Spark plug with rounded insulator base section

Description

State of the art

[0001] The invention relates to a spark plug according to claim 1 and a pre-chamber spark plug according to claim 8.

[0002] In modern automotive internal combustion engines, the trend is toward ever higher pressures and, consequently, higher temperatures in the combustion chambers. These highly turbocharged engines achieve higher power outputs. However, the increased pressures and temperatures in the combustion chamber also raise the demands placed on individual components, such as the spark plug. The spark plug is subjected not only to the pressure, temperature, and chemical conditions of normal engine operation, but also to more extreme conditions during irregular events. One such irregular event is pre-ignition.

[0003] During normal operation of an internal combustion engine, the spark plug initiates the ignition of the air-fuel mixture at a specific engine stroke. This ignition creates a desired pressure and temperature increase, which is used, for example, to drive the wheels of a motor vehicle or a generator to produce electricity. The combustion chamber temperatures of a highly turbocharged internal combustion engine are significantly higher than those of a naturally aspirated engine. This results in greater thermal stress on the spark plug and the challenge of designing the spark plug in such a way as to prevent irregular combustion events, such as pre-ignition, at the spark plug itself.

[0004] In another irregular combustion event, pre-ignition, the air-fuel mixture ignites in the combustion chamber before the actual ignition point during any given cylinder stroke, at a location that is hotter than the spark plug. This pre-ignition creates a flame front that spreads through the combustion chamber. This flame front is accompanied by a pressure increase and a corresponding temperature increase. This temperature and pressure increase can lead to so-called mega-knocking in the engine. During a mega-knock event, the pressure spikes to many times the maximum pressure during normal combustion, stressing the spark plug and other components in the combustion chamber.These pressure spikes during the mega-knock event can cause the spark plug insulator to break, leading to a failure of the cylinder in which the damaged spark plug is installed.

[0005] In current technology, there are various approaches to making the spark plug itself more robust so that it can withstand the pressure spikes during mega-knocking.

[0006] An example of a spark plug is disclosed in patent application DE 10 2012 012 210 A1, in which the spark plug has a ring-shaped shoulder at the combustion chamber-side end of the housing, thereby reducing the entrance of the breathing chamber and preventing pressure peaks from a mega-knock event from entering the breathing chamber of the spark plug, thus protecting the insulator.

[0007] EP 289 064 B1 discloses a spark plug in which the insulator seat geometry has been improved to such an extent that a smaller preload is sufficient for a gas-tight mounting of the insulator in the housing, thus resulting in a lower overall tensile stress on the insulator during mounting and improved flexural strength in the event of mega-knock events.

[0008] Spark plugs are known from DE 10 2015 207886 A1, DE 29 38 986 A1 and DE 699 24 786 T2, and a pre-chamber spark plug is known from US 2012 / 240890 A1.

[0009] The object of the invention is to provide a spark plug in which the tendency to unwanted pre-ignition is minimized or eliminated, and which is additionally protected against the mega-knocks resulting from the pre-ignition events. Advantage of the invention / Disclosure of the invention

[0010] This problem is solved by the spark plug according to the invention. The spark plug comprises a housing, an insulator partially arranged in the housing, a center electrode, and a ground electrode. The housing has a longitudinal axis extending from the combustion chamber-side end to the end of the housing opposite the combustion chamber, and the housing has a bore along its longitudinal axis, thus giving the housing an inner surface. On its inner surface, the housing has a shoulder. The insulator, partially arranged in the housing bore, has a longitudinal axis extending from the combustion chamber-side end to the end of the insulator opposite the combustion chamber. The insulator has an insulator collar that is radially surrounded by the housing, an insulator base that is the combustion chamber-side end of the insulator and has a smaller diameter than the insulator collar, and a transition area that connects the insulator collar and the insulator base and rests on the shoulder of the housing.The center electrode is located within the insulator. The ground electrode is positioned at the combustion chamber end of the housing, with the ground electrode and center electrode arranged to form an ignition gap. Furthermore, the spark plug has a breathing chamber located at the combustion chamber end of the spark plug. This breathing chamber is bounded by a section of the housing and a section of the insulator base and has an opening to the combustion chamber.

