HEAT INSULATING CERAMIC ARTICLES FOR USE IN EXHAUST CHANNELS IN INTERNAL COMBUSTION ENGINES AND A PROCESS FOR PRODUCING THE SAME

Abstract: A heat insulating ceramic insert-cast article for an internal combustion engine, comprising a ceramic liner (1) and a metallic member (2) enclosing the outer periphery of the ceramic liner. The ceramic liner contacts the exhaust gases. The joint boundary between the ceramic liner and the metallic member is constituted by contact faces (3) at which the ceramic liner contacts the metallic member and heat insulating air layers (4) at which the ceramic liner and the metallic member do not contact each other. To produce such an article, first, the outer periphery of the ceramic liner is covered with a heat insulating layer which is partially formed with cuts, and then the ceramic liner is enclosed with a molten metal. Thus, the metal flows into the cuts and solidifies therein to form the contact faces (3) between the ceramic liner and the metal. Portions at which the metal does not flow inside due to the presence of the layer are converted to heat insulating layers.

Description: The present invention relates to heat insulating ceramic articles for use in exhaust passages in internal combustion engines and a process for producing the same. In a system for purifying exhaust gases from internal combustion engines such as gasoline engines and diesel engines and for preventing air pollution with such exhaust gases, it is desired that the temperature of the exhaust gases is maintained high, because reduction in the temperature of the exhaust gases lowers the purifying performance of the catalyst. For this reason, as described in Japanese patent publication No. 51-16,168, it is proposed that an exhaust port in a cylinder head or an exhaust manifold is lined with a heat insulating ceramic liner. Although such a ceramic liner is enclosed with a metal such as aluminum or the like, its heat insulating effect is not sufficient because the thickness of the liner is about 2 to 3 mm. As a result, the temperature of exhaust gases merely increases by about 20°C as compared with a case where no liner is provided. Under the circumstances, a trial is made, in which fibres made of a refractory material are wound around the outer periphery of a ceramic liner, and then the ceramic liner is enclosed so that heat insulation is enhanced by the fiber layer. However, since the fibers are gradually damaged due to thermal shocks on starting and stopping of engines or due to engine vibrations and heat, they lose ability to hold the ceramic liner. Ultimately, there is the possibility that the ceramic liner slips out from the fibers. Thus, US-A-4604779 illustrates a cylinder head in which an insert-cast ceramic liner is separated from the metal by a covering layer of fibers. Furthermore, as shown in Japanese Utility Model Registration Application Laidopen No. 54-56,010, a space is formed by arranging a metallic plate around the outer periphery of a ceramic liner so that heat insulating property possessed by air may be utilized. However, since the structure is complicated and the production cost is great and since the ceramic liner is held at opposite ends only, there is a problem in that forces for holding the ceramic liner are likely to be lost. It is an object of the present invention to solve the above-mentioned problems, and to provide a heat insulating ceramic article for an exhaust passage in an internal combustion engine, which article can keep the temperature of exhaust gases significantly higher than in known articles, is free from reduction in forces for holding a ceramic liner, is not structurally complicated, and can be inexpensively manufactured. Another object of the present invention is to provide a process for producing such a heat insulating ceramic article. The present invention provides a heat insulating ceramic insert-cast article for an internal combustion engine exhaust passage, as set out in claim 1. Such a heat insulating ceramic insert-cast article may be produced by the method set out in claim 9. Embodiments of the invention will be given in the following description when taken in conjunction with the attached drawings, with the understanding that modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains. In the attached drawings : Fig. 1 is a sectional view of an exhaust manifold as a first embodiment according to the present invention; Fig. 2 is a sectional view of Fig. 1 taken along a line II-II; Fig. 3 is a sectional view of a cylinder head exhaust port liner as a second embodiment according to the present invention; and Fig. 4(A) through Fig. 4(D) are perspective views of diagrammatically illustrating joining boundaries between ceramic liners and metallic members. The present invention will be explained in more detail with reference to the attached drawings: In Fig. 1 is sectionally shown an exhaust manifold as an embodiment to which the present invention is applied. Reference numerals 1 and 2 are a ceramic liner for an exhaust manifold and a metallic member, respectively. The metallic member is made of for example an aluminum alloy, and encloses the outer periphery of the ceramic liner. The ceramic liner 1 is made of a ceramic material, for instance, containing not less than 65% of aluminum titanate as a crystalline phase, and having the average particle size of not less than 10 µm, a Young's modulus of 500 to 20000 MPa (50 to 2,000 kgf/mm²), a compression strength of 50 to 400 MPa (5 to 40 kgf/mm²), a porosity of 5 to 35%, and a heat conductivity of about 0.8 to 50x10³ cal·cm·sec.