Patent Publication Number: US-8981634-B2

Title: Spark plug with increased mechanical strength

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Ser. No. 61/645,020 filed on May 9, 2012, the entire contents of which are incorporated herein. 
    
    
     TECHNICAL FIELD 
     This invention generally relates to spark plugs and other ignition devices for internal combustion engines and, more particularly, to spark plugs with increased mechanical strength to withstand various axial and/or radial stresses 
     BACKGROUND 
     Spark plugs for vehicle engines are designed to seal the combustion chamber so that exhaust gases cannot vent directly into the atmosphere, but instead must pass through an appropriate vehicle exhaust system. 
     With reference to  FIGS. 1-1B , there is shown a cross-sectional view of a spark plug  10  having a conventional arrangement that includes a shell  12 , insulator  14 , center electrode assembly  16 , and ground electrode  18 . An external seal is established between shell  12  and the cylinder head (not shown) when the spark plug is installed and screwed into the cylinder head so that a conical shell seat or a separate external gasket  20  is compressed against a seat portion in the cylinder head. An internal seal, on the other hand, is established between insulator  14  and shell  12  and is typically achieved with a separate internal gasket or gasket ring  22 , which is located between a seat portion  30  of the shell and a shoulder portion  32  of the insulator. According to this design, internal gasket  22  is a tapered ring that contacts seat and shoulder portions  30 ,  32  with side surfaces  40 ,  42  of the gasket, respectively, as opposed to contacting such portions with end surfaces  44 ,  46  of the gasket. In order to ensure that the internal seal sufficiently seals or blocks off exhaust gases that are under pressure in the combustion chamber, the insulator, gasket ring and shell are usually pre-loaded or compressed in the axial direction so that a good seal is formed. Axially or compressively pre-loading these components, however, can introduce an axial stress AS into insulator  14 . 
     One area of insulator  14  that tends to be vulnerable to stress and breaking is the area of the insulator between positions B and C in  FIGS. 1-1B . This is particularly true if the axial stress AS from the pre-loading is coupled with a radial or bending stress RS that is exerted against the insulator core nose  36  in an area between positions A and B. A potential source of the radial stress RS is a pressure wave resulting from engine knock or other misfiring events. If the overall or combined stress (e.g., stresses AS+RS) exceeds the internal strength of insulator  14 , which is usually made from a somewhat brittle ceramic material, then the insulator can crack, break or otherwise fail. 
     SUMMARY 
     According to one aspect, there is provided a spark plug, comprising: a metallic shell having an internal surface with a seat portion; an insulator having an external surface with a shoulder portion and being at least partially located within the metallic shell; a gasket having upper and lower axial ends and being at least partially located between the metallic shell and the insulator; a center electrode being at least partially located within the insulator; and a ground electrode being attached to the metallic shell. The gasket upper axial end has a mating surface that contacts the insulator shoulder portion and the gasket lower axial end has a mating surface that contacts the shell seat portion so that the insulator and metallic shell are sealed together. 
     According to another aspect, there is provided a spark plug, comprising: a metallic shell having an internal surface with a seat portion; an insulator having an external surface with a shoulder portion and being at least partially located within the metallic shell; an annular cavity being formed between the metallic shell internal surface and the insulator external surface and being substantially enclosed; a gasket having upper and lower axial ends and being located within the substantially enclosed annular cavity; a center electrode being at least partially located within the insulator; and a ground electrode being attached to the metallic shell. The gasket is compressed in the axial direction between the insulator shoulder portion and the shell seat portion so that the gasket expands in the radial direction and presses against the insulator external surface and the shell internal surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred exemplary embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein: 
         FIG. 1  is a cross-sectional view of a spark plug with a conventional insulator and internal gasket arrangement, and  FIGS. 1A-1B  are enlarged insets of  FIG. 1 ; 
         FIG. 2  is a cross-sectional view of a spark plug with an exemplary insulator, shell and gasket arrangement that uses a generally cylindrical shaped gasket to improve the mechanical strength of the plug, and  FIGS. 2A-2B  are enlarged insets of  FIG. 2 ; 
         FIG. 3  is a cross-sectional view of a spark plug with another exemplary insulator, shell and gasket arrangement that uses a gasket with cylindrical and flange portions to improve the mechanical strength of the plug, and  FIGS. 3A-3B  are enlarged insets of  FIG. 3 ; 
