PISTON OF AN INTERNAL COMBUSTION ENGINE

Abstract

A piston of an internal combustion engine may include a piston shaft and a piston head. The piston head may be provided with a closed cooling channel with a cooling medium arranged therein. The piston shaft may have a spherically round cross-sectional shape, wherein a deviation from the roundness with respect to a piston diameter may be less than 0.5 per thousand.

Claims

1. A piston of an internal combustion engine, comprising a piston shaft and a piston head provided with a closed cooling channel with a cooling medium arranged therein; wherein the piston shaft has a spherically round cross-sectional shape, wherein a deviation from the roundness with respect to a piston diameter is less than 0.5 per thousand.

2. The piston according to claim 1, wherein the cooling channel in a region of a piston base expands radially outwards in a direction of a top land.

3. The piston according to claim 1, comprising an upper portion and a lower portion connected thereto, wherein the cooling channel is formed partially in the upper portion and partially in the lower portion.

4. The piston according to claim 3, wherein the cooling channel expands in the direction of the piston shaft by at least one of milling and bores.

5. The piston according to claim 1, further comprising ribs protruding from a lower piston side and arranged in a region of the cooling channel at the lower piston side.

6. The piston according to claim 5, wherein the ribs are produced by one of stamping and forging.

7. The piston according to claim 5, wherein at least one of: the ribs extend substantially in a radial direction with respect to a piston axis; and recesses are arranged between the ribs, wherein a volume of the ribs, which protrude from the lower piston side, corresponds to a volume of the recesses, which are stamped in the lower piston side.

8. The piston according to claim 1, wherein at least one of: the piston is constructed as one of a steel piston or a cast piston of grey cast iron; and the cooling medium has at least one of sodium and one of potassium and water.

9. An internal combustion engine comprising an engine block having at least one cylinder in which a piston is arranged, the piston including: a piston shaft and a piston head provided with a closed cooling channel with a cooling medium arranged therein; wherein the piston shaft has a spherically round cross-sectional shape, wherein a deviation from the roundness with respect to a piston diameter is less than 0.5 per thousand.

10. The internal combustion engine according to claim 9, wherein the cooling channel in a region of a piston base expands radially outwards in a direction of a top land.

11. The internal combustion engine according to claim 9, wherein the piston includes an upper portion and a lower portion connected thereto, wherein the cooling channel is formed partially in the upper portion and partially in the lower portion.

12. The internal combustion engine according to claim 11, wherein the cooling channel expands in the direction of the piston shaft by at least one of milling and bores.

13. The internal combustion engine according to claim 9, wherein the piston includes ribs protruding from a lower piston side and arranged in a region of the cooling channel at the lower piston side.

14. The internal combustion engine according to claim 13, wherein the ribs are produced by one of stamping and forging.

15. The internal combustion engine according to claim 13, wherein at least one of: the ribs extend substantially in a radial direction with respect to a piston axis; and recesses are arranged between the ribs, wherein a volume of the ribs, which protrude from the lower piston side, corresponds to a volume of the recesses, which are stamped in the lower piston side.

16. The internal combustion engine according to claim 9, wherein at least one of: the piston is constructed as one of a steel piston or a cast piston of grey cast iron; and the cooling medium has at least one of sodium and one of potassium and water.

17. A piston of an internal combustion engine, comprising: a piston shaft and a piston head provided with a closed cooling channel with a cooling medium arranged therein; a plurality of ribs protruding from a lower piston side and arranged in a region of the cooling channel at the lower piston side; and recesses are arranged between the ribs, wherein a volume of the ribs, which protrude from the lower piston side, corresponds to a volume of the recesses, which are stamped in the lower piston side; wherein the piston shaft has a spherically round cross-sectional shape, wherein a deviation from the roundness with respect to a piston diameter is less than 0.5 per thousand.

18. The piston according to claim 17, wherein the cooling channel in a region of a piston base expands radially outwards in a direction of a top land.

19. The piston according to claim 17, wherein the ribs are produced by one of stamping and forging.

20. The piston according to claim 17, wherein the ribs extend substantially in a radial direction with respect to a piston axis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the schematic drawings:

[0018] FIG. 1 is a sectioned illustration through a piston according to the invention with a plane of section which is different in the left half of the image and the right half of the image,

[0019] FIG. 2 is an illustration of the generically spherically round cross-sectional shape with different views,

[0020] FIG. 3 is an illustration as in FIG. 1, but with different planes of section,

[0021] FIG. 4 is a view from below of a piston according to the invention with stamped ribs.

