Cylinder head unit and method for producing a valve seat

Abstract: A cylinder head unit for an internal combustion engine comprises a cylinder head body (11), an air intake system communicating with a combustion chamber (12) at an intake port opening (13a) and an exhaust system communicating with the combustion chamber (11) at an exhaust port opening (14a). Said intake and exhaust port openings (13a,14a) are operable by respective intake and exhaust valves (17,18) guided by respective valve guides (15,16) which are accommodated in respective valve guide holes (4a). In addition, valve seats are provided at the intake and exhaust port openings. Said valve seats (19) are metallurgically bonded to said cylinder head body (11) and the following inequation is fulfilled: D < D o < D c where D is the outside diameter of the respective intake and exhaust valves (17,18) coming into contact with the respective valve seats (19), D o is the outside diameter of the respective valve seats (19), and D c is the diameter of said intake and exhaust port openings (13a,14a) adjacent said combustion chamber (12).

Description: This invention relates to a cylinder head unit for an internal combustion engine, comprising a cylinder head body, an air intake system communicating with a combustion chamber at an intake port opening, an exhaust system communicating with the combustion chamber at an exhaust port opening, said intake and exhaust port openings are operable by respective intake and exhaust valves guided by respective valve guides accommodated in respective valve guide holes, and valve seats provided at the intake and exhaust port openings and a method for producing a valve seat within a cylinder head unit of an internal combustion engine. The conventional cylinder head body for engines is typically made of aluminum alloy, and areas of the cylinder head body with which the intake valve and exhaust valve come in contact are provided with valve seats. Since those valve seats are repeatedly contacted by the intake and exhaust valves and subjected to high temperatures, they are made of iron-based sintered alloy which is excellent in wear resistance and high temperature strength. As shown in FIG. 20, they are press fit, to be integral with the cylinder head body, in the recesses formed in the combustion chamber side opening areas of the intake and exhaust ports of the cylinder head, and finished with grinding process. FIG. 20 shows an enlarged cross section of a portion of the conventional cylinder head where a valve seat is press fit. The figure also shows a cylinder head body (1), a press fit valve seat (2), and a recess (3) for press fitting the valve seat. Due to the fact that the thermal conductance of the iron-based sintered alloy for the valve seat (2) is lower than that of aluminum alloy used for the cylinder head body (1), that the valve seat (2) has a certain thickness to prevent deformation when it is press fit, and that minute gaps are present between the valve seat (2) and the cylinder head body (1), thermal resistance increases when heat is conducted from the intake and exhaust valve faces and exhaust gas to the cylinder head body (1). This may result in insufficient cooling of the cylinder head, in abnormal combustion, and in excessive rise in the valve temperature. To solve the problems associated with the press fitting of the valve seat (2), a process has been proposed (for instance Japanese laid-open patent publication Sho-62-150014) in which a valve seat material of favorable heat resistance, wear resistance, and corrosion resistance is melted by laser heat and deposited (cladded) to the valve seat attaching areas on the cylinder head body (1), and the cladded areas are machined. A valve seat formed by such a laser cladding process is shown in FIG. 21. FIG. 21 shows an enlarged cross section of a valve seat area of a cylinder head in which a valve seat is formed by the laser cladding process, showing a valve seat area (4), a joining boundary surface (5) of the valve seat area (4), and melt reaction layers (6) and (7) formed in the vicinity of the joining boundary surface (5). Still another process has been proposed to improve melt adhesion between the cladded layer and the cylinder head body (for instance Japanese laid-open patent publication Hei-2-196117) in which the surface of the cylinder head body to be cladded is plastically deformed by rolling or the like prior to the laser cladding. However, the valve seat formed by the laser cladding has a problem in the joining strength because the valve seat material is heated and melted. This is because that the area around the joining boundary surface (5) of the cylinder head body (5) melts when the valve seat material is heated and melted. In other words, when the above-mentioned area of the cylinder head body (1) melts and then solidifies, gas is produced and remains as blow holes in the melt reaction layer (7) in the vicinity of the joining boundary surface. When molten aluminum alloy solidifies, pores may be produced in the melt reaction layer (7) by solidification shrinkage. The laser cladding process has another problem that it is likely to be affected by pores produced when cast and impurities contained in the material. Another problem is that the strength given by age-hardening treatment of the joining area made by melting and solidification is likely to lower when the engine is operated for a long period of time under conditions which raise the valve seat temperature. EP-A-0 092 683 (FIG. 1,4) discloses a cylinder head unit of an internal combustion engine, comprising a cylinder head body, an air intake/exhaust system with intake/exhaust valves 3 communicating with a combustion chamber at intake/exhaust port openings; intake/exhaust port openings are operated by respective intake/exhaust valves guided by respective valve guides accommodated in respective valve guide holes, and valve seats 18 are provided at the intake and exhaust port openings, wherein according to fig. 4 the valve seats 18 are metallurgically bonded (by a laser melting process;) to the cylinder head body and wherein according to fig. 1 the following relation is fulfilled: D, the outside diameter of the respective intake and exhaust valves 3 coming into contact with the respective valve seats is smaller than Do, the outside diameter of the respective valve seats , which itself is smaller than Dc, the diameter of the intake/exhaust port openings adjacent the combustion chamber thus D< Do < Dc. Accordingly, it is an objective of the present invention to provide an improved cylinder head unit for an internal combustion engine as indicated above which prevents overheating of the area around the valve seat and simultaneously enhances the joining strength between a valve seat and a cylinder head body. According to the invention, this objective is solved with a cylinder head according to claim 27. It is a further objective of the present invention to provide an improved method for producing a valve seat as indicated above facilitating the prevention of overheating and an improvement of joining strength between a valve seat and a cylinder head body. According to the present invention, this objective is solved by a method according to claim 1. According to an advantageous embodiment of the present invention, the following inequation is fulfilled: D < D o < D + 5mm < D c . Advantageous results of joining strength are achievable when the metal of said cylinder head body is an Al-Si-Mg-based aluminium alloy and the metal of said valve seats is an iron based sintered alloy containing melt-impregnated copper. A still further enhancement of the joining strength is achievable when said metallurgical bonding of said valve members comprises: (a) placing a valve seat