BRPI0713451A2 - spark plug - Google Patents

Abstract

Spark plug. A spark plug insulator having high dielectric strength, high density, and an optical property that allows light to pass through.

Description

"SPARK PLUG"

CROSS-REFERENCE REQUEST REFERENCE

This application claims the benefit of provisional application US 60 / 815,946 filed June 23, 2006, the entire disclosure of the provisional application being considered part of the disclosure of this application and incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention is directed to a spark plug insulator having a high dielectric intensity, high density, fine grain, and an optical property that allows light to pass through.

An example of a spark plug is illustrated in fig. 1 having an outer shell 20 attached to an insulator 30. Central electrode 50 and terminal 40 are at least partially secured within insulator 30. Isolator 30 isolates charged terminal 40 and central electrode 50 from ground electrode 22. Isolator 30 also separates terminal 40 and central electrode 50 from external electrical interference.

As manufacturers continually increase complexity and reduce the size of internal combustion engines, there is a strong need for smaller spark plugs that take up less space. Currently, the size of the spark plug, particularly the diameter of the spark plug, is limited in further reduction due to the required dielectric strength of the insulator over the service life of the spark plug.

The dielectric intensity is directly related to the thickness required for the insulator walls. Another factor limiting downsizing is that more manufacturers are demanding longer spark plug life, requiring 160,934 kilometers, 24,440 kilometers and 281635 kilometers of spark plugs. The longer the desired service life, the greater the dielectric strength required. In the past, the common way to increase the service life or dielectric intensity of a spark plug has been to increase the thickness of the insulator walls. However, current demand for more compact spark plugs for modern engines prevents the use of thicker wall insulators.

The recent move to electromechanical valve actuators in place of cam shafts to improve fuel efficiency and power is also expected to increase demand for thinner and smaller spark plugs. Demand for higher dielectric intensity is also being driven by recent developments and trends in modern internal combustion engines. To improve fuel economy and increase performance, engines are being engineered with higher compression and turbocharged engines are becoming more common. Making the spark bounce the spark gap under higher compression requires higher voltage and therefore higher dielectric intensity in an insulator. Therefore, an insulator is required for a spark plug having increased dielectric strength having reduced wall thickness and size.

Modern spark plugs are typically formed of an alumina composition with other additives unique to a particular spark plug manufacturer. When formed, alumina with the unique components typically forms a ceramic insulator that is white and non-transparent. In fact, the inventors are not aware of any current spark plugs where the insulator permits light transmission, so that the insulator visibly shows the spark inside the combustion chamber under normal lighting, much less in sunlight. Although some past insulators formed from an amorphous material, such as glass, were transparent, these insulators do not meet the dielectric strength required by modern internal combustion engines and especially the current size requirements as required. a much larger wall thickness of these earlier transparent insulators even under reduced dielectric intensity requirements. In diagnosing various engine problems, it would be desirable to see in the cylinder or in a minimal view the combustion to determine if there is a spark problem occurring in the cylinder. Currently, to determine if there is a problem with electric ignition, spark plug wires or spark plugs on a vehicle require special testing. Therefore, it is desirable to see the spark occur in the cylinder combustion chamber while still retaining a spark plug having high dielectric strength and desirable mechanical qualities, including the diameter of the insulator.

The insulator 30 is traditionally fused and glazed to provide a smooth surface over the terminal portion 12 of the insulator 30. Enameling is required to prevent unloading which may occur on unglazed insulators 30 over the terminal portion 12. To reduce costs and As manufacturing time is desirable, it is desirable not to enamel the insulators, however, currently, without further processing steps, the outer surface of the insulator cannot be made to the uniformity required to prevent discharge and must therefore be enamelled.

The firing end portion or core nose 14 of the insulator 30 is typically unglazed due to the cost of the glazing process, as discharge is generally not a problem for that spark plug portion. Therefore, the firing end portion or core nose of insulator 30 traditionally has a roughened surface that can attract deposits from the combustion process in the cylinder, which can detrimentally affect spark plug firing. In engines that are not properly tuned, that are not operating efficiently, or that have mechanical problems, such as oil leaking into the cylinder during the combustion process, these deposits can quickly and significantly form to eventually create an electrical conduit between the central electrode 50 and the lower strength housing 20, thereby bouncing the spark plug gap, which in turn prevents the spark plug from providing the proper spark profile to efficiently ignite the gases in the cylinder .

