View Full Version : Dumby Question - How do jet engines start?

2nd Sep 2002, 22:57
Hi there :) ,

This is a bit of dumb question but ive always wondered how jets start. Obviously its not like a car with big starter motor in it to whizz the fans round lol.

Does something blast compressed air in it to ignite and start the whole thing turning.

TIA :)

(the IL-2 Sturmovik ace!)

2nd Sep 2002, 23:11
Erm....many turbine engines DO use a starter motor. Others use compressed air supplied from some other source eg another engine.

Once the starter has the engine turning fast enough, fuel is introduced & ignited. The burning fuel helps to accelerate the engine until it can eventually sustain itself. After this RPM is reached the starter motor can be switched off.

3rd Sep 2002, 00:20
Just some more info -

On 737-400 (for gnd running purposes atleast) we have pnuematic starter, this can be spun over by air from gnd cart, APU, or air from already running engine, all selected from pnuematic panel o/head. When start selected air directed into start duct and eng will start to motor over (n2) once predetermined n2 speed reached the fuel/igniton is introduced and light up should happen almost instantaneously, at this point EGT becomes the gauge to watch along with n2, oil press, and fuel flow (alot to take in very quickly - and act on if not right!!) at approx 46% the starter should automatically cut out and the engine continue to run up to idle parameters, a quick throttle check is then carried out to establish control of the engine.

This is just a brief idea of what happens and I HAVE LEFT LOTS OUT, i.e talking to the guy on the head set outside, pre-start FOD checks, fuel levels for runs, ATC permission, temp and px info, what to do when it goes wrong and a whole host of other thing equally important - but I had to mention this incase I get flamed by some one for not having done this, that or the other!!!

Hey, its as close as I get to being a "Jet-Jockey" and most definetly the best perk of my job - unfortunately I dont get to do it often enough!!

wryly smiling
3rd Sep 2002, 00:44
lots of different ways
pneumatic driven starter motor
electrical starter motor
compressed air
even a explosive cartridge to drive a starter motor

3rd Sep 2002, 03:42
The three S


Hopefully in that order or there may be big problems.

To start one, you need to pressurize the burner. before you energize an ignitor (spark type plug) and introduce the fuel so it gets atomized into a mist fine enough to burn *in the burner*

In order to pressurize the burner you need to spin up the compressor rotor that feeds the burner. This requires either an air driven motor of some kind or lots of ram air speed coming in the front (air start)

So far you've got lots of good answers hollar if you want more.

3rd Sep 2002, 08:07
Generally speaking any modern jet engine is started by mechanically driving the inner spool of the engine (there may be up to three on RR-Engines). This is done via a bevel gear set, a shaft leading to the outer side of the engine and a gearbox attached to it. It depends what drives the gearbox, it might be an electrical starter (mostly found on small business jets), a combined starter and generator (rare), an hydraulic starter (also rare) or a pneumatic starter (widely used).
These starters use really high rpm, so the effectivity of electric starters is low (high speeds on the brushes, high inductive losses), same is for hydraulic starters (high flow velocity of the hydraulic fluid) pneumatic starters take much more rpm without getting uneconomically.
If inner spool rpm is high enough (normally in the order of 30%) the combustion chamber pressure is high enough to make it work (vaporizing and burning the fuel), the outer spool(s) is(are) driven by airflow via the according turbine.

I once cranked a small gas turbine (military water pump) by hand ! Very interesting thing, you can feel the torque needed to drive the engine, feel it increase with rpm (more air blown through), feel it decreasing if fuel and ignition is switched on, feel it further decreasing the faster you drive it until it runs on its own at roundabout 40000 rpm. (you have to be FAST, because turbine temperature rapidly rises if you ignite it, and stabilizes only above 50000 rpm !)
Very interesting experience !

3rd Sep 2002, 10:26
cheers, thats cleared things up! :)

(The IL-2 Sturmovik Ace!)

4th Sep 2002, 04:12
Volume, starter/generators are not that rare. Most if not all turbine helicopters use them. Ok, Ok so it's not a big jet:D

Check 6
4th Sep 2002, 15:30
Jet engines are like Trolley Dollies, they automatically starting "whining" when you leave the gate.

Sorry, couldn't resist.

:D :D :D

5th Sep 2002, 10:47
whats a trolly dolly?

Check 6
5th Sep 2002, 11:20
Cabin dragon, flight attendant.

:D :D :D

5th Sep 2002, 11:23
In Chuck Yeager's auto-biography her talks about parking one aircraft with its engine running in front of the one to be started and using the efflux to turn the engine of the rear a/c. Can't remember the type but it was something of the F86/F100 vintage.

Check 6
5th Sep 2002, 11:41
I have a pilot friend who flew as a First Officer on Martin 404's based in Floriday during the late 1970's.

One of their routes was to an airport in Cuba. He flew frequently with a very salty Captain known as ROCKETMAN .

On one departure from Cuba, they loaded the pax, but the left starter failed.

They unloaded the pax, started the right engine and departed. After airborne they "airstarted" the left engine and returned to the airport.

The pax were then loaded and they returned to Florida.

:D :D :D

5th Sep 2002, 14:04
>In Chuck Yeager's auto-biography her talks about parking one aircraft with its engine running in front of the one to be started and using the efflux to turn the engine of the rear a/c. Can't remember the type but it was something of the F86/F100 vintage.

Here I thought this thread was now dead with all those good answers above. But isn't the F86 a centrifugal compressor powered jet? If so, how do you air start one of these using ram air in the front?

Somehow I can't see it pressuirizing those burner cans enough to hold the flame in front of the turbine wheel.

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