Ceramic port liners

Abstract: The outer peripheral surface of a port liner body is worked to give an uneven surface where the liner is joined to a metal in insert-casting. Further, the outer peripheral surface of the ceramic port liner body may be covered with a ceramic sheet which is irreversibly expandable by heating.

Description: The present invention relates to ceramic port liners used to line inner surfaces of exhaust ports in gasoline engines, diesel engines or the like. Environmental pollution with exhaust gases from automobiles has become a serious social problem, and poisonous substances contained therein have mainly been removed by catalysts. From the standpoint of resources and costs, use amounts of noble metals such as Pt or Rh used as these catalysts have been demanded to be reduced. In addition, recently increasing 4 valve type engines have a problem in that purifying performance of catalysts is deteriorated due to reduction in temperatures of exhaust gases. As one of techniques for solving these problems, it has conventionally been proposed that temperatures of exhaust gases are raised by thermally insulating action of ceramic port liners which are lined to inner surfaces of engine exhaust ports. In general, in order to fit ceramic port liners to engine exhaust ports, the ceramic port liners are insert-cast with a metal such as aluminum simultaneously when engine cylinder heads are to be produced. However, great compression stress acts due to shrinkage through solidification of the molten metal. Thus, there is a problem in that when stress locally concentrate upon even a high strength ceramic material, the ceramic portion is broken. On the other hand, Japanese patent publication Nos. 46 26,330 and 51-16,168 propose ceramic materials used for hollow tubular members to be insert-cast with a metal. Japanese patent publication No. 46-26,330 relates to an invention in which a ceramic member made of high strength alumina-based or silicon carbide based ceramic material is made porous at its surface portion to withstand shrinkage of a metal during cooling. However, such a composite material is in a yielded state in which fine cracks are latently present. Consequently, if such a composite material is used at a portion undergoing vigorous vibration in such as an engine, it is feared that the cracks proceed to conspicuously deteriorate durability. Japanese patent publication No. 51-16,168 relates to an invention for producing heat insulating cast products aiming at low elastic modulus. The cast products are made of flexible ceramic semi-products consisting of a refractory filler and alumina cement, and their values of elastic modulus are 9500 MPa (950 kg/mm²)at the minimum. Thus, they may be broken due to compression stress during insert-casting. In addition to the above-mentioned prior art methods, many attempts have been made to mitigate compression stress due to a molten metal during shrinkage in solidifying by coating the outer periphery of a ceramic member with a layer of asbesto, ceramic fibers made of alumina or the like to be interposed as a buffer member between the metal and the ceramic member by utilizing an elastic force possessed by the layer (for instance, Japanese patent application Laid-open Nos. 52-121,114 and 59-175,693, EP-A-119 499 and US patent 4 604 779). However, since the outer periphery of the ceramic member is coated with the ceramic fibers, a final product is more costly due to an increased number of production steps. Furthermore, this method has a serious problem in that if elasticity of the ceramic fiber layer is lost due to high temperatures and vibrations during engine operation, the port liner is peeled off from the metal member. Further, Japanese patent application Laid-open No. 60-169,655 proposes hollow tubular ceramic members in which at least one high pressure-proof reinforcement is provided around the outer peripheral surface thereof. Since such ceramic members use tialite (aluminum titanate) and zirconia as ceramic bodies and the reinforcement, respectively, differences in thermal expansion and Young's modulus between them are so great that the ceramic members cannot withstand thermal shock and stress and are broken when being insert-cast with aluminum. Moreover, many attempts have also been proposed to prevent such breakage by making port liners themselves of a high strength material. However, since an extremely great stress is concentrated upon a depressed portion such as a central portion of a two barrel type port liner used in a four valve type engine, in which the outer surface is depressed inside, cracking cannot completely be prevented there. The present invention aims at least partly to solve the above-mentioned problems encountered by the prior art techniques, and to provide ceramic port liners which are free from cracking due to compression stress developed during insert-casting, do not peel off during use, and can easily be insert-cast. Further, the present inventors have found through repeated researches to solve the above-mentioned problems that breakage of the ceramic member due to compression stress during insert-casting is more conspicuous when the port liner is peeled from the insert-casting metal and that the stress concentration is mitigated by preventing the peeling to remarkably reduce occurrence of cracks. According to the present invention, there is provided a ceramic port liner as set out in claim 1. Features, and advantages of the present invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attache d drawings, with the understanding that modifications, variations, and changes of the same may be made by the skilled person in the art to which the invention pertains. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a perspective view of a port liner for use in a four valve type engine; Fig. 2 is a sectional view of the port liner insert-case into an engine head; and Fig. 3 is a ceramic port liner according to a third embodiment of the present invention. As shown in Figs. 1 and 2, the conventional port liner 1 for a four valve type engine comprises two ports 2,2, and a single exhaust opening 3, and a depressed portion 4 is formed at an intermediate portion between the ports 2,2. Fig. 3 shows a port liner for a four valve type engine as an embodiment of the present invention. The port liner comprises a port liner body 1' with two ports 2, 2 and a single exhaust opening 3. The port liner body 1' of this embodiment is made of a ceramic material containing aluminum titanate as a main ingredient and having low Young's modulus. More specifically, the ceramic material contains not less than 65% of aluminum titanate as a crystalline phase, the average particle diameter of crystals thereof is not less than 10 µm, and the ceramic material has Young's modulus of 500 to 20000 MPa (50 to 2,000 kgf/mm²), compression strength of (50 to 400 MPa) (5 to 40 kgf/mm²), and porosity of 5 to 35%. The outer peripheral surface of the port liner body 1' is covered with a ceramic sheet 8 which irreversibly expands by heating. As the ceramic sheet 8 exhibiting such a behavior, a mixture of a thermally expandable material such as vermiculite stone or vermiculite, ceramic fibres and a fine amount of an organic binder may be used. For instance, a product commercially available under a trademark " Interam mat " from 3M may be used. Such a ceramic sheet having a thickness of 3 mm is expanded to a thickness of 5.5 mm by heating it at 700°. After that, even when the ceramic sheet is cooled down to 200°C, it maintains the thickness of not less than 4.5 mm. Further, the ceramic sheet exhibits an irreversible behavior. That is, even when it is subjected to repeated cycles between temperature rising up to 700°C and cooling down to 200°C, the thickness does not substantially change and is stabilized in a range from 4.5 to 5.0 mm. In the illustrated embodiment, the ceramic sheet is applied onto the entire outer peripheral surface of the port liner body 1'. However, such a ceramic sheet may be applied to only a portion of the port liner body, such as a depressed portion at an intermediate portion between two ports 2, 2 which particularly has weak strength because the outer peripheral surface is depressed. When the ceramic port liner thus constituted is insert-cast with a molten metal such as aluminum in the production of a cylinder head, the ceramic sheet 8 applied around the outer peripheral surface of the port liner body 1' is expanded with heat possessed by the molten metal (about 700°C in the case of aluminum), and firmly holds the port liner body 1' by its reaction force. As a result, a buffer layer is formed around the outer peripheral surface of the insert-cast port liner body 1' through expansion of the ceramic sheet, which prevents the port liner body 1' from being broken due to shrinkage force generated when the metal solidifies. As mentioned above, since the ceramic sheet 8 also firmly holds the port liner body 1', the phenomenon that the port liner body 1' locally peels from the insert-casting metal to concentrate tensile stress thereon can be prevented. The stress concentration is likely to occur particularly at the depressed portion of the outer peripheral surface of the port liner body 1', and is a major factor to cause cracking. According to this embodiment of the present invention, since the port liner body 1' is firmly held by a holding force as reaction in the expansion of the ceramic sheet, the port liner body 1' is prevented from cracking due to this stress concentration. According to the present invention, the buffer layer formed by irreversible expansion of the ceramic sheet 8 is extremely stable, it will not lose its elasticity due to high temperatures and vibrations of an engine under operation. Thus, the port liner body 1' will not peel off. Further, according to the present invention, since a layer having excellent thermal insulation is formed around the outer peripheral surface of the port liner body 1', temperature drop of exhaust gases can be extremely effectively prevented. As is clear from the aforegoing explanation,the present invention has many merits that it can of course fully exhibit heat insulating effects of port liner themselves, can completely prevent breakage due to compression stress during insert-casting, is free from peeling-off during use, and facilitates insert-casting. Thus, since the present invention provides ceramic port liners which solve the problems possessed by the prior art techniques, the invention has a practically great value.

Claims: A ceramic port liner (1) comprising a port liner body (1') made of a ceramic material, and a ceramic fibre sheet (8) coated around an outer peripheral surface of the port liner body (1'), characterized in that said ceramic fibre sheet (8) is irreversibly expandable by heating. A ceramic port liner (1) according to claim 1, wherein said ceramic fibre sheet (8) is made of vermiculite, ceramic fibres and an organic binder. A ceramic port liner (1) according to claim 1 or claim 2, wherein said port liner body (1') is made of an aluminum titanate-based ceramic material having low Young's modulus in the range of 500 to 20000 Mpa (50 to 2000 kgf/mm²).

Inventors: HAMANAKA TOSHIYUKI, HARADA TAKASHI, HATTORI FUMIO, HAMANAKA, TOSHIYUKI, HARADA, TAKASHI, HATTORI, FUMIO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: F02F1
IPC codes: F01N7, F02F1, F02B1, F02B77, F02F7, F02B3, C04B35