Gaseous-fuelled internal combustion engine with a multi-position laser ignition system

Abstract

An internal combustion engine 100 comprising a laser ignition system 200B for igniting gaseous fuel (hydrogen or methane) within a combustion chamber 110 of the engine. The laser ignition system 200B comprises a laser source 210 and optics 201B. The optics 201B comprise a first focuser 251, and a first optical window 261 at a boundary of the combustion chamber, the focuser 251 and the optical window 261 arranged to focus and transmit laser light from the laser source apparatus 210 to ignite the gaseous fuel position 271 in the combustion chamber. The optics 201B further comprise a second focuser 252, and a second optical window 262 similarly arranged to focus and transmit laser light from the laser source apparatus 210 to ignite the fuel at a second position 272. There may be up to six ignition positions within the combustion chamber, and the engine may be used in a vehicle.

Description

TECHNICAL FIELD The present disclosure relates to a gaseous-fuelled internal combustion engine with a multi-position laser ignition system. Aspects of the invention relate to the internal combustion engine and to a vehicle. BACKGROUND Lean burning of gaseous fuels (‘gas’), such as hydrogen or methane, along with moderate levels of exhaust gas recirculation, provides the advantages of high brake thermal efficiency, low levels of nitrogen oxides, and reduction of abnormal combustion. Although the laminar flame speed of gaseous fuels is faster than that of liquid fuels such as gasoline, it is not fast enough to prevent slow burning in very lean gas-air mixtures. This could lead to misfiring, poor combustion stability, and increased emissions of unburned gaseous fuel. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide an internal combustion engine and a vehicle as claimed in the appended claims. According to an aspect of the present invention there is provided an internal combustion engine comprising a laser ignition system for igniting gaseous fuel within a combustion chamber of the internal combustion engine, the laser ignition system comprising a laser source apparatus and optics, the optics comprising: a first focuser, and a first optical window at a boundary of the combustion chamber, the first focuser and the first optical window arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at a first position in the combustion chamber; and a second focuser, and a second optical window at the boundary of the combustion chamber, the second focuser and the second optical window arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at a second position in the combustion chamber. The laser ignition system can therefore be described as a multi-position (multi-point) laser ignition system with separate optical windows for the different positions (ignition positions). An advantage is enabling a faster burn rate of a lean gas-air mixture, due the ability to ignite the gas-air mixture at multiple locations inside the combustion chamber during a single combustion event. An advantage of using the laser ignition system rather than spark plugs is that hot spots inside the combustion chamber are avoided, reducing the risk of auto-ignition. Due to the low ignition energy requirement of gaseous fuels, low laser fluence would be sufficient to initiate the combustion. Advantages of using separate optical windows with separate focusers is that these optics are easier to package within the confined space proximal to the combustion chamber, and that there is a high degree of freedom over the locations of the ignition positions within the combustion chamber. If separate laser emitters are used for the ignition positions, the characteristics of the laser fluence at each ignition position can be controlled. The focusers may be in the form of focusing lenses or focusing mirrors. The optical windows may be in the form of quartz windows. The optical windows may be located in an engine block of the internal combustion engine, or in a cylinder head of the internal combustion engine. The focusers may be separate devices than the optical windows, or the optical windows themselves may be focusing windows, the focusers being curved focusing surfaces of the focusing windows. The optics may further comprise a third focuser, and a third optical window at the boundary of the combustion chamber, the third focuser and the third optical window arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at a third position in the combustion chamber. The optics may further comprise a fourth focuser, and a fourth optical window at the boundary of the combustion chamber, the fourth focuser and the fourth optical window arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at an optional fourth position in the combustion chamber. The optics may provide a total number of ignition positions in the combustion chamber consisting of two to six, or four to six, ignition positions. The fifth position may be defined by a fifth focuser and a fifth optical window at the boundary of the combustion chamber, together arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at the fifth position in the combustion chamber. If there is a sixth position, the sixth position may be defined by a sixth focuser and a sixth optical window at the boundary of the combustion chamber, together arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at the sixth position in the combustion chamber. An advantage of the upto six positions is faster and more complete combustion. Adding more than six positions may result in a diminishing combustion benefit, while adding mass to the vehicle. Some, most, or all of the positions may be nonlinearly spatially distributed over an area within the combustion chamber. Some, most, or all of the positions may be circumferentially equispaced. Some, most, or all of the positions may be located away from a central piston axis of the combustion chamber. Some, most, or all of the positions may be approximately equidistant from the central piston axis. Some, most, or all of the positions may be at a distance no greater than 0.6R or no greater than 0.8R from the central piston axis, wherein R is a radius of the combustion chamber. An advantage of the positions being distributed in such a manner is that the positions provide more complete coverage of the cross-sectional