JP2012026375A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control system for an internal combustion engine that prevents a captured reducing agent from being discharged to the downstream side even when a drastic temperature change occurs in a selective reduction catalyst.SOLUTION: The exhaust emission control system includes: a first selective reduction catalyst that purifies exhaust under the presence of ammonia; a urea injection device that injects urea water to the upstream side of the first selective reduction catalyst; a catalyst-temperature prediction part 53 that predicts a predicted temperature equivalent to a temperature of the first selective reduction catalyst after the prediction time TPRE from the current time; a storage amount estimating part 56 that estimates a storage amount of the selective reduction catalyst; a target storage amount calculating part 55 that calculates a target storage amount on the basis of the estimated predicted temperature T; and a storage injection amount calculating part 52 that calculates a storage injection amount G, being a part of a urea injection amount Ginjected by the urea injection device, so that the estimated storage amount STmay become the target storage amount ST.

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 gas purification system, urea water 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 the exhaust gas. NOx is selectively reduced. 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. Further, as described in Patent Document 1, this limit amount has a characteristic of decreasing as the temperature of the selective reduction catalyst increases. Therefore, for example, when the temperature rises in a state where an amount of ammonia close to the limit amount is adsorbed to the selective reduction catalyst, a part of the adsorbed ammonia is discharged downstream. For this reason, the adsorption amount of ammonia in the selective reduction catalyst needs to be controlled to an appropriate amount according to the temperature so as not to exceed the limit amount.

JP 2009-293444 A

As described above, although the adsorption amount of ammonia can be controlled by increasing / decreasing urea water, the rate of change of the adsorption amount at this time is compared with the change rate of the limit amount due to the temperature change of the selective reduction catalyst. If the change is sudden, the change rate of the limit amount may be faster.
For example, when the selective reduction catalyst becomes hot, the limit amount decreases, so the adsorption amount is decreased faster than the reduction amount of the limit amount so that ammonia previously adsorbed on the selective reduction catalyst is not discharged downstream. There is a need. However, when the temperature suddenly rises as described above, the rate of decrease of the limit amount wins over the rate of decrease of the amount of adsorption, and ammonia that exceeds the limit amount is exhausted downstream without being completely adsorbed. It will end up.

  An object of the present invention is to provide an exhaust gas purification system for an internal combustion engine that can prevent the captured reducing agent from being discharged downstream even when the selective reduction catalyst undergoes a rapid temperature change. And

In order to achieve the above object, the present invention is provided in an exhaust system (for example, an exhaust passage 11 described later) of an internal combustion engine (for example, an engine 1 described later), purifies exhaust gas in the presence of a reducing agent, and performs this reduction. A selective reduction catalyst (for example, a first selective reduction catalyst 231 described later) that captures the agent, and a reducing agent (for example, ammonia) or an additive that is a source of the reducing agent upstream of the selective reduction 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) including a reducing agent supply means (for example, a urea injection device 25 to be described later) for supplying (for example, urea water) is provided. The exhaust purification system includes a temperature prediction unit (for example, a catalyst temperature prediction unit 53 described later) for estimating a predicted temperature corresponding to a temperature after a predetermined time (TPRE) from the present time of the selective reduction catalyst, and a selective reduction catalyst. Based on the storage amount estimation means (for example, a storage amount estimation unit 56 described later) that estimates the storage amount corresponding to the amount of the reducing agent that is captured, and the estimated temperature (T _{CAT — PRE} ), the storage amount Target storage amount calculation means for calculating a target storage amount corresponding to the target value of the target storage amount (for example, a target storage amount calculation unit 55 described later), and the estimated storage amount (ST _UREA ) is the calculated target storage amount ( so that the ST _{UREA - TRGT),} the supply amount of reducing agent or additive according to the reducing agent supply means _{(G UREA,} G Supply amount calculating means for calculating a _{REA_ST)} (e.g., includes the urea injection control unit 5 and a storage injection amount calculation section 52) described below, a.

  In the present invention, a predicted temperature corresponding to a temperature after a predetermined time from the present time of the selective reduction catalyst is estimated, a target storage amount is calculated based on the predicted temperature, and the reducing agent is supplied so that the storage amount becomes the target storage amount. The supply amount by means is calculated. Therefore, for example, when it is predicted that the temperature will suddenly rise after a predetermined time from the present time, the target storage amount is decreased according to this, and the storage amount is reduced according to the target storage amount. be able to. Therefore, since the actual storage amount can be reduced faster than the actual limit amount decreases after the predetermined time, it is possible to suppress the trapped reducing agent from being discharged from the selective reduction catalyst to the downstream side. it can.

