WO2015022568A1

WO2015022568A1 - Control system and control method for internal combustion engine

Info

Publication number: WO2015022568A1
Application number: PCT/IB2014/001486
Authority: WO
Inventors: Keiichiro Aoki; Tatsuhiro Hashida; Toyoharu Kaneko; Keigo Mizutani
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2013-08-14
Filing date: 2014-08-08
Publication date: 2015-02-19
Also published as: JP2015036538A

Abstract

A control system for an internal combustion engine includes a limiting current sensor and an electronic control unit. The electronic control unit is configured to (i) execute sulfur poisoning recovery control for increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage, and (ii) determine that recovery of sulfur poisoning of the limiting current sensor has completed when an output current that reflects an extent of the sulfur poisoning of the limiting current sensor is smaller than or equal to a predetermined determination value, the output current being an output current from the limiting current sensor at the time when the sulfur poisoning recovery control is executed.

Description

CONTROL SYSTEM AND CONTROL METHOD FOR INTERNAL COMBUSTION

ENGINE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a control system and control method for an internal combustion engine.

2. Description of Related Art

[0002] Japanese Patent Application Publication No. 3- 100454 (JP 3-100454 A) describes a degradation recovery method for a limiting current oxygen sensor. In the degradation recovery method, degradation of the oxygen sensor (that is, degradation of the oxygen sensor due to S0₂) is recovered by gradually increasing a monitoring voltage of the limiting current oxygen sensor in an atmosphere not containing S0₂.

SUMMARY OF THE INVENTION

[0003] In the method described in JP 3-100454 A, it is not determined whether degradation of the oxygen sensor has been sufficiently recovered. Therefore, there is a possibility that recovery of degradation of the oxygen sensor is ended before the degradation of the oxygen sensor is recovered. Therefore, it is desired to determine whether degradation of the oxygen sensor has been sufficiently recovered. Particularly, when recovery of degradation of the oxygen sensor is carried out in an atmosphere containing a large amount of S0₂, there is a high possibility that recovery of degradation of the oxygen sensor is ended before the degradation of the oxygen sensor is recovered. Therefore, in this case, it is particularly desired to determine whether degradation of the oxygen sensor has been sufficiently recovered.

[0004] This also applies to the case where recovery of sulfur poisoning of a sulfur-poisoned sensor (that is, a sensor degraded by S0₂) is carried out. The invention provides a technique for accurately determining whether recovery of sulfur poisoning of a sensor has completed.

[0005] A first aspect of the invention provides a control system for an internal combustion engine. The control system includes: a limiting current sensor; and an electronic control unit configured to (i) execute sulfur poisoning recovery control for increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage, and (ii) determine that recovery of sulfur poisoning of the limiting current sensor has completed when an output current that reflects an extent of the sulfur poisoning of the limiting current sensor is smaller than or equal to a predetermined determination value, the output current being an output current from the limiting current sensor at the time when the sulfur poisoning recovery control is executed. With this configuration, ti is possible to accurately determine whether recovery of sulfur poisoning of the limiting current sensor has completed.

[0006] When the sulfur poisoning recovery control is executed once, the output current that reflects the extent of the sulfur poisoning may be an output current in a period in which the applied voltage is reduced at the time when the sulfur poisoning recovery control is executed.

[0007] When the sulfur poisoning recovery control is executed twice, the output current that reflects the extent of the sulfur poisoning may be an output current in a period in which the applied voltage is reduced at the time when the second sulfur poisoning recovery control is executed, and the predetermined determination value may be a value that is determined from an output current in a period in which the applied voltage is reduced at the time when the first sulfur poisoning recovery control is executed.

[0008] A second aspect of the invention provides a control system for an internal combustion engine. The control system includes: a limiting current sensor; and an electronic control unit configured to (i) execute sulfur poisoning recovery control for increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage, and (ii) detect a parameter regarding a specific component (hereinafter, "specific component parameter") in test gas by using an output current in a period in which the applied voltage is reduced at the time when the sulfur poisoning recovery control is executed.

[0009] With this configuration, the specific component parameter is detected by the limiting current sensor subjected to recovery of sulfur poisoning, so it is possible to accurately detect the specific component parameter. In addition, the specific component parameter is detected by using the output current during execution of the sulfur poisoning recovery control, so it is possible to detect the specific component parameter further early after recovery of the sulfur poisoning. Particularly, even when the influence of the specific component on output current at the time when the applied voltage is kept at a constant voltage or the influence of the specific component on output current at the time when the applied voltage is increased is smaller than the influence of the other components on the output current, but when the influence of the specific component on output current at the time when the applied voltage is reduced from the parameter detection voltage is larger than the influence of the other components on the output current, it is possible to accurately detect the specific component parameter with the limiting current sensor that is usable in detecting the concentration of oxygen in test gas.

[0010] A third aspect of the invention provides a control system for an internal combustion engine. The control system includes: a limiting current sensor; and an electronic control unit configured to (i) execute sulfur poisoning recovery control for increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage, (ii) execute voltage control for reducing the applied voltage from a parameter detection voltage, and (iii) detect a parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when the voltage control is executed.

[0011] With this configuration, the specific component parameter is detected by the limiting current sensor subjected to recovery of sulfur poisoning, so it is possible to accurately detect the specific component parameter. In addition, the specific component parameter is detected independently of recovery of sulfur poisoning, so it is possible to further accurately detect the specific component parameter. Particularly, even when the influence of the specific component on output current at the time when the applied voltage is kept at a constant voltage or the influence of the specific component on output current at the time when the applied voltage is increased is smaller than the influence of the other components on the output current, but when the influence of the specific component on output current at the time when the applied voltage is reduced from the parameter detection voltage is larger than the influence of the other components on the output current, it is possible to accurately detect the specific component parameter with the limiting current sensor that is usable in detecting the concentration of oxygen in test gas.

[0012] The electronic control unit may be configured to detect a parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when voltage control for reducing the applied voltage from a parameter detection voltage is executed, and the electronic control unit may be configured to execute the sulfur poisoning recovery control when an output current is larger than or equal to a sulfur poisoning recovery determination value, the output current is output at the time when the voltage control is executed. With this configuration, only when there is no possibility that the detection accuracy of the limiting current sensor decreases because of sulfur poisoning, the specific component parameter is detected. Therefore, it is possible to further accurately detect the specific component parameter.

[0013] The electronic control unit may be configured to detect a parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when voltage control for reducing the applied voltage from a parameter detection voltage is executed, and the electronic control unit may be configured to issue an alarm when an output current is larger than or equal to an alarm determination value, the output current is output at the time when the voltage control is executed. With this configuration, when there is a possibility that the fuel property is abnormal, a user of a detecting device that detects the specific component parameter is allowed to know that there is a possibility that the fuel property is abnormal.

[0014] The specific component may be SOx. In this case, it is possible to detect a parameter regarding SOx.

[0015] The sulfur poisoning recovery voltage may be higher than or equal to 0.8 V. With this configuration, it is possible to further reliably recover sulfur poisoning.

[0016] In the sulfur poisoning recovery control, the applied voltage at the timing of an end of reducing the applied voltage from the sulfur poisoning recovery voltage may be lower than or equal to 0.7 V. With this configuration, it is possible to further reliably recover sulfur poisoning.

[0017] The parameter detection voltage may be higher than or equal to 0.8 V. With this configuration, it is possible to obtain an output current that accurately corresponds to the specific component parameter, and, by extension, it is possible to accurately detect the specific component parameter.

[0018] The electronic control unit may be configured to set the frequency of a change in voltage to 100 Hz or lower at the time when the applied voltage is reduced from the sulfur poisoning recovery voltage. With this configuration, it is possible to further reliably recover sulfur poisoning.

[0019] The electronic control unit may be configured to set the frequency of a change in voltage to 100 Hz or lower at the time when the applied voltage is increased to a sulfur poisoning recovery voltage. With this configuration, it is possible to further reliably recover sulfur poisoning.

[0020] The internal combustion engine is, for example, a gasoline engine. The gasoline engine is operated at a stoichiometric air-fuel ratio in almost all the engine operation range. Thus, the concentration of oxygen in exhaust gas that is the test gas is low. Therefore, the specific component parameter is easily detected.

[0021] The electronic control unit may be configured to normally apply the limiting current sensor with an ordinary voltage lower than the sulfur poisoning recovery voltage, and detect a concentration of oxygen in test gas by using an output current from the limiting current sensor at the time when the ordinary voltage is applied to the limiting current sensor. With this configuration, it. is possible to detect the concentration of oxygen in the test gas. [0022| The electronic control unit may be configured to normally apply the limiting current sensor with an ordinary voltage lower than the sulfur poisoning recovery voltage, and detect a concentration of oxygen in test gas by using the output current of the limiting current sensor at the time when the ordinary voltage is applied to the limiting current sensor. With this configuration, it is possible to detect the concentration of oxygen in the test gas.

[0023] The electronic control unit may be configured to detect a parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when the applied voltage is reduced from a parameter detection voltage.

(0024] With this configuration, it is possible to detect the specific component parameter. Particularly, even when the influence of the specific component on output current at the time when the applied voltage is kept at a constant voltage or the influence of the specific component on output current at the time when the applied voltage is increased is smaller than the influence of the other components on the output current, but when the influence of the specific component on output current at the time when the applied voltage is reduced from the parameter detection voltage is larger than the influence of the other components on the output current, it is possible to accurately detect the specific component parameter with the limiting current sensor that is usable in detecting the concentration of oxygen in test gas.

[0025] The electronic control unit may be configured to use a peak value of an output current at the time when the applied voltage is reduced from the parameter detection voltage as an output current for detecting the parameter. The peak value is a minimum output current (or a maximum output current) that is output in the period in which the applied voltage is reduced. Thus, the peak value is an output current that accurately corresponds to the specific component parameter. Therefore, by using the peak value as the output current for detecting the parameter, it is possible to further accurately detect the specific component parameter.

[0026] The electronic control unit may be configured to normally apply a voltage lower than the parameter detection voltage in advance, and use an output cun-ent, which is output at the time when the applied voltage is increased to the parameter detection voltage and is then reduced, as an output cun-ent for detecting the parameter. In this case, the voltage that is applied to the limiting cun-ent sensor in advance before the applied voltage is reduced is lower than the parameter detection voltage. Therefore, in comparison with the case where the parameter detection voltage is applied to the limiting cunent sensor in ad ance before the applied voltage is reduced, it is possible to reduce electric power that is consumed in detecting the specific component parameter.

[0027] The applied voltage at the timing of an end of reducing the applied voltage from the parameter detection voltage may be lower than or equal to 0.7 V. With this configuration, it is possible to obtain an output cun-ent that accurately corresponds to the specific component parameter, and, by extension, it is possible to accurately detect the specific component parameter.

[0028] The electronic control unit may be configured to set the frequency of a change in voltage to 100 Hz or lower at the time when the applied voltage is reduced from the parameter detection voltage. With this configuration, it is possible to reliably obtain an output current that accurately corresponds to the specific component parameter, and, by extension, it is possible to accurately detect the specific component parameter.

[0029] The electronic control unit may be configured to set the frequency of a change in voltage to 100 Hz or lower at the time when the applied voltage is increased to the parameter detection voltage. With this configuration, it is possible to reliably obtain an output cun-ent that accurately conesponds to the specific component parameter, and, by extension, it is possible to accurately detect the specific component parameter.

[0030] A fourth aspect of the invention provides a control method for an internal combustion engine, the internal combustion engine including a limiting current sensor. The control method includes: carrying out recovery of sulfur poisoning by increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage; acquiring an output current that reflects an extent of the sulfur poisoning of the limiting cun-ent sensor, the output current being an output current from the limiting current sensor at the time when the recovery of the sulfur poisoning is carried out; and determining that the recovery of the sulfur poisoning has completed when the acquired output current is smaller than or equal to a predetermined determination value. With this configuration, it is possible to accurately determine whether recovery of sulfur poisoning of the limiting current sensor has completed.

[0031 J A fifth aspect of the invention provides a control method for an internal combustion engine, the internal combustion engine including a limiting current sensor. The control method includes: carrying out recovery of sulfur poisoning by increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage; acquiring an output current that reflects an extent of the sulfur poisoning of the limiting current sensor, the output current being an output current from the limiting current sensor at the time when the recovery of the sulfur poisoning is carried out; and detecting a parameter regarding a specific component in test gas using the output current of the limiting current sensor in the period in which the applied voltage is reduced at the time when the recovery of the sulfur poisoning is carried out in the case where the acquired output current is smaller than or equal to the predetermined determination value.

[0032] With this configuration, the specific component parameter is detected by the limiting current sensor subjected to recovery of sulfur poisoning, so it is possible to accurately detect the specific component parameter. In addition, the specific component parameter is detected by using the output current at the time when recovery of sulfur poisoning is executed, it is possible to detect the specific component parameter further early after recovery of the sulfur poisoning.

[0033] A sixth aspect of the invention provides a control method for an internal combustion engine, the internal combustion engine including a limiting current sensor. The control method includes: carrying out recovery of sulfur poisoning by increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage; acquiring an output current that reflects an extent of the sulfur poisoning of the limiting current sensor, the output current being an output current from the limiting current sensor at the time when the recovery of the sulfur poisoning is can-ied out; reducing the applied voltage from a voltage that is used to detect a parameter, when the acquired output current is smaller than or equal to the predetermined deteiTnination value; and detecting the parameter regarding a specific component in test gas by using the output current from the limiting current sensor at the time when the applied voltage is reduced.

[0034] With this configuration, the specific component parameter is detected by the limiting current sensor subjected to recovery of sulfur poisoning, so it is possible to accurately detect the specific component parameter. In addition, the 'specific component parameter is detected independently of recovery of sulfur poisoning, so it is possible to further accurately detect the specific component parameter.

[0035) When the recovery of the sulfur poisoning is carried out once, the output current that reflects the extent of the sulfur poisoning may be an output current in a period in which the applied voltage is reduced at the time when the recovery of the sulfur poisoning is carried out.

[0036] When the recovery of the sulfur poisoning is carried out twice, the output current that reflects the extent of the sulfur poisoning may be an output current in a period

^"I

in which the applied voltage is reduced at the time when the second recovery of the sulfur poisoning is carried out, and the predetermined determination value may be a value that is determined from an output current in a period in which the applied voltage is reduced at the time when the first recovery of the sulfur poisoning is carried out.

[0037] The control method may further include reducing the applied voltage from a voltage that is used to detect a parameter; and detecting the parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when voltage control is normally executed, wherein the recovery of the sulfur poisoning may be carried out when the output current, which is output at the time when the voltage control is normally executed, is larger than or equal to a sulfur poisoning recovery determination value. With this configuration, only when there is no possibility that the detection accuracy of the limiting current sensor decreases because of sulfur poisoning, the specific component parameter is detected. Therefore, it is possible to further accurately detect the specific component parameter.

[0038] The control method may further include reducing the applied voltage from a voltage that is used to detect a parameter; detecting the parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when voltage control is normally executed; and issuing an alarm when the output current, which is output at the time when the voltage control is normally executed, is larger than or equal to an alarm determination value. With this configuration, when there is a possibility that the fuel property is abnormal, it is allowed to know that there is a possibility that the fuel property is abnormal.

[0039] In the control method, the specific component may be SOx. In this case, it is possible to detect a parameter regarding SOx.

[0040] In the control method, the sulfur poisoning recovery voltage may be higher than or equal to 0.8 V. With this configuration, it is possible to further reliably recover sulfur poisoning.

[0041] In the control method, when the recovery of the sulfur poisoning is carried out, the applied voltage at the timing of an end of reducing the applied voltage from the sulfur poisoning recovery voltage may be lower than or equal to 0.7 V. With this configuration, it is possible to further reliably recover sulfur poisoning.

