Patent Publication Number: US-2007101964-A1

Title: Valve seat for engine method of manufacturing the valve seat, and cylinder head for engine

Description:
CROSS-REFERENCE TO PRIOR APPLICATION  
      This is a U.S. National Phase Application under 35 U.S.C. §371 of International Patent Application No. PCT/JP2004/014334 filed Sep. 30, 2004, and claims the benefit of Japanese Patent Application Nos. 2003-341541 filed Sep. 30, 2003; 2003-341542 filed Sep. 30, 2003 and 2003-341543 filed Sep. 30, 2003, all of which are incorporated by reference herein. The International Application was published in Japanese on Apr. 7, 2005 as WO 2005/031126 a1 under PCT Article 21(2). 
    
    
     TECHNICAL FIELD  
      The present invention relates to an engine valve seat, a manufacturing method thereof, and an engine cylinder head.  
     BACKGROUND ART  
      As disclosed in Japanese Unexamined Patent Publication No. H7-133705, Japanese Unexamined Patent Publication No. S58-77114, and Japanese Unexamined Patent Publication No. S58-77116, such a conventional type is known which can improve wear resistance and further reduce a mutual damage by forming an iron oxide film essentially consisting of Fe 3 O 4  on the surface of a valve seat made of an iron-based sintered alloy, so that it can be applied to, particularly, an intake valve seat of a high power internal combustion engine, such as a diesel engine or an LPG engine, which does not likely generate combustion products. Also known is a valve seat obtained by performing an infiltration process on a primary sintered alloy, processing it to an almost finished size, and performing oxidation, and is therefore applicable to an LPG engine or a hydrogen engine which improves thermal conductivity by an infiltrated plating layer and wear resistance by an oxide film on a valve abutment surface. Further known is a valve seat applicable to an LPG engine, a hydrogen engine, a high-lead gasoline engine, or an exhaust gas recirculation apparatus (E.G.R).The valve seat is made of a sintered alloy press-molded, sintered and processed to a predetermined size is mounted in a cylinder head and is then subjected to a steam treatment to form an oxide film.  
      Recently, an engine fuel that essentially consists of alcohol which generates a relatively clean exhaust gas became popular. Such a fuel, together with air, is introduced into a combustion chamber through an air inlet, and the fuel is burned after an intake valve seat provided at the air inlet is closed by an intake valve, thereby acquiring power. Subsequently, an exhaust valve seat provided at an air outlet is opened by an exhaust valve, thereby exhausting an exhaust gas.  
     DISCLOSURE OF THE INVENTION  
     Problems to be Solved by the Invention  
      The engine fuel essentially consisting of alcohol may contain a relatively large amount of water in comparison with conventional gasoline and diesel oil. The water can possibly penetrate into a clearance between the intake valve seat and the mounting reception portion of a cylinder head in which the valve seat is mounted when the engine fuel essentially consisting of alcohol is introduced together with air into the cylinder through the air inlet having the intake valve seat. If there is water in the clearance between the intake valve seat and the mounting reception portion in this manner, galvanic corrosion may occur in a case of dissimilar metal contact. For example, the intake valve seat can be an iron-based metal and, the mounting reception portion and therefore the cylinder head are aluminum-based metals. That is, according to the galvanic corrosion, when dissimilar metals contact with each other with water being present therebetween, electricity is generated, so that a naturally charged metal is corroded. In a case of aluminum and iron, aluminum is negatively charged and is corroded. As a result, for example, a hole which leads from the mounting reception portion to a cooling channel may be formed in the cylinder head.  
      The conventional technologies form an iron oxide film on the valve seat to improve wear resistance but, it cannot prevent galvanic corrosion occurred in an engine which uses the engine fuel essentially consisting of alcohol.  
      Such galvanic corrosion may also occur at the mounting reception portion of the air outlet side and the exhaust valve seat.  
      An object of the invention is to provide an engine valve seat, a manufacturing method thereof, and an engine cylinder head which can prevent galvanic corrosion even if an intake or exhaust valve seat and a mounting reception portion are a combination of dissimilar metals.  
     MEANS FOR SOLVING THE PROBLEMS  
      A first aspect of the present invention proposes an engine valve seat with a plating layer formed on a surface of a valve seat main body provided at a mounting reception portion provided at an air inlet or air outlet of a cylinder head.  
      A second aspect of the present invention proposes an engine valve seat with a plating layer formed on at least that surface of a valve seat main body, provided at a mounting reception portion provided at an air inlet or air outlet of a cylinder head, which faces the receiving portion.  
      A third aspect of the present invention proposes the engine valve seat in that the standard electrode potential of the plating layer is set between the electrode potential of the valve seat main body and the electrode potential of the mounting reception portion.  
      A forth aspect of the present invention proposes an engine cylinder head having a valve seat provided at a mounting reception portion provided at an air inlet or air outlet of the cylinder head, characterized in that a plating layer is formed on a surface of the mounting reception portion.  
      A fifth aspect of the present invention proposes an engine cylinder head having a valve seat provided at a mounting reception portion provided at an air inlet or air outlet of the cylinder head, characterized in that a plating layer is formed on that surface of the mounting reception portion which faces the valve seat.  
      A sixth aspect of the present invention proposes the engine cylinder head in that the standard electrode potential of the plating layer is set between the electrode potential of the valve seat and the electrode potential of the mounting reception portion.  
