The Effects of Fuel Characteristics on Stoichiometric Spark-Assisted HCCI

The characteristics of fuel lean homogeneous charge compression ignition (HCCI) operation using a variety of fuels are well known and have been demonstrated using different engine concepts in the past. In contrast, stoichiometric operation of HCCI is less well documented. Recent studies have highlighted the benefits of operating at a stoichiometric condition in terms of load expansion combined with the applicability of three way catalyst technology to reduce NO_x emissions. In this study the characterization of stoichiometric HCCI using gasoline-like fuels was undertaken. The fuels investigated are gasoline, a 50% volume blend of iso-butanol and gasoline (IB50), and an 85% volume blend of ethanol and gasoline (E85). A single cylinder engine operating with direct injection (DI) and spark assist combined with a fully variable hydraulic valve actuation system allowed a wide range of operating parameters to be studied. The resultant fuel properties, which differed in terms of octane rating, fuel oxygenation, and heat of vaporization, show that stoichiometric HCCI is possible using a range of fuels but that these fuel characteristics do have some effect on the combustion characteristics. How these fuel properties can enable an increased engine operating envelope to be achieved, in comparison with both fuel lean HCCI and conventional spark ignited combustion, is then discussed.