Patent Publication Number: US-11394178-B2

Title: Spark plug including rounded insulator base section

Description:
FIELD 
     The present invention relates to a spark plug and to a prechamber spark plug. 
     BACKGROUND INFORMATION 
     In today&#39;s internal combustion engines in the automobile sector, there is a trend toward increasingly higher pressures, and thus also toward higher temperatures in the respective combustion chambers of the cylinders. Higher performances are achieved as a result of these highly supercharged internal combustion engines. Due to the higher pressures and the higher temperatures in the combustion chamber, the requirements with regard to the individual components, such as for example with regard to the spark plug, are also increasing. For example, the spark plug is not only subject to the pressure, temperature, and chemical conditions during normal operation of the internal combustion engine, but also to more extreme conditions during irregular events in the internal combustion engine. One of these irregular events is the preignition. 
     During the regular ignition during normal operation of the internal combustion engine, the ignition of the air/fuel mixture is initiated by the spark plug during a certain operating cycle of the internal combustion engine. The ignition results in a desired pressure and temperature increase which is used, for example, for driving the wheels in the case of a motor vehicle or a generator for power generation. The combustion chamber temperatures of a highly supercharged internal combustion engine are considerably higher than in a low-pressure supercharged internal combustion engine, resulting in a higher thermal load on the spark plug as well as the challenge of designing the spark plug in such a way that no irregular combustion events, such as self-ignition, occur at the spark plug. 
     During another irregular combustion event, the preignition, the air/fuel mixture ignites in any location having an excessively high temperature other than at the spark plug in the combustion chamber prior to the actual ignition timing, during an arbitrary operating cycle of the cylinder. As a result of this preignition of the air/fuel mixture, a flame front arises, which propagates in the combustion chamber. This flame front is accompanied by a pressure increase and a corresponding temperature increase. This temperature increase and the pressure increase may result in the so-called mega knocking in the engine. Within a mega knock event, the pressure in a pressure peak increases to a multiple of the maximum pressure during a regular combustion, causing the spark plug and the other components in the combustion chamber to experience loads. Due to these pressure peaks during the mega knock event, the insulator of the spark plug may break and result in a failure of the cylinder in which the damaged spark plug is installed. 
     There are different approaches in the related art as to how to design the spark plug itself to be more robust, so that it withstands the pressure peaks during mega knocks. 
     German Patent Application No. DE 10 2012 012 210 A1 describes one example for a spark plug in which the spark plug includes an annular projection at the combustion chamber-side end of the housing, by which the inlet of the breathing space is reduced and the pressure peaks from a mega knock event cannot penetrate into the breathing space of the spark plug, by which the insulator is to be protected. 
     European Patent No. EP 2789064 B1 describes a spark plug in which the insulator seat geometry was improved to the effect that a smaller preload is sufficient for a gas-tight installation of the insulator in the housing, and thus overall a lower tensile stress acts on the insulator during the installation, and the insulator has an improved flexural strength during mega knock events. 
     It is an object of the present invention to provide a spark plug in which the tendency toward undesirable preignition is minimized or eliminated, and which is additionally protected against the mega knocks resulting from the preignition events. 
     SUMMARY 
     The object may be achieved by a spark plug according to an example embodiment of the present invention. In accordance with an example embodiment of the present invention, the spark plug includes a housing, an insulator partially situated in the housing, a center electrode, and a ground electrode. The housing has a longitudinal axis which extends from the combustion chamber-side end to the end of the housing facing away from the combustion chamber, the housing including a borehole along its longitudinal axis, as a result of which the housing has an inner side. The housing includes a projection on its inner side. The insulator partially situated in the housing borehole has a longitudinal axis which extends from the combustion chamber-side end to the end of the insulator facing away from the combustion chamber. The insulator has an insulator collar, which is radially surrounded by the housing, an insulator base, which is the combustion chamber-side end of the insulator and has a smaller diameter than the insulator collar, and a transition area, which connects the insulator collar and the insulator base to one another and rests on the projection of the housing. The center electrode is situated in the insulator. The ground electrode is situated at a combustion chamber-side end of the housing, the ground electrode and the center electrode being situated in such a way that, together, they form an ignition gap. The spark plug furthermore includes a breathing space, which is configured at the combustion chamber-side end of the spark plug, the breathing space being delimited by a section of the housing and a section of the insulator base and having an opening to the combustion chamber. 
