering unit mounted?
Front face of the gearbox and is attached to the front of the fuel pump
What does the EEC do to start or stop fuel flow?
Sends a signals to the minimum pressure and shutoff valve in the fuel metering unit
What is the primary fuel system?
Stores the fuel
What is the secondary fuel system?
Supplies fuel to the engine
Engine fuel systems can be spilt into 2 subsystems what are they?
Fuel distribution subsystem, fuel control subsystem
The engine fuel distribution system has 3 main tasks what are they?
To safely supply the fuel from the aircraft fuel system to the combustion chamber
To pressurise the fuel sufficiently so that it can be vaporised in the combustion chamber
To heat the fuel. This makes sure that fuel flow to the fuel nozzles is free of any ice build-up
Where does the fuel system usually start?
Directly behind the low pressure fuel shutoff valve
What does the low pressure fuel shutoff valve do?
Feeds fuel into the main fuel supply line
What pressure does the over pressure relief valve run at?
1250psi
What does the low pressure fuel pump increase fuel to?
175psi
At maximum engine speed what does the high pressure fuel pump increase pressure to?
900 psi
What does the metering section allow?
Only metered fuel to pass through the nozzles
What does the metering section allow?
Only metered fuel to pass through the nozzles
What is the dual function of the oil cooler?
The cold fuel cools the oil of the engine oil system which in turn heats the fuel to a temperature above the freezing point of water
What does the fuel control unit do?
meters the fuel that is needed for combustion. It is also responsible for the supply and shut-off of fuel to the fuel nozzles at the combustion chamber
What does the fuel flow transmitter do?
The fuel flow transmitter measures the actual fuel flow and transmits signals to the flight deck for the fuel flow and fuel used indication
What do main fuel pumps do?
Deliver a continuous supply of fuel at the correct pressure and throughout the operation of the aircraft engine
What are the two engine driven fuel pumps?
Constant displacement and non-constant displacement
Where is a non-constant displacement pump used?
At the inlet of the engine driven pump to provide positive flow to the second stage pump
How is the fuel pump lubricated?
By the fuel passing through the pump
Where is a low pressure filter installed?
Between the supply tanks and the engine fuel systems to protect the engine driven fuel pumps
Where is a high pressure filter installed?
Between the fuel pump and the fuel control to protect the fuel from any containments that could come for the low pressure pump
What are the three most common filters?
Micron filter, water screen filter, plain screen filter
What are filtering actions rated in?
Microns
What are the most widely used filters?
200 mesh and 35 mesh micron filters
Where are 200 mesh and 35 mesh micron filters used?
Fuel pumps, fuel controls and between the fuel pump and fuel control where the removal of micronic particles is needed
What are these filters made of?
Fine mesh steel wire
What can the heat exchanger use as a source of heat?
Bleed air or engine oil
What do fuel nozzles do?
Inject fuel into the combustion area in highly atomised spray pattern so burning is completed evenly
What does the fuel manifold do?
Distributes the fuel to the individual fuel nozzles
What do shrouds do?
Used in fluid line that are routed through critical areas of the engine to catch fluid leaks
What are two methods to protect the connections of the fluid lines against leaks
Shrouded connections, double sealed connections
What are the two types of fuel nozzles?
Simplex and duplex
What does a duplex nozzle require?
requires a dual manifold and a pressurising valve or flow divider for dividing primary and secondary (main) fuel flow
What does a simplex nozzle require?
simplex nozzle requires only a single manifold for proper fuel delivery
What are the fuel nozzles constructed of and how are they installed?
External mounting wherein a mounting pad is provided for attachment of the nozzles to the case or the inlet air elbow, with the nozzle near the dome
Internal mounting at the liner dome, in which the chamber cover must be removed for replacement or maintenance of the nozzle
What is a disadvantage of the simplex nozzle?
Can only give good atomisation in a small fuel flow range
How do you get a good atomisation across a wide fuel flow range with simplex nozzles
Two sets of simplex nozzles
What nozzles do modern aircraft use?
Duplex nozzles which combine two nozzles in one component
How xo you prevent carbon formation?
Check valve
