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Date: Mon, 03 Apr 2006 13:46:05 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Erratic mixture
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Bill Dube wrote:

> The difference in pressure across the injectors is what matters, not 
> the absolute fuel pressure.  If you hold the difference in pressure 
> constant, the injectors deliver the same amount of fuel per pulse, 
> regardless of manifold pressure changes. If you run a fuel pressure 
> that is not referenced to the manifold, you will have less and less 
> difference in pressure as the boost comes on. This will make your 
> situation worse, not better.


Agreed.  But with a fuel pressure over 50 PSI, it will take a very 
significant turbo boost to cut off the flow altogether.  The two 
question that then remain are, "Do the injectors have the time to 
dribble in enough fuel at full boost?" and (on the other end) "Can we 
turn the firehose off quick enough not to flood the engine?"

>    The stability issue is a subtle one, but important. Let's pick high 
> manifold pressure (boost) as an example. You open the throttle. 
> Pressure instantly goes up. The difference in pressure across the 
> injectors goes down. The fuel per pulse goes down. The fuel flow 
> instantly goes down before the injection computer can compensate. The 
> engine goes lean. The engine slows down a bit. The manifold pressure 
> is starting to go up a bit more because the engine slowed down a bit. 
> The difference in pressure across the injectors now goes down a bit 
> more. They instantaneously deliver less fuel. The engine goes even 
> leaner. The computer is now catching up. It calls for more fuel. The 
> injectors open for longer. The engine starts to speed up. The manifold 
> pressure goes down. The difference in pressure across the injectors 
> increases. They instantaneously deliver more fuel. The engine speed up 
> more. The manifold pressure drops more. The injectors deliver even 
> more fuel per pulse, instantaneously. The computer now catches up and 
> calls for less fuel. ............
>
> Bill Dube'

This reasoning hinges on the assumption that the mechanical manifold to 
fuel pressure reference system is instantaneous and that the computer 
will be lagging behind.  I'm not sure I can swallow that pill unless 
there is a lot of unnecessary hysteresis built in front of the computers 
pressure sensor (perhaps from using an overly large hose).  I don't 
think I've ever heard of a problem tracked back to the EC2 not being 
able to keep up.

-- 
         ,|"|"|,               Ernest Christley     |
----===<{{(oQo)}}>===----     Dyke Delta Builder    |
        o|  d  |o          www.ernest.isa-geek.org  |