TECHNICAL FIELD

The invention relates to a piston crown for a piston in an internal combustion arrangement comprising a cylinder, the piston crown having a piston bowl surface adapted for facing a combustion chamber in said cylinder, wherein the piston bowl surface comprises:

a circumferential rim portion, a floor portion connected to and surrounded by said circumferential rim portion, a plurality of circumferentially spaced protrusions in the circumferential rim portion, and at least one spray impingement portion, located between two adjacent protrusions.

The invention can be applied in internal combustion arrangements, in particular in a diesel engine, for all types of vehicles. As such, the invention may be applied in an internal combustion arrangement for a heavy vehicle such as a lorry or truck, a construction equipment vehicle, a marine vessel or a car.

BACKGROUND

In the field of internal combustion arrangements, numerous efforts are made to accomplish efficient combustion which is also satisfactory in view of residual products, in particular soot particle and NOx emissions, although also carbon monoxide emissions, and hydrocarbon emissions may naturally be considered.

A combustion process in which the fuel is injected directly into the cylinder and is ignited by increased temperature and pressure in the cylinder is generally referred to as a diesel process. When the fuel is ignited in the cylinder, combustion gases present in the cylinder undergo turbulent mixing with the burning fuel, so that a mixture-controlled diffusion flame is formed. The combustion of the fuel/gas mixture in the cylinder gives rise to heat generation which causes the gas in the cylinder to expand and which hence causes the piston to move in the cylinder. Depending on a number of parameters, such as the injection pressure of the fuel, the quantity of exhaust gases recirculated to the cylinder, the time of injection of the fuel and the turbulence prevailing in the cylinder, different engine efficiency and emission values are obtained. The production of soot particles during combustion is a complicated process, where the quantity of soot particles is the net difference between formed soot and oxidised soot. Generally, combustion performed with a fuel-rich fuel/air mixture, insufficient mixing and at high temperature produces high soot formation. However, the formation of soot particles may be counteracted if soot oxidation may be promoted.

During combustion, nitrogen oxides (NOx) are formed from the nitrogen content in the air in a thermal process having a strong temperature dependency, and which inter alia depends on the size of the heated-up volume and the duration of the process.

Also, carbon monoxide emissions (CO) and hydrocarbon emissions (HC) may appear, in particular if unburnt fuel ends up in relatively cool regions of the combustion chamber, such as close to the cylinder wall or in cavities between piston and cylinder lining. To control and in particular to reduce emissions from the combustion process in a combustion engine, it has been proposed to utilise the shape of the piston bowl surface facing towards the combustion chamber. The piston bowl surface is part of a piston crown of the reciprocating piston in a cylinder. To this end, the piston bowl surface may be designed so as to affect various parameters inside the combustion chamber such as flame propagation, mixing energy, kinetic energy distribution, and/or swirl. Generally, it has been desired to minimise the occurrence of stagnation zones in the combustion chamber.

For certain applications, it has been proposed to use piston crowns comprising circumferentially arranged protrusions or ridges for influencing the conditions inside the combustion chamber.

For example, WO 201 1/101 154 describes a piston arranged for reciprocal movement in a combustion engine cylinder, wherein, half way between flame plume impingement areas and in a plane perpendicular to said reciprocal movement, there are arranged protrusions or ridges protruding into the combustion chamber.

US 8 499 735 describes another piston, wherein, between spray/flame plume impingement areas and in a plane substantially perpendicular to reciprocal piston movement, there are arranged a first type of protrusions protruding into the combustion chamber, having a smooth form for preserving kinetic energy in the flame and for redirecting circumferential flame progress mainly towards a center axis of the piston with minimal flame-to-flame interaction. Moreover, protrusions of a second type are arranged in the impingement areas, which are adapted for redirecting the flame towards a circumferential flame progress direction.

There is a need to further improve the control of the combustion process in a combustion engine arrangement, or to provide a useful alternative. SUMMARY

An object of the invention is to provide a piston crown fulfilling the above-mentioned need. The object is achieved by a piston crown according to claim 1 . Hence, there is provided a piston crown for a piston in an internal combustion arrangement, the piston crown having a piston bowl surface adapted for facing a combustion chamber in said cylinder, wherein the piston bowl surface comprises: a circumferential rim portion, a floor portion connected to and surrounded by said circumferential rim portion and a plurality of circumferentially spaced protrusions in the circumferential rim portion, and at least one spray impingement portion, located between two adjacent protrusions.