area of the combustion chamber, and therefore faster and more complete combustion. However, they are not too close to the combustion chamber wall, therefore reducing wall heating. Some, most, or all of the positions may be above and aligned with a piston bowl of a piston of the internal combustion engine. The piston bowl is a combustion cavity formed in a piston crown of the piston. The piston bowl may have curved walls. The piston bowl may be surrounded by an upper surface portion of the piston crown. Alignment with the piston bowl may comprise the positions being above the piston bowl and between the upper surface portion and the central piston axis. Alignment with the piston bowl may comprise the positions being aligned with the curved walls of the piston bowl, the curved walls joining the piston bowl to the upper surface portion. An advantage is that the positions are aligned with the location where the gas-air mixture is most homogeneous due to the mixing effect of the piston bowl, and therefore easier to ignite at lean mixtures. The positions are also away from the combustion chamber wall, reducing wall heating. Some, most, or all of the positions may be located to both an intake side and an exhaust side of the combustion chamber. Different ones of the positions may be proximal to different valve ports formed in the internal combustion engine. Different ones of the positions may be proximal to different valve pockets formed in a piston of the internal combustion engine. The valve pockets may be formed in a piston crown of the piston. The valve pockets may be formed in an upper surface portion of the piston crown. The laser source apparatus may comprise a laser emitter configured to emit a beam of laser light, the optics comprising a beam splitter arrangement configured to direct a first portion of the beam to the first focuser, and to direct a second portion of the beam to the second focuser. The beam splitter arrangement may comprise a first beam splitter to direct the first portion of the beam, and a second beam splitter to direct the second portion of the beam. The first and second portions may be reflected portions of the beam. The second beam splitter may be configured to receive a transmitted portion of the beam from the first beam splitter. Where the third and fourth positions may be provided, the beam splitter arrangement may be further configured to direct a third portion of the beam to the third focuser, and to direct a fourth portion of the beam to the fourth focuser. The beam splitter arrangement may comprise a third beam splitter to direct the third portion of the beam, and a fourth beam splitter to direct the fourth portion of the beam. The third and fourth portions may be reflected portions of the beam. The third beam splitter may be configured to receive a transmitted portion of the beam from the second beam splitter. The fourth beam splitter may be configured to receive a transmitted portion of the beam from the third beam splitter. The optics may comprise mirrors between the beam splitter arrangement and the focusers, the mirrors configured to steer the respective portions to the respective focusers. The optics may comprise a beam blocker arranged to absorb a further portion of the beam from the beam splitter arrangement at a location external to the combustion chamber. The further portion may be a transmitted portion of the beam from a last beam splitter of the beam splitter arrangement. An advantage is that the number of laser emitters is minimised. For example, a single laser emitter can produce a single beam that is split by the beam splitter arrangement. Although a last beam splitter will receive the lowest fluence due to transmission losses through intermediate beam splitters, a low energy laser emitter can still produce at least enough energy at the last position to ignite a gas-air mixture. The beam splitters and the beam blocker may be linearly aligned. An advantage of the linear alignment is that the beam splitters and beam blocker can be conveniently aligned along an optical waveguide (e.g., manifold) extending across the combustion chamber, externally from the combustion chamber. In other examples, the laser source apparatus may comprise a plurality of laser emitters for the combustion chamber. The plurality of laser emitters may be configured to supply laser light to the positions within the combustion chamber. An advantage is that transmission losses are reduced because the number of beam splitters in series can be reduced or eliminated, and that the laser fluence at different ones of the positions can be controlled separately, if required. The plurality of laser emitters for the combustion chamber may comprise individual laser emitters for individual ones of the plurality of positions. The number of laser emitters may be equal to the number of the positions. Beam splitters may therefore not be required. The optics may comprise fibre optic cables to couple the laser emitters to the focusers. In other examples, the number of laser emitters for each combustion chamber may be fewer than the number of the positions. For example, the optics may comprise a first beam splitter arrangement configured to direct a first portion of laser light from a first of the plurality of laser emitters to one of the focusers, and to direct a second portion of the laser light from the first laser emitter to another of the focusers, and the optics further comprising a second beam splitter arrangement configured to direct a first portion of laser light from a second of the plurality of laser emitters to a further one of the focusers. If there are one or more further focusers and corresponding positions, the second beam splitter arrangement may also be configured to direct a further one or more portions of the laser light from the second laser emitter to the further one or more of the focusers. Where multiple laser emitters and beam splitter arrangements may be provided, the optics may comprise a plurality of beam blockers. A first beam blocker may be arranged to absorb a further portion of the beam from the first beam splitter arrangement at a location external to the combustion chamber. The further portion may be a transmitted portion of the beam from a last beam splitter of the first beam splitter arrangement. A second beam blocker may be arranged to absorb a further portion of