member onto a surface of said openings of said cylinder head unit, and (b) pushing an electrode against the end face of said valve seat base material opposite to said cylinder head body with a pushing direction matched with an axis of said intake or exhaust valve, whereby said electrode being adapted to apply electricity to said cylinder head body through said valve seat member. The ease of correct positioning of a valve seat member onto the valve opening is further improved by advancing a guide rod coaxially aligned with said electrode such that said guide rod enters said valve guide hole and simultaneously guides said electrode for matching the pushing direction with the axis (C) of said valve, whereby said guide rod is fixed to or separated from said electrode. It is advantageous when the pressing force and/or said electricity are applied according to a predetermined pattern which may comprise a first pushing force being applied at an early stage of the bonding process and then a second pushing force being applied with a certain higher value till bonding is completed. Thereby, it is advantageous when the pattern of the applied electricity starts when a time has lapsed after application of the first pushing force whereby the value of the electricity first increases, then decreases near to zero and after this increases again before reduced to zero during the time said second pushing force is still applied. The applicants have proposed a technique in which annular valve seat members made of iron-based sintered alloy are heated, pressed, and joined to the openings in the cylinder head body (European patent application number 96 100 938.8). According to that technique, since the cylinder head body is made of aluminum alloy which is easily caused to flow plastically by electric heating, the valve seat member is sunk into the valve seat member by heating and pressing. In that case, atoms on the pressed contact boundary surface between the two parts diffuse mutually and both parts are tightly secured without a gap. In that case, since the valve seat and the cylinder head body melt little, defects in materials as described above are prevented from occurring. The engine cylinder head according to the invention is a further improvement of the previous proposal to prevent the cylinder head from deforming. That is to say, it is generally known that alluminum alloy is easily deformed by a very small stress at temperatures above the aging temperature, and its creep strength is also low. Therefore, the cylinder head body should be prevented from deforming. With the arrangement according to the invention, the valve seat is prevented from being deformed, damaged, or sunk into the cylinder head body by the repeated striking of the valve at high temperatures against the valve seat, by satisfying the following relationship D < D o < D + 5 mm where, D o is the outside diameter of the valve seat, and D is the outside diameter of the valve which comes into contact with the valve seat. The basis of the numerical limits above will be described below. As shown in FIG. 12(A), when the valve seat member is heated and pressed, the sinking amount (d) increases, periphery of the valve seat expands, and the projected area of the valve seat as seen in the valve axis direction increases as shown in FIG. 12(B). If the repeated maximum load by the valve seating load is assumed to be constant, the relationship between the projected area of the valve seat and the surface pressure acting on the boundary surface between the valve seat and the cylinder head is as shown in FIG. 13A). It has been confirmed that the relationship between the surface pressure and the outside diameter (D o ) of the valve seat is as shown in FIG. 13(B). In FIG. 13(B), the horizontal broken line indicates the compression limit strength of the cylinder head made of aluminum alloy material (CH by JIS Code). When the surface pressure exceeds the limit, deformation occurs and causes trouble in use. Therefore, the outside diameter (D o ) of the valve seat must be determined so that the surface pressure on the boundary surface is below the compression limit strength. The applicants have confirmed from various tests that the minimum value of the outside diameter (D o ) of the valve seat is equal to the outside diameter of the valve. Therefore, it has been determined that the outside diameter of the valve (D) is smaller than the outside diameter (D o ) of the valve seat (D < D o ). As shown in FIG. 14(A) which shows the cross section of the exhaust (or intake) port, the space between the inside diameter (D c ) of the exhaust port and the outside diameter (D) of the valve serves as the outflow port of the exhaust gas (or inflow port of the mixture). When the outside diameter (D) of the valve is assumed to be constant, as shown in FIG. 14(B), the greater the inside diameter (D c ) of the exhaust port, the smaller becomes the gas flow resistance. Therefore, the inside diameter (D c ) of the exhaust or intake port should be as great as possible. However, there is a limitation because of the space for the adjacent exhaust port and intake port. It has been confirmed by experiments that, as shown in FIG. 14(B), when the inside diameter (D c ) of the exhaust or intake port is gradually increased, the gas flow resistance does not decrease significantly when the value (D c ) is about (D) + 5 mm or greater. Therefore, the inside diameter (D c ) of the exhaust or intake port is sufficient if it is (D) + 5 mm. In view of the above, the outside diameter (D o ) of the valve seat is determined to be (D) + 5 mm which is smaller than the inside diameter (D c ) of the exhaust or intake port. Next, the effect of the arrangement according to another embodiment of the invention will be described. The applicants have taken note of the fact that in the engine cylinder head, the exhaust port reaches higher temperature than that the intake port reaches, and strengthened the exhaust port only. This makes it possible to minimize the deformation of part of the cylinder head material near the valve seat and moreover the manufacturing cost of the cylinder head is reduced. More specifically, plastic flow of the valve seat material is restricted by increasing the valve seat width as seen generally in the direction normal to the seating surface of the valve seat, thereby decreasing the surface pressure on the cylinder head material. The projected area of the valve seat after forming is increased by increasing the sinking dimension of the valve seat material when it is heated and pressed (Refer to FIGs. 9 and 10). Furthermore, the valve seat may be prevented from being deformed or damaged by increasing the valve seat thickness in the direction generally normal to the seating surface of the valve seat, thereby securing the rigidity of the valve seat (19). The temperatures around the openings of the cylinder head body become uneven depending on various conditions such as the arrangement of the intake and exhaust ports. With the engine cylinder head according to a further embodiment of the invention, since the center of the inside circle constituting the inside diameter of the valve seat is displaced from the center of the outside circle constituting the outside diameter of the valve seat, and the valve seat width in plan view is varied, the wider portion of the valve seat may be located to the position subjected to higher temperatures. This makes it possible to secure the projected area of required part of the valve seat and reduce the surface pressure of the compression force exerted on the cylinder head body. Other preferred embodiments of the present invention are laid down in further dependent claims. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: FIG. 1 is a cross-sectional side view of a valve seat as the first embodiment of this invention; FIG. 2 is a cross-sectional side view in the state of a valve seat base material being placed over the port opening; FIG. 3 is a plan view of a press machine for joining the valve seat to the port opening of the cylinder head; FIG. 4 is a side view of a press machine for joining the valve seat to the port opening of the cylinder head; FIG. 5 is a cross-sectional side view in the state of an electrode being in contact with the valve seat base material; FIG. 6 is a graph of pressurizing pattern, current value pattern, and sinking amount; FIG. 7 is a cross-sectional side view in the state of an alloy layer being produced from the metallic material of the film on the valve seat base material and the metallic material of the cylinder head body; FIG. 8 is a cross-sectional side view in the state of the material metal of the cylinder head body causing plastic flow; FIG. 9 is a cross-sectional side view in the state of the valve seat base material being sunk in the cylinder head body; FIG. 10 is a cross-sectional side view in the state of the valve seat being finish machined; FIG. 11 shows the second embodiment of the invention, with the intake and exhaust ports seen from the combustion chamber side; FIG. 12(A) shows a cross section of the valve seat for explaining the effect of the invention; FIG. 12(B) is a graph of relationship between the sinking amount and the projected area of the valve seat when the valve seat is heated and pressed against the cylinder head body for explaining the effect of the invention; FIG. 13(A) is a graph of relationship between the projected area of the valve seat and the surface pressure caused by the compression force exerted on the boundary surface between the valve seat and the cylinder head body for explaining the effect of the invention; FIG. 13(B) is a graph of relationship between the outside diameter of the valve seat and the surface pressure on the boundary surface of the valve seat for explaining the effect of the invention; FIG. 14(A) is a cross-sectional view of the valve seat for explaining the effect of the invention; FIG. 14(B) is a graph of relationship between the inside diameter of the exhaust port and gas flow resistance; FIG. 14(C) shows the intake and exhaust ports seen from the combustion chamber side; FIGs. 15(A), 15(B) and 15(C) show the third embodiment of this invention; FIG. 16 is a plan view to show the fourth embodiment of this invention; FIG. 17 is a cross-sectional side view to show the fifth embodiment of this invention; FIGs. 18(A) through 18(E) are cross-sectional side views to show the sixth embodiment of this invention; FIGs. 19(A) through 19(F) are cross-sectional side views to show the seventh embodiment of this invention; FIGs. 19(G) through 19(I) are cross-sectional side views to show the eighth embodiment of this invention; FIG. 20 is a cross-sectional side view of a conventional valve seat; and FIG. 21 is a cross-sectional side view of another example conventional valve seat. A. First Embodiment An embodiment of this invention will be described in detail in reference to FIGs. 1 to 10. (1) Overall Constitution of the Cylinder Head FIG. 1 shows a cross section of a valve seat area of a cylinder head of this invention. FIG. 2 shows a partially enlarged cross section of a state in which a valve seat member is placed on the port opening. In those figures, a cylinder head body (11) for a four cycle engine is made of a cast aluminum alloy material with a dome-shaped recess (12) for a downward opening combustion chamber, an intake port (13) opening to the recess (12), and an exhaust port (14) opening to the recess (12). The aluminum alloy used for the cylinder head body (11) is an Al-Si-Mg-based aluminum alloy specified in JIS (Japanese Industrial Standard) as AC4C. The reason for using that material is the strongest joining strength of the valve seat in comparison with other aluminum alloys. An intake valve (17) and an exhaust valve (18) are installed in the upper wall areas of the intake port (13) and the exhaust port (14) through valve guides (15) and (16), respectively. A valve seat (19), to be described later, is joined to each of openings of ports (13) and (14). The valve guides (15) and (16) are secured as they are press fit into valve guide holes (11a) formed in the cylinder head body (11). The valve guide holes (11a) are formed so that their axes (C) align with the axes of the openings (13a) and (14a) of the intake port (13) and the exhaust port (14). Each of the valve seats (19) shown in FIG. 1 is made by press joining an annularly formed valve seat member to the port openings (13a) and (14a) under heated state, followed by finish machining. The valve seat member is shown as (20) in FIG. 2. The valve seat member (20) is made of an iron-based sintered material member formed in an annular shape (21) and covered with a copper coating (22). As the material for the annular member (21) in this embodiment, an iron-based sintered material member in which copper is melt-impregnated is used from the viewpoint of minimizing internal resistance heat during energization which will be described later. The copper coating (22) is formed by electroplating the annular member (21) with a coating thickness of 0.1 - 30 micrometers. As shown in FIG. 2, the valve seat member (20) is formed so that part of its circumference, when placed on each of the openings (13a) and (14a) of the intake port (13) and the exhaust port (14), faces the inside of each of the openings (13a) and (14a). By the way, FIG. 2 shows the cylinder head body (11) upside down, with the underside (opening side of the recess (12) for the combustion chamber) facing upward. To describe in detail, the outer circumferential surface (20a) of the valve seat member (20) is sloped down toward the center of the valve seat member (20) and the bottom surface (20b) is sloped with a smaller gradient than that of the surface (20a). An outside surface where the outer circumferential surface (20a) and the bottom surface (20b) meet each other is formed in a convex curved surface. In FIG. 2, the convex curved surface is shown as (20c). A raised portion (23) is formed on part of each of the openings (13a) and (14a) which faces the convex curved surface (20c). In other words, when the valve seat member (20) is placed as shown in FIG. 2 on each of the openings (13a) and (14a), the convex curved surface (20c) comes in contact with the raised portion (23) of the cylinder head body (11). The inside surface of the valve seat member (20) comprises a surface (20d) sloped down toward the center of the valve seat member (20), and an axially extended surface (20e) axially extending from the inside circumferential end of the sloped surface (20d). (2) Valve Seat Joining Procedure To join the valve seat member (20) formed as described above to each of the openings (13a) and (14a) of the cylinder head body (11), a press machine (24) shown in FIGs. 3 and 4 is used. The press machine (24) comprises a base (25), a lower platen (26) secured to the lower part of the base (25), and an upper platen (27) vertically movable relative to the lower platen (26). The upper platen (27) is attached to the lower end of a rod (28a) which is a working end of a cylinder device (28) attached, with its axis directed vertically, to the upper part of the base (25). A power supply (not shown) supplies power to the lower platen (26) and the upper platen (27) through conductor members (26a) and (27a), respectively. The conductor member (27a) connected to the upper platen (27) is constituted to flexibly deform or move according to the vertical movements of the upper platen (27). This embodiment is constituted so that the upper platen (27) is the positive electrode and the lower platen (26) is the negative electrode. In the upper part of the base (25) supporting the cylinder device (28) is provided a displacement meter (30) which emits laser beams to a reflector member (29) fixed to the front part of the upper platen (27) to measure the displacement of the upper platen (27) by measuring the distance to the reflector member (29) using the laser beams reflected from the reflector member (29). To join the valve seat member (20) by using the press machine (24), first the lower electrode (31) is fixed on the lower platen (26), and the cylinder head body (11) is placed on and fixed to the lower electrode (31). Here, the cylinder head body (11) is placed with its recess (12) for the combustion chamber facing upward and positioned so that the axis through the port opening to which the valve seat member (20) is to be joined aligns with the axis of the rod (28a) of the cylinder device (28). Next, as shown in FIG. 5, a guide rod (32) is inserted from the recess (12) side into a valve guide hole (11a) of the port to which the valve seat member (20) is joined. The guide rod (32) is made of a metallic round bar (32a) with its outside surface coated with an insulation material (32b) such as alumina, and its length is such that it projects up from the combustion chamber side end surface of the cylinder head body (11). In this embodiment, the insulation material (32b) is made by flame-spraying ceramic material such as alumina on the round bar (32a), followed by polishing. After that, the valve seat member (20) is placed over the port opening, and the upper electrode (33) is placed on the valve seat member (20). The upper electrode (33) is provided with a centered through-hole (33a) into which the guide rod (32) is inserted, and its lower end is formed with a tapered surface (33b) for coming into tight contact with the sloped surface (20d) (FIG. 2) of the valve seat member (20) and with a circumferential positioning surface (33c) for coming into tight contact with the axially extended surface (20e). A magnetic member (33d) is fixed to the lower end of the upper electrode (33) so that the valve seat member (20) is magnetically attracted. In other words, the upper electrode (33) is positioned coaxially with the port opening of the cylinder head body (11) as the guide rod (32) is fit into the through hole (33a), and the valve seat member (20) is coaxially positioned with the port opening as the tapered surface (33b) and the circumferential surface (33c) are brought in tight contact with the valve seat member (20). After the electrode (33) is placed on the valve seat member (20), the upper electrode (33) is rotated to check whether the valve seat member (20) is securely fit. After that, the cylinder device (28) is driven so that the upper platen (27) is lowered and brought in tight contact with the upper electrode (33). Here, the underside of the upper platen (27) is made parallel to the upper surface of the upper electrode (33). Next, the cylinder device (28) is driven to lower the upper platen (27) and to press the valve seat member (20) with a constant pressing force through the upper electrode (33) against the cylinder head body (11). Here, since the moving direction of the upper electrode (33) is restricted by the guide rod (32), the direction of the pressing force applied to the valve seat member (20) is aligned with the axis of each of the port openings (13a) and (14a). Therefore, the valve seat member (20) is pressed in the state of its axis aligned with the axis of each of the port openings (13a) and (14a) along the axis. The pressing force is changed according to the pressing force pattern shown with a solid line in FIG. 6. That is, a relatively small, constant pressing force (P1) is applied during the first period of the joining process, and thereafter a relatively large, second pressing force (P2) is applied to the end of the process. When the upper platen (27) is stabilized after the pressing with the first pressing force (P1) is started, the distance between the laser displacement meter (30) and the reflector member (29) is measured with the laser displacement meter (30), and the measured distance is recorded as a descent start position of the upper platen (27). After a time (T1) shown in FIG. 6 has elapsed from the start of the pressing with the first pressing force P1, a voltage is applied between the upper platen (27) and the lower platen (26) so that a current flows between the two platens or through the valve seat member (20), the cylinder head body (11), and the lower electrode (31). Here, the current flows from the upper electrode (33) toward the cylinder head body (11). The current value here is also changed according to the current value pattern shown with a broken line in FIG. 6. In other words, after the current is increased, the current is lowered to almost zero, and again increased, and in the middle of the final stage of joining while the pressing force is held unchanged, the current is lowered to zero. Here, as shown in FIG. 2, the convex surface (20c) of the valve seat member (20) is in contact with the raised portion (23) of the cylinder head body (11). Since the contact area between those components is very small, when the current is applied as described above, the electric resistance in the contact area increases and heat is produced in the contact area. The heat is conducted throughout the contact boundary surface between the valve seat member (20) and the cylinder head body (11). When the temperature on the contact boundary surface between the valve seat member (20) and the cylinder head body (11) rises, atoms in the material metals which are in pressing contact with each other in solid state (copper in the copper coating 22 and aluminum alloy in the cylinder head body 11) begin active movements and diffuse among each other. When the mutual diffusion of atoms occurs as described above, the composition in the vicinity of the boundary surface becomes eutectic alloy of copper constituting the copper coating (22) and aluminum alloy of the cylinder head body (11) so that it can transform from solid state to liquid state at a lower temperature in comparison with the melting point of pure copper. The state at this time in the vicinity of the boundary surface is schematically shown in FIG. 7. In FIG. 7, the area where mutual diffusion of atoms has occurred and the eutectic layer has been produced is shown with a symbol A. When the temperature in the vicinity of the boundary surface further rises and part of the eutectic alloy layer transforms into liquid phase, the diffusion phenomenon of atoms becomes more active, the eutectic alloy layer grows, and accordingly the boundary surface between solid and liquid phases expands. Along with the progress in the transformation of the eutectic alloy layer into the liquid phase, plastic flow (plastic deformation) occurs in the aluminum alloy in the cylinder head body (11) adjacent to the eutectic alloy layer, because it is pressed by the valve seat member (20) and its temperature is raised by the resistance heat. Since the plastic flow occurs generally symmetrically in upward and downward directions with respect to the first contact area in FIG. 7, the eutectic alloy layer which has transformed into liquid phase is pushed out from the contact area along with the plastic flow as shown in FIG. 8. In FIG. 8, the areas where the eutectic alloy layer has been forced out are shown with a symbol B. Here, part of the copper coating (22) on the valve seat member (20) is also transformed into eutectic alloy and forced out from the contact area, and part of the annular member (21) comes in contact with aluminum alloy, causing atom diffusion phenomenon between the