[0042] In the control method, the parameter detection voltage may be higher than or equal to 0.8 V. With this configuration, it is possible to obtain an output current that accurately corresponds to the specific component parameter, and, by extension, it is possible to accurately detect the specific component parameter.

[0043] In the control method, when the recovery of the sulfur poisoning is carried out, the frequency of a change in voltage may be set to 100 Hz or lower at the time when the applied voltage is reduced from the sulfur poisoning recovery voltage. With this configuration, it is possible to further reliably recover sulfur poisoning.

[0044] In the control method, when the recovery of the sulfur poisoning is carried out, the frequency of a change in voltage at the time when the applied voltage is increased to the sulfur poisoning recovery voltage and is then reduced may be set to 100 Hz or lower. With this configuration, it is possible to further reliably recover sulfur poisoning.

[0045] In^' the control method, the internal combustion engine is, for example, a gasoline engine. The gasoline engine is operated at a stoichiometric air-fuel ratio in almost all the engine operation range. Thus, the concentration of oxygen in exhaust gas that is the test gas is low. Therefore, the specific component parameter is easily detected.

[0046j The control method may further include: normally applying the limiting current sensor with an ordinary voltage lower than the sulfur poisoning recovery voltage, and detecting a concentration of oxygen in test gas by using an output current from the limiting current sensor at the time when the ordinary voltage is applied to the limiting current sensor. With this configuration, it is possible to detect the concentration of oxygen in the test gas.

[0047] The control method may further include: normally applying the limiting current sensor with an ordinary voltage lower than a parameter detection voltage, and detecting a concentration of oxygen in test gas by using an output current from the limiting current sensor at the time when the ordinary voltage is applied to the limiting current sensor. With this configuration, it is possible to detect the concentration of oxygen in the test gas.

[0048] The control method may further include: reducing the applied voltage to a voltage that is used to detect a parameter; acquiring an output cun-ent of the limiting current sensor; and detecting the parameter regarding a specific component in test gas by using the acquired output current.

[0049] With this configuration, it is possible to detect the specific component parameter. Particularly, even when the influence of the specific component on output current at the time when the applied voltage is kept at a constant voltage or the influence of the specific component on output current at the time when the applied voltage is increased is smaller than the influence of the other components on the output cutTent, but when the influence of the specific component on output current at the time when the applied voltage is reduced from the parameter detection voltage is larger than the influence of the other components on the output current, it is possible to accurately detect the specific component parameter with the limiting current sensor that is usable in detecting the concentration of oxygen in test gas.

[0050] In the control method, when the specific component parameter is detected, a peak value of the output current of the limiting current sensor may be acquired. The peak value is a minimum output current (or a maximum output current) that is output in the period in which the applied voltage is reduced. Thus, the peak value is an output current that accurately corresponds to the specific component parameter. Therefore, by using the peak value as the output current for detecting the parameter, it is possible to further accurately detect the specific component parameter.

[0051] The control method may further include: normally applying a voltage lower than the parameter detection voltage in advance; and increasing the applied voltage to the parameter detection voltage before the applied voltage is reduced. In this case, the voltage, which is applied to the limiting current sensor in advance before the applied voltage is reduced, is lower than the parameter detection voltage. Therefore, in comparison with the case where the parameter detection voltage is applied to the limiting current sensor in advance before the applied voltage is reduced, it is possible to reduce electric power that is consumed in detecting the specific component parameter.

[0052] In the control method, when the applied voltage is reduced, the applied voltage at the timing of an end of reducing the applied voltage from the parameter detection voltage may be lower than or equal to 0.7 V. With this configuration, it is possible to obtain an output current that accurately corresponds to the specific component parameter, and, by extension, it is possible to accurately detect the specific component parameter.

[0053] In the control method, when the applied voltage is reduced, the frequency of a change in voltage may be 100 Hz or lower at the time when the applied voltage is reduced from the parameter detection voltage. With this configuration, it is possible to reliably obtain an output current that accurately corresponds to the specific component parameter, and, by extension, it is possible to accurately detect the specific component parameter.

[0054] In the control method, when the applied voltage is increased, the frequency of a change in voltage may be 100 Hz or lower at the time when the applied voltage is increased to the parameter detection voltage. With this configuration, it is possible to reliably obtain an output current that accurately corresponds to the specific component parameter, and, by extension, it is possible to accurately detect the specific component parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 shows a limiting current sensor (dual cell limiting current sensor) according to an embodiment of the invention;

FIG. 2 shows the output characteristics of the limiting current sensor shown in FIG. 1 ;

FIG. 3 shows the output characteristics of the limiting current sensor shown in FIG. 1 ; FIG. 4 shows the correlation between a concentration of SOx and a peak value of output current;

FIG. 5 shows a limiting current sensor (single cell limiting current sensor) according to another embodiment of the invention;

FIG. 6 shows the output characteristics of the limiting current sensor shown in FIG. 5;

FIG. 7 shows an internal combustion engine including an SOx concentration detecting system having the limiting current sensor shown in FIG. 1 or the limiting current sensor shown in FIG. 5;

FIG. 8 shows a time chart that shows an output current corresponding to a change in applied voltage according to a first embodiment;

FIG. 9A and FIG. 9B show modes of an increase and reduction in applied voltage at the time when the concentration of SOx is detected;

FIG. 10A shows an example of a circuit that is employed in the limiting current sensor shown in FIG. 1 ;

FIG. 10B shows an example of a circuit that is employed in the limiting current sensor shown in FIG. 5;

FIG. 1 1 shows an example of an SOx concentration detecting flowchart according to the first embodiment;

FIG. 12 shows a time chart that shows an output current corresponding to a change in applied voltage at the time when sulfur poisoning recovery control is executed according to the first embodiment;

FIG. 1 3 shows an example of a sulfur poisoning recovery control and sulfur poisoning recovery completion determination flowchart according to the first embodiment;

FIG. 14 shows an example of a sulfur poisoning recovery control and sulfur poisoning recovery completion detennination flowchart according to a second embodiment;

FIG. 15 shows an example of a sulfur poisoning recovery control and sulfur poisoning recovery completion detennination flowchart according to a third embodiment;

FIG. 16 shows an example of a sulfur poisoning recovery control and sulfur poisoning recovery completion determination flowchart according to a fourth embodiment;

FIG. 17A and FIG. 17B show modes of an increase and reduction in applied voltage at the time when it is determined that recovery of sulfur poisoning has completed;

FIG. 18 shows an example of an SOx concentration detecting flowchart according to a fifth embodiment;

FIG. 19 shows an example of an SOx concentration detecting flowchart according to a sixth embodiment; and

FIG. 20 shows an example of an SOx concentration and air-fuel ratio detecting flowchart according to a seventh embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0056] The control system for an internal combustion engine according to the invention will be described with reference to the accompanying drawings. Hereinafter, embodiments of the invention will be described by way of an example in which exhaust

*

gas that is emitted from an internal combustion engine is employed as test gas, sulfur oxides (hereinafter, "SOx") in exhaust gas are employed as a specific component and the concentration of SOx is employed as a specific component parameter.

[0057] FIG. 1 shows one of limiting current sensors according to a first embodiment of the invention. The limiting current sensor shown in FIG. 1 is a dual cell li m iting current sensor. The limiting current sensor 10 shown in FIG. 1 includes a first solid electrolyte layer 1 1 A, a second solid electrolyte layer 1 I B, a first alumina layer 12A, a second alumina layer 12B, a third alumina layer 12C, a fourth alumina layer 12D, a fifth alumina layer 12E, a sixth alumina layer 12F, a diffusion-controlling layer 13, a heater 14, a pump cell 15, a first pump electrode 15A, a second pump electrode 15B, a pump cell voltage source 15C, a sensor cell 16, a first sensor electrode 1 6A, a second sensor electrode 16B, a sensor cell voltage source 16C, a first air introducing passage 1 7A, a second air introducing passage 17B and an internal space 18.

[0058] The solid electrolyte layers 1 1 A, 1 I B are made of zirconia, or the like, and have an oxygen ion conductivity. The alumina layers 12A to 12F are layers made of alumina. The diffusion-controlling layer 13 is a porous layer, and allows exhaust gas to pass therethrough. In the limiting current sensor (hereinafter, also simply referred to as sensor) 10, the layers are laminated from the lower side in FIG. 1 in order of the sixth alumina layer 12F, the fifth alumina layer 12E, the fourth alumina layer 12D, the second solid electrolyte layer 1 1 B, the diffusion-controlling layer 13 and the third alumina layer 12C, the first solid electrolyte layer 1 1 A, the second alumina layer 12B, and the first alumina layer 12 A. The heater 14 is arranged between the fifth alumina layer 12E and the sixth alumina layer 12F.

[0059] The first air introducing passage 17A is defined by the first alumina layer

12A, the second alumina layer 12B and the first solid electrolyte layer 1 1 A. Part of the first air introducing passage 17A is open to the air. The second air introducing passage 1 7B is defined by the second solid electrolyte layer 1 1 B, the fourth alumina layer 12D and the fifth alumina layer 12E. Part of the second air introducing passage 17B is open to the air. The internal space 18 is defined by the first solid electrolyte layer 1 1 A, the second solid electrolyte layer 12B. the diffusion-controlling layer 13 and the third alumina layer 1 2C. Part of the internal space 18 communicates with the outside of the sensor via the diffusion-controlling layer 13.

[0060] Each of the first pump electrode 15A and the second pump electrode 1 5B is made of a platinum-group element, such as platinum and rhodium, or an alloy of the platinum-group element. The first pump electrode 15A is arranged on one wall surface of the second solid electrolyte layer 1 1 B (that is, a wall surface of the second solid electrolyte layer I I B, which defines the internal space 18). The second pump electrode 15B is arranged on the other wall surface of the second solid electrolyte layer 1 1 B (that is, a wall surface of the second solid electrolyte layer 1 1 B, which defines the second air introducing passage 17B). These electrodes 15 A. 15B and the second solid electrolyte layer I I B constitute the pump cell 15. The sensor 10 is configured to be able to apply voltage from the pump cell voltage source 15C to the pump cell 15. Specifically, the pump cell. 15 is configured to be able to apply voltage from between the first pump electrode 15A and the second pump electrode 15B. The first pump electrode 15A is a negative electrode-side electrode. The second pump electrode 15B is a positive electrode-side electrode.

[0061] When voltage is applied to the pump cell 15, oxygen inside the internal space 18 becomes an oxygen ion on the first pump electrode 15A at the time when the oxygen contacts the first pump electrode 15 A. The oxygen ion moves inside the second solid electrolyte layer 1 I B toward the second pump electrode 15B. At this time, a current directly proportional to the amount of oxygen ions that move inside the second solid electrolyte layer 11 B flows between the first pump electrode 15A and the second pump electrode 15B. When an oxygen ion reaches the second pump electrode 15B, the oxygen ion becomes oxygen at the second pump electrode 15B, and the oxygen is released to the second air introducing passage 17B. That is, the pump cell 15 is able to decrease the concentration of oxygen in exhaust gas by releasing oxygen in exhaust gas to the air from exhaust gas through pumping. The pumping capacity of the pump cell 15 increases as the voltage that is applied from the pump cell voltage source 15C to the pump cell 15 increases.

[0062] Each of the first sensor electrode 16A and the second sensor electrode 16B is made of a platinum-group element, such as platinum and rhodium, or an alloy of the platinum-group element. The first sensor electrode 16A is arranged on one wall surface of the first solid electrolyte layer 1 1 A (that is, a wall surface of the first solid electrolyte layer 1 1 A, which defines the internal space 18). The second sensor electrode 16B is arranged on the other wall surface of the first solid electrolyte layer 1 1 A (that is, a wall surface of the first solid electrolyte layer 1 1 A, which defines the first air introducing passage 17A). These electrodes 16A, 16B and the first solid electrolyte layer 1 1 A constitute the sensor cell 16. The sensor 10 is configured to be able to apply voltage from the sensor cell voltage source 16C to the sensor cell 16. Specifically, the sensor 10 is configured to be able to apply voltage from between the first sensor electrode 16A and the second sensor electrode 16B. The first sensor electrode 16A is a negative electrode-side electrode. The second sensor electrode 16B is a positive electrode-side electrode.

[0063] When voltage is applied to the sensor cell 16, SOx inside the internal space 18 are decomposed on the first sensor electrode 16A when SOx contact the first sensor electrode 16A, and oxygen in the SOx becomes oxygen ions. The oxygen ions move inside the first solid electrolyte layer 1 1 A toward the second sensor electrode 16B. At this time, a current directly proportional to the amount of oxygen ions that move inside the first solid electrolyte layer 1 1 A flows between the first sensor electrode 16A and the second sensor electrode 16B. When oxygen ions reach the second sensor electrode 16B, the oxygen ions become oxygen at the second sensor electrode 16B, and the oxygen is released to the first air introducing passage 17A.

[0064] FIG. 2 shows the correlation between a pump cell applied voltage and a pump cell output current in the dual cell limiting current sensor according to the first embodiment. The pump cell applied voltage is a voltage that is applied to the pump cell 15 by the pump cell voltage source 15C. The pump cell output current is a current that flows between the first pump electrode 15A and the second pump electrode 15B. In FIG. 2, the line indicated by A/F = 12 shows a change in output current for a change in pump cell applied voltage when the air-fuel ratio of exhaust gas is 12. Similarly, the lines indicated by A/F = 13 to A/F = 18 respectively show changes in output current for a change in pump cell applied voltage when the air-fuel ratio of exhaust gas is 13 to 18.

[0065] As shown in FIG. 2, for example, when the air-fuel ratio of exhaust gas is 18. within the range in which the pump cell applied voltage is lower than a certain value Vth, (i) the absolute value of the pump cell output, cuiTent reduces as the pump cell applied voltage increases when the pump cell output cuiTent is a negative value; whereas (ii) the absolute value of the pump cell output cuiTent increases as the pump cell applied voltage increases when the pump cell output cuiTent is a positive value. Within a constant range in which the pump cell applied voltage is higher than or equal to the certain value Vth, the pump cell output current is a constant value irrespective of the pump cell applied voltage.

[0066] The above correlation between the pump cell applied voltage and the pump cell output current similarly holds when the air-fuel ratio of exhaust gas is 12 to 1 7. As is apparent from FIG. 2, in all the air-fuel ratios to be detected, when the voltage at which the pump cell output current is constant irrespective of the pump cell applied voltage is applied to the pump cell 15, it is possible to detect the air-fuel ratio of exhaust gas on the basis of the pump cell output current that is detected at that time. That is, the dual cell limiting current sensor 10 according to the first embodiment is usable in detecting the air-fuel ratio of exhaust gas. The air-fuel ratio of exhaust gas is a parameter that correlates with the concentration of oxygen in exhaust gas, so, in principle, the dual cell limiting cuiTent sensor according to the first embodiment is able to detect the concentration of oxygen in exhaust gas.

[0067] The correlation between the sensor cell applied voltage and the sensor cell output current in the dual cell limiting cuiTent sensor according to the first embodiment is also the same as the correlation shown in FIG. 2. Thus, in a state where the pump cell applied voltage is set to zero (that is, in a state where the pump cell 15 is not functioning), when the voltage is applied to the sensor cell 16 so as to keep the sensor cell output current constant at all the air-fuel ratios to be detected irrespective of the sensor cell applied voltage, it is possible to detect the air-fuel ratio of exhaust gas on the basis of the sensor cell output current that is detected at that time. That is, the dual cell limiting current sensor 10 according to the first embodiment is usable in detecting the air-fuel ratio of exhaust gas. The sensor cell applied voltage is a voltage that is applied to the sensor cell 16 by the sensor cell voltage source 16C. The sensor cell output current is a current that flows between the first sensor electrode 16A and the second sensor electrode 16B.

