JP2015155665A - Control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control device of an internal combustion engine which can accurately detect an SOx concentration of an exhaust gas of the internal combustion engine.

SOLUTION: An SOx concentration of an exhaust gas is detected by using a limit current type sensor which can detect an oxygen concentration (air-fuel ratio) of the exhaust gas of an internal combustion engine. Sweeping processing is performed by gradually lowering an application voltage to the sensor to a second voltage from a first voltage, and the SOx concentration of the exhaust gas is detected on the basis of a relationship between a waveform of an output current of the sensor during the execution of the sweeping processing and a prescribed reference value while controlling an engine so as to maintain the oxygen concentration of the exhaust gas at a constant value. Here, the first voltage is an application voltage which is obtained by accumulating sulfur generated by reducing SOx at the sensor in the sensor, the second voltage is an application voltage which is obtained by oxidizing sulfur at the sensor, and the reference value is the limit current value of the sensor corresponding to the oxygen concentration of the constant value.

COPYRIGHT: (C)2015,JPO&INPIT

Description

  The present invention relates to an apparatus for detecting the SOx concentration of exhaust gas using a limit current sensor (for example, a so-called A / F sensor) capable of detecting the oxygen concentration of exhaust gas of an internal combustion engine.

  Conventionally, as a sensor for detecting the oxygen concentration of exhaust gas, a solid electrolyte capable of conducting oxygen ions and a pair of electrodes (a pair of electrodes consisting of a measurement electrode and a reference electrode) provided so as to sandwich the solid electrolyte, There is known a limiting current type sensor having a diffusion rate-determining layer (diffusion resistance layer) provided so as to cover a measurement electrode.

  The principle of detecting the oxygen concentration of the exhaust gas using the sensor is as follows. First, when a voltage is applied to the electrode pair so that a predetermined potential difference is generated between the electrode pair, oxygen in the exhaust gas is ionized at the measurement electrode. The oxygen ions pass through the solid electrolyte and move to the reference electrode, and then recombine at the reference electrode to return to oxygen. The movement of electrons resulting from this series of electrochemical reactions is output as an electric current from the electrode pair. Since the diffusion control layer controls the movement of oxygen to the measurement electrode, the magnitude of the current output from the electrode pair and the oxygen concentration of the exhaust gas correspond one-to-one. Therefore, when the current output from the electrode pair is measured, the oxygen concentration of the exhaust gas is specified (detected) based on the measured current.

One of the conventional detection devices (hereinafter referred to as “conventional device”) utilizes the fact that the molecules decomposed by the measurement electrode differ when the magnitude of the voltage applied to the electrode pair is different. The concentration of the component (for example, H ₂ O concentration, that is, humidity) is detected. Specifically, the conventional apparatus uses a sensor having two electrode pairs (an upstream electrode pair and a downstream electrode pair). In the conventional apparatus, first, a voltage of a magnitude that decomposes oxygen is applied to the upstream electrode pair, and oxygen in the exhaust gas is discharged to the outside of the sensor. Next, the conventional apparatus applies a voltage of a magnitude that decomposes H ₂ O to the downstream electrode pair, and measures the current output from the electrode pair. The conventional apparatus, based on the current, identifies of H _{2 O} concentration in the exhaust gas. Thus, the conventional apparatus, since it measured and distinguished from the current output due to current output due in H 2 _O in oxygen, can be accurately detected of H 2 _O concentration (e.g., Patent Document 1 See).

  Hereinafter, a limiting current type sensor capable of detecting the oxygen concentration of exhaust gas is referred to as an “oxygen concentration sensor”, and a voltage applied to an electrode pair of the sensor is referred to as an “applied voltage to the sensor”. The current output from the pair is referred to as “sensor output current”. The internal combustion engine is referred to as an “engine”.

JP-A-2-122255

  The amount of sulfur component contained in engine fuel is generally extremely small. However, depending on the region where the engine is used, a fuel having a relatively high sulfur concentration may be used. In that case, SOx generated at the time of combustion may cause white smoke of the exhaust gas. Therefore, it is required to measure the sulfur concentration of the fuel. The inventor of the present invention studied to detect the SOx concentration of exhaust gas as a parameter related to the sulfur concentration of fuel.

  Specifically, the inventor of the present invention examined whether or not the SOx concentration of the exhaust gas can be detected using the method employed in the conventional apparatus. As a result of this study, when a voltage large enough to decompose SOx (in other words, reduce SOx to sulfur) is applied to the electrode pair of the oxygen concentration sensor, not only SOx but also components in exhaust gas other than SOx are decomposed. It was found that it is not easy to distinguish and measure only the output current caused by SOx. That is, it is difficult to accurately detect the SOx concentration of the exhaust gas depending on the method adopted in the conventional apparatus.

  The objective of this invention is providing the control apparatus which can detect the SOx density | concentration of the exhaust gas of an engine accurately.

In order to solve the above problems, a control device according to the present invention provides:
The present invention is applied to an internal combustion engine having a limiting current type sensor capable of detecting the oxygen concentration of exhaust gas, and includes a processing unit for detecting the SOx concentration of exhaust gas.

This processing unit
While controlling the engine to maintain the oxygen concentration of the exhaust gas at a constant value,
The voltage applied to the sensor gradually decreases from a first voltage at which sulfur produced by SOx reduction by the sensor is accumulated in the sensor to a second voltage at which sulfur is oxidized to SOx by the sensor. And execute the sweep process
The SOx concentration of the exhaust gas is detected based on the waveform of the output current of the sensor during the execution of the sweep process and the reference value that is the limit current value of the sensor corresponding to the constant value of oxygen concentration.

  According to experiments and considerations by the inventor of the present invention, when the “sweep process” for reducing the applied voltage to the sensor from the “first voltage” to the “second voltage” is executed, The “waveform” includes an output component resulting from the re-oxidation of sulfur to SOx by the sensor (an output component resulting from the “exhaust SOx concentration”) and an output component resulting from the “exhaust oxygen concentration”. However, it does not include output components that are substantially attributable to other components.

  Therefore, the apparatus of the present invention performs the sweep process while “maintaining the oxygen concentration of the exhaust gas at a constant value”, thereby making the output component due to the oxygen concentration constant, Prevents disturbing the “waveform”. The apparatus of the present invention analyzes the waveform of the output current in consideration of the output component (that is, the “reference value”) due to the oxygen concentration. As a result, the apparatus of the present invention can distinguish and measure only the output component resulting from the SOx concentration from the output current of the sensor.

