Patent Publication Number: US-2017350343-A1

Title: Piston for internal combustion engine, and process and device for producing said piston

Description:
TECHNICAL FIELD 
     The present invention relates to an internal combustion engine piston formed by casting, and a process and device for producing the internal combustion engine piston. 
     BACKGROUND ART 
     As is well known, as a spark-ignition gasoline internal combustion engine, a so-called direct-injection (GDI) internal combustion engine is provided, which is aimed to improve fuel economy by lean burn and enhance output power by homogeneous combustion. 
     In such an engine, it is known that provision of a thermal insulating material partly in a crown surface of a piston made of aluminum alloy, wherein the crown surface forms a combustion chamber, produces an effect to promote atomization of injected fuel. However, it is difficult to tightly bind the thermal insulating material to the aluminum alloy base material. 
     Therefore, as described in following patent document 1 for which the applicant applied above, a low thermal conductivity part which is lower in thermal conductivity than the aluminum alloy base material is provided at a predetermined location of the crown surface of the piston. The low thermal conductivity part has a structure where a porous member made of a glass material lower in thermal conductivity than the aluminum alloy base material is impregnated with aluminum alloy material which is a piston base material. According to the structure, both high thermal insulation performance and high bonding strength between the piston base material and the low thermal conductivity part are satisfied. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Publication No. 2014-25418 
       
