HEAT-INSULATING PISTON

Abstract: A heat-insulating piston comprises a ceramic cylindrical member (4) having the lower end surface thereof in contact with the outer peripheral upper end surface of a piston skirt member (2), a head base member (1) having the peripheral portion thereof in contact with an inner peripheral step portion (12) of the cylindrical member (4) and fixed to the piston skirt member (2), a heat-insulating member (3) disposed on the head base member (1) and a ceramic thin film member (5) disposed on the heat-insulating member (3) and having the peripheral portion (13) thereof bonded to the cylindrical member (4), this invention relating to a heat-insulating piston which has metallic heat-resistant members (10) between the inner peripheral step (12) portion of the cylindrical member (4) and the peripheral portion (13) of the head base member (1) in order to prevent the occurrence of a gap between the thin film member (5) and the heat-insulating member (3).

Description: Field of the Invention This invention relates to a heat-insulating piston consisting of a composite structure containing a ceramic member and a heat-insulating member Description of the Prior Art Conventionally, a heat-insulting piston is disclosed in Japanese Patent Laid-Open No. 302164/1988. This heat-insulating piston will be explained with reference to Fig. 6 of the accompanying drawings. This heat-insulating piston comprises a piston head portion 41 having a fitting boss portion 44 at is center and made of a material having a thermal expansion coefficient substantially equal to that of a ceramic material, and a metallic piston skirt portion 42 having a center fitting hole 52 to which the fitting boss portion 44 is fitted at its center. The fitting boss portion 44 of the piston head portion 41 and the center fitting hole 52 of the piston skirt portion 42 are fixed to each other by metal flow of a metallic ring 51. A heat-insulating buffer material 48 as a heat-insulating gasket is interposed under a push state at the center contact portion between the piston head portion 41 and the piston skirt portion 42. A heat-insulating air layer 49 is defined between the piston head portion 41 and the piston skirt portion 42. Furthermore, a ceramic thin sheet 45, which is formed to an extremely small thickness in order to reduce thermal capacity, is disposed on the piston head portion 41 through a heat-insulating material 43 in such a manner as to face a combustion chamber. A ceramic ring 46 made of the same material as the ceramic thin sheet 45 is fitted to the outer peripheral portion of the latter, and these ceramic thin sheet 45 and ceramic ring 46 are bonded at the contact portion by CVD (Chemical Vapor Deposition) as described in Japanese Patent Laid-Open no. 302164/1988 (U.S. Patent Specification no. 4,848,291), for example. A step portion 56 is formed on the inner peripheral surface of the ceramic ring 46 and the outer peripheral portion of the piston head-portion 41 fits to the ceramic ring 46 so as to come into contact with the step portion 56 of the ceramic ring 46. A heat-insulating material 43 is sealed into the space defined by the ceramic thin sheet 45, the ceramic ring 46 and the piston head portion 41. This heat-insulating material 43 is made of a material such as potassium titanate whiskers, zirconia fibers. When the piston head portion 41 is fitted to the piston skirt portion 42 under the push state, the outer peripheral portion of the piston head portion 41 is pushed to the step portion 56 of the ceramic ring 46 and the ceramic ring 46 is pushed to the peripheral portion of the piston skirt portion 42. A carbon seal 47 as a gasket is interposed in order to provide sealing between the ceramic ring 46 and the piston skirt portion 42. In a heat-insulating engine member using a ceramic material as a heat-insulating or heat-resistant material such as a piston, it is extremely difficult to obtain sufficient heat-insulating characteristics. The ceramic material is kept under the state where it is exposed to high temperatures on the combustion chamber side and there exist the problems, therefore, that the ceramic material receives a thermal shock and its strength is not sufficient. If the thickness of the ceramic material on the wall surface is increased for the purpose of heat insulation, a thermal capacity becomes greater and there occur the problems that intake air receives a greater quantity of heat from the combustion chamber and is heated to high temperatures during an intake stroke, its heat affects the intake air, suction efficiency drops and air cannot be sucked, whereas the heat-insulating property must be improved in an expansion stroke, on the contrary. To solve these problems, the structure of the heat-insulating piston disclosed in Japanese Patent Laid-Open No. 302164/1988 is as described above in order to obtain extremely high heat-insulating property, to minimize the thermal capacity of the surface portion of the piston head which is exposed to the combustion gas and reaches high temperature, to improve intake efficiency and cycle efficiency, to eliminate the occurrence of the problems of strength even when a thermal shock is applied, to improve heat resistance, corrosion resistance and deformation resistance, to obtain a stable fitting state and to receive under a preferred state the pressure which acts on the piston head at the time of explosion. Further, it improves the seal function between the piston head and the piston skirt. In the heat-insulating piston described above, the heat-insulating material disposed between the head base portion and the ceramic thin sheet disposed on the combustion chamber side is composed of whiskers or fibers of mullite, alumina, possium titanate, zirconia, or the like, and the ceramic thin sheet and the ceramic ring are made of a ceramic material