CN101858276A - Pressure reducing device for reducing the air pressure on a piston ring set of a reciprocating piston internal combustion engine - Google Patents

Abstract

The invention relates to a decompression device for reducing the air pressure on a piston ring set of a reciprocating piston internal combustion engine. The piston comprises a piston crown adjoining the ring region, which in the mounted state faces the combustion chamber. In the operating state, the piston is arranged reciprocatingly along the longitudinal axis of the cylinder in such a way that the piston crown of the piston cooperates with a scraping device arranged at the cylinder wall of the cylinder near the top dead center of its movement in the cylinder. It acts for the controlled scraping of combustion residues, wherein the pressure relief device is designed in the form of a recess, so that a controlled deposition of combustion residues on the pressure relief device can be achieved. According to the invention, the scraper device and the piston crown are designed and arranged relative to each other in such a way that they are at least partially inclined relative to each other at a predeterminable angle at least at the standstill temperature, so that, thus, on the top dead Near the point, under the joint action of the piston crown and the scraper device, the reduction of the air pressure acting on the piston ring set can be automatically adjusted.

Description

Pressure reducing device for reducing the air pressure on a piston ring set of a reciprocating piston internal combustion engine

technical field

The invention relates to a pressure relief device (Reduziereinrichtung), a piston, a detachable (demontierbaren) for reducing the air pressure on a piston ring set (Kolbenringpaket) of a reciprocating piston internal combustion engine (Hubkolbenbrennkraftmaschine) according to the preamble of the independent claims Scraper rings (Abstreifring) and cylinders (Zylinder), and reciprocating piston internal combustion engines, especially large two-stroke diesel engines.

Background technique

Pistons known from the prior art for reciprocating piston internal combustion engines (such as two-stroke large diesel engines), in particular for longitudinally scavenging two-stroke large diesel engines, are generally equipped with a plurality of piston ring grooves each in a piston ring groove. A group of piston rings arranged one above the other in the center. Typically, depending on the size, power or construction of the reciprocating piston internal combustion engine or depending on the requirements and the particular operating conditions under which the machine operates, known piston ring sets comprise at least two piston rings, In most cases, however, three, four or even five piston rings are involved.

Here, the piston rings fulfill completely different functions, such as the distribution and/or scraping of lubricant on the running surfaces of the cylinder, the sealing of the combustion chamber relative to the crankcase, or in longitudinally scavenged two-stroke large diesel engines In case of sealing against the receiver chamber, etc.

In longitudinally scavenged two-stroke large diesel engines, the use of piston ring sets consisting of four or five piston rings for the sealing of the combustion chamber against the bottom side of the piston, which faces the gas storage, is typical At the beginning of the scavenging phase, fresh air flows from this storage chamber into the combustion chamber of the cylinder through scavenging ports. Depending on the annular gap, the lower piston ring is loaded differently. Since there are too many piston rings with a similar or identical function, various instabilities can arise between the underlying piston rings, which can lead, for example, to pressure fluctuations which in turn Causes instability in piston operation.

Large diesel engines are frequently used as drive units for ships or even in stationary operation, for example for driving large generators for generating electrical energy. In this case, the engine is generally operated in continuous operation over a considerable period of time, which places high demands on operational reliability and availability. Therefore, for the operator (Betreiber), in particular long maintenance intervals (Wartungsintervalle), low wear and economical handling of fuel and production raw materials (Umgang) are important criteria for machine operation. In particular, the piston running behavior of such large-bore low-speed diesel engines is decisive for the length of the maintenance intervals, the availability and (likewise directly via lubricant consumption) the operating costs and thus the economy. the elements of. The complex issue of cylinder lubrication is therefore also becoming more and more important for engines, wherein, especially in large diesel engines, cylinder lubrication takes place via lubrication devices in the reciprocating pistons or via Lubricating oil nozzle to achieve.

One point that repeatedly causes problems in the operation of internal combustion engines is combustion residues, which can deposit (ablagern) at various points in the engine. In particular, in two-stroke large diesel engines, which are usually operated on heavy oil, combustion residues represent a huge problem because the propellant used (ie heavy oil) is loaded with many possible solid, liquid and gaseous Substances of various types of combustion residues, which can in particular settle at the piston, at the piston ring groove, especially also at the piston crown or at the cylinder, preferably also near top dead center .

