Diesel fuel lubricity and the benefits of blending with Biodiesel

Lubricity is the ability of a liquid to provide hydrodynamic and/or boundary lubrication to prevent wear between moving parts. Lubricity can also be defined as the ability to reduce friction between solid surfaces in relative motion or the quality that prevents wear when two moving metal parts come in contact with each other.

Although the viscosity of diesel fuel was believed to be related to lubricity, many researchers suggested that the lubricity of the fuel is not provided by fuel viscosity. Researchers found that lubricity is provided by other components of the fuel such as “polycyclic aromatic types with sulfur, oxygen, and nitrogen content.” Oxygen and nitrogen were shown to impart natural lubricity in diesel fuel. Oxygen definitely contributes to the natural lubricity of diesel fuel, but that nitrogen is a more active lubricity agent than oxygen. Diesel fuels that are high in sulfur but low in nitrogen exhibit poor lubricity.

Lowering sulfur or aromatics might not lower fuel lubricity. However, hydrotreating is documented as lowering the lubricity of diesel fuel. The special hydrotreating that is used to reduce the sulfur content of diesel fuel also lowers the lubricity of the diesel fuel. The components: oxygen and nitrogen may be rendered ineffective as a result of severe hydrotreatment to desulfurize the fuel.

It is important to note that some fuel injection system diesel engines rely entirely upon diesel fuel to lubricate the moving parts that operate with close tolerances under high temperatures and high pressure. Rotary distributor injection pumps manufactured by several companies are most susceptible to boundary lubrication wear.

The ways to evaluate the lubricity of a fuel include the following:

(i) vehicle test,

(ii) fuel-injection test equipment bench test, and

(iii) a laboratory test.

The least expensive and most time-efficient of these tests is the laboratory lubricity test.

Fuel-injection equipment tests require 500–1000 h of closely monitored operations. On road vehicle tests require a similar period of time (500–1000 h). The laboratory lubricity test provides a low-cost, accurate evaluation, in <1 wk.

The ASTM D 975 standard specification for diesel-fuel oils does not include a specification for lubricity.

The ASTM D 6078 standard for lubricity is agreed upon by some engine manufacturers in Europe. These companies have selected test procedures to evaluate the lubricating quality of diesel fuel.

The addition of biodiesel, even in very small quantities, has been shown to provide increases in fuel lubricity using a variety of bench scale test methods. Even a small amount of Biodiesel means cleaner emissions and better engine lubrication. Just 1% Biodiesel added to petro-diesel will increase lubricity by 65%, reducing mechanical problems and enhancing the life and efficiency of the engine.