TECHN ICAL FI ELD OF TH E INVENTION

The present invention relates to a piston for an internal combustion engine accordi ng to the preamble of clai m 1 and to an internal combustion engine comprisi ng at least one cylinder with such a piston . It also relates to a motor vehicle comprising an internal combustion engine and to a method for creati ng a swirl motion in a combustion chamber of a cylinder in an internal combustion engine.

Whi le the piston is primarily discussed with respect to diesel engines, it is to be u nderstood that the piston may be used i n any kind of internal combustion engine in which fuel or other fluids is/are di rectly injected into a combustion chamber, for example by means of injection of fuel followed by fuel ignition to provide piston movement. The piston may e.g . be used in two-stroke and four- stroke engi nes such as Otto engines, homogeneous charge compression ig nition (HCCI) engi nes, and reactivity controlled compression ignition (RCCI) engines.

The internal combustion engine may also be a stationary i nternal combustion engine used in e.g . a pump or an electric generator.

BACKG ROUN D AN D PRIOR ART

Internal combustion engines such as diesel engi nes, also known as compression-ig nition engi nes, and Otto engi nes, or spark- ignition engines, are commonly used in different types of motor vehicles, such as trucks and buses, cars, vessels, etc. Internal combustion engines are also used in many industrial applications. Internal combustion engines, hereinafter also referred to as engines, may be driven by a plurality of different types of fuel, such as diesel, petrol, ethanol, gaseous fuel, and biofuel. The engines have a number of cylinders in which a reciprocating piston is provided. In an upper end of the piston, a piston bowl is provided. Together with an upper part of the cylinder and a cylinder head, the piston bowl forms a combustion chamber in which fuel is injected and combusted. The piston bowl is designed to contribute to mixing of air and fuel and to create a flow pattern influencing combustion and emission formation within the combustion chamber.

In a diesel engine, the fuel is normally injected during a powerstroke of the piston. The fuel is ignited by the compression heat and combusted almost immediately following injection. Air and fuel must therefore be mixed in a very short time, and it is desirable to ensure that the mixing is efficient and that the fuel becomes well-distributed within the combustion chamber so as to achieve a complete combustion. During the combustion, large amounts of soot are created due to the lack of oxygen in the non- pre-mixed diesel flame. The time between end of injection and exhaust valve opening is thereby critical to oxidise remaining soot particles in the cylinder. Four important parameters control the soot oxidation during this so called post-oxidation phase, namely time, temperature, oxygen and turbulence. The amount of soot particles present i n the exhaust gases can thereby be m inimized by controlling those parameters.

Increasi ngly stringent emission regulations, relating pri marily to soot and nitrogen oxide (NOx) emissions, make it necessary to ai m at further improving the emission control of internal combustion engines. However, more efficient emission control often require more complex and energy demanding aftertreatment and combustion systems, contributing to an increased fuel consu mption . For example, a swirl motion may be created to form turbulence in the combustion chamber and efficiently mix fuel with air. The swirl motion is a large scale swirling motion around the axis of the cylinder, which is typically created during the i ntake stroke of the piston by the i ntake ports. The swirl motion improves the combustion conditions and increases the turbulence i n the post-oxidation phase, thereby reduci ng the emission levels. However, the creation of swirl motion duri ng the intake stroke is energy demandi ng and also i ncreases heat transfer to the wal ls of the combustion chamber duri ng the subsequent compression stroke. Thus, the creation of swirl motion by intake ports during the i ntake stroke generally reduces the efficiency of the engine. On the other hand , if the i ntake ports are designed not to create swirl , the soot emission levels may increase and thereby the demands on e.g . higher injection pressure or diesel particulate filters ( DPF) provided downstream of the engine. Such hig her injection pressures and filters general ly i ncrease production costs and fuel consu mption .

There are known solutions for i mproving the mixture of air/fuel by modification of the piston bowl . For example, FR2898638 discloses a piston for an internal combustion engine, having a piston bowl in which a propeller-like bottom structure is provided. The bottom structure creates a swi rl motion i n the combustion chamber upon i njection of fuel . However, there is a risk that fuel gets obstructed within cavities in the bottom structure, thus leading to insufficient mixi ng of air and fuel . Soot and hydrocarbons (HC) may thereby be created and increase the emission levels.

