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Old 06-05-2008, 04:22 PM   #210 (permalink)
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Originally Posted by Njay View Post
I hooked up at a sentence in an engine manual (in Italian...): "the valve makes the pressure difference constant". If you think about it, for the fuel to go through the injector opening at the same speed in all conditions, the pressure *difference* from one side of the injector hole (the injector's fuel chamber) and the other side (the cylinder entrance) must the constant... this is my suspicion. I'll have to take the car for revision soon, maybe I'll try to talk to someone there at Ford.
Very good point. I didn't think of that. So the term "constant pressure" can be misleading, unless we specify what we mean. Constant pressure relative to the atmosphere is not the same thing as constant pressure relative to the intake manifold.

I have a feeling that a random person at the dealer won't be a good source for a reliable, knowledgeable answer.

Now that I investigate some more, I think there actually isn't a problem. Looking at the service manual for my car ('95 Civic VX, p. 11-105), I see that the fuel pressure regulator has a hose connecting to the intake manifold. Fuel pressure is supposed to be 40-47 psi with the hose disconnected, and 31-38 psi with the hose connected (and engine idling). So the system is designed to drop fuel pressure when manifold pressure drops. So the system keeps fuel pressure more-or-less constant relative to manifold pressure, not relative to atmosphere. (I say "more-or-less" because I don't know for sure that the relationship is purely proportional. Although I bet it is.)

Anyway, I have a feeling that my car is just like your car, in this regard. And I have a feeling that this is the prevailing design, for EFI. Which should mean that pulse width is a good proxy for true fuel flow, after all.

It actually makes a lot of sense that engineers designing injectors would want to be able to assume a constant relative pressure. This would make is easier to predict, with a great deal of refinement, the exact mechanics of how the injector will behave.

So dcb, I have a feeling we've been spinning your wheels on this point, and this is something we actually don't need to worry about.
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