JP2017003298A - OUTPUT CORRECTION DEVICE OF NOx SENSOR - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an output correction device of an NOx sensor that can accurately output a value corresponding to the concentration of NOx and NH3 which are contained in exhaust gas.SOLUTION: An output correction device of an NOx sensor is applied to an NOx sensor 77 which is disposed downstream an NOx purification catalyst in an exhaust passage of an internal combustion engine, and outputs a detection value corresponding to the concentration of NOx and NHcontained in exhaust gas. A control part 100 that the device includes corrects a detection value by the NOx sensor to a larger value as a molar ratio between NO and NHcontained in exhaust gas which reaches the NOx sensor becomes closer to 1:1. Thus, the device can appropriately correct the detection value by the NOx sensor, and accurately acquire the concentration of NOx and NHcontained in exhaust gas.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a NOx sensor output correction device that corrects an output value of a NOx sensor that detects a concentration of nitrogen oxide (NOx) in exhaust gas discharged from a combustion chamber of an internal combustion engine.

  Conventionally, NOx sensors that detect the concentration of nitrogen oxide (NOx) in the exhaust gas of an internal combustion engine are known. In many cases, this NOx sensor is disposed at a position downstream of the NOx purification catalyst such as an SCR catalyst and an NSR catalyst.

The SCR catalyst performs selective catalytic reduction (SCR). SCR refers to reducing and purifying NOx (nitrogen oxide) in exhaust gas to nitrogen (N ₂ ) by NH ₃ (ammonia) generated from a reducing agent (urea) added upstream of the SCR catalyst. .

On the other hand, the NSR catalyst performs NOx storage reduction (NSR: NOx Storage-Reduction). The NSR oxidizes nitric oxide (NO) in the exhaust gas in a lean combustion state to nitrogen dioxide (NO ₂ ), and stores it as a metal nitrate using an alkaline earth metal. When a rich combustion state is created instantaneously (when a so-called “rich spike” is introduced), the NSR catalyst decomposes the nitrate and reduces and purifies NOx to nitrogen (N ₂ ).

By the way, in the SCR catalyst, ammonia (NH ₃ ) flows out downstream of the catalyst (so-called “ammonia”, for example, due to the addition of an excessive reducing agent and / or a decrease in activity accompanying the deterioration of the catalyst). Slip ") may occur. It is known that NH ₃ is also generated in the NSR catalyst by the above-described rich spike. In this way, NH ₃ in the exhaust also affects the detection value of the NOx sensor. Specifically, it is known that the detected value of the NOx sensor corresponds to “the concentration of NOx and the concentration of NH ₃ ” in the exhaust gas (see, for example, Patent Documents 1 and 2).

JP 2009-156229 A JP 2013-213466 A

In order to perform these accurately, such as when performing a self-diagnosis of whether the NOx sensor is normal and performing various controls of the internal combustion engine using the output value of the NOx sensor, the NOx sensor is It is important to accurately output values corresponding to the concentrations of “NOx and NH ₃ ” contained in the exhaust gas.

The present inventor has determined that the NOx sensor is more effective than the “detected value (estimated value) that the NOx sensor will output” estimated based on the concentration of “NOx and NH ₃ ” contained in the exhaust gas that reaches the NOx sensor. It has been found that the actual detection value (actual measurement value) may be small. Furthermore, the present inventor has found that the magnitude of the difference between the estimated value and the actually measured value varies depending on the molar ratio of NOx and NH ₃ in the exhaust gas. However, conventionally, no technology has been proposed that can accurately obtain values corresponding to the concentrations of “NOx and NH ₃ ” contained in the exhaust gas in consideration of this molar ratio.

From the above, one of the objects of the present invention is to accurately adjust the concentration of “NOx and NH ₃ ” based on the detection value of the NOx sensor even in a situation where “NOx and NH ₃ ” is contained in the exhaust gas. An object of the present invention is to provide a NOx sensor output correction device capable of obtaining a well-matched output value.

In view of the above points, the NOx sensor output correction device according to the present invention (hereinafter sometimes referred to as “the present invention device”) is interposed in the exhaust passage of the internal combustion engine and is exhausted. NOx contained in the exhaust gas that is disposed downstream of the NOx purification catalyst that reduces and purifies NOx contained in the exhaust gas and that is made of a porous material through which the exhaust gas can pass. Thus, the present invention is applied to a NOx sensor that has an electrode portion that converts to N ₂ and outputs a value corresponding to the amount of NO reduced at the electrode portion as a detection value.

  The device of the present invention includes a control unit that obtains the corrected detection value as an output value by correcting the detection value. The control unit includes an estimation unit and a correction unit.

The estimation unit estimates each concentration of NO and NH _{3 in} the exhaust gas flowing out from the NOx purification catalyst based on an acquired value other than the detection value.

The correction unit corrects the value detected by the NOx sensor to a larger value as the molar ratio between NO and NH ₃ in the exhaust gas derived from the estimated NO concentration and NH ₃ concentration is closer to 1: 1. To do. In other words, the correction unit increases the detection value after correction with respect to the detection value as the molar ratio of NH ₃ to NO derived from the estimated NO concentration and the estimated NH ₃ concentration is closer to 1. The corrected detection value is obtained by correcting the detection value so that the ratio becomes a larger value. Specifically, for example, the correction unit performs a ratio of the detection value after correction to the detection value when the difference between the “molar ratio of NH _{3 to} NO” and 1 is a first value. The first correction ratio is greater than a second correction ratio that is a ratio of the detection value after correction to the detection value when the magnitude of the difference is a second value that is larger than the first value. The detected value is corrected so as to be a value. Accordingly, even if the detected value is the same, the larger the detected “molar ratio of NH _{3 to} NO” is, the larger the detected value after correction is obtained.

According to the apparatus of the present invention, the concentration of “NOx and NH ₃ ” contained in the exhaust gas can be obtained with high accuracy from the detection value obtained by the NOx sensor.

By the way, as described above, the closer the molar ratio between NO and NH ₃ in the exhaust gas reaching the NOx sensor is closer to 1: 1 (the smaller the difference between the molar ratio of NH _{3 to} NO and 1 is). The detection value actually output by the NOx sensor is smaller than the detection value estimated based on the “NOx concentration and NH ₃ concentration” in the exhaust gas. The cause of such a phenomenon is that a reaction represented by the following chemical reaction formula (1) occurs in the diffusion layer provided in the NOx sensor, and NO and NH ₃ in the exhaust gas have a molar ratio of 1: 1. Is consumed.

The yield of the above reaction is not only the molar ratio of NO and NH ₃ in the exhaust, but also the operation of the internal combustion engine, such as the flow rate (exhaust flow rate), temperature (exhaust temperature) and oxygen concentration of the exhaust gas reaching the NOx sensor. Can be affected by the situation.

Specifically, the reaction occurs while NO and NH ₃ contained in the exhaust gas pass through the diffusion layer. Therefore, the shorter the time required for the exhaust gas to pass through the diffusion layer, the shorter the time during which the reaction can occur, and the less NO and NH ₃ are consumed by the reaction. That is, the higher the speed of the exhaust gas passing through the diffusion layer, the smaller the amount of decrease in the detected value by the NOx sensor due to the reaction. Therefore, when the molar ratio between NO and NH ₃ in the exhaust gas reaching the NOx sensor is constant, it is desirable to obtain a smaller corrected detection value as the exhaust gas flow rate increases.

  Therefore, in the output correction apparatus for a NOx sensor according to one embodiment of the present invention, the control unit may further include an exhaust flow rate detection unit that detects an exhaust flow rate that is a flow rate of exhaust gas that reaches the NOx sensor. As will be described in detail later, the exhaust flow rate can be acquired based on, for example, an intake air amount detected by an air flow meter. In this case, the exhaust flow rate detection unit includes an air flow meter. Further, the correction unit corrects the detection value so that the difference between the first corrected detection value and the second corrected detection value decreases as the detected exhaust flow rate increases. It may be configured to obtain a corrected detection value.

Based on the above, it is possible to more appropriately correct the detection value by the NOx sensor in accordance with the change in consumption due to the above reaction of NO and NH ₃ due to the change in the exhaust flow rate. As a result, the concentration of “NOx and NH ₃ ” contained in the exhaust gas can be obtained with higher accuracy from the detection value obtained by the NOx sensor.

By the way, the higher the temperature of the diffusion layer that is the reaction field of the reaction represented by the chemical reaction formula (1), the higher the reaction occurs, and the more NO and NH ₃ are consumed by the reaction. Become. For example, the temperature of the diffusion layer becomes higher as the temperature of the exhaust gas passing through the diffusion layer is higher. That is, the higher the temperature of the exhaust gas that reaches the NOx sensor (exhaust temperature), the greater the amount of decrease in the detected value by the NOx sensor due to the reaction. Therefore, when the molar ratio between NO and NH ₃ contained in the exhaust gas reaching the NOx sensor is constant, it is desirable to obtain a larger corrected detection value as the exhaust gas temperature is higher.

  Therefore, in the NOx sensor output correction apparatus according to another embodiment of the present invention, the control unit may further include an exhaust temperature detection unit that detects an exhaust temperature that is the temperature of the exhaust that reaches the NOx sensor. As will be described in detail later, the exhaust temperature can be detected by a temperature sensor disposed in an exhaust pipe (exhaust passage) upstream of the NOx sensor. In this case, the exhaust temperature detection unit includes a temperature sensor. Furthermore, the correction unit corrects the detection value so that the difference between the detection value after the first correction and the detection value after the second correction increases as the detected exhaust gas temperature increases. It may be configured to obtain a corrected detection value.

