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We attempt to improve the feeling of acceleration by considering the change in instantaneous torque when the throttle is opened in motorcycles equipped with parallel two cylinder engines. Feeling of acceleration and vibrations for motorcycles with five types of 400cc engines, including 3 types of engines with different crankpin phase angles varying by 90° each along with two variations in the firing interval were assessed by sensory evaluation and instruments. As the result, it was found that there is a correlation between the feeling of acceleration and the change in instantaneous torque caused by alteration of the crankpin phase angle. It was therefore determined that the feeling of acceleration could be improved by using a crankpin phase angle of 270° relative to crankpin phase angle of 180 or 360° conventionally used in parallel two cylinder engines.

This paper presents the characteristics of a Variable- Radius/Length engine (V-R/L engine) which has an eccentric on the connecting rod big end. This is intended to improve the power output of short-stroke multi-valve engines. The V-R/L engine makes it possible to improve power output through high volumetric efficiency and to delay the ignition timing for best torque (MBT) through refined combustion-chamber shape. However, more improvements are required to apply the mechanism to high-speed engines because of increased friction losses.