Multipart piston

Abstract

The present invention relates to a multipart piston (1) for a combustion piston engine, comprising at least a first piston part (2) and a second piston part (3) and, in particular, to a multipart piston (1) comprising at least one fluid channel (14) in fluid communication with a fluid supply of the combustion piston engine. Further, the present invention relates to a piston-rod assembly (38), to a method of cooling a multipart piston (1), to a combustion piston engine comprising the piston (1) or the piston-rod assembly (38) of the present disclosure as well as to a method for replacing a second piston part (3) of at least one piston (1) of such an engine. Embodiments of the invention have been particularly developed for cooling a multipart piston (1) during operation of the combustion piston engine and will be described hereinafter with reference to this application.

Description

[0001] MULTIPART PISTON

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a multipart piston for a combustion piston engine, comprising at least a first piston part and a second piston part and, in particular, to a multipart piston comprising at least one fluid channel in fluid communication with a fluid supply of the combustion piston engine.

[0004] Further, the present invention relates to a piston-rod assembly, to a method of cooling a piston, to a combustion piston engine comprising the piston or the piston-rod assembly of the present disclosure as well as to a method for replacing a second piston part of at least one piston of such an engine. Embodiments of the invention have been particularly developed for cooling a piston during operation and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

[0005] BACKGROUND OF THE INVENTION

[0006] Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.

[0007] Known combustion piston engines, for example diesel engines or petrol engines, work according to the reciprocating piston principle, wherein at least one piston is disposed slidably along a stroke axis in a cylinder bore delimiting a working chamber in the form of a combustion chamber. The piston may be connected to a crankshaft by means of a connecting rod, enabling the piston to reciprocate between two maximal positions (i.e. the top dead center and the bottom dead center of the stroke), thereby changing the volume of the combustion chamber. For operating the combustion piston engine, a gas mixture is periodically injected into the combustion chamber and ignited. The expansion of the ignited gas mixture generates forces that are used to actuate the piston and the crankshaft. As a result, the reciprocating motion of the piston is converted into a rotational motion of the crankshaft, which can for example be used to power a drive train of a vehicle.

[0008] During use of a piston in a combustion piston engine, the piston is subjected to high mechanical and thermal loading causing the piston to degrade over time. Overheating of a combustion piston engine, especially of the piston, and the lack of sufficient lubrication may drastically accelerate the degradation. This may result in seizure of the circumferential surfaces of the piston or the piston rings that are in contact with the cylinder. Furthermore, wear, erosion, fracture or melting of the piston head, especially of the top side of the piston facing the combustion chamber, may occur. Accordingly, for wear and tear resistance, known pistons are customarily manufactured in a single piece by known casting or forging methods as the primary manufacturing process.

[0009] To counteract overheating, the heat dissipation of a piston may be improved by specifically adapting a piston’s geometry. However, the choice of the geometry of a piston is regularly restricted by the manufacturing process of the piston. Known processes such as machining previously casted or forged one-piece pistons do not readily allow for the production of pistons with complex geometries, such as pistons with undercuts, internal cavities, recesses, complex curved surfaces and / or deep and narrow channels.

[0010] Furthermore, for replacing a damaged and / or degraded piston, currently, a combustion piston engine has to be disassembled to the point that the connection between the piston and the connecting rod is exposed. Subsequently, a piston pin circlip (also known as a wrist pin clip or gudgeon pin circlip) and a piston pin must be removed to release the piston from the connecting rod. It is a well-known risk that, during this step, the piston pin circlip may fall into the crankcase of the combustion piston engine, such that the inner surface of the cylinder bore may be damaged - either during retrieval of the piston pin circlip or during subsequent operation of the combustion piston engine. Ultimately, the damaged and / or degraded piston is removed and replaced with a new piston such that the combustion piston engine can be reassembled. Overall, such a replacement of a damaged and / or degraded piston is very time-consuming and, therefore, regularly very costly. There is a need in the art for improved pistons for combustion piston engines, in particular for combustion piston engines, and for methods of their production, maintenance and replacement.

[0011] DE 102016202420 A1 discloses a two-part piston, the parts of which may be bonded together by an adhesive bond. The multipart piston of this disclosure may also comprise a substantially circumferential cooling channel for cooling the adhesive bond.

[0012] EP 1 859 155 B1 discloses a two-part piston with inner and outer cooling channels, which may receive oil as a coolant. The cooling oil may exit the cooling channel to the underside of the piston.

[0013] US 1 ,368,938 describes a piston with a detachable piston head and a piston body, which allows for the assembly of a piston where the piston body and the piston head may be made of differing materials.

[0014] CN 117823294 A discloses a multipart piston comprising a self-locking, annular inner cooling ring / channel insert as one part of the multipart piston. The cooling ring insert is described as being a closed structure.

[0015] EP 1 878 902 A discloses a multipart piston comprising an internal, closed cooling channel system comprising an inner cooling channel as well as an annular cooling channel, wherein both are connected to allow exchange of a coolant between the channels within the closed channel system.

[0016] US 10,202, 935 B2 discloses multi-piece piston for an internal combustion engine where the assembled piston comprises a cooling duct, preferably an axial or circumferential duct, which may receive and discharge coolant through oil bores facing the underside of the piston in the form of spray oil.

[0017] US 4,343,229 discloses a multipart piston, which may partly consist of ceramic materials.

[0018] FR 917 647 discloses a multi-piece piston with multiple internal cooling channels for an internal combustion engine. In the embodiments described, the piston is connected to the small end of the connecting rod by a conventional piston pin connection. During operation of the engine, a valve system arranged at the big end of the connecting rod regulates oil flow into a delivery channel of the connecting rod, which is guided into the internal channel system of the piston across the piston pin at the small end of the connecting rod. The valve system allows for oil to flow out of the piston’s channel system when the piston moves over the Top Dead Centre (TDC). The outflowing oil exits the piston through bores in the underside of the piston crown.

[0019] US 2,702,219 A1 discloses a one-piece piston with recesses disposed on the piston skirt, wherein such recesses are configured to that the piston skirt can comprise recesses for collecting oil from the general oil reservoir by forming a chamber with the cylinder wall. During operation the collected oil becomes pressurized to some degree within the chamber formed such that it may be guided through a channel to the bearing of the piston pin connecting the piston to the connecting rod small end such as to lubricate the connection.

[0020] GB 1479175 discloses a multipart piston assembly, comprising a cooling channel system, where engine oil delivered to the piston’s channel system from the engine’s general oil supply via the connecting rod in a continuous manner during operation.

[0021] US 4,363,293 also discloses a multipart piston assembly with an oil coolant system comprising several channels, which is delivered to the piston’s channel system through a central oil channel in the connecting rod in a continuous manner during operation. The outflowing oil exits the piston’s channel system through an outlet channel at the underside of the piston crown.

[0022] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. In particular, it is an object of the present invention to provide an improved piston and a combustion piston engine with improved manufacturability and maintainability. Furthermore, it is an object of the present invention to provide a simpler method for replacing the piston crown. SUMMARY OF THE INVENTION

[0023] In a first aspect, the present invention relates to a piston for a combustion piston engine, comprising at least a first piston part and a second piston part, wherein said first piston part comprises a receiving portion for attaching a connecting rod and wherein said second piston part comprises a top side, wherein said top side of said second piston part, when said piston is arranged in a cylinder of said combustion piston engine, delimits a working chamber of said cylinder, wherein said first and second piston parts are connectable, in particular detachably connectable, to each other to assemble said piston, characterized in that said first and / or second piston parts comprise at least one fluid channel, and wherein said at least one fluid channel is arranged to be in fluid communication with a fluid supply of said combustion piston engine when said piston moves between the regions of the dead centers in said cylinder but wherein said at least one fluid channel is sealed off from said fluid supply when said piston moves in the regions of the dead centers.

[0024] In a second aspect, the present invention relates to a piston-rod assembly for a combustion piston engine comprising a connecting rod attached via its connecting rod small end to said receiving portion of a piston according to the first aspect, wherein said connecting rod comprises a fluid channel with an inlet opening in fluid communication with a fluid supply of said combustion piston engine and an outlet opening, characterized in that said piston-rod assembly comprises a valve arrangement for sealing off fluid flow from said fluid supply through said fluid channel and through said outlet opening into said piston via said inlet opening of said fluid channel whenever said piston moves into the region of the top or bottom dead center of said piston’s stroke.

[0025] In a third aspect, the present invention relates to a method of cooling a piston for a combustion piston engine according to the first aspect, comprising

[0026] (a) directing a first volume of cooling fluid sufficient to at least fill all internal interspaces and fluid channels of said piston from a fluid supply of said combustion piston engine into said piston via said inlet opening of said at least one fluid channel; (b) filling all internal interspaces and fluid channels of said piston with said first volume of cooling fluid when said piston moves between the regions of the dead centers;

[0027] (c) sealing off said inlet opening of said at least one fluid channel from said fluid supply when said piston moves in the regions of the dead centers;

[0028] (d) opening said inlet opening of said at least one fluid channel and again bringing said at least one fluid channel into fluid communication with said fluid supply of said combustion piston engine;

[0029] (e) directing a second volume of cooling fluid sufficient to at least fill all internal interspaces and fluid channels of said piston from said fluid supply of said combustion piston engine into said piston via said inlet opening of said at least one fluid channel; and

[0030] (f) replacing said first volume of cooling fluid with said second volume of cooling fluid when said piston moves between the regions of the dead centers.

[0031] In a fourth aspect, the present invention relates to a combustion piston engine comprising:

[0032] - one or more cylinders; and

[0033] - a corresponding number of one or more pistons according to the first aspect or a corresponding number of one or more piston-rod assemblies according to the second aspect, wherein each of said one or more pistons or each piston of said one or more piston- rod assemblies are disposed slidably along a stroke axis in said cylinder.

[0034] In a fifth aspect, the present invention relates to a method for replacing a second piston part of at least one piston of a combustion piston engine of the fourth aspect, comprising the steps of:

[0035] - providing said piston disposed in said cylinder with said first and second piston parts connected to each other;

[0036] - disconnecting said second piston part from said first piston part;

[0037] - removing said disconnected second piston part from said cylinder;

[0038] - inserting a replacement second piston part into said cylinder; and

[0039] - connecting said replacement second piston part to said first piston part disposed in said cylinder to assemble said piston. DETAILED DESCRIPTION OF THE INVENTION

[0040] The inventor has recognized that, advantageously, the design freedom of a piston of the first aspect with regard to the choice of materials and geometry can be greatly enhanced.

[0041] Specifically, it has been recognized that a piston with an intricate system of internal cooling interspaces and channels can be manufactured in multiple parts, which are connectable for assembly into a piston of sufficient strength and durability for use in a combustion piston engine.

[0042] The advantageous design freedom is attributed to the fact that the process of manufacturing a piston with a complex geometry, having undercuts, internal cavities, recesses, complex curved surfaces and / or deep and narrow channels, is greatly facilitated by manufacturing several piston parts, which in turn are connectable such as to assemble the piston. A multipart piston allows for easier customization, enabling manufacturers to meet specific engine requirements, which are hardly achievable by known piston production processes for one-piece pistons. In addition, the process of producing pistons is simplified because the need for entirely new tooling for each variation is abolished.

[0043] The inventor has further realized that the individual parts of a multipart piston can be manufactured with much higher precision such that, for example, each piston part may comprise a bore, where the bores, upon assembly axially to define a continuous channel extending through both piston parts. Similarly, the individual piston parts may be manufactured to comprise recesses, which form cavities in the assembled piston. When the parts comprise corresponding recesses, cavities with larger volumes within the piston may be formed when the piston is assembled. Accordingly, the inventor has developed a multipart piston with an intricate system of internal interspaces and channels for improved cooling of the piston, in particular of the piston crown, during operation in a combustion piston engine.

[0044] Furthermore, it has been recognized that a multipart piston allows for the replacement of only the damaged sections, such as the second piston part with the top side, rather than the entire piston. This targeted repair reduces material costs and labor time, as it does not require the entire connecting rod or cylinder components to be disassembled. Additionally, it minimizes the downtime of the combustion piston engine, improving maintenance speed and efficiency and reduces the risk of cylinder or engine damage. The above-mentioned advantages especially come into effect in high-performance applications where wear tends to affect specific parts of the piston more heavily. Such applications may comprise racing engines, turbocharged and supercharged engines, marine engines and heavy-duty diesel engines.

[0045] It is emphasized that in accordance with the invention the piston of the first aspect comprises at least a first and a second piston part. It is conceivable, that the piston comprises more than these two parts. For example, the piston may also comprise a third piston part or, additionally, a fourth piston part and so forth.

[0046] In order to provide a clear and consistent understanding of the specification and claims, and the scope to be given such terms, the following definitions are provided.

