JP2019035380A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

To early eliminate expansion of an ammonia adsorption amount estimation error without deteriorating fuel economy.SOLUTION: An exhaust emission control device for an internal combustion engine 1 includes: a selective reduction type NOx catalyst 24 provided in an exhaust passage 4; a reducing agent injection valve 25 provided upstream of the NOx catalyst; an estimation section 100 for estimating ammonia adsorption amount of the NOx catalyst; a control section 100 for controlling the reducing agent injection amount injected from the reducing agent injection valve on the basis of the ammonia adsorption amount estimated by the estimation section; a determination section 100 for determining occurrence of ammonia slip in the NOx catalyst; and a change section 100 for changing an estimation value of the ammonia adsorption amount to a predetermined reference value when the determination section determines that the ammonia slip occurs. The change section calculates the reference value on the basis of the estimation value and a target value of the ammonia adsorption amount, a flow rate and a temperature of exhaust gas flowing into the NOx catalyst and NOx amount flowing into the NOx catalyst.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus provided with a selective reduction type NOx catalyst.

2. Description of the Related Art An exhaust gas purification device for an internal combustion engine is known that includes a selective reduction type NOx catalyst (so-called SCR) that reduces and purifies NOx in exhaust gas. On the upstream side of the NOx catalyst, urea water as a reducing agent is injected, and the urea water is hydrolyzed to generate ammonia (NH ₃ ). Ammonia reacts with NOx on the NOx catalyst to reduce and purify NOx.

  The NOx catalyst has an ammonia adsorption ability, and exhibits higher NOx purification performance as more ammonia is adsorbed. For this reason, the ammonia adsorption amount of the NOx catalyst is estimated, and the urea water injection amount is controlled so that the estimated adsorption amount approaches the target adsorption amount.

  Due to the expansion of the ammonia adsorption amount estimation error, the urea water injection amount may become excessive, and ammonia slip may occur in which ammonia flows out downstream of the catalyst. Conventionally, when this ammonia slip occurs, it has been proposed to raise the temperature of the NOx catalyst and initialize the estimated adsorption amount to zero (see, for example, Patent Document 1).

JP 2014-88800 A

  However, according to the prior art method, additional fuel injection is required to raise the temperature of the NOx catalyst, resulting in a deterioration in fuel consumption. Although it is possible to raise the temperature of the NOx catalyst by using an opportunity such as filter regeneration, it is difficult to eliminate the expansion of the ammonia adsorption amount estimation error at an early stage because such an opportunity is limited.

  Accordingly, the present invention has been made in view of the above circumstances, and an object thereof is to provide an exhaust gas purification apparatus for an internal combustion engine that can quickly eliminate an increase in an ammonia adsorption amount estimation error without deteriorating fuel consumption.

According to one aspect of the invention,
A selective reduction type NOx catalyst provided in the exhaust passage;
A reducing agent injection valve provided in the exhaust passage upstream of the NOx catalyst;
An estimation unit for estimating an ammonia adsorption amount of the NOx catalyst;
Based on the ammonia adsorption amount estimated by the estimation unit, a control unit for controlling the reducing agent injection amount injected from the reducing agent injection valve;
A determination unit for determining that ammonia slip is occurring in the NOx catalyst;
A change unit that changes the estimated value of the amount of adsorbed ammonia to a predetermined reference value when the determination unit determines that ammonia slip is occurring,
The changing unit calculates the reference value based on the estimated value and target value of the ammonia adsorption amount, the flow rate and temperature of the exhaust gas flowing into the NOx catalyst, and the NOx amount flowing into the NOx catalyst. An exhaust emission control device for an internal combustion engine is provided.

  Preferably, the changing unit calculates a base value of the reference value based on the estimated value and target value of the ammonia adsorption amount, and calculates a first correction coefficient based on the flow rate and temperature of the exhaust gas flowing into the NOx catalyst. The second correction coefficient is calculated based on the amount of NOx flowing into the NOx catalyst and the temperature of the exhaust gas flowing into the NOx catalyst, and the base value is multiplied by the first correction coefficient and the second correction coefficient to obtain the reference value. calculate.

  According to the present invention, the expansion of the ammonia adsorption amount estimation error can be eliminated at an early stage without deteriorating the fuel consumption.

It is the schematic which shows the structure of an internal combustion engine. It is a graph which shows the ammonia adsorption | suction characteristic of a NOx catalyst. The map for calculating a correction coefficient is shown. It is a graph which shows the relationship of each quantity with respect to urea water injection quantity. It is a time chart at the time of performing urea water injection amount control of this embodiment. It is a flowchart of a control routine. 7A shows a base map, FIG. 7B shows a first map, and FIG. 7C shows a second map.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. However, it should be noted that the present invention is not limited to the following embodiments.

  FIG. 1 shows an internal combustion engine to which the exhaust emission control device of this embodiment is applied. An internal combustion engine (also referred to as an engine) 1 is a multi-cylinder engine mounted on a vehicle (not shown). In the present embodiment, the vehicle is a large vehicle such as a truck, and the engine 1 as a vehicle power source mounted on the vehicle is an in-line four-cylinder diesel engine. However, there are no particular limitations on the types, types, applications, and the like of the vehicle and the internal combustion engine. For example, the vehicle may be a small vehicle such as a passenger car, and the engine 1 may be a gasoline engine.

