An experimental study on high pressure sprays and on the combustion and sooting tendencies of oxygenated fuels

Abstract

Carbon dioxide (CO2) is a global threat, so continuing to improve technologies such as diesel engines that already emit low CO2 levels is vital to ensure the sustainable future of transportation. Internal combustion engines (ICEs) are powerful, but the depletion of fossil fuels and the detrimental effect of emissions on health, the environment, and engine efficiency, are driving innovations in combustion. An emission of concern is soot. Advancing the understanding of injection and combustion processes, especially of less-characterised transients, can help optimise combustion to reduce soot. The use of oxygenated fuels is also promising, but studies of long carbon-chain ones are still needed. The aim of this research was to address these two areas to contribute to improving ICEs, focusing on soot. Experiments were performed in a constant volume chamber, and data were collected using high-speed imaging techniques. The first part of this thesis explored the evolution of diesel transients under inert and reactive conditions. Early injection and end of injection (EOI) transients were investigated under inert conditions for nine pilot-dwell-main combinations. During early injection stages, the main spray’s penetration was governed by collision and wake effects; increasing the dwell generally reduced collision. Changes in dwell also affected EOI expulsions after the pilot. For reactive conditions, soot was characterised for single injections after the EOI (aEOI) at ambient temperatures between 860 K and 1310 K. Near-nozzle soot aEOI formed faster as temperature increased. It was revealed that aEOI, up to 75% of soot formed recessively from the flame base and the rest formed progressively from the nozzle. The second part of this thesis identified that C8 oxygenates have similar thermo-physical properties to diesel, showing potential to be alternative fuels. Their combustion was studied neat and as drop-ins. Spatio-temporal flame data were obtained using two-colour pyrometry; the results showed that all fuels had similar flame temperatures, between 1700 K and 1800 K. The sooting tendency increased in the order of ester < alcohol ≤ ketone < diesel, with reductions relative to diesel between 83% and 71%. Dilution and oxygen effects had a larger influence on soot reduction than moiety-specific effects. The C8 oxygenates were also blended with dodecane to have a matching oxygen content of 2.5% and a cetane number of ~ 52. The sooting tendency increased in the order of ketone ≈ alcohol < ester < aldehyde < diesel, with reductions relative to diesel between 55% and 41%. Octanone and octanol had the highest soot reduction potential, but all oxygenates constrained high sooting regions to smaller areas and all reduced soot aEOI. The results showed that even as drop-ins, all oxygenates reduced soot significantly relative to diesel: they show potential to further drive the improvement of ICEs.