US201 1 /0253095 discloses a piston for an internal combustion engine, having a piston bowl in wh ich a plurality of protrusions are provided, protrudi ng from a side wall of the piston bowl . Fuel is injected into the combustion chamber and is redirected upon impacting with the protrusions such that a rotational motion may be induced .

However, there is a need for a further improved piston that enables a combination of efficient combustion , a li mited heat transfer to the walls of the combustion chamber, and reduced em ission levels. SUMMARY OF TH E INVENTION

It is a first objective of the present invention to provide a piston for an i nternal combustion engine which has a piston bowl configured to improve the efficiency of the combustion and post-oxidation phase, to reduce emission levels and to enable reduction of heat transfer to the walls of the combustion chamber. A second objective is to provide an internal combustion engi ne with an improved efficiency and reduced em ission levels, thus reduci ng the need for extremely hig h injection pressures, diesel particulate filters and simi lar. A third objective is to provide an in at least some aspect improved and less energy consumi ng method for creating a swi rl motion in a combustion chamber that survives into the post- oxidation phase. At least the first objective is achieved by means of the initially defined piston for an internal combustion engi ne, characterised i n that the piston bowl further comprises:

- an annular ridge formed i n a transition between the an nular upper side wall portion and the annular lower side wal l portion, projecting toward the central axis,

- a plurality of angularly spaced protrusions, protruding toward the central axis from the annular upper side wall portion , each protrusion having a concave surface portion ,

wherein the piston bowl is configured so that a fluid spray i njected toward a target position located below one of said angu larly spaced protrusions is split by the an nular ridge i nto an upper flow portion and a lower flow portion , wherei n the upper flow portion is deflected by the concave surface portion of the protrusion located above the target position so that it contributes to creation of a swirl motion in the combustion chamber.

The piston according to the invention has a piston bowl configured to deflect a portion of the fluid spray such that a swirl motion in the piston bowl is created and the mixi ng of air and fuel is improved. The swirl motion may hereby be i nduced during injection of e.g . fuel into the combustion chamber, and the swirl motion wi ll survive into the post-oxidation phase of the combustion . The swi rl motion may be created independently of the design of the intake ports. Thus, the energy requ ired to create the swirl motion may be reduced. Moreover, heat transfer to the walls of the combustion chamber may be reduced duri ng the compression stroke of the cylinder when swirl is not created during the intake stroke. The improved mixing of fuel and air also leads to lower emission levels including less soot particles, and may thereby reduce the need for expensive and energy demanding i ncreased injection pressures and/or diesel particulate filters. Moreover, the angularly spaced protrusions, which may be identical , del imit the flame and reduce its size, thereby deli miti ng the surface area of the flame which is exposed to unburned oxygen and n itrogen . Since NOx gases are pri marily created at the surface of the flame, the smaller flame results in reduced levels of NOx emissions. The proposed piston is thereby useful for i mproving energy efficiency and reduci ng both soot and NOx emission levels of an internal combustion eng ine. The configu ration with the ann ular ridge located below the protrusions makes it possible to, upon injection , direct the fl uid spray such that the fl uid spray, or, in the case where the fluid spray is a fuel spray wh ich is ignited upon injection , the flame, may be split on the annular ridge. The upper flow portion of the fl uid spray/flame is deflected toward the concave surface portion of the above protrusion , and the lower flow portion of the fl uid spray/flame may be deflected i nto an annular channel deli mited by the ann ular bottom and the ann ular lower side wal l portion . The upper flow portion of the fluid spray/flame, which is deflected by the concave surface portion , may be deflected toward a position above the annular ridge, so that a swirl motion is induced in an upper part of the combustion chamber. This may further improve the m ixing of fuel and air due to velocity g radients. During the post oxidation phase, the large scale swi rl created i n the combustion chamber may be fractu red into small scale turbulence leading to accelerated soot oxidation and thereby lower soot emissions. The fracture of the large scale swirling motion to small scale turbulence is possible thanks to the velocity differences i n the piston bowl , created by the large scale swirl motion during the combustion phase.

Since the swirl motion may with the proposed piston be created upon injection of fuel , the rotational speed of the fuel/air mixture becomes proportional to the fuel i njection pressure. Th us, the swirl motion scales with the fuel injection pressure and is thereby adapted to the operating conditions of the internal combustion engine.