      A seventh aspect of the present invention proposes an engine cylinder head having a valve seat provided at a mounting reception portion provided at an air inlet or air outlet of the cylinder head, characterized in that plating layers are formed on both the surface of the mounting reception portion and the surface of the valve seat.  
      An eighth aspect of the present invention proposes an engine cylinder head having a valve seat provided at a mounting reception portion provided at an air inlet or air outlet of the cylinder head, characterized in that a plating layer is formed on that surface of the mounting reception portion which faces the valve seat, and a plating layer is formed on that surface of the valve seat which faces the mounting reception portion of the valve seat.  
      A ninth aspect of the present invention proposes the engine cylinder head in that a material for the plating layer of the mounting reception portion and a material for the plating layer of the valve seat are provided in such a manner that electrode potentials equal or approximately equal to each other. Alternately, the electrode potential of the aluminum-based cylinder head, the electrode potential of the plating layer of the mounting reception portion, the electrode potential of the plating layer of the valve seat, and the electrode potential of the iron-based valve seat, increase in that order.  
      A tenth aspect of the present invention proposes an engine valve seat with an insulating layer formed on a surface of a valve seat main body provided at a mounting reception portion provided at an air inlet or air outlet of a cylinder head, characterized in that the valve seat main body is made of an iron-based alloy, and the insulating layer is an iron oxide film.  
      An eleventh aspect of the present invention proposes an engine valve seat with an insulating layer formed on that surface of a valve seat main body, provided at a mounting reception portion provided at an air inlet or air outlet of a cylinder head, which faces the mounting reception portion, characterized in that the valve seat main body is made of an iron-based alloy, and the insulating layer is an iron oxide film.  
      A twelfth aspect of the present invention proposes a method of manufacturing a valve seat provided at a mounting reception portion provided at an air inlet or air outlet of a cylinder head, characterized in that after an insulating layer is formed on an entire surface of a valve seat main body, the valve seat main body is mounted in the mounting reception portion after which the insulating layer on a seat surface of the valve seat main body is removed, and the seat surface is processed.  
      A thirteenth aspect of the present invention proposes the valve seat manufacturing method having the steps of making the valve seat main body of an iron-based alloy, and an iron oxide film is formed as the insulating layer by steaming the surface of the valve seat main body.  
      A fourteenth aspect of the present invention proposes an engine valve seat with a coating layer for electrical insulation formed on a surface of a valve seat main body provided at a mounting reception portion provided at an air inlet or air outlet of a cylinder head.  
      A fifteenth aspect of the present invention proposes an engine valve seat with a coating layer for electrical insulation formed on at least that surface of a valve seat main body, provided at a mounting reception portion provided at an air inlet or air outlet of a cylinder head, which faces the mounting reception portion.  
      A sixteenth aspect of the present invention proposes an engine cylinder head having a valve seat provided at a mounting reception portion provided at an air inlet or air outlet of a cylinder head, characterized in that a coating layer for electrical insulation is formed on the surface of the concaved mounting reception portion.  
      A seventeenth aspect of the present invention proposes the engine valve seat in that the coating layer is a ceramic coating layer.  
      An eighteenth aspect of the present invention proposes the engine cylinder head wherein the coating layer is a ceramic coating layer.  
      An nineteenth aspect of the present invention proposes the engine valve seat in that the coating layer is a polytetrafluoroethylene resin layer.  
      A twentieth aspect of the present invention proposes the engine cylinder head according to claim  16 , characterized in that the coating layer is a polytetrafluoroethylene resin layer.  
      A twenty-first aspect of the present invention proposes the engine cylinder head in that the cylinder head is made of an aluminum alloy, and the coating layer is an alumite treated layer.  
     EFFECTS OF THE INVENTION  
      According to the invention as set forth in the first aspect, because the plating layer provided on the surface of the valve seat main body intervenes between the mounting reception portion of the cylinder head and the valve seat main body, a potential difference between the mounting reception portion and the valve seat main body is reduced through water, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the second aspect, because the plating layer is provided on at least that surface of the of the valve seat main body which faces the mounting reception portion, the plating layer is provided on that surface where the mounting reception portion of the cylinder head and the valve seat main body possibly contact each other, so that a potential difference between the mounting reception portion and the valve seat main body, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the third aspect, the plating layer provided on the valve seat reduces a potential difference between the plating layer and the mounting reception portion as much as possible.  
      According to the invention as set forth in the fourth aspect, because the plating layer provided on the surface of the mounting reception portion intervenes between the mounting reception portion of the cylinder head and the valve seat main body, a potential difference between the mounting reception portion and the valve seat main body is reduced through water, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the fifth aspect, because the plating layer is provided on that surface of the of the mounting reception portion which faces the valve seat, the plating layer is provided on that surface where the mounting reception portion of the cylinder head and the valve seat possibly contact each other, so that a potential difference between the mounting reception portion and the valve seat, thereby preventing galvanic corrosion.  
      The invention as set forth in the sixth aspect reduces a potential difference between the plating layer and the valve seat as much as possible.  
