US2470568A - Spark plug - Google Patents

Description

May 17, 1949.

J. D. MCCRUMM SPARK PLUG 2 Sheets-Sheet l Filed April 6, 1946 ;lll y INVENTOR.

Mv //-www J. D. McCRUMM May 17, 1949.

SPARK mm1 2 Sheets-Sheet 2 Filsi April e. 194e VEN TOR. 6em/M /f/peA/fy Patented May 17, 1949 SPARK PLUG John D. McCrumm,

Douglas Aircraft Calif.

Inglewood, Calif., aulgnor to Company, Inc., Santa Monica,

Application April 6, 1946, Serial No. 660,065 4 Claims. (Cl. 175-115) This invention deals with ignition devices and is particularly concerned with spark plugs. The present application is a continuation, in part, of the co-pending application of John D. McCrumm and Ernst H. Plesset entitled Method and apparatus for igniting combustible mixtures, Serial No. 653,384, led March 9, 1946.

Although the presently preferred embodiment of the invention assumes the form of a spark plug for igniting combustible mixtures of liquid fuels and air, particularly in internal combustion heaters, it is equally well adapted, as will be made manifest, for employment in ilring any type of combustion device for converting potential chemical energy into heat energy, either of the potential or of the kinetic variety.

In such devices, the prime requisites are (1) insta-ntaneous ignition, and (2) continuous firing .over a long period of service including cyclic, or

off-and-on, operation of the device. However, the conventional spark plug, including a narrow spark gap constituted by two electrodes arranged side by side and close together, is not capable of meeting satisfactorily either of these requirements. For, at or near extremes of atmospheric conditions, the gaseous density of the combustible mixture between the electrodes falls to such a W value that not only is it impossible to achieve instantaneous ignition because of the low electronic kinetic energy produced in the spark gap of such plugs, but, if the gaseous density is low enough, the mixture cannot be ignited at any 4time-rate. Again, fouling of the electrodes and even of the entire ignition head, occurs with such regularity as to be considered commonplace. Such fouling renders it diillcult, if not impossible, to operate the ignition head cyclically and eventually terminates ignition.

The present invention provides a single-electrode spark plug which is adapted to be employed in widely spaced relationship with an extraneously mounted ground-electrode in combustiontype thermal energy-conversion devices of substantially all kinds to effectuate instantaneous and continuous ignition therein of substantially all types of combustible mixtures. The plug will perform these functions under all extremes of' atmospheric conditions likely to be encountered throughout a long service life and without consequential fouling.

The plug is rendered capable of effecting instantaneous ignition by the incorporation of means for enabling the creation of an extra-l ordinarily high potential difference between the plug and its associated ground, before spark-gap breakdown, without causing the plug to break down under the concomitant high electrical and mechanical stresses set up therein. Electrical discharge from the plug is, in fact, inhibited until a critically elevated potential difference has been created in the spark-gap-one which will be adequate, on spark-gap breakdown, to kinetically energize the ions in the gap sufilciently to trigger an instantaneous chain reaction in a combustible having the lowest gaseous density that is likely to be encountered in the normal use of such devices. f

Fouling of the electrode, of the remainder of the spark plug, and of the ignition head` are precluded by the provision of means for repelling or positively precluding, the deposition of products of combustion thereon. These means include instrumentalities for maintaining the sparking tip constantly at an elevated temperature and configurations for inducing turbulence of the combustible mixture at the tip and in the rest of the spark gap.

Brieily considered, the means for enabling the plug to withstand the high electrical stress inci- `dent upon building up the aforementioned critically elevated potential diilerence essentially comprise the features of (1) so mounting the electrode rod with relation to the insulator thereof and (2) so mounting the insulator with reference to the body of the plug, that (1) thermal shock cannot be transmitted to the insulator from the suddenly heated or cooled rod or from the metallic shell and that (2) mechanical, mounting-induced stresses which would tend to cause electrical breakdown between the rod, insulatorand plug body, are entirely obviated, in at least the preferred embodiment, and are minimized in all other embodiments.

