Fuel pump questions

[Frederic Breitwieser]

>So you mean to tell me that you can just as easily pump water up a 3/8"
>pipe 75' straight up and into the bottom of a water tower and that 3/8"
>line will have LESS pressure on it than the same line going to the top
>of the water tower and dripping in through the top?  I've been wrong
>before, but I really don't think this is right.
>
>If you put the fuel return line into the bottom of the tank, wouldn't
>that be just like pinching the line and creating more pressure?  Or the
>same as using a smaller fuel line diameter?

Howdy Andy - irregardless what was said, its like this:

The fuel regulator is about 25-26" off the ground.  The gas tank is
entirely below this measurement in most OEM vehicles that I know of, so
whether the return line feeds to the top of the bottom of the tank, it
makes absolutely no difference, since the tank is below the regulator/fuel
rail, etc.

If you have one of the old fashioned tea-kettles, where the pot is one
part, and the spout is a seperate piece that connects to the bottom of the
pot, you can demonstrate this to yourself.  Fill the tea-kettle 2/3 of the
way up, so the spout inlet (outlet!) is completely submerged below the
water line within the pot.  Pour a glass of water into the spout, and the
water level of the tea-kettle will rise, and the two level off.

Here is how the system works:

The fuel tank is filled, and when closed, rests at atmospheric pressure.
When the car runs, the fuel pump draws in fuel, pumping at 55PSI (or so)
above atmospheric pressure, moving fuel into the fuel rail, and into the
injectors as they open and close.  If the fuel rail experiences more
pressure than the regulator allows (45 PSI or so), it opens slightly to
allow the excess pressure to "bleed off".  This higher pressure fuel then
follows the return line back to the fuel tank.  As long as the regulator is
open entirely or partially, fuel will flow back into the tank.  Its
pressurized coming back, so it would go back to a fuel tank mounted on the
roof (don't try this at home <Grin>).

Since the majority of the fuel is dumped through the injectors into the
cylinders, the return line *ABSOLUTELY CAN BE* narrower in diameter than
the fuel line.

Think about it... if the engine is not running, and the fuel pump is, then
the regulator will open, and the fuel will go around in a complete circle.
Therefore, if the fuel line is 1/2", then the return line doesn't need to
be any larger, as it would easily flow the same amount of fuel.

I hope that jumble of text I just typed clears things up.  Typically, the
return line is 1/2 of cross sectional area of what the sending line is.  So
remember that a .5" fuel line will flow significantly more than double what
a .25" fuel line would flow.

Cheers!


Frederic Breitwieser
Homebrew Automotive Mailing List
Bridgeport, CT 06606
http://www.geocities.com/MotorCity/Downs/4605/index.html
(Portable Webserver Link Address)

1989 AG Hummer 4-Door
1993 Supercharged Lincoln Continental
2000 Mid-Engine Sports Car <smile>

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