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Date: Mon, 03 Apr 2006 11:05:39 -0600
From: Bill Dube <william.p.dube@noaa.gov>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Erratic mixture
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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.

They reference the fuel pressure to the manifold to make the fuel 
delivery proportional to the duty cycle of the injector pulse at all 
manifold pressures. It makes the whole system more stable. It also 
extends the range of the injectors, at low speeds and high speeds.

If you allow the difference in pressure across the injector to vary, it 
will put out more fuel per pulse at low manifold pressure and less fuel 
per pulse at high manifold pressure. This is exactly what you don't want 
to happen.

    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'


Ernest Christley wrote:

> Russell Duffy wrote:
>
>> Think about how a fuel injector works.  The fuel rail has a high 
>> pressure in it, and the intake end of the injector is in a lower 
>> pressure area.  To actuate the injector, all you're doing is opening 
>> it.  The rate of fuel flow is controlled by the difference between 
>> the pressure in the fuel rail, and the pressure in your intake.   
>> As you can see, if you were to set your fuel pressure at a fixed 
>> point, then run lots of boost, there would be very little flow across 
>> the injector, because the differential pressure would be low.  This 
>> will make your injector appear to get smaller as boost increases, and 
>> eventually you will go lean.  Recall also that lean + boost = rebuild 
>> :-(
>
>
>
> But doesn't the computer sense the high manifold pressure and increase 
> the open time on the injector to allow more fuel in?  Is the manifold 
> pressure so high that it is creating enough backpressure on the 
> injectors that there is no fuel at all flowing?  Wouldn't the proper 
> fix in that case be to increase the fix point of fuel pressure?  Is it 
> that there isn't enough adjustment range using injector pulse timing 
> to accomodate both an idle opening time and max boost opening time? 
> Wouldn't the issue be less critical in a normally aspirated engine 
> since there isn't nearly as much of a manifold pressure range to 
> accomodate with varying injector open times?
>
> I don't know the proper numbers to apply, but lets see if I can 
> describe my understanding of the situation.
>
> In a normally aspirated engine, you need 1 drop of fuel at idle and 50 
> drops full throttle.  In the turbo, you need 1 drop at idle and 100 at 
> full boost.  With the system regulated at 30lbs of fuel pressure, the 
> computer can meter out 1 drop of fuel at a time, and it has 40 
> increments available per intake cycle.  Everything is good at idle, 
> but neither system will never get fuel at full power.
>
> The solution is to increase the fuel pressure to 45lbs at full 
> throttle, where the metering resolution of the computer is 1.5drops.   
> The timing is the same, but more fuel is getting sprayed for each 
> millisecond.  Now both engines will run rich at idle, the NA will be 
> fine at WOT, but the turbo will still be running lean unless boost in 
> limited.  The turbo needs to increase the fuel pressure to 60lbs at 
> high power settings so that the computer delivers 2 drops at at time, 
> and it can get the fuel it needs at full power.  But then the mixture 
> is so rich that it won't run at idle, because the computer doesn't 
> have the resolution to lean it down.
>
> I'm attempting to implement a returnless system that will be 
> referenced to atmospheric pressure.  It allows for very simple system 
> with the pumps located where they can get the highest head pressure 
> and far away from any heat sources.  It may also give me autmatic 
> leaning with altitude.  I'll be able to add surgical tubing later on 
> to get the manifold reference back to the regulator if necessary.  The 
> big question is, "Will there be enough timing adjustment range in the 
> EC2 to allow the injector pressure to remain constant?"
> That's what test stands are for.
>