The spray impingement portion comprises a reflection surface, being defined by that each possible normal to the reflection surface is directed towards a central axis (A) of the piston, and forming an angle being in the range of a constant angle ± 1 0 ⁵ with said central axis.

Further, the constant angle is at least 50 ⁵ . As implied by the above-mentioned definition, the reflection surface is defined as a portion of the rim surface displaying a variation of ± 10 ^Q about a constant angle. Also, it is implied in the term "surface" that the reflection surface should have at least some extension along the central axis and circumferentially about the central axis. The reflection surface in the spray impingement portion is the surface onto which a spray originating from an orifice in an injector is intended to impinge, causing the spray to be reflected. By affecting the way in which the spray is reflected, the shape of the reflection surface will impact the distribution of the kinetic energy inside the combustion chamber.

The provision of a reflection surface as defined in the above in the spray impingement portion may be made in an arrangement of the piston crown together with an injector for injecting a spray along at least one spray vector. Hence, the spray vector will form an angle with the central axis. The reflection surface should be designed in correspondence with the desired spray vector, such that the constant angle of the reflection surface corresponds to the angle which the spray vector forms with the central axis.

In this case, the reflection surface will be generally perpendicular to the spray vector, resulting in the impinging spray being reflected back towards the opposite direction of the spray vector. A reflection angle being defined between the spray vector impinging upon the reflection surface, and a general reflected spray direction, will hence be approximately zero.

This is believed to be particularly advantageous in combination with the spray impingement area being neighbored by protrusions as described in the above.

Indeed, with such a piston crown in an internal combustion arrangement, results have been achieved indicating advantageous fuel consumption properties as seen in relation to the compression volume of the internal combustion arrangement. Moreover, such an arrangement has been shown to be advantageous in view of soot emissions.

The constant angle being at least 50 ⁵ implies that the spray vector may form an angle of at least 50 ⁵ with the central axis. Accordingly, use of a relatively large angle between the central axis and the injecting spray vector is envisaged. This is believed to be particularly advantageous in combination with the protrusions and the spray impingement areas as described herein, allowing for favorable combustion conditions. Optionally, the constant angle may be at least 60 ⁵ , preferably at least 70 ⁵ , most preferred between 70 and 80 ⁵ .

Optionally, the constant angle may be less than 85 ⁵ , preferably less than 80 ⁵ .

According to embodiments, the reflection surface may be defined by that each possible normal to the reflection surface is directed towards a central axis of the piston, and forming an angle being in a range of a constant angle ± 5 ⁵ with said central axis, preferably forming an angle being in the range of a constant angle ± 2- with said central axis.

If such a piston crown is arranged with an injector providing a spray vector forming said constant angle with the central axis, this means that a reflection angle being defined between the spray vector impinging upon the reflection surface, and a general reflected spray direction will be limited to 5 ⁵ , or 2-, respectively (considering the entire reflection surface).

In particular, the reflection surface may be defined by that each possible normal to the reflection surface is forming an angle being the constant angle. In this case, in combination with a spray vector likewise forming the constant angle with the central axis, a reflection angle may be approximately 0 ⁵ , all over the reflection surface.

In a second aspect, the above-mentioned object is achieved by an internal combustion engine arrangement comprising an engine cylinder, a piston crown having a piston bowl surface facing a combustion chamber in said cylinder, the piston bowl surface comprising: a circumferential rim portion, a floor portion connected to and surrounded by said circumferential rim portion, a plurality of circumferentially spaced protrusions in the rim portion, at least one spray impingement portion, located between two adjacent protrusions, and an injector comprising at least one orifice, said orifice being arranged to inject fuel along a fuel spray vector towards said spray impingement portion.

In the internal engine arrangement, the spray impingement portion comprises a reflection surface, the reflection surface being defined by that each possible normal to the reflection surface forms an angle γ with the spray vector of the corresponding injector orifice, as seen in a plane through the central axis, the angle being less than 10 ⁵ . Accordingly, a theoretical reflection angle being the difference between the spray vector injected towards the reflection surface, and a spray leaving the reflection surface after having impinged towards it, should be less than 10 ⁵ .

Advantageously, the angle between each possible normal to the reflection surface and the spray vector of the corresponding injector orifice, as seen in a plane through the central axis is less than 5 ⁵ , preferably less than 2°. In particular, the angle between each possible normal to the reflection surface and the spray vector of the corresponding injector orifice, as seen in a plane through the central axis may be essentially zero. In this case, the reflection surface will be formed as a part of a conical surface, for which all possible normals are parallel to the spray vector. For both aspects of the invention, whether considering the piston crown only or considering an internal combustion engine arrangement, the following features may be provided.

