US2680432A - Spark plug - Google Patents

Description

R. E. RAND June 8, 1954 SPARK PLUG Filed Sept. 13, 1951 INVENTOR. Foam-r E. R AND HTT'O RNEY Patented June 8, 1954 UNITED STATES FPATENT OFFICE SPARK PLUG Robert E-.'Rand, Philadelphia, Pa. Application September 13, 1951, Serial No. 246,364 11 Claims. (Cl. 123 169) The invention relates in general to spark plu for firing internal combustion engines and particularly relates to shielded spark plugs for use in high compression aircraft engines. More exactly classified, the invention relates to an improvement in spark plugs of the type which comprises an outer tubular steel shell externally threaded at its lower end for mounting in the engine cylinder and whose upper portion above the engine is air-cooled, and in which shell is fitted a core assembly comprising a core of ceramic material or porcelain and a metal spindle extending axially through the core and terminating in a head forming an inner electrode coacting with an outer electrode extending from the shell to form one or more firing gaps.

It is usual in this art, as shown in the patent to Lewis F. Hahn, No. 2,445,777, July 27, 1948, to interpose between a tapered portion of ceramic core and a similarly tapered portion of the steel shell a copper sleeve or jacket forming a gasket seat for receiving the core and which copper sleeve also defines the upper limits of a gas-heating chamber exposed to the firing chamber at the firing end of the plug.

The invention specifically relates in a similar form of spark plug to an improvement in the manner of utilizing the heat from the inner electrode to pre-heat the fuel gases in the gas-heatingohamber before they are ignited and to dissipate all surplus heat not needed for this gas preheating in order to keep the firing chamber cool to the extent of preventing pre-ignition of the gases, irrespective of the temperatures developed the firing chamber.

It is well known in this art that the temperature of the fuel gases at the firing end of spark plugs must be just right for ideal firing condrtions. They must be neither so cold as to defeat complete combustion of all of the fuel present and thus precipitate carbon, nor so hot as to pro duce pre-ignition and knocking. While spars plugs undoubtedly have been custom-made to prhvide this ideal thermal condition of ust enough and not too much heat present during the compression and firing cycles in any one case, it has been practicallyimpossible heretofore to produce such plugs in large quantities as a fac tory proposition and with each plug having within operative limits the same heat rating as every other plug. As is well known in this art, the heat rating or" any spark plug is for the most part in direct proportion to the amount of ceramic core exposed to the combustion flame present as compared with the heat transferability of the ceramic core;

In general, the present disclosure features in the design of the spark plug'illustrated a nicety of balance between those heat radiating parts which tend to keep the optimum firing temperature and those heat conductive parts which tend to abstract from the firing chamber all heat which cessively the ideal gas firing temperature.

.As thermal expansion and contraction, especially in the metalparts of spark plugs, do change their thermal rating, theeffect of such variations in temperatures must be minimized in cases as herein where the objective is to produce repeatedly as a factory produced article a standard form of plug,

Broadly, this is attained, first, by fixing the volumetric capacity of the gas-fuel heating chamber and by fixing the radiating area through which heat is discharged'into the chamber, thus insuring that the amount of gas to be heated remains constant, and, second, byproviding a heat path for conducting oil from the area of heat generation the surplus heatand including in the highly conductive part of the heat path control means automatically responsive to the temperature of the heat passing at the moment for varying the conductivity of the path and thus more rapidly conducting heat awayfrom the firing chamber with increase-in temperature.

The fixing of the volumetric capacity and heat radiating area of the gas-heating chamber is attained overknown similar forms'of spark plugs as exemplified by the: above-noted Hahn patent by restraining any tendency sleeve or gasket from expanding axially down-- ward in the direction towards the firing chamber, sleeve to expand freely in the opposite direction, and in this way keep the While permitting the gas chamber volume fixed as originally designed and incidentally increase-its area of heat trans ierence into'the upperairi cooled shell with increase in' the temperature of the heat conducte' lhe heat path for gettingthe heat away from the hot inner electrode? to'the air-cooled exposed area-of; the outer shellincludes a fixedccre-seating outer sleeve or gasket at the discharge end of the path and athin-walled inner sleeve of copper or equivalent highly conductive metal in endwise engagement with the inner electrode and forming the heat intake or' path and with a por'tio'nof the ceramic core located between the overlapping'ends of the inner and outer sleeves. I I