the beam from the second beam splitter arrangement at a location external to the combustion chamber. The further portion may be a transmitted portion of the beam from a last beam splitter of the second beam splitter arrangement. The laser source apparatus may be configured to generate an output of pulsed laser light. The laser source apparatus may be configured to generate the output in the ultraviolet, visible, or infrared spectrum. The optics may comprise first and second beam directing devices arranged to direct laser light from the laser source apparatus to the first and second focusers, respectively. Each beam directing device may comprise a mirror or a fibre optic cable. For example, where individual laser emitters may be provided for individual ones of the plurality of positions, the earlier-described fibre optic cables may couple the laser emitters to the focusers of the respective positions. Or, where one or more beam splitter arrangements are provided, the earlier-described mirrors may be provided to steer the laser light from the beam splitter arrangements) to the respective focusers. According to another aspect of the invention, there is provided a vehicle comprising the internal combustion engine. According to a further of the present invention there is provided a laser ignition system for a gaseous-fuelled internal combustion engine, the laser ignition system comprising a laser source apparatus and optics, collectively defining: a first optical path between the laser source apparatus and a combustion chamber, to provide a first laser ignition position within a combustion chamber; and a second optical path between the laser source apparatus and the combustion chamber, to provide a second laser ignition position within the combustion chamber. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination that falls within the scope of the appended claims. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination that falls within the scope of the appended claims, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 illustrates a schematic view illustrating an example of a vehicle; FIG. 2 illustrates a schematic side cross-section view illustrating an example of a combustion chamber; FIG. 3 illustrates a schematic view illustrating an example of a laser ignition system; FIG. 4 illustrates a schematic view illustrating another example of a laser ignition system; FIG. 5 illustrates a schematic view illustrating a further example of a laser ignition system; and FIG. 6 illustrates a schematic view illustrating a portion of a further example of a laser ignition system. DETAILED DESCRIPTION A vehicle 1 in accordance with an embodiment of the present invention is described herein with reference to the accompanying FIG. 1. In some, but not necessarily all examples, the vehicle 1 is a passenger vehicle, also referred to as a passenger car or as an automobile. In other examples, embodiments of the invention can be implemented for other applications, such as commercial vehicles or static plant and machinery. A gaseous-fuelled internal combustion engine 100 (‘engine’) is schematically labelled in FIG. 1, such as a hydrogen engine or a methane engine. The engine 100 may be a reciprocating piston engine, comprising one or more combustion chambers containing pistons. The engine 100 may be operable in a four-stroke combustion cycle, for example. A gaseous-fuelled engine 100 (e.g., hydrogen or methane engine) differs from a liquid-fuelled engine (e.g., gasoline or diesel) in several ways. A gaseous-fuelled engine 100 can include, among other things: hardened and stronger moving parts and head gaskets; and gas fuel injectors rather than liquid fuel injectors. Unlike gasoline or diesel engines, a controller of a hydrogen-fuelled engine may be mapped to enable air-gas mixtures leaner than the hydrogen stoichiometric ratio of 34:1. For example, the controller of a hydrogen-fuelled engine may be mapped to enable air-gas mixtures leaner than 50:1, or leaner than 70:1, or leaner than 100:1. A cross-section of a combustion chamber 110 (cylinder) of the engine 100 is schematically illustrated in FIG. 2. The engine 100 comprises an engine block 102 and a cylinder head 104. The cylinder head 104 comprises a plurality of valve ports 106,108 for the combustion chamber 110, including intake valve ports 106 and exhaust valve ports 108. A piston 120 is arranged to move in a reciprocating motion within the combustion chamber 110, along a central piston axis A of the combustion chamber 110. The engine 100 comprises a cylinder liner, the inside surface of which defines a combustion chamber wall 112. Piston rings (not shown) of the piston 120 are arranged to slide along the combustion chamber wall 112. A gas injector (not shown) may be arranged for direct injection and may be centrally mounted or side mounted with respect to the combustion chamber 110. Alternatively, the gas injector may be arranged for indirect injection and may be a port fuel injector. The piston 120 comprises a piston crown 122, referring to the upper surface of the piston 120 facing the cylinder head 104. In FIG. 2, the piston crown 122 has a complex geometry, including a piston bowl 124. The piston bowl 124 is a combustion cavity formed in the piston crown 122 of the piston 120. The piston bowl 124 has curved walls 126. The curved walls 126 of the piston bowl 124 are connected to an outer upper surface 6 portion 128 (uppermost surface portion) ofthe piston crown 122, which surrounds the piston bowl 124, In other examples, the piston crown 122 is flatter, not having a piston bowl 124. The later FIGS 3-6 illustrate that the upper surface portion 128 ofthe piston crown 122 can further comprise valve pockets 131, 132, 133, 134, which are recessed portions ofthe upper surface portion 128. The valve pockets 131, 132, 133,134 are aligned with poppet valve heads to prevent contact between the poppet valve heads and the piston crown 122. Some engines do not require valve pockets in the piston 120. Turning now to FIGS. 3-6, the engine 100 comprises a laser ignition system 200A, 200B, 200C instead of conventional spark plugs. FIGS. 3-6 show different example implementations ofthe laser ignition system 200A, 200B, 200C. First, the common features are defined. FIGS. 3-6 each illustrate an engine 100 comprising a laser ignition system 200A, 200B, 200C for igniting