[0068] Next, the output characteristics of the dual cell limiting current sensor will be described. According to the researches of the inventors of the present application, it is newly found that a current corresponding to the concentration of SOx in exhaust gas is obtained from the limiting current sensor by reducing the voltage applied to the dual cell limiting current sensor from a predetermined voltage (hereinafter, "SOx concentration detection voltage"). The voltage applied to the dual cell limiting current sensor is specifically a voltage applied from the sensor cell voltage source 16C to the sensor cell 16. In the following description, the output current is a current that is output from the sensor cell 16.

[0069] FIG. 3 shows a change in output current when the applied voltage is gradually increased from 0.1 V to 1.0 V and is then gradually reduced from 1.0 V to 0.1 V. The abscissa axis of FIG. 3 represents applied voltage, and the ordinate axis of FIG. 3 represents output current. While the applied voltage is changed in this way, a voltage for setting the concentration of oxygen in exhaust gas inside the internal space 18 to zero (or substantially zero) is applied to the pump cell 15.

[0070] In FIG. 3, the continuous line LU0 shows a change in output current at the time when the applied voltage is increased from 0.1 V to 1.0 V in the case where no SOx are contained in exhaust gas. The case where no SOx are contained in exhaust gas is the case where the concentration of SOx in exhaust gas is zero. On the other hand, the continuous line LDO shows a change in output current at the time when the applied voltage is reduced from 1.0 V to 0.1 V similarly in the case where no SOx are contained in exhaust gas. In FIG. 3, the alternate long and short dashed line LU1 shows a change in output current at the time when the applied voltage is increased from 0.1 V to 1.0 V in the case where SOx are contained in exhaust gas. The alternate long and short dashed line LD1 shows a change in output cuiTent at the time when the applied voltage is reduced from 1 .0 V to 0. 1 V similarly in the case where SOx are contained in exhaust gas.

[0071 ] In the case where no SOx are contained in exhaust gas, when the applied voltage is increased from 0.1 V to about 0.2 V, the output cuiTent steeply increases to about 4 μΑ as shown by the continuous line LU0 of FIG. 3. In the period in which the applied voltage increases from about 0.2 V to about 0.85 V, the output current is substantially constant at about 4 μΑ. When the applied voltage exceeds about 0.85 V, the output current starts increasing. In the period in which the applied voltage is increased from about 0.85 V to 1 .0 V, the output current gradually increases, and reaches about 7 μΑ at the time when the applied voltage has reached 1 .0 V.

[0072] After that, when the applied voltage is gradually reduced from 1.0 V toward 0.4 V, the output current gradually reduces from about 7 μΑ and becomes substantially constant at about 3.5 μΑ in the period from when the applied voltage becomes lower than about 0.85 V to when the applied voltage reaches 0.4 V as shown by the continuous line LD0 in FIG. 3.

[0073] On the other hand, in the case where SOx are contained in exhaust gas, when the applied voltage is increased from 0.1 V to about 0.2 V, the output current steeply increases to, about 4 μΑ as shown by the alternate long and short dashed line LU 1 in FIG. 3. In the period in which the applied voltage increases from about 0.2 V to about 0.6 V, the output current is substantially constant at about 4 μΑ. When the applied voltage exceeds about 0.6 V, the output current starts increasing. In the period in which the applied voltage is increased from about 0.6 V to 1.0 V, the output cuiTent gradually increases, and reaches about 7 μΑ at the time when the applied voltage has reached 1 .0 V.

[0074] . After that, when the applied voltage is gradually reduced from 1 .0 V toward 0.4 V, the output current gradually reduces from about 7 μΑ, steeply reduces to invert its flow direction and then reaches about -5 μΑ in the period from when the applied voltage becomes lower than about 0.8 V to when the applied voltage reaches about 0.7 V as shown by the alternate long and short dashed line LD 1 in FIG. 3. In the period in which the applied voltage is further reduced from about 0.7 V to 0.4 V, the output cuiTent steeply increases to return to the original flow direction, and becomes about 3.5 μΑ when the applied voltage reaches 0.4 V.

[0075) Thus, in the case where SOx are contained in exhaust gas, when the applied voltage is increased from 0.4 V to 0.8 V and is then reduced from 0.8 V to 0.4 V, the output current steeply decreases and then steeply increases in the period in which the applied voltage is reduced. That is, when the applied voltage is reduced from 0.8 V to 0.4 V. the output current exhibits a change having a minimum value (that is a peak value). Here, when the applied voltage has reached about 0.7 V, the output current becomes a peak value.

[0076) The output current in the period from when the applied voltage exceeds about 0.6 V to when the applied voltage reaches 1 .0 V in the case where SOx are contained in exhaust gas is larger than the output current in the period from when the applied voltage exceeds about 0.6 V to when the applied voltage reaches 1 .0 V in the case where no SOx are contained in exhaust gas.

[0077J According to the researches of the inventors of the present application, it is found that there is the correlation shown in FIG. 4 between the peak value of output current and the concentration of SOx at the time when the applied voltage is reduced from 0.8 V to 0.4 V as described above in the dual cell limiting current sensor. That is, it is found that the concentration of SOx in exhaust gas increases as the difference between a reference current (that is, an output current at the timing at which the applied voltage has reached 0.8 V) and the peak value increases. The dual cell limiting current sensor according to the first embodiment is usable in detecting the concentration of oxygen in exhaust gas (by extension, the air-fuel ratio of exhaust gas). Thus, with the dual cell limiting current sensor according to the first embodiment, it is possible to calculate (that is, detect) the concentration of SOx by using the peak value with a sensor that is usable in detecting the concentration of oxygen in exhaust gas.

[0078) FIG. 5 shows another one of the limiting current sensors according to the first embodiment of the invention. A limiting current sensor 30 shown in FIG. 5 is a single cell limiting current sensor. The limiting current sensor 30 shown in FIG. 5 includes a solid electrolyte layer 3 1 , a first alumina layer 32A, a second alumina layer 32B, a third alumina layer 32C, a fourth alumina layer 32D, a fifth alumina layer 32E, a diffusion-controlling layer 33, a heater 34, a sensor cell 35, a first sensor electrode 35A, a second sensor electrode 35B, a sensor cell voltage source 35C, an air introducing passage 36, and an internal space 37.

[0079] The solid electrolyte layer 31 is made of zirconia, or the like, and has an oxygen ion conductivity. The alumina layers 32A to 32E are made of alumina. The diffusion-controlling layer 33 is a porous layer, and allows exhaust gas to pass therethrough. In the sensor 30, the layers are laminated from the lower side in FIG. 5 in order of the fifth alumina layer 32E, the fourth alumina layer 32D, the third alumina layer 32C, the solid electrolyte layer 3 1 , the diffusion-controlling layer 33 and the second alumina layer 32B, and the first alumina layer 32A. The heater 34 is arranged between the fourth alumina layer 32D and the fifth alumina layer 32E.

[0080] The air introducing passage 36 is a space defined by the solid electrolyte layer 3 1 , the third alumina layer 32C and the fourth alumina layer 32D. Part of the air introducing passage 36 is open to the air. The internal space 37 is a space defined by the first alumina layer 32A, the solid electrolyte layer 31 , the diffusion-controlling layer 33 and the second alumina layer 32B. Part of the internal space 37 communicates with the outside of the sensor via the diffusion-controlling layer 33.

[0081] Each of the first sensor electrode 35 A and the second sensor electrode 35B is made of a platinum-group element, such as platinum and rhodium, or an alloy of the platinum-group element. The first sensor electrode 35A is arranged on one wall surface of the solid electrolyte layer 31 (that is, a wall surface of the solid electrolyte layer 3 1 , which defines the internal space 37). The second sensor electrode 35B is arranged on the other wall surface of the solid electrolyte layer 3 1 (that is, a wall surface of the solid electrolyte layer 3 1 , which defines the air introducing passage 36). These electrodes 35 A, 35B and the solid electrolyte layer 3 1 constitute the sensor' cell 35. The sensor 30 is configured to be able to apply voltage from the sensor cell voltage source 35C to the sensor cell 35. Specifically, the sensor 30 is configured to be able to apply voltage from the sensor cell voltage source 35C to between the first sensor electrode 35A and the second sensor electrode 35B. The first sensor electrode 35A is a negative electrode-side electrode. The second sensor electrode 35B is a positive electrode-side electrode.

[0082] When voltage is applied to the sensor cell 35, SOx inside the internal space 37 are decomposed on the first sensor electrode 35 A when SOx contact the first sensor electrode 35A, and oxygen in the SOx becomes oxygen ions. The oxygen ions move inside the solid electrolyte layer 31 toward the second sensor electrode 35B. At this time, a current directly proportional to the amount of oxygen ions that move inside the solid electrolyte layer 3 1 flows between the first sensor electrode 35A and the second sensor electrode 35B. When oxygen ions reach the second sensor electrode 35B, the oxygen ions become oxygen at the second sensor electrode 35B, and the oxygen is released to the air introducing passage 36.

[0083] The correlation between the sensor cell applied voltage and the sensor cell output current in the single cell limiting current sensor according to the present embodiment is the same as the correlation shown in FIG. 2. Thus, when the voltage is applied to the sensor cell 35 so as to keep the sensor cell output current constant at all the air-fuel ratios to be detected irrespective of the sensor cell applied voltage, it is possible to detect the air- fuel ratio of exhaust gas on the basis of the sensor cell output current that is detected at that time. That is, the single cell limiting current sensor 30 according to the present embodiment is usable in detecting the air-fuel ratio of exhaust gas. The air-fuel ratio of exhaust gas is a parameter that con-elates with the concentration of oxygen in exhaust gas, so, in principle, the single cell limiting current sensor according to the present embodiment is able to detect the concentration of oxygen in exhaust gas. The sensor cell applied voltage is a voltage that is applied to the sensor cell 35 by the sensor cell voltage source 35C. The sensor cell output current is a current that flows between the first sensor electrode 35A and the second sensor electrode 35B.

[0084] Next, the output characteristics of the single cell limiting current sensor will be described. According to the researches of the inventors of the present application, it is found that a current corresponding to the concentration of SOx in exhaust gas is obtained from the limiting current sensor by reducing the voltage applied to the single cell limiting current sensor from a predetermined voltage (hereinafter, "SOx concentration detection voltage") as in the case of the dual cell limiting cun-ent sensor. The voltage applied to the single cell limiting current sensor is specifically a voltage applied from the sensor cell voltage source 35C to the sensor cell 35. In the following description, the output current is a cuirent that is output from the sensor cell 35, and the concentration of oxygen in the exhaust gas is constant at 1 %.

[0085] FIG. 6 shows a change in output current in the case where the applied voltage is gradually increased from 0.1 V to 1.0 V and is then gradually reduced from 1.0 V to 0.1 V. The abscissa axis of FIG. 6 represents applied voltage, and the ordinate axis of FIG. 6 represents output current.

[0086] In FIG. 6, the alternate long and short dashed line LU l shows a change in output current at the time when the applied voltage is increased from 0.1 V to 1.0 V in the case where SOx are contained in exhaust gas. In FIG. 6, the alternate long and short dashed line LD1 shows a change in output current at the time when the applied voltage is reduced from 1.0 V to 0.1 V similarly in the case where SOx are contained in exhaust gas.

[0087] On the other hand, in the case where SOx are contained in exhaust gas, when the applied voltage is increased from 0.1 V to about 0.2 V, the output current steeply increases to about 100 μΑ as shown by the alternate long and short dashed line LU l in FIG. 6. In the period in which the applied voltage increases from about 0.2 V to about 0.6 V, the output current is substantially constant at about 100 μΑ. When the applied voltage exceeds about 0.6 V, the output current starts increasing. In the period in which the applied voltage is increased from about 0.6 V to 1 .0 V, the output current slightly gradually increases, and reaches about 105 μΑ at the time when the applied voltage has reached 1.0 V.

[0088] After that, when the applied voltage is gradually reduced from 1.0 V toward 0.4 V, the output cun-ent gradually decreases from about 105 μΑ, steeply decreases and reaches about 80 μΑ in the period from when the applied voltage becomes lower than about 0.8 V to when the applied voltage reaches about 0.7 V as shown by the alternate long and short dashed line LD 1 in FIG. 6. In the period in which the applied voltage is reduced from about 0.7 V to 0.4 V, the output current steeply increases and becomes about 100 μΑ when the applied voltage reaches 0.4 V.

[0089] Thus, in the case where SOx are contained in exhaust gas, the applied voltage is increased from 0.4 V to 0.8 V. After that, when the applied voltage is reduced from 0.8 V to 0.4 V, the output current steeply reduces and then steeply increases in the period in which the applied voltage is reduced. That is. when the applied voltage is reduced from 0.8 V to 0.4 V, the output current exhibits a change having a minimum value (that is a peak value). Here, when the applied voltage has reached about 0.7 V, the output current becomes a peak value.

[0090] According to the researches of the inventors of the present application, it is found that there is the same correlation as the correlation shown in FIG. 4 between the peak value of output current and the concentration of SOx at the time when the applied voltage is reduced from 0.8 V to 0.4 V as described above in the single cell limiting current sensor. That is, it is found that the concentration of SOx in exhaust gas increases as the difference between a reference current (that is, an output current at the timing at which the applied voltage has reached 0.8 V) and the peak value increases. The single cell limiting current sensor according to the first embodiment is usable in detecting the concentration of oxygen in exhaust gas (by extension, the air-fuel ratio of exhaust gas). Thus, with the single cell limiting current sensor according to the first embodiment, which is usable in detecting the concentration of oxygen in exhaust gas, it is possible to calculate (that is, detect) the concentration of SOx by using the peak value.

[0091] Next, an SOx concentration detecting system according to the first embodiment will be described. FIG. 7 shows an internal combustion engine including the SOx concentration detecting system having the limiting current sensor 10 shown in FIG. 1 or the limiting current sensor 30 shown in FIG. 5. The internal combustion engine 50 shown in FIG. 7 is a spark ignition internal combustion engine (so-called gasoline engine). However, the invention is also applicable to a compression self-ignition internal combustion engine (so-called diesel engine). The internal combustion engine shown in FIG. 7 is operated at a stoichiometric air-fuel ratio in almost all the engine operation range.

[0092] The internal combustion engine 50 including the SOx concentration detecting system, shown in FIG. 7, includes the limiting current sensor 10 (see FIG. 1 ) or the limiting current sensor 30 (see FIG. 5), a cylinder head 51 , a cylinder block 52, a combustion chamber 53, a fuel injection valve 54, an ignition plug 55, a fuel pump 56, a fuel supply tube 57, a piston 60, a connecting rod 61. a crankshaft 62, a crank angle sensor 63, an intake valve 70, an intake port 71 , an intake manifold 72, a surge tank 73, a throttle valve 74, an intake pipe 75, an air flow meter 76, an air filter 77, an exhaust valve 80, an exhaust port 81 , an exhaust manifold 82, an exhaust pipe 83, an electronic control unit (ECU) 90, an accelerator pedal 101 , and an accelerator pedal depression amount sensor 102.

[0093] The fuel injection valve 54, the ignition plug 55, the throttle valve 74, the crank angle sensor 63, the air flow meter 76, the accelerator pedal depression amount sensor 102 and the limiting current sensor 10 or the limiting current sensor 30 are electrically connected to the ECU 90. The ECU 90 transmits signals for operating the fuel injection valve 54, the ignition plug 55 and the throttle valve 74 to them. The ECU 90 receives signals from the crank angle sensor 63, the air flow meter 76 and the accelerator pedal depression amount sensor 102. A signal corresponding to the rotation speed of the crankshaft 62 is output from the crank angle sensor 63. The ECU 90 calculates an engine rotation speed on the basis of the signal received from the crank angle sensor 63. A signal corresponding to the flow rate of air passing through the intake pipe 75 (by extension, the flow rate of air taken into the combustion chamber 53) is output from the air flow meter 76. The ECU 90 calculates an intake air amount on the basis of the signal received from the air flow meter 76. A signal corresponding to the depression amount of the accelerator pedal 101 is output from the accelerator pedal depression amount sensor 102. The ECU 90 calculates an engine load on the basis of the signal received from the accelerator pedal depression amount sensor 102.

