Piston blank, piston, and method

Abstract

Disclosed is a piston blank (39) for a piston (8), said piston blank (39) being made at least in part of a steel alloy containing 0.5 to 2 percent by weight chromium and 2.5 to 3.5 percent by weight silicon.

Description

[0001] PISTON BLANK, PISTON AND PROCESS

[0002] The present invention relates to a piston blank for a piston, a piston with such a piston blank and a method for producing such a piston with the aid of such a piston blank.

[0003] Pistons for internal combustion engines can be made from steel alloys, such as the heat-treatable steel 42CrMo4 or the micro-alloyed steel 38MnVS6. Such pistons can either be constructed as a single piece or be assembled from a piston crown and piston lower section, which can be joined together using a joining process. Such a piston can be cooled using a cooling oil jet, which is injected into a circumferential annular cooling channel of the piston using an injection nozzle. A specific minimum volume flow of cooling oil is required to ensure appropriate heat dissipation and keep highly stressed areas of the piston below a critical temperature for piston scaling.

[0004] Due to their chemical composition, the aforementioned materials exhibit a predetermined and similar thermal conductivity. This results in a certain combustion chamber surface temperature, depending on the piston cooling design. This combustion chamber surface temperature cannot be increased any further, as the aforementioned materials have limited scaling resistance. A further increase in the combustion chamber surface temperature can lead to scaling-related cracking and thus to piston failure.

[0005] However, a higher combustion chamber surface temperature, i.e., hotter combustion caused by a reduction in heat loss to the piston crown, can fundamentally increase the thermodynamic efficiency of engine combustion. This can result in advantages in terms of fuel consumption and CO2 emissions. To date, increased demands regarding fuel consumption and CO2 reduction have often been met by developing friction-optimized piston systems. These include, for example, optimized piston skirt profiles and installation clearances, as well as cost-intensive ring packages with special coatings or complex surface optimizations for cylinder liners.

[0006] WO 2014 / 198896 A1 describes a piston, in particular a steel piston, for an internal combustion engine with a piston crown which is part of a combustion chamber, wherein at least the piston crown has an oxidation protection layer.

[0007] Against this background, it is an object of the present invention to provide an improved piston blank for a piston.

[0008] Accordingly, a piston blank for a piston is proposed. The piston blank is made, at least in sections, from a steel alloy with a chromium content of 0.5 to 2 percent by weight and a silicon content of 2.5 to 3.5 percent by weight.

[0009] Because the steel alloy contains the aforementioned chromium and silicon content, the steel alloy is particularly resistant to scaling. This allows the combustion chamber surface temperature of a piston made from the piston blank to be increased without the piston becoming scaled. This leads to an increase in the thermodynamic efficiency of engine combustion. This allows compliance with more stringent requirements regarding fuel consumption and emissions, particularly CO2 emissions. In this context, the "combustion chamber surface temperature" can be understood in particular as a temperature of a surface of a piston crown of the piston. Furthermore, the "combustion chamber surface temperature" can be understood more generally as a temperature of a surface of a combustion chamber associated with the piston. The surface of the piston crown can be part of the combustion chamber.

[0010] The piston blank differs from the piston in that, in comparison to the piston blank, the piston is machined, for example, using a machining and / or forming process. The piston blank can also differ from the piston in that a lower piston part of the piston blank and an upper piston part of the piston blank are not yet firmly connected to one another. The piston blank or the piston can in particular be assigned a symmetry or central axis, to which the piston blank or the piston can be constructed essentially rotationally symmetrically. This aforementioned central axis can in particular be formed by a central axis of a cylinder which envelops surfaces of a piston skirt of the piston and thereby has a minimal diameter, wherein the central axis of the cylinder is arranged perpendicular to a pin bore of the piston.

[0011] The piston blank or piston is also assigned a coordinate system with a width direction (x-direction), a height direction (y-direction), and a depth direction (z-direction). The y-direction can also be referred to as the axial direction. The terms "y-direction" and "axial direction" are therefore interchangeable. The directions are oriented perpendicular to each other. The central axis coincides with the y-direction or is oriented parallel to the y-direction. The piston blank or piston is also assigned a radial direction. The radial direction is oriented perpendicular to the central axis and points away from it. The steel alloy is preferably a so-called low-alloy steel alloy. The steel alloy can, for example, be formed into the piston blank as a semi-finished product using a forging process.The piston blank, or the aforementioned piston lower part and the aforementioned piston upper part, can therefore be forged components. However, this does not preclude the piston blank from being machined using a material-removing manufacturing process, such as milling, turning, and / or eroding. Furthermore, the piston blank, or the piston lower part and the piston upper part, can also be cast components or cast components reworked using a forging process.

[0012] The fact that the piston blank is made "at least in sections" of the steel alloy means in this case, in particular, that at least part of the piston blank can be made of the steel alloy. However, this does not preclude the entire piston blank from being made of the steel alloy. In particular, the steel alloy is provided at least in the region of a combustion chamber bowl of the piston blank or the piston. In the event that the piston blank has a piston lower part and a piston upper part as mentioned above, for example, only the piston upper part can be made of the steel alloy.

[0013] Compared to the previously mentioned steel alloys 42CrMo4 or 38MnVS6, the steel alloy from which the piston blank is made exhibits lower thermal conductivity. This results in less heat being dissipated from the combustion chamber of an internal combustion engine during piston operation. This increases the combustion chamber surface temperature, which leads to an increase in the thermodynamic efficiency of engine combustion. In addition, the alloy components chromium and silicon prevent scaling at the elevated combustion chamber surface temperature. "Scaling" or "oxidation wear" can be understood here as the high-temperature corrosion of metals caused by direct chemical reaction with hot, oxygen-containing gases.Due to a reduction in the strength of steel alloys with little or no scale resistance, scaling can lead to scale-related cracking and thus to piston failure. This is reliably prevented by the aforementioned alloying components of the steel alloy, which significantly increase scale resistance. In addition to the elements iron, chromium, and silicon, the steel alloy may contain the elements carbon, manganese, phosphorus, sulfur, molybdenum, titanium, lead, antimony, aluminum, nitrogen, copper, tin, nickel, and boron. Oxygen and hydrogen may also be present in small amounts in the steel alloy.

