US2186039A - Radio shielded spark plug - Google Patents

Description

Jam 9, 1940- M. F. PETERS ETAL RADIO SHIELDED SPARK PLUG Filed Aug. 2l, 1935 5 Sheets-Sheet l I INVENToRs M F. Peters and (W. Asrn Figa ATTORNEY Jan. 9, 1940. M PE1-ERS ET AL 2,186,039

RADIO SHIELDED SPARK PLUG Filed Aupa, 1935 s sheets-sheet 2 al L@ if? @d 3 S8 gg/ sa F q 7 mvENToRs M.F. PeTer-s and AV. AsTn BY ATTORNEY M. F. PETERS ET' AL RADIO SHIELDED SPARK PLUG Jgn. 9, 1940.

Filed Aug. 2l, 1935 5 Sheets-Sheet 3 mvENToRs M. F. Peters and A.V.Asn BY F913 www Figla Jan. 9, 1940.

M. F. PETERS ET AL 2,186,039

RADIO SHIELDED SPARK PLUG Filed Aug. 21, 1935 5 Sheets-Sheet 4 y lNvEN-ToRs M. F.`Pefers' and AV. Ashn ATTORNEY Jan. 9, 1940. M. F. PETERS I-:r AL 2,186,039

RADIO SHIELDED SPARK PLUG Filed Aug. 21, 1935 5 Sheets-Sheet 5 VARIATION OF NOISE WITH SIZE OF OPENING IN PLUG- sHIELDING. (COMPLETELY OPEN READING Iso) i NOISE READING n ou I 2 RATIO OF SLOT WIDTH T0 STRIP WIDTH FIB. 20 5 I I L I L. NoIsE VARIATION WITH NUMBER E 3 oI= oPENINGs TOTAL wIoTI-I 0F 5 oPENING= TOTAL wInTI-I oF I cLosING STRIP Ld 0 2 I I l l I 5 G I 1 NUMBER OF OPENINGS .aI MEIQIIIIEIIIRVMI BY y ATTORNEY Patented Jan. 9, 1940 UNITED STATES vPATENT OFFICE Melville F. Peters and Allen V. Astin, Washington, D. C.

Application August 21, 1935, Serial No. 37,176

27 Claims.

(Cl. 12S-169) (Granted under the act of March 3, 1883, as

amended April 30,

This invention relates to spark plugs and particularly to spark plugs which are provided with a shielding to reduce or eliminate the disturbance that their operation produces on nearby radio receiving sets.

Among the objects of this invention are: To provide an adequately radio shielded spark plug; to provide a shielded spark plug with air circulation around the upper assembly of the plug; to provide a shielded spark plug that reduces the heating of the high tension ignition cable connected to the plug; to provide a shielded spark plug of reduced size and weight; to provide a spark plug in which the upper terminal and contacts are hermetically sealed; to provide a spark plug which uses a new type of fin, where the ns serve for radio shielding, to provide a spark plug which cools the ignition harness, to provide cooling of shell electrodes, thus increasing the life; to provide a radio shielded plug in which the danger of flash-over due to accumulation of dirt and moisture is reduced; to provide a radio shielded plug of high anti-preignition properties without increasing fouling tendencies.

In the drawings:

Fig. l is a longitudinal section of a shielded spark plug embodying our invention;

Fig, 2 is a side elevation thereof on a reduced scale;

Fig. 3 is a longitudinal section of an element for connecting the shield of the plug to the shieldingT harness of the cable: l

Fig. 4 is a longitudinal section of a spark plug embodying the principles of our invention in a somewhat different structural form;

Fig. 5 is a partial longitudinal section of a spark plug wherein the principal part of spark plug shield is continuous but is connected to the spark rlug base 'by a. plurality of short bars;

Fig. 6 is an element for the same purpose as that in Fig. 3 but having different structural form:

Fig. '7 is a longitudinal section cf acenter electrode of a spark plug partly adapted for use with our novel shielding;

Fg. 8 is a View, partially in longitudinal secii'vn, of a still dilerent embodiment of the principles of the present invention;

il is. an inverse sectional View of one form of spark plug shielding member;

Fig. lil is top plan view of the element which connects the spark plug shield in Fig. 8 to the cable shielding harness;

Fig. 11 is a longitudinal sectional view illustrating the use of a heat radiating and air bailling member in connection with a spark plug shield;

Figs.- l2 and 13 illustrate two means of mounting a connecting element on a circumferentially corrugated flexible tube to decrease the length of the means for connecting the spark plug shield to the shield harness proper.

Figs. 14 and l5 are respectively a transverse section and a fragmentary elevation partly in section, of the shield shown in Fig. 1 modified by providing it with wrench lugs;

Fig. 16 illustrates a spark plug shield provided4 with circumferential fins to augment the heat radiating properties of the shield;

Fig. 1'7 discloses a shield having the longitudinally extending bars transversely corrugated to increase the length of the path of heat transfer between the ends of the shield;

Figs. 18 and 19 are respectively a plan View and n Fig. 2l shows the relation between noise and o the number of openings in the spark plug shield, where the total width of opening is equal to the total Width of the strips connecting the ends of the spark plug shield.

