Abstract:
A glow plug assembly includes an integrated, internal pressure sensor. In order to reduce loading on the center electrode, improve sensor responsiveness, and provide better thermal performance, the pressure sensor assembly is housed in a canister which forms a containment capsule and rigidly connects inside the glow plug shell near its seat area. The pressure sensor makes direct contact with the base end of the heater probe so that movements of the heater probe caused by fluctuations in gas pressure lead directly to changing force on the sensor stack.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    None. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a glow plug, and more particularly toward a glow plug having an integrated pressure sensing device for use in an internal combustion engine. 
         [0004]    2. Related Art 
         [0005]    Glow plugs are typically used in applications where a source of intense heat is required to either directly initiate or to aid in the initiation of combustion. As such, glow plugs are used in space heaters, industrial furnaces and diesel engines to name a few. 
         [0006]    In the field of compression ignition engines, there are trends toward ever greater output and efficiency, as well as toward the use of flexible fuels, which together have increased the demand for and usage of various types of combustion sensors necessary to enable enhanced control of the engine and combustion processes. Combustion sensors, particularly combustion pressure sensors, have in the past been discrete sensors that are inserted into the combustion chamber through special threaded bores created just to accommodate these sensors. The sensors themselves have generally been used only in engine and engine control development, and not in mass production owing to their high cost and the additional demands they place on space around the cylinder head. 
         [0007]    Several examples of glow plugs with integrated pressure sensors can be found in the prior art. A particular problem or concern with many, if not all, pressure sensing glow plug designs relates to the undesirable stresses introduced into the glow plug components, and particularly to the electrode itself, in order to adequately preload the pressure sensor. Joint strength between the electrode and heater probe components is quite often challenged by prior art designs which incorporate a pressure sensing device into the glow plug shell. 
         [0008]    One prior art example may be found in US Publication No. 2007/0095811 published May 3, 2007. According to this design, a pressure sensing unit is preinstalled on the heater probe and pretensioned through an external support tube that is subsequently joined to the glow plug shell at its upper end. A particular drawback of this arrangement lies in the way its flexible membrane element between the glow plug shell and heater probe (to accommodate pressure fluctuations) is compressed along its length. A further drawback of this design resides in the location of its sensor element which protrudes into the combustion gas relatively far away from the cylinder head seat, and is thus subject to rapid thermal shock. These features lead to reduced working life and less than optimal functionality. 
         [0009]    Taken as a whole, prior art glow plugs with integrated pressure sensors tend to place the center electrode or other force transmitting member in tension, with the shell components in compression.  FIG. 2  provides an illustration of one such prior art glow plug design. The joint between the center electrode and the heater probe needs a tensile strength which is not required in normal glow plug operations, and which is very difficult to achieve. Furthermore, preloads or pretensioning on the sensor must be high enough to ensure that load always stays on the sensor under all conditions, even as changes in the sensed pressure reduce the preload. Doubtless, some random examples do exist where the center electrode is not tensioned, such as in the above-noted US 2007/0095811. However, these examples are prone to distortions and other design defects. Prior art designs also have a certain minimum length required for all necessary components, and rely on forces transmitted through the long and thin center electrode which can give problems of thermal performance and reduced sensitivity. Furthermore, manufacturing issues related to the assembly of a sensor stack, i.e., the stack of components which together function as a sensor assembly, complicate the necessary electrical connections. 
         [0010]    Accordingly, there is a need for a glow plug with integrated pressure sensor that avoids placing unnecessary stress on the center electrode component, enables lower starting loads, better thermal performance, higher sensitivity, and does not require a strong bond from center electrode to heater probe. Furthermore, there is a need for such a glow plug and pressure sensor assembly that is more easily assembled in the context of high volume production. 
       SUMMARY OF THE INVENTION 
       [0011]    The subject invention addresses the shortcomings exhibited in prior art designs by providing a glow plug assembly for an internal combustion engine, wherein the assembly has an integrated internal pressure sensor. The assembly comprises a shell having an axially extending bore, and an elongated heater probe. The heater probe has a base end disposed within the bore in electrical contact with the shell. An electrode is in electrical contact with the base end of the heater probe while being electrically insulated from the shell. A pressure sensor is disposed within the shell. The pressure sensor is supported against the base end of the heater probe and is adapted to measure pressure fluctuations when the glow plug assembly is installed in an engine. A canister is disposed within the shell and surrounds the pressure sensor. The canister extends between first and second ends, with its first end operatively fixed to the shell while its second end is in pressing contact with the pressure sensor. The canister establishes a compressive preload force on the pressure sensor without transmitting transient distortions that may occur in the shell to the pressure sensor. 
