Air-fuel ratio control apparatus for engine

Abstract

An engine has an upstream catalyst and a downstream catalyst in an exhaust system. An air-fuel ratio control apparatus has an oxygen sensor that outputs signal indicative of an oxygen storage amount in the downstream catalyst. The apparatus has an air supply device for supplying the air into upstream the downstream catalyst. When the signal of the oxygen sensor indicates a shortage of the oxygen storage amount in the downstream catalyst, the air supply device is activated to supply the air to the downstream catalyst. Thus, the downstream catalyst can recover the oxygen storage amount sufficient to keep its catalysis.

What is claimed is:  
     
         1 . An air-fuel ratio control apparatus for an engine comprising: 
 a first catalyst disposed in an exhaust passage of the engine;    a second catalyst disposed in the exhaust passage downstream the first catalyst;    an air-fuel ratio sensor disposed in the exhaust passage upstream the first catalyst;    second catalyst condition detecting means for detecting a condition of the second catalyst;    feedback control means for controlling an air-fuel ratio to maintain a detected air-fuel ratio by the air-fuel ratio sensor in a target air-fuel ratio by using a feedback control method, the target air fuel ratio being set in accordance with an output of the second catalyst condition detecting means; and    oxygen storage recovery means for recovering an oxygen storage in the second catalyst when the second catalyst condition detecting means outputs a predetermined level.    
     
     
         2 . The air-fuel ratio control apparatus for an engine according to  claim 1 , wherein the second catalyst condition detecting means detects an air-fuel ratio of the exhaust gas in the exhaust passage downstream the second catalyst, or a rich condition and a lean condition of the exhaust gas in the exhaust passage downstream the second catalyst.  
     
     
         3 . The air-fuel ratio control apparatus for an engine according to  claim 1 , wherein the oxygen storage recovery means increases an oxygen concentration in the exhaust gas supplied to the second catalyst when the second catalyst condition detecting means outputs a signal that indicates a rich condition of the exhaust gas.  
     
     
         4 . The air-fuel ratio control apparatus for an engine according to  claim 3 , further comprising an air supply device that supplies the air to the second catalyst, wherein the oxygen storage recovery means increases an oxygen concentration in the exhaust gas supplied to the second catalyst by activating the air supply device.  
     
     
         5 . The air-fuel ratio control apparatus for an engine according to  claim 3 , wherein the oxygen storage recovery means increases an oxygen concentration in the exhaust gas supplied to the second catalyst by at least reducing a fuel amount supplied to the engine.  
     
     
         6 . The air-fuel ratio control apparatus for an engine according to  claim 5 , wherein the fuel amount is reduced by adjusting the air-fuel ratio of the exhaust gas supplied to the second catalyst in a lean condition.  
     
     
         7 . The air-fuel ratio control apparatus for an engine according to  claim 6 , wherein the air-fuel ratio is adjusted in a lean condition by setting the target air-fuel ratio in the feedback control means in a lean value.  
     
     
         8 . The air-fuel ratio control apparatus for an engine according to  claim 5 , further comprising means for suspending a fuel supply for all or partial cylinder and resuming the fuel supply when an engine speed reaches to a resuming engine speed, wherein the fuel amount is reduced by adjusting the resuming engine speed lower than a normal value.  
     
     
         9 . The air-fuel ratio control apparatus for an engine according to  claim 1 , wherein the feedback control means comprises: 
 main feedback means for maintaining the detected air-fuel ratio in the target air-fuel ratio by using the feedback control method; and    sub feedback means for setting the target air fuel ratio in accordance with the output of the second catalyst condition detecting means.