Spark plug with inductive suppressor

Abstract

Disclosed herein is a spark plug having a wire wound inductive suppressor in its center bore for suppressing radio frequency interference. The suppressor utilizes a conductive, rather than a resistive wire as in many prior art suppressors. In addition, the suppressor employs a core of ferromagnetic materials rather than a ceramic or insulative core. This facilitates the attainment of high inductance values with relatively large diameter wire of comparatively few winding turns, with resistance kept at low values. The inductance produced by the suppressor tends to damp out radio frequency oscillations produced by the ignition system of which the spark plug is a part.

Description

BACKGROUND OF THE INVENTION

The invention relates to ignition circuit radio frequency interference suppressors, and more particularly to inductive suppressors of low resistance for use in the central bore of a spark plug.

Radio frequency interference (RFI) suppressors, either in the high voltage ignition cable or in the spark plug of an automotive ignition circuit, have been known for many years. The suppressors, until recently, comprised resistance elements with carbon resistors being generally used.

Other methods of RFI suppression are now employed. High resistance carbon resistors and other high resistance suppressors cannot be depended upon in many capacitor discharge (CD) ignition systems, which are in increasing use particularly in two-cycle engines, because the high resistance may inhibit plug firing. A fast ignition pulse risetime across the spark plug gap is typical of CD systems. Accordingly, RFI suppression in CD systems is often difficult. Depending on a number of variables, many CD systems cannot tolerate a high resistance in the secondary circuit. The most significant effect of a high resistance in these systems is a reduction in magnitude of current flow through the spark plug gap. High resistance suppressors such as carbon resistors have thus been found to have the effect of limiting current flow across the spark gap as well as slowing the ignition risetime. The result in many CD systems is a tendency to inhibit plug firing. Although carbon resistors of low ohmic value have been tried, they have generally been found not to provide the required noise suppression. Experimentation with resistors of wound resistance wire, however, has indicated better RFI suppression for a given value of resistance. This result is of course due to the wire windings which produce an inductance.

Increased inductance increases the impedance of the ignition circuit without increasing resistance. The effect of the impedance is to impede or damp out high frequency oscillations (10 MHz to 1000 MHz).

U.S. Pat. No. 3,518,606, which deals with RFI suppression by the inclusion in series of a wire winding in an ignition cable, discloses the use in the core of the winding of a binding layer including ferritic materials. The ferritic core would increase the impedance of the ignition cable. However, the resistance of the wire is the primary suppression means.

U.S. Pat. No. 3,267,325 is concerned with the generation of oscillations across a spark plug gap by means of multiple spark gaps and added capacitance and inductance. A wire winding embedded in a ferritic core is shown inside the bore of a spark plug, to produce inductance. However, the purpose of the inductor is to generate high frequency oscillations across the spark plug gap, with the addition of added capacitance and internal spark gaps. This would produce a strong RFI, so that such inductor usage directly opposes the object of the instant invention, as will be seen below.

SUMMARY OF THE INVENTION

The present invention is an improved RFI suppressor spark plug including a conductive wire wound suppressor of low resistance connected in series in the center electrode of the spark plug. The suppressor has a ferromagnetic core to facilitate the attainment of high inductance values with relatively few wire winding turns. Thus, the wire may be of comparatively large diameter. This is advantageous in several ways. Even lower resistance is obtained with large diameter wire of minimal length. Construction costs are lower in producing such an inductor than in producing one of many turns of very small wire, which must be very carefully handled. Problems of providing effective termination of small wire are eliminated with the use of the larger diameter wire.

