Abstract: An air-fuel ratio deviation calculated by an air-fuel ratio deviation calculation part is inputted to a cylinder-by-cylinder air-fuel ratio estimation part. A cylinder-by-cylinder air-fuel ratio is estimated in the cylinder-by-cylinder air-fuel ratio estimation part. In the cylinder-by-cylinder air-fuel ratio estimation part, attention is paid to gas exchange in an exhaust collective part of an exhaust manifold, and a model is created. In this model, a detection value of an A/F sensor is obtained by multiplying histories of the cylinder-by-cylinder air-fuel ratio of an inflow gas in the exhaust collective part and histories of the detection value of the A/F sensor by specified weights respectively and by adding them. The cylinder-by-cylinder air-fuel ratio is estimated on the basis of the model.

Abstract: An ECU calculates the air-fuel ratio of each cylinder based on a sensor signal from an A/F sensor arranged at the collecting portion of the exhaust manifold and feedback controls the fuel injection quantity for each cylinder by using the individual cylinder air-fuel ratio obtained. A collecting portion fuel quantity is calculated from the air-fuel ratio at the exhaust collecting portion based on from the A/F sensor signal, and the gas flow rate at the exhaust collecting portion is calculated using the gas flow rate history of each cylinder. An observer using the individual cylinder fuel quantity as a variable is constructed by a model in which the collecting portion fuel quantity is associated with the individual cylinder fuel quantity, so that the individual cylinder fuel quantity is estimated from the result of observation by the observer. Each cylinder's air-fuel ratio is calculated from the estimated individual cylinder fuel quantity.

Abstract: An ECU calculates the air-fuel ratio of each cylinder based on a sensor signal from an A/F sensor arranged at the collecting portion of the exhaust manifold and feedback controls the fuel injection quantity for each cylinder by using the individual cylinder air-fuel ratio obtained. A collecting portion fuel quantity is calculated from the air-fuel ratio at the exhaust collecting portion based on from the A/F sensor signal, and the gas flow rate at the exhaust collecting portion is calculated using the gas flow rate history of each cylinder. An observer using the individual cylinder fuel quantity as a variable is constructed by a model in which the collecting portion fuel quantity is associated with the individual cylinder fuel quantity, so that the individual cylinder fuel quantity is estimated from the result of observation by the observer. Each cylinder's air-fuel ratio is calculated from the estimated individual cylinder fuel quantity.

Abstract: An occluded oxygen quantity in a catalyst is estimated when fuel cut is executed. Then, upon return from the fuel cut, a target air-fuel ratio is set to significantly richer value. When it is detected on the basis of an output of an oxygen sensor that oxygen occluded by an upstream-side catalyst has been consumed, the target air-fuel ratio is switched to slightly richer value. Lastly, when the occluded oxygen quantity has become 0, a return is made to a normal air-fuel ratio feedback control. It is possible to consume the oxygen occluded by the catalyst quickly, and simultaneously it is possible to diminish emission released to the atmosphere even if an estimated value of the occluded oxygen quantity is deviated from an actual value.

Abstract: An exhaust gas cleaning system has two catalysts and three sensors. The target voltage of a second sensor arranged upstream of a downstream catalyst is set according to the output voltage of the second sensor and the output voltage of a third sensor. Thus, even if the output voltage of the second sensor shows a stoichiometric condition and the output voltage of the third sensor shows a rich condition, it is possible to restrict the unnecessary correction of the target voltage and to optimize the cleaning rate of the downstream catalyst. Therefore, it is possible to reduce emission from being impaired by an expected variation in the air-fuel ratio.

Abstract: An occluded oxygen quantity in a catalyst is estimated when fuel cut is executed. Then, upon return from the fuel cut, a target air-fuel ratio is set to significantly richer value. When it is detected on the basis of an output of an oxygen sensor that oxygen occluded by an upstream-side catalyst has been consumed, the target air-fuel ratio is switched to slightly richer value. Lastly, when the occluded oxygen quantity has become 0, a return is made to a normal air-fuel ratio feedback control. It is possible to consume the oxygen occluded by the catalyst quickly, and simultaneously it is possible to diminish emission released to the atmosphere even if an estimated value of the occluded oxygen quantity is deviated from an actual value.

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.

