EP3234348A1

EP3234348A1 - An ignition assembly and a method of igniting a combustible fuel mixture in a combustion chamber of an internal combustion piston engine

Info

Publication number: EP3234348A1
Application number: EP14805958.7A
Authority: EP
Inventors: Petri Saari; Jingzhou Yu
Original assignee: Wartsila Finland Oy
Current assignee: Wartsila Finland Oy
Priority date: 2014-11-12
Filing date: 2014-11-12
Publication date: 2017-10-25
Also published as: KR101932367B1; WO2016075357A1; KR20170080676A; CN107002625A

Abstract

Invention relates to an ignition assembly configured to ignite a combustible fuel mixture in a combustion chamber of an internal combustion piston engine, comprising a prechamber assembly (14) adapted for attachment in an opening of a cylinder head (10), the prechamber assembly has at least one orifice (18) to provide a flow communication between the prechamber and the combustion chamber of the engine (100), the ignition assembly further comprising a pulsed plasma plug assembly having at least one electrode in communication with the prechamber space and adapted to generate a plurality of plasma streamers between at least two electrodes, and a control unit provided with executable instructions to activate the pulsed plasma plug assembly in order to generate pulses. It is characteristic to the invention that the control unit (62) is provided with pulse information for setting variables of a train of pulses used during each ignition when activating the pulsed plasma plug assembly, and the control unit is further provided with executable instructions to activate the pulsed plasma plug assembly using the pulse information to generate the pulses.

Description

An ignition assembly and a method of igniting a combustible fuel mixture in a combustion chamber of an internal combustion piston engine

Technical field

[001 ] The present invention relates to an ignition assembly according to the preamble of claim 1 .

[002] The present invention relates also to a method of igniting a com- bustible fuel mixture in a combustion chamber of an internal combustion piston engine by according to the preamble of independent method claim.

Background art [003] 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.

[004] 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 accu- rately. Therefore, the timing and the strategy of the ignition plays very important role.

[005] 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. [006] 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.

[007] FR 2 886 689 A1 proposes also corona ignition in a precombus- tion chamber, by way of which actual ignition is then effected in the com- bustion 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 precom- bustion 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.

[008] US 201 1/0100322 A1 discloses approach to ignite the fuel-air mix- ture 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 comprises 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.

[009] 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 pre- chamber.

[0010] US 2014/0109886 A1 discloses a system for and a method of providing pulsed power to improve the performance efficiency of the en- gine. Pulsed power that converts a low-power into a high-power and long-time input into a short-time output is also employed to improve the fuel efficiency.

[001 1 ] An object of the invention is to provide an ignition assembly in which the performance is considerably improved compared to the prior art solutions.

Disclosure of the Invention

[0012] Object of the invention is substantially met by an ignition assembly configured to ignite a combustible fuel mixture in a combustion chamber of an internal combustion piston engine, comprising a prechamber assembly adapted for attachment in an opening of a cylinder head, the prechamber assembly has at least one orifice to provide a flow communication between the prechamber and the combustion chamber of the en- gine, the ignition assembly further comprising a pulsed plasma plug assembly having at least one electrode in communication with the prechamber space and adapted to generate a plurality of plasma streamers between the at least two electrodes, and a control unit provided with executable instructions to activate the pulsed plasma plug assembly in or- der to generate pulses. It is characteristic to the invention that the control unit is provided with pulse information for setting variables of a train of pulses used during each ignition when activating the pulsed plasma plug assembly, and the control unit is further provided with executable instructions to activate the pulsed plasma plug assembly using the pulse infor- mation to generate the pulses. [0013] This provides an ignition assembly for igniting a combustible fuel mixture for which performance is considerably improved by using pulsed power. The ignition of the fuel mixture in the combustion prechamber is fast and more reliable compared to prior art solutions. The ignition assembly enhances 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 plasma streamers are very reactive and the streamers are taken place at several places at a time in the combustion prechamber. In addition, a leaner burn capability is enhanced. Streamers are a generic mode of electric breakdown. They play a role in the initial stages of sparks. Streamers are characterized by a self-generated field enhancement at the head of the growing discharge channel. [0014] The sequence of successive pulses used in one ignition is called here 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 during one ignition, length or duration of each pulse and interval or time between two successive pulses in the train of pulses.

