DK174378B1 - Piston for an internal combustion engine - Google Patents

Description

in DK 174378 B1

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a plunger for an internal combustion engine, having several cooling devices located by recesses in the piston, each cooling device containing a primary cooling fluid in a cavity and having an upper portion closest to the upper side of the piston and a lower section projecting into a space. for a secondary cooling fluid.

US-A-5,454,351 discloses such a piston for a trunk motor wherein separate elongated cooling elements 10 containing the primary cooling fluid are circumferentially embedded in the piston near the piston ring grooves. The cooling elements extend from the area around the upper piston ring to the area around the bottom of the piston skirt. Within the piston skirt, the cooling elements 15 are freely depressing and are cooled during engine operation by splash cooling. The primary cooling fluid is, for example, water which, during the up and down movement of the piston, is shaken vigorously around the cavities.

US-A-4,470,375 also discloses a piston for a trunk motor, with an integrated cooling element containing a primary cooling fluid. The cooling element is composed of three pieces of pipe, which together form a U-shape, where the two pieces of pipe extend from top to bottom of the piston and are connected at the top by the third pipe 25, which runs diametrically below the top wall of the piston. The cooling element is adapted to keep the piston approximately isothermal during engine operation in order to control the piston's expansion and thereby achieve better hydrodynamic relationship between piston and cylinder wall.

In the known pistons with separate cooling elements which are pressed or molded into the material of the piston crown, insulating air pockets may exist between the cooling elements and the material of the piston, which reduces the ability of the cooling elements to conduct heat away from the piston top.

Furthermore, there is a risk of strength-reducing stresses between such embedded cooling elements and the piston's material due to various heat exchanger coefficients of the cooling inserts and the piston's material, respectively.

The present invention has for its object to provide a piston having better cooling properties 10 and greater strength stability than the above-mentioned known pistons.

To this end, the invention is characterized in that the material of the piston in the upper sections of the cooling devices is exposed in the cavities and that the lower sections of the cooling devices are upwardly open, tubular cooling inserts, which at their outer sides are fluid-sealed mounted in the corresponding recesses. .

In this way, an optimal heat conduction is obtained from the piston material to the primary cooling fluid, since the cooling fluid is in direct contact with the piston material and therefore the heat transfer is not hindered by any intermediate wall with any air pockets between a cooling element wall and the piston material. Therefore, exposing the piston material to the cavities enables effective heat dissipation from the thermal hardest loaded areas near the piston top with a relatively small area of the cooling fluid contact area, while retaining the mechanical strength of the piston.

The heat transfer from the primary cooling fluid to the secondary cooling fluid takes place through the wall of the tubular cooling inserts, and these may have a contact area much larger than the area of the contact surface between the piston material and the primary cooling fluid. In this way, a secondary cooling fluid having less thermal conductivity than the primary cooling fluid can be used, for example, the primary cooling fluid may be water and the secondary oil.

It is also an advantage that the outer sides of the tubular cooling inserts only need a small contact surface with the piston material to be fluid-sealed mounted in the recesses, since any problems with stresses between the inserts and the piston material appear only at the contact surface.

In one embodiment, the space for the secondary cooling fluid is a closed chamber located in the plunger with cooling and inlet and outlet openings. This is particularly advantageous for larger motors with a great need for cooling the piston, for example cross-head motors, because the cooling inserts carried in the chamber can be cooled more safely and efficiently when the piston continuously contains a quantity of secondary cooling fluid in the chamber immediately around the inserts. The amount and circulation of cooling fluid can be affected through the location of the inlet and outlet openings and the pumping of 20 cooling fluid through the inlet opening. In the chamber, cooling of the cooling insert is typically done by so-called splash cooling, with air usually present in the chamber and the secondary cooling fluid being shaken vigorously back and forth as the piston operates.

In a preferred embodiment, the upper portions of the cooling devices extend at least to the top of the piston. In modern high-performance engines, the components of the cylinder are subjected to large thermal loads, and the location of the upper sections of the cooling devices off the top land, ie. above the piston ring area, the temperature level of the piston rings decreases, which is advantageous for both traditional and high top country pistons.

In one embodiment, the cavities in the cooling devices have an upper boundary which is located at a distance from the upper side of the piston top of no more than 0.15 times the diameter of the piston. For example, for a piston diameter of 500 mm, this distance may be 40 mm.