.lnthe instant device'the automatic control feature i's'irih'e'rentin the-inner sleeve'in that it gases at the relatively low' tends to raise exof the core-seating receiving end of the is not fixed to either the ceramic core or to the electrode spindle and is thus free to expand and contract thermally. As it expands under the influence of increase in temperature it elongates and in elongating more fully laps the outer sleeve and in this way widens the heat path across the ceramic core and leading to the outer sleeve and thus functions as a heat valve to permit more and more heat to cross the ceramic and into the outer sleeve with increase in temperature of the heat being conducted along the path. As the temperature drops the inner sleeve contracts in its axial length and thus at lower temperatures less heat is conducted away from the gas-heating chamber by reason of the resulting narrowing of the band forming that part of the heat path which traverses the ceramic core in its air-cooled portion.

Still another object of the invention is to provide a form of such automatic heat-conductive control means which can be factory-set to give an intended rate of heat discharge within prescribed limits, but which can be readily replaced in the plug by a similar form of control means pre-rated to give a different rate of heat discharge.

This objective is attained by reason of the fact that it is possible with the design illustrated and before the core assembly is fabricated to select from a set of inner tubes of slightly different lengths that length of sleeve which experimentally has shown to be the proper length to give the optimum rate of heat withdrawal from the hot region of the plug.

As above noted the heat rating of spark plugs of the type above indicated is directly dependent, all other conditions being the same, upon the area of the ceramic core exposed to the fuel gases flowing about the same just before they are ignited.

The primary object of the invention is to keep this heat-distributing area constant, or substantially so, under the variable thermal conditions under which such spark plugs operate.

It is well known that an enormous amount of heat is generated in the firing chamber at each firing cycle, some of which heat is drained off as usual directly through the outlining shell, but even so the larger proportion of heat is taken up by the inner or central electrode which tends quickly to make the central electrode too hot for ideal firing conditions. The present disclosure features a heat-conductive path fashioned for withdrawing most but not all of the heat away from the center electrode as fast as the heat accumulates in the firing chamber of the spark plug.

As hereinafter noted, the outer and inner heat conductive and radiating sleeves herein featured are made substantially of commercially pure copper. However, the firing temperature of the high compression engines for which the instant spark plug is designed is quite apt to be sufficiently high to melt or in time to disintegrate the portion of such copper sleeve as may be directly exposed to the flaming gases present.

Accordingly, another object of the invention is to protect these sleeves and shield them from flame contact, and this is particularly true in the case of the inner sleeve which must be located in the area of greatest temperature to per form its intended function, and even in the case of the more remote and thus less heated outer sleeve care is exercised in its design to restrict to the optimum amount the area of the lower seated edge exposed to the flaming gases.

Various other objects and advantages of the invention will be in part obvious from an inspection of the accompanying drawings and in part will be more fully set forth in the following particular description of one form of spark plug embodying the invention, and the invention also consists in certain new and novel features of construction and combination of parts hereinafter set forth and claimed.

In the drawings,

Fig. l is a view in axial cross section of a shielded spark plug forming a preferred embodiment of the invention, with parts broken out to save space, and taken in the plane indicated by the line |-I of Fig. 3 and grossly enlarged to bring out structural details;

Fig. 2 is a transverse sectional view taken on the line 2-2 of Fig. l and looking upwardly as indicated by the arrows; and

Fig. 3 is a bottom end view looking upwardly past the pair of outer wire electrodes into the firing and gas-heating chambers.

The spark plug includes two main elements, a one-piece long, tubular steel shell if) having a bore H extending axially therethrough from end to end, and a prefabricated core assembly l2 demountably fitted in the bore. The lower portion of the shell forming the firing end l3 of the plug is externally threaded as shown at M for insertion in the engine cylinder and when mounted in position is preferably inset about one-eighth of an inch from the inner wall of the engine cylinder. The portion if: of the shell above the threaded portion is exposed for aircooling. The air-cooled portion includes an enlarged hex forming part It and thereabove a long upstanding shielding barrel ll with its upper end externally threaded to receive the usual harness for supplying high tension current to the spark plug.

The core assembly includes a molded core l8 of ceramic or porcelain material, the lower portion of which is of slightly tapered frusto-conical form and forms a long tapered nose is. The upper portion of the tapered nose is fitted snugly and in gas-tight engagement with the similarly tapered bore 25 forming a core assembly seat of an outer gasket-forming sleeve or ferrule 2i, hereinafter sometimes referred to as the outer sleeve or radiator, in turn fitted in the cylindrical bore of the shell.

