GB2300224A

GB2300224A - Vaporising injected i.c.engine fuel

Info

Publication number: GB2300224A
Application number: GB9508693A
Authority: GB
Inventors: Esmail Razzagh Karimi
Original assignee: Perkins Ltd
Current assignee: Perkins Ltd
Priority date: 1995-04-28
Filing date: 1995-04-28
Publication date: 1996-10-30
Anticipated expiration: 2015-04-28
Also published as: ZA963061B; AU5282296A; GB9508693D0; WO1996034189A1; GB2300224B

Abstract

Within a chamber defined by a ceramic or metal wall 14 with heating elements 38 and integral with or connected through a thermally insulating washer (46,Fig.8) to a fuel injector 32 injected fuel is vaporised. The plumes 40 of vaporised fuel are directed radially (Fig.5) or at an angle to the radial (Figs.2b,3) by wall outlets 16 into the combustion chamber 10 which may have pockets (42,Fig.6) or grooves (44,Fig.7) in its wall. The injector may provide a pilot injection.

Description

AN INTERNAL COMBUSTION ENGINE INCLUDING A FUEL VAPORISING CHAMBER The present invention relates to an internal combustion (i.c.) engine including a fuel vaporising chamber. The present invention is intended particularly, but not exclusively, for compression ignition engines.

A conventional indirect injection (IDI) diesel engine has a separated combustion chamber. In such an engine, a pre-combustion chamber is located in the engine cylinder head remote from a main combustion chamber which is contiguous with a top of an engine piston. The pre-combustion chamber is connected by a throat or transfer passage to the main combustion chamber. Fuel is sprayed or injected directly into the pre-combustion chamber into a swirling flow of hot air pushed into the chamber during the engine compression stroke. On igniting in this chamber a mixture of fuel vapours, gases and oxidation products is injected into the main combustion chamber.

It is advantageous to make the throat or transfer passage of a narrow diameter since this will decrease engine knocking by ensuring that pressure in the main combustion chamber rises less rapidly. However, a disadvantage is that, even with a narrow diameter throat, the blow-down of fuel vapours etc. into the main combustion chamber may still cause such a rapid combustion in it that knock occurs anyway. It is therefore necessary to carefully design and dimension the throat and to also accurately control timing of fuel injection events. If fuel injection occurs too early, injected fuel impinges on a strong current of air entering the pre-combustion chamber and a fierce explosion can occur. This results in inefficient fuel consumption, increased engine noise and increased production of undesirable emissions including particulates.

A further disadvantage is that much of the injected fuel is not vaporised and enters the main combustion chamber in liquid droplet form. Thus, a proportion of fuel combustion will occur under stoichiometric conditions which provides the largest contribution to undesirable nitrous oxide (NOx) generation during combustion. The poor vaporisation of fuel in the precombustion chamber is, in part, due to the normal cooling of the cylinder head which causes an undesirable cooling of the hot air being urged into the pre-combustion chamber on the engine compression stroke.

United States patent specification No US 4421079 describes an IDI diesel engine including means within the pre-combustion chamber for overcoming the problem of poor fuel vaporisation in said chamber.

A centrally located tubular member is positioned within the chamber and is supplied internally with hot air through a secondary throat or transfer passage. Fuel is sprayed against the outer surface of the heated tubular member to be vaporised.

Vaporised fuel mixes with air upon passage of the air through transpiration holes in the member and passes to the main combustion chamber via a primary throat.

Whilst this arrangement goes some way to improving fuel vaporisation and thus reducing the generation of particulates, heating losses still occur in the cylinder head and some of the aforementioned problems associated with having a narrow (primary) throat linking the precombustion chamber with the main combustion chamber may still occur.

In conventional direct injection (DI) diesel engines, liquid fuel is injected directly into the (main) combustion chamber. The liquid fuel is injected into the combustion chamber around top dead centre (TDC) and disintegrates to form discrete droplets. The droplets penetrate into a hot gas field within the chamber and evaporation takes place. Fuel vapour and air mix due to air and fuel momentum and fuel combustion occurs. However, the dynamics of fuel injection and subsequent evaporation of fuel in the hot gas field are such that some fuel droplets remain during fuel combustion. Fuel combustion is a complex process comprising a number of identifiable phases which are not all conducive to efficient fuel combustion and low emission/particulate generation.

