Ignition coil

Abstract

The present invention relates to an ignition coil for generating a high-voltage pulse with a superimposed high-frequency voltage. The ignition coil comprises a first coil arranged on the primary side, a second coil arranged on the secondary side, a magnetic core and a third coil. The windings of the first coil and of the second coil are wound around the magnetic core. The second coil and the third coil are electrically connected to one another. A high-frequency terminal, which receives the high-frequency voltage, is electrically connected to the second coil and to the third coil.

Claims

1. An ignition coil, comprising: a ground terminal; an output terminal; a first coil; a second coil; a third coil; and a capacitor, wherein said second coil is electromagnetically coupled with said first coil, a first end of said second coil is electrically conductively connected to said ground terminal, a second end of said second coil is electrically conductively connected to a first end of said third coil, a second end of said third coil is electrically conductively connected to said output terminal, a first contact of said capacitor is electrically conductively connected to said second end of said second coil, and said first contact is electrically conductively connected to said second end of said third coil.

2. The ignition coil of claim 1, comprising: an input terminal, wherein a first end of said first coil is electrically conductively connected to said ground terminal, and a second end of said first coil is electrically conductively connected to said input terminal.

3. The ignition coil of claim 1, wherein: said third coil is electromagnetically coupled with said first coil.

4. The ignition coil of claim 1, wherein: said third coil is substantially orthogonal to said first coil and said second coil.

5. The ignition coil of claim 1, wherein: an electromagnetic coupling of said third coil to said first coil is substantially less than an electromagnetic coupling of said second coil to said first coil.

6. The ignition coil of claim 1, wherein: said first coil comprises a first number of windings, and said second coil comprises a second number of windings that is at least 10 times said first number of windings.

7. The ignition coil of claim 1, comprising: a magnetic core, wherein said first coil comprises first windings wound around said magnetic core, and said second coil comprises second windings wound around said magnetic core.

8. The ignition coil of claim 7, wherein: said third coil comprises third windings wound around said magnetic core.

9. The ignition coil of claim 7, comprising: a housing; and a dielectric resin inside said housing, wherein said dielectric resin encases said magnetic core, said first coil, said second coil, said third coil and said capacitor.

10. The ignition coil of claim 7, wherein: at least part of said third coil is situated within an imaginary, minimally-sized rectangular parallelepiped cuboid enclosing said magnetic core, said first coil, and said second coil.

11. A system, comprising: a magnetic core; a first, primary-side coil; a second, secondary-side coil; a third coil; and a capacitor, wherein said first coil comprises first windings wound around said magnetic core, said second coil comprises second windings wound around said magnetic core, said second coil and said third coil constitute at least part of a first electrical path from a first node to a second node, a first contact of said capacitor is connected to said first electrical path at a common node intermediate said second coil and said third coil, and said capacitor does not constitute part of said first electrical path.

12. The system of claim 11, wherein: said capacitor and said third coil constitute at least part of a second electrical path from a second contact of said capacitor to said second node.

13. The system of claim 11, comprising: a spark plug that constitutes at least part of a third electrical path from said second node to ground.

14. The system of claim 11, comprising: a high-frequency signal generator that generates an AC signal having a frequency greater than 100 kHz, wherein an output node of said high-frequency signal generator is connected to a second contact of said capacitor.

15. The system of claim 11, comprising: a spark plug; and a high-frequency signal generator, wherein said high-frequency signal generator generates an AC signal having a frequency greater than 100 kHz, and said high-frequency signal generator, said capacitor, said third coil, and said spark plug are connected in series.

16. The system of claim 15, wherein: said spark plug and said first electrical path are connected in series.

17. The system of claim 11, comprising: a DC voltage source; and a switch, wherein said DC voltage source, said switch, and said first coil are connected in series.

18. The system of claim 11, comprising: a dielectric resin that encases said magnetic core, said first coil, said second coil, said third coil and said capacitor.

19. The system of claim 11, wherein: at least part of said third coil is situated within an imaginary, minimally-sized rectangular parallelepiped cuboid enclosing said magnetic core, said first coil, and said second coil.

20. A method, comprising: energizing a magnetic core using a flow of DC current through a first coil at least partially wound around said magnetic core, creating a voltage pulse in a second coil at least partially wound around said magnetic core by ceasing said flow of DC current, superimposing an AC signal onto said voltage pulse using a bandpass filter, and feeding said voltage pulse superimposed with said AC signal to a spark plug, wherein said spark plug comprises a first electrode and a second electrode, said bandpass filter comprises a third coil and a capacitor, said second coil, said third coil, said first electrode, and said second electrode are connected in series, and said capacitor, said third coil, said first electrode, and said second electrode are connected in series.