The core 18 is provided with a bore 25'. extend ing axially therethrough from end to end. The core assembly also includes a one-piece core conductive element 23 formed of manganese nickel, and which comprises a long shank or spindle 24 of cylindrical form and which terminates at its lower end in a disk-like head forming an inner or central electrode underlapping and in firm-heat-transferring relation to the exposed face 25 forming the lower end of the ceramic nose E9.

The portion of the core I8 above the nose and above the sleeve 2! is reduced in its external diameter to form an upwardly facing shoulder 21 on which rests a lower cushioning gasket 23. The barrel l1 in the part thereof above the gasket 28 is lined with a one-piece tubular sleeve 29 of insulating material such as an aluminum oxide molded with a bonding compound, and is held in place by an upper, cushioning gasket 3?} in turn held in place by an inturned flange 35 formed by machine-rolling the upper edge of the barrel I'l into a bearing engagement therewith held to an exact tolerance to effect the desired compression of the gaskets 28 and 30.

The core conductive element 23 is held in place in the core it by an out-turned flange 32 formed on the upper end of the spindle and in bearing engagement through spring washer or clip 33 with the upper end of the core. For reasons hereinafter noted it is suggested that the upper end of the core be trued, as by grinding the ceramic to insure a fiat surface for receiving the spring washer 33. The upper end of the core is recessed to provide a pocket 34 in which packing material 34 is contained and operative to defeat gas leakage through the bore 22. The packing material illustrated is a silicon rubber base which can accept 600 degrees F. before distortion.

The bore it through the shell is made wide open at the firing end 35 of the plug and between the lower end of the outer sleeve 2! and the firing end forms an annular gas-heating chamber 35 outlined on its outer side by the steel shell and outlined in part on its inner side by the lower portion of the nose l9 and by the perimeter of the inner electrode 25. The lower end of the chamber 35 below the inner electrode and exposed to the engine cylinder will be referred to hereinafter as a firing chamber 31. The portion of the chamber 35 about the head 25 and between the same and the shell forms a slightly constricted annular passageway 38, placing the firing charm her 3'! in wide-open fluid communication with the upper portion of the gas-heating chamber, that is, the portion outlined on its inner side by the lower portion of the nose l9. The fiat electrode head 25 is off-centered slightly, say, about .065 of an inch from the axis of the spindle, so that one side of the passageway 38, for instance, the upper side as viewed in plan in Fig. 3, is of slightly less clearance than the opposite or lower side, which eccentricity is grossly exaggerated in Fig. 3.

A long sleeve of highly conductive metal such as electrolytic copper is located about the lower portion of the shank 26 with the lower end of the sleeve bearing in heat-transferring relation to the upper side of the usually hot head 25 at its juncture with the shank.

In order to point out the several distinctively novel features of the disclosure hereinafter mentioned, it may -e assumed that the spark plug as thus far disclose 1, and with some form of outer or ground electrode in the firing chamber, is somewhat conventional and the preceding description at is st in its broad aspects is that of known forms 0 spark plugs.

It is a novel feature of the present disclosure, stemming from the desire to make the volume of the gas-heating chamber 35, as well as its inner heat-radiating surface and its outer heat-dispensing Or cooling areas, preset and constant, irrespective of temperature conditions present, to design the structure in such way as will, tend to defeat any possibility of the outer sleeve 2% from expanding into the chamber 35 and in this way to restrict the volumetric capacity of the chamber. For instance, in the above-identified patent to Hahn, No. 2,445,777, the soft metal gasket (33) has its lower end exposed to the flowing gases and is free to distend downwardly into the portion (25) of the bore, thus progressively reducing the axial dimension of this gas-heating area with increase of temperature. This disclosure herein features the reducing of the di ameter of the cylindrical bore It in upwardly spaced relation to the firing chamber to provide in the shell an upwardly facing shoulder 40 which provides a seat on which rests the outer marginal edge portion of the lower end of the outer sleeve 2]. This shoulder acts to defeat any possibility of the sleeve 2| creeping thermally downward into the gas-heating chamber 35. The sleeve 2| is a greater cross section of material at its lower seated end than at its upper exposed end. The sleeve 2! is a casting formed of leaded copper or electrolytic copper annealed dead soft. Its periphery is cylindrical with a driven tight fit in the portion of the cylindrical bore above the shoulder 40. The sleeve is molded with its bore 20 or conical form in which the core l8 snugly fits when the core is jammed forced into the sleeve 2| with the mounting of the element 23 in the shell. The inner, ring-like portion 4| of the lower seated end of the sleeve 2i forms the upper end of the gas-heating chamber 35 and it is a feature of this disclosure that the portion 4| is of limited area and is directly exposed to the hot gases within the chamber 35 and thus acts as a heat-receiving end to the sleeve 2! when functioning as a radiator to remove heat from the chamber 35 or as a source of heat supplied by the gases in the chamber 35, depending upon the temperature in the chamber at any instant of time.

