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Title:
A MICROWAVE PLASMA IGNITION ASSEMBLY
Document Type and Number:
WIPO Patent Application WO/2017/093598
Kind Code:
A1
Abstract:
Invention relates to a microwave plasma ignition assembly configured to ignite a combustible fuel mixture in a combustion chamber of an internal combustion piston engine, comprising a prechamber assembly (14) removably adapted for attachment to a cylinder head (10), the prechamber assembly comprising a prechamber space (15) and at least one orifice (18) to provide a flow communication between the prechamber space (15) and the combustion chamber of the engine (100) when assembled to the cylinder head, the microwave plasma ignition assembly further comprising a microwave plasma plug assembly having an microwave antenna in communication with the specially designed prechamber space which acts as a cavity resonator, and a control unit (62) provided with executable instructions to activate the microwave plasma plug assembly in order to generate microwave pulses, the control unit (62) is provided with pulsed power input information for setting variables of a train of microwave pulses used during each ignition when activating the microwave plasma plug assembly, and that the control unit is further provided with executable instructions to activate the microwave plasma plug assembly using the pulsed power input information to generate microwave pulses.

Inventors:
SAARI, Petri (Tarhaajantie 2, Vaasa, FI-65380, FI)
SILLANPÄÄ, Hannu (Tarhaajantie 2, Vaasa, FI-65380, FI)
YU, Jingzhou (Tarhaajantie 2 Vaasa,Finland, FI-65380 Vaasa, FI-65380, FI)
Application Number:
FI2015/050853
Publication Date:
June 08, 2017
Filing Date:
December 04, 2015
Export Citation:
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Assignee:
WÄRTSILÄ FINLAND OY (Tarhaajantie 2, Vaasa, FI-65380, FI)
International Classes:
F02P23/04; F02B19/10; F02B19/12; F02B19/16; F02B43/00; F02M21/02; H01T13/02
Domestic Patent References:
WO2010009489A12010-01-28
WO2004036012A22004-04-29
WO2009058339A12009-05-07
Foreign References:
EP2065592A12009-06-03
JP2012089289A2012-05-10
KR20020075978A2002-10-09
DE102010027943A12011-10-20
EP2097629B12012-09-26
FR2886689A12006-12-08
US20110100322A12011-05-05
US20140109886A12014-04-24
CN103470427A2013-12-25
US4446826A1984-05-08
Attorney, Agent or Firm:
GENIP OY (Heikinkatu 7, Kotka, 48100, FI)
Download PDF:
Claims:
Claims

1 . A microwave plasma ignition assembly (106) configured to ignite a combustible fuel mixture in a combustion chamber of an internal combustion piston engine, comprising a prechamber assembly (14), the prechamber assembly comprising a prechamber space (15) and at least one orifice (18) so as to provide a flow communication between the prechamber space (15) and the combustion chamber of the engine (100), the micro- wave plasma ignition assembly further comprising a microwave plasma plug assembly (22) having a microwave antenna (23) in communication with the prechamber space (15), and a control unit (62) provided with executable instructions to activate the microwave plasma plug assembly (22) in order to generate microwave plasma, the control unit (62) is provided with pulsed power input information for setting variables of a train of microwave pulses used and a pulse duration during each ignition when activating the microwave plasma plug assembly, and the control unit is further provided with executable instructions to activate the microwave plasma plug assembly using the pulsed power input information to generate mi- crowave pulses, characterized in that the prechamber assembly (14) is removably adapted for an attachment to a cylinder head (10), and that the microwave antenna is adapted to generate microwave plasma between the microwave antenna and a wall of the prechamber space (15) wherein the prechamber space forms a cavity resonator by having a circular cross section at each location in the direction of the central longitudinal axis along the microwave antenna towards the second end of the prechamber space (15), and that the wall of the prechamber space (15) is arranged to curve rotationally symmetrically with a distance of the first radius (Re) in respect to an end of the microwave antenna (23) so as to form a spherical section (20.3) to the prechamber space (15).

2. The microwave plasma ignition assembly according to claim 1 , characterized in that the control unit is adapted to form the pulsed power input information as a function of one or more operational parameter of the engine obtained during one or more previous ignitions. 3. The microwave plasma ignition assembly according to claim 1 , characterized in that the control unit is provided with executable instructions to activate the microwave plasma plug assembly generating microwave pulses.

4. The microwave plasma ignition assembly according to claim 1 , characterized in that the number of microwave pulses is -1 -5 pulses per one ignition.

5. The microwave plasma ignition assembly according to claim 1 , characterized in that the pulsed power input information comprises a map of distinct values of number of microwave pulse for predetermined opera- tional parameters of the engine.

6. The microwave plasma ignition assembly according to claim 1 , characterized in that the prechamber assembly (14) is provided with a controllable gaseous fuel inlet (26) for feeding gaseous fuel into the prechamber space. 7. The microwave plasma ignition assembly according to claim 6, characterized in that the control unit is provided with executable instructions to control the pressure of the gaseous fuel feed to provide a predetermined operational conditions in the prechamber.