[0014] According to one embodiment, the chromium content is 0.9 to 1.2 percent by weight and / or the silicon content is 2.85 to 3 percent by weight.

[0015] With this previously mentioned chromium content and this silicon content, a particularly high scaling resistance can be achieved.

[0016] According to a further embodiment, the steel alloy has a carbon content of 0.35 to 0.5, in particular 0.4 to 0.44, percent by weight.

[0017] Due to this low carbon content, the steel alloy is easily formable, which means that the piston blank can be manufactured and / or machined using a forging process.

[0018] According to a further embodiment, the steel alloy has a manganese content of 0.5 to 0.9, in particular 0.6 to 0.8, percent by weight. For example, one property of the manganese content in the steel alloy is that it increases the hardenability of the steel alloy.

[0019] According to a further embodiment, the steel alloy has a titanium content of 0.005 to 0.015 percent by weight.

[0020] The titanium content gives the steel alloy high toughness, strength and ductility.

[0021] According to a further embodiment, the steel alloy has a molybdenum content of 0.1 to 0.3, in particular 0.15 to 0.2, percent by weight.

[0022] The molybdenum content leads to an increase in the tempering resistance and the high-temperature strength of the steel alloy.

[0023] According to a further embodiment, the steel alloy has a silicon content of 2.5 to 3.5, in particular 2.85 to 3, percent by weight.

[0024] The silicon content leads to an increase in tensile strength and yield strength due to solid solution strengthening and, as a diffusion barrier for oxygen, increases the scaling resistance of the steel alloy.

[0025] According to a further embodiment, the steel alloy has an increased scaling resistance at 550 to 650 °C, in particular at 580 to 600 °C.

[0026] "Scale resistance" or "scaling resistance" refers to resistance to scaling. The terms "scaling resistance" and "scaling resistance" can be interchanged here. Scale resistance can be determined by measuring the weight of the piston blank or piston, or additionally by measuring the thickness of the scale layer, age-hardening or annealing the piston blank or piston at a specific temperature, then measuring the weight of the piston blank or piston, and finally determining the degree of oxidation based on a change in the weight of the piston blank or piston. The increased scaling resistance enables the piston to be used at higher temperatures, thereby increasing the combustion chamber surface temperature with the aforementioned advantages.

[0027] According to a further embodiment, the piston blank has a piston lower part and a piston upper part, wherein the piston upper part is made of the steel alloy, wherein the piston lower part is made of the steel alloy or of a further material different from the steel alloy, and wherein the further material in particular has a higher thermal conductivity than the steel alloy.

[0028] This means that preferably at least the piston upper part is made of the steel alloy. Thermal conductivity or thermal conductivity coefficient is a material property that determines heat flow through a material due to thermal conduction. The lower the thermal conductivity, the better the thermal insulation. The additional material can be, for example, the previously mentioned tempering steel 42CrMo4 or the micro-alloyed steel 38MnVS6. The piston lower part and the piston upper part are joined together using a joining process. For example, the piston lower part and the piston upper part are joined together to form an intermediate component of the piston, from which the finished piston is manufactured. In integral joints, the joining partners are held together by atomic or molecular forces.Material-locking connections are non-detachable connections that can only be separated by destroying the connecting elements and / or the connecting partners. For example, the piston lower section and the piston upper section are welded together, in particular friction-welded. The intermediate component can be further machined using an abrasive manufacturing process, in particular a machining process, to form the piston from the intermediate component. Alternatively or additionally, the piston upper section and the piston lower section can be connected to one another in a form-fitting manner. A form-fitting connection is created when two connecting partners engage with one another or behind one another. For example, the piston lower section and the piston upper section can be screwed together.

[0029] According to a further embodiment, the piston blank is a one-piece component made entirely of the steel alloy.

[0030] In this case, the piston blank does not have a separate piston lower and upper sections. "Single-piece" or "one-piece" in this context means that the piston blank is not composed of different sub-components, but rather forms a single component. The piston blank can, in particular, be constructed from a single piece of material. "Single-piece" means that the piston blank is made entirely of the same material, namely the steel alloy.

[0031] Furthermore, a piston with such a piston blank is proposed.

[0032] As previously mentioned, the piston differs from the piston blank in that, in the piston, the piston lower part and the piston upper part are firmly connected to one another. The piston may further differ from the piston blank in that the piston blank is machined to form the piston. Machining may be performed, for example, using a forging process and / or a material-removing process, such as turning, milling, eroding, or the like. The piston is part of the aforementioned internal combustion engine. The internal combustion engine may comprise multiple pistons.

[0033] According to one embodiment, the piston comprises a combustion chamber bowl and a cooling channel which runs at least partially around the combustion chamber bowl, wherein an average first wall thickness of a first wall provided between the combustion chamber bowl and the cooling channel is greater than 5 percent, preferably greater than 6 percent, preferably greater than 7 percent, of a piston diameter of the piston.

[0034] The combustion chamber bowl may already be formed on the piston blank, in particular on the piston crown. The combustion chamber bowl can be formed and / or reworked using a machining process or a forging process. The cooling channel runs in a ring around the center axis of the piston. A cooling oil, in particular engine oil, can be passed through the cooling channel to dissipate heat from the piston. The cooling oil can, for example, be injected into the cooling channel using an injection nozzle through bores provided on the piston. Because the first wall thickness is greater than 5 percent of the piston diameter, the dissipation of heat by the cooling oil in the combustion chamber bowl area can be further reduced. This leads to an additional increase in the combustion chamber surface temperature and thus in the thermodynamic efficiency.According to internal company knowledge, the respective wall thickness between the cooling channel and the combustion chamber bowl, as well as between an inner shape of the piston and the combustion chamber bowl, is usually designed at 3.5 percent of the piston diameter. By increasing the wall thickness to over 5 percent of the piston diameter, heat dissipation can be reduced. Furthermore, by adapting the geometry, in particular the cross-sectional geometry, of the cooling channel, an increase in the combustion chamber surface temperature and the thermodynamic efficiency can be achieved. In this case, the cooling channel can be equipped with a smaller cross-sectional geometry compared to cooling channels known in-house, which also reduces heat dissipation from the combustion chamber bowl. This measure is also advantageous with regard to the dimensions and overall height of the piston.Alternatively, the cooling channel can be designed as an open cooling channel, which is sprayed with cooling oil via a freely accessible inner surface using the injection nozzle. Alternatively, the piston can be designed entirely without a cooling channel. As a further measure, the amount of cooling oil used to cool the piston can be reduced. This also increases the combustion chamber surface temperature and thus the thermodynamic efficiency. Furthermore, an additional efficiency advantage results from reducing the power loss of a required oil pump, which indirectly contributes to fuel savings. The "piston diameter" in this context refers to the diameter of the smallest cylinder, which includes a so-called piston skirt of the piston.The first wall thickness of the first wall is defined in particular as a smallest distance between the combustion chamber bowl, in particular a rounding of the combustion chamber bowl, and the cooling channel, in particular a wall of the cooling channel.

