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A periodic impulsive sound at idle is occasionally described as ‘disagreeable’ in two-stroke engines. The relation between combustion conditions, piston vibrations, and the disagreeable sound is analyzed to clarify the phenomena. Some means to alleviate disagreeable sound are then proposed through stabilized combustion, high rigidity sound transfer systems, and refined skirt profiles. Experimental results are shown for the effects on main three factors evaluating disagreeable sound-loudness, impulsiveness, and frequency characteristics. In addition, piston behavior is measured, and the relation between piston motion and disagreeable sound is discussed in this paper.

A new noise evaluation meter has been developed, which provides a new index for the noisiness of idling engines. In this research, auditory sensations of motorcycle engine noises have been analyzed using multidimensional scaling techniques. As a result, it has been found that the principal hearing factors are loudness, impulsiveness, and frequency characteristics of noise. These factors are quantified with an A-weighted sound level, a fluctuation level, and a high frequency level, respectively. An engine noise evaluation meter has been made, on the assumption that a total evaluation level is obtained by a linear combination of the physical quantities. The functions of the meter are to measure the physical quantities of noise, to calculate the total evaluation level, and to indicate the results. Then, actual engine noises have been evaluated by the meter, and there is a high correlation between the sensory evaluation values and the total evaluation levels.