Engine Fuel and Fuel Metering

Question 1

What are some of the sources used to heat the fuel in a turbine engine fuel system?

Answer 1

Bleed air and engine lubricating oil.

Question 2

What are the most common types of fuel metering systems used on small reciprocating engines?

Answer 2

Float-type carburetors, pressure-injection carburetors, and direct fuel injection systems.

Question 3

What function does the mixture control perform?

Answer 3

Controls the fuel/air mixture.

Question 4

What generally causes spark plug fouling?

Answer 4

Operating the engine with an excessively rich mixture at idle.

Question 5

What is the purpose of an accelerating system?

Answer 5

It provides an immediate but brief increase in fuel flow in the venturi to enrich the mixture.

Question 6

What is a carburetor economizer system?

Answer 6

It is a power enrichment system that provides a richer mixture at high power settings, where the excess fuel aids in engine cooling. This system functions at throttle settings above cruise power settings.

Question 7

When the mixture is placed in the idle-cutoff position, a slight rise in rpm is noted prior to the engine decelerating. What does this indicate?

Answer 7

The idle mixture is set correctly.

Question 8

Why should engine rpm be accelerated periodically when making carburetor adjustments?

Answer 8

To clear the engine.

Question 9

What maintains the fuel pressure in a pressure injection carburetor?

Answer 9

An engine-driven fuel pump.

Question 10

What happens if the manual mixture control of a pressure injection carburetor is moved to the idle cutoff position?

Answer 10

The engine stops.

Question 11

How does an automatic mixture control function?

Answer 11

A sealed brass bellows, connected to the fuel metering system, expands and contracts with changes in pressure and temperature, adjusting the mixture accordingly.

Question 12

What are some of the advantages of fuel injection systems over carburetor systems?

Answer 12

Less danger of induction icing, better acceleration, better fuel distribution, better fuel economy, reduced overheating of individual cylinders.

Question 13

What purpose do turbine engine fuel control units have?

Answer 13

They automatically meter fuel to the engine.

Question 14

What are the two basic types of turbine engine fuel control units?

Answer 14

Hydromechanical and electronic.

Question 15

What are the engine variables detected by the fuel control unit?

Answer 15

Power lever position, engine rpm, compressor inlet temperature, compressor inlet pressure, compressor discharge pressure, burner pressure.

Question 16

What are the two major components of a supervisory electronics engine control?

Answer 16

The electronic control unit, and the hydromechanical fuel control.

Question 17

What are some of the advantages of a full-authority digital engine control (FADEC) over a hydromechanical fuel control?

Answer 17

Better fuel economy, improved starts, requires no engine trimming, provides engine limit protection, provides constant idle speeds regardless of atmospheric conditions or bleed air requirements, fully modulates the active clearance control, and allows more repeatable engine transients.

Question 18

What do turbine engine fuel spray nozzles do?

Answer 18

They inject fuel into the combustion area.

Question 19

What are the different engine indications used for trimming a turbine engine?

Answer 19

Either EPR or rpm is used depending on the particular engine.

Question 20

Which maintenance adjustments are normally allowed on an installed hydromechanical fuel control?

Answer 20

Specific gravity for fuel, idle rpm, and maximum rpm (or maximum EPR).

Question 21

What type of pump can vary the amount of fuel discharged regardless of speed?

Answer 21

A variable displacement pump.

Question 22

What is used to keep water, sediment, and foreign matter out of the carburetor?

Answer 22

The main fuel strainer.

Question 23

What is the purpose of a pressure relief valve in a constant displacement pump?

Answer 23

It returns excess fuel that is not required by the engine to the inlet side of the pump.

Question 24

What type of engine-driven pump is widely used other than gear or piston pumps?

Answer 24

A rotary vane type.

Question 25

What function do fuel boost pumps perform?

Answer 25

They supply pressurized fuel to the fuel pump, which helps prevent vapor lock and cavitation.

Question 26

What is a commonly used type of fuel boost pump?

Answer 26

The centrifugal type.

Question 27

What causes vapor lock and why is it of concern?

Answer 27

Vapor lock is caused by insufficient fuel pressure, high fuel temperatures and excessive fuel turbulence, which may completely block any fuel flow resulting in engine failure.

Question 28

What function does the engine-driven pump on a turbine engine perform?

Answer 28

It provides a continuous supply of fuel at the proper pressure while the engine is running.

Question 29

Name the two categories of turbine engine pumps.

Answer 29

Constant and variable displacement.

Question 30

What category is a gear-type pump classified in?

Answer 30

Constant displacement.

Question 31

When performing an external inspection of an engine-driven fuel pump, what should you look for?

Answer 31

Leaks and security of mounting.

Question 32

What is the purpose of a shear section of the driveshaft in a dual element constant displacement pump?

Answer 32

If one element seizes, a portion of the driveshaft seizes, allowing the other element to continue to operate.

Question 33

Where in a turbine engine fuel system is ice formation likely to occur and how is it prevented?

Answer 33

The fuel filter is most susceptible to ice formation, so a fuel heater is used that consists of a heat exchanger using either engine oil or bleed air to warm the fuel.

Question 34

What is the purpose of a bypass valve in an engine fuel system micron filter?

Answer 34

It is a safety feature that allows fuel to flow to the engine if the filter becomes blocked.

Question 35

What is the purpose of a duplex fuel nozzle?

Answer 35

A duplex fuel nozzle discharges two different spray patterns. The spray pattern is wider during start up and acceleration and narrows when engine speed increases above idle.

Question 36

What are the functions of the pressurizing and dump portions of a pressurizing and dump valve?

Answer 36

The pressurizing portion provides primary and secondary fuel flow to dual-line duplex fuel nozzles, and the dump portion allows fuel to drain from the manifolds after engine shutdown.

Question 37

What purpose do fuel selector valves serve?

Answer 37

They allow tank and engine selection and provide a means for shutting off fuel flow.