Kinematic viscosity is an important parameter of diesel, biodiesel. It reflects flow ability and lubricity of diesel–biodiesel ‘B’ blends.

Viscosity is very complex property of fluids, especially fluid mixtures such as hydrocarbon fuels, thus no be explained in a few sentences. Avoiding definition and analysis of Newton’s and non-Newton’s fluids, this blog post is attended on viscosity influences related to diesel and biodiesel fuel.Term Viscosity refers to two quantities: Kinematic Viscosity and Dynamic (Absolute) Viscosity (and many subs of them). Basically, those are same, but different in practical analysis of fuel quality. Kinematic viscosity is the measure of fluids’ resistance to flow and shear under the forces of gravity. It is value of dynamic viscosity divided by density. Diesel composed by paraffin as well as biodiesel by methyl esters. It means that each diesel, fossil or bio is composed by large hydrocarbon molecules. As larger molecules as flow resistance gets grater. It means that kinematic viscosity becomes higher with molecule size increasing. It often makes a mass that viscosity depends on density. Quantities are related, but not depended. For example, diesel has about 4 times higher kinematic viscosity then water, although water has higher density. Also, diesel has higher viscosity and higher density then gasoline. No rules. Both, Kinematic and Dynamic Viscosity strongly depends on temperature. Increasing the temperature decreases the viscosity. Viscosity dependence on temperature is defined and known for pure substances, but diesels are most mixture of varying of compounds. No rules, equations and linear dependences for calculations. It is being determined by laboratory measurement for particular fuel sample.
Diesel and Biodiesel kinematic viscosity limits below are in accordance with ISO international standardized specifications. It may slightly vary in national standards.

Kinematic viscosity of Diesel

Diesel is mainly composed by paraffines and aromatic hydrocarbons. Composition creates kinematic viscosity 2.5 – 4.0 mm2/ s at 40°C. A value lower the minimum limit probably means that diesel has bed lubricity characteristics. Value higher the maximum probably means that diesel will have bad flow characteristics at lower temperatures, and high pure point. It may make the problems in cold climate areas.

Kinematic viscosity of Biodiesel

Biodiesel is mainly composed by methyl and/ or ethyl esters. Esters create higher biodiesel kinematic viscosity then fossil diesel. There are differences in national specification related to biodiesel density. For example, ASTM is more flexible in this case. They specified 1.9 - 6.0 mm2/s while European EN 14214 specified 3.5 - 5.0 mm2/s. Both limits are set at 40°C. Obviously, biodiesel blends B10, B20, B50 and B100 have kinematic viscosity 2 – 6 mm2/s at 40°C. It depend on amount of diesel and biodiesel particularly and their compositions.

Kinematic Viscosity Measurement

Kinematic viscosity measurement has to conform to the requirements in each: ASTM D445, D446, D7279, IP 71 or ISO 3104, 3105. Kinematic viscosity of diesel and biodiesel can be measured by glass capillary tubes. All organizations for standardization set standard measurement temperature 40°C. It requires laboratory water bath able to maintain constant temperature. Every temperature oscillating will cause error in recorded values. By multiplying the time taken by the factor of the viscometer, the viscosity value is obtained. It is manual kinematic viscosity measurement. The price of measurement set is about US$1000.

Kinematic Viscosity can be measured by automatic viscometers in purpose to obtain more accurate results. The principle is same, thought temperature, time record and all measurement preparation is done and controlled by software. The measurement operation needs not human monitoring. Average price of automatic viscometer device is about US$30.000.