JP2009115032A - Exhaust gas purifying apparatus for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purifying apparatus for internal combustion engine which can accurately detect NOx quantity in the downstream side of an NOx-purifying catalyst and can fairly calculate an NOx purification rate or the like, by extension. <P>SOLUTION: An exhaust pipe 22 is provided with an oxidation catalyst 41, an SCR catalyst 42 and an ammonia slip catalyst 43 in sequence from its upstream side. In the exhaust pipe 22, the urea water-adding valve 44 is provided between the oxidation catalyst 41 and the SCR catalyst 42. A NOx sensor 47 which detects the NOx quantity in the exhaust gas is provided downstream of the SCR catalyst 42 regarding the exhaust after passing through the SCR catalyst. An ECU 50 controls the urea water-adding valve 44 to add the urea water to the exhaust gas while changing the urea water adding amount so as to be different from each other. While the urea water is added, the ECU successively obtains an output of the NOx sensor 47 in each adding amount, and computes an adding amount command value in which the sensor output becomes minimum. <P>COPYRIGHT: (C)2009,JPO&INPIT

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 that is suitably employed in a selective reduction (SCR: Selective Catalytic Reduction) type exhaust gas purification system using an ammonia-based reducing agent such as an aqueous urea solution.

  In recent years, in an internal combustion engine (particularly a diesel engine) applied to automobiles and the like, a urea SCR system has been developed as an exhaust purification device that purifies NOx (nitrogen oxide) in exhaust gas with a high purification rate. It has been put to practical use. The following configuration is known as a urea SCR system.

  That is, in the urea SCR system, a selective reduction type NOx purification catalyst (SCR catalyst) is provided in an exhaust pipe connected to the engine body, and urea water (urea aqueous solution) as a reducing agent is disposed upstream of the exhaust pipe in the exhaust pipe. A urea water addition valve to be added to is provided. In such a system, urea water is added to the exhaust pipe by the urea water addition valve, whereby urea water is supplied to the NOx purification catalyst together with the exhaust gas, and the exhaust gas is purified by a NOx reduction reaction on the NOx purification catalyst. The During the reduction of NOx, ammonia (NH3) is generated by hydrolyzing urea water with exhaust heat, and the NOx purification catalyst performs exhaust gas purification by selectively reducing NOx with ammonia. .

Further, a NOx sensor is provided on the downstream side of the NOx purification catalyst, and the NOx concentration on the downstream side of the catalyst is detected by this NOx sensor. Then, the NOx purification rate is calculated based on each NOx sensor output. For example, in the exhaust purification device of Patent Document 1, the NOx sensors are provided before and after the NOx purification catalyst, and the NOx purification rate is calculated based on the outputs of these NOx sensors. Further, during the steady operation of the engine, the supply state and non-supply state of the reducing agent are switched, and the difference in the NOx purification rate between when the reducing agent is supplied and when it is not supplied is calculated. Based on the difference, the ammonia adsorption amount and the reducing agent addition amount of the NOx purification catalyst are calculated.
JP 2003-314256 A

  However, the NOx sensor generally has a structure having a sensor element using a solid electrolyte body and a pair of electrodes, and has detection sensitivity not only for NOx but also for ammonia (NH3). Therefore, when the ammonia surplus in the NOx purification catalyst is discharged to the downstream side of the catalyst (so-called ammonia slip occurs), the NOx sensor generates an output corresponding to the ammonia reaction. In such a case, when the NOx purification rate or the like is calculated using the NOx sensor output, the NOx purification rate or the like may be erroneously calculated. If the calculation accuracy of the NOx purification rate decreases in this way, the ammonia adsorption amount and the reducing agent addition amount calculated based on the NOx purification rate also become inaccurate. As a result, the NOx purification rate in the NOx purification catalyst decreases and ammonia slip There is concern about an increase in the amount.

  The main object of the present invention is to provide an exhaust purification device for an internal combustion engine that can correctly detect the amount of NOx on the downstream side of the NOx purification catalyst and thereby appropriately calculate the NOx purification rate and the like. is there.

  Hereinafter, means for solving the above-described problems and the effects thereof will be described.

  In the exhaust purification apparatus of the present invention, a reducing agent (for example, an ammonia-based reducing agent such as an aqueous urea solution) is provided by a reducing agent adding means on the exhaust upstream side of the NOx purification catalyst (for example, ammonia selective reduction catalyst) in the exhaust passage of the internal combustion engine. In addition, the NOx purification catalyst performs NOx purification by the reaction between the reducing agent and NOx. Then, the NOx amount is detected by the NOx sensor on the exhaust downstream side of the NOx purification catalyst, and the NOx purification rate is calculated based on the detected value.

  Further, in the invention described in claim 1, in particular, the reducing agent is added by the reducing agent addition means while changing to different addition amounts, and the sensor output of the NOx sensor is acquired for each addition amount when the reducing agent is added. At the same time, the addition amount command value is calculated from the reducing agent addition amount that makes the sensor output the minimum value among the acquired sensor outputs.

  In short, in the NOx sensor, in addition to the NOx in the exhaust, the reducing agent surplus in the NOx purification catalyst is similarly detected. Here, assuming that the urea aqueous solution as the reducing agent is gradually increased, the NOx concentration on the downstream side of the NOx purification catalyst gradually decreases with the increase of the reducing agent, and further the ammonia concentration (ammonia concentration by increasing the reducing agent). (Slip amount) increases. In this case, as shown in FIG. 4, the NOx sensor output has a downward convex characteristic with respect to the reducing agent addition amount (urea water addition amount), and the reducing agent addition amount at which the NOx sensor output becomes the minimum value in the same characteristic is Further, both the NOx concentration and the ammonia concentration on the downstream side of the NOx purification catalyst are small, which corresponds to a reducing agent addition amount that maximizes the NOx purification rate.

In this regard, according to the above configuration of claim 1,
・ Adding a reducing agent while changing to a plurality of different addition amounts,
Obtaining a NOx sensor output for each addition amount at the time of addition of the reducing agent, and calculating an addition amount command value based on a reducing agent addition amount at which the sensor output is a minimum value among the obtained sensor outputs;
Thus, the reducing agent addition amount that minimizes the excess amount of the reducing agent (ammonia slip amount) and maximizes the NOx purification rate can be found as the addition amount command value. As a result, the NOx amount on the downstream side of the NOx purification catalyst can be correctly detected, and the NOx purification rate and the like can be calculated appropriately.

  Here, as described in claim 2, the reducing agent addition amount may be changed to at least one of the increase side or the decrease side with reference to each addition amount command value. According to such a configuration, it is possible to appropriately grasp whether or not the addition amount command value for each time is an optimum value, that is, whether or not the addition amount at which the NOx sensor output becomes the minimum value.

