Autoignition and combustion characteristics of biomass-derived fuel molecules in a spark-ignited engine

Lignocellulosic biomass is a promising renewable feedstock for the production of future transportation biofuels. However, fuels derived from biomass are vastly different in composition to traditional fossil-derived hydrocarbon fuels; e.g. the presence of oxygen and variation in molecular structure, so that little is known about the combustion behaviour and knock tendency of these fuels in spark-ignition engines. This research focused on experimentally investigating the effect of utilising a wide range of lignocellulosic biomass-derived fuel molecules, producible from hemicellulose and lignin with different chemical composition and properties, as biofuels or fuel blending components on autoignition and combustion characteristics in spark-ignition engines. The study was conducted on a single cylinder variable compression engine (Ricardo E6). For all tests, the engine was operated in spark-ignition mode at operating conditions similar to the international standard test method for Research Octane Number (RON) of spark-ignition engine fuels (ASTM D2699 – 19); fixed operating conditions of engine speed and ignition timing, 600 rpm and 13 CAD BTDC respectively, and controlled inlet coolant and oil temperatures, while air temperature before carburettor entry was intentionally varied from day-to-day to compensate for changes in atmospheric pressure. During experiments, the engine compression ratio (CR) was varied in order to investigate the effect of the molecules investigated on knock resistance of the fuel blends during lambda λ sweeps from 1.00 to 0.8. It was found that unsaturated cyclic molecules with single, or more but isolated (non-adjacent) attached oxygenated functional groups, such as anisole and 4-ethylguaiacol (4-EG), respectively, have increased resistance to knock relative to unsaturated or saturated cyclic molecules with adjacent attached oxygenated functional groups, such as guaiacol, veratrole and methylcyclohexanones. The results also showed that the ortho effect in oxygenated molecules is sensitive to the type of attached oxygenated functional groups. Moreover, for the saturated 5 carbon ring structure, the presence of an ether linkage, such as in 2-MTHF, decreased knock resistance, while the addition of a carbonyl group, such as in GVL, reversed this effect and increased it significantly