Exhaust purification system of internal combustion engine

An exhaust purification system of an internal combustion engine comprising:

• a catalyst arranged in an exhaust passage of the internal combustion engine and able to store oxygen;
• an air-fuel ratio sensor arranged at a downstream side of the catalyst and detecting an air-fuel ratio of an outflowing exhaust gas flowing out from the catalyst;
• a current detection device detecting an output current of the air-fuel ratio sensor;
• a voltage application device applying voltage to the air-fuel ratio sensor;
• a voltage control part configured to control the voltage applied to the air-fuel ratio sensor;
• and
• an air-fuel ratio control part configured to control the air-fuel ratio of an inflowing exhaust gas flowing into the catalyst, wherein
• the air-fuel ratio control part is configured to control the air-fuel ratio of the inflowing exhaust gas so that the output current of the air-fuel ratio sensor becomes a predetermined target value,
• the voltage control part is configured to perform at least one of voltage lowering control setting the applied voltage to a first voltage lower than a reference voltage when the air-fuel ratio of the outflowing exhaust gas detected by the air-fuel ratio sensor becomes higher than a lean judged air-fuel ratio leaner than a stoichiometric air-fuel ratio, and a voltage raising control setting the applied voltage to a second voltage higher than the reference voltage when the air-fuel ratio of the outflowing exhaust gas detected by the air-fuel ratio sensor becomes lower than a rich judged air-fuel ratio richer than the stoichiometric air-fuel ratio, and
• the reference voltage is a voltage at which the output current of the air-fuel ratio sensor becomes zero when the air-fuel ratio of the outflowing exhaust gas is the stoichiometric air-fuel ratio.