Engine Fuel and Fuel Metering Systems Flashcards
Gasoline and other liquid fuels do not burn at all unless they are mixed with what other element?
Air
Where are the fuel spray nozzles located in a turbine engine?
Externally or internally on the combustion chamber
Name the heat sources for the operation of fuel heaters used in turbine engines.
Heat Exchangers, Bleed Air, Engine Lubrication Oil
What engine variables are sensed by a hydro mechanical fuel control?
Throttle, RPM, EGT
In an electrical fuel control system, what are the two input components?
Electrical and Fuel
What are the two most commonly used carburetors in small reciprocating engines?
Float and Pressure type
What type of fuel control unit is used in a turbine engine?
Hydromechanical
What is meant by trimming a turbine engine?
Adjusting the fuel control for the proper specific gravity of the fuel and for the proper idle and full throttle RPM
When should a turbine engine be trimmed?
Loss in engine power, or the manufacturer specifies
What specifications are used to trim a turbine engine?
Data plate on the engine
What are the three types of electronic fuel controls for turbine engines?
Supervisory EEC, FADEC and Mechanical
What is a FADEC?
A high precision digital electronic fuel control that utilizes an EEC and the FMC to prevent over-speed and over-temp conditions
Does a turbine engine controlled by a FADEC require manual trimming?
No
What is the purpose of a mixture control in a float type carburetor?
To control the fuel-air mixture as air density changes with altitude changes.
What is the purpose of an accelerating system on a float type carburetor?
Temporarily enrich the mixture in the venturi
Describe the purpose and operation of a venturi.
Proportions the fuel-air mixture, lowers the pressure at the discharge nozzle, and limits airflow at full throttle. It measures differential pressure
What is the function of a metering jet?
Limit the fuel flow when the throttle valve is wide open
What is the fuel metering force in a float type carburetor?
Differential pressure between the pressure in the float chamber and at the nozzle
What will be the result of using excessively rich idle mixtures?
Carbon fouling of the spark plug
What is the purpose of the economizer system in a float carburetor?
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%
Where does a pressure injection carburetor obtain fuel pressure?
Boost pump for starting and engine driven fuel pump for normal operations
What operates an automatic mixture control (AMC) on a pressure carburetor?
The movement of the bellow operates a tapered needle that controls the impact air pressure into the “A” chamber of the carburetor
In what position do you place the mixture control of a pressure injection carburetor to stop the engine?
Idle cutoff position
What RPM indication should result when the mixture control is placed in the idle cutoff position when the idle mixture is set correctly?
There should be a slight increase in RPM, followed by a rapid drop of RPM to zero
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 prior to installation?
To soften the diaphragms and make them as pliable as they were when the carburetor was originally calibrated
What are three types of carburetor icing?
Fuel Evaporation, Throttle, Impact
What is the typical fuel pressure in a float type carburetor?
3 to 5 PSI
Name some advantages of direct fuel injection systems over conventional carburetor systems.
Improved acceleration, reduced overheating of cylinders, better fuel economy, improved fuel distribution, less danger of induction system icing
What is the purpose of filters in a fuel system?
To prevent foreign matter from entering the carburetor
Why do carburetors in aircraft have a mixture control?
The mixture control provides a means to adjust for the correct mixture at different altitudes
What two items are adjusted when setting idling conditions on a float carburetor?
Throttle stop for idling RPM and the idle mixture through the idle needle valve
What is the purpose of fuel selector valves?
Shutting off fuel flow, cross-feed for tank and engine selection and for fuel transfer
What should be looked for when inspecting an engine driven fuel pump?
Fuel leaks and security of mounting
What is the purpose of an engine driven fuel pump?
Always delivers a continuous supply of fuel at the proper pressure during engine operations
What type of fuel pump is generally used with large reciprocating engines?
Positive displacement rotary-vane type pump
What happens to excess fuel not required by the engine in a constant displacement pump?
Pressure relief valve opens and the fuel is routed back to the inlet side of the pump
What is the most common type of fuel boost pump?
Electrically driven centrifugal type pump
What are the functions of the boost pump?
Supplies fuel to the engine pump for starting, maintains fuel line pressure to prevent the fuel from boiling at altitude and allows fuel to bypass the pump in case of a pump failure
What is the purpose of the bypass valve in the engine driven fuel pump?
Provides a path around the pump vanes for starting the engine and to allow fuel to bypass the pump in case of a pump failure
What is the purpose of using boost pumps in a fuel system?
Supply fuel to a pressure carburetor during engine starting and in case of engine driven pump failure. The main function of a boost pump is to keep the pressure on the suction side of the engine driven pump from becoming low enough to permit the fuel to boil when operating at high altitudes
What are the three general causes of vapor lock?
Low fuel pressure, high fuel temperatures, and excessive fuel turbulence
Turbine engine fuel pumps may be divided into what two distinct system categories?
Constant and variable displacement
What category is a turbine engine driven gear type pump?
Constant displacement
What type of fuel pump can be made to vary the fuel flow at any speed?
Variable displacement
What part of a gas turbine engine fuel system is very susceptible to the formation of ice?
Fuel filters
How is the engine fuel system protected from ice formation?
Using fuel heaters
Why is a micron fuel filter provided with a bypass valve as a necessary safety factor?
The small openings in this type of filter make it very susceptible to clogging
Where are fuel filters located?
At the fuel pump in line with the system
Can you use automotive gasoline in a reciprocating engine in place of Avgas?
Yes, if you have a STC
What is the purpose of the pressurizing portion of a pressurizing and dump valve?
Provide primary and secondary fuel flow to dual line duplex fuel nozzles
What do the divided fuel flows of a turbine engine duplex fuel nozzle produce in the fuel?
Two spray patterns
What is stoichiometric ratio?
15 to 1
What are the three components of an EEC?
Hydromechanical Fuel Control
Electronic Fuel Control Unit
Aircraft Mounted Power Lever Angle Potentiometer
What meters fuel in a FADEC system?
EEC.
How long must you wait to adjust idling controls on a float type carburetor?
5 minutes.