The manner in which the above objects are obtained by our invention will be better understood after consideration of the theory involved in the shielding of spark plugs.

An ignition system for automotive engines comprises a high tension insulated wire connecting a source of ,current at high potential to the insulated center electrode of a spark plug. As increasing voltage is applied to the plug, the gap breaks down and the current passes across the gap between the spark plug points and returns to the source by a path that includes the engine. If the high tension portion of the circuit is surrounded by a metal casing, the greater portion of the current, after passing through the spark gap, returns through the metal casing or shielding harness. A portion of the electro-magnetic l'ield set up between current in the high tension part of the circuit is neutralized by the electro-magnetic field of the current flowing in the high tension portion of the circuit. The flow of the major portion of return current through the shielding harness is due to the fact that this tends to make the total or resultant magneti; field a minimum. This is known as the proximity effect. I any portion of the shield circuit does not completely surround the adjacent portion of the high tension circuit, the electro-magnetic eld produced by the current flowing in the high tension ignition cable will be distorted by the current returning through the partly surrounding shield so as to vtend to produce neutralizatiton of the radiating elds.

The shielding harness surrounding the ignition cable is long in comparison with the shielding around the spark plug. If the shielding around the spark plug is removed, but means are provided for lthe current to return through the shield around the cable, the shielding will be effective. Furthermore, ifthe path provided for the return current from the spark plug shield to the cable shielding'harness is placed near the shank of the spark plug, the two fields will be displaced to make the resultant eld a minimum.

In view of the foregoing, one of the important aspects of the present invention consists in providing a connection between the shell of the spark plug and the cable shielding harness in which the path in this portion of the return circuit is near and parallel to but insulated from the core of the spark plug and in which the average radial angle at the core subtended by the metal of the path is greaterthan zero degrees and less than 360 degrees.

The relative effectiveness of the described method of suppressing radio interference depends in the general case upon the ratio of the total angle subtended by the conducting strip (or strips) at the axis of the center electrode to the total angle subtended by the openings. The relative effectiveness of the interference suppression depends further on the distribution of the conducting strips around the center electrode. For any given arrangement of the strips there is little change in the effectiveness of shielding as long as the ratio of the angle subtended by the strips to the angle subtended by the openings is equal to or greater than one. Cooling of the plug and cable increases as the openings are increased so from this point of view it is desirable to have the openings as large as possible. An optimum value, from both points of view, would be to have the openings approximately equal to the strips. The effectiveness of the suppression of noise increases, for any given total angle subtended by the strips, as the number of strips increases. The greatest changes are from one to two and two to three. As the number of strips is increased beyond three the change in shielding effectiveness is much more gradual and beyond Vsix is almost negligible. Since the cost of plug manufacture is likely to increase as the number of strips is increased it does not seem worth while to increase the number of strips to much beyond six.

Tn certain special arrangements of ignition systems the gains in shielding effectiveness byl distributing the .conducting strips around the center electrode and decreasing the size of the opening as compared to strips may be quite negligible. This is true in systems where the bonds between the engine and the cable shield are critically adjusted or where a large mass of metal (Cowling, etc.) effectively screens the antenna ofV radio set from the plugs. In nsuch cases a two strip shield with openings 2 or, 3times the width of the strips provides adequate shielding.

This present invention has several features in common with the copending application of M. F.'

. Peters, Ser. No. '756,43l, led Dec.. '7, 1934.

Referring toFig. l, I4 designates a sparklplug shell to which are fixed sparking points I having in them a'core I6 of copper to increase the heat conducting properties of the points and so reduce the ,rate of wear of the points due to high temperature. Engaged with shell I4 is a gland nut I1 within which is a bushing I8,of metal preferably of high thermal conductivity. The center electrode I9 comprises a shank 2D carrying sparking terminal 2| disposed between the points I5, the shank 20 being of reduced cross sectional area from the nose of the electrode to a point adjacent to the outer face of clamp nut I'I and around this reduced portion is disposed a sheath ZIa of metal having high thermal conductivity. Insulating material 22, which may be mica washers, is held in position betwen the nose of the center electrode and bushing I8. The center electrode I9 has in it an axial bore 23 from its outer end to a point adjacent the outer end oi sheath 2 Ia and in the outer portion of the base is a plug 24 which is one means of preventing collapse of the hollow tube when assembling. The plug 24 is not necessary and under certain conditions may be left out to advantage. Other methods of assembly do not require it. The terms outer and inner as used herein refer to the relative positionsof the parts with respect to the combustion chamber of an engine in which the plugs will be used. A mica cigarette 24 is disposed around sheath 2Ia and the outer portion of center electrode I 9 to a point adjacent the inner end of filling 24 and mica washers 25 are disposed around .the cigarette between the outer face of nut I'I and externally threaded member 25 of good thermal conductivity that is secured to the outer end of center electrode I9.

