JP5198520B2 - Exhaust gas purification system for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control system capable of suppressing the emission of a reducing agent while maintaining high NOx elimination performance even while a temperature of a selective reduction catalyst following regeneration of an exhaust purification filter is high. <P>SOLUTION: The exhaust emission control system includes a DPF collecting PMs in exhaust, an urea injection device supplying urea to upstream sides of first and second selective reduction catalysts, and a limit injection amount calculating part 57 calculating an NO reducible amount QNO<SB POS="POST">RED_MAX</SB>and an NO<SB POS="POST">2</SB>reducible amount QNO2<SB POS="POST">RED_MAX</SB>equivalent to an NO amount and an NO<SB POS="POST">2</SB>amount reducible by the first and second selective catalysts on the basis of feed NOx QNOX, and an upper limit injection amount G<SB POS="POST">UREA_ULIM</SB>equivalent to a limit value with respect to a supply amount of urea water during the regeneration of the DPF on the basis of the calculated NO reducible amount QNO<SB POS="POST">RED_MAX</SB>and the NO<SB POS="POST">2</SB>reducible amount QNO2<SB POS="POST">RED_MAX</SB>. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

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 purification system including a selective reduction catalyst that reduces NOx in exhaust 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. Thus, for example, Patent Document 1 discloses a technique for calculating a NOx purification rate in a selective reduction catalyst and controlling the amount of adsorption of ammonia in accordance with the calculated NOx purification rate.

  By the way, in the exhaust purification system provided with the filter (DPF (Diesel Particulate Filter)) which collects the particulate matter (PM (Particulate Matter)) in the exhaust gas in addition to the selective reduction catalyst as described above, the PM deposited on the filter. In order to burn off, it is necessary to raise the temperature of the filter to a PM combustion temperature of about 600 ° C. On the other hand, it is known that the ability of the selective reduction catalyst to adsorb ammonia decreases as the temperature increases. Therefore, when the temperature of the filter is increased in order to burn and remove PM, ammonia that has been adsorbed before the regeneration of the filter is started may be discharged as the temperature of the selective reduction catalyst increases.

  Therefore, in Patent Document 2, it is determined whether or not ammonia adsorbed from the selective reduction catalyst is discharged when starting the temperature increase of the filter. There has been proposed a technique for increasing the amount of NOx flowing into the selective reduction catalyst in order to wait and promote consumption of ammonia adsorbed on the selective reduction catalyst.

Japanese Patent No. 3951774 JP 2010-90852 A

  According to the technique of Patent Document 2, although it is possible to suppress the ammonia adsorbed so far from being discharged when the temperature rise of the filter is started and the temperature of the selective reduction catalyst rapidly rises, the filter During the regeneration after raising the temperature, it has not been studied deeply. Normally, the regeneration of the filter is performed over several tens of seconds. During this time, the temperature of the selective reduction catalyst is also kept at a high temperature, so that the selective reduction catalyst cannot adsorb ammonia. Therefore, the ammonia supplied to the selective reduction catalyst is basically discharged to the downstream side of the selective reduction catalyst except for that consumed for NOx reduction. As described above, during the regeneration of the filter, the ability of the selective reduction catalyst to adsorb ammonia is remarkably reduced. Therefore, an appropriate amount of urea water is used so as to suppress the discharge of ammonia while maintaining high NOx purification performance. It has become particularly difficult to supply.

  The present invention has been made in view of the above-described problems, and the amount of reducing agent discharged while maintaining high NOx purification performance even while the selective reduction catalyst is at a high temperature accompanying regeneration of the exhaust purification filter. It is an object of the present invention to provide an exhaust gas purification system for an internal combustion engine that can suppress the above.

In order to achieve the above object, the present invention is provided in an exhaust system (for example, exhaust pipe 11 described later) of an internal combustion engine (for example, engine 1), and purifies exhaust gas in the presence of a reducing agent (for example, ammonia described later). And a selective reduction catalyst (for example, a first selective reduction catalyst 231 and a second selective reduction catalyst 232 described later) for capturing (adsorbing) the reducing agent, and a particulate matter in the exhaust is provided in the exhaust system. A filter to be collected (for example, DPF 22 described later) and a reducing agent supply means for supplying a reducing agent or a precursor thereof (for example, urea water described later) to the upstream side of the selective reduction catalyst in the exhaust system (for example, An exhaust gas purification system for an internal combustion engine (for example, an exhaust gas purification system 2 to be described later) is provided. The exhaust purification system combusts and removes particulate matter accumulated by raising the temperature of the filter, and regenerates the filter, for example, an exhaust system temperature control unit 4 to be described later, and the selective reduction catalyst. Based on the NOx amount acquisition means (for example, a NOx sensor 28 and a unit converter 571 described later) and the acquired NOx amount (for example, a feed NOx amount QNOX described later), which acquire the NOx amount in the exhaust gas flowing into the exhaust gas, The NO amount and NO ₂ amount that can be reduced by the selective reduction catalyst are calculated, and the calculated NO amount (for example, NO reduction possible amount QNO _{RED_MAX} or NO reduction request amount QNO _{RED_REQ} described later) and NO ₂ amount (for example, Based on the NO ₂ reducible amount QNO2 _{RED_MAX} or the NO ₂ reduction request amount QNO2 _{RED_REQ} described later) Limit for the supply amount of the reducing agent or precursor during the regeneration of the serial filter (e.g., upper-limit injection amount _{G UREA_ULIM} or lower injection amount _{G UREA_LLIM} below) limit value calculating means for calculating a (e.g., limit injection amount of the later And a calculation unit 57).

In the present invention, it obtains the amount of NOx in the exhaust gas flowing into the selective reduction catalyst, based on the obtained NOx amount to calculate the amount of NO and NO ₂ amount which can be reduced with a selective reduction catalyst, further calculated amount of NO and Based on the NO ₂ amount, a limit value for the supply amount of the reducing agent or the precursor during regeneration of the filter is calculated. In this way, by setting a limit value for the supply amount of the reducing agent or precursor during regeneration of the filter, the supply amount of the reducing agent or precursor becomes excessive or insufficient relative to the required amount. As a result, it is possible to suppress the discharge amount of the reducing agent while maintaining the NOx purification performance of the selective reduction catalyst high.

By the way, in the selective reduction catalyst, NO and NO ₂ constituting NOx in the exhaust gas are reduced by a reducing agent under different reactions. In the present invention, the limit value of the supply amount of the reducing agent or precursor is calculated based on the NO amount and NO ₂ amount that can be reduced by the selective reduction catalyst. Therefore, since the amount of reducing agent necessary for reducing NO and NO ₂ in the exhaust gas can be calculated with high accuracy, the amount of reducing agent discharged can be reduced while maintaining high NOx purification performance during regeneration of the filter. The limit value of the optimum supply amount of the reducing agent or precursor can be calculated so as to suppress it. By dividing into NO and NO ₂ as described above, the required amount of reducing agent is accurately calculated, and the limit value is calculated accordingly. During the regeneration of the filter, the ability to adsorb the reducing agent is significantly reduced. Then it is particularly effective.

