Fuel evaporation rate and vented tanks?

I guess everybody already knows this........."Addition of ethanol increases the volatility of gasoline, potentially worsening the problem." http://en.wikipedia.org/wiki/Gasoline

 

One night I left the cap off a gallon of laquer thinner. I'd tell you the next morning it was mostly gone, but it was definitely right there in the air.

Maybe I should've adapted an OT fuel vaporization management system complete with 40' of rubber hose and a plastic - no make that billet -charcoal canister.

 

On my board track racer I have a very simple copper vent. If I leave it in the sun at a show, it will easily lose half a tank of gas in 8 hours. Which is about a 1/3 gallon.

 

I have a 67 Alpine. It uses a gas cap that will allow pressure to build to a few pounds before venting. Does not vent unless the temps are very high and I'm driving. The valve in the cap opens under a vacuum. Think of a radiator cap designed to work with a coolant recovery system. Won't they fit some of the old tanks?

Also, with carbs the garage always smelled like gasoline until the bowl was dry. Went to EFI and the gasoline odor disappeared.

Bill

 

Wait.. I thought the cut off for the HAMB was mid sixties?

I am bailing on this one... I have stepped over the HAMB line too much already..

All I will say on this subject is "If you can't afford to put gas in it, give to me and I will." My hose with a plug is about the only "HAMB friendly" solution I see. I won't discuss evap systems on the open HAMB forum anymore..

Back on topic... Who just bought a duece with a banger??

 

You say you don't believe they're venting that fast, and then you say how you prevent yours from venting that fast.

 

How about a simple on/off valve that opens up a vent tube when the ignition is on? This 12Volt 1/4" valve can be found at Mcmaster.com (PN 4916k2 $40).




Or, how about a one-way valve?

This is from Auto Zone (PN 47149). It's supposed to go on a vacuum line. It could be used on a gas tank's vent tube to let air in but not out. I'm not sure how much vacuum it would take to open but if it doesn't work they are only $5.00.

 

^ would those 2 valves be a problem if you just gassed up full from cool underground tanks, then parked it in summer temps?

Would it cause enough pressure in the tank to blow a rubber line off? (clamped hose)


My twin Rochester AA carbs smell up the shop pretty good, but I don't see a cure for bowl evaporation.

I thought I remembered seeing a carb decades ago that had some sort of external bowl vent cover that opened with the throttle, but I could be wrong. There still may have been another open vent tube above the venturi inside?

 

That sounds like a good plan to me

 

I'm wondering, after reading all of the posts on this topic, if this has regional variables.
That guy from Iowa sounds like he is still getting vintage style pump gas.
I think a way to keep this on topic is to figure out simple solutions.
Driving down the road, I'm not concerned about evap because the tank has a negitive atmosphere.

I think the carb bowl vent connected to the tank vent with a tee and a brass shut off would do fine.

 

Two comments;
As for evaporation rate, I had this conversation recently with a tech guy at a major carb manufacturer. He told me they poured a pint of current pump gas into a pan and a pint of race gas into an identical pan. Left them sitting out over the weekend in the shop and on Monday the pump gas was all gone, the race gas had barely lost any.

Charcoal canister. It won't control the evaporation (it might slow it down slightly), it will only control the fumes in to the atmosphere. We do a bit of emissions testing with the California Air Resources Board (CARB) and the level of testing we do in a sealed room, with the air sucked out is staggering. But to relate it to HAMB cars, I had an issue with my Ranchero smelling like fumes all the time (as well as my 66 VW) So I got a non vented cap, pulled the filler neck out and drilled it for a barb fitting and used a Harley Davidson Charcoal canister (which you can get FREE at many Harley shops as every Yay Hoo with a Harley pulls them off). They are about 3" round and 6" long, made of plastic and don't weigh much of anything. I think they are only on California Harley's though.
No More Raw Fuel Smell!!!!!

 

Good stuff! except....

I don't understand how it can not control the evaporation, if it is indeed capturing all the fuel vapors. What is there in fuel, that is not fuel?

 

Every micron of evap is horsepower into thin air, that you are paying for!

Thanks for the race fuel comparison!

 

The "ignition on" system won't help because pressure builds up from temperature differentials whether the car is running or not. As an example, my red plastic gas cans in the garage blow up like balloons on a hot day.

Same for the one way (check) valves. The pressure has to be able to vent in both directions, outward as the fuel pressurizes and inward as fuel is sucked from the tank. Again, my thoughts were to restrict venting to only pressurized events, and eliminate regular atmospheric evaporation.

 

It gets sucked into the charcoal. It doesn't turn around and go back into the tank.

 

How does it get out of the charcoal?

 

As I understand it there are a couple of different systems. They way that I described using it, was just capturing the fuel vapors that were escaping through evaporation to get rid of the smell. Eventually this will become saturated as it is not getting the gas sucked back out.
In the standard Vacuum system that comes on cars it gets more complicated; there are chambers in the canister running in sequence from the intake to the outtake. The interior of the canister is filled with charcoal or carbon pellets. The input of the canister connects to the gas tank's vent port, while the output connects to the purge valve in the side of the vehicle's intake manifold.
When the car is shut off, there is a pressure imbalance within the fuel tank caused by fuel being siphoned out, but no air being let back in to take up the empty space. The lower pressure in the gas tank promotes a greater rate of evaporation, letting some of the fuel become a gas. Eventually the tank's internal pressure equalizes, at which point the gas leaves the tank through the vent port and goes into the carbon canister. It's trapped there by the properties of the carbon or charcoal within the canister, keeping it from escaping into the air. When the vehicle's engine starts, the sudden suction created along the intake manifold opens up the purge valve and pulls all the gaseous fuel out of the canister and burns it in the engine. Depending on whether it is carb's or injected the hook ups and controls will be quite different and the modern systems are much more elaborate than the early ones of the 70's.

