15.11 Fuel Systems

Question 1

How is power increased or decreased in the engine?

Answer 1

By varying the amount of fuel flow to the combustion chamber.

Question 2

What is a ‘Rich blowout’?

Answer 2

Excess fuel cools the air-fuel mixture below combustion temperature.

The amount of oxygen in the air supply is insufficient to support combustion.

Question 3

What’s the 3 main functions of an engine fuel system?

Answer 3

• Pressurise the fuel.
• Meter the fuel flow.
• Deliver atomised fuel flow to the combustion chamber.

Question 4

What are the three main types of fuel control system?

Answer 4

• Hydromechanical.
• Hydromechanical/electronic.
• FADEC.

Question 5

What’s the main fact about a ‘Pure hydromechanical system’?

Answer 5

It has no electronic interface in assisting the computing and metering of fuel.

Question 6

What’s true about the accuracy of the fuel metering function in a pure hydromechanical system?

Answer 6

It’s not as accurate as an electronic system.

Question 7

What are the two main sections of a fuel control system?

Answer 7

• Computing section.
• Metering section.

Question 8

What be can added to a hydromechanical unit to improve fuel flow metering?

Answer 8

An electronic system.

Question 9

What’s the 3 advantages of adding an electronic system to a hydromechanical unit?

Answer 9

• Can receive more inputs.
• Is more accurate.
• The hydromechanical section can then act as a backup if the electronic system fails.

Question 10

What is an EEC?

Answer 10

Electronic engine control.

Question 11

What does a hydromechanical/electronic system consist of?

Answer 11

A hydromechancal unit, and a remotely located EEC.

Question 12

What are the two modes of a Hydromechanical/electronic system?

Answer 12

• Automatic mode.
• Manual mode.

Question 13

What is automatic mode in a Hydromechanical/electronic system?

Answer 13

The EEC is in control of fuel flow metering.

Question 14

What is manual mode in a Hydromechanical/electronic system?

Answer 14

The hydromechanical unit takes over control of fuel flow metering.

Question 15

In normal operation, what does the Engine Fuel Control Unit control? (4)

Answer 15

• Thrust setting.
• Speed governing.
• Acceleration.
• Deceleration.

Question 16

What happens if the electronic unit fails, in a Hydromechanical/electronic system?

Answer 16

Manual mode is activated and allows the engine to operated at a reduced power under full control of the hydromechanical unit.

Question 17

Is the solenoid energised or de-energised, when in automatic mode?

Answer 17

The solenoid is energised.

Question 18

What type of filter is the fuel supplied to the unit through?

Answer 18

A 200 micron inlet filter screen.

Question 19

What determines the position of the metering valve, in terms of a specific ratio?

Answer 19

Fuel flow (wf) / Compressor discharge pressure (P3).

(wf/P3 ratio).

Question 20

What does a fixed wf/P3 ratio give, in terms of thrust?

Answer 20

A minimum of 90% thrust, up to 30000ft, surge-free.

Question 21

What drains the fuel nozzles and manifold after engine shutdown?

Answer 21

Drain valve assy.

Question 22

Where is the Engine Fuel Control Unit usually located?

Answer 22

Remotely located and airframe mounted.

Question 23

How is the interface between the remotely located EFCU and the engine/aircraft, achieved?

Answer 23

Branched wiring harness.

Question 24

What does FADEC stand for?

Answer 24

Full Authority Digital Engine Control.

Question 25

Does the FADEC system have a hydromechanical backup?

Answer 25

Nope.

Question 26

What is the primary component of the FADEC system?

Answer 26

The EEC.

Question 27

What does the FADEC use to provide information to the EEC?

Answer 27

Electronic sensors.

Question 28

How does the EEC calculate fuel flow for the metering valve, in a FADEC system?

Answer 28

The EEC uses the signals received from the FADEC's electronic sensors.

Question 29

How does the metering valve know how to control the fuel flow, in a FADEC system?

Answer 29

It reacts to the commands received from the EEC's calculations. (The EEC makes its calculations based on information from the FADEC's sensors).

Question 30

What is the construction of the EEC's channels?

Answer 30

It has a two channel system (A and B) that are physically independent from eachother.

Question 31

What does one channel of the EEC's two channel system, consist of?

Answer 31

It's an independent computer that can completely control engine operation.

Question 32

How do the EEcs channels communicate to eachother?

