Patent Publication Number: US-2019186325-A1

Title: SOx CONCENTRATION ACQUIRING APPARATUS OF INTERNAL COMBUSTION ENGINE

Description:
BACKGROUND 
     Field 
     The invention relates to a SOx concentration acquiring apparatus of an internal combustion engine. 
     Description of the Related Art 
     There is known a SOx concentration acquiring apparatus for acquiring a concentration of sulfur oxide included in an exhaust gas just discharged from an internal combustion engine (for example, see JP 2015-17931 A). The known SOx concentration acquiring apparatus (hereinafter, will be referred to as “the known apparatus”) comprises a limiting current sensor. The limiting current sensor includes solid electrolyte layers, a diffusion-limited layer, a first sensor electrode, and a second sensor electrode. The first and second sensor electrodes are provided such that one of the solid electrolyte layers is positioned between the first and second sensor electrodes. The limiting current sensor includes an interior space defined by the solid electrolyte layers. The exhaust gas enters into the interior space through the diffusion-limited layer. The first sensor electrode is provided such that the first sensor electrode exposes to the interior space. Hereinafter, the concentration of the sulfur oxide will be referred to as “the SOx concentration”, and the concentration of the sulfur oxide included in the exhaust gas just discharged from the internal combustion engine will be referred to as “the exhaust SOx concentration”. 
     The known apparatus increases a voltage applied to the second sensor electrode so as to procude an electric potential difference with respect to the first sensor electrode and then, decreases the voltage. The known apparatus acquires the exhaust SOx concentration on the basis of a current flowing between the first and second sensor electrodes while the known apparatus decreases the voltage. Hereinafter, the voltage applied to the second sensor electrode will be referred to as “the sensor voltage”, and the current flowing between the first and second sensor electrodes will be referred to as “the sensor current”. 
     As described above, in the limiting current sensor of the known apparatus, the exhaust gas enters into the interior space through the diffusion-limited layer. While the exhaust gas moves through the diffusion-limited layer, at least a part of the SOx included in the exhaust gas adheres to the diffusion-limited layer. On the other hand, while the known apparatus increases the sensor voltage for acquiring the exhaust SOx concentration, the SOx decomposes at the first sensor electrode. Therefore, the SOx concentration in the interior space decreases temporarily. As a result, the SOx adhering to the diffusion-limited layer may remove from the diffusion-limited layer and enter into the interior space. Also, the exhaust gas including the SOx continuously flows from an outside of the limiting current sensor into the interior space through the diffusion-limited layer. 
     Therefore, while the sensor voltage increases, the SOx concentration in the interior space may deviate from the exhaust SOx concentration. Accordingly, the sensor current acquired while the sensor voltage decreases after the sensor voltage increases, may not represent the exhaust SOx concentration accurately. 
     There is known a sensor provided with a protection layer covering sensor elements in order to prevent condensed water from adhering to the sensor elements such as the solid electrolyte layers and the diffusion-limited layer, thereby preventing the sensor elements from cracking. In this sensor, the exhaust gas flows into the interior space through the protection layer and the diffusion-limited layer. Therefore, at least a part of the SOx included in the exhaust gas adheres to the protection layer and the diffusion-limited layer. Thus, the SOx may remove from the protection layer and the diffusion-limited layer, thereby flowing into the interior space when the known apparatus increases the sensor voltage for acquiring the exhaust SOx concentration. 
     In this case, an amount of the SOx flowing into the interior space in this sensor, is larger than the amount of the SOx flowing into the interior space in a sensor not provided with the protection layer. Therefore, in the sensor provided with the protection layer, the SOx concentration in the interior space may be considerably different from the exhaust SOx concentration while the sensor voltage is increased. As a result, the sensor current acquired while the sensor voltage is decreased, may be unlikely to represent the exhaust SOx concentration accurately. 
     SUMMARY 
     The invention has been made for solving the above-mentioned problems. An object of the invention is to provide a SOx concentration acquiring apparatus of the internal combustion engine which can acquire the exhaust SOx concentration accurately. 
     A SOx concentration acquiring apparatus of an internal combustion engine ( 50 ) according to the invention comprises a sensor cell ( 15 ,  26 ), a diffusion-limited layer ( 13 ,  23 ), a sensor cell voltage source ( 15 C,  26 C), an interior space ( 17 ,  28 ), and an electronic control unit ( 90 ). 
     The sensor cell ( 15 ,  26 ) is formed by a solid electrolyte layer ( 11 ,  21 A), a first sensor electrode ( 15 A,  26 A), and a second sensor electrode ( 15 B,  26 B). The first sensor electrode ( 15 A,  26 A) is provided on one of opposite surfaces of the solid electrolyte layer ( 11 ,  21 A). The second sensor electrode ( 15 B,  26 B) is provided on the other surface of the solid electrolyte layer ( 11 ,  21 A). The sensor cell voltage source ( 15 C,  26 C) applies a voltage to the sensor cell ( 15 ,  26 ). The interior space ( 17 ,  28 ) is defined by the solid electrolyte layer ( 11 ,  21 A) and the diffusion-limited layer ( 13 ,  23 ) such that an exhaust gas discharged from the internal combustion engine ( 50 ) flows into the interior space ( 17 ,  28 ) through the diffusion-limited layer ( 13 ,  23 ), and the first sensor electrode ( 15 A,  26 A) exposes to the interior space ( 17 ,  28 ). The electronic control unit ( 90 ) controls a sensor voltage (Vss) which is a voltage applied to the sensor cell ( 15 ,  26 ) from the sensor cell voltage source ( 15 C,  26 C). 
     The electronic control unit ( 90 ) is configured to execute a first voltage control for increasing the sensor voltage (Vss) from a voltage lower than an oxygen increasing voltage (Vox_in) to a first high voltage equal to or higher than the oxygen increasing voltage (Vox_in) and then, decreasing the sensor voltage (Vss) from the first high voltage to a first low voltage lower than an oxygen decreasing voltage (Vox_de) (see a process of a step  830  in  FIG. 8 ). The oxygen increasing voltage (Vox_in) is a voltage, at which an amount of oxygen component produced by SOx decomposing to sulfur component and the oxygen component is larger than the amount of the oxygen component consumed by the sulfur component being oxidized by the oxygen component to the SOx. The oxygen decreasing voltage (Vox_de) is a voltage, at which the amount of the oxygen component consumed by the sulfur component being oxidized by the oxygen component to the SOx is larger than the amount of the oxygen component produced by the SOx decomposing to the sulfur component and the oxygen component. 
     The electronic control unit ( 90 ) is further configured to execute a second voltage control for increasing the sensor voltage (Vss) from the first low voltage to a second high voltage equal to or higher than the oxygen increasing voltage (Vox_in) and then, decreasing the sensor voltage (Vss) from the second high voltage to a second low voltage lower than the oxygen decreasing voltage (Vox_de) (see a process of a step  840  in  FIG. 8 ). 
     The electronic control unit ( 90 ) is further configured to acquire a current (Iss) flowing through the sensor cell ( 15 ,  26 ) as a SOx concentration current (Iss_sox) while the electronic control unit ( 90 ) decreases the sensor voltage (Vss) in the second voltage control (see a process of a step  1035  in  FIG. 10 ). The electronic control unit ( 90 ) is further configured to acquire a SOx concentration (Csox) of the exhaust gas on the basis of the SOx concentration current (Iss_sox) (see a process of a step  1050  in  FIG. 10 ). 
     The electronic control unit of the SOx concentration acquiring apparatus according to the invention executes the first voltage control before the electronic control unit executes the second voltage control. Then, the electronic control unit acquires the SOx concentration of the exhaust gas on the basis of the current flowing through the sensor cell while the electronic control unit decreases the sensor voltage in the second voltage control. 
     At least a part of the SOx adhering to the diffusion-limited layer, removes from the diffusion-limited layer while the sensor voltage is increased in the first voltage control. Thus, the amount of the SOx removing from the diffusion-limited layer is small while the sensor voltage is increased in the second voltage control. As a result, the SOx concentration in the interior space corresponds to or generally corresponds to the SOx concentration of the exhaust gas while the second voltage control is executed. Thus, the current flowing through the sensor cell represents the SOx concentration of the exhaust gas accurately while the second voltage control is executed. Therefore, the SOx concentration of the exhaust gas can be acquired accurately. 
     According to an aspect of the invention, the electronic control unit ( 90 ) may be further configured to acquire a peak value (Ipeak) of the current (Iss) flowing through the sensor cell ( 15 ,  26 ) as the SOx concentration current (Iss_sox) while the electronic control unit ( 90 ) decreases the sensor voltage (Vss) in the second voltage control. 
     The electronic control unit of the SOx concentration acquiring apparatus according to this aspect acquires the peak value of the current flowing through the sensor cell while the electronic control unit decreases the sensor voltage in the second voltage control. The peak value is a current which has changed to a largest extent after the electronic control unit starts to decrease the sensor voltage. Thus, a change of the SOx concentration of the exhaust gas reaching the first sensor electrode is represented accurately by a change of the peak value. Therefore, the SOx concentration of the exhaust gas can be acquired accurately by acquiring the peak value as the SOx concentration current. 
     According to another aspect of the invention, the SOx concentration acquiring apparatus may further comprise a protection layer ( 19 ,  29 ). In this case, the protection layer ( 19 ,  29 ) is formed of a material, through which the exhaust gas can flow. Further, the protection layer ( 19 ,  29 ) is provided covering the solid electrolyte layer ( 11 ,  21 A) and the diffusion-limited layer ( 13 ,  23 ). 
     When the protection layer is provided covering the solid electrolyte layer and the diffusion-limited layer, the exhaust gas flows into the interior space through the protection layer and the diffusion-limited layer. Therefore, the SOx adheres to the protection layer. The SOx may remove from the protection layer when the sensor voltage is increased. In this connection, at least a part of the SOx removes from the protection layer while the sensor voltage is increased in the first voltage control. Thus, the amount of the SOx removing from the protection layer is small while the sensor voltage is increased in the second voltage control. As a result, the SOx concentration in the interior space corresponds to or generally corresponds to the SOx concentration of the exhaust gas while the second voltage control is executed. Thus, the current flowing through the sensor cell represents the SOx concentration of the exhaust gas accurately while the sensor voltage control is executed. Therefore, the SOx concentration of the exhaust gas can be acquired accurately. 
     According to further another aspect of the invention, the electronic control unit ( 90 ) may be further configured to execute the first voltage control and the second voltage control when an operation of the internal combustion engine ( 50 ) is in one of a steady operation state and an idling operation state. 
     According to further another aspect of the invention, the electronic control unit ( 90 ) may be further configured to execute a constant voltage control for controlling the sensor voltage (Vss) to a constant voltage lower than the oxygen increasing voltage (Vox_in) before the electronic control unit ( 90 ) executes the first voltage control after the electronic control unit ( 90 ) executes the second voltage control (see a process of a step  850  in  FIG. 8 ). In this case, the electronic control unit ( 90 ) may be further configured to acquire an oxygen concentration (Coxy) of the exhaust gas on the basis of the current (Iss) flowing through the sensor cell ( 15 ,  26 ) while the electronic control unit ( 90 ) executes the constant voltage control (see a process of a step  870  in  FIG. 8 ). 
     Thereby, the exhaust oxygen concentration as well as the SOx concentration of the exhaust gas can be acquired. 
