M15.11 Fuel systems

Question 1

What are the two fuel subsystems?

Answer 1

Distribution and control

Question 2

The high pressure fuel system down stream of the HP fuel pump features a …?

Answer 2

Pressure relief valve to protect the high pressure components

Question 3

Explain the oil cooler mounted downstream of the HP pump

Answer 3

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

Question 4

What is the pressure on the output side of the LP pump?

Answer 4

175 PSI

Question 5

What is the purpose of boosting fuel pressure to 175 PSI

Answer 5

To prevent the fuel system from drawing fuel from the tank, it also prevents cavitation at the inlet of the HP pump

Question 6

What is the HP pump outlet at maximum engine speed?

Answer 6

900 PSI, With an overpressure relief valve set to trigger at 1250 PSI, releasing fuel back to the inlet port of the HP pump

Question 7

Explain how the pump provides flow?

Answer 7

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

Question 8

What is the typical boost pump pressure?

Answer 8

40 PSI

Question 9

Explain bypass fuel IDG coolers

Answer 9

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.

Question 9

Explain what happens with fuel under low power conditions

Answer 9

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.

Question 10

How are fuel pumps usually assembled?

Answer 10

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.

Question 11

What type of fuel pumps are used?

Answer 11

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

Question 12

Explain piston fuel pumps

Answer 12

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.

Question 13

Explain the gear type pump

Answer 13

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

Question 14

Explain a filter with two outlets?

Answer 14

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

Question 15

What is the servo fuel filter like?

Answer 15

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.

Question 16

What are the main types of heat exchangers?

Answer 16

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

Question 17

Where is the servo fuel heater typically located?

Answer 17

Mounted directly onto the engine oil cooler

Question 18

What are fuel shrouds?

Answer 18

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

Question 19

Explain the ways in which fuel leaks can be prevented?

Answer 19

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.

Question 20

Explain the fuel manifolds on an older engine

Answer 20

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.

Question 21

What is the most important factors for good atomization?

Answer 21

Rate of swirl of the fuel and the fuel pressure

Question 22

What is a primary nozzle?

Answer 22

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

Question 23

What is a secondary nozzle?

Answer 23

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.

Question 24

On a duplex fuel nozzle how is secondary fuel flow controlled?

Answer 24

the secondary fuel flow is controlled by a spring loaded pressurizing
valve. This valve which is also called the flow divider valve, is usually an internal component of the fuel nozzles. It opens by the increasing fuel pressure when the engine speed increases, and it closes when the engine speed decreases.
The flow divider valve can also be a single component as on some old aircraft engines and on many APUs. It then supplies the primary and the secondary fuel manifold.

Question 25

What is the purpose of air flowing in a fuel nozzle?

Answer 25

The two types of fuel spray nozzles also pick up some of the airflow that enters the combustion chamber. This airflow passes through slots and holes in the outer shell of the nozzle tip, which cools the material of the fuel nozzle and blows away all residual fuel from the orifices when the engine is shut down. This prevents the formation of carbon at the orifices. Another method to prevent carbon formation is by check valves in the fuel nozzle. The check valve closes when the fuel pressure in the fuel manifold decreases after the HP fuel shut-off valve is closed. This makes sure that no
fuel enters the combustion chamber from the manifold by gravity

Question 26

Explain an air spray fuel nozzle

Answer 26

Another method of fuel atomization is used in the air-spray nozzle. It uses a high velocity of air instead of a high velocity of fuel to cause atomization. This method gives good atomization even at low fuel flows, but it always needs a high airflow through the nozzle.
The air-spray nozzle uses a large inner airflow, which rotates after it passes through the inner swirler. When this airflow meets the fuel at the tip of the nozzle it tears the solid fuel flow into small droplets. This effect is further improved by an outer airflow which passes through an outer swirler. In this nozzle the fuel flow is atomized by the airflow which passes through it. The air-spray nozzle is always a single flow nozzle because it can supply sufficient fuel for all operating conditions of the engine. In the same way as
on the fuel spray nozzle the bypassing airflow cools the nozzle and prevents formation of carbon at the tip of the nozzle

Question 27

What are the two operating conditions of an engine?