German patent specification No DE 2416804 describes a diesel engine in which at least one embodiment (figure 1) has been designed in an attempt to marry the advantages offered by IDI diesel engines, in which fuel is partially vaporised in a remote pre-combustion chamber, and DI diesel engines which are more responsive to injection events. This engine has a main combustion chamber contiguous to a face of an engine piston, a vaporising chamber thermally adjacent but separated from said main combustion chamber by a wall, and means for introducing fuel into said vaporising chamber, wherein thermal energy present in the vaporising chamber at least partially vaporises introduced fuel, some of the resulting fuel vapour passing through the structure of the wall into the main combustion chamber.

Fuel is injected to strike the inner surface of the wall which has a largely gas permeable structure. The wall may be formed of either a porous sintered ceramic material or an open-pored foamed metallic material. Thermal energy stored in the wall causes a part of the injected fuel to vaporise and ignite. The subsequent increase in temperature and pressure within the vaporising chamber causes vaporisation of remaining injected fuel which passes into the main combustion chamber via capillary passages in the material of the wall.

The wall structure also acts as a filter to prevent soot particles passing into the main combustion chamber.

The structure of the wall is such that it substantially reduces the flow rate of vaporised fuel passing from the vaporising chamber to the main combustion chamber. Therefore, to obtain good mixing of emergent vaporised fuel with compressed air in the main chamber, it is essential that the distance between the surface of the piston crown defining the main combustion chamber and the outer surface of the wall is small at all points. Thus the main combustion chamber has a shape generally conforming with that of the vaporising chamber and has a not substantially greater volume.

This arrangement has a number of disadvantages.

In particular, the requirement that the main combustion chamber shape must conform closely with that of the vaporising chamber leads to a taller than normal engine piston because of the deeper than normal main combustion chamber.

Also, the globe shaped vaporising chamber wall has a diameter considerably larger than the diameter of the injection nozzle to which it attaches. Thus, the injection nozzle and wall comprising separate components must be assembled together from opposite sides of the cylinder head.

More importantly, the porous structure of the wall has a detrimental effect on the dynamics of fuel/air mixing in the main combustion chamber leading to poorer fuel combustion efficiency than that envisaged.

It is an object of the present invention to obviate and mitigate the aforesaid disadvantages.

According to one aspect of the present invention, there is provided a method of burning fuel in an internal combustion engine, comprising the step of introducing fuel into a vaporising chamber thermally adjacent but separated from a main combustion chamber by a wall having at least one aperture formed therein, wherein thermal energy present in the vaporising chamber at least partially vaporises said introduced fuel, some of the resulting fuel vapour being directed through the aperture(s) into the main combustion chamber containing hot air for combustion thereof.

Preferably, the method includes injecting the fuel into the vaporising chamber.

Preferably also, the method includes injecting the fuel into the vaporising chamber at high pressure.

Preferably, in the method the thermal energy for partially vaporising said injected fuel is stored in the wall and/or air present in the vaporising chamber.

Preferably further, the thermal energy is provided by a prior combustion event.

Alternatively, the thermal energy is provided by heating elements located within the wall.

Preferably also, the air present in the vaporising chamber enters said chamber from the main combustion chamber on the compression stroke of the engine.

Preferably, the method includes directing the vaporised fuel to discrete locations in the main combustion chamber.

Preferably also, the method includes directing the vaporised fuel into the main combustion chamber in such a manner that it creates swirling of said vaporised fuel within the main chamber.

The method may also include the step of injecting a small pilot charge of fuel into the vaporising chamber prior to the injection of a main fuel charge in order to initiate fuel combustion, whereby combustion of the pilot charge creates a hot mixing zone within the vaporising chamber into which the main fuel charge is injected.

According to a second aspect of the present invention, there is provided an internal combustion engine having a main combustion chamber contiguous to a face of an engine piston, a vaporising chamber thermally adjacent but separated from said main combustion chamber by a wall having at least one aperture formed therein, means for introducing fuel into said vaporising chamber, wherein thermal energy present in the vaporising chamber at least partially vaporises introduced fuel, some of the resulting fuel vapour being directed through the aperture (s) into the main combustion chamber containing hot air for combustion thereof.

Preferably, the means for introducing fuel into the vaporising chamber comprises a fuel injection means.

