Exhaust Emission Control Device

Abstract

Increase of NO x emission is minimized even under a condition that NO x depuration system has malfunction. An exhaust emission control device is disclosed which has selective reduction catalyst 10 incorporated in an exhaust pipe 9 , urea water 17 (reducing agent) being added to exhaust gas upstream of the selective reduction catalyst 10 by urea water adding means 18 (reducing agent adding means) so as to depurate NO x through reduction. The exhaust emission control device has NO x sensors 24 and 25 respectively arranged on entry and exit sides of the selective reduction catalyst 10 for detecting NO x concentrations, and an NOx deputation control device 23 (control device) which judges abnormality, on the basis of detection signals 24 a and 25 a from the NO x sensors 24 and 25 , by non-attainment of a predetermined No x reduction ratio and, when such abnormality is judged, restriction of an amount of fuel to be injected is commanded.

1 . An exhaust emission control device wherein selective reduction catalyst is incorporated in an exhaust pipe, a reducing agent being added upstream of said catalyst by reducing-agent adding means to depurate NO x  through reduction, comprising NO x  sensors arranged respectively on entry and exit sides of the selective reduction catalyst for detecting NO x  concentrations, and a control device for judging abnormality on the basis of detection signals from both of the NO x  sensors by non-attainment of a predetermined NO x  reduction ratio and for commanding restriction of an amount of fuel to be injected when said abnormality is judged. 
   
   
       2 . An exhaust emission control device as claimed in  claim 1 , wherein the reducing agent is urea water, the selective reduction catalyst having a characteristic of capable of selectively reacting NO x  with ammonia even in the presence of oxygen.