(1) Field of the Invention
The present invention relates to an internal combustion engine piston in which a piston cap constituted by a plurality of ceramic pieces is fixed to the top portion of a piston body made of a metal, and a method of producing the same.
(2) Description of the Invention
Since ceramic materials have excellent mechanical strength, heat resistance, oxidation resistance, and heat insulating properties, when a portion of a piston, a cylinder liner, a cylinder head and so on of an internal combustion engine, which portion is to be brought into contact with a high temperature combustion gas, is constituted by the ceramic material, an increase in combustion temperature and reduction in heat loss can be attained to result in improved engine efficiency and reduction in harmful components in the exhaust gas can be expected. Thus, here have been proposed internal combustion engine pistons in which a piston cap made of ceramics is fixed to the top portion of a metal piston body.
The engine pistons in which the ceramic piston cap is fixed to the top portion of the metal piston body are disclosed in, for instance, Japanese Patent Application Laid-Open No. 74,853/1983, U.S. Pat. No. 4,404,935 and U.S. Pat. No. 4,245,611. Among them, Japanese Patent Appllication Laid-Open No. 74,853/1983 and U.S. Pat. No. 4,404,935 disclose pistons in which a ceramic piston cap having an integral structure is fixed to the top portion of a metal piston body by casting or engagement.
In general, since the ceramic material has a lower heat conductivity, there occurs a large difference in temperature between the high temperature combustion gas-contacting surface and the metal piston body-contacting surface of the ceramic piston cap which is fixed to the top portion of the metal piston body. Therefore, the ceramic piston cap with such an integrated structure has the drawback that the piston cap tends to be broken due to thermal stress caused by the large temperature difference, and this tendency becomes conspicuous as the size of the piston increases.
On the other hand, U.S. Pat. No. 4,245,611 discloses a piston in which a piston cap constituted by a plurality of ceramic pieces is fixed to the top portion of a metal piston body through casting. According to this piston, a cushioning layer composed of ceramic fibers is provided around the piston cap to mitigate the stress caused through the casting. However, this structure has the drawback that the fixing of the ceramic piston cap to the metal piston body becomes insufficient due to the presence of the cushioning layer so that the piston cap becomes loose when the piston is in use. Further, Japanese Patent Utility Model registration Publication No. 48,327/1983 and Japanese Patent Application Laid Open No. 60,306/1979 disclose pistons in which a molten metal is directly cast around a piston cap constituted by a plurality ceramic pieces. However, the method by which the piston cap, split into the plurality of ceramic pieces is insert cast and fixed to the top portion of the metal piston body has the drawback that the assembling of the piston cap into a mold is complicated, and therefore this method is unsuitable for mass production. Further, the piston in which the ceramic piston cap is fixed to the metal piston body through casting has the drawback that the ceramics fracture when thermal treatment such as quenching is effected.