Based on the above, it is possible to more appropriately correct the detected value by the NOx sensor in accordance with the change in consumption due to the above reaction of NO and NH ₃ due to the change in the exhaust temperature. As a result, the concentration of “NOx and NH ₃ ” contained in the exhaust gas can be obtained with higher accuracy from the detection value obtained by the NOx sensor.

By the way, oxygen (O ₂ ) is contained in the reaction product of the chemical reaction formula (1). Therefore, the greater the amount of O ₂ contained in the exhaust gas that reaches the NOx sensor, the higher the rate of the reaction, and the more NO and NH ₃ are consumed by the reaction. That is, the larger the amount of O ₂ contained in the exhaust gas that reaches the NOx sensor, the greater the amount of decrease in the detected value by the NOx sensor due to the reaction. Therefore, when the molar ratio between NO and NH ₃ in the exhaust gas that reaches the NOx sensor is constant, it is desirable to obtain a larger corrected detection value as the amount of O ₂ contained in the exhaust gas increases.

Therefore, in the output correction device for a NOx sensor according to still another embodiment of the present invention, the control unit detects an oxygen concentration that is an oxygen concentration that is a concentration of O ₂ contained in the exhaust gas that reaches the NOx sensor. A detection part may be further provided. As will be described in detail later, the oxygen concentration can be detected by an O ₂ sensor disposed in an exhaust pipe (exhaust passage) upstream of the NOx sensor. In this case, the oxygen concentration detection unit includes an O ₂ sensor. Further, the correction unit corrects the detection value so that the difference between the first corrected detection value and the second corrected detection value increases as the detected oxygen concentration increases. It may be configured to obtain a corrected detection value.

According to the above, the detection value by the NOx sensor is more appropriately corrected in accordance with the change in consumption due to the above reaction of NO and NH ₃ caused by the change in the concentration of O ₂ contained in the exhaust gas that reaches the NOx sensor. can do. As a result, the concentration of “NOx and NH ₃ ” contained in the exhaust gas can be obtained with higher accuracy from the detection value obtained by the NOx sensor.

By the way, the NOx purification catalyst is a NOx purification catalyst capable of discharging NH ₃ downstream thereof. More specifically, the NOx purification catalyst includes one or both of an SCR catalyst and an NSR catalyst.

  Other objects, other features, and attendant advantages of the present invention will be readily understood from the description of each embodiment of the present invention described with reference to the following drawings.

FIG. 1 is an overall view of an internal combustion engine to which an output correction device (first device) for a NOx sensor according to a first embodiment of the present invention is applied. FIG. 2 is a diagram showing the internal structure of the downstream NOx sensor. FIG. 3 is a diagram showing an electrochemical reaction of NOx in the downstream NOx sensor. FIG. 4 is a diagram showing an electrochemical reaction of NH ₃ inside the downstream NOx sensor. Figure (a) of 5 is a schematic graph showing the relationship between the magnitude of the molar ratio and the sensor current IS of the NH ₃ in the total amount of NO and NH ₃ contained in the exhaust, (b) is the NH ₃ 6 is a schematic graph showing the relationship between the molar ratio of the sensor and the magnitude of the correction coefficient k for appropriately correcting the sensor current IS. FIG. 6 is a schematic diagram illustrating a configuration of a control unit included in the first device. FIG. 7 is a flowchart showing a detection value correction routine executed by the CPU of the ECU functioning as the first device. FIG. 8A is a schematic graph showing how the relationship between the molar ratio of NH ₃ and the magnitude of the sensor current IS in the total amount of NO and NH ₃ contained in the exhaust gas changes depending on the operating state of the internal combustion engine. (B) is a schematic graph showing how the relationship between the molar ratio of NH ₃ and the magnitude of the correction coefficient k for appropriately correcting the sensor current IS changes depending on the operating condition of the internal combustion engine. is there. FIG. 9 is a schematic diagram illustrating a configuration of a control unit included in the second device. FIG. 10 is a flowchart showing a detection value correction routine executed by the CPU of the ECU functioning as the second device.

<First Embodiment>
Hereinafter, an NOx sensor output correction apparatus (hereinafter also referred to as “first apparatus”) according to a first embodiment of the present invention will be described with reference to the drawings.

<Configuration of internal combustion engine>
The first device is applied to the internal combustion engine (engine) 10 shown in FIG. The engine 10 is a multi-cylinder (in this example, in-line four cylinders), four-cycle, piston reciprocating type, and diesel engine. The engine 10 includes an engine body 20, a fuel supply system 30, an intake system 40, an exhaust system 50, and an EGR system 60.

  The engine main body 20 includes a main body 21 including a cylinder block, a cylinder head, a crankcase, and the like. The main body 21 is formed with four cylinders (combustion chambers) # 1 to # 4. A fuel injection valve (injector) 23 is provided above each of the cylinders # 1 to # 4. Each fuel injection valve 23 opens in response to an instruction from an engine ECU (electronic control unit) 80 described later, and directly injects fuel into the corresponding cylinders # 1 to # 4.

  The fuel supply system 30 includes a fuel pressurization pump (supply pump) 31, a fuel delivery pipe 32, and a common rail (pressure accumulation chamber) 33. The discharge port of the fuel pressurization pump 31 is connected to the fuel delivery pipe 32. The fuel delivery pipe 32 is connected to the common rail 33. The common rail 33 is connected to the fuel injection valve 23.

  The fuel pressurizing pump 31 pressurizes after pumping up fuel stored in a fuel tank (not shown), and supplies the pressurized high-pressure fuel to the common rail 33 through the fuel delivery pipe 32.

  The intake system 40 includes an intake manifold 41, an intake pipe 42, an air cleaner 43, a compressor 44a of a supercharger 44, an intercooler 45, a throttle valve 46, and a throttle valve actuator 47.

  The intake manifold 41 includes “branches connected to the respective cylinders # 1 to # 4” and “collected portions in which the branches are gathered”. The intake pipe 42 is connected to the collecting portion of the intake manifold 41. The intake manifold 41 and the intake pipe 42 constitute an intake passage.

  In the intake pipe 42, an air cleaner 43, a compressor 44a, an intercooler 45, and a throttle valve 46 are sequentially arranged from the upstream side to the downstream side of the flow of intake air. The throttle valve actuator 47 changes the opening of the throttle valve 46 in accordance with an instruction from the ECU 80.

  The exhaust system 50 includes an exhaust manifold 51, an exhaust pipe 52, a turbine 44b of the supercharger 44, an exhaust purification device 53, and a urea water addition device 54.

  The exhaust manifold 51 includes “branches connected to the respective cylinders # 1 to # 4” and “collected portions in which the branches are gathered”. The exhaust pipe 52 is connected to a collecting portion of the exhaust manifold 51. The exhaust manifold 51 and the exhaust pipe 52 constitute an exhaust passage.

  A turbine 44b and an exhaust purification device 53 are disposed in the exhaust pipe 52 from the upstream side to the downstream side of the exhaust flow.

The exhaust purification device 53 includes an oxidation catalyst 53a, a catalyst-carrying diesel particulate filter (hereinafter referred to as “DPR”) 53b, a selective reduction type NOx catalyst (hereinafter referred to as “SCR catalyst”) 53c, and NH ₃ oxidation catalyst (ASC) 53d is included.

The oxidation catalyst 53 a is disposed in the exhaust pipe 52 at a position downstream of the turbine 44 b of the supercharger 44. The DPR 53b is disposed in the exhaust pipe 52 at a position downstream of the oxidation catalyst 53a. The SCR catalyst 53c is disposed in the exhaust pipe 52 at a position downstream of the DPR 53b. The NH ₃ oxidation catalyst 53d is disposed in the exhaust pipe 52 at a position downstream of the SCR catalyst 53c.

  The urea water addition device 54 includes a urea water tank 55, a first connection pipe 56, a urea water pressurization device 57, a second connection pipe 58 and a urea water injection valve 59. The first connection pipe 56 connects the urea water tank 55 and the urea water pressurizing device 57. The second connection pipe 58 connects the urea water pressurizing device 57 and the urea water injection valve 59. The urea water injection valve 59 is disposed in the exhaust pipe 52 upstream of the SCR catalyst 53c.

  The supercharger 44 is a known variable displacement supercharger, and a plurality of nozzle vanes (variable nozzles) (not shown) are provided in the turbine 44b. The opening degree of the nozzle vane is changed according to an instruction from the ECU 80, and as a result, the supercharging pressure is changed (controlled).

  The EGR system 60 includes an exhaust gas recirculation pipe 61, an EGR control valve 62, and an EGR cooler 63.

  The exhaust gas recirculation pipe 61 communicates an exhaust passage (exhaust manifold 51) upstream of the turbine 44b with an intake passage (intake manifold 41) downstream of the throttle valve 46. The exhaust gas recirculation pipe 61 constitutes an EGR gas passage.

