JP2012036799A - Exhaust emission control system of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control system of an internal combustion engine, capable of accurately estimating the amount of reducing agents captured in a selected reduction catalyst by discriminating between ammonium and urea water.SOLUTION: The exhaust emission control system includes an NOx sensor and a unit conversion unit for acquiring a feed NOx amount QNOX equivalent to an exhaust NOx amount flowing into a first selected reduction catalyst, a first conversion rate calculation unit 511 for calculating a first NHconversion rate RNU1 equivalent to the rate of urea water converted into NHin urea water supplied on the basis of a first equivalent rate αwhich is a ratio of a supplied urea water amount QUREA to the feed NOx amount QNOX, a first NHsupplied amount estimation unit 512 for estimating a first NHsupply amount NH3which is a NHsupply amount to the first selected reduction catalyst on the basis of the first NHconversion rate RNU1, and a first storage amount estimation unit for estimating a storage amount equivalent to NHcaptured in the first selected reduction catalyst.

Description

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

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

  Such a selective reduction catalyst has an ability to adsorb ammonia that has not been used to reduce NOx in the exhaust gas. That is, when the amount of urea water supplied is larger than the amount of NOx flowing into the selective reduction catalyst, the excess ammonia that is not used for NOx reduction is adsorbed by the selective reduction catalyst, and conversely flows into the selective reduction catalyst. When the supply amount of urea water is smaller than the NOx amount, ammonia adsorbed on the selective reduction catalyst is used for NOx reduction. Therefore, the adsorption amount of ammonia in the selective reduction catalyst can be controlled by increasing / decreasing the supply amount of urea water.

  From the viewpoint of NOx purification, it is preferable that as much ammonia as possible be adsorbed on the selective reduction catalyst, but the amount of ammonia that can be adsorbed by the selective reduction catalyst is limited. If ammonia exceeding this limit amount is supplied to the selective reduction catalyst, ammonia that cannot be adsorbed will be discharged downstream. For this reason, the adsorption amount of ammonia in the selective reduction catalyst needs to be controlled to an appropriate amount so as not to exceed the limit amount.

  Patent Document 1 discloses a technique for estimating the ammonia adsorption amount in the selective reduction catalyst in controlling the ammonia adsorption amount in this way. In this technique, the ammonia inflow amount is calculated based on the output of an ammonia sensor provided upstream of the selective reduction catalyst, and the ammonia adsorption amount in the selective reduction catalyst is estimated based on the ammonia inflow amount.

JP 2008-303759 A

  By the way, in the urea addition type system, urea water is supplied to the upstream side of the selective reduction catalyst in the exhaust pipe. The urea water supplied here always flows into the selective reduction catalyst after all of it is converted to ammonia in the exhaust pipe. However, there is a case where a part thereof flows into the selective reduction catalyst in a state of urea water. Since the selective reduction catalyst has the ability to adsorb ammonia but not the ability to adsorb urea water, the urea water that has flowed in is converted into ammonia inside the selective reduction catalyst and is adsorbed or consumed in the state of ammonia, or It is discharged from the downstream of the selective reduction catalyst in the state of urea water.

  Thus, in the urea addition type system, urea water flowing into the selective reduction catalyst is potentially adsorbed as ammonia inside the selective reduction catalyst or consumed for NOx reduction. In the technique of Patent Document 1, it is not assumed that urea water flows into the selective reduction catalyst, and as a result, there is a possibility that an error may occur in the estimation of the adsorption amount of ammonia in the selective reduction catalyst.

  The present invention has been made in view of the above-described problems, and is capable of accurately estimating the amount of reducing agent trapped in the selective reduction catalyst by distinguishing between the reducing agent and its precursor. The purpose is to provide a system.

In order to achieve the above object, the present invention is provided in an exhaust system (for example, an exhaust pipe 11 described later) of an internal combustion engine (for example, an engine 1 described later), and exhausts in the presence of a reducing agent (for example, ammonia described later). And a selective reducing catalyst that captures the reducing agent (for example, a first selective reducing catalyst 231 or a second selective reducing catalyst 232 described later) and a reducing agent upstream of the selective reducing catalyst in the exhaust system. An exhaust gas purification system for an internal combustion engine (for example, an exhaust gas purification system 2 to be described later) provided with a precursor supply means (for example, an urea injection device 25 to be described later) for supplying a precursor (for example, urea water to be described later). To do. The exhaust purification system includes NOx amount acquisition means (for example, a NOx sensor 28, a unit conversion unit 8 and a first NOx emission amount estimation unit 53 described later) for acquiring the NOx amount of exhaust flowing into the selective reduction catalyst; The acquired NOx amount (for example, a feed NOx amount QNOX, which will be described later, a first NOx emission amount QNOX _{1ST → 2ND,} etc.) and a precursor supply amount (for example, a supply amount QUARE, which will be described later, a first urea slip amount). QUREA etc. _1ST) and the ratio of (e.g., the first equivalent ratio alpha _1ST described later, based on the first such 2 equivalent ratio alpha _2ND), corresponds to the ratio of the converted precursor to the reducing agent of the supplied precursors reducing agent conversion to (e.g., a 1N H ₃ conversion rate RNU1 described below, such as the 2NH ₃ conversion rate RNU2) conversion rate calculating means for calculating the (e.g., first below換率calculator 511, and the like 2 conversion rate calculation unit 551), on the basis of the calculated reducing agent conversion, the reducing agent supply amount to the selective reduction catalyst (e.g., a 1N H ₃ supply amount below QNH3 _1ST , second NH ₃ supply amount QNH3 _2ND, etc.) reducing agent supply amount estimation means (for example, first NH ₃ supply amount estimation unit 512, second NH ₃ supply amount estimation unit 552 described later), and the estimated reduction Storage amount estimation means for estimating a storage amount corresponding to the amount of reducing agent trapped by the selective reduction catalyst (for example, a first storage amount estimation unit 52 and a second storage amount estimation described later) based on the agent supply amount Part 56).

  In the present invention, the NOx amount of the exhaust gas flowing into the selective reduction catalyst is acquired, and based on the ratio between the acquired NOx amount and the precursor supply amount, the precursor supplied to the selective reduction catalyst is converted into a reducing agent. The reducing agent conversion rate corresponding to the ratio of the precursor thus obtained is calculated. Further, the reducing agent supply amount to the selective reduction catalyst is estimated based on the reducing agent conversion rate, and the storage amount of the reducing agent in the selective reduction catalyst is estimated based on the estimated reducing agent supply amount. Therefore, among the precursors supplied from the upstream side of the selective reduction catalyst, the amount supplied to the selective reduction catalyst in the state of the reducing agent is distinguished from the amount supplied to the selective reduction catalyst in the state of the precursor. Therefore, the storage amount in the selective reduction catalyst can be estimated with high accuracy.

  In this case, it is preferable that the conversion rate calculating means calculates the reducing agent conversion rate based on the ratio and the temperature of the exhaust system (for example, an exhaust temperature TGAS described later).

  The reducing agent conversion rate varies depending on the temperature of the exhaust system. Therefore, in the present invention, the reducing agent supply amount to the selective reduction catalyst is accurately estimated by calculating the reducing agent conversion rate based on the exhaust system temperature in addition to the ratio of the NOx amount and the precursor supply amount. As a result, the storage amount estimation accuracy can be further improved.

  In this case, it is preferable that the conversion rate calculating means calculates the reducing agent conversion rate based on the ratio and a space velocity of the exhaust system (for example, a space velocity SV described later).

