ADDITION-AMOUNT CONTROLLER FOR EXHAUST GAS PURIFYING AGENT AND EXHAUST EMISSION CONTROL SYSTEM

Abstract

troller 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 NO_x 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 NO_x 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 NO_x (nitrogen oxides) in the exhaust gas are classified into two important trends, namely, the above-mentioned urea-SCR system, and a NO_x storage-reduction catalyst. The urea-SCR system is already put into practical use in large trucks, and known to have a high purification ratio of a maximum of about "90%". Presently, the general urea-SCR systems which are now studied for application to diesel engines are designed to reduce (purify) NO_x in the exhaust gas by means of NH₃ (ammonia) generated from a urea ((NH₂)₂CO) aqueous solution (hereinafter referred to as a urea water).

[0004]Conventionally, the system disclosed in JP-A-2003-293739 is known as a specific example of such a urea-SCR system. This system mainly includes a catalyst for promoting a specific exhaust gas purifying reaction (reduction reaction of NO_x), an exhaust pipe for guiding the exhaust gas discharged from an exhaust gas generating source (for example, an internal combustion engine) to the catalyst, and a urea water addition valve disposed at a midway point of the exhaust pipe for injecting and adding the urea aqueous solution (additive) to the exhaust gas flowing in the exhaust pipe. The system with this arrangement is configured to inject and add the urea aqueous solution into the exhaust gas by the urea water addition valve, and to supply the urea aqueous solution to the catalyst on the downstream side together with the exhaust gas, using a flow of the exhaust gas. The urea aqueous solution thus supplied is hydrolyzed by exhaust gas heat or the like to generate NH₃ (ammonia), as represented by the following chemical equation: (NH₂)₂CO+H₂O→2NH₃+CO₂. This leads to a reduction reaction of NO_x by the NH₃ on the catalyst, through which the exhaust gas is purified.

[0005]However, the catalyst used in such purification of the exhaust gas generally promotes the reduction reaction of NO_x in a temperature range exceeding an activation temperature (critical reaction temperature) inherent to the catalyst, that is, a temperature range having the activation temperature as the lower limit. Thus, the system as disclosed in JP-A-2003-293739 cannot have a sufficient capacity of purifying the exhaust gas when the catalyst is at a low temperature below the activation temperature.

SUMMARY OF THE INVENTION

[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.

[0007]As a result, it is found that an exhaust gas purification capacity largely changes according to timing of storing NH₃ by addition of the exhaust gas purifying agent. For example, when purifying the exhaust gas from the vehicle internal combustion engine while a rotation velocity of an output shaft of the internal combustion engine is sufficiently high, the temperature of portions around the internal combustion engine including the catalyst become sufficiently high due to the combustion caused by previous acceleration of the engine. Even when the engine is decelerated, the temperature of the catalyst remains at a temperature above the activation temperature for a while. 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.

[0008]The present invention has been made in view of the forgoing facts, and it is an object of the invention to provide an addition-amount controller for an exhaust gas purifying agent, which can obtain a high exhaust gas purification capacity in response to more conditions, and an exhaust emission control system which can exhibit the high exhaust gas purification capacity by using the addition-amount controller.

[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 NO_x (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.

[0010]According to a first aspect of the present invention, the addition-amount controller includes: mode selection means for selecting one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO_x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; and condition determining means for determining whether a rotation speed of an output shaft of the internal combustion engine is decelerated from a higher speed state than an allowable level. Furthermore, the execution condition of the storage control mode is set to be satisfied after a predetermined dissatisfaction time period from start of the deceleration when the condition determining means determines that the rotation speed is decelerated from the higher speed state.

[0011]Alternatively, according to a second aspect of the present invention, the addition-amount controller includes: mode selection means for selecting one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO_x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; and condition determining means for determining whether an amount of fluctuation in a load on the output shaft of the internal combustion engine toward a negative side is larger than an allowable level. Furthermore, the execution condition of the storage control mode is set to be satisfied after a predetermined dissatisfaction time period from the fluctuation when the condition determining means determines that the amount of fluctuation in the load toward the negative side is larger than the allowable level.

[0012]Alternatively, according to a third aspect of the present invention, the addition-amount controller includes: mode selection means for selecting one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO_x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; and condition determining means for determining whether an amount of fluctuation in the rotation speed of the output shaft of the internal combustion engine toward the negative side is larger than an allowable level. Furthermore, the execution condition of the storage control mode is set to be satisfied after a predetermined dissatisfaction time period from the fluctuation when the condition determining means determines that the amount of fluctuation in the rotation speed toward the negative side is larger than the allowable level.

[0013]Alternatively, according to a fourth aspect of the present invention, the addition-amount controller includes: mode selection means for selecting one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO_x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; and condition determining means for determining whether fuel cut is performed in the internal combustion engine. Furthermore, the execution condition of the storage control mode is set to be satisfied after a predetermined dissatisfaction time period from start of the fuel cut when the condition determining means determines that the fuel cut is performed.

[0014]Generally, the amount of NH₃ for effectively purifying NO_x 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 NO_x. 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.

[0015]In general, the temperature of the catalyst has an influence on the property of the catalyst (for example, a limit NH₃ storage amount, or the like). Further, it is known that the higher the temperature of the catalyst, the less the limit storage amount of NH₃ (limit NH₃ storage amount). Thus, when the catalyst temperature is sufficiently low, an allowance degree up to the limit NH₃ storage amount is large, which may require more storage of NH₃. As mentioned above, when the catalyst is at a low temperature, the activation temperature (critical reaction temperature) of the catalyst is strongly required to be decreased. In view this point, the addition-amount controller may be further provided with catalyst temperature determination means for determining whether the temperature of the catalyst at that time is lower than an allowable level. In this case, the predetermined dissatisfaction time period has end timing at which the catalyst temperature determination means determines that the temperature of the catalyst is lower than the allowable level.

[0016]For example, when the temperature of the catalyst at that time is in a predetermined temperature range having the critical reaction temperature or a predetermined temperature smaller than the critical reaction temperature as an upper limit boundary value, the catalyst temperature determination means determines that the temperature of the catalyst at that time is lower than the allowable level.

[0017]Alternatively, the execution condition of the purification control mode may be set to be satisfied when the execution condition of the storage control mode is not satisfied. In this case, the mode selection means switches between two types of the control modes of the purification control mode and the storage control mode according to satisfaction or dissatisfaction of the execution condition.

[0018]According to a fifth aspect of 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 NO_x (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. Furthermore, the addition-amount controller includes: operating mode determination means for determining whether the operating mode of the internal combustion engine is a preset operating mode in which a load on the output shaft of the internal combustion engine is controlled to be decreased when the catalyst is at a high temperature exceeding the critical reaction temperature; and prohibition means for prohibiting addition of NH₃ or the additive serving as the generating source of NH₃ by the addition valve to the catalyst in the amount of storing NH₃, during a predetermined time period, when the operating mode determination means determines that the operating mode at that time is the preset operating mode.

[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 NO_x due to the execution of the unnecessary storage of NH₃.