In particular, in order to scrape off deposits of combustion residues of the piston crown, it is known, for example, in Waertsilae two-stroke engines, that a so-called anti-wear ring (Antipolishing Ring) is arranged in the upper part of the cylinder liner (Zylinderliner) . The ring can be designed, for example, as a thin-walled bushing with a rectangular cross section, which generally has a smaller inner diameter than the inner diameter of the cylinder itself. The narrowing of the diameter produces a scraping effect for scraping off deposits on the piston crown. In this case, the inner diameter of the wear ring depends on the diameter of the piston crown, more specifically on the largest diameter of the piston crown during engine operation. Therefore, in the known anti-wear rings, the goal is to design the inner diameter so narrow that the gap between the piston crown and the anti-wear ring is as small as possible, but the piston crown is not mechanically in direct contact. Clean the anti-friction rings.

Obviously, the design of the wear ring is subject to many compromises. At low (tiefen) loads and cooler piston crowns, the gap (ie the distance between piston crown and wear ring) is already greater than at high loads solely due to thermal expansion effects. In addition, it must be taken into account here that generally speaking, the various components in the cylinder, such as the cylinder running surface, the piston, in particular the piston ring, the piston ring groove and the piston crown, all depend on the completed operating hours. wear differently in the circumferential direction and in the longitudinal direction and therefore do not have a fixedly defined diameter.

It is therefore clear that, in engines known from the prior art, wear rings and piston crowns do not co-operate optimally under all operating conditions and not over the entire service life of the components involved.

In particular, an optimum sealing effect between the wear ring and the piston crown is not ensured in all cases (while the two would otherwise cooperate in a sealing manner near top dead center). That is, in addition to the scraping off of combustion residues, an important task of the anti-wear ring/piston crown pair is to seal the pole against the underside of the piston and thus in particular with respect to the piston ring set below the piston crown. The high air pressure that is built up in the cylinder near the top dead center of the piston during the compression stroke or by ignition of the propellant-air mixture in the combustion chamber.

Therefore, in particular, it should be ensured that the pressure generated in the combustion chamber in the vicinity of the top dead center can be utilized as completely as possible without partly unused flowing out next to the piston in the form of gas flow to the lower part of the cylinder. The neutralization flows out into the air storage chamber. In addition, in the case of an insufficient seal between the wear ring and the piston crown, there is always the risk that individual particles will be uncontrolled from the deposits of combustion residues due to the gas flow flowing downwards at the piston. Therefore, these particles can be deposited on the cylinder running surface, for example in the lower part of the cylinder, which can also contaminate the cylinder lubricating oil and in the worst case can lead to damage, such as wear and tear to be feared.

Contents of the invention

It is therefore the object of the present invention to provide a device by means of which an improved sealing between the combustion chamber and the bottom side of the piston can be achieved, in particular in the vicinity of the top dead center of the piston. In particular, the invention should reduce the air pressure acting on the piston ring set of the piston in the operating state, wherein the upper deadening of the piston can be realized automatically and constantly independently of the operating state or the wear state of the components involved. The best seal between the piston crown and wear ring around the point.

The object of the invention which achieves this object is characterized by the features of the independent claims.

The dependent claims relate to particularly advantageous embodiments of the invention.

The present invention therefore relates to a pressure relief device for reducing the gas pressure acting on a piston ring set arranged in the ring region of a piston in the operating state of a reciprocating piston internal combustion engine. In this case, the piston includes a piston crown adjoining the ring region, which in the assembled state faces the combustion chamber of the reciprocating piston internal combustion engine. The piston is arranged in such a way that it can reciprocate (hin-und herbewegbar) along the longitudinal axis of the cylinder in the operating state, that is, the piston crown of the piston is arranged on the cylinder wall of the cylinder near the top dead center of its movement in the cylinder The scraping device (Abstreifeinrichtung) at the joint function for the controlled (kontrollierte) scraping of combustion residues, wherein the pressure relief device is designed in the shape of a recess (Ausnehmung), so that it can be realized in the pressure relief device controlled deposition of combustion residues. According to the invention, the scraper device and the piston crown are designed and arranged relative to each other in such a way that they are at least partially (abschnittsweise) inclined relative to each other at a predeterminable angle at least at a standstill temperature (Stillstandstemperatur), so that , near the top dead center, under the joint action of the piston crown and the scraper device, the reduction of the air pressure acting on the piston ring set can be automatically adjusted (automatisch einstellbar).