The radio shielding' for the spark plug proper comprises a plurality of longitudinally extending bars 2B which are preferably separated from each other by spaces having substantially the same width as the bars themselves. These bars may be formed integrally with the spark plug shell I4 or they may be secured thereto in any suitable manner such as by welding, brazing, threading or screws. The number of barsused depends somewhat upon the particular conditions of operation to which the spark plug will be subjected, the length of leads and position and method of bonding, the essential feature being that spaces are left open through the shielding around the spark plug to permit of air circulation to dissipate heat from the outer assembly of the spark plug. The bars also serve as iins for 'cooling the core of the plug. While our shield is shown made with longitudinal bars it is satisfactory in many cases to use a continuous tubular member 21 Fig. 5) connected to the base of the spark plug by bars 28 or the spark plug shield may be of conventional form with more or less irregularly distributed apertures formed therein.

The copper cores I6 in spark points I5 conduct heat rapidly from the extremities of the points to shell I4, whence, it is conducted outwardly through the shell into the engine and to the bars 26; yalso the high conducting sheath 2Ia rapidly transfers heat from the nose of center electrode I9 to the bushing I8 and thence to clamp nut I'I and shell I4. The shank 20 is bored to diminish the cross sectional area in the upper assembly as shown at 23, which reduces the thermal conductivity of the electrode and thus lessens the quantity'of heat carried to the top terminal and minimizes cable burning, thus decreasing the thermal expansion of the shank in the upper coreassembly. However, the heat that is transmitted along the electrode and the mica. Washers in the upper core'assembly' to the tube terminal 26 is conducted through the insulator 32 which in turn is cooled by the circulation of air. The heat is, therefore, dissipated before reaching the cable.

The bars 26 may bev made of materials having low thermal conductivities or high thermal conductivities or of different materials in different portions of their length; that is, the inner onethird ory inner one-half, or other fractions of the length of each bar may be made of a poor conductor, examples of which are stainless steel or nickel-chrome alloy and the remainder of each bar be made of copper or aluminum or other good conductor of heat. Or in some cases, the inner portion may be of a good thermal conductor and the outer portion of a poor thermal conductor, the relative positions of the two materials of different thermal conductivity being determined by the flow of heat in the particular service for which the shielded plug is to be used, or the I bars may be made of poor thermal conductivity materials and metals plated on the base to produce any desired thermal conductivity.

The mica washers or other insulation used in lieu thereof are formed with a shoulder 29 upon which is seated a sealing gasket 30 against which is disposed shoulder 3| of insulating sleeve 32. A nut 33 of good thermal conducting material is disposed within sleeve 32 and engaged with the threads on member 25. the nut 33 having longitudinally extending flanges 34 at its ends to provide a large heat transfer surface in contact with sleeve 32 to insure efficient transmission of heat from the outer end of the center electrode through the sleeve 32. All edges 33 and 34 may be rounded to prevent large electrical gradients in the insulation and around the spring, which prevents formation of ozone. A tubular member 35 of insulating material is disposed in the outer end of sleeve 32 and extends somewhat beyond the outer end of the sleeve, the inner end of the member 35 resting upon electrical contact means which, in the instance shown. comprise a metal disc 36 adapted to be contacted by the ignition cable and a spring 31.

Connected to the outer end of the spark plug shield is an electrically conducting member 38 provided with threads to engage a securing element 39 whereby the member 40 through which the ignition cable passes and which forms the intermediate connection between the spark plug shield and the cable shielding harness is secured to the spark plug shield. A sealing gasket 4| is held under compression by a flange 42 on the member and thus by means of the gasket 4| and the gasket 30 the outer end of the center electrode is effectively protected against access of moisture and dirt. The member 40 may be of stainless steel to provide a path of high resistance to heat transfer or it may be of relatively high thermal conductivity depending upon the flow of heat when in use. At the present time, practice recommends it be given a large cross section area and be made of a good conductor of heat. It should be chromium plated, for protection against chemical reaction due to its surroundings, or cadmium plated or otherwise.

The spark plug shield above described eliminates one great objection to previously known shields lin that the free circulation of the air therethrough carries away any particles of gas. liquid, o solid matter that be forced through the spark plug by the pressure in the engine cylinder. Hence. the plug does not become dirty and thereby acquire a strong tendency to loss of current and flash-over. While a single connection between the shell of the spark plug and the shielding harness of the cable will in most cases prevent any observable interference with radio, a stronger shield and more satisfactory service un 'der all conditions is attained when a plurality of the symmetrically spaced bars 26 are used.