[0035] According to a further embodiment, the average first wall thickness is at least 5 millimeters.

[0036] In this context, the "average" first wall thickness is understood in particular to mean that the first wall thickness, viewed along its direction of extension or main direction of extension, is on average at least 5 millimeters or greater than 5 percent of the piston diameter. The "direction of extension" or "main direction of extension" can be understood as a direction along which the first wall has its greatest geometric extent. In particular, the "direction of extension" or "main direction of extension" can be understood as a course of the first wall along a surface of the combustion chamber bowl. This surface can be referred to as the combustion chamber bowl surface. This means that the first wall thickness can be less than the aforementioned 5 percent of the piston diameter or 5 mm in some regions or locally.However, viewed across the entire extension direction or main extension direction of the first wall, i.e., globally, the first wall thickness averages at least 5 millimeters, or the first wall thickness is greater than 5 percent of the piston diameter. The terms "extension direction" and "main extension direction" are interchangeable in this context.

[0037] According to a further embodiment, an average second wall thickness of a second wall provided between the combustion chamber bowl and an inner shape of the piston is greater than 5 percent, preferably greater than 6 percent, particularly preferably greater than 7 percent, of the piston diameter.

[0038] The combustion chamber bowl preferably has a combustion chamber bowl base facing the combustion chamber and the inner shape facing away from the combustion chamber. The combustion chamber bowl base and the inner shape can each be conical or tapered. Facing the combustion chamber, the second wall forms the combustion chamber bowl base. Facing away from the combustion chamber, the second wall forms the inner shape. The first wall and the second wall merge into one another. The second wall thickness of the second wall is defined in particular as a smallest distance between the combustion chamber bowl, in particular the combustion chamber bowl base of the combustion chamber bowl, and the inner shape. The first wall merges in particular at the aforementioned rounding of the combustion chamber bowl into the second wall or vice versa. This means in particular that the first wall is connected to the second wall. According to a further embodiment, the average second

[0039] Wall thickness at least 5 millimeters.

[0040] Here too, the second wall may be at least 5 millimeters or 5 percent of the piston diameter in some areas or locally. On average, however, the second wall thickness is always greater than 5 millimeters or is at least 5 percent of the piston diameter. Here, too, it is particularly important that the second wall thickness, viewed along a direction of extension or main direction of extension of the second wall, is on average at least 5 millimeters or is at least 5 percent of the piston diameter. The use of the steel alloy with low thermal conductivity leads to a reduction in heat dissipation from the combustion chamber bowl to the cooling channel and thus to an increase in the temperature of a surface of the combustion chamber bowl and the combustion chamber.Design measures such as increasing wall thickness, adjusting the geometry of the cooling channel, and / or reducing the amount of cooling oil have a similar effect, leading to a higher combustion chamber surface temperature by reducing heat dissipation. As a result, the thermodynamic efficiency of combustion can be increased, thus reducing fuel consumption. Furthermore, CO2 emissions can be reduced. A prerequisite for this is sufficient scaling resistance. In addition to low thermal conductivity, the alloying constituents of the steel alloy lead to increased scaling resistance. This means that the threshold temperature above which technically relevant scaling occurs can be shifted to a higher temperature.Due to the high scaling resistance of the steel alloy, additional combustion-related measures can be taken to increase the combustion chamber surface temperature and improve thermodynamic efficiency. By using the highly scaling-resistant and simultaneously low-thermal-conductivity steel alloy, at least for the piston crown and / or in the combustion bowl area, the thermodynamic efficiency of the combustion engine can be significantly increased. This can result in fuel consumption benefits and a reduction in CO2 emissions. This allows the constantly increasing demands of legislation and the market to be met. Increasingly strict limits regarding exhaust gases, fuel consumption, and emissions, especially CO2 emissions, can be met.

[0041] Furthermore, a method for producing such a piston blank is proposed, wherein the piston blank is made of a steel alloy having a chromium content of 0.5 to 2 percent by weight and a silicon content of 2.5 to 3.5 percent by weight.

[0042] The piston blank can be a cast component. The piston blank can also be a forged component. Furthermore, the piston blank can also be a cast component that is reworked using a forging process. In this process, the piston lower part and the piston upper part can be manufactured separately. The piston lower part and the piston upper part are firmly connected to one another, in particular welded, to form the aforementioned intermediate component or the piston. To form the piston from the intermediate component, the intermediate component can be machined using a machining and / or forming process.

[0043] The embodiments and features described for the proposed piston blank apply to the proposed piston and the proposed method accordingly and vice versa.

[0044] "One" in this case is not necessarily limited to a single element. Rather, multiple elements, such as two, three, or more, may be included. Any other counting term used here should not be understood as implying a limitation to the exact number of elements mentioned. Rather, numerical deviations upwards and downwards are possible, unless otherwise stated.

[0045] Further possible implementations of the piston blank, the piston, and / or the method also include combinations of features or embodiments described above or below with respect to the exemplary embodiments that are not explicitly mentioned. In this case, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the piston blank, the piston, and / or the method.

[0046] Further advantageous embodiments and aspects of the piston blank, the piston, and / or the method are the subject of the dependent claims and the exemplary embodiments of the piston blank, the piston, and / or the method described below. The piston blank, the piston, and / or the method are explained in more detail below using preferred embodiments with reference to the accompanying figures.