  The engine 1 includes an engine body 2, an intake passage 3 and an exhaust passage 4 connected to the engine body 2, a turbocharger 14, and a fuel injection device 5. The engine body 2 includes structural parts such as a cylinder head, a cylinder block, and a crankcase, and movable parts such as a piston, a crankshaft, and a valve housed therein.

  The fuel injection device 5 includes a common rail fuel injection device, and includes a fuel injection valve, that is, an injector 7 provided in each cylinder, and a common rail 8 connected to the injector 7. The injector 7 is an in-cylinder injector that directly injects fuel into the cylinder 9, that is, into the combustion chamber. The common rail 8 stores the fuel injected from the injector 7 in a high pressure state.

  The intake passage 3 is mainly defined by an intake manifold 10 connected to the engine body 2 (particularly a cylinder head) and an intake pipe 11 connected to the upstream end of the intake manifold 10. The intake manifold 10 distributes and supplies the intake air sent from the intake pipe 11 to the intake ports of each cylinder. The intake pipe 11 is provided with an air cleaner 12, an air flow meter 13, a compressor 14 </ b> C of the turbocharger 14, an intercooler 15, and an electronically controlled intake throttle valve 16 in order from the upstream side. The air flow meter 13 is a sensor for detecting an intake air amount per unit time of the engine 1, that is, an intake flow rate, and is also referred to as a mass air flow (MAF) sensor or the like.

  The exhaust passage 4 is mainly defined by an exhaust manifold 20 connected to the engine body 2 (particularly a cylinder head) and an exhaust pipe 21 connected to the downstream side of the exhaust manifold 20. The exhaust manifold 20 collects exhaust gas sent from the exhaust port of each cylinder. A turbine 14 </ b> T of the turbocharger 14 is provided between the exhaust pipe 21 or between the exhaust manifold 20 and the exhaust pipe 21. In the exhaust passage 4 on the downstream side of the turbine 14T, an oxidation catalyst 22, a filter 23, a selective reduction type NOx catalyst (SCR) 24, and an ammonia oxidation catalyst 26 are provided in this order from the upstream side. These form post-processing members that perform exhaust post-processing. The exhaust passage 4 between the filter 23 and the NOx catalyst 24 is provided with a urea injector 25 as a reducing agent injection valve that injects urea water as a reducing agent into the exhaust passage 4.

  The oxidation catalyst 22 oxidizes and purifies unburned components (hydrocarbon HC and carbon monoxide CO) in the exhaust, and heats the exhaust gas with the reaction heat at this time. The filter 23 is a so-called continuous regeneration type diesel particulate filter that collects particulate matter (also referred to as PM) contained in the exhaust gas and continuously removes the collected PM by reacting with the precious metal. To do. The filter 23 is a so-called wall flow type in which the openings at both ends of the honeycomb structure base material are alternately closed in a checkered pattern.

The NOx catalyst 24 reduces and purifies NOx by reacting ammonia obtained by hydrolyzing the urea water injected from the urea injector 25 with NOx in the exhaust gas. The NOx catalyst 24 is a material in which a noble metal such as Pt is supported on the surface of a base material such as zeolite or alumina, a material in which a transition metal such as Cu is ion-exchanged on the surface of the base material, and titania on the surface of the base material. / Vanadium catalyst (V ₂ O ₅ / WO ₃ / TiO ₂ ) supported and the like. The ammonia oxidation catalyst 26 oxidizes and purifies excess ammonia discharged from the NOx catalyst 24.

  The engine 1 also includes an EGR device 30. The EGR device 30 includes an EGR passage 31 for returning a part of exhaust gas (referred to as EGR gas) in the exhaust passage 4 (especially in the exhaust manifold 20) to the intake passage 3 (particularly in the intake manifold 10), and EGR. An EGR cooler 32 that cools the EGR gas flowing in the passage 31 and an EGR valve 33 for adjusting the flow rate of the EGR gas are provided. The EGR device 30 is for executing external EGR.

  Further, the present embodiment includes an electronically controlled exhaust throttle valve 37 and an exhaust injector 38 provided in the exhaust passage 4 respectively. In the present embodiment, these are provided in the exhaust passage 4 between the turbine 14 </ b> T and the oxidation catalyst 22, and an exhaust injector 38 is disposed downstream of the exhaust throttle valve 37. However, these installation positions can be changed. The exhaust throttle valve 37 is a valve for adjusting the exhaust flow rate. The exhaust injector 38 is an injector for injecting fuel into the exhaust passage 4 mainly when the filter 23 is regenerated.

