Vapor/Air Mixture in Fuel Tanks

Modern fuel tanks are actually under higher than atmospheric pressure.

The amount of fuel in the tank is unimportant.

Don't worry.

Thanks Lyn. That's what I suspected, but I've not been able to tell whether the 'whoosh' on opening the gas cap was due to pressure or to partial vacuum in the tank.

Now I still don't know what portion of the pressure is due to air and what portion is due to fuel vapors. I presume the pressure is created by pumping air into the tank. Is that so? I don't know how else you would pressurize the tank...

I think it all depends on the temperature. This is based on my observation of my 5 gal plastic gas container for my lawn mower. When it is hot outside, the tank expands. When I take the cap off, pressure whooshes out. When it's cold outside the tank is actually under a vacuum, which is observable. The walls of the tank are sucked in. When I remove the cap, air rushes in. I would assume that automotive gas caps are designed to allow air both in, and out, to prevent structural damage to the tank.

I have heard that it's better to fill up in the cooler morning hours. I don't know if that's true.........well it probably is true, but pretty insignificant. I just fill up when I get low. It's been working for years.

There may be some merit to not letting your tank get low because doing so would create more room for evaporated gas fumes............but again, probably not significant enough to worry about.

Wait a second. Modern fuel tanks operate above atmospheric?

I have no idea how the modern stuff works. The older evap canisters were tied into a vacuum line, pulling a vacuum on the tank to prevent the release of vapors into the atmosphere.

Based on my old school knowledge, I would have to amend my previous post, and say that gas caps are should be designed to allow air in, but not out.

I don't know how else to get the vapors to the canister without a vacuum.

But, I'm not familiar with the new fangled stuff.

I've noticed the pressure in my gas can as well. My understanding is that the molecules will leave the liquid gasoline until the number is the air space above reaches a concentration so that equilibrium is reached, i.e., the same number are leaving and reentering the liquid. This partial pressure of each constituent of the liquid (septane, octane, etc.) is a function of temperature, and these pressures add to the 14.7 psi of the air already present. So when you put gasoline in the can and close it up, pressure builds up.

Yep. And since it's completely enclosed, when the temp gets cool, the gas contracts, creating a vacuum.

This is an interesting read. It looks like it is a good idea to fill up when it's cooler, if possible.

It is my understanding that, at least here in Arizona, gasoline dispensers have a compensation device to protect the retailer from any loss due to contraction of fuel, but no device to protect the consumer from hot dispensing of fuel.

The oil companies say it would cost too much to "modify" their pumps to protect the consumer. Go figure.

The storage tanks over here are buried deep, 2 meters or more, so they don't change their temperature much over the whole year.

Same here! If I'm not mistaken, most underground fuel tanks here are double-walled, so if the inner tank leaks, the outer one will trap the leaked fuel. The gap between the tanks has got to insulate the inner one significantly, making it even harder for the fuel to be heated by the ground.

The concern then becomes the temperature of the fuel at delivery time. I have no idea whether fuel delivery trucks have double-walled tanks, but I suspect not. I presume that any intelligent trucking company would avoid afternoon delivery in hot weather, just to keep the pressure down, but then that adjective may not always apply.

Yeah. The formula is given in the link above that the wholesalers use for hot gas, ( above 60°F).

There used to be a sticker on the pumps too, here in NC, that gave the breakdown of the gas price, and how much tax was going to the state and federal government. It was substantial. I checked the other day...............they're gone.

If I remember correctly, the last time I checked, it added up to about .60 per gallon.

Here we go. As of 2009.

http://en.wikipedia.org/wiki/Fuel_taxes_in_the_United_States

I'm ready for a workable free energy machine to show up on here.

_{I'm going to regret saying that.}

the whoose you hear is from the vacume formed in the tank, this is a safety feature so if there is a small punture as in an accident the fuel stays in the tanks

It is not pressurised ( sorry Lyn) if it was and the tank split there would be fuel every where in seconds

No need to apologize.

It is my understanding that MODERN fuel caps do not allow fuel vapors to vent to atmosphere. They do allow small amounts of extrenal air into the tank so that it does not collapse. I believe that the caps are tested, under pressure, during some states emissions tests.