[0011] According to the invention, the section of the insulator base that defines the breathing space has a rounded shape. In cross-section, this rounded shape has a first leg length and a second leg length angled relative to the first leg length. The first leg length is longer than the second leg length. The first leg length extends between the intersection of the two leg lengths and a first endpoint of the rounded shape, and the second leg length extends between the intersection of the two leg lengths and a second endpoint of the rounded shape. The resulting contour of the insulator base offers the advantage that the breathing space between the housing and the insulator base is well purged under normal operating conditions, resulting in good heat distribution and dissipation within the insulator base. This prevents undesirable pre-ignition at the spark plug.

[0012] Investigations by the applicant on spark plugs according to the invention have revealed a second effect. In the presence of pre-ignition and the associated critical pressure and temperature conditions, at approximately 110 bar and approximately 850 K, the gas mixture in the breathing chamber heats up so intensely that a further (second) ignition occurs in the area of ​​the insulator base. The spark plug is then surrounded by a combusted air-fuel mixture, and the spark plug's breathing chamber is filled with it. This combusted air-fuel mixture acts as a protective cushion for the spark plug against the pressure peaks of the mega-knock event. The pressure peaks are subject to greater dispersion and damping in the combusted air-fuel mixture than in the unburned air-fuel mixture; therefore, the pressure peaks are more effectively dampened in the spark plug's protective cushion and reach the spark plug in a significantly weakened form or are even completely dampened.This reduces the stress on the spark plug, especially the insulator, from the pressure peaks of the Megs knock event. The pressure and temperature conditions at which this second effect becomes dominant depend on the specific combination of spark plug and engine.

[0013] Advantageous further developments of the invention are the subject of the dependent claims.

[0014] In an advantageous embodiment of the invention, the first leg length is at least 1.5 times, preferably at least 2 times, and particularly preferably 5 times, the length of the second leg. This ensures that the rounding at the insulator foot section is large enough to achieve the technical effects described above.

[0015] In further training, it is additionally or alternatively stipulated that the first leg length is a maximum of 10 times, preferably 7 times, the length of the second leg.

[0016] According to the invention, the first leg length extends parallel to the longitudinal axis of the insulator and the second leg length extends perpendicular to the longitudinal axis of the insulator. This results, particularly in combination with the upper limit for the maximum first leg length, in the breathing chamber having a sufficiently large width (distance between the insulator base and the inside of the housing, perpendicular to the longitudinal axis at the opening of the breathing chamber towards the combustion chamber) in relation to its length (measured parallel to the longitudinal axis). This ensures that, on the one hand, the breathing chamber can be adequately purged under normal operating conditions to prevent the insulator base from overheating, and on the other hand, that the secondary ignition, which protects the spark plug, can also occur.

[0017] Advantageously, the rounding can be described by the two leg lengths L1 and L2 and the two angles α1 and α2, wherein the angle α1 spans between the tangent of the rounding at the second endpoint of the rounding and a first parallel to the longitudinal axis of the insulator passing through the second endpoint of the rounding, and wherein the angle α2 spans between the tangent of the rounding at the first endpoint of the rounding and a second parallel to the longitudinal axis of the insulator passing through the first endpoint of the rounding, wherein the angle α1 has a value greater than or equal to 0° and less than or equal to arctan (L2 / L1) and / or the angle α2 has a value greater than or equal to arctan (L2 / L1) and less than or equal to 90°, and wherein the second endpoint of the rounding is closer to the combustion chamber-side end of the spark plug than the first endpoint of the rounding.

[0018] In particular, the rounding is a concave rounding at the insulator base. This means that the rounding curves towards the longitudinal axis of the insulator. This directs the flow particularly well and with minimal turbulence within the breathing chamber. This effect effectively flushes the hot gases (residual gas) out of the breathing chamber, resulting in good heat distribution and dissipation at the insulator base.

[0019] In a further development of the spark plug according to the invention, the rounding extends over the entire section of the insulator base that defines the breathing chamber. This can be particularly advantageous for spark plugs with a low heat range and thus a short insulator base, as well as for pre-chamber spark plugs.