[0015] According to an embodiment of the invention the control unit comprises instructions to generate the number of pulses used during one ig- nition depending of operational parameters of the engine.

[0016] According to an embodiment of the invention the pulse information comprises information of pulse duration and/or pulse interval of each pulse or of a cluster of pulses.

[0017] According to an embodiment of the invention the control unit is provided with executable instructions to activate the pulsed plasma plug assembly in order to generate pulses, each pulse having duration less than 100ns.

[0018] According to an embodiment of the invention the used number of pulses is 2 - 40 pulses per one ignition.

[0019] According to an embodiment of the invention the number of pulses is less than 30 pulses per one ignition. [0020] According to an embodiment of the invention the number of pulses is less than 20 pulses per one ignition.

[0021 ] According to an embodiment of the invention the pulse information is formed as a function of one or more operational parameter of the en- gine obtained during one or more previous.

[0022] According to an embodiment of the invention the pulse 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 acti- vating the pulsed plasma plug assembly.

[0024] 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 executa- ble instructions to control the fuel feed to provide lean fuel mixture in the combustion chamber of the engine and lean fuel mixture in the prechamber. [0025] Another object of the invention is met by a method of igniting a combustible fuel mixture in a combustion chamber of an internal combustion piston engine by means of a plasma pulsed ignition, comprising the steps of: igniting a combustible fuel mixture in a combustion chamber of an internal combustion piston engine initiated by means of a pre- chamber assembly with a prechamber space, and activating a pulsed plasma plug assembly to generate pulses thus generating a plurality of plasma streamers between the at least one electrode and a wall of the prechamber space, in which the pulsed plasma plug assembly is activat- ed with a controllable train of pulses and generate plurality of the pulses.

[0026] In order to minimize of avoid the transition from plasma to spark formation, the duration of the voltage pulses are substantially short. According to an embodiment of invention the method comprises the step of producing number of pulses each pulse having duration of 10 ns - 100 ns in the train of pulses. This also prevents additional heat generated and temperature level to be adequate during the combustion thereby preventing pollutant nitrogen oxides to be generated. Due to the high- energy electron created by the pulsed plasma plug assembly, the com- bustion of the air-fuel mixture is enhanced.

[0027] According to an embodiment of invention the method comprises the step of producing a rise time of the fast rising voltage pulse that is less than 100 nanoseconds.

[0028] According to an embodiment of the invention, the number of pulses in the train of pulses is controlled as a function of one or more operational parameter of the engine. [0029] According to an embodiment of the invention the fuel is introduced into the engine such that lean fuel (lamda λ >1 ,4) mixture is provided into the prechamber of the engine by controlling fuel fed by into the prechamber via the prechamber gas fuel and leaner (lambda being for ex- ample even >1 ,4) fuel mixture is provided into the combustion chamber of the engine. Generally, the air-fuel equivalence ratio, λ (lambda), is the ratio of actual air-fuel ratio to stoichiometry for a given mixture. λ= 1 .0 is at stoichiometry, rich mixtures λ < 1 .0, and lean mixtures λ > 1 .0.

[0030] This provides the effect of simultaneously decreasing NOx emission and improving overall engine performance which ensuring effective and stable ignition of the gaseous fuel. [0031 ] According to an embodiment of the invention the pulse 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 20 pulses per ignition.