Description

CONTENTS OF THE DRAWINGS

(1) The present invention will also be explained in more detail on the basis of the exemplary embodiments disclosed in the schematic figures of the drawings. In the drawings:

(2) FIG. 1A shows a circuit diagram of a first embodiment of the ignition coil,

(3) FIG. 1B shows a circuit diagram of a second embodiment of the ignition coil,

(4) FIG. 2A shows a three-dimensional illustration of the first embodiment of the ignition coil,

(5) FIG. 2B shows a three-dimensional illustration of a further implementation of the first embodiment of the ignition coil,

(6) FIG. 2C shows a three-dimensional illustration of an arrangement which is integrated in a housing and is composed of an ignition coil and a bandpass filter,

(7) FIG. 3A shows a three-dimensional illustration of a first subvariant of the second embodiment of the ignition coil,

(8) FIG. 3B shows a three-dimensional illustration of a second subvariant of the second embodiment of the ignition coil,

(9) FIG. 3C shows a three-dimensional illustration of an extension of the second subvariant of the second embodiment of the ignition coil,

(10) FIG. 3D shows a three-dimensional illustration of a third subvariant of the second embodiment of the ignition coil,

(11) FIG. 4A shows a three-dimensional illustration of an ignition coil with a first implementation for minimizing the electrical coupling of the HF voltage into the primary side of the ignition coil,

(12) FIG. 4B shows a three-dimensional illustration of an ignition coil with a second implementation for minimizing the electrical coupling of the HF voltage into the primary side of the ignition coil,

(13) FIG. 4C shows a three-dimensional illustration of an ignition coil with a third implementation for minimizing the electrical coupling of the HF voltage into the primary side of the ignition coil, and

(14) FIG. 5 shows a cross-sectional illustration of an engine block with an integrated ignition coil.

(15) The appended figures of the drawings are intended to impart further understanding of the disclosed embodiments. They illustrate embodiments and serve, in conjunction with the description, to clarify principles and concepts of the invention. Other embodiments and many of the specified advantages become apparent by considering the drawings. The elements of the drawings are not necessarily shown true to scale with respect to one another.

(16) In the figures of the drawings, identical, functionally identical and identically acting elements, features and components are respectively provided with the same reference numbers unless stated otherwise.

(17) In the text which follows, the figures are described coherently and comprehensively.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

(18) Before the geometric arrangement of the individual components in an ignition coil is explained in detail with reference to FIGS. 2A, 2B, 3A, 3B, 3C, 3D, 4A, 4B, 4C and 5, the electrical connection of the individual components of an ignition coil and an arrangement for integrating an ignition coil and a bandpass filter according to the present disclosure will be presented in the following with reference to the circuit diagrams in FIGS. 1A and 1B:

(19) In the circuit diagram in FIG. 1A, an arrangement for integrating a first embodiment of an ignition coil according to the present disclosure with a bandpass filter is illustrated:

(20) The first coil 1 is connected at one end to the electrode of a DC voltage source 4, preferably a battery, via a DC voltage terminal 2 of the ignition coil, a switch 3. The other electrode of the DC voltage source 3 is connected to a ground potential. The further electrode of the first coil 1 is also connected to a ground potential via a ground terminal 5 of the ignition coil. In the phase before the ignition of the spark plug 6, which is connected to the ignition coil, the switch 3 is closed. A DC current, which is driven by the DC voltage of the DC voltage source 5, flows through the first coil 1 of the ignition coil.

(21) In order to fire the spark plug 5, the switch 3 is opened, and therefore the flow of current through the first coil 1 is interrupted. This interruption of the flow of current induces a voltage pulse in the first coil 1. The voltage level of the voltage pulse is dependent on the inductivity of the first coil 1 and the change in current in the first coil 1, and therefore indirectly on the voltage level of the DC voltage source 4. The voltage level of the voltage pulse is therefore in the order of magnitude of several 100 V and is therefore not sufficient for igniting the fuel/air mixture within the combustion chamber by means of the spark plug 6. In order to amplify the voltage pulse induced in the first coil 1, a transformer with a magnetic core 7 is provided in the ignition coil, around which transformer the windings of the first coil 1 are wound on the primary side, and the windings of a second coil 8 and of a and of a third coil 9 are wound on the secondary side.

(22) If the number of windings in the two coils which are arranged on the secondary side is a multiple of the number of windings in the coil which is arranged on the primary side, the voltage pulse which is induced in the first coil 1 is transformed into a high-voltage pulse in the two coils which are arranged on the secondary side. In order to generate a secondary-side high-voltage pulse of several 10 kV from the primary-side voltage pulse of the level of several 100 V, a ratio between the windings of the first coil 1 and the windings of the second coil 8 and the third coil 9 is to be typically provided between 10 windings and several 100 windings.

