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How Do Jet Engines Start Up?

Mon Mar 25, 2002 11:37 am

While pushing out aircraft and watching them, I see that the captain "spins" the engine, then after it gets going fast enough, it ignites and makes a loud pop, then starts full on. How does this ignition of the engine work? Does the captain ignite the engine or does he turn it on and it ignites itself? What is the line of events going on in the engine as it starts?

SJCguy  Smile
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RE: How Do Jet Engines Start Up?

Mon Mar 25, 2002 11:53 am

I believe the APU supplies power to the engines to startup. Well, a jet engine is ignited through the ignition of jet fuel. The engines stabilizes at approximately 20-25% N1 (I think that's right)

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RE: How Do Jet Engines Start Up?

Mon Mar 25, 2002 12:52 pm

It all depends on the engine. Some engines are more automated than others. Here is a typical start sequence for a "classic" jet engine: 1) a start valve is opened to provide bleed air (from an APU, ground power source, or another engine that is running) to an air turbine starter. The air turbine starter is connected to the compressor spool, which turns and compresses the air. The speed of this spool is known as N1. When the N1 reaches a certain speed, ignitors are turned on (usually automatically) and fuel is introduced by the pilot (sometimes automatically). Combustion takes place shortly after this sequence. After combustion, the engine becomes "self sustaining," meaning that the power turbine section can turn the compressor section without the air turbine starter. The engine start sequence is complete when the engine indications are stabilized and the generators are brought on-line. In aircraft such as the ERJ-145, the entire start sequence is automated - it only requires you to turn the start knob...thats it!
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RE: How Do Jet Engines Start Up?

Mon Mar 25, 2002 1:03 pm

Actually, the compressor spool speed is known as N2. Also, to expand, the ignitors are similar to the spark plugs in your car.
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RE: How Do Jet Engines Start Up?

Mon Mar 25, 2002 1:37 pm

the compressor is also know as N1 too. N1 refers to the fan and the low pressure compressor which is attached to the low pressure turbine by a shaft. N2 refers to the high pressure turbine and high pressure compressor
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RE: How Do Jet Engines Start Up?

Mon Mar 25, 2002 3:01 pm

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RE: How Do Jet Engines Start Up?

Mon Mar 25, 2002 8:39 pm

N1, N2, Ng - different names, same thing. It's the gas generator RPM.

Large turbines almost exclusively use an air starter, as described. Smaller turbines can use electrical starters. Sometimes, this is achieved by running the generator 'in reverse'. A less common method are hydraulic starters, where a hydraulically driven engine does the initial cranking. Again, often this is a hydraulic pump going 'in reverse'.

On military engines which have to be startable without depending on external air sources etc, you might see cartridge starters or iso-propyl-nitrate starters, where the combustion gases from IPN in a chamber or a cartridge are sent through a turbine to crank the engine.

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