WO2006075344A2

WO2006075344A2 - Lost foam casting method, in particular for an engine cylinder head

Info

Publication number: WO2006075344A2
Application number: PCT/IT2005/000017
Authority: WO
Inventors: Bruno Bassi
Original assignee: Meccanica Bassi S.P.A.
Priority date: 2005-01-14
Filing date: 2005-01-14
Publication date: 2006-07-20
Also published as: EP1836014B1; EP1836014A2; WO2006075344A3; DE602005014571D1; ATE431766T1

Abstract

The object of the present invention is a casting method for obtaining a casting in a metal material, comprising the manufacture of a model of the casting in a material adapted for dissolving in contact with the molten metal material, the laying of said coated model with the pouring gate in a container packed with sand and adapted for receiving the molten metal material, the casting of the molten metal material in said container so that said metal material dissolves the material forming the gate casting model replacing it, wherein during the moulding or prior to the coating, at least one insert in a material resistant to the heat of the molten metal material in incorporated into said model, so that said at least one insert is incorporated in the casting at the end of the casting step.

Description

DESCRIPTION "Lost foam casting method, in particular for an engine cylinder head"

[0001] . The object .of the present invention is a casting method, in particular for obtaining an engine cylinder head. More in particular, the invention relates to a casting technology known as "lost foam" .

[0002] . As known by the man skilled in the art, this technology provides for the manufacture of a 'model of the part to be obtained by casting, made of a material , such as polystyrene, adapted for dissolving, for example by evaporation, in contact with the molten metal . Once moulded, for example by pressing, the model is coated and laid with the pouring gate in a container filled with sand and adapted for receiving the molten metal , which exactly replaces the material forming the model , and solidifies as it cools down.

[0003] . Engine cylinder heads have already been manufactured using this technology. An example of use of the lost foam technology to manufacture a cylinder head is described, for example, in document US 5 449 033.

[0004] . Once the head has been molten, by the process mentioned above or in any other way, the casting is not complete yet as all the necessary mechanical machining has to be performed, comprising those needed to insert the valve seats and the valve guides for intake and discharge valves .

[0005] . These machining operations are especially heavy due to the limited tolerances needed to insert said valve seats and guides . The insertion (assembly) of these parts in the cylinder head, which have perfectly cylindrical outer diameters, is carried out by driving, using presses with the necessary power. Such operation is particularly delicate and heavy since to have sufficient driving, as needed to prevent such parts from moving in the respective seats if the head casting undergoes thermal expansion due to overheating, besides the exact machining tolerances, they must have a calibrated interference between the outer diameters of the valve guides and of the valve seats and that of the respective driving seats . For this reason and to facilitate the operation, the outer diameters must be contracted, bringing valve seats and guide at very low temperature, for example in the range of -150⁰C and below, using liquid nitrogen baths .

[0006] . Even though the valve seats are made of steel or of composite or ceramic material , during the driving they can easily undergo breakage and make the completed cylinder head mechanical processing casting to be discarded. Moreover, in order to obtain an optimum heat exchange between the valve seat and the casting body, the valve seats should have the least inner/outer diameter ratio (ring thickness) and a maximum height . However, this contradicts the object of having a high shaft/seat interference to have a good driving as the valve seat thus shaped would be fragile in said step and therefore subject to easy breakage .

[0007] . Moreover, the operation for inserting the valve seats requires the casting thickness in the combustion chamber cap to be larger than what needed so as to not damage the combustion chamber in the driving step . The same concept applies to valve guides, where for the same reason, the containment bosses are normally oversized. In fact , this operation may cause cracking in the cylinder head casting due to the press force, which has sufficient power to overcome the interference existing between the diameter of the machined seats on the casting and the diameter of the valve seats and of the valve guides . Such increase of thickness of the casting is to the detriment of the cooling fluid flow section.

[0008] . Another typical disadvantage of the lost foam technology is that the finish of the inner surface of the intake and discharge ducts of the molten part is substantially defined by the quality of the material forming the model . Since this is usually made of a foamed material , such as polystyrene, even though it is suitably coated to oppose the inside and outside thrust of sand, the inner surface of the ducts always appears quite rough and with some flaws . Such anomalies will be reproduced on the molten part, thus interfering with the mixture flow towards the combustion chambers and the outwards flow of exhaust gases . Further flaws arise from the fact that the model has to be divided into sectors that are then glued in order to obtain the inside of the ducts . The connection of the sectors and the necessary adhesive could cause deformities and blowholes to the casting. [0009] . The object of the present invention is to propose a casting method, in particular for an engine cylinder head, which allows remedying the disadvantages mentioned above .

