JP2012189007A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control system for an internal combustion engine capable of utilizing NOx purification function by a second selective reduction catalyst at a downstream side to the maximum while suppressing discharge of a reduction agent from a tail pipe.SOLUTION: This exhaust emission control system includes: a first selective reduction catalyst and a second selective reduction catalyst, provided in series in an exhaust pipe; a second consumption amount estimation part 51 estimating an NOx amount flowing into the second selective reduction catalyst and an NOx purification rate of the second selective reduction catalyst and estimating an NHamount consumed for purifying the flowing amount of NOx by the second selective reduction catalyst on the basis of the NOx purification rate and the NOx amount; a target NHconcentration determination part 52 determining a target value VNH3for an NHsensor so that NHin the amount equivalent to the estimated NHconsumption amount is re-adsorbed in the second selective reduction catalyst; and a controller 53 determining a corrected injection amount G_so that an output value VNH3 of the sensor becomes the determined target value VNH3.

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine. More specifically, the present invention relates to an exhaust gas purification system including a selective reduction catalyst that purifies nitrogen oxides (NOx) in exhaust gas in the presence of a reducing agent.

Conventionally, as one of exhaust purification systems for purifying NOx in exhaust, a system in which a selective reduction catalyst for selectively reducing NOx in exhaust with a reducing agent such as ammonia (NH ₃ ) is provided in the exhaust passage has been proposed. ing. For example, in a urea addition type exhaust purification system, urea water, which is a precursor of ammonia, is supplied from the upstream side of the selective reduction catalyst, and ammonia is generated from the urea water by thermal decomposition or hydrolysis with the heat of exhaust gas. This ammonia selectively reduces NOx in the exhaust gas. In addition to such a urea addition type system, for example, a system in which ammonia is generated by heating an ammonia compound such as ammonia carbide and this ammonia is directly added has also been proposed. Hereinafter, a urea addition type system will be described.

  Such a selective reduction catalyst has the capability of purifying NOx in the exhaust in the presence of ammonia and adsorbing ammonia that has not been used for NOx reduction. That is, when the amount of urea water supplied is larger than the amount of NOx flowing into the selective reduction catalyst, the excess ammonia that is not used for NOx reduction is adsorbed by the selective reduction catalyst, and conversely flows into the selective reduction catalyst. When the supply amount of urea water is smaller than the NOx amount, ammonia adsorbed on the selective reduction catalyst is used for NOx reduction. Therefore, the storage amount of ammonia in the selective reduction catalyst can be controlled by increasing or decreasing the supply amount of urea water.

  In order to increase the NOx purification rate of the selective reduction catalyst, it is preferable that as much ammonia as possible is adsorbed, but there is a limit to the amount of ammonia that can be adsorbed by the selective reduction catalyst. When ammonia exceeding this limit amount is supplied to the selective reduction catalyst, ammonia that has not been adsorbed is discharged downstream. In recent years, an exhaust gas purification system has been proposed that aims to achieve both improvement in the NOx purification rate and suppression of ammonia slip.

  For example, in the exhaust purification system of Patent Document 1, two selective reduction catalysts, a first selective reduction catalyst and a second selective reduction catalyst, are provided in series in the exhaust passage, and the output value of an ammonia sensor provided between these catalysts is set. Based on this, the injection amount of urea water is determined. According to this system, while maintaining the state in which the amount of ammonia that can be stored in the first selective reduction catalyst is maintained and the NOx purification performance is maximized, the ammonia discharged from the first selective reduction catalyst is reduced downstream. By adsorbing with the second selective reduction catalyst or consuming it for the reduction of NOx, it is possible to suppress the discharge of ammonia from the tail pipe.

International Publication No. 2009/128169

By the way, in the system shown in Patent Document 1, as shown in FIG. 26, when the estimated value NH3 _{2BED_ST} of the storage amount of the second selective reduction catalyst is equal to or less than the predetermined value NH3 _{CONS_OPT} , the output value of the ammonia sensor In order to suppress an excessive increase in the storage amount of the second selective reduction catalyst when the target value for NH3 _{CONS is} set to a predetermined optimum value NH3 _{CONS_OPT} and the estimated value NH3 _{2BED_ST} of the storage amount _becomes larger than the NH3 _{CONS_OPT} , the target value to reduce the amount of ammonia flowing into the second selective reduction catalyst is set to a smaller value _{NH3 CONS -} than the _{NH3 CONS - OPT.}

  As described above, in the system disclosed in Patent Document 1, when the storage amount of the second selective reduction catalyst becomes large, the target value of the ammonia sensor is set small, and the amount of ammonia discharged from the first selective reduction catalyst is reduced. By reducing the amount, it is possible to suppress an excessive increase in the storage amount of the second selective reduction catalyst, and in turn, discharge of ammonia from the tail pipe. However, the state of the second selective reduction catalyst cannot always be maintained so that an optimal amount of ammonia is always stored only by switching the target value approximately in binary according to the storage amount as in this technique. Therefore, the NOx purification performance of the second selective reduction catalyst was not fully utilized.

  An object of the present invention is to provide an exhaust gas purification system for an internal combustion engine that can fully utilize the NOx purification performance of the downstream second selective reduction catalyst while suppressing discharge of the reducing agent from the tail pipe. And

In order to achieve the above object, the present invention is provided in an exhaust passage (for example, an exhaust pipe 11 described later) of an internal combustion engine (for example, an engine 1 described later), purifies NOx in the exhaust in the presence of a reducing agent, and A first selective reduction catalyst (for example, a first selective reduction catalyst 231 to be described later) that adsorbs the reducing agent, and a second selective reduction catalyst (for example, provided downstream of the first selective reduction catalyst in the exhaust passage) A second selective reduction catalyst 232) to be described later, and a reducing agent supply means for supplying a reducing agent or a precursor thereof upstream of the first selective reduction catalyst in the exhaust passage (for example, a urea injection device 25 to be described later). A second NOx inflow amount acquisition means (for example, a second NOx inflow amount estimation unit 511 described later or a NOx sensor) that estimates or detects the amount of NOx that has flowed into the second selective reduction catalyst, and an operating condition of the internal combustion engine An exhaust gas purification system for an internal combustion engine (for example, described later), comprising: a second NOx purification rate estimating means (for example, a second NOx purification rate estimating unit 516 described later) for estimating the NOx purification rate of the second selective reduction catalyst based on An exhaust gas purification system 2) is provided. The exhaust purification system calculates the inflow amount of NOx based on the NOx purification rate (η _2nd ) in the second selective reduction catalyst and the NOx amount (QNO _2nd , QNO2 _2nd ) that flows into the second selective reduction catalyst. A second reducing agent consumption estimating means (for example, a second NH ₃ consumption estimating unit 515 described later) for estimating the amount of reducing agent consumed for purification by the second selective reduction catalyst, and the estimated reduction The amount of reducing agent or precursor supplied by the reducing agent supply means based on the amount of consumption so that an amount of reducing agent equivalent to the amount of agent consumed (QNH3 _cons ) is re-adsorbed to the second selective reduction catalyst ( Reducing agent supply amount determining means for determining G _{UREA_FB (} and hence G _UREA ) (for example, a target NH ₃ concentration determining unit 52 and a sliding mode controller 53 described later) And comprising.

