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This paper describes a measurement method using laser induced fluorescence we have developed for simple simultaneous measurements of piston ring oil film thickness at plural points for internal combustion engines. The findings obtained by the measurements of oil film thickness on both thrust and anti-thrust sides of the piston for a mono-cylinder compact diesel engine using this new measurement method are also discussed in this paper. One of main findings is that the oil film thickness of each ring on both sides differs markedly in terms of the absolute value and the stroke- to-stroke variation. It is found that this difference in oil film thickness is caused by the difference in the amount of lubricating oil supplied to the oil ring, and the effect is greater than that of engine speed or load.

This paper describes measurements of oil film thickness of piston ring packages which have different oil control rings. The oil film thickness measurements were taken at three points, namely, the piston thrust side, front side and rear side, by the Laser Induced Fluorescence Method(LIF). One of the main findings is that the oil film thickness on the thrust side varies greatly from cycle to cycle, while cyclic variations are smaller on the front and rear sides. This difference is due to the smaller inclination of the oil control rings on the front and rear sides, compared with that on the thrust side. It is also found that oil consumption has a good correlation with oil film thickness on the thrust side and that the thrust side oil film thickness becomes thinner as the oil ring becomes narrower.

Offsetting the crankshaft axis with respect to the cylinder axis has been thought to be a method to reduce piston side force[1]. Hence the piston friction is expected to be reduced. An automotive manufacturer has already used the crankshaft offset for a production gasoline engine to improve fuel economy. The authors have conducted research into the effect of crankshaft offset on the piston friction. A single-cylinder engine was modified to have a crankshaft offset. Piston frictional force was measured in real-time by using a floating liner method. In addition, laser-induced fluorescence (LIF) technique was employed to measure oil film thickness on the piston skirt area, and a gap sensor was used to measure piston motion. As a result, the authors concluded that the effect of crankshaft offset on piston friction could not be explained only by its effect on the piston side force. In accordance with the measurement results, crankshaft offset changed piston slap motion.

This study has been aimed at the reduction of the intense piston skirt friction force that appears in the expansion stroke out of all piston friction forces generated in gasoline engines. The friction characteristics at the piston skirt have been analyzed according to the measured results at piston friction forces and the shapes of wears at the piston skirt in actual engine operations. It is found from the above that the majority of the side force working on each piston is supported by the oil film on the skirt, while only some of the side force is supported by the portion in metallic contact with the cylinder. It is also found through experiments that the metallic contact portion has a great effect on the friction force at the skirt. The effect of piston posture in expansion stroke on the friction force has been also analyzed based on the measured results of piston slap motions.