(23) The embodiment of the magnetic core 7 and the arrangement of the first coil 1, the second coil 8 and the third coil 9 will be explained in more detail below.

(24) The one end of the second coil 8 and the one end of the third coil 9 are electrically connected to one another. The other end of the second coil 8 is connected to a ground potential by a further ground terminal 10 of the ignition coil.

(25) The other end of the third coil 9 is electrically connected to an electrode of the spark plug 6 via a high-voltage terminal 11 of the ignition coil. The other electrode of the spark plug 6 is connected to the ground potential.

(26) In order to generate a high-voltage pulse with a superimposed HF voltage, an HF terminal 12 which is associated with the ignition coil and has the purpose of feeding in an HF voltage is electrically connected to the second coil 8 and the third coil 9. This HF voltage is superimposed additively on the high-voltage pulse which is transformed into the second coil 8 and into the third coil 9. Instead of an HF voltage, an HF current can also be impressed or fed in at the HF terminal 12. The HF voltage is generated in an HF voltage source 13.

(27) In order to form a bandpass filter 14, which is implemented as a series resonant circuit composed of a coil and a capacitor, a capacitor 15 is connected between the HF source 13 and the HF terminal 12. The third coil 9 serves as a coil of the series resonant circuit and/or of the bandpass filter 15.

(28) The capacitor 15 serves at the same time as a high-pass filter. Its capacitance is dimensioned in such a way that the harmonic portions of the high-voltage pulse generated in the second coil 8 occur in the low-frequency stopband of the high pass filter, and are therefore blocked before the HF voltage source 13. Finally, the capacitor 15 also blocks the DC portion of the high-voltage pulse which is generated in the second coil 8. In the second parameterization step, the inductivity of the third coil 9 is configured in such a way that, in combination with the capacitance of the capacitor 15 which is defined in the first parameterization step, a resonance frequency of the series resonant circuit, and therefore a central frequency of the bandpass filter 14, is present at which the frequency of the generated HF voltage occurs. In this way the bandpass filter 14 is transmissive for the generated HF voltage, while it has a blocking effect for the relatively high-frequency ignition noise.

(29) With the ignition coil according to FIG. 1A, an ignition coil is therefore provided which generates a high-voltage pulse with a superimposed HF voltage, and at the same time integrates the coil of the bandpass filter with low expenditure. In the first embodiment of an ignition coil according to the present disclosure, illustrated in FIG. 1A, the coil of the bandpass filter is implemented as part of the secondary-side winding of an ignition coil. The secondary-side winding of the ignition coil is therefore composed of the serial connection of the second coil 8 and the third coil 9. The present disclosure also covers the alternative case in which the secondary-side winding of the ignition coil is implemented as a single coil which is arranged on the secondary side and comprises two coil regions which are connected to one another in a serial fashion. In this context, a so-called central contact or central terminal for feeding in the HF voltage is provided in the connecting region between the two coil regions. The integration of the coil of the bandpass filter into the secondary-side winding of the ignition coil advantageously also brings about a reduction in the overall volume of the arrangement composed of the ignition coil and bandpass filter.

(30) In a second embodiment of the ignition coil according to the present disclosure, the third coil 9 is located outside the magnetic core 7 of the ignition coil. Only the windings of the first coil 1 and of the second coil 8 are wound around the magnetic core 7. The magnetic flux is guided and concentrated in the magnetic core 7 between the first coil 1 arranged on the primary side and the second coil 8 arranged on the secondary side. A large part of the inductive coupling is therefore implemented only between the first coil 1 and the second coil 8. In the second embodiment of the ignition coil, the third coil 9 is instead arranged in the direct vicinity of the magnetic core 7 and of the first and second coils 1 and 8. The inductive coupling between the first coil 1 and the third coil 9 is therefore significantly reduced in comparison with the first embodiment. The inductive coupling between the first coil 1 and the third coil 9 is carried out here only by means of the flux leakage.

(31) The second embodiment of the ignition coil does not differ from the first embodiment in other details. A repeated description of the features and components which are identical to those in the first embodiment is therefore not given at this point.

(32) FIG. 2A presents an arrangement of a first embodiment of the ignition coil:

(33) The magnetic core 7 is constructed here from layered pieces of sheet metal, between each of which layers of electrically insulating material are arranged. The layered pieces of sheet metal are manufactured from a soft-magnetic material, preferably from iron. Eddy currents in the longitudinal direction of the magnetic core 7 are prevented by the layering of the pieces of sheet metal.