  The EGR control valve 62 is disposed in the exhaust gas recirculation pipe 61. The EGR control valve 62 can change the exhaust amount (EGR gas amount) recirculated from the exhaust passage to the intake passage by changing the cross-sectional area of the EGR gas passage in accordance with an instruction from the ECU 80. It has become.

  The EGR cooler 63 is interposed in the exhaust gas recirculation pipe 61 so that the temperature of the EGR gas passing through the exhaust gas recirculation pipe 61 is lowered.

  The ECU 80 is an electronic circuit including a known microcomputer, and includes a CPU, a ROM, a RAM, a backup RAM, an interface, and the like. The ECU 80 is connected to sensors described below, and receives (inputs) signals from these sensors. Further, the ECU 80 sends instruction (drive) signals to various actuators (the fuel injection valve 23, the urea water injection valve 59, etc.).

  The ECU 80 includes an air flow meter 71, a throttle valve opening sensor 72, an EGR control valve opening sensor 73, a crank angle sensor 74, a water temperature sensor 75, a NOx sensor 76, a NOx sensor 77, an accelerator operation amount sensor 78, a vehicle speed sensor 79, a temperature. The sensor 81 and the temperature sensor 82 are connected.

  The air flow meter 71 is disposed in the intake pipe 42. The air flow meter 71 measures the mass flow rate (intake air amount) of intake air passing through the intake passage and outputs a signal representing the intake air amount Ga.

  The throttle valve opening sensor 72 detects the opening (throttle valve opening) of the throttle valve 46 and outputs a signal representing the throttle valve opening TA.

  The EGR control valve opening sensor 73 detects the opening of the EGR control valve 62 and outputs a signal representing the opening.

  The crank angle sensor 74 is disposed in the engine body 20. The crank angle sensor 74 outputs a signal corresponding to a rotational position (that is, crank angle) of a crankshaft (not shown) of the engine 10.

  The ECU 80 acquires the crank angle (absolute crank angle) of the engine 10 based on the compression top dead center of a predetermined cylinder based on signals from the crank angle sensor 74 and a cam position sensor (not shown). Further, the ECU 80 acquires the engine speed NE based on the signal from the crank angle sensor 74.

  The water temperature sensor 75 detects the temperature of the cooling water (cooling water temperature) of the engine 10 and outputs a signal representing the cooling water temperature THW.

  The NOx sensor 76 is disposed in the exhaust pipe 52 at a position downstream of the DPR 53 b and upstream of the urea water injection valve 59. This sensor (hereinafter also referred to as “upstream side NOx sensor”) 78 measures the NOx concentration in the exhaust gas reaching the sensor 78 and outputs a signal (current) representing the NOx concentration, as will be described later.

The NOx sensor 77 is disposed in the exhaust pipe 52 at a position downstream of the SCR catalyst 53c and upstream of the NH ₃ oxidation catalyst 53d. This sensor (hereinafter also referred to as “downstream NOx sensor”) 77 measures the “NOx and NH ₃ concentration” in the exhaust gas reaching the sensor 77, as will be described later, and the “NOx and NH ₃ concentration”. A signal (current) representing is output.

The accelerator operation amount sensor 78 detects an operation amount of an accelerator pedal (not shown) and outputs a signal representing the operation amount Accp.
The vehicle speed sensor 79 detects the traveling speed of the vehicle on which the engine 10 is mounted, and outputs a signal representing the traveling speed (vehicle speed) SPD.

  The temperature sensor 81 is disposed on the SCR catalyst 53c. The temperature sensor 81 detects the temperature of the SCR catalyst 53c and outputs a signal representing the temperature TSCR.

The temperature sensor 82 is disposed in the exhaust pipe 52 at a position downstream of the SCR catalyst 53c and upstream of the NH ₃ oxidation catalyst 53d. The temperature sensor 82 detects the temperature (exhaust temperature) of the exhaust gas flowing out from the SCR catalyst 53c, and outputs a signal representing the exhaust temperature TEX.

<Basic operation of oxidation catalyst>
The oxidation catalyst 53a oxidizes unburned hydrocarbons (unburned HC) and carbon monoxide (CO) in the exhaust gas flowing into the oxidation catalyst 53a. In addition, the oxidation catalyst 53a also has a function of increasing the NO ₂ concentration in the exhaust gas flowing into the oxidation catalyst 53a.

<Basic operation of DPR>
The DPR 53b is a filter that collects particulates (soot) in the exhaust, and has a noble metal supported on the surface thereof. The particulate matter collected in the DPR 53b is burned by the catalytic action of the noble metal when the temperature of the DPF 53b increases.

<Basic operation of SCR catalyst>
The urea water injection valve 59 injects urea water in the urea water tank 55 into the exhaust pipe 52 in response to an instruction from the ECU 80. Thereby, urea water is supplied to the SCR catalyst 53c. The urea water supplied to the SCR catalyst 53c is converted into NH ₃ through a hydrolysis reaction shown in the following chemical reaction formula (2).

NOx is generated with combustion in each cylinder (each combustion chamber) # 1 to # 4. This NOx is discharged into the exhaust passage and flows into the SCR catalyst 53c. NOx flowing into the SCR catalyst 53c is reduced by the SCR catalyst 53c through any of the chemical reactions shown in the following chemical reaction formulas (3) to (5) using NH ₃ generated from the urea water as a reducing agent. Purified.

<Basic operation of NH ₃ oxidation catalyst>
However, if the appropriate amount of urea water (raw material NH ₃₎ is supplied to the SCR catalyst 53c, since the NH ₃ generated from the urea water is used to purify the NOx, the NH ₃ flows out from the SCR catalyst 53c There is nothing to do. However, if an excessive amount of urea water is supplied to the SCR catalyst 53c, NH ₃ may flow out from the SCR catalyst 53c.

NH ₃ oxidation catalyst 53d, during its activity, oxidizing the reaction shown a NH ₃ flowing out of the SCR catalyst 53c as described above the following chemical equation (6).

<Urea water injection control>
As described above, the NOx that has flowed into the SCR catalyst 53c uses the chemical reaction represented by the chemical reaction formulas (3) to (5) using NH ₃ generated from the urea water injected into the exhaust pipe 52 as a reducing agent. Through either of these, the SCR catalyst 53c is reduced and purified. Therefore, the more urea water is injected into the exhaust pipe 52, the higher the purification rate of NOx by the SCR catalyst 53c. However, if excessive urea is injected into the exhaust pipe 52, “so-called ammonia slip” in which NH ₃ flows out from the SCR catalyst 53c occurs.

Therefore, in this example, during the operation of the engine 10, the ECU 80 has the amount of NH ₃ adsorbed on the SCR catalyst 53c (hereinafter referred to as “NH ₃ adsorption amount”) QNH3 “a predetermined range amount”. The amount of urea water injected from the urea water injection valve 59 is controlled so that this state is maintained. The “amount in the predetermined range” is set to a value such that the NOx purification rate by the SCR catalyst 53c is within an allowable range and ammonia slip does not occur. In this example, the upper limit value QNH3upper of the predetermined range is set to a value that does not cause ammonia slip. On the other hand, the lower limit value QNH3lower of the predetermined range is set to a value at which the NOx purification rate RNOX of the SCR catalyst 53c is the minimum required purification rate.

On the other hand, the NH ₃ adsorption amount QNH3 decreases as NH ₃ is consumed for the reduction purification of NOx by the SCR catalyst 53c. Therefore, the ECU 80 estimates “the amount of NH ₃ consumed for the reduction of NOx by the SCR catalyst 53c (hereinafter referred to as“ NH ₃ consumption ”) dQNH3d”.

  More specifically, the ECU 80 determines the “SCR based on the“ NOx concentration in the exhaust gas flowing into the SCR catalyst 53c ”(hereinafter referred to as“ inflow NOx concentration ”) CNOXin” and “intake air amount Ga”. The amount of NOx flowing into the catalyst 53c (hereinafter referred to as “inflowing NOx amount”) QNOX is estimated. The inflow NOx concentration CNOXin is measured by the upstream NOx sensor 76. The intake air amount Ga is measured by the air flow meter 71.

  Further, the ECU 80 determines the NOx concentration of the SCR catalyst 53c based on the “inflow NOx concentration CNOXin” and the “NOx concentration in the exhaust gas flowing out from the SCR catalyst 53c (hereinafter referred to as“ outflow NOx concentration ”) CNOXout”. The purification rate RNOX is estimated. The outflow NOx concentration CNOXout is measured by the downstream NOx sensor 77.

The ECU 80 estimates the NH ₃ consumption dQNH3d based on the “inflowing NOx amount QNOX” and the “NOx purification rate RNOX of the SCR catalyst 53c”. The ECU 80 newly sets a value obtained by subtracting the estimated NH ₃ consumption amount dQNH3d from the “current NH ₃ adsorption amount QNH3” as the current NH ₃ adsorption amount QNH3.

When the “current NH ₃ adsorption amount QNH3” obtained as described above is smaller than the predetermined range, the ECU 80 causes NH ₃ to be injected into the SCR catalyst 53c by the urea water injection from the urea water injection valve 59. It is newly adsorbed and the NH ₃ adsorption amount QNH3 is increased. More specifically, the ECU 80 is necessary to make the NH ₃ adsorption amount QNH3 an amount within the predetermined range based on “the difference ΔQNH3 between the current NH ₃ adsorption amount QNH3 and the amount within the predetermined range”. The amount of urea water is calculated, and the calculated amount of urea water is injected from the urea water injection valve 59.

