JP5553629B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

The present invention relates to an exhaust purifying apparatus for an internal combustion engine, and more particularly relates to an exhaust purifying apparatus for an internal combustion engine provided with a selective reduction catalyst for reducing NO _x in the exhaust passage in the presence of a reducing agent.

2. Description of the Related Art Conventionally, as one of exhaust purification apparatuses that purify NO _x in exhaust gas, an apparatus has been proposed in which a selective reduction catalyst that selectively reduces NO _x in exhaust gas by adding a reducing agent is provided in an exhaust passage. Yes. For example, urea addition type of the selective reduction catalyst using urea water as the reducing agent, the added urea is hydrolyzed to produce ammonia (NH _3), to selectively reduce NO _x in the exhaust by NH ₃ .

In such a selective reduction catalyst, if less than the injection amount is optimum amount of the reducing agent, the reduction rate of the NO _x by NH ₃ to be consumed in the reduction of the NO _x is insufficient is reduced, optimal When the amount is larger than the amount, NH ₃ surplus in the reduction of NO _x is discharged. For this reason, it is important to appropriately control the injection amount of the reducing agent in the exhaust emission control device including the selective reduction catalyst.

Patent Document 1 discloses an exhaust gas purification apparatus for an internal combustion engine using two selective reduction catalysts. In this exhaust purification apparatus, in accordance with the NH ₃ concentration NH ₃ sensor provided between two of the selective reduction catalyst is detected, feedback injection amount of the reducing agent on the upstream side of the selective reduction catalyst (F / B) control To do.

International Publication WO2009 / 128169

The selective reduction catalyst has an ability to adsorb NH ₃ , and this NH ₃ adsorptionable amount tends to decrease as the temperature of the selective reduction catalyst increases. In the exhaust purifying apparatus according to patent document 1 performs the F / B control the temperature is NH ₃ sensor from a low state (for example, 160 ° C.) according to the NH ₃ concentration detection of the selective reduction catalyst. Therefore, when the temperature of the selective reduction catalyst rises to a high temperature (for example, 250 ° C.) at which the purification performance can be maximized, the amount of NH _{3 that} can be adsorbed by the selective reduction catalyst decreases, and the amount of NH _{3 that} can be adsorbed exceeds There is a problem that a large amount of NH ₃ is discharged (slipped).

Accordingly, the present invention is, in the exhaust purification system for an internal combustion engine having a selective reduction catalyst, and an object thereof is to suppress the NH ₃ adsorption capacity more abundant NH ₃ slip from the selective reduction catalyst.

Another object of the present invention is to quickly perform suppression control of the NH ₃ slip.

The present invention provides an exhaust emission control device for an internal combustion engine. The exhaust purification device is a selective reduction catalyst that is provided in an exhaust passage of an internal combustion engine, captures NH ₃ as a reducing agent, and reduces NO _x flowing through the exhaust passage using the captured NH ₃ . A selective reduction catalyst including a first selective reduction catalyst and a second selective reduction catalyst provided downstream of the first selective reduction catalyst in the exhaust passage, and a reducing agent upstream of the selective reduction catalyst in the exhaust passage. Reducing agent supply means for supplying, NH ₃ detection means for detecting the NH ₃ concentration between the first selective reduction catalyst and the second selective reduction catalyst in the exhaust passage, and the amount of NH ₃ in the first selective reduction catalyst. A first correction amount that sets the first correction amount of the reducing agent supply amount so that the difference between the calculating means and the calculated NH ₃ amount and the maximum NH ₃ storage amount in the first selective reduction catalyst decreases. Setting means, detected NH ₃ concentration and target NH And a second correction amount setting means for setting a second correction amount of the reducing agent supply amount so that a difference from the _three concentrations decreases, and the reducing agent supply amount setting means includes NH in the first selective reduction catalyst. Until the ₃ amount reaches the maximum NH ₃ storage amount, the supply amount of the reducing agent is set using only the first correction amount, and when the maximum NH ₃ storage amount is reached, the second correction amount is used. Sets the supply amount of reducing agent.

