JP7433581B2 - Piston for internal combustion engine and its manufacturing method - Google Patents

Description

The present invention relates to a method of manufacturing a piston for an internal combustion engine by casting using a metal mold, that is, a method of manufacturing a piston for an internal combustion engine. The invention also relates to a piston for an internal combustion engine formed by casting using a mold.

Pistons used in internal combustion engines such as engines are formed by casting. Specifically, the piston is formed by filling a mold with molten metal such as aluminum, aluminum alloy, etc., and then cooling and solidifying the molten metal.

During casting, the surface of the mold comes into contact with the molten metal to be filled, and comes into contact with the cast surface of the piston formed after the molten metal is cooled and solidified. Therefore, it is necessary to protect the surface of the mold, to prevent the molten metal from solidifying during filling, and to ensure heat retention to ensure the fluidity of the molten metal. A mold coating agent is applied to the surface of the mold for the purpose of reliably releasing the mold from the mold. A layer made of this mold coating agent (hereinafter referred to as a "coating layer") is formed on the surface of the mold. (For example, see paragraph [0029] of Patent Document 1.)

JP2018-178848A

However, when casting is repeated continuously using the above mold, the coating layer may peel off locally. In this case, the mold is locally depressed, and the piston is raised to correspond to the depression. Such a protrusion of the piston causes variations in the volume of the combustion chamber of the internal combustion engine defined by the piston and the cylinder.

In view of these circumstances, in a method of manufacturing a piston for an internal combustion engine, it is desirable to suppress variations in the shape of the piston even when casting is repeated. Furthermore, in pistons for internal combustion engines, it is desirable to suppress variations in shape.

In order to solve the above problems, a method for manufacturing a piston for an internal combustion engine according to one aspect is a method for manufacturing a piston for an internal combustion engine, the method comprising: manufacturing a piston for an internal combustion engine by casting using a mold. The piston has a piston top forming an intake valve recess, and the intake valve recess tapers from an intake bottom wall forming at least a bottom thereof and an outer portion of the intake bottom wall located near an outer peripheral edge of the piston top. and an intake outer side wall that extends above the piston top so as to form an arc shape protruding toward the outer periphery of the piston top. The method includes a surface roughening treatment step of performing shot peening treatment on a region of the mold corresponding to an intermediate portion in the extending direction of the piston, and a casting step of forming the piston top by casting using the mold.

In order to solve the above problems, a piston for an internal combustion engine according to one embodiment is a piston for an internal combustion engine formed by casting a metal alloy using a mold, the piston having a piston top formed with an intake valve recess, and The valve recess tapers up from an intake bottom wall forming at least the bottom of the intake bottom wall and an outer part of the intake bottom wall located near the outer periphery of the piston top, and extends above the piston top and extends toward the outer periphery of the piston top. In a piston for an internal combustion engine having an intake outer side wall extending to form an arc shape protruding toward the inner combustion engine, the surface roughness of the surface of the metal alloy at an intermediate portion in the extending direction of the intake outer side wall is Ra is 4 μm or less.

In the method for manufacturing a piston for an internal combustion engine according to one embodiment, variations in the shape of the piston can be suppressed even when casting is repeated. Further, in the piston for an internal combustion engine according to one aspect, variations in the shape of the piston can be suppressed.

FIG. 1 is a plan view schematically showing the top of a piston according to an embodiment. FIG. 2 is a cross-sectional view schematically showing a piston according to one embodiment and a mold corresponding to the top of the piston, taken along line AA in FIG. 1. FIG. 3 is an enlarged view of part B in FIG. FIG. 4 is an enlarged view of section C in FIG. FIG. 5 is a bottom view schematically showing a mold corresponding to the top of the piston according to one embodiment. FIG. 6 is an enlarged view of section D in FIG. FIG. 7 is an enlarged view of section E in FIG. FIG. 8 is a flowchart for explaining a piston manufacturing method according to one embodiment. FIG. 9 is a graph showing the surface roughness of measurement points 1 to 4 of the piston in Examples and Comparative Examples. FIG. 10(a) is a graph showing the relationship between the variation rate of the combustion chamber volume at the top of the piston and the number of shots in which the piston was cast using a mold in the example. FIG. 10(b) is a graph showing the relationship between the variation rate of the combustion chamber volume at the top of the piston and the number of shots in which the piston was cast using a mold in a comparative example.