  Therefore, the apparatus of the present invention having the above configuration can accurately detect the SOx concentration of the exhaust gas.

  By the way, as described above, the apparatus of the present invention detects an output component resulting from the SOx concentration in the process of detecting the SOx concentration of the exhaust gas. Since this output component has a one-to-one correspondence with the SOx concentration, it substantially represents the SOx concentration. Therefore, the “SOx concentration” in the present invention can be restated as at least one of the SOx concentration itself of the exhaust gas and the value corresponding to the SOx concentration on a one-to-one basis.

  The apparatus of the present invention does not simply measure the output current for each applied voltage as in the conventional apparatus, but only the output current component caused by SOx in the exhaust gas by a process unique to the present invention called a sweep process. It can be distinguished and measured. As a result, it was difficult for the conventional apparatus to distinguish between SOx and other components in the exhaust gas, whereas the apparatus of the present invention can detect the SOx concentration of the exhaust gas with high accuracy. Thus, the present invention has an advantageous effect compared with the conventional apparatus.

  Next, a plurality of modes (modes 1 to 3) of the control device of the present invention will be described.

・ Mode 1
The “reference value” may be acquired when the sweep process is actually executed, or may be acquired with reference to a map or the like determined in advance through experiments or the like.

  When the reference value is acquired when the sweep process is actually executed, if the acquisition of the reference value is excessively separated from the execution of the sweep process, the detection accuracy of the SOx concentration is improved when the output characteristics of the sensor change over time. May be reduced. Therefore, in order to detect the SOx concentration as accurately as possible, it is preferable that the acquisition of the reference value and the execution of the sweep process are performed at timings as close as possible.

Therefore, the control device of this aspect is
The second voltage matches the applied voltage used when detecting the oxygen concentration of the exhaust gas;
The reference value is a value of the output current of the sensor at the time when the voltage applied to the sensor in the sweep process drops to the second voltage (that is, a voltage for detecting the oxygen concentration).
Can be configured as follows.

  According to the inventors' experiments and the like, when the sweep process is executed, the second voltage is normally accumulated in the sensor before reaching the “applied voltage used when detecting the oxygen concentration of the exhaust gas”. All sulfur reoxidation is complete. Therefore, although the second voltage itself is a voltage at which sulfur oxidation can occur, the output current when the applied voltage is the second voltage substantially includes only an output component due to oxygen.

  Therefore, the apparatus of this aspect uses the output current when the applied voltage drops to the second voltage as the reference value. Furthermore, since the apparatus of this aspect acquires the reference value when the sweep process is completed, the acquisition of the reference value and the sweep process are performed substantially continuously. Therefore, the apparatus of this aspect can detect the SOx concentration of the exhaust gas with higher accuracy than when the acquisition of the reference value and the execution of the sweep process are performed at different timings.

・ Aspect 2
On the other hand, when the reference value is acquired with reference to a map or the like determined in advance by experiments or the like, for example, “correspondence between oxygen concentration and limit current” prepared for measuring oxygen concentration can be used. .

Therefore, the control device of this aspect is
The processing unit stores in advance a correspondence relationship between the oxygen concentration of the exhaust gas and the value of the limit current,
The reference value is a limit current value obtained by applying the constant oxygen concentration to the correspondence relationship;
Can be configured as follows.

  According to the above configuration, although it is necessary to know the oxygen concentration (a constant value) of the exhaust gas during the sweep process, it is not necessary to separately perform measurement or the like for determining the reference value as in the first aspect. Therefore, the apparatus of this aspect can easily and quickly detect the SOx concentration. Furthermore, since the “correspondence” is stored in advance, the apparatus of this aspect reliably determines the reference value even if the oxygen concentration (constant value) of the exhaust gas during the sweep process differs for each SOx concentration measurement. be able to.

・ Aspect 3
According to the inventor's experiment and the like, the waveform of the output current of the sensor during the execution of the sweep process has an extreme value (peak value of the output current) that changes corresponding to the SOx concentration of the exhaust gas (for example, FIG. reference.). The apparatus of this aspect calculates | requires SOx density | concentration from the absolute value of the difference of this extreme value and a reference value as one aspect | mode which detects the SOx density | concentration of waste gas.

Specifically, the control device of this aspect is
The processor uses the “extreme value of the output current of the sensor during execution of the sweep process” as a value indicating the characteristic of the waveform, and the larger the absolute value of the difference between the extreme value and the reference value is, Detect the SOx concentration of the exhaust gas so that the SOx concentration becomes high,
The second voltage is an applied voltage smaller than an applied voltage at which the extreme value is output;
Can be configured as follows.

  According to the above configuration, the SOx concentration of the exhaust gas can be detected without requiring complicated calculation. The correspondence between the absolute value of the difference between the extreme value and the reference value and the SOx concentration of the exhaust gas can be determined in advance by experiments or the like.

FIG. 1 is a schematic diagram illustrating a schematic configuration of an internal combustion engine to which a control device according to an embodiment of the present invention is applied. FIG. 2 is a schematic diagram showing a schematic configuration of a limiting current type oxygen concentration sensor (one cell sensor). FIG. 3 is a schematic diagram showing output characteristics of a limiting current type oxygen concentration sensor. FIG. 4 is a schematic diagram illustrating an example of the waveform of the output current of the sensor when the sweep process is executed. FIG. 5 is a schematic diagram showing the relationship between the SOx concentration of exhaust gas and the output of the sensor. FIG. 6 is a schematic diagram illustrating an example of the waveform of the output current of the sensor when the sweep process is executed. FIG. 7 is a schematic diagram illustrating an example of the waveform of the output current of the sensor when the sweep process is executed. FIG. 8 is a time chart showing an example of the relationship among the air-fuel ratio of exhaust gas (oxygen concentration of exhaust gas), the voltage applied to the sensor, and the output current of the sensor. FIG. 9 is a time chart showing an example of the relationship among the air-fuel ratio of exhaust gas (oxygen concentration of exhaust gas), the voltage applied to the sensor, and the output current of the sensor. FIG. 10 is a flowchart illustrating a routine executed by the control device according to the embodiment. FIG. 11 is a schematic diagram showing a schematic configuration of a limiting current type oxygen concentration sensor (2-cell sensor).

<Embodiment>
[Outline of device]
FIG. 1 shows a schematic configuration of an internal combustion engine 10 to which a control device (hereinafter referred to as “implementation device”) according to an embodiment of the present invention is applied.

