Fuel evaporation rate and vented tanks?

Has anyone studied the rate of evaporation of what we call gasoline today?

The other night I took the top off a carb to adjust the floats. Left it overnight to finish the next day. More than half the fuel was gone.

A few months ago I put 3 gallons of gas in a project car's dead-empty and recently cleaned gas tank. The car has been started about a dozen times and run around the block about a half dozen times, a total of less than 4-5 miles driving. Last night it ran out of gas on a test run. The car has a vented cap and I'm thinking that quite a bit may have evaporated away.

Another question is how do I vent a tank in such a way to slow/stop evaporation of fuel?

 

You don't happen to live in the Bronx, do ya?

Half a tank of gas gone overnight? Certainly cannot be due to evaporation. If it is, I recommend you refrain from striking a match in your garage!

 

No. I suggest you re-read the original post. Half a carb bowl lost overnight. Couple of gallons lost over months.

 

I have a 66 Pontiac junk around car, I never keep much gas in it because it sits outside all the time. If i do not mess with it for a couple months it will be bone dry, 1-3 gallons. It has been especially rough this year its been extreamly hot here this year and the gas seems to disapear quicker than it has in previous years.

 

I had the same quandry this spring. I split a 5 gallon jug between a 51 chevy truck and a 54 210...same deal, ran them both for a total of 15-20 minutes each over the winter...this spring both were dry and of course checking the fuel level was the last thing I thought of trying to get them started!!
Some think I got syphoned but I highly doubt it...

 

I've been considering putting a charcoal canister on the fuel system of one of my old trucks, just to see what happens. Thats the one truck that has a newer tank with an unvented cap and a separate vent tube coming off the tank.

I think you're on to something. Car makers figured it out in the early 70s, but only after the feds made it a law.

 

Aren't gas vapours heavier than air? If that is the case the gas would evaporate until it filled the empty space in the tank and then stop because it would not rise into the air at any significant rate especially through a tiny vent hole. Now if it was hot out those gases would continue to expand and force thenselves out of the tank increasing the amount lost. Keep your tank full and there is almost no room for the evaporation to occur put 1 gallon in and you have a huge empty space to fill up with evaporating gas vapour.

 

The fuel still evaporates the filter is just supposed to limit the amount of hydrocarbons that reach the environment from the evaporation. I don't believe it affects the performance much other then added weight but it is not going to stop your fuel from evaporating.

I suppose that if one was worried about it one could put a non vented cap on a ventless tank over night and then swap the cap out when the vehicle is in use, then the only time that evaporation would be a real problem would be when the car was running. You still may loose the same amount as vaopr when you open the cap but if you park in the shade it will be less.

 

My Son Don doesn't get to use his T bucket much because of his work schedule and one time he called me and asked if I had siphoned some gas out of the tank of it for some reason. I told him no and he said the last time he drove it he filled it up and parked it and now the tank was almost empty when he went to use it today. It had been parked for a few months. I got sort of pissed because I felt he was accusing either Dan or me of taking it.

Then one tiime later I went to use my roadster and although I usually drive it often this time it had sat for maybe a month. My tank was also lower than I ever keep it and told him I think we both were victims of evaporation. Our shop is usually in the high 90's every day and even at night gets only down into the high 80's, so there is sure enough heat to cause that to happen.

This thread kind of confirms what I suspected. As for maybe putting a tight cap on the tank while stored, I think that might cause some problems too. I have two of those plastic 5 gallon gas cans in the shop and they are swollen to about 7 gallon size due to the fumes expanding in the tightly sealed can. Possibly there would be enough pressure generated to force gas out of the fuel line and into the carb if the tank were capped tight.

Don

 

uh....fuel IS hydrocarbons. The canister traps the fuel vapor (hydrocarbons) and the car burns it the next time you drive when it "purges" the canister.

It's pretty obvious to someone with a garage that late model cars don't stink like gas, but older cars do.

 

Your post wasn't clear about that. But OK, you lost a few ounces overnight. That sounds normal to me.

 

I've got an older GMC motorhome that's always hard to get started after sitting a while. I found that the fuel in the carb evaporates through the vents. Somewhere I read that the new fuel does evaporate much faster than the old stuff. A non vented cap won't stop it since the carb vents are open.

J.

 

My thoughts were that a restricted vent-- one that would blow open when pressurized in either direction--- rather than a free open vent might minimize losses. I'm thinking that freely vented systems would tend to lose more.

Question is how/where do you find such a restrictor? Maybe just a small diameter hose?