 

So....as long as you have a purge system, the fuel that evaporates is captured by the charcoal, and later burned, to make the car go.

It's interesting that here we are talking about the second major "emissions control" system that was put on cars, after the adoption of PCV. PCV turned out to be an OK thing and adopted by most of the hot rodding community.....I wonder if evaporative emission controls ever will be?

Thanks for the info, too! It pretty much agrees with what I thought I knew about the subject.

 

The 70s VW air cooled bug uses a long plastic vapor condenser running along the top edge of the dash by the hood opening. I assume that lets whatever vapor that does turn to liquid, return to the tank. Then one small hose runs off the highest point on the condenser, back to the charcoal can under the Rt rear fender.

The reason the can is there, is how the cannister gets purged when running: There are two 1/2" ID rubber hoses on opposite ends of the can; one goes to the air cleaner, and the other goes to the engine cooling fan shroud.

When the engine cooling fan is running, air gets forced through the charcoal can and then escapes back into the air cleaner to be burned.


There were no systems to recover bowl vent fumes; they vented inside the carb top/air cleaner.
I do believe there was a replacement schedule on the can itself, X amount of miles or whatever.

 

I would assume you are still losing some gas with the purge style canisters as the charcoal has to be retaining some of what it is absorbing.

 

The charcoal does, indeed, absorb gas fumes. Those that get past the charcoal get sucked into the intake or air cleaner (depending on the make, model, and year of 1970s/early '80s car we are talking about). Some of my cars have vented to the air cleaner and some right to the intake manifold. Either way, the excess fumes don't really get used" by the motor except maybe to ever so slightly richen the air/fuel mixture on fireup.

I have driven a couple vehicles (1973 Cutlass, 1985 Chevy 1/2 ton pickup) with every hose except the one to the gas tank open and it still cuts way, way down on the gas smell compared to a vented gas cap.
Personally on those two cars I just ran a hose from the canister outlet to somewhere outside the engine compartment so gas fumes could not build up under my hood.
Can't get more simple than that! (Hose from tank to charcoal canister, hose from canister to outside the car, done.)

Yes, I have owned way too many 1973-1982 cars in my life.

 

A bowl vent solonoid was used in OT type vehicles.

 

I am very interested in this discussion however my fear is that this thread is too far off topic for the HAMB.

I have a 55 Olds convertible that has a big problem with fuel evaportion as well. I have rigged up a small electric pump back by the tank to combat the carb evaporation so I don't burn up the starter when I go to drive it after it has set for a few days or a week. The carb is dry so I just press a button and pump the carb full then start it up and the mechanical pump pulls fine through the electric one. However I keep thinking of how to combat the gas that is venting out the cap from the tank as well. I have some ideas but don't want to discuss here. Honestly... I thought this thread would have been deleted by now due to subject matter..

What moderator determines if this is an OK topic or not? My understanding of the HAMB makes this whole topic questionable on this board in my mind.

BTW... Hotroddon did a very good job of describing some of the early systems. However, in my opinion, those early type won't help much with the modern evaporation issue. You (we) really need a more modern version that is truly sealed to stop loosing your expensive modern pump gas.

 

interesting about how the HAMB works, as far as what is allowed, and what is not allowed. Bags, alternators, and 5 speed transmissions seem to be required equipment on a HAMB ride....so it does not surprise me at all that discussing adding EVAP systems to old cars is allowed too.

Don't worry about it....trying to solve old car problems is OK here.

 

Interesting discussion so far . I have run into similar issues with old cars around our shop.

Does a fuel additive like Stabil slow this evaporation issue ? I'm thinking this could be the easy fix to the problem as these products are meant to stabilize fuel for extended periods of time .

 

So... Can I take it from this post that we are OK discussing this topic?

BB<--- still leary

 

The online research I've done since this topic came up shows not much is being done about it.
Not sure if I can post links to outside websites, but I found that our general feelings about this are concurrent with other vintage cars buffs.

Some of the old school ways of treating today's old cars have changed.

Seems like the old way of leaving a tank full to overwinter is out,
And running it dry is in.
Lucky my Chevy has a drain built in, not that I want to put in storage ever again.

 

If you install a charcoal canister it must me installed and plumbed correctly. Even manufacturers have had problems with them catching fire due to poor design and defective components.

 

I think your idea of venting only pressurized gasses is a good one. It would certainly stop free flowing evaporation. The added pressure would also raise the temperature needed to convert the remaining liquid gasoline into gas vapors (evaporation) - just like the pressure in our radiators raises the boiling point of water.

I don't have any idea how much pressure is needed to raise the boiling point of gasoline above the temperature of a hot summer day, but it would be nice if the gas tank could handle it. Then there would be no evaporation at all.

That seems to be what is going on with Garcoal's T'bird. He said, "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'm too young to remember, so correct me if I'm wrong, but I think the fuel tanks and caps were sealed on cars built before the '70's. If this is so, then it stands to reason that gas tanks have been built that can withstand the pressures created by sealing them, in spite of your plastic gas cans being not quite up to the same task. If gas tanks in cars can withstand the pressure of a hot summer day, then the, "ignition on vent" would be ideal. If they can't, then it's a horrible idea because it will result in a pressurized tank full of gas spewing it's contents.

So, the question is, "Were there ever any cars built that didn't vent positive gas pressure?" I have to plead ignorance because I'm only a child of 46 years old.

Perhaps a better questions are, "1. How much pressure does a tank full of gasoline make on a hot summer day? 2. How much pressure will my gas tank withstand?" If the latter is more than the former, seal it. If not, seal it to a lesser pressure that is safe, just as you suggested.

I would think both of these questions could be answered with a little experimentation and a pressure gauge.