Fuel vapor treatment system for internal combustion engine

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

• a canister that is connected to a fuel tank through a vapor introduction passage and has an adsorbent for temporarily adsorbing fuel vapor, the fuel vapor being produced in the fuel tank and being introduced into the canister through the fuel vapor introduction passage;
• a purge passage introducing a desorbed fuel vapor from the adsorbent into an intake pipe of the engine;
• a purge valve provided in the purge passage, the purge valve controlling a flow rate of fuel vapor flowing through the purge passage;
• a first throttle provided in the purge passage;
• a first pressure detecting means for detecting a variation in pressure of a purge gas passing through the first throttle;
• a second throttle provided in a gas passage of a positive crankcase ventilation apparatus that recirculates a blow-by gas into the intake pipe;
• a second pressure detecting means for detecting a variation in pressure of a gas passing through the second throttle;
• a first quantity calculation means for calculating a quantity of fuel vapor in an air-fuel mixture introduced into the intake pipe from the canister based on the variation in pressure detected by the first pressure detecting means and the variation in pressure detected by the second pressure detecting means;
• and
• a second quantity calculation means for calculating a quantity of air in an air-fuel mixture introduced into the intake pipe from the canister based on the variation in pressure detected by the first pressure detecting means and the variation in pressure detected by the second pressure detecting means.