[0047] Fig. 1 shows a schematic side view of an embodiment of a vehicle!

[0048] Fig. 2 shows a schematic sectional view of an embodiment of a piston for an internal combustion engine;

[0049] Fig. 3 shows the detailed view III according to Fig. 2;

[0050] Fig. 4 shows a schematic perspective partial sectional view of the piston according to Fig. 2; Fig. 5 shows a schematic sectional exploded view of an embodiment of a piston blank for the piston according to Fig. 2;

[0051] Fig. 6 shows a schematic sectional view of an intermediate component for the piston according to Fig. 2; and

[0052] Fig. 7 shows a schematic block diagram of an embodiment of a method for producing the piston blank according to Fig. 2.

[0053] In the figures, identical or functionally equivalent elements have been given the same reference numerals unless otherwise stated.

[0054] Fig. 1 shows a schematic side view of an embodiment of a vehicle 1. The vehicle 1 is a motor vehicle, in particular a passenger car. The vehicle 1 can also be a commercial vehicle, for example a truck, a harvester, or a construction machine. Furthermore, the vehicle 1 can also be a military vehicle. Furthermore, the vehicle can also be an aircraft, a watercraft, or a rail vehicle. However, it is assumed below that the vehicle 1 is a motor vehicle, in particular a passenger car.

[0055] The vehicle 1 comprises a body 2, which encloses a passenger compartment or vehicle interior 3 of the vehicle 1. A driver and passengers can be located in the vehicle interior 3. The body 2 separates an environment 4 of the vehicle 1 from the vehicle interior 3. The vehicle interior 3 is accessible from the environment 4 by means of doors.

[0056] The vehicle 1 comprises a chassis with several wheels 5, 6. The number of

[0057] Wheels 5, 6 are basically arbitrary. Preferably, the vehicle 1 has four

[0058] Wheels 5, 6. However, vehicle 1 may, for example, have six wheels 5, 6. Wheels 5, 6 are part of a chassis of vehicle 1. Only two wheels 5, 6 may be driven. However, all wheels 5, 6 may also be driven. In this case, vehicle 1 is an all-wheel drive vehicle.

[0059] The vehicle 1 comprises an internal combustion engine or a combustion engine 7. The combustion engine 7 can be a diesel engine or a gasoline engine. The vehicle 1 can be powered solely by the combustion engine 7. However, the vehicle 1 can also be a hybrid vehicle. In this case, the vehicle 1 has at least one electric motor in addition to the combustion engine 7. The combustion engine 7 comprises an engine block and a plurality of pistons accommodated in piston bores of the engine block. For example, the combustion engine 7 can have three, four, five, six, or more than six pistons.

[0060] Fig. 2 shows a schematic sectional view of an embodiment of a piston 8 for the internal combustion engine 7. Fig. 3 shows the detailed view III according to Fig. 2. Fig. 4 shows a schematic perspective partial sectional view of the piston 8. In the following, reference is made simultaneously to Figs. 2 to 4.

[0061] The piston 8 can be part of a vehicle 1 as previously explained, in particular the internal combustion engine 7. However, the piston 8 is particularly preferably part of a commercial vehicle. In this case, the vehicle 1 is a commercial vehicle. The internal combustion engine 7 and thus the piston 8 can be used in any vehicle 1, ship, machine, or the like. Furthermore, the internal combustion engine 7 or the piston 8 can also be used for stationary applications, such as for generators, power, heat, or the like. The piston 8 can comprise a symmetry or central axis 9, to which the piston 8 can be constructed essentially rotationally symmetrically. The piston 8 is assigned a coordinate system with a width direction or x-direction x, a height direction or y-direction y, and a depth direction or z-direction z. The y-direction y can also be referred to as the axial direction.The terms "y-direction" and "axial direction" are therefore interchangeable. The x, y, and z directions are oriented perpendicular to each other. The central axis 9, in particular, coincides with the y-direction y or is oriented parallel to it. Furthermore, a radial direction R is assigned to the piston 8. The radial direction R is oriented perpendicular to the central axis 9 and points away from it.

[0062] The piston 8 has a piston base or piston skirt 10 and a piston head 11. Viewed along the central axis 9, the piston skirt 10 is arranged below the piston head 11. The piston skirt 10 has a piston hub with a pin bore 12, in which a pin (not shown) for coupling the piston 8 to a connecting rod (not shown) of the internal combustion engine 7 can be received. A symmetry or central axis 13 of the pin bore 12 intersects the central axis 9 or is offset from it. Furthermore, the central axis 13 is oriented perpendicular to the central axis 9. The central axis 13 coincides with the z-direction z or is oriented parallel to it.

[0063] In the orientation of Fig. 2, a skirt section 14, 15 is provided on each side of the piston hub. A first skirt section 14 and a second skirt section 15 are provided. The skirt sections 14, 15 can be cylindrical in section. In other words, the skirt sections 14, 15 can form part of a cylinder that is rotationally symmetrical to the central axis 9. The skirt sections 14, 15 together form what is known as a piston skirt of the piston 8. The skirt sections 14, 15 can be rotationally symmetrical to the central axis 9 in section. In this case, however, the skirt sections 14, 15 do not, in particular, form a complete cylinder. One of the skirt sections 14, 15 forms a pressure side of the piston 8, while the other of the skirt sections 14, 15 forms a counter-pressure side of the piston 8.

[0064] The skirt sections 14, 15 are connected to one another by means of wall sections 16, 17. A first wall section 16 and a second wall section 17 are provided. The radial direction R points away from the central axis 9 in the outward direction of the skirt sections 14, 15. The pin bore 12 penetrates the wall sections 16, 17. The skirt sections 14, 15 and the wall sections 16, 17 enclose an interior space 18 of the piston skirt 10. The interior space 18 is open downwards in the orientation of Figs. 2 to 4. The aforementioned pin for coupling the piston 8 to the connecting rod runs along the central axis 13 through the interior space 18.

