Field of the Invention

The present invention relates to an ignition device which gives improved conversion of chemical energy to useful mechanical work. In particular, although not limited thereto, the device is suitable for inclusion in petrol and diesel motors for conventional vehicles, but also finds application in marine vessels and aircraft. Additionally, the device also finds application in areas such as heating and cooling systems and also power generation.

Background to the Invention

The use of controlled vapour phase explosions to provide power is well-known, forming the basis for the internal combustion engine, as well as other applications. The basic principle is relatively simple in that an ignition source is housed within a chamber. The chamber is then supplied with a fuel in the form of a vapour or gas and the ignition source activated to initiate combustion or other chemical reaction of the fuel. The energy thereby released is converted into mechanical energy to provide motive or other mechanical power.

Usually the energy is in the form of expansion of the hot gases formed by the combustion. This manner of energy transformation can however be inefficient. Due to energy losses even the most efficient petrol engines rarely run at an efficiency of over 40%. Much of the loss is due to heat losses or increasing entropy. Given that the availability of carbonaceous fuel, especially obtained from fossil fuels is a finite and dwindling resource, it would obviously be of benefit to be able to increase the efficiency of the combustion processes. This would not only prolong the availability of the fuel resource for use, but would moreover also reduce the rate of carbon dioxide emissions due to the overall decreased fuel consumption. Further, there would be a reduction in soot, ΝΟχ, ground level ozone emission and other pollutants.

Many different routes are known in the art to increase the efficiency of fuel use. However, it is widely recognised that using conventional incremental improvements, there is a diminishing return for further innovations.

It is an object of the current invention to provide an improved combustion method, to increase the efficiency of fuel usage. It is a further object of the current invention to provide a device which will provide the increased efficient fuel usage. Summary of the Invention

In a first aspect, the invention may broadly be said to consist of an interface for an engine fuel ignition system, comprising:

a housing, configured to connect and form a passage between an ignition means and a combustion chamber such that the ignition means is offset from the combustion chamber, the passage formed to provide fluidic communication between the ignition means and the combustion chamber, at least a portion of the passage shaped as an elongate cone frustum.

Preferably, the ratio of the cone section length/height, to the difference between the inlet- outlet dimension, is substantially between 3 and 5, further preferably the ratio is substantially 3.5, and yet further preferably the ratio is substantially 4.

Preferably the ignition means comprises a spark plug.

Optionally, the combustion chamber comprises a chamber formed in a cylinder head. Conveniently the housing further comprises at least one baffle. Further conveniently the at least one baffle extends at least partway across the passage. Yet further conveniently, multiple baffles are formed in at least one ring around the passage. Still further conveniently, multiple baffles are formed in a stacked configuration with the top baffle of the stack aligned towards the combustion chamber. Still yet further conveniently, the stack comprises at least two rows.

In a second aspect of the invention there is provided an engine fuel ignition system, comprising: a housing, configured to connect and form a passage between an ignition means and a combustion chamber such that the ignition means is offset from the combustion chamber, the passage formed to provide fluidic communication between the ignition means and the combustion chamber, the passage comprising at least one baffle.

Conveniently, at least a portion of the passage is shaped as an elongate cone frustum. Further conveniently, the ratio of the cone section length/height, to the difference between the inlet-outlet dimension, is substantially between 3 and 5, yet further conveniently the ratio is substantially 3.5, and still yet further conveniently the ratio is substantially 4.

Preferably, the ignition means comprises a spark plug.

Preferably, the combustion chamber comprises a chamber formed in a cylinder head. According to a third aspect of the invention, there is provided a vehicle, comprising an interface in accordance with the first aspect of the invention.

According to a fourth aspect of the invention, there is provided a vehicle comprising an interface in accordance with the second aspect of the invention.

According to a fifth aspect of the invention, there is provided an engine for a vehicle, comprising an interface in accordance with the first aspect of the invention.