(34) The magnetic core 7 is composed of a main limb 16, two return limbs 17.sub.1 and 17.sub.2 and two yokes 18.sub.1 and 18.sub.2, which connect the two return limbs 17.sub.1 and 17.sub.2 to the main limb 16. The windings of the first coil 1, of the second coil 8 and of the third coil 9 are wound around the main limb 16. The windings of the first coil 1, of the second coil 8 and of the third coil 9 are therefore each guided through two feedthroughs in the magnetic core 7 which are respectively arranged between the main limb 16, one of the two return limbs 17.sub.1 and 17.sub.2 and in each case one region of the two yokes 18.sub.1 and 18.sub.2 in the longitudinal direction of the magnetic core 7.

(35) In addition to this embodiment of the ignition coil, which is also referred to as a shell-type transformer, an embodiment of the ignition coil is also conceivable in which the magnetic core 7 only has a single return limb. However, a greater degree of compactness of the ignition coil is implemented in this embodiment at the cost of higher flux leakage. The implementation of the ignition coil as a core-type transformer with two main limbs and two yokes connecting the two main limbs to one another is also conceivable. The windings of the first coil 1 are wound around the one main limb here, and the windings of the second and third coils 8 and 9 are wound around the other main limb. However, more compact winding of the windings which are arranged on the primary side and the windings which are arranged on the secondary side, around the associated main limb, and therefore a shorter longitudinal extent of the ignition coil requires a greater transverse extent of the ignition coil here owing to the provision of two main limbs.

(36) As illustrated in FIG. 2A, the windings of the first coil 1 preferably surround the main limb 16, firstly adjacent to the main limb 16, while the windings of the second and third coils 8 and 9 surround the windings of the first coil 1. The windings of the second and of the third coils 8 and 9 are arranged adjacent to one another in the direction of their longitudinal extent in the first implementation illustrated in FIG. 2A. The transverse extent of the second and third coils 8 and 9 and therefore also the transverse extent of the ignition coil are minimized in this implementation.

(37) The first coil 1, the second coil 8 and the third coil 9 are each wound around a winding body made of an electrically insulating material, not illustrated in FIG. 2A for reasons of clarity. Each of the winding bodies respectively serves as a spacer element between the magnetic core 7, the first coil 1, the second coil 8 and the third coil 9. The individual winding bodies are preferably connected to one another. In this way, the magnetic core 7, the first coil 1, the second coil 8 and the third coil 9 can be respectively positioned and oriented with respect to one another. In particular, an arrangement with minimized intermediate distances and therefore minimized installation space is possible with such winding bodies and all spacer elements.

(38) FIG. 2A shows the electrical connection between the second coil 8 and the third coil 9, which connection is connected to the HF terminal 12. The two ground terminals 5 and 10 of the first coil 1, and respectively the second coil 8, the DC voltage terminal 2 connected to the first coil 1 and the high-voltage terminal 11 connected to the output of the third coil 9 can be seen in FIG. 2A.

(39) According to FIG. 2C, the ignition coil is preferably arranged in a housing 19. This housing 19, indicated by dashed lines in FIG. 2C, is preferably manufactured from electrically conductive material, for example aluminum, in order to achieve a good electromagnetic shielding effect. In this way, the HF voltage which is coupled into the ignition coil does not penetrate the exterior space of the housing 19, and therefore does not have a negative effect on or cause the disruption of electronics which are arranged in the engine compartment of a vehicle. On the other hand, as a result of the shielding housing, HF electronics which are arranged in the engine compartment of a vehicle do not have adverse effects on the high-voltage pulse which is generated in the ignition coil and on the control electronics (not illustrated in FIG. 2C) of the ignition coil.

(40) The capacitor 15 is integrated into the housing 19 of the ignition coil, and therefore the bandpass filter 14 is completely integrated with it. This gives rise to a compact design of an arrangement for integrating an ignition coil and bandpass filter. In order to bring about particularly space-saving positioning within the housing 19, the capacitor 15 is, as indicated in FIG. 2C, arranged in a space, not yet occupied, within the housing 19, at a lateral distance from an end face of the magnetic core 7. Alternatively, the capacitor 15 can, however, also be arranged outside the housing 19.

(41) All the terminals of the ignition coil are, as indicated in FIG. 2C, led out of the housing 19. Respective suitable connectors, preferably housing connectors can preferably be formed for the individual terminals of the ignition coil. In this context it is to be noted that the HF terminal 12 of the ignition coil, which terminal is electrically connected to the second coil 8 and to the third coil 9, is moved to the other terminal of the capacitor 15, owing to the integration of the capacitor 15 into the housing 19, and said HF terminal 12 is therefore led out of the housing 19 as HF terminal 12′.