The upper portion of the gas-heating chamber is defined on its inner side by the portion of the perimeter of the core nose l9 which liesbetween the radiator end 4| of the outer sleeve 2| and the electrode head 25 and forms a heatdispensing, radiating surface 42 of fixed area to heat the gases in contact therewith in the fixed volum chamber 35.

As it is within the scope of the disclosure to form the shell and outer sleeve 2! integral, these parts will be referred to sometime hereinafter as an outer member having a shoulder 4i facing the firing chamber and forming the upper end of the gas-heating chamber 35.

It is noted that the lower portion of shank 24 is not, cemented in place in the ceramic core as is usual in such spark plugs. It has been found that under the high temperatures present in the lower portion of spark plugs all known forms of cement eventually disintegrate, leaving heat pockets in the space or, rather, spaces formerly occupied by the cement, which pockets ruin the pro-fixed heat rating of the plug.

The disclosure particularly features the inner sleeve 39 in its function of receiving heat at its lower end from the hot head 25 and from the ad jacent hot lower end of the shank 2d, and for dis charging the heat into the nose E9 of the core, both in. the portions thereof which are outlined by the surface 52 and by sleeve 2 i. In order to minimize any resistance to the passage of heat across the joints formed between the inner sleeve and both the metal spindle and the ceramic nose it follows that the surfaces forming the joints must be smooth and closely fitting to avoid the formation of even minute pockets in these joints. No particular difiiculty is had in machining either the bore of the sleeve 33 or the perimeter of the shank 2-4, as both are of metal in order to effect a rather close but slidable fit of the sleeve on the shank and at the same time to provide an extensive area of heat-transferring contact between the ceve 39 and the shank. As a matter of fact the inner sleeve 39 may be purchased as a standsize of copper tubing and out off to the desired length and used without any-special machining.

However, in molding the core l8 to its desired configuration it is practically impossible to form its bore 22 to fit snugly the outer perimeter of the pre-sized sleeve 39 and at the sam time to permit the sleeve in expanding thermally to maintain the desired continuity of extensive heat-transferring joint between the sleeve 39 and the nose l9, especially as no cement can be used in this joint. Accordingly, the lower end of the ceramic wall forming the bore 22 is drilled to a true cylinder and lapped to a tolerance of about 0.002 inch relative to the external diameter of the sleeve 39, so that the lower portion of bore 22 has a larger diameter at the nose end by the cross section of material of sleeve 39 than at its midlength portion between the pocket 34 and the drilled portion. In this way heat received from the firing chamber 3? into the head 25 is conducted at least part way up the adjacent lower end 01 the shank 2B; is discharged therefrom radially into the highly conductive sleeve 39 and into the ceramic forming the nose 19, without any particularly noticeable barrier either at the joint between the shank and sleeve or at the joint between the sleeve and ceramic core. The joints formed between the inner sleeve and the ceramic nose and between the inner sleeve and the shank are formed by smooth surfaces and thus free of any resistance to movement of the sleeve relative to either shank or core.

It is a particular feature of novelty in the device disclosed that the upper end of the inner sleeve 39 must extend above the plane which defines the lower end of the outer sleeve 2| to form in the nose an annular band 43 of the ceramic matter operative to conduct heat radially from the upper end of the inner sleeve outwardly to the lower end of radiator sleeve 2| and hence to the air-cooled portion of the outer shell.

It has been found experimentally that if, on the contrary, the inner sleeve 39 terminates short of, that is, below the outer sleeve 2| there is formed therebetween in the core a hot pocket which, due to the high temperature of heat present, disintegrates any cement in its vicinity into a powder and this, of course, induces erratic heat rating qualities.