[0032] The prechamber assembly according to an embodiment of the invention is adaptable in a cylinder head of an internal combustion piston engine, and the prechamber assembly comprises a prechamber, and a pulsed plasma plug assembly which is provided with at least one elec- trode extending into the prechamber 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 and a main combustion chamber of the engine in the cylinder. The prechamber has a circular cross section at each location in the direction of the central longitudinal axis along the electrode towards the second end of the prechamber, and the inner surface of the prechamber is arranged to curve rotationally symmetrically with a distance of the first radius in respect to an end of the electrode i.e. being the center point, so as to form a spherical section to the prechamber. [0033] Advantageously the prechamber has a greater circular cross section at first end of the prechamber than at the longitudinal location of the end of the electrode. [0034] According to an embodiment of the invention the radius of the prechamber 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 electrode.

Brief Description of Drawings

[0035] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which

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

Figure 2 illustrates an ignition assembly according to another embodiment of the invention,

Figure 3 illustrates an ignition assembly according to another embodiment of the invention,

Figure 4 illustrates an ignition assembly according to another embodiment of the invention,

Figure 5 illustrates an ignition assembly according to another embodi- ment of the invention,

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

Figure 7 illustrates an ignition assembly according to another embodiment of the invention, and

Figure 8 illustrates an ignition assembly according to still another embodiment of the invention. Detailed Description of Drawings

[0036] 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 an 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. According to the embodi- ment of figure 1 , the ignition assembly comprises a prechamber assembly 14 which has at least one orifice 18 to provide a flow communication between the prechamber and the combustion chamber of the engine 100. The combustion chamber of the engine 100 may be called as a main combustion chamber. The prechamber assembly is provided with a chamber space in which the ignition is initiated. The ignition assembly further comprises a pulsed plasma plug assembly 22 having at least one electrode 23 in communication with the prechamber space 15. In this embodiment the electrode 23 extends into the prechamber 20. The ignition assembly is adapted to generate a plurality of plasma streamers be- tween at least two electrodes adapted to the assembly. In the embodiment of figure 1 the ignition assembly is adapted to generate a plurality of plasma streamers between the at least one electrode and a wall of the prechamber space, which wall operates as one of the at least two electrodes. The approach of the present invention is to apply a so-called pulsed plasma ignition. Pulsed plasma is a non-equilibrium plasma. It is also referred to as non-thermal or low temperature plasma. In such a pulsed-plasma ignition system, after activating the assembly, a number of high voltage pulses is generated, which pulses have a duration from 5 nanoseconds to 100 nanoseconds and each of the pulses end before the arc occurs. The pulsed plasma plug assembly generates and delivers fast rise, ultra-short, high-voltage pulses effecting in the prechamber space. The pulsed plasma plug assembly 22 is connected to a voltage source 60 that supplies energy that is the source of voltage pulses generated in the pulsed plasma plug assembly 22.

[0037] The ignition assembly comprises further a control unit 62 ar- ranged to control the operation of, among other possible entities, a voltage source 60 of the ignition assembly. The control unit 62 of the ignition assembly 106 is provided with executable instructions to activate, or in other words trigger, the pulsed plasma plug assembly 106, comprising a step of successively applying energy in order to generate pulses, each pulse having a duration less than 100ns. Figure 6 illustrates an activation procedure complying with executable instructions according to an embodiment of the invention.

[0038] The control unit 62 is provided with pulse information for setting a variable of a train of pulses to be used during each ignition when activating the pulsed 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 pulse 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 pulsed plasma plug assembly using the pulse information to generate plurality of pulses. 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. [0039] The pulse 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 pulses is advantageously 2-40 pulses per one ignition. [0040] According to an embodiment of the invention the storage unit 63 comprises a map of distinct values of number of pulses relating to one or more predetermined operational parameters of the engine.