[0020] In an alternative embodiment, the insulator base can also have several sections, each section of which can in turn have several segments. The insulator base has a section that delimits the breathing space, this section having at least one segment with the rounding according to the invention. In addition to the segment with the rounding, the section of the insulator base delimiting the breathing space can also have one or more segments with a cylindrical and / or conical shape or rounding. The segments with the different shapes blend seamlessly into one another. The other sections of the insulator base can have a cylindrical and / or conical shape or also a rounding. The sections with the different shapes blend seamlessly into one another.

[0021] Alternatively or additionally to the rounding according to the invention, a layer can be applied, at least partially, to the section of the insulator base that defines the breathing space and / or to the inner surface of the housing that defines the breathing space. This layer is designed to trigger a second ignition at the spark plug in the event of irregular combustion in the combustion chamber. The layer is, for example, a catalytic layer that undergoes an exothermic chemical reaction above a certain pressure and / or temperature, thereby initiating a second ignition of the air-fuel mixture, which in turn forms the protective cushion around the spark plug.

[0022] Alternatively or additionally, a piezoelectric element can be arranged on the spark plug, which releases an electrical impulse from a certain pressure, thereby initiating the second ignition and forming the protective cushion around the spark plug.

[0023] These two alternatives to the insulator base with a rounded section can also be applied to spark plugs where, for whatever reason, the section of the insulator base limiting the breathing space cannot have a rounding with two different leg lengths, so that the advantageous second technical effect can also be achieved in these spark plugs.

[0024] Furthermore, the second technical effect, the creation of a protective cushion in the event of irregular combustion in the combustion chamber, can also be achieved by a different geometric design of the insulator base.

[0025] Furthermore, the invention also relates to a pre-chamber spark plug. The pre-chamber spark plug has a housing with a longitudinal axis extending from the combustion chamber end to the end of the housing furthest from the combustion chamber. The housing has a bore along its longitudinal axis, thus giving the housing an inner surface. The housing has a shoulder on its inner surface. An insulator is partially arranged in the housing bore, the insulator having a longitudinal axis extending from the combustion chamber end to the end of the insulator furthest from the combustion chamber, and the insulator having an insulator collar extending radially from the housing, an insulator foot which is the combustion chamber end of the insulator and has a smaller diameter than the insulator collar, and a transition area connecting the insulator collar and the insulator foot and resting on the shoulder of the housing.Furthermore, the spark plug also has a center electrode arranged in the insulator, a cap arranged at a combustion chamber-side end of the housing which forms a pre-chamber, a ground electrode arranged on the housing or on the cap, wherein the ground electrode and the center electrode are arranged so that together they form an ignition gap, and a breathing chamber formed at the combustion chamber-side end of the spark plug, wherein the breathing chamber is bounded by a section of the inside of the housing and a section of the insulator base and has an opening to the combustion chamber volume enclosed by the cap.According to the invention, the section of the insulator foot that limits the breathing space has a rounding, wherein the rounding, viewed in cross-section, has a first leg length and a second leg length angled to the first leg length, wherein the first leg length is greater than the second leg length, wherein the first leg length extends between the intersection of the leg lengths with each other and a first endpoint of the rounding, and the second leg length extends between the intersection of the leg lengths with each other and a second endpoint of the rounding.

[0026] This has the advantage that the breathing space between the housing and the insulator base is well purged under normal operating conditions, resulting in good heat distribution and heat dissipation in the insulator base, which prevents unwanted pre-ignition at the spark plug.

[0027] The pre-chamber spark plug according to the invention can also be further developed with the features of the above-described further developments of the spark plug according to the invention. Brief description of the drawings

[0028] An embodiment of the invention is described in detail below with reference to the accompanying drawing. The drawing shows: Figure 1 a well-known spark plug Figure 2 a section of the insulator with a rounding according to the invention at the insulator base, Figure 3 a schematic representation of the breathing space for two embodiments, and Figure 4 shows a variety of embodiments of the rounding according to the invention at the insulator base. embodiment of the invention

[0029] Figure 1Figure 1 shows a well-known spark plug in a semi-cut view. This overview figure serves to introduce and differentiate the various components and sections of the spark plug. The following are shown in the Figures 2 to 4 The illustrated embodiments of an insulator base with a rounding according to the invention can be used in a spark plug according to Figure 1 be applied.

