Impact of octane sensitivity and thermodynamic conditions on combustion process of spark-ignition to compression-ignition through an optical rapid compression machine

Abstract

Spark assistance is an effective method to improve combustion stability of homogeneous charge compression ignition (HCCI) with lean mixture. Ethanol, a renewable energy, blended with commercial fuels is a promising method to deal with problems of energy safety and greenhouse gas (GHG) emission. However, the influence of ethanol blends on spark ignition to compression ignition is not clear. To this end, this study presents experimentally fundamental investigation on the compression ignition (with and without spark assistance) of ethanol-blended gasoline surrogate fuel in an optical rapid compression machine at the equivalence ratio of 0.5. Three fuels with different octane sensitivities (S) comprising n-heptane/iso-octane/ethanol were formulated and S is the difference between research octane number (RON) and motor octane number (MON). Effective temperature of experiments ranges from 730 K to 860 K, overlapping most of the temperature region with negative temperature coefficient (NTC) of iso-octane at the corresponding equivalence ratio, and the pressure is consistence with engine-relevant operating conditions. The results show that more fuel reactivity of ethanol than iso-octane at 860 K is not attributed to the NTC behavior of iso-octane but to the reactive species generation in ethanol oxidation. There is always a positive correlation between effective pressure and knock intensity (KI). At lower temperature (<785 K), higher S results in the lower KI, while at higher temperature (860 K), the heat amount released by auto-ignition instead of S has a dominant impact on KI. Moreover, buffer gas dilution tolerance is not merely affected by laminar speed itself but determined by the intensity of heat release ahead of auto-ignition. The intensity of these heat release is proportional to flame speed and lower heating value (LHV). High S fuel is more sensitive to extra dilution at a fixed thermodynamic condition due to lower energy content, while the overall ignition delay (OID) requirement of no auto-ignition in the end gas is similar reflecting certain independence of fuel type. Medium S fuel has the better relationship between the timescale of flame propagation and auto-ignition, which is more suitable for spark-ignition to compression-ignition (SICI). This investigation provides a reference to fuel design for real engine applications.