Patent Publication Number: US-10763006-B2

Title: Ion probe

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2017-154105 filed on Aug. 9, 2017 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates generally to an ion probe, and relates more specifically to an ion probe including a metal wire, a metal sheath covering the metal wire, and insulation powder provided between the metal wire and the metal sheath. 
     2. Description of Related Art 
     As described in, for example, “Development of Combustion Behavior Analysis Technique in the Ultra High Engine Speed Range” written by Takehiko Kato et al. (DENSO Corporation) and published in DENSO Technical Review, Vol. 13, No. 1, pp. 58-63 (May, 2008) (hereinafter, simply referred to as “Development of Combustion Behavior Analysis Technique in the Ultra High Engine Speed Range”), ion probes are used to detect the flame propagation behavior in a combustion chamber of an engine. A flame is plasma. Therefore, when a flame reaches a distal end of an ion probe, an ion current flows through the ion probe, whereby the flame is detected. Therefore, as described in “Development of Combustion Behavior Analysis Technique in the Ultra High Engine Speed Range”, providing a plurality of ion probes in a combustion chamber makes it possible to detect the flame propagation behavior. 
     As described in “Development of Combustion Behavior Analysis Technique in the Ultra High Engine Speed Range”, in general, an ion probe is configured such that an insulation layer is provided between a metal wire having a distal end to be exposed to flames and a metal sheath covering the metal wire. The insulation layer is usually made of insulation powder. 
     SUMMARY 
     The inventor et al. have found the following issues related to ion probes. Insulation powder provided between a metal wire and a metal sheath absorbs moisture generated in a combustion chamber. Hence, repeated flame detection, that is, repeated use of an ion probe, reduces the insulation resistance between the metal wire and the metal sheath, resulting in reduction in the accuracy of flame detection. 
     In “Development of Combustion Behavior Analysis Technique in the Ultra High Engine Speed Range”, the insulation layer made of insulation powder is covered with a ceramic adhesive layer provided at a distal end of the ion probe, which is exposed to flames. However, the ceramic adhesive layer is porous, and thus moisture permeates through the ceramic adhesive layer. The insulation powder then absorbs the moisture, and therefore the foregoing issues cannot be effectively resolved. On the other hand, when an organic adhesive layer is used instead of a ceramic adhesive layer, burnout of the organic adhesive layer due to flames or deterioration of the organic adhesive layer due to heat may occur, and therefore the foregoing issues cannot be effectively resolved. 
     The disclosure provides an ion probe configured to restrain reduction in the insulation resistance between a metal wire and a metal sheath due to repeated use of the ion probe. 
     An aspect of the disclosure relates to an ion probe including: a metal wire; a metal sheath covering the metal wire; insulation powder provided between the metal wire and the metal sheath; and a ceramic capillary through which a portion of the metal wire projecting from a distal end of the metal sheath is passed. The ceramic capillary is bonded to the distal end of the metal sheath by an organic adhesive layer. A part of the insulation powder located at the distal end of the metal sheath is covered with the organic adhesive layer. 
     In the ion probe according to the above aspect of the disclosure, the ceramic capillary through which the portion of the metal wire projecting from the distal end of the metal sheath is passed is bonded to the distal end of the metal sheath by the organic adhesive layer. Thus, the organic adhesive layer is covered with the ceramic capillary. Therefore, even if the ion probe is repeatedly used, burnout of the organic adhesive layer due to flames and deterioration of the organic adhesive layer due to heat can be restrained. In addition, in the ion probe according to the above aspect of the disclosure, a part of the insulation powder located at the distal end of the metal sheath is covered with the organic adhesive layer. Thus, it is possible to restrain moisture from being absorbed into the insulation powder. That is, it is possible to restrain reduction in the insulation resistance between the metal wire and the metal sheath due to repeated use of the ion probe. 
     In the above aspect, an outer diameter of the ceramic capillary and an outer diameter of the metal sheath may be equal to each other. The feature that the outer diameter of the ceramic capillary and the outer diameter of the metal sheath are equal to each other includes not only a state where the outer diameter of the ceramic capillary and the outer diameter of the metal sheath are precisely equal to each other but also a state where the outer diameter of the ceramic capillary and the outer diameter of the metal sheath are substantially equal to each other. In this case, the state where the outer diameter of the ceramic capillary and the outer diameter of the metal sheath are substantially equal to each other can be regarded as the state where the outer diameter of the ceramic capillary and the outer diameter of the metal sheath are equal to each other, based on the technical knowledge in this technical field. With this configuration, it is possible to facilitate the assembly of the ion probe to a flame detection target member. 