[0065] The piston 8 has a cooling channel 19 that completely surrounds the central axis 9 and is preferably rotationally symmetrical thereto. The cooling channel 19 is, in particular, toroidal. The cooling channel 19 has a wall 20 that defines a geometry or a cross-sectional geometry of the cooling channel 19. A cooling oil, in particular engine oil, can be conducted through the cooling channel 19 in order to dissipate heat Q introduced into the piston 8 during operation. For this purpose, the cooling oil can be injected into the cooling channel 19 using an injection nozzle arranged below the piston 8 in the orientation shown in Figs. 2 to 4.

[0066] The cooling channel 19 is in fluid communication with the interior 18 by means of a plurality of bores 21, 22. The number of bores 21, 22 is fundamentally arbitrary. Preferably, a plurality of bores 21, 22 are provided, which can be arranged evenly distributed around the central axis 9. The bores 21, 22 can also be arranged unevenly distributed around the central axis 9. For example, during operation of the piston 8 in the orientation of Fig. 2 to 4, cooling oil can be injected from below into the interior 18 with the aid of the aforementioned injection nozzle. At least a portion of the cooling oil flows through the bores 21, 22 into the cooling channel 19 and out again from there. Heat Q is then dissipated from the piston 8 with the cooling oil.

[0067] The piston head 11 has a piston crown 23, which faces a cylinder head of the internal combustion engine 7. A large part of the heat Q is also introduced into the piston crown 23. The piston crown 23 faces, in particular, a combustion chamber 24 of the internal combustion engine 7. The piston crown 23 comprises an annular piston crown section 25, which defines a plane oriented perpendicular to the central axis 9. Furthermore, the piston crown 23 has a combustion chamber recess 26, which is set back from the piston crown section 25. Viewed along the central axis 9 or along the y-direction y, the combustion chamber recess 26 is thus offset or recessed from the piston crown section 25.

[0068] The combustion chamber bowl 26 can have any desired geometry. In the present case, the combustion chamber bowl 26 has a shoulder 27 which runs around the central axis 9 and is set back from the piston crown section 25 when viewed along the y-direction y. A combustion chamber bowl edge 28 of the combustion chamber bowl 26 projects radially into the combustion chamber bowl 26 when viewed opposite to the radial direction R. Adjacent to the combustion chamber bowl edge 28 is a rounded portion 29 running around the central axis 9. The rounded portion 29 merges into a combustion chamber bowl bottom 30, which is in particular conical or tapered and extends upwards when viewed along the y-direction y. However, the combustion chamber bowl bottom 30 ends below the shoulder 27 when viewed along the y-direction y. A first wall 31 (Fig. 3) is provided between the combustion chamber bowl 26 and the cooling channel 19. The first wall 31 fluidically separates the cooling channel 19 from the combustion chamber bowl 26.The first wall 31 extends completely around the central axis 9. The first wall 31 has a first wall thickness w31. The first wall thickness w31 is at least 5 mm. In particular, an average of the first wall thickness w31 over the entire first wall 31 is at least 5 mm. This means that the first wall 31 can also have a smaller first wall thickness w31 than 5 mm in some regions or locally. However, viewed along a main extension direction of the first wall 31, in this case the y-direction y, the first wall thickness w31 is always at least 5 mm on average. The "main extension direction" is understood here to mean a direction, in this case the y-direction y, along which the first wall 31 has its greatest geometric extent.

[0069] A second wall 32 (Fig. 3) separates the combustion chamber bowl 26 from the interior 18. The second wall 32 forms the combustion chamber bowl bottom 30 on the front side. On the rear side, i.e. facing the interior 18, the second wall 32 forms a so-called inner shape 33 of the piston 8. The inner shape 33 can be conical or tapered. The second wall 32 has a second wall thickness w32. The second wall thickness w32 is also at least 5 mm. In particular, an average of the second wall thickness w32 over the entire second wall 32 is at least 5 mm. This means that the second wall 32 can also have a smaller second wall thickness w32 than 5 mm in some regions or locally. However, viewed along a main direction of extent of the second wall 32, in this case the x-direction x, the second wall thickness w32 is always at least 5 mm on average.

[0070] The piston 8 has a piston diameter d8. The piston diameter d8 is defined as a diameter of the smallest possible cylinder that encloses the piston skirt, i.e. the skirt sections 14, 15. This cylinder is oriented perpendicular to the central axis 13. The first wall thickness w31 is at least on average greater than 5% of the piston diameter d8. Preferably, the first wall thickness w31 is at least on average greater than 6% of the piston diameter d8. Particularly preferably, the first wall thickness w31 is at least on average greater than 7% of the piston diameter d8. However, the first wall thickness w31 is at least on average 5 mm. The second wall thickness w32 is also at least on average greater than 5% of the piston diameter d8. Preferably, the second wall thickness w32 is at least on average greater than 6% of the piston diameter d8. Particularly preferably, the second wall thickness w32 is at least on average greater than 7% of the piston diameter d8.However, the minimum second wall thickness w32 is 5 mm on average.

[0071] Any number of cutting planes E (Fig. 3) can be laid through the central axis 9. The central axis 9 lies in each of these cutting planes E. For each cutting plane E, in particular, the first wall thickness w31 and / or the second wall thickness w32 is on average at least 5%, preferably 6%, more preferably 7%, of the piston diameter d8 and / or is at least 5 mm. For example, the average wall thicknesses w31, w32 can each be calculated separately from one another. The respective average wall thickness w31, w32 can be calculated along a cutting contour of the first wall 31 and / or the second wall 32 located in the respective cutting plane E with a constant step size or a constant increment of not more than 1 mm.In particular, the calculation of the wall thicknesses w31, w32 is carried out along a line formed by the aforementioned cutting contour, which is formed by a section of the cutting plane E with surfaces of the combustion chamber bowl 26 and the piston crown section 25.

[0072] The first wall thickness w31 of the first wall 31 is defined in particular as a smallest distance between the combustion chamber bowl 26, in particular the rounded portion 29, and the cooling channel 19, in particular the wall 20 of the cooling channel 19. The second wall thickness w32 of the second wall 32 is defined in particular as a smallest distance between the combustion chamber bowl 26, in particular the combustion chamber bowl bottom 30, and the inner shape 33. The first wall 31 merges into the second wall 32 at the rounded portion 29, or vice versa. This means, in particular, that the first wall 31 is connected to the second wall 32.