  In this case, the exhaust purification system further includes exhaust system temperature control means (for example, an exhaust system temperature control unit 4 described later) for controlling the temperature of the exhaust system, and the temperature prediction means is the exhaust system temperature control means. It is preferable to set the target temperature of the selective reduction catalyst at the time of temperature control by the predicted temperature.

  In the present invention, while controlling the exhaust system temperature by the exhaust system temperature control means, the target temperature of the selective reduction catalyst at this time is set as the predicted temperature, the target storage amount is calculated based on the predicted temperature, and this target storage amount is calculated. Control the amount of storage so that In this way, by coordinating the control of the storage amount of the selective reduction catalyst with the temperature control of the exhaust system, the selective reduction catalyst captures as much reducing agent as possible and suppresses the reducing agent from being discharged downstream. be able to.

  In this case, the exhaust purification system further includes an exhaust purification filter (for example, DPF 22 described later) that is provided in the exhaust system and collects particulate matter in the exhaust, and the exhaust system temperature control means includes the exhaust system temperature control means. The temperature of the purification filter is raised, and the collected particulate matter is burned and removed to regenerate the exhaust purification filter. The temperature predicting means is a filter that has been learned in advance when the exhaust purification filter is regenerated. It is preferable to estimate the predicted temperature based on the temperature change mode of the selective reduction catalyst during regeneration.

  In the present invention, when the exhaust purification filter is regenerated, the temperature of the selective reduction catalyst is rapidly increased by estimating the predicted temperature based on the temperature change mode of the selective reduction catalyst at the time of filter regeneration learned in advance. I can predict that. Therefore, since the supply amount of urea water can be reduced before the temperature of the selective reduction catalyst rises according to this prediction, the reducing agent captured by the selective reduction catalyst before regeneration of the exhaust purification filter is It is possible to suppress discharge during reproduction.

  In this case, when the internal combustion engine is started, the temperature predicting means preferably estimates the predicted temperature based on the previously learned temperature change mode of the selective reduction catalyst when the internal combustion engine is started.

  In the present invention, when the internal combustion engine is started, it is possible to predict that the temperature of the selective reduction catalyst will increase by estimating the predicted temperature based on the temperature change mode of the selective reduction catalyst that was learned in advance when the internal combustion engine was started. . Therefore, since the supply amount of urea water can be reduced before the temperature of the selective reduction catalyst rises according to this prediction, the reducing agent captured by the selective reduction catalyst before the start of the internal combustion engine is reduced. It can suppress discharging at the time of starting.

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 the selective reduction catalyst which concerns on the said embodiment, and the temperature of a selective reduction catalyst. It is a block diagram which concerns on calculation of the urea injection quantity by the urea injection apparatus which concerns on the said embodiment. It is an example of the map for calculating the target storage amount which concerns on the said embodiment. It is a schematic diagram which shows the concept of the storage model of the selective reduction catalyst which concerns on the said embodiment. It is a time chart which shows the example of control of the exhaust gas purification system 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 with an oxidation catalyst 21 provided in the exhaust passage 11 of the engine 1 and a particulate matter (hereinafter referred to as “PM (Particulate Matter)”) provided in the exhaust passage 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 passage 11 and flows through the exhaust passage 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 as a reducing agent upstream of the urea selective reduction catalyst 23 in the exhaust passage 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). The urea injection valve 253 is connected to the ECU 3, operates in accordance with a control signal from the ECU 3, and injects urea water into the exhaust passage 11 in accordance with this control signal. That is, urea injection control is executed.

The oxidation catalyst 21 is provided on the upstream side of the DPF 22 in the exhaust passage 11 and oxidizes NO in the exhaust to convert it into NO ₂ , thereby promoting NOx reduction in the urea selective reduction catalyst 23. Further, the DPF 22 on the downstream side is heated by burning the unburned fuel supplied by performing post injection during DPF regeneration described later.
The DPF 22 is provided on the downstream side of the oxidation catalyst 21 in the exhaust passage 11, and when the exhaust passes through the fine holes in the filter wall, the PM mainly composed of carbon in the exhaust 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 passage 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 amount of ammonia adsorbed on the selective reduction catalyst is referred to as a storage amount, and the limit of the storage amount is referred to as a 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, in the selective reduction catalyst, when ammonia is generated exceeding the maximum storage capacity, the produced ammonia is discharged to the downstream side of the selective reduction catalyst. In this way, the fact that ammonia is not adsorbed by the selective reduction catalyst and is discharged downstream is referred to as “ammonia slip”. In the present embodiment, as will be described in detail later, by performing urea injection control so that the storage amount of the first selective reduction catalyst is maintained at a predetermined target value, the NOx purification rate of the urea selective reduction catalyst 23 as a whole is increased. While maintaining high, the occurrence of ammonia slip is suppressed as much as possible.

