WO1996007226A1

WO1996007226A1 - Spark plug for an internal combustion engine

Info

Publication number: WO1996007226A1
Application number: PCT/DE1995/001072
Authority: WO
Inventors: Walter Benedikt; Hans-Dieter Herde; Manfred Vogel; Dittmar Klett; Werner Herden; Rudolf Pollner; Dietrich Trachte; Richard Adamczuk; Andreas Niegel; Bernd Mueller; Roland Mueller
Original assignee: Robert Bosch Gmbh
Priority date: 1994-09-01
Filing date: 1995-08-16
Publication date: 1996-03-07
Also published as: DE4431143B4; DE4431143A1

Abstract

The proposal is for harder-wearing spark plugs for an internal combustion engine. One feature concerns the burn-off resistor (13) of the plug which projects toward the spark path (20) of the plug in order that the current from the charge stored in the ignition capacitor is given off in a limited manner. A further modification relates to the diameter of the inner conductor (11, 16) which is reduced, thus making it possible to reduce the capacitance of the plug. A further modification concerns the electrode head (14) of the inner conductor (11, 16), where it is proposed to coat the electrode head (14) with a noble metal and to use a highly heat-conductive material for the electrode head (14) which acts as a heat buffer.

Description

Spark plug for an internal combustion engine

State of the art

The invention relates to a spark plug for internal combustion engines according to the preamble of the main claim. Such a spark plug is already known from EP 0 101 547 B1. In this known spark plug, a cylindrical metal tube is provided, in which a tubular ceramic insulator is arranged. The ceramic insulator contains an inner conductor, the inner conductor consisting of two parts, namely firstly a metal body serving as a contact pin and secondly a platinum rod which represents the end of the inner conductor on the combustion chamber side. At the connection-side end of the inner conductor there is a cavity in the ceramic insulator, which is filled with an electrically conductive casting compound. This potting compound contains layers that act as electrical resistance. A metallic connecting bolt then protrudes into the electrically conductive casting compound on the connection side, which also protrudes from the ceramic insulator and can be contacted with the ignition cable in a known manner.

The function of the layer acting as electrical resistance is mainly that the erosion of the electrodes of the spark plug is reduced.

This wear process is described in the publication Bosch, Technical Instruction, Spark Plugs, Robert Bosch GmbH 1985, page 18.

This reduction in erosion is no longer sufficient for today's requirements regarding the service life of spark plugs or the extension of maintenance intervals. Advantages of the invention

The spark plug according to the invention with the characterizing features of the main claim has the advantage that the service life (service life) of the spark plug is increased by the advance of the resistance in the direction of the spark gap and / or by the reduction in the diameter of the inner conductor compared to conventional inner conductors. The increase in service life is ultimately based on the fact that the spark plug according to the invention has reduced electrode wear.

The measures listed in the subclaims enable advantageous developments and improvements of the spark plug specified in the main claim. It is particularly advantageous that the resistance is arranged in the rod-shaped inner conductor in the region of the insulator foot. The resistance is then simultaneously protected from the combustion chamber atmosphere by the insulator and no ductile and weldable materials are required. The outer shape of the spark plug remains unaffected. Advantageously, the material for the resistance is e.g. an electrically conductive casting compound, in particular a glass melt with an addition of electrically conductive particles, such as soot or graphite particles. Another possibility for realizing the resistance is to coat a thermally conductive carrier material, in particular AlN, with a resistance layer applied using thick-film technology.

Another advantage of the spark plug according to claim 5 is that the noble metal tip has an enlarged diameter compared to the known spark plug from EP 0 101 547. This measure also increases the service life the spark plug increased again. Another advantage of this spark plug results from the fact that at least two ground electrodes are provided and the ground electrodes are bent toward the inner conductor in such a way that an air gap is created, the outer surface of the ground electrodes in particular being flush with the upper edge of the insulator. The fact that there is more than one ground electrode and the arrangement from ground electrode to insulator results in very good flame core formation, so that even engines with a lean mixture can be ignited well. This also improves the exhaust gas behavior of these engines. The arrangement of the ground electrodes in relation to the insulator creates an air sliding spark gap, which causes the spark plug to burn very well and thus further increases the service life of the spark plug. The fact that the wear of the spark plug is reduced overall ensures that the same voltage requirement for the spark plug is present over the entire service life.

