1. Field of the Invention
The present invention relates in general to an evaporative emission control system of an internal combustion engine, and more particularly to a carbon canister which is practically employed in the evaporative emission control system.
2. Description of the Related Art
Hitherto, for suppressing atmospheric pollution from motor vehicles powered by internal combustion engines, various evaporative emission control systems have been proposed and put into practical use. Some of them are of a type which employs a carbon canister to capture any fuel vapors (viz., HC) coming from the fuel tank. That is, the carbon canister prevents the vapors from escaping into the atmosphere. The carbon canister generally comprises a canister case which is filled with activated charcoal mass which adsorbs the fuel vapors. The canister case is formed at one end with an atmospheric air inlet port and at the other with both a fuel vapor inlet port and a fuel vapor outlet port. These three ports are communicated through flow passages defined in the activated charcoal mass.
Upon stopping of the engine, fuel vapors from the fuel tank are led into the canister through the fuel vapor inlet port and adsorbed (or trapped) by the activated charcoal mass. Only air that has left the fuel vapors therefrom is discharged to the atmosphere through the atmospheric air inlet port.
While, under operation of the engine with a canister purging mode, a certain negative pressure is applied to the interior of the canister from an intake system of the engine through the fuel vapor outlet port. With this, atmospheric air is led into the canister through the atmospheric air inlet port to pick up the trapped fuel vapors and carry the same to an intake manifold of the intake system of the engine through the fuel vapor outlet port. The fuel vapors thus led to the intake manifold become part of the air/fuel mixture entering the engine cylinders to burn. The action of clearing the trapped fuel vapors from the canister is called “purging”. The air used for purging the canister (more specifically, the activated charcoal mass received therein) is called “purging air”.
Due to inherent construction of the carbon canister, the trapped fuel vapors therein have such a concentration distribution characteristic that the fuel vapor concentration lowers as approaching the atmospheric air inlet port. However, because of the shape of the canister wherein the activated carbon is packed in a continuous space in the canister case, a so-called vapor migration phenomenon takes place wherein due to adsorption equilibrium, the trapped fuel vapors diffuse and move toward a lower concentration zone, that is, toward the atmospheric air inlet port. Thus, undesired leakage of the fuel vapors into the atmosphere increases with passing of time.