°C. Thereby, the ceramic liner 1 can be prevented from being cracked during casting even in a complicated shape such as a bifurcated port liner. As shown in Fig. 1, the joining between the ceramic port liner 1 and a metallic member 2 is constituted by contact regions 3 at which the ceramic liner 1 directly contacts the metallic member, while there are heat insulating air layers 4 at which they do not contact. As diagrammatically shown in Fig. 4(A) through Fig. 4(D), the contact faces 3 may be designed in linear, spiral, or dotted fashion. It is preferable that the contact faces 3 are uniformly distributed over the entire outer periphery of the ceramic liner 1. However this is not an essential restriction, and e.g. slight deviation therefrom is acceptable. The contact regions 3 serve to hold the ceramic liner, and it is preferable to decrease the contact regions 3 and to increase the area of the heat insulating air layers 4 so that the heat insulation may be improved. Practically, the total area of the contact faces is not more than 35% of the entire area of the boundary face, and preferably not more than 10%. The thickness of the heat insulating air layer is preferably 0.5 to 5 mm. The interior of the heat insulating air layer is void, or may be filled with fibers or the like. Fig. 3 shows an embodiment in which the present invention is applied to a port liner. The structure of a joining boundary is the same as the above exhaust manifold. Since the ceramic liner 1 itself not only has heat insulating property and but also the heat insulating air layers 4 exhibit excellent heat insulating effect, the temperature of exhaust gases can be maintained high by such a heat insulating ceramic insert-cast article according to the present invention. Furthermore, since the outer periphery of the ceramic liner 1 is directly supported by the contact regions 3 formed with accuracy according to the present invention, it is not feared that the forces for holding the ceramic liner 1 are reduced. Next, the example of the process for producing hear insulating ceramic inser-cast article according to the present invention will be explained. First, the outer periphery of the ceramic liner 1 is covered with a layer which can withstand a molten metal during insert casting. It is preferable to use a water-soluble glass powder or a water-soluble glass fiber sheet which can be removed after the casting, or a cast sand solidified by a cold box process. The glass fiber sheet is made of glass containing not less than 30% by weight of boron oxide, and preferably not less than 50% by weight. This glass fiber sheet can sufficiently withstand the temperature at which the ceramic liner is enclosed with aluminum, which is terminated in short time, and can readily be dissolved off with hot water containing an alkaline compound such as NaOH. On the other hand, the cold box process is a process in which an isocyanate resin is added to cast sand as a binder, and is cured with an amine gas after shaping. After the heat treatment at around 500°C, the resin can easily be broken by light vibrations. Such a layer is partially formed with cuts. When the ceramic liner 1 is enclosed with the molten metal in the sate that the liner is covered with this layer, the molten metal flows into the cuts formed in the layer, and solidifies there to form contact regions between the ceramic liner 1 and the metallic member 2. On the other hand, portions at which the molten metal is prevented by the layer from flowing inside are converted to the heat insulating air layers as voids by removing the layer with hot water or by heat treatment. As the material of the layer, besides the above materials, a general fiber sheet may be used. In this case, the sheet may mechanically be scraped out after the solidification. When the porosity of the layer itself is large, the layer may be retained as a heat insulating layer 4 without being removed. The heat insulating ceramic insert-cast article according to the present invention may easily and inexpensively be produced by the above process. In order to confirm the effect of the articles according to the present invention, the following experiment was conducted. A cylinder having an outer diameter of 30 mm, a thickness of 4 mm, and a length of 300 mm was prepared from a ceramic material containing aluminum titanate as main crystals and having a Young's modulus of 200 kgf/mm². Next, a water-soluble glass fiber sheet was fitted around the outer periphery of the cylinder, which was then insert cast with aluminum. The sheet was formed with a number of 3 mm diameter holes to give an area ratio of the contact faces of 10% or 20%. As a result, an insert-cast product having an outer diameter of 45 mm and a length of 300 mm was obtained. After cooling, the water-soluble glass fiber sheet was removed by washing with water. Exhaust gases from an engine at 700°C were led to each of the thus obtained test pieces, and the heat insulating effect thereof was evaluated. As compared with an exhaust pipe made of a stainless steel with no ceramic cylinder, the temperature of the exhaust gases could be maintained higher by 70°C and 60°C in the case of the aluminum insert-cast test pieces having the area ratio of the contact faces 3 being 10% and 20%, respectively. Furthermore, in the case of a test piece in which a ceramic fiber sheet having a larger porosity and holes formed therein to give an area ratio of the contact regions 3 being 20% was used and not removed after insert-casting, the temperature of the exhaust gases could be maintained higher by about 40°C. The temperature of the exhaust gases could be maintained higher by about 20°C in the case of a ceramic liner directly enclosed with aluminum in which the area ratio of the contact face 3 was 100%. As is clear from the foregoing explanation, the present invention has various merits. That is, heat insulating effect is excellent, and the temperature of exhaust gases can be maintained sufficiently higher as compared with known articles. Further, it is not feared that the force for holding the ceramic liner will lower, the structure is simple, and production can inexpensively be done. Thus, the ceramic insert-cast article according to the present invention is preferable for the cylinder head exhaust port or the exhaust manifold.

