WO2011024721A1

WO2011024721A1 - Exhaust gas purification device for internal combustion engine

Info

Publication number: WO2011024721A1
Application number: PCT/JP2010/064080
Authority: WO
Inventors: 久夫羽賀; 橋本　英史
Original assignee: 本田技研工業株式会社
Priority date: 2009-08-28
Filing date: 2010-08-20
Publication date: 2011-03-03
Also published as: DE112010003455T5

Abstract

An exhaust gas purification device for an internal combustion engine, provided with a selective reduction catalyst, the exhaust gas purification device being capable of maintaining a high rate of NOx reduction and minimizing the discharge of ammonia to the downstream side of the selective reduction catalyst. An exhaust gas purification device (2) wherein a urea-based selective catalytic reduction catalyst (23) comprises a first selective catalytic reduction catalyst (231) and a second selective catalytic reduction catalyst (232) which is provided in the exhaust path (11) at a position downstream of the first selective catalytic reduction catalyst (231). Also, the exhaust gas purification device (2) is provided with a urea injection device (25) which supplies a reduction agent to the exhaust path (11) to a position upstream of the urea-based selective catalytic reduction catalyst (23), and also with an ammonia sensor (26) which detects the amount of ammonia in the exhaust path (11) at a position between the first selective catalytic reduction catalyst (231) and the second selective catalytic reduction catalyst (232). The amount (G_UREA) of injection of urea by the urea injection device (25) is determined so that the value of the amount (NH3_CONS) of ammonia detected by the ammonia sensor (26) is greater than "0".

Description

Exhaust gas purification device for internal combustion engine

The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus for an internal combustion engine that includes a selective reduction catalyst that reduces NOx in exhaust gas in the presence of a reducing agent.

Conventionally, as one of exhaust purification devices for purifying NOx in exhaust gas, a device in which a selective reduction catalyst for selectively reducing NOx in exhaust gas by adding a reducing agent is provided in an exhaust passage has been proposed. For example, in a urea addition type selective reduction catalyst using urea water as a reducing agent, ammonia is generated from the added urea, and NOx in the exhaust gas is selectively reduced by this ammonia.

In such a selective reduction catalyst, when the injection amount of the reducing agent is smaller than the optimum amount, the NOx reduction rate is lowered due to a shortage of ammonia consumed for the reduction of NOx. When there is a large amount, ammonia surplus in the reduction of NOx 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. In view of this, Patent Document 1 and Patent Document 2 describe a method of estimating the NOx reduction rate in the selective reduction catalyst and controlling the injection amount of the reducing agent based on this estimation.

In the exhaust emission control device of Patent Document 1, the NOx concentration downstream of the selective reduction catalyst is detected, and the detected NOx concentration and the composition of the exhaust gas flowing into the selective reduction catalyst from the operating state of the internal combustion engine, more specifically, Estimates the ratio of NO to NO ₂ . Further, the NOx reduction rate of the selective reduction catalyst is estimated based on the composition of the exhaust, and the injection amount of the reducing agent is controlled.
Further, in the exhaust purification device of Patent Document 2, the temperature of the catalyst is detected as an amount related to the NOx reduction rate in the selective reduction catalyst, and the amount of reducing agent injected is controlled based on this temperature.

However, the NOx reduction rate in the selective reduction catalyst varies depending not only on the exhaust composition and the temperature of the selective reduction catalyst as described above, but also on the deterioration state of the selective reduction catalyst. Moreover, the purification performance varies among individuals. In addition to this, when ammonia is stored in the selective reduction catalyst, the optimum amount of the reducing agent is different, so that the NOx reduction rate in the selective reduction catalyst apparently changes. Therefore, it is difficult to always optimally control the injection amount of the reducing agent in the exhaust purification devices as shown in

Patent Documents

1 and 2.

Therefore, in the following, a technique for more directly detecting the NOx reduction rate in the selective reduction catalyst and controlling the injection amount of the reducing agent based on this will be examined.

FIG. 26 is a schematic diagram showing a configuration of a conventional exhaust purification device 80.
As shown in FIG. 26, in the exhaust passage 82 of the engine 81, the oxidation catalyst 83 and urea water as a reducing agent stored in the urea tank 84 are placed in the exhaust passage 82 in order from the upstream side to the downstream side. A urea injection valve 85 that injects and a selective reduction catalyst 86 that reduces NOx in the exhaust in the presence of urea water are provided. Further, a temperature sensor 87 for detecting the temperature of the selective reduction catalyst 86 and a NOx sensor 88 for detecting the NOx concentration downstream of the selective reduction catalyst 86 are provided for monitoring the purification performance of the selective reduction catalyst.

In this exhaust purification device 80, for example, the NOx concentration of exhaust exhausted from the engine 81 is estimated from a preset map, and urea injection is performed based on this NOx concentration and the catalyst temperature detected by the temperature sensor 87. The amount of urea water injected by the valve 85 is determined. In particular, here, the deterioration state of the selective reduction catalyst 86 can be estimated based on the difference between the NOx concentration detected by the NOx sensor 88 and the estimated NOx concentration of the exhaust gas. In this exhaust purification device, it is possible to correct the injection amount of urea water in accordance with the deterioration state of the selective reduction catalyst 86 estimated as described above.

JP 2006-274986 A JP 2004-100700 A

FIG. 27 is a diagram showing the relationship between the NOx concentration and ammonia concentration of the exhaust downstream of the selective reduction catalyst and the output value of the NOx sensor in the above-described conventional exhaust purification device. Specifically, FIG. 27 shows, in order from the top, the relationship between the NOx concentration in the exhaust downstream of the selective reduction catalyst, the ammonia concentration in the exhaust downstream of the selective reduction catalyst, the output value of the NOx sensor, and the urea water injection amount. .

When the injection amount of urea water is increased, ammonia generated in the selective reduction catalyst is also increased, so that the NOx reduction rate in the selective reduction catalyst is increased. For this reason, as shown in FIG. 27, the NOx concentration downstream of the selective reduction catalyst decreases as the urea water injection amount increases. When the urea water injection amount indicated by the star is exceeded, the NOx concentration becomes substantially constant regardless of the urea water injection amount. That is, the urea water in an amount exceeding the asterisk is surplus for reducing the produced NOx.

Also, the ammonia generated from the urea water surplus here is not consumed for the reduction of NOx, but is stored in the selective reduction catalyst or discharged downstream of the selective reduction catalyst. Therefore, as shown in FIG. 27, the ammonia concentration in the exhaust downstream of the selective reduction catalyst increases when it exceeds the injection amount of urea water indicated by an asterisk. Note that the ammonia generated in this way is not stored in the selective reduction catalyst but is discharged downstream thereof, hereinafter referred to as “ammonia slip”.

As described above, the urea water injection amount indicated by an asterisk in FIG. 27 is the optimal injection amount in the exhaust gas purification apparatus because both the NOx concentration and the ammonia concentration can be minimized.

However, as shown in FIG. 27, the output value of the NOx sensor shows a downwardly convex characteristic with the output value at the optimum injection amount as the minimum point. This is because the existing NOx sensor is sensitive not only to NOx but also to ammonia due to its detection principle.
Therefore, it is not possible to determine whether the urea water injection amount is insufficient or excessive with respect to the optimal injection amount only by the output value from the NOx sensor. For this reason, it is difficult to suppress the discharge of ammonia while continuing to supply an optimal amount of urea water to maintain a high NOx reduction rate in the selective reduction catalyst.

The present invention has been made in consideration of the above-described points. In an exhaust gas purification apparatus for an internal combustion engine equipped with a selective reduction catalyst, ammonia is discharged downstream of the selective reduction catalyst while maintaining a high NOx reduction rate. An object of the present invention is to provide an exhaust purification device for an internal combustion engine that can be suppressed.

In order to achieve the above object, the present invention is provided in the exhaust passage (11) of the internal combustion engine (1), generates ammonia in the presence of a reducing agent, and selects the NOx flowing through the exhaust passage with this ammonia. An exhaust gas purification device (2) for an internal combustion engine comprising a reduction catalyst (23) is provided. The selective reduction catalyst includes a first selective reduction catalyst (231) and a second selective reduction catalyst (232) provided downstream of the first selective reduction catalyst in the exhaust passage, The exhaust purification device includes a reducing agent supply means (25) for supplying a reducing agent to the upstream side of the selective reduction catalyst in the exhaust passage, and the first selective reduction catalyst and the second selective reduction in the exhaust passage. Ammonia detection means (26) for detecting the amount of ammonia between the catalyst and the amount of ammonia (NH3 _CONS ) detected by the ammonia detection means is controlled to be a value greater than “0”. The first control input calculating means (3, 4, 42) for calculating the control input, and the reducing agent supply amount (G _UREA ) by the reducing agent supplying means are the control inputs calculated by the first control input calculating means. Reducing agent supply amount determining means (3, 7) for determining including force (G _{UREA_FB} ).

According to this invention, the first selective reduction catalyst and the second selective reduction catalyst are provided in the exhaust passage in order toward the downstream side, and the reducing agent is further introduced from the upstream side of the first selective reduction catalyst and the second selective reduction catalyst. A reducing agent supplying means for supplying and an ammonia detecting means for detecting the amount of ammonia between the first selective reduction catalyst and the second selective reduction catalyst were provided. Therefore, a control input for controlling the ammonia amount detected by the ammonia detection means to be a value larger than “0” is calculated, and the supply amount of the reducing agent by the reducing agent supply means is set to such a control input. Decided to include.
Thereby, the state in which ammonia has flowed out from the first selective reduction catalyst, that is, the state in which ammonia is sufficiently stored in the first selective reduction catalyst, can be maintained, and a high NOx reduction rate can be maintained. In particular, even in the case where a large amount of NOx is temporarily generated due to a sudden change in the operating state of the internal combustion engine, and the generation of ammonia for reducing this NOx is not in time, it is stored in the first selective reduction catalyst. Due to the ammonia, the NOx reduction rate during the transition until the generation of ammonia is completed can be maintained high.
Further, in this case, although ammonia slip occurs in the first selective reduction catalyst, the discharged ammonia is stored in the second selective reduction catalyst or consumed for NOx reduction in the second selective reduction catalyst. . Thereby, it can suppress that ammonia discharge | emits to the most downstream of a selective reduction catalyst, maintaining a high NOx reduction rate.

By the way, when the operating state of the internal combustion engine changes, the exhaust flow rate also changes. At this time, for example, even when a constant amount of reducing agent is supplied by the reducing agent supply means, if the exhaust gas flow rate changes, the exhaust gas between the first selective reduction catalyst and the second selective reduction catalyst is changed. The ammonia concentration also changes. That is, the ammonia concentration changes according to the exhaust gas flow rate. For this reason, for example, detection means for detecting the ammonia concentration between the first selective reduction catalyst and the second selective reduction catalyst is used so that the ammonia concentration detected by the detection means becomes a predetermined value larger than “0”. When the control input is determined, the amount of ammonia flowing into the second selective reduction catalyst deviates from an appropriate amount depending on the operating state of the internal combustion engine, and as a result, ammonia may slip to the most downstream side.
On the other hand, according to the present invention, the ammonia detection means detects the amount of ammonia between the first selective reduction catalyst and the second selective reduction catalyst, and the detected ammonia amount becomes a predetermined value greater than “0”. Thus, the control input is calculated. As a result, an appropriate supply amount of the reducing agent can be determined regardless of the operating state of the internal combustion engine, so that ammonia can be prevented from slipping to the most downstream side.

