US2921239A

US2921239A - Electric igniters for use with fluent fuel burners and in sparking plugs

Info

Publication number: US2921239A
Application number: US507287A
Authority: US
Inventors: Thomas F Dryden
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1955-05-10
Filing date: 1955-05-10
Publication date: 1960-01-12
Anticipated expiration: 1977-01-12

Description

Jan. 12, 1960 T. F. DRYDEN 2,921,239

ELECTRIC IGNITERS FOR USE WITH FLUENT FUEL BURNERS AND 1N SPARKING PLUGS 2 Sheets-Sheet 1 Filed May l0, 1955 ATTORNEY FIG.4

Jan. 12, 1960 Filed May l0, 1955 T. ELECTRIC IGNITERS F' F DRYDEN OR USE WITH FLUENT FUEL BURNERS AND IN SPARKING PLUGS 2 Sheets-Sheet 2 FIG. 5

INVENTOR.

THOMAS F. DRYDEN ATTORNEYl United States Patent@ ELECTRIC IGNITERS FR USE WIIH FLUENT FUEL BURNERS AND IN SPARKING PLUGS Thomas F. Dryden, London, England, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Application May 10, 1955, Serial No. 507,287

6 Claims. (Cl. 317-83) This invention relates to electric igniters for use with uent fuel burners and to sparking plugs and to a method of manufacture of such sparking plugs. The methods of lighting up commonly used in small furnace charnbers fired with fluent fuel are unsuitable for application to large furnace chambers such as those found in modern central station boilers and the practice has arisen of utilising automatic apparatus for effecting lighting up. Such apparatus involves electrical ignition means and in order to obtain the necessary energy in the spark a high tension of some thousands of volts has commonly been employed. 'In such a high voltage system the insulators are prone to breakage or to failure due to dirty surfaces. In addition danger to personnel may arise. An object of the invention is the provision of an improved relatively low voltage ignition system for effecting ignition in a furnace chamber.

In orderv to be effective, the electrodes of an ignition system must be properly positioned in relation to the stream of fluent fuel to be ignited, but when the spark has effected ignition of the burner with which the system is associated, the electrodes are usually withdrawn from the heat of the llame. If the burner itself is'an ignition burner for a pluverised fuel red furnace, the burner as a whole must be withdrawn at the termination of the lighting up period. In some forms of burner a circular series of air directing vanes is provided adjacent the burner tip and the electrodes must be projected beyond the Vanes in order to ignite the burner and then withdrawn behind the vanes and a further object o-f the invention is the provision of an igniter in which this may be accomplished by a simple linear movement yof the electrodes and without making a large aperture in the vanes.

A still further object of the invention is the provision of a simple but effective form of sparking plug including a surface adapted during operation to be traversed by spark discharges and an economical method of manufacture of the plug.

The present invention includes an electric igniter for use with a fluent fuel burner comprising a sparking plug arranged to be projected to and retracted from an operative position and comprising an inner electrode and an outer electrode surrounding the inner electrode, separated therefrom by insulating material and extending in or substantially in the direction of movement between the projected and retracted positions, the insulating material providing an annular surface disposed at the forward end of the outer electrode between `that electrode and the inner electrode and adapted, during operation of the plug, to be traversed by spark discharges between the electrodes.

The invention also includes a sparking plug having an inner electrode, an outer electrode of tubular form surrounding the inner electrode and separated therefrom by insulating material which affords an annular surface disposed between the electrodes at one end of the outer Patented Jan. 12, 1960 ice be traversed by spark discharges between the electrodes, at least an end portion of the insulating material adjacent the annular surface being compressed between the electrodes.

The invention further includes an electric igniter for use with .a uent fuel burner comprising a sparking plug arranged to be projected to and retracted from an operative position, a capacitance, an electric charging circuit for the capacitance and an electric discharge circuit including in series the capacitance, the sparking plug and a switch arranged periodically to close the discharge circuit.

