Microwave pre-chamber ignition for a combustion engine

Abstract

The invention relates to a microwave ignition device (1) for igniting an ignitable fluid in a combustion chamber of an internal combustion engine. The invention also relates to a method for igniting an ignitable fluid in a combustion chamber of an internal combustion engine and an internal combustion engine having at least one combustion chamber.

Description

[0001] The invention relates to a microwave ignition device for igniting an ignitable fluid in a combustion chamber of an internal combustion engine. Furthermore, the invention relates to a method for igniting an ignitable fluid in a combustion chamber of an internal combustion engine and to an internal combustion engine with at least one combustion chamber.

[0002] Ignition devices and ignition methods for igniting an ignitable fluid or fluid mixture in the combustion chamber of an internal combustion engine, which include a prechamber, are known in various configurations from the prior art. A prechamber is essentially a small combustion chamber adjacent to, but separated from, the actual combustion chamber of an internal combustion engine. The volume enclosed by a prechamber, the interior of the prechamber or the inner cavity of the prechamber, is specifically designed and configured to hold an ignitable fluid and is typically significantly smaller than the volume of the actual combustion chamber.Between a pre-chamber and the actual combustion chamber, a connection exists, for example, in the form of at least one bore or channel through the outer wall of the pre-chamber into its interior, for the exchange of fluid with the combustion chamber, in particular so that ignitable fluid can pass from the combustion chamber into the pre-chamber. In passive ignition devices, a small portion of ignitable fluid already present in a combustion chamber passes through the at least one bore during a compression process in the combustion chamber, driven by the increase in pressure in the combustion chamber. In alternative active designs of ignition devices and corresponding ignition methods, it is provided that (additionally) ignitable fluid is introduced directly into the pre-chamber or is provided within the pre-chamber.The at least one bore also serves to allow hot, volatile combustion products to escape from the interior of the pre-chamber into the combustion chamber after ignition of ignitable fluid in the pre-chamber.

[0003] When an ignitable fluid is ignited in the combustion chamber of an internal combustion engine comprising an ignition system with a pre-chamber, the ignitable fluid is first ignited in the inner cavity of the pre-chamber. This ignition is typically achieved with a conventional spark plug, as is also used for the direct ignition of an ignitable fluid in the combustion chamber of an internal combustion engine. The ignition and subsequent combustion of a fluid in the inner cavity of the pre-chamber leads to the propagation of jets of hot, volatile combustion products through at least one bore from the inner cavity of the pre-chamber into the combustion chamber. This then ignites any ignitable fluid present in the combustion chamber.The ignition of the ignitable fluid in the combustion chamber occurs, particularly when several torch jets propagate (if several bores are present in the pre-chamber wall for this purpose) from the inner cavity of the pre-chamber into the combustion chamber in various spatial areas of the interior of the combustion chamber, in contrast to ignition by means of a spark from a conventional spark plug or the like, not only at the location of the spark, but spatially distributed at many points in the combustion chamber virtually simultaneously.

[0004] Igniting an ignitable fluid in the combustion chamber of an internal combustion engine using a pre-chamber can significantly improve the ignition behavior and combustion process (including ignition and flame front propagation) of lean air-fuel mixtures. The excess air in lean air-fuel mixtures leads to unfavorable ignition conditions and thus to insufficient ignition of the air-fuel mixture with conventional ignition using a local spark. The decreasing flame speed results in a longer burn time and consequently in incomplete combustion, meaning that the combustion may be incomplete or prematurely terminate.By igniting lean air-fuel mixtures using an ignition system comprising a pre-chamber, the ignition and combustion behavior of such lean ignitable fluids can be significantly improved, thereby improving the efficiency of an internal combustion engine with such an ignition system when burning lean fluids and reducing pollutant emissions, especially CO emissions.

[0005] Prior art ignition devices and methods for igniting a fluid or fluid mixture in a combustion chamber of an internal combustion engine, which incorporate passive and / or active pre-chamber ignition devices, include conventional spark plugs for igniting the ignitable fluid inside the pre-chamber. Ignition is typically initiated by a spark in the region of the pre-chamber floor. The pre-chamber floor is defined as the end of the interior or inner cavity of a pre-chamber that is opposite the other end, in the vicinity of which at least one bore is provided for the exchange of fluid with, and for the passage of, hot, volatile combustion products from the inner cavity of the pre-chamber into an associated combustion chamber. Ignition thus occurs at a location inside the pre-chamber that is farther away from the at least one bore compared to almost all other locations inside.This type of ignition often fails, particularly at low loads or idle speeds, because combustion products (residual gas or similar) accumulate in the pre-chamber floor, hindering reliable ignition. Purging the pre-chamber to remove these combustion products could remedy this, but such a solution would significantly increase the complexity of both the pre-chamber ignition device and the corresponding pre-chamber ignition process for an internal combustion engine. Another disadvantage of known ignition devices and processes that utilize pre-chambers is the short service life of the spark plugs within them. The spark plugs are insufficiently cooled and are therefore regularly exposed to high temperatures for extended periods. These high temperatures reduce their lifespan and contribute to the short service life of these known ignition devices.

[0006] US 2020 / 0182217 A1, US 7,644,698 B2 and US 3,934,566 A show conventional ignition devices with special structures for spark or plasma arc generation. Microwave ignition devices, however, are not discussed.

[0007] Therefore, the invention is based on the objective of proposing an ignition device and an ignition method for an internal combustion engine which has a significantly increased reliability and stability compared to solutions known from the prior art and enables safe and reliable ignition of the ignitable fluid intended for the operation of the internal combustion engine for all possible operating conditions of an internal combustion engine.