    
     SUMMARY OF THE INVENTION 
     However, in the prior art described in Patent Document 1, the aluminum alloy molten metal is not sufficiently impregnated into the whole of each pore of the porous thermal insulting material due to viscosity, etc. of the aluminum alloy molten metal. Thereby, not a few vacant pores remain. 
     Therefore, in a driving of the internal combustion engine, unburnt gas gets into the remaining pores and is discharged as it is as exhaust gas, so there is a risk of deteriorating exhaust emission performance. 
     It is an object of the present invention to provide a piston for internal combustion engine to be able to prevent vacant pores of a porous member from remaining by improving sealing treatment and to prevent deterioration of exhaust emission performance, while securing high thermal insulation performance and high bonding strength in a piston base material by a low thermal conductivity part. 
     In particular, the invention described in claim  1  is characterized in that the low thermal conductivity part comprises a porous member having thermal conductivity lower than that of the piston base material and an impregnant with which pores of the porous member is impregnated, wherein the impregnant is held in the pores under usage environment of the piston during the driving of the engine. 
     According to the present invention, it is possible to prevent the pores of the porous member from remaining by improving the sealing treatment to prevent the deterioration of the exhaust emission performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a longitudinal sectional view of an internal combustion engine piston according to the present invention.  FIG. 1B  is an enlarged view of an A-part shown in  FIG. 1A . 
         FIG. 2A  is a longitudinal sectional view of a porous member employed in the present embodiment.  FIG. 2B  is an enlarged view of a B-part shown in  FIG. 2A . 
         FIG. 3  is a characteristic diagram showing vacant pores (porosity) and residual sodium chloride in relation to a volume ratio of sodium chloride in the porous member. 
         FIG. 4  is a characteristic diagram showing a relationship between volume of sodium chloride and thermal conductivity. 
         FIG. 5  is a longitudinal sectional view showing a casting mold device employed in the present embodiment. 
         FIG. 6  is a longitudinal sectional view showing a piston shaped body immediately after being cast by the casting mold device. 
         FIG. 7  is a schematic view showing a varnish impregnation device employed in the present embodiment. 
         FIG. 8  is a schematic view of a varnish impregnation step by the varnish impregnation device, wherein  FIG. 8A  shows a situation where varnish is provided into a vacuum vessel under a vacuum state,  FIG. 8B  shows a situation where to provide the varnish has finished, and  FIG. 8C  shows a situation where the varnish has been impregnated into each of pores. 
         FIG. 9  shows a schematic view showing a varnish impregnation device employed in the second embodiment, wherein  FIG. 9A  shows a situation where a negative pressure introduction mechanism and a varnish providing mechanism are not connected with the vacuum vessel, and  FIG. 9B  shows a situation where these mechanisms are connected with the vacuum vessel. 
         FIG. 10  is a schematic view of a varnish impregnation device employed in the present embodiment. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     The following describes in detail an internal combustion engine piston according to the present invention, and a process and device for producing the internal combustion engine piston according to the present embodiments, with reference to the drawings. The piston employed in the present embodiments is applied to a spark-ignition direct-injection gasoline engine. 
     The whole of the piston  1  is integrally cast of an AC8A Al—Si based aluminum alloy as a base material. As shown in  FIG. 1A , the piston  1  includes: a crown part  2  formed into a substantially cylindrical shape, and defining a combustion chamber by a crown surface  2   a ; a thrust-side skirt part  3  and an anti-thrust-side skirt part  3  in a pair, each of which is integrally formed with an outer peripheral edge of a lower end of the crown part  2 , and has a circular arc shape; and a pair of apron parts  4  coupled to both ends of each skirt part  3  in its circumferential direction. The apron parts  4  are integrally formed with respective pin boss portions  4   a ,  4   a  for supporting both ends of a piston pin (not shown). The pin boss portions  4   a ,  4   a  include pin holes  4   b ,  4   b.    
     The crown part  2  has a disc shape formed relatively thick. The crown surface  2   a  defining the combustion chamber is formed in a substantially uneven shape in a cross section, and is formed partly with a recessed portion  2   b  having a large flat surface area. A low thermal conductivity part  5  lower in thermal conductivity than a piston base material  1 ′ is embedded in a predetermined location of an upper surface of the recessed portion  2   b . Further, the outer periphery of the crown part  2  is formed with three piston ring grooves  2   c.    
     The low thermal conductivity part  5  is embedded in the location of the recessed portion  2   b  receiving direct injection of fuel from an injector in the form of a fuel injection valve provided in a cylinder head (not shown). The low thermal conductivity part  5  is integrally embedded in the recessed portion  2   b  during casting of the piston  1  described below. As shown in  FIG. 1B , in the low thermal conductivity part  5 , a part of aluminum alloy material  1   a  of the piston base material  1 ′ is impregnated into the inside of a porous member  6  which is made of a glass material having lower thermal conductivity than the piston base material  1 ′. 
     That is, the low thermal conductivity part  5  is composed of the porous member  6 , the aluminum alloy material  1   a , and varnish  39 . The porous member  6  is basically formed in a protrusive disk shape by a production process described later separately from the piston  1  and formed of the glass material. The aluminum alloy material  1   a  is a part of the piston base material  1 ′ which is impregnated into pores  9   a  after water-soluble salt of the porous member  6  impregnated in advance has been dissolved. The varnish  39  is impregnated into the pores  9   a , and it is described later. 
     &lt;Production Process for Porous Member&gt; 
     The following describes the outline of the production process for the porous member  6 . First, a first powder  8  and a second powder  9  are mixed together to produce a mixed powder, wherein the first powder  8  is a powder of glass insoluble in water, and the second powder is a soluble powder (sodium chloride powder). The mixed powder is placed into a mold, and pressure-formed at a predetermined pressure, and thereafter sintered at a predetermined sintering temperature TB. The sintering temperature TB is lower than the sintering temperature TA of the second powder  9 . 