The term “standstill temperature” is understood here to mean the temperature that the engine assumes after a sufficiently long wait in a non-running state. The standstill temperature is therefore essentially understood to be the ambient temperature when the engine has been switched off for a sufficiently long time and approximately assumes the ambient temperature. Thus, distinguished from the ambient temperature is the operating temperature of the engine or of different parts of the engine, wherein, as is undoubtedly known to those skilled in the art, the operating temperature is the temperature of the engine or of the engine in the operating state. The temperature exhibited by each part.

Therefore, the advantage of the varying cross-section between the cylinder or the scraper or pressure relief device and the piston crown is important to the invention.

Preferably, the components such as the scraper device or pressure relief device and the piston crown are shaped such that they are not parallel to one another in the cold state, that is to say at standstill temperature. Here, too, the scraping device or the pressure relief device and the opposing surfaces of the piston crown can be arranged, for example, in a spherical, undulating or stepped shape.

The piston crown preferably has a conical shape that tapers toward the combustion chamber, and the wiper ring has a shape that closes toward the combustion chamber, or vice versa, depending on the deformation tendency . In a further embodiment, it is obviously also possible that only one of the two opposing faces has a varying cross section, wherein preferably the piston crown has a varying cross section.

A particular advantage of the invention is that the part heats up during operation, so that the surface is implemented with or without a profile in the shape of a recess, for example in the shape of a groove (Nut). They are optimally facing one another and thus produce the desired sealing effect, for example in a labyrinth of recesses with or without deposited contaminants.

In this case, the closed, in particular narrowed cross section towards the combustion chamber leads to a better seal without deposits, since the smallest cross section is located closest to the combustion chamber.

Conversely, an open, in particular enlarged cross section towards the combustion chamber leads to a rapid deposition of carbon black particles, which deposit due to the predictably higher temperature and larger cross section.

The surface of the reduced or enlarged cross-section preferably generally has an angle of approximately 0.238° to 0.7° relative to the vertical, ie with reference to the longitudinal axis of the cylinder.

In particular, according to the invention, the inner diameter of the wear ring is provided with a certain number of circumferential grooves or threads of varying fineness. The function of such or geometrically designed recesses is in particular that a labyrinth effect can be produced which has a sealing effect in a known manner and thus at least strongly reduces the pressure along the piston crown towards the piston ring set and furthermore The volume flow of combustion gases entering the area below the piston. As a result, the maximum pressure acting on the piston ring set is likewise reduced.

Furthermore, the grooved surface (which can also be provided not only on the wear ring but alternatively or additionally also in the region of the piston crown) by the recess can accelerate the deposition of combustion residues. The surface of the piston crown and/or wear ring that is thus coated with combustion residues reduces the inner diameter of the wear ring or the outer diameter of the piston crown and thus reduces the distance (i.e. between the wear ring and the piston crown). the gap between). Thus, the distance between the wear ring and the piston crown is automatically adjusted to an optimum value by the invention in the operating state.

For example, in the case of a power increase and the resulting increase in the outer diameter of the piston crown or a reduction in the inner diameter of the wear ring, the combustion residues at the wear ring and/or at the piston crown The relatively soft deposits are scraped off relative to each other by their relative movement in such a way that between the wear ring and the piston crown, or between the surfaces of the deposits on one or both of these parts The optimal spacing between them appears automatically. Thus, independently of the operating state of the reciprocating piston internal combustion engine and independently of the wear state of the engine components involved, an optimum distance between the anti-wear ring and the piston crown is produced automatically and consistently, so that, as a result, An optimal sealing effect and thus the greatest possible reduction of the pressure on the piston ring set likewise always occurs automatically.

Furthermore, due to such a reduced distance between the wear ring and the piston crown, the volume flow of the combustion gases exiting the combustion chamber through the piston and across the piston ring set is greatly reduced.

As already mentioned, in a particularly preferred embodiment, the recess is provided on the scraper device, which is preferably a wear ring. Here, additionally or alternatively, the recess according to the invention can advantageously also be provided on the piston crown.

Depending on requirements, the recesses can in practice be designed quite differently here. The recesses may then, for example, comprise areas of indentations which may be arranged on the piston ring crown or scraper (i.e. wear ring) either in an orderly pattern or even in a random, preferably uniform distribution. place.

In a particularly practical embodiment, the recess is formed in the shape of a groove that runs around with reference to the longitudinal axis of the cylinder, and/or the recess is designed as a groove that runs helically around with reference to the longitudinal axis of the cylinder. shape, wherein both types of grooves can obviously also be provided in combination in the same example. It is evident here that a plurality of circumferential grooves are preferably provided.

In particular in order to simplify maintenance work on the cylinder and thus save costs, the scraper device is arranged on the cylinder wall as a detachable (ie replaceable) scraper ring.