According to the present invention, the NH ₃ amount in the first selective reduction catalyst is based on the calculated NH ₃ amount instead of the second correction amount based on the detected NH ₃ concentration until the NH ₃ storage amount becomes equal to or greater than the maximum NH ₃ storage amount. since setting the supply amount of the reducing agent by using the first correction amount, it can be controlled lower the feed rate of the reducing agent until the above maximum NH ₃ storage amount, the excess NH after maximum NH ₃ storage amount ₃ slip amount can be suppressed.

According to one aspect of the present invention, the reducing agent supply amount setting means continues to use the first correction amount after the NH ₃ amount in the first selective reduction catalyst reaches the maximum NH ₃ storage amount. Set the supply amount.

According to an aspect of the present invention, after the NH ₃ amount in the first selective reduction catalyst reaches the maximum NH ₃ storage amount, the supply of the reducing agent is performed using not only the second correction amount but also the first correction amount. Since the amount is set, rapid control (convergence) to the target NH ₃ concentration becomes possible.

It is a mimetic diagram showing composition of an engine and its exhaust gas purification device according to one example of the present invention. And concentration of NO _x in the selective reduction catalyst, and NH ₃ concentration is a diagram showing the relationship between the NH ₃ storage amount. Is a graph showing the relationship between the temperature and the NH ₃ storage amount of the selective reduction catalyst. It is a block diagram which shows the structure (function) of ECU3 of FIG. NH ₃ is a diagram showing a flow of a first correction amount setting calculated reducing agent supply amount of the estimated value of the adsorption amount. It is a figure which shows the control flow which leads to calculation of the 1st and 2nd correction amount of urea water. And the catalyst temperature is a diagram showing the urea water correction amount by NH ₃ sensor F / B, and NH ₃ concentrations, the relationship between the NH ₃ storage amount. And the catalyst temperature is a diagram showing the urea water correction amount by NH ₃ sensor F / B, and the urea water correction amount according to the estimated storage F / B, and NH ₃ concentrations, the relationship between the NH ₃ storage amount.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and its exhaust purification device 2 according to an embodiment of the present invention. The engine 1 is a gasoline engine or a diesel engine, and is mounted on a vehicle (not shown).

The exhaust purification device 2 includes an oxidation catalyst 21 provided in the exhaust passage 11 of the engine 1 and a nitrogen oxide (hereinafter referred to as “hereinafter referred to as“ nitrogen oxide ”) provided in the exhaust passage 11 provided downstream of the oxidation catalyst 21 in the exhaust passage 11. NO _x ”) in the presence of a reducing agent, a selective reduction catalyst 23, a urea water injection device 25 for supplying urea water as a reducing agent to the upstream side of the selective reduction catalyst 23 in the exhaust passage 11, and electronic control A unit (hereinafter referred to as “ECU”) 3 is included.

  The ECU 3 is a computer that includes a central processing unit (CPU) and a memory. The memory can store a computer program for realizing various controls of the vehicle and data, tables, and maps necessary for executing the program. As will be described later, the ECU 3 receives data sent from each part (sensor or the like) of the vehicle, performs calculation, and generates a control signal for controlling each part of the vehicle.

  The urea water injection device 25 includes a tank 251 and an injector 253. The tank 251 stores urea water, and is connected to the injector 253 via the urea water supply path 254 and a pump (not shown). The tank 251 is provided with a urea water level sensor 255. This sensor 255 detects the water level of the urea water in the tank 251 and outputs a detection signal corresponding to this water level to the ECU 3. The injector 253 is connected to the ECU 3 and injects urea water into the exhaust passage 11 in accordance with a control signal from the ECU 3. That is, the injector 253 injects a predetermined amount of urea water into the exhaust passage 11 at an injection time (seconds / shot) set by the ECU 3 and an injection interval (seconds, period (Hz)).

The oxidation catalyst 21 is provided on the upstream side of the selective reduction catalyst 23 and the injector 253 in the exhaust passage 11, and converts NO occupying most of the NO _{x in} the exhaust gas into NO ₂ , thereby the selective reduction catalyst. to promote the reduction of the NO _x in 23.

