Abstract:
A sheathed-type glow plug is provided, for example, for starting a self-igniting combustion engine, including a heating pin engaging in a combustion chamber having an ignitable fuel-air mixture, which includes an electrically conductive ceramic, and which may be heated to an ignition temperature by being connected to a voltage source, the sheathed-type glow plug surrounding an integrated temperature sensor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a sheathed-element glow plug, for example, for starting a self-igniting combustion engine. 
     BACKGROUND INFORMATION 
     It is believed that sheathed-element glow plugs are known. To start a self-igniting combustion engine, an initial ignition of a fuel-air mixture may be required, which may be supplied by sheathed-element glow plugs positioned in a wall of a combustion chamber. The sheathed-element glow plugs include a heating pin, which may contact the fuel-air mixture to be ignited. 
     The heating pin may be produced from electrically conductive ceramic. In this context, the heating pin may have a defined electrical resistance, so that a heating current will flow when the heating pin is connected to a voltage source, which may produce a specific temperature in heating the heating pin, and which may be sufficient to ignite the fuel-air mixture. 
     For monitoring and controlling the operation of the self-igniting combustion engine, it may be advantageous to determine the heating pin temperature. For this purpose, the temperature of the heating pin may be derived from a measurement of the heating current flowing through the heating pin. The electrically conductive ceramics, of which the heating pins may be made, may have a positive temperature coefficient. Thus, since increasing temperature causes the resistance to increase, the heating current decreases, given a constant supply voltage. From this, the instantaneous temperature of the heating pin may be determined from the time characteristic of the heating current. However, it is believed to be disadvantageous that the temperature distribution over the length of the heating pin may vary considerably at equal heating current. For example, the temperature distribution may be a function of a rotatory speed, a load condition and/or cooling of the combustion engine. Temperature differences of up to, for example, 200° C., may occur. 
     SUMMARY OF THE INVENTION 
     An exemplary sheathed-element glow plug according to the present invention permits a direct temperature measurement at the tip of a heating pin, without impairing the actual glowing function of the sheathed-element glow plug. Since the sheathed-element glow plug includes an integrated temperature sensor, the temperature of the heating pin may be determined both during active operation of the sheathed-element glow plug and during the passive set-up of the sheathed-element glow plug. This may permit an accurate measurement of the temperature, independently of the operating state of the self-igniting combustion engine. 
     In another exemplary embodiment according to the present invention, the temperature sensor is integrated directly into the heating pin. The heating pin may include, for example, a bore hole extending essentially axially, for accommodating the temperature sensor. The integration of the temperature sensor into the sheathed-element glow plug may be simple, and no additional construction space for the temperature sensor may be required, since the sensor is integrated inside the heating pin. 
     In yet another exemplary embodiment according to the present invention, the bore hole, which accommodates the temperature sensor, is positioned inside an insulating core of the heating pin, thereby permitting the temperature sensor to be positioned, without impairment of the actual glowing function of the heating pin. 
     In still another exemplary embodiment according to the present invention, the bore hole of the heating pin, which accommodates the temperature sensor, includes a groove with an open edge. This may permit the temperature sensor to be positioned directly adjacent to an outer circumferential wall of the heating pin, so that the temperature may be exactly measured, since the arrangement in the open edged recess obviates the need to consider the thermal transition resistance of the ceramic material of the heating pin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view through a first exemplary sheathed-element glow plug according to the present invention. 
     FIG. 2 is a schematic view of a temperature sensor. 
     FIG. 3 is a schematic sectional view through a heating pin. 
     FIG. 4 is a sectional view through a second exemplary sheathed-element glow plug according to the present invention. 
     FIGS. 5 &amp; 6 are schematic views of an exemplary heating pin according to the second exemplary embodiment variant of the present invention. 
     FIGS. 7 &amp; 8 are schematic views of another exemplary heating pin according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a sectional view through a first exemplary sheathed-element glow plug  10 , which may be used to start a self-igniting combustion engine. Sheathed-element glow plug  10  includes a plug housing  12 , which is essentially formed in the shape of a hollow cylinder and accommodates a heating pin  14 . Plug housing  12  may be sealingly mounted in a wall of a cylinder housing (not shown), so that heating pin  14  may extend into the combustion chamber. Heating pin  14  is electrically and conductively connected to a contact stud  18  via a contact spring  16 . Contact stud  18  may be connected to a voltage source, e.g., an automotive battery in a motor vehicle, so that a voltage may be applied to heating pin  14  via contact stud  18  and a contact element, such as contact spring  16 . Contact pin  14  may be made of, for example, a ceramic, electrically conductive material. Sheathed-element glow plug  10  includes further components, of which seals  20  and  22 , a ceramic sleeve  24 , a metal ring  26 , and a tension element  28  are marked. Sheathed-element glow plug  10  also includes an integrated temperature sensor  30 , which extends over essentially the entire length of sheathed-element glow plug  10  along a longitudinal axis  32 . 
     It is believed that the design and function of sheathed-element glow plugs are known and, as such, they are not described in greater detail. 
     During normal use of sheathed-element glow plug  10 , a voltage U is applied to heating pin  14 , which causes current I to begin to flow. The size of heating current I depends on the electrical resistance R of heating pin  14 , which may be designed so that it functions as a heating element (glow element). In this context, the distribution of electrical resistance R may vary over the length of heating pin  14 . For example, in the region of a heating pin tip  34 , a higher electrical resistance R may be concentrated, so that a higher voltage U drops lower, and heating inside heating pin tip  34  is greater than in the remaining region of heating pin  14 . 
