EP0016647A1

EP0016647A1 - Ignition plugs

Info

Publication number: EP0016647A1
Application number: EP80300888A
Authority: EP
Inventors: John Ernest Orrin; Felix Giri Prof. Weinberg; Ivan Michael Dr. Vince
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1979-03-27
Filing date: 1980-03-21
Publication date: 1980-10-01

Abstract

A plasma ignition plug intended for use in association with a cylinder of an internal combustion engine and includes a body 11 containing a plasma medium flow system 15, 17 (27) said system comprising a cavity 15, a recess 17 (27) which communicates with the cavity 15 and from which, in use, plasma medium is forced into the cavity 15 by the piston of the engine during its compression stroke, an orifice 16 in the wall of the cavity, first and second electrodes 13,14 between which, in use, a spark is struck in the cavity 15 to generate plasma in plasma medium contained in the cavity 15, said plasma issuing, in use, from the cavity 15 by way of said orifice 16, and, a conduit 19 in said body whereby plasma medium is supplied, in use, to said flow system.

Description

This invention relates to ignition plugs particularly but not exclusively for use in internal combustion engines.
For some time the desirability of operating an' internal combustion engine using a fuel/air mixture weaker that that which is currently considered to be usual has been recognised. However, it has also been recognised that a problem exists in igniting and ensuring efficient combustion of a weak mixture.
Successful combustion of weaker mixtures requires increased rates of flame propogation within the mixture and this in turn requires more effective ignition than can be provided by conventional spark discharge ignition plugs. It has been suggested that the ignition and combustion of a weak mixture can be improved by using a plasma jet or stream of radicals, free atoms and/or other excited species as the ignition source, and such a concept is disclosed in an article in the magazine "Nature" Volume 272 No. 5651 pages 341-343 March 23rd 1978. This article discloses also a plasma ignition plug used for experimental purposes. This disclosed plug has a small cavity bounded in part by electrodes between which an electrical spark is struck to generate plasma from the medium in the cavity. The cavity is filled with the plasma medium, prior to striking the spark, by way of a feed capillary and the plasma generated in the medium issues as a jet from the orifice of the cavity. The plug described in the aforementioned article is designed for use in conjunction with a test chamber filled with air/fuel mixture at atmospheric pressure, and is unsuitable for use in an internal combustion engine. Firstly the generation of plasma in a medium compressed during the compression stroke of the engine requires a reduced electrode gap and moreover a smaller cavity would be required if, for a spark of the same energy, the temperature of the plasma is to be maintained in the much denser (by virtue of the compression) plasma medium.
The use of a smaller cavity, probably less than 3 cubic millimeters, makes it extremely difficult to provide a supply system and supply capillary having a volume negligible by comparison with that of the cavity and so during compression the plasma medium in the cavity would be forced back out of the cavity into the supply system. Even in the event that this reverse flow during compression does not occur the medium will be compressed in the cavity so that assuming an engine of normal compression ratio, at the end of the compression stroke the medium originally filling the cavity will have been compressed to approximately 10% of its original volume the remainder of the cavity being filled with the air/fuel mixture-of the engine charge. In a previously proposed plasma plug for an internal combustion engine the plasma cavity was filled by the action of the engine piston with compressed weak air/fuel mixture from the cylinder by way of the cavity orifice. Thus the plasma medium was the weak air/fuel mixture itself and this proved not to be such a satisfactory medium for the generation of a plasma capable of promoting efficient combustion of the mixture in the cylinder. It is an object of the present invention to provide a plasma ignition plug suitable for use in an internal combustion engine.
A plasma ignition plug according to the invention is intended for use in association with a cylinder of an internal combustion engine and includes a body containing a plasma medium flow system, said system comprising a cavity a recess which communicates with the cavity and from which, in use, plasma medium is forced into the cavity by the piston of the engine during its compression stroke, an orifice in the wall of the cavity, first and second electrodes between which, in use, a spark is struck in the cavity to generate plasma in plasma medium contained in the cavity, said plasma issuing, in use, from the cavity by way of said orifice, and, a conduit in said body whereby plasma medium is supplied, in use, to said flow system.
Conveniently said recess lies between said cavity and the engine cylinder in use, and communicates with said cavity by way of said orifice, the open end of said recess remote from the cavity being provided with flow restriction means whereby loss of plasma medium from the recess by way of said open end of the recess particularly during the compression stroke of the engine is resisted.
Conveniently said conduit opens into said recess.
Desirably said means restricting the bpen end of the recess is an area of gauze having an aperture whereby the plasma jet issues into the cylinder, having passed through the recess.
Alternatively said means restricting the open end of the recess is a perforated baffle plate fixed to said body, and having an aperture whereby the plasma jet issues-into the cylinder having passed through the recess.
Alternatively the means restricting the open end of the recess is a baffle member movable relative to the body under the influence of pressure rise in the engine cylinder during the compression stroke to sweep plasma medium from the recess into the cavity, the baffle member having an aperture through which the plasma jet issues.
Conveniently the supply conduit is provided with non-return means which prevents reverse flow of plasma medium during the engine compression stroke.
The invention further resides in an ignition system for an internal combustion engine including an ignition plug as specified above and a plasma medium supply device for supplying plasma medium to the recess and the cavity by way of said conduit.