such as silicon nitride. Therefore, since the materials are different between the heat-insulating material, the ceramic thin sheet around the former and the ceramic ring and their thermal expansion coefficients are therefore different, the difference of thermal expansion occurs between the different materials after the ceramic thin sheet and the ceramic ring are bonded mutually and a gap develops between the ceramic thin sheet as the surface of the piston head and the heat-insulating material. This is structurally disadvantageous to the explosion force at the time of combustion and results in the breakdown of the ceramic thin sheet. Moreover, if the bond portion between the ceramic thin sheet and the ceramic ring is bonded by chemical vapor deposition or coating, the bond portion does not have the strength sufficient to keep the bonded state against the explosion force at the time of combustion, so that the bond portion between the ceramic thin sheet and the ceramic ring peels or cracks develops. The present invention seeks to solve the problems described above. According to the present invention there is provided a heat-insulating piston comprising: a piston skirt member having a peripheral upper end surface; a ceramic annular member having a lower end surface thereof contacting said peripheral upper end surface of said piston skirt member; said annular member having a step portion extending inwardly in a radial direction, on the inner peripheral surface thereof; a head base member fitted to said piston skirt member, having a peripheral portion thereof in contact with said step portion of said annular member; a heat-insulating member disposed on the upper surface of said head base member; and a flat, ceramic thin film member disposed on the upper surface of said heat-insulating member and having a peripheral portion thereof bonded to an upper end portion of said annular member; characterised in that metallic heat-resistant members are inserted into a void defined between said inner peripheral step portion of said annular member and said head base member, in the region of the peripheral portion thereof, so as to urge together, through said head base member, a lower surface of said thin film member and the upper surface of said heat-insulating member to prevent the occurrence of any gap therebetween. This heat-insulating piston provides a piston head portion with very high insulation property. The thin film member as the surface portion of the piston head which is exposed to a combustion gas, and which reaches a high temperature and faces a combustion chamber, is preferably formed of a ceramic material such as silicon nitride (Si₃N₄), silicon carbide (SiC), in order to secure heat-resistance of the thin film member. This thin film member allows minimisation of its thermal capacity, so as to improve follow-up property to the gas temperature and hence, suction efficiency. The inserts of metallic heat-resistant members between a step portion of the annular member constituting a portion of the slide surface and made of a ceramic material and the peripheral portion of a head base portion, are pushed into the heat-insulating member to the thin film member in order to prevent the occurrence of a gap between the thin film member in order to prevent the occurrence of a gap between the thin film member and the heat-insulating member, to prevent the occurrence of a bending stress in the flat plate-like thin film member against the explosion force at the time of combustion, to prevent the breakage of the thin film member, to prevent the occurrence of peel and crack between the thin film member and the cylindrical member and to improve the strength thereof. The metallic heat-resistant members may be disposed in the voids between the inner peripheral step portion of the cylindrical member and the peripheral portion of the head base member after the cylindrical member and the thin film member are bonded to each other. Preferably, the void defined between the inner peripheral step portion of the annular member and the peripheral portion of the head base member consists of a groove formed in the inner peripheral step portion of the annular member and a groove formed in the peripheral portion of the head base member, the metallic heat-resistant members being softened and then pushed into the grooves after the annular member and the thin film member are bonded together. The metallic heat-resistant members can be inserted into the voids easily, reliably and sufficiently, and the fixed state of the metallic heat-resistant members can be stabilized and kept rigidly in place. The heat-insulating member can be brought into strong contact with the ceramic thin film member through the lower end surface of the cylindrical member and the outer peripheral upper end surface of the piston skirt member, that is, through the head base member, the close contact state between the peripheral portion of the head base portion and the heat-insulating member can be kept firmly, so that the close contact state between the thin film member as the surface exposed to the combustion gas and the heat-insulating member can be kept under a satisfactory state, no bending stress acts on the thin film member due to the explosion force, and high strength with high reliability can be secured. This piston structure can secure a high heat-insulating property by means of the heat-insulating member, the thickness of the thin film member positioned on the surface portion of the piston head exposed to the combustion gas reaching a high temperature can be minimised, allowing minimisation of the thermal capacity of the thin film member. The piston structure allows improvement in suction efficiency, heat-insulation, deformation resistance and corrosion resistance. A preferred embodiment of the structure of a heat-insulating piston in