In this case, the limit value calculation means is a parameter (for example, a selective reduction catalyst temperature T _SCR described later) and a parameter (for example, a selective reduction catalyst temperature T _SCR described later) correlated with the acquired NOx amount (for example, a feed NOx amount QNOX described later) and an operating state of the internal combustion engine. Calculating the amount of NO that can be reduced and the amount of NO ₂ based on the NOx purification rate (for example, the NOx purification rate η _MAX described later) of the selective reduction catalyst calculated according to the space velocity SV). preferable.

In the present invention, in addition to the obtained NOx amount, based on the NOx purification rate calculated in accordance with a correlation to the operating state of the internal combustion engine parameter, by calculating a reducible NO amount and NO ₂ amount, Since the minimum amount of reducing agent required to reduce NO and NO ₂ in the exhaust gas can be calculated with higher accuracy, as a result, the NOx purification performance of the selective reduction catalyst is more reliably maintained while maintaining the NOx purification performance high. Emission can be suppressed.

  In this case, the exhaust purification system includes an exhaust gas recirculation means (for example, an EGR device 17 described later) for recirculating a part of the exhaust gas in the exhaust system to the intake system of the internal combustion engine, and immediately after the start of regeneration of the filter. It is preferable to further include correction means (for example, an intake control unit 6 described later) for correcting the fresh air amount of the internal combustion engine to the increase side by the exhaust gas recirculation means while the temperature of the filter is raised.

  In the present invention, immediately after the start of regeneration of the filter, while the temperature of the filter is raised, the amount of NOx in the exhaust gas flowing into the selective reduction catalyst is corrected by correcting the fresh air amount of the internal combustion engine to the increase side by the exhaust gas recirculation means. Therefore, it is possible to actively consume the reducing agent that has been adsorbed until the temperature of the filter starts to rise. Therefore, it can be suppressed that the reducing agent is not consumed for the reduction of NOx with the start of the temperature rise of the filter and is discharged to the downstream side of the selective reduction catalyst.

In this case, while the temperature of the filter is raised, the correction means discharges the reducing agent from the selective reduction catalyst (for example, a temperature rising NH ₃ slip speed VNH3 _SLIP described later) and the selective reduction. Compared with the consumption rate of the reducing agent in the catalyst (for example, NH ₃ consumption rate VNH3 _RED at the time of temperature increase, which will be described later), when the discharge rate of the reducing agent is faster than the consumption rate, the fresh air amount of the internal combustion engine is It is preferable to correct to the increase side.

  In the present invention, when the discharge rate of the reducing agent from the selective reduction catalyst is higher than the consumption rate of the reducing agent in the selective reduction catalyst while the temperature of the filter is raised, the fresh air amount is corrected to the increase side. By increasing the amount of NOx flowing into the selective reduction catalyst, the consumption rate of the reducing agent in the selective reduction catalyst can be temporarily increased, so that a large amount of reducing agent is discharged from the selective reduction catalyst while the temperature of the filter is raised. Can be suppressed.

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 the relationship between the NOx purification rate in a selective reduction catalyst, and a catalyst temperature. It is a figure which shows the NOx purification rate during temperature rising and regeneration of DPF. It is a block diagram which concerns on determination of the urea injection quantity by the urea injection apparatus which concerns on the said embodiment. It is a block diagram which shows the structure of the normal injection amount calculation part which concerns on the said embodiment. It is a block diagram which shows the structure of the restriction | limiting injection amount calculation part which concerns on the said embodiment. It is a block diagram which concerns on calculation of the EGR valve opening degree for controlling the fresh air quantity of an engine to predetermined | prescribed target fresh air quantity among the intake control parts which concern on the said embodiment. It is a block diagram which shows the structure of the slip speed calculation part which concerns on the said embodiment. It is a block diagram which shows the structure of the consumption speed calculation part which concerns on the said embodiment. It is a flowchart which shows the procedure of the urea injection control by the urea injection control part which concerns on the said embodiment. It is a figure which shows a part of procedure of the fresh air quantity control by the intake control part which concerns on the said 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 is provided in an oxidation catalyst 21 provided in the exhaust pipe 11 of the engine 1, a DPF 22 provided in the exhaust pipe 11 as a filter for collecting PM in the exhaust, and provided in the exhaust pipe 11. A urea selective reduction catalyst 23 that purifies nitrogen oxide (NOx) in the exhaust gas flowing through the exhaust pipe 11 in the presence of ammonia (NH ₃ ) as a reducing agent, and upstream of the urea selective reduction catalyst 23 in the exhaust pipe 11. On the side, urea injection device 25 for supplying urea water as a precursor of ammonia, EGR device 17 for recirculating a part of the exhaust gas in exhaust pipe 11 into intake pipe 12, and an electronic control unit (hereinafter referred to as “ECU”) And 3).

  The EGR device 17 includes an EGR pipe 18 that communicates the upstream side of the oxidation catalyst 21 and the intake pipe 12 in the exhaust pipe 11, and an EGR valve 19 that controls the flow rate of the exhaust gas flowing through the EGR pipe 18. Is done. The EGR valve 19 is connected to the ECU 3, operates according to a control signal from the ECU 3, and in response to this control signal, the flow rate of exhaust gas recirculated into the intake pipe 12 through the EGR pipe 18, and thus the new engine 1. Control your volume.

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

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

The oxidation catalyst 21 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. Further, the downstream DPF 22 is heated by burning the unburned fuel supplied by executing post-injection during the temperature rising and regeneration of the DPF, which will be described later.
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 and the maximum storage capacity of the first selective reduction catalyst 231 are referred to as the first storage amount and the first maximum storage capacity, and the storage amount and the maximum storage capacity of the second selective reduction catalyst 232 are referred to as the second storage. Referred to as volume and second maximum storage capacity. 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 reduction 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”.

The ECU 3 is connected to an NH ₃ sensor 26, an exhaust temperature sensor 27, a NOx sensor 28, an air flow meter 29, a crank angle position sensor 14, an accelerator opening sensor 15, 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. 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 29 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 14 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 15 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 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 engine speed NE. Hereinafter, the required torque TRQ is a load parameter that represents the load of the engine 1.

The temperature of the exhaust system, such as the temperature of the oxidation catalyst 21 (hereinafter referred to as “oxidation catalyst temperature”) T _DOC and the temperature of the first and second selective reduction catalysts (hereinafter referred to as “selective reduction catalyst temperature”) T _SCR , for example, The ECU 3 calculates the temperature TGAS, the engine speed NE, and the intake air amount QA. Further, the exhaust space speed SV is calculated by the ECU 3 by searching a predetermined map based on the engine speed NE and the intake air amount QA.

  The ECU 3 shapes input signal waveforms from various sensors, corrects the voltage level to a predetermined level, converts an analog signal value into a digital signal value, and a central processing unit (hereinafter “ CPU ”). In addition, the ECU 3 includes a storage circuit that stores various calculation programs executed by the CPU, calculation results, and the like, and an output circuit that outputs control signals to the engine 1, the urea injection valve 253, the EGR valve 19, and the like.

  Hereinafter, the exhaust system temperature control unit 4 related to the exhaust system temperature control, the urea injection control unit 5 related to urea water injection control, and the intake control unit 6 related to control of the intake system, which are configured in the ECU 3, will be described. The configuration will be described in order.