     In the above aspect, the insulation powder may contain magnesium oxide powder. With this configuration, it is possible to increase the insulation resistance between the metal wire and the metal sheath. 
     In the above aspect, the portion of the metal wire projecting from the distal end of the metal sheath may be passed through a through-hole of the ceramic capillary. The through-hole may have an increased-diameter portion having a diameter that is increased toward a butt surface of the ceramic capillary, which butts against the distal end of the metal sheath. The organic adhesive layer may be disposed in the increased diameter portion. 
     The disclosure provides the ion probe configured to restrain reduction in the insulation resistance between the metal wire and the metal sheath due to repeated use of the ion probe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a circuit diagram of a flame detecting system including ion probes according to an embodiment; 
         FIG. 2  is a plan view illustrating an example of how ion probes are arranged in a combustion chamber of a cylinder head; 
         FIG. 3  is a longitudinal sectional view of the ion probe according to the embodiment; 
         FIG. 4  is a longitudinal sectional view of an ion probe according to a comparative example to be compared with the embodiment; 
         FIG. 5  illustrates graphs indicating a temporal change in an output voltage Vout of the ion probe according to the comparative example; 
         FIG. 6  illustrates graphs indicating a temporal change in an output voltage Vout of an ion probe according to an example of the embodiment; 
         FIG. 7  is a graph indicating the definition of a signal-to-noise (abbreviated as “SN”) ratio of each of the ion probe according to the example of the embodiment and the ion probe according to the comparative example; and 
         FIG. 8  is a graph indicating a change in the SN ratio of the ion probe according to the example of the embodiment and a change in the SN ratio of the ion probe according to the comparative example, with respect to the exhaust gas recirculation rate. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an example embodiment of the disclosure will be described in detail with reference to the accompanying drawings. Note that the disclosure is not limited to the following example embodiment. For the sake of clear understanding, the following description and the drawings will be provided in an appropriately simplified manner. 
     Example Embodiment 
     Configuration of Flame Detecting System 
     First, a flame detecting system including ion probes according to an example embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a circuit diagram of the flame detecting system including the ion probes according to the present embodiment. The flame detecting system illustrated in  FIG. 1  is used to detect a flame in a combustion chamber CC of an engine. As illustrated in  FIG. 1 , the flame detecting system includes ion probes  10 , a direct-current (DC) power source DPS, a resistor R 1 , a resistor R 2 , a capacitor C, and an output terminal OT. 
     As illustrated in  FIG. 1 , a first end of the DC power source DPS is connected to a first end of the capacitor C via the resistor R 1 . Further, a second end of the DC power source DPS is connected to a second end of the capacitor C via the resistor R 2 . That is, the resistor R 1 , the capacitor C, and the resistor R 2  are connected in series between both terminals of the DC power source DPS, thereby constituting a closed circuit. Because the capacitor C is connected in series to the DC power source DPS in this manner, normally, no current flows through the closed circuit. Thus, the first end of the capacitor C is at the same electric potential as that at the first end of the DC power source DPS, and the second end of the capacitor C is at the same electric potential as that at the second end of the DC power source DPS. 
     As illustrated in  FIG. 1 , the ion probe  10  is connected to the first end of the capacitor C to which the resistor R 1  is connected. The ion probe  10  includes a metal wire  11  and a metal sheath  12 . The metal wire  11  is connected to the first end of the capacitor C. Hence, a distal end of the metal wire  11  is at the same electric potential as that at the first end of the DC power source DPS. The distal end of the metal wire  11  projects from the metal sheath  12 , so that the distal end of the metal wire  11  is exposed to flames. When a flame reaches the distal end of the metal wire  11 , an ion current flows through the metal wire  11 . Although an output voltage of the DC power source DPS, that is, a voltage applied to the metal wire  11 , is not limited to any specific voltages, it may be, for example, approximately 300 V or approximately −300 V. 
     The metal sheath  12  is a protective tube that covers the metal wire  11 . The metal sheath  12  is electrically insulated from the metal wire  11 . The metal sheath  12  is inserted into a cylinder head CH so as to be in contact with the cylinder head CH. Hence, as illustrated in  FIG. 1 , the metal sheath  12  is electrically connected to the cylinder head CH that is grounded. That is, the metal sheath  12  is grounded via the cylinder head CH. The details of the configuration of the ion probe  10  according to the present embodiment will be described later. 
