Chapter 7 Engine fuel and fuel metering

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?

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 settings.

What does a slight rise in r.p.m. prior to engine deceleration indicate when the mixture is in idle-cutoff position?

The idle mixture is set correctly.

Why should engine r.p.m. be accelerated periodically when making carburetor adjustments?

To clear the engine.

What maintains the fuel pressure in a pressure-injection carburetor?

An engine-driven fuel pump.

What happens if the manual mixture control of a pressure injection carburetor is moved to the idle cutoff position?

The engine stops.

How does an automatic mixture control (AMC) function?

A sealed brass bellows expands and contracts with changes in pressure and temperature, adjusting the mixture.

What are some advantages of fuel injection systems over carburetor systems?

Less danger of induction icing, better acceleration, better fuel distribution, better fuel economy, 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?

Hydromechanical and electronic.

What engine variables are detected by the fuel control unit?

Power lever position, engine r.p.m., compressor inlet temperature, compressor inlet pressure, compressor discharge pressure, burner pressure.

What are the two major components of a supervisory electronics engine control?

The electronic control unit (computer) and the hydromechanical fuel control.

What are some advantages of a full-authority digital engine control (FADEC) over a hydromechanical fuel control?

Better fuel economy, improved starts, requires no engine trimming, provides engine limit protection, constant idle speeds, fully modulates active clearance control.

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 r.p.m. is used depending on the particular engine.

What maintenance adjustments are normally allowed on an installed hydromechanical fuel control?

Specific gravity for fuel, idle r.p.m., and maximum r.p.m.

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 pump is widely used other than gear or piston pumps?

A rotary vane type.

What function do fuel boost pumps perform?

They supply pressurized fuel to the fuel pump, preventing 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 of concern?

Vapor lock is caused by insufficient fuel pressure, high fuel temperatures, and excessive fuel turbulence, which may block 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 a shear section of the driveshaft in a dual element constant displacement pump?

If one element seizes, it allows the other element to continue to operate.

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 to warm the fuel.

What is the purpose of a bypass valve in an engine fuel system micron filter?

It 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.

What are the functions of the pressurizing and dump portions of a pressurizing and dump valve?

The pressurizing portion provides fuel flow to dual-line duplex fuel nozzles, and the dump portion allows fuel to drain after engine shutdown.

What purpose do fuel selector valves serve?

They allow tank and engine selection, and provide a means for shutting off fuel flow.