Frost, James; (2022) A systematic study of how molecular structure influences combustion and pyrolysis of novel renewable fuels in a diesel engine and high temperature reactor. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Internal combustion technology has been employed as the primary mode of propulsion for transportation since the early 1900s, with continual improvements in efficiency ever since. Despite this, issues associated with the fuels combusted have caused a shift towards electrification, but this is not suitable for all applications and geographies. It is therefore necessary to seek sustainable fuels, in conventional engines, for the short to medium-term alleviation of CO2 emissions, fossil fuel usage and toxic pollution detrimental to air quality. The work presented uses a compression ignition (CI) engine to test a range of novel fuels, with proven sustainable sources, to detect their impact on exhaust emissions and combustion. Initially, a range of 2nd generation furanic biofuels were tested, in order to understand the effect of molecular structure on combustion. Saturated cyclic molecules with carbonyl side chains possessed strong ignitability, while alleviating particulate mass (PM); these molecular attributes are indicative of lactones. Lactones were therefore tested within the CI engine, with a subsequent investigation utilising a pyrolytic reactor to help understand soot formation from these fuels, since soot is initially formed from gaseous precursors. Gas Chromatography- Flame Ionisation Detection (GC-FID) analysis was used to detect soot-precursor concentrations in the breakdown of a lactone compared to other C10 fuels. It was determined that, despite minor differences in species present and differing precursor abundance, the pattern of pyrolysis products was similar for all fuels; ethylene was most prevalent, with lower levels of C3s (propene) and C4 (1,3-butadiene) gases. Preliminary investigations were undertaken to detect molecular breakdown of fuels in the engine itself, employing in-cylinder sampling to collect cylinder contents. Minor differences were noted between engine and reactor, likely due to air-derived oxygen in the engine. Finally, an investigation was conducted to gauge the impact of a hydrogen fuel carrier (ammonium hydroxide) on diesel combustion. This is less hazardous than ammonia itself, but the presence of water was shown to be detrimental to combustion efficiency at lower engine loads, while the fuel- bound nitrogen appeared to contribute to greater NOx emissions at comparable engine loads.

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