Hi Vince,
Can your "returnless" PWM system have a return line.....of sorts.
Since
the purpose of the return is only to "burp" the system, you need to
pass
air or gas bubbles. Copper tubing for refrigeration purposes is
available
in very small inside diameters. On the order of .020 to .040, and in
lengths
to 20 feet. The idea in refrigeration is a combination of dia and
length
yields a known restriction the freon must pass thru. If a size and
length
could be chosen to pass "air" and gasoline at an extreemly small
rate--say
a pint an hour at max system pressure-- the return line could be
routed to
any tank. A four hour flight would then put an additional HALF
gallon into
a tank, likewise on that four hour flight your "in use" tank would
be short
a half gallon. Flying to envelop limits that are going to be
hindered by a
half gallon over a four hour flight is just not prudent anyway.

Dave M

Dave M -

I like your terminology: Burping.  We wouldn't just be "burping"
though, we'd also be "stirring".  We'd BURP for hot start issues
(vapors).  We'd STIR for contamination (water and oil).

To address all three of Dave Leonard's issues, the "return-less" PWM
EFI Fuel Pump system would need to purge 1) vapors, 2) slugs of water,
and 3) slugs of 2-cycle oil.  The builder's choice would purge back to
the fuel tank(s), a waste tank, or overboard.  Personally, I'd go with
purging back to the "main" tank, AKA stirring.

I mentioned your "leaky" option as an alternative before, but proposed
returning through a small orifice instead.  The motorcycle carb jets
others are discussing sounds like a place to start.

My concern with a FIXED orifice size is that the three things we want
to purge will pass at different rates.  Vapors will whisk right
through, where oil may clog small orifices.  Or refrigerant lines.

A ball valve will QUICKLY purge a fuel rail under pressure.  A quick
open-close should move contaminants right along.  In fact, it could be
part of the start-up procedure:
...
- Open Fuel Purge Valve
- Turn ON right fuel pump; Stir for 10 seconds; Turn OFF right fuel pump
- Turn ON left fuel pump; Stir for 10 seconds
- Close Fuel Purge Valve
- Check Fuel Pressure (should be 45-60 PSI)
...
(takes off on LEFT tank)

This is my leading design at the moment.  For a "returnless" PWM EFI
system it lowers power consumption and extends pump life, but still
provides a purge option for Dave Leonard's nightmares.  After all, he
DID say to trust him on this one.  :)

If other ideas are out there, let's hear them.  By the way, I'm still
looking for a fuel pressure sensor for EFI fuel systems - any
suggestions?

-Vince Orton

Hi Vince,
Unfortunately, you are correct. You should probably eliminate the 'small
orifice' idea from further consideration. Oil routinely flows thru a
refrigeration system with the freon, it's needed to lube the compressor.
But WATER in a small orifice will have a tendency to freeze and presto
the return flow is lost. So much for the KISS principle  : )  Dave M