Piston for an internal combustion engine and method for its manufacture

Abstract

Piston (10) for an internal combustion engine with a piston head (11) and a piston skirt (12), wherein the piston head (11) has a piston crown (13) and a circumferential top land (15), wherein the piston (10) is provided with a heat-insulating silicon-containing coating (23) which has a closed-cell pore structure, characterized in that the pore structure is formed substantially from vacuum pores.

Description

[0001] The present invention relates to a piston for an internal combustion engine having a piston head and a piston skirt, wherein the piston head has a piston crown and a circumferential top land, wherein the piston is provided with a heat-insulating coating having a closed pore structure. The present invention further relates to a method for producing such a piston.

[0002] It is desirable to equip pistons for modern internal combustion engines with effective thermal insulation, particularly in the area of the piston crown, in order to avoid overheating under the high thermal loads typical of modern internal combustion engines.

[0003] From DE 10 2012 025 283 A1 a coating with a closed-cell pore structure is known, wherein the pores are filled with a gas.

[0004] The object of the present invention is to further develop a generic piston and a method for its production in such a way that an effective thermal insulation layer is obtained using the simplest possible means.

[0005] The solution is that the pore structure is essentially formed from vacuum pores.

[0006] The method according to the invention is characterized by the following method steps: (a) cleaning the surface area of a piston to be coated; b) providing a matrix comprising at least one silicon-containing material and elemental magnesium; (c) applying the matrix to the surface area of the piston (10) to be coated; (d) heating the matrix to a temperature of at least 400°C, so that a heat-insulating silicon-containing coating (23) with a closed-cell pore structure is formed, which essentially has vacuum pores; (e) cooling the coating (23).

[0007] It has surprisingly been found that the reaction of elemental magnesium with the silicon-containing material according to the formula 2 Mg + SiO2 → 2MgO + Si A reduction in the volume of the reactants, magnesium and silicon-containing material, of up to 35% occurs as soon as the reactants are heated to a temperature above 400°C. The result is a closed-cell pore structure, whereby the pores are not filled with a gas, as is known in the prior art, but rather have a vacuum. This significantly reduces the thermal conductivity of the coating according to the invention compared to the prior art and correspondingly improves its thermal insulation effect.

[0008] Advantageous further training results from the subclaims.

[0009] The coating provided according to the invention preferably has a porosity between 10 vol.% and 20 vol.%. This value represents a good compromise between the thermal insulation effect of the closed-cell pore structure and the compressive strength of the coating required during engine operation.

[0010] In a further advantageous embodiment, the coating according to the invention has a thermal conductivity of at most 1 W / mK, preferably less than 1 W / mK, in order to optimize the thermal insulation effect of the coating.

[0011] Preferably, an adhesive layer is provided between the coated surface area of the piston and the coating in order to optimize the adhesion between the coating provided according to the invention and the piston material.

[0012] Suitable materials for the bonding layer include MeCrAlY compounds, where the symbol Me stands for nickel, iron, and cobalt. Bonding layers made of these materials can be applied to the surface area of the piston to be coated, for example, using flame spraying. Al2O3 is also a suitable material for the bonding layer and is applied to the surface area of the piston to be coated, for example, using a sol-gel process.

[0013] An embodiment of the present invention is explained in more detail below with reference to the accompanying drawings. The only Fig. 1 shows a schematic, not to scale representation of a first embodiment of a piston according to the invention in section.

[0014] Fig. Figure 1 shows a first embodiment of a piston 10 according to the invention. In this embodiment, the piston 10 is a one-piece box-type piston and has a piston head 11 and a piston skirt 12. The piston 10 can be made, for example, from a material based on cast iron, steel, or aluminum. The piston according to the invention can, of course, also be designed as a multi-part piston.

[0015] The piston head 11 has a piston crown 13 with a combustion bowl 14, a circumferential top land 15, and a circumferential ring section 16 for receiving piston rings (not shown). Furthermore, a circumferential, closed cooling channel 17 is formed between the combustion bowl 14 and the ring section 16.

