JP7530574B2 - Piston for internal combustion engine and manufacturing method thereof - Google Patents

Description

The present invention relates to a piston for an internal combustion engine having an outer periphery that is slidable relative to an inner periphery of a cylinder bore. The present invention also relates to a method for manufacturing such a piston for an internal combustion engine.

In an internal combustion engine such as an engine installed in a vehicle such as an automobile, a piston reciprocates in a cylinder bore extending along a linear longitudinal axis in a direction along the longitudinal axis. At this time, the outer periphery of the piston slides against the inner periphery of the cylinder bore. Typically, the piston includes a piston body having an outer periphery that can slide against the inner periphery of the cylinder bore, and two skirts that extend from the outer periphery of the piston body to the bottom side of the cylinder bore, and further, a coating layer is formed on the outer periphery of each skirt in the piston to reduce the frictional resistance of the outer periphery of the piston against the inner periphery of the cylinder bore.

However, in pistons, it is desirable to obtain an even greater reduction in frictional resistance in addition to the reduction in frictional resistance provided by the coating layer. For this reason, various pistons have been proposed to obtain even greater reductions in frictional resistance.

One example of such a piston is a piston that has been formed by casting, and that has had shot material such as hard particles uniformly sprayed by shot peening to form multiple dimples on the outer periphery of the two skirts, and then that has had a resin coating applied to the outer periphery of the two skirts to form a coating layer. In this piston, multiple recesses are formed on the outer surface of the coating layer on the outer periphery of the two skirts to imitate the multiple dimples on the outer periphery of the two skirts, and the multiple recesses are all equal in size and are uniformly distributed throughout the entire coating layer. (See, for example, Patent Document 1.)

JP 2016-160293 A

However, in a piston, the load acting on the skirt on the thrust side of the two skirts is greater than the load acting on the skirt on the anti-thrust side. Therefore, in a piston in which the sizes of the multiple recesses in the coating layers of the two skirts are equal and the multiple recesses are uniformly distributed throughout the coating layer, as in the example piston above, after a long period of use, wear in certain areas of the coating layer of the skirt on the thrust side, especially the central area, will occur more quickly than wear in other areas. In this case, it will be difficult to maintain the initial sliding performance of the piston for a long period of time.

For example, in such a piston, after a long period of use, the balance of the load acting on the two skirts may be lost, which may accelerate wear in the specific area more quickly than in other areas, increasing the vibrations that occur when the piston reciprocates.

In addition, in the example of the piston described above, there is a risk that the shot material sprayed by shot peening may remain on the outer periphery of the skirt. It is difficult for the coating layer to adhere to the outer periphery of such a skirt. In this case, there is a risk that the frictional resistance of the piston may increase and the sliding performance of the piston may decrease.

Furthermore, when casting an aluminum alloy piston, silicon may crystallize on the surface of the piston rough material. Therefore, when manufacturing such pistons, a mixed acid treatment is performed to remove the crystallized silicon. However, the mixed acid treatment may dissolve not only the silicon but also the piston rough material. In this case, the shape precision of the piston rough material may decrease, which may increase the frictional resistance of the piston and reduce the sliding performance of the piston.

In light of this situation, it is desirable to efficiently improve the sliding performance of pistons in internal combustion engines and their manufacturing methods.

In order to solve the problem, a piston of an internal combustion engine according to one embodiment comprises a piston body having an outer periphery configured to be slidable in a longitudinal direction along a linear longitudinal axis relative to an inner periphery of a cylinder bore extending along the longitudinal axis, and two skirts that face each other in the radial direction of the piston body and extend from the outer periphery of the piston body toward the bottom of the cylinder bore, and the two skirts are arranged in a skirt facing direction, that is, a first skirt located on the thrust side in the skirt facing direction, and a second skirt located on the counter-thrust side in the skirt facing direction. A piston for an internal combustion engine, which comprises a first coating layer formed on the outer periphery of the first skirt facing the inner periphery of the cylinder bore and a second skirt located on the side of the first skirt, and a second coating layer formed on the outer periphery of the second skirt facing the inner periphery of the cylinder bore, the first coating layer has more recesses formed therein than the second coating layer, and the size of the area occupied by the recesses in a unit area in a specific portion of the first coating layer is larger than the size of the area occupied by the recesses in a unit area in the remaining portion of the first coating layer other than the specific portion.