  According to the third aspect of the present invention, at least three stages of reducing agent addition are executed by changing the reducing agent addition amount within a predetermined range, and an intermediate amount of the reducing agent is added among the at least three stages of reducing agent addition. When the sensor output at the time of addition becomes the minimum value, the reducing agent addition is executed again after changing the reducing agent addition amount after reducing the change width.

  In short, when the reducing agent addition amount is changed and at least three levels of reducing agent addition are executed, the NOx sensor output at the time of adding an intermediate amount of the reducing agent is the minimum value when the addition amount is changed. It means that the optimum value of the addition amount command value (that is, the reducing agent addition amount at which the NOx sensor output becomes the minimum value) exists between the addition amount and the maximum addition amount. In such a case, the optimum value of the addition amount command value can be obtained with high accuracy by executing the reducing agent addition accompanying the change of the reducing agent addition amount again after reducing the change width as described above.

  In the invention according to claim 4, based on the sensor output before the change of the reducing agent addition amount and the sensor output after the change, is the reducing agent addition amount at which the sensor output becomes the minimum value on the reduction side? Whether the amount is on the increase side is predicted, and the reducing agent addition amount is increased or decreased based on the prediction result. For example, when the NOx sensor output increases with respect to the change in the reducing agent addition amount, the increase / decrease direction is reversed. Alternatively, if the NOx sensor output decreases with respect to the change in the reducing agent addition amount, the reducing agent addition amount is changed in the same increase / decrease direction.

  According to the fourth aspect, it is possible to cause the reducing agent addition amount to be increased or decreased only on the side with the reducing agent addition amount at which the sensor output becomes the minimum value (either the reduction side or the increase side). Therefore, the increase / decrease process of the reducing agent addition amount can be simplified.

  Further, as described in claim 5, it is preferable to set a range of change of the reducing agent addition amount based on the output value of the NOx sensor. For example, the change width of the reducing agent addition amount is increased as the sensor output value is increased. When the sensor output value is relatively large, the rate of change of the sensor output value with respect to the change of the reducing agent addition amount is relatively large, and when the sensor output value is relatively small, the sensor output value with respect to the change of the reducing agent addition amount Is relatively small (see FIG. 4). Therefore, in order to find the minimum value of the NOx sensor output, it is desirable to set the change amount of the reducing agent addition amount based on the output value of the NOx sensor as described above.

  As described in claim 6, if the difference between the minimum value and the maximum value of the sensor output acquired with the change of the reducing agent addition amount is within a predetermined value, the addition amount command value is not newly calculated. Is desirable. That is, in the vicinity where the NOx sensor output becomes the minimum value, the NOx sensor output may hardly change even if the reducing agent addition amount is increased or decreased (see FIG. 4). In this case, it is not necessary to calculate (update) the addition amount command value, and it is possible to stop calculating (updating) the unnecessary addition amount command value.

  According to the seventh aspect of the present invention, when increasing or decreasing the reducing agent addition amount, the period during which the addition amount is reduced is longer than the period during which the addition amount is increased. That is, when the amount of reducing agent added is increased and when it is decreased, the response speed of the sensor output is different, and the latter is slower. This is because the reducing agent consumption rate in the NOx purification catalyst is smaller than the reducing agent adsorption rate (ammonia adsorption rate) in the catalyst. In this regard, according to the configuration of the seventh aspect, it is possible to appropriately obtain the value of the NOx sensor output accompanying the change in the addition amount regardless of whether the reduction agent addition amount is increased or decreased. . In addition, by providing a difference in the length of the period between the increase and the decrease, the minimum necessary period can be set in any case.

  The addition amount command value at which the NOx sensor output becomes the minimum value does not change sequentially, and becomes a steady value if the operating state of the internal combustion engine is constant. Therefore, as described in claim 8, the addition amount command value calculated by the command value calculation means is stored in the backup memory as a learning value, and the learning value is updated at any time (if necessary). Is desirable. Thereby, the addition amount command value at which the NOx sensor output becomes the minimum value may be calculated with the minimum necessary frequency. For example, the addition amount command value is calculated only once every time power is supplied to the control device (ECU).

  It is considered that the characteristics of the NOx sensor output with respect to the reducing agent addition amount vary depending on the operating state of the internal combustion engine. Therefore, as described in claim 9, the addition amount command value calculated by the command value calculation means is stored in the memory in association with the operating state of the internal combustion engine during the addition amount control by the addition amount control means. Is desirable. Thereby, even if the operating state of the internal combustion engine changes, an optimum addition amount command value can be set each time.

  In the invention according to claim 10, an oxidation catalyst for reducing agent purification (for example, ammonia slip catalyst) provided on the downstream side of the NOx purification catalyst, and an activity determination means for determining whether or not the oxidation catalyst is in an active state. With. When the activation determination unit determines that the oxidation catalyst is in an inactive state, the command value calculation unit calculates the addition amount command value.

  In short, if the oxidation catalyst on the downstream side of the NOx purification catalyst is inactive, if the reducing agent is discharged to the downstream side of the NOx purification catalyst, the reducing agent cannot be properly purified. In this respect, when the oxidation catalyst is inactive as described above, the command value calculation means calculates the addition amount command value, thereby suppressing the discharge of the reducing agent to the downstream side of the NOx purification catalyst, and thus reducing the reducing agent. Air emissions can be suppressed.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, an embodiment of the invention will be described with reference to the drawings. In this embodiment, an engine control system is constructed with a multi-cylinder diesel engine for a vehicle as a control target. In the control system, various controls of the engine are performed with an electronic control unit (hereinafter referred to as ECU) as a center. The In this embodiment, a common rail fuel injection system is adopted as the fuel injection system, and a urea SCR system is adopted as the exhaust purification system. First, an overall outline of the present system will be described with reference to FIG.

  The engine 10 has an engine body 11 having a reciprocating engine structure. As a basic structure, the engine 10 is provided in a piston 12 reciprocating in a cylinder, and each port on the intake side and the exhaust side, and is individually opened and closed. An intake valve 13 and an exhaust valve 14 are provided. As the piston 12 reciprocates, the crankshaft 15 rotates. The cylinder head is provided with a fuel injection valve 16 for each cylinder. Fuel is directly injected into the combustion chamber 17 by the fuel injection valve 16, and the injected fuel is used for combustion in the combustion chamber 17.

  The crankshaft 15 is provided with a crank angle sensor 18 for detecting the rotation of the crankshaft 15. Further, the cylinder block is provided with a water temperature sensor 19 for detecting the temperature of the engine cooling water.