8. The microwave plasma ignition assembly according to claim 6, characterized in that the control unit is provided with executable instructions to control amount of the gaseous fuel to be fed to the prechamber space so as to provide a predetermined operational conditions in the prechamber.

9. The microwave plasma ignition assembly according to claim 6, characterized in that the control unit is provided with executable instructions to control the pressure of the gaseous fuel to be fed to the prechamber space in response of an engine load so as to provide a predetermined operational conditions in the prechamber.

10. The microwave plasma ignition assembly according to claim 6, characterized in that the control unit is provided with executable instructions to control the pressure of the gaseous fuel to be fed to the prechamber space in response of an engine speed so as to provide a predeter- mined operational conditions in the prechamber.

1 1 . The microwave plasma ignition assembly according to anyone of claims 8-9, characterized in that the air content in the gaseous fuel to be fed to the prechamber space is in practical circumstances zero.

12. The microwave plasma ignition assembly according to claim 1 , characterized in that the prechamber space (15) has a greater circular cross section at first end of the prechamber space (15) than at the longitudinal location of the end of the microwave antenna (23).

13. The microwave plasma ignition assembly according to claim 1 , characterized in that the radius of the prechamber space (15) is arranged to chamber linearly from the radius at the location of the first end of the prechamber space (15) to the radius at the longitudinal location of the end of the microwave antenna (23).

Description:
A microwave plasma ignition assembly

Technical field [001 ] The present invention relates to a microwave plasma ignition assembly according to the preamble of claim 1 .

Background art

[002] The operational requirements of combustion engines are becoming more and more demanding. The combustion engines need to have low specific fuel consumption and simultaneously they also need to meet very stringent emission requirements. Generally, when the temperature rises in the combustion chamber, the amount of formation of the nitrogen oxides increases. The combustion temperature can be decreased by using leaner fuel mixture in which the air/fuel ratio is high. In some circumstances, the combustion of lean fuel mixtures may be incomplete.

[003] In order to cope with emission requirements there are various techniques available by means of which the gaseous emissions may be controlled when the engine is running. On the other hand, it is not desirable that the overall performance of the engine will suffer resulted from actions aiming to reduce the emissions. It is crucial to combust the combustible fuel mixture in a cylinder of the engine very efficiently and accurately. Therefore, the timing and the strategy of the ignition plays very important role. [004] There exists several ways to ignite a fuel-air mixture in the combustion chamber i.e. in the cylinder of the engine operating with otto-cycle. Typically, spark plugs are used to generate an electric arc or electric spark and the spark plug may be arranged in the cylinder so as to ignite the fuel- air mixture therein. The disadvantage of using the conventional spark plugs, which may also be called as arc discharges, is that the length of the electric arc is substantially limited in size. Therefore, there can be a number of conventional spark plugs in larger combustion chambers to ignite the combustible fuel mixture more efficiently over a wider area. In addition, in case of standard spark plugs, an electrical pulse igniting the fuel-air mixture may have duration of several microseconds which initiates an electrical breakdown between the electrodes. The electrical breakdown creates a single conducting channel between the two electrodes and a current, i.e. the spark, starts flowing until the energy in the ignition coil is depleted. The standard spark plug may be placed in the cylinder or in a prechamber which is in connection with the main combustion chamber.

[005] For example, EP 2097629 B1 discloses a prechamber arrangement for a piston engine wherein a mixture of fuel and combustion air is ignited by a spark plug arranged in the prechamber. In the case of the corona discharge ignition, the fuel-air mixture is ignited by high electric field strengths without so-called arc discharge. Prechambers are typically used in lean burn Otto-cycle engines using gas as a fuel. The prechambers may be supplied with a richer fuel mixture whereas the combustion chamber is supplied with a leaner fuel mixture.

[006] FR 2 886 689 A1 proposes also corona ignition in a precombustion chamber, by way of which actual ignition is then effected in the combustion chamber. Injection of the fuel is effected in such a way that a given proportion of the fuel can pass into the precombustion chamber by way of the openings. An internal combustion engine operated in that way tends to ignition misfires in operation. Precombustion chamber ignition is based on the concept that a first fuel/air mixture is ignited in the precombustion chamber and that fuel/air mixture ignited in that way can pass by way of transfer openings into the combustion chamber of the internal combustion engine where ignition of the actual fuel/air mixture takes place. [007] US 201 1 /0100322 A1 discloses approach to ignite the fuel-air mixture using also corona discharge ignition in the cylinder of the engine. US 201 1 /0100322 A1 discloses a device for igniting a fuel-air mixture in the combustion chamber of an internal combustion engine. The device com- prises an electrode connected to a voltage source and extending into a combustion prechamber wherein a corona discharge takes place. The fuel-air mixture ignited in the prechamber transfer to the combustion chamber to ignite the fuel-air mixture therein. The documents has its focus on different shapes of the prechamber, the cross-sectional internal surface of which is smaller in the region of the at least one opening than the cross- sectional internal surface in the region in which the electrode passes into the precombustion chamber. Additionally, the document discloses a fluid inlet opening into the precombustion chamber. A fluid can be let in by way of the fluid inlet to flush the precombustion chamber. In the preferred case the fluid inlet is connected to a fuel source as in that way fuel or a fuel/air mixture can be let into the precombustion chamber and the supply of fuel is effected independently of the conventional inlet valves.