[0020]For example, the addition valve may be adapted to inject and add a urea aqueous solution as the additive to the exhaust gas on an upstream side with respect to the catalyst. In this case, the urea aqueous solution is injected and added to the exhaust gas on the upstream side with respect to the catalyst, so that the urea is hydrolyzed by exhaust gas heat or the like until the urea reaches the catalyst to form NH₃. This can supply more NH₃ (purifying agent) to the catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments when taken together with the accompanying drawings. In which:

[0022]FIG. 1 is a schematic diagram showing an addition-amount controller for an exhaust gas purifying agent, and an exhaust emission control system with the addition-amount controller, according to one embodiment of the invention;

[0023]FIG. 2 is a flowchart showing control processing for controlling an amount of addition of urea water;

[0024]FIG. 3 is a flowchart showing control processing for determining start timing of an engine deceleration time period;

[0025]FIG. 4 is a flowchart showing control processing for determining end timing of the engine deceleration time period;

[0026]FIG. 5 is a graph showing an example of a map used for calculation of a limit NH₃ storage amount;

[0027]FIG. 6 is a graph showing an example of a relationship between the critical reaction temperature of a SCR catalyst and the NH₃ storage amount;

[0028]FIG. 7 is a graph showing an example of a purifying property of the SCR catalyst;

[0029]FIGS. 8A to 8C are timing charts showing one form of urea water addition control according to the embodiment; and

[0030]FIG. 9 is a flowchart showing another example of control processing regarding mode selection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031]An addition-amount controller for an exhaust gas purifying agent and an exhaust emission control system according to one embodiment of the invention will be described below with reference to the accompanying drawings. The exhaust emission control system of this embodiment has the basic structure used in a general urea-SCR (selective reduction) system, as an example. With the structure shown in FIG. 1, NH₃ (ammonia) generated from a urea ((NH₂)₂CO) aqueous solution (hereinafter referred to as a urea water) reduces (purifies) NO_x in exhaust gas.

[0032]Referring to FIG. 1, the structure of the exhaust emission control system will be described in detail below. FIG. 1 is a diagram schematically showing the structure of a urea-SCR system (exhaust gas purification device) according to this embodiment.

[0033]As shown in FIG. 1, this system is adapted to purify exhaust gas emitted from a diesel engine (exhaust gas generating source) mounted on, for example, a four-wheeled vehicle (not shown). The system mainly includes various actuators and sensors for purifying the exhaust gas, and an ECU (electronic control unit) 40. The engine of this embodiment (engine of interest) is supposed to be a multi-cylinder engine (for example, inline four-cylinder engine) mounted on the four-wheeled vehicle (for example, an automatic car). Each cylinder is provided with an injector having a fuel injection valve. Fuel supplied to each cylinder by the injector burns off in the corresponding cylinder. The engine is the so-called four stroke (4×piston stroke) reciprocating diesel engine (internal combustion engine) which is designed to convert energy generated by combustion of the fuel into a rotational operation to rotate an output shaft (crankshaft). In other words, in this engine, the cylinder of interest at that time is sequentially determined by a cylinder determination sensor (electromagnetic pickup) provided in a cam shaft of an air intake and exhaust valve. One combustion cycle consisting of four strokes, namely, suction, compression, combustion, and exhaust, is performed in a cycle of "720° CA" at each of four cylinders #1 to #4. Specifically, for example, the respective combustion cycles for the four cylinders are sequentially executed at the cylinders #1, #3, #4, and #2 in that order by shifting the cycle between one cylinder and the next cylinder by 180° CA".

[0034]Specifically, various exhaust gas purification devices are disposed in the exhaust emission control system to form an exhaust gas purification system. The exhaust gas purification devices include a diesel particulate filter (DPF) 11, an exhaust gas pipe (exhaust gas passage) 12, a SCR catalyst 13, an exhaust gas pipe (exhaust gas passage) 14, and a NH₃ catalyst (for example, oxidation catalyst) 15 disposed from the upstream side of the exhaust gas (on the engine side which is an exhaust gas generating source) in that order. Onto a wall surface of the passage at a midway point of the exhaust gas pipe 12, a urea water addition valve 16 is disposed such that an injection port 16a opens toward the downstream side of the exhaust gas. Therefore, an injection port 16a is difficult to be dirty with the exhaust gas. The urea water addition valve 16 is adapted to add (inject and supply) the urea water pressure-fed into a urea water tank 17a to the downstream part with respect to the DPF 11. In this embodiment, the urea water addition valve 16 is a so-called electromagnetic driven injection valve whose driving is electrically controlled by the ECU 40. The addition valve 16 is controlled by the ECU 40 so that the urea water serving as an additive is injected and supplied by a desired addition amount to the exhaust gas flowing in the exhaust gas pipe 12 between the DPF 11 and the SCR catalyst 13. Thus, the urea water added (or NH₃ after decomposition) is supplied to the SCR catalyst 13 on the downstream side together with the exhaust gas using the flow of exhaust gas (exhaust gas flow).

[0035]That is, in this system, addition of the urea water through the urea water addition valve 16 generates the NH₃ (purifying agent) based on the urea water as indicated by the following decomposition reaction (formula 1) in the exhaust gas. The following NO_x reduction reaction (as indicated by the following formula 2) is performed by use of NH₃ on the SCR catalyst 13, thereby purifying the exhaust gas (purifying NO_x) to be purified.

(NH₂)₂CO+H₂O→2NH₃+CO₂ (Formula 1)

NO+NO₂+2NH₃→2N₂+3H₂O (Formula 2)

[0036]The excessive NH₃ (surplus NH₃) not consumed in the above reduction reaction (indicated in the formula 2) and flowing into the downstream side of the SCR catalyst 13 (exhaust pipe 14) is purified through the reaction (indicated by the formula 3) by the NH₃ catalyst 15, and thereby the amount of NH₃ emitted into the atmosphere is decreased. The temperature of the exhaust gas on the downstream side of the SCR catalyst 13 and the amount of NO_x (i.e., NO_x emission amount) contained in the exhaust gas can be detected (specifically, can be calculated by the ECU 40 based on outputs from the sensors) by an exhaust gas sensor 14a (incorporating therein a NO_x sensor and a temperature sensor) provided in the exhaust gas pipe 14.

4NH₃+3O₂→2N₂+6H₂O (Formula 3)

[0037]Next, each of the above-mentioned exhaust gas purification devices constituting the exhaust gas purification system of the exhaust emission control system according to this embodiment will be described in detail below.

[0038]First, the DPF 11 is a continuously regenerated filter for particulate matter PM removal, that is, for collecting particulate matter (PM) in the exhaust gas. For example, the DPF 11 can be continuously used by repeatedly burning and removing (corresponding to a regeneration process) the collected PM in post injection or the like after main injection for mainly generating torque. The DPF 11 supports a platinum-based oxidation catalyst not shown (in this example, the DPF and the oxidation catalyst are integrally formed with each other, but may be formed separately). This can remove HC and CO together with soluble organic fraction (SOF), which is one of the PM components, and also oxidize a part of NO_x (as the ratio of NO to NO₂ ("NO:NO₂") is closer to "1:1", the purification ratio of NO_x becomes higher as indicated by the above reaction formula 2).

[0039]The SCR catalyst 13 is formed of catalytic metal, such as vanadium oxide (V₂O₅), supported on, for example, a honeycomb structural catalyst carrier. The SCR catalyst 13 has a catalytic action for promoting the reduction reaction (exhaust gas purification reaction) of NO_x, that is, the reaction indicated by the above formula 2.