Preferably, the fuel injection means is a high pressure fuel injection means.

Preferably, the vaporising chamber is immediately adjacent the main combustion chamber.

Preferably, the vaporising chamber extends into the main combustion chamber.

Preferably, the vaporising chamber is contained completely within the main combustion chamber.

Preferably, the vaporising chamber wall has two or more apertures.

Preferably, the apertures are dimensioned and positioned so as to direct fuel vapour from the vaporising chamber to discrete locations in the main combustion chamber.

Preferably, the discrete locations comprise pockets formed in the face of the engine piston.

Preferably, the number of apertures in the wall equals the number of pockets in the piston crown face.

The vaporising chamber wall may be made from a heat retentive material such as a ceramic material.

The vaporising chamber wall may be connected to a face of an engine cylinder head in which the fuel injection means is mounted.

Alternatively, the vaporising chamber wall may be connected to an injection nozzle part of the fuel injection means.

The vaporising chamber wall may have an outer diameter less than or equal to a largest diameter of a body part of the fuel injector means which inserts into a bore in the cylinder head.

The vaporising chamber wall may be formed integrally with the nozzle part of the fuel injection means.

The vaporisation chamber wall may be separated from the cylinder head/nozzle part of the fuel injector means by a thermally insulating means.

The thermally insulating means may comprise a washer formed of thermally insulating material.

According to a third aspect of the present invention, there is provided a fuel injector for an i.c. engine having an injector nozzle and a wall arranged to surround said nozzle to define a fuel vaporising chamber between said nozzle and an inner surface of the wall, wherein the wall has at least one aperture formed therein.

Preferably, the vaporising chamber wall has an outer diameter less than or equal to a largest diameter of a body part of the fuel injector which, in situ, inserts into a bore of an engine cylinder head.

Preferably, the wall is formed integrally with the injector nozzle.

Alternatively, the wall is separated from the injector nozzle by a thermally insulating means such as a thermally insulating washer.

The foregoing and further features of the present invention will be more readily understood from the following description of preferred embodiments, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a typical heat release rate diagram for fuel combustion in a diesel engine; Figure 2a is a cross-sectional view in a vertical plane of a piston crown and vaporising chamber arrangement according to a first embodiment of the invention; Figure 2b is a view on horizontal section line IIB to IIB of figure 2a and illustrates a fuel vapour flow pattern; Figure 3 is a section view similar to figure 2(b) of a fuel vapour flow pattern for a second embodiment of the invention; Figure 4 is a view of a horizontal section through a vaporising chamber wall for use in the embodiment of figure 3;; Figure 5 is a section view similar to figure 3 of a fuel vapour flow pattern for a third embodiment of the invention; Figure 6 is a cross-sectional view in a vertical plane of a piston crown for use in the embodiments of figures 3 and 5; Figure 7 is a cross-sectional view in a vertical plane of another piston crown for use in the embodiments of figures 3 and 5; Figure 8 is a cross-sectional view in a vertical plane of a vaporising chamber and injector nozzle combination according to a fourth embodiment of the invention; Figure 9 is a cross-sectional view in a vertical plane of a vaporisation chamber and pintle type injector nozzle according to a fifth embodiment of the invention; Figure 10 is a cross-sectional view in a vertical plane of a vaporising chamber wall for use in embodiments of the invention; and Figure 11 is a cross-sectional view in a vertical plane of another vaporising chamber wall for use in embodiments of the invention.

One problem with combustion systems in current diesel engines is that they rely heavily on the optimisation of the fuel injection system with inlet port geometry and combustion chamber shape to achieve good air/fuel mixing and combustion characteristics. As legislated emissions levels become increasingly stringent worldwide the fuel injection system requires ever more accurate microlevel optimisation and matching with the combustion chamber shape, in particular. For each new engine specification a development programme is required for the optimisation and matching of the fuel injection system and the combustion chamber parameters. Such programmes are extremely costly and time consuming.

Another problem with current combustion systems is their inability to reduce NOx levels below 4.0 g/bhph despite high injection pressures (1500 bar) and retarded injection timing (30 after TDC (ATDC)).

The use of exhaust gas recirculation (EGR) and NOx catalyst technologies to clean exhaust gas is costly and adds further complexity/sensitivity to the combustion system.

