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In order to reduce the weight of an engine, aluminum has been increasingly used as a material for the intake manifold --an engine component formerly made of a heavier metallic material. Recently, with the aim of further reducing the engine weight, more and more intake manifolds are being made of plastics. To bring the plastic manifold into practical use, a number of methods have been proposed and put into practice; these methods include melt core molding, injection molding and blow molding. Formerly, the melt core molding process had to be used to obtain a solid plastic intake manifold port section having a large curvature. The newly developed intake manifold does not need that process to form its solid port section having a large curvature. Instead, it employs a new process for solid injection molding in which the core is drawn out while it is rotated.

A fuel control method to reduce the harmful exhaust gas from SI engines is proposed. As is well known, both the amplitude and the frequency of the limit cycle in a conventional air-fuel ratio control system are determined uniquely by parameters in the system. And this limits our making full use of the oxygen storage effect of TWC. A simple model of TWC reaction revealed the relationship between maximum conversion efficiency and both the amplitude and the frequency in a air fuel control system. It also revealed that TWC conversion efficiency attained to maximum levels when both the amplitude and the frequency of the limit cycle are selected so as to make full use of the oxygen storage effect of TWC. In order to achieve this, it is necessary to vary both the amplitude and the frequency arbitrarily.