         [0012]    According to another aspect of this invention, a method for manufacturing a glow plug assembly is provided. The method comprises the steps of: forming a shell having an axially extending bore, forming an elongated heater probe having a base end and a heating tip opposite the base end, supporting the base end of the heater probe within the bore of the shell so as to establish electrical conductivity between the shell and the heater probe, electrically connecting an electrode to the base end of the heater probe while maintaining electrical insulation between the electrode and the shell, providing a canister having first and second ends, attaching the first end of the canister to the shell, providing a pressure sensor, placing the pressure sensor inside the canister so that the pressure sensor rests against the base end of the heater probe, and compressing the pressure sensor with the second end of the canister to establish a preload force on the pressure sensor. 
         [0013]    The subject invention, addresses the prior art shortcomings in that it does not depend on the electrode to transmit forces to or from the pressure sensor. Rather, the electrode passes through the pressure sensor generally untouched. This is distinguished from prior art systems like that depicted in  FIG. 2 . According to this invention, the pressure sensor is effectively placed directly onto the base end of the heater probe and pressed against it using a canister which surrounds the pressure sensor to form a containment capsule. The canister is rigidly connected to the shell preferably very near to its seat. Because the heater probe moves in response to pressure fluctuations but the shell does not, movement of the heater probe leads directly to changing forces on the pressure sensor. As force increases with applied gas pressure, the initial preload force can be relatively low. The force carrying elements can thus have a short length and high cross-sectional area, giving high stiffness and hence a high degree of sensor sensitivity. The canister can be designed with a short length and, owning to its enclosed nature, gives additional benefits of good thermal performance due to reduced differences in thermal expansion, which further contributes to a low starting preload. Electrical connections can be brought out as an easily accessible, coaxial configuration if desired. Manufacturing is aided by the alignment inherent in the assembly being inside the canister. The connection of the canister to the shell of the glow plug is arranged, preferably, to be very close to the end of the shell, i.e., near the seat, thereby minimizing signal distortion due to changing forces in the external shell of the glow plug. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein: 
           [0015]      FIG. 1  is a side elevation view of a prior art glow plug assembly; 
           [0016]      FIG. 2  is a fragmentary cross-sectional view of a prior art glow plug assembly including an integrated pressure sensing device, wherein the center electrode is placed in tension when the sensor assembly is preloaded; 
           [0017]      FIG. 3  is a fragmentary perspective view of a glow plug assembly according to the subject invention shown in quarter-section; 
           [0018]      FIG. 4  is a partial cross-sectional view of the glow plug assembly of  FIG. 3 ; 
           [0019]      FIGS. 5A-D  depict an assembly operation wherein the subject glow plug is assembled; 
           [0020]      FIG. 6  is a glow plug assembly according to a first alternative embodiment of the subject invention; 
           [0021]      FIG. 7  is a cross-sectional view of a second alternative embodiment; 
           [0022]      FIG. 8  is a cross-section of a third alternative embodiment; and 
           [0023]      FIG. 9  is fragmentary perspective view of the electrode according to the third alterative embodiment shown in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, a glow plug according to the prior art is generally shown at  10  in  FIGS. 1 and 2 . The glow plug  10  includes an annular metal shell  12  having a bore  14  which extends along an imaginary longitudinal axis A. The shell  12  may be formed from any suitable metal, such as various grades of steel. The shell  12  may also incorporate a plating or coating layer, such as a nickel or nickel alloy coating over some or all of its surfaces including the exterior surface  16  and within the bore  14  so as to improve its resistance to high temperature oxidation and corrosion. The shell  12  includes external wrenching flats  18  or other suitably configured tool-receiving portion to advance screw threads  20  into an appropriately tapped hole in an engine cylinder head, pre-ignition chamber, intake manifold or the like. A tapered seat  22  bears against a complimentary-shaped pocket in the mating feature to perfect a pressure-tight seal in operation. 
         [0025]    The glow plug assembly  10  includes a heater probe, generally indicated at  24 . The heater probe  24  may be of the metallic or ceramic type. A metallic type heater probe  24  commonly includes a resistance heating element, powder packing material, and a seal. In the case of ceramic construction technology, the heater probe  24  will be constructed according to known ceramic designs. Regardless of a metallic or ceramic construction, the heater probe  24  will have a base end  26  ( FIG. 2 ), supported in the shell  12 , and a heating tip  28  opposite the base end  26 . An electrode  30  makes electrical contact with the base end  26  of the heater probe  24  while maintaining electrical isolation from the shell  12 . In the example of  FIG. 2 , the electrode  30  is formed with a tapering tip that seats within a mating socket formed in the base end  26  of the heater probe  24 . Other joint designs are known in the art and can be used with effectiveness in this invention providing they are properly configured. A pressure sensor, generally indicated at  32 , is disposed inside the shell  12  to form a fully integrated pressure sensing glow plug  10 . 