  The ECU 3 is connected to a NOx sensor 28, a crank angle position sensor 14, an accelerator opening sensor 15, an ignition switch 16, and the like.

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

  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 rotational speed of the engine 1 is calculated by the ECU 3 based on this pulse signal. The accelerator opening sensor 15 detects a depression amount of an accelerator pedal (not shown) of the vehicle (hereinafter referred to as “accelerator opening”) 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 ignition switch 16 is provided in a driver's seat of a vehicle (not shown), and transmits a signal for instructing start or stop of the engine to the ECU 3.

  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.

  Hereinafter, the configuration of the exhaust system temperature control unit 4 related to the execution of the exhaust system temperature control and the urea injection control unit 5 related to the execution of the urea injection control coordinated with the temperature control of the exhaust system, which is configured in the ECU 3. 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 in the selective reduction catalyst and the temperature of 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 PM accumulated in the DPF 22 increases, the pressure loss increases and the fuel consumption of the engine 1 deteriorates or the output decreases.

Accordingly, the exhaust system temperature control unit 4 integrates the PM accumulation amount QPM in the DPF 22 from the fuel injection amount, the engine speed, and the like, and regenerates the DPF 22 when the PM accumulation amount QPM exceeds a predetermined threshold value QPM _{REG_START.} It is determined that the time has come, and the DPF 22 is heated to regenerate the DPF by burning and removing the PM collected by the DPF 22. At this time, the exhaust system temperature control unit 4 sets about 600 ° C., which is the combustion temperature of PM, as the target temperature of the DPF 22, and the ignition timing of the engine 1 so as to maintain the temperature of the DPF 22 at this target temperature for a predetermined period. Or change the target air-fuel ratio or perform post-injection.

[Yurea injection control unit 5]
Next, the urea injection control unit 5 will be described.
FIG. 3 is a block diagram relating to calculation of the urea injection amount G _UREA by the urea injection device.
The urea injection control unit 5 includes a feedforward injection amount calculation unit 51, a catalyst temperature prediction unit 53, a catalyst temperature estimation unit 54, a target storage amount calculation unit 55, a storage amount estimation unit 56, and a storage injection amount calculation unit. 52.

As shown in FIG. 3 and the following formula (1), the urea injection amount _{G UREA} has a feedforward injection amount _{G UREA - FF} calculated by the feedforward injection amount calculation unit 51, the catalyst temperature prediction unit 53, the catalyst temperature estimating unit 54 And the storage injection amount G _{UREA_ST} calculated by the target storage amount calculation unit 55, the storage amount estimation unit 56, and the storage injection amount calculation unit 52.

Hereinafter, as will be described in detail, the feedforward injection amount _{GUREA_FF} corresponds to the urea injection amount necessary to reduce NOx discharged from the engine without excess or deficiency, and the storage injection amount _{GUREA_ST} is the first selection. This corresponds to a urea injection amount for maintaining the storage amount of the reduction catalyst at a target value described later.

  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.

Feedforward injection amount calculation unit 51 according to the calculation of the feedforward injection amount _{G UREA - FF,} as shown in the following formula (2), the detected value _{NOX CONS} of the NOx sensor, by multiplying the conversion factor _{K CONV_NOX_UREA,} feedforward The injection amount _{GUREA_FF} is determined. In the following equation (2), the conversion coefficient K _{CONV_NOX_UREA} is a conversion coefficient for converting from the NOx amount to the urea injection amount. More specifically, the conversion coefficient K _{CONV_NOX_UREA} is a urea injection amount necessary for reducing a predetermined amount of NOx.

Hereinafter, the _{configurations} of the catalyst temperature estimation unit 54, the catalyst temperature prediction unit 53, the target storage amount calculation unit 55, the storage amount estimation unit 56, and the storage injection amount calculation unit 52 related to the calculation of the storage injection amount G _{UREA_ST} will be described in order. .