Hereby, the thickness of the portion of the piston top wall 5 which separates the primary cooling fluid from the very hot top surface of the piston can be minimized, thereby optimizing the thermal conductivity of this "bottleneck" in the piston cooling while at the same time taking into account the strength characteristics of the piston top.

The individual recess may comprise a bore and terminated upwardly in a cavity portion with greater cross-sectional area than the bore. The tubular cooling insert can be easily attached to the bore and may have a relatively small diameter, while the contact surface 15 between the piston material and the primary cooling fluid may be advantageously large, the cavity portion of the recess above the bore having relatively large dimensions. Thus, it is possible to remove material precisely at the places where maximum cooling is required.

Furthermore, the recesses can be rounded upwards to avoid stress concentrations in the highest loaded piston material.

In a further embodiment, the recesses 25 are elongated substantially parallel to the axis of the piston and are disposed circumferentially in an annular region between a cavity centrally located in the piston top and the outer wall of the piston. The parallel recesses simplify the piston manufacture, and radially within the recesses there may be a relatively solid rib-like region integral with the piston top for power transmission between the piston top and the piston rod. The upper part of the cooling devices may be located in the corner area between the top land and the top side of the piston 35 and may possibly extend obliquely in relation to the axis of the piston. The secondary cooling fluid can be supplied or discharged via the central cavity, in which the cooling fluid can also splash cool the central part of the top wall of the piston.

In an advantageous embodiment, at least one channel is drilled between the upper portion of the central cavity and the upper portion of the individual recess, which is blocked relative to the central cavity. These additional cooling channels extend partially through the rib-shaped region near 10 at the top wall of the piston and provide improved cooling of the relatively large material concentration in this region.

Since the ducts are blocked, there is no flow of cooling fluid through the ducts, but as a result of the piston's movements, the cooling fluid will flow into the cooling ducts and then, possibly by evaporation, leave the ducts again in the same way as it entered.

In a further advantageous embodiment, each cooling insert at its upwardly open end has an outer circumferential mounting region which is fixed in the corresponding recess at a distance from its lower end, and the cooling insert has less outer diameter under its mounting area than the inner diameter of the recess. , so that between the cooling insert and the recess there is a circular chamber for the secondary cooling fluid. By positioning the mounting region at a suitable distance from the lower end of the recess, the ratio between the effective heat exchange area in the upper section of the cooling devices in which the plunger is cooled and the effective heat exchange area in the lower section of the cooling devices, where a heat transfer occurs from the primary cooling fluid to the secondary cooling fluid, a current piston construction is adapted, for example the ratio of these areas can be from three to five. A chamber for the secondary cooling fluid extends about 35 cooling inserts upwards to the mounting area.

6 DK 174378 B1

Each cooling insert may have cooling ribs on its exterior which increase the effective heat exchange area between the cooling inserts and the secondary cooling fluid and allow the entire cooling insert to be reduced, for example shorter for a given heat exchange area, which is a particular advantage of pistons for a cross main engine. , where the piston in its lower dead center position is immediately above an intermediate bottom which limits the piston height.

The cooling inserts can furthermore be elastically mounted in the recesses, for example by means of a bellows seal, such as a folded collar, which is attached to the recess along its outer edge. In this way, any deformations due to different coefficient of thermal expansion of the piston material and cooling insert can be compensated for.

The cooling inserts can advantageously be mounted in the recesses by soldering, brazing or welding, which can provide a good and reliable seal.

In addition, the cooling inserts may advantageously be at least partially of aluminum or copper alloys which have a good thermal conductivity and relatively high softness relative to the iron-based piston material.

In one embodiment, each cooling insert has at least one sensor for detecting whether the cooling device cavity contains an amount of cooling fluid. In this way, it is possible to check whether the cooling devices contain at least a desired minimum amount of cooling fluid, without having to separate the piston.

In a further development thereof, the primary cooling fluid is electrically conductive, and the sensor in each cooling insert comprises at least one electrically conductive element which is freely available within the cooling insert and is in contact with the cooling fluid when there is a sufficient amount. B1 thereof, whereby the presence of cooling fluid in the cooling insert can be ascertained by simple ohmic measurement.

In an alternative embodiment, the primary cooling fluid is electrically insulating, and the sensor comprises a capacitor means which is freely available within the cooling insert. An amount of cooling fluid within the cooling insert can then be detected by capacitance measurement.