[0008] The object of the invention is achieved by a microwave ignition device for igniting an ignitable fluid in a combustion chamber of an internal combustion engine according to claim 1, an internal combustion engine according to claim 6, and a method for igniting an ignitable fluid in a combustion chamber of an internal combustion engine according to claim 7. Further advantageous embodiments of the invention can be found in the respective dependent claims.

[0009] Accordingly, a microwave ignition device according to the invention comprises at least one hollow body with a preferred longitudinal axis and two opposite ends spaced apart along the preferred longitudinal axis, wherein the hollow body comprises at least one cavity, wherein the cavity is at least partially configured as a prechamber for receiving an ignitable fluid in an interior of the prechamber, wherein the prechamber adjoins the first end of the hollow body and an outer wall of the hollow body bounding the prechamber / cavity has at least one bore extending completely from an inside to an outside through the wall in a vicinity of the first end, which enables an exchange of fluid between the prechamber and an outside space and is additionally configured to allow hot volatile combustion products to escape from the prechamber into an outside space.wherein the cavity is at least partially configured as a cavity resonator for microwave radiation, into which microwave radiation can be coupled from the second end of the hollow body from the outside, and the cavity resonator at least partially coincides with the antechamber, and wherein at least two means for local field enhancement are arranged or configured in a vicinity of the first end on the inside circumferential side around an outer wall of the cavity resonator / cavity.

[0010] The hollow body can be formed in one or more parts, or segmented – provided that the individual parts or segments from which the hollow body is formed all connect successively along the preferred longitudinal axis. The preferred longitudinal axis, along which the hollow body extends between its two longitudinal ends, can be an axis of symmetry for individual segments, i.e., for individual sections of the hollow body along the preferred longitudinal axis, or even for the entire hollow body, although this is by no means mandatory.Furthermore, symmetry with respect to the preferred longitudinal axis can also be limited to individual aspects of the hollow body's design. For example, the external design (such as the outer circumferential shell or the like) of the hollow body may be approximately symmetrical in a segment, while the cavity or partial cavity located within this segment may deviate from the symmetry of the external design in that segment, or vice versa. For instance, the external design of a segment of the hollow body may be cylindrical or rotationally symmetrical, while the internal cavity or partial cavity in that segment may be elongated cuboid. Additionally, the preferred longitudinal axis of the cavity or partial cavity (if one can be defined) need not coincide with the preferred longitudinal axis of the hollow body.The preferred longitudinal axis of the cavity or partial cavity can run parallel to the preferred longitudinal axis of the hollow body at approximately a distance from it, or it can form an angle with it.

[0011] The very term "hollow body" makes it immediately clear that said hollow body has at least one internal cavity. The hollow body may also have several cavities. If, in the case of multiple cavities within the hollow body, some or all of the cavities are interconnected, possibly indirectly via at least one other cavity, it may be advantageous to consider this as a single, continuous cavity with various partial cavities or cavity segments (if the relevant cavities connect successively along the preferred longitudinal axis of the hollow body). In addition to one or more cavities, partial cavities, or cavity segments, the hollow body has a shell or the like as an outer boundary separating it from an external space, other (hollow) spaces, other bodies, objects, devices, apparatus, etc.If the hollow body is formed from several parts and / or segments, then the shell is at least divided and / or segmented accordingly.

[0012] The hollow body is preferably made of a material that is at least heat- and pressure-resistant and has good electrical conductivity, preferably metallic. In a multi-part design of the hollow body, different parts and / or segments of the hollow body can be made of different materials. Inner walls, inner surfaces, or the like, forming the immediate boundary of the cavity, cavities, and / or partial cavities of the hollow body, can be at least partially provided with coatings that (further) improve the electrical conductivity.

[0013] At the first end of the hollow body, which is also the first end of the pre-chamber of the hollow body, the interior of the pre-chamber is preferably designed such that fluid from the combustion chamber can flow into the pre-chamber from the combustion chamber through at least one bore during a compression process, in a flow-optimized manner. At the same time, the pre-chamber at the first end is particularly preferably also designed such that hot, volatile combustion products are further accelerated as they pass out of the pre-chamber through the at least one bore, in order to achieve the widest possible propagation of the hot, volatile combustion products in the form of torch jets, flame jets, or the like.If several bores are provided in the wall of the hollow body bordering the pre-chamber at the first end, these are preferably arranged and aligned in such a way that hot volatile combustion products in the form of torch jets can spread through each of the bores into different spatial areas of a combustion chamber, in order to be able to ignite ignitable fluid located in the combustion chamber quasi-simultaneously at as many points as possible inside the combustion chambers by means of the torch jets.

[0014] The antechamber is at least partially designed as a cavity resonator for microwave radiation. The essential characteristic of the antechamber is that it is designed and intended to hold an ignitable fluid, which is ignited or can be ignited within the antechamber and burns at least partially within it. The essential characteristic of the cavity resonator, on the other hand, is that microwave radiation can be coupled into it, and that when microwave radiation is coupled into the cavity resonator, a standing electromagnetic wave forms within it. The essential characteristics of the antechamber and the cavity resonator, as well as their resulting configurations, can be combined at least partially (in sections, segments) or even completely. The antechamber can therefore, for example, completely enclose the cavity resonator.The resonator may extend beyond the cavity resonator on both sides, particularly along the preferred longitudinal axis of the hollow body in the axial direction. The cavity resonator may also coincide with the antechamber, making it difficult to distinguish between the antechamber and the cavity resonator. This means that the interior (i.e., the respective internal cavity) of the antechamber and the cavity resonator may occupy essentially the same volume element within the hollow body. Similarly, the cavity resonator may extend beyond the antechamber, particularly along the preferred longitudinal axis of the hollow body, towards the other end of the hollow body.In this case, a boundary, wall, border or the like of the antechamber towards the second end of the hollow body must therefore, on the one hand, represent an impenetrable barrier, especially for ignitable fluid contained or capable of being contained in the antechamber, but at the same time must not block the microwave radiation, since otherwise the cavity resonator cannot fulfill its intended function.

