PISTON FOR AN INTERNAL COMBUSTION ENGINE HAVING LIQUID METAL COOLING

Abstract

A piston for an internal combustion engine may include a piston crown having a closed circumferential cooling channel, a piston skirt, a first metallic coolant arranged in the cooling channel and having a metal or metal alloy with a melting point below 250° C., and a second nonmetallic coolant arranged in the cooling channel and having a melting point below 40° C. and a density which is lower than a density of the first coolant.

Claims

1. A piston for an internal combustion engine having, comprising: a piston crown having a closed circumferential cooling channel; a piston skirt; a first metallic coolant arranged in the cooling channel and having a metal or metal alloy with a melting point below 250° C.; and a second nonmetallic coolant arranged in the cooling channel and having a melting point below 40° C. and a density which is lower than a density of the first coolant.

2. The piston as claimed in claim 1, wherein the metal or metal alloy of the first coolant does not ignite spontaneously.

3. The piston as claimed in claim 1, wherein the first coolant does not comprise any alkali metals or heavy metals.

4. The piston as claimed in claim 1, wherein the first coolant comprises at least one of tin, bismuth, gallium, and silver.

5. The piston as claimed in claim 1, wherein at least one of: the first coolant comprises a tin-bismuth alloy, and the first coolant comprises a tin-silver alloy.

6. The piston as claimed in claim 1, wherein the second coolant is thermally stable up to 300° C.

7. The piston as claimed in claim 1, wherein the second coolant comprises biphenyl and diphenyl ether.

8. The piston as claimed in claim 1, wherein the second coolant comprises silicone oil.

9. The piston as claimed in claim 1, wherein the second coolant comprises silicone oil, biphenyl and diphenyl ether.

10. The piston as claimed in claim 1, wherein the second coolant comprises water, in particular salt-containing water.

11. The piston as claimed in claim 1, wherein the density of the first coolant is at least 5 times the density of the second coolant (26).

12. The piston as claimed in claim 1, wherein a volume of the first coolant and a volume of the second coolant together occupy at least 10% by volume of a volume of the cooling channel.

13. The piston as claimed in claim 12, wherein the volume of the first coolant and the volume of the second coolant occupy from 20 to 40% by volume of the volume of the cooling channel.

14. The piston as claimed in claim 1, wherein a volume of the second coolant is less than a volume of the first coolant and greater than half the volume of the first coolant.

15. The piston as claimed in claim 1, wherein a volume of the second coolant is greater than a volume of the first coolant and less than three times the volume of the first coolant.

16. The piston as claimed in claim 6, wherein the second coolant is thermally stable up to 400° C.

17. The piston as claimed in claim 16, wherein the second coolant is thermally stable up to 500° C.

18. The piston as claimed in claim 7, wherein the second coolant comprises a eutectic mixture of biphenyl and diphenyl ether.

19. The piston as claimed in claim 10, wherein the water is salt-containing water.

20. The piston as claimed in claim 11, wherein the density of the first coolant is at least 7 times the density of the second coolant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings show, in each case schematically,

[0027] FIG. 1 a sectional view through a piston according to the invention,

[0028] FIG. 2 a perspective partial sectional view through the piston of FIG. 1.

DETAILED DESCRIPTION

[0029] A first embodiment, as depicted in FIGS. 1 and 2, of a piston 10 has a piston crown 12 and a piston skirt 14. The piston crown 12 has a piston top 15 in which a piston bowl 16 is formed. Furthermore, there is a circumferential ring belt 18 into which piston rings can be inserted. At the transition between the ring belt 18 and the piston top 15 there is a top land 20. Furthermore, the piston crown 12 has a closed circumferential cooling channel 22 in which a first coolant 24 and a second coolant 26 are arranged.

[0030] The piston skirt 14 adjoins the piston crown 12 in the axial direction. The piston skirt 14 has a boss 28 having two wrist pin holes 30 into which a wrist pin can be inserted in order to attach the piston 10 to a connecting rod of the internal combustion engine. Furthermore, the piston skirt 14 has two running surfaces 32 and 34 which each cover a partial circumference of a cylindrical surface. The two running surfaces 32 and 34 join the two bosses 28.

