Exhaust emission purifying apparatus for engine

An exhaust emission purifying apparatus for an engine, comprising:

• a reduction catalytic converter which is disposed in an engine exhaust system and reductively purifies nitrogen oxides in the exhaust emission;
• a reducing agent tank which is provided for storing therein a liquid reducing agent;
• a pump which carries out suction and supply of the liquid reducing agent by pumping out of the reducing agent tank;
• a flow control valve which controls a flow rate of the liquid reducing agent supplied by pumping of the pump;
• an injection nozzle which injection-supplies the liquid reducing agent having flown from the flow control valve by which flow rate of the liquid reducing agent is controlled, to the exhaust emission at a position upstream of the reduction catalytic converter;
• and
• a control unit incorporating therein a computer,
• wherein the control unit computes in real time an injection amount of the liquid reducing agent injection-supplied by the injection nozzle based on engine operating conditions and also, determines that a reducing agent supply system arranged on the downstream of the pump brings about clogging, when a reduction in a pressure of the liquid reducing agent remaining in the downstream of the pump and also in the upstream of the flow control valve by an amount equal to or more than a first predetermined pressure does not occur until a cumulative injection amount obtained by sequentially integrating the injection amount computed in real time reaches a predetermined amount after the pump is temporarily stopped.