Piston for internal combustion engine and method for manufacturing the same

Abstract

The present invention relates to a piston for an internal combustion engine and a method for manufacturing the same. The piston (100) for an internal combustion engine has: two piston pin seats (120) designed to accommodate piston pins and having a substantially constant cross-section, wherein the piston is substantially cylindrical, and wherein a reference plane of the piston is perpendicular to an axis (70) of the cylinder, wherein a center line (g1, g2) of the piston pin seat (120) is inclined relative to the reference plane.

Description

[0001] This application is a divisional application of the invention patent application with the original application number 201980068100.9 and the international application number PCT / EP2019 / 078682 (application date: October 22, 2019, invention name: Piston for internal combustion engine and its manufacturing method). Technical Field

[0002] The present invention relates to a piston for an internal combustion engine (i.e. a diesel engine or a gasoline engine) and a method for manufacturing the same. It also relates to a device for manufacturing the piston. Background Art

[0003] Pistons for gasoline and diesel engines are generally provided with a profiled hole in the piston pin boss to avoid excessive loads on the inner edge of the piston pin boss top side caused by ignition pressure and piston bending. The lower part of the piston pin boss is subjected to centrifugal forces. In order to reduce weight and to use a tapered connecting rod, a pin boss that is truncated into a cone is generally used, that is, the length of the lower pin boss is shorter than the length of the upper pin boss (here, the lower side refers to the side away from the piston top, and the upper side refers to the side facing the piston top).

[0004] In the prior art, the piston pin seat is realized in the form of a continuous hole to ensure the coaxiality of the two pin seat sides. The corresponding profiled hole is realized by turning, wherein in the case of a conventional lathe, the shape of the lower pin seat must correspond to the shape of the upper pin seat, i.e., the pin seat surface is formed by the rotation of the contour curve around the hole axis, and the cut perpendicular to the hole axis produces a circle with a radius defined by the contour curve. Due to the thermal deformation of the piston, the central axes of the two pin seats in the hot working state are respectively rotated toward the piston top, whereby the thermal contour of the lower piston pin seat is completely different from the thermal contour of the upper piston pin seat. Since the design of the piston pin seat is mainly carried out under ignition pressure load for strength considerations, the lower piston pin seat can have dangerous loads at the edge under centrifugal load, as the inventors have determined. This is to be avoided.

[0005] Known prior art

[0006] Examples of the prior art are DE 10 2005 020 501 A1, WO 2007 / 115527 A1, DE 10 137 437 A1, WO 2007 / 025733 A1, DE 10 2009 056 920 A1, DE 10 2009 036 784 A1 and WO 2005 / 078321 A1.

[0007] By using a special precision lathe with a magnetic spindle, the deflection can be limited variably depending on the angle. In principle, non-rotationally symmetrical special-shaped holes can be processed, so that the shape of the upper pin seat and the shape of the lower pin seat can be independent of each other. The corresponding manufacturing method is described, for example, in WO2007 / 025733A1. With this technology, a specific ellipticity in the horizontal and vertical directions can be formed as needed, while a suitable superposition function between the shape of the upper pin seat part and the shape of the lower pin seat part is adopted in the circumferential direction. JP2018-119444A discloses the features that fall into the preamble of the first aspect of the invention. Summary of the invention

[0008] The inventors have noticed that in the prior art methods, only inadequate compensation of thermal expansion occurs. This leads to higher side loads on the bottom side of the pin seat in the operating state, which should be avoided or reduced.

[0009] The object of the present invention is to alleviate said disadvantages.

[0010] The present invention is achieved by the subject matter of the first aspect of the present invention. Preferred embodiments are defined in further aspects.

[0011] According to the invention, a piston for an internal combustion engine has two piston pin seats. Such a piston can be a piston for a diesel engine, but can also be a piston for a gasoline engine. The piston pin seat is the part of the piston which is designed to accommodate a piston pin and connect the piston to a connecting rod when such a piston is in use. The piston pin seat has a substantially constant cross section. This means that the piston pin seat has a cross section in the part accommodating the piston pin, which cross section does not significantly widen or narrow in the direction of the piston pin seat or changes in any other way. The substantially constant cross section does not have to extend through the entire piston pin seat, it is sufficient that the substantially constant cross section extends only through the area by means of which the piston pin is accommodated and held when the piston is in use.

