Exhaust purification system for internal combustion engine

An exhaust purification system for an internal combustion engine comprising:

• a selective reduction catalyst that is provided in an exhaust channel of the internal combustion engine and purifies NOx in exhaust under the presence of NH3;
• a reducing agent supply means for supplying NH3 or a precursor thereof to the selective reduction catalyst;
• and
• an exhaust gas sensor that detects a concentration of NH3 in exhaust on a downstream side from the selective reduction catalyst, wherein gain of the exhaust gas sensor changes under the presence of O2;
• an upstream-side detection means for detecting or estimating an O2 concentration of exhaust on an upstream side from the selective reduction catalyst;
• a variation calculating means for calculating a variation from a predetermined base value of an output value of the upstream-side detection means;
• a downstream-side estimation means for modeling an error component value of the exhaust gas sensor with a value obtained by multiplying a predetermined correction coefficient by the variation calculated by way of the variation calculating means;
• a transient extraction filter that blocks a stationary component and at least allows a frequency band corresponding to a increase-decrease request of drive power from a driver to pass from the output value of the exhaust gas sensor and the variation calculated by way of the variation calculating means, and calculates a filter value of each;
• an identification means for identifying a value of the correction coefficient so that error between the filter value of the output value of the exhaust gas sensor and a value calculated by multiplying the correction coefficient by the filter value of the variation calculated by way of the variation calculating means becomes a minimum;
• and
• a supply amount determination means for determining a supply amount of the reducing agent supply means based on an NH3 concentration estimated value obtained by removing the error component value calculated by way of the downstream-side estimation means from the output value of the exhaust gas sensor.