two components to occur. The area where the diffusion phenomenon is taking place is shown with a symbol C in FIG. 8. As described above, since part of the eutectic alloy layer is forced out from the contact area and the aluminum alloy flows plastically, the valve seat member (20) starts to sink into the cylinder head body (11) at the time (T2) shown in FIG. 6. After the valve seat member (20) starts to sink as described above, at the time (T3) in FIG. 6, the pressing force is increased to the second pressing force (P2). As the pressing force is increased, the plastic flow amount of the aluminum alloy increases, and the amount of the eutectic alloy forced out also increases. As a result, eutectic alloy consisting of copper-aluminum alloy is additionally produced in part of the contact area where the eutectic reaction has not occurred. The above phenomenon is repeated and the eutectic alloy layer is transformed into liquid phase and additional amount is forced out from the contact area. Along with the above process, the area in the boundary surface between iron-based sintered alloy of the annular member (21) and aluminum alloy where mutual diffusion of atoms occurs also widens. After pressing with the second pressing force (P2), at the time (T4) in FIG. 6, the current value is once reduced to near zero and increased again to the previous value. By the temporary reduction in the current value, heat generation is temporarily restricted, removal of the eutectic alloy and the plastic flow are temporarily restricted, and the rate of increase in the sinking amount of the valve seat member (20) is temporarily reduced as shown in FIG. 6. The temporary reduction in the current value is made to prevent undesirable melting of aluminum alloy by the heat. After the current value is raised to the previous value as described above, it is gradually lowered to zero from the time (T5) to the time (T6). The above reaction occurs while the current is flowing as a matter of course. However, even after the current is stopped, until the temperature lowers such that the above reaction is impossible, the above reaction continues, and the above phenomenon of production of the eutectic alloy, transformation into liquid phase, and removal by plastic flow occur along with the phenomenon of mutual atom diffusion between the iron-based alloy and the aluminum alloy, while the valve seat member (20) continues to sink, and is finally embedded in the cylinder head body (11) as shown in FIG. 9. When the increase in the sinking amount almost stops at the time (T7) in FIG. 6, pressing with the cylinder device (28) is stopped, the final position of the upper platen (27) is obtained by the laser displacement meter (30) from the distance between the laser displacement meter (30) and the reflector member (29), and the upper platen (27) is raised to remove the cylinder head body (11) from the press machine (24). By the way, average current value and total energization time are obtained by the end of the entire process. Next, total sinking amount of the valve seat member (20) is obtained by calculating the height difference between the descent start position and the final position of the upper platen (27). If that amount is not within a predetermined range (D) in FIG. 6, the joining is determined as unacceptable. The allowable range (D) in this embodiment is about 0.5 mm to about 2 mm. While the range (D) varies with the material of the cylinder head body (11), a range of about 1 mm to 1.5 mm is preferable. The final processing of the cylinder head is carried out by removing unnecessary part from the cylinder head body (11) in FIG. 9 to which the valve seat member (20) has been joined by grinding for instance as shown in FIG. 10. By the final processing, unnecessary part of the annular body (21) and the copper coating (22) are removed and a valve seat (19) is obtained which is joined to the cylinder head body (11) through the atom diffusion area shown with a symbol (C) in FIG. 10. Here, the configuration is such that a relationship D < D o < D + 5 holds where it is assumed that (D o ) is the outside diameter of the valve seat (19), and (D) is the outside diameter of the intake valve (17) or the exhaust valve (18). (3) Effect of the Embodiment In the cylinder head of the above constitution, the valve seat (19) and the cylinder head body (11) are firmly secured to each other by diffusion of atoms without gap. Therefore, thermal resistance of the two parts is small and the cooling performance of the cylinder head is improved. Furthermore, as described above, since the cylinder head body (11) does not melt during the manufacturing process, no blow holes or shrinkage pores are produced during solidification. In particular, with this invention, since the projected area of the valve seat (19) is secured by the numerical limits as described above, the valve seat is prevented from being deformed or damaged, or sinking into the cylinder head body (11) by repeated striking of the intake and exhaust valves (17, 18) at high temperatures. Furthermore, press-in force can be restricted when the valve seat base material (20) is heated and pressed against the cylinder head body (11). B. Second Embodiment Next, the second embodiment will be described in reference to FIG. 11. FIG. 11 shows an intake port (13) and an exhaust port (14) as seen from the combustion chamber side. The figure also shows an ignition plug attachment hole (8), a coolant circulation hole (10), and a hole (11b) for attaching a cylinder head body to an engine body (not shown). Temperature of the hatched area around the exhaust port (14) becomes high. However, since the exhaust port (14) is smaller in inside diameter than the intake port (13), the distance between the exhaust ports (14) can be made greater than the distance between the intake ports (13) and therefore at least one of the three constitutions enumerated below may be employed to reinforce the exhaust port (14) side of the cylinder head body(11). (1) Referring to FIGs. 14(A) and 14(C), the width (W) of the valve seat (19) of the exhaust port (14) as seen in the direction generally normal to the seating surface of the valve seat (19) is greater than the width of the seating surface of the valve seat (19) of the intake port (13). This reduces the surface pressure when the valve seat (19) presses the cylinder head body (11) and restricts plastic flow of the base material of the cylinder head body (11). (2) Refer to FIGs. 9 and 10), the sinking depth of the exhaust port (14) when the valve seat base material (20) is heated and pressed against the cylinder head body (11) is greater than the sinking depth of the valve seat base material (20) of the intake port (13). This increases the projected area of the valve seat (19) after forming. (3) Referring to FIG. 14(A), the thickness (T) of the exhaust port (14) in the direction generally normal to the seating surface of the valve seat (19) is thicker than the thickness of the valve seat (19) of the intake port (13). This secures the rigidity of the valve seat (19) and prevents it from being deformed or damaged. C. Third Embodiment Next, referring to FIG. 15, the third embodiment of this invention will be described. In the cylinder head shown in FIGs. 15(A) and 15(B), the taper angle () between the peripheral surface (20a) and the bottom surface (20b) of the valve seat (19) is made 120° or greater so that the maximum stress transmitted through the valve seat (19) to the cylinder head body (11) is restricted. More specifically, in the cylinder head shown in FIG. 15(A), the angle (α) between the bottom surface (20b) of the valve seat (19) and a plane normal to the axis of the port opening (13a) or (14a) is set to 30°, and the angle (β) between the peripheral