The influence of molecule size and structure on the lubricity of liquids: An experimental study

In the 1980s, the sulphur content of diesel oil was reduced for environmental reasons. As a result of this, breakdowns of diesel engine injection equipment due to extensive wear were observed. It appeared that the cause of this was the contribution of sulphur containing species to the lubricity of the fuel.

The lubricity of a liquid is defined as its ability to protect the surfaces in the boundary lubrication regime. The wear was reduced, as it is today, by addition of anti-wear agents to the fuel. In order to establish the adequate amount of additive, laboratory tests appeared based on a steel ball sliding on a steel disk, the contact only being lubricated by the tested fuel.

The result of a test is the measure of the size of the wear scar on the ball. In 1995 a new clean fuel for diesel engines appeared: Dimethyl Ether (DME). DME is a volatile fuel meaning that it has to be pressurised in order to stay liquid. Recently a lubricity test capable of handling DME was developed [1], as well as a volatile fuel viscometer [2].

As a result of this development it has become possible to test the lubricity of small hydrocarbons such as propane and butane as well as liquids with larger molecules. The results of these tests are very well suited for comparison with the outcome of friction and wear models [3], as only simple molecules can be included in these today.