JP6460333B2 - Piston for internal combustion engine and surface treatment method for piston of internal combustion engine - Google Patents

Description

  The present invention relates to a piston for an internal combustion engine.

  2. Description of the Related Art Conventionally, a piston for an internal combustion engine in which a crown portion is cooled by contact of oil with a portion opposite to the combustion chamber with respect to the crown surface is known. In the piston described in Patent Document 1, in order to suppress accumulation of carbon deposits on the surface in contact with oil, a non-adhesive coating material is attached to the above-mentioned part.

Special table 2014-533805 gazette

  An object of this invention is to improve the cooling efficiency in the surface which oil contacts.

  The coating at the site preferably contains graphite and polytetrafluoroethylene.

  Therefore, the cooling efficiency in the surface which oil contacts can be improved.

The longitudinal section of the piston of Embodiment 1 is shown. The longitudinal section of the piston of Embodiment 1 is shown. The experimental apparatus which measures the temperature change of the piston at the time of cooling is shown. The change of the cooling rate ratio according to the composition of the cooling film in the experimental results is shown. The preferable range of the composition of a cooling film according to an experimental result is illustrated. The rotation apparatus in the manufacturing method of the piston of Embodiment 2 is shown. The mode of the painting process of the piston in Embodiment 2 is shown. The mode of the painting process of the piston in Embodiment 3 is shown. It is a front view of the cooling film of Embodiment 5. It is a front view of the cooling film of Embodiment 6. The longitudinal cross-section of the piston of Embodiment 7 is shown.

  Hereinafter, embodiments for carrying out the piston and the surface treatment method of the present invention will be described with reference to the drawings.

[Embodiment 1]
First, the configuration will be described. FIG. 1 shows a cross section of a piston 1 of an internal combustion engine (hereinafter referred to as an engine) of the present embodiment taken along a plane that includes the axis 10 of the piston 1 and is orthogonal to the axis 11 of the piston pin hole 51. FIG. 2 shows a cross section of the piston 1 taken along a plane including the axis 10 and the axis 11. The engine is a direct injection 4-stroke diesel engine. The engine 1 may be a sub-chamber type, a two-stroke engine or a gasoline engine. The piston 1 is accommodated in a cylinder formed in the cylinder block so as to be able to reciprocate. A cylinder head is installed in the cylinder block so as to close the opening of the cylinder. The piston 1 has a piston body 2 and a coating 3. The piston body 2 is made of a base material by casting using an aluminum alloy (for example, Al-Si AC8A) as a material. The piston body 2 may be made of iron, or the base material of the piston body 2 may be formed by forging. The piston main body 2 has a bottomed cylindrical shape, and includes a crown portion (head portion) 4, an apron portion (pin boss portion) 5, and a skirt portion 6. Hereinafter, the direction in which the axis 10 extends is referred to as the piston axial direction. In the piston axial direction, the crown portion 4 side with respect to the apron portion 5 and the skirt portion 6 is referred to as one axial side, and the opposite side is referred to as the other axial side. The radial direction of the piston 1 is referred to as the piston radial direction.

  The crown portion 4 has a crown surface portion 40, a back surface portion 41, and a land portion 42. The crown surface portion 40 is on one axial side of the crown portion 4 and has a crown surface 400. The crown surface 400 is substantially circular when viewed from one side in the axial direction. A combustion chamber is defined between the crown surface 400 (at the top dead center of the piston), the cylinder inner wall, and the cylinder head. The crown surface 400 is directly exposed to the combustion gas in the combustion chamber. The crown surface portion 40 has a cavity 401. The cavity 401 is a concave portion formed at a substantially central portion of the crown surface 400, and defines a combustion chamber. The shape of the cavity 401 is a deep dish shape (reentrant shape), and the bottom surface of the cavity 401 is a curved surface that protrudes toward one side in the axial direction toward the axial center 10 side. The shape of the cavity 401 is not limited to the above and is arbitrary. The back surface portion 41 has a back surface 410. The back surface 410 is the surface of the crown portion 4 on the side opposite to the combustion chamber (on the other side in the axial direction) with respect to the crown surface 400 and excluding the pin boss 50. The back surface 410 is on the back side of the crown 4 and forms a part of the inner peripheral surface of the piston body 2. The land portion 42 extends from the outer peripheral side of the crown surface portion 40 to the other side in the axial direction. The outer peripheral surface of the land part 42 has three annular grooves (ring grooves). These ring grooves 421, 422, 423 extend in the circumferential direction of the piston 1 (the direction around the axis 10). A compression ring is installed in each of the ring grooves 421 and 422 on the side close to the crown surface 400 (one axial side), and an oil ring is installed in the ring groove 423 on the side far from the crown surface 400 (the other axial side).

  The apron portion 5 and the skirt portion 6 extend from the crown portion 4 to the other side in the axial direction. The inner peripheral side of the skirt part 6 and the apron part 5 is hollow. There are a pair of apron portions 5 on both sides in the piston radial direction. The outer peripheral surface of the apron portion 5 is closer to the axis 10 than the outer peripheral surface of the crown portion 4 (skirt portion 6). Each apron section 5 has a pin boss 50. Each pin boss 50 has a piston pin hole 51. The piston pin hole 51 extends through the pin boss 50 in the piston radial direction. A pair of skirt portions 6 are provided on both sides in the piston radial direction. The skirt portion 6 is sandwiched between the apron portions 5 and 5 in the circumferential direction of the piston 1. Both skirt parts 6 and 6 are connected by the apron part 5 through the transition part. The skirt part 6 is thinner than the apron part 5. The skirt portion 6 slides against the cylinder inner wall. The piston pin end 51 is fitted into the piston pin hole 51. The piston 1 is connected to one end side (small end portion) of a connecting rod (connecting rod) via a piston pin. The other end side (large end portion) of the connecting rod is connected to the crankshaft. In an engine lubrication system including an oil pan and an oil pump, engine oil (hereinafter referred to as oil) is pumped from the main gallery to the large end of the connecting rod. Oil is supplied to the small end from the large end.

  The crown portion 4 includes a plurality of oil relief holes (drain holes) 43, a first cooling passage 411, and a second cooling passage (cooling channel) 44. The oil escape hole 43 extends inside the crown portion 4 and opens to the bottom surface side of the ring groove 423 and the back surface 410, and connects the inside of the ring groove 423 and the space on the inner peripheral side of the piston 1. The oil scraped off from the cylinder inner wall by the oil ring is discharged from the inside of the ring groove 423 to the space on the inner peripheral side of the piston 1 through the oil relief hole 43. The oil relief hole 43 may be opened not on the back surface 410 but on the inner peripheral surface of the second cooling passage 44 described later.

  The first cooling passage 411 is a region on the back side of the bottom surface of the cavity 401 (on the other side in the axial direction) on the back surface 410 and sandwiched between the pin bosses 50. The first cooling passage 411 is a curved concave portion that follows the shape of the bottom surface of the cavity 401. A projection (entire) of the surface of the first cooling passage 411 in the piston axial direction overlaps the bottom surface of the cavity 401. The first cooling passage 411 faces the outer peripheral surface of the small end portion of the connecting rod. The first cooling passage 411 has a bowl shape, and is deepest on the side of the axis 10 and gradually becomes shallower as the distance from the axis 10 increases. In other words, the bottom surface of the first cooling passage 411 is located on the other side in the axial direction of the piston 1 (downward in FIGS. 1 and 2) as it goes radially outward from the axial center 10 in a plane including the axial center 10. Raise. The first cooling passage 411 extends in the piston radial direction orthogonal to the axis 11. In other words, the dimension of the first cooling passage 411 in the piston radial direction orthogonal to the direction in which the piston pin hole 51 extends is larger than the dimension of the first cooling passage 411 in the direction in which the piston pin hole 51 extends. Both ends of the first cooling passage 411 in the piston radial direction orthogonal to the piston pin hole 51 are connected to a planar region 412 on the back surface 410 (on the back side of the second cooling passage 44 described later). The oil escape hole 43 opens in the region 412. In addition, the shape of the 1st cooling channel | path 411 is not restricted above, For example, according to the shape of the cavity 401, it can change suitably.

