This issue has been worked over
pretty thoroughly, but bear with me through some comments and possible
clarifications if this subject has your interest.
1.)
In general I don’t think it is
ever a good idea to expect fuel, particularly mogas, to be “sucked”. The
ability to draw any liquid is limited by its vapor pressure, the pressure at
which it will flash to vapor rather to remain as a liquid. Even water at
room temp can only be sucked to a head of about 30 ft (about 14 psi) beyond
which it will be vapor. At the boiling point, obviously it can’t be lifted or
drawn at all, all you get is vapor. Auto fuel at room temp can only be
drawn to something like a 6 ft head, less than 2 psi, or it will flash to
vapor. Increase its temp a bit and pretty soon you can’t suck it at all;
all you get is vapor. So any system in which you expect to apply
negative pressure to move the fuel is at risk. It may work fine at
normal temps, but when it warms up, look out.
2.)
As a corollary to this, in a
gravity fed system it is not a good idea to have a fuel filter (or even
gascolator with a fine screen) upstream from the EFI pump, because it causes
restriction to flow, especially after it has picked up some dirt. Yes; you do
not want any water getting through the pump, and one good reason for having a
sump or header tank is the have the exit to the EFI pump above the bottom so
any water and dirt will be collected, and be drained with a sample
drain. If an upstream filter or gascolator is in the engine compartment,
so much the worse, because it causes a dwell time in a hot place for the fuel
to heat. To draw it from there to the pump, as in Paul’s case, can be a setup
for vapor lock.
3.)
Normally you want the fuel to
recirc through the fuel rail, with the pressure regulator bypass going back to
some tank upstream from the EFI pump; the reasons all relate to keeping down
the temp of the fuel to the fuel rail. As Leon
points out, it is has been the proven way. Yes, you can plumb a one-way
path from between the pump and the regulator to the fuel rail so only fuel
burned by the engine goes to the engine compartment, and normally it may work
fine, but the residence time of the fuel in the hot environment can result in
very high fuel temps. It can cause variation in performance depending on
the temp as the density of the fuel varies. Even though the pressure
downstream from the EFI pump significantly reduces the likelihood of vapor
forming; I don’t know how hot the fuel would need to get to boil at 40 psi; I
can imagine the case after engine shutdown on a hot day that won’t restart
because the fuel in the line and rail have filled with vapor; possibly
bleeding back through the pump.
4.)
The bypass fuel return from the
pressure regulator can indeed be under pressure if it is restricted. The
regulator produces a differential pressure across a restriction. If you
restrict the bypass flow (back pressure) the pressure in the fuel rail will
increase, possibly up to the limit of the pump. So in John’s case it is
to be expected that if the tank the return fuel is going to is full, the
engine will lose power because the mixture will get very
rich.
5.)
It seems to me the sump/header
tank must be vented in some way to allow air/vapor return to the main tank, or
out. There can be different circumstances for different
configurations. The case that Jim mentions for not having the vent in a
gravity feed system is to overcome a specific issue of loss of flow due to
tank cap leaks which caused the low pressure above the wing (strake) to offset
the rather small gravity pressure head. His solution of a controlled
vent is a good one, unless you are confident that your caps will always be
leaktight.
6.)
Whether or not the Paul’s sump
tank became full of vapor is not known, but a vent is needed. The vent line
must extend upward to a level above the level of the fuel in the main tanks
before going down and out to avoid fuel being lost overboard. It is also
important that the vent lines from the main tanks and the sump tank go off the
highest point in the tank. If the exit is submerged, expanding fuel can result
in fuel going overboard.
Ok; this got longer and maybe
more boring than I had hoped, but like everyone else, I’m trying to be
helpful.
Al
Subject:
[FlyRotary] Vapor Lock
Hi, Perry....One question....Does
your unburned fuel (from the fuel rail) go back into your sump tank
also? Or do you return the unused fuel to one of your fuel
tanks? If I were returning fuel to one of the main tanks, I would think
that venting the sump would be appropriate, but since I am sending fuel undere
pressure back into the sump tank, I am concerned that once the sump tank is
full, the additional fuel pumped back into the sump tank would take the path
of least resistance....out the vent instead of back up the fuel line into the
tank? Thanks for your input. Paul Conner
One other comment Paul: the fuel
return from the fuel rail is not pressurized. The pressure regulator maintains
HP on the fuel rail side, but the fuel return output back to the sump is not
under high pressure.