(42) When the ignition coil is mounted in the housing 19, a liquid sealing compound 20 composed of electrically insulating material, preferably a casting resin 20, particularly preferably polyurethane, is introduced between the housing 19 and the ignition coil and its intermediate spaces. After the curing of the sealing compound 20, the intermediate space between the housing 19 and the ignition coil is completely filled with the cured sealing compound 20. In this way, the high-voltage strength of the ignition coil between its individual components—magnetic core 7, first coil 1, second coil 8 and third coil 9—and also between the individual components of the ignition coil and the electrically conductive housing 19 is additionally increased. Moreover, the spacing between the third coil 9 which is embodied as an HF coil and the electrically conductive housing 19 and between the third coil 9 and the typically grounded magnetic core 7 is to be configured by means of the sealing compound 20 in such a way that the parasitic capacitances of the third coil 9 are at a relatively low level. The high-voltage strength of the third coil 9 which is embodied as an HF coil can be additionally improved by not only the insulation but also by the sealing compound 20 by means of an insulated HF coil, for example by means of an HF coil which is manufactured with an enameled copper wire. The first coil 1 and the second coil 8 can also be wound with an enameled copper wire in order to increase the high-voltage strength.

(43) In a second implementation of the first embodiment of the ignition coil according to FIG. 2B, the third coil 9 is not arranged, when viewed in the direction of its longitudinal extent, adjacent to the second coil 8 but rather surrounds the second coil 8. The third coil 9 is therefore arranged, when viewed in the direction of its transverse extent, adjacent to the second coil 8. The third coil 9 can be wound here onto a winding body. In order to reduce the magnetic coupling between the third coil 9 and the first coil 1 as well as the second coil 8, a foil 26 made of an easily magnetizable material, preferably made of a Mu metal, is arranged between the third coil 9 and the second coil 8. Alternatively, it is also possible to arrange a copper foil in which eddy currents are excited by the HF current flowing in the third coil 9, and therefore the electromagnetic field between the third coil 9 and the second coil 8 or the first coil 1 is damped. In order to provide electrical insulation, a foil made of a dielectric material, preferably made of a plastic, in particular made of polyurethane, is respectively arranged between the foil 26 made of magnetizable material or the copper foil, and the third coil 9 as well as the second coil 8.

(44) In the first implementation of the first embodiment of an ignition coil according to FIG. 2A it is also possible to respectively arrange, for the sake of a more compact design, a dielectric plastic film, instead of winding bodies, between the first coil 1 and the second coil 8 or the third coil 9.

(45) In both implementations of the first embodiment of an ignition coil according to FIGS. 2A and 2B, the third coil 9 can be configured like the second coil 8 in respect of its transmission characteristic, in particular its HF transmission characteristic. However, since an HF current which is driven by the applied HF voltage is to flow through the third coil 9 in as optimum a way as possible, while electrical coupling of the HF current into the second coil 8 is to be minimized as far as possible, high-frequency technical optimization of the third coil 9 is to be aimed at, as is presented below:

(46) In a first technical measure, for this purpose the distances between respective successive windings of the third coil 9 are configured to be larger than the distances between respective successive windings of the second coil 8. The parasitic capacitances, which occur, in particular, between two successive windings, in the third coil 9 are therefore minimized in comparison with the second coil 8, and in this way the HF transmission characteristic of the third coil 9 is optimized in comparison with the second coil 8.

(47) In a second technical measure, the parasitic capacitances in the third coil 9 are minimized by a particular way of winding the electrical conductor. The third coil 9 is wound, for example, to form a honeycomb coil, a basket coil, star coil or flat coil. In this way, the HF transmission behavior of the third coil 9 can be optimized in comparison with the second coil 8. An additional improvement of the HF transmission behavior for the third coil 9 is achieved by the winding of an HF braded conductor as an electrical conductor for the third coil 9.

(48) In a third technical measure, the wire diameter, i.e. the diameter of the electrical conductor, of the third coil 9 is configured to be larger than the wire diameter of the second coil 8. The HF current flows only on the surface of the electrical conductor of a coil owing to the skin effect, and said current penetrates, starting from the surface of the electrical conductor, only as far as a specific penetration depth, which depends inter alia on the frequency of the HF current and on material parameters of the electrical conductor, into the electrical conductor of the coil. Therefore, in the case of an electrical conductor with a relatively large diameter and an identical penetration depth, the cross-sectional area of the electrical conductor of the coil in which the HF current flows is larger owing to the relatively large circumference than in an electrical conductor with a relatively small diameter. The electrical impedance of the third coil 9, which acts on the HF current, is therefore smaller than in the case of the second coil 8, by virtue of the second technical measure. The HF transmission characteristic is therefore improved in the third coil 9 in comparison with the second coil 8.

