Re: Surge on GE90

From: (TriStar500)
Organization: America Online, Inc. (1-800-827-6364)
Date:         15 Jun 95 14:25:24 
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>I have a question... When an engine such as the GE90
>is starting, what controls which direction the combusted
>material is expelled from the combustion chamber?  I guess
>the fan is electrically spun-up with "starter", but when
>ignition first takes place as in any "explosion", how do
>you control which direction the expanded gases are expelled
>from the engine?  Is there some sort of mecahanism which
>governs the fan & compressor from turning in one direction

Differential air pressure controls which way things are moving. Remember
that the compressor, at the front of the engine and in front of the
combustion chamber, is raising the pressure of the air at the front and is
pushing everything out that back. There are valves in the compression
section that open to 'bleed' air off of compressor during start because
the compressors a little too efficient during start. After engine start
these valves are closed. One way to tell if these valves haven't closed is
if you get a compressor stall while pushing the throttle off of the idle
stop (Off Idle Stall).

The starter on these big engines are pneumatic turbines that use bleed air
from the APU or ground cart to spin the engine. This is why the air
conditioning goes off when engines are started. The APU can't put out
enough air to run the AC and starter at the same time. This is also why
reading lights and movies/audio go off during start, the APU generator is
being unloaded of things that aren't needed for engine start. Since there
is enough air from a 777 APU to start both engines at the same time I
suspect you can start one engine and keep the AC going - a welcome

You'll only see electric starters on smaller jets and turboprop. Those
electric starters are also used as generators.

>Also, are there any books available to the general public
>regaarding how the modern turbo-fan engines work.  I've been
>to the Nat'l Air and Space Museum in DC but there just wasn't
>enough information available other than the generic diagrams
>indicating what takes place in the various engine stages (fan,
>compressors, combustion chamber, etc).

Don't know of any books, but these big engines work the same way that the
first jets did. The basic principal is that air is drawn into the inlet
and is compressed. The air then passes through the combustion chamber
where it is mixed with fuel and ignited. This air then passes through the
turbine section which turns the compressor section.

All we've since the first jets is the addition of compressor sections,
separated into spools (N1 and N2 on most and N1, N2 and N3 on Rolls-Royce)
, big fans and electronic engine controls that are simply there to improve
efficiency and performance. All these different type of jet engines all
work the same way though.