[Exhaust system temperature control unit 4]
First, the exhaust system temperature control unit 4 will be described.
FIG. 2 is a graph showing the relationship between the NOx purification rate and the catalyst temperature in the selective reduction catalyst. The NOx purification rate in the selective reduction catalyst shows an upward characteristic with respect to the catalyst temperature. Therefore, the selective reduction catalyst has an optimum temperature for maintaining a high NOx purification rate. In the example shown in FIG. 2, this optimum temperature is about 250 ° C.

  Returning to FIG. 1, the exhaust system temperature control unit 4 sets the optimum temperature as the target temperature of the first selective reduction catalyst 231 in order to maintain the NOx purification performance of the selective reduction catalysts 231 and 232 at a high level. The ignition timing and target air-fuel ratio of the engine 1 are changed so that the temperature of the reduction catalyst 231 is maintained at the target temperature, or post injection (fuel injection during the exhaust process) is executed.

  By the way, if the amount of accumulated PM in the DPF 22 increases, the pressure loss increases, and the fuel consumption of the engine 1 deteriorates or the output decreases. Therefore, it is necessary to regenerate the DPF 22 by burning and removing the PM accumulated in the DPF 22. .

Therefore, the exhaust system temperature control unit 4 integrates the PM accumulation amount in the DPF 22 from the fuel injection amount and the rotational speed NE of the engine 1, and when the PM accumulation amount exceeds a predetermined threshold, the timing for regenerating the DPF 22 The DPF is regenerated by the following procedure.
First, the exhaust system temperature control unit 4 sets a DPF temperature increase flag F _RISE indicating that the regeneration temperature of the DPF 22 is being increased to “1” in response to determining that it is time to regenerate the DPF 22. At the same time, about 600 ° C., which is the combustion temperature of PM, is set as the target temperature of the DPF, and the temperature of the DPF 22 is raised to this target temperature. Further, the exhaust system temperature control unit 4 resets the DPF temperature increase flag F _RISE to “0” in response to the temperature of the DPF 22 reaching the target temperature, and indicates that the DPF 22 is being regenerated. The regeneration flag F _DPFREG is set to “1”, and the temperature of the DPF 22 is kept at this target temperature until it can be determined that the PM deposited on the DPF 22 has been burned and removed. Thereafter, the exhaust system temperature control unit 4 resets the DPF regeneration flag F _DPFREG to “0” in response to determining that PM has been burned and removed, and ends the regeneration of the DPF. The exhaust system temperature control unit 4 changes the temperature of the DPF 22 to the target temperature by changing the ignition timing and the target air-fuel ratio of the engine 1 or by performing post injection (fuel injection during the exhaust process). Increase or maintain temperature.

[Yurea injection control unit 5]
Next, before describing the configuration of the urea injection control unit 5, the purification performance of the selective reduction catalyst of the conventional exhaust purification system during the regeneration of the DPF as described above will be examined.
FIG. 3 is a diagram showing the NOx purification rate during the temperature rise and regeneration of the DPF. More specifically, the lower thin line indicates the vehicle speed [km / h], and the lower thick line indicates the equivalent ratio α of urea water supplied from the urea injection device. The middle thick line indicates the output of the NH ₃ sensor, that is, the NH ₃ concentration [ppm] between the first selective reduction catalyst and the second selective reduction catalyst, and the middle thin line indicates the NH ₃ concentration of the exhaust discharged from the tail pipe, The NH ₃ concentration [ppm] on the downstream side of the 2-selective reduction catalyst is shown. The upper row shows the NOx purification rate [%] combining the first and second selective reduction catalysts.

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”.

<DPF regeneration temperature rise>
During the regeneration temperature increase of the DPF, the temperature of the first and second selective reduction catalysts rises in a short time as the temperature of the DPF increases, and at the same time, the maximum storage capacity of the first and second selective reduction catalysts also decreases in a short time. descend. For this reason, as indicated by the solid line in the middle stage, NH ₃ adsorbed on the first selective reduction catalyst is discharged, and thereafter, NH ₃ is discharged from the downstream side of the second selective reduction catalyst. Therefore, during the regeneration temperature increase of the DPF, even if the injection of urea water is suppressed as shown in FIG. 3, the NH ₃ adsorbed so far is released, so that excessive NH ₃ slip is likely to occur. It has become.
<DPF regeneration in progress>
During the regeneration of the DPF, the first and second selective reduction catalysts become high temperature, and thus the ability to adsorb NH ₃ is lost. For this reason, when urea water is injected so that the equivalence ratio α becomes “1”, the NOx purification rate increases accordingly, but NH ₃ slip in the second selective reduction catalyst is also likely to occur. Conversely, if the injection amount of urea water is reduced, NH ₃ slip can be suppressed, but the NOx purification rate is reduced. As described above, during the regeneration of the DPF, the first and second selective reduction catalysts have no ability to adsorb NH ₃ , so an appropriate amount is set so as to suppress NH ₃ slip while maintaining a high NOx purification rate. It is particularly difficult to supply the urea water.

Hereinafter, the configuration of the urea injection control unit 5 will be described with reference to FIGS. 4 to 6.
FIG. 4 is a block diagram relating to determination of the urea injection amount G _UREA by the urea injection device.
The urea injection control unit 5, a normal injection quantity calculating section 51 for calculating the normal injection amount _{G UREA_ORD,} the limit injection amount calculating unit 57 for calculating an upper limit injection amount _{G UREA_ULIM} and the lower limit injection amount _{G UREA_LLIM,} these injection amount _{G UREA_ORD} , G _{UREA_ULIM} , G _{UREA_LLIM} , and an injection amount limiting unit 59 that determines the final urea injection amount G _UREA based on the _{UREA_LLIM} .

Hereinafter, as will be described in detail, the normal injection amount calculation unit 51 calculates the normal injection amount G _{UREA_ORD} based on the detection value NH3 _CONS of the NH ₃ sensor and the detection value NOX _{CONS of the} NOx sensor, and the limiting injection amount calculation unit 57 calculates the detected value _{NOX CONS} of the NOx sensor, the oxidation catalyst temperature _{T DOC,} selective reduction catalyst temperature _{T SCR,} the upper-limit injection amount _{G UREA_ULIM} and the lower limit injection amount _{G UREA_LLIM} based on such space velocity SV. When the DPF regeneration is not being performed (F _DPFREG = "0"), the injection amount restriction unit 59 determines the normal injection amount G _{UREA_ORD} as the urea injection amount G _UREA , and when the DPF regeneration is being performed (F _DPFREG = "" 1 ") in determines that limiting the normal injection amount _{G UREA_ORD} within the upper-limit injection amount _{G UREA_ULIM} and the lower limit injection amount _{G UREA_LLIM} as the urea injection amount _{G uREA.}

FIG. 5 is a block diagram illustrating a configuration of the normal injection amount calculation unit 51.
The normal injection amount calculation unit 51 includes a feedback controller 52, a feedforward controller 53, and a storage correction input calculation unit 54.
In the normal injection amount calculation unit 51, as shown in the following equation (1), the FB injection amount G _{UREA_FB} calculated by the feedback controller 52, the FF injection amount G _{UREA_FF} calculated by the feedforward controller 53, and storage correction The normal injection amount _{GUREA_ORD} is determined by adding the corrected injection amount _{GUREA_ST} calculated by the input calculation unit 54.