[0073] A ring section or ring field 34 is provided on the piston head 11. The ring field 34 forms, in particular, a substantially cylindrical outer surface of the piston head 11, which can be constructed rotationally symmetrically to the central axis 9. The ring field 34 has a plurality of ring grooves 35 arranged one above the other as viewed along the y-direction y, of which only one is provided with a reference symbol in Fig. 2. The ring grooves 35 are suitable for receiving piston rings. For example, two or three such ring grooves 35 are provided. A top land 36 adjoining the piston crown 23 is part of the ring field 34. However, the top land 36 does not have a ring groove 35 for receiving a piston ring as mentioned above.

[0074] The piston 8 is two-part and comprises a lower piston part 37 and an upper piston part 38. The lower piston part 37 and the upper piston part 38 are two separate components which are joined together to form the piston 8. In integrally bonded connections, the connecting partners are held together by atomic or molecular forces. Integral connections are non-detachable connections that can only be separated by destroying the connecting means and / or the connecting partners. For example, integral connections can be achieved by gluing, soldering, or welding. For example, the lower piston part 37 is welded to the upper piston part 38, in particular friction welded.

[0075] Piston 8 is designed to achieve a higher combustion chamber surface temperature in combustion chamber 24 compared to conventional pistons, thus resulting in hotter combustion. Increasing the combustion chamber surface temperature can increase the thermodynamic efficiency of engine combustion. This can result in advantages in terms of fuel consumption and CO2 emissions.

[0076] For conventional pistons, especially for conventional steel pistons, materials such as 42CrMo4 heat-treatable steel or 38MnVS6 micro-alloyed steel can be used. These pistons can either be constructed as a single piece or comprise a piston lower and upper sections joined together by a joining operation. Typically, the entire piston, even in two-piece designs, is made of the same material.

[0077] Such a piston is cooled by a cooling oil jet injected into a circumferential, annular cooling channel via an injection nozzle. A specific minimum volume flow of cooling oil is required to ensure appropriate heat dissipation and keep the highly stressed areas, particularly the combustion bowl rim, below the critical temperature for piston scaling. Both of the above-mentioned materials, 42CrMo4 and 38MnVS6, have a thermal conductivity determined by their chemical composition and similar for both materials. This results in a certain combustion chamber surface temperature with standard piston cooling. This temperature cannot be increased any further, as both materials have limited scaling resistance. Therefore, an increase in the combustion chamber surface temperature could lead to scaling-related cracking and thus to piston failure.The increasing demands for reducing fuel consumption and CO2 emissions are therefore often met by developing friction-optimized piston systems. For example, optimized piston skirt profiles and installation clearances are used, as well as cost-intensive ring packages with special coatings such as amorphous carbon (Diamond Carbon, DLC), or complex surface optimizations for cylinder liners. The reduction of fuel consumption and CO2 emissions is to be improved with the help of the previously discussed piston 8.

[0078] For this purpose, a steel alloy with high scaling resistance and low thermal conductivity is used, at least for the area of ​​the combustion chamber bowl 26 of piston 8. Due to the lower thermal conductivity, the combustion chamber surface temperature increases even with standard piston cooling, which leads to an increase in the thermodynamic efficiency of engine combustion.

[0079] A low-alloy steel alloy with the following chemical composition is particularly suitable for piston 8:

[0080] Carbon C- 0.35 to 0.5 percent by weight, in particular 0.4 to 0.44 percent by weight.

[0081] Silicon Si: 2.5 to 3.5 weight percent, in particular 2.85 to 3 weight percent.

[0082] Chromium CE 0.5 to 2% by weight, in particular 0.9 to 1.2% by weight.

[0083] Manganese Mn: 0.5 to 0.9 weight percent, especially 0.6 to 0.8 weight percent. Titanium Ti: 0.005 to 0.015 weight percent.

[0084] Molybdenum Mo: 0.1 to 0.3 weight percent, in particular 0.15 to 0.2 weight percent.

[0085] In addition to other alloying components, the steel alloy primarily contains the element iron (Fe). The steel alloy exhibits increased scaling resistance at 550 to 650 °C, especially at 580 to 600 °C.

[0086] In the event that the piston 8 is a one-piece component and thus not divided into the piston lower part 37 and the piston upper part 38, the entire piston 8 is made of this steel alloy. In the event that the piston 8 is two-piece and has the piston upper part 38 separate from the piston lower part 37, only the piston upper part 38, which has the combustion chamber bowl 26, can be made of the steel alloy. For the piston lower part 37, the heat-treatable steel 42CrMo4 or the micro-alloyed steel 38MnVS6 are then preferably used.

[0087] Through design measures, the increase in the combustion chamber surface temperature can be further increased by reducing the cooling effect in the area of ​​the combustion chamber bowl 26. This is possible because the steel alloy, due to its higher scaling resistance, can withstand a higher surface temperature in the combustion chamber bowl 26 without causing failure of the piston 8. This increase in the combustion chamber surface temperature can be achieved by the measures already explained and combined with each other. Firstly, by increasing the wall thicknesses w31, w32 in the area of ​​the combustion chamber bowl 26, the dissipation of heat Q by the cooling oil is further reduced and thus leads to an additional increase in the combustion chamber surface temperature and thus the thermodynamic

[0088] efficiency.

[0089] By adapting the geometry of the cooling channel 19, an increase in the combustion chamber surface temperature and the thermodynamic efficiency can also be achieved. The cooling channel 19 can be equipped with a smaller cross-section compared to known pistons, thereby reducing the dissipation of heat Q. This measure is also advantageous with regard to the dimensions and height of the piston 8. Alternatively, the cooling channel 19 can be designed as an open cooling channel, which is sprayed with cooling oil via a freely accessible inner surface. Furthermore, it is also possible to dispense with the cooling channel 19 entirely.

[0090] Furthermore, the amount of cooling oil used to cool piston 8 can be reduced. This also increases the combustion chamber surface temperature and thus the thermodynamic efficiency. Furthermore, an additional efficiency advantage results from the reduction of the power loss of an oil pump used to deliver the cooling oil, which indirectly contributes to fuel savings.