DETAILED DESCRIPTION

[0022] According to FIGS. 1 and 3, a piston 1 according to the invention of an internal combustion engine 2 which is illustrated only in a highly schematic manner in FIG. 3 has a piston shaft 3 and a piston head 4, wherein a closed cooling channel 5 with a cooling medium 6 which is arranged therein is provided in the piston head 4. A combustion chamber bowl 7 is further arranged in the piston head 4 itself. According to the invention, the piston shaft 3 now has a spherically round cross-sectional shape (cf. also FIG. 2), wherein a piston axis is indicated with the reference numeral 8 and wherein a deviation from the roundness with respect to a piston diameter D is smaller than 0.5 per thousand. The term “spherical” is intended to mean in this instance that the piston 1 is constructed along the piston axis 8 thereof in the manner of a barrel, that is to say that a diameter D of the piston 1 in the region of the piston head 4 and in the region at a lower end of the piston shaft 3 is smaller than therebetween. The deviation of the roundness is in this instance always intended to be considered in a plane transverse relative to the piston axis 8. Over the height H, the radii R therefore differ as a result of the roundness whilst they are identical in a plane.

[0023] If the left-hand illustration in FIG. 2 is considered, it can be seen that the spherical shape with respect to the piston axis 8 in the upper end and at a lower end 9 of the piston shaft 3 has a smaller diameter D than, for example, in a central region 10 of the piston shaft 3. In the right-hand illustration of FIG. 2, for example, the spherically round cross-sectional shape in comparison with a spherically oval cross-sectional shape known from the prior art is illustrated. The spherically round cross-sectional shape is in this instance indicated with a continuous line, whilst the spherically oval cross-sectional shape, as known from piston shafts of pistons from the prior art is illustrated with a broken line. The spherically round cross-sectional shape of the piston shaft 3 according to the invention is consequently distinguished by a circular cross-sectional shape having a radius R which is constant at a respective height level H.

[0024] As a result of the spherical round cross-sectional shape of the piston shaft 3 according to the invention, it is not only possible to achieve improved sliding of the piston shaft 3 on a cylinder wall 10 (cf. FIG. 3) or on a cylinder liner 11, if such a liner is arranged in the cylinder, but especially an abutment face is increased, that is to say, a contact face between the piston shaft 3 and the cylinder wall 10 or the cylinder liner 11 which may be arranged in the cylinder, whereby improved heat transfer can be achieved. With pistons which have previously been known from the prior art and which have a spherically oval cross-sectional shape, these were in contact with only a small portion of the shaft face thereof with a cylinder wall or cylinder liner, whereby only a small amount of heat could be discharged from the piston to the cylinder and consequently also significantly reduced cooling of the piston was possible.

[0025] If FIGS. 1 and 3 are considered further, it can be seen that the cooling channel 5 is expanded outwards in a radial direction in the region of the piston base 13, that is to say, in the direction of a top land 14, or has such an expansion 15. The temperature in a first annular groove 16 can thereby be reduced by up to 10 K, whereby the problem of oil carbon formation and in particular also the coking of the oil which is required for lubrication of the piston 1 in the cylinder can be prevented but at least significantly reduced. Additionally or alternatively, such an expansion 15′ may also be provided radially inwardly in the direction of the combustion bowl 7.

[0026] If FIGS. 1 and 3 are considered further, it can be seen that the piston 1 is constructed in multiple parts, in this instance in two parts, with an upper portion 17 and a lower portion 18 which is connected thereto, in particular welded thereto, wherein the cooling channel 5 is formed partially in the upper portion 17 and partially in the lower portion 18. The upper portion 17 and the lower portion 18 are in this instance connected to each other along a joining plane 19, for example, friction or laser welded. In order to be able to expand the cooling channel 5 in a downward direction, there may additionally be provided bores 20 which extend as far as a location close to the lower end 9 of the piston shaft 3 (cf. FIG. 1) and thereby bring about an improved heat discharge to the region of the piston shaft 3. The cooling channel 5 may additionally or alternatively also be expanded by means of milling, for example, hollow milling, whereby, at a cooling channel base 21, the undulating shape which is typical at that location is produced as a result of the process. As a result of such an undulating shape at the cooling channel base 21, the surface can be increased and thereby a heat transfer can also be improved.

[0027] Additionally or alternatively, there may be arranged at a lower piston side 22 (cf. FIGS. 1, 3 and 4) ribs 25 which preferably extend only over a region between inner shaft walls and the connection thereof to the piston base 13. These ribs 25 have in this instance, on the one hand, the function of increasing the surface, at least by 1.2 times to 2 times, whereby the heat transfer to the oil which is injected from below is also increased. On the other hand, the ribs 25 guide the injected oil over the centre axis (piston axis 8) towards the opposite side. As a result of oblique positioning of an injection nozzle, a movement of an impact location of the oil jet depending on the position of the piston 1 can further be achieved in accordance with the arrow 24 illustrated according to FIG. 4, whereby a particularly uniform cooling can be achieved by means of continuous back-and-forth movement of the oil jet 4 over the ribs 25. The movement of the oil jet is in this instance brought about by the upward and downward movement of the piston 1 between the top dead centre OT and the bottom dead centre UT thereof. The ribs 25 may in this instance preferably be stamped or forged by means of stamping with a corresponding forging or stamping die during the heat shaping process. In this instance, it has been found that this is achieved in a particularly good and simple manner as long as the ribs 25 are constructed in a rounded manner or follow a sinusoidal shape.