[0010] . Said object and the related advantages are achieved with a casting method and with the equipment according to the following claims .

[0011] . Examples of embodiments of the method and of the equipment according to the present invention shall be described in more detail hereinafter, with reference to the attached indicative and non-limiting drawings, wherein:

[0012] . - figure 1 shows a section view of a model of foamed material with pouring gate of a cylinder head of an engine, immersed in the filling sand before casting the molten metal , with the valve seats incorporated and the valve guides inserted;

[0013] . - figure 2 shows an alternative embodiment of the model of figure 1, where the intake and exhaust ducts of the cylinder head are coated with tubular inserts incorporated in the model ;

[0014] . - figure 3 shows an enlarged detail of the model of figure 2 ;

[0015] . - figure 4 shows a partly axial sectioned view of a valve seat; [0016] . - figure 4a shows a top plan view of the valve;

[0017] . - figure 5 shows a section view of a die for manufacturing a first sector of foamed material of the model of figure 1 or 2 with the valve seat incorporated therein;

[0018] . - figure 6 shows a section view of a die for manufacturing a second sector in two parts of the model of figure 1 ;

[0019] . - figure 7 shows a section view of a die for manufacturing a third sector of the model of figure 1 ; [0020] . - figure 8 shows a section view of a die for manufacturing a second sector of the model with tubular inserts incorporated therein of figure 2 ; [0021] . - figure 9 shows a section view of a die for manufacturing a further sector for the model of figure 1 or 2 ;

[0022] . - figure 10 shows a cutaway view of a second example of model of foamed material for an engine cylinder head, equivalent to figure 1 ; and

[0023] . - figure 11 shows a view similar to the previous one and equivalent to figure 2 , wherein the model incorporates tubular inserts for coating the intake and discharge ducts . [0024] . The present invention relates to a casting method of an engine cylinder head using the technology known as "lost foam" ..

[0025] . This technology provides for the manufacture of a model of the part to be obtained by casting, made of a material adapted for dissolving, for example by evaporation, in contact with the molten metal . Preferably, the model is made of a foamed material , such as polystyrene . In the following description, the term "foamed material" will be used for simplicity to denote any material adapted for dissolving in contact with the molten metal .

[0026] . Once moulded, for example by pressing, the model with the pouring gate associated to it , is coated and laid in a container adapted for being filled with sand or other material resistant to the heat generated by the molten metal . The sand is made - for example by vibrating the container - to fill any recess or hole in the model of foamed material . Afterwards, through the gate also made of foamed material, the molten metal is cast in the container . The molten metal exactly replaces the foamed material and, once cooled, it solidifies . Finally the sand, along with the coat, are removed from the metal casting thus obtained. [0027] . The casting method of the present invention provides for the incorporation, in the model of foamed material , of at least one insert made of a material resistant, that is , inert, to contact with molten metal , so that said insert is embedded in proper position into the casting when the molten metal replaces the foamed material .

[0028] . Incorporation into the model means both a complete burying in the foamed material forming the model, and a partial incorporation, that is, an insertion or embedding of only a part on the insert in the foamed material, while another part protrudes from the model . [0029] . Moreover, incorporation of an insert into the model means both an embedding of the insert into the model as the latter is manufactured, as for example is the case for valve seats as it will be described hereinafter, and an insertion of part or all the insert into a special seat into the model , after the manufacture of the latter, as for example is the case for valve guides . [0030] . The choice of incorporating an insert into the model at the same time as or after the manufacture of the latter, may depend for example on reasons related to the manufacture technology of the model itself . For example, if the model is obtained by pressing, an insert having a certain inclination relative to the die shakeout direction, such as for example the valve guides , cannot be incorporated in the model during the pressing step and must be inserted afterwards .

[0031] . In any case, the model of foamed material also serves as support and handling element for at least one insert during the various steps of the casting method, up to the casting of the molten metal .

[0032] . Advantageously, relative to the model it is incorporated into, the insert is arranged in the same position it must take when it is incorporated in the casting.

[0033] . Material resistant or inert to the contact with the molten metal means a material that does not substantially change in shape at the temperature of the molten material , or that does not undergo phase transitions or variations in its molecular structure at such temperature . For example, if the insert is made of a metallic material, such as a pure compound or an alloy, its casting temperature must be higher than that of the cast metal .

