Plumbing the EFI fuel return for twin tanks - need ideas

I've been wrestling with the best method for balancing the return fuel from an EFI system to the two fuel tanks. My main fear is that one tank will get filled faster than the other.

Solution 1: Build two completely separate systems. Two tanks, filters, pumps, fuel rails, and bypass valves.

Pros: No possible cross-flow.

Cons: A pressure difference from side to side could cause one bank to run lean. Also two points of failure for one system. One fuel pump fails and unless I tell the ECU to only fire on one bank, I DNF.

Solution 2: Fabricate a BIG balance line between the tanks and plumb the return with equal length return lines.

Pros: It'll balance.

Cons: Where to run a 1 inch balance line in an ERA.

And for your added pleasure: plumbing the return lines poses a challenge in the ERA.

Solution 1: Run a 3/8 steel line from the rear to the front and fabricate an "AN" boss in the filler neck.

Best option but a pain to run.

Solution 2: Dump the return to a "T" on the inlet side of the fuel pump.

This brings up an interesting prospect. Most of the circulating fuel is in a loop from the inlet through the pump, around the rails, and through the bypass valve and back to the pump. This fuel will get hotter than if it was circulating to the tank and back.

This is my plight. If any of you have done this and know the "best" way I would appreciate your opinion.

Thanks,

Mark

Use three pumps. One for each tank. Plumb one pump from each tank into a third mixing pot. Use a third pump to pull from the mixing pot to the engine.

This is the way lots of people have done it in the past and the way the RF design is from the start. The RF car has the third pot built into the tank, some of the GTD cars have just used an external tank as the mixing pot.

As for the return from the rail I think you plumb it into the mixing pot. I'm sure someone here can help out with the details but this is the way I think dual setups are commonly used so as to avoid the presure problems and balance problems.

The two feed pumps to the pot can be low pressure since they are not directly feeding the engine. The third pump from the mixing chamber must be able to supply whatever pressure you are using at the motor.

Mark, this is an interesting problem. I agree you don't want fuel running around in a circle and getting hot...it will affect the performance adversely and it's dangerous as well...
You're going to have one high-pressure pump drawing through a valve that lets you select which tank you pull from, right? I think what you need is TWO valves, ganged so that you also switch the return line at the same time; this way overflow fuel would go back to the tank it came from rather than both or the wrong one. I believe that ganged valves such as that are easily available in the hydraulic control market (Northern Inc might have them).
Another option is to send all overflow fuel to a collector or sump tank (say a quart or two) which then drains back to both tanks through a set of one-way valves. The problem with a balance tube is that fuel will slosh from one tank to the other, plus you have a one-inch pipe with fuel in it running crossways and possibly through the passenger space. If you had a return collector tank with one-way valves the fuel would distribute equally back to the two tanks and couldn't get from one to the other.
This is very much a topic of general interest because all of us who are thinking about EFI systems for a GT40 sooner or later need to address the two-tank/one-engine problem. If other folks have looked at this, I would like to know how they solved the problem.
(Interestingly, Ferraris have often treated the ignition and induction/fuel supply systems on their vee engines as two separate engines on one crankshaft- this goes back quite a ways in their engineering. They usually have one large tank, though, not two.)

Mark,

Ron's post describes the system but Jim's comments are very valid.

Our system is as follows :

There are THREE tanks which are independant of eachother. RF use a swirl pot tank at the rear of the right hand tank ( opposite the battery position ) and positioned in the sill.

There are THREE pumps, two LP transfer pumps and one HP injection pump, and FOUR fuel filters, one prior to all three pumps and the remaining filter AFTER the injection pump. These filters are all important.

Fuel is transfered to the swirl pot using the transfer pumps which are controlled by a switch on the dash.

The injection system draws from and returns to the swirl pot.

A spill / over flow returns the excess fuel through a soliniod valve to the tank that is supplying the swirl pot. This solenoid valve is also controlled by the switch that controls the two transfer pumps.

I agree with Jim in that a build up of temperature will cause problems. These problems include vapourisation in the fuel rails and the obvious change in calerific values of the charge. The circulation of excess fuel back to the main tanks eliminates these problems. Another problem that the system above eliminated is a pressurising of the swirl pot due to different feed rates for the transfer pumps as compared to demand from the engine at different rev rates. There is also an expansion of the fuel in the swirl pot due to temperature rise. My system equalises the pressure in all three tanks.

A word of warning !!!!

