Addition-amount controller for exhaust gas purifying agent and exhaust emission control system

Abstract

In an addition-amount controller for an exhaust gas purifying agent to be used for an exhaust emission control system of an engine, one mode is executed based on satisfaction of an execution condition for each mode, from among a plurality of control modes. The control modes includes a purification control mode in which an addition amount of NH3 or an additive serving as a generating source of the NH3 is determined according to a predetermined parameter associated with an amount of NOx in the exhaust gas, and a storage control mode in which the addition amount is set to be larger than that in the purification control mode. When a rotation speed of an output shaft of the engine is determined to be decelerated from a higher speed state than an allowable level, the execution condition of the storage control mode is satisfied after a predetermined dissatisfaction time period from start of the deceleration.

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]This application is based on Japanese Patent Application No. 2007-168405 filed on Jun. 27, 2007, the contents of which are incorporated herein by reference in its entirety.FIELD OF THE INVENTION[0002]The present invention relates to an addition-amount controller for an exhaust gas purifying agent, for controlling an amount of addition of NH3 for purifying exhaust gas by reaction with NOx in the exhaust gas. The invention also relates to an exhaust emission control system e.g., a urea-SCR system, for purifying exhaust gas by an exhaust gas purifying reaction based on NH3 on a catalyst.BACKGROUND OF THE INVENTION[0003]In recent years, urea-SCR (selective reduction) systems have been developed in electric power plants, various factories, vehicles, and the like. Particularly, in the field of vehicles (especially, a diesel engine vehicle), post treatment techniques of exhaust gas for purifying and reducing NOx (nitrogen oxides) in the exhaust g...

AI Technical Summary

Benefits of technology

[0006]The inventors of the present application take into consideration the facts that the general catalyst for purifying the exhaust gas used in a vehicle-mounted internal combustion engine or the like can store NH3 and that the larger the amount of the stored NH3, the lower the activation temperature (critical reaction temperature) of the catalyst. Thus, the inventors now conduct various studies on and have developed an addition-amount controller for an exhaust gas purifying agent so as to improve an exhaust gas purification capacity using a decrease in activation temperature (critical reaction temperature) due to the storage of NH3.

[0009]According to the present invention, an addition-amount controller for an exhaust gas purifying agent is configured to be applied to an exhaust emission control system for purifying exhaust gas emitted from an internal combustion engine. The exhaust emission control system includes a catalyst for promoting a specific exhaust gas purification reaction in a temperature range having a critical reaction temperature as a lower limit, and an addition valve for adding an additive of NH3 (ammonia) or an additive serving as a generating source of the NH3 to the catalyst itself or the exhaust gas on an upstream side with respect to the catalyst, the additive being adapted to purify NOx (nitrogen oxides) in the exhaust gas by the exhaust gas purification reaction on the catalyst. The addition-amount controller is adapted to control an amount of addition by the addition valve, and the catalyst has properties of storing NH3 and further decreasing the critical reaction temperature as the amount of NH3 storage is increased.

[0014]Generally, the amount of NH3 for effectively purifying NOx in the exhaust gas (and an amount of addition by the addition valve) is different from an amount of NH3 that is appropriate to be stored (and an amount of addition by the addition valve, appropriate to be stored). This is because when storing of NH3, the amount of NH3 to be further stored is needed in addition to the amount of NH3 to be consumed for purifying the NOx. Any one controller of the above first to fourth aspects of the invention is made taking into consideration this fact with the above-mentioned property of the catalyst that the larger the amount of stored NH3, the lower the critical reaction temperature. Such a controller according to any one of the first to fourth aspects of the present invention can enhance the purification capacity of the catalyst at a low temperature by decreasing the critical reaction temperature of the catalyst through execution of the above-mentioned storage control mode. Further, at the time of deceleration or the like when the catalyst is supposed to be in a high temperature state, or in a similar situation thereto, the mode selection means does not start storing of NH3 immediately in deceleration of the internal combustion engine to be purified, in a decrease of load on the engine, or in start of the fuel cut. However, the mode selection means starts the storing of NH3 after a predetermined dissatisfaction time period that can be set, for example, by the time, the temperature of the catalyst, or the like. Accordingly, the controller according to any one aspect of the above first to fourth aspects of the present invention can suitably suppress the deterioration of the emission characteristics, while reducing the deterioration of the emission characteristics due to the exertion of the unnecessary storage of the NH3.

[0019]In an operating mode in which the load applied to the output shaft of the internal combustion engine is controlled to be decreased, for example, in a deceleration operation, a fuel cut operation, or a reduced-cylinder operation, the temperature of the catalyst is supposed to be at the high temperature. In this regard, the storage of NH3 (specifically, addition for the purpose of storage) is prohibited for the predetermined time period, for example, while the catalyst temperature is sufficiently high. This can suitably suppress deterioration of the emission characteristics, while reducing a decrease in purification ratio of NOx due to the execution of the unnecessary storage of NH3.

Problems solved by technology

Thus, even if storing of the NH3 is started at the same time as start of deceleration, the storing of the NH3 may be not effective, and the remaining NH3 not stored may lead to deterioration of emission characteristics.

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