Evaporative Emission Control Canister: Description and Operation

PURPOSE/OPERATION
The charcoal canister is filled with activated charcoal granules, which can hold up to one-third their own weight in fuel vapors. Activated charcoal is used as a vapor trap because of its great surface area. Each gram of activated charcoal has a surface area of 1,100 square meters, or more than a quarter acre. The canister absorbs and stores fuel and vapors from the fuel system, until they can be purged and burned in the combustion process. The canister is connected to the fuel tank by a vent line. Within the vent line is a two-way check valve, which opens to allow excess fuel tank pressure and vapors into the canister for storage, until the engine is started. During the canister purge cycle fresh air is drawn through the filter at the bottom of the canister, purging the charcoal of fuel vapors.