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The field of diesel combustion research is producing numerous reports on studies of premixed combustion, which promises simultaneous reduction of both NOx and soot, in order to meet increasingly stringent regulations on harmful emissions from automobiles. However, although premixed combustion can simultaneously reduce both NOx and soot, certain issues have been pointed out, including the fact that it emits greater quantities of unburned HC and CO gases and the fact that it limits the operating range. Furthermore, this combustion method sets the ignition delay longer with the aim of promoting the mixing of fuel and air. This raises issues with the product due to the combustion instability and sensitivity to the uneven fuel properties that are found on the market, the capability of the engine response under transient conditions, the deterioration in combustion noise, and so on.

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.