  The reducing agent conversion rate varies depending on the space velocity of the exhaust system. Therefore, in the present invention, the reducing agent supply rate to the selective reduction catalyst is accurately estimated by calculating the reducing agent conversion rate based on the space velocity of the exhaust system in addition to the ratio of the NOx amount and the precursor supply amount. As a result, the estimation accuracy of the storage amount can be further improved.

In this case, the reducing agent is ammonia, a precursor thereof is urea water, and the exhaust purification system is configured to generate a CO ₂ generation amount (for example, described later) corresponding to the amount of carbon dioxide generated together with ammonia from the urea water. CO ₂ generation amount estimation means for estimating the first CO ₂ generation amount QCO2 _1ST , the _second CO ₂ generation amount QCO2 _2ND, etc. (for example, a first CO ₂ generation amount estimation unit 513, a _second CO ₂ generation amount estimation unit 553, which will be described later) And a precursor slip amount (for example, described later) which is a precursor amount discharged downstream from the selective reduction catalyst by subtracting the estimated reducing agent supply amount and CO ₂ production amount from the precursor supply amount. Of the first urea slip amount QUARE _1ST , the second urea slip amount QUARE _2ND, etc. A first slip amount estimating unit 514, a second slip amount estimating unit 554, and the like).

When ammonia is used as the reducing agent in the selective reduction catalyst, urea water is often used as the precursor. When urea water is hydrolyzed in a high-temperature exhaust system, carbon dioxide is generated together with ammonia. Therefore, in the present invention, the amount of CO ₂ produced corresponding to the amount of carbon dioxide produced together with ammonia from urea water is estimated. Then, by subtracting the estimated reducing agent supply amount and CO ₂ generation amount from the precursor supply amount to the selective reduction catalyst, a precursor slip amount, which is a precursor amount discharged downstream from the selective reduction catalyst, is obtained. presume. When urea water is discharged to the downstream side of the selective reduction catalyst, the supplied urea water is not only wasted, but it can also cause a strange odor. In the present invention, by estimating the amount of precursor slip, the amount of urea water supplied from the precursor supply means can be limited at an appropriate timing so that the above-described problems do not occur.

It is a mimetic diagram showing composition of an engine and its exhaust gas purification system concerning one embodiment of the present invention. It is a figure which shows what ratio the urea water injected from the urea injection apparatus changes. It is a figure which shows what ratio the urea water injected from the urea injection apparatus changes. It is a figure which shows what ratio the urea water injected from the urea injection apparatus changes. It is a figure which shows what ratio the urea water injected from the urea injection apparatus changes. Is a diagram showing an equivalent ratio and NH ₃ conversion of the urea water. It is a block diagram which shows the module regarding determination of the urea injection quantity of the urea injection apparatus comprised by ECU which concerns on the said embodiment. It is a block diagram which shows the structure of the storage amount estimation part which concerns on the said embodiment. It is a figure which shows typically the calculation in the storage amount estimation part which concerns on the said embodiment. It is a block diagram which shows the structure of the 1st supply amount estimation part which concerns on the said embodiment. It is a block diagram which shows the structure of the 1st storage amount estimation part which concerns on the said embodiment. It is a block diagram which shows the structure of the 1st NOx discharge | emission amount estimation part which concerns on the said embodiment. It is a block diagram which shows the structure of the 2nd supply amount estimation part which concerns on the said embodiment. It is a block diagram which shows the structure of the 2nd storage amount estimation part which concerns on the said embodiment. It is a flowchart which shows the procedure of the urea injection control performed by ECU which concerns on the said embodiment. Is a block diagram showing the configuration of the NH ₃ slip amount estimating section of the embodiment.

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

  The exhaust purification system 2 collects the oxidation catalyst 21 provided in the exhaust pipe 11 of the engine 1 and the particulate matter (hereinafter referred to as “PM (Particulate Matter)”) provided in the exhaust pipe 11. An exhaust purification filter (hereinafter referred to as “DPF (Diesel Particulate Filter)”) 22 and a nitrogen oxide (hereinafter referred to as “NOx”) in the exhaust gas that is provided in the exhaust pipe 11 and circulates through the exhaust pipe 11 are reduced. A urea selective reduction catalyst 23 that purifies in the presence of ammonia, a urea injection device 25 that supplies urea water, which is a precursor of ammonia, to the upstream side of the urea selective reduction catalyst 23 in the exhaust pipe 11, and electronic control And a unit (hereinafter referred to as “ECU”) 3.

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

The oxidation catalyst 21 is provided on the upstream side of the DPF 22 in the exhaust pipe 11 and oxidizes NO in the exhaust to convert it to NO ₂ , thereby promoting NOx reduction in the urea selective reduction catalyst 23.
The DPF 22 is provided on the downstream side of the oxidation catalyst 21 in the exhaust pipe 11, and when the exhaust gas passes through fine holes in the filter wall, the PM mainly containing carbon in the exhaust gas is converted into the surface of the filter wall and the filter. Collect by depositing in holes in the wall.

  The urea selective reduction catalyst 23 includes a first selective reduction catalyst 231 and a second selective reduction catalyst 232 provided downstream of the first selective reduction catalyst 231 in the exhaust pipe 11. These selective reduction catalysts 231 and 232 selectively reduce NOx in the exhaust gas in an atmosphere in which a reducing agent such as ammonia exists. Specifically, when urea water is injected by the urea injection device 25, this urea water is thermally decomposed or hydrolyzed by the heat of the exhaust to generate ammonia as a reducing agent. The produced ammonia is supplied to the selective reduction catalysts 231 and 232, and NOx in the exhaust is selectively reduced by these ammonia.

  By the way, these selective reduction catalysts 231 and 232 have a function of reducing NOx in the exhaust gas with ammonia generated from urea water, and also have a function of adsorbing a predetermined amount of the generated ammonia. In the present embodiment, the ammonia amount adsorbed by the selective reduction catalysts 231 and 232 is referred to as a storage amount, and the limit of the storage amount is referred to as a maximum storage capacity. More specifically, the storage amount of the first selective reduction catalyst 231 is referred to as a first storage amount, and the storage amount of the second selective reduction catalyst 232 is referred to as a second storage amount. The ammonia adsorbed in this way is also consumed as appropriate in the reduction of NOx in the exhaust. For this reason, as the storage amount increases, the NOx purification rate in the selective reduction catalyst increases. Further, when the supply amount of urea water is small with respect to the amount of NOx discharged from the engine, the adsorbed ammonia is consumed for the reduction of NOx so as to compensate for the shortage of urea water.

  Here, when the selective reduction catalysts 231 and 232 generate ammonia exceeding the maximum storage capacity, the generated ammonia is discharged to the downstream side of the selective reduction catalysts 231 and 232. In this manner, ammonia that is not adsorbed by the selective reduction catalysts 231 and 232 but is discharged downstream is referred to as “ammonia slip”. In this embodiment, as will be described in detail later, the first storage amount and the second storage amount are accurately estimated, and the urea injection amount is determined based on this estimation, so that the NOx as the entire urea selective reduction catalyst 23 is determined. Generation of ammonia slip in the second selective reduction catalyst 232 is suppressed as much as possible while maintaining a high purification rate.

  These selective reduction catalysts 231 and 232 have the ability to adsorb ammonia, but do not have the ability to adsorb urea water. Therefore, part of the urea water supplied from the urea injection device 25 may be discharged to the downstream side of the selective reduction catalysts 231 and 232 as it is. Hereinafter, discharge of urea water to the downstream side of the selective reduction catalysts 231 and 232 in this way is referred to as “urea slip”. In this embodiment, as will be described in detail later, a urea slip in the first selective reduction catalyst 231 and the second selective reduction catalyst 232 is estimated, and the urea injection amount is limited based on this estimation, whereby the second selective reduction is performed. Generation of urea slip in the catalyst 232 is suppressed as much as possible.