In the present invention, the NOx purification rate of the second selective reduction catalyst that changes in accordance with the operating state of the internal combustion engine is estimated, the amount of NOx flowing into the second selective reduction catalyst is estimated or detected, and further, the NOx purification rate and The consumption of the reducing agent in the second selective reduction catalyst is estimated based on the amount of inflowing NOx. Then, the supply amount of the reducing agent or the precursor is determined based on the estimated consumption amount so that the reducing agent equivalent to the estimated consumption amount of the reducing agent is re-adsorbed to the second selective reduction catalyst. . Thus, the amount of the reducing agent in the second selective reduction catalyst is determined by determining the supply amount of the reducing agent or precursor to be newly supplied by supplementing the amount actually consumed in the second selective reduction catalyst. It can be suppressed that the storage amount increases excessively and the reducing agent is discharged downstream. Moreover, by re-adsorbing only the consumed amount, it is possible to suppress the reduction amount of the reducing agent in the second selective reduction catalyst from being excessively reduced and the NOx purification rate from being reduced.
In addition, since the second selective reduction catalyst is provided on the downstream side of the first selective reduction catalyst, the reducing agent does not flow into the second selective reduction catalyst until it exceeds the amount that can be stored in the first selective reduction catalyst. Will be. Therefore, the amount of reduction that can be stored in the first selective reduction catalyst is determined by determining the supply amount of the reducing agent or the precursor so that the consumed amount of the reducing agent is re-adsorbed with respect to the second selective reduction catalyst. Since the agent can continue to be stored, the NOx purification rate in the first selective reduction catalyst can also be maintained high.

In this case, the second storage amount estimation means for estimating the storage amount of the reducing agent in the second selective reduction catalyst (for example, a second storage amount estimation unit 521 described later) and the second storage amount estimation unit based on the operating state of the internal combustion engine. A second target storage amount setting unit (for example, a second target storage amount setting unit 523 described later) that sets a target storage amount of the reducing agent in the two-selective reduction catalyst, and the reducing agent supply amount determination unit includes: When the estimated amount of storage of the reducing agent (ST _2nd ) is less than the set target storage amount (ST _{2nd_CMD} ), the amount of reduction equivalent to the estimated amount of reducing agent consumption (QNH3 _cons ) agent as is re-adsorbed on the second selective reduction catalyst, a reducing agent by the reducing agent supply unit based on the consumption (QNH3 _cons) the The supply amount of precursor _{(G UREA,} G _{UREA_FB)} determines, wherein when the storage amount of the estimated reducing agent _{(ST 2nd)} is the set target storage amount _{(ST 2nd_CMD)} above, the estimated The amount of reducing agent or precursor supplied by the reducing agent supply means (G _UREA , so that an amount of reducing agent less than the consumed amount of reducing agent (QNH 3 _cons ) is re-adsorbed to the second selective reduction catalyst. _Preferably, G _{UREA_FB} ) is determined.

  In the present invention, the storage amount of the reducing agent in the second selective reduction catalyst is estimated, and the target storage amount with respect to the storage amount is set based on the operating state of the internal combustion engine. If the storage amount is less than the target storage amount and it can be determined that the storage amount is sufficient, as described above, the reducing agent or the precursor can be re-adsorbed so that the same amount of reducing agent as the consumption amount is re-adsorbed. Determine the supply. In addition, when the storage amount is equal to or greater than the target storage amount and it can be determined that there is no room for the storage amount, the supply amount of the reducing agent or precursor is reduced so that a smaller amount of the reducing agent than the consumption amount is re-adsorbed. By determining, the storage amount of the second selective reduction catalyst is decreased toward the target storage amount. As a result, the storage amount of the second selective reduction catalyst can be controlled in the vicinity of the target storage amount, so that the NOx purification rate can be maintained high, and the excessive amount of reducing agent can be prevented from being discharged from the tail pipe. Can do.

In this case, when the estimated storage amount of the reducing agent (ST _2nd ) is equal to or _larger than the set target storage amount (ST _{2nd_CMD} ), the amount of the reducing agent that is re-adsorbed on the second selective reduction catalyst. It is preferable to determine the supply amount (G _UREA , G _{UREA_FB} ) of the reducing agent or the precursor by the reducing agent supply means based on the consumption amount so that the value becomes 0 or a predetermined amount in the vicinity of 0.

  According to the present invention, when the storage amount of the second selective reduction catalyst is equal to or greater than the target storage amount, the amount of reducing agent re-adsorbed on the second selective reduction catalyst becomes 0 or a predetermined value near zero. That is, that is, the supply amount of the reducing agent or the precursor is determined so that the storage amount of the second selective reduction catalyst is surely lowered. As a result, the storage amount can be quickly reduced so that the reducing agent is not discharged from the tail pipe.

In this case, the exhaust purification system includes a reducing agent detection means (for example, an NH ₃ sensor described later) that detects a reducing agent concentration or a reducing agent amount of exhaust gas between the first selective reduction catalyst and the second selective reduction catalyst. 26) further wherein the reducing agent supply amount determination means, wherein the reducing agent by the reducing agent supply means so that the output value (VNH3) becomes a predetermined target value (VNH3 _CMD) of the reducing agent detection means or precursor A target value setting for setting a target value for the output value of the feedback control means (for example, sliding mode controller 53 described later) and the reducing agent detection means to a value larger than 0 corresponding to the consumption. Means (for example, a target NH ₃ concentration determination unit 52 described later).

  According to the present invention, by performing feedback control based on the output value of the reducing agent detection means provided between the first selective reduction catalyst and the second selective reduction catalyst, the exhaust gas flowing into the second selective reduction catalyst Since the amount of ammonia can be grasped more accurately, it is possible to more reliably maintain the NOx purification rate in the second selective reduction catalyst at a high level and suppress the discharge of ammonia from the tail pipe.

It is a mimetic diagram showing composition of an engine and its exhaust gas purification system concerning one embodiment of the present invention. It is a block diagram which concerns on determination of the urea water injection amount comprised by ECU. It is a block diagram which shows the structure of the 2nd consumption estimation part of a correction | amendment injection quantity determination part. It is a block diagram showing the configuration of a target NH ₃ concentration determination section of the correction injection amount determining unit. In the second consumption estimating unit and the target NH ₃ concentration determination section is a time chart showing an example of change in each parameter. The estimated value of the NH ₃ storage amount of the second selective reduction catalyst is a time chart showing the relationship between the target value determined by the target NH ₃ concentration determination unit. It is a figure which shows the setting table of the switching function setting parameter VPOLE. It is a flowchart which shows the procedure which determines the urea water injection amount.

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

The exhaust purification system 2 includes an oxidation catalyst 21 provided in the exhaust pipe 11 of the engine 1, a CSF (Catalyzed Soot Filter) 22 provided in the exhaust pipe 11 for collecting soot in the exhaust, and provided in the exhaust pipe 11. A urea selective reduction catalyst 23 for purifying nitrogen oxide (NOx) in the exhaust gas flowing through the exhaust pipe 11 in the presence of ammonia (NH ₃ ) as a reducing agent, and a urea selective reduction catalyst in the exhaust pipe 11. A urea injection device 25 that supplies urea water, which is a precursor of NH ₃ , and an electronic control unit (hereinafter referred to as “ECU”) 3 are configured on the upstream side of 23.