In this case, the amount of ammonia that can be stored in the first selective reduction catalyst is a first storage capacity, the amount of ammonia that can be stored in the second selective reduction catalyst is a second storage capacity, and the second storage capacity is the first storage capacity. It is preferably larger than the difference between the maximum and minimum capacity.

Incidentally, the storage capacity of the selective reduction catalyst varies depending on the temperature of the selective reduction catalyst. Specifically, the storage capacity decreases as the temperature of the selective reduction catalyst increases. Accordingly, when the temperature is rapidly increased in a state where ammonia is sufficiently stored in the first selective reduction catalyst as described above, the first storage capacity is rapidly decreased, and the stored ammonia is reduced to the second level. Released to the selective reduction catalyst.
According to the present invention, the second storage capacity of the second selective reduction catalyst is made larger than the difference between the maximum time and the minimum time of the first storage capacity of the first selective reduction catalyst. Thereby, for example, when the operating state of the internal combustion engine shifts from the low load operating state to the high load operating state, the temperature of the first selective reduction catalyst suddenly rises, and ammonia flows from the first selective reduction catalyst to the downstream side thereof. Even when is released, this ammonia can be stored in the second selective reduction catalyst. Thereby, it can further suppress that ammonia discharges to the most downstream side of the selective reduction catalyst.

In this case, the exhaust emission control device further includes target ammonia amount setting means (3, 41) for setting the target value of the ammonia amount (NH3 _CONS ) detected by the ammonia detection means to a value larger than “0”. The first control input calculating means preferably calculates the control input so that the ammonia amount detected by the ammonia detecting means falls within a predetermined range including the target value (NH3 _{CONS_TRGT} ).

By the way, the NOx reduction rate in the selective reduction catalyst has a smaller response delay with respect to the supply amount of the reducing agent and higher sensitivity than the ammonia slip in the selective reduction catalyst. That is, for example, when the supply amount of the reducing agent is reduced to suppress ammonia slip, there is a problem that the NOx reduction rate in the selective reduction catalyst is remarkably lowered.
According to the present invention, the target value of the ammonia amount detected by the ammonia detection means is set to a value larger than “0”, and further, the detected ammonia amount falls within a predetermined range including this target value. The control input was calculated, and the supply amount of the reducing agent was calculated including this control input.
That is, by controlling the supply amount of the reducing agent so that the ammonia amount between the first selective reduction catalyst and the second selective reduction catalyst falls within a predetermined range including the target value, the fluctuation of the supply amount of the reducing agent is changed. Can be reduced. Thereby, the NOx reduction rate in the NOx reduction catalyst can be maintained high. The above control based on the premise that ammonia slip occurs in the first selective reduction catalyst is particularly effective in the present invention in which the second selective reduction catalyst is provided downstream of the first selective reduction catalyst. is there.

In this case, the first control input calculation means is configured to be able to execute response designation type control capable of setting a convergence speed of the ammonia amount (NH3 _CONS ) detected by the ammonia detection means to the target value, The convergence rate when the ammonia amount detected by the detection means is included in the predetermined range (RNH3 _{CONS_TRGT} , NH3 _{CONS_LMTL} to NH3 _{CONS_LMTH} ) is the convergence rate when the ammonia amount detected by the ammonia detection means is within the predetermined range (RNH3 _{CONS_TRGT} , NH3 _{CONS_LMTL} to NH3 _{CONS_LMTH} ) is preferably set slower than the convergence speed.

According to the present invention, the response designation type control that can designate the convergence speed to the target value as the control input for controlling the ammonia amount detected by the ammonia detecting means to be within a predetermined range including the target value. Calculated by Here, the convergence speed when the detected ammonia amount falls within the above range is set to be slower than the convergence speed when it falls outside the above range.
As a result, when the detected ammonia amount falls outside the above range, the occurrence of excessive ammonia slip and the decrease in the NOx reduction rate are quickly suppressed, and when the detected ammonia amount falls within the above range. Thus, it is possible to prevent a large change in the supply amount of the reducing agent and to prevent the NOx reduction rate from being significantly reduced.

In this case, it is preferable that the target ammonia amount setting means sets the target value to a smaller value as the temperature of the exhaust gas of the internal combustion engine or the temperature of the selective reduction catalyst is higher.

A general selective reduction catalyst has a characteristic that the amount of ammonia that can be stored decreases as the temperature increases. According to the present invention, the target value of the ammonia amount is set smaller as the temperature of the exhaust gas of the internal combustion engine or the temperature of the selective reduction catalyst is higher. As a result, the amount of ammonia flowing into the second selective reduction catalyst can be appropriately controlled in accordance with the storable amount of ammonia in the selective reduction catalyst, thus further preventing ammonia from slipping to the most downstream side. it can.

In this case, the reducing agent further comprises second control input calculating means (5) for calculating a control input based on the rotational speed (NE) of the internal combustion engine and a load parameter (TRQ) representing a load of the internal combustion engine. The supply amount determining means preferably determines the supply amount (G _UREA ) of the reducing agent supplied by the reducing agent supply means further including the control input (G _{UREA_FF} ) calculated by the second control input calculating means.

According to this invention, the control input is calculated based on the rotational speed of the internal combustion engine and the load parameter indicating the load of the internal combustion engine, and the supply amount of the reducing agent is determined including this control input. Since the amount of NOx in the exhaust gas changes in accordance with the operating state such as the rotational speed and load of the internal combustion engine, the amount of NOx in the exhaust flows into the selective reduction catalyst by determining the supply amount of the reducing agent including such control input. An appropriate amount of reducing agent according to the amount of NOx in the exhaust can be supplied. Thereby, the NOx reduction rate in the selective reduction catalyst can be maintained high.
At the same time, by keeping the NOx reduction rate high, it is possible to prevent a large fluctuation in the supply amount of the reducing agent, and to prevent the occurrence of ammonia slip and the reduction in the NOx reduction ratio due to this fluctuation.

In this case, the amount of ammonia stored in the first selective reduction catalyst is used as a first storage amount, and the first storage amount is estimated. The estimated first storage amount (ST _{UREA_FB} ) is a predetermined target storage amount. Third control input calculating means (6) for calculating a control input (G _{UREA_ST} ) for controlling to converge to (ST _{UREA_TRGT} ) is further provided, and the reducing agent supply amount determining means is controlled by the reducing agent supply means. It is preferable to determine the supply amount (G _UREA ) of the reducing agent further including the control input (G _{UREA_ST} ) calculated by the third control input calculating means.

By the way, when the supply of the reducing agent is started from a state where the first storage amount of the first selective reduction catalyst is smaller than the first storage capacity, the first selective reduction is performed until the first storage amount reaches the first storage capacity. Until the ammonia in the catalyst is saturated, the NOx reduction rate decreases. Further, after the ammonia is saturated, ammonia slip occurs in the first selective reduction catalyst. Here, when ammonia slip occurs, the supply amount of the reducing agent is reduced in order to suppress this, and there is a possibility that the NOx reduction rate is lowered again.
According to the present invention, the first storage amount of the first selective reduction catalyst is estimated, a control input for controlling the estimated first storage amount so as to converge to the predetermined target storage amount is calculated, The supply amount of the reducing agent is determined including such control input.
Thereby, when the first storage amount is smaller than the first storage capacity, for example, by increasing the supply amount of the reducing agent, the time to reach the first storage capacity is shortened, and the NOx reduction rate is promptly increased. Can be increased.
Further, immediately before the first storage amount reaches the first storage capacity, for example, by reducing the supply amount of the reducing agent, it is possible to prevent the occurrence of ammonia slip in the first selective reduction catalyst. As a result, when ammonia slip occurs as described above, it is possible to prevent a reduction in the NOx reduction rate that occurs when the supply amount of the reducing agent is reduced for the purpose of suppressing this.

In this case, in addition to the deviation (E _ST ) between the estimated first storage amount (ST _{UREA_FB} ) and the target storage amount (ST _{UREA_TRGT} ), the third control input calculation means, It is preferable to calculate a control input (G _{UREA_ST} ) based on the differentiation of the first storage amount.

According to the present invention, when calculating the control input for controlling the estimated first storage amount to converge to the predetermined target storage amount, in addition to the deviation between the estimated first storage amount and the target storage amount, Thus, the control input is calculated based on the differential of the deviation or the estimated differential of the first storage amount.
In particular, here, the first storage amount is calculated by sequentially integrating the amount of ammonia stored in the first selective reduction catalyst, so that the dynamic characteristic exhibits an integral elemental behavior. If the control input is calculated based only on the deviation between the first storage amount and the predetermined target storage amount, the control input may vibrate, and as a result, a periodic ammonia slip may occur. According to this invention, in addition to the deviation between the estimated first storage amount and the target storage amount, the control input is calculated based on the derivative of this deviation or the differentiation of the first storage amount. Can be prevented from vibrating.

In order to achieve the above object, the present invention is provided in the exhaust passage (11) of the internal combustion engine (1), generates ammonia in the presence of a reducing agent, and selects the NOx flowing through the exhaust passage with this ammonia. A reduction catalyst (23); and a reducing agent supply means (25) for supplying a reducing agent to the upstream side of the selective reduction catalyst in the exhaust passage, wherein the selective reduction catalyst is a first selective reduction catalyst (231). ) And a second selective reduction catalyst (232) provided on the downstream side of the first selective reduction catalyst in the exhaust passage, a control method for the exhaust purification device is provided. provide. In the control method, the ammonia detection step for detecting the ammonia amount between the first selective reduction catalyst and the second selective reduction catalyst, and the value of the ammonia amount (NH3 _CONS ) detected in the ammonia detection step are: A first control input calculation step for calculating a control input for controlling to be a value larger than “0”, and a reducing agent supply amount (G _UREA ) by the reducing agent supply means are calculated in the first control input. _And a reducing agent supply amount determination step that includes the control input (G _{UREA_FB} ) calculated in the step.

In this case, the amount of ammonia that can be stored in the first selective reduction catalyst is a first storage capacity, the amount of ammonia that can be stored in the second selective reduction catalyst is a second storage capacity, and the second storage capacity is the first storage capacity. It is preferably larger than the difference between the maximum and minimum capacity.
In this case, the control method further includes a target value setting step of setting a target value of the ammonia amount (NH3 _CONS ) of the first selective reduction catalyst and the second selective reduction catalyst to a value larger than “0”, In the first control input calculation step, it is preferable to calculate the control input so that the ammonia amount detected in the ammonia detection step falls within a predetermined range including the target value (NH3 _{CONS_TRGT} ).
In this case, in the first control input calculation step, the control input is calculated based on response designation type control that can set a convergence speed of the ammonia amount (NH3 _CONS ) detected in the ammonia detection step to the target value. In addition, the convergence rate in the case where the ammonia amount detected in the ammonia detection step is included in the predetermined range (RNH3 _{CONS_TRGT} , NH3 _{CONS_LMTL} to NH3 _{CONS_LMTH} ), the ammonia amount detected in the ammonia detection step is the predetermined amount. It is preferable to set it slower than the convergence speed when it is included outside the range (RNH3 _{CONS_TRGT} , NH3 _{CONS_LMTL} to NH3 _{CONS_LMTH} ).

