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aviation maintenance powerplant chapter 7
oral questions engine fuel and fuel metering
37
Aviation
Undergraduate 2
02/16/2014

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Term
What are some sources used to heat the fuel in a turbine engine fuel system
Definition
Bleed air or
Lubricating oil
Term
What are the most common types of fuel metering systems used on small reciprocating engines
Definition
Float type carburetor
Pressure injection carburetor or
Direct fuel injection system
Term
What function does the mixture control perform
Definition
Controls fuel air mixture
Term
What generally causes spark plug fouling
Definition
Operating engine with an excessively rich mixture at idle
Term
What is the purpose of an accelerating system
Definition
Provides a immediate but brief increase in fuel flow to the venturi to enrich the mixture
Term
What is a carburetor economizer system
Definition
A power enrichment system that provides a richer mixture at high power settings
Term
When the mixture is placed in the idle cutoff position, a slight rise in r.p.m. is noted prior to the engine decelerating, what does this indicate?
Definition
The idle mixture is set correctly
Term
Why should engine r.p.m. be accelerated periodically when making carburetor adjustments?
Definition
To clear the engine
Term
What maintains fuel pressure in a pressure-injection carburetor
Definition
Engine driven fuel pump
Term
What happens if the manual mixture control of a pressure injection carburetor is moved to the idle cutoff position
Definition
The engine stops
Term
How does the automatic mixture control function
Definition
A sealed brass bellows, connected to the fuel metering system, expands and contracts
w/changes in pressure and temperature, adjusting the mixture accordingly
Term
What are some advantages of fuel injection over carburetor systems
Definition
Less induction icing
Better acceleration
Better fuel distribution
Better fuel economy
Reduced over heating of individual cylinders
Term
What purpose do turbine engine fuel control units serve
Definition
They automatically meter fuel to the engine
Term
What are 2 basic types of turbine engine fuel control units
Definition
Hydromechanical
Electronic
Term
What are the engine variables detected by the fuel control unit
Definition
Power lever position
Engine r.p.m.
Compressor inlet temp
Compressor inlet pressure
Compressor discharge pressure
Burner pressure
Term
what are the 2 major components of a supervisory electronics engine control
Definition
electronic control unit,
hydro-mechanical fuel control
Term
what are some of the advantages of a full-authority digital engine control over a hydro-mechanical fuel control
Definition
-better fuel economy
-improved starts
-no engine trimming
-provides engine limit protection
-provides constant idle speeds
-fully modulates the active clearance controls
-allows more repeatable engine transients
Term
what do turbine engine fuel spray nozzles do
Definition
they inject fuel into the combustion area
Term
what are the different engine indications used for trimming a turbine engine
Definition
EPR or r.p.m is used depending on the particular engine
Term
what maintenance adjustments are normally allowed on an installed hydro-mechanical fuel control
Definition
-specific gravity for fuel
-idle r.p.m.
-max. r.p.m.
Term
what type of pump can vary the amount of fuel discharged regardless of speed
Definition
variable displacement pump
Term
what is used to keep water, sediment, and foreign matter out of the carburetor
Definition
main fuel strainer
Term
what is the purpose of a pressure relief valve in a constant displacement pump
Definition
returns excess fuel that is not required by the engine to the inlet side of the pump
Term
what type of engine-driven fuel pump is widely used other than gear or piston pumps
Definition
rotary vane type
Term
what function do fuel boost pumps perform
Definition
supply pressurized fuel to the fuel pump, which helps prevent vapor lock and cavitation
Term
what is a commonly used type of fuel boost pump
Definition
centrifugal type
Term
what causes vapor lock and why is it of concern
Definition
caused by insufficient fuel pressure, high fuel temp., and excessive fuel turbulence, which may block any fuel resulting in engine failure
Term
what function does the engine-driven pump on a turbine engine perform
Definition
provides a continuous supply of fuel at the proper pressure while the engine is running
Term
name the 2 categories of turbine engine pumps
Definition
constant and variable displacement
Term
what category is a gear-type pump classified in
Definition
constant displacement
Term
when performing an external inspection of an engine-driven fuel pump, what should you look for
Definition
leaks and security of mounting
Term
what is the purpose of a shear section of the drive shaft in a dual element constant displacement pump
Definition
if one element seizes, a portion of the drive shaft seizes, allowing the other element to continue to operate
Term
where in a turbine engine fuel system is ice formation likely to occur and how is it prevented
Definition
fuel filter- a heat exchanger using oil or bleed air to warm fuel
Term
what is the purpose of a bypass valve in an engine fuel system micron filter
Definition
it is a safety feature that allows fuel to flow to the engine if the filter becomes blocked
Term
what is the purpose of a duplex fuel nozzle
Definition
discharges 2 different spray patterns, a wider one during start up and a narrow one when engine speed increases above idle
Term
what are the functions of the pressurizing and dump portions of a pressurizing dump valve
Definition
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
Term
what purpose do fuel selector valves serve
Definition
allow tank and engine selection, and provide a means for shutting off fuel flow
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