Answer 32

Via crosstalking.

Question 33

What does the crosstalking logic do?

Answer 33

Compares the data from the two channels, to determine which data is best to control the output driver for the torque motor, or the solenoid bank.

Question 34

What does the primary channel control?

Answer 34

All of the output drivers.

Question 35

What two types of memory does the EEC have?

Answer 35

- Volatile. - Non-volatile.

Question 36

What are the two thrust control modes for the EEC?

Answer 36

- Normal mode. - Alternate mode.

Question 37

What controls thrust in EEC normal mode?

Answer 37

The thrust is set by the EPR (Engine Pressure Ratio).

Question 38

What controls alternate thrust in EEC alternate?

Answer 38

The thrust is set by N1.

Question 39

What does the programming plug in the EEC do?

Answer 39

Selects the correct software in the EEC for the thrust rating of the engine.

Question 40

What happens to the programming plug if the EEC is removed?

Answer 40

It remains with the engine, its attached to the fan case with a lanyard.

Question 41

What is the primary fuel system?

Answer 41

It stores the fuel.

Question 42

What is the secondary fuel system?

Answer 42

Supplies the fuel to the engine.

Question 43

What are the two main purposes of the secondary fuel system?

Answer 43

- Supply fuel to the combustion chamber. - Control quantity of fuel necessary for correct engine operation.

Question 44

What two sub-systems is the engine fuel control system split into?

Answer 44

- Distribution System. - Control system.

Question 45

What are the 3 main tasks of the distribution system?

Answer 45

- Safely supply fuel to the combustion chamber. - Pressurise fuel for vaporisation. - Heat the fuel.

Question 46

What's the first component in the fuel distribution system?

Answer 46

Low pressure fuel pump.

Question 47

What is the purpose of the LP fuel pump?

Answer 47

Receives fuel from the tanks and increases fuel pressure to 175psi. Prevents cavitation at the HP pump.

Question 48

What does the fuel flow through, before it reaches the HP pump?

Answer 48

Through the oil cooler (to heat the fuel) and the inlet filter.

Question 49

The fuel then reaches the HP Pump. What happens here?

Answer 49

Fuel pressure is increased to 900psi at max engine speed.

Question 50

What other component does the HP pump have, to protect against overpressure?

Answer 50

A pressure relief valve that opens at 1250psi.

Question 51

What does the HP pump supply then fuel to?

Answer 51

The metering valve.

Question 52

What is true about the quantity of fuel that is supplied to the metering section from the HP pump?

Answer 52

It always supplies a greater quantity of fuel to the metering section, than is actually required.

Question 53

What happens in the metering section with regards to how the fuel is metered?

Answer 53

The metering section only supplies the metered fuel to the fuel nozzles. The un-metered (excess) fuel flows back to the pump via the bypass return line.

Question 54

What is the purpose of the un-metered fuel?

Answer 54

The un-metered fuel is used to cool the IDG oil, on its way back to the LP pump.

Question 55

In low-power conditions, where is the un-metered fuel from the metering section sent to?

Answer 55

Back to the fuel tanks, via the fuel recirculation valve.

Question 56

What's the purpose of sending un-metered fuel through the fuel recirculation valve, in low power conditions?

Answer 56

It uses colder fuel to help cool the hot un-metered fuel before it flows back into the fuel tank. This is because the hot fuel could cause the LP pump to overheat.

Question 57

What type of fuel pump is normally used at the inlet of the engine driven pump?

Answer 57

Non-constant displacement fuel pump.

Question 58

What type of fuel pump is normally used at the main stage of the engine driven fuel pump?

Answer 58

Positive displacement pump.

Question 59

How are fuel pumps lubricated?

Answer 59

By the flow of fuel passing through them.

Question 60

When should you never operate a fuel pump?

Answer 60

If there is no fuel supply to it. It will burn out very quickly.

Question 61

Where are low pressure filters usually installed?

Answer 61

Between the supply tanks and the engine fuel system, to protect the EDP.

Question 62

Where are high pressure filters usually installed?

Answer 62

Between the fuel pump and fuel control system, to protect the system from contaminants from the LP pump.

Question 63

What are the 3 most common types of fuel filter?

Answer 63

- Micron filter. - Wafer screen filter. - Plain screen mesh filter.

Question 64

What is the most commonly used type of filter?