[0034] . According to a preferred embodiment , wherein the part to melt is an engine cylinder head, said insert is a valve seat 2 for an intake or exhaust valve . [0035] . According to a preferred embodiment, said insert is a valve guide 10 for an intake or exhaust valve . [0036] . According to a further advantageous embodiment, said insert is a tubular element 7 , 8 for coating the intake 30 and/or exhaust 31 ducts . [0037] . According to a preferred embodiment of the method according to the present invention, the model of foamed material is obtained by pressing. When there are holes or recesses in the part to be molten, as in the case of an engine cylinder head disclosed here, the model cannot be obtained by pressing in a single piece, but it consists of a plurality of portions , or sectors, as it will be defined in the following description, each obtained in a special die and then glued to one another to form the model . In other words, once moulded, the model exhibits a layered structure, that is, consisting of a plurality of layers, or "slices" , glued to one another .

[0038] . In an embodiment aimed at obtaining a model of a cylinder head incorporating valve seats 2 and valve guides 10 , but not the tubular elements 7 , 8 (figures 1 and 10) , the sectors of foamed material that make up the model and that are obtained in special dies are :

[0039] . - a lower sector 1 that forms the bottom part of the model of the cylinder head, adjacent to the top of the combustion chamber;

[0040] . - two sectors 3 and 4 that define the bottom part of the intake 30 and exhaust 31 ducts ; [0041] . - a sector 5 that defines the top part of the intake 30 and exhaust 31 ducts ; [0042] . - an upper sector 9 to be arranged on sector 5.

[0043] . The cylinder head model is completed by an upper part 11 and by pouring gates 12 and 12 ' which, not being directly involved in the method of the invention, are not described in detail and are only partly shown in the drawings .

[0044] . In this embodiment, the method provides for obtaining the foamed sectors 1 in a specific die 20 (Figure 5) . Such die exhibits special references 16 to arrange, prior to the injection of the foamed material, the valve seats 2 for the intake and discharge valves . Seats 2 will then be incorporated in the foamed material and then constrained to the lower sector 1. [0045] . For simplicity of description, the valve seats 2 and the valve guides 10 are indicated with a single reference number but, even though they are structurally identical, they usually differ from one another in their dimensions . [0046] . To obtain proper and safe incorporation of the valve seats 2 to the foamed sector 1, valve seats 2 and die 20 must be specifically shaped.

[0047] . According to a preferred embodiment, the valve seats 2 (Figures 3 , 4 , 4a) present an annular structure that develops around an axis x which, with valve mounted on, is inclined by a certain angle α relative to the vertical axis Y of the engine head as shown in the drawings . The valve seats 2 are made, for example, of metal or ceramic material or of sintered material . [0048] . Preferably, each valve seat 2 exhibits an inner side surface comprising two annular portions having taper opposed to one another. A first lower portion 14 narrows from bottom upwards; the second upper portion 15 widens upwards .

[0049] . The lower conical surface 14 is adapted for settling on a first reference boss with truncated cone shape 16 that extends from one of the parts forming die 20 (Figure 5) . A second part of die 20 , adapted for closing on the first one, exhibits a second boss with truncated cone shape 15 ' adapted for settling on the upper conical surface 15 of the valve seat 2 , locking it into position. Truncated cone boss means a circular-section projection that extends on a bottom surface and that preferably exhibits a mean diameter considerably larger than its height .

[0050] . The upper conical surface 15 is also adapted for receiving the lower end 7" and 8" of the tubular elements 7 and 8 coating the intake 30 and exhaust 31 ducts , when provided (Figures 2 , 8 , 11) . When said tubular elements 7 and 8 are not provided, said surface 15 will exhibit a smaller diameter, and therefore adjacent to the lower conical surface 14 , thus forming mechanical machining allowance of the valve seat 2. [0051] . Preferably, each of the two conical portions 14 , 15 extends by half the height of the valve seat 2. Advantageously, moreover, the taper of the upper annular portion 15 is equal too or larger than angle α between axis X of the engine valves and vertical axis Y. In this way, it is possible to carry out the automated introduction of the lower end 7" , 8" of the tubular elements 7 , 8 coating ducts 30 and 31 , when provided, in the valve seats 2 , when moulding the foamed model . [0052] . Advantageously, moreover, the taper of the lower annular portion 14 is equal to or larger than angle α between axis X of the engine valves and the vertical axis Y. In this way it is possible to carry out the automated introduction of the valve seats 2 in the die 20 on the reference bosses 16 without the tangent of the taper towards axis X being in undercut . Moreover, said taper approaches what will be the finished mechanical machining seat, therefore with less machining allowance to remove . [0053] . According to a further aspect of the invention, each valve seat 2 exhibits a substantially cylindrical outer side surface 17.

