A charcoal canister is part of the evaporative emission control (EVAP) system in many modern automobiles, which is designed to keep gasoline from evaporating into the atmosphere. Rather than venting a gas tank to the atmosphere, an EVAP system can allow fumes to flow into a canister that contains activated carbon and is therefore capable of adsorbing the fuel vapor. This process, coupled with the closed fuel system, can help ensure that a minimal amount of fuel vapor is allowed to reach the atmosphere.
The adsorption process used by these canisters makes use of the unique properties of activated carbon. Activated carbon is most often created from charcoal, though it may also be obtained by processing other materials, such as peat and wood. The activation process results in carbon with a very porous structure and tremendous surface area in comparison to its physical size. Fuel vapors may then be contained on the surface of this activated carbon. Unlike absorption or chemisorption, this can allow the fuel vapor to be trapped while remaining free to be used at a later time. In adsorption, one material can be bound to the surface of another material, while absorption involves one material being bound inside the volume of another material. Chemisorption can involve an actual chemical reaction at the surface of a material, as seen in corrosion.
By containing fuel vapors that would otherwise be vented to the atmosphere, the charcoal canister can have a positive environmental impact. Rather than being vented to the atmosphere or absorbed and disposed of, the fuel vapors can be reintroduced to the fuel system and burnt. During such circumstances, air can be passed through the canister, freeing some of the vapor and carrying it to the intake. This can allow vapors that would otherwise have been lost to be reclaimed and used during combustion. To that end, all automobiles manufactured in the US since the early 1970s have had closed fuel systems, and similar systems were also introduced to many markets throughout the world around that same time.
As part of the EVAP system, the charcoal canister is generally located somewhere between the fuel tank and the intake. They can often be found in the engine compartment and may be identified by the line that connects them to the fuel tank. Additionally, other lines will often be connected to them, such as manifold and ported vacuum sources. In carbureted models, there can also be a large hose that connects the canister to the carburetor bowl. In addition to automotive applications, these devices may be used in other vehicles, such as motorcycles.