     According to further another aspect of the invention, the SOx concentration acquiring apparatus may comprise the solid electrolyte layer as a first solid electrolyte layer. In this case, the SOx concentration acquiring apparatus may further comprise a pump cell ( 25 ) and a pump cell voltage source ( 25 C). In this case, the pump cell ( 25 ) is formed by a second solid electrolyte layer ( 21 B), a first pump electrode ( 25 A), and a second pump electrode ( 25 B). The first pump electrode ( 25 A) is provided on one of opposite surfaces of the second solid electrolyte layer ( 21 B). The second pump electrode ( 25 B) is provided on the other surface of the second solid electrolyte layer ( 21 B). The pump cell voltage source ( 25 C) applies a voltage to the pump cell ( 25 ). The interior space ( 28 ) is defined by the first solid electrolyte layer ( 21 A), the second solid electrolyte layer ( 21 B), and the diffusion-limited layer ( 23 ) such that the first pump electrode ( 25 A) exposes to the Interior space ( 28 ). 
     In this case, the SOx concentration acquiring apparatus may further comprise a protection layer ( 29 ). In this case, the protection layer ( 29 ) is formed of a material, through which the exhaust gas can flow. Further, the protection layer ( 29 ) is provided covering the first solid electrolyte layer ( 21 A), the second solid electrolyte layer ( 21 B), and the diffusion-limited layer ( 23 ). 
     Alternatively, the pump cell is formed by the first solid electrolyte layer ( 21 A), the first pump electrode, and the second pump. In this case, the first pump electrode is provided on one of the opposite surfaces of the first solid electrolyte layer ( 21 A) such that the first pump electrode exposes to the interior space ( 28 ). The second pump electrode is provided on the other surface of the first solid electrolyte layer ( 21 A). 
     According to this aspect, the electronic control unit ( 90 ) may be further configured to execute a pump voltage control for applying a voltage (Vpp) capable of decreasing an oxygen concentration of the exhaust gas to generally zero to the pump cell ( 25 ). The electronic control unit ( 90 ) may be further configured to execute a constant voltage control for controlling the sensor voltage (Vss) to a constant voltage lower than the oxygen increasing voltage (Vox_in) (see a process of a step  1550  in  FIG. 15 ). The electronic control unit ( 90 ) may be further configured to acquire a NOx concentration (Cnox) of the exhaust gas on the basis of the current (Iss) flowing through the sensor cell while the electronic control unit ( 90 ) executes the pump voltage control and the constant voltage control (see a process of a step  1560  in  FIG. 15 ). Thereby, the NOx concentration of the exhaust gas as well as the SOx concentration of the exhaust gas can be acquired. 
     According to further another aspect of the invention, the electronic control unit ( 90 ) may be further configured to acquire the oxygen concentration (Coxy) of the exhaust gas on the basis of a current (Ipp) flowing through the pump cell ( 25 ) while the electronic control unit ( 90 ) executes the pump voltage control (see a process of a step  1565  in  FIG. 15 ). Thereby, the exhaust oxygen concentration as well as the SOx concentration of the exhaust gas can be acquired. 
     In this case, the first pump electrode ( 25 A) may be positioned upstream of the first sensor electrode ( 26 A) in a direction along a flow of the exhaust gas in the interior space ( 28 ). 
     In the above description, for facilitating understanding of the present invention, elements of the present invention corresponding to elements of an embodiment described later are denoted by reference symbols used in the description of the embodiment accompanied with parentheses. However, the elements of the present invention are not limited to the elements of the embodiment defined by the reference symbols. The other objects, features and accompanied advantages of the present invention can be easily understood from the description of the embodiment of the present invention along with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view for showing an internal combustion engine provided with a SOx concentration acquiring apparatus according to a first embodiment of the invention. 
         FIG. 2  is a view for showing an inner configuration of a limiting current sensor of the SOx concentration acquiring apparatus according to the first embodiment. 
         FIG. 3  is a view for showing a relationship among a voltage applied to a sensor cell of the limiting current sensor of the SOx concentration acquiring apparatus according to the first embodiment, a current flowing through the sensor cell, and an oxygen concentration of the exhaust gas just discharged from the Internal combustion engine. 
         FIG. 4A  is a view for showing a relationship between the voltage applied to the sensor cell and the current flowing through the sensor cell. 
         FIG. 4B  is a view for showing a relationship between the voltage applied to the sensor cell and the current flowing through the sensor cell. 
         FIG. 5  is a view for showing a relationship between a peak current difference and a SOx concentration of the exhaust gas just discharged from the internal combustion engine. 
         FIG. 6  is a view for showing a time chart Illustrating changes of the voltage applied to the sensor cell and the current flowing through the sensor cell. 
         FIG. 7  is a view for showing manners of increasing and decreasing the voltage applied to the sensor cell by the SOx concentration acquiring apparatus according to the first embodiment. 
         FIG. 8  is a view for showing a flowchart illustrating a routine executed by a CPU of an ECU of the SOx concentration acquiring apparatus according to the first embodiment. 
         FIG. 9  is a view for showing a flowchart illustrating a routine executed by the CPU. 
         FIG. 10  is a view for showing a flowchart illustrating a routine executed by the CPU. 
         FIG. 11  is a view for showing a flowchart illustrating a routine executed by the CPU. 
         FIG. 12  is a view for showing the internal combustion engine provided with the SOx concentration acquiring apparatus according to a second embodiment of the invention. 
         FIG. 13  is a view for showing an inner configuration of a limiting current sensor of the SOx concentration acquiring apparatus according to the second embodiment. 
         FIG. 14  is a view for showing a relationship between the current flowing through the sensor cell of the sensor of the SOx concentration acquiring apparatus according to the second embodiment and a NOx concentration of the exhaust gas just discharged from the internal combustion engine. 
         FIG. 15  is a view for showing a flowchart illustrating a routine executed by the CPU of the ECU of the SOx concentration acquiring apparatus according to the second embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Below, a SOx concentration acquiring apparatus of an internal combustion engine according to embodiments of the invention will be described with reference to the drawings. The SOx concentration acquiring apparatus according to a first embodiment of the invention is applied to the internal combustion engine shown in  FIG. 1 . Hereinafter, the SOx concentration acquiring apparatus according to the first embodiment will be referred to as “the first embodiment apparatus”. 
     The internal combustion engine  50  is a spark-ignition internal combustion engine (i.e., a so-called gasoline engine). In this connection, the invention may be applied to a compression-ignition internal combustion engine (i.e., a so-called diesel engine). The internal combustion engine  50  shown in  FIG. 1  operates at a stoichiometric air-fuel ratio in a substantial engine operation region. 
     In  FIG. 1 , a reference sign  51  denotes a cylinder head,  52  denotes a cylinder block,  53  denotes combustion chambers,  54  denotes fuel injectors,  55  denotes spark plugs,  56  denotes a fuel pump,  57  denotes a fuel supply pipe,  60  denotes pistons,  61  denotes connecting rods,  62  denotes a crank shaft,  63  denotes a crank angle sensor,  70  denotes intake valves,  71  denotes intake ports,  72  denotes an intake manifold,  73  denotes a surge tank,  74  denotes a throttle valve,  75  denotes an intake pipe,  76  denotes an air-flow meter,  77  denotes an air filter,  80  denotes exhaust valves,  81  denotes exhaust ports,  82  denotes an exhaust manifold,  83  denotes an exhaust pipe,  90  denotes an electronic control unit,  91  denotes an acceleration pedal, and  92  denotes an acceleration pedal operation amount sensor. Hereinafter, the electronic control unit  90  will be referred to as “the ECU  90 ”. 
     The fuel injectors  54 , the ignition plugs  55 , the throttle valve  74 , the crank angle sensor  63 , the air-flow meter  76 , the acceleration pedal operation amount sensor  92 , and a limiting current sensor  10  are electrically connected to the ECU  90 . 
     The ECU  90  is an electronic control circuit including as a main component a microcomputer including a CPU, a ROM, a RAM, an interface, etc. The CPU realizes various functions by executing instructions or routines stored in a memory (i.e., the ROM). 
     The ECU  90  is configured to send signals to the fuel injectors  54 , the ignition plugs  55 , and the throttle valve  74  for activating the fuel injectors  54 , the ignition plugs  55 , and the throttle valve  74 , respectively. The ECU  90  receives signals from the crank angle sensor  63 , the air-flow meter  76 , and the acceleration pedal operation amount sensor  92 . The crank angle sensor  63  outputs a signal corresponding to a rotation speed of the crank shaft  62 . The ECU  90  calculates an engine speed (i.e., a rotation speed of the internal combustion engine  50 ) on the basis of the signals output from the crank angle sensor  63 . The air-flow meter  76  outputs a signal corresponding to a flow rate of an air passing the air-flow meter  76 , that is, a flow rate of the air flowing into the combustion chambers  53 . The ECU  90  calculates an intake air amount (i.e., an amount of the air flowing into the combustion chambers  53 ) on the basis of the signals output from the air-flow meter  76 . The acceleration pedal operation amount sensor  92  outputs a signal corresponding to an operation amount of the acceleration pedal  91 . The ECU  90  calculates an engine load KL (i.e., a load of the internal combustion engine  50 ) on the basis of the signals output from the acceleration pedal operation amount sensor  92 . 
     The first embodiment apparatus includes the limiting current sensor  10 , a sensor cell voltage source  15 C, a sensor cell ammeter  15 D, a sensor cell voltmeter  15 E, and the ECU  90 . The sensor  10  is a single-cell type limiting current sensor. The sensor  10  is provided on the exhaust pipe  83 . 
     As shown in  FIG. 2 , the sensor  10  includes a solid electrolyte layer  11 , a first alumina layer  12 A, a second alumina layer  12 B, a third alumina layer  12 C, a fourth alumina layer  12 D, a fifth alumina layer  12 E, a diffusion-limited layer  13 , a protection layer  19 , a heater  14 , a sensor cell  15 , a first sensor electrode  15 A, a second sensor electrode  15 B, an atmospheric air introduction passage  16 , and an interior space  17 . 
     The solid electrolyte layer  11  is a layer formed of zirconia or the like and has oxygen ion conductive property. The alumina layers  12 A to  12 E are layers formed of alumina, respectively. The diffusion-limited layer  13  is a porous layer, through which an exhaust gas discharged from the combustion chambers  53  of the engine  50  can flow. In the sensor  10 , the layers are laminated such that the fifth alumina layer  12 E, the fourth alumina layer  12 D, the third alumina layer  12 C, the solid electrolyte layer  11 , the diffusion-limited layer  13  and the second alumina layer  12 B, and the first alumina layer  12 A are positioned in order from the lower side of  FIG. 2 . The heater  14  is positioned between the fourth and fifth alumina layers  12 D and  12 E. 
     The atmospheric air introduction passage  16  is a space defined by the solid electrolyte layer  11 , the third alumina layer  12 C, and the fourth alumina layer  12 D, and a part of the atmospheric air introduction passage  16  opens to the atmosphere. The interior space  17  is a space defined by the first alumina layer  12 A, the solid electrolyte layer  11 , the diffusion-limited layer  13 , and the second alumina layer  12 B, and a part of the interior space  17  communicates with the outside of the sensor  10  via the diffusion-limited layer  13 . The exhaust gas discharged from the engine  50  flows into the interior space  17  through the diffusion-limited layer  13 . 