[0012] According to the invention, such a piston is substantially cylindrical and has an axis extending along the cylinder axis. A plane perpendicular to the axis is defined as the reference plane of the piston.

[0013] According to the invention, the center line of the piston pin boss is inclined with respect to the reference plane. In this case, the center line of the piston pin boss is understood to be a line defined by that portion of the piston pin boss which has a substantially constant cross section. The center lines extend through the center of gravity of the cross section. The center line is preferably inclined in such a way that the piston pin boss tilts away from the top of the piston when one moves along the piston pin boss from the outside of the piston to the inside of the piston.

[0014] The inventors have noticed that the stresses acting under centrifugal force in the lower piston pin boss during hot operation can be reduced by a corresponding inclination of the piston pin boss centerline. In this case, the inclination of the piston pin boss pointing upstream, i.e. toward the piston top, is fully or partially compensated for thermal expansion.

[0015] It is particularly preferred that the inclination angle α of the piston pin boss center line relative to the reference plane is in the range of 0.02° to 1°, particularly preferably in the range of 0.02° to 0.5°. Simulations have shown that this angle range is very suitable.

[0016] The invention is considered to be applicable to all types of pistons, i.e. not only for gasoline engine pistons but also for diesel engine pistons. The main application is gasoline engine pistons which are almost always made of aluminum alloys. In fact, it can also be applied to diesel engine pistons which are generally made of steel or aluminum alloys.

[0017] It is also preferred that the two center lines of the two piston pin bosses enclose the same angle α with the reference plane.

[0018] It is also preferred that the center lines of the two piston pin bosses are mirror-symmetrical to each other about a plane that contains the axis of the piston, such as a plane that extends centrally between the two piston pin bosses and is perpendicular to the two piston pin bosses.

[0019] Optionally, the cross section of the piston pin seat has an oval profile. With this oval profile, the stresses occurring in the piston can be reduced, which is beneficial for the wear characteristics of the piston.

[0020] Mathematically, the piston according to the invention for an internal combustion engine is defined as follows in a coordinate system KS0:

[0021] The axis z0 corresponds to the axis of the cylinder circumscribing the piston and points to the piston crown. The axis x0 points to the pressure side and the axis y0 points to the front side. A pin plane is defined which is displaced in the x0 direction parallel to the y0z0 plane by the dimension d, i.e. the pin misalignment. The x0y0 plane is called the ring plane (pin seat plane) and the x0z0 plane is called the running plane.

[0022] The intersection straight line between the pin plane and the ring plane defines the axis of the piston pin boss in a conventional piston. Two points A and B are defined on the straight line, each of which is spaced from the running plane by half the pin length. The points represent the end point of the piston pin boss and the start point of the groove for the locking ring.

[0023] In contrast to the prior art, a piston is proposed in which the two holes of the piston pin seat are not located on a continuous horizontal axis AB, but are defined by two axes inclined at the same angle α relative to the ring plane. The axes extend through points A and B and intersect at a further point C, which lies in the motion plane due to symmetry. A cylindrical coordinate system KS1, KS2 is defined on each of the two axes, wherein the z-axis points from A or B to C and the r-axis lies in the pin plane and points to the piston top.

[0024] The motivation for introducing suitable bore axes comes from the complete or partial compensation of thermal expansion in the hot working state, so that the two bore axes of the hot piston pin boss are basically coaxial, thereby eliminating the disadvantage of edge loads at the lower piston pin boss.

[0025] In order to achieve a fine profile of the pin hole, the special-shaped holes are arranged at intervals [0,L B ] is defined, where L B The shaped hole extends from the starting point of the two pin holes A and B to C and defines the local radial extension R in the form of a continuous convex function f of the axial deflection:

[0026] R1=f(z1), R2=f(z1), (1)

[0027] In this case, z1 and z2 define the respective position along the respective seat axis. R represents the distance of the respective seat surface from the line AC or BC.