gaseous fuel within a combustion chamber 110 ofthe engine 100, the laser ignition system 200A, 200B, 200C comprising a laser source apparatus 210 and optics 201 A, 201B, 201C, the optics 201A, 201B, 201C comprising: a first focuser 251, and a first optical window 261 at a boundary ofthe combustion chamber 110, the first focuser 251 and the first optical window 261 arranged to focus and transmit a beam of laser light from the laser source apparatus 210 to ignite the gaseous fuel at a first position (ignition position) 271 in the combustion chamber 110; a second focuser 252, and a second optical window 262 at the boundary ofthe combustion chamber 110, the second focuser 252 and the second optical window 262 arranged to focus and transmit laser light from the laser source apparatus 210 to ignite the gaseous fuel at a second ignition position 272 in the combustion chamber 110; optionally, a third focuser 253, and a third optical window 263 at the boundary of the combustion chamber 110, the third focuser 253 and the third optical window 263 arranged to focus and transmit laser light from the laser source apparatus 210 to ignite the gaseous fuel at an optional third ignition position 273 in the combustion chamber 110; optionally, a fourth focuser 254, and a fourth optical window 264 at the boundary of the combustion chamber 110, the fourth focuser 254 and the fourth optical window 264 arranged to focus and transmit laser light from the laser source apparatus 210 to ignite the gaseous fuel at an optional fourth ignition position 274 in the combustion chamber 110; optionally (FIG. 6 only), a fifth focuser 255, and a fifth optical window 265 at the boundary of the combustion chamber 110, the fifth focuser 255 and the fifth optical window 265 arranged to focus and transmit laser light from the laser source apparatus 210 to ignite the gaseous fuel at an optional fifth ignition position 275 in the combustion chamber 110; and optionally (FIG. 6 only), a sixth focuser 256, and a sixth optical window 266 at the boundary ofthe combustion chamber 110, the sixth focuser 256 and the sixth optical window 266 arranged to focus and transmit laser light from the laser source apparatus 210 to ignite the gaseous fuel at an optional sixth ignition position 276 in the combustion chamber 110. An advantage of the plurality of ignition positions 271,272, 273, 274, 275, 276 is enabling a faster burn rate of a lean gas-air mixture due the ability to ignite the gas-air mixture at multiple locations inside the combustion chamber 110 during a single combustion event. An advantage of using the laser ignition system 200A, 200B, 200C rather than spark plugs is that hot spots inside the combustion chamber 110 are avoided, reducing the risk of pre-ignition, i.e., abnormal combustion. Due to the low ignition energy requirement of gaseous fuels, low laser fluence would be sufficient to initiate the combustion. Therefore, a low-power laser source apparatus 210 can be used. Four to six ignition positions 271,272, 273, 274, 275, 276 are shown. Depending on the implementation, the number of ignition positions may be selected from the range two to six ignition positions. An advantage of increasing the number of ignition positions is faster and more complete combustion, allowing for leaner mixtures and further increases of exhaust gas recirculation. However, adding more than four to six ignition positions may result in a diminishing rate of improvement of combustion efficiency, while adding mass to the vehicle 1 as well as complexity. Advantages of using multiple optical windows 261,262, 263, 264, 265, 266 with separate focusers 251,252, 253, 254, 255, 256 is that the focusers are easier to package within the confined space proximal to the combustion chamber 110, and that there is a high level of freedom over the locations of the ignition positions 271,272, 273, 274, 275, 276 within the combustion chamber 110. The focusers 251,252, 253, 254, 255, 256 are illustrated as focusing lenses. In other examples, the focusers 251,252, 253, 254, 255, 256 may be focusing mirrors. Each focuser 251,252, 253, 254, 255, 256 is illustrated as being a convex lens configured to reduce the beam diameter from the laser source apparatus 210, and having a focal length configured to focus the beam at a predetermined focal point within the combustion chamber 110. The ignition position 271,272, 273, 274, 275, 276 is defined as the focal point defined by the focuser 251,252, 253, 254, 255, 256. The laser fluence reaches a peak at the focal point, to provide sufficient laser energy for ignition at the ignition position 271,272, 273, 274, 275, 276, In the illustrated example, the number of focusers 251,252, 253, 254, 255, 256 equals the number of ignition positions 271, 272, 273, 274, 275, 276. Each focuser 251, 252, 253, 254, 255, 256 is a monofocal lens. In other examples, a multifocal lens may be used to produce a plurality of the ignition positions 271, 272, 273, 274, 275, 276. Therefore, for example, a subset of the ignition positions (e.g., 271,274) may be different focal points of a single multifocal lens, and another subset of the ignition positions (e.g., 272, 273) may be different focal points of another multifocal lens. The focusers 251,252, 253, 254, 255, 256 are located outside the combustion chamber 110, therefore, optical windows 261, 262, 263, 264, 265, 266 are provided between the focusers 251,252, 253, 254, 255, 256 and the ignition positions 271, 272, 273, 274, 275, 276. The optical windows 261,262, 263, 264, 265, 266 are located at a boundary of the combustion chamber 110, and provide an optically transparent window from outside the engine 100 to inside the combustion chamber 110. The boundary of the combustion chamber 110 refers to the combustion chamber wall 112 and the interior surface of the cylinder head 104. The optical windows 261, 262, 263, 264, 265, 266 may be located at the boundary at various locations around the boundary of the combustion chamber 110. The optical windows 261, 262, 263, 264, 265, 266 and ignition positions 271, 272, 273, 274, 275, 276 may be at a constant or similar elevation, close to top dead centre. Due to the high combustion pressures and temperatures inside the combustion chamber 110, each optical window 261,262, 263, 264, 265, 266 may be a quartz window due to its low coefficient of thermal expansion and high thermal