The selective reduction catalyst 23 includes a first selective reduction catalyst 231 and a second selective reduction catalyst 232 provided downstream of the first selective reduction catalyst 231 in the exhaust passage 11. The first and second selective reduction catalysts 231 and 232 selectively reduce NO _x in the exhaust gas in an atmosphere in which ammonia as a reducing agent exists. Specifically, when urea water is injected by the urea water injection device 25, ammonia (hereinafter referred to as “NH ₃ ”) is generated from urea by hydrolysis, and the NO _x in the exhaust gas in the selective reduction catalyst 23 is generated by this NH _3. (NO and NO ₂ ) are selectively reduced. The selective reduction catalyst 23 also has a function of storing NH _3, NO _x is reduced and purified by NH ₃ that is stored.

In addition to the NH ₃ sensor 26, the catalyst temperature sensor 27, and the NO _x sensor 28, the ECU 3 is connected with a crank angle position sensor 14, an accelerator opening sensor 15, and a urea water remaining amount warning lamp 16.

The NH ₃ sensor 26 detects the concentration of ammonia in the exhaust gas between the first selective reduction catalyst 231 and the second selective reduction catalyst 232 in the exhaust passage 11 (hereinafter referred to as “NH ₃ concentration”), and detects the detected NH A detection signal corresponding to the _three concentrations is sent to the ECU 3.

The catalyst temperature sensor 27 detects the temperature of the first selective reduction catalyst 231 (hereinafter referred to as “catalyst temperature”), and sends a detection signal corresponding to the detected catalyst temperature to the ECU 3. The NO _x sensor 28 detects the concentration of NO _{x in} the exhaust gas flowing into the first selective reduction catalyst 231 and sends a detection signal corresponding to the detected NO _x to the ECU 3.

  The crank angle position sensor 14 detects the rotation angle of the crankshaft of the engine 1, generates a pulse at every crank angle, and sends the pulse signal to the ECU 3. The ECU 3 calculates the rotational speed NE of the engine 1 based on this pulse signal. The crank angle position sensor 14 further generates a cylinder identification pulse at a predetermined crank angle position of the specific cylinder and sends it to the ECU 3.

  The accelerator opening sensor 15 detects the amount of depression of an accelerator pedal (not shown) of the vehicle (hereinafter referred to as “accelerator opening”), and sends a detection signal corresponding to the detected accelerator opening to the ECU 3. In the ECU 3, the required torque of the engine 1 is calculated according to the accelerator opening and the rotational speed.

  The urea water remaining amount warning lamp 16 is provided, for example, on a meter panel of the vehicle, and lights up when the remaining amount of urea water in the tank 251 is less than a predetermined remaining amount. As a result, the driver is warned that the remaining amount of urea water in the tank 251 has decreased.

FIG. 2 is a diagram illustrating the relationship among the NO _x concentration, the NH ₃ concentration, and the storage amount of NH ₃ in the selective reduction catalyst 23 of FIG. Graphs 30 and 31 in FIG. 2 indicate that as NO _x is reduced and the NO _x concentration decreases, NH ₃ of the selective reduction catalysts 231 and 232 is consumed and the NH ₃ concentration also decreases.

In FIG. 2, NH ₃ is almost saturated in the first selective reduction catalyst 231, that is, a state of maximum NH ₃ storage amount (reference numeral 32). The broken line indicated by reference numeral 35 indicates the minimum NH ₃ storage amount (lower limit storage amount) required to reduce NO _x in a low load (low rotation) operation state such as idling. In the second selective reduction catalyst 232, NH ₃ slipped from the first selective reduction catalyst 231 is stored (adsorbed) by an amount indicated by reference numeral 33. A broken line 34 indicates that this storage amount varies depending on the catalyst temperature.

FIG. 3 is a diagram showing the relationship between the temperature and the NH ₃ storage amount in the selective reduction catalyst. The amount of NH ₃ storage tends to increase at low temperatures and decrease with increasing temperatures.

FIG. 4 is a block diagram showing a configuration (function) of the ECU 3 in FIG. The function of each block is realized by the CPU included in the ECU 3 reading and executing a control program stored in a memory (not shown). The NH ₃ amount calculation means 301 of the first selective reduction catalyst calculates the amount of NH ₃ (estimated amount) captured (adsorbed) by the first selective reduction catalyst 231 in FIG. The first correction amount setting unit 302 for the reducing agent supply amount has a difference between the NH ₃ amount calculated by the NH ₃ amount calculating unit 301 of the first selective reduction catalyst and the maximum NH ₃ storage amount in the first selective reduction catalyst. The first correction amount of the supply amount of the reducing agent (urea water) is set so as to be zero.