The urea injection device 25 includes a urea tank 251 and a urea injection valve 253.
The urea tank 251 stores urea water, and is connected to the urea injection valve 253 via a urea supply path 254 and a urea pump (not shown). This urea tank 251 is provided with a urea level sensor 255. The urea level sensor 255 detects the water level of the urea water in the urea tank 251 and outputs a detection signal substantially proportional to the water level to the ECU 3. The urea injection valve 253 is connected to the ECU 3, operates in accordance with a control signal from the ECU 3, and injects urea water into the exhaust pipe 11 in accordance with this control signal. That is, urea injection control is executed.

The urea injection device 25 includes a urea tank 251 and a urea injection valve 253.
The urea tank 251 stores urea water, and is connected to the urea injection valve 253 via a urea supply path 254 and a urea pump (not shown). This urea tank 251 is provided with a urea level sensor 255. The urea level sensor 255 detects the water level of the urea water in the urea tank 251 and outputs a detection signal substantially proportional to the water level to the ECU 3. The urea injection valve 253 is connected to the ECU 3, operates in accordance with a control signal from the ECU 3, and injects urea water into the exhaust pipe 11 in accordance with this control signal. That is, urea injection control is executed.

The oxidation catalyst 21, CSF 22 provided upstream of the of the exhaust pipe 11, another for oxidizing HC and CO in the exhaust gas purification, to convert the NO in the exhaust to NO _2.
The CSF 22 is provided on the downstream side of the oxidation catalyst 21 in the exhaust pipe 11, and when the exhaust gas passes through the fine holes in the filter wall, the PM mainly composed of carbon in the exhaust gas is converted into the surface of the filter wall and the filter. Collect by depositing in holes in the wall. In addition, since an oxidation catalyst is applied to the filter wall, the filter wall has a function of oxidizing CO, HC, and NO in the exhaust similarly to the oxidation catalyst 21 described above.
By providing the oxidation catalyst 21 and the CSF 22 on the upstream side of the urea selective reduction catalyst 23, the NO ₂ -NOx ratio of the exhaust gas flowing into the urea selective reduction catalyst 23 (the molar ratio of NO ₂ to NOx that combines NO and NO _2). ) Is controlled in the vicinity of an optimum value (for example, 0.5) at which the NOx purification rate becomes maximum.

The urea 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 pipe 11. Each of these selective reduction catalysts 231 and 232 selectively reduces NOx in the exhaust in an atmosphere in which a reducing agent such as NH ₃ exists. Specifically, when urea water is injected by the urea injection device 25, the urea water is thermally decomposed or hydrolyzed by the heat of the exhaust to generate NH ₃ as a reducing agent. The produced NH ₃ is supplied to the selective reduction catalysts 231 and 232, and NOx in the exhaust is selectively reduced by these NH ₃ .

By the way, these selective reduction catalysts 231 and 232 have a function of reducing NOx in the exhaust with NH ₃ generated from urea water, and also have a function of adsorbing the generated NH ₃ and storing a predetermined amount. In the present embodiment, the amount of NH ₃ adsorbed by the selective reduction catalysts 231 and 232 is referred to as a storage amount, and the limit of the storage amount is referred to as a maximum storage capacity. The NH ₃ adsorbed in this way is also consumed as appropriate for the reduction of NOx in the exhaust gas. For this reason, as the storage amount increases, the NOx purification rate in the selective reduction catalyst increases.

Further, when the supply amount of urea water is small relative to the amount of NOx discharged from the engine, the adsorbed NH ₃ is consumed for the reduction of NOx so as to compensate for the shortage of urea water. When NH ₃ is generated exceeding the maximum storage capacity, the generated NH ₃ is discharged downstream of the selective reduction catalysts 231 and 232.

In order to detect the states of the engine 1 and the exhaust purification system 2 as described above, the ECU 3 is connected with an NH ₃ sensor 26, an exhaust temperature sensor 27, an NOx sensor 28, an air flow meter 29, a crank angle position sensor 14, and the like. Yes.

The NH ₃ sensor 26 detects the NH ₃ concentration of the exhaust gas between the first selective reduction catalyst 231 and the second selective reduction catalyst 232 in the exhaust pipe 11, and outputs an output signal VNH3 substantially proportional to the detected NH ₃ concentration. The ECU 3 is supplied. The exhaust temperature sensor 27 detects the temperature of the exhaust downstream of the second selective reduction catalyst 232 and supplies an output signal to the ECU 3 that is substantially proportional to the detected temperature. The NOx sensor 28 detects the NOx concentration of the exhaust gas flowing into the first selective reduction catalyst 231 and supplies the ECU 3 with an output signal VNOx that is substantially proportional to the detected NOx concentration. The air flow meter 29 detects an intake air amount (mass flow rate) flowing through an intake passage (not shown), and supplies an output signal to the ECU 3 that is substantially proportional to the detected intake air amount.

  The crank angle position sensor 14 detects the rotation angle of the crankshaft of the engine 1, generates a pulse at every predetermined crank angle, and supplies the pulse signal to the ECU 3. The engine speed NE of the engine 1 is calculated by the ECU 3 based on this pulse signal. The temperature of the exhaust system such as the temperature of the oxidation catalyst 21 and the CSF 22 (hereinafter collectively referred to as the CSF temperature) TCSF, the temperature of the first and second selective reduction catalysts (hereinafter collectively referred to as the selective reduction catalyst temperature) TSCR, For example, it is calculated by the ECU 3 based on the output value of the exhaust temperature sensor and the input such as the engine speed NE. Further, the exhaust flow rate Gex is calculated by the ECU 3 based on the output value of the air flow meter, the engine speed NE, and the like that are substantially proportional to the intake air amount.

  The ECU 3 shapes input signal waveforms from various sensors, corrects the voltage level to a predetermined level, converts an analog signal value into a digital signal value, and a central processing unit (hereinafter “ CPU ”). In addition, the ECU 3 includes a storage circuit that stores various calculation programs executed by the CPU, calculation results, and the like, and an output circuit that outputs control signals to the engine 1, the urea injection valve 253, and the like.

FIG. 2 is a block diagram relating to determination of the urea water injection amount G _UREA configured in the ECU.
In the present embodiment, as shown in the following equation (1), the corrected injection amount G _{UREA_FB} calculated by the corrected injection amount determination unit 5 is added to the FF injection amount G _{UREA_FF} calculated by the feedforward injection amount determination unit 4. By doing so, the urea water injection amount _GUREA is determined.

  Here, the symbol (k) is a symbol indicating the discretized time, and indicates that the data is detected or calculated every predetermined control period. That is, when the symbol (k) is data detected or calculated at the current control timing, the symbol (k-1) indicates data detected or calculated at the previous control timing. In the following description, the symbol (k) is omitted as appropriate. The unit time in the following description corresponds to one control cycle in the ECU.