  The second cooling passage 44 is annular and extends in the circumferential direction of the piston 1 so as to surround the cavity 401 inside the crown portion 4. The second cooling passage 44 has a tubular shape (sealed) in which communication with the space on the inner peripheral side of the piston 1 is blocked except for the inlet 413 and the outlet (not shown). Of the inner peripheral surface of the second cooling passage 44, the surface on the one axial side is on the back side (the other axial side) of the crown surface 400 (on the outer peripheral side of the cavity 401). A projection of the surface on the one axial side of the second cooling passage 44 in the piston axial direction (the whole) overlaps with the crown surface 400 (on the outer peripheral side of the cavity 401). Of the inner peripheral surface of the second cooling passage 44, the surface on the other side in the axial direction is on the back side (one side in the axial direction) of the back surface 410. A projection of the surface on the other axial side of the second cooling passage 44 in the piston axial direction (the whole) overlaps the back surface 410. A region 412 on the back side (the other side in the axial direction) of the second cooling passage 44 on the other side in the axial direction in the back surface 410 is planar. In the region 412, the inlet 413 and the outlet of the second cooling passage 44 open, for example, at positions approximately opposite to each other across the axis 10. Of the inner peripheral surface of the second cooling passage 44, the outer surface in the piston radial direction is on the back side (inner side in the piston radial direction) of the outer peripheral surface of the land portion. A projecting (outside) surface of the second cooling passage 44 on the outer side in the piston radial direction overlaps with the three ring grooves 421, 422, and 423. Of the inner peripheral surface of the second cooling passage 44, the inner surface in the piston radial direction has a curved surface shape that follows the shape of the outer surface in the piston radial direction in the cavity 401. The surface on the inner side in the piston radial direction of the second cooling passage 44 is on the back side (outer side in the piston radial direction) of the side surface of the cavity 401. The surface of the second cooling passage 44 projected in the piston radial direction in the piston radial direction (part thereof) overlaps the side surface of the cavity 401.

The film 3 has a lubricating film 30 and a cooling film 31. The lubricating film 30 includes a solid lubricant and a binder resin, and covers the outer peripheral surface of the skirt portion 6. The solid lubricant has a function of reducing friction between the outer peripheral surface of the skirt portion 6 and the cylinder inner wall. Graphite (hereinafter referred to as C) is used as the solid lubricant. The lubricant film 30 is a solid lubricant, together with or in place of C, other solid lubricants such as molybdenum disulfide (hereinafter referred to as MoS ₂ ) or polytetrafluoroethylene (hereinafter referred to as PTFE). May be included.). The binder resin is a resin as a binder (binder) having adhesiveness to other materials, and has a function of fixing a solid lubricant to the piston body 2 that is an object to be coated. As the binder resin, a resin excellent in heat resistance and wear resistance, for example, a polyamideimide resin (hereinafter referred to as PAI) is used. The lubricating coating 30 is made of another binder resin such as a polyimide resin (hereinafter referred to as PI) or an epoxy resin (hereinafter referred to as EP) as the binder resin together with or instead of PAI. At least one of them may be included. PI, like PAI, is excellent in heat resistance and wear resistance. PAI, PI, and EP are excellent in adhesion.

The cooling film 31 covers at least a part, preferably all, of the surface of the first cooling passage 411, or covers at least a part, preferably all, of the inner peripheral surface of the second cooling passage 44. The cooling film 31 may or may not cover the region 412 on the back surface 410, the inner peripheral surface of the skirt portion 6, and the inner peripheral surface of the apron portion 5 (excluding the piston pin hole 51). The cooling film 31 includes an additive and a binder resin. Additives include both C and PTFE. The cooling film 31 may contain other additives such as boron nitride (hereinafter referred to as BN) or MoS ₂ together with C and PTFE. The binder resin has a function of fixing the additive to the piston body 2 that is an object to be coated. For example, PAI is used. The cooling film 31 may contain at least one of other binder resins, for example, PI or EP, as a binder resin, together with or in place of PAI. Hereinafter, the weight (mass)% is expressed as wt%. The cooling coating 31 has a binder resin content of 50 wt% or more and 90 wt% or less, and an additive content of 10 wt% or more and 50 wt% or less. Preferably, the binder resin content is 50 wt% or more and 70 wt% or less, and the additive content is 30 wt% or more and 50 wt% or less. More preferably, the content of the binder resin is 50 wt% or more and 60 wt% or less, and the content of the additive is 40 wt% or more and 50 wt% or less. The C content in the total (mixture) of C and PTFE in the cooling film 31 is 30 wt% or more and 80 wt% or less, preferably 40 wt% or more and 70 wt% or less.

  The manufacturing method of piston 1 has a casting process, a heat treatment process, a machining process, a lubricating film forming process, and a cooling film forming process. In manufacturing the piston 1, a casting process, a heat treatment process, a machining process, and a lubricating film forming process are performed in this order. Further, the cooling film forming step is performed at an appropriate timing. In the casting process, a molten aluminum alloy is poured into a mold and solidified to mold the base material of the piston 1. Here, the base material refers to the piston body 2 (coarse material) in a rough shape before machining or surface treatment. In this step, the inner peripheral surface (including the back surface 410) of the piston body 2 and the second cooling passage 44 are formed. The method for forming the second cooling passage 44 is not limited to the method in which the second cooling passage 44 is integrally formed with the crown portion 4. The crown part 4 is divided into two parts, each having an inner wall part of the second cooling passage 44, and the second cooling passage 44 is formed by integrating the two divided crown parts 4 by welding or the like. Also good. In the heat treatment step, the properties of the base material (hereinafter referred to as piston base material) are improved by heat treatment, and adjusted to appropriate strength and hardness. In the machining process, the piston base material is machined by a lathe or the like. For example, the piston pin hole 51 and the spigot portion 60 of the skirt portion 6 are processed using the casting surface as a reference. Next, the ring grooves 421, 422, and 423 are processed using the piston pin hole 51 and the spigot portion 60 as a reference, and the outer diameter of the piston body 2 such as the outer peripheral surface of the crown portion 4 and the outer peripheral surface of the skirt portion 6 is finished. Further, the crown surface 400 (cavity 401) and the oil relief hole 43 are processed.

  In the lubricating film forming step, the lubricating film 30 is formed on the outer peripheral surface of the skirt portion 6. The lubricating film forming process includes a painting process and a baking process. In the painting process, a paint in which a solid lubricant is dispersed in a binder resin solution is applied to the outer peripheral surface of the skirt portion 6 by, for example, screen printing. The coating may be performed by printing other than screen printing, spraying of a paint by spraying or the like, or dipping in the paint. In the firing step, the coating film is fired by baking to form the lubricating film 30. Specifically, the coating film is baked under predetermined baking conditions. Firing conditions are, for example, 190 ± 10 ° C. for 30 minutes. Since low temperature firing at 200 ° C. or less is also possible, firing can be easily applied even when the material of the piston body 2 is an aluminum alloy. By baking, volatile components are removed from the coating film, and a tough lubricating film 30 is formed, and the solid lubricant is fixed on the outer peripheral surface of the skirt portion 6 through the binder resin.

  In the cooling film forming step, the cooling film 31 is formed on the back surface 410 of the crown 4 or the inner peripheral surface of the second cooling passage 44. The cooling film forming process includes a painting process and a baking process. In the coating process, a coating material in which an additive is dispersed in a binder resin solution is applied to at least a part of the back surface 410 (the surface of the first cooling passage 411) or the inner peripheral surface of the second cooling passage 44. In addition, in order to improve the adhesiveness of the cooling film 31 or the like before the coating process, a process such as removing oil or dirt from the surface to be coated may be performed. To prepare a paint, for example, blend a binder resin (PAI) and additives (C and PTFE) in an organic solvent, add other additives to the solution as necessary, and use a bead mill or the like. What is necessary is just to mix and disperse. The paint can be diluted with a solvent as required. In the baking step, the coating film is baked by baking. Specifically, the coating film is baked under the same baking conditions as in the lubricating film forming process. By baking, the volatile component (organic solvent) is removed from the coating film, and a tough cooling film 31 is formed, and the additive is fixed on the surface via the binder resin. In addition, you may provide the cooling process which cools piston 1 (film | membrane 3) after the baking process of a lubricating film formation process, or the baking process of a cooling film formation process.