[0031] The piston 10 has a plurality of holes 36 which run essentially axially and open into the cooling channel 22. As a result, the coolant present in the cooling channel 22 can cover a larger distance in the axial direction due to the up and down motion of the piston 10, so that heat transport in the axial direction is improved.

[0032] Furthermore, a wall 38 which delimits the cooling channel 22 radially outward and which bears the ring belt 18 is inclined. In particular, the wall 38 is thicker in the vicinity of the piston top 15 than in a region which is closer to the piston skirt 14. As a result, coolant which has been heated up at the piston top 15 does not come into contact with the wall 38 on its way downward, which prevents the wall 38 and thus the ring belt 18 from being heated. Only when the coolant moves from the bottom upward, i.e. out of the holes 36, can it contact the wall 38. However, the coolant which moves from the bottom upward out of the holes 36 has cooled down, so that the wall 38 and the ring belt 18 can be cooled.

[0033] The first coolant 24 comprises a metal or a metal alloy which has a melting point which is less than 250° C., preferably less than 200° C. and particularly preferably less than 150° C. When the coolant is solid, it contributes only little to cooling. On the other hand, when the first coolant 24 is liquid, the coolant is moved in the axial direction as a result of the up and down motion of the piston 10, so that the coolant at the piston top 15 can take up heat from the piston top 15 and can transport this away in a downward direction due to the motion. The first coolant 24 can then transfer its heat to the piston skirt 14 in the region of the holes 36. The heat transfer from the piston top 15 to the piston skirt 14 is greatly increased by the convective movement of the first coolant 24. Since the first coolant 24 comprises metal, which has a high thermal conductivity and high heat capacity, the convective heat transfer is very high.

[0034] It has been found that when first metallic coolants 24 which have a melting point above 150° C. are used, convective cooling commences too late. This means that in the case of a cold start the piston top 15 which is initially cooled only slightly by conduction of heat can become heated to such an extent that it is damaged before the first coolant 24 melts and can contribute to cooling by convection.

[0035] There are metals and metal alloys which have melting points below 100° C. However, these alloys suffer from the problem that metals which ignite spontaneously, are toxic or are very expensive are present therein. The costs of production of such a piston are therefore increased. The use of metals which do not ignite spontaneously, are not toxic and have an acceptable purchase price would reduce the costs of the piston 10.

[0036] The second coolant 26 is arranged as auxiliary coolant in the cooling channel 22. The second coolant 26 has a melting point below 40°, preferably below 30° and particularly preferably below 20° C. The second coolant 26 is preferably nonmetallic, so that the second coolant 26 does not form a metallic alloy with the first coolant 24 and therefore would not solidify together with the first coolant 24.

[0037] Due to the lower melting point the second coolant 26 can contribute to convective cooling of the piston top 15 even immediately after a cold start of the internal combustion engine. The main task of the second coolant 26 is, however, to ensure that the first coolant 24 melts in good time. Since the second coolant 26 is liquid even in the initial phase, heat energy can be transferred from the piston top 15 to the first coolant 24 and heat the latter quickly enough for the first coolant 24 to ensure sufficient cooling of the piston 10.

[0038] Suitable materials for the second coolant 26 are, for example, mixtures of biphenyl and diphenyl ether, preferably eutectic mixtures. As an alternative or in addition, it is also possible to use silicone oils. These compounds have a satisfactory thermal stability of at least 400° C.

[0039] To obtain a very low gas pressure during operation of the piston 10, the cooling channel 22 can be either evacuated or filled with dry air, and to decrease the air pressure alkali metals, for example sodium, potassium and/or lithium, can be added in a small amount as alloying constituent to the first coolant 24. Alkali metals react with atmospheric oxygen and lithium also reacts with atmospheric nitrogen to form lithium nitride, so that both the oxygen and the nitrogen are bound firmly in chemical form and the amount of gas in the cooling channel 22 is reduced.

[0040] Possible metals or metal alloys for the first coolant 24 are, for example, tin, bismuth and silver. For example, a eutectic mixture of tin and bismuth has a melting point of 138° C. A eutectic mixture of tin and silver has a melting point of 221° C.