Two-cycle engines can also form deposits due to oil in the cylinder during combustion. Therefore, it is desirable to develop a material for use as a spark plug insulator that has a smoother surface that does not require glazing or other finishing processes.

SUMMARY OF THE INVENTION

The present invention is directed to a spark plug insulator having a high dielectric intensity, excellent mechanical properties and optical properties that allow light to pass from the spark plug firing end to the terminal portion. It has been found that: (1) a somewhat transparent spark plug insulator that allows for the spark and combustion process to be shown in the chamber by light transmission, and (2) a spark plug insulator having high dielectric intensity. and excellent mechanical properties allowing a thinner spark plug can be created by using a material to form the insulator which is at least 99% polycrystalline Al2O3 with other trace materials and metal oxides. To form these high purity Al2O3 insulators, metal oxides act as a sintering aid and a grain growth inhibitor.

The additional scope of applicability of the present invention will become apparent from the following detailed description, claims and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, as various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art. in technique.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be more fully understood from the detailed description given hereinafter, the appended claims, and the accompanying drawings in which:

fig. 1 is a sectional view of a typical spark plug; and

fig. 2 is a sectional view of the spark plug insulator portion in fig. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A spark plug 10 is illustrated in a sectional view in fig. 1. Spark plug 10 includes an outer shell 20 attached to an insulator 30, said outer shell 20 includes a ground electrode 22.

Although the spark plug 10 is illustrated as having a center hole 32 over the insulator 30 in which a terminal 40, a conductive material 46, a sealing material 48, and a central electrode 50, any spark plug configuration, are located; or igniter having an insulator, may use the insulator 30 of the present invention. References to spark plug and spark plug isolator also generally refer to igniters and spark isolators. The central electrode 50 includes a tip 56 facing the ground electrode 22. As further illustrated in FIG. 2, insulator 30 extends between a firing end 13 and a terminal end 11. Starting from the terminal end 11 and extending toward the firing end 13, the spark plug insulator 30 includes a terminal portion 12, a large shoulder 16, a small shoulder 18 and a trigger nose or core nose portion 14. The insulator 30 is further formed with varying wall thicknesses between the inner surface 34 of the central bore 32 and the outer surface 36. The inner surface 34 also defines a counterbore seat 38 against which the central electrode 50 rests. Although the spark plug isolator illustrated in the figures and described herein is a typical spark plug used in an internal combustion engine, as for use in an automobile engine, one skilled in the art would readily recognize that the insulator 30 may be formed in A variety of shapes, sizes and configurations, depending on the desired application. For example, in some embodiments, shoulders 16 and 18 may be omitted.

The insulator 30 is formed from a material having approximately at least 99 wt% Al 2 O 3 and more preferably at least 99,4 wt% Al 2 O 3. The desirable characteristics of insulator 30 may also be obtained by forming insulator 30 from approximately 99.97% or more of Al 2 O 3 by weight. To optimize the sintering process as well as improve the electrical and mechanical properties, and spark plug durability, spark plug insulator 30 is made from a material containing one or more of several metal oxides such as oxides. of the Group IIIB transition metals, the oxides of the lanthanide series, and MgO. Such metal oxides are typically present individually or in combination in amounts of up to approximately 1,000 ppm. Depending on the desired characteristics, metal oxides may be present in the amount of approximately 50 to 675 ppm and, in some examples, between 100 and 600 ppm. Examples of metal oxides include Y 2 O 3, La 2 O 3, Yb 2 O 3 and MgO. Examples of transition metal oxides selected from Group IIIB include Se2O3, Y2O3 and La2O3. Examples of oxides from the lanthanide series include metal oxides selected from the oxide group containing the oxides of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Table 1 below provides Examples 1-8 showing the composition of exemplary insulators and, in particular, the amount of Al 2 O 3, MgO, and other metal oxides included in the insulator. Table 1