In the accompanying drawings Figure 1 is a diagramatic sectional view of part of a plasma ignition plug in accordance with a first example of the invention,
Figure 2 is a view similar to Figure 1 of a plug in accordance with a second example of the invention, and
Figure 3 is a view similar to Figures 1 and 2 of a plug in accordance with a third example of the invention.

Referring first to Figure 1 of-the drawings, the plug is basically similar in construction to a conventional .spark ignition plu in that it includes an externally screw threaded metal sleeve 11 within which is received an insulating member 12 through which extends an electrode member 13. Beyond this basic construction however there is no great similarity. At the end of the plug which will be exposed within the cylinder of the engine in use, the metal sleeve 11 is partially closed by an integral radially inwardly extending flange 14.
The electrode 13 which extends in a bore in the insulating member 12 terminates short of the end of the insulating member 12 so that a cavity 15 is defined at the end of the insulating member 12 adjacent the flange 14. Part of the wall of the cavity 15 is defined by the end face of the electrode 13, and the flange 14 adjacent the cavity 15 is shaped to define a restricted orifice 16 (approximately 0.25 mm diameter). The outer surface of the flange 14 is shaped to define a generally conical recess 17 communicating with the chamber 15 by way of the orifice 16. An annulus 18 of metal gauze partially closes the open face of the recess 17, the centrally disposed aperture of the annulus 18 being aligned with the orifice 16 in the direction of the axis of the plug. It is to be understood that alternatives to the gauze 18 exist, for example, an annular baffle plate could be substitued for the gauze, the plate being perforated.
A conduit 19 is provided in the plug, the conduit 19 terminating at one end at the recess 17, and terminating at the other end at a supply union (not shown) which is provided on a part of the plug external to the cylinder of the engine in use. A non-return valve 21 is provided at the end of the conduit 19 adjacent the recess 17. Conveniently the non-return valve is in the form of a simple ball member movable against a shaped seating in the conduit 19 to prevent flow of fluid from the recess 17 back along the conduit 19.
When the plug is utilized in an internal combustion engine the plug is screwed into a threaded bore in the cylinder head of the engine in a manner similar to a conventional spark ignition plug. Thus the end of the plug carrying the recess 17 is exposed to the interior of the respective engine cylinder, and a part of the plug carrying the supply union for the conduit 19, and a terminal assembly for the electrode 13 is exposed on the exterior of the engine.
Generally conventional high voltage electrical connections are made to the electrode 13 and the sleeve 11, and a supply device for supplying plasma medium is connected to the supply union of the conduit 19. A wide variety of plasma mediums can be utilized as will be apparent to those familiar with plasma technology. However, it has been found that vapourised engine fuel is a suitable medium, and clearly since the vehicle equipped with the internal combustion engine will carry a supply of fuel for the internal combustion engine then it would seem expedient to use the same fuel as the plasma medium wherever possible.
In use therefore vapourised fuel is supplied under pressure through the conduit 19 to issue into the recess 17. During the compression stroke in the respective cylinder the cloud of fuel vapour in the recess 17 is compressed into the chamber 15, it being appreciated that the gauze annulus 18 restricts dispersion of the cloud out of the open end of the recess thus maintaining a cloud of fuel vapour to be compressed through the orifice 16 into the chamber 15.
At a predetermined point around the top dead centre position of the piston of the respective cylinder a high voltage is applied between the electrode 13 and the sleeve 11, so that a spark discharge occurs between the electrode 13 and the flange 14, the spark discharge thus occurring in the chamber 15. In known manner the spark discharge within the chamber 15 generates plasma in the medium occupying the chamber 15, and a jet of extremely hot plasma issues from the chamber 15 by way of the orifice 16 and the recess 17 and passes into the fuel air charge compressed in the cylinder thus igniting the charge to drive the piston of the cylinder downwardly in normal manner.
The plasma jet issuing from the plug promotes efficient ignition and combustion of weaker fuel/air mixtures than can successfully be ignited by a conventional spark discharge plug.
- It will be recognised that at some point during the compression stroke the non-return valve 21 will close, and assuming that the compression ratio of the engine is approximately 10 to 1, then the rate of supply of plasma medium to the recess 17 must be so controlled that prior to the closure of the valve 21 a volume of plasma medium at least 10 times the volume of the cavity 15 (measured at atmospheric pressure) must flow into the recess 17 to ensure that the cavity 15 is filled with plasma medium at the end of the compression stroke.
The plug illustrated in Figure 2 differs from the plug of Figure 1 in the nature of the recess and the means for restricting dispersion of the plasma medium from the recesss. The plug again includes an outer metal sleeve 11 containing an insulating member 12 through which extends. an electrode 13. The electrode 13 again terminates short of the end of the member 12 to define in the member 12 a cavity 15, the cavity 15 being provided with an orifice 16. The end region of the sleeve 11 defines a substantially cylindrical recess 27 communicating with the cavity 15 by way of the orifice 16. Slidably disposed within the recess 27 is an annular baffle plate 28 the central aperture of which is aligned with the orifice 16. A very low rate compression spring 29 acts between the end of the insulating member 12 and the baffle plate 28 to urge the baffle plate 28 away from the insulating member 12. The baffle plate 28 and the sleeve 11 include mating abutment surfaces 31 limiting movement of the baffle plate 28 under the action of the spring 29.
