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The idling stop system for scooters employs an ACG starter connected directly to the crankshaft without reduction gears; therefore, it is possible to start the engine silently. The system is designed to stop the engine automatically when the scooter stops, and the engine restarts simply by opening the throttle. Scooters with 50 cm₃ engines have already been employing the idling stop system. This system for 125 cm₃ engines was developed to enlarge the application range of the idling stop system. It needs a large ACG starter because the cranking torque is higher than that of 50 cm₃ engines, so the cranking torque was reduced by using a decompression device. The ACG starter was designed with a thin and large diameter construction in order to minimize the increase in engine width. The new idling stop system was developed with an integrated control of the fuel injection system and the ACG starter.

Exhaust gas recirculation (EGR) systems are used as a technology to reduce NOx emissions. However, the charging of the fresh air-fuel mixture decreases due to the inflow of the burnt gas from the EGR system, and the drivability deteriorates. Therefore, an internal EGR system that uses the thermal energy of the burnt gas is generally designed. Such a system employs a variable valve timing mechanism using the hydraulic or the electromagnetic to reduce the exhaust emissions and the fuel consumption and to obtain favorable drivability, generally. However, because such a system is expensive, there are few small motorcycles with the internal EGR system. In the present study, the developed internal EGR system that slightly opens the exhaust valve at the beginning of the compression stroke or the end of the intake stroke has been designed. With this opening and closing timing, the charging rate of fresh air-fuel mixture decreases very little because the intake stroke nearly completes.