A piston and a method for manufacturing the same according to one embodiment will be described below. The piston according to this embodiment is formed by casting using a metal mold. That is, the piston is a cast product. Further, the piston according to this embodiment is used in an automobile engine. However, the piston may also be used in internal combustion engines other than automobile engines. For example, the piston may be used in a general-purpose engine installed in a motorcycle engine, an outboard motor, a snowmobile engine, a small lawn mower, or the like. In the manufacturing method according to this embodiment, such a piston is manufactured by casting using a mold.

"About the outline of pistons"
First, the outline of the piston 1 according to the present embodiment will be explained with reference to FIGS. 1 to 4. As shown in FIGS. 1 to 3, the piston 1 has a piston top 2 located at its top end. An intake valve recess 3 is formed in the piston top 2. The intake valve recess 3 is recessed from the piston top 2 to avoid the intake valve (not shown).

In FIGS. 1 and 3, as an example, two intake valve recesses 3 are formed in the piston top 2, each corresponding to two intake valves (not shown). However, the present invention is not limited thereto, and the piston top may be formed with one intake valve recess corresponding to one intake valve, or the piston top may be formed with three intake valve recesses, each corresponding to three or more intake valves. The above intake valve recess may be formed.

As shown in FIG. 3, each intake valve recess 3 has an intake bottom wall 31 forming at least its bottom. Each intake valve recess 3 has an intake outer side wall 32 that tapers and rises from an outer portion 31a of an intake bottom wall 31 of the intake valve recess 3 located near the outer peripheral edge 2a of the piston top 2. As shown in FIGS. 1 and 3, the intake outer side wall 32 extends on the piston top 2 so as to form an arc shape that projects toward the outer peripheral edge 2a of the piston top 2. As shown in FIGS.

Furthermore, the outline of the piston 1 according to this embodiment is preferably as follows. As shown in FIGS. 1 to 4, an exhaust valve recess 4 is formed in the piston top 2. As shown in FIGS. Exhaust valve recess 4 is recessed from piston top 2 to avoid an exhaust valve (not shown).

In FIGS. 1 and 4, as an example, two exhaust valve recesses 4 are formed in the piston top 2, each corresponding to two exhaust valves (not shown). However, the present invention is not limited thereto, and the piston top may be formed with one exhaust valve recess corresponding to one exhaust valve, or the piston top may be formed with three exhaust valve recesses, each corresponding to three or more exhaust valves. The above exhaust valve recess may be formed.

As shown in FIG. 4, each exhaust valve recess 4 has an exhaust bottom wall 41 forming at least the bottom thereof. Each exhaust valve recess 4 has an exhaust outer side wall 42 that tapers and rises from an outer portion 41a of an exhaust bottom wall 41 of the exhaust valve recess 4 located near the outer peripheral edge 2a of the piston top 2. As shown in FIGS. 1 and 4, the exhaust outer side wall 42 extends on the piston top 2 so as to form an arc shape that projects toward the outer peripheral edge 2a of the piston top 2. As shown in FIGS.

As shown in FIGS. 1, 3 and 4, in the case where the piston top 2 has a plurality of intake valve recesses 3 and a plurality of exhaust valve recesses 4, the piston top 2 It has an intake connecting wall 5 that connects the intake outer side walls 32 of the matching intake valve recesses 3. The piston top 2 also has an exhaust connection wall 6 that connects the exhaust outer side walls 42 of adjacent exhaust valve recesses 4 among the plurality of exhaust valve recesses 4 .

"About the details of the piston"
Details of the piston 1 according to this embodiment will be described with reference to FIGS. 1 to 4. That is, the details of the piston 1 are preferably as follows. The material of the piston 1 is preferably an aluminum alloy. However, the material of the piston is not limited to this. For example, the material of the piston may be iron, aluminum, titanium, titanium alloy, magnesium, magnesium alloy, or the like.

As shown in FIGS. 1 and 2, the outer circumferential surface of the piston 1 is formed into a substantially cylindrical shape with the piston axis 1a as the center and surrounding the piston axis 1a. The top of the piston 1 is located on one side (indicated by arrow P1) in the piston axis direction along the piston axis 1a. The bottom of the piston 1 is located on the other side (indicated by arrow P2) in the piston axial direction along the piston axis 1a. As shown in FIG. 2, the piston top 2 is arranged along a virtual piston top surface 1b that is substantially orthogonal to the piston axis 1a and located at the top of the piston 1. Although not particularly illustrated, the piston 1 reciprocates within the cylinder of the engine along the piston axis 1a.