  A control device for controlling the engine 1 is mounted on the vehicle. The control device has an electronic control unit (referred to as ECU) 100 that forms a control unit or a controller. ECU 100 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like. The ECU 100 is configured and programmed to control the in-cylinder injector 7, the intake throttle valve 16, the urea injector 25, the EGR valve 33, the exhaust throttle valve 37, and the exhaust injector 38. Unless otherwise specified, it is assumed that the intake throttle valve 16 and the exhaust throttle valve 37 are controlled to be fully opened.

  The control device also has the following sensors. Regarding these sensors, in addition to the air flow meter 13 described above, a rotational speed sensor 40 for detecting the rotational speed of the engine, specifically a rotational speed per minute (rpm), and an accelerator opening degree are detected. An accelerator opening sensor 41 is provided. Further, exhaust temperature sensors 42, 43, 44 for detecting the exhaust temperature are provided at upstream side inlet portions of the oxidation catalyst 22, the filter 23 and the NOx catalyst 24. An exhaust temperature sensor 46 for detecting the exhaust temperature is provided at the downstream outlet of the NOx catalyst 24. Further, a differential pressure sensor 45 for detecting a differential pressure between the exhaust pressure at the inlet and the outlet of the filter 23 is provided.

  Further, an upstream side NOx sensor 47 and a downstream side NOx sensor 48 for detecting NOx in the exhaust gas are provided at the upstream side inlet portion and the downstream side outlet portion of the NOx catalyst 24, respectively. These NOx sensors 47 and 48 emit an output correlated with the NOx concentration of the exhaust gas. However, the NOx sensors 47 and 48 can also detect ammonia. The upstream NOx sensor 47 is provided on the upstream side of the urea injector 25. Output signals from the above sensors are sent to the ECU 100.

  Next, the contents of control executed by the ECU 100 will be described.

  First, an outline of control of the urea water injection amount injected from the urea injector 25 will be described. The urea water injection amount M is generally expressed as a sum of a first injection amount MA, a second injection amount MB, and a third injection amount MC, which will be described later, and is expressed by an equation: M = MA + MB + MC. The ECU 100 calculates the urea water injection amount M and causes the urea injector 25 to inject an amount of urea water equal to the calculated urea water injection amount M.

  First, the first injection amount MA corresponding to the NOx amount flowing into the NOx catalyst 24 (inflow NOx amount Min) is calculated. The inflow NOx amount Min is represented by the product of the NOx concentration detected by the upstream NOx sensor 47 and the exhaust gas flow rate. The exhaust gas flow rate is calculated based on the value of the intake air amount detected by the air flow meter 13. For example, the exhaust gas flow rate is calculated as a value equal to the intake air amount. A predetermined relationship between the inflow NOx amount Min and the first injection amount MA, specifically, a map (which may be a function, the same applies hereinafter) is stored in the ECU 100, and the ECU 100 refers to this map to determine the inflow NOx. A first injection amount MA corresponding to the amount Min is calculated. Here, the minimum injection amount required to reduce and purify the inflow NOx, in other words, the injection amount with an equivalence ratio of 1 to the inflow NOx amount Min is calculated as the first injection amount MA.

  The upstream NOx sensor 47 may be provided at a position upstream of the exhaust passage 4. The inflow NOx amount may be estimated by the ECU 100 based on the engine operating state (for example, the engine speed and the fuel injection amount of the in-cylinder injector 7). Further, the exhaust gas flow rate may be directly detected by a flow sensor provided in the exhaust passage 4.

  Second, a second injection amount MB for calculating the ammonia adsorption amount of the NOx catalyst 24 to be close to the target adsorption amount is calculated. That is, the NOx catalyst 24 has ammonia adsorption ability, and exhibits higher NOx purification performance as more ammonia is adsorbed. Therefore, the ammonia adsorption amount of the NOx catalyst 24 is estimated, and the reducing agent injection amount is controlled based on the difference between the estimated adsorption amount and the target adsorption amount. The reason for estimating the ammonia adsorption amount is that it is difficult to actually measure it.

  FIG. 2 shows the ammonia adsorption characteristics of the NOx catalyst 24. The line a indicates the upper limit value or adsorption limit of the ammonia adsorption amount obtained through experiments or the like, and this upper limit value tends to decrease as the catalyst temperature of the NOx catalyst 24 increases. Note that if ammonia is supplied when the actual ammonia adsorption amount is the upper limit value, the ammonia cannot be adsorbed to the NOx catalyst 24, so it flows out downstream of the NOx catalyst 24 and causes ammonia slip.

  A target value on the low adsorption amount side by a predetermined margin from the line a is determined as a line b, and this line b is stored in the ECU 100 in the form of a map.

  The ECU 100 estimates the catalyst temperature of the NOx catalyst 24 based on the detected value of at least one of the exhaust temperature sensors 44 and 46. For example, any one of the detected values may be regarded as the catalyst temperature, or an average value of both the detected values may be regarded as the catalyst temperature. Then, a target value Wt (a value at point c in FIG. 2) of the ammonia adsorption amount corresponding to the estimated catalyst temperature (Tc1 in FIG. 2) is calculated from the map. The catalyst temperature may be detected directly. Estimation and detection are collectively referred to as acquisition.

