US3693607A

US3693607A - Make-and-break spark plug

Info

Publication number: US3693607A
Application number: US105403A
Authority: US
Inventors: Max Pasbrig
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-01-11
Filing date: 1971-01-11
Publication date: 1972-09-26
Anticipated expiration: 1989-09-26

Abstract

There is disclosed a make-and-break type of spark plug for piston engines comprising a spark plug housing equipped with a counter electrode and a movable electrode positioned to cooperate with said counter electrode to form therebetween a short-circuit path. An armature is movably arranged within the spark plug housing and a magnetic coil acts upon said armature. Means serve to connect the armature with the movable electrode, and said armature includes an end face directed away from said movable electrode with a pressure equalization compartment being located within said spark plug housing at the region of said end face of said armature facing away from said movable electrode. At least one bore communicates the pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said at least one bore extending through said armature, said connecting means for said armature with said movable electrode and through both of said electrodes.

Description

Elnited States Patent Pasbrig 1 1451 Sept. 26, 1972 {54] MAKE-AND-BREAK SPARK PLUG [72] Inventor: Max Pasbrig, Casa Luce, Via Eco,

6644 Orselina, Switzerland [22] Filed: Jan. 11, 1971 [21] Appl. No.: 105,403

52 U.S. c1 ..123/143 R, 123/1465 R, 123/169 R, 123/169 CL, 123/169 EL, 313/152 51 Int, Cl ..F02p 23/00 [58] Field of Search ..l23/l43 R, 146.5 R, 169 R, 123/169 CL, 169 EL, 169 EA; 13/4; 313/152 Primary Examiner-Laurence M. Goodridg Attorney-Werner W. Kleeman [5 7] ABSTRACT There is disclosed a make-and-break type of spark plug for piston engines comprising a spark plug housing equipped with a counter electrode and a movable electrode positioned to cooperate with said counter electrode to form therebetween a short-circuit path. An armature is movably arranged within the spark plug housing and a magnetic coil acts upon said armature. Means serve to connect the armature with the movable electrode, and said armature includes an end face directed away from said movable electrode with a pressure equalization compartment being located within said spark plug housing at the region of said end face of said armature facing away from said movable electrode. At least one bore communicates the pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said at least one bore extending through said armature, said connecting means for said armature with said movable electrode and through both of said electrodes.

12 Claims, 3 Drawing Figures MAKE-AND-BREAK SPARK PLUG BACKGROUND OF THE INVENTION The present invention relates to a new and improved make-and-break type spark plug for piston engines, which is of the type where a magnetic coil acts upon an armature actuating a movable electrode and at which an ignition pulse passes through the magnetic coil and a short-circuit path betweenthe electrode and a counter electrode (ground electrode), whereby the magnetic field building up in the magnetic coil brings about a socalled tearing up or breaking away of the short-circuit path under corresponding spark formation.

Make-and-break spark plugs possess the advantage in contrast to conventionally employed spark plugs with a fixed electrode that they can produce powerful ignition sparks at relatively low voltages. At a time where the nowadays employed high ignition voltages are still poorly controlled attempts have again been made to employ in practice make-and-break spark plugs, yet without these efforts having brought about any extensive success. The reason therefore is probably based upon the fact that the problems arising by virtue of the burn-off, the charring or pitting of the interrupter contacts, the soot build-up and similar disturbances could only be partially overcome, and in particular great difficulties arose because of the pronounced and periodic altering pressure. Above all, these difficulties are predicated upon the fact that the movable electrode must be torn away by the armature of the magnetic coil from the fixed or stationary electrode in a rapid time sequence, and specifically corresponding to the rotational speed of the piston engine, so that the additional difficulty arose that the breaking or tearing away movement must occur against the compression force, requiring the armature and the magnetic coil to exert relatively high forces.

SUMMARY OF THE INVENTION Accordingly, there is still present a real need in the art for an improved make-and-break spark plug which is not associated with the aforementioned drawbacks of the prior art constructions. Therefore, a primary objective of the present invention is to provide such type spark plug which effectively and reliably fulfills the existing need and is not associated with the previously explained drawbacks.

In consideration of the foregoing explained problem of the state-of-the-art spark plugs of this type a further specific object of the present invention therefore is concerned with reducing the forces which must be exerted by the armature for the purpose of actuating the movable electrode and to that end especially making use of the pressure in the internal cylinder compartment, that is to say, the compression pressure.

