Cold, green and clean: reduction of parasitic losses in zero emission cryogenic expanders

Stead, Iestyn M N ORCID: 0000-0003-0573-1761 (2021). Cold, green and clean: reduction of parasitic losses in zero emission cryogenic expanders. University of Birmingham. Ph.D.

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Abstract

Dearman were a start up company manufacturing engines utilising a waste product, liquid nitrogen, to extract clean and cold power. The test case of the Dearman engine is a truck refrigeration unit (TRU), used on refrigerated delivery lorries; accounting for a large proportion of the total particulate emissions in refrigerated transport. The Dearman Engine reduces the total emissions from these refrigerated lorries however there is room for improvement and refinement to advance the technology closer to the maximum potential efficiency of the engine. This will reduce emissions from the system and the impact of refrigerated transportation on the environment. This thesis is focused on the tribology of
the engine, reducing the parasitic losses through a three pronged approach: new materials, new lubricants and new designs. The Dearman Engine has a low power output compared to an internal combustion engine (ICE) and as such parasitic losses have the potential to
absorb a significant amount of the power generated.

The cylinder liner — piston seal interaction was identified as the key contributor to friction in the engine. A material replacement investigation of the cylinder liner was undertaken through tribological specimen testing. A poly-tetrafluoroethylene (PTFE) cylinder, representative of the piston seal, was reciprocated against a number of polymer matrix composites (PMCs). The investigation compared the PMC to the current material used in the Dearman Engine: honed aluminium. The benchmark coefficient of friction at 12 C under a contact pressure and reciprocating frequency representative of the Dearman Engine was 0.14. After investigation it was determined that unhoned poly-oxymethylene (POM) was the most beneficial polymer to implement in this area with a coefficient of friction of 0.08 at 12 C. Unhoned POM also outperformed the benchmark in terms of settling time and percentage overshoot: summary statistics that were linked to the wear of the PTFE cylinder.

The potential of a laminated polymer as an alternative to the current piston seal, a composite, was also investigated: providing a saving in manufacturing costs and required energy of manufacture. Samples were tested under contact pressures representative of the pressures in the Dearman Engine. There was no significant difference between the coefficient of friction and wear of a PTFE-PEEK composite and a PTFE-PEEK laminate at the same ratio of PTFE:PEEK (20%).

In an attempt to negate the need for an oil pump two fluids were examined as lubricants and potential heat exchange fluids (HEFs); one based on pectin and the other on bovine serum albumin (BSA). HEF is the working fluid that ensures efficient expansion of N2 within the engine. It was shown that both fluids formed lubricating films and had the potential to be successful industrially viable lubricants. Pectin solutions were more sensitive to the concentration, where as BSA demonstrated less variation in the coefficient of friction between the two concentrations. At high loads BSA produced the lowest coefficient of friction; although at both loads tested 5mg/ml pectin was capable of producing low coefficients of friction and as such in a complex system may be a better solution. The study identified that these lubricants have the potential to be a replacement
for hydrocarbon based lubricants under the conditions tested.

Data analysis also led to improvements in the processing and potential insights inferred from tribological data: utilising a higher sample rate to investigate the effects of viscoelastic properties of polymers and lubricants on friction. Traditionally data is collected at a lower sample rate; this is a brute force approach to tribology and increasing this sample rate gives insights far exceeding the information present in traditional data analysis. The key feature that is missed in reciprocating tests is the area at the extremes of the stroke where a potential lubricant layer is broken down and there is stick-slip behavior resulting from asperity contact and a potential increased wear rate.

Combining a biomimetic lubricant/heat exchange fluid with a POM cylinder liner and a laminated PTFE/POM piston seal has the potential to reduce the parasitic losses and overall weight of the TRU. New analytical techniques will assist in future testing of viscoelastic materials and lubricants. This thesis has provided the initial steps in developing the Dearman Engine from a tribological perspective: potentially advancing the technology’s industrial readiness and ensuring that the environmental ethos of the company is maintained.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):