As shown in FIG. 1, the outer peripheral edge 2a of the piston top 2 is formed into a substantially circular shape. The intake and exhaust valve recesses 3 and 4 are arranged to face each other in one radial direction of the piston top 2 (hereinafter referred to as "recess opposing direction"). The plurality of intake valve recesses 3 may be arranged so as to face the plurality of exhaust valve recesses 4, respectively, in the recess opposing direction.

The intake valve recess 3 is located on one side (indicated by arrow Q1) of the boundary line 2b of the piston top 2 in a direction facing the recess. The exhaust valve recess 4 is located on the other side (indicated by arrow Q2) of the boundary line 2b of the piston top 2 in the direction in which the recess is opposed. Note that the boundary line 2b is defined as a straight line that is substantially perpendicular to the direction in which the recess faces and passes through the substantially center of the piston top portion 2.

As shown in FIG. 2, the intake bottom wall 31 is inclined so that the farther it is from the boundary line 2b in the direction facing the recess, the closer it is to the bottom of the piston 1. This intake bottom wall 31 forms the bottom of the intake valve recess 3 and a wall connected to the piston top 2 on the boundary line 2b side with respect to this bottom. The intake outer side wall 32 is inclined so that the farther it is from the outer part 31a of the intake bottom wall 31 toward the outer peripheral edge 2a of the piston top 2, the closer it is to the top of the piston 1. The inclination of the intake outer side wall 32 with respect to the piston virtual top surface 1b is greater than the inclination of the intake bottom wall 31 with respect to the piston virtual top surface 1b.

The exhaust bottom wall 41 is inclined so that the farther it is from the boundary line 2b in the direction facing the recess, the closer it is to the bottom of the piston 1. The exhaust bottom wall 41 forms the bottom of the exhaust valve recess 4 and a wall connected to the piston top 2 on the boundary line 2b side with respect to this bottom. The exhaust outer side wall 42 is inclined so that the farther it is from the outer part 41a of the exhaust bottom wall 41 toward the outer peripheral edge 2a of the piston top 2, the closer it is to the top of the piston 1.

The slope of the exhaust outer side wall 42 with respect to the piston virtual top surface 1b is greater than the slope of the exhaust bottom wall 41 with respect to the piston virtual top surface 1b. The slope of the exhaust outer side wall 42 with respect to the piston virtual top surface 1b can also be made smaller than the slope of the intake outer side wall 32 with respect to the piston virtual top surface 1b. However, the slope of the exhaust outer sidewall relative to the piston virtual top surface may also be approximately equal to the slope of the intake outer sidewall relative to the piston virtual top surface, or greater than the slope of the intake outer sidewall relative to the piston virtual top surface. You can also do it.

As shown in FIGS. 1 and 3, the intake connecting wall 5 extends substantially linearly on the piston top 2 between the ends of the adjacent intake outer side walls 32 in the extending direction. The intake connecting wall 5 is inclined so that the farther it is from the boundary line 2b in the direction facing the recess, the closer it is to the top of the piston 1. Although not specifically illustrated, the inclination of the intake connecting wall 5 with respect to the piston virtual top surface 1b is smaller than the inclination of the intake outer side wall 32 with respect to the piston virtual top surface 1b.

As shown in FIGS. 1 and 4, the exhaust connection wall 6 extends substantially linearly on the piston top 2 between the ends of the adjacent exhaust outer side walls 42 in the extending direction. The exhaust connection wall 6 is inclined so that the farther it is from the boundary line 2b in the direction facing the recess, the closer it is to the top of the piston 1. Although not specifically illustrated, the slope of the exhaust connection wall 6 with respect to the piston virtual top surface 1b is smaller than the slope of the exhaust outer side wall 42 with respect to the piston virtual top surface 1b.

"About the mold"
The mold 101 used in manufacturing the piston 1 according to this embodiment will be described with reference to FIGS. 2 and 5 to 8. The material of the mold 101 is a material equivalent to SKD61. However, the material of the mold is not limited to this. For example, the material of the mold can be tempered steel. The material of the mold may be SC material, SCМ material, SUS material, etc.

