Gas Turbine - Fuel Systems Flashcards
Specific Fuel Consumption for turbojet engine is
The mass of fuel burned per hour divided by the thrust produced
Fuel Oil Heat Exchanger or Fuel cooled oil cooler
Hot oil cooled and cold fuel is warmed - could indicate failure if oil contents rising and oil pressure is falling
Primary means of determining the thrust of jet engine
Rate of fuel flow
Spar Valve in fuel system of gas turbine
(Low pressure shut off valve)
Located in engine pylon isolates the ac fuel supply from the engine
Route of fuel into the engine
Enter fuel oil heat exchanger (warm cold fuel) then filtered and delivered under a LP pump to a HP pump. Fed to fuel controller (determined by thrust)
Fuel control systems x3
Hydro mechanical unit (mechanical fuel control unit) - old
Hydro mechanical engine controller - basic control of speed core
FADEC - 2 channel engine electronic controller
FADEC System - Why do they have two Engine controllers
2 channels for redundancy
Fuel System Major Components
Low pressure fuel pump
High pressure fuel pump
High pressure shut off valve (HP Cock)
Pressure raising shut off valve (PRSOV)
Low pressure fuel pump
Ensure positive fuel pressure feed to the HP pump
HP Fuel Pump x2 types are
Spur Gear fixed volume pump
Variable Volume Type (Swash Plate)
Spur Gear Fuel Pump - Spur Gear
Light/simple/minimal heat transfer
Pump supplies excess fuel to fuel metering units
Variable Volume - High Pressure Fuel Pump
Swash plate/control piston/governor to control pumps output
Stroke volume limited by angle of swash plate via control piston from inputs from engine over speed governor and the FCU
HP Fuel pumps use what liquid for lubrication
Fuel
Fuel Oil Heat Exchanger
Heat from oil used to warm the fuel.
Oil is always at a higher pressure than the fuel
Prevents fuel entering the oil system
NON FADEC Control - Fuel control unit
Adjust fuel flow to keep engine RPM/temp in limits
Each engine has a dedicated FCU
Ensures correct ratio of fuel and air
Controls piston in HP fuel pump
FCU senses
Thrust level
Air temp/pressures
Rapid acceleration/deacceleration
Engine speed
Exhaust gas temperature
Compressor delivery pressure
Acceleration Control Unit
Limit fuel flow to prevent over fuelling when thrust lever advanced
Altitude sensing unit
Compensates for changes in intake pressure caused by changes of airspeed or variation in pressure
FADEC basic principals
EEC engine electronic controllers computers than sense a wide variety of engine parameters and send control commands to FMU which regulates HP pump output pressure to deliver correct fuel to engine
FADEC functions
Deliver correct amount of fuel to the engine
Maintain sufficient HP pump output at low RPM to power the FMUs internal components
Fuel shut off when commanded by the pilot
Over speed protection
Control compressor airflow
Ensure manifold fuel draining on shutdown and returning excess fuel to the tanks
Fuel used in gas turbines
AVTUR - SG 0.62 to 0.88
Jet A/A1/A-1
Jet A
USA
Freezing point -40 degrees
JET A1
Most countries - additives to inhibit oxidation/corrosion/anti static/FSII
-47 degrees more suitable for long haul flights
Fuel Waxing
Very cold environment leads to clogging filters and pipes
Slowly builds up and fuel does not freeze at one temperature
Slush
HP Cock
Shuts off fuel to the combustor
Fuel Metering Unit
Regulates HP pump output pressure and flow to fuel nozzles
Electronic Engine Controller
FADEC system
Digital computer sending commands to FMU
Also send control commands to engines airflow to control system for preventing compressor stalls
Where is fuel measured
Between the fuel control unit and engine burners
Where is pressure measured in gas turbine
Between booster pump and engine
Auxiliary gearbox
Driven by HP compressor via shaft via an internal gearbox to an external gearbox
AC generator through its constant speed drive
Oil pumps
Hydraulic pumps
High pressure fuel pumps
Fuel Oil Heat Exchanger leak
If normal the oil is at higher pressure so would leak into the fuel and be burnt out
- low oil pressure and oil level dropping
If fuel system is at higher pressure than oil then fuel will leak into the oil system.
Rising oil level and low oil pressure
Hydro mechanical unit operation
Uses fuel used in combustion as hydraulic fluid
Fuel control system that uses the following signals for FCU
- N2
- Compressor discharge pressure
- Compressor inlet temperature
- Fuel shut off
- Thrust Lever angle
Thermal efficiency in jet engine
Around 30%
Describes the efficiency with which fuel energy is converted to kinetic energy
Propulsive efficiency
Kinetic energy translated into propulsive force
Specific Range
Distance flown per unit of fuel in nautical miles
NM/KG
