In general, there is known a NOx catalyst for purifying NOx (nitrogen oxide) contained in an exhaust gas as an exhaust purifying apparatus arranged in an exhaust system for an internal combustion engine such as a diesel engine or a lean burn gasoline engine. Various types of NOx catalysts are known as this type of NOx catalysts, and among them, there is well known a NOx storage reduction (NSR) catalyst for absorbing and removing NOx contained in the exhaust gas. The NOx storage reduction catalyst has an absorption/desorption function of absorbing NOx in the exhaust gas when an air-fuel ratio of an exhaust gas to be supplied is leaner (that is, oxygen excessive atmosphere) than a predetermined value (typically, theoretical air-fuel ratio) and desorbing the absorbed NOx when the air-fuel ratio of the exhaust gas to be supplied is richer (that is, oxygen-lacking atmosphere) than the predetermined value to reduce the NOx to N2.
On the other hand, for example, in a case of an engine boarded on an automobile, on-board diagnosis (OBD) on degradation of a catalyst is required by regulations of various countries for preventing the automobile from running in a state where the exhaust gas is deteriorated. Therefore, there are provided various conventional technologies for diagnosing the degradation in regard to the NOx storage reduction catalyst.
Degradation of the NOx storage reduction catalyst causes reduction in capabilities of absorbing NOx, that is, a NOx amount which the catalyst can absorb. Therefore, as a representative method of detecting degradation of a NOx catalyst, there is provided a method of detecting a NOx absorption capability of the NOx catalyst, which is compared with a predetermined degradation determination value.
In regard to the related technology, for example, Japanese Patent No. 3316066 discloses the technology in which a NOx concentration sensor is arranged downstream of a NOx catalyst and NOx emission amounts based upon an actual NOx concentration of the NOx concentration sensor are integrated for a predetermined time to diagnose presence/absence of a failure of the NOx catalyst with this integral value. In addition, Japanese Patent No. 3589179 discloses the technology in which when reducers are supplied for desorbing NOx from a NOx catalyst (NOx absorber), the extra reducers which are not used for desorbing the NOx are discharged downstream of the NOx catalyst in the form of ammonium, thereby detecting a degradation degree of the NOx catalyst. In this case, an ammonium concentration downstream of the NOx catalyst is detected and also a representative value of an amount of the extra reducers is found from a change of the ammonium concentration, detecting the degradation degree of the NOx catalyst based upon this representative value. The ammonium concentration downstream of the NOx catalyst is detected by a sensor which can detect both of a NOx concentration and an ammonium concentration in the exhaust gas.
Incidentally, under the present circumstance where exhaust gas regulations for automobiles have become tightened, the NOx emission regulation value is made very small and on the other hand, a value of the NOx emission by which the NOx catalyst should be determined to be degraded, that is, an OBD regulation value is also made very small. Particularly, according to U.S. SULEV (Super Ultra Low Emission Vehicle), not only the emission regulation value is tight, but also the OBD regulation value is 1.75 times as tight as the emission regulation value, as compared to Europe STEP IV or the like. That is, a difference in emission level and in catalyst degradation degree from the emission regulation value to the OBD regulation value is small and the small difference is required to be distinguished. Therefore, higher diagnosis precision is required also in regard to degradation diagnosis of the NOx catalyst.
For example, according to the technology described in Japanese Patent No. 3316066, the NOx emission amount is integrated based upon an output value of the NOx concentration sensor downstream of the NOx catalyst to diagnose presence/absence of a failure of the NOx catalyst based upon this integral value. However, the NOx emission amount herein means an emission amount of NOx which is not processed by the NOx catalyst and simply passes through the NOx catalyst (called “passing NOx”) as it is and the output value itself of the NOx concentration sensor is minutely small. In general, as an output value of the sensor is smaller, an error rate of the sensor is larger. Therefore, in consideration of the error of the sensor, there is the possibility that a value containing many error amounts is integrated, which is used for failure diagnosis. This technology is not necessarily sufficient in terms of securing high diagnosis precision
In addition, according to the technology described in Japanese Patent No. 3589179, the extra amount of the reducers among the reducers supplied to the NOx catalyst is decreased to calculate an amount of the reducers which is necessary and just enough for NOx desorption from the NOx catalyst. When this appropriate amount of the reducers is below a predetermined value, it is determined that the NOx catalyst is degraded by assuming that the NOx absorption capability of the NOx catalyst is sufficiently lowered (refer to paragraphs 0065 to 0068, P9) That is, the technology described in Japanese Patent No. 3589179 is adapted to measure the NOx absorption capability of the NOx catalyst and compare this value with a predetermined degradation determination value.
In this technology, the extra amount of the reducers is used for calculating an appropriate amount of the reducers and the amount of reducers to be supplied is feedback-controlled to be always appropriate regardless of whether the appropriate amount of the reducers are large or small (that is, even if the NOx absorption capability is high or low). In consequence, the extra amount of the reducers is generated relatively frequently without a failure, and an excessive amount of the reducers leads to deterioration of fuel consumption.