[0016] The piston skirt 12 has, in a conventional manner, hubs 18 that are connected to the underside of the piston head 11 via hub connections 19. The hubs 18 are provided with hub bores 21 for receiving a piston pin (not shown). The hubs 18 are connected to one another in a conventional manner via running surfaces 22.

[0017] According to the invention, in the Fig.In the embodiment shown in Figure 1, a heat-insulating coating 23 with a closed-cell pore structure is applied in the area of the piston crown 13. In this embodiment, an adhesive layer 24 is applied between the coated surface area of the piston 10 and the heat-insulating coating 23.

[0018] According to the invention, the heat-insulating coating 23 comprises elemental silicon and magnesium oxide, wherein the closed-cell pore structure is characterized in that the pores each enclose a vacuum.

[0019] The heat-insulating coating 23 can be applied, for example, to the surface area of the piston 10 to be coated as follows.

[0020] First, the surface area of the piston 10 to be coated is cleaned in a conventional manner, in particular degreased and freed of other contaminants. Optionally, a bonding layer 24 can be applied to the surface area to be coated. Materials of the MeCrAlY type are applied to the surface area of the piston 10 to be coated, for example, by flame spraying. Aluminum oxide is applied to the surface area of the piston 10 to be coated, for example, by a sol-gel process in a conventional manner.

[0021] After cleaning or applying the adhesion promoter layer 24, a suspension containing at least one silicon-containing material and elemental magnesium is applied, for example, by spraying or printing, to the surface area to be coated. In the exemplary embodiment, the suspension consists of a silicone resin and elemental magnesium powder. To adjust a viscosity suitable for spraying or printing, an alcohol, in particular ethanol, can additionally be added. After the coating process, in the exemplary embodiment, the coated component or coating is first heated to a temperature of 100°C to 300°C. The silicone resin cures under the formation of inorganic silicon dioxide. The resulting silicon dioxide forms a matrix that encloses the powder particles of elemental magnesium. The coated component or coating is then heated to a temperature of approximately 400°C.At this temperature, the reaction 2 Mg + SiO2 → 2MgO + Si is observed between the powder particles of elemental magnesium and the silicon dioxide, which causes the desired volume reduction, causing the formation of a closed-cell pore structure with vacuum pores.

Claims

[1] Piston (10) for an internal combustion engine with a piston head (11) and a piston skirt (12), wherein the piston head (11) has a piston crown (13) and a circumferential top land (15), wherein the piston (10) is provided with a heat-insulating silicon-containing coating (23) having a closed-cell pore structure, characterized by that the pore structure is essentially formed from vacuum pores. [2] Piston according to claim 1, characterized by that the coating (23) has a thermal conductivity of at most 1 W / mK. [3] Piston according to claim 1, characterized by that the coating (23) contains elemental silicon and magnesium oxide. [4] Piston according to claim 1, characterized by that an adhesion promoter layer (24) is provided between the coated surface area of the piston (10) and the coating (23). [5] Piston according to claim 4, characterized bythat the adhesion promoter layer (24) is selected from the group comprising Al2O3 and MeCrAIY with Me = Ni, Fe, Co. [6] Method for coating a piston (10) for an internal combustion engine with a piston head (11) and a piston skirt (12), wherein the piston head (11) has a piston crown (13) and a circumferential top land (15), characterized by the following procedural steps: (a) cleaning the surface area of a piston (10) to be coated; (b) providing a matrix on the surface area of the piston to be coated, which matrix comprises at least one silicon-containing material and elemental magnesium; (c) heating the matrix to a temperature of at least 400°C to form a heat-insulating silicon-containing coating (23) having a closed-cell pore structure which essentially has vacuum pores; (d) Cooling the coating (23). [7] Method according to claim 6, characterized by that before step (b) an adhesion promoter layer (24) is applied to the surface area to be coated. [8] Method according to claim 6, characterized by that in step (b) a suspension containing a silicone resin and elemental magnesium powder is applied to the surface area to be coated and then heated to a temperature of 100°C to 300°C. [9] Method according to claim 8, characterized by that an alcohol is added to the suspension. [10] Method according to claim 6, characterized by that in step (d) a coating (23) is formed which contains elemental silicon and magnesium oxide.

Images