Engine Fuel and Fuel Metering Systems

Question 1

Where does the fuel metering system of a turbine engine discharge its fuel?

Answer 1

Through spray nozzles into the combustion chambers.

Question 2

What is adjusted when the fuel control unit of a turbine engine is trimmed?

Answer 2

The idle speed and the maximum-thrust speed.

Question 3

What are two locations water may be injected into a turbine engine?

Answer 3

At the compressor inlet and at the inlet to the diffuser section.

Question 4

What is meant by a duplex nozzle in a turbine engine fuel metering system?

Answer 4

A duplex fuel nozzle is one that has two fuel discharge passages. A flow divider sends fuel for low-pressure operation through one discharge pas-sage, and when the engine demands more fuel, it is sprayed out through the second discharge passage.
The spray pattern keeps the flame centered in the burner for all operating conditions.

Question 5

What is the function of a float carburetor?

Answer 5

The float carburetor’s function is to measure airflow through the engine induction system and dispense the appropriate amount of gasoline into the airflow for all engine operating parameters and conditions. It must also provide the fuel in a state that is as vaporized as possible by the time ignition occurs in the engine cylinders.

Question 6

Does the air-fuel mixture provided by a float carburetor become richer or leaner as the aircraft goes up in altitude?

Answer 6

If the mixture is not adjusted, it will become richer as the aircraft gains in altitude.

Question 7

Does the air-fuel mixture provided by a float carburetor become richer or leaner when carburetor heat is applied?

Answer 7

The mixture becomes richer when carburetor heat is applied.

Question 8

What would happen to the air-fuel mixture ratio in a float carburetor if the main air bleed were to become plugged?

Answer 8

The mixture would become excessively rich.

Question 9

How does the automatic mixture control in a pressure carburetor keep the air-fuel mixture constant as the aircraft changes altitude?

Answer 9

it automatically, and progressively, bleeds the air between the two sides of the air diaphragm and decreases the air metering force. This leans the mixture as the aircraft goes up in altitude.

Question 10

What are the two purposes of the manifold valve (or flow divider in the fuel injection system of a horizontally opposed aircraft engine?

Answer 10

It provides a constant discharge fuel pressure for idling, and it provides a positive shutoff for the fuel when the engine is shut down.

Question 11

How does the mixture control change the air-fuel mixture ratio on the Teledyne-Continental fuel injection system?

Answer 11

When the mixture control is in its FULL RICH posi-tion, all fuel goes to the manifold valve. When it is in the IDLE CUTOFF position, all fuel is returned to the pump inlet. Intermediate positions vary the amount of fuel that goes to the engine.

Question 12

Why must the diaphragms of a pressure carburetor be soaked before the carburetor is ready for flight?

Answer 12

Soaking the diaphragms of a pressure carburetor restores them to the condition of flexibility that they had when the carburetor was calibrated.

Question 13

What are two basic types of fuel controls for aircraft turbine engines?

Answer 13

Hydro Pneumatic and electro-hydromechanical.

Question 14

How does the exhaust gas temperature of a reciprocating engine tell anything about the air-fuel mixture being burned by the engine?

Answer 14

A stoichiometric mixture (a mixture in which all of the constituents of the air-fuel mixture are burned) produces the highest exhaust gas temperature. The mixture is adjusted to get the highest EGT, and then it is enriched to place it on the rich side of stoichio-metric.

Question 15

What two things are adjusted when adjusting the idling of a reciprocating engine?

Answer 15

The throttle stop, to get the proper idling RPM, and the idle mixture control, to get the smoothest operation.

Question 16

What is controlled by the adjustment of the orifice in the fuel pump of a Teledyne-Continental fuel injection system?

Answer 16

The high unmetered fuel pressure.

Question 17

What is the significance of the letters that are stamped on the flats of a fuel injection nozzle?

Answer 17

These letters designate the relative size of the orifice in the injector nozzle. The lower the letter in the alphabet, the smaller the amount of fuel the nozzle will flow.

Question 18

What is the function of the derichment valve in a pressure carburetor used on an aircraft engine with an antidetonation injection system?

Answer 18

The derichment valve automatically closes to lean the air-fuel mixture when ADI fluid is flowing. As soon as the ADI fluid stops flowing, the derichment valve opens and the mixture returns to its rich condition.

Question 19

What is the function of the economizer system in an aircraft engine carburetor?

Answer 19

The economizer (or power enrichment) system allows the engine to operate with an economically lean mixture for all conditions other than full power.
When the throttle is opened for full power, the power enrichment system automatically enriches the mixture to remove some of the heat that is developed during this type of operation.

Question 20

In what position is the mixture control placed for starting an engine that is equipped with a pressure carburetor?