         [0026]    Referring still to  FIG. 2 , during assembly the electrode  30  is placed in tension to put the pressure sensor  32  into compression. Increased pressure acting on the heater probe  24  causes displacement of a flexible membrane  34  which allows movement of the center electrode  30 . This, in turn, moves an upper retainer  36  in an upward direction, which has the effect of reducing the preloaded compressive force on the pressure sensor  32 . Therefore, initial load, i.e., preload, in the electrode  30  must be enough to accommodate this fall in load plus any changes due to thermal effects in the pressure sensor assembly  32 . Considering the long length of this assembly and its open nature, which leads to greater thermal differences, thermal effects can be substantial. Therefore, a large initial preload is needed in practice. This has the undesirable effect of separating the joint between the base end  26  of the heater probe  24  and the electrode  30 . 
         [0027]    Referring now to  FIGS. 3 ,  4 , and  5 A-D a glow plug assembly according to the present invention is generally shown at  110 . In  FIGS. 3-5D , which illustrate one embodiment of the subject invention, reference numbers corresponding to those presented in  FIGS. 1 and 2 , but offset by  100 , are used as a matter of convenience. As shown in these views, a canister, generally indicated at  138 , is disposed within the shell  112  and surrounds the pressure sensor  132 . The canister  138  extends between first  140  and second  142  ends thereof, such that the first end  140  is operatively affixed to the shell  112 , whereas the second end  142  (acting through a cap member  146 ) is in pressing contact with the pressure sensor  132 . The canister  138  is effective to establish a compressive preload force on the pressure sensor  132  without transmitting transient distortions occurring in the shell  112  to the pressure sensor  132 . Furthermore, the canister  138  isolates the center electrode  130  from any preload forces, so that its connection to the heater probe  124  is not stressed by the preloading operation of the pressure sensor  132 . 
         [0028]    The shell  112  has an upper end adjacent its wrenching flats (not shown in  FIGS. 3-5D ) and a lower end adjacent the seat  122 . The flexible pressure-sensitive membrane  134  is adapted for exposure to pressure fluctuations when installed in an engine and is preferably disposed at the lower end of the shell  112 . The first end  140  of the canister  138  is directly joined to the shell  112  adjacent the pressure-sensitive membrane  134 . As perhaps best shown in  FIGS. 3 and 4 , the canister  138  includes a generally cylindrical sidewall  144  and a cap member  146  that extends inwardly from the sidewall  144 . The sidewall  144  is directly joined to the shell  112 , whereas the cap member  146  bears in pressing engagement against the top of the pressure sensor  132 . The cap member  146  can be brazed or welded to the sidewall  144 , as indicated by the weld line visible in  FIG. 3 . 
         [0029]    The specific joint design between the heater probe  124  and the electrode  130  can vary from one design to the next. In the disclosed embodiment, however, the heater probe  124  is shown including a probe contact  148  generally overlying its base end  126  for transmitting compressive preload forces from the pressure sensor  132  to the heater probe  124 . As can be seen therefore, the center electrode  130  establishes electrical contact and connection to the contact pad  148 , which in turn transmits electricity to the appropriate resistive elements contained within the heater probe  124 . 
         [0030]    The pressure-sensitive membrane  134  may take many forms, but in the preferred embodiment is integrally formed with a lower portion of the shell  112  such that it contains the annular seat  122 . The pressure-sensitive membrane  134  may also include a rim section  150  that extends upwardly from the seat  122  a short distance. The rim section  150  has a mating interface for coupling directly to the first end  140  of the canister  138 . In this example, the mating interface takes the form of a counter-bore which receives the first end  140  of the canister  138  in tight fitting, e.g., interference fit, manner. The pressure-sensitive membrane  134  also includes a thin flexible membrane section that extends radially inwardly from the rim section  150  to a sleeve portion  152 . The sleeve portion  152  directly engages the outer surface of the heater probe  124  for transferring pressure induced movements of the heater probe  124  into the flexible membrane section. 
         [0031]    Referring now to the pressure sensor  132 , several components are stacked or assembled together to form the overall pressure sensing device. These elements include a lower insulation pad  154  disposed between the probe contact pad  148  and the pressure sensor  132 . Similarly, an upper insulation pad  156  is disposed between the cap member  146  and the pressure sensor  132 . Respective upper  158  and lower  160  sensor contacts directly abut the respective upper  156  and lower  154  insulation pads, on opposite sides of the pressure sensor  132 . These contacts  158 ,  160  transmit electrical signals to and from the pressure sensor  132  for use in the engine management system, without touching either the charged electrode  130  or the grounded shell  112 . 