Connected to the ECU 3 are an NH ₃ sensor 26, an exhaust temperature sensor 27, a NOx sensor 28, an air flow meter 15, a crank angle position sensor 29, an accelerator opening sensor 14, a urea remaining amount warning lamp 16, and the like.

NH ₃ sensor 26 detects the concentration _{NH3 CONS} of exhaust ammonia between the first selective reduction catalyst 231 in the exhaust pipe 11 and the second selective reduction catalyst 232, detected substantially proportional to the detected ammonia concentration _{NH3 CONS} A signal is supplied to the ECU 3. The exhaust gas temperature sensor 27 detects the exhaust gas temperature (hereinafter referred to as “exhaust gas temperature”) TGAS on the downstream side of the second selective reduction catalyst 232 and supplies a detection signal substantially proportional to the detected exhaust gas temperature TGAS to the ECU 3. In the following, the temperatures estimated based on the exhaust gas temperature TGAS are used as the temperatures of the first and second selective reduction catalysts 231 and 232 and the oxidation catalyst 21, but the present invention is not limited to this. NOx sensor 28 detects a concentration _{NOX CONS} of exhaust NOx flowing into the first selective reduction catalyst 231, and supplies a detection signal substantially proportional to the detected NOx concentration _{NOX CONS} the ECU 3. The air flow meter 15 detects an intake air amount (mass flow rate) QA flowing through an intake passage (not shown) and supplies a detection signal substantially proportional to the detected intake air amount QA to the ECU 3.

  The crank angle position sensor 29 detects the rotation angle of the crankshaft of the engine 1, generates a pulse at every crank angle, and supplies the pulse signal to the ECU 3. The engine speed NE of the engine 1 is calculated by the ECU 3 based on this pulse signal. The accelerator opening sensor 14 detects a depression amount (hereinafter referred to as “accelerator opening”) of an accelerator pedal (not shown) of the vehicle, and supplies a detection signal substantially proportional to the detected accelerator opening to the ECU 3. In the ECU 3, the required torque of the engine 1 is calculated according to the accelerator opening and the engine speed. The urea remaining amount warning lamp 16 is provided, for example, on a meter panel of the vehicle, and lights up when the remaining amount of urea water in the 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.

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

Next, referring to FIG. 2 to FIG. 6, how the urea water injected from the urea injection device changes in the process of passing through the exhaust pipe and the first and second selective reduction catalysts in the exhaust pipe. explain.
Urea water (CO (NH ₂ ) ₂ + H ₂ O) injected from the urea injection device is thermally decomposed or hydrolyzed in the exhaust pipe and in the first and second selective reduction catalysts, and finally the following formula (1 ), Ammonia (NH ₃ ) and carbon dioxide (CO ₂ ) are produced.

2 to 5, the horizontal axis is the distance from the injection point of urea water, the temperature at each part in the exhaust pipe is shown in the upper stage, and the urea water injected in the lower stage is converted to NH ₃ , CO ₂ or HCNO. The ratio of the urea water thus produced and the NOx purification rate in each part are shown.
In the examples shown in FIGS. 2 to 5, urea water having a concentration of 32.5% was used, and the urea injection amount was 50 [mg / sec]. In the example shown in FIG. 2, the space velocity is about 12000 [h ⁻¹ ] and the exhaust temperature is about 200 [° C.], and in the example shown in FIG. 3, the space velocity is about 20000 [h ⁻¹ ] and the exhaust temperature is about In the example shown in FIG. 4, the space velocity is about 12000 [h ⁻¹ ] and the exhaust temperature is about 250 [° C.]. In the example shown in FIG. 5, the space velocity is about 20000 [h ⁻¹ ]. And the exhaust temperature was about 250 [° C.].

As shown in FIGS. 2 to 5, the urea water injected from the urea injection device upstream of the first selective reduction catalyst is partly from the injection point to the lower end of the second selective reduction catalyst. Decomposed to produce NH ₃ and CO ₂ . In addition, HCNO is transiently generated from the injection point to the lower end side of the first selective reduction catalyst, but the amount is small compared with NH ₃ and CO ₂ and as it goes downstream. Decrease. Therefore, in this embodiment, the generation of HCNO is ignored.

In the present embodiment, NH ₃ conversion rate is defined as a parameter corresponding to the ratio of urea water converted to NH ₃ in the supplied urea water. More specifically, a urea water supply point (for example, an injection point by the urea injection device or a lower end of the first selective reduction catalyst) is set, and the urea water supplied from this supply point is a predetermined target point. The ratio of conversion to NH ₃ until reaching the lower end of the first selective reduction catalyst (for example, the lower end of the first selective reduction catalyst) is defined as the NH ₃ conversion rate at the target point.

As shown in FIGS. 2 to 5, the NH ₃ conversion rate tends to increase toward the downstream side regardless of the exhaust gas temperature and the space velocity.
By comparing FIG. 2 and FIG. 3 or FIG. 4 and FIG. 5, it can be seen that the NH ₃ conversion rate tends to increase as the space velocity increases.
Further, comparing FIG. 2 with FIG. 4, it can be seen that when the space velocity is relatively low, the NH ₃ conversion rate does not tend to increase even if the exhaust gas temperature increases. On the other hand, comparing FIG. 3 with FIG. 5, it can be seen that when the space velocity is relatively high, the NH ₃ conversion rate tends to increase as the exhaust gas temperature increases.

FIG. 6 is a diagram showing the equivalent ratio α of urea water and the NH ₃ conversion rate.
Here, the equivalent ratio α of urea water refers to the inflow amount of urea water per unit time (including the amount that flows in as NH ₃ ) and the inflow amount of NOx per unit time for the target selective reduction catalyst. This ratio (urea water inflow amount / NOx inflow amount) is equal to (1) when the amount of urea water that can reduce NOx without excess or deficiency flows into the inflowing NOx. That is, when the amount of urea water necessary for reducing the inflowing NOx is not supplied to the NOx flowing into the target selective reduction catalyst, the equivalence ratio α becomes smaller than “1” and flows in. When more urea water than the amount necessary for reducing NOx is supplied, the equivalent ratio α becomes a value larger than “1”.

FIG. 6 shows that when the equivalent ratio α in the first selective reduction catalyst is “4” or “5”, that is, four times or five times the amount necessary to reduce NOx flowing into the first selective reduction catalyst. in the case of supplying the urea water, indicating the NH ₃ conversion at the second lower end of the selective reduction catalyst.
As can be seen from FIG. 6, it can be seen that the NH ₃ conversion rate changes not only according to the space velocity and the exhaust temperature, but also according to the equivalent ratio α of urea water.

As described above, as shown in FIGS. 2 to 6, the NH ₃ conversion rate in each portion in the exhaust pipe changes according to the equivalent ratio of urea water, the space velocity, and the exhaust temperature, and therefore, based on these three parameters. Can be estimated.

FIG. 7 is a block diagram showing modules relating to determination of the urea injection amount _GUREA of the urea injection device configured in the ECU. In this embodiment, the unit of the urea injection amount G _UREA is a mass flow rate, that is, the mass of urea water injected per unit time. The block relating to the determination of the urea injection amount _GUREA includes a base injection amount calculation unit 4, a storage amount estimation unit 5, a corrected injection amount calculation unit 6, and a slip restriction unit 7.