According to a sixth aspect of the invention, there is provided an engine for a vehicle, comprising an interface in accordance with the second aspect of the invention.

According to a seventh aspect of the invention, there is provided a spark plug and engine interface, the interface being in accordance with the first aspect of the invention. According to an eighth aspect of the invention, there is provided a spark plug and engine interface, the interface being in accordance with the second aspect of the invention. According to an ninth aspect of the invention, there is provided ignition device providing an ignition source to generate a localised high temperature region, and a housing to support said source;

a containment means secured to the housing; the ignition source being partially surrounded by the containment means,

the containment means including one of more apertures allowing fluid flow between the internal volume space within the containment means and the external space surrounding the containment means.

Preferably the ignition source comprises an ignition spark generating portion and support therefor, the portion being connectable to a high voltage source. Further preferably, the containment means is substantially cylindrical, a first open end of the cylinder being secured to the housing in touching arrangement with the support, a second open end of the cylinder providing a single aperture to concentrate outflow of fluid through said aperture. Yet further preferably, the cylinder wall is convoluted. Still yet further preferably, the second open end of the containment means extends inwardly into the inner volume of the containment means.

Optionally, the containment means includes a plurality of apertures through which fluid can flow following combustion and enables combusting and combustion fluid to be directed to all regions of a chamber housing the ignition device.

Preferably, the containment means is formed of an unprotected metal or metal alloy. Further preferably, the metal or metal alloy is selected from titanium, a nickel-bearing alloy such as those marketed under the trade names Incomel™ or Hastelloy™.

Optionally, the containment means is formed of a glass or ceramic material.

Preferably, the surface of the containment means is coated to further enhance resistance to degradation. Further preferably, the coating is a refractory coating to increase resistance of the containment means towards oxidation.

Optionally, the interior surface of the containment means includes one or more baffles. Brief Description of the Drawings

The invention is now described with reference to and as illustrated by the accompanying drawings which show by way of example only, four embodiments of an ignition device.

In the drawings:

Figure 1 is a perspective view of a first embodiment of an ignition device; Figure 2 is a side view of the embodiment of Figure 1;

Figure 3 is a perspective view of a second embodiment of an ignition device; Figure 4 is a side view of the embodiment of Figure 3;

Figure 5 is a perspective view of a third embodiment of an ignition device; and Figure 6 is a side view of the embodiment of Figure 5. Figures 7a to 7g show schematic cutaway side views of an engine combustion chamber and a shroud or interface that locates and forms a passage between the spark gap of a spark plug, and the inlet to the combustion chamber in which the spark plug would normally locate, figure 7a showing the default or normal, known combustion chamber with the spark gap at the top end, figures 7b to 7g showing the spark gap offset within a narrower passage outside the combustion chamber of the engine cylinder, figure 7b and 7e showing a frustoconical passage arrangement, figures 7d, 7f, 7g, and 7h showing baffles located in the passage that act to disrupt the fluid flow through the passage.

Figures 8a to 8d show side views through the cylinder combustion chamber of the propagation of the flame front from the spark gap to the top surface of the piston with time, the views rotated through 90 degrees counter-clockwise so that the piston top surface is on the far right and the spark plug and interface are on the left, the default or known embodiment of figure 7a shown in the top left, the embodiment of figure 7b top- right, the embodiment of figure 7e bottom-left, and the embodiment of figure 7h bottom- right, the flame shown at four discrete time steps of 0.0015 seconds, 0.0030 seconds, 0.0045 seconds, and 0.0065 seconds from ignition, in figures 8a to 8d respectively.

Figure 9 shows a graph of combustion progress for the default or normal engine of figure 7a and the frustoconical engines of figures 7b and 7e, with time plotted on the x-axis versus percentage of fuel burnt on the y-axis.