The upper and relatively cool portion of the core assembly, that is, the portion spaced above the band 43, contains the usual cement Al filling the space between the upper end of the core and spindle to assist gasket 3 3 in defeating gas leakage through the bore 22.

As the head 25 tends to become quite hot in actual practice, especially in the region of its jointure with the shank 23, there is a possibility of the adjacent end of the ceramic core distintegrating in the region of its face 26 unless special care is exercised to remove excessive high temperature heat from this point as quickly as pos sible. As the sleeve 39 is highly conductive and for its entire length is exposed in heat-transferring relation to the core, the heat disintegration of the lower end of the nose is avoided in the instant disclosure. It is also noted that no part of the copper sleeve 39 is directly exposed to the flaming gases, either in the firing chamber 3'! or in the gas-heating chamber 5. In this way the copper of the sleeve 39 is shielded at its lower end by the manganese nickel head 25 and on its outer long side by the insulating ceramic and is thus protected from being oxidized by the flaming gases in the firing end of the plug.

Assuming all other conditions fixed as herein indicated, the thermal rating of the plug may be varied as desired simply by providing in stock sleeves 39 of slightly different lengths and inserting in place a sleeve of that length which tests have shown to give the required heat rating.

In order to insure the maximum possible efficiency of heat transfer between the head and the end or the nose at the firing end of the plug, it is suggested that the face 25 be ground fiat and engage for its entire area over the area of the upper side of the head exteriorly of the inner sleeve. Both the upper and lower ends of the core are ground fiat to extend in planes at right angles to the axis of the core to provide fiat surfaces for engagement respectively by a current-supplying attachment and by the inner electrode 25.

The firing end of the plug is provided with a pair of thin wire outer or ground electrodes 44 and 45 square in cross section and whose root ends are inserted with a driven fit into the portion of the shell which encircles the firing chamber 31. The electrodes extend in parallel relation in approximately radial directions on opposite sides of the axis of the core assembly and their free ends underlap the head 25 and coact therewith to form a pair of firing gaps, one of which is shown at 45 in Fig. 1. These electrodes are made of manganese nickel and project into the firing chamber from opposite sides thereof and preferably from the portions of the firing chamber which is below the average cross section of the passageway 38.

It is noted that, except for the parts of the electrodes which cross the area immediately below annular passageway 38, the lower end of the shell bore is otherwise free of any obstruction to the passage of gases between the firing chamber and the upper part of the gas-heating chamber 35 and in this way there is practically no interference to the whirling action of the gases as they pass into and from the chamber 35 durmg the compression, scavenging and firing cycles of the engine operation.

Using two electrodes of high erosion-resistant material and with relatively small cross section and set non-concentrically in relation to the center line of the spindle as herein disclosed imposes less interference to the desired freedom of movement of the flow of gases into and out of the passageway 33 than would be the case were a single electrode or the necessarily larger diameter used.

In the device illustrated there is no leakage of gas pressure in the firing chamber. This is due in part to the fiat machined joint at 26; to the long joints between the inner sleeve in its snug engagement with both the metal shank 24 and the ceramic core; to the long cylinder of cement ll and to the packing compressed in the pocket 34. It is understood that the core is held under axial compression by the rugged shank 24 and squeezed between the wide bearing head 25 and the compressed washer 33, which, in turn, is squeezed between the outwardly rolled flange 32 and the machined top end of the core 18.

Described more in detail, the spring washer 33 is a one-piece metal stamping of annular form, bent back upon itself to form an upper ring 48 and a lower ring 49 with the rings integrally connected at their perimeters. The outer portion of the lower ring bears on the machined fiat face of the core and its inner portion is dished upwardly at an angle of about twenty-five degrees to form a pocket 50 continuous with the pocket 34 to receive the upper portion of the packing material 34 which projects above the core. The upper side of the inner portion of ring 49 provides a seat for the flange 32 which is contained in the opening provided therefor in the upper ring 48. The top surface of the flange 32 and the top flat surface of the upper ring coact to form a flat surface SE for engagement by the contact spring of a spark plug lead connection.

These parts are assembled by means of a fiatfaced tool operating with a downward rotating pressure stroke to squeeze the material 34' into the t-wo pockets 34 and 50, to roll over the upper end of the shank to form the flange 32, and to compress the lower ring 48 firmly against the ground top end of the core [8.