     The first and second sensor electrodes  15 A and  15 B are electrodes formed of material having a high reducing property, for example, platinum group element such as platinum and rhodium or alloy of the platinum group element. The first sensor electrode  15 A is positioned on one of opposite surfaces of the solid electrolyte layer  11  (that is, the surface of the solid electrolyte layer  11  which defines the interior space  17 ). Thus, the first sensor electrode  15 A exposes to the interior space  17 . The second sensor electrode  15 B is positioned on the other surface of the solid electrolyte layer  11  (that is, the surface of the solid electrolyte layer  11  which defines the atmospheric air introduction passage  16 ). The first sensor electrode  15 A, the second sensor electrode  15 B, and the solid electrolyte layer  11  form the sensor cell  15 . 
     The sensor  10  is configured to be able to apply a voltage from the sensor cell voltage source  15 C to the sensor cell  15  (in particular, to the second sensor electrode  15 B so as to produce an electric potential difference with respect to the first sensor electrode  15 A). The sensor cell voltage source  15 C is configured to apply a direct voltage to the sensor cell  15 . It should be noted that the first sensor electrode  15 A is a cathode side electrode, and the second sensor electrode  15 B is an anode side electrode when the sensor cell voltage source  15 C applies the direct voltage to the sensor cell  15 . 
     The protection layer  19  is a porous layer formed of material including at least one of lanthanum (La), calcium (Ca), and magnesium (Mg). The exhaust gas can flow through the protection layer  19 . The protection layer  19  is provided such that the protection layer  19  covers an outer surface of the first alumina layer  12 A, end surfaces of the diffusion-limited layer  13 , end surfaces of the solid electrolyte layer  11 , end surfaces of the first alumina layer  12 A, end surfaces of the second alumina layer  12 B, end surfaces of the third alumina layer  12 C, end surfaces of the fourth alumina layer  12 D, end surfaces of the fifth alumina layer  12 E, and an outer surface of the fifth alumina layer  12 E. 
     The protection layer  19  prevents condensed water included in the exhaust gas from adhering to the solid electrolyte layer  11 , the alumina layers  12 A to  12 E, and the diffusion-limited layer  13 , thereby preventing the solid electrolyte layer  11 , the alumina layers  12 A to  12 E, and the diffusion-limited layer  13  from cracking. In addition, the protection layer  19  traps components included in the exhaust gas, which components may deteriorate the sensor  10 , thereby preventing the sensor  10  from deteriorating. 
     The heater  14 , the sensor cell voltage source  15 C, the sensor cell ammeter  15 D, and the sensor cell voltmeter  15 E are electrically connected to the ECU  90 . 
     The ECU  90  controls an activation of the heater  14  to maintain a temperature of the sensor cell  15  at a sensor activating temperature, at which the sensor  10  is activated. 
     In addition, the ECU  90  controls a voltage of the sensor cell voltage source  15 C to apply a voltage set as described later to the sensor cell  15  from the sensor cell voltage source  15 C. 
     The sensor cell ammeter  15 D detects a current Iss flowing through a circuit including the sensor cell  15  and outputs a signal representing the detected current Iss to the ECU  90 . The ECU  90  acquires the current Iss on the basis of the signal. Hereinafter, the current Iss will be referred to as “the sensor current Iss”. 
     The sensor cell voltmeter  15 E detects a voltage Vss applied to the sensor cell  15  and outputs a signal representing the detected voltage Vss to the ECU  90 . The ECU  90  acquires the voltage Vss on the basis of the signal. Hereinafter, the voltage Vss will be referred to as “the sensor voltage Vss”. 
     &lt;Summary of Operation of First Embodiment Apparatus&gt; 
     &lt;Acquisition of Exhaust SOx Concentration&gt; 
     When the voltage is applied to the sensor cell  15 , and SOx (i.e., sulfur oxide) included in the exhaust gas flowing into the interior space  17  contacts the first sensor electrode  15 A, the SOx is reduced and decomposed on the first sensor electrode  15 A, oxygen component of the SOx becomes oxygen ion and then, the oxygen ion moves toward the second sensor electrode  15 B through the solid electrolyte layer  11 . At this time, an electric current proportional to an amount of the oxygen ion, which has moved through the solid electrolyte layer  11 , flows between the first and second sensor electrodes  15 A and  15 B. Then, when the oxygen ion reaches the second sensor electrode  15 B, the oxygen ion becomes oxygen on the second sensor electrode  15 B and then, is discharged to the atmospheric air introduction passage  16 . 
     A relationship among the sensor voltage Vss, the sensor current Iss, and an air-fuel ratio A/F of the exhaust gas just discharged from the engine  50 , is shown in  FIG. 3 . The sensor voltage Vss is a voltage applied to the sensor cell  15  by the sensor cell voltage source  15 C. The sensor current Iss is an electric current flowing between the first and second sensor electrodes  15 A and  15 B when the voltage is applied to the sensor cell  15 . The air-fuel ratio A/F of the exhaust gas corresponds to an air-fuel ratio of a mixture formed in the combustion chambers  53 . Hereinafter, the air-fuel ratio A/F of the exhaust gas will be referred to as “the exhaust air-fuel ratio A/F”. 
     In  FIG. 3 , a line denoted by A/F=12 shows a change of the sensor current Iss relative to a change of the sensor voltage Vss in case that the exhaust gas air-fuel ratio A/F is 12. Similarly, lines denoted by A/F=13 to A/F=18 show changes of the sensor current Iss relative to changes of the sensor voltage Vss in case that the exhaust air-fuel ratios A/F are 13 to 18, respectively. 
     For example, in case that the exhaust gas air-fuel ratio A/F is 18, and the sensor voltage Vss is within a range lower than a predetermined value Vth, when the sensor current Iss is a negative value, an absolute value of the sensor current Iss decreases as the sensor voltage Vss increases. On the other hand, when the sensor current Iss is a positive value, the absolute value of the sensor current Iss increases as the sensor voltage Vss increases. Further, in case that the sensor voltage Vss is within a constant range higher than or equal to the predetermined value Vth, the sensor current Iss is a constant value, independently of the sensor voltage Vss. 
     Similarly, this relationship between the sensor voltage Vss and the sensor current Iss is established in case that the exhaust gas air-fuel ratios A/F are 12 to 17, respectively. 
     From a study, the inventors of this application have a nwe knowledge that the sensor current Iss changes as shown in  FIG. 4A  while gradually increasing the sensor voltage Vss from 0.2 V to 0.8 V and then, gradually decreasing the sensor voltage Vss from 0.8 V to 0.2 V when the exhaust gas including no SOx and having a constant oxygen concentration reaches the first sensor electrode  15 A. 
     As shown by a line LU 1  in  FIG. 4A , the sensor current Iss is about 0.4 mA when the sensor voltage Vss is 0.2 V. When the sensor voltage Vss starts to increase from 0.2 V, the sensor current Iss starts to increase from about 0.4 mA. While the sensor voltage Vss increases after the sensor voltage Vss reaches about 0.4 V, the sensor current Iss decreases slightly. While the sensor voltage Vss increases after the sensor voltage Vss reaches about 0.6 V, the sensor current Iss increases slightly. While the sensor voltage Vss increases after the sensor voltage Vss reaches about 0.7 V, the sensor current Iss decreases. When the sensor voltage Vss reaches 0.8 V, the sensor current Iss reaches about 0.5 mA. 
     When the sensor voltage Vss starts to decrease from 0.8 V, the sensor current Iss starts to decrease from about 0.5 mA as shown by a line LD 1  in  FIG. 4A . While the sensor voltage Vss decreases after the sensor voltage Vss reaches about 0.6 V until the sensor voltage Vss reaches about 0.25 V, the sensor current Iss is generally constant at 0.3 mA. When the sensor voltage Vss reaches about 0.25 V, the sensor current Iss starts to increase. When the sensor voltage Vss reaches 0.2 V, the sensor current Iss reaches about 0.4 mA. 
     On the other hand, the inventors of this application have a new knowledge that the sensor current Iss changes as shown in  FIG. 4B  while gradually increasing the sensor voltage Vss from 0.2 V to 0.8 V and then, gradually decreasing the sensor voltage Vss from 0.8 V to 0.2 V when the exhaust gas including the SOx and having the constant oxygen concentration reaches the first sensor electrode  15 A. 
     Similar to an example shown in  FIG. 4A , as shown by a line LU 1  in  FIG. 4B , when the sensor voltage Vss is 0.2 V, the sensor current Iss is about 0.4 mA. When the sensor voltage Vss starts to increase from 0.2 V, the sensor current Iss starts to increase from about 0.4 mA. While the sensor voltage Vss increases after the sensor voltage Vss reaches about 0.4 V, the sensor current Iss decreases moderately. While the sensor voltage Vss increases after the sensor voltage Vss reaches about 0.6 V, the sensor current Iss increases moderately. While the sensor voltage Vss increases after the sensor voltage Vss reaches about 0.7 V, the sensor current Iss decreases. When the sensor voltage Vss reaches 0.8 V, the sensor current Iss reaches about 0.5 mA. 
     When the sensor voltage Vss starts to decrease from 0.8 V, the sensor current Iss starts to decrease from about 0.5 mA as shown by a line LD 1  in  FIG. 4B . While the sensor voltage Vss decreases after the sensor voltage Vss reaches about 0.6 V until the sensor voltage Vss reaches about 0.52 V, the sensor current Iss is generally constant at 0.3 mA. When the sensor voltage Vss reaches about 0.52 V, the sensor current Iss starts to increase. When the sensor voltage Vss reaches about 0.3 V, the sensor current Iss starts to increase. That is, when the sensor voltage Vss reaches about 0.3 V, the sensor current Iss reaches a minimum value. When the sensor voltage Vss reaches 0.2 V, the sensor current Iss reaches about 0.4 mA. 
     The change of the sensor current Iss shown in  FIG. 4B  while the sensor voltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas including the SOx reaches the first sensor electrode  15 A, is different from the change of the sensor current Iss shown in  FIG. 4A  while the sensor voltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas including no SOx reaches the first sensor electrode  15 A. 
     In particular, the sensor current Iss while the sensor voltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas including the SOx reaches the first sensor electrode  15 A, is generally lower than the sensor current Iss while the sensor voltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas including no SOx reaches the first sensor electrode  15 A. 
     In particular, the sensor current Iss reaches the minimum value (that is, a peak current Ipeak) while the sensor voltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas including the SOx reaches the first sensor electrode  15 A. As described above, in this embodiment, the sensor current Iss reaches the peak current Ipeak when the sensor voltage Vss reaches about 0.3 V. 
     In the sensor  10 , there is a phenomenon that the sensor current Iss is low while the sensor voltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas includes the SOx, compared with when the exhaust gas includes no SOx. In addition, there is a phenomenon that the peak current Ipeak appears while the sensor voltage Vss decreases from 0.8 V to 0.2 V. The inventors of this application have understood reasons for the phenomena as described below. 
     When the sensor voltage Vss exceeds a certain value while the sensor voltage Vss increases from 0.2 V to 0.8 V, the SOx reaching the first sensor electrode  15 A decomposes to sulfur component and oxygen component at the first sensor electrode  15 A. The oxygen component changes to the oxygen ion and moves toward the second sensor electrode  15 B through the solid electrolyte layer  11 . The sulfur component adheres to the first sensor electrode  15 A. 