     As illustrated in  FIG. 1 , an output voltage Vout, that is, a voltage across both terminals of the resistor R 2 , is output from the output terminal OT. As described above, normally, no current flows through the resistor R 2  and thus the output voltage Vout is 0V. When a flame reaches the distal end of the metal wire  11  and an ion current flows through the metal wire  11 , a current temporarily flows also through the resistor R 2 . As a result, the output voltage Vout temporarily fluctuates, so that a peak appears in the output voltage Vout. 
     As described above, the flame detecting system illustrated in  FIG. 1  can detect a flame, because a peak appears in the output voltage Vout when the flame reaches the distal end of the metal wire  11  of the ion probe  10 . 
       FIG. 2  is a plan view illustrating an example of how the ion probes  10  are arranged in the combustion chamber CC of the cylinder head CH. Specifically,  FIG. 2  is a plan view of the cylinder head CH as viewed from its mating surface-side. The mating surface of the cylinder head CH is a surface thereof to be fitted to a cylinder block. As illustrated in  FIG. 2 , a combustion chamber CC having a circular shape in a plan view is provided at a center portion of the cylinder head CH. The combustion chamber CC is, for example, a pent-roof combustion chamber. In the combustion chamber CC, two intake valves IN 1 , IN 2  and two exhaust valves EX 1 , EX 2  are provided such that the intake valve IN 1  and the exhaust valve EX 2  are opposed to each other with respect to a central axis of the combustion chamber CC and the intake valve IN 2  and the exhaust valve EX 1  are opposed to each other with respect to the central axis of the combustion chamber CC. A spark plug SP is provided at the center of the combustion chamber CC. In the example illustrated in  FIG. 2 , twelve ion probes  10  are disposed such that the distal ends thereof are arranged at equal intervals along the circumference of the combustion chamber CC. Providing a plurality of the ion probes  10  in the combustion chamber CC in this manner makes it possible to detect the flame propagation behavior in the combustion chamber CC. 
     Configuration of Ion Probe According to Present Embodiment 
     Next, with reference to  FIG. 3 , the configuration of the ion probe  10  according to the present embodiment will be described in detail.  FIG. 3  is a longitudinal sectional view of the ion probe  10  according to the present embodiment. As illustrated in  FIG. 3 , the ion probe  10  according to the present embodiment includes the metal wire  11 , the metal sheath  12 , insulation powder  13 , a ceramic capillary  14 , and an organic adhesive layer  15 . 
     The metal wire  11  is a core wire of the ion probe  10 . For example, a nickel-chrome (i.e., nichrome) wire having a diameter of approximately 0.3 mm may be used as the metal wire  11 . As illustrated in  FIG. 3 , the distal end of the metal wire  11  projects from the metal sheath  12  and the ceramic capillary  14 , and the distal end of the metal wire  11  is disposed in the combustion chamber CC so as to be exposed to flames when the ion probe  10  is used. When a flame reaches the distal end of the metal wire  11 , an ion current flows through the metal wire  11 . 
     The metal sheath  12  is a protective tube that covers the metal wire  11 . For example, a metal tube having a diameter of approximately 1 mm may be used as the metal sheath  12 . As illustrated in  FIG. 3 , the metal sheath  12  is inserted into the cylinder head CH when the ion probe  10  is used. The insulation powder  13  is provided between the metal wire  11  and the metal sheath  12  (i.e., a space between the metal wire  11  and the metal sheath  12  is filled with the insulation powder  13 ), so that the metal sheath  12  and the metal wire  11  are insulated from each other by the insulation powder  13 . For example, magnesium oxide (MgO) powder, which has a high resistance and a high electric insulation performance, may be used as the insulation powder  13 . 
     The ceramic capillary  14  is a protective tube made of ceramic. The ceramic capillary  14  covers a portion of the metal wire  11 , which projects from a distal end of the metal sheath  12 . As illustrated in  FIG. 3 , the portion of the metal wire  11 , which projects from the distal end of the metal sheath  12 , is passed through the ceramic capillary  14 . In order to make it easier to pass the metal wire  11  through the ceramic capillary  14 , the diameter of a through-hole of the ceramic capillary  14  is gradually increased toward a butt surface thereof, which butts against the distal end of the metal sheath  12 . That is, the through-hole of the ceramic capillary  14  has an increased-diameter portion having a diameter that is increased toward the butt surface of the ceramic capillary  14 . The ceramic capillary  14  is inserted into the cylinder head CH when the ion probe  10  is used. For example, in order to facilitate the assembly of the ceramic capillary  14  and the metal sheath  12  to a flame detection target member, such as the cylinder head CH, the ceramic capillary  14  may preferably have substantially the same diameter as that of the metal sheath  12 . 