Now, in order to implement these and still further objects of the invention, which will become more readi- By virtue of the pressure equalization afforded by the teachings of the invention the armature forces required for tearing away of the movable electrode are considerably reduced. The requisite armature force must essentially only apply the necessary inertia force for shifting away the movable electrode and essentially need not overcome any additional pressure forces since such are almost completely balanced out by the pressure equalization chamber and the pressure equalization bore means.

The pressure increasing to a relatively high value within the cylinder should, on the one hand, lightly press the electrode against the counter electrode so that a pronounced magnetic field builds up at the magnetic coil before the armature raises the electrode against the action of this force and the stress of an associated spring from the counter electrode. However, on the other hand this pressure action should not reach a magnitude where lifting or raising of the electrode is questionable. At low voltages a small gap between both electrodes constitutes a relatively high electrical resistance. Since the magnetic coil and this gap are located in series this means that the current delivered to the spark plug drops quite markedly already at the beginning of the tearing away or contact break movement. Therefore, the magnetic field should be completely built-up already at the beginning of such contact-break movement and have reached its optimum value. However, when the contact-break movement has once been started, then, the compressive force built-up in the cylinder, and if necessary, together with the initiated combustion, should support the contact-break movement, so that the movable electrode can assume, as rapidly as possible, its favorable spacing from the fixedelectrode. The collapsing magnetic field and, as the case may be in conjunction with the still flowing current then delivers a series of pronounced ignition sparks which distribute over the ring-shaped electrode surface. The tearing away or contact-break movement therefore should occur, as contemplated by the invention, in the form of a catapulting movement.

The thermal value or thermal coefficient does not play any, or only a very slight, role for the inventive spark plug. It need not be feared that the spark plugs of the engine, which normally require a small thermal coefficient, will be soiled, while, on the other hand, the spark plug is also suitable for motors with a high thermal coefficient, without having to fear overheating. This is predicated upon the fact that the electrode is ring-shaped and in comparison to conventional spark plugs or to heretofore known make-and-break spark plugs is quite large. The forceful current flow prior to the contact-break movement, the catapulting of the electrode, and the relatively forceful rebound takes care of possible combustion residues. The thermal conductivity between the electrode and the counter electrode affords rapid heat withdrawal.

The movement of the armature and the electrode occurs axially with respect to the cylinder opening, the movable electrode upon tearing-away or breaking the short circuit path being catapulted or displaced in the direction of the interior of the cylinder away from the counter electrode.

According to a preferred embodiment of the invention the electrode extends past the periphery of the electrode support providing the connection with the armature and bears with its protruding marginal portion, in its rest position at a neighboring end surface of the spark plug housing forming the counter electrode.

In each instance excitation of the magnetic coil is periodically brought about by current pulses controlled by a thyristor as a function of the rotational speed of the engine, so that a series of ignition or firing sparks which continuously wander upon the surface of the electrode appear between the movable electrode and the neighboring end surface of the spark plug housing.

The electrode should be formed of a suitable materi- -al possessing a very high molecular weight and a melting point exceeding 3,000 C. It is preferred to use molybdenum or a molybdenum alloy having a melting point of about 3,4l C. I 4

According to a preferred embodiment of the invention the contact surface between the movable electrode and the counter electrode, which contact surface forms the short-circuit path is constructed to possess a substantially ring-shaped configuration, the external diameter of the ring-shaped contact surface possessing, for instance, a value of at least millimeters. The largest contact surface furthermore can possess an area of at least 20 mm.

The voltage of the ignition pulse preferably amounts to 30 to 50 volts whereas the current intensity is between 0.3 and 3.4 amperes. Additionally, the invention is concerned with an ignition system for piston engines consisting of a battery or another current source, a distributor and a number of spark plugs corresponding to the number of cylinders of the engine. The combination of the spark plugs as make-and-break spark plugs and that each spark plug has associated therewith a thyristor which periodically delivers a low voltage pulse to the associated spark plug is considered to be novel.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now the drawings, FIG. 1 illustrates a preferred embodiment of inventive spark plug which will be seen to comprise a housing or shell 1 formed of an electrically conductive non-magnetic metal and essentially possesses the external configuration of a stepped circular cylinder. Housing 1 encloses a continuous hollow bore or compartment la which widens at its upper region into a compartment or chamber 2.

The lower housing portion possesses an external threading 3 by means of which housing 1, and specifically by further using a sealing ring 4 as employed with conventional spark plugs, can be threadably connected into a threaded bore provided at the cylinder head of the engine. The lowermost portion of the housing bore la is considerably narrowed so that there only remains a central throughpassage 5.