[0091] The use of the steel alloy with low thermal conductivity leads to a reduction in the dissipation of heat Q from the combustion chamber bowl 26 to the cooling channel 19 and thus to an increase in the surface temperature of the combustion chamber bowl 26 and the combustion chamber 24. The steel alloy has a thermal conductivity that is approximately 20 W / m*K lower than that of the materials 42CrMo4 or 38MnVS6. Simulations have shown that the combustion chamber bowl edge temperature at the combustion chamber bowl edge 28 increases by 2 K for every 1 W / m*K of reduced thermal conductivity. The additional design measures listed above, such as increasing the wall thicknesses w31, w32, adapting the cooling channel geometry of the cooling channel 19, or reducing the amount of cooling oil, have an analogous effect, leading to a higher combustion chamber surface temperature by reducing the dissipation of heat Q.As a result, the thermodynamic efficiency of combustion can be increased, thus saving fuel and reducing CO2 emissions. This requires sufficient scaling resistance of the steel alloy.

[0092] In addition to its low thermal conductivity, the steel alloy is also highly resistant to scaling, meaning that the threshold temperature at which technically relevant scaling occurs can be shifted to a higher temperature by at least 70 K. Due to the steel alloy's high scaling resistance, additional combustion-related measures can also be taken to increase the combustion chamber surface temperature and improve thermodynamic efficiency.

[0093] By using the highly scaling-resistant and simultaneously low-thermal-conductive steel alloy, at least as a material for the piston crown 38, the thermodynamic efficiency of the internal combustion engine 7 can be increased. This allows for fuel consumption advantages and CO2 emissions benefits to be realized.

[0094] Fig. 5 shows a schematic sectional exploded view of an embodiment of a piston blank 39 for the piston 8.

[0095] The piston 8 can be manufactured using the piston blank 39. The piston blank 39 comprises a piston lower part 37 as mentioned above and a piston upper part 38 as mentioned above. The piston blank 39 differs from the piston 8 in that the piston lower part 37 is not yet connected to the piston upper part 38. The piston blank 39 can additionally differ from the piston 8 in that the piston 8 is machined using a removal process after the piston lower part 37 is welded to the piston upper part 38. Possible removal processes include, for example, erosion, milling, turning, or the like. Furthermore, the piston blank 39 can also be formed using a forming manufacturing process, for example a forging process, to form the piston 8.

[0096] The center axis 9 can be assigned to the piston blank 39. The previously mentioned coordinate system with the directions x, y, z can also be assigned to the piston blank 39. Furthermore, the radial direction R can also be assigned to the piston blank 39.

[0097] The cooling channel 19 is formed partially on the piston lower part 37 and partially on the piston upper part 38. In particular, a first cooling channel section 19A is provided on the piston lower part 37. A second cooling channel section 19B can be provided on the piston upper part 38. The cooling channel sections 19A, 19B together form the cooling channel 19. The piston upper part 38 has the combustion chamber bowl 26, which can be machined to produce the piston 8 from the piston blank 39 using an abrasive or forming manufacturing process in order to produce the final geometry of the combustion chamber bowl 26 shown in Figs. 2 to 4.

[0098] As previously mentioned, the piston lower part 37 and the piston upper part 38 are two separate components which can be joined to one another in a materially bonded manner, in particular by welding. For this purpose, the piston lower part 37 has a first joining surface 40 which runs annularly around the central axis 9, and a second joining surface 41 which runs annularly around the central axis 9. Viewed along the radial direction R, the second joining surface 41 is positioned within the first joining surface 40. Correspondingly, the piston upper part 38 has a first joining surface 42 which runs annularly around the central axis 9, and a second joining surface 43 which runs annularly around the central axis 9. Viewed along the radial direction R, the second joining surface 43 is positioned within the first joining surface 42.

[0099] The piston lower part 37 can further have a circumferential shoulder 44, which extends radially outward from the piston lower part 37 when viewed along the radial direction R. The shoulder 44 is optional. The piston lower part 37 and the piston upper part 38 are each integral, in particular single-piece, components. "Integral" or "one-piece" in this context means that the piston lower part 37 and the piston upper part 38 are not composed of different sub-components, but rather each form a single component.

[0100] At least the piston crown 38 is made, at least in sections, from the aforementioned highly scaling-resistant steel alloy. In particular, in the area of ​​the combustion chamber bowl 26, the piston crown 38 is made from the steel alloy. The piston crown 37 can be made, for example, from the materials 42CrMo4 or 38MnVS6. Alternatively, however, the piston crown 37 can also be made from the same highly scaling-resistant steel alloy from which the piston crown 38 is made. The piston blank 39 can also be a one-piece component. In this case, the piston crown 37 and the piston crown 38 are not two separate components that are subsequently joined together. The piston blank 39 is then made entirely from the highly scaling-resistant steel alloy.

[0101] "Integral" means that the piston lower part 37 and the piston upper part 38 are each made entirely of the same material. The piston 8 itself, or the piston blank 39, however, is made of multiple parts. The piston lower part 37 is preferably a cast component. The piston upper part 38 can also be a cast component. Furthermore, the piston lower part 37 can also be a forged component. The piston upper part 38 can also be a forged component. If the piston lower part 37 and / or the piston upper part 38 are each cast components, they can be reworked using a forging process. However, the piston lower part 37 and / or the piston upper part 38 can also be manufactured or machined using a machining process.

[0102] Fig. 6 shows a schematic sectional view of an intermediate component 45 for the piston 8.

[0103] To form the intermediate component 45, the piston lower part 37 and the piston upper part 38 are joined together at their joining surfaces 40 to 43, forming joining planes 46, 47. The joining planes 46, 47 can be weld seams, in particular friction weld seams. The first joining surfaces 40, 42 and the second joining surfaces 41, 43 are each firmly connected to one another. The intermediate component 45 differs from the piston blank 39 in that the piston lower part 37 is firmly connected, in particular welded, to the piston upper part 38. Friction welding, for example, is a suitable welding process.

