diesel thermal expansion - one more time?

[crazy paul]

I know this has been discussed more or less ad nauseum in the past, but please bear with me. I was posting in the French Quarter (Quebec discussion group)...expounding on the advantages of the Ventectomy, and how diesel will not expand like gas. Well, a gas station attendent came back and said the correction factors they used for diesel and gas were very close.

Well, after a little research on the web, I came up with the following (sorry about the metric Yanks...)
Coefficient of thermal expansion/degree Celcius:
Gas=.000945
Diesel=.000824

Therefore, for a 55 litre fillup (14.5 US Gal), assuming a 15 C (27F) degree change in temperature(ie: from 15 in the ground to 30 in the tank some time later), the gas would expand to 55.78 litres and the diesel to 55.68 litres. That is only .1 of a litre difference (.21 oz). /images/graemlins/shocked.gif

If these numbers are correct, and with all of these Ventectominers filling up to the cap, why isn't there any anecdotal evidence of fuel expansion messes? /images/graemlins/confused.gif

Please no flaming, just polite, adult discussion...

 

[Ted_Grozier]

Your numbers are probably correct but there is still air in the tank after fillup -- you can hear the fuel sloshing around.

 

[tadc]

It's not just about the expansion of the liquid fuel.. it's also about the vapor pressure of the fuel. Gas=lots, diesel=almost nil.

 

[Paul_]

It's a simple physics problem so clearly no one has had the combination of a 15 deg C rise in a tank with less than .68 L space remaining. Plus the tank will swell slightly under the hydrostatic pressure. Is there a pressure relief valve anywhere in the system?

 

[weedeater]

It's open to the outside when the cap is put on.

 

[dieselimblut]

Can you say...... Molitof Cocktail? Oh yah, thats why the cap is now attached to the filler. Besides, when I fill, I usually drive somewhere using up any negative expansion space

Isnt Physics fun???

The subject matter of this reply is not necessarily the expressed views of the writer

 

[Brock_from_WI]

I can tell you I have stored gasoline for quite some time and almost no matter what you do the plastic tanks swell up and bulge and they always vent lots of fumes when you open them. Now that I have had diesel, I never see this happening, at all, nothing. I usually squeeze the plastic tanks, then close them. The gas is bulged out within a month, while the diesel is still squeezed in after a month.

So I think it has to do with what tadc said. Gasoline turns to a vapor really easily and diesel just doesn’t do that on it’s own. I can tell you being bored I have filled my tank to the brim and cleaned my windshield, then added more, then cleaned the inside, then added more, until it wouldn’t take any more. I have never had a problem.

 

[jck66]

[ QUOTE ]
crazy paul said:
If these numbers are correct, and with all of these Ventectominers filling up to the cap, why isn't there any anecdotal evidence of fuel expansion messes? /images/graemlins/confused.gif

[/ QUOTE ]

Keep doing the math and you'll see why it has not happened yet. Forgive my discomfort with the metric system - 0.68 liters of fuel is around 0.2 gallons, and at roughly 50 mpg, you only have to go 10 miles to vacate that space.

Plus, I think there's still space in the tank even after venting. One time I filled "completely", drove a mile home from the station, and opened the tank to check. I couldn't see any liquid in the filler neck anymore.

Just my two cents... /images/graemlins/smile.gif

 

[Drivbiwire]

It's not an issue here is why...

You have a large mass of fuel that is being pumped into your tank probably at an average temperature perhaps warmer than ambient if it's stored under ground, meaning it's going to cool once it gets into your tank reducing the volume slightly. Also since your are going to use some fuel driving home from the station you will continue to increase the amount of usable space in the tank as soon as you drive away. How fast will the fuel change temp? not very again we are dealing with a substantial mass, probably decreasing in volume considerably faster than it's increasing due to cuel usage.

The only time that fuel expansion has ever been an issue for me is when I land after cruising at 39,000 feet with 25,000#'s of cold soaked fuel then have the wings topped off with warm fuel. The expansion will cause fuel to run out the vents ONLY if I am on the ground for more than 2 hours and even then we are talking about a drip or two initally... never more than a steady drip at most. we are talking about a tank capacity of about 52,000 gallons /images/graemlins/eek.gif

Worrying about venting fuel over in a TDI is not even worth the time, it just won't happen with the small volumes we are dealing with.

Like what was mentioned, the system has places that fuel can expand into, you never use 100% of the total volume of the tank no matter how hard you try to top it off. I believe ther is a way to vent even more air, but this requires driving around in circles to slosh the air to the top, then finishing the top off, even with this trick it only gets you another 1/2 gallon or so.

DB

 

[MrMopar]

The only time I ever worried about fuel expansion is when I would completly fill up my fuel tank in my diesel pickup on long trips. I used to take a small ramp that I would store in the bed of my Dodge Ram and when I was at the filling station I would park the left-rear wheel on top of this ramp so that part of the truck would sit about 5 inches higher than the other 3 wheels. Doing this, I could fit an extra 4.5 gallons of fuel in the tank and there was minimal air space left over after that. I would do this on long trips because that meant about an extra 80 or 90 miles of driving before I'd have to fuel up again. Later on I bought a 100 gallon fuel cube to put in the bed flush up against the cab and plumbed in into the fuel system as a second tank. After that I could pretty much go coast to coast without fillups so I never worried about topping off anymore.

 

[crazy paul]

OK.....I can buy the fact that diesel is much less volatile than gas. Volatility meaning the vapour pressure exerted by the product. From my research on the web, typically, gas exerts around 8 psi, while diesel only .0074.