[0047] Definitions

[0048] In the context of the present disclosure, the term "combustion piston engine" is to be understood in the broadest sense and refers to an engine that uses at least one piston reciprocatingly moving in a cylinder to convert pressure into motion or vice versa. The piston and the corresponding cylinder form the working chamber, in which a fluid, for example a gas, a liquid or an aerosol, may be pressurized. Typically, the working chamber is a combustion chamber.

[0049] In the context of the present disclosure, the term "piston" is to be understood in the broadest sense and refers to a component reciprocatingly movable between two maximal positions in a cylinder (i.e., the top dead center (TDC) and bottom dead center (BDC)), wherein a volume of the working chamber changes due to the reciprocating movement. The piston can be disposed slidably along a stroke axis in the cylinder of the combustion piston engine. The piston typically comprises a receiving portion that may be adapted for rotatably attaching a connecting rod, wherein the connecting rod may connect the piston to a crankshaft of a combustion piston engine. In the context of the present disclosure, the terms "top side" or “piston crown” are used interchangeable and are to be understood in their broadest sense, referring to a surface of the piston facing towards the working chamber - regardless of the spatial orientation of the cylinder in which the piston moves. In a combustion piston engine, the top side or piston crown faces the combustion chamber. Therefore, the pressure forces resulting from the expansion of the ignited fuel gas mixture are exerted on the top side / crown of the piston causing the movement of the piston. The top side therefore also refers to a surface of a piston facing towards the working chamber, wherein a stroke axis of the piston is arranged in the cylinder other than vertically, for example arranged substantially horizontally in a boxer type engine.

[0050] In the context of the present disclosure, the terms “region of a dead center” or “regions of the dead centers” are to be understood to refer to the small interval of crankshaft rotation around top or bottom dead center where piston displacement changes only minimally despite crank rotation. At these positions, the piston, connecting rod and crank are nearly aligned, so the piston’s motion slows to a stop before reversing direction. Because of this geometry, several degrees of crankshaft movement near TDC or BDC correspond to only a very small fraction of the total stroke. This “flat” motion region is important for valve timing, ignition, and lubrication events, which often occur when the piston is in or near a dead center, i.e. in the region of a dead center. Typically, the region of a dead center extends ±5 to 10° around the TDC and BDC.

[0051] In the context of the present disclosure, the term “sealed off” means that a fluid flow between at least two fluid carrying compartments (such as a fluid supply and the interspaces, cavities and / or channels of a piston according to the invention) is blocked by a fluid-tight barrier. Typically, the fluid-tight barrier is provided by a valve arrangement, which, in a closed position is configured to seal off the fluid supply, thereby preventing pressurized fluid flow from the fluid supply.

[0052] In the context of the present disclosure, the phrase “the receiving portion is configured to directly receive the small end of a respective connecting rod” is to be understood to mean that the connection between the connecting rod and the receiving portion of the piston is “direct” in the sense that the connection does not comprise an intermediate bearing and / or piston pin such that the piston and connecting rod form an integral connection.

[0053] In the context of the present disclosure, the term “semi-circumferential interspace” refers to an interspace extending along only a portion of the circumference of the piston, less than 360°, such as approximately 180°, so as to form a partial circumferential interspace rather than a continuous circumferential interspace. Two semi-circumferential interspaces may be arranged such that, together, they extend over substantially the entire circumference of the piston.

[0054] With regard to methods for connecting the first and the second piston part to each other to assemble the piston and for disconnecting the first and the second piston part from each other to disassemble the piston, it has been recognized, that a plug-rotate- mechanism is very well suited for connecting or disconnecting the piston parts in a cylinder with very limited operating space. Furthermore, plug-rotate-mechanism allows for a very easy and quick connection of the piston parts. In the context of the present disclosure, the term "plug-rotate-mechanism" is to be understood in the broadest sense and refers to a mechanism for providing a form-fit connection between the first and the second piston part that inhibits a movement of the piston parts relative to each other at least in the directions of motion of the piston in the cylinder along the stroke axis. For establishing this connection, the first and the second piston part are engaged with one another by means of a translational motion of at least one of the piston parts in a direction towards one another and a rotational motion of at least one of the piston parts relative to each other. For loosening this connection, the above- mentioned steps may be performed in reverse order. Consequently, the plug-rotate- mechanism provides a connection, wherein no additional fastening or connecting means are required. The rotational motion may interlock the first and the second piston part with each other, so that the translational motion is inhibited. Furthermore, a force-fit connection in the form of a frictional connection between the first and the second piston part may be provided by means of the rotational motion that inhibits a rotational movement of the piston part relative to each other during operation of the combustion piston engine.

[0055] In addition to the above definitions, and unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

[0056] Further, reference throughout this specification to “one embodiment”, “some embodiments” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in some embodiments” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0057] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

[0058] As used herein, the term “exemplary” is used in the sense of providing examples, as opposed to indicating quality. That is, an “exemplary embodiment” is an embodiment provided as an example, as opposed to necessarily being an embodiment of exemplary quality.

[0059] Further features, advantages and preferred embodiments are disclosed or may become apparent in the following.

[0060] Embodiments of the invention

[0061] As indicated above, the inventor has developed a multipart piston with an intricate system of internal interspaces and channels for improved cooling of the piston, in particular of the piston crown, during operation in a combustion piston engine.

[0062] Importantly, the intricate system of internal interspaces and channels is arranged such that a cooling fluid, for example engine oil from the general oil supply (i.e. the oil sump), is guided through the system under pressure such as to completely fill all cavities and channels with cooling fluid while removing all gaseous volumes from the system of internal interspaces and channels.

[0063] Accordingly, in a first aspect, the invention relates to piston for a combustion piston engine comprising at least a first piston part and a second piston part, wherein the first piston part comprises a receiving portion for attaching a connecting rod, and wherein the second piston part comprises a top side, wherein the top side of the second piston part, when the piston is arranged in a cylinder of the combustion piston engine, delimits a working chamber of the cylinder, wherein the first and second piston parts are connectable, in particular detachably connectable, to each other to assemble the piston, characterized in that the first and / or second piston parts comprise at least one fluid channel, and wherein the at least one fluid channel is arranged to be in fluid communication with a fluid supply of the combustion piston engine when the piston moves between the regions of the dead centers in the cylinder but wherein the at least one fluid channel is sealed off from the fluid supply when the piston moves in the regions of the dead centers.

[0064] The piston of the first aspect provides for superior cooling of the piston during operation as the at least one fluid channel allows for a cooling fluid (such as, for example, engine oil delivered to the piston from the oil sump through a central channels of the connecting rod) to flow though the piston only when the piston moves between the regions of the dead centers, i.e. during the upward or downward movement of the piston in the cylinder at crank angles of but not when the piston moves in the regions of the dead centers. A typical 4-stroke cycle typically requires two full rotations of the crankshaft such that the cycle from beginning to end spans crank angles from 0 to 720° of rotation. Within each cycle, the piston moves through its highest point within the cylinder (top dead center; TDC) twice. The first occurs at the end of the compression stroke, just before ignition and the beginning of the power stroke, and the second occurs at the end of the exhaust stroke, immediately before the intake stroke begins. Equally, the piston passes through its lowest point within the cylinder (bottom dead center; BDC) twice, namely, at the end of the intake stroke, just before the compression stroke begins, and the second occurs at the end of the power stroke, immediately before the exhaust stroke starts. As such, the piston moves through the TDC at 0 / 720° and at 360° crank angle and through the BDC at 180° and 540° crank angle. The region of a dead center typically spans about 5 to 10° crank angle on either side of the dead center.

[0065] As such, if the region of a dead center extends for 10° on either side, the regions of the TDC in a 4-stroke cycle are from 710° to 10° crank angle as well as from 350° to 370° crank angle, while the regions of the BDC are from 170° to 190° crank angle as well as from 530° to 550° crank angle. Consequently, the piston moves between those regions at crank angles of 10 to 170° (intake stroke), 190 to 350° (compression stroke), 370 to 530° (power stroke) and 550 to 710° (exhaust stroke).

[0066] In embodiments where the region of a dead center extends for 5° on either side, the regions of the TDC in a 4-stroke cycle are from 715° to 5° crank angle as well as from 355° to 365° crank angle, while the regions of the BDC are from 175° to 185° crank angle as well as from 535° to 545° crank angle. Consequently, the piston moves between those regions at crank angles of 5 to 175° (intake stroke), 185 to 355° (compression stroke), 365 to 535° (power stroke) and 545 to 715° (exhaust stroke).

[0067] The arrangement of the at least one fluid channel such that it is in fluid communication with a fluid supply of the combustion piston engine when the piston moves between the regions of the dead centers in the cylinder but is sealed off from the fluid supply when the piston moves in the regions of the dead centers is instrumental in achieving overall efficiency during operation. Firstly, the fluid communication between the fluid supply and the fluid channels allows pressurized cooling fluid to repeatedly enter (and exit) the cooling channel during the upward or downward movement of the piston in each stroke. Without wanting to be bound by theory, the repeated stopping and starting of pressurized fluid flow throughout the internal cavities and channels may generate turbulent fluid flow conditions, which allow for much greater heat transfer to the fluid than laminar fluid flow conditions. Secondly, the sealing off ensures that a defined volume of the cooling fluid, which is more or less equal to the internal volume of the cooling channel and any associated cavities throughout the piston, remains within the piston during the time interval that the piston moves through the regions of the dead centers. Having a constant volume of fluid within the piston during this time interval is sufficient for the fluid to absorb heat from the piston to achieve efficient piston cooling as indicated above, but also ensures a constant weight of the piston. Thirdly, as all internal cavities and channels of the piston are filled with fluid, the piston crown volume is substantially incompressible. However, despite having a substantially incompressible piston crown volume (similar to that of a solid piston without any internal cavities or channels), when fluid-filled, the piston of the present disclosure remains significantly lighter than a solid piston since the density (g / cm3) of the fluid is typically between 3 to 10 times lower than typical piston materials. For example, aluminum alloy is about 3x denser / heavier than engine oil, titanium is about 5x denser / heavier than engine oil, cast iron is about 8x denser / heavier than engine oil and steel is about 9x denser / heavier than engine oil. Fourthly, as the internal channels and cavities of the piston are repeatedly filled during operation, the fluid volume present within the piston from the previous filling is replaced with fresh / cooler fluid from the fluid supply every time the fluid channel comes in fluid communication with the fluid supply again. While this further ensures continuous cooling of the piston, the fluid volume expelled from the piston can advantageously be used for lubricating the piston within the cylinder, for example for lubricating the piston skirt.

[0068] In certain embodiments, the first and / or second piston parts comprise two fluid channels, wherein both channels are alternatingly in fluid flow communication with a fluid supply of the combustion piston engine when the piston moves between the regions of the dead centers in the cylinder. Providing the piston with two fluid channels can ensure that these two channels come into fluid communication with the fluid supply alternatingly during each stroke in the 4-stroke cycle. One example of such an embodiment can be realized when the inlet opening of each fluid channel is provided in the receiving portion of the piston and the fluid is provided to each of these channels via a central channel of the connecting rod exiting at the connecting rod small end. During operation the fluid communication between the fluid supply and the fluid channel in the receiving portion of the first piston is established when the connecting rod pivots relative to the stroke axis, i.e. during each downward or upward movement of the piston within the cylinder.

[0069] As indicated generally above, in certain embodiments, when the piston moves between the regions of the dead centers in the cylinder, the at least one fluid channel is in fluid communication:

[0070] (a) with the receiving portion of the first piston part; and (b) with a first interspace formed between the first and second piston parts in a connected state, delimited by a bottom side of the second piston part and a top side of the first piston part.

[0071] This configuration ensures that fluid can be delivered to the internal system of channels and cavities of the piston through a channel of the connecting rod, which exits into the receiving portion of the piston at the small end of the connecting rod. Typically, the first interspace forms a central cavity of the piston, which is defined by at least one fluid recess in the top side of the first piston part and / or in the bottom side of the second piston part. The first interspace of the piston parts in a connected state (i.e. of the assembled piston) can be precisely formed by corresponding recesses in the first and second piston parts. During manufacture of the individual piston parts, the exact configuration of the recesses can be inspected prior to assembly of the piston. When filled with fluid, the first interspace (i.e. the central cavity) of the assembled piston can greatly improve the heat dissipation capacity of the piston and, as both piston parts are in direct contact with the fluid, efficient cooling of both piston parts is ensured.

[0072] Further, in some embodiments, when the piston moves between the regions of the dead centers in the cylinder, the at least one fluid channel is further in fluid communication:

[0073] (c) via one or more inlet fluid channels, with a second interspace formed between the first and second piston parts in a connected state; and / or

[0074] (d) via one or more outlet fluid channels, with one or more corresponding recesses of at least one lateral guiding surface of the piston, wherein the at least one lateral guiding surface is for guiding the piston inside the cylinder.