  A difference ΔW between the target adsorption amount Wt and the estimated adsorption amount We is obtained by an expression: ΔW = Wt−We, and a second injection amount MB corresponding to the difference ΔW is calculated. The larger the difference ΔW is, the larger the second injection amount MB is calculated.

  For example, when the estimated amount of adsorption We is smaller than the target amount of adsorption Wt, as shown at point d in FIG. 2, the difference ΔW is positive, so a positive second injection amount MB on the injection amount increasing side is calculated. By injecting the two injection amounts MB, the estimated adsorption amount We increases and gradually approaches the target adsorption amount Wt. On the other hand, when the estimated adsorption amount We is larger than the target adsorption amount Wt, for example, as shown at point e in FIG. 2, the difference ΔW is negative, and therefore the zero or negative second injection amount MB is calculated. As a result, the ammonia adsorbed on the NOx catalyst 24 is consumed for the reduction of NOx, the estimated adsorption amount We decreases, and gradually approaches the target adsorption amount Wt.

  Various methods including a publicly known method can be adopted as a method for estimating the ammonia adsorption amount. In this embodiment, a mathematical model is constructed based on a chemical reaction formula representing the reaction between ammonia and NOx in the NOx catalyst 24, and the ammonia adsorption amount is accurately estimated by the ECU 100 based on the model. At this time, the ECU 100 determines the urea water injection amount M, the catalyst temperature of the NOx catalyst 24, the exhaust gas flow rate, the detected values of the upstream and downstream NOx sensors 47 and 48, and engine parameters (engine speed, fuel injection amount) representing the engine operating state. Etc.), etc., and the ammonia adsorption amount is estimated.

  Third, a third injection amount MC commensurate with the amount of NOx flowing out from the NOx catalyst 24 (outflow NOx amount) is calculated. Specifically, when the output (sensor output) V of the downstream NOx sensor 48 is equal to or less than a predetermined upper limit value Vup, the third injection amount MC that is zero is calculated assuming that the outflow NOx amount is within the allowable range. On the other hand, when the sensor output V exceeds the upper limit value Vup, since the outflow NOx amount is outside the allowable range, the positive third injection amount MC is calculated in order to increase the urea water injection amount and suppress the outflow NOx amount. The

  At this time, a difference ΔV (= V−Vup) between the sensor output V and the upper limit value Vup is calculated, and a third injection amount MC corresponding to the difference ΔV is calculated. Thus, the urea water injection amount M is feedback-controlled or feedback-corrected based on the sensor output V.

  Here, in the present embodiment, the third injection amount MC is represented by a correction coefficient K (≧ 1). That is, in the case of the present embodiment, the previous equation M = MA + MB + MC is represented by M = K × MA + MB (= MA + MB + (K−1) × MA), and MC = (K−1) × MA. A map as shown in FIG. 3 is stored in the ECU 100, and a correction coefficient K greater than 1 is calculated as the difference ΔV increases from zero. When the difference ΔV becomes equal to or greater than the limit value ΔV1 (> 0), the increase of the correction coefficient K is suppressed and limited to the limit value K1 (> 1). The correction coefficient K is 1 when the difference ΔV is less than or equal to zero.

  When the sensor output V exceeds the upper limit value Vup, a correction coefficient K (> 1) corresponding to the difference ΔV is calculated, and the first injection amount MA that is the base injection amount is increased and corrected by the correction coefficient K. As a result, The urea water injection amount M is corrected to increase.

  Although an example of simple feedback control is shown here, the feedback control may be more complicated using a known technique such as PID control. Alternatively, the third injection amount MC, which is an addition term irrelevant to the first injection amount MA, may be calculated according to the difference ΔV, and the urea water injection amount M may be calculated by the equation M = MA + MB + MC.

  By the way, the downstream side NOx sensor 48 can detect not only NOx but also ammonia, and cannot detect both of them separately. For this reason, the relationship between the NOx catalyst downstream NOx amount with respect to the urea water injection amount, the sensor output of the downstream NOx sensor 48, and the ammonia amount (outflow ammonia amount) flowing out downstream of the NOx catalyst is shown in FIG. As shown.

  As shown in the vicinity of the left end of the figure, when the urea water injection amount is relatively small and insufficient with respect to the inflow NOx amount, the NOx catalyst 24 cannot reduce all the inflow NOx. Occur. The amount of outflow NOx increases and the NOx sensor output also increases. As the urea water injection amount increases, the urea water injection amount gradually matches the inflow NOx amount, so the outflow NOx amount gradually decreases and the NOx sensor output also gradually decreases.

  However, when the urea water injection amount is further increased, the urea water injection amount becomes excessive with respect to the inflow NOx amount, and an ammonia slip in which excess ammonia flows out from the NOx catalyst 24 occurs. As the urea water injection amount increases, the outflow ammonia amount also increases. Since the NOx sensor 48 detects this ammonia, the NOx sensor output gradually increases as the urea water injection amount increases.

