WO2011027214A2

WO2011027214A2 - Internal combustion engine and method for manufacturing thermal insulator for internal combustion engine

Info

Publication number: WO2011027214A2
Application number: PCT/IB2010/002213
Authority: WO
Inventors: Takashi Sasajima; Daisaku Sawada; Eiichi Kamiyama; Narihito Tatsuda; Kazuhisa Yano
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2009-09-03
Filing date: 2010-09-02
Publication date: 2011-03-10
Also published as: JP2011052630A; WO2011027214A3

Abstract

An internal combustion engine includes a structural component having a thermal insulator composed of an aggregate of spherical porous materials that have a uniform diameter with a mean particle diameter of 0.1 to 3 μm, and that have fine pores with a mean pore diameter of 1 to 10 nm, and a substrate disposed adjacently to the thermal insulator. A method for manufacturing the thermal insulator includes: obtaining a mixture by mixing, in a liquid state, spherical porous materials in which fine pores are filled with a masking substance and a reactive binder; obtaining a molded body by compressively molding the mixture; inducing a reaction of the reactive binder in the molded body; and removing the masking substance by firing the molded body in which the reactive binder has reacted.

Description

INTERNAL COMBUSTION ENGINE AND METHOD FOR MANUFACTURING THERMAL INSULATOR FOR INTERNAL COMBUSTION ENGINE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to an internal combustion engine, and more particularly to an internal combustion engine using a thermal insulator in some of structural components constituting a combustion chamber or the like, and to a method for manufacturing the thermal insulator.

2. Description of the Related Art

[0002] Thermal insulators used in structural components of internal combustion engines are available to reduce cooling loss on the structural components and increase thermal efficiency. For example, Japanese Patent Application Publication No. 6-10757 (JP-A-6-10757) discloses a cylinder in which a liner portion of a cylinder block is formed from a ceramic material having a porous structure. Such ceramic material demonstrates high thermal insulation capability apparently because it is a porous material. As a result, thermal efficiency of the internal combustion engine can be increased by using such a cylinder and therefore output and fuel economy can be expected to increase.

[0003] However, structural components of internal combustion engines require even better thermal insulation capability. One of the means for attaining this object is to increase the porosity of thermal insulation portions of structural component. Increasing the porosity of thermal insulation portions means increasing internal voids in the thermal insulation portion. Since thermal conductivity of air is comparatively small, thermal insulation capability is increased by increasing the internal voids. However, where the internal voids are increased, the thermal insulator decreases in strength per unit volume. In addition, where the internal voids are increased, uniform distribution of voids constituting the thermal insulator is difficult to ensure. For these reasons, when a high-strength thermal insulator is required, the internal voids in the thermal insulator T IB2010/002213

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cannot be increased and thermal insulation ability of high uniformity cannot be obtained.

SUMMARY OF INVENTION

[0004] The invention provides an internal combustion engine including a structural component with increased thermal insulation ability and a method for manufacturing a thermal insulator provided in the internal combustion engine.

[0005] The first aspect of the invention relates to an internal combustion engine. The internal combustion engine includes a structural component having a thermal insulator composed of an aggregate of spherical porous materials that have a uniform diameter with a mean particle diameter of 0.1 to 3 μπι, and that have fine pores with a mean pore diameter of 1 to 10 nm, and a substrate disposed adjacently to the thermal insulator.

