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Fuel Metering Systems

1. Where does the fuel metering system of a turbine engine discharge its fuel?
The fuel is discharged through spray nozzles into the combustion chambers.
2. What is adjusted when the fuel control unit of a turbine engine is trimmed?
The idle speed and the maximum-thrust speed.
3. What are two locations water may be injected into a turbine engine?
At the compressor inlet and at the inlet to the diffuser section.
4. What is meant by a duplex nozzle in a turbine engine fuel metering system?
A duplex fuel nozzle is one that has two fuel discharge passages. A flow divider sends fuel for low-pressure operation through one discharge passage 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.
5. Does the fuel-air mixture provided by a float carburetor become richer or leaner as the aircraft goes up in altitude?
If the mixture is not adjusted it will become richer as the aircraft gains in altitude.
6. Does the fuel-air mixture provided by a float carburetor become richer or leaner when carburetor heat is applied?
The mixture becomes richer when carburetor heat is applied.
7. What would happen to the fuel-air mixture ratio in a float carburetor if the main air bleed were to become plugged?
The mixture would become excessively rich.
8. How does the automatic mixture control in a pressure carburetor keep the fuel-air mixture constant as the aircraft changes altitude?
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.
9. What are two purposes of the manifold valve (or flow divider) in the fuel injection system of a horizontally opposed aircraft engine?
It provides a constant discharge fuel pressure for idling and it provides a positive shutoff for the fuel when the engine is shut down.
10. How does the mixture control change the fuel-air mixture ratio on the Teledyne-Continental fuel injection system?
When the mixture control is in its Full Rich position 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.
11. Why must the diaphragms of a pressure carburetor be soaked before the carburetor is ready for flight?
Soaking the diaphragms of a pressure carburetor restores them to the condition of flexibility that they had when the carburetor was calibrated.
12. What are two basic types of fuel controls for aircraft turbine engines?
Hydropneumatic and electro-hydromechanical.
13. How does the exhaust gas temperature of a reciprocating engine tell anything about the fuel-air mixture being burned by the engine?
A stoichiometric mixture (a mixture in which all of the constituents of the fuel-air 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 stoichiometric.
14. What two things are adjusted when adjusting the idling of a reciprocating engine?
The throttle stop to get the proper idling RPM-and the idle mixture control to get the smoothest operation.
15. What is controlled by the adjustment of the orifice in the fuel pump of a Teledyne-Continental fuel injection system?
The high unmetered fuel pressure.
16. What is the significance of the letters that are stamped on the flats of a fuel injection nozzle?
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.
17. What is the function of the derichment valve in a pressure carburetor used on an aircraft engine with an antidetonation injection system?
The derichment valve automatically closes to lean the fuel-air 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.
18. What is the function of the economizer system in an aircraft engine carburetor?
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.
19. In what position is the mixture control placed for starting an engine that is equipped with a pressure carburetor?
In the Idle Cutoff position. The engine is started with fuel from the primer system.
20. In what position is the carburetor heat control placed when starting an aircraft engine?
In the Cold position.
21. Why is an aircraft reciprocating engine killed by placing the mixture control in the Cutoff position
rather than by using the magneto switch?,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.
22. Where is the fuel from a continuous-flow fuel injection system discharged?
It is discharged through injector nozzles screwed into the cylinder heads near the intake valves.
23. In which direction relative to the wind should a turbine-powered aircraft be positioned when the fuel control is being trimmed?
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.
24. What engine parameters are sensed by the fuel control unit of a turbine engine?
Engine RPM inlet air pressure compressor discharge pressure burner can pressure and inlet air temperature.
25. When does the acceleration system operate on an aircraft carburetor?
When the throttle is suddenly opened.