     Thereafter, the sintered product is immersed in water or hot water that has been stirred, so that the second powder  9  in the sintered product is dissolved away by the water or hot water, to form many pores  9   a  and thereby form the porous member  6  shown in  FIG. 2 . The thermal conductivity of the porous member  6  is sufficiently lower than that of the base material  1 ′ that is a molten metal. 
     The first powder  8  is a glass powder as described above, and is a hard and transparent substance based on silicate, borate, phosphate which is a non-crystalline solid exhibiting a glass transition phenomenon with rising temperature. Chemically, the first powder  8  mainly contains a silicate compound (silicate mineral) which becomes glassy state. The oxide constituting the glass is SiO 2 , Al 2 O 3 , B 2 O 3 , BaO, Bi 2 O 3 , Li 2 O, MgO, P 2 O 5 , PbO, SnO, TiO 2 , ZnO, R 2 O (R is an abbreviation of alkali metal: Li, Na, K), or RO (R is an abbreviation of alkaline-earth metal: Mg, Ca, Sr, Ba). 
     The temperature at which the first powder  8  is softened (softening point) is lower than the melting point of the second powder  9 , wherein the first powder  8  has a melting point higher than or equal to 700° C. 
     The glass transition point, which is a temperature at which the glass structure changes, wherein the viscosity is about 1013.3 poise. The softening point, which is a temperature at which the glass is softened and deformed by its own weight, wherein the viscosity is about 107.6 poise. 
     On the other hand, the second powder  9  contains a water-soluble salt such as sodium chloride, potassium chloride, magnesium chloride, calcium chloride, calcium carbonate, sodium carbonate, sodium sulfate, magnesium sulfate, potassium sulfate, sodium nitrate, calcium nitrate, magnesium nitrate, potassium nitrate, or sodium tetraborate. The second powder  9  may be one of them or a mixed salt of two or more of them. 
     It is desirable that the salt is a water-soluble salt having a melting point exceeding 700° C., such as sodium chloride, potassium chloride, magnesium chloride, calcium chloride, calcium carbonate, sodium carbonate, sodium sulfate, magnesium sulfate, potassium sulfate, or sodium tetraborate. In the present embodiment, sodium chloride is employed. 
     Example 
     The following describes a specific process for producing the porous member  6 . 
     First, the first powder  8  is mixed with the second powder  9  while stirred, wherein the first powder  8  is borosilicate glass (glass powder ASF1898, produced by Asahi Glass Co., Ltd.), and the second powder  9  is sodium chloride. 
     The mixing ratio of the first powder  8  and the second powder  9  was set so that the first powder was 40 to 20 vol % and the second powder  9  was 60 to 80 vol %. The first powder  8  and the second powder  9  were mixed to produce a mixed powder, wherein the first powder  8  and the second powder  9  were in a weight ratio of 54:46 (mixing step). 
     The particle size of each powder was set so that the first powder  8  had an average particle size of 4.5 μm, and the second powder  9  had a particle size where 75 to 180 μm was 70% or more. 
     Then, the mixed powder was set in a mold and pressure-formed, and burnt by heating at a temperature of 650 to 750° C. for a period of 20 to 40 minutes. In this example, the mixed powder was heated at a temperature of 700° C. for a period of 30 minutes, to obtain a sintered product (burning step). 
     The sintered product was immersed in stirred hot water (liquid) at 55° C. so that the inside second powder  9  (sodium chloride) was dissolved and extracted from the sintered product to obtain a porous member  6  having many pores  9   a  (dissolving step). In the dissolving step, the second powder  9  is subjected to dissolution in hot water at 50 to 95° C. for a period of 30 minutes to 3 hours. 
     As shown in  FIG. 2A , the porous member  6  includes a disk-shaped base portion  6   a , and a projecting portion  6   b , wherein the projecting portion  6   b  has a small-diameter cylindrical shape, and is integrally formed with the upper surface of the base portion  6   a . Further, as shown in  FIG. 2B , major part of the second powder  9  is dissolved and removed from the porous member  6 , and the first powder  8  (glass) remains in the porous member  6 , so that many pores  9   a  are formed around the first powder  8 . 
     In the mixing step and the burning step described above, heating the molded body of the mixed powder of the first powder  8  (glass powder) and the second powder  9  (sodium chloride) causes the glass powder to surround and cover the particles of sodium chloride. Accordingly, the formed configuration of the porous member  6  varies depending on the mixing ratio of the first powder  8  and the second powder  9 . 
     That is, the inventor(s) of the present application made an experiment in which the mixing ratio of the first powder  8  and the second powder  9  was variously changed, and got a result shown in  FIGS. 3 and 4 . 
     Specifically, for example, when the powder of sodium chloride is at 80 vol % or more, and the glass powder is at 20 vol % or less, the glass powder particles are not melt-bonded to each other by heating. Therefore, a molded body can be produced, so that the form of the body is lost when dissolved in water or hot water. 
     When the powder of sodium chloride is less than 60 vol %, and the glass powder is more than 40 vol %, the glass powder particles are melt-bonded to each other easily by heating, resulting in surrounding and covering the sodium chloride powder particles. Accordingly, when the powder of sodium chloride is dissolved out in water or hot water thereafter, the water or hot water cannot contact the sodium chloride powder, so that the porous member  6  cannot be formed. 
     When the powder of sodium chloride is at 60 to 80 vol %, and the glass powder is at 40 to 20 vol %, open pores  9   a  (pores communicating from the surface to the inside) are obtained. All of the sodium chloride powder is not dissolved, but part of the sodium chloride powder is brought into closed state by being covered with the glass powder. The quantity of sodium chloride powder in the closed state is determined by the mixing ratio of the sodium chloride powder (second powder  9 ) and the glass powder (first powder  8 ). 
     When the sodium chloride (second powder  9 ) is at 80 vol %, there is no residual sodium chloride after the dissolution. As the volume percent of the second powder  9  decreases, the volume percent of the residual sodium chloride increases. Then, when the second powder  9  is at 60 vol %, the residual sodium chloride powder is at 25 vol %. The residual sodium chloride powder is surrounded by the first powder  8  that is a glass powder, and functions as a thermal insulating material. On the other hand, when the porous material  6  thus obtained is impregnated with a piston cast alloy (a part of aluminum alloy material  1   a ) described below, and the impregnated part is finished by cutting, the residual sodium chloride appears in the cut surface. 