The portions of the plug which are exposed to products of combustion are made deposit-repellent by the employment of materials which are resistant to corrosion and erosion and by providing a relatively long thermal path of very poor thermal conductivity whereby to maintain the extremity of the plug hot. The plug is, further, so configured and mounted as to create or maintain turbulence in the combustible mixture, both in that portion which lies in the spark gap, and those portions thereof which lie between the spark plug and the ignition head, whereby to furnish a positive preventive of deposition.

Although productive of the aforementioned im-I provedresults, which adapt the plug to employment in such widely separated ilelds as internal combustion heaters, jet engines, and in the gas generators of gas turbine engines, the plug is quite compact and includes but va small number of parts. It is thus adapted to large-scale production, enabling it to be sold ata relatively low price. Installation and maintenance of the plug are simplified over that of the ordinary spark plug, the plug requiring practically no attention, such as cleaning orgap-adjustment, after it has been easily and quickly .screwed into the ignition head. In many instances. the plug outlasts the other components of the ignition head.

Several of the presently. preferred embodiments of the foregoing and other inventive concepts are illustrated in the accompanying drawings and described hereinafter, by way of example only, it being understood that the invention is limited in the embodiments which it can assume, only by the scope of the accompanying claims.

In these drawings,

Figure 1 is a longitudinal half-section of one embodiment of the invention;

Figure 2 is a transverse section on line 2-2 of Figure 1;

Figure 3 is a fragmentary longitudinal halfsection of another embodiment of the invention;

Figure 4 is a transverse section on line 4--4 of Figure 3;

Figure 5 is` a view similar to Figure 3 of still another embodiment; and

Figure 6 is a transverse section thereof on line 6-6 of Figure 5. l

` In all embodiments of the invention, the construction generally comprises an outer shell-like body encompassing a single, elongated electrode mounted coaxially of the body, and dielectrically separated radially from the body around the adjacent peripheries of both for its full extent within the body. The rod is so mounted in the dielectric shielding, which in turn is so 4mounted in the body, that no consequential mountingstresses are set up in the shielding, the mounting at thel same time providing the maximum feasible length of breakdown path between the rod and the shell-like metallic body. In the preferred embodiment, in fact, all so-called shortpaths are eliminated, there being provided an unusually elongated, radially extending dielectric y path between the closest points of the rod and the mounting-portion of the body, and adequately long such paths between the rodv and all other portions of the body. The inner end of the rod is longitudinally spaced from the adjacent end of the body to provide a conductor receptacle, and the other end of the rod projects a considerable distance from the body. Most of this projecting portion is delectrically shielded, longitudinally and radially, from the adjacent end of the body, and an annular gap is-provided between the rodand the shield, to obviate thermal shock.

The body is adapted, at the inner end thereof, to disengageably engage in a wall of an ignition head or the like, and at the outer end is adapted to disengageably engage the anchoring means terminating a conventional armored conductor having conductive means projecting therefrom into the hollow shell and therein establishing electric contact with the single electrodes of the plug.

In the particular'- construction illustrated in Figures 1 and 2, the plug comprises a metallic shell I0 provided with screw threads II at the inner end for disengageably mounting the plug through the wall of an ignition head, the other end of the shell preferably being provided with screw threads I2 for engagedly receiving the anchoring means of a conventional armored conductor, not shown, but of the type including a conductive projection extending inwardly of the plug sufficiently to establish an electrical contact therein which is hereinafter particularized. The shell also incorporates a plurality of polygonally arranged wrench-receiving flats I3 and a polygonally contoured sealing shoulder Ila. The shell includes a substantially cylindrical, multistepped bore I4 extending longitudinally therethrough. In the outer, largest-diameter portion thereof is mounted a substantially cylindrical insulating liner I5 preferably composed of an aluminum oxide ceramic, and bearing an annular groove I6 at its inner end-portion. The radial face l6a of this groove is longitudinally spaced from the confronting face IBb of the step of the next largest diameter of the bore I4, in order to enable lengthwise adjustment of the sleeve in the bore, whereby to effectuate pulverization vand packing of pulverulent dielectric material, as hereinafter described.