Jaguar have used a system similar to mine in their XJ12 cars fitted with twin side tanks and injection engines. The "shuttle valves" used to dirrect fuel to and from the 'selected' tank were prone to failure and the engine would be feeding from one tank and returning to the other tank. This resulted in overflowing tanks and lots of spilt fuel.

The trick with this system is the choice of parts and the R&D to verify them.

I am happy to sell systems, please contact Gordon etc.

Best wishes,

Robert

I have already thought about this too, and planned on a 3 pump system with a swirl pot also.

I've been thinking of using a valve from a ford truck. My F250 deisel has dual tanks, and there is a valve mounted on the frame which handles the feed/return to the correct tank.
The valve is electric and has 6-hose connections on it.
A feed & return for each tank, and a feed/return for the engine.
This is the valve I was going to use.

The only downside I saw, is that the valve is plastic and there is no way to use AN type hose fittings on it.

Oh, and on your swirlpot, make sure and "gravity feed" the HP injection pump.
They are made to push much better than they pull.

here is a page with info on making your own and other helpful info.
web page

[ November 30, 2002: Message edited by: Alain Vanhollebeke ]

For those who subscribe to the KISS principle, here is how I solved this problem. Since many trucks are now using EFI and trucks often use an extra tank, the industry is now producing tank selection valves that also control which tank the return line goes to and can handle EFI fuel pressures. I employed a single fuel pump with one of these valves, which cost $50-60. The other thing these selector valves do is select which sender signal is sent forward. What this allowed me to do is use a single fuel level gauge and a fuel pressure gauge where the second fuel level gauge would go normally. I am not the least bit concerned with deciding how to use the tanks as I have a lot of experience in Mooney airplane which is a low wing aircraft with twin tanks, so I know the procedure. Also, the engine doesn't even burp when switching tanks, so that I can always flip the switch to check the condition of the other tank.

One thing that you need to be careful of with EFI fuel pumps is DON'T LET THEM RUN DRY. My pump has port on the bottom that lets the fuel flow into it, but shuts off as soon as there is pressure on it. Having the return line to tie this into is very handy and assures that the pump won't run dry; that, and the fact that the pumps intake is level with the sump port on the fuel tanks, so there should always be fuel in it.

Mark,
I plan to use two pumps on my EFI system with a checkvalve downstream of each pump before the lines "Y" together. The only path for fuel to flow is from whichever pump, to the engine on the supply side of the fuel rails. The checkvalves prevents backflow to either tank. A single pressure regulator is downstream of the fuel rails. It will feed a single line to a motorized 3 way fuel valve much like Lynn and others above have refered to that trucks with dual tanks use. The motorized valve will be set to send the fuel return back to the proper tank via cockpit switch (which also selects which pump to run). I bought the valve from JC Whitney for about $50 (If memory serves me correctly!). The valve body has two sets of ports on it, a 3/8" ID set and a maybe 1/4" set. I plan to use only the 3/8" set for the return lines. The valve body is plastic of some kind, so AN fittings won't work, but a SS braided line (rubber, not teflon lined!) can be put on the fitting and secured with a simple SS hose clamp or one of the fancier clamps that looks line an AN fitting. Since the motorized valve is on the non pressurized return side, fuel pressure will be low, so normal clamps should work fine. A simple flip of the switch in the car will select which tank/pump will run from the ECU, and which tank the fuel will be returned to.

[ November 30, 2002: Message edited by: DaveWharran ]

Lynn is your valve also plastic? Do you have a part number for the valve?

Thanks
Gary

Very good information, especially from Lynn. I would use a purpose-built valve such as the one he mentions, definitely over a hydraulic valve which may be rated for higher pressures but is not rated for fuel. Anything that is approved for use in an airplane will be safe, and then some, for a GT40, where you have the luxury of stopping when you are out of fuel- rather than finding a place to land.

Gary, yes the housing is plastic. To clarify for Jim, the valve is not an aircraft valve, it is made for EFI powered trucks where an auxiliary fuel tank is desired. It is designed for fuel and EFI pressures of up to 65psi though. The valve is made by Pollak and available from J.C Whitney for $54 (81UV2705U) and is the 6 port model. It has 3 3/8" ports for primary flow and 3 5/16" ports for return flow.

The reference to aircraft was with respect to the procedures followed in use of the fuel from two tanks separate tanks. As you state, fuel management is serious business in an aircraft and so is the balance of the aircraft. Our procedure was to fly for 1 hour on the first tank (in a GT40, I would presume this to be the passenger side). Then you switch to the opposite tank and run it down until it is effectively empty, keeping track of the time it provided. At this point you know you have the time the second tank lasted minus 1 hour. Fuel gauges in an aircraft were only rough indicators not to be trusted, the clock was considered the primary indicator of fuel remaining.