[0006] In the internal combustion engine, since the spherical porous materials have a mean particle diameter within a range of 0.1 to 3 uin and the thermal insulator is an aggregate of the spherical porous materials with a uniform diameter, the thermal insulator has a comparatively uniform structure. The thermal insulator has a high strength not only because of a high strength of the spherical porous materials themselves, but also because the particles of uniform diameter are aggregated. Further, when a thermal insulator is used in a combustion chamber of an internal combustion engine, a high-pressure field is formed inside the combustion chamber, thermal conductivity of gas molecules present therein increases, and the amount of heat transferred by the gas molecules inside the fine pores cannot be ignored. In addition, a mean free path itself of the gas decreases in the high-pressure field and therefore the fine pores should be sufficiently reduced in size. In the internal combustion engine according to the invention, the diameter of fine pores in the spherical porous materials is 1 to 10 nm, which is sufficiently less than the mean free path of gas in the high-pressure field. Therefore, heat transfer by gas molecules inside the fine pores can be inhibited. In addition, since the gaps between the particles are decreased, part of this effect can be also 002213

used. Therefore, with the internal combustion engine according to the first aspect of the invention, thermal insulation capability can be improved over that when thermal insulation is performed by simply ensuring the voids inside a thermal insulator.

[0007] In the internal combustion engine, the fine pores may be formed radially from a central portion to a surface portion of the spherical porous materials, and the thermal insulator may be at least part of an inner wall of a flow-through path of a gas taken in by the internal combustion engine.

[0008] With such internal combustion engine, a stress applied to spherical porous materials having radial fine pores can be effectively dispersed. In addition, since the thermal insulator, which is the aggregate of the spherical porous materials, can be constituted by disposing the particles of uniform diameter with a high density, the thermal insulator has high rigidity.

[0009] In the internal combustion engine, the flow-through path may be a combustion chamber of the interna] combustion engine, an exhaust passage for discharging a gas discharged from the combustion chamber, and a reflow passage for returning part of the gas flowing in the exhaust passage to an intake passage.

[0010] In the internal combustion engine, a surface of the thermal insulator that is in contact with the gas and a surface of the substrate that is in contact with the thermal insulator may communicate with each other via gaps between particles of the spherical porous materials and via the fine pores, and a thickness of the thermal insulator may be equal to or less than 0.5 mm.

[0011] In such internal combustion engine, a surface of the thermal insulator that is in contact with the gas and a surface of the substrate that is in contact with the thermal insulator can be caused to communicate with each other via gaps between particles of the spherical porous materials and via the fine pores thereof. Therefore, a structural member having a thin thermal insulator with a thickness of equal to or less than 0.5 mm can be obtained, such a structural member being adaptable to the environment with a variable pressure of gas taken into an internal combustion engine. [0012] In the internal combustion engine, the thermal insulator may be at least part of an inner wall of a flow-through path of a gas taken in by the internal combustion engine, a surface of the thermal insulator that is in contact with the gas and a surface of the substrate that is in contact with the thermal insulator may communicate with each other via gaps between particles of the spherical porous materials and via the fine pores, and a thickness of the thermal insulator may be equal to or less than 0.5 mm.

[0013] In the internal combustion engine, the spherical porous materials may be spherical mesoporous silica.

[0014] In this case, it is possible to provide an internal combustion engine having a structural component with thermal insulation capability improved by the spherical mesoporous silica.

[0015] In the internal combustion engine, a monodispersity of the spherical porous materials may be equal to or less that 10%. In the internal combustion engine, each particle of the spherical porous materials in the thermal insulator may be joined together by a jointing material, and a surface area of the jointing material may be equal to or less than 1/4 of a surface area of the spherical porous materials. In the internal combustion engine, the surface area of the jointing material may be equal to or less than 1/10 of the surface area of the spherical porous materials. In the internal combustion engine, each particle of the spherical porous materials in the thermal insulator may be joined together at contact points.

[0016] The second aspect of the invention relates to a method for manufacturing a thermal insulator for an internal combustion engine. The manufacturing method includes: obtaining a mixture by mixing, in a liquid state, spherical porous materials in which fine pores are filled with a masking substance and a reactive binder; obtaining a molded body by compressively molding the mixture; inducing a reaction of the reactive binder in the molded body, and removing the masking substance by firing the molded body in which the reactive binder has reacted. 213

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BRIEF DESCRIPTION OF DRAWINGS

[0017] The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of exampole emboidments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 illustrates an internal combustion engine of an embodiment of the invention;

FIG 2 is an enlarged view of part of the exhaust passage shown in FIG 1; and

FIG 3 illustrates the relationship between the size of voids and thermal conductivity at an air temperature of 1000°C and pressure of 1 MPa.