(49) In a fourth technical measure, the third coil 9 is coated, while the second coil 8 remains without a coating. The coating of the third coil 9 has a lower electrical impedance than the basic material of the third coil 9. Therefore, the coating is manufactured from a coating material which has a higher electrical conductivity and/or lower permeability than the basic material. The HF current, which flows in the surface region of the electrical conductor of the coil owing to the skin effect, consequently experiences a better HF transmission characteristic in the third coil 9 than in the second coil 8.

(50) At this point is to be noted that the inductivity of the basic material of the second coil 2 is larger by a multiple than the total inductivity of the basic material and coating material of the third coil 9, with the result that the HF current preferably flows through the third coil 9 owing to the significantly higher impedance of the second coil 8.

(51) In the second embodiment of an ignition coil, which is presented in the following with reference to FIGS. 3A, 3B, 3C and 3D, the third coil 9 does not have a magnetic core and is therefore implemented as an air coil. In a suitably selected orientation of the third coil 9 with respect to the magnetic core 7, it is possible to significantly minimize the magnetic and inductive coupling between the third coil 9 and the first coil 1 by means of the magnetic flux which is guided and concentrated in the magnetic core 7. Magnetic and inductive coupling to the first coil 1 is implemented only via the flux leakage which occurs in a significantly weaker form. In contrast to the first embodiment of an ignition coil, the magnetic and inductive coupling of the HF voltage from the secondary side into the primary side of the ignition coil is significantly minimized.

(52) In the first subvariant of the second embodiment of an ignition coil according to FIG. 3A, the third coil 9 which is embodied as an air coil is positioned at a lateral distance from an end face 21 of the magnetic core 7. Moreover, the third coil 9 surrounds, with its windings, at least one region of the first coil 1 and of the third coil 8, which region corresponds to the region, projecting out of the magnetic core 7, of the first coil 1 and of the third coil 8.

(53) Therefore, the third coil 9 takes up the still unused space to the side of the magnetic core 7, which space is not used by the first coil 1 and the second coil 8. However, in order to achieve a compact design of the ignition coil, the third coil 9 is positioned near to the magnetic core 7 and at the first and second coils 1 and 8. In this way, a compact design is implemented for the ignition coil. Of course, in the arrangement of an ignition coil illustrated in FIG. 3A, the third coil 9 can be arranged not only above the magnetic core 7 but also below the magnetic core 7.

(54) Finally, the cross-sectional face of the third coil 9 is oriented parallel to the end face 21 of the magnetic core 7. As a result of this orientation of the third coil 9 with respect to the magnetic core 7, the magnetic field of the third coil 9 runs orthogonally with respect to the direction of the magnetic flux of the first and second coils 1 and 8 within the magnetic core 7. Only in the junction region between the main limb and the two yokes of the magnetic core 7 is the orthogonality in the orientation of the magnetic field of the third coil 9 with respect to the magnetic flux within the magnetic core 7 not given to a slight extent. However, since this junction region is very small and is not located at the maximum of the magnetic field strength of the third coil, magnetic and inductive coupling between the third coil 9 and the two other coils of the ignition coil, in particular the first coil 1, is minimized as far as possible.

(55) In a second subvariant of the second embodiment of an ignition coil, the third coil 9 is also positioned at a lateral distance from an end face 21 of the magnetic core 7. The third coil 9 is arranged here laterally adjacent either to one of the two yokes or to one of the two return limbs of the magnetic core 7. Therefore, in the second subvariant, the third coil 9 also takes up the still unused space to the side of the magnetic core 7, which space is not used by the first coil 1 and the second coil 8. In this case a compact design for the ignition coil is also achieved.

(56) In the second subvariant, the cross-sectional face of the third coil 9 is positioned perpendicularly with respect to an end face 21 of the magnetic core 7. In the second subvariant, the magnetic field of the third coil 9 is also oriented within the magnetic core 7 orthogonally with respect to the direction of the magnetic flux of the first and second coils 1 and 8, which is guided in the magnetic core 7. Only in the junction region between the main limb and the two yokes of the magnetic core 7 is the orthogonality between the magnetic field of the third coil 9 and the magnetic flux, guided in the magnetic core, of the first and second coils 1 and 8 not given to a slight extent. Since the coil length is typically greater than the wire diameter of the third coil 9, the orthogonality between the magnetic field of the third coil 9 and the magnetic flux, guided in the magnetic core, of the first and second coils 1 and 8 in the junction region between the main limb and the two yokes of the magnetic core 7 is implemented to a slightly less well in the second subvariant than in the first subvariant. However, since the junction region is also comparatively very small here and is not located at the maximum of the magnetic field strength of the third coil 9, the magnetic coupling between the third coil 9 and the first and second coils 1 and 8 is also reduced in the second subvariant of the second embodiment.