The invention moreover includes the method of forming a sparking plug which includes assembling an inner rod electrode member within a tubular outer electrode member with insulation between the members and swaging the outer electrode member and thereby compressing insulating material between the electrode members, firmly uniting the electrode members and reducing the diameter of the end of the outer electrode member bounding the annular surface which is afforded by the insulation at one end thereof and is adapted, during operation of the plug, to be traversed by spark discharges between the electrodes.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a longitudinal sectional elevation to a relativeiy large scale, of a sparking plug;

Figure 2 is a longitudinal sectional elevation of an electrically-operated igniter assembly which includes the sparking vplug of Figure 1;

Figure 3 is a partly diagrammatic, sectional side View of an oil burner in its operative position, the oil burner, certain parts of which have been omitted, constituting an ignition burner for a pulverised fuel fired furnace and employing the igniter shown in Figure 2;

Figure 4 is a sectional plan view at right angles to Figure 3 the burner being shown in this view in its retracted position;

Figure 5 is a sectional View illustrating a detail of the burner and taken on the line V-V of Figure 3;

Figure 6 is a fragmentary sectional View of a detail ofdthe burner taken on the line VI-VI of Figure 4; an

Figure 7 is a schematic view of electrical circuit means associated with the igniter assembly.

Referring to Figure l, a sparking plug 1 for use in an electric igniter of an oil fired burner constituting an ignition burner for a pulverised fuel tired furnace, includes an inner electrode 3 in the form of a cylindrical mild steel rod having a diameter of 5%;2 inch and a length of 5% inches which is disposed within an outer, co-axial tubular, mild steel electrode 5 having an overall diameter of 5/16 inch, a thickness of 0.031 inch and a length of 411/16 inches. The forward end of the electrode 3 extends a distance of 3/16 inch beyond the forward end of the electrode 5 from which end a tube`7 of mica mounted in closely fitting relationship upon the inner electrode 3 and made from sheet mica of 0.015 inch thickness extends rearwardly for about 1 inch and is compressed over a forward section 9 thereof between the

electrodes

3 and 5. To this end the diameter of a forward end portion 11 of the electrode 5, which portion is about 1/2 inch long, is reduced by swaging of the outer electrode 5 over the said portion 11 thereby compressing the forward section 9 of the tube 7 between, and firmly uniting the

electrodes

3 and 5. At its forward end the mica tube 7 provides `electrode and adapted, during .operation of the plug, to

an annular surface 13 which, during operation of the plug, is traversed by spark discharges between the

electrodes

3 and 5. The thickness of the tubular electrode is so selected that, during operation of the'plug, erosion of that electrode takes place at a rate which is approximately the same as that-at which the mica tube 7 is eroded. Thus any tendency for an annularga'p to' appear, during 'operation of theplug, between the

electrodes

3 and 5 rearwardly of theforward end of the electrode 5 is reduced thereby rendering the-spark discharges between the electrodes eifective to promote ignition over theY full length thereof.

Around its rear end the tubular electrode 5 is provided witha closely tting, mild steel ring which is soldered to the electrode 5 and includes a rear portion 17 of uniform external diameter and a front, forwardly tapering portion 19. Mounted upon the inner electrodeA 3 rearwardly of the electrode 5 is a bushing 21 of asbestos cement insulating vmaterial and including a forward section 23 of reduced external diameter which ts within the vrear end of the tubular electrode 5 and a rearward section 25 which has an external diameter equal to that of the ring portion 17 and, at its forward end, provides a shoulder 27 which is held against the rear end of the electrode 5 and ring 15 by means of a nut 29 which engages a rear, externally threaded end part 31 of the electrode 3. j

'l`he sparking plug l is formed 'by mountingk upon andsoldringt'o the electrode S'the ring 15, locating the mica tube 7 upon the electrode 3, arranging the electrode 3 in position'within theelectrodeS swaging the forward portion A141 ofthe electrode 5 down upon the tube 7 thereby compressing the tube 7 between and rmly uniting the

electrodes

3 and 5, threading the bushing 21 onto the-rear end of the electrode 3 and tightening up the nut l29 to fix vthe'bushing 21 in position.

The electrodes 3 v'and 5 are cut from `stock material so that thelabour involved 'win their manufacture is minimised. It Awillbeobserved that the swaging isrestricted to `thefforward end portion 11 of the louter :electrode 5 so that the workrrequired in forming the plug is minimised and the full diameter of the electrode 5 is available betweenthe swaged portion and the rear end `of that electrode, a feature which makes for improved rigidity of the plug under operating conditions.

, The sparking plug 1 which vsomewhat resembles a long `pencil islreadily replaceable in relation to a reciprocable holder 33, shown in Figure 2, which is associated, yin a ymarmer hereinafter described, with pneumatic means for effecting the reciprocating movement in relation to an associated oil burner which constitutes the ignition burner for a pulverised fuel red furnace, the ignition burner itself being movable between a projected operating position and a retracted position.