At this time, the ECU 80 “based on the amount of urea water injected from the urea water injection valve 59“ amount of NH ₃ newly adsorbed on the SCR catalyst 53 c (hereinafter referred to as “new NH ₃ adsorption amount”). Estimate dQNH3i. Then, the ECU 80 newly sets a value obtained by adding the estimated new NH ₃ adsorption amount dQNH3i to the “current NH ₃ adsorption amount QNH3” as the current NH ₃ adsorption amount QNH3.

<Internal structure of downstream NOx sensor>
Next, the internal structure of the downstream NOx sensor 77 will be described with reference to FIG. The internal structure of the upstream NOx sensor 76 is the same as the internal structure of the downstream NOx sensor 77 described below.

  The downstream NOx sensor 77 has a sensor element 77a therein. As shown in FIG. 2, the sensor element 77a includes two solid electrolyte layers 90a and 90b, six alumina layers 91a to 91f, one heater layer 92, one diffusion resistance layer 93, and five electrodes 94a to 94e. As well as a protective layer (trap layer) 99.

  The solid electrolyte layers 90a and 90b are thin plate bodies having oxide ion conductivity made of zirconia or the like. The alumina layers 91a to 91f are dense layers (gas impermeable plates) made of alumina. These layers are formed of an alumina layer 91f, an alumina layer 91e, an alumina layer 91d, a solid electrolyte layer 90b, an alumina layer 91c, a solid electrolyte layer 90a, an alumina layer 91b, and an alumina layer 91a from bottom to top in FIG. They are stacked in order.

  The diffusion resistance layer 93 is a porous layer and can pass exhaust gas. The diffusion resistance layer 93 is disposed between the solid electrolyte layer 90a and the solid electrolyte layer 90b.

  Between the “solid electrolyte layers 90a and 90b”, the “diffusion resistance layer 93”, and the “alumina layer 91c”, a space 98a is defined by the wall surfaces of these layers. Exhaust gas flows into the space 98a from the outside of the sensor element 77a through the diffusion resistance layer 93 (hereinafter, this space is referred to as “exhaust chamber”).

  Furthermore, a space 98b is defined between the “solid electrolyte layer 90a” and the “alumina layers 91a and 91b” by the wall surfaces of these layers. This space 98b flows into the atmosphere (hereinafter, this space is referred to as “atmosphere chamber”).

  In addition, a space 98c is defined between the “solid electrolyte layer 90b” and the “alumina layers 91d and 91e” by the wall surfaces of these layers. This space 98c flows into the atmosphere as the reference gas (hereinafter, this space is referred to as “atmosphere chamber” or “reference gas chamber”).

  The heater layer 92 is disposed between the alumina layer 91e and the alumina layer 91f. The heater layer 92 generates heat in response to a drive (instruction) signal from the ECU 80, and raises the temperature of the sensor element 77a.

  The protective layer 99 is a porous layer through which exhaust can pass. The protective layer 99 is disposed on the outer surface of the alumina layer 91a, the end surfaces of the “solid electrolyte layers 90a and 90b, the alumina layers 91a to 91f, the diffusion resistance layer 93”, and the outer surface of the alumina layer 91f.

  The protective layer 99 is formed by the condensed water mixed in the exhaust gas adhering to the “solid electrolyte layers 90a and 90b, the alumina layers 91a to 91f, the alumina layers 91d to 91f, and the diffusion resistance layer 93”. Is prevented from occurring.

  Furthermore, the protective layer 99 prevents deterioration of the sensor element 77a by capturing “a component that deteriorates the sensor element 77a” contained in the exhaust gas.

  The electrode 94b is disposed on one wall surface of the solid electrolyte layer 90a so as to be disposed in the exhaust chamber 98a. The electrode 94a is disposed on the other wall surface of the solid electrolyte layer 90a so as to face the electrode 94b with the solid electrolyte layer 90a interposed therebetween. The electrode 94a is disposed in the atmospheric chamber 98b.

  The electrode 94a is connected to the positive electrode of the voltage source 95b via the wiring 95d. In the wiring 95d, an electric resistance 95c and a current measuring device 95a are interposed in this order from the electrode 94a to the voltage source 95b. The current measuring device 95a is connected to the ECU 80, and a current (a “pump current” described later) measured by the current measuring device 95a is sent to the ECU 80. On the other hand, the electrode 94b is connected to the negative electrode of the voltage source 95b via the wiring 95e.

  These “electrodes 94 a and 94 b, solid electrolyte layer 90 a, current measuring device 95 a, voltage source 95 b, electrical resistance 95 c, and wirings 95 d and 95 e” constitute a pump unit 95.

  The electrode 94c is disposed on one wall surface of the solid electrolyte layer 90b so as to be disposed in the exhaust chamber 98a on the downstream side of the electrode 94b along the flow direction of the exhaust gas. The electrode 94d is disposed on the other wall surface of the solid electrolyte layer 90b so as to face the electrode 94c with the solid electrolyte layer 90b interposed therebetween. The electrode 94d is disposed in the atmospheric chamber 98c.

  The electrode 94c is connected to the voltage measuring device 96a through the wiring 96b. On the other hand, the electrode 94d is connected to the voltage measuring device 96a through the wiring 96c. The voltage measuring device 96a is connected to the ECU 80, and the voltage measured by the voltage measuring device 96a is sent to the ECU 80. These “electrodes 94c and 94d, voltage measuring device 96a, and wirings 96b and 96c” constitute an oxygen concentration detection unit 96.

  The electrode 94e is disposed on one wall surface of the solid electrolyte layer 90b so as to be disposed in the exhaust chamber 98a on the downstream side of the electrode 94c along the exhaust flow direction. The electrode 94e is disposed on the wall surface of the solid electrolyte layer 90b so as to face the electrode 94d with the solid electrolyte layer 90b interposed therebetween.

  The electrode 94d is connected to the positive electrode of the voltage source 97b through the wiring 97c. On the other hand, the electrode 94e is connected to the negative electrode of the voltage source 97b via the wiring 97d. A current measuring device 97a is interposed in the wiring 97d. The current measuring device 97a is connected to the ECU 80, and a current (a “sensor current” described later) measured by the current measuring device 97a is sent to the ECU 80. These “electrodes 94d and 94e, current measuring instrument 97a, voltage source 97b, and wirings 97c and 97d” constitute a sensor unit 97.

<NOx concentration measurement>
Next, measurement of NOx concentration in exhaust gas by the downstream NOx sensor 77 will be described with reference to FIG. The measurement principle of the NOx concentration in the exhaust gas by the upstream NOx sensor 76 is also the same as the “measurement of the NOx concentration in the exhaust gas by the downstream NOx sensor 77” described below.

The exhaust gas that has reached the downstream side NOx sensor 77 flows into the exhaust chamber 98a through the “protective layer 99 and the diffusion resistance layer 93” in order. That is, the “protective layer 99 and the diffusion resistance layer 93” correspond to the “diffusion layer”. A voltage is applied between the “electrode 94a and the electrode 94b” constituting the pump unit 95 by the voltage source 95b. Thereby, as shown in FIG. 3, oxygen (O ₂ ) in the exhaust gas receives electrons (e ⁻ ) at the electrode 94b and becomes oxide ions (O ²⁻ ). This O ²⁻ reaches the electrode 94a through the solid electrolyte layer 90a. O ^{2− that} has reached the electrode 94a emits electrons at the electrode 94a to become oxygen (O ₂ ), and is released into the atmospheric chamber 98b of the sensor element 77a.

In this way, current flows through the pump unit 95 as O ²⁻ passes through the solid electrolyte layer 90a.

Further, when nitrogen dioxide (NO ₂ ) is contained in the exhaust gas, this NO ₂ is reduced at the electrode 94b to become NO. O ²⁻ is produced by the reduction of NO ₂ . This O ²⁻ also reaches the electrode 94a through the solid electrolyte layer 90a. O ²⁻ that has reached the electrode 94a also emits electrons (e ⁻ ) at the electrode 94a to become O ₂ , and is released into the atmospheric chamber 98b of the sensor element 77a.

Thus, the current flows through the pump unit 95 also when O ²⁻ produced due to the reduction of NO ₂ passes through the solid electrolyte layer 90a.

The current measuring device 95a detects the current IP flowing through the pump unit 95 as described above. This current (hereinafter also referred to as “pump current”) IP increases as the amount of O ²⁻ passing through the solid electrolyte layer 90a increases. That is, the pump current IP increases as the oxygen concentration in the exhaust gas flowing into the exhaust chamber 98a increases, and increases as the NO ₂ concentration in the exhaust gas increases.

  A voltage having a value at which the current flowing between these electrodes 94a and 94b becomes the oxygen limit current (voltage corresponding to the oxygen limit current region) is applied between the electrodes 94a and 94b. For this reason, the oxygen concentration in the exhaust gas flowing into the exhaust chamber 98a from the outside of the sensor element 77a through the “protective layer 99 and the diffusion resistance layer 93” (diffusion layer) is a very small constant concentration (this In the example, it is reduced to a concentration of several ppm, which can be regarded as approximately “0 (zero)”.

