Piston for an internal combustion engine and method for its production

Abstract

The present invention relates to a piston for use in internal combustion engines, having a piston crown, an annular cooling channel and in each case two skirt and box walls which adjoin the piston crown, wherein a skirt wall extends in each case between two box walls, wherein the box walls have pin bores which are designed to receive gudgeon pins, wherein the cooling channel has inlets and outlets for coolant, and wherein at least one flat support extends in the region of the inlets and outlets for coolant between an outer wall of the pin bore and the adjacent skirt walls, which support contains the respective inlet and outlet, and the surface of which, which faces away from the piston crown, has a largely flat portion which extends substantially in the radial direction with respect to the centre axis of the pin bore.

Description

[0001] Piston for an internal combustion engine and method for its production

[0002] Technical area

[0003] The present invention relates to a piston for use in internal combustion engines and a method for its production.

[0004] State of the art

[0005] Pistons are used in internal combustion engines to convert the thermal energy from fuel combustion into mechanical energy to move a vehicle. The pistons are therefore used in an environment in which they are exposed to high pressures and, at the same time, high temperatures. For modern and efficient diesel engines, steel pistons are currently designed for pressures of 250 to 300 bar. Such pistons require a piston geometry that can withstand these loads with an acceptable stress distribution and without plastic deformation, and that has a high level of operational strength. High stresses occur in particular in the area where the force is introduced from the piston crown to the piston boss, the so-called block support.Furthermore, a bore near the block support is required for cooling the piston via the annular cooling channel, which introduces a further stress concentration in this already highly stressed area.

[0006] Typically, the fillet radius in the block support between the pin boss and the lower edge of the cooling channel is increased if the stress concentration severely compromises the strength of the piston. Sufficient strength cannot always be achieved by increasing the radius, as the installation space in the area of ​​the inlet and outlet bores of the cooling channel is limited, and increasing the radius is not the most efficient design change for stress reduction.

[0007] State of the art which describes pistons is described in WO 2017 / 102 252 Al, DE 3 917 755 C2, DE 10 2007 018 932 Al, DE 10 2018 109 205 Al, DE 10 2019 204 559 Al, DE 10 2020 007 484 Al and US D886 155 S.

[0008] Description of the invention

[0009] The inventors have discovered that a special design of the piston in the area of ​​the block support leads to increased strength and dissipates stresses particularly efficiently. The invention is defined by the independent claims. Preferred embodiments are described in the respective dependent claims.

[0010] According to claim 1, a piston for use in internal combustion engines has a piston skirt and two skirt and box walls which are arranged alternately along the circumference of the piston, so that two skirt walls enclose a box wall (and correspondingly two box walls also enclose a skirt wall). While the skirt wall typically follows the outer contour of the piston, the box walls are slightly set back. Due to the position of the connecting rod, different levels of lateral forces act on the two skirt walls. The side which is loaded during the power stroke is referred to as the pressure side, the opposite side as the counterpressure side.

[0011] The box walls are provided with pin bores designed to accommodate piston pins, i.e., into which such piston pins can be inserted. Typically, these pin bores have a round cross-section, possibly with a shaped bore, and are accordingly largely cylindrical. The axis of the pin bore can intersect the cylinder axis at a single point, but can also be at a certain distance from it, the so-called pin offset. The outer wall of the pin bore, i.e., the wall that encloses the pin bore, is called the pin boss.

[0012] The inventors have found that due to the sliding contact between the pin and the pin bore, essentially only normal forces can be transferred from the piston to the pin, which means that the force flow must be in a radial direction onto the pin bore. Due to the ovalization of the pin, the maximum contact stresses in the pin bore are at a certain angle ß to the apex of the pin bore, which depends on the ignition pressure and the pin geometry. Typical values ​​for ß are between 30 ° and 60 °. In a conventional design with a fillet radius on the block support between the pin eye and the lower edge of the cooling channel, the radial force flow is disrupted and a stress concentration arises which is critical at higher ignition pressures.

[0013] According to the invention, the transition between a pin boss and the lower edge of the cooling channel is formed at least at one point by a flat support, which is preferably located in the region of the inlet and / or outlet of the cooling channel on the pressure side of the piston. The flat support extends between the outer wall of the pin bore and the associated skirt wall. The support has a largely flat section on the surface facing away from the piston crown. According to the invention, this largely flat section extends essentially in the radial direction with respect to the cylindrical surface of the pin bore. The support is delimited to the connecting rod pivot plane by a further surface, which essentially represents a continuation of the surface above the piston boss.Compared to DE 10 2019 204559 A1, the inventive design differs in that it features a flat support that contains the bore of the cooling channel inlet, instead of a rib positioned next to the cooling channel inlet. This shape is better suited for the forging manufacturing process, as the material can flow better and less wear is generated on the forging die.