The engine 10 is a spark ignition internal combustion engine (so-called gasoline engine). The engine 10 has the following configuration.
・ Main body 20
The fuel injection valve 21, ignition plug 22, combustion chamber 23, fuel pump 24, fuel supply pipe 25, piston 26, connecting rod 27, and crankshaft 28 belong to the main body 20.
・ Intake system 30
The intake valve 31, the intake port 32, the intake manifold 33, the throttle valve 34, the intake pipe 35, and the air cleaner 36 belong to the intake system 30.
・ Exhaust system 40
The exhaust valve 41, the exhaust port 42, the exhaust manifold 43, the exhaust gas purification catalyst 44, and the exhaust pipe 45 belong to the exhaust system 40.
・ Accelerator pedal 51
・ Several sensors 61 to 64
A limiting current type oxygen concentration sensor 61, a crank position sensor 62, an air flow meter 63, and an accelerator opening sensor 64 are included in the plurality of sensors.
Electronic control device 71

  As shown in FIG. 2, a limiting current type oxygen concentration sensor 61 (hereinafter referred to as “sensor 61”) includes a sensor cell 61D including a solid electrolyte layer 61A, a measurement electrode 61B, and a reference electrode 61C, a diffusion rate-determining method. A layer 61E, a sensor control unit 61F, a first alumina layer 61G, a second alumina layer 61H, a third alumina layer 61I, a fourth alumina layer 61J, a fifth alumina layer 61K, and a heater 61L are provided. Further, the sensor 61 is formed with an air introduction path 61M and an internal space 61N. The sensor 61 is a “one cell sensor” having one sensor cell 61D.

  The sensor 61 is provided in the exhaust manifold 43 so that the front end portion (the front end portion on the side where the diffusion-controlling layer 61E exists) is exposed to the exhaust gas. In consideration of the possibility that the exhaust gas purification catalyst 44 may change the SOx concentration of the exhaust gas, the sensor 61 is provided upstream of the exhaust gas purification catalyst 44.

  The solid electrolyte layer 61A is made of zirconia or the like that can conduct oxygen ions. The measurement electrode 61B and the reference electrode 61C are made of a platinum group element such as platinum and rhodium or an alloy containing them. The measurement electrode 61B and the reference electrode 61C are arranged so as to sandwich the solid electrolyte layer 61A. The measurement electrode 61B is disposed on the wall surface on one side of the solid electrolyte layer 61A (specifically, on the wall surface defining the internal space 61N), and the reference electrode 61C is disposed on the other side of the solid electrolyte layer 61A. It is arranged on the wall surface (specifically, on the wall surface that defines the air introduction path 61M).

  The sensor control unit 61F is connected to the sensor cell 61D so that the measurement electrode 61B is a cathode and the reference electrode 61C is an anode. The sensor control unit 61F is connected to the heater 61L so that electric power can be supplied to the heater 61L. The sensor control unit 61F is connected to the electronic control device 71.

  The sensor control unit 61F receives an instruction signal from the electronic control device 71, applies a voltage corresponding to the instruction signal to the sensor cell 61D, and transmits a current value output from the sensor cell 61D to the electronic control device 71. The sensor control unit 61F receives an instruction signal from the electronic control device 71 and supplies power corresponding to the instruction signal to the heater 61L.

  The application of voltage to the sensor cell 61D is for measurement so as to generate a potential difference according to the instruction signal from the electronic control unit 71 between the measurement electrode 61B and the reference electrode 61C (between the electrode pair). This is performed by applying a voltage to the electrode 61B and the reference electrode 61C.

  The internal space 61N is a space defined by the solid electrolyte layer 61A, the diffusion control layer 61E, the first alumina layer 61G, and the second alumina layer 61H. The internal space 61N is separated from the outside of the sensor (inside the exhaust manifold 43) by the diffusion rate controlling layer 61E. The diffusion-controlling layer 61E has a porous structure and controls the movement of exhaust gas from the exhaust manifold 43 to the internal space 61N (and thus diffusion of the exhaust gas to the solid electrolyte layer 61A). The atmosphere introduction path 61M is open to the atmosphere outside the sensor 61.

  The sensor 61 is normally used for detecting the oxygen concentration of the exhaust gas flowing in the exhaust manifold 43. Specifically, when a voltage for measuring the oxygen concentration (hereinafter referred to as “normal voltage”) is applied to the sensor cell 61D, oxygen contained in the exhaust gas existing in the internal space 61N is ionized by the measurement electrode 61B. Is done. Oxygen ions move from the measurement electrode 61B through the solid electrolyte layer 61A to the reference electrode 61C. The oxygen ions that have reached the reference electrode 61C are recombined to return to oxygen, and are released to the atmosphere introduction path 61M. The movement of electrons due to this series of electrochemical reactions is measured by the sensor control unit 61F as an output current from the sensor cell 61D. Due to the action of the diffusion-controlling layer 61E, the magnitude of the output current (in other words, the amount of oxygen ions moving between the electrode pair) and the oxygen concentration of the exhaust gas have a one-to-one relationship. That is, the output current from the sensor cell 61D has a magnitude corresponding to the oxygen concentration of the exhaust gas. This output current is generally referred to as “limit current”.

  By the way, the “oxygen concentration of exhaust gas” is determined mainly depending on the air-fuel ratio of the air-fuel mixture before combustion. Conversely, the air-fuel ratio of the air-fuel mixture can be estimated based on the oxygen concentration of the exhaust gas. Therefore, the oxygen concentration of the exhaust gas is also referred to as “the exhaust gas air-fuel ratio”. According to this designation, for example, the oxygen concentration of the exhaust gas generated by combustion of the stoichiometric air-fuel mixture is substantially zero, and the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio. Further, hereinafter, the applied voltage to the sensor cell 61D is referred to as “applied voltage to the sensor 61”, and the output current from the sensor cell 61D is referred to as “the output current of the sensor 61”.

  3A is a schematic diagram showing the relationship among the air-fuel ratio A / F of exhaust gas, the voltage Vs applied to the sensor 61, and the output current Is from the sensor 61. FIG. As shown in the figure, the air-fuel ratio A / F of the exhaust gas and the magnitude of the output current Is (limit current) have a one-to-one relationship when the applied voltage Vs is within a specific range. Therefore, the voltage within this range is used as the normal voltage V0 (see the alternate long and short dash line in the figure). The normal voltage V0 is a fixed value (eg, 0.4 V) generally determined based on experiments or the like. FIG. 3B is a schematic diagram showing the relationship between the air-fuel ratio A / F of the exhaust gas and the output current Is when the applied voltage Vs is the normal voltage V0. As shown in the figure, the air-fuel ratio A / F of the exhaust gas is uniquely specified based on the output current Is.