Fig. 9 shows a modification of the arrangement of bars 26 wherein the corresponding bars 26 are so turned that the major faces thereof lie at an acute angle to a radius of the shield instead of the said major faces defining substantially cylindrical surfaces. The bars in this position deflect the air into the shield and cause more rapid flow of air over the spark plug itself.

In Fig. 4 the parts corresponding to the elements previously described in Fig. 1 are designated by the same reference characters. However, in this figure the bore 45 in center electrode i9 extends throughout the length of the shank of the electrode and instead of having an external sheath of material having high thermal conductivity the inner part of the bore contains a filling 46, of material such as copper, silver, aluminum, or other metal having high thermal conductivity, to a point adjacent the outer face of the nut The insulating material 48 extending from nut |1 to the outer extremity of electrode |9, which material is shown as composed of mica washers, has embedded in it an outwardly facing metallic cup 49 that ts closely the mica cigarette 24. Seated upon a sealing gasket 50 in the cup 49 is one end of the hollow element 5| which serves to connect the spark plug shield 52 with the cable shielding harness. The element 5| has in it insulating material 53 which is shaped to receive and fit closely the insulating material 48 outwardly of cup 49 and also has in it a recess 54 to receive the ignition cable.

Connection between electrode I9' and a cable in recess 54 is effected by a screw 55 which may be turned to penetrate the cable and secure it firmly in the element 5|, the screw being in contact with a cup 56 which embraces and contacts cap 51 secured to electrode I9'. A coupling 56 of the well known union type may be used to secure the element 5| in operative relation with the plug shield 52. The bars 26 of shield 52 may be of metals of different thermal ccnductivities as above described, and likewise the metallic element 5| may be of either low or high thermal conductivity depending uponheat flow conditions. In all this discussion it is the factor KA which is important where K is the average thermal conductivity of the cross sectional area A. Whether the good conductor is placed inside or Aoutside the poor conductor is unimportant.

In Fig. 8 Aa grid-like skeleton 59 of material such a steel is secured to shell I4 of the plug and the plug shield 6U of softer metal is die-cast to embed the skeleton 59 therein. The shield 69 may or may not consist of longitudinal bars, access of air thereto being afforded through the member 6| mounted on the outer end thereof, which member is provided with openings 62 as as shown in Fig, 10. The member 6| has a cylindrical portion 63 that lies co-axially with the center electrode of the spark plug and extends sorne distance into the plug shield 6U. An insulating sleeve 64 disposed in the cylindrical portion of member 5| has threaded engagement with the insulation 65 around the outer portion of the center electrode which thus protects from moisture and dirt the conducting eements connected to that electrode. Secured to the member 6| is a 15 flexible member 66 for connecting the plug shield to the cable harness. As is shown in the drawings, the member 66 is made of circumferentially corrugated flexible metal tubing, which is readily bent to adapt the position of the outer connecting member 81 carried thereby for connection to the harness. Member 68 may be made more conductive of heat by plating it with copper, which may be protected from attack by flashing it with chromium. An effective seal is obtained by means of the gasket 88 clamped by the flange 69 secured to the member 66 and the outer faces of sleeve 84 and member 6|. between the center electrode and the cable is effected through disk |00, stem and spring |02. Metal bosses |03 on metal sleeve or strips |04 carry heat from stem |0| and transfer it through insulating sleeve 64 to cylindrical portion 63 and so reduce the heat imparted, by conduction, to the cable.

It has been found that for use on some engines the method of connecting the flexible tubing B6 to the spark plug shield and the harness as disclosed in Fig. 8 requires more room than is available and, therefore, we have devised the means shown in Figs. 12 and 13 for associating a coupling member with the flexible tubing 66. This consists in securing angular washers 1| to a radially extending face of a corrugation and when so constructed the length of the assembly is very materially decreased.

Fig. 3 shows a connecting element adapted for use with a. known type of spark plug shield and cable harness. The exterior metal member 'l2 is substantially L-shaped and has in it insulating material 13 wherein are bores 14 and 15 substantially co-axial with the two limbs of the element. In the base of bore| 14 is a metal cup 16 which is in electrically conducting relation with a spring 11 and a metal disc 18 at the other end of"sp'ring 11. Insulating sleeve 19 extends into bore 14 and the opening through sleeve 19 is outwardly tapered at 80 to receive a frusto-conical member 8| with which is engaged a rubber gasket 82. When the ignition cable is inserted into sleeve A19 to contact the disc 18 and the harness around such cable is connected to the element 12 the rubber gasket 82 is brought into sealing contact with the end of 12. 'I'he insulation in the other limb of element 12 extends beyond the outer metal portion and when in operative assembly is received in theV spark plug shield. A metal member 83 is secured against the end of the protruding insulation 84 by the pin 85 that has screw threaded engagement with the insulation as shown at 86. Coupling element 81 effects operative engagement with the spark plug shield and presses gasket 88 into sealing relation with the end of such shield. The pin 85 is preferably made of stainless steel or other poor thermal conductor and while serving as an effective connecting element between the center electrode of the plug and metal cup 16 it reduces. the transfer of heat to the cup 16. The limb of element 12 that surrounds pin 85 is also of poor thermal conducting metal while that part of element 12 around the other limb is a good conductor of heat whereby the transmission of heat to the shielding around the cable is largely prevented and such heatas does reach that portion of the shielding is rapidly conducted away and Y dissipated. Or the conductivity of the limbs may be reversed or made both good conductorvor both poor conductor.