     When the appeared sodium chloride powder is dissolved and removed with water or hot water again, the cut surface becomes a composite structure of cast alloy of the piston base material  1 ′ and the glass that is the porous member  6 . As the quantity of the sodium chloride powder increases, the dissolved quantity increases, which increases the unevenness of the surface and thereby increases the area of the surface. 
     In view of the foregoing, in this embodiment, the second powder  9  (sodium chloride powder) was set at 60 to 80 vol %, and the first powder  8  (glass powder) was set at 40 to 20 vol %. 
     Next, most of the second powder  9  is removed, and the porous member  6  composed mainly of the first powder  8  (glass) is placed in a vacuum casting mold  10  described later, thereafter, the molten metal of aluminum alloy is injected into the mold  10  to mold the piston  1 . In addition, part of the molten metal of the base material  1 ′ is impregnated into each pore  9   a  of the porous member  6  during the molding of the piston  1 , to embed the low thermal conductivity part  5  integrally in the recessed portion  2   b  of the crown surface  2   a  (injection step). 
     The vacuum casting mold  10  is briefly explained, as it is identical to the one described in Japanese Patent Application Publication No. 2014-25418, which is cited as the prior art. As shown in  FIG. 5 , a mold  11  includes a core  15  in a lower part of the mold  11  wherein the core  15  is formed as a combination of a plurality of split cores, such as a center core  12 , and a Philip core  13  and a side core arranged around the center core  12 . Furthermore, the vacuum casting mold  10  is provided with left and right wrist pins (not shown) extending horizontally and facing each other for forming a cooling passage for circulating cooling water therein. The distal end of the wrist pin is detachably engaged with a hole formed in the side core. 
     The vacuum casting mold  10  further includes a top core  19  in the upper part, which is removable from the mold  11 . The top core  19  includes an outer top core  21  and an inner top core  23 , wherein the outer top core  21  has a space as an example of a vacuum vent section  20 , and the inner top core  23  is integrally provided with the outer top core  21 . 
     The outer top core  21  is provided with an adapter  25  in the upper end part for sealing the vacuum vent section  20 , and is provided with a first communication pipe  27  substantially in the center of the adapter  25 . The first communication pipe  27  communicates with the vacuum vent section  20 , and is connected to a negative pressure generator such as a vacuum pump (not shown). Accordingly, the inside of the vacuum vent section  20  can be depressurized to a negative pressure by operation of the negative pressure generator. 
     The inner top core  23  is for forming a cavity  22  between the core  15  and the mold  11 , and is formed as an air-permeable mold (porous mold) made of a porous material. 
     A cavity surface  23 A, which is a lower surface of the inner top core  23 , is formed as a transfer surface for transferring the crown surface  2   a  of the piston  1  and formed as a finished surface by electrical discharge machining. Accordingly, the inner top core  23  is excellent in heat resistance and wear resistance to a molten aluminum alloy, and no galling occurs. 
     Moreover, in the cavity surface  23 A of the inner top core  23 , a part  23 B is formed to have a thickness greater than 2 mm and less or equal to 12 mm, wherein the part  23 B corresponds to a delicate portion and an edge portion of a crown combustion chamber of the crown surface  2   a  of the piston  1  as a product. 
     As shown in  FIG. 5 , the predetermined location of the inner top core  23  is provided with a second communication pipe  28  which is a metal pipe, and extends in the vertical direction through the inner top core  23 , the vacuum vent section  20 , and the adapter  25 . The lower end portion of the second communication pipe  28  is formed with a retaining recess  23   c  for retaining the porous member  6 . Namely, the porous member  6  is retained in the predetermined location in the cavity surface  23 A of the inner top core  23  in advance, and the projecting portion  6   b  is fitted and retained by press-fitting in the lower end portion of the second communication pipe  28 , and the base portion  6   a  is retained in contact with the peripheral surface of the retaining recess  23   c.    
     The upper end portion of the second communication pipe  28  is connected to a negative pressure generator such as a vacuum pump (not shown), similar to the first communication pipe  27 . Accordingly, by operation of the negative pressure generator, the inside of the porous member  6  retained in the retaining recess  23   c  in advance is depressurized to a negative pressure, so that molten metal of the aluminum alloy  1   a  described below is impregnated into the many pores  9   a.    
     Therefore, when the vacuum vent section  20  is brought into a negative pressure state, gas in the cavity  22  is sucked through the inner top core  23  to the vacuum vent section  20  and then vented to the outside. Furthermore, the molten aluminum alloy injected into the cavity  22  is sucked into direct contact with the cavity surface  23 A (transfer surface) of the inner top core  23 , so that the shape of the cavity surface  23 A is transferred as it is. 
     In addition, when the vacuum vent section  20  is brought into the negative pressure state to suck and vent the gas in the cavity  22 , and the molten metal in the cavity  22  is sucked into direct contact with the cavity surface  23 A of the inner top core  23 , it is possible to effectively carry out the suction of the part corresponding to the delicate portion or edge portion of the product, and thereby transfer the shape of the cavity surface  23 A of the inner top core  23  accurately, even if the product includes the delicate portion or edge portion. 
     The mold  11  is further provided with a runner  29  for injecting the molten metal into the cavity  22 , wherein the runner  29  is communicated with the lower portion of the cavity  22 . 
     &lt;Casting Process of Piston&gt; 
     Accordingly, in order to cast the piston  1  with the mold  10 , the molten metal of aluminum alloy is injected into the cavity  22  through the runner  29  of the mold  11  (injecting step), and the vacuum vent section  20  is subjected to the negative pressure. The provision of the molten metal into the cavity  22  is performed at the lower side of the cavity  22 , and the vacuum vent section  20  is depressurized to the negative pressure, so that the gas in the cavity  22  passes through the inner top core  23 , and is vented to the outside. 