As shown in FIG. 7 and the following equation (2), the urea injection amount G _UREA includes a base injection amount G _{UREA_BS} calculated by a base injection amount calculation unit 4 described later, a storage amount estimation unit 5 described later, and a corrected injection amount. It is calculated by adding the corrected injection amount _{GUREA_COR} calculated by the calculation unit 6 and the urea slip restriction unit 7.

  The symbol (k) is a symbol indicating the discretized time, and indicates that the data is detected or calculated for each predetermined control period. That is, when the symbol (k) is data detected or calculated at the current control timing, the symbol (k-1) indicates data detected or calculated at the previous control timing. In the following description, the symbol (k) is omitted as appropriate.

The base injection amount calculating unit 4 will be described below, in operation of the storage amount estimating unit 5 and the correction injection amount calculation section 6, substantially the detected value _{NH3 CONS} and the NOx concentration of the NH ₃ sensor 26 is substantially proportional to the NH ₃ concentration As the detected value NOX _CONS of the proportional NOx sensor 28, the value converted by the unit converter 8 into the NH ₃ amount QNH3 _MID and the NOx amount QNOX corresponding to the mass flow rate is used. In addition, since NH ₃ in the exhaust system is generated from urea water injected by the urea injection device, in the present embodiment, the weight of NH ₃ is converted into the corresponding weight of urea water for convenience of calculation. However, the present invention is not limited to this. The amount of ammonia and the amount of urea water necessary to generate this amount of ammonia are mutually multiplied by a coefficient corresponding to the concentration of urea in the urea water stored in the urea tank 251. Can be converted to

Based on the intake air amount QA, the unit conversion unit 8 converts the detected values NH3 _CONS and NOX _CONS corresponding to the NH ₃ concentration and NOx concentration in the exhaust gas into an NH ₃ amount QNH3 _MID and an NOx amount QNOX. Since the NH ₃ amount QNH _{3 MID} calculated by the unit converter 8 corresponds to the NH ₃ amount slipped from the first selective reduction catalyst, it is hereinafter referred to as “first NH ₃ slip amount”. Further, the NOx amount QNOX calculated by the unit conversion unit 8 corresponds to the NOx amount discharged from the engine and supplied to the first selective reduction catalyst, and is hereinafter referred to as “feed NOx amount”.

  In the calculations performed by the base injection amount calculation unit 4, the storage amount estimation unit 5, and the corrected injection amount calculation unit 6, which will be described below, the exhaust space velocity SV is determined by the space velocity calculation unit 9 in the engine speed NE and intake air What is calculated by searching a predetermined map based on the quantity QA is used.

The base injection amount calculation unit 4 calculates a base injection amount G _{UREA_BS} by multiplying the feed NOx amount QNOX by a conversion coefficient K _{CONV_NOX_UREA} as shown in the following equation (3). In the following equation (3), the conversion coefficient K _{CONV_NOX_UREA} is a conversion coefficient for converting the NOx amount into the urea injection amount. By multiplying the conversion coefficient K _{CONV_NOX_UREA} by the NOx amount QNOX, the base injection amount G _{UREA_BS} that is the amount of urea water necessary for reducing the amount of NOx flowing into the selective reduction catalyst can be calculated. it can. That is, the base injection amount G _{UREA_BS} corresponds to an injection amount with an equivalence ratio α of “1”.

The storage amount estimation unit 5 estimates the first storage amount ST _{NH3 — 1ST} and the second storage amount based on parameters such as the feed NOx amount QNOX, the first NH ₃ slip amount QNH3 _MID , the space velocity SV, and the exhaust temperature TGAS. The value ST _{NH3_2ND} is calculated. Hereinafter, the configuration of the storage amount estimation unit 5 will be described in detail with reference to FIGS.

FIG. 8 is a block diagram illustrating a configuration of the storage amount estimation unit 5.
The storage amount estimation unit 5 includes a first supply amount estimation unit 51, a first storage amount estimation unit 52, a first NOx emission amount estimation unit 53, a second supply amount estimation unit 55, and a second storage amount estimation unit 56. And a urea slip determination unit 57, which calculate the estimated value ST _{NH3_1ST} of the first storage amount and the estimated value ST _{NH3_2ND} of the second storage amount and update the urea slip flag F _{UREA_SLIP} described later. To do.

FIG. 9 is a diagram schematically illustrating the calculation in the storage amount estimation unit 5.
Part of the urea water supplied from the upstream injection point of the first selective reduction catalyst 231 into the exhaust pipe with the supply amount QUARE is from the injection point to the lower end of the first selective reduction catalyst 231. It is converted to NH ₃ and supplied to the first selective reduction catalyst 231. NH ₃ supplied to the first selective reduction catalyst 231 is consumed for the reduction of NOx, adsorbed to the first selective reduction catalyst 231, or discharged downstream of the first selective reduction catalyst 231. Of the urea water supplied into the exhaust pipe with the supply amount QUARE, the portion that has not been converted to NH ₃ before reaching the lower end of the first selective reduction catalyst 231 is the first in the state of urea water. It is discharged from the selective reduction catalyst 231 downstream.
Under the above situation, the first supply amount estimation unit 51 of the storage amount estimation unit 5 uses the first NH ₃ supply amount QNH3 _1ST , which is the NH ₃ amount supplied to the first selective reduction catalyst 231, and the first selection. A first urea slip amount QUARE _1ST which is the amount of urea water discharged from the reduction catalyst 231 is estimated. In addition, the first storage amount estimation unit 52 estimates the first storage amount ST _{NH3 — 1ST} based on the estimated first NH ₃ supply amount QNH3 _1ST .

On the other hand, in addition to NH ₃ discharged from the first selective reduction catalyst 231, the second selective reduction catalyst 232 includes urea water supplied from the first selective reduction catalyst 231 at the first urea slip amount QUARE _1ST . NH ₃ converted before reaching the lower end of the second selective reduction catalyst 232 is supplied. The NH ₃ supplied to the second selective reduction catalyst 232 is consumed for NOx reduction, is adsorbed by the second selective reduction catalyst 232, or is discharged downstream of the second selective reduction catalyst 232. Of the urea water supplied from the first selective reduction catalyst 231 with the first urea slip amount QUARE _1ST , the amount that has not been converted to NH ₃ before reaching the lower end of the second selective reduction catalyst 232 is: The urea water is discharged from the second selective reduction catalyst 232 in the downstream state.
Under the above situation, the second supply amount estimation unit 55 of the storage amount estimation unit 5 uses the second NH ₃ supply amount QNH3 _2ND , which is the NH ₃ amount supplied to the second selective reduction catalyst 232, and the second selection. A second urea slip amount QUARE _2ND that is the amount of urea water discharged from the reduction catalyst 232 is estimated. Further, the second storage amount estimation unit 56 estimates the second storage amount ST _{NH3_2ND} based on the estimated second NH ₃ supply amount QNH3 _2ND .

FIG. 10 is a block diagram illustrating a configuration of the first supply amount estimation unit 51.
The first supply amount estimation unit 51 includes a first conversion rate calculation unit 511, a first NH ₃ supply amount estimation unit 512, a first CO ₂ generation amount estimation unit 513, and a first slip amount estimation unit 514. Thus, the first NH ₃ supply amount QNH3 _1ST and the first urea slip amount QUARE _1ST are calculated.

The first conversion rate calculation unit 511 includes a first equivalent ratio calculation unit 511a, a first temperature factor calculation unit 511b, and a first space velocity factor calculation unit 511c, and thereby, NH ₃ conversion in the first selective reduction catalyst. The first NH ₃ conversion rate RNU1, which is the rate, is calculated. More specifically, the first NH ₃ conversion rate RNU1 is converted to NH ₃ until the urea water supplied to the first selective reduction catalyst at the supply amount QUARE reaches the lower end of the first selective reduction catalyst. Defined as a percentage.