In order to solve the problem, a manufacturing method of a piston for an internal combustion engine according to one embodiment comprises a raw material including a piston body having an outer periphery configured to be slidable in a longitudinal direction along a longitudinal axis relative to an inner periphery of a cylinder bore extending along a linear axis, and two skirts that face each other in a radial direction of the piston body and extend from the outer periphery of the piston body toward a bottom of the cylinder bore, the two skirts being comprised of a first skirt located on the thrust side in the skirt facing direction and a second skirt located on the anti-thrust side in the skirt facing direction, and the first and second skirts being comprised of a first skirt located on the thrust side in the skirt facing direction and a second skirt located on the anti-thrust side in the skirt facing direction, A method for manufacturing a piston for an internal combustion engine in which a coating layer is formed on the outer periphery of each of the first skirts, the method including the steps of: pouring molten metal from an outlet of the die arranged to correspond to a portion of the outer periphery of the first skirt into a cavity of the die formed to fit the shape of the piston rough material, removing the shaped piston rough material from the cavity of the die, removing silicon exposed on the surface of the removed piston rough material by water jet spray, and forming the coating layer on the surface of the piston rough material from which the silicon has been removed.

In one embodiment of a piston for an internal combustion engine and a method for manufacturing the same, the sliding performance of the piston can be efficiently improved.

FIG. 1 is a partially sectional front view that shows a schematic front view of a piston and a vertical cross section of a cylinder bore according to an embodiment. FIG. 2 is a thrust side view that shows a schematic diagram of a piston according to an embodiment. FIG. 3 is a schematic anti-thrust side view of a piston according to an embodiment. FIG. 4 is a flowchart illustrating a method for manufacturing a piston according to an embodiment. FIG. 5 is a cross-sectional view that illustrates an example of a casting apparatus used in a casting step in a manufacturing method of a piston according to an embodiment. FIG. 6 is a front view showing a schematic diagram of a water jet spraying device used in a silicon removal step, together with a piston blank, in a method for manufacturing a piston according to an embodiment. Figure 7(a) is an enlarged view showing a schematic view of the surface of the outlet-corresponding portion of the first skirt in the piston rough material before the silicon removal process in a piston manufacturing method of one embodiment, and Figure 7(b) is an enlarged view showing a schematic view of the surface of the outlet-corresponding portion of the first skirt in the piston rough material after the silicon removal process in a piston manufacturing method of one embodiment. Figure 8(a) is a cross-sectional view showing a schematic representation of the outlet-corresponding portion of the first skirt in the piston rough material before a surface treatment process in a piston manufacturing method according to one embodiment, and Figure 8(b) is a cross-sectional view showing a schematic representation of the outlet-corresponding portion of the first skirt in the piston rough material after a surface treatment process in a piston manufacturing method according to one embodiment.

A piston for an internal combustion engine according to one embodiment and a method for manufacturing the same are described below. The piston according to this embodiment is used in an automobile engine. However, the piston may also be used in an internal combustion engine other than an automobile engine. For example, the piston may be used in an outboard motor engine, or a general-purpose engine mounted on a small lawn mower or generator, etc.

"Outline of the piston"
First, an outline of the piston 1 according to this embodiment will be described with reference to Figures 1 to 3. That is, the piston 1 is generally configured as follows. As shown in Figures 1 to 3 in particular, the piston 1 is configured to be able to reciprocate in the longitudinal direction (indicated by two one-sided arrows P1, P2 in Figures 1 to 3) along the longitudinal axis L (indicated by a dashed line) in a cylinder bore 2 extending along a linear longitudinal axis L. Note that in Figures 1 to 3, the longitudinal top side facing the top side of the cylinder bore 2 is indicated by a one-sided arrow P1, and the longitudinal bottom side facing the bottom side of the cylinder bore 2 is indicated by a one-sided arrow P2.

The piston 1 has a piston body 11 having an outer periphery 11a configured to be able to slide longitudinally relative to the inner periphery 2a of the cylinder bore 2 within the cylinder bore 2. The piston 1 has two skirts 12, 13 that face each other in one radial direction of the piston body 11. The two skirts 12, 13 extend from the outer periphery 11a of the piston body 11 toward the bottom of the cylinder bore 2. In the following, the radial direction is defined as referring to the radial direction of the piston body 11.