surface (20a) of the valve seat (19) and the normal line of the above-mentioned plane is set to 15° so that the taper angle () is set to 135°. In the cylinder head shown in FIG. 15(B), the angles are set as α = 30°, β = 30°, and  = 150°. In the cylinder head shown in FIG. 15(C), the bottom surface (20b) of the valve seat (19) is at right angles to the axis of the port opening (13a) or (14a), and the angle (α) is set to 0°. As a result, a compression force only is exerted on the joining portion which is at right angles to the axis, shearing force on the joining portion is reduced, and plastic flow of aluminum alloy is restricted. By the way, in this embodiment, the angles are set as β = 15° and  = 105°. D. Fourth Embodiment Next, referring to FIGs. 16 and 17, the fourth embodiment of this invention will be described. The characteristic of the fourth embodiment is that the axis (0 a ) of the inside circle (19a) constituting the inside diameter of the valve seat (19) is displaced from the axis (0 b ) of the outside circle (19b) constituting the outside diameter of the valve seat (19). As shown in FIG. 16, when the intake and exhaust ports (13, 14) are located close to each other, temperature of the valve seat (19) and its surrounding area becomes higher on the side facing the opposing port (13 or 14). Therefore, the projected area of part of the valve seat (19) on the opposing port (13 or 14) side is increased to reduce the surface pressure caused by compression force on the cylinder head body. That is to say, since the surface pressure in the part which reaches a higher temperature is reduced, the cylinder head body (11) is prevented from being deformed. To displace the axis (0 a ) of the inside circle (19a) from the axis (0 b ) of the outside circle (19b), those axes may be displaced from each other while remaining parallel to each other (Refer to FIG. 17(A)), or may be tilted relative to each other (Refer to FIG. 17(B)). F. Sixth Embodiment FIG. 18 shows the sixth embodiment of this invention. This embodiment is characterized in that the bottom surface (20b) and peripheral surface (20a) of the valve seat (19) are formed with raised stripes (19c). While the raised stripe (19c) shown in the drawing is extended over the entire circumference in a single location, it may be provided in plural locations. FIGs. 18(A) through 18(C) show various position settings of the raised stripes (19c). FIGs. 18(D) and 18(E) shows examples of the raised stripes provided in plural locations. With these embodiments, since plastic flow of aluminum alloy in the shearing direction is hindered by the raised stripe (19c), the valve seat (19) is prevented from sinking. C. Seventh Embodiment FIGs. 19(A) through 19(E) show examples in which a reinforced texture is interposed between the valve seat (19) and the cylinder head body (11). In the first embodiment described before, the valve seat base material (20) is melt-impregnated with copper and provided with a copper film (22). The copper film (22) reacts with the aluminum alloy of the cylinder head body (11) to constitute eutectic alloy having a melting point lower than that of pure copper, and the eutectic alloy transforms into liquid phase by the resistance heat described before. The liquefied eutectic alloy is discharged through the contact portion along with the plastic flow of the aluminum alloy constituting the cylinder head (11). The embodiment shown in FIG. 19(A) is characterized in that the eutectic alloy is prevented from being discharged to remain in the joining portion by devising the resistance heating method. With such a constitution, a valve seat (50) is obtained which has a higher strength than that of aluminum alloy constituting the cylinder head body (11) and a greater projected area than that of the valve seat (19), and resistance against deformation and creep strength of the valve seat (19) is increased. By the way, the copper film (22) may be replace with plating of Zn, Sn, Ag, or Al-Si alloy. FIG. 19(C) shows an example in which a metal for impregnating the valve seat base material (20) or metal for plating its surface (these metals are hereinafter called 'insert material') is diffused in aluminum alloy of the cylinder head body (11). With this example, since a hardened solid solution texture layer (51) is formed around the joined portion, the same effect as described above is obtained. As shown in FIG. 19(B), it is also possible to form a solid solution texture layer (52) in which the insert material is diffused in uneven depths. Further in examples shown in FIGs. 19(B) and 19(C), an intermetallic compound of the insert material and aluminum alloy may be formed in the joining portion between the valve seat (19) and the cylinder head body (11). This restricts plastic flow in the shearing direction of the joining portion of aluminum alloy. FIG. 19(D) shows an example in which a fine texture layer (53) is formed by making fine the texture around the joining portion between the valve seat (19) and the cylinder head body (11). FIG. 19(E) shows an example in which a deposition reinforced texture layer (54) is formed by causing a compound to deposit and diffuse; or by implanting, metal-diffusing, and solidifying ions of Fe or Ni in the texture. FIG. 19(F) shows an example in which a compound texture layer (55) is formed by dispersing metallic particles and fibers in the texture. When a compound is made to deposit, grain boundary slip in the texture may be restricted by causing the compound to deposit on the crystal grain boundary. H. Eighth Embodiment FIGs. 19(G) through 19(I) show the eighth embodiment of this invention, with three examples in which flange portion (60 or 61) is formed on the entire circumferential edge of the valve seat (19) so that the projected area of the valve seat (19) increases and that surface pressure due to compression force transmitted to the cylinder head body (11) is reduced and the amount of heat from high temperature gas to the cylinder head body (11) is restricted by covering the cylinder head body (11). I. Modified Examples This invention is not limited to the embodiments described above but may be modified in various ways as described below. (1) In the first embodiment, plastic flow of aluminum alloy around the joining portion may be restricted by making the surface roughness (Ra) on the joining portion side of the valve seat (19) 10 or greater. (2) When the cylinder head body (11) is made by casting, strontium may be used as an improvement treatment material so that deformation resistance and creep strength of aluminum alloy are increased. (3) This invention may be employed to almost any kinds of engines for automobiles and motorcycles. According to the invention described above, since the projected area of the valve seat is secured, the surface pressure caused by the compression force exerted on the cylinder head body when the valve strikes the valve seat is reduced. Since only the exhaust port side of the valve seat subjected to higher temperatures is reinforced, plastic flow of aluminum alloy in the joining portion is restricted while restricting increase in costs. Furthermore, since rigidity of the valve seat is secured, the valve seat is prevented from being deformed or damaged. Since the axis of the inside circle constituting the inside diameter of the valve seat is displaced from the axis of the outside circle constituting the outside diameter of the valve seat to vary the width of the valve seat in plan view, the wider wall portion of the valve seat may be located in the position subjected to higher temperatures, so that the projected area of the required position in the valve seat is secured and the surface pressure caused by compression force exerted on the cylinder head body is reduced.