  The configuration of the fuel supply system will be briefly described (however, since it is well-known, description by illustration is omitted). A high-pressure pump and a common rail (pressure accumulation pipe) are provided as a configuration of the fuel supply system, and the fuel in the fuel tank is increased in pressure by the high-pressure pump and is pumped to the common rail. A high pressure fuel of about several tens to 200 MPa is stored in the common rail, and this high pressure fuel is supplied to the fuel injection valve 16 of each cylinder. Note that the fuel pressure in the common rail is appropriately adjusted according to the engine operating condition and the like.

  An intake pipe (including a manifold portion) 21 is connected to the intake port of the engine body 11, and an exhaust pipe (including a manifold portion) 22 is connected to the exhaust port. The intake pipe 21 is provided with a throttle actuator 23 having an electrically driven throttle valve. The intake passage in the intake pipe 21 and the exhaust passage in the exhaust pipe 22 are connected by an EGR passage 24, and an EGR valve 25 and an EGR cooler 26 are provided in the EGR passage 24. An air cleaner 27 is provided at the most upstream portion of the intake pipe 21.

  Further, in this system, a turbocharger 30 is provided as a supercharging device. The turbocharger 30 includes an intake compressor 31 provided in the intake pipe 21 and an exhaust turbine 32 provided in the exhaust pipe 22, and the exhaust turbine 32 rotates by the exhaust gas flowing through the exhaust pipe 22. The force is transmitted to the intake compressor 31 via the shaft 33. Then, the intake air flowing through the intake pipe 21 is compressed by the intake compressor 31 and supercharging is performed. The air supercharged by the turbocharger 30 is cooled by the intercooler 34 and then fed to the downstream side of the intake pipe 21.

  The intake pipe 21 is provided with sensors such as an intake air amount sensor (air flow meter), an intake air pressure sensor, an intake air temperature sensor, etc., but description thereof is omitted for convenience.

  Next, an exhaust purification system provided in the exhaust system will be described. In the exhaust pipe 22, an oxidation catalyst 41, an SCR catalyst (ammonia selective reduction catalyst) 42, and an ammonia slip catalyst 43 are disposed in order from the upstream side. The SCR catalyst 42 corresponds to a NOx purification catalyst. Further, a urea water addition valve 44 for adding and supplying urea water (urea aqueous solution) as a reducing agent into the exhaust pipe 22 is provided between the oxidation catalyst 41 and the SCR catalyst 42 in the exhaust pipe 22. . The urea water addition valve 44 has substantially the same configuration as an existing fuel injection valve (electromagnetically driven injector), and the tip injection hole of the urea water addition valve 44 is opened by a valve opening operation associated with an electrical control command. The urea water is injected from the part. Note that the urea water in the urea water tank is sequentially supplied to the urea water addition valve 44 by a pressure feeding operation of the urea water pump (not shown).

  In the exhaust purification system having the above configuration, when urea water is added and supplied into the exhaust pipe 22 by the urea water addition valve 44 during engine operation, urea water is supplied to the SCR catalyst 42 together with the exhaust gas in the exhaust pipe 22. The exhaust gas is purified by performing a reduction reaction of NOx in the SCR catalyst 42.

Specifically, the urea water injected from the urea water addition valve 44 is hydrolyzed by exhaust heat,
(NH2) 2CO + H2O → 2NH3 + CO2 (Formula 1)
Ammonia (NH3) is generated by the reaction as described above. When the exhaust gas passes through the SCR catalyst 42, NOx in the exhaust gas is selectively reduced and purified by ammonia. At that time, NOx is reduced and purified by the following reduction reaction.
4NO + 4NH3 + O2 → 4N2 + 6H2O (Formula 2)
6NO2 + 8NH3 → 7N2 + 12H2O (Formula 3)
NO + NO2 + 2NH3 → 2N2 + 3H2O (Formula 4)
In this way, when the reduction and purification of NOx by ammonia is performed, if ammonia does not react with NOx and becomes surplus, the surplus ammonia is mixed with the exhaust and released downstream of the exhaust. In such a case, surplus ammonia is removed by an ammonia slip catalyst 43 (for example, an oxidation catalyst) on the downstream side of the SCR catalyst.

  Further, an oxygen concentration sensor 45 and an exhaust temperature sensor 46 are provided between the oxidation catalyst 41 and the SCR catalyst 42 in the exhaust pipe 22, and based on the output of each of these sensors, the oxygen concentration in the exhaust gas and The exhaust temperature is detected. Further, on the downstream side of the SCR catalyst 42, there is provided a NOx sensor 47 for detecting the NOx amount (NOx concentration) in the exhaust with the exhaust after passing through the SCR catalyst as a detection target. Based on this, the NOx purification rate of the SCR catalyst 42 is detected.

  Although omitted in FIG. 1, a DPF (diesel particulate filter) is installed in the exhaust pipe 22, and PM (particulate matter) in the exhaust is collected by the DPF. Yes.

  The ECU 50 includes a known microcomputer (not shown) including a CPU, a ROM, a RAM, and the like. The ECU 50 includes detection signals from the various sensors described above and fuel pressure (rail pressure in the common rail). Detection signals are sequentially input from a rail pressure sensor for detecting), an accelerator sensor for detecting an accelerator operation amount (accelerator opening) by a driver, and the like. Then, the ECU 50 executes fuel injection control, fuel pressure control (rail pressure control), and the like based on engine operation information such as engine rotation speed and accelerator operation amount. As a result, the fuel injection operation of the fuel injection valve 16 and the fuel pumping operation by the high-pressure pump are controlled. In addition, the ECU 50 appropriately executes control of the throttle actuator 23, the EGR valve 25, and the like based on each engine operating state.

  The ECU 50 is provided with an EEPROM 51 as a backup memory. In the EEPROM 51, various learning values, diagnosis data (failure diagnosis data), and the like are appropriately stored and continuously stored. Note that a standby RAM in which the power supply state is maintained even after the power supply to the ECU 50 is cut off can be used as the backup memory.

  Further, the ECU 50 calculates the NOx amount on the downstream side of the SCR catalyst 42 or the NOx purification rate based on the output of the NOx sensor 47. Further, the urea water addition amount is controlled based on the NOx purification rate. Incidentally, the NOx purification rate X1 is calculated based on the NOx emission amount Y1 from the engine and the NOx amount Y2 on the downstream side of the SCR catalyst 42 (X1 = (Y1-Y2) / Y1). At this time, the NOx emission amount Y1 is calculated by a map or a mathematical formula based on the respective engine operating state (engine speed, fuel injection amount). Further, the NOx amount Y2 on the downstream side of the SCR catalyst 42 is calculated from the NOx sensor output.