     According to an example embodiment of the present invention, it is provided that the section of the insulator base delimiting the breathing space includes a rounding, the rounding, as viewed in the cross-section, having a first leg length and a second leg length angled with respect to the first leg length, the first leg length being greater than the second leg length, the first leg length extending between the intersecting point of the leg lengths with one another and a first end point of the rounding, and the second leg length extending between the intersecting point of the leg lengths with one another and a second end point of the rounding. The contour of the insulator base resulting from the rounding yields the advantage that the breathing space between the housing and the insulator base is flushed well during normal operating conditions, so that a good heat distribution and heat dissipation arise in the insulator base. This prevents undesirable preignitions at the spark plug. 
     Examinations of the applicant on spark plugs according to the present invention have shown a second effect. When a preignition and the associated critical pressure and temperature conditions, at approximately 110 bar and approximately 850 K, are present, the gas mixture in the breathing space heats so drastically that a further (second) ignition occurs in the area of the insulator base. The spark plug is thereafter surrounded by a burned air/fuel mixture, and the breathing space of the spark plug is filled with it. This burned fuel/air mixture acts like a protective cushion for the spark plug against the pressure peaks of the mega knock event. The pressure peaks are subject to a greater dispersion and attenuation in the burned air/fuel mixture than in the unburned air/fuel mixture, which is why the pressure peaks are attenuated more strongly in the protective cushion of the spark plug, and the pressure peaks reach the spark plug in a considerably weakened form, or are even completely attenuated. In this way, the spark plug, in particular, the insulator, experiences less drastic loading from the pressure peaks of the mega knock event. The pressure and temperature conditions starting at which the second effect becomes dominant specifically depend on the combination of the spark plug and the engine. 
     Advantageous refinements of the present invention are described herein. 
     In one advantageous refinement of the present invention, it is provided that the first leg length is at least 1.5 times, preferably at least 2 times, particularly preferably 5 times the second leg length. In this way, it is ensured that the rounding at the section of the insulator base is large enough for the above-described technical effects to arise. 
     In one refinement of the present invention, it is additionally or alternatively provided that the first leg length is no more than 10 times, preferably 7 times, the second leg length. 
     Advantageously, the first leg length extends in parallel to the longitudinal axis of the insulator, and the second leg length extends perpendicularly to the longitudinal axis of the insulator. This results, in particular also in combination with the upper limit for the maximum first leg length, in the breathing space having a sufficiently large width (distance between insulator base and housing inner side perpendicular to the longitudinal axis at the opening of the breathing space toward the combustion chamber) in relation to its length (measured in parallel to the longitudinal axis) so that, on the one hand, the breathing space may be flushed sufficiently well during normal operating conditions, so that the insulator base does not become too hot, and, on the other hand, also the second ignition protecting the spark plug may take place. 
     Advantageously, in accordance with an example embodiment of the present invention, the rounding may be described by the two leg lengths L1 and L2 as well as the two angles α 1  and α 2 , angle α 1  spanning between the tangent of the rounding in the second end point of the rounding and a first parallel, passing through the second end point of the rounding, to the longitudinal axis of the insulator, and angle α 2  spanning between the tangent of the rounding in the first end point of the rounding and a second parallel, passing through the first end point of the rounding, to the longitudinal axis of the insulator, angle α 1  having a value of greater than or equal to 0° and smaller than or equal to arctan (L2/L1) and/or angle α 2  having a value of greater than or equal to arctan (L2/L1) and smaller than or equal to 90°, and the second end point of the rounding being closer to the combustion chamber-side end of the spark plug than the first end point of the rounding. 
     In particular, the rounding is a concave rounding at the insulator base. This means that the rounding is curved toward the longitudinal axis of the insulator. In this way, the flow is guided particularly well and with low turbulence in the breathing space. This effect flushes the hot gases (residual gas) effectively out of the breathing space, by which a good heat distribution and heat dissipation arise in the insulator base. 
     In one refinement of the spark plug according to an example embodiment of the present invention, it is provided that the rounding extends across the entire section of the insulator base which delimits the breathing space. This may, in particular, be advantageous in the case of spark plugs having a low heat rating, and thus a short insulator base, as well as in the case of prechamber spark plugs. 
     In one alternative embodiment of the present invention, the insulator base may also include multiple sections, it also being possible, in turn, for one section to include multiple segments. The insulator base has a section which delimits the breathing space, this section having at least one segment which includes the rounding according to the present invention. In addition to the segment including the rounding, the section of the insulator base which delimits the breathing space may also include one or multiple segment(s) having a cylindrical and/or conical shape or rounding. The segments having the different shapes steadily transition into one another. The other sections of the insulator base may have a cylindrical and/or conical shape or also a rounding. The sections having the different shapes steadily transition into one another. 
