Fuel Expansion

[Nate]

Is there a way to calculate fuel expansion??



I'm building a aux fuel tank, and I'm wondering how much the fuel will expand when the tank gets hot??



I was planning on painting the tank black too.

 

[DaveN]

This isn't scientific, but off the top of my head if you plan for 3~5% this should give you plenty of room. If you want specifics, let me know and I can do the math for you.



Problem is - if you build the tank for X gallons, and you fill it to the top - it doesn't really matter...

 

[Nate]

I'm not sure what you mean, if I fill it to the top it wouldn't matter?

 

[cojhl2]

I'm not sure what you mean, if I fill it to the top it wouldn't matter?

I think what he means if it is completely full, and the fuel expands it's gonna come out the filler. It has no choice.



Unless you build an airpocket in it and vent that thru some kind of popoff valve I guess.



I'm gonna keep on this thread,, methinks there is an education waiting!!

 

[Spooled-up]

I had to take this into account when I invented the vent kit. Scroll to the bottom of this page and the thermal expansion formula is there.



Here's the quote if you don't feel like scrolling:



THERMAL EXPANSION

Like all liquids, diesel fuel slightly expands in volume as its temperature increases. The coefficient of thermal expansion measures the rate of the expansion. A typical value of the coefficient of thermal expansion for diesel fuel is 0. 00046 per degree Fahrenheit. Using this value, 1. 000 gallon of diesel fuel at 20°F will expand to 1. 037 gal-lons at 100°F.



I calculated that if I add 35 gallons of fuel in my tank at 50 degrees F, and park my truck in the desert sun until the fuel heats to 100 degrees F, that I will have expanded the fuel by . 8 gallons.



50 degrees temp increase

x

. 00046 expansion coefficient

_________________________

. 023 expansion



Then take . 023 X 35 gallons and you get . 805 gallons

 

[cojhl2]

So what this means, ignoring the increase in tank volume due to expansion of the tank itself, you should not fill the tank to more than 0. 805 gal from the top.



However, if again ignoring tank volume change, filling the tank to the top of the filler will cause overflow as the liquid warms up.



Back to Nate's statement?



So it seems this info is important when designing and filling a storage tank where there is some control to space at the top of the tank,, but on vehicles we just fill to the point of overflow.

 

[steved]

On a side note, mainly because I'm to lazy to go crawl under my truck right now... but if you actually "fill" a non-vented OE tank (by non-vented I mean no aftermarket vent kit), there should still be enough air space trapped in the tank to allow expansion correct??



My tank setup is capable of filling the main tank completely into the filler neck, nut I am only adding fuel to the neck and also have a vent line back to the aux cell. I was thinking about this the other day... and figured I would watch this thread... I would hate to have almost 38 gallons of fuel barf out onto the ground from a split tank...



steved

 

[Spooled-up]

So what this means, ignoring the increase in tank volume due to expansion of the tank itself, you should not fill the tank to more than 0. 805 gal from the top.



However, if again ignoring tank volume change, filling the tank to the top of the filler will cause overflow as the liquid warms up.

there should still be enough air space trapped in the tank to allow expansion correct??

You're correct, and I think the reason I've never had a drop leak is that the tank volume does indeed expand by the sides swelling. It's either that or all of the many air pockets that can be left at the top of the tank that the vent kit can't even vent.



Here's the inside of one:



[/IMG]#ad

 

[cojhl2]

Air pockets can't let that work the way intuition leads one to believe.



When the tank is filled the airpockets already are prepressured to what ever the head is on the filler neck. So if the fuel expands,, and the tank does not then out the filler it comes.

 

[JFlagg]

Hey Spooled up, is that really what the insides of our tanks look like? I think I've seen that pic before and it had Sigourney Weaver crawling around in it! :-laf

 

[brods]

Steved, if you have the stock fuel cap then you should be ok. The stock cap has both a vacuum relief and a pressure relief valve built in. The excess pressure would be relieved by fuel flowing out the cap.



cojhl2, that is why compressing the air would work. The air has to compress to a high enough pressure to overcome the pressure relief valve in the cap before fuel would come out.



If you want I can take some pictures of the cap I took apart. I removed the vacuum relief spring on my cap and then just plugged the rollover vent fittings on the tank.



Nice pict Spooled-up, is that a long bed tank or shorty?

 

[Nate]

My idea for the aux tank was to just use a vented cap.



steved you tied the vent for the main tank to the vent of the aux tank?? How did you go about doing that?





I'm hoping to prevent any bugs in my system before I actually build the tank. I was planning on painting the tank black, but I may do it in red instead (truck is red)

 

[steved]

My idea for the aux tank was to just use a vented cap.



steved you tied the vent for the main tank to the vent of the aux tank?? How did you go about doing that?





I'm hoping to prevent any bugs in my system before I actually build the tank. I was planning on painting the tank black, but I may do it in red instead (truck is red)

I decided to use a 22-gallon fuel cell from Summit Racing (an RCI cell) that has a vent in it...



I did this for two reasons... two "supposedly" sealed systems would allow air displaced from the main tank (from filling it with a pump) to replace the volume of fuel I just pumped out of the cell... essentially cancelling any possible overpressure scenario (I had envisioned a 14psi pump potentially splitting that plastic tank under that kind of pressure). Secondly, if I happened to overfill the tank, the excess fuel would simply be pushed back up the "vent" line to the cell... thereby preventing it from pushing fuel out some unknown orifice in the tank...