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

The feedback controller 52 includes a target ammonia concentration setting unit 521 and a sliding mode controller 522.
The target ammonia concentration setting unit 521 sets a target value NH3 _{CONS_TRGT} of the detection value NH3 _CONS of the NH ₃ sensor. More specifically, the target value NH3 _{CONS_TRGT} is set to a value slightly larger than “0”, for example.
The sliding mode controller 522 calculates the FB injection amount G _{UREA_FB} so that the detection value NH 3 _CONS of the NH ₃ sensor converges to the set target ammonia concentration NH 3 _{CONS_TRGT} .

The feedforward controller 53 determines the engine speed NE and the engine speed NE so that the NOx purification rate in the first and second selective reduction catalysts is maintained at the maximum value according to the amount of NOx in the exhaust gas that varies depending on the operating state of the engine. The FF injection amount _{GUREA_FF} is calculated based on the load parameter TRQ representing the engine load. That is, the FF injection amount _{GUREA_FF} corresponds to the urea water injection amount calculated so that the equivalence ratio α is “1”.

The storage correction input calculation unit 54 estimates the first storage amount of the first selective reduction catalyst, and calculates the corrected injection amount G _{UREA_ST} so that the estimated first storage amount converges to a predetermined target storage amount. The storage correction input calculation unit 54 estimates the maximum storage capacity of the first selective reduction catalyst that changes according to the temperature of the selective reduction catalyst, and is equal to or slightly more than the estimated storage capacity of the target storage amount. Set to a small value.

Therefore, by injecting an amount of urea water corresponding to the normal injection amount G _{UREA_ORD} determined as described above, the first selective reduction catalyst has adsorbed an amount of NH ₃ close to its maximum storage capacity. Thus, the NOx purification rate in the first selective reduction catalyst can be maintained high. Further, by setting the target value NH3 _{CONS_TRGT} of the detection value of the NH ₃ sensor to a value slightly larger than “0”, the amount of NH ₃ flowing into the second selective reduction catalyst can be reduced, so that the second The storage amount can be kept small. Therefore, even if the temperature of the first selective reduction catalyst rises due to high load operation or the like and NH ₃ adsorbed on the first selective reduction catalyst slips, it can be adsorbed by the second selective reduction catalyst. Excessive ammonia slip from the tail pipe can be suppressed.

It should be _noted that the specific method for calculating the normal injection amount _{GUREA_ORD} as described above, that is, the specific configuration of the feedback controller 52, the feedforward controller 53, and the storage correction input calculation unit 54, and the first selective reduction catalyst. Since a specific method for estimating the storage amount and the maximum storage capacity is described in detail in International Publication No. 2008/57628 by the applicant of the present application, further detailed description is omitted here.

FIG. 6 is a block diagram illustrating a configuration of the limited injection amount calculation unit 57.
The limit injection amount calculation unit 57 includes a unit conversion unit 571, an upper limit calculation unit 572 for calculating an upper limit injection amount _{GUREA_ULIM} corresponding to an upper limit value for the urea injection amount during regeneration of the DPF, and a lower limit value for the normal injection amount. A lower limit value calculation unit 576 that calculates a lower limit injection amount G _{UREA_LLIM} corresponding to.

Based on the intake air amount QA, the unit converter 571 converts the NOx sensor detected value NOX _CONS corresponding to the NOx concentration in the exhaust gas into the NOx amount QNOX corresponding to the mass flow rate. Since the NOx amount QNOX calculated by the unit converter 571 corresponds to the NOx amount discharged from the engine and flowing into the first selective reduction catalyst, it is hereinafter referred to as “feed NOx amount”.

The upper limit calculation unit 572 includes a reducible NOx amount calculation unit 573, a consumed NH ₃ amount calculation unit 574, and an injection amount conversion unit 575.

The reducible NOx amount calculation unit 573 includes a NO ₂ / NO amount calculation unit 573a and a NOx purification rate calculation unit 573b, so that the NO amount that can be reduced by the first and second selective reduction catalysts out of the inflowing NOx. corresponding NO reducible amount _{QNO red_max} and NO ₂ reducible amount _{QNO2 red_max} corresponding to NO ₂ amount is calculated.

The NO ₂ / NO amount calculation unit 573a searches a predetermined map based on the feed NOx amount QNOX and the oxidation catalyst temperature T _DOC , and thereby the NO inflow amount QNO _IN corresponding to the NO amount flowing into the first selective reduction catalyst. If, to calculate the NO ₂ inflow QNO2 _iN corresponding to NO ₂ amount flowing in the first selective reduction catalyst.
NOx purification ratio calculating unit 573b, by searching a predetermined map based on the selective reduction catalyst temperature T _SCR and the space velocity SV of the parameter correlated with the operating state of the engine, in the first, second selective reduction catalyst The NOx purification rate η _MAX is calculated.
The above-described NO reducible amount QNO _{RED_MAX} and NO ₂ reducible amount QNO2 _{RED_MAX} are represented by the following formulas (2-1) and (2-2): NO inflow amount QNO _IN and NO ₂ inflow amount QNO2 _IN , By multiplying by the NOx purification rate η _MAX .

The consumption NH ₃ amount calculation unit 574 assumes that NO and NO ₂ are reduced by NH ₃ under the reactions as shown in the following formulas (3-1) to (3-3), so that the above NO reducible amount _{QNO red_max} NO, the and NO ₂ reducible amount _{QNO2 red_max} of NO ₂ to calculate the amount of NH ₃ is consumed for the reduction, which is referred to as NH ₃ maximum consumption _{QNH3 rED_MAX.} This NH ₃ maximum consumption amount QNH3 _{RED_MAX} corresponds to the maximum value of the NH ₃ amount that can be consumed by the first and second selective reduction catalysts.

The injection amount conversion unit 575 calculates the upper limit injection amount G _{UREA_ULIM} by converting the NH ₃ maximum consumption amount QNH3 _{RED_MAX} into the mass of urea water. More specifically, urea water (CO (NH ₂ ) ₂ + H ₂ O) having a predetermined concentration injected from the urea injection device is thermally decomposed or hydrolyzed in the exhaust pipe and in the first and second selective reduction catalysts. Finally, as shown in the following formula (4), by assuming that NH ₃ and carbon dioxide (CO ₂ ) are generated, the upper limit injection amount G _{UREA_ULIM is obtained} from the NH ₃ maximum consumption amount QNH _{3 RED_MAX.} Is calculated.

The lower limit calculation unit 576 includes a reduction request NOx amount calculation unit 577, a consumed NH ₃ amount calculation unit 578, and an injection amount conversion unit 579.

The reduction request NOx amount calculation unit 577 includes a request purification rate calculation unit 577b that outputs a smaller value than the above-described NOx purification rate calculation unit 573b, and thereby the first and second selective reduction catalysts out of the inflowing NOx. calculating the NO ₂ reduction demand _{QNO2 RED_REQ} corresponding to NO reduction demand _{QNO RED_REQ} and NO ₂ amount equivalent to amount of NO to be minimally reduced is required.