  Between the “electrode 94c and electrode 94d” constituting the oxygen concentration detection unit 96, there are “the oxygen concentration in the exhaust gas whose oxygen concentration has been reduced by the pump unit 95” and “the oxygen concentration in the atmospheric chamber 98c”. A voltage (electromotive force) is generated due to the difference. The voltage measuring device 96a measures this voltage. The ECU 80 estimates “the oxygen concentration in the exhaust gas whose oxygen concentration has been reduced by the pump unit 95” based on the measured voltage. This oxygen concentration is used for “correction of sensor current” when calculating (estimating) the NOx concentration as will be described later.

A constant voltage is applied between the “electrode 94 d and electrode 94 e” constituting the sensor unit 97 by the voltage source 97 b of the sensor unit 97. Accordingly, when NO is contained in the exhaust gas that has reached the electrode 94e, this NO is reduced at the electrode 94e to become nitrogen (N ₂ ).

O ²⁻ is produced by the reduction of NO. This O ²⁻ reaches the electrode 94d through the solid electrolyte layer 90b. The O ^{2− that} has reached the electrode 94d emits electrons (e ⁻ ) from the electrode 94d to become O ₂ , and is released into the atmospheric chamber 98c of the sensor element 77a.

Thus, the current IS flows through the sensor unit 97 as O ²⁻ generated due to the reduction of NO passes through the solid electrolyte layer 90b. The current measuring device 97a detects this current IS. This current (hereinafter also referred to as “sensor current”) IS is larger as the amount of O ²⁻ passing through the solid electrolyte layer 90b is larger. That is, the sensor current IS increases as the NO concentration (NOx concentration) in the exhaust gas reaching the electrode 94e increases.

<NH ₃ inflow>
By the way, if excessive urea water is supplied to the SCR catalyst 53c, NH ₃ may flow into the exhaust chamber 98a. As shown in FIG. 4, NH ₃ in the exhaust gas flowing into the exhaust chamber 98a is contained in a large amount in the exhaust gas at the electrode 94b of the pump unit 95 (hereinafter also referred to as “pump electrode 94b”). It reacts with some of the O ₂ that is converted to nitric oxide (NO) and water (H ₂ O).

Further, NO generated from NH ₃ then reaches the sensor electrode 94e and is reduced to N ₂ at the sensor electrode 94e as described above. Then, O ²⁻ is generated by the reduction of NO. This O ²⁻ reaches the electrode 94d through the solid electrolyte layer 90b. The O ^{2− that} has reached the electrode 94d emits electrons at the electrode 94d to become O ₂ , and is emitted to the atmospheric chamber 98c of the sensor element 77a.

As described above, when O ²⁻ generated by reduction of NO generated from NH ₃ passes through the solid electrolyte layer 90b, a current (sensor current) IS flows through the sensor unit 97. The sensor current IS increases as the amount of O ²⁻ passing through the solid electrolyte layer 90b increases. That is, the sensor current IS increases as the amount of NO in the exhaust gas reaching the sensor electrode 94e increases, in other words, as the amount of NH ₃ in the exhaust gas flowing into the exhaust chamber 98a increases.

As described above, the sensor current IS flowing through the sensor unit 97, ^{O 2} produced by the reduction of NO derived from the respective downstream NOx sensor 77 NO contained in the exhaust gas that has reached, NO ₂ and NH ₃ ^- includes current resulting from the.

<Oxygen concentration estimation>
By the way, when O ₂ is contained in the exhaust gas that has reached the electrode 94e (hereinafter referred to as “sensor electrode 94e”) of the sensor unit 97, in the sensor electrode 94e, O ₂ in the exhaust gas is also O ^{2 −.} This O ²⁻ also passes through the solid electrolyte layer 90b and reaches the electrode 94d. In this case, the sensor current IS, in addition to the "current due to O ^2- generated by reduction of NO", "current due to O ₂ originally contained in the exhaust gas reaches the electrodes 94e" also Also included.

On the other hand, the amount of O ₂ originally contained in the exhaust gas that has reached the sensor electrode 94e can be estimated based on the oxygen concentration detected by the oxygen concentration detector 96 described above. Therefore, in this example, the ECU 80 subtracts (corrects) the “sensor current generated due to O ₂ originally contained in the exhaust gas reaching the sensor electrode 94e” from the sensor current IS. As a result, current due to only O ²⁻ generated by reduction of NO derived from NO, NO _2, and NH ₃ contained in the exhaust gas that has reached the downstream NOx sensor 77 is supplied to the sensor unit 97. It can be detected as a flowing sensor current IS. That is, it is possible to estimate (measure) the “concentration of NOx and NH _{3 in} the exhaust gas flowing out from the SCR catalyst 53c” based on the sensor current IS. That is, the sensor current IS corresponds to a “detected value” by the NOx sensor 77.

  However, as described above, the oxygen concentration in the exhaust gas flowing into the exhaust chamber 98 a through the “protective layer 99 and the diffusion resistance layer 93” is lowered to a very small constant concentration by the pump unit 95. Therefore, the correction amount of the sensor current IS based on the oxygen concentration detected by the oxygen concentration detection unit 96 can be set to a minute constant value, and such correction may not be performed depending on the case.

  Similarly, the upstream NOx sensor 76 estimates (measures) the NOx concentration in the exhaust gas flowing into the SCR catalyst 53c.

<Correction of detection value by first device>
As described above, the downstream NOx sensor 77 can detect “the concentrations of NOx and NH _{3 in} the exhaust gas flowing out from the SCR catalyst 53c” based on the sensor current IS that is the “detection value”. . However, as described above, as a result of the study by the present inventors, the sensor current IS (estimated as a detection value estimated based on the respective concentrations of NOx and NH ₃ contained in the exhaust gas that reaches the downstream NOx sensor 77 is estimated. Sensor current IS (actually measured value) as a detected value actually output by the NOx sensor may be smaller than (value).

Therefore, in order to accurately obtain the concentrations of NOx and NH ₃ contained in the exhaust gas from the sensor current IS as the detection value by the downstream NOx sensor 77, it is necessary to correct the detection value by the downstream NOx sensor 77. . Specifically, for example, as shown in the following calculation formula (1), the correction sensor current ISc can be obtained by multiplying the sensor current IS detected by the downstream NOx sensor 77 by the correction coefficient k. it is conceivable that.

However, the present inventor has a case where the difference between the estimated value and the actual measurement value of the sensor current IS is not constant, and the correct correction sensor current ISc cannot be obtained by adopting a fixed value as the correction coefficient k. I noticed that there is. More specifically, the present inventor has found that the difference between the estimated value of the detection value by the downstream NOx sensor 77 and the actual measurement value is the molar ratio of NO and NH ₃ in the exhaust gas that reaches the downstream NOx sensor 77. It was found that the closer to 1: 1, the larger.

That is, as shown in FIG. 5 (a), even if the total molar concentration of NO, NO ₂ and NH ₃ contained in the exhaust gas reaching the downstream NOx sensor 77 is constant, the NO in the exhaust gas The closer the molar ratio of NH ₃ to NH ₃ , the smaller the magnitude of the sensor current IS. That is, the closer the molar ratio of NH ₃ to the total amount of NO and NH ₃ contained in the exhaust gas (NH ₃ molar ratio MRNH3) is 50%, the magnitude of the sensor current IS is reduced, due to the downstream side NOx sensor 77 The difference between the estimated value of the detected value and the actually measured value increases.

As described above, in the above phenomenon, the reaction expressed by the following chemical reaction formula (1) occurs in the diffusion layer (protective layer 99 and diffusion resistance layer 93) provided in the downstream NOx sensor 77, and NO in the exhaust gas And NH ₃ are considered to be consumed at a molar ratio of 1: 1.

Therefore, in order to accurately obtain the concentrations of NOx and NH ₃ contained in the exhaust gas from the sensor current IS as a detection value by the downstream NOx sensor 77, NO and NH in the exhaust gas that reaches the downstream NOx sensor 77 are obtained. It is desirable to correct the sensor current IS to a larger value as the molar ratio with ₃ is closer to 1: 1.

<Configuration of control unit>
Therefore, the first device includes a control unit that corrects the detection value (sensor current IS) from the downstream NOx sensor 77. As shown in FIG. 6, the control unit 100 includes an estimation unit 110 and a correction unit 120. In the first device, the control unit 100 is implemented as a function exhibited by the ECU 80. Specifically, control unit 100 includes a CPU (hereinafter simply referred to as “CPU”) included in ECU 80 in accordance with a program stored in a ROM included in ECU 80 based on the input signals from the various sensors described above. It is configured to execute processes and operations as described below.

The estimation unit 110 estimates the concentration of NO and the concentration of NH ₃ contained in the exhaust gas flowing out from the SCR catalyst 53c, which is a NOx purification catalyst. Specifically, the CPU detects “inflow NOx concentration CNOXin” measured by the upstream NOx sensor 76, “urea water amount QNY” that is the amount of urea water added from the urea water addition device 54, and the temperature sensor 81. The “temperature TSCR” which is the temperature of the SCR catalyst 53c to be obtained is acquired.

Then, the CPU applies “inflow NOx concentration CNOXin”, “urea water amount QNY”, and “temperature TSCR” to each lookup table stored in advance in the ROM. Thereby, the CPU acquires the concentrations of NO and NH ₃ contained in the exhaust gas flowing out from the SCR catalyst 53c (“NO concentration CNO_S” and “NH ₃ concentration CNH3_S”, respectively).