[008] WO 2009058339 A1 discloses a radio frequency igniter having combustion prechamber. The igniter may have an electrode extending partially into the combustion prechamber. The electrode is configured to direct current having a voltage component in the radio frequency range wherein the current creates a corona within the integral combustion prechamber.

[009] US 2014/0109886 A1 discloses a system for and a method of providing pulsed power to improve the performance efficiency of the engine. Pulsed power that converts a low-power into a high-power and longtime input into a short-time output is also employed to improve the fuel efficiency. [0010] CN 103470427 A discloses a microwave plasma ignition combustion system of an internal combustion engine. The microwave plasma ignition combustion system comprises a microwave ignition device arranged to feed microwave pulse with preset frequency into the combustion cham- ber and the resonator effect is realized in the combustion chamber.

[001 1 ] US 4,446,826 discloses an ignition system for an internal combustion engine in which combustion chambers are shaped in such a manner that a microwave resonance easily causes a plasma discharge. Microwaves are supplied from a microwave oscillator through respective coaxial cables to all the combustion chambers so that the combustion chambers resonate whenever the microwave power is injected, or so that only when the volume of the combustion chambers reaches to a resonatable condition, is the microwave power injected into the combustion chambers from the microwave oscillator thereby causing plasma discharge to occur in the combustion chambers.

[0012] An object of the invention is to provide a microwave plasma ignition assembly in which the performance is considerably improved compared to the prior art solutions.

Disclosure of the Invention

[0013] Object of the invention is substantially met by a microwave plasma ignition assembly configured to ignite a combustible fuel mixture in a combustion chamber of an internal combustion piston engine, comprising a prechamber assembly, the prechamber assembly comprising a precham- ber space and at least one orifice so as to provide a flow communication between the prechamber space and the combustion chamber of the engine, the microwave plasma ignition assembly further comprising a microwave plasma plug assembly having a microwave antenna in communica- tion with the prechamber space, and a control unit provided with executable instructions to activate the microwave plasma plug assembly in order to generate microwave pulses, the control unit is provided with pulsed power input information for setting variables of a train of microwave pulses used and a pulse duration during each ignition when activating the microwave plasma plug assembly, and the control unit is further provided with executable instructions to activate the microwave plasma plug assembly using the pulsed power input information to generate microwave pulses. It is characteristic to the invention that the prechamber assembly is remov- ably adapted for an attachment to a cylinder head and that the microwave antenna is adapted to generate microwave plasma between the microwave antenna and a wall of the prechamber space wherein the prechamber space forms a cavity resonator by having a circular cross section at each location in the direction of the central longitudinal axis along the mi- crowave antenna towards the second end of the prechamber space, and that the wall of the prechamber space is arranged to curve rotationally symmetrically with a distance of the first radius in respect to an end of the microwave antenna so as to form a spherical section to the prechamber space.

[0014] This provides a microwave plasma ignition assembly for igniting a combustible fuel mixture for which performance is considerably improved by using high frequency (at GHz level) and high pulsed power (at kW level) and an advantageous prechamber design. Advantageously, the form of the prechamber space enables a microwave resonance effect to occur therein efficiently. In other words, the ignition assembly can be called as a prechamber microwave resonance plasma ignition wherein the prechamber space acts as a cavity resonator. This would significantly improve the performance of the prehchamber ignition system since the resonance field could substantially enhance plasma generation by utilizing the entire prechamber as a cavity resonator, and then improve the flame kernel for- mation and propagation during ignition phase. Furthermore, the precham- ber assembly is easy to assemble to the cylinder head. The ignition of the gaseous fuel in the combustion prechamber is fast and more reliable compared to prior art solutions. The microwave plasma ignition assembly en- hances the fuel consumption and generates far less pollutant emissions. Thus the overall combustion process is improved. This is partially due to the fact that the microwave plasma is very reactive and a plasma cloud is taken place around the microwave antenna in the combustion prechamber. In addition, a leaner burn capability is enhanced. The active radicals generated by the microwave resonance around the microwave antenna play a role in the initial stages of ignitions. When microwave is used in a combustion environment, energy is coupled into electrons in gas and then non-thermal plasma can be generated to initiate the flame kernel and enhance the combustion. This provides the effect of simultaneously decreas- ing NOx emission and improving overall engine performance which ensuring effective and stable ignition of the gaseous fuel.

