The present invention relates to an electrode arrangement for igniting fuel-air mixtures in the compression chamber of a cylinder in an internal combustion engine, particularly an Otto cycle engine, in which in addition to being delimited by the cylinder wall the compression chamber is also delimited by a piston moveable in said cylinder and a wall of a cylinder head, there being mounted in the cylinder head a sparking plug which includes at least one electrode and which may comprise a part of a fluid injection device that functions as a sparking plug. Such a device may be intended for gas injection for instance air injection, and/or fuel injection.

The invention also relates to a method of the kind defined in the preamble of Claim 1.

DESCRIPTION OF THE BACKGROUND ART SE-B-8500324-2 (Publication No. 449 125) primarily relates to a method of controlling the spark ignition of a fuel-air mixture in the combustion chamber of an Otto cycle engine. The publication also teaches an Otto cycle engine which has fixedly mounted in each combustion chamber a sparking plug electrode for spark ignition and an earth electrode which coacts with said ignition electrode and which is fixedly connected to the piston in the cylinder concerned and therewith moveable in relation to said fixedly mounted sparking plug electrode.

This known engine construction is concerned with a single-pole sparking plug which is fixed in the cylinder head of the engine and the single pole of which is comprised of a central electrode. This type of sparking plug thus lacks side electrodes.

The invention according to the aforesaid patent publication is aimed to enable the ignition to be governed so that well-controlled combustion will be obtained in widely different operating conditions, regardless of whether the engine is working at a low or a high load.

GB A 2 322 672 teaches a method of injecting fuel directly into the combustion chamber of internal combustion engines, where the fuel-air mixture is ignited with an ignition spark that jumps across the ignition gap between the central electrode of a single electrode sparking plug attached in the cylinder head on the one hand, and an earth electrode fixedly mounted on the top of the piston on the other hand. Because the size of the spark gap changes as the piston moves in the cylinder, it is possible to select the most suitable possible ignition points and consequently corresponding spark lengths, which enables the extent of incomplete combustion, e. g. ignition failure, to be reduced, and then particularly in the partial load range. This enables the ignition and combustion conditions to be improved.

OBJECT OF THE INVENTION As will be evident from the aforegoing, ignition electrode arrangements that include a fixed and a moveable electrode in each cylinder of an internal combustion engine are thus known to the art. Because the fixed electrode can conveniently consist of the central electrode of a constructively simple, single-pole sparking plug (= single electrode sparking plug), the sparking plug can be manufactured at a lower cost than a conventional two-pole sparking plug, which has both a central electrode and a side electrode that is firmly screwed into an attachment part of the plug.

However, one drawback with an electrode arrangement in which the size of the spark gap varies is that it is not always possible to achieve positive ignition (a effective ignition spark), particularly in the case of large spark gaps. For different reasons, it is not possible in practice to work with ignition voltages above a certain level, for instance 30-40 kV.

For instance, in respect of ignition advance (where ignition shall take place at a given number of degrees of crankshaft rotation prior to the upper dead centre position), it may be problematic to achieve positive ignition because the spark gap (the distance between the piston-mounted electrode and the sparking plug electrode attached to the cylinder head) is excessively large with respect to the available ignition voltage. There is a danger that the spark will choose the"wrong path"and jump between the sparking plug electrode and the nearest surrounding outer surface of the cylinder head, i. e. the surface area adjacent to the

sparking plug instead of the piston electrode. This can result in the spark being insufficient to provide positive ignition, and also in damage to the insulator (ceramic) surrounding the sparking plug electrode.

Among other things, the present invention aims at eliminating the aforesaid problems associated with an electrode arrangement that includes a spark gap which varies with the position of the piston in the cylinder. A general object of the invention is to provide an ignition electrode arrangement that has properties with which: -improved combustion conditions can be achieved; -good ignition quality can be obtained; -the limits are extended with respect to the range within which acceptable ignition combustion conditions can be achieved, also when concerning engine operation with the use of a gas mixture in which the gasified fuel constitutes a small part of the gas mass taken into the compression chamber for ignition; -improved fuel economy is achieved; -the useful life/durability of the sparking plug is increased; -the quantities of harmful emission products generated in the combustion process are reduced; and -with the fuel-air mixture can be ignited even when the spark gap dependent on the position of the piston is too large to enable positive ignition with a spark to the piston-mounted electrode.

SUMMARY OF THE INVENTION These objects are achieved in accordance with the present invention with an electrode arrangement of the kind defined in the introduction that includes the features set forth in the characterising clause of Claim 1, and by means of the method comprising the steps set forth in the characterising clause of Claim 9.

A primary characteristic feature of the inventive electrode arrangement is thus that it includes at least two separate earth electrodes, of which the first electrode is mounted on the top of the piston whereas the second earth electrode is mounted in the cylinder head

wall adjacent the compression chamber or is attached to the sparking plug or forms a separate intermediate part between the sparking plug and said wall.