The feedforward injection amount determination unit 4 searches a map set in advance based on the output value VNOx of the NOx sensor so that the NOx purification rate in the first and second selective reduction catalysts is maintained at the maximum value. The FF injection amount _{GUREA_FF} is determined. That is, the FF injection amount G _{UREA_FF} corresponds to the urea water injection amount calculated so that the equivalence ratio α is 1.

Here, the equivalent ratio α of urea water is the ratio of the inflow amount of urea water per unit time (including the amount that flows in as NH ₃ ) to the inflow amount of NOx per unit time (urea water inflow amount / NOx inflow amount). When the amount of urea water that can reduce NOx without excess or deficiency flows into NOx flowing into the selective reduction catalyst, the equivalent ratio α is 1. That is, when the amount of urea water necessary for reducing the inflowing NOx is not supplied to the NOx flowing into the target selective reduction catalyst, the equivalence ratio α becomes a value smaller than 1, and the inflowing NOx is reduced. When more urea water than the amount necessary for reduction is supplied, the equivalent ratio α becomes a value larger than 1.

The corrected injection amount determination unit 5 includes a second consumption amount estimation unit 51, a target NH ₃ concentration determination unit 52, and a sliding mode controller 53.
The second consumption estimation unit 51 estimates the amount of NH ₃ consumed by the second selective reduction catalyst for purifying NOx flowing into the second selective reduction catalyst. Target NH ₃ concentration determination unit 52, based on consumption of NH ₃ that is estimated by the second consumption amount estimation unit 51, a target value of the NH ₃ concentration in exhaust gas flowing into the second selective reduction catalyst, i.e. NH ₃ sensor The target value VNH3 _CMD for the output value VNH3 is determined. The sliding mode controller 53 determines the corrected injection amount G _{UREA_FB} so that the output value VNH3 of the NH ₃ sensor converges in the vicinity of the determined target value VNH3 _CMD . Hereinafter, the configuration of each block constituting the corrected injection amount determination unit 5 will be described in detail in order.

FIG. 3 is a block diagram illustrating a configuration of the second consumption amount estimation unit 51.
As shown in FIG. 3, the second consumption amount estimation unit 51, the NOx amount discharged from the first selective reduction catalyst to the downstream side thereof, that is, the NOx amount that flows into the second selective reduction catalyst per unit time (NO amount and and the 2NOx inflow amount estimating unit 511 for estimating the quantity) of the combination of the NO ₂ content, the amount of NH ₃ consumed per unit time in order to purify NOx in the inflow amounts by the second selective reduction catalyst And a second NH ₃ consumption estimation unit 515 for estimation.

The 2NOx inflow amount estimating unit 511, a first 1NOx inflow amount estimating unit 512 for estimating the amount of NO introduced into the first selective reduction catalyst and NO ₂ amount, NOx purification of the first selective reduction catalyst based on the operating state of the engine And a first NOx purification rate estimation unit 513 that estimates the rate.

The first NOx inflow amount estimating unit 512 estimates the NO ₂ -NOx ratio of the exhaust gas flowing into the first selective reduction catalyst, and the NOx flowing into the first selective reduction catalyst per unit time with the estimated NO ₂ -NOx ratio. based like amount _{QNOx 1st,} we calculate an estimated value _{QNO2 1st} estimate _{QNO 1st} and NO ₂ of NO quantity flowing into the first selective reduction catalyst per unit time. Here, the NO ₂ -NOx ratio of the exhaust gas flowing into the first selective reduction catalyst is calculated by, for example, the CSF temperature TSCF, the oxidation catalyst, and the exhaust space velocity SV in the CSF. Here, as the space velocity SV of the exhaust gas in the oxidation catalyst and the CSF, for example, a value calculated based on the exhaust flow rate Gex and the capacities of the oxidation catalyst and the CSF is used. Further, the NOx amount QNOx _1st flowing into the first selective reduction catalyst per unit time is set to a concentration [ppm] by multiplying the NOx sensor output value VNOx substantially proportional to the NOx concentration of the exhaust by the exhaust flow rate Gex. A value obtained by unit conversion from a value in units to a value in units of quantity [g] is used.

The first NOx purification rate estimation unit 513 is determined in advance based on the selective reduction catalyst temperature TSCR, the exhaust space velocity SV in the selective reduction catalyst, the estimated value ST _1st of the NH ₃ storage amount of the first selective reduction catalyst, and the like. By searching the map, an estimated value η _1st of the NOx purification rate of the first selective reduction catalyst is calculated. The estimated value η _1st of the NOx purification rate is set to a smaller value as the space velocity SV becomes higher, and is set to a larger value as the estimated value ST _1st of the NH ₃ storage amount becomes larger. The selective reduction catalyst temperature TSCR is set to a large value when it is within or near the predetermined active temperature range, and is set to a small value as the distance from the active temperature range is increased. Here, as the exhaust space velocity SV in the selective reduction catalyst, for example, the one calculated by the exhaust flow rate Gex and the capacity of the selective reduction catalyst is used. Further, as the estimated value ST _1st of the NH ₃ storage amount of the first selective reduction catalyst, a value estimated based on a conventionally known algorithm is used. More specifically, for example, an algorithm described with reference to FIGS. 16 to 21 in International Publication No. 2009/128169 by the present applicant is used.

The estimated value QNO _2nd of the NO amount flowing into the second selective reduction catalyst per unit time is calculated from the estimated value QNO _1st of the NO amount flowing into the first selective reduction catalyst per unit time, as shown in FIG. It is calculated by subtracting the amount of NO reduced by the first selective reduction catalyst per hour (QNO _1st × η _1st ). Further, the estimated value QNO2 _2nd of the NO ₂ amount flowing into the second selective reduction catalyst per unit time is calculated from the estimated value QNO2 _1st of the NO ₂ amount flowing into the first selective reduction catalyst per unit time per unit time. It is calculated by subtracting the amount of NO ₂ reduced by the first selective reduction catalyst (QNO2 _1st × η _1st ).

The second NH ₃ consumption estimation unit 515 includes a second NOx purification rate estimation unit 516 that estimates the NOx purification rate of the second selective reduction catalyst based on the operating state of the engine, and a reduction reaction calculation unit 517. The

The second NOx purification rate estimation unit 516 searches a predetermined map based on the selective reduction catalyst temperature TSCR, the exhaust space velocity SV in the selective reduction catalyst, and the NH ₃ storage amount ST _2nd of the second selective reduction catalyst. Thus, the estimated value η _2nd of the NOx purification rate of the second selective reduction catalyst is calculated. The estimated value η _2nd of the NOx purification rate is set to a smaller value as the space velocity SV becomes higher, and is set to become a larger value as the estimated value ST _2nd of the NH ₃ storage amount becomes larger. The selective reduction catalyst temperature TSCR is set to a large value when it is within or near the predetermined active temperature range, and is set to a small value as the distance from the active temperature range is increased. In the second NOx purification rate estimation unit 516, the specific value of the estimated value ST _2nd of the NH ₃ storage amount of the second selective reduction catalyst is calculated by, for example, a target NH ₃ concentration determination unit 52 described later. A value is used.

