Development of Technologies for Improving Fuel Economy of Small Motorcycle Engines

A single cylinder gasoline engine of displacement 125 cm³ for prototype was developed, and the fuel economy of this engine was improved by reducing friction and improving combustion. For reducing friction, various methods were innovated; enhancement of the oil film retention by modifying the striation finish on piston skirt, adding the needle bearing to the rocker arm shaft, press-fitting the bush into the small end of connecting rod, reducing contact pressure with the piston ring, and spray coating molybdenum disulfide onto the shift fork. By innovating these friction reduction methods, the friction of the engine was reduced by 6% compared to the previous model which was already employed the roller rocker arm and the offset cylinder. For improving combustion, the swirl control valve (SCV) was installed into the intake port. By the effect of SCV, both improvement of engine power and improving combustion under low load was achieved. As a result, the fuel economy in ECE40 mode was improved by 7%, and the maximum power was improved by 8% compared to the previous model.