The two skirts 12, 13 consist of a first skirt 12 located on the thrust side (indicated by a one-sided arrow Q1 in FIG. 1) in the skirt opposing direction, which is the direction in which they face each other, and a second skirt 13 located on the anti-thrust side (indicated by a one-sided arrow Q2 in FIG. 1) in the skirt opposing direction. The skirt opposing direction is one radial direction. The first and second skirts 12, 13 each have an outer periphery 12a, 13a that faces the inner periphery 2a of the cylinder bore 2. In FIG. 1, the skirt opposing direction is indicated by two one-sided arrows Q1, Q2.

As shown in Figures 1 and 2, a first coating layer 20 is formed on the outer periphery 12a of the first skirt 12. As shown in Figures 1 and 3, a second coating layer 30 is formed on the outer periphery 13a of the second skirt 13. More recesses 21 are formed in the first coating layer 20 than in the second coating layer 30.

As shown in FIG. 2, the size of the area occupied by the recesses 21 in a unit area in the specific portion 20a of the first coating layer 20 is larger than the size of the area occupied by the recesses 21 in a unit area in the remaining portion 20b of the first coating layer 20 other than the specific portion 20a. The specific portion 20a of the first coating layer 20 can be arranged in the longitudinal direction at a reference portion 1a having a reference diameter defined on the outer peripheries 12a, 13a of the first and second skirts 12, 13. Note that in FIG. 2, the specific portion 20a is surrounded by a virtual line, i.e., a two-dot chain line.

"Details about pistons"
1 to 3, the piston 1 according to this embodiment can be configured in detail as follows. The piston 1 has a blank 10 which is a metal casting. The metal constituting the blank 10 can be an aluminum alloy. In particular, the metal can be an Al (aluminum)-Si (silicon) based aluminum alloy. However, the metal of the blank is not limited thereto. For example, the metal of the blank can be aluminum, iron, an iron alloy, titanium, a titanium alloy, magnesium, a magnesium alloy, or the like.

The piston 1 has two sidewalls 14, 15 formed to connect two skirts 12, 13 in the circumferential direction of the piston body 11. The piston 1 has two pin bosses 16, 17 configured to hold a piston pin (not shown). The two pin bosses 16, 17 are provided on the two sidewalls 14, 15, respectively. In the following, the circumferential direction is defined as referring to the circumferential direction of the piston body 11.

In such a piston 1, the blank 10 can include a piston body 11, first and second skirts 12, 13, two sidewalls 14, 15, and two pin bosses 16, 17.

Furthermore, in the piston 1, the outer periphery 11a of the piston body 11 is formed in a generally cylindrical shape. It has a crown portion 11b located at the top end in the longitudinal direction. The piston body 11 has a ring land portion 11c located on the bottom side in the longitudinal direction relative to the crown portion 11b. The piston body 11 has a plurality of ring grooves 11d formed in the ring land portion 11c so as to be recessed from the outer periphery 11a. The plurality of ring grooves 11d are spaced apart from one another in the longitudinal direction. A piston ring (not shown) is fitted into each ring groove 11d.

The outer periphery 12a, 13a of the first and second skirts 12, 13 are each formed along a curved surface of a barrel shape. The reference diameter is the reference outer diameter of the piston 1. The reference diameter is an outer diameter determined at a specified height in the longitudinal direction of the skirts 12, 13 based on the thrust side end area and the anti-thrust side end area provided on the outer periphery 12a, 13a of the skirts 12, 13 along the skirt opposing direction in the fit relationship between the piston 1 and the cylinder bore 2. In Figures 1 to 3, a reference plane C located at this specified height is shown.

The thrust side of the skirt facing direction where the first skirt 12 is located is the direction in which lateral pressure is received from the inner periphery 2a of the cylinder bore 2 immediately after the piston 1 passes top dead center when the engine is running. The anti-thrust side of the skirt facing direction where the second skirt 13 is located is the direction facing the opposite side from the thrust side in the skirt facing direction.

When viewed in the longitudinal direction, each of the first and second skirts 12, 13 is formed approximately symmetrically in the circumferential direction with respect to a thrust axis M (indicated by a dashed line) that extends in the direction in which the skirts face each other. The thrust axis M extends linearly so as to pass through approximately the circumferential center of the skirts 12, 13. The thrust axis M is located at a specified height in the longitudinal direction of the skirts 12, 13.

