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A hybrid system combining a 4-stroke, 50cc-gasoline-engine with an electric motor was developed, and the maximum vehicle speed achieved by the electric motor is 30 km/h. Either an electric motor or engine is selected as the power source according to the running condition, and it is switched automatically. A parallel hybrid system, the Modulated Hybrid System (MHS), was adopted. Therefore, the energy source of the electric motor can be charged by an external or an internal power source. The switch mechanism of the driving power source is simple by using a one-way-clutch. The driving force, the vehicle speed, and the remaining battery energy are the parameters for switching control of the driving power sources. In order to achieve smooth switching and quick responses, the electric motor output is controlled by the feedback of the driving torque.

With the interest in global environmental issues growing in recent years, the demand for the reduction of exhaust gas emission and improvement in fuel consumption for small motorcycles has increased greatly. Recently, small motorcycles have been marketed equipped with an electronically controlled fuel injection system effective in reducing emissions and enhancing fuel consumption by accurately controlling the air-fuel ratio. The small motorcycles' market comprises mainly ASEAN countries, and the majority of the motorcycles consist of reasonably priced models with air-cooled engines. Fuel injection systems have already been adopted for motorcycles equipped with water-cooled engines in the markets of advanced countries, mostly in EU. Given the above situation, two issues must be addressed to adopt a successful fuel injection system for air-cooled, low-priced small motorcycles.