  Specifically, regarding the urea water addition by the urea water addition valve 44, the ECU 50 outputs a valve opening command pulse of a predetermined cycle to the urea water addition valve 44, and the drive unit of the urea water addition valve 44 is accompanied by the pulse output. A drive current flows through the (solenoid part). Then, with the energization, the urea water addition valve 44 is opened and urea water is added (injected). At this time, the urea water addition amount is increased or decreased by variably adjusting the output period (or output frequency) of the valve opening command pulse, and an outline thereof is shown in FIG. When reducing the urea water addition amount with respect to the base addition amount shown in (a), the output period of the valve opening command pulse is increased as shown in (b). When increasing the urea water addition amount, the output period of the valve opening command pulse is reduced as shown in (c). In addition, when decreasing the urea water addition amount, the valve opening drive of the urea water addition valve 44 may be temporarily stopped.

  Next, the configuration of the NOx sensor 47 will be described with reference to FIG. FIG. 3 is a cross-sectional view showing a cross-sectional structure of the sensor element 60 constituting the NOx sensor 47. The sensor element 60 has a so-called three-cell structure including a pump cell, a sensor cell, and a monitor cell, and each cell is laminated and configured. Since the monitor cell has a function of discharging oxygen in the gas like the pump cell, the monitor cell may be referred to as an auxiliary pump cell or a second pump cell.

  In the sensor element 60, solid electrolyte bodies 61 and 62 made of an oxygen ion conductive material such as zirconia are formed in a sheet shape, and are stacked at a predetermined interval in the upper and lower directions of the figure via spacers 63 made of an insulating material such as alumina. ing. Among these, an exhaust inlet 61a is formed in the solid electrolyte body 61 on the upper side of the figure, and exhaust around the sensor element is introduced into the first chamber 64 through the exhaust inlet 61a. The first chamber 64 communicates with the second chamber 66 through the throttle portion 65. On the upper surface of the solid electrolyte body 61 in the figure, a porous diffusion layer 67 for taking in and out the exhaust gas with a predetermined diffusion resistance is provided, and an insulating layer 69 for forming an air passage 68 is provided.

  Further, an insulating layer 71 made of alumina or the like is provided on the lower surface of the solid electrolyte body 62 in the figure, and an atmospheric passage 72 is formed by the insulating layer 71.

  The solid electrolyte body 62 on the lower side of the figure is provided with a pump cell 81 so as to face the first chamber 64, and the pump cell 81 takes in and out oxygen in the exhaust gas introduced into the first chamber 64. The residual oxygen concentration in the chamber 64 is adjusted to a predetermined concentration. The pump cell 81 has a pair of upper and lower electrodes 82 and 83 provided with the solid electrolyte body 62 sandwiched therebetween, and in particular, the electrode 82 on the first chamber 64 side is a NOx inactive electrode (an electrode that is difficult to decompose NOx). Yes. The pump cell 81 decomposes oxygen present in the first chamber 64 in a state where a voltage is applied between the electrodes 82 and 83 and discharges it from the electrode 83 to the atmosphere passage 72 side.

  Further, a monitor cell 84 and a sensor cell 85 are provided in the upper solid electrolyte body 61 in the drawing so as to face the second chamber 66. After the surplus oxygen is discharged by the pump cell 81 described above, the monitor cell 84 generates a current output in accordance with the electromotive force or voltage application according to the residual oxygen concentration in the second chamber 66. The sensor cell 85 detects the NOx concentration from the gas in the second chamber 66.

  The monitor cell 84 and the sensor cell 85 are arranged side by side at positions close to each other, have electrodes 86 and 87 on the second chamber 66 side, and have a common electrode 88 on the atmosphere passage 68 side. That is, the monitor cell 84 is constituted by the solid electrolyte body 61, the electrode 86 and the common electrode 88 arranged to face each other, and the sensor cell 85 is similarly arranged to face the solid electrolyte body 61 and the electrode 87 arranged to sandwich it. And a common electrode 88. The electrode 86 (electrode on the second chamber 66 side) of the monitor cell 84 is made of a noble metal such as Au—Pt that is inactive to NOx, whereas the electrode 87 (electrode on the second chamber 66 side) of the sensor cell 85 is active for NOx. It consists of noble metals such as platinum Pt and rhodium Rh. For convenience, in the drawing, the monitor cell 84 and the sensor cell 85 are shown side by side with respect to the flow direction of the exhaust gas. However, in actuality, these cells 84 and 85 are arranged at the same position with respect to the flow direction of the exhaust gas. It is like that.

  Further, a heater (heating element) 73 for heating the entire sensor is embedded in the insulating layer 71. The heater 73 generates heat energy by supplying power from a battery power source or the like so as to activate the entire sensor element including the pump cell 81, the monitor cell 84, and the sensor cell 85.

  In the sensor element 60 (NOx sensor 47) configured as described above, the exhaust is introduced into the first chamber 64 through the porous diffusion layer 67 and the exhaust introduction port 61a. Then, when this exhaust gas passes in the vicinity of the pump cell 81, a decomposition reaction occurs by applying a pump cell applied voltage between the pump cell electrodes 82 and 83, and the exhaust cell passes through the pump cell 81 according to the oxygen concentration in the first chamber 64. Oxygen is taken in and out. At this time, since the electrode 82 on the first chamber 64 side is a NOx inert electrode, NOx in the exhaust gas is not decomposed in the pump cell 81, but only oxygen is decomposed and discharged from the electrode 83 to the atmospheric passage 72. By the action of the pump cell 81, the inside of the first chamber 64 is maintained in a predetermined low oxygen concentration state.

  The gas that has passed in the vicinity of the pump cell 81 flows into the second chamber 66, and the monitor cell 84 generates an output corresponding to the residual oxygen concentration in the gas. The output of the monitor cell 84 is detected as a monitor cell current when a predetermined monitor cell application voltage is applied between the monitor cell electrodes 86 and 88. Further, when a predetermined sensor cell applied voltage is applied between the sensor cell electrodes 87 and 88, NOx in the gas is reduced and decomposed, and oxygen generated at that time is discharged from the electrode 88 to the atmospheric passage 68. At this time, the concentration of NOx contained in the exhaust gas is detected by the current (sensor cell current) flowing through the sensor cell 85.

  Incidentally, when urea water is added (injected) by the urea water addition valve 44, the NOx purification rate of the SCR catalyst 42 is calculated based on the output of the NOx sensor 47. However, in such a case, the NOx sensor 47 configured as described above reacts not only with NOx in the exhaust gas but also with ammonia (NH3). Therefore, if there is surplus ammonia due to ammonia slip on the downstream side of the SCR catalyst 42, it will be based on ammonia detection. Sensor output is generated. As a result, the NOx purification rate is erroneously calculated, and as a result, the urea water addition amount cannot be optimally controlled.