<table> table see original document page 8 </column> </row> <table>

To the extent that metal oxides are present during the firing of the Al2O3 insulator, the insulator may include aluminum metal oxides in some examples. For example, the chemical composition of the metal oxide may be M 3 Al 5 O 12, where M is from the group comprising Se, Y, La, Ce, Pr, Nd, Pm, Sm, I, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. More generally, the metal oxides may be of the chemical composition MxAlyOz, where M is from the same group as listed above and χ is greater than or equal to 1, y is greater than or equal to 1 but less than which, or, equal to 2, and z is greater than or equal to 3, but less than or equal to 4. Aluminum joins metal oxide from Al2O3 due to the high percentage of Al2O3 in relation to metal oxides and any other impurities.

It has been found that an insulator 30 formed according to the materials listed above has a dielectric strength equal to or at least 17 kV (RMS) per millimeter when measured using a 60 Hz alternating voltage source of a complete sine wave over a 1.27 mm thick specimen. To allow the spark plug to be thinner by reducing the thickness of the insulator, it is desirable that the dielectric strength be at least 17 kV and more preferably 17 kV (RMS) per millimeter when used using a current source. of 60 Hz alternating voltage with a full sine waveform on a 1.27 mm thick specimen. The increase in dielectric strength allows a reduction in thickness between inner surface 34 and outer surface 36 on insulator 30 and thereby allows insulator 30 to be made smaller with thinner walls. A smaller insulator creates a smaller spark plug, thereby allowing room for more components surrounding the spark plug. High dielectric strength also provides a longer service life.

The spark plug insulator 30 made of the materials listed above also has the desirable property of being both translucent and transparent. The Al2O3 in the spark plug forms a polycrystalline structure that is different from the previous amorphous structure of the transparent glass insulators. Although some spark plugs were previously made of glass, which allows light to transfer from one end to the other, as modern spark plugs have moved away from glass for ceramic materials to increase dielectric strength and reduce size, the insulators have been a white color and are not transparent. A spark plug insulator 30, formed from the materials listed above, permits light transmission from the firing end 13 to the terminal end 11 or terminal portion 12. The spark plug insulator material 30 has been selected to provide a optical transmission of approximately 50% or more for a 1mm thick specimen. It has been found that an insulator 30 made of the materials described above may have an optical transmission rate of more than 80% for a thick Imm specimen. Although a transmission rate of over 80% is not required, the higher the optical transmission, the easier it is to see the spark and combustion inside the combustion chamber. Applicants have found that in order to produce the desired light transmission from the core nose portion 14 to the terminal portion 12, it is desirable that the optical transmission rate of the isolator be greater than or equal to 50% for a specimen of Imm thick. Of course, by using the materials listed above, much higher optical transmission rates can be obtained. Depending on the materials selected, the polycrystalline insulator 30 may have a transparent, opaque or translucent quality.

Porosity is known to have an effect on optical transmission. Conventional spark plug insulators made of 95% alumina ceramic compositions contain about 4% porosity and have optical transmission of about 5% or less for 1mm thick specimens, making the alumina ceramics white or opaque. When spark plug insulators are made from high purity alumina (over 99.5% alumina) and contain 0.3% porosity, the optical transmission remains at about 5% or less and the material is opaque. Significant increases in optical transmission of alumina ceramics require porosity to be 0.1% or less. Spark plug insulators made in accordance with the present invention have 99.9% purity polycrystalline alumina and less than 0.1% porosity, which gives an optical transmission of over 80% for 1mm thick specimens.

