Science Questions

How does a jet engine get started?

Sat, 20th Aug 2011

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Question

Titus Theseira, via Facebook asked:

How does a jet engine get started?

Answer

Dave -   A gas turbine jet engine is quite interesting in that it just doesnít work unless itís spinning - like a normal car engine.  So basically, you've got to spin it up fairly fast, so you attach an electric motor into it - either itís integrated in there or you stick on the outside and spin it up.  Then it gets going pretty fast, you inject some fuel, then you essentially have something like a spark plug inside it which ignites the fuel, then it starts it going and it basically then starts running, spins up to full speed, and you have a jet engine.

Chris -   So itís not dissimilar to a diesel engine in a sense that you've got to get the engine spun up to start compressing air to get it hot so you can inject fuel to start it burning, and then once itís going, itís got its own ability to carry itself.

Dave -   Yeah, pretty much! Also, there's nothing really to make the air explosion coming out of the back if the air is not coming in the front so you've got to have everything spinning for it to work at all.

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or, you can simply plug it into a convenient leaf blower

http://www.youtube.com/watch?v=qHfoRwIKaXc

(DO NOT TRY THIS AT HOME!) Geezer, Wed, 24th Aug 2011

I watched that video but you can't actually see anything! chris, Thu, 25th Aug 2011



If I'd been holding the camera, I don't think I would have wanted to get any closer either! (It's the thing to the left connected by a large diameter flexible plastic hose.)

You could also use a hair drier to preheat it, then use compressed air to spin it up, as demonstrated in this fine piece of cinematography:

http://www.youtube.com/watch?v=3kfjpc75cTg

(Note the highly sophisticated, and critically important, lubrication system.) Geezer, Thu, 25th Aug 2011

That's not a hair drier, it's a hot air gun. There's a difference. Bored chemist, Fri, 26th Aug 2011



I realized that, but I thought hair drier was a bit more in keeping with the overall arrangement. Geezer, Fri, 26th Aug 2011

And you can jump start them.......... Not recommended though, as you just point the exhaust of another running one into the inlet of the non runner to spin it up enough to start. SeanB, Fri, 26th Aug 2011



I suppose if you run them downhill very fast they might start too, but I've never actually tried that, and I hope I'm never in a plane where they have to try it either 
Geezer, Fri, 26th Aug 2011

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