Generally, the rim portion defines a maximum level of said piston bowl surface along the central axis, and the floor portion defines a minimum level of said piston bowl along the central axis, the piston bowl having a piston bowl extension along said central axis being the difference between said maximum level and said minimum level, and wherein said reflection surface has a reflection surface extension along the central axis. According to an embodiment, the reflection surface extension is at least 10 % of said piston bowl extension, preferably at least 15 % of said piston bowl extension.

According to an embodiment, the reflection surface extension is between 15 and 30% of said piston bowl extension, preferably between 20 and 30%, most preferred about 25%.

The axial extension of the reflection surface (the reflection surface extension) should preferably be such that the spray vector will impinge upon the reflection surface during at least a main portion of the injection period. Preferably, the spray vector will impinge upon the reflection surface during the entire injection period. Accordingly, a suitable axial extension of the reflection surface may be selected by considering the injection period and the distance along the central axis which the piston crown will move in relation to an injector orifice during said injection period.

According to an embodiment, the reflection surface extension is extending at a first 5 distance from the minimum level of said piston bowl along the central axis, said first distance being at least 15% of said piston bowl extension, preferably at least 25 % of said piston bowl extension, most preferred at least 30%. Advantageously, the first distance may be less than 50% of said piston bowl extension, preferably less than 40%.

10 The reflection surface will furthermore have an extension along the circumferential direction of the rim portion. The extension along the circumferential direction may be expressed in terms of an angle about the central axis, in a plane perpendicular to said central axis.

15 The circumferential extension of the reflection surface should generally be selected such that the area provided by the reflection surface is sufficient for the entire spray jet to impinge thereupon.

In an embodiment, the reflection surface may have a circumferential extension of at least 20 5 ⁵ about said central axis, in a plane perpendicular to said central axis. Preferably, the circumferential extension is at least 10 ⁵ , most preferred in the range 10 to 40 ⁵ . Optionally, the circumferential extension is less than 50 ⁵ , preferably less than 40 ⁵ .

In an embodiment, the rim portion may comprise at least two reflection surfaces, more 25 preferred 2 to 10 reflection surfaces, more preferred 4 to 8, most preferred 5 to 7. The amount of reflection surfaces may naturally be adapted to the number of spray jets, i.e. the number of injector orifices of a corresponding injector.

In an embodiment, the reflection surfaces may be located at regular angular intervals 30 about the central axis. Also the location of the reflection surface may naturally be adapted so as to correspond to the location of injector orifices of a corresponding injector.

In an embodiment, all reflection surfaces of the piston crown are uniform. In an embodiment, at least some of the protrusions are uniform, preferably all of said protrusions of the piston crown are uniform.

In an embodiment, at least the portion that is located closest to the central axis of at least one of said protrusions forms a surface, for which each possible normal forms the same angle as the reflection surface.

Advantageously, the inner shape of the piston bowl may be designed such that the reflection surfaces join smoothly with the protrusions. For example, the reflection surfaces may be seen as portions on a circumferentially extending band, having the axial extension Da and being arranged so as to form a part of the inner contour of the piston bowl, extending around the circumference thereof. Such a band would hence be applied so as to form the contour of the protrusions as well as the reflection surfaces in between the protrusions.

The protrusions will extend towards the central axis A at least a distance Ddiff from the reflection surfaces. For example, the distance Ddiff may be at least 10 % of the maximum inner radius of the piston bowl surface, advantageously at least 20 %. Preferably, the distance Ddiff may be less than 50% of the maximum inner radius of the piston bowl surface.

Also, the protrusions will have an extension along the circumference of the piston bowl. The extension may be at least 5 ⁵ , preferably at least 10 ⁵ , most preferred between 10 and 30 ⁵ .

In an embodiment, an inner circumferential wall of said piston bowl may consist of said protrusions and reflection surfaces.

In another aspect, the present disclosure relates to a piston comprising a piston crown in accordance with the above.

In another aspect, the present disclosure relates to an internal combustion engine arrangement comprising an engine cylinder and a piston positioned for reciprocal movement in the engine cylinder along a central axis (A) between a bottom dead center position and a top dead center position, wherein the piston comprises a piston crown as described in the above.