In addition, among NO and NO ₂ flowing into the second selective reduction catalyst, the amount of NO and NO ₂ reduced in the second selective reduction catalyst per unit time flowed into the second selective reduction catalyst per unit time. It is calculated by multiplying the estimated values QNO _2nd and QNO2 _2nd of the NO amount and NO ₂ amount by the estimated value η _2nd of the NOx purification rate.

The reduction reaction calculation unit 517 assumes that NO and NO ₂ are reduced by NH ₃ under the reactions shown in the following formulas (2-1) to (2-3), and flows into the second selective reduction catalyst. The estimated value QNH3 _{cons of the} amount of NH ₃ consumed per unit time for reducing the NO and NO ₂ by the second selective reduction catalyst is used as the amount of NO and NO reduced by the second selective reduction catalyst per unit time. Calculate from ₂ quantities. The reaction shown in Formula (2-1) is a reaction that simultaneously reduces NO and NO ₂ in the exhaust gas, and is called Fast SCR (Selective Catalytic Reduction). The reaction shown in Formula (2-2) is a reaction that reduces only NO in the exhaust gas, and is called Standard SCR. The reaction shown in Formula (2-3) is a reaction that reduces only NO _{2 in} the exhaust, and is called Slow SCR. Note that the reaction rates of these three types of reactions are different, the Fast SCR of Formula (2-1) being the fastest, and the Standard SCR of Formula (2-2) being the slowest.

In the reduction reaction calculation unit 571, for example, when the amount of NO reduced by the second selective reduction catalyst is equal to the amount of NO _2, it is assumed that NH ₃ is consumed only by the reaction shown in Formula (2-1). To calculate the estimated value QNH3 _cons . When the amount of NO consumed by the second selective reduction catalyst is larger than the amount of NO ₂ , it is assumed that the equivalent amount of NO and NO ₂ has consumed NH ₃ by the reaction shown in Formula (2-1), As for the surplus of NO, an estimated value QNH3 _cons is calculated by assuming that NH ₃ has been consumed by the reaction shown in Formula (2-2). In addition, when the amount of NO ₂ consumed by the second selective reduction catalyst is larger than the amount of NO, it is assumed that the equivalent amount of NO and NO ₂ has consumed NH ₃ by the reaction shown in Formula (2-1). Further, the surplus of NO ₂ is calculated as an estimated value QNH 3 _cons by assuming that NH ₃ is consumed by the reaction shown in Formula (2-3).

As described above, in the present embodiment, the NOx amount flowing into the second selective reduction catalyst per unit time is estimated by the second NOx inflow amount estimating unit 511. However, the present invention is not limited to this, and it may be detected using a NOx sensor. . That is, instead of the second NOx inflow amount estimating unit 511, a NOx sensor for detecting the NOx concentration of exhaust gas between the first selective reduction catalyst and the second selective reduction catalyst is newly provided, and based on the output value of the NOx sensor. Thus, the amount of NOx flowing into the second selective reduction catalyst per unit time may be calculated. Incidentally, NOx sensors extant, since it has also detectability for NH ₃ not only NOx, it is preferable to performing processing such as subtracting the output value of the NH ₃ sensor from the output value of the NOx sensor.

FIG. 4 is a block diagram showing a configuration of the target NH ₃ concentration determination unit 52.
As shown in FIG. 4, the target NH ₃ concentration determination unit 52 is based on the second storage amount estimation unit 521 that calculates the estimated value ST _2nd of the NH ₃ storage amount of the second selective reduction catalyst, and the operating state of the engine. A second target storage amount setting unit 523 that sets a target value ST _{2nd_CMD} for the NH ₃ storage amount of the second selective reduction catalyst, and an NH ₃ sensor according to the estimated value ST _2nd of the NH ₃ storage amount and the target value ST _{2nd_CMD.} And an NH ₃ sensor target value determining unit 527 that determines a target value VNH3 _CMD for the output value.

The second storage amount estimation unit 521 calculates an estimated value QNH3 _{cons of the} amount of NH ₃ consumed by the second selective reduction catalyst per unit time from the NH ₃ amount QNH3 _2nd flowing into the second selective reduction catalyst per unit time. The estimated value ST _2nd of the NH ₃ storage amount of the second selective reduction catalyst is calculated by integrating the subtracted values QNH 3 _2nd -QNH 3 _cons . Note that, in the integration calculation in the second storage amount estimation unit 521, the maximum storage capacity ST _{2nd_max} of the second selective reduction catalyst calculated by the second maximum storage capacity estimation unit 524 described later is _set as the upper limit value, and 0 is set as the lower limit value. And The NH ₃ amount QNH3 _2nd flowing into the second selective reduction catalyst per unit time is obtained by multiplying the output value VNH3 of the NH ₃ sensor approximately proportional to the NH ₃ concentration of the exhaust by the exhaust flow rate Gex, etc. A value obtained by unit conversion from a value in units of ppm] to a value in units of amount [g] is used.

The second target storage amount setting unit 523 includes a second maximum storage capacity estimation unit 524 and an allowable factor setting unit 525.
The second maximum storage capacity estimation unit 524 calculates an estimated value ST _{2nd_max} of the maximum NH ₃ storage capacity of the second selective reduction catalyst by searching a predetermined map based on the selective reduction catalyst temperature TSCR. The estimated value ST _{2nd_max} is set to become a smaller value as the selective reduction catalyst temperature TSCR becomes higher.

The allowable factor setting unit 525 defines a ratio of the amount of NH ₃ actually stored with respect to the maximum amount of NH ₃ that can be stored in the selective reduction catalyst as a storage rate, and is determined in advance based on the selective reduction catalyst temperature TSCR. By searching the map, the upper limit value of the storage rate allowable for the second selective reduction catalyst is set. That is, in the second selective reduction catalyst, when an amount of NH ₃ exceeding the maximum storage capacity flows in and is discharged to the downstream side, it is directly discharged from the tail pipe to the outside of the vehicle. For this reason, unlike the first selective reduction catalyst, it is preferable that the NH ₃ storage amount has a margin by setting the upper limit value of the storage rate as described above. Further, in the allowable factor setting unit 525, the selective reduction catalyst temperature TSCR is set so as to increase as the maximum NH ₃ storage capacity decreases as described above.

The target value ST _{2nd_CMD} for the NH ₃ storage amount of the second selective reduction catalyst is obtained by multiplying the estimated value ST _{2nd_max} of the maximum NH ₃ storage capacity by the upper limit value of the storage rate set by the allowable factor setting unit 525. Calculated.

The NH ₃ sensor target value determination unit 527 has a NH ₃ concentration VNH3 of exhaust that needs to be supplied to the second selective reduction catalyst in order to resorb the amount of NH ₃ consumed per unit time in the second selective reduction catalyst. _cons and a predetermined lower limit value VNH3 _L (0 <VNH3 _L <VNH3 _cons ) set to a value smaller than this NH ₃ concentration VNH3 _{cons and} greater than 0, either of which is a target value VNH3 _CMD Determine as. The NH ₃ concentration NH3 _cons is expressed in units of the amount [g] by dividing the exhaust gas flow rate Gex by the estimated value QNH3 _{cons of the} amount of NH ₃ consumed in the second selective reduction catalyst per unit time. A value obtained by converting the value into a value with the concentration [ppm] as a unit is used.