Figures 10a to 10c show the actual velocity of the flame front as measured within and across the combustion chamber, the view rotated through 90 degrees counter-clockwise so that the piston top surface is on the far right, and the spark plug and interface are on the left, and the default or known embodiment of figure 7a is shown in the top left in each figure, with the embodiment of figure 7b top-right, the embodiment of figure 7e bottom- left, and the embodiment of figure 7h bottom-right, figure 10a showing the velocities at 0.002 seconds from ignition, figure 10b showing the velocities at 0.0035 seconds from ignition, and figure 10c showing the velocities at 0.005 seconds from ignition.

Figures 11a and 11 b show the pressures as measured within and across the combustion chamber, the view rotated through 90 degrees counter-clockwise so that the piston top surface is on the far right, and the spark plug and interface are on the left, and the default or known embodiment of figure 7a is shown in the top left in each figure, with the embodiment of figure 7b top-right, the embodiment of figure 7e bottom-left, and the embodiment of figure 7h bottom-right, the pressure measured at 0.004 seconds and 0.005 seconds after ignition, these time intervals shown in figures 11a and 11b respectively; Figures 12a and 12b show the temperature gradients within the combustion chamber during fuel burn, the view rotated through 90 degrees counter-clockwise so that the piston top surface is on the far right, and the spark plug and interface are on the left, and the default or known embodiment of figure 7a is shown in the top left in each figure, with the embodiment of figure 7b top-right, the embodiment of figure 7e bottom-left, and the embodiment of figure 7h bottom-right, the temperatures taken for two time steps post- ignition at 0.0035 and 0.0055 seconds, for figures 12a and 12b respectively;

Figure 13 shows a schematic side view of the cylinder combustion chamber and shroud or interface of the preceding figures, showing a line representative of a plane across the chamber at which the measurements of velocity and pressure were taken for the plots of figures 10 and 1 1, the line located substantially at the mid-point of the chamber so as to bisect the combustion chamber; Figure 14 is a section through an embodiment of an ignition device suitable for use in accordance either inserted into a combustion chamber as a conventional spark plug as described with reference to figures 1 - 6 or also in accordance with the manner described in figures 7 - 13; Figures 15a - 15d illustrate the shroud utilised in the embodiment of Figure 14: Figure15a is an end view of the shroud, Figure 15b a section through B-B of Figure 15a, and Figures 15c, 15d perspective views.

Detailed Description of the Invention

The present invention is intended for use incorporated into an apparatus or a process in which an ignition source is initially surrounded by a fuel. Usually the fuel is in the form of a vapour or gas. In this document the term 'vapour' is used in its conventional form to refer to a dispersion of liquid, usually droplets forming a disperse phase in a gaseous or vacuum phase. The ignition source subsequently ignites the fuel to cause a controlled reaction of the fuel to release chemical energy which is converted into mechanical energy to drive a machine. The principal exemplified embodiment shown herein is that of a spark plug such as is present in a conventional internal combustion engine using petrol as a fuel. This example illustrates the features of the current invention which are applicable more broadly to other devices in which ignition of a vapour or gaseous fuel is accomplished from a localised ignition source, with the fuel conversion then being self-sustaining through the body of the fuel, remote from the immediate region of the ignition source. For example the invention is applicable in diesel engines which have been adapted or are designed to run with the glow plugs retained in a heated state. The shroud surrounds the ignition point of the glow plug thus initiating combustion within the shroud. So, for example, in the illustrated examples, the ignition source comprises a spark plug of conventional design. The spark plug includes an electrode having at one end, a spark gap. The spark gap extends into a chamber in which fuel, either in vapour or gaseous form, formed by a hydrocarbon/oxygen mixture is present under higher than ambient pressure. Application of a voltage across the spark gap, typically 10 to 25 kV, causes a spark between the electrodes and the extremely high temperature of the spark initiates (ignites) the fuel mixture.

