Evaporative Emission Control Canister: Description and Operation

Charcoal Canister, Typical:

PURPOSE
To trap and reclaim fuel vapor that would normally escape into the atmosphere due to expansion within the fuel system.

OPERATION
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.

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.

CONSTRUCTION
It is filled with activated charcoal granules which will hold up to one-third their own weight in fuel vapors.