shock resistance. The optical windows 261, 262, 263, 264, 265, 266 and corresponding window sockets (not labelled) in the boundary of the combustion chamber 110 may be shaped (e.g., tapered) to require the optical windows 261,262, 263, 264, 265, 266 to be inserted into the sockets from outside the combustion chamber 110, during manufacture. The optical windows 261,262, 263, 264, 265, 266 may be secured to the surfaces to which they are mounted by adhesive, and gaskets may separate the optical windows 261,262, 263, 264, 265, 266 from the surfaces to which they are mounted. A window holder (not shown) may be secured, e.g., bolted, to the outside of the engine to further secure the optical windows 261,262, 263, 264, 265, 266. In one implementation, the optical windows 261,262, 263, 264, 265, 266 are located in the engine block 102, proximal to or located at the top edge of the engine block 102. Therefore, the optical windows 261,262, 263, 264, 265, 266 extend from outside the engine block 102 to the combustion chamber wall 112. However, if the position of the piston 120 would obstruct line of sight from the optical windows 261,262, 263, 264, 265, 266 to the ignition positions 271, 272, 273, 274, 275, 276 at the desired moment of initiation of ignition, one or more of the optical windows 261,262, 263, 264, 265, 266 may be alternatively located in the cylinder head 104 ofthe engine 100, e.g., between valve seats. If the focusers 251,252, 253, 254, 255, 256 are external to the engine block 102 and cylinder head 104, the system 200A, 200B, 200C may comprise waveguides (e.g., light tubes) to optically couple each focuser 251, 252, 253, 254, 255, 256 to the associated optical window 261,262, 263, 264, 265, 266, to reduce the chance of unintentional beam obstruction. Or, the focusers 251, 252, 253, 254, 255, 256 may be located in one or more cavities ofthe engine block 102 or cylinder head 104. In the illustrated example, the focusers 251,252, 253, 254, 255, 256 and optical windows 261,262, 263, 264, 265, 266 are separate devices. However, in other implementations, the optical windows 261,262, 263, 264, 265, 266 and focusers 251,252, 253, 254, 255, 256 may be the same device. A single focusing window 261, 262, 263, 264, 265, 266 may be a convex lens, the convex surfaces ofthe focusing window functioning as the focuser 251,252, 253, 254, 255, 256. The illustrated focusers 251, 252, 253, 254, 255, 256, optical windows 261, 262, 263, 264, 265, 266, and ignition positions 271, 272, 273, 274, 275, 276 are spatially distributed around the central piston axis A (FIG. 9 2), They may form a ring of positions. The illustrated ignition positions 271,272,273,274, 275, 276 are spatially distributed over an area within the combustion chamber 110. This means that they are nonlinear, meaning that the ignition positions 271, 272, 273, 274, 275, 276 are not all aligned along one single axis. As shown, the ignition positions 271,272, 273, 274, 275, 276 may be symmetrically distributed around the central piston axis A, which is advantageous for combustion stability. Whereas a spark plug is often coaxial with or proximal to the central piston axis A, in a central area of the cylinder head 104 between the intake and exhaust valve ports 106, 108, the illustrated ignition positions 271, 272, 273, 274, 275, 276 are located away from the central piston axis A. To limit wall heating, the ignition positions 271, 272, 273, 274, 275, 276 may be separated from the central piston axis A by a distance no greater than 0.6R or no greater than 0.8R from the central piston axis A, where R is the radius of the combustion chamber 110. The optical windows 261, 262, 263, 264, 265, 266 may correspondingly be located outboard of the central area of the cylinder head 104, for example between the intake and exhaust valve seats, or located in the combustion chamber wall 112 of the engine block 102. Some, most, or all of the ignition positions 271, 272, 273, 274, 275, 276 may be approximately equidistant from the central piston axis A. The ignition positions 271, 272, 273, 274, 275, 276 are above and aligned with the piston bowl 124 of the piston crown 122. The ignition positions 271,272, 273, 274, 275, 276 are inboard of the upper surface portion 128 of the piston crown 122, meaning between the upper surface portion 128 and the central piston axis A. The four or six laser ignition positions 271,272, 273, 274, 275, 276 are illustrated as being located above the curved walls 126 of the piston bowl 124, i.e., closer to a rim of the piston bowl 124 than to the central piston axis A. It is advantageous to align the ignition positions 271,272, 273, 274, 275, 276 directly above the curved walls 126 of the piston bowl 124 because a more homogeneous gas-air mixture occurs towards the rim of the piston bowl 124, and because the ignition positions 271,272, 273, 274, 275, 276 are near equidistant between the central piston axis A and the combustion chamber wall 112 to provide a ring of uniform coverage around the combustion chamber 110. In examples, one or more of the ignition positions 271,272 are located at the intake side of the combustion chamber 110, proximal to the intake valve pockets 131,132 and intake valve ports 106 / seats. One or more of the ignition positions 273, 274 are located at the exhaust side of the combustion chamber 110, proximal to the exhaust valve pockets 133, 134 and exhaust valve ports 108 / seats. It is advantageous to have at least some ignition positions 271,272, 273, 274 at the intake and exhaust sides of the combustion chamber 110 because combustion kernel and flame development is not symmetric in all planes and it is common for the centre of combustion to migrate to one side of the chamber 110 during combustion itself. This can result in knock / abnormal combustion of the end-gas at the opposing side of the combustion chamber 110. Having ignition sites on the intake and exhaust sides of the combustion chamber 110 will seek to minimise knock as a result of combustion asymmetry. FIG. 3 illustrates a first implementation of a laser ignition system 200A and optics 201 A, in this case arranged to create four ignition positions 271,272, 273, 274 within the combustion chamber 110. The number of ignition positions may vary. The laser ignition system 200A comprises a laser source apparatus 210 in the form of a single laser emitter 211. The