[0094] The limiting current sensor 10 or the limiting current sensor 30 is attached to the exhaust pipe 83. Thus, gas (that is, test gas) that is intended to be detected by the limiting current sensor 10 or the limiting cun-ent sensor 30 is exhaust gas that is discharged from the combustion chamber 53. A current conesponding to the concentration of SOx in exhaust gas coming to the limiting cunent sensor 10 or the limiting current sensor 30 is output from the limiting current sensor 10 or the limiting cun-ent sensor 30. The ECU 90 calculates the concentration of SOx on the basis of the cunent received from the limiting current sensor 10 or the limiting current sensor 30. The details of this calculation method will be described later.

[0095) Detecting the concentration of SOx according to the first embodiment will be described with reference to FIG. 8. In the first embodiment, the applied voltage is steadily kept at 0.4 V (see the period before time TO in FIG. 8). That is, the voltage of 0.4 V is steadily applied to the sensor. In detecting the concentration of SOx according to the first embodiment, the applied voltage is increased from 0.4 V to 0.8 V (the period from time TO to time Tl in FIG. 8) and then the applied voltage is reduced from 0.8 V to 0.4 V (the period from time Tl to time T2 in FIG. 8). At this time, the ECU calculates (detects) the concentration of SOx by using the peak value of the output current input to the ECU in the period from when the applied voltage is reduced from 0.8 V to 0.4 V, and the reference cunent. At this time, as the difference between the reference cun-ent and the peak value increases, the concentration of SOx to be calculated increases.

[0096] When the concentration of SOx is calculated by using the difference between the peak value and the reference current (hereinafter, "current difference"), for example, the concentration of SOx corresponding to the cun-ent difference is obtained through an experiment, or the like, in advance for each current difference, these obtained concentrations of SOx are stored in the ECU in form of a map as a function of the current difference, and the concentration of SOx conesponding to the cunent difference that is calculated during detecting the concentration of SOx is read from the map. Thus, the concentration of SOx is calculated.

[0097] The limiting current sensor of the SOx concentration detecting system according to the first embodiment is usable in detecting the concentration of oxygen in exhaust gas (by extension, the air-fuel ratio of exhaust gas). Thus, with the SOx concentration detecting system according to the first embodiment, it is possible to detect the concentration of SOx in exhaust gas with the sensor that is usable in detecting the concentration of oxygen in exhaust gas. That is, the inventors of the present application obtained findings that, although the influence of SOx on output current at the time when the applied voltage is kept at a constant voltage (for example, 0.4 V) or the influence of SOx on output current at the time when the applied voltage is increased is smaller than the influence of the other components (for example, 0₂ and NOx) on the output current, the influence of SOx on output current at the time when the applied voltage is reduced from a parameter detection voltage (for example, 0.8 V) is larger than the influence of the other components on the output current. Therefore, with the SOx concentration detecting system according to the first embodiment, it is possible to accurately detect the concentration of SOx with the sensor that is usable in detecting the concentration of oxygen in exhaust gas.

[0098] The peak value is an output current that is maximally different from the output current in the case where the concentration of SOx is zero and that is output in the period in which the applied voltage is reduced. Thus, the peak value is an output current that accurately corresponds to the concentration of SOx. Therefore, by using the peak value as the output current for detecting the concentration of SOx, it is possible to further accurately detect the concentration of SOx.

[0099] In the first embodiment, the voltage of 0.4 V is applied to the sensor in advance before the applied voltage is reduced. Thus, the voltage is lower than 0.8 V that is the applied voltage at the timing of a start of reducing the applied voltage. Therefore, according to the first embodiment, in comparison with the case where the voltage of 0.8 V is applied to the sensor in advance before the applied voltage is reduced, it is possible to reduce electric power that is consumed in detecting the concentration of SOx.

[0100] In detecting the concentration of SOx according to the first embodiment, the applied voltage at the timing of a state of increasing the applied voltage (that is, the applied voltage that is steadily applied to the sensor in advance) is not limited to 0.4 V. The applied voltage may be a voltage that causes a change in output current having a peak value at the time when the applied voltage is increased and is then reduced. For example, the applied voltage at the timing of a start of increasing the applied voltage just needs to be lower than or equal to 0.6 V, and is desirably 0.4 V.

[0101] The applied voltage at the timing of an end of increasing the applied voltage is not limited to 0.8 V. The applied voltage may be a voltage that causes a change in output current having a peak value at the time when the applied voltage is increased and is then reduced or a voltage higher than or equal to the maximum voltage of an output stable voltage range (that is, in the case where the concentration of SOx is zero, the range in which the output current is substantially constant irrespective of the applied voltage), and may be, for example, higher than or equal to 0.8 V.

[0102] The applied voltage at the timing of an end of reducing the applied voltage is not limited to 0.4 V, and just needs to be lower than or equal to the applied voltage corresponding to the peak value. For example, the applied voltage at the timing of an end of reducing the applied voltage just needs to be lower than or equal to 0.7 V, and is desirably 0.4 V. Thus, the applied voltage at the timing of a start of increasing the applied voltage may be the same as or may be different from the applied voltage at the timing of an end of reducing the applied voltage.

[0103] In detecting the concentration of SOx according to the first embodiment, the peak value is used. Instead, the output current in the range in which the output current steeply reduces or the range in which the output current steeply increases in the period in which the applied voltage is reduced from 0.8 V to 0.4 V may be used.

[0104] The concentration of oxygen in exhaust gas flowing into the internal space of the sensor may change in the period in which the applied voltage is reduced. In this case, when the fact that a certain time is required to reduce the applied voltage is taken into consideration, the output current at the time when the applied voltage is 0.4 V more accurately reflects the concentration of oxygen in exhaust gas inside the internal space of the sensor at the timing at which the peak value is output than the output current at the time when the applied voltage is 0.8 V. Therefore, in detecting the concentration of SOx according to the first embodiment, in the case where the applied voltage is reduced from 0.8 V to 0.4 V, instead of the reference current, the output current at the timing at which the applied voltage has reached 0.4 V (or the output current after a lapse of a predetermined time from that timing) may be used as a reference current. With this configuration, even when the concentration of oxygen in exhaust gas changes in the period in which the applied voltage is reduced, it is possible to accurately detect the concentration of SOx.

[0105] In the first embodiment, instead of calculating the concentration of SOx by using the peak value and the reference current, the concentration of SOx may be calculated by using the peak value and a conversion coefficient. At this time, as the peak value increases in the negative direction, the concentration of SOx to be calculated becomes higher. The conversion coefficient is a coefficient by which the peak value is converted to the concentration of SOx in accordance with the correlation shown in FIG. 4. Of course, in the case where the peak value appears as a positive value, as the peak value increases in the positive direction, the concentration of SOx to be calculated becomes higher.

[0106] In detecting the concentration of SOx according to the first embodiment, if the rate of increase or rate of decrease (sweep rate) in applied voltage is too high, there is a possibility that the peak value is not output or the peak value that sufficiently corresponds to the concentration of SOx is not output even when the applied voltage is reduced. Therefore, in detecting the concentration of SOx according to the first embodiment, it is desirable that the rate of increase and rate of decrease in applied voltage at which the peak value that sufficiently corresponds to the concentration of SOx is output at the time when the applied voltage is reduced be selected.

[0107] Specifically, as shown in FIG. 9A, it is desirable that the applied voltage be increased such that the rate of increase in applied voltage gradually decreases and then the applied voltage be reduced such that the rate of decrease in applied voltage gradually increases. Alternatively, as shown in FIG. 9B, it is desirable that the applied voltage be increased such that the rate of increase in applied voltage is kept constant and then the applied voltage be reduced such that the rate of decrease in applied voltage is kept constant. [0108] In addition, specifically, in detecting the concentration of SOx according to the first embodiment, where a change in applied voltage that is increased from 0.4 V to 0.8 V and is then reduced from 0.8 V to 0.4 V is expressed by frequency, the frequency is desirably lower than or equal to 100 Hz. In other words, a time from when a start of increasing the applied voltage to when an end of reducing the applied voltage is desirably longer than or equal to 0.01 seconds.

[0109] When the internal combustion engine includes the limiting current sensor (dual cell limiting current sensor) shown in FIG. 1 , for example, the circuit shown in FIG. 10A is employed as an SOx detection circuit. The SOx detection circuit shown in FIG. 10A includes the limiting current sensor (the limiting current sensor shown in FIG. 1 ) 10, the heater 14, the pump cell 15, the sensor cell 16, the ECU 90, an applied voltage instruction unit 91 , a parameter calculation unit 92, a heater control unit 93, applied voltage control circuits 94P, 94S, and output current detection circuits 95P, 95 S.

[0110] The applied voltage instruction unit 91 , the parameter calculation unit 92 and the heater control unit 93 are component elements of the ECU 90.

[0111] The applied voltage instruction unit 91 transmits a command regarding the voltage applied to the pump cell 15 to the applied voltage control circuit 94P, and transmits a command regarding the voltage applied to the sensor cell 16 to the applied voltage control circuit 94S.

[0112] The parameter calculation unit 92 receives a signal corresponding to a pump cell output current from the output current detection circuit 95P. The parameter calculation unit 92 calculates the pump cell output current on the basis of the signal received from the output current detection circuit 95P. The parameter calculation unit 92 calculates the air-fuel ratio of exhaust gas (or the concentration of oxygen in exhaust gas) on the basis of the calculated output current. The parameter calculation unit 92 receives a signal corresponding to a sensor cell output current from the output current detection circuit 95S, and calculates the sensor cell output current on the basis of the received signal. The parameter calculation unit 92 calculates the concentration of SOx in exhaust gas on the basis of the calculated output current. In addition, the parameter calculation unit 92 calculates the impedance of the circuit in the sensor 10 on the basis of the signals received from the output cun-ent detection circuits 95P, 95S, and transmits information regarding the calculated impedance to the heater control unit 93. The heater control unit 93 transmits a control signal to the heater 14. The control signal is used to control the heater 14 on the basis of information about the impedance received from the parameter calculation unit 92.

[0113] The applied voltage control circuit 94P controls the pump cell applied voltage on the basis of the command received from the applied voltage instruction unit 91 . Alternatively, the applied voltage control circuit 94P controls the pump cell applied voltage on the basis of the command received from the applied voltage instruction unit 91 and the signal corresponding to the pump cell output current that is provided from the output current detection circuit 95P.

[0114] The output current detection circuit 95P detects the pump cell output current, and transmits a signal corresponding to the detected output current to the parameter calculation unit 92 and the applied voltage control circuit 94P.

[0115] The applied voltage control circuit 94S controls the sensor cell applied voltage on the basis of the command received from the applied voltage instruction unit 91. Alternatively, the applied voltage control circuit 94S controls the sensor cell applied voltage on the basis of the command received from the applied voltage instruction unit 91 and the signal corresponding to the sensor cell output current that is provided from the output current detection circuit 95S.

[0116] The output current detection circuit 95 S detects the sensor cell output current, and transmits a signal corresponding to the detected output current to the parameter calculation unit 92 and the applied voltage control circuit 94S.

[0117] On the other hand, for example, the circuit shown in FIG. 10B is employed as the SOx detection circuit in the case where the internal combustion engine includes the limiting current sensor (single cell limiting current sensor) shown in FIG. 5. The SOx detection circuit shown in FIG. 10B includes the limiting current sensor (the limiting current sensor shown in FIG. 5) 30, the heater 34, the sensor cell 35, the ECU 90, the applied voltage instruction unit 91 , the parameter calculation unit 92, the heater control unit 93, an applied voltage control circuit 94 and an output current detection circuit 95.

[0118] The applied voltage instruction unit 91 , the parameter calculation unit 92 and the heater control unit 93 are component elements of the ECU 90.

[0119] The applied voltage instruction unit 91 transmits a command regarding the voltage applied to the sensor cell 35 to the applied voltage control circuit 94.

[0120] The parameter calculation unit 92 receives a signal corresponding to the sensor cell output current from the output current detection circuit 95, calculates a sensor cell output current on the basis of the received signal, and calculates the air-fuel ratio of exhaust gas (or the concentration of oxygen in exhaust gas) or the concentration of SOx in exhaust gas on the basis of the calculated output current. The parameter calculation unit 92 calculates the impedance of the circuit in the sensor 30 on the basis of the signal received from the output current detection circuit 95, and transmits information regarding the calculated impedance to the heater control unit 93. The heater control unit 93 transmits a control signal to the heater 14. The control signal is used to control the heater 34 on the basis of information regarding the impedance received from the parameter calculation unit 92.

[0121] The applied voltage control circuit 94 controls the sensor cell applied voltage on the basis of the command received from the applied voltage instruction unit 91. Alternatively, the applied voltage control circuit 94 controls the sensor cell applied voltage on the basis of the command received from the applied voltage instruction unit 91 and the signal corresponding to the sensor cell output current that is provided from the output current detection circuit 95.

[0122] The output current detection circuit 95 detects the sensor cell output current, and transmits a signal corresponding to the detected output current to the parameter calculation unit 92 and the applied voltage control circuit 94.

[0123] An example of an SOx concentration detecting flowchart according to the above-described first embodiment will be described with reference to FIG. 1 1.

[0124] When the flowchart shown in FIG. 1 1 is started, the applied voltage is kept at 0.4 V. In step 10, the applied voltage Vs is increased from 0.4 V toward 0.8 V. Subsequently, in step 1 1 , it is detemiined whether the applied voltage Vs has reached 0.8 V (Vs = 0.8 V). When it is detemiined that Vs is equal to 0.8 V, the flowchart proceeds to step 12. On the other hand, when it is detemiined that Vs is not equal to 0.8 V, the flowchart returns to step 10. Thus, until it is determined in step 1 1 that Vs is equal to 0.8 V, the applied voltage Vs continues to be increased.

[0125] In step 12, the applied voltage Vs is reduced from 0.8 V toward 0.4 V, and the output current Is is detected. Subsequently, in step 13, it is determined whether the applied voltage Vs has reached 0.4 V (Vs is equal to 0.4 V). When it is determined that Vs is equal to 0.4 V, the flowchart proceeds to step 14. On the other hand, when it is determined that Vs is not equal to 0.4 V, the flowchart returns to step 12. Thus, until it is detemiined in step 13 that Vs is equal to 0.4 V, the applied voltage Vs continues to be reduced, and the output current Is continues to be detected.

[0126] In step 14, an SOx concentration Csox is calculated on the basis of the peak value of the output current Is detected in step 12, after which the flowchart ends.

[0127] When a catalyst that purifies components in exhaust gas is provided in the exhaust pipe, there is a possibility that SOx in exhaust gas is trapped by the catalyst. In this case, when the limiting current sensor is attached to the exhaust pipe at a portion downstream of the catalyst, there is a possibility that the concentration of SOx is not accurately detected. Therefore, in the above-described embodiment, when the catalyst is provided in the exhaust pipe, the limiting current sensor is desirably attached to the exhaust pipe at a portion upstream of the catalyst.

[0128] Next, sulfur poisoning recovery control according to the first embodiment will be described. This control is control for recovering the sulfur-poisoned sensor 10 or the sulfur-poisoned sensor 30. Sulfur poisoning is degradation of the sensor 10 or the sensor 30 (more specifically, the first sensor electrode 16A or the first sensor electrode 35A) due to SOx in exhaust gas.

[0129] In the first embodiment, the applied voltage is steadily kept at 0.4 V. That is, the voltage of 0.4 V is steadily applied to the sensor. When a request to recover sulfur poisoning is issued, the'applied voltage is increased from 0.4 V to 0.8 V and then the applied voltage is reduced from 0.8 V to 0.4 V. With this configuration, sulfur poisoning of the sensor reduces, and finally sulfur poisoning of the sensor is recovered as a result of repeating this control.