The invention outlined herein is also suitable for other types of ignition source, such as the glow plugs utilised in igniting a volume of heated compressed air underlying the function of a diesel engine. The utilisation of the energy thereby released and its conversion into mechanical energy is usually fairly inefficient however, and the invention set out below aims to improve the efficiency. In accordance with one aspect of the invention therefore there is provided an ignition source in which said ignition source is contained within or partially surrounded by a flow modifying shroud or containment means. The shroud modifies the flow of ignited fuel through the chamber housing the source. Because of the modified flow, the combustion of the fuel and the spread of the combustion front over time, throughout the body of the fuel, non-useful energy conversion is reduced, leaving more available for mechanical work.

Referring initially to Figures 1 and 2, these illustrate a first embodiment of an ignition source, generally referenced 10. The ignition source 10 comprises a first part in the form of a conventional spark plug 1 1 of a type typically used within a petrol engine. The spark plug 1 1 has a terminal 12 of electrically conducting material to enable the plug 1 1 to be connected to an ignition system. At the other end of the spark plug 11, and usually in-use extending into a fuel chamber or the like (not illustrated), is the main electrode 13 which extends from the terminal 12 and through an electrically insulating sleeve, of for example porcelain. The electrically insulating sleeve separates different parts of the electrically conducting elements of the spark plug, so preventing short circuits, provides support for the elements of the spark plug, as well as allowing the spark plug to be fitted into a fuel chamber without imparting charge thereto. The main electrode 13 has a spark gap across which electrons pass when the electrode is subjected to a sufficiently high voltage. As the electrons pass across the gap the molecules therein are heated to a high temperature and emit light, producing the "spark".

Deployed about the main electrode is a shroud 14. In the Figures 1 - 4, the shroud 14 is for convenience shown as transparent to enable the elements within the shroud 14 to be seen. The shroud 14 is formed of a heat-resistant material and in the case where the fuel combustion reaction is oxidative, the material is also oxidation resistant. Typical materials from which the shroud 14 can be formed are metal or metal alloys such as titanium, or nickel-bearing alloys such as those marketed under the trade names Incomel™ or

Hastelloy™. Alternatively, the shroud 14 can be formed of a ceramic or glass material or plastics material. In order, where required, to further improve the resistance of the shroud 14 to chemical and physical degradation, a surface coating can be applied. Such a coating can include an oxide of a metal from which the shroud is formed. Alternatively, a refractory coating can be applied to increase resistance towards oxidation.

The shroud 14 as shown in Figure 1 is substantially ovoid in shape and is open at both narrow ends having apertures 15a, 15b. The ovoid shape shown in Figures 1 and 2 exemplifies a generally cylindrical shape suitable for shrouds according to the invention. The aperture 15a is so sized to fit about the body of the spark plug 1 1 to at least touch the body along the circumference of the aperture 15a and preferably to form a fluid-resistant or fluid-tight seal. The other aperture 15b defined by the shroud 14 is arrayed

perpendicularly to the axis of the spark plug 11. The shroud 14 therefore firstly provides a barrier to expansion of hot gases in a direction laterally away from the electrode 13.

Moreover, the combustion reaction producing the hot gases is also hindered from lateral propagation. The only direction available for expansion therefore is through the aperture 15b.

Without being bound by theory, it is believed that the shroud acts to change the flow characteristics in such a way as to introduce a delay in propagation of the combustion reaction, which takes the form of a wave front away from the ignition source, which delay can in turn allow the combustion materials and energy release within the shroud to buildup prior to their exit through the aperture. In a prior art spark plug therefore, the fuel is ignited and the localised combustion wave immediately spreads out away from the source. In the present invention, the delay firstly results in a build-up of energy within the shroud prior to the main propagation. Second, the energy of propagation is then released through the restricting aperture which concentrates the energy and also causes faster movement away from the ignition source thus giving a more efficient conversion of the chemical energy released into mechanical energy. In the example of an internal combustion engine therefore, the more efficient use of the chemical energy could mean that less fuel is required to drive the piston within the cylinder in which the spark plug is housed. This results in higher fuel efficiency. Alternatively, the piston can be driven more strongly to produce greater power to the drive systems of the vehicle.