In NH ₃ sensor target value decision unit 527, as shown in the following formula (3), compares the target value _{ST 2Nd_CMD} for NH ₃ storage amount of the second selective reduction catalyst, the estimated value _{ST 2nd} of NH ₃ storage amount When the estimated value ST _2nd is less than the target value ST _{2nd_CMD} , the NH ₃ concentration VNH3 _cons calculated by unit conversion of the consumed amount QNH3 _cons as described above is used as the target value VNH3 _CMD of the NH ₃ sensor. decide. After setting the target value VNH3 _CMD as described above, the consumption amount is determined by determining the corrected injection amount _{GUREA_FB} so that the output value VNH3 of the NH3 sensor converges to the target value VNH3 _CMD by a sliding mode controller described later. The amount of NH ₃ equal to the amount obtained can be re-adsorbed on the second selective reduction catalyst.
When the estimated value ST _2nd is equal to or _larger than the target value ST _{2nd_CMD} , the lower limit value VNH3 _L set to a value smaller than the NH ₃ concentration VNH3 _cons and larger than 0 as described above is used as the target value VNH3 of the NH ₃ sensor. Determine as _CMD . Thereby, an amount of NH3 smaller than the consumed amount QNH3 _cons is re-adsorbed by the second selective reduction catalyst, and therefore the estimated value ST _2nd of the NH ₃ storage amount of the second selective reduction catalyst is directed toward the target value ST _{2nd_CMD.} Can be gradually reduced. Particularly, by setting the lower limit value VNH3 _L to the amount in the neighborhood of 0, since the amount of NH ₃ to be re-adsorbed by the second selective reduction catalyst can be in the vicinity of 0 or 0, NH ₃ in the second selective reduction catalyst The estimated value ST _2nd of the storage amount can be quickly decreased toward the target value ST _{2nd_CMD} .

FIG. 5 shows the parameters (NOx amount QNOx _1st flowing into the first selective reduction catalyst, QNOx _1st and first selective reduction catalyst) in the second consumption estimation unit 51 and the target NH ₃ concentration determination unit 52 configured as described above. 6 is a time chart showing an example of changes in NOx purification rate η _1st , NO amount QNO _2nd and NO ₂ amount QNO2 _2nd , NH ₃ sensor target value VNH3 _CMD ) slipping from the first selective reduction catalyst to the second selective reduction catalyst.

As shown in FIG. 5, when the engine shifts from a medium load to a high load, the amount of NOx flowing into the first selective reduction catalyst increases (QNOx _1st increase). Further, at this time, the NOx purification rate of the first selective reduction catalyst decreases (η _1st decrease) due to an increase in the space velocity of the exhaust gas. By NOx amount is increased and purification rate flowing into the first selective reduction catalyst is decreased, the amount of NOx slipping from the first selective reduction catalyst in the second selective reduction catalyst is increased _{(QNO 2nd, QNO2} 2nd increase ).
Thus, when the amount of NOx flowing into the second selective reduction catalyst increases, the amount of NH ₃ consumed by the second selective reduction catalyst to reduce the amount of Qx3 _cons also increases, so that the NH ₃ consumed here Is re-adsorbed to the second selective reduction catalyst, the target value VNH3 _CMD for the output value VNH3 of the NH ₃ sensor is set to a larger value.

Conversely, when the engine shifts from a medium load to a low load, the amount of NOx flowing into the first selective reduction catalyst decreases (QNOx _1st decrease), and the NOx purification rate of the first selective reduction catalyst increases ( η _1st increase). By NOx amount is reduced and the purification rate flowing into the first selective reduction catalyst increases, the amount of NOx slipping from the first selective reduction catalyst in the second selective reduction catalyst decreases _{(QNO 2nd, QNO2} 2nd reduction ).
Thus, when the amount of NOx flowing into the second selective reduction catalyst decreases, the NH ₃ amount QNH3 _cons consumed by the second selective reduction catalyst to reduce this also decreases, so the output value VNH3 of the NH ₃ sensor The target value VNH3 _{CMD for} is set to a smaller value.

FIG. 6 is a time chart showing the relationship between the estimated value ST _2nd (lower stage) of the NH ₃ storage amount of the second selective reduction catalyst and the target value VNH 3 _CMD (upper stage) determined by the target NH ₃ concentration determination unit. .
As shown in FIG. 6, the target value ST _{2nd_CMD} of the NH ₃ storage amount of the second selective reduction catalyst is set to a smaller value by the _product of the maximum storage capacity ST _{2nd_max} and the upper limit value of the storage rate. As described above, when an excess amount of NH ₃ is discharged from the first selective reduction catalyst due to rapid acceleration, rapid temperature increase, or the like by providing a margin between the target value ST _{2nd_CMD} and the maximum storage capacity ST _{2nd_max.} Even so, it can be adsorbed by the second selective reduction catalyst so as not to be discharged from the tail pipe.

Further, as shown in the above formula (3), when the estimated value ST _2nd of the NH ₃ storage amount of the second selective reduction catalyst is less than the target value ST _{2nd_CMD} , the amount consumed in the second selective reduction catalyst The target value VNH3 _CMD of the NH ₃ sensor is set to a value corresponding to the NH ₃ consumption QNH3 _cons per unit time so that NH ₃ is adsorbed again. Then, when the estimated value ST _2nd of the NH ₃ storage amount of the second selective reduction catalyst is equal to or greater than the target value ST _{2nd_CMD} , the NH ₃ sensor target value VNH _{3 CMD} is NH to reduce the NH ₃ storage amount. _{3 The} lower limit value VNH3 _L is set regardless of the consumption QNH3 _cons .

Returning to FIG. 2, the configuration of the sliding mode controller 53 will be described.
Sliding mode controller 53, the output value VNH3 of NH ₃ sensor, so as to converge in the vicinity of the set target value _{VNH3 CMD,} determines the correction injection amount _{G UREA - FB.} As will be described in detail below, in the present embodiment, the corrected injection amount _{GUREA_FB} is determined by response designating control that can set the convergence speed of the output value VNH3 of the NH ₃ sensor to the target value VNH3 _CMD . A conventionally known feedback algorithm may be used.

First, as shown in the following equation (4), the deviation between the detected value VNH3 of the NH3 sensor and the target value VNH3 _CMD is calculated and defined as the slip amount deviation ENH3.

Next, a switching function setting parameter VPOLE corresponding to the output value VNH3 is calculated based on a predetermined VPOLE setting table. Further, as shown in the following equation (5), the sum of the product of this VPOLE (k) and the previous value ENH3 (k-1) of the slip amount deviation and the current value ENH3 (k) of the slip amount deviation is calculated. This is defined as the switching function σ (k).

  For example, when the phase plane is defined with the horizontal axis representing the previous value ENH3 (k−1) of the deviation and the vertical axis representing the current value ENH3 (k) of the error, the switching function σ (k) defined by Equation (5) is defined. The combination of the deviations ENH3 (k) and ENH3 (k-1) for which becomes 0 becomes a switching line having a slope of -VPOLE. On this switching straight line, by setting -VPOLE to a value smaller than 1 and larger than 0, ENH3 (k-1)> ENH3 (k) is satisfied, so that the deviation ENH3 converges to 0. . That is, the switching function setting parameter VPOLE is a parameter that specifies the convergence of the error ENH3.