[0130] A sulfur poisoning recovery completion determination according to the first embodiment will be described. This determination is a determination as to whether recovery of sulfur poisoning of the sensor has completed as a result of executing the above-described sulfur poisoning recovery control.

[0131] In this determination, when the above-described sulfur poisoning recovery control is executed once, the area of the region Ar shown in FIG. 12 is calculated as "reference area", and the area of the region As shown in FIG. 12 is calculated as "sulfur poisoning area". The area of the region Ar is surrounded by the output current before the applied voltage is increased and the trajectory of the output current that is larger than or equal to the output current before the applied voltage is increased and that is an output current in the period from when the applied voltage is increased from 0.4 V to 0.8 V and is then reduced to 0.4 V. The area of the region As is surrounded by the output current before the applied voltage is increased and the trajectory of the output current that is smaller than the output current before the applied voltage is increased and that is an output current in the period from when the applied voltage is increased from 0.4 V to 0.8 V and is then reduced to 0.4 V. When the ratio of the sulfur poisoning area to the reference area (= Sulfur poisoning area/Reference area) is smaller than or equal to a predetermined determination value, it is determined that recovery of sulfur poisoning has completed.

[0132] With the sulfur poisoning recovery completion determination according to the first embodiment, it is possible to accurately determine whether recovery of sulfur poisoning of the sensor has completed. Hereinafter, the reason will be described.

[0133] When the sensor is exposed to exhaust gas from the internal combustion engine, the sensor may be subjected to sulfur poisoning. In the case where the sensor is subjected to sulfur poisoning, when the applied voltage is increased from 0.4 V to 0.8 V and is then reduced to 0.4 V, the reference area is almost not different from the reference area in the case where the sensor is not subjected to sulfur poisoning as shown in FIG. 12. On the other hand, at this time, it is found through the researches of the inventors of the present application that the sulfur poisoning area is larger than the sulfur poisoning area in the case where the sensor is not subjected to sulfur poisoning. That is, it is found that the ratio of the sulfur poisoning area to the reference area (hereinafter, "area ratio" ) in the case where the sensor is subjected to sulfur poisoning is larger than the area ratio in the case where the sensor is not subjected to sulfur poisoning.

[0134] Thus, when the predetermined determination value is set to an appropriate value, it is possible to accurately determine whether recovery of sulfur poisoning has completed by determining that recovery of sulfur poisoning has completed when the area ratio is smaller than or equal to the predetermined determination value.

[0135] The fact that the area ratio is smaller than or equal to the predetermined determination value means, in other words, that the sulfur poisoning area is smaller than or equal to a value that is determined from the reference area. Thus, with the sulfur poisoning recovery completion determination according to the first embodiment, when the sulfur poisoning area is smaller than or equal to the predetermined determination value (in the first embodiment, the value that is determined from the reference area) in the case where sulfur poisoning recovery control is executed once, it is determined that recovery of sulfur poisoning has completed.

[0136] In this case, the predetermined determination value is determined from the reference area; however, when it is possible to previously set the predetermined determination value for accurately determining that recovery of sulfur poisoning has completed on the basis of the sulfur poisoning area irrespective of the reference area, it is possible to determine whether recovery of sulfur poisoning has completed by using the predetermined determination value and the sulfur poisoning area. Thus, with the sulfur poisoning recovery completion determination according to the first embodiment, broadly speaking, when the sulfur poisoning area is smaller than or equal to the predetermined determination value at the time when the sulfur poisoning recovery control is executed once, it is determined that recovery of sulfur poisoning has completed.

[0137] The sulfur poisoning area is an area that reflects the extent of sulfur poisoning of the sensor. Alternatively, the sulfur poisoning area is an area that reflects the degree of sulfur poisoning of the sensor or an area on which the influence of sulfur poisoning of the sensor is larger than the reference area. The sulfur poisoning area is calculated by using the output current in the period in which the applied voltage is reduced (hereinafter, "voltage-reducing output current"). Thus, the voltage-reducing output current is also an output current that reflects the extent of sulfur poisoning of the sensor. Alternatively, the output current that reflects the extent of sulfur poisoning of the sensor is an output current that reflects the degree of sulfur poisoning of the sensor or an output current on which the influence of' sulfur poisoning of the sensor is larger than a voltage-increasing output current. Thus, when it is possible to previously set the predetermined determination value for accurately determining that recovery of sulfur poisoning has completed on the basis of the voltage reducing output current, it is possible to determine whether recovery of sulfur poisoning has completed by using the predetermined determination value and the voltage-reducing output current. Thus, with the sulfur poisoning recovery completion determination according to the first embodiment, broadly speaking, when the output current that reflects the extent of sulfur poisoning of the sensor is smaller than or equal to the predetermined determination value (or when the output current that reflects the degree of sulfur poisoning of the sensor is smaller than or equal to the predetermined determination value or when the voltage-reducing output current is smaller than or equal to the predetermined determination value (this predetermined determination value is a value that is determined from the voltage-increasing output current) under the same condition) at the time when the sulfur poisoning recovery control is executed once, it is determined that recovery of sulfur poisoning has completed.

[0138] An example of a sulfur poisoning recovery control and sulfur poisoning recovery completion determination flowchart according to the first embodiment will be described with reference to FIG. 13.

[0139] When the flowchart shown in FIG. 13 is started, the applied voltage is kept at 0.4 V. In step 20, it is determined whether a sulfur poisoning recovery request flag Fs is set (Fs = 1 ). This flag Fs is set at the time when there is a request to execute sulfur poisoning recovery control, and is reset at the time when recovery of sulfur poisoning has completed. In step 20, when it is determined that Fs is not equal to 1 , the flowchart directly ends. On the other hand, when it is determined that Fs is equal to 1 , the flowchart proceeds to step 21 , the applied voltage Vs is increased from 0.4 V toward 0.8 V, and the output current Is is detected. Subsequently, in step 22, it is determined whether the applied voltage Vs has reached 0.8 V (Vs = 0.8 V). When it is determined that Vs is equal to 0.8 V, the flowchart proceeds to step 23. On the other hand, when it is determined that Vs is not equal to 0.8 V, the flowchart returns to step 2 1 . Thus, until it is determined in step 22 that Vs is equal to 0.8 V, the applied voltage Vs continues to be increased.

[0140] In step 23, the applied voltage Vs is reduced from 0.8 V toward 0.4 V, and the output current Is is detected. Subsequently, in step 24, it is determined whether the applied voltage Vs has reached 0.4 V (Vs = 0.4 V). When it is determined that Vs is equal to 0.4 V, the flowchart proceeds to step 25. On the other hand, when it is determined that Vs is not equal to 0.4 V, the flowchart returns to step 23. Thus, until it is determined in step 24 that Vs is equal to 0.4 V, the applied voltage Vs continues to be reduced, and the output current Is continues to be detected.

[0141] In step 25, it is determined whether the area ratio Rs is smaller than or equal to the predetermined determination value Rsth (Rs < Rsth). The area ratio Rs is the ratio of the sulfur poisoning area Sd to the reference area Si (= Sd/Si). The reference area Si is calculated by using the output current Is detected in step 21 . The sulfur poisoning area Sd is calculated by using the output current Is detected in step 23. When it is determined in step 25 that Rs is smaller than or equal to Rsth, the flowchart proceeds to step 26, the sulfur poisoning recovery request flag Fs is reset, and then the flowchart ends. On the other hand, when it is determined that Rs is not smaller than or equal to Rsth, the flowchart returns to step 21 . Thus, until it is determined in step 25 that Rs is smaller than or equal to Rsth, the applied voltage Vs is increased and reduced. That is, the sulfur poisoning recovery control is executed. [0142] Next, a second embodiment of the invention will be described. In the following description of the second embodiment, for components and controls that are the same as the components and controls of the first embodiment or components or controls that are naturally derived from the components or controls of the first embodiment in light of the components or controls of the second embodiment that will be described below, the description thereof is omitted.

[0143] With a sulfur poisoning recovery completion determination according to the second embodiment, when the above-described sulfur poisoning recovery control is executed once, the peak value (the output current Ispi in FIG. 12) of output current in the period in which the applied voltage is increased from 0.4 V to 0.8 V is calculated as "voltage-increasing peak value", and the peak value (the output current Ispd shown in FIG, 12) of output current in the period in which the applied voltage is reduced from 0.8 V to 0.4 V is calculated as "voltage-reducing peak value". When the absolute value of the ratio of the voltage-reducing peak value to the voltage-increasing peak value (= Voltage-reducing peak value/Voltage-increasing peak value) is smaller than or equal to a predetermined determination value, it is determined that recovery of sulfur poisoning has completed.

[0144] With the sulfur poisoning recovery completion determination according to the second embodiment, it is possible to accurately determine whether recovery of sulfur poisoning of the sensor has completed. Hereinafter, the reason will be described.

[0145] In the case where the sensor is subjected to sulfur poisoning, it is found through the researches of the inventors of the present application that, when the applied voltage is increased from 0.4 V to 0.8 V and is then reduced to 0.4 V, although the voltage-increasing peak value is not so different from the voltage-increasing peak value in the case where the sensor is not subjected to sulfur poisoning, the absolute value of the voltage-reducing peak value is larger than the absolute value of the voltage-reducing peak value in the case where the sensor is not subjected to sulfur poisoning as shown in FIG. 12. That is, it is found that the absolute value of the ratio of the voltage-reducing peak value to the voltage-increasing peak value (hereinafter, "peak ratio") in the case where the sensor is subjected to sulfur poisoning is larger than the absolute value of the peak ratio in the case where the sensor is not subjected to sulfur poisoning.

[0146] Thus, when the predetermined determination value is set to an appropriate value, it is possible to accurately determine whether recovery of sulfur poisoning has completed by determining that recovery of sulfur poisoning has completed in the case where the absolute value of the peak ratio is smaller than or equal to the predetermined determination value.

[0147] The fact that the absolute value of the peak ratio is smaller than or equal to the predetemiined determination value means, in other words, that the absolute value of the voltage-reducing peak value is smaller than or equal to a value that is determined from the voltage-increasing peak value. Thus, with the sulfur poisoning recovery completion determination according to the second embodiment, when the sulfur poisoning recovery control is executed once, in the case where the absolute value of the voltage-reducing peak value is smaller than or equal to the predetemiined determination value (in the second embodiment, the value that is determined from the voltage-increasing peak value), it is determined that recovery of sulfur poisoning has completed.

[0148] In this case, the predetermined determination value is determined from the voltage-increasing peak value; however, when it is possible to previously set the predetemiined determination value for accurately determining that recovery of sulfur poisoning has completed on the basis of the absolute value of the voltage-reducing peak value irrespective of the voltage-increasing peak value, it is possible to determine whether recovery of sulfur poisoning has completed by using the predetemiined determination value and the absolute value of the voltage-reducing peak value. Thus, with the sulfur poisoning recovery completion determination according to the second embodiment, broadly speaking, when the sulfur poisoning recovery control is executed once, in the case where the absolute value of the voltage-reducing peak value is smaller than or equal to the predetemiined determination value, it is determined that recovery of sulfur poisoning has completed.

[0149] The absolute value of the voltage-reducing peak value is a value that reflects the extent of sulfur poisoning of the sensor. Alternatively, the absolute value of the voltage-reducing peak value is a value that reflects the degree of sulfur poisoning of the sensor or a value on which the influence of sulfur poisoning of the sensor is larger than the voltage-increasing peak value. The voltage-reducing peak value is an output current in the period in which the applied voltage is reduced (hereinafter, "voltage-reducing output current" ). Thus, the voltage-reducing output current is also an output- current that reflects the extent of sulfur poisoning of the sensor. Alternatively, the voltage-reducing output current is an output current that reflects the degree of sulfur poisoning of the sensor or an output current on which the influence of sulfur poisoning of the sensor is larger than the voltage-increasing output current. Thus, when it is possible to previously set the predetermined determination value for accurately determining that recovery of sulfur poisoning has completed on the basis of the voltage-reducing output current, it is possible to determine whether recovery of sulfur poisoning has completed by using the predetermined determination value and the voltage-reducing output current. Thus, with the sulfur poisoning recovery completion determination according to the second embodiment, broadly speaking, when the output current that reflects the extent of sulfur poisoning of the sensor is smaller than or equal to the predetermined determination value (or when the output current that reflects the degree of sulfur poisoning of the sensor is smaller than or equal to the predetermined determination value or when the voltage-reducing output current is smaller than or equal to the predetermined determination value (this predetermined determination value is a value that is determined from the voltage-increasing output current) under the same condition) at the time when the sulfur poisoning recovery control is executed once, it is determined that recovery of sulfur poisoning has completed.

[0150] An example of a sulfur poisoning recovery control and sulfur poisoning recovery completion determination flowchart according to the second embodiment will be described with reference to FIG. 14. Step 30 to step 34 in the flowchart shown in FIG. 14 are the same as step 20 to step 24 in the flowchart shown in FIG. 13, so the description of these steps is omitted. [0151] In step 35, it is determined whether the absolute value of the peak ratio |Rp| is smaller than or equal to the predetermined determination value Rpth (|Rp| < Rpth). The peak ratio Rp is the ratio of the voltage-reducing peak value Ispd to the voltage-increasing peak value Ispi (= Ispd/Ispi). When it is determined in step 35 that |Rp| is smaller than or equal to Rpth, the flowchart proceeds to step 36, the sulfur poisoning recovery request flag Fs is reset, and then the flowchart ends. On the other hand, when it is determined that |Rp| is not smaller than or equal to Rpth. the flowchart returns to step 31. Thus, until it is determined in step 35 that |Rp| is smaller than or equal to Rpth, the applied voltage Vs is increased and reduced. That is, the sulfur poisoning recovery control is executed.

[0152] Next, a third embodiment will be described. In the following description, for components and controls that are the same as the components and controls of the above-described embodiments or components or controls that are naturally derived from the components or controls of the above-described embodiments in light of the components or controls of the third embodiment that will be described below, the description thereof is omitted.

[0153] With the sulfur poisoning recovery completion determination according to the third embodiment, when the above-described sulfur poisoning recovery control is executed multiple times, the reference area (the area of the region Ar in FIG. 12) in the first one of the above-described successive two sulfur poisoning recovery controls is calculated as "first reference area", and the sulfur poisoning area (the area of the region As in FIG. 12) in the first sulfur poisoning recovery control is calculated as "first sulfur poisoning area". The ratio of the first sulfur poisoning area to the first reference area (= First sulfur poisoning area/First reference area) is calculated as "first area ratio".

[0154] In addition, the reference area (the area of the region Ar in FIG. 12) in the second sulfur poisoning recovery control is calculated as "second reference area", and the sulfur poisoning area (the area of the region As in FIG. 12) in the second sulfur poisoning recovery control is calculated as "second sulfur poisoning area". The ratio of the second sulfur poisoning area to the second reference area (= Second sulfur poisoning area/Second reference area) is calculated as "second area ratio".

[0155] When a variation from the first area ratio to the second area ratio (= First area ratio - Second area ratio) is smaller than or equal to a predetermined determination value, it is determined that recovery of sulfur poisoning has completed.

[0156] With the sulfur poisoning recovery completion determination according to the third embodiment, it is possible to accurately determine whether recovery of sulfur poisoning of the sensor has completed. Hereinafter, the reason will be described.