The Fig. 6 is an alternative embodiment of our Electrical connection invention which may be used in the same manner as that shown in Fig. 3 but is simpler to manufacture. metal member 98 is recessed at 9| to receive the ignition cable and in the bottom of this recess is a spring 92 in contact with a metal cup 93 connected to a wire 94 that has 10W thermal conductivity and is adapted to contact the center electrode of a spark plug through the exposed screw head 95 on the end of the protruding insulation at 96.

An additional heat dissipating element for use with a shield of the type shown in Fig. 1 is illustrated in Fig. 11. A disc 91 of metal that is a. good conductor of heat is clamped between the member 38 and the spark plug shield having bars 26, the lower face of disc 91 is polished to reflect radiant heat and the upper face thereof is blackened to make it an eicient radiator of the heat received by conduction from the parts of the shielding equipment. This disc further serves as a deector of heat where rising from the parts below it and so tends to prevent heating of the cable harness by such currents of air. All parts of the spark plug may be made black to increase radiation or polished to reflect radiation depending upon the surroundings.

As heretofore pointed out one of the features of the present invention is the attaining of efficient heat radiation, conduction and convection from the spark plug in engine operation under maximum loads with the radio shielding and one of the important features by which-this result is secured is providing the center electrode of the spark plug with a portion that is a good conductor of heat from the nose of the center electrode to the exposed portions of the shell and clamp nut, which good conductor is shown in Fig. 1 as an external sheath a of metal having a high thermal conductivity. To prevent buckling of the sheath 2|a due to unequal increase in length of the sheath 2|a and the center electrode I9 we provide a gap as indicated at 98 in Fig. '7 to allow the sheath 2 |a freedom of expansion without abutting the adjacent shoulder on .the electrode shank. The gap may be of the order of im of an inch.

The form of our invention shown in Fig. 1 is screwed into the engine block by engaging suitable lugs on a wrench in the slots between the bars 26, but when those bars are either partially or wholly of one of the softer metals they may be distorted, and we may therefore provide the lugs |00 on the base of the plug, as shown in Figs. 14 and 15, to be engaged by the wrench.

When indicated as desirable by a study of heat fiow conditions in anycase. we may increase the radiation from the plug shield by adding circumferential ns |6| (Fig. 16) to the top of the shield. flns |02 to the bottom thereof. or both. The above mentioncdflns are preferably made of metal having high thermal conductivity and plated with chromium or. the like to prevent chemical Acorrosion thereof.

Heat flow from the base of the spark plug to the high tension cable may be greatly retarded by lengthening the path traversed between those points by means of transverse corrugations |03 in the strips 26' as disclosed in.. Fig. 17. This is effected not only by the increase in distance froml base to cable but also by the augmented radiation from the greater surficial area of the strips. It is to be vunderstood that the metal used in the strips 28' will be varied to suit thermal conditions during operation, as heretofore explained.

The insulating material 89 in The quantity of heat conducted through the spark plug shield from the plug base to the cable is further limited by substituting the connecting nipple 38 shown in Figs. 18 and 19 for the nipple 38 in Fig. l. The nipple 38 is solid throughout its length, whereas the threaded end portions IM and are separated except for the connecting bars |06. It is obvious that the latter form not only offers much greater resistance to the ilow of heat from one end to the other than does the former, but also it improves the air circulation through the spark plug shield.

Figs. and 21 give in graphic form the results of measurements on the variation of the effectiveness of shielding with changes in the ratio of slot width to strip width, and in the number of slots. respectively.

In Fig. 20 the ordinates represent the noise `(expressed in arbitrary units) from an ignition system and the abscissae represent the ratio of the total width of the slots between the bars-or strips to the total width of the bars themselves. The numbers adjacent the several curves indicate the number of the connecting bars 'of the shield upon which the measurements were made for each curve. The curves IA and 2A represent results obtained when the bars were somewhat farther from the spark plug corethan were the bars in the other cases in this figure. In all cases involving a plurality of bars, the bars were symmetrically disposed around the core, which arrangement has been found to give a minimum of noise for any size or number of bars. It will be observed that the noise increases with increase of ratio between Width of slots to width of bars and with decrease in the number of bars, but that no practical benefits are obtained by using more than six bars when the ratio of slot width to bar width is one or less.