[0040]The structure of the urea water addition valve 16 is based on that of a fuel injection valve (injector) commonly used in supply of fuel to an engine for a vehicle (internal combustion engine). The structure of the urea water addition valve 16 is well known, and thus will be briefly described below. That is, for convenience of explanation, illustration of an inside structure of the addition valve 16 will be omitted. The urea water addition valve 16 incorporates in a valve body, a needle driving portion formed of an electromagnetic solenoid or the like, and a needle driven by the needle driving portion and reciprocating (moving vertically) in the valve body (housing). The needle is adapted to open and close a necessary number of injection holes formed in an injection port 16a at the tip of the valve body, or a circulation route to these injection holes. When the electromagnetic solenoid is energized, the urea water addition valve 16 with this arrangement (each element) moves in the direction of opening the valve by driving the needle by use of the electromagnetic solenoid according to an electric signal from the ECU 40 (for example, a pulse signal by PWM (Pulse Width Modulation) control), that is, according to an injection command from the ECU 40. Thus, the injection port 16a at the tip of the valve body is opened, specifically, at least one of the injection holes at the injection port 16a is opened, so that the urea water is added (injected) toward the exhaust gas flowing through the exhaust pipe 12. At this time, the amount of addition of the urea water (injection amount) is determined based on an energization time of the electromagnetic solenoid (for example, corresponding to a pulse width of a pulse signal by the ECU 40).

[0041]On the other hand, a urea water supply system for pressure-feeding the urea water to the urea water addition valve 16 mainly includes a urea water tank 17a, and a pump 17b. That is, the urea water stored in the urea water tank 17a is pumped by the pump 17b disposed in the tank 17a, and then pressure-fed toward the urea water addition valve 16. The pressure-fed urea water is sequentially supplied to the urea water addition valve 16 through a pipe 17c for supply of the urea water.

[0042]At this time, foreign matter contained in the urea water is removed by a barrier filter 17f provided on the upstream side with respect to the addition valve 16 before the urea water is supplied to the urea water addition valve 16. The pressure of supply of the urea water to the addition valve 16 is controlled by a urea water pressure regulator 17d. Specifically, when the supply pressure exceeds a predetermined value, a mechanical device using a spring or the like allows the urea water in the pipe 17c to return to the urea water tank 17a. In the present system, the supply pressure of the urea water is controlled to remain at the predetermined value (set pressure) based on the action of the regulator 17d. The supply pressure of the urea water is not controlled precisely to be kept at the set pressure even by the action of such a regulator 17d. In this system, the supply pressure of the urea water can be detected by the urea water pressure sensor 17e (specifically, calculated by the ECU 40 based on the sensor output) provided in a predetermined detection position (for example, on the downstream side of the regulator 17d where a fuel pressure is stabilized through the pressure control by the regulator 17d).

[0043]A section for mainly performing control associated with the exhaust gas purification as an electronic control unit in such a system is the ECU 40 (for example, the ECU for control of the purification of exhaust gas connected to an ECU for control of the engine via a CAN or the like), that is, the addition-amount controller for an exhaust gas purifying agent according to this embodiment. The ECU 40 includes a well-known microcomputer (not shown), and operates various types of actuators, such as the urea water addition valve 16, based on detection signals from the various sensors to perform various types of control operations associated with the exhaust gas purification in the optimal form according to the condition of each time. The microcomputer installed on the ECU 40 basically includes a CPU (central processing unit) for performing various computations, a RAM (random access memory) serving as a main memory for temporarily storing therein data in the middle of the computation, the result of computation, or the like, and a ROM (read-only memory) serving as a program memory. The microcomputer also includes an EEPROM (electrically erasable and programmable read-only memory; electrically erasable programmable nonvolatile memory) serving as a memory for data storage, and a backup RAM (RAM fed by a backup power source, such as a vehicle-mounted battery). Further, the microcomputer includes signal processors, including an A/D converter and a clock generation circuit, various computation devices, such as an input/output port, for inputting and outputting signals with the external element, a storage device, a communication device, and a power supply circuit. The ROM previously stores therein various programs and a control map associated with the control of the exhaust gas purification, including a program associated with control of an addition amount of the exhaust gas purifying agent. The memory for storing data (for example, EEPROM) previously stores therein various kinds of control data or the like, including design data for the engine.

[0044]In the above description, the structure of the exhaust emission control system of this embodiment has been described in detail. That is, in this embodiment with this arrangement, NH₃ serving as the purifying agent is added to the exhaust gas in the form of urea aqueous solution (urea water) by the urea aqueous addition valve 16. Thus, the urea water is decomposed in the exhaust gas to form NH₃, and the NO_x reduction reaction (indicated by the formula 2) is performed on the SCR catalyst 13 based on the thus-generated NH₃ to purify the exhaust gas (exhaust gas from the engine) to be purified. Furthermore, in this embodiment, the processing shown in FIG. 2 is carried out as the control of an addition amount of the urea water. This processing can obtain the high exhaust gas purification capacity in response to more conditions. The control of the addition amount of the urea water will be described with reference to FIGS. 2 to 8.

[0045]FIG. 2 is a flowchart showing the addition-amount control of the urea water. A series of control steps in the processing shown in FIG. 2 is basically performed repeatedly at intervals of a predetermined processing time while a predetermined condition is satisfied by executing the program stored in the ROM by means of the ECU 40, for example, during the time from the startup of the engine to the stopping of the engine. Values of various parameters used in the processing shown in FIG. 2 are stored in the storage device, such as the RAM or EEPROM mounted on the ECU 40, as occasion arises, and updated at any time if necessary.

[0046]As shown in FIG. 2, in the control of the urea water addition amount, at step S10, it is determined whether or not the engine is being accelerated, that is, it is determined whether or not the timing at that time of step S10 is in an engine deceleration time period.

[0047]The engine deceleration time period is set by repeatedly performing a routine processing other than the processing shown in FIG. 2, that is, the series of steps in the processes shown in FIGS. 3 and 4, stored in the ROM of the ECU 40, at intervals of the predetermined processing time.

[0048]The processing shown in FIG. 3 is to determine the start timing of the engine deceleration time period.

[0049]As shown in FIG. 3, in determination of the start timing, at step S31, it is determined whether or not the engine is driven at a high speed, specifically, whether or not a rotation speed (engine rotation speed) of the output shaft of the engine is larger than a predetermined determination value (engine rotation speed>determination value). Furthermore, at step S31, it is also determined whether or not the engine is being decelerated, specifically, whether or not an accelerator pedal is in a non-operation state (an amount of operation of the accelerator≈0). The determination at step S31 is repeatedly performed. When it is determined in the same step S31 that the engine is driven at the high speed and the accelerator pedal is in the non-operation state (that is, the driver's foot releases the accelerator pedal), the timing at that time is set as the start timing of the engine deceleration time period in the subsequent step S32. The above-mentioned engine deceleration time period corresponds to a time period from when the start timing is set at step S32 to when the end timing is set. During the engine deceleration time period, fuel cut (stopping processing of fuel injection associated with generation of torque) is performed in all cylinders of the engine.

[0050]In contrast, the processing shown in FIG. 4 is to determine the end timing of the engine deceleration time period.

[0051]As shown in FIG. 4, in determination of the end timing, first, at step S41, it is determined whether or not the engine is being decelerated, that is, whether or not the start timing of the engine deceleration time period is already set in the previous step S32 shown in FIG. 3. Only when it is determined that the engine is being decelerated at step S41, the processes at step S42 and the following steps are carried out.