Usually an inductor coil must be insulated from a conductive core to eliminate flashover or shorting through the core. A ferromagnetic or ferritic core, however, may have sufficient insulative properties without a separate insulative shield if an appropriate binder is used in the core. Thus, an RFI suppressor spark plug according to the present invention may comprise a conductive wire wound directly on a ferromagnetic core, connected in series in the center electrode assembly of a spark plug.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially sectioned elevational view of a spark plug according to the invention;

FIG. 2 is a perspective view of an inductive suppressor incorporated in the spark plug of FIG. 1;

FIG. 3 is a sectional view taken along the line 3--3 of FIG. 2; and

FIG. 4 is a sectional elevational view of a spark plug including a modified form of the inductive suppressor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 of the drawing, a spark plug 10 is shown having a ceramic insulator 11 with a central bore 12 therein and a metallic outer body or shell portion 13 encasing the ceramic insulator 11 and having secured to it a ground electrode 14. Within the center bore 12 is a center electrode assembly generally indicated by the reference number 15 extending through the length of the plug. Included within the center electrode assembly 15 is an external terminal 16 which extends outside the spark plug 10 for contact with a high voltage ignition cable. Below and in electrical contact with the external terminal 16 is a wire wound RFI inductive suppressor 17. The suppressor 17 is engaged either above or below (as shown) by a spring 18 which makes electrical contact with the suppressor 17 and assures the maintanence of good contact or electrical continuity in the center electrode assembly 15 during thermal expansion and contraction of the spark plug 10. Also in contact with the spring 18 is a lower electrode portion 19 which extends to and outside of the lower end or nose of the ceramic insulator 11 to define a spark gap 20 with the ground electrode 14.

Turning to FIG. 2, the inductive suppressor 17 is shown removed from the spark plug 10, while FIG. 3 shows the suppressor 17 in cross-section. The suppressor 17 includes terminal caps 21, each being in electrical contact with an end of a wound wire 22. Within the wire winding 22 is a core 23, the composition of which includes ferromagnetic materials. An insulation sheath 24 may be provided around the core 23 to insulate the wire winding 22 from the core 23, thereby preventing flashover along the core 23. The terminal caps 21 may likewise be insulated from the core 23. The ends 25 of the wire 22 are secured in electrical contact with the respective terminal caps 21 by soldering, welding, or any other suitable electrical connection. Thus, the current in the ignition circuit between the external terminal 16 and the spring 18 will travel through the wound wire 22 but not directly through the ferromagnetic core 23.

The winding 22 is preferably of a conductive wire such as copper. Its size is preferably about 40 gauge or larger. The insulation sheath 24 may be of any suitable material and may comprise a total continuous enclosure of the ferromagnetic core 23 to provide the required insulation between the core 23 and the terminal caps 21 as well as between the core 23 and the wire winding 22. However, the composition of the ferromagnetic core 23 may be such that the need for an insulation sheath 24 is obviated. Such a composition would include a suitable binder material mixed with the ferromagnetic particles before pressing to provide the needed insulative quality. The binding material may, for example, be a solution of polyvinyl alcohol, a phenol formaldehyde resin, polystyrene, or a glass.

The use of ferromagnetic materials in the core 23 gives the inductor 17 sufficient flux to facilitate the attainment of high inductance values with comparatively few turns of wire winding. Thus, the relatively large diameter wire discussed above is suitable, and this high-inductance suppressor may be made compact enough to fit in the small space afforded in the central bore 12 of the spark plug, such as the spark plug 10 of FIG. 1.

To eliminate the spring 18, an inductive suppressor 31 may be constructed and assembled within a spark plug 32 as shown in FIG. 4. The wire winding of the suppressor 31 may comprise a coil of relatively heavy wire 33, with coils 34 and 35 extending beyond both ends of a core 36. The ferromagnetic core 36 may be assembled within the wound coil 33 or cast in situ therein. Thus, the wire winding 33 would act as a coil spring of greater length than the core 36, engaging the lower electrode portion 19 and the external terminal 16, and the suppressor 31 would also serve as a spring. Assembly and material costs would be thereby reduced, decreasing the cost of producing the spark plug.