In this case, in the target value setting step, the target value is preferably set to a smaller value as the temperature of the exhaust gas of the internal combustion engine or the temperature of the selective reduction catalyst is higher.
In this case, the control method further includes a second control input calculation step of calculating a control input based on a rotational speed (NE) of the internal combustion engine and a load parameter (TRQ) representing a load of the internal combustion engine, In the reducing agent amount determining step, it is preferable to determine the reducing agent supply amount (G _UREA ) by the reducing agent supply means further including the control input (G _{UREA_FF} ) calculated in the second control input calculating step. .
In this case, the amount of ammonia stored in the first selective reduction catalyst is used as a first storage amount, and the first storage amount is estimated. The estimated first storage amount (S _{TUREA_FB} ) is a predetermined target storage amount. A third control input calculating step for calculating a control input for controlling to converge to (ST _{UREA_TRGT} ), and in the reducing agent supply amount determining step, a reducing agent supply amount by the reducing agent supply means ( G _UREA ) is preferably determined by further including the control input (G _{UREA_ST} ) calculated in the third control input calculation step.
In this case, in the third control input calculation step, in addition to the deviation (E _ST ) between the estimated first storage amount (ST _{UREA_FB} ) and the target storage amount (ST _{UREA_TRGT} ), It is preferable to calculate a control input (G _{UREA_ST} ) based on the differentiation of the first storage amount.

According to the above control method invention, the same effect as the above-described exhaust gas purification apparatus for an internal combustion engine can be obtained.

1 is a schematic diagram illustrating a configuration of an internal combustion engine and an exhaust purification device thereof according to an embodiment of the present invention. It is a figure which shows the relationship between the amount of NOx in the selective reduction catalyst which concerns on the said embodiment, the amount of ammonia, and the storage amount of ammonia. It is a figure which shows the relationship between the storage capacity and temperature of the selective reduction catalyst which concerns on the said embodiment. It is a block diagram which shows the structure of the module which calculates the urea injection quantity by the urea injection valve which concerns on the said embodiment. It is a figure which shows the change of a NOx reduction rate when the urea injection amount is controlled so that the output value of the ammonia sensor strictly converges to the target ammonia amount. It is a figure for demonstrating the concept of the control in the sliding mode controller which concerns on the said embodiment. It is a figure which shows the phase plane of the slip amount deviation at the time of last control, and the slip amount deviation at the time of this control. It is a figure which shows the relationship between the switching function setting parameter which concerns on the said embodiment, and the convergence time of slip amount deviation. It is a figure which shows the structure of the VPOLE setting table which concerns on the said embodiment. It is a figure which shows the change of NOx reduction rate when urea injection control is performed using the sliding mode controller which concerns on the said embodiment. It is a figure which shows the relationship of the engine load at the time of performing urea injection control only by the sliding mode controller which concerns on the said embodiment, NOx amount upstream of a selective reduction catalyst, detected ammonia amount, urea injection amount, and NOx reduction rate. It is a figure which shows an example of the control map for determining FF injection quantity which concerns on the said embodiment. The figure which shows the relationship of the engine load, the NOx amount upstream of the selective reduction catalyst, the detected ammonia amount, the urea injection amount, and the NOx reduction rate when urea injection control is executed using the feedforward controller according to the embodiment. is there. It is a figure which shows the relationship between the NOx reduction | restoration rate in the case where urea injection control is started from the state where the ammonia stored in the selective reduction catalyst is unsaturated, the urea injection amount, the detected ammonia amount, and the ammonia storage amount. It is a schematic diagram which shows the concept of the ammonia storage model in the storage correction input calculation part which concerns on the said embodiment. It is a block diagram which shows the structure of the 1st form of the storage correction input calculation part which concerns on the said embodiment. It is a figure which shows the time change of the 1st storage amount estimated by the 1st form of the storage correction input calculation part which concerns on the said embodiment. It is a block diagram which shows the structure of the 2nd form of the storage correction input calculation part which concerns on the said embodiment. It is a block diagram which shows the structure of the 3rd form of the storage correction input calculation part which concerns on the said embodiment. It is a figure which shows the relationship between NOx reduction rate, urea injection amount, detected ammonia amount, and ammonia storage amount at the time of performing urea injection control using the storage correction input calculation part which concerns on the said embodiment. It is a figure which shows the time change of the 1st storage amount estimated by the storage correction input calculation part which concerns on the said embodiment. It is a figure which shows an example of the search map of the target ammonia amount which concerns on the said embodiment. It is a figure which shows the change of the ammonia amount between the 1st selective reduction catalyst and the 2nd selective reduction catalyst in the conventional exhaust gas purification apparatus. It is a figure which shows the change of the ammonia amount between the 1st selective reduction catalyst and the 2nd selective reduction catalyst in the exhaust gas purification apparatus which concerns on the said embodiment. It is a flowchart which shows the procedure of the urea injection control process performed by ECU which concerns on the said embodiment. It is a schematic diagram which shows the structure of the conventional exhaust gas purification apparatus. It is a figure which shows the relationship between the NOx discharge | emission amount and ammonia discharge | emission amount downstream of a selective reduction catalyst, and the output of a NOx sensor in the said conventional exhaust purification apparatus.

Hereinafter, an embodiment 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 lean burn operation type gasoline engine or diesel engine, and is mounted on a vehicle (not shown).

The exhaust purification device 2 is provided with an oxidation catalyst 21 provided in the exhaust passage 11 of the engine 1 and nitrogen oxide (hereinafter referred to as “NOx”) in the exhaust gas provided in the exhaust passage 11 and flowing through the exhaust passage 11. A urea selective reduction catalyst 23 that purifies in the presence of a reducing agent, a urea injection device 25 that supplies urea water as a reducing agent to the upstream side of the urea selective reduction catalyst 23 in the exhaust passage 11, and an electronic control unit (hereinafter referred to as an electronic control unit). 3) (referred to as “ECU”).

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 passage 11 in accordance with this control signal. That is, urea injection control is executed.

The oxidation catalyst 21 is provided on the upstream side of the urea selective reduction catalyst 23 and the urea injection valve 253 in the exhaust passage 11 and converts NO in the exhaust gas into NO ₂ , thereby the NOx in the urea selective reduction catalyst 23 is converted. Promote reduction.

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 passage 11. Each of the first selective reduction catalyst 231 and the second selective reduction catalyst 232 selectively reduces NOx in the exhaust in an atmosphere in which urea water exists. Specifically, when urea water is injected by the urea injection device 25, ammonia is generated from urea in the first selective reduction catalyst 231 and the second selective reduction catalyst 232, and NOx in the exhaust is selectively reduced by this ammonia. Is done.
The detailed configuration of the urea selective reduction catalyst 23 will be described in detail later with reference to FIGS.

The ECU 3 is connected to the crank angle position sensor 14, the accelerator opening sensor 15, and the urea remaining amount warning light 16, in addition to the ammonia sensor 26, the catalyst temperature sensor 27, and the NOx sensor 28.

The ammonia sensor 26 detects the amount of ammonia (hereinafter referred to as “ammonia amount”) NH3 _CONS in the exhaust passage 11 between the first selective reduction catalyst 231 and the second selective reduction catalyst 232, and detects the detected ammonia. A detection signal substantially proportional to the amount NH3 _CONS is supplied to the ECU 3.

The catalyst temperature sensor 27 detects the temperature (hereinafter referred to as “catalyst temperature”) T _SCR of the first selective reduction catalyst 231, and supplies a detection signal substantially proportional to the detected catalyst temperature T _SCR to the ECU 3.

The NOx sensor 28 detects the amount of NOx in the exhaust gas flowing into the first selective reduction catalyst 231 (hereinafter referred to as “NOx amount”) NOX _CONS , and supplies the ECU 3 with a detection signal substantially proportional to the detected NOx amount NOX _CONS. To do.

The crank angle position sensor 14 detects the rotation angle of the crankshaft of the engine 1, generates a pulse every crank angle, and supplies 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 supplies it to the ECU 3.

The accelerator opening sensor 15 detects a depression amount (hereinafter referred to as “accelerator opening”) AP of an accelerator pedal (not shown) of the vehicle, and supplies a detection signal substantially proportional to the detected accelerator opening AP to the ECU 3. In the ECU 3, the required torque TRQ of the engine 1 is calculated according to the accelerator opening AP and the rotational speed NE. Hereinafter, the required torque TRQ is a load parameter that represents the load of the engine 1.

The urea remaining amount warning lamp 16 is provided, for example, on the meter panel of the vehicle, and lights up when the remaining amount of urea water in the urea 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 urea tank 251 has decreased.

[Detailed configuration of urea selective reduction catalyst]
By the way, in the urea selective reduction catalyst 23 described above, the first selective reduction catalyst 231 and the second selective reduction catalyst 232 each have a function of reducing NOx in the exhaust gas with ammonia generated from urea, and the generated ammonia is reduced. It also has a function of storing a predetermined amount.
Hereinafter, the amount of ammonia stored in the first selective reduction catalyst 231 is referred to as a first storage amount, and the amount of ammonia that can be stored in the first selective reduction catalyst 231 is referred to as a first storage capacity. Further, the ammonia amount stored in the second selective reduction catalyst 232 is defined as a second storage amount, and the ammonia amount that can be stored in the second selective reduction catalyst 232 is defined as a second storage capacity.

The ammonia stored in this way is also consumed as appropriate for the reduction of NOx in the exhaust. For this reason, as the first and second storage amounts increase, the NOx reduction rate in the

selective reduction catalysts

231 and 232 increases. Further, when the supply amount of urea water is small with respect to the generated NOx amount, the stored ammonia is consumed for the reduction of NOx so as to make up for the shortage of urea water.

Here, when ammonia is generated exceeding the storage capacity in each

selective reduction catalyst

231, 232, the generated ammonia is discharged to the downstream side of each

selective reduction catalyst

231, 232. In this way, ammonia that is not stored in the

selective reduction catalysts

231 and 232 but is discharged downstream is referred to as “ammonia slip”.

In such

selective reduction catalysts

231 and 232, in order to keep the NOx reduction rate high, these

selective reduction catalysts

231 and 232 maintain a state in which an amount of ammonia close to the respective storage capacity is stored. It is preferable to continue.
However, in such a state where an amount of ammonia close to the storage capacity is stored, ammonia slip is likely to occur, and ammonia may be discharged outside the vehicle. In particular, it is preferable to prevent ammonia slip in the second selective reduction catalyst 232 as much as possible.

In view of these points, optimum forms of the first selective reduction catalyst 231 and the second selective reduction catalyst 232 for preventing ammonia from being discharged outside the vehicle while maintaining a high NOx reduction rate are shown in FIG. A description will be given with reference to FIG.

FIG. 2 is a diagram illustrating the relationship among the NOx amount, the ammonia amount, and the ammonia storage amount in the selective reduction catalyst. Specifically, FIG. 2A shows the above relationship in a comparative example (1BED + NOx sensor layout) in which a NOx sensor is provided on the downstream side of one selective reduction catalyst, and FIG. shows the relationship in a comparative example in which the ammonia sensor provided downstream of the selective reduction catalyst (1BED + NH ₃ sensor layout) of FIG. 2 (c), two selective reduction catalyst (first selective reduction catalyst and the second selective reduction representing the relationship in the present embodiment in which a ammonia sensor (2BED + MID-NH ₃ sensor layout) between the catalyst).

FIG. 3 is a diagram showing the relationship between the storage capacity of the selective reduction catalyst and the temperature. In FIG. 3, the solid line 3a shows the relationship between the storage capacity and the catalyst temperature in the catalyst before deterioration, and the broken line 3b shows the relationship between the storage capacity and the catalyst temperature in the catalyst after deterioration.