Thus, according to the basic principle of the invention the spark ignition system utilises an electrode configuration which comprises a combination of the fixed sparking plug electrode with a moveable earth electrode on the top of the piston and also with a further earth electrode that is fixedly mounted in the cylinder head. The spark gap in respect of the second earth electrode, i. e. the distance between said electrode and the central electrode of the single-pole sparking plug, may be 4.5 mm, i. e. much larger than the spark gap in the case of a conventional two-pole sparking plug where the size of the spark gap does not normally exceed 1.0-1.5 mm The presence of the additional fixedly-mounted second earth electrode provides a much greater possibility of changing the engine ignition setting, particularly at low loads, since the ignition can be heightened (= advanced ignition) without risk of spark tracking in the insulator around the sparking plug electrode due to the excessively large distance between this latter electrode and the earth electrode mounted on the top of the piston.

If it is assumed, for instance, that the spark gap between the sparking plug electrode and the side electrode that has the form of a separate, fixedly mounted second earth electrode can suitably reach 4.5 mm for example, it could be expected that the ignition spark will typically jump to the new side electrode (instead of to the earth electrode on the top of the piston) at advanced ignition settings in the range of 20-30 degrees of crank shaft rotation prior to the upper dead centre point.

When the ignition spark jumps between the sparking plug electrode and the separate second earth electrode there is obtained a powerful"voluminous spark"which gives good ignition quality even in low load operating conditions.

A relatively large electrode distance is desirable at low load, where ignition problems often exist. Such a distance is achieved by means of the second earth electrode, which is mounted at the cylinder head at a relatively large distance from the sparking plug electrode. However, this electrode placement results in an excessively high ignition voltage

requirement at heavy loads, and consequently the electrode arrangement will also include the first earth electrode mounted on the top of the piston, said first electrode being located relatively close to the sparking plug electrode at the moment of ignition in the case of high loads, i. e. late ignition advance. The ignition voltage requirement is reduced to a reasonable level in this way.

Different practical embodiments of the inventive electrode arrangement may include the features set forth in Claims 2-8.

The second earth electrode may either be a separate electrode mounted in the cylinder head and distanced from the sparking plug, or an electrode that is co-ordinated with the sparking plug. In the former case (= separate electrode), the electrode may either be permanently fixed to the cylinder head or may have the form of a removable/replaceable component. In the latter case (= co-ordinated electrode), the electrode may either be a fixed/integral part of the sparking plug or may be fixedly attached to a separate attachment part mounted at/on or around the sparking plug. The earth electrode may then comprise, for instance, an electrode on a separate sleeve or an annulus fitted on and around the front part of the sparking plug.

When the second earth electrode constitutes a fixed/integral part of the sparking plug, the plug will thus form a two-pole sparking plug.

For space reasons (packing reasons of a technical nature), it will probably often be preferred in practice for the sparking plug to be a single pole plug (= single electrode sparking plug) which is positioned in the cylinder head so that the plug electrode will be located on the symmetry axis of the compression chamber. This embodiment will probably be particularly convenient when using so-called four-valve technology. It will often be preferred to place the piston-mounted earth electrode centrally on the top of the piston. In those instances when the piston-mounted earth electrode can be allowed to"overlap"the sparking plug electrode, the piston-mounted earth electrode is placed on the top of the piston in a position to one side of the centre of the piston top. In the case of this embodiment, the piston- mounted earth electrode will preferably extend in the axial direction of the piston so that the end portion of the electrode will overlap with the single electrode of the sparking plug when the piston is in its upper dead centre position.

Regardless of whether the sparking plug electrode is placed centrally in the cylinder or not, it is still necessary to position the separate side electrode mounted in the cylinder head on one side of the sparking plug. In order to obtain a well-defined voluminous spark, it may be suitable to provide the separate side electrode with an angled or curved outer part that faces in towards the symmetry axis of the compression chamber.

The first earth electrode that is fixedly connected to the piston is suitably formed as an integral part of the piston top and may, for instance, have the form of a tapering nipple that projects out from the bottom surface of a recess in the top of the piston.

Although the second earth electrode fixedly mounted in the cylinder head is comprised of a separate side electrode which is completely separated from the sparking plug in the aforedescribed embodiments, it is conceivable to use instead some type of two-pole sparking plug where the second earth electrode is connected to the sparking plug attachment part or is mounted at the front part of the sparking plug and thereby form a side electrode connected to the sparking plug.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further illustrated and explained with reference to a number of embodiments thereof and also with reference to the accompanying drawings, in which Fig. 1 is an axial longitudinally sectioned view of an engine cylinder that includes one embodiment of an inventive electrode arrangement ; Figs. 2a-2e are similar sectional views, although in larger scale than Fig. 1, showing the relative positions of sparking plug electrode and piston electrode for different configurations, although the side electrode is not shown for the sake of simplicity; Fig. 3 is a sectional view similar to the view of Fig. 1, showing part of the cylinder head with a single-pole sparking plug mounted therein, and a separate side electrode; and Fig. 4 is an axial longitudinal sectional view of part of a cylinder head and an associated cylinder with a piston arranged therein, in accordance with an alternative embodiment of an inventive electrode arrangement.