[0015] In the vicinity of the first end of the hollow body, at least two means for local field enhancement are arranged or formed on the inside, circumferentially around an outer wall of the cavity resonator. Preferably, three or more means for local field enhancement are provided. The means for local field enhancement influence the distribution of the electric field in the cavity resonator when microwave radiation is coupled into the cavity resonator and a standing electromagnetic wave forms within it.The means for local field enhancement preferably extend from the outer circumferential wall of the cavity resonator a certain distance into the interior / inner cavity of the cavity resonator, essentially in the direction of the center of the cross-section of the cavity resonator at the respective position where the respective means for local field enhancement is arranged or formed with respect to the preferred longitudinal direction of the cavity body, and / or predominantly in a radial direction. According to the peak effect, the means for local field enhancement produce particularly high electric field strengths, especially in the vicinity of each free end of each means for local field enhancement. For this to occur, it is essential that the means for local field enhancement are arranged or formed at this location.These are situations where, depending on the characteristics (especially wavelength) of the intended microwave radiation, particularly high electric field strengths are present in the cavity resonator during the formation of the standing electromagnetic wave in the cavity resonator by coupling in microwave radiation. It should be noted that the means for local field enhancement may alter the field distribution in the cavity resonator compared to an identical cavity resonator without such means. The means for local field enhancement can have the basic form of a more or less pronounced point projecting from the inner wall of the cavity resonator into the interior of the cavity resonator. However, such a design is by no means necessary or even mandatory.

[0016] Due to the placement of the local field enhancement devices in the vicinity of the first end of the hollow body, these devices are located in a region or segment of the cavity resonator that coincides with the antechamber (or vice versa). If ignitable fluid is present in the antechamber and microwave radiation is coupled into the cavity resonator, resulting in the formation of a standing electromagnetic wave within the resonator, at least one or more flashovers or arcs will form between the local field enhancement devices due to the ignitable fluid. These flashovers or arcs ignite the ignitable fluid in the antechamber. The precise formation of these flashovers or arcs depends on many factors, including the detailed design of the local field enhancement devices, their number, and their arrangement / distribution around the cavity resonator. An asymmetric orAn uneven arrangement / distribution of the means for local field enhancement, circumferentially around the outer wall of the cavity resonator, enables the use of a broader frequency range of microwave radiation. Alternatively, the means for local field enhancement can be designed differently for this purpose, for example, by projecting to varying depths into the cavity resonator.Due to the arrangement of the means for local field enhancement in the vicinity of (near) the first end of the hollow body and thus simultaneously the first end of the pre-chamber, where at least one bore is also provided, it is ensured that sufficient (fresh) ignitable fluid is always present in the area of ​​the means for local field enhancement and that ignition of the fluid in the pre-chamber, as well as the immediately subsequent ignition of ignitable fluid in a combustion chamber of an internal combustion engine associated with the microwave ignition system according to the invention, can occur safely and reliably. This applies in particular even during extended continuous operation of the microwave ignition device or the engine with such a microwave ignition device.

[0017] The first end of the cavity resonator can coincide with the first end of the antechamber and thus also with the first end of the cavity body. However, the first end of the cavity resonator can also be located within the antechamber at a certain axial distance from the first end of the antechamber / cavity body as a whole. This should not be interpreted, however, as meaning that a wall or similar structure extends transversely through the antechamber at this point, which, while it would serve as a reflector for microwave radiation, would also divide the antechamber into two separate chambers, which would be rather impractical.In the region of the first end, the prechamber can be designed, for both fluid dynamic reasons (optimizing the flow of ignitable fluid from an associated combustion chamber into the prechamber) and thermodynamic reasons (optimizing the propagation of hot, volatile combustion products from the prechamber into the combustion chambers), roughly like a nozzle with a cross-section that tapers towards the first end. Depending on the wavelength of the intended microwave radiation, this cross-section may become too small for the microwave radiation to propagate further towards the first end of the prechamber (see cutoff frequency), thus representing a kind of virtual (reflective) wall and a corresponding virtual first end of the cavity resonator.

[0018] According to a preferred embodiment of a microwave ignition system according to the invention, the prechamber is spatially separated from the rest of the cavity and / or from all other (partial) cavities of the hollow body by a barrier that is at least partially microwave-transparent. The microwave-transparent barrier closes off the prechamber towards the second end of the hollow body and, in particular, prevents ignitable fluid and hot volatile combustion products from flowing out of the prechamber towards the second end of the hollow body and from penetrating into other parts, areas of the cavity segments that are not part of the prechamber, and / or into other (partial) cavities of the hollow body.Since the pre-chamber always coincides at least partially with the cavity resonator, it is essential that the barrier does not further impede the intended microwave radiation, i.e., that it is transparent to the intended microwave radiation, as otherwise the cavity resonator cannot fulfill its function. The microwave-transparent barrier is also preferably resistant to strong, sudden temperature and pressure changes and other stresses that are to be expected in the immediate vicinity of a combustion chamber for the combustion of ignitable fluids. The barrier is preferably made of a ceramic material that is transparent to the intended microwave radiation.

[0019] In a preferred embodiment of a microwave ignition system according to the invention, microwave radiation is coupled into the cavity resonator eccentrically and / or asymmetrically. This breaks the symmetry of the cavity resonator and results in many resonance modes coupling within the cavity resonator. This also significantly reduces the problems associated with microwave radiation being coupled into a microwave feed to the cavity resonator, from the microwave radiation entering the cavity resonator, and potentially even traveling back to a microwave source or being reflected back.