     When the sensor voltage Vss decreases below a certain value while the sensor voltage Vss decreases from 0.8 V to 0.2 V, the sulfur component adhering to the first sensor electrode  15 A is oxidized by the oxygen, thereby returning to the SOx. At this time, a decomposing reaction of the SOx to the sulfur component and the oxygen component at the first sensor electrode  15 A, may occur. However, an oxidizing reaction of the sulfur component adhering to the first sensor electrode  15 A, is more dominant than the decomposing reaction. As a result, an amount of the oxygen component consumed by the oxidizing reaction in the interior space  17  is larger than an amount of the oxygen component produced from the SOx by the decomposing reaction. Thus, the amount of the oxygen ion moving toward the second sensor electrode  15 B through the solid electrolyte layer  11  decreases. Therefore, the sensor current Iss decreases. Thus, the sensor current Iss is low while the sensor voltage Vss decreases from 0.8 V to 0.2 V when the exhaust gas includes the SOx, compared with when the exhaust gas includes no SOx. 
     The amount of the oxygen consumed by the oxidizing reaction of the sulfur component while the sensor voltage Vss decreases from 0.8 V to 0.2 V, becomes a maximum value when the sensor voltage Vss is a certain value. Thus, the peak current Ipeak appears. 
     In this embodiment, the voltage of 0.8 V is employed as a voltage suitable for causing a decomposing amount of the SOx at the first sensor electrode  15 A to reach a large amount sufficient for acquiring a concentration of the SOx included in the exhaust gas just discharged from the engine  50  exactly while the sensor voltage Vss increases from 0.2 V to 0.8 V. Hereinafter, the concentration of the SOx will be referred to as “the SOx concentration”, and the concentration of the SOx included in the exhaust gas just discharged from the engine  50  will be referred to as “the exhaust SOx concentration”. Further, the voltage sensor Vss at a point of time when the sensor voltage Vss stops to increase, in this embodiment, the voltage of 0.8 V, will be referred to as “the increasing end voltage Vup_end”. The increasing end voltage Vup_end is, for example, a voltage capable of causing reactions such as a decomposing reaction of water included in the exhaust gas at the first sensor electrode  15 A other than the decomposing reaction of the SOx to occur to the minimum extent. 
     Further, in this embodiment, the voltage of 0.2 V is employed as a voltage suitable for causing an oxidizing amount of the sulfur component adhering to the first sensor electrode  15 A to reach a large amount sufficient for acquiring the exhaust SOx concentration exactly while the sensor voltage Vss decreases from 0.8 V to 0.2 V. Hereinafter, the sensor voltage Vss at a point of time when the sensor voltage Vss stops to decrease, in this embodiment, the voltage of 0.2 V, will be referred to as “the decreasing end voltage Vdown_end”. 
     Further, in the following description, the sensor voltage Vss for causing an amount of the oxygen produced by the decomposing reaction of the SOx to the sulfur component and the oxygen component to become larger than the amount of the oxygen consumed by the oxidizing of the sulfur component to the SOx, will be referred to as “the oxygen increasing voltage Vox_in”. In this embodiment, the oxygen increasing voltage Vox_in is 0.6 V. Further, in the following description, the sensor voltage Vss for causing the amount of the oxygen consumed by the oxidizing of the sulfur component to the SOx to become larger than the amount of the oxygen produced by the decomposing reaction of the SOx to the sulfur component and the oxygen component, will be referred to as “the oxygen decreasing voltage Vox_de”. In this embodiment, the oxygen decreasing voltage Vox_in is 0.6 V. 
     As shown in  FIG. 5 , the inventors of this application have a knowledge that there is a relationship among a reference current Iref, a peak current difference dIss, and the exhaust SOx concentration. The reference current Iref is a current at or immediately before the sensor voltage Vss starts to increase. The peak current difference dIss is a difference between the reference current Iref and the peak current Ipeak (dIss=Iref−Ipeak). The exhaust SOx concentration increases as the peak current difference dIss increases. 
     The exhaust gas flows into the interior space  17  of the sensor  10  through the protection layer  19  and the diffusion-limited layer  13 . A part of the SOx included in the exhaust gas adheres to the protection layer  19  and the diffusion-limited layer  13 . While the first embodiment apparatus increases the sensor voltage Vss for acquiring the exhaust SOx concentration, the SOx decomposes at the first sensor electrode  15 A and thus, the SOx concentration in the interior space  17  decreases temporarily. Therefore, the SOx adhering to the protection layer  19  and the diffusion-limited layer  13  may remove from the protection layer  19  and the diffusion-limited layer  13 , and the removed SOx may flow into the interior space  17 . In addition, the exhaust gas including the SOx continuously flows into the interior space  17  from the outside of the sensor  10  through the protection layer  19  and the diffusion-limited layer  13 . 
     Therefore, while the sensor voltage Vss increases, the SOx concentration in the interior space  17  may not correspond to the exhaust SOx concentration. Therefore, while the sensor voltage Vss decreases after the sensor voltage Vss increases, the sensor current Iss may not represent the exhaust SOx concentration exactly. 
     Accordingly, as shown in  FIG. 6 , the first embodiment apparatus executes a constant voltage control for controlling the sensor voltage Vss to maintain the sensor voltage Vss at a constant value lower than the oxygen increasing voltage Vox_in when the exhaust SOx concentration is not requested to be acquired, that is, in a time period before a point of time t 0 . In this embodiment, the constant value lower than the oxygen increasing voltage Vox_in is 0.4 V. The first embodiment apparatus acquires the sensor currents Iss while the first embodiment apparatus executes the constant voltage control. The first embodiment apparatus stores the acquired sensor currents Iss in the RAM. 
     When the exhaust SOx concentration is requested to be acquired and an engine operation (that is, an operation of the engine  50 ) is in a steady operation state or an idling operation state, the first embodiment apparatus executes a first voltage control including a first voltage increasing control and a first voltage decreasing control described below. 
     The exhaust SOx concentration is requested to be acquired, for example, when a vehicle equipped with the engine  50  moves for a predetermined distance after fuel is supplied to a fuel tank which stores the fuel to be supplied to the fuel injectors  54 . Alternatively, the exhaust SOx concentration is requested to be acquired when the vehicle equipped with the engine  50  moves for the predetermined distance after the fuel is supplied to the fuel tank and thereafter, the exhaust SOx concentration is requested to be acquired each time the vehicle moves for the predetermined distance or another predetermined distance. 
     The steady operation state is a state that the engine speed NE and the engine load KL are constant or generally constant, respectively. That is, when the engine operation is in the steady operation state, a concentration of the oxygen included in the exhaust gas just discharged from the engine  50  is constant or generally constant. Hereinafter, the concentration of the oxygen included in the exhaust gas just discharged from the engine  50  will be referred to as “the exhaust oxygen concentration”. The idling operation state is a state that the operation amount AP of the acceleration pedal is zero and thus, a minimum amount of the air required to maintain the operation of the engine  50  is caused to flow into the combustion chambers  53 , and the fuel injectors  54  are caused to inject the fuel. Therefore, the exhaust oxygen concentration is constant or generally constant when the engine operation is in the idling operation state. 
     When the first embodiment apparatus starts to execute the first voltage control, the first embodiment apparatus starts to execute the first voltage increasing control for increasing the sensor voltage Vss from 0.4 V with an increasing rate of the sensor voltage Vss decreasing gradually (see the point of time t 0  in  FIG. 6 ). When the sensor voltage Vss reaches the increasing end voltage Vup_end (in this embodiment, 0.8 V), the first embodiment apparatus stops executing the first voltage increasing control (see a point of time t 1  in  FIG. 6 ). Thereby, the first embodiment apparatus increases the sensor voltage Vss from 0.4 V to 0.8 V. 
     Thereafter, the first embodiment apparatus starts to execute the first voltage decreasing control for decreasing the sensor voltage Vss from the increasing end voltage Vup_end (in this embodiment, 0.8 V) with a decreasing rate of the sensor voltage Vss increasing gradually (see the point of time t 1  in  FIG. 6 ). When the sensor voltage Vss reaches the decreasing end voltage Vdown_end (in this embodiment, 0.2 V), the first embodiment apparatus stops executing the first voltage decreasing control (see a point of time t 2  in  FIG. 6 ). Thereby, the first embodiment apparatus decreases the sensor voltage Vss from 0.8 V to 0.2 V. 
     In this embodiment, the first embodiment apparatus changes the sensor voltage Vss in the first voltage increasing control such that a period of time from a point of time of starting to increase the sensor voltage Vss to a point of time of stopping increasing the sensor voltage Vss, is 0.1 seconds (=100 ms). In this connection, the period of time from the point of time of starting to increase the sensor voltage Vss to the point of time of stopping increasing the sensor voltage Vss in the first voltage increasing control of the first embodiment, is not limited to 0.1 seconds. 
     Further, in this embodiment, the first embodiment apparatus changes the sensor voltage Vss in the first voltage decreasing control such that a period of time from a point of time of starting to decrease the sensor voltage Vss to a point of time of stopping decreasing the sensor voltage Vss, is 0.1 seconds (=100 ms). In this connection, the first embodiment apparatus may be configured to change the sensor voltage Vss in the first voltage decreasing control such that the period of time from the point of time of starting to decrease the sensor voltage Vss to the point of time of stopping decreasing the sensor voltage Vss, corresponds to a period of time longer than 0.1 seconds and equal to or shorter than 5 seconds. 
     The first embodiment apparatus executes a second voltage control including a second voltage increasing control and a second voltage decreasing control after the first embodiment apparatus stops executing the first voltage control. 
     When the first embodiment apparatus starts to execute the second voltage control, the first embodiment apparatus starts to execute the second voltage increasing control for increasing the sensor voltage Vss from the decreasing end voltage Vdown_end (in this embodiment, 0.2 V) with the increasing rate of the sensor voltage Vss decreasing gradually (see the point of time t 2  in  FIG. 5 ). When the sensor voltage Vss reaches the increasing end voltage Vup_end (in this embodiment, 0.8 V), the first embodiment apparatus stops executing the second voltage increasing control (see a point of time t 3  in  FIG. 6 ). Thereby, the first embodiment apparatus increases the sensor voltage Vss from 0.4 V to 0.8 V. 
     Thereafter, the first embodiment apparatus starts to execute the second voltage decreasing control for decreasing the sensor voltage Vss from the increasing end voltage Vup_end (in this embodiment, 0.8 V) with the decreasing rate of the sensor voltage Vss increasing gradually (see the point of time t 3  in  FIG. 6 ). When the sensor voltage Vss reaches the decreasing end voltage Vdown_end (in this embodiment, 0.2 V), the first embodiment apparatus stops executing the second voltage decreasing control (see a point of time t 4  in  FIG. 6 ). Thereby, the first embodiment apparatus decreases the sensor voltage Vss from 0.8 V to 0.2 V. 
     In this embodiment, the first embodiment apparatus changes the sensor voltage Vss in the second voltage increasing control such that a period of time from a point of time of starting to increase the sensor voltage Vss to a point of time of stopping increasing the sensor voltage Vss, is 0.1 seconds (=100 ms). In this connection, the period of time from the point of time of starting to increase the sensor voltage Vss to the point of time of stopping increasing the sensor voltage Vss in the second voltage increasing control of the first embodiment, is not limited to 0.1 seconds. 
     Further, in this embodiment, the first embodiment apparatus changes the sensor voltage Vss in the second voltage decreasing control such that a period of time from a point of time of starting to decrease the sensor voltage Vss to a point of time of stopping decreasing the sensor voltage Vss, is 0.1 seconds (=100 ms). In this connection, the first embodiment apparatus may be configured to change the sensor voltage Vss in the second voltage decreasing control such that the period of time from the point of time of starting to decrease the sensor voltage Vss to the point of time of stopping decreasing the sensor voltage Vss, corresponds to a period of time longer than 0.1 seconds and equal to or shorter than 5 seconds. 
