Re: Engine start process questions

Date:         16 Jun 97 21:35:39 
From:         Ron Adams <ronadams@randomc.com>
Organization: Random Access Inc. +1 (800) 910-1190
References:   1
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Rich Nute wrote:
> A jet engine is started by:
>
> 1.  Spin the turbine up to some rpm.
>
>     How many rpm?

It depends on the engine, but generally something above 20 percent N2.
BTW, exact RPMs aren't shown, unlike reciprocating engines. It's shown
as a percentage of max RPM. The early jets had RPM but since it is
typically over 10,000, percentage is much more reasonable.

>     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?

Not usually.

>     What is the approximate gear ratio between the
>     electric motor and the turbine?

Sorry, you have me there.

>     How is the spin air injected into the engine?

It's not. It goes to an air-driven starter motor.

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

Through the fuel control unit, something like a fuel injector
controller, but more complex. Fuel is then injected into the burner
"cans" that are in the "hot section", arranged symetrically around the
engine core. These cans are more like sieves, in that they have holes
for the air to come in.  The can actually holds the flame in place.

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

Igniters, like spark plugs, located in some of the burner cans. They run
until the engine is at the proper idle RPM (usually between 50 & 60
percent).

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

Depends on the cockpit and how sophisticated it is.

For the earlier types, one turns on the ignition switch, holds a start
switch while the engine turns up and then at the proper time turns on
the fuel control switch. Once the engine reaches the proper RPM, the
start switch is released and the ignition switch turned off.

In later versions, one pushes the start switch, waits for the proper RPM
and turns on the fuel control switch. At the proper RPM, the start
switch pops out on its own.

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

No, on the fuel control switch. The first step in fighting an engine
fire is to turn off the fuel control switch. If necessary, the fire
extinguishing system is the next step.

> Thanks,
> Richard Nute
> San Diego

 You're quite welcome.

Ron Adams, Atlanta