Speed of sound in pure fatty acid methyl esters and biodiesel fuels

The property changes associated with the differences in chemical composition of biodiesel may change the fuel injection timing which in turn cause different exhaust emissions and performance of engines. The property that has an important effect on the fuel injection timing is the speed of sound (related with isentropic bulk modulus). Despite the speed of sound of pure fatty acid (methyl and ethyl) esters being reasonably known in a wide range of temperature the experimental data for biodiesel are very scarce in the literature. In this work the speed of sound of six fatty acid methyl esters (FAME = laurate (MeC12:0), myristate (MeC14:0), palmitate (MeC16:0), stearate (MeC18:0), oleate (MeC18:1), linoleate (MeC18:2)) and six biodiesel fuel samples were measured using a non-intrusive ultrasonic methodology. The measurements for FAMEs were made at atmospheric pressure from a minimum of 288.15 K to a maximum of 353.15 K, and in the temperature range 298.15-353.15 K for biodiesel samples. The uncertainty of the measurements was estimated as less than ±1 m s−1. The speed of sound data combined with available density data from literature was used to calculate the isentropic compressibility and the molecular compressibility for the FAMEs and for the biodiesel samples. The results for molecular compressibility evidenced that this property is almost independent of the temperature in the temperature range of calculations both for FAMES and biodiesel. Linear relationships were established between the molar compressibility and the molecular weight for FAMES and biodiesel. The before mentioned behavior of molar compressibility face to temperature and molecular weight make it possible to develop prediction methods for the calculation of the speed of sound.