Claims: Method for producing a valve seat within a cylinder head unit having a cylinder head body of an internal combustion engine, comprising the steps of: (a) placing a valve seat member (20) onto the surface of an opening within said cylinder head body (11), (b) pressing said valve seat member (20) against said cylinder head body (11) and then impressing a voltage between the abutting surfaces of said valve seat member (20) and said cylinder head body (11), so that said valve seat member (20) and said cylinder head body (11) are metallurgically bonded with each other, and (c) applying a finishing treatment to said bonded pieces such that the following inequation is fulfilled: D < D o < D c where D is the outside diameter of the respective intake and exhaust valves (17, 18) coming into contact with the respective valve seats (19), D o is the outside diameter of the respective valve seats (19), and D c is the diameter of said intake and exhaust port openings (13a, 14a) adjacent said combustion chamber (12). Method according to claim 1, characterized in that the following inequation is fulfilled: D < D o < D + 5mm < D c . Method according to claim 1 or 2, characterized in that the position for said valve seat members (20) is dependent from the axial direction (C) of valve guide holes (11a). Method according to claim 3, characterized in that valve guides (15, 16) are inserted into the respective valve guide holes (11a) prior to or after the bonding process of said valve seat members (20). Method according to at least one of claims 1 to 4, characterized in that said metallurgical bonding of said valve members (20) comprises: (a) placing a valve seat member (20) onto a surface of said openings (13a, 14a) of said cylinder head body (11), and (b) pushing an electrode (33) against the end face of said valve seat base material (20) opposite to said cylinder head body (11) with a pushing direction matched with an axis (C) of said intake or exhaust valve (17, 18), whereby said electrode (33) being adapted to apply electricity to said cylinder head body (11) through said valve seat member (20). Method according to one of claims 1 to 5, characterized by advancing a guide rod (32) coaxially aligned with said electrode (33) such that said guide rod (32) enters said valve guide hole (11a) and simultaneously guides said electrode for matching the pushing direction with the axis (C) of said valve (17, 18), whereby said guide rod (32) is fixed to or separated from said electrode (33). Method according to claim 5 or 6, characterized in that either the electrode or the cylinder head body (11) or both are moved towards each other. Method according to at least one of claims 5 to 7, characterized in that the pressing force andlor said electricity are applied according to a predetermined pattern. Method according to at least one of claims 5 to 8, characterized in that in step (a) said valve seat member (20) and said opening (13a, 14a) contact each other along a circumferential line and that this line of contact is provided by a convex portion (20c) of said valve seat base material (20) and/or a raised portion (23) of said opening (13a, 14a). Method according to one of claims 5 to 9, characterized in that during step (a) said electrode magnetically attracts said valve seat member (20) for placing said valve seat member (20) on the surface of said valve opening (13a, 14a). Method according to at least one of claims 5 to 10, characterized in that the pattern for the pressing force comprising a first pushing force (P1) being applied at an early stage of the bonding process and then a second pushing force (P2) being applied with a certain higher value till bonding is completed. Method according to claim 11, characterized in that the pattern of the applied electricity starts when a time has lapsed after application of the first pushing force (P1) whereby the value of the electricity first increases, then decreases near to zero and after this increases again before reduced to zero during the time said second pushing force (P2) is still applied. Method according to claim 11 or 12, characterized in that the second pushing force (P2) is applied when it is recognized that the valve seat member (20) has begun to sink. Method according to one of claims 5 to 13, characterized in that the magnitude of sinking of the valve seat member (20) into the opening (13a, 14a) is measured continuously during the whole bonding process. Method according to claim 14, characterized in that said magnitude of sinking of the valve seat member (20) into the valve opening (13a, 14a) is controlled, in particular on the basis of said measured sinking value. Method according to one of claims 5 to 15, characterized in that said valve seat member (20) is made of an Fe-based sinter alloy being provided with a coating (22) of a metal or metal alloy being capable of forming an eutectic alloy with that cylinder head body (11). Method according to at least one of claims 1 to 16, characterized in that the metal of said cylinder head body (11) is an Al-Si-Mg-based aluminium alloy and that the metal of said valve seats (19) is an iron based sintered alloy containing melt-impregnated copper. Method according to at least one of claims 1 to 17, characterized in that the diameter of said exhaust port opening (14a) is smaller than the diameter of said intake port opening (13a). Method according to at least one of claims 1 to 18, characterized in that a width (W) of said valve seat (19) at said exhaust port opening (14a) is larger than the width of the seating surface of the valve seat (19) at said intake port opening (13a) as seen in a direction generally normal to the seating surface of said valve seats (19). Method according to at least one of claims 1 to 19, characterized in that a sinking depth of the valve seat (19) at said exhaust port opening (14a) is larger than the sinking depth of the valve seat (19) at said intake port opening (13a). Method according to at least one of claims 1 to 20, characterized in that a thickness (T) of the valve seat (19) at said exhaust port opening (14a) is larger than a thickness (T) of the valve seat (19) at said intake port opening (13a) as seen in a direction generally normal to the seating surface. Method