Towards a plastic engine: Low—temperature tribology of polymers in reciprocating sliding

Fundamental tribological studies of a range of engineering polymers, including a bio-derived Polytrimethylene-terephthalate (PTT) have been conducted at low temperatures (12
and −8
). The aim of the study was to replace a conventional honed aluminium liner/polytetrafluoroethylene (PTFE) seal interface with a polymer combination, within a Dearman Engine. The Dearman Engine is a novel piston engine driven by the expansion of a cryogenic fluid, either liquid nitrogen or liquid air, to produce emissions-free combined cooling and power. A tribometer was used to simulate engine conditions.
The efficacy of the polymers relative to the honed aluminium benchmark was assessed in terms of a projected material transfer layer. The magnitude of which was determined using a combination of scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDS) and Abbott—Firestone curves, a descriptor of surface texture. The concept of a frictional overshoot, correlated with the severity of wear measured on the PTFE specimen, is introduced.
For the engine, a low coefficient of friction (CoF) leads to reduced parasitic losses and low wear will ensure the integrity of the piston seal is maintained, and so to the sealing capacity of the engine. Material transfer, generally from seal to liner specimens was detected in all but the unhoned polyoxymethylene (POM) and unhoned Aluminium samples. These two combinations had the lowest frictional overshoots and the lowest levels of wear. These are properties that were also linked to a low surface roughness and a low gradient of the interquartile range of the Abbott-Firestone curve. These materials both outperformed the benchmark seal/liner combination and the POM provides the potential for an all polymer expansion chamber.