The influence on combustion and emissions of alkyl chain length in gamma and delta lactones in a heavy-duty diesel engine

• A series of bio-derived gamma and delta lactones were successfully tested as standalone fuels in a heavy-duty diesel engine. • Gamma lactones exhibit shorter ignition delays than delta lactone isomers. • γ-dodecalactone exhibits a shorter ignition delay than diesel with combustion phasing more dominated by the mixing-controlled flame. • A set of lactones representable of all those tested emitted a lower particle number than fossil diesel, further decreasing with lactone size. • Deca-, undeca- and dodecalactones compared to fossil diesel all exhibit lower nitrogen oxide emissions and higher carbon monoxide emissions. Heavy-duty engines used in transport are particularly challenging to decarbonise as they cannot be easily replaced by alternative powertrains, and it remains likely that many industries will remain reliant on conventional fuels for the foreseeable future. To reduce the emissions of CO 2 from heavy-duty engines, the approach of using biofuels is of interest as this would require little modification to existing vehicle fleets. Biodiesels (fatty acid methyl esters) are already widely used as a drop in biofuel for diesel, but continual growth in supply is limited by feedstock scarcity, as vegetable oils compete with the food supply. Lactones are a class of cyclic esters which can be produced from second generation biomass, a globally abundant feedstock. As esters their chemical reactivity and stability is similar to that of common biodiesels, and previous literature suggests these compounds have desirable ignition qualities and as oxygenates lower particulate emissions. In this study, the combustion and emissions characteristics of a series of isomeric gamma and delta lactones with increasing alkyl chain lengths, from C4 to C8, as single component test fuels in a heavy-duty compression ignition engine. It was found that increasing the length of the alkyl substituent chain adjacent to the ring bound oxygen within the lactones reduced the duration of ignition delay, resulting in a smaller premixed burn fraction during combustion and decreasing emissions of nitrogen oxides, unburnt hydrocarbons and carbon monoxide. For the majority of the lactones an equivalent isomer was also tested, allowing for comparisons on between equivalent carbon count or substituent alkyl chain length. Gamma lactones compared to delta lactones of the same carbon number exhibited a shorter duration of ignition delay and apparent peak heat release rate. When comparing lactones of different ring size with the same length of the substituent alkyl chain, the delta lactones initially exhibited a shorter duration of ignition delay, longer duration of combustion and lower unburnt hydrocarbons and carbon monoxide emissions.