The catalyst temperature estimation unit 54 estimates a current catalyst temperature _TCAT corresponding to the current temperature of the first selective reduction catalyst, based on a predetermined heat conduction model of the exhaust system in which the engine 1 and its exhaust are regarded as heat sources. As the heat conduction model, for example, an exhaust system model formulated according to Newton's cooling law as described in Japanese Patent Application Laid-Open No. 2006-250945 and Japanese Patent No. 4373909 by the applicant of the present application is used. . Here, as the input to the heat conduction model, parameters indicating the running state of the vehicle such as the engine speed, fuel injection amount, fuel injection timing, intake air amount, EGR amount, outside air temperature, and vehicle speed are used.

The catalyst temperature prediction unit 53 includes a first predicted temperature calculation unit 531, a second predicted temperature calculation unit 532, a third predicted temperature calculation unit 533, a fourth predicted temperature calculation unit 534, and a predicted temperature selector 535. Thus, a predicted temperature T _{CAT_PRE} corresponding to the temperature of the first selective reduction catalyst after a predetermined predicted time _TPRE from the _present is estimated. More specifically, among the first predicted temperature T _{CAT_PRE_1} , the second predicted temperature T _{CAT_PRE_2} , the third predicted temperature T _{CAT_PRE_3} , and the fourth predicted temperature T _{CAT_PRE_4} calculated by the predicted temperature calculation units 531 _,. The most suitable predicted temperature is selected by the predicted temperature selector 535, and this is output as the predicted temperature _{TCAT_PRE} .

The first predicted temperature calculation unit 531 outputs the target temperature of the first selective reduction catalyst at the time of temperature control by the exhaust system temperature control unit as the first predicted temperature T _{CAT_PRE_1} . As described above, the exhaust system temperature control unit basically controls the temperature of the first selective reduction catalyst to the target temperature except at the time of DPF regeneration. It can be said that this corresponds to the temperature of the first selective reduction catalyst in FIG.

The second predicted temperature calculation unit 532 estimates the temperature of the first selective reduction catalyst after the predicted time TPRE from the current time based on a model equivalent to the heat conduction model in the catalyst temperature prediction unit 53, and uses this as the second predicted temperature. _Output as _{TCAT_PRE_2} . Here, in order to predict the temperature of the first first selective reduction catalyst in the future based on the heat conduction model, the parameter indicating the traveling state of the vehicle as described above from the present time to after the predicted time TPRE is required. Become. Therefore, in the second predicted temperature calculation unit 532, assuming that the current traveling state is maintained until after the predicted time TPRE, a parameter indicating the traveling state from the present to the predicted time TPRE is assumed, By repeatedly calculating the heat conduction model based on this input, the temperature of the first selective reduction catalyst after the predicted time _TPRE is estimated from the present time, and this is output as the second predicted temperature _{TCAT_PRE_2} .

The third predicted temperature calculation unit 533 estimates the temperature of the first selective reduction catalyst after the predicted time TPRE from the current time based on the temperature increase mode of the first selective reduction catalyst at the time of DPF regeneration learned in advance. It outputs as 3rd estimated temperature _{TCAT_PRE_3} .

The fourth predicted temperature calculation unit 534 estimates the maximum temperature of the first selective reduction catalyst at the time of engine start according to the deviation between the target temperature and the temperature at the time of engine start estimated according to the operating state at the time of engine start. At the same time, the temperature of the first selective reduction catalyst after the predicted time TPRE from the present time is estimated based on the temperature learning mode of the first selective reduction catalyst at the time of engine start learned in advance and the estimated maximum temperature. 4 Output as predicted temperature _{TCAT_PRE_4} .
As will be described in detail later with reference to FIG. 6, the first selective reduction catalyst immediately after engine startup is in a state substantially equal to the outside air temperature sufficiently lower than the target temperature (about 250 ° C.). It is necessary to raise the temperature as quickly as overshoot occurs from the temperature and to activate the purification performance. The maximum temperature of the first selective reduction catalyst at the time of starting the engine is estimated in consideration of the occurrence of this overshoot.