Continuing, it will be seen that a cylindrical insulating sleeve 6 is mounted within the housing 1, above which sleeve there is located a mica insulating disc 7. A soft iron body member 8 constructed in the form of a stepped cylinder body bears against the insulating disc 7, the lower portion of soft iron body 8 being surrounded by the upper portion of the housing 1. The uppermost edge 9 of the housing wall is inwardly flexed so that the soft iron body member 8 is fixedly clamped within the housing 1. An insulating ring 10 as well as a further insulating disc 11 prevents all electrical contact between the soft iron body member 8 and the housing 1.

A continuous bore 8a piercingly extends through the soft iron body member 8, this continuous bore 8a having an upper somewhat widened bore section 12. in which there is accommodated a helical spring 13. The lower end of the helical spring 13 bears upon the substantially ring-shaped stepped surface of the bore. In the assembled condition of the device as shown in FIG. 2, the upper end of the helical spring 13 continuously presses against the lower ring-shaped surface of a circular substantially cylindrical armature 14 fabricated from soft iron. This armature 14 is rigidly connected with a substantially circular or round rod member 15 fabricated from a non-magnetizable yet electrically conductive material. Rod member 15 extends through the respective coaxial central bores 8a and 6a of the soft iron body member 8 and the insulating sleeve 6 and carries at its lower end an electrode 16. In the illustration of FIG. 1, the circular rod 15 with the armature 14 has been omitted to preserve clarity in illustration. The electrode 16 is constructed in the form of a hollow cylinder, from the inner wall of which there extends up to its central region a web 17, and at that location is rigidly connected with the lower end of the rod member 15. The substantially ring-shaped end surfaces of the shell or housing 1 and the electrode 16 which confront one another are preferably ground flat and size-matched to one another.

The armature 14, the circular rod member 15 and the electrode 16 possess a pressure equalization bore means 31 which communicates the interior of the engine cylinder with a pressure equalization chamber or compartment 21 disposed above the armature 14. The pressure increase brought about by the piston ascending within its cylinder acts, on the one hand, from below upon the lower ring-shaped surface of the electrode 16 and, on the other hand, acts from above upon the end face of the armature 14. If the electrode 16 is raised from its counter electrode or seat 1b then the pressure equalizes from the inside via the ring-shaped opening 30 in the electrode 16 and, of course, also from the outside, so that only the pressure differential of the cross-section of the throughbored circular rod member 15 acts upon the electrode 16, whereby the pressure effect is maintained at the armature 14.

The uppermost portion or section of the soft iron body member 8 extends into the internal compartment of a magnetic coil 18 which is mounted in a coil support 19 and closed towards the outside by a protective cap or sleeve 20. The coil support 19 consists of an electrically conductive, yet non-magnetizable metal and is threadablyconnected, while interposing a ring-shaped seal 27 with the upwardly extending section 12 of the soft iron body member 8 equipped with external threading 12a. The pressure equalization compartment remaining above the armature 14 possesses a height h which is greater than the height of the electrode 16, so that even if there occurs an extensive erosion or burning-off of the electrode 16 it still possesses a sufficient size in the sense of the described pressure play.

The coil winding 18 is connected at location 22 to the metallic coil support 19. The other end of this coil winding 18 is connected via a connection pin 23 and a plate 24.riveted therewith to a connection pin 25. The plate 24 is electrically insulated by an insulating disc 26 with respect to the coil support 19.

The length of the hollow circular rod is dimensioned such that in the illustrated rest portion of the electrode a certain amount of play a exists between the lower edge of the armature 14 and the upper edge of the therebelow situated soft iron body member 8, this play a corresponding to the maximum size of the spark gap.

The mode of operation of the complete ignition system is as follows:

