Although portable electric devices have become increasingly sophisticated and compact, the amount of energy required for their operation has increased and device performance may be restrained by the energy source in the near future. A small power source is also needed for mobile robots for home usage. Small, portable distributed power sources with higher energy density than the conventional battery are required. Since the energy density of hydrocarbon or hydrogen fuel is quite high compared to the battery, combustion micro engines that use these fuels are promising for this purpose. In this research, we focused on the small glow-ignition engine developed for model airplanes. Since the glow engine does not require an auxiliary electric circuit for ignition, it is suitable for lightweight miniature engines. However, unlike the spark-ignition engine, there is no equipment that controls the ignition timing. In the glow-ignition system, heat is supplied by the following three methods for ignition of the air-fuel mixture: (1) Heat produced from the compression of the air-fuel mixture; (2) Heat remaining in the element from the last explosion; and (3) Heat produced by the catalytic action of an ignition element. In such an ignition system, the combustion state may differ between cycles. In order to clarify the combustion process, a miniature pressure sensor was built into the cylinder head, and pressure fluctuation was measured. Analysis was conducted to detect cycle-to-cycle variation. The base engine used was a commercial glow-ignition four-stroke engine with a swept volume of 4.89 cc. This is a high-speed engine with a maximum engine speed exceeding 14,000 rpm. Although the nominal output was 368 W, the value measured by this research was much lower. Fuel consumption was measured by placing a fuel tank on an electronic balance. The fuel used for the experiment consisted of mainly methanol, nitromethane, and lubricant. Weight ratio of these components and air/fuel ratio were changed independently, and the effect on the combustion pressure was evaluated.

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