  In the implementation apparatus, the value of the output current Is of the sensor 61 is measured by the sensor control unit 61F and transmitted to the electronic control unit 71. Then, the electronic control unit 71 specifies (detects) the air-fuel ratio A / F of the exhaust gas by applying the received value of the output current Is of the sensor 61 to the correspondence relationship shown in FIG.

  As described above, the sensor 61 included in the engine 10 to which the implementation apparatus is applied is a sensor having a characteristic of outputting a current (limit current) corresponding to the oxygen concentration of exhaust gas on a one-to-one basis, and is a solid capable of conducting oxygen ions. An electrolyte, a measurement electrode and a reference electrode (electrode pair) provided so as to sandwich the solid electrolyte, and a diffusion-controlling layer provided so as to cover the measurement electrode are included. In the sensor having this configuration, the voltage applied to the sensor can be rephrased as a voltage applied to the electrode pair in order to cause a potential difference between the electrode pair. Further, the output current of the sensor can be rephrased as a current output from the electrode pair.

  Referring again to FIG. 1, the crank position sensor 62 is configured to output a signal representing the rotational position of the crankshaft 28. The air flow meter 63 is configured to output a signal representing the amount of air per unit time (intake air amount) sucked into the engine 10. The electronic control unit 71 calculates the amount of air introduced into the combustion chamber 23 based on these signals. The accelerator opening sensor 64 outputs a signal indicating the opening of the accelerator pedal 51. The electronic control unit 71 determines the output required for the engine 10 based on this signal.

  The electronic control unit 71 is an electronic circuit mainly composed of a known microcomputer including a CPU, a ROM, a RAM, and the like. A CPU (hereinafter referred to as “CPU”) of the electric control device 71 transmits an instruction signal to the fuel injection valve 21, the throttle valve 34, the sensor 61, and the like, and receives signals output from the plurality of sensors 61 to 64. Is configured to do.

[SOx concentration detection method]
A method of detecting the SOx concentration in the implementation apparatus will be described with reference to FIGS. The implementation apparatus uses the sensor 61 to detect the SOx concentration of the exhaust gas. According to experiments and considerations by the inventor, when the applied voltage Vs to the sensor 61 is gradually decreased from the specific first voltage V1 to the specific second voltage V2, the output current Is of the sensor 61 becomes the SOx of the exhaust gas. It was found that a unique waveform corresponding to the concentration was drawn. Hereinafter, the process of gradually decreasing the applied voltage Vs to the sensor 61 from the first voltage V1 to the second voltage V2 is referred to as a “sweep process”.

  First, an outline of the sweep process will be described. FIG. 4 is a schematic diagram illustrating an example of the waveform of the output current of the sensor 61 when the sweep process is performed. The waveform of the output current Is when the applied voltage Vs drops from the first voltage V1 to the second voltage V2 (the waveform represented by the solid line in the figure, from point A through point B to point C) is It changes corresponding to the SOx concentration. However, this waveform changes in accordance with not only the SOx concentration of the exhaust gas but also the air-fuel ratio (oxygen concentration) of the exhaust gas during the sweep process. Therefore, the implementation apparatus extracts a value representing only the SOx concentration of the exhaust gas from the waveform of the output current Is, and detects the SOx concentration of the exhaust gas based on the value. In order to perform this detection with high accuracy, the execution apparatus controls the engine 10 so as to maintain the air-fuel ratio of the exhaust gas during the sweep process at a constant value. The output current Is corresponding to this constant value is used as an output component (reference value Iref) corresponding to the air-fuel ratio of the exhaust gas.

  As illustrated, the first voltage V1 and the second voltage V2 are higher than the normal voltage V0 for measuring the oxygen concentration. Therefore, when the oxygen concentration is measured using the sensor 61, when the sweep process is started, the implementation apparatus changes the applied voltage Vs from the normal voltage V0 to the first voltage V1 (indicated by a broken line in the drawing). (See waveform from point D to point A). In addition, when the sweep process is finished, the implementation apparatus drops the applied voltage Vs from the second voltage V2 to the normal voltage V0 (see the waveform from point C to point D indicated by a broken line in the figure). In the example shown in the figure, for convenience, the execution apparatus not only performs the sweep process (period of point A-point B-point C) but also the period before and after the sweep process (period of point D-point A). And the period between point C and point D), the air-fuel ratio of the exhaust gas is maintained at a constant value.

  Specifically, when the sensor 61 is used to detect the air-fuel ratio of exhaust gas, the applied voltage Vs to the sensor 61 is the normal voltage V0 (see point D in the figure). When executing the sweep process, the execution apparatus changes the applied voltage Vs from the normal voltage V0 to the first voltage V1 (see point A in the figure). Thereby, SOx is reduced to sulfur by the sensor 61 (specifically, on the surface of the measurement electrode 61B, etc.), and the sulfur generated by this reduction is transferred to the sensor 61 (on the surface of the measurement electrode 61B, etc.). Accumulated. The amount of sulfur accumulated in the sensor 61 corresponds to the SOx concentration of the exhaust gas.

  The reason for this is that sulfur (S) is solid under the temperature at which the sensor 61 is normally used and accumulates in the sensor without volatilizing from the sensor. Furthermore, similarly to the fact that the limit current value in the sensor 61 increases as the oxygen concentration in the exhaust gas increases, the amount of SOx accumulated in the sensor 61 per unit time increases as the SOx concentration in the exhaust gas increases.

Since the first voltage V1 is larger than the normal voltage V0, when the applied voltage Vs is the first voltage V1, oxygen in the exhaust gas is also ionized by the sensor 61, and components other than SOx in the exhaust gas (for example, Hx ₂ O) can also be decomposed by the sensor 61. However, oxygen ions are exhausted from the sensor 61 according to the oxygen measurement principle (see above) of the sensor 61, and the substance (for example, H ₂ ) generated by decomposition of components other than SOx is generally a gas at the above temperature. 61 is not accumulated. Therefore, when the applied voltage Vs is the first voltage V1, only the sulfur is substantially accumulated in the sensor 61.