By referring to FIG. 3 it will be understood that the voltage of a conventional automobile battery 32 is stepped up to a value exceeding volts by means of a suitable transformer 33. A number of thyristors 34 corresponding to the number of cylinders of the engine are electrically connected on the one hand with the transformer 33 and, on the other hand, with the connection pin 25 of each respective associated spark plug. The thyristors 34 are controlled by a distributor 35 which, in known manner, for four-cycle engines delivers, for each second engine revolution, a square-wave control pulse to each thyristor 34. Normally the square-wave control pulse will possess a pulse width of one threethousandths of a second. Triggering of the pulses occurs in known manner, for instance by a revolving magnet. Mechanically moved contacts are generally not desired, yet could be possibly utilized in the existing installation. Upon the arrival of a squarewave pulse the corresponding thyristor 34 opens or fires so that a current of about 0.5 to 2 amperes flows via the connection pin 25 through the magnetic coil 18 and via the soft iron body member 8 as well as the circular rod member 15 to the electrode 16 and via the short-circuit path between the electrode 16 to the spark plug body to ground. The magnetic field which builds-up produces a downwardly directed force in the showing of the drawing at the armature 14 by means of the soft iron body member 8. When this force overcomes the pressure of the helical spring 13 and the already described difference of the compression force, then, the electrode 16 is catapulted in the direction of the interior of the engine cylinder. As a result, there generally occurs a series of ignition sparks which distribute themselves about the ring-shaped surface of the electrode 16 and which are supplied from the still flowing current and from the collapsing magnetic field of the magnetic coil 18. Upon collapse of the magnetic field the helical spring 13 again forces the armature 14 back into its starting position, where the electrode 16 again bears against the counter electrode formed by the spark plug housing.

A particular advantage of the spark plug of the invention resides in the ability to be able to choose the relationship of an automatic regulation of the firing time as a function of the pressure play. The point of time of tearing away of the short-circuit path between the electrode 16 and the counter electrode, on the one hand, depends upon the occurrence with time of the control pulses at the associated thyristor 34, however, also, on the other hand upon the course of the pressure build-up in the cylinder of the motor, which can be used for improved combustion.

Experiments have shown that when using spark plugs designed according to the teachings of the present invention there exists a considerable reduction in the content of the carbon monoxide in the exhaust gases, so that when inspecting the automobile no significant thought need be given about permitting an engine equipped with spark plugs designed according to the invention to run in a closed area.

The bum-ofi' or erosion at the electrode is still nonetheless acceptable notwithstanding the pronounced spark formation. During a trial run it was found that upon travelling 15,000 kilometers there existed a bum-off or erosion of 2.5 millimeters at the electrode 16 and 2.5 millimeters at the counter electrode.

During replacement the upper portion of the housing is threaded away from the external threading at the section 12 of the soft iron body member 8 and the lower portion of the spark plug is exchanged.

Those skilled in the art will be readily capable of undertaking suitable modifications and accommodating the spark plugs to particular conditions.

By virtue of the fact that the contact surfaces 1b and 16a or short-circuit path between the movable electrode 16 and the counter electrode formed by surface 1b are constructed to possess a substantially ringshaped configuration there results a particularly uniform erosion or bum-off. If any given location is somewhat more pronouncedly burned-off, then at that location ignition sparks no longer form for a certain time whereas protruding parts constitute preferred locations for the spark formation. Moreover, the saving in fuel with engines equipped with spark plugs designed according to the present invention is considerable and in certain instances exceeds 10 percent.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. A make-and-break spark plug for piston engines comprising a spark plug housing equipped with means providing a counter electrode, a movable electrode positioned to cooperate with said counter electrode, said movable electrode and counter electrode forming therebetween a short-circuit path, an armature movably arranged within said spark plug housing, a magnetic coil for acting upon said armature, means for connecting said armature with said movable electrode, said armature including an end face directed away from said movable electrode, means providing a pressure equalization compartment within said spark plug housing at the region of said end face of said armature facing away from said movable electrode, the height of said pressure equalization compartment being greater than the height of said movable electrode, and means providing at least one bore for communicating said pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said means providing said at least one bore extending through said armature, said connecting means for said armature with said movable electrode, and through both of said electrodes.

2. A make-and-break sparkplug for piston engines comprising a spark plug housing equipped with means providing a counter electrode, a movable electrode positioned to cooperate with said counter electrode, said movable electrode and counter electrode forming therebetween a short-circuit path, an armature movably arranged within said spark plug housing, a magnetic coil for acting upon said armature, means for connecting said armature with said movable electrode, said armature including an end face directed away from said movable electrode, means providing a pressure equalization compartment within said spark plug housing at the region of said end face of said armature facing away from said movable electrode, and means providing at least one bore for communicating said pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said means providing said at least one bore extending through said armature, said connecting means for said armature with said movable electrode, and through both of said electrodes, said movable electrode and said counter electrode possessing confronting contact surfaces forming therebetween said short-circuit path and possessing a substantially ring-shaped configuration.

3. The spark plug as defined in claim 1, further including an ignition circuit for such sparkplug embodying thyristor means for producing ignition pulses.