The sensors in the cooling inserts in a piston may be interconnected and connected to a measuring point for 10 common measurement of whether all the cooling inserts in the piston contain a predetermined minimum amount of cooling fluid. The target location may either be accessible after the engine has stopped, or even during operation.

In addition, the cooling inserts may be made or assembled from a material which melts at temperatures greater than about 300 ° C. If a leakage should occur in a cooling device so that the primary cooling fluid penetrates, then the temperature of the cooling device will rise and the cooling insert will melt by exceeding the design melting temperature. Thereafter, the secondary cooling fluid will have access to the recess and cool the plunger as necessary, thereby avoiding overheating of the plungers, 25 and it may be possible to operate the engine at reduced power.

The primary cooling fluid may further be added to a tracer which is detectable outside the cooling devices in the event of a cooling device leakage.

This allows monitoring of the cooling devices also during operation of the engine, for example by sampling from the secondary cooling fluid, in which any leaked tracer may occur.

8 DK 174378 B1

The invention will now be explained in more detail by way of examples of embodiments with reference to the schematic drawing, in which FIG. 1 shows an embodiment of the piston according to the invention, partly in radial section; FIG. 2 shows the piston top of the piston of FIG. 2, seen from below before mounting on the piston rod and with mounted cooling inserts in the recesses; and FIG. 3 is a sectional view of one embodiment of an elastic attachment of a tubular cooling insert in a recess in a piston.

In FIG. 1 shows a piston 1 according to the invention. The piston shown is for a two-stroke cross-head motor and, depending on the motor size, can be made in various sizes of diameters, typically ranging from 250 mm to 1000 mm.

During combustion in the engine, the cylinder pressure above the piston rises to a maximum combustion pressure, which can be, for example, between 160 and 200 bar, and at the same time the temperature in the upper end of the cylinder rises to more than 500 ° C. The upper surface of the piston is severely stressed in areas, and great demands are placed on both the piston's cooling system and its strength properties. The piston 1 has a piston top 2 which is mounted on a central piston rod 3 and a cylindrical skirt 4 which is mounted below and in extension of the piston top 2 and thus forms the lower part of the outer surface of the piston 5. The piston top 2 is usually made of steel and with an upper welded, not shown coating of a particularly heat-resistant material, and it can be made either by machining a solid, cylindrical workpiece or a molded workpiece, casting being preferably suitable for larger pistons, e.g. diameter over 600 mm.

9 DK 174378 B1

The piston top 2 is configured as an integral member comprising a top wall 6 with concave top face, a downwardly extending cylindrical outer wall 7 which forms the upper part of the outer surface 5 of the piston and in the lower 5 part several piston rings 8 are mounted in a known manner. in circular grooves, and a downwardly cylindrical rib 9 extending within the outer wall 7 surrounding a central cavity 19 located in the piston top 2 immediately below the top wall 6. The 10 cylindrical rib 9 has a lower horizontal abutment surface which known in the art by means of several bolts not shown, is tensioned against an upper abutment surface of the piston rod 3. The rib 9 is therefore the supporting element which transmits the large axial forces between the piston 1 and the piston rod 3 during the operation of the motor.

The rib 9 is relatively high, thereby enabling a high outer wall 7 of the piston top 2, and thus a high top land 10, which constitutes the free portion of the outer surface of the piston 8 which is located between the upper side of the piston and the upper piston ring 8. relatively far down on the piston 1, and therefore they avoid the very large heat effect and the consequent risk of coke deposition at the top of the piston.

It is further apparent that the space between the outer wall 7 of the piston top and the rib 9 is filled in material integrally with the piston top 2, and that in this material there are a plurality of recesses evenly distributed in the circumferential direction of the piston in the form of bores parallel to the piston axis 11. which has upwardly rounded blind ends 12 at the top wall 6 of the piston top 2 and extends down to an annular chamber 13 formed partly in the lower part of the material in the space between the outer wall 7 and the rib 9, as well as in the piston skirt 4. The bores 11 are In the drawing shown, they may also be of a different shape, for example conical. The solid material which lies between and adheres to the outer wall 7 and the rib 9 stiffens the outer wall 7 considerably and thus enables a solid outer wall 7 despite the high top 10 mentioned above.