[0075] Typically, the at least one lateral guiding surface (acting as the piston skirt) is arranged on the second piston part. The at least one lateral guiding surface moves adjacent to the cylinder wall and is consequently subjected to wear. Therefore, in addition to the top side of a piston, once damaged or worn, the at least one lateral guiding surface can also be easily replaced by replacing the second piston part.

[0076] According to a further preferred embodiment of the invention, the at least one lateral guiding surface comprises at least one fluid recess. The at least one fluid recess serves as a reservoir for oil lubricating the at least one lateral guiding surface and the running surfaces of the cylinder wall.

[0077] Like the first interspace, the second interspace of the assembled piston can be precisely formed by providing a recess in the first and / or second piston parts. The exact configuration of the recesses can be inspected prior to assembly of the piston.

[0078] In certain embodiments the piston comprises a lateral sealing surface with at least one, preferably three, circumferential grooves for arranging piston rings therein. Piston rings provide a seal between the piston and cylinder wall, preventing combustion gases from leaking into the crankcase while controlling oil consumption by scraping excess oil off the cylinder walls. In addition, the heat dissipation can be improved by transferring heat from the piston to the cylinder wall via the sealing surface and / or the piston rings. The circumferential grooves may comprise reservoirs in which excess fluid that is collected by the piston rings may be collected during operation. Typically, the circumferential grove reservoirs have outlet openings, through which the excess fluid may drain to the inner side of the piston during operation. Typically, the diameter of the outlet openings is smaller than the diameter of the circumferential groove reservoirs such that the excess fluid that is collected in the reservoirs drains therefrom and, as such, remains for further lubrication of the piston rings.

[0079] Generally, the lateral sealing surface is arranged on the second piston part. Typically, the second interspace is a substantially semi-circumferential interspace extending almost around the entire circumference of the assembled piston part such that the fluid therein can absorb heat from the lateral sealing surface of the second piston part, also the second interspace can greatly improve the heat dissipation capacity of the piston.

[0080] During operation, fluid can be transported from the second interspace, via the one or more outlet fluid channels, to the at least one lateral guiding surface for improving the lubrication of corresponding running surfaces in these areas, i.e. the running surfaces of the piston skirt. Furthermore, heated fluid, in particular in the form of engine oil which has absorbed heat during its residence within the piston, can be drained from the piston. In the case of an oil as a fluid in a combustion piston engine, the oil can return to the oil supply (oil sump) of the combustion piston engine.

[0081] To avoid an overpressure within and throughout the system of internal channels and cavities of the assembled piston, in particular the fluid channels must be configured such that the internal pressure does not significantly exceed the general pressure under which the fluid is fed into the piston from the fluid supply. To this end, the area of the inlet opening of one of the at least one fluid channels may be substantially equal to the combined area of all inlet openings the one or more inlet fluid channels as well as to the combined area of all inlet openings the one or more outlet fluid channels.

[0082] In other words, in embodiments where the cooling fluid is fed into the piston via a single inlet opening of the fluid channel in the receiving portion but from the first interspace to two separate second interspaces, via two inlet openings of two inlet fluid channels leading to the second interspace, the areas of those two inlet openings are each half of the area of the single inlet opening of the fluid channel in the receiving portion. Further, if the fluid is then drained from each of the two second interspaces, through two inlet openings of the fluid outlet channels leading to the recesses in the guiding surface (piston skirt) of the second piston part, i.e. in sum via four outlet fluid channels, the area of each of the four inlet openings of the outlet fluid channels is one quarter of the aera of the single inlet opening of the fluid channel in the receiving portion. For illustration only, if the area of the single inlet opening of the fluid channel in the receiving portion were 4 mm2, the area of each of the two inlet openings of two inlet fluid channels leading to the second interspace would be 2 mm2and the area of each of the four inlet openings of the four outlet fluid channels leading to the second interspace would be 1 mm2.

[0083] Importantly, in all embodiments of the piston of the first aspect described, the receiving portion may comprise an undercut acting in a tensile and compressive direction, so that the receiving portion is configured to directly receive, in a form-fitting manner and pivotable about a pivot axis, a small end of the respective connecting rod corresponding to the undercut of the receiving portion. For the sake of clarity, and as outlined above, in operation the direct connection between the receiving portion of the piston and the small end of the connecting rod does not comprise an intermediate bearing and / or piston pin and, therefore, the piston and connecting rod form an integral connection.

[0084] According to a preferred embodiment of the piston of the first aspect, the first and second piston parts are connectable by means of a form-fit and / or force-fit connection, preferably comprising a plug-rotate-mechanism and / or a screwed connection and / or a friction connection.

[0085] A form-fit and / or force-fit connection allows for a very easy and quick connection of the piston parts. A form-fit and / or force-fit connection may easily be detachable. With regard to the plug-rotate-mechanism, it is referred to the above-made statements to avoid repetition. A screwed connection may have the advantage that it provides a high connection strength and minimal connection play. A friction connection is selflocking without the need for additional locking mechanisms and provides a high resistance against loosening under operational loads of the piston.

[0086] According to a further preferred embodiment of the piston of the first aspect, the first and second piston parts are connectable by means of a bonded connection, preferably comprising an adhesively bonded connection and / or a welded connection. A bonded connection provides enhanced load distribution across the bonding surface, minimizing stress concentrations and reducing the risk of local bond failures. Furthermore, a bonded connection of the piston parts shows a high fatigue resistance under operational loads of the piston.

[0087] According to a further preferred embodiment of the piston of the first aspect, the first and second piston parts are connectable by means of a plug-rotate-mechanism, for example a bayonet mechanism. To avoid repetition, it is referred to the above-made statements with regard to the plug-rotate-mechanism. The bayonet mechanism may comprise a twist-and-lock action, wherein one of the piston parts comprises at least one recess and the other piston part comprises at least one protruding pin or lug that corresponds to the at least one recess. By inserting the piston part with the at least one protruding pin or lug into the other piston part and rotating it, the at least one protruding pin or lug slides into the at least one recess, interlocking the two piston parts securely. A bayonet mechanism allows for quick attachment and detachment of the first and second piston part. According to a further preferred embodiment of the invention, the plug-rotate- mechanism comprises, the first and second piston parts being engagable with each other by means of a plugging mechanism with a direction of motion along a stroke axis of the piston, preferably wherein the first piston part is insertable into the second piston part, and / or the first and second piston parts being interlockable with each other by means of a rotational movement relative to each other, wherein optionally a rotational axis of the movement is substantially aligned with the stroke axis. A plugging mechanism facilitates the correct positioning of the piston parts relative to each other, especially when the piston parts must be connected inside a cylinder with low or none visibility. A rotational movement for interlocking the piston parts with each other may further facilitate the connecting process, especially when the first and second piston parts are connected to each other inside the cylinder.

[0088] According to a further preferred embodiment of the invention, the first and second piston parts comprise corresponding interlocking parts, each comprising a respective connecting surface, wherein in the interlocked state the connecting surfaces of the interlocking parts form a friction connection between the first and second piston parts. Thereby, a high resistant connection can be achieved with easy means.

[0089] According to a further preferred embodiment of the invention, the connecting surfaces comprise a tangential gradient of at least 0.1 ° and a maximum of 3.0°, more preferably of 1.5°, with regard to the rotational axis. Such a tangential gradient enables the connection between the piston parts to be securely locked simultaneously with the rotational movement of the piston parts relative to each other as part of the plug- rotate-mechanism. Therefore, the connection can be established easily and quickly.

[0090] Notwithstanding, even when in a securely locked position, the interlocking parts may be further fixed against any rotational movement relative to each other.

[0091] Specifically, in another preferred embodiment, the corresponding interlocking parts are connectable to each other by means of a screwed connection and / or an adhesive bond. The screwed connection is detachable and therefore facilitates disconnection of the piston parts. An adhesive bond provides a uniform force transmission from one piston part to the other piston part leading to a high connection resistance, especially against fatigue loading, and additionally provides a sealing of the contact areas between the piston parts.

[0092] According to a further preferred embodiment of the invention, the top side of the second piston part comprises a tool receiving portion, preferably comprising at least one indentation and / or at least one elevation, for receiving a piston assembling tool. Such a receiving portion enables the use of a piston assembly tool for facilitating the connection and / or disconnection of the first and second piston part.

[0093] Generally, the tool receiving portion is adapted for a torsional moment to be transmitted, wherein a rotational axis of the torsional moment is substantially aligned with a stroke axis of the piston. Thereby, the first and second piston parts can be connected or disconnected particularly well, when the piston is disposed in the cylinder of the combustion piston engine with limited operating space. As is well known in the art, the top surface of the piston crown may be designed to comprise three-dimensional structure, including protrusions into the working chamber of the cylinder such that the pressure values during a compression stroke can be influenced, for example increased.

[0094] According to a further preferred embodiment of the invention, the first and second piston parts are made of differing materials. By providing piston parts with differing materials, the piston may be adapted to specific requirements depending on the use case.

[0095] According to a further preferred embodiment of the invention, the first and / or second piston parts, preferably the second piston part, are made of a polymer, in particular a fiber-reinforced polymer. A piston part made of a polymer comprises a low weight and therefore a low moment of inertia when being in operation in a combustion piston engine. This way, the efficiency of the combustion piston engine can be improved. Furthermore, due to the low thermal expansion coefficient of polymer, a piston part made of polymer shows a low thermal expansion, which leads to good sealing behavior to the combustion chamber. Consequently, engine emissions can be greatly reduced and engine efficiency can be enhanced. A piston part made of a fiber- reinforced polymer has a very high strength as well as a very high resistance against wear, improving the lifetime of the piston, in particular of a piston with a second piston part made of a fiber-reinforced polymer, and the combustion piston engine.

[0096] According to a further preferred embodiment of the invention, the at least one fluid channel, the one or more inlet fluid channels and the one or more outlet fluid channels are created by casting, forging and / or machining processes such as boring or milling As already described above, by providing fluid channels and a fluid supply, the fluid can flow through the piston during operation. This way, the piston can be actively cooled during operation in the combustion piston engine, reducing the risk of overheating and damage of the piston and the combustion piston engine. The fluid may be a liquid, in particular comprising a liquid comprising oil or water, or may be an aerosol, in particular an aerosol comprising oil and / or air. For driving the fluid through the fluid channels, the combustion piston engine may comprise a pump to pressurize the fluid such as to generate a pressure driven fluid flow. The fluid supply may be an oil supply of a combustion piston engine. In a multipart piston, a fluid channel and / or an internal cavity can be manufactured more easily by casting, forging and / or machining processes such as boring or milling than in a one-piece piston.

[0097] Furthermore, the piston of the first aspect may be substantially round, i.e. circular, rather than oval shaped as are conventional pistons. As such, and according to a further preferred embodiment of the invention, the piston may comprise a plurality of radially arranged substantially planar cross-sectional areas extending through a stroke axis of the piston, characterized in that the piston is formed in such a way, that a first and a second cross-sectional area of the plurality of cross-sectional areas have a size which differs from one another by less than 10%, preferably less than 5%, and are positioned offset from each other at an angle of 90° with regard to the stroke axis, preferably wherein the first or second cross-sectional area extends through a pivot axis of a connection of the first piston part with a connecting rod. More preferably, the piston is designed to be fully axially symmetric with respect to the stroke axis. In other words, the pieces of the piston, which result from any arbitrary radial cut through the stroke axis of the piston, have substantially the same weight. Without wanting to be bound by theory, such a dimensioning of the piston results in a near-uniform thermal expansion in a radial direction with regard to the stroke axis of the piston. In a combustion piston engine, this can lead to an improved sealing between the combustion chamber and the crankcase, resulting in reduced blow-by and, therefore, in a reduction of undesirable engine emissions and an enhanced engine efficiency. Alternatively or additionally, the at least one lateral guiding surface and / or the lateral sealing surface may comprise a circular outer contour. This can result in improved sealing properties. In a combustion piston engine of the fourth aspect, preferably in a combustion piston engine, improved sealing properties can in particular be achieved compared to an oval piston when the engine has not yet reached a normal operation temperature, for example shortly after the start of a cool engine. The disclosure of PCT-Application No. PCT / EP 2022 / 087653, filed on December 22, 2022, is hereby incorporated by reference in its entirety. In particular, paragraphs

[0005] and

[0019] to

[0036] of PCT-Application No. PCT / EP 2022 / 087653, filed on December 22, 2022, are hereby incorporated by reference.

[0098] As already indicated above, the construction of the receiving portion of the piston of the first aspect allows for its direct connection to the small end of a connecting rod having a fluid channel, such that the system of internal channels and cavities of the piston can be brought into fluid communication with an engine’s fluid supply through the connecting rod. Typically, the connection between the receiving portion of the piston and the small end of the connecting rod comprises a valve arrangement for opening or sealing off the fluid communication.

