> >
> Al Gietzen wrote:
>
>> 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

>>
>> 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.
>>



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