The U.S. Renewable Fuel Standard requires an increase in the production of ethanol and advanced biofuels up to 36 billion gallons by 2022. Ethanol will be limited to 15 billion gallons, which leaves 21 billion gallons to come from other sources. Due to its high octane number, renewable character, and minimal toxicity, ethanol was believed to be one of the most favorable alternative fuels to displace gasoline in spark ignition engines.
Replacing gasoline with ethanol results in a substantial reduction in vehicle range, and high ethanol content blends can cause material compatibility issues and require adaptive engine calibrations. In addition, ethanol is fully miscible in water which requires blending at distribution sites instead of the refinery. Higher carbon number alcohols, on the other hand, have a higher energy density and lower affinity for water than ethanol, which could mitigate some of the above mentioned issues. However, little information is available on the combustion characteristics of a majority of the longer-chain alcohols.
This study evaluates the combustion properties of higher carbon number alcohols, ranging from ethanol (C2) to hexanol (C6) in a direct-injection, spark-ignition engine. Test fuels are created by splash blending alcohols at a volumetric concentration of 50% with a blendstock for oxygenate blending. Combustion characteristics are evaluated by comparing overall efficiencies as well as heat release characteristics and emissions for a set of representative steady-state operating points. Results suggest that combustion properties of blends of alcohols with carbon numbers from two to six are similar to those of the reference fuel at low and medium engine loads. Properties of blends of alcohols with carbon numbers from two to four are similar to those of the reference fuel even at high loads. However, due to their reduced knock resistance, the suitability of longer chain alcohols, specifically C5 and longer, as blending agents at increased levels is questionable.