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Date: Sun, 13 Feb 2005 09:11:39 -0600
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Vapor Lock
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Gravity feed from two tanks is almost never equal from each tank. Slight =

differences in fuel line length & bends & mostly differences in vent=20
positioning make it very hard to get equal flow. Ask any single engine=20
Cessna driver.

Charlie
(following local convention)

Jim Sower wrote:

> Al,
> <... The case that Jim mentions for not having the vent in a gravity=20
> feed system is to overcome a specific issue of loss of flow due to=20
> tank cap leaks which caused the low pressure above the wing (strake)=20
> to offset the rather small gravity pressure head. His solution of a=20
> controlled vent is a good one, unless you are confident that your caps =

> will always be leaktight ...>
>
> Actually, it wasn't leaky caps. I taped up the caps thoroughly=20
> airtight and it didn't change a thing. To this day I don't know what=20
> caused the assymmetric gravity feed, but I fixed it by de-selecting=20
> the tank that is transferring too well.
>
> But that wouldn't work at all with a vented sump ... Jim S.
>
>
> Al Gietzen wrote:
>
>> This issue has been worked over pretty thoroughly, but bear with me=20
>> through some comments and possible clarifications if this subject has =

>> your interest.
>>
>> 1.) In general I don=92t think it is ever a good idea to expect fuel, =

>> particularly mogas, to be =93sucked=94. The ability to draw any liquid=
 is=20
>> 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=20
>> be sucked to a head of about 30 ft (about 14 psi) beyond which it=20
>> will be vapor. At the boiling point, obviously it can=92t be lifted or=
=20
>> drawn at all, all you get is vapor. Auto fuel at room temp can only=20
>> be drawn to something like a 6 ft head, less than 2 psi, or it will=20
>> flash to vapor. Increase its temp a bit and pretty soon you can=92t=20
>> suck it at all; all you get is vapor. So any system in which you=20
>> 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=20
>> idea to have a fuel filter (or even gascolator with a fine screen)=20
>> upstream from the EFI pump, because it causes restriction to flow,=20
>> 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=20
>> 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=20
>> compartment, so much the worse, because it causes a dwell time in a=20
>> hot place for the fuel to heat. To draw it from there to the pump, as =

>> in Paul=92s case, can be a setup for vapor lock.
>>
>> 3.) Normally you want the fuel to recirc through the fuel rail, with=20
>> the pressure regulator bypass going back to some tank upstream from=20
>> the EFI pump; the reasons all relate to keeping down the temp of the=20
>> fuel to the fuel rail. As Leon points out, it is has been the proven=20
>> way. Yes, you can plumb a one-way path from between the pump and the=20
>> regulator to the fuel rail so only fuel burned by the engine goes to=20
>> the engine compartment, and normally it may work fine, but the=20
>> residence time of the fuel in the hot environment can result in very=20
>> high fuel temps. It can cause variation in performance depending on=20
>> the temp as the density of the fuel varies. Even though the pressure=20
>> downstream from the EFI pump significantly reduces the likelihood of=20
>> vapor forming; I don=92t know how hot the fuel would need to get to=20
>> boil at 40 psi; I can imagine the case after engine shutdown on a hot =

>> day that won=92t restart because the fuel in the line and rail have=20
>> filled with vapor; possibly bleeding back through the pump.
>>
>> 4.) The bypass fuel return from the pressure regulator can indeed be=20
>> under pressure if it is restricted. The regulator produces a=20
>> differential pressure across a restriction. If you restrict the=20
>> bypass flow (back pressure) the pressure in the fuel rail will=20
>> increase, possibly up to the limit of the pump. So in John=92s case it=
=20
>> is to be expected that if the tank the return fuel is going to is=20
>> full, the engine will lose power because the mixture will get very ric=
h.
>>
>> 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=20
>> different circumstances for different configurations. The case that=20
>> Jim mentions for not having the vent in a gravity feed system is to=20
>> 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=20
>> small gravity pressure head. His solution of a controlled vent is a=20
>> good one, unless you are confident that your caps will always be=20
>> leaktight.
>>
>> 6.) Whether or not the Paul=92s sump tank became full of vapor is not =

>> known, but a vent is needed. The vent line must extend upward to a=20
>> 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=20
>> highest point in the tank. If the exit is submerged, expanding fuel=20
>> can result in fuel going overboard.
>>
>> Ok; this got longer and maybe more boring than I had hoped, but like=20
>> everyone else, I=92m trying to be helpful.
>>
>> Al
>>
>> Subject: [FlyRotary] Vapor Lock
>>
>> Hi, Perry....One question....Does your unburned fuel (from the fuel=20
>> rail) go back into your sump tank also? Or do you return the unused=20
>> fuel to one of your fuel tanks? If I were returning fuel to one of=20
>> the main tanks, I would think that venting the sump would be=20
>> appropriate, but since I am sending fuel undere pressure back into=20
>> the sump tank, I am concerned that once the sump tank is full, the=20
>> 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=20
>> into the tank? Thanks for your input. Paul Conner
>>
>> One other comment Paul: the fuel return from the fuel rail is not=20
>> pressurized. The pressure regulator maintains HP on the fuel rail=20
>> side, but the fuel return output back to the sump is not under high=20
>> pressure.
>>