In one form of the invention where the plug was designed to have a heat rating of about 300 pounds indicated mean effective pressure, the steel shell had a length of three and threesi'xt'ee'nths of an inch; the core had an all-over length of one and threesixteenths of an inch; the lengths of the inner and outer sleeves were equal, and each one half inch. The length of the outer sieeve it was held rather close to seventeen thirty-seconds of the length of the nose measured axially from the top of the sleeve 2! to the top of the head 25, and thus the length of the gas chamber 35 in its upper wide portion above the head was about fifteen sixty-fourths of an inch.

The ratio of length of the outer sleeve to the length of the gas chamber between the outer sleeve and the top of the inner electrode appears-to be a critical factor in setting the indicated mean effective pressure, that is, the heat rating of the plug.

In the instant case it was found, experimentally, that increasing the length of th sleeve 2| by one sixteenth of an inch, that is, from sixteen thirty-secon'ds to seventeen thirty-seconds, gave a heat rating of 325 pounds mean effec-' tive pressure, and increasing the length of the sleeve to twenty thirty-seconds gave a rating of 471 pounds mean effective pressure. The 325 pounds rating is sufficiently cold to avoid pro-- ignition in the modern types of aircraft engines.

Another controlling factor in the designing of such plugs is that when the sleeve is lengthened materially beyond the prescribed one-half inch While it is the intent in this disclosure to re place a worn-out spark plug in the engine cylinder with an entirely new plug, it is suggested that, in so far as it is the core element which is most likely to become worn, it is possible by first lifting the flange 3! to remove the contents of the shell by driving the core assembly upwardlyfrom the shell and a fresh core assembly is then placed in the old shell in place of the worn core element so removed.

In operation and assuming that the parts are in the positions described, and further assuming that a fuel charge in the engine has just been fired, it will be obvious that the flaming burning gases in the chamber will heat the head 25 most highly and will heat the portion of the chamber almost as high. The expanding wave-front of the burning gases will sweep into the chamber 35 most easily'through the part of the passageway 33 of greatest clearance and then through the part of least clearance with a whirling action of the gases which tends Cir to sweep the electrodes free of carbon. some of the heat in the chamber 35 will, of course, pass directly outward into the threaded portion of the shell; The greater part of the high temperature heat will pass into the head 25 and into the lower part of the ceramic core, tending to heat them quite quickly to the high temperature of combustion. This heat is conducted most easily up through the inner metal sleeve and less easily up through the insulating ceramic nose. As hereinbefore noted, heat is conducted from the upper portion of the inner sleeve radialiy across the ceramic band 63 as indicated by the wavy arrows to the outer sleeve or radiator 2| and hence to the air-cooled shell. It is understood that the parts are so designed in their heat=conductive capacity that all of the heat is not conducted off and, on the contrary, the head and ceramic nose is at that temperature during the gas scavenging step as is set by the rate at which the heat is conducted away from this area and is set to give the desired pro-heated temperature best suited for the current firing condi tions.

It is noted that the cement 47 is confined to the upper relatively cool portion of the plug and sleeve towards the firing chamber.

As fresh fuel gases are ejected into the engine cylinder following the scavenging step they flow past the pair of electrodes; through the annular core nose, so that desired tempera two firing gaps.

the gases are preheated to the ture prior tobeing fired at the I claim: 1. A spark plug comprising an outer tubular metallic externally threaded at its lower ceive heat therefrom, and said assembly including acore" conductive element provided with a disk-like head defining. the lower end of the core assembly, forming an inner electrode constitutinga heat absorbing shield coextensive with the enthe lower face of the core assembly and outer electro'de'extending into the firing chamber fromthe shell and under-lapping the inner electrode.

2. The spark plug defined in claim 1, wherein the radiator is a casting formed of electrolytic copper annealed dead soft, of greater cross section of material at its lower end facing the annular chamber than at its upper end, said radiator being free to expand axially in the direction away from the shoulder and which radiator has a length slightly greater than the length of the annular heat-distributing space between the radiator and the disk-like head, both measured axially of the plug and regulating by reason of its length the capacity of the plug to keep cool.

