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Date: Fri, 22 Sep 2006 09:23:18 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Fuel Pressure Regulator Vacuum or Boost??
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Blake Lewis wrote:

> I am just learning this stuff. This is my line of thoughts on the 
> subject.
>
>    The amount of fuel that will flow through the injector during a
> pulse width, is proportional to the pressure differential across the
> injector.
> (Delta Pressure) * (Pulse Time)  =  constant * Volume
>   Without the "Vacuum Regulator", at Idle when you have a vacuum
> (less pressure) the Delta Pressure would be bigger, flowing more fuel
> per pulse time.
>   I guess the computer wants a constant flow rate.

Not really.  The computer has a lookup table that matches injector open 
time to manifold pressure.  The issue is, "How much adjustment range 
does the computer have for modifying the open time?"

Pulling some numbers from the air:

You need 1GPH to get a good idle.  But not just any 1GPH, you need a 
nice steady 1GPH.  The computer has to do this by commanding a 10% duty 
cycle on the injectors.  But in the real world, there is no way to 
perfectly maintain a  steady 10% cycle with all the electromechanical 
stuff going on.  Sometimes it wanders to 11 and sometimes it drops to 9. 
That 1 point change is a tenth of the whole duty cycle. What you get is 
a rough idle. The problem is even worse if you're turboed, and injectors 
are sized to flow enough fuel at full boost.  By the time you get to 
idle, those injectors may have to drop to a 5% duty cycle to get down to 
1GPH.

Now, if the computer could command a 15 or even 20% duty cycle to get 
that 1GPH, then the wandering would be much less of the total open 
time.  How could we make the computer have to use a longer duty cycle?  
How about we drop the delta pressure across the injector opening?  Good 
solution.  Enter the vacuum referenced pressure regulator.  As manifold 
pressure goes down, so does the fuel pressure.  The injectors have to 
stay open longer.

Is that the only solution?  We could accept the rough idle as the cost 
of a simpler solution.  We could use a much smaller injector for the 
primary which will be used at idle.  The primaries could be sized to 
need a 30% cycle to get that 1GPH, and monster secondaries would insure 
that there is enough flow for WOT.   That complicates the software, but 
the hardware remains essentially the same.  There is the option of 
higher quality injectors that will wander less (Oops! That's sounding 
expensive.  Sorry I mentioned that one.)

There may be other solutions, but given Ed's experience with nothing 
more than a rough idle, I think staged injectors and the acceptance of a 
little roughness at idle will let you use any pressure regulator you choose.

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