     The first embodiment apparatus acquires the sensor current Iss and stores the acquired sensor current Iss as a SOx concentration current Iss_sox in the RAM while the first embodiment apparatus decreases the sensor voltage Vss from 0.8 V to 0.2 V in the second voltage control. After the first embodiment apparatus stops executing the second voltage control, the first embodiment apparatus acquires the peak current Ipeak from the stored SOx concentration currents Iss_sox. In addition, the first embodiment apparatus acquires, as the reference current Iref, the sensor current Iss stored in the RAM immediately before the first embodiment apparatus starts to execute the first voltage control. The first embodiment apparatus acquires, as the peak current difference dIss, the difference between the reference current Iref and the peak current Ipeak (dIss=Iref−Ipeak). 
     The first embodiment apparatus applies the acquired peak current difference dIss to a look-up table Map1Csox(dIss) to acquire the exhaust SOx concentration Csox. The look-up table Map1Csox(dIss) is prepared previously on the basis of experiments, etc. for determining a relationship between the peak current difference dIss and the exhaust SOx concentration in the sensor  10 . The exhaust SOx concentration Csox acquired from the look-up table Map1Csox(dIss) increases as the peak current difference dIss increases. 
     After the first embodiment apparatus stops executing the second voltage control, the first embodiment apparatus starts to execute the constant voltage control, thereby increasing the sensor voltage Vss from 0.2 V to 0.4 V and maintaining the sensor voltage Vss at 0.4 V. 
     As described above, the first embodiment apparatus executes the first voltage control before the first embodiment apparatus executes the second voltage control. Therefore, a large part or at least a part of the SOx which may remove from the protection layer  19  and the diffusion-limited layer  13  due to increasing of the sensor voltage Vss, removes from the protection layer  19  and the diffusion-limited layer  13  due to executing of the first voltage increasing control of the first voltage control. Thus, when the sensor voltage Vss increases in the second voltage control, an amount of the SOx removing from the protection layer  19  and the diffusion-limited layer  13  is small. As a result, the SOx concentration in the interior space  17  generally corresponds to or is close to the exhaust SOx concentration while the second voltage control is executed. Thus, the sensor current Iss represents the exhaust SOx concentration accurately while the sensor voltage Vss decreases in the second voltage control. Therefore, the first embodiment apparatus can acquire the exhaust SOx concentration accurately. 
     It should be noted that even when the sensor  10  does not include the protection layer  19 , the SOx adheres to the diffusion-limited layer  13  and thus, the first embodiment apparatus may be applied to a sensor which does not include the protection layer  19 . 
     Further, as shown in  FIG. 7 , the first embodiment apparatus may be configured to increase the sensor voltage Vss from 0.4 V to 0.8 V in the first voltage increasing control such that the increasing rate of the sensor voltage Vss is constant. In addition, as shown in  FIG. 7 , the first embodiment apparatus may be configured to decrease the sensor voltage Vss from 0.8 V to 0.2 V in the first voltage decreasing control such that the decreasing rate of the sensor voltage Vss is constant. 
     Similarly, the first embodiment apparatus may be configured to increase the sensor voltage Vss from 0.2 V to 0.8 V in the second voltage increasing control such that the increasing rate of the sensor voltage Vss is constant. In addition, the first embodiment apparatus may be configured to decrease the sensor voltage Vss from 0.8 V to 0.2 V in the second voltage decreasing control such that the decreasing rate of the sensor voltage Vss is constant. 
     Further, the sensor voltage Vss at the point of time of starting to increase the sensor voltage Vss in the first voltage increasing control, that is, the sensor voltage Vss applied to the sensor cell  15  constantly, is not limited to 0.4 V. The sensor voltage Vss at the point of time of starting to increase the sensor voltage Vss in the first voltage increasing control may be a voltage lower than the oxygen increasing voltage Vox_in. For example, the sensor voltage Vss at the point of time of starting to increase the sensor voltage Vss in the first voltage increasing control, may be 0.2 V. 
     Further, the sensor voltage Vss at the point of time of stopping increasing the sensor voltage Vss in the first and second voltage increasing controls, that is, the increasing end voltage Vup_end, is not limited to 0.8 V. The sensor voltage Vss at the point of time of stopping increasing the sensor voltage Vss in the first and second voltage increasing controls, may be a voltage higher than the oxygen increasing voltage Vox_in. 
     Further, the sensor voltage Vss at the point of time of stopping decreasing the sensor voltage Vss in the first and second voltage decreasing controls, is not limited to 0.2 V. The sensor voltage Vss at the point of time of stopping decreasing the sensor voltage Vss in the first and second voltage decreasing controls, may be a voltage lower than the oxygen decreasing voltage Vox_de. 
     Further, the first embodiment apparatus uses the peak current Ipeak for acquiring the exhaust SOx concentration Csox. In this connection, the first embodiment apparatus may be configured to use the sensor current Iss decreasing or increasing rapidly while the sensor voltage Vss decreases from 0.8 V to 0.2 V in place of the peak current Ipeak. 
     Further, the first embodiment apparatus acquires the exhaust SOx concentration Csox by using the peak current Ipeak and the reference current Iref. Alternatively, the first embodiment apparatus may be configured to acquire the exhaust SOx concentration Csox by multiplying the peak current Ipeak by a conversion coefficient Kconvert (Csox=Ipeak*Kconvert). In this case, the conversion coefficient Kconvert is set to a value capable of acquiring the exhaust SOx concentration Csox which increases as the peak current Ipeak decreases. 
     Further, if an influence of the oxygen included in the exhaust gas reaching the sensor cell  15  A to the peak current Ipeak in the second voltage decreasing control, can be eliminated, the first embodiment apparatus may be configured to execute the first and second voltage controls and acquire the exhaust SOx concentration Csox when the exhaust SOx concentration is requested to be acquired although the engine operation is not in any of the steady operation state and the idling operation state. 
     &lt;Acquisition of Exhaust Oxygen Concentration&gt; 
     As understood referring to  FIG. 3 , in the sensor  10 , there is a limiting current range which is a range of the sensor voltage Vss in which the sensor current Iss is constant, independently of the sensor voltage Vss when the exhaust oxygen concentration (i.e., the exhaust gas air-fuel ratio A/F) is constant. Therefore, the exhaust oxygen concentration (i.e., the exhaust gas air-fuel ratio A/F) can be acquired by using the sensor current Iss when a voltage within the limiting current range for the exhaust oxygen concentration to be acquired, is applied to the sensor cell  15 . 
     As described above, the first embodiment apparatus executes the constant voltage control for controlling the sensor voltage Vss to 0.4 V when the exhaust SOx concentration is not requested to be acquired. In this embodiment, the voltage of 0.4 V is the voltage within the limiting current range for the range of the exhaust oxygen concentration to be acquired. 
     Accordingly, the first embodiment apparatus acquires the sensor current Iss as an oxygen concentration current Iss_oxy while the first embodiment apparatus executes the constant voltage control. Then, the first embodiment apparatus applies the oxygen concentration current Iss_oxy to a look-up table MapCoxy(Iss_oxy), thereby acquiring the exhaust oxygen concentration Coxy. 
     The look-up table MapCoxy(Iss_oxy) is prepared previously on the basis of experiments, etc. for determining a relationship between the sensor current Iss and the exhaust oxygen concentration when the sensor voltage Vss is controlled to 0.4 V. The exhaust oxygen concentration Coxy acquired from the look-up table MapCoxy(Iss_oxy) increases as the oxygen concentration current Iss_oxy increases. 
     Thereby, the first embodiment apparatus can acquire the exhaust oxygen concentration as well as the exhaust SOx concentration. 
     &lt;Concrete Operation of First Embodiment Apparatus&gt; 
     Next, a concrete operation of the first embodiment apparatus will be described. The CPU of the ECU  90  of the first embodiment apparatus is configured or programmed to execute a routine shown in  FIG. 8  each time a predetermined time elapses. 
     Therefore, at a predetermined timing, the CPU starts a process from a step  800  in  FIG. 8  and proceeds with the process to a step  810  to determine whether a value of a SOx concentration acquiring request flag Xsox is “1”. The value of the SOx concentration acquiring request flag Xsox is set to “1” when the exhaust SOx concentration is requested to be acquired and is set to “0” when the exhaust SOx concentration is acquired. 
     When the value of the SOx concentration acquiring request flag Xsox is “1”, the CPU determines “Yes” at the step  810  and then, proceeds with the process to a step  815  to determine whether the engine operation is in the steady operation state or the idling operation state. 
     When the engine operation is in the steady operation state or the idling operation state, the CPU determines “Yes” at the step  815  and then, proceeds with the process to a step  820  to determine whether a value of a first voltage control end flag Xalt is “0”. The value of the first voltage control end flag Xalt is set to “1” when the first voltage control ends and is set to “0” when the second voltage control ends after the first voltage control ends. Immediately after the exhaust SOx concentration is requested to be acquired, the first voltage control has not been executed and thus, the value of the first voltage control end flag Xalt is “0”. 
     When the value of the first voltage control end flag Xalt is “0” at a time of executing a process of the step  820 , the CPU determines “Yes” at the step  820  and then, proceeds with the process to a step  830  to execute a routine shown by a flowchart in  FIG. 9 . 
     Therefore, when the CPU proceeds with the process to the step  830  in  FIG. 8 , the CPU starts a process from a step  900  in  FIG. 9  and then, proceeds with the process to a step  905  to determine whether a value of a voltage increasing end flag Xup 1  is “0”. The value of the voltage increasing end flag Xup 1  is set to “1” when the first voltage increasing control ends and is set to “0” when the first voltage decreasing control ends after the first voltage increasing control ends. 
     When the value of the voltage increasing end flag Xup 1  is “0” at a time of executing a process of the step  905 , the CPU determines “Yes” at the step  905  and then, executes a process of a step  910  described below. Then, the CPU proceeds with the process to a step  915 . 
     Step  910 : The CPU starts to execute the first voltage increasing control when the CPU has not executed the first voltage increasing control. On the other hand, the CPU continues to execute the first voltage increasing control when the CPU already executes the first voltage increasing control. When the CPU executes the process of the step  910  immediately after the CPU first determines “Yes” at the step  905 , the CPU has not executed the first voltage increasing control. In this case, the CPU starts to execute the first voltage increasing control. The CPU continues to execute the first voltage increasing control until the CPU determines “Yes” at the step  915 . 
     When the CPU proceeds with the process to the step  915 , the CPU determines whether the sensor voltage Vss reaches 0.8 V, that is, the sensor voltage Vss is equal to or higher than 0.8 V. When the sensor voltage Vss is lower than 0.8 V, the CPU determines “No” at the step  915  and then, proceeds with the process to a step  895  in  FIG. 8  via a step  995  to terminate this routine once. 
     On the other hand, when the sensor voltage Vss is equal to or higher than 0.8 V, the CPU determines “Yes” at the step  915  and then, executes processes of steps  920  and  925  described below. Then, the CPU proceeds with the process to the step  895  in  FIG. 8  via the step  995  to terminate this routine once. 
     Step  920 : The CPU stops executing the first voltage increasing control. 
     Step  925 : The CPU sets the value of the voltage increasing end flag Xup 1  to “1”. Thereby, when the CPU proceeds with the process to the step  905 , the CPU determines “No” at the step  905 . 