Fig. 21 demonstrates that twelve bars, when total slot width equals total strip width, give as effective shielding as does a complete inclosure of the plug in an electrically conductive shield, and in addition there is derived the benet of cooling and the prevention of fouling as above set forth. This figure shows also that from the standpoint of commercial production six bars and slots of equal total width are probably most desirable.

The data in the curves were taken with the pick-up antenna of the noise indicator one foot from the high tension cable. 'I'he relative differences between any observation and that for the completely inclosed plug decrease as the -antenna 'is moved to a greater distance from the cable and metallic objects are interposed between the antenna and the spark plug, so that considerable practical shielding is obtained when a. single small bar connects the base of the plug to the cable shield.

The invention described herein may be manufactured and used by o r for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

We claim:

1. In combination, a spark plug including a shell and a center electrode having in it an axial bore extending from its outer end to a point adjacent its inner end and a filling of material of high thermal conductivity in said bore extending from the inner end of said bore over a distance between one-fourth and one-half of the length of said bore, insulating material around said inner electrodev extending inwardly from the outer end of said electrode over a substantial por- Y tion of its length, an outwardly facing cup embedded in said insulating material, a metal cage mounted on said shell and surrounding the outer portion of said plug, said cage including longitudinally extending bars separated from each other by spaces substantially equal in width to the width of the bars, the said bars being made of material of high thermal conductivity for substantially one-half their length and of material of low thermal conductivity for the remainder of the length thereof, and means for electrically connecting the shielding harness of an ignition cable to said cage and a cable to said inner electrode, said means having a portion seated in said cup. A

2. In combination, a spark plug including a. shell and a center electrode having in it an axial bore extending from its outer end to a point adjacent its inner end and a filling of material of high thermal conductivity in said bore extending from the inner end of said bore over a distance between one-fourth and one-half of the length of said bore, insulating material around said inner elect-rode extending inwardly from the outer end of said electrode over a substantial portion of its length, an outwardly facing cup embedded in said insulating material, a metal cage mounted on said shell and surrounding the outer portion of said plug, said cage including longitudinally extending bars separated from each other by spaces substantially equal in width to the width of the bars, the said bars being made of material of high thermal conductivity for substantially the inner half of their length and of material of low thermal conductivity for the remainder of their length, and means for electrically connecting the shielding harness of an ignition cable to said cageand a cable to said inner electrode, said means having a portion seated in said cup.

3. In combination, a spark plug including a shell and a center electrode having in it an axial bore extending from itsI Aouter end to a point adjacent its' inner end and a lling of material of high thermal conductivity in said bore extending,

from the inner end of said bore over a distancef between one-fourth and one-half of the length of said bore, insulating material around said inner electrode extending inwardly from the outer end of said electrode over a substantial portion of its length, an outwardly facing cup embedded in said insulating material, a metal cage mounted on said shell and surrounding the outer portion of said plug, said cage including longitudinally extending bars separated from each other by spaces substantially equal in width to the width of the bars, the said bars being made of material of thermal conductivity for substantially theouter half of their length and of material of low thermal conductivity for the remainder of their length, and means for electrically connecting the shielding harness of a cable to said cage and a cable to said inner electrode, said means having a portion seated in said cup.

4. In combination, a spark plug including a shell and a center electrode having in it an axial bore extending from its outer end to a point adjacent its inner end and a filling of material of high thermal conductivity in said bore extending from the inner end of said bore over a distance between one-fourth and one-half of the length of said bore, insulating material around said inner electrode extending inwardly from the outer end of said electrode over a substantial 75 i portion oi its length, an outwardly facing cup embedded in said insulating material, a metal cage mounted on said shell andsurrounding the outer portion of said plug, said eige including longitudinally extending bars separated from each other by spaces substantially equal in width to the width of the bars, and means for electrically connecting the shielding harness of a cable to said cage and a cable to said inner electrode, said means having a portion seated in said cup.

5. In combination, a spark plug including a shell and a center electrode having .in it an axial bore extending from its outer end to a point adjacent its inner end and a filling of material of high thermal conductivity in said bore extending from the inner end of said bore over a distance between one-fourth and one-half of the length of said bore, insulating material around said inner electrode extending inwardly from the outer end of said electrode over a substantial portion of its length, a metal cage mounted on said shell and surrounding the outer portion of said plug, said cage including longitudinally extending bars separated from each other by spaces substantially equal in width to the width of the bars, the said bars being made of material of high thermal conductivity for substantially one-half their length and of material of low thermal conductivity for .the remainder of the length thereof, and means for electrically connecting the shielding harness of an ignition cable to said cage and a cable to said inner electrode.

6. A shield for spark plugs, comprising a metal member disposable around the outer portion of the plug having a plurality of spaced apart longitudinally extending bars, the width of any one of said bars not exceeding the diameter of said outer portion, said member being adapted to have electrical connection with the grounded shell of a spark plug and with the shielding harness of an ignition cable connected to said plug.

