Patent Application
20170145950
This invention relates to a method for manufacturing a piston for a direct injection engine.
A method in which the top face of a piston made from an aluminum alloy is subjected to an anodizing treatment to form an anodic oxide coating, and the surface of the anodic oxide coating that is formed as a result is subjected to a sealing treatment is already known. For example, in Japanese Patent Laid-Open No. 2012-72745, a method for manufacturing a piston is disclosed that includes a step of forming a porous layer by subjecting the top face of a piston made of an aluminum alloy to an anodizing treatment, and a step of forming a coating layer that covers the surface of the porous layer by plasma spraying of a Y2O3-stabilized ZrO2 powder onto the porous layer. Similarly to a common anodic oxide coating, the porous layer has a large number of pores that are formed during the course of the anodizing treatment, and has a lower thermal conductivity and also a lower thermal capacity per unit volume than a conventional ceramic-based heat-shielding coating. Further, the coating layer is formed so as to block the openings of the pores of the porous layers, and the Y2O3-stabilized ZrO2 has a lower thermal conductivity than the aluminum alloy. Therefore, according to the heat-shielding coating constituted by the coating layer and the porous layer, a low thermal conductivity and a low thermal capacity per unit volume can be realized.
Further, in Japanese Patent Laid-Open No. 2010-249008, a technique is disclosed that forms a silicon oxide coating on the top face of a piston by coating an organic silicon solution onto the outermost surface of an anodic oxide coating formed on the top face of the piston, and performing a heat treatment. Similarly to the above described coating layer, openings of pores in the anodic oxide coating can be blocked by the silicon oxide coating. A low thermal conductivity as well as a low thermal capacity per unit volume can also be realized by this kind of heat-shielding coating constituted by a silicon oxide coating and an anodic oxide coating.
PTL 1: Japanese Patent Laid-Open No. 2012-72745
PTL 2: Japanese Patent Laid-Open No. 2010-249008
An aluminum alloy that is commonly used in a piston includes additives for improving the mechanical properties thereof. However, there is the problem that such additives (mainly silicon) inhibit the formation of an anodic oxide coating, and cause minute irregularities to arise on the surface of the anodic oxide coating that is formed. If irregularities arise on the surface of the anodic oxide coating, the heat transfer area increases and therefore an effect of improving a heat shielding property by means of the anodic oxide coating is lost. Further, if irregularities arise on the surface of the anodic oxide coating, the fluidity of a flame (growth rate of the flame) that is produced by combustion of fuel decreases and the combustion efficiency deteriorates. In this respect, according to the aforementioned sealing coating such as a coating layer or a silicon oxide coating, since the sealing coating can cover irregularities on the surface of the anodic oxide coating and smoothen the surface of the heat-shielding coating, there is the advantage that a decrease in the fluidity of a flame can be suppressed in comparison to a heat-shielding coating constituted by only an anodic oxide coating.
However, the present inventor found that the following problem arises when a piston that has the above described kind of sealing coating formed on the top face thereof is applied to a diesel engine or some types of gasoline engines. That is, in a direct injection engine which injects fuel directly into a cavity portion formed in a concave shape in the top face of a piston, a flame arises as a result of injecting fuel at high pressure into the cavity portion. Consequently, there is the problem that the sealing coating formed in the cavity portion is liable to be locally damaged by the penetrating force of the fuel that is injected at high pressure. If the sealing coating is locally damaged, there is a high possibility that such damage will affect the fluidity of a flame that arises. Furthermore, if the damage progresses and a fragment of the sealing coating detaches from the cavity portion, there is a risk that the cylinder bore will be damaged by the detached fragment or that the detached fragment will bite into a piston ring groove and cause the engine performance to deteriorate.
The present invention has been made in consideration of the above described problem. That is, an object of the present invention is, in a piston having a sealing coating formed on the top face thereof that is used for a direct injection engine, to suppress the occurrence of damage to the sealing coating that is caused by injected fuel.
A first invention is a method for manufacturing a piston for a direct injection engine that directly injects fuel into a cavity portion that is formed in a concave shape in a piston top face, comprising:
a piston preparation step of preparing a piston that has the cavity portion and that is made of an aluminum alloy having an aluminum purity of less than 99.0%;
an aluminum coating formation step of forming an aluminum coating having an aluminum purity of 99.0% or more over an entire area of a surface of the cavity portion;
an anodic oxide coating formation step of, after formation of the aluminum coating, forming an anodic oxide coating having pores over an entire area of the piston top face by subjecting the piston top face to an anodizing treatment; and
a sealing coating formation step of, after formation of the anodic oxide coating, forming a sealing coating that seals the pores of the anodic oxide coating on an outer side relative to the cavity portion of the piston top face.
A second invention is the method for manufacturing a piston for a direct injection engine according to the first invention, wherein:
the aluminum coating formation step is a step of forming the aluminum coating to a predetermined thickness, and
the anodic oxide coating formation step is a step of subjecting the piston top face to an anodizing treatment under conditions such that an aluminum alloy that is on an inner side relative to the aluminum coating that is formed to the predetermined thickness is not anodized.