The two pin bosses 16, 17 face each other in a boss facing direction (indicated by two one-sided arrows R1, R2 in Figs. 2 and 3), which is a radial direction separate from the thrust facing direction. Note that in Figs. 2 and 3, one side of the bore facing direction is indicated by a one-sided arrow R1, and the other side of the bore facing direction is indicated by a one-sided arrow R2. The boss facing direction is approximately perpendicular to the longitudinal direction and the thrust facing direction. The thrust facing direction is also approximately perpendicular to the longitudinal direction and the boss facing direction.

The outer peripheries 14a, 15a of the two sidewalls 14, 15 are positioned radially closer to the center than the outer peripheries 12a, 13a of the first and second skirts 12, 13. When viewed in the longitudinal direction, each of the two sidewalls 14, 15 is formed approximately symmetrically in the circumferential direction with respect to a bore axis N (indicated by a dashed line) extending in the bore facing direction. The bore axis N is located a predetermined compression height away from the reference position of the crown portion 11b of the piston body 11 in the longitudinal direction.

The two pin bosses 16, 17 have through holes 16a, 17a that extend along the bore axis N. The through holes 16a, 17a are formed so that a piston pin (not shown) can fit into them.

Each of the first and second coating layers 20, 30 contains a resin material. This resin material can be a resin material having low friction resistance and high heat resistance, such as PAI (polyamideimide), PI (polyimide), PTFE (polytetrafluoroethylene), etc. However, the resin material is not limited to this.

The first and second coating layers 20, 30 may each contain the above-mentioned resin material, a solid lubricant such as graphite, molybdenum disulfide, etc., and microparticles such as titanium oxide, etc. The first and second coating layers 20, 30 may be composed of similar components. However, the first and second coating layers may also be composed of different components.

Referring to Figures 1 and 2, the first coating layer 20 has a plurality of recesses 21. Each recess 21 in the first coating layer 20 is formed so as to recess from the outer surface of the first coating layer 20 toward the outer periphery 12a of the first skirt 12.

The radial center of the specific portion 20a in the first coating layer 20 can be approximately aligned with the thrust axis M. The longitudinal center of the specific portion 20a can be approximately aligned with the reference plane C. The thrust axis M is located along the reference plane C. The recesses 21 of the first coating layer 20 are disposed in both the specific portion 20a and the remaining portion 20b. The recesses 21 located in the specific portion 20a are larger than the recesses 21 located in the remaining portion 20b. However, the recesses of the first coating layer can be disposed only in the specific portion. The recesses located in the specific portion can also be substantially the same size as the recesses located in the remaining portion.

Referring to Figures 1 and 3, the second coating layer 30 has a plurality of recesses 31. Each recess 31 in the second coating layer 30 is formed to recess from the outer surface of the second coating layer 30 toward the outer periphery 13a of the second skirt 13. The recesses 31 located in the second coating layer 30 are smaller than the recesses 21 located in the specific portion 20a of the first coating layer 20. However, the recesses in the first coating layer may be disposed only in the specific portion. The recesses located in the second coating layer may be substantially the same size as the recesses located in the specific portion of the first coating layer.

"Outline of piston manufacturing method"
Next, an outline of a method for manufacturing the piston 1 according to this embodiment will be described with reference to Figures 4 to 8. That is, the method for manufacturing the piston 1 is generally as follows. With reference to Figures 4 and 5, the method for manufacturing the piston 1 includes a step (casting step) S1 of pouring a molten metal E from an outlet 41 of a die 40 into a cavity portion 42 of the die 40, as shown by a one-sided arrow F in Figure 5, in order to form a blank 10 of the piston 1.

As shown in FIG. 5, the outlet 41 of the mold 40 used in the casting process S1 is positioned to correspond to a portion 12b (hereinafter referred to as the "outlet corresponding portion" as necessary) of the outer periphery 12a of the first skirt 12 in the raw material 10. This outlet corresponding portion 12b corresponds to a specific portion 20a of the first coating layer 20. The cavity portion 42 of the mold 40 is shaped to fit the shape of the raw material 10.

As shown in FIG. 4, the method for manufacturing the piston 1 includes a step (removal step) S2 of removing the shaped rough material 10 of the piston 1 from the cavity portion 42 of the mold 40. With reference to FIGS. 4, 6, and 7, the method for manufacturing the piston 1 includes a step (silicon removal step) S5 of removing the silicon G exposed on the surface of the removed rough material 10 by water jet spray. With reference to FIGS. 4 and 8, the method for manufacturing the piston 1 includes a step (surface treatment step) S6 of forming the first and second coating layers 20, 30 on the surface of the rough material 10 of the piston 1 from which the silicon G has been removed.