The required purification rate calculation unit 577b searches for a predetermined map based on the exhaust gas temperature TGAS and the space velocity SV, thereby obtaining a request corresponding to the minimum required value as the NOx purification rate in the first and second selective reduction catalysts. The purification rate η _REQ is calculated. Thus, since the required purification rate η _REQ corresponds to the minimum required NOx purification rate, the NOx purification rate calculated by the NOx purification rate calculation unit 573b under the same exhaust temperature TGAS and space velocity SV. The value is smaller than the rate η _MAX .

The above-described NO reduction request amount QNO _{RED_REQ} and NO ₂ reduction request amount QNO2 _{RED_REQ} are expressed as NO inflow amount QNO _IN and NO ₂ inflow amount QNO2 _IN as shown in the following equations (5-1) and (5-2). , By multiplying the required purification rate η _REQ .

Consumption NH ₃ amount calculation unit 578, similarly to the consumption NH ₃ amount calculating unit 574 described above, is consumed NO of NO reduction demand _{QNO RED_REQ,} and NO ₂ reduction demand _{QNO2 RED_REQ} of NO ₂ to reduction NH ₃ amount is calculated, and this is set as NH ₃ required consumption amount QNH _{3 RED_REQ} . This NH ₃ required consumption amount QNH3 _{RED_REQ} corresponds to the NH ₃ amount that needs to be supplied at a minimum in order to maintain the NOx purification rate in the first and second selective reduction catalysts higher than the required purification rate η _REQ .

The injection amount conversion unit 579 calculates the lower limit injection amount G _{UREA_LLIM} by converting the NH ₃ required consumption amount QNH3 _{RED_REQ} to the mass of urea water in the same manner as the injection amount conversion unit 575 described above.

Returning to FIG. 4, the injection amount restriction unit 59 will be described.
While the DPF regeneration flag F _DPFREG is “0”, that is, when the DPF regeneration is not being performed, the injection amount limiting unit 59 sets the normal injection amount G _{UREA_ORD} to the urea injection amount G _{UREA as} shown in the following equation (6). Determine as.

Further, when the DPF regeneration flag F _DPFREG becomes “1”, that is, during the DPF regeneration, as shown in the following equation (7), the upper limit injection amount G _{UREA_ULIM} and the lower limit injection amount G _{UREA_LLIM} are limited. The normal injection amount G _{UREA_ORD} is determined as the urea injection amount G _UREA .

[Intake control unit 6]
Next, a block related to the fresh air amount control of the engine 1 in the intake control unit 6 will be described.
FIG. 7 is a block diagram related to the determination of the EGR valve opening VO for controlling the fresh air amount of the engine to a predetermined target fresh air amount QNEW in the intake air control unit 6.

As shown in FIG. 7, the valve opening degree VO includes a normal opening degree VO _ORD calculated by the normal opening degree calculation unit 61 and a closing side correction opening degree VO _CLO calculated by the closing side correction opening degree calculation unit 62. Are selected by the opening selector 63.

The normal opening calculation unit 61 receives the target fresh air amount QNEW, which is the target value of the engine fresh air amount, calculates an appropriate EGR valve opening according to the target fresh air amount QNEW, and normally opens it. Output as degree VO _ORD . This target fresh air amount QNEW is determined by a process (not shown) according to the driving state of the vehicle.

The close side correction opening calculation unit 62 is more than the normal opening VO _ORD based on the NH ₃ slip speed VNH3 _SLIP at the time of temperature increase, the NH ₃ consumption speed VNH3 _{RED at the time of} temperature increase, and the target fresh air amount QNEW. The closed side corrected opening degree VO _CLO corrected to the closed side is calculated.
Here, the NH ₃ slip speed VNH3 _SLIP at the time of temperature increase is the first and second selective reduction in the process of increasing the regeneration temperature of the DPF and the temperature of the first and second selective reduction catalysts is rapidly increased. This corresponds to the amount of NH ₃ discharged from the catalyst to the downstream side per unit time, and is calculated by the slip speed calculation unit 64 (see FIG. 8 described later).
Further, the NH ₃ consumption rate VNH3 _RED at the time of temperature increase refers to the reduction of inflowing NOx in the first and second selective reduction catalysts in which the temperature of the DPF is increasing and the temperature is rapidly increasing. This corresponds to the amount of NH ₃ that can be consumed by the unit of time, and is calculated by the consumption rate calculation unit 65 (see FIG. 9 described later).

The closed side corrected opening degree calculation unit 62 includes a surplus NH ₃ amount calculation unit 621, a reaction model calculation unit 622, an additional fresh air amount calculation unit 623, a target new air amount correction unit 624, and an opening degree calculation unit 625. It is comprised including.
Surplus NH ₃ amount calculation unit 621, as shown in the following formula (8), by subtracting the in heating NH ₃ slip speed _VNH3 during heating NH ₃ consumption rate from _{SLIP VNH3 RED,} excess amount of NH ₃ DerutaNH ₃ Is calculated. The surplus NH ₃ amount ΔNH3 calculated here corresponds to the amount of NH ₃ per unit time that can be exhausted to the downstream side of the first and second selective reduction catalysts that cannot be consumed only by NOx that is currently flowing into the first and second selective reduction catalysts. . That is, in order to prevent the adsorbed NH _{3 from} being discharged in the process in which the temperature of the first and second selective reduction catalysts rapidly increases during the regeneration temperature increase of the DPF, it is necessary to consume extra. This corresponds to a certain amount of NH ₃ per unit time.

The reaction model calculation unit 622 assumes that the excess NH ₃ is reduced by assuming that NO and NO ₂ are reduced by NH ₃ under the reactions shown in the above formulas (3-1) to (3-3). The amount of NOx required to consume the amount ΔNH3 of NH ₃ is calculated, and this is set as the additional NOx amount ΔNOX.

  The additional fresh air amount calculation unit 623 calculates an additional amount of fresh air necessary to add NOx supplied to the first and second selective reduction catalysts by an amount corresponding to the additional NOx amount ΔNOX. This is defined as an additional fresh air amount ΔQNEW.

The target fresh air amount correction unit 624 adds the additional fresh air amount ΔQNEW to the target fresh air amount QNEW as shown in the following formula (9), thereby correcting the new fresh air amount corresponding to the target fresh air amount correction value. QNEW _COR is calculated.

The opening calculation unit 625 receives the corrected fresh air amount QNEW _COR , calculates an appropriate EGR valve opening corresponding to the corrected fresh air amount QNEW _COR , and outputs this as a closed correction opening VO _CLO . When the temperature of the DPF is started and the temperature of the first and second selective reduction catalysts rises, the NH ₃ adsorbed up to that time is discharged, so that the surplus NH ₃ amount ΔNH ₃ becomes a positive value, As a result, the additional fresh air amount ΔQNEW may be a positive value. At this time, the corrected fresh air amount QNEW _COR becomes a value larger than the target fresh air amount QNEW before being corrected, so that the closed-side corrected opening degree VO _CLO becomes the target fresh air amount QNEW before being corrected. Accordingly, the normal opening degree VO _ORD calculated according to is corrected to the closing side.