      According to the invention as set forth in the seventh aspect, because the plating layers are provided on both the mounting reception portion of the cylinder head and the valve seat, a potential difference between the mounting reception portion and the valve seat main body is reduced through water, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the eighth aspect, the plating layers are respectively provided on those surfaces of the mounting reception portion and valve seat which face with each other, a potential difference between the mounting reception portion and the valve seat main body is reduced, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the ninth aspect, because the plating layers respectively provided on the mounting reception portion and the valve seat are provided in such a manner that the electrode potential of the same or similar material or aluminum-based cylinder head, the electrode potential of the plating layer of the mounting reception portion, the electrode potential of the plating layer of the valve seat, and the electrode potential of the iron-based valve seat increase in that order, a potential difference between the plating layer of the mounting reception portion and the plating layer of the valve seat is reduced as much as possible.  
      According to the invention as set forth in the tenth aspect, because the insulating layer intervenes between the mounting reception portion of the cylinder head and the valve seat main body, no dissimilar metal contact through water by the mounting reception portion and the valve seat main body occurs, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the eleventh aspect, the insulating film is provided on that surface where the mounting reception portion of the cylinder head and the valve seat main body possibly contact each other, no dissimilar metal contact by the mounting reception portion and the valve seat main body occurs, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the twelfth aspect, because removal of the iron oxide film on the seat surface of the valve seat main body and processing of the seat surface can be carried out at the same time, manufacturing without a loss at a manufacturing process is ensured.  
      The invention as set forth in the thirteenth aspect facilitates formation of the insulating layer as the iron oxide film by seaming.  
      According to the invention as set forth in the fourteenth aspect, because the coating layer provided on the surface of the valve seat main body intervenes between the mounting reception portion of the cylinder head and the valve seat main body, no dissimilar metal contact through water by the mounting reception portion and the valve seat main body occurs, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the fifteenth aspect, because the coating layer is provided on at least that surface of the of the valve seat main body which faces the mounting reception portion, the plating layer is provided on that surface where the mounting reception portion of the cylinder head and the valve seat main body possibly contact each other, so that no dissimilar metal contact by the mounting reception portion and the valve seat main body occurs, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the sixteenth aspect, because the coating layer provided on the mounting reception portion intervenes between the cylinder head and the valve seat, no dissimilar metal contact through water by the mounting reception portion and the valve seat occurs, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the seventeenth and eighteenth aspects, because the coating layer as the ceramic coating layer intervenes between the mounting reception portion of the cylinder head and the valve seat main body, no dissimilar metal contact through water by the mounting reception portion and the valve seat main body occurs, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the nineteenth and twentieth aspects, because the coating layer as the polytetrafluoroethylene resin layer intervenes between the mounting reception portion of the cylinder head and the valve seat main body, no dissimilar metal contact through water by the mounting reception portion and the valve seat main body occurs, thereby preventing galvanic corrosion.  
      According to the invention as set forth in the twenty-first aspect, as the alumite treated layer as the coating layer intervenes between the mounting reception portion of the cylinder head and the valve seat main body, no dissimilar metal contact through water by the mounting reception portion and the valve seat main body occurs, thereby preventing galvanic corrosion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view illustrating an embodiment of the invention.  
       FIG. 2  is a cross-sectional view of relevant parts illustrating the embodiment as illustrated in  FIG. 1  of the invention.  
       FIG. 3  is an explanatory table of standard electrode potentials illustrating the embodiment of the invention of  FIG. 1 .  
       FIG. 4  is a cross-sectional view illustrating another embodiment of the invention.  
       FIG. 5  is a cross-sectional view of relevant parts illustrating the embodiment as illustrated in  FIG. 4  of the invention.  
       FIG. 6  is a cross-sectional view illustrating a further embodiment of the invention.  
       FIG. 7  is a cross-sectional view of relevant parts illustrating the embodiment as illustrated in  FIG. 6  of the invention.  
       FIG. 8  is a cross-sectional view illustrating an embodiment of the invention.  
       FIG. 9  is a cross-sectional view of relevant parts illustrating the fourth embodiment as illustrated in  FIG. 8  of the invention.  
       FIG. 10  is a cross-sectional view of the manufacturing step illustrating the embodiment as illustrated in  FIG. 8  of the invention.  
       FIG. 11  is a cross-sectional view of relevant parts of the manufacturing step illustrating the embodiment as illustrated in  FIG. 8  of the invention.  
       FIG. 12  is a perspective view of the examination of the iron oxide films by a steam treatment.  
       FIG. 13  is a cross-sectional view illustrating another embodiment of the invention.  
       FIG. 14  is a cross-sectional view of relevant parts illustrating the embodiment of the invention as illustrated in  FIG. 13 .  
       FIG. 15  is a cross-sectional view illustrating a further embodiment of the invention.  
       FIG. 16  is a cross-sectional view of relevant parts illustrating the embodiment of the invention as illustrated in  FIG. 15 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Individual embodiments of an engine valve seat, manufacturing method thereof, and engine cylinder head according to the invention will be explained below.  
      FIGS.  1  to  3  illustrate an embodiment, and a cylinder head  2  fixed to a cylinder  1  in which a non-illustrated piston reciprocates, is made of an aluminum alloy, and has an intake port  3  provided on the one side thereof, and an exhaust port  4  on the other side thereof. An air inlet  6  of the intake port  3  that faces a combustion chamber  5  is provided with an intake valve seat  7  which is opened and closed by an intake valve  8 . Likewise, an air outlet  9  of an exhaust port  4  that faces the combustion chamber  5  is provided with an exhaust valve seat  10  which is opened and closed by an exhaust valve  11 . The cylinder head  2  further has a cooling channel  12  provided between the intake port  3  and the exhaust port  4 .  
