Exhaust gas purifying device for an internal combustion engine

Abstract

An exhaust gas purifying device according to the present invention has an exhaust emission control catalyst provided in an exhaust passage of an internal combustion engine, a reducing agent adding unit provided in the exhaust passage at a position upstream of the exhaust emission control catalyst for adding a reducing agent, a control unit for controlling the reducing agent adding unit, and an air-fuel ratio estimating unit for estimating an air-fuel ratio prior to addition of the reducing agent. The control unit performs control such that addition of the reducing agent by the reducing agent adding unit is stopped when the air-fuel ratio estimated by the air-fuel ratio estimating unit reaches a stop reference value. When the air-fuel ratio is leaner than the stop reference value, the reduction efficiency of the exhaust emission control catalyst is low, so fuel serving as the reducing agent is wasted. However, since addition of the reducing agent is stopped when the estimated air-fuel ratio reaches the stop reference value, the reducing agent can be prevented from being added wastefully.

1 . An exhaust gas purifying device for an internal combustion engine, comprising: 
 an exhaust emission control catalyst provided in an exhaust passage of the internal combustion engine;    reducing agent adding means provided in the exhaust passage at a position upstream of the exhaust emission control catalyst, for adding a reducing agent;    control means for controlling the reducing agent adding means; and    air-fuel ratio estimating means for estimating an air-fuel ratio prior to addition of the reducing agent,    wherein the control means performs control so that addition of the reducing agent by the reducing agent adding means is stopped when the air-fuel ratio estimated by the air-fuel ratio estimating means reaches a stop reference value.    
   
   
       2 . An exhaust gas purifying device for an internal combustion engine according to  claim 1 , characterized in that: 
 the air-fuel ratio estimating means comprises: 
 target intake air amount calculating means for calculating a target intake air amount sucked into the internal combustion engine in accordance with a driving condition of a vehicle; and  
 intake air amount detecting means for detecting an actually measured intake air amount,  
   wherein the air-fuel ratio estimating means estimates the air-fuel ratio prior to addition of the reducing agent based on a ratio between the target intake air amount calculated by the target intake air amount calculating means and the actually measured intake air amount detected by the intake air amount detecting means.    
   
   
       3 . An exhaust gas purifying device for an internal combustion engine according to  claim 1 , characterized in that: 
 the air-fuel ratio estimating means comprises: 
 target fuel injection amount calculating means for calculating a target fuel injection amount injected into the internal combustion engine in accordance with a driving condition of a vehicle; and  
 intake air amount detecting means for detecting an actually measured intake air amount,  
   wherein the air-fuel ratio estimating means estimates the air-fuel ratio prior to addition of the reducing agent based on a ratio between the target fuel injection amount calculated by the target fuel injection amount calculating means and the actually measured intake air amount detected by the intake air amount detecting means.