Received-SPF: none
 receiver=logan.com; client-ip=216.93.66.206; envelope-from=dsbarno@vbe.com
Message-ID: <007e01c511f1$e5ec8240$3c40fea9@u036fr3za011>
From: "Barnhart" <dsbarno@vbe.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-727168@logan.com>
Subject: Re: [FlyRotary] Re: Vapor Lock
Date: Sun, 13 Feb 2005 11:31:48 -0600
MIME-Version: 1.0
Content-Type: text/plain;
	charset="Windows-1252"
Content-Transfer-Encoding: 8bit

Al,

I know what you mean about fuel being better pushed than sucked.  My tech
counselor/dar made it very clear that he wanted the sump at the lowest
point. My low point is behind the rear seats and is a taildragger. Picked up
a 1/2 inch unit from Andair and have 3 feet of head from both sides feeding
it. Keeping the fuel pumps about  12 inches away. Also have a plan b....

Barny
MGDQ 20bt (don't really have dial up here, more like wind up)


> > >
> > 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
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.
> >>
>
>
>
> >>  Homepage:  http://www.flyrotary.com/
> >>  Archive:   http://lancaironline.net/lists/flyrotary/List.html
>
>
> _____________________________________________________
> This message scanned for viruses by CoreComm
>
>