[0015] Advantageously, the prechamber is specifically designed as a microwave resonator cavity based on the microwave resonator theory. Ac- cording to the microwave resonator theory, the prechamber diameter is decided by the resonating frequency, and it is equivalent to ¼ of the wavelength at resonating frequency. The microwaves at high frequency (GHz level) are made to resonate in a quarter wave resonating cavity and results in amplified voltage near the tip of antenna allows breakdown of the sur- rounding gas and generates a plasma cloud. According to an embodiment of the invention, the prechamber is a microwave resonator cavity.

[0016] According to an embodiment of the invention, the microwave pulse duration is ranging from 1 microsecond level to 2 milliseconds level, and the number of pluses is ranging from one to five, depending of operational parameters of the engine. Typically, the pulsed power input to the microwave generator has a very high peak power (1 kW-100kW) but with lower average power (10W - 1 kW). The total microwave energy input can be varied by adjusting the total pulse duration of the energy input.

[0017] According to an embodiment of the invention the control unit comprises instructions to generate the number of pulsed power inputs used during one ignition depending of operational parameters of the engine. In other words, a multiple or split microwave pulses can be delivered by the microwave generator in every engine combustion cycle.

[0018] According to an embodiment of the invention the pulsed power input information comprises information of pulse duration and/or pulse interval of each pulse or of a cluster of pulses.

[0019] According to an embodiment of the invention the control unit is provided with executable instructions to activate the microwave plasma plug assembly in order to generate pulses. [0020] According to an embodiment of the invention the used number of pulses is 1 - 5 pulses per one ignition.

[0021 ] According to an embodiment of the invention the pulsed power in- put information is formed as a function of one or more operational parameter of the engine obtained during one or more previous ignitions.

[0022] According to an embodiment of the invention the pulsed power input information comprises a map of distinct number of pulse values for predetermined operational parameters of the engine.

[0023] According to an embodiment of the invention the prechamber assembly is provided with a controllable gaseous fuel inlet for feeding fuel into the prechamber space and the control unit is provided with executable instructions to control the fuel feed in synchronized manner with activating the microwave plasma plug assembly.

[0024] According to an embodiment of the invention the prechamber as- sembly is provided with a controllable gaseous fuel inlet for feeding gaseous fuel into the prechamber space and the control unit is provided with executable instructions to control the gaseous fuel feed to provide lean fuel mixture in the combustion chamber of the engine and lean fuel mixture in the prechamber.

[0025] According to an embodiment of the invention, the prechamber assembly is provided with a controllable gaseous fuel inlet for feeding gaseous fuel into the prechamber space and that the control unit is provided with executable instructions to control amount of the gaseous fuel to be fed to the prechamber space so as to provide a predetermined operational conditions in the prechamber.

[0026] According to an embodiment of the invention, the control unit is provided with executable instructions to control the pressure of the gase- ous fuel to be fed to the prechamber space in response of an engine load so as to provide a predetermined operational conditions in the prechamber.

[0027] According to an embodiment of the invention, the control unit is provided with executable instructions to control the pressure of the gaseous fuel to be fed to the prechamber space in response of an engine speed so as to provide a predetermined operational conditions in the prechamber. [0028] According to an embodiment of the invention, the air content in the gaseous fuel to be fed to the prechamber space is in practical circumstances zero. Advantageously, the gaseous fuel which does not contain air is introduced into the prechamber space and gas-air mixture is fed to the cylinder. Therefore, the conditions in the prechamber space can be controlled more accurately than in the prior art solutions. This provides advantageous conditions for microwave plasma to occur in the prechamber space.

[0029] According to an embodiment of the invention, the microwave plasma ignition assembly is provided with the prechamber space that has a greater circular cross section at first end of the prechamber space than at the longitudinal location of the end of the microwave antenna.

[0030] According to an embodiment of the invention, the radius of the prechamber space is arranged to chamber linearly from the radius at the location of the first end of the prechamber space to the radius at the longitudinal location of the end of the microwave antenna. According to an embodiment of the invention, in order to achieve the required electromagnetic field in the prechamber space, the size of the prechamber and the microwave antenna are specifically designed. According to the microwave resonator theory, the prechamber diameter and height is designed as a cylindrical cavity resonator to generate the electric field in the pre- chamber space.

[0031 ] According to an embodiment of the invention, the number of pulses in the train of microwave pulses and the pulse duration are controlled as a function of one or more operational parameter of the engine.