[0052]That is, when the engine is being decelerated, the procedure will proceed to step S42. At step S42, it is determined whether or not the engine rotation speed at that time is smaller than an allowable level, specifically, whether or not the engine rotation speed is smaller than a predetermined determination value (engine rotation speed<determination value). The predetermined determination value at step S42 is not necessarily the same as the determination value at step S31. The determination process at step S42 is repeatedly performed. When the engine rotation speed is determined to be smaller than the allowable level (predetermined determination value) at step S42, the engine is determined to be at the low speed, and in the subsequent step S43, the timing at that time is set as the end timing of the engine deceleration time period. The engine deceleration time period described above is determined in this way.

[0053]When the time is determined to be in the engine deceleration time period at step S10, the processes at step S11 and the following steps will be carried out. When the time is determined not to be in the engine deceleration time period in the same step S10, the procedure will proceed to step S19a.

[0054]In this embodiment, the addition-amount controller selects one of a purification control mode and a storage control mode to be carried out. In the purification control mode, an amount of addition of the urea water by the urea water addition valve 16 is determined according to a predetermined parameter about an NO_x amount in the exhaust gas, specifically, the engine rotation speed and the fuel injection amount. In the storage control mode, an addition amount of the urea water by the urea water addition valve 16 is set to be larger than that in the purification control mode (specifically, only by increasing an amount required to cover a shortfall with respect to a target value of the NH₃ storage amount). That is, while one of the control modes is not performed, the other is performed. The selection of the control mode (i.e., switching between these control modes) is performed based on the result of determination by the processes in steps S10 and S13. More specifically, when the necessary condition is determined not to be satisfied in at least one of steps S10 and S13, the storage of NH₃ is determined to be unnecessary, and thus the purification control mode is performed through the processes at steps S19a and S20. On the other hand, when the necessary conditions are determined to be satisfied in both steps S10 and S13, the storage control mode is performed through the processes in steps S14 to S20 so as to store the NH₃ on the SCR catalyst 13.

[0055]That is, when the time is determined not to be in the engine deceleration time period at step S10, the purification control mode is performed through the processes in steps S19a and S20. Specifically, at step S19a, an addition amount Q of the urea water is obtained according to the engine rotation speed and the fuel injection amount using a reference map (or a mathematical formula) for calculation of a predetermined addition amount of the urea water. This reference map has suitable values (optimal values) of the urea water addition amount Q previously determined and written therein by experiments or the like according to (or in an appropriate manner to) respective values of the engine rotation speed and the fuel injection amount. The map is stored, for example, in the ROM or the like in the ECU 40. This can obtain the high NO_x purification ratio. In the subsequent step S20, the urea water addition valve 16 is driven (energized only for a time period according to the urea water addition amount Q) based on the urea water addition amount Q thus obtained.

[0056]On the other hand, when the time is determined to be in the engine deceleration time period at the previous step S10, an exhaust gas temperature Tex is detected in the subsequent step S11. For example, the exhaust gas temperature Tex can be actually measured by the exhaust gas sensor 14a. In next step S12, the temperature of the SCR catalyst 13 (catalyst temperature Tc) is calculated based on the detected exhaust gas temperature Tex. The catalyst temperature Tc is calculated using, for example, a predetermined map (or a mathematical formula).

[0057]Then, at step S13, it is determined whether or not the catalyst temperature Tc calculated in the previous step S12 is smaller than a predetermined determination value Ts (Tc<Ts). The determination value Ts is variably set, for example, to an occasional critical reaction temperature of each time. When the catalyst temperature Tc is determined not to be smaller than the determination value Ts at step S13, the procedure will proceed to next step S19a, in which the purification control mode described above is performed. That is, even when the engine is decelerated, NH₃ is not stored on the SCR catalyst 3 while the catalyst remains at the high temperature (Tc≧Ts).

[0058]In contrast, when the catalyst temperature Tc is determined to be smaller than the determination value Ts at step S13, the storage control mode is performed through the processes in the following steps S14 to S20 so as to store NH₃ in the SCR catalyst 13.

[0059]Specifically, at step S14, first, a present NH₃ storage amount ST1 which is the NH₃ storage amount at that time of the SCR catalyst 13 is obtained. At this time, the present NH₃ storage amount ST1 is calculated by another routine. Specifically, an amount of increase or decrease in NH₃ storage amount ΔNH₃ of the SCR catalyst 13 of each time is determined based on a difference between the NH₃ amount supplied to the SCR catalyst 13 and the amount of consumption of NH₃ on the SCR catalyst 13. And the occasional amounts of increase or decrease of the respective times are subsequently summed to be set as the above-mentioned present NH₃ storage amount ST1 (ST1 (present value)=ΣST1 (previous value)+ΔNH₃). The above NH₃ amount supplied to the SCR catalyst 13 is calculated based on, for example, the addition amount of urea water by the urea water addition valve 16. In contrast, the consumption amount of NH₃ on the SCR catalyst 13 is calculated mainly based on the NO_x amount emitted from the engine and the purification capacity of the catalyst 13. Among them, the NO_x amount emitted from the engine can be calculated based on the predetermined parameter (for example, the engine rotation speed and the fuel injection amount) associated with the operating condition of the engine. On the other hand, the purification capacity of the SCR catalyst 13 (reaction rate of the NH₃) can be calculated, for example, using a control model of the SCR catalyst 13. The control model for use can be, for example, one or a combination of the following models: a property model showing a relationship between parameters as to a predetermined property; a transfer function showing a correspondence relationship between respective inputs and outputs regarding a level ratio, a frequency-amplitude ratio, a phase difference, a proportion element, a differential element, an integral element, and a delay element (=output signal/input signal); and a mathematical model in which a predetermined natural phenomenon is mathematically described.

[0060]Then, at step S15, a limit NH₃ storage amount ST21 is calculated based on the catalyst temperature Tc calculated in the previous step S12. FIG. 5 shows an example of a map used for calculation of the limit NH₃ storage amount ST21. This map has suitable values (optimal values) previously written therein by experiments. As shown in FIG. 5, the limit NH₃ storage amount ST21 tends to decrease (a NH₃ storage capacity tends to decrease) with an increasing of the catalyst temperature Tc.

[0061]Then, at step S16, a necessary NH₃ storage amount (required NH₃ storage amount ST22, for example, a fixed value) is obtained so as to obtain a desired temperature as the critical reaction temperature (activation temperature) of the SCR catalyst 13. The required NH₃ storage amount ST22 is determined based on the relationship between the critical reaction temperature of the SCR catalyst 13 and the NH₃ storage amount as shown in FIG. 6 (one example provided by experiments or the like by the inventors). As indicated by the solid line RT in FIG. 6, the critical reaction temperature of the SCR catalyst 13 tends to decrease with increasing NH₃ storage amount. In the example indicated by the solid line RT, the desired temperature is supposed to be a critical reaction temperature T1 with respect to the critical reaction temperature T0 when NH₃ is not stored. For example, the critical reaction temperature T1 is a temperature lower than "140° C." which is the catalyst temperature supposed in idling, more specifically, for example, one temperature in a range of "50 to 120° C.". At this time, the critical reaction temperature (activation temperature) of the SCR catalyst 13 is an important parameter for determining the purification property of the SCR catalyst 13. FIG. 7 is a graph showing an example of the purification property of the SCR catalyst 13. As shown in FIG. 7, the NO_x purification ratio of the SCR catalyst 13 largely changes at the boundary of the critical reaction temperature. That is, on the low temperature side with respect to the critical reaction temperature, the NO_x purification ratio is set to substantially "0", and the NH₃ storage amount is larger than the NH₃ consumption amount consumed by the purification reaction with NO_x. In contrast, on the high temperature side with respect to the critical reaction temperature, the NO_x purification ratio basically becomes larger as increasing catalyst temperature (in particular, drastically changes at a temperature near the critical reaction temperature RT).

