Final answer:
Viscosity is relevant to the use of biodiesel as it affects the flow and lubrication in engines. Biodiesel with high viscosity may cause performance issues, while low viscosity may not provide sufficient lubrication.
Step-by-step explanation:
Viscosity is a measure of the thickness or resistance to flow of a liquid. When it comes to the use of biodiesel, viscosity is relevant because it affects the flow of the fuel in engines. Biodiesel with a high viscosity may have difficulty flowing smoothly through the engine, leading to performance issues. On the other hand, biodiesel with a low viscosity may not provide sufficient lubrication. Therefore, it is important to consider the viscosity of biodiesel when using it as a fuel.