3. A spark plug including an outer tubular metallic shell provided'externally with threads for engaging in an engine cylinder and whose bore is provided with a shoulder facing in the direction away from the firing end of the plug, an outer sleeve fitted to the bore and at all times in endwise engagement with the shoulder and free to expand in the direction away from the shoulder, a core assembly intruded telescopically into the shell and including an outer core of ceramic material fitted snugly in the bore of the outer sleeve, the lower portion of the core below the outer sleeve forming a conical nose centered within and spaced from the shell to form an annular heat-distributing chamber, a metal shank extending axially through the core and provided at its lower end with a head underlapping the core for its entire area to protect the core and forming the inner electrode of the plug, a long inner cylindrical sleeve of a highly conductive metal with its lower end contacting the head in heat-transferring relation thereto with its upper end terminating within the portion of the shell which is provided with the threads and located between the shank and core in thermal transferring relation therewith for the entire length of the inner sleeve, the outer sleeve being of high conductive metal fitted between the core and shell in thermal transferring relation therewith for the entire length of the outer sleeve, the lower end of the outer sleeve inwardly of the shoulder forming a ring-like portion exposed to the annular heat-distributing chamber to receive heat therefrom, and an annular band-forming portion of the ceramic core in the part thereof between the lower end of the outer sleeve and the upper portion of the inner sleeve and thus within the threaded portion of the shell encircled by the outer sleeve, encircling the inner sleeve and coacting therewith to form a thermal path leading from the inner electrode upwardly through the inner sleeve, radially from the upper portion thereof through said annular band-forming portion and lengthwise through the outer sleeve to distribute its heat along a material area of the bore of the shell in an air-cooled portion thereof above its threaded portion.

l. In a spark plug, the combination consisting of a shell whose upper portion is air-cooled, a core assembly including a core and a spindle within the shell, said spindle provided at its lower end with a head forming an inner electrode, means forming a thermal path for conducting heat from the head to the shell, includ ing an inner sleeve of highly conductive metal and of solid cross section of material with a preformed bore fitted to the lower portion of the spindle, with one end in direct heat-transferring contact with the head and forming the receiving end of the path, a portion of the core spaced from the head constituting a mid-length portion of the path, and an outer sleeve of highly conductive metal fitted in the shell between said mid-length portion of the core and the shell and in spaced relation to the electrode head and in heat-transferring relation to both the core and the shell and constituting the heat-discharging end of the path, and said inner sleeve functioning by reason of its length to form a heat valve for regulating the amount of heat conducted therethrough to the core.

5. A spark plug including an outer tubular shell with its lower end portion externally threaded and with its bore at its lower end open to form a firing chamber and which bore is stepped in spaced relation to the firing chamber to form an annular heat-distributing chamber therebetween and to form an upwardly facing stop shoulder within the region of the externally threaded end portion and thus within the portion of the spark plug intended to be in contact with the engine cylinder, an outer sleeve of a highly conductive metal with a driven tight fit to the portion of the bore above the shoulder and with its lower end engaging the shoulder and whose bore provides a seat of conical form, a ceramic core of conical form fitted to said seat and located thereby relative to the shell, the lower portion of the core being spaced from the part or" the shell encircling the same to form an annular gas chamber open to the firing chamber and of pre-fixed volumetric capacity, the lower end of the outer sleeve being exposed to the heatdistributing chamber to receive heat therefrom. said outer sleeve having a length equal to about seventeen thirty-seconds of the distance measured from the top of the outer sleeve to the bottom of the ceramic core and forming the heatdistributing chamber, both measurements taken axially or" the plug.

6. In a spark plug, the combination of a tubular metallic shell externally threaded at one end and provided with a bore extending axially therethrough and wide open at its firing end so threaded to form a firing chamber, a core of insulating material centered in the bore and with its lower end tapered and forming a nose terminating in spaced relation to the firing chamber, said core provided with a bore extending axially therethrough, said bore having a larger diameter at the nose end than at its midportion, said bore in its part of larger diameter being machined to a true cylinder, a core-conductive element having a spindle in the bore of the core and provided at one end of the spindle with a disk-like head lapping the exposed end of the nose and forming an inner electrode in bearing engagement with the nose, a sleeve of conductive metal slidably fitted in the cylindrical core portion of large diameter and capable of sliding by reason of axial thermal expansion and contraction on the spindle, said sleeve having one end bearing in heat-receiving relation with the disk-like inner electrode, the nose coacting with the portion of the shell surrounding the same to form an annular heat-distributing chamber, and said electrode coacting with the portion of the shell surrounding the inner electrode to form an annular chamber with one end open to the firing chamber and the other end open to the heat-distributing chamber, and an outer wire electrode extending from the shell into the firing chamber in underlapping relation to the inner electrode spaced therefrom, and forming a firing gap between the side of the wire electrode and the fiat under side of the inner electrode.