DETAILED DESCRIPTION OF EMBODIMENTS

[0018] Embodiments of the invention will be described below with reference to the appended drawings. In the drawings, identical or corresponding components will be assigned with identical reference numerals and explanation thereof will be simplified or omitted.

[0019] FIG 1 illustrates the configuration of the internal combustion engine that is an embodiment of the invention. A internal combustion engine 10 includes a cylinder 12 and a cylinder head 14 as constituent components. A piston 16 is inserted and disposed inside the cylinder 12 so that the piston can slide in the up-down direction. A combustion chamber 18 is formed by the inner circumferential surface of the cylinder 12, the lower surface of the cylinder head 14, and the top surface of the piston 16.

[0020] An intake passage 20 for introducing gas sucked in from the outside into the combustion chamber 18 is linked to the cylinder head 14. An exhaust passage 22 for releasing the gas discharged from the combustion chamber 18 to the outside is also linked to the cylinder head 14. A reflow passage 24 for returning part of the gas flowing in the exhaust passage 22 to the intake passage 20 is connected to the intermediate section of the exhaust passage 22.

[0021] Further, as shown by a heavy line in FIG 1, a thermal insulator 26 is provided at the inner wall of the combustion chamber 18. The thermal insulator 26 can be also effectively provided on part of the inner surface of the exhaust passage 22 and the inner wall of the reflow passage 24. The thermal insulator 26 provided on each inner wall has a thickness of 0.2 to 0.5 mm.

[0022] (Thermal Insulator 26)

FIG. 2 shows part (enlarged) of cross section of the exhaust passage 22 shown in FIG. 1. As shown in FIG 2, the thermal insulator 26 is disposed adjacently with the inner wall of the exhaust passage 22. High-temperature working gas (exhaust gas) flows along a flow path formed by the thermal insulator 26. In the thermal insulator 26, Mesoporous-Silica-Sphere (MSS) particles 26a are stacked in a state of being closely packed together by a jointing material 26b. The jointing material 26b is disposed to join the MSS particles 26a together in contact points thereof.

[0023] Since the MSS particles 26a are of uniform diameter, the particles can be undisposed uniformly, while leaving but small gaps between the particles, tiin a field with a variable gas pressure, the gas typically penetrates into the gaps between the particles due to a pressure drop, but because the particles are arranged uniformly with small gaps therebetween, the gas on the side where heat should be retained does not reach the surface of the base material and does not transfer the heat directly thereto, whereby thermal insulation ability is ensured. The expression "of uniform diameter" as used herein is assumed to mean that when a plurality (preferably 20 or more) of particles manufactured under identical conditions are observed under a microscope, the monodispersity thereof is equal to or less than 10%. The monodispersity is represented by a value obtained by dividing a standard deviation of particle diameter by a mean particle diameter. A particle diameter is found by analyzing the image observed under a scanning electron microscope (SEM).

[0024] (MSS Particles 26a)

As the name thereof suggests, the MSS particles 26a constituting the thermal insulator 26 are spherical silica particles having mesopores 26c. In addition to being spherical, the particles are extremely uniform in diameter. Therefore, the particles can be densely stacked without gaps therebetween and the exhaust passage 22 imparted with rigidity can be formed. The term "spherical" as used herein means that an average value of sphericity of the particles is equal to or less than 13% when a plurality of particles (preferably 20 or more) particles manufactured under identical conditions are observed under a microscope. The term "sphericity" as used herein is an index representing the degree of deviation of the external shape of each particle from a circle. The sphericity is represented by a ratio (= Ar_max x 100/r₀ (%)) of a maximum value (Ar_max) of a radial distance between a circumscribing circle points on the particle surface to a radius (ro) of the smallest circumscribing circle that is in contact with the particle surface.

