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How Do Jet Engines Start Up?  
User currently offlineSJCguy From United States of America, joined Apr 2001, 579 posts, RR: 1
Posted (14 years 2 months 1 week 3 days 13 hours ago) and read 39384 times:

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

6 replies: All unread, jump to last
User currently offlineKFRG From , joined Dec 1969, posts, RR:
Reply 1, posted (14 years 2 months 1 week 3 days 13 hours ago) and read 39353 times:

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)


User currently offlineRamper@IAH From United States of America, joined Dec 2001, 240 posts, RR: 1
Reply 2, posted (14 years 2 months 1 week 3 days 12 hours ago) and read 39323 times:

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!

User currently offlineJsuen From United States of America, joined Aug 2001, 211 posts, RR: 0
Reply 3, posted (14 years 2 months 1 week 3 days 12 hours ago) and read 39302 times:

Actually, the compressor spool speed is known as N2. Also, to expand, the ignitors are similar to the spark plugs in your car.

User currently offlineJsf119 From United States of America, joined Jul 2001, 196 posts, RR: 0
Reply 4, posted (14 years 2 months 1 week 3 days 11 hours ago) and read 39280 times:

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

User currently offlineYeeha From Australia, joined Apr 2001, 70 posts, RR: 0
Reply 5, posted (14 years 2 months 1 week 3 days 10 hours ago) and read 39254 times:

try this site...http://www.howstuffworks.com/turbine.htm

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (14 years 2 months 1 week 3 days 4 hours ago) and read 39209 times:

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.


I thought I was doing good trying to avoid those airport hotels... and look at me now.
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