Grain size also influences optical transmission and, in particular, inline optical transmission. The interfaces between the grains in sintered alumina cause the scattered light transmitted. This is because alumina is a bi-refringent material - the refractive index of alumina is different depending on the crystallographic orientation. Since the alumina grains in an insulator will be aligned somewhat randomly, light will be scattered as it passes from one grain to the other. This impacts inline optical transmission. Although two alumina samples may have identical optical transmission, one sample with higher inline optical transmission will show a clearer image through the material. In comparison, a clear glass window has high inline optical transmission, while a frosted glass window has low inline optical transmission. Aligned alumina optical transmission increases as grain size decreases. For 5 micron grain size alumina, the inline optical transmission will typically be less than 5%. For the 2 micron grain size, the inline optical transmission may be as high as 20%, but in the present invention the inline optical transmission is approximately 50% with a grain size of less than 1 micron.

For some spark plug applications, a high degree of inline optical transmission may be desirable. For example, it may be desirable to obtain a clear picture of the spark and subsequent development of the combustion front in the engine cylinder. In other cases, a low degree of inline optical transmission may be preferred. For example, it may be desirable to detect light from spark and combustion, but not aesthetically appealing to the consumer to see internal components of the spark plug, such as sealing and glass terminal. In this case, a low optical line transmission would be preferable. Therefore, the present invention may modify optical transmission by modifying grain size and may have low in-line optical transmission, such as 20% or even as low as 5%.

The above materials are selected to also have high mechanical strength when synthesized having a grain size of about 30 microns or less. Mechanical strength is higher for material with the finest grain size. For example, a 30 micron grain size insulator generally has a mechanical strength of about 350 MPa, while a 10 micron grain size insulator has a mechanical resistance of about 450 MPa, and when grain size is smaller than 1 micron, the mechanical resistance may exceed 600 MPa.

The materials selected above also allow the spark plug insulator 30 to be formed with a surface that does not require glazing. The surface formed when the insulator 30 is made of the above materials in the quantities listed has an average roughness of 0.40μm or less without additional processing steps such as enameling. A typical unglazed prior art insulator has an average roughness of 1.6 μm or more, which can cause dirt or other contaminants to adhere to the insulator surface over the terminal end, which promotes discharge of the electrical spark from the terminal to the wrapper. Therefore, standard insulators always require enamelling to prevent dirt or other contaminants from adhering. The outer surface 36 of the insulator 30 of the present invention has the unexpected benefit of an average roughness of approximately 0.40 μm or less, which eliminates the need to enamel the spark plug and reduces the possibility of unwanted discharge. It has also been found that using the materials selected above in the quantities listed, a spark plug comprising an unglazed insulator can have an average external surface roughness of 0.20 μm or less, thereby further preventing discharge.

An insulator made in accordance with the materials listed above and the amount listed has a high dielectric strength and a high mechanical strength. This allows the spark plug insulator to be formed to a thickness between outer surface 36 and inner surface 34 at the small shoulder portion 18, approximately equal to or less than 2.54 mm. In addition, it has been found that spark plugs can be formed with sufficient dielectric strength having a thickness of approximately 1.9 mm or less between inner surface 34 and outer surface 36 and, more surprisingly, thickness. may be reduced to approximately equal to or less than 1.3 mm while maintaining sufficient dielectric strength and mechanical strength under current requirements.

The spark plug can be formed in a traditional manner that is well known in the art. In particular, the spark plug insulator 30 is first formed by preparing a suspension of alumina powder, particle size of about 0.4 microns or less, and other additives in water. These additives include inorganic dopants such as MgO5 Y2O3, MgAl2O4 and the other materials listed above, and other organic binders such as polyvinyl alcohol, polyethylene glycol and acrylic emulsion. The suspension is then spray dried to produce a free flowing powder with an average grain size between 50 and 100 microns. This spray dried powder is then isostatically pressed into a die at a pressure of about 200 MPa to form a piece to be pressed. The outer surface of the part to be pressed is formed using an abrasive polishing wheel to form the desired outer profile of the insulator. The insulator 30 is then melted at a temperature between 1450 and 1800 degrees Celsius to synthesize the powder into a dense, homogeneous body. It may be desirable to fuse in a vacuum or in a controlled atmosphere, such as oxygen or hydrogen, to achieve a highly dense, substantially pore-free ceramic. The process of forming the insulator is similar to traditional methods, but uses different starting materials, higher pressing pressure, and different firing conditions.

