1. Field of the Invention
This invention relates to an evaporative emission control system for an internal combustion engine, which is adapted to control emission of evaporative fuel generated from a fuel tank connected to the engine.
2. Prior Art
Among evaporative emission control systems for controlling emission of evaporative fuel generated from a fuel tank into the atmosphere, there is conventionally known a type that evaporative fuel generated in the fuel tank is temporarily adsorbed by an activated carbon in a charcoal canister, before it is purged into the intake system of the engine.
Further, when the engine is started in a cold condition, it is desirable to supply a large amount of evaporative fuel to improve the startability of the engine. To this end, it has already been proposed, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 3-97560 to heat fuel adsorbed in the canister to thereby promote evaporation of the fuel.
However, according to this prior art, only the amount of fuel spontaneously evaporated in the fuel tank is heated, and hence it is impossible to generate evaporative fuel in large quantities. Therefore, evaporative fuel cannot be supplied to the engine in a desired or sufficient amount which is required e.g. at the start of the engine in a cold condition.
On the other hand, in evaporative emission control systems of the above-mentioned type, the engine is supplied with both fuel injected from fuel injection valves and evaporative fuel purged from the canister, when purging of evaporative fuel is carried out. It has already been proposed e.g. by U.S. Pat. No. 4,641,623 to control the amount of fuel injected from the fuel injection valves according to the flow rate of evaporative fuel purged while evaporative fuel is being purged into the intake system, to thereby control the air-fuel ratio of an air-fuel mixture supplied to the engine to a desired value.
Further, in the U.S. Patent, it is also disclosed to control the amount of evaporative fuel purged into the intake system to a predetermined value when the amount of evaporative fuel adsorbed in the canister exceeds a predetermined value.
However, in this prior art, the amount of evaporative fuel adsorbed in the canister is not quantitatively detected. This makes it difficult to constantly secure a predetermined amount of or more evaporative fuel to be purged into the intake system.