AbstractČUPERA, J., SEDLÁK, P., KARAFIÁT, P.: Diff erences in rheological profi le of regular diesel and bio-diesel fuel. Acta univ. agric. et silvic. Mendel. Brun., 2010, LVIII, No. 5, pp. 67-74 Biodiesel represents a promising alternative to regular fossil diesel. Fuel viscosity markedly infl uences injection, spraying and combustion, viscosity is thus critical factor to be evaluated and monitored. This work is focused on quantifying the diff erences in temperature dependent kinematic viscosity regular diesel fuel and B30 biodiesel fuel. The samples were assumed to be Newtonian fl uids. Vis cosi ty was measured on a digital rotary viscometer in a range of 0 to 80 °C. More signifi cant diff erence between minimum and maximum values was found in case of diesel fuel in comparison with biodiesel fuel. Temperature dependence of both fuels was modeled using several mathematical modelspolynomial, power and Gaussian equation. The Gaussian fi t off ers the best match between experimental and computed data. Description of viscosity behavior of fuels is critically important, e.g. when considering or calculating running effi ciency and performance of combustion engines. The models proposed in this work may be used as a tool for precise prediction of rheological behavior of dieseltype fuels. diesel fuel, biodiesel, kinematic viscosity, modeling Diesel fuel viscosity has a signifi cant impact on how well fuel (either regular or biodiesel) burns. As viscosity increases, fuel molecules cluster together. The molecules on the outside of the cluster mix well with oxygen during combustion and burn well. The molecules on the inside of the cluster do not mix as well and do not burn completely. Knowledge of fuel viscosity is important for several reasons, as it determines the temperature for handling, the size of the centrifuges and the temperature at which the fuel is injected into the engine. It is well known that as the temperature of the fuel is increased, the viscosity is reduced, but diff erences between individual blends or e.g. regular and biodiesel can be expected. Some fuel suppliers publish temperature/viscosity charts which are based on the average data of a large number of samples. However, estimations from the charts cannot be regarded as precise since the exact relationship depends on the source and composition of the fuel. The viscosity usually quoted is the kinematic viscosity expressed in centiStokes (cSt) at some reference temperature. Although ISO 8217 has a reference temperature of 100 °C, it is anticipated that this will be changed to 50 °C at the next revision. This is a value widely used throughout the industry. A few extra degrees of preheating can o en make a fuel suitable for injection equipment. Because of the viscosity/temperature relationship, a few degrees change could make a big diff erence to the injection viscosity. In practical terms, this means that if the actual fuel viscosity is greater than that ordered, it is likely that the fuel heater can accommodate this.Biodiesel refers to a vegetable o...