Signature Matching by Concept Contraction and Concept Abduction

An NOx sensor 27 detects the concentration of NOx released from an occlusion type NOx catalyst 25 into the atmosphere. Based on the output of the NOx sensor 27, a total NOx discharge A is computed. When the total NOx discharge A reaches an NOx emission judgment amount A0, corresponding to an NOx emission judgment amount designated by NOx emission regulations, before a distance traveled C reaches a predetermined distance traveled C0, an exhaust air-fuel ratio is changed to a rich air-fuel ratio to release NOx from the occlusion type NOx catalyst 25 efficiently. Then, the exhaust air-fuel ratio is changed to a stoichiometric air-fuel ratio to purify and reduce NOx by a three-way catalyst function.