FIG. 7 is a diagram showing a setting table for the switching function setting parameter VPOLE. Here, the horizontal axis indicates the output value VNH3 (k) of the NH3 sensor, and the vertical axis indicates the switching function setting parameter VPOLE (k). In the present embodiment, the range [VNH3 _{CMD_L} , VNH3 _{CMD_H} ] is set in a region larger than 0 with respect to the output value VNH3 so as to include the determined target value VNH3 _CMD . When the output value VNH3 is within the above range, the output value VNH3 is within the above range by setting the switching function setting parameter VPOLE to a low convergence value near -1 (for example, -0.95). Control to drift. Further, when the output value VNH3 is larger than the upper limit _{VNH3 CMD_H} the above range, the switching function setting parameter VPOLE, the low convergence value convergence value among which is set to a value greater than (e.g., -0.4) to set, the output value VNH3 when the lower limit _{VNH3 CMD_L} less than the above range, the switching function setting parameter VPOLE, high convergence value that is further set to a value greater than the in convergence value (e.g., -0.2 ), The output value VNH3 is controlled so as to quickly converge within the above range.

Based on the switching function σ (k) calculated as described above, the reaching law input U _RCH (k), the nonlinear input U _NL (k), and the adaptive law input U _ADP (k) are calculated. As shown in (6), the sum of these U _RCH (k), U _NL (k), and U _ADP (k) is calculated and defined as a corrected injection amount G _{UREA_FB} (k).

The reaching law input U _RCH (k) is an input for placing the deviation state quantity on the switching straight line. As shown in the following equation (7), the switching function σ (k) has a predetermined reaching law control gain K _RCH. It is calculated by multiplying.

The nonlinear input U _NL (k) is an input for suppressing the nonlinear modeling error and placing the deviation state quantity on the switching straight line. As shown in the following equation (8), sign (σ (k)) It is calculated by multiplying by a predetermined nonlinear input gain _KNL . Here, sign (σ (k)) is a sign function, and is 1 when σ (k) is a positive value, and is −1 when σ (k) is a negative value.

The adaptive law input U _ADP (k) is an input for suppressing the influence of modeling error and disturbance and placing the deviation state quantity on the switching line. As shown in the following equation (9), the switching function σ (k ) Multiplied by a predetermined adaptive law gain K _ADP and the previous value U _ADP (k−1) of the adaptive law input.

FIG. 8 is a flowchart showing a procedure for determining the urea water injection amount _GUREA . This process is executed by the ECU as described above for each control cycle.
In S1, it is determined whether or not the exhaust purification system is normal. More specifically, for example, whether or not the urea injection device is normal, whether or not the first and second selective reduction catalysts have deteriorated or failed, and the remaining amount of urea water in the urea tank is a specified value. Whether or not warming up after engine startup has been completed, whether or not various sensors such as the NH ₃ sensor, NOx sensor, exhaust temperature sensor have failed, and these sensors have become active. Or not. If the determination in S1 is NO and it is determined that the apparatus is not normal, the process _proceeds to S2, and the supply of urea water is stopped, that is, the urea water injection amount _GUREA is set to 0, and then this process ends. If the determination in S1 is YES, the process proceeds to S3.

In S3, it is determined whether or not urea water injection is possible. Here, when urea water injection is not possible, for example, when the temperature of the selective reduction catalyst is low and urea water is injected, NOx reduction by the selective reduction catalyst cannot be expected, or when the exhaust temperature is low and urea water is injected. etc. If not hydrolyzed and the like in the NH ₃ also. Even when this determination is NO, the process proceeds to S2, the supply of urea water is stopped, and this process is terminated.

If the determination in S3 is YES, the FF injection amount _{GUREA_FF} is determined in S4 (see the feedforward injection amount determination unit 4 in FIG. 2), and the NOx purification rates of the first and second selective reduction catalysts are determined in S5. Estimated values η _1st , η _{2nd and the} like are calculated (see the first NOx purification rate estimation unit 513 and the second NOx purification rate estimation unit 516 in FIG. 3), and consumed in the second selective reduction catalyst in unit time in S6. The estimated value QNH3 _cons of the NH ₃ amount is calculated (see the reduction reaction calculation unit 517 in FIG. 3).

In S7, it is determined whether or not the estimated value ST _2nd of the NH ₃ storage amount of the second selective reduction catalyst is less than the target value ST _{2nd_CMD} (see the above formula (3)). When this determination is YES and the estimated value ST _2nd of the NH ₃ storage amount is less than the target value ST _{2nd_CMD} , the _{process proceeds} to S8, and the target value VNH3 _CMD of the NH ₃ sensor is consumed in the second selective reduction catalyst. The concentration VNH3 _cons is set according to the NH ₃ amount QNH3 _cons . If this determination is NO and the estimated value ST _2nd of the NH ₃ storage amount is equal to or _larger than the target value ST _{2nd_CMD} , the _{process proceeds} to S9, and the target value VNH3 _CMD of the NH ₃ sensor is set to the lower limit value VNH3 _L.

After setting the target value _{VNH3 CMD} of the NH ₃ sensor as described above, the output value VNH3 of NH ₃ sensor to the target value _{VNH3 CMD} calculates the correction injection amount _{G UREA - FB} so as to converge at S10 (in FIG. 2 In S11, the sum of the corrected injection amount G _{UREA_FB} and the FF injection amount G _{UREA_FF} is determined as the urea water injection amount G _UREA , and this process is terminated.

According to this embodiment, the following effects can be obtained.
(1) In the present embodiment, the estimated value η _2nd of the NOx purification rate of the second selective reduction catalyst that changes according to the operating state of the engine is calculated, and the NO amount and NO ₂ amount that have flowed into the second selective reduction catalyst Estimated values QNO _2nd and QNO _{2 2nd} are calculated, and further, an estimated value QNH 3 _cons of NH ₃ consumption in the second selective reduction catalyst is calculated based on these estimated values η _2nd , QNO _2nd and QNO _{2 2nd} . And here so that consumption and an equivalent amount of NH ₃ in NH ₃ estimated is re-adsorption in the second selective reduction catalyst, the correction injection amount based on the estimated value _{QNH3 cons G UREA_FB} hence urea water injection amount G _{Determine UREA} . In this way, the amount of NH ₃ storage in the second selective reduction catalyst is determined by determining the urea water injection amount G _UREA to be newly supplied by making up for the amount actually consumed in the second selective reduction catalyst again. It can be suppressed that NH ₃ is excessively increased and discharged to the downstream side. Moreover, by re-adsorbing only the consumed amount, it is possible to suppress the NH ₃ storage amount of the second selective reduction catalyst from being excessively reduced and the NOx purification rate from being reduced.
In addition, since the second selective reduction catalyst is provided on the downstream side of the first selective reduction catalyst, NH ₃ does not flow into the second selective reduction catalyst until it is supplied beyond the amount that can be stored in the first selective reduction catalyst. Will be. Therefore, by determining the urea water injection amount _GUREA so that the consumed NH ₃ is re-adsorbed with respect to the second selective reduction catalyst, a storable amount of the reducing agent is contained in the first selective reduction catalyst. Since it can be kept stored, the NOx purification rate in the first selective reduction catalyst can also be maintained high.