The fuel spray may typically be ig nited a very short ti me after injection into the combustion chamber, i .e. after an ignition delay. Upon ign ition , a flame is formed . However, the injected fluid spray does not necessarily have to be a fuel spray, but may also be a mixture of fuel/gas or a liqu id spray which is injected primari ly to create a swirl motion , either during the compression stroke or during the power stroke of the piston . For example, injection of fluid in the form of water may reduce the combustion temperature and thereby reduce NOx emissions. I n e.g . an Otto engi ne, a fuel/air mixture may be injected during the compression stroke such that a swirl motion is created before ignition of the fuel/air mixture using a spark plug . The fl uid spray may also be injected during the power stroke to improve the conditions during the post- oxidation phase. Of cou rse, it is possible to combine the piston according to the invention with i ntake ports configured to create a swirl motion during the intake stroke, in order to further increase the turbulence in the combustion chamber. The swi rl created by the intake ports can either be i n the same direction as the fl uid injection induced swi rl , or be i n the form of a cou nterflow.

Accordi ng to one embodiment, the concave surface portion of each of the angularly spaced protrusions is configured to face radially inward. Fuel can thereby be efficiently redirected to create a swirl motion .

Accordi ng to one embodiment, the concave surface portion of each of the angularly spaced protrusions is configured so that at least a part of the upper flow portion is redirected toward a position above the an nular ridge. The swirl motion may thereby be induced in an upper part of the combustion chamber located above the an nular ridge. If the lower flow portion is deflected from the annular ridge toward an an nular chan nel discussed above at the bottom of the piston bowl , the m ixing of fuel and air is further i mproved.

According to one embodiment, each of the angularly spaced protrusions has an in nermost poi nt located at a sim ilar radial distance from the central axis as the an nular ridge. By "in nermost point" is herein intended the point closest to the central axis, i .e. the point that protrudes the most from the an nular upper side wall portion . By "sim ilar radial distance" is herein intended a distance that does not differ by more than 1 0% from the radial distance between an innermost poi nt of the an nular ridge to the central axis. In this embodiment, the ignition delay may be optimized for reduction of both NOx gases and soot particles. It is also possible to make the protrusions extend to a position closer to the central axis, i n which case a shorter ignition delay can be expected, with a resulting reduced amou nt of NOx gases and an i ncreased amount of soot. If the protrusions are instead made to extend to a position further away from the central axis, the opposite can be expected.

According to one embodiment, the an nular lower side wall portion is i n the form of a concave surface free from protrusions. This reduces the risk that fuel gets obstructed i n the lower part of the piston bowl and improves the conditions for mixing of fuel/air in the lower part of the combustion chamber with the rotating fuel/air within the upper part of the combustion chamber .

According to one embodiment, each of the angularly spaced protrusions further comprises a convex surface portion located opposite the concave su rface portion . The upper flow portion may thereby be spl it on an innermost edge of the protrusion and while one part of the upper flow portion follows the concave surface portion , another part follows the convex su rface portion , both parts contributi ng to the creation of the swi rl motion .

According to one embodiment, the central bottom portion has a highest poi nt located on the central axis, from which hig hest point the central bottom portion slopes downward toward the an nular bottom portion . This configuration increases the compression achieved i n the combustion chamber during the compression stroke. Furthermore, air is pressed into the periphery of the combustion chamber where combustion takes place. The central bottom portion preferably has a con ical or an essentially conical shape. The highest point of the central portion may be located at an axial level on or above an axial level of the annular ridge. At least the second objective is achieved by means of an internal combustion engine comprising at least one cylinder with the proposed piston . Advantages and advantageous featu res of such a combustion engine appear from the above description of the proposed piston . Of course, the i nternal combustion engine may comprise a plurality of cyli nders havi ng the proposed piston . The internal combustion engi ne may be adapted for use within a motor veh icle or withi n a stationary machine such as a pu mp or an electrical generator. Accordi ng to one embodiment, the internal combustion engine further comprises an injector configured to i nject and di rect a fluid spray toward a pl urality of target positions, wherein each target position is located below one of said angularly spaced protrusions. Each target position is associated with one of the angu larly spaced protrusions and is located directly below the associated protrusion . The internal combustion engine is thereby configured to create a swirl motion upon i njection of fluid in the form of e.g . fuel , air, water or mixtures thereof. Preferably, the injector may be positioned on the central axis.