  Thus, the first voltage V <b> 1 is an applied voltage at which sulfur generated by SOx being reduced to sulfur by the sensor 61 is accumulated in the sensor 61. In this example, the first voltage V <b> 1 is a voltage (for example, 1.0 V) that is confirmed in advance to cause the same reduction and accumulation by experiments or the like, and is recorded in the ROM of the electronic control unit 71.

  When the applied voltage Vs is changed from the normal voltage V0 to the first voltage V1, the output current Is of the sensor 61 increases as illustrated due to the reduction of SOx described above (see point A in the figure). .) The output current Is at this time is the first value I1.

  Next, the implementation apparatus performs a sweep process while maintaining the air-fuel ratio of the exhaust gas at a constant value, and gradually lowers the applied voltage Vs from the first voltage V1. During the execution of the sweep process, the sensor 61 oxidizes sulfur to SOx, decomposes oxygen, and decomposes components other than SOx in the exhaust gas. Therefore, the output current Is includes output components due to each of them. However, according to the inventor's experiment and the like, the output component caused by components other than SOx is extremely small and can be ignored from the viewpoint of detecting the SOx concentration. Therefore, the output component Is during execution of the sweep process substantially includes an output component due to sulfur (specifically, SOx) and an output component due to oxygen.

  Therefore, if the oxygen concentration of the exhaust gas fluctuates excessively during the execution of the sweep process, the output current Is greatly changes to reflect the fluctuation, and the accuracy of measuring the output component due to sulfur (specifically, SOx) is high. May decrease. Therefore, the implementation apparatus performs the sweep process while maintaining the air-fuel ratio of the exhaust gas at a constant value. As a result, the output component due to oxygen contained in the output current Is is maintained constant (maintained at the limit current value corresponding to a constant value of oxygen concentration), and thus is caused by fluctuations in oxygen concentration. The output current Is is not disturbed.

  As a result, the output current Is draws a unique waveform corresponding to the SOx concentration of the exhaust gas. Specifically, as illustrated, the output current Is draws a waveform that once decreases from the first value I1 and then increases again (see solid lines connecting points A, B, and C in the figure). ). For example, a minimum value (hereinafter referred to as “peak value Ipeak”) in this waveform has a value corresponding to the SOx concentration of the exhaust gas. That is, the waveform of the output current Is has a peak value Ipeak corresponding to the SOx concentration of the exhaust gas.

  The execution apparatus ends the sweep process at a predetermined time after the peak value Ipeak is output (see point C in the figure). The applied voltage Vs at the time when the sweep process is finished is the second voltage V2. Thus, the second voltage V2 is an applied voltage for obtaining a unique waveform corresponding to the SOx concentration of the exhaust gas described above, and is an applied voltage at which sulfur is oxidized (reoxidized) to SOx by the sensor 61. . Note that the output current Is at the time when the sweep process ends is the second value I2.

  Thereafter, the implementation apparatus returns the applied voltage Vs to the normal voltage V0 (see point D in the figure). As described above, in the example shown in the figure, the execution apparatus maintains the air-fuel ratio of the exhaust gas at the same constant value as the air-fuel ratio during the sweep process until the applied voltage Vs is returned to the normal voltage V0. Accordingly, the implementation apparatus uses the output current Is when the applied voltage Vs is returned to the normal voltage V0 as the reference value Iref.

  The implementation apparatus detects the SOx concentration of the exhaust gas based on the reference value Iref and the waveform (solid line in the figure) of the output current Is obtained during the sweep process. Thereby, the implementation apparatus can analyze the output component caused by the SOx concentration of the exhaust gas separately from the output component caused by other components.

  As an example of this detection, the implementation apparatus specifies (detects) the SOx concentration of the exhaust gas based on “the absolute value DEFis of the difference between the peak value Ipeak and the reference value Iref” as illustrated. Specifically, according to the inventors' experiment and the like, as shown in FIG. 5, the larger the absolute value DEFis of this difference, the higher the SOx concentration Csox. Therefore, the implementation apparatus calculates the absolute value DEFis of the difference from the waveform of the output current Is during the sweep process and the reference value Iref, and applies the absolute value DEFis of the difference to the correspondence in FIG. The SOx concentration Csox is specified (detected). 5 is determined in advance by experiments or the like, and is recorded in the ROM of the electronic control unit 71.

  As described above, the reference value Iref is used to extract only the output component resulting from the SOx concentration from the output current Is. Therefore, the reference value Iref is, for example, “When the output current of the sensor depends only on the oxygen concentration of the exhaust gas and the engine is controlled so that the oxygen concentration of the exhaust gas is the above-mentioned constant value, In other words, “output current”.

  Note that the principle that the peak value Ipeak of the output current occurs when the sweep process is executed is not clear at this time. However, in cyclic voltammetry, a general method for measuring the electrochemical properties of an object, it is known that a response current peak occurs near the redox potential due to the rate-limiting phenomenon of electrons or oxides. Yes. It is considered that a phenomenon similar to this phenomenon occurs during the sweep process of the present invention.

[Other detection methods of SOx concentration]
The implementation apparatus does not necessarily need to detect the SOx concentration of the exhaust gas based on the absolute value DEFis of the difference, and can employ other various detection methods. That is, the implementation apparatus can analyze the waveform of the output current by various methods.

  For example, as shown in FIG. 6, the implementation apparatus identifies (detects) the SOx concentration of the exhaust gas based on the “area AREAis of the region surrounded by the waveform of the output current Is during the sweep process and the reference value Iref”. ) Specifically, the larger the area AREAis, the higher the SOx concentration Csox. Therefore, the implementation apparatus detects the area AREAis of the region from the waveform of the output current Is, and detects the SOx concentration Csox of the exhaust gas by applying the area AREAis of the region to a predetermined map or the like (not shown). Can do.

  Further, for example, as illustrated in FIG. 7, the implementation apparatus may be configured such that “the time when the output current Is is the peak value Ipeak (point B in the drawing) and the time when the output current Is is the reference value Iref (shown in the drawing). The SOx concentration of the exhaust gas can be specified (detected) based on the absolute value tan θis of the change rate of the output current Is between the point E). Specifically, the larger the absolute value tanθis of the change rate, the higher the SOx concentration Csox. Therefore, the implementation apparatus calculates the absolute value tanθis of the change rate from the waveform of the output current Is, and applies the absolute value tanθis of the change rate to a predetermined map or the like (not shown), thereby obtaining SOx of the exhaust gas. The concentration Csox can be detected.

