US2894161A - Method and apparatus for electric ignition - Google Patents

Description

July 7, 1 5. J. SHEHE EN METHOD AND APPARATUS FOR ELECTRIC IGNITION Filed Dec. 6, 1955 INVENTOR. SHAY J. SHEHEEN ATTORNEY U i d t s Patc l Q METHOD APPARATUS FOR ELECTRIC IGNITION Shay J. Sheheen, Ithaca, N.Y., assignor to General Laboratories Associates, 'Inc., Norwich, N.Y., a corporation of New York i Application December 6, 1955, Serial No. 551,380 A 3 Claims. (Cl. 313-431) The present invention relates to a method and apparatus for electric ignition, and is particularly concerned with a method and apparatus for producing an electric spark in a fluid medium, e.g., a fuel or a fuel mixture, 'whichmedium may be either electrically conductive or non-conductive.

In certain types of modern engines, for example, rocket engines, the fuel may contain one or more electrically conductive components, for example, nitric acid. Further-' more, it may be that the component in question is conductive inone state, e.g., the liquid state, and non-conductive or discontinuous in another state, e.g., the vapor state, and both states may be present in the fuel to be ignited; Such electrically conductive components efiectively' shunt the electrodes of conventional spark plugs, thereby preventing proper ignition of the fuel by the plug.

Conventional fixed gap spark plugs in current use may be classified as either jump gap plugs or surface gap plugs.

A jump gap plug comprises two sparking points separated by an air gap. While the gap is nominally termed an air.

gap, under operating conditions it is filled by an electrically non-conductive mixture of air and fuelgas or vapor,

which mixture is ignited by a spark between the points of the plug. A jump gap plug must be supplied with electrical energy from a source having a potential high enough to break down the insulating material between the points of the gap. That potential is commonly termed the breakdown voltage of the gap, and is much higher than the potential required to maintain the spark after the gap has broken down; p

A surface gap plug comprises two electrodes separated by a member having a layer of surface conductive material. Such surface conductive materials are usually materials of the class generally known as semi-conductors. When a surface gapis subjected to asufiiciently high voltage, an are forms across the surface of the material, producing a sparkwhich is communicated to the fuel.

(Spark plugs with gaps of suchsurfac'e conductive mate rial are in common use in jet engines.) The potential of the source supplying such a plug is commonlymuch lower, for a given gap width, thanthe breakdown voltage apparatus described herein by providing a spark plughaving a surface conductive gap and a jump gap in parallel with it. The plug is providedwith electrical energy from a low impedance source typical of those commonly used in connection with surface conductive plugs. the potential of the source is lower than the 2,894,161 PatntedJuly'l, 1959 I breakdown voltage of the jump gap and the current capacity of the source is substantially higher than that commonly used with jump gaps. i 3

Such a plug in combination with such a source will produce a spark whether the fluid medium to which the plug is exposed is electrically conductive and completely shunts the plug or whether that fluid medium is nonconductive, for example, by reason of being in a discontinuous state such as a vapor. If the fluid medium is nonconductive, the spark appears only at the. surface gap and i if the fluid medium is conductive the spark appears only at thejump gap. 1,; p j

Other objects and advantagesof the. invention will become apparent from a consideration of the following description and claims, taken together, with the accompanying drawing.

In the drawing:

Fig. l is an electrical wiringdiagram of acircuit for energizing a spark plug in accordance with the invention; and i i i Fig. 2 is a central cross-sectional view througha spark plug embodying the invention.

The circuit illustrated inFig. l is a simplified version of the ignition system shown and claimed in the patent to John V. McNulty, No. 2,716,720, dated August 30, 1955,

. entitled Engine Ignition Apparatus and Procedure. Sys-" tems of this type have the characteristic that charges of electricity for the formation'of sparks are delivered to the spark plngsin carefully measured and regulated quantities of energy,.so that uniform sparks may be produced at the plug, regardless of variations which may occur in the ener gy supply system, for example, in the potential of. the source of electrical energy; i