     As an alternative or in addition to the rounding according to an example embodiment of the present invention, a layer may be at least partially applied to the section of the insulator base which delimits the breathing space and/or to the housing inner side delimiting the breathing space, which is configured to trigger a second ignition at the spark plug during irregular combustions in the combustion chamber. The layer is a catalytic layer, for example, which undergoes an exothermic chemical reaction starting at a certain pressure and/or starting at a certain temperature, by which a second ignition of the air/fuel mixture is initiated, the protective cushion in turn forming around the spark plug. 
     As an alternative or in addition, a piezoelectric element may be situated at the spark plug, which releases an electrical pulse starting at a certain pressure, by which, in turn, the second ignition is initiated, and the protective cushion forms around the spark plug. 
     These two alternatives to the insulator base including a rounded section may also be employed in spark plugs in which, for whatever reason, the section of the insulator base which delimits the breathing space cannot include a rounding having two different leg lengths, so that the advantageous second technical effect may also be implemented in these spark plugs. 
     The second technical effect, the generation of a protective cushion during irregular combustions in the combustion chamber, may furthermore also be achieved by a different geometric configuration of the insulator base. 
     The present invention furthermore also relates to a prechamber spark plug. In accordance with an example embodiment of the present invention, the prechamber spark plug includes a housing, which has a longitudinal axis extending from the combustion chamber-side end to the end of the housing facing away from the combustion chamber. The housing includes a borehole along its longitudinal axis, as a result of which the housing has an inner side. The housing includes a projection on its inner side. An insulator is partially situated in the housing borehole, the insulator having a longitudinal axis which extends from the combustion chamber-side end to the end of the insulator which faces away from the combustion chamber, and the insulator including an insulator collar, which is radially surrounded by the housing, an insulator base, which is the combustion chamber-side end of the insulator and has a smaller diameter than the insulator collar, and a transition area, which connects the insulator collar and the insulator base to one another and rests on the projection of the housing. The spark plug furthermore also includes a center electrode situated in the insulator, a cap, which is situated at a combustion chamber-side end of the housing and forms a prechamber, a ground electrode situated at the housing or at the cap, the ground electrode and the center electrode being situated in such a way that, together, they form an ignition gap, and a breathing space, which is formed at the combustion chamber-side end of the spark plug, the breathing space being delimited by a section of the inner side of the housing and a section of the insulator base and having an opening to the combustion chamber volume enclosed by the cap. According to an example embodiment of the present invention, the section of the insulator base which delimits the breathing space includes a rounding, the rounding, as viewed in the cross-section, having a first leg length and a second leg length angled with respect to the first leg length, the first leg length being greater than the second leg length, the first leg length extending between the intersecting point of the leg lengths with one another and a first end point of the rounding, and a second leg length extending between the intersecting point of the leg lengths with one another and a second end point of the rounding. 
     This may yield an advantage that the breathing space between the housing and the insulator base is flushed well during normal operating conditions, so that a good heat distribution and heat dissipation arise in the insulator base, achieving that undesirable preignitions do not occur at the spark plug. 
     The prechamber spark plug according to the present invention may also be refined with the features of the above-described refinements of the spark plug according to the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention are described in detail hereafter with reference to the figures. 
         FIG. 1  shows a generally conventional spark plug. 
         FIG. 2  shows a section of the insulator including a rounding according to an example embodiment of the present invention at the insulator base. 
         FIGS. 3 a  and 3 b    show schematic representations of the breathing space for two exemplary embodiments of the present invention. 
         FIGS. 4 a -4 i    show a plurality of embodiments of the rounding according to an example embodiment of the present invention at the insulator base. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
       FIG. 1  shows a generally conventional spark plug  1  in a semi-sectional view. This overview figure is used to introduce the different components and sections of the spark plug and to delimit them from one another. The exemplary embodiments for an insulator base including a rounding according to the present invention shown in  FIGS. 2 through 4  may be employed in a spark plug according to  FIG. 1 . 