  A balance point at which the NOx slip and the ammonia slip are balanced, that is, the urea water injection amount that can minimize both the outflow NOx amount and the outflow ammonia amount as much as possible is indicated by Mh in the figure. A small injection amount side from Mh is defined as a NOx slip region, and a large injection amount side from Mh is defined as an ammonia slip region.

  The NOx sensor output draws a curve that has a minimum value at the balance point. Therefore, it is impossible to determine whether the NOx sensor output is in the NOx slip region (whether NOx slip is occurring) or in the ammonia slip region (whether ammonia slip is occurring) only by the NOx sensor output. .

  On the other hand, the urea water injection amount is forcibly increased or decreased, and it is detected whether the NOx sensor output changes in accordance with the increase or decrease, and based on the result, it is determined in which region the NOx sensor output is located. It is possible to determine. For example, if the NOx sensor output decreases when the urea water injection amount is increased, it is determined that the NOx slip region is present (NOx slip is occurring), and the NOx sensor output increases when the urea water injection amount is increased. If it is, it is determined that it is in the ammonia slip region (ammonia slip is occurring).

  Incidentally, both the NOx slip and the ammonia slip are the results that occur because the urea water injection amount control is not suitably executed. The present inventor has come to think that one of those causes is an increase in the ammonia adsorption amount estimation error, and has come up with a urea water injection amount control suitable for eliminating NOx slip and ammonia slip. Therefore, the urea water injection amount control of this embodiment will be described in detail below.

  FIG. 5 shows a time chart when the urea water injection amount control of the present embodiment is executed. Such control is executed by the ECU 100. In the figure, (A) is the sensor output V of the downstream NOx sensor 48, (B) is the correction coefficient K, (C) is the ammonia adsorption amount (estimated adsorption amount We) of the NOx catalyst 24 estimated by the ECU 100, (D ) Indicates the urea water injection amount M.

  Before time t1, the sensor output V is increasing below the upper limit value Vup, the difference ΔV (= V−Vup) is negative, and the correction coefficient K is 1 (see FIG. 3). At time t1, the sensor output V reaches the upper limit value Vup, and thereafter the sensor output V increases. Along with this, the difference ΔV also increases with a positive value, and the correction coefficient K gradually increases from 1.

  At time t2, the difference ΔV reaches the limit value ΔV1, and the correction coefficient K also reaches the limit value K1. Thereafter, the sensor output V and the difference ΔV increase, but the correction coefficient K is limited to and held by the limit value K1.

  Since the urea water injection amount is corrected to be increased by feedback control based on the sensor output V, if NOx slip actually occurs, it will eventually disappear, and the sensor output V will drop below the upper limit value Vup as shown by the broken line a. there's a possibility that. However, this is not solved in the illustrated example.

  The first elapsed time is counted from the time when the sensor output V exceeds the upper limit value Vup (immediately after t1). If the sensor output V has not yet decreased below the upper limit value Vup even at the time t4 when the first elapsed time has reached the predetermined first threshold time Δts1, the estimation error of the estimated adsorption amount We has increased more than expected. Therefore, the value of the estimated adsorption amount We is changed or switched. That is, when the state where the sensor output V exceeds the upper limit value Vup continues for the first threshold time Δts1, the value of the estimated adsorption amount We is changed.

  At this time, the value of the estimated adsorption amount We is forcibly and stepwise changed from We1 immediately before the change to a smaller predetermined decrease side reference value We2, and is switched or reset. The amount of decrease from We1 to We2 is relatively large. As a result, the urea water injection amount M is changed more greatly than in the case of feedback correction, and is greatly increased from M1 immediately before the change to M2.

  Thus, the urea water injection amount M is increased by decreasing the estimated adsorption amount We. The reason for this is to suspect that there is a possibility that an estimated adsorption amount We that is considerably larger than the true adsorption amount is calculated (that is, the estimated adsorption amount We is excessive), and to suppress the error. If the estimated adsorption amount We is excessively large, the urea water injection amount M is inevitably small, so the urea water injection amount M is insufficient and the tendency of NOx slip increases. Therefore, at this stage, it is substantially determined that the NOx slip occurs within the first threshold time Δts1, and the urea water injection amount M is increased by decreasing the estimated adsorption amount We. If the NOx slip actually occurs within the first threshold time Δts1, the NOx slip can be reliably suppressed by the urea water injection amount increase control here.

  The decrease side reference value We2 is a value that is relatively lower than the target adsorption amount Wt (see FIG. 2). Thereby, the difference ΔW between the target adsorption amount Wt and the estimated adsorption amount We increases, the second injection amount MB increases, and the urea water injection amount M increases.

  If the sensor output V did not decrease within the first threshold time Δts1 (due to NOx slip) due to an excessive amount of the estimated adsorption amount We (if the above suspicion was correct), the above As a result of the urea water injection amount increase measure, the sensor output V decreases to the upper limit value Vup or less (NOx slip is eliminated) as shown by the broken line b. However, in the illustrated example, it is not lowered.