[0020] According to a preferred embodiment of a microwave ignition system according to the invention, a further part of the cavity and / or a further cavity of the hollow body extending between the second end of the hollow body and the cavity resonator is designed as a microwave waveguide into which microwave radiation can be coupled at the second end. In this embodiment, the microwave ignition source thus comprises, in addition to the pre-chamber and cavity resonator, a section of microwave waveguide for guiding microwave radiation from the second end of the hollow body to the cavity resonator, more precisely: to the second end of the cavity resonator. In this case, the second end of the cavity resonator and the second end of the hollow body are spaced apart from each other in the axial direction.In contrast, in alternative designs, especially those without the additional cavity / microwave waveguide, the second ends of the cavity resonator and the hollow body can coincide, possibly also with the second end of the antechamber.

[0021] According to a preferred embodiment of a microwave ignition system according to the invention, at least one bore has a diameter between 0.2 mm and 1.3 mm. This refers to a bore with a circular cross-section and corresponding cross-sectional area. For bores with a different (non-circular) cross-sectional shape, a corresponding equivalent (identically sized) circular area is to be used. Such particularly small bores are advantageous for improving the ignition behavior of a microwave ignition device according to the invention in combination with an internal combustion engine in which the use of highly charged ignitable fluids is intended. Hydrogen-air (gas) mixtures are cited as an example of highly charged ignitable fluids or fluid mixtures. In highly charged internal combustion engines, the pressure in the combustion chamber can exceed 150 bar.The high pressure is linked to a high compression of the ignitable fluid in the combustion chamber of a highly turbocharged internal combustion engine during a compression process in the combustion chamber, with a seamless transition to the (subsequent) ignition of the ignitable mixture. This high compression typically results in a significantly higher dielectric strength of the ignitable fluid. When using conventional spark plugs for ignition, this inhibits the formation of sparks between the spark plug electrodes. Highly turbocharged internal combustion engines with prior art ignition systems therefore suffer from efficiency losses and insufficient smooth running due to frequent misfires.

[0022] Due to the small cross-section of the bore, the pressure rise in the pre-chamber relative to the combustion chamber of an internal combustion engine associated with the microwave ignition device according to the invention is delayed during a compression process. The maximum pressure in the pre-chamber, or in the portion of ignitable fluid contained in the pre-chamber, can therefore be significantly lower than in the combustion chamber immediately before ignition, compared to the combustion chamber. Accordingly, the dielectric strength of the portion of ignitable fluid contained in the pre-chamber is also considerably lower than that of the (main) portion of ignitable fluid contained in the combustion chamber. Thus, even highly turbocharged internal combustion engines can be ignited safely and reliably using a microwave ignition device according to the invention.

[0023] An internal combustion engine according to the invention, comprising at least one combustion chamber, has a microwave ignition system according to one of the embodiments of a microwave ignition system according to the invention described above, or any combination of these embodiments. Preferably, at least one microwave ignition system according to the invention is assigned to each combustion chamber of such an internal combustion engine.

[0024] A method according to the invention for igniting an ignitable fluid in a combustion chamber of an internal combustion engine comprises at least the following steps: a. Provision of a cavity, wherein a first spatial region of the cavity is at least partially designed to receive an ignitable fluid and a further, second spatial region of the cavity is configured such that a standing electromagnetic wave is formed in the second region upon introduction of microwave radiation, wherein the second region coincides at least partially with the first region, wherein at least one connection between the first region and the combustion chamber enables an exchange of fluid between the first region and the combustion chamber as well as the passage of hot volatile combustion products from the first region into the combustion chamber, wherein at least two means for local field enhancement are provided at locations in the immediate vicinity of the end of the connection to the first region on the inside circumferentially around the second region.a. where particularly high electric field strengths are to be expected in the second region when a standing electromagnetic wave is present; b. introduction of ignitable fluid from the combustion chamber into the first region through the connection during a compression process relating to the combustion chamber; c. introduction of microwave radiation into the second region, wherein, due to the peak effect, at least one or more electrical discharges are formed between the at least two means for local field enhancement by the ignitable fluid, which ignite the ignitable fluid inside the first region and, as a result, a flame jet propagates through the connection into the combustion chamber and ignites the ignitable fluid located in the combustion chamber.

[0025] The inventive method for igniting an ignitable fluid in a combustion chamber of an internal combustion engine enables safe and reliable ignition, particularly at low load and speed ranges. This is achieved in particular by providing for the ignition of an ignitable fluid in the first spatial region of the cavity in the immediate vicinity of at least one connection to the associated combustion chamber of an internal combustion engine. Thus, at the point(s) or in the region where ignition of the ignitable fluid takes place in the first spatial region, a sufficient amount of ignitable fluid is present at the time of ignition.

[0026] In a privileged embodiment of a method according to the invention for igniting an ignitable fluid in a combustion chamber of an internal combustion engine, the at least one connection has a sufficiently small cross-section that, during a compression process, the pressure increase in the first region of the cavity is delayed compared to the pressure increase in the combustion chamber, and the maximum pressure immediately before the ignition of a fluid in the first region is no more than one tenth, preferably no more than one twentieth, of the pressure in the combustion chamber.The small cross-section of the connection thus inhibits the flow of ignitable fluid from the combustion chamber into the first area of ​​the cavity during the compression process with respect to the combustion chamber to such an extent that, up to a point immediately before ignition, sufficient ignitable fluid for reliable ignition has reached the first area from the combustion chamber, but the pressure in the first area, and thus in the fluid located there, as well as its compression, is still considerably lower than in the combustion chamber and the (main) part of the ignitable fluid located there.