      A mounting reception portion  13  for the intake valve seat  7  is provided at the air inlet  6 . The mounting reception portion  13  is so formed in a concaved manner as to have a slightly larger diameter than the diameter of the intake port  3 , and the intake valve seat  7  is fitted into the mounting reception portion  13 .  
      The iron-based intake valve seat  7  is formed in a ring-like shape in such a way that its external diameter is the same size as the diameter of the mounting reception portion  13 , and its internal diameter is the same size as the diameter of the intake port  3 , and is made of an iron-based sintered alloy. That surface of the intake valve seat  7  which faces the mounting reception portion is covered with a plating layer  14 . The standard electrode potential of the plating layer  14  is set between the electrode potential of a later-described valve seat main body  17  and the electrode potential of the mounting reception portion  13 . That is, an electrode potential is a voltage between an electrode and a solution or electrolyte in which the electrode is soaked, and is normally compared with a standard electrode like a hydrogen electrode.  FIG. 3  illustrates standard electrode voltages of metals. The standard electrode potential (E H ) of the cylinder  2  made of an aluminum alloy is almost −1.3 V or so because the standard electrode voltage (E H ) of aluminum is −1.337 V, and the standard electrode potential of the iron-based valve seat main body  17  is −0.42 V or so, so that it is preferable that the plating layer  14  should be made of an aluminum or aluminum alloy whose standard electrode potential becomes almost the same as that of the mounting reception portion  12 , Zn (standard electrode potential is −0.76 V or so) or Cr (standard electrode potential is −0.56 V or so) whose standard electrode potential is set in the middle, or Al—Zn, Cr-based composite plating, or Zn-based composite plating. Meanwhile, the Cr-based composite plating is a plating in which ceramic particles of Al 2 O 3  or the like, or resin particles of PTFE (PolyTetraFluoroEthylene) or the like in a metallic plating film are dispersed, and has both corrosion resistance and wear resistance. Its thickness is 50 angstroms to 100 μm, and preferably 1 to 100 μm, and is formed on an outer periphery portion  14   a  formed at the outer periphery surface of the intake valve seat  7  and an abutment portion  14   b  located at the intake port  3  side, as well as a chamfered angle portion  14   c  formed at the edge at the mounting reception portion  13  side. The surface of the intake valve seat  7  which faces the combustion chamber  5  is tapered with the intake valve  8  being the shaft center and is formed as a seat surface  15 , and the plating layer  14  is not formed on the seat surface  15 , and an inner periphery surface  16  may be or may not be provided with the plating layer  14 .  
      Next, an intake valve seat manufacturing method and an attaching method will be explained. With respect to the intake valve seat  7 , mixed well with Fe powders are all of or part of Fe—Si powders, Ni powders, Co powders, Fe—W powders, Fe—Cr powders, Cu powders, Fe—Nb powders, Fe—V powders, and C powders, powder compacting by molds is performed on the obtained mixed powders, and the obtained mold powder compacting body is sintered under a normal condition, thereby manufacturing the valve seat main body  17  with an ingredient composition substantially the same as the mixture composition.  
      The plating layer  14  is formed on the outer periphery portion  14   a , abutment portion  14   b , and the chamfered angle portion  14   c  of the valve seat main body  17 . Next, the intake valve seat  7  is mounted in the cylinder head  2 . This mounting is carried out by press, shrink fit, or expansion fit of the intake valve seat  7  to the mounting reception portion  13 . Next, the seat surface  15  is machined in such a way that the intake valve  8  closely contacts the seat surface  15  of the mounted intake valve seat  7 .  
      Next, the effect of the above-described structure will be explained. When an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in the cylinder  1  together with air through the intake port, if the water penetrates a clearance S between the intake valve seat  7  and the mounting reception portion  13  and remains there, the cylinder head  2  and the intake valve seat  7  contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. The plating layer  14  which is made of the same or homogeneous material as that of the cylinder head  2  is, however, formed on that surface of the intake valve seat main body  17  which faces the mounting reception portion  13 , thus preventing occurrence of galvanic corrosion.  
      As explained above, according to the embodiment, as the plating layer  14  is formed on the surface of the iron-based valve seat main body  17  provided at the mounting reception portion  13  formed at the air inlet  6  of the cylinder head  2  made of an aluminum alloy, the plating layer  14  intervenes between the cylinder head  2  and the valve seat main body  17 , a potential difference between the dissimilar metals for the mounting reception portion  13  and the valve seat main body  17  is reduced through water, thereby preventing galvanic corrosion.  
      Because the plating layer  14  is provided on at least that surface of the valve seat main body which faces the mounting reception portion  13 , the plating layer  14  is provided on the surface of valve seat main body  17  which can contact the mounting reception portion  13 , and the potential difference between the dissimilar metals for the mounting reception portion  13  and the valve seat main body  17  is reduced, thereby preventing galvanic corrosion.  
      Further, as the electrode potential of the plating layer  14  provided on the valve seat main body  17  is almost the same as the electrode potential of the mounting reception portion  13 , or is set between the electrode potential of the valve seat main body  17  and the electrode potential of the mounting reception portion  13 , so that the potential difference between the plating layer  14  and the mounting reception portion  13  is reduced.  
