Ignition Devices

Abstract

An ignition device for an internal combustion engine includes a chamber having a wall, a hole in the wall through which a medium to be ignited may communicate with the inside of the chamber, and means to produce a plasma flame within the chamber of sufficient energy to project through the hole. The means to produce the plasma flame includes first and second electrodes spaced apart by a gap. A potential from a first source is applied across the electrodes which is insufficient by itself to cause electrical breakdown of the gap, and a higher potential from a second source is applied across the gap, or a part of the gap, which is sufficient to cause the potential from the first source to be discharged across the gap.

Description

This invention relates to ignition devices.

The invention consists in an ignition device for an internal combustion engine including a chamber having a wall, a hole in the wall through which a medium to be ignited may communicate with the inside of the chamber, and means to produce a plasma flame within the chamber of sufficient energy to project through the hole wherein the means to produce the plasma flame includes first and second electrodes spaced apart by a gap such that a potential from a first source applied across the electrodes is insufficient to cause the breakdown of the gap and a higher potential from a second source applied across the gap or a part of the gap is sufficient to cause the potential from the first source to be discharged across the gap.

The discharge due to the first potential may be of substantially greater energy than that due to the second, higher, potential.

Means may be provided to cause the plasma flame to occur at precisely timed intervals. The ignition device may therefore be employed in providing ignition in reciprocating internal combustion engines.

The invention may also be employed where precise timing of the ignition is not required.

A number of embodiments of the invention will now be described by way of example with reference to the accompanying drawings, of which

FIG. 1 is a diagram of a first embodiment of the invention

FIG. 2 is a diagram of a second embodiment,

FIG. 3 is a diagram of a third embodiment

FIG. 4 is a diagram of a further embodiment suitable for use in an internal combustion engine, and

FIG. 5 is a circuit diagram showing the application to a multi-cylinder engine.

Referring to FIG. 1, the ignition device includes a disc-shaped front electrode 11 formed with a central orifice 12. The disc may be 1 inch diameter and the orifice 1/32 inch diameter, and the electrode may be 1/16 inch thick. Coaxial with the orifice 12 is a rod-shaped rear electrode 13 which may be 1/4 inch diameter and have a smaller-diameter tungsten tip 14. The bodies of the electrodes 11, 13 are of copper, or other suitable electrically-conducting material, and the rod-shaped electrode 13 may be screwed into a rear plate 15 of copper, or other conducting material.

An annular spacer 16 of electrically-insulating material is interposed between front electrode 11 and plate 15, and defines a chamber 17 which is substantially closed, that is it is closed except for the orifice 12. A third electrode 18 projects through spacer 16 into the chamber 17, so that its tip lies in the region between the front electrode 11 and the tip 14 of the rear electrode.

A capacitor 19 is connected across the electrodes 11, 13 and a D.C. voltage V, which may be for example 100-200 volts, is applied across the capacitor, through resistance 20. The latter may be, for example, 1,000 chms. An inductor 21 is incorporated in the circuit between the capacitor 19 and the electrode 11, which inductor may have a value of 20.mu.H, and as shown the rear electrode 13 is connected to earth. The polarity of the electrodes 11, 13, may, however, be opposite to that shown.

The third electrode 18 is connected to provide a precisely timed EHT pulse, and for example a conventional ignition coil and distributor may be employed to supply a pulse to the third electrode at the required point in the cycle of an internal combustion engine. The potential may be of the order of 10 kV.

The D.C. voltage V is not normally high enough to cause breakdown of the gap between electrodes 11, 13, and the EHT pulse supplied to the third electrode 18 causes a spark to occur between it and the tip 14 of the rear electrode 13. This causes sufficient ionisation in the chamber 17 to cause the gap between the front electrode 11 and rear electrode 13 to break down, and the capacitor 19 discharges across the gap. The energy of this discharge is such as to cause a plasma arc to occur, the gas within chamber 17 rapidly heating up and expanding, causing the plasma arc flame to project through orifice 12. The discharge continues until either the voltage across the electrodes 11, 13 has dropped below that required to sustain it, or until the rapid expansion of the gases within chamber 17 "blows out" the discharge.

The embodiment of FIG. 2 is similar to that of FIG. 1, and the same reference numerals are used for similar parts. Instead of using a third electrode 18, however, the EHT potential (derived for example from an ignition coil and distributor system) is applied across the electrodes 11, 13, the value of the inductor 21 which is not shown in FIG. 2 but which couples the voltage V in a similar manner to that shown in FIG. 1 being such as to avoid adverse effects on the capacitor 19, which would otherwise be shortcircuited. The impedance of the inductor 21 is such as to allow an RF signal across the gap between electrodes 11, 13, which thus breaks down and allows discharge of the capacitor across the gap.