Preferably, the piston crown is arranged in relation to an injector of said combustion engine arrangement, said injector comprising at least one orifice, said orifice being arranged to inject fuel along a fuel spray vector towards said spray impingement portion such that each possible normal to the reflection surface forms an angle with the spray vector of the corresponding injector orifice, as seen in a plane through the central axis, the angle being less than 10 ⁵ , preferably less than 5 ⁵ , preferably less than 2-, most preferred substantially 0 ⁵ .

As already described in the above, an internal combustion engine arrangement wherein the angle between the normal to the reflection surface and the spray vector of the corresponding injector orifice is less than 10 ⁵ , preferably less than 5 ⁵ , preferably less than 2-, most preferred substantially 0 ⁵ may optionally possibly be achieved using a piston crown where the constant angle is more than or equal to 50 ⁵ .

In another aspect, there is provided a vehicle, preferably a truck, comprising an internal combustion engine arrangement comprising a piston crown as described in the above and/or an internal combustion arrangement as described in the above. It will be understood that the present disclosure may also be applied to other engines than to a vehicle engine in a truck, e.g. to an engine in a bus, excavator, loader, or other vehicle, or to an engine for marine applications, for providing back-up power etc. Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

In the drawings: Fig. 1 is a schematic cross-sectional view of an engine arrangement including a piston comprising a piston crown in accordance with an embodiment of the invention;

Fig. 2a is a top view of an embodiment of a piston crown in accordance with an embodiment of the invention;

Fig, 2b is a perspective view of the piston crown of Fig. 2a

Fig. 3 is a side view as seen from a cross-sectional longitudinal plane through a piston crown in accordance with Fig. 2a;

Fig 4 is a schematic cross-sectional view of a portion of a piston crown in accordance with Fig. 2a;

Fig. 5 is another schematic cross-sectional view of a portion of a piston crown in accordance with Fig. 2a, wherein the cross-section at a protrusion is compared to the cross-section at a spray impingement area; and

Fig. 6 is a schematic view of a vehicle in which an engine arrangement in accordance with Fig. 1 may be arranged.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Fig. 6 illustrates a vehicle, for example a truck 100, in which an engine 101 comprising an engine arrangement 1 arrangement including a piston crown 3 as described in the below may be arranged.

Fig. 1 illustrates schematically an engine arrangement for an internal combustion engine.

The engine arrangement 1 comprises a cylinder 2 wherein a piston 5 is arranged for reciprocal movement along a central axis A. The piston 5 is mechanically connected to a crankshaft 4 of the engine, so that the piston 5 is movable in the cylinder 2 between an upper and a lower dead centre position.

One end of the cylinder cavity is closed by a engine cylinder head 14.

A combustion chamber facing portion of the piston 5 comprises a piston crown 3, having a piston bowl surface 6 facing the combustion chamber.

Hence, in Fig. 1 , the piston bowl surface 6 forms a combustion chamber 7 with the inner surface of the cylinder head 14, and the side wall of the cylinder 2. In the cylinder head 14, one or more induction ports 9 with corresponding induction valves 10 are arranged. Also arranged in the cylinder head are one or more exhaust ports 1 1 with corresponding exhaust valves 12.

Moreover, in the cylinder head 14, there is disposed at least one fuel injector 13, through which fuel is injected into the cylinder 2 as a fuel spray. The fuel is preferably injected with a pressure in the range 600 to 3000 bar. Generally, for an engine system using EGR, about 1000 to 2500 bar may be preferred, without EGR about 800 to 1400 bar.

Ignited fuel spray forms a plume in the combustion chamber 7.

The injector 13 comprises at least one, preferably a plurality of injection orifices for permitting the pressurised fuel to flow into the combustion chamber. The injected fuel will thereby provide kinetic energy into the combustion chamber 7, so as to induce thorough mixing of the fuel with the air contained therein.

The injector 13 may be any suitable type of injector capable of injecting fuel. The fuel injector 13 is preferably centrally disposed in the cylinder head so that a geometrical central axis A of the fuel injector coincides with a geometrical central axis A of the cylinder, which is also an axis of reciprocation of the piston 3.

The combustion engine may advantageously be a four-stroke engine, comprising a plurality of cylinders 2, each provided with a piston 5, wherein each piston 5 for instance may be connected to a common crankshaft 4.

In the illustrated embodiments, the piston crown 3 forms an integral portion of a piston 5. However, it is also conceivable to provide the piston crown 3 as a separate unit, to be attached to one or more piston base portions, so as to form a complete piston 5.