"Details about piston manufacturing methods"
4 to 8, the manufacturing method of the piston 1 can be described in detail as follows. With reference to Fig. 4 and Fig. 5, the molten metal E used in the casting step S1 is made of the same metal as the metal constituting the above-mentioned blank 10. In the casting step S1, a gravity casting method is used.

When using such gravity casting, natural cooling is performed without forcibly circulating cooling water in the die 40. Therefore, in many cases, the part of the raw material 10 of the piston 1 that finally cools and solidifies is the part 12b that faces the outlet 41 through which the high-temperature molten metal E flows toward the cavity portion 42 of the die 40, i.e., the outlet-corresponding part 12b. In this part, for example, silicon G, which is a component in Al-Si-based aluminum alloys, and in particular primary crystal silicon G, tend to coarsen and crystallize at high density.

As shown in FIG. 4, the manufacturing method of the piston 1 includes a step S3 (heat treatment step) of subjecting the raw material 10 to a heating treatment, cooling treatment, etc., in order to adjust the strength, hardness, etc., of the raw material 10 removed from the die 40 in the removal step S2. For example, the heat treatment step S3 can be a T5 heat treatment that performs only artificial age hardening, or a T6 heat treatment that performs solution treatment and artificial age hardening, etc.

The manufacturing method of the piston 1 includes a step (machining step) S4 of cutting the rough material 10 to adjust the shape of the rough material 10 that has been subjected to the heat treatment step S3. For example, in the machining step S4, the rough material 10 can be finished by cutting using a precision lathe or the like to improve the dimensional accuracy of the inner diameter, outer diameter, and surface roughness of the first and second skirts 12, 13 of the rough material 10, and the shape accuracy of the ring groove 11d, the first and second skirts 12, 13, etc. of the rough material 10. By the cutting process in the machining step S4, the surfaces of the first and second skirts 12, 13 are shaped to have striations.

As shown in FIG. 7(a), silicon G, particularly primary crystal silicon G, is crystallized on the surfaces of the outer peripheries 12a, 13a of the first and second skirts 12, 13 in the raw material 10 of the piston 1 before the silicon removal process S5, particularly on the surface of the outlet-corresponding portion 12b of the first skirt 12, as shown by the dotted line.

As shown in FIG. 6, in the water jet spraying of the silicon removal process S5, water W is sprayed from the nozzle 50 of the water jet sprayer substantially perpendicularly onto such a surface. As an example, the spray pressure of the water W in the water jet spraying can be about 100 MPa to about 200 MPa. However, the spray pressure is not limited to this. The spray pressure can be set appropriately depending on the distance between the nozzle and the rough material, etc.

As a result, as shown in Figures 7(b) and 8(a), on the surface of the outer periphery 12a of the first skirt 12 in the raw material 10 of the piston 1 after the silicon removal process S5, particularly on the surface of the outlet-corresponding portion 12b of the first skirt 12, a recess 12c (shown in Figures 2, 7(a), 7(b), 8(a), and 8(b)) is formed in the portion from which the silicon G has been removed. The recess 12c is formed in an anchor shape. A recess 13b (shown in Figure 3) is also formed on the surface of the outer periphery 13a of the second skirt 13 in the portion from which the silicon G has been removed.

Referring to Figures 4 and 8, in the surface treatment step S6, the first and second coating layers 20, 30 can be formed by printing a chemical agent, in which a solid lubricant and fine particles are dispersed in a resin material as a binder, on the outer periphery 12a, 13a of the first and second skirts 12, 13 by a screen method, a spray method, or the like, and then baking them. However, in the surface treatment step, the first and second coating layers can also be formed by other steps.

As shown in FIG. 8(b), the surface treatment step S6 forms a recess 21 in the first coating layer 20 following the recess 12c in the outer periphery 12a of the first skirt 12. In particular, the recess 21 in the specific portion 20a of the first coating layer 20 is formed following the recess 12c in the outlet-corresponding portion 12b of the first skirt 12. The first coating layer 20 adheres to the first skirt 12 due to the anchor effect provided by the anchor-shaped recess 12c.

