Ignition Devices

Abstract

An ignition device for an internal combustion engine has a chamber provided with a hole through which the chamber communicates with a medium to be ignited and includes first and second electrodes defining between them a first gap transverse to the hole, and a third electrode defining a second gap between itself and one of the first and second electrodes. A potential is applied between the first and second electrodes which is insufficient by itself to cause electrical breakdown of the first gap, and a higher potential is applied across the second gap so as to cause breakdown of the first gap, and thereby to cause the potential applied to the first and second electrodes to discharge across the first gap.

Description

This invention relates to ignition devices, particularly for internal combustion engines.

According to the present invention, an ignition device for an internal combustion engine includes a chamber having a wall, a hole in the wall through which the chamber is in communication with a medium to be ignited, and means to produce a plasma flame which projects through said hole to ignite said medium, said means including first and second electrodes defining between them a first gap transverse to said hole, said electrodes being adapted to receive a first potential across them which is insufficient by itself to cause electrical breakdown of said gap, a third electrode defining a second gap between itself and one of said first and second electrodes, said second gap being adapted to receive a second higher potential across said second gap, whereby to cause breakdown of said first gap, and thereby to cause a first potential applied to said first and second electrodes to discharge across said first gap.

The first and second electrodes may be aligned, so that the first gap is between their ends, or they may be in staggered relationship so that the first gap is between their ends but at right angles to the longitudinal axes of the electrodes.

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

FIG. 1 is a diagrammatic cross-sectional view of one embodiment of an ignition device and the associated electrical circuit,

FIG. 2 is a section on the line A--A of FIG. 1,

FIG. 3 is a view similar to FIG. 2 of an alternative electrode arrangement,

FIG. 4a is a sectional view of a further embodiment of ignition device,

FIG. 4b is a front view of the device of FIG. 4a, and

FIG. 5 is a circuit diagram for the ignition of a four-cylinder engine.

Referring to FIGS. 1 and 2, the ignition device comprises a body 16 of ceramic or other insulating material having a chamber 17 adjacent one end. Two electrodes 13, 14 of rectangular section and which are electrically insulated from one another are embedded in the ceramic or other suitable insulating material 16. The electrodes have their free ends spaced apart to form a gap within the chamber 17 formed in the insulating material. The chamber is closed except for a restricted orifice 12 in its front wall, near the gap between the electrodes. A rod-shaped electrode 15 is mounted to project into the chamber 17 coaxially with the orifice 12 and serves as a trigger electrode, defining a second gap between itself and electrode 14.

A capacitor 19 is connected across the electrodes 13, 14, and a D.C. voltage V, which is not sufficient by itself electrically to break down the gap between the electrodes 13, 14, is applied across the capacitor 19 through a resistance 20. The voltage may, for example, be 100-500 volts. The value of resistance 20 may be 1,000 ohms. The trigger electrode 15 is connected, in the case of an ignition device for a reciprocating internal combustion engine, through a series gap 18, to a conventional ignition coil 22 and distributor (not shown) to provide a timed extra high voltage pulse across the second gap at a potential of, say, 10 kV. This pulse applied to electrodes 14, 15, produces a spark across the gap between them, which in turn causes ionisation of the gap between the electrodes 13, 14, so that the capacitor 19 discharges across this latter gap. The configuration of chamber 17, orifice 12, and the gap between the electrodes 13, 14 is such that this causes an arc plasma to occur, the gas within chamber 17 rapidly heating up and expanding, causing the plasma arc flame to project through the orifice 12. The discharge continues until either the voltage across the electrodes 13, 14 has dropped below that required to sustain it, or until the rapid expansion of the gases within chamber 17 "blows out" the discharge. The purpose of the series gap 18, which is wider than the gap between electrodes 14, 15, is to break down when the extra high voltage is applied across it, and thereby to cause a very rapid rise in voltage across the gap between electrodes 14, 15.

Where the ignition device is employed e.g. for starting a gas turbine, a trembler coil may be employed instead of the coil 22 and distributor.