The conduit 19 again opens into the recess 27 and is provided with a non-return valve 21. The moveable plate 28 is electrically connected to the sleeve 11 and constitutes the second electrode. The aperture of the plate 28 is so sized that the baffle plate, in part, overlies directly the electrode 13.
In operation plasma medium is again supplied by way of the conduit 19 to the recess 27 and during the compression stroke the rising pressure in the cylinder of the engine causes the baffle plate 28 to move against the action of the spring 29 thus sweeping the plasma medium in the recess 27 into the cavity 15. It will be recognised that the rate of the spring 29 and the inertia of the baffle plate 28 are arranged to be small enough for the baffle plate to move substantially immediately the pressure in the cylinder starts to rise during the compression stroke. Again, at a predetermined point around the top dead centre position of the piston of the cylinder a spark is struck between the electrode 13 and the second electrode (the moveable baffle plate 28) and plasma is generated by the spark in the cavity 15 and issues as a hot plasma jet by way of the orifice 16, the recess 27 and the central aperture of the baffle plate 28 into the cylinder to ignite the compressed, weak air/fuel mixture in the cylinder. It will be recognised also that the baffle plate 28 will be returned to its rest position under the action of the spring 29 during the induction stroke of the engine.
It will be recognised that in such an arrangement the gap between the electrodes, across which the spark is struck, varies during the compression stroke, and if desired it can be arranged that the point of least separation between the baffle plate 28 and the electrode 13 occurs at the point around the top dead centre position of the piston where ignition is to be initiated.
The rate of supply of plasma medium via the conduit 19 is again so controlled that prior to closure of the valve 21 the volume of plasma-supplied to the recess 27 is sufficient to ensure that the cavity 15 is filled with plasma medium at the end of the compression stroke. The baffle plate 28 fulfils a function similar to the gauze annulus 18 of the example shown in Figure 1 in restricting dispersion of the plasma medium from the recess 27 into the cylinder of the engine during the compression stroke.
In a modification of both of the examples described above the non-return valve 21 is omitted, and the rate of supply of plasma medium to the recess of the plug is increased to compensate for the reverse flow of plasma medium which will occur from the recess 17 back along the conduit 19 during the compression stroke.
In a further modification of the example shown in Figure 2 the baffle plate 28 is not electrically connected to the sleeve 11, and an extension of the sleeve 11 constitutes the second electrode. However, a more practical version of such an arrangement is shown in Figure 3 where a lightweight moveable baffle plate 28A is shown.
It will be recognised that with both of the plugs described above we are seeking to ensure that at the point at which ignition is to be initiated the cavity 15 of the plug is filled completely with plasma medium, and it will be recognised that such an effect may be obtained by other structural arrangements which supply and store the plasma medium for compression into the cavity 15 during the compression stroke. The arrangement must of course minimise dispersion of the stored medium which will occur due, inter alia, the gas flow and turbulence generated by movement of the advancing piston during the compression stroke.
While the arrangements described above are described with reference to the use of engine fuel as the plasma medium it is to be recognised that other media can be utilized, and by way of example media such as hydrogen gas, nitrogen gas, water vapour, methane, and fuel/air mixture could be utilized. However, pure fuel vapour has been found to give very good results in igniting and promoting combustion of weak air/fuel mixtures.
It is to be recognised that the supply of plasma medium to the plug is to be so arranged that at the time of sparking the cavity 15 is full of plasma medium. A low pressure supply system (two atmospheres or less) could be utilized, its flow rate being such that at the commencement of the compression stroke both recess 27 and cavity 15 are full, having being filled during.the previous exhaust, and induction strokes. Alternatively a high pressure supply system could be used. The nature of the supply system whether substantially continuous or using timed high pressure pulses is not critical to the basic invention provided that it enares that the cavity 15 is-full of the desired plasma medium at the time of sparking.
It will be noted that particularly in the event of using fuel as the plasma medium the fuel is required in vapourized form. It is found that fuel can be supplied to the plug in liquid form where it is vapourized as a result of the heat of the plug. Clearly the plug retains heat bet- combustion strokes of the engine and the obviating of the need for a vapourizing mechanism reduces the mechanial complexity of the system. During starting from cold it is believed that the first few operations will result in little plasma emission but the heat of the spark will rapidly raise the plug temperature sufficiently to vapourize the fuel supplied thereto so that cranking the engine for a few revolutions by means of the starter motor will be sufficient to start the engine.

