OK, from a technical/scientific standpoint, I just don't understand the charcoal filter. I'm a science geek, and understand how activated charcoal works on a molecular level. But, the carbon has to be periodically "regenerated" to keep working. The contaminants that activated charcoal removes have to be baked off or otherwise removed for it to continue to work.

I have raised tropical fish for many, many years, and the activated carbon in the fish tank filters need to be changed every few months.

What "regenerates" the carbon in these filters? Why don't they go bad quickly (like in a tank or two of gas)? Do they need periodic charcoal replacement?

Any other science geeks here that can explain this to me?

Thanks!

The carbon canister absorbs hydocarbon vapors as the fuel tank vents from normal temperature changes. When your engine starts, it draws fresh air through the carbon canister and purges the hydrocarbons.

Some people have emptied the later canisters (plastic ones) and replaced them with the fish tank charcoal to refresh them.

According to a "service manual item" (I think it was on Bowlby's site), these cannisters are to be refreshed/replaced every 50,000 miles.

I took mine (1974 2.0L with a plastic one) apart a while ago. Looks like PO had refreshed it at some point with charcoal from the local pet store. Easy job for sure...

Activated carbon tightly adsorbs organic contaminants. I don't think they can be de-sorbed just by fresh air flowing past the carbon.

Activated carbon's efficiency is based mainly on it's massive surface area, and some of the cheaper fish tank carbon has a much lower surface area per volume than quality activated carbon. But, it's very inexpensive, so easy to swap out frequently.

I'd love to get comments from someone who has detailed technical knowledge in this area. Any of you got any industry contacts?

Interesting question. I was just removing all the parts from the front trunk last night, and was pondering the functionality of a 40-year old charcoal canister myself.

I'm a mechanical engineer for the electrical utility here, and it turns out we're in the process of installing an activated carbon injection system for mercury capture even as I type this. Next time the carbon vendor guys are here, I'll see if I can get some answers.

^^^ That's be great!.

Yeah, I was thinking that a company that sells carbon to high-end users such as your firm might know. Or better yet, a company that makes the carbon.

From a scientific perspective, I find it nearly impossible to believe that a 40 year old canister is doing anything at all.

I think that by this point, it is simply routing the fuel vapors into the intake. Which is a lot better than dumping them out of the car, at least.

--DD

Gasoline is highly volatile so the majority of it will evaporate out of the carbon, especially with forced air flowing through it. Think of putting lighter fluid on your charcoal, the charcoal will absorb it but it will release the naptha and other volatile elements back into the atmosphere after awhile. If you waited too long you'd have a hard time getting it started.

Activated carbon absorbs about 1/3 of its weight. It can off gas collected vapors after a while as 914GT mentioned. And no fresh air does not magically clean it. Nothing cleans it - you must dispose of it if you want clean activated carbon.

My employer makes air cleaning products and deals with activated carbon. We actually tested aviation fuel and its vapors with activated carbon - it works very well.

Same goes for my paint mask. 90 days then I throw them away. Now my 40 YO canister; that's another story.

By no means is the carbon completely cleaned by air flow through the canister, but much of the vapor is purged out and burned. That's how the system works. Otherwise the carbon would quickly saturate with gas vapor and need to be discarded after a short time. Newer cars have much more sophisticated systems than the 914 did, and regulate the amount pulled back out of the canister depending on engine load and operating conditions. As far as I know these systems are designed to meet EPA regulations for the life of the car and don't require routine canister replacement.