Abstract: If conditions for measuring a storage quantity of O2 in a catalyst are satisfied, a target air-fuel ratio is switched to a rich value after the end of a fuel-cut state. The quantity of a rich component contained in an exhaust gas flowing into a catalyst during a period of time period is calculated. The quantity of a rich component required to consume all O2 absorbed by the catalyst in the fuel-cut state is calculated. The computed quantity of a rich component is taken as a storage quantity of O2 in the catalyst. Since the air-fuel ratio of exhaust gas is changed to a rich value, to improve exhaust gas cleaning, during the computation of a storage quantity of O2, the storage quantity of O2 in the catalyst can be computed without worsening emission of exhaust gas.

Abstract: An upstream catalyst and a downstream catalyst are disposed in series in an exhaust pipe, and first through third sensors for detecting the air-fuel ratio or an adsorption amount of hazardous components on the rich/lean side of exhaust gases are disposed on the upstream and downstream sides of the upstream catalyst and the downstream side of the downstream catalyst, respectively. An ECU for controlling an engine controls the air-fuel ratio so that when the adsorption amount of the hazardous components on the rich side of the downstream catalyst is large, that of the hazardous components on the lean side of the upstream catalyst is large. Similarly, the ECU controls the air-fuel ratio so that when the adsorption amount of the hazardous components on the lean side of the downstream catalyst is large, that of the hazardous components on the rich side of the upstream catalyst is large.

Abstract: Exhaust gas sensors are provided at the upstream side and the downstream side of a catalyst, respectively. An intermediate target value is set on the basis of the output of the downstream-side exhaust gas sensor of preceding computation time and a final target value that is a final downstream-side target air-fuel ratio. The compensation amount of the upstream-side target air-fuel ratio is calculated on the basis of the deviation between the present output of the downstream-side exhaust gas sensor and the intermediate target value. At least one of an update amount and an update rate of the intermediate value, a control gain, a control period and a control range of a sub-feedback control is varied.

Abstract: A plurality of catalysts are installed in an exhaust pipe, air-fuel ratio sensors or oxygen sensors are installed upstream and downstream of each catalyst, and the air-fuel ratio of the exhaust gas is feedback controlled to a target air-fuel ratio based on the output of the air-fuel ratio sensor located upstream of the upstream catalyst. In this the exhaust gas is sufficiently purified with the upstream catalyst alone when the exhaust gas flow rate is small, the oxygen sensor located downstream of the upstream catalyst is used as the downstream sensor for setting a target air-fuel ratio. Furthermore, when the exhaust gas flow rate increases, the amount of exhaust gas components passing through without purification in the upstream catalyst is increased. Therefore, the downstream sensor used for setting the air-fuel ratio is switched to the oxygen sensor located downstream of the downstream catalyst.

Abstract: Exhaust gas sensors are provided at the upstream side and the downstream side of a catalyst, respectively. An intermediate target value is set on the basis of the output of the downstream-side exhaust gas sensor of preceding computation time and a final target value that is a final downstream-side target air-fuel ratio. The compensation amount of the upstream-side target air-fuel ratio is calculated on the basis of the deviation between the present output of the downstream-side exhaust gas sensor and the intermediate target value. At least one of an update amount and an update rate of the intermediate value, a control gain, a control period and a control range of a sub-feedback control is varied.

Abstract: An upstream catalyst and a downstream catalyst are disposed in series in an exhaust pipe, and first through third sensors for detecting the air-fuel ratio or an adsorption amount of hazardous components on the rich/lean side of exhaust gases are disposed on the upstream and downstream sides of the upstream catalyst and the downstream side of the downstream catalyst, respectively. An ECU for controlling an engine controls the air-fuel ratio so that when the adsorption amount of the hazardous components on the rich side of the downstream catalyst is large, that of the hazardous components on the lean side of the upstream catalyst is large. Similarly, the ECU controls the air-fuel ratio so that when the adsorption amount of the hazardous components on the lean side of the downstream catalyst is large, that of the hazardous components on the rich side of the upstream catalyst is large.

Abstract: An air-fuel ratio control system has a catalyst and controls the air-fuel ratio properly in accordance with the in-catalyst state. The in-catalyst state amount variation is calculated based on the deviation between the actual excess fuel factor detected by means of an air-fuel ratio sensor on the upstream side of the catalyst and the target excess fuel factor, and then accumulated to obtain the current in-catalyst state amount. At that time, the previous air flow derived earlier by a lag time corresponding to the time period from the time of fuel injection to the time of detection of the excess fuel factor of exhaust gas is used as the air flow. The calculated in-catalyst state amount value is subjected to guard processing, and thereafter the target excess fuel factor on the upstream side of the catalyst is calculated by use of various gains and control parameters.