[0062]In the following step S17, by comparing the limit NH₃ storage amount ST21 obtained at step S15 with the required NH₃ storage amount ST22 obtained at step S16, it is determined whether or not the required NH₃ storage amount ST22 is smaller than the limit NH₃ storage amount ST21 (ST21>ST22). When the relation of ST21>ST22 is determined to be satisfied at step S17, then at the following step S171, the above required NH₃ storage amount ST22 is set as a target NH₃ storage amount ST2. On the other hand, when the relation of ST21>ST22 is determined not to be satisfied at step S17, then at step S172, the above limit NH₃ storage amount ST21 is set as the target NH₃ storage amount ST2.

[0063]Then, at step S18, a difference between the present NH₃ storage amount ST1 and the target NH₃ storage amount ST2 is calculated as a shortfall of the NH₃ storage amount ΔST (an amount of storage that is lacking as compared to the target NH₃ storage amount ST2) (ΔST=ST2-ST1).

[0064]Then, a urea water addition amount Q is obtained using the reference map for calculation of the predetermined urea water addition amount (the same one as that used at step S19a) and the NH₃ storage amount shortfall ΔST at step S19b. Specifically, the urea water addition amount Q in the storage control mode is a urea water addition amount increased so as to cover the NH₃ storage amount shortfall ΔST, as compared to the urea water addition amount in the purification control mode. In the following step S20, the urea water addition valve 16 is driven (energized only for a time corresponding to the urea water addition amount Q) based on the urea water addition amount Q thus obtained.

[0065]FIGS. 8A to 8C are timing charts showing the urea water addition control by taking as an example a case where the vehicle equipped with the exhaust gas purification device and the exhaust emission control system of this embodiment is decelerated from a high speed state. FIG. 8A shows the transition of the engine rotation speed. FIG. 8B shows the transition of the temperature of the SCR catalyst 13. FIG. 8C shows the presence or absence of execution of the storage control mode (ON=execution, OFF=non-execution). At this time, a device for starting storing of NH₃ at the same time as the start of deceleration (for example, see Japanese Patent Application No. 2000-556137) is used as a comparative example. The operating form of the comparative example is indicated by the solid line L1 in FIG. 8C, and the operating form of this embodiment is indicated by the solid line L2 in FIG. 8C. The following description will be given by comparing both operating forms with each other.

[0066]As shown in FIGS. 8A-8C, the vehicle is driven in the steady state at a high speed until the timing t1 shown, and starts to decelerate when the driver's foot releases the accelerator pedal at the timing t1. The accelerator pedal is determined to be in a non-operating state during the high-speed operation at step S31 shown in FIG. 3. Then, in the sequent step S32, the timing of the determination is set as start timing of the engine deceleration time period.

[0067]In the comparative example (indicated by the solid line L1 in FIG. 8C), storing of NH₃ is started at this timing t1. In contrast, in this embodiment (indicated by the solid line L2 in FIG. 8C), storing of NH₃ is not started until the catalyst temperature Tc is determined to be smaller than the allowable level (Tc<Ts) at step S13 shown in FIG. 2 (that is, for a predetermined dissatisfaction time period). Therefore, the present embodiment (L2) can suppress the deterioration of the emission characteristics due to emission of NH₃ as compared to the comparative example (L1).

[0068]At this timer the temperature of the SCR catalyst 13 starts to decrease at the timing t2 that is slightly delayed from the deceleration described above (timing t1). In this embodiment, when the temperature of the SCR catalyst 13 continues to decrease, and then becomes lower than the determination value Ts at timing t3, the catalyst temperature Tc is determined to be smaller than the allowable level at step S13 shown in FIG. 2. At timing t3, the control mode is switched from the purification control mode to the storage control mode, so that the storage of NH₃ on the SCR catalyst 13 is started. The end timing of the engine deceleration time period (set at step S43 in FIG. 4) is timing after the timing t3.

[0069]In this way, in this embodiment, the series of control steps in the processing shown in FIG. 2 is repeatedly carried out, so that the execution of the storage control mode increases the NH₃ storage amount of the SCR catalyst 13 by the shortfall.

[0070]Furthermore, the activation temperature of the catalyst 13 (critical reaction temperature) is controlled to an appropriate one (for "ST21>ST22", to the critical reaction temperature T1). The execution of the storage control mode is not started immediately after the deceleration of the engine, but started after the predetermined dissatisfaction time period. This 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.

[0071]As mentioned above, the addition-amount controller for an exhaust gas purifying agent and the exhaust emission control system according to this embodiment obtain the following excellent effects and advantages.

[0072](1) The addition-amount controller can be suitably applied to the exhaust emission control system for purifying the exhaust gas emitted from the internal combustion engine (engine). In this case, the addition-amount controller includes the SCR catalyst 13 having properties of storing NH₃ and further decreasing the critical reaction temperature (activation temperature) as the amount of NH₃ storage is increased (see FIG. 6). The SCR catalyst 13 is adapted to promote a specific exhaust gas purification reaction in a temperature range having the critical reaction temperature as the lower limit. The addition-amount controller also includes the urea water addition valve 16 for adding the additive (urea water) serving as a NH₃ (ammonia) generating source to the exhaust gas on the upstream side with respect to the SCR catalyst 13. The additive is adapted to purify the exhaust gas by the above exhaust gas purification reaction with NO_x (nitrogen oxides) in the exhaust gas on the catalyst 13. Furthermore, the addition-amount controller is adapted to control the amount of addition of the urea water by the urea water addition valve 16. Such an addition-amount controller for an exhaust gas purifying agent (ECU 40) includes a control program (mode selection means, corresponding to steps S10 and S13 in FIG. 2) for selecting one mode to be executed at that time based on satisfaction of the execution condition for each mode, from among a plurality of control modes, including a purification control mode and a storage control mode. In the purification control mode, the addition amount of the urea water by the urea water addition valve 16 is determined according to a predetermined parameter associated with the NO_x amount of the exhaust gas. In the storage control mode, the addition amount of the urea water by the urea water addition valve 16 is set to be larger than that in the purification control mode. The addition-amount controller also includes a control program (condition determining means, corresponding to step S31 in FIG. 3) for determining whether or not a rotation speed of an output shaft of the engine (engine rotation speed) is decelerated from a higher speed state than an allowable level. When the rotation speed is determined to be decelerated from the high speed state by the process at step S31, the execution condition of the storage control mode is satisfied after a predetermined dissatisfaction time period (set by the process at step S13 shown in FIG. 2) from start of the deceleration. This can enhance the purification capacity of the catalyst by decreasing the critical reaction temperature of the catalyst through execution of the above-mentioned storage control mode. At the deceleration time when the catalyst is supposed to be at a high temperature, the storing of NH₃ is not started immediately after the start of deceleration, but after the predetermined dissatisfaction time period (after a time period in which the catalyst temperature becomes lower than the allowable level). This 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.