[0030] The spark plug 1 comprises a housing 2. An insulator 3 is inserted into the housing 2. The housing 2 and the insulator 3 each have a bore and each has a longitudinal axis that coincides with the central axis 8 of the spark plug. A center electrode 4 is inserted into the insulator 3. Furthermore, a terminal bolt 5 is inserted into the insulator 3. A resistive element 6 is located in the insulator 3 between the center electrode 4 and the terminal bolt 5. The resistive element 6 electrically connects the center electrode 4 to the terminal bolt 5. A ground electrode 7 is electrically connected to the housing 2 on the side facing the combustion chamber. The corresponding ignition spark is generated between the ground electrode 7 and the center electrode 4. The spark plug 1 extends around the central axis 8.

[0031] The housing 2 has a shaft 9. This shaft 9 features a polygonal section 10, a shrink groove 11, and a thread 12. The thread 12 is used to screw the spark plug 1 into an internal combustion engine.

[0032] The connecting bolt 5 comprises a bolt shaft 14 extending along the central axis 8 and a collar 13. The collar 13 rests on the insulator 3.

[0033] The insulator 3 has an insulator head 31, an insulator collar 32, and an insulator foot 34. The insulator head 31 is the end of the insulator 3 facing away from the combustion chamber and projects out of the housing 2 on the side of the spark plug 1 facing away from the combustion chamber. The insulator foot 34 is the end of the insulator 3 facing the combustion chamber. The insulator collar 32 is arranged between the insulator head 31 and the insulator foot 34. The insulator collar 32 is radially surrounded by the housing 2. Between the insulator collar 32 and the insulator foot 34, there is a transition area 33 with which the insulator 3 rests on the shoulder 22 of the housing 2. Figure 1 The transitions 33a, 33b from the insulator bundle 32 to the transition area 33 and from the transition area 33 to the insulator foot 34 are marked.

[0034] The insulator foot 34 extends from the foot groove 33b, which is the transition from the transition area 33 to the insulator foot 34 and is typically shaped as a rounded area, to the insulator foot tip, which is the combustion chamber-side end of the insulator foot 34. The insulator foot 34 of the spark plug 1 in Figure 1It has a conical shape and can be subdivided into two sections 341 and 348. The first section 341 of the insulator foot 34 adjoins the base groove 33b directly. The first section 341 of the insulator foot 34 is radially surrounded by a projection 23 located on the inside of the housing 2. The projection 23 is bounded on the side facing away from the combustion chamber by the shoulder 22, on which the insulator 3 rests, and on its side facing the combustion chamber by a section 22b, where the inner diameter of the housing increases again. The projection 23 itself has a substantially constant inner diameter. Together with this projection 23, the first section 341 forms a narrow gap 51, a so-called bottleneck. This narrow gap 51 has a significantly smaller width and thus a significantly smaller volume than the breathing space 50 and is not part of the breathing space 50 within the scope of this application.The breathing space 50 extends from the combustion chamber-side end of the gap 51 to the combustion chamber-side end face of the housing 2. Furthermore, the breathing space 50 is bounded by a section 24 of the housing and a second section 348 of the insulator foot 34.

[0035] Alternatively, the housing 2 can also have only the shoulder 22 on which the insulator 2 rests, and have a constant or conically increasing inner diameter towards the combustion chamber. In this case, there is no narrow gap 51 and the breathing space 50 begins directly at the combustion chamber-facing end of the foot groove 33b.

[0036] Not shown in Figure 1 is an internal seal which can be arranged, for example, between the shoulder 22 of the housing 2 and the transition area 33 of the insulator 3, thus sealing the gap between housing 2 and insulator 3.