Referring to the drawing, there'is shown a battery 1- which supplies electrical.enengytoia motor 2, drivingly connected to a multiple lobe cam 3 and a single lobe cam 4.. i The multiple lobe cam 3 operates a switch 5 connected in series between the. battery 1 and the primary section 6 of the winding of an auto-transformer 7. The

secondary section 8 of the winding of the autmtransformer 7 is connected in series with a circuit which includes a,

rectifier or other asymmetric element 9, and a condenser 10. An output circuit is connected across the condenser 10, and includes a trigger switch 11 operated by the single lobe cam 4, and a spark plug 18 including a jump gap 19 anda' parallel surface gap 20. i i i The motor 2 has a voltage-speed characteristic such that the speed varies directly, and linearly .as the voltage.

Consequently, the time ofclosure of the switch Sbythe lobe of the cam 3 varies inversely with the voltage of the source 1. Regardless of variations in the voltage of the source, each closure of the switch 5 therefore delivers a fixed predetermined quantity of electricity to the primary section 6 of the winding of the auto-trasformer 7. These measured quantities are stepped up in potential by the auto-transformer, and pass through the rectifier 9 andare stored on the condenser 10. The condenser 10 is discharged each time that the single lobe cam 4 actuates the trigger switch 11. Sincethe single lobe cam 4 is driven by the same motor 2 which drives the multiple lobe cam 3, it will be apparent that each discharge of the condenser 10 is preceded by the same number of charging pulses, determined by the number of lobes on the periphery of the cam 3.

Referring to Fig. 2, there is shown in detail the spark plug 18 including an outer cylindrical'casing 12, an annular ceramic body 13 having a coating 14 of semiconductive material on one face and an inner threaded post 15 concentric with the casing 10.

'Attached to the outer casing 12 near the end thereof" is a rod 16 of electrically conductive material having its outer end bent over toward the axis of thec asing 12,

as shown at 164. "Another rod 17 is welded or otherwise suitably fastened to a flange 15a carried at the end of the post 15 and projects into close proximity with the tip of rod 16a. The tips of the rods 16 and 17 define between them" the jump gap 19. a

The post- 15 'supports a conductive washer 20a which is held in conductive contact with the face of the semiconductive coating 14 by the flange 15a of the post, the shank of the post 15 being threaded into the main inner conductive member 15b the end of which bears on the opposite face of the ceramic member 13. An annular ring 20b disposed between a shoulder of the casing 12 and the face of the semi-conductive coating 14 also is held in conductive contact with this coating. The outer diameter of the washer 20a and the inner diameter of the ring 20b are such that the annular space therebetween determines the surface gap 20. This spacing and the spacing of the flange 15a from the casing 12 are greater than the jump gap 19.

When used on a rocket engine, the plug described above may at times be completely immersed in electrically conductive liquid fuel. At other times, it may be immersed in a spray comprising particles of such fuel separated by regions of vapor. At still other times, it may be immersed in vapor. In either of the latter two cases, the vapor path between the electrodes of the plug may be continuous or discontinuous. At any of these several times, a spark may be required at the plug, for the purpose of igniting the fuel.

The gap 19 is constructed so that its breakdown potential in air or other non-conductive gaseous fluids is substantially greater than the potential to which the condenser is charged. Consequently, when the spark plug 18 receives a pulse of electrical energy at a time when the plug is immersed in a non-conductive fluid, there is no spark generated at the jump gap 19, but a spark is generated at the surface gap 20, in the conventional manner for surface gap spark plugs.

When the plug is immersed in a conductive liquid, surface gap 20'is effectively shunted by that liquid, so that the current flow through the semi-conductive layer 14 is too small to initiate a spark. The lowest impedance path between the outer electrode or shell 12 of the plug and the inner electrode or post is then the path through the rods 16, -17 and the conductive liquid at the jump/gap 19. The current therefore tends to concent rate in that path. The energy discharge from the condenser 10 is substantially greater than that normally used in connection with gap plugs and issufiicient when passing through the liquid in the jump gap 19 to generate gas .or vapor'in'the gap.