What I found is that the vent is almost not needed... although the overfill prevention part is cool and does work, if this had been a gravity feed only type setup (no pump), it would not be needed, as long as the cell is vented to atmosphere.



The main tank pulls a lot of vaccum on the cell via the vent and would cause it to partially collapse... requiring me to vent the cell as well. It would definately pull the fuel out of the cell, and does continue to siphon after I shut the pump off.



To date I have ran arounf 200-gallons through it without one spilled drop... as for the expansion thing, I guess it is a moot point since it can alway expand back into my cell as well as the main tank...



steved

 

[steved]

Air pockets can't let that work the way intuition leads one to believe.



When the tank is filled the airpockets already are prepressured to what ever the head is on the filler neck. So if the fuel expands,, and the tank does not then out the filler it comes.

But air can still compress a lot, unlike a fluid.



steved

 

[steved]

On a side note, if you use a pump to transfer, you cannot simply watch your fuel guage... mine did this again last night.



I know my pump runs about 1 gallon/minute, therefore in 22 minutes, it should drain the cell. I was at around 3/8 tank last night and decided that I had better get some fuel while it is relatively cheap at $2. 55/gallon. So I flipped the valve for the ride home and proceded to pump from the cell to the tank.



The guage quite moving at around 5/8 tank, and I could hear the pump "cavitating" in my CB. So I shut it off thinking to myself that it should be almost full.



Drove another 35 miles to home and the station before topping off... I always pump through the cell even though it takes a little longer. The main tank was completely full and upon stopping the guage had started to read full!!!



Go figure that one... the guage seems to be on a delay!!



steved

 

[Spooled-up]

Nice pict Spooled-up, is that a long bed tank or shorty?

Yup, it's a pic from inside my '07 Mega Cab short bed tank. It's a 34 gallon as opposed to the 35 gallon long bed tank. I managed to fill it with over 36 gallons of fuel after the vent mod was done. They obviously don't include the reserve in the fuel capacity.

Hey Spooled up, is that really what the insides of our tanks look like? I think I've seen that pic before and it had Sigourney Weaver crawling around in it!

:-laf :-laf :-laf

 

[Nate]

Good info.



Where did you tie the aux tank into the main tank? Someone told me something about there being an "aux" port on the top of the tank??



Other setups I've seen people have tapped into the vent line on the fuel fill.

I decided to use a 22-gallon fuel cell from Summit Racing (an RCI cell) that has a vent in it...



I did this for two reasons... two "supposedly" sealed systems would allow air displaced from the main tank (from filling it with a pump) to replace the volume of fuel I just pumped out of the cell... essentially cancelling any possible overpressure scenario (I had envisioned a 14psi pump potentially splitting that plastic tank under that kind of pressure). Secondly, if I happened to overfill the tank, the excess fuel would simply be pushed back up the "vent" line to the cell... thereby preventing it from pushing fuel out some unknown orifice in the tank...



What I found is that the vent is almost not needed... although the overfill prevention part is cool and does work, if this had been a gravity feed only type setup (no pump), it would not be needed, as long as the cell is vented to atmosphere.



The main tank pulls a lot of vaccum on the cell via the vent and would cause it to partially collapse... requiring me to vent the cell as well. It would definately pull the fuel out of the cell, and does continue to siphon after I shut the pump off.



To date I have ran arounf 200-gallons through it without one spilled drop... as for the expansion thing, I guess it is a moot point since it can alway expand back into my cell as well as the main tank...



steved

 

[steved]

Good info.



Where did you tie the aux tank into the main tank? Someone told me something about there being an "aux" port on the top of the tank??



Other setups I've seen people have tapped into the vent line on the fuel fill.

On the 3rd gens, there is no port on top of the tank... that must be a 1st or 2nd gen thing... I looked for one and there are only the two 1/4" barbs for the rollover.



Also on 3rd gens, there must be a check valve in the tank at some point as if you try to put fuel in the vent on the main tank, it will flow out the top and then barf fuel out the fill (with the cap off) as it builds air pressure (since the tank can't vent)...



Because I had issues putting fuel into the vent hose, I pulled the return manifold from my FASS, drilled it, tapped it, installed a 3/8" hose barb, and reinstalled. Putting fuel into the fill line eliminated the "barfing fuel" problem.



steved

 

[brods]

Yup, it's a pic from inside my '07 Mega Cab short bed tank. It's a 34 gallon as opposed to the 35 gallon long bed tank. I managed to fill it with over 36 gallons of fuel after the vent mod was done. They obviously don't include the reserve in the fuel capacity.







:-laf :-laf :-laf

It appears the fuel tanks and fuel caps are the same for gassers, where the gasoline vapors need to be contained for pollution controls. So maybe the extra air space in the tank is needed due to the higher volatility of gasoline and DCX saw no reason to modify the tank to hold an extra couple of gallons when its used with the diesel?

 

[DiTrani]

FYI:



THERMAL EXPANSION

Like all liquids, diesel fuel slightly expands in volume as its temperature increases. The coefficient of thermal expansion measures the rate of the expansion. A typical value of the coefficient of thermal expansion for diesel fuel is 0. 00046 per degree Fahrenheit. Using this value, 1. 000 gallon of diesel fuel at 20°F will expand to 1. 037 gal-lons at 100°F.