[0027] When ignitable fluids are highly compressed and therefore under high pressure (over 150 bar) immediately before ignition during the compression process in the combustion chamber of an internal combustion engine, problems regularly arise with regard to reliable ignition of the ignitable fluid using ignition methods known from the prior art. This is due to the increased dielectric strength of an ignitable fluid at high compression, which, for example, inhibits the formation of an ignition spark between the electrodes of the spark plugs when using conventional spark plugs to ignite a highly compressed fluid. This particularly affects hydrogen-air (gas) mixtures as ignitable fluids or fluid mixtures.When using ignition methods known from the prior art, highly turbocharged internal combustion engines suffer from efficiency losses and insufficient smooth running due to regular misfires.

[0028] By delaying the pressure increase in the first region of the cavity relative to the combustion chamber during a compression process in a method according to the invention for igniting an ignitable fluid in a combustion chamber of an internal combustion engine, the pressure in the first region of the cavity at the time of maximum compression in the combustion chamber immediately before ignition can be significantly lower than the pressure in the combustion chamber. For example, with a pressure of 100 bar in the combustion chamber at maximum compression, the pressure in the first region of the cavity can be limited to less than 10 bar. The portion of the ignitable fluid located in the first region of the cavity, with its significantly lower compression and pressure compared to the (main) portion of the ignitable fluid in the combustion chamber, also exhibits considerably lower dielectric strength.This allows highly compressed ignitable fluids such as hydrogen-air mixtures to be ignited even more reliably and safely using a method according to the invention for igniting an ignitable fluid in a combustion chamber of an internal combustion engine.

[0029] The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the figure description and / or shown in the figures alone, can be used not only in the combinations specified, but also in other combinations or individually. It is not necessary to implement all features of independent claims 1 and 7 to realize the invention. Individual features of independent claims 1 and 7 can also be replaced by other disclosed features or combinations of features.

[0030] Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings, in which identical or functionally equivalent elements are provided with identical reference numerals. These drawings show: Fig. 1 a schematic representation of a first embodiment of a microwave ignition device according to the invention in a first longitudinal sectional view; Fig. 2 the microwave ignition device according to the invention Figure 1 in a second longitudinal sectional view; Fig. 3 a schematic representation of a second embodiment of a microwave ignition device according to the invention in a first longitudinal sectional view; Fig. 4 the microwave ignition device according to the invention Figure 3in a second longitudinal sectional view; Fig. 5 a third embodiment of a microwave ignition device according to the invention in a longitudinal sectional view; Fig. 6 an interior view towards the first end (left) and a corresponding longitudinal sectional view (right) of a segment of the hollow body of a microwave ignition device according to the invention in the region of the first end based on the embodiment according to Figures 1 and 2 with a design variant concerning the means for local field elevation; Fig. 7 a view accordingly Figure 6 with an alternative design concerning the means for local field elevation; Fig. 8 a view corresponding to the Figures 6 and 7 with a further alternative design concerning the means for local field elevation; Fig. 9 a view corresponding to the Figures 6 to 8with an additional alternative design concerning the means for local field elevation; Fig. 10 a view according to the Figures 6 to 9 with yet another alternative design concerning the means for local field elevation; and Fig. 11 a view corresponding to the Figures 6 to 10 with yet another alternative design concerning the means for local field elevation.

[0031] In Figure 1 A embodiment of a microwave ignition device 1 according to the invention is shown schematically in a first longitudinal section view. Figure 2 The microwave ignition device 1 shows Figure 1 in a further longitudinal section view, wherein the section plane of this longitudinal section view is orthogonal to the section plane of the longitudinal section view from Figure 1The microwave ignition device 1 comprises, as its basic element, an elongated hollow body 2 extending between a first or front end 3 and a second or rear end 4 along a preferred longitudinal axis (not explicitly marked in the figures). The alternative designation of the first end 3 of the hollow body 2 as the front end and the second end 4 as the rear end is without limitation of generality. The basic external shape, i.e., the outer shell or outer surface(s), of the hollow body 2 is essentially rotationally symmetrical about the preferred longitudinal axis. This basic external shape allows the microwave ignition device 1 to be used as a direct replacement for known ignition devices, both without a pre-chamber, such as conventional spark plugs, and with a pre-chamber, without major modifications to existing internal combustion engine designs.The hollow body 2 has two distinct cavities, 5 and 6, which extend through different regions of the hollow body 2 along the preferred longitudinal axis and meet at a point along the preferred longitudinal axis, thus being directly connected to each other. Therefore, cavities 5 and 6 could also be considered as two sub-cavities of a single, continuous cavity within the hollow body.

[0032] The first cavity 5 of the hollow body 2 has a rotationally symmetrical basic shape with the preferred longitudinal axis of the hollow body 2 as its axis of symmetry. The first cavity 5 is partially configured as an antechamber 9 and partially as a cavity resonator 8, with the antechamber 9 and cavity resonator 8 also partially coinciding. The prechamber 9 extends from the first or front end 3 of the hollow body 2, which is also the first or front end of the prechamber 9, to a pressure window 10. The pressure window 10 is fitted into the first cavity 5 and forms a spatial barrier between the prechamber 9 and a further subsection of the first cavity 5, the intermediate cavity 11, which follows the prechamber 9 towards the second end of the hollow body 2, and the second cavity 6 that adjoins it. The prechamber 9 is designed and configured to hold ignitable fluid inside.In the front area of ​​the pre-chamber 2 in the vicinity of the first end 3 of pre-chamber 9 or hollow body 2, several bores 12 are formed in the wall of the pre-chamber 9 or hollow body 2, wherein in the . Figures 1 and 2Similarly, for the sake of clarity, only some of the bores 12 are explicitly referenced in all subsequent figures. The bores 12 allow the passage or inflow of ignitable fluid from a combustion chamber of an internal combustion engine, to which the microwave ignition system 1 is assigned or assignable, into the prechamber 9. Furthermore, the bores 12 allow hot, volatile combustion products to exit the prechamber 9 into the associated combustion chamber after the ignitable fluid in the prechamber has been ignited. The front part of the prechamber 9, which comprises approximately one quarter of its total length, tapers towards its first or front end 3 compared to the rest of the prechamber 9.This serves, in a manner comparable to a nozzle, to improve the flow characteristics during the passage of hot, volatile combustion products from the pre-chamber 9 through the bores 12, which are also arranged in the front area of ​​the pre-chamber 9, into an associated combustion chamber of an internal combustion engine.

