Exhaust gas purifying apparatus for internal combustion engine

Abstract

An exhaust pipe is provided with an oxidation catalyst, a SCR catalyst (ammonia selective reduction catalyst), and an ammonia slip catalyst. In the exhaust pipe, the urea water adding valve is provided between the oxidation catalyst and the SCR catalyst. A NOx sensor which detects the NOx quantity in an exhaust gas is provided downstream of the SCR catalyst. An ECU controls the urea water adding valve to add the urea water to the exhaust gas. While the urea water is added to the exhaust gas, the ECU successively obtains a NOx sensor output, and computes an adding quantity command value in which the NOx sensor output becomes minimum.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application is based on Japanese Patent Application No. 2007-290828 filed on Nov. 8, 2007, the disclosure of which is incorporated herein by reference.FIELD OF THE INVENTION[0002]The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, which is preferably applied to an exhaust gas purifying system of Selective Catalytic Reduction (SCR) type using ammonia reducing agent such as urea aqueous solution (urea water).BACKGROUND OF THE INVENTION[0003]A urea Selective Catalytic Reduction (SCR) system has been developed as an exhaust gas purifying apparatus which purifies nitrogen oxide (NOx) in an exhaust gas exhausted from an internal combustion engine, especially from a diesel engine. The urea SCR system has following configuration:[0004]That is, in the urea SCR system, a selective catalytic reduction type NOx reduction catalyst (SCR catalyst) is provided in an exhaust gas pipe connected to an e...

AI Technical Summary

Benefits of technology

[0007]The present invention is made in view of the above matters, and it is an object of the present invention to provide an exhaust gas purifying apparatus for an internal combustion engine, which can correctly detect NOx quantity downstream of the NOx reduction catalyst so that the NOx purifying ratio can be appropriately computed.

[0012]Thereby, the surplus quantity of the reducing agent (ammonia slip quantity) is reduced and the NOx purifying ratio becomes maximum. As the result, the NOx quantity downstream of the NOx reduction catalyst is correctly detected, so that the NOx purifying ratio can be properly computed.

[0015]That is, there is a possibility that the most appropriate adding quantity command value may exists between the maximum adding quantity and the minimum adding quantity, in which the NOx sensor output becomes minimum. In such a case, the reducing agent addition is performed again while the variation width is made small, whereby the optimum value of the adding quantity command value can be accurately obtained.

[0017]According to this configuration, the reducing agent adding quantity is varied only in a direction where the minimum sensor output exists. Thus, the increase / decrease process of the reducing agent adding quantity can be simplified.

[0019]According to another aspect of the invention, it is desirable that the adding quantity command value is not newly computed when a difference between a minimum value of the sensor output and a maximum value of the sensor output, which are obtained due to a variation in adding quantity of the reducing agent, is within a specified value. That is, at vicinity where the NOx sensor output is minimum, the NOx sensor output hardly varies even if the reducing agent adding quantity is varied. Hence, unnecessary computation (update) of the adding quantity command value can be avoided.

[0020]According to another aspect of the invention, a period during which the reducing agent adding quantity is decreased is longer than a period during which the reducing agent adding quantity is increased. That is, comparing a case that the urea water adding quantity is increased with a case that the urea water adding quantity is decreased, a response speed of the NOx sensor differs. The response speed is slow in the latter case. This is because the reducing agent consuming speed in the NOx reduction catalyst is slower than the reducing agent adsorbing speed (ammonia adsorbing speed) in the NOx reduction catalyst. According to the above configuration, the NOx sensor output can be appropriately obtained even in both cases where the reducing agent adding quantity is increased or decreased.

Problems solved by technology

In such a case, there is a possibility that the NOx purifying ratio is erroneously computed based on the NOx sensor output.

If the accuracy of the NOx purifying ratio is deteriorated, the accuracy of the ammonia adsorbed quantity and the reducing agent added quantity is also deteriorated, which may cause a decrease of the NOx purifying ratio and an increase of the ammonia slip quantity.

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