Referring now to Figures 3 and 4, these illustrate an alternative embodiment of ignition source 30 in which the shroud 34 inverts at the aperture 35b, and whereby a generally cylindrical extension 36 extends inwardly towards the main electrode 33. The extension 36 again alters the flow characteristics of the initially combusted fuel and enables a more complete fuel consumption and a higher energy concentration to build-up within the shroud 34 prior to the combustion products' release from the shroud 34 and into the main fuel-containing chamber. The combustion products continue, as in other embodiments to combust as they leave the shroud and cause the fuel outside the shroud, within the fuel- containing chamber, to combust. Again therefore the energy release via combustion of the fuel is higher than with prior art ignition sources and also is more efficiently converted to mechanical energy.

The shroud can also have the form as described below and as shown in Figures 14 and 15, and including the frusto-conical passage shown in that embodiment.

The shroud as described above can include within the shroud wall, one or more apertures which allow combustion to propagate laterally through the wall (e.g. 34) of a shroud into the combustion chamber. It is envisaged however, that in such embodiments the combined area of the aperture or apertures will be relatively small compared to the overall area of the shroud wall itself. Typically the area of the shroud wall is greater than 50%, preferably >60%, further preferably >70% and especially preferably >80%. Such apertures can take the form of circular apertures, elliptical apertures, polygonal apertures or elongate apertures, which elongate apertures can be parallel to the main body of the device, arrayed laterally across or at an oblique angle.

In the third embodiment of the ignition device shown in Figures 5 and 6, the main features with the exception of the shroud are the same as in the previously described

embodiments. The shroud 54 in this embodiment is in the form of a cage structure 51. The cage structure 51 is fixed at a first end 52 to the main body of the ignition source 50, and extends over and around the ignition source 53. Additionally, the cage structure 51 extends completely around the ignition source 50, including the end 55 of the cage structure 51 opposed to the ignition source 50.

In this embodiment, access to the ignition source by the fuel is far easier and provides for more rapid achievement and maintenance of an equilibrium mixture of fuel around the ignition source 53. The cage structure 51 also enables the propagation of combustion to proceed more evenly in all three spatial dimensions, and may be of application when the combustion chamber is more spherical or at least extends more equally in the lateral directions as compared with perpendicularly to these dimensions.

The invention defined herein is suitable to be directly installed in place of already in-situ ignition sources or instead of such sources as the fitment (or connector 12) is the same as for prior art sources. So, for example, where the ignition source is a spark plug, existing plugs can be replaced by plugs in accordance with the current invention. Additionally it is envisaged that prior art spark plugs or other ignition sources can be adapted in accordance with the invention by the installation of a shroud or cage as described above about the ignition source of the prior art plug to provide a plug or ignition source in accordance with the invention.

As such, the shroud can either be fitted onto an existing ignition source using a connection already incorporated into a shroud, or through the use of known connector types.

Alternatively, the ignition source can fit to a shroud using a screw-thread connection or a push-fit connection. A sealant and/or adhesive can be supplied to ensure the fit is fluid tight. It will be recognised that a number of other features, not illustrated can be included without departing from the scope of the invention.

Additionally or alternatively, the internal surface of the shroud can include one or more baffles to partially delay the propagation of the combustion. The wall of the shroud, pertaining to the embodiments of Figures 1 - 4 can include convolutions to introduce chaotic flow into the flow of combusted fuel.

Further embodiments of the present invention are shown in figures 7 to 12. The embodiments shown in these figures and as described below is an interface or shroud for use in a conventional internal combustion engine that uses petrol fuel. However, it should be noted that the features of the current invention are also applicable more broadly to other types of internal combustion engine, such as to diesel engines, or to other devices in which ignition of a vapour or gaseous fuel is accomplished from a localised ignition source, with the fuel conversion then being self-sustaining through the body of the fuel, remote from the immediate region of the ignition source.

