Dimethyl Ether: New Advances in Wear Testing: Theoretical and Experimental Results

The issues addressed in this paper are investigation of the wear mechanisms present in the standard lubricity test for diesel oil: The High frequency reciprocating Rig (HFRR). The HFRR is a laboratory wear test using a ball on disk configuration. The result of a test is the wear scar diameter (WSD) on the ball.

Up to now, all analyses indicated that fuel viscosity influences the wear scar size and fuel performance in full-scale pumps. The wear scar size could then be a result of hydrodynamic lubrication (at least a significant part of it) and not of boundary lubrication as it was the original intention of the test.

The appearance of an excellent volatile fuel for diesel engines, Dimethyl Ether (DME), has resulted in new wear tests such as the Medium Frequency Pressurised Reciprocating Rig (MFPRR), a pressurised version of the HFRR. DME has a about 25 times lower viscosity than diesel oil so the MFPRR viscosity sensibility issue is seriously aggravated for this fuel.

Molecular dynamics calculations involving straight alkanes with lengths from 3 to 14 carbon atoms have been performed. The model is based on simple inter-atomic and surface interactions and it simulates an asperity contact between curved surfaces with long-range elasticity. This last property has enabled the model to correlate well with experimental results.

The outcome of the alkane calculations indicates that the longer ones lubricate better than the shorts ones but not necessarily because of viscosity differences. The reason is more likely that the long alkanes are able to squeeze more atoms into a contact than a short one when the lubrication film has a thickness similar to a few molecular diameters.

Recent MFPRR wear tests involving short straight alkanes and their branched isomers show that different wear scar diameters are obtained for a straight and a branched alkane isomers although they have almost the same viscosity. This result indicates that other fundamental properties than the viscosity, such as the molecule structure, can influence the lubricity outcome of the MFPRR.

The inclusion of atoms into the contacts has the potential of explaining this phenomenon and, in the near future, a search for optimising this atom inclusion mechanism will be initiated both by experiments in the MFPRR and molecular dynamics calculations.