accordance with the present invention will be described in detail, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a sectional view showing a heat-insulating piston structure in accordance with one embodiment of the present invention; Fig. 2 is an enlarged view of the portion represented by symbol A in Fig. 1 before a heat-insulating member is inserted; Fig. 3 is an enlarged view showing the state after insertion of Fig. 2; Fig. 4 is an enlarged view of the portion represented by symbol A in Fig. 1 and shows another example before insertion of the heat-insulating member; Fig. 5 is an enlarged view showing the state after insertion of Fig. 4; and Fig. 6 is a sectional view showing an example of the structure of a conventional heat-insulating piston. Fig. 1 shows the structure of the heat-insulating piston in accordance with an.embodiment of the present invention. This heat-insulating piston comprises primarily a piston skirt member 2, an annular 4 coming partially into contact with the piston skirt member 2, a head base member 1 fixed to the skirt member 2, a heat-insulating member 3 disposed on the head base member 1, a thin film member 5 disposed on the heat-insulating member 3 and bonded around its periphery to the annular member 4, and metallic heat-resistant members 10 disposed in the spaces between the annular member 4 and the head base member 1. In this heat-insulating piston, the piston skirt member 2 is made of a metallic material. The annular member 4 whose lower end surface is pushed to the upper end surface of the outer periphery of the piston skirt member 2 and the head base member 1 fixed to the piston skirt member 2 while its peripheral portion is pushed and brought into contact with an inner peripheral step portion 12 of the annular member 4 are made of a ceramic material such as silicon nitride (Si₃N₄), silicon carbide (SiC). Further, the heat-insulating member 3 disposed on the head base is made of a whisker fired material of a ceramic material such as silicon nitride (Si₃N₄), Silicon carbide (SiC). The thin film member 5 which is disposed on the heat-insulating member 3 and whose peripheral portion is bonded to the annular member 4 is made of a ceramic material such as silicon nitride (Si₃N₄) and silicon carbide (SiC). The metallic heat-resistant members 10 disposed between the inner peripheral step portion 12 of the annular member 4 and the peripheral portion 13 of the head base member 1 are made of a heat-resistant alloy such as a nickel alloy. A combustion chamber is not formed in this head base member 1 itself and the portion of the head base member 1 on the combustion chamber side is shaped flat. The head base member 1 and the piston skirt member 2 are fixed to each other by fitting the fitting boss portion 8 disposed at the center of the head base member 1 into the fitting hole 11 formed at the center of the piston skirt member 2 and disposing a metal ring 9 by metal flow into the groove portion defined between them. In this case, the inner peripheral step portion 12 is formed on the annular member 4 constituting the upper portion of the slide surface of the piston, the peripheral portion 13 of the head base member 1 is engaged with this inner peripheral step portion 12 and moreover, the lower end surface of the outer periphery of the annular member 4 and the upper end surface of the piston skirt member 2 are brought into pressure contact while interposing a seal member 7 between them. The heat-insulating member 3 is disposed in a cylindrical hole portion defined by the head base member 1 and the annular member 4. A heat-insulating air layer 6 is defined between the lower surface of the head base member 1 and the piston skirt member 2. In this structure of the heat-insulating piston, the thin film member 5 disposed on the outer surface of the heat-insulating member 3 is made of the same ceramic material as the heat-insulating member 3 such as silicon nitride (Si₃N₄), silicon carbide (SiC), and can be disposed on the heat-insulating member 3 by bonding it to the side of the heat-insulating member 3 exposed to the combustion gas, that is, on its surface on the combustion chamber side, by CVD (Chemical Vapor Deposition) or coating. Accordingly, since this thin film member 5 provides the surface exposed to the combustion chamber and moreover, can be formed as thin as possible, the thermal capacity of the surface exposed to the combustion gas can be reduced and the structure can be made highly heat-resistant. This heat-insulating member 3 exhibits the heat-insulating function and at the same time, can function as a structural member which receive the pressure acting on the ceramic thin film member 5 at the time of explosion. In this structure of the heat-insulating piston, the compressive force due to explosion must be received uniformly by the heat-insulating member 3 and to this end, too, the upper surface of the head base member 1 and the thin film member 5 are shaped in a flat form. Particularly in order to insert the metallic heat-resistant members 10 into the voids between the inner peripheral step portion 12 of the annular member 4 and the peripheral portion 13 of the head base member 1, they are softened and pushed into the voids defined by the groove 15 formed in the inner peripheral step portion 12 of the annular member 4 and the groove 14 formed in the peripheral portion 13 of the head base member 1 or pushed into the voids defined between the inner peripheral step portion 12 and the peripheral portion 13 of the head base member 1, as will be described later, after the annular member 4 and the thin film member 5 are mutually bonded at the joint portion 24 of the peripheral portion but before the head base member 1 and the piston skirt member 2 are fixed to each other. When the metallic heat-resistant members 10 are inserted into the voids between the