Claims

1. A plasma ignition plug, for an internal combustion engine, including a body containing a cavity receiving plasma medium from a conduit in said body and first and second electrodes between which, in use, a spark is struck in said cavity to generate plasma in the medium contained in the cavity, the cavity having an orifice through which the plasma generated in the cavity issues, characterized in that the body of the plug contains a plasma medium flow system including said cavity 15 and recess 17 (27) which communicates with the cavity 15 and from which, in use, plasma medium is forced into the cavity 15 by the piston of the internal combustion engine during its compression stroke, said conduit 19 delivering plasma medium to said flow system 15, 17 (27).

2. A plug as claimed in claim 1 characterized in that said recess 17 (27) lies between said cavity 15 and the engine cylinder in use, and communicates with said cavity 15 by way of said orifice 16, the open end of said recess 17 (27) remote from the cavity 15 being provided with flow restriction means 18 (28, 28a) whereby loss of plasma medium from the recess 17 (27) by way of said open end of the recess 17 (27) particularly during the compression stroke of the engine is resisted.

3. A plug as claimed in claim 1 or claim 2 characterized in that said conduit 19 opens into said recess 17, (27).

4. A plug as claimed in any one of claims 1 to 3 characterized in that said means 18 (28, 28a) restricting the open end of the recess 17 (27) is an area of gauze 18 having an aperture whereby the plasma jet issues into the cylinder, baving passed through the recess 17 (27).

5. A plug as claimed in any one of claims to 3 characterized in that said means 18 (28, 28a) restricting the open end of the recess 17 (27) is a perforated baffle plate fixed to said body 11, and having an aperture whereby the plasma jet issues into the cylinder having passed through the recess.

6. A plug as claimed in any one of claims 1 to 3 characterized in that said means 18 (28, 28a) restricting the open end of the recess 17 (27) is a baffle member 28 (28a) movable relative to the body 11 under the influence of pressure rise in the engine cylinder during the compression stroke to sweep plasma medium from the recess 17 (27) into the cavity 15, the baffle member 28 (28a) having an aperture through which the plasma jet issues.

7. A plug as claimed in any one of claims 1 to 6 characterized in that the supply conduit 19 is provided with non-return means 21 which prevents reverse flow of plasma medium during the engine compression stroke.

8. An ignition system for an internal combustion engine characterized in that it includes an ignition plug as claimed in any one of claims 1 to 7 and a plasma medium supply device for supplying plasma medium to the recess 17 (27) and the cavity 15 by way of said conduit 19.