  By the way, from the viewpoint of the detection accuracy of the SOx concentration, in the detection method shown in FIG. 6, it is preferable that the second voltage V2 coincides with the normal voltage V0. On the other hand, in the detection methods shown in FIGS. 4 and 7, it is sufficient that the peak value Ipeak is acquired, and therefore the second voltage V2 does not necessarily need to coincide with the normal voltage V0. Thus, the implementation apparatus may use the applied voltage specified in advance as the second voltage V2 when oxidation from sulfur to SOx occurs in the sensor 61 by experiments or the like, and the sweep process is actually executed. The applied voltage when it is confirmed based on the output current Is that sulfur oxidation has occurred may be used.

  The execution device may simply decrease the voltage Vs applied to the sensor 61 “gradually” in the sweep process, and may employ various modes of reduction. For example, the applied voltage Vs may be continuously dropped, or may be dropped discretely at appropriate intervals in terms of analyzing the waveform of the output current Is (see above). Further, the rate of decrease of the applied voltage Vs (the amount of decrease of the applied voltage Vs per unit time) is the speed at which reoxidation from sulfur to SOx occurs in the sensor 61, and the sensitivity for detecting the change in output current accompanying reoxidation. And can be determined in consideration of the detection accuracy required for the implementation apparatus. In addition, the voltage may be decreased at the same rate from the start to the end of the sweep process, or the voltage may be decreased at different rates. However, in order to increase the detection accuracy of the SOx concentration, it is preferable that the rate of decrease of the applied voltage Vs is not different for each measurement (each measurement is performed at the same rate of decrease).

  The execution apparatus may use a target value dedicated to the sweep process, which is predetermined by experiment or the like, as the air-fuel ratio (a constant value) of the exhaust gas during the sweep process, or the actual value when the sweep process is actually started. An air-fuel ratio may be used.

  The implementation apparatus does not necessarily need to use a voltage that has been confirmed in advance to reduce SOx by experiments or the like as the first voltage V1. For example, the first voltage V1 is gradually applied from an applied voltage (for example, a normal voltage V0 for detecting the oxygen concentration) that is known to be lower than the first voltage V1 before the actual sweep process is performed. The applied voltage Vs when the voltage Vs is increased and the reduction of SOx is confirmed based on the output current Is can be used.

  The implementation apparatus detects values (DEFis in FIG. 4, AREAis in FIG. 6, and tan θis in FIG. 7) resulting from the SOx concentration in the process of detecting the SOx concentration in the exhaust gas. Since this value has a one-to-one correspondence with the SOx concentration, it substantially represents the SOx concentration. Therefore, the target (SOx concentration) detected by the implementation apparatus can be paraphrased as at least one of the SOx concentration itself of the exhaust gas and the value corresponding to the SOx concentration on a one-to-one basis.

[Actual detection example]
A detection example when the above-described detection method is actually applied to the engine 10 will be described with reference to FIGS. In this example, the execution apparatus performs a sweep process during a specific period (between times t1 and t2) after the time when the condition (details will be described later) for detecting the SOx concentration of the exhaust gas is satisfied, The SOx concentration is detected.

  Specifically, at time t0 in FIG. 8, the air-fuel ratio of the exhaust gas changes based on the required output to the engine 10 and the like regardless of the detection of the SOx concentration. In order to measure the air-fuel ratio of the exhaust gas using the sensor 61 at time t0, the applied voltage Vs to the sensor 61 is set to the normal voltage V0. The output current Is of the sensor 61 at time t0 is a value I0 corresponding to the air-fuel ratio of the exhaust gas.

  Thereafter, at a predetermined timing, the execution apparatus increases the applied voltage Vs from the normal voltage V0 to the first voltage V1 in order to start the sweep process. Then, at time t1 in the figure, the applied voltage Vs reaches the first voltage V1 (see point A in the figure). In this example, the implementation apparatus performs the sweep process while maintaining the air-fuel ratio af1 of the exhaust gas at time t1. That is, the implementation apparatus controls the engine 10 so as to maintain the air-fuel ratio of the exhaust gas during the execution of the sweep process at a constant value af1 (for example, maintain the operating state of the engine 10 at time t1 as it is). Note that point A, point B, and point C in the figure correspond to point A, point B, and point C in FIGS. 4, 6, and 7, respectively. However, unlike the examples shown in FIGS. 4, 6, and 7, in this example, the air-fuel ratio at a time other than during the execution of the sweep process is not controlled to a particularly constant value.

  The execution apparatus performs the sweep process in a period from time t1 to time t2. Specifically, the implementation apparatus gradually decreases the applied voltage Vs from the first voltage V1 to the second voltage V2 at a constant speed while maintaining the air-fuel ratio of the exhaust gas at a constant value af1. The execution apparatus records the waveform of the output current Is during the sweep process (a waveform connecting point A, point B, and point C in the figure) in the RAM of the electronic control unit 71. In this example, the second voltage V2 is set to the same voltage as the normal voltage V0.

  The implementation apparatus calculates the peak value Ipeak based on the waveform of the output current Is recorded in the RAM (see point B in the figure). Furthermore, the implementation apparatus uses the output current Is at time t2 when the sweep process ends as the reference value Iref (see point C in the figure). The execution apparatus calculates the absolute value DEFis of the difference between the peak value Ipeak and the reference value Iref. The implementation apparatus then maps the absolute value DEFis of the difference to the absolute value DEFis of the difference and the correspondence relationship between the SOx concentration of the exhaust gas (recorded in the ROM of the electronic control unit 71, see FIG. 5). To detect the SOx concentration Csox of the exhaust gas.

  After the sweep process is completed (after time t2), the applied voltage Vs is maintained at the normal voltage V0, and the air-fuel ratio of the exhaust gas is detected again. After time t2, the air-fuel ratio of the exhaust gas changes again based on the required output to the engine 10 and the like.

  Note that after the absolute value DEFis of the difference is calculated, the execution apparatus does not calculate the SOx concentration Csox, and performs some processing (for example, a high value corresponding to the SOx concentration on a one-to-one basis). It may be used for a warning for informing the SOx concentration and measurement of the sulfur concentration of the fuel.

[Other detection examples]
The implementation apparatus does not necessarily need to detect the SOx concentration as shown in FIG. For example, as shown in FIG. 9, the implementation apparatus may set the second voltage V2, which is the applied voltage Vs that ends the sweep process, to an applied voltage that does not match the normal voltage V0. Specifically, in this example, the execution apparatus monitors the output current Is recorded in the RAM every moment during the sweep process. Then, the execution apparatus ends the sweep process at an arbitrary time after confirming that the peak value Ipeak has been output. In this way, the applied voltage Vs when the execution device ends the sweep process is the second voltage V2 (see point C in the figure).