Referring to FIGS. 2 and 5 to 7, the mold 101 has a piston top region 102 that corresponds to the piston top 2. As shown in FIGS. The piston top region 102 has an intake valve recess region 103 corresponding to the intake valve recess 3 . The intake valve recess region 103 has an intake bottom wall region 131 corresponding to the intake bottom wall 31 . Intake valve recess region 103 also has an intake outer sidewall region 132 that corresponds to intake outer sidewall 32 .

The mold 101 has an exhaust valve recess area 104 corresponding to the exhaust valve recess 4 . The exhaust valve recess region 104 has an exhaust bottom wall region 141 that corresponds to the exhaust bottom wall 41 . Exhaust valve recess region 104 also has an exhaust outer sidewall region 142 that corresponds to exhaust outer sidewall 42 .

The mold 101 has an intake connecting wall region 105 corresponding to the intake connecting wall 5 . The mold 101 has an exhaust connection wall region 106 corresponding to the exhaust connection wall 6 .

The piston top region 102 of such a mold 101 is partially or entirely subjected to shot peening treatment. Therefore, the mold 101 has a treatment area subjected to such shot peening treatment. The surface roughness of the treated area is preferably approximately Rzjis 20 μm to 50 μm. However, the surface roughness of the treated area is not limited to this. The treated surface of the piston 1, particularly the piston top 2, corresponding to the treated area is formed to have a surface roughness corresponding to the surface roughness of the treated area. The surface roughness of the treated surface of the piston 1, particularly the piston top 2, is preferably about Ra 4 μm or less. In particular, a treated surface with a surface roughness of about Ra 4 μm or less may be produced by transferring a treated area with a surface roughness of about Rzjis 20 μm to 50 μm. However, the surface roughness of the treated surface is not limited to this.

Referring to FIGS. 6 and 7, the treatment region may be an intermediate section 132a of the outer intake sidewall region 132 corresponding to an intermediate section 32a of the outer intake sidewall 32 in the extension direction. In this case, the intermediate portion 32a of the outer intake sidewall 32 will have a treated surface that is a transfer of the intermediate section 132a of the outer intake sidewall region 132.

The processing regions are intermediate sections 132a and 142a in the extending direction of the intake and exhaust outer sidewall regions 132 and 142, respectively, corresponding to intermediate portions 32a and 42a in the extending direction of the intake and exhaust outer sidewalls 32 and 42. I can do it. In this case, the intermediate portions 32a, 42a of the outer intake and exhaust sidewalls 32, 42 will have a treated surface that is a transfer of the intermediate section 132a, 142a of the outer intake and exhaust sidewall regions 132, 142.

The treatment area may be the entire intake and exhaust outer sidewall areas 132, 142. In this case, the entire intake and exhaust outer sidewalls 32, 42 will have a treated surface that is a transfer of the entire intake and exhaust outer sidewall regions 132, 142.

The treatment area may be the entire intake and exhaust outer sidewall areas 132, 142 and the intake and exhaust connecting wall areas 105, 106. In this case, the entire intake and exhaust outer side walls 32, 42 and the intake and exhaust connecting walls 5, 6 cover the entire intake and exhaust outer side wall regions 132, 142 and the intake and exhaust connecting wall regions 105, 106. It will have a transferred treated surface.

The treatment areas may be intake and exhaust valve recess areas 103, 104 and intake and exhaust connection wall areas 105, 106. In this case, the intake and exhaust valve recesses 3, 4 and the intake and exhaust connection walls 5, 6 have a treated surface onto which the intake and exhaust valve recess areas 103, 104 and the intake and exhaust connection wall areas 105, 106 are transferred. Become.

The treatment area may be the piston top area 102. In this case, the piston top 2 will have a treated surface onto which the piston top region 102 has been transferred.

"About the piston manufacturing method"
With reference to FIG. 8, a method for manufacturing the piston 1 according to this embodiment will be described. That is, the method for manufacturing the piston 1 using the mold 101 is as follows. Shot peening treatment is performed on the treatment area of the mold 101 (surface roughening treatment step S1). Piston top 2 is formed by casting aluminum alloy using mold 101 (casting step S2).