  A plurality of manufacturing methods can be adopted depending on the timing at which the cooling film forming process is performed and the manner in which the coating is applied (coating) in the cooling film forming process. The manufacturing method of this embodiment performs a cooling film formation process after a heat treatment process and before a machining process. Moreover, it coats by immersing a piston base material in a coating material at the coating process of a cooling film formation process. Specifically, the entire piston base material is immersed in the paint in the tank, and the paint is attached to the entire surface of the piston base material (including the inner peripheral surface of the second cooling passage 44) and then pulled up. Thereby, a coating film is formed on the inner peripheral surface including the back surface 410 of the piston body 2, the outer peripheral surface (before machining) and the crown surface 400, and the entire inner peripheral surface of the second cooling passage 44. The thickness (film thickness) of the coating film can be appropriately adjusted depending on the immersion conditions. A machining step is performed after the cooling film forming step. The coating film is removed from the part to be cut or ground, and the coating film remains in the other part. As a result, the cooling film 31 is formed on the back surface 410, the inner peripheral surface of the skirt portion 6 (excluding the spigot portion 60), and the inner peripheral surface of the apron portion 5 (excluding the piston pin hole 51). When the crown surface 400 is partially machined (for example, only the cavity 401 is machined), the cooling film 31 is also formed on a portion of the crown surface 400 that is not processed (cast surface).

  Next, the function and effect will be described. First, the function and effect of the piston 1 will be described. There is an increasing demand to improve engine efficiency and performance. The improvement in efficiency and performance is, for example, improvement in fuel combustion efficiency. Combustion efficiency is improved by bringing the mixing ratio of fuel and air to complete combustion, but in this case, the combustion temperature rises, so the piston temperature also rises. Therefore, it is necessary to maintain the temperature of the piston below an allowable temperature. If it cannot be maintained, abnormal combustion such as knocking occurs. Or, oxidation and corrosion of the piston surface proceed. Alternatively, ring seizure (stick) in the ring groove and ring groove wear occur. Alternatively, excessive tempering at the center of the crown surface that directly receives the combustion gas can reduce the mechanical properties of the piston material and cause cracks to form in areas of high stress. Therefore, the crown 4 is cooled by oil when the engine is operated. The parts that need to be cooled are the center part of the crown surface 400 where the temperature is highest, the lip 402 between the inner wall of the cavity 401 and the outer peripheral side of the crown surface 400, and the stick of the ring. The land portion 42 includes ring grooves 421, 422, and 423. The cooling of these parts is performed mainly by the oil adhering to the piston surface on the back side of these parts and the transfer of heat from these parts to the oil. Low temperature oil adheres to the hot piston surface and absorbs heat. The oil whose temperature has risen due to heat removal moves from the spot and leaves the piston surface. Oil (new low temperature) again adheres to the piston surface. Cooling is performed by repeating this cycle.

  The cooling of the central portion of the crown surface 400 or the cooling of the inner wall on the bottom surface side of the cavity 401 is performed mainly by oil adhering to the surface of the first cooling passage 411 which is the piston surface on the back side of these portions. Is called. The adhesion of oil to the surface of the first cooling passage 411 is, for example, the scattering of oil from the crankshaft, the injection of oil from the oil jet holes provided on the outer peripheral surface of the large end portion or the small end portion of the connecting rod, the cylinder This is performed by jetting oil from the oil jet (nozzle, etc.) installed on the block (cylinder inner wall) toward the back surface 410. The scattered or sprayed oil adheres to the first cooling passage 411 on the back surface 410 and then flows down from the first cooling passage 411 and returns to the oil pan. Heat exchange is performed by the oil flowing while wetting the surface of the first cooling passage 411, and the central portion of the crown surface 400 in the crown portion 4 (between the center portion of the crown surface 400 and the back surface 410) or the cavity 401. The bottom side (between the cavity 401 and the back surface 410) is cooled. Since the first cooling passage 411 has a bowl shape, the flow of oil is promoted. That is, the oil adhering to any part in the first cooling passage 411 tends to flow outward in the radial direction by its own weight.

  The cooling of the land portion 42 and the cooling of the inner wall on the side surface side of the cavity 401 radially adjacent to the land portion 42 are mainly performed on the inner peripheral surface of the second cooling passage 44 that is the piston surface on the back side of these portions. This is done by attaching oil. The oil jetted from the oil jet toward the back surface 410 is introduced into the second cooling passage 44 from the inlet 413 of the second cooling passage 44, flows through the second cooling passage 44, and then flows out from the outlet. . Thereafter, the oil falls and returns to the oil pan. If the oil jet is adjusted or arranged to inject oil toward the inlet 413 of the second cooling passage 44, the oil flows more smoothly through the second cooling passage 44. Heat exchange is performed by the oil flowing while wetting the inner peripheral surface of the second cooling passage 44, and the bottom side of the ring grooves 421, 422, 423 in the crown portion 4 (between the ring grooves 421, 422, 423 and the second cooling passage 44) and the cavity 401 The side surface (between the cavity 401 and the second cooling passage 44) is cooled. The second cooling passage 44 also cools the crown surface portion 40 (between the outer circumferential crown surface 400 and the second cooling passage 44) on the outer peripheral side of the cavity 401. The second cooling passage 44 is in the form of a groove that communicates with the space on the inner peripheral side of the piston 1 in the entire range on the other side in the axial direction of the piston 1 (downward in FIGS. 1 and 2), like the first cooling passage 411. (Open type) may be used. In this case, the second cooling passage 44 constitutes a part of the back surface 410, like the first cooling passage 411. If the oil jet is adjusted or arranged so that the oil is injected along the circumferential direction in which the second cooling passage 44 extends, the oil flows more smoothly in the second cooling passage 44.

  By adjusting the shape of the back surface 410, it is possible to easily generate an oil flow and to promote the detachment of the oil (which has once adhered to the site). The shape of the first cooling passage 411 is an example. Further, by providing a protrusion structure such as a fin on the back surface 410, the contact area with the oil may be increased, and the efficiency of heat exchange (cooling efficiency) by the oil may be increased. The piston 1 improves the efficiency of heat exchange with oil (cooling efficiency) by the surface treatment of providing the cooling film 31 on the back surface 410. The cooling film 31 has a function of actively increasing the cooling efficiency on the spot where the oil exchanges heat with the piston surface. The improvement of the cooling efficiency by the cooling film 31 is the above-mentioned heat transfer or separation (in other words, the oil adhesion to the piston surface, the heat transfer from the piston surface to the oil, the oil removal from the piston surface) (in other words, This is achieved by promoting the attachment of new oils at low temperatures. The cooling film 31 includes a binder resin and an additive, and the additive includes C and PTFE. These additives define the characteristics of the cooling film 31 when exposed to the surface of the cooling film 31. C has a property of improving the wettability of oil on the surface of the cooling film 31. By improving the wettability, the heat transfer is performed more efficiently, and the heat transfer is promoted. PTFE has the property of promoting the above-described release, that is, the property of improving oil release. By including C and PTFE as additives, the cooling film 31 actively increases the cooling efficiency on the spot where the oil exchanges heat with the piston surface.

  Improving the cooling efficiency on the spot does not mean that the flow of oil as a whole piston surface is promoted by adjusting the piston surface shape, etc. This refers to enhancing the cooling efficiency by promoting the heat transfer to the oil and the detachment of the oil in the micro area. In addition, positively increasing the cooling efficiency indicates the following. Conventionally, a piston is known in which a non-adhesive film material such as chrome is attached to a cooling passage in contact with oil (for example, Patent Document 1). The oil decomposes and oxidizes over time when it comes into contact with the hot piston surface. This forms a carbon deposit. If the accumulation of carbon deposits continues, an insulating layer is formed on the piston surface, so that the cooling efficiency of the piston surface with oil decreases. The non-stick coating material suppresses the accumulation of carbon deposits on the top surface. Thereby, suppression of the fall of cooling efficiency is achieved. In such a conventional technique, the surface treatment of adhesion of the non-adhesive film material is performed in order to suppress the cooling efficiency from being lowered, that is, to approach the cooling efficiency in the original state where the surface treatment is not performed. It is not performed to form a film having a positive effect of further improving the cooling efficiency in the original state. In this respect, the cooling film 31 of the piston 1 of the present embodiment has a positive effect of increasing the cooling efficiency as compared with the original state where the surface treatment is not performed. Further, the non-adhesive film material in the above prior art is to improve the cooling efficiency over time by suppressing the accumulation of carbon deposits over time, whereas the cooling film 31 in the present embodiment, It improves the cooling efficiency from time to time (in this sense "in place") when the oil adheres to the piston surface.