Answer 20

In the IDLE CUTOFF position. The engine is started with fuel from the primer system.

Question 21

In what position is the carburetor heat control placed when starting an aircraft engine?

Answer 21

In the COLD position.

Question 22

Why is an aircraft reciprocating engine killed by placing the mixture control in the Cutoff position, rather than by using the magneto switch?

Answer 22

By shutting off the fuel to the cylinders, the combustion chambers are left full of air with no fuel. This makes the engine less likely to fire if the propeller is pulled through with the ignition switch accidentally on.

Question 23

Where is the fuel from a continuous-flow fuel injection system discharged?

Answer 23

It is discharged through injector nozzles screwed into the cylinder heads near the intake valves.

Question 24

In which direction relative to the wind should a turbine-powered aircraft be positioned when the fuel control is being trimmed?

Answer 24

If the wind velocity is less than 10 miles per hour, it can be faced in any direction. If the wind velocity is between 10 and 25 MPH, it should be trimmed facing into the wind. The engine should not be trimmed when the wind velocity is more than 25 MPH.

Question 25

What engine parameters are sensed by the fuel control unit of a turbine engine?

Answer 25

Engine RPM, inlet air pressure, compressor discharge pressure, burner can pressure, and inlet air temperature.

Question 26

When does the acceleration system operate on an aircraft carburetor?

Answer 26

When the throttle is suddenly opened.

Question 27

What is the purpose of the bypass valve in an engine-driven fuel pump?

Answer 27

The bypass valve allows fuel to flow around the engine-driven pump for starting and for emergency operation if the engine-driven pump should fail.

Question 28

On which side of the firewall is the engine fuel shutoff valve located?

Answer 28

The shutoff valve must be on the side of the firewall away from the engine.

Question 29

What are three purposes for the boost pumps in an aircraft fuel system?

Answer 29

To provide fuel pressure for starting the engine, to pressurize the fuel lines to prevent vapor lock, and to transfer fuel from one tank to another.

Question 30

What kind of boost pump is used in most aircraft fuel systems?

Answer 30

Electrically operated centrifugal pumps.

Question 31

Where does an engine-driven fuel pump direct the excess fuel from its pressure relief valve?

Answer 31

Back to the inlet side of the pump.

Question 32

What causes a vapor lock in an aircraft fuel system?

Answer 32

The fuel becomes hot enough that it boils. Vapors are released from the liquid fuel, and these vapors block the fuel lines so the liquid fuel cannot flow to the engine.

Question 33

What is done in most aircraft fuel systems to prevent vapor lock?

Answer 33

Boost pumps in the fuel tank pressurize the fuel in the lines and force the fuel into the fuel metering system.

Question 34

What is meant by a compensated relief valve in an engine-driven fuel pump?

Answer 34

A pressure relief valve that is acted upon by a diaphragm as well as a spring. Atmospheric pressure acting on the diaphragm varies the pump discharge pressure so that it will remain a given amount higher than the pressure of the air entering the carburetor.

Question 35

What is the purpose of the pressurizing and dump valve in the fuel system for a turbine engine?

Answer 35

For normal engine operation, the pressurizing and dump valve acts as a flow divider, directing the fuel into the main or pilot manifold so it will be discharged from the proper orifice in the duplex fuel nozzle.
When the engine is shut down, the dump function of the valve dumps all of the fuel from the manifold.

Question 36

What should be done to an aircraft reciprocating engine if it has been operated on turbine engine fuel?

Answer 36

All of the turbine fuel should be drained out and the system filled with the proper grade of aviation gasoline. The engine should be given a compression check, and all of the cylinders should be inspected with a borescope. The oil should be drained and the filters carefully examined. The engine should be given a complete run-up check.

Question 37

What are the two basic types of turbine engine fuel?

Answer 37

Jet A and A-1, which are a special type of kerosine-base fuel. This is similar to the military JP-5.
Jet B is a gasoline-based fuel similar to military JP-4.
38. Why do some turbine engine fuel systems incorporate a fuel heater?
The fuel heater keeps the fuel warm enough that any water that precipitates out of the fuel will not freeze on the filters.

Question 38

What are two reasons Prist is used in turbine engine fuel?

Answer 38

Prist is an antifreeze agent that lowers the freezing point of any water that precipitates out of the fuel.
It also acts as a biocidal agent that kills the microbial growth that forms scum in fuel tanks. This scum traps and holds water against the aluminum alloy in the fuel tanks and causes corrosion.

Question 39

Why do some aircraft fuel filters have a built-in relief valve?

Answer 39

This relief valve will open and allow unfiltered fuel to flow to the fuel control device if the filter should become plugged with ice or other contaminants.