(57) In the second subvariant, the third coil 9 has a lower cross-sectional face than in the first subvariant, and therefore has a lower inductivity. As has already been mentioned above, for the configuration of the bandpass filter 14, a comparatively high inductivity is necessary for the third coil 9 at a given frequency of HF voltage and in the case of a comparatively low capacitance of the capacitor 15.

(58) For this purpose, in an extension of the second subvariant of the second embodiment of an ignition coil according to FIG. 3C, a plurality of third coils 9.sub.1, 9.sub.2, 9.sub.3 and 9.sub.4 are connected in series. With each third coil which is additionally connected in series the total inductivity of such a serial connection of third coils is increased by the inductivity of a single third coil.

(59) Since a third coil 9 can be respectively positioned at a lateral distance at each yoke and at each return limb of the magnetic core 7 and at each of the two end faces 21 of the magnetic core 7, up to eight third coils can be positioned and connected in the ignition coil. In this way, the total inductivity of such a serial connection of third coils can be multiplied by a factor of eight in comparison with the inductivity of a single third coil.

(60) In the first subvariant, the inductivity of the third coil 9 can also be doubled if a third coil is respectively positioned at a lateral distance from the two end faces 21 of the magnetic core 7, and the two third coils are connected in series with respect to one another.

(61) In a third subvariant of the second embodiment of an ignition coil according to FIG. 3D, the third coil 9 is positioned to the side of the lateral face of the first coil 1 and of the second coil 8, preferably to the side of the lateral face of the second coil 8 which is arranged on the outside. Owing to the lateral positioning of the third coil 9 with respect to the first and second coil 1 and 8, the design of the ignition coil in the third subvariant of the second embodiment is degraded to a certain extent over all the subvariants and embodiments presented until now. However, in the third subvariant, owing to the greater distance between the third coil 9 and the magnetic core 7 it is possible to implement lower eddy current losses in the magnetic core 7, i.e. lower HF losses of the third coil 9 through which an HF current flows, at the cost of the smaller degree of compactness of the ignition coil. The magnetic and inductive coupling between the third coil 9 and the two coils of the ignition coil, in particular the first coil 1, is also reduced owing to the larger distance between the third coil 9 and the magnetic core 7. Finally, in the third subvariant it is possible to implement a greater degree of inductivity for the third coil 9, since free spaces are provided for lengthening the third coil 9 and for increasing the size of the cross-sectional face of the third coil 9.

(62) In addition to the minimizing of the magnetic coupling between the third coil 9 and the two other coils of the ignition coil, in particular the first coil 1, the electrical coupling of the HF voltage from the HF terminal 12 into the second coil 8 is to be additionally minimized. The minimization of the electrical coupling of the HF voltage from the HF terminal 12 into the second coil 8 is explained in detail in the following with reference to FIGS. 4A to 4C:

(63) in a first variant for minimizing the electrical coupling of the HF voltage from the HF terminal 12 into the second coil 8 according to FIG. 4A, an ohmic resistor 22 is connected between the HF terminal 12 and the second coil 8. In order to achieve a design for the ignition coil which is compact as possible, the ohmic resistor 22 is preferably to be positioned to the side of one of the two end faces 21 of the magnetic core 7, in a space which is not yet used by the first coil 1, the second coil 8 or the third coil 9.

(64) The ohmic resistor 22 is dimensioned in such a way that an HF current which is driven by the HF voltage at the HF terminal 12 is damped in such a way that only a comparatively low HF current flows through the second coil 8. The ohmic resistor 22 is, moreover, to be dimensioned in relation to the ohmic resistor within the second coil 8 in such a way that the HF voltage level at the junction between the second coil 8 and the ohmic resistor 22 is significantly lower than at the HF terminal 12.

(65) The ohmic resistor 22 also damps, as an additional positive effect, spark plug current which is driven by the high-voltage pulse. A relatively high-frequency interference current, which is caused by the ignition process, is superimposed on this spark plug current which brings about ignition of fuel/air mixture in the combustion chamber. The relatively high-frequency interference current which is superimposed in the spark plug current is disadvantageously output from the spark plug as EMC interference and irradiated in the feedline of the spark plug. Since the level of the relatively high-frequency interference current is dependent on the level of the spark plug current, the EMC irradiation can be effectively reduced by the damping of the spark plug current by means of the ohmic resistor 22.