[0054] . Advantageously, at least one annular projection 18 extends from said outer surface 17, defining an undercut adapted for serving as axial locking means of the valve seat 2 to the foamed material , when the latter has polymerised and afterwards, to the casting metal when the latter has solidified.

[0055] . Moreover, at least one longitudinal groove 19 serving as anti-rotation means for seat 2 is obtained in said annular projection 18.

[0056] . The two sectors 3 and 4 that define the bottom part of the intake and exhaust ducts are obtained with a single die 21 which forms the two parts (figure 6) . [0057] . The sector 5 that defines the top part of the ducts is obtained with a die 22 (figure 7) . According to the invention, said die 22 is provided with mobile pins 23 , each adapted for obtaining a lower seat 10 ' in sector 5 , wherein the lower end of a valve guide 10 is afterwards introduced.

[0058] . The upper sector 9 is obtained with a special die 25. According to the invention, also this die is provided with mobile pins 26 , each adapted for obtaining an upper seat 10" , coaxial to the lower seat 10 ' , in sector 9 , wherein the lower end of a valve guide 10 is afterwards introduced.

[0059] . Once the various foamed sectors have been moulded, they are assembled by glueing for forming the model of figure 1 or 10. At this point, the valve seats 2 are already incorporated in the model .

[0060] . After moulding the model, the valve guides 10 are introduced, for example prior to the model coating, in the special seats 10 ' and 10" . With the following addition of the top part 11, the cylinder head model is complete and ready for the optional coating.

[0061] . Advantageously, the valve guides 10 exhibit a plurality of circular grooves 35 and of longitudinal slots 36 adapted for ensuring higher stability in the casting, preventing any rotations or vertical movements of the valve guides when the molten metal of the cylinder head fully incorporates them on the diameter and when the contraction of said molten metal caused by the shrinkage after the solidification is complete . [0062] . According to an alternative embodiment shown in figures 2 , 8 and 11, the casting method according to the invention provides for the incorporation of tubular elements 7 and 8 coating the intake 30 and exhaust 31 ducts in the cylinder head model . [0063] . The method relating to this alternative embodiment differs from what described above with reference to a cylinder head without tubular elements 7 and 8 only in that a single sector 6 obtained in a die 24 may be used in place of the foamed material sectors 3 , 4 and 5 and of the relevant dies 21 and 22 (figure 8) . In practice, the presence of the tubular elements 7 and 8 allows joining sectors 3 , 4 and 5 in a single sector 6 enclosing said tubular elements 7 and 8. [0064] . Before carrying out the injection of foamed material, tubular elements 7 and 8 are laid in die 24. Advantageously, at the ends of the elements 7 and 8 , die 24 exhibits side seats 7' , 8 ' and lower conical seats 7a, 8a suitable for serving as guiding means for proper positioning of elements 7 and 8.

[0065] . Also die 24 is provided with mobile pins 23 ' adapted for obtaining the lower seats 10 ' in sector 6 for the passage of the valve guides 10. [0066] . It should be noted that the tubular elements 7 and 8 have respective holes 32 , 33 obtained in the upper side surface for allowing the passage and positioning of the lower ends of the intake and exhaust valve guides 10. Advantageously, the mobile pins 23 ' partly enter into the tubular elements 7 and 8 passing through said holes 32 , 33 , so as to prevent the foamed material from entering into elements 7 and 8 while concurring to their centring in die 24.

[0067] . Moreover, it should be noted that, as shown in figure 3 , in the presence of at least one of the tubular elements 7, 8 , the corresponding valve seat 2 will have an upper conical annular portion 15 with larger diameter than the lower one, substantially by an amount equal to twice the thickness of said tubular elements, for defining a support step for the lower end of said elements . In this case, the valve seats 2 also serve as guiding means cooperating with the tubular elements 7, 8 for proper centring of sector 6 on sector 1.

[0068] . Once moulded, the foamed material model fitted with the inserts is coated by dipping. After drying, the model is fully coated with a compact coating film having suitable thickness for standing the vibrations for the settling of the filling sand 13 and afterwards, the metallostatic pressure . [0069] . The coating also covers the visible (exposed) portions of the valve seats 2 , of the ends of the valve guides 10 and also the ends and the inner portion of the tubular elements 7 and 8 , if provided. Since the coating is compact, along with the packed sand, it has the further function of keeping valve seats, valve guides and tubular elements for the intake and exhaust ducts into position, preventing any collapsing of said parts during the casting step .