A third invention is the method for manufacturing a piston for a direct injection engine according to the first or the second invention, wherein:
the aluminum coating formation step is a step of forming the aluminum coating to a predetermined thickness; the method further comprising, between the piston preparation step and the aluminum coating formation step, a grinding step of grinding inwardly a surface of the cavity portion by an amount corresponding to the predetermined thickness.
A fourth invention is the method for manufacturing a piston for a direct injection engine according to any one of the first to the third inventions, wherein the direct injection engine is a diesel engine.
According to the first invention, after an aluminum coating having an aluminum purity of 99.0% or more is formed over the entire area of the surface of a cavity portion of a piston made of an aluminum alloy having an aluminum purity of less than 99.0%, an anodic oxide coating having pores can be formed over the entire area of the top face of the piston, and thereafter a sealing coating that seals the pores of the anodic oxide coating on the outer side relative to the cavity portion can be formed. Therefore, an anodic oxide coating that is derived from an aluminum coating can be formed on the inner side of the cavity portion, and a heat-shielding coating that is constituted by an anodic oxide coating derived from an aluminum alloy and a sealing coating can be formed on the outer side of the cavity portion. Since the aluminum coating includes almost no additives, the occurrence of minute irregularities on the surface of the anodic oxide coating that is derived from the aluminum coating can be suppressed. Therefore, a decrease in the fluidity of a flame can be suppressed even without forming a sealing coating on the surface of the anodic oxide coating that is derived from the aluminum coating. Further, since a sealing coating need not be formed on the surface of the anodic oxide coating that is derived from the aluminum coating, the problem that a sealing coating formed in the cavity portion is damaged by the penetrating force of injected fuel can be fundamentally solved.
In a case where an aluminum alloy that is on an inner side relative to an aluminum coating is anodized, minute irregularities arise on the surface of an anodic oxide coating derived from the aluminum alloy that is on the inner side, and hence there is the problem that the surface of the anodic oxide coating that is derived from the aluminum alloy becomes rough. In this respect, according to the second invention, the top face of the piston can be subjected to an anodizing treatment under conditions such that an aluminum alloy that is on an inner side relative to an aluminum coating that is formed to a predetermined thickness is not anodized. That is, it is possible to anodize only the aluminum coating, and the surface of the anodic oxide coating that is derived from the aluminum coating can be reliably smoothened.
According to the third invention, the surface of the cavity portion can be ground inwardly by an amount corresponding to a predetermined thickness between a piston preparation step and an aluminum coating formation step. Therefore, the occurrence of a difference in level between the inside and outside of the cavity portion after formation of the aluminum coating can be suppressed.
In general, because the injection pressure of fuel is higher in a diesel engine than in a gasoline engine, a sealing coating formed in a cavity portion in a piston of a diesel engine is liable to be damaged. In this respect, according to the fourth invention, even in the case of applying the invention to a diesel engine, damage of a sealing coating formed in a cavity portion can be suppressed.
Hereunder, an embodiment of a method for manufacturing a piston for a direct injection engine will be described referring to
Three piston ring grooves 18 are formed on the side face of the crown portion 14. A cavity portion 20 is provided in a concave shape in the center of the upper face (hereunder, also referred to as “piston top face”) of the crown portion 14. The cavity portion 20 is constituted by a side wall portion 22 that is formed so as to face the inner part of the crown portion 14 from an opening edge 20a of the cavity portion 20, and a truncated cone-shaped ridge portion 24 that is formed to rise upward from a deepest part of the side wall portion 22. A squish portion 26 of the same height as an outer edge 14a of the crown portion 14 is formed on the outside of the cavity portion 20. A recess for preventing contact with an intake valve and an exhaust valve is provided as necessary in the surface (hereunder, also referred to as “squish face”) of the squish portion 26.
The method for manufacturing the piston will now be described with reference again to
After step S2, masking of the squish face is performed (step S3). The masking technique is not particularly limited, and for example a masking tape may be attached to the squish face, or a masking member that is formed to conform to the shape of the piston 10 may be pushed against the squish face.
Following step S3, a high-purity aluminum coating is formed on the cavity face (step S4). The high-purity aluminum coating can be formed by an electroplating method, an evaporation method, a thermal spraying method or a cold spraying method, and forming the high-purity aluminum coating by an electroplating method or an evaporation method in which it is difficult for the intervention of impurities such as aluminum oxide to occur is preferable. The aluminum purity of the high-purity aluminum coating formed by the process in step S4 is 99.0% or more, and preferably is 99.5% or more. Further, it is preferable to make the coating thickness TAl of the high-purity aluminum coating a thickness that is equal to one half or more of a target coating thickness TTARGET of an anodic oxide coating (described in detail later) that is to be formed on the cavity face, and it is more preferable to make the coating thickness TAl a thickness that is equal to one half of the thickness of the target coating thickness TTARGET.