Specifically, the casting process S2 is as follows. Molten aluminum alloy is poured into a mold 101. The mold 101 is filled with molten metal so as to fill the entire piston top region 102. The mold 101 filled with the molten metal is left in place for a certain period of time until the molten metal is cooled and solidified so that the shape of the piston top 2 is determined. Thereafter, the shaped piston 1 is taken out from the mold 101.

Further, after the surface roughening treatment step S1 and before the casting step S2, a surface treatment for suppressing wear may be performed on the treatment region of the mold 101 (surface treatment step). For example, the surface treatment can be carbonization treatment using molten salt, CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), DLC (Diamond-Like Carbon) coating, or the like. Such surface treatment can prevent substances based on the material of the piston 1, such as aluminum alloy, from adhering to parts of the mold 101 that are difficult to release. As a result, the frequency of cleaning, reprocessing, etc. of the mold 101 can be reduced.

As described above, in the method for manufacturing the piston 1 according to the present embodiment, the piston top region 102 of the mold 101 is partially or entirely subjected to shot peening treatment, and the mold 101 undergoes the shot peening treatment. Has an area. Furthermore, in the piston 1 according to the present embodiment, the surface of the piston top 2 is a treated surface onto which the piston top region 102 partially or entirely shot peened is transferred. Therefore, even when casting is continuously repeated using the mold 101, the coating layer of the mold does not peel off in the processing area of the mold 101, unlike in the conventional case. Therefore, deformation of the mold 101 can be prevented, and as a result, variations in the shape of the piston 1 manufactured can be suppressed.

Here, the present inventor has arrived at the following new knowledge regarding the principle of peeling of the mold coating layer when a mold coating agent is applied to the surface of a mold as in the past. Generally, a concave portion is formed at the top of the piston, and a convex portion corresponding to the concave portion at the top of the piston is formed on the surface of the mold. In particular, the side wall rising from the bottom wall of the concave portion at the top of the piston is sloped to provide a draft angle, etc., and the corresponding side wall of the convex portion of the mold is also sloped. As the inclination of the side wall surface of the convex portion of the mold increases, it becomes more difficult to apply a mold coating agent to the side wall of the convex portion of the mold. As a result, the coating layer on the side wall of the convex portion of the mold may be thinner than the coating layer on other portions of the mold. Moreover, it becomes difficult to make the thickness of the mold layer constant, and there is a possibility that the thickness of the mold layer becomes non-uniform.

In this case, the heat insulating performance of the coating layer decreases on the sidewalls of the convex portions, and the temperature of the sidewalls of the convex portions rises more easily than the temperature of other portions. In particular, since the contact area with the molten metal in the convex part of the mold is larger than the contact area with the molten metal in other parts of the mold, the temperature of the convex part tends to be higher than the temperature of other parts. When casting is repeated continuously using such a mold, the adhesion between the side wall of the convex part of the mold and the coating layer decreases, and due to this decrease in adhesion, the convex part of the mold The coating layer on the side wall of the mold is likely to peel off. In addition, undercuts may occur in thin coating layers on the side walls of the convex portions of the mold, coating layers with uneven thickness, etc., and such undercuts may cause the piston to release from the mold. At times, there is a risk that resistance may occur that may cause the coating layer to peel off.

A comparison of the piston 1 having the above-mentioned external shape, particularly the inclination within the piston top 2, is as follows. The inclinations of the intake and exhaust outer side walls 32, 42 and the inclinations of the intake and exhaust connection walls 5, 6 at the piston top 2 are greater than the inclinations of other parts of the piston top 2. The inclination of the intake and exhaust outer side walls 32, 42 is greater than the inclination of other parts of the piston top 2. The inclination of the intermediate portions 32a, 42a of the intake and exhaust outer side walls 32, 42 in the extending direction is greater than the inclination of other portions of the piston top 2. The inclination of the intermediate portion 32a of the intake outer side wall 32 in the extending direction is greater than the inclination of other portions of the piston top portion 2.

Based on any of these slope comparisons, when the large slope part of the piston top 2 is defined as a part having a larger slope than other parts of the piston top 2, when a coating agent is applied to the piston top 2, The mold layer formed by the mold coating agent in the large slope portion peels off more easily than the mold layer in other portions.