In this case, the detachable wiper device can have a constant cross-section with respect to the longitudinal axis of the cylinder, that is to say the cross-section of the wear ring is particularly advantageously rectangular. In special cases, however, it is also possible for the anti-wear ring to be adapted, for example, to the geometry of the piston crown and to have a non-constant, varying cross-section. For example, it is possible that the anti-wear ring has a cross-sectional shape corresponding to the cross-section of the piston crown, for example an approximately triangular cross-sectional shape or another suitable cross-sectional shape.

Likewise, for cost reasons or to simplify servicing work on the engine, a recess can be provided on the detachable piston crown ring. This means that, corresponding to the anti-wear ring, an exchangeable piston crown ring can be arranged on the piston crown, which is for example welded to the piston crown, clamped on the piston crown under high mechanical stress, or otherwise The appropriate way is reliably fixed at the piston crown. In certain cases it is then sufficient to replace only the piston crown ring and not the entire piston, which saves in particular spare parts and thus further costs.

Preferably, the recesses according to the invention have a maximum depth of 0.1 mm to 5 mm, preferably between 0.5 mm and 3 mm, wherein, for example, in the case of groove-shaped recesses the number of recesses lies between 1/cm and 15/cm cm, especially between 4/cm and 10/cm, or wherein, in the case of planarly distributed recesses, the number is between ¹ /cm and 15/cm, especially between ⁴ between /cm ² and 10/cm ² .

Furthermore, the invention relates to a piston, a detachable wiper ring and a detachable piston crown ring with a decompression device according to the invention, as well as a cylinder and a piston, each with a detachable scraper ring or removable piston crown ring.

Furthermore, the present invention relates to a reciprocating piston internal combustion engine, in particular to a two-stroke large diesel engine, with pressure reducing means and/or with pistons and/or cylinders (as described in detail in that application) .

A rather special advantage of the invention is mainly also that older engines, especially Large diesel engines, which in some cases have an extremely high lifetime, can likewise be retrofitted very simply according to the invention without complex changes being made at the engine.

Description of drawings

Below, the present invention will be further described with the help of drawings. Among them, in a schematic way:

Fig. 1 a shows the cylinder assembly (Zylinderanordnung) with the anti-wear ring of the present invention;

Figure 1b shows the cylinder assembly of Figure 1a with deposits of combustion residues at the wear ring;

Figure 2a shows a cylinder assembly according to the invention with a piston crown having a helically surrounding groove;

Figure 2b shows the cylinder assembly of Figure 2a with deposits of combustion residues at the piston crown;

Figure 3 shows a cylinder assembly according to the invention with a detachable wear ring and a detachable piston crown ring;

Figures 4a-4c show three further preferred embodiments of the invention.

Detailed ways

FIGS. 1 a to 3 each show in a schematic illustration partly and in section a cylinder assembly with a pressure relief device according to the invention, which is designated as a whole below with the reference numeral 1 .

In addition to the pressure reducing device 1,100 according to the invention, all the embodiments of Figures 1a to 3 show a known cylinder assembly of a reciprocating piston internal combustion engine, in this particular example a two-stroke crosshead large diesel engine , with a cylinder 4 in which a piston 2 is arranged, on which a piston ring pack 22 with three piston rings is arranged in a ring region 21 . Obviously, the ring set 22 can also comprise more or less than three piston rings. The air pressure P built up in the combustion chamber 3 during the compression stroke of the piston 2 or by ignition of the combustion mixture in the combustion chamber 3 acts in the operating state on the piston ring set 22 , which is arranged according to the diagram Under the piston crown 23 facing the combustion chamber 3 . The piston 2 is arranged in a known manner so that it can be moved back and forth in the operating state along the longitudinal axis L of the cylinder 4 such that the piston crown 23 of the piston 2 is at the top dead center of the movement of the piston 2 in the cylinder 4 The vicinity of OT cooperates with the scraper device 5 arranged on the cylinder wall 41 of the cylinder 4 to scrape off combustion residues 6 .

Now, in contrast to the known prior art, the decompression device 1 in the form of a recess 100 is arranged and designed according to the invention in such a way that a controlled reduction of the combustion residues 6 at the decompression device can be achieved. deposition, and, in the vicinity of the top dead center OT, under the joint action of the piston crown 23 and the scraper device 5 (here it is the anti-wear ring 5), the reduction of the air pressure P acting on the piston ring set 22 can be automatically reduced is adjusted.