That is, when the ammonia is surplus in the SCR catalyst 42 and the surplus ammonia is discharged downstream of the catalyst, the NOx sensor 47 causes the gas containing ammonia to flow in the order of the first chamber 64 → the second chamber 66 and enter the sensor cell 85. A chemical reaction of ammonia occurs. Specifically, the sensor cell 85 undergoes an oxidation reaction according to the following equation, and the NOx sensor output increases with the oxidation reaction.
4NH3 + 5O2 → 4NO + 6H2O (Formula 5)
FIG. 4 shows a correspondence relationship between the NOx concentration on the downstream side of the catalyst, the NH3 concentration, and the NOx sensor output with respect to the urea water addition amount.

  Assuming that the urea water addition amount is gradually increased in the temperature range where the SCR catalyst 42 is activated, NOx is reduced because it is purified by the SCR catalyst 42, but the ammonia emission amount is the NOx purification rate. Becomes excessive from the point at which it becomes saturated, and increases with an increase in the amount of urea water added (see FIGS. 4A and 4B). At this time, as described above, since the NOx sensor 47 reacts not only to NOx but also to ammonia, the output of the NOx sensor 47 increases as NOx increases and increases as ammonia increases. That is, the NOx sensor output has a characteristic that shows a downwardly convex shape with respect to the urea water addition amount on the horizontal axis (see FIG. 4C).

  Here, in the NOx sensor output shown in FIG. 4C, there is a urea water addition amount at which the output is minimized. For example, if the urea water addition amount is “A1” in the figure, the NOx sensor output becomes the minimum value. Become. In this case, in the region where the urea water addition amount <A1, the NOx sensor output decreases as the NOx purification rate increases as the urea water addition amount increases. Further, in the region of urea water addition amount> A1, the smaller the urea water addition amount, the smaller the ammonia slip and the smaller the NOx sensor output. As described above, the urea water addition amount at which the NOx sensor output becomes the minimum value is the urea water addition amount at which the ammonia slip amount is small and the NOx purification rate is maximized. In view of this, by controlling the urea water addition amount so that the NOx sensor output becomes the minimum value, it is possible to perform the urea water addition control so that the NOx purification rate is high and the ammonia slip is minimized.

  In the characteristic shown in FIG. 4C, the change gradient (the characteristic gradient) of the NOx sensor output with respect to the change in the urea water addition amount is small near the minimum value of the NOx sensor output, and becomes large when moving away from the minimum value. It has become. Depending on the sensor, there may be a substantially flat region near the minimum value of the NOx sensor output.

  In this embodiment, the urea water addition amount is increased or decreased to add the urea water, and the NOx sensor output is acquired for each addition amount, and the sensor output becomes the minimum value among the acquired sensor outputs. An addition amount command value is calculated from the urea water addition amount. Then, the urea water addition amount is controlled using the calculated addition amount command value as a target value.

  Next, the control procedure of the urea water addition amount will be described with reference to the flowchart of FIG. The processing in FIG. 5 is repeatedly executed by the ECU 50 at a predetermined time period or crank angle period.

  In FIG. 5, first, in step S101, it is determined whether or not the SCR catalyst 42 is in an active state. Specifically, for example, it is determined whether the exhaust temperature is higher than a predetermined value (for example, 150 ° C.). Then, the process proceeds to the subsequent step S102 on condition that the SCR catalyst 42 is in an active state.

  In steps S102 to S104, an execution condition for determining whether or not to perform the increase / decrease process of the urea water addition amount is determined. Specifically, in step S102, it is determined whether or not the change amount ΔNE that is the difference between the previous value and the current value for the engine speed NE is equal to or less than a predetermined value. In step S103, it is determined whether or not the change amount ΔQ, which is the difference between the previous value and the current value, of the fuel injection amount Q is equal to or less than a predetermined value. Further, in step S104, it is determined whether or not the NOx purification rate at that time is smaller than a predetermined value. Here, in steps S102 and S103, it is determined whether or not the engine operating state is in a steady state. In step S104, it is determined whether or not the addition amount command value needs to be updated based on the NOx purification rate each time.

  If any of steps S102 to S104 is NO, the process proceeds to step S105, and if all of steps S102 to S104 are YES, the process proceeds to step S106. In step S105, urea water addition based on each addition amount command value is executed. This corresponds to a normal urea water addition process. At this time, urea water addition is performed using the addition amount command value calculated so far.

  On the other hand, in step S106, the urea water addition amount increasing / decreasing process is executed to update the addition amount command value. The addition amount increasing / decreasing process will be described in detail with reference to FIG. In such an increase / decrease process, the urea water addition amount is changed in the order of the addition amount command value → the addition amount increase value → the addition amount decrease value, and the NOx sensor output is acquired each time. Hereinafter, for convenience of explanation, the urea water addition period based on the addition amount command value is “first period”, the urea water addition period based on the addition amount increase value is “second period”, and the urea water addition period based on the addition amount decrease value. Is referred to as a “third period”.

  In FIG. 6, in step S <b> 201, it is determined whether it is currently the first period (period of urea water addition based on the addition amount command value). And if it is the 1st period, urea water addition by an addition amount command value will be performed at Step S202, and NOx sensor output V1 at that time will be memorized at subsequent Step S203.

  If it is not the first period, the process proceeds to step S204, and it is determined whether or not it is currently the second period (a period of urea water addition based on the added amount increase value). If it is the second period, the process proceeds to step S205, where the addition amount command value is changed to the increase side by a predetermined increase / decrease width (change width) α, and at the addition amount increase value (addition amount command value + α). Perform urea water addition. Subsequently, in step S206, the NOx sensor output V2 when the addition amount is increased is stored.

  If the period is not the second period, the present period can be regarded as the third period (the period of urea solution addition based on the added amount reduction value). In this case, the process proceeds to step S207, and the addition amount command value is changed to the decrease side by the increase / decrease width α, and urea water addition is executed with the addition amount decrease value (addition amount command value−α). Subsequently, in step S208, the NOx sensor output V3 when the addition amount is reduced is stored.

  Returning to the description of FIG. 5, in step S107, it is determined whether or not the addition amount increasing / decreasing process has been completed. At this time, in the processing of FIG. 6, urea water addition is executed by the addition amount command value, the addition amount increase value, and the addition amount decrease value, respectively, and the NOx sensor outputs V1 to V3 at the time of each urea solution addition ( If (memory) has been completed, the process proceeds to step S108, and otherwise the process is temporarily terminated.

  In step S108, a sensor output minimum value Vmin that is a minimum value is calculated from the NOx sensor outputs V1 to V3 described above. In the subsequent step S109, the sensor output maximum value Vmax that is the maximum value is calculated from the NOx sensor outputs V1 to V3.