  The cooling film 31 may cover at least a part of the crown part 4 on the other side in the axial direction than the crown surface 400 (a part on the opposite side of the combustion surface with respect to the crown surface 400). For example, the cooling film 31 may be provided on the inner peripheral surface of the second cooling passage 44 without being provided in the first cooling passage 411. In this case, if the cooling film 31 covers at least a part of the inner peripheral surface of the second cooling passage 44, the above-described effects can be obtained in the covered region. If the entire inner peripheral surface of the second cooling passage 44 is covered, the above-described effects can be obtained over the entire range of the inner peripheral surface of the second cooling passage 44. Similarly, the cooling film 31 may not be provided on the inner peripheral surface of the second cooling passage 44. If at least a part of the first cooling passage 411 is covered, the above-described effects can be obtained in the covered region. . If the entire first cooling passage 411 is covered, the above-described effects can be obtained over the entire range of the first cooling passage 411. Of the crown 4, the surface of the first cooling passage 411 and the inner peripheral surface of the second cooling passage 44 are located on the other axial side of the crown surface 400 (the portion opposite to the combustion chamber with respect to the crown surface 400). Of these, the axially one side (upper side in FIGS. 1 and 2) and the inner surface in the piston radial direction are on the back side of the crown surface 400. If the cooling film 31 is formed on these surfaces, the above-mentioned effects can be obtained immediately behind the crown surface 400, so that the crown surface 400 can be cooled more efficiently. The cooling film 31 is not limited to the first cooling passage 411 or the like, but is a region 412 on the back surface 410, a transition region between the inner peripheral surface of the skirt portion 6 or the inner peripheral surface of the apron portion 5 and the back surface 410, or the skirt. The inner peripheral surface of the portion 6 or the inner peripheral surface of the apron portion 5 (excluding the piston pin hole 51) may be provided. In this case, the efficiency of cooling with oil is improved in each region covered by the cooling film 31, whereby the entire piston 1 is cooled more efficiently, and as a result, the cooling efficiency of each part in the crown portion 4 is further improved.

  The additive content in the cooling film 31 is 10 wt% or more (the binder resin content is 90 wt% or less). Thus, by suppressing the binder resin content below a certain level and securing the C and PTFE content above a certain level, it is possible to obtain at least a certain level of the above-described effect of increasing the cooling efficiency. The binder resin content in the cooling film 31 is 50 wt% or more (the additive content is 50 wt% or less). Thus, the adhesive force of the cooling film 31 is high by suppressing the content of the additive to a certain value or less and ensuring the amount of the binder resin to some extent. Therefore, since the adhesiveness between the cooling film 31 and the piston body 2 is good, peeling of the cooling film 31 is suppressed. Therefore, the above-described effect of increasing the cooling efficiency can be obtained more reliably. Further, the cooling film 31 is cured by being baked. Thereby, peeling of the cooling film 31 from the piston main body 2 is more reliably suppressed. When the cooling film 31 includes PAI or PI as a binder resin, PAI and PI are excellent in wear resistance and heat resistance, and therefore, the peeling of the cooling film 31 from the piston body 2 is more reliably suppressed.

  The present inventor believes that in the total (mixture) of C and PTFE, if the proportion of C is too large, the above-mentioned detachability should be suppressed, and if the proportion of PTFE is too large, the above-mentioned wettability should be suppressed, and cooling efficiency From this point of view, it was predicted that there would be an optimum range of the content (ratio) of C and PTFE in the above total. In order to find the optimum range, the following experiment was conducted.

試料1〜35の塗料を各別にピストン１の少なくとも背面410（第１冷却通路411）に塗布し、冷却皮膜31を形成した。焼成工程における焼成条件は190±10℃で30分だった。各試料1〜35の塗料による冷却皮膜31を背面410に有する各ピストン１につき、その背面410にオイルを噴射しつつ、冠面400（キャビティ401）の温度変化を測定した。図３は、用いた測定装置８を模式的に示す。図１と同様のピストン１の断面を示す。ディスペンサ（液体定量吐出装置）81を用い、オイル溜まり80のオイルを、ピストン１の背面410の中央（第１冷却通路411）を狙って、ニードル82から噴射させた。オイルの噴射開始直前からのキャビティ401の表面の温度変化を、赤外線温度測定器83により測定した。温度測定（オイルの噴射開始）の前に、ピストン１を予め200℃程度に加熱した。使用したオイルは、粘度分類が5W-30であり温度が25℃〜28℃だった。噴射量は80ml/minだった。なお、温度測定の精度を向上するため、予めピストン１のキャビティ401の表面に黒体84を塗布した。 (experimental method)
The experimental method was as follows. A paint was prepared in which the binder resin was PAI, the additives were C and PTFE (additives other than C and PTFE were not included), and the solvent was N-methylpyrrolidone NMP or γ-butyrolactone GBL. Hereinafter, the ratio of the total of C and PTFE (content of C and PTFE) in the cooling film 31 is expressed as α. The proportion of C in the total of C and PTFE is expressed as β. α corresponds to the proportion of the total of C and PTFE in the entire composition excluding the solvent of the paint (that is, the total of PAI, C, and PTFE). β is the ratio of C to the sum of C and PTFE in the unit area of the cooling film 31 (wt%) unless there is a significant deviation in the density of C and PTFE (mass per unit area) between the various parts of the cooling film 31 ). As shown in Table 1, for each case where α is 50 wt%, 40 wt%, 30 wt%, 20 wt%, 10 wt%, β is 0 wt%, 10 wt%, 30 wt%, 50 wt%, 70 wt%, 80 wt%, 100 wt% Samples 1 to 35 of the paint were prepared.

The coating materials of Samples 1 to 35 were individually applied to at least the back surface 410 (first cooling passage 411) of the piston 1 to form a cooling film 31. Firing conditions in the firing process were 190 ± 10 ° C. for 30 minutes. For each piston 1 having a cooling film 31 of paint of each sample 1-35 on the back surface 410, the temperature change of the crown surface 400 (cavity 401) was measured while injecting oil onto the back surface 410. FIG. 3 schematically shows the measurement apparatus 8 used. The cross section of the piston 1 similar to FIG. 1 is shown. The oil in the oil reservoir 80 was sprayed from the needle 82 with the aim of the center (the first cooling passage 411) of the back surface 410 of the piston 1 using a dispenser (liquid dispensing device) 81. The temperature change of the surface of the cavity 401 immediately before the start of oil injection was measured by the infrared temperature measuring device 83. Prior to temperature measurement (start of oil injection), the piston 1 was heated to about 200 ° C. in advance. The oil used had a viscosity classification of 5W-30 and a temperature of 25 ° C to 28 ° C. The injection amount was 80 ml / min. In order to improve the accuracy of temperature measurement, a black body 84 was applied to the surface of the cavity 401 of the piston 1 in advance.

(Experimental result)
The temperature change rate (cooling rate) was calculated for the result of the above measurement for each piston 1. As a comparison object, the temperature change was measured in the same manner as described above for the piston 1 that did not have the cooling film 31 (the surface treatment was not performed), and the cooling rate was calculated. Table 2 shows the cooling rate ratio of each piston 1 subjected to the surface treatment when the cooling rate of the piston 1 not having the cooling film 31 is set as a reference (1). The larger the value of the cooling rate ratio, the higher the cooling rate of the piston 1, and the higher the cooling efficiency of the cavity 401 by the cooling film 31 in the piston 1.