In addition, the surface treatment step S6 forms a recess 31 in the second coating layer 30 following the recess 13b in the outer periphery 13a of the second skirt 13. The second coating layer 30 adheres to the second skirt 13 due to the anchor effect brought about by the anchor-shaped recess 13b.

As described above, the piston 1 of the internal combustion engine according to this embodiment includes a piston body 11 having an outer periphery 11a configured to be slidable in the longitudinal direction along the longitudinal axis L relative to the inner periphery 2a of the cylinder bore 2 within the cylinder bore 2 extending along a linear longitudinal axis L, and two skirts 12, 13 that face each other in the radial direction of the piston body 11 and extend from the outer periphery 11a of the piston body 11 toward the bottom of the cylinder bore 2, and the two skirts 12, 13 are arranged in a skirt opposing direction, that is, a first skirt 12 located on the thrust side and a second skirt 12 located on the anti-thrust side in the skirt opposing direction. A piston 1 for an internal combustion engine, which is composed of a first skirt 12 and a second skirt 13, a first coating layer 20 is formed on the outer periphery 12a of the first skirt 12 facing the inner periphery 2a of the cylinder bore 2, and a second coating layer 30 is formed on the outer periphery 13a of the second skirt 13 facing the inner periphery 2a of the cylinder bore 2, and the first coating layer 20 has more recesses 21 formed therein than the second coating layer 30, and the size of the area occupied by the recesses 21 in a unit area in a specific portion 20a of the first coating layer 20 is larger than the size of the area occupied by the recesses 21 in a unit area in the remaining portion 20b of the first coating layer 20 other than the specific portion 20a.

In such a piston 1, the oil that accumulates in the recesses 21 of the first coating layer 20 on the first skirt 12, particularly in the recesses 21 of the specific portion 20a of the first coating layer 20, is greater than the oil that accumulates in the recesses 31 of the second coating layer 30 on the second skirt 13. Therefore, even if the load acting on the first skirt 12 on the thrust side becomes greater than the load acting on the second skirt 13 on the anti-thrust side, the seizure resistance of the first coating layer 20 on the thrust side can be made greater than the seizure resistance of the second coating layer 30 on the anti-thrust side.

Even if such a piston 1 is used for a long period of time, the amount of wear of the first coating layer 20 on the thrust side and the amount of wear of the second coating layer 30 on the anti-thrust side can be made substantially equal. Therefore, the initial sliding performance of the piston 1 can be maintained for a long period of time. Therefore, in the piston 1 according to this embodiment, the sliding performance of the piston 1 can be efficiently improved.

In the piston 1 according to this embodiment, the specific portion 20a of the first coating layer 20 is disposed in the longitudinal direction at a reference portion 1a having a reference diameter defined on the outer periphery 12a, 13a of the first and second skirts 12, 13.

In such a piston 1, the oil pool that accumulates in the specific portion 20a of the first coating layer 20, which is located in the reference portion 1a that receives a larger load than other portions, can be made larger than the oil pools in other portions. Therefore, the seizure resistance of the specific portion 20a of the first coating layer 20 on the thrust side can be made higher than the seizure resistance of the remaining portion 20b of the first coating layer 20 and the second coating layer 30 on the anti-thrust side. Therefore, the initial sliding performance of the piston 1 can be maintained for a long period of time. In other words, the sliding performance of the piston 1 can be efficiently improved.

The manufacturing method of the piston 1 of the internal combustion engine according to this embodiment is a method for manufacturing a piston 1 of an internal combustion engine, comprising: a piston body 11 having an outer periphery 11a configured to be slidable in the longitudinal direction along the longitudinal axis L with respect to an inner periphery 2a of the cylinder bore 2 within the cylinder bore 2 extending along a linear longitudinal axis L; and a raw material 10 including two skirts 12, 13 that face each other in the radial direction of the piston body 11 and extend from the outer periphery 11a of the piston body 11 toward the bottom of the cylinder bore 2, the two skirts 12, 13 being composed of a first skirt 12 located on the thrust side in the skirt facing direction and a second skirt 13 located on the anti-thrust side in the skirt facing direction, and the outer peripheries 12a, 13a of the first and second skirts 12 being formed by forming the first and second skirts 12 in a first and second direction. A manufacturing method for a piston 1 for an internal combustion engine, in which coating layers 20, 30 are formed on the surface of the piston 1, includes a step S1 of pouring molten metal E from an outlet 41 of a die 40 arranged to correspond to a portion 12b of the outer periphery 12a of the first skirt 12 into a cavity portion 42 of the die 40 formed to fit the shape of the piston 1 rough material 10, a step S2 of removing the shaped piston 1 rough material 10 from the cavity portion 42 of the die 40, a step S5 of removing silicon G exposed on the surface of the removed piston 1 rough material 10 by water jet spray, and a step S6 of forming the coating layers 20, 30 on the surface of the piston 1 rough material 10 from which the silicon G has been removed.