  Thereafter, in step S110, it is determined whether or not the difference (Vmax−Vmin) between the sensor output maximum value Vmax and the sensor output minimum value Vmin is greater than a predetermined value. At this time, if Vmax−Vmin ≦ predetermined value, it is determined that there is no need to update the addition amount command value, and this processing is ended as it is (that is, without updating the addition amount command value). If Vmax−Vmin> predetermined value, the process proceeds to subsequent step S111.

  In step S111, it is determined whether or not the NOx sensor output V1 among the NOx sensor outputs V1 to V3, that is, the NOx sensor output at the time of urea water addition based on the original addition amount command value (intermediate value) that has not increased or decreased is the minimum value Vmin. Determine. If step S111 is NO, the process proceeds to step S112, and if YES, the process proceeds to step S113.

  In step S112, the urea water addition amount corresponding to the sensor output minimum value Vmin is stored in the EEPROM 51 as the addition amount command value. At this time, either the addition amount increase value (addition amount command value + α) or the addition amount decrease value (addition amount command value−α) described above is stored as a new addition amount command value. The new addition amount command value corresponds to the learning value. Thereby, the update (learning) of the addition amount command value is completed.

  Further, in step S113, an increase / decrease range with respect to the addition amount command value is reduced, and an instruction is given to execute the addition amount increase / decrease process (the process of FIG. 6) again. When the addition amount increase / decrease process is re-executed, the addition amount command value is updated based on the result (steps S108 to S112).

  Even when the step S111 is repeated YES a predetermined number of times, the increase / decrease range with respect to the addition amount command value is reduced, and the addition amount increase / decrease process (the process of FIG. 6) is commanded again. Good. That is, when the increase / decrease process of the urea water addition amount and the minimum value search of the NOx sensor output are repeatedly executed, the NOx sensor output at the time of urea water addition based on the original addition amount command value becomes the minimum value Vmin (addition amount command). When the same value is repeated a predetermined number of times, the amount of increase / decrease with respect to the addition amount command value is reduced, and the addition amount increase / decrease process (the process of FIG. 6) is commanded again.

  Here, the addition amount command value at which the NOx sensor output becomes the minimum value does not change sequentially, and becomes a steady value if the engine operating state is constant. Therefore, the addition amount command value (learning value) does not need to be repeatedly calculated in the same operation state, and may be calculated with a minimum frequency. For example, the addition amount command value may be calculated only once every time power is supplied to the control device (ECU).

  Further, the characteristics of the NOx sensor output with respect to the urea water addition amount vary depending on the engine operating state. More specifically, the characteristics of the NOx sensor output change when the exhaust amount changes or the exhaust temperature changes with a change in the engine operating state. For example, as shown in FIG. 7, when the exhaust amount increases, the sensor output characteristic changes from L1 to L2 (the same applies when the exhaust temperature decreases). Therefore, it is desirable to learn the addition amount command value calculated from FIG. 5 in association with the engine operating state when the addition amount increases or decreases. For example, the engine load (accelerator operation amount) and the engine rotation speed may be used as operating state parameters, and the addition amount command value may be learned in association with these parameters.

  In addition to learning the addition amount command value in association with the engine load and the rotation speed, it is also possible to learn the addition amount command value in association with the exhaust state such as the exhaust amount and the exhaust temperature.

  Next, the procedure for searching for the increase / decrease of the addition amount and the sensor output minimum value based thereon will be described more specifically. FIG. 8 is a time chart showing the transition of the NOx sensor output when the addition amount is increased or decreased, and FIGS. 9 and 10 are diagrams schematically showing the characteristics of the NOx sensor output.

  In FIG. 8, as three addition periods with different urea water addition amounts, a first period T1 (a urea solution addition period based on an addition amount command value) and a second period T2 (a urea solution addition period based on an addition amount increase value) The third period (period of urea water addition based on the added amount reduction value) is shown. FIGS. 9 and 10 correspond to FIG. 4C described above. In FIGS. 9 and 10, for convenience of explanation, the characteristics of the NOx sensor output with respect to the urea water addition amount are shown in a V shape. This makes it easy to understand the magnitude relationship of the NOx sensor output. 9 and 10, the numbers 1, 2, and 3 indicate the order in which the urea water addition amount is changed. The urea water addition by the addition amount command value, the urea water addition by the addition amount increase value, and the addition amount decrease, respectively. This corresponds to urea water addition depending on the value.

  When the urea water addition amount is increased or decreased as described above, the addition amount command value for each time is larger than the urea water addition amount corresponding to the sensor output minimum value due to the characteristics of the NOx sensor output (see FIG. 4C). The change mode of the NOx sensor output at the time of increase / decrease of the addition amount changes depending on how small or small it is.

  For example, in the case of FIG. 9A, the addition amount command value, the addition amount increase value, and the addition amount decrease value each time are larger than the urea water addition amount corresponding to the sensor output minimum value. In this case, the NOx sensor output V3 at the time of urea water addition by the addition amount reduction value becomes the minimum value. Therefore, the addition amount command value is updated with the urea water addition amount (addition amount reduction value) corresponding to the NOx sensor output V3.

  The time chart in the case of FIG. 9A is FIG. In FIG. 8A, the urea water addition amount is increased or decreased by changing the output cycle of the valve opening command pulse of the urea water addition valve 44. As the urea water addition amount increases or decreases, V1, V2, and V3 are acquired as NOx sensor outputs, and the NOx sensor output V3 is obtained as the minimum value Vmin.

  In FIG. 8A, when the urea water addition amount is increased or decreased, the period T3 during which the addition amount is reduced is longer than the period T2 during which the addition amount is increased. That is, when the amount of urea water addition is increased and when it is decreased, the response speed of the NOx sensor output is different, and the latter is slower. This is due to the fact that the consumption rate of ammonia in the SCR catalyst 42 is smaller than the ammonia adsorption rate in the catalyst 42. As described above, the sensor output value can be appropriately obtained at any time when the addition amount is increased or decreased by making a difference in the length of the period between when the urea water is increased and when the amount is decreased. In any case, the minimum necessary period can be set.

  Further, when the NOx sensor output V3 is obtained as the minimum value Vmin as shown in FIG. 8A and FIG. 9A, the same addition amount increase / decrease process is continuously performed to search for the minimum value Vmin. Will be described. In such a case, as shown in FIGS. 8B and 9B, urea water addition is performed using the previously calculated addition amount command value (V3 in FIG. 9A) as the current value of the addition amount command value. After the NOx sensor output V1 ′ is acquired, urea water addition by the addition amount increase value and urea water addition by the addition amount decrease value are executed. Then, NOx sensor outputs V2 'and V3' are acquired. At this time, the NOx sensor output V1 'becomes the minimum value among the NOx sensor outputs V1' to V3 ', and the addition amount command value remains unchanged. For example, in FIG. 8B, “V2′−V1 ′” corresponds to the change width (Vmax−Vmin), and if the change width is equal to or less than a predetermined value, the addition amount command value is not updated. It is like that.