[0032] According to an embodiment of the invention the pulsed power input information is obtained from a map comprising distinct values of the number of pulses for predetermined operational parameters of the engine. As an example, it may be defined for example that at loads more than 20 % of nominal maximum power the control system uses 2 pulses per ignition. Loads less than 20 % of nominal maximum power, the control system uses 5 pulses per ignition. [0033] The prechamber assembly according to an embodiment of the invention is removably adaptable in a cylinder head of an internal combustion piston engine, and the prechamber assembly comprises a prechamber having a prechamber space therein, and a microwave plasma plug assembly which is provided with an microwave antenna extending into the prechamber space from its first end along a central longitudinal axis of the prechamber. The prechamber is provided with at least one orifice at the second end of the prechamber in order to provide a flow communication between the prechamber space and a main combustion chamber of the engine in the cylinder when assembled to the cylinder head. The prechamber has a circular cross section at each location in the direction of the central longitudinal axis along the microwave antenna towards the second end of the prechamber, and the wall of the prechamber space is arranged to curve rotationally symmetrically with a distance of the first radius in re- spect to an end of the microwave antenna i.e. being the center point, so as to form a spherical section to the prechamber. The wall of the prechamber space can be called in other words as an inner surface of the prechamber. [0034] Advantageously the prechamber space has a greater circular cross section at first end of the prechamber space than at the longitudinal location of the end of the microwave antenna.

[0035] According to an embodiment of the invention the radius of the pre- chamber space is arranged to chamber linearly from the radius at the location of the first end of the prechamber to the radius at the longitudinal location of the end of the microwave antenna.

Brief Description of Drawings [0036] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which

Figure 1 illustrates a microwave plasma ignition assembly according to a an embodiment of the invention,

Figure 2 illustrates a microwave plasma ignition assembly according to another embodiment of the invention,

Figure 3 illustrates a microwave plasma ignition assembly according to another embodiment of the invention,

Figure 4 illustrates a microwave plasma ignition assembly according to another embodiment of the invention,

Figure 5 illustrates a microwave plasma ignition assembly according to another embodiment of the invention, and

Figure 6 illustrate the activation procedure according to an embodiment of the invention.

Detailed Description of Drawings

[0037] Figure 1 show schematically an internal combustion piston engine 100 which comprises one or more cylinders 102 and cylinder heads 104, as is known in the art. The engine is provided with a microwave plasma ignition assembly 106 which is configured to ignite a combustible fuel mixture of gaseous fuel in a combustion chamber of the internal combustion piston engine 100 making use of a prechamber assembly. Advanta- geously, the prechamber assembly 14 is removably adapted for an attachment to the cylinder head 104. This makes it easy to replace and maintain. According to the embodiment of figure 1 , the microwave plasma ignition assembly comprises the prechamber assembly 14 comprising a prechamber element 20 having a prechamber space 15 therein in which the ignition is initiated. The prechamber assembly 14 has at least one orifice 18 in the prechamber element 20 so as to provide a flow communication between the prechamber space 15 and the combustion chamber of the engine 100 when assembled to the cylinder head 104 as illustrated in Figure 1 . The combustion chamber of the engine 100 may be called as a main combustion chamber. The microwave plasma ignition assembly further comprises a microwave plasma plug assembly 22 having a microwave antenna 23 in communication with the prechamber space 15. In this embodiment the microwave antenna 23 extends into the prechamber space 15 of the prechamber element 20. The microwave plasma ignition assembly is adapted to generate microwave plasma. In the embodiment of figure 1 the microwave plasma ignition assembly is adapted to generate microwave plasma between the microwave antenna and a wall 24 of the prechamber space 15. As can be seen, the wall 24 of the prechamber space 15 forms an inner surface of the prechamber element 20. The microwave plasma plug assembly generates and delivers microwave plasma around the microwave antenna effecting in the prechamber space 15. The microwave plasma plug assembly 22 is connected to a voltage source 60 that supplies microwave energy that is the source of voltage pulses generated in the microwave plasma plug assembly 22.

[0038] The microwave plasma ignition assembly comprises further a control unit 62 arranged to control the operation of, among other possible entities, a voltage source 60 of the microwave plasma ignition assembly. The control unit 62 of the microwave plasma ignition assembly 106 is provided with executable instructions to activate, or in other words trigger, the microwave plasma plug assembly 22, comprising a step of successively applying energy in order to generate microwave pulses. Figure 6 illustrates an activation procedure complying with executable instructions according to an embodiment of the invention.

[0039] The control unit 62 is provided with pulsed power input information for setting a variable of a train of microwave pulses to be used during each ignition when activating the microwave plasma plug assembly. The sequence of pulses used in one ignition is called as a pulse train. The pulse train is a variable which is arranged controllable during the operation of the engine. The pulse train itself comprises individual variables which are independently adjustable. The individual variables comprise number of pulses, length or duration of each pulse and interval or time between two successive pulses. The pulsed power input information is made available to the control unit 62 for example such that it is arranged in communication with a storage unit 63 into which the information has been stored. Additionally, the control unit 62 is provided with executable instructions to activate the microwave plasma plug assembly using the pulsed power input information to generate microwave plasma. In this connection it is meant by one ignition that a charge intended to combust during one cycle of the cylinder is ignited in one or more stages. It should be understood that the control unit may be a part of engines electric operating system 64. [0040] The pulsed power input information may be formed as a function of one or more operational parameter of the engine obtained during one or more previous ignitions of the same or all of the cylinders of the engine. This way the ignition is adapted to be suitable for each ignition of a cylinder of the engine. The number of microwave pulses is advantageously 1 -5 pulses per one ignition.