The i nvention also relates to a motor veh icle comprising the proposed internal combustion engine. The motor veh icle may be a heavy motor vehicle such as a truck or a bus, but it may also be e.g . a passenger car or another motor vehicle. At least the third objective is ach ieved by means of a method for creating a swi rl motion in a combustion chamber of a cyli nder in the proposed internal combustion engine, comprising :

- providing a flow of air into the combustion chamber duri ng an intake stroke of the piston ,

- during or after a compression stroke of the piston , injecting a fl uid spray toward the pl urality of target positions, so that the fl uid spray is at each one of the target positions spl it by the annular ridge i nto an upper flow portion and a lower flow portion , wherein the upper flow portion is deflected by the concave surface portion of the protrusion located above the target position so that a swirl motion is created in the combustion chamber. Advantages of the method appear from the above description of the proposed piston .

Accordi ng to one embodiment, i njecting a fluid spray comprises, following a compression stroke of the piston , injecting a fuel spray so that when the fuel spray is ig nited and a flame is formed , at least an upper flow portion of the flame is deflected by the concave surface portion so that a swirl motion is created in the combustion chamber. This is applicable for diesel engines, i n which the fuel spray is ignited almost immediately upon injection .

Accordi ng to one embodiment, the flow of ai r i nto the combustion chamber is provided without creating a swirl motion . The swirl motion is thereby created entirely upon injection of fluid and the amount of energy required for creati ng the swirl motion is reduced. It is however also possible to combi ne creation of a swirl motion during the intake stroke with creation of a swirl motion upon injection of fluid, for further enhancing turbulence in the combustion chamber.

Further advantages as well as advantageous features of the present invention will appear from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will in the following be described with reference to the appended drawings, in which: Fig. 1 schematically shows an axial section of a cylinder of an internal combustion engine according to an embodiment,

Fig.2 is a perspective view of a piston according to a first embodiment,

Fig.3 is an upper end view of the piston in fig.2,

Fig.4 is a section taken along the line IV- IV in fig.2,

Fig.5 is a section taken along the line V-V in fig.2,

Fig.6 is a perspective view of a piston according to a second embodiment,

Fig.7 is an upper end view of the piston in fig.6,

Fig.8 is a section taken along the line VIM-VIM in fig.6, Fig.9 is a section taken along the line IX-IX in fig.6, and Fig. 10 is a cross section taken along the line X-X in fig. 9 shown together with a diagram showing rotational speed within a combustion chamber as a function of distance from a central axis.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig. 1 shows a section taken along a central axis C of a cylinder 1 of an internal combustion engine in the form of a diesel engine according to an embodiment of the invention. In the cylinder 1, a piston 2 configured to reciprocate within the cylinder along the common central axis C is provided. A piston bowl 3 is formed in the piston 2, which together with internal walls of the cylinder 1 and an internal surface of a cylinder head 4 creates a combustion chamber 5. A fuel injector 6 is positioned on the central axis C above the piston bowl 3. An intake port 7 is provided in the cylinder head 4 for supply of air into the combustion chamber 5 via an intake valve 8. Furthermore, an exhaust port 9 is provided in the cylinder head 4 for evacuation of exhaust gases via an exhaust valve 10.

The piston 2 according to the embodiment shown in fig.1 is shown in closer detail in figs. 2-5. A piston 2 according to a second embodiment is shown in figs. 6-10. Common elements of the piston 2 according to the first and the second embodiment will in the following be described using common reference numerals.

The piston 2 according to both embodiments has the basic shape of a right circular cylinder with an upper end 11 and a lower end 12, between which a central axis C and a peripheral envelope surface 13 extend. The upper end 11 comprises an annular top surface 14 defining an upper plane Pu. The piston bowl 3 is recessed with respect to the upper plane Pu defined by the top surface 14. An annular bottom portion 15 defines a lowest level of the piston bowl 3. Radially inside of the annular bottom portion 15, a central bottom portion 16 which is elevated with respect to the lowest level is provided. The central bottom portion 16 is cone shaped with a rounded top 17, which top 17 is recessed with respect to the upper plane Pu. An annular upper side wall portion 18 extends downward and radially inward from the top surface 11. An annular lower side wall portion 19 extends upward from the annular bottom portion 15 toward the upper side wall portion 18. Between the upper side wall portion 18 and the lower side wall portion 19, an annular ridge 20 is formed, projecting toward the central axis C. Together, the annular bottom portion 15 and the lower side wall portion 19 delimit an annular channel 26 surrounding the central bottom portion 16.