Figs 2a, 2b and 3 illustrate the piston crown 3 in more detail. The piston bowl surface 6 facing the combustion chamber 7 when in an engine arrangement 1 as the one exemplified in Fig. 1 , comprises a circumferential rim portion 34 and a floor portion 31 connected to and surrounded by the circumferential rim portion 34. As may be gleaned from Fig. 3 and 4, the floor portion 31 may be generally dome-shaped with a central apex coinciding with the central axis A of the piston 3. The floor portion 31 may form dome side surfaces 32 extending circumferentially from said dome-shape, and forming a dome angle being 2 x the angle a between them. Said angle a may advantageously be between 60 and 85 ⁵ . In the illustrated embodiment, the angle a is about 75 ⁵ .

The circumferential rim portion 34 may comprise a plurality of circumferentially spaced protrusions 331 . Each protrusion 331 extends from the adjacent sides of the rim portion 34 toward the central axis A of the piston crown, forming an apex towards the central axis A.

In the illustrated embodiment, the rim portion 34 comprises a total of six protrusions 331 , equally distributed around the circumference of the piston crown. However, as explained in the above, other numbers of protrusions 331 are conceivable.

As illustrated in Fig. 2, alternately with the protrusions 331 , the rim portion 34 forms reflection surfaces 330, intended to constitute spray impingement areas. Hence, in the illustrated embodiment, six spray impingement areas are formed.

Generally, it may be preferred to arrange protrusions 331 alternately with the reflection surfaces 330, resulting in an equal number of protrusions 331 and reflection surfaces 330. However, reflection surfaces 330 may also be provided between only some of the possible pairs of protrusions 331 formed in the rim 34. For example, with six protrusions 331 only three reflection surfaces 330 may be arranged.

The suitable number and location of reflection surfaces 330 may naturally be determined so as to correspond to the number and location of spray orifices 330 in an injector to be used in combination with the piston crown.

The purpose of the reflection surfaces 330 being, as described in the above, to enhance kinetic energy distribution in the combustion chamber L al. by directing flow towards the protrusions, implies that each reflection surface 330 should be located between two such protrusions 331 .

Fig. 4 is a partial cross-section through the piston crown 3 at a reflection surface 330, and illustrates the reflection surface 330 in more detail. According to the definition thereof, the reflection surface 330 is a portion of the rim 34 wherein all the angles (β') formed between the all normals N to the rim surface 34 and the center axis A lie in a range of ± 10 ^Q from a selected constant angle, and where said selected constant angle is at least 50 ⁵ . If considering a spray vector v, illustrating the direction of a fuel spray being injected from an injector orifice 130, such a spray vector v will impinge on the rim portion 34. It will be understood, that since all normals N to the reflection surface 330 form essentially the same (± 10 ⁵ ) angle to the spray vector v, the spray vector v will be reflected in the same manner regardless of where along the essentially conical portion the collision with the rim portion 34 takes place.

Preferably, the extension Da along central axis A of the essentially conical portion 30 is such that a spray injected via the orifice 130 impinges somewhere along the essentially conical portion throughout the injection period.

As illustrated in Fig. 4, the spray vector v forms a spray angle β with the central axis A, said spray angle β being determined by the spray orifice 130. The constant angle should be adjusted to the spray impingement angle β such that the constant angle is equal to the spray impingement angle β. In this case, a reflection angle γ being an angle formed between the constant angle and the spray impingement angle β, should be relatively small, preferably within the range of ± 5 ⁵ , most preferred substantially zero.

E.g. zero reflection angle γ corresponds to the spray being directed back towards the spray vector angle after impact on the rim portion 24 of the piston crown 3. This leads to an enhanced distribution of kinetic energy, and in particular to the effect of the protuberances in the combustion chamber being enhanced.

It follows from the term "reflection surface" that the section must have a certain axial extension Da along the central axis A and a certain circumferential extension Dc along a circumference about the central axis A, or else there would not be any reflection surface. The amount of these extensions may be determined having the intended operation of the combustion engine in mind, i.e. such that spray injected by the injector will be able to impinge upon the reflection surface at least during the injection period, while the piston will be moving in relation to the injector.

Hence, an appropriate axial extension of the reflection surface Da may be set considering the injection period, start of injection, location and speed of the piston in relation to the injector during the fuel injection.