      Other embodiments are explained below. For the other embodiments, the same portions as the above embodiment are denoted by the same reference numerals to omit their detailed explanations.  
       FIGS. 4 and 5  illustrate another embodiment. Provided at the air inlet  6  formed at the cylinder head  2  is a mounting reception portion  22  for an intake valve seat  21 , and the intake valve seat  21  is fitted into the mounting reception portion  22 . The intake valve seat  21  is made of an iron-based sintered alloy, and is formed in a ring-like shape in such a way that its external diameter is the same size as the diameter of the mounting reception portion  22 , and its internal diameter is the same size as the diameter of the intake port  3 .  
      The mounting reception portion  22  is provided with a plating layer  23 . The plating layer  23  is formed on the surfaces opposite to the intake valve seat  21 , i.e., formed on an inner periphery surface portion  23   a  and a bottom surface portion  23   b . The electrode potential of the plating layer  23  is set between the electrode potential of the valve seat  21  and the electrode potential of the mounting reception portion  22 . That is, the standard electrode potential (E H ) of the cylinder  2  made of an aluminum alloy is −1.3 V or so, and the standard electrode potential of the iron-based valve seat  21  is −0.42 V or so, so that it is preferable that the plating layer  23  should be made of Zn (standard electrode potential is −0.76 V or so), Cr (standard electrode potential is −0.56 V or so) whose standard electrode potentials are set in the middle, or Al—Zn, Cr-based composite plating, or Zn-based composite plating.  
      Therefore, when an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in the cylinder  1  together with air through the intake port, if the water penetrates the clearance S between the intake valve seat  7  and the mounting reception portion  13  and remains there, the cylinder head  2  and the intake valve seat  7  contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. As the plating layer  23  which reduces the electrode difference is formed on the surface of the mounting reception portion  22 , however, occurrence of galvanic corrosion is suppressed.  
      As explained above, in the embodiment, as the plating layer  23  is formed on the mounting reception portion  23  provided at the air inlet  6  of the cylinder head  2  made of an aluminum alloy and the valve seat  21  is provided through the plating layer  23 , the plating layer  23  intervenes between the cylinder head  2  and the valve seat  21 , the potential difference originating from dissimilar metal contact between the mounting reception portion  22  and the valve seat  21  is reduced, thereby ensuring prevention of galvanic corrosion.  
      Because the plating layer  23  is provided on that surface of the mounting reception portion  22  which faces the valve seat  21 , the plating layer  23  is provided on the surface of the mounting reception portion  22  which possibly contacts the valve seat  21 , and the potential difference between the dissimilar metals for the mounting reception portion  22  and the valve seat  21  is reduced, thereby preventing galvanic corrosion.  
      Further, as the electrode potential of the plating layer  23  provided on the valve seat  21  is set between the electrode potential of the valve seat  21  and the electrode potential of the mounting reception portion  22 , so that the potential difference between the plating layer  23  and the mounting reception portion  22  is possibly reduced.  
       FIGS. 6 and 7  illustrate a further embodiment. Provided at the air inlet  6  formed at the cylinder head  2  is a mounting reception portion  32  for an intake valve seat  31 , and the intake valve seat  31  is fitted into the mounting reception portion  32 . The intake valve seat  31  is made of an iron-based sintered alloy, and is formed in a ring-like shape in such a way that its external diameter is the same size as the diameter of the mounting reception portion  22 , and its internal diameter is the same size as the diameter of the intake port  3 .  
      Plating layers  33 ,  34  are respectively provided on that surface of the valve seat  31  which faces the mounting reception portion  32 , and that surface of the mounting reception portion  32  which faces the valve seat  31 . The plating layer  33  is formed on an outer periphery portion  33   a  formed at the outer surface of the intake valve seat  31  and an abutment portion  34   b  located at the intake port  3  side, as well as a chamfered angle portion  33   c  formed at the edge at the mounting reception portion  32  side. The plating layer  34  is provided on the surface opposite to the intake valve seat  31 , that is, an inner periphery surface portion  34   a  and a bottom surface portion  34   b , and the materials of the plating layers  33 ,  34  are the same or similar materials so that electrode potentials becomes the same or approximately the same. The materials of the plating layers  33 ,  34  are provided in such a manner that the electrode potential of the cylinder head  2  made of an aluminum alloy (−1.33 V), the electrode potential of the Zn-based plating layer  34  of the mounting reception portion  34  (−0.76 V), the electrode potential of the Cr-based plating layer  33  of the valve seat  33  (−0.56 V), and the electrode potential of the iron-based valve seat  33  (−0.42 V), increase in that order.  
      Therefore, when an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in the cylinder  1  together with air through the intake port, if the water penetrates a clearance S between the intake valve seat  31  and the mounting reception portion  33  and remains there, the cylinder head  2  and the intake valve seat  31  contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. As the plating layers  33 ,  34  which reduce the electrode difference are formed on the surfaces of the mounting reception portion  32 , however, occurrence of galvanic corrosion is suppressed.  
      As explained above, according to the embodiment, as the plating layers  33 ,  34  are respectively formed on the valve seat  31  and the mounting reception portion  32 , the plating layers  33 ,  34  are present between the cylinder head  2  and the valve seat  31 , the potential difference originating from dissimilar metal contact between the mounting reception portion  32  and the valve seat  31  is reduced, thereby ensuring prevention of galvanic corrosion.  
