A prototype of measurement device that compresses a sample of engine oil at constant temperature and calculates its void fraction from the magnitude of volume change and pressure was proposed. During compression, the oil sample was pressurized to several hundreds of kPa above atmospheric pressure. Because the gas can be regarded as an ideal gas at this pressure level, the estimation of void fraction can be based on a simple formula derived from the ideal gas law, the law of conservation of mass and Henry’s law. The calibration line is represented by a linear equation of the void fraction, and from the coefficient of void fraction or the constant term the volume fraction of the dissolved gas in the initial state can be known. That is, by experimentally determining the calibration line, not only void fraction but also the volume fraction of the dissolved gas in the initial state can be known. Then, the results of measurement principle confirmation tests were given. Initial tests have revealed that the output of the device is highly repeatable over a range of void fractions from 0.05 to 0.30, and sample oil temperatures from 20 °C to 95 °C. A temperature dependence of the calibration lines was observed, although the effects were found to be minimal above 40 °C, particularly at higher void fractions. Finally, the measurement void fractions of the engine oil in the passage that is diverted from the sump bottom of the running engine (motored) were showed. The void fractions were less than 0.05. Measurements of void fraction were carried out in two ways: analysis of bubbles in the engine oil image data and the proposed method.
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