Theoretical question - how to plumb twin fuel tanks?

[JohnD]

Say, just for argument, that you wanted to fit twin fuel tanks.     How to link, fill and pump from them?

I've looked it up on Google and all the sites are boating ones that say, have a balance tube from one to t'other, running under the tanks - IMHO not the best idea in a car., that has nasty rough road under it, not a nice strong hull.

A balance tube that ran above the tanks would be fine, as long as it stayed full of fuel.    So one with a T in it with the pump on the upright would ensure that.   BUT, inevitably it would draw from one tank preferentially, as the resistance of each arm of the T will not be equal.

So, have the pump draw from one tank, that has no vent, so fuel is drawn from the other, which does have a vent, and the balance tube stays full - BUT then, you would not be able to fill both tanks.

So, both must have filler tubes, but the tank that the pump draws from must have a completely sealing cap on it.    Then fuel can be drawn until both are empty.

More complex solutions with dual pumps, but they need switching valves, one way vents, dual gauges etc. etc.     Any simple ideas that are better, please?

John

[thebrookster]

The easiest method is to have two have separate fillers and suction lines. Valve on each suction, to a T-piece. Then when one tank runs out, close that tank and open other side.

To run together has many hazards. A balance pipe is okay, except each tank must be same height, suction must be the same, then consider liquid movement (cornering). If you are consuming from both tanks same time and you lose suction on one, you lose suction!

For me, I would keep tanks and filling separate, and look at a solenoid valve to switch tank suction. Fit a swirl pot AFTER the valve, then you can switch on the fly.

Sucking from one tank through another is not recommended, the power required to suck with that line up would be huge. Nick is the best guy to advise on that one, but I can't think of any situation off hand that has tanks in series like that. Now, fit a booster pump between the tanks, and have second tank filling main tank, with overflow back to second tank. This would work, but then you need some method to stop the booster pump when the second tank hits low level. Also, unless you run some big pipes you will pressurise the primary tank!

But all the long distance and off-road setups I have ever seen all have separate fillers and manual valves on each tank. They simply stop and switch when required. Simple system, easy to fix, very little to break.

Phil

[mpbarrett]

I think the early V12 XJ6 (very thirsty)  had twin tanks with twin fillers and a switch on the dash to select which tank to use.
I had a friend who had one, he use to get ~12MPG around town so it really did need the twin tanks!

mike

[Nick Jones]

IMO About the only ways to acceptably run two tanks are

- As the boat folks suggest, with a low level link pipe (and high level vent link) which allow function as a single tank with two compartments. I agree that packaging that in an automotive context could be tricky (could be done in a Vitesse though)

- As Phil suggests, two separate tanks with separate fill and outlets with either a single pump and switchable valves or twin pumps. 
 

Other solutions exist with in-tank pumps transferring fuel between tanks as you might find on aircraft or bigger boats. Over complex in a car I’d say.  A sort of of example of this can be found in various Audi’s where packaging the Quattro equipment results in a very awkward shaped tank. The D2 A8 for example has a saddle tank. It has two in-tank pumps, but only one feeds the engine with the other just to send fuel over the saddle back. The stroke of design genius (!) is to have the gauge float on the side opposite the engine feed pump so if the transfer pump fails it’s entirely possible to run out of fuel with the gauge showing just under half a tank...... Apparently if you know what’s going on, a bit of violent slaloming may slosh enough fuel over the hump to get you to the next filling station ....... just don’t do it with a traffic officer watching..... (Story from A8 forum)

[Escadrille Ecosse]

1 hour ago, mpbarrett said:

I think the early V12 XJ6 (very thirsty)  had twin tanks with twin fillers and a switch on the dash to select which tank to use.
I had a friend who had one, he use to get ~12MPG around town so it really did need the twin tanks!

mike

This. A dentist friend of my dad had one back in the late '70s.

I would suggest that a car installation is very similar to a low wing single engine light aircraft.

I flew SA Bulldog T-1s with the University Air Squadron. They had twin fuel tanks and a large selector valve for off, left, right, both. Still remember one of the landing checks was to ensure the selector was to 'both'. Going clockwise from the the throttle - RPM max, mixture fully rich, flaps to inter, fuel to both, booster pumps on, parking brake off, harness tight, canopy latched.