     As illustrated in  FIG. 3 , the ceramic capillary  14  is bonded to the distal end of the metal sheath  12  by an organic adhesive layer  15 . In this case, for example, a part of the insulation powder  13  located at the distal end of the metal sheath  12  is covered with the organic adhesive layer  15  having a water-proof property. That is, the part of the insulation powder  13  which is covered with the organic adhesive layer  15  is located at the distal end of the metal sheath  12 . Hence, it is possible to restrain moisture generated in the combustion chamber CC from being absorbed into the insulation powder  13 . Further, the organic adhesive layer  15  is covered with the ceramic capillary  14 . Thus, it is possible to restrain burnout of the organic adhesive layer  15  due to flames and deterioration of the organic adhesive layer  15  due to heat, even if flame detection is repeatedly performed. 
     Thus, even if flame detection is repeatedly performed, the organic adhesive layer  15  continuously restrains the moisture from being absorbed into the insulation powder  13 . That is, it is possible to restrain the reduction in the insulation resistance between the metal wire  11  and the metal sheath  12  due to repeated use of the ion probe  10 , thereby restraining the reduction in the accuracy of flame detection. 
     Configuration of Ion Probe According to Comparative Example 
     Next, the configuration of an ion probe according to a comparative example to be compared with the present embodiment will be described with reference to  FIG. 4 .  FIG. 4  is a longitudinal sectional view of the ion probe according to the comparative example to be compared with the present embodiment. As illustrated in  FIG. 4 , the ion probe according to the comparative example includes the metal wire  11 , the metal sheath  12 , and the insulation powder  13 . However, the ion probe according to the comparative example does not include the ceramic capillary  14  and the organic adhesive layer  15 , which are included in the ion probe  10  according to the present embodiment illustrated in  FIG. 3 . 
     As illustrated in  FIG. 4 , in the ion probe according to the comparative example, a part of the insulation powder  13  located at the distal end of the metal sheath  12  is exposed, and thus the insulation powder  13  absorbs moisture generated in the combustion chamber CC. Consequently, due to repeated use of the ion probe, the insulation resistance between the metal wire  11  and the metal sheath  12  is reduced, resulting in the reduction in the accuracy of flame detection. 
     Temporal Change in Output Voltage of Ion Probe According to Comparative Example 
     Hereinafter, a temporal change in the output voltage of the ion probe according to the comparative example illustrated in  FIG. 4  will be described with reference to  FIG. 5 .  FIG. 5  illustrates graphs indicating a temporal change in the output voltage Vout of the ion probe according to the comparative example. An upper graph indicates a temporal change in the output voltage Vout in a first flame detection test. A lower graph indicates a temporal change in the output voltage Vout in a twelfth flame detection test. In each of the upper and lower graphs, the abscissa axis represents time, and the ordinate axis represents the output voltage Vout and the output from a pressure sensor configured to detect a pressure inside the combustion chamber CC. As illustrated in  FIG. 5 , setting a threshold for the output voltage Vout of the ion probe makes it possible to determine whether a flame has reached the distal end of the metal wire  11  and to determine whether the flame has receded from the distal end of the metal wire  11 . In addition, comparing the upper and lower graphs in  FIG. 5  with each other makes it possible to check a temporal change in the output voltage Vout of the ion probe according to the comparative example. 
     As indicated in the upper graph in  FIG. 5 , in the first flame detection test, the peaks in the output from the pressure sensor substantially coincided with the peaks in the output voltage Vout. At this time, the insulation resistance between the metal wire  11  and the metal sheath  12  was 29 MΩ. 
     On the other hand, as indicated in the lower graph in  FIG. 5 , in the twelfth flame detection test, the width of each peak in the output voltage Vout was greater than that in the first flame detection test, and the peaks were lost in the noise. Therefore, the accuracy of flame detection was obviously lower than that in the upper graph. Specifically, the value of the output voltage Vout was higher than the threshold even after each peak in the output from the pressure sensor. This indicates that the flame did not recede from the distal end of the metal wire  11 , so that it was not possible to accurately detect a flame. At this time, the insulation resistance between the metal wire  11  and the metal sheath  12  was reduced to 0.13 MΩ. As described above, in the ion probe according to the comparative example, the insulation resistance between the metal wire  11  and the metal sheath  12  was reduced due to repeated use of the ion probe, so that the accuracy of flame detection was reduced. 