[0025] The MSS particles 26a have an infinite number of mesopores 26c formed from the central portion of the particles to the surface thereof. The infinite number of mesopores 26c formed in the particles makes it possible to realize a high porosity of equal to or higher than 70% when a molded body is obtained,;: Therefore, the exhaust passage 22 with high thermal insulation ability can be constituted. Further, since the infinite number of mesopores 26c are formed from the central portion of the particles to the surface thereof, that is, the mesopores 26c are formed radially, uniformity of thermal insulation capability can be maintained even at a level of individual particles. Further, since the mesopores 26c are formed radially, the outer force is received in the longitudinal direction of the pores, regardless of the direction of deformation, and the particles demonstrate high rigidity. Therefore, the decrease in strength per unit volume caused by increase in porosity can be compensated.

[0026] However, the up-down movement of piston and rapid combustion of fuel mixture cause pressure fluctuations in the combustion chamber of the internal combustion engine. In particular, when the pressure is high, the mean free path of gas molecules present inside the engine is shortened.

[0027] FIG. 3 shows the relationship between the size of voids and thermal conductivity, for example, at an air temperature of 1000°C and a pressure of 1 MPa. The size (nm) of voids is plotted against the abscissa and thermal conductivity (W/m ) is plotted against the ordinate. As shown in FIG 3, as the size of voids decreases, thermal conductivity of the air decreases. Where the size of voids is equal to or less than the mean free path (28 nm) of the air, thermal conductivity of the air can be rather small.

[0028] Gas components inside the combustion chamber in the present embodiment are generally identical to those of the air, and the pore diameter of mesopores 26c in the MSS in accordance with the invention is sufficiently small to obtain the effect of decreasing the thermal conductivity of gas molecules. More specifically, the mean diameter of mesopores 26c is 1 to 10 nm and is equal to or less than the mean free path, and thermal conduction by gas molecules present in the internal voids in the thermal insulator can be inhibited even in a high-pressure environment such as inside the combustion chamber. The pore diameter can be found by a method of calculating from a fine pore distribution curve determined by a nitrogen gas adsorption method.

[0029] As described above, each of the MSS particles 26a has mesopores 26c formed in the radial direction, and thermal insulation ability of the particles themselves is high. In addition, the particles are joined together in the contact points thereof by the jointing material 26b and the entire structure has very high thermal insulation ability. Therefore, sufficient thermal insulation ability can be ensured even when the thermal insulator is thin. Since a thin thermal insulator can be used, a load placed on the thermal insulator by inflow and outflow of gas to^" and from the gaps between the particles of the thermal insulator in a pressure fluctuation field inside the combustion chamber is decreased and fatigue fracture of the thermal insulator caused by gas flow can be avoided.

[0030] (Jointing material 26b)

The jointing material 26b constituting the thermal insulator 26 is composed of silica.

However, the jointing material 26b also may be an oxide of a transition metal element or a typical metal element, for example, alumina, titania, magnesia, and zirconia. The jointing material 26b may be also a mixture including two or more components such as 213

silica and oxides of the aforementioned metals.

[0031] Since the jointing material 26b joins the MSS particles 26a together in the contact points, the shape thereof is not particularly limited. However, in order to join the particles together, it is desirable that the surface area of the jointing material be less than the surface area of the MSS particles 26a. For the thermal insulator 26 to demonstrate a low thermal conductivity and high rigidity, it is desirable that the ratio of the surface area of the jointing material 26b to the surface area of the MSS particles 26a be equal to or less than 1/4, more desirably equal to or less than 1/10.

