Air-fuel ratio control apparatus and air-fuel ratio control method for internal combustion engine

Abstract

A fuel injection amount is controlled so that the air-fuel ratio of exhaust gas flowing into a catalyst (6) oscillates around a stoichiometric air-fuel ratio. When it is estimated that the level of oxygen storage capacity of the catalyst (6) is lower than a predetermined reference capacity, the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio is reduced by controlling the fuel injection amount. The level of the oxygen storage capacity of the catalyst (6) is estimated based on the amplitude of the output signal from an oxygen sensor (14) disposed downstream of the catalyst (6).

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The invention relates to an air-fuel ratio control apparatus and an air-fuel ratio control method for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity.[0003]2. Description of the Related Art[0004]Japanese Patent Application Publication No. 5-321721 (JP-A-5-321721) describes an example of an air-fuel ratio control apparatus that includes a linear air-fuel ratio sensor disposed upstream of a catalyst, and an oxygen sensor disposed downstream of the catalyst. The air-fuel ratio control apparatus controls an air-fuel ratio based on signals output from the two sensors. The linear air-fuel ratio sensor has a linear output characteristic in which an output linearly changes in proportion to the air-fuel ratio. The oxygen sensor outputs a signal according to the concentration of oxygen in gas. The oxygen sensor has an output characteristic in which the outp...

AI Technical Summary

Benefits of technology

[0011]The invention provides an air-fuel ratio control apparatus and an air-fuel ratio control method for an internal combustion engine, which suppress the discharge of emissions even when the level of oxygen storage capacity of a catalyst is decreased.

[0015]The fuel injection amount control means may control the amount of fuel injected to the internal combustion engine by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio. The amplitude reduction means may reduce the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio on the feedback control by reducing a gain used to correct the amount of fuel injected to the internal combustion engine or by limiting a correction amount based on the difference between the signal output from the air-fuel ratio sensor and the reference value corresponding to the stoichiometric air-fuel ratio.

[0016]The fuel injection amount control means may control the amount of fuel injected to the internal combustion engine by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio while the fuel injection amount control means corrects the amount of fuel injected to the internal combustion engine based on a difference between the output signal from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio. The amplitude reduction means may reduce the amplitude of the oscillation of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio by reducing a gain used to correct the amount of fuel injected to the internal combustion engine or by limiting a correction amount based on the difference between the signal output from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio.

[0017]The air-fuel ratio control apparatus for the internal combustion engine may further include amplitude reduction stop means for stopping reducing the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when one of fuel supply cutoff control and fuel injection amount increase control is executed.

[0020]A second aspect of the invention relates to an air-fuel ratio control method for an internal combustion engine. The air-fuel ratio control method includes: controlling an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into a catalyst disposed in an exhaust passage for the internal combustion engine, oscillates around a stoichiometric air-fuel ratio; estimating a level of oxygen storage capacity of the catalyst; and reducing an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when it is estimated that the level of the oxygen storage capacity of the catalyst is lower than a predetermined reference capacity.

Problems solved by technology

However, recently, it has been found that the air-fuel ratio of the exhaust gas flowing into the catalyst as well as the deterioration of the catalyst influences the decrease in the level of the oxygen storage capacity of the catalyst.

However, it takes time to sufficiently recover the level of the oxygen storage capacity.

Thus, until the level of the oxygen storage capacity is sufficiently recovered, excessive emissions beyond the purification capacity of the catalyst are discharged into the atmosphere.

However, there is a possibility that emissions may be discharged from the catalyst due to the decrease in the level of the oxygen storage capacity of the catalyst when the air-fuel ratio fluctuates to some extent.

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