Several different variations of interface are described and shown. The interface is generally designated as shroud or interface 100, with the variations described as 100b, 100c, etc. Other features also follow a similar numbering convention, for example outlet 1 10 is designated as outlet 1 10b on interface 100b, outlet 1 10c on interface 100c, etc.

In use, the interface 100 connects between the main body of the engine and an ignition means, in this case a spark plug. The interface 100 connects to the cylinder block or cylinder head 102 by locating in and connecting to the cylinder head 102 in the spark plug port or plug hole 103. The spark plug 104 that would normally locate in the port 103 which connects to the interface 100 on the outer side (i.e. away from the engine), the spark from the plug 104 igniting the vapour in the combustion chamber of the cylinder in a similar manner to conventional use. 'Vapour' in the context of this embodiment indicates a mixture of fuel and air within the combustion chamber, and where applicable and as described below, within the interface 100, the fuel mixed with the air as droplets forming the disperse phase. The spark plug 104 in this example is of conventional design, with an electrode at one end (the inner end, within the interface 100), and a spark gap. In use, with the spark plug 104 connected to the interface 100, the spark gap extends into the interface 100 so that the fuel/air vapour mixture can be ignited via application of a voltage across the spark gap, typically 10 to 25 kV, which causes a spark between the electrodes, the extremely high temperature of the spark igniting the fuel mixture. The interface substantially surrounds the electrode and provides fluidic communication with the combustion chamber, but otherwise isolating the electrode. Without being bound by theory it is believed the interface enables a volume of ignited vapour to initially build up within the interface which ignited vapour then exits to the main combustion chamber volume providing increased and faster combustion compared to that provided by a standard spark plug.

A number of different potential configurations for the interface are shown in figures 7a - 7h. These figures show a schematic cross-sectional view of the interfaces 100b - 100h of the further embodiments, each connecting to an engine block so as to fluidly connect with a combustion chamber 105 of an engine block - that is, the combustion chamber formed in an engine cylinder head. In these figures, the engine is shown in a 'conventional' orientation, with the spark plug port top centre. The lower end of the combustion chamber is closed by the top surface of the piston 108. In the other figures (8, 10 - 13) relating to these embodiments, the figures are shown rotated through 90 degrees counterclockwise so as to more clearly display relevant technical detail. The interface of these embodiments of the present invention has a main hollow body that forms a passage between two open ends. The spark plug 104 screws into one end so that the spark gap is located in the passage through the hollow body. The other end connects with the engine block so that a fluidic connection is made between the combustion chamber and the passage in the hollow body of the interface.

Figure 7a shows the combustion chamber with an interface 100a arranged so that the spark gap of the plug 104 is located directly at the edge of the combustion chamber 105. This is a known conventional arrangement, with the spark plug seating at the top of an engine cylinder. Figures 7b - 7h show the internal passage of the interface to the side/slightly outside of the combustion chamber 105. Figures 7b and 7e show passages that are frustoconical, narrowing towards the opening 106 with the combustion chamber 105. That is, the shape of the passage is that of a cone frustum. The passages for these embodiments are dimensioned so as to be elongate. The embodiment shown in figure 7b is dimensioned so that the length of the frustoconical section is substantially 40mm, with a (wider) inlet diameter of substantially 15mm, and a (narrower) outlet diameter of substantially 5mm. In the embodiment shown in figure 7e, the length of the frustoconical section is substantially 70mm, with a (wider) inlet diameter of substantially 35mm, and a (narrower) outlet diameter of substantially 15mm. It can be seen that the ratio of the length, to the difference between the inlet-outlet dimension, is between 3.5 and 4 (the inlet-outlet difference for the embodiment of figure 7b is 10mm (15mm - 5mm), and the length is 40mm, giving a ratio of 4:1, and the inlet-outlet difference for the embodiment of figure 7e is 20mm (35mm - 15mm), and the length is 70mm, giving a ratio of 3.5:1). Figures 7c, 7d, 7f, 7g, and 7h show passages that are parallel-sided (i.e. no narrowing along the length of the passage). The passages of the embodiments of figures 7d - 7h include baffles 107 that act to disrupt the flow along the passages. In the embodiment of figure 7h, there are six baffles formed in a 'stacked' or triangular configuration (that is, each line adds another baffle so the arrangement is: 1, 1 -2, 1-2-3, 1 -2-3-4, etc, the baffles arranged so that they form a triangular shape or stack). In the embodiment of figure 7h, the stack is formed so that the line of three baffles is furthest from the opening 106, and the single baffle 107 is closest to the opening 106, slightly behind the opening 106. Each baffle is circular in cross-section and has a diameter of substantially 5mm. The baffles 107 in all of these embodiments extend into/and at least partially or fully across the passage to act as obstacles for propagation of the flame from the spark gap along the passage and into the combustion chamber of the cylinder.

