Controller of exhaust gas purifying agent and exhaust gas purifying system

Abstract

A quantity of a purifying agent that is added into an exhaust gas, reacts with a specific component in the exhaust gas, and purifies the exhaust gas is controlled. A controller is configured so as to be provided with a program to carry out either increment control for increasing the quantity of urea water sequentially or decrement control for decreasing the quantity of the urea water sequentially while monitoring, in NOx in the exhaust gas, the quantity not purified through the reaction with NH3 (the discharged NOx quantity); and switch the quantity control from the currently adopted increment or decrement control to the other increment or decrement control in response to whether the feature of the change of the discharged NOx quantity comes to a predetermined feature.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application is based on Japanese Patent Application No. 2007-46062 filed on Feb. 26, 2007, the disclosure of which is incorporated herein by reference.FIELD OF THE INVENTION[0002]The present invention relates to an exhaust gas purifying agent controller to control a quantity of a purifying agent, for example ammonia (NH3) added in the form of a urea aqueous solution or the like, to purify an exhaust gas by reacting with a specific component, for example nitrogen oxide (NOx) or the like, in the exhaust gas. The present invention also relates to an exhaust gas purifying system to purify an exhaust gas by activating exhaust gas purifying reaction with a predetermined purifying agent on a catalyst as represented by a urea Selective Catalytic Reduction (SCR) system.BACKGROUND OF THE INVENTION[0003]In power plants, various kinds of factories, automobiles, and others, a urea SCR system has been being developed. In the case of an automobile (...

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Benefits of technology

[0012]That is, as stated above, the quantity of a purifying agent takes a so-called optimum value when the quantity (or the purifying rate) of a purified specific component, for example NOx, in the specific component to be purified is the maximum and the quantity of the redundant purifying agent (a redundant NH3 quantity) or the quantity of the discharged purifying agent (a discharged NH3 quantity) is the minimum. The minimum quantity required for making the quantity of a purified specific component maximum (that is, making the unpurified specific component quantity minimum) corresponds to the aforementioned optimum value, and the following results have been obtained with regard to the relationship between the quantity of a purifying agent and the quantity of a purified specific component. That is, when a purified quantity approaches the maximum value from the state of a low purified quantity level while the quantity is increased sequentially, the state of the change (the feature of the change) of the purified quantity before the purified quantity comes to the maximum is largely different from the state after the purified quantity comes to the maximum. Meanwhile, when a purified quantity is reduced by sequentially decreasing the quantity from the state where the purified quantity is the maximum, the state of the change (the feature of the change) of the purified quantity when the purified quantity is the maximum is largely different from the state where the purified quantity is lower than the maximum. With regard to a purified quantity, the quantity of a purifying agent at the turning point of the feature of the change corresponds to the minimum quantity required for making an unpurified specific component quantity minimum (the purified quantity is the maximum), namely the optimum value of the quantity. According to the present invention, even when the quantity of a purifying agent is in an excessive region or an insufficient region, it is possible to bring the quantity of a purifying agent close to an optimum value by either the first or second quantity control. Even when the quantity of the unpurified specific component itself is not confirmed, it is possible to control the quantity of a purifying agent to an optimum value by the feature of the change of an unpurified specific component quantity. As a result, even when the optimum value of the quantity varies due to aging or the like, the accuracy (convergence to an optimum value) of the quantity control can be maintained at a high level.

[0017]By adopting either the first or second quantity control, it is possible to bring the quantity of a purifying agent close to an optimum value from an excessive region or an insufficient region. As already mentioned, accuracy in quantity control (convergence to an optimum value) can be maintained at a high level even when aging or the like exists.

[0018]The aspect (trend) of the variation of an unreacted purifying agent quantity or a discharged purifying agent quantity against the change of the quantity is opposite the aspect (trend) of the variation of an unpurified specific component quantity. Consequently, it is effective to configure that either the first quantity control or the second quantity control is the control to decrease the quantity of the purifying agent sequentially at a predetermined rate and to stop when the quantity of an unreacted purifying agent exceeds an allowable range and does not change any more even when the quantity is decreased; and the other quantity control is the control to increase the quantity of the purifying agent sequentially at a predetermined rate and to stop when the quantity of the unpurified specific component exceeds an allowable range and changes by the increase of the quantity. It is possible to control a quantity easily in an appropriate manner.

[0026]When the practicability of the above invention is considered in view of the demand from society (public interest), as represented by the aforementioned urea SCR (Selective Catalytic Reduction) system, a NOx purifying device that uses a urea aqueous solution (urea water) as an additive is expected as an exhaust gas purifying system to purify NOx (nitrogen oxide) in an exhaust gas at a high purifying rate. The reason why NH3 is added in the form of a urea aqueous solution is that a urea aqueous solution is easier to handle (less poisonous) than NH3. Consequently, considering the practicability of the current situation, it is effective to configure a controller such that the specific component is NOx and the purifying agent is NH3 produced by being added in the form of a urea aqueous solution and decomposing in an exhaust gas. Further, when such a urea SCR system is adopted in an automobile and a controller is mounted on a vehicle or the like having a diesel engine for example, it is possible to realize the improvement of fuel consumption and the reduction of PM while allowing NOx to be generated during combustion. This is a technology that largely contributes to the improvement in the performance of an automobile and the cleaning of an exhaust gas that is a key to the prevalence of diesel engines.

Problems solved by technology

Consequently, it is difficult to obtain an optimum purifying agent quantity from those parameters.

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