'7. A core assembly for use in a spark plug,

comprising a core of ceramic material provided with a bore extending axially therethrough, a element extending through jacent end of the core in contact therewith and forming an inner electrode, the ceramic wall of said bore in the part thereof terminating at said adjacent end being machined to a true cylinder, a preformed cylindrical sleeve of highly conductive metal of less length than the metallic conductive element having a loose fit in said drilled portion of the bore and having a sliding on the conductive element and thus free to move relative to the conductive element by reason of its thermal expansion and contraction and with one end of the sleeve in endwise heat-transferring engagement with the inner electrode and with its other end free to move axially.

8. In a spark plug, the combination of an outer tubular member with its bore open at one end to form a firing chamber, a core of ceramic material fixed in said bore and at the end adjacent the firing chamber spaced from the outer tubular member to form an annular heating chamber, said core having a bore at the end adjacent the firing chamber machined to a true cylinder, a cylindrical sleeve of highly conductive material having a close but slidable fit in said machined portion of the core bore, a onepiece core-conductive element having a shank fitted in the bore of the sleeve and terminating in a head forming an inner electrode in snug, fiat-faced, heat-transferring engagement with the end of the core adjacent the firing chamber and said sleeve completely filling the annular space between the shank and said end of the core whereby the cylindrical joints formed between the sleeve and shank and sleeve and core are free of any heat-retaining pockets.

9. An article of manufacture constituting the core element of a spark plug including a molded core of ceramic material with one end forming a conical nose provided ing axially therethrough, the portion of the bore conductive metal slidably fitted in said machined portion of greater diameter forming a heat valve for regulating the amount of heat transmitted therethrough per unit of time, and a core conductive element including a shank having its lower portion of true cylindrical form slidably fitted Within the bore of the sleeve and in heat transferring relation thereto and the portion of the shank thereabove loosely fitted in the portion of the core bore of less diameter, a filling of cement between said last-named portion and the core, and said element provided with a fiat head at one end of the shank forming an inner with a bore extend ing end of the spark plug and extending to the air-exposed part of the shell, a tubular core of material fitted in the outer sleeve and having electric insulating properties and at least a limited degree of heat-conductivity, a metal shank in the bore of the core and provided with a head forming an inner electrode, of an inner preformed sleeve having a solid cross section of material and formed of a highly conductive metal and having a pre-fixed length approximately two to three times its diameter, located between the shank and core, with its lower end bearing in heat-transferring relation to the inner electrode and with its upper end extending above the level of the lower end of the outer sleeve and free to expand upwardly and axially and thus by reason of its thermal expansion to control the extent to which it projects above said level in its lapping of the outer sleeve, and said inner sleeve functioning by reason of its length as a heat valve for regulating the amount of heat which is withdrawn thereby per unit of time from the vicinity of the inner electrode and which is discharged from the upper end of the inner sleeve across the core to the outer sleeve and then to the air-exposed portion of the shell.

11. In a spark plug designed to have a heat rating of about 300 pounds indicated mean effective pressure including a steel shell having an all-over length of about three and three-sixteenths of an inch provided with a bore extending axially therethrough and open at one end to form a firing chamber, an outer copper sleeve fitted in the bore in fixed spaced relation to said open end, a core of ceramic material fitted in the sleeve and projecting therefrom towards the firing chamber, a metallic core conductive element including a spindle extending through the core and terminating in a head forming an inner electrode at the firing chamber, said head underlapping the adjacent end of the core and in heattransferring relation thereto, a long preformed copper sleeve of materially less length than the core fitted between the shank and ceramic core and in heat-transferring relation to the portions of both the shank and core adjacent the firing chamber, the portion of the core between the annular gas chamber open to the firing chamber, said inner and outer sleeves each having a length of one-half inch and the length of said gas chamber being about fifteen sixty-fourths of an inch, all measurements taken axially of the plug.

References Cited in the file of this patent UNITED STATES PATENTS

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