The fuel injector 6 is configured for injecting fuel into the cylinder 1 as a fuel spray 25 so that the fuel is mixed with air compressed in the cylinder 1 to form a fuel/air mixture. The fuel/air mixture is after an ignition delay ignited by compression heat generated in the cylinder 1. The ignited part of the fuel spray 25 forms a flame. The fuel can be injected with different injection pressures, from low to very high pressures. The fuel injector 6 includes a plurality of small injection orifices (not shown), formed in the lower end of a nozzle assembly of the fuel injector 6 for permitting the high pressure fuel to flow from a nozzle cavity of the fuel injector 6 into the combustion chamber 5 with high pressure to induce thorough mixing of the fuel with the hot compressed air within the combustion chamber 5. It should be understood that the fuel injector 6 may be any type of fuel injector capable of injecti ng high pressure fuel through a pl urality of i njector orifices into the combustion chamber 5. Also, the fuel injector need not necessarily be positioned on the central axis C.

In other embodi ments, in which the i nternal combustion engine is e.g . an Otto engine, the fuel injector may be configured to inject a mixture of fuel and air into the combustion chamber. The injector may also be configured to inject other fluids such as gases or liquids, e.g . water, which are not combusted but are primarily used to induce a swirl motion .

In the first embodi ment shown in figs. 2-5, a plurality of identical and ang ularly spaced protrusions 21 protrude toward the central axis C from the upper side wall portion 1 8 above target positions located on the ridge 20. Each protrusion 21 is wedge shaped with a first concave surface portion 22, which in the radial direction extends from the upper side wall portion 1 8 to a curved innermost edge 23 of the protrusion 21 , whose innermost point is located closest to the central axis C at approxi mately the same distance from the central axis C as the annular ridge 20. In the axial direction , the protrusion extends from the top surface 1 4 to the ridge 20. The first concave surface portion 22 is directed so that no part of the concave surface portion 22 is h idden behi nd any part of the protrusion 21 as seen from the central axis C. The first concave surface portion 22 has a curvature both as seen in an upper end view such as in fig . 3 and in a sectional view across the protrusion 21 , such as shown in fig . 5. Each protrusion 21 further has a smaller second concave su rface portion 24 located opposite the first concave surface portion 22 and a planar upper surface portion 27 at the level of the upper plane P u .

The i njection orifices of the fuel i njector 6 are arranged so that the fuel spray 25 is i njected toward target positions on , above or below the annu lar ridge 20, wh ich target positions are located below the first concave surface portions 22 of the protrusions 21 . It should be noted that the piston 2 is moving along the central axis C as the fuel spray 25 is injected , and therefore the exact target positions i n the axial direction wil l vary. The target position ai med for in the axial direction also depends on e.g . load and injection timing . As the ignited fuel spray 25, i .e. the flame, strikes the target positions, the flame is split on the an nular ridge 20 into an upper flow portion 25a and a lower flow portion 25b. The upper flow portion 25a of the flame is deflected upward, toward the concave surface portion 22. The lower flow portion 25b of the flame is deflected downward, i nto the an nular channel 26 and toward the central bottom portion 1 6. As the upper flow portion 25a impinges on the concave surface portion 22, it is deflected toward a position in an upper part of the combustion chamber 5, above the ann ular ridge 20, wh ich position is angularly spaced from the concave surface portion 22 by which the flame was deflected . The deflected upper flow portions 25a of the flames thereby together i nduce a swirl motion in the upper part of the combustion chamber 5, i .e. a large scale rotation i n the direction of rotation R arou nd the central axis C. Between a lower part of the combustion chamber 5, below the an nular ridge 20, and the upper part of the combustion chamber 5, tu rbulence may be created as the rotati ng flow of fuel/air mixture in the upper part of the combustion chamber 5 interacts with the fuel/air m ixture in the lower part of the combustion chamber 5, which rotates with an axis of rotation perpendicular to or essentially perpendicular to the central axis C.