In each bore 11 is a coaxially inserted tubular cooling insert 14 which is open upwards and closed downwards. The upwardly closed bore 11 and the downwardly closed cooling insert 14 together form a cooling device 29 which encloses an elongated cavity 15 partially filled with a primary cooling fluid 16. The primary cooling fluid 16 is a fluid having a relatively high thermal conductivity. for example water. Thus, in the upper portion of the cooling device 29, the piston material lies freely in the cavity 15, which in this area is bounded upwardly by the blind end 12 and radially of the inside of the bore 11, and in the lower section of the cooling device 29, the cavity 15 is delimited by the inside of the cooling insert 14.

The number of cooling devices 29 can be varied according to the current conditions, for example between 10 and 20 may be appropriate.

The cooling inserts 14 can also be arranged in pistons where the bores 11 are distributed differently than shown in the figures, for example, the cooling inserts 14 can be mounted in bores which originate from a central cavity of the piston, as is the case in pistons of the type Sulzer RTA. In this case, the cooling inserts 14 can project into the central heat exchange cavity with the secondary cooling fluid.

In the illustrated embodiment, the cooling insert 14 is mounted in the bore 11 with its upper edge 17 approximately opposite the upper piston ring 8. The upper section of the cooling device 29, where the primary cooling fluid 16 cools the piston material exposed in the cavity 15, is therefore located 11 further, in the upper portion of the cooling device 29, each cavity 15 is expanded by two additional cooling channels 27 produced by drilling from the central cavity 19 in the transition area between the top wall 6 and the rib 9, so that the channels 27 open at the top of the blind ends 12, which upwardly delineate the cavities 15. The additional cooling ducts 27 have been closed after the bore at their opening in the central cavity 19 with a plug 28, 10 so that they only communicate with the cavities 15 in the cooling devices 29. The ducts 27 , which is seen in section in FIG. 1 and 3, and from the end of FIG. 2, extends in the region where the rib 9 is integrated with the top wall 6, and serves to provide additional cooling of this region where 15 relatively high mass of material is concentrated. As the channels 27 are relatively thin, a good strength of the piston is maintained in this area. These additional cooling ducts 27 may be omitted, and there may also be only one or optionally more than two ducts 27 for each cooling device. In addition, it is possible to increase the effective heat exchange area between the piston material and the primary cooling fluid 16 in other ways than with the shown extra channels 27, for example, the blind ends 12 may have a larger diameter than the 25 bores 11 located below, or each cavity 15 may be upwards. bounded by several adjacent side-up, rounded bottom holes, instead of a blinding 12.

Along its upper rim 17, the cooling insert 14 is secured fluid-sealing to the inside of the bore 11, and below the rim-shaped mounting area 17, the cooling insert 14 has a slightly smaller diameter than the bore 11, so that a circular chamber 18 is formed between the cooling insert 14 and the bore 1 , which below 35 opens into the annular chamber 13. The tubular 12 cooling insert 14 projects into the annular chamber 13 and extends immediately above the bottom thereof, and has longitudinal cooling ribs 22 for heat exchange with the secondary cooling fluid. . In Fig. 2, the cooling inserts 14 are seen mounted in the bores 11 in the piston top 2, seen from below before mounting on the piston rod 3. Where the area available for heat exchange in the upper sections of the cooling devices 29 is very limited, it is clear, that there are no immediate restrictions on the size of the effective heat exchange area in the lower section of the cooling devices 29, the cooling inserts 14 being able to be extended downwards by the appropriate design of the annular chamber 13 and the piston skirt 4.

The tubular cooling insert 14 is made of a material which has a good thermal conductivity, for example aluminum or copper, thereby obtaining the best effect of the cooling ribs 22. However, it is also possible to use other materials for the cooling inserts 14, for example steel. The rim-shaped mounting area 17 of the cooling insert 14 is preferably fixed by soldering or welding in the bore 11, as problems with stresses in the joint in this way can be minimized, but it is also possible to carry out the joint 25 by threading or by pressing. However, stresses on the joint can also be prevented by elastic shaping of the joint, for example as a bellows joint. An example of this is shown in FIG. 3, wherein a tubular cooling insert 14 is provided at its upper edge 30 with a folded collar 26 sufficiently resilient to be able to compensate for deformations due to various heat expansion coefficients of the cooling insert 14's and the piston 1's material, respectively.