  Incidentally, when the addition amount increase / decrease process of FIGS. 9 (a) and 9 (b) is continued, when the addition amount increase / decrease shown in FIG. 9 (b), the NOx sensor output is increased on the increase side from the processing result shown in FIG. 9 (a). It is known beforehand that V will increase (V2 ′> V1 ′). Therefore, it is good also as a structure which changes the urea water addition amount only to the amount reduction side.

  Next, the case where the addition amount increase / decrease process is performed for each addition amount command value and then the increase / decrease range is reduced and the addition amount increase / decrease process is executed again will be described with reference to FIGS. .

  In FIG. 10 (a), NOx sensor outputs V1 to V3 are acquired as shown in the figure as the urea solution is added in three stages according to the respective addition amount command value, addition amount increase value, and addition amount decrease value. At this time, among the NOx sensor outputs V1 to V3, the NOx sensor output V1, that is, the NOx sensor output at the time of urea water addition by the original addition amount command value (intermediate value) that has not increased or decreased is the minimum value Vmin. In such a case, a more desirable appropriate value of the addition amount command value (that is, the NOx sensor output is smaller) between the maximum addition amount (addition amount increase value) and the minimum addition amount (addition amount decrease value) when urea water is increased or decreased. The amount of urea water added) may be present. Therefore, as shown in FIG. 10B, the increase / decrease process with respect to the addition amount command value is reduced and the addition amount increase / decrease process is executed again.

  In FIG. 10B, among the newly acquired NOx sensor outputs V1 ′ to V3 ′, the NOx sensor output V3 ′ at the time of urea water addition by the addition amount reduction value becomes the minimum value. The addition amount command value is updated by the urea water addition amount (addition amount reduction value) corresponding to the output V3 ′.

  According to the embodiment described in detail above, the following excellent effects can be obtained.

  The urea water addition is performed while changing to different urea water addition amounts, and the NOx sensor output is acquired for each addition amount, and the addition amount command value is determined by the urea water addition amount at which the sensor output is the minimum value among them. Since the ammonia slip amount is small and the NOx purification rate is maximized, the urea water addition amount can be found as the addition amount command value. As a result, the NOx amount on the downstream side of the SCR catalyst 42 can be correctly detected, and the NOx purification rate and the like can be calculated appropriately.

  Further, since the urea water addition by the urea water addition valve 44 is executed with the addition amount command value calculated as described above as a target value during engine operation, the state where the NOx purification rate is maximized during engine operation is maintained. it can. Moreover, it is possible to continue the state in which ammonia is discharged to the downstream side of the SCR catalyst 42 as much as possible. That is, both of the NOx purification rate and the reduction of ammonia slip can be realized.

  When the addition amount command value is increased / decreased by a predetermined increase / decrease range and the urea solution addition in three stages is executed, and the NOx sensor output at the time of adding an intermediate amount of urea solution among the additions of the urea solution in the three stages becomes the minimum value In addition, since the increase / decrease range is reduced and the urea water addition amount is changed again, the optimum value of the addition amount command value can be obtained with high accuracy.

  The addition amount command value is updated only when the NOx purification rate is relatively low. Therefore, when the NOx purification rate is at an appropriate level and it is not necessary to update the addition amount command value, calculation (update) of the unnecessary addition amount command value can be stopped.

  Since the addition amount command value is not newly calculated when the difference (Vmax−Vmin) between the sensor output maximum value Vmax and the sensor output minimum value Vmin when the urea water increases / decreases is smaller than a predetermined value, an unnecessary addition amount Command value calculation (update) can be stopped.

  Since the addition amount command value at which the NOx sensor output becomes the minimum value is stored in the EEPROM 51 (backup memory) as a learning value, it is possible to control the urea water addition while constantly grasping the characteristics of the NOx sensor output. It can be suitably reflected. Further, it is only necessary to calculate the addition amount command value at which the NOx sensor output becomes the minimum value with the necessary minimum frequency, and the calculation load for calculating the addition amount command value can be reduced.

  Further, by configuring the addition amount command value in the EEPROM 51 in association with the engine operating state (including the exhaust state), even when the engine operating state changes during the urea water addition by the urea water addition valve 44. The optimum addition amount command value can be set each time.

  The present invention is not limited to the description of the above embodiment, and may be implemented as follows, for example.

  -In the above embodiment, when the urea water addition amount is increased or decreased, the change amount is constant, and the addition amount command value is set as the center value, and the urea water addition amount is added across the addition amount command value. Although it was set as the structure changed in order of an amount increase value-> addition amount decrease value, this may be changed. For example, as shown in FIG. 11A, the urea water addition amount may be changed in one direction on the increase side (or on the decrease side). In FIG. 11A, as indicated by the circled numbers 1 to 4, the urea water addition amount is increased by a certain amount to obtain the NOx sensor output every time. Or as shown in FIG.11 (b), you may make it increase urea water addition amount (it is also possible to reduce it), changing a change width. In FIG. 11 (b), the change width is increased when the addition amount is changed as indicated by the circle numbers 1 → 2 → 3, and the change width is reduced when the addition amount is changed as indicated by the subsequent circle numbers 3 → 4 → 5.

  -The change width of the urea water addition amount may be variably set based on the output value of the NOx sensor. For example, the greater the NOx sensor output value, the greater the change amount of the urea water addition amount. As can be seen from FIG. 4, when the NOx sensor output is relatively large, the change gradient (change rate) of the NOx sensor output with respect to the change in the urea water addition amount is relatively large, and the NOx sensor output is relatively small. The change gradient (change rate) of the NOx sensor output with respect to the change of the urea water addition amount is relatively small. Therefore, it is possible to appropriately find the minimum value of the NOx sensor output by setting the change width of the urea water addition amount based on the output value of the NOx sensor as described above.

  -Based on the NOx sensor output before the change of the urea water addition amount and the NOx sensor output after the change, predict whether the urea water addition amount at which the NOx sensor output becomes the minimum value is on the decrease side or the increase side The urea water addition amount may be increased or decreased based on the prediction result. For example, the increase / decrease direction is reversed when the NOx sensor output increases with respect to the increase / decrease in the urea water addition amount. Alternatively, when the NOx sensor output decreases with the increase / decrease change of the urea water addition amount, the urea water addition amount is changed in the same increase / decrease direction. According to this configuration, the urea water addition amount can be increased or decreased only on the side where the urea water addition amount at which the sensor output becomes the minimum value is present (either the reduction side or the increase side). Therefore, the increase / decrease process of the urea water addition amount can be simplified.