[0033] Unlike the prechamber 9, the cavity resonator 8, in addition to the first cavity 5 of the hollow body 2, also includes the pressure window 10 and the intermediate cavity 11. The cavity resonator 8 serves to generate a standing electromagnetic wave within it. For this purpose, microwave radiation can be coupled into the cavity resonator 8, with microwave radiation initially being coupled into the second cavity 6 of the hollow body 2, which is designed as a microwave waveguide 7, at its second or rear end 4. Since the pressure window 10, while representing a spatial barrier or wall, particularly for ignitable fluid contained in the prechamber and also for hot volatile combustion products, and thus forming the second or rear end of the prechamber 9, is simultaneously located in the middle of the cavity resonator 8, the pressure window 10 must only impede the intended microwave radiation to a minimal extent.The pressure-resistant 10 is therefore made of a microwave-transparent ceramic material that also permanently withstands the conditions in the immediate vicinity of a combustion of an ignitable fluid, such as strong, sudden pressure and temperature fluctuations as well as high peak pressures and peak temperatures.

[0034] The microwave ignition system 1 in the configuration variant according to the Figures 1 and 2This is designed as an example for microwave radiation from the Ka-band (frequency range from 26.5 GHz to 40 GHz). The microwave waveguide 7 is a WR34 rectangular waveguide. The microwave waveguide 7 extends from the second or rear end 4 of the hollow body 2, which is also the second or rear end of the microwave waveguide 7, to the first cavity 5, where the microwave waveguide 7 terminates at its first or front end at the second or rear end of the cavity resonator 8 and simultaneously in the first cavity 5 of the hollow body 5. The termination of the microwave waveguide 7 in the cavity resonator 8 is slightly eccentric to the preferred longitudinal axis of the hollow body 2, which in turn is also the preferred longitudinal axis or (rotational) axis of symmetry of the cavity resonator 8.At the second or rear end 4, the center of the rectangular cross-section of the microwave waveguide 7 coincides with the preferred longitudinal axis; at the first or front end, at the opening into the cavity resonator 8, the center of the rectangular cross-section of the microwave waveguide 7 is at a distance from the preferred longitudinal axis. This is shown in the illustration in . Figure 1Looking into the narrow side of the microwave waveguide 7, the first end of the microwave waveguide 7, at its opening into the cavity resonator 8, lies slightly lower than at the second or rear end 4. If the microwave waveguide 7 is assigned its own preferred longitudinal axis, the preferred longitudinal axis of the hollow body 2 and the preferred longitudinal axis of the microwave waveguide 7 form a small acute angle and intersect at a point at the second or rear end of the microwave waveguide 7 and the hollow body 2, respectively. The eccentric opening of the microwave waveguide 7 into the cavity resonator 8, which deviates from the symmetry of the cavity resonator 8, enables the coupling of many resonant modes of microwave radiation within the cavity resonator 8 and simultaneously significantly reduces the problem of microwave radiation being reflected back from the cavity resonator 8 into the microwave waveguide 7.

[0035] Near the first or front end of the cavity resonator 8, and thus in the vicinity of the first or front end 3 of the antechamber 9 or the hollow body 2, means for local field enhancement 13 are formed on the inside, circumferentially around the outer wall of the cavity resonator 8, which is simultaneously the outer wall of the antechamber 9 and the cavity 5. These means for local field enhancement 13 project into the interior, i.e., the inner cavity, of the cavity resonator 8. The means for local field enhancement 13 influence the distribution of the electric field in the cavity resonator 8 when microwave radiation is introduced or coupled into the cavity resonator 8 and a standing electromagnetic wave forms within it.The means for local field enhancement 13, according to the peak effect, produce particularly high electric field strengths at each free end of each means for local field enhancement 13 projecting into the cavity resonator 8. For this to occur, it is essential that the means for local field enhancement 13 are arranged or formed where, depending on the characteristics (especially wavelength) of the intended microwave radiation, particularly high electric field strengths are present in the cavity resonator 8 during the standing electromagnetic wave that forms in the cavity resonator 8 when microwave radiation is coupled in. It should be noted that the means for local field enhancement may alter the field distribution in the cavity resonator 8 compared to an identical cavity resonator 8 without such means for local field enhancement 13.This also applies to the design of the pressure window 10 with regard to the material used for the pressure window 10 and the interaction of the intended microwave radiation with the material, the thickness of the pressure window 10, i.e., its extent along the preferred longitudinal axis, and its precise arrangement in the first cavity 5 / the cavity resonator 8 along the preferred longitudinal axis of the hollow body 2. The already mentioned space 11 between the opening of the microwave waveguide 7 into the cavity resonator 8 and the pressure window 10, which is located in the... Figures 1 and 2 In the illustrated embodiment of a microwave ignition system according to the invention, part 1 of the cavity resonator 8 serves in particular to match the impedance of microwave radiation at the transition from the microwave conductor 7 to the cavity resonator 8.