The holder 33 comprises a mild steel, tubular member 35 having an internal diameter which is slightly greater than the external diameter of the ring 15 and the bushing '2.1`and within the forward end of which is welded a ring member'37, the latter including a forward section 39 which fits upon the tubular electrode 5 and a rearward section 41 ytheV inner surface of which is complementary with and engages the forwardly tapering surface of the portion'19 of lthe ring 15. A rear end portion 43 of Vthe tubular member 35 is welded within a forward section 45,' of 'reduced internal diameter, of a tubular part 47, having a` rear section 49. The tubular part 35 provides a 'housing' for a resilient electrical terminal vdevic'e`51 'which is spring-biasedv to a forward position in which Yit is urged into contact with the rear end 31 of the electrode'3 in the mannerV described below.

The terminal device51 comprises a rod 53`having a diameter approximately equal to that of the electrode "3 andextending axially within the tubular' member 35, thek rear end ofthe rod 53-projectin`g beyond the rear end of the tubular part 47 whilst upon the forward end of the rod 53 is mounted a bushing 55 formed from asbestoscementinsulating material and having a bore 57 a rearlward section 59 of which is of reduced diameter and the adjacent endsurface of the electrode 3.

Disposed within the tubular part 47 ,is a housing -69` for a coil spring 71 which is mounted on the rod 53. The housing is formed from resin-impregnated insulating material and comprises four

annular elements

73, 75, 77 and 79.l The forward element73 of the housing 69 has a rearward part 81 which fits within the front end of the rear section 49 of the part 47 and a forward part 83 which engages within the rear end of theA tubular member 35, the arrangement being suchthat anannular shoulder 85 formed at thefront of the part S1 abuts a complementary shoulder 87 formed 'by' the rear end surface ofthe `tubular'member 35 andthe annular surface between the sections 45 and 49 of the part 47; A central `bore89 of "the element 73 includes a forward portion'91whichLts-the'rod 53 so as to permit sliding of vthe rod therethrough anda rearward portion 93of increaseddiameter. lThe elements 75 and'77 of thehousing69 have the same external diameter Vand. engage within the section 49 ofthe tubular part 47,v the internaldiame'ter Vofeachof the elements `75.and 77 being equal to that of the portion 93 of the element73. The kannular element 79thas an external diameter equal .to that of the element'77 and tits Vwithin the end ofthe annular part 47. A central bore^95 of the elementj79includes a forward section 97 of diameter equal to that of the internal'diameter of .the elementsi75 and77 and a rearward vsection`9`9 co-axial with and of the "samefdiameter as the boreportion 91 of the element 73.

l"For retaining'the housing 69v within the tubular part `47a union nut v101 is mounted upon an externally threaded end part 1'03 of the tubular part 47 Land provides a radially inwardly extendingannular flange between which andthe shoulder 87 are clamped 'the elements of the housing 69. f

The rod 53 projects rearwardly beyond the flange 105 and has an externally threaded end portion 107 adapted for kconnection to an electrical supply.

lThe internal bores vof the elements 75 and 77 and the borejportions 93 and 97 respectively of the

elements

73 and 79 together constitutea cylindrical cavity'109 within which is disposed the coil spring 71 which is adapted to bias therod 53 to a forward position. To this end thel rod 53 is provided with a pin 113 which extends through the rod 53 normal to the axis thereof. Rearwardly of the pin 113 is provided a washer 1'15 mounted on the rod 53 and free to slide within the cylindrical cavity 109. The coil spring 71 is arrangedin compression between the washer ,and an annular shoulder 117 formed at thelrearward. end of the bore portion 97 of the element 79, the arrangement being such that rearward movement of: the rod 53 vplaces thespring 71 in greater compression. u

Between the vrod 53 and the tubularrnember 35, at suitable intervals alongthe length of the member 35 are .provided porcelain insulators, such asthe insulator V119, ofannular form. It will be appreciated that the electrode`5, ring 15,"ring member-37, tubular member 35 .and tubular part 47 constitute an yelectrically conductive path and that the resilience of the spring 71 of the electrical terminal devicef51 ensures that ,contact between .the electrode:3.and the terminalmember 63 is maintained notwithstanding.differential expansion of the parts of theyigniter which occurs during'service thereof.