[0070] . From what said above it is clear that the casting method according to the invention surprisingly allows obviating the disadvantages mentioned with reference to the prior art, as the inserts, in the first place valve seats and valve guides, that before had to be introduced by mechanical machining in the casting after melting, now are incorporated (concurrently casted) with the casting itself , that is, are already incorporated in the casting at the end of the casting step .

[0071] . The advantages obtainable thanks to the method of the invention are numerous .

[0072] . As opposing the driving force of the inserts is not needed anymore, it is possible to reduce the thickness of the combustion chamber cap, thus creating higher circulation of the cooling fluid due to the increase of the fluid passage section. The release of the heat from the combustion chamber is thus facilitated, thus increasing the cooling, and thereby the life, of the valve seats , of the valve themselves, of the spark plug and of all distribution gears . [0073] . It is therefore possible to eliminate the bosses normally provided inside the ducts in the lower ends of the valve guides, as the presence of such bosses currently serves only for creating a plane perpendicular to the axis of the valve guides, so that the tool that generates the machining seat in the casting for the valve guides, at the beginning of the machining, does not meet an inclined plane determined by the shape of the duct, which could deviate the tool . [0074] . Other advantages arise from the possibility of incorporating the tubular elements coating the intake and exhaust ducts in the casting (concurrent casted) : [0075] . - for the tubular elements it is possible to use different materials from those formed by the base casting, for example for varying the heat conductivity based on the application;

[0076] . - irrespective of the type of material used to obtain the tubular elements, the latter have very smooth inner surfaces so as to improve the flow of the mixture towards the combustion chambers and the outwards flow of exhaust gases . The use of composite materials allows obtaining surfaces with an excellent finish, certainly better than those obtainable with a foamed model; [0077] . - the use of a specific composite material as duct coating material for obtaining a good heat insulation allows transferring the heat of the discharge duct, produced by the combustion gases coming from the combustion chamber, directly to the exhaust manifold, . rather than being partly absorbed by the head cooling fluid, thus preventing the engine from getting overheated during its operation for any reason. By avoiding the overheating of the engine, also many of the reasons that normally cause considerable damages to the engine are prevented. Moreover, by bringing the exhaust gases still at high temperature as not decreased by the cooling fluid directly into the exhaust manifold, the combustion products can be better eliminated using suitable known devices .

[0078] . - As the engine distribution gears operate at a lower temperature than what currently occurs, they benefit from better lubrication, thus obtaining less consumption of lubricant . In short, all the fluid cooling systems can be set with other parameters as they need lower cooling capacity.

[0079] . - By the incorporation of the tubular elements, all the surface flaws that could currently occur in the foamed model are eliminated as the inner surface of the ducts, which is critical in a casting for engine cylinder heads, now consists of a material differing from the foamed, and in a single block, that is, without joints produced by gluing foamed sectors . Said joints, in fact, could impair the casting quality. At present , heat- meltable adhesives are used for gluing, which even though they are designed for the specific use, could increase the blowing and porosity levels in the casting . [0080] . - The incorporation of the tubular elements in a casting method for Lost-Foam technology further allows having one less die to realise, thus one sector less to mould and glue, as the foamed material coating around the tubular element will be in a single block instead of two half sectors (lower and upper) .

Claims

1. Casting method for obtaining a casting in a metal material , comprising the steps of :

obtaining a model of the casting in a material adapted for dissolving in contact with the molten metal material ;

laying said model in a container adapted for receiving the molten metal material ;

casting the molten metal material into said container so that said metal material dissolves the material forming the casting model, replacing it,

characterised in that, prior to the casting step, it provides for an incorporation, into said model, of at least one insert made of a material resistant , that is, inert, to contact with the molten metal material , so that said at least one insert is embedded into the casting at the end of the casting step .

2. Casting method of an engine cylinder head according to claim 1 , wherein said insert is at least one valve seat for an intake and/or exhaust valve .

3. Casting method of an engine cylinder head according to claim 1 or 2 , wherein said at least one insert is a valve guide for an intake and/or exhaust valve .

4. Casting method of an engine cylinder head according to claim 1, 2 or 3 , wherein said at least one insert is a hollow tubular element coating the intake and/or exhaust duct .

5. Casting method according to any one of the previous claims, wherein said model of the casting is made of a foamed material , such as polystyrene .