[0099] Accordingly, in a second aspect, the present invention further relates to a piston-rod assembly for a combustion piston engine comprising a connecting rod attached via its connecting rod small end to the receiving portion of a multipart piston according to the first aspect, wherein the connecting rod comprises a fluid channel with an inlet opening in fluid communication with a fluid supply of the combustion piston engine and an outlet opening, characterized in that the piston-rod assembly comprises a valve arrangement for sealing off fluid flow from the fluid supply through the fluid channel and through the outlet opening into the piston via the inlet opening of the fluid channel whenever the piston moves into the region of the top or bottom dead center of the piston’s stroke.

[0100] In some embodiments of the second aspect, the valve arrangement comprises a control pocket created as a recess in the receiving portion of the piston surrounding the inlet opening of the fluid channel of the receiving portion and / or in a corresponding surface of the connecting rod small end surrounding the outlet opening of the fluid channel of the connecting rod. The control pocket ensures that fluid can flow not only when the outlet opening of the fluid channel of the connecting rod overlaps with the inlet opening of the fluid channel in the receiving portion of the piston but across a larger area. The extension of the recess forming the pocket in a direction perpendicular to the pivot axis of the connecting rod defines the duration during which fluid communication between the fluid supply is possible when the piston moves between the regions of the dead centers. The longer extension of the pocket in a direction perpendicular to the pivot axis of the connecting rod, the shorter the time interval within which the fluid channel of the piston is sealed off from the fluid supply when the piston moves in the regions of the dead centers.

[0101] As such, the recess is dimensioned to allow fluid flow from the fluid channel of the connecting rod into the piston via the fluid channel when the piston is moving outside of the region of the top and bottom dead centers. Conversely, the recess is dimensioned to seal off fluid flow from the fluid channel of the connecting rod into the piston via the fluid channel during a 4-stroke cycle at crank angles from 710° to 10°, from 170° to 190°, from 350° to 370° and 530° to 550°.

[0102] In some embodiments, the outlet opening of the fluid channel of the connecting rod defines an area which is larger than the area of the inlet opening of the fluid channel. This has the advantageous effect that more fluid is supplied to the piston-rod connection than can flow into the fluid channel of the receiving portion of the first piston part. The excess fluid advantageously lubricates the connection during operation. This is even more important as the small end of the connecting rod may be attached directly to the receiving portion in a form-fitting manner and pivotable about a pivot axis.

[0103] As already indicated above, the design and configuration of the intricate system of internal cavities and channels throughout the multipart piston of the first aspect allows for the complete filling of and complete removal of gases from the system, thereby allowing for more efficient piston cooling during operation in a combustion piston engine. Accordingly, in a third aspect, the present invention also relates to a method of cooling a multipart piston for a combustion piston engine according to any the first aspect, comprising

[0104] (a) directing a first volume of cooling fluid sufficient to at least fill all internal interspaces and fluid channels of the piston from a fluid supply of the combustion piston engine into the piston via the inlet opening of the at least one fluid channel;

[0105] (b) filling all internal interspaces and fluid channels of the piston with the first volume of cooling fluid when the piston moves between the regions of the dead centers;

[0106] (c) sealing off the inlet opening of the at least one fluid channel from the fluid supply when the piston moves in the regions of the dead centers;

[0107] (d) opening the inlet opening of the at least one fluid channel and again bringing the at least one fluid channel into fluid communication with the fluid supply of the combustion piston engine;

[0108] (e) directing a second volume of cooling fluid sufficient to at least fill all internal interspaces and fluid channels of the piston from the fluid supply of the combustion piston engine into the piston via the inlet opening of the at least one fluid channel; and

[0109] (f) replacing the first volume of cooling fluid with the second volume of cooling fluid when the piston moves between the regions of the dead centers.

[0110] Specifically, the filling step (b) may comprise a sequential flow of the first volume of cooling fluid into all internal interspaces and fluid channels of the piston in the following order:

[0111] (1 ) through one fluid channel into the first interspace;

[0112] (2) from the first interspace through one or more inlet openings into the one or more inlet fluid channels and through one or more outlet openings of the inlet fluid channel into the second interspace; and

[0113] (3) from the second interspace through one or more inlet openings into the one or more outlet fluid channels and through one or more outlet openings of the outlet fluid channel into a corresponding fluid recess of the lateral guiding surface. As indicated, typically, all internal interspaces and fluid channels of the piston are configured such that the sequential flow of the second volume of cooling fluid cooling fluid ensures

[0114] (a) the removal of any gaseous volumes from the internal interspaces and fluid channels; and

[0115] (b) the complete filling of all internal interspaces and fluid channels with the cooling fluid.

[0116] In certain embodiments, the replacing step (f) comprises a sequential flow of the second volume of cooling fluid into all internal interspaces and fluid channels of the piston mirroring that of the first volume of cooling fluid as defined above and wherein the fluid flow of the second volume of cooling fluid into the piston expels the first volume of cooling fluid from the piston through the one or more outlet openings of the outlet fluid channel thereby lubricating the lateral guiding surface of the piston with the first volume of cooling fluid.

[0117] During operation of the combustion piston engine, the steps (a) to (e) are continuously repeated such that a fresh volume of cooling fluid replaces a respective previous volume of cooling fluid every time the piston moves between the regions of the dead centers.

[0118] As indicated above, the combustion piston engine of the fourth aspect, comprises

[0119] - one or more cylinders; and

[0120] - a corresponding number of one or more pistons according to the first aspect or a corresponding number of one or more piston-rod assemblies according to the second aspect, wherein each of the one or more pistons or each piston of the one or more piston-rod assemblies are disposed slidably along a stroke axis in the cylinder.

[0121] Typically, in the combustion piston engine of the fourth aspect, the crankshaft is rotatably connected to the receiving portion of the first piston part of each piston by a connecting rod, characterized in that each connecting rod comprises at least one fluid channel and the crankshaft comprises an internal fluid supply, wherein the at least one fluid channel of each connecting rod is in fluid communication with the internal fluid supply. The at least one fluid channel of the piston may be in fluid communication with the at least one fluid channel of each connecting rod and the internal fluid supply of the crankshaft.

[0122] According to a further preferred embodiment of the invention, the at least one fluid channel of each connecting rod is, at least partially during a piston stroke, preferably when the piston moves between the regions of the dead centers of a crank drive, in fluid communication with the receiving portion and the at least one fluid channel of the piston. Thereby, heated fluid inside the piston may be efficiently replaced by cooler fluid, in particular from the fluid supply.

[0123] As described above, according to a preferred embodiment of the invention, the connecting rod and / or the piston comprises a valve arrangement for controlling a fluid flow to and / or from the receiving portion and the at least one fluid channel of the piston, in particular by a pivoting of the connecting rod about a pivot axis. The valve arrangement may be provided on at least one connecting rod end, preferably on the connecting rod end received by the receiving portion of a piston of the first aspect. Consequently, the fluid quantity flowing through the fluid channels of the piston can be actively influenced by means of the valve arrangement, in particular, the fluid quantity is influenced depending on the angular position of the connecting rod relative to its pivoting axis. For example, fluid, in particular oil, can only flow when the piston moves between the respective regions of the dead centers of the piston stroke.

[0124] According to a further preferred embodiment of the invention, the valve arrangement comprises a control pocket or a plurality of control pockets formed on a surface of a connecting rod end, so that the fluid flow is limited or prevented when the piston is about to reach either its top dead center (TDC) or its bottom dead center (BDC) and / or is in a substantially rectilinear arrangement with the connecting rod and a stroke axis of the piston in the cylinder bore. Such a control pocket can be introduced by simple mechanical means, for example by a milling process, an erosion process or during casting of the connecting rod and / or the piston, and is therefore inexpensive to manufacture. It is conceivable that the connecting rod comprises a part of the control pocket and the piston comprises another corresponding part of the control pocket. The geometry of the control pocket may be designed in such a way that, for example, it is determined purely mechanically that a fluid flow is prevented in the region of a respective dead center of the piston in the cylinder bore and / or in the region of a substantially rectilinear arrangement of the connecting rod in relation to stroke axis of the piston, and the fluid flow is then released when the connecting rod pivots about the pivot axis. This way, an active and controlled lubrication and / or cooling, depending on the fluid, of the connection between the receiving portion of the piston and the connecting rod and of the piston can be realized, whereby, in particular, the heat dissipation from a combustion chamber of a combustion piston engine can be optimized.

[0125] Therefore, the piston can be designed with little thermal reserves and thus be very light. This significantly increases the efficiency of a corresponding combustion piston engine. The control pocket may comprise the form of a groove on an outer surface of the connecting rod end. Furthermore, the valve arrangement, in particular the control pocket, may be arranged and / or designed in such a way that the fluid flow is restricted for an angle of ±20°, ±15°, ±10° and / or ±5° between a longitudinal axis of the connecting rod and the stroke axis of the piston in the cylinder bore. This design may determine corresponding angles in such a way that, in particular, a full revolution of a crankshaft of a combustion piston engine is used to exploit corresponding control regions of the valve arrangement and / or the control pocket in accordance with the invention. Here, angle specifications refer to a full angle of 360°. The disclosure of PCT-Application No. PCT / EP 2022 / 087641 , filed on December 22, 2022, is hereby incorporated by reference in its entirety. In particular, paragraphs

[0006] and

[0021] to

[0057] of PCT-Application No. PCT / EP 2022 / 087641 , filed on December 22, 2022, are hereby incorporated by reference.

[0126] In conventional internal combustion engines, combustion of the fuel-air mixture is inherently incomplete, resulting in limited efficiency and resulting pollutant emissions. In contrast, the embodiments of the first to fourth aspects of the present invention are particularly suited for increasing efficiency of a combustion piston engine.

[0127] As explained above, the piston and piston-rod assemblies of the first and second aspects, respectively, are specifically configured for the repeated opening and sealing off of the valve arrangement in the connection between the receiving portion of the piston and the small end of the connecting rod, which allows for the method of cooling the piston of the third aspect to be performed during operation of the combustion piston engine of the fourth aspect. As a consequence, during operation of the combustion piston engine of the fourth aspect combustion proceeds much more efficiently and, therefore, more completely, enabling the engine to deliver the same output power with a reduced fuel input. Without wanting to be bound by theory, the intricate system of internal cavities and channel of the piston of the first aspect, which can be completely filled without any gas pockets remaining, in combination with the repeated complete pressure-driven expulsion of the entire volume of heated cooling fluid from the system for complete replacement with fresh cooling fluid, allows for such efficient heat transfer from the piston that combustion efficiency can be increased compared to a comparable engine with a conventional piston.

[0128] Furthermore, the complete geometrical symmetry of the assembled multipart piston as well as the uniform mass distribution throughout the piston with respect to the stroke axis, both in an empty but also in a fluid-filled state, allows for a near-uniform thermal expansion as the piston reaches operational temperatures. Due to this symmetry, the piston runs substantially without any significant imbalances within the cylinder and piston runs very smoothly, with only very little vibration. This allows for the piston (as well as the cylinder) of an engine of the fourth aspect to be manufactured much more precisely than in the engine technologies currently being employed in commercially available combustion engine driven motor vehicles.

[0129] Specifically, the piston can be manufactured with a comparatively reduced pistoncylinder clearance. In this context, the expulsion of heated cooling fluid through a recess of the guiding surface (piston skirt) provides additional, yet targeted lubrication of the running surfaces of the piston within the cylinder. As will be readily appreciated, during operation, a piston having a comparatively reduced piston-cylinder clearance provides improved sealing efficiency of the piston rings, whereby the ingress of lubricating oil into the combustion chamber is minimized, resulting in lower oil consumption and reduced deposit formation within the working chamber. The improved sealing further maintains higher compression pressure, enabling more efficient combustion so that a given power output can be achieved with reduced fuel input. In addition, the reduced clearance limits blow-by of combustion gases into the crankcase, thereby lowering the emission of hydrocarbons, particulates, and other pollutants. This is particularly useful in engine applications that require frequent cold starts or need to accommodate frequently changing load conditions, such as agricultural or forestry machines or stationary on-demand generators. As will be appreciated, coating the top surface of the piston crown provides several functional advantages. Such a coating may act as a thermal barrier, reducing heat transfer into the piston body and lowering the thermal load on the piston. The coating may also increase wear resistance against fuel spray impingement and combustion erosion, reduce the formation of carbon deposits and, thereby, extend piston service life and improve engine efficiency. However, these advantages are often accompanied by technical problems encountered in engine operation. In particular, thermal barrier coatings are subject to high cyclic thermal stresses, which can lead to fissures, cracking, or spalling of the coating surface. Importantly, differences in thermal expansion between the coating and the piston substrate can cause delamination under load. These drawbacks have complicated the realization of the intended advantages in current engine technologies, since durability and adhesion of the coating must be ensured under extreme temperature and pressure cycles typical for combustion engines. As outlined above, the particular configuration of the piston allows a near-uniform thermal expansion as the engine reaches operational temperatures, while simultaneously cooling the piston such that even the uniform expansion is slowed down in comparison to an uncooled piston heating up. Accordingly, the piston of the first aspect of the present invention is particularly well suited for coating the top surface of the piston crown such that all of the known advantages of such a coating can readily be realized, while avoiding many of the known problems.