(2) In the present embodiment, the estimated value ST _2nd of the NH ₃ storage amount in the second selective reduction catalyst is calculated, and the target storage amount ST _{2nd_CMD} for the storage amount ST _2nd is set based on the operating state of the engine. When the storage amount ST _2nd is less than the target storage amount ST _{2nd_CMD} and it can be determined that the storage amount is sufficient, the NH ₃ equivalent to the estimated amount of consumption QNH3 _cons is re-adsorbed as described above. Thus, the corrected injection amount _{GUREA_FB} is determined. In addition, when the storage amount ST _2nd is _{equal to} or _larger than the target storage amount ST _{2nd_CMD} and it can be determined that there is no room for the storage amount, an amount of NH ₃ smaller than the estimated consumption amount QNH3 _cons is re-adsorbed. By determining the corrected injection amount G _{UREA_FB} , the storage amount ST _2nd of the second selective reduction catalyst is decreased toward the target storage amount ST _{2nd_CMD} . As a result, the storage amount ST _2nd of the second selective reduction catalyst can be controlled in the vicinity of the target storage amount ST _{2nd_CMD} , so that an excessive amount of reducing agent is discharged from the tail pipe while maintaining the NOx purification rate high. Can be suppressed.

(3) According to the present embodiment, when the storage amount ST _2nd of the second selective reduction catalyst is equal to or _larger than the target storage amount ST _{2nd_CMD} , the amount of NH ₃ re-adsorbed on the second selective reduction catalyst is 0. Alternatively, the corrected injection amount G _{UREA_FB} is determined so as to be a predetermined value near 0, that is, the storage amount ST _2nd of the second selective reduction catalyst is surely decreased. As a result, the storage amount ST _2nd can be quickly reduced so that the reducing agent is not discharged from the tail pipe.

(4) According to the present embodiment, the feedback control based on the output value VNH3 of the NH ₃ sensor provided between the first selective reduction catalyst and the second selective reduction catalyst allows the flow into the second selective reduction catalyst. Since the amount of NH ₃ in the exhaust gas to be obtained can be grasped more accurately, it is possible to more reliably maintain the NOx purification rate in the second selective reduction catalyst at a high level and suppress the discharge of ammonia from the tail pipe.

In the above embodiment, an example in which the present invention is applied to a urea addition type exhaust gas purification system that uses ammonia as a reducing agent and supplies urea water as a precursor thereof is shown, but the present invention is not limited thereto.
For example, ammonia may be supplied directly without supplying urea water and generating ammonia from the urea water. The ammonia precursor is not limited to urea water, and other additives may be used. Further, the reducing agent for reducing NOx is not limited to ammonia. The present invention can also be applied to an exhaust purification system using, for example, hydrocarbons instead of ammonia as a reducing agent for reducing NOx.

1. Engine (internal combustion engine)
2 ... Exhaust purification system 231 ... 1st selective reduction catalyst 232 ... 2nd selective reduction catalyst 25 ... Urea injection device (reducing agent supply means)
26 ... NH ₃ sensor (reducing agent detection means)
3 ... ECU
DESCRIPTION OF SYMBOLS 5 ... Correction injection amount determination part 51 ... 2nd consumption estimation part 511 ... 2nd NOx inflow amount estimation part (2nd NOx inflow amount acquisition means)
515 ... second 2NH ₃ consumption estimation unit (second reducing agent consumption estimating means)
516 ... 2nd NOx purification rate estimation part (2nd NOx purification rate estimation means)
52 ... Target NH ₃ concentration determination unit (reducing agent supply amount determination means, target value setting means)
521 ... Second storage amount estimation unit (second storage amount estimation means)
523 ... Second target storage amount setting unit (second target storage amount setting means)
53. Sliding mode controller (reducing agent supply amount determining means, feedback control means)

Claims (4)

A first selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine, purifies NOx in the exhaust in the presence of the reducing agent, and adsorbs the reducing agent;
A second selective reduction catalyst provided downstream of the first selective reduction catalyst in the exhaust passage;
Reducing agent supply means for supplying a reducing agent or a precursor thereof upstream of the first selective reduction catalyst in the exhaust passage;
Second NOx inflow amount acquisition means for estimating or detecting the amount of NOx flowing into the second selective reduction catalyst;
An exhaust gas purification system for an internal combustion engine, comprising: a second NOx purification rate estimating means for estimating a NOx purification rate of the second selective reduction catalyst based on an operating state of the internal combustion engine,
Based on the NOx purification rate in the second selective reduction catalyst and the amount of NOx flowing into the second selective reduction catalyst, the amount of reducing agent consumed to purify the inflowing amount of NOx by the second selective reduction catalyst. Second reducing agent consumption estimation means for estimating the amount;
The supply amount of the reducing agent or the precursor by the reducing agent supply means based on the consumption amount so that the reduced agent equivalent to the estimated consumption amount of the reducing agent is re-adsorbed to the second selective reduction catalyst. An exhaust gas purification system for an internal combustion engine, comprising: a reducing agent supply amount determining means for determining Second storage amount estimation means for estimating a storage amount of the reducing agent in the second selective reduction catalyst;
Second target storage amount setting means for setting a target storage amount of the reducing agent in the second selective reduction catalyst based on an operating state of the internal combustion engine,
The reducing agent supply amount determination means includes
When the estimated storage amount of the reducing agent is less than the set target storage amount, an amount of reducing agent equivalent to the estimated amount of reducing agent consumption is re-adsorbed on the second selective reduction catalyst. And determining the supply amount of the reducing agent or precursor by the reducing agent supply means based on the consumption amount,
When the estimated storage amount of the reducing agent is greater than or equal to the set target storage amount, a smaller amount of reducing agent than the estimated reducing agent consumption is re-adsorbed on the second selective reduction catalyst. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the supply amount of the reducing agent or precursor by the reducing agent supply means is determined.   When the estimated storage amount of the reducing agent is equal to or larger than the set target storage amount, the amount of the reducing agent re-adsorbed on the second selective reduction catalyst is 0 or a predetermined amount in the vicinity of 0. The exhaust gas purification system for an internal combustion engine according to claim 2, wherein the supply amount of the reducing agent or the precursor by the reducing agent supply means is determined based on the consumption amount. A reducing agent detecting means for detecting a reducing agent concentration or a reducing agent amount of exhaust gas between the first selective reduction catalyst and the second selective reduction catalyst;
The reducing agent supply amount determination means includes
Feedback control means for determining a supply amount of the reducing agent or precursor by the reducing agent supply means so that an output value of the reducing agent detection means becomes a predetermined target value;
4. A target value setting means for setting a target value for an output value of the reducing agent detection means to a value larger than 0 corresponding to the consumption amount, according to any one of claims 1 to 3. An exhaust purification system for an internal combustion engine.

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