[0041 ] In the embodiment of figure 1 the prechamber assembly 14 is provided with a gaseous fuel conduit 25 and an inlet 26 for feeding 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 fuel flow into the prechamber space. When the engine is operating the combustible fuel mixture is ignited in the prechamber 20, then the ignited fuel mixture passes via the orifices 18 into the combustion chamber 40 and there ignite the actual combustible fuel mixture. 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 fuel directly to the prechamber is arranged suitably in respect to the activating the pulsed plasma plug assembly. The valve member 27 of the conduit 25 is controlled to open prior to or simultaneously with activating the pulsed plasma plug assembly. The control unit is provided with executable instructions to control the timing of commencing the fuel feel and duration of the fuel feed in synchronized manner with activating the pulsed plasma plug assembly. In this manner it is possible to have optimized circumstances in the prechamber for ignition of fuel. Furthermore, the wall 24 of the combustion prechamber 20 may act as one of the electrodes so that an electric field is produced between the pulsed plasma plug assembly 22 and the wall 24 that is grounded when the voltage source 60 supplies the pulsed plasma plug assembly 22 with voltage. [0042] 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. [0043] Figure 2 illustrates a prechamber assembly 14 according to an embodiment of the invention. The prechamber assembly 14 is adapted for 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 gas- eous fuel is combusted in the combustion chamber and the engine is operating using a lean-burn Otto cycle.

[0044] The prechamber assembly 14 comprises a pulsed plasma plug assembly 22 which is provided with an electrode 23. The electrode 23 extends a distance into the prechamber 20 from its first end 20.1 along a central longitudinal axis CL of the prechamber 20. A wall 24 of the prechamber 20 is adapted to act as a cathode for the electrode 23 of the pulsed plasma plug assembly. The prechamber 20 is provided with at least one orifice 18 at the second end 20.2 of the prechamber 20 so as to provide a flow communication between the prechamber 20 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. [0045] The prechamber 20 has a circular cross section at each longitudinal location X in the direction of the central longitudinal axis CL along the electrode 23 towards the second end of the prechamber 20, perpendicu- lar 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 electrode and the inner wall 24 of the prechamber are coaxially arranged with each other. This applies to longitudinal section beginning from the first end 20.1 of prechamber 20 and ending to the free end of the electrode 23 in the chamber. The prechamber 20 is advantageously rotationally symmetrical with respect to a central axis CL.

[0046] Additionally, the prechamber 20, specifically the inner wall 24, is arranged to curve rotationally symmetrically with a distance of the first radius Re from an end of the electrode 23 forming a to some degree spherical section 20.3 to the prechamber 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 electrode 23.

[0047] In this embodiment the prechamber 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 pre- chamber 20 is arranged to the tip section 20.4 of the prechamber 20 to provide a flow communication between the prechamber 20 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 20 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.

[0048] The form of the prechamber 20 may also be described by consid- ering that is 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 electrode than the cross-sectional area in a region at which the electrode 23 passes into the prechamber 20. This means that radiuses defined from a central axis CL of the prechamber 20 to an inner surface of the prechamber are smaller in a region at the end of the electrode than radiuses at the first end 20.1 . For example, the cross sectional area of the prechamber 20 may be tapering when moving along the central axis CL from the cap surface 16 towards the second end 20.2.