FIG. 5 is a diagram illustrating an example of the flow of calculating the estimated value of the NH ₃ adsorption amount and setting the first correction amount of the reducing agent supply amount. The estimated value Vs of the NH ₃ adsorption amount is based on the supplied NH ₃ amount V1, the NH ₃ amount V2 consumed in the NO _x reduction reaction, and the NH ₃ amount V3 slipping the first selective reduction catalyst 231. It is calculated by the following formula. The symbol Σ in the following equation means that the estimated value Vs is obtained as an integrated value per unit time. In FIG. 5, the loop 37 of the block “1 / Z” means that this integration is performed.

Vs = Σ [V1-V2-V3]

The supplied NH ₃ amount V1 is calculated from the amount of urea water injected from the injector 253 using the hydrolysis reaction equation shown in FIG. The NH ₃ amount V2 consumed in the NO _x reduction reaction is determined as follows. It is converted into the amount of NO and NO ₂ amount using the NO _x conversion map the amount of NO _x detected by the NO _x sensor 28 is stored in advance in ECU3 memory. Similarly, purification (reduction) rates F1 and F2 for NO and NO ₂ are obtained from the NO _x purification rate map stored in the memory of the ECU 3. F1, F2 and NO amount, by multiplying each of the NO ₂ amount, and calculates the reduced amount of NO and reducing NO ₂ amount. The amount of NH ₃ corresponding to the amount of reduced NO and the amount of reduced NO ₂ is determined from each of the three reduction reaction formulas (1) to (3) in FIG. The total amount of NH ₃ obtained for each formula is the NH ₃ amount V2 consumed in the NO _x reduction reaction.

The NH ₃ amount V3 slipping the first selective reduction catalyst 231 is obtained as the NH ₃ weight by multiplying the NH ₃ concentration detected by the NH ₃ sensor 26 by a parameter such as the volume of the selective reduction catalyst obtained in advance.

In FIG. 5, the NH ₃ storage capacity Va of the first selective reduction catalyst is obtained from the NH ₃ storage capacity map stored in the memory of the ECU ₃ . This NH ₃ storage possible amount map corresponds to, for example, the relationship diagram between the catalyst temperature and the NH ₃ storage amount in FIG. The difference (Va−Vs) between the NH ₃ storage capacity Va and the calculated NH ₃ adsorption amount estimated value Vs becomes zero, in other words, the NH ₃ adsorption amount estimated value Vs can be stored in NH _3. A first correction amount (first FB correction amount) of the urea water supply amount is calculated as a control input for feedback (PI) control so as to converge to the amount Va. In FIG. 5, the corrected urea water amount is calculated.

Returning to FIG. 4, the second correction amount setting means 303 of the reducing agent supply amount, so that the difference between the NH ₃ concentration and the target NH ₃ concentration detected by the NH ₃ sensor 26 of FIG. 1 is zero, the reduction A second correction amount of the agent supply amount is set. More specifically, as is detected NH ₃ concentration converges to a preset target NH ₃ concentrations were, as a control input to the feedback control, the second correction amount of the supply amount of the urea water (second FB correction amount).

  The means 304 for setting the reducing agent supply amount from the first correction amount and the second correction amount is configured to determine the urea water supply amount by the reducing agent supply means, that is, the urea water amount injected from the injector 253 in FIG. It is set using the first correction amount and the second correction amount.

  FIG. 6 is a diagram illustrating an example of a control flow for calculating the first and second correction amounts of the urea water supply amount according to the present invention. This control flow is executed every predetermined control cycle by calling a control program stored in the memory by the ECU 3.

  In step S1, it is determined whether or not the failure flag of the urea water injection device 25 is “1”. This failure flag is set to “1” when it is determined in the determination process (not shown) that the urea water injection device has failed, and is set to “0” otherwise. If this determination is Yes, the process moves to step S14, and the urea water injection amount is set to “0”, and then this process ends. If this determination is No, the process proceeds to step S2.