[0037] In Figure 2A schematic representation of the insulator foot 34 is shown. This representation serves to clarify the different sections of the insulator foot 34, as well as to illustrate the leg lengths L1, L2 and the angles α1, α2. In this example, the insulator foot 34 can be divided into two sections 341, 348. The first section 341 has a cylindrical shape and could, for example, define the narrow gap 51 below the foot groove 33b. The second section 348 has two segments 342, 343. The first segment 342 has the rounding 345 according to the invention. The second segment 343 has a conical shape and a smaller outer diameter than the first section 341.

[0038] The fillet 345 has its first endpoint 346 at the transition point to the first section 341 of the insulator base 34. In this figure, the transition point is shown as a corner. The fillet 345 has its second endpoint 347 at the transition point to the second segment 343. This transition point results from the angle α1 to a parallel of the longitudinal axis 8 of the insulator 3, which passes through the second endpoint 347, becoming and remaining minimal, being minimal, or changing sign. In the example shown here, α1 = 0 and remains at 0, since the second segment 343 has a cylindrical shape. The leg lengths L1, L2 extend parallel and perpendicular, respectively, to the longitudinal axis 8 of the insulator 3. A leg length is always measured between the intersection of the legs and the first or second endpoint 346, 347 of the fillet 345.

[0039] For example, the first leg length L1 can be greater than or equal to 3 mm and less than or equal to 20 mm. The second leg length L2 then has a value of, for example, greater than or equal to 0.6 mm and less than or equal to 3 mm.

[0040] In Figure 3Two examples are shown, illustrating the section 24 of the housing 2, which defines the breathing chamber 50, and the section 348 of the insulator base 34, which also defines the breathing chamber 50, and thus the resulting breathing chamber 50. The projection 23 on the inside of the housing 2 and the first section 341 of the insulator base 34, which together with the projection 23 forms the narrow gap 51, are indicated. Adjacent to the narrow gap 51 towards the combustion chamber is the breathing chamber 50, which is bounded by a second section 348 of the insulator base 34 and a housing section 24. It can be seen, by way of example, that the edges and corners of the housing 2 and the insulator 3 are angular, conical, or rounded.

[0041] In Figure 3a and 3bThe second section of the insulator foot 34 always has a second segment 343, which has a convex rounding. The rounding 345 according to the invention has a concave shape. The second endpoint 347 of the rounding 345 according to the invention is located at the point where the angle α1 becomes minimal. Figure 3a α1 = 0°, L1 is 4.2 mm and L2 is 1.2 mm. For the embodiment according to Figure 3a This results in a ratio of L1 / L2 of 3.5.

[0042] At Figure 3bα1 becomes minimal and is not equal to 0°. For example, L1 = 2 mm and L2 = 1 mm, resulting in a ratio of L1 to L2 of 2. In the second segment 343, the angle α1 for a tangent along the surface of the second segment 343 increases again. In other words, the radius of curvature of the fillet 345 of the first segment 342 has a different sign than the radius of curvature of the fillet of the second segment 343. The point at which the radius of curvature changes its sign is the second endpoint 347 of the fillet 345 according to the invention. The fillet 345 according to the invention does not have to end in a straight line.

[0043] Figure 4Figure 1 shows a series of different embodiments of the insulator foot 34; this list is not exhaustive. In all embodiments shown here, the insulator foot 34 has a first section 341, which is formed between the foot groove 33b and the section 348 of the insulator foot 34 with the rounding 345 according to the invention. This first section 341 can, together with a projection 23 formed on the housing 2, form the narrow gap 51, or, measured parallel to the longitudinal axis, it can be so short that this section is practically negligible. Furthermore, all embodiments show the second section 348 of the insulator foot 34, which has a first segment 342 with the rounding 345 according to the invention and, in some cases, a second segment 343 without the rounding according to the invention. For all embodiments, the leg lengths L1 and L2 and the approximate position of the first and second endpoints 346, 347 of the rounding 345 according to the invention are shown.

[0044] In Figure 4a The first section 341 has a cylindrical shape. The rounding 345 according to the invention has α2 = 90° at its first endpoint 346 and α1 = 0° at its second endpoint 347. At the second endpoint 347, the rounding 345 transitions into a straight line, which then merges into the cylindrical shape of the second segment 343.

