Koivisto, EK; (2016) Ignition and combustion of future oxygenated fuels in compression-ignition engines. Doctoral thesis , UCL (University College of London).

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Abstract

Biodiesel and ethanol-diesel blends are currently commercially available but further engine research is required so as to develop new, more efficient and environmentally friendly, alternative fuels for diesel engines. Although natural sources of feedstock can provide a wide range of potential fuel molecules, their combustion characteristics are generally not yet well understood. This PhD project contributes to the development of alternative fuels by systematically investigating in a single cylinder direct injection diesel engine the fundamental effects of chemical and physical fuel properties on combustion characteristics, exhaust gas formation and engine thermal efficiency of over 70 potential alternative fuel molecules. These molecules included oxygen-bearing ones, alkylbenzenes and diesel refinery streams. Chemical kinetic studies available in literature were applied to explain the differences in ignition delays of different molecules. The results showed that oxygenated fuels have longer ignition delays, higher levels of NOx and lower particulate emissions compared to hydrocarbon fuels. It was concluded that the shorter ignition delays of biodiesel, compared to fossil diesel fuel, which have been reported by several previous studies, are caused by the longer carbon chain of the biofuel molecules despite the oxygenated fuel structure, as well as by the lower content of aromatic and cyclic compounds. Additionally, it was observed that although adding oxygen into the fuel molecular structure lowers the overall particulate emissions, it increases the number of small particulates. Furthermore, it was found, based on experimental engine tests and a zero dimensional thermodynamic simulation, that development of alternative fuels could provide more opportunities for reducing pollutant exhaust gas emissions than improving efficiency, because molecular fuel structure does not appear to have a substantial effect on engine thermal efficiency. The results of this work can be used in the selection and development of new alternative fuels, with environmentally friendly combustion characteristics and improved thermal efficiency.

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