As described above, the CPU determines the concentrations of NO and NH ₃ (“NO concentration CNO_S” and “NH ₃ concentration CNH3_S”) in the exhaust gas flowing out from the NOx purification catalyst (SCR catalyst 53c) from the downstream NOx sensor. 77 based on the obtained values other than the detected value (“inflow NOx concentration CNOXin”, “urea water amount QNY”, and “temperature TSCR”). That is, the CPU functions as the estimation unit 110 that constitutes the control unit 100 included in the first device. Each lookup table used for obtaining the NO concentration CNO_S and the NH ₃ concentration CNH3_S from the inflow NOx concentration CNOXin, the urea water amount QNY, and the temperature TSCR is obtained by, for example, experiments and stored in the ROM of the ECU 80. deep.

Alternatively, the estimation unit 110 (the CPU functioning as the CPU) may estimate the NO concentration CNO_S and the NH ₃ concentration CNH3_S based on, for example, a theoretical model of the SCR catalyst 53c.

On the other hand, the correction unit 120 (the CPU functioning as) calculates the molar ratio of NO and NH ₃ in the exhaust based on the NO concentration CNO_S and the NH ₃ concentration CNH3_S acquired as described above. Then, as the molar ratio is closer to 1: 1 (as the NH ₃ molar ratio MRNH3 is closer to 50%), the CPU corrects the sensor current IS as a detection value by the downstream NOx sensor 77. Set to a larger value.

In this example, the CPU is acquired as described above based on the correspondence relationship between the NH ₃ molar ratio MRNH3 and the correction coefficient k for correcting the sensor current IS as shown in FIG. A specific value of the correction coefficient k is obtained from the NH ₃ molar ratio MRNH3 calculated from the NO concentration CNO_S and the NH ₃ concentration CNH3_S. Then, the CPU multiplies the acquired correction coefficient k by the sensor current IS to calculate a correction sensor current ISc. The CPU corrects the detection value by the downstream NOx sensor 77 by correcting the measured value of the sensor current IS so as to approach the true value in this way.

As described above, the CPU detects the detected value (by the downstream NOx sensor 77) as the molar ratio of NH ₃ to NO derived from the estimated NO concentration and the estimated NH ₃ concentration is closer to 1 (by the downstream NOx sensor 77). The detected value (sensor current IS) (by the downstream NOx sensor 77) is corrected so that the ratio of the corrected detected value to the current IS) becomes a larger value. Specifically, the first is the ratio of the corrected detection value to the detection value (sensor current IS) when the difference between the “molar ratio of NH _{3 to} NO” and 1 is the first value. The correction ratio is the ratio of the corrected detection value to the detection value (sensor current IS) when the magnitude of the difference (between the molar ratio and 1) is a second value larger than the first value. The detection value (sensor current IS) is corrected so as to be a value larger than the second correction ratio. As a result, even if the magnitude of the detected value (sensor current IS) is the same, the larger the detected “molar ratio of NH _{3 to} NO” is, the larger the detected value (sensor current ISc) after correction is obtained. It is done.

That is, the CPU also functions as the correction unit 120 that constitutes the control unit 100 included in the first device. The molar ratio of NO and NH ₃ in the exhaust gas reaching the downstream NOx sensor 77 and flows out from the SCR catalyst 53c (or NO and the molar ratio of NH ₃ to the total amount of NH _3), corrects the sensor current IS The correspondence relationship between the correction coefficient k and the correction coefficient k is determined by, for example, experiments, and stored in the ROM of the ECU 80 as data such as a lookup table.

In this example, as described above, the molar ratio of NO and NH ₃ in the exhaust gas (specifically, the NH ₃ molar ratio MRNH3) is calculated from the concentration of NO estimated by the estimation unit and the concentration of NH _3. And the correction unit performed a process of obtaining a specific value of the correction coefficient k from the NH ₃ molar ratio MRNH3. However, the estimation unit may perform both or one of these processes.

  Further, in this example, the function of the control unit 100 as described above is exhibited by the ECU 80 that controls the engine 10. However, the function of the control unit 100 may be exhibited by a separate control device other than the ECU 80 that controls the engine 10, or may be distributed between such separate control devices and the ECU 80.

<Specific operation of first device>
Here, a specific operation of the first device in correction of the detection value by the NOx sensor will be described with reference to the attached flowchart. The CPU executes the detection value correction routine shown by the flowchart in FIG. 7 every elapse of a predetermined time. Therefore, when the predetermined timing comes, the CPU starts processing from step 700 in FIG. 7 and proceeds to step 710 to determine whether or not the detection condition is satisfied.

The above "detection conditions"
(1) The temperature of the engine 10 is equal to or higher than a predetermined temperature (the engine 10 has been warmed up),
(2) The SCR catalyst 53c is activated (the SCR catalyst 53c has been warmed up), and
(3) The downstream side NOx sensor 77 is activated (the downstream side NOx sensor 77 has been warmed up),
Sometimes true.

  If the detection condition is not satisfied at the time when the CPU executes the process of step 710, the CPU makes a “No” determination at step 710 to proceed to step 795 to end the present routine tentatively.

  On the other hand, when the detection condition is satisfied at the time when the CPU executes the process of step 710, the CPU makes a “Yes” determination at step 710 and sequentially executes the processes of steps 720 to 750 described below. Run.

Step 720: The CPU estimates “NO concentration CNO_S and NH ₃ concentration CNH3_S” of the exhaust gas that flows out of the SCR catalyst 53c and reaches the downstream NOx sensor 77. Specifically, the CPU applies “inflow NOx concentration CNOXin”, “urea water amount QNY”, and “temperature TSCR” to each lookup table stored in advance in the ROM described above. Thereby, the CPU acquires the concentrations of NO and NH ₃ contained in the exhaust gas flowing out from the SCR catalyst 53c (“NO concentration CNO_S” and “NH ₃ concentration CNH3_S”, respectively).

Step 730: CPU, based on the estimated "NO concentration CNO_S and NH ₃ concentrations CNH3_S", to calculate the molar ratio of NH ₃ (NH ₃ molar ratio MRNH3) to the total amount of NO and NH ₃ contained in the exhaust .
Step 740: The CPU corrects the sensor current IS, which is a value detected by the downstream NOx sensor 77, from the calculated NH ₃ molar ratio MRNH3 and the lookup table shown in FIG. 5B. To get.
Step 750: The CPU calculates the correction sensor current ISc by multiplying the acquired correction coefficient k by the sensor current IS (corrects the detection value by the downstream NOx sensor 77).

  After executing the processing of step 720 to step 750, the CPU proceeds to step 795 to end the present routine tentatively.

As described above, according to the first device, the downstream side caused by the reaction represented by the above-described chemical reaction formula (1) in the diffusion layer (the protective layer 99 and the diffusion resistance layer 93) included in the downstream side NOx sensor 77. A change in the detected value from the NOx sensor 77 can be corrected appropriately. As a result, according to the first device, the downstream NOx sensor 77 can be used to accurately acquire values corresponding to the concentrations of NOx and NH ₃ contained in the exhaust gas that reaches the downstream NOx sensor 77. it can.

Second Embodiment
By the way, as described above, the detected value corresponding to the “NOx and NH ₃ concentration” of the exhaust gas detected by the NOx sensor is smaller than the original value in the diffusion layer included in the NOx sensor. This is probably because the reaction represented by the reaction formula (1) occurs. Furthermore, as described above, the yield of the reaction is affected by the operating conditions of the internal combustion engine, such as the flow rate of exhaust gas (exhaust flow rate), temperature (exhaust gas temperature), and oxygen concentration reaching the NOx sensor.

Therefore, the NOx sensor output correction device (hereinafter also referred to as “second device”) according to the second embodiment of the present invention takes into consideration the influence on the yield of the reaction due to the operating state of the internal combustion engine. Thus, the concentration of “NOx and NH ₃ ” contained in the exhaust gas is obtained more accurately by correcting the detection value by the NOx sensor more appropriately. Specifically, the second device takes into account the influence of the exhaust flow rate (exhaust flow rate), temperature (exhaust temperature), and oxygen concentration on the yield of the reaction, which is detected by the NOx sensor. Is more appropriately corrected, and the concentrations of “NOx and NH ₃ ” contained in the exhaust gas are obtained with higher accuracy.

<Configuration of internal combustion engine>
Similarly to the first device, the second device is also applied to the internal combustion engine (engine) 10 shown in FIG. However, in this example, the engine 10 further includes an O ₂ sensor 83 (not shown) disposed at a position downstream of the SCR catalyst 53 c in the exhaust pipe 52 and upstream of the downstream NOx sensor 77. Yes.

<Internal structure of downstream NOx sensor>
Since the configurations and operations of the upstream NOx sensor 76 and the downstream NOx sensor 77 included in the second device have also been described in detail in the description relating to the first device, description thereof will not be repeated here.

<Correction of detection value by second device>

In the second apparatus, as indicated by the solid line in FIG. 8B, the closer the molar ratio of NO to NH ₃ in the exhaust gas that reaches the downstream NOx sensor 77 is to be closer to 1: 1 (that is, NH The correction coefficient k for correcting the sensor current IS is set to a larger value as the ₃ molar ratio MRNH3 is closer to 50%. In this respect, the second device is similar to the first device.

