Experimental investigation of butanol isomer combustion in Homogeneous Charge Compression Ignition (HCCI) engines

Longer chain alcohols, such as butanol, possess major physiochemical advantages over ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. In this study, two butanol isomers (n-butanol and isobutanol) are investigated as potential fuels for Homogeneous Charge Compression Ignition (HCCI) engines. Wide ranges of intake pressure and equivalence ratio are investigated and the results are presented in comparison to ethanol and gasoline as reference fuels. Under all tested conditions, the butanol isomers require lower intake temperatures for a fixed combustion phasing, indicating higher HCCI reactivity. Both isomers show single-stage ignition behavior at all test points and behave similarly in regard to the combustion stability. Engine operation using n-butanol is slightly more stable under all conditions and misfiring occurs slightly later under very lean and naturally aspirated conditions. Similar to gasoline, n-butanol shows a higher heat release rate (HRR) at the beginning of combustion. The intermediate temperature heat release (ITHR) lowers the coefficient of variation (CoV) of IMEPg (gross indicated mean effective pressure), especially at retarded combustion timing and lean mixtures. However, the knock resistance of n-butanol is lower compared to isobutanol and the other tested fuels. The exhaust emissions of the two butanol isomers are in the same range as the two reference fuels. Overall, the results indicate that butanol is suited for use as a fuel in HCCI engines, either in neat form or in blend with gasoline.