Subject M - Fuel Metering Systems

Question 1

Where are the fuel spray nozzles located in a turbine engine?

Answer 1

They are located either externally or internally on the combustion chamber in such a way that the fuel can be sprayed into the combustion area.

Question 2

Name the heat sources for the operation of fuel heaters used in turbine engines?

Answer 2

Either bleed air or engine lubricating oil.
if bleed air is used it is call an air-to-liquid heat exchanger.
When oil is used its called liquid-to-liquid heat exchanger.
Turbine engines use heat exchangers to heat the fuel and prevent the possibility of ice crystal formations.

Question 3

What engine variables are sensed by a hydro mechanical fuel control?

Answer 3

The fuel control senses power lever position
rpm
compressor inlet pressure temperature,
burner pressure or compressor discharge pressure,
and EGT( exhaust gas temp) or IGT (inlet gas temp)

Question 4

What are the two most commonly used carburetors in small reciprocating engines?

Answer 4

They are the float-type carburetors and pressure-type carburetors.

Question 5

What type of fuel control unit is used in a turbine engine?

Answer 5

Both hydro mechanical and electronic fuel control units are in use today. However the most common is the hydro mechanical type

Question 6

What is meant by trimming a turbine engine?

Answer 6

Adjusting the fuel control for the proper specific gravity of the fuel and for the proper idle and full throttle RPM.

Question 7

When should a turbine engine be trimmed?

Answer 7

Whenever there is a loss in engine power, or the manufacturer specifies

Question 8

What specifications are used to trim a turbine engine?

Answer 8

The data plate on the engine typically contains the trimming specifications.

Question 9

What are two types of electronic fuel controls for turbine engines?

Answer 9

Supervisory EEC (electronic engine control) and FADEC( Full-Authority Digital Electronic Control.

Question 10

What is a FADEC?

Answer 10

A Full-Authority Digital Electronic Control. It is a high-precision digital electronic fuel control that utilizes an Electronic Engine Control and the Flight Management computer to measure the fuel nozzle that prevents over-speed and over-temp conditions. The FADEC also send information to indication systems for the aircraft crew.

Question 11

Does a turbine engine controlled by a FADEC require manual trimming?

Answer 11

No, the FADEC is self trimming.

Question 12

What is the purpose of a mixture control in a float type carburetor?

Answer 12

It is to control the fuel- air mixture as air density changes and altitude changes.

Question 13

What is the purpose of an accelerating system on a float-type carburetor?

Answer 13

When the throttle is opened rapidly, the fuel-air mixture will lean out temporarily. to overcome this tendency, a charge of fuel from the accelerating pump will temporarily enrich the mixture in the venturi.

Question 14

Describe the purpose and operation of a venturi?

Answer 14

The venturi performs three functions. It proportions the fuel-air mixture, lowers the pressure at the discharge nozzle, and limits airflow at full throttle.
The principle operation of the carburetor depends on the differential between the atmospheric pressure, and the decreased pressure offered by the venturi.

Question 15

What is the function of a metering jet?

Answer 15

Its purpose is to limit the fuel flow when the throttle is wide open.
The main metering jet is placed in the fuel passage between the float carburetor and the discharge nozzle.

Question 16

What is the fuel metering force in a a float-type carburetor?

Answer 16

It is differential pressure between the pressure in the float chamber and at the nozzle.

Question 17

What will be result of using excessively rich idle mixtures?

Answer 17

Excessively rich idle mixtures will cause carbon fouling of the spark plug

Question 18

What is the purpose of the economizer system in a float carburetor?

Answer 18

The economizer is essentially a valve which is closed at throttle settings below 60-70 % of rated power, but provides additional fuel for cooling the engine to prevent detonation at higher throttle settings above 60-70 %.

Question 19

Where does a pressure injection carburetor obtain fuel pressure?

Answer 19

It obtains fuel pressure from the boost pump for starting and the engine driven fuel pump for normal operation.

Question 20

What operates an automatic mixture contol?

Answer 20

The AMC contains sealed bellows that expand or contract with changes in atmospheric pressure. The movement of the bellow operates a tapered needle that controls the impact air pressure into the “A” chamber of the carburetor.

Question 21

In what position do you place the mixture control of a pressure injection carburetor to stop the engine?

Answer 21

In the idle cutoff position

Question 22

What RPM indication should result when the mixture control is placed in the idle cutoff position when the idle mixture is set correctly?

Answer 22

There should be a slight increase in RPM, followed by a rapid drop of RPM to zero.

Question 23

What is the purpose of filling a pressure injection carburetor full of fuel and allowing it to soak for a period of about eight hours to installation.

Answer 23

to soften the diaphragms and make them as pliable as they were when the carburetor was originally calibrated.

Question 24

What are the three types of carburetor icing?

Answer 24

Throttle icing, impact icing, and fuel evaporation icing.

Question 25

What is the typical fuel pressure in a float type carburetor?

Answer 25

3-5 psi.