  FIG. 4 is a line graph showing, for each α, how the numerical value of the cooling rate ratio in Table 2 changes according to the change in β. The following can be read from this graph. When the C and PTFE content α in the cooling film 31 is 10 wt% or more (the binder resin content is 90 wt% or less), the piston without the cooling film 31 in the range where β is larger than 10 wt% and smaller than 100 wt% The cooling efficiency is higher than 1. Also, the cooling efficiency tends to increase as α increases. For example, when α is 30 wt% or more (the binder resin content is 70 wt% or less), the cooling efficiency is higher than when α is less than 30 wt% (for example, α is 10 wt%). Similarly, when α is 40 wt% or more (the binder resin content is 60 wt% or less), the cooling efficiency is higher than when α is less than 40 wt% (for example, α is 30 wt%). When α is around 50 wt%, the cooling efficiency is the highest. When compared with the same α, the cooling efficiency tends to increase as the C content β in the total of C and PTFE in the cooling film 31 approaches the range of 50 wt% to 60 wt%. For example, when β is 30 wt% or more and 80 wt% or less, the cooling efficiency is higher than when β is 30 wt% or less or 80 wt% or more. Similarly, when β is 40 wt% or more and 70 wt% or less, the cooling efficiency is substantially higher than when β is 40 wt% or less (for example, β is 30 wt%) or 70 wt% or more (for example, β is 80 wt%). Expensive. When β is 50 wt% or more and 60 wt% or less, the cooling efficiency is the highest.

  FIG. 5 is a triangular graph (triangular diagram) showing the relative proportions of PAI, C, and PTFE. Each vertex of the entire triangle corresponds to 100 wt% of the component described at the vertex, and the side facing the vertex in the entire triangle corresponds to 0 wt% of the component described at the vertex. As described above, if β is greater than 10 wt% and less than 100 wt% and α is 10 wt% or more, an improvement in cooling efficiency can be expected. On the other hand, in order to ensure the adhesive strength of the cooling film 31, α needs to be 50 wt% or less. Therefore, the shaded range 91 by the halftone dots in FIG. 5 is appropriate as the composition of the cooling film 31. If β is 30 wt% or more and 80 wt% or less, the cooling efficiency is higher. Therefore, among the range 91, a shaded range 92 with a downward slanting oblique line is preferable. If α is 30 wt% or more, the cooling efficiency is higher. Therefore, among the range 92, a shaded range 93 with a left-down diagonal line is more preferable. When β is 40 wt% or more and 70 wt% or less, the cooling efficiency is higher. Therefore, among the range 93, the shaded range 94 by horizontal lines is more preferable. If α is 40 wt% or more, the cooling efficiency is higher. Therefore, among the range 94, a shaded range 95 with vertical lines is more preferable. Cooling efficiency is highest when α is in the vicinity of 50 wt% and β is in the range of 50 wt% to 60 wt%. Therefore, in the range 95, the closer to the portion 96 where β is 50 wt% to 60 wt% on the line where α is 50 wt%, it is more preferable.

When the cooling film 31 contains other additives such as BN or MoS ₂ together with C and PTFE, the content of the other additives in the cooling film 31 is X (wt%). If α + X is 50 wt% or less, the binder resin content is 50 wt% or more. Therefore, the adhesiveness between the cooling film 31 and the piston body 2 is good as described above. In this case, by adopting the above numerical ranges for α and β, it is possible to obtain the operational effects corresponding to the numerical ranges. For example, if α is 30 wt% or more, the cooling efficiency is higher than when α is less than 30 wt%. When compared with the same α, the cooling efficiency is higher when β is 30 wt% or more and 80 wt% or less than when β is 30 wt% or less or 80 wt% or more.

  Next, the effect of the manufacturing method of piston 1 is demonstrated. In the manufacturing method of this embodiment, a cooling film formation process is performed before a machining process. In the cooling film forming step, coating is performed by dipping. Therefore, the cooling film 31 can be formed on the entire inner peripheral surface of the second cooling passage 44, the entire range of the back surface 410 including the first cooling passage 411, and the substantially entire range of the inner peripheral surfaces of the skirt portion 6 and the apron portion 5. Therefore, the cooling efficiency of the piston 1 can be further improved. After the cooling film 31 is formed, machining is performed. Parts such as the piston pin hole 51 and the piston outer peripheral surface formed by machining are originally parts that do not require the cooling coating 31. These parts are newly formed by machining, or the coating film (formed in the cooling film forming process) is removed by machining. Therefore, in the cooling film forming step, masking for preventing the formation of the cooling film 31 at the above-described site is unnecessary. Thereby, the manufacturing method of piston 1 can be simplified.

Hereinafter, effects exhibited by the piston 1 of the present embodiment or the surface treatment method thereof will be listed.
(1) A piston 1 of an internal combustion engine having a crown 4 on one side in the axial direction, the crown 4 having a crown surface 400 on one side in the axial direction, and the other in the axial direction than the crown surface 400 The cooling film 31 includes a binder resin (polyamideimide resin PAI) and an additive, and the content of the binder resin in the cooling film 31 is 50 wt% or more and 90 wt% or less. Additives include graphite C and polytetrafluoroethylene PTFE.
Therefore, since the cooling film 31 includes C that improves oil wettability and PTFE that improves oil detachability, the other side in the axial direction than the crown surface 400 in the crown 4 (the first cooling passage 411 and the first cooling channel 31). 2) the cooling efficiency on the surface of the cooling passage 44 and the like) can be positively improved. Therefore, each site | part of the crown part 4 can be cooled effectively. Moreover, since the content of the binder resin is 50 wt% or more, peeling of the cooling film 31 can be suppressed.

(2) The content β of C in the mixture of C and PTFE in the cooling film 31 is 30 wt% or more and 80 wt% or less.
Therefore, by setting β in the range of 30 wt% or more and 80 wt% or less, the cooling efficiency can be improved as compared with the case where β is set in the other range.

(3) The content α of the mixture of C and PTFE in the cooling film 31 is 30 wt% or more and 50 wt% or less.
Therefore, when α is in the range of 30 wt% or more, the cooling efficiency can be improved as compared with the case where α is in the range of 30 wt% or less.

(4) The C content β in the mixture of C and PTFE is 40 wt% or more and 70 wt% or less.
Therefore, by setting β in the range of 40 wt% or more and 70 wt% or less, the cooling efficiency can be improved as compared with the above (2).

(5) The content α of the mixture of C and PTFE in the cooling film 31 is 40 wt% or more and 50 wt% or less.
Therefore, by setting α to a range of 40 wt% or more, the cooling efficiency can be improved more than the above (3).

(6) The additive contains boron nitride BN or molybdenum disulfide MoS ₂ , and the content α of the mixture of C and PTFE in the cooling film 31 is 30 wt% or more.
Therefore, even when the additive contains BN or the like, the cooling efficiency can be further improved as in (3) above by setting α to a range of 30 wt% or more.

(7) The cooling film 31 is on the back side of the crown surface 400.
Therefore, the crown surface 400 that defines the combustion chamber can be efficiently cooled by the cooling film 31.

(8) It has a pair of skirt part 6 extended from the crown part 4 to the other side of an axial direction, and a pair of apron part 5 which connects a pair of skirt part 6, The inner peripheral surface of the skirt part 6 and the apron part 5 Has a cooling film 31.
Therefore, the whole piston 1 can be cooled more efficiently, and the cooling efficiency of each part in the crown part 4 can be further improved.

(9) A surface treatment method for a piston 1 of an internal combustion engine having a crown portion 4 on one side in the axial direction, and the crown portion 4 having a crown surface 400 on one side in the axial direction. A cooling film 31 is formed by paint on at least a part of the other side in the axial direction from 400. The paint contains C and PTFE, and the binder resin (PAI) content is 50 wt% or more and 90 wt% or less. is there.
Therefore, since the cooling efficiency on the surface of the part on the back side of the crown part 4 (the first cooling passage 411, the second cooling passage 44, etc.) can be positively improved, the same effect as the above (1) can be obtained.

(10) The content β of C in the mixture of C and PTFE in the paint is 30 wt% or more and 80 wt% or less.
Therefore, the same effect as the above (2) can be obtained.