     When the value of the voltage increasing end flag Xup 1  is “1” at a time of executing a process of the step  905 , the CPU determines “No” at the step  905  and then, executes a process of a step  930  described below. Then, the CPU proceeds with the process to a step  935 . 
     Step  930 : The CPU starts to execute the first voltage decreasing control when the CPU has not executed the first voltage decreasing control. On the other hand, the CPU continues to execute the first voltage decreasing control when the CPU already executes the first voltage decreasing control. When the CPU executes the process of the step  930  immediately after the CPU first determines “No” at the step  905 , the CPU has not executed the first voltage decreasing control. In this case, the CPU starts to execute the first voltage decreasing control. The CPU continues to execute the first voltage decreasing control until the CPU determines “Yes” at the step  935 . 
     When the CPU proceeds with the process to the step  935 , the CPU determines whether the sensor voltage Vss reaches 0.2 V, that is, the sensor voltage Vss is equal to or lower than 0.2 V. When the sensor voltage Vss is higher than 0.2 V, the CPU determines “No” at the step  935  and then, proceeds with the process to the step  895  in  FIG. 8  via the step  995  to terminate this routine once. 
     On the other hand, when the sensor voltage Vss is equal to or lower than 0.2 V, the CPU determines “Yes” at the step  935  and then, executes processes of steps  940  and  945  described below. Then, the CPU proceeds with the process to the step  895  in  FIG. 8  via the step  995  to terminate this routine once. 
     Step  940 : The CPU stops executing the first voltage decreasing control. 
     Step  945 : The CPU sets the value of the first voltage control end flag Xalt to “1”. Thereby, when the CPU proceeds with the process to the step  820  in  FIG. 8 , the CPU determines “No” at the step  820 . In addition, the CPU sets the value of the voltage increasing end flag Xup 1  to “0”. 
     When the value of the first voltage control end flag Xalt is “1” at a time of executing a process of the step  820  in  FIG. 8 , the CPU determines “No” at the step  820  and then, proceeds with the process to a step  840  to execute the second voltage control shown by a flowchart in  FIG. 10 . 
     Therefore, when the CPU proceeds with the process to the step  840 , the CPU starts a process from a step  1000  in  FIG. 10  and then, proceeds with the process to a step  1005  to determine whether a value of a voltage increasing end flag Xup 2  is “0”. The value of the voltage increasing end flag Xup 2  is set to “1” when the second voltage increasing control ends and is set to “0” when the second voltage decreasing control ends after the second voltage increasing control ends. 
     When the value of the voltage increasing end flag Xup 2  is “0” at a time of executing a process of the step  1005 , the CPU determines “Yes” at the step  1005  and then, executes a process of a step  1010  described below. Then, the CPU proceeds with the process to a step  1015 . 
     Step  1010 : The CPU starts to execute the second voltage increasing control when the CPU has not executed the second voltage increasing control. On the other hand, the CPU continues to execute the second voltage increasing control when the CPU already executes the second voltage increasing control. When the CPU executes the process of the step  1010  immediately after the CPU first determines “Yes” at the step  1005 , the CPU has not executed the second voltage increasing control. In this case, the CPU starts to execute the second voltage increasing control. The CPU continues to execute the second voltage increasing control until the CPU determines “Yes” at the step  1015 . 
     When the CPU proceeds with the process to the step  1015 , the CPU determines whether the sensor voltage Vss reaches 0.8 V, that is, the sensor voltage Vss is equal to or higher than 0.8 V. When the sensor voltage Vss is lower than 0.8 V, the CPU determines “No” at the step  1015  and then, proceeds with the process to the step  895  in  FIG. 8  via a step  1095  to terminate this routine once. 
     On the other hand, when the sensor voltage Vss is equal to or higher than 0.8 V, the CPU determines “Yes” at the step  1015  and then, executes processes of steps  1020  and  1025  described below. Then, the CPU proceeds with the process to the step  895  in  FIG. 8  via the step  1095  to terminate this routine once. 
     Step  1020 : The CPU stops executing the second voltage increasing control. 
     Step  1025 : The CPU sets the value of the voltage increasing end flag Xup 2  to “1”. Thereby, when the CPU proceeds with the process to the step  1005 , the CPU determines “No” at the step  1005 . 
     When the value of the voltage increasing end flag Xup 2  is “1” at a time of executing a process of the step  1005 , the CPU determines “No” at the step  1005  and then, executes processes of steps  1030  and  1035  described below. Then, the CPU proceeds with the process to a step  1040 . 
     Step  1030 : The CPU starts to execute the second voltage decreasing control when the CPU has not executed the second voltage decreasing control. On the other hand, the CPU continues to execute the second voltage decreasing control when the CPU already executes the second voltage decreasing control. When the CPU executes the process of the step  1030  immediately after the CPU first determines “No” at the step  1005 , the CPU has not executed the second voltage decreasing control. In this case, the CPU starts to execute the second voltage decreasing control. The CPU continues to execute the second voltage decreasing control until the CPU determines “Yes” at the step  1040 . 
     Step  1035 : The CPU acquires the sensor current Iss and stores the acquired sensor current Iss as the SOx concentration current Iss_sox in the RAM. 
     When the CPU proceeds with the process to the step  1040 , the CPU determines whether the sensor voltage Vss reaches 0.2 V, that is, the sensor voltage Vss is equal to or lower than 0.2 V. When the sensor voltage Vss is higher than 0.2 V, the CPU determines “No” at the step  1040  and then, proceeds with the process to the step  895  in  FIG. 8  via the step  1095  to terminate this routine once. 
     On the other hand, when the sensor voltage Vss is equal to or lower than 0.2 V, the CPU determines “Yes” at the step  1040  and then, executes processes of steps  1045  to  1055  described below. Then, the CPU proceeds with the process to the step  895  in  FIG. 8  via the step  1095  to terminate this routine once. 
     Step  1045 : The CPU stops executing the second voltage decreasing control. 
     Step  1050 : The CPU acquires the peak current Ipeak from the SOx concentration currents Iss_sox stored in the RAM and calculates the difference between the reference current Iref and the peak current Ipeak as the peak current difference dIss. Then, the CPU applies the peak current difference dIss to the look-up table Map1Csox(dIss) to acquire the exhaust SOx concentration Csox. 
     Step  1055 : The CPU sets the values of the SOx concentration acquiring request flag Xsox, the first voltage control end flag Xalt, and the voltage increasing end flag Xup 2  to “0”, respectively. 
     When the value of the SOx concentration acquiring request flag Xsox is “0” at a time of executing a process of the step  810  in  FIG. 8 , and the engine operation is not in any of the steady operation state and the idling operation state at a time of executing a process of the step  815  in  FIG. 8 , the CPU determines “No” at any of the steps  810  and  815  and then, executes processes of steps  850  to  870 . Then, CPU proceeds with the process to the step  895  to terminate this routine once. 
     Step  850 : The CPU starts to execute the constant voltage control for controlling the sensor voltage Vss to 0.4 V when the CPU has not executed the constant voltage control. On the other hand, the CPU continues to execute the constant voltage control when the CPU already executes the constant voltage control. 
     Step  860 : The CPU acquires the sensor current Iss as the oxygen concentration current Iss_oxy. 
     Step  870 : The CPU applies the oxygen concentration current Iss_oxy to the look-up table MapCoxy(Iss_oxy) to acquire the exhaust oxygen concentration Coxy. 
     The first embodiment apparatus can acquire the exhaust SOx concentration and the exhaust oxygen concentration by executing the processes described above. 
     Further, when the exhaust SOx concentration is equal to or lower than an upper limit concentration Csox_limit designated by law but is near the upper limit concentration Csox_limit, it is desired to determine that the exhaust SOx concentration is near the upper limit concentration Csox_limit in order to inform that the exhaust SOx concentration is near the upper limit concentration Csox_limit. 
     Accordingly, the CPU of the ECU  90  of the first embodiment apparatus is configured or programmed to execute a routine shown by a flowchart in  FIG. 11  each time a predetermined time elapses. Therefore, at a predetermined timing, the CPU starts a process from a step  1100  in  FIG. 11  and proceeds with the process to a step  1110  to determine whether the exhaust SOx concentration Csox acquired at the step  1050  in  FIG. 10  is larger than an upper limit concentration Cth. The upper limit concentration Cth is a permissible upper limit value of the exhaust SOx concentration. 
     When the exhaust SOx concentration Csox is larger than the upper limit concentration Cth, the CPU determines “Yes” at the step  1110  and then, proceeds with the process to a step  1120  to determine that the exhaust SOx concentration is larger than the upper limit concentration Cth. Then, the CPU proceeds with the process to a step  1195  to terminate this routine once. 
     On the other hand, when the exhaust SOx concentration Csox is equal to or smaller than the upper limit concentration Cth, the CPU determines “No” at the step  1110  and then, proceeds with the process to a step  1130  to determine that the exhaust SOx concentration is equal to or smaller than the upper limit concentration Cth. Then, the CPU proceeds with the process to the step  1195  to terminate this routine once. 
     Second Embodiment 
     Next, the SOx concentration acquiring apparatus of the internal combustion engine according to a second embodiment of the invention will be described. The SOx concentration acquiring apparatus according to the second embodiment of the invention is applied to the internal combustion engine  50  shown in  FIG. 12 . The internal combustion engine  50  shown in  FIG. 12  is the same as the internal combustion engine  50  shown in  FIG. 1 . Hereinafter, the SOx concentration acquiring apparatus according to the second embodiment will be referred to as “the second embodiment apparatus”. 
     The second embodiment apparatus Includes a limiting current sensor  20  having an inner configuration shown in  FIG. 13 , a pump cell voltage source  25 C, a sensor cell voltage source  26 C, a pump cell ammeter  25 D, a sensor cell ammeter  26 D, a sensor cell voltmeter  26 E, and the ECU  90 . The sensor  20  is a two-cell type limiting current sensor. The sensor  20  is provided on the exhaust pipe  83 . 
     As shown in  FIG. 13 , the sensor  20  includes a first solid electrolyte layer  21 A, a second solid electrolyte layer  21 B, a first alumina layer  22 A, a second alumina layer  22 B, a third alumina layer  22 C, a fourth alumina layer  22 D, a fifth alumina layer  22 E, a sixth alumina layer  22 F, a diffusion-limited layer  23 , a protection layer  29 , a heater  24 , a pump cell  25 , a first pump electrode  25 A, a second pump electrode  25 B, a sensor cell  26 , a first sensor electrode  26 A, a second sensor electrode  26 B, a first atmospheric air introduction passage  27 A, a second atmospheric air introduction passage  27 B, and an interior space  28 . 
     Each of the solid electrolyte layers  21 A and  21 B is a layer formed of zirconia or the like and has the oxygen ion conductive property. The alumina layers  22 A to  22 F are layers formed of alumina, respectively. The diffusion-limited layer  23  is a porous layer, through which the exhaust gas can flow. In the sensor  20 , the layers are laminated such that the sixth alumina layer  22 F, the fifth alumina layer  22 E, the fourth alumina layer  22 D, the second solid electrolyte layer  21 B, the diffusion-limited layer  23  and the third alumina layer  22 C, the first solid electrolyte layer  21 A, the second alumina layer  22 B, and the first alumina layer  22 A are positioned in order from the lower side of  FIG. 13 . The heater  24  is positioned between the fifth and sixth alumina layers  22 E and  22 F. 