As the piston 1 moves up and down 35 during engine operation, the primary cooling fluid 16 is strongly shaken up and 13 within the cavities 15 of the cooling devices 29. In the upper sections of the cooling devices 29, the primary cooling fluid 16 comes into direct contact with the the material of the piston as it is freely in the cavities 15, and therefore a good heat transfer from the piston material to the cooling fluid 16. In the lower sections of the cooling devices 29, the cooling fluid 16 gives off heat via the cooling inserts 14 to the secondary cooling fluid. By appropriate selection of the primary cooling fluid 16, it is possible to evaporate this 10 upon contact with the hot plunger material in the upper sections of the cooling devices 29, and then to allow the cooling fluid 16 to condense upon cooling in the cooling inserts 14 in the lower sections of the cooling devices 29, but it is often sufficient. to dissipate the heat without evaporation of the cooling fluid.

The outside of each cooling insert 14 is cooled by the secondary cooling fluid, which, during operation of the piston, is fed through a central channel 21 of the piston rod 3 to the central cavity 19 of the piston top 2, 20 from which it is distributed to the cooling devices 29 through channels 20 extending from at the top of the central cavity 19 and inclined downward and radially outwardly through the rib 9 to open into the circumferential chambers 18 between the cooling inserts 14 and the bores 11. In the total compartment 25 formed by the circumferential chambers 18 around the cooling inserts 14 and the adjacent and below annular annular chamber 13, the secondary cooling fluid is shaken vigorously back and forth during the up and down movement of the piston, whereby the exterior of the tubular cooling insert 14 and the piston material located around the bore 11 is cooled by so-called splash cooling.

From the annular chamber 13, the secondary cooling fluid is passed through radial channels 23 in the piston rod 35 to an annular cooling fluid channel 24 which in the piston rod extends coaxially around the axial channel 21 which is separated from the channel 24 thin-walled pipe element 25.

During operation of the piston, the secondary cooling fluid, usually oil, is pumped through the piston, providing a pressure difference of, for example, from 1 to 3 bar between the piston cooling fluid inlet and outlet, i.e., channels 21, 24 of the piston rod 3. In the above, channel 21 described as access channel and channel 24 as exit channel, but the reverse flow direction is also possible, just as channels for supplying and diverting secondary cooling fluid to and from the annular chamber 13 can be designed in various ways in different ways without moving without for the scope of the invention.

It is possible to retrofit existing pistons with cooling devices 29 according to the invention. In pistons which have elongated cooling recesses with blind ends located below the top wall of the piston, the recesses can be drilled deeper, so that the top wall above the recesses becomes thinner and provides better heat conduction. When using conventional direct cooling of the recesses, oil is used as a cooling fluid, the thickness of the top wall must not be too small. When retrofitting a piston 25, the top wall can be thinned if, for example, the primary cooling fluid is water which does not disintegrate at higher temperatures.

The above-described embodiments of the plunger may be combined and the plunger may have a different design than shown in the drawing. For example, the cooling insert 14 may be mounted in the bore 11 with its upper rim 17 above or above the lower piston ring 8 of the piston. Also, it is also possible to mount the cooling inserts 14 in a traditional piston with relatively low top land.

In FIG. 2, in one of the dotted line cooling devices 29, a sensor 30 is indicated to determine whether the cavity 15 in the cooling device 29 contains an amount of cooling fluid 16. The sensor 30 may comprise two 5 electrical poles which may, for example, be in the cooling device's cavity 15 freely accessible elements. . If the primary cooling fluid 16 is electrically conductive, a quantity of cooling fluid can be ascertained by ohmic measurement across the two poles, and if the cooling fluid is electrically insulating, a quantity of fluid can be measured by capacitance measurement across the poles.

In ohmic measurement, great resistance is measured if the poles are not in contact with the cooling fluid and the height of the poles above the bottom of the cavity 15 is therefore chosen such that the desired minimum amount of fluid just touches the lower edge 15 of the poles. It may also be possible to use other types of sensors 30, such as, for example, ultrasonic sensors.