[0049] The prechamber assembly 14 comprises an ignition assembly 12 for igniting a combustible fuel mixture in the combustion chamber 40 of the internal combustion piston engine. The ignition assembly 12 is provided with a pulsed plasma plug assembly 22 having at least one electrode 23. The ignition assembly is capable of generating fast rising voltage pulses which creates a plurality of plasma streamers in the prechamber without formation of a conventional spark as is described in connection with figure 1 . The wall surfaces 24 include a cap surface 16 and a portion of the electrode 23 extends into the prechamber 20 through the cap surface 16. Advantageously, the pulsed plasma plug assembly 22 and the prechamber assembly 14 may be replaceably assembled to the head 10 of the engine. The pulsed plasma plug assembly 22 is provided with an insulator 21 so as to at least partially surround the electrode 23. [0050] As illustrated in Fig. 2, the pulsed plasma plug assembly 22 is connected to a voltage source 60 that supplies energy that is the source of voltage pulses to the pulsed plasma plug assembly 22. The pulsed 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 opera- tion of the pulsed plasma plug assembly 22. The control unit 62 and the voltage source 60 are designed to generate fast application of voltage pulses, as explained above in connection with figures 1 and 6. [0051 ] The electrode 23 in the prechamber 20 of the pulsed plasma plug assembly 22 extends at least partially into the prechamber 20 wherein the plasma streamers generated for igniting the fuel. The control unit 62 is arranged to control the operation of the pulsed plasma plug assembly 22 so that the plasma pulsed ignition with the plasma streamers takes place in the prechamber 20. Furthermore, the inner wall 24 of the prechamber 20 preferably acts as a counterpart electrode so that an electric field is produced between the pulsed plasma plug assembly 22 and the wall 24 that is grounded when the voltage source 60 supplies the pulsed plasma plug assembly 22 with voltage. In other words, the electrode 23 of the pulsed plasma plug assembly 22 acts as an anode whereas the wall 24 of the prechamber 20 acts as a cathode. Streamers then occur between the electrode 23 of the pulsed plasma plug assembly 22 and the inner walls 24 of the prechamber 20. This ensures ignition in the prechamber 20 simultaneously at various locations so as to improving combustion stability in the prechamber 20 as well as in the combustion chamber 40. [0052] The prechamber 20 is provided with a conduit 25 for introducing gaseous fuel or the mixture of fuel and air into the prechamber 20. The conduit 25 is arranged to open into the prechamber 20. 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 20. Fur- thermore, the conduit 25 is arranged in flow communication with a fuel source 70.

[0053] When the engine is operating the combustible fuel mixture is ignited in the prechamber 20, then the ignited fuel mixture 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. [0054] Figure 3 discloses another embodiment of the invention. The pre- chamber assembly 14 in the figure 3 comprises elements corresponding to those shown in figure 1 and 2 except that the prechamber 20 is of dif- ferent form. Also in this embodiment the prechamber assembly 14 comprises a pulsed plasma plug assembly 22 with an electrode 23 which extends a distance into the prechamber 20 from the first end 20.1 of the prechamber along a central longitudinal axis CL of the prechamber 20. [0055] The prechamber 20 in this embodiment has a cylindrical cross section in the direction of the central longitudinal axis CL along the electrode 23 towards the second end of the prechamber 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 20 and ending to tip or end of the electrode 23. The prechamber 20 is advantageously rotation- ally symmetrical with respect to a central axis CL.

[0056] Additionally, the prechamber 20, specifically the inner wall 24, is arranged to curve rotationally symmetrically with a distance of the first radius Re from an end of the electrode 23 forming a to some degree of spherical section 20.3 to the prechamber 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 electrode 23 is equal to the radius R2 and radius R1 .

[0057] Also in this embodiment the prechamber 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 20 is arranged to the tip section 20.4 of the prechamber 20 to provide a flow communication between the prechamber 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. [0058] The prechamber assembly 14 comprises an ignition assembly 12 for igniting a combustible fuel mixture in the combustion chamber 40 of the internal combustion piston engine. The ignition assembly 12 is provided with a pulsed plasma plug assembly 22 having at least one electrode 23. The ignition assembly is capable of generating fast rising volt- age pulses which creates a plurality of plasma streamers in the prechamber between the electrode and the prechamber wall. Advantageously, the pulsed plasma plug assembly 22 and the prechamber assembly 14 may be replaceably assembled to the head 10 of the engine. The pulsed plasma plug assembly 22 is provided with an insulator 21 so as to at least partially surround the electrode 23 as explained in connection with figures 1 and/or 2.