Other methods may be used to form the ceramic insulator. For example, the powder may be dispersed in a cement by means of a high energy mill such as a vibrating mill or friction mill using high purity grinding media to prevent contamination. A ball mill could also be used as long as it had high purity grinding media. For conventional spark plugs, a ball mill is used and the grinding media are of similar composition to the insulator material, ie 90% to 95% alumina. However, in order to obtain the high purity and low porosity insulator, it is desirable to filter or otherwise remove the oversized particles from the suspension. Conventional spark plug suspensions are sieved through a 45 53 micron sieve. In order to obtain the transparent alumina of the present invention, it may be desirable to use a fiber filter to remove particles above 10 microns. Another method for removing large particles is to allow them to settle outside the suspension under the force of gravity.

The firing process may also be modified, for example, during the early stages of the firing process of a traditional insulator, the binder is removed by thermal methods, however, in the transparent alumina of the present invention, it may be desirable to conduct such thermal process to removal of binder in an atmosphere other than that of the subsequent high temperature sintering process. More specifically, it may be desirable to remove the binder in an inert atmosphere such as argon or nitrogen to prevent the formation of stable carbon compounds that may form as a result of binder combustion and may persist in the final ceramic, and reduce optical transmission. of the insulator. Insulators may also be formed by other methods known to those skilled in the art, such as injection molding or extrusion.

The foregoing examination reveals and describes an example embodiment of the present invention. One skilled in the art will readily recognize from this examination, and from the accompanying drawings and claims, that various changes, modifications, and variations may be made herein without departing from the true spirit and clear scope of the invention as defined by the appended claims.

Claims (86)