First, when the ignition switch 16 is operated and an engine start request is detected, the predicted temperature selector 535 detects the start request and then detects the start temperature until the temperature of the first selective reduction catalyst converges near the target temperature. The higher one of the two predicted temperatures T _{CAT_PRE_2} and the fourth predicted temperature T _{CAT_PRE_4} is selected, and this is output as the predicted temperature T _{CAT_PRE} .
Thereafter, the predicted temperature selector 535 determines whether or not the PM deposition amount QPM exceeds a threshold value QPM _{REG_PRE} that is smaller than the above-described threshold value QPM _{REG_START} for determining the start of DPF regeneration, and the PM deposition amount QPM When the value exceeds the threshold value QPM _{REG_PRE} , it is predicted that DPF regeneration will start after the predicted time _TPRE , and the third predicted temperature T _{CAT_PRE_3} is selected according to this, and this is output as the predicted temperature T _{CAT_PRE} .
On the other hand, when the PM accumulation amount QPM is _{equal to} or less than the threshold value QPM _{REG_PRE} , the higher one of the first predicted temperature T _{CAT_PRE_1} and the second predicted temperature T _{CAT_PRE_2} is selected, and this is output as the predicted temperature T _{CAT_PRE} .

The target storage amount calculation unit 55 is estimated by the storage amount estimation unit 56 based on the predicted temperature T _{CAT_PRE} estimated by the catalyst temperature prediction unit 53 and the current catalyst temperature T _CAT estimated by the catalyst temperature estimation unit 54. It calculates a target storage amount _{ST UREA - TRGT} corresponding to the target value of the storage amount _{ST UREA.}

More specifically, by any higher of predicted temperature _{T CAT_PRE} and current catalyst temperature _{T CAT} defined as the temperature parameter _{T MAP,} searching a predetermined map based on the temperature parameter _{T MAP,} target storage The quantity ST _{UREA_TRGT} is calculated. In this way, by calculating the target storage amount ST _{UREA_TRGT} based on the higher one of the predicted temperature T _{CAT_PRE} and the current catalyst temperature T _CAT , the target storage amount can be estimated in consideration of the occurrence of the above-described overshoot. Since ST _{UREA_TRGT} can be calculated, it is possible to suppress the occurrence of ammonia slip even if the catalyst temperature actually increases thereafter.

FIG. 4 is an example of a map for calculating the target storage amount ST _{UREA_TRGT} . The solid line is the map value of the maximum storage capacity of the first selective reduction catalyst, and the broken line is the map value of the target storage amount. According to this map, the target storage amount is set to a value slightly smaller than the maximum storage capacity. Thus, the maximum storage capacity decreases as the catalyst temperature increases. Along with this, the map value of the target storage amount also decreases.

Returning to FIG. 3, the storage amount estimation unit 56 estimates a storage amount ST _UREA corresponding to the amount of ammonia adsorbed on the first selective reduction catalyst, based on the storage model of the selective reduction catalyst shown below.

FIG. 5 is a schematic diagram showing the concept of the storage model of the selective reduction catalyst.
This ammonia storage model is a model for estimating a change in the storage amount of ammonia in the selective reduction catalyst according to the urea injection amount with respect to the NOx amount of the exhaust gas flowing into the selective reduction catalyst. Specifically, the state of change of the storage amount in the selective reduction catalyst includes a state where the urea injection amount is appropriate with respect to a predetermined NOx amount (see FIG. 5A) and a state where the urea injection amount is excessive ( These are classified into three states, that is, a state in which the urea injection amount is insufficient (see FIG. 5C).

  As shown in FIG. 5A, when the urea injection amount is in an appropriate state with respect to NOx flowing into the selective reduction catalyst, that is, the amount of ammonia that can most efficiently reduce NOx in the exhaust gas, When the amount of ammonia generated from the supplied urea water substantially matches, there is no change in the storage amount.

  As shown in FIG. 5B, when the urea injection amount is excessive with respect to NOx flowing into the selective reduction catalyst, that is, the amount of ammonia generated from the supplied urea water is reduced in the exhaust gas. When the amount of NOx is larger than the amount that can be most efficiently reduced, this excess ammonia is adsorbed by the selective reduction catalyst. Therefore, in such an over-dosing state, the storage amount increases.

  As shown in FIG. 5C, when the urea injection amount is insufficient with respect to NOx flowing into the selective reduction catalyst, that is, the amount of ammonia generated from the supplied urea water is reduced in the exhaust gas. If the amount of NOx is less than the amount that can be most efficiently reduced, this deficiency is compensated by the adsorbed ammonia. Accordingly, in such an under-supply state, the storage amount decreases.

Returning to FIG. 3, the storage amount estimation unit 56 estimates the storage amount ST _UREA based on the above storage model. More specifically, it is calculated based on the following formulas (3) to (6).

First, the urea injection amount G _{UREA_IDEAL} (k) necessary for reducing NOx flowing into the selective reduction catalyst is calculated based on the detected value NOX _CONS of the NOx sensor as shown in the following equation (3). The

The excess urea injection amount D _UREA (k) that causes the storage amount to increase or decrease is _{calculated from} the actual urea injection amount G _UREA (k) as shown in the following equation (4). Calculated by subtracting _{UREA_IDEAL} (k).