As an example to show the effects of a ferromagnetic core in an inductive suppressor for use in the center bore of a spark plug, a calculation was made to compare a wire wound suppressor of known resistance and inductance having a hollow ceramic core with a similar suppressor having a ferromagnetic core. The winding was approximately 293 turns of 0.00157 diameter copper wire. The coil had a length of 0.450 inches between terminal caps and a diameter of 0.111 inches. The ceramic core suppressor had a known inductance and resistance of 39 microhenries and 40 ohms, respectively.

To calculate the inductance of the same suppressor having a ferromagnetic core, the following equation was used (the equation appears in several technical textbooks, including Electronic Designers' Handbook, Landee, Davis and Albrecht, p. 14-4 (McGraw-Hill, 1957)): ##EQU1## wherein L is inductance in henries,

N is the number of turns,

A is the cross sectional area of the core in square inches,

.mu..sub.e is the effective a-c permeability of the core and air gap (CGS units), and

l.sub.e is the length of the core in inches.

A ferromagnetic core according to this invention was prepared and tested in a simpler inductive suppressor. Using the above equation, its permeability was found to be 44.8. Using this permeability figure, the inductance of a suppressor similar to that above but having the tested ferromagnetic core was calculated, again using the above equation. The calculation indicated an inductance of 2750 microhenries for the same 293 turns of wire, compared with the above inductance figure of 39 microhenries without the ferromagnetic core.

Permeability figures for ferromagnetic cores are known in the inductor art to vary from about 16 to about 4000. From the above equation it can be seen that if a core were used having a permeability of 4000, the inductance of the example inductive suppressor would be increased nearly one hundred fold. Regardless of the core used, the coil's resistance would remain at about 40 ohms.

In a CD ignition system it is generally best to keep resistance as low as possible while inductance may vary anywhere from about 40 or 50 microhenries upward. Ferromagnetic core inductive suppressors according to the invention having inductance of 160 microhenries and a resistance of about 4 ohms have been prepared. Suppressors of well over 200 microhenries, with little difference in resistance also may be prepared for use in a spark plug.

The above described preferred embodiment provides an RFI suppressor spark plug with a low resistance but high inductance center electrode which is particularly useful in two stroke cycle engines having CD ignition systems. Various other embodiments and changes in the preferred embodiment will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the following claims.

Claims

What I claim is:

1. A spark plug having a ceramic insulator with a central bore therein, a center electrode assembly within said bore, and a wire wound radio interference inductive suppressor within such center electrode assembly, said suppressor comprising a core of ferromagnetic materials and a conductive wire wound around said core and connected in series in such center electrode assembly.

2. The spark plug of claim 1 wherein said conductive wire is insulated from said core for reducing flashover.

3. The spark plug of claim 1 wherein said conductive wire has a diameter of about 0.003 inch.

4. The spark plug of claim 1 wherein said inductive suppressor has an inductance of at least about 50 microhenries and a resistance below about 10 ohms.

5. The spark plug of claim 1 wherein said inductive suppressor has an inductance of at least about 150 microhenries and a resistance below about 5 ohms.

6. The spark plug of claim 1 wherein said conductive wire is copper.

7. The spark plug of claim 1 wherein said wound wire of said suppressor defines a conductive coil spring extending longitudinally within said center electrode assembly and having a length greater than that of said core.

8. A wire wound radio interference inductive suppressor for connection in the center electrode assembly within the bore of a spark plug insulator, comprising a core of ferromagnetic materials and a conductive wire of about 40 gauge wound around said core for connection in series in the center electrode assembly, said suppressor having an inductance of at least about 50 microhenries and a resistance of below about 10 ohms.

9. The suppressor of claim 8 wherein said wire is insulated from said core.

10. The suppressor of claim 8 wherein said core contains a polyvinyl alcohol binder solution, whereby said core is insulative.

11. The suppressor of claim 8 wherein said wound wire defines a coil spring extending longitudinally within such center electrode assembly, said spring being of greater length than said core.