  In step S2, it is determined whether or not the deterioration flag of the selective reduction catalyst is “1”. This catalyst deterioration flag is set to “1” when it is determined in the determination process (not shown) that one of the first selective reduction catalyst 231 and the second selective reduction catalyst 232 in FIG. 1 has failed, and otherwise. Set to “0”. If this determination is Yes, the process moves to step S14, and the urea water injection amount is set to “0”, and then this process ends. If this determination is No, the process proceeds to step S3.

  In step S3, it is determined whether or not the urea water remaining amount is less than a predetermined value. The remaining amount of urea water indicates the remaining amount of urea water in the urea water tank 251 in FIG. 1, and is calculated based on the output of the level sensor. If this determination is Yes, the process moves to step S4, and if No, the process moves to step S5.

  In step S4, a warning light for the remaining amount of urea water is turned on, the process proceeds to step S14, and after the urea water injection amount is set to “0”, this process is terminated.

  In step S5, it is determined whether or not the warm-up timer value of the oxidation catalyst 21 in FIG. 1 is greater than a predetermined value. This catalyst warm-up timer value measures the warm-up time of the oxidation catalyst 21 after the engine is started. If this determination is Yes, the process proceeds to step S6. When this determination is No, the process proceeds to step S14, and after setting the urea water injection amount to “0”, this process is terminated.

In step S6, it is determined whether or not the sensor failure flag is “0”. This sensor failure flag is set to “1” when it is determined that at least one of the NH ₃ sensor 26 and the catalyst temperature sensor 27 in FIG. 1 has failed in a determination process (not shown), and “0” otherwise. "Is set. If this determination is Yes, the process proceeds to step S7. When this determination is No, the process proceeds to step S14, and after setting the urea water injection amount to “0”, this process is terminated.

In step S7, it is determined whether or not the activation flag of the NH ₃ sensor 26 is 1. The NH ₃ sensor activation flag is set to “1” when it is determined in the determination process (not shown) that the NH ₃ sensor 26 has reached the active state, and is set to “0” otherwise. If this determination is Yes, the process proceeds to step S8. When this determination is No, the process proceeds to step S14, and after setting the urea water injection amount to “0”, this process is terminated.

In step S8, it is determined whether the urea water is hydrolyzed and NH ₃ can be generated. Specifically, it is determined whether or not the temperature of the selective reduction catalyst 23 is equal to or higher than a predetermined temperature at which urea water can be hydrolyzed (for example, about 160 ° C.). When this determination is Yes, it is determined that the urea water is hydrolyzed and NH ₃ can be generated, and the process proceeds to step S9. When this determination is No, the process proceeds to step S14, and the urea water injection amount is set to “0”, and then this process ends.

In step S9, calculation of the urea water injection amount is started. First, in step S10, the estimated storage NH ₃ amount captured by the first selective reduction catalyst 231 is calculated. This calculation method is as described in the description of the calculation of the estimated amount Vs in FIG.

In step S11, the first corrected supply amount of urea water is calculated using the estimated storage NH ₃ amount. The calculation method of the first correction supply amount is as described in the calculation of the corrected urea water (first FB correction amount) in FIG.

In step S12, it is determined whether or not the NH ₃ concentration that is the output of the NH ₃ amount sensor 26 is obtained. If this determination is Yes, it is determined that NH ₃ has been stored up to the maximum storage NH ₃ amount (for example, reference numeral 32 in FIG. 2) in the first selective reduction catalyst 231, and the process proceeds to step S13. If this determination is No, this process ends. That is, until the NH ₃ is stored up to the maximum storage NH ₃ amount in the first selective reduction catalyst 231, only the calculated first corrected supply amount of urea water is used as the F / B control input of the urea water supply amount. .

In step S <b> 13, the second corrected supply amount of urea water is calculated using the NH ₃ concentration that is the output of the NH ₃ amount sensor 26. The method for calculating the second correction supply amount is as described in the description of the calculation of the second F / B correction amount by the second correction amount setting means 303 for the reducing agent supply amount in FIG. Thus, in the present invention, after the NH ₃ concentration is detected, in other words, after the NH ₃ is stored up to the maximum storage NH ₃ amount in the first selective reduction catalyst 231, the second corrected supply amount is calculated. Therefore, after the maximum storage, both the second correction supply amount and the previous first correction supply amount can be used as control inputs for the urea water supply amount.