On the other hand, in the manufacturing method according to the present embodiment, shot peening treatment is performed on the large slope portion of the piston top 2 instead of applying a mold coating agent. Further, in the piston 1 according to the present embodiment, the surface of the large slope portion of the piston top 2 is a treated surface that is a transfer of the piston top region 102 that has been subjected to shot peening treatment. Therefore, even when casting is repeated continuously using a mold, it is possible to prevent the shape of the mold from changing due to peeling of the coating layer of the mold as in the past. , it is possible to suppress variations in the shape of the piston 1 due to changes in the shape of the mold.

Further, in general, when a mold coating agent is not applied to the region of the mold, a heat insulating layer is formed by unevenness caused by the roughness of the region of the mold. Therefore, if the surface roughness of the mold region is too small, this heat insulating layer will not be sufficiently formed, and as a result, there is a risk that poor flow of the metal will occur during casting. On the other hand, in the manufacturing method according to the present embodiment, the surface roughness of the processing area of the mold 101 is set to approximately Rzjis 20 μm or more, so that a sufficient heat insulating layer can be formed, and as a result, the It is possible to prevent the occurrence of poor water circulation.

Furthermore, generally, when casting is repeated continuously using a mold, the surface roughness of the region of the mold deteriorates, and as a result, the surface roughness of the casting surface of the piston may deteriorate. On the other hand, in the manufacturing method according to the present embodiment, the surface roughness of the processing area of the mold 101 is set to approximately Rzjis 50 μm or less, so even if casting is repeated continuously using the mold 101, Also, it is possible to prevent the surface roughness of the treated area of the mold 101 from deteriorating as the surface roughness of the casting surface of the piston 1 would deteriorate.

In addition, in the piston 1 according to the present embodiment, the surface roughness of the treated surface of the piston 1, particularly the piston top 2, can be set to about Ra 4 μm or less. Therefore, even when casting shots are continuously repeated using the mold 101 for the piston 1, deterioration of the mold 101 is prevented, and thereby the shape variation of the piston 1 is reduced to a small range. It can be suppressed. In particular, when forming a coating layer on the surface of a mold as in the past, it was difficult to reduce the surface roughness of a piston cast using such a mold to approximately Ra4μm or less; Therefore, the surface roughness of the treated surface of the piston 1 according to the present embodiment, particularly the piston top 2, can be set to about Ra4 μm or less, and as a result, the shape variation of the piston 1 according to the present embodiment can be reduced more than before. It can be kept within a small range.

Note that the surface roughness of the treated surface of the piston 1, particularly the piston top 2, can be defined as being at least a limit value that can be obtained by transferring the surface of a mold that has been subjected to shot peening treatment. . The surface roughness of the treated surface of the piston 1, particularly the piston top 2, can also be defined as being greater than about 0 (zero).

Although the embodiments of the present invention have been described so far, the present invention is not limited to the above-described embodiments, and the present invention can be modified and changed based on the technical idea thereof.

"Example"
An example will be explained. In the example, the piston 1 according to the above embodiment was manufactured using the mold 101 according to the above embodiment. Specifically, even when casting the piston 1 using the mold 101 for 4,500 shots, the treated surface of each of the two intake valve recesses 3 and the two exhaust valve recesses 4 in the piston top 2 of the piston 1 is Shot peening treatment was performed on the two intake valve recess regions 103 and the two exhaust valve recess regions 104 in the piston top region 102 of the mold 101 with the aim of maintaining the surface roughness at about Ra 4 μm or less. 4500 shots of casting were performed using such a mold 101, thereby manufacturing 4500 pistons 1.

In such an example, the surface roughness of the treated surface in each of the two intake valve recesses 3 and the two exhaust valve recesses 4 was measured on the piston top 2 of the piston 1 obtained by casting the 4500th shot. Note that in the following, one of the two intake valve recesses 3 will be referred to as "measurement site 1", the other of the two intake valve recesses 3 will be referred to as "measurement site 2", and one of the two exhaust valve recesses 4 will be referred to as "measurement site 3". ", and the other of the two exhaust valve recesses 4 is called the "measurement site 4." Regarding the measurement conditions of the surface roughness, the measurement was carried out based on "JIS B 0633:2001". Regarding further conditions in the measurement of surface roughness, the round (R) of the tip of the stylus used for measurement is 2 μm, the feeding speed of the stylus is 0.5 mm/sec, the cutoff value λc is 0.8 mm, And the measurement length was 2.5 mm.