     The first atmospheric air introduction passage  27 A is a space defined by the first alumina layer  22 A, the second alumina layer  22 B, and the first solid electrolyte layer  21 A, and a part of the first atmospheric air introduction passage  27 A opens to the atmosphere. The second atmospheric air introduction passage  27 B is a space defined by the second solid electrolyte layer  21 B, the fourth alumina layer  22 D, and the fifth alumina layer  22 E, and a part of the second atmospheric air introduction passage  27 B opens to the atmosphere. The interior space  28  is a space defined by the first solid electrolyte layer  21 A, the second solid electrolyte layer  21 B, the diffusion-limited layer  23 , and the third alumina layer  22 C, and a part of the interior space  28  communicates with the outside of the sensor  20  via the diffusion-limited layer  23 . The exhaust gas discharged from the engine  50  flows into the interior space  28  through the diffusion-limited layer  23 . 
     The first and second pump electrodes  25 A and  25 B are electrodes formed of material having low reducing performance (for example, an alloy of gold and platinum), respectively. The first pump electrode  25 A is positioned on one of opposite surfaces of the second solid electrolyte layer  21 B (that is, a surface of the second solid electrolyte layer  21 B which defines the interior space  28 ). The second pump electrode  25 B is positioned on the other surface of the second solid electrolyte layer  21 B (that is, a surface of the second solid electrolyte layer  21 B which defines the second atmospheric air introduction passage  27 B). The first pump electrode  25 A, the second pump electrode  25 B, and the second solid electrolyte layer  21 B form the pump cell  25 . 
     The sensor  20  is configured to be able to apply the direct voltage from the pump cell voltage source  25 C to the pump cell  25  (in particular, to the second pump electrode  25 B so as to produce an electric potential difference with respect to the first pump electrode  25 A). It should be noted that the first pump electrode  25 A is a cathode side electrode, and the second pump electrode  25 B is an anode side electrode when the pump cell voltage source  25 C applies the direct voltage to the pump cell  25 . 
     When the voltage is applied to the pump cell  25 , and the oxygen in the interior space  28  contacts the first pump electrode  25 A, the oxygen becomes the oxygen ion on the first pump electrode  25 A and then, the oxygen ion moves toward the second pump electrode  25 B through the second solid electrolyte layer  21 B. At this time, the electric current proportional to the amount of the oxygen ion, which has moved through the second solid electrolyte layer  21 B, flows between the first and second pump electrodes  25 A and  25 B. Then, when the oxygen ion reaches the second pump electrode  25 B, the oxygen ion becomes the oxygen on the second pump electrode  25 B and then, is discharged to the second atmospheric air introduction passage  27 B. Therefore, the pump cell  25  can discharge the oxygen from the exhaust gas to the atmosphere by a pumping function, thereby decreasing the oxygen concentration in the interior space  28 . An ability of the pumping function of the pump cell  25  increases as the voltage applied to the pump cell  25  from the pump cell voltage source  25 C increases. 
     The first and second sensor electrodes  26 A and  26 B are electrodes formed of material having high reducing performance (for example, platinum group element such as platinum and rhodium or alloy of the platinum group element). The first sensor electrode  26 A is positioned on one of opposite surfaces of the first solid electrolyte layer  21 A (that is, a surface of the first solid electrolyte layer  21 A which defines the Interior space  28 ). Therefore, the first sensor electrode  26 A exposes to the interior space  28 . The second sensor electrode  26 B is positioned on the other surface of the first solid electrolyte layer  21 A (that is, a surface of the first solid electrolyte layer  21 A which defines the first atmospheric air introduction passage  27 A). The first sensor electrode  26 A, the second sensor electrode  26 B, and the first solid electrolyte layer  21 A form the sensor cell  26 . 
     The sensor  20  is configured to be able to apply the voltage from the sensor cell voltage source  26 C to the sensor cell  26  (in particular, to the second sensor electrode  26 B so as to produce an electric potential difference with respect to the first sensor electrode  26 A). The sensor cell voltage source  26 C is configured to apply the direct voltage to the sensor cell  26 . It should be noted that the first sensor electrode  26 A is a cathode side electrode, and the second sensor electrode  26 B is an anode side electrode when the sensor cell voltage source  26 C applies the direct voltage to the sensor cell  26 . 
     The protection layer  29  is a porous layer formed of material including at least one of lanthanum (La), calcium (Ca), and magnesium (Mg). The exhaust gas can flow through the protection layer  29 . The protection layer  29  is provided such that the protection layer  29  covers an outer surface of the first alumina layer  22 A, end surfaces of the diffusion-limited layer  23 , end surfaces of the first solid electrolyte layer  21 A, end surfaces of the second solid electrolyte layer  21 B, end surfaces of the first alumina layer  22 A, end surfaces of the second alumina layer  22 B, end surfaces of the third alumina layer  22 C, end surfaces of the fourth alumina layer  22 D, end surfaces of the fifth alumina layer  22 E, end surfaces of the sixth alumina layer  22 F, and an outer surface of the sixth alumina layer  22 F. 
     The protection layer  29  prevents the condensed water included in the exhaust gas from adhering to the first solid electrolyte layer  21 A, the second solid electrolyte layer  21 B, the alumina layers  22 A to  22 F, and the diffusion-limited layer  23 , thereby preventing the first solid electrolyte layer  21 A, the second solid electrolyte layer  21 B, the alumina layers  22 A to  22 F, and the diffusion-limited layer  23  from cracking. In addition, the protection layer  29  traps the components included in the exhaust gas, which components may deteriorate the sensor  20 , thereby preventing the sensor  20  from deteriorating. 
     When the voltage is applied to the sensor cell  26 , and the SOx in the interior space  28  contacts the first sensor electrode  26 A, the SOx decomposes on the first sensor electrode  26 A, the oxygen component of the SOx becomes the oxygen ion and then, the oxygen ion moves toward the second sensor electrode  26 B through the first solid electrolyte layer  21 A. At this time, the electric current proportional to the amount of the oxygen ion, which has moved through the first solid electrolyte layer  21 A, flows between the first and second sensor electrodes  26 A and  26 B. When the oxygen ion reaches the second sensor electrode  26 B, the oxygen ion becomes the oxygen on the second sensor electrode  26 B and then, is discharged to the atmospheric air introduction passage  27 A. 
     The heater  24 , the pump cell voltage source  25 C, the sensor cell voltage source  26 C, the pump cell ammeter  25 D, the sensor cell ammeter  26 D, and the sensor cell voltmeter  26 E are electrically connected to the ECU  90 . 
     The ECU  90  controls an activation of the heater  24  to maintain a temperature of the sensor cell  26  at the sensor activating temperature, at which the sensor  20  is activated. 
     In addition, the ECU  90  controls the voltage of the pump cell voltage source  25 C to apply the voltage set as described later to the pump cell  25  from the pump cell voltage source  25 C. 
     In addition, the ECU  90  controls the voltage of the sensor cell voltage source  26 C to apply the voltage set as described later to the sensor cell  26  from the sensor cell voltage source  26 C. 
     The pump cell ammeter  25 D detects a current Ipp flowing through a circuit including the pump cell  25  and outputs a signal representing the detected current Ipp to the ECU  90 . The ECU  90  acquires the current Ipp on the basis of the signal. Hereinafter, the current Ipp will be referred to as “the pump current Ipp”. 
     The sensor cell ammeter  26 D detects a current Iss flowing through a circuit including the sensor cell  26  and outputs a signal representing the detected current Iss to the ECU  90 . The ECU  90  acquires the current Iss on the basis of the signal. Hereinafter, the current Iss will be referred to as “the sensor current Iss”. 
     The sensor cell voltmeter  26 E detects a voltage Vss applied to the sensor cell  26  and outputs a signal representing the detected voltage Vss to the ECU  90 . The ECU  90  acquires the voltage Vss on the basis of the signal. Hereinafter, the voltage Vss will be referred to as “the sensor voltage Vss”. 
     &lt;Summary of Operation of Second Embodiment Apparatus&gt; 
     &lt;Acquisition of Exhaust SOx Concentration&gt; 
     Similar to the sensor  10 , the inventors of this application have obtained following knowledge about the sensor current Iss in the sensor  20 . While the sensor voltage Vss decreases from 0.8 V to 0.2 V after the sensor voltage Vss increases from 0.4 V to 0.8 V with the voltage Vpp capable of reducing the oxygen concentration in the interior space  28  to zero (or generally zero) being applied to the pump cell  25 , the peak current Ipeak appears. In addition, the peak current difference dIss which is the difference between the reference current Iref and the peak current Ipeak (dIss=Iref−Ipeak) increases as the exhaust SOx concentration increases. 
     Further, also in the sensor  20 , the exhaust gas flows into the interior space  28  of the sensor  20  through the protection layer  29  and the diffusion-limited layer  23 . The SOx may adhere to the protection layer  29  and the diffusion-limited layer  23  and thus, when the second embodiment apparatus increases the sensor voltage Vss for acquiring the exhaust SOx concentration, the SOx may remove from the protection layer  29  and the diffusion-limited layer  23  and flow into the interior space  28 . Therefore, while the sensor voltage Vss decreases after the sensor voltage Vss increases, the sensor current Iss may not represent the exhaust SOx concentration exactly. 
     Accordingly, the second embodiment apparatus executes a constant voltage control for controlling the sensor voltage Vss to maintain the sensor voltage Vss at 0.4 V with the voltage Vpp capable of reducing the oxygen concentration in the interior space  28  to zero (or generally zero) being applied to the pump cell  25 . The second embodiment apparatus acquires the sensor currents Iss while the second embodiment apparatus executes the constant voltage control. The second embodiment apparatus stores the acquired sensor currents Iss in the RAM. 
     When the exhaust SOx concentration is requested to be acquired, and the engine operation is in the steady operation state or the idling operation state, the second embodiment apparatus executes the above-described first voltage control. After the second embodiment apparatus stops executing the first voltage control, the second embodiment apparatus executes the above-described second voltage control. 
     The second embodiment apparatus acquires the sensor current Iss and stores the acquired sensor current Iss as the SOx concentration current Iss_sox in the RAM while the second embodiment apparatus decreases the sensor voltage Vss from 0.8 V to 0.2 V in the second voltage control. After the second embodiment apparatus stops executing the second voltage control, the second embodiment apparatus acquires the peak current Ipeak from the stored SOx concentration currents Iss_sox. In addition, the second embodiment apparatus acquires, as the reference current Iref, the sensor current Iss stored in the RAM immediately before the second embodiment apparatus starts to execute the first voltage control. The second embodiment apparatus acquires, as the peak current difference dIss, the difference between the reference current Iref and the peak current Ipeak (dIss=Iref−Ipeak). 
     The second embodiment apparatus applies the acquired peak current difference dIss to a look-up table Map2Csox(dIss) to acquire the exhaust SOx concentration Csox. The look-up table Map2Csox(dIss) is prepared previously on the basis of experiments, etc. for determining a relationship between the peak current difference dIss and the exhaust SOx concentration in the sensor  20 . The exhaust SOx concentration Csox acquired from the look-up table Map2Csox(dIss) increases as the peak current difference dIss increases. 
     After the second embodiment apparatus stops executing the second voltage control, the second embodiment apparatus starts to execute the constant voltage control, thereby increasing the sensor voltage Vss from 0.2 V to 0.4 V and maintaining the sensor voltage Vss at 0.4 V. 
     Similar to the first embodiment apparatus, the second embodiment apparatus executes the first voltage control before the second embodiment apparatus executes the second voltage control. The second embodiment apparatus acquires the exhaust SOx concentration Csox on the basis of the sensor current Iss acquired while the second embodiment apparatus executes the second voltage control. Therefore, the second embodiment apparatus can acquire the exhaust SOx concentration accurately. 