In the embodiment of FIG. 3, a third electrode 28 is positioned so that the current path for the EHT potential across electrodes 28, 13 is via the front electrode 11. Thus the third electrode 28 is mounted in insulating spacer 16 with a gap between it and the front electrode. When the EHT pulse is applied, the gap between third electrode 28 and front electrode 11, and that between front electrode 11 and the tip 14 of rear electrode 13, are in series and both break down. The ionisation of the latter gap allows the capacitor 19 to discharge across it.

The value of the inductor 21 need not be such that its impedance is large enough for the EHT pulse to cause a visible spark between front electrode 11 and rear electrode 13; too large a value of the inductor tends to reduce the current in the arc resulting from the capacitor discharge, and thus reduces the energy of the plasma.

In FIG. 4, the ignition device has a front electrode 31 which is cylindrical and externally screw-threaded for insertion in the spark plug aperture of an internal combustion engine. An orifice 32 in the electrode 31 communicates with the combustion space of the engine cylinder (in the case of a reciprocating engine). The rear electrode 33 is again rod-shaped, and is spaced from the hexagonal boss 35 of the front electrode by an insulating spacer 36, which may be of a ceramic material, e.g. alumina. Since the front electrode 31 will be effectively earthed in this arrangement, the polarity of the D.C. voltage to be applied to it is reversed as compared with that shown in FIGS. 1 - 3. A chamber 37 is formed within the electrode 31.

A third electrode 38 which is shown as a rod of similar diameter to that of an extension of the rear electrode 33, is supported with a suitable gap from the extension by an insulating sleeve 39. An EHT pulse applied between the third electrode 38 and the electrode 31 breaks down the gap between electrodes 38, 33 and the gap between the electrodes 33, 31, the latter breakdown causing discharge of the capacitor 19, and thus causing a plasma arc flame in the chamber 37 and through the orifice 32 in a similar manner to that described above.

The gap between the electrodes 11 and 28 in FIG. 2 and between the electrodes 33 and 38 in FIG. 4 is provided so as to prevent the rising EHT voltage being passed through inductor 21 and capacitor 19 to the other electrode 14 or 31; and thereby reducing the potential difference across the electrodes 11-14 or 31-33. The breakdown of the gap gives a sudden high frequency connection of electrode 11 to electrode 28, or of electrode 33 to electrode 38 which is not passed by the inductor 21. Therefore the potential difference across electrodes 11-14 or 31-33 attains a high enough value to cause breakdown across these electrodes.

In place of the ignition coil and distributor, where the ignition device is to be employed, e.g. for starting a gas turbine engine, or for ignition of a gas burner, where precise timing of the spark is not required, a trembler coil may be employed.

In FIG. 5 there is shown diagrammatically a circuit for providing timed ignition in a four-cylinder reciprocating internal combustion engine. This is shown as employing four ignition devices as shown in FIG. 4. The electrodes 31 are each connected to earth, and the electrodes 33 are each continuously connected through inductor 21 to the high-potential side of the capacitor 19, across which the D.C. voltage V is connected.

The third electrode 38 of each device is connected through a distributor 40, such as conventionally used in ignition systems, to a coil 41 for the provision of the EHT supply.

Claims

We claim:

1. An ignition device for an internal combustion engine including a body of insulating material, a first electrode, a second electrode, said body together with said first and second electrodes defining a substantially closed chamber, said first electrode closing one end of said chamber and being formed with an orifice therethrough, said second electrode being rod-shaped and extending part-way towards said first electrode, whereby to define a first gap between said first and second electrodes, a first source of electrical potential connected across said first and second electrodes, said potential being insufficient to cause electrical breakdown of said first gap, and means, including a second source of electrical potential, at a substantially higher potential than said first source, to apply said substantialy higher potential across at least part of said first gap, whereby to ionize said part of said first gap and thereby to cause the lower potential from the first source to be discharged across said first gap, the energy of the discharge being such as to cause a plasma arc to occur and rapidly heat up and expand the gas within said substantially closed chamber, thereby causing a plasma arc flame to project through said orifice.

2. An ignition device as claimed in claim 1, wherein said means for applying said substantially higher potential across at least part of said first gap is connected across said first and second electrodes to apply said substantially higher potential across the whole of said first gap.

3. An ignition device as claimed in claim 1, including also a third electrode, and wherein said means for applying said substantially higher potential across at least part of said first gap is connected across said first and third electrodes.

4. An ignition device as claimed in claim 3, wherein said third electrode is aligned with and spaced from said second electrode, and is on the side of said second electrode remote from said first electrode.

5. An ignition device as claimed in claim 3, wherein said third electrode projects through said body of insulating material, into said substantially closed chamber, between said first electrode and said second electrode.

6. An ignition device as claimed in claim 1, wherein said first source of electrical potential connected across said first and second electrodes includes capacitor means, and wherein said means for applying said substantially higher potential across at least part of said first gap includes an ignition coil and a distributor.