1. Field of the Invention
This invention relates to a fuel evaporation control or vapor recovery system including a charcoal canister, and, more particularly, to a system of the type described, which allows the supply of gasoline vapor to pass to the engine during engine deceleration, thereby improving the efficiency of the charcoal canister.
2. Description of the Prior Art
A fuel evaporation control or vapor recovery system, which prevents the emission of gasoline vapor from the fuel tank into the atmosphere, having a charcoal canister having activated charcoal housed therein, is known. In the prior art fuel evaporation control system, gasoline vapor, which is accumulated or adsorbed within the charcoal canister, is supplied to an internal combustion engine under the following conditions: when the engine is loaded over a given load level; while the supply of gasoline vapor is interrupted; and when the engine is loaded below a given load level.
Recently, government regulations for controlling fuel evaportion from fuel tanks have become more strict, and hence it is desirable to improve the efficiency of charcoal canisters. According to the prior art, it is an easy task to provide systems, which may increase the amount of gasoline vapor to be supplied to an engine by increasing the size of the charcoal canister. However, the amount of oxygen in an exhaust system is limited in high load conditions, so that an increase in amount of gasoline vapor would lead to an increase in amount of harmful emission from an engine. An increase in size of the charcoal canister to offset this is not desirable from a design viewpoint.
Meanwhile, it has become common practice to use a secondary air supply means for an exhaust system for treating harmful constituents of exhaust gases (for instance, hydrocarbons and carbon monoxide). The secondary air is supplied to an exhaust system from an air pump or through a reed valve means, which operates in response to a variable vacuum in the exhaust manifold. Meanwhile, the concentration of oxygen contained in exhaust gases is relatively high, during engine deceleration or in a negative output condition of the engine. This discovery has been overlooked in solving the aforesaid problem.