The opening selector 63 is configured to calculate the normal opening calculated by the normal opening calculation unit 61 in accordance with the NH ₃ slip speed VNH3 _SLIP at the time of temperature increase, the NH ₃ consumption speed VNH3 _RED at the time of temperature increase, and the DPF temperature increase flag F _RISE. One of the opening degree VO _ORD and the closing side correction opening degree VO _CLO calculated by the closing side correction opening degree calculation unit 62 is selected and determined as the EGR valve opening degree VO.

More specifically, when the DPF temperature increase flag F _RISE is “0”, the opening selector 63 sets the temperature increase NH ₃ slip speed VNH3 _SLIP and the temperature increase NH ₃ consumption speed VNH3 _RED regardless of the values. The normal opening VO _ORD is determined as the EGR valve opening VO.
On the other hand, when the DPF temperature increase flag F _RISE is “1”, that is, while the regeneration temperature of the DPF is being increased and the temperature of the first and second selective reduction catalysts is rapidly increasing, ₃ slip speed VNH3 _SLIP is compared with NH ₃ consumption speed VNH3 _RED at the time of temperature increase, and when the NH ₃ slip speed VNH3 _{SLIP at the time} of temperature increase is equal to or less than the NH ₃ consumption speed VNH3 _{RED at the time of} temperature increase, the normal opening VO _ORD is The EGR valve opening degree VO is determined, and when the temperature rising NH ₃ slip speed VNH3 _SLIP is higher than the temperature rising NH ₃ consumption speed VNH3 _RED , the closing side correction opening degree VO _CLO is determined as the EGR valve opening degree VO.

FIG. 8 is a block diagram showing a configuration of the slip speed calculation unit 64.
Slip speed calculating section 64, a NH ₃ sensor base calculation unit 641 which calculates a slip speed based on the detected value _{NH3 CONS} of the NH ₃ sensor, catalyst temperature based calculation for calculating a slip rate based on the selective reduction catalyst temperature _{T SCR} And a comparator 643 that compares the output of the NH ₃ sensor base calculation unit 641 with the output of the catalyst temperature base calculation unit 642 and outputs the larger one.

The NH ₃ sensor base calculation unit 641 converts the detected value NH 3 _CONS corresponding to the NH ₃ concentration in the exhaust gas into the NH ₃ amount QNH ₃ by the unit conversion unit, and, as shown in the following formula (10), this NH ₃ amount calculating a differential value of QNH3, which is referred to as NH ₃ sensor base slip speed DQNH3.
Incidentally, since the NH ₃ sensor detects the NH ₃ concentration between the first selective reduction catalyst and the second selective reduction catalyst, the NH ₃ sensor base slip speed dQNH ₃ is selected from the first selective reduction catalyst. This corresponds to the amount of NH ₃ discharged per unit time toward the reduction catalyst. In the present embodiment, as described above, the first selective reduction catalyst performs control to suppress the amount of NH ₃ flowing into the second selective reduction catalyst as much as possible while maintaining the storage amount close to the maximum storage capacity. In the second selective reduction catalyst, a large amount of NH ₃ is not adsorbed. Thus, during playback heating the DPF, NH ₃ discharged from the entire the combined first selective reduction catalyst and the second selective reduction catalyst, and many are NH _3, which has been adsorbed by the first selective reduction catalyst it is conceivable that. Therefore, NH ₃ sensor base slip speed dQNH3 of NH ₃ following equation which is calculated based on the detected value _{QNH3 CONS} of the sensor (10) is in the playing heating of the DPF, the first selective reduction catalyst and the second selective reduction This is considered to correspond to the slip speed of NH ₃ discharged from the whole catalyst.

The catalyst temperature base calculation unit 642 calculates a maximum storage capacity ST _{NH3_MAX} of the first selective reduction catalyst by searching a predetermined map based on the selective reduction catalyst temperature T _SCR and, as shown in the following formula (11), A differential value of the maximum storage capacity ST _{NH3_MAX} is calculated and is set as a catalyst temperature base slip speed dST _{NH3_MAX} .
As described above, in the present embodiment, the first selective reduction catalyst performs control to suppress the amount of NH ₃ flowing into the second selective reduction catalyst as much as possible while maintaining the storage amount close to the maximum storage capacity. In the second selective reduction catalyst, a large amount of NH ₃ is not adsorbed. Therefore, during the regeneration temperature increase of the DPF, NH ₃ discharged from the whole of the first selective reduction catalyst and the second selective reduction catalyst increases the temperature of the first selective reduction catalyst and decreases its maximum storage capacity. As a result, NH ₃ is considered to be no longer able to adsorb. For this reason, the catalyst temperature base slip speed dST _{NH3_MAX} of the following equation (11) calculated based on the selective reduction catalyst temperature T _SCR indicates that the first selective reduction catalyst and the second selective reduction catalyst are used during the regeneration temperature _increase of the DPF. This is considered to correspond to the slip speed of NH ₃ discharged from the whole.

The comparator 643 compares the NH ₃ sensor base slip speed dQNH3 with the catalyst temperature base slip speed dST _{NH3_MAX,} and outputs the larger one as the temperature rising NH ₃ slip speed VNH3 _SLIP . In this way, by estimating the slip speed by a plurality of different methods and using the larger one of them, it is possible to more reliably suppress excessive ammonia slip at the time of the DPF regeneration temperature increase.

FIG. 9 is a block diagram illustrating a configuration of the consumption speed calculation unit 65.
The consumption rate calculation unit 65 includes a unit conversion unit 654, a NOx amount calculation unit 651, and a reaction model calculation unit 652, and calculates the NH ₃ consumption rate VNH3 _RED during temperature rise using these units.
The unit conversion unit 654 calculates the feed NOx amount QNOX in the same manner as the unit conversion unit 571 described above.

The temperature increase NOx amount calculation unit 651 includes a NO ₂ / NO amount calculation unit 651a and a temperature increase NOx purification rate calculation unit 651b, and thereby, the first NOx that flows in during the regeneration temperature increase of the DPF, to calculate a second selection time reduction catalyst by raising the temperature corresponding to the reducible NO amount NO reducible amount _{QNO RED_RISE} and during heating corresponding to NO ₂ amount NO ₂ reducible amount _{QNO2 RED_RISE.}

The NO ₂ / NO amount calculation unit 651a searches for a predetermined map based on the feed NOx amount QNOX and the oxidation catalyst temperature T _DOC , and thereby the NO inflow amount QNO _IN corresponding to the NO amount flowing into the first selective reduction catalyst. If, to calculate the NO ₂ inflow QNO2 _iN corresponding to NO ₂ amount flowing in the first selective reduction catalyst.
During heating the NOx purification ratio calculating unit 651b, by searching a predetermined map based on the selective reduction catalyst temperature _{T SCR} and space velocity SV, the first playing heating the DPF, NOx in the second selective reduction catalyst The purification rate η _RISE is calculated.
During heating NO reducible amount above _{QNO RED_RISE} and in heating NO ₂ reducible amount _{QNO2 RED_RISE} the following formulas (12-1), as shown in (12-2), NO inflow _{QNO IN} and and NO ₂ is calculated by multiplying the inflow amount QNO2 _IN by the NOx purification rate η _{RISE during} temperature increase.