     Simultaneously, the porous member  6  is depressurized to the negative pressure through the second communication pipe  28  by the vacuum pump, so the molten metal supplied to the cavity  22  is sucked into direct and intimate contact with the cavity surface  23 A (transfer surface) of the inner top core  23 , because the vacuum vent section  20  is at negative pressure. 
     Specifically, when the molten metal of aluminum alloy is injected into the cavity  22  through the runner  29 , and the sprue is closed by the molten metal of aluminum alloy, a motor for depressurization (not shown) is driven to vent air from the vacuum vent section  20 , and thereby depressurize the vacuum vent section  20 . When this depressurization causes a differential pressure between the vacuum vent section  20  and the cavity  22 , the gas in the cavity  22  is vented through the pores of the inner top core  23 , which is a breathable mold (porous mold), to the outside. 
     When the molten metal in the cavity  22  rises gradually to be into contact with the cavity surface  23 A of the inner top core  23 , the molten metal is sucked into intimate contact with the cavity surface  23 A because the vacuum vent section  20  is depressurized. When the piston  1  is formed, the unevenness of the cavity surface  23 A is transferred to the piston crown surface. The configuration that the part  23 B of the recessed portion  23 C of the cavity surface  23 A, which corresponds to the projecting part of the piston crown surface, is formed thinner than the remaining part, makes it possible to effectively perform the suction and intimate contact of the molten metal at this part, and precisely form a part of the crown surface  2   a  even if the shape of the part of the crown surface  2   a  is hard to appear. 
     Since the inside of the porous member  6  is at negative pressure, part of the molten metal of aluminum in the cavity  22  is sucked into the porous member  6 , and is made to permeate and fill the many pores  9   a  from which sodium chloride has been dissolved. As a result, as shown in  FIG. 6 , the low thermal conductivity part  5  impregnated with the aluminum alloy material  1   a  that is the piston base material  1 ′ is embedded integrally in and fixed to the piston base material  1 ′. The pores  9   a  are impregnated with the aluminum alloy material  1   a , wherein a small quantity of the second powder  9  (sodium chloride) remains. 
     Thereafter, the piston base material  1 ′, which is integrated with the low thermal conductivity part  5 , is taken out from the cooled vacuum casting mold  10 . In an upper surface of the recessed portion  2   b  of the piston base material  1 ′, a cylindrical part  2   d  is integrally formed on the outer peripheral side of the low thermal conductivity part  5  and formed so that the height of the cylindrical part  2   d  is an approximately same height as height of the low thermal conductivity. 
     Thereafter, a varnish  39  is impregnated into the pores  9   a  of an upper surface of the low thermal conductivity part  5  (porous member  6 ) of the piston base material  1 ′ by a varnish impregnation device. 
     That is, as a lot of pores  9   a  (1-10% of all porosity) are generated on, especially, an upper surface  6   c  of the porous member  6 , there is a risk of deteriorating exhaust emission performance due to fuel gas generated in a lot of pores  9   a , as mentioned above. Therefore, a sealing treatment is conducted. As a general sealing treatment, an organic sealing agent is infiltrated and impregnated into the pores  9   a . As major types of the sealing agent, it is possible to cite epoxy resin, phenolic resin, vinyl resin, butyral resin, derivative of organic amine, etc. As a sealing agent (impregnant), one having low viscocity to be capable of easily impregnating into the pores is desirable, and used in means such as brush coating, dip coating, and spray coating. 
     However, in these sealing agents, their heat resisting temperatures are low, so they cannot endure a thermal environment where the temperature of the crown surface of the piston  1  is 350° C. In the impregnation into the pores  9   a , sufficient impregnation into the pores  9   a  can&#39;t be expected from the brush coating, etc. Furthermore, it is possible to impregnate it into gaps of several μm by applying, that is to say, a vacuum impregnation method. However, as the whole components are impregnated in a general vacuum impregnation method, it is necessary to clean other part than impregnated part after the treatment. Therefore, it is poor in workability. 
     So, in the present embodiments, an impregnating property of the varnish  39  into each of pores  9   a  has been improved by the following varnish impregnation device which is vacuum-assisted. 
     The varnish  39  must be what can endure combustion heat of an injection fuel after curing as a sealing agent. In the present embodiments, it is polyimide precursor or polyamide imide precursor, which has a glass transition temperature of 350° C. or higher, and it is solved in a solvent mainly including DMA, DMF, or GBL. This has been selected from the following Tables 1 and 2 which are based on the results of experiments by the inventor(s) of the present application. In addition, as to the varnish  39 , a polyimide solution having already formed into polyimide can be used, not a precursor. 
     In the experiments, two types (1) and (2) of polyimide (PI) and two types (1) and (2) of polyamide imide (PAI) are respectively prepared as the varnishes  39 , and the above solvents are added to them and dissolved. Furthermore, these are provided into the pores  9   a  of the porous member  6  by the above mentioned device. In the state, the experiments has been conducted. 
     First, in Table 1, in order to volatilize the solvent of the varnish, they have been heated under atmosphere at 100 to 200° C. for 30 minutes and for 60 minutes. One where the solvent wasn&#39;t volatilized by curing the surface of the varnish and the surface swelled was regarded as x, and one where there were no change in the surface was regarded as ∘. 
     Next, in Table 2, continuously one having regarded as ∘ was heated under atmosphere at 300° C. for 30 minutes. One which swelled by volatilization of residual solvent or which was carbonized by thermal decomposition was regarded as x, and one which remained as a solid matter with small volume change was regarded as ∘. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Solvant Volatilization 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 min 
                 100 
                 110 
                 120 
                 130 
                 140 
                 150 
                 160 
                 170 
                 180 
                 190 
                 200 
               
