Patent Publication Number: US-2011056925-A1

Title: Pressure Measuring Glow Plug

Description:
The invention is directed to a glow plug having the features given in the preamble of claim  1 . A glow plug of this type is known from DE 10 2004 044 727 A1 and includes a pressure measuring device for measuring combustion chamber pressure, and therefore glow plugs of this type are also referred to as pressure measuring glow plugs. 
     In the case of the glow plug known from DE 10 2004 044 727 A1, a metallic sealing membrane in the form of a bellows is disposed between the glow plug body and the heating rod; the metallic sealing membrane prevents combustion gasses from escaping the combustion chamber through the plug housing. The metallic sealing membrane can easily withstand the temperatures of combustion gasses in the combustion chamber, and ensures good axial mobility of the heating rod, thereby enabling the combustion chamber pressure to be transferred via the heating rod to a pressure measuring device. 
     An object of the invention is to provide a way to more precisely measure combustion chamber pressure using a glow plug. 
     This problem is solved according to the invention by a glow plug having the features in claim  1 . Advantageous refinements of the invention are the subject matter of the dependent claims. 
     Within the scope of the invention it was recognized that the accuracy with which combustion chamber pressure can be measured using the pressure measuring device is limited by natural oscillations of the glow plug. Given that an annular or sleeve-shaped vibration damper is disposed between the glow plug body and the heating rod, radially acting natural oscillations of the glow plug can be damped, thereby increasing the measurement accuracy. Advantageously, a vibration damper can be used for damping and to shift the natural oscillations toward higher frequencies which are excited less easily and therefore have smaller oscillation amplitudes. 
     A vibration damper composed of perfluoroelastomers has good temperature resistance and an advantageously low friction coefficient. Therefore the axial mobility of the heating rod in the glow plug housing is hardly impaired at all by such a vibration damper. Perfluoro rubber is particularly well-suited. A well-suited perfluoro rubber is commercially available under the name Kalrez, 
     In the case of a glow plug according to the invention, in the particularly significant frequency range of 1 kHz to 20 kHz, in particular 3 kHz to 5 kHz, natural oscillations having a radial amplitude can be damped and shifted toward higher frequencies. 
     The vibration damper is preferably designed as an O-ring. Surprisingly, the natural oscillations of the heating rod can be clearly damped using such a simple measure. The vibration damper is preferably disposed behind the sealing membrane, as viewed from the combustion chamber, and is therefore protected by the sealing membrane against direct exposure to hot combustion gasses. The sealing membrane is preferably composed of metal and can be designed e.g. as a bellows-type sealing element or corrugated tube-type sealing element. 
     A natural oscillation can be damped particularly efficiently by situating the vibration damper on the antinodal point of the natural oscillation. The vibration damper is preferably situated on an antinodal point of the strongest radially acting harmonic. Surprisingly, this is typically the third or fourth natural oscillation, thereby enabling radial oscillatory motions of the heating rod to be damped particularly efficiently in that the vibration damper acts on an antinodal point of the third or fourth natural oscillation. 
     Preferably, the vibration damper encloses a constriction of the heating rod. The heating rod therefore has a greater thickness on either side of the constriction than at the constriction itself. Advantageously, the position of the vibration damper can therefore be easily specified. The constriction is preferably limited on one side by an annular surface that extends obliquely to the longitudinal direction of the heating rod. In this manner, a vibration damper can be easily slid into position. On its other side, the constriction is preferably limited by an annular surface that extends perpendicularly to the longitudinal direction of the heating rod. This perpendicular annular surface advantageously forms a stop for the vibration damper. It is preferable for the oblique surface to be situated on the side of the constriction that is closer to one end of the heating rod. 
     The heating rod is preferably enclosed by at least two vibration dampers. This measure has the advantage that it amplifies the damping effect. In particular, different natural oscillations can be damped efficiently by using a plurality of vibration dampers. The vibration dampers are preferably situated on the two strongest antinodal points of radially acting harmonics. 
     Within the scope of the present invention, a heating rod is understood to mean not only its heated section on its end near the combustion chamber but the entire rod which extends in the housing tube to the pressure measuring device, i.e. also a section of the inner pole that is not enclosed by a housing tube, and which can be used to supply a heating element in the heated section of the heating rod with current. 
    
    
     
       Further details and advantages of the invention are explained using embodiments, with reference to the attached drawings. Parts that are identical or similar are labeled using the same reference numerals. The drawings show: 
         FIG. 1 : an embodiment of a glow plug according to the invention, in a partially exposed view; 
         FIG. 2 : a detailed view of  FIG. 1 ; 
         FIG. 3 : a further detailed view of  FIG. 1 ; 
         FIG. 4 : a detailed view of a modified embodiment; and 
         FIG. 5 : a further embodiment of a glow plug according to the invention. 
     