DESCRIPTION OF EMBODIMENTS Fig. 1 illustrates part of a cylinder head 2 and an associated cylinder 4 of an internal combustion engine, particularly an Otto cycle engine. A piston 6 is moveable in the cylinder 4. Provided on the outside of the upper part of the piston are a number of piston ring grooves 8 for receiving piston rings (not shown in Fig. 1) that are intended to seal the piston against the inner surface 10 of the cylinder wall 12. The piston is provided with a gudgeon pin hub 14 for coaction with a gudgeon pin, not shown. The piston is shown in Fig. 1 in its upper dead centre position.

The compression chamber 20 for the fuel-air mixture present in the cylinder is defined between the upper side of the piston top wall 16 and a wall surface 18 on the underside of the cylinder head 2. Naturally, the compression chamber 20 will also be defined by that part of the inner surface 10 of the cylinder wall 12 that is located above the piston top wall 16 prior to the piston 6 having reached the upper dead centre position shown in Fig. 1.

A single-pole sparking plug 22 is sealingly mounted in a through-penetrating plug opening 24 in the cylinder head 2, in a conventional manner. In the illustrated case, the sparking plug 22 is placed in the cylinder head so that the single electrode (central electrode) 26 of the sparking plug will be located on the symmetry axis A of the compression chamber 20.

The side of the piston top wall 16 that lies proximal to the compression chamber 20 includes a ring-shaped recess or cavity 28 that forms a combustion chamber around a first earth electrode 30 fixedly mounted in the centre of the piston top (see Fig. 1 and Fig. 2a).

In order to ensure that positive ignition (effective sparking) will also be achieved when the spark between the central electrode 26 of the single-pole sparking plug 22 and the piston- carried, and therewith moveable, earth electrode 30 is still excessively large (as in the case of certain ignition advance for instance)-and there is consequently a risk that the spark will instead jump to the nearest area of the wall surface 18 of the cylinder head 2-there is provided in the compression chamber 20 an additional second earth electrode 32 which is fastened in the wall part 34 of the cylinder head 2. In this case, the curved or angled earth electrode 32 extending inwardly of the symmetrv axis of the compression chamber 20 is

fastened in said wall part 34 at a radial distance from the sparking plug electrode 26 and is situated in a diametric plane of the cylinder 4 that contains the symmetry axis A.

The tops of pistons are often of a much more sophisticated designed than the piston top illustrated in Fig. 1, particularly in the case of internal combustion engines in which the finely divided fuel is injected directly into the compression chambers of the cylinders. This may make it necessary to choose another position for the piston-carried earth electrode 30 on the piston top than the central position shown in Fig. 1 and Fig. 2a. Figs. 2b-2e and Fig. 4 illustrate examples of such placements where the earth electrode 30 on the piston top is positioned on one side of the centre of the piston top, i. e. at a radial distance e from the sparking plug electrode 26.

Figs. 2e, 2d and 2c show piston tops where the earth electrode 30 is so positioned and so dimensioned that the electrode overlaps the central electrode 26 of the sparking plug to a greater or lesser extent when the piston 6 is located in the illustrated upper dead centre position (see the measurements t3, t2, tl).

Referring now to Fig. 3 which illustrates an embodiment of the earth electrode 32 mounted in the cylinder head 2 that is slightly different to the embodiment shown in Fig. 1. In this case, the earth electrode 32 is angled inwardly of the symmetry axis A and extends slightly down beneath the flat underside 36 of the cylinder head. The top of the engine piston moveable in an associated cylinder must then have a combustion chamber cavity that will provide space for the downwardly extending end part 38 of the earth electrode. The central electrode 26 of the sparking plug 22 is enclosed in an insulating body, the forward part of which located in the compression chamber 20 is referenced 40.

Fig. 4 shows a further embodiment of an inventive electrode arrangement. In the case of this embodiment, the engine piston 6 includes a combustion chamber cavity 42 having a non- centrally positioned earth electrode 30. The other earth electrode 44 is mounted in the same transit opening in the cylinder head 2 as the sparking plug 22 in this case. The electrode 44 can either form an integral part of the sparking plug or a part of an auxiliary component mounted on the front portion of the plug.

When a separate auxiliary component is used, said component may have the form of a sleeve-like part with the electrode 44 projecting out parallel with the single-pole sparking plug from the edge of the sleeve that faces towards the compression chamber. The sleeve- like part will then surround the front portion of the sparking plug and may suitably be provided with a side projection that will prevent rotation of the sleeve and that ensures that said sleeve will be held in a given position in the transit opening and therewith give the earth electrode of the sleeve part functioning as a side electrode a specific setting in the combustion chamber. This enables a piston having a non-centrally placed earth electrode (piston electrode) to be used without the danger of"earth electrode collision". A side electrode that is an"integrated part"of the sparking plug does not provide this possibility, since the side electrode can become situated in any rotational position whatsoever in the combustion chamber.