The spark plug with the characterizing features of the independent claim 6 also has the advantage that the service life of this spark plug is increased compared to conventional spark plugs. The fact that the area of the inner conductor coming into contact with the combustion chamber is coated with a layer of noble metal effectively prevents corrosion of the inner conductor. It is also advantageous that only a thin layer of noble metal is used, so that expensive noble metal can be saved compared to the solution with a solid noble metal rod. The provision of a highly thermally conductive material which is in direct contact with the noble metal layer has the advantage that the heat which arises from the ignition spark can be dissipated very quickly from the noble metal layer and thus the melting of the surface of the noble metal layer is reduced. drawing

Embodiments of the invention are shown in the drawing and explained in more detail in the following description. Show it

Figure 1 is a schematic representation of the inner part of a spark plug known from the prior art; Figure 2 is a schematic representation of the inner part of a

Advance resistance spark plug;

Figure 3 is a schematic representation of the inner part of a spark plug with a protective layer of precious metal;

Figure 4 is a schematic representation of the inner part of a spark plug, in which the resistance is the combustion chamber side

End of the center electrode and with a

Precious metal layer is coated;

Figure 5 is a schematic representation of the inner part of a spark plug, in which both the resistance is preferred and a thermally conductive material from one

Precious metal layer is coated and

Figure 6 is a schematic representation of the combustion chamber side

Part of a spark plug showing the arrangement of

Ground electrodes and insulator.

Description of the embodiments

The basic structure of a spark plug is sufficiently known from the prior art and can be found, for example, in EP 0 101 547. In Figure 1, the inner part of such a known spark plug is shown. The insulator is identified by the reference number 10. The reference number 11 denotes the inner conductor of the spark plug. It consists of two parts, the contact pin 16 and the precious metal rod 17. The connection-side cavity 25 of the insulator 10 is filled with a casting compound. It contains the layers that act as electrical resistance. A connecting bolt 26 also projects into the cavity 25. The one with the reference number 27 Provided portion of the insulator 10 is hereinafter referred to as the insulator base. In the following, the differences between the spark plugs according to the invention and these conventional spark plugs are dealt with essentially.

In FIG. 2, reference number 10 again designates the insulator. This consists of a ceramic, such as Alumina. The inner conductor 11 is embedded in the insulator 10. It consists of the three parts metal rod 12, erosion resistor 13 and electrode head 14. For simplicity, the metal rod 12 consists of the same material as the contact pin of the spark plug known from the prior art. The electrode head 14 can e.g. consist of a nickel-based alloy. The erosion resistance is preferred in the direction of the spark gap of the spark plug. In this embodiment, it is integrated into the inner conductor for the sake of simplicity. However, it could also be integrated into the metal tube (not shown) which surrounds the insulator and would then have to be attached as close as possible to the ground electrode or possibly even within the ground electrode.

The preference for the erosion resistance is based on the knowledge of the applicant based on corresponding test series that in the event of a sparkover, the charged capacity (a few picofarads) of the part of the spark plug is discharged "unbraked" in the range of a few nanoseconds after the possibly existing erosion resistance. The metal tube of the spark plug and the inner conductor 11 inside the insulator create a cylindrical capacitor with a capacitance of a few pF. Since the discharge of this capacitor takes place within a few nanoseconds, currents flow in the range from 10 to a few hundred amperes. The current can increase up to a few hundred amperes if there is no burning resistance in the candle. The on the energy converted in the breakthrough phase heats the spark base far above the melting point, so that material is sputtered off. This problem is solved in accordance with the invention in that the erosion resistance, which may have a size of 1 kOhm to lOkOhm, for example, is arranged as close as possible to the spark plug gap in order to largely avoid the relevant capacitance.