6. Casting method according to any one of the previous claims, wherein at least one portion of said model of the casting is obtained by pressing.

7. Casting method according to claim 6 , wherein said at least one insert is arranged in a die for pressing at least one portion of the casting model, prior to the injection in said die of the material adapted for dissolving in contact with the molten metal material .

8. Casting method according to any one of the previous claims, wherein the step for obtaining the casting model comprises the manufacture of at least one seat in said model adapted for receiving a corresponding insert after the model has been moulded.

9. Casting method according to any one of the previous claims, wherein the model of the casting is obtained by gluing a plurality , of portions or sector to one another, each obtained in advance in a special die .

10. Equipment for ^' obtaining a casting in a metal material by casting, comprising:

means for obtaining a model of the casting in a material adapted for dissolving in contact with the molten metal material ;

means for incorporating at least one insert in said model made of a material resistant to the heat of the molten metal material;

- means for laying said model in a container adapted for receiving the molten metal material ;

means for casting the molten metal material into said container so that said metal material dissolves the material forming the casting model, replacing it .

11. Equipment according to claim 10 for obtaining an engine cylinder head, comprising at least one die for obtaining at least one portion of said model by pressing.

12. Die of an equipment according to claim 11 , defining at least one seat for arranging a valve seat therein for an intake and/or exhaust valve .

13. Die according to claim 12 , comprising at least two portions adapted for closing onto one another, wherein said at least one seat is defined by bosses shaped as truncated cone obtained in said two portions of the die .

14. Die of an equipment according to claim 11, characterised in that it exhibits guide seats adapted for receiving the ends of at least one tubular element coating the intake and/or exhaust duct .

15. Die of an equipment according to claim 11, characterised in that it is provided with at least one mobile pin adapted for obtaining a respective seat for introducing a valve guide in the model for the intake and/or exhaust valve .

16. Valve seat for an engine cylinder head, characterised in that it exhibits an inner side surface comprising at least one conical annular portion adapted for settling on a respective reference boss shaped as truncated cone obtained in a die according to claim 13.

17. Valve seat according to claim 16, wherein the inner side surface further exhibits an upper conical surface adapted for receiving a conical end of a tubular element coating the intake or exhaust duct .

18. Valve seat according to claim 17, wherein said upper annular conical portion exhibits a larger diameter than the lower conical portion, substantially by an amount equal to twice the thickness of said tubular elements, for defining a support step for the lower end of said elements .

19. Valve seat according to claim 17 or 18 , wherein the taper of the two upper and lower annular portions are opposed to one another.

20. Valve seat according to any one of the previous claims, wherein the lower conical portion narrows from bottom upwards .

21. Valve seat according to any one of the previous claims, wherein each of the conical annular portions substantially extends by half the height of the valve seat .

22. Valve seat according to any one of the previous claims, wherein the taper of the upper annular portion is equal to or larger than, the angle of the engine valves .

23. Valve seat according to any one of the previous claims, wherein the taper of the lower annular portion is equal to or larger than, the angle of the engine valves .

24. Valve seat according to any ^• one of the previous claims, characterised in that it exhibits a substantially cylindrical outer side surface .

25. Valve seat according to claim 24 , wherein axial locking means to the material wherein the valve is incorporated are provided in said outer surface .

26. Valve seat according to claim 25 , wherein said axial locking means comprise at least one annular projection extending from said side surface for defining at least one undercut with it .

27. Valve seat according to claim 23 _/. wherein angular locking means to the material wherein the valve is incorporated are provided in said outer surface .

28. Valve seat according to claim 27, wherein the outer side surface exhibits at least one annular projection and wherein said angular locking means comprise at least one longitudinal milling obtained in said projection.

29. Tubular element adapted for coating the intake or exhaust duct of an engine cylinder head.

30. Tubular element according to claim 29 , characterised in that it exhibits a perfectly smooth inner surface .

31. Tubular element according to claim 29 or 3O ₇ characterised in that it is made of a material resistant to the contact with a molten metal material .

32. Tubular element according to claim 31, characterised in that it is made of a composite material .

33. Tubular element according to any one of the previous claims, characterised in that it is made of a refractory material .

34. Tubular element according to any one of the previous claims, characterised in that it exhibits a conical end facing the corresponding valve seat of the engine cylinder head.

35. Tubular element according to any one of the previous claims, characterised in that it exhibits a hole in the side surface for the passage of a valve guide .

36. Engine cylinder head, characterised in that it incorporates at least one tubular element coating the intake or discharge duct .