The reaction model calculation unit 652 assumes that NO and NO ₂ are reduced by NH ₃ under the reactions shown in the above formulas (3-1) to (3-3). NO reducible amount _{QNO RED_RISE} NO, the and by calculating the amount of NH ₃ is consumed to reduce in heating NO ₂ reducible amount _{QNO2 RED_RISE} of NO _2, the in heating NH ₃ consumption rate _{VNH3 RED} calculate.

FIG. 10 is a flowchart showing a procedure of urea injection control by the urea injection control unit described above. This urea injection control is to determine the urea injection amount G _UREA by the above-described method, and is executed every predetermined control cycle.

In S1, it is determined whether or not the apparatus related to urea injection control is normal. More specifically, for example, whether or not the urea injection device is normal, whether or not the first and second selective reduction catalysts are deteriorated and failed, and the remaining amount of urea water in the urea tank is a specified value. Whether or not warming up after engine startup has been completed, whether or not various sensors such as the NH ₃ sensor, NOx sensor, exhaust temperature sensor have failed, and these sensors have become active. Whether or not the temperature of the first and second selective reduction catalysts is equal to or higher than a specified temperature. If the determination in S1 is NO and it is determined that the apparatus is not normal, the _{process proceeds} to S2, and the urea injection amount _GUREA is set to “0”, and then this process ends. If the determination in S1 is YES and it is determined that the apparatus is normal, the process proceeds to S3.

In S3, it is determined whether or not the DPF temperature increase flag F _RISE is “1”. If this determination is YES and the regeneration temperature of the DPF is being increased, the _{process proceeds} to S2 in order to actively consume NH ₃ adsorbed on the first and second selective reduction catalysts, and the urea injection amount G _UREA. Is set to “0”. If the determination in S3 is NO, the process moves to S4.

In S4, the normal injection amount _{GUREA_ORD} is calculated, and the _{process proceeds} to S5. In S5, it is determined whether or not the DPF regeneration flag F _DPFREG is “1”. When this determination is NO, the _{process proceeds} to S6, and the normal injection amount G _{UREA_ORD} is determined as the urea injection amount G _UREA . If the determination in S5 is YES, the process moves to S7.

In S7, an upper limit injection amount _{GUREA_ULIM} and a lower limit injection amount _{GUREA_LLIM} are calculated, and the _{process proceeds} to S8. In S8, it is determined whether or not the normal injection amount G _{UREA_ORD} is larger than the upper limit injection amount G _{UREA_ULIM} . If this determination is YES, it is determined that there is a possibility that NH ₃ slip may occur if urea water is injected in an amount corresponding to the normal injection amount G _{UREA_ORD} during DPF regeneration. G _{UREA_ULIM} is determined as the urea injection amount G _UREA . If the determination in S8 is NO, the process moves to S10.

In S10, it is determined whether or not the normal injection amount G _{UREA_ORD} is smaller than the lower limit injection amount G _{UREA_LLIM} . If this determination is YES, it is determined that there is a possibility that the NOx purification rate may be excessively reduced if the amount of urea water corresponding to the normal injection amount G _{UREA_ORD} is injected during DPF regeneration, and the process _proceeds to S11. The lower limit injection amount G _{UREA_LLIM} is determined as the urea injection amount G _UREA . If the determination in S10 is NO, the process proceeds to S5 described above.

FIG. 11 is a diagram showing a part of a flowchart showing a procedure of new air amount control by the intake control unit described above. This new air amount control is to determine the EGR valve opening VO by the above-described method, and is executed every predetermined control cycle.
In S21, it is determined whether or not the DPF temperature increase flag F _RISE is “1”, that is, whether or not the DPF regeneration temperature is being increased. If this determination is YES, the process moves to S22, and if NO, the process moves to S23. In S22, it is determined whether or not the temperature rising NH ₃ slip speed VNH3 _SLIP is higher than the temperature rising NH ₃ consumption speed VNH3 _RED . If this determination is YES, the process proceeds to S24, and if NO, the process proceeds to S23.

In S23, the normal opening degree VO _ORD calculated in accordance with the target fresh air amount QNEW determined by a process (not shown) is determined as the EGR valve opening degree VO.
On the other hand, in S24, when it is determined that the regeneration temperature of the DPF is being increased and excessive NH ₃ slip may occur in the first and second selective reduction catalysts, _In order to suppress ₃ slips, the closed side corrected opening degree VO _CLO calculated according to the corrected fresh air quantity QNEW _COR obtained by correcting the target fresh air quantity to the increasing side is determined as the EGR valve opening degree VO. As a result, the amount of NOx flowing into the first and second selective reduction catalysts increases, and excessive NH ₃ slip is suppressed.

According to this embodiment, there are the following effects.
(1) In the present embodiment, by setting the upper-limit injection amount _{G UREA_ULIM} that the upper limit value for the urea injection amount _{G UREA} during regeneration DPF, and a lower limit injection amount _{G UREA_LLIM} as the lower limit, the urea injection Since the amount G _UREA can be prevented from becoming excessive or conversely insufficient with _respect to the required amount, ammonia slip can be suppressed as a result while maintaining high NOx purification performance in the selective reduction catalyst.

In the present embodiment, the upper-limit injection amount _{G UREA_ULIM,} to a 1, NO ₂ reducible amount corresponding to NO reducible amount _{QNO red_max} and NO ₂ amount corresponding to reducible NO amount in the second selective reduction catalyst _{QNO2 red_max} Calculate based on Further, the lower limit injection amount _{G UREA_LLIM,} a 1, NO ₂ reduction request corresponding to the NO reduction demand _{QNO RED_REQ} and NO ₂ amount equivalent to amount of NO to be minimally reduced as required by the second selective reduction catalyst Calculate based on the quantity QNO2 _{RED_REQ} . Therefore, since the amount of reducing agent necessary for reducing NO and NO ₂ in the exhaust gas can be calculated with high accuracy, ammonia slip is suppressed while maintaining high NOx purification performance during regeneration of the DPF. In addition, the optimum upper limit injection amount G _{UREA_ULIM} and the lower limit injection amount G _{UREA_LLIM} can be calculated. The required amount of NH ₃ by dividing the NO and NO ₂ was accurately calculated as described above, accordingly possible to calculate the upper-limit injection amount _{G UREA_ULIM} and the lower limit injection amount _{G UREA_LLIM} adsorbs NH ₃ This is particularly effective during the regeneration of DPFs whose capabilities are significantly reduced.

Further, in the exhaust purification system of the present embodiment, the normal injection amount G _{UREA_ORD} is determined based on the output of the NH ₃ sensor provided between the first selective reduction catalyst and the second selective reduction catalyst. By performing feedback control using such an NH ₃ sensor, as described above, excessive ammonia slip from the second selective reduction catalyst is suppressed while maintaining the NOx purification rate of the first selective reduction catalyst high. be able to. However, during regeneration of the DPF, the ability of the first selective reduction catalyst to adsorb ammonia is lost, so the NH ₃ sensor reacts more sensitively than during the period during which the DPF is not being regenerated, so the normal injection amount G _{UREA_ORD} Tends to be corrected to the weight reduction side, and as a result, the NOx purification rate during regeneration of the DPF may decrease. Therefore, in the exhaust purification system of this embodiment that determines the urea injection amount based on the output of such an NH ₃ sensor, setting the lower limit injection amount G _{UREA_LLIM} as described above is during the regeneration of the DPF. Is particularly effective for maintaining a high NOx purification rate.