In a typical DI diesel engine combustion system, the liquid fuel injected into the combustion chamber around TDC disintegrates and forms discrete droplets. The droplets penetrate a hot gas field within the chamber and evaporation takes place.

Fuel vapour and air mix due to air and fuel momentum. Once the concentration of the mixture is within the flammability limit and the mixture has reached the spontaneous ignition temperature (i.e.

around 600 OC) ignition will occur. The period from the start of injection to start of combustion is often referred to as the "ignition delay"(figure 1).

During the ignition delay period a quantity of air and fuel is prepared and immediately after start of combustion this prepared mixture will burn at a very fast rate causing a rapid pressure rise in the cylinder. This phase of combustion is referred to as "pre-mixed burning" which is mainly, but not solely, responsible for NOx generation. Injecting the fuel at near TDC, when the air within the engine cylinder has reached significantly higher temperatures (e.g. 1050 OK) and pressures (e.g. 100 bar), will result in a faster evaporation rate and reduce the ignition delay period. The promotion of rapid mixing of air and fuel by swirl or other air motions will also reduce the ignition delay period.

Therefore, reducing the ignition delay period will reduce the quantity of prepared mixture and decrease the amount of premixed burning and thus NOx generation.

Once the prepared mixture has burned, the fuel will continue burning at a rate controlled by the availability of oxygen in the cylinder. This phase of burning is referred to as "diffusion burning" (figure 1). The diffusion burning phase involves the orderly diffusion of gaseous fuel and oxygen accompanied by smooth and controlled combustion of the fuel. However, in an engine it is desirable to have the combustion proceed as rapidly as possible so as to enhance the thermal efficiency by releasing the heat energy at the earliest practical time in the expansion part of the cycle (i.e. close to but after TDC), thereby increasing the work output of the engine. Rapid diffusion burning of vaporised fuel and air is the most desirable way of attaining complete and efficient combustion with a minimum of unwanted side effects.

Reducing the ignition delay period to nearly zero (0.06 milliseconds (ms), for example can be achieved by known high pressure fuel injection systems) can reduce or eliminate pre-mixed burning but does not create a condition where any fuel injected subsequent to start of combustion would burn as it enters the combustion chamber. In fact it is evident from both experimental and predictive results that after the start of combustion a significant amount of fuel is still in liquid phase within the chamber and as long as there are liquid fuel droplets present, a large proportion of the burning will take place under stoichiometric conditions. This creates the highest temperature and makes the biggest contribution towards NOx generation. NOx generation is dependent on the partial pressure of 2 and on temperature within the cylinder.The temperature, however, has a more dominant effect. The presence of liquid fuel droplets during combustion creates undesirable local high temperature zones or "hot spots" within the combustion chamber.

It is believed that NOx generation is essentially a local temperature and fluid phase problem and not just a pre-mixed phase or peak cylinder pressure problem. Therefore, it is believed that the ability of known systems to reduce NOx generation by using higher fuel injection pressures and further optimisation of the fuel injection system is very limited. The use of higher injection pressures, EGR and NOx catalyst aftertreatment of exhaust gas to reduce particulates and NOx generation is costly and adds to development time and design sensitivities.

It has surprisingly been found that a combustion system meeting the following requirements: minimised ignition delay period to prevent pre-mixed burning; rapid evaporation of fuel to reduce or eliminate droplet phase burning; and fast, mixing controlled, combustion to enable lean gas phase burning can readily be provided without undue structural complexity and at the same time provide a combustion system more tolerant to design variables.

A preferred embodiment of an internal combustion (i.c.) engine in accordance with the present invention is illustrated in figure 2 (a and b).

This comprises a vaporisation chamber 10 completely contained within, but separated from a main combustion chamber 12 by a wall 14. The wall 14 has a number of apertures 16 which afford communication between the enclosed vaporising chamber volume and the surrounding volume of the main combustion chamber 12. The main combustion chamber 12 is formed in the crown 18 of an engine piston 20 mounted for reciprocal movement within a cylinder 22 formed in an engine block 24. The cylinder 22 is capped by a cylinder head 26, an inner surface 28 of which defines the upper limit of the main combustion chamber 12.