      The plating layers  33 ,  34  are made of the same or similar materials, and provided in such a manner that the electrode potential of the aluminum-based cylinder head, the electrode potential of the plating layer of the mounting reception portion, the electrode potential of the plating layer of the valve seat, and the electrode potential of the iron-based valve seat, increase in that order, thus possibly reducing the potential difference between the plating layers  33  and  34 .  
      FIGS.  8  to  12  illustrate another embodiment. The air inlet  6  of the intake port  3  that faces the combustion chamber  5  is provided with an intake valve seat  40  which is opened and closed by the intake valve  8 . A mounting reception portion  41  for the intake valve seat  40  is provided at the air inlet  6 . The mounting reception portion  41  is so formed in a concaved manner as to have a slightly larger diameter than the diameter of the intake port  3 , and the intake valve seat  40  is fitted into the mounting reception portion  41 .  
      The intake valve seat  40  is formed in a ring-like shape in such a way that its external diameter is the same size as the diameter of the mounting reception portion  41 , and its internal diameter is the same size as the diameter of the intake port  3 . That surface of the intake valve seat  40  which faces the mounting reception portion  41  is covered with an iron oxide film  42  which is essentially composed of Fe 3 O 4  as an insulating layer by a seam (vapor) treatment. The iron oxide film  42  has a thickness of 1 to 50 μm, preferably 3 to 20 μm, and is formed on an outer periphery portion  42   a  formed at the outer periphery surface of the intake valve seat  40  and an abutment portion  42   b  located at the intake port  3  side, as well as a chamfered angle portion  42   c  formed at the edge at the mounting reception portion  41  side. The surface of the intake valve seat  7  which faces the combustion chamber  5  is tapered with the intake valve  8  being the shaft center and is formed as a seat surface  43 , and the iron oxide film is not formed on the seat surface  43 , and an inner periphery surface  44  may be or may not be provided with the iron oxide film.  
      Next, an intake valve seat manufacturing method will be explained. With respect to the intake valve seat  40 , mixed well with Fe powders are all of or part of Fe—Si powders, Ni powders, Co powders, Fe—W powders, Fe—Cr powders, Cu powders, Fe—Nb powders, Fe—V powders, and C powders, powder compacting by molds is performed on the obtained mixed powders, and the obtained mold powder compacting body is sintered under a normal condition, thereby manufacturing the valve seat main body  45  with an ingredient composition substantially the same as the mixture composition. If needed, press work by a correcting press is carried out, and a valve seat main body  45  is manufactured.  
      Further, a steam treatment at a predetermined temperature in a range of, for example, 500 to 550° C. is performed on those valve seat main bodies  45  for 30 to 120 minutes, and the iron oxide film  42  is formed on the outer periphery portion  42   a , the abutment portion  42   b , the chamfered angle portion  42   c , as well as the seat surface  43  and the inner periphery surface  44 . The iron oxide film  42  has characteristics such that the density is 6.8 g/cm 3 , the hardness HRB is 50, the tensile strength is 400 N/mm 2 , and the elongation is 1.5%.  
      Next, the intake valve seat  40  is mounted in the cylinder head  2 . This mounting is carried out by press, shrink fit, or expansion fit of the intake valve seat  40  to the mounting reception portion  41 . Next, the seat surface  43  of the mounted intake valve seat  40  is machined in such a manner as to closely contact the intake valve  8 . This machining is carried out with a cutting tool  48  that integrally has an intake valve guide cutting portion  46  and a valve seat cutting portion  47  which respectively perform coaxial cutting to valve guides  8   a  and valve seats  40  of the intake valves  8  plurally provided at the cylinder head  2 , and this tool is rotated around the Z axis, so that the intake valve guide cutting portion  46  machines the inner periphery surface of the valve guide  8   a  and the valve seat cutting portion  47  eliminates the iron oxide film  42  on the seat surface  43  at the same time.  
      Next, the effect of the above-described structure will be explained. When an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in the cylinder  1  together with air through the intake port  3 , if the water penetrates a clearance S between the intake valve seat  40  and the mounting reception portion  41  and remains there, the cylinder head  2  and the intake valve seat  40  contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. As the iron oxide film  42 , which has an electrical insulation property is formed on that surface of the intake valve seat main body  45  which faces the mounting reception portion  41 , however, occurrence of galvanic corrosion is suppressed.  
      Next, regarding an examination for iron oxide films by a steam treatment will be explained with reference to  FIG. 12 . In the examination, an iron-based sintered body  49  having a 60 mm diameter and a 20 mm thickness with iron oxide films  42  formed on both surfaces by a steam treatment, and one without an iron oxide film were prepared, one electrode  50  was provided on one surface of each test piece and other electrode  51  was provided by providing a piece of copper sheet on the other surface, an insulation resistance meter  52  was connected between both electrodes, and a resistance was measured. As a result, the resistance of one without an iron oxide film was 0.3 to 1.0Ω, while the resistance of the other with the iron oxide films  42  was 10 to 40Ω, and electrical insulation was confirmed.  