Answer 64

Micron filter, because it has the greatest filtering action.

Question 65

What size particles can a micron filter, filter out?

Answer 65

10-25 microns.

Question 66

What's the disadvantage or micron filters, and how is it overcome?

Answer 66

They are susceptible to clogging. Overcome by pressure bypass valve.

Question 67

What is the benefit of using a Porous Cellulose material in the filter element?

Answer 67

It can also absorb some of the water that may be present in the fuel.

Question 68

What are the most common mesh ratings of filters used in fuel systems?

Answer 68

200 mesh and 35 mesh.

Question 69

Which type of filter element is replaceable?

Answer 69

Wafer screen type.

Question 70

What two sources of heat can a heat exchanger use to heat the fuel?

Answer 70

- Engine bleed air. - Hot oil.

Question 71

What's the name for a heat exchanger that uses bleed air to heat the fuel?

Answer 71

Air-to-liquid exchanger.

Question 72

What's the name for a heat exchanger that uses hot oil to heat the fuel?

Answer 72

Liquid-to-liquid exchanger.

Question 73

What supplies fuel to the fuel manifold?

Answer 73

The fuel control unit.

Question 74

What does the fuel manifold supply fuel to?

Answer 74

The fuel nozzles.

Question 75

What is the purpose of using shrouds around the fuel manifold?

Answer 75

They protect against high pressure fuel leaks in the hot environment of the compressor and combustion case.

Question 76

How do the shrouds protect against fuel leaks?

Answer 76

They catch any leaked fuel and carry it to the engine drain system.

Question 77

What do double-sealed connections between the fuel manifold and fuel nozzles, provide?

Answer 77

Two layers of protection against fuel leaks between the fuel manifold and nozzles.

Question 78

What creates the inner seal in a double-sealed connection?

Answer 78

A pair of O-rings.

Question 79

What creates the outer seal in a double-sealed connection?

Answer 79

A conical seal between the manifold and nozzle.

Question 80

What is the main purpose of the fuel nozzles?

Answer 80

To atomise the fuel.

Question 81

What are the two types of fuel nozzles?

Answer 81

- Simplex. - Duplex.

Question 82

What's inside a simplex fuel nozzle?

Answer 82

A swirl chamber.

Question 83

What's the disadvantage of a simplex fuel nozzle?

Answer 83

Only provides good atomisation in a small fuel-flow range.

Question 84

How is the disadvantage of simplex fuel nozzles overcome?

Answer 84

By using two different sets of simplex nozzles. (primary and secondary).

Question 85

What is the function of the primary simplex nozzles?

Answer 85

They are used for engine start up to idle speed.

Question 86

What is the function of the secondary simplex fuel nozzles?

Answer 86

They are used for acceleration up to full power.

Question 87

Are simplex fuel nozzles normally used on modern GTEs?

Answer 87

Nope.

Question 88

What do Duplex fuel nozzles achieve?

Answer 88

They combine the two systems of the simplex fuel nozzles, into one.

Question 89

What does each duplex nozzle consist of?

Answer 89

Each nozzle has 'dual-flow'; - Two discharge orifices. - Two internal supply lines.

Question 90

What is the structure of the primary orifice in a duplex nozzle?

Answer 90

It's smaller and located in the centre of the nozzle.

Question 91

What is the structure of the secondary orifices in a duplex nozzle?

Answer 91

They are larger, and they surround the primary orifice.

Question 92

What divides the flow of fuel through the duplex nozzle into the different orifices?

Answer 92

Flow divider valve. It's spring loaded.

Question 93

What happens to the flow divider valve, as engine speed increases?

Answer 93

The valve opens to increase fuel pressure.

Question 94

What else do the duplex nozzles do, in relation to airflow and cooling?

Answer 94

They pick-up some of the airflow from the combustion chamber, to help cool the nozzle. Also helps blow-away residual fuel on the nozzle tip after engine shutdown.

Question 95

How do air-spary nozzles atomise fuel?

Answer 95

They use high velocity airflow that flows through an inner and outer air swirler.

Question 96

What's the advantage of air-spray nozzles?

Answer 96

They provide good atomisation even at lower engine speeds.

Question 97

What's the disadvantage of air-spray nozzles?

Answer 97

They require a constant high-velocity airflow.

Question 98

What type of flow do air-spray nozzles always provide?

Answer 98

Single flow.