The kinematic viscosity is a measurement of a fluid’s thickness or resistance to flow over time. For biodiesel, the viscosity is an important factor when it comes to the performance of the fuel in an engine. Both low and high viscosities can have detrimental effects on an engine’s performance, with low viscosities not providing sufficient lubrication for the precision fit of fuel injection pumps (resulting in leakage or increase wear), and high viscosities leading to the formation of large droplets on injection (resulting in poor combustion, increased exhaust smoke and emissions). 
Significance
ASTM test method D445 is the preferred method of measuring the kinematic viscosity of biodiesel. A biodiesel sample is placed inside a calibrated viscometer, which is then placed inside a constant temperature kinematic viscosity bath set to 40°C. Once in the bath, the time it takes for a fixed volume of liquid to flow under gravity through the capillary of the viscometer is measured. Using the time and the calibration constant of the viscometer, the kinematic viscosity is calculated. The acceptable viscosity range for biodiesel according to ASTM D6751 is between 1.9 and 6.0 mm2/s. The upper limit for biodiesel is higher than the upper limit for regular diesel fuel (4.1 mm2/s) and should be taken into account when looking at blends.