[0130] Accordingly, the piston crown surface of the piston of the first aspect can readily be coated with various types of coatings in order to withstand even higher thermal and mechanical loads present during combustion. Metallic coatings, such as chromium- based or nickel-based layers, provide high hardness, wear resistance, and corrosion protection. Ceramic coatings, including alumina- and zirconia-based thermal barrier coatings, are configured to reduce heat transfer into the piston body and thereby lower the thermal load. Carbon-based coatings, such as diamond-like carbon (DLC), provide extreme hardness and low friction, while conversion coatings chemically modify the aluminum substrate to improve corrosion resistance.

[0131] A particularly suitable chromium-based metallic coating for application to the piston crown surface of the second piston part of the piston of the first aspect is AL-COAT® (thin dense chromium on aluminum), which is deposited by electrolytic means, and provides a hardness of about 70-72 Rockwell C. Together with high wear resistance, low friction and strong adhesion to aluminum substrates, these properties render the AL-COAT® 70-72 Rockwell C coating particularly suitable for further protecting the piston crown surface of the piston of the present invention from combustion-induced thermal and mechanical stresses.

[0132] As such, the embodiments of the present invention allow for the construction of highly efficient combustion piston engines that are particularly suited to overcome thermal challenges. As will be appreciated, particularly in high-performance applications such as racing, combustion piston engines are faced with significant thermal challenges as elevated rotational speeds and extreme specific power outputs generate sustained high combustion temperatures that accelerate piston crown fatigue and valve erosion. Similar conditions arise in turbocharged and supercharged engines, where elevated intake pressures increase peak combustion pressures and temperatures, creating a risk of piston crown melting, lubricant breakdown, and knock. Heavy-duty diesel engines also operate at high mean effective pressures over prolonged duty cycles, resulting in piston crown temperatures that may exceed 400 °C. Aviation piston engines present further challenges due to sustained high loads and limited airflow for cooling, necessitating special measures to protect piston crowns and valve seats.

[0133] These thermal challenges are regularly compounded in engines operated with alternative or so-called “green” fuels such as methanol, ethanol or hydrogen, which exhibit combustion characteristics that can lead to particularly high flame temperatures and abnormal ignition phenomena. While technical measures such as the use of rich fuel mixtures or reinforced piston and cylinder materials can mitigate these effects, such measures are accompanied by their own drawbacks. Rich mixtures increase fuel consumption and thereby reduce efficiency, while the use of thicker or stronger structural materials raises engine weight and compromises overall performance. Accordingly, engines designed for operation with alternative fuels must balance the thermal challenges presented by elevated combustion temperatures against the secondary disadvantages associated with conventional countermeasures, which - to date - makes them expensive and / or uneconomic and, therefore, commercially unattractive (if not non-viable). Due to the immense cooling effect that can be realized through the construction of the piston of the first aspect, multipart piston of the present invention has the potential to change this current paradigm, as the combustion piston engine of the fourth aspect if well suited to be adapted for use in the above applications, where thermal challenges persist to date.

[0134] In addition, the configuration of the receiving portion of the first piston part of the multipart piston of the first aspect and of the small end of the connecting rod in the piston-rod assembly of the second aspect allows for drastic weight reduction, compared to a similar engine operating with a conventional piston pin connection of the connecting rod. A piston-rod assembly having a reduced overall weight relative to a comparator engine provides significant performance benefits. The lower reciprocating mass decreases inertial forces at the dead centers, thereby reducing mechanical stresses and frictional losses. This reduction in inertial loading enables smoother operation at higher engine speeds and contributes to improved durability of associated components. In addition, the decreased mass allows the engine to achieve a higher power output for a given fuel input, thereby increasing overall efficiency and reducing specific fuel consumption.

[0135] As a consequence of the improved combustion process in an engine of the fourth aspect of the present invention as outlined above, the amount of pollutant emissions per absolute fuel quantity is necessarily lowered, and this reduction is further enhanced by the decreased fuel demand required to achieve a given power output. Accordingly, the engine of the fourth aspect of the present invention is therefore particularly efficient both in terms of fuel utilization and emission performance.

[0136] In particular embodiments, substituting the piston rod assemblies of an existing combustion piston engine with piston-rod assemblies of the second aspect may lead to a reduction in fuel consumption of the so-modified engine, in direct comparison to the same, un-modified engine of at least 5%, such as of at least 10%, such as of at least 15%, such as of at least 20%, such as of at least 25%, such as of at least 30%, such as of at least 35%, such as of at least 40%, such as of at least 45%. In particular embodiments, the reduction in fuel consumption may range from 5 to 60%, such as from 10 to 55% or from 15 to 50% or from 20 to 45% or from 25 to 45% or from 30 to 45% or from 35 to 45% or from 40 to 45%. In particular embodiments, substituting the piston rod assemblies of an existing combustion piston engine with piston-rod assemblies of the second aspect may lead to a reduction in CO2 emission of the so-modified engine, in direct comparison to the same, un-modified engine of at least 4%, such as of at least 5%, such as of at least 6%, such as of at least 7%, such as of at least 8%, such as of at least 9%, such as of at least 10%, such as of at least 15%, such as of at least 20%, such as of at least 25%, such as of at least 30%. In particular embodiments, the reduction in CO2 emission may range from 4 to 40%, such as from 5 to 35% or from 8 to 35% or from 10 to 35% or from 15 to 35% or from 20 to 35% or from 20 to 30% or from 20 to 25%.

[0137] Importantly, in practice, the inventor has successfully implemented the present invention in a real-life situation by replacing the standard piston-rod assemblies of a Ford F 150 Lariat 5.0 V8 engine with piston-rod assemblies according to the second aspect. Importantly, no modifications of the electronic engine management system (EMS) were made in the test vehicle. Only the necessary “mechanical” modifications were made to accommodate the piston-rod assemblies of the second aspect within the engine block of the vehicle.

[0138] During certified emission measurements and analysis as well as during certified fuel consumption measurements using the test vehicle with the modified engine (tests conducted independently by TUV SUD Germany, as well as by TUV NORD Germany) a significant reduction in fuel consumption and pollutant emissions was observed and reported / certified by TUV SUD Auto Service GmbH.

[0139] For a particular test drive, TUV SUD Auto Service GmbH certified the following:

[0140] A distance of 203 km with an elevation profile of 1179 m was covered.

[0141] The route covered included urban sections, motorway sections, and interurban routes roads.

[0142] The rules of the German Road Traffic Act were complied with.

[0143] The fuel consumed was refilled in each case at the Shell filling station on Rupertistrafte in Traunstein using the temperature-compensated pump Zp01 , terminal no. 70012851. This pump has been tested and calibrated in accordance with Directive 2004 / 22 / EC, ensuring a permissible calibration error of the delivery volume of ±0.5%. By using the same pump throughout, the error introduced by the automatic termination of refueling via the nozzle shut-off mechanism can be disregarded. The required data were read out or recorded from the vehicle and made available by the authorized personnel.

[0144] This yields:

[0145] 23.091 L I 203 km = 0.1137 L / km = 11.37 L / 100 km, which corresponds to 20.69 mpg.

[0146] Summary:

[0147] The consumption measurement under everyday operating conditions resulted in 11.37 L / 100 km.

[0148] Reported consumption values for this vehicle type in Europe vary by source between 14.3 L / 100 km and 17.6 L / 100 km.

[0149] Comparing these fuel consumption figures with the test results, a fuel consumption reduction of at least 2.93 L / 100 km was achieved. Based on the lowest published fuel consumption figure for the European market (i.e. compared to 14.3 L / 100 km) this corresponds to a fuel saving of 20.49%.

[0150] Based on the highest published fuel consumption figure for the European market (i.e. compared to 17.6 L / 100 km, the measured fuel consumption was reduced by 6.23 L / 100 km, corresponding to a fuel saving of 35.4%.

[0151] Comparing the measured fuel consumption figures with the official figures from the U.S. Government source for fuel economy information for the above vehicle, the fuel consumption was reduced by 42.1 %.

[0152] In short, the only mechanically modified FORD F 150 Lariat 5.0 L V8 engine of the test vehicle consumed between 20.49% and 42.1 % less fuel in the test, depending on the reference value.

[0153] With respect to the emission analysis conducted for the test vehicle, TUV NORD, conducted and documented an emission measurement that was carried out on a dynamometer specially constructed at TUV NORD for such measurements. The measurement tables were evaluated by TUV SUD. In the report, TUV SUD states that the reduction in CO2 emissions compared to the European average for individual measurement no. 2025012303-02 was 8%. A direct comparison with the corresponding US averages was not possible in the reported measurement / analysis cycle but according to an expert at TUV SUD can be extrapolated to a comparative saving of around 30%.

[0154] Again, the above-described reductions in emissions were achieved by only mechanically replacing the piston-rod assemblies. It will be readily appreciated that far greater fuel and emission savings can be realized if further adaptations to the engine are made, which can be made due to the improved abilities of the Pistons of the present invention to cope with thermal challenges.

[0155] In a fifth aspect, the present invention relates to a method for replacing a second piston part of at least one piston of a combustion piston engine of the fourth aspect, comprising the steps of:

[0156] - providing the piston disposed in the cylinder with the first and second piston parts connected to each other;

[0157] - disconnecting the second piston part from the first piston part;

[0158] - removing the disconnected second piston part from the cylinder;

[0159] - inserting a replacement second piston part into the cylinder; and

[0160] - connecting the replacement second piston part to the first piston part disposed in the cylinder to assemble the piston.

[0161] In conventional combustion piston engines, for example in engines having a piston pin connection between the piston and the small end of the connecting rod, replacement of one or more pistons due to wear of the piston crown and / or piston skirt requires removal of the entire piston-rod assembly, which is labor-intensive, partintensive and, therefore, expensive. In contrast, the multipart piston of the first aspect is configured such that the second piston part, comprising the piston crown and the piston skirt can be detached from the first piston part while the first piston part remains connected to the small end of the connecting rod. This configuration enables the method of the fifth aspect described above. As will be readily appreciated, this configuration greatly simplifies engine / piston maintenance, reduces the number of replacement parts, shortens service time and, thereby, significantly lowers overall maintenance / repair costs.

[0162] There are several ways to design and further develop the teaching of the present invention in an advantageous way. To this end, it is to be referred to the patent claims below on the one hand and to the following explanation of preferred examples of embodiments of the invention, illustrated by the drawings on the other hand. In connection with the explanation of the preferred embodiments of the invention by the aid of the drawing, generally preferred embodiments and further developments of the teaching will be explained.

[0163] In the drawings

[0164] Figs. 1 -3 show in schematic representations with different views, an assembled piston of the first aspect according to one embodiment of the present invention;

[0165] Figs. 4-6 show in schematic representations with different views, a first piston part of the piston of the first aspect according to the embodiment shown in Fig. 1 -3;

[0166] Figs. 7-9 show in schematic representations with different views, a second piston part of the piston of the first aspect according to the embodiment shown in Fig. 1 -3;

[0167] Figs. 10-11 b show in schematic representations, steps of connecting the first and second piston parts to assemble the piston of the first aspect according to the embodiment shown in Fig. 1 -3;

[0168] Figs. 12-16 show in schematic sectional representations with different views, the assembled piston of the first aspect according to the embodiment shown in Fig. 1 -3; Figs. 17-20 show in schematic representations with different views the assembled piston of the first aspect according to the embodiment shown in Fig. 1 - 3 with an attached connecting rod;

[0169] Fig. 21 shows in a schematic sectional representation, the assembled piston of the first aspect and the attached connecting rod according to the embodiment shown in Fig. 17-20 in a dead center, i.e. for example at a crank angle of 0°;

[0170] Fig. 22 shows in a schematic sectional representation, the assembled piston of the first aspect and the attached connecting rod according to the embodiment shown in Fig. 17-20 offset from either dead center, i.e. during the pistons upward or downward movement during operation with a pivot angle of the connecting rod relative to the stroke axis of about 20°;

[0171] Fig. 23 shows in schematic representation, the control pocket of the connecting rod according to the embodiment shown in Fig. 17-20;

[0172] Fig. 24 shows steps of a method according to the third aspect of the present invention, namely of a method of cooling a piston of the first aspect for a combustion piston engine.