     Since temperature sensor  30  is integrated into sheathed-element glow plug  10 , an instantaneous temperature may be ascertained directly in the region of heating pin tip  34 . 
     Temperature sensor  30  is schematically shown in FIG.  2 . Temperature sensor  30  may be made, for example, of a combination of two electrically conductive materials, which produce a voltage proportional to the temperature acting upon the temperature sensor  30 . For example, a thermoelement of platinum-platinum/rhodium may form temperature sensor  30 . This electrical conductor  36  is placed as a conductor loop inside temperature sensor  30  and may be connected to an evaluation circuit via outer contacts  38 . Temperature sensor  30  is made of an electrically nonconductive, temperature-stable ceramic, and includes a double capillary tube (not shown) for accommodating the conductor loops. Temperature sensor  30  is guided through contact stud  18  in an insulating manner. For this purpose, contact stud  18  has a bore hole  40  extending through the longitudinal extension of the sheathed-element glow plug. Since the outer circumference of temperature sensor  30  is made of electrically insulating ceramic, a short-circuit with contact stud  24  may be prevented, or at least made less likely. 
     Inside heating pin  14 , temperature sensor  30  extends directly into heating pin tip  34 . Heating pin  14  may be made of the electrically conductive ceramic, which surrounds an insulating core  42 , resulting in the formation of the U-shaped conductor loop of the electrically conductive ceramic material of heating pin  14 . Temperature sensor  30  is positioned inside insulating core  42 , or may itself form insulating core  42 , since the outer portion of temperature sensor  30  may have electrically insulating properties. The distance between temperature sensor  30  and the electrically conductive region of heating pin  14  may be, for example, about 0.2 mm. 
     FIG. 3 shows heating pin  14 , which has an accommodation  44  running along longitudinal centerline  32 , into which temperature sensor  30  may be inserted. Accommodation  44  extends to heating pin tip  34 . Accommodation  44  may be formed, for example, by a blind-end bore  45 . 
     Accommodation  44  may be formed, for example, when the ceramic is still a blank. This may avoid chipping (or the like) during the formation of accommodation  44 . 
     FIG. 4 shows a second exemplary sheathed-element glow plug  10  according to the present invention, the same parts as in FIG. 1 being given the same reference numerals. Except for the differences described below, the design and function of the second exemplary embodiment are similar to those of the first exemplary embodiment described above with respect to FIG.  1 . 
     As shown in FIG. 4, temperature sensor  30  is positioned inside heating pin  14  along an orientation deviating from longitudinal centerline  32 . The positioning of temperature sensor  30  is selected so that, with increasing approximation to heating pin tip  34 , the radial distance from longitudinal centerline  32  increases until temperature sensor  30  intersects circumferential surface  46  of heating pin  14 . In this regard, heating pin  14  is shown in FIGS. 5 through 8 in two different exemplary embodiments according to the present invention, respectively. 
     FIG. 5 shows a top view of the heating pin  14  shown in FIG. 4, as seen from the right. FIG. 6 shows a sectional view of FIG. 5 rotated by 90°. Accommodation  44 , for the accommodation of temperature sensor  30 , is formed by a bore hole  47 , which, starting from longitudinal centerline  32 , proceeds at an angle α from longitudinal centerline  32 . The angle α is selected so that, with respect to overall length  1  of heating pin  14 , bore hole  47  opens on circumferential surface  46  at about ½ the length, and changes to an open-edged recess  48 . The depth of open-edged recess  48  is adapted to the diameter of temperature sensor  30 , so that the latter does not radially protrude above circumferential surface  46  of heating pin  14 . 
     FIGS. 7 and 8 show a further exemplary embodiment according to the present invention, in which accommodation  44  is formed by a radial slit  50 , which over length  1  of heating pin  14 , declines in depth up to ½ the length and then forms recess  48  open at the edge, as shown in FIG.  6 . By forming slit  50 , the temperature sensor  30  may be set radially into heating pin  14 , whereas, according to the exemplary embodiment described above with respect to FIGS. 5 and 6, the temperature sensor  30  is threaded into bore hole  47 , so that it may be positioned into open-edge recess  48 . 
     Both bore hole  47  according to the exemplary embodiment described above with respect to FIGS. 5 and 6, groove  50  according to the exemplary embodiment described above with respect to FIGS. 7 and 8, and open-edge recess  48 , which is common to both exemplary embodiments, are positioned in a region of heating pin  14 , which is made of an insulating material. Heating pin  14  may be made of a layered construction, with an insulating ceramic embedded in the U-shaped conductor loop made of electrically conductive ceramic. Thus, impairment of the electrically conductive ceramic may be avoided, such as of the cross section of the electrically conductive layer. The temperature sensor  30  may be fastened in bore hole  47  or groove  50 , and open-edge recess  48  by glazing using a glass ceramic. In this context, the heat expansion behavior of this glass ceramic, the ceramic material of temperature sensor  30  and the insulating ceramic material of heating pin  14  may be adjusted to one another, so that, when the overall layer composite is heated, an essentially equal heat expansion behavior results.