M15.11 Fuel systems Flashcards
What are the two fuel subsystems?
Distribution and control
The high pressure fuel system down stream of the HP fuel pump features a …?
Pressure relief valve to protect the high pressure components
Explain the oil cooler mounted downstream of the HP pump
This arrangement has the advantage that it requires less external fuel lines, because the low pressure pump and the high pressure pump are usually in one housing, but the danger of fuel leaks in the oil cooler is much higher than in a low pressure system
What is the pressure on the output side of the LP pump?
175 PSI
What is the purpose of boosting fuel pressure to 175 PSI
To prevent the fuel system from drawing fuel from the tank, it also prevents cavitation at the inlet of the HP pump
What is the HP pump outlet at maximum engine speed?
900 PSI, With an overpressure relief valve set to trigger at 1250 PSI, releasing fuel back to the inlet port of the HP pump
Explain how the pump provides flow?
The pump supplies more fuel than is required for combustion, only in the metered section does the fuel flow to the nozzles, any unused fuel returns to the fuel pump via the bypass return line
What is the typical boost pump pressure?
40 PSI
Explain bypass fuel IDG coolers
On some engines this bypass fuel is used to cool the IDG oil cooler, as shown in this example. When the bypass fuel passes through the IDG oil cooler, it takes the heat from the IDG oil.
Explain what happens with fuel under low power conditions
In low engine power conditions the hot bypass return fuel heats up the fuel from the low pressure pump too much, so that the engine oil can not be cooled sufficiently. In this situation the bypass return fuel is fed back to the fuel tank by the fuel recirculation system. Some cold fuel from the low pressure stage is added to the hot bypass fuel so that the fuel that enters the tank is not too hot.
How are fuel pumps usually assembled?
Engine fuel pumps are usually made up of two stages: the low pressure stage and the high pressure stage. The two stages are usually combined in a common housing so that they need only one drive shaft. The low pressure fuel pumps are usually impeller type pumps driven by the accessory gearbox. Impeller type pumps look similar to radial compressors. They have an impeller wheel in the pump housing with an axial inlet port and radial outlet ports. These pumps can supply a continuous flow of fluid, but they cannot create very high pressures.
What type of fuel pumps are used?
High pressure fuel pumps are always positive displacement pumps. Two basic types of pump are used on gas turbine engines:
* The gear type pump
* The piston type pump
Explain piston fuel pumps
The piston type pump has a pump housing with many pistons in a rotor assembly. It usually also has a variable Swash plate (Wobble plate), which changes the stroke of the pistons.
Piston type pumps are used where very high pressures are needed. They can create pressures of more than 2000 psi.
In the piston type pump the output depends on the engine speed and the stroke of the pistons. The stroke of the pistons can be controlled via the Swash plate (Wobble plate) by a servo signal from the fuel control unit. Therefore this pump is also used to meter the fuel for the combustion chamber.
Explain the gear type pump
Gear type fuel pumps are most commonly used on turbofan engines. They have two counter-rotating gears in a pump housing. The teeth of the gears carry the fluid from the pump inlet to the outlet. You have seen that the gear type pumps and piston type pumps are used as high pressure pumps in engine fuel systems. The next segment describes the build-up and operation of fuel filters and heat-exchangers
Explain a filter with two outlets?
On some engines you can find fuel filters with two outlet ports:
* The main outlet port to the fuel control unit
* A servo outlet port to the servo fuel heater
What is the servo fuel filter like?
The filter screen for the servo fuel flow is usually finer than the normal filter element. It is often called a wash screen. This is because the normal discharge flow washes away all particles which are caught by the screen when the servo fuel passes through it.
What are the main types of heat exchangers?
The heat exchangers that can be found in an engine fuel system are:
* The engine oil cooler
* The servo fuel heater
* The IDG oil cooler
All these heat exchangers are usually fuel/oil heat exchangers
Where is the servo fuel heater typically located?
Mounted directly onto the engine oil cooler
What are fuel shrouds?
To prevent fuel leaks in the hot environment of the HP compressor and combustion case, the fuel lines are protected by shrouds.
Shrouds are used on fluid lines that are routed through critical areas of the engine to catch fluid leaks and carry them away to the engine drain mast
Explain the ways in which fuel leaks can be prevented?
The highest risk of leaks is at the connections of fluid lines. Therefore, on modern engines only the connections of the fuel lines have special leak protection. There are two methods to protect the connections of fluid lines
against leaks:
* Shrouded connections
* Double sealed connections
Shrouded connections cannot prevent leaks, but they can catch leaks and carry them away by the engine drain system. The O-rings inside the shroud make sure that a leakage from the coupling nut cannot get onto the hot combustion case.
A double sealed connection gives double safety against leakage. A small transfer tube fits between the fuel manifold and the fuel nozzle. This connection is equipped with an inner seal which is a pair of O-rings. A conical seal between the fuel nozzle and the fuel manifold is the outer seal in this double sealed connection.
Explain the fuel manifolds on an older engine
On some older aircraft engines you can find two fuel supply lines and two fuel manifolds instead of one.
This design method was necessary because the combustion chamber was equipped with two types of fuel nozzles.
The primary manifold supplies the primary fuel nozzles and the secondary manifold supplies the secondary fuel nozzles.
On modern aircraft you find this design method on APUs only.
What is the most important factors for good atomization?
Rate of swirl of the fuel and the fuel pressure
What is a primary nozzle?
One set with small discharge orifices is used for engine start up to
approximately idle speed.
These nozzles are usually called the primary fuel nozzles
What is a secondary nozzle?
A second set of fuel nozzles have larger discharge orifices to supply fuel for the acceleration up to maximum power.
These nozzles are called the secondary fuel nozzles.
The primary and secondary fuel nozzles are usually supplied by independent primary and secondary fuel manifolds.
