M15.15 Power augmentation Flashcards
How can a loss in power be compensated for?
Increasing the engine speed
Increasing the fuel flow
Increasing the air density
What are the issues with increasing fuel flow?
Mechanical limitations such as overspeed.
Older engines may not be able to cope with the higher temperatures.
Methods of power augmentation are?
Water methanol for turbo prop and turbo shaft
Water injection for turbo jets
Afterburner
How does water injection increase thrust?
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
With water injection, how much thrust can be increased?
10 - 30%
What type of water must be used?
De-mineralized water-this prevents impurities building up on the blades
How does the combustion chamber injection work?
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.
What must be done with the water stored on the aircraft?
Must be used in full during takeoff to prevent the water freezing, unless a non freezing fluid is used
What type of compressor typically uses inlet injection systems?
Centrifugal
What is a typical mix of methanol to water?
60% water - 40% methanol
What is the purpose of methanol?
Anti freeze and to use as fuel to compensate for the temp drop
What type of compressor is best suited to axial flow compressor?
Axial
How is water injected into the combustion chamber on axial flow compressors?
Through drillings in the fuel nozzles
How is the flow and activation of the water injection system controlled?
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.
How much of the air in the engine is actually combusted?
25%
What is the temperature of the flame in the combustor?
1700 degrees
How are the fuel burners arranged?
The fuel burners are arranged so that the flame is concentrated around the axis of the jet pipe.
How is the ignited afterburner flame maintained?
A flame holder is fitted that creates some turbulence that slows the airflow to ensure the flame does not blow out.
What temperature is required to ignite the flame?
800 degrees
What are the three methods of afterburner ignition?
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.
What is the construction of the jet pipe of an afterburning engine?
Has additional insulation
Also has a double skin construction that has a cooling layer of air flowing between them
Why must the propelling nozzle expand when afterburner is in use?
Because the additional gas is expanded to prevent damage
What is a thrust decay system with regards to afterburning?
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.
What is screech?
Screech is a loud noise caused by intense fluctuations caused by cyclic vibrations created by the unsteady burning process.