In the piston 1 produced by this manufacturing method, the oil that accumulates in the recesses 21 of the first coating layer 20 on the first skirt 12, particularly in the recesses 21 of the specific portion 20a of the first coating layer 20, is greater than the oil that accumulates in the recesses 31 of the second coating layer 30 on the second skirt 13. Therefore, even if the load acting on the first skirt 12 on the thrust side becomes greater than the load acting on the second skirt 13 on the anti-thrust side, the seizure resistance of the first coating layer 20 on the thrust side can be made greater than the seizure resistance of the second coating layer 30 on the anti-thrust side.

Even if such a piston 1 is used for a long period of time, the amount of wear of the first coating layer 20 on the thrust side and the amount of wear of the second coating layer 30 on the anti-thrust side can be made substantially equal. Therefore, the initial sliding performance of the piston 1 can be maintained for a long period of time. Therefore, in the piston 1 according to this embodiment, the sliding performance of the piston 1 can be efficiently improved.

Furthermore, the water jet spray used to remove the silicon G crystallized on the surface of the rough material 10 of the piston 1 can cause less damage to the rough material 10 of the piston 1 compared to shot peening, mixed acid treatment, etc. Therefore, the shape precision of the piston 1 can be improved. Therefore, in the manufacturing method of the piston 1 according to this embodiment, the sliding performance of the piston 1 can be efficiently improved.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-mentioned embodiments, and the present invention can be modified and changed based on its technical concept.

"Example"
An example will be described. In the example, the piston 1 according to the above embodiment made of an Al-Si aluminum alloy was manufactured by the manufacturing method according to the above embodiment. As a further condition, in the casting step S1, the molten metal E was an Al-Si aluminum alloy molten metal E. In the heat treatment step S3, a T6 heat treatment was performed. In the silicon removal step S5, water jet injection was performed only on the first and second skirts 12, 13. The injection pressure of the water W in the water jet injection was set to about 160 MPa.

In this embodiment, the surfaces of the first and second skirts 12, 13 of the raw material 10 of the piston 1 before the surface treatment step S6 were observed with an optical microscope set at a magnification of about 200 times. Furthermore, the dimensions of the multiple recesses 12c formed in the outlet corresponding portion 12b of the first skirt 12 and the multiple recesses 13b formed in the second skirt 13 were obtained by the diameter derived from three measurement points arbitrarily extracted from the contour of the recess 13b, and the average value of these dimensions was calculated. The surfaces of the first and second coating layers 20, 30 on the first and second skirts 12, 13 of the piston 1 after the surface treatment step S6 were observed with an SEM (scanning electron microscope) set at a magnification of about 40 times.

"Comparative Example"
A comparative example will now be described. In the comparative example, a piston rough material made of an Al-Si aluminum alloy was manufactured by the same manufacturing method as in the example, except that the silicon removal process was not performed. In this comparative example, the surfaces of the first and second skirts of the piston rough material before the surface treatment process were observed with an optical microscope set at a magnification of about 200 times. The surfaces of the first and second coating layers on the first and second skirts of the piston after the surface treatment process were observed with an SEM set at a magnification of about 40 times.

Comparing the results of the optical microscope observations in the examples and comparative examples, the following was confirmed. In the examples, the presence of numerous recesses 12c on the surface of the first skirt 12 and the presence of numerous recesses 13b on the surface of the second skirt 13 were confirmed. On the other hand, in the comparative examples, it was confirmed that the surfaces of the first and second skirts were smooth. In other words, in the comparative examples, the presence of recesses on the surfaces of the first and second skirts could not be confirmed. Therefore, it was confirmed that the surface treatment process using water jet spraying can reliably form the recesses 12c of the first skirt 12 and the recesses 13b of the second skirt 13 so as to form the recesses 21, 31 of the first and second coating layers 20, 30.