This will not be the first time if I am proven wrong. It will not be the last time, either.

You are right.

In fact, older cars with the wrong cap (no hole) can actually collapse the tank......this does not happen with modern tanks EXCEPT if something is wrong!!

So hearing a "whoosh" of air INTO the tank should ring warning bells for anyone hearing it....that is a possible problem about to happen.....

There could possibly be a slight whoosh of air leaving the tank on opening could easily happen on a warm day.......but only slightly.....

Andy, it sounds like you are contradicting yourself! Still 'ere's point about fuel spraying out of a pressurized and perforated tank is a VERY valid one. That makes me believe there must normally be a partial vacuum in the fuel tank. If that is true, you would expect a small 'whoosh' on opening the tank (I do hear it frequently on my Passat), and don't think that is any kind of danger warning.

Now it's true that a parked car, especially one parked for some time in the sun, as mine commonly is, could develop some pressure in the tank. But I always drive at least to the gas station (≈2km to the closest one) before removing the cap. I assume that in that distance, the fuel system will have normalized the pressure.

BTW, I still have no information on my original question: What is the (range of) fuel vapor/air ratio(s) in the space above the liquid fuel in an automotive fuel tank?

Of course if the tank is under partial vacuum, then opening the cap will only let air in, not let fuel vapor out. In that case any fuel recovered by the gas station's vapor recovery system will depend on the fueling time, so a rapid fueling would be desirable (contrary to another point of the original email).

I'll just remind you that atmospheric pressure of 14.7PSI must be overcome before anything would be vented. The higher pressure to which I referred was something on the order of +.25 psi.

The tank is not pressurized to enhance fuel flow. That happens inside the fuel delivery system. I'm talking about pressure in the fuel storage system. (Tank)

The concentration of fuel vapors is dependent on the gas laws.

In a modern car, the way the tank is organised with the charcoal filter means that you will have relatively little pressure of vacuum.

The atmosphere inside the tank will usually be overwhelming petrol vapour, so much so that it won't even burn until mixed with more air.

Years ago the tank "Sender" was an open (and sparking!) rheostat attached to a float, inside the tank!!!!! Linked to ground at one end.....or the metal tank I should say......it looked dangerous, but wasn't. Sometimes it was in the gas and sometimes under the petrol when full....

What they have today I have no idea.....

Underground storage tanks stay pretty much at the same temperature day and night so the time of filling a tank is unimportant.

If much vapor is generated during tank filling, it is sucked back into the fueling system. But we are talking very little liquid volume when the vapor condenses.

Modern tank systems are vented out through the charcoal filter and that filter is also connected to the fuel metering system and is regularly purged into the engine.

Fuel tank caps only allow air in. In fact, in California, we now have a test where they close off the line to the charcoal filter and they put a pressure in the gas tank through a special cap. The tank system has to hold pressure for a certain amount of time or the vehicle will not pass the smog test.

I definitely agree with your first statement, and already said so to the source of the email in question. I have an underground wine cellar, which unfortunately I have essentially abandoned due to excessive moisture and actually flowing water in the winter. I left a max/min thermometer in there for many years, and it never got above 65°F nor below 55°F. (northern California, shaded by trees in the summer)

I also agree with #2, assuming it is talking about the gas station tank.

I'm ignorant about #3. I assume that this one is talking about the vehicle tank. I gather there are two lines connecting the tank to the engine, one for liquid and one for vapor. Where is this carbon filter? If, as Lyn indicated, the vehicle tank is somewhat pressurized, then some gas (gaseous material, not liquid) must be pumped into the top of the tank. I can imagine that gas to be air, although that might make an explosive mixture in the 'empty' part of the tank. I can also imagine cooled exhaust gas to be used for pressurizing, as it would not lead to an explosive mixture. I'm still anxious to be further educated.

Thanks!

Dick

GA for a great post....thats exactly how it works!!!

It also saves us money by not blowing of huge amounts of fuel when its hot from millions of cars!!!

I usually fill my tank up when it gets down to around one-half tank as I believe the fuel pump is submerged in the fuel to keep it cool. Your fuel pump may not last as long if you continually wait until you are near empty to refuel.