[0073](2) In the storage control mode, at step S20 shown in FIG. 2, the NH₃ storage amount of the SCR catalyst 13 is controlled to be the target NH₃ storage amount ST2 by compensating for the shortfall of the NH₃ storage amount corresponding to a difference between the target NH₃ storage amount ST2 and the present NH₃ storage amount ST1 (i.e., NH₃ storage amount shortfall ΔST) by the processing at step S19b. Thus, in the storage control mode, the shortfall of the NH₃ storage amount (NH₃ storage amount shortfall ΔST) is compensated, so that the NH₃ storage amount of the SCR catalyst 13 can be set to the target NH₃ storage amount.

[0074](3) In step S20 shown in FIG. 2, while a predetermined condition (conditions in steps S10 and S13) is satisfied, the control of the NH₃ storage amount at steps S14 to S20 described above is repeatedly performed. With this arrangement, the NH₃ storage amount of the SCR catalyst 13 can be continuously controlled to an appropriate amount with high accuracy while the predetermined condition at steps S10 and S13 is satisfied. Thus, the activation temperature (i.e., critical reaction temperature) of the catalyst 13 is controlled to an appropriate temperature.

[0075](4) The addition-amount controller further includes a control program (catalyst temperature determination means, step S13 shown in FIG. 2) for determining whether or not the temperature of the SCR catalyst 13 at that time (catalyst temperature Tc) is lower than the allowable level. The timing when the temperature of the SCR catalyst 13 is determined to be lower than the allowable level at step S13 (timing t3 shown in FIG. 8) is set as the end timing of the predetermined dissatisfaction time period. At this end timing, the storage control mode is started. Accordingly, it is possible to store the NH₃ in a limited way in the more demanding condition, that is, when the catalyst temperature is sufficiently low (lower than the allowable level).

[0076](5) The determination value Ts used at step S13 shown in FIG. 2 can be variably set to the critical reaction temperature of each time. That is, at step S13, when the catalyst temperature Tc is in a predetermined temperature range (Tc<Ts) in which the critical reaction temperature (determination value Ts) is the upper limit boundary value, the catalyst temperature Tc is determined to be lower than the allowable level. In this way, the start timing of the NH₃ storage as described above (timing t3 shown in FIGS. 8A to 8C) can be set to a more preferable time.

[0077](6) The execution condition of the purification control mode is satisfied when the execution condition of the storage control mode is not satisfied. That is, at steps S10 and S13 shown in FIG. 2, two types of control modes, namely, the purification control mode and the storage control mode are switched according to the satisfaction or dissatisfaction of these execution conditions. This can more easily and accurately achieve the control of the exhaust gas purification.

[0078](7) At step S14 shown in FIG. 2, the amount of increase or decrease in NH₃ storage amount of the SCR catalyst 13 of each time ΔNH₃ is determined based on the difference between the NH₃ amount supplied to the SCR catalyst 13 and the NH₃ consumption amount on the catalyst 13. Further, the increase or decrease amounts of the respective times are subsequently summed (ST1 (value at this time)=ΣST1 (previous value)+ΔNH₃), thereby detecting the present NH₃ storage amount ST1 described above. This arrangement makes it possible to accurately calculate the amount of increase or decrease in NH₃ storage amount of each time and the present NH₃ amount ST1 by determination that the remaining NH₃ is stored on the SCR catalyst 13 based on the revenue and expenditure of the NH₃ amount.

[0079](8) At step S14 shown in FIG. 2, the amount of consumption of NH₃ on the SCR catalyst 13 is determined based on a predetermined parameter associated with the operating condition of the engine (for example, the engine rotation speed and the fuel injection amount). With this arrangement, the NO_x amount emitted from the engine, and further the NH₃ consumption amount on the SCR catalyst 13 can be detected more easily and accurately.

[0080](9) The addition-amount controller also includes a control program (limit storage amount detection means, corresponding to step S15 shown in FIG. 2) for detecting the limit storage amount of NH₃ that can be stored in the SCR catalyst 13 at that time (limit NH₃ storage amount ST21). The addition-amount controller further includes a control program (steps S17, S171, and S172 shown in FIG. 2) for setting a variable range of the target NH₃ storage amount ST2 by using the limit NH₃ storage amount ST21. The limit NH₃ storage amount ST21 is detected by the process at step S15 and is set as the upper limit value (guard value). Thus, it is possible to set the limit NH₃ storage amount ST21 as the upper limit, so as to prevent (or suppress) the supply of the excess NH₃.

[0081](10) At step S15 shown in FIG. 2, the limit NH₃ storage amount ST21 is detected based on the exhaust gas temperature on the downstream side with respect to the catalyst 13, which corresponds to the temperature of the SCR catalyst 13. Thus, it can detect (estimate) the temperature of the SCR catalyst 13 with high accuracy.

[0082]A temperature lower than the catalyst temperature of "140° C." supposed in idling, and an NH₃ storage amount corresponding to the temperature are set as the critical reaction temperature T1, and further as the required NH₃ storage amount ST22 (see FIG. 6 for both), respectively. This can surely purify the exhaust gas even when starting to accelerate from the idling state.

[0083](12) The urea water addition valve 16 is configured to inject and add the urea aqueous solution as the additive for acting as the NH₃ generating source, to the exhaust gas on the upstream side (exhaust pipe 12) with respect to the SCR catalyst 13 (that is, to achieve the so-called urea SCR system). Thus, the urea aqueous solution is injected and added to the exhaust gas on the upstream side with respect to the SCR catalyst 13. Therefore, until the urea water reaches the catalyst 13, the urea is hydrolyzed by exhaust gas heat or the like to form NH₃. This can supply more NH₃ (purifying agent) to the SCR catalyst 13.

[0084](13) The above urea SCR system is installed on the vehicle equipped with the diesel engine (four-wheeled vehicle in this embodiment). This can improve the fuel efficiency and decrease the PM by allowing the generation of NO_x during the combustion process. This can achieve the cleaner diesel vehicle having the high exhaust gas purification capacity.

[0085](14) In contrast, the exhaust emission control system includes the SCR catalyst 13 and the urea water addition valve 16 together with each program (ECU 40), and a urea water supply device (the urea water tank 17a, the pump 17b, and the like) for supplying the urea aqueous solution to the addition valve 16. The exhaust emission control system with this arrangement achieves the exhaust gas purification system having the higher exhaust gas purification capacity.

[0086]The above-mentioned embodiment may be changed in the following way.

[0087]In the above-mentioned embodiment, when the catalyst temperature Tc is in the predetermined temperature range (Tc<Ts) using the critical reaction temperature (determination value Ts) as the upper limit boundary value, the catalyst temperature To is determined to be lower than the allowable level (step S13 shown in FIG. 2), but the invention is not limited thereto. A predetermined temperature (a temperature lower than the critical reaction temperature of each time only by a predetermined temperature, or a sufficiently low fixed value) lower than the critical reaction temperature may be set as the determination value Ts. Even in this case, when the catalyst temperature Tc is in a predetermined temperature range ("Tc<Ts" or "Tc≦Ts") using the determination value Ts as the upper limit boundary value, the catalyst temperature Tc may be determined to be lower than the allowable level at step S13 described above. In such a case, an effect based on the effect described in the paragraph (5) can be obtained.

[0088]According to the applications of the exhaust emission control system, the control process at step S15, S17, S171, or S172 shown in FIG. 2 may be omitted. In this case, at step S18, the required NH₃ storage amount ST22 is effectively set to the target NH₃ storage amount ST2 as it is.

