Summary
Highlights
Engines are designed for fuels with specific viscosities. Kinematic viscosity, which is the ratio of absolute viscosity divided by fluid density, is crucial for drop-in biofuels to perform similarly to petroleum-based fuels. It's measured in centistokes or millimeters squared per second, and is more relevant for understanding flow behavior.
Biodiesel typically has a slightly higher kinematic viscosity (around 4.5 centistokes) compared to petroleum diesel (around 3 centistokes). The acceptable range for biodiesel according to ASTM D 6751 is between 1.9 and 6.0 mm²/s, which is higher than the 4.1 mm²/s upper limit for normal diesel fuel.
Low viscosity fuels can cause leaks through seals, while high viscosity fuels are difficult to pump and can lead to poor atomization, resulting in inefficient combustion and increased harmful emissions.
Temperature and the feedstocks used for biodiesel production both influence kinematic viscosity. Kinematic viscosity for both biodiesel and diesel decreases as temperature increases. Although the rates are similar, there's a greater difference between biodiesel and petroleum diesel at lower temperatures. Blending biodiesel with petroleum diesel reduces the kinematic viscosity compared to pure biodiesel, which is recommended for colder temperatures.
To measure kinematic viscosity, first clean the viscometer with suitable solvents and dry it with filtered air. To charge the sample, invert the instrument, apply suction to the large tube while immersing the small tube in the liquid sample, and draw the liquid to the second mark. Clean and return the instrument to its vertical position.
Place the viscometer in a holder, insert it into a constant temperature bath (40°C or 100°C), and allow 10-15 minutes for the sample to reach the bath temperature. Apply suction or pressure to draw the liquid slightly above the top mark. Measure the efflux time as the liquid flows freely past the second mark down to the first mark.
Calculate the kinematic viscosity by multiplying the efflux time (in seconds) by the viscometer constant. Repeat the test, calculate again, and then average the values to report the final kinematic viscosity of the fuel.