[0032] The thermal insulator 26 can be joined to the piston 16 by a variety of methods. For example, the thermal insulator can be joined after the piston has been machined. Such a manufacturing method includes: (1) a substrate molding step; (2) a substrate machining step; (3) a thermal insulator fabrication step, and (4) a joining step.

[0033] In the substrate molding step (1), a casting mold corresponding to the predetermined shape of the piston substrate is prepared, and a molten metal such as an aluminum alloy is poured into the casting mold. The piston substrate is then obtained by cooling for a predetermined time and removing from the mold.

[0034] Then, in the substrate machining step (2), the piston substrate obtained in the substrate molding step (1) is subjected to the predetermined machining.

[0035] Separately from the substrate molding step (1) and substrate machining step (2), the thermal insulator 26 is fabricated (thermal insulator fabrication step (3)). This step includes a mixing step (3-1), a molding step (3-2), a reaction step (3-3), and a removal step (3-4).

[0036] In the mixing step (3-1), a small amount of a reactive binder and a third component are mixed with MSS having the fine pores thereof filled with a masking substance. In this case, a surfactant or a pore-increasing _. agent that are used in the below-described manufacture of MSS can be used as they are as the masking substance, but other substances may be also used after the surfactants etc. have been removed. Octadecyltrimethyl ammonium chloride (CisTMACl) and trimethylbenzene (TMB) are T IB2010/002213

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used as the masking substance, but the QeTMACl may be also used alone, or other organic substances, for example, furfuryl alcohol may be used. In other words, the masking substance is not particularly limited, provided it can be decomposed and removed. Further, tetraethoxysilane (TEOS) is used as the reactive binder, but compounds that have a functional group bondable to a silanol group on the silica surface and can form a metal oxide upon polymerization under the effect of external excitation such as heat and light, e.g. starting materials for silica such as tetramethoxysilane (TMOS) and alkoxides including a metal element such as titanium tetraisopropoxide (((CH₃)₂CHO)₄Ti) may be also used. The reactive binder is mixed in a liquid state or in a state of solution including the reactive binder. As a result, the reactive binder is dispersed between the MSS and can join the MSS particles in contact points thereof. It is preferred that during mixing, the amount of the reactive binder be adjusted so that the jointing material 26b be contained in an amount of equal to or less than 20 parts by weight per 100 parts by weight of MSS. Further, polytetrafluoroethylene (PTFE) is used as the third component. The PTFE is used as a binder for temporarily bonding the MSS particles to each other before the MSS particles are strongly joined together by the reactive binder. The third component is selected in an optimum amount for mixing.

[0037] In the molding step (3-2), the mixture obtained in the mixing step (3-1) is cast into a metal form of a predetermined shape and compression molded. The compression pressure is 20 MPa, but it also may be for example 60 MPa. The molding is conducted under optimum conditions determined correspondingly to the material composition, dimensions of the metal mold, and molding method.

[0038J In the reaction step (3-3), the molded body obtained in the molding step (3-2) is subjected to external excitation such as heat or light to induce the reaction of the reactive binder dispersed between the MSS particles. More specifically, the TEOS is polymerized and the MSS particles are bonded together by heating the obtained molded body.

[0039] In the removal step (3-4), the molded body obtained in the reaction step (3-3) is fired to remove the masking material from the MSS particles. The above-described steps make it possible to fabricate the thermal insulator 26 having disposed therein the jointing material that joins the MSS particles together in contact points thereof. The molded body is fired in the air for 6 h at a temperature of 550°C, but the optimum firing temperature, time, atmosphere and the like can be selected correspondingly to the type of masking substance.

[0040] A thermal insulator in which the MSS particles of uniform diameter are stacked in a closely packed state can be fabricated by optimizing the production conditions in the above-described steps (3-1) to (3-4). The thermal insulator thus fabricated has excellent thermal insulation capability with a thermal conductivity being equal to or less than 0.1 W/mK.