[0014] It is preferred that the substantially flat portion extends substantially in a radial direction forming an angle a of 50-70°, preferably 60°, to the apex of the pin bore. Such a piston exhibits a significant reduction in stress and thus a greatly improved service life.

[0015] Preferably, the flat section of the support extends over a radius into the shaft wall and / or the lower edge of the cooling channel and over a further radius into the bolt eye. The two radii can be selected independently of each other.

[0016] Optionally, the flat section of the support can deviate from the radial direction and be offset parallel to it by a certain amount, namely up to ±5% of the piston diameter.

[0017] The piston according to the invention has a cooling channel which is provided below the piston crown. Such a cooling channel is typically annular and is formed, for example, by welding an upper and lower part. Friction welding can be used as the welding process. However, other welding processes are also conceivable. The cooling channel can also be created using salt cores (e.g. in the case of aluminum pistons). The cooling channel has inlets and outlets for coolant, which is typically oil. The coolant is injected through the inlet, then flows through the cooling channel and exits the outlet, thereby cooling the piston. The inlets and outlets are arranged in the space enclosed by the skirt and box walls.At least one, preferably both, of the inlets and outlets are formed in the area of ​​the planar support, meaning that the planar support forms part of the enclosure of the inlet and / or outlet. Such a design has proven particularly useful for reasons of operational strength.

[0018] The position of the cooling channel outlet can be freely selected. The position of the cooling channel inlet, however, is determined by the position of the oil spray nozzle. If this is positioned too far toward the connecting rod pivot plane of the piston, the wall thickness of the reinforcing rib is reduced, resulting in further stress concentration. Therefore, a minimum wall thickness of 40-60% of the diameter of the cooling channel inlet bore is preferred.

[0019] If the installation space is severely restricted by a wide connecting rod, the minimum wall thickness in the area of ​​the cooling channel inlet can be reduced. To compensate for the stress concentration at the apex of the boundary line between the inlet bore and the flat support, a local thickening in the normal direction can be used. This local thickening should amount to 10-20% of the diameter of the inlet bore at the apex and should merge tangentially into the contour of the flat support at a distance of the radius of the inlet bore. At very high ignition pressures, the minimum wall thickness and local thickening measures can be combined. This leads to a particularly effective reduction in stress in the area of ​​the cooling channel inlet. It is also preferred that the geometric shape of the flat support can be demolded inwards along the bolt axis. This leads to better manufacturability.

[0020] Furthermore, a method according to the invention in which a piston is produced as described above is provided. At least one of the inlets and outlets is formed by drilling starting from the cooling channel to be formed. By drilling the inlet and outlet of the cooling channel from the side of the cooling channel, the outlet surface does not have to be flat and can thus be designed optimally to absorb the force flow. This provides increased flexibility in the design of the piston.

[0021] Short description of the drawings

[0022] Figure 1 shows a sectional view through a piston according to the invention according to a first embodiment in a section perpendicular to the pin bore.

[0023] Figure 2 shows a bottom view of the piston of Figure 1 .

[0024] Figure 3 shows a detailed view of an outlet of the cooling channel of the piston of Figure 1 .

[0025] Figure 4 shows the piston of Figure 1 in an oblique view.

[0026] Figure 5 shows a detailed view of the outlet of the cooling channel of the piston of Figure 1 .

[0027] Figure 6 shows a detailed view of the support according to the first embodiment.

[0028] Figure 7 shows a detailed view of the support according to the second embodiment. Figures 8 to 15 show simulation results for an exemplary embodiment of a piston according to the prior art and the present invention.

[0029] Detailed description of the characters

[0030] A first embodiment of the invention is described below with reference to Figures 1-6.

[0031] Figure 1 shows a sectional view of a piston 10 according to one embodiment of the invention. The piston 10, which in this case is formed by welding two components, has a piston crown 12 containing a combustion chamber bowl 14. Radially outside the combustion chamber bowl 14, an annular cooling channel 16 is provided, within which welding beads 17 extend as artifacts due to the manufacturing process of the piston 10. Further radially outward on the radial outside of the piston, an annular field 18 is provided for receiving piston rings.

[0032] The piston 10 has skirt walls 26 and box walls 28. The box walls 28 each have a pin bore 22 with a surround 32. The pin bore 22 is essentially cylindrical, with the center point M of the bore lying in a plane which is offset by a pin offset d from the plane containing the center axis of the piston 10.

[0033] The enclosure 32 of the bolt bore 22 is connected to the associated shaft wall 26 via a flat support 30. The flat support 30 has a flat section 31 (see Fig. 5), i.e. a section which is not curved or not significantly curved. This section extends, as can be seen from Figure 1, along a radial direction R which extends radially from the center Z of the bolt bore 22 to the outside of the shaft wall 26. The flat section 31 merges with two radii of curvature RI, R2 into the enclosure 32 of the bolt bore 22 (radius RI) and into the shaft wall 26 (radius R2).

