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.