Why do I have an "Activated Carbon Cannister" in my unleaded/petrol car?
What is it's purpose?
How does it work?
Do I need to do anything to maintain it?
I would like a general answer, but my car is a 2003 Opel Agila if you want to be specific!
Gasoline is a pretty toxic liquid. And it evaporates pretty easily. The Carbon Cannister is a system to help keep toxic fumes from gasoline out of the environment. So here's how it works.
You fill your car up with gasoline at the gas station. You add a gas cap to seal off the tank. Cool, no evil fumes can evaporate from your gas tank and contaminate the atmosphere. But... have you ever seen the science experiment in elementary school where they suck a vacuum on a Cola can? And the can goes totally flat because of the vacuum? Well imagine your gas tank as the soda can. As you drive, the gasoline gets consumed by the engine. If you had a tight seal on the tank (remember we don't want to vent gasoline fumes) then your gas tank would squish flat. And that's no fun.
The carbon canister is a tool that allows us to vent clean air into the gas tank to prevent the tank from going flat. Clean air comes in to replace the gasoline. Gas vapors that evaporate inside your gas tank get trapped by the carbon canister. These vapors accumulate in the carbon canister and eventually get consumed by the engine. I'm not sure of the exact drive cycle, but the engine computer, at periodic intervals, opens up the valving to the carbon canister and sucks the extra gasoline fumes stored there to the engine. I thought this happens four or five times per hour but don't quote me on that. A lot depends on your drive cycle, speed, etc... Generally they want the gas tank at a very slight vacuum to atmospheric.
Does that make more sense?
One note: Because the carbon canister is design to accommodate gasoline fumes and not liquid gasoline, you never ever ever want to go to the gas station, fill up the tank, then fill to click, fill to click, fill to click, fill to click. That is a bad thing to do. Just let it click once (and twice if you must) but never more than that. If you overfill the tank, the liquid gasoline goes into the carbon canister and messes with it bad. The only fix is to replace the carbon canister. $$ ouch.