In the layout shown in FIG. 2A, for example, by controlling the supply amount of urea water based on the output from the NOx sensor, the NOx reduction rate in the selective reduction catalyst can be kept high. When such control is performed, the amount of NOx discharged from the engine and the supply amount of urea water necessary for the reduction of this NOx are in a generally balanced state, so ammonia generated from urea water Is consumed in the reduction of NOx, and ammonia stored in the selective reduction catalyst and ammonia slip in the selective reduction catalyst are both small. For this reason, the storage amount of ammonia in the selective reduction catalyst has little change and tends to be small with respect to the storage capacity.

That is, when the supply amount of urea water as described above is appropriately controlled, the storage amount in the selective reduction catalyst is kept substantially constant. However, for example, when the operating state of the engine suddenly changes, when the selective reduction catalyst deteriorates over time, or when the temperature changes suddenly, the supply amount of urea water deviates from an appropriate amount, and the storage amount May become “0” and the NOx reduction rate may decrease, or the storage amount may be saturated and excessive ammonia slip may occur.

Therefore, in the layout shown in FIG. 2A, it is difficult to control the storage amount of ammonia, and it is difficult to keep both the improvement of the NOx reduction rate and the prevention of ammonia slip.

In the layout shown in FIG. 2B, when controlling the supply amount of urea water so as to maintain the NOx reduction rate high based on the output from the ammonia sensor, in order to obtain the output from the ammonia sensor, It is necessary to maintain a state in which minute ammonia slip has occurred. For this reason, the storage amount of ammonia in the selective reduction catalyst is always saturated as shown in FIG.
When the storage amount is maintained in a saturated state in this way, for example, in a state where a large amount of NOx is temporarily generated by suddenly accelerating the vehicle, and generation of ammonia for reducing this NOx is not in time. Even if it exists, the NOx reduction rate in the transition time until the production | generation of ammonia is completed can be kept high with the stored ammonia.

By the way, as shown in FIG. 3, the storage capacity of the selective reduction catalyst changes according to the catalyst temperature. Specifically, the storage capacity decreases as the catalyst temperature increases.
Therefore, in the layout shown in FIG. 2B described above, since the storage amount is maintained saturated, for example, the vehicle shifts from the low load operation state to the high load operation state, and the catalyst temperature is When shifting from a low temperature (for example, 200 ° C.) state to a high temperature (for example, 500 ° C.) state, there is a possibility that an excessive ammonia slip occurs according to this temperature difference.

Returning to FIG. 2, in the layout of this embodiment shown in FIG. 2C, the ammonia sensor 26 is provided between the first selective reduction catalyst 231 and the second selective reduction catalyst 232.
In this layout, the supply amount of urea water is controlled so that the value of the ammonia amount detected by the ammonia sensor 26 is larger than “0”, whereby the layout shown in FIG. Similarly to the above, the state in which ammonia is saturated from the first selective reduction catalyst 231 can be maintained. Thereby, a high NOx reduction rate in the first selective reduction catalyst 231 can be maintained.

Further, even if the reduction of NOx in the first selective reduction catalyst 231 is insufficient, the remaining NOx and the ammonia slipped to the second selective reduction catalyst 232 are reacted in the second selective reduction catalyst 232, so that FIG. 2A, the NOx reduction rate can be maintained high for the first selective reduction catalyst 231 and the second selective reduction catalyst 232 as a whole.
In addition, by making the first selective reduction catalyst 231 saturated with ammonia, the generation of ammonia is completed when the vehicle suddenly accelerates as described above, as in the layout shown in FIG. It is possible to keep the NOx reduction rate at the time of transition until high.
In addition, although ammonia slip occurs in the first selective reduction catalyst 231 as described above, the ammonia discharged from the first selective reduction catalyst 231 is stored in the second selective reduction catalyst 232 or is selected by the second selection. The reduction catalyst 232 consumes NOx reduction. As a result, it is possible to suppress the discharge of ammonia downstream of the second selective reduction catalyst 232 while maintaining a high NOx reduction rate for the first selective reduction catalyst 231 and the second selective reduction catalyst 232 as a whole.

In order to prevent excessive ammonia slip from occurring when the low load operation state is shifted to the high load operation state and the catalyst temperature is shifted from the low temperature to the high temperature, the second storage capacity is the first storage capacity. It is preferable to design larger than the difference between the maximum and minimum times. By designing in this way, the ammonia released from the first selective reduction catalyst 231 can be stored in the second selective reduction catalyst 232. Thereby, it is possible to further suppress the discharge of ammonia downstream of the second selective reduction catalyst 232.

As shown in FIG. 3, the storage capacity decreases as the catalyst deteriorates. For this reason, the amount of ammonia released in response to a sudden change in the operating state and the catalyst temperature is larger before the deterioration than after the deterioration. Therefore, the second storage capacity is particularly preferably designed to be larger than the difference (maximum capacity difference) between the maximum time and the minimum time of the first storage capacity of the first selective reduction catalyst before deterioration. As a result, ammonia slip in the second selective reduction catalyst can be prevented more reliably.

Returning to FIG. 1, the ECU 3 reshapes 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 the like. And an arithmetic processing unit (hereinafter referred to as “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 a control signal to the engine 1, the urea injection valve 253, and the like.

FIG. 4 is a block diagram showing a configuration of a module for calculating a urea injection amount G _UREA (amount of urea water supplied) by the urea injection valve. The function of this module is realized by processing executed by the ECU 3.

4 includes a feedback controller 4, a feedforward controller 5, a storage correction input calculation unit 6, and an adder 7.

The feedback controller 4 includes a target ammonia amount setting unit 41 and a sliding mode controller 42.

The target ammonia amount setting unit 41, as will be described in detail later with reference to FIGS. 22 to 24, is a target value (hereinafter referred to as “ammonia amount”) detected by an ammonia sensor (hereinafter referred to as “detected ammonia amount”) NH3 _CONS . NH3 _{CONS_TRGT (referred} to as “target ammonia amount”) is set. As will be described in detail later, the target ammonia amount NH3 _{CONS_TRGT} is set to a value larger than “0”.

As will be described in detail later with reference to FIGS. 5 to 10, the sliding mode controller 42 controls the detected ammonia amount NH3 _CONS so as to converge to the set target ammonia amount NH3 _{CONS_TRGT} . feedback injection amount for the urea injection amount _{G uREA} (hereinafter, referred to as "FB injection _amount") is calculated _{G uREA - FB.}

As will be described in detail later with reference to FIGS. 11 to 13, the feedforward controller 5 sets the maximum NOx reduction rate in the selective reduction catalyst in accordance with the amount of NOx in the exhaust gas that changes depending on the operating state of the engine. as a control input for controlling to maintain, a feedforward injection amount for the urea injection amount _{G uREA} (hereinafter, referred to as "FF injection _amount") is calculated _{G uREA - FF.}

As will be described in detail later with reference to FIGS. 14 to 21, the storage correction input calculation unit 6 estimates the first storage amount of the first selective reduction catalyst, and the estimated first storage amount is a predetermined target storage. as a control input for controlling so as to converge to the amount _{ST uREA - TRGT,} it calculates the correction injection amount _{G uREA - ST} for the urea injection amount _{G uREA.}

As shown in the following equation (1), the adder 7 includes an FB injection amount G _{UREA_FB} calculated by the feedback controller 4, an FF injection amount G _{UREA_FF} calculated by the feedforward controller 5, and a storage correction input calculation unit 6. The urea injection amount _GUREA is determined by adding the corrected injection amount _{GUREA_ST} calculated by the above.

Here, the symbol (k) is a symbol indicating the discretized time, and indicates that the data is detected or calculated every predetermined control cycle. That is, when the symbol (k) is data detected or calculated at the current control timing, the symbol (k−1) indicates that the data is detected or calculated at the previous control timing. In the following description, the symbol (k) is omitted as appropriate.

[Configuration of sliding mode controller]
The detailed configuration of the sliding mode controller will be described with reference to FIGS.
As described above, the sliding mode controller calculates the FB injection amount G _{UREA_FB} so that the detected ammonia amount NH 3 _CONS converges to the target ammonia amount NH 3 _{CONS_TRGT} set by the target ammonia amount setting unit. Two problems that the present inventor has focused on when performing feedback control based on the output value NH3 _CONS of the ammonia sensor will be described.

(1) Detection Resolution of Ammonia Sensor As described above, existing NOx sensors are sensitive not only to NOx but also to ammonia due to its detection principle. On the other hand, it is known that an ammonia sensor that is sensitive to only ammonia without being sensitive to NOx can be developed. However, such an ammonia sensor has a limit in detection resolution, and the detection resolution varies depending on the individual or changes due to aging. For this reason, it is difficult to strictly control the output value NH3 _CONS from the ammonia sensor to the target ammonia amount NH3 _{CONS_TRGT} .

(2) Disagreement between NOx reduction rate and ammonia slip responsiveness Even if the above-described problem relating to the decomposition detection ability of the ammonia sensor can be solved, the NOx reduction rate and ammonia slip urea injection amount _GUREA in the selective reduction catalyst There is a problem of responsiveness mismatch. Specifically, the NOx reduction rate in the selective reduction catalyst has a smaller response delay with respect to the urea injection amount _{GUREA and higher} sensitivity than the ammonia slip in this selective reduction catalyst.

FIG. 5 is a graph showing a change in the NOx reduction rate when the urea injection amount _GUREA is controlled so that the output value NH3 _CONS of the ammonia sensor strictly converges to the target ammonia amount NH3 _{CONS_TRGT} .
As shown in FIG. 5, when the output value NH3 _CONS of the ammonia sensor rapidly increases and the occurrence of ammonia slip is detected, if the urea injection amount _GUREA is decreased to suppress this ammonia slip, the ammonia slip is reduced. Before being suppressed, the NOx reduction rate is significantly reduced. At this time, if the urea injection amount _GUREA is continuously reduced so that the detected ammonia amount NH3 _CONS strictly converges to the target ammonia amount NH3 _{CONS_TRGT} , the NOx reduction rate further decreases.

In view of the two problems as described above, in the present embodiment, control based on the concept as described below is executed.
FIG. 6 is a diagram for explaining the concept of control in the sliding mode controller. In FIG. 6, the horizontal axis indicates time, and the vertical axis indicates the detected ammonia amount NH 3 _CONS .

In the present embodiment, the target ammonia amount set by the target ammonia amount setting unit _{NH3 CONS_TRGT} (> 0), the target defined by than the target ammonia amount _{NH3 CONS - TRGT} small lower _{NH3 CONS - LMTL} a large upper _{NH3 CONS - LMTH} The ammonia slip range RNH3 _{CONS_TRGT} is set, and the FB injection amount G _{UREA_FB} is calculated so that the detected ammonia amount NH3 _{CONS_TRGT} falls within the target ammonia slip range RNH3 _{CONS_TRGT} . Here, the target ammonia slip range RNH3 _{CONS_TRGT} is preferably set in consideration of the detection resolution of the ammonia sensor.

Furthermore, after setting the target ammonia slip range RNH3 _{CONS_TRGT} as described above, when the detected ammonia amount NH3 _CONS is at the value A (when NH3 _{CONS_LMTH} ≦ NH3 _CONS ), when it is at the value B (NH3 _{When CONS} <NH3 _{CONS_LMTL} ), and when the value is C (NH3 _{CONS_LMTL} ≦ NH3 _CONS <NH3 _{CONS_LMTH} ), the FB injection amount G _{UREA_FB} is calculated so as to exhibit the following behavior.