      As explained above, according to the embodiment, as the iron oxide film  42  is formed on the surface of the iron-based valve seat main body  45  provided at the mounting reception portion  41  formed at the air inlet  6  of the cylinder head  2  made of an aluminum alloy, the iron oxide film  42  intervenes between the cylinder head  2  and the valve seat main body  45 , no dissimilar metal contact through water by the mounting reception portion  41  and the valve seat main body  45  occurs, thus preventing galvanic corrosion.  
      Because the iron oxide film  42  is provided on at least that surface of the valve seat main body which faces the mounting reception portion  41 , the iron oxide film  42  is provided on the surface of the valve seat main body  45  which possibly contacts mounting reception portion  41 , no dissimilar metal contact by the mounting reception portion  41  and the valve seat main body  45  occurs, thus preventing galvanic corrosion.  
      Further, as the valve seat main body  45  is made of an iron-based alloy and the insulating layer is the iron oxide film  42 , formation of the insulating layer is relatively easy.  
      As the valve seat main body  45  is mounted in the mounting reception portion  41  after the iron oxide film  42  as the insulating layer is formed on the entire surface of the valve seat main body  45 , and then the iron oxide film  42  on the seat surface  43  of the valve seat main body  45  is removed and the seat surface  43  is processed, the valve seat main body  45  with the iron oxide film  42  on the entire surface can be directly mounted in the mounting reception portion  41 , and the iron oxide film  42  on the seat surface  43  can be removed when the seat surface  43  is processed in a shape coaxial to the valve  8 , so that elimination of the iron oxide film  42  on the seat surface  43  and the finish processing of the seat surface  43  can be carried out together, thereby ensuring manufacturing without a loss during a manufacturing process.  
      As the valve seat main body  45  is made of an iron-based alloy, and a steam treatment is applied to its surface to form the iron oxide film  42  as the insulating layer, formation of the insulating layer is relatively easy.  
     FIFTH EMBODIMENT  
       FIGS. 13 and 14  illustrate a further embodiment. The air inlet  6  of the intake port  3  that faces the combustion chamber  5  is provided with an intake valve seat  60  which is opened and closed by the intake valve  8 . A mounting reception portion  61  for the intake valve seat  60  is provided at the air inlet  6 . The mounting reception portion  13  is so formed in a concaved manner as to have a slightly larger diameter than the diameter of the intake port  3 , and the intake valve seat  60  is fitted into the mounting reception portion  61 .  
      The intake valve seat  60  is formed in a ring-like shape in such a way that its external diameter is the same size as the diameter of the mounting reception portion  61 , and its internal diameter is the same size as the diameter of the intake port  3 , and is made of an iron-based sintered alloy. That surface of the intake valve seat  60  which faces the mounting reception portion  61  is covered with a ceramic coating layer  62  for electric insulation. The ceramic coating layer  62  is formed by vapor coating technique, such as plasma CVD (Chemical Vapor Deposition), or ion plating, and is, for example, a PVD coating of Diamond-Like Carbon (DLC), TiCN, or TiAlN, or a CVD coating of Al 2 O 3 , SiC, Si 3 N 4 , or Al 2 O 3 , and its thickness is 50 angstroms to 100 μm, and preferably 1 to 50 μm, and is formed on an outer periphery portion  62   a  formed at the outer periphery surface of the intake valve seat  60  and an abutment portion  62   b  located at the intake port  3  side, as well as a chamfered angle portion  62   c  formed at the edge at the mounting reception portion  61  side. The surface of the intake valve seat  60  which faces the combustion chamber  5  is tapered with the intake valve  8  being the shaft center and is formed as a seat surface  63 , and the coating layer  62  is not formed on the seat surface  63 , and an inner periphery surface  16  may be or may not be provided with the coating layer  62 .  
      Next, an intake valve seat manufacturing method and an attaching method will be explained. With respect to the intake valve seat  60 , mixed well with Fe powders are all of or part of Fe—Si powders, Ni powders, Co powders, Mo powders, Fe—W powders, Fe—Cr powders, Cu powders, Fe—Nb powders, Fe—V powders, and C powders, powder compacting by molds is performed on the obtained mixed powders, and the obtained mold powder compacting body is sintered under a normal condition, thereby manufacturing the valve seat main body  65  with an ingredient composition substantially the same as the mixture composition.  
      The coating layer  62  is formed on the outer periphery portion  62   a , abutment portion  62   b , and chamfered angle portion  62   c  of the valve seat main body  65 . Next, the intake valve seat  60  is mounted in the cylinder head  2 . This mounting is carried out by press, shrink fit, or expansion fit of the intake valve seat  60  to the mounting reception portion  61 . Next, the seat surface  63  is machined in such a way that the intake valve  8  closely contacts the seat surface  63  of the mounted intake valve seat  60 .  
      Next, the effect of the above-described structure will be explained. When an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in the cylinder  1  together with air through the intake port, if the water penetrates a clearance S between the intake valve seat  60  and the mounting reception portion  61  and remains there, the cylinder head  2  and the intake valve seat  60  contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. As the coating layer  62  for electrical insulation is formed on that surface of the intake valve seat main body  65  which faces the mounting reception portion  61 , however, occurrence of galvanic corrosion is prevented.  
      As explained above, according to the embodiment, as the coating layer  62  for electrical insulation is formed on the surface of the iron-based valve seat main body  63  provided at the mounting reception portion  61  formed at the air inlet  6  of the cylinder head  2  made of an aluminum alloy, the coating layer  62  intervenes between the cylinder head  2  and the valve seat main body  65 , no dissimilar metal contact through water by the mounting reception portion  13  and the valve seat main body  65  occurs, thus preventing galvanic corrosion.  