The wall 14 defining the vaporisation chamber 10 is generally bulb-shaped and surrounds a nozzle part 30 of a fuel injector 32. The wall 14 is integrally formed with the nozzle part 30 of the injector 32 and has a largest external diameter equal to or smaller than a largest diameter of a body part 34 of the injector 32 which inserts through a bore 36 in the cylinder head 26. Thus the injector/wall combination can readily be mounted in the cylinder head 26 and can be replaced without having to remove the cylinder head 26.

The wall 14 is formed of a heat insulating ceramic material although it is envisaged that a wall formed from metal would suffice. A wall made from metal may be more resistant to the extreme temperatures and pressures experienced in the cylinder 22 and might be less expensive to manufacture.

Heating elements 38 (shown only in figure 2) are housed within the chamber wall 14 to increase the vaporising chamber temperature, when required. In use, a small amount of fuel, a pilot charge, is injected into the vaporising chamber 10, at a predetermined time, immediately before the main fuel injection charge to increase by combustion of the pilot charge the temperature and turbulence within the vaporising chamber. The main fuel charge is then injected into the resulting high temperature turbulent zone in the vaporising chamber. The main fuel charge is vaporised and at least the majority of it passes through the apertures 16 in the wall 14 to mix with hot compressed air in the main combustion chamber 12 for completion of the burning process. Thus, diffusion burning of the fuel is promoted.

The apertures 16 in the wall are positioned and dimensioned to direct emergent fuel vapour from the vaporising chamber 10 to discrete locations in the main combustion chamber 12. The vaporising chamber 10 has a smaller volume than the surrounding volume of the main combustion chamber 12. This volume is, in fact, in the order of several times smaller.

The apertures 16 are angled with respect to the vertical axis of the vaporising chamber 10 in order to create rapid motion in air entering the chamber 10 on the compression stroke and in fuel vapour subsequently emerging from the chamber 10. Due to the heat from burning of the pilot injection, heat from the heating elements 38 and rapid local air motion within chamber 10 the liquid phase droplet size of injected fuel will reduce drastically.

The wall 14 of the vaporising chamber 10 absorbs and retains heat from previous combustion cycles after engine start up which increases fuel evaporation rate and reduces the ignition delay period. It is envisaged that the heating elements 38 will only be required on engine start-up.

In this embodiment, the vaporising chamber wall 14 has four apertures 16 equally spaced on the same horizontal plane around the wall 14. The flow pattern of vaporised fuel into the main combustion chamber 12 can be seen in figure 2. This comprises four plumes 40 emerging to create a counterclockwise swirl (indicated by the arrows) within the main chamber 12. This swirl may enhance swirl already induced in air admitted to the engine cylinder 22 prior to the compression stroke or counter it to create turbulent flow.

Figure 3 illustrates a vaporised fuel flow pattern for a second embodiment of the invention.

The flow pattern is similar to that of figure 2, but is directed in a clockwise direction and is created by eight fuel vapour plumes 40. Like numerals are used to denote like parts. An aperture arrangement suitable to achieve this pattern is shown in figure 4 in which eight apertures 16 are arranged around the wall 14 in the same horizontal plane.

Figure 5 illustrates a vaporised fuel flow pattern for a third embodiment. This flow pattern is similar to that of figure 3 with the exception that no swirl in the horizontal plane is induced in the emergent fuel vapour. An aperture arrangement suitable to achieve such a flow pattern is similar to that of figure 4, but the axes of the apertures 16 pass through a centre point of the vaporising chamber 10.

Cross-sectional views in a vertical plane of alternative piston crowns 18 adapted for use with the second and third embodiments are illustrated in figures 6 and 7, respectively. The piston crown 18 of figure 6 has a number of pockets 42 formed therein toward which, in use, emergent fuel vapour is directed. The pockets 42 are there to break down any bulk air motion into local turbulence zones to enhance rapid mixing of fuel vapour and air. The pockets 42 are not shown as being on the same plane.

It will be understood that the spatial arrangement of pockets 42 can be such as to mirror the arrangement of apertures 16 in the wall 14. These need not be in the same horizontal plane but could have a pin-cushion type arrangement. In fact, in the piston crown 18 of figure 7, pockets formed on different horizontal planes have been joined to provide a number of concentric grooves 44 at respective horizontal planes.