FIG. 7 is a diagram showing the relationship between the catalyst temperature, the urea water correction amount by the NH ₃ sensor F / B, the NH ₃ concentration, and the NH ₃ storage amount in the first selective reduction catalyst 231. (A) is a conventional example shown for comparison, and an example in which urea water correction by the NH ₃ sensor F / B is applied to the entire region, that is, before and after NH ₃ full (maximum) storage in the first selective reduction catalyst 231. It is. (B) is an embodiment of the present invention, which is an example of applying urea water correction by the NH ₃ sensor F / B after NH ₃ full (maximum) storage in the first selective reduction catalyst 231. First, the conventional example (a) will be described.

In FIG. 7A, when the catalyst temperature rises and reaches a temperature at which urea water can be hydrolyzed (for example, 160 ° C.), supply of urea water from the injector 253 is started. In response to the start of the urea water supply, the urea water correction by the NH ₃ sensor F / B control, that is, the correction by the second FB correction amount described above is started. Until the NH ₃ sensor reaction, that is, the NH ₃ full (maximum) storage in the first selective reduction catalyst 231 is obtained, the supply amount of urea water is corrected to the maximum (1). Accordingly, the NH ₃ storage amount in the first selective reduction catalyst 231 increases at the maximum storage speed (2). NH ₃ from the sensor response was obtained, although the detected NH ₃ concentration is in the F / B control to converge to a preset target NH ₃ concentrations were, NH ₃ in the first selective reduction catalyst 231 full ( (Maximum) Since the supply amount of urea water was corrected to the maximum extent to the storage, the NH ₃ concentration greatly exceeded the target NH ₃ concentration. That is, a large amount of NH ₃ slip occurs as the NH ₃ storage capacity of the first selective reduction catalyst 231 decreases as the temperature of the catalyst increases (3).

In FIG. 7B, as in the case of FIG. 7A, when the catalyst temperature reaches a temperature at which urea water can be hydrolyzed (for example, 160 ° C.), supply of urea water from the injector 253 is started. However, unlike the case of (a), urea water correction by NH ₃ sensor F / B control is not performed, and instead, NH ₃ sensor reaction from the start of urea water supply, that is, NH ₃ full in the first selective reduction catalyst 231 is performed. Until the (maximum) storage is obtained, F / B correction (correction by the first FB correction amount) using the NH ₃ estimated storage amount of FIG. 5 is performed. In this case, the NH ₃ storage amount in the first selective reduction catalyst 231 increases at a slower storage speed than in the case (a) (1). After the NH ₃ sensor reaction is obtained, urea water correction (correction by the second FB correction amount) by NH ₃ sensor F / B control is started (2). NH ₃ storage amount in order to have increased moderately, NH ₃ concentration without exceed greatly the target NH ₃ concentration, is rapidly controlled such that it converges to the target NH ₃ concentration (3). As a result, the NH ₃ slip amount decreases and converges more quickly than in the case of (a) (4).

Further, the F / B correction (correction by the first FB correction amount) using the previous NH ₃ estimated storage amount is continuously used even after the NH ₃ full (maximum) storage. The reason is that the NH ₃ sensor F / B control has a relatively slow concentration F / B response, so the urea water injection amount overshooting the NH ₃ adsorbable amount is less likely to return to that possible amount. Therefore, it is for speeding up the movement by the combined use. The correction based on the first FB correction amount may be stopped after NH ₃ full (maximum) storage depending on the situation.

FIG. 8 shows the catalyst temperature, the urea water correction amount by the NH ₃ sensor F / B, the urea water correction amount by the F / B correction (correction by the first FB correction amount) using the NH ₃ estimated storage amount, FIG. 4 is a diagram showing the relationship between the NH ₃ concentration and the NH ₃ storage amount in the first selective reduction catalyst 231. (A) is an example in the case where F / B correction using the NH ₃ estimated storage amount is not used after NH ₃ full (maximum) storage, and (b) is an example in the case of using it together. First, the case of (a) will be described.

In FIG. 8A, when the catalyst temperature rises and reaches a temperature at which urea water can be hydrolyzed (for example, 160 ° C.), supply of urea water from the injector 253 is started. From the start of urea water supply until the NH ₃ sensor response is obtained, F / B correction using the estimated NH ₃ storage amount (correction by the first FB correction amount) is performed (1). The amount of NH ₃ storage in the first selective reduction catalyst 231 increases at a relatively slow storage speed during that time (2).