Furthermore, in the example, the combustion chamber volume of the piston top 2 of the piston 1 obtained by casting the 500th, 1000th, 1500th, 2000th, and 2500th shot was measured using a laser measuring machine, and the combustion chamber volume was compared to the design value of the combustion chamber volume. The ratio of these combustion chamber volume measurements (hereinafter referred to as "combustion chamber volume variation rate" as necessary) was calculated.

"Comparative example"
A comparative example will be explained. In a comparative example, a piston was manufactured using a conventional mold. Specifically, as in the prior art, mold coating layers were formed in two intake valve recess areas and two exhaust valve recess areas in the piston top area of the mold. Using this mold, 4500 shots were cast, thereby manufacturing 4500 pistons.

In such a comparative example, the surface roughness of the treated surface in each of the two intake valve recesses and the two exhaust valve recesses was measured at the top of the piston obtained by casting the 100th shot. Note that in Comparative Example 1 as well, the names "measurement site 1" to "measurement site 4" are used as in Example 1. Furthermore, the conditions for measuring surface roughness in Comparative Example 1 are the same as those for measuring surface roughness in Example 1.

Furthermore, in a comparative example, the combustion chamber volume at the top of the piston of the piston obtained by casting the 500th, 1000th, 1500th, 2000th, and 2500th shot was measured using a laser measuring machine, and these values were compared to the design value of the combustion chamber volume. The ratio of the measured values of the combustion chamber volume, that is, the variation rate of the combustion chamber volume was calculated.

The measurement results of the surface roughness of such Examples and Comparative Examples are shown in the graph of FIG. In the graph of FIG. 9, the vertical axis Ra indicates the measured value of surface roughness (μm), and the horizontal axis M1, M2, M3, and M4 indicate the measurement sites 1, 2, 3, and 4, respectively. shows.

As shown using diagonally hatched bar elements in the graph of FIG. 9, the measured values of surface roughness at measurement sites 1, 2, 3, and 4 in the example are Ra2.452 μm and Ra2.386 μm, respectively. , Ra 3.503 μm, and Ra 3.137 μm. On the other hand, as shown by the white bar elements in the graph of FIG. They were Ra4.620μm, Ra5.567μm, and Ra4.543μm. Therefore, in the comparative example, it was confirmed that it was difficult to maintain the surface roughness of the measurement parts 1 to 4 at 4 μm or less, and on the other hand, it was difficult to maintain the surface roughness of the piston 1 at 4 μm or less. It was confirmed that it is effective to use the mold 101 subjected to shot peening treatment as in the example.

Next, the measurement results of the surface roughness of Examples and Comparative Examples are shown in the graphs of FIGS. 10(a) and 10(b). In the graphs of FIGS. 10(a) and 10(b), the vertical axis Y represents the variation rate of the combustion chamber volume, and the horizontal axis X represents the number of casting shots using the mold.

As shown by the solid line L1 in the graph of FIG. 10(a), even when 2,500 shots of casting were performed using the mold 101 in the example, the combustion of the piston 1 obtained by such casting The chamber volume variation rate was within the range of 0.995 to 1.005. On the other hand, as shown by the solid line L2 in the graph of FIG. The volume variation rate was within the range of 0.990 to 1.010. Therefore, it has been confirmed that the shape variation of pistons cast using shot peened molds can be smaller than that of pistons cast using conventional molds with a coating layer formed. did it.

DESCRIPTION OF SYMBOLS 1... Piston 2... Piston top, 2a... Outer periphery 3... Intake valve recess, 31... Intake bottom wall, 31a... Outer part, 32... Intake outer side wall, 32a... Intermediate part 4... Exhaust valve recess, 41... Exhaust bottom wall , 41a...Outer part, 42...Exhaust outer side wall, 42a...Intermediate part 5...Intake connecting wall 6...Exhaust connecting wall 101...Mold S1...Roughening treatment process S2...Casting process

Claims (10)