However, as described above, the yield of the reaction represented by the chemical reaction formula (1) described above is the internal combustion rate such as the flow rate of exhaust gas (exhaust flow rate), temperature (exhaust gas temperature), and oxygen concentration reaching the NOx sensor. It is affected by the operating conditions of the engine. Therefore, as will be described in detail below, the degree to which the detected values corresponding to the concentrations of NOx and NH ₃ contained in the exhaust gas detected by the downstream side NOx sensor 77 are smaller than the original values is also reduced. Influenced by driving conditions.

  Here, the influence of the operating state of the internal combustion engine on the detection value by the NOx sensor and the correction of the detection value by the second device will be described in detail below. In the following description, for easy understanding, each of the exhaust flow rate, the exhaust temperature, and the oxygen concentration will be described individually.

(Effect of exhaust flow rate)
As described above, the higher the exhaust gas flow rate, the higher the speed of the exhaust gas passing through the diffusion layer (the protective layer 99 and the diffusion resistance layer 93) included in the downstream NOx sensor 77, and the exhaust gas passes through the diffusion layer. The time required is shortened. Therefore, the time during which the reaction represented by the chemical reaction formula (1) can occur is shortened, and NO and NH ₃ consumed by the reaction are reduced.

As a result, the extent to which the detected values corresponding to the concentrations of NOx and NH ₃ contained in the exhaust gas detected by the downstream side NOx sensor 77 are smaller than the original values also decreases as the exhaust gas flow rate increases. Specifically, as indicated by the broken line in FIG. 8A, the sensor current IS increases as the exhaust gas flow rate increases (open arrow).

  Therefore, as shown by the broken line in FIG. 8B, the second device sets the correction coefficient k for correcting the sensor current IS to a smaller value as the exhaust gas flow rate increases (open arrow). ). Accordingly, in the second device, the correction unit 120 corrects the detection value so that the difference between the first corrected detection value and the second corrected detection value becomes smaller as the detected exhaust flow rate is larger. To obtain the corrected detection value.

Conversely, as the exhaust gas flow rate decreases, the extent to which the detected values corresponding to the concentrations of NOx and NH ₃ contained in the exhaust gas detected by the downstream side NOx sensor 77 become smaller than the original values increases. Specifically, as indicated by the dotted line in FIG. 8A, the sensor current IS decreases as the exhaust flow rate decreases (black arrow).

  Therefore, as indicated by the dotted line in FIG. 8B, the second device sets the correction coefficient k for correcting the sensor current IS to a larger value as the exhaust flow rate decreases (black arrow). ). Thereby, in the second device, the correction unit 120 corrects the detection value so that the difference between the first corrected detection value and the second corrected detection value increases as the detected exhaust flow rate decreases. To obtain the corrected detection value.

As described above, the second device reflects the influence of the exhaust flow rate on the yield of the reaction represented by the above-described chemical reaction formula (1) in the diffusion layer included in the downstream NOx sensor 77, while the downstream NOx sensor 77. Thus, the concentration of “NOx and NH ₃ ” contained in the exhaust gas can be obtained more accurately by correcting the detection value obtained by the above.

(Effect of exhaust temperature)
As described above, the higher the exhaust gas temperature, the higher the temperature of the diffusion layer (the protective layer 99 and the diffusion resistance layer 93) included in the downstream NOx sensor 77. Therefore, the speed of the reaction represented by the chemical reaction formula (1) is increased, and NO and NH ₃ consumed by the reaction are increased.

As a result of the above, the degree to which the detected values corresponding to the concentrations of NOx and NH ₃ contained in the exhaust gas detected by the downstream side NOx sensor 77 are smaller than the original values also increases as the exhaust gas temperature increases. Specifically, as indicated by the dotted line in FIG. 8A, the sensor current IS decreases as the exhaust temperature increases (black arrow).

  Therefore, as indicated by the dotted line in FIG. 8B, the second device sets the correction coefficient k for correcting the sensor current IS to a larger value as the exhaust temperature becomes higher (black arrow). ). Thereby, in the second device, the correction unit 120 corrects the detection value so that the difference between the first corrected detection value and the second corrected detection value increases as the detected exhaust gas temperature increases. To obtain the corrected detection value.

Conversely, the lower the exhaust gas temperature, the smaller the detected values corresponding to the concentrations of NOx and NH ₃ contained in the exhaust gas detected by the downstream NOx sensor 77 are smaller than the original values. Specifically, as indicated by the broken line in FIG. 8A, the sensor current IS increases as the exhaust gas temperature decreases (open arrow).

  Therefore, as indicated by the broken line in FIG. 8B, the second device sets the correction coefficient k for correcting the sensor current IS to a smaller value as the exhaust temperature becomes lower (open arrow). ). Accordingly, in the second device, the correction unit 120 corrects the detection value so that the difference between the first corrected detection value and the second corrected detection value becomes smaller as the detected exhaust gas temperature is lower. To obtain the corrected detection value.

As described above, the second device reflects the influence of the exhaust temperature on the yield of the reaction represented by the above-described chemical reaction formula (1) in the diffusion layer included in the downstream NOx sensor 77, while the downstream NOx sensor 77. Thus, the concentration of “NOx and NH ₃ ” contained in the exhaust gas can be obtained more accurately by correcting the detection value obtained by the above.

(Effect of oxygen concentration)
As described above, since the reaction product of the chemical reaction formula (1) contains oxygen (O ₂ ), the concentration of O ₂ (oxygen concentration) contained in the exhaust gas that reaches the downstream NOx sensor 77 increases. The reaction rate represented by the chemical reaction formula (1) becomes faster. Therefore, the higher the oxygen concentration in the exhaust gas that reaches the downstream NOx sensor 77, the more NO and NH ₃ are consumed by the reaction.

As a result, the extent to which the detected values corresponding to the concentrations of NOx and NH ₃ contained in the exhaust gas detected by the downstream NOx sensor 77 are smaller than the original values also increases as the oxygen concentration increases. Specifically, as indicated by the dotted line in FIG. 8A, the sensor current IS decreases as the oxygen concentration increases (black arrow).

  Therefore, as indicated by the dotted line in FIG. 8B, the second device sets the correction coefficient k for correcting the sensor current IS to a larger value as the oxygen concentration becomes higher (blacked). Arrow). Accordingly, in the second device, the correction unit 120 corrects the detection value so that the difference between the first corrected detection value and the second corrected detection value increases as the detected oxygen concentration increases. To obtain the corrected detection value.

Conversely, the lower the oxygen concentration, the smaller the extent to which the detected values corresponding to the concentrations of NOx and NH _{3 in} the exhaust detected by the downstream NOx sensor 77 are smaller than the original values. Specifically, as indicated by the broken line in FIG. 8A, the sensor current IS increases as the oxygen concentration decreases (open arrow).

  Therefore, the second device sets the correction coefficient k for correcting the sensor current IS to a smaller value as the oxygen concentration is lower as indicated by the broken line in FIG. Arrow). Thereby, in the second device, the correction unit 120 corrects the detection value so that the difference between the first corrected detection value and the second corrected detection value becomes smaller as the detected oxygen concentration is lower. To obtain the corrected detection value.

As described above, the second device reflects the influence of the oxygen concentration on the yield of the reaction represented by the chemical reaction formula (1) described above in the diffusion layer provided in the downstream NOx sensor 77, while the downstream NOx sensor. The detection value by 77 can be corrected more appropriately, and the concentrations of “NOx and NH ₃ ” contained in the exhaust gas can be acquired with higher accuracy.

<Configuration of control unit>
The control unit 200 included in the second device basically has the same configuration as the control unit 100 included in the first control unit. However, as illustrated in FIG. 9, the control unit 200 further includes various detection units (1) to (3) listed below.
(1) An exhaust flow rate detection unit for detecting the exhaust flow rate,
(2) An exhaust temperature detection unit that detects the exhaust temperature, and (3) an oxygen concentration detection unit that detects the oxygen concentration.

  In this example, the CPU of the ECU 80 acquires the flow rate (exhaust flow rate) SV of the exhaust gas that reaches the downstream side NOx concentration sensor 77 based on the intake air amount Ga detected by the air flow meter 71 described above. That is, in this example, the air flow meter 71 constitutes an exhaust flow rate detection unit. This exhaust flow rate SV is acquired by a process separately executed by the CPU.

  Further, the exhaust temperature TEX is acquired by the CPU of the ECU 80 based on the output signal from the temperature sensor 82 disposed in the exhaust pipe 52 at the downstream position of the SCR catalyst 53c and upstream of the downstream NOx sensor 77. To do. That is, in this example, the temperature sensor 82 constitutes an exhaust temperature detection unit. The exhaust temperature TEX is also acquired by a process separately executed by the CPU.

In addition, the oxygen concentration COX is output from an O ₂ sensor 83 (not shown) provided by the CPU of the ECU 80 in the exhaust pipe 52 that is downstream of the SCR catalyst 53c and upstream of the downstream NOx sensor 77. Get based on. That is, in this example, the CPU and the O ₂ sensor constitute an oxygen concentration detection unit. This oxygen concentration COX is also acquired by a process separately executed by the CPU and stored in the backup RAM of the ECU 80.