  In this example, the output current Is at the end of the sweep process t2 may include an output component due to the SOx concentration as well as the air-fuel ratio of the exhaust gas. Therefore, the output current Is at the end of the sweep process (time t2 in FIG. 8) cannot be used as the reference value Iref as in the detection example in FIG. Therefore, in this example, the reference value Iref is estimated from the output current during the sweep process measured by another oxygen concentration sensor (not shown) different from the sensor 61 or the operating state of the engine 10 during the sweep process. An output current obtained by applying the air-fuel ratio of the exhaust gas to the map of FIG. 3B is used. When other sensors are used, the output characteristics of the other sensors and the output characteristics of the sensor 61 are the same, or the output current of the other sensor is set to the sensor 61 in order to increase the SOx concentration detection accuracy. It is preferable to use after converting (correcting) the output current.

  In this detection example, the process of detecting the SOx concentration can be complicated compared to the detection example of FIG. However, the present detection example has an advantage that the length of time for executing the sweep process can be shortened as compared with the detection example of FIG.

  Furthermore, the implementation apparatus can use an air-fuel ratio other than the air-fuel ratio af “other than” the exhaust gas at the start of the sweep process (time t1) as the air-fuel ratio (oxygen concentration) of the exhaust gas during the sweep process. For example, a single fixed value set in advance or a value selected from a plurality of fixed values prepared in advance may be used as the air-fuel ratio of the exhaust gas during the sweep process.

  As described above, the waveform of the output current Is at the time of the sweep process is generated when sulfur accumulated in the sensor 61 is reoxidized. Therefore, strictly speaking, the SOx concentration of the exhaust gas detected by the implementation device represents the SOx concentration of the exhaust gas at the time when sulfur is accumulated in the sensor 61 (at the start of the sweep process). On the other hand, depending on the configuration and arrangement of the sensor 61, a certain amount of time may be required to accumulate a sufficient amount of sulfur to detect the SOx concentration. In that case, the SOx concentration detected by the implementation device represents an average value of the SOx concentration of the exhaust gas during the period in which sulfur is accumulated in the sensor.

[Operation of the device]
The actual operation of the implementation apparatus will be described with reference to FIG. In the implementation apparatus, the CPU in the electronic control unit 71 executes the routine shown in FIG. 10 and performs a sweep process while controlling the engine 10 so as to maintain the air-fuel ratio (oxygen concentration) of the exhaust gas at a constant value. The SOx concentration of the exhaust gas is detected.

  Specifically, the CPU executes the “SOx concentration detection” routine of FIG. 10 every time a predetermined time elapses. When the processing of this routine is started, the CPU proceeds to step 1005 to determine whether or not the “condition for detecting the SOx concentration” is satisfied at the present time. In this example, the conditions are as follows: (1) the engine 10 is in steady operation, and (2) the SOx concentration is detected once since the vehicle on which the engine 10 is mounted has been refueled most recently. This is true if not.

  If the above condition is satisfied, the CPU makes a “Yes” determination at step 1005 to proceed to step 1010. In this step, the CPU transmits an instruction signal for increasing the applied voltage Vs to the sensor 61 to the first voltage V1 to the sensor 61 (to the sensor control unit 61F). In accordance with this instruction signal, the sensor 61 increases the applied voltage Vs to the first voltage V1. Note that the CPU transmits an instruction signal for maintaining the applied voltage Vs to the normal voltage V0 to the sensor 61 before transmitting this instruction signal.

  Next, the CPU proceeds to step 1015. In this step, the CPU starts “control for maintaining the oxygen concentration of the exhaust gas at a constant value”. In this example, the CPU adjusts the fuel injection amount while considering the intake air amount so as to maintain the air-fuel ratio (oxygen concentration) of the exhaust gas at the current time (starting of the sweep process), and maintains the air-fuel ratio of the exhaust gas. To do. Therefore, the constant value in this example is the value of the oxygen concentration of the exhaust gas at the time when the sweep process is started. Since this control stabilizes the oxygen concentration (air-fuel ratio) of the exhaust gas, this control is also referred to as “stabilization control”.

  Next, the CPU proceeds to step 1020. In this step, the CPU records the waveform of the output current Is received from the sensor 61 in the RAM while transmitting to the sensor 61 an instruction signal for gradually decreasing the applied voltage Vs from the first voltage V1 to the second voltage V2. To do. In this example, the second voltage V2 is set to the same voltage as the normal voltage V0.

  Next, the CPU proceeds to step 1025. In this step, the CPU records the output current Is when the applied voltage Vs reaches the second voltage V2 (= normal voltage V0) in the RAM as the reference value Iref. Thereafter, the CPU proceeds to step 1030 to end the stabilization control.

  Next, the CPU proceeds to step 1035. In this step, the CPU calculates the peak value Ipeak of the output current Is based on the waveform of the output current Is during the sweep process recorded in the RAM. For example, the CPU calculates the rate of change of the output current Is every predetermined time from the start of the sweep process, and the output current Is when the rate of change is zero (when the rate of change is reversed from positive to negative). Is calculated as a peak value Ipeak. Alternatively, for example, the CPU may calculate the minimum value of the output current Is (sample value) recorded in the RAM as the peak value Ip.

  Next, the CPU proceeds to step 1040. In this step, the CPU calculates the absolute value DEFis of the difference between the peak value Ipeak and the reference value Iref. Then, the CPU proceeds to step 1045. In this step, the CPU stores the absolute value DEFis of the calculated difference in a look-up table (Map (DEFis) in the drawing) representing “a relationship between the SOx concentration Csox and the absolute value DEFis of the difference”. By applying, the SOx concentration Csox is specified. That is, at step 1045, the SOx concentration Csox is detected.

  Thereafter, the CPU proceeds to step 1095 to end the present routine tentatively. If “the condition for detecting the SOx concentration” is not satisfied when step 1005 of this routine is executed, the CPU makes a “No” determination at step 1005 to directly proceed to step 1095 to execute this routine. Exit.