The fuel selector is the on the floor in front of the left hand (lead pilot's) seat.

There was a diagram of the fuel system in the Pilot's Notes but I couldn't find on t'internet. The system on the DHC Chipmunk is very similar though but needs extra lever as it's tandem seating. The non-return valves prevent cross flow from on to the other when doing out of balance aerobatics or in the even of a leak in one tank

With a carburated engine there is enough time to switch tanks when you hear the engine miss slightly, assuming you haven't been keeping an eye on the fuel gauges (naughty naughty).

Found a picture on the web of one XX559 in Universities of Glasgow and Strathclyde markings which was one of the aircraft I flew.

 

[JohnD]

Gosh!    The U of Sideways Common Room delivers the goods, as always!  Knowledge, experience, ideas and concrete examples, what more could I ask (I'll think of something!)

Thank you all!

John

[Escadrille Ecosse]

John, if ever there was a man capable of thinking up interesting/unusual/off the wall questions it is your good self.

[JohnD]

Confession time - this is not for the Vitesse, but for the No.2 Project, the GT40.    I wrote about this in 2008(!) when Jon and I were still working on it, but then he emigrated, I started bulding engines for the Vit, and it rather stalled.    But in Lockdown, and with the latest Vit engine in limbo by my machine shop (their classic expert is part retired), I've gone back to the 40.    It looks like this now, but with a proper ROPS bar fitted:

It now has a handbrake, and a pedal box to be fitted shortly (I'll show you that when it's finished) and in an idle moment, I wondered about fuel tanks.    The original sites, in the sills, do NOT attract!   Far too dangerous, even if I lash out on bag tanks, as original.    The volume in the space frame between the radius arms will take 25L a side, not really enough for a thirsty Grand Tourer, but 50L would be fine, hence twin tanks.   There's room under the front of the engine, in the middle, for another 30L, not enough by itself, and would be a bit vulnerable, without a heavy plate to protect it, quote apart from the complexity of three tanks!

John

[Escadrille Ecosse]

Ooooh. Nice!

[morgan33]

If you’re looking for a simple way to link twin fuel tanks without complex switching valves and dual pumps, one effective approach is to use a balance tube running above the tanks with a properly placed vent system. Instead of relying on a single sealed tank, you could install a one-way check valve in the balance tube to prevent uneven draw while ensuring fuel can flow freely between the tanks. Additionally, using a Y-connector instead of a T in the fuel line may help equalize resistance from both tanks, reducing the tendency to draw from one preferentially. This setup is often seen in fuel tank marine applications, where maintaining a balanced fuel supply is crucial, but it can be adapted for rough road conditions by securing lines properly to avoid damage.

[JohnD]

Thank you, Morgan!

I've had a more recent discussion on here, with Paul who runs a Jaguar with twin tanks, some models of which had a solenoid changeover valve, before a single pump.  See: Jaguar XJ6 Series 1 - Page 4 - Other Marques - Sideways Technologies , so I have a plan.

You suggested a balance tube above the tanks.    That would require the tube to be full of fuel at all times to maintain the syphon, wouldn't it?     A low tank and some hearty cornering would uncover the bottom end of one or the other, and lose the syphon. wouldn't it?     Or had you something else in mind?

John

 

[Kiwifrog]

I used to own a Sumbeam Alpine series V Holbay with twin tanks, it just had a single low down balance tube between the tanks as per the attached age

 

[Escadrille Ecosse]

Elegant simplicity.

[RedRooster]

And a right bar steward to fit

3 hours ago, Kiwifrog said:

I used to own a Sumbeam Alpine series V Holbay with twin tanks, it just had a single low down balance tube between the tanks as per the attached age

 

And a right bar steward to fit

[JohnD]

Yes, but as I said previously, a balance tube under the car is just what I don't want.   Too vulnerable.

At the start of this thread, I mentioned the pedal box.   This took a lot longer to design and  fabricate than expected, as the best position for the pdeasl put the master cylinders right in the middle of a main strut of the space frame.    Here's my solution:

  

The two brackets above are for the remote reservoirs.