[0041] In the joining step (4), the piston substrate obtained in the substrate machining step (2) and the thermal insulator 26 obtained in the thermal insulator fabrication step (3) are joined together. The joining may be conducted by pasting the thermal insulator 26 onto the piston substrate by using an appropriate adhesive. The piston 16 provided with the thermal insulator 26 on the surface can be manufactured by the present step.

[0042] A method for manufacturing MSS will be explained below. Thus, in the MSS manufacturing process, (1) starting materials including a silica starting material and a surfactant are mixed in a solvent and reacted under the predetermined temperature conditions. During the reaction, the surfactant forms rod micella due to self-fibrillation and the rod micella are oriented radially, while maintaining hexagonal symmetry. This becomes a casting mold for silica formation. (2) A pore-increasing agent is then added to the precursor particles including the surfactant, fine pores in the precursor particles are enlarged under the predetermined temperature conditions, and (3) the surfactant and pore-increasing agent are removed from the precursor particles. By optimizing the production conditions of the above-described steps (1) to (3), it is possible to manufacture MSS particles of uniform diameter and a mean particle size of 0.1 to 3.0 μτη in which fine pores with a mean pore diameter of 1 to 10 nm are formed radially.

Claims

WHAT IS CLAIMED IS:

1. An internal combustion engine comprising: a structural component having a thermal insulator composed of an aggregate of spherical porous materials that have a uniform diameter with a mean particle diameter of 0.1 to 3 μτη, and that have fine pores with a mean pore diameter of 1 to 10 run, and a substrate disposed adjacently to the thermal insulator.

2. The internal combustion engine according to claim 1, wherein

the fine pores are formed radially from a central portion to a surface portion of the spherical porous materials, and the thermal insulator is at least part of an inner wall of a flow-through path of a gas taken in by the internal combustion engine.

3. The internal combustion engine according to claim 2, wherein

the flow-through path is a combustion chamber of the internal combustion engine, an exhaust passage for discharging a gas discharged from the combustion chamber, and a reflow passage for returning part of the gas flowing in the exhaust passage to an intake passage.

4. The internal combustion engine according to claim 2 or 3, wherein

a surface of the thermal insulator that is in contact with the gas and a surface of the substrate that is in contact with the thermal insulator communicate with each other via gaps between particles of the spherical porous materials and via the fine pores, and a thickness of the thermal insulator is equal to or less than 0.5 mm.

5. The internal combustion engine according to claim 1, wherein

the thermal insulator is at least part of an inner wall of a flow-through path of a gas taken in by the internal combustion engine, a surface of the thermal insulator that is in contact with the gas and a surface of the substrate that is in contact with the thermal insulator communicate with each other via gaps between particles of the spherical porous materials and via the fine pores, and a thickness of the thermal insulator is equal to or less than 0.5 mm.

6. The internal combustion engine according to any one of claims 1 to 5, wherein the spherical porous materials are spherical rnesoporous silica.

7. The internal combustion engine according to any one of claims 1 to 6, wherein a monodispersity of the spherical porous materials is equal to or less that 10%.

8. The internal combustion engine according to any one of claims 1 to 7, wherein each particle of the spherical porous materials in the thermal insulator is joined togetheivby a jointing material, and a surface area of the jointing material is equal to or less than 1/4 of a surface area of the spherical porous materials.

9. The internal combustion engine according to claim 8, wherein

the surface area of the jointing material is equal to or less than 1/10 of the surface area of the spherical porous materials.

10. The internal combustion engine according to any one of claims 1 to 7, wherein each particle of the spherical porous materials in the thermal insulator is joined together at contact points.

11. A method for manufacturing a thermal insulator for an internal combustion engine, comprising:

obtaining a mixture by mixing, in a liquid state, spherical porous materials in which fine pores are filled with a masking substance and a reactive binder; obtaining a molded body by compressively molding the mixture;

inducing a reaction of the reactive binder in the molded body, and removing the masking substance by firing the molded body in which the binder has reacted.