The particular mechanism by which this gas or vapor is produced is not known with certainty. However, most of the liquids on which tests have been made have been electrolytically conductive, and it is considered most probable that the gas or vapor is generated by electrolysis of the liquid, with the rods 16, 17 acting as anode and cathode. It is possible, however, that with some liquids, the gas or vapor may be formed by some other physical phenomenon-40m example, a portion of the liquid may become heated above its'boiling point. This vapor appears first only in a very small volume within the gap, so that the initial volume of vapor has a very low breakdown potential and a spark readily starts in the vapor as soon as it is formed. The spark generates further heat and the volume of vapor rapidly expands between the points of the gap, until the spark completely fills the jump gap. A spark of substantial volume and'energy is obtained even under liquids which are highly conductive.

A jump gap 19 should be small in area and short in length as compared to the surface conductive gap. The maximum length of jump gap which may be'used is a function of the gap conductivity and the energy of the source; a

The length of the surface conductive gap, i.'e., the

radial distance between the shell 12 and the post 15, also depends on the energy of the source- Experiments have been successfully conducted using a plug and an energy source of the type described with a potential of approximately 1000' volts and an energy per spark discharge of 2 joules, with a jump gap 0.030 in. long and a surface gap 0.070 in. long.

Whi e I ha Sh wn an described a Pre rr d o ment of my invention, other modifications thereof will electrodes are immersed in said electrically non-conduo tive fluid medium, and a pair of points remote from said member, respectively electrically connected to said electrodes and spaced apart by a distance substantially smaller than the spacing between said electrodes and constituting a jump gap, said jump gap having a breakdown potential in said non-conductive medium substantially greater than that of said discharge path, said jump gap being effective when said points are immersed in said electrically conductive fluid medium to provide a second electrical energy discharge path between said electrodes of lower impedance than said first path, said jump-gap having a limited cross-sectional area and being elfective upon a discharge of current therethrough to change a portion of said conductive medium adjacent said gap to a gaseous state, and to create a spark discharge within said changed portion of the medium.

2. A spark plug for producing a spark in either electrically non-conductive or conductive fluid media, comprising a cylindrical shell of electrically conductive material, an annular body having at least one end of semiconductive material within and contacting said shell and concentric therewith, with said one end disposed toward and spaced inwardly from an end of the shell, a central post of electrically conductive material extending axially of and contacting said semi-conductive material and having an end projecting beyond said body, means .to connect said shell and said post to the opposite terminals of a source of "electrical energy having a potential sufficient when the plug is immersed in said non-conductive medium to produce a spark between said end portion of said post and said shell and across the surface of said one end of Y said body, a first electrically conductive rod attached to said post and extending axially therefrom beyond the end of said shell, a second electrically conductive rod attached to said shell and extending beyond the end thereof and parallel to said first rod, at least one of said rods being bent to bring the rod ends toward one another to define between said rod ends a jump gap having a breakdown potential in said non-conductive medium greater than the potential of said source, and a breakdown potential in said conductive medium at least as low as the potential of said source.

3. A spark plug for producing a spark in either electrically non-conductive or conductive fluid media, comprising a cylindrical shell of electrically conductive material, an annular body having at least one end of semiconductive material within and contacting said shell and concentric therewith, with said one end disposed toward and inwardly from an end of the shell, a central post of electrically conductive material extending axially of and contacting said semi-conductive material and having an end portion projecting beyond said body, said post, shell and semi-conductive material defining a first electrical energy discharge path having a predetermined breakdown potential when immersed in an electrically non-conductive fluid medium, said discharge path being recessed inwardly from the end of said shell, a first electrically conductive rod attached to said post and extending axially therefrom beyond the end of said shell, a second electrically conductive rod attached to said shell and extending beyond the end thereof and parallel to said first rod, at least one of said rods being bent to bring the rod ends toward one another to define between said :rod ends a jump gap having a breakdown potential in said non-conductive medium greater than the breakdown potential of said first path, said gap having a spacing narrower than the: spacing between said post and she'll adjacent said body.

References Cited in the file of this patent UNITED STATES PATENTS Smits Oct. 24, Togn'ola Oct. 16, Sackett et a1. Apr. 8, Devaux Sept. 1, Laird Feb. 7, Tognola May 22,

FOREIGN PATENTS Great Britain Nov. 1,

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