An insulating washer I1, preferably composed of a heat-resistant, mechanical-stress dissipating material, such as an asbestos composition, is provided between the outer end of the liner I5 and the outer end of the shell I0, being restrained from lengthwise outward displacement by an inturned annular flange I8.

Mounted co-axially of the inner end of the liner I5 and separated dielectrically therefrom and from the inner end `of the body I0,'is an elongate electrode I9, which, generally considered, is constructed and configured to provide a relatively long thermally conductive path in a spark plug of this capacity and body-size, so aS to tend to retain heat and maintain a high Iheathead at the sparking tip thereof. The electrode 4 is contemplated as essentially composed of a material which will have a low absolute thermal conductivity to assist in maintaining this high heat head. To further assist in keeping'the electrode hot, the greater portion thereof is conceived of as shielded, by a dielectric construction described hereinafter, from heat loss to the shell or body by radiation, conduction or convection, and the plug as an entirety is adapted to be so mounted in the ignition head that the greater portion of the electrode rod is shileded from heat loss to the ambient atmosphere.

The electrode I9 consists of a solid cylindrical rod uniformly circular in cross-section throughout its length, except at the' inner and outer ends and in a substantially medial location. The rod I9 is preferably composed of a chromium alloy such as Chromel A, although Chromel P and Chromel D are contemplated as lying within the scope of the invention, as well as the 25-12 stainless steels. derstood, however, that any material which has a low coeicient of thermal conductivity is contemplated as suitable for the electrode, and that the mentioned substances are illustrative, merely. The relative diameter, the shape, and the composition of the electrode rod, including its tip, are such as to prevent discharge from the sparking tip until an electrical stress has been built up which will be adequate, on breakdown. to kinetically highly energize the gaseous mixture at the tip sufficiently to cause instantaneous ignition. The sparking tip 20 of this electrode, instead of being elongate or pointed as is conventional, is, as compared to the conventional tip, of a decidedly blunt and It is to be unrounded, relatively short and broad configura-.n tion. such as to prevent discharge until the` aforesaid high voltage has been built up. In

otherfconditionsfbut -thefconstruction of"y the" plugA and the lrod 'is suchthat by suitably va'ryi Y ing the'thickness and .other dimensions of the dielectricafthe present configuration of plug can be adapted to withstand electrical breakdown under impressed voltagesfof as highk asseveral v.

hundred thousandsA of volts.

Mounting means are provided'for supporting and positioningthe electrode concentrieally `of muy to 'parvenza` same into powder, form'. The space between parts lea and |6b permits a wide the bodywhich at the same time'pre'vent electrical 'breakdown between' the rod and the 'me-"f" tallicshell under high" electric stress.'

the passage oLvoltage from the yelectrode to the shell, -the mountingl also beingrsuchas lto y The, means present yno short-path". at any kpoint for Y Il, occupying the verticallyA extending space be-y securely position theelectrode-without applying'` any consequential mechanical stress to therod its outerend extends'a substantial distancebeA yondthe? shell, there including a cylindrical skirtfsurrounding the' cylindrical rod Vprojecting therefrom.;I 'he rod, although supported and positionedlby;thesleeve. isradially sepa-VV rated therefrom throughout its length, the two being spaced iny their upper halvesV by a pul-l verulent dielectric andy in their lower. halves,y by

an annular'elongate ai'r gap 22, both as here-'f' inafter detailed.

The sleeve 2| isv composedof an improved rigid ceramic, and the rod is supported therein at two prevent egress of the insulating material be- 1 tween therod |,9 and the sleeve 2| during and 6 spending annular shoulder 2l located on the inner-wall of the plug near its lower end.