optics 201A of the laser ignition system 200A further comprise a beam splitter arrangement 220A of beam splitters 221,222, 223, 224 to divide the single beam B1 into a plurality of beam portions B1R1, B1R2, B1R3, B1R4 which are each directed to the focusers 251, 252, 253, 254 via beam directing devices, in this case mirrors 241,242,243,244. The beam portions B1R1, B1R2, B1R3, B1R4 represent a plurality of different optical paths from the laser source apparatus 210 to the combustion chamber 110. The laser emitter 211 is orientated parallel or mostly parallel to a crankshaft axis of the engine 100, and perpendicular to the central piston axis A, so that its beam B1 passes between the intake and exhaust sides of the combustion chamber 110. A beam blocker 281 is located after the last beam splitter 224, to absorb a final incident portion of the beam. If the beam splitter arrangement 220Aand the beam blocker 281 are external to the engine block 102 and cylinder head 104, they may be contained within a suitable waveguide, such as a manifold with outputs for the reflected beam portions B1R1, B1R2, B1R3, B1R4. If internal, they may be contained within a passage extending along the engine block 102 or cylinder head 104. The illustrated beam splitter arrangement 220A forms a plurality of 90-degree reflected beam portions B1R1, B1R2, B1R3, B1R4, and the mirrors 241,242, 243, 244 steer (reflect) the beam portions B1R1, B1R2, B1R3, B1R4 towards the focusers 251,252, 253,254. The illustrated mirrors 241,242, 243, 244 are angled at approximately 45 degrees to the incident beam portions B1R1, B1R2, B1R3, B1R4 to reflect the beam portions B1R1, B1R2, B1R3, B1R4 by approximately 90 degrees, to a direction approximately parallel to the beam B1 of the laser emitter 211. In other examples the mirrors 241,242, 243, 244 reflect the beam portions B1R1, B1R2, B1R3, B1R4 towards the focusers 251,252, 253, 254 at a different angle, for example mostly radially or radially towards the central piston axis A. The illustrated beam splitter arrangement 220A comprises four beam splitters 221, 222, 223, 224 in series between the laser emitter 211 and the beam blocker 281. The number of beam splitters in this example correspond to the variable number of ignition positions 271,272, 273, 274. The first beam splitter 221 receives the incident beam B1 from the laser emitter 211, and splits the beam B1 into a first reflected portion B1R1 and a first transmitted portion B1T1. The first reflected portion B1R1 is directed to the first mirror 241 which reflects the portion B1R1 to the first focuser 251. The first transmitted portion B1T1 is directed to the second beam splitter 222. The second beam splitter 222 receives the first transmitted portion B1T1 from the first beam splitter 221, and splits the first transmitted portion B1T1 into a second reflected portion B1R2 and a second transmitted portion 11 B1T2. The second reflected portion B1R2 is directed to the second mirror 242 which reflects the portion B1R2 to the second focuser 252. The second transmitted portion B1T2 is directed to the third beam splitter 223. The third beam splitter 223 receives the second transmitted portion B1T2 from the second beam splitter 222, and splits the second transmitted portion B1T2 into a third reflected portion B1R3 and a third transmitted portion B1T3. The third reflected portion B1R3 is directed to the third mirror 243 which reflects the portion B1R3 to the third focuser 253. The third transmitted portion B1T3 is directed to the fourth beam splitter 224. The fourth beam splitter 224 receives the third transmitted portion B1T3 from the third beam splitter 223, and splits the third transmitted portion B1T3 into a fourth reflected portion B1R4 and a fourth transmitted portion B1T4. The fourth reflected portion B1R4 is directed to the fourth mirror 244 which reflects the portion B1R4 to the fourth focuser 254. The fourth transmitted portion B1R4 is directed to the beam blocker 281. The first to fourth reflected portions B1R1, B1R2, B1R3, B1R4 of the beam may be contained within suitable waveguides extending from the beam splitter arrangement 220A to the mirrors 241, 242, 243, 244. Suitable waveguides may be provided between the mirrors 241,242, 243, 244 and the focusers 251,252, 253, 254. In another implementation, the focusers 251,252, 253, 254 themselves are focusing mirrors, and the mirrors 241, 242, 243, 243 are not required. However, an advantage of having mirrors 241, 242, 243, 244 separate from focusers 251, 252, 253, 254 is that there is greater freedom over how the beam splitter arrangement 220A are focusers 251, 252, 253, 254 can be packaged relative to each other, because the beam can be steered as required by the mirrors 241,242, 243, 244. The laser emitter 211 is configured to generate the beam B1 in the ultraviolet, visible, or infrared spectrum. The laser emitter 211 may be configured to generate a pulsed output. A base pulse frequency of the laser emitter 211 may be controlled by a controller via Q-switching, to synchronise with engine speed. An encoder signal synchronised to engine speed and a controller can be used to generate a pulse for every firing cycle (combustion event) of the combustion chamber 110. The controller can also advance and retard the pulses within individual firing cycles. The first laser emitter 211 may be a fundamental beam (1064nm), or from a second (532nm), third (355 nm), or fourth (266nm) harmonic generator. A 1064nm laser from a low-power laser emitter 211 can provide approximately WOOrnJ (1V) of energy, which is more than enough to produce the required energy at the ignition positions 271, 272, 273, 274. Although transmission losses through the beam splitters 221,222, 223, 224 will reduce the energy of the beam at the fourth ignition position 274, a minimum energy of 8-12mJ / pulse at the ignition position would be achievable and sufficient to trigger the ignition due to the low ignition energy requirement of gaseous fuels (e.g., hydrogen). FIGS. 4-5 illustrate second and third example laser ignition systems 200B, 200C in which the laser source apparatus 210 comprises a plurality of laser emitters 211,212, 213, 214, or 211,212, for the same combustion 12 chamber 110. An advantage is that transmission losses are reduced because the number of beam splitters is reduced or eliminated, and the laser fluence at separate