[0028] The special-shaped holes R1(z1) and R2(z2) are arranged at intervals [0,L B ] is a convex function, so for all u,v∈[0,L B ] and t∈[0,1] are applicable to:

[0029] R(tu+(1-t)v)≤tR(u)+(1-t)R(v) (2)

[0030] As an extension of the rotationally symmetrical hole in a suitable coordinate system, the ellipticity a(z) is introduced in the horizontal direction, where a(z) describes the increase in diameter:

[0031]

[0032] This means that the diameter in the horizontal direction is 2*R(z)+a(z), while in the vertical direction there is a diameter 2*R(z).

[0033] Similarly, the vertical ellipticity is introduced:

[0034]

[0035] Alternatively, the general ellipticity in a suitable coordinate system can be described by the definition of a Fourier series:

[0036]

[0037] According to the present invention, a method for manufacturing a piston according to a ninth aspect of the present invention is also provided. The piston according to the present invention can be manufactured by means of such a method.

[0038] In this case, according to the invention, a piston blank is placed on a turntable. This piston blank is the object from which the finished piston is to be manufactured. The piston blank is machined (e.g. by turning) from one side of the blank (the circumferential side of the finished piston) to form one of the two piston pin seats. The piston blank is then rotated about the axis of the piston to be manufactured (generally by 180°). It is then subtractively machined from the outside to the inside in a second position to form the second of the two piston pin seats. This method can be implemented more easily because only a single device for subtractive machining of the piston blank is required.

[0039] Alternatively, it is also possible to place the piston blank on a platform that can be pivoted about a pivot axis that is perpendicular to the axis of the piston to be manufactured and to the straight line connecting the two piston pin seats in the finished piston. Now, in a first step of the subtractive machining, the first and second piston pin seats are formed, for which the piston blank is rotated about the pivot axis at an angle α. The subtractive machining extends essentially in the direction that extends along the straight line connecting the two piston pin seats in the finished piston. Once the tool has reached the piston center axis, the piston is pivoted back about the pivot axis of the platform at an angle of -2α. This is followed by subtractive machining of the piston blank to form the second of the two piston pin seats. The subtractive machining is also essentially carried out in this direction (the subtractive machining was completed in this direction during the first step). That is, the direction of the subtraction does not differ between the production of the first piston pin seat and the production of the second piston pin seat, except for the angle α. Such a method is easy to implement, but a corresponding "long" tool is required for the subtractive machining here. The pivoting is carried out at an angle of ±α, that is, at an inclination angle of the piston pin seat relative to the reference plane.

[0040] Another manufacturing method according to the invention comprises placing a piston blank on a platform. The platform can be moved along the axis of the piston to be manufactured. Subsequently, the piston blank is subtractively machined in a direction perpendicular to the axis of the piston to be manufactured to form one of the two piston pin seats. At the same time, the piston blank is moved along the axis of the piston to be manufactured to form a first piston pin seat. The combination of the displacement movement along the piston axis and the subtractive machining perpendicular to the piston axis generally results in an inclined orientation of the piston pin seat, as claimed herein. Once the tool has reached the central axis of the piston, the direction of movement of the tool is maintained, but the direction of movement of the piston blank is reversed, thereby forming a second piston pin seat.

[0041] The invention is also realized by a device designed to carry out the method according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The piston according to the invention is shown in an overall view.

[0043] Figure 2 The piston according to the invention is shown in cross section.

[0044] Figure 3 Another cross-sectional view shows Figure 2 The piston.

[0045] Figure 4 A piston according to the prior art is shown in an operating state, with temperature zones and thermal deformations shown in elevation.

[0046] Figure 5 The thermal clearance of a piston according to the prior art is shown.

[0047] Figure 6 and Figure 7 Shown in cold state ( Figure 6 ) and working status ( Figure 7 ) of the upper and lower special-shaped holes of the first piston according to the present invention, respectively subtract the cold gap or the hot gap.