That explains why a sealed plastic jerry can full of gas is all bloated out after a short time, while another plastic can of diesel is not.

...but it does not explain what appears to be an urban legend about liquid gas expanding much more than diesel. The facts seem to indicate that these two liquides expand at very similar rates.

In other words, a full tank of gas should not any more expand out of the filler pipe than one full of diesel. /images/graemlins/wink.gif

Just want to try and set the record straight.

 

[tadc]

[ QUOTE ]
crazy paul said:
OK.....I can buy the fact that diesel is much less volatile than gas. Volatility meaning the vapour pressure exerted by the product. From my research on the web, typically, gas exerts around 8 psi, while diesel only .0074.

That explains why a sealed plastic jerry can full of gas is all bloated out after a short time, while another plastic can of diesel is not.

...but it does not explain what appears to be an urban legend about liquid gas expanding much more than diesel. The facts seem to indicate that these two liquides expand at very similar rates.

In other words, a full tank of gas should not any more expand out of the filler pipe than one full of diesel. /images/graemlins/wink.gif

[/ QUOTE ] I think you're still missing something here. If the filler pipe is full of gasoline, the "air" space trapped in the tank will become pressurized by the vapor pressure (moreso as it warms) and perhaps force fuel out of the filler neck via the cap seal. If my physics are right, the "fuller" the tank (less airspace), the more force the vapor pressure will exert. So, the tank is designed to allow a fairly large airspace to remain, so the vapor pressure will not exert as much force to eject fuel from the filler neck.

On a diesel, there is negligible vapor pressure so this isn't a problem.

 

[jck66]

Vapor pressure is a function of temperature ONLY for a given material. Headspace is immaterial. This may not seem intuitive but it is true. The vapor pressure is the force.

And unless you have some funky narrow geometry in your plumbing, the headspace should always be at the filler neck. You'd have to be a pretty dumb automotive engineer to design a fuel tank that could eject fuel from the filler neck due to thermal expansion. /images/graemlins/rolleyes.gif

 

[Dean_S]

I remember seeing a diesel fuel tank on a truck leaking from the filler cap as things warmed up in the sun. That was in the 60's and it was a steel tank. I asked about it and was told that it was not the first time. And there were no things like air capture pockets in the tanks then to defeat ones ability to easily fill the tank competely.

With a plastic tank, the tank it self will expand with rising temperatures. So the tank will get bigger to some extent as the fuel expands. So some things of this nature may not occur with plastic tanks that might otherwise with steel tanks.

 

[crazy paul]

[ QUOTE ]
RiceEater....
It's this higher pressure of the gas that forces the soda pop out of the bottle.


jck66....
The vapor pressure is the force.


[/ QUOTE ]

Thanks boys. I think we have put this one to bed.

So, lets give this a more formalized look.....

Diesel`s thermal expansion is very similar to gas, but because of its very low vapour pressure, is not as suceptible to overflow given high temperatures /images/graemlins/grin.gif

Lets hear it for teamwork!!

 

[Deception]

I vent every time I fill up, to the point where fuel is visible from the filler. Never had a single issue about fuel expansion, ever. Even in the summer.

Don't worry about it. /images/graemlins/laugh.gif

 

[Reidler]

Thanks Deception!

Glad to hear that. You live in the same environment as me so I'll take that for what it worth. I used to only add about 5 liters after venting but next time i'm going to "Fill-er-up"! /images/graemlins/eek.gif /images/graemlins/grin.gif

Gary

 

[Deception]

No problem! Venting is really handy as it lets you stay away from the station for a few extra days. /images/graemlins/smile.gif

I read a whole bunch of threads about diesel fuel expansion a while back and one user topped up his TDI and parked right under the California sun for a few hours afterwards, and no fuel leaked or expanded out.

 

[Paul_]

The fuel cap seems to have pressure relief vent like any other car - my guess is it's set to relieve at less than 3 psi, and in any vacuum condition. I don't see any open vent including the one below the cap, please correct me if I'm wrong.

The vapor pressure is *not* relevant to the original question as it's relatively low, and the same for any given temperature at *any* fill level. The tank is clearly designed to withstand it up to the cap relief pressure.

The issue is the change in volume of the *liquid* only, due to temperature changes. If the tank was 100% full (nearly impossible in practice,) the tank would have no choice but to simply follow the fuel volume change since it has a much lower modulus of elasticity.

In practice, I don't think the tank would have any trouble surviving this over practical temperature changes.

Lastly, surely the design purpose of the expansion volume in the tank is *soley* to contain the foam and therefore reduce the chance of it spilling out of the neck before the pump clicks off? My '82 Rabbit would do that if I wasn't careful.
Much less space would be needed if the reason was fuel expansion.

 

[AVE_ENG]

[ QUOTE ]
Drivbiwire said:
It's not an issue here is why...

You have a large mass of fuel that is being pumped into your tank probably at an average temperature perhaps warmer than ambient if it's stored under ground, meaning it's going to cool once it gets into your tank reducing the volume slightly. ....


[/ QUOTE ]

Warmer fuel in the ground? I think that depends on your geography. In most places that experience seasonal temperature changes, I would expect the fuel inground to be cooler than the ambient and hardly ever warmer than the air.

 

[scooperhsd]

Down here in North Carolina ? - Yeah I can believe that the fuel in the ground can be warmer than ambient - they only put waterlines down 6 inches and don't have a freeze problem. It would be more accurate to say that the fuel in the ground doesn't have the same extremes of temperature that ambient does.