15.11 - Fuel Systems

Question 1

Describe the stoichiometric ratio.

Answer 1

The fuel-air mixture ratio that, when burned, leaves no uncombined oxygen nor any free carbon. It releases the maximum amount of heat, and therefore produces the highest exhaust gas temperature. The ideal proportion (stoichiometric ratio) is 15 pounds of air for 1 pound of jet fuel. 15:1

Question 2

Briefly describe FADEC.

Answer 2

Full Authority Digital Engine Control (FADEC) uses a computerised system such as an ECU (Electronic Control Unit) which uses electronic sensors for it’s inputs and meters fuel with electronic output signals.

This system utilises a HMU (Hydro Mechanical Unit) and a Torque Motor to drive engine valves and actuators.

This electronically controlled fuel control is very accurate in scheduling fuel by sensing many of the engine parameters

Question 3

How is continuous operation of the engine maintained if a fuel filter becomes blocked?

Answer 3

A by-pass valve around the filter accompanied by a ‘Filter Blocked’ or ‘’Low Fuel Pressure’’’ warning to the flight deck via a differential pressure switch.

Question 4

What is the purpose of fuel heaters?

Answer 4

The purpose of the fuel heater is to melt ice crystals to stop them blocking fuel filters. Ice crystals can form by water in the fuel freezing.

Question 5

List the two types of atomizing fuel nozzles:

Answer 5

1. Simplex
2. Duplex

Question 6

What is the standard Stoichiometric Ratio of air/fuel?

Answer 6

The ideal proportion of air and jet fuels which is 15:1

This is called the Stoichiometric ratio

Question 7

What can sudden deceleration of an engine cause?

Answer 7

Engine Flame-out

FCU (Fuel Control Unit) must accelerate by ‘‘Controlled over-fuelling’’ and decelerate by ‘‘Controlled Under Fuelling’’

Question 8

List the common engine parameters needed for delivering the proper fuel ratio?

Answer 8

• Power lever position (throttle lever)
• Compressor RPM
• Inlet temperature
• Compressor delivery temperature
• Aircraft altitude
• Airspeed (ram recovery)

Question 9

Describe the purpose of an FCU?

Answer 9

Fuel Control Unit (FCU) must deliver the correct amount of fuel

The FCU accelerates the engine by means of a ‘‘controlled over-fuelling’’ and decelerate it by means of a ‘‘controlled under-fuelling’’

Question 10

Define FADEC, what device meters the fuel?

Answer 10

FADEC = Full Authority Digital Engine Control

Usually a sharp edged metering device

Question 11

What is an EEC?

Answer 11

EEC = Electronic Engine Control

Pratt & Whitney call their FADEC computer an EEC

(whereas CFM call their engine control = ECU

Question 12

List the two main types of fuel pump

Answer 12

1. Plunger type Fuel Pump (variable delivery)
2. Spur Gear type Fuel Pump ( Constant delivery)

Both are positive displacement pumps

Question 13

What types of delivery do the two types of fuel pumps have?

Answer 13

• Plunger Type Fuel Pump = Variable delivery
• Spur Gear Type Fuel Pump = Constant delivery

Question 14

How can a Fuel filter bypass be indicated?

Answer 14

By-pass valve around filter is used to ensure continuous fel delivery to engine

These systems always incorporate a ‘‘Filter Blocked’’ or ‘‘Low Fuel Pressure’’ warning to the flight deck via a differential pressure switch

Question 15

What is the purpose of a fuel heater?

Answer 15

Ice crystals which occur when water entrained (‘‘free water’’) is in the fuel freezes can equally be able to block a fuel filter. (as well as solid contaminants)

Fuel heaters are installed to prevent this

Question 16

List the two types of fuel nozzles?

Answer 16

1. Pressure Atomising Nozzle
2. Vaporising

(Fuel nozzles are fuel distributors)

Question 17

What is a P&D valve?

Answer 17

P&D valve = Pressurising and Dump Valves

The dump vavle is quite lightly sprung to the open or ‘‘dump’’ position on engine shutdown.

When the engine starts to rotate, rising fuel pressure will overcome the spring loading to close off the dump valve and allow normal operation until the next shut-down.