Internal combustion engine system

A control method for an internal combustion engine system that includes:

• a turbocharger that has a turbine that is disposed in an exhaust passage of an internal combustion engine;
• a bypass passage that is provided in the exhaust passage and that bypasses the turbine;
• a wastegate valve that opens and closes the bypass passage;
• and
• an air-fuel ratio sensor provided on a downstream side of a discharge port of the bypass passage,
• the control method comprising
• changing a proportion of an amount of exhaust gas that directly strikes the air-fuel ratio sensor after being discharged from the discharge port to an amount of exhaust gas discharged from the discharge port, depending on a degree of opening of the wastegate valve,
• wherein when the internal combustion engine is in a cold-start state, the proportion of the amount of exhaust gas that directly strikes the air-fuel ratio sensor after being discharged from the discharge port of the bypass passage to the amount of exhaust gas discharged from the discharge port of the bypass passage is made smaller than when the internal combustion engine is in a warm-start state.