     Spark plug  1  includes a housing  2 . An insulator  3  is inserted into housing  2 . Housing  2  and insulator  3  each have a borehole and each have a longitudinal axis, which coincide with center axis  8  of the spark plug. A center electrode  4  is inserted into insulator  3 . Furthermore, a connecting bolt  5  is inserted into insulator  3 . A resistance element  6  is situated in insulator  3  between center electrode  4  and connecting bolt  5 . Resistance element  6  connects center electrode  4  to connecting bolt  5  in an electrically conducting manner. A ground electrode  7  is connected to housing  2  in an electrically conducting manner on the side facing the combustion chamber. The corresponding ignition spark is generated between ground electrode  7  and center electrode  4 . Spark plug  1  extends around center axis  8 . 
     Housing  2  includes a shank  9 . A polygon  10 , a shrink stitch  11 , and a thread  12  are configured at this shaft  9 . Thread  12  is used to screw spark plug  1  into an internal combustion engine. 
     Connecting bolt  5  includes a bolt shank  14 , which extends along center axis  8 , and a collar  13 . Connecting bolt  5  rests with collar  13  on insulator  3 . 
     Insulator  3  includes an insulator head  31 , an insulator collar  32 , and an insulator base  34 . Insulator head  31  is the end of insulator  3  which faces away from the combustion chamber and protrudes from housing  2  on the side of spark plug  1  which faces away from the combustion chamber. Insulator base  34  is the end of insulator  3  which faces the combustion chamber. Insulator collar  32  is situated between insulator head  31  and insulator base  34 . Insulator collar  32  is radially surrounded by housing  2 . There is a transition area  33  between insulator collar  32  and insulator base  34 , with which insulator  3  rests on projection  22  of housing  2 . Transitions  33   a ,  33   b  from insulator collar  32  to transition area  33  as well as from transition area  33  to insulator base  34  are identified in  FIG. 1 . 
     Insulator base  34  extends from base fillet  33   b , which is the transition from transition area  33  to insulator base  34  and is typically configured as a rounding, to insulator base tip, which is the combustion chamber-side end of insulator base  34 . Insulator base  34  of spark plug  1  in  FIG. 1  has a conical shape and may be divided into two sections  341 ,  348 . First section  341  of insulator base  34  directly abuts root fillet  33   b . First section  341  of insulator base  34  is radially surrounded by a protrusion  23  situated on the inner side of housing  2 . Protrusion  23  is delimited on the side facing away from the combustion chamber by projection  22 , on which insulator  3  rests, and on its side facing the combustion chamber by a section  22   b  in which the housing inside diameter increases again. Protrusion  23  itself has an essentially constant inside diameter. Together with this protrusion  23 , first section  341  forms a narrow gap  51 , a so-called bottle neck. This narrow gap  51  has a considerably smaller width, and thus a considerably smaller volume, than breathing space  50  and, within the scope of the present application, does not belong to breathing space  50 . Breathing space  50  extends from the combustion chamber-side end of gap  51  to the combustion chamber-side end face of housing  2 . Breathing space  50  is furthermore delimited by a section  24  of the housing and a second section  348  of insulator base  34 . 
     As an alternative, housing  2  may also only include projection  22  on which insulator  2  rests, and may have a constant or conically increasing inside diameter in the direction of the combustion chamber. In this case, there is no narrow gap  51 , and breathing space  50  begins directly at the end of base fillet  33   b  which faces the combustion chamber. 
     Not shown in  FIG. 1  is an inner seal, which may, for example, be situated between projection  22  of housing  2  and transition area  33  of insulator  3 , and thus seals the space between housing  2  and insulator  3 . 
       FIG. 2  shows a schematic representation of insulator base  34 . This representation is used to illustrate the different sections of insulator base  34  as well as the representation of leg lengths L1, L2 as well as angles α 1 , α 2 . In this example, insulator base  34  may be divided into two sections  341 ,  348 . First section  341  has a cylindrical shape and could, for example, delimit narrow gap  51  beneath base fillet  33   b . Second section  348  has two segments  342 ,  343 . First segment  342  includes rounding  345  according to the present invention. Second segment  343  has a conical shape and a smaller outside diameter than first section  341 . 
     Rounding  345  has its first end point  346  at the transition point to first section  341  of insulator base  34 . The transition point is shown as a corner in this figure. Rounding  345  has its second end point  347  at the transition point to second segment  343 . This transition point arises from angle α 1  to a parallel of longitudinal axis  8  of insulator  3  which extends through second end point  347  becoming minimal, and remaining or being minimal or changing the sign. In the example shown here, α 1 =0 and remains at 0 since second segment  343  has a cylindrical shape. Leg lengths L1, L2 extend in parallel or perpendicularly to longitudinal axis  8  of insulator  3 . In the process, a leg length is always measured between the intersecting point of the legs with one another and first and second end point  346 ,  347  of rounding  345 . 