[0157] The second area ratio is calculated on the basis of an output current after the first sulfur poisoning recovery control is executed. Thus, when the second area ratio is calculated, the extent of sulfur poisoning of the sensor is lower than that when the first area ratio is calculated. In addition, as the extent of sulfur poisoning of the sensor at the time when the first sulfur poisoning recovery control is executed increases, the extent of sulfur poisoning of the sensor significantly decreases as a result of executing the first sulfur poisoning recovery control. That is, as the extent of sulfur poisoning of the sensor at the time when the first sulfur poisoning recovery control is executed increases, a variation from the first area ratio to the second area ratio (hereinafter, "area ratio variation") increases. In other words, when the extent of sulfur poisoning of the sensor at the time when, the first sulfur poisoning recovery control is executed is significantly low, the area ratio variation is significantly small. When the extent of sulfur poisoning of the sensor at the time when the first sulfur poisoning recovery control is executed is significantly low, there is a significantly high possibility that recovery of sulfur poisoning of the sensor completes as a result of executing the second sulfur poisoning recovery control.

[0158] Thus, when the predetermined determination value is set to an appropriate value, it is possible to accurately determine whether recovery of sulfur poisoning has completed by determining that recovery of sulfur poisoning has completed when the area ratio variation is smaller than or equal to the predetermined determination value.

[0159] The fact that the area ratio variation is smaller than or equal to the predetermined determination value means, in other words, that the second area ratio is smaller than or equal to a value that is determined from the first area ratio. Thus, with the sulfur poisoning recovery completion determination according to the third embodiment, when the sulfur poisoning recovery control is executed twice, in the case where the second area ratio is smaller than or equal to the predetennined determination value (in the third embodiment, the value that is determined from the first area ratio), it is determined that recovery of sulfur poisoning has completed.

[0160] In this case, the predetermined determination value is determined from the fi rst area ratio; however, when it is possible to previously set the predetermined determination value for accurately determining that recovery of sulfur poisoning has completed on the basis of the second area ratio irrespective of the first area ratio, it is possible to determine whether recovery of sulfur poisoning has completed by using the predetermined determination value and the second area ratio. Thus, with the sulfur poisoning recovery completion determination according to the third embodiment, broadly speaking, when the sulfur poisoning recovery control is executed twice, in the case where the second area ratio is smaller than or equal to the predetermined determination value, it is determined that recovery of sulfur poisoning has completed.

[0161] The second area ratio is an area ratio that reflects the extent of sulfur poisoning of the sensor. Alternatively, the second area ratio is a value that reflects the degree of sulfur poisoning of the sensor or a value that indicates the latest influence of sulfur poisoning of the sensor as compared to the first area ratio under the same condition. The second area ratio is calculated by using the output current in the period in which the applied voltage is reduced (hereinafter, "second voltage-reducing output current"). Thus, the second voltage-reducing output current is also an output current that reflects the extent of sulfur poisoning of the sensor (or an output current that reflects the degree of sulfur poisoning of the sensor or an output current that indicates the latest influence of sulfur poisoning of the sensor as compared to the first voltage-reducing output current (that is, the voltage-reducing output current that is used to calculate the first area ratio) under the same condition. Thus, when it is possible to previously set the predetermined determination value for accurately determining that recovery of sulfur poisoning has completed on the basis of the voltage-reducing output current, it is possible to determine whether recovery of sulfur poisoning has completed by using the predetermined determination value and the voltage-reducing output current. Thus, with the sulfur poisoning recovery completion determination according to the third embodiment, broadly speaking, when the output current that reflects the extent of sulfur poisoning of the sensor is smaller than or equal to the predetermined determination value (or when the output current that reflects the degree of sulfur poisoning of the sensor is smaller than or equal to the predetermined determination value or when the second voltage-reducing output current is smaller than or equal to the predetermined determination value (this predetermined determination value is a value that is determined from the first voltage-reducing output current) under the same condition) at the time when the sulfur poisoning recovery control is executed twice, it is determined that recovery of sulfur poisoning has completed.

[0162] In the third embodiment, when an air-fuel ratio variation is larger than or equal to a predetermined value or when a variation in output current is larger than or equal to a predetermined value, recovery of sulfur poisoning may be carried out through control other than the above-described sulfur poisoning recovery control for controlling the applied voltage. The variation in output current is a variation from the voltage-reducing peak value at the time when the first sulfur poisoning recovery control is executed to the voltage-reducing peak value at the time when the second sulfur poisoning recovery control is executed. This control is, for example, particularly effective in the case where the air-fuel ratio variation is larger than or equal to the predetermined value although the above-described sulfur poisoning recovery control for controlling the applied voltage is executed multiple times.

[0163] An example of a sulfur poisoning recovery control and sulfur poisoning recovery completion determination flowchart according to the third embodiment will be described with reference to FIG. 15. Step 40 to step 44 in the flowchart shown in FIG. 15 are the same as step 20 to step 24 in the flowchart shown in FIG. 13, so the description of these steps is omitted.

[0164] In step 45, a sulfur poisoning recovery control counter N is counted up. The counter N is counted up each time the sulfur poisoning recovery control is executed once, the counter N is counted down after the sulfur poisoning recovery control is executed twice successively, and the counter N is reset when it is determined that recovery of sulfur poisoning has completed.

[0165] Subsequently, in step 46, it is determined whether the counter N has reached "2" (N = 2). When it is determined that N is equal to 2. the flowchart proceeds to step 47. On the other hand, when it is determined that N is not equal to 2, the flowchart returns to step 41 . Thus, until it is determined in step 46 that N is equal to 2, the applied voltage Vs is increased and reduced. That is, the sulfur poisoning recovery control is executed.

[0166] In step 47, the counter N is counted down. Subsequently, in step 48, it is determined whether the area ratio variation ARs is smaller than or equal to the predetermined determination value ARsth (ARs < ARsth). The area ratio variation ARs is a variation from the first area ratio Rs l to the second area ratio Rs2 (= Rs l - Rs2). The first area ratio Rsl is calculated by using the output current Is detected in step 41 and step 43 at the time when the last but one sulfur poisoning recovery control is executed. The second area ratio Rs2 is calculated by using the output current Is detected in step 41 and step 43 at the time when the last sulfur poisoning recovery control is executed. When it is determined in step 48 that ARs is smaller than or equal to ARsth, the flowchart proceeds to step 49, the sulfur poisoning recovery request flag Fs and the counter N are reset, and then the flowchart ends. On the other hand, when it is determined that ARs is not smaller than or equal to ARsth, the flowchart returns to step 41 . Thus, until it is determined in step 48 that ARs is smaller than or equal to ARsth, the applied voltage Vs is increased and reduced. That is, the sulfur poisoning recovery control is executed.

[0167] A fourth embodiment will be described. In the following description, for components and controls that are the same as the components and controls of the above-described embodiments or components or controls that are naturally derived from the components or controls of the above-described embodiments in light of the components or controls of the fourth embodiment that will be described below, the description thereof is omitted. (0168] With the sulfur poisoning recovery completion determination according to the third embodiment, when the above-described sulfur poisoning recovery control is executed multiple times, the voltage-increasing peak value (the output current Ispi in FIG. 12) in the first one of the above-described successive two sulfur poisoning recovery controls is calculated as "first voltage-increasing peak value", and the voltage-reducing peak value (the output current Ispd in FIG. 12) in the first sulfur poisoning recovery control is calculated as "first voltage-reducing peak value". The ratio of the first voltage-reducing peak value to the first voltage-increasing peak value (= First voltage-reducing peak value/First voltage-increasing peak value) is calculated as "first peak ratio".

[0169] In addition, the voltage-increasing peak value (the output current Ispi in FIG. 12) in the second sulfur poisoning recovery control is calculated as "second voltage-increasing peak value", and the voltage-reducing peak value (the output current Ispd in FIG. 12) in the second sulfur poisoning recovery control is calculated as "second voltage-reducing peak value". The ratio of the second voltage-reducing peak value to the second voltage-increasing peak value (= Second voltage-reducing peak value/Second voltage-increasing peak value) is calculated as "second peak ratio".

[0170] When a variation from the first peak ratio to the second peak ratio (= First peak ratio - Second peak ratio) is smaller than or eqiial to a predetermined determination value, it is determined that recovery of sulfur poisoning has completed.

[0171] With the sulfur poisoning recovery completion determination according to the fourth embodiment, it is possible to accurately determine whether recovery of sulfur poisoning of the sensor has completed. Hereinafter, the reason will be described.

[0172] The second peak ratio is calculated on the basis of the output current after the first sulfur poisoning recovery control is executed. Thus, when the second peak ratio is calculated, the extent of sulfur poisoning of the sensor is lower than that when the first peak ratio is calculated. In addition, as the extent of sulfur poisoning of the sensor at the time when the first sulfur poisoning recovery control is executed increases, the extent of sulfur poisoning of the sensor decreases by a larger amount as a result of executing the first sulfur poisoning recovery control. That is, as the extent of sulfur poisoning of the sensor at the time when the first sulfur poisoning recovery control is executed increases, a variation from the first peak ratio to the second peak ratio (hereinafter, "peak ratio variation") increases. In other words, as the extent of sulfur poisoning of the sensor at the time when the first sulfur poisoning recovery control is executed is significantly low, the peak ratio variation is significantly small. When the extent of sulfur poisoning of the sensor at the time when the first sulfur poisoning recovery control is executed is significantly low, there is a significantly high possibility that recovery of sulfur poisoning of the sensor completes as a result of executing the second sulfur poisoning recovery control.

[0173] Thus, when the predetermined detennination value is set to an appropriate value, it is possible to accurately determine whether recovery of sulfur poisoning has completed by determining that recovery of sulfur poisoning has completed in the case where the peak ratio variation is smaller than or equal to the predetermined detennination value.

[0174] The fact that the peak ratio variation is smaller than or equal to the predetermined detennination value means, in other words, that the second peak ratio is smaller than or equal to a value that is detennined from the first peak ratio. Thus, with the sulfur poisoning recovery completion determination according to the fourth embodiment, when the sulfur poisoning recovery control is executed twice, in the case where the second peak ratio is smaller than or equal to the predetermined determination value (in the fourth embodiment, the value that is detennined from the first peak ratio), it is detennined that recovery of sulfur poisoning has completed.

[0175] In this case, the predetermined detennination value is determined from the first peak ratio; however, when it is possible to previously set the predetermined determination value for accurately detennining that recovery of sulfur poisoning has completed on the basis of the second peak ratio irrespective of the first peak ratio, it is possible to determine whether recovery of sulfur poisoning has completed by using the predetermined detennination value and the second peak ratio. Thus, with the sulfur poisoning recovery completion detemiination according to the fourth embodiment, broadly speaking, when the sulfur poisoning recovery control is executed twice, in the case where the second peak ratio is smaller than or equal to the predetermined determination value, it is determined that recovery of sulfur poisoning has completed.

[0176] The second peak ratio is a value that reflects the extent of sulfur poisoning of the sensor. Alternatively, the second peak ratio is a value that reflects the degree of sul fur poisoning of the sensor or a value that indicates the latest influence of sulfur poisoning of the sensor as compared to the first peak ratio under the same condition. The second peak ratio is calculated by using the output current in the period in which the applied voltage is reduced (hereinafter, "voltage-reducing output current"). Thus, the voltage-reducing output current is also an output current that reflects the extent of sulfur poisoning of the sensor. Thus, when it is possible to previously set the predetermined determination value for accurately determining that recovery of sulfur poisoning has completed on the basis of the voltage-reducing output current, it is possible to determine whether recovery of sulfur poisoning has completed by using the predetermined determination value and the voltage-reducing output current. Thus, with the sulfur poisoning recovery completion detemiination according to the fourth embodiment, broadly speaking, when the output current that reflects the extent of sulfur poisoning of the sensor is smaller than or equal to the predetermined determination value (or when the output current that reflects the degree of sulfur poisoning of the sensor is smaller than or equal to the predetermined determination value or when the second voltage-reducing output ^'current is smaller than or equal to the predetermined determination value (this predetermined determination value is a value that is determined from the first voltage-reducing output current) under the same condition) at the time when the sulfur poisoning recovery control is executed twice, it is determined that recovery of sulfur poisoning has completed.

[0177] In the fourth embodiment, when a peak ratio variation is larger than or equal to a predetermined value or when a variation in output current is larger than or equal to a predetermined value, recovery of sulfur poisoning may be carried out through control other than the above-described sulfur poisoning recovery control for controlling the applied voltage. The variation in output current is a variation from the voltage-reducing peak value at the time when the first sulfur poisoning recovery control is executed to the voltage-reducing peak value at the time when the second sulfur poisoning recovery control is executed. This control is_< for example, particularly effective in the case where the peak ratio variation is larger than or equal to the predetermined value although the above-described sulfur poisoning recovery control for controlling the applied voltage is executed multiple times.

[0178] An example of a sulfur poisoning recovery control and sulfur poisoning recovery completion determination flowchart according to the fourth embodiment will be described with reference to FIG. 16. Step 50 to step 57 in the flowchart shown in FIG. 16 are the same as step 40 to step 47 in the flowchart shown in FIG. 15, so the description of these steps is omitted.

[0179] In step 58, it is determined whether the peak ratio variation ARp is smaller than or equal to the predetermined determination value ARpth (ARp < ARpth). The peak ratio variation ARp is a variation from the first peak ratio Rp l to the second peak ratio Rp2 (= Rp l - Rp2). The first peak ratio Rpl is calculated by using the peak value of the output current Is detected in step 51 and step 53 at the time when the last but one sulfur poisoning recovery control is executed. The second peak ratio Rp2 is calculated by using the peak value of the output current Is detected in step 5 1 and step 53 at the time when the last sulfur poisoning recovery control is executed. When it is determined in step 58 that ARp is smaller than or equal to ARpth, the flowchart proceeds to step 59, the sulfur poisoning recovery request flag Fs and the counter N are reset, and then the flowchart ends. On the other hand, when it is determined that ARp is not smaller than or equal to ARpth, the flowchart returns to step 5 1 . Thus, until it is determined in step 58 that ARp is smaller than or equal to ARpth, the applied voltage Vs is increased and reduced. That is, the sulfur poisoning recovery control is executed.

[0180] In the sulfur poisoning recovery completion determination (that is, sulfur poisoning recovery control) according to the above-described embodiments, if the rate of increase or rate of decrease (sweep rate) in applied voltage is too high, there is a possibility that the peak value is not output or the peak value that sufficiently coiTesponds to the concentration of SOx is not output even when the applied voltage is reduced. In the sulfur poisoning recovery completion determination according to the above-described embodiments, it is desirable that the rate of increase and rate of decrease in applied voltage at which the peak value that sufficiently corresponds to the extent of sulfur poisoning at the time when the applied voltage is reduced be selected.

[0181] Specifically, as shown in FIG. 17A, desirably, after the applied voltage is increased such that the rate of increase in applied voltage gradually decreases, the applied voltage is reduced such that the rate of decrease in applied voltage gradually increases. Alternatively, as shown in FIG. 17B, desirably, after the applied voltage is increased such that the rate of increase in applied voltage is kept constant, the applied voltage is reduced such that the rate of decrease in applied voltage is kept constant.

[0182] In addition, specifically, in the sulfur poisoning recovery completion determination according to the above-described embodiments, when a change in applied voltage in the period in which the applied voltage is increased from 0.4 V to 0.8 V and is reduced from 0.8 V to 0.4 V is expressed by frequency, the frequency is desirably lower than or equal to 100 Hz. In other words, a time from when a start of increasing the applied voltage to when an end of reducing the applied voltage is desirably longer than or equal to 0.01 seconds.

[0183] While the sulfur poisoning recovery control according to the above-described embodiments is being executed, detection of the concentration of SOx is stopped. Therefore, when it is detennined that recovery of sulfur poisoning has completed, the concentration of SOx may be calculated (detected) by using the voltage-reducing peak value used at the time when it is detennined that recovery of sulfur poisoning has completed. Alternatively, the concentration of SOx may be calculated (detected) by using the voltage-reducing peak value at the time when the applied voltage is increased from 0.4 V to 0.8 V and is then reduced from 0.8 V to 0.4 V after it is determined that recovery of sulfur poisoning has completed.