In the examples, the following was confirmed by comparing the average dimensions of the recesses 12c in the outlet-corresponding portion 12b of the first skirt 12 and the recesses 13b in the second skirt 13. The average dimension of the recesses 12c in the outlet-corresponding portion 12b of the first skirt 12 was about 44 μm. The average dimension of the recesses 13b in the second skirt 13 was about 33 μm. Furthermore, according to the results of observation with an optical microscope in the examples, it was confirmed that the multiple recesses 12c formed in the outlet-corresponding portion 12b of the first skirt 12 were more densely packed than the multiple recesses 13b in the second skirt 13. Therefore, in the examples, it was confirmed that the recesses 12c can be reliably formed in the outlet-corresponding portion 12b of the first skirt 12 so as to form the recesses 21 that provide good oil storage in the specific portion 20a of the first coating layer 20.

Comparing the SEM observation results of the examples and comparative examples, the following was confirmed. In the examples, the presence of numerous recesses 21 in the first coating layer 20 and the presence of numerous recesses 31 in the second coating layer 30 were confirmed. In the comparative examples, it was confirmed that the surfaces of the first and second coating layers were smooth. That is, in the comparative examples, the presence of recesses in the first and second coating layers could not be confirmed. Therefore, it was confirmed that the recesses 21, 31 in the first and second coating layers 20, 30 can be reliably formed by the surface treatment process using water jet spraying.

1...piston, 1a...reference portion 2...cylinder bore, 2a...inner periphery 10...raw material, 11...piston body, 11a...outer periphery, 12...first skirt (skirt), 12a...outer periphery, 12b...part (part corresponding to outlet), 13...second skirt (skirt), 13a...outer periphery 20...coating layer, first coating layer, 20a...specific portion, 20b...remaining portion, 21...recess 30...coating layer, second coating layer 40...mold, 41...outlet, 42...cavity portion L...longitudinal axis, E...molten metal, G...silicon, primary crystal silicon S1...casting process (process), S2...removal process (process), S5...silicon removal process (process), S6...surface treatment process (process)

Claims (2)

a piston body having an outer periphery configured to be slidable in a longitudinal direction along a linear longitudinal axis relative to an inner periphery of the cylinder bore within the cylinder bore;
two skirts that face each other in the radial direction of the piston body and extend from the outer periphery of the piston body toward the bottom of the cylinder bore;
The two skirts are composed of a first skirt located on the thrust side in a skirt facing direction in which the two skirts face each other, and a second skirt located on the anti-thrust side in the skirt facing direction,
a first coating layer is formed on an outer periphery of the first skirt facing an inner periphery of the cylinder bore;
A piston for an internal combustion engine, comprising: a second coating layer formed on an outer periphery of the second skirt facing an inner periphery of the cylinder bore,
a recess in the first coating layer is formed to correspond to the recess in the outer periphery of the first skirt;
a recess in the second coating layer is formed to correspond to the recess in the outer periphery of the second skirt;
The first coating layer has more recesses formed therein than the second coating layer,
A piston for an internal combustion engine, wherein the size of the area occupied by the recess per unit area in a specific portion in the center of the first coating layer is larger than the size of the area occupied by the recess per unit area in the remainder of the first coating layer other than the specific portion, and the specific portion is surrounded by the remainder . A method for manufacturing a piston for an internal combustion engine, comprising: a blank including a piston body having an outer periphery configured to be slidable in a longitudinal direction along a linear longitudinal axis relative to an inner periphery of a cylinder bore extending along the longitudinal axis; and two skirts facing each other in a radial direction of the piston body and extending from the outer periphery of the piston body toward a bottom of the cylinder bore, the two skirts being comprised of a first skirt located on the thrust side in the skirt facing direction and a second skirt located on the anti-thrust side in the skirt facing direction, the method comprising the steps of: forming a coating layer on the outer periphery of each of the first and second skirts;
A process for forming a blank of the piston by pouring a molten metal from an outlet of the die arranged to correspond to a part of the outer periphery of the first skirt into a cavity portion of the die formed to fit the shape of the blank of the piston;
removing the shaped piston blank from the cavity of the die;
removing silicon exposed on the surface of the removed piston blank by water jet spray;
forming the coating layer on the surface of the piston raw material from which the silicon has been removed.