[0089]In the above-mentioned embodiment, the dissatisfaction time period (the non-execution time period of the storage control mode) is set when the rotation speed of the output shaft of the engine (engine rotation speed) is decelerated from the high speed state in which the rotation speed is higher than the allowable level, but the invention is not limited thereto. For example, when larger fluctuations in the load applied to the output shaft of the engine toward the negative side (decrease side) occur as compared to an allowable level, the dissatisfaction time interval may be set. That is, in this case, for example, at step S31, it is determined whether or not the amount of fluctuation in an accelerator operation amount (corresponding to a required torque) toward the negative side (non-operation side) is larger than a predetermined determination value. Here, the amount of fluctuation is the amount of change per unit of time, for example. When the amount of fluctuation in the accelerator operation amount is determined to be larger (it is determined that the amount of pushing the accelerator quickly becomes smaller) at step S31, the procedure will proceed to step S32. Not only an estimated value based on such an accelerator operation amount, but also, for example, an actually measured value by a cylinder inner pressure sensor can be used as the detection value of the load on the engine.

[0090]Alternatively, when larger fluctuations of the rotation speed of the output shaft of the engine toward the negative side (deceleration side) occur as compared to the allowable level, the dissatisfaction time interval may be set. That is, in this case, for example, it is determined whether or not the amount of fluctuation in the engine rotation speed toward the negative side (deceleration side) is larger than the predetermined determination value. When the amount of fluctuation in the engine rotation speed is determined to be larger at step S31, the procedure will proceed to step S32.

[0091]Alternatively, when the fuel for the engine is cut, the dissatisfaction time period may be set. That is, in this case, for example, at step S31, it is determined whether the fuel cut is being performed or not. When it is determined that the fuel cut is being performed at step S31, the procedure will proceed to step S32.

[0092]Also, these structures can obtain an effect based on the effect described in the paragraph (1).

[0093]The addition-amount controller includes a control program (operating mode determination means) for determining whether the operating mode of the engine (internal combustion engine) at that time is a preset operating mode or not. The preset operating mode is a mode in which the load applied to the output shaft of the engine is controlled to be decreased when the temperature of the SCR catalyst 13 is in the high temperature state exceeding the critical reaction temperature (activation temperature). Specifically, it is determined whether the operating mode at that time is the above-mentioned preset operating mode or not at step S31 shown in FIG. 3. With this arrangement, when, for example, the operating mode at that time is determined to be the preset operating mode, the procedure will proceed to step S32. Thus, in the preset operating mode, the addition of urea water by the urea water addition valve 16 (storage control mode) for the purpose of storing NH₃ on the SCR catalyst 13 by the amount corresponding to the addition amount by the valve 16 is prohibited for a predetermined time period (i.e., a time period until the catalyst temperature becomes lower than the allowable level). Also this arrangement can obtain an effect based on the effect described in the paragraph (1). It is effective that the preset operating modes set can include, for example, the deceleration operation, the fuel cut operation, the reduced-cylinder operation, and the like.

[0094]The dissatisfaction time period set is not limited to the time period associated with the catalyst temperature, but can be any other time period. For example, the dissatisfaction time period may be set based on the time. Specifically, the timing after a predetermined time from the start of the deceleration may be set as the end timing of the dissatisfaction time period. Here, the end timing of the dissatisfaction time period corresponds to the start timing of NH₃ storage.

[0095]Although in the above-mentioned embodiment, two control modes of the purification control mode and the storage control mode are switched, the invention is not limited thereto. Adding another control mode to these control modes enables selection of one to be executed at that time from among three or more types of control modes based on satisfaction of the execution condition of each mode.

[0096]For example, mode selection may be performed through the processing exemplified as shown in the flowchart of FIG. 9. In this example, the use of a value of a urea water addition control flag F ("0 to 2") selects one of three types of the purification control mode, the storage control mode, and the urea water non-addition mode.

[0097]As shown in FIG. 9, in this example, it is determined whether or not a predetermined execution condition associated with the storage control mode (storage control execution condition) is satisfied at step S101. When the storage control execution condition is determined to be satisfied at step S101, the urea water addition control flag F is set to "2" at the subsequent step S103.

[0098]In contrast, when the storage control execution condition is determined not to be satisfied at step S101, it is determined whether or not a predetermined execution condition associated with the purification control mode (purification control execution condition) is satisfied in the subsequent step S102. When the purification control execution condition is determined to be satisfied at step S102, the urea water addition control flag F is set to "1" at the subsequent step S105. On the other hand, when the purification control execution condition is determined not to be satisfied at step S102, the urea water addition control flag F is set to "0" at the subsequent step S104.

[0099]This can select one to be executed at that time among the three or more types of control modes.

[0100]Although in the above-mentioned embodiment, the NH₃ storage amount is set corresponding to the predetermined critical reaction temperature T1 lower than the catalyst temperature of "140° C." supposed in idling, as the required NH₃ storage amount ST22 for use in determination of the target NH₃ storage amount ST2, the invention is not limited thereto. For example, the required NH₃ storage amount ST22 can be effectively set to a boundary value (convergent point) at which the critical reaction temperature is not decreased even by increasing the NH₃ storage amount of the SCR catalyst 13. This arrangement can suitably prevent (or suppress) the excess storage of NH₃ not contributing to the critical reaction temperature.

[0101]In the above-mentioned embodiment, the required NH₃ storage amount ST22 is set as the fixed value, but the invention is not limited thereto. The required NH₃ storage amount ST22 may be variably set according to the condition of each time. For example, the storage amount may be set to differ between at the startup time of the engine and the idling time. Alternatively, the required NH₃ storage amount ST22 may be variably set according to a target value of the critical reaction temperature or a target value of the NO_x purification ratio on the SCR catalyst 13.

[0102]In the above-mentioned embodiment, the catalyst temperature Tc is determined based on the exhaust gas temperature. However, the temperature of the catalyst itself is not determined, and the exhaust gas temperature may be used as a substitute for the catalyst temperature.

[0103]The NO_x amount in the exhaust gas can be determined not only by estimation from the engine operating state, but also, for example, by the actually measured value (sensor output) by an NO_x sensor or the like. Furthermore, for example, the NO_x amount in the exhaust gas can be estimated based on the state of the exhaust gas (e.g., exhaust gas temperature detected, for example, by the exhaust gas temperature sensor or the like) or components (for example, an oxygen concentration detected by an oxygen concentration sensor or the like).

[0104]The kind of the exhaust gas generating source to be purified or the system structure can be arbitrarily changed according to the used conditions or the like.

[0105]For example, when the exhaust gas from the engine for a vehicle is an object to be purified, the invention can be applied not only to a compression ignition diesel engine, but also a spark ignition gasoline engine or the like. Since the compression ignition engine, such as the diesel engine, has the low exhaust gas temperature as compared to that in the spark ignition engine, the invention is effectively applied to the compression ignition engine, thereby enhancing the purification capacity when the catalyst temperature is low. The invention can also be applied to a rotary engine or the like other than a reciprocating engine. Furthermore, the invention can also be applied to purification of exhaust gas from sources other than the vehicle, that is, for example, purification of exhaust gas from an electric power plant, various factories, or the like.

[0106]On the other hand, the system structure may be changed in the following way. For example, as shown in FIG. 1, the additive (urea water) is added to the exhaust gas on the upstream side with respect to the catalyst 13 to deliver the additive to the catalyst 13 by the exhaust gas flow, but the invention is not limited thereto. Alternatively, the additive may be directly added (for example, injected) to the catalyst itself. For example, when the amount of emission of NH₃ is sufficiently decreased in the structure shown in FIG. 1, the NH₃ catalyst 15 can be omitted from the structure.