To yassemble the plug :1 .within the lholder 33, the ring member 37. is .welded'.Withinthesforward end. of .the tubular member 35 and the tubular part 47 is mounted upon the rear end of the member 35. lThe plug 1 is vthen inserted within the rear end of the'tubular member 35 after which the rod 53, upon which are mounted the bushing 55, terminal member 63, insulators 119, housing element 73, pin 113 and washer 115, is inserted in the rear end of the tubular member 35 and moved forwardly to push the plug 1 through to its operative p0- sition at the forward end of the member 35. The housing elements 75 and 77 are then inserted within the tubular part 47, the spring 71 and housing element 79 are threaded on to the rear end of the rod 53 and urged forwardly until the element 79 is disposed within the rear end ofthe section 49 of the part 47 whereupon the union nut 101 is mounted on the threaded part 103 of the tubular member 47 and tightened up.

The plug 1 and holder 33 constitute an igniter assembly which, as shown in Figures 3, 4, 5 and 6 is utilised for effecting ignition in an oil-tired burner, generally indicated at 131, the burner being suitable for use as a starting burner for a pulverised fuel fired furnace. The burner 131 includes a mechanical, fuel atomizer unit 133 having a barrel 135 at the forward end of which is provided an atomizer head 137 and at the rear end of which are provided fuel supply means (not shown). The barrel 135 can be forwardly projected axially to an operative position and rearwardly retracted axially to a withdrawn position. For effecting such movement there are provided two double-acting

air cylinders

143 and 145 mounted on the rearward side of an end plate 147 of the burner. The

cylinders

143 and 145 which are disposed at the same horizontal level and on opposite sides of the barrel 135, are respectively provided with pistons (not shown) and connecting

rods

149 and 151 which extend forwardly through

apertures

153 and 155 in the plate 147 and at their forward ends are connected to a yoke 157 which is welded to a tube 159 within which the barrel 135 of the atomizer unit 133 is secured. The tube 159 and barrel 135 extend rearwardly through a sleeve 161 which is secured to the plate 147 by welding within a central aperture formed in that plate and constitutes a support within which the tube 159 slidably ts.

Towards its forward end the barrel 135 is secured to an inner, movable, tubular casing 163 of the burner by a support 165 in the form of a spider which includes vertically disposed lower and

upper members

167 and 169 of which the lower member 167 extends between and is welded at its ends respectively to the inner surface of the casing 163 and a lower part of the barrel 135, whilst the upper member 169 is welded at its ends respectively to the inner surface of the casing 163 and an upper part of the barrel 135. The support 165 also includes two horizontally disposed members 171 respec tively arranged on opposite sides of the barrel 135`and each having an inner end welded to the barrel 135 and an outer end welded to a vertical plate 173 which in turn is welded to the inner surface of the tubular casing 163.

The upper member 169 of the support 165 comprises two parallel plates 168 between which is welded an -angle bracket 175 which serves, in a manner hereinafter described, as a forward support for the igniter assembly.

Around the atomizer head 137 is disposed a series of fixed vanes 177 arranged to impart a whirling motion to combustion yair supplied, during operation, to the burner..

The vanes 177 are connected at their outer edges to a sleeve 178 which is secured within the forward end of the casing 163.

The casing 163 is free to move with the unit 133 upon operation of the

air cylinders

143 and 145. To this end there are provided in the horizontal diametrical plane of the casing 163. rollers 179 respectively mounted at opposite sides of the casing 163 and arranged to engage horizontal rails 181 provided upon diametrically opposite arcuate plates 183 which extend axially of the burner are disposed outwardly of the casing 163, the plates 183 being welded at their forward ends to an annular"V support 185 surrounding the inner casing 163 and resting upon the inner surface of an outer tubular casing 187 of the burner provided with an inlet 188 for combustion air under pressure. The rails 181 extend rearwardly from the support 185 to a ring 189 which is provided with slots (not shown) for the passage therethrough of the plates 183. The ring 189 is formed around its periphery with a slot within which is disposed an asbestos gasket 191 for preventing, during operation of a furnace associated with the burner, rearward flow of hot gases between the inner and

outer casings

163 and 187. The plates 183 at their rearward ends are connected by welding to inwardly extending projections 193 of an annular flange 195 which is disposed rearwardly of a ange 197 to which the rearward end of the outer casing 187 of the burner is welded.

A tubular casing 199 is provided at the rear end of the burner and includes at its front end a plate 201 formed with an aperture 203 through which the barrel extends. The casing 199 is Welded at its rear end to -an annular plate 205 disposed rearwardly of the flange 195 and spaced therefrom by an asbestos gasket 207. The

iianges

195 and 197, the gasket 207 and the plate 205 are secured by bolts 208 both to a rear wall 209 of a windbox for the burner land to the rear plate 147, the plate 205 being spaced from the plate 147 by cylindrical elements 211 the arrangement being such that all the parts of the burner Within the casing 199 are exposed to atmospheric air and are thereby maintained cool. So as to prevent the flow into the interior of the casing 199 of combustion air which enters the burner through the inlet 188, there is provided around the barrel 135 adjacent the front plate 201 of the casing 199 a gland 213.