As described with reference to FIGS. 2 to 6, the first equivalence ratio calculation unit 511a is one of the parameters correlated with the NH ₃ change rate, and the equivalent ratio of urea water (hereinafter, “ Α _{1ST (referred} to as “first equivalence ratio”) is calculated by searching a predetermined map based on the supply amount QUARE of urea water and the feed NOx amount QNOX.
Here, since the urea water injected from the urea injection device is supplied to the first selective reduction catalyst with a predetermined time lag, the current value QUARE (k) of the supply amount of urea water is, for example, the above formula The past value G _UREA (k−1) of the urea injection amount determined in (2) is used.

As described above, the first temperature factor calculation unit 511b is based on the first equivalent ratio α _1ST and the exhaust gas temperature TGAS to consider the influence of the exhaust gas temperature, which is one of the parameters correlated with the NH ₃ change rate. The first temperature factor K _{1ST_TGAS} is calculated by searching a predetermined map.
In the present embodiment, the exhaust gas temperature TGAS is used to calculate the first temperature factor K _{1ST_TGAS. However} , the present invention is not limited to this, and the temperature of the first selective reduction catalyst may be used. However, since the reaction in the first selective reduction catalyst is basically not accompanied by heat generation, the exhaust temperature TGAS and the temperature of the first selective reduction catalyst are considered to be substantially equal.

The first space velocity factor calculation unit 511c is based on the first equivalent ratio α _1ST and the space velocity SV in order to consider the influence of the exhaust temperature, which is one of the parameters correlated with the NH ₃ change rate as described above. The first space velocity factor K _{1ST_SV} is calculated by searching a predetermined map.
The first NH ₃ conversion rate RNU1 is calculated by multiplying the first temperature factor K _{1ST_TGAS} and the first space velocity factor K _{1ST_SV} as shown in the following equation (4).

The first NH ₃ supply amount estimation unit 512 estimates the first NH ₃ supply amount QNH3 _1ST by multiplying the supply amount QUREA by the first NH ₃ conversion rate RNU1, as shown in the following equation (5-1). Here, as described above, the past value G _UREA (k−1) of the urea injection amount is used for the current value QUARE (k) of the urea water supply amount (see the following formula (5-2)). .

In the first CO ₂ generation amount estimation unit 513, the first CO ₂ generation amount QCO2 _1ST corresponding to the amount of CO ₂ generated together with NH ₃ from the urea water until reaching the lower end side of the first selective reduction catalyst, Estimated based on 1NH ₃ supply amount QNH3 _1ST .

The first slip amount estimation unit 514 subtracts the first NH ₃ supply amount QNH3 _1ST and the first CO ₂ generation amount QCO2 _1ST from the supply amount QUARE, as shown in the following formula (6), whereby the first selective reduction catalyst. 1st urea slip amount QUARE _1ST corresponding to the amount of urea water discharged to the downstream second selective reduction catalyst is estimated.

FIG. 11 is a block diagram illustrating a configuration of the first storage amount estimation unit 52.
The first storage amount estimation unit 52 includes a first NH ₃ consumption amount estimation unit 521, a first adsorption amount estimation unit 522, and a first integration calculation unit 523, thereby estimating the first storage amount. The value ST _{NH3_1ST} is calculated.

The first NH ₃ consumption estimation unit 521 includes a first NO ₂ / NO calculation unit 521a, a first NOx purification rate calculation unit 521b, and a first reaction model calculation unit 521c, which supply the first selective reduction catalyst. calculating a second 1N H ₃ consumption _{QNH3 1ST_RED} corresponding to the amount of NH ₃ that is consumed in the reduction of NOx of the NH ₃ which is.
The first NO ₂ / NOx calculating unit 521a searches the predetermined map based on the feed NOx amount QNOX and the temperature of the oxidation catalyst estimated based on the exhaust gas temperature TGAS, thereby flowing into the first selective reduction catalyst. A first NO inflow amount QNO _{1ST_IN} corresponding to the NO amount to be _calculated and a first NO ₂ inflow amount QNO2 _{1ST_IN} corresponding to the NO ₂ amount flowing into the first selective reduction catalyst are calculated.
The first NOx purification rate calculating unit 521b calculates a NOx purification rate η _1ST in the first selective reduction catalyst by searching a predetermined map based on the exhaust temperature TGAS and the space velocity SV.
Of the inflowing NOx, the first NO reduction amount QNO _{1ST_RED} and the first NO ₂ reduction amount QNO2 _{1ST_RED} corresponding to the NO amount and NO ₂ amount respectively reduced by the first selective reduction catalyst are expressed by the following equations (7-1), ( 7-2), the first NO inflow amount QNO _{1ST_IN} and the first NO ₂ inflow amount QNO2 _{1ST_IN} are multiplied by the purification rate η _1ST .

The first reaction model calculation unit 521c assumes that NO and NO ₂ are reduced by NH ₃ under the reactions shown in the following formulas (8-1) to (8-3). 1NO reduction amount QNO _{1ST_RED} NO and first NO ₂ reduction amount QNO2 The amount of NH ₃ required to reduce NO ₂ of _{1ST_RED} is calculated, and this is set as the first NH ₃ consumption amount QNH3 _{1ST_RED} .

The first adsorption amount estimation unit 522 subtracts the first NH ₃ consumption amount QNH3 _{1ST_RED} and the first NH ₃ slip amount QNH3 _MID from the first NH ₃ supply amount QNH3 _1ST as shown in the following formula (9). A first NH ₃ adsorption amount ΔQNH3 _1ST corresponding to the amount of NH ₃ adsorbed on the one selective reduction catalyst is calculated.

The first integration calculation unit 523 calculates the estimated value ST _{NH3 — 1ST} of the first storage amount by integrating the first NH ₃ adsorption amount ΔQNH3 _1ST . Here, the first integration calculation unit 523 sequentially calculates the maximum storage capacity of the first selective reduction catalyst based on the exhaust temperature TGAS, and the first storage amount is set so that the integration value does not exceed the maximum storage capacity. Estimated value ST _{NH3 — 1ST} is calculated. In addition, the first integration calculation unit 523 _sets “0” as a lower limit value, and calculates an estimated value ST _{NH3 — 1ST} of the first storage amount so as not to fall below this lower limit value.

FIG. 12 is a diagram illustrating a configuration of the first NOx emission amount estimation unit 53.
As shown in FIG. 12, the first NOx emission amount estimation unit 53 is configured to include a first NO ₂ / NO amount calculation unit 531 and a first NOx purification rate calculation unit 532, and discharges downstream from the first selective reduction catalyst. the first 1NOx emissions _{Qnox 1ST → 2ND} corresponding to the amount of NOx is calculated by divided into NO and NO ₂ which constitute the NOx.

The first NO ₂ / NO amount calculation unit 531 searches the predetermined map based on the feed NOx amount QNOX and the temperature of the oxidation catalyst estimated based on the exhaust gas temperature TGAS, so that the first NO inflow amount QNO _{1ST_IN} And the first NO ₂ inflow amount QNO2 _{1ST_IN} .
The first NOx purification rate calculation unit 532 calculates a NOx purification rate η _1ST in the first selective reduction catalyst by searching a predetermined map based on the exhaust temperature TGAS and the space velocity SV.