If the vapour mixture within the combustion chamber is fully combusted or burnt faster, this is advantageous. A faster burn, or faster progress of the combustion front from the point of ignition (flame velocity), is advantageous as this produces increased pressure on the head of the piston and more force on the piston. A fuller burn (a greater percentage of the fuel fully ignited and burnt) will also result in increased pressure within the combustion chamber, which increases the pressure on the head of the piston and produces more force on the piston. This also tends to produce a more even pressure distribution, which is also advantageous. A more effective burn within a particular size of engine (engine cylinder) means that less fuel can be used in order to produce the same result.

Experimentally, the embodiments of the interfaces 100 have been shown to be

advantageous over the standard arrangement shown in figure 7a. The experimental results relating to the embodiments of figures 7b, 7e, and 7h will now be described in relation to the standard arrangement of figure 7a.

Figures 8a to 8d show propagation of the flame front 120 from the spark gap to the piston top surface 108. In each of these figures, the view is rotated through 90 degrees counterclockwise, as outlined above, so that the piston top surface 108 is on the far right, and the spark plug and interface are on the left. In each of these figures, the default or known embodiment (figure 7a) is shown in the top left, the embodiment of figure 7b top-right, the embodiment of figure 7e bottom-left, and the embodiment of figure 7h bottom-right. Figures 8a to 8d show the flame at four discrete time steps, these being 0.0015 seconds, 0.0030 seconds, 0.0045 seconds, and 0.0065 seconds from ignition, respectively.

As can be seen for all of the embodiments, in comparison to the default of figure 7a, the flame front propagates further/faster over the same time frame, despite ignition taking place outside the combustion chamber. It can also be seen in relation to the embodiment of figure 7h that the baffles have the effect of 'flattening' the flame front so that it is roughly parallel to the plane of the top surface of the piston 108, and the flame front will reach the plane of the top surface of the piston substantially simultaneously at all points across the surface, or at least in a lesser time interval between first and last contact than is the case with the default of figure 7a.

Combustion progress is shown in the graph of figure 9, for the default or normal engine of figure 7a (line 109), and the frustoconical engines of figures 7b and 7e (lines 1 12, and 1 11 respectively). Time is plotted on the x-axis versus percentage of fuel burnt on the y-axis. It can be seen that the line representing the default engine is less steep than those of the two frustoconical engines, indicating that the fuel is burning slower than for the frustoconical engines. The actual velocity of the flame front is shown in figures 10a, 10b and 10c, which show the velocity measured within the combustion chamber 105. The position at which the velocities are measured is shown by plane 109 in figure 13, the plane 109 extending across the combustion chamber, substantially bisecting the combustion chamber.