[0059] The electrode 23 in the prechamber 20 of the pulsed plasma plug assembly 22 extends at least partially into the prechamber 20 wherein a plasma pulsed application with the plasma streamers is practised for igniting the fuel. The streamers then occur between the electrode 23 of the pulsed plasma plug assembly 22 and the walls 24 of the prechamber 20. This ensures ignition in the prechamber 20 simultaneously at various locations so as to improving combustion stability in the prechamber 20 as well as in the combustion chamber 40.

[0060] 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 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 in which the electrode 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 implementa- tion of the embodiment

[0061 ] Figure 5 discloses another embodiment of the invention. The prechamber assembly 14 in the figure 5 comprises elements corresponding to those shown in figure 1 except that the prechamber 20 is of different construction. As can be seen in the figure 5, here the ignition assembly 12 is provided with a body part 40 through which the electrode and the 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 or the prechamber 20. This embodiment provides easily interchangeable and ser- viceable electrode.

[0062] 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 pulsed plasma plug assembly 106 to successively apply energy to the electrodes and thus generate pulses 77. As can be seen, the triggering act causes a generation of a burst of sequence of pulses 76 during one ignition. In the figure there is exemplary shown the on/off (1 ,0) status the triggering 74 and the pulses 76. Each of the pulses is controlled to have duration D advanta- geously less than 100ns. The actual duration of one or more of the 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 in- struction to provide several 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.

[0063] Figure 7 shows schematically an internal combustion piston en- gine 100 which comprises one or more cylinders 102 and cylinder heads 104. The engine is provided with an ignition assembly 106 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 14. The prechamber assembly 14 has at least one orifice 18 to provide a flow communication between the prechamber and the combustion chamber of the engine 100. The prechamber assembly is provided with a chamber space in which the ignition is initiated. The ignition assembly further comprises a pulsed plasma plug assembly 22 having at least one electrode 23 in communication with the prechamber space 15.

[0064] In this embodiment the pulsed plasma plug assembly 22 comprises a removably assembled plug 722. The plug 722 comprises a body 724 by means of which the plug 722 may be attached to the prechamber 20. The body 722 is provided with a cavity 726 at its one end which is arranged to open into the prechamber space 15 when installed into the prechamber 20. The body is advantageous provided with outer thread which co-operated with an inner thread in the prechamber 20 by means of which the body and thus the plug 722 may be attached to the pre- chamber. The plug 722 is also provided with an electrode 23 and an insulator 21 to support and at least partially surround the electrode 23. The electrode 23 is electrically insulated from the body 724, and the prechamber as well, and the body 724 operate as a ground electrode of the assembly. [0065] The electrode 23 extends into the cavity 726 of the body and is arranged to it central axis. The cavity 726 is delimited by the wall of the cavity such that the cavity is cylindrical having circular cross section. The length of the cavity 726 and the length of the electrode 23 are substan- tially equal to each other. In other words the electrode extends from a bottom of the cavity to an edge of the cavity. The cavity is formed inside a sleeve-like part at the end of the body. The ignition assembly is adapted to generate a plurality of plasma streamers between the electrode and the cylindrical wall of the cavity 726. In the embodiment of fig- ure 7 the ignition assembly is adapted to generate a plurality of plasma streamers into the cavity space of the plug. The pulsed plasma plug assembly generates and delivers fast rise, ultra-short, high-voltage pulses effecting in the prechamber space via the plug cavity. The pulsed plasma plug assembly 22 is connected to a voltage source 60 that supplies en- ergy that is the source of voltage pulses generated in the pulsed plasma plug assembly 22.

[0066] In the embodiment of figure 7 the body 722 and the electrode 23 are arranged to extend into the prechamber space 15 which is beneficial for improving ignition of the fuel/air mixture in the prechamber 20. This is due to the fact that the location of plasma streamers is favourable for ignition. The sleeve-like part of the body 724, which extends into the prechamber, is according to an embodiment of the invention provided with porous or perforated wall section 725. This enhances the fluid communi- cation between the prechamber space 15 and the cavity 726 e.g. improving the flame propagation.