Alternatively or in addition to this, the capacitance can also be reduced by reducing the diameter of the inner conductor 11. For inner conductors according to the prior art, the diameter is approximately 2.5 mm. To reduce the relevant capacity, this diameter is preferably reduced to <2 mm. A preferred range is 0.3 to 1.5 mm.

The burning resistance must be good heat dissipation. The inner conductor is particularly suitable for the installation location of the erosion resistor, since the resistor is then protected from the combustion chamber atmosphere by the insulator and no ductile and weldable materials are required.

The relevant resistance values can be realized, for example, by a homogeneous, good heat-conducting material, such as silicon carbide (SiC), as a mass resistance, whose desired electrical conductivity (approx. 10 ² Ωcm) can be set by means of doping. As an alternative to this, such a resistor can also be applied to a highly thermally conductive carrier, for example to aluminum nitride (A1N) using thick-film technology. A material that is also considered for resistance is also called Panat.

In FIG. 3, the same reference numerals designate the same parts as in FIG. 2. In contrast to FIG. 2, the burn-up resistance in FIG. 3 is not preferred or not at all unavailable. Instead, the electrode head 14 is coated with a protective metal layer 15. The metal rod 12 is designed as a two-substance metal rod. It can consist partly of copper and partly of nickel. The electrode head 14 is provided as a heat buffer. For this purpose, it is proposed that it consists of an electrically conductive material with a high melting point and the highest possible product of thermal conductivity, specific heat and density. Such materials are in particular the metals Cu, Ag, Au, W, but also Co, Mo, Ni, Cr. Alloys from these metals are also suitable as materials for this. Another suitable material for this material are conductive ceramic materials such as silicon carbide (Sie). The heat capacity of these heat buffers can be greater by factors than "the inside" of a candle electrode made from conventional, corrosion-resistant, homogeneous materials, such as Ni-based alloys or from solid platinum (Pt).

The noble metal layer can in particular consist of Pt, Rh or Ir. The heat transfer from the precious metal coating to the core should be as optimal as possible. Alloy or diffusion areas between the partners are desirable. The provision of the precious metal layer not only serves to improve the corrosion protection of the electrode head. It has been shown in the test series already mentioned that when the ignition lines are discharged and the capacity of the ignition coil, high currents also flow through the electrodes in the region of a few amperes, even if there is a burn-off resistor. Erosion occurs in this phase of the discharge. These high current components and thus arcing phases also occur in the post-discharge phase (spark tail), especially in the case of secondary sparks and glow-arc transitions. In the area of the continuously burning spark tail, at high pressure, arc discharges also occur with currents in the area of some 10 milliamperes, which can melt the surface of the electrodes.

For the mechanisms of erosion described here, the question of how quickly and to what extent the energy converted at the spark base is dissipated from this geometrical area, and thus the

Melting of the surface can be reduced. Therefore, the invention is a thin one

Precious metal layer in its function as

Use corrosion protection agents against the combustion chamber atmosphere and poor heat dissipation e.g. to replace the platinum with a solid core made of a very good heat-dissipating material.

In FIG. 4, the same reference numbers also denote the same as in FIGS. 2 and 3. The difference compared to FIGS. 2 and 3 is that in this case the electrode head 14 has been dispensed with entirely and the erosion resistor 13 forms the end of the inner conductor 11 on the combustion chamber side. The outer edge of the erosion resistor 13 is additionally covered with a noble metal layer 15. The erosion resistance is made of the highly thermally conductive material SiC, with appropriate doping for the desired electrical conductivity.

Another embodiment of the invention is shown in FIG. Here, too, the same reference numerals designate the same as in the preceding FIGS. 2 to 4. In this exemplary embodiment, the electrode head 14 is designed as a heat buffer. For this purpose it consists of one of the materials which have already been described as suitable for this. The electrode head 14 is additionally coated with a protective metal layer 15. In this case, the erosion resistor 13 consists of a Carrier material of aluminum nitride (A1N) with an applied thick film resistor.