When NH3 _CONS is a value A, an excessive ammonia slip has occurred with _respect to the target ammonia amount NH3 _{CONS_TRGT.} Therefore, the detected ammonia amount NH3 _CONS is quickly and without overshooting the target ammonia amount NH3 _{CONS_TRGT} . The FB injection amount _{GUREA_FB} is calculated so as to converge.
When NH3 _CONS is a value B, since the ammonia slip is insufficient with _respect to the target ammonia amount NH3 _{CONS_TRGT} , the detected ammonia amount NH3 _CONS is quickly and without overshoot to the target ammonia amount NH3 _{CONS_TRGT} . The FB injection amount _{GUREA_FB} is calculated so as to converge.

When NH3 _CONS is a value C, an ammonia slip that is not excessive or insufficient with _respect to the target ammonia amount NH3 _{CONS_TRGT} has occurred, so that the detected ammonia amount NH3 _CONS gradually converges to the target ammonia amount NH3 _{CONS_TRGT.} The FB injection amount G _{UREA_FB} is calculated. That is, the FB injection amount G _{UREA_FB} is calculated so as to constrain the detected ammonia amount NH 3 _CONS within the target ammonia slip range RNH 3 _{CONS_TRGT} .

Here, in particular, when NH3 _CONS is at value A or B and when it is at value C, the convergence speed of detected ammonia amount NH3 _CONS with respect to target ammonia amount NH3 _{CONS_TRGT} when value C is Set slower than the convergence speed in the case of A or value B.

In the present embodiment, the behavior of the detected ammonia amount NH3 _CONS as described above is realized by response designation control that can set the convergence speed of the detected ammonia amount NH3 _{CONS to} the target ammonia amount NH3 _{CONS_TRGT} . This response designation type control refers to control that can designate both the convergence speed and convergence behavior of a deviation based on a function that defines the convergence behavior of the deviation.
Hereinafter, the operation of the sliding mode controller configured to be able to execute this response designation control will be described.

First, as shown in the following formula (2), a deviation between the ammonia amount NH3 _CONS (k) detected by the ammonia sensor and the target NH3 _{CONS_TRGT} (k) is calculated, and this is set as a slip amount deviation E _NH3 (k). Define.

Next, a switching function setting parameter VPOLE (k) corresponding to the detected ammonia amount NH3 _CONS (k) is calculated based on a predetermined VPOLE setting table as shown in FIG. Further, as shown in the following equation (3), the product of this VPOLE (k) and the slip amount deviation E _NH3 (k−1) at the previous control and E _NH3 (k) is calculated, Is defined as a switching function σ (k).

Here, the relationship between the switching function setting parameter VPOLE (k) and the convergence speed of the slip amount deviation E _NH3 (k) will be described.
FIG. 7 is a diagram showing a phase plane in which the horizontal axis is the slip amount deviation E _NH3 (k−1) at the previous control and the vertical axis is the slip amount deviation E _NH3 (k) at the current control.
In this phase plane, the combination of slip amount deviations E _NH3 (k) and E _NH3 (k−1) satisfying σ (k) = 0 is a straight line having a slope of −VPOLE (k) as shown in FIG. Become. In particular, this straight line is called a switching straight line. Further, as shown in FIG. 7, by setting −VPOLE to a value smaller than “1” and larger than “0”, E _NH3 (k−1)> E _NH3 (k) is satisfied, so that the slip amount deviation E _{NH 3} (k) will converge to “0”. The sliding mode control is control that focuses on the behavior of the deviation E _NH3 (k) on the switching line.

That is, a combination of the slip amount deviation E _NH3 (k) in the current control and the slip amount deviation E _NH3 (k−1) in the previous control (hereinafter referred to as “deviation state quantity”) is placed on this switching line. By performing the control in this manner, it is possible to realize a control that is robust against disturbances and modeling errors, and to converge the detected ammonia amount NH3 _CONS without overshooting the target ammonia amount NH3 _{CONS_TRGT} .

FIG. 8 is a diagram showing the relationship between the switching function setting parameter VPOLE and the convergence time of the slip amount deviation E _NH3 . Here, the horizontal axis represents the convergence time of the slip amount deviation _{E NH3,} the vertical axis represents the slip amount deviation _{E NH3.} FIG. 8 shows cases where VPOLE is “−1”, “−0.95”, “−0.7”, and “−0.4”, respectively.
As shown in FIG. 8, when VPOLE is brought close to “0”, the slip amount deviation E _NH3 exhibits an exponential decay behavior with respect to “0”, and the convergence speed thereof is increased. Further, when VPOLE is brought close to “−1”, the convergence speed decreases while maintaining an exponential decay behavior. In particular, when VPOLE is set to “−1”, E _NH3 is maintained at the initial deviation E _NH3 (k = 0) at the start of control.

FIG. 9 is a diagram showing the configuration of the VPOLE setting table. Here, the horizontal axis represents the detected ammonia amount NH3 _CONS (k), and the vertical axis represents the switching function setting parameter VPOLE (k). The VPOLE setting table shown in FIG. 9 is set to realize the behavior control described with reference to FIG. 6 described above. As a specific example, four VPOLE setting tables shown in FIG. 9 include four

lines

9a, 9b, 9c, and 9d. A VPOLE setting table is shown.

As described above, when the detected ammonia amount NH3 _CONS is _{equal to} or _larger than NH3 _{CONS_LMTL and} smaller than NH3 _{CONS_LMTH} (when NH3 _{CONS_LMTL} ≦ NH3 _CONS <NH3 _{CONS_LMTH} ), the detected ammonia amount NH3 _CONS is _{equal to} or higher than _{MT3 CONS_} It is set slower than the convergence speed in some cases (when NH3 _{CONS_LMTH} ≦ NH3 _CONS ) and when the detected ammonia amount NH3 _CONS is smaller than NH3 _{CONS_LMTL} (when NH3 _CONS <NH3 _{CONS_LMTH} ).

Therefore, as shown in FIG. 9, when NH3 _{CONS_LMTL} ≦ NH3 _CONS <NH3 _{CONS_LMTH} , VPOLE is set in the vicinity of “−1” (specifically, VPOLE≈−0.95), and NH3 _{CONS_LMTH} ≦ NH3 _In the case of _CONS and NH3 _CONS <NH3 _{CONS_LMTH} , VPOLE is set in the vicinity of “0” (specifically, VPOLE≈−0.4).

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 (4), the sum of these U _RCH (k), U _NL (k), and U _ADP (k) is calculated and defined as the FB 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 (5), 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 (6), 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 “−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 (7), the switching function σ (k ) Multiplied by a predetermined adaptive law gain K _ADP and the sum of the adaptive law input U _ADP (k−1) at the previous control.

The reaching law input U _RCH (k), the non-linear input U _NL (k), and the adaptive law input U _ADP (k) are respectively calculated as deviation state quantities under the control policy detailed with reference to FIG. Is set to an optimum value on the basis of experiments so that is stably placed on the switching straight line.

FIG. 10 is a diagram showing a change in the NOx reduction rate when urea injection control is executed using the sliding mode controller of the present embodiment as described above. Specifically, in FIG. 10, the upper part shows the time change of the detected ammonia amount NH3 _CONS , the middle part shows the time change of the urea injection amount _GUREA , and the lower part shows the time change of the NOx reduction rate.
In FIG. 10, the solid line indicates the control result of the present embodiment, and the broken line indicates the control result when urea injection control is performed so that the detected ammonia amount NH3 _CONS converges strictly to the target ammonia amount NH3 _{CONS_TRGT.} Indicates.

According to the present embodiment, the urea injection amount G _UREA is calculated so that the detected ammonia amount NH 3 _CONS drifts within the target ammonia slip range RNH 3 _{CONS_TRGT} . Thereby, the fluctuation _| variation of the urea injection amount _GUREA can be made small.
In particular, when control is performed so that the detected ammonia amount strictly converges to the target ammonia amount as shown by the broken line, if an excessive ammonia slip occurs, the urea injection amount is greatly reduced to suppress this ammonia slip. As a result, the NOx reduction rate may be significantly reduced. According to the present embodiment, it is possible to reduce the amount of decrease in the urea injection amount G _UREA when such an excessive ammonia slip occurs, thereby maintaining a high NOx reduction rate.

Further, according to this embodiment, the detected ammonia amount _{NH3 CONS} is, the convergence rate when within the target ammonia slip range _{RNH3 CONS - TRGT,} so slower than the convergence rate in a case that is outside the target ammonia slip range _{RNH3 CONS - TRGT} Set.
As a result, when the detected ammonia amount NH3 _CONS is outside the target ammonia slip range RNH3 _{CONS_TRGT} , the occurrence of excessive ammonia slip and the decrease in the NOx reduction rate are promptly suppressed. Further, when the detected ammonia amount NH3 _CONS is within the target ammonia slip range RNH3 _{CONS_TRGT} , it is possible to prevent a large change in the urea injection amount _GUREA and to prevent the NOx reduction rate from being significantly reduced.

[Configuration of feedforward controller]
Next, a detailed configuration of the feedforward controller will be described with reference to FIGS.
As shown in the above problem (2), the NOx reduction rate in the selective reduction catalyst and the response to the urea injection amount _GUREA of the ammonia slip are different. Specifically, the ammonia slip in the selective reduction catalyst has a larger response delay with respect to the urea injection amount _GUREA than the NOx reduction rate in the selective reduction catalyst. The subject which this inventor focused on when performing urea injection control with respect to such a selective reduction catalyst is demonstrated.

(3) Decrease in NOx reduction rate due to change in engine operating state FIG. 11 shows engine load, NOx amount upstream of the selective reduction catalyst, detected ammonia amount NH3 when urea injection control is executed only by the sliding mode controller described above. It is a figure which shows the relationship between _CONS , urea injection amount _GUREA , and NOx reduction rate.

As shown in FIG. 11, when the engine load increases from time t ₁ to time t ₂ , the NOx amount on the upstream side of the selective reduction catalyst increases as the load increases. In this case, in order to prevent the NOx reduction rate from decreasing, it is necessary to increase the urea injection amount _GUREA as the NOx amount increases. However, in the above-described sliding mode controller, feedback control based on the output value NH3 _CONS of the ammonia sensor having a response delay larger than the NOx reduction rate is performed, so that the increase in the urea injection amount _GUREA is delayed from the ideal case. End up. For this reason, the NOx reduction rate may decrease.
Further, when feedback control based on the output value NH3 _CONS of the ammonia sensor having a large response delay is performed, vibrational behavior such as overshoot or undershoot is likely to occur in the sensor output value NH3 _CONS . For this reason, the urea injection amount G _UREA also vibrates, and a reduction in the NOx reduction rate due to undershoot as shown in FIG. 11 is likely to occur.

In view of the above problems, in this embodiment, the feedforward controller calculates the FF injection amount _{GUREA_FF} corresponding to the operating state of the engine. Specifically, in this feedforward controller, the FF injection amount _{GUREA_FF} is determined by map search, for example, based on the engine speed NE and the load parameter TRQ _{indicating the} engine load as the engine operating state.

FIG. 12 is a diagram illustrating an example of a control map for determining the FF injection amount _{GUREA_FF} .
As shown in FIG. 12, in this control map, the FF injection amount _{GUREA_FF} is determined to be a larger value as the engine speed NE or the load parameter TRQ increases.
This is because the larger the engine load parameter TRQ, the higher the combustion temperature of the air-fuel mixture and the higher the NOx emission amount. The higher the engine speed NE, the higher the NOx emission amount per unit time. Because.

