Thermophysical properties of rapeseed oil methyl esters (RME) at high pressures and various temperatures evaluated by ultrasonic methods

Investigation of the high-pressure thermophysical properties of biofuels, e.g., bulk modulus, surface tension, and viscosity is of paramount importance in fuel injection systems in diesel engines. Another crucial and dangerous phenomenon that may occur in biofuels at high pressures is phase transition (solidification), which can drastically increase the viscosity of the biofuel. This effect may hamper proper operation of the engine, especially under cold-start conditions. Unfortunately, the availability of high-pressure thermophysical properties of biofuels is still limited. The goal of this paper is to investigate the impact of high pressures on thermophysical properties of biofuels on the example of rapeseed fatty acid methyl esters (RME) in a wide range of pressures (0.1to250MPa) and temperatures (5to20°C). To this end we employed innovative ultrasonic techniques, i.e., the Bleustein-Gulyaev surface acoustic waves for measuring RME viscosity, and ultrasonic bulk compressional waves for measuring sound velocity in RME and consequently evaluating RME thermophysical parameters, e.g., bulk modulus and surface tension. The viscosity of the measured RME displayed an abrupt increase at pressures: 260 MPa (t=20°C), 230 MPa (t=15°C), 190 MPa (t=10°C), and 130 MPa (t=5°C). Evidently it was a signature of the phase transition (solidification) occurring in the RME. The discovered high viscosity high-pressure phase in RME can be very detrimental for operation of modern common rail Diesel engines. Therefore, the results of research presented in this paper should be interesting for engineers and designers working with modern common rail Diesel engines using biofuels.