As above, in each of these figures, the view is rotated through 90 degrees counterclockwise so that the piston top surface 108 is on the far right, and the spark plug and interface are on the left. Also as above, in each of these figures, the default or known embodiment (figure 7a) is shown in the top left, the embodiment of figure 7b top-right, the embodiment of figure 7e bottom-left, and the embodiment of figure 7h bottom-right. Figure 10a shows the velocities at 0.002 seconds from ignition, figure 10b shows the velocities at 0.0035 seconds from ignition, and figure 10c shows the velocities at 0.005 seconds from ignition. As can be seen, for all of the new embodiments, in comparison to the default of figure 7a, the velocities are far higher.

The effect of the higher velocities results in increased pressure. This is shown graphically in figures 11a and 11b. As above, the views have been rotated through 90 degrees counterclockwise, and the default or known embodiment (figure 7a) is shown in the top left, the embodiment of figure 7b top-right, the embodiment of figure 7e bottom-left, and the embodiment of figure 7h bottom-right. Also as above, the pressure was measured at the plane 109 as shown in figure 13.

The pressure was measured at two time intervals: 0.004 seconds, and 0.005 seconds after ignition. As can be seen in the plotted results, the measured pressure was far higher for the embodiments of figures 7b, 7e, and 7h, as compared to the default of figure 7a. This translates to a higher force acting on the piston.

The temperature gradients within the combustion chamber during fuel burn are shown graphically in figures 12a and 12b. As above, the views have been rotated through 90 degrees counter-clockwise, and the default or known embodiment (figure 7a) is shown in the top left, the embodiment of figure 7b top-right, the embodiment of figure 7e bottom- left, and the embodiment of figure 7h bottom-right. The two time steps, for figures 12, and 12b respectively, are 0.0035 and 0.0055 seconds after ignition. It can be seen from all of the experimental results outlined above that offsetting the spark gap within a narrower passage outside the cylinder is advantageous in comparison to the known or default arrangement of having the spark gap at the top of the cylinder and using normal cylinder geometry. This arrangement results in higher temperatures, faster burn, and greater pressure on the piston for the same cylinder size and geometry, and for the same fuel/air vapour mix. The same result as for the default arrangement of figure 7a could therefore be achieved using less fuel. The result is an increase in fuel efficiency.

It can be seen that it is most advantageous to have the spark gap offset within a passage having the shape of an elongate cone frustum. This ensures that the walls are angled to provide a smooth, rather than an abrupt, transition, allowing the flame to propagate rapidly. It has been found experimentally that the best results can be achieved by having a ratio of the cone section length/height, to the difference between the inlet-outlet dimension, which is between 3 and 5, and most preferably substantially 3.5.

An embodiment of an ignition device, described in the previous paragraph, suitable for use as described in the embodiment of figures 7 - 13 but also in the manner described for figures 1 - 6 is shown in Figure 14, with the shroud of the device shown in Figures 15. The ignition device 140 has an ignition source 141, similar to a conventional spark plug, having an electrode gap 142 across which a spark is generated. Extending away from the electrode gap 142 is a shroud 143 having an internal passage 144 in the shape of a truncated frustum and narrowing away from the electrode gap142. In use, the end of the shroud is located either as part of the combustion chamber wall or extending into the combustion chamber.

The ignition source is seated securely inside the first cylindrical section 151 (see Figures 15). The ignition device is then secured to the combustion chamber by means of suitable, usually threaded attachment means on the outside of the cylindrical section 152. The internal passage 144 is housed within the cylindrical section 152 and the location of, for example, the threaded section determines how far the internal passage 144 extends into the combustion chamber. It will of course be understood that the invention is limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention. For example, in variations of the arrangements described above, the size and shape of the baffles can be altered. For example, the cross-sectional diameter can be changed, or the cross-sectional shape (e.g. to an oval cross-section or similar). As described above, the baffles extend into the passage. These can extend al the way across the passage, or partway.

It should also be noted that the embodiments described above are for a shroud or interface that connects between the cylinder head and an ignition means such as a spark plug. The cylinder head of the engine could be formed so as to have an integral interface/shroud that is essentially the same as the separate interface/shroud described above.