Engine Fuel and Fuel Metering Flashcards
What are some of the sources used to heat the fuel in a turbine engine fuel system?
Bleed air and engine lubricating oil
What are the most common types of fuel metering systems used on small reciprocating engines?
Float type carburetors, pressure injection carburetors and direct fuel injection systems
What function does the mixture control perform?
It controls the fuel air mixture
What generally causes spark plug fouling?
Operating the engine with an excessively rich mixture at idle
What is the purpose of an accelerating system?
It provides an immediate but brief increase in fuel flow in the Venturi to enrich the mixture
What is a carburetor economizer system?
It is a power enrichment system that provides a richer mixture at high power setting where the excess fuel aid in engine cooling. This system functions as throttle setting above cruise power setting.
When the mixture is placed in the idle cut off position, a slight rise in rpm is noted prior to the engine decelerating, what does this indicate?
The idle mixture is set correctly
What maintains the fuel pressure in a pressure injector carburetor?
An engine driven fuel pump
Why should engine rpm be accelerated periodically when making carburetor adjustments?
To clear the engine
What happens if the manual mixture control of a pressure injector carburetor is moved to the idle cut off position?
The engine stops
How does an automatic mixture control function?
A sealed brass bellows connected to the fuel metering system expands and contracts with the changes in pressure and temperature, adjusting the mixture accordingly
What are some of the advantages of fuel injection systems over carbureted systems?
Less danger of induction icing, better acceleration, better fuel distribution, better fuel economy and reduced overheating of individual cylinders
What purpose do turbine engine fuel control units serve?
They automatically meter fuel to the engine
What are the two basic types of turbine engine fuel control units?
Hydro mechanical and electronic
What are the engine variables detected by the fuel control unit?
Power lever position, engine rpm, compressor inlet temperature, compressor inlet pressure, compressor discharge pressure and burner pressure
What are the two major components of a supervisory electronics engine control?
The electronic control unit computer and the hydro mechanical fuel control
What are some of the advantages of a full authority digital engine control over a hydro mechanical fuel control?
Better fuel economy, improved starts, requires no engine trimming, provides engine limit protection, provides constant idle speeds regardless of atmospheric conditions or bleed air requirements, fully modulates the active clearance control and allows more repeatable engine transients
What do turbine engine fuel spray nozzles do?
They inject fuel into the combustion area
What are the different engine indications used for trimming a turbine engine?
Either epr or rpm is used depending on the particular engine
What maintenance adjustments are normally allowed on an installed hydro mechanical fuel control?
Specific gravity for fuel, idle rpm and maximum rpm or maximum epr
What type of pump can vary the amount of fuel discharged regardless of speed?
A variable displacement pump
What is used to keep water, sediment and foreign matter out of the carburetor?
The main fuel strainer
What is the purpose of a pressure relief valve in a constant displacement pump?
It returns excess fuel that is not required by the engine to the inlet side of the pump
What type of engine driven fuel is widely used other than geared or piston pumps?
A rotary vane type
What function do fuel boost pumps perform?
They supply pressurized fuel to the fuel pump, which helps prevent vapor lock and cavitation
What is a commonly used type of fuel boost pump?
The centrifugal type
What causes vapor lock and why is it a concern?
Vapor lock is caused by insufficient fuel pressure, high fuel temperatures and excessive fuel turbulence which may completely block any fuel flow, resulting in engine failure
What function does the engine driven pump on a turbine engine perform?
It provides a continuous supply of fuel at the proper pressure while the engine is running
Name the two categories of turbine engine pumps
Constant and variable displacement
What category is a gear type pump classified in?
Constant displacement
When performing an external inspection of an engine driven fuel pump, what should you look for?
Leaks and security of mounting
What is the purpose of the sheer section of the driveshaft in a dual element constant displacement pump?
If one of the elements seizes, a portion of the driveshaft seizes, allowing the other element to continue the operation
Where in a turbine engine fuel system is ice formation likely to occur and how is it prevented?
The fuel filter is most susceptible to ice formation, so a fuel heater is used consisting of heat exchanger using either engine oil or bleed air to warm the fuel
What is the purpose of a bypass valve in an engine fuel system micron filter?
It is a safety feature that allows fuel to flow to the engine if the filter becomes blocked
What is the purpose of a duplex fuel nozzle?
A duplex fuel nozzle discharges two different spray patterns. The spray pattern is wider during start up and acceleration and narrows when the engine speed increases above vital
What are the functions of the pressurizing and dump portions of a pressurizing and dump valve?
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 shut down
What purpose do fuel selector valves serve?
They allow tank and engine selection and allow a means for shutting off fuel flow
