Sender: "Marvin Kaye" <marv@lancaironline.net>
To: lml@lancaironline.net
Date: Wed, 22 Dec 2004 09:08:54 -0500
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From: "Gary Casey" <glcasey@adelphia.net>
Subject: pressure tank (air/fuel separator)
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<<We're proposing this:  We are adding a +/- 3.5 gallon, pressurized
aluminum fuel header tank, to the system.  We are relocating the
continuous duty, electric fuel pump currently supplied by Lancair to a
location between the fuel tank (where fuel is being picked up) and this
pressure tank so the fuel is pumped (pushed) through the pressure tank
at about 32 psi to create what is essentially a fuel/air separator.  The
pressure tank is vented at the very top, and can be equipped with an
optical fuel sensor or fuel probe.>>

The problem that I see is that you can't combine the ideas of a pressurized
and vented tank.  Without the vent the collected vapor won't go away and
with the vent you can't pressurize the tank.  Also, it would be nice to run
the fuel return line to this tank rather than all the way back to the wing
tank except you don't want to pressurize the return line.  What we did at
Mercury Marine was to incorporate a float valve just like in a carburetor
where the float opened the vent when the fuel level dropped, expelling the
vapor.  If you did it this way and the electric pump failed the float would
open the vent, allowing the engine pump to draw all the fuel from the header
tank.  Otherwise, the failure would have to be such that vapor (or liquid)
could be drawn from the wing tanks.  If you use a vent system and the
electric pump failed open - the "normal" failure mode - the vent would allow
air to enter the header tank, making all the remaining fuel unusable.  If
there was a blockage that caused the interruption the engine pump would be
unable to pull fuel from the header tank without the header tank being
vented.  With an unvented tank it would likely prevent fuel delivery
interruption during fuel tank changes, which I believe was the original
intent.  With no vent where would the trapped air go?  Most likely after the
engine was shut off the pressure would bleed off and the air would go out
through the engine pump and back to the fuel tank, but only after a whole
tank-full of air was collected.  With the tank originally full of air and
then pressurized to 35 psi it would remain about 1/3 full of air while
running, which does provide an accumulator effect, making up for momentary
flow interruptions in the supply.  In summary, I'm not sure the header tank
buys you enough to justify the cost, complexity and weight, although engine
power interruption during fuel tank changes is also a bad thing.

Gary Casey