ignition positions 271,272, 273, 274 can be controlled separately by controlling the laser emitters 211,212, 213, 214, or 211,212, separately. FIG. 4 illustrates the second example laser ignition system 200B and optics 201B. FIG. 4 illustrates individual laser emitters 211, 212, 213, 214 transmitting (outputting) respective beams B1, B2, B3, B4 of laser light to individual ones of the ignition positions 271,272, 273, 274, via optics 201B. The number of laser emitters 211, 212, 213, 214 is equal to the number of the ignition positions 271, 272, 273, 274, which may vary. Beam splitters are therefore not be required. The laser emitters 211,212, 213, 214 in this example are fibre-coupled laser emitters, wherein beam directing devices in the form of fibre optic cables 231, 232, 233, 234 optically couple each laser emitter 211,212, 213, 214 to the respective focuser 251,252, 253, 254. In the illustrated implementation, beams B1, B2, B3, B4 from four pulsed fibre-coupled laser emitters 211,212, 213, 214 (within the infrared range) are directed to four ignition positions 271, 272, 273, 274 inside the combustion chamber 110, via the focusers 251, 252, 253, 254 and optical windows 261, 262, 263, 264. The beams B1, B2, B3, B4 represent a plurality of different optical paths from the laser source apparatus 210 to the combustion chamber 110. This robust system will eliminate some of the optical components such as the beam splitters 231,232, 233, 234 and mirrors 241,242, 243, 244. The laser emitters 211,212, 213, 214 may be external to the engine block 102 and cylinder head 104. In FIG. 4, two laser emitters 211,212 are shown located to a first side (e.g., intake side) of the combustion chamber 110, and two laser emitters 213, 214 are shown located to the second opposite side (e.g., exhaust side) of the combustion chamber 110. However, the fibre optic cables 231,232, 233, 234 may be flexed, and / or mirrors may be provided, so that the laser emitters 211,212, 213, 214 can be relocated wherever suitable packaging space can be found. In other examples, the fibre optic cables 231,232, 233, 234 may be changed for different types of straight or curved or flexible waveguides. FIG. 5 illustrates the third example laser ignition system 200C and optics 201C, being a variant of FIG. 3. The laser source apparatus 210 comprises a pair of laser emitters 211,212, and the optics 201C comprise a pair of beam splitter arrangements 220B, 220C and a pair of beam blockers 281,282. The first laser emitter 211, first beam splitter arrangement 220B, focusers 251,252, and optical windows 261, 262 are configured to generate and direct laser energy to a first subset of the ignition positions 271,272. The second laser emitter 212, second beam splitter arrangement 220C, focusers 253, 254, and optical windows 263, 264, are configured to generate and direct laser energy to a second different subset of the ignition positions 273, 274. The first and second laser emitters 211,212 may be external to the engine block 102 and cylinder head 104. The first and second laser emitters 211,212 are shown opposite each other and facing each other. The first 13 laser emitter 211 is located to one side (e.g., intake side) of the combustion chamber 110 and the second laser emitter 212 is located to the other side (e.g., exhaust side) of the combustion chamber 110. An initial beam splitter 221 of the first beam splitter arrangement 220B receives the beam B1 from the first laser emitter 211, and splits the beam B1 into a first reflected portion B1R1 and a first transmitted portion B1T1. The first reflected portion B1R1 is directed to the first mirror 241 which reflects the portion B1R1 to the first focuser 251. The first transmitted portion B1T1 is directed to another beamsplitter 222 of the first beam splitter arrangement 220B. The other beam splitter 222 of the first beam splitter arrangement 220B receives the first transmitted portion B1T1 from the first beam splitter 221, and splits the first transmitted portion B1T1 into a second reflected portion B1R2 and a second transmitted portion B1T2. The second reflected portion B1R2 is directed to the second mirror 242 which reflects the portion B1R2 to the second focuser 252. The second transmitted portion B1T2 is directed to the first beam blocker 281. The second beam splitter arrangement 220C is similar. An initial beam splitter 224 of the second beam splitter arrangement 220C receives the beam B2 from the second laser emitter 212, and splits the beam B2 into a first reflected portion B2R1 and a first transmitted portion B2T1. The first reflected portion B2R1 is directed to the mirror 244 which reflects the portion B2R1 to the focuser 254. The first transmitted portion B2T1 is directed to another beam splitter 223 of the second beam splitter arrangement 220C. The other beam splitter 223 of the second beam splitter arrangement 220C receives the first transmitted portion B2T1 from the initial beam splitter 224, and splits the first transmitted portion B2T1 into a second reflected portion B2R2 and a second transmitted portion B2T2. The second reflected portion B2R2 is directed to the mirror 243 which reflects the portion B2R2 to the focuser 253. The second transmitted portion B2T2 is directed to the second beam blocker 282. In this example, an advantage is that transmission losses from the first beam splitter to the last beam splitter of a beam splitter arrangement 220B, 220C is minimised, so the energy at each ignition position 271,272, 273, 274 is more equal. This can mean that weaker laser emitters 211, 212 can be used, reducing the size and complexity of the laser emitters and reducing any chance of burning the initial beam splitter. FIG. 6 schematically illustrates the fifth and sixth focusers 255, 256, fifth and sixth optical windows 265, 266, and fifth and sixth ignition positions 275, 276, where six ignition positions are provided. The manner in which laser light is transmitted to the fifth and sixth focusers 255, 256 can be in accordance with any of the laser ignition systems 200A, 200B, 200C described in relation to the preceding FIGS. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. The different approaches of FIGS. 3-6 may be combined so that different ones of the approaches may be used for different ones of the ignition positions. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Claims