(11) The content α of the mixture of C and PTFE in the paint is 30 wt% or more and 50 wt% or less.
Therefore, the same effect as the above (3) can be obtained.

(12) The cooling film 31 is formed by immersing the base material of the piston 1 in the paint.
Therefore, the cooling film 31 can be formed over the entire inner peripheral surface of the second cooling passage 44, the entire range of the back surface 410 including the first cooling passage 411, and the substantially entire range of the inner peripheral surfaces of the skirt portion 6 and the apron portion 5. Therefore, the cooling efficiency can be further improved.

[Embodiment 2]
The manufacturing method of the piston 1 of this embodiment performs the cooling film formation process after the heat treatment process and before the machining process, as in the first embodiment. In the coating process of the cooling film forming process, the coating is performed by spraying (spraying) the paint. Specifically, in the painting step, paint is sprayed toward the inlet 413 of the first cooling passage 411 and the second cooling passage 44 in the back surface 410. The paint sprayed toward the inlet 413 of the second cooling passage 44 flows into the second cooling passage 44 from the inlet 413 and flows out from the outlet. As a result, a coating film is formed on the inner peripheral surface of the piston 1 including the back surface 410 (first cooling passage 411) or the inner peripheral surface of the second cooling passage 44. The thickness (film thickness) of the coating film can be appropriately adjusted depending on the spraying conditions.

  6 and 7 schematically show an apparatus 7 for performing the coating process. FIG. 7 shows a cross section of a portion surrounded by a dotted line in FIG. The cross section similar to FIG. The device 7 has a rotating device and a coating device. The rotating device has a holding plate 70 and a rotating motor. As shown in the perspective view of FIG. 6, the holding plate 70 is circular. Three small circular holding holes 700 pass through the holding plate 70. These holding holes 700 are arranged substantially symmetrically with respect to the central axis 71 of the holding plate 70. The diameter of the holding hole 700 is substantially equal to the diameter of the crown portion 4 of the piston 1. The holding plate 70 is driven by a rotation motor and rotates around the central axis 71 in one direction. The holding plate 70 is rotated every 120 degrees. That is, the holding plate 70 is rotated by 120 degrees by one rotation operation and stopped. There are three stop positions of the holding hole 700. When attention is paid to a certain holding hole 700, the holding hole 700 returns to the first position by three rotations. The three stop positions are an input position, a painting position, and an extraction position. Of these stop positions, the portion surrounded by the dotted line in FIG. 6 corresponds to the painting position, and the painting apparatus is installed at this painting position.

  As shown in FIG. 7, the painting apparatus includes a paint hopper 71, an airless painting machine 72, and a painting gun 73. The paint hopper 71 is a liquid tank that stores paint. The airless coating machine 72 applies pressure to the paint supplied from the paint hopper 71 via the pipe line 74, and supplies this high-pressure paint to the paint gun 73 via the pipe line 75. The coating gun 73 is disposed on a substantially central axis of the coating hopper 71 and is directed to the holding plate 70 (the holding hole 700 at the coating position). The coating gun 73 sprays the supplied high-pressure paint toward the holding hole 700 in the form of a mist. The piston base material after the heat treatment step is installed in the holding hole 700 that is stopped at the charging position. The piston base material moves to the coating position while being held by the holding plate 70 in accordance with the rotation operation of the holding plate 70. Therefore, while the holding plate 70 is stopped, the paint is sprayed toward the back surface 410 of the piston base material by the coating device. Thereby, a coating film is formed on the inner peripheral surface of the piston including the back surface 410. Excess paint falls from the inner peripheral surface of the piston by its own weight and returns to the paint hopper 71. The coating apparatus is not limited to airless spray coating, and may perform air spray coating. In addition, when the holding hole 700 (piston base material) is stopped at the painting position, the position where the painting gun 73 is oriented is the center side of the back surface 410 (first cooling passage 411) and the inlet 413 side of the second cooling passage 44. You may make it change with. Accordingly, the paint can be efficiently applied to both the back surface 410 (first cooling passage 411) and the inner peripheral surface of the second cooling passage 44. The piston 1 that has been painted is moved to the take-out position by the next rotation of the holding plate 70. Therefore, when the holding plate 70 is stopped, the piston 1 is taken out from the holding hole 700 at the take-out position. The above operation is repeated for each holding hole 700. As a result, the coating process is continuously performed at relatively short intervals.

  In order to easily hold the piston base material in the holding hole 700, a preliminary machining process is performed after the heat treatment process and before the coating process. In this machining process, the outer diameter of the piston base material is roughened. At this time, the land portion 42 is provided with a step 420 with a difference in outer diameter. That is, the outer peripheral surface of the piston 1 is cut to some extent while leaving a part of the land portion 42. As a result, a step 420 is formed between the part of the land portion 42 and the portion on the skirt portion 6 side on the outer peripheral surface of the piston 1. The step 420 is brought into contact with the peripheral edge of the holding hole 700 and is caught, whereby the piston base material is held by the holding plate 70. In the state of being held in this way, the crown surface 400 of the piston 1 is cut off from the paint sprayed from the coating gun 73 by the holding plate 70. Therefore, no film is formed on the crown surface 400 (before machining). As described above, the holding plate 70 also functions as a masking plate. Other configurations are the same as those of the first embodiment.

  Next, the function and effect will be described. In the manufacturing method of the present embodiment, as in the first embodiment, the cooling film forming step is performed before the machining step, so that masking of the piston pin holes 51 and the like is unnecessary. Therefore, the manufacturing method of piston 1 can be simplified. In the case where the cooling film 31 is formed on the crown surface 400 before the machining process, when the entire crown surface 400 is not machined (for example, only the central portion of the crown surface 400 or the cavity 401 is machined), the crown surface 400 is formed. In this case, the cooling film 31 remains on the portion that is not machined. On the other hand, in the manufacturing method of the present embodiment, the crown surface 400 is masked by the holding plate 70 in the cooling film forming step. Therefore, the cooling film 31 is not formed on the crown surface 400 even when the entire crown surface 400 is not machined. In the cooling film forming step, painting is performed by spraying. Therefore, the cooling film 31 can be formed on the entire inner peripheral surface of the second cooling passage 44, the entire range of the back surface 410 including the first cooling passage 411, and the substantially entire range of the inner peripheral surfaces of the skirt portion 6 and the apron portion 5. Therefore, the cooling efficiency of the piston 1 can be further improved. By using the device 7, the painting process of the plurality of pistons 1 can be performed continuously without interruption, so that the productivity of the pistons 1 can be improved. In addition, the piston 1 can be held and masked at the same time by causing the holding plate 70 of the rotating device to function as a masking plate. Therefore, the productivity of the piston 1 can be further improved. Other functions and effects are the same as those of the first embodiment.

[Embodiment 3]
The manufacturing method of piston 1 of this embodiment performs a cooling film formation process after a machining process and before a lubricating film formation process. In addition, you may perform a cooling film formation process after a lubricating film formation process. In the coating process of the cooling film forming process, the coating is performed by spraying as in the second embodiment. FIG. 8 is a cross-sectional view similar to FIG. 7, schematically showing an apparatus 7 for performing the coating process. The coating apparatus is the same as that in the second embodiment. The rotating device is the same as that of the second embodiment (FIG. 6) except that the holding plate 70 has an inlay receiving portion 701 at the periphery of each holding hole 700. As shown in FIG. 8, the spigot receiving portion 701 is an annular convex portion that surrounds the periphery of the holding hole 700 and protrudes at a predetermined height toward the side opposite to the coating apparatus. The piston body 2 is held in the holding hole 700 by fitting the inlay portion 60 of the skirt portion 6 to the outer periphery of the inlay receiving portion 701. The crown surface 400 and the outer peripheral surface (ring grooves 421, 422, 423) of the piston body 2 are shielded from the paint sprayed from the coating gun 73 by the holding plate 70. Therefore, the adhesion of the paint to these surfaces is suppressed. As described above, the holding plate 70 also functions as a masking plate.

