Fuel vapor treatment system for internal combustion engine

A fuel vapor treatment system for an internal combustion engine comprising:

• a canister containing an adsorbing material for temporarily adsorbing fuel vapor conducted thereto from the interior of a fuel tank through an inlet passage;
• a purging passage for conducting an air-fuel mixture containing fuel vapor desorbed from the adsorbing material into an intake pipe of the internal combustion engine and purging the fuel vapor;
• a purge control valve disposed in the purging passage to adjust the purge flow rate based on the result of measurement of a fuel vapor concentration in the air-fuel mixture;
• a measurement passage having an orifice;
• gas flow producing means for producing a gas flow within and along a measurement passage;
• measurement passage switching means for switching the measurement passage between a first concentration measurement state in which the measurement passage is open to the atmosphere at both ends thereof, allowing an air to flow through the measurement passage, and a second concentration measurement state in which the measurement passage is put in communication at both ends thereof with the canister, allowing the air-fuel mixture fed from the canister to flow through the measurement passage;
• differential pressure detecting means for detecting a pressure at both ends of the orifice;
• and
• fuel vapor concentration calculating means for calculating a fuel vapor concentration based on a pressure difference detected in the first concentration measurement state and a pressure difference detected in the second concentration measurement state.