Exhaust emission purifying apparatus for engine

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.