After the NH ₃ sensor reaction is obtained, urea water correction by the NH ₃ sensor F / B control (correction by the second FB correction amount) is started as in the case of FIG. 7B. The correction amount is initially toward the increase (plus) side, but shifts to the decrease side because the actual concentration of NH ₃ of the NH ₃ sensor exceeds the target concentration (3). However, even if the urea water correction amount is reduced, the NH ₃ sensor response is slow, and thus a concentration equal to or higher than the NH ₃ target concentration is continued (4). That is, a relatively large amount of NH ₃ slip occurs as the NH ₃ storage capacity of the first selective reduction catalyst 231 decreases as the catalyst temperature rises (5).

In FIG. 8B, the process from the start of urea water supply until the reaction of the NH ₃ sensor is obtained is the same as in the case of FIG. The difference from (a) is that F / B correction using the NH ₃ estimated storage amount is continued even after the reaction of the NH ₃ sensor is obtained, and combined with urea water correction by NH ₃ sensor F / B control. It is. The urea water correction amount by the NH ₃ sensor F / B control is initially directed to the increase (plus) side, but shifts to the decrease side because the actual concentration of NH ₃ of the NH ₃ sensor exceeds the target concentration (1). At the same time, the F / B correction using the NH ₃ estimated storage amount also exceeds the storage possible amount, so that the correction amount shifts to the decrease side (2). The combined use of both F / B corrections increases the effect of suppressing the deviation of the NH ₃ concentration from the target concentration (3). As a result, even if there is a decrease in the NH ₃ storage capacity at the first selective reduction catalyst 231 due to the temperature rise of the catalyst, the NH ₃ slip amount is smaller than in the case of (a) and converges quickly (4 ).

The above-described embodiment is an example, and the present invention is not limited to this. The present invention is applicable to an engine having an arbitrary number of cylinders. The present invention can also be applied to an engine such as a direct injection gasoline engine. Furthermore, the present invention can be applied not only to the urea water supply that is the above-described embodiment but also to the case where gaseous NH ₃ is directly supplied as the reducing agent supply. For example, when using the embodiment of FIG. 1, NH ₃ gas can be directly supplied by using an NH ₃ gas supply device instead of the urea water injection device 25.

Claims (2)

An exhaust purification device for an internal combustion engine,
A selective reduction catalyst provided in an exhaust passage of an internal combustion engine for capturing NH ₃ as a reducing agent and reducing NO _X flowing through the exhaust passage using the captured NH ₃ , comprising: a first selective reduction catalyst; A selective reduction catalyst including a second selective reduction catalyst provided downstream of the first selective reduction catalyst in the exhaust passage;
Reducing agent supply means for supplying the reducing agent to the upstream side of the selective reduction catalyst in the exhaust passage;
And NH ₃ detection means for detecting the NH ₃ concentration between the second selective reduction catalyst and the first selective reduction catalyst of the exhaust passage,
Means for calculating the amount of NH ₃ in the first selective reduction catalyst;
First correction amount setting means for setting a first correction amount of the reducing agent supply amount so that a difference between the calculated NH ₃ amount and the maximum NH ₃ storage amount in the first selective reduction catalyst decreases. ,
A second correction amount setting means for setting a second correction amount of the reducing agent supply amount so that a difference between the detected NH ₃ concentration and the target NH ₃ concentration decreases ;
After calculating starting the NH ₃ amount in said first selective reduction catalyst, wherein the first distance from the NH ₃ amount of the selective reduction in the catalyst is equal to or greater than ₃ storage amount maximum NH, using only the first correction amount Exhaust gas comprising: a reducing agent supply amount setting means for setting the reducing agent supply amount and setting the reducing agent supply amount using the second correction amount after the maximum NH ₃ storage amount is set. Purification equipment. The reducing agent supply amount setting means continues to use the first correction amount to reduce the supply amount of the reducing agent even after the NH ₃ amount in the first selective reduction catalyst reaches the maximum NH ₃ storage amount. The exhaust emission control device according to claim 1, which is set.

Images