     For example, first leg length L1 may be greater than or equal to 3 mm and smaller than or equal to 20 mm. Second leg length L2 then, for example, has a value of equal to or greater than 0.6 mm and smaller than or equal to 3 mm. 
       FIGS. 3 a  and 3 b    show two examples in which section  24  of housing  2  which delimits breathing space  50  and section  348  of insulator base  34  which delimits breathing space  50 , and thus also the resulting breathing space  50 , are shown. Indicated are projection  23  on the inner side of housing  2  as well as first section  341  of insulator base  34 , which together with projection  23  forms narrow gap  51 . In the direction of the combustion chamber, breathing space  50  abuts narrow gap  51 , which is delimited by a second section  348  of insulator base  34  and a housing section  24 . By way of example, it is apparent that in the case of housing  2  and in the case of insulator  3  edges and corners are designed to be angular, conical or with roundings. 
     In  FIGS. 3 a  and 3 b   , the second section of insulator base  34  always has a second segment  343 , which includes a convex rounding. Rounding  345  according to the present invention has a concave shape. Second end point  347  of rounding  345  according to the present invention arises at the point when angle α1 becomes minimal. In  FIG. 3 a   , thus α1=0°, L1 is 4.2 mm, and L2 is 1.2 mm. A ratio of L1/L2 of 3.5 results for the exemplary embodiment according to  FIG. 3   a.    
     In  FIG. 3 b   , al becomes minimal and is dissimilar to 0°. For example, L1=2 mm and L2=1 mm may apply, thereby resulting in a ratio of L1 to L2 of 2. In second segment  343 , angle α1 increases again for a tangent along the surface of second segment  343 . In other words, the radius of curvature of rounding  345  of first segment  342  has a different sign than the radius of curvature of the rounding of second segment  343 . The point at which the radius of curvature changes its sign is second end point  347  of rounding  345  according to the present invention. Rounding  345  according to the present invention does not have to end in a straight line. 
       FIGS. 4 a -4 i    show a number of different embodiments of insulator base  34 , the enumeration not being exhaustive. In all example embodiments here, insulator base  34  has a first section  341 , which is designed with rounding  345  according to the present invention between base fillet  33   b  and section  348  of insulator base  34 . Together with a projection  23  formed at housing  2 , this first section  341  may form narrow gap  51  or may, measured in parallel to the longitudinal axis, be so short that this section is essentially negligible. Furthermore, all specific embodiments show second section  348  of insulator base  34 , which has a first segment  342  including rounding  345  according to the present invention and partially a second segment  343  without the rounding according to the present invention. For all exemplary embodiments, leg lengths L1 and L2 as well as the approximate positions of first and second end points  346 ,  347  of rounding  345  according to the present invention are plotted. 
     In  FIG. 4 a   , first section  341  has a cylindrical shape. Rounding  345  according to the present invention has α2=90° at its first end point  346 , and α1=0° at its second end point  347 . At second end point  347 , rounding  345  transitions into a straight line, which transitions into the cylindrical shape of second segment  343 . 
       FIG. 4 b    differs from  FIG. 4 a    in that first end point  346  of rounding  345  according to the present invention is radially further to the inside, and does not rest directly on the edge and transition to first section  341  of insulator base  34 . In this example, rounding  345  also transitions into a straight line in its first end point  346 . 
       FIG. 4 c    differs from  FIG. 4 a    in that, in first end point  346 , the tangent has an angle α2 of smaller than 90° and greater than 45°. 
       FIG. 4 d    differs from  FIG. 4 a    in that first section  341  has a conical shape. 
       FIG. 4 e    differs from  FIG. 4 a    in that second section  348  of insulator base  34  only includes first segment  342  including rounding  345  according to the present invention. 
       FIG. 4 f    differs from  FIG. 4 a    in that, in second end point  347 , angle α1 is greater than 0° and smaller than 45°. 
       FIG. 4 g    differs from  FIG. 4 a    in that second segment  343  has a conical shape. 
       FIG. 4 h    differs from  FIG. 4 d    in that, in first end point  346  of rounding  345  according to the present invention, angle α2 is smaller than 90° and greater than 45°. 
       FIG. 4 i    differs from  FIG. 4 f    in that second segment  343  has a conical shape. 
     All shown edges may also be chamfered or have small convex roundings. 
     The specific embodiments shown here for an insulator base including a rounding according to the present invention may also be used in a prechamber spark plug.