Answer 27

Steady state
Transient operation

Question 28

How does the computing section in the metering unit compensate for bleed air demand?

Answer 28

It receives the compressor discharge pressure

Question 29

How does the fuel control unit switch between ground and flight idle?

Answer 29

To switch from minimum idle to approach idle, the fuel control unit receives signals about the aircraft configuration which either comes from the air ground logic or from other suitable configuration switches.

Question 30

How much fuel at idle does the pump supply?

Answer 30

Aprox three times more fuel than needed

Question 31

What parameters determine the rates of deceleration?

Answer 31

These parameters again are:
* The engine speed
* The compressor inlet temperature
* The compressor discharge pressure

Question 32

How is constant speed maintained?

Answer 32

The fuel control unit must keep the demanded speed constant, because changing ambient conditions or load changes of the bleed load, electrical load, or hydraulic load can affect the speed of the engine. The fuel control unit senses the speed change via the speed signal from the accessory gearbox and changes the fuel flow to bring the engine speed back to the demanded speed

Question 33

Explain constant thrust control

Answer 33

Modern engines have an additional power management system, which is either a separate computer, or an integral part of the fuel control unit.
The power management system makes sure that the engine always supplies the same constant thrust for the given thrust lever position.
The power management system uses the thrust lever position as the thrust demand signal. This position is usually called the thrust lever angle (TLA). Each thrust lever angle always represents a fixed thrust value, as shown in this
diagram. The power management system uses the thrust lever angle to calculate which fan rotor speed N1, or on other engines which engine pressure ratio is needed
to get the selected thrust. This calculated thrust is the thrust command signal
that is given to the computing section.

Question 34

Explain constant thrust control using N1

Answer 34

The following example shows how constant thrust is controlled on an engine
that uses the fan speed N1 to set the thrust. With a constant fan speed,
the thrust changes with the ambient conditions. Therefore, the power
management system needs information about the ambient pressure and the
ambient temperature to correct the N1 thrust command.
For a given thrust lever angle the power management calculates the N1
command as shown.
When the ambient conditions change the density of the air changes. The
power management system increases the N1 command if the air density is
low, and it decreases the N1 command if the density is high. The computing
section in the fuel control unit compares the N1 command signal with the
actual N1 speed signal and, if necessary, it sends control signals to the
metering section to get the commanded N1, which is equal to the commanded
thrust.

Question 35

What are the two types of fuel control units?

Answer 35

There are two different types of fuel control units:
* Speed governed fuel control units, which you usually find on older engine
types
* Constant thrust fuel control units used on more modern turbofan engines
Speed governed fuel control units mainly use the N2 demand signal and the
N2 speed feedback signal for steady state fuel metering.
Constant thrust fuel control units use similar signals like the speed governed
fuel control units and they additionally use thrust feedback signals like N1
speed, ambient pressure and temperature signals.

Question 36

Explain how an fuel control unit works

Answer 36

This fuel control unit has many pipes and push-pull cables connected to
it. Some of them are needed to transmit the throttle demand signal, the
temperature signal, the pressure signal and the speed feedback signals for
fuel metering and others are needed to control the engine compressor.
The thrust lever signal is usually a mechanical deflection of a small power
lever at the fuel control unit. This is either done by a rack and pinion
transmission or by rods and levers. The N2 speed feedback signal is usually
transmitted via a mechanical drive shaft from the main fuel pump.
Some fuel control units even get N1 speed feedback signals. This signal is a
fuel pressure signal which comes from a hydromechanical N1 speed sensor.
The temperature signals like CIT or fan inlet temperature come from
hydromechanical temperature sensors. These temperature sensors convert
the air temperatures to fuel pressure signals.
The fuel control unit receives pressure signals like CDP or ambient pressure
via air sense lines. Pressure sensors in the fuel control unit convert the
pressure signals into mechanical signals.
In summary, hydromechanical fuel control units use hydraulic, mechanical,
and pneumatic signals for their operation.

Question 37

What are the three main tasks carried out within a fuel control unit?

Answer 37

Metering
Governing
Limiting