FIG. 13 shows the engine load, the NOx amount upstream of the selective reduction catalyst, the detected ammonia amount NH3 _CONS , and the urea injection amount G _UREA when urea injection control is executed using the feedforward controller of the present embodiment as described above. It is a figure which shows the relationship between NOx reduction rate.
In FIG. 13, the solid line indicates the control result of this embodiment, and the broken line indicates the control result when urea injection control is performed only by the sliding mode controller.

As shown in FIG. 13, when the engine load increases from time t ₁ to time t ₂ , the NOx amount on the upstream side of the selective reduction catalyst increases as the load increases. Here, when the engine load increases, the feedforward controller calculates the FF injection amount G _{UREA_FF} appropriately set in accordance with the increase in NOx, thereby making the urea injection amount G _UREA ideal without any delay. Can be maintained at a reasonable injection amount. Thereby, the NOx reduction rate can be maintained at the highest value.
In addition, by maintaining the NOx reduction rate high in this way, it is possible to prevent large fluctuations in the urea injection amount G _UREA and to prevent the occurrence of ammonia slip and the reduction in the NOx reduction ratio due to this fluctuation. .

[Configuration of storage correction input calculation unit]
Next, a detailed configuration of the storage correction input calculation unit will be described with reference to FIGS.
As described above, the first selective reduction catalyst and the second selective reduction catalyst have a function of storing ammonia. In executing urea injection control for such a selective reduction catalyst, three problems that the present inventors have focused on will be described.

(4) Reduction in NOx reduction rate when storage amount is not saturated FIG. 14 shows a state in which ammonia stored in the selective reduction catalyst is unsaturated, that is, a state in which the storage amount in the selective reduction catalyst is less than its storage capacity. It is a figure which shows the relationship between the NOx reduction _{| restoration} rate at the time of starting urea injection control from, the urea injection amount _GUREA , the detected ammonia amount NH3 _CONS, and the ammonia storage amount. In the example shown in FIG. 14, urea injection control is started from the state where the storage amount of ammonia is “0” at time t = 0, and the storage amount reaches the storage capacity at time t = t ₁ . .
As shown in FIG. 14, between the time t = 0 ~ t _1, since the storage amount of ammonia is less than the storage capacity, the NOx reduction rate of the selective reduction catalyst, and reduction of the NOx reduction rate at saturation End up.

(5) As shown in generation 14 of excessive ammonia slip by reducing the delay of the urea injection amount, between the time t = 0 ~ t _1, since the storage amount of ammonia is less than the storage capacity, the ammonia slip occurs do not do. Thus, between time t = 0 ~ _{t 1,} the output value _{NH3 CONS} of the ammonia sensor is "0". During this time, the urea injection amount G _UREA is set to the maximum value in order to shorten the period during which the NOx reduction rate is reduced as much as possible in response to the output value NH3 _CONS of the ammonia sensor being “0”. Set to.

In this case, although the control for reducing the urea injection amount G _UREA is executed in response to the storage amount reaching the storage capacity at the time t = t ₁ , the detection delay of the ammonia sensor and the urea injection amount are increased from the maximum values. Due to the delay caused by the reduction, it takes time to actually reduce the urea injection amount. For this reason, the detected ammonia amount NH3 _CONS greatly overshoots from the target ammonia amount NH3 _{CONS_TRGT} , and an excessive ammonia slip occurs.

(6) Reduction of NOx reduction rate due to generation of excessive ammonia slip When excessive ammonia slip occurs as described above, it is necessary to further reduce the urea injection amount _GUREA in order to suppress this ammonia slip. . However, in this case, the NOx reduction rate decreases again.

In order to solve these three problems, it is necessary to execute urea injection control according to the following policy.
That is, in order to solve the problem (4), the urea injection amount _GUREA is increased until the storage amount of ammonia reaches the storage capacity, thereby shortening the period during which the NOx reduction rate is reduced. Also, (5) and in order to solve the problems of (6), after increasing the amount of urea injection amount G _UREA As described above, the urea injection amount G _UREA before ammonia saturated ammonia slip occurs Reduce.

In order to realize urea injection control according to such a policy, in this embodiment, the storage correction input calculation unit estimates the first storage amount of the first selective reduction catalyst based on an ammonia storage model described later. first storage amount _{ST uREA - FB} that this estimate is a predetermined target storage amount _{ST uREA - TRGT,} quickly and to converge without overshoot, it calculates the correction injection amount _{G uREA - ST} in the urea injection amount _{G uREA.}

Here, the target storage amount ST _{UREA_TRGT} is set to the same value as the first storage capacity ST _{UREA_MAX1} of the first selective reduction catalyst by a setting unit (not shown), but is not limited thereto. For example, in order to suppress the occurrence of an excessive ammonia slip, the target storage amount ST _{UREA_TRGT} may be set in the vicinity of the first storage capacity ST _{UREA_MAX1} and smaller than this ST _{UREA_MAX1} .

FIG. 15 is a schematic diagram illustrating a concept of an ammonia storage model in the storage correction input calculation unit.
This ammonia storage model is a model for estimating a change in the storage amount of ammonia in the selective reduction catalyst according to the urea injection amount with respect to the NOx amount of the exhaust gas flowing into the selective reduction catalyst. Specifically, the state of change of the storage amount in the selective reduction catalyst includes a state in which the urea injection amount is appropriate with respect to a predetermined NOx amount (see FIG. 15A), and a state in which the urea injection amount is excessive ( The state is classified into three states, that is, a state in which the urea injection amount is insufficient (see FIG. 15C).

As shown in FIG. 15A, when the urea injection amount is in an appropriate state with respect to NOx flowing into the selective reduction catalyst, that is, the amount of ammonia that can most effectively reduce NOx in the exhaust, When the amount of ammonia generated from the supplied urea water substantially matches, there is no change in the storage amount.

As shown in FIG. 15B, when the urea injection amount is excessive with respect to NOx flowing into the selective reduction catalyst, that is, the amount of ammonia generated from the supplied urea water is reduced in the exhaust gas. When the amount of NOx is larger than the amount that can be most efficiently reduced, this excess ammonia is stored in the selective reduction catalyst. Therefore, in such an over-dosing state, the storage amount increases.

As shown in FIG. 15C, when the urea injection amount is insufficient with respect to NOx flowing into the selective reduction catalyst, that is, the amount of ammonia generated from the supplied urea water is reduced in the exhaust gas. If the amount of NOx is less than the amount that can be reduced most efficiently, this deficiency is compensated by the stored ammonia. Therefore, in such an under-supply state, the storage amount decreases.

Next, the configuration of the storage correction input calculation unit that calculates the above-described correction injection amount G _{UREA_ST} based on the above storage model will be described with reference to FIGS. In addition, as a specific configuration of such a storage correction input calculation unit, three modes will be described below.

FIG. 16 is a block diagram showing the configuration of the first form of the storage correction input calculation unit.
The storage correction input calculation unit includes a control object 61 configured based on the ammonia storage model as described above and a controller 62 of the control object 61.

The control target 61 uses a surplus urea injection amount D _UREA that indicates the amount of urea water that is surplus when reducing NOx in the exhaust as a control input, and a first storage amount ST _{UREA_FB} of the first selective reduction catalyst as a control output. To do. Specifically, the control target 61 sequentially adds the stored ammonia amount or sequentially subtracts the consumed ammonia amount based on the surplus urea injection amount _DUREA , so that the first selective catalytic reduction catalyst The integrator 611 estimates the first storage amount ST _{UREA_FB} .

First, the surplus urea injection amount D _UREA (k) is obtained from the urea injection amount G _UREA (k) by the adder 63 from the urea injection amount G _UREA (k) as NOx of exhaust flowing into the first selective reduction catalyst. It is calculated by subtracting the ideal urea injection amount G _{UREA_IDEAL} (k), which is the urea injection amount necessary for reduction. The urea injection amount G _UREA (k) is added to the corrected injection amount G _{UREA_ST} (k) calculated by the controller 62 by the adder 64 and the FB injection amount G _{UREA_FB} (k) and the FF injection amount G _{UREA_FF} (k). It is calculated by adding.

Here, the ideal urea injection amount G _{UREA_IDEAL} (k) reduces the NOx amount NOx _{CONS of the} exhaust gas flowing into the first selective reduction catalyst detected by the NOx sensor and NOx as shown in the following equation (9). _Therefore , it is calculated by multiplying by a conversion coefficient K _{CONV_NOX_UREA} for conversion to an injection amount necessary for this.

Here, when there is no NOx sensor for detecting the NOx amount of the exhaust gas flowing into the first selective reduction catalyst, the FF injection amount G _{UREA_FF} (k) may be set as the ideal urea injection amount G _{UREA_IDEAL} (k).

The integrator 611 integrates the surplus urea injection amount D _UREA (k) with respect to time k as shown in the following equation (10) based on the surplus urea injection amount D _UREA (k) increasing or decreasing the first storage amount. The first storage amount ST _{UREA_FB} (k) is estimated by combining the calculation and the limit processing for the first storage amount as shown in the following equation (11).

In particular, in Equation (11), a lower limit process for the first storage amount ST _{UREA_FB} (k), that is, a process in which ST _{UREA_FB} (k) becomes “0” at the minimum is performed. That is, in Expression (11), the upper limit process for the first storage amount ST _{UREA_FB} (k), that is, the process that _makes ST _{UREA_FB} (k) the maximum storage capacity ST _{UREA_MAX1} is not performed.
This is because the problem shown in (5) above may not be solved. That is, when the target first storage amount ST _{UREA_TRGT} is set to the same value as the first storage capacity ST _{UREA_MAX1} as described above, if the upper limit process is performed, the first storage amount is reduced without reducing the urea injection amount G _UREA. This is because the amount ST _{UREA_FB} is limited to the first storage capacity ST _{UREA_MAX1} , and it becomes difficult to perform control to suppress ammonia slip.

The controller 62 _calculates the corrected injection amount G _{UREA_ST} (k) in the urea injection amount G _UREA by PI control so that the estimated first storage amount ST _{UREA_FB} (k) converges to the target first storage amount ST _{UREA_TRGT.} To do.

In the controller 62, as shown in the following equation (12), the adder 621 subtracts the target first storage amount ST _{UREA_TRGT} (k) from the estimated first storage amount ST _{UREA_FB} (k), It is defined as a storage amount deviation E _ST (k).

Next, as shown in the following equation (13), the multiplier 622 multiplies the first storage amount deviation E _ST (k) by the proportional gain KP _ST to calculate the proportional term G _{UREA_ST_P} (k).

Further, as shown in the following equation (14), the integral term G _{UREA_ST_I is} obtained by multiplying the time integral value of the first storage amount deviation E _ST (k) by the integral gain KI _ST by the integrator 623 and the multiplier 624. (K) is calculated.

Next, as shown in the following formula (15), the adder 625 calculates the sum of the proportional term G _{UREA_ST_P} (k) and the integral term G _{UREA_ST_I} (k), and uses this as the corrected injection amount G _{UREA_ST} (k). Define.

FIG. 17 is a diagram showing a temporal change of the first storage amount ST _{UREA_FB} estimated by the first form of the storage correction input calculation unit as described above.
As shown in FIG. 17, the first storage amount ST _{UREA_FB} _exhibits a vibration behavior with respect to the target first storage amount ST _{UREA_TRGT} , and ammonia slip occurs periodically.
This is because the control object 61 as the storage model described above has a structure including the integrator 611. That is, in this case, the proportional term G _{UREA_ST_P} of the controller 62 becomes an integral term, and the integral term G _{UREA_ST_I} becomes an integral term with respect to the integral value. In particular, the integral term G _{UREA_ST_I} shows an oscillatory behavior.
Therefore, hereinafter, a second mode and a third mode of the storage correction input calculation unit that solve such a problem will be described.

