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Ed Anderson wrote:
Ernest,
One thing that you might want to consider ref the "jet" pump is the effect
that high altitude might have on the operation of the pump. I have not
looked into it but wonder whether the lesser air density would affect the
amount of pressure differential produced. Whether there might be a point
where the jet just ceases to pump.
Yep. It's nothing more than a Venturi pump. Creates a low pressure at the venturi and depends on atmosphic pressure to push fuel though. I doubt it would generate more than a couple inches of head pressure, but it barely has to produce any at all. Start with A and B full, the transfer line empty. As the engine sucks fuel from A, it's level slowly drops. For a 20 gallon tank, it'll be half empty in an hour. The jet pump has been slowly creating a vacuum in the transfer line over that hour. Arbitrarily choosing 6" high tanks, the fuel level in A is 3" lower than B. The jet pump now has an hours worth of vacuum AND 3" of head working in it's favor. One way to look at it is all the 'pump' really does is keep the siphon tube primed.
On another topic, looking at the pages on fuel pressure regulators. They mention 3 types. The one we've been discussing, wherein the fuel rail pressure referenced to the manifold, a constant pressure type, that is just reference to a spring, and the one I found intriguing, the atmospheric referenced type.
It seems like the atmospheric type would work best for an airplane.
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