Figure 8 illustrates another embodiment of the invention in which the fuel injector 32 is separated from the vaporising chamber wall 14 by a thermally insulating washer 46. The washer acts to limit heat conduction from the wall 14 to the cooled cylinder head 26.

In figure 9, another embodiment of the invention is illustrated in which a pintle type fuel injector 48 is employed.

Figures 10 and 11 illustrate further aperture arrangements for the wall 14 in which the apertures 16 are evenly spaced around the wall in the same vertical plane. The apertures 16 in figure 11 are angled to enhance swirling motion of ingoing air and emergent fuel vapour.

In prior art IDI diesel engines, a significant problem encountered is the cooling and pumping losses associated with having a pre-combustion chamber mounted within the cylinder head and linked to the main combustion chamber by a narrow throat.

In the present invention, the vaporising chamber is located thermally adjacent the main combustion chamber. "Thermally adjacent" defines the arrangement of the vaporising chamber wall 14 with respect to the cylinder head 2b such that said wall 14 or hot air entering the vaporising chamber 10 bounded by said wall 14 is not cooled by the cylinder head 26.

In a further embodiment (not shown), the vaporising chamber is provided within the cylinder head surrounding the injector nozzle. The chamber is immediately adjacent the main combustion chamber and is arranged such that thermal energy stored in the chamber wall and air present therein is not readily lost to the cooled cylinder head.

The present invention differs from that of DE 2416804 insofar that the vaporising chamber wall has at least one aperture for allowing fuel vapour to pass into the main combustion chamber. However, the essential difference of the present invention over this prior art is the recognition that it is not essential to have a totally enclosed vaporising chamber in which communication between the chamber volume and surrounding volume of the combustion chamber is via the porous structure of the wall.

Thus, it is possible with the present invention to provide a combustion system which has a high tolerance to design variables.

The present invention essentially recognises that, in designing engine combustion systems to meet ever stricter legislated emissions levels, it is necessary not to confine solutions to the present teaching of employing faster/higher pressure injection systems matched at a micro-level to combustion chamber shape. The provision of a vaporising chamber in the manner of the present invention offers many benefits including reduced engine noise and NOx generation. Surprisingly, it has been found that the provision of a vaporising chamber in accordance with the present invention dramatically increases engine design tolerances, so much so that it is possible to employ the combustion system of the present invention to upgrade nonemission compliant engines without modifying the main combustion chamber shape. The invention provides a further advantage in that it may be possible to relax design constraints employed in designing piston combustion chamber shapes to achieve the low emissions attained hitherto.

Whilst the above description of the present invention refers only to diesel i.c. engines, it will be understood that the combustion system of the present invention can be employed in i.c. engines of any type including gas fuelled i.c. engines, for example.

Claims

1. A method of burning fuel in an internal combustion engine, comprising the step of introducing fuel into a vaporising chamber thermally adjacent but separated from a main combustion chamber by a wall having at least one aperture formed therein, wherein thermal energy present in the vaporising chamber at least partially vaporises said introduced fuel, some of the resulting fuel vapour being directed through the aperture(s) into the main combustion chamber containing hot air for combustion thereof.

2. A method as claimed in claim 1, wherein it includes injecting the fuel into the vaporising chamber.

3. A method as claimed in claim 2, wherein it includes injecting the fuel at high pressure into the vaporising chamber.

4. A method as claimed in any one of claims 1 to 3, wherein the thermal energy for vaporising the fuel is provided by a prior combustion event.

5. A method as claimed in any preceding claim, wherein the thermal energy is provided by heating the wall by means of heating elements located within the wall.

6. A method as claimed in any preceding claim, wherein the air present in the vaporising chamber enters said chamber from the main combustion chamber on the compression stroke of the engine.

7. A method as claimed in any preceding claim, wherein it includes directing the vaporised fuel to discrete locations in the main combustion chamber.

8. A method as claimed in any preceding claim, wherein it includes directing the vaporised fuel into the main combustion chamber in such a manner that it creates swirling of said vaporised fuel within the main chamber.

9. A method as claimed in any preceding claim, wherein it includes the step of injecting a small pilot charge of fuel into the vaporising chamber prior to the injection of a main fuel charge in order to initiate fuel combustion, whereby combustion of the pilot charge creates a hot mixing zone within the vaporising chamber into which the main fuel charge is injected.