         FIG. 4  is a cross-sectional view of a spark plug with an exemplary insulator, shell and gasket arrangement that uses a generally annular shaped gasket to improve the mechanical strength of the plug, and  FIGS. 4A-4B  are enlarged insets of  FIG. 4 ; and 
         FIGS. 5 and 6  are graphs showing the results of stress reduction using finite element analysis (FEA), where the  FIG. 5  graph is for a conventional spark plug and the  FIG. 6  graph is for one of the exemplary spark plugs of the present application. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The spark plug embodiments described below have particular configurations that increase the mechanical strength of the spark plug and help prevent breaking, cracking and/or other failures in the insulator. According to the exemplary arrangements shown in  FIGS. 2-4 , spark plug  50  includes a metallic shell  52 , insulator  54 , center electrode  56 , and ground electrode  58  and is designed such that the shell, the insulator and a gasket  62 ,  62 ′,  62 ″ work together to provide better support for the insulator against axial and/or radial stresses. This improved support offsets certain stresses, such as radial stress RS that can be exerted against the insulator core nose  66  when the engine experiences knocking or misfiring and can lead to cracking, breaking or other failures of the insulator. If insulator  54  is damaged, high-voltage electrical current may flow directly from center electrode assembly  56  to shell  52  during operation such that it bypasses the intended spark gap; this, in turn, can result in improper combustion and engine operation.  FIGS. 2-4  show three different potential embodiments, although others are certainly possible, and will be described in the following paragraphs. 
     Turning now to  FIGS. 2-2B , there is shown a spark plug  50  that has an insulator  54  with increased or improved radial support in between positions B and D, which corresponds to an area between seat portion  70  of the shell and shoulder portion  72  of the insulator where the insulator is sometimes prone to weakness. In this particular embodiment, gasket  62  is a sleeve-like cylindrical gasket that has upper and lower axial ends  80 ,  82  separated by an axial length X and inner and outer radial sides  84 ,  86  separated by a radial width Y. The sleeve-like design allows cylindrical gasket  62  to brace supported portion  68  across an axial length so that the insulator is supported at a location below shoulder portion  72 . The cylindrical gasket  62  may be aligned upright so that a cross-section of the cylindrical gasket has a longitudinal axis LB that is generally parallel to a longitudinal axis LA of the spark plug. 
     Upper and lower axial ends  80  and  82  may be angled or tapered and include mating surfaces so that they can tightly mate with corresponding angled surfaces of shoulder portion  72  and seat portion  70 , respectively. This arrangement—where gasket  62  is located within an annular cavity  90  formed between an internal surface  76  of the metallic shell and an external surface  78  of the insulator—can seal the insulator and the shell together and can also provide better support for the insulator for improved mechanical strength. As best illustrated in  FIG. 2B , the walls of the annular cavity  90  surround the entire gasket  62  so that the cavity is substantially enclosed; if the walls of the cavity contact all of the sides of the gasket, yet allow for a small opening, like opening  92 , this is deemed to be “substantially enclosed.” In this particular embodiment, upper and lower axial ends  80 ,  82  are angled in a generally parallel manner to one another and inner and outer radial sides  84 ,  86  are straight in a generally parallel manner to one another. This results in a cross-sectional shape of the gasket that is a parallelogram, however, other configurations are possible so long as an adequate seal is formed. As shown in  FIG. 2B , the mating surface of the upper axial end  80  can be angled and form an obtuse angle θ with inner radial side  84 , however, this is optional. 
     Inner and outer radial sides  84  and  86  of the gasket are designed to flushly contact and seal up against a supported portion  68  of the insulator and a supportive portion  64  of the shell, respectively. In the embodiment of  FIGS. 2-2B , both the supported and supportive portions  68 ,  64  are generally straight and parallel to one another, which results in the inner and outer radial sides  84 ,  86  of the gasket also being straight and parallel to one another, as well as being parallel to a longitudinal axis LA of the spark plug. The axial length X of gasket  62  is equal to or greater than its radial width Y so that it can act as an elongated supportive sleeve to brace insulator portion  68  in the radial direction; the insulator is sometimes most vulnerable or susceptible to radial bending and breaking in the area of supported portion  68 , which in this case is just above shell seat portion  70 . Gasket  62  may be comprised of any suitable spark plug seal or gasket material, including compressed glass/metal powder. 
