Lubricity

Lubricity is the measure of the reduction in friction of a lubricant. The study of lubrication and mechanism wear is called tribology.

Measurement of lubricity
Lubricity of a material cannot be directly measured, so tests are performed to quantify a lubricant's performance. This is done by determining how much wear is caused to a surface by a given friction-inducing object in a given amount of time. Other factors such as surface size, temperature, and pressure are also specified. The greater the wear scar the worse the lubricity. For this reason lubricity is also termed a substance's anti-wear property.

Examples of test setups include "Ball-on-cylinder" and "Ball-on-three-discs" tests.

Lubricity in diesel engines
In a modern diesel engine, the fuel is part of the engine lubrication process. Diesel fuel naturally contains sulfur compounds that provide good lubricity, but because of regulations in many countries (such as the US and the EU) sulfur must be removed. Reformulated diesel fuel has a lower lubricity and requires lubricity improving additives to prevent excessive engine wear.