Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2001-10-03
2003-04-01
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...
C060S276000, C060S299000
Reexamination Certificate
active
06539707
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This application is based on and incorporates herein by reference Japanese Patent Application Nos. 2000-308001 filed on Oct. 3, 2000, 2001-31532 filed on Feb. 7, 2001, 2001-65962 filed on Mar. 9, 2001, 2001-77396 filed on Mar. 19, 2001, and 2001-83964 filed on Mar. 23, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exhaust emission control system for an internal combustion engine, in which a plurality of catalysts or a plurality of catalyst groups are disposed in series in an exhaust passage.
2. Description of Related Art
In recent years, to increase the capability of reducing hazardous substances in exhaust gas of an engine, two catalysts for exhaust emission control are disposed in series at some midpoint of an exhaust pipe of the engine. According to the method, an air-fuel ratio sensor (or oxygen sensor) is disposed on each of the upstream side of an upstream catalyst and the downstream side of a downstream catalyst. An air-fuel ratio closed loop control is performed by detecting the air-fuel ratio of exhaust gas flowing in the upstream catalyst by the upstream sensor and making the detected air-fuel ratio coincide with a target air-fuel ratio. The air-fuel ratio of the exhaust gas passed through the downstream catalyst is detected by the downstream sensor, and the target air-fuel ratio on the upstream side is corrected so that the air-fuel ratio detected on the downstream side coincides with a predetermined value.
Generally, conversion efficiency of a catalyst varies according to a state of adsorbing hazardous components which are generated in a state where the air-fuel ratio is lean (hereinbelow, called components on the lean side) and hazardous components which are generated in a state where the air-fuel ratio is rich (hereinbelow, called components on the rich side) of the catalyst. At and around the stoichiometric air-fuel ratio, the catalyst reduces both components on the rich side (HC, CO, and the like) and components on the lean side (NOx and the like) in the exhaust gas most efficiently, and the highest catalytic conversion efficiency can be obtained. In the conventional air-fuel ratio feedback system, however, there is a tendency that when the amount of adsorbing the components on the rich side of the upstream catalyst is large, that of the downstream catalyst is also large. When the amount of adsorbing the components on the lean side of the upstream catalyst is large, that of the downstream catalyst is also large. As a result, there is a tendency that the states of both the upstream and downstream catalysts are controlled in the same way. Thus, the exhaust gases cannot be treated by efficiently using the two catalysts. Considering that two catalysts are used, an effect of improving the catalytic conversion efficiency is not so great.
In the above-described system, it is desirable to set the adsorption state of both of the upstream and downstream catalysts to a stoichiometric state as much as possible during the engine operation. However, depending on the driving conditions, in order to save the fuel or to prevent an excessive increase in the engine rotation, there is a case such that the fuel cut is executed. Since oxygen in the air taken in the cylinders is not used for combustion but is exhausted as it is to the exhaust pipe during the fuel cut, the lean-side components (oxygen) in the exhaust gases entering the catalysts largely increase, and the lean-side component adsorption amount of the catalysts largely increases. Thus, JP-A-6-200803 and JP-A-8-193537 disclose the techniques such that when the fuel cut is finished and the fuel injection is restarted, the air-fuel ratio is set temporarily to the rich side to make the lean-side components (oxygen) adsorbed by a catalyst react with the rich-side components (HC, CO, and the like) in the exhaust gases, thereby promptly decreasing the lean-side component adsorption amount of the catalyst.
In each of the two publications, only one catalyst is disposed in the exhaust pipe. It can be considered to apply the technique of JP-A-6-200803 to a system having two catalysts as follows. When the fuel cut is finished and the fuel injection is re-started, the rich-side control for setting the air-fuel ratio temporarily to the rich side by about 5-10% is performed to reduce the lean-side component adsorption amount of the catalysts. By the operation, when the output of an air-fuel ratio sensor (or oxygen sensor) on the downstream side changes to a rich output, the rich-side control is stopped and the program returns to the normal control.
However, as the lean-side component adsorption amount of the catalysts decreases during the rich-side control, the amount of rich-side components necessary to reduce the lean-side components also decreases. If the degree of richness in the air-fuel ratio during the rich-side control is fixed, the setting of the air-fuel ratio to the rich side is insufficient when the lean-side component adsorption amount of the catalysts is large at an initial stage of the rich-side control. On the contrary, as the lean-side component adsorption amount of the catalyst becomes small at the end of the rich-side control, the setting of the air-fuel ratio to the rich side becomes excessive, and a rich-side component exhaust amount to the atmosphere increases.
In order to solve the drawback, in JP-A-8-193537, an oxygen adsorption amount of the catalyst during the rich-side control is estimated and the degree of richness is changed according to the oxygen adsorption amount. However, since the maximum oxygen adsorption amount changes by the change with time of each of the catalysts, it is difficult to estimate the oxygen adsorption amount of each catalyst with high accuracy. It is accordingly difficult to properly change the degree of richness in association with the change in the actual oxygen adsorption amount of each of the catalysts during the rich-side control.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide an exhaust emission control system of an internal combustion engine with increased catalytic conversion efficiency, capable of efficiently reducing hazardous components in exhaust gas by efficiently using a plurality of catalysts (or catalyst groups) disposed in series in an exhaust passage.
According to a first aspect of the present invention, in an exhaust emission control system of an internal combustion engine, a state of a catalyst or a catalyst group disposed on the upstream side (hereinbelow, called “upstream catalyst”) is detected or estimated by upstream catalyst state detecting means, and a state of a catalyst or a catalyst group disposed on the downstream side (hereinbelow, called “downstream catalyst”) is detected or estimated by downstream catalyst state detecting means. As shown in
FIG. 6
, an air-fuel ratio is controlled by air-fuel ratio control means so that one of the states of the upstream and downstream catalysts is that an adsorption amount of hazardous components on the rich side is large and the other one is that an adsorption amount of hazardous components on the lean side is large.
For example, when the adsorption amount of the components on the rich side of the upstream catalyst is large, the conversion efficiency of the components on the lean side (NOx and the like) in exhaust gases of the upstream catalyst is high but the conversion efficiency of the components on the rich side (HC, CO, and the like) of the upstream catalyst is relatively low. Consequently, the amount of the components on the rich side in the exhaust gases flowing from the upstream catalyst becomes relatively large. In this case, it is controlled so that the adsorption amount of the components on the lean side of the downstream catalyst is large. Therefore, the components on the rich side which cannot be reduced by the upstream catalyst can be efficiently reduced by the downstream catalyst in which the adsorption amount of the components on the lean side is large. On the other hand, when the
Iida Hisashi
Ikemoto Noriaki
Shimizu Kouichi
Yamashita Yukihiro
Denion Thomas
Denso Corporation
Nixon & Vanderhye P.C.
Tran Diem
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