Re: Engine start process questions

Date:         16 Jun 97 21:35:39 
From:         Iain Stuart <>
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In article <airliners.1997.1226@ohare.Chicago.COM>, Rich Nute
<> writes
>A jet engine is started by:
>1.  Spin the turbine up to some rpm.
>    How many rpm?
On RB211's about 25% of max HP speed - About 3,000 rpm
The other shafts of a multi-shaft engine are obviously slower.
>    How is the spin air injected into the engine?

There's an external gearbox fitted with a air starter motor, and
generators and pumps. Pressurised air is fed in (from other APU, engines
or ground carts) driving the starter, driving the gearbox, a shaft into
the core of the engine, and driving the HP system. During the start, the
starter disengages once the ngine is self-sustaining.   Once running
above idle, the HP system drives the gearbox, providing electrical and
hydraulic power to the aircraft.

>2.  Inject fuel.
>    How is this done?

Thru' fuel injectors feeding fuel directly into the combustion chamber.

>3.  Ignite fuel.
>    How is this done?

Ignitor plugs ( big spark plugs ) but once started, they're not required
and the combustor is self sustaining. (You can switch on continuous
ignition if you have a nervous pilot.  ;-)  )

>In the cockpit, what are the engine start controls,
>and what are the pilots' jobs during engine start?

With modern aircraft systems, press the autostart button, and wander off
for a few mins while the electronics take care of everything.

>Doesn't one pilot keep a hand on the engine fire extinguisher?

Only if he's a nervous wuss ;-)

>Richard Nute

Welcome !

ps.   Rolls-Royce's UK Homepage has a "How A Jet Engine Works" section.
Iain Stuart

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