               
                   
               
               
                 PI 
                 (1) 
                 30 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                 x 
                 x 
                 x 
                 x 
               
               
                   
                 (2) 
                 60 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                 x 
                 x 
                 x 
                 x 
               
               
                 PAI 
                 (1) 
                 30 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                 x 
                 x 
                 x 
                 x 
               
               
                   
                 (2) 
                 60 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                 x 
                 x 
                 x 
                 x 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Heating under atmosphere 300° C. × 30 min 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 min 
                 100 
                 110 
                 120 
                 130 
                 140 
                 150 
                 160 
                 170 
                 180 
                 190 
                 200 
               
               
                   
               
               
                 PI 
                 (1) 
                 30 
                 x 
                 x 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                   
                   
                   
                   
                   
               
               
                   
                 (2) 
                 60 
                 x 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                   
                   
                   
                   
                   
               
               
                 PAI 
                 (1) 
                 30 
                 x 
                 x 
                 x 
                 x 
                 ∘ 
                 ∘ 
                   
                   
                   
                   
                   
               
               
                   
                 (2) 
                 60 
                 x 
                 x 
                 x 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
               
            
           
         
       
     
     According to Table 1, in both of (1), (2) in PI and PAI, regarding heating in the range of 100 to 150° C. for 30 to 60 minutes, there were no change in the surface of varnish, so they were regarded as ∘. However, in higher than 150° C., the solvent wasn&#39;t volatilized by curing the surface of varnish, and the surface swelled, so they were regarded as x. 
     Thereafter, in case of heating the varnishes regarded as ∘ in Table 1 under atmosphere at 300° C. for 30 minutes, as shown in Table 2, one heated at low temperatures such as 100 to 110° C. in (1) and (2) of PI, one heated at 100 to 130° C. in (1) of PAI, and one heated at low temperatures such as 100 to 120° C. in (2) were not available, because swelling, which is caused by the residual solvents, occurred by heating under atmosphere at 300° C. 
     On the other hand, as to one heated at relatively high temperatures such as 130 to 150° C. in Table 1, the varnish  39  impregnated into each of pores  9   a  didn&#39;t change largely even if they were further heated under atmosphere at 300° C. Therefore, it was found that sufficient impregnating effects can be obtained. 
     Accordingly, judging from this experiment results, it has been found that the varnish  39  heated at 130 to 150° C. for 30 to 60 minutes is especially suitable. That is, the varnish  39  obtained in the temperature condition and the heating time condition is excellent in terms of heat-resisting property and durability. 
     [Varnish Impregnation Device] 
     As shown  FIG. 7 , the varnish impregnation device is composed of a vacuum vessel  30 ; a negative pressure introduction mechanism  31 ; and a varnish providing mechanism  32 . The vacuum vessel  30  is mounted and held on an upper end face of the cylindrical part  2   d  formed on the recessed portion  2   b  of the piston base material  1 ′, and is a cup shaped member having a cylindrical shape with a cover. The negative pressure introduction mechanism  31  makes the inside of the vacuum vessel  30  be in a negative pressure condition. The varnish providing mechanism  32  provides varnish for an outside surface of the low thermal conductivity part  5 . 
     The vacuum vessel  30  is integrally formed of stainless steel based metal material which has a relative thick wall and high rigidity and arranged so as to cover an upper surface of the low thermal conductivity part  5 . Furthermore, the vacuum vessel  30  is mainly composed of a cylindrical wall  30   a  mounted on the upper surface of the cylindrical part  2   d ; and an upper wall part  30   b  which is disk-shaped and integrally formed on an upper end part of the cylindrical wall  30   a.    
     The cylindrical wall  30   a  is formed so that its outer diameter is fractionally smaller than that of the cylindrical part  2   d  and so that its inner diameter is larger than that of the cylindrical part  2   d . Furthermore, the cylindrical wall  30   a  is mounted and held on the upper surface of the cylindrical part  2   d . In addition, in the entire underside, annular sealing material  33  which seals a gap between the cylindrical wall  30   a  and the upper surface of the cylindrical part  2   d  is integrally provided. 
     Regarding the upper wall part  30   b , a first fixing hole  30   c  where a vacuum pipe  35  of the negative pressure introduction mechanism  31  is inserted and fixed is penetratingly formed at a predetermined location in the outer circumferential side of the upper wall part  30   b . Furthermore, a second fixing hole  30   d  where a varnish providing pipe  41  of the varnish providing mechanism  32  is inserted and fixed is penetratingly formed at a location opposite to the fixing hole  30   c  in a radial direction. 
     The negative pressure introduction mechanism  31  is mainly composed of a vacuum pump  34 ; a vacuum pipe  35 ; a first switching valve  36 ; and a third switching valve  38 . The vacuum pump  34  is a negative pressure generator which generates negative pressure. The vacuum pipe  35  is a negative pressure introduction passage wherein its one end part  35   a  is connected with the vacuum pump  34  and its other end part  35   b  is connected with the vacuum vessel  30  through the fixing hole  30   c . The first switching valve  36  is disposed on the way of the vacuum pipe  35 , and it communicates with or block the inside of the vacuum pipe  35 . The third switching valve  38  is disposed in an atmospheric pressure introduction pipe  37  which diverges from a downstream side of the first switching valve  36  and communicates with atmosphere. 