1. Spark plug, characterized in that it comprises: an insulator having a density greater than or equal to approximately 3,95 g / cm3. 2. Spark plug, characterized in that it comprises: an insulator comprising at least 99% by weight of Al2O3. Spark plug according to claim 2, characterized in that said insulator includes approximately 99.9% by weight of Al2O3. Spark plug according to claim 3, characterized in that said insulator contains less than 1,000 ppm Y 2 O 3. Spark plug according to Claim 4, characterized in that said insulator contains approximately less than 500 ppm Y2O3. Spark plug according to claim 3, characterized in that said insulator contains more than 10 ppm Y2O3. Spark plug according to claim 6, characterized in that said insulator contains more than approximately 100 ppm Y2O3. Spark plug according to claim 3, characterized in that said insulator contains less than 1,000 ppm La 2 O 3. Spark plug according to claim 8, characterized in that said insulator contains approximately less than 500 ppm La 2 O 3. Spark plug according to claim 3, characterized in that said insulator contains more than 10 ppm La 2 O 3. Spark plug according to claim 10, characterized in that said insulator contains more than approximately 100 ppm La 2 O 3. Spark plug according to claim 3, characterized in that said insulator contains less than 1,000 ppm Yb 2 O 3. Spark plug according to Claim 12, characterized in that said insulator contains approximately less than 500 ppm Yb 2 O 3. Spark plug according to claim 3, characterized in that said insulator contains more than 10 ppm Yb 2 O 3. Spark plug according to claim 14, characterized in that the insulator contains more than approximately 100 ppm Yb 2 O 3. Spark plug according to claim 3, characterized in that said insulator contains less than 1,000 ppm MgO. Spark plug according to claim 16, characterized in that said insulator contains approximately less than 500 ppm MgO. Spark plug according to claim 3, characterized in that said insulator contains more than 10 ppm MgO. Spark plug according to claim 18, characterized in that said insulator contains more than approximately 100 ppm MgO. Spark plug according to claim 3, characterized in that said insulator includes less than 1000 ppm MgO and less than 1,000 ppm Y2O3. Spark plug according to claim 20, characterized in that said insulator includes approximately less than 500 ppm MgO and approximately less than 500 ppm Y2O3. Spark plug according to claim 21, characterized in that said insulator includes more than 10 ppm MgO and more than 10 ppm Y 2 O 3. Spark plug according to claim 21, characterized in that said insulator includes approximately more than 100 ppm MgO and approximately more than 10 ppm Y2O3. Spark plug according to Claim 2, characterized in that said insulator comprises approximately 99.94 wt.% Al 2 O 3. Spark plug according to claim 24, characterized in that said insulator includes approximately 100-500 ppm MgO. Spark plug according to Claim 25, characterized in that said insulator includes approximately 300 ppm MgO. Spark plug according to claim 2, characterized in that said insulator includes approximately 99.97% by weight of Al2O3. Spark plug according to claim 27, characterized in that said insulator includes approximately 100-500 ppm MgO. Spark plug according to Claim 27, characterized in that said insulator comprises approximately 300 ppm MgO. Spark plug according to claim 27, characterized in that said insulator includes approximately 120 ppm MgO. Spark plug according to Claim 2, characterized in that said insulator comprises approximately 99.99% by weight of Al2O3. Spark plug according to claim 2, characterized in that said insulator additionally includes approximately 50-1000 ppm metal oxides selected from the group comprising MgO, transition metal oxides and oxides of the series. of lanthanides. Spark plug according to claim 2, characterized in that said insulator additionally includes approximately 100-600 ppm metal oxides selected from the group comprising MgO, transition metal oxides and oxides of the series. of lanthanides. Spark plug according to claim 32, characterized in that said insulator additionally includes approximately 50-1000 ppm metal oxides selected from the group consisting essentially of MgO, transition metal oxides and oxides. from the lanthanide series. Spark plug according to claim 33, characterized in that said insulator additionally includes approximately 100-600 ppm metal oxides selected from the group consisting essentially of MgO, transition metal oxides and oxides. from the lanthanide series. Spark plug according to claim 34, characterized in that said insulator additionally includes approximately 50-1000 ppm metal oxides selected from the group consisting of MgO, transition metal oxides and oxides of the Lanthanides series. Spark plug according to claim 35, characterized in that said insulator additionally includes approximately 100-600 ppm metal oxides selected from the group consisting of MgO, transition metal oxides and oxides of the Lanthanides series. Spark plug according to Claim 32, characterized in that said transition metal oxides are selected from Group IIIB metal oxides from the periodic table. Spark plug according to claim 32, characterized in that said transition metal oxides are selected from the group comprising SC2O3, Y2O3 and La2O3. Spark plug according to Claim 32, characterized in that said transition metal oxides are selected from the group consisting essentially of Sc2C> 3, Y2O3 and La2O3. Spark plug according to claim 32, characterized in that said transition metal oxides are selected from the group consisting of Sc2O3, Y2O3 and La2O3. Spark plug according to Claim 32, characterized in that said oxides of the lanthanide series are selected from the group of oxides comprising the oxides of La, Ce, Pr, Nd, Sm, Eu, Gd. , Tb, Dy, Ho, Er, Tm Yb, and Lu. Spark plug according to Claim 32, characterized in that said oxides additionally include aluminum. Spark plug according to claim 43, characterized in that said oxides including aluminum include MgAl2O4 and M3AI5O12, where M is selected from the group comprising Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Me, Gd, Tb, Dy, Ho, Er, Tm, Ybe