The annular space so defined between the insulating. sleeves ligand 2|@ ,and the inner o wall V of the shell kis occupledby an insulating rna-MI terial lil.' lmaterial preferably consists4 of a friable, butfplasticlzable, dielectric V'such'as the composition knownas Sillimanite The space' is preferably illledby inserting` a cylin.l der of thismaterial through the outer end of theA plug body with'the sleeve I5 absent, whereafter the sleeve |5is inserted and urged against the cylinder ofinsulating material sulliciently forcerange of pulverizing force to be applied.v -The powdered material thereafter agglomer'ates, and? upon drying, again forms a more yor less rigid cylinder.y VA' similar cylinder of similar material is inserted between the rod I9V and sleeve tween annular shoulder 25, and the inner wall of member 2|. Itis then reduced to powdered form 30a' in amanner similar to that afore-f described.' The annular shoulderl 2`5'serves to after loading, and a removable ring 3|, shown in phantom, may be temporarily applied inthe inner endof the sleeve `bore to hold the insulating material in the space between the sleeve 2| and the shell fora similar purpose, being removed after the space has vbeen1 filled.

The sleeves i5 and 2| are. of course, contemplatedas vbeing `of suclrradial thicknesses, and

are so spaced from the shell I0 at their closest points, -as to inhibit and substantially prevent v4l) insulating materialsuch as-an aluminum oxide locationsfthereof; To constitute the one sup i port, the :innermost end oi! ltheelectrode rod ispeenedy koutwardlyanl downwardly against a gland-like member 2?.'rtkdisposed radially and vlongitudirrally.between thefrod end and the'fend of the sleeve. The `glandfmernber 22a in turn f' is forcefullyxseated against the inner-endof the. l

insulating sleeve 2| over anl insulating and forcedissipating' washerV 23, 'composed lof la. ,heat-re-V "sistant material "such'as an asbe'silos-composition. To-constitute the" other -supportfor 'fthe rod;I thesleeve,l is provided, substantially-medii' any: ofthe length .of its1nteriyperiphery'lwithl'l sleeve 2|, an annular air gap 22, as aforemenpgeglgilgr rgil annular clearance ofthe order of not less than r-withf an annularrprojfection, @or flange :25, h adapted ytobe seated in the annular' :shoulder: 24...

mounting for the 'romand-sleeve unit in; they n; b0dy.ihe Outer surfacevyof. thefinsultinglslifeve- .2| integrally "beersubsteiiailr mediana .i its; length,"A jan" annular projection or flange 2,-'whlch e il him 'W 'is and 'the f fciruxnferenuer groevev 11| s fon the sleeve 'iis f initially` longitudinallyspaced rel-1,

atively quiteia distance from an annulanshoul- ,i

der- 5b oni-the inner faceiof the :.shell..` The outer,

facezl; cartine annuler nrbietioa ylfprefereblr extends parallel to, and spaced fromgao rre electrical breakdown between the electrode and the shell at any point of either.'Y

Thefouter periphery of the lower portion of the sleeve 2|1 is annularly spaced from the inner periphery ofthe lower end of the metallic shell III in this` critical region sufdciently to inhibit electrical breakdown betweeny the rod in this region and'this end of the shell. Although the plug is'thus endowed with two concentric longitudinally extending gaps at its lower or inner end, the barrier afforded by the dielectricsviil l and 30a 'is` sumcient to prevent the ignitedv charge from .forcing its way outwardly through the sparkplugeven,should a backfire occur.