  Therefore, next, the estimated adsorption amount We is suspected to be too small. That is, the second elapsed time is counted from the time point t4 when the estimated adsorption amount We is decreased. If the sensor output V still does not fall below the upper limit value Vup even at the time t5 when the second elapsed time reaches the predetermined second threshold time Δts2, the estimated adsorption amount We is now considerably smaller than the true adsorption amount. It is suspected that there is a possibility of being calculated (that is, the estimated adsorption amount We is too small), and the value of the estimated adsorption amount We is increased in order to suppress the error. Thus, the value of the estimated adsorption amount We is also changed when the sensor output V exceeds the upper limit value Vup for the second threshold time Δts2. If the estimated adsorption amount We is too small, the urea water injection amount M inevitably becomes larger, so the urea water injection amount M becomes excessive and the tendency of ammonia slip increases. Therefore, at this stage, it is substantially determined that ammonia slip occurs within the first threshold time Δts1, and the urea water injection amount M is decreased by increasing the estimated adsorption amount We. If ammonia slip actually occurs, the ammonia slip can be reliably suppressed by reducing the urea water injection amount here.

  At this time, the value of the estimated adsorption amount We is forced and stepped from a value We2 ′ immediately before the change to a predetermined increase-side reference value We3 that is larger than this and is larger than the value We1 before the estimated adsorption amount is decreased. Changed to, switched or reset. The amount of increase from We1 to We3 is also relatively large. As a result, the urea water injection amount M is changed to be larger than that in the case of feedback correction, and is greatly reduced from the value M2 'immediately before the change to M3 smaller than this and smaller than M1.

  The increase side reference value We3 is a value higher than the target adsorption amount Wt (see FIG. 2). As a result, the difference ΔW between the target adsorption amount Wt and the estimated adsorption amount We is negative, the second injection amount MB is zero or negative, and the urea water injection amount M is decreased.

  If the sensor output V did not decrease within the first threshold time Δts1 (because the above suspicion was correct) if the estimated adsorption amount We was too small (due to ammonia slip), As a result of the urea water injection amount reduction measure, the sensor output V should be reduced to the upper limit value Vup or less (ammonia slip is eliminated) as shown by the broken line c. However, in the illustrated example, it is not lowered.

  Therefore, in this case, an abnormality due to a cause other than the estimation error expansion of the estimated adsorption amount We is considered, and thus an abnormality determination that the apparatus is abnormal is performed. That is, the third elapsed time is counted from the time t5 when the estimated adsorption amount We is increased. Even when the third elapsed time reaches a predetermined third threshold time Δts3, if the sensor output V still does not fall below the upper limit value Vup, an abnormality is determined and a warning device (check lamp or the like) not shown is used. Start and prompt the user for inspection and maintenance. Thus, when the state where the sensor output V exceeds the upper limit value Vup continues for the third threshold time Δts3, abnormality determination is performed.

  When the sensor output V decreases as shown by the broken line b due to the decrease in the estimated adsorption amount to We2, it is assumed that the excessive error increase in the estimated adsorption amount and NOx slip have been eliminated, and the estimated adsorption amount continues from the value after the decrease. Is calculated. Further, when the sensor output V decreases as indicated by the broken line c due to the increase in the estimated adsorption amount to We3, it is assumed that the underestimated error expansion and ammonia slip of the estimated adsorption amount are eliminated, and the estimated adsorption amount continues from the increased value. Is calculated.

  The first to third threshold times Δts1 to Δts3 can be arbitrarily set, and may be the same value or different values. However, it is necessary to set the sensor output V to an appropriate length so that the state where the sensor output V exceeds the upper limit value Vup does not continue for a long time.

  On the other hand, in this embodiment, when the state in which the sensor output V exceeds the upper limit value Vup continues for the second threshold time Δts2, it is substantially determined that ammonia slip has occurred, and the value of the estimated adsorption amount We is It is changed to the increase side reference value We3. At this time, the increase side reference value We3 is appropriately set based on each state quantity. Specifically, the increase side reference value We3 is optimal based on the estimated adsorption amount We and the target adsorption amount Wt, the flow rate and temperature of the exhaust gas flowing into the NOx catalyst 24, and the NOx amount flowing into the NOx catalyst 24. Set to

  More specifically, the ECU 100 has a base map as shown in FIG. 7 (A), a first map as shown in FIG. 7 (B), and a second map as shown in FIG. 7 (C). Stored in advance. Then, the ECU 100 calculates the base value B of the increase side reference value We3 from the base map based on the estimated adsorption amount We and the target adsorption amount Wt.

  Further, the ECU 100 calculates the first correction coefficient F1 from the first map based on the exhaust gas flow rate Ge calculated based on the value of the intake air amount and the exhaust temperature Tin detected by the exhaust temperature sensor 44. Further, the ECU 100 calculates the second correction coefficient F2 from the second map based on the inflow NOx amount Min and the exhaust gas temperature Tin.

  The ECU 100 calculates the increase-side reference value We3 by multiplying the base value B by the first correction coefficient F1 and the second correction coefficient F2.