The igniter assembly which includes the plug 1 and holder 33 is secured within a tube 215 which extends rearwardly in a direction parallel with the atomizer unit 133 from a location adjacent the forward end of the holder 33. The tube 215 is supported Itowards its forward end within an aperture 217 formed in the bracket and, rearwardly of the bracket 175, the tube 215 passes through an aperture 219 formed in the plate 201, there being provided a gland 221 for preventing rearward ow of combustion air through the aperture 219 into the casing 199. At its rear end the tube 215 passes through an aperture 223 formed in the plate 147. For effecting forward and rearward movement of the igniter there is provided a double-acting air cylinder 225 mounted at the rear side of the plate 147 and having, associated with a piston (not shown), a connecting rod which extends forwardly through an aperture in the plate 147 and is connected at its forward end to the tube 215 by means of a yoke 227.

As shown in Figure 3 with the atomizer unit 133 and the sparking plug 1 in their operative positions, the sparking plug extends through an aperture 229 formed in the vanes 177. In this position the forward end of the electrode 3 is arranged adjacent the surface of a conically shaped jet of atomizedy fuel indicated by the broken lines 231 which, during operation of the burner, is discharged from the atomizer head 137. It will be appreciated that the size of the aperture 229 is determined by the maximum diameter of the electrode 5 and that in view of the pencil-like form of the plug 1 an aperture of small diameter can be used so that in operation the symmetry of the air flow from the vanes is little disturbed and the shape of the ame from the burner is substantially unaffected.

The burner is provided with suitable doors (not shown) which are arranged during forward movement of the casing 163 to open and during rearward movement of the casing 163 to close. In their closed position the doors serve both to prevent ow of air forwardly through the casing 163 to the associated furnace chamber and to 7 protect the movable'parts of the burner from To commenceoperationof the burner 131, air under pressure is supplied yto the

cylinders

143, 145y and 225 so as to project bothfthe atomizer unit 133 and the igniter forwardly from therposition indicatedin Figure 4 to that indicated in Figure 3. forwardly, electrical circuit, means are operated and sparkingof the plugl 1 between the

electrodes

3 and 5 and across the annular surface 13 commences. When the atomizer unit 133 reaches the position indicated in Figure 3,- fuel oil-under pressure is supplied thereto and theconical jet of atomizedr fuel discharged from the atomizer head 137 is ignited-by the spark discharges -between the electrodes of the sparking plug 1. Upon ignition of the atomizer fuel -oil jet discharged from the head 137, theair cylinder 225 operated so as to eiect withdrawal of the sparking plug 1 through the aperture 229 in the vanes 177. to a position rearwardly of those vanes. The ignitedfuel jet issuing from the atomizerhead 137 is used to effect ignition ofl a jet or jets of pulverisedfuel and air discharged from a burner or from respective burners disposed adjacent the burner 131 and, upon ignition of the pulverised fuel and air jet or jets, the supply of fuel oil to the atomizer unit 133 is cut off whereupon the cylinders 143 and v145 are operated to effect withdrawal of the unit 133 to the position indicated in Figure 4.

With a sparking plug such as that described above, the sparking always occurs across the annular surface 13 Which-is of small external diameter so that the region of sparking may be accurately located with respect to the conical oil jet discharged from the atomizer head 137 and may be intimately applied to that jet. Since the area of yfree movement` of the spark is small the tolerance in the position of the sparking plug relatively to the oil jet in order to obtain reliable ignition is relatively great.