Claims: A heat insulating ceramic insert-cast article suitable for use in an exhaust passage in an internal combustion engine, comprising a ceramic liner (1) and a metallic member (2) enclosing the outer periphery of the ceramic liner, said ceramic liner (1) being adapted to contact exhaust gases and being thermally insulated from the metallic member (2), characterised in that an air insulation layer region or regions (4) are present between the ceramic liner (1) and the metal member (2), and there is direct supporting contact between the outer periphery of the ceramic liner (1) and the metal member (2) at direct contact regions (3) existing (a) at both ends of the liner (1) and (b) at a plurality of locations spaced apart between said ends or at one or more direct contact regions (3) extending continuously from one end to the other end of the ceramic liner, said direct contact regions (3) being at most 35% of the whole area of the outer periphery of the ceramic liner. A heat insulating ceramic insert-cast article according to claim 1, wherein the area of said direct contact regions (3) amounts to 30% of the whole area of the periphery of the ceramic liner (1). A heat insulating ceramic insert-cast article according to claim 1 or 2, wherein the ceramic liner (1) is made of a ceramic material having a Young's modulus of not more than 20000 MPa (2,000 kgf/mm²). A heat insulating ceramic insert-cast article according to any one of claims 1 to 3 wherein said air insulation layer regions (4) are a plurality of separate regions each extending from one end of the liner (1) to the other. A heat insulating ceramic insert-cast article according to any one of claims 1 to 3 wherein said air insulation layer region (4) is a spirally extending region. A heat insulating ceramic insert-cast article according to any one of claims 1 to 3 wherein said contact regions (3) are a plurality of discrete regions distributed longitudinally and laterally of said periphery of the liner (1). A heat insulating ceramic insert-cast article according to any one of claims 1 to 3 wherein said contact regions (3) include both ends of the liner (1) and a plurality of spaced regions between said ends. A heat insulating ceramic insert-cast article according to any one of claims 1 to 3 and 7 suitable for an exhaust manifold. A process for producing a heat insulating ceramic insert-cast article according to claim 1, for use in an exhaust passage in an internal combustion engine, comprising the steps of covering an outer periphery of the ceramic liner (1) with a heat insulating layer having apertures and casting a molten metal around the liner (1) so that the metal flows into the apertures of the layer and solidifies therein to form said direct contact regions (3) between the ceramic liner (1) and the resultant metal member (2), and subsequently converting the heat insulating layer into said air insulation layer region or regions (4). A process according to claim 9, wherein the heat insulating layer is made of a material selected from a water-soluble glass powder and a water-soluble glass fiber sheet, and is removed by means of hot water containing an alkaline compound after the casting of the metal. A process according to claim 9, wherein the heat insulating layer is formed from cast sand by a cold box process, and is removed by imparting vibrations to the layer after the casting.

Inventors: HAMANAKA, TOSHIYUKI, HARADA, TAKASHI, YAMADA, SHUNICHI
Applicants: NGK INSULATORS, LTD.
Application year: 1989
Main classification: F02B77
IPC codes: C04B37, F02F7, F16J10, F02B1, B23K9, F02B77, F01N13