      Because the coating layer  62  is provided on at least that surface of the valve seat main body  65  which faces the mounting reception portion  61 , the coating layer  62  is provided on the surface of the valve seat main body  65  which possibly contacts the mounting reception portion  61 , no dissimilar metal contact by the mounting reception portion  61  and the valve seat main body  65  occurs, thus preventing galvanic corrosion.  
      Furthermore, as the coating layer  62  is made of ceramic, electrical insulation between the cylinder head  2  and the valve seat main body  65  can be ensured.  
       FIGS. 15 and 16  illustrate another embodiment. Provided at the air inlet  6  formed at the cylinder head  2  is a mounting reception portion  70  for an intake valve seat  70 , and the intake valve seat  70  is fitted into the mounting reception portion  71 . The intake valve seat  21  is made of an iron-based sintered alloy, and is formed in a ring-like shape in such a way that its external diameter is the same size as the diameter of the mounting reception portion  71 , and its internal diameter is the same size as the diameter of the intake port  3 .  
      A coating layer  72  made of a ceramic for electrical insulation similar to the fifth embodiment is provided at the mounting reception portion  71 . The coating layer  72  is provided on the surfaces opposing to the intake valve seat  70 , that is, an inner periphery surface portion  72   a  and a bottom surface portion  72   b.    
      Therefore, when an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in the cylinder  1  together with air through the intake port, if the water penetrates a clearance S between the intake valve seat  70  and the mounting reception portion  71  and remains there, the cylinder head  2  and the intake valve seat  7  contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. As the coating layer  72  for electrical insulation is formed on the surfaces of the mounting reception portion  71 , however, occurrence of galvanic corrosion is prevented.  
      As explained above, according to the embodiment, as the coating layer  72  is formed at the mounting reception portion  71  formed at the air inlet  6  of the cylinder head  2  made of an aluminum alloy, and the iron-based valve seat  70  is provided via the coating layer  72 , the coating layer  72  made of a ceramic intervenes between the cylinder head  2  and the valve seat  70 , no dissimilar metal contact by the mounting reception portion  71  and the valve seat  70  occurs, thus preventing galvanic corrosion.  
      Because the coating layer  72  is provided on that surface of the mounting reception portion  71  which faces the valve seat  70 , the coating layer  72  is provided on the surface of the mounting reception portion  71  which possibly contacts the valve seat  70 , no dissimilar metal contact by the mounting reception portion  72  and the valve seat  70  occurs, thus preventing galvanic corrosion.  
      Furthermore, as the coating layer  72  is made of ceramic, electrical insulation between the cylinder head  2  and the valve seat  70  can be ensured.  
      In an embodiment, the coating layer of the valve seat of one of the previous embodiments is formed by a PolyTetraFluoroEthylene (PTFE) resin layer, instead of ceramic. The polytetrafluoroethylene resin layer has superior characteristics, such as an electrical insulating property, and a fire resistance.  
      Therefore, when an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in a cylinder together with air through an intake port, if the water penetrates a clearance between an intake valve seat and a mounting reception portion and remains there, a cylinder head and the intake valve seat contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. As the polytetrafluoroethylene resin layer for electrical insulation is formed on the intake valve seat, however, occurrence of galvanic corrosion is prevented.  
      As explained above, formation of the polytetrafluoroethylene resin layer according to the embodiment does not cause dissimilar metal contact between the cylinder head and the valve seat, so that galvanic corrosion can be prevented.  
      The coating layer in another embodiment of the mounting reception portion of the previous embodiment is formed by a PolyTetraFluoroEthylene (PTFE) resin layer, instead of ceramic. The polytetrafluoroethylene resin layer has superior characteristics, such as an electrical insulating property, and a fire resistance.  
      Therefore, when an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in a cylinder together with air through an intake port, if the water penetrates a clearance between an intake valve seat and a mounting reception portion and remains there, a cylinder head and the intake valve seat contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. As the polytetrafluoroethylene resin layer for electrical insulation is formed on the intake valve seat, however, occurrence of galvanic corrosion is prevented.  
      As explained above, formation of the polytetrafluoroethylene resin layer according to the embodiment does not cause dissimilar metal contact between the cylinder head and the valve seat, so that galvanic corrosion can be prevented.  
      In a further embodiment, the coating layer of the mounting reception portion provided at the cylinder head  2  made of an aluminum alloy in the sixth embodiment is formed by an alumite treated layer (anodized layer) for electrical insulation, instead of ceramic.  
      Therefore, when an engine fuel essentially consisting of alcohol which may contain a relatively large amount of water is introduced in a cylinder together with air through an intake port, if the water penetrates a clearance between an intake valve seat and a mounting reception portion and remains there, a cylinder head and the intake valve seat contact each other through the water, and this results in dissimilar metal contact, so that galvanic corrosion may occur. As the alumite treated layer for electrical insulation is formed on the intake valve seat, however, occurrence of galvanic corrosion is prevented.  
      As explained above, formation of the alumite treated layer according to the embodiment does not cause dissimilar metal contact between the cylinder head and the valve seat, so that galvanic corrosion can be prevented.