Referring to FIG. 3, the electrodes 13A, 14A, are in staggered relationship, instead of being aligned as in FIG. 2, so that the gap is between their free ends but at right angles to their longitudinal axes. The gap is near orifice 12, as in the previous embodiment, and the trigger electrode 15 is also shown as coaxial with the chamber 17.

FIGS. 4a and 4b show a construction of an ignition device which may be inserted in the threaded spark plug aperture of an internal combustion engine. The device comprises a generally cylindrical body 26 of insulating material, e.g. ceramic, provided with an axial trigger electrode 25 terminating at one end in the chamber 27 and coaxial with the outlet orifice 22. An outer metal member 24 surrounds a part of the ceramic body 26 and is externally threaded at 21 to engage the threads of the spark plug aperture. The member 24 also incorporates the electrode 24A extending towards the orifice 22 and which is the earthed electrode when the device is fitted to an engine. The opposite electrode 23A is formed at one end of a conducting rod 23 embedded in the ceramic body 26 and terminating at its other end in a terminal or tab 23B to which an external connection can be made from the non-earthed side of the voltage source V (FIG. 1). The connection to the trigger electrode 25 is made via terminal 29 and the series gap is shown at 28. The metal member 24 is provided with an inturned shoulder 24B to grip the ceramic body 26. Packing 30 may be provided between the shoulder 24B and the body 26. The device of FIG. 4 may be manufactured according to techniques well known in the art of manufacturing spark plugs.

FIG. 5 shows a circuit diagram for providing timed ignition in a four cylinder reciprocating internal combustion engine employing four ignition devices as shown in FIG. 4. The electrodes 24A are each connected to earth and the electrodes 23A are each continuously connected through inductor 42 to the high-potential side of the capacitor 19 across which the D. C. voltage V is connected via resistor 20. The trigger electrode 25 of each device is connected through a distributor 40, such as is conventionally used in ignition systems, to a coil 41 for the provision of the very high voltage supply.

Claims

We claim:

1. An ignition device for an internal combustion engine having means for providing first and second potential differences, said ignition device comprising:

a chamber having a wall perforated by a hole through which said chamber is in communication with a medium to be ignited, and

means to produce a plasma flame which projects through said hole to ignite said medium, said means including

first and second electrodes defining therebetween a first gap transverse to said hole and having a breakdown potential too great to break down when only said first potential difference is applied across said electrodes, and

a third electrode defining between itself and one of said first and second electrodes a second gap having a breakdown potential such that it breaks down when said second potential difference is applied across it, thus causing ionization in said first gap when said first potential is simultaneously applied across said first gap, with a consequent electrical discharge across said first gap which causes said plasma flame to occur.

2. An ignition device as claimed in claim 1 in which the second potential difference is greater than the first.

3. An ignition device as claimed in claim 1, including a further gap in series with said third electrode.

4. An ignition device as claimed in claim 3, wherein said series gap is wider than said second gap.

5. An ignition device as claimed in claim 1 wherein the first and second electrodes are aligned, so that the first gap is between their ends.

6. An ignition device as claimed in claim 1, wherein the first and second electrodes are in staggered relationship so that the first gap is between their ends but at right angles to the longitudinal axes of the electrodes.

7. An ignition device as claimed in claim 1, wherein the third electrode is coaxial with the hole.

8. An ignition device as claimed in claim 1, wherein the chamber is formed in a body of insulating material and the first and second electrodes form at least part of a wall of the chamber.

9. An ignition device as claimed in claim 8, wherein the first and second electrodes are exposed at a surface of the device adjacent the hole.

10. An ignition device as claimed in claim 8, wherein the device includes an outer threaded portion for mating with the threads of a spark plug aperture in an internal combustion engine.

11. An ignition device as claimed in claim 10, wherein the threaded portion is formed on a metal member which surrounds the body of insulating material, said metal member also having a part defining one of said first and second electrodes.