Control device for an internal combustion engine

1. A control device for an internal combustion engine, comprising:

failure determining means for making a failure determination of a three-way catalyst of an internal combustion engine on the basis of an output of air-fuel ratio detecting means disposed upstream and downstream of said catalyst;

deterioration determining means for determining a deterioration of said catalyst before said failure determining means makes a failure determination; and

low NOx control means for controlling said internal combustion engine to be operated in an operating state where NOx emissions are lowered when said deterioration determining means determines a deterioration of the catalyst.

2. The control device for an internal combustion engine according to claim 1 , wherein:

said failure determining means carries out the failure determination of said catalyst when a reverse frequency ratio, which is the ratio between a reverse frequency of the output of said upstream air-fuel ratio detecting means and a reverse frequency of the output of said downstream air-fuel ratio detecting means, exceeds a given failure determination value; and

said deterioration determining means carries out the deterioration determination of said catalyst when said reverse frequency ratio exceeds a deterioration determination value that is set as a smaller value than said failure determination value.

3. The control device for an internal combustion engine according to claim 1 , wherein:

said low NOx control means changes a control amount according to a deterioration degree of said catalyst, which is determined by said deterioration determining means.

4. The control device for an internal combustion engine according to claim 1 , wherein:

said low NOx control means implements at least one of enrichment of an air-fuel ratio of said internal combustion engine, an increase of an EGR amount, and retard of ignition timing, to thereby control said internal combustion engine to be operated in said operating state where NOx emissions are lowered.

5. The control device for an internal combustion engine according to claim 1 , wherein:

the low NOx control means controls said internal combustion engine to be operated in the operating state where NOx emissions are lowered by controlling a fuel injection.

6. The control device for an internal combustion engine according to claim 1 , wherein:

the low NOx control means controls said internal combustion engine to be operated in the operating state where NOx emissions are lowered by controlling an EGR.

7. The control device for an internal combustion engine according to claim 1 , wherein:

said low NOx control means lowers the NOx emissions by using a correction coefficient determined based on a reverse frequency ratio, which is a ratio between a reverse frequency of the output of said upstream air-fuel ratio detecting means and a reverse frequency of the output of said downstream air-fuel ratio detecting means, and

the correction coefficient is determined based on a map.

8. A control device for an internal combustion engine, comprising:

failure determining means for making a failure determination of a three-way catalyst of an internal combustion engine on the basis of an output of air-fuel ratio detecting means disposed upstream and downstream of said catalyst;

deterioration determining means for determining a deterioration of said catalyst before said failure determining means makes a failure determination; and

low NOx control means for controlling said internal combustion engine to be operated in an operating state where NOx emissions are lowered when said deterioration determining means determines a deterioration of the catalyst, wherein:

said failure determining means finds the number of times the output of said downstream air-fuel ratio detecting means passes reverse reference values for failure determination, which are set on an increase side and a decrease side on the basis of a given neutral value, as a first downstream reverse frequency, and carries out the failure determination on the basis of the ratio between said first downstream reverse frequency and an upstream reverse frequency of the output of said upstream air-fuel ratio detecting means; and

said deterioration determining means finds the number of times the output of said downstream air-fuel ratio detecting means passes a reverse reference value for deterioration determination, which is set to be closer to a neutral value side than to said reverse reference values for failure determination, as a second downstream reverse frequency, and carries out the deterioration determination on the basis of the ratio between said second downstream reverse frequency and the upstream reverse frequency.

9. A control device for an internal combustion engine, comprising:

failure determining means for making a failure determination of a three-way catalyst of an internal combustion engine on the basis of an output of air-fuel ratio detecting means disposed upstream and downstream of said catalyst;

deterioration determining means for determining a deterioration of said catalyst before said failure determining means makes a failure determination; and

low NOx control means for controlling said internal combustion engine to be operated in an operating state where NOx emissions are lowered when said deterioration determining means determines a deterioration of the catalyst, wherein:

said low NOx control means controls said internal combustion engine to be operated in said operating state where the NOx emissions are lowered, when said internal combustion engine is stopped and restarted after said deterioration determining means determines a deterioration of the catalyst.