Patent ID: 7842267

Claim:
A method of purifying an exhaust gas of an engine, which adds a reducing agent for NOx to the exhaust gas thereby reducing the NOx in the exhaust gas, the method comprising the steps of: providing a tank that stores the reducing agent to be added to the exhaust gas, or a precursor thereof, in an aqueous solution state; detecting a predetermined state parameter of the aqueous solution corresponding to a thermal characteristic of the aqueous solution stored in the tank; and performing a predetermined calculation on abnormalities of the aqueous solution based on the detected state parameter, wherein the step of performing the calculation comprises: detecting predetermined abnormalities on the aqueous solution when the detected state parameter is within respective abnormal regions other than a predetermined region defined as a normal region; making a first abnormality judgment or a second abnormality judgment upon an establishment of a predetermined determinate condition, wherein a time lag exists between the detection of the abnormality and the establishment of the determinate condition; and generating a signal corresponding to the first or second abnormality judgment, wherein a first abnormality is detected with respect to a concentration of the aqueous solution when the detected state parameter is within a first abnormal region on one side of the normal region, and a second abnormality is detected when the detected state parameter is within a second abnormal region on the opposite side to the first abnormal region across the normal region, wherein the first abnormality judgment is made in association with the first abnormality detection, and the second abnormality judgment is made in association with the second abnormality detection, and wherein the step of making the first abnormality judgment or the second abnormality judgment comprises: canceling the first abnormality judgment with deviation of the detected state parameter from the first abnormal region after making the first abnormality judgment; canceling the second abnormality judgment with deviation of the detected state parameter from the second abnormal region after making the second abnormality judgment; and maintaining, when the first abnormality is detected after making the second abnormality judgment as a result of the detected state parameter being directly shifted from the second abnormal region to the first abnormal region, the second abnormality judgment for a predetermined period of time from the detection of the first abnormality.