[0045] Figure 4b differs from Figure 4a by the fact that the first endpoint 346 of the rounding 345 according to the invention is radially further inwards and does not lie directly on the edge and transition to the first section 341 of the insulator foot 34. In this example, the rounding 345 also transitions into a straight line at its first endpoint 346.

[0046] Figure 4c differs from Figure 4a by the fact that at the first endpoint 346 the tangent has an angle α2 of less than 90° and greater than 45°.

[0047] Figure 4d differs from Figure 4abecause the first section 341 has a conical shape.

[0048] Figure 4e differs from Figure 4a by the fact that the second section 348 of the insulator foot 34 only has the first segment 342 with the rounding 345 according to the invention.

[0049] Figure 4f differs from the Figure 4a by the fact that at the second endpoint 347 the angle α1 is greater than 0° and less than 45°.

[0050] Figure 4g differs from Figure 4 a because the second segment 343 has a conical shape.

[0051] Figure 4h differs from Figure 4d by the fact that at the first endpoint 346 of the rounding 345 according to the invention the angle α2 is less than 90° and greater than 45°.

[0052] Figure 4i differs from Figure 4f because the second segment 343 has a conical shape.

[0053] All edges shown may also be chamfered or have small convex roundings.

[0054] The embodiments shown here for an insulator base with a rounding according to the invention can also be used with a pre-chamber spark plug.

Claims

1. Spark plug (1), having • a housing (2), wherein the housing (2) has a longitudinal axis (8) which extends from the combustion chamber-side end to the end of the housing (2) remote from the combustion chamber, and wherein the housing (2) has a bore along its longitudinal axis (8), as a result of which the housing (2) has an inner face, wherein the housing (2) has a shoulder (22) on its inner face, • an insulator (3) partially arranged in the housing bore, wherein the insulator (3) has a longitudinal axis (8) which extends from the combustion chamber-side end to the end of the insulator (3) remote from the combustion chamber, and wherein the insulator (3) has an insulator collar (32), which is radially provided by the housing (2), an insulator foot (34), which is the combustion chamber-side end of the insulator (3) and has a smaller diameter than the insulator collar (32), and a transition region (33), which connects the insulator collar (32) and the insulator foot (34) to each other and rests on the shoulder (22) of the housing (2), wherein the insulator foot (34) is divided into a first section (341) and a second section (348), the first section (341) of which is radially surrounded by a projection (23) which is arranged on the inner face of the housing (2) and is delimited by the shoulder (22) on the side remote from the combustion chamber, • a centre electrode (4) arranged in the insulator (3), • an earth electrode (7) arranged at a combustion chamber-side end of the housing (2), wherein the earth electrode (7) and the centre electrode (4) are arranged such that they together form a spark gap, and • a breathing space (50) which is formed at the combustion chamber-side end of the spark plug, wherein the breathing space (50) is delimited by a section (24) of the housing (2) and a section (348) of the insulator foot (34) and has an opening to the combustion chamber, wherein the second section (348) of the insulator foot (34) delimiting the breathing space (50) has a rounded portion (345), wherein the rounded portion (345), as viewed in cross section, has a first limb length L1 and a second limb length L2 that is angled in relation to the first limb length L1, wherein the first limb length L1 extends parallel to the longitudinal axis (8) of the insulator (3) and the second limb length L2 extends perpendicular to the longitudinal axis (8) of the insulator (3) and the first limb length L1 is the length of the rounded portion projected parallel to the longitudinal axis (8) of the insulator (3) and the second limb length L2 is the length of the rounded portion projected perpendicular to the longitudinal axis (8) of the insulator (3), and therefore the first limb length L1 extends between the point of intersection of the limb lengths with each other and a first end point (346) of the rounded portion (345) and the second limb length L2 extends between the point of intersection of the limb lengths with each other and a second end point (347) of the rounded portion (345), characterized in that the first limb length L1 is greater than the second limb length L2.