Lu. Spark plug according to claim 43, characterized in that said oxides including aluminum include MgAl 2 O 4 and MxAlyOz, where M is selected from the group comprising Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Me, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and χ is greater than or equal to 1, y is greater than or equal to 1, but less than or equal to 2, and z is greater than or equal to 3, but less than or equal to 4. Spark plug according to claim 2, characterized in that said insulator has a dielectric intensity of at least 17 kV (RMS) per millimeter. Spark plug according to claim 2, characterized in that said insulator has a dielectric intensity of approximately more than 17.5 kV (RMS) per millimeter. Spark plug according to Claim 2, characterized in that said insulator is transparent, translucent or opaque. Spark plug according to claim 2, characterized in that said insulator allows light to pass from a core portion to a terminal portion. Spark plug according to claim 2, characterized in that said insulator is unglazed. Spark plug according to Claim 50, characterized in that said unglazed insulator is sufficiently discharge resistant. Spark plug, characterized in that it comprises a polycrystalline insulator having a core nose and an end portion and wherein said insulator is capable of allowing light to pass from said core nose to said end portion. Spark plug according to Claim 52, characterized in that it comprises an opaque or translucent polycrystalline insulator. Spark plug according to claim 52, characterized in that said insulator includes a large shoulder and a small shoulder between said core nose and an end portion and where the thickness of the insulator in said small shoulder portion It is smaller than 2.54 mm. Spark plug according to claim 52, characterized in that said insulator has a grain size of approximately less than 30 microns. Spark plug according to Claim 52, characterized in that said insulator has an optical transmission rate of more than 50% per millimeter in the range of visible light. Spark plug according to Claim 56, characterized in that said insulator has an optical transmission rate of more than 70% per millimeter in the range of visible light. Spark plug according to Claim 56, characterized in that said insulator has an optical transmission rate of more than 80% per millimeter in the range of visible light. Spark plug according to claim 52, characterized in that said insulator has a grain size of approximately less than 20 microns. Spark plug according to claim 52, characterized in that said insulator has a grain size of approximately less than 10 microns. Spark plug according to claim 52, characterized in that said insulator has a grain size of approximately less than 5 microns. Spark plug according to claim 52, characterized in that said insulator has a grain size of approximately less than 2 microns. Spark plug according to claim 62, characterized in that said insulator has an inline optical transmission of more than 15%. 64. Spark plug according to claim 52, characterized in that said insulator has a grain size of approximately 1 micron. Spark plug according to Claim 64, characterized in that said insulator has an inline optical transmission of more than 40%. 66. Spark plug comprising an unglazed insulator, characterized in that said insulator has an outer surface and said outer surface has an average roughness of 0.40 micrometer or less. Spark plug according to claim 66, characterized in that said insulator has a grain size of approximately less than 30 microns. Spark plug according to claim 66, characterized in that said insulator has a grain size of less than or equal to approximately 15 microns. Spark plug according to claim 66, characterized in that said insulator has a grain size of less than or equal to approximately 5 microns. Spark plug according to claim 69, characterized in that said insulator has an inline optical transmission of more than 5%. 71. Spark plug according to claim 70, characterized in that said insulator has an inline optical transmission greater than or equal to 20%. Spark plug according to Claim 71, characterized in that said insulator has a grain size of approximately less than or equal to 2 microns. Spark plug according to Claim 72, characterized in that said insulator has an inline optical transmission greater than or equal to approximately 50%. Spark plug according to claim 73, characterized in that said insulator has a grain size of less than or equal to approximately 1 micron. 75. Spark plug according to claim 52, characterized in that said insulator has a porosity of less than or equal to approximately 0.1%. 76. Spark plug, characterized in that it comprises an insulator comprising a large shoulder and a small shoulder between said core nose and an end portion and where the thickness of the insulator in said small shoulder portion is less than 2, 54 mm. Spark plug according to claim 76, characterized in that said thickness is approximately less than or equal to 1.9 mm. Spark plug according to Claim 76, characterized in that said thickness is approximately less than or equal to 1.3 mm. Spark plug according to Claim 76, characterized in that said insulator has a porosity of less than or equal to approximately 0.3%. 80. Spark plug according to claim 79, characterized in that said insulator has a light transmission of more than 5% per millimeter. 81. Spark plug according to claim 76, characterized in that said insulator has a porosity of less than or equal to approximately 0.1%. Spark plug according to Claim 81, characterized in that said insulator has a light transmission of more than 40% per millimeter. Spark plug according to Claim 81, characterized in that said insulator has light transmission of more than 60% per millimeter. Spark plug according to Claim 81, characterized in that said insulator has light transmission of more than 75% per millimeter. Spark plug according to Claim 81, characterized in that said insulator has light transmission of more than 80% per millimeter. 86. Spark plug, characterized in that it comprises an unglazed insulator, wherein said insulator has an outer surface and said outer surface has an average roughness of 0.20 micrometers or less.