  The device 7 has a piston pin hole masking member 76 and an oil relief hole masking device 77. The piston pin hole masking member 76 is a stopper made of, for example, silicon rubber, and masks each piston pin hole 51. Thereby, adhesion of the paint to the inner peripheral surface of the piston pin hole 51 is suppressed. The oil relief hole masking device 77 has an annular shape as a whole, the outer diameter side and both axial sides are closed, and the inner diameter side is opened. While the piston body 2 is stopped at the painting position, the oil relief hole masking device 77 is installed in the crown portion 4 so as to surround the land portion 42. An annular closed space is formed between the outer peripheral surface of the land portion 42 and the inner peripheral surface of the oil relief hole masking device 77. While the paint is sprayed from the paint gun 73, air is supplied from the outside to the inside of the closed space. The air in the closed space is blown out to the inner peripheral side of the piston through the oil relief hole 43 opened at the bottom of the ring groove 423. Air flow is indicated by arrows. As a result, the paint sprayed from the coating gun 73 is prevented from adhering to the inner peripheral surface of the oil escape hole 43 or blocking the oil escape hole 43. Other configurations are the same as those of the first embodiment.

  Next, the function and effect will be described. In the manufacturing method of this embodiment, a cooling film formation process is performed after a machining process. Therefore, by continuously performing the lubricating film forming process and the cooling film forming process, the baking process of the lubricating film forming process and the baking process of the cooling film forming process can be made common. That is, after the coating process of the lubricating film forming process and the coating process of the cooling film forming process are completed, both the lubricating film 30 and the cooling film 31 can be baked simultaneously by performing the firing process once. Thereby, the manufacturing method of piston 1 can be simplified. In the cooling film forming step, similarly to the second embodiment, the cooling film 31 can be formed on the inner peripheral surface of the skirt portion 6 and the like by painting by spraying. Therefore, the cooling efficiency of the piston 1 can be further improved. Similar to the second embodiment, the productivity of the piston 1 can be improved by using the device 7 and by causing the holding plate 70 to function as a masking plate. Instead of the oil relief hole masking device 77, an oil relief hole masking member that is a plug made of, for example, silicon rubber may be used. If the oil relief hole masking device 77 is used, it is possible to save the effort of plugging the oil relief holes 43 having a small diameter and a large number one by one with a stopper, thereby reducing man-hours and improving the production speed. Other functions and effects are the same as those of the first embodiment.

(13) The cooling film 31 is formed after machining the base material of the piston 1.
Therefore, when the lubricating film 30 is provided in addition to the cooling film 31, the firing process in the process of forming these films 3 can be made common.

[Embodiment 4]
The manufacturing method of piston 1 of this embodiment performs a cooling film formation process after a machining process and before a lubricating film formation process like Embodiment 3. In addition, you may perform a cooling film formation process after a lubricating film formation process. In the coating process of the cooling film forming process, coating is performed by pad printing, roller coating, brush coating, or the like. For example, a paint is applied to the first cooling passage 411 in the back surface 410 to form a coating film. In pad printing, printing (application) is also possible on the curved first cooling passage 411 with a soft pad. Overprinting is also possible. When the second cooling passage 44 has an open groove shape, the paint can be applied to the inner peripheral surface of the second cooling passage 44 by pad printing or the like. Other configurations are the same as those of the first embodiment. In the manufacturing method of the present embodiment, since the cooling film forming process is performed after the machining process as in the third embodiment, the firing process of the lubricating film forming process and the firing process of the cooling film forming process can be made common. Therefore, the manufacturing method of piston 1 can be simplified. Since the coating is performed by pad printing or the like in the cooling film forming step, the coating can be performed only on a portion where the cooling film 31 is necessary. Therefore, masking for preventing the formation of the cooling film 31 is unnecessary, and paint can be saved. Other functions and effects are the same as those of the first embodiment.

[Embodiment 5]
In the present embodiment, the cooling film 31 is formed by separately applying the two additives C and PTFE. That is, instead of mixing and dispersing C and PTFE in the paint in advance as in the first embodiment and applying them, the cooling film 31 may be formed by separately applying C and PTFE in the same range. In this case, C and PTFE may be applied while being shifted in time (for example, by spraying or printing, the upper layer may be reapplied so that the lower layer additive is exposed on the piston surface), or C and PTFE may be applied simultaneously. (For example, spraying simultaneously from another direction toward the same range by spraying). Both additives C and PTFE are exposed on the piston surface adjacent to each other in such a manner that the areas are slightly shaded or have a mesh or streak pattern, and the areas are more or less distinguished from each other. Yes. In this case, instead of the content (wt%), β can be defined by an area ratio or the like. For example, β can be defined by the ratio of the exposure amount of C to the sum of the exposure amount of C and the exposure amount of PTFE in a predetermined range of the cooling film 31. In other words, the definition of β in wt% as in the first embodiment is an example instructing the ratio of the exposure amount of C, and when C and PTFE are mixed and dispersed in one paint in advance. Is particularly useful.

  In the manufacturing method of the present embodiment, the cooling film 31 is formed by applying C and PTFE to the same range of the piston surface while being shifted in time. At that time, the first paint and the second paint are used. The first paint contains PAI as a binder resin and C as an additive (without PTFE). The second paint contains PAI as a binder resin and PTFE as an additive (C is not included). For example, the first and second paints are sequentially applied using the method of the fourth embodiment. FIG. 9 schematically shows the cooling film 31 of the present embodiment formed by applying a paint in a lattice pattern in a predetermined range 310. In the coating process of the cooling film 31, first, the first coating is applied to the surface of the piston body 2 (for example, the surface of the first cooling passage 411) by pad printing in a plurality of lines arranged in parallel at predetermined intervals. ) Apply on top. After the first paint is dried, pad printing is performed to form a plurality of lines that are substantially perpendicular to the first paint line (hereinafter referred to as the first line) 311 and are arranged at predetermined intervals. The second paint is applied on the surface where the first lines 311 are arranged. On the surface of the piston, a line (hereinafter referred to as the second line) 312 made of the second paint is exposed as it is, while a part of the first line 311 is covered with the second line 312 and the remaining part of the first line 311 is exposed. A part is exposed from between the adjacent second lines 312. On the piston surface, PTFE is exposed on the second line 312 and C is exposed in a region sandwiched between the second lines 312. The areas where PTFE is exposed and the areas where C is exposed are staggered in a staggered pattern. Other configurations are the same as those of the first embodiment.

  If the C content (ratio) in the first paint and the PTFE content (ratio) in the second paint are substantially the same, the first line in the predetermined range 310 of the piston surface on which the cooling film 31 is formed. The ratio of the exposed area S1 of 311 and the exposed area S2 of the second line 312 can be regarded as the ratio of the exposed amount of C and the exposed amount of PTFE. Therefore, β can be defined by the ratio of S1 to the sum of S1 and S2. For example, the width of the second line 312 and the interval between the second lines 312 are designed so that S1 and S2 are equal. In this case, β is 50% in area ratio, and a relatively high cooling efficiency is obtained. That is, for β defined by the area ratio, the same operational effects as for β of the first embodiment can be obtained by setting the same numerical range as β of the first embodiment defined by wt%.

  If the PAI content in the first paint is 50 wt% or more (the C content is 50 wt% or less), the adhesion between the first wire 311 and the piston body 2 is improved. If the PAI content in the second paint is 50 wt% or more (PTFE content is 50 wt% or less), the adhesion between the second wire 312 and the first wire 311 or the piston body 2 is improved. Therefore, peeling of the cooling film 31 is suppressed. If the PAI content in the first paint is 90 wt% or less (the C content is 10 wt% or more), at least the effect of improving oil wettability by C in the region where the first line 311 is exposed to the piston surface will be described. It becomes possible to obtain a certain degree. If the PAI content in the second paint is 90 wt% or less (PTFE content is 10 wt% or more), at least the effect of improving the oil detachment property by PTFE in the region where the second line 312 is exposed to the piston surface. It becomes possible to obtain a certain degree. Therefore, it is possible to obtain at least a certain level of the above-described effect of increasing the cooling efficiency.