10. An internal combustion engine having a main combustion chamber contiguous to a face of an engine piston, a vaporising chamber thermally adjacent but separated from said main combustion chamber by a wall having at least one aperture formed therein, and means for introducing fuel into said vaporising chamber, wherein thermal energy present in the vaporising chamber at least partially vaporises introduced fuel, some of the resulting fuel vapour being directed through the aperture(s) into the main combustion chamber containing hot air for combustion thereof.

11. An engine as claimed in claim 10, wherein the means for introducing fuel into the vaporising chamber comprises a fuel injection means.

12. An engine as claimed in claim 11, wherein the fuel injection means is a high pressure fuel injection means.

13. An engine as claimed in any one of claims 10 to 12, wherein the vaporising chamber is immediately adjacent the main combustion chamber.

14. An engine as claimed in claim 13, wherein the vaporising chamber extends into the main combustion chamber.

15. An engine as claimed in claim 14, wherein the vaporising chamber is contained completely within the main combustion chamber.

16. An engine as claimed in any one of claims 10 to 15, wherein the vaporising chamber wall has two or more apertures.

17. An engine as claimed in claim 16, wherein the apertures are dimensioned and positioned so as to direct fuel vapour from the vaporising chamber to discrete locations in the main combustion chamber.

18. An engine as claimed in claim 17, wherein the discrete locations comprise pockets formed in the face of the engine piston.

19. An engine as claimed in claim 18, wherein the number of apertures in the wall equals the number of pockets in the piston crown face.

20. An engine as claimed in any one of claims 10 to 19, wherein the vaporising chamber wall is made from a heat retentive material such as a ceramic material.

21. An engine as claimed in any one of claims 10 to 20, wherein the vaporising chamber wall is connected to a face of an engine cylinder head in which the fuel injection means is mounted.

22. An engine as claimed in any one of claims 10 to 20, wherein the vaporising chamber wall is connected to an injection nozzle part of the fuel injection means.

23. An engine as claimed in claim 22, wherein the vaporising chamber wall has an outer diameter less than or equal to a largest diameter of a body part of the fuel injector means which inserts into a bore in the cylinder head.

24. An engine as claimed in claim 22 or claim 23, wherein the vaporising chamber wall is formed integrally with the nozzle part of the fuel injection means.

25. An engine as claimed in any one of claims 21 to 23, wherein the vaporisation chamber wall is separated from the cylinder head/nozzle part of the fuel injector means by a thermally insulating means.

26. An engine as claimed in claim 25, wherein the thermally insulating means comprises a washer formed of thermally insulating material.

27. A fuel injector for an i.c. engine having an injector nozzle and a wall arranged to surround said nozzle to define a fuel vaporising chamber between said nozzle and an inner surface of the wall, wherein the wall has at least one aperture formed therein.

28. A fuel injector as claimed in claim 27, wherein the vaporising chamber wall has an outer diameter less than or equal to a largest diameter of a body part of the fuel injector which, in situ, inserts into a bore of an engine cylinder head.

29. A fuel injector as claimed in claim 27 or claim 28, wherein the wall is formed integrally with the injector nozzle.

30. A fuel injector as claimed in claim 27 or claim 28, wherein the wall is separated from the injector nozzle by a thermally insulating means such as a thermally insulating washer.

31. A method substantially as hereinbefore described with reference to figure 2 (a and b).

32. A method substantially as hereinbefore described with reference to figures 3 and 4.

33. A method substantially as hereinbefore described with reference to figure 5.

34. A method substantially as hereinbefore described with reference to figure 8.

35. A method substantially as hereinbefore described with reference to figure 9.

36. An engine substantially as hereinbefore described with reference to figure 2 (a and b).

37. An engine substantially as hereinbefore described with reference to figures 3 and 4.

38. An engine substantially as hereinbefore described with reference to figure 5.

39. An engine substantially as hereinbefore described with reference to figure 8.

40. An engine substantially as hereinbefore described with reference to figure 9.

41. An injector substantially as hereinbefore described with reference to figure 2 (a and b).

42. An injector substantially as hereinbefore described with reference to figures 3 and 4.

43. An injector substantially as hereinbefore described with reference to figure 5.

44. An injector substantially as hereinbefore described with reference to figure 8.

45. An injector substantially as hereinbefore described with reference to figure 9.