[0173] Figs. 25-26 illustrate in a schematic sectional representation, the fluid flow through the fluid channel of the attached connecting into the assembled piston of the first aspect according to the embodiment shown in Fig. 22, i.e. with the valve arrangement being in fluid communication with the fluid channel filling the central cavity of the piston.

[0174] Fig. 27 illustrates in a schematic sectional representation, the fluid flow through the inlet fluid channel into the central cavity and onwards into the circular segment cavity of the assembled piston of the first aspect according to the embodiment shown in Fig. 13. Fig. 28 illustrates in a simplified schematic top view diagram, the fluid flow through the inlet fluid channel, into the central cavity and onwards into both arms of the circular segment cavity of the assembled piston of the first aspect according to the embodiment shown in Fig. 13 towards the outlet fluid channels positioned at the end of each arm of the circular segment cavity.

[0175] Fig. 29 illustrates in a schematic sectional representation, the fluid flow through the inlet fluid channel, into the central cavity, onwards into the circular segment cavity and to the outlet fluid channels positioned at the end of an arm of the circular segment cavity showing that the fluid exits the assembled piston of the first aspect according to the embodiment shown in Fig. 13 into the fluid recess of the lateral guiding surface.

[0176] Fig. 30 shows steps of a method for replacing a second piston part of at least one piston of the first aspect of a combustion piston engine of the fourth aspect according to an embodiment of the present invention.

[0177] Figs. 1 -3 show in schematic representations with different views, an assembled piston of the first aspect according to one embodiment of the present invention.

[0178] Figs. 4-6 show in schematic representations with different views, a first piston part of the piston of the first aspect according to the embodiment shown in Fig. 1 -3.

[0179] Figs. 7-9 show in schematic representations with different views, a second piston part of the piston of the first aspect according to the embodiment shown in Fig. 1 -3.

[0180] The piston 1 of the first aspect, typically for a combustion piston engine of the fourth aspect, comprises a first piston part 2 and a second piston part 3, wherein the first piston part 2 comprises a receiving portion 4 for attaching a connecting rod 5 and wherein the second piston part 3 comprises a top side 6. The receiving portion 4 comprises an undercut 37 acting in a tensile and compressive direction, so that the receiving portion 4 is configured to directly receive, in a form-fitting manner and pivotable about a pivot axis 36, a small end 30 of the respective connecting rod 5 corresponding to the undercut 37 of the receiving portion 4. When the piston 1 is arranged in a cylinder of the combustion piston engine, the top side 6 of the second piston part 3, delimits a combustion chamber of the cylinder, wherein the first and second piston parts 2, 3 are detachably connectable to each other to assemble the piston 1 . In accordance with the invention, the first and / or second piston parts 2,3 comprise at least one fluid channel 14. The inlet opening 39 of the at least one fluid channel 14 and, thereby, the at least one fluid channel 14 itself is arranged to be in fluid communication with a fluid supply of the combustion piston engine when the piston 1 moves between the regions of the dead centers in the cylinder but wherein the at least one fluid channel 14 is sealed off from the fluid supply when the piston 1 moves in the regions of the dead centers. The first and / or second piston parts 2,3 comprise two fluid channels 14, wherein, during operation, both channels are alternatingly in fluid flow communication with a fluid supply of the combustion piston engine when the piston 1 moves between the regions of the two dead centers in the cylinder.

[0181] When the piston 1 moves between the regions of the dead centers in the cylinder, the at least one fluid channel 14 is in fluid communication:

[0182] (a) with the receiving portion 4 of the first piston part 2; and

[0183] (b) with a first interspace 15 formed between the first and second piston parts 2,3 in a connected state, delimited by a bottom side 25 of the second piston part 3 and a top side 26 of the first piston part 2.

[0184] Also, when the piston 1 moves between the regions of the dead centers in the cylinder, the at least one fluid channel 14 is further in fluid communication:

[0185] (c) via one or more inlet fluid channels 14', with a second interspace 16 formed between the first and second piston parts 2,3 in a connected state; and / or

[0186] (d) via one or more outlet fluid channels 14", with one or more corresponding recesses 10 of at least one lateral guiding surface 9 of the piston, wherein the at least one lateral guiding surface 9 is for guiding the piston 1 inside the cylinder.

[0187] The first interspace 15 forms a central cavity 27 of the piston, which is defined by at least one fluid recess 28 in the top side 26 of the first piston part 2 and / or in the bottom side 25 of the second piston part 3. The second interspace 16 is a substantially semi-circumferential interspace formed at least in part by the semi-circumferential fluid recess 28'.

[0188] The area of the inlet opening 39 of one of the at least one fluid channels 14 is substantially equal to the combined area of all inlet openings 41 the one or more inlet fluid channels 14' as well as to the combined area of all inlet openings 43 of the one or more outlet fluid channels 14".

[0189] Furthermore, the piston 1 comprises a lateral guiding surface 9 for guiding the piston

[0190] 1 inside the cylinder, wherein the lateral guiding surface 9 is arranged on the second piston part 3. The lateral guiding surface 9 comprises four fluid recesses 10, which serve as a reservoir for oil lubricating the lateral guiding surface 9 and the running surfaces of the cylinder wall.

[0191] In addition, the piston 1 comprises a lateral sealing surface 11 with three circumferential grooves 12 for arranging piston rings therein, wherein the lateral sealing surface 11 is arranged on the second piston part 3. The lateral guiding surface 9 and the lateral sealing surface 11 comprise a circular outer contour. As shown, the circumferential grove reservoirs 12' have outlet openings 12", through which the excess fluid drains to the inner side of the piston during operation. The diameter of the outlet openings 12" is smaller than the diameter of the circumferential groove reservoirs 12' such that the excess fluid that is collected in the reservoirs 12' drains slowly therefrom and, as such, remains for further lubrication of the piston rings.

[0192] The piston 1 may be disposed slidably along a stroke axis 18 in a cylinder of a combustion piston engine. When arranged in a combustion piston engine, the top side 6 faces the combustion chamber and may receive pressure forces resulting from the expansion of the ignited gas mixture causing the movement of the piston 1 .

[0193] The first and second piston parts 2, 3 are connectable by means of a plug-rotate- mechanism. The plug-rotate-mechanism comprises, the first and second piston parts 2, 3 being engagable with each other by means of a plugging mechanism with a direction of motion along a stroke axis 18 of the piston 1 , wherein the first piston part

[0194] 2 is insertable into the second piston part 3. Furthermore, the plug-rotate-mechanism comprises the first and second piston parts 2, 3 being interlockable with each other by means of a rotational movement relative to each other, wherein a rotational axis 19 of the movement is substantially aligned with the stroke axis 18.

[0195] The first and second piston parts 2, 3 comprise corresponding interlocking parts 7, 8, each comprising a respective connecting surface 20', 20", wherein in the interlocked state the connecting surfaces 20', 20" of the interlocking parts 7, 8 form a friction connection between the first and second piston parts 2, 3. The corresponding interlocking parts 7, 8 comprise four first interlocking elements 21 ' that each extend from the second piston part 3 in a radially inward direction with regard to the rotational axis 19 of the rotational movement. In addition, the corresponding interlocking parts 7, 8 comprise four second interlocking elements 21 ". Each of the second interlocking elements 21 " forms part of the first piston part 2 and comprises a recess 22 with a shape corresponding to one of the four first interlocking elements 21 '. The first and second interlocking elements 21 ', 21 " each comprise one of the connecting surfaces 20. In a connected state, each connecting surface 20' assigned to one of the first interlocking elements 21 ' is in the friction connection with one connecting surface 20" assigned to one corresponding second interlocking element 21 ".

[0196] The connecting surfaces 20', 20" comprise a tangential gradient with regard to the rotational axis 19 of between 0.1 and 3.0°, such as, for example of 0.25 to 2.75°, 0.5 to 2.0°, 0.75 to 1 .75° or1 .0 to 1 .75° or of 1 .5°.

[0197] The corresponding interlocking parts 7, 8 when connected to each other can further be fixed against rotational movement relative to each other by means of a screwed connection and an adhesive bond (not shown). Each screwed connection of the corresponding interlocking parts 7, 8 may comprise a blind hole with a thread extending from the top surface 6 of the second piston part 3, through the first and second interlocking elements 7, 8. The thread may be arranged in the first and / or second piston part 2, 3. The thread may for example be arranged in at least one of the corresponding interlocking parts 7, 8, in particular in the first and / or the second interlocking element 21 ', 21 ". The screwed connection may further comprise a screw or a bolt with a drive head, wherein after connecting the corresponding interlocking parts 7, 8 the drive head may be removed and the screw or bolt may be ground plane to a surface of the top side 6. Fig. 10-11 b show in schematic representations, steps of connecting the first and second piston parts to assemble the piston of the first aspect according to the embodiment shown in Fig. 1 -3.

[0198] In a first step, the first and second piston parts 2, 3 are engaged with each other by means of a plugging mechanism with a direction of motion along a stroke axis 18 of the piston 1 (see Fig. 10), wherein the first piston part 2 is inserted into the second piston part 3. In a second step, the first and second piston parts 2, 3 are interlocked with each other by means of a rotational movement relative to each other, wherein a rotational axis 19 of the movement is substantially aligned with the stroke axis 18. In Fig. 11 a the first piston part 2 is plugged into the second piston part 3. Fig. 11 b shows the piston 1 after the first and second piston parts 2, 3 have been interlocked with each other by means of a rotational movement relative to each other. In the interlocked state the connecting surfaces 20', 20" of the interlocking parts 7, 8 form the friction connection between the first and second piston parts 2, 3.

[0199] For disconnecting the first and second piston parts 2, 3 from each other to disassemble the piston 1 , the first and second piston parts 2, 3 are unlocked from each other by means of a rotational movement relative to each other, wherein the rotational axis 19 of the movement is substantially aligned with the stroke axis 18 of the piston 1 . Furthermore, the first and second piston parts 2, 3 are disconnected from each other with a direction of motion along the stroke axis 18. Therefore, for disconnecting the first and second piston parts 2, 3 from each other the steps for connecting the first and second piston parts 2, 3 with each other are to be performed in reverse order.

[0200] The top side 6 of the second piston part 3 comprises a tool receiving portion 13 comprising a plurality of indentations 22 and elevations 23 for receiving a piston assembling tool for facilitating the connection and disconnection of the first and second piston part 2, 3 (see Fig. 1 and 7). The tool receiving portion 13 is adapted for a torsional moment to be transmitted, wherein a rotational axis 24 of the torsional moment is substantially aligned with the stroke axis 18 of the piston 1 . Fig. 12-16 show in schematic sectional representations with different views, the assembled piston of the first aspect according to the embodiment shown in Fig. 1 -3.

[0201] The first and second piston parts 2, 3 comprise a plurality of fluid channels 14, wherein the fluid channels 14 are in fluid communication with a fluid supply of the combustion piston engine when the piston 1 is arranged in the cylinder. The fluid preferably is oil from an oil reservoir of a crank case of a combustion piston engine.

[0202] Typically, the fluid channels 14 are in fluid communication with (a) the four recesses 10 of the lateral guiding surface 9, and / or (b) with a first interspace 15 formed between the first and second piston parts 2, 3 in a connected state, delimited by a bottom side 25 of the second piston part 3 and a top side 26 of the first piston part 2, and / or (c) with a second interspace 16 formed between the first and second piston parts 2, 3 in a connected state, wherein the second interspace 16 is a circumferential or semi- circumferential, interspace, and / or (d) with the receiving portion 4 of the first piston part 2. The first interspace 15 comprises a central cavity 27 defined by two fluid recesses 28, wherein one of the fluid recesses 28 is arranged in the top side 26 of the first piston part 2 and the other one of the fluid recesses 28 is arranged in the bottom side 25 of the second piston part 3. Similarly, the second interspace 16, which is a substantially circumferential or semi-circumferential interspace and comprises two semi-circular segment cavities 29, is defined by a semi-circumferential fluid recess 28' in the bottom side 25 of the second piston part 3 and by the top side 26 of the first piston part 2. The fluid channels 14 are designed to enable fluid flow from the receiving portion 4, through the first interspace 15, in particular through the central cavity 27, through the second interspace 16, in particular through the semi-circular segment cavities 29, into the recesses 10 of the lateral guiding surface 10. Therefore, fluid may flow from the receiving portion 4, through the piston 1 and to the four recesses 10 of the lateral guiding surface 10, thereby acting as lubricant as well as coolant of the piston 1 during operation.