After step S4 the entire area of the piston top face is subjected to an anodizing treatment (step S5). More specifically, first, the masking is removed from the squish face and the piston 10 is placed in an electrolyzer. The electrolyzer includes an electrolytic bath containing an electrolyte therein, a cathode and a power source (none of which are shown in the drawings). Next, the cathode and the piston 10 as an anode are placed in the electrolyte, and an anodic oxide coating is formed by passing a current between the two poles.
The details of the anodizing treatment in step S5 will now be described referring to
As shown in
On the other hand, in the cavity portion 20, as shown in
Preferably, the anodizing treatment in step S5 is performed under conditions such that the aluminum alloy on the inner side of the high-purity aluminum coating is not oxidized. It is known from empirical knowledge that the coating thickness of an alumite coating is proportional to the current density and the electrolysis time. In step S5, an electrolyte containing 20% sulfuric acid is used, and constant-current electrolysis is performed for 45 minutes at a current density of 51.6 A/cm2 while maintaining the piston temperature (or electrolyte temperature) at 10±5° C. By this means, only the high-purity aluminum coating is oxidized without oxidizing the aluminum alloy on the inner side of the high-purity aluminum coating, and an alumite coating having a coating thickness that is approximately equal to the target coating thickness TTARGET is formed.
In the present embodiment, by making the coating thickness TAl of the high-purity aluminum coating formed in step S4 equal to or greater than one half of the target coating thickness TTARGET and also performing anodizing treatment under the conditions described above in step S5, it is possible to transform only the high-purity aluminum coating into the alumite coating 30 (
Accordingly, it is preferable that, after taking into account the coating thickness TAl of the high-purity aluminum coating formed in step S4, the anodizing treatment in step S5 is performed under conditions (electrolyte composition, piston temperature, current density and electrolysis time) such that the aluminum alloy on the inner side of the high-purity aluminum coating is not exposed.
The description of the method for manufacturing a piston will now be continued referring again to
After step S6, the squish face is subjected to a sealing treatment (step S7). More specifically, first, a sealing agent is coated onto the squish face. A silicon-based polymer solution that includes silicon in a main chain backbone (more specifically, a polymer solution containing polysilazane or polysiloxane) is used as the sealing agent. The polymer solution may include an additive as necessary. A method for coating the sealing agent is not particularly limited, and a spray method, a blade coating method, a spin coating method and a brush application method may be mentioned as examples thereof. After coating the sealing agent, the sealing agent is dried and baked to form a sealing coating. The conditions (temperature, time period and the like) for drying and baking the sealing agent are appropriately adjusted in accordance with the coating thickness of the sealing agent.
After the sealing coating 32 has been formed, the entire area of the piston top face is polished as necessary after removing the masking from the cavity face. Prior to polishing the piston top face, it is preferable to grind the opening edge 20a so that the squish face and the cavity face are continuous with respect to each other. Thus, by performing the above described steps, the piston for a diesel engine of the present embodiment is manufactured.
Note that, in the above described embodiment, step Si corresponds to a “piston preparation step” in the above described first invention, steps S3 and S4 correspond to an “aluminum coating formation step” in the first invention, step S5 corresponds to an “anodic oxide coating formation step” in the first invention, and steps S6 and S7 correspond to a “sealing coating formation step” in the first invention. Further, step S2 corresponds to a “grinding step” in the above described third invention.
In this connection, in the above described embodiment, after the entire area of the cavity face is ground in step S2 in
Naturally, the heat-shielding coatings 34 and 36 both have lower thermal conductivity as well as lower thermal capacity per unit volume than the aluminum alloy. The heat-shielding coatings 34 and 36 also have lower thermal conductivity and lower thermal capacity per unit volume than a conventional ceramic-based heat-shielding coating. According to the heat-shielding coatings 34 and 36, rather than constantly maintaining the surfaces on which the coatings are formed at a high temperature as in the case of the ceramic-based heat-shielding coating, it is possible to cause the temperature of the surfaces on which the coatings are formed to follow the temperature of gas that fluctuates during the cycle of the engine. That is, the temperature of the surfaces on which the coatings are formed can be made a low temperature during a period from an intake stroke to a compression stroke (the upstroke in the case of a two-cycle engine), and made a high temperature during a period from an expansion stroke to an exhaust stroke (the downstroke in the case of a two-cycle engine). Accordingly, by applying the piston on which the heat-shielding coatings 34 and 36 are formed to a direct injection engine, since not only the thermal efficiency of the engine but also the air intake efficiency thereof can be improved, advantageous effects of improving the fuel consumption and reducing the amount of NOx emissions can be obtained.
Further, high pressure injection of fuel from an injector 38 shown in
As described above referring to
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-163583 | Aug 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/002998 | 6/16/2015 | WO | 00 |
