High-Pressure Viscosity of Biodiesel from Soybean, Canola, and Coconut Oils

Current and future injector designs for diesel engines approach pressures of greater than 100 MPa. However, the high-pressure physical properties, such as viscosity, of biologically derived diesel fuel (biodiesel) arc nearly absent in the literature. This study focuses on the viscosity of biodiesel samples, fatty acid methyl esters (FAMEs), derived from soybean oil, soybean oil from Vistive soybeans, canola oil, recycled canola oil that has been used in cooking and frying, and coconut oil from 283.15 to 373.15 K and pressures up to 131 MPa. Petroleum-derived diesel (ultra-low sulfur, number 2 diesel) has also been investigated to compare to the biodiesel samples. The viscosity of the samples increases linearly with pressure until approximately 35 MPa, followed by a higher order response to pressure. Except for coconut-oil-derived biodiesel, the biodiesel samples have viscosities that are greater than petroleum-derived diesel at both ambient and elevated pressures. However, at lower temperatures and high pressures, the diesel and biodiesel samples become more similar. The viscosity of the biodiesel samples with pressure can increase nearly 300% over the pressure range investigated over their respective ambient-pressure viscosity; number 2 diesel increases up to similar to 400% over similar pressures. The biodiesel samples at 283.15 K were found to experience pressure-induced cloud points (solid-liquid equilibrium) from 70 to 100 MPa, which significantly increases their viscosity. The Tait-Litovitz equation was found to correlate the data very well over the large range of both temperature and pressure.