inner peripheral step portion 12 of the annular member 4 and the peripheral portion 13 of the head base member 1, the heat-insulating member 3 can be brought into contact with the thin film member 5 through the head base member 1 and the occurrance of any gap between the thin film member 5 and the heat-insulating member 3 can be prevented. In this structure of the heat-insulating piston, for example, the groove 15 is formed in the inner peripheral step portion 12 of the annular member 4 and the groove 14 is formed in the peripheral portion 13 of the head base member 1 in such a manner as to define the voids of the metallic heat-resistant members 10, that is, their accommodation portion 16, as shown in Fig. 2. The metallic heat-resistant members 10 can be disposed in this accommodation portion 16 in the following way. As shown in Fig.2, the metallic heat-resistant members 10 are first disposed at part of the accommodation portion 16 and then the heat-resistant metal is locally heated by a radio frequency heater and softened. Then, it is pushed completely into the accommodation portion 16 by use of a jig 17 as represented by arrow B in Fig. 3 and is thereafter hardened. Alternatively, the metallic heat-resistant members 10 can be disposed in the voids between the inner peripheral step portion 12 of the annular member 4 and the peripheral portion 13 of the head base member 1 in the following way. The metallic heat-resistant members 10 are fabricated to the thickness of the void between the inner peripheral step portion 12 of the annular member 4 and the peripheral portion 13 of the head base member 1 as shown in Fig. 4 and are placed on the lower surface 21 of the head base member 1. A push jig 22 having a taper surface 23 is put onto the side surface of the heat-resistant members 10. Then, another push jig 18 having a taper surface 20 which comes into sliding contact with the taper surface 23 of the push jig 22 is pushed in the direction represented by arrow C, so that the push jig 22 is moved in the direction represented by arrow D and can push the heat-resistant member 10 into the void between the inner peripheral step portion 12 of the annular member 4 and the peripheral portion 13 of the head base member 1.

Claims: A heat-insulating piston comprising: a piston skirt member (2) having a peripheral upper end surface; a ceramic annular member (4) having a lower end surface thereof contacting said peripheral upper end surface of said piston skirt member (2); said annular member (4) having a step portion (12) extending inwardly in a radial direction, on the inner peripheral surface thereof; a head base member (1) fitted to said piston skirt member (2), having a peripheral portion (13) thereof in contact with said step portion (12) of said annular member (4); a heat-insulating member (3) disposed on the upper surface of said head base member (1); and a flat, ceramic thin film member (5) disposed on the upper surface of said heat-insulating member (3) and having a peripheral portion thereof bonded (24) to an upper end portion of said annular member (4); characterised in that metallic heat-resistant members (10) are inserted into a void (16) defined between said inner peripheral step portion (12) of said annular member (4) and said head base member (1), in the region of the peripheral portion (13) thereof, so as to urge together, through said head base member (1), a lower surface of said thin film member (5) and the upper surface of said heat-insulating member (3) to prevent the occurrence of any gap therebetween. A heat-insulating piston according to claim 1, wherein said void (16) defined between said inner peripheral step portion (12) of said annular member (4) and said peripheral portion (13) of said head base member (1) consists of a groove (15) formed in said inner peripheral step portion (12) of said annular member (4) and a groove (14) formed in said peripheral portion (13) of said head base member (1), said metallic heat-resistant members (10) having been pushed into said grooves (14,15) in a softened state. A heat-insulating piston according to claim 2, wherein said heat-resistant members (10) are pushed into said groove (14) in said head base member (1) and softened by heating locally only said heat-resistant members (10), said softened heat-resistant members (10) then being pushed into said groove (15) in said inner peripheral step portion. A heat-insulating piston according to claim 1, wherein said metallic heat-resistant members (10) are pushed into said void (16) defined between said inner peripheral step portion (12) of said annular member (4) and said peripheral portion (13) of said head base member (1). A heat-insulating piston according to any preceding claim, wherein said heat-resistant members are made of a heat-resistant alloy. A heat-insulating piston according to any preceding claim, wherein said heat-resistant members (10) are made of a nickel alloy. A heat-insulating piston according to any preceding claim, wherein the upper end portion of said annular member (4) and the peripheral portion of said thin film member (5) are mutually bonded together by chemical vapor deposition. A heat-insulating piston according to any preceding claim, wherein said ceramic thin film member (5) is composed of a sheet-like member whose entire surface is flat. A heat-insulating piston according to any preceding claim, wherein said thin film member (5) is made of silicon nitride. A heat-insulating piston according to any one of claims 1 to 8, wherein said thin film member (5) is made of silicon carbide. A heat-insulating piston according to any one of claims 1 to 8, wherein said heat-insulating member (3) is made of mullite fibers.

Inventors: HIRAI, KATSUNORI, REOPARESU DAIKAN 202
Applicants: ISUZU MOTORS LIMITED
Application year: 1990
Main classification: F02F3
IPC codes: F02F3, F16J1