Return-Path: Received: from fed1rmmtao04.cox.net ([68.230.241.35] verified) by logan.com (CommuniGate Pro SMTP 4.3c1) with ESMTP id 726441 for flyrotary@lancaironline.net; Sat, 12 Feb 2005 14:19:12 -0500 Received-SPF: none receiver=logan.com; client-ip=68.230.241.35; envelope-from=ALVentures@cox.net Received: from BigAl ([68.7.14.39]) by fed1rmmtao04.cox.net (InterMail vM.6.01.04.00 201-2131-117-20041022) with ESMTP id <20050212191826.JRBI19250.fed1rmmtao04.cox.net@BigAl> for ; Sat, 12 Feb 2005 14:18:26 -0500 From: "Al Gietzen" To: "'Rotary motors in aircraft'" Subject: RE: [FlyRotary] Vapor Lock Date: Sat, 12 Feb 2005 11:18:31 -0800 Message-ID: <000001c51137$a3b26c60$6400a8c0@BigAl> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0001_01C510F4.95919D60" X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.6626 Importance: Normal In-Reply-To: X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 This is a multi-part message in MIME format. ------=_NextPart_000_0001_01C510F4.95919D60 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable This issue has been worked over pretty thoroughly, but bear with me = through some comments and possible clarifications if this subject has your = interest. =20 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. =20 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. =20 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. =20 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. =20 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. =20 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. =20 Ok; this got longer and maybe more boring than I had hoped, but like everyone else, I'm trying to be helpful. =20 Al =20 =20 =20 =20 =20 Subject: [FlyRotary] Vapor Lock =20 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. ------=_NextPart_000_0001_01C510F4.95919D60 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

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.

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