M15.15 Power augmentation

Question 1

How can a loss in power be compensated for?

Answer 1

Increasing the engine speed
Increasing the fuel flow
Increasing the air density

Question 2

What are the issues with increasing fuel flow?

Answer 2

Mechanical limitations such as overspeed.
Older engines may not be able to cope with the higher temperatures.

Question 3

Methods of power augmentation are?

Answer 3

Water methanol for turbo prop and turbo shaft
Water injection for turbo jets
Afterburner

Question 4

How does water injection increase thrust?

Answer 4

The water rapidly vaporizes and thereby cools the air in its vicinity. This increases the air density and also decreases the air temperature into the combustion chamber. The decrease in combustor air inlet temperature has the positive effect that more fuel can be burnt before the turbine inlet temperatures are reached. This increases the engine power and recovers the power loss at low air densities

Question 5

With water injection, how much thrust can be increased?

Answer 5

10 - 30%

Question 6

What type of water must be used?

Answer 6

De-mineralized water-this prevents impurities building up on the blades

Question 7

How does the combustion chamber injection work?

Answer 7

Water is injected into the combustion chamber inlet, this increases the mass flow through turbine relative to that through the compressor, the pressure and temperature drop result in an increased jet pipe pressure.
This results in cooler turbine inlet temperature being lower allowing for more fuel and higher engine speeds.

Question 8

What must be done with the water stored on the aircraft?

Answer 8

Must be used in full during takeoff to prevent the water freezing, unless a non freezing fluid is used

Question 9

What type of compressor typically uses inlet injection systems?

Answer 9

Centrifugal

Question 10

What is a typical mix of methanol to water?

Answer 10

60% water - 40% methanol

Question 11

What is the purpose of methanol?

Answer 11

Anti freeze and to use as fuel to compensate for the temp drop

Question 12

What type of compressor is best suited to axial flow compressor?

Answer 12

Axial

Question 13

How is water injected into the combustion chamber on axial flow compressors?

Answer 13

Through drillings in the fuel nozzles

Question 14

How is the flow and activation of the water injection system controlled?

Answer 14

Either by torque pressure-indicates when power is low and coolant is required, will also determine the quantity.

Ambiant air pressure-to ensure power that power is restored to ISA power not necessarily full sea level power.

Engine RPM-to ensure that it is only available at maximum power.

Question 15

How much of the air in the engine is actually combusted?

Answer 15

25%

Question 16

What is the temperature of the flame in the combustor?

Answer 16

1700 degrees

Question 17

How are the fuel burners arranged?

Answer 17

The fuel burners are arranged so that the flame is concentrated around the axis of the jet pipe.

Question 18

How is the ignited afterburner flame maintained?

Answer 18

A flame holder is fitted that creates some turbulence that slows the airflow to ensure the flame does not blow out.

Question 19

What temperature is required to ignite the flame?

Answer 19

800 degrees

Question 20

What are the three methods of afterburner ignition?

Answer 20

Igniter plug
Catalytic ignition-creates a flame due to the chemical reaction of fuel air mixture being sprayed on heated platinum/rhodium elements.
hot shot ignition-extra fuel is injected into the combustion section that flows past the turbine igniting the afterburner flame.

Question 21

What is the construction of the jet pipe of an afterburning engine?

Answer 21

Has additional insulation
Also has a double skin construction that has a cooling layer of air flowing between them

Question 22

Why must the propelling nozzle expand when afterburner is in use?

Answer 22

Because the additional gas is expanded to prevent damage

Question 23

What is a thrust decay system with regards to afterburning?

Answer 23

On the ground at idle, the nozzle will open up and expand, this kill all the thrust coming from the engine, allowing the engine idle at a higher but more optimal setting.

Question 24

What is screech?

Answer 24

Screech is a loud noise caused by intense fluctuations caused by cyclic vibrations created by the unsteady burning process.

Question 25

How is screech controlled?

Answer 25

Use of a circular heat shield called a screech liner is used, this corrugated liner absorbs the fluctuations and stops large cyclic vibrations of a larger amplitude.

Question 26

How is afterburning controlled?

Answer 26

The lever is moved forward of take-off thrust, this mechanically send a signal to the the fuel pump which determines the fuel requirement. When the fuel in the pipe is ignited the pressure increases (p6) thus changing the pressure ratio across the turbine. this is what is used to adjust the nozzle area until the correct ratio of p3 to p6 is maintained.
A safety feature is installed so that if there is a malfunction, then afterburning is inhibited.