[0028] Preferably, for example, sodium and/or potassium is used as a cooling medium 6 in the cooling channel 5, wherein there are also considered in particular admixtures thereof which become liquid, for example, at −12° C. and during operation of the internal combustion engine 2 are shaken back and forth by the back-and-forth movement of the piston 1 and thereby absorb heat from the piston base 13 and discharge it to the piston shaft 3. Alternatively, water can also be used as a cooling medium 6. Water affords the advantage that it is very cost-effective and a far less complex filling installation can be used for it. Furthermore, it is available everywhere and does not pose any risk to humans and the environment. The operating principle is in this instance based on the use of evaporation and condensation enthalpy of the cooling medium 6. The water evaporates in the upper region of the cooling channel 5 which faces the piston base 13 and the combustion bowl 7 and condenses in the lower portion of the cooling channel 5, where the heat is discharged, for example, to the piston shaft 3. The operating principle functions in this instance in a similar manner to a heatpipe with which large quantities of heat can be transferred. Such a “heatpipe” uses the evaporation and condensation enthalpy of the cooling medium (operating medium). When water is used as a cooling medium 6, precise attention must be paid to the filling quantity since water in comparison with sodium/potassium is a worse heat conductor and the evaporation and condensation enthalpy is the only important aspect. In order, where possible, not to impede the transport of heat through the water, it is therefore advantageous if there is substantially only so much water available in the cooling channel 5 that the maximum energy which is introduced into the piston 1 during an operating cycle evaporates the majority of the water present to the greatest possible extent. A filling quantity typically of from 0.01% to 10% of the volume of the cooling channel 5 should accordingly already be sufficient to transport the heat from the hot locations of the piston 1 into colder regions. The function of this method is in this instance connected with the physical properties of water, according to which, during the transition from the liquid phase into the gas phase, heat is absorbed and, vice versa when the water vapour is condensed, heat is discharged to the environment. The function is accordingly limited in an upward direction to a maximum temperature of 374° C. (critical temperature) since above the critical temperature, there occurs no phase jump. In a downward direction, the melting point of the water at 0° C. has a limiting action. It has been found that in particular for steel pistons during operation of the engine, this temperature range is not left. Typically, temperatures from 100 to 300° C. are observed. The extent of the expansion of the cooling channel 5 under pressure naturally has to be taken into account during the configuration which may lead to greater wall thicknesses in the region of the cooling channel 5. The pressure varies in this instance typically between 50 and 100 bar, depending on the respective engine concept.

[0029] At high specific power levels, it has additionally been found that as a result of the addition of salt or highly thermally conductive powders (for example, based on copper, aluminium or silicon carbide or low-melting metals, such as tin, an SnBi-eutectic, bismuth or gallium), the boiling power of the water is significantly increased and the film boiling which otherwise occurs from a heat flow density of approximately 1000 kW/m.sup.2 can be displaced to higher heat flow densities.

[0030] When FIG. 4 is considered, it can be seen that the ribs 25 extend substantially in a radial direction with respect to the piston axis 8, and that recesses 23 are arranged between the individual ribs 25, wherein a volume of the ribs 25 which protrude from the lower piston side 22 corresponds to the volume of the recesses 23 stamped in the lower piston side 22. During the stamping or forging of the ribs, a flowing of the material occurs only locally, whereby only a very small loading or no additional loading at all is produced for the forging tool and the service-life of the tool is not influenced or is influenced only in an insignificantly negative manner. The ribs 25 or the recesses 23 can consequently be introduced in a cost-neutral manner to the greatest extent.

[0031] With the piston 1 according to the invention and the spherically round cross-sectional shape thereof, in the region of the piston shaft an abutment face against a cylinder liner 11 or a cylinder wall 10 of the internal combustion engine can be increased, whereby improved heat transfer and consequently also improved cooling of the piston 1 can be achieved, which is a great advantage, in particular for highly supercharged diesel engines with a specific power of over 60 kW per litre cubic capacity.

[0032] In addition to the spherically round cross-sectional shape of the piston shaft 3, other measures which promote the cooling of the piston 1, such as, for example, the ribs 25, the bores 20, the expansions 15 can be applied cumulatively or alternatively.