In the above-mentioned types of electrical measurement, it is possible to mount a sensor 30 in each cooling device 29 and connect all of these sensors 30 in series so that the resistance or capacitance of all sensors 30 can be measured over two contact elements 32, such as e.g. may be mounted on the outer wall of the piston 7. The connections 31 between the sensors and the contact elements 32 are indicated by dashed lines in FIG. 2nd

These contact members 32 can be accessed through purging air ports 33 in the cylinder liner 34 of the piston 1, so that it is possible to check if there is a quantity of cooling fluid 16 in all the cooling devices 29 without having to separate the engine. The rinsing air ports 33 and cylinder liner 34 are also indicated by dotted lines in FIG. 2nd

Claims (19)

1. A combustion engine piston (1), having several cooling devices located at recesses in the piston, each cooling device containing a primary 5 cooling fluid in a cavity having an upper section adjacent to the upper side of the piston and a lower section projecting into the a space for a secondary cooling fluid, characterized in that the material of the piston (1) in the upper sections of the cooling devices (29) is exposed in the cavities (15) and that the lower sections of the cooling devices (29) are open, tubular cooling inserts (14) which are fluid-sealed at their outer sides in the corresponding recesses. Piston (1) according to claim 1, characterized in that the space for the secondary cooling fluid is a closed chamber (13, 18) located in the piston (1) with cooling and fluid inlet and outlet openings. Piston (1) according to claim 1 or 2, characterized in that the upper portions of the cooling devices (29) extend at least at the top (10) of the piston (1). Piston (1) according to one of the preceding claims, characterized in that the cavities (15) in the cooling devices (29) have an upper bounding (12) located at a distance from the piston top (2). upper side of not more than 0.15 times the diameter of the piston (1). Piston (1) according to one of the preceding claims, characterized in that the individual recess 30 comprises a bore (11) and is closed upwards in a cavity part with a larger cross-sectional area than the bore (11). Piston (1) according to one of the preceding claims, characterized in that the recesses are rounded upwards. DK 174378 B1 Piston (1) according to one of the preceding claims, characterized in that the recesses are elongated substantially parallel to the axis of the piston (1) and are disposed circumferentially in an annular region between a centrally located in the piston (2). cavity (19) and piston outer wall (7). Piston (1) according to claim 7, characterized in that at least one channel (27) is blocked between the upper part of the central cavity (19) and the upper part of the individual recess (27) which is blocked relative to the central cavity ( 19). Piston (1) according to one of the preceding claims, characterized in that each cooling insert (14) has at its upwardly open end an outer circumferential mounting region (17) mounted in the corresponding recess at a distance from its the lower end, and that the cooling insert (14) below its mounting region (17) has a smaller outer diameter than the inner diameter of the recess, so that there is between the cooling insert (14) and the recess 2. a circular chamber (18) for the secondary cooling fluid. Piston (1) according to one of the preceding claims, characterized in that each cooling insert (14) on its outer side has cooling ribs (22) which are preferably one-way. Piston (1) according to one of the preceding claims, characterized in that the cooling inserts (14) are elastically mounted in the recesses. Piston (1) according to one of the preceding claims, characterized in that the cooling inserts (14) are mounted in the recesses by soldering, brazing or welding. Piston (1) according to one of the preceding claims, characterized in that the cooling inserts (14) are at least partly of aluminum or copper alloys. Piston (1) according to one of the preceding claims, characterized in that each cooling insert (14) 5 has at least one sensor (30) for determining whether the cavity (15) of the cooling device (29) contains an amount of cooling fluid (16). . Piston (1) according to claim 14, characterized in that the primary cooling fluid (16) is electrically conductive and the sensor in each cooling insert (14) comprises at least one electrically conductive element which is freely accessible within the cooling insert (14). ). Piston (1) according to claim 14, characterized in that the primary cooling fluid (16) is electrically insulating and the sensor (30) comprises a capacitor means which is freely accessible within the cooling insert (14). Piston (1) according to one of claims 14 to 16, characterized in that the sensors (30) in the cooling inserts (14) of a piston (1) are interconnected and connected to a measuring point (32) for common measurement of whether all the cooling inserts (14) in the piston (1) contain a predetermined minimum amount of cooling fluid (16). Piston (1) according to one of the preceding claims, characterized in that the cooling inserts (14) are made or assembled from a material which melts at temperatures greater than about 300 ° C. Piston (1) according to one of the preceding claims, characterized in that the primary cooling fluid (16) is provided with a tracer which is detectable outside the cooling devices (14) in the event of a leakage of a cooling device (29). 35