, In orderv to bar, isolate, and exclude thermal shocks, either of' internal or of external origin,

of the'rod and the surrounding portion of the sleeve'2|,..thevupper boundarylof the gapbeing limltedonlyfby the requirement that ity be not so. large'astopejr'mit backring discharge out of the tively l'long pathy between: the sparking tip of the electrode andthe'rl'owermost portion' of the'insulator lskirtrfa stilllonger path .being` provided j between-fthe nearest pointof the4 metallieshell and the tip of the electrode;';. f f

into the'. flowing l medium `all l have al cylindrical The embodiment graphically disclosed in Fig--v ures 3 and 4 is, in essence, the equivalent of the embodiment described supra, dilering therefrom chiey in the employment, to support and mount the electrode-carrying sleeve 2| in the casing, of both the friable plastic dielectric 30 and a metallic annulus 33. The mounting member,

preferably comprises a copper washer disposed between the outer face 21 of the radial projection 260i the outer periphery of the sleeve 2| and the complementary shoulder 28 on the adjacent end of the shell l0. The washer is inserted in the shell prior tothe insertion thereinto the sleeve 2| and seats firmly oriv thev annular shoulder 28, being of such a cross-sectional conguration and internal diameter as to properly space and center the 'electrode-carrying sleeve while supporting same.

Although this metallic 4mounting may,' as compared to the previous form,' somewhat reduce the length of the dielectric path between the rod and the shell at one point, and may, under certain conditions, set up. somewhat larger mechanical stresses thereat, it does introduce certain refinements and simplications in manufacturing'procedures and lowers the cost of the plug.

Also, since it is quite adequate despite the shortened dielectric path and higher mechanical stresses, to withstand electric stresses up to the order of 'about 75,000 volts, it is quite acceptable .for certain'types of service, and particularly in automobile engines, domestic heaters and similar usages where the gaseous density is usually at the optimum.

The embodiment illustrated in Figures 5 and 6 is essentially the same as that illustrated in Figures 3 and 4 except that instead of employing but one metallic mounting ring, disposed between the outer or lower face 21 of the annular projection 26 and the shoulder 28 on the inner wall of the shell, an additional such ring 34 is employed and is located between the junction with the sleeve wall of the upper inner face of the annular projection, and the radially adjacent wall of the shell l0.

The essential purpose of the ring 34 is to prevent the venting, through and out of the plug, of gases which may have by-passed the ring 33. Although such by-passing is likely to occur only upon excessive back-firing in the combustion device, this modication particularly adapts the -vplug to serve in automobile engines.

body adapted near the one end for mounting in an ignition space and adapted near the other end for connection to an electrical conductor; a frangible dielectric sleeve, bearing an annular projection, disposed coaxially in the rst said end of said body and longitudinally coextensive therewith for the greater portion of the length of said sleeve, the outer end of the sleeve extending outwardly of said body end; a dielectric powder loosely and unstressedly interposed between said sleeve and said body and surrounding said projection so as to floatingly support said sleeve in said body, said powder being disposed along a major portion of the coextent of said sleeve and said body so as to separate said sleeve from all physical, electrical and thermal contact with said body throughout said coextent; an electrode of low thermal conductivity, and bearing an annular projection, disposed coaxially in said sleeve in longitudinally substantially coextensive correspondence therewith and projecting from the inner end thereof, said electrode operating at an ultra-high heat level and under a high electric potential; and a dielectric powder loosely and unstressedly interposed between said electrode and said sleeve and surrounding said projection so as to iioatingly support said electrode in said oatingly mounted sleeve; whereby to relieve said sleeve of substantially all mechani- .cal stresses and to exclude therefrom electric and thermal shocks originatingl in said electrode .and in said plug-body.