[0048] Figure 8 and Fig. 9 Shown in cold state ( Figure 8 ) and working status ( Fig. 9 ) of the upper and lower special-shaped holes of the second piston according to the present invention, respectively, the cold gap or the hot gap is reduced.

[0049] Fig.10 A first manufacturing method for a piston according to a first embodiment of the present invention is shown.

[0050] Fig.11 A second manufacturing method for a piston according to one embodiment of the present invention is shown.

[0051] Fig.12 A third embodiment of a method for producing a piston according to one embodiment of the present invention is shown. DETAILED DESCRIPTION

[0052] Figure 1 Shown is an overall view of a piston according to the invention. The piston is also Figure 2 and Figure 3 is shown in a cross-sectional view.

[0053] The piston 100 has a piston crown 110, which forms the lower boundary of the combustion chamber in use. The piston generally has an annular groove that can accommodate piston rings. Two piston pin seats 120 designed to accommodate piston pins (not shown) are located below (i.e., further away from the piston crown 110).

[0054] If you can Figure 2 As can be seen in the drawing, the piston pin bosses have grooves 130 at their radial ends, respectively, which are used to accommodate locking rings that hold the piston pins in the piston pin bosses 120 .

[0055] exist Figure 3 1 shows how the piston pin boss will pivot about the horizontal plane. The "classic" piston pin boss extends along a straight line connecting points A and B. Points A and B represent the center points of the cross section of the piston pin boss at the points where the inner edge of the groove for the locking ring is located. According to the invention, each individual piston pin boss is inclined downwards with respect to this line, as shown by the lines g1 and g2. They extend from point A or B through point C, which is below the line between A and B. This design of the piston pin boss can be implemented by conventional fine machining with a rotationally symmetrical profiled hole, respectively. The angle of inclination α is generally significantly less than 1°.

[0056] Due to the small angle of inclination, the profiled hole expanding towards the inside of the pin seat and the play between the pin and the pin seat, the pin can also be mounted although the axes of the pin seats are no longer arranged coaxially as is common in the prior art.

[0057] Reference Figures 4 to 5 , the advantages of this design become clear. In this case, Figure 4 Shown (extremely exaggerated, exaggeration factor 50) is a finite element simulation of a piston according to the prior art with a horizontal piston pin boss in the operating state.

[0058] As can be clearly seen from the piston deformation, when the cylindrical straight pin is inserted, stresses occur especially on the inner side in the lower area of ​​the piston pin seat. Figure 5 The upper solid line represents the deformation of the top side of the piston pin seat along the pin axis, while the lower solid line extending substantially horizontally along the X-axis represents the top side of the piston pin. The upper dashed line represents the bottom side of the piston pin extending substantially parallel to the top side of the pin, while the lower dashed line represents the lower side of the piston pin seat. It can be clearly seen that the piston pin seat and the piston pin have different orientations, so stresses originating from different shapes appear.

[0059] Figure 6 and Figure 7 It is shown that the present invention at least partially compensates for this effect. These figures show the cold state ( Figure 6 ) and in working state ( Figure 7) The operating state refers to the temperature that occurs in the engine when this piston is used. In this case, the piston pin seat is symmetrically inclined at an angle of α = 0.114°. Figure 6 The upper solid line in the figure shows the top side of the piston pin seat, and the lower solid line shows the bottom side of the piston pin seat. The dotted line represents the pin seat axis. Figure 7 It is shown in its operating state, ie at the temperature which occurs in the operating state. It can be clearly seen that in particular the axis runs parallel to the X axis in the operating state, thus avoiding stresses.

[0060] Figure 8 and Fig. 9 Basically shows Figure 6 and Figure 7 The same content of , but here a different inclination angle of 0.057° was chosen. In this case, there is also less clamping.

[0061] Figures 10 to 12 A method for producing a piston according to the invention is schematically shown. Fig.10 The piston blank 150 is shown arranged on a turntable (not shown). The piston pin boss 120' is produced by means of an inclined feed movement V of a subtractive tool. After the piston pin boss 120' has been formed, the piston blank 150 is rotated 180° (rotation D). Subsequently, a second piston pin boss is produced by means of the same subtractive machining V.

