1.  Field of the Invention
This invention relates to an apparatus for supplying fuel from a canister to an intake passage in an internal combustion engine having a supercharger adapted to compress air to increase the charging efficiency of the engine and thereby improve the engine output. 
2.  Description of the Prior Art
In the prior art, apparatus is known for preventing evaporation of fuel from an internal combustion engine.  It includes a canister adapted to absorb the fuel evaporated from a fuel tank or the like.  The absorbed fuel is separated from the canister by the drawing force of a negative pressure created in the intake system of the engine during its operation, and released into the intake system to be burned in the engine. 
In an internal combustion engine having a supercharger, however, a positive pressure prevails between an air compressor and a throttle valve.  In order to separate fuel from a canister by utilizing a negative pressure, it is necessary to connect a fuel pipe between the canister and an intake passage downstream of the throttle valve or upstream of the compressor.  If the fuel pipe is connected to the intake passage downstream of the throttle valve, however, it follows that in the event the throttle valve has a small degree of opening, a high negative pressure prevailing downstream thereof causes a large quantity of fuel to be released from the canister.  Conversely, a low negative pressure causes a small quantity of fuel to be released in the event the throttle valve has a large degree of opening.  Stated differently, a large quantity of fuel is released from the canister when a small quantity of air is introduced into the engine, and a small quantity of fuel is released when a large quantity of air is introduced.  This may result in an increase in the quantity of harmful matter such as hydrocarbon and carbon monoxide in the exhaust gas of the engine. 
If the fuel pipe is connected to the intake passage upstream of the compressor, it is impossible to obtain a desired quantity of fuel from the canister.  This is because only a low negative pressure prevails in the intake passage, though the quantity of fuel released from the canister may be proportional to the quantity of air introduced into the engine.