'l'. A shield for spark plugs, comprising a metal member having longitudinally extending bars oi less Width than the diameter of a spark plug separated from each other by spaces equal in width to the width of the bars, said member being adapted to have electrical connection with the grounded shell of a spark plug and with the shielding harness of an ignition cable connected to said plug.

8. A shield for spark plugs, comprising a metal member having longitudinally extending bars separated from each other by spaces equal in width to the width of the bars, said member being adapted to have electrical connection with the grounded shell of a spark plug and with the shielding harness of an ignition cable connected to said plug, said bars being made` of material having low thermal conductivity for substantially one-half their length and of a diierent material having high thermal conductivity for the remainder-of their length.

9. A shield for spark plugs, comprising a. metal member disposable around the outer portion of the plug, said member having a portion electrically connectible to the grounded shell of a spark plug, a portion electrically connectible to the shielding harness of anignition cable, and an intervening portion in which apertures are formed, said member being made of material having thermal conductivity of the order of the thermal conductivity of stainless steel for at least a portion of its length.

10. 'Ihe combination with a spark plug having a shell, a center electrode carried thereby and insulating material around the outer portion of said electrode, of a member partially embedded in said insulating material and extending outwardly beyond the outer surface of said material, a laterally apertured shielding member disposed around the outer portion of the spark plug in electrical connection .with said shell, an electrically conducting sleeve extending into said yshielding member to seat against said partially embedded member, means mechanically and electrically connecting said sleeve to said shielding member, insulating material in said sleeve extending throughout the length of said sleeve and having in it ,spaces to receive the outer end of the spark plug and to receive a cable, and means to connect electrically a cable in said insulating material to said center electrode.

1l. The combination with a spark plug having a shell, a center electrode carried thereby and insulating material around the outer portion of said electrode, of a member partially embedded in said insulating material and extending outwardly beyond the outer surface of saidmaterial, a laterally apertured shielding member disposed around the outer portion of the spark plug in electrical connection with said shell, an electrically conducting sleeve extending into said, shielding member to seat against said partially embedded member, means to hold said sleeve in electrical contact with said shielding member,

insulating material in said sleeve extending throughout the length of said sleeve and having in it spaces to receive the outer end of the spark plug and to receive a cable, and means to connect electrically a cable in said insulating mater rial to said center electrode.

12. In combination, a spark plug having a shell, a center electrode hollow except at its inner end and insulating material around the outer portion of the said electrode, a filling of material in the hollow of said electrode, said filling having high thermal conductivity and extending from the inner-end of the hollow to substantially the inner limit of said insulating material, an electrically shielding member disposed around the outer portion of said plug, the wall of said member being apertured to permit the dissipation of heat from the spark plug structure and the material over at least a portion of the length of said member being of low thermalconductivity, an electrically conducting sleeve connected to said member in electrically conducting relation therewith, insulating `material in said sleeve having in it a cavity to t around the outer portion of said insulating material and a cavity to receive a cable, and means to form electrical connection between said center electrode and a cable in said last mentioned cavity.

13, In combination, a spark plug including a shell and a center electrode mounted therein, the inner portion of said electrode being of. high thermal conductivity as compared with the outer portion thereof, and an electrically shielding member disposed around the outer portion of said plug in electrically conducting relation with said shell, said member having apertures through the wall thereof. f

said shell, said member having between its ends a portion including longitudinally extending bars separated by spaces, the radially inner and radially outer faces of-each individual bar being set at an acute angle to a line from the center of said electrode to said individual bar.

l5. An electrically shielding member for the outer portion of a spark plug, comprising a hollow member having end portions and an intermediate portion including longitudinally extending bars separated by spaces, the said bars each having a. pair of substantially parallel faces set at an acute angle to a perpendicular from the longitudinal axis of said member to such faces.

16.I In combination, a spark plug including a center electrode and insulating material around the outer portion thereof, an outwardly facing cup member carried by and extending laterally beyond said insulating material, a metallic sleeve member having an end seated in said cup member, insulating material in said sleeve having therein a cavity to fit on said insulating material, and means to form an electrical connection between said electrode and a cable extending into said sleeve.

17. In combination, a spark plug including a center electrode and insulating material around the outerv portion thereof, an outwardly facing cup member carried by and extending laterally beyond said insulatingmaterial, a metallic sleeve member having an end seated in said cup member, insulating material in said sleeve having therein a cavity to fit on said insulating material, means to form an electrical connection between said electrode and a cable extending into said sleeve, and means to ground said sleeve'fV 18. A shielding device for a spark plug, comprising a body member disposable around the outer portion of a spark plug in electrical connection with a grounded member of the plug, and an outer member securable to the outer portion of the said body member, said outer member having a portion to lit in the outer end of said body member and a cylindrical portion of less diameter than the said body member connected to the first mentioned portion by a perforated portion, the said outer member being of material having low thermal conductivity.