(66) In a second variant for minimizing the electrical coupling of the HF voltage from the HF terminal 12 into the second coil 8 according to FIG. 4B, a further coil 23, which is referred to in the following as the fourth coil 23, is connected between the HF terminal 12 and the second coil 8. This fourth coil 23 is embodied as an HF coil and is therefore implemented as an air coil in view of minimizing the HF losses. The fourth coil 23 is preferably embodied as an inductor and damps, with its inductive impedance, the HF voltage fed in at the HF terminal 12. At the junction between the fourth coil 23 and the second coil 8 there is consequently an HF voltage level which is reduced in comparison with the voltage level of the HF voltage at the HF terminal 12.

(67) With a view to achieving a compact design of the ignition coil, the fourth coil 23 which is implemented as an air coil is positioned, in a way analogous to the third coil 9 in the first subvariant of the second embodiment of an ignition coil, at a lateral distance from an end face 21 of the magnetic core 7 and surrounds the region, projecting out of the magnetic core 7, of the first coil 1 and of the second coil 8. According to FIG. 4B, the third coil 9 and the fourth coil 23 are each positioned at a lateral distance from two different end faces 21 of the magnetic core 7, with the result that an ignition coil with the highest possible degree of compactness is implemented.

(68) The cross-sectional face of the fourth coil 23 is oriented, in a way analogous to the cross-sectional face of the third coil 9, parallel to an end face 21 of the magnetic core 7. In this way, the magnetic field both of the third coil 9 and of the fourth coil 23 are respectively oriented orthogonally with respect to the direction of the magnetic flux of the first coil 1 and of the second coil 8 within the magnetic core 7. The magnetic and inductive coupling of the third coil 9 and also of the fourth coil 23 with respect to the first coil 1 and with respect to the second coil 8 is therefore reduced.

(69) According to FIG. 4C the fourth coil 23 can be positioned, in a way analogous to the third coil in the second subvariant of the second embodiment of an ignition coil, at a lateral distance from an end face 21 of the magnetic core 7 and at the same time can be oriented with its cross-sectional face perpendicularly with respect to an end face 21 of the magnetic core 7. The third coil 9 and the fourth coil 23 can, according to FIG. 4C, each be positioned at a lateral distance from two different end faces 21 of the magnetic core 7.

(70) In a way analogous to the extension of the second subvariant of the second embodiment of an ignition coil, it is possible, with a view to increasing the inductivity of the fourth coil 23, to connect a plurality of fourth coils 23 in series and to arrange them in a space-optimized fashion within the ignition coil.

(71) In a third embodiment of an ignition coil which is illustrated in FIG. 5, the third coil 9 is arranged in the connecting shaft 24 of an engine block 25 with a view to achieving a compact design. The third coil 9 is positioned here to the side of the lateral face of the first coil 1 and of the second coil 8, preferably to the side of the lateral face of the second coil 8 which is arranged on the outside.

(72) The cross-sectional face of the third coil 9 is oriented here parallel to an end face 21 of the magnetic core 7. In this way, the magnetic field of the third coil 9 is oriented orthogonally with respect to the magnetic flux of the first coil 1 and of the second coil 8, which magnetic flux is guided in the magnetic core 7. Therefore, the magnetic and inductive coupling between the third coil 9 and the first coil 1 is minimized with the exception of the coupling by the flux leakage.

(73) The housing 19 of the ignition coil, which is indicated by dashed lines in FIG. 5, is configured in such a way that it contains all the components of the ignition coil and can be introduced into the connecting shaft 24 of the engine block 25.

(74) Although the present invention has been described completely above on the basis of preferred exemplary embodiments, it is not limited thereto but rather can be modified in a variety of ways.

LIST OF REFERENCE NUMERALS

(75) 1 first coil 2 DC voltage terminal 3 switch 4 DC voltage source 5 ground terminal 6 spark plug 7 magnetic core 8 second coil 9 third coil 9.sub.1, 9.sub.2, 9.sub.3, 9.sub.4 third coil 10 mass terminal 11 high-voltage terminal 12,12′ high-frequency terminal 13 high-frequency voltage source 14 bandpass filter 15 capacitor 16 main limb 17.sub.1, 17.sub.2 return limb 18.sub.1, 18.sub.2 yoke 19 housing 20 sealing compound 21 end face 22 ohmic resistor 23 fourth coil 24 connecting shaft 25 engine block 26 foil