The effects of fuel characteristics on stoichiometric spark-assisted HCCI

The characteristics of fuel lean 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 NOx emissions. In this study the characterization of stoichiometric HCCI using gasoline-like fuels was undertaken. The fuels investigated are gasoline, a 50 vol% blend of iso-butanol and gasoline (IB50), and an 85% vol blend of ethanol and gasoline (E85). A single cylinder engine operating with direct injection and spark assist combined with a fully variable hydraulic valve actuation system allowed a wide range of operating parameters to be studied. This included the effects of negative valve overlap duration, intake valve closing and valve lift. Furthermore, the interaction between fuel injection timing and spark and how they can affect the required valve timing to achieve stoichiometric HCCI combustion are also studied. A comprehensive combustion and emissions analysis is conducted using gasoline, IB50 and E85 at an engine speed of 2000rpm over a range of operating loads. 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.