As illustrated in Fig. 4, the floor portion 31 of the piston bowl surface 6 defines a minimum level of the piston bowl surface 6, as seen along the axis A, and the rim portion 34 defines a maximum level of the piston bowl surface 6. The distance between the maximum level and the minimum level is referred to as a piston bowl extension Dmax.

The axial extension Da of the reflection surface may be expressed in relation to said piston bowl extension Dmax, and may be at least 10% of the piston bowl extension. In the illustrated embodiment, the axial extension Da of the reflection surface is about 25% of the piston bowl extension Dmax.

As illustrated in Fig. 4, the reflection surface 330 may be located so as to begin its extension at a distance D1 from the minimum level of the piston bowl surface 6. This may be preferred since the distance D1 provides a space between the reflection surface 330, where the fuel spray is to impinge, and the floor portion 31 of the piston bowl surface 6. Such a space might be important for the creation of turbulence in the spray.

To this end, the reflection surface 330 may be located so as to begin its extension at a distance D1 being at least 15 % of the piston bowl extension Dmax. In the illustrated embodiment, D1 is about 20% of the piston bowl extension Dmax.

Similarly, the reflection surface 330 may be located so as to end its extension at a distance D2 from the maximum level of the piston bowl surface 6. The distance D2 may be at least 10% of the piston bowl extension Dmax, preferably between 15 % and 60%. As indicated over one of the exemplary reflection surfaces 330 in Fig. 2, the reflection surface 330 may have a circumferential extension (Dc) about the central axis (A). The circumferential extension (Dc) may be expressed as a circumferential extension angle (δ), as seen in a plane perpendicular to the central axis A.

The circumferential extension angle (δ) may be at least 5 ⁵ , preferably at least 10 ⁵ , most preferred between 10 and 40 ⁵ , or even between 20 and 35 ⁵ . In the illustrated embodiment, the circumferential extension angle (δ) is about 30 ⁵ . In the embodiment illustrated in Fig. 2, the reflection surfaces 330 are located at regular angular intervals about the central axis (A). For the location of a reflection surface 330 to be determined, the centre of such a section should be considered, i.e. the intersection between a midpoint of the axial extension Da and the circumferential extension Dc. It will be understood that the locations about the central axis (A) of the reflection surfaces 330 should be determined so as to correspond with the locations of the spray orifices 130 of an injector 13 to be used with the piston crown.

In the embodiment illustrated in Fig. 2, the reflection surfaces 330 are distributed around the central axis A with 60 ⁵ intervals.

Preferably, and as illustrated in Fig. 2, all of the reflection surfaces 330 are uniform. In other words, the reflection surfaces 330 display the same selected angle (β'), with the same variation, they display the same axial extension Da and circumferential extension Dc.

Moreover, it is preferred that the reflection surfaces 330 constitute all of the impingement areas of the piston crown. In other words, when in an arrangement with an injector 13, all fuel sprays injected by the injector 13 will impinge upon a reflection surface 330.

Each protrusion 331 forms an apex towards the central axis A in accordance with previously known technology. It has been found to be advantageous, if at least a portion of the apex of the protrusions 331 forms a surface for which the normal forms the same selected angle to the central axis A as the reflection surfaces 330. As illustrated in Fig. 5, this would mean that a normal to the protrusion apex would extend essentially in parallel to a normal of the reflection surface 330.

The axial extension and location of such a surface of the protrusion 331 may preferably correspond to the axial extension Da and location of the reflection surface 330, or at least overlap thereto.

As also illustrated in Fig. 5, the protrusions will extend towards the central axis A at least a radial distance Ddiff from the reflection surfaces. For example, the radial distance Ddiff may be at least 10 % of the maximal inner radius of the piston bowl surface 6, advantageously at least 20 %. Preferably, the radial distance Ddiff may be less than 50% of the maximal inner radius of the piston bowl surface 6.

Also, the protrusions will have an extension along the circumference of the piston bowl. The extension may be at least 5 ⁵ , preferably at least 10 ⁵ , most preferred between 10 and 30 ⁵ .

In the illustrated embodiment, the inner circumferential wall of the piston bowl surface 6 consists of said protrusions 331 and reflection surfaces 330.

As indicated in the Figures, it is understood that the angles α, β, β' referred to herein, which relate to angles formed with the central axis A of the piston crown 3, are all measured as seen towards the direction of the piston 5 of the central axis A. It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. In particular, and as outlined in the above, the number of reflection surfaces and protrusions may be varied. Moreover, the dimensions of various features may be adapted to suit particular circumstances.