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This paper deals with the generation mechanism of exhaust port clogging from carbon deposit and visible exhaust smoke as well as the method to reduce them. Consequently, it was found that the selection of appropriate exhaust port shape and the reduction of distillate of high boiling point mineral oil in the engine oil were effective to reduce exhaust port clogging from carbon deposit. While, increased exhaust gas temperature at the rear outlet end of the muffler and reduced reaction energy (measured by D.S.C.-a method for thermal analysis) in the engine oil were found to be effective to reduce visible exhaust smoke. It is desired that not only engine and related components but also oil should be improved from now on.

Effect of base stock material, especially polybutene, on two-cycle engine oil performance was studied. At first, visible smoke tests were conducted, using test oils of different base stocks. Polybutene showed the best performance among the test oils. Then, piston detergency tests were performed. Test oil of higher molecular weight polybutene showed poorer results. In addition, piston deposits were analyzed and it was found that polybutene molecular weight was increased during the test. Piston detergency was discussed in relation with the increase in viscosity caused by increase in molecular weight. Finally, engine startability tests were carried out. Good correlation with viscosity was found only in test oils of same mineral base stock concentration. The increase in engine start torque was attributed to increase in polybutene viscosity caused by selective evaporation of the lower molecular weight portion.

Two-stroke gasoline engine is lightweight, compact and has numerous advantages. However, because of its inherent engine configuration, engine oil is used as once-through system and is discarded, resulting in exhaust smoke in some occasions. In order to reduce the exhaust smoke, it is imperative to make improvement not only in engine itself but also in oil used. No standard method was available to evaluate engine smoke performance. This study was initiated to devise a simple evaluation method. In two studies reported here, evaluation of oil alone and evaluation of oil performance in actual engines are given. Following two facts were found as a result. The first item is the fact that oil can be evaluated by any actual engine performance evaluation method once the engine used and the method used are specified. The simplest and the most accurate method among the actual engine performance methods is the method to use the small two-stroke electric generator.