     The vacuum pump  34  is general one using oil, etc. and creates a vacuum condition by sucking the inside of the vacuum vessel  30  at a predetermined suction pressure. 
     The first switching valve  36  is conducted to opening/closing operation during the following impregnation work of varnish. It is opened during operating the vacuum pump  34  and, it is closed when the operation stops. Furthermore, after the closing operation, the third switching valve  38  is conducted to the opening operation. 
     The varnish providing mechanism  32  is mainly composed of a storage tank  40 ; a varnish providing pipe  41 ; and a second switching valve  42 . The storage tank  40  is bottomed cylindrical shaped and stores the varnish  39 , which is an impregnant, therein. The varnish providing pipe  41  is an impregnant introduction passage wherein its one end part  41   a  is connected with the storage tank  40  and its other end part  41   b  is connected with the second fixing hole  30   d  of the vacuum vessel  30 . The second switching valve  42  is disposed on the way of the varnish providing pipe  41 . 
     The storage tank  40  has a heater  43  in the entire periphery. This heater  43  holds temperature of the varnish  39  at a fixed value in order to stabilize the viscosity of the internal varnish  39 . 
     The second switching valve  42  is what opens and closes the varnish providing pipe  41  during the following varnish filling (providing) operation. When the pressure of the vacuum vessel  30  is reduced by the predetermined pressure by the vacuum pump  34 , it performs opening operation to provide the varnish  39  being in the storage tank  40  for a side of an upper surface  6   c  of the porous member  6 . 
     Furthermore, the negative pressure introduction mechanism  31  and the varnish providing mechanism  32  are in a state that they are connected with the vacuum vessel  30  in advance. 
     Hereinafter, operation process of impregnating the varnish  39  into each of pores  9   a  of the porous member  6  is explained. 
     First, as shown in  FIG. 7 , through the sealing member  33 , the vacuum vessel  30  is mounted and fixed on the upper surface of the cylindrical part  2   d  which is on the crown part  2  of the piston base material  1 ′ molded by the producing process mentioned above. 
     Thereafter, the other end part  35   b  of the vacuum pipe  35  of the vacuum pump  34  is connected with and fixed on the first fixing hole  30   c , and a lower end part of the varnish providing pipe  41  where the storage tank  40  is installed is connected with the second fixing hole  30   d.    
     At this time, all of the switching valves (the first switching valve  36 , the second switching valve  42 , the third switching valve  38 ) are in a state of closing operation. Therefore, a communication of upstream and downstream with regard to each of the vacuum pipe  35  and varnish providing pipe  41  is blocked, and a communication between the inside of the vacuum pipe  35  and atmosphere is also blocked. 
     Next, a predetermined amount of the varnish  39  is put into the storage tank  40 , and the varnish  39  in the storage tank  40  is heated by the predetermined temperature while turning on the switch of the heater  43  in order to stabilize the viscosity of the varnish  39 . 
     Thereafter, the first switching valve  36  is opened, and the vacuum pump  34  is operated. When the pressure in the vacuum vessel  30  becomes 0.01 MPa or less in a degree of vacuum, the first switching valve  36  is closed. 
     Thereafter, as shown in  FIG. 8A  and  FIG. 8B , the varnish providing pipe  41  is communicated with the vacuum vessel  30  by opening the second valve  42 , and the varnish  39  is supplied into the vacuum vessel  30 . Thereby, the upper surface  6   c  of the porous member  6  is made into a state of being covered with the varnish  39 . Thereafter, the second switching valve is closed after the provision volume of the varnish  39  had gotten proper. Thereby, the upper surface  6   c  of the porous member  6  gets the state of being covered with the varnish  39 . 
     Subsequently, as shown in  FIG. 8C , the third valve  38  is conducted to the opening operation in the state where the first switching valve  36  remains to close. Thereby, atmospheric pressure is supplied into the vacuum vessel  30  through the downstream side of the vacuum pipe  35  from the atmospheric pressure introduction pipe  37 . Thereby, the varnish  39  is impregnated into each of pores  9   a , which is in vacuum condition, in a state the varnish  39  is strongly sucked by the atmospheric pressure. 
     After the varnish  39  has been impregnated into each of pores  9   a  of the porous member  6 , the vacuum vessel  30 , negative pressure introduction mechanism  31 , and varnish providing mechanism  32  are removed from the piston base material  1 ′. 
     Thereafter, after the varnish  39  impregnated into each of pores  9   a  of the porous member  6  is cured, casting fins, each piston ring groove  2   c , etc. which are formed on the outer peripheral surface of the piston base material  1 ′ are conducted to cutting. Furthermore, the crown surface  2   a , the cylindrical part  2   d , the projecting portion  6   b  of the low thermal conductivity part  5  (porous member  6 ) being convex shaped, etc. are conducted to cutting, thereby making them into the same surface as the upper surface of the recessed portion  2   b  (cutting work). By those series of molding process, the molding work of the piston  1  is to be completed. 
     As described above, in the present embodiment, the low thermal conductivity part  5  is provided at the part of the crown surface  2   a  of the piston  1  to which fuel is directly injected, wherein the low thermal conductivity part  5  is composed of the porous member  6  as a main structure, and the porous member  6  is made of borosilicate glass having a lower thermal conductivity than the aluminum alloy material. This produces an excellent thermal insulation property, and thereby promotes sufficiently atomization of the fuel, to enhance the combustion performance, and enhance the fuel efficiency. 
     With regard to the thermal conductivity of the low thermal conductivity part  5 , as the porosity of the pores  9   a  of the porous member  6  decreases, the quantity of impregnation of the aluminum alloy material  1   a  of the piston  1  in the pores  9   a  decreases so that the thermal conductivity decreases because the total volume ratio of the first powder  8  (glass powder) and the residual sodium chloride powder increases. 
