Analytical Assessment of C2–C8 Alcohols as Spark-Ignition Engine Fuels

The U.S. Renewable Fuel Standard (RFS2) requires a drastic increases in production of advanced biofuels up to 36 billion gallons over the next decade while corn-based ethanol will be capped at 15 billion gallons. Currently ethanol is the predominant alternative fuel and is widely distributed at 10 vol % blends in gasoline (E10). However, certain properties of ethanol make it less desirable as a blending agent in particular at higher blend levels. Therefore the engine- and vehicle-related properties of longer chain alcohols are evaluated in comparison to gasoline to determine their suitability as blending agents for spark-ignition engine fuels. This analytical study aims at providing comprehensive property data for a range of alcohol isomers with a carbon count up to C8. Relevant physical property data is used to determine the general suitability of longer chain alcohol isomers as blending agents based on factors such as melting point and boiling. Based on initial findings the scope of the study was narrowed down to alcohols in the C2–C6 range. It was determined that the engine- and combustion-relevant information is missing from the literature for a wide range of longer chain isomers. Thus fuel testing for engine-relevant properties such as lower heating value, knock resistance (RON, MON) and Reid Vapour Pressure (RVP) for alcohols up to C6 was performed as part of this study. Data suggests that the melting point of alcohols increases with increasing carbon count and all C7 and C8 isomers exhibit melting points in excess of −40 °C making their use as vehicle fuel questionable. Boiling points increase with increasing carbon count and n-structures generally have slightly higher boiling points than their respective iso-structures. Latent heat of vaporization decreases with carbon count, the mass-specific value for ethanol is triple that of gasoline, the energy specific ratio increases to a factor of 5. Alcohol fuels generally have a significantly lower RVP than gasoline, RVP decreases with increasing carbon count. Stoichiometric air demand and fuel energy content increase with carbon count. Knock resistance expressed as Research Octane Number (RON) and Motor Octane Number (MON) decreases significantly with increasing carbon count, iso-structures show increased knock resistance compared to their respective n-structures. This study is limited to analytical results as well as fuel property testing according to ASTM standards. Only properties of neat alcohols are evaluated in comparison to gasoline certification fuel, gasoline blend stock for ethanol blending and E10. The analysis of the reported properties is further focused on spark-ignition engine applications only. Future phases of this project will include the assessment of properties of multi-component blends as well as efficiency, performance and emissions testing on a modern direct-injection engine. While data for a limited number of commonly used alcohols such as ethanol and iso-butanol is available in the literature, little or no data is available for a majority of other alcohols and their isomers. In addition, engine-related data published in the past occasionally disregards the significant differences between alcohol isomers of the same chain length. This study offers a comprehensive review of physical properties of alcohols and their common isomers in the C2–C8 range as they relate to in-vehicle use and spark-ignition combustion engine application. Data presented in this paper suggests that higher alcohols have certain physical properties that might be desirable for blending with gasoline. Due to their oxygen content all alcohols have an inherent disadvantage in terms of energy content compared to non-oxygenated fuels. While this disadvantage becomes less pronounced with increasing carbon count, other less desirable properties such as a low RVP and reduced knock resistance become more dominant with longer chain length alcohols. In addition to merely evaluating properties, the selection of promising alcohols and blend levels will ultimately depend on the introduction scenario and target properties.