  -It is determined whether or not the ammonia slip catalyst 43 provided downstream of the SCR catalyst 42 is in an active state, and when it is determined that the ammonia slip catalyst 43 is in an inactive state, the urea water addition amount is increased or decreased And updating the addition amount command value. Specifically, the ECU 50 determines the active state of the ammonia slip catalyst 43 based on the temperature (actually measured value or estimated value) of the ammonia slip catalyst 43, the elapsed time after engine start, and the like. Then, increase / decrease processing of the added amount of urea water is executed. According to such a configuration, when the ammonia slip catalyst 43 is in an inactive state, it is possible to suppress the discharge of ammonia to the downstream side of the SCR catalyst 42 and thus suppress the discharge of ammonia to the atmosphere.

  In the above embodiment, the urea water addition amount is controlled by adjusting the output period of the valve opening command pulse to the urea water addition valve 44. However, instead of this, the opening to the urea water addition valve 44 is controlled. The urea water addition amount may be controlled by adjusting the pulse length (energization time) of the valve command pulse.

  A NOx sensor using a sensor element other than the structure shown in FIG. 3 can be applied. For example, it is good also as a structure (2 cell structure) which has a pump cell and a sensor cell and does not have a monitor cell (2nd pump cell). Alternatively, the oxygen pumping may be performed between the sensor element and the surroundings of the sensor element in place of the configuration in which oxygen pumping (pumping and filling) is performed with the atmosphere chamber (atmosphere passage 72 in FIG. 3) in the pump cell. .

  As the reducing agent addition means, it is possible to adopt a configuration other than the urea water addition valve 44 (corresponding to an electromagnetically driven injector). For example, a urea water addition nozzle that simultaneously ejects urea water as a reducing agent and pressurized air from the nozzle tip can be employed as the reducing agent addition means.

The block diagram which shows the outline of the engine control system in embodiment of invention. The time chart for demonstrating the output form of a valve opening command pulse. Sectional drawing which shows the cross-section of the sensor element which comprises a NOx sensor. The figure which shows the correspondence of NOx density | concentration of the downstream of a catalyst with respect to urea water addition amount, NH3 density | concentration, and NOx sensor output. The flowchart which shows the control procedure of urea water addition amount. The flowchart which shows addition amount increase / decrease processing. The figure which shows the characteristic of NOx sensor output. The time chart which shows transition of the NOx sensor output at the time of addition amount increase / decrease. The figure which shows the characteristic of NOx sensor output typically. The figure which shows the characteristic of NOx sensor output typically. The figure which shows the characteristic of NOx sensor output typically.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Engine, 22 ... Exhaust pipe, 42 ... SCR catalyst (NOx purification catalyst), 43 ... Ammonia slip catalyst (oxidation catalyst), 44 ... Urea water addition valve (reducing agent addition means), 47 ... NOx sensor, 50 ... ECU (Addition amount control means, command value calculation means, learning means, activity determination means), 51 ... EEPROM, 60 ... sensor element, 61,62 ... solid electrolyte body, 81 ... pump cell, 84 ... monitor cell, 85 ... sensor cell.

Claims (11)

A NOx purification catalyst provided in the exhaust passage of the internal combustion engine, a reducing agent addition means for adding a reducing agent to the exhaust upstream side of the NOx purification catalyst, a NOx sensor for detecting the NOx amount on the exhaust downstream side of the NOx purification catalyst, An exhaust gas purification apparatus for an internal combustion engine that executes addition of a reducing agent by the reducing agent addition means, with each addition amount command value as a target value,
An addition amount control means for adding a reducing agent by the reducing agent addition means while changing to different addition amounts,
The sensor output of the NOx sensor is acquired for each addition amount at the time of addition of the reducing agent by the addition amount control means, and the addition is performed according to the reducing agent addition amount at which the sensor output becomes a minimum value among the obtained sensor outputs. Command value calculating means for calculating a quantity command value;
An exhaust emission control device for an internal combustion engine, comprising:   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the addition amount control means changes the reducing agent addition amount to at least one of an increase side and a decrease side on the basis of each addition amount command value.   The addition amount control means executes at least three stages of reducing agent addition by changing the reducing agent addition amount within a predetermined change range, and at the time of adding an intermediate amount of the reducing agent among the at least three stages of reducing agent addition. 3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein when the sensor output in the engine reaches a minimum value, the reducing agent addition is performed again with the change of the reducing agent addition amount after reducing the change width. . Based on the sensor output before the change of the reducing agent addition amount and the sensor output after the change, a means for predicting whether the reducing agent addition amount at which the sensor output becomes the minimum value is on the decrease side or the increase side With
The exhaust purification device for an internal combustion engine according to any one of claims 1 to 3, wherein the addition amount control means increases or decreases the amount of addition of the reducing agent based on a prediction result by the prediction means.   The exhaust purification device for an internal combustion engine according to any one of claims 1 to 4, wherein the addition amount control means sets a range of change of the reducing agent addition amount based on an output value of the NOx sensor.   If the difference between the minimum value and the maximum value of the sensor output acquired with the change of the reducing agent addition amount by the addition amount control means is within a predetermined value, the command value calculation means newly sets the addition amount command value. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 5, wherein the exhaust gas purification device is not calculated.   The internal combustion engine according to any one of claims 1 to 6, wherein when the reducing agent addition amount is increased or decreased by the addition amount control means, a period during which the addition amount is reduced is longer than a period during which the addition amount is increased. Exhaust purification device.   The internal combustion engine according to any one of claims 1 to 7, further comprising learning means for storing the addition amount command value calculated by the command value calculation means as a learning value in a backup memory and updating the learning value as needed. Engine exhaust purification system.   9. The learning unit according to claim 8, wherein the learning unit stores the addition amount command value calculated by the command value calculation unit in the memory in association with an operation state of the internal combustion engine at the time of addition amount control by the addition amount control unit. An exhaust purification device for an internal combustion engine. An exhaust gas purification apparatus for an internal combustion engine comprising an oxidation catalyst for purifying a reducing agent provided on the downstream side of the NOx purification catalyst, and an activity determination means for determining whether or not the oxidation catalyst is in an active state;
The internal combustion engine according to any one of claims 1 to 9, wherein when the activation determination unit determines that the oxidation catalyst is in an inactive state, the addition amount command value is calculated by the command value calculation unit. Exhaust purification equipment.   The exhaust purification apparatus for an internal combustion engine according to any one of claims 1 to 10, wherein the NOx sensor includes a sensor element having a solid electrolyte body and a NOx detection electrode provided on the solid electrolyte body. .

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