Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
1999-06-08
2001-05-01
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S286000, C060S295000, C060S301000
Reexamination Certificate
active
06223525
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an air-fuel ratio controlling device for an internal combustion engine, and particularly to an air-fuel ratio controlling device for an internal combustion engine having an exhaust system which is provided with a NOx (nitrogen oxide) absorbent catalyst.
2. Description of the Related Art
When the lean operation is executed in such a way that the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine is set to be on the lean side with respect to a theoretical air-fuel ratio, the output amount of NOx tends to be increased.
Conventionally, a technique is therefore known in which a NOx absorbent catalyst that absorbs NOx is disposed in an exhaust gas passage of an engine so as to purify an exhaust gas. Such a NOx absorbent catalyst has properties such that, in a state where the air-fuel ratio is set to be on the lean side with respect to a theoretical air-fuel ratio and the oxygen concentration in the exhaust gas is relatively high (NOx is rich) (hereinafter, referred to as “exhaust gas-lean state”), the catalyst absorbs NOx, and, on the other hand, in a state where the air-fuel ratio is set to be on the rich side with respect to the theoretical air-fuel ratio, the oxygen concentration in the exhaust gas is low, and the HC and CO components are large (hereinafter, referred to as “exhaust gas-rich state”), the catalyst releases NOx which has been absorbed.
Conventionally, an air-fuel ratio controlling device is known in which such a NOx absorbent catalyst is disposed together with a three-way catalyst in an exhaust gas passage of an engine and the air-fuel ratio of an air-fuel mixture is periodically controlled to be on the leaner side and the richer side with respect to a theoretical air-fuel ratio (Japanese Patent No. 2,605,579). In the device, in view of a phenomenon that, when the air-fuel ratio is set to be on the leaner side with respect to the theoretical air-fuel ratio, the NOx purification rate of the three-way catalyst is lowered, the NOx absorbent catalyst is placed downstream from the three-way catalyst so as to absorb NOx.
In a device of the related arts, however, no consideration is given on reduction (release) of NOx which has been absorbed by the NOx absorbent catalyst, and hence there is the following problem.
When the air-fuel ratio is to be controlled so as to be in the vicinity of the theoretical air-fuel ratio, an air-fuel ratio on the richer side with respect to the theoretical air-fuel ratio is set a value at which HC and CO can be purified by means of the purification property of the three-way catalyst. On the downstream side of the three-way catalyst, therefore, the exhaust gas-rich state is not attained usually. Consequently, the reduction of the absorbed NOx in the NOx absorbent catalyst is insufficiently performed, thereby there is such a problem that the NOx absorption ability is gradually lowered and the output amount of NOx is increased.
SUMMARY OF THE INVENTION
The invention has been conducted in order to solve the problem. It is an object of the invention to provide an air-fuel ratio controlling device which, when an air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine with a NOx absorbent catalyst, being disposed in an exhaust system, is controlled by setting said ratio in the vicinity of a theoretical air-fuel ratio, the NOx absorption ability of the NOx absorbent catalyst can be maintained so as to prevent the output amount of NOx from being increased.
In order to achieve the object, one of the embodiments of this invention is an air-fuel ratio controlling device for an internal combustion engine, comprising: a NOx absorbent catalyst which is disposed in an exhaust system of an internal combustion engine, and which, in an exhaust gas-lean state, absorbs NOx in an exhaust gas, and, in an exhaust gas-rich state, reduces NOx which has been absorbed; air-fuel ratio detecting means which is disposed in the exhaust system; and air-fuel ratio controlling means for changing an air-fuel ratio of an air-fuel mixture to be supplied to the engine, to a leaner air-fuel ratio or a richer air-fuel ratio with respect to a theoretical air-fuel ratio, and is characterized in that the air-fuel ratio controlling device further comprises: NOx amount estimating means for, based on an output of the air-fuel ratio detecting means, estimating an amount of NOx absorbed in the NOx absorbent catalyst; and air-fuel ratio enriching means for, when the NOx amount estimated by the NOx amount estimating means reaches a preset value, enriching the air-fuel ratio of the air-fuel mixture to a value which is richer than the richer air-fuel ratio.
In the specification, the term “preset value” means a value corresponding to the NOx amount in the case where a NOx absorbent catalyst absorbs NOx to the limit of its NOx absorption ability, or that which is slightly smaller than the value.
According to this configuration, the air-fuel ratio of the air-fuel mixture to be supplied to the engine is set to a leaner air-fuel ratio or a richer air-fuel ratio with respect to a theoretical air-fuel ratio, and the amount of NOx absorbed in the NOx absorbent catalyst is estimated based on the output of the air-fuel ratio detecting means. When the estimated NOx amount reaches the preset value, the air-fuel ratio of the air-fuel mixture is set to be made richer than the richer air-fuel ratio. Therefore, NOx which has been absorbed by the NOx absorbent catalyst is appropriately reduced, so that the output amount of NOx can be prevented from being increased, while maintaining the NOx absorption ability.
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Denion Thomas
Honda Giken Kabushiki Kaisha
Sughrue Mion Zinn Macpeak & Seas, PLLC
Tran Diem
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