The 1NO ₂ emissions _{QNO2 1ST → 2ND,} as shown in FIG. 12, from the 1NO ₂ inflow _{QNO2 1ST_IN,} by subtracting the product of the first 1NO ₂ inflow _{QNO2 1ST_IN} and the 1NOx purification rate eta _1ST Calculated.
The 1NO emissions _{QNO 1ST → 2ND,} as shown in FIG. 12, is calculated by the first 1NO inflow _{QNO 1ST_IN,} subtracts the product of the first 1NO inflow _{QNO 1ST_IN} and the 1NOx purification rate eta _1ST .
Further, the 1NOx emissions _{Qnox 1ST → 2ND} is calculated by summing a first 1NO ₂ emissions _{QNO2 1ST → 2ND,} a first 1NO emissions _{QNO 1ST → 2ND,} the.

FIG. 13 is a block diagram illustrating a configuration of the second supply amount estimation unit 55.
The second supply amount estimation unit 55 includes a second conversion rate calculation unit 551, a second NH ₃ supply amount estimation unit 552, a second CO ₂ generation amount estimation unit 553, and a second slip amount estimation unit 554. The second NH ₃ supply amount QNH _{3 2ND} and the second urea slip amount QUREA _2ND are calculated from these.

The second conversion rate calculation unit 551 includes a second equivalent ratio calculation unit 551a, a second temperature factor calculation unit 551b, and a second space velocity factor calculation unit 551c, and thereby, NH ₃ conversion in the second selective reduction catalyst. The second NH ₃ conversion rate RNU2, which is the rate, is calculated. More specifically, the second NH ₃ conversion rate RNU2 is calculated as NH ₃ until the urea water supplied to the second selective reduction catalyst at the first urea slip amount QUARE _1ST reaches the lower end of the second selective reduction catalyst. Defined as the percentage converted to.

As described with reference to FIGS. 2 to 6, the second equivalent ratio calculation unit 551a performs the equivalent ratio of urea water (hereinafter referred to as “the second selective reduction catalyst”), which is one of the parameters correlated with the NH ₃ change rate. Α _{2ND (referred} to as “second equivalence ratio”) is calculated by searching a predetermined map based on the first urea slip amount QUARE _1ST and the first NOx emission amount QNOX _{1ST → 2ND} .

The second temperature factor calculation unit 551b is based on the second equivalent ratio α _2ND and the exhaust gas temperature TGAS in order to consider the influence of the exhaust gas temperature, which is one of the parameters correlated with the NH ₃ change rate as described above. The second temperature factor K _{2ND_TGAS} is calculated by searching a predetermined map.
In the present embodiment, the exhaust gas temperature TGAS is used to calculate the second temperature factor K _{2ND_TGAS. However} , the present invention is not limited to this, and the temperature of the second selective reduction catalyst may be used. However, since the reaction in the second selective reduction catalyst is basically not accompanied by heat generation like the first selective reduction catalyst, the exhaust temperature TGAS and the temperature of the second selective reduction catalyst are considered to be substantially equal. It is done.

The second space velocity factor calculation unit 551c is based on the second equivalent ratio α _2ND and the space velocity SV in order to consider the influence of the exhaust temperature, which is one of the parameters correlated with the NH ₃ change rate as described above. The second space velocity factor K _{2ND_SV} is calculated by searching a predetermined map.
The second NH ₃ conversion rate RNU2 is calculated by multiplying the second temperature factor K _{2ND_TGAS} and the second space velocity factor K _{2ND_SV} as shown in the following formula (10).

The second NH ₃ supply amount estimation unit 552 adds the first NH ₃ slip amount QNH3 _MID to the product obtained by multiplying the first urea slip amount QUREA _1ST by the second NH ₃ conversion rate RNU2 as shown in the following equation (11). Thus, the second NH ₃ supply amount QNH3 _2ND is estimated. The first term on the right side of the following formula (11) corresponds to the amount that is supplied in the form of urea water and converted to NH ₃ in the second selective reduction catalyst, and the second term on the right side is the value in the state of NH ₃ . This corresponds to the amount supplied to the two-selective reduction catalyst.

In the _second CO ₂ generation amount estimation unit 553, the second CO ₂ generation amount QCO2 _2ND corresponding to the amount of CO ₂ generated together with NH ₃ from the urea water until reaching the lower end side of the second selective reduction catalyst, This is estimated based on the amount of NH ₃ converted from urea water in the two-selective reduction catalyst (the first term on the right side of the formula (11)).

In the second slip amount estimation unit 554, as shown in the following equation (12), the NH ₃ amount converted from the urea water in the second selective reduction catalyst from the first urea slip amount QUARE _1ST (in the above equation (11)). By subtracting the first term on the right side) and the _second CO ₂ generation amount QCO2 _2ND , the second urea slip amount QUARE _2ND corresponding to the amount of urea water discharged downstream from the second selective reduction catalyst is estimated.

FIG. 14 is a block diagram illustrating a configuration of the second storage amount estimation unit 56.
The second storage amount estimation unit 56 includes a second NH ₃ consumption amount estimation unit 561, a second adsorption amount estimation unit 562, and a second integration calculation unit 563, thereby estimating the second storage amount. The value ST _{NH3_2ND} is calculated.

The second NH ₃ consumption estimation unit 561 includes a second NOx purification rate calculation unit 561b and a second reaction model calculation unit 561c, which reduce NOx out of NH ₃ supplied to the second selective reduction catalyst. calculating a second 2NH ₃ consumption _{QNH3 2ND_RED} corresponding to the amount of NH ₃ consumed.
The second NOx purification rate calculation unit 561b calculates a NOx purification rate η _2ND in the second selective reduction catalyst by searching a predetermined map based on the exhaust temperature TGAS and the space velocity SV.
Of the inflowing NOx, the second NO reduction amount QNO _{2ND_RED} and the _second NO ₂ reduction amount QNO2 _{2ND_RED} corresponding to the NO amount and NO ₂ amount respectively reduced by the second selective reduction catalyst are _expressed by the following equations (13-1), ( as shown in 13-2), is calculated by multiplying the purification rate eta _2ND to the 1NOx second 1NO ₂ emissions estimated by the discharge amount estimating unit 53 _{QNO2 1ST → 2ND} and a 1NO emissions _{QNO 1ST → 2ND} The

Similarly to the first reaction model calculation unit 521c, the second reaction model calculation unit 561c performs the reaction shown in the above formulas (8-1) to (8-3) so that NO and NO ₂ are generated by NH _3. Assuming that the _second NO reduction amount QNO _{2ND_RED} and the _second NO ₂ reduction amount QNO2 _{2ND_RED} NO ₂ are reduced, the amount of NH ₃ required for reducing NO ₂ is calculated and calculated as the second NH ₃ consumption amount. Let QNH3 _{2ND_RED} .

The second adsorption amount estimation unit 562 is adsorbed by the second selective reduction catalyst by subtracting the second NH ₃ consumption amount QNH3 _{2ND_RED} from the second NH ₃ supply amount QNH3 _2ND as shown in the following formula (14). the 2NH corresponding to NH ₃ of ₃ calculates the adsorption amount ΔQNH3 _2ND.

Returning to FIG. 8, the urea slip determination unit 57 sets the urea slip flag F _{UREA_SLIP} indicating that the urea water is being discharged from the second selective reduction catalyst according to the second urea slip amount QUARE _2ND. Set to “1” or “0”. More specifically, the urea slip determination unit 57 sets the flag F _{UREA_SLIP} to “1” when the second urea slip amount QUARE _2ND is larger than a threshold set in the vicinity of “0”, and is equal to or less than the above threshold. If it is, the flag F _{UREA_SLIP} is reset to “0”.