  In this way, when the estimated adsorption amount We is changed or reset to the increase side reference value We3, it is not necessary to raise the temperature of the NOx catalyst 24 or to perform additional fuel injection, so that deterioration of fuel consumption can be avoided. Further, since the change is made regardless of the opportunity for filter regeneration or the like, it is possible to eliminate the increase in the ammonia adsorption amount estimation error at an early stage.

  The decrease side reference value We2 may be a constant value, but may be variably set similarly to the increase side reference value We3.

  As described above, in this embodiment, when it is determined that ammonia slip is occurring, the estimated adsorption amount We is changed to the increase side reference value We3, and the increase side reference value We3 is changed to the estimated adsorption amount We and the target adsorption amount. Calculation is made based on Wt, the exhaust gas flow rate Ge, the exhaust temperature Tin, and the inflow NOx amount M1. For this reason, the expansion of the ammonia adsorption amount estimation error can be eliminated at an early stage without deteriorating the fuel consumption.

  In this embodiment, when the state in which the sensor output V exceeds the upper limit value Vup continues for the threshold time (first threshold time Δts1 or second threshold time Δts2), the estimated adsorption amount We is changed (decreased or increased). As a result, the reducing agent injection amount M is changed (increased or decreased) (t4 or t5), so that NOx slip and ammonia slip due to the expansion of the ammonia adsorption amount estimation error can be suitably eliminated. At the same time, the estimation error of the estimated adsorption amount We can be reduced to bring the estimated adsorption amount We close to the true adsorption amount, and the urea water injection amount control is appropriately corrected to NOx emission (NOx emission amount to the atmosphere). Can be suppressed. Even if such an estimated error expansion state is left unattended, it is not easily resolved, but it is possible to eliminate the estimation error expansion at an early stage by providing a means for forcibly changing the estimated adsorption amount We as in this embodiment. It is.

  In this embodiment, when the state where the sensor output V exceeds the upper limit value Vup continues for the first threshold time Δts1 (t4), the urea water injection amount M is increased by decreasing the estimated adsorption amount We. Thereafter, when the state in which the sensor output V exceeds the upper limit value Vup continues for the second threshold time Δts2 (t5), the reducing agent injection amount M is decreased by increasing the estimated adsorption amount We.

  That is, first, the urea water injection amount is increased by suspecting NOx slip and the estimated adsorption amount is decreased, and then the urea water injection amount is decreased by suspecting the ammonia slip and the estimated adsorption amount is increased. If this order is reversed, if the NOx slip actually occurs, the urea water injection amount is decreased by increasing the estimated adsorption amount first, so that the outflow NOx amount increases and NOx emission deteriorates. Therefore, in this embodiment, first, the urea water injection amount is increased by decreasing the estimated adsorption amount, and after ensuring that the NOx slip is not occurred, the urea water injection amount is decreased by increasing the estimated adsorption amount by suspecting ammonia slip. Thereby, NOx emission deterioration at the time of eliminating NOx slip and ammonia slip can be effectively suppressed.

  In the present embodiment, after the urea water injection amount is decreased (after t5), when the state where the sensor output V exceeds the upper limit value Vup continues for the third threshold time Δts3 (t6), the abnormality determination is performed. Do. That is, if the sensor output V does not decrease even when the estimated adsorption amount We is changed greatly and the reducing agent injection amount M is changed greatly, an abnormality occurs due to a cause other than the estimation error increase of the estimated adsorption amount We. Therefore, the user is judged to be abnormal and prompts the user to perform maintenance. Thereby, while detecting an abnormal condition reliably, the early resolution can be aimed at.

  Next, with reference to FIG. 6, the control routine of the present embodiment for realizing the above control will be described. The illustrated routine is repeatedly executed by the ECU 100 at every predetermined calculation cycle τ (for example, 10 msec). Note that first to third flags F1 to F3 described later are OFF in the initial state.

  In step S101, it is determined whether or not the first flag F1 is on (ON). When the flag is not on (off), the process proceeds to step S102, and when it is on, the process proceeds to step S108.

  In step S102, it is determined whether or not the second flag F2 is on. If not, the process proceeds to step S103. If it is on, the process proceeds to step S112.

  In step S103, it is determined whether or not the third flag F2 is on. If not, the process proceeds to step S104. If it is on, the process proceeds to step S115.

  In step S104, it is determined whether or not the sensor output V exceeds the upper limit value Vup (V> Vup). If so, the process proceeds to step S105, and if not, the current routine is terminated.

  In step S105, it is determined whether or not the state in which the sensor output V exceeds the upper limit value Vup continues for the first threshold time Δts1. If it continues, the process proceeds to step S106, and if it does not continue, the routine ends.

  In step S106, the first flag F1 is turned on. Next, in step S107, the estimated adsorption amount We is decreased, and thereby the reducing agent injection amount M is increased. Thus, the routine ends.

  In step S108, it is determined whether or not the sensor output V exceeds the upper limit value Vup (V> Vup). If so, the process proceeds to step S109, and if not, the process proceeds to step S108A.

  In step S108A, the first flag F1 is turned off, and then the routine is terminated.

  In step S109, it is determined whether or not the state in which the sensor output V exceeds the upper limit value Vup continues for the second threshold time Δts2. If it continues, the process proceeds to step S110, and if it does not continue, the routine ends.

