Air-fuel ratio control apparatus for exhaust gas from...

Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...

Reexamination Certificate

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Details

C060S276000, C060S277000, C123S679000, C701S103000, C701S109000

Reexamination Certificate

active

06351943

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for controlling the air-fuel ratio of an exhaust gas emitted from an internal combustion engine, and more particularly to an apparatus for controlling the air-fuel ratio of an exhaust gas that is purified by a catalytic converter of the nitrogen-oxide-absorption type that is disposed in the exhaust passage of an internal combustion engine.
2. Description of the Related Art
The applicant of the present application has proposed a technique for controlling the air-fuel ratio of an exhaust gas that enters a catalytic converter, or more specifically the air-fuel ratio of a combusted air-fuel mixture which, when burned, enters as an exhaust gas into a catalytic converter and is recognized as the concentration of oxygen in the exhaust gas, as disclosed in Japanese laid-open patent publication No. 11-93740, for example.
According to the disclosed system, an exhaust gas sensor (O
2
sensor) for detecting the concentration of a certain component, e.g., oxygen, of the exhaust gas that has passed through the catalytic converter is disposed downstream of the catalytic converter, and the air-fuel ratio of the exhaust gas that enters the catalytic converter is controlled depending on the output of the exhaust gas sensor, i.e., the detected value of the concentration of oxygen.
Specifically, the purifying capability of a catalytic converter, i.e., the ability of a catalytic converter to purify NOx (nitrogen oxide), HC (hydrocarbon), CO (carbon monoxide), etc. is optimum irrespectively of the deteriorated state of the catalytic converter when the air-fuel ratio of the exhaust gas that enters the catalytic converter is close to a stoichiometric air-fuel ratio and the output of the O
2
sensor as the exhaust gas sensor is settled to a certain output value. According to the above proposed technique, therefore, the certain output value is used as a target value for the output of the O
2
sensor, and the air-fuel ratio of the exhaust gas that enters the catalytic converter is controlled according to a feedback control process in order to converge the output of the O
2
sensor to the target value.
An exhaust system ranging from an upstream side of the catalytic converter to the O
2
sensor disposed downstream of the catalytic converter, i.e., a system for generating the output of the O
2
sensor from the air-fuel ratio of the exhaust gas that enters the catalytic converter, generally has a relatively long dead time owing to the catalytic converter included in the exhaust system. Stated otherwise, when the air-fuel ratio of the exhaust gas that enters the catalytic converter is changed, a relatively long dead time is required until the output of the O
2
sensor reflects the change in the air-fuel ratio. According to the above proposed technique, data representing an estimated value of the output of the O
2
sensor after the dead time of the exhaust system is sequentially determined. Then, a manipulated variable defining an air-fuel ratio for the exhaust gas entering the catalytic converter, i.e., a target air-fuel ratio for the exhaust gas, is sequentially generated in order to converge the estimated value of the output of the O
2
sensor which is represented by the above data to the target value, and the air-fuel ratio of an air-fuel mixture actually combusted by the internal combustion engine is manipulated depending on the target air-fuel ratio. In this manner, the effect of the dead time is compensated for, and the control process for converging the output of the O
2
sensor to the target value is stably carried out.
Some generally known internal combustion engines mounted on automobiles or the like, i.e., so-called lean-burn engines, are operated such that the air-fuel ratio of an air-fuel mixture combusted by the internal combustion engine and hence the air-fuel ratio of an exhaust gas entering a catalytic converter are controlled at a lean air-fuel ratio, which represents less fuel than at the stoichiometric air-fuel ratio, depending on operating conditions (rotational speed, intake pressure, demanded load, etc.) of the internal combustion engine in order to reduce the fuel consumption and also minimize the amount (absolute amount) of harmful gases contained in the exhaust gas.
While the internal combustion engine is being operated to control the air-fuel ratio at the lean air-fuel ratio, however, it is not possible to control the air-fuel ratio of the exhaust gas that enters the catalytic converter in order to converge the output of the O
2
sensor disposed downstream of the catalytic converter to the target value according to the above proposed technique. Under some operating conditions of the internal combustion engine, it is not possible or not preferable to operate the internal combustion engine to control the air-fuel ratio at the lean air-fuel ratio.
If the above proposed technique for achieving the optimum purifying capability of the catalytic converter is applied to the above internal combustion engine, then the internal combustion engine is operated in different modes including an operation. mode (hereinafter referred to as “stoichiometric operation mode”) in which the air-fuel ratio of the exhaust gas that enters the catalytic converter is controlled at an air-fuel ratio close to the stoichiometric air-fuel ratio in order to converge the output of the O
2
sensor disposed downstream of the catalytic converter to the target value, and an operation mode (hereinafter referred to as “lean operation mode”) in which the air-fuel ratio of the exhaust gas that enters the catalytic converter is controlled at a lean air-fuel ratio. Control processes of these operation modes are selectively carried out depending on operating conditions of the internal combustion engine.
While an internal combustion engine is operating in a lean operation mode, the amount of NOx contained in the exhaust gas emitted from the internal combustion engine is generally relatively large. Therefore, the internal combustion engine is combined with an NOx-absorption catalytic converter.
The NOx-absorption catalytic converter comprises a three-way catalyst and an NOx absorbent. NOx absorbents that are available includes an occlusion-type NOx absorbent for occluding NOx when the air-fuel ratio of the exhaust gas entering the catalytic converter is a lean air-fuel ratio and the oxygen concentration in the exhaust gas is relatively high, i.e., NOx in the exhaust gas is relatively high, and an adsorption-type NOx absorbent for adsorbing NOx in the exhaust gas when the air-fuel ratio of the exhaust gas entering the catalytic converter is a lean air-fuel ratio. Irrespectively of whether it is of the occlusion type or the adsorption type, an NOx adsorbent reduces NOx that has been absorbed (occluded or adsorbed) at the lean air-fuel ratio when the air-fuel ratio of the exhaust gas that enters the catalytic converter is a stoichiometric air-fuel ratio or a rich air-fuel ratio (at which the fuel is more than at the stoichiometric air-fuel ratio) and the oxygen concentration in the exhaust gas is relatively low.
More specifically, when the air-fuel ratio of the exhaust gas that enters the catalytic converter becomes a stoichiometric air-fuel ratio or a rich air-fuel ratio, the occlusion-type NOx absorbent discharges the occluded NOx, and the discharged NOx is reduced by a reducing agent such as CO, H
2
, or the like in the exhaust gas. When the air-fuel ratio of the exhaust gas that enters the catalytic converter becomes a stoichiometric air-fuel ratio or a rich air-fuel ratio, the adsorbed NOx in the adsorption-type NOx absorbent is reduced by the reducing agent in the exhaust gas, and the reduced nitrogen gas is discharged from the NOx absorbent.
The occlusion-type NOx absorbent comprises barium oxide (BaO), and the adsorption-type NOx absorbent comprises sodium (Na), titanium (Ti), or strontium (Sr).
When the internal combustion engine with the NOx-absorption catalytic converter in the exhaust passage is operating in the lean ope

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