Therefore, the estimated value ST _UREA (k) of the storage amount has the maximum storage capacity ST _{UREA_MAX} (k) as an upper limit value, and, as shown in the following formulas (5) and (6), the surplus amount of the urea injection amount D _UREA ( k).

Here, the maximum storage capacity ST _{UREA_MAX} (k) is set by searching the map as shown in FIG. 4 described above according to the catalyst temperature T _CAT .

Storage injection amount calculation unit 52, as estimated storage amount _{ST UREA} becomes the target storage amount _{ST UREA - TRGT} Thus, storage injection amount _{G UREA - ST} by proceeding as shown in the following formula (7) - (10) (K) is calculated.

First, as shown in the following equation (7), a deviation E _ST (k) between the estimated storage amount ST _UREA (k) and the calculated target storage amount ST _{UREA_TRGT} (k) is calculated.

Next, the product of the deviation E _ST (k) and the integral gain KI _ST is defined as an integral term G _{UREA_ST_I} (k) as shown in the following equation (8).

On the other hand, a differential value ST _UREA (k) −ST _UREA (k−1) of the estimated value of the storage amount is calculated, and this differential value is multiplied by the proportional gain KP _ST as shown in the following formula (9). , Defined as the proportional term G _{UREA_ST_P} (k).

Next, as shown in the following formula (10), the sum of the proportional term G _{UREA_ST_P} (k) and the integral term G _{UREA_ST_I} (k) is calculated and determined as the storage injection amount G _{UREA_ST} (k).

FIG. 6 is a time chart showing a control example of the exhaust purification system configured as described above. In FIG. 6, the upper row shows a target temperature (solid line) related to temperature control by the exhaust system temperature control unit, a predicted temperature T _{CAT_PRE} (broken line) estimated by the catalyst temperature prediction unit, and a current catalyst estimated by the catalyst temperature estimation unit. The time change of temperature _TCAT (a dashed-dotted line) is shown. The middle row shows the target storage amount (solid line) in the exhaust purification system of the present embodiment that calculates the target storage amount based on the predicted temperature T _{CAT_PRE} and the current catalyst temperature T _CAT , and only the current catalyst temperature T _CAT unlike the present embodiment. The time change with the target storage amount (broken line) in the exhaust purification system of the comparative example which calculates the target storage amount based on FIG. Moreover, the lower stage shows the time change of the ammonia slip amount (solid line) of the present embodiment and the ammonia slip amount (broken line) of the comparative example. In the following description, the engine is divided into three parts: at the time of engine start between times t0 and t3, at high load operation between times t4 and t6, and at DPF regeneration after time t8.

<When starting the engine>
When the ignition switch is turned on at time t0 and engine start is started, the exhaust system temperature control unit sets the target temperature to the optimum temperature at which NOx can be purified most efficiently by the selective reduction catalyst, and the first selective reduction catalyst. Is raised to this target temperature. In particular, since the temperature of the first selective reduction catalyst at time t0 immediately after engine startup is considerably lower than the target temperature, rapid temperature increase is performed. For this reason, the actual temperature of the first selective reduction catalyst overshoots the target temperature at time t2, and then converges to the target temperature at time t3 (see the upper dashed line).
In the catalyst temperature prediction unit, the fourth predicted temperature T _{CAT_PRE_4} calculated based on the temperature increase mode and the maximum temperature of the first selective reduction catalyst at the time of engine start learned in advance at the time of engine start and the second set as the target temperature. the predicted temperature _{T CAT_PRE} any higher of the predicted temperature _{T CAT_PRE_2.} Between time t0 and t1, since 2nd predicted temperature _{TCAT_PRE_2} is higher than 4th predicted temperature _{TCAT_PRE_4} (refer the thin broken line of the upper stage), target temperature becomes predicted temperature _{TCAT_PRE} . Thereafter, in the period from time t1 to t3, in response to the fourth predicted temperature _{T CAT_PRE_4} is higher than the target temperature, the fourth predicted temperature _{T CAT_PRE_4} is predicted temperature _{T CAT_PRE.} The temperature increase mode of the first selective reduction catalyst until the temperature of the first selective reduction catalyst at the start of the engine converges to the target temperature is unlikely to change greatly every time the engine is started. Even if it is calculated based on the maximum temperature, it is possible to predict the temperature after the predicted time TPRE from the present with a relatively high accuracy (see the upper broken line).
At this time, in the comparative example, the target storage amount is calculated based only on the current catalyst temperature _TCAT . Thus, at time t0 to t3, but gradually decreasing the target storage amount in accordance with the current catalyst temperature T _CAT rise (see a broken line of the middle), it can be reduced quickly in response to the actual storage amount to Absent. During this reason, the time t2 to t3, with the increase overshoot current catalyst temperature T _CAT is the target temperature, there are cases where a large ammonia slip occurs in the first selective reduction catalyst (see the broken line in the lower).
On the other hand, in the present embodiment, the target temperature is set to the predicted temperature T _{CAT_PRE} from time t0 to t1, and the target storage amount is calculated based on the predicted temperature (see the solid line in the middle stage), so that sufficient storage is _achieved. Along with securing the amount, ammonia slip can be suppressed to the maximum (see the solid line at the bottom). Further, after the fourth predicted temperature T _{CAT_PRE_4} exceeds the target temperature at the time t1, the target storage amount is calculated based on the predicted temperature T _{CAT_PRE} , thereby exceeding the current catalyst temperature T _CAT generated after the predicted time _TPRE. In preparation for the chute, the target storage amount can be set to an appropriate value (see the middle solid line), so that ammonia slip can be suppressed to the maximum (see the lower solid line).