[0062] Fig.11 A second embodiment of the invention is shown. In this case, a piston blank 160 is placed on a pivotable platform (not shown). The platform is first tilted at a tilting angle α and a piston pin seat 120 ′ is produced by subtractive machining V. After the first piston pin seat 120 ′ has been formed, the piston is tilted back by a tilting angle of −2α (tilting movement K). Subsequently, a second piston pin seat 120 ″ is produced.

[0063] Fig.12 A third method according to the invention for producing a piston is shown. Here, a piston blank 160 is placed on a platform (not shown) and is formed into a piston pin boss 120 ′ by means of a subtractive machining V, while the piston blank is moved in a displacement direction F. The subtractive machining takes place completely horizontally here. Subsequently, the piston is also subtractively machined in the same subtractive direction V to produce a second piston pin boss 120 ″, for which only the displacement direction F is reversed.

Claims

1. A piston (100) for an internal combustion engine, the piston (100) comprising: Two piston pin bosses, each having a pin hole (120) designed to receive a piston pin, wherein: The piston is basically cylindrical, and wherein the reference plane of the piston is perpendicular to the axis (z0) of the cylinder, wherein the center line (g1, g2) of the pin hole (120) is inclined relative to the reference plane, characterized in that the center lines (g1, g2) of the two pin holes enclose the same angle α with the reference plane and are mirror-symmetrical about the plane containing the axis of the cylinder.

2. The piston (100) for an internal combustion engine according to claim 1, wherein: The inclination angle α of the center line (g1, g2) of the pin hole (120) is in the range of 0.02° to 1°, preferably in the range of 0.02° to 0.5°.

3. The piston (100) according to any one of the preceding claims, wherein: The pin hole (120) of the piston pin seat is a special-shaped hole described by the following formula: R1=f(z1), R2=f(z1).

4. The piston (100) according to any one of the preceding claims, wherein: The cross section of the pin hole (120) has an elliptical profile.

5. The piston (100) according to claim 4, wherein: The pin hole (120) has a profile described by the following formula:

6. The piston (100) according to claim 4, wherein: The pin hole (120) has a profile described by the following formula:

7. The piston (100) according to claim 3, wherein: The pin hole (120) has a profile described by the following formula:

8. A method for producing a piston according to any one of the preceding claims, comprising the following steps: The piston blank (150) is placed on a turntable. The piston blank is processed (V) in a direction of material reduction from the peripheral surface side of the piston to be manufactured inward to form one of the two pin holes, rotating (D) the piston blank about the axis of the piston to be manufactured, and Next, the piston blank is subjected to material reduction processing in an inward direction from the peripheral surface side of the piston to be manufactured to form a second pin hole of the two pin holes.

9. A method for manufacturing a piston according to any one of claims 1 to 7, the method comprising the following steps: The piston blank (160) is placed on a platform which can be pivoted about a pivot axis which is perpendicular to the axis (70) of the piston to be manufactured and to a straight line connecting two pin holes in the finished piston, A first step of subtractively machining (V) the piston blank to form one of the two pin holes (120), wherein: The subtractive machining is performed along a direction extending along a straight line connecting two pin holes in the finished piston, A second step of subtractive machining the piston blank to form a second one of the two pin holes, wherein the subtractive machining is performed along the direction along which the subtractive machining was performed during the first step, preferably with the platform pivoting (K) by an angle 2α between the first step and the second step of the subtractive machining.

10. A method for manufacturing a piston according to any one of claims 1 to 7, the method comprising the following steps: The piston blank (160) is placed on a platform that can move along the axis (70) of the piston to be manufactured. subtractively machining (V) the piston blank in a direction perpendicular to the axis of the piston to be manufactured to form one of the two pin holes, and at the same time, moving (F) the piston blank parallel to the axis of the piston to be manufactured to form a first pin hole, Subsequently, the piston blank is subtractively machined in the same tool movement direction, while at the same time, the piston blank is moved (F) in the opposite direction to the initial movement direction of the piston blank to form a second pin hole.

11. A device designed to carry out the method according to any one of claims 8 to 10.

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