     Temporal Change in Output Voltage of Ion Probe According to Present Example 
     Hereinafter, a temporal change in the output voltage of the ion probe  10  according to an example of the present embodiment illustrated in  FIG. 3  (hereinafter, referred to as “present example”) will be described with reference to  FIG. 6 .  FIG. 6  illustrates graphs indicating a temporal change in the output voltage Vout of the ion probe  10  according to the present example. An upper graph indicates a temporal change in the output voltage Vout in a first flame detection test. A lower graph indicates a temporal change in the output voltage Vout in a twenty-fifth flame detection test. In each of the upper and lower graphs, the abscissa axis represents time, and the ordinate axis represents the output voltage Vout and the output from a pressure sensor configured to detect a pressure inside the combustion chamber CC. As illustrated in  FIG. 6 , setting a threshold for the output voltage Vout of the ion probe  10  makes it possible to determine whether a flame has reached the distal end of the metal wire  11  and to determine whether the flame has receded from the distal end of the metal wire  11 . In addition, comparing the upper and lower graphs in  FIG. 6  with each other makes it possible to check a temporal change in the output voltage Vout of the ion probe  10  according to the present example 
     As indicated in the upper graph in  FIG. 6 , in the first flame detection test, the peaks in the output from the pressure sensor substantially coincided with the peaks in the output voltage Vout. At this time, the insulation resistance between the metal wire  11  and the metal sheath  12  was equal to or higher than 2000 MΩ, which is a measurement limit. As indicated in the lower graph in  FIG. 6 , in the twenty-fifth flame detection test as well as in the first flame detection test, the peaks in the output from the pressure sensor substantially coincided with the peaks in the output voltage Vout. At this time, the insulation resistance between the metal wire  11  and the metal sheath  12  was maintained to be equal to or higher than 2000 MΩ, which is the measurement limit. 
     As described above, in the ion probe  10  according to the present example, it is possible to restrain the reduction in the insulation resistance between the metal wire  11  and the metal sheath  12  due to repeated use of the ion probe  10 , thereby restraining the reduction in the accuracy of flame detection. 
     SN Ratio of Ion Probe According to Present Example and SN Ratio of Ion Probe According to Comparative Example 
     Next, a signal-to-noise (abbreviated as “SN”) ratio of the ion probe  10  according to the present example and a SN ratio of the ion probe according to the comparative example will be described with reference to  FIG. 7  and  FIG. 8 .  FIG. 7  is a graph indicating the definition of the SN ratio of each of the ion probe  10  according to the present example and the ion probe according to the comparative example.  FIG. 8  is a graph indicating a change in the SN ratio of the ion probe  10  according to the present example and a change in the SN ratio of the ion probe according to the comparative example, with respect to the exhaust gas recirculation (abbreviated as “EGR”) rate. 
     First, the definition of the SN ratio of each of the ion probe  10  according to the present example and the ion probe according to the comparative example will be described with reference to  FIG. 7 . As illustrated in  FIG. 7 , a peak voltage with respect to an average of the background noise (abbreviated as “BGN”) of the output voltage Vout of the ion probe is defined as a signal S. A value obtained by tripling a standard deviation σ of the BGN, that is, 3σ, is defined as noise N. 
     Next, with reference to  FIG. 8 , description will be provided on a change in the SN ratio of the ion probe  10  according to the present example and a change in the SN ratio of the ion probe according to the comparative example, with respect to the EGR rate. As illustrated in  FIG. 8 , at every EGR rate, the SN ratio of the ion probe  10  according to the present example was higher than the SN ratio of the ion probe according to the comparative example, and thus the accuracy of flame detection in the present example was higher than that in the comparative example. As the EGR rate increases, the ion density in a flame is reduced and therefore the peak voltage of the output voltage Vout is reduced and the SN ratio is also reduced. 
     As illustrated in  FIG. 5 , in the ion probe according to the comparative example, even in the first flame detection test, the insulation resistance between the metal wire  11  and the metal sheath  12  was 29 MΩ. In contrast to this, as illustrated in  FIG. 6 , in the ion probe  10  according to the present example, the insulation resistance between the metal wire  11  and the metal sheath  12  was equal to or higher than 2000 MΩ. As described above, the ion probe  10  according to the present example has a higher insulation resistance than that of the ion probe according to the comparative example. Therefore, it can be considered that the SN ratio of the ion probe  10  according to the present example is higher than that of the ion probe according to the comparative example. 
     The disclosure is not limited to the foregoing embodiment, and various changes and modifications may be made to the foregoing embodiment as appropriate within the technical scope of the disclosure.