Further, at times t3 to t4, the first selective reduction catalyst is heated to near the target temperature, and in response to the completion of the engine start, the catalyst temperature prediction unit sets the first temperature that is the target temperature in the exhaust system temperature control unit. 1 Predicted temperature _{TCAT_PRE_1} is _assumed to be predictive temperature _{TCAT_PRE} (see the solid line in the upper stage).

<During high load operation>
At time t4, the high load operation is started by the operation of the driver. At this time, the actual temperature of the first selective reduction catalyst begins to rise from time t5 with a predetermined delay, as shown in FIG. 6 (see the upper one-dot chain line). At this time, the catalyst temperature prediction unit sets the second predicted temperature T _{CAT_PRE_2} as the predicted temperature T _{CAT_PRE} . Since the second predicted temperature T _{CAT_PRE_2} is estimated based on the heat conduction model, the temperature increase of the first selective reduction catalyst after time t5 can be predicted from time t4 before the predicted time TPRE ( (See the upper dashed line).
At this time, in the comparative example, since the target storage amount is calculated based only on the current catalyst temperature _TCAT , the target storage amount is decreased after time t5 (refer to the broken line in the middle stage). Therefore, the control for decreasing the storage amount is performed. Starts after time t5. For this reason, after time t5, the temperature of the first selective reduction catalyst rises and the maximum storage capacity decreases, whereas ammonia that cannot be adsorbed slips (see the broken line in the lower stage).
In contrast, in this embodiment, the larger of the current catalyst temperature _{T CAT} and predicted temperature _{T CAT_PRE,} i.e. calculates a target storage amount based on the predicted temperature _{T CAT_PRE.} For this reason, it is possible to start the control for decreasing the target storage amount from time t4 (see the solid line in the middle stage) and reducing the storage amount accordingly. For this reason, even after the time t5, even if the temperature of the first selective reduction catalyst rises and the maximum storage capacity starts to decrease, much of the ammonia already adsorbed at this time is consumed for the reduction of NOx. Unlike the comparative example, a large ammonia slip does not occur (see the lower solid line).

Also, from time t6 to t7, the temperature of the first selective reduction catalyst is maintained at the target temperature by the exhaust system temperature control unit. As a result, the catalyst temperature prediction unit sets the first predicted temperature T _{CAT_PRE_1} , which is the target temperature in the exhaust system temperature control unit, as the predicted temperature T _{CAT_PRE} (see the upper broken line).