according to at least one of claims 1 to 21, characterized in that said valve seat (19) comprising a peripheral surface (20a) facing said intake or exhaust port opening (13a, 14a), respectively, and a bottom surface (20b) joined to said peripheral surface (20a) defining a first tapered angle () between both surfaces (20a, 20b) which is set within a range from 100°-160°, that a second angle (α) between said bottom surface (20b) and a plane normal to the axis of the respective one of said intake and exhaust port openings (13a, 14a) is set within a range from 0°-30°, and that a third angle (β) between the peripheral surface (20a) and said normal plane is set within a range from 15°-30°. Method according to at least one of claims 1 to 22, characterized in that an axis (O a ) of the inside circle (19a) constituting the inside diameter of the valve seats (19) is displaced towards the peripheral edge of said combustion chamber (12) from an axis (O b ) of the outside circle (19b) constituting the outside diameter of the valve seats (19). Method according to at least one of claims 1 to 23, characterized in that a peripheral surface (20a) of said valve seat (19) facing said intake or exhaust port opening (13a, 14a), respectively, adjacent said combustion chamber (12) and/or a bottom surface (20b) of said valve seat (19) joined to said peripheral surface (20a) are provided with at least one raised stripe (19c) extending over the entire circumference of said valve seat (19). Method according to at least one of claims 1 to 24, characterized by a reinforced texture interposed between said valve seat (19) and the cylinder head body (11). Method according to at least one of claims 1 to 25, characterized in that one or both circumferential edges of said valve seat (19) are provided with flange portions (60, 61). A cylinder head unit for an internal combustion engine, comprising a cylinder head body (11), an air intake system communicating with a combustion chamber (12) at an intake port opening (13a), an exhaust system communicating with the combustion chamber (11) at an exhaust port opening (14a), said intake and exhaust port openings (13a, 14a) are operable by respective intake and exhaust valves (17, 18) guided by respective valve guides (15, 16) accommodated in respective valve guide holes (11a), and valve seats (19) provided at the intake and exhaust port openings (13a, 14a), said valve seats (19) are metallurgically bonded to said cylinder head body (11) and that the following inequation is fulfilled: D < D o < D c where D is the outside diameter of the respective intake and exhaust valves (17, 18) coming into contact with the respective valve seats (19), D o is the outside diameter of the respective valve seats (19), and D c is the diameter of said intake and exhaust port openings (13a, 14a) adjacent said combustion chamber (12), characterized in that said valve seats (19) are electrical-metallurgically bonded to said cylinder head body (11) by impressing a voltage thereto, and a peripheral surface (20a) of said are electrical-metallurgically bonded valve seat (19) facing said intake or exhaust port opening (13a, 14a), respectively, adjacent said combustion chamber (12) and/or a bottom surface (20b) of said electrical-metallurgically bonded valve seat (19) joined to said peripheral surface (20a) are provided with at least one raised stripe (19c) extending over the entire circumference of said electrical-metallurgically bonded valve seat (19). A cylinder head unit according to claim 27, characterized in that the following inequation is fulfilled: D < D o < D + 5mm < D c . A cylinder head unit according to claim 27 or 28, characterized in that the metal of said cylinder head body (11) is an Al-Si-Mg-based aluminium alloy and that the metal of said valve seats (19) is an iron based sintered alloy containing melt-impregnated copper. A cylinder head unit according to at least one of claims 27 to 29, characterized in that the diameter of said exhaust port opening (14a) is smaller than the diameter of said intake port opening (13a). A cylinder head unit according to at least one of claims 27 to 30, characterized in that a width (W) of said valve seat (19) at said exhaust port opening (14a) is larger than the width of the seating surface of the valve seat (19) at said intake port opening (13a) as seen in a direction generally normal to the seating surface of said valve seats (19). A cylinder head unit according to at least one of claims 27 to 31, characterized in that a sinking depth of the valve seat (19) at said exhaust port opening (14a) is larger than the sinking depth of the valve seat (19) at said intake port opening (13a). A cylinder head unit according to at least one of claims 27 to 32, characterized in that a thickness (T) of the valve seat (19) at said exhaust port opening (14a) is larger than a thickness (T) of the valve seat (19) at said intake port opening (13a) as seen in a direction generally normal to the seating surface. A cylinder head unit according to at least one of claims 27 to 33, characterized in that said valve seat (19) comprising a peripheral surface (20a) facing said intake or exhaust port opening (13a, 14a), respectively, and a bottom surface (20b) joined to said peripheral surface (20a) defining a first tapered angle () between both surfaces (20a, 20b) which is set within a range from 100°-160°, that a second angle (α) between said bottom surface (20b) and a plane normal to the axis of the respective one of said intake and exhaust port openings (13a, 14a) is set within a range from 0°-30°, and that a third angle (β) between the peripheral surface (20a) and said normal plane is set within a range from 15°-30°. A cylinder head unit according to at least one of claims 27 to 34, characterized in that an axis (O a ) of the inside circle (19a) constituting the inside diameter of the valve seats (19) is displaced towards the peripheral edge of said combustion chamber (12) from an axis (O b ) of the outside circle (19b) constituting the outside diameter of the valve seats (19).

Inventors: ADACHI SHUHEI, INAMI JUNICHI, ADACHI, SHUHEI, INAMI, JUNICHI
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1996
Main classification: F01L3
IPC codes: F01L1, F01L3, F02F1