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