2. An aviation spark plug substantially imstresses and shocks and to fouling, comprising: a hollow metallic body adapted near the one end for mounting in an ignition space and adapted near the opposite end for connection with a conductor of ultra-high, cyclically and continuously applied, electric potentials; an electrode, having a medially located annular projection, disposed along the longitudinal center line of said body and having a composition conferring thereupon a relatively high potential storage capacity and Joule-capacity and an absolutely low thermal conductivity suiiicient in themselves to elevate at least the firing tip portion of said electrode to anv ultra-high temperature under application of said ultra-high potentials, thereby to prevent fouling of said tip tending to eiiec't radial expansion thereof; a dielectric sleeve longitudinally substantially coextensive with said electrode disposed intermediate said electrode and said body; a dielectric powder loosely interposed between the upper portion of said electrode and said sleeve and supported by the latter and surrounding said projection so as to iloatingly and unstressingly support said electrode in said sleeve, the lower portion of said electrode being radially separated from said sleeve an air-gap distance substantially greater than that necessary to fit said electrode in said sleeve, said powder and said air gap thereby precluding all physical, thermal and electrical contact of said electrode with said sleeve; whereby to adapt said plug to both continuously and cyclically generate a high potential spark at ultra high potentials and at ultra high temperatures in highly carbonaceous combustibles having a low gaseous density at high altitudes without rupturing said sleeve or fouling said plug.

3. An aeronautical spark plug, comprising: a hollow metallic' body adapted near the one end for mounting in an ignition space and adapted near the other end for connection to an electrical conductor; a frangible dielectric sleeve, bearing an annular projection located substantially medially of its length, disposed coaxially in the first-said end and longitudinally coextensive with 9, v said body for the greater portion of the length of the sleeve; a comminuted, flowable dielectric medium interposed between said sleeve and said body and surrounding said annular projection and organized with said body and said sleeve to full-floatingly support said sleeve in said body and separate said sleeve from direct physical, electrical and thermal contact with said body; an electrode, having an annular projection, disposed coaxially in said sleeve in longitudinally substantially coextensive correspondence therewith and projecting from the inner end thereof; and a comminuted, owable dielectric material loosely and unstressedly interposed between said electrode and said sleeve around said projection and so organized with said sleeve and said electrode as to ull-loatingly support the latter in the former without direct physical, electrical or thermal contact between them; whereby to substantially obviate breakdown'of said plug under the electrical', vmechanical and thermal stresses originating in ultra-high electrical potentials applied'to said electrode.

Q4. An aeronautical spark plug substantially immune to rupture and other damage from ultrahigh electrically and thermally generated stresses e and shocks and from fouling, comprising: an elecltricallyconductive shell adapted near its one end for mounting in an ignition space and adapt'- ed near its opposite end for connection to an electrical conductor; an elongate electrode coaxiallycoextensive with the'greater part of the length of said shell, thereby to provide an ei!ec 4tively attenuated thermal path through said plug, saidelectrode protruding sumciently from said plug to form a discharge tip and having-a com- "position conferring thereon a relatively high elec'- trical'capacity and a lowA specic thermal nf .'f vductivity sunlcient in themselves toelevate lsaid electrode, including said tip. to a temperature 'fr g ladequate to prevent incrementsY in the diameter v electrical contact with said sleeve andthe sleeve thereby being segregated from direct physical, electrical and thermal contact with said shell; whereby to adapt said plug to both continuously and cyclically generate a high potential spark at ultra-high potentials and ultra-high temperatures of the plug in'highly carbonaceous combustibles having a low gaseous-density at high altitudes without rupturing said sleeve or fouling said tip. A

- JOHN D. McCRUMM.

REFERENCES CITED The following references are of record in the `ille of this patent:

UNITED s'ra'rns PA'I'EN'rs Number Name Date Re. 14,862 DuiIy May 25, 1920 1,354,126 Olson Sept.`28, 1920 1,677,516 Farquharson July 17, 1928 1,761,865 Berthold June 3, 1930 2,001,888 Randolph May 21, 1935 2,020,966 Rohde Nov. 12, 1935 2,071,645. McNeil Feb. 23,1937 v2,090,039 Goddard Aug. 17, 1937 A.

2,266,318 Heller Dec. 16, 1941 2,351,029 Feichter June 13,` 1944 2,377,213 Danziger May 29, 1945

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