Monitoring apparatus for a multi-cylinder internal combustion engine

A monitoring apparatus for an internal combustion engine, applied to a multi-cylinder internal combustion engine having a plurality of cylinders comprising:

• a fuel injector for injecting fuel;
• a catalytic converter disposed in an exhaust passage of said engine and at a position downstream of an exhaust gas aggregated portion into which exhaust gases discharged from combustion chambers of a plurality of said cylinders of said engine merge;
• an upstream air-fuel ratio sensor, disposed in said exhaust passage and at said exhaust gas aggregated portion or between said exhaust gas aggregated portion and said catalytic converter, and outputting an output value corresponding to an air-fuel ratio of a gas flowing at a position at which said upstream air-fuel sensor is disposed;
• a downstream air-fuel ratio sensor, disposed in said exhaust passage at a position downstream of said catalytic converter, and outputting an output value corresponding to an air-fuel ratio of a gas flowing at said position at which said downstream air-fuel sensor is disposed;
• sub feedback amount calculation means for calculating a sub feedback amount to make an air-fuel ratio represented by said output value of said downstream air-fuel ratio sensor coincide with a stoichiometric air-fuel ratio, every time a predetermined first update timing arrives;
• fuel injection control means for controlling an injection amount of fuel injected from said fuel injector every time a predetermined second update timing arrives based on at least said output value of said upstream air-fuel ratio sensor and said sub feedback amount in such a manner that an air-fuel ratio of an air-fuel mixture supplied to said engine coincides with the stoichiometric air-fuel ratio;
• learning means for changing a learning value of said sub feedback amount every time a predetermined third update timing arrives in such a manner that said learning value of said sub feedback amount comes closer to an amount corresponding to a steady-state component of said sub feedback amount;
• monitoring means for performing an abnormality determination as to whether or not an abnormality state of said engine is occurring based on a first parameter for abnormality determination which varies in accordance with said learning value;
• learning value changing speed setting means for setting a changing speed of said learning value at any one of a first changing speed, a second changing speed smaller than said first changing speed, and a third changing speed smaller than said second changing speed;
• and
• monitoring control means for allowing or prohibiting said monitoring means to perform said abnormality determination based on said set changing speed of said learning value.