[0036] In the Figures 3 and 4A second embodiment of a microwave ignition system 1 according to the invention is shown in a respective schematic longitudinal section view, wherein the section plane of the longitudinal section view is in Figure 4 orthogonal to the cutting plane of the longitudinal section view in Figure 3 is. Compared to the one in the Figures 1 and 2 In the first embodiment shown, the eccentricity / asymmetry of the microwave waveguide 7 is even more pronounced at the junction with the first cavity 5 or the cavity resonator 8. In the second embodiment, the microwave waveguide 7 also has a rectangular cross-section, which, however, tapers continuously (linearly) from the second end 4 to the first end, i.e., towards the junction with the cavity resonator 8. This is particularly evident from Figure 4with a view into the broad side of the microwave waveguide 7. The design of the tapering of the first cavity 5 or the antechamber 9 towards the first or front end 3 is slightly different in the second embodiment. In the first embodiment according to the Figures 1 and 2 In the second embodiment, an S-shaped narrowing (in longitudinal section) of the first cavity 5 or the pre-chamber 9 towards the first or front end 3 is provided, in the Figures 3 and 4 In contrast, the tapering is conical. A further difference from the first embodiment is that, in the second embodiment of a microwave ignition system 1 according to the invention, the means for local field enhancement are not integrally formed with the outer wall of the first cavity 5 / the cavity resonator 8 / the prechamber 9. The means for local field enhancement 13 are, in the second embodiment according to Figures 3 and 4The means for local field enhancement 13 are inserted into the outer wall from the outside through recesses or bores (not explicitly referenced in the figures) and extend radially a short distance into the cavity resonator 8 and thus also into the antechamber 9. The means for local field enhancement 13 are cylindrical and therefore do not have a pronounced pointed shape.

[0037] In Figure 5Figure 1 shows a longitudinal sectional view of a third embodiment of a microwave ignition system 1 according to the invention. The third embodiment features a simple rectangular microwave waveguide 7 for guiding microwave radiation from the second or rear end of the microwave waveguide 7 or the hollow body 2 to the opening of the microwave waveguide 7 at its first end into the second end of the first cavity 5. The microwave waveguide 7 does not open eccentrically / asymmetrically into the first cavity 5, nor does the cross-section of the microwave waveguide 7 change between the two ends. In the [figure / ... Figure 5In the third embodiment of a microwave ignition system 1 according to the invention, the prechamber 9 completely encloses the cavity resonator 8. The second end of the cavity resonator 9 forms an eccentric or asymmetric aperture 14, i.e., a (conductive / metallic) partition inserted into the first cavity 5 with a recess arranged eccentrically or asymmetrically to the preferred longitudinal axis of the hollow body 2 and simultaneously to the rotational symmetry axis of the cavity resonator 8, which allows microwave radiation to enter the cavity resonator 8. The aperture 14 fulfills the same function as an eccentrically or asymmetrically arranged opening of the microwave waveguide 7. In the third embodiment of a microwave ignition system 1 according to the invention, Figure 5The first end of the microwave waveguide 7 does not lead directly into the cavity resonator as in the two design variants according to the Figures 1 and 2 or 3 and 4, but into the first cavity 5 of the hollow body 2. As mentioned, the cavity resonator 8 already terminates at the aperture 14 as its second end and thus still within the antechamber 9, the second end of which, as in the other two embodiments, is formed by the interface of the pressure disk 10 facing the first end 3 of hollow body 2 / first cavity 5 / antechamber 9. Between the opening of the microwave waveguide 7 into the first cavity 5 and the pressure disk 10, the intermediate cavity 11 again serves for impedance matching with respect to microwaves at the transition from the microwave waveguide 7 into the first cavity 5.

[0038] In Figure 6The left of the two illustrations shows a view along the preferred longitudinal axis of a hollow body 2 of a microwave ignition system 1 according to the invention into the interior of the hollow body 2 in the region of its first or front end 3, and thus into the interior of the first hollow body 5 / the cavity resonator 8 / the prechamber 9. The right of the two illustrations shows a longitudinal sectional view of an end region or section of the hollow body 2 at its first or front end 3. The basic shape of the hollow body 2 at its first or front end 3 corresponds to that of the [unclear - possibly "the" or ... Figures 1 and 2 The first embodiment of a microwave ignition system according to the invention is shown. 1. The focus of the illustration is Figure 6 The focus is on the design of the means for local field elevation 13, which is why, particularly in the longitudinal section view, only a front end section of the hollow body 2 is shown. In the exemplary embodiment according to the Figure 6 The means for local field enhancement 13 are designed as rectangular ribs with an overhang. Six identical means of local field enhancement 13 of this design are arranged equidistantly spaced around the inner surface of the outer wall of the first cavity 5 / cavity resonator 8 / antechamber 9.

[0039] The further Figures 7 to 11 based on the representation in Figure 6 and show based alternative designs of means for local field enhancement 13. The means for local field enhancement are all identically designed and arranged equidistantly spaced around the outer wall of first cavity 5 / cavity resonator 8 / prechamber 9 on the inside.

[0040] In the version according to Figure 7 Six means designed as round bridges with overhangs for local field elevation 13 are provided.

[0041] In the version according to Figure 8Six means designed as rectangular narrow walkways with overhang are provided for local field elevation 13.

[0042] In the version according to Figure 9 Six means designed as rectangular ribs for local field enhancement 13 are provided, wherein the ribs taper to a point at the axial end pointing away from the first or front end 3 of first cavity 5 / cavity resonator 8 / prechamber 9.

[0043] In the version according to Figure 10 twelve means designed as narrow rectangular bridges with overhangs for local field elevation 13 are provided.