[0041 ] According to an embodiment of the invention the storage unit 63 comprises a map of distinct values of number of microwave pulses relating to one or more predetermined operational parameters of the engine.

[0042] In the embodiment of figure 1 the prechamber assembly 14 is provided with a gaseous fuel conduit 25 and an inlet 26 for feeding gaseous fuel into the prechamber space. The conduit is here arranged in communication with a source of gaseous fuel 70. The conduit 25 is provided with a valve member 27 which controls the gaseous fuel flow into the prechamber space 15. When the engine is operating the combustible gase- ous fuel is ignited in the prechamber element 20 i.e. in the prechamber space 15, then the ignited gaseous fuel passes via the orifices 18 into the combustion chamber 40 and there ignite the actual combustible fuel mixture. The actual combustible fuel mixture comprises advantageously air and gaseous fuel. The valve member 27 is arranged to be controlled by the control unit 63, so that the operation of the valve member 27, i.e. feeding of the gaseous fuel directly to the prechamber is arranged suitably in respect to the activating the microwave plasma plug assembly. The valve member 27 of the conduit 25 is controlled to open prior to or simultaneously with activating the microwave plasma plug assembly. [0043] Advantageously, the air content of the gaseous fuel is zero in practical circumstances. Therefore, the conditions in the prechamber space 15 can be controlled by means of the gaseous fuel (e.g. controlling pressure of the gaseous fuel feed, duration of the gaseous fuel feed and/or amount of the gaseous fuel). The control unit is provided with executable instructions to control the timing of commencing the gaseous fuel feed and duration of the gaseous fuel feed in synchronized manner with activating the microwave plasma plug assembly. In this manner it is possible to have optimized circumstances in the prechamber space 15 for ignition of gaseous fuel. [0044] As can been seen figure 1 the cylinder head 10 comprises an inlet channel 50 for introducing air into the combustion chamber 40. Preferably, the major part of the gaseous fuel used in the engine is mixed with the air in the inlet channel 50. As is known in the art the inlet channel 50 is provided with at least one inlet valve 25 so as to control a fuel flow into the cylinder 40 of the engine. The cylinder head 10 comprises also an exhaust gas channel 52 which is provided with at least one exhaust gas valve, respectively 46.

[0045] Figure 2 illustrates a prechamber assembly 14 according to an em- bodiment of the invention. The prechamber assembly 14 is adapted re- movably for an attachement in an opening of a cylinder head 10 of a combustion engine to operate as ignition source for the engine. This kind of a prechamber assembly is specifically advantageous for fuel ignition when the gaseous fuel is combusted in the combustion chamber and the engine is operating using a lean-burn Otto cycle.

[0046] The prechamber assembly 14 comprises a microwave plasma plug assembly 22 which is provided with a microwave antenna 23. The microwave antenna 23 extends a distance into the prechamber space 15 of the prechamber element 20 from its first end 20.1 along a central longitudinal axis CL of the prechamber element 20. The prechamber 20 is provided with at least one orifice 18 at the second end 20.2 of the prechamber element 20 so as to provide a flow communication between the prechamber space 15 and a combustion chamber 40 of the engine. The first end and the second end refer to a direction of the central longitudinal axis CL. The number of orifices may vary according to the actual application.

[0047] The prechamber space 15 has a circular cross section at each longitudinal location X in the direction of the central longitudinal axis CL along the microwave antenna 23 towards the second end of the prechamber element 20, perpendicular to the axis CL. As an example, the cross sections at two locations are denoted by radiuses R1 and R2, as an example. Thus the microwave antenna and a wall 24 of the prechamber space 15 are coaxially arranged with each other. This applies to longitudinal section beginning from the first end 20.1 of prechamber element 20 and ending to the free end of the microwave antenna 23 in the chamber. The prechamber element 20 is advantageously rotationally symmetrical with respect to a central axis CL. [0048] Additionally, the inner surface of the prechamber element 20, specifically the wall 24 of the prechamber space 15, is arranged to curve rotationally symmetrically with a distance of the first radius Re from an end of the microwave antenna 23 forming a to some degree spherical section 20.3 to the prechamber space 15 of the prechamber element 20. The spherical section 20.3 is longitudinally located as an extension of the inner wall 24 joining to the tangent of the spherical section 20.3. The first radius Re is equal to the radius R2 at the longitudinal location at tip of the microwave antenna 23.