[0034] The angle a formed by the radius R and the axial direction of the piston is in the range of 50° to 70° and is preferably 60°. This has proven particularly advantageous for the stability of the piston and the reduction of stresses.

[0035] Figure 2 shows a bottom view of the piston shown in Figure 1. Figure 1 is a section along line BB of Figure 2. It can be seen how an outlet 34 is designed for the cooling channel 16. An inlet 35 is arranged approximately diagonally opposite. A similar view is shown in Figure 4. Here it can also be seen that the planar support 30 is essentially flat, i.e. only slightly curved.

[0036] Figure 3 shows a further detailed view, which can be seen as a plan view of detail Z of Figure 2. Here, too, it can be seen how the flat support 30 forms part of the enclosure of the outlet of the cooling channel 34 and adjoins the enclosure 32 of the bolt bore 22.

[0037] Figure 5 shows a further detailed view of the outlet 34 shown in Figure 3. From the figure, it can be seen that the planar support 30 has a material thickness in the direction of the pin bosses in the range of 40% to 60% of the diameter of the associated inlet 34.

[0038] Figure 6 shows the planar support 30 of the piston 10 according to the first embodiment of the invention in detail. From the figure it can be seen that a large part of the surface of the support extends essentially flat. Figure 7 shows the planar support 30' in a piston according to a second embodiment of the invention. The details of the piston are otherwise identical to the piston according to the first embodiment. It can be seen in Figure 7 that a thickening 34' (thickening) is provided on the essentially flat, planar support 30'. Such a thickening 34' leads to a further reduced stress within the piston material.

[0039] A flat support is understood here as a rib-like material projection whose surface is at least partially flat, i.e., flat. The inlet s for the cooling channel is not recessed from this surface, but rather intersects it.

[0040] A largely flat curve is understood to be a curve that deviates from a completely flat curve by a maximum of 10% in the axial direction of the piston.

[0041] The invention was analyzed using extensive finite element simulations. Figures 8 to 15 show the amplitude stress for the fatigue strength calculation for different geometric variants of an exemplary piston, each at the same scale and in two different views.

[0042] Figures 8 and 9 show a piston according to the state of the art. The highest stress (here designated as 100% as a reference value) occurs at the cooling channel inlet. An engineering improvement with a circumferential reinforcement (see Figures 10 and 11) reduces the stress moderately to 92% and shifts the position of the maximum to the radius between the pin eye and the circumferential reinforcement. Only the inventive flat support with integrated cooling channel inlet according to Figures 12 and 13 results in a significant stress reduction to 71%, the position of the maximum is again at the apex of the cooling channel.

[0043] Entrance .

[0044] A further reduction to 54 % is achieved by the local thickening in the normal direction at the apex according to the invention ( see Figures 14 and 15 ) , whereby the stress at the edge line of the inlet is equalized and thus reduced .

Claims

Claims 1. Piston (10) for use in internal combustion engines, comprising a piston crown (12), an annular cooling channel (16), an annular field (18), and two skirt and box walls (26, 28) each adjoining the piston crown (12), wherein the box walls (28) have pin bores (22) designed to receive piston pins, wherein the cooling channel (16) has inlets and outlets (34, 35) for coolant, and wherein in the region of at least one of the inlets and outlets (34, 35) for coolant, at least one planar support (30) extends between an outer wall (32) of the pin bore (22) and the adjacent skirt walls (26), which contains the respective inlet and outlet (34, 35) and whose surface facing away from the piston crown (12) has a largely flat section (31) which is in Extends substantially in the radial direction to the central axis of the bolt bore (22).

2. Piston (10) according to claim 1, wherein the substantially flat portion (31) extends substantially in a radial direction to the center axis of the pin bore and the angle between a tangential plane to the planar support and a plane through the center axis of the pin bore and the apex of the pin bore (22) is between 50° and 70°, preferably 60°.

3. Piston (10) according to one of the preceding claims, wherein the flat section (31) merges over a radius (RI) into the skirt wall (26) and / or a further radius (R2) into the outer wall (32) of the pin bore (22).

4. Piston (10) according to one of the preceding claims, wherein the flat portion (31) is parallel to a radial direction within a range of ± 5% of the piston diameter.

5. Piston according to one of the preceding claims, wherein one, preferably both, of the inlets and outlets (34, 35) have a surround which has a minimum wall thickness in the range of 40-60% of the diameter of the associated inlet and outlet.

6. Piston according to one of the preceding claims, wherein one, preferably both, of the inlets and outlets (34, 35) at the apex (36) has a local thickening in the normal direction, which amounts to 10-20% of the diameter of the respective bore and merges tangentially into the contour of the planar support.

7. Piston according to one of the preceding claims, wherein the piston (10) is a steel piston.

8. A method for producing a piston according to one of the preceding claims, wherein at least one of the inlets and outlets (34, 35) is formed by drilling starting from the cooling channel (16) to be formed.

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