As described above, the implementation apparatus performs the sweep process while controlling the engine 10 so as to maintain the oxygen concentration (air-fuel ratio) of the exhaust gas at a constant value, and the waveform and reference of the output current Is of the sensor 61 during the sweep process. Based on the value Iref, the SOx concentration Csox of the exhaust gas is specified (detected). Thereby, the implementation apparatus can eliminate the influence of components other than SOx in the exhaust gas (for example, oxygen and H ₂ O) as much as possible, and can accurately detect the SOx concentration Csox of the exhaust gas.

<Other embodiments>
The present invention is not limited to the above embodiment, and various modifications can be employed within the scope of the present invention. For example, the sensor 61 included in the engine 10 to which the implementation apparatus is applied is a one-cell sensor having one sensor cell 61D, but the engine 10 is a sensor having a plurality of sensor cells 61D instead of the sensor 61 (for example, two cells). -A sensor) may be provided.

  FIG. 11 is a schematic diagram showing a schematic configuration of a two-cell sensor. As shown, the two-cell sensor 62 includes a sensor cell 62D1 including a solid electrolyte layer 62A1, a measurement electrode 62B1, and a reference electrode 62C1, and a solid electrolyte layer 62A2, a measurement electrode 62B2, and a reference electrode 62C2. Sensor cell 62D2. The sensor cell 62D1 is also referred to as an upstream cell, and the sensor cell 62D2 is also referred to as a downstream cell.

  The two-cell sensor 62 further includes a diffusion rate controlling layer 62E, a sensor control unit 62F, a first alumina layer 62G, a second alumina layer 62H, a third alumina layer 62I, a fourth alumina layer 62J, a fifth alumina layer 62K, It has 6 alumina layers 62L and a heater 62M. In addition, the two-cell sensor 62 has an air introduction path 62N and an internal space 62O. The two-cell sensor 62 may be provided in the exhaust manifold 43 in the same manner as the sensor 61 (see FIG. 2).

  The two-cell sensor 62 can detect the SOx concentration of the exhaust gas using the downstream cell 62D2 after releasing the oxygen in the exhaust gas to the outside of the internal space 61N using the upstream cell 62D1. Therefore, when the SOx concentration is detected using the two-cell sensor 62, the output current due to oxygen is substantially not included in the output current of the downstream cell 62D2, so the one-cell sensor is used. Compared to the case of the case (in the case of the implementation apparatus), the influence of the change in the oxygen concentration of the exhaust gas on the detection of the SOx concentration is small. However, even in the two-cell sensor 62, oxygen in the exhaust gas is not completely removed in the upstream cell 62D1, and the exhaust gas heading toward the downstream cell 62D2 may slightly contain oxygen. Therefore, by applying the control device or method of the present invention to an engine equipped with the two-cell sensor 62, the SOx concentration can be detected with high accuracy.

  Furthermore, although the engine 10 to which the implementation apparatus is applied is a gasoline engine, the control apparatus or method of the present invention can also be applied to a diesel engine.

  The execution device sets the operating state of the engine 10 at the time of starting the sweep process as the control (stabilization control) for maintaining the air-fuel ratio (oxygen concentration) of the exhaust gas during the sweep process at a constant value. (See step 1015 in FIG. 10). According to this method, it is not always necessary to acquire the air-fuel ratio of exhaust gas during the sweep process. However, the apparatus of the present invention may control the engine 10 as feedback control of the air-fuel ratio of the exhaust gas while acquiring the air-fuel ratio of the exhaust gas during the sweep process every moment as stabilization control. For example, the implementation apparatus may use a method of feedback-controlling the fuel injection amount so that the air-fuel ratio of the exhaust gas matches a certain value after the intake air amount is determined based on the required output to the engine 10. Furthermore, when the engine 10 is a diesel engine, the apparatus according to the present invention is configured so that the air-fuel ratio of the exhaust gas coincides with a constant value after the fuel injection amount is determined based on the required output to the engine 10 as stabilization control. Thus, a method of adjusting the intake air amount, the EGR amount, and the like can be used. In addition, when the engine 10 is used in combination with a motor or the like (for example, when the engine 10 is mounted on a hybrid vehicle), the apparatus of the present invention maintains the air-fuel ratio of the exhaust gas at a constant value. A method of compensating for excess or deficiency of output by a motor while controlling the motor can be used.

  Furthermore, the apparatus of the present invention can be configured to estimate the amount of sulfur component (sulfur concentration) contained in the fuel based on the SOx concentration of the exhaust gas. The sulfur concentration of fuel is generally determined depending on the type of fuel. Therefore, from the viewpoint of estimating the sulfur concentration of the fuel, it is only necessary to acquire the SOx concentration of the exhaust gas every time fuel is supplied to the engine.

DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 21 ... Fuel injection valve, 22 ... Spark plug, 41 ... Exhaust manifold, 61 ... Limit current type oxygen concentration sensor (air-fuel ratio sensor), 71 ... Electronic control unit

Claims (4)

A control device that is applied to an internal combustion engine having a limiting current type sensor capable of detecting the oxygen concentration of exhaust gas, and that includes a processing unit for detecting the SOx concentration of exhaust gas,
The processor is
While controlling the engine to maintain the oxygen concentration of the exhaust gas at a constant value,
The voltage applied to the sensor gradually decreases from a first voltage at which sulfur produced by SOx reduction by the sensor is accumulated in the sensor to a second voltage at which sulfur is oxidized to SOx by the sensor. And execute the sweep process
Detecting the SOx concentration of the exhaust gas based on the waveform of the output current of the sensor during the execution of the sweep process and a reference value which is a value of a limit current of the sensor corresponding to the oxygen concentration of the constant value;
Control device for internal combustion engine. The control device according to claim 1,
The second voltage matches the applied voltage used when detecting the oxygen concentration of the exhaust gas;
The reference value is the value of the output current of the sensor at the time when the applied voltage to the sensor drops to the second voltage in the sweep process.
Control device for internal combustion engine. The control device according to claim 1,
The processing unit stores in advance a correspondence relationship between the oxygen concentration of the exhaust gas and the value of the limit current,
The reference value is a limit current value obtained by applying the constant oxygen concentration to the correspondence relationship;
Control device for internal combustion engine. In the control device according to any one of claims 1 to 3,
The processing unit uses the extreme value of the output current of the sensor during the execution of the sweep process as a value indicating the characteristic of the waveform, and the SOx concentration increases as the absolute value of the difference between the extreme value and the reference value increases. Detect the exhaust gas SOx concentration so that the
The second voltage is an applied voltage smaller than an applied voltage at which the extreme value is output;
Control device for internal combustion engine.

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