Pressure Effect on the Speed of Sound of Waste Cooking Oil Biodiesel

The knowledge of speed of sound for biodiesel mixed with diesel at high pressures and in a wide range of temperatures is important for optimizing the injection process in diesel engine operation. The experimental speed of sound data in the literature about biodiesel is scarce. For that reason, the authors tried to evaluate the influence of temperature and pressure on the speed of sound of used cooking oil biodiesel (UCO) over the temperature range of 303-343 K and pressure varying from atmospheric up to 25 MPa. For that purpose, an acoustic cell with two ultrasonic probes working in through-transmission was used for speed of sound measurement. The uncertainty for the measurements was estimated as 1.4 m.s(-1). New measurements for 1-butanol, used as a calibration test substance, are reported with good agreement with literature data, with 0.2% differences. Correlations for the speed of sound for 1-butanol and biodiesel were developed for pressure and temperature ranges with accuracy better than the measurement uncertainty. A very recent predictive method for the speed of sound of biodiesel was tested with the data of this work, resulting in average percent deviations of only 0.12%. Differences in the speed of sound between biodiesel and diesel are presented and discussed.