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Hybrid vehicles (HVs) are becoming more widely used. Since HVs supplement engine drive with motor power, the lubricant oil temperature remains at a lower level than in a conventional gasoline vehicle. This study analyzed the effect of cylinder bore temperature and lubricant oil temperature on engine friction. The results showed that, although the lubricant oil temperature was not relevant, the bore temperature had significant effect on piston friction. It was found that raising the temperature of the middle section of the cylinder bore was the most effective way of reducing piston friction.

A study of diesel fuel as a lubricant for diesel engines was conducted with the aim of dramatically reducing engine friction and eliminating the need to change the lubricating oil. A prototype single-cylinder engine modified for diesel fuel lubrication was made, and it was confirmed that firing operation is possible. Piston friction during the firing operation was reduced by modifying the shape of the cylinder liner surface to improve the retention of the lubricating oil. The study produced valid findings concerning engine lubrication, not only with diesel fuel, but also with ultra-low viscosity oil.

A newly developed fluorescent diagnostic system can measure the oil film thickness distribution through two-dimensional images with high sensitivity and quick response. This system consists of a filtered Xe-flash lamp, a filtered charge-coupled-device video camera, a personal-computer-based signal analyzer and lubricating oil with the fluorescent dye added. We have installed the system in a single cylinder research engine and measured the oil film thickness distribution around the piston under various operating conditions. One example of the results is that an oil spout from a gap in the first compression ring, which agrees with the measured oil consumption rate of the engine, was clearly observed at high engine speed.