Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
2001-04-10
2004-12-28
Elve, M. Alexandra (Department: 1725)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S168000
Reexamination Certificate
active
06835357
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control system for an internal combustion engine, and particularly to an exhaust emission control system which includes an NOx removing device for removing NOx (nitrogen oxides) and an oxygen concentration sensor arranged downstream of the NOx removing device, and determines deterioration of the NOx removing device and abnormality of the oxygen concentration sensor.
An exhaust emission control system in which a three-way catalyst for purifying exhaust gases is arranged in an exhaust system of an internal combustion engine and two oxygen concentration sensors are respectively provided upstream and downstream of the three-way catalyst is well known. In this system, an air-fuel ratio of an air-fuel mixture to be supplied to the engine is feedback-controlled according to outputs of the two oxygen concentration sensors. With respect to this exhaust emission control system, a method for determining abnormality of the downstream oxygen concentration sensor is disclosed in Japanese Patent Publication No. 2826564.
In this abnormality determining method, the air-fuel ratio is changed to the rich region with respect to the stoichiometric ratio and is held in the rich region when an output of the oxygen concentration sensor arranged downstream of the three-way catalyst exhibits a lean air-fuel ratio. If the output of the downstream oxygen concentration sensor exhibits a lean air-fuel ratio over a predetermined time period, while the output of the oxygen concentration sensor arranged upstream of the three-way catalyst exhibits a rich air-fuel ratio, it is determined that the downstream oxygen concentration sensor is abnormal.
When the air-fuel ratio of the air-fuel mixture supplied to an internal combustion engine is set to the lean region with respect to the stoichiometric ratio, that is, when a so-called lean operation is carried out, the amount of NOx exhausted from the engine tends to increase. To cope with the NOx, a technique for reducing exhaust emission has been known, in which an NOx removing device containing an NOx absorbent for absorbing NOx is provided in an exhaust system of the engine, to thereby reduce the exhaust emission. Another technique is known where an oxygen concentration sensor provided downstream of an NOx removing device determines the deterioration of the NOx removing device according to an output of the downstream oxygen concentration sensor (Japanese Patent Laid-open No. Hei 10-299460).
In the operation of an engine having an NOx removing device, a lean operation is carried out for a long period of time. Therefore, the output of an oxygen concentration sensor arranged downstream of the NOx removing device, in the exhaust system, exhibits a lean air-fuel ratio for a long period of time. Further, when the air-fuel ratio is set to a rich region, NOx absorbed in the NOx removing device is reduced.
Accordingly, if the technique disclosed in the above document, Japanese Patent Publication No. 2826564, is applied to the oxygen concentration sensor arranged downstream of the NOx removing device, it is difficult to appropriately execute the abnormality determination at a suitable timing when the output of the oxygen concentration sensor changes. Specifically, in order to positively change an output of an oxygen concentration sensor provided downstream of an NOx removing device to a rich region, it may often required to continue to operate in the rich region over a long period of time. This may cause an adverse effect on the exhaust emission characteristic and the engine drivability.
Further, in the operation of an engine having an NOx removing device, a large proportion of time is spent in lean operation. Therefore, it may be expected that an abnormality in a downstream oxygen concentration sensor can be rapidly determined.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an exhaust emission control system for an internal combustion engine that is capable of executing abnormality determination of an oxygen concentration sensor provided downstream of an NOx removing device with suitable timing, to suppress the adverse effects of the abnormality determination on the exhaust emission characteristic and the engine drivability.
To achieve the above object, according to an aspect of the present invention, there is provided an exhaust emission control system for an internal combustion engine, including: nitrogen oxides removing means provided in an exhaust system of the engine for removing nitrogen oxides in exhaust gases in a condition where an air-fuel ratio of an air-fuel mixture supplied to the engine is set to a lean region with respect to a stoichiometric ratio; an oxygen concentration sensor provided downstream of the nitrogen oxides removing means for detecting the concentration of oxygen in the exhaust gases; deterioration determining means for determining deterioration of the nitrogen oxides removing means on the basis of an output of the oxygen concentration sensor after the air-fuel ratio has been changed from the lean region to a rich region with respect to the stoichiometric ratio; and abnormality determining means for determining abnormality of the oxygen concentration sensor on the basis of an output of the oxygen concentration sensor during a period in which the air-fuel ratio is kept in the rich region with respect to the stoichiometric ratio immediately after execution of the deterioration determination by the deterioration determining means.
With this configuration, the deterioration determination of the nitrogen oxides removing means is executed on the basis of the output of the oxygen concentration sensor downstream of the nitrogen oxides removing means after the air-fuel ratio of an air-fuel mixture to be supplied to the engine has been changed from the lean region to the rich region, and the abnormality of the oxygen concentration sensor is determined on the basis of the output of the oxygen concentration sensor during a period in which the air fuel-ratio is kept in the rich region with respect to the stoichiometric ratio immediately after execution of the deterioration determination. Since the air-fuel ratio is changed to the rich region by executing the deterioration determination of the NOx removing means and thereby the amount of NOx absorbed in the NOx removing means is reduced, the oxygen concentration on the downstream side of the NOx removing means can be positively lowered by keeping the air-fuel ratio in the rich region only for a very short period of time after the deterioration determination of the NOx removing means. Accordingly, the abnormality determination of the oxygen concentration sensor can be accurately performed by monitoring the output of the oxygen concentration sensor in the above very short period of time in which the air-fuel ratio is kept in the rich region after the deterioration determination. That is, by performing the abnormality determination of the oxygen concentration sensor immediately after the deterioration determination of the NOx removing means, it is possible to minimize the period of time in which the enrichment of the air-fuel ratio is kept for the abnormality determination, and hence to suppress the adverse effect of the enrichment on the exhaust emission characteristic and the drivability of the engine.
In the above exhaust emission control system, preferably, the abnormality determination by the abnormality determining means is executed if the output of the oxygen concentration sensor is unchanged before the end of the deterioration determination by the deterioration determining means.
With this configuration, the abnormality determination by the abnormality determining means is executed if the output of the oxygen concentration sensor is unchanged before the end of the deterioration determination by the deterioration determining means. In other words, if the output of the oxygen concentration sensor has changed before the end of the deterioration determination by the deterioration deter
Hashimoto Akira
Miyashita Kotaro
Arent Fox
Elve M. Alexandra
Honda Giken Kogyo Kabushiki Kaisha
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