John

[Escadrille Ecosse]

1 minute ago, JohnD said:

Yes, but as I said previously, a balance tube under the car is just what I don't want.   Too vulnerable.

Are the tanks going to be that low?

[JohnD]

It's a GT40!  Low as low is!

[Escadrille Ecosse]

1 hour ago, JohnD said:

It's a GT40!  Low as low is!

True dat.

[PaulAA]

John

Strikes me that wholly independent tanks, either with a switchover solenoid ahead of a single pump or two separate pumps with non-return valves, would give you (i) built in redundancy, (ii) security in the event of contamination or damage, and (iii) more reliable information on contents(?)

But I'm just sitting in an armchair, waiting for a bit of rain to wash the remaining salt off the roads...

[Hamish]

I understood the gt40’s were initially 2 filler near independent tanks and later a single filler with the fuel tanks in the cills the broad section below the doors you sit on to slide in. 
there were 2 balance pipes on the single fill system one a quite large bore pipe that connected the 2 tank uprights for air escape/equalisation under the dash  ( the fillers were in the top of the front wings)

and a massive drain pipe to connect the 2 cill  tanks that ran under the front edge of the seats. Through the cabin !!

Think I would go with 2 separate tanks and engine feeds but being drawn down at the same time. No switching of tanks and the fuel gauge in one should be the same fore each side. 
and no big fuel pipes in the cabin.

[JohnD]

Thank you, Hamish!  Never seen those pics, where are they from, please?    But they describe why I don't want sill tanks or a balance tube!  And would the pump draw fuel equally?    The flow resistance from either side will not be the same, even if the hoses are equal length.

Paul, us simians think alike!

John

[Hamish]

https://www.gt40s.com/threads/mk2-balance-pipe-fuel-tanks.34919/
 

here you go John. 
but I’ve had a love of the gt40 since my dad got shafted by a “mate” for an original le man car. It was in the 1970’s and prices were relative. 
 

Dad asked said mate to have a look for him as he was going to buy it. Said mate bought the car for himself !!!!

thankfully sideways isn’t like that and Craig helped me buy the Alvis. 

[JohnD]

Thanks again, Hamish!   I thought I knew all the GT40 websites, but have missed this one.  Looks interesting, contemporary and with lots of builders!.

John

Edited March 20, 2025 by JohnD

[Escadrille Ecosse]

9 hours ago, JohnD said:

But they describe why I don't want sill tanks or a balance tube!  And would the pump draw fuel equally?    The flow resistance from either side will not be the same, even if the hoses are equal length.

The large bore balance tube effectively makes the two tanks into one while at the same time acting as a baffle to reduce fuel slosh on cornering. So a single pump is all thats needed to draw evenly from the two tanks.

Filling the sills with petrol does seem a little sub-optimal' for a road car but the big tube across the floor isn't really any different from the petrol tank in a Spitfire or any of the pre-TR7 TRs.

9 hours ago, JohnD said:

Paul, us simians think alike!

This isn't meant to be rude but this is just the reliability by duplication falacy. If one tank is filled with contaminated fuel, or suffering from internal corrosion then it's odds on that the other one is exactly the same. Unless you always fill them from different petrol stations and ensure that the fuel they're selling doesn't come from the same terminal (because odds on it does).

Similarly if you have two pumps each feeding from their own tank, how do you know if one has already failed and the other one is about to? Do you have a test/switching routine to check each pump/tank. Like they do on aircraft. And you can think of a few other common mode failure examples.

Essentially it's just a way of making things more complicated and expensive, and ironically often less reliable.

When NASA sent the astronauts to the moon the two most critical pieces of equiment (aside from basic breathing air) were the motor of the service module (to get the guys back from the moon and into earth orbit) and the lunar ascent motor. To get the lunar module off the moon and rendezvous with the command module. Just one of each.

Even the breathing air stuff was singled up. The only duplication there, just like safety relief valves on boilers, etc, was to allow for changing out consumables (filters) or sending stuff to get maintained (relief valves) where the equipment had to be able to keep running in the meantime.

Edited March 20, 2025 by Escadrille Ecosse

[JohnD]

No argument.   It was oxygen tank no.TWO that exploded on Apollo 13!

John