[ December 01, 2002: Message edited by: Lynn Larsen ]

Thanks a million for all this input.

I've seen the swirl tank high/low pressure designs on dragsters. Got a local Kinsler expert. I'm worried about malfunction of the control valve for the return lines and wonder if the Ferrari method might be the better option. It was what I came up with first also.

What would be the drawbacks of two separate systems? Left tank feeds the left bank, right tank feeds the right bank.

1. You'd need to be sure to get the pressure balanced from side to side and check it regularly.

2. A partial pump failure on one side could cause a lean condition that could damage the engine. Total pump failure on one side would give you a four cylinder engine.

Two separate systems would give me no chance of any crossflow. I could return the fuel to the front of the tank without the use of a vapor separator tank. I could even put in a manual valve to feed the other side in case one pump failed. This would be the most simple (KISS) and lightest way to go.

So am I out in the ozone layer or did Enzo's boys have the best method? Would I be hating life if I did this?

Thanks again,

Mark

Seems to my Lynn's method is simplier than the two pumps/two systems method. The valve is an off the shelf component that works well, uses one pump, no balance problems, no leaning problems or danger of engine damage. You could even make Lynn's system redundant with a second pump that can be activated in case of failure - but it isn't really necessary. These aren't airplanes, they aren't race cars with $millions on a win, and OEM factory EFI pumps have a high MTBF.

R

Agree Ron, to a point.
In fact, I'm using the same valve as Lynn. What worried me though was the small port sizes on the valve. Maybe alright for a car or truck making 250 or 300 HP, but marginal for something making 400+?

The pump I'm using (Aeromotive)has an AN10 suction line and AN8 discharge. (I'd be afraid to use anything smaller than AN8 for a suction from the tanks with these pumps.) The AN8 line "Y"s into two AN6 lines, one for each fuel rail. AN6 from each rail to the regulator. AN6 from the regulator to the valve and then on to the tanks.

That's why I'm going the two pump route. Large lines on the pump suctions and supply to the fuel rails, AN6 return through the larger ports on the valve. The smaller ports would be unused. Overkill? Probably, but I'd rather do it once. Thus, 2 pumps and larger lines won out in my case. Additonal hardware was one pump, one check valve, one suction filter, and associated hoses and fittings. Not a tremendous amount in the grand scheme of things. Yup, I'm an engineer..electrical, but still like to oversize things!

Dave

One thing to remember is the purpose of the swirl/surge tank. EFI engine systems don't have a float bowl and will lose fuel pressure if the fuel inlet on the tanks gets exposed to air. It can damage the fuel pumps and the engine runs lean or stops in extreme cases.

Surge tanks tanks are usually tall and thin and are kept full.

The "main" tanks on a GT40 are long which gives a possibility that in a long down hill run for instance the fuel pickup (if it is at the rear)will become exposed to air. The low pressure fuel pumps can cope with running dry and the surge tank is used to supply the engine until the fuel pickup in the main tanks can find some fuel again.

If you don't use a surge tank you may have fuel starvation problems at low fuel levels.

BTW. I'm using the system described by Robert and Ron

Regards

John

As Dave stated: In fact, I'm using the same valve as Lynn. What worried me though was the small port sizes on the valve. Maybe alright for a car or truck making 250 or 300 HP, but marginal for something making 400+?

Me too. I saw those lines and thought not for 500+hp.

RF Robert & Gordon, I know you probably don't want to tell me which valve you guys sell but is it the one mentioned by Lynn? Can you guys give me a quote on the cost of your system. Any pics?

Thanks,

Mark

Update:

I've settled on a Pollak. I found one that is for marine use. Stainless nipples and some kind of corrosion proof coating on a cast metal body. Of course "marine" = more dollars but it's a substantial looking item. Sorry I don't have the part number handy but if someone needs it, email me and I'll send it.

What size are the inlets and outlets on this model? I am concerned for my application having small return lines (5/16").

Thanks
Gary

Try this. http://www.pollakaftermarket.com/sc_app/default.asp?code=
#42-103
John

[ May 06, 2003: Message edited by: John Schneider ]

Mark
So I assume you are using two 3-port models. One for supply and one for return? Or are you using two pumps and check valves for supply?

Thanks
Gary

The marine model has 3/8" stainless fittings. They are glued, screwed and tatooed in so you don't have the option of AN is you so wish. I figured the returns won't have much pressure and a stainless screw clamp will do.