  In step S110, the second flag F2 is turned on and the first flag F1 is turned off. Next, in step S111, the estimated adsorption amount We is increased, and thereby the reducing agent injection amount M is decreased. Thus, the routine ends.

  In step S112, it is determined whether or not the sensor output V exceeds the upper limit value Vup (V> Vup). If so, the process proceeds to step S113, and if not, the process proceeds to step S112A.

  In step S112A, the second flag F2 is turned off, and then the routine is terminated.

  In step S113, it is determined whether or not the state in which the sensor output V exceeds the upper limit value Vup continues for the third threshold time Δts3. If it is continued, the process proceeds to step S114. If it is not continued, the routine is terminated.

  In step S114, the third flag F3 is turned on and the second flag F2 is turned off. Next, in step S115, an abnormality determination is made. Thus, the routine ends.

  When this control routine is executed, since the first to third flags F1 to F3 are initially off, the process proceeds to step S104. When steps S104 and S105 become yes, the first flag F1 is turned on in step S106. In step S107, the estimated adsorption amount We is decreased, and the reducing agent injection amount M is increased.

  At the subsequent calculation time, the first flag F1 is on, so that the process proceeds from step S101 to step S108 and waits for steps S108 and S109 to become yes as described above. If step S108 or S109 is YES, the second flag F2 is turned on and the first flag F1 is turned off in step S110, the estimated adsorption amount We is increased in step S111, and the reducing agent injection amount M is decreased.

  Since the first flag F1 is turned off and the second flag F2 is turned on at the subsequent calculation timing, the process proceeds from step S102 to step S112 and waits for steps S112 and S113 to become yes as described above. If steps S112 and S113 are YES, the third flag F3 is turned on and the second flag F2 is turned off in step S114, and an abnormality is determined in step S115.

  At the subsequent calculation time, the first and second flags F1 and F2 are off and the third flag F3 is on. Therefore, the process proceeds from step S103 to step S115, and the abnormality determination is continued.

  As understood from the above description, the ECU 100 of the present embodiment corresponds to an estimation unit, a control unit, a determination unit, and a change unit in the claims.

  Although the embodiments of the present invention have been described in detail above, various other embodiments of the present invention are possible.

  (1) For example, the first correction amount is calculated based on the exhaust gas flow rate Ge and the exhaust temperature Tin, and the second correction amount is calculated based on the inflow NOx amount M1 and the exhaust temperature Tin. The increase side reference value We3 may be calculated by adding the amount to the base value B. Further, the base value B and the correction value (the first and second correction coefficients or the first and second correction amounts) for correcting the base value B may be calculated by using other state quantities or parameters additionally. Good.

  (2) Other methods can be used for determining ammonia slip. For example, when the NOx sensor output V increases when the exhaust gas temperature rises, it may be determined that ammonia that has been adsorbed by the NOx catalyst temperature rises and flows out, and that ammonia slip has occurred.

  (3) Reverse the order of the decrease and increase in the estimated adsorption amount We (that is, increase and decrease in the urea water injection amount M) when the sensor output V exceeds the upper limit value Vup for the first threshold time Δts1. First, the estimated adsorption amount We may be increased (decrease in the urea water injection amount M), and then the estimated adsorption amount We may be decreased (increase in the urea water injection amount M). For example, this method is effective when ammonia slip is to be stopped preferentially.

  (4) The reducing agent may be other than urea water as long as it has an equivalent function.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

1 Internal combustion engine
4 Exhaust passage 24 NOx catalyst 25 Urea injector 100 Electronic control unit (ECU)

Claims (2)

A selective reduction type NOx catalyst provided in the exhaust passage;
A reducing agent injection valve provided in the exhaust passage upstream of the NOx catalyst;
An estimation unit for estimating an ammonia adsorption amount of the NOx catalyst;
Based on the ammonia adsorption amount estimated by the estimation unit, a control unit for controlling the reducing agent injection amount injected from the reducing agent injection valve;
A determination unit for determining that ammonia slip is occurring in the NOx catalyst;
A change unit that changes the estimated value of the amount of adsorbed ammonia to a predetermined reference value when the determination unit determines that ammonia slip is occurring,
The changing unit calculates the reference value based on the estimated value and target value of the ammonia adsorption amount, the flow rate and temperature of the exhaust gas flowing into the NOx catalyst, and the NOx amount flowing into the NOx catalyst. An exhaust gas purification apparatus for an internal combustion engine characterized by the above. The changing unit calculates a base value of the reference value based on the estimated value and target value of the ammonia adsorption amount, calculates a first correction coefficient based on the flow rate and temperature of the exhaust gas flowing into the NOx catalyst, and the NOx A second correction coefficient is calculated based on the amount of NOx flowing into the catalyst and the temperature of exhaust gas flowing into the NOx catalyst, and the reference value is calculated by multiplying the base value by the first correction coefficient and the second correction coefficient. Item 6. An exhaust emission control device for an internal combustion engine according to Item 1.

Images