[0104] The piston 8 differs from the intermediate component 45 in that, in contrast to the intermediate component 45, the piston 8 is reworked using a machining and / or forming process. For example, to produce the piston 8 from the intermediate component 45, the combustion chamber bowl 26 is machined, the annular zone 34 is formed onto the intermediate component 45, the shoulder 44 is removed, and protruding beads of the joining plane 46 are removed. Furthermore, a cylindrical outer surface 48 of the intermediate component 45 can be machined to produce the annular zone 34. Fig. 7 shows a schematic block diagram of an embodiment of a

[0105] Method for producing the piston blank 39.

[0106] In the method, in a step S1, the piston blank 39 is manufactured from the steel alloy having a chromium content of 0.5 to 2 percent by weight and a silicon content of 2.5 to 3.5 percent by weight. Step S1 can comprise casting, forming, and / or machining the steel alloy. Furthermore, in step S1, the piston lower part 37 and the piston upper part 38 can be manufactured as separate components. In this case, at least the piston upper part 38 is manufactured from the steel alloy. Alternatively, the piston blank 39 can also be manufactured as a single-piece component in step S1. In this case, the piston lower part 37 and the piston upper part 38 are not two separate components.

[0107] The method may include a step S2 in which the piston lower part 37 and the piston upper part 38 are joined or assembled to form the intermediate component 45. If the piston lower part 37 and the piston upper part 38 are joined together, the piston lower part 37 and the piston upper part 38 are bonded together, in particular welded, at the joining surfaces 40 to 43. Preferably, the piston lower part 37 and the piston upper part 38 are friction welded together at the joining surfaces 40 to 43.

[0108] Although the present invention has been described using exemplary embodiments, it is capable of being modified in many ways.

[0109] 1 vehicle

[0110] 2 Body

[0111] 3 Vehicle interior

[0112] 4 Surroundings

[0113] 5 wheel

[0114] 6 wheel

[0115] 7 Combustion engine

[0116] 8 pistons

[0117] 9 Central axis

[0118] 10 piston skirt

[0119] 11 Piston head

[0120] 12 bolt hole

[0121] 13 Central axis

[0122] 14 Shaft cut

[0123] 15 shaft cut

[0124] 16 wall section

[0125] 17 wall section

[0126] 18 Interior

[0127] 19 Cooling channel

[0128] 19 A Cooling channel section

[0129] 19B Cooling channel section

[0130] 20 wall

[0131] 21 Hole

[0132] 22 Hole

[0133] 23 Piston crown

[0134] 24 combustion chamber

[0135] 25 Piston crown section

[0136] 26 Combustion chamber bowl 27 Paragraph

[0137] 28 Combustion chamber bowl edge

[0138] 29 Rounding

[0139] 30 combustion chamber bowl floor

[0140] 31 Wall

[0141] 32 Wall

[0142] 33 Inner shape

[0143] 34 Ringfeld

[0144] 35 ring groove

[0145] 36 Firebridge

[0146] 37 piston lower part

[0147] 38 piston top

[0148] 39 piston blank

[0149] 40 joining surface

[0150] 41 Joining surface

[0151] 42 joining surface

[0152] 43 Joining surface

[0153] 44 paragraph

[0154] 45 Intermediate component

[0155] 46 joining level

[0156] 47 joining level

[0157] 48 Outer surface d8 Piston diameter

[0158] E cutting plane

[0159] Q Heat

[0160] R Radial direction

[0161] 51 steps

[0162] 52 Step w31 Wall thickness w32 Wall thickness x x-direction y y-direction z z-direction

Claims

PATENT CLAIMS 1. Piston blank (39) for a piston (8), wherein the piston blank (39) is made at least partially from a steel alloy having a chromium content of 0.5 to 2 wt% and a silicon content of 2.5 to 3.5 wt%.

2. Piston blank according to claim 1, characterized in that the chromium content is 0.9 to 1.2 percent by weight and / or the silicon content is 2.85 to 3 percent by weight.

3. Piston blank according to claim 1 or 2, characterized in that the steel alloy has a carbon content of 0.35 to 0.5, in particular of 0.4 to 0.44, by weight percent.

4. Piston blank according to one of claims 1 - 3, characterized in that the steel alloy has a manganese content of 0.5 to 0.9, in particular of 0.6 to 0.8, by weight percent.

5. Piston blank according to one of claims 1 - 4, characterized in that the steel alloy has a titanium content of 0.005 to 0.015 percent by weight.

6. Piston blank according to one of claims 1-5, characterized in that, that the steel alloy has a molybdenum content of 0.1 to 0.3, in particular of exhibits 0.15 to 0.2 percent by weight.

7. Piston blank according to one of claims 1 - 6, characterized in that the steel alloy has an increased resistance to scaling at 550 to 650 °C, in particular at 580 to 600 °C.

8. Piston blank according to one of claims 1 - 7, characterized in that the piston blank (39) has a piston lower part (37) and a piston upper part (38), wherein the piston upper part (38) is made of the steel alloy, wherein the piston lower part (37) is made of the steel alloy or of a further material different from the steel alloy, and wherein the further material in particular has a higher thermal conductivity than the steel alloy.

9. Piston blank according to one of claims 1 - 7, characterized in that the piston blank (39) is a one-piece component made entirely of the steel alloy.

10. Piston (8) with a piston blank (39) according to one of claims 1 - 9.

11. Piston according to claim 10, characterized by a combustion chamber bowl (26) and a cooling channel (19) circumferencing at least partially around the combustion chamber bowl (26), wherein an average first wall thickness (w31) of a first wall (31) provided between the combustion chamber bowl (26) and the cooling channel (19) is greater than 5 percent, preferably greater than 6 percent. percent, especially preferably greater than 7 percent, of a piston diameter (d8) of the piston (8).

12. Piston according to claim 11, characterized in that the average first wall thickness (w31) is at least 5 millimeters.

13. Piston according to claim 11 or 12, characterized in that an average second wall thickness (w32) of a second wall (32) provided between the combustion chamber recess (26) and an inner form (33) of the piston (8) is greater than 5 percent, preferably greater than 6 percent, particularly preferably greater than 7 percent, of the piston diameter (d8).

14. Piston according to claim 13, characterized in that the average second wall thickness (w32) is at least 5 millimeters.

15. Method for producing a piston blank (39), wherein the piston blank (39) is made of a steel alloy having a chromium content of 0.5 to 2 wt% and a silicon content of 2.5 to 3.5 wt%.

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