In this example, as described above, the oxygen concentration COX is obtained by separately providing the O ₂ sensor constituting the oxygen concentration detection unit. However, for example, when the engine 10 includes an air-fuel ratio sensor, the oxygen concentration When there is an existing component that can be used for acquiring COX, the oxygen concentration COX may be acquired using the existing component.

  As described above, the control unit 200 included in the second device further includes various detection units (1) to (3), and the operation status of the engine 10 (specifically, the exhaust flow rate SV, the exhaust temperature TEX, and the oxygen concentration). The detection value is corrected by reflecting the influence of the COX) on the sensor current IS. Except for this point, the control unit 200 is also implemented as a function exhibited by the ECU 80, similarly to the control unit 100 included in the first control unit.

Specifically, the estimation unit 210 included in the control unit 200 is similar to the estimation unit 110 included in the control unit 100. The “NO concentration (CNO_S) and NH _{3 of} the exhaust gas flowing out from the SCR catalyst 53c that is the NOx purification catalyst” Concentration (CNH3_S) ". Similarly to the correction unit 120 included in the control unit 100, the correction unit 220 included in the control unit 200 also uses the NH ₃ molar ratio MRNH3 and the correction coefficient calculated from the NH ₃ molar ratio MRNH3 calculated from the NO concentration CNO_S and the NH ₃ concentration CNH3_S. A specific value of the correction coefficient k is acquired based on the correspondence relationship with k.

However, the correction unit 220 is based on the “correspondence relationship between the NH ₃ molar ratio MRNH3 and the correction coefficient k” corresponding to the operation state of the engine 10 (specifically, the exhaust flow rate SV, the exhaust temperature TEX, and the oxygen concentration COX). Thus, a specific value of the correction coefficient k is acquired. Then, the correction unit 220 calculates the correction sensor current ISc by multiplying the acquired correction coefficient k by the sensor current IS. The second device corrects the detected value of the downstream side NOx sensor 77 more appropriately by correcting the measured value of the sensor current IS to be closer to the true value in this way, and is included in the exhaust gas. The concentration of “NOx and NH ₃ ” can be obtained with higher accuracy.

  Similarly to the control unit 100, the control unit 200 also requires the CPU to correct the sensor current IS by the second device in accordance with a program stored in the ROM provided in the ECU 80 based on the input signals from the various sensors described above. It is configured to execute various processes and operations. That is, the CPU functions as the estimation unit 210 and the correction unit 220 that configure the control unit 200 included in the second device.

The correspondence relationship between the NH ₃ molar ratio MRNH3 and the correction coefficient k in various operating conditions of the engine 10 (specifically, the exhaust flow rate SV, the exhaust temperature TEX, and the oxygen concentration COX) is obtained in advance by, for example, experiments or the like. . Each correspondence relationship is stored in the ROM of the ECU 80 as data such as a lookup table in association with, for example, the operation status of the engine 10 (specifically, the exhaust flow rate SV, the exhaust temperature TEX, and the oxygen concentration COX). Keep it.

<Specific operation of the second device>
Here, the specific operation of the second device in the correction of the detection value by the NOx sensor will be described with reference to the attached flowchart. The CPU executes the detection value correction routine shown by the flowchart in FIG. 10 every elapse of a predetermined time. The flowchart shown in FIG. 10 is the same as the flowchart shown in FIG. 7 except for the following points.

After executing the processing of step 730, the CPU proceeds to step 735, and acquires the exhaust flow rate SV, the exhaust temperature TEX, and the oxygen concentration COX. Next, the CPU proceeds to step 740 to store the NH ₃ molar ratio MRNH3 calculated in step 730 and the exhaust flow rate SV, exhaust temperature TEX and oxygen concentration COX acquired in step 735 into the ROM of the ECU 80. The correction coefficient k is obtained based on the look-up table.

  Thereafter, similarly to the flowchart shown in FIG. 7, the CPU proceeds to step 750 to calculate the correction sensor current ISc by multiplying the acquired correction coefficient k by the sensor current IS, thereby detecting by the downstream NOx sensor 77. Correct the value.

As described above, the second device acquires the correction coefficient k according to the NH ₃ molar ratio MRNH3 in the operating state of the engine 10. Thereby, according to the 2nd apparatus, the downstream NOx sensor resulting from the reaction represented by the above-mentioned chemical reaction formula (1) in the diffusion layer (protective layer 99 and diffusion resistance layer 93) with which the downstream NOx sensor 77 is provided. The change in the detected value from 77 can be corrected more appropriately. As a result, according to the second device, the concentration of “NOx and NH ₃ ” contained in the exhaust gas can be obtained with higher accuracy using the downstream NOx sensor 77.

<Modification>
In the embodiment described above, the reaction between NO and NH ₃ expressed by the chemical reaction formula (1) in the protective layer 99 and the diffusion resistance layer 93 of the downstream side NOx sensor 77 is taken into consideration. However, even when the reaction occurs only in one of the protective layer 99 and the diffusion resistance layer 93 of the downstream NOx sensor 77 and when the NOx sensor does not include a protective layer, the NOx sensor is the same as in the above-described embodiment. The detected value from can be corrected appropriately.

  Further, the output correction device according to the above-described embodiment is applied to a NOx sensor disposed in the engine 10 including the supercharger 44 and the EGR system 60. However, as a matter of course, the NOx sensor output correction device (the device of the present invention) according to the present invention is also applied to an NOx sensor disposed in an internal combustion engine that does not include one or both of the supercharger and the EGR system. Can be applied.

  In addition, the output correction device according to the above-described embodiment is applied to a NOx sensor disposed in the engine 10 that is a multi-cylinder (in this example, in-line four cylinders), four-cycle, piston reciprocating, and diesel engines. . However, as a matter of course, the device of the present invention can also be applied to a NOx sensor disposed in a diesel engine of a type other than the above. Furthermore, the device of the present invention can also be applied to NOx sensors disposed in various types of gasoline engines.

By the way, as described above, the device of the present invention appropriately corrects a change in sensor current due to the reaction between NO and NH ₃ in the diffusion layer (protective layer and diffusion resistance layer) of the NOx sensor, The concentration of “NOx and NH ₃ ” contained is obtained with high accuracy. Therefore, the device of the present invention exhibits its effect when NH ₃ can be contained in the exhaust gas that reaches the NOx sensor. In other words, the present invention device is preferably applied to an NOx sensor disposed downstream of the catalyst in an internal combustion engine having a NOx purification catalyst that may discharge NH ₃ downstream thereof.

As described at the beginning of this specification, as described above, specific examples of the NOx purification catalyst that may discharge NH ₃ downstream thereof include, for example, an SCR catalyst and an NSR catalyst. .

  In the above, for the purpose of explaining the present invention, several embodiments and modifications having specific configurations have been described with reference to the accompanying drawings. However, the scope of the present invention is not limited to these illustrative examples. It should be understood that the present invention should not be construed as being limited to the embodiments and the modifications, and that appropriate modifications can be made within the scope of the matters described in the claims and the specification.

  DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 52 ... Exhaust passage (exhaust pipe), 53c ... SCR catalyst, 54 ... Urea water addition device, 76 ... Upstream NOx sensor, 77 ... Downstream NOx sensor, 80 ... Engine electronic control unit (ECU), And 100 ... a control unit.

Claims (4)

A diffusion layer made of a porous material that is disposed in the exhaust passage of the internal combustion engine and disposed downstream of the NOx purification catalyst that reduces and purifies NOx contained in the exhaust, and is made of a porous material through which the exhaust can pass. A NOx sensor having an electrode part that reduces NO contained in exhaust gas that has passed through the diffusion layer and converts it into N _2, and outputs a value corresponding to the amount of NO reduced in the electrode part as a detection value An output correction device,
A control unit for obtaining the corrected detection value as an output value by correcting the detection value;
The controller is
An estimation unit that estimates each concentration of NO and NH _{3 in} the exhaust gas flowing out from the NOx purification catalyst based on an acquired value other than the detected value;
As the molar ratio of NH ₃ to NO derived from the estimated NO concentration and the estimated NH ₃ concentration is closer to 1, the ratio of the corrected detection value to the detection value becomes larger. A correction unit for obtaining the detection value after correction by correcting the detection value;
Comprising
Output correction device for NOx sensor. The output correction apparatus according to claim 1,
The control unit further includes an exhaust flow rate detection unit that detects an exhaust flow rate that is a flow rate of exhaust reaching the NOx sensor,
The correction unit corrects the detection value by correcting the detection value so that the difference between the detection value after the first correction and the detection value after the second correction becomes smaller as the detected exhaust flow rate is larger. Configured to determine the value,
Output correction device. The output correction apparatus according to claim 1,
The control unit further includes an exhaust gas temperature detection unit that detects an exhaust gas temperature that is the temperature of the exhaust gas that reaches the NOx sensor,
The correction unit corrects the detection value by correcting the detection value so that the difference between the detection value after the first correction and the detection value after the second correction increases as the detected exhaust gas temperature increases. Configured to determine the value,
Output correction device. The output correction apparatus according to claim 1,
The control unit further includes an oxygen concentration detection unit that detects an oxygen concentration that is a concentration of O ₂ contained in the exhaust gas that reaches the NOx sensor,
The correction unit corrects the detection value by correcting the detection value so that the difference between the detection value after the first correction and the detection value after the second correction increases as the detected oxygen concentration increases. Configured to determine the value,
Output correction device.

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