[0067] The ignition assembly 12 comprises further a control unit 62 and a storage unit arranged to operate similar manner to the embodi- ment of figure 1 . Thus the operation and the components of the assembly corresponds to what is mentioned the reference to figure 1 above. [0068] Figure 8 shows an ignition assembly 106 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 14, which is otherwise similar to that shown in figure 7 except that in the embodiment of figure 8 the body 722 and the electrode 23 are arranged to extend to be flush with the inner surface of the prechamber space 15 which is beneficial for improving corrosion resistance and cooling the body 722. [0069] The ignition assembly 12 comprises further a control unit 62 and a storage unit arranged to operate similar manner to the embodiment of figure 1 . Thus the operation and the components of the assembly corresponds to what is mentioned the reference to figure 1 above.

[0070] While the invention has been described herein by way of exam- pies 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 ap- pended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.

Claims

1 . An ignition assembly configured to ignite a combustible fuel mix- ture in a combustion chamber of an internal combustion piston engine, comprising a prechamber assembly (14) adapted for attachment in an opening of a cylinder head (10), the prechamber assembly has at least one orifice (18) to provide a flow communication between the prechamber and the combustion chamber of the engine (100), the ignition as- sembly further comprising a pulsed plasma plug assembly having at least one electrode in communication with the prechamber space and adapted to generate a plurality of plasma streamers between at least two electrodes, and a control unit provided with executable instructions to activate the pulsed plasma plug assembly in order to generate pulses, characterized in that the control unit (62) is provided with pulse information for setting variables of a train of pulses used during each ignition when activating the pulsed plasma plug assembly, and the control unit is further provided with executable instructions to activate the pulsed plasma plug assembly using the pulse information to generate the pulses .

2. The ignition assembly according to claim 1 , characterized in that the control unit is adapted to form pulse information as a function of one or more operational parameter of the engine obtained during one or more previous ignitions.

3. The ignition assembly according to claim 1 , characterized in that the control unit is provided with executable instructions to activate the pulsed plasma plug assembly generating pulses each of which having duration less than 100ns.

4. The ignition assembly according to claim 1 , characterized in that the number of pulses is 2 - 40 pulses per one ignition.

5. The ignition assembly according to claim 4, characterized in that the number of pulses is less than 30 pulses per one ignition.

6. The ignition assembly according to claim 4, characterized in that the number of pulses is less than 20 pulses per one ignition.

7. The ignition assembly according to claim 1 , characterized in that the pulse information comprises a map of distinct values of number of pulse for predetermined operational parameters of the engine.

8. The ignition assembly according to claim 1 , characterized in that the prechamber assembly (14) is provided with a controllable gaseus fuel inlet (26) for feeding fuel into the prechamber space and that the control unit is provided with executable instructions to control the fuel feed to provide lean fuel mixture in the combustion chamber of the engine and lean fuel mixture in the prechamber.

9. A method of igniting a combustible fuel mixture in a combustion chamber of an internal combustion piston engine by means of a plasma pulsed ignition, comprising the steps of igniting a combustible fuel mixture in a combustion chamber of an internal combustion piston engine initiated by means of a prechamber assembly (14) with a prechamber space, and activating a pulsed plasma plug assembly to generate pulses for generating a plurality of plasma streamers between the at least two electrodes, in which method the pulsed plasma plug assembly is activated with a controllable train of pulses and generate plurality of the pulses.

10. Method according to claim 9, characterized in that in the train of pulses the number pulses is controlled as a function of one or more op- erational parameter of the engine.

1 1 . Method according to claim 10, characterized in that the number of pulses is less than 40 pulses per one ignition.

12. Method according to claim 10, characterized in that the number of pulses is less than 30 pulses per one ignition.

13. Method according to claim 10, characterized in that the number of pulses is less than 20 pulses per one ignition.

14. Method according to claim 10, characterized in that the number of pulses is obtained from a map comprising distinct number of pulse values for predetermined operational parameters of the engine.