     It should be noted that even when the sensor  20  does not include the protection layer  29 , the SOx adheres to the diffusion-limited layer  23  and thus, the second embodiment apparatus may be applied to a sensor which does not include the protection layer  29 . 
     &lt;Acquisition of Exhaust NOx Concentration&gt; 
     When the exhaust gas includes nitrogen oxide (hereinafter, will be referred to as “NOx”), the NOx is reduced by the sensor cell  26  with the sensor voltage Vss being maintained at 0.4 V and is decomposed to nitrogen and the oxygen. The oxygen produced by the NOx being decomposed, becomes the oxygen ion at the sensor cell  26 . The oxygen ion moves toward the second sensor electrode  26 B through the first solid electrolyte layer  21 A. 
     Even when the voltage Vpp capable of reducing the oxygen concentration in the interior space  28  to zero or generally zero, is applied to the pump cell  25 , the NOx included in the exhaust gas is unlikely to be reduced since the first and second pump electrodes  25 A and  25 B forming the pump cell  25  are made of the material having the low reduction property. In addition, when the voltage Vpp capable of reducing the oxygen concentration in the interior space  28  to zero or generally zero, is applied to the pump cell  25 , almost no oxygen is included in the exhaust gas reaching the sensor cell  26 . 
     Therefore, when the voltage Vpp capable of reducing the oxygen concentration in the interior space  28  to zero or generally zero, is applied to the pump cell  25 , and the sensor voltage Vss is maintained at 0.4 V, the sensor current Iss output in proportion to the amount of the oxygen ion moving through the first solid electrolyte layer  21 A, is proportional to a concentration of the NOx included in the exhaust gas just discharged from the engine  50 . Hereinafter, the concentration of the NOx will be referred to as “the NOx concentration”, and the concentration of the NOx included in the exhaust gas just discharged from the engine  50  will be referred to as “the exhaust NOx concentration”. There is a relationship shown in  FIG. 14  between the sensor current Iss and the exhaust NOx concentration. Therefore, the exhaust NOx concentration can be acquired by using the sensor current Iss. 
     Accordingly, the second embodiment apparatus executes a pump voltage control for applying the voltage Vpp capable of reducing the oxygen concentration in the interior space  28  to zero or generally zero to the pump cell  25  and the constant voltage control for controlling the sensor voltage Vss to 0.4 V. The second embodiment apparatus acquires the sensor current Iss as a NOx concentration current Iss_nox while the second embodiment apparatus executes the pump voltage control and the constant voltage control. Then, the second embodiment apparatus applies the NOx concentration current Iss_nox to a look-up table MapCnox(Iss_nox), thereby acquiring the exhaust NOx concentration Cnox. The look-up table MapCnox(Iss_nox) is prepared previously on the basis of experiments, etc. for determining a relationship between the sensor current Iss and the exhaust NOx concentration in the sensor  20 . The exhaust NOx concentration Cnox acquired from the look-up table MapCnox(Iss_nox) increases as the NOx concentration current Iss_nox increases. 
     &lt;Acquisition of Exhaust Oxygen Concentration&gt; 
     There is a relationship as shown in  FIG. 3  between the voltage Vpp applied to the pump cell  25  from the pump cell voltage source  25 C and the pump current Ipp. Accordingly, the second embodiment apparatus executes the pump voltage control for applying the voltage Vpp capable of reducing the oxygen concentration in the interior space  28  to zero or generally zero to the pump cell  25 . The second embodiment apparatus acquires the pump current Ipp as an oxygen concentration current Ipp_oxy while the second embodiment apparatus executes the pump voltage control. Then, the second embodiment apparatus applies the oxygen concentration current Ipp_oxy to a look-up table MapCoxy(Ipp_oxy), thereby acquiring the exhaust oxygen concentration Coxy. The look-up table MapCoxy(Ipp_oxy) is prepared previously on the basis of experiments, etc. for determining a relationship between the pump current Ipp and the exhaust oxygen concentration Coxy in the sensor  20 . The exhaust oxygen concentration Coxy acquired from the look-up table MapCoxy(Ipp_oxy) increases as the oxygen concentration current Ipp_oxy increases. Hereinafter, the voltage Vpp applied to the pump cell  25  from the pump cell voltage source  25 C will be referred to as “the pump voltage Vpp”. 
     Thereby, the second embodiment apparatus can acquire the exhaust oxygen concentration as well as the exhaust SOx concentration and the exhaust NOx concentration. 
     It should be noted that a relationship between the sensor voltage Vss and the sensor current Iss is the same as the relationship shown in  FIG. 3 . Therefore, the second embodiment apparatus may be configured to acquire the sensor current Iss as an oxygen concentration current Iss_oxy while the second embodiment apparatus controls the sensor voltage Vss to 0.4 V and the pump voltage Vpp to 0 V and apply the oxygen concentration current Iss_oxy to a look-up table MapCoxy(Iss_oxy), thereby acquiring the exhaust oxygen concentration Coxy. The exhaust oxygen concentration Coxy acquired from the look-up table MapCoxy(Iss_oxy) increases as the oxygen concentration current Iss_oxy increases. 
     &lt;Concrete Operation of Second Embodiment Apparatus&gt; 
     Next, a concrete operation of the second embodiment apparatus will be described. Similar to the first embodiment apparatus, the CPU of the ECU  90  of the second embodiment apparatus is configured or programmed to execute the routine shown in  FIG. 8  each time the predetermined time elapses. 
     When the CPU of the second embodiment apparatus executes the routine shown in  FIG. 8 , the CPU applies the peak current difference dIss to the look-up table Map2Csox(dIss) to acquire the exhaust SOx concentration Csox at the step  1050  in  FIG. 10 . 
     Further, the CPU of the second embodiment apparatus executes processes of steps  1550  to  1565  shown in  FIG. 15  in place of executing the processes of the steps  850  to  870  shown in  FIG. 8 . 
     In addition, the CPU of the second embodiment apparatus controls the activation of the pump cell voltage source  25 C to apply the pump voltage Vpp capable of reducing the oxygen concentration in the interior space  28  to zero (or generally zero) to the pump cell  25 . 
     When the value of the SOx concentration acquiring request flag Xsox is “0” at the time of executing the process of the step  810  in  FIG. 8 , the CPU of the second embodiment apparatus determines “No” at the step  810  and then, executes the processes of the steps  1550  to  1565  in  FIG. 15  described below. Also, when the engine operation is not in any of the steady operation state and the idling operation state at the time of executing the process of the step  815  in  FIG. 8 , the CPU of the second embodiment apparatus determines “No” at the step  815  and then, executes the processes of the steps  1550  to  1565  in  FIG. 15  described below. Then, the CPU of the second embodiment apparatus proceeds with the process to the step  895  in  FIG. 8  via the step  1095  in  FIG. 10  to terminate this routine once. 
     Step  1550 : The CPU of the second embodiment apparatus starts to execute the constant voltage control for controlling the sensor voltage Vss to 0.4 V when the CPU has not executed the constant voltage control. On the other hand, the CPU of the second embodiment apparatus continues to execute the constant voltage control when the CPU already executes the constant voltage control. 
     Step  1555 : The CPU of the second embodiment apparatus acquires the pump current Ipp as the oxygen concentration current Ipp_oxy and the sensor current Iss as the NOx concentration current Iss_nox. 
     Step  1560 : The CPU of the second embodiment apparatus applies the NOx concentration current Iss_nox to the look-up table MapCnox(Iss_nox) to acquire the exhaust NOx concentration Cnox. 
     Step  1565 : The CPU of the second embodiment apparatus applies the oxygen concentration current Ipp_oxy to the look-up table MapCoxy(Ipp_oxy) to acquire the exhaust oxygen concentration Coxy. 
     The concrete operation of the second embodiment apparatus has been described. The second embodiment apparatus can acquire the exhaust SOx concentration, the exhaust NOx concentration, and the exhaust oxygen concentration by executing the routine shown in  FIG. 8 . 
     It should be noted that the present invention is not limited to the aforementioned embodiment and various modifications can be employed within the scope of the present invention. 
     For example, the embodiment apparatuses execute the second voltage control after the embodiment apparatuses execute the first voltage control once and acquire the exhaust SOx concentration Csox by using the peak current Ipeak which the embodiment apparatuses acquire while the embodiment apparatuses execute the second voltage control. In this connection, the embodiment apparatuses may be configured to execute the second voltage control after the embodiment apparatuses execute the first voltage control twice and acquire the exhaust SOx concentration Csox by using the peak current Ipeak which the embodiment apparatuses acquire while the embodiment apparatuses execute the second voltage control. 
     Further, in the above-described embodiments, the sensor voltage Vss at the point of time of stopping increasing the sensor voltage Vss in the first voltage increasing control is 0.8 V, and the sensor voltage Vss at the point of time of stopping increasing the sensor voltage Vss in the second voltage increasing control is 0.8 V. Thus, the sensor voltage Vss at the point of time of stopping increasing the sensor voltage Vss in the first voltage increasing control and the sensor voltage Vss at the point of time of stopping increasing the sensor voltage Vss in the second voltage increasing control are the same. In this connection, the sensor voltage Vss at the point of time of stopping increasing the sensor voltage Vss in the first voltage increasing control and the sensor voltage Vss at the point of time of stopping increasing the sensor voltage Vss in the second voltage increasing control may be different from each other. 
     Further, in the above-described embodiments, the sensor voltage Vss at the point of time of stopping decreasing the sensor voltage Vss in the first voltage decreasing control is 0.2 V, and the sensor voltage Vss at the point of time of stopping decreasing the sensor voltage Vss in the second voltage decreasing control is 0.2 V. Thus, the sensor voltage Vss at the point of time of stopping decreasing the sensor voltage Vss in the first voltage decreasing control and the sensor voltage Vss at the point of time of stopping decreasing the sensor voltage Vss in the second voltage decreasing control are the same. In this connection, the sensor voltage Vss at the point of time of stopping decreasing the sensor voltage Vss in the first voltage decreasing control and the sensor voltage Vss at the point of time of stopping decreasing the sensor voltage Vss in the second voltage decreasing control may be different from each other. 
     Further, the embodiment apparatuses acquire the exhaust SOx concentration Csox by using the peak current difference dIss which is the difference between the reference current Iref and the peak current Ipeak. In this connection, the embodiment apparatuses may be configured to acquire the exhaust SOx concentration Csox by using the peak current Ipeak directly. In this case, the acquired exhaust SOx concentration Csox increases as the peak current Ipeak decreases. 
     Further, the embodiment apparatuses may be configured to acquire the exhaust SOx concentration Csox by using a changing amount of the sensor current Iss per unit time or a changing amount of the sensor current Iss per unit changing amount of the sensor voltage Vss while the embodiment apparatuses execute the second voltage decreasing control. In this case, the acquired exhaust SOx concentration Csox increases as the changing amount of the sensor current Iss per unit time increases. Also, the acquired exhaust SOx concentration Csox increases as the changing amount of the sensor current Iss per unit changing amount of the sensor voltage Vss increases. 
     Further, the embodiment apparatuses may be configured to execute the second voltage control several times, acquire the peak current Ipeak each time the embodiment apparatuses execute the second voltage control, and acquire the difference between an average Ipeak_ave of the peak currents Ipeak and the reference current Iref as the peak current difference diss (=Iref−Ipeak_ave).