     To achieve a strong radial support of the insulator in the area of supported portion  68 , the cylindrical gasket  62  may be radially press-fit between the insulator and shell at portions  64  and  68 . However, during engine operation the temperatures of the individual components of the spark plug increase differently and expand and contract at different rates, which can lead to a relaxation of the radial press-fit of gasket  62 . To counteract this phenomenon, it may be helpful to press-fit or otherwise assembly gasket  62  in a heated condition onto a cool insulator  54 . If gasket  62  is compressed in the axial direction between shoulder portion  72  and seat portion  70 , the inner and outer radial sides  84  and  86  can expand away from one another and press against supported portion  68  and supportive portion  64 , respectively. Other techniques may be employed as well. 
     The stress reduction effect of the exemplary spark plug design was modeled in a finite element analysis (FEA) and the comparison to a conventional spark plug is shown in  FIGS. 5 and 6 . Both calculations consider the same assembly loads and the same external bending force. While the conventional spark plug shows a maximum principal stress of approximately 485N/mm 2  due to the super-position of pre-loaded axial stress and a radial bending stress in the same area (shown in  FIG. 5 ), the stress shown is reduced to about 381N/mm 2  when spark plug  50  is subject to the same forces. The stress can be further reduced by shortening the bending arm of the insulator, as described next. The pre-loaded axial stress mentioned above leads to a pre-stressing of the insulator just below insulator shoulder portion  72 , between positions B and C. The cylindrical and sleeve-like gasket  62  can act as a radial support for the insulator between positions B and D. Put differently, the configuration of the insulator, gasket and shell may result in a partitioning of the axial and radial stresses (AS, RS) so that they are not superimposed or focused in the same area between positions C and B, as was the case with spark plug  10  in  FIG. 1 . By partitioning these stresses, as opposed to allowing them to concentrate in a small, unsupported area, spark plug  50  is able to reduce the risk of the insulator cracking, breaking or otherwise failing. 
     With reference to  FIGS. 3-3B , there is shown another example of a spark plug  50  that provides increased radial support of an insulator  54  in an area that can be susceptible to multiple stresses. In this embodiment, where like reference numerals refer to the same components as the previous embodiment, gasket  62 ′ has a somewhat different configuration than that of the last embodiment and is largely located below seat portion  70  of the shell, as opposed to above it. Here, gasket  62 ′ has a cylindrical portion  100  that is integrally formed with a flange or collar portion  102  at its upper end. Cylindrical portion  100  is somewhat sleeve-like and tightly surrounds and gives radial support to portion  68  of the insulator, where flange portion  102  flares out and extends away from the cylindrical portion so that it receives shoulder portion  72  of the insulator. Flange portion  102  helps maintain gasket  62 ′ in its proper position. By locating gasket  62 ′ mostly below shell seat portion  70 , this embodiment is able to reduce the bending arm which results in a further reduction of axial or tensile stress in the area between positions B and C. As with the last embodiment, the axial length X of gasket  62 ′ is preferably greater than or equal to its radial width Y (in the illustrated example of  FIG. 3 , the radial width Y is quite thin so that axial length X is several times larger than Y, although this is not mandatory). 
     Turning now to  FIGS. 4-4B , there is shown yet another example of a spark plug  50  with a particular configuration designed to increase the support for insulator  54  in an area that can be vulnerable to various stresses. According to this exemplary embodiment, where like reference numerals refer to the same components as the previous embodiments, supported portion  68  of the insulator is radially supported or braced not by the gasket alone, as in the past embodiments, but by the inner radial side of the gasket and the supportive portion  64  of the shell which together form a unified supporting surface that supports or braces the insulator. From  FIGS. 4A-B , it can be seen that the external surface of the insulator directly contacts or abuts the internal surface of the shell, and does so in a manner so that gives the insulator radial support all along its supported portion  68  between positions B and D. Gasket  62 ″ is much like that of the  FIG. 2  embodiment, only it is shorter in the axial direction and is flushly aligned with supportive portion  68  along its inner radial side. In this particular case, the axial length X is approximately equal to the radial thickness Y such that the annular gasket is more ring-like than sleeve-like. Gasket  62 ″ is located between insulator shoulder portion  72  and shell seat portion  70  and seals the combustion chamber at this point. Generally speaking, the gasket  62 ″ addresses or at least mitigates some of the axial stresses AS resulting from pre-loading or compressing the relevant components, and supportive portion  64  of the shell addresses the radial stresses RS by radially supporting the insulator. 
     It is to be understood that the foregoing is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims. 
     As used in this specification and claims, the terms “for example,” “e.g.,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.