[0049] In this embodiment the prechamber element 20 comprises a tip section 20.4 which is adapted to extend into the combustion chamber 40 of the engine when installed. Particularly, the at least one orifice 18 of the prechamber element 20 is arranged to the tip section 20.4 of the prechamber element 20 so as to provide a flow communication between the prechamber space 15 and a combustion chamber 40 of the engine. The tip section is arranged as an extension of the spherical section 20.3 having a smaller cross sectional area than the prechamber space 15 at the location of radius R2, which provides increased gas velocity and improved mixing of the gases. The actual shape of the tip section may vary depending on the case and it is illustrated here to have a cylindrical form with a rounded end, as an example.

[0050] The form of the prechamber element 20 may also be described by considering that it has a first region and a second region in the longitudinal direction. The regions have different cross-sectional areas so that the cross-sectional area perpendicular to the central axis CL is smaller in a region at the free end of the microwave antenna than the cross-sectional area in a region at which the microwave antenna 23 passes into the prechamber space 15. This means that radiuses defined from a central axis CL of the prechamber element 20 to a wall of the prechamber space 15 are smaller in a region at the end of the microwave antenna than radiuses at the first end 20.1 . For example, the cross sectional area of the prechamber space 15 may be tapering when moving along the central axis CL from the cap surface 16 towards the second end 20.2. [0051 ] The prechamber assembly 14 comprises a microwave plasma ignition assembly 12 for igniting a combustible fuel mixture in the combustion chamber 40 of the internal combustion piston engine. The microwave plasma ignition assembly 12 is provided with a microwave plasma plug assembly 22 having a microwave antenna 23. The microwave plasma ignition assembly is capable of generating microwave pulse with high power (at kW level) and high frequency (at GHz level),which create microwave plasma in the prechamber space 15 around the microwave antenna 23 as is described in connection with figure 1 . The wall 24 of the prechamber space 15 include a cap surface 16 and a portion of the microwave antenna 23 extends into the prechamber space 15 through the cap surface 16. Advantageously, the microwave plasma plug assembly 22 and the prechamber assembly 14 may be replaceably assembled to the head 10 of the engine. The microwave plasma plug assembly 22 is provided with an insulator 21 so as to at least partially surround the microwave antenna 23.

[0052] As illustrated in Fig. 2, the microwave plasma plug assembly 22 is connected to a voltage source 60 that supplies energy that is the source of voltage pulses to the microwave plasma plug assembly 22. The microwave plasma plug assembly 22 may be provided with a control unit 62 that controls an operation of the voltage source 60 and thus also the operation of the microwave plasma plug assembly 22. The control unit 62 and the voltage source 60 are designed to generate pulsed power with high fre- quency and control the duration of microwave energy input, as explained in connection with figures 1 and 6.

[0053] The microwave antenna 23 of the microwave plasma plug assembly 22 extends at least partially into the prechamber space 15 wherein the plasma generated for igniting the fuel. The control unit 62 is arranged to control the operation of the microwave plasma plug assembly 22 so that the microwave plasma takes place in the prechamber space 15. The specially designed prechamber acts as a cavity resonator, which confines electromagnetic fields in the microwave region of spectrum. Once the electromagnetic fields are built-up high enough to exceed the breakdown threshold of gas mixture, microwave plasma then occurs between the microwave antenna 23 of the microwave plasma plug assembly 22 and the walls 24 of the prechamber space 15. This ensures ignition in the prechamber 20 simultaneously at various locations so as to improving combustion stability in the prechamber element 20 as well as in the combus- tion chamber 40.

[0054] The prechamber element 20 is provided with a conduit 25 for introducing gaseous fuel into the prechamber space 15. The conduit 25 is arranged to open into the prechamber space 15. Obviously, the conduit 25 is provided with a valve member even if not shown in figure 2 so as to opening or closing the gas flow into the prechamber space 15. Furthermore, the conduit 25 is arranged in flow communication with a gaseous fuel source 70. [0055] When the engine is operating the combustible gaseous fuel is ignited in the prechamber space 15, then the ignited fuel passes via the orifices 18 into the combustion chamber 40 and there ignite the actual combustible fuel mixture. The fuel combusted in the combustion chamber 40 is preferably gaseous fuel.

[0056] Figure 3 discloses another embodiment of the invention. The prechamber assembly 14 in the figure 3 comprises elements corresponding to those shown in figure 1 and 2 except that the prechamber element 20 is of different form. Also in this embodiment the prechamber assembly 14 comprises a microwave plasma plug assembly 22 with a microwave antenna 23 which extends a distance into the prechamber space 15 from the first end 20.1 of the prechamber along a central longitudinal axis CL of the prechamber 20.