In the second embodiment shown i n figs. 6-1 0, a plurality of mutually identical and angularly spaced protrusions 31 protrude toward the central axis C from the upper side wall portion 1 8 above target positions located on the ridge 20. The protrusions 31 are in the form of fins having a concave surface portion 32, which i n the radial direction extends from the upper side wall portion 1 8 to a curved innermost edge 33 of the protrusion 31 , whose innermost poi nt is located at the level of the upper plane P u , closest to the central axis C at approximately the same distance from the central axis C as the annu lar ridge 20. In the axial direction , the protrusion extends from the top surface 1 4 to the ridge 20. The concave surface portion 32 is directed so that no part of the concave su rface portion 32 is hidden behind any part of the protrusion 31 as seen from the central axis C. The concave surface portion 32 has a curvature both as seen in a transverse cross sectional view such as i n fig . 1 0 and in an axial sectional view across the protrusion 31 , such as shown in fig . 9. Each protrusion 31 further has a convex surface portion 34 located opposite the fi rst concave surface portion 32, extending from the upper side wall portion 1 8 to the i nnermost edge 33. The protrusion 31 has an upper edge 35 extendi ng in the upper plane P u . An incl ined surface 36 extends from the upper edge 35 to a curved edge 37 defini ng a transition between the inclined su rface 36 and the concave surface portion 32.

The injection orifices of the fuel injector 6 are i n the second embodiment arranged so that fuel spray 25 is injected toward target positions on , below or above the ann ular ridge 20, which target positions are located below the protrusions 31 , i n the shown embodiment below the innermost edge 33. As the ign ited fuel spray 25, i .e. the flame, strikes the target positions, the flame is spl it on the annu lar ridge 20 into an upper flow portion 25a and a lower flow portion 25b. The upper flow portion 25a of the flame is deflected upward, toward the concave surface portion 32. The lower flow portion 25b of the flame is deflected downward, into the an nular channel 26 and toward the central bottom portion 1 6. As the upper flow portion 25a of the flame i mpinges on the innermost edge 33 of the protrusion 31 , it is spl it into a first portion 25a' following the convex surface portion 34 and a second portion 25" following the concave surface portion 32. Both portions 25a', 25a" are deflected toward a position withi n the upper part of the combustion chamber 5 above the ann ular ridge 20 , wh ich position is angularly spaced from protrusion 31 on which the flame was deflected. The deflected upper flow portions of the flames thereby together induce a swirl motion in the direction of rotation R in the upper part of the combustion chamber 5. Between the lower part of the combustion chamber 5, below the annular ridge 20, and the upper part of the combustion chamber 5, tu rbulence may be created as the rotati ng flow of fuel/air m ixture in the upper part of the combustion chamber 5 interacts with the fuel/ai r mixtu re in the lower part of the combustion chamber 5, which rotates with an axis of rotation perpendicular to or essentially perpendicular to the central axis C.

In the embodi ment shown in figs. 6-1 0, the ign ition delay may be expected to be relatively short due to the relatively narrow protrusions 21 . The relatively short ignition delay is expected to result in a reduced amount of NOx gases created duri ng combustion , but i nstead the soot emissions may be somewhat increased in comparison with longer ignition delays. In a method according to an embodiment of the present invention , carried out in the i nternal combustion eng ine described with reference to fig . 1 , a flow of air is provided i nto the combustion chamber 5 duri ng an i ntake stroke of the piston 2 via the intake port 7 and the i ntake valve 8. During a subsequent compression stroke of the piston 2, a fuel spray 25 is injected by the fuel i njector 6 toward the pl ural ity of target positions, so that the fuel spray 25 is at each one of the target positions split by the ann ular ridge 20 into an upper flow portion 25a and a lower flow portion 25b. The upper flow portion 25a is deflected by at least the concave surface portion 22, 32 of the protrusion 21 , 31 located above the target position , so that a swirl motion is created in the combustion chamber 5.

Fig . 1 0 shows rotational velocity ω as a function of distance r from the central axis C of the piston 2 at the end of i njection . As can be seen , the large scale swirl motion created du ring fuel i njection leads to large variations in the rotational velocity ω depending on the distance r from the central axis. During the post oxidation phase, the large scale swi rl motion created i n the combustion chamber 5 may be fractured into small scale turbulence leading to accelerated soot oxidation and thereby lower soot emissions.

The invention is of course not i n any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof wi ll be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.