[0203] Fig. 12 shows cross sections of two fluid channels that are in fluid communication with the receiving portion and the central cavity of the first interspace, allowing a fluid flow from the receiving portion to the central cavity. Fig. 13 shows cross sections of two fluid channels that are in fluid communication with the central cavity and the two circular segment cavities, allowing a fluid flow from the central cavity to the two circular segment cavities.

[0204] Fig. 14 shows further cross sections of one of the two fluid channels that connect the receiving portion with the central cavity and of one of the two fluid channels that connect the central cavity with the circular segment cavities.

[0205] Fig. 15 shows a cross section of one of four fluid channels that are in fluid communication with the two circular segment cavities and the four recesses of the lateral guiding surface, allowing a fluid flow from the circular segment cavity to the four recesses of the lateral guiding surface.

[0206] Fig. 16 shows further cross sections of one of the two fluid channels that connect the receiving portion with the central cavity and of one of the four fluid channels that connect the circular segment cavities with the four recesses of the lateral guiding surface.

[0207] Figs. 17-20 show in schematic representations with different views a piston-rod assembly of the second aspect with the assembled piston of the first aspect according to the embodiment shown in Fig. 1 -3 attached to a connecting rod.

[0208] Figs. 21 -22 show in schematic sectional representations, the assembled piston of the first aspect and the attached connecting rod according to the embodiment shown in Figs. 17-20 with different pivot angles of the connecting rod.

[0209] Fig. 23 shows in schematic representation, the control pocket of the connecting rod according to the embodiment shown in Figs. 17-20.

[0210] The piston-rod assembly 38 for a combustion piston engine comprising a connecting rod 5 attached via its connecting rod small end 30 to the receiving portion 4 of a multipart piston 1 according to the first aspect, wherein the connecting rod 5 comprises a fluid channel 17 with an inlet opening 45 in fluid communication with a fluid supply of the combustion piston engine and an outlet opening 46, characterized in that the piston-rod assembly comprises a valve arrangement 31 for sealing off fluid flow from the fluid supply through the fluid channel 17 and through the outlet opening 46 into the piston via the inlet opening 39 of the fluid channel 14 whenever the piston moves into the region of the top or bottom dead center of the piston’s stroke.

[0211] The valve arrangement comprises a control pocket 32 created as a recess in the receiving portion 4 of the piston 1 surrounding the inlet opening 39 of the fluid channel 14 of the receiving portion 4 and / or in a corresponding surface of the connecting rod small end 33 surrounding the outlet opening 46 of the fluid channel 17 of the connecting rod 5. The recess is dimensioned to allow fluid flow from the fluid channel 17 of the connecting rod 5 into the piston via the fluid channel 14 when the piston is moving outside of the region of the top and bottom dead centers, specifically, the recess is dimensioned to seal off fluid flow from the fluid channel 17 of the connecting rod 5 into the piston via the fluid channel 14 during a 4-stroke cycle at crank angles from 710° to 10°, from 170° to 190°, from 350° to 370° and 530° to 550°.

[0212] The outlet opening 46 of the fluid channel 17 of the connecting rod 5 defines an area which is larger than the area of the inlet opening 39 of the fluid channel 14.

[0213] The small end 30 of the connecting rod 5 is attached directly to the receiving portion

[0214] 4 in a form-fitting manner and pivotable about a pivot axis 36.

[0215] The receiving portion 4 of the piston 1 is adapted for rotatably attaching the connecting rod 5, wherein the connecting rod 5 may connect the piston 1 to a crankshaft of a combustion piston engine. The connecting rod 5 comprises a connecting rod end 30 adapted for being directly received by and / or attached to the receiving portion 4. Generally, the shape of the connecting rod end 30 partially corresponds to a shape of the receiving portion 4. A separate bushing is not required.

[0216] As is apparent from Fig. 21 -22, the connecting rod 5 comprises a fluid channel 17, wherein the fluid channel 17 of the connecting rod 5 may be in fluid communication with an internal fluid supply of a crankshaft. The fluid channel 5 of the connecting rod

[0217] 5 is in fluid communication with the fluid channels 14 of the piston 1.

[0218] When the piston 1 moves between the dead centers of a piston stroke, the fluid channel 17 of the connecting rod 5 is in fluid communication with the receiving portion 4 and the fluid channels 14 of the piston 1 . When the piston 1 is positioned in the dead centers, the fluid channel 17 of the connecting rod 5 is not in such fluid communication. For achieving this, the connecting rod 5 comprises a valve arrangement 31 for controlling a fluid flow to the receiving portion 4 and the fluid channels 14 of the piston 1 by means of a pivoting of the connecting rod 5 about the pivot axis 36. The valve arrangement 31 is provided on the connecting rod end 30 assigned to the receiving portion 4. The valve arrangement 31 comprises a control pocket 32 formed on a surface 33 of the connecting rod end 30, so that the fluid flow is limited or prevented in particular in the region of an upper pressure point and in the region of a lower pressure point of the piston 1 in the cylinder bore, or in other words in a substantially rectilinear arrangement of the connecting rod 5 in relation to the stroke axis 18 of the piston 1 in the cylinder bore (see Fig. 23). The control pocket 32 comprises the form of a groove on the surface 33 of the connecting rod end 30. The surface 33 is an outer surface that is in contact with the receiving portion 4. Furthermore, the control pocket 32 is arranged and designed in such a way that the fluid flow is restricted for an angle 34 of between about 5° and about 25° between a longitudinal axis 35 of the connecting rod 5 and the stroke axis 18 of the piston 1 in the cylinder bore, preferably the angle 34 is about 5°, about 10°, about 15°, about 20° or about 25°.

[0219] Furthermore, the piston 1 of the first aspect is substantially round, i.e. circular rather than oval shaped as are conventional pistons. As such, the piston 1 of the first aspect comprises a plurality of radially arranged substantially planar cross-sectional areas extending through the stroke axis 18 of the piston 1 , and is formed in such a way, that a first and a second cross-sectional area of the plurality of cross-sectional areas have a size, which differs from one another by only less than 10%, while being positioned offset from each other at an angle of 90° with regard to the stroke axis 18, wherein the first or second cross-sectional area extends through the pivot axis 36 of the connection of the first piston part 2 with the connecting rod 5.

[0220] Fig. 24 shows steps of cooling a multipart piston according to an embodiment of the present invention.

[0221] In a step S1 , a first volume of cooling fluid sufficient to at least fill all internal interspaces and fluid channels of the piston 1 is directed from a fluid supply of the combustion piston engine into the piston 1 via the inlet opening 39 of the at least one fluid channel 14.

[0222] In a step S2, all internal interspaces and fluid channels of the piston 1 are filled with the first volume of cooling fluid when the piston 1 moves between the regions of the dead centers.

[0223] In a step S3, the inlet opening 39 of the at least one fluid channel 14 is sealed off from the fluid supply when the piston 1 moves in the regions of the dead centers.

[0224] In a step S4, the inlet opening 39 of the at least one fluid channel 14 is opened and the at least one fluid channel 14 is again brought into fluid communication with the fluid supply of the combustion piston engine.

[0225] In a step S5, a second volume of cooling fluid sufficient to at least fill all internal interspaces and fluid channels of the piston 1 is again directed from the fluid supply of the combustion piston engine into the piston 1 via the inlet opening 39 of the at least one fluid channel 14.

[0226] In a step S6, the first volume of cooling fluid is replaced with the second volume of cooling fluid when the piston 1 moves between the regions of the dead centers

[0227] Figs 25 to 29 illustrate the flow of fluid through the system of internal cavities and channels of the multipart piston of the invention.

[0228] The filling step S2 comprises a sequential flow of the first volume of cooling fluid into all internal interspaces and fluid channels of the piston 1 in the following order:

[0229] (1 ) through one fluid channel 14 into the first interspace 15;

[0230] (2) from the first interspace 15 through one or more inlet openings 41 into the one or more inlet fluid channels 14' and through one or more outlet openings 42 of the inlet fluid channel 14' into the second interspace 16; and

[0231] (3) from the second interspace 16 through one or more inlet openings 43 into the one or more outlet fluid channels 14" and through one or more outlet openings 44 of the outlet fluid channel 14" into a corresponding fluid recess 10 of the lateral guiding surface 9. All internal interspaces and fluid channels of the piston 1 are configured such that the sequential flow of the second volume of cooling fluid cooling fluid ensures

[0232] (a) the removal of any gaseous volumes from the internal interspaces and fluid channels; and

[0233] (b) the complete filling of all internal interspaces and fluid channels with the cooling fluid.

[0234] The replacing step S6 comprises a sequential flow of the second volume of cooling fluid into all internal interspaces and fluid channels of the piston 1 mirroring that of the first volume of cooling fluid as defined above and wherein the fluid flow of the second volume of cooling fluid into the piston 1 expels the first volume of cooling fluid from the piston 1 through the one or more outlet openings 44 of the outlet fluid channel 14" thereby lubricating the lateral guiding surface 9 of the piston 1 with the first volume of cooling fluid.

[0235] During operation of the combustion piston engine, the steps S1 to S5 are continuously repeated such that a fresh volume of cooling fluid replaces a respective previous volume of cooling fluid every time the piston 1 moves between the regions of the dead centers.

[0236] Fig. 30 shows steps of a method for replacing a second piston part of at least one piston of a combustion piston engine according to an embodiment of the present invention.

[0237] In a step S1 the piston 1 is provided disposed in the cylinder with the first and second piston parts 2, 3 connected to each other.

[0238] In a step S2', the second piston part 3 is disconnected from the first piston part 2.

[0239] In a step S3', the disconnected second piston part 3 is removed from the cylinder.

[0240] In a step S4', a replacement second piston part is inserted into the cylinder. In a step S5', the replacement second piston part is connected to the first piston part 2 disposed in the cylinder to assemble the piston 1 of the first aspect..

[0241] Many modifications and other embodiments of the invention set forth herein will come to mind to the one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing description and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0242] L i s t o f r e f e r e n c e s i g n s

[0243] 1 piston

[0244] 2 first piston part

[0245] 3 second piston part

[0246] 4 receiving portion

[0247] 5 connecting rod

[0248] 6 top side of the second piston part , 8 interlocking parts

[0249] 9 lateral guiding surface (piston skirt)

[0250] 10 fluid recess of the lateral guiding surface

[0251] 11 lateral sealing surface

[0252] 12 circumferential groove (ring land) 2' circumferential groove reservoir " circumferential groove outlet opening

[0253] 13 tool receiving portion

[0254] 14 fluid channel 4' inlet fluid channel " outlet fluid channel

[0255] 15 first interspace

[0256] 16 second interspace

[0257] 17 fluid channel of the connecting rod

[0258] 18 stroke axis

[0259] 19 rotational axis 0' connecting surface assigned to a first interlocking element " connecting surface assigned to a second interlocking element 1 ' first interlocking element 1 " second interlocking element

[0260] 22 indentation

[0261] 23 elevation

[0262] 24 rotational axis

[0263] 25 bottom side of the second piston part 26 top side of the first piston part

[0264] 27 central cavity

[0265] 28 fluid recesses

[0266] 28' semi-circumferential fluid recess

[0267] 29 semi-circular segment cavity

[0268] 30 connecting rod small end

[0269] 31 valve arrangement

[0270] 32 control pocket

[0271] 33 surface of the connecting rod small end

[0272] 34 pivot angle

[0273] 35 longitudinal axis of the connecting rod

[0274] 36 pivot axis

[0275] 37 undercut

[0276] 38 piston-rod assembly

[0277] 39 inlet opening of fluid channel 14

[0278] 40 outlet opening of fluid channel 14

[0279] 41 inlet opening of inlet fluid channel 14'

[0280] 42 outlet opening of inlet fluid channel 14'

[0281] 43 inlet opening of outlet fluid channel 14"

[0282] 44 outlet opening of outlet fluid channel 14"

[0283] 45 inlet opening of fluid channel 17

[0284] 46 outlet opening of fluid channel 17

[0285] S1-S6 Steps of the method of cooling a multipart piston 1 for a combustion piston engine1 '-S5' Steps of the method for replacing a second piston part 2

Claims

C l a i m s1 . Piston (1 ) for a combustion piston engine comprising at least a first piston part (2) and a second piston part (3), wherein said first piston part (2) comprises a receiving portion (4) for attaching a connecting rod (5), and wherein said second piston part (2) comprises a top side (6), wherein said top side (6) of said second piston part (2), when said piston (1 ) is arranged in a cylinder of said combustion piston engine, delimits a working chamber of said cylinder, wherein said first and second piston parts (2,3) are connectable, in particular detachably connectable, to each other to assemble said piston (1 ), characterized in that said first and / or second piston parts (2,3) comprise at least one fluid channel (14), and wherein said at least one fluid channel (14) is arranged to be in fluid communication with a fluid supply of said combustion piston engine when said piston (1 ) moves between the regions of the dead centers in said cylinder but wherein said at least one fluid channel (14) is sealed off from said fluid supply when said piston (1 ) moves in the regions of the dead centers.