FIG. 6 also shows a specific embodiment of a spark plug which is designed for a particularly long service life. The same reference numbers are used here as in the previous Figures 2 to 5, so that these reference numbers do not have to be repeated. In addition, the cylindrical metal tube 18 is shown, in which the insulator 10 is embedded. The inner conductor 11 of this spark plug consists of the two parts contact pin 16 and noble metal rod 17. The noble metal rod 17 is flush with the insulator surface on the combustion chamber side. The noble metal rod 17 is sintered into the ceramic of the insulator 10, for example. The erosion resistance connects to the contact pin 16 and is arranged in the insulator foot 27. Here too, the erosion resistance 13 is preferred over the spark plugs known from the prior art in the direction of the spark gap. For example, silicon carbide (SiC) with appropriate doping is used as the resistance material for the erosion resistor 13. The connecting bolt 26 connects to the erosion resistor 13. In the side view of FIG. 5, two ground electrodes 19 are additionally shown. The ground electrodes 19 are bent towards the center electrode. They are bent at an angle of 90 °. Here, the bent part of the ground electrode is bent so that the outer surface of the bent part of the ground electrode lies flush with the upper edge of the insulator. The resulting spark pattern is designated by the reference number 20. It can be seen that, starting from the noble metal rod 17, the spark glides past the insulator surface and runs over an air gap to the end face of the ground electrode. Such a spark is called an air sliding spark. In order to improve the stability, the diameter of the precious metal rod, which is made here from platinum, is enlarged. The diameter is, for example, in the range from 0.3 to 1.5 mm.

The contact pin 16 is reduced in its diameter in order to additionally reduce the erosion when the spark plug capacity is discharged. The diameter of the contact pin 16 is preferably in the range from 0.3 to 1.5 mm.

Although only two ground electrodes 19 are shown in the drawing, it is within the scope of this invention to have more than two, e.g. provide three or four ground electrodes.

Claims

Expectations

1. Spark plug for an internal combustion engine with a cylindrical metal tube, in which a rod-shaped inner conductor is inserted, which is surrounded by an insulator and with a current-limiting resistor in the circuit of the spark plug, characterized in that the resistor (13) is arranged so that it in the direction of the spark gap (20) of the spark plug, up to the spark gap (20), and / or that the diameter of the inner conductor (11, 16) is smaller or equal

Is 2 mm.

2. Spark plug according to claim 1, characterized in that the resistor (13) together with the rod-shaped inner conductor (11, 16) is arranged in the region of the insulator base (27).

3. Spark plug according to claim 1 or 2, characterized in that the resistor (13) consists of an electrically conductive material, in particular a glass melt with an addition of electrically conductive particles, in particular soot or graphite particles.

4. Spark plug according to claim 1 or 2, characterized in that the resistor (13) consists of a heat-conducting carrier material, in particular aluminum nitride (AIN) and a resistor applied in thick-film technology.

5. Spark plug according to one of the preceding claims, characterized in that the rod-shaped inner conductor (11, 16) on the combustion chamber side in a noble metal rod (17) ends that the noble metal rod (17) has a diameter in the range from 0.3 to 1.5 mm, that at least two earth electrodes (19) are provided, and that the earth electrodes (19) are bent towards the noble metal rod (17), that an air gap is created between the insulator surface and the ground electrode (19).

6. Spark plug for an internal combustion engine with a cylindrical metal tube, in which a rod-shaped inner conductor is inserted, which is surrounded by an insulator, characterized in that at least the area of the inner conductor (11) that comes into contact with the ignition spark has a noble metal layer, in particular Pt, Rh-Ir layer is coated and that the inner conductor (11) at least at the end facing the noble metal layer (15) made of a heat-conducting material, in particular one of the metals Cu, Ag, Au, W, Co, Mo, Ni , Cr or alloys of these metals or from a thermally conductive ceramic material such as SiC.