 

YES!
The new formulations of automotive fuel definately evaporate much quicker today,
But they don't leave as much varnish behind if that helps.(specific gravity )
I've been considering some type gizmo for the bowl vent for awhile, maybe hooked to a charcoal canister and the tank.
This is a very real issue now and more so in the future, stay ahead of the curve of the Greenbergs

 

Ha I got an idea, there are 100,000+ of us, lets buy our own refinery and pruduce and sell fuel in a coop. There is bound to be a loophole that we can hit that will allow us to use gasoline instead of a blend.

 

porknbeaner I like the way you think.

 

Loophole=Lawyers


Modern cars have sealed fuel systems, all we have to do is seal our systems.

 

Non ethanol gas avsilsbility is catching on big time here in my small GA town. We have maybe a half dozen stations selling a non ethanol 90 octane instead of premium in some cases and in others it is in addition to premium.
Not sure of any effect on evaporation rate, however, just wish it had a little more octane. Wondering about blending it with 100 low lead aviation fuel for my hot rod.
Dave

 

Considering what we pay for gas, the evaporation losses are worse than my dad's liquor bottles when I was a teen.

 

Well at least you claimed evaporation instead of watering it down!

 

My kid brother pulled that stunt once and Dad was more pissed off that he ruined what was left of the booze than that he drank some of it!

 

Mine too, dad kept leaving the caps loose.
Then for punishment he bought a case of beer to store in my closet.

 

I plan to add a charcoal canister of some sort to my next car when I get to that point - let us know if you find a good, easily adapted one. I can't stand that smell anymore.

I think Pork'n'beaners point is that you still "lose" the gas vapor, only instead of going into your garage air it goes into your engine's air stream the next time you start it? They usually vent to a vacuum line hooked to your motor. But I can see how the restriction of the charcoal filter might actually slow the evaporation in the first place?

 

I actually work on very modern OT vehicles and it is a requirement that modern vehicles capture all fuel vapors from evaporation and burn them. You still have to be able to vent the tank since the liquid fuel that leaves has to be replaced by outside air. It is all very controlled and the Feds require all automakers to be able to detect a leak in the system of a hole that is less than .020" in size and turn on the "check engine" light.

You read that right... less than 20 thou of an inch.

That is why you don't smell gas fumes in your garage when you keep you OT car in there.

And Yes... today's fuel evapoates at a much higher rate than it did 20-30 years ago. It can because the modern fuel systems are sealed when shut off.

Adapting a modern evap system to a hot rod is complicated. Take a look at Chevy's crate engine kit (E-Rod) if you want to see how it is done. Uses an engine control module. I don't see adapting something similar to a carbed engine very easily.

 

How about adapting a 1970s evap system? They were pretty simple...and they worked with carbs.

 

You are right, it did.... but the early ones simply sucked fumes into the engine from the tank when running and still used a vented cap. Once the vented cap went a way (about the time of ECMs) then it became a whole different game. Again... you need to seal the tank when sitting but still allow air in when fuel is burned off while running. You also need to allow pressure to escape when the tank and fuel get hot while not running (swelled gas can mentioned earlier). I am telling you.. It isn't easy and I have thought about it. I ain't saying it is impossible, just not as easy as it sounds at first.

Sucking the fumes in while running (while being slightly more eco-friendly) won't really help your evaporation issue while parked.

Probably one of the easiest things you could do is adapt a sealed cap and a seperate vent hose. If you know the car is going to sit for a while, plug the hose and seal it up. I would suggest putting a note on the inside of the windshield to remind yourself to unplug it before driving or it could shut off or collapse the tank.

My gasser has a sealed cap on the Moon tank and I plug the vent hose because the alky will evaporate in a couple days if I don't. But... I leave myself a note on the inside of the windshield....

 

I've been scratching my head all summer trying to figure out where my gas was going. I have no leaks and a vented cap on a moon tank. I drive my '30 PU on weekends and it is always empty the next Saturday morning. There must be something to this.

 

I think your history is missing about ten years....starting in the early 70s, cars got pressure-vacuum caps, and charcoal canisters, and the purge was controlled by ported vacuum. Computer control of the system started in the early 80s.

The 1970s system is what could likely be adapted to older cars

 

im sorry im finding it tough to believe that your gas tanks are venting off that fast. my tbird sits months on end and the full tank i left is still full. i try real hard to leave them full so the fuel doesnt go south as fast. if that much fuel evaporated you would have the inner side of your fuel tank coated with a huge thick layer of that brown stuff gas will leave. making the car unrunnable. our cars on average dont get the best mpg. just admit you forgot to fill it, or when you hit the gas its just like flushing a toilet