Electrical circuit means for effecting energisation of the sparking plugl Yare illustrated in Figures 6 and 7 which show the plug 1 connected in series witha capacitor 241 and switching device 243, the inner electrode 3 of the sparking plug being connected by way of the terminal and rod 53 to one side of the switching device 243, whilst the electrode 5 of the. sparking plug is connected by way of the electrically conductive path including the ring 15, ring member 37', tubular member 35 and tubular part 47 of the igniter to the positive side of the capacitor 241 which side is maintained at earth potential. The capacitor 241`Which has a capacitance of l2. micro-farads is further arranged in series with a charging circuit comprising a voltage doubler unit 247, which is arranged to be energised by a transformer 249 supplied from a low tension, alternating current source 251 of voltage 250 volts R.M.S. and frequency 50 cycles per second, and a pre-set resistor 245 whichis adjusted to a value between 500Y and`l1,000 ohms tov produce a. charging current of 120 milliarnperes approximately. The primary coil 253 of the vtransformer 249 is connected in series with a switch 255 across thev terminals of the supply 251 whilst the secondary coil 257 of the transformer which is a-dapted to yield a secondary voltage of 215 volts R.M.S. is connected vin series with the voltage doubler unit 247 which is of well known form and comprises a pair of rectiers 259 and a pair of capacitors 261veach having a capacitance ofv 16 microfarads.Vv The voltage doubler unit 247 is adapted to establish acrosskk the pair of capacitors 261 a unidirectional output voltage of value lessy than twice the peak voltage-developed across the secondary coil 257 ofthe transformer byl anl amount which depends upon and increases with increase of thercurrent drawn from Ythe voltage doubler unit. The output voltage from the voltage doubler unit247 iis applied tothe series combination of theresistor'id" and capacitor '241 to effect As the igniter moves.

8 charging of the capacitor 24,1the latter being periodical# ly ,disqhargedacrossithe electrodes of the sparking plug 1 through Operation o f'the .Switching devise 243; Thus the seriesucombination of the plug 1 and switching device 243. constitutes a v-discharge circuit` for the capacitor.`

-.'Ihe switching device 243 comprises a four lobe cam 263,keyed' to thelshaft v265 of an electric motor (not shown) which is arranged to be energised upon closurev of the switch 255;v Associated with the cam 263 is a. pivotally mounted follower 267 to which is connectedja contact Varm 2,6 9v provided with an electrical contact element 271, `the element 2,71 being biased towards contact with a Xed contact element 273 by means of a leaf spring l275. The contact element 271 is in electrical connection withthe electrode 3 ofthe plug 1 whilst the element 273 is inV electrical connection with the negative sideofthe capacitor 241.

Whemforward movement of the plug 1 is initiated', duringoperation ofy the burner 131, by supplying air to the cylinder 225, the switch 255 is closed and the motor associated withthe cam 263 is simultaneously energise'd.

UponV rotation of the ,cam 263 the contact elements 271W and 273 are closed four times per second to provide, atY

- burnerand arranged forprojection and retraction to and fromV an operative position, said igniter comprising, in

combination, Va relatively elongated inner electrode; a relatively elongated tubular outer electrode surrounding said inner electrode and radially spaced substantially there from throughout the major portion of the length of said tubular electrode to provide rigidity for the igniter;- said electrodes being elongated in the direction of such projection` and retraction and the sparking end of ther innerelectrode projecting a short distance beyond the sparking end of the outer electrode; and insulation ma-v terial disposed between said inner and outer electrodes and including an insulating sleeve having an outer end flush with the sparking end of the outer electrode to provide an annular surface of insulating material traversed' by spark discharges between the electrodes when the igniter is energized; said outer electrode being reduced in diameter adjacent itssparking end to compress the outer portion of said sleeve between said electrodes; the length of said sleeve vbeing a minor fraction of the length of said electrodes but at least equal to the length of the reduced portion of said outer electrode.

2. An igniter as claimed in claim l, wherein the outer electrodecomprises a length of tubing of smaller diameter than that of al reciprocable tubular holder from which the outer electrode projects.

3. An igniter as claimed lin claim 2, wherein the spark` igniter is readily replaceable in relation to the reciprocable holder. v

4. An igniter as claimed in claim 2, wherein the spark igniter is mounted within and projects from the holder and the inner electrode projects rearwardly beyond the outer electrode in order'to engage a resilient electrical .terminal` device associated with the holder.

5. An igniter as claimed in claim 1, wherein the inner electrode is in the form of a slender rod and the outer electrode comprises a length of tubing.

6. An igniter as claimed in claim l, wherein the insulation material is erodible by spark'discharges, the outer r andthe thickness ofthe outer electrode, atleast at the:

References Cited inthe le of this patent UNITED STATES PATENTS 1,824,806 Dubilier Sept. 29, 1931 1,845,305 Macrae Feb. 16, 1932 2,069,951 Hastings Feb. 9, 1937 10 Sabol Oct. 26, 1943 Hastings Aug. 15, 1944 Smits May 1, 1.951 Thomas Mar. 2, 1954 Shoobert Mar. 2, 1954 Burger Mar. 12, 1957 FOREIGN PATENTS France Aug. 2, 1932 Great Britain Mar. 21, 1940