6 ms·

> In the second module an air compressor takes air in and puts it under high pressure, which shrinks the volume of the air and allows the engine to be smaller (

> In the second module an air compressor takes air in and puts it under high pressure, which shrinks the volume of the air and allows the engine to be smaller (because compressed air takes up less space). Does this reporter know what they're talking about? I always thought compressors were an essential part of a jet engine (compression heats the air, then fuel is sprayed in which auto-ignites, expanding, and turning the turbine blades). Not just because compressed air takes up less space.

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WalterBright 10y ago
The compressor does not heat the air to the ignition point. The flame that is already in the engine provides the ignition, and the flame is prevented from blowing out the back by "flame holders". https://en.wikipedia.org/wiki/Flame_holder https://en.wikipedia.org/wiki/Flame_holder The ignition at engine start is done with spark plugs. The purpose of the compressor is to provide dense enough air to support combustion, and the pressure is needed to prevent the exhaust gases from blowing out the front of the engine.
- AceyMan 10y ago
  Of tangential relevance: On the aircraft I was trained on, it was SOP to set the igniters = ON when encountering some significant amount of precipitation in flight, as a defense against a water induced snuff-out. (I forget exactly how the precip threshold was determined.) Otherwise, as parent said, the flame persists following engine start, and so the "spark plugs" (igniters) are not kept active.
- mschuster91 10y ago
  > and the pressure is needed to prevent the exhaust gases from blowing out the front of the engine Out of curiosity: has that ever happened and how does such an event look like?
  - M_Grey 10y ago
    I'm pretty sure that you'd need a pretty catastrophic failure of the engine for that to happen, and the event might start to strongly resemble an explosion.
  - jabl 10y ago
    See "compressor stall", https://en.wikipedia.org/wiki/Compressor_stall https://en.wikipedia.org/wiki/Compressor_stall A compressor stall can be severe enough to damage or even destroy the engine, though I don't think there's (usually, at least?) actually exhaust blowing out the front.
    - Scramblejams 10y ago
      I read a book in which the author related the story of an F-8 Crusader which suffered an in-flight failure during refueling, causing flame to shoot forward from the intake. Apparently there was photographic evidence, which came in handy when the manufacturer insisted that was impossible. Believe it was one of D.K. Tooker's books, probably "The Second Luckiest Pilot." Very entertaining read, highly recommended if you like vignettes of danger in the skies.
  - pjc50 10y ago
    Sure: https://www.youtube.com/watch?v=MQWYhsYfMxE https://www.youtube.com/watch?v=MQWYhsYfMxE (rather bad quality, good explanation) There's a supersonic variant called an "unstart" which is very dangerous.
    - mschuster91 10y ago
      Thanks for the video. Amazing that an aircraft engine is attached to the wing in only such a small area and is not sheared off by the power of such an event...
      - WalterBright 10y ago
        The engine is designed to shear off if it starts vibrating past a certain amount. It's better than wrecking the wing. The engine mounts are very carefully designed to not be too strong nor too weak. They're very much like a fuse in an electrical circuit. In fact, they're called "fuse pins". The concept is not without controversy: https://www.washingtonpost.com/archive/politics/1993/06/18/boeing-abandons-747-jumbo-jets-breakaway-engine-design/9ad385d4-64d9-447a-ad5d-dcbee4da3f2c/?utm_term=.cff5ca8849b2 https://www.washingtonpost.com/archive/politics/1993/06/18/b...
- CarVac 10y ago
  The purpose of the compressor is simply to compress the air. It's one step of the Brayton Cycle which is used in gas turbines to convert heat energy into mechanical energy. https://en.wikipedia.org/wiki/Brayton_cycle https://en.wikipedia.org/wiki/Brayton_cycle
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WalterBright 10y ago

The compressor does not heat the air to the ignition point. The flame that is already in the engine provides the ignition, and the flame is prevented from blowing out the back by "flame holders". https://en.wikipedia.org/wiki/Flame_holder https://en.wikipedia.org/wiki/Flame_holder The ignition at engine start is done with spark plugs. The purpose of the compressor is to provide dense enough air to support combustion, and the pressure is needed to prevent the exhaust gases from blowing out the front of the engine.

AceyMan 10y ago
Of tangential relevance: On the aircraft I was trained on, it was SOP to set the igniters = ON when encountering some significant amount of precipitation in flight, as a defense against a water induced snuff-out. (I forget exactly how the precip threshold was determined.) Otherwise, as parent said, the flame persists following engine start, and so the "spark plugs" (igniters) are not kept active.
mschuster91 10y ago
> and the pressure is needed to prevent the exhaust gases from blowing out the front of the engine Out of curiosity: has that ever happened and how does such an event look like?
- M_Grey 10y ago
  I'm pretty sure that you'd need a pretty catastrophic failure of the engine for that to happen, and the event might start to strongly resemble an explosion.
- jabl 10y ago
  See "compressor stall", https://en.wikipedia.org/wiki/Compressor_stall https://en.wikipedia.org/wiki/Compressor_stall A compressor stall can be severe enough to damage or even destroy the engine, though I don't think there's (usually, at least?) actually exhaust blowing out the front.
  - Scramblejams 10y ago
    I read a book in which the author related the story of an F-8 Crusader which suffered an in-flight failure during refueling, causing flame to shoot forward from the intake. Apparently there was photographic evidence, which came in handy when the manufacturer insisted that was impossible. Believe it was one of D.K. Tooker's books, probably "The Second Luckiest Pilot." Very entertaining read, highly recommended if you like vignettes of danger in the skies.
- pjc50 10y ago
  Sure: https://www.youtube.com/watch?v=MQWYhsYfMxE https://www.youtube.com/watch?v=MQWYhsYfMxE (rather bad quality, good explanation) There's a supersonic variant called an "unstart" which is very dangerous.
  - mschuster91 10y ago
    Thanks for the video. Amazing that an aircraft engine is attached to the wing in only such a small area and is not sheared off by the power of such an event...
    - WalterBright 10y ago
      The engine is designed to shear off if it starts vibrating past a certain amount. It's better than wrecking the wing. The engine mounts are very carefully designed to not be too strong nor too weak. They're very much like a fuse in an electrical circuit. In fact, they're called "fuse pins". The concept is not without controversy: https://www.washingtonpost.com/archive/politics/1993/06/18/boeing-abandons-747-jumbo-jets-breakaway-engine-design/9ad385d4-64d9-447a-ad5d-dcbee4da3f2c/?utm_term=.cff5ca8849b2 https://www.washingtonpost.com/archive/politics/1993/06/18/b...
CarVac 10y ago
The purpose of the compressor is simply to compress the air. It's one step of the Brayton Cycle which is used in gas turbines to convert heat energy into mechanical energy. https://en.wikipedia.org/wiki/Brayton_cycle https://en.wikipedia.org/wiki/Brayton_cycle

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