[0044] In the version according to Figure 11Six rectangular narrow ribs with overhangs are designed as means for local field enhancement 13 and additionally a central dome 15 are provided, which also and in combination with the means for local field enhancement 13 influence the field distribution in the cavity resonator 8 to generate arcs between the means for local field enhancement 13, the means for local field enhancement 13 and the dome 15 and / or optionally additionally between the means for local field enhancement 13, the dome 15 and / or the outer wall of the cavity resonator 8, provided that microwave radiation is coupled into the cavity resonator 8.In this respect, the dome 15 is also to be regarded as a means for local field elevation 13, whereby the dome 15, due to its different structure compared to all other means for local field elevation 13 shown so far in the figures as examples and without limiting the generality, has been assigned its own designation and its own reference number.

[0045] As already mentioned in a previous section, the basic external shape allows the microwave ignition devices 1 according to the invention, as exemplified in the figures, to be used in internal combustion engines without major modifications to known or existing internal combustion engine designs. The fundamental design of the exemplary embodiments of microwave spark plugs 1 shown in the figures can be adapted to any size and / or power class of internal combustion engine.

[0046] In the above embodiment, microwave radiation from the Ka band (frequency range from 26.5 GHz to 40 GHz) was used. With appropriate adjustment, another suitable microwave radiation from a different band, for example the X band (frequency range from 7.0 GHz to 11.2 GHz), can be used. Reference symbol list

[0047] 1 Microwave ignition device 2 Hollow body 3 First or front end of hollow body 4 Second or rear end of hollow body 5 First cavity 6 Second cavity 7 Microwave waveguide 8 Cavity resonator 9 Antechamber 10 Pressure window 11 Intermediate cavity 12 Bore 13 Means of local field enhancement 14 Aperture 15 Dome

Claims

1. Microwave ignition device (1) for igniting an ignitable fluid in a combustion chamber of an internal combustion engine, comprising at least one hollow body (2) with a preferred longitudinal axis and two opposite ends (3, 4) spaced apart along the preferred longitudinal axis, wherein the hollow body (2) comprises at least one cavity (5), wherein the cavity (5) is at least partially formed as a prechamber (9) for receiving an ignitable fluid in an interior of the prechamber (9), wherein the prechamber (9) directly adjoins the first end (3) of the hollow body and an outer wall of the hollow body (2) bordering the prechamber (9) has, in a vicinity of the first end, at least one bore (12) extending completely from an inner side to an outer side through the wall, the bore enabling an exchange of fluid between the prechamber (9) and an outer space and being additionally configured to allow hot volatile combustion products to exit the prechamber (9) into an outer space, wherein the cavity (5) is at least partially formed as a cavity resonator (8) for microwave radiation, into which microwave radiation can be coupled from the second end (4) of the hollow body (2) from the outside so that a standing electromagnetic wave is formed in the cavity resonator (8), and the cavity resonator (8) at least partially coincides with the prechamber (9), and wherein, in an area surrounding the first end (3), at least two means for local field enhancement (13) are arranged or formed circumferentially on the inside around an outer wall of the cavity resonator (8) / the cavity (5), to influence the distribution of an electric field in the cavity resonator (8) when microwave radiation is coupled into the cavity resonator (8).

2. Microwave ignition device according to claim 1, characterized in that the prechamber (9) is spatially separated from the rest of the cavity (5) and / or from all further cavities (6) of the hollow body (2) by an at least partially microwave-transparent barrier (10).

3. Microwave ignition device according to claim 1 or 2, characterized in that the coupling of microwave radiation into the cavity resonator (8) takes place eccentrically and / or asymmetrically.

4. Microwave ignition device according to any one of claims 1 to 3, characterized in that a further part of the cavity and / or a further cavity (6) of the hollow body (2) extends between the second end (4) of the hollow body (2) and the cavity resonator (8), is formed as a microwave waveguide (7) into which microwave radiation can be coupled at the second end (4).

5. Microwave ignition device according to any one of claims 1 to 4, characterized in that the at least one bore (12) has a diameter between 0.2 mm and 1.3 mm.

6. Internal combustion engine comprising at least one combustion chamber, characterized by a microwave ignition system (1) according to at least one of claims 1 to 5.

7. Method for igniting an ignitable fluid in a combustion chamber of an internal combustion engine by means of a microwave ignition device according to any one of the preceding claims 1-5, comprising at least the following steps: a. Providing a cavity (5), wherein a first spatial region of the cavity (9) is at least partially configured to receive an ignitable fluid and a further, second spatial region of the cavity (8) is configured such that, upon introduction of microwave radiation, a standing electromagnetic wave is formed in the second region (8), wherein the second region (8) at least partially coincides with the first region (9), wherein at least one connection (12) between the first region (8) and the combustion chamber enables an exchange of fluid between the first region (9) and the combustion chamber as well as the transfer of hot volatile combustion products from the first region (9) into the combustion chamber, wherein, in a vicinity of the end of the connection (12) into the first region (9), at least two means for local field enhancement (13) are provided circumferentially on the inside around the second region (8) at locations where particularly high electric field strengths are expected when a standing electromagnetic wave is present in the second region (8); b. Introducing ignitable fluid from the combustion chamber into the first region (9) through the connection (12) in the course of a compression process relating to the combustion chamber; c. Introducing microwave radiation into the second region (8), wherein, due to the tip effect, at the at least two means for local field enhancement (13), one or more electric flashovers form through the ignitable fluid at least between the means for local field enhancement (13), which ignite the ignitable fluid inside the first region (9) and, as a consequence, a flame jet propagates through the connection (12) into the combustion chamber and ignites the ignitable fluid present in the combustion chamber.

8. Method according to claim 7, characterized in that the at least one connection (12) has a sufficiently small cross-section such that, during a compression process relating to the combustion chamber, the pressure increase in the first region (9) of the cavity (5) occurs with a delay relative to the pressure increase in the combustion chamber, and the maximum pressure immediately before the ignition of a fluid in the first region (9) does not exceed one tenth, preferably not more than one twentieth, of the pressure in the combustion chamber.

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