Module 15.13 Start & Ignition

Question 1

How are gas turbine engines started?

Answer 1

By rotating the high-pressure compressor to provide sufficient air to support combustion in the combustion section.

Question 2

Once ignition and fuel have been introduced what must continue running?

Answer 2

The starter

Question 3

Why must the starter keep running after fuel have been introduced?

Answer 3

To assist the engine until the engine reaches a self-sustaining speed.

Question 4

When is the ignition turned on?

Answer 4

As soon as the starter has accelerated the compressor sufficiently to get enough airflow through the engine.

Question 5

What would happen if assistance from the starter was cut off below the self-accelerating speed?

Answer 5

The engine would either fail to accelerate to idle speed or might even decelerate (a hot or hung false start).

Question 6

What is a cartridge starter?

Answer 6

A small impulse-type turbine that is driven by high velocity gases from a burning cartridge.

The power output of the turbine is passed through a reduction gear and an automatic disconnect mechanism to rotate the engine.

Question 7

What type of starter is used on most commercial jet engines?

Answer 7

The air starter.

Question 8

How does an air starter work?

Answer 8

It transmits power through a reduction gear and clutch to the starter output shaft which is connected to the engine.

Question 9

Where does an air starter get air from?

Answer 9

External ground supply, auxiliary power unit (A.P.U.) or cross-feed from a running engine.

Question 10

What psi of air is used at a time in an air starter?

Answer 10

30-50 psi

Question 11

When does the sprag clutch in an air starter engage and disengage?

Answer 11

Engages automatically as soon as the rotor starts to turn, but disengages as soon as the drive coupling (engine) turns more rapidly than the rotor side.

Question 12

How is the engine started if the air starter valve fails to open?

Answer 12

There is usually some form of manual valve override to allow engine start.

Question 13

How many engineers are need to manually one the air starter valve?

Answer 13

Two

Question 14

What are the two types of electric starting systems?

Answer 14

Direct cranking electrical systems

Starter-generator systems

Question 15

What is a e direct cranking electric starter?

Answer 15

A direct current (D.C.) electric motor coupled to the engine.

Question 16

What engines use a direct cranking electric starter?

Answer 16

Small turbine engines, such as Auxiliary Power Units (APUs).

Question 17

What is a starter generator starting system?

Answer 17

A starter system that acts like a direct cranking electrical starter system, except after that, they switch to a generator once the engine has reached a self-sustaining speed.

Question 18

What does the ignitor do?

Answer 18

Ignites the fuel in the combustion chamber.

Question 19

When is the ignitor switched off?

Answer 19

After the fuel has been ignited and there’s a constant flame.

Question 20

When is the continuous ignition mode used?

Answer 20

If there a possibility of engine flame out.

Question 21

What type of ignition system do most turbine engines use?

Answer 21

A high-energy capacitor-type.

Question 22

What are ignition systems cooled by?

Answer 22

Fan air

Question 23

When is cooling of the ignition system important?

Answer 23

When the continuous ignition is used for a period of time.

Question 24

What are the three power sources of the ignition system?

Answer 24

Battery

115 AC

Permanent magnet generator

Question 25

Where is ignitor energy stored?

Answer 25

In capacitors

Question 26

How man capacitors does each ignition discharge circuit incorporate?

Answer 26

Two

Question 27

Where are the capacitors for the ignition discharge circuit located?

Answer 27

In the exciter unit.

Question 28

What is an exciter?

Answer 28

A dual unit that produces sparks at each of the two igniter plugs.

Question 29

How many ignitor plug are there?

Answer 29

Two

Question 30

What is an annular gap igniter plug sometimes referred to?

Answer 30

A long-reach igniter.

Question 31

Why are annular gap igniter referred to as long-reach igniters?

Answer 31

Because they protrude slightly into the combustion chamber liner to produce a more effective spark.

Question 32

Why does a constrained gap ignition plug operate at much cooler temperatures?

Answer 32

It does not project into the combustion chamber liner.