A method for manufacturing a piston for an internal combustion engine, comprising manufacturing a piston for an internal combustion engine by casting using a metal mold, wherein the piston for an internal combustion engine has a piston top portion forming an intake valve recess, and the intake valve recess has a bottom portion thereof. an air intake bottom wall forming at least an outer peripheral edge of the piston top, and an arc that tapers up from an outer part of the intake bottom wall located near the outer peripheral edge of the piston top, and projects above the piston top toward the outer peripheral edge of the piston top. In a method of manufacturing a piston for an internal combustion engine, the piston has an intake outer side wall extending to form a shape.
a surface roughening treatment step of performing shot peening treatment on a region of the mold corresponding to an intermediate portion in the extending direction of the intake outer side wall;
A method of manufacturing a piston for an internal combustion engine, comprising: a casting step of forming the top of the piston by casting using the mold. An exhaust valve recess is further formed in the piston top, and the exhaust valve recess tapers from an exhaust bottom wall forming at least the bottom thereof and an outer portion of the exhaust bottom wall located near the outer peripheral edge of the piston top. an exhaust outer side wall that rises and extends above the piston top so as to form an arcuate shape protruding toward the outer peripheral edge of the piston top;
2. The method for manufacturing a piston for an internal combustion engine according to claim 1, wherein, in the surface roughening treatment step, a shot peening treatment is performed on a region of the mold corresponding to an intermediate portion in the extending direction of the exhaust outer side wall. The method for manufacturing a piston for an internal combustion engine according to claim 2, wherein the shot peening treatment in the surface roughening treatment step is performed on a region of the mold corresponding to the entire intermediate portion of the intake and exhaust outer side walls. . the piston top has a plurality of the intake valve recesses and a plurality of the exhaust valve recesses;
The piston top portion includes an intake connection wall that connects the intake outer side walls of adjacent intake valve recesses among the plurality of intake valve recesses, and an exhaust connection wall that connects the exhaust outer side walls of adjacent exhaust valve recesses among the plurality of exhaust valve recesses. having a wall;
4. The method for manufacturing a piston for an internal combustion engine according to claim 2, wherein in the surface roughening treatment step, a shot peening treatment is performed on a region of the mold corresponding to the intake and exhaust connection walls. According to any one of claims 1 to 4, the shot peening treatment in the surface roughening treatment step is such that the surface roughness of the region of the mold to which the shot peening treatment is applied is Rzjis 20 μm to 50 μm. A method of manufacturing a piston for an internal combustion engine. A piston for an internal combustion engine formed by casting a metal alloy using a mold,
Equipped with a piston top that forms an intake valve recess,
The intake valve recess tapers up from an intake bottom wall that forms at least the bottom of the intake valve recess and an outer part of the intake bottom wall that is located near the outer periphery of the piston top, and extends above the piston top from the outside of the piston top. A piston for an internal combustion engine, the piston having an intake outer side wall extending to form an arcuate shape protruding toward the periphery,
A piston for an internal combustion engine, wherein a surface roughness of a surface of the metal alloy at an intermediate portion in an extending direction of the intake outer side wall is Ra of 4 μm or less. The piston for an internal combustion engine according to claim 6, wherein the metal alloy is an aluminum alloy. An exhaust valve recess is further formed in the top of the piston,
The exhaust valve recess tapers up from an exhaust bottom wall forming at least the bottom of the exhaust valve recess and an outer part of the exhaust bottom wall located near the outer peripheral edge of the piston top, and extends above the piston top from the outside of the piston top. an exhaust outer side wall extending to form an arc shape protruding toward the periphery;
The piston for an internal combustion engine according to claim 6 or 7 , wherein the surface roughness of the surface of the metal alloy at an intermediate portion in the extending direction of the exhaust outer side wall is Ra of 4 μm or less. The piston for an internal combustion engine according to any one of claims 6 to 8, wherein the surface roughness of the surface of the metal alloy in the entire intermediate portion of the intake and exhaust outer side walls is Ra of 4 μm or less. the piston top has a plurality of the intake valve recesses and a plurality of the exhaust valve recesses;
The piston top portion includes an intake connection wall that connects the intake outer side walls of adjacent intake valve recesses among the plurality of intake valve recesses, and an exhaust connection wall that connects the exhaust outer side walls of adjacent exhaust valve recesses among the plurality of exhaust valve recesses. having a wall;
The piston for an internal combustion engine according to any one of claims 6 to 9 , wherein the surface roughness of the metal alloy in the intake and exhaust connecting walls is Ra of 4 μm or less.