These general principles of the invention, which are obviously specific to all embodiments, can be realized in different embodiments as impressively illustrated with reference to FIGS. 1 a to 3 .

In FIG. 1 a , the pressure relief device 1 according to the invention is designed in the shape of a groove 100 which runs helically around a longitudinal axis L. In FIG. In this case, FIG. 1 a shows the state before the first start-up of the cylinder 4 after repair. Consequently, neither on the piston crown 23 nor on the scraper 5 with the helical groove 100 are there any deposits of combustion residues 6 .

Here, FIG. 1 b shows the cylinder assembly of FIG. 1 a after a certain number of operating hours. As can be clearly seen, combustion residues 6 have now clearly deposited on the detachable wiper ring 5 . In this case, the combustion residues 6 are shaped at the surface facing the piston crown 23 by interaction with the piston crown 23 during the reciprocating movement of the piston 2 in the vicinity of top dead center OT in such a way that the combustion residues 6 The surface of the piston crown 23 cooperates more or less form-fittingly with the corresponding surface of the piston crown 23 . On the one hand, an optimal sealing effect between wiper ring 5 and piston crown is thereby achieved. On the other hand, this optimal sealing is also maintained if the outer diameter of piston crown 23 and/or the inner diameter of wiper ring 5 changes, for example due to load fluctuations or temperature fluctuations. For example, if the distance between the wiper ring 5 and the piston crown 23 increases over time, more combustion residues 6 will deposit, so that the gap will reclose. Conversely, if the distance between the piston crown 23 and the wiper ring 5 is reduced, during the scraping movement of the piston crown 23 along the surface of the relatively soft deposits on the wiper ring 5, exactly So many combustion residues 6 are worn away, that is, until the optimum distance reappears.

It should be clearly appreciated here that the sealing effect between piston crown 23 and wiper ring 5 is compared to The existing technology has been significantly improved. This is because the surrounding groove 100 acts as a labyrinth seal and the pressure on the piston ring set 22 is already considerably reduced by this alone.

The embodiment of FIGS. 2 a and 2 b differs from FIGS. 1 a and 1 b mainly in that the pressure relief device 1 according to the invention is not arranged at the scraper device 5 but at the piston crown 23 . The recesses 100 here are not in the form of circumferential grooves but in the form of more or less evenly distributed recesses 100 , for example in the form of small dimples 100 distributed on the surface of the piston crown 23 . structure.

Consequently, deposits of combustion residues 6 first adhere to the piston crown 23 . The sealing effect thus achieved is produced exactly analogously to that in the example of FIGS. 1 a and 1 b.

Finally, FIG. 3 shows a very particular embodiment of the invention in which a pressure relief device 1 , 100 according to the invention in the shape of a recess 100 is provided both at the wiper ring 5 and at the piston crown 23 . Furthermore, a detachable piston crown ring 7 is provided at the piston crown 23 so that, for example, when the pressure relief device 1,100 at the piston crown 23 is worn, the piston crown 7 does not have to be replaced but the entire piston 2 . Here, the cylinder assembly of FIG. 3 is not yet in operation; therefore, there are also no deposits of combustion residues 6 yet. In the operating state, combustion residues gradually accumulate on the piston crown 23 and on the wiper ring 5 until finally a form-fit seal is formed.

Here, the example of FIG. 3 is particularly well suited for applications where relatively few deposits are formed, so that the doubly active labyrinth seal (which is arranged both on the piston crown 23 is arranged at the wiper ring 5) to act particularly strongly, so that, even without significant deposits of combustion residues 6, the distance between the piston crown 23 and the wiper ring 5 is significantly greater than that of the prior art A greatly improved sealing effect has been ensured.

Finally, Figures 4a to 4c show three further particularly preferred embodiments of the invention.

Therefore, as already mentioned, the advantage of the varying cross-section between the cylinder 4 or the scraper device 5 or the pressure relief device 1 and the piston crown 23 is important for the invention.

As can be seen particularly well from FIGS. 4a-4c (which show the cylinders at standstill temperature), components such as the scraper device 5 or the decompression device 1 and the piston crown 23 are shaped in such a way that They are not parallel to one another, especially in the cold state. In this case, the opposing surfaces of the scraper device 5 or the pressure relief device 1 and the piston crown 23 can likewise be arranged, for example, in a convex-spherical, wave-like or stepped shape.

According to the embodiment of FIG. 4 a , the piston crown 23 has a conically tapered shape towards the combustion chamber 3 and the scraper ring 5 has a closed shape towards the combustion chamber 3 , but alternatively, depending on the deformation tendency, it The reverse is also possible in another exemplary embodiment not shown in detail here.