2. Spark plug (1) according to Claim 1, characterized in that the first limb length L1 is at least 1.5 times, preferably at least 2 times, particularly preferably 5 times, the second limb length L2.

3. Spark plug (1) according to Claim 1 or Claim 2, characterized in that the first limb length L1 is at most 10 times, preferably 7 times, the second limb length L2.

4. Spark plug (1) according to any of the preceding claims, characterized in that the rounded portion (345) is described by the two limb lengths L1, L2 and the two angles α1 and α2, wherein the angle α1 stretches between the tangent of the rounded portion in the second end point (347) of the rounded portion and a first parallel to the longitudinal axis of the insulator passing through the second end point (347) of the rounded portion, and wherein the angle α2 stretches between the tangent of the rounded portion in the first end point (346) of the rounded portion and a second parallel to the longitudinal axis (8) of the insulator passing through the second end point (346) of the rounded portion, wherein the angle α1 has a value of greater than or equal to 0° and less than or equal to arctan (L2 / L1) and / or the angle α2 has a value of greater than or equal to arctan (L2 / L1) and less than or equal to 90°, and wherein the second end point (347) of the rounded portion is closer to the combustion chamber-side end of the spark plug (1) than the first end point (346) of the rounded portion.

5. Spark plug (1) according to any of the preceding claims, characterized in that the rounded portion (345) is a concave rounded portion at the insulator foot (34).

6. Spark plug (1) according to any of the preceding claims, characterized in that the rounded portion (345) extends over the entire section (342) of the insulator foot delimiting the breathing space.

7. Spark plug (1) according to any of the preceding claims, characterized in that a layer is at least partially applied to the section (348) of the insulator foot (34) delimiting the breathing space (50), the layer being designed to trigger ignition at the spark plug (1) in the event of irregular combustion in the combustion chamber.

8. Pre-chamber spark plug, having • a housing, wherein the housing has a longitudinal axis which extends from the combustion chamber-side end to the end of the housing remote from the combustion chamber, and wherein the housing has a bore along its longitudinal axis, as a result of which the housing has an inner face, wherein the housing has a shoulder on its inner face, • an insulator partially arranged in the housing bore, wherein the insulator has a longitudinal axis which extends from the combustion chamber-side end to the end of the insulator remote from the combustion chamber, and wherein the insulator has an insulator collar, which is radially provided by the housing, an insulator foot, which is the combustion chamber-side end of the insulator and has a smaller diameter than the insulator collar, and a transition region, which connects the insulator collar and the insulator foot to each other and rests on the shoulder of the housing, wherein the insulator foot (34) is divided into a first section (341) and a second section (348), the first section (341) of which is radially surrounded by a projection (23) which is arranged on the inner face of the housing (2) and is delimited by the shoulder (22) on the side remote from the combustion chamber, • a centre electrode arranged in the insulator, • a cap which is arranged at a combustion chamber-side end of the housing and forms a pre-chamber, • an earth electrode arranged on the housing or on the cap, wherein the earth electrode and the centre electrode are arranged such that they together form a spark gap, and • a breathing space which is formed at the combustion chamber-side end of the spark plug, wherein the breathing space is delimited by a section of the housing and a section of the insulator foot and has an opening to the combustion chamber volume enclosed by the cap, characterized in that the second section of the insulator foot delimiting the breathing space has a rounded portion, wherein the rounded portion, as viewed in cross section, has a first limb length and a second limb length that is angled in relation to the first limb length, wherein the first limb length is greater than the second limb length, wherein the first limb length L1 extends parallel to the longitudinal axis (8) of the insulator (3) and the second limb length L2 extends perpendicular to the longitudinal axis (8) of the insulator (3) and the first limb length L1 is the length of the rounded portion projected parallel to the longitudinal axis (8) of the insulator (3) and the second limb length L2 is the length of the rounded portion projected perpendicular to the longitudinal axis (8) of the insulator (3), and therefore the first limb length extends between the point of intersection of the limb lengths with each other and a first end point of the rounded portion and the second limb length extends between the point of intersection of the limb lengths with each other and a second end point of the rounded portion.