    
    
     The glow plug depicted in  FIGS. 1 through 3  has a glow plug body  1  comprising an external thread  2  for threading into a threaded hole of a diesel engine, a heating rod  3  that is displaceable in the glow plug body  1  in axial direction, and comprises a heatable section  3   a  that has a reduced diameter, extends out of glow plug body  1  and, in the embodiment shown, extends to a housing  4  for electronic circuitry on the end of glow plug body  1  that normally faces away from the combustion chamber. 
     Heating rod  3 , which can be displaced in a sliding manner in the axial direction relative to glow plug body  1 , transfers a pressure that is present in the combustion chamber of a diesel engine to a pressure measuring device that is not depicted. The greater the combustion chamber pressure, the further heating rod  3  is pressed against a restoring force into the glow plug housing. The pressure-dependent displacement of heating rod  3  relative to glow plug body  1  is preferably between 0.05 μm/bar and 0.25 μm/bar. 
     In the embodiment shown, the pressure measuring device for measuring a combustion chamber pressure acting on the heating rod is disposed in housing  4  and can contain e.g. a piezo sensor or a strain gauge to measure pressure. The pressure measuring device can also be disposed in a cylindrical section of glow plug body  1  that is situated in front of housing  4 , as viewed from the combustion chamber. 
     A metallic sealing membrane  5  is disposed between glow plug body  1  and heating rod  3 ; metallic sealing membrane  5  is designed as a bellows-type sealing element and is shown in particular in the detailed view of  FIG. 2 . Sealing membrane  5  protects the pressure measuring device against direct exposure to combustion gasses in the combustion chamber while allowing heating rod  3  to move axially relative to glow plug body  1 . 
     To dampen radial oscillations of heating rod  3 , an annular or sleeve-shaped vibration damper  6  is disposed between glow plug body  1  and heating rod  3 . The damper  6  presses against heating rod  3  and glow plug body  1 . When an oscillation of the heating rod acts in the radial direction, the vibration damper is compressed, thereby damping vibrations. To achieve the greatest possible damping effect, vibration damper  6  is preferably situated on an antinodal point of the strongest radially acting harmonic. This is typically the third or fourth natural oscillation of heating rod  3 . 
     Vibration damper  6  is composed of a perfluoroelastomer, in particular of perfluoro rubber, and has a temperature resistance greater than 300° C., preferably greater than 320° C. A suitable material is commercially available under the name Kalrez. In the simplest case, vibration damper  6  can be designed as an O-ring. 
     One embodiment of vibration damper  6  is shown, in particular, in the detailed view of  FIG. 3 . Preferably, heating rod  3  includes a narrow point in which vibration damper  6  is disposed. The narrow point makes it easier to position vibration damper  6  during production. 
     To improve vibration damping, heating rod  3  can also be enclosed by one or more additional vibration dampers  6 . In that case, the vibration dampers are preferably situated on the two strongest antinodal points of radially acting harmonics. 
     A vibration damper  6  can also be placed on bellows-type sealing membrane  6 , as shown in  FIG. 4  as an example. 
     In the embodiment shown, sealing membrane  5  designed as a bellows is situated in front of external thread  2  of the glow plug, as viewed from the combustion chamber. In this manner, screw-in effects and temperature influences of the material of glow plug  3  can be largely eliminated. 
       FIG. 5  shows a further embodiment of a glow plug comprising a bellows-type sealing membrane  5  that is disposed on the end of glow plug body  1  near the combustion chamber. In this embodiment, vibration damper  6  is therefore disposed behind sealing membrane  5 , as viewed from the combustion chamber. A further difference from the embodiment presented in  FIGS. 1 through 4  is that the outer sleeve of heating rod  3  does not extend entirely to pressure measuring device  7 . Inner pole  3   b  of heating rod  3  is therefore visible in  FIG. 5 . A first vibration damper  6 , in the form of an O-ring, is disposed between inner pole  3   b  of heating rod  3  and glow plug body  1 , and a second vibration damper encloses bellows-type sealing membrane  5 . 
     Inner pole  3   b  is an electrical conductor that supplies a helical heating wire in heatable section  3   a  of heating rod  3  with current. To provide electrical insulation, inner pole  3   b  can be enclosed by a ceramic insulator e.g. magnesium or aluminum oxide. An earth connection of the helical heating wire can be realized using glow plug body  1  and the outside of the section of heating rod  3  that extends out of glow plug body  1 . 
     The signals from pressure measuring device  7  are directed via connecting lines to an electronic circuit in housing  4 . The signals that are received can be evaluated directly in the electronic circuit in housing  4 . It is not necessary for the signals to be evaluated in the glow plug; instead, they can be transmitted directly to an engine control unit. 
     REFERENCE NUMERALS 
     
         
           1  Glow plug body 
           2  External thread 
           3  Heating rod 
           3   a  Heatable section of the heating rod 
           3   b  Inner pole of the heating rod 
           4  Housing 
           5  Sealing membrane 
           6  Vibration damper 
           7  Pressure measuring device