Returning to FIG. 7, the corrected injection amount calculation unit 6 includes the estimated values ST _{NH3 — 1ST} and ST _{NH3 — 2ND} of the respective storage amounts of the first and second selective reduction catalysts calculated by the storage amount estimation unit 5 and the first NH ₃ slip. Based on the amount QNH3 _{MID and the} like, a correction injection amount corresponding to the correction value for the base injection amount G _{UREA_BS} is calculated, and this is set as a provisional value G _{UREA_COR_TEMP} of the correction injection amount. For example, the corrected injection amount calculation unit 6 sets target values for the respective storage amounts in accordance with the states of the first and second selective reduction catalysts, and the estimated values ST _{NH3 — 1ST} and ST _{NH3 — 2ND of} the storage amounts are the target values. The corrected injection amount is calculated so as to match.

As shown in the following equation (15), the urea slip limiting unit 7 _corrects the calculated provisional value G _{UREA_COR_TEMP} without changing the value when the urea slip flag F _{UREA_SLIP} is “0”. Output as quantity G _{UREA_COR} . On the other hand, when the flag F _{UREA_SLIP} is “1”, the correction injection amount G _{UREA_COR} is _set to forcibly limit the urea injection amount to the minimum necessary amount in order to prevent the urea water from being continuously discharged. Output as “0”.

Next, a specific procedure of urea injection control will be described with reference to FIG.
FIG. 15 is a flowchart showing a procedure of urea injection control executed by the ECU. This urea injection control is to calculate the urea injection amount G _UREA by the above-described method, and is executed every predetermined control cycle (for example, 50 msec).

In 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. When this determination is YES, the _{process proceeds} to S9, and after setting the urea injection amount _GUREA to “0”, this process is ended. If this determination is NO, the process proceeds to S2.

In 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 one of the first and second selective reduction catalysts has failed, and is set to “0” otherwise. The When this determination is YES, the _{process proceeds} to S9, and after setting the urea injection amount _GUREA to “0”, this process is ended. If this determination is NO, the process proceeds to S3.

In S3, it is determined whether the urea remaining amount Q _{UREA_TANK} is less than a predetermined value Q _REF . This urea remaining amount Q _{UREA_TANK} 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 moves to S4, and if NO, the process moves to S5.
In S4, the urea remaining amount warning lamp is turned on, the _{process proceeds} to S9, and after the urea injection amount _GUREA is set to “0”, this process is terminated.

In S5, it is determined whether or not 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 S6. When this determination is NO, the _{process proceeds} to S9, and after setting the urea injection amount _GUREA to “0”, this process is terminated.

In 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 NH ₃ sensor or NOx sensor has failed in a determination process (not shown), and is set to “0” otherwise. If this determination is YES, the process moves to S7. When this determination is NO, the _{process proceeds} to S9, and after setting the urea injection amount _GUREA to “0”, this process is terminated.

In S7, it is determined whether or not the ammonia sensor activation flag F _NH3ACT is 1. The NH ₃ sensor activation flag F _NH3ACT is set to “1” when it is determined that the NH ₃ 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 moves to S8. When this determination is NO, the _{process proceeds} to S9, and after setting the urea injection amount _GUREA to “0”, this process is terminated.

In 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 proceeds to S10. When 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 _{process proceeds} to S9, where the urea injection amount _GUREA is _set to “0”. After setting to, this process ends. The temperature T _SCR of the first selective reduction catalyst is estimated based on, for example, the exhaust gas temperature TGAS.

In S10, the base injection amount calculation unit described above calculates the base injection amount _{GUREA_BS} based on the above equation (3), and the _{process proceeds} to S11.
In S11, the storage amount estimation unit estimates the first storage amount ST _{NH3_1ST} and the second storage amount ST _{NH3_2ND} based on the above formulas (4) to (14).
In S12, the provisional value G _{UREA_COR_TEMP} of the correction injection amount is calculated by the above-described correction injection amount calculation unit.
In S _<b> 13, the slip limiter described above determines whether the urea slip flag F _{UREA_SLIP} is “1”. When this determination is YES, that is, when it is determined that urea water is discharged downstream of the second selective reduction catalyst, the process proceeds to S14, and the corrected injection amount G is calculated based on the above equation (15). _{UREA_COR} is forcedly set to “0”, and the process proceeds to S16. On the other hand, if the determination in S13 is NO, then move to S15, based on the equation (15), the provisional value _{G UREA_COR_TEMP} calculated in S12 and the correction injection amount _{G UREA_COR,} proceeds to S16.
In S16, the urea injection amount _GUREA is calculated based on the equation (2), and the urea injection control is terminated.

According to this embodiment, the following effects can be obtained.
(1) According to this embodiment, of the urea water supplied from the upstream side of the first and second selective reduction catalysts 231 and 232, the amount supplied in the ammonia state and the state supplied as the urea water Therefore, the amount of storage in the selective reduction catalysts 231 and 232 can be estimated with high accuracy.

(2) According to this embodiment, in addition to the first equivalent ratio alpha _1ST and second equivalent ratio alpha _2ND, based on the exhaust gas temperature TGAS, calculates a second 1N H ₃ conversion rate RNU1 and a 2NH ₃ conversion rate RNU2 This makes it possible to accurately estimate the first NH ₃ supply amount QNH3 _1ST and the second NH ₃ supply amount QNH3 _2ND, and as a result, the storage amount estimation accuracy can be further improved.

(3) In addition to the first equivalent ratio alpha _1ST and second equivalent ratio alpha _2ND, based on the space velocity SV, by calculating a second 1N H ₃ conversion rate RNU1 and a 2NH ₃ conversion rate RNU2, a 1N H ₃ supply Since the amount QNH3 _1ST and the second NH ₃ supply amount QNH3 _2ND can be estimated with high accuracy, the estimation accuracy of the storage amount can be further improved as a result.

(4) According to the present embodiment, by subtracting the _second NH ₃ supply amount QNH3 _2ND and the _second CO ₂ generation amount QCO2 _2ND from the first urea slip amount QUARE _1ST , the second selective reduction catalyst 232 moves downstream. A second urea slip amount QUARE _2ND that is the amount of discharged urea water is estimated. Thereby, the supply amount of urea water from the urea injection device can be limited at an appropriate timing.

The present invention is not limited to the embodiment described above, and various modifications can be made.
In the above-described embodiment, the exhaust purification system including two selective reduction catalysts including the first selective reduction catalyst 231 and the second selective reduction catalyst 232 provided on the downstream side thereof has been described as an example. The present invention is not limited to this, and it is effective even when applied to an exhaust purification system having only one selective reduction catalyst.

  In the above embodiment, the feed NOx amount QNOX corresponding to the NOx amount discharged from the engine and supplied to the first selective reduction catalyst is estimated based on the output of the NOx sensor 28. However, the present invention is not limited to this. In addition, for example, the feed NOx amount may be estimated based on a parameter indicating the operating state of the engine such as the engine speed and the required torque.

In the above embodiment, the second 1N H ₃ slip amount _{QNH3 MID} corresponding to the amount of NH ₃ slipped from the first selective reduction catalyst has been estimated based on the output of the NH ₃ sensor 26 is not limited thereto. Even if the NH ₃ sensor 26 is not used, the first NH ₃ slip amount is not limited to the first NH ₃ supply amount QNH _{3 1ST} estimated by the first supply amount estimation unit 51, but also the exhaust temperature TGAS, the feed NOx amount QNOX, and the space velocity SV It can be estimated based on the above. Referring to FIG. 16, description will be given of a configuration of NH ₃ slip amount estimating unit 59 for estimating a second 1N H ₃ slip amount _{QNH3' MID} without using the output of the NH ₃ sensor.