4. The sparkplug as defined in claim 1, wherein said armature and said movable electrode are arranged for substantially coaxial movement.

5. The spark plug as defined in claim 4, wherein said movable electrode is displaced away from said counter electrode in a direction away fromsaid spark plughousing to break-up said short-circuit path.

6. A make-and-break spark plug for piston engines comprising a spark plug housing equipped with means providing a counter electrode, a movable electrode positioned to cooperate with said counter electrode, said movable electrode and counter electrode forming therebetween a shorbcircuit path, an armature movably arranged within said spark plug housing, a magnetic coil for acting upon said armature, means for connecting said armature with said movable electrode, said armature including an end face directed away from said movable electrode, means providing a pressure equalization compartment within said spark plug housing at the region of said end face of said armature facing away from said movable electrode, and means providing at least one bore for communicating said pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said means providing said at least one bore extending through said armature, said connecting means for said armature with said movable electrode, and through both of said electrodes, said movable electrode being displaced away from said counter electrode in a direction away from said spark plug housing to breakup said short-circuit path, said movable electrode extending past the periphery of said means for connecting said armature with said movable electrode, said movable electrode incorporating a protruding marginal portion which in the rest position of said movable electrode bears against a neighboring end contact surface of said spark plug housing providing said counter electrode, said bore being provided at said armature, at said connecting means and at said movable electrode to permit the pressure prevailing in the engine cylinder to be transmitted to said pressure equalization'compartment and to thus support the axial movement of said movable electrode, and .ignition circuit means for delivering thyristor-controlled current pulses as a function of the rotational speed of the engine to-the magneticcoil, so that between the movable electrode and said neighboring end contact face of said spark plug housing there exists an ignition spark which continuously wanders upon the surface of said movable electrode.

7. A make-and-break spark plug for piston engines comprising a spark plug housing equipped with means providing a counter electrode, a movable electrode positioned to cooperate with said counter electrode, said movable electrode and counter electrode forming therebetween a short-circuit path, an armature movably arranged within said spark plug housing, a magnetic coil for acting upon said armature, means for connecting said armature with said movable electrode, said armature including an end face directed away from said movable electrode, means providing a pressure equalization compartment within said spark plug housing at the region of said end face of said armature facing away from said movable electrode, and means providing at least one bore for communicating said pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said means providing said at least one bore extending through said armature, said connecting means for said armature with said movable electrode, and through both of said electrodes, said armature being provided with a central bore, said connecting means comprising a through-bored rod member seated in said central bore and connected with said movable electrode.

8. The spark plug as defined in claim 7, wherein said movable electrode consists of a substantially ringshaped member having a radially extending web to which there is secured said rod member.

9. A make-and-break spark plug for piston engines comprising a spark plug housing equipped with means providing a counter electrode, a movable electrode positioned to cooperate with said counter electrode, said movable electrode and counter electrode forming therebetween a short-circuit path, an armature movably arranged within said spark plug housing, a magnetic coil for acting upon said armature, means for connecting said armature with said movable electrode, said armature including an end face directed away from said movable electrode, means providing a pressure equalization compartment within said spark plug hous ing at the region of said end face of said armature facing away from said movable electrode, and means providing at least one bore for communicating said pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said means providing said at least one bore extending through said armature, said connecting means for said armature with said movable electrode, and through both of said electrodes, said electrode being formed of a high molecular weight material having a melting point exceeding 3,000 C.

10. The spark plug as defined in claim 9, wherein said electrode is formed of a material selected from molybdenum and molybdenum alloys.

11. The spark plug as defined in claim 1, wherein said housing of said spark plug is constructed as a bipartite member, one part of which houses said magnetic coil, whereas the other part of which is an exchangeable housing portion which can be threadably connected with the engine cylinder and can be replaced after pronounced bum-off of the movable electrode.

12. An ignition system for piston engines comprising a power source, a distributor in circuit with said power source, a number of spark plugs corresponding to the number of cylinders of the engine, transformer means for stepping up the voltage of the power source so as to exceed 30 volts, said spark plugs comprising make-andbreak spark plugs, each such spark plug including a magnetic coil and a movable electrode and a cooperating counter electrode forming therebetween a short circuit path, a thyristor in circuit with the magnetic coil of each spark plug and said transformer means for periodically connecting in pulse-like fashion as a function of the rotational speed of the engine said transformer means with the series circuit consisting of the magnetic coil of the associated spark plug and the breakable short-circuit path between the movable electrode and the counter electrode.

