Control apparatus for internal combustion engine

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

an exhaust system of the internal combustion engine;

a catalytic converter disposed in the exhaust system, for purifying an exhaust gas;

upstream-side air-fuel ratio detecting means for detecting an air-fuel ratio upstream of the catalytic converter;

downstream-side air-fuel ratio detecting means for detecting an air-fuel ratio in the exhaust gas downstream of the catalytic converter;

air-fuel ratio adjusting means for adjusting the air-fuel ratio through adjustment of an amount of fuel supplied to the internal combustion engine;

downstream-side target air-fuel ratio storing means for storing and retaining a target value for the air-fuel ratio downstream of the catalytic converter;

downstream-side control means for calculating an upstream-side target air-fuel ratio through proportional calculation so that the downstream-side air-fuel ratio detected by the downstream-side air-fuel ratio detecting means coincides with the downstream-side target air-fuel ratio stored and retained by the downstream-side target air-fuel ratio storing means;

upstream-side control means for controlling the air-fuel ratio adjusting means so that the upstream-side air-fuel ratio detected by the upstream-side air-fuel ratio detecting means coincides with the upstream-side target air-fuel ratio calculated by the downstream-side control means;

fuel supply cut means for cutting the fuel supply performed by means of the air-fuel ratio adjusting means; and

proportional gain changeover means for making a changeover in a proportional gain for proportional calculation in the downstream-side control means for a predetermined period from a normal proportional gain to a post-fuel-cut proportional gain set larger than the normal proportional gain by a predetermined value, when the fuel cutoff means starts cutting the fuel supply.

2. A control apparatus for an internal combustion engine according to claim 1 , wherein:

the air-fuel ratio adjusting means adjusts the fuel supply amount based on the upstream-side target air-fuel ratio calculated by the downstream-side control means while the proportional gain changeover means maintains the proportional gain at the post-fuel-cut proportional gain.

3. A control apparatus for an internal combustion engine according to claim 1 , further comprising:

an air amount detecting sensor for detecting an intake air amount, wherein:

the proportional gain changeover means calculates an integrated value of an amount of air sucked into the internal combustion engine, and makes a changeover in the proportional gain of the downstream-side control means from the post-fuel-cut proportional gain to the normal proportional gain when the integrated value of the amount of air sucked into the internal combustion engine reaches a predetermined criterial air amount after the fuel cutoff means has stopped cutting the fuel supply.

4. A control apparatus for an internal combustion engine according to claim 3 , further comprising:

catalytic degradation degree calculating means for calculating a degree of degradation of the catalytic converter, wherein:

the integrated value of the amount of air sucked into the internal combustion engine is reduced by a predetermined value in accordance with the degradation degree of the catalytic converter calculated by the catalytic degradation degree calculating means.

5. A control apparatus for an internal combustion engine according to claim 1 , wherein:

the post-fuel-cut proportional gain is set larger than the normal proportional gain by a predetermined value on a lean side with respect to a predetermined air-fuel ratio.

6. A control apparatus for an internal combustion engine according to claim 5 , wherein:

the predetermined air-fuel ratio is the downstream-side target air-fuel ratio.