FIG. 16 is a block diagram illustrating a configuration of the NH ₃ slip amount estimation unit 59.
The NH ₃ slip amount estimation unit 59 includes a first NH ₃ consumption amount estimation unit 591, a surplus NH ₃ amount calculation unit 592, an integration operation unit 593, an NH ₃ slip determination unit 594, a maximum storage capacity calculation unit 595, And a first NH ₃ slip amount calculation unit 596.

The surplus NH ₃ amount calculation unit 592 converts the first NH ₃ supply amount QNH3 _1ST corresponding to the NH ₃ amount supplied to the first selective reduction catalyst to the NH ₃ amount consumed for NOx reduction in the first selective reduction catalyst. By subtracting the corresponding first NH ₃ consumption amount QNH3 _{1ST_RED} , a surplus NH ₃ amount QNH3 _EXT corresponding to the NH ₃ amount that can be adsorbed by the first selective reduction catalyst or discharged from the first selective reduction catalyst is calculated. . Incidentally, the feed amount of NOx Qnox, the configuration of the 1N H ₃ consumption estimating unit 591 for estimating a second 1N H ₃ consumption _{QNOX 1ST_RED} based such as the exhaust temperature TGAS and space velocity SV, the described with reference to FIG. 11 is the same as that of the 1N H ₃ consumption estimation unit 521, a detailed description thereof is omitted.

The integration calculation unit 593 calculates the integrated value ΣNH3 _1ST of the surplus NH ₃ amount QNH3 _EXT under a predetermined lower limit value and upper limit value. Here, by setting the lower limit value to “0” and the upper limit value to the maximum storage capacity of the first selective reduction catalyst, this integrated value ΣNH3 _1ST can be regarded as the estimated value of the first storage amount. Also, in this integration calculation unit 593, the integration value ΣNH3 _1ST reaches the upper limit value when a first NH ₃ slip flag FNH3 _{1ST_SLIP} updated by the NH ₃ slip determination unit 594 described later is set to “1”. Judge that you did.

The maximum storage capacity calculation unit 595 searches the predetermined map based on the exhaust temperature _TGAS to calculate the estimated value ST _{NH3_MAX} of the first maximum storage capacity corresponding to the maximum storage capacity of the first selective reduction catalyst.

The NH ₃ slip determination unit 594 _compares the estimated value ST _{NH3_MAX} of the first maximum storage capacity with the integrated value ΣNH3 _1ST calculated by the integration calculating unit 593, thereby determining the presence or absence of ammonia slip in the first selective reduction catalyst. judge. More specifically, NH ₃ slip determination unit 594, when the integrated value ShigumaNH3 _1ST is the estimated value _{ST NH3 - MAX} above, the 1N H ₃ slip indicating that ammonia slip has occurred in the first selective reduction catalyst When the flag FNH3 _{1ST_SLIP} is set to “1” and the integrated value ΣNH3 _1ST is smaller than the estimated value ST _{NH3_MAX} , the first NH ₃ slip flag FNH3 _{1ST_SLIP} is reset to “0”.

When the NH ₃ slip determination unit 594 determines that no ammonia slip has occurred, the first NH ₃ slip amount calculation unit 596 outputs “0” as the first NH ₃ slip amount QNH3 ′ _MID , If it is determined by the _three- slip determination unit 594 that ammonia slip has occurred, the surplus NH ₃ amount QNH3 _EXT is output as the first NH3 slip amount QNH3 ′ _MID .

Further, the first NH ₃ slip amount QNH3 ′ _MID estimated as described above is converted into units by the unit conversion unit 597 based on the intake air amount QA as shown in FIG. It is also possible to estimate the NH ₃ concentration NH3 ′ _CONS corresponding to the estimated value of the output of the NH ₃ sensor.

DESCRIPTION OF SYMBOLS 1 ... Engine (internal combustion engine), 11 ... Exhaust pipe (exhaust system), 2 ... Exhaust purification system, 23 ... Urea selective reduction catalyst, 231 ... 1st selective reduction catalyst (selective reduction catalyst), 232 ... 2nd selective reduction catalyst (Selective reduction catalyst), 25 ... urea injection device (precursor supply means), 28 ... NOx sensor (NOx amount acquisition means), 3 ... ECU, 4 ... base injection amount calculation unit, 5 ... storage amount estimation unit, 51 ... 1st supply amount estimation part, 511 ... 1st conversion rate calculation part (conversion rate calculation means), 511a ... 1st equivalence ratio calculation part, 511b ... 1st temperature factor calculation part, 511c ... 1st space velocity factor calculation part, 512 ... second 1N H ₃ supply quantity estimating section (reducing agent supply quantity estimating means), 513 ... first 1 CO ₂ generation amount estimation unit (CO ₂ generation amount estimation means), 514 ... first slip amount estimating unit (precursor slip amount estimating Means), 52. Rage amount estimator (storage amount estimator), 53 ... first NOx emission estimator (NOx amount acquirer), 55 ... second supply amount estimator, 551 ... second conversion rate calculator (conversion rate calculator), 551a: first equivalence ratio calculation unit, 551b: first temperature factor calculation unit, 551c ... first space velocity factor calculation unit, 552 ... second NH ₃ supply amount estimation unit (reducing agent supply amount estimation means), 553 ... second CO ₂ generation amount estimation unit (CO ₂ generation amount estimation unit), 554 ... second slip amount estimation unit (precursor slip amount estimation unit), 56 ... second storage amount estimation unit (storage amount estimation unit), 57 ... urea slip Determination unit, 6 ... corrected injection amount calculation unit, 7 ... slip limiting unit, 8 ... unit conversion unit (NOx amount acquisition means)

Claims (4)

A selective reduction catalyst that is provided in an exhaust system of an internal combustion engine, purifies exhaust gas in the presence of a reducing agent, and captures the reducing agent;
An exhaust gas purification system for an internal combustion engine, comprising: a precursor supply means for supplying a precursor of a reducing agent to an upstream side of the selective reduction catalyst in the exhaust system,
NOx amount acquisition means for acquiring the NOx amount of exhaust flowing into the selective reduction catalyst;
Based on the ratio of the acquired amount of NOx and the amount of precursor supplied to the selective reduction catalyst, a reducing agent conversion rate corresponding to the proportion of the precursor converted into a reducing agent among the supplied precursors is calculated. Conversion rate calculation means;
Reducing agent supply amount estimating means for estimating a reducing agent supply amount to the selective reduction catalyst based on the calculated reducing agent conversion rate;
Exhaust gas of an internal combustion engine, comprising: a storage amount estimating means for estimating a storage amount corresponding to the amount of reducing agent trapped by the selective reduction catalyst based on the estimated reducing agent supply amount Purification system.   2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the conversion rate calculating means calculates the reducing agent conversion rate based on the ratio and the temperature of the exhaust system.   2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the conversion rate calculating means calculates the reducing agent conversion rate based on the ratio and a space velocity of the exhaust system. The reducing agent is ammonia, and its precursor is urea water,
The exhaust purification system includes:
And CO ₂ generation amount estimation means for estimating the CO ₂ generation amount corresponding to the amount of carbon dioxide produced together with the ammonia from the urea water,
A precursor slip that estimates a precursor slip amount that is a precursor amount discharged downstream from the selective reduction catalyst by subtracting the estimated reducing agent supply amount and CO ₂ generation amount from the precursor supply amount. An exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 3, further comprising: an amount estimation unit.

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