  The C content (ratio) in the first paint and the PTFE content (ratio) in the second paint may be different. In this case, α1 is obtained by multiplying the content of C in the first line 311 by the ratio of S1 to the sum of S1 and S2. Α2 is obtained by multiplying the PTFE content in the second line 312 by the ratio of S2 to the sum of S1 and S2. α1 and α2 are obtained by weighting the ratio (content) occupied by C and PTFE in each paint (each line 311, 312) by the ratio of the exposed area. For the value obtained by adding α1 and α2, the same lower limit value as for α in the first embodiment can be set. For example, if the sum of α1 and α2 is 30 wt% or more, the cooling efficiency is higher than when the sum is less than 30 wt%. Speaking of β, S1 * is obtained by multiplying S1 by the content of C in the first paint. S2 * is the product of S2 multiplied by the PTFE content in the second paint. S1 * and S2 * are obtained by weighting the exposed areas by the contents of C and PTFE in the respective paints (each line 311 and 312), and correspond to the substantial exposed amounts of C and PTFE in the predetermined range 310, respectively. Β can be defined by the ratio of S1 * to the sum of S1 * and S2 *. With respect to β defined in this way, the same operational effects as those of β in the first embodiment can be obtained by setting a numerical range similar to β in the first embodiment defined in wt%. Other functions and effects are the same as those of the first embodiment.

[Embodiment 6]
In the present embodiment, similarly to the fifth embodiment, the cooling film 31 is formed by separately applying C and PTFE to the same range of the piston surface while being shifted in time. FIG. 10 schematically shows the cooling film 31 of the present embodiment formed by applying the paint in a polka dot pattern in the predetermined range 310. In the coating process of the cooling film 31, first, the first paint is applied on the surface of the piston body 2 (for example, the surface of the first cooling passage 411) in a uniform plane by pad printing, roller coating, or the like. After the first coating material is dried, the second coating material is formed into a shape in which a plurality of dots 314 (having a predetermined diameter) are regularly arranged at substantially equal intervals in a predetermined direction by pad printing. It applies | coats on the coating surface 313 by a 1st coating material. On the piston surface, the dots 314 of the second paint are exposed as they are, and the coating surface 313 is exposed from between the adjacent dots 314. Other configurations are the same as those of the first embodiment.

  As in the fifth embodiment, if the C content (ratio) in the first paint and the PTFE content (ratio) in the second paint are substantially the same, the predetermined range 310 on the piston surface (cooling coating 31). Β can be defined by the ratio of the exposed area of the coating surface 313 to the sum of the exposed area of the coating surface 313 and the exposed area of the dots 314 (in other words, the area of the predetermined range 310). For example, the diameter of the dots 314 and the interval between the dots 314 are designed so that the exposed area of the coating surface 313 and the exposed area of the dots 314 are equal. In this case, β is 50% in area ratio, and a relatively high cooling efficiency is obtained. The same applies to the content of PAI (in other words, C and PTFE) in each paint. Other functions and effects are the same as those of the first embodiment.

  In the fifth and sixth embodiments, the first coating applied first may include PTFE, and the second coating applied later may include C. In addition to the fifth and sixth embodiments, various patterns of the cooling film 31 are conceivable. Alternatively, the first and second paints may be applied by dividing into regions so that they do not overlap each other.

[Embodiment 7]
In the present embodiment, the shape of the piston body 2 is different from that of the first embodiment. FIG. 11 shows the same cross section as FIG. 1 of the piston 1 of this embodiment. Illustration of the film 3 is omitted. The crown 4 does not have the second cooling passage 44. The shape of the cavity 401 is a shape that guides the tumble flow generated in the intake process, and promotes the generation of the tumble flow. Of the back surface 410, the region between the bottom surfaces of the cavity 401 (excluding the pin boss 50) and sandwiched between both pin bosses 50 is a planar shape that follows the shape of the bottom surface of the cavity 401. ) 414 to groove (recess) 415 are provided (hereinafter, these are collectively referred to as fins 414). The fins 414 extend in the direction around the axis 10 of the piston 1, and a plurality of fins 414 are provided side by side in the piston radial direction. The cooling film 31 is provided on at least the back surface 410 (including the region where the fins 414 are provided). Other configurations are the same as those of the first embodiment. In the piston 1 of this embodiment, the local wettability is enhanced by the cooling film 31 as in the first embodiment. In addition, in the macro shape of the back surface 410, the contact area between the oil and the piston surface is widened by the fins 414, so that the cooling efficiency of the crown 4 is further improved. Other functions and effects are the same as those of the first embodiment.

[Other Embodiments]
As mentioned above, although the form for implementing this invention was demonstrated, even if there is a design change etc. of the range which does not deviate from the summary of invention, the concrete structure of this invention is not limited to embodiment, It is included in the present invention. For example, the material of the piston is not limited to an aluminum alloy but may be iron or the like. PAI, PI, and EP are excellent in adhesiveness and can be applied regardless of the piston material.

Hereinafter, technical ideas other than those described in the scope of claims, which are grasped from the embodiments, are listed.
(6) In the piston of the internal combustion engine according to claim 2,
The additive includes boron nitride or molybdenum disulfide,
The piston of the internal combustion engine, wherein the content of the mixture in the coating is 30% by weight or more.
(8) In the piston of the internal combustion engine according to claim 1,
A pair of skirt portions extending from the crown portion to the other side in the axial direction;
A pair of apron portions connecting the pair of skirt portions;
A piston for an internal combustion engine, characterized in that the coating is provided on inner peripheral surfaces of the skirt portion and the apron portion.
(12) In the surface treatment method for a piston of an internal combustion engine according to claim 7,
A surface treatment method for a piston of an internal combustion engine, wherein the coating is formed by immersing a base material of the piston in the paint.
(13) In the method for surface treatment of a piston of an internal combustion engine according to claim 7,
A surface treatment method for a piston of an internal combustion engine, wherein the coating is formed after machining the base material of the piston.

1 piston 31 cooling film 4 crown 400 crown 5 apron 6 skirt

Claims (3)

A piston for an internal combustion engine comprising a piston body having a crown portion and a cooling film formed on at least a part of the surface of the piston body ,
The crown has a crown surface, a back surface, and a cooling passage,
The crown faces the combustion chamber in the cylinder of the internal combustion engine;
In the cross section perpendicular to the direction in which the piston moves in the cylinder of the internal combustion engine, the back surface passes through the center of the crown surface and is parallel to the direction in which the piston moves. In the direction of the piston axis, provided on the back side of the crown surface; and
The cooling passage is provided inside the crown portion so as to extend in the circumferential direction in the direction of the piston axis, and has an opening on the back surface.
The cooling coating is provided on at least a part of the back surface and / or the cooling passage , and has a binder resin and an additive,
The binder resin has a content in the cooling coating of 50 wt% or more and 70 wt% or less,
The additive has a mixture of graphite and polytetrafluoroethylene , the content of the mixture in the cooling coating is 30 wt% or more and 50 wt% or less, and the ratio of the graphite to the total of the mixture is A piston for an internal combustion engine that is 50% or more and 60% or less . The piston of the internal combustion engine according to claim 1 ,
The piston of the internal combustion engine, wherein the content of the mixture in the cooling film is 40 wt% or more and 50 wt% or less. A surface treatment method for a piston of an internal combustion engine,
The piston includes a piston body having a crown,
The crown has a crown surface, a back surface, and a cooling passage,
The crown faces the combustion chamber in the cylinder of the internal combustion engine;
In the cross section perpendicular to the direction in which the piston moves in the cylinder of the internal combustion engine, the back surface passes through the center of the crown surface and is parallel to the direction in which the piston moves. In the direction of the piston axis, provided on the back side of the crown surface; and
The cooling passage is provided inside the crown portion so as to extend in the circumferential direction in the direction of the piston axis, and has an opening on the back surface.
The surface treatment method includes a step of forming a cooling film with a paint on at least a part of the back surface and / or the cooling passage ,
The paint has a binder resin and an additive,
The binder resin has a content in the cooling coating of 50 wt% or more and 70 wt% or less,
The additive has a mixture of graphite and polytetrafluoroethylene, the content of the mixture in the cooling coating is 30 wt% or more and 50 wt% or less, and the ratio of the graphite to the total of the mixture is A surface treatment method for a piston of an internal combustion engine that is 50% or more and 60% or less .

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