1. An internal combustion engine comprising a laser ignition system for igniting gaseous fuel within a combustion chamber of the internal combustion engine, the laser ignition system comprising a laser source apparatus and optics, the optics comprising:a first focuser, and a first optical window at a boundary of the combustion chamber, the first focuser and the first optical window arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at a first position (271) in the combustion chamber; anda second focuser, and a second optical window at the boundary of the combustion chamber, the second focuser and the second optical window arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at a second position in the combustion chamber.

2. The internal combustion engine of claim 1, the optics further comprising:a third focuser, and a third optical window at the boundary of the combustion chamber, the third focuser and the third optical window arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at a third position in the combustion chamber.

3. The internal combustion engine of claim 2, wherein the first, second, and third positions are nonlinearly spatially distributed over an area within the combustion chamber.

4. The internal combustion engine of claim 2 or 3, wherein the first, second, and third positions are located away from a central piston axis of the combustion chamber.

5. The internal combustion engine of any preceding claim, the optics further comprising:a fourth focuser, and a fourth optical window at the boundary of the combustion chamber, the fourth focuser and the fourth optical window arranged to focus and transmit laser light from the laser source apparatus to ignite the gaseous fuel at a fourth position in the combustion chamber.

6. The internal combustion engine of any preceding claim, the optics providing a total number of ignition positions in the combustion chamber consisting of 4 to 6 ignition positions.

7. The internal combustion engine of any preceding claim, wherein each of the positions are above and aligned with a piston bowl of a piston of the internal combustion engine.

8. The internal combustion engine of any preceding claim, the laser source apparatus comprising a laser emitter configured to emit a beam of laser light, the optics comprising a beam splitter arrangement configured to direct a first portion of the beam to the first focuser, and to direct a second portion of the beam to the second focuser.

9. The internal combustion engine of claim 8, comprising a beam blocker arranged to absorb a further portion of the beam from the beam splitter arrangement at a location external to the combustion chamber.

10. The internal combustion engine of any one of claims 1 to 7, the laser source apparatus comprising a plurality of laser emitters configured to supply laser light to a plurality of positions within the combustion chamber including the first and second positions.

11. The internal combustion engine of claim 10, the plurality of positions comprising one or more further positions within the combustion chamber, wherein the plurality of laser emitters comprises individual laser emitters for individual ones of the plurality of positions.

12. The internal combustion engine of any preceding claim, wherein the laser source apparatus is configured to generate an output of pulsed laser light.

13. The internal combustion engine of any preceding claim, wherein the optics comprises first and second beam directing devices arranged to direct laser light from the laser source apparatus to the first and second focusers, respectively.

14. The internal combustion engine of claim 13, wherein each beam directing device comprises a mirror or a fibre optic cable.

15. A vehicle comprising the internal combustion engine of any one of the preceding claims.17

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