[0057] The prechamber space 15 in this embodiment has a cylindrical cross section in the direction of the central longitudinal axis CL along the microwave antenna 23 towards the second end of the prechamber element 20, perpendicular to the axis CL. The equal cross sections at two locations are denoted by radiuses R1 and R2, as an example. This applies to longitudinal section beginning from the first end 20.1 of prechamber el- ement 20 and ending to tip or end of the microwave antenna 23. The prechamber element 20 is advantageously rotationally symmetrical with respect to a central axis CL.

[0058] Additionally, an inner wall of the prechamber element 20, specifi- cally the wall 24 of the prechamber space 15, is arranged to curve rotationally symmetrically with a distance of the first radius Re from an end of the microwave antenna 23 forming to some degree of spherical section 20.3 to the prechamber element 20. The spherical section 20.3 is longitudinally located as an extension of the inner wall 24 joining to the tangent of the spherical section 20.3. In this embodiment the first radius Re at the longitudinal location at tip of the microwave antenna 23 is equal to the radius R2 and radius R1 .

[0059] Also in this embodiment the prechamber element 20 comprises a tip section 20.4 which is adapted to extend into the combustion chamber 40 of the engine when installed. Particularly, the at least one orifice 18 of the prechamber element 20 is arranged to the tip section 20.4 of the prechamber element 20 to provide a flow communication between the prechamber element 20 and a combustion chamber 40 of the engine. The actual shape of the tip section may vary depending on the case and it is illustrated here to have a cylindrical form with a rounded end, as an example. [0060] The prechamber assembly 14 comprises a microwave plasma ignition assembly 12 for igniting a combustible fuel mixture in the combustion chamber 40 of the internal combustion piston engine. The microwave plasma ignition assembly 12 is provided with a microwave plasma plug assembly 22 having a microwave antenna 23. The microwave plasma ignition assembly is capable of generating microwave pulses with high power (at kW level) and high frequency (at GHz level) which create microwave plasma in the prechamber space between the microwave antenna and the wall of prechamber space. Advantageously, the microwave plasma plug assembly 22 and the prechamber assembly 14 may be re- placeably assembled to the head 10 of the engine. The microwave plasma plug assembly 22 is provided with an insulator 21 so as to at least partially surround the microwave antenna 23 as explained in connection with fig- ures 1 and/or 2.

[0061 ] The microwave antenna 23 of the microwave plasma plug assembly 22 extends at least partially into the prechamber space 15 wherein a microwave plasma application is practised for igniting the fuel. The plasma cloud then occurs between the microwave antenna 23 of the microwave plasma plug assembly 22 and the walls 24 of the prechamber space 15. This ensures ignition in the prechamber element 20 simultaneously at various locations so as to improving combustion stability in the prechamber element 20 as well as in the combustion chamber 40.

[0062] Figure 4 discloses another embodiment of the invention. The prechamber assembly 14 in the figure 4 comprises elements corresponding to those shown in figure 2 except that the prechamber element 20 is of different construction. As can be seen in the figure 4, the prechamber body 30 is formed of two main parts 31 , 32, namely an end part 31 and main part 32. The end part comprises the spherical section 20.3 and the tip section 20.4, whereas the main part 32 comprises the prechamber space in which the microwave antenna is located. The tip section 20.4 and the spherical section are separate unit so that it is possible, for example use one type of spherical section with several type of tip sections, supporting modular implementation of the embodiment.

[0063] Figure 5 discloses another embodiment of the invention. The pre- chamber assembly 14 in the figure 5 comprises elements corresponding to those shown in figure 1 except that the prechamber element 20 is of different construction. As can be seen in the figure 5, here the microwave plasma ignition assembly 12 is provided with a body part 40 through which the microwave antenna and the gaseous fuel conduit 25 are brought into the chamber, and by means of which the assembly may be releasably attached to the first end 20.1 of the prechamber element 20. This embodiment provides easily interchangeable and serviceable microwave an- tenna.

[0064] Figure 6 illustrates an activation procedure complying with executable instructions according to an embodiment of the invention. There is shown, in the horizontal axis a top dead center (TDC) situation where the triggering 74 or commencing of the activation procedure is performed at a predetermined moment. The actual triggering timing may be set to be at the top dead center or advanced or retarded to some extent in respect to the TDC position depending e.g. on the operational circumstances of the engine. The triggering act causes the microwave plasma plug assembly 106 to successively apply energy to the microwave antenna and thus generate microwave pulses 76. As can be seen, the triggering act causes a generation of one microwave pulse or a burst of sequence of microwave pulses 76 during one ignition. In the figure there is exemplary shown the on/off (1 ,0) status the triggering 74 and the microwave pulses 76. Each of the microwave pulses is controlled to have duration D. The actual duration of one or more of the microwave pulses in the burst may however vary. Also the interval I between the pulses may vary. The executable instructions to the control unit 62 comprise instruction to provide one or several microwave pulses during one ignition. In this connection it is meant by one ignition that a charge intended to combust during one cycle of the cylinder is ignited in one or more stages.

[0065] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.