[0107]When various modifications are made to the structures in the above embodiments, the details of various processes (programs) described above are preferably changed (have designs changed) to the respective optimal forms according to the actual structure if necessary.

[0108]Actually, the main demand for the invention comes from the urea-SCR (selective reduction) system. The invention, however, can also be used for other applications as long as the exhaust gas is purified on a catalyst using the same purifying agent (NH₃) for purifying the same specific component of interest.

[0109]In the above-mentioned and modified examples, various types of software (programs) are supposed to be used, but hardware, such as a dedicated circuit, may be used to achieve the same function.

[0110]Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.

Claims

1. An addition-amount controller for an exhaust gas purifying agent, the controller being applied to an exhaust emission control system for purifying exhaust gas emitted from an internal combustion engine, the exhaust emission control system including 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 NO_x (nitrogen oxides) in the exhaust gas by the exhaust gas purification reaction on the catalyst, the addition-amount controller being adapted to control an amount of addition by the addition valve, the catalyst having properties of storing NH₃ and further decreasing the critical reaction temperature as the amount of NH₃ storage is increased, the addition-amount controller comprising:mode selection means for selecting one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO_x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; andcondition determining means for determining whether a rotation speed of an output shaft of the internal combustion engine is decelerated from a higher speed state than an allowable level,wherein the execution condition of the storage control mode is set to be satisfied after a predetermined dissatisfaction time period from start of the deceleration when the condition determining means determines that the rotation speed is decelerated from the higher speed state.

2. An addition-amount controller for an exhaust gas purifying agent, the controller being applied to an exhaust emission control system for purifying exhaust gas emitted from an internal combustion engine, the exhaust emission control system including 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 NO_x (nitrogen oxides) in the exhaust gas by the exhaust gas purification reaction on the catalyst, the addition-amount controller being adapted to control an amount of addition by the addition valve, the catalyst having properties of storing NH₃ and further decreasing the critical reaction temperature as the amount of NH₃ storage is increased, the addition-amount controller comprising:mode selection means for selecting one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO_x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; andcondition determining means for determining whether an amount of fluctuation in a load on the output shaft of the internal combustion engine toward a negative side is larger than an allowable level,wherein the execution condition of the storage control mode is set to be satisfied after a predetermined dissatisfaction time period from the fluctuation when the condition determining means determines that the amount of fluctuation in the load toward the negative side is larger than the allowable level.

3. An addition-amount controller for an exhaust gas purifying agent, the controller being applied to an exhaust emission control system for purifying exhaust gas emitted from an internal combustion engine, the exhaust emission control system including 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 NO_x (nitrogen oxides) in the exhaust gas by the exhaust gas purification reaction on the catalyst, the addition-amount controller being adapted to control an amount of addition by the addition valve, the catalyst having properties of storing NH₃ and further decreasing the critical reaction temperature as the amount of NH₃ storage is increased, the addition-amount controller comprising:mode selection means for selecting one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO_x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; andcondition determining means for determining whether an amount of fluctuation in the rotation speed of the output shaft of the internal combustion engine toward the negative side is larger than an allowable level,wherein the execution condition of the storage control mode is set to be satisfied after a predetermined dissatisfaction time period from the fluctuation when the condition determining means determines that the amount of fluctuation in the rotation speed toward the negative side is larger than the allowable level.

4. An addition-amount controller for an exhaust gas purifying agent, the controller being applied to an exhaust emission control system for purifying exhaust gas emitted from an internal combustion engine, the exhaust emission control system including 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 NO_x (nitrogen oxides) in the exhaust gas by the exhaust gas purification reaction on the catalyst, the addition-amount controller being adapted to control an amount of addition by the addition valve, the catalyst having properties of storing NH₃ and further decreasing the critical reaction temperature as the amount of NH₃ storage is increased, the addition-amount controller comprising:mode selection means for selecting one mode to be executed at that time based on satisfaction of an execution condition for each mode, from among a plurality of control modes, the control modes including a purification control mode in which the addition amount by the addition valve is determined according to a predetermined parameter associated with an amount of NO_x in the exhaust gas, and a storage control mode in which the addition amount by the addition valve is set to be larger than that in the purification control mode; andcondition determining means for determining whether fuel cut is performed in the internal combustion engine,wherein the execution condition of the storage control mode is set to be satisfied after a predetermined dissatisfaction time period from start of the fuel cut when the condition determining means determines that the fuel cut is performed.

5. The addition-amount controller for an exhaust gas purifying agent according to claim 1, further comprising catalyst temperature determination means for determining whether the temperature of the catalyst at that time is lower than an allowable level,wherein the predetermined dissatisfaction time period has end timing at which the catalyst temperature determination means determines that the temperature of the catalyst is lower than the allowable level.

6. The addition-amount controller for an exhaust gas purifying agent according to claim 5, wherein, when the temperature of the catalyst at that time is in a predetermined temperature range having the critical reaction temperature or a predetermined temperature smaller than the critical reaction temperature as an upper limit boundary value, the catalyst temperature determination means determines that the temperature of the catalyst at that time is lower than the allowable level.

7. The addition-amount controller for an exhaust gas purifying agent, according to claim 1, wherein the execution condition of the purification control mode is set to be satisfied when the execution condition of the storage control mode is not satisfied, andwherein the mode selection means switches between two types of the control modes of the purification control mode and the storage control mode according to satisfaction or dissatisfaction of the execution condition.

8. The addition-amount controller for an exhaust gas purifying agent, according to claim 1, further comprising:operating mode determination means for determining whether the operating mode of the internal combustion engine is a preset operating mode in which a load on the output shaft of the internal combustion engine is controlled to be decreased when the catalyst is at a high temperature exceeding the critical reaction temperature; andprohibition means for prohibiting addition of NH₃ or the additive serving as the generating source of NH₃ by the addition valve to the catalyst in the amount of storing NH₃, during a predetermined time period, when the operating mode determination means determines that the operating mode at that time is the preset operating mode.

9. An addition-amount controller for an exhaust gas purifying agent, the controller being applied to an exhaust emission control system for purifying exhaust gas emitted from an internal combustion engine, the exhaust emission control system including 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 NO_x (nitrogen oxides) in the exhaust gas by the exhaust gas purification reaction on the catalyst, the addition-amount controller being adapted to control an amount of addition by the addition valve, the catalyst having properties of storing NH₃ and further decreasing the critical reaction temperature as the amount of NH₃ storage is increased, the addition-amount controller comprising:operating mode determination means for determining whether the operating mode of the internal combustion engine is a preset operating mode in which a load on the output shaft of the internal combustion engine is controlled to be decreased when the catalyst is at a high temperature exceeding the critical reaction temperature; andprohibition means for prohibiting addition of NH₃ or the additive serving as the generating source of NH₃ by the addition valve to the catalyst in the amount of storing NH₃, only during a predetermined time period, when the operating mode determination means determines that the operating mode at that time is the preset operating mode.

10. The addition-amount controller for an exhaust gas purifying agent according to claim 1, wherein the addition valve is adapted to inject and add a urea aqueous solution as the additive to the exhaust gas on an upstream side with respect to the catalyst.

11. An exhaust emission control system comprising:the addition-amount controller as in claim 10;the catalyst and the addition valve; anda urea water supply device for supplying the urea aqueous solution to the addition valve.

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