Exhaust emission purifying apparatus for engine

An exhaust emission purifying apparatus for an engine, comprising:

• a reduction catalytic converter that reductively purifies nitrogen oxides in the exhaust emission by using a reducing agent;
• a reducing agent tank that stores a liquid reducing agent or its precursor;
• a reducing agent dosing device that doses the liquid reducing agent or its precursor stored in the reducing agent tank to the exhaust emission upstream side of the reduction catalytic converter;
• a level measuring device that measures a level of the liquid reducing agent or its precursor stored in the reducing agent tank;
• and
• a control unit, including a computer, to execute:
• a dosing flow rate computing process of computing a dosing flow rate of the liquid reducing agent or its precursor according to engine operating conditions,
• a reducing agent dosing process of controlling the reducing agent dosing device, based on the dosing flow rate computed in the dosing flow rate computing process,
• an integrated amount computing process of computing an integrated amount obtained by sequentially integrating the dosing flow rate computed in the dosing flow rate computing process,
• a consumed amount computing process of computing a consumed amount of the liquid reducing agent or its precursor, based on the level measured by the level measuring device, and
• a failure judging process of judging whether or not a supply system of the liquid reducing agent or its precursor is failed, based on a ratio between the integrated amount computed in the integrated amount computing process and the consumed amount computed in the consumed amount computing process.