FIG. 18 is a block diagram showing the configuration of the second form of the storage correction input calculation unit. The storage correction input calculation unit of the second form is different from the first form shown in FIG. 16 described above in the configuration of the controller 62A.
As will be described in detail later, the controller 62A is a controller that uses an expanded system PI control in which the integrator 611 of the control target 61 is regarded as a part of the controller.

In the controller 62A, as shown in the following equation (16), the adder 621 subtracts the target first storage amount ST _{UREA_TRGT} (k) from the estimated first storage amount ST _{UREA_FB} (k), It is defined as a storage amount deviation E _ST (k).

Further, in this controller 62A, in order to solve the above-described problem, the integrator 611 of the control target 61 is regarded as a part of the controller, and the proportional term G _{UREA_ST_P} ( k) and the integral term G _{UREA_ST_I} (k) are each calculated in consideration of later integration.

Specifically, a differential value E _ST (k) −E _ST (k−1) of the first storage amount deviation is calculated by the delay calculator 626 and the adder 627, and a proportional gain is calculated by the multiplier 622. A product obtained by multiplying KP _ST is defined as a proportional term G _{UREA_ST_P} (k) as shown in the following equation (17).
Also, the product of the first storage amount deviation E _ST (k) multiplied by the integral gain KI _ST by the multiplier 624 is defined as an integral term G _{UREA_ST_I} (k) as shown in the following equation (18).

FIG. 19 is a block diagram showing the configuration of the third form of the storage correction input calculation unit. The storage correction input calculation unit of the third form is different from the second form shown in FIG. 18 described above in the configuration of the controller 62B.
The controller 62B recognizes the integrator 611 of the controlled object 61 as a part of the controller in the same manner as the controller 62A described above, and gives an expanded system IP that gives the first storage amount deviation E _ST (k) only to the integral term. It is a controller using control.

In the controller 62B, as shown in the following equation (20), the adder 621 subtracts the target first storage amount ST _{UREA_TRGT} (k) from the estimated first storage amount ST _{UREA_FB} (k), It is defined as a storage amount deviation E _ST (k).

Next, the product of the first storage amount deviation E _ST (k) multiplied by the integral gain KI _ST by the multiplier 624 is defined as an integral term G _{UREA_ST_I} (k) as shown in the following equation (21).

On the other hand, the differential value ST _{UREA_FB} (k) −ST _{UREA_FB} (k−1) of the first storage amount is calculated by the delay computing unit 268 and the adder 629, and this differential value is multiplied by the proportional gain KP _ST by the multiplier 622. This is defined as a proportional term G _{UREA_ST_P} (k) as shown in the following formula (22).

Next, as shown in the following equation (23), the adder 625 calculates the sum of the proportional term G _{UREA_ST_P} (k) and the integral term G _{UREA_ST_I} (k), and uses this as the corrected injection amount G _{UREA_ST} (k). Define.

FIG. 20 shows the relationship between the NOx reduction rate, the urea injection amount _GUREA , the detected ammonia amount NH3 _CONS, and the ammonia storage amount when urea injection control is executed using the storage correction input calculation unit as described above. FIG. In the example shown in FIG. 20, urea injection control is started from the state where the storage amount of ammonia is “0” at time t = 0, and the storage amount reaches the storage capacity at time t = t ₁ . .
In FIG. 20, the solid line indicates the control result of the present embodiment, and the broken line indicates the control result when urea injection control is performed without estimating the first storage amount.

According to this embodiment, the first storage amount _{ST UREA - FB} estimated, by feedback control so the first storage amount _{ST UREA - FB} converges to the target first storage amount _{ST UREA - TRGT,} first storage amount first The time to reach the storage capacity can be shortened. Thereby, the time until the ammonia is saturated in the first selective reduction catalyst can be shortened, and the NOx reduction rate can be quickly increased.

Further, the estimating the first storage amount _{ST UREA - FB,} by feedback control so the first storage amount _{ST UREA - FB} converges to the target first storage amount _{ST UREA - TRGT,} actually ammonia in the first selective reduction catalyst saturation Before _starting , the reduction of the urea injection amount _GUREA can be started. That is, the delay in reducing the urea injection amount can be eliminated. Thereby, generation | occurrence | production of an excessive ammonia slip can be prevented.
Further, by preventing the occurrence of such an excessive ammonia slip, the amount of reduction in the urea injection amount for the purpose of suppressing the ammonia slip can be reduced. Thereby, the fall of a NOx reduction rate can be prevented.

FIG. 21 is a diagram showing a temporal change in the first storage amount ST _{UREA_FB} estimated by the storage correction input calculation unit as described above. FIG. 21A shows a control result according to the first form using PI control, FIG. 21B shows a control result according to the second form using expanded PI control, and FIG. (C) shows a control result according to the third mode using the expanded system IP control.

As shown in FIG. 21B, when the expanded PI control is used, the periodic vibration of the first storage amount ST _{UREA_FB} generated when the PI control is used is eliminated, and the target first storage It quickly converges to the quantity ST _{UREA_TRGT} . This also suppresses the occurrence of periodic ammonia slip.

As shown in FIG. 21C, when the expanded system _IP control is used, the periodic vibration of the first storage amount ST _{UREA_FB} is further increased as compared with the case where the above-described expanded system PI control is used. As a result, the occurrence of ammonia slip can be further suppressed.
This is because, as shown in the equation (22), the proportional term _{G UREA - ST - P,} the first storage amount deviation _E instead _ST, because calculated based on the first storage amount _{ST UREA - FB.} In this case, the proportional term _{G UREA - ST - P,} rather than acting as _the first storage amount deviation _{E ST} becomes _"0", act to _{ST UREA - FB} becomes "0", _thereby, over the _{ST UREA - FB} Shooting is suppressed.

However, comparing the case where the enlarged system IP control is used with the case where the enlarged system PI control is used, the overshoot as described above is suppressed when the enlarged system IP control is used. although, the time until the first storage amount _{ST UREA - FB} reaches the target first storage amount _{ST UREA - TRGT} becomes long. For this reason, it is preferable to use the expanded system IP control or the expanded system PI control depending on the configuration of the exhaust purification device.

[Configuration of target ammonia amount setting unit]
Next, a detailed configuration of the target ammonia amount setting unit will be described with reference to FIGS.
The storage capacity of the first and second selective reduction catalysts varies depending on the state. When the storage capacity decreases rapidly, ammonia that cannot be held by these selective reduction catalysts slips downstream. Therefore, in order to suppress the slipping of ammonia to the most downstream side, the target ammonia amount NH3 _{CONS_TRGT} corresponding to the target value of the ammonia amount flowing into the second selective reduction catalyst is appropriately set according to the state of these selective reduction catalysts. Must be set to On the other hand, the target ammonia amount setting unit of the present embodiment sets the target ammonia concentration NH3 _{CONS_TRGT} based on the detection value T _SCR of the catalyst temperature sensor.

FIG. 22 is a diagram illustrating an example of a search map for the target ammonia amount NH3 _{CONS_TRGT} . In Figure 22, the horizontal axis represents the detected value _{T SCR} of the catalyst temperature sensor, the vertical axis indicates the target amount of ammonia _{NH3 CONS - TRGT.}
As described in detail with reference to FIG. 3, the storage capacity of the selective reduction catalyst has a characteristic that it decreases as the catalyst temperature increases. Therefore, the target ammonia amount NH3 _{CONS_TRGT} is set to a smaller value as the catalyst temperature T _SCR increases so that the amount of ammonia flowing into the second selective reduction catalyst decreases as the catalyst temperature increases and the storage capacity decreases.

Next, a control example by the conventional exhaust purification device shown in FIG. 23 regarding the change in the amount of ammonia between the first selective reduction catalyst and the second selective reduction catalyst, that is, the amount of ammonia flowing into the second selective reduction catalyst, Comparison is made with an example of control by the exhaust purification system of this embodiment shown in FIG. Here, unlike the exhaust gas purification apparatus of the present embodiment, the conventional exhaust gas purification apparatus uses an ammonia sensor that detects the ammonia concentration, and controls the detected value of the ammonia concentration to coincide with a predetermined target ammonia concentration. Shows the case.
Here, unlike the exhaust purification device of the present embodiment, the conventional exhaust purification device uses a sensor that detects the ammonia concentration between the first selective reduction catalyst and the second selective reduction catalyst, and this ammonia concentration. That is, urea injection control is performed so that the detected value coincides with a predetermined target value.

FIG. 23 is a diagram showing a change in the amount of ammonia between the first selective reduction catalyst and the second selective reduction catalyst in the conventional exhaust purification device.
FIG. 24 is a diagram showing a change in the amount of ammonia between the first selective reduction catalyst and the second selective reduction catalyst in the exhaust gas purification apparatus of the present embodiment. 23 and 24, in order from the upper stage to the lower stage, the engine load, the exhaust flow rate, the ammonia concentration between the first selective reduction catalyst and the second selective reduction catalyst, and the first selective reduction catalyst, The relationship with the amount of ammonia between 2nd selective reduction catalysts is shown.

As shown in FIG. 23, when the engine load increases rapidly, the exhaust flow rate and the exhaust temperature increase rapidly. At this time, the ammonia concentration in the exhaust gas relatively decreases as the exhaust gas flow rate increases. However, in the conventional exhaust gas purification device based on the ammonia concentration, the urea injection amount is set so that the detected value of the ammonia concentration matches the target value. Incremental control is performed. Accordingly, as shown in FIG. 23, the ammonia concentration between the first selective reduction catalyst and the second selective reduction catalyst coincides with the target value, but the ammonia amount deviates from an appropriate amount and increases. For this reason, an amount of ammonia exceeding the storage capacity flows into the second selective reduction catalyst, and as a result, ammonia slip may occur.

On the other hand, since the exhaust purification apparatus of the present embodiment performs control based on the ammonia amount, the urea injection amount does not increase with an increase in the exhaust gas flow rate. For this reason, as shown in FIG. 24, the ammonia concentration between the first selective reduction catalyst and the second selective reduction catalyst decreases as the exhaust flow rate increases.
Further, at this time, if the engine load is increased, the catalyst temperature also rises as the exhaust temperature rises, so that the storage capacity of the second selective reduction catalyst decreases. As described above, in the exhaust purification system of this embodiment, the target ammonia amount NH3 _{CONS_TRGT} is determined according to the catalyst temperature. For this reason, as shown in FIG. 24, the target ammonia amount NH3 _{CONS_TRGT} is set so as to decrease as the storage capacity of the second selective reduction catalyst decreases. Therefore, ammonia slip can be introduced into the second selective reduction catalyst according to the state, and ammonia slip can be suppressed.

Next, urea injection control processing executed by the ECU will be described with reference to FIG.
FIG. 25 is a flowchart showing a procedure of urea injection control processing executed by the ECU.
This urea injection control process is to calculate the urea injection amount G _UREA by the above-described method, and is executed at predetermined control cycles.

In step S1, it is determined whether the urea failure flag F _UREANG is “1”. The urea failure flag F _UREANG is set to “1” when it is determined in the determination process (not shown) that the urea injection device has failed, and is set to “0” otherwise. If this determination is YES, the process moves to step S9, and after setting the urea injection amount G _UREA to “0”, this process ends. If this determination is NO, the process proceeds to step S2.