<DPF regeneration>
At time t8, DPF regeneration is started. At this time, the exhaust system temperature control unit performs control to raise the temperature of the DPF to the combustion temperature of PM, and the target temperature of the first selective reduction catalyst also rapidly increases. Further, when the DPF regeneration is started, the temperature of the first selective reduction catalyst rapidly rises faster than the speed at the time of the high load operation or the engine start described above (see the upper one-dot chain line).
In contrast, the catalyst temperature prediction unit predicts that DPF regeneration starts at time t7, which is before the prediction time TPRE of time t8, and accordingly, the first selective reduction catalyst at the time of DPF regeneration learned in advance. The third predicted temperature T _{CAT_PRE_3} calculated based on the temperature rise mode is _set as the predicted temperature T _{CAT_PRE} . The temperature increase mode of the first selective reduction catalyst until the temperature of the first selective reduction catalyst at the time of DPF regeneration reaches a target temperature near the combustion temperature of PM is unlikely to change greatly every time the DPF regeneration is performed. Even if the calculation is based on the learned temperature increase mode, the temperature after the prediction time TPRE can be predicted from the present with a relatively high accuracy (see the broken line in the upper stage).
At this time, in the comparative example, the target storage amount is calculated based only on the current catalyst temperature T _CAT that gradually rises above the predicted temperature T _{CAT_PRE} . Therefore, after the time t8, the target storage amount is decreased according to the current catalyst temperature T _CAT that rapidly increases (see the broken line in the middle stage), and the storage amount is reduced accordingly, but the storage amount is decreased. The rate at which the maximum storage capacity decreases is faster than the rate at which it is caused to occur, and thus excessive ammonia slip occurs (see the dashed line at the bottom).
In contrast, in this embodiment, the larger of the current catalyst temperature _{T CAT} and predicted temperature _{T CAT_PRE,} i.e. calculates a target storage amount based on the predicted temperature _{T CAT_PRE.} For this reason, the target storage amount is quickly set to be small from time t7 before the predicted time TPRE from time t8 when DPF regeneration actually starts (see the solid line in the middle stage). As described above, in this embodiment, since the target storage amount can be quickly set to be small before the DPF regeneration is started, the storage amount can be quickly reduced in accordance with the target storage amount. Thus, an excessive ammonia slip does not occur (see the solid line at the bottom).

The present invention is not limited to the embodiment described above, and various modifications can be made.
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 an additive that is the source of the reducing agent has been shown. It is not limited.
For example, ammonia may be supplied directly without supplying urea water and generating ammonia from the urea water. Moreover, as an additive used as the origin of ammonia, you may use not only urea water but another additive. Further, the reducing agent for reducing NOx is not limited to ammonia. The present invention can also be applied to an exhaust purification system using, for example, hydrocarbons instead of ammonia as a reducing agent for reducing NOx.

1. Engine (internal combustion engine)
11 ... Exhaust passage (exhaust system)
2 ... Exhaust gas purification system 22 ... DPF (Exhaust gas purification filter)
23 ... urea selective reduction catalyst 231 ... first selective reduction catalyst (selective reduction catalyst)
25 ... Urea injection device (reducing agent supply means)
3 ... ECU
4. Exhaust system temperature control unit (exhaust system temperature control means)
5. Urea injection control unit (supply amount calculation means)
52... Storage injection amount calculation unit 52 (supply amount calculation means)
53 ... Catalyst temperature prediction section (catalyst temperature prediction means)
55. Target storage amount calculation unit (target storage amount calculation means)
56: Storage amount estimation unit (storage amount estimation 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 reducing agent supply means for supplying a reducing agent or an additive that is a source of the reducing agent to the upstream side of the selective reduction catalyst in the exhaust system,
Temperature prediction means for estimating a predicted temperature corresponding to a temperature after a predetermined time from the present of the selective reduction catalyst;
Storage amount estimation means for estimating a storage amount corresponding to the amount of reducing agent trapped by the selective reduction catalyst;
A target storage amount calculating means for calculating a target storage amount corresponding to a target value of the storage amount based on the estimated predicted temperature;
A supply amount calculating means for calculating a supply amount of the reducing agent or additive by the reducing agent supply means so that the estimated storage amount becomes the calculated target storage amount. Engine exhaust purification system. An exhaust system temperature control means for controlling the temperature of the exhaust system;
2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the temperature prediction unit sets a target temperature of the selective reduction catalyst at the time of temperature control by the exhaust system temperature control unit as a predicted temperature. An exhaust purification filter that is provided in the exhaust system and collects particulate matter in the exhaust;
The exhaust system temperature control means raises the temperature of the exhaust purification filter, regenerates the exhaust purification filter by burning and removing the collected particulate matter,
The said temperature estimation means estimates the estimated temperature based on the temperature change aspect of the selective reduction catalyst at the time of filter regeneration learned in advance when the exhaust purification filter is regenerated. Exhaust gas purification system for internal combustion engines.   The said temperature prediction means estimates the estimated temperature based on the temperature change mode of the selective reduction catalyst at the start of the internal combustion engine learned in advance when the internal combustion engine is started. Exhaust gas purification system for internal combustion engines.

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