Exhaust emission control system for internal combustion engine

An exhaust emission control system for an internal combustion engine, comprising:

• a first catalyst disposed in an exhaust gas passage of the engine, said first catalyst functioning to trap NOx in an atmosphere having an air-fuel ratio leaner than a stoichiometric level and to release NOx in an atmosphere having an air-fuel ratio richer than the stoichiometric level and reduce released NOx in presence of HC and CO;
• a second catalyst disposed in the exhaust gas passage upstream of said first catalyst and having an oxygen trapping ability;
• and
• a control unit that accomplishes an enrichment treatment by enriching stepwise an air-fuel ratio of an air-fuel mixture to be supplied to the engine to an enrichment degree at a timing at which a lean operation is changed to a stoichiometric operation in the engine, and recovers the enriched air-fuel ratio to the stoichiometric level at a recovery rate immediately after enriching the air-fuel ratio,
• wherein the control unit estimates the oxygen trapping ability of said second catalyst, and
• corrects the enrichment degree in the stepwise enriching to decrease in correlation to a decrease in estimated oxygen trapping ability.

An exhaust emission control system for an internal combustion engine, comprising:

• a first catalyst disposed in an exhaust gas passage of the engine, said first catalyst functioning to trap NOx in an atmosphere having an air-fuel ratio leaner than a stoichiometric level and to release NOx in an atmosphere having an air-fuel ratio richer than the stoichiometric level and reduce released NOx in presence of HC and CO;
• a second catalyst disposed in the exhaust gas passage upstream of said first catalyst and having an oxygen trapping ability;
• means for accomplishing an enrichment treatment by enriching stepwise an air-fuel ratio of an air-fuel mixture to be supplied to the engine to an enrichment degree at a timing at which a lean operation is changed to a stoichiometric operation in the engine, and recovering the enriched air-fuel ratio to the stoichiometric level at a recovery rate immediately after the enriching the air-fuel ratio;
• means for estimating the oxygen trapping ability of said second catalyst;
• means for correcting the enrichment degree in stepwise enriching to decrease in correlation to a decrease in the estimated oxygen trapping ability;
• and
• means for restricting the corrected enrichment degree to a combustion stability limit when the corrected enrichment degree exceeds the combustion stability limit on a side of a rich air-fuel mixture, and correcting the recovery rate in such a manner that amount of HC and CO to be supplied to the second catalyst when the corrected enrichment degree is restricted is generally equal to an amount of HC and CO to be supplied to the second catalyst when the corrected enrichment degree is not restricted.

An exhaust emission control system for an internal combustion engine, comprising:

• a first catalyst disposed in an exhaust gas of the engine, said first catalyst functioning to trap NOx in an atmosphere having an air-fuel ratio leaner than a stoichiometric level and to release NOx in an atmosphere having an air-fuel ratio richer than the stoichiometric level reduce released NOx in the presence of HC and CO;
• a second catalyst disposed in the exhaust gas passage upstream of said first catalyst
• and having an oxygen trapping ability;
• and
• a control unit that accomplishes an enrichment treatment by enriching stepwise an air-fuel ratio of an air-fuel mixture to be supplied to the engine to an enrichment degree at a timing at which a lean operation is changed to a stoichiometric operation in the engine, and recovering the enriched air-fuel ratio to the stoichiometric level at a recovery rate after enriching the air-fuel ratio,
• wherein the control unit estimates an oxygen trapping ability of said second catalyst and corrects the enrichment degree in stepwise enriching to decrease in correlation to a decrease in the estimated oxygen trapping ability, and
• wherein the control unit further restricts the corrected enrichment degree to a combustion stability limit when the corrected enrichment degree exceeds the combustion stability limit on a side of a rich air-fuel mixture, and corrects the recovery rate in a manner that an amount of HC and CO to the supplied to the second catalyst when the corrected enrichment degree is restricted is generally equal to the amount of HC and CO to be supplied to the second catalyst when the corrected enrichment degree is not restricted.

A method of controlling exhaust emission of an internal combustion engine provided with an exhaust emission control system including a first catalyst disposed in an exhaust gas passage of the engine, said first catalyst functioning to trap NOx in an atmosphere having an air-fuel ratio leaner than a stoichiometric level and to release NOx in an atmosphere having an air-fuel ratio richer than the stoichiometric level and reduce released NOx in presence of HC and CO, and a second catalyst disposed in the exhaust gas passage upstream of said first catalyst and having an oxygen trapping ability, the method comprising:

• accomplishing an enrichment treatment by enriching stepwise an air-fuel ratio of an air-fuel mixture to be supplied to the engine to an enrichment degree at a timing at which a lean operation is changed to a stoichiometric operation in the engine, and recovering the enriched air-fuel ratio to the stoichiometric level at a recovery rate immediately after enriching the air-fuel ratio;
• estimating the oxygen trapping ability of said second catalyst;
• and
• correcting the enrichment degree to decrease in correlation to a decrease in the estimated oxygen trapping ability.