19. A shielding device for a spark plug, comprising a body member disposable varound the outer portion of a spark plug in electrical connection with a grounded member of the plug, and an outer member securable to the outer portion of the said body member, said outer member having a portion to t in the outer end of said body member and a cylindrical portion of less diameter than the said body member connected to the rst mentioned portion by a perforated portion.

20. The combination with a spark plug having a shell and a center electrode, of a skeleton of rigid material carried by said shell and a shielding member of material softer than that in said skeleton disposed around the said plug outwardly of said shell with said skeleton embedded in said softer material.

21. The combination with a spark plug having a shell and a center electrode with insulating material around said electrode outwardly of said shell, the outer end of said material having external screw threads, of a shielding member disposed around said plug outwardly of said shell and in electrically conducting relation therewith, an element having a cup-shaped portion intertting With and seated against the outer end of said member and a cylindrical portion coaxial with said electrode, an insulating sleeve in said cylindrical portion engaged with the threads on said insulating material, and means to conduct heat from the outer end of said electrode to said cylindrical portion through said sleeve.

22. The combination with a spark plug having a shell and a center electrode with insulating material around said electrode outwardly of said shell, the o-uter end of said material having external screw threads, of a shielding member disposed around said plug outwardly of said shell and in electrically conducting relation therewith, an element having a cup-shaped portion interiitting with and seated against the outer end of said member and a cylindrical portion coaxial vwith said electrode, and an insulating sleeve in said cylindrical portion engaged with the threads on said insulating material.

23. In combination, a spark plug shell, a gland nut internally engaged with said shell, a bushing of material having good thermal conductivity in said nut, an insulating cigarette disposed in said bushing, a center electrode mounted in said cigarette, a portion of said electrode extending inwardly from the outer end of said nut being of greater thermal conductivity than a portion next outwardly thereof, insulating material around said center electrode substantially coterminous with said portion next outwardly of said nut, said material being provided with an outwardly facing shoulder, means associated with the outer end of said electrode to increase the thermal conductivity thereof, an externally threaded member having good thermal conductivity disposed around and secured to the outer end of said electrode, a sleeve of insulating material cooperating with the shoulder on said insulating material around the center electrode to form a 4seal at that point and extending outwardly beyond the end of said electrode, an heat conducting element engaged with the externally threaded member and in heat transferring relation with said sleeve, a cylindrical element disposed in said sleeve and extending beyond the outer end thereof, a sealing element around said cylindrical element in contact with the outer end of said sleeveradio-shielding means disposed around the outer end of said electrode, an element of a shielding harness for an ignition cable, and means to secure said last named element in cooperative relation with said radio-shielding means and to maintain an eiectiveseal at said sealing element and at the'slioulder on the insulating material around said center electrode.

24. In combination, a spark plug shell, a gland nut internally engaged with said shell, a bushing of material having good thermal conductivity in said nut, an insulating cigarette disposed in said bushing, a center electrode mounted in said cigarette, a portion of said electrode extending inwardly from the outer end of said nut being of greater thermal conductivity than a portion next outwardly thereof, insulating material around said center electrode substantially coterminous with said portion next outwardly of said nut, said material being provided with an outwardly 'facing shoulder, means associated with the outer end of said electrode to increase the thermal conductivity thereof, an externally threaded member having good thermal conductivity disposed around and secured to the outer end of said electrode, a sleeve of insulating material cooperating with the shoulder on said insulating material around the center electrode to form a seal at that point and extending outwardly beyond the end of said electrode, a heat conducting element engaged with said externally threaded member and in heat transferring relation with said sleeve, a cylindrical element disposed in said sleeve and extending beyond the outer end thereof, a sealing element around said cylindrical element in contact with the outer end of said sleeve, laterally apertured radio-shielding means disposed around the outer end of said electrode, an element of a shielding harness for an ignition cable, and means to secure said last named element in cooperative relation With said radio-shielding means and to maintain an effective seal at said sealing element and at the shoulder on the insulating material around said center electrode.

' 25. A shield for spark plugs, comprising a metal member disposable around the outer portion of the plug, said member having a portion electrically connectible to the grounded shell of a spark plug, a portion ,electrically connectible to the shielding harness of an ignition cable, and an intervening portion comprising longitudinally extending spaced apart Abars, the said bars being formed ofmaterial having low thermal conductivity and the width of each bar not exceeding the diameter of the spark plug.

26. A shield for spark plugs, comprising a metal member disposablearound the outer portion of the plug, said member having a-portion electrcally connectible to the grounded shell of a' radio shielding member in electrically conducting 3 relation with the base of said plug, said member' being circumferentially discontinuous over a por.. tion only of its length, and elements associated with the outer end of said member coacting with the outer end of said plug to form a moisture proof enclosure for the outer terminal of the plug and the ignition circuit parts contacting said terminal.

MELVILLE F. PETERS. ALLEN V. ASTIN.

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