         [0032]    Because the canister  138  avoids placing any stress on the electrode  130  during the preload operation, there is no requirement that the electrode  130  be sufficiently rigid to carry compressive loads. Therefore, if desired, the electrode  130  may comprise a flexible cable, although a rigid electrode  130  is equally within the scope of design choice for this invention. Another advantage of this invention is realized by the closed format afforded by the canister  138 , thereby leading to much lower initial preloads being required and more even temperature characteristics. Because the canister  138  has a substantially larger cross-section and smaller length than the center electrode  130 , it is able to achieve higher measurement sensitivity than prior art designs which relied upon loads carried through the electrode. Furthermore, a reduction in the electrical noise in the system can be realized when the canister  138  acts as a grounded screen, via its direct connection to the grounded shell  112 . Also, connection of the canister  138  to the shell  112  at its lower end, close to the membrane  134 , means that changes in forces acting upon the shell  112  through the seat area  122  can be arranged to cause minimal changes in loads transferred to the pressure sensor  132 . 
         [0033]      FIGS. 5A-D  illustrate a possible assembly process for the glow plug assembly  110 . In this example,  FIG. 5A  shows the sidewall  144  portion of the canister  138  first attached to the rim section  150  of the pressure-sensitive membrane  134 , which forms part of the shell  112 . This connection can be accomplished by interference fit, welding, brazing or by other means. Also in this step, center electrode  130 , in the form of a flexible cable, is directly connected to the contact pad  148  of the heater probe  124 . Of course, if a rigid style electrode  130  is preferred, it can be used in place of the flexible cable.  FIG. 5B  shows the sensor components  154 ,  160 ,  132 ,  156 ,  158  assembled inside the sidewall  144 , on top of the probe contact pad  148 . In  FIG. 5C , the cap member  146  is placed on top of the sensor stack and force is applied to provide the correct preload to the sensor  132 . A rigid joint is made between the cap member  146  and the sidewall  144  of the canister  138 , such as by welding, brazing or by other means. In  FIG. 5D , the remaining electrical connections are made and the upper portion of the shell  112  is attached to the rim section  150  by an appropriate method such as welding or brazing. 
         [0034]      FIG. 6  shows a first alternative embodiment of the subject glow plug assembly  210 , wherein like or corresponding parts to those previously introduced are distinguished by the prefix  2 . In this example, the variation to the electrical connection is shown, wherein only a lower sensor contact  260  is used together with a lower insulator pad  254 . Both the upper insulator pad and upper sensor contact have been eliminated in this design, with electrical connection occurring directly through the cap member  246 . Furthermore, in this first alternative embodiment, the construction of the shell  212  is changed, with a flange  262  extending outwardly from the sidewall  244  of the canister  238 , and interposed between the rim section  250  and the upper portion of the shell  212 . 
         [0035]    A second alternative embodiment of the glow plug assembly is generally shown at  310  in  FIG. 7 . Like or corresponding parts are here identified by common reference numerals beginning with the number  3 . This second alternative embodiment is similar in many respects to the first alternative embodiment shown in  FIG. 6 , but in this instance the lower insulator pad and lower sensor contact have been eliminated. An upper insulator pad  356  and an upper sensor contact  358  are used. The canister  338  is integrated together with the pressure sensing membrane  334 , such that the sidewall  344  is formed integrally with the rim section  350 . The cap member  346  is attached to the sidewall  344  in a fashion similar to that described above. Of course, those of skill in the art will envision other configurations for the canister and its associated components without departing from the spirit of the invention. 
         [0036]      FIGS. 8 and 9  illustrate yet another, third alternative embodiment of this invention, generally indicated at  410 , with like or corresponding parts identified by familiar reference numerals beginning with  4 . In the case of this third alternative embodiment, electrical contacts to the pressure sensor  432  are not brought out of the canister  438 , but rather the electrode  430  is specially configured to route the necessary electrical connections. More specifically, the electrode  430  is provided with an insulated cover  464 . The electrode  430  in this example is rigid, although a flexible cable design may also be used. Here, the upper sensor contact  458  has a shaped wire configuration as shown in  FIG. 9 , and is supported in a groove on the outer surface of the cover  464 . Likewise, the lower sensor contact  460  is supported in a groove on the cover  464 . These sensor contacts  458 ,  460  have a bent circular configuration at the appropriate points of contact with respective upper  466  and lower  468  disk-like terminals positioned on opposite sides of the pressure sensor  432 . In this design, the canister  438  is also uniquely shaped. The sidewall  444  and cap member  446  are formed as an integral unity without a subsequent joining operation being required. Preloading is accomplished when the canister  438  is seated in the rim section  450  of the pressure sensitive membrane  434 , and joined thereto such as by interference fit, welding, brazing or other fixation technique. 
         [0037]    The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention, which is defined by the following claims.