In another embodiment, as schematically shown by way of example in FIG. , the piston crown 23 has a varying cross-section.

As also mentioned, a special advantage for the present invention is that the components heat up during operation, so that it can be implemented with or without the contour 100 (the contour 100 is in the shape of a recess 100, for example a groove 100) face each other in an optimal manner and thus, for example, a desired sealing effect occurs in the maze of recesses 100 with or without deposited contaminants 6 .

Here, in the absence of deposits 6, the closed, in particular narrowed cross section towards the combustion chamber 3, as shown in FIG. Next to combustion chamber 3.

In contrast, a cross section that is open towards the combustion chamber 3 , in particular that becomes larger, leads to a rapid deposition 6 of carbon black particles, which deposit due to the predictably higher temperature and larger cross section.

The reduced or enlarged cross-sectional faces in Figs. 4a-4c preferably have an angle with respect to the vertical (ie with reference to the longitudinal axis L of the cylinder 4) of the order of 0.238° to 0.7°.

It will be clear to a person skilled in the art that the previously described examples of implementation forms of the invention are to be understood as exemplary, and in particular that the invention also includes all suitable combinations of the illustrated embodiments.

Claims (15)

1. A pressure reducing device for reducing the air pressure (P) which, in the operating state of a reciprocating piston internal combustion engine, acts on the set of piston rings arranged in the ring area (21) of the piston (2) (22), wherein said piston (2) comprises a piston crown (23) adjoining said ring region (21), which in the installed state faces the combustion chamber (3) of said reciprocating piston internal combustion engine, And the piston (2) is arranged reciprocally movable along the longitudinal axis (L) of the cylinder (4) in the operating state in such a way that the piston crown (23) of the piston (2) rests on the piston (2) ) near the top dead center (OT) of the movement in said cylinder (4) cooperates with a scraper (5) provided at the cylinder wall (41) of said cylinder (4) for combustion controlled scraping of residues (6), wherein the decompression device is designed in the shape of a recess (100), so that a controlled deposition of combustion residues (6) at the decompression device can be achieved, It is characterized in that the scraper device (5) and the piston crown (23) are designed and arranged relative to each other in such a way that they lie at least partially at a predeterminable angle (α) at the standstill temperature lower relative to each other so that, in the vicinity of the top dead center (OT), under the combined action of the piston crown (23) and the scraper (5), the piston ring set (22) The reduction of the air pressure (P) on can be adjusted automatically. 2. The pressure reducing device according to claim 1, characterized in that the piston crown (23) has a shape that tapers towards the combustion chamber (3). 3. The pressure reducing device according to claim 1 or 2, characterized in that the piston crown (23) has a shape that expands towards the combustion chamber (3). 4. The pressure reducing device according to any one of the preceding claims, characterized in that the scraping device (5) has a shape that tapers towards the combustion chamber (3). 5. The pressure reducing device according to any one of the preceding claims, characterized in that the scraping device (5) has a shape that expands towards the combustion chamber (3). 6. The decompression device according to any one of the preceding claims, characterized in that the recess (100) is arranged at the scraping device (5). 7. The pressure relief device according to any one of the preceding claims, characterized in that the recess (100) is provided at the piston crown (23). 8. The pressure-relief device according to any one of the preceding claims, characterized in that the recess (100) is configured in the shape of a groove (100) that runs around with reference to the longitudinal axis, in particular configured with reference to The shape of the groove (100) helically encircling the longitudinal axis (L). 9. The pressure relief device according to any one of the preceding claims, characterized in that a plurality of circumferential grooves (100) are provided. 10. The pressure reducing device according to any one of the preceding claims, characterized in that the scraper device (5) is arranged at the cylinder wall (41 ) as a detachable scraper ring (5). 11. The decompression device according to any one of the preceding claims, characterized in that the recess (100) is arranged at the detachable piston crown ring (7). 12. A piston with a pressure relief device (1, 100) according to any one of claims 1 to 11. 13. A detachable scraper ring with a pressure relief device (1, 100) according to any one of claims 1 to 11. 14. A cylinder with a detachable scraper ring (5) according to claim 13. 15. A reciprocating piston internal combustion engine, especially a two-stroke large diesel engine, with the decompression device (1, 100) according to any one of claims 1 to 11 and/or with the pressure reducing device according to claim 12 described piston and/or with a cylinder according to claim 14.

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