     After the residual sodium chloride appearing on the surface is dissolved and removed with water or hot water, the surface area becomes larger than the surface area constituted by the first powder  8  and the residual sodium chloride before the removal, because only the glass component of the first powder  8  is left to form irregularities after the removal. 
     As described above, as the thermal conductivity of the low thermal conductivity part  5  decreases, the quantity of accumulated heat in the low thermal conductivity part  5  increases, so that the accumulated heat serves to promote atomization of the fuel, and the increase of the surface area also serves to transmit heat to the fuel, and thereby promote atomization of the fuel. 
     Moreover, the configuration that the low thermal conductivity part  5  is impregnated through the many pores  9   a  with the aluminum alloy material  1   a  that is identical to the piston base material  1 ′, serves to enhance the fusion resistance between the aluminum alloy material  1   a  and the piston base material  1 ′, and thereby enhance the bond strength therebetween. 
     As a result, it is possible to simultaneously achieve high thermal insulation and high bond strength between the piston base material  1 ′ and the low thermal conductivity part  5 . 
     In particular, the configuration that the many pores  9   a  of the porous member  6  are impregnated with the aluminum alloy material  1   a  of the piston  1 , serves to increase the interface strength between the porous member  6  and the cast alloy of the piston  1 . 
     Furthermore, in the present embodiment, as the varnish  39  is impregnated into each of pores  9   a  formed in the porous member  6  using the above-mentioned varnish impregnation device which is vacuum-assisted, impregnating ability of the varnish  39  to each of pores  9   a  can be improved. Especially, the varnish  39  is directly supplied to the upper surface  6   c  of the porous member  6  and impregnated into each of pores  9   a  by vacuum drawing and atmospheric pressure. Therefore, the impregnating effect of the varnish  39  improves. 
     As the result, the remaining of the pores  9   a , in particular, in the upper surface  6   a  side of the porous member  6  can sufficiently be prevented by improving the sealing treatment. Therefore, it is possible to sufficiently prevent exhaust emission property from worsening. 
     Second Embodiment 
       FIG. 9  shows the second embodiment. In  FIG. 9 , a pipe connecting connector  44  is installed in advance in the first fixing hole  30   c  formed on the upper wall part  30   b  of the vacuum vessel  30 . On the other hand, a support member  45   a  and a support piece  45   b  are disposed on an upper part of a hole edge of the second fixing hole  30   d . The support member  45   a  is cylindrical. The support piece  45   b  is mounted and fixed on an upper end surface of the support member  45   a , and in its center, a support hole where a lower end part of the varnish providing pipe  41  inserted and supported is installed. Furthermore, the support member  45   a  and the support piece  45   b  liquidtightly contact the vacuum vessel  30  by a predetermined sealing member, and a gap between the hole edge of the support hole and a lower end part of the varnish providing pipe  41  is also sealed. 
     Therefore, in order to connect the vacuum vessel  30  with the negative pressure introduction mechanism  31  and the varnish providing mechanism  32 , as shown in  FIG. 9B , the connector  44  installed on the first fixing hole  30   c  in advance is connected with the other end part  35   b  of the vacuum pipe  35 . Furthermore, the support part  45   a  where the support piece  45   b  is fixed is mounted and fixed on an upper part of the second fixing hole  30   d , and the tip part of the lower end part of the varnish providing pipe  41  is faced to the inside of the vacuum vessel  30  while inserting the lower end part into the support hole of the support piece  45   b . Thereby, the connection of the mechanisms  31  and  32  to the vacuum vessel  30  is completed. 
     In this connection state, each of pores  9   a  of the porous member  6  is filled (impregnated) with the varnish  39  by the same processes as the above-mentioned filling and impregnating processes of the varnish. Therefore, the same effects as in the first embodiment can be obtained. 
     Furthermore, after having completed the impregnation work of the varnish  39 , the negative pressure introduction mechanism  31  and the varnish providing mechanism  32  are removed from the vacuum vessel  30 . Thereafter, the vacuum vessel  30  is removed from the cylindrical part  2   d . In this way, as each of mechanisms  31  and  32  is removed from the vacuum vessel  30 , the varnish  39  never run out to a surrounding area. 
     Third Embodiment 
       FIG. 10  shows the third embodiment. In the first and second embodiments, the varnish impregnation device is connected with individual piston base material  1 ′. However, in this embodiment, four vacuum vessels  30  integrally combined in advance are simultaneously mounted and fixed on upper surfaces of each cylindrical part  2   d  of four piston base materials  1 ′ arranged in parallel. Furthermore, the first fixing holes  30   c  of each vacuum vessel  30  are connected with the other end parts  35   b  of the vacuum pipe  35 , which is branched into four branches, of the negative pressure introduction mechanism  31 ; and the lower end parts  41   b  of the varnish providing pipe  41 , which is branched into four branches, of the varnish providing mechanism  32 . 
     Therefore, according to this embodiment, as its basic configuration is the same as the first and second embodiments, the same effects can be obtained. Especially, it is possible to conduct vacuum drawing, provision of varnish  39 , etc. at the same time to each porous member  6  of the four piston base materials  1 ′, so working efficiency of the sealing treatment can be improved. 
     The present invention is not limited to the structures of each embodiment. For example, a lot of piston base materials  1 ′ are continuously arranged on the manufacturing line, and the vacuum vessels  30  are mounted and fixed in succession on the cylindrical parts  2   d  of each piston base material  1 ′, and continuous sealing treatments can be conducted. 
     Furthermore, in the present embodiments, a materials including polyimide or polyamide imide, which are organic resin materials, has been used as a sealing agent (impregnant). However, it may be what is capable of enduring under thermal environment of the crown surface  2   a  of the piston  1 . For example, sodium silicate, which is inorganic material, alkyl silicate, organosiloxane, dichromate, etc. may be used.