Re: Engine start process questions

Date:         16 Jun 97 21:35:40 
From:         dahler@gte.net (Chris Dahler)
Organization: GTE Intelligent Network Services, GTE INS
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In article <airliners.1997.1226@ohare.Chicago.COM>,
richn@sdd.hp.com says...
>A jet engine is started by:
>
>1.  Spin the turbine up to some rpm.
>
>    How many rpm?

Generally, the RPM is given as a percent rather than a
total number.  This is far easier than trying to quickly
comprehend some number like "10450 RPM"; conceptually, it
is easier to understand "20%".  The turbines on most
airliners are spun to anywhere from 20 to 25 percent
before fuel injection.

>    The spin is either by an electric motor (for small
>    engines) or compressed air (for larger engines).
>
>    Does the electric motor also serve as the generator?
>    What is the approximate gear ratio between the
>    electric motor and the turbine?

It depends.  On most corporate jets with engines that are
started electrically, the starter is also the generator.
There are no airliner jet engines that are started
electrically due to the excessive electrical loads that
would be required to spin such relatively large turbines.
The APU on most airliners is a small turbine which is
started electrically from battery power, and the starter
for these APUs generally is not the APU generator.

>    How is the spin air injected into the engine?

A valve opens which allows compressed air from either the
APU or an external air source to blow onto a small turbine
wheel, which in turn drives the shaft of the engine.

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

A valve opens which allows fuel under pressure from boost
pumps located in the fuel tanks to be sprayed through
injector nozzles into the burner cans, where it mixes with
the compressed air from the front of the engine and
provides a continuous combustion.

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

There are igniters located around the circumference of the
burner cans which fire just like the spark plugs in a
car's engine.  This provides the initial ignition to the
fuel/air mixture.  Once the fire is started, it will keep
burning as long as fuel and air are continuously supplied,
so the igniters are only used to get the combustion
started or as a backup safety measure when flying in rain.

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

It varies from one airline to another, but generally the
copilot will open the start valve allowing the air into
the starter, and the captain will raise the start lever or
switch allowing fuel into the engine.

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

No, but the controls are located centrally for quick
access should a problem occur.

Chris Dahler
dahler@gte.net