In step S2, it is determined whether or not the catalyst deterioration flag F _SCRNG is “1”. The catalyst deterioration flag F _SCRNG is set to “1” when it is determined in the determination process (not shown) that either the first selective reduction catalyst or the second selective reduction catalyst has failed, and “0” otherwise. Set to If this determination is YES, the process moves to step S9, and after setting the urea injection amount G _UREA to “0”, this process ends. If this determination is NO, the process proceeds to step S3.

In step S3, it is determined whether the urea remaining amount Q _UREA is less than a predetermined value Q _REF . This urea remaining amount Q _UREA indicates the remaining amount of urea water in the urea tank, and is calculated based on the output of the urea level sensor. If this determination is YES, the process proceeds to step S4, and if NO, the process proceeds to step S5.
In step S4, the urea remaining amount warning lamp is turned on, the _{process proceeds} to step S9, the urea injection amount _GUREA is set to “0”, and then this process ends.

In step S5, it is determined whether the catalyst warm-up timer value T _MAST is greater than a predetermined value T _MLMT . This catalyst warm-up timer value T _{MAST measures} the warm-up time of the urea selective reduction catalyst after engine startup. If this determination is YES, the process proceeds to step S6. When this determination is NO, the _{process proceeds} to step S9, and after setting the urea injection amount _GUREA to “0”, this process is ended.

In step S6, it is determined whether or not the sensor failure flag F _SENNG is “0”. This sensor failure flag F _SENNG is set to “1” when it is determined that the ammonia sensor or the catalyst temperature sensor has failed in a determination process (not shown), and is set to “0” otherwise. If this determination is YES, the process proceeds to step S7. When this determination is NO, the _{process proceeds} to step S9, and after setting the urea injection amount _GUREA to “0”, this process is ended.

In step S7, it is determined whether or not the ammonia sensor activation flag F _NH3ACT is 1. The ammonia sensor activation flag F _NH3ACT is set to “1” when it is determined that the ammonia sensor has reached an active state in a determination process (not shown), 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 S9, and after setting the urea injection amount _GUREA to “0”, this process is ended.

In step S8, it is determined whether or not the temperature T _SCR of the first selective reduction catalyst is higher than a predetermined value T _{SCR_ACT} . If this determination is YES, it is determined that the first selective reduction catalyst has been activated, and the routine goes to Step S10. If this determination is NO, it is determined that the first selective reduction catalyst has not yet been activated and urea injection should be stopped, and the _{routine proceeds} to step S9, where the urea injection amount _GUREA is _set to “0”. This processing is terminated.

In step S10, the target ammonia amount setting unit described above, calculates the target amount of ammonia _{NH3 CONS - TRGT} based on the catalyst temperature _{T SCR,} it proceeds to step S11.
In step S11, the FF injection amount _{GUREA_FF} is calculated by the above-described feedforward controller, and the _{process proceeds} to step S12.

In step S12, the storage correction input calculation unit described above calculates the corrected injection amount G _{UREA_ST} based on the equations (8) to (23), and the _{process proceeds} to step S13.
In step S13, the above-described sliding mode controller calculates the FB injection amount G _{UREA_FB} based on the equations (2) to (7), and the _{process proceeds} to step S14.
In step S14, the urea injection amount _GUREA is calculated based on the equation (1) by the above-described adder, and this process is terminated.

In the present embodiment, the ammonia sensor 26 constitutes an ammonia detection means, and the ECU 3 controls the first control input calculation means, the second control input calculation means, the third control input calculation means, the reducing agent supply amount determination means, and the target ammonia. A quantity setting means is configured. Specifically, the feedback controller 4 and the sliding mode controller 42 of the ECU 3 constitute a first control input calculation means, the feed forward controller 5 of the ECU 3 constitutes a second control input calculation means, and a storage correction input calculation unit of the ECU 3 6 constitutes the third control input calculating means, the adder 7 of the ECU 3 constitutes the reducing agent supply amount determining means, and the feedback controller 4 and the target ammonia amount setting unit 41 of the ECU 3 constitutes the target ammonia amount setting means. .

The present invention is not limited to the embodiment described above, and various modifications can be made.
In the above embodiment, based on the detected value _{T SCR} of the catalyst temperature sensor for detecting the temperature of the first selective reduction catalyst has been calculated target ammonia amount _{NH3 CONS - TRGT,} not limited to this. For example, the target ammonia amount may be calculated based on a detection value of an exhaust temperature sensor that detects the temperature of the exhaust.

1. Engine (internal combustion engine)
11 ... Exhaust passage (exhaust passage)
2 ... Exhaust purification device 23 ... Urea selective reduction catalyst (selective reduction catalyst)
231 ... 1st selective reduction catalyst 232 ... 2nd selective reduction catalyst 25 ... Urea injection device (reducing agent supply means)
26. Ammonia sensor (ammonia detection means)
28 ... NOx sensor 3 ... Electronic control unit (first control input calculating means, second control input calculating means, third control input calculating means, reducing agent supply amount determining means, target ammonia amount setting means)
4 ... Feedback controller (first control input calculating means, target ammonia amount setting means)
41 ... Target ammonia amount setting section (target ammonia amount setting means)
42... Sliding mode controller (first control input calculating means)
5 ... Feed forward controller (second control input calculating means)
6 ... Storage correction input calculation unit (third control input calculation means)
7. Adder (reducing agent supply amount determining means)

Claims

In an exhaust gas purification apparatus for an internal combustion engine provided with a selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine, generates ammonia in the presence of a reducing agent, and reduces NOx flowing through the exhaust passage with this ammonia.
The selective reduction catalyst includes a first selective reduction catalyst and a second selective reduction catalyst provided on the downstream side of the first selective reduction catalyst in the exhaust passage,
Reducing agent supply means for supplying a reducing agent to the upstream side of the selective reduction catalyst in the exhaust passage;
Ammonia detection means for detecting an ammonia amount between the first selective reduction catalyst and the second selective reduction catalyst in the exhaust passage;
First control input calculating means for calculating a control input for controlling the ammonia amount detected by the ammonia detecting means to be a value greater than “0”;
Exhaust gas from an internal combustion engine, comprising: a reducing agent supply amount determining means for determining a supply amount of the reducing agent by the reducing agent supply means including a control input calculated by the first control input calculating means. Purification equipment.
The amount of ammonia that can be stored in the first selective reduction catalyst is defined as a first storage capacity,
The amount of ammonia that can be stored in the second selective reduction catalyst is defined as a second storage capacity,
2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the second storage capacity is larger than a difference between a maximum time and a minimum time of the first storage capacity.
A target ammonia amount setting means for setting a target value of the ammonia amount detected by the ammonia detection means to a value larger than “0”;
The first control input calculating means includes
The exhaust purification device for an internal combustion engine according to claim 1 or 2, wherein the control input is calculated so that an ammonia amount detected by the ammonia detection means falls within a predetermined range including the target value. .
The first control input calculating means includes
It is configured to be able to execute response designation control capable of setting a convergence speed of the ammonia amount detected by the ammonia detection means to the target value,
The convergence speed when the ammonia amount detected by the ammonia detection means falls within the predetermined range is greater than the convergence speed when the ammonia amount detected by the ammonia detection means falls outside the predetermined range. 4. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the exhaust gas purification apparatus is set late.
5. The internal combustion engine according to claim 3, wherein the target ammonia amount setting means sets the target value to a smaller value as the temperature of the exhaust gas of the internal combustion engine or the temperature of the selective reduction catalyst is higher. Exhaust purification device.
A second control input calculating means for calculating a control input based on a rotation parameter of the internal combustion engine and a load parameter representing a load of the internal combustion engine;
The said reducing agent supply amount determination means determines the supply amount of the reducing agent by the said reducing agent supply means further including the control input calculated by the said 2nd control input calculation means from Claim 3 characterized by the above-mentioned. 6. An exhaust emission control device for an internal combustion engine according to any one of 5 above.
The amount of ammonia stored in the first selective reduction catalyst is defined as a first storage amount,
And a third control input calculating means for calculating a control input for controlling the estimated first storage amount so that the estimated first storage amount converges to a predetermined target storage amount,
The said reducing agent supply amount determination means determines the supply amount of the reducing agent by the said reducing agent supply means further including the control input calculated by the said 3rd control input calculation means from Claim 3 characterized by the above-mentioned. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 6 to 10.
The third control input calculating means includes
In addition to the deviation between the estimated first storage amount and the target storage amount,
8. The exhaust gas purification apparatus for an internal combustion engine according to claim 7, wherein the control input is calculated based on the differential of the deviation or the differential of the first storage amount.
A selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine, generates ammonia in the presence of a reducing agent, and reduces NOx flowing through the exhaust passage with the ammonia;
Reductant supply means for supplying a reductant to the upstream side of the selective reduction catalyst in the exhaust passage,
The selective reduction catalyst includes an exhaust purification device configured to include a first selective reduction catalyst and a second selective reduction catalyst provided downstream of the first selective reduction catalyst in the exhaust passage. A control method for an exhaust purification device,
An ammonia detection step of detecting an ammonia amount between the first selective reduction catalyst and the second selective reduction catalyst;
A first control input calculating step for calculating a control input for controlling the ammonia amount detected in the ammonia detecting step to be a value greater than “0”;
An exhaust purification device comprising: a reducing agent supply amount determining step for determining a reducing agent supply amount by the reducing agent supply means including a control input calculated in the first control input calculating step. Control method.
The amount of ammonia that can be stored in the first selective reduction catalyst is defined as a first storage capacity,
The amount of ammonia that can be stored in the second selective reduction catalyst is defined as a second storage capacity,
The method for controlling an exhaust emission control device according to claim 9, wherein the second storage capacity is larger than a difference between a maximum time and a minimum time of the first storage capacity.
A target value setting step of setting a target value of the ammonia amount of the first selective reduction catalyst and the second selective reduction catalyst to a value larger than “0”;
11. The control input is calculated in the first control input calculation step so that the ammonia amount detected in the ammonia detection step falls within a predetermined range including the target value. A control method for the exhaust emission control device according to claim 1.
In the first control input calculation step,
The control input is calculated based on response designation control capable of setting a convergence speed of the ammonia amount detected in the ammonia detection step to the target value, and the ammonia amount detected in the ammonia detection step is the predetermined amount. 12. The convergence speed when it is included in the range is set slower than the convergence speed when the ammonia amount detected in the ammonia detection step is outside the predetermined range. Control method of exhaust emission control device.
The exhaust gas purification apparatus according to claim 11 or 12, wherein, in the target value setting step, the target value is set to a smaller value as the temperature of the exhaust gas of the internal combustion engine or the temperature of the selective reduction catalyst is higher. Control method.
A second control input calculating step of calculating a control input based on a rotation parameter of the internal combustion engine and a load parameter representing a load of the internal combustion engine;
14. The reducing agent amount determining step includes determining the amount of reducing agent supplied by the reducing agent supply means further including the control input calculated in the second control input calculating step. A method for controlling an exhaust emission control device according to any one of the above.
The amount of ammonia stored in the first selective reduction catalyst is defined as a first storage amount,
A third control input calculating step for estimating the first storage amount and calculating a control input for controlling the estimated first storage amount so as to converge to a predetermined target storage amount;
12. The reducing agent supply amount determination step determines the supply amount of the reducing agent by the reducing agent supply means further including the control input calculated in the third control input calculation step. The control method of the exhaust gas purification apparatus in any one of 14.
In the third control input calculating step,
In addition to the deviation between the estimated first storage amount and the target storage amount,
The control method of the exhaust emission control device according to claim 15, wherein the control input is calculated based on the differential of the deviation or the differential of the first storage amount.