2. The piston (1 ) according to claim 1 , characterized in that said first and / or second piston parts (2,3) comprise two fluid channels (14), wherein both channels are alternatingly in fluid flow communication with a fluid supply of said combustion piston engine when said piston (1 ) moves between the regions of the dead centers in said cylinder.

3. The piston (1 ) according to claim 1 or claim 2, characterized in that, when said piston (1 ) moves between the regions of the dead centers in said cylinder, said at least one fluid channel (14) is in fluid communication:(a) with said receiving portion (4) of said first piston part (2); and(b) with a first interspace (15) formed between said first and second piston parts (2,3) in a connected state, delimited by a bottom side (25) of said second piston part (3) and a top side (26) of said first piston part (2).

4. The piston (1 ) according to claim 3, characterized in that, when said piston (1 ) moves between the regions of the dead centers in said cylinder, said at least one fluid channel (14) is further in fluid communication:(c) via one or more inlet fluid channels (14'), with a second interspace (16) formed between said first and second piston parts (2,3) in a connected state; and / or(d) via one or more outlet fluid channels (14"), with one or more corresponding recesses (10) of at least one lateral guiding surface (9) of said piston, wherein said at least one lateral guiding surface (9) is for guiding said piston (1 ) inside said cylinder.

5. The piston (1 ) according to claim 4, characterized in that said second interspace (16) is a substantially semi-circumferential interspace.

6. The piston (1 ) according to claim 4, characterized in that the area of the inlet opening of one of said at least one fluid channels (14) is substantially equal to the combined area of all inlet openings said one or more inlet fluid channels (14') as well as to the combined area of all inlet openings of aid one or more outlet fluid channels (14").

7. The piston (1 ) according to any one of claims 1 to 6, characterized in that said first interspace (15) forms a central cavity (27) of said piston, which is defined by at least one fluid recess (28) in said top side (26) of said first piston part (2) and / or in said bottom side (25) of said second piston part (3).

8. The piston (1 ) according to any one of claims 2 to 7, characterized in that said at least one lateral guiding surface (9) is arranged on said second piston part (3).

9. The piston (1 ) according to any one of claims 1 to 8, characterized in that said piston (1 ) comprises a lateral sealing surface (11 ) with at least one, preferably three, circumferential grooves (12) for arranging piston rings therein.

10. The piston according to claim 9, wherein said lateral sealing surface (11 ) is arranged on said second piston part (3).

11. The piston according to any one of claims 1 to 10, characterized in that said receiving portion (4) comprises an undercut (37) acting in a tensile and compressive direction, so that said receiving portion (4) is configured to directly receive, in a formfitting manner and pivotable about a pivot axis (36), a small end (30) of the respective connecting rod (5) corresponding to the undercut (37) of said receiving portion (4).

12. The piston (1 ) according to any one of claims 1 to 11 , characterized in that: said first and second piston parts (2,3) are connectable by means of a form-fit and / or force-fit connection; and / or said first and second piston parts (2,3) are connectable by means of a bonded connection; and / or said first and second piston parts (2,3) are connectable by means of a plug- rotate-mechanism.

13. The piston (1 ) according to claim 12, characterized in that said form-fit and / or force-fit connection comprises a plug-rotate-mechanism, a screwed connection and / or a friction connection.

14. The piston according to claim 12, characterized in that said bonded connection comprises an adhesively bonded connection and / or a welded connection.

15. The piston according to claim 12, characterized in that said plug-rotate- mechanism comprises a bayonet mechanism.

16. The piston (1 ) according to claim 12, characterized in that said first and second piston parts (2,3) are engagable with each other by means of a plugging mechanism with a direction of motion along a stroke axis (18) of said piston (1 ), preferably wherein said first piston part (2) is insertable into said second piston part (3).

17. The piston (1 ) according to claim 12 or claim 16, characterized in that said first and second piston parts (2,3) are interlockable with each other by means of a rotational movement relative to each other, wherein optionally a rotational axis (19) of said movement is substantially aligned with said stroke axis (18).

18. The piston (1 ) according to claim 12, 16 or 17, characterized in that said first and second piston parts (2,3) comprise corresponding interlocking parts (7,8), each comprising a respective connecting surface (20', 20"), wherein in the interlocked state said connecting surfaces (20', 20") of said interlocking parts (7, 8) form a friction connection between said first and second piston parts (2, 3).

19. The piston (1 ) according to claim 18, characterized in that: said connecting surfaces (20', 20") comprise a tangential gradient of at least 0.1 ° and a maximum of 3.0°, more preferably of 1 .5°, with regard to the rotational axis (19); and / or said corresponding interlocking parts (7,8) are connectable to each other by means of a screwed connection and / or an adhesive bond.

20. The piston (1 ) according to any one of claims 1 to 19, characterized in that: said first and second piston parts (2,3) are made of differing materials; and / or said first and / or second piston parts (2,3), preferably said second piston part (3), are made of a polymer, in particular a fiber-reinforced polymer.21 . The piston (1 ) according to any one of claims 1 to 20, characterized in that said at least one fluid channel (14), said one or more inlet fluid channels (14') and said one or more outlet fluid channels (14") are created by casting, forging and / or machining processes such as boring or milling.

22. The piston (1 ) according to any one of claims 1 to21 , wherein said piston (1 ) comprises a plurality of radially arranged substantially planar cross-sectional areas extending through a stroke axis (18) of said piston (1 ), characterized in that said piston (1 ) is formed in such a way, that a first and a second cross-sectional area of said plurality of cross-sectional areas have a size which differs from one another by less than 10% and are positioned offset from each other at an angle of 90° with regard to said stroke axis (18), preferably wherein said first or second cross-sectional area extends through a pivot axis (36) of a connection of said first piston part (2) with a connecting rod (5).

23. A piston-rod assembly (38) for a combustion piston engine comprising a connecting rod (5) attached via its connecting rod small end (30) to said receivingportion (4) of a multipart piston (1 ) according to any one of claims 1 to 22, wherein said connecting rod (5) comprises a fluid channel (17) with an inlet opening (45) in fluid communication with a fluid supply of said combustion piston engine and an outlet opening (46), characterized in that said piston-rod assembly comprises a valve arrangement (31 ) for sealing off fluid flow from said fluid supply through said fluid channel (17) and through said outlet opening (46) into said piston via said inlet opening (39) of said fluid channel (14) whenever said piston moves into the region of the top or bottom dead center of said piston’s stroke.

24. The piston-rod assembly (38) according to claim 23, characterized in that said valve arrangement comprises a control pocket (32) created as a recess in the receiving portion (4) of said piston (1 ) surrounding said inlet opening (39) of said fluid channel (14) of the receiving portion (4) and / or in a corresponding surface of said connecting rod small end (33) surrounding said outlet opening (46) of said fluid channel (17) of the connecting rod (5).

25. The piston-rod assembly (38) according to claim 24, characterized in that said recess is dimensioned to allow fluid flow from said fluid channel (17) of the connecting rod (5) into said piston via said fluid channel (14) when said piston is moving outside of the region of the top and bottom dead centers.

26. The piston-rod assembly (38) according to claim 25, characterized in that said recess is dimensioned to seal off fluid flow from said fluid channel (17) of the connecting rod (5) into said piston via said fluid channel (14) during a 4-stroke cycle at crank angles from 710° to 10°, from 170° to 190°, from 350° to 370° and 530° to 550°.

27. The piston-rod assembly (38) according to any one of claims 23 to 26, characterized in that said outlet opening (46) of said fluid channel (17) of the connecting rod (5) defines an area which is larger than the area of the inlet opening (39) of said fluid channel (14).

28. The piston-rod assembly (38) according to any one of claims 23 to 27, characterized in that said small end (30) of the connecting rod (5) is attached directlyto said receiving portion (4) in a form-fitting manner and pivotable about a pivot axis (36).

29. A method of cooling a multipart piston (1 ) for a combustion piston engine according to any one of claims 1 to 22, comprising(a) directing (S1 ) a first volume of cooling fluid sufficient to at least fill all internal interspaces and fluid channels of said piston (1 ) from a fluid supply of said combustion piston engine into said piston (1 ) via said inlet opening (39) of said at least one fluid channel (14);(b) filling (S2) all internal interspaces and fluid channels of said piston (1 ) with said first volume of cooling fluid when said piston (1 ) moves between the regions of the dead centers;(c) sealing off (S3) said inlet opening (39) of said at least one fluid channel (14) from said fluid supply when said piston (1 ) moves in the regions of the dead centers;(d) opening (S4) said inlet opening (39) of said at least one fluid channel (14) and again bringing said at least one fluid channel (14) into fluid communication with said fluid supply of said combustion piston engine;(e) directing (S5) a second volume of cooling fluid sufficient to at least fill all internal interspaces and fluid channels of said piston (1 ) from said fluid supply of said combustion piston engine into said piston (1 ) via said inlet opening (39) of said at least one fluid channel (14); and(f) replacing (S6) said first volume of cooling fluid with said second volume of cooling fluid when said piston (1 ) moves between the regions of the dead centers.

30. The method of cooling a multipart piston (1 ) according to claim 29, wherein said filling step (b) comprises a sequential flow of said first volume of cooling fluid into all internal interspaces and fluid channels of the piston (1 ) in the following order:(1 ) through one fluid channel (14) into said first interspace (15);(2) from said first interspace (15) through one or more inlet openings (41 ) into said one or more inlet fluid channels (14') and through one or more outlet openings (42) of said inlet fluid channel 14' into said second interspace (16); and(3) from said second interspace (16) through one or more inlet openings (43) into said one or more outlet fluid channels (14") and through one or more outletopenings (44) of said outlet fluid channel (14") into a corresponding fluid recess (10) of the lateral guiding surface (9).31 . The method of cooling a multipart piston (1 ) according to claim 30, wherein all internal interspaces and fluid channels of said piston (1 ) are configured such that said sequential flow of said second volume of cooling fluid cooling fluid ensures(a) the removal of any gaseous volumes from said internal interspaces and fluid channels; and(b) the complete filling of all internal interspaces and fluid channels with said cooling fluid.

32. The method of cooling a multipart piston (1 ) according to claim 30 or 31 , wherein said replacing step (f) comprises a sequential flow of said second volume of cooling fluid into all internal interspaces and fluid channels of the piston (1 ) mirroring that of said first volume of cooling fluid as defined in claim 30 and wherein said fluid flow of said second volume of cooling fluid into said piston (1 ) expels said first volume of cooling fluid from said piston (1 ) through said one or more outlet openings (44) of said outlet fluid channel (14") thereby lubricating said lateral guiding surface (9) of said piston (1 ) with said first volume of cooling fluid.

33. The method of cooling a multipart piston (1 ) according to any one of claims 29 to 32, wherein, during operation of said combustion piston engine, said steps (a) to (e) are continuously repeated such that a fresh volume of cooling fluid replaces a respective previous volume of cooling fluid every time said piston (1 ) moves between the regions of the dead centers.

34. A combustion piston engine comprising:- one or more cylinders; and- a corresponding number of one or more pistons (1 ) according to any one of claims 1 to 22 or a corresponding number of one or more piston-rod assemblies (38) according to any one of claims 23 to 28, wherein each of said one or more pistons (1 ) or each piston of said one or more piston-rod assemblies (38) are disposed slidably along a stroke axis (18) in said cylinder.

35. The combustion piston engine according to claim 34, comprising a crankshaft, wherein said crankshaft is rotatably connected to said receiving portion (4) of said first piston part (2) of each piston (1 ) by a connecting rod (5), characterized in that each connecting rod (5) comprises at least one fluid channel (17) and said crankshaft comprises an internal fluid supply, wherein said at least one fluid channel (17) of each connecting rod (5) is in fluid communication with said internal fluid supply.

36. The combustion piston engine according to claim 35, characterized in that said at least one fluid channel (17) of each connecting rod (5) is, at least partially during a piston stroke, preferably when said piston (1 ) moves between the regions of the dead centers, in fluid communication with said receiving portion (4) and said at least one fluid channel (14) of said first piston part (2).

37. Method for replacing a second piston part (3) of at least one piston (1 ) of a combustion piston engine according to any one of claims 34 to 36, comprising the steps of:- providing (S1 ') said piston (1 ) disposed in said cylinder with said first and second piston parts (2,3) connected to each other;- disconnecting (S2') said second piston part (3) from said first piston part (2);- removing (S3') said disconnected second piston part (3) from said cylinder;- inserting (S4') a replacement second piston part (3) into said cylinder; and- connecting (S5') said replacement second piston part (3) to said first piston part (2) disposed in said cylinder to assemble said piston (1 ).

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