(2) In the present embodiment, in addition to the feed amount of NOx Qnox, based on the NOx purification rate calculated in accordance with the selective reduction catalyst temperature _{T SCR} and space velocity SV, the above-described NO reduction can amount _{QNO red_max,} NO ₂ By _calculating the reducible amount QNO2 _{RED_MAX} , the NO reduction request amount QNO _{RED_REQ} , and the NO ₂ reduction request amount QNO2 _{RED_REQ} , the minimum amount of NH ₃ required to reduce NO and NO ₂ in the exhaust gas is further increased. Since it can be calculated with accuracy, as a result, ammonia slip can be suppressed more reliably while maintaining the NOx purification performance of the selective reduction catalyst high.

(3) In the present embodiment, the NOx amount of the exhaust gas flowing into the first and second selective reduction catalysts is increased by correcting the fresh air amount of the engine to the increase side by the EGR device during the regeneration temperature increase of the DPF. Therefore, it is possible to actively consume NH ₃ that has been adsorbed before the start of the temperature rise of the DPF. Therefore, it can be suppressed that NH ₃ is not consumed for the reduction of NOx with the start of the temperature rise of the DPF and is discharged to the downstream side of the selective reduction catalyst.

(4) In the present embodiment, the NH ₃ discharge rate (NH ₃ slip rate VNH _{3 SLIP at the time of} temperature increase) from the first and second selective reduction catalysts during the regeneration temperature increase of the DPF is the first and second selections. When the consumption rate of NH _{3 in} the reduction catalyst is higher than the NH ₃ consumption rate (NH ₃ consumption rate VNH3 _{RED at the time of} temperature increase), the fresh air amount is corrected to the increase side and the NOx amount flowing into the first and second selective reduction catalysts is increased. As a result, the consumption rate of NH ₃ in the first and second selective reduction catalysts can be temporarily increased, so that excessive ammonia slip during the temperature increase of the DPF can be suppressed.

The present invention is not limited to the embodiment described above, and various modifications can be made.
In the above embodiment, the oxidation catalyst temperature T _DOC and the selective reduction catalyst temperature T _SCR are estimated based on the exhaust gas temperature TGAS, the engine speed NE, and the like. However, the present invention is not limited to this, and may be detected directly by a temperature sensor. .

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

In the above embodiment, the urea injection amount _GUREA is set to “0” when the regeneration temperature of the DPF is being increased in S3 of FIG. 10, but the present invention is not limited to this. During the regeneration temperature increase of the DPF, the temperatures of the first and second selective reduction catalysts also rise, and as a result, the respective maximum storage capacities also decrease. In the storage corrected input calculating unit 54 shown in FIG. 5, since calculates the correction injection amount G _{UREA - ST} as the urea injection amount G _UREA to suit reduction of such maximum storage capacity is corrected to reduction side, regeneration of the DPF During the temperature increase, as described above, the urea injection amount _GUREA is corrected to be “0” as a result even if it is not forced to be “0”.

1. Engine (internal combustion engine)
2 ... Exhaust gas purification system 11 ... Exhaust pipe (exhaust system)
12 ... Intake pipe (intake system)
17 ... EGR device (exhaust gas recirculation means)
22 ... DPF (filter)
23 ... urea selective reduction catalyst 231 ... first selective reduction catalyst (selective reduction catalyst)
232: Second selective reduction catalyst (selective reduction catalyst)
25 ... Urea injection device (reducing agent supply means)
26 ... NH ₃ sensor 28 ... NOx sensor (NOx amount acquisition means)
3 ... ECU
4. Exhaust system temperature controller (filter regeneration means)
5 ... Urea injection control unit 57 ... Limit injection amount calculation unit (limit value calculation means)
571 ... Unit converter (NOx amount acquisition means)
6 ... Intake control unit (correction means)

Claims (5)

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;
A filter provided in the exhaust system for collecting particulate matter in the exhaust;
Reducing agent supply means for supplying a reducing agent or a precursor thereof upstream of the selective reduction catalyst in the exhaust system;
An exhaust gas purification system for an internal combustion engine , comprising: a reducing agent sensor that outputs a detection signal substantially proportional to the concentration of the reducing agent in the exhaust downstream of the selective reduction catalyst ,
Filter regeneration means for burning and removing the particulate matter deposited by raising the temperature of the filter, and regenerating the filter;
Normal supply amount calculating means for calculating a normal supply amount of the reducing agent or precursor so that the value of the detection signal of the reducing agent sensor becomes a predetermined target value;
NOx amount acquisition means for acquiring the NOx amount in the exhaust gas flowing into the selective reduction catalyst;
Based on the acquired NOx amount, the NO amount and NO ₂ amount that can be reduced by the selective reduction catalyst are calculated, and further, the reducing agent during regeneration of the filter is calculated based on the calculated NO amount and NO ₂ amount. Or an upper limit calculating means for calculating an upper limit for the supply amount of the precursor;
Based on the obtained NOx amount, the reduction by the selective reduction catalyst as a small amount of reduction than reducible NO amount and NO ₂ amount is calculated amount of NO and NO ₂ amount is determined, reduction determined that the calculated Lower limit value calculating means for calculating a lower limit value for the supply amount of the reducing agent or precursor during regeneration of the filter, based on the NO amount and NO ₂ amount to be obtained ,
The reducing agent supply means is configured to regenerate the filter, when the normal supply amount is larger than the upper limit value, when the normal supply amount is smaller than the lower limit value. An exhaust gas purification system for an internal combustion engine, characterized by supplying an amount corresponding to the lower limit value, and otherwise supplying the normal supply amount of the reducing agent or precursor . The upper limit calculating means is configured to reduce the NOx that can be reduced based on the acquired amount of NOx and the NOx purification rate of the selective reduction catalyst calculated according to a parameter correlated with the operating state of the internal combustion engine. _2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the amount and the NO2 amount are calculated. The lower limit calculating means calculates a NOx purification rate required for the selective reduction catalyst as a purification rate lower than the calculated NOx purification rate based on a parameter correlated with the operating state of the internal combustion engine, Based on the acquired NOx amount and the calculated NOx purification rate required for the selective reduction catalyst, the NO amount and NO required for the reduction are determined. ₂ The exhaust gas purification system for an internal combustion engine according to claim 2, wherein the amount is calculated. Exhaust gas recirculation means for recirculating a part of the exhaust gas in the exhaust system to the intake system of the internal combustion engine;
The correction means for correcting the fresh air amount of the internal combustion engine to the increase side by the exhaust gas recirculation means while the temperature of the filter is raised immediately after the start of regeneration of the filter. 2. An exhaust gas purification system for an internal combustion engine according to 1. The correction means compares the reducing agent discharge rate from the selective reduction catalyst with the reducing agent consumption rate in the selective reduction catalyst while raising the temperature of the filter, and reduces the reducing agent discharge rate. 5. The exhaust gas purification system for an internal combustion engine according to claim 4 , wherein the amount of fresh air of the internal combustion engine is corrected to an increase side when the speed is higher than a consumption speed.

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