Claims

2. A make-and-break spark plug for piston engines comprising a spark plug housing equipped with means providing a counter electrode, a movable electrode positioned to cooperate with said counter electrode, said movable electrode and counter electrode forming therebetween a short-circuit path, an armature movably arranged within said spark plug housing, a magnetic coil for acting upon said armature, means for connecting said armature with said movable electrode, said armature including an end face directed away from said movable electrode, means providing a pressure equalization compartment within said spark plug housing at the region of said end face of said armature facing away from said movable electrode, and means providing at least one bore for communicating said pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said means providing said at least one bore extending through said armature, said connecting means for said armature with said movable electrode, and through both of said electrodes, said movable electrode and said counter electrode possessing confronting contact surfaces forming therebetween said short-circuit path and possessing a substantially ring-shaped configuration.

3. The spark plug as defined in claim 1, further including an ignition circuit for such spark plug embodying thyristor means for producing ignition pulses.

4. The spark plug as defined in claim 1, wherein said armature and said movable electrode are arranged for substantially coaxial movement.

5. The spark plug as defined in claim 4, wherein said movable electrode is displaced away from said counter electrode in a direction away from said spark plug housing to break-up said short-circuit path.

6. A make-and-breaK spark plug for piston engines comprising a spark plug housing equipped with means providing a counter electrode, a movable electrode positioned to cooperate with said counter electrode, said movable electrode and counter electrode forming therebetween a short-circuit path, an armature movably arranged within said spark plug housing, a magnetic coil for acting upon said armature, means for connecting said armature with said movable electrode, said armature including an end face directed away from said movable electrode, means providing a pressure equalization compartment within said spark plug housing at the region of said end face of said armature facing away from said movable electrode, and means providing at least one bore for communicating said pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said means providing said at least one bore extending through said armature, said connecting means for said armature with said movable electrode, and through both of said electrodes, said movable electrode being displaced away from said counter electrode in a direction away from said spark plug housing to break-up said short-circuit path, said movable electrode extending past the periphery of said means for connecting said armature with said movable electrode, said movable electrode incorporating a protruding marginal portion which in the rest position of said movable electrode bears against a neighboring end contact surface of said spark plug housing providing said counter electrode, said bore being provided at said armature, at said connecting means and at said movable electrode to permit the pressure prevailing in the engine cylinder to be transmitted to said pressure equalization compartment and to thus support the axial movement of said movable electrode, and ignition circuit means for delivering thyristor-controlled current pulses as a function of the rotational speed of the engine to the magnetic coil, so that between the movable electrode and said neighboring end contact face of said spark plug housing there exists an ignition spark which continuously wanders upon the surface of said movable electrode.

8. The spark plug as defined in claim 7, wherein said movable electrode consists of a substantially ring-shaped member having a radially extending web to which there is secured said rod member.

9. A make-and-break spark plug for piston engines comprising a spark plug housing equipped with means providing a counter electrode, a movable electrode positioned to cooperate with said counter electrode, said movable electrode and counter electrode forming therebetween a short-circuit path, an armature movably arranged within said spark plug housing, a magnetic coil for acting upon said armature, means for coNnecting said armature with said movable electrode, said armature including an end face directed away from said movable electrode, means providing a pressure equalization compartment within said spark plug housing at the region of said end face of said armature facing away from said movable electrode, and means providing at least one bore for communicating said pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said means providing said at least one bore extending through said armature, said connecting means for said armature with said movable electrode, and through both of said electrodes, said electrode being formed of a high molecular weight material having a melting point exceeding 3,000* C.

11. The spark plug as defined in claim 1, wherein said housing of said spark plug is constructed as a bipartite member, one part of which houses said magnetic coil, whereas the other part of which is an exchangeable housing portion which can be threadably connected with the engine cylinder and can be replaced after pronounced burn-off of the movable electrode.

12. An ignition system for piston engines comprising a power source, a distributor in circuit with said power source, a number of spark plugs corresponding to the number of cylinders of the engine, transformer means for stepping up the voltage of the power source so as to exceed 30 volts, said spark plugs comprising make-and-break spark plugs, each such spark plug including a magnetic coil and a movable electrode and a cooperating counter electrode forming therebetween a short-circuit path, a thyristor in circuit with the magnetic coil of each spark plug and said transformer means for periodically connecting in pulse-like fashion as a function of the rotational speed of the engine said transformer means with the series circuit consisting of the magnetic coil of the associated spark plug and the breakable short-circuit path between the movable electrode and the counter electrode.