[0184] When the concentration of SOx is detected by using the voltage-reducing peak value used at the time when it is determined that recovery of sulfur poisoning has completed, the concentration of SOx is detected by the sensor of which sulfur poisoning has been recovered, so it is possible to accurately detect the concentration of SOx. In addition, because the concentration of SOx is detected by using the peak value during - execution of sulfur poisoning recovery control (that is, during the sulfur poisoning recovery completion determination), it is possible . to detect the concentration of SOx further early after recovery of sulfur poisoning (that is, after the sulfur poisoning recovery completion determination).

[0185] On the other hand, when the concentration of SOx is detected by using the voltage-reducing peak value at the time when the applied voltage is increased from 0.4 V to 0.8 V and is then reduced from 0.8 V to 0.4 V after it is determined that recovery of sulfur poisoning has completed, the concentration of SOx is detected by the sensor of which sulfur poisoning has been recovered, so it is possible to accurately detect the concentration of SOx. In addition, the concentration of SOx is detected independently of sulfur poisoning recovery control (that is, independently of the sulfur poisoning recovery completion determination), so it is possible to further accurately detect the concentration of SOx.

[0186] In the case where the concentration of SOx is detected by using the voltage-reducing peak value used at the time when it is determined that recovery of sulfur poisoning has completed, the concentration of SOx may be detected by using the voltage-reducing peak value used to calculate the area ratio, the peak ratio, the area ratio variation or the peak ratio variation, which is smaller than or equal to the corresponding predetermined determination value, without determining . whether recovery of sulfur poisoning has completed when the area ratio, the peak ratio, the area ratio variation or the peak ratio variation is smaller than or equal to the corresponding predetermined determination value. In this case, the concentration of SOx is detected because recovery of sulfur poisoning has completed, so, in this case as well, it is allowed to understand that it is determined that recovery of sulfur poisoning has substantially completed.

[0187] Similarly, in the case where the concentration of SOx is detected by using the voltage-reducing peak value at the time when the applied voltage is increased from 0.4 V to 0.8 V and is then reduced from 0.8 V to 0.4 V after it is detemiined that recovery of sulfur poisoning has completed, the concentration of SOx may be detected by using the voltage-reducing peak value at the time when the applied voltage is increased from 0.4 V to O.8 V and is then reduced from 0.8 V to 0.4 V, without detennining whether recovery of sulfur poisoning has completed when the area ratio, the peak ratio, the area ratio variation or the peak ratio variation is smaller than or equal to the corresponding predetermined determination value. In this case, the concentration of SOx is detected because recovery of sulfur poisoning has completed, so, in this case as well, it is allowed to understand that it is determined that recovery of sulfur poisoning has substantially completed.

[0188] A fifth embodiment will be described. Components and controls of the fifth embodiment, which will not be described below, are respectively the same as the components and controls of the above-described embodiments or components or controls that are naturally derived from the components or controls of the above-described embodiments in light of the components or controls of the fifth embodiment that will be described below.

[0189] In the fifth embodiment, the applied voltage is steadily kept at 0.4 V. In detecting the concentration of SOx according to the fifth embodiment, the applied voltage is increased from 0.4 V to 0.8 V and is then reduced from 0.8 V to 0.4 V. At this time, the ECU determines whether the absolute value of the peak value of output current input to the ECU in the period in which the applied voltage is reduced from 0.8 V to 0.4 V is larger than or equal to a sulfur poisoning recovery determination value. When the absolute value of the peak value is larger than or equal to the sulfur poisoning recovery determination value, the ECU executes sulfur poisoning recovery control. On the other hand, when the absolute value of the peak value is smaller than the sulfur poisoning recovery determination value, the ECU calculates (detects) the concentration of SOx by using the peak value and the reference current.

[0190] The sulfur poisoning recovery determination value according to the fifth embodiment is, for example, set as follows. The sulfur content of SOx in exhaust gas may adhere to the first sensor electrode. It is found through the researches of the inventors of the present application that, as the amount of adhesion sulfur (that is, the amount of sulfur that adheres to the first sensor electrode) increases, the absolute value of the peak value increases. When the amount of adhesion sulfur is significantly large, there is a possibility that the detection accuracy of the limiting current sensor (particularly, the detection accuracy of the concentration of SOx) decreases. Thus, when the amount of adhesion sulfur is large, it is desirable to remove sulfur that adheres to the first sensor electrode (that is, to execute sulfur poisoning recovery control). Therefore, the sulfur poisoning recovery determination value according to the fifth embodiment is, for example, set to the absolute value of the peak value in the case where it is required to execute sulfur poisoning recovery control (that is. the absolute value of the peak value of output current input to the ECU in the period in which the applied voltage is reduced from 0.8 V to 0.4 V).

[0191) With the SOx concentration detecting system according to the fifth embodiment, when there is a possibility that the detection accuracy of the sensor decreases because of sulfur poisoning, sulfur poisoning recovery control is executed. In other words, only when there is no possibility that the detection accuracy of the sensor decreases because of sulfur poisoning, the concentration of SOx is detected. Therefore, with the SOx concentration detecting system according to the fifth embodiment, it is possible to further accurately detect the concentration of SOx.

[0192] An example of an SOx concentration detecting flowchart according to the fifth embodiment will be described with reference to FIG. 18.

[0193] When the flowchart shown in FIG. 18 is started, the applied voltage is kept at 0.4 V. In step 60, the applied voltage Vs is increased from 0.4 V toward 0.8 V. Subsequently, in step 61 , it is determined whether the applied voltage Vs has reached 0.8 V (Vs = 0.8 V). When it is determined that Vs is equal to 0.8 V, the flowchart proceeds to step 62. On the other hand, when it is determined that Vs is not equal to 0.8 V, the flowchart returns to step 60. Thus, until it is determined in step 61 that Vs is equal to 0.8 V, the applied voltage Vs continues to be increased. [0194] In step 62, the applied voltage Vs is reduced from 0.8 V toward 0.4 V, and the output current Is is detected. Subsequently, in step 63, it is determined whether the applied voltage Vs has reached 0.4 V (Vs is equal to 0.4 V). When it is determined that Vs is equal to 0.4 V, the flowchart proceeds to step 64. On the other hand, when it is determined that Vs is not equal to 0.4 V, the flowchart returns to step 62. Thus, until it is determined in step 63 that Vs is equal to 0.4 V, the applied voltage Vs continues to be reduced, and the output current Is continues to be detected.

[0195] In step 64, it is determined whether the absolute value |Isp| of the peak value of the output current Is detected in step 62 is larger than or equal to the sulfur poisoning recovery determination value Ispths (|Isp| > Ispths). When it is determined that |Isp| is larger than or equal to Ispths, the flowchart proceeds to step 65, the sulfur poisoning recovery request flag Fs is set, and then the flowchart ends. In this case, because the sulfur poisoning recovery request flag Fs is set, sulfur poisoning recovery control is executed. On the other hand, when it is determined that |Isp| is not larger than or equal to Ispths, the flowchart proceeds to step 66, the SOx concentration Csox is calculated on the basis of the peak value of the output current Is detected in step 62, and then the flowchart ends.

[0196] When sulfur poisoning recovery control is executed, as the intake air amount increases, the rate of recovery of sulfur poisoning increases. The intake air amount is the amount of air that is taken into the combustion chamber, and this amount is equal to the amount of exhaust gas that reaches the sensor. Therefore, in the above-described sulfur poisoning recovery completion determination, the area ratio of the first embodiment, the peak ratio of the second embodiment, the area ratio variation of the third embodiment or the peak ratio variation of the fourth embodiment may be corrected by the intake air amount, and the sulfur poisoning recovery completion determination may be carried out by using the corrected area ratio, peak ratio, area ratio variation or peak ratio variation.

[0197] More specifically, in the first embodiment, for example, a value obtained by dividing the area ratio by the intake air amount (= Area ratio/Intake air amount) may be used as an area ratio, a value obtained by dividing the reference area by the intake air amount (= Reference area/Intake air amount) may be used as a reference area or a value obtained by dividing the sulfur poisoning area by the intake air amount (= Sulfur poisoning area/Intake air amount) may be used as a sulfur poisoning area.

[0198] In the second embodiment, for example, a value obtained by dividing the peak ratio by the intake air amount (= Peak ratio/Intake air amount) may be used as a peak ratio, a value obtained by dividing the voltage-increasing peak value by the intake air amount (= Voltage-increasing peak value/Intake air amount) may be used as a voltage-increasing peak value, or a value obtained by dividing the voltage-reducing peak value by the intake air amount (= Voltage-reducing peak value/Intake air amount) may be used as a voltage-reducing peak value.

[0199] In the third embodiment, for example, a value obtained by dividing the area ratio variation by the intake air amount (= Area ratio variation/Intake air amount) may be used as an area ratio variation, a value obtained by dividing the first area ratio by the intake air amount (= First area ratio/Intake air amount) may be used as a first area ratio, or a value obtained by dividing the second area ratio by the intake air amount (= Second area ratio/Intake air amount) may be used as a second area ratio.

[0200) In the fourth embodiment, for example, a value obtained by dividing the peak ratio variation by the intake air amount (= Peak ratio variation/Intake air amount) may be used as a peak ratio variation, a value obtained by dividing the first peak ratio by the intake air amount (= First peak ratio/Intake air amount) may be used as a first peak ratio, or a value obtained by dividing the second peak ratio by the intake air amount (= Second peak ratio/Intake air amount) may be used as a second peak ratio.

[0201] A total displacement during execution of sulfur poisoning recovery control may be employed as a displacement or an average displacement during execution of sulfur poisoning recovery control may be employed as a displacement.

[0202] In the above-described embodiments, when "ratios" are used to determine whether recovery of sulfur poisoning has completed, differences in the rate of recovery of sulfur poisoning based on the intake air amount are cancelled, so the above-described correction, based on the intake air amount does not need to be carried out.

[0203] In the third embodiment and the fourth embodiment, the intake air amount at the time when the first sulfur poisoning recovery control is executed may be different from the intake air amount at the time when the second sulfur poisoning recovery control is executed. Thus, in these embodiments, it is desirable to carry out the above-described correction based on the intake air amount.

[0204] By carrying out the above-described correction based on the intake air amount, it is possible to further accurately determine whether recovery of sulfur poisoning has completed.

[0205] In detecting the concentration of SOx according to the above-described embodiments, it is presumable that the reason why a current corresponding to the concentration of SOx is output from the sensor at the time when the applied voltage is reduced is that a reaction regarding SOx occurs in the sensor cell On the other hand, this reaction is significantly influenced by the temperature of the sensor cell. Thus, in consideration that the concentration of SOx in exhaust gas is significantly low, the temperature of the sensor cell is desirably kept constant. Therefore, in the above-described embodiments, when the concentration of SOx is detected, the heater may be controlled such that the temperature of the sensor cell is kept constant. With this configuration, the concentration of SOx is further accurately detected.

[0206] Similarly, in the sulfur poisoning recovery control or sulfur poisoning recovery completion determination according to the above-described embodiments, it is desirable that the temperature of the sensor cell be kept constant.

[0207] Next, a sixth embodiment will be described. Components and controls of the sixth embodiment, which will not be described below, are respectively the same as the components and controls of the above-described embodiments or components or controls that are naturally derived from the components or controls of the above-described embodiments in light of the components or controls of the sixth embodiment that will be described below.

[0208] In the sixth embodiment, the applied voltage is steadily kept at 0.4 V. In detecting the concentration of SOx according to the sixth embodiment, the applied voltage is increased from 0.4 V to 0.8 V and is then reduced from 0.8 V to 0.4 V. At this time, the ECU determines whether the absolute value of the peak value of output current input to the ECU in the period in which the applied voltage is reduced from 0.8 V to 0.4 V is larger than or equal to an alarm determination value. In the present embodiment, when the absolute value of the peak value is larger than or equal to the alarm determination value, the ECU alarms an abnormality of fuel property. On the other hand, when the absolute value of the peak value is smaller than the alarm determination value, the ECU calculates (detects) the concentration of SOx by using the peak value and the reference current.

[0209] The alarm determination value according to the sixth embodiment is, for example, set as follows. As described above, the sulfur content of SOx in exhaust gas may adhere to the first sensor electrode. It is found through the researches of the inventors of the present application that the absolute value of the peak value increases as the amount of adhesion sulfur increases. When the amount of adhesion sulfur is significantly large, there is a possibility that the detection accuracy of the limiting current sensor (particularly, the detection accuracy of the concentration of SOx) decreases. One of factors that increase the amount of adhesion sulfur is that the concentration of SOx in exhaust gas is high. When the concentration of sulfur content in fuel is high, the concentration of SOx in exhaust gas is high. When the concentration of sulfur content in fuel is high to such an extent that the concentration is not allowed, that is, there is a possibility that fuel property is abnormal, it is desirable to alarm the abnormality.

[0210] The alarm determination value according to the sixth embodiment is, for example, set to an appropriately selected value larger than or equal to the minimum value of the absolute value of the peak value in the case where the fuel property falls outside an allowable range (particularly, when the concentration of sulfur in fuel is higher than an allowable concentration) (that is, the absolute value of the peak value of output current input to the ECU in the period in which the applied voltage is reduced from 0.8 V to 0.4 V).

[0211] The alarm determination value according to the sixth embodiment may be the same as the sulfur poisoning recovery deteimination value according to the fifth embodiment or may be a different value.

[0212] With the detection of the concentration of SOx according to the sixth embodiment, when there is a possibility that the fuel property is abnormal, the possibility of the abnormality is alarmed, so a user of the SOx concentration detecting system is allowed to know that there is a possibility that the fuel property is abnormal.

[0213) An example of an SOx concentration detecting flowchart according to the sixth embodiment will be described with reference to FIG. 19. Step 70 to step 73 in the flowchart shown in FIG. 19 are the same as step 60 to step 63 in the flowchart shown in FIG. 18, so the description of these steps is omitted.

[0214] In step 74, it is determined whether the absolute value of the peak value of the output current Is detected in step 72 is larger than the alarm determination value Isptha (|Isp| > Isptha). When it is determined that |Isp| is larger than Isptha, the flowchart proceeds to step 75, an abnormality of the fuel property is alarmed, and then the flowchart ends. On the other hand, when it is determined that |Isp| is not larger than Isptha, the flowchart proceeds to step 76, the SOx concentration Xsox is calculated on the basis of the peak value of the output current Is detected in step 72, and then the flow chart ends.

[0215] A seventh embodiment will be described. In the following description, for components and controls that are the same as the components and controls of the above-described embodiments or components or controls that are naturally derived from the components or controls of the above-described embodiments in light of the components or controls of the seventh embodiment that will be described below, the description thereof is omitted.

[0216] In the seventh embodiment, the applied voltage is steadily kept at 0.4 V. That is, 0.4 V is steadily applied to the sensor cell. The voltage of 0.4 V is a voltage higher than or equal to a voltage Vth shown in FIG. 2, and is a voltage at which the sensor cell output cunent is constant in-espective of the sensor cell applied voltage in the case where the air- fuel ratio of exhaust gas is constant.

[0217] In detecting the concentration of SOx and the air-fuel ratio according to the seventh embodiment, the ECU calculates (detects) the air-fuel ratio from the correlation shown in FIG. 2 on the basis of the sensor cell output current at the time when the voltage of 0.4 V is steadily applied to the sensor cell.

(0218] On the other hand, when there is a request to detect the concentration of SOx, the sensor cell applied voltage is increased from 0.4 V to 0.8 V and is then reduced from 0.8 V to 0.4 V. At this time, the ECU calculates (detects) the concentration of SOx by using the peak value of output current input to the ECU in the period in which the sensor cell applied voltage is reduced from 0.8 V to 0.4 V, and the reference current.

[0219] After the sensor, cell applied voltage is reduced from 0.8 V to 0.4 V, the ECU calculates (detects) the air-fuel ratio from the correlation shown in FIG. 2 on the basis of the sensor cell output current. At this time, the sensor cell applied voltage is kept at 0.4 V.

[0220] In the case where the dual cell limiting current sensor is utilized in detecting the concentration of SOx and the air-fuel ratio according to the seventh embodiment, when the air-fuel ratio is detected, the applied voltage of the pump cell 15 is set to zero.

[0221] According to the seventh embodiment, it is possible to detect the air-fuel ratio of exhaust gas and the concentration of SOx in exhaust gas with the single sensor.

[0222] In the period in which the applied voltage is reduced, the temperature that is required for a sufficient reaction regarding SOx in the sensor cell is higher than the temperature that is required to accurately detect the air-fuel ratio. The temperature that is required for a sufficient reaction regarding SOx in the sensor cell is a temperature that is required to accurately detect the concentration of SOx, to sufficiently recover sulfur poisoning and to accurately determine whether recovery of sulfur poisoning has completed. Therefore, in the above-described embodiments, the temperature while the applied voltage is being reduced at the time when the concentration of SOx is detected, sulfur poisoning recovery control is executed and the sulfur poisoning recovery completion determination is carried out may be controlled to a temperature higher than the temperature at the time when the air-fuel ratio is detected. More specifically, for example, the temperature at the time when the air-fuel ratio is detected may be set to 600°C, and the temperature in the period in which the applied voltage is reduced at the time when the concentration of SOx is detected, sulfur poisoning recovery control is executed and the sulfur poisoning recovery completion determination is earned out may be set to 750°C.

[0223] In the case where the sensor is utilized to detect the air-fuel ratio, it is not possible to detect the air-fuel ratio with the sensor during detection of the concentration of SOx, execution of sulfur poisoning recovery control and the sulfur poisoning recovery completion determination. Thus, in the case where the air-fuel ratio that is detected by the sensor is utilized in air-fuel ratio control over the internal combustion engine, it is not possible to control the air-fuel ratio of the internal combustion engine as desired during detection of the concentration of SOx, execution of sulfur poisoning recovery control and the sulfur poisoning recovery completion determination.

[0224] For example, after an engine start, when the sensor temperature has reached a warm-up temperature, the concentration of SOx may be detected, sulfur poisoning recovery control may be executed and the sulfur poisoning recovery completion determination may be canned out. Alternatively, after an engine start, after the sensor temperature has reached a warm-up temperature and before an activation determination of the sensor is carried out, the concentration of SOx may be detected, sulfur poisoning recovery control may be executed and the sulfur poisoning recovery completion determination may be carried out. Alternatively, after an engine start, after the sensor temperature has reached a warm-up temperature and an activation determination of the sensor has been carried out, when fuel-cut operation (that is, operation in which supply of fuel to the combustion chamber is stopped) is carried out, the concentration of SOx may be detected, sulfur poisoning recovery control may be executed and the sulfur poisoning recovery completion determination may be canned out.

[0225] An example of an SOx concentration and air-fuel ratio detecting flowchart according to the seventh embodiment will be described with reference to FIG. 20. Step 81 to step 84 in the flowchart shown in FIG. 20 are the same as step 10 to step 13 in the flowchart shown in FIG. 13, so the description of these steps is omitted. [0226] When the flowchart shown in FIG. 20 is started, the applied voltage is kept at 0.4 V. In step 80, it is determined whether an SOx concentration detection flag Fsox is set (Fsox = 1 ). The flag Fsox is set when there is a request to detect the concentration of SOx in exhaust gas and is reset when detection of the concentration of SOx in exhaust gas has completed. In step 80, when it is determined that Fsox is equal to 1 , the flowchart proceeds to step 81 . On the other hand, when it is determined that Fsox is not equal to 1 , the flowchart proceeds to step 87.

[0227] In step 87, the output current Is is detected. Subsequently, in step 88, the air-fuel ratio A/F is calculated on the basis of the output current Is detected in step 87, and then the flowchart ends.

[0228J In step 85, the SOx concentration Csox is calculated on the basis of the peak value of the output current Is detected in step 83. Subsequently, in step 86, the SOx concentration detection flag Fsox is reset, and the flowchart ends.

[0229] The above-described embodiments are embodiments in the case where the concentration of SOx in exhaust gas is detected. However, the concept of the above-described embodiments is broadly applicable to the case of detecting a parameter regarding a specific component that correlates with an output current in a period in which the applied voltage is reduced from a predetermined voltage. In this case, there is a condition that it is possible to distinguish the output current that correlates with a specific component parameter to be detected from the output current that correlates with a parameter regarding the other specific components.

[023.0] In other words, the concept of the above-described embodiments is also applicable to the case of detecting a parameter regarding a specific component that does not correlate with (or that extremely slightly correlates with) the output current at the time when the applied voltage is kept at a constant voltage or that does not correlate with (or that extremely slightly correlates with) the output current at the time when the applied voltage is increased but that correlates with the output current at the time when the applied voltage is reduced from a predetermined voltage.

[0231] The above-described embodiments are embodiments to detect the concentration of SOx by using the minimum value of the output current at the time when the applied voltage is reduced. However, the concept of the above-described embodiments is also broadly applicable to the case where a parameter regarding a specific component is detected by using the maximum value of the output current at the time when the applied voltage is reduced.

[0232] In detecting the concentration of SOx according to the above-described embodiments, the applied voltage is increased before the applied voltage is reduced. However, as long as the applied voltage is reduced, even when the applied voltage is not increased before that, no small advantage of the above-described embodiments is obtained. The control system for an internal combustion engine according to the above-described embodiments is, broadly speaking, a control system for an internal combustion engine including a limiting current sensor (for example, the sensor 10 or the sensor 30). The control system includes a control unit (for example, the ECU 90) that executes sulfur poisoning recovery control for increasing an applied voltage, applied to the sensor, to a sulfur poisoning recovery voltage (for example, a voltage higher than or equal to 0.8 V, particularly, 0.8 V) and is reduced. The control unit determines that recovery of sulfur poisoning of the sensor has completed when an output current that reflects the extent of the sulfur poisoning of the sensor (for example, an area ratio, a peak ratio, an area ratio variation or a peak ratio variation) is smaller than or equal to a predetermined determination value, the output current being an output current from the sensor at the time when the sulfur poisoning recovery control is executed.

[0233] With more limited expression, in the case where the sulfur poisoning recovery control is executed once, the output current that reflects the extent of the sulfur poisoning is, for example, an output current (for example, a sulfur poisoning area or a voltage-reducing peak value) in the period in which the applied voltage is reduced at the time when the sulfur poisoning recovery control is executed.

[0234] Alternatively, with more limited expression, in the case where the sulfur poisoning recovery control is executed twice, the output current that reflects the extent of the sulfur poisoning is. for example, an output current (for example, a second area ratio or a second peak ratio ) in the period in which the applied voltage is reduced at the time when the second sulfur poisoning recovery control is executed, and the predetermined detemiination value is, for example, determined from an output current (for example, a first area ratio or a first peak ratio) in the period in which the applied voltage is reduced at the time when the first sulfur poisoning recovery control is executed.

[0235] The control unit of the control system for an internal combustion engine according to the above-described embodiments detects a parameter regarding a specific component in test gas (for example, the concentration of SOx in exhaust gas) by using an output current (for example, a voltage-reducing peak value) in the period in which the applied voltage is reduced at the time when the sulfur poisoning recovery control is executed instead of determining whether recovery of the sulfur poisoning of the sensor has completed when an output current that reflects the extent of the sulfur poisoning of the sensor (for example, an area ratio, a peak ratio, an area ratio variation or a peak ratio variation) is smaller than or equal to a predetermined detemiination value, the output current being an output current from the sensor at the time when the sulfur poisoning recovery control is executed.

[0236| The control unit of the control system for an internal combustion engine according to the above-described embodiments executes voltage control for reducing the applied voltage from a parameter detection voltage (for example, a voltage higher than or equal to 0.8 V, particularly, 0.8 V) instead of determining whether recovery of the sulfur poisoning of the sensor has completed in the case where an output current that reflects the extent of the sulfur poisoning of the sensor (for example, an area ratio, a peak ratio, an area ratio variation or a peak ratio variation) is smaller than or equal to a predetermined detemiination value, the output current being an output current from the sensor at the time when the sulfur poisoning recovery control is executed, and detects a parameter regarding a specific component in test gas (for example, the concentration of SOx in exhaust gas) by using an output current from the sensor (for example, a peak value) at the time when the voltage control is executed.

[0237] The control system for an internal combustion engine according to the above-described embodiments is a control method for an internal combustion engine including a limiting current sensor (for example, the sensor 10 or the sensor 30). The control method includes a sulfur poisoning recovery step, an output current acquisition step and a determination step. In the sulfur poisoning recovery step, an applied voltage, applied to the sensor, is increased to a sulfur poisoning recovery voltage (for example, a voltage higher than or equal to 0.8 V, particularly, 0.8 V) and is then reduced. In the output cun-ent acquisition step, an output current that reflects the extent of sulfur poisoning of the sensor (for example, an area ratio, a peak ratio, an area ratio variation or a peak ratio variation) is acquired from an output current from the sensor in the sulfur poisoning recovery step. In the determination step, it is determined that recovery of the sulfur poisoning has completed when the output current acquired in the output current acquisition step is smaller than or equal to a predetermined determination value.

[0238] The control system for an internal combustion engine according to the above-described embodiments executes the control method including a specific component parameter detection step instead of the determination step. In the specific component parameter detection step, when the output current acquired in the output cun-ent acquisition step is smaller than or equal to the predetermined determination value, a parameter regarding a specific component in test gas (for example, the concentration of SOx in exhaust gas) is detected by using an output current in the period in which the applied voltage is reduced at the time when the sulfur poisoning recovery step is executed.

[0239] The control system for an internal combustion engine according to the above-described embodiments executes the control method including a voltage control step and a specific component parameter detection step instead of the determination step. In the voltage control step, when the output current acquired in the output cun-ent acquisition step is smaller than or equal to the predetermined determination value, the applied voltage is reduced from a parameter detection voltage (for example, a voltage higher than or equal to 0.8 V, particularly, 0.8 V). In the specific component parameter detection step, a parameter regarding a specific component in test gas (for example, the concentration of SOx in exhaust gas) is detected by using the output cun-ent from the sensor at the time when the voltage control step is executed.

Claims

CLAIMS:

1 . A control system for an internal combustion engine, the control system comprising:

a limiting current sensor; and

an electronic control unit configured to

( i ) execute sulfur poisoning recovery control for increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage, and

(ii) determine that recovery of sulfur poisoning of the limiting current sensor has completed when an output current that reflects an extent of the sulfur poisoning of the limiting cun-ent sensor is smaller than or equal to a predetermined determination value, the output current being an output current from the limiting current sensor at the time when the sulfur poisoning recovery control is executed.

2. The control system according to claim 1 , wherein

when the sulfur poisoning recovery control is executed once, the output current that reflects the extent of the sulfur poisoning is an output current in a period in which the applied voltage is reduced at the time when the sulfur poisoning recovery control is executed.

3. The control system according to claim 1 or 2, wherein

when the sulfur poisoning recovery control is executed twice, the output current that reflects the extent of the sulfur poisoning is an output current in a period in which the applied voltage is reduced at the time when a second sulfur poisoning recovery control is executed, and

the predetermined determination value is a value that is determined from an output current in a period in which the applied voltage is reduced at the time when a first sulfur poisoning recovery control is executed.

4. A control system for an internal combustion engine, the control system comprising:

a limiting current sensor; and

an electronic control unit configured to

(i) execute sulfur poisoning recovery control for increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage, and

(ii) detect a parameter regarding a specific component in test gas by using an output current in a period in which the applied voltage is reduced at the time when the sulfur poisoning recovery control is executed.

5. A control system for an internal combustion engine, the control system comprising:

a limiting current sensor; and

an electronic control unit configured to

(i) execute sulfur poisoning recovery control for increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage,

(ii) execute voltage control for reducing the applied voltage from a parameter detection voltage, and

(iii) detect a parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when the voltage control is executed.

6. The control system according to any one of claims 1 to 5, wherein

the electronic control unit is configured to detect a parameter regarding a specific component by using an output current from the limiting current sensor at the time when voltage control for reducing the applied voltage to a parameter detection voltage, and

the electronic control unit is configured to execute the sulfur poisoning recovery control when an output current is larger than or equal to a sulfur poisoning recovery determination value, the output current is output at the time when the voltage control is executed.

7. The control system according to any one of claims 1 to 6, wherein

the electronic control unit is configured to detect a parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when voltage control for reducing the applied voltage from a parameter detection voltage is executed, and

the electronic control unit is configured to issue an alarm when an output current is larger than or equal to an alarm determination value, the output . current is output at the time when the voltage control is executed.

8. The control system according to any one of claims 4 to 7, wherein

the specific component is SOx.

9. The control system according to any one of claims 1 to 8, wherein

the sulfur poisoning recovery voltage is higher than or equal to 0.8 V.

10. The control system according to any one of claims 1 to 9, wherein

in the sulfur poisoning recovery control, the applied voltage at the timing of an end of reducing the applied voltage from the sulfur poisoning recovery voltage is lower than or equal to 0.7 V.

1 1 . The control system according to any one of claims 5 to 7, wherein

the parameter detection voltage is higher than or equal to 0.8 V.

12. The control system according to any one of claims 1 to 1 1 , wherein

the electronic control unit is configured to normally apply the limiting current sensor with an ordinary voltage lower than the sulfur poisoning recovery voltage, and detect a concentration of oxygen in test gas by using an output current from the limiting current sensor at the time when the ordinary voltage is applied to the limiting cun-ent sensor.

1 3. The control system according to any one of claims 5 to 7. wherein

the electronic control unit is configured to normally apply the limiting current sensor with an ordinary voltage lower than the parameter detection voltage, and detect a concentration of oxygen in test gas by using an output cun-ent from the limiting current sensor at the time when the ordinary voltage is applied to the limiting cun-ent sensor.

14. A control method for an internal combustion engine, the internal combustion engine including a limiting current sensor, the control method comprising:

carrying out recovery of sulfur poisoning by increasing an applied voltage, applied to the limiting current sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage;

acquiring an output current that reflects an extent of the sulfur poisoning of the limiting current sensor, the output current being an output current from the limiting current sensor at the time when the recovery of the sulfur poisoning is carried out; and

determining that the recovery of the sulfur poisoning has completed when the acquired output current is smaller than or equal to a predetermined determination value.

15. A control method for an internal combustion engine, the internal combustion engine including a limiting current sensor, the control method comprising:

carrying out recovery of sulfur poisoning by increasing an applied voltage, applied to the limiting cun-ent sensor, to a sulfur poisoning recovery voltage and then reducing the applied voltage;

acquiring an output current that reflects an extent of the sulfur poisoning of the limiting cun-ent sensor, the output current being an output current from the limiting current sensor at the time when the recovery of the sulfur poisoning is carried out; and detecting a parameter regarding a specific component in test gas by using an output current from the limiting current sensor in a period in which the applied voltage is reduced at the time when the recovery of the sulfur poisoning is carried out in the case where the acquired output current is smaller than or equal to a predetermined determination value.

16. A control method for an internal combustion engine, the internal combustion engine including a limiting current sensor, the control method comprising:

acquiring an output current that reflects an extent of the sulfur poisoning of the limiting current sensor, the output current being an output current from the limiting current sensor at the time when the recovery of the sulfur poisoning is carried out;

reducing the